This chapter reviews the normal physiology of neurotransmission, the molecular action and biochemistry of several major neurotransmitters and their receptors, and the toxicologic mechanisms by which numerous xenobiotics act at the molecular level. Acetylcholine, norepinephrine, epinephrine, dopamine, serotonin, γ-aminobutyric acid (GABA), γ-hydroxybutyrate (GHB), glycine, glutamate, and adenosine are neurotransmitters and neuromodulators of toxicologic interest discussed herein.

When examining molecular actions of xenobiotics on neurotransmitter systems, it quickly becomes apparent that xenobiotics rarely possess single pharmacologic actions. As examples, doxepin, in part, antagonizes voltage-gated sodium channels, voltage-gated potassium channels, histaminic H₁ and H₂ receptors, α-adrenergic receptors, muscarinic acetylcholine receptors, dopamine D₂ receptors, and GABA_A receptors; and inhibits norepinephrine, serotonin, and adenosine reuptake. Similarly, carbamazepine blocks voltage-gated sodium channels; inhibits the reuptake of norepinephrine, adenosine, and serotonin; antagonizes adenosine and muscarinic receptors; activates GABA_B receptors; and binds to benzodiazepine-binding sites on mitochondria. For obvious reasons, then, this chapter cannot include every action of every xenobiotic on the nervous system. Nor is it meant to be a complete discussion of toxidromes produced by various xenobiotics, as these are discussed in specific chapters. Rather, this chapter provides a general and basic understanding of mechanisms of action of various xenobiotics affecting neurotransmitter function and receptors, especially in the central nervous system. With this focus, the clinical effects produced are more easily understood and predicted, and specific treatments can be rationally undertaken. Given the complexity of the nervous system and the numerous actions of a given xenobiotic, it is not always clear which neurotransmitter system is producing an observed effect. Therefore, specific xenobiotics are found in several sections. An attempt is made to note the major mechanism of action of a xenobiotic, although other actions are noted when possible.

Membrane Potentials, Ion Channels, and Nerve Conduction

Membrane-bound sodium, potassium adenosine triphosphatase (ATPase) moves 3 sodium ions (Na⁺) from inside the cell to the interstitial space while pumping 2 potassium ions (K⁺) into the cell. Because the cell membrane is not freely permeable to large, negatively charged intracellular molecules, such as proteins, an equilibrium results in which the inside of the neuron is negative with respect to the outside. This typical neuronal resting membrane potential is –65 mV.

Sodium, calcium (Ca²⁺), K⁺, and chloride (Cl⁻) ions move into and out of neurons through ion channels. When moving through ion channels, which are long polypeptides comprising several subunits that span the plasma membrane multiple times, ions always move passively down electrochemical gradients. Many different ion channels are structurally comparable, sharing similar amino acid sequences.²⁹ Channels for a specific ion also vary in structure, depending on the specific subunits that have combined to form the channel. Because of structural similarity of different channels, it is not surprising that many xenobiotics are able to bind to more than one type of ion channel.

More than 40 different ion channels have been described in various nerve terminals,¹⁸² and it is estimated that a human being contains hundreds of different varieties of ion channels for Na⁺, Cl⁻, Ca²⁺, and K⁺. Most ion channels fall into 2 general classes: voltage-gated (voltage-dependent) ion channels and ligand-gated ion channels.¹⁸² Voltage-gated channels open or close in response to changes in membrane potential. Ligand-gated channels open or close when a ligand (eg, neurotransmitter) binds to and changes the conformation of the channel.

A commonly accepted model describes voltage-gated Na⁺ channels and some other voltage-gated ion channels in 3 possible states. Using Na⁺ channels as an example, the Na⁺ channel is closed at rest and impermeable to Na⁺, preventing Na⁺ from moving into the cell. When the channel undergoes activation, the channel opens, allowing Na⁺ to move intracellularly, down its electrochemical gradient. The channel then undergoes a third conformational change by becoming inactivated, preventing further influx of Na⁺. The term recovery describes the conversion of inactive channels back to the resting state, a process that requires repolarization of the cell membrane.

Depolarization of a neuron usually results from an initial inward flux of cations (Na⁺ or Ca²⁺), or prevention of K⁺ efflux. The fall in membrane potential (movement toward 0 mV) results in further activation of these voltage-gated Na⁺ channels, allowing yet a greater influx of cations. When the membrane potential falls to threshold, Na⁺ channels are activated en masse, and there is a large influx of Na⁺.

Depolarization of a segment of the neurolemma causes the adjacent neuronal membrane to reach threshold, resulting in the propagation of an action potential down the neuron. Sodium channel activation is quickly followed by inactivation, ending depolarization. Over the short term, repolarization of the neuron subsequently occurs mainly from efflux of K⁺ and some influx of Cl⁻.

Neurotransmitter Release

Neurotransmitters are chemicals that are released from nerve endings into the synapse, where they produce effects by binding to receptors on postsynaptic or presynaptic cell membranes. The receptors are also found on other neurons or effector organs such as smooth muscle. Concentrations of neurotransmitters in cytoplasm are kept low to prevent passive movement out of the nerve ending. To provide a source of neurotransmitters that is protected from cytoplasmic enzymatic degradation and that can rapidly be released, neurotransmitters are concentrated and stored within vesicles in the nerve terminal. As a wave of depolarization from Na⁺ influx reaches the nerve ending, membrane depolarization opens voltage-gated Ca²⁺ channels, allowing Ca²⁺ to move rapidly into the cell. This influx of Ca²⁺ triggers exocytosis of vesicle contents into the synapse via the snare complex. The voltage-gated Ca²⁺ channels responsible for inward Ca²⁺ currents that trigger neurotransmitter release are members of the Ca_v2 subfamily (N, P/Q, and R subtypes).^188,233 Ziconotide, is a conotoxin derivative that is used for analgesia blocking N-type Ca²⁺ channels on nociceptive neurons in the dorsal root to prevent neurotransmitter release. Cardiovascular calcium channel blockers used in clinical practice do not block these subtypes of voltage-dependent Ca²⁺ channels, but block the L-subtype. However, L-subtype Ca²⁺ channels reside elsewhere on neurons.

Vesicle Transport of Neurotransmitters

The pH inside neurotransmitter vesicles is about 5.5, which is lower than that in the cytoplasm. A vacuolar ATPase in the vesicular membrane is responsible for movement of protons into the vesicular lumen at the expense of ATP hydrolysis. Vesicular uptake pumps (transporters) that move neurotransmitters or their precursors from the cytoplasm into the vesicle lumen, in turn, are powered by the electrochemical H⁺ gradient. That is, the movement of an H⁺ out of the vesicle into the cytoplasm is coupled to the movement of a neurotransmitter from the cytoplasm into the vesicle.

Various vesicular transporters for neurotransmitters have been sequenced to date. Vesicular GABA transporter (VGAT) (also known as VIAAT) transports GABA and glycine. Vesicular monamine transporter (VMAT2) transports all 3 monoamines, dopamine, norepinephrine, and serotonin (VMAT1 transports monoamines into non-neuronal vesicles). Vesicular acetylcholine transporter (VAChT) is responsible for acetylcholine (ACh) transport, and 3 vesicular glutamate transporter (VGluTs) (VGluT1-3) move glutamate into vesicles.

Neurotransmitters are confined within the vesicle, to a great extent, by ion trapping, as they are more ionized and less able to diffuse back out of the vesicle at the lower pH. Anything that causes a decrease in the pH gradient across the vesicle membrane results in the movement of neurotransmitters into the cytoplasm.²⁷⁴ For example, amphetamine moves into vesicles, where it buffers protons, causing the movement of monoamine neurotransmitters out of vesicles, and raising cytoplasmic concentrations of neurotransmitters, and ultimately raising the synaptic monoamine concentrations.^274,275

Neurotransmitter Reuptake

Although ACh is inactivated in the synapse by enzymatic degradation, other neurotransmitters have their synaptic effects terminated by active uptake into neurons or glial cells. These plasma membrane neurotransmitter transporters are distinct from those transporters responsible for movement of neurotransmitters into vesicles. Cell membrane transporters for different neurotransmitters are Na⁺-dependent transport proteins, during which the uptake of neurotransmitters is accompanied by the movement of Na⁺ across the synaptic membrane.⁵ These uptake transporters are commonly known as either uptake or reuptake pumps; the term “reuptake” will be used in this chapter for purposes of consistency.

Neurotransmitter reuptake transporters are subdivided into 2 main families.⁵ One family (SLC6) includes structurally similar uptake pumps for GABA, glycine, norepinephrine, dopamine, and serotonin. They generally comprise 600 to 700 amino acids and form loops spanning the plasma membrane 12 times. Four GABA reuptake transporters (GAT-1 through GAT-4) transport GABA into neurons and glial cells. Reuptake of dopamine, serotonin, and norepinephrine is achieved by DAT, SERT, and NET respectively. Glycine uptake into neurons and astrocytes is achieved by GLYT-1 and GLYT-2.

The second family (SLC1) comprises 5 glutamate reuptake transporters (excitatory amino acid transporters; EAATs), which appear to traverse the plasma membrane 10 times and move glutamate from the synapse into glial cells and neurons.

Several properties make transporter proteins of particular toxicologic significance. First, they are capable of moving neurotransmitters in either direction; when cytoplasmic neurotransmitter concentrations are significantly elevated, neurotransmitters are transported back into the synapse. Second, these transporters are not always completely specific. For instance, the uptake transporter for norepinephrine moves dopamine and other bioactive amines into the neuron. Third, a xenobiotic that acts at the level of the membrane transporter affects functions of several different neurotransmitters, depending on its specificity for a particular transporter. As an example, fluoxetine is fairly specific at inhibiting reuptake of serotonin, whereas cocaine inhibits the reuptake of serotonin, norepinephrine, and dopamine.

Neurotransmitter Receptors

Channel Receptors

The first general class of neurotransmitter receptors comprises ligand-gated ion channels (LGICs; channel receptors; ionotropic receptors), in which the receptor for the neurotransmitter is part of an ion channel. These channels comprise multiple subunits that combine in various combinations to create channels that vary in response to a given neurotransmitter or other agonist/antagonist. Ligand-gated ion channels for neurotransmitters discussed in this chapter are divided into 2 main groups, based on structure of subunits and assemblies. Most neurotransmitter LGICs are pentameric and display 4 transmembrane helices. Ligand-gated ion channels for glutamate, however, are tetrameric and comprise 3 transmembrane helices.¹⁷³

By binding to its LGIC receptor, the neurotransmitter allosterically changes the configuration of the ion channel so that ions traverse the channel in greater quantities per unit time. As an example, the acetylcholine nicotinic ACh receptor at the neuromuscular junction is a ligand-gated Na⁺ channel. When acetylcholine binds to the nicotinic receptor, the configuration of the channel changes, allowing Na⁺ to move into the cell and trigger an action potential. The action potential then propagates down muscle via voltage-gated Na⁺ channels. Table 13–1 lists other examples of channel receptors.

G Protein–Coupled Receptors

The second general class of neurotransmitter receptors are linked to G proteins, which are part of a superfamily of proteins with guanosine triphosphatase (GTPase) activity responsible for signal transduction across plasma membranes.²⁶⁴ G proteins comprise 3 polypeptide subunits: α, β, and γ chains. These chains span the plasma membrane several times, and they associate with a separately transcribed neurotransmitter receptor that spans the cell membrane 7 times, with an external binding site for neurotransmitters. Some receptors (eg, GABA_B receptor) coupled to G proteins are obligatory heterodimers comprising 2 separate proteins, both of which must be present for activity.

The α chain normally binds guanosine diphosphate (GDP) in the cytoplasm and is inactive. When a neurotransmitter binds to its receptor externally on the cell membrane, GDP dissociates from the α chain and guanosine triphosphate (GTP) binds in its place, activating the α subunit. The activated α subunit then dissociates from the receptor and from the β-γ chains. Both the activated α subunit and β-γ subunits modulate effectors in the plasma membrane.²⁶⁴ The effector influenced by α or β-γ subunits is frequently an enzyme that the subunits stimulate or inhibit (eg, adenylate cyclase) or an ion channel that is opened or closed directly or through other chemical reactions (eg, channel phosphorylation).⁴⁸ Intrinsic GTPase activity in the α chain eventually converts the GTP to GDP, inactivating the α subunit and allowing it to reassociate with the β-γ chains and the neurotransmitter receptor, terminating the consequences of neurotransmitter binding.²⁶⁴

The type of the α chain that the G proteins contain determines the categorization. The 3 main families of G proteins coupled to neurotransmitter receptors are G_s (containing α_s), G_i/o (containing α_i or α_o), and G_q (or G_q/11, containing α_q). G_s stimulates membrane-bound adenylate cyclase; activation of a neurotransmitter receptor coupled to G_s causes a rise in intracellular 3′,5′-cyclic adenosine monophosphate (cAMP) concentration. Neurotransmitter receptors activating G_i may inhibit adenylate cyclase or modulate K⁺ and Ca²⁺ channels. Receptors coupled to G_q act through membrane-bound phospholipase C to increase intracellular calcium concentrations. See Table 13–1 for a list of the neurotransmitter receptors coupled to G proteins. A given neurotransmitter can activate different classes of receptors (eg, ion-channel and G protein) or different types of receptors in the same class. For example, GABA_A receptors are Cl⁻ channels, whereas GABA_B receptors are coupled to G proteins. Dopamine D₁–like receptors (D₁ and D₅) are linked to G_s, whereas D₂-like receptors (D₂, D₃, and D₄) are most commonly linked to G_i or G_o.

Importantly, some single G protein–coupled receptors activate more than one type of G protein in the same cell, depending on various circumstances, including duration of receptor activation. Evidence continues to accumulate indicating that G protein–coupled receptors for the same or different neurotransmitters form both heterodimers and oligomers, resulting in yet more variation in response to neurotransmitter binding. Examples include combinations of various dopamine receptors, combinations of dopamine and 5-HT receptors, and combinations of dopamine and glutamate receptors.^189,191,245

Receptor downregulation occurs at various levels. For example, prolonged receptor activation results in receptor phosphorylation by G protein kinases (GPKs), causing receptor binding to a member of the arrestin family of proteins. This, in turn, prevents further activation of the G protein by the receptor and eventually triggers receptor endocytosis.^7,150,259

Neuronal Excitation and Inhibition

Excitatory neurotransmitters usually act postsynaptically by causing Na⁺ or Ca²⁺ influx, or by preventing K⁺ efflux, triggering depolarization and an action potential (Fig. 13–1). These effects usually are mediated by either ion channel or G protein–coupled receptors.

Postsynaptic inhibition results from actions on ion channel receptors or receptors coupled to G proteins (Fig. 13–1). Inhibition is usually accomplished by neuronal influx of Cl⁻ or efflux of K⁺ to hyperpolarize the neuron and move membrane potential farther away from threshold, making it more difficult for a given stimulus to depolarize the membrane to threshold voltage.

Presynaptic inhibition, the prevention of neurotransmitter release, is usually mediated by receptors coupled to G proteins, but ligand-gated ion channels such as kainate glutamate receptors are found on neuronal terminals, as well. When a neurotransmitter released from a neuron binds to a receptor on that same neuron to limit further neurotransmitter release, the receptor is termed an autoreceptor.²⁵² Autoreceptors reside on dendrites, cell bodies, axons, and presynaptic terminals. Activation of autoreceptors on dendrites and cell bodies (somatodendritic autoreceptors) usually impairs neurotransmitter release by increasing K⁺ efflux, thereby hyperpolarizing the neuron away from threshold (Fig. 13–2). However, activation of autoreceptors found on presynaptic terminals (terminal autoreceptors) usually limits increases in intracellular Ca²⁺ concentration by limiting Ca²⁺ influx or preventing Ca²⁺ release from intracellular stores, impairing exocytosis of neurotransmitter vesicles (Fig. 13–2). Types of neurotransmitter receptors that serve as autoreceptors also usually reside postsynaptically, where they may mediate different physiologic effects.

Inhibition of neurotransmitter release from nerve terminals is not limited to actions by autoreceptors. Presynaptic terminal inhibitory receptors for various neurotransmitters are sometimes found on a single neuron (heteroreceptors). For example, stimulation of presynaptic α₂ receptors found on postganglionic parasympathetic nerve terminals prevents ACh release.

Finally, stimulation of receptors on presynaptic nerve endings sometimes enhances, rather than inhibits, neurotransmitter release. Such receptors also are usually coupled to G proteins. For example, activation of β₂ receptors on adrenergic nerve terminals enhances norepinephrine release.

Acetylcholine (ACh) is a neurotransmitter of the central and peripheral nervous system. Centrally, it is found in both brain and spinal cord; cholinergic fibers project diffusely to the cerebral cortex. Peripherally, ACh serves as a neurotransmitter in autonomic and somatic motor fibers (Fig. 13–3).

Synthesis, Release, and Inactivation

Acetylcholine is synthesized from acetylcoenzyme A and choline by the enzyme choline acetyltransferase. Acetylcholine moves into synaptic vesicles via the vesicular membrane transporter, VAChT, where it is stored before release into the synapse by Ca²⁺-dependent exocytosis. Acetylcholine undergoes enzymatic degradation in the synapse to choline and acetic acid by acetylcholinesterase. A Na⁺-dependent transporter in the neuronal membrane (ChT) then pumps choline back into the cytoplasm to be used again as a substrate for ACh synthesis (Fig. 13–4). Pseudocholinesterase (also known as plasma cholinesterase or butyryl cholinesterase) is made in the liver and plays no role in the degradation of synaptic ACh. However, it does metabolize some xenobiotics, including cocaine and succinylcholine.

Acetylcholine Receptors

Nicotinic Receptors

After release from cholinergic nerve endings, ACh activates 2 main types of receptors: nicotinic and muscarinic.¹⁵¹ Nicotinic receptors (nAChRs) reside in the CNS (mainly in spinal cord), on postganglionic autonomic neurons (both sympathetic and parasympathetic), in the adrenal medulla, and at skeletal neuromuscular junctions, where they mediate muscle contraction (Fig. 13–3).

Nicotinic receptors at neuromuscular junctions (NMJ nAChRs) are part of a Na⁺ channel made from 5 protein subunits. Stimulation of these receptors by ACh results mainly in Na⁺ influx, depolarization of the endplate, and triggering of an action potential that is propagated down the muscle by voltage-gated Na⁺ channels.

Nicotinic receptors on central or peripheral neurons or in the adrenal gland are termed neuronal nAChRs. Neuronal nAChRs are also ion channels, although in some cases Ca²⁺ influx through the receptor may be more important than Na⁺ influx. Neuronal nAChRs also comprise 5 subunits.

Muscarinic Receptors

Muscarinic receptors reside in the CNS (mainly in the brain), on end organs innervated by postganglionic parasympathetic nerve endings, and at most postganglionic sympathetically innervated sweat glands (Fig. 13–3). Five subtypes of muscarinic receptors, M_1–5, are recognized and linked to several G proteins. For example, in the heart, ACh released from the vagus nerve binds to M₂ receptors linked to G_i in pacemaker cells. G_i opens K⁺ channels, allowing efflux of K⁺ down its concentration gradient, which makes the inside of the cell more negative and more difficult to depolarize, slowing the heart rate. Different subtypes of muscarinic receptors also act as autoreceptors in various locations, M₁ being the most common.

Xenobiotics

Table 13–2 provides examples of xenobiotics that affect cholinergic neurotransmission.

Modulators of Acetylcholine Release

Figure 13–4 illustrates sites of actions of numerous xenobiotics that influence the cholinergic nervous system. Botulinum toxins, some neurotoxins from pit vipers, and elapid β-neurotoxins prevent release of ACh from peripheral nerve endings.¹⁰¹ This results in ptosis, other cranial nerve findings, weakness, and respiratory failure. Hypermagnesemia also inhibits ACh release, probably by inhibiting Ca²⁺ influx into the nerve endings.¹⁵¹

Guanidine, aminopyridines, and black widow spider venom enhance the release of ACh from nerve endings. Aminopyridines, in part, block voltage-gated K⁺ channels to prevent K⁺ efflux; the resultant action potential widening (delayed repolarization) causes prolongation of Ca²⁺-channel activation, enhancing influx of Ca²⁺, which promotes neurotransmitter release. Aminopyridines are used therapeutically in Lambert–Eaton syndrome, myasthenia gravis, multiple sclerosis, and experimentally in Ca²⁺-channel channel blocker overdose.

Black widow spider venom causes ACh release by opening neuronal Ca²⁺– channels with resultant muscle cramping and diaphoresis.¹²

Nicotinic Receptor Agonists and Antagonists

Xenobiotics that bind to and activate nicotinic receptors stimulate postganglionic sympathetic and parasympathetic neurons, skeletal muscle end-plates, the adrenal medulla, and/or neurons within the CNS (Fig. 13–3). Prolonged depolarization at the receptor eventually causes diminution of responses to receptor occupancy.²¹⁴ For example, poisoning by nicotine, both a neuronal and NMJ nAChR agonist, produces hypertension, tachycardia, vomiting, diarrhea, muscle fasciculations, and convulsions, followed by hypotension, bradydysrhythmias, paralysis, and coma. Succinylcholine is a neuromuscular blocker that initially stimulates and then blocks muscular activity through prolonged depolarization of NMJ nAChRs.

Xenobiotics that block NMJ nAChRs without stimulation at skeletal neuromuscular junctions produce weakness and paralysis. Examples include tubocurarine and atracurium. α-Neurotoxins from elapids (eg, α-bungarotoxin) directly antagonize NMJ nAChRs, producing ptosis, weakness, and respiratory failure from paralysis.²⁹⁶

Peripheral neuronal nAChR blockade produces autonomic ganglionic blockade. Trimethaphan was used as a pharmacologic ganglionic blocker; however, it is not entirely specific for neuronal nAChRs. Occasionally, trimethaphan caused weakness and paralysis from NMJ nAChR blockade.

The functions of neuronal nAChRs are modulated by a variety of xenobiotics that do not bind to the ACh binding site, but bind instead to a number of distinct allosteric sites. For example, aside from their ability to inhibit acetylcholinesterase, physostigmine, tacrine, and galantamine bind to a noncompetitive allosteric activator site on neuronal nAChRs to enhance channel opening and ion conductance. Furthermore, a diverse range of xenobiotics, including chlorpromazine, phencyclidine, ketamine, local anesthetics, and ethanol bind to a noncompetitive negative allosteric site(s) on nAChRs to inhibit inward ion fluxes without directly affecting ACh binding. Some steroids desensitize neuronal nAChRs by binding to yet an additional allosteric site.²¹⁹

Muscarinic Receptor Agonists and Antagonists

Peripheral muscarinic agonists produce bradycardia, miosis, salivation, lacrimation, vomiting, diarrhea, bronchospasm, bronchorrhea, and micturition. Central muscarinic agonists produce sedation, extrapyramidal dystonias, rigidity, coma, and convulsions. The anticholinergic toxidrome results from blockade of muscarinic receptors and is more appropriately referred to as an antimuscarinic toxidrome.²⁵⁵ Central nervous system muscarinic blockade produces confusion, agitation, myoclonus, tremor, picking movements, abnormal speech, hallucinations, and coma. Peripheral antimuscarinic effects include mydriasis, anhidrosis, tachycardia, and urinary retention. Muscarinic antagonists number in the hundreds (Table 13–2 for common examples).

Acetylcholinesterase Inhibition

Xenobiotics inhibiting acetylcholinesterase (ie, anticholinesterase) raise ACh concentrations at both nicotinic and muscarinic receptors, producing a variety of CNS, sympathetic, parasympathetic, and skeletal muscle signs and symptoms.⁵⁰ Anticholinesterases include organic phosphorus compounds and N-methylcarbamates. Organic phosphorus compounds are usually encountered as insecticides, although topical medicinal organic phosphorus compounds are used for the treatment of glaucoma and lice. N-Methyl-carbamates are found as insecticides and pharmaceuticals. Medicinal N-methylcarbamates include physostigmine, pyridostigmine, rivastigmine, and neostigmine. Edrophonium, galantamine, tacrine, donepezil, and metrifonate are non-carbamate, reversible anticholinesterases.

α₂-Adrenergic Receptor Agonists and Antagonists

Agonists and antagonists of α₂-adrenergic receptors are discussed in detail below. Briefly, stimulation of presynaptic α₂-adrenergic receptors on postganglionic parasympathetic nerve endings decreases ACh release. Conversely, presynaptic α₂ antagonism increases ACh release (Fig. 13–4).

Norepinephrine (NE), epinephrine (EPI), dopamine (DA), and serotonin (5-hydroxytryptamine {5-HT}) are similar in many respects. Neurotransmitter synthesis, vesicle transport and storage, uptake, and degradation share many enzymes and structurally similar transport proteins. Cocaine, reserpine, amphetamine, and monoamine oxidase inhibitors (MAOIs) affect all four types of neurons. In addition, these xenobiotics produce several different effects in the same system. For example, in the noradrenergic neuron, amphetamines work mainly by causing the release of cytoplasmic NE, but they also inhibit NE reuptake, and their metabolites inhibit monoamine oxidase. Actions of xenobiotics that affect all bioactive amine neurotransmitters are described in the most detail for noradrenergic neurons. For the sake of brevity, similar mechanisms of action are simply noted in discussions of dopaminergic and serotonergic neurotransmission.

Norepinephrine is released from postganglionic sympathetic terminals (Fig. 13–3). The adrenal gland, acting as a modified sympathetic ganglion, releases EPI and lesser amounts of NE in response to stimulation of neuronal nAChRs. Epinephrine-containing neurons also reside in the brain stem.

The locus ceruleus is the main noradrenergic nucleus and resides in the floor of the fourth ventricle on each side of the pons. Axons radiate from this nucleus out to all layers of the cerebral cortex, to the cerebellum, and to other structures. Norepinephrine demonstrates both excitatory and inhibitory actions in the CNS. Norepinephrine released from locus ceruleus projections in the hippocampus increases cortical neuron activity through β-adrenergic receptor activation and G protein–mediated inhibition of K⁺ efflux. Norepinephrine released in outer cortical areas produces inhibitory effects mediated by α₂-adrenergic receptor activation. Consistent with this, NE demonstrates antiepileptic actions in animals. The antiepileptic action of carbamazepine may partly result from inhibition of NE uptake.⁷⁶ Despite antagonistic actions on different cortical neurons, electrical stimulation of the locus ceruleus produces widespread cortical activation and excitation. This overall effect explains a great deal of the hyperattentiveness and lack of fatigue that accompanies use of xenobiotics that mimic or increase noradrenergic activity in the brain. Neuronal firing in the locus ceruleus increases during waking and dramatically falls during sleep.

Synthesis, Release, and Reuptake

Figure 13–5 is a representation of a noradrenergic neuron. Tyrosine hydroxylase is the rate-limiting enzyme in NE synthesis and is sensitive to negative feedback by NE. This enzyme requires Fe²⁺ as a cofactor and exists as a homotetramer that is upregulated by chronic exposure to caffeine and nicotine. Under normal dietary conditions, tyrosine hydroxylase is completely saturated by tyrosine, and increasing dietary tyrosine does not appreciably increase dopa synthesis. Dopa undergoes decarboxylation by L-amino acid decarboxylase (DOPA decarboxylase) to DA. DOPA decarboxylase is not specific for DOPA. For example, it also catalyzes the formation of serotonin from 5-HT.

About one-half of cytoplasmic DA is actively pumped into vesicles by VMAT2. The remaining DA is quickly deaminated. In the vesicle, DA is converted to NE by dopamine-β-hydroxylase. Vesicles isolated from peripheral nerve endings contain DA, NE, dopamine-β-hydroxylase, and ATP, and all of these substances are released into the synapse during Ca²⁺-dependent exocytosis triggered by neuronal firing. In neurons containing EPI as a neurotransmitter, NE is released from vesicles into the cytoplasm, where it is converted to EPI by phenylethanolamine-N-methyl-transferase. Epinephrine is then transported back into vesicles before synaptic release.¹⁵¹

Norepinephrine is removed from the synapse, mainly by reuptake into the presynaptic neuron by the NE transporter (NET). Although this transporter has great affinity for NE, it also transports other amines, including DA, tyramine, MAOIs, and amphetamines. Once pumped back into the cytoplasm, NE is either transported back into vesicles for further storage and release, or quickly degraded by monoamine oxidase (MAO), an enzyme expressed on the outer mitochondrial membrane.

Monoamine oxidase (MAO) is present in all human tissues except red blood cells. It exists as 2 isoenzymes, MAO-A and MAO-B,¹⁸¹ each with partially overlapping substrate affinities (Table 13–3). Neuronal MAO, bound to the outer mitochondrial membrane, oxidatively deaminates cytoplasmic amines, including neurotransmitters, to attenuate elevated cytoplasmic concentrations of bioactive amines. Hepatic and intestinal MAO prevents large quantities of dietary bioactive amines from entering the circulation and producing systemic effects.

Catechol-O-methyltransferase (COMT) mainly exists as a membrane-bound enzyme widely distributed throughout the body, including the central nervous system, which is responsible for metabolism of DA, L-dopa, NE, and EPI. It remains controversial as to whether enzymatic activity is focused on extracellular (synaptic) or cytoplasmic degradation of catecholamines. In extraneuronal tissue, COMT metabolizes catecholamines, including those that have entered the systemic circulation.

Adrenergic Receptors

The 2 main types of adrenergic receptors are α-adrenergic receptors and β-adrenergic receptors. All adrenergic receptors are coupled to G proteins.²²²

β-Adrenergic Receptors

β-Adrenergic receptors are divided into 3 major subtypes (β₁, β₂, and β₃), depending on their affinity for various agonists and antagonists.²⁵⁷ β₁-adrenergic receptors and β₂-adrenergic receptors are linked to G_s, and their stimulation raises cAMP concentration and/or activates protein kinase A, which, in turn, produces several effects, including regulation of ion channels. At least some β₃-adrenergic receptors are coupled to G_i/o proteins. Upregulation of β₃ receptors responsible for a negative inotropic effect in diseased myocardium appears to protect against adverse effects of chronic excessive catecholamine stimulation. β₃-receptor activation by nebivolol probably is responsible for nitric oxide production in some vascular beds.⁸⁸

The β-adrenergic receptors are polymorphic, with genetic variation in humans.^41,121 Polymorphism influences response to medications, regulation of receptors, and clinical course of disease.^41,123,156 In general, peripheral β₁-adrenergic receptors are found mainly in the heart and in the juxtaglomerular apparatus. β₂-adrenergic receptors also reside in the heart as well as other organs where their activation mediates additional adrenergic effects such as bronchodilation and vasodilation.¹²³ Presynaptic β₂-adrenergic receptor activation causes release of NE from nerve endings (positive feedback). β₃-adrenergic receptors reside mainly in fat, but they also reside in skeletal muscle, gallbladder, and colon where they regulate metabolic processes. Polymorphism of β₃-adrenergic receptors is associated with clinical expressions of non–insulin-dependent diabetes and obesity.^41,273,298

α-Adrenergic Receptors

α-Adrenergic receptors are linked to G proteins that inhibit adenylate cyclase to affect cAMP concentrations, affect ion channels, increase intracellular Ca²⁺ through inositol triphosphate and diacylglycerol production, or produce other actions.²²² These receptors are divided into 2 main types, α₁ and α₂, and at least 6 subtypes—α_1A, α_1B, α_1D, α_2A, α_2B, and α_2C.^68,110,257 Most α₁-adrenergic receptors are coupled to G_q, whereas most α₂-adrenergic receptors are coupled to G_i/o, although at least some α_2A-adrenergic receptors are coupled to G_s. The roles of α₂ receptor subtypes are not fully elucidated. Receptors of the α_2A type are the predominant form found in the CNS and are responsible mediating sedative, hypotensive and analgesic effects from α₂ agonists.^120,213

In peripheral tissue, α₁-adrenergic receptors reside on the postsynaptic membrane in continuity with the synaptic cleft. Stimulation of these receptors on blood vessels results in vasoconstriction.

Central and peripheral α₂-adrenergic receptors are found at presynaptic and postganglionic sites. Presynaptic α₂-adrenergic receptor activation mediates negative feedback, limiting further release of NE (Fig. 13–5). Postganglionic parasympathetic neurons also contain presynaptic α₂-adrenergic receptors that, when stimulated, prevent release of ACh (Fig. 13–4).

Postsynaptic α₂-adrenergic receptors on vasculature also mediate vasoconstriction.²²² Initially, it was suggested that postsynaptic α₂-adrenergic receptors resided mainly outside of the synapse and mediated vasoconstrictive responses to circulating α-adrenergic agonists such as NE, whereas postsynaptic α₁-adrenergic receptors responded to NE released from nerve endings. However, in at least some tissues (eg, saphenous vein), NE released following nerve stimulation produces vasoconstriction through action at α₂-adrenergic receptors, making the previous differentiation not as distinct.^68,135 Because both α₁-adrenergic receptors and α₂-adrenergic receptors on non-cerebral vasculature mediate vasoconstriction, a patient with hypertension from high concentrations of circulating catecholamines (eg, pheochromocytoma or clonidine withdrawal) or from extravasation of NE from an intravenous line commonly requires both α₁-adrenergic receptor and peripheral α₂-adrenergic receptor blockade to vasodilate adequately (eg, phentolamine).

Activation of postsynaptic α₂-adrenergic receptors in the brain stem inhibits sympathetic output and produces sedation. α_2A-Adrenergic heteroreceptor activation on non-noradrenergic cells is important in explaining sedation, antinociception, hypothermia, effects on cognition, and centrally mediated bradycardia and hypotension in response to administration of α₂ agonists (Fig. 13–6).⁹³ Dexmedetomidine, an imidazole and potent α_2A-adrenergic receptor agonist, is used for sedation in intensive care patients, although hypotension and bradycardia occur as expected side effects.²⁷

Xenobiotics

Xenobiotics producing pharmacologic effects that result in or mimic increased activity of the adrenergic nervous system are sympathomimetics (Table 13–4). Those with the opposite effect are sympatholytics (Table 13–5).

Sympathomimetics

Direct-Acting

Xenobiotics whose sympathomimetic actions result from direct binding to α-adrenergic receptors or β-adrenergic receptors are called direct-acting sympathomimetics. Most do not cross the blood–brain barrier in significant quantities.

Indirect-Acting

Xenobiotics that produce sympathomimetic effects by causing the release of cytoplasmic NE from the nerve ending in the absence of vesicle exocytosis are called indirect-acting sympathomimetics. Amphetamine is the prototype of indirect-acting sympathomimetics and is used for the discussion of what is known about their mechanisms of action.⁴⁵ In general, mechanisms of indirect release of NE by amphetamines, cocaine, phencyclidine, MAOIs, and mixed-acting xenobiotics noted in Table 13–4 are similar in that their actions depend on their ability to produce elevated cytoplasmic NE concentrations.

Amphetamine and structurally similar indirect-acting sympathomimetics move into the neuron mainly by the membrane transporter that transports NE into the neuron. Lipophilic indirect-acting sympathomimetics frequently move into the neuron by diffusion. From the cytoplasm, amphetamine is transported into neurotransmitter vesicles, where it buffers protons to raise intravesicular pH. As noted earlier, much of the ability of the vesicle to concentrate NE (and other neurotransmitters) is a result of ion trapping of NE at the lower pH. The rise in intravesicular pH produced by amphetamine causes NE to leave the vesicle and move into the cytoplasm.^274,275 This movement results from diffusion or reverse transport of NE by VMAT2. In the cytoplasm, amphetamine also competes with NE and DA for transport into vesicles, which further contributes to elevated cytoplasmic NE concentrations. In the case of amphetamine, the rise in cytoplasmic concentrations of NE is further enhanced by the ability of amphetamine metabolites to inhibit MAO, which impairs NE degradation.

Every time the Na⁺-dependent uptake transporter, NET, moves a bioactive amine (eg, tyramine) into the neuron from which it is released, a binding site for NE on NET conceptually transiently faces inward and becomes available for reverse transport of NE out of the neuron. The normally low concentration of cytoplasmic NE prevents significant reverse transport. In the face of elevated cytoplasmic NE concentrations produced by indirect-acting sympathomimetics, NET moves NE out of the neuron and back into the synapse, where the neurotransmitter activates adrenergic receptors (indirect action). This process is sometimes referred to as facilitated exchange diffusion or displacement of NE from the nerve ending. Evidence supporting reverse transport produced by amphetamine is that inhibitors of the transporter (eg, tricyclic antidepressants) prevent amphetamine induced NE release.

While all indirect-acting sympathomimetics cause reverse NE transport by increasing cytoplasmic NE concentrations, those that move into the neuron by the membrane transporter (eg, amphetamines, MAOIs, DA, tyramine) further enhance reverse transport because their uptake causes more NE-binding sites on NET to face inward per unit time.

Although cocaine inhibits NET, it also causes some NE release. In fact, cocaine similarly lessens pH gradients across vesicle membranes to raise cytoplasmic concentrations of NE.²⁷⁵ That cocaine produces less NE release than amphetamines is partly explained by cocaine induced inhibition of the membrane transporter and possibly by the fact that cocaine does not move into the neuron by active uptake (ie, does not increase the number of NE-binding sites facing inward), but diffuses into the neuron. In fact, most of the severe sympathomimetic effects of cocaine appear to result from action on the brain rather than peripheral nerve endings.²⁸⁴

Phencyclidine (PCP) is a hallucinogen that possesses multiple pharmacologic actions. Like toxicity from many hallucinogens, PCP toxicity is accompanied by increased adrenergic activity, which results, in part, from PCP induced decreases in pH gradients across the vesicle membrane²⁷⁵ and indirect release of NE. Like cocaine, PCP moves into the neuron by diffusion rather than uptake through the membrane transporter, at least partly explaining less PCP induced NE release than which contains typically occurs in amphetamine poisoning.

In addition to causing ACh release, black widow spider venom α-latrotoxin causes vesicle exocytosis of NE, producing hypertension and diaphoresis over the palms, soles, upper lip, and nose. All of the aforementioned indirectly acting sympathomimetics, except α-latrotoxin, enter the CNS.

Mixed-Acting

Mixed-acting sympathomimetics act directly and indirectly. For example, ephedrine indirectly causes NE release and directly activates adrenergic receptors. Intravenously administered DA indirectly causes NE release, explaining most of its vasoconstricting activity, but also directly stimulates dopaminergic and β-adrenergic receptors. Direct α-agonism occurs at high doses. Except for DA, these xenobiotics cross the blood–brain barrier to produce central effects.

Reuptake Inhibitors

Inhibitors of NE reuptake raise concentrations of NE in the synapse to produce excessive stimulation of adrenergic receptors.

There are 2 main means by which inhibitors of bioactive amine inhibit reuptake: competitive and noncompetitive. Noncompetitive inhibitors, such as cyclic antidepressants, carbamazepine, venlafaxine, methylphenidate, and cocaine, bind at or near the carrier site on NET to prevent NET from moving NE and similar xenobiotics into or out of the neuron. Various xenobiotics used for their antimuscarinic effects also block NET noncompetitively. These include atomoxetine, benztropine, diphenhydramine, trihexyphenidyl, and orphenadrine.¹⁹³

The second mechanism, competitive inhibition of NET, characterizes most indirect-acting sympathomimetics, including amphetamine and structurally similar xenobiotics (eg, mixed-acting agents, MAOIs). These xenobiotics prevent NE reuptake by competing with synaptic NE for binding to the carrier site on NET, a mechanism by which they move into the neuron. In fact, an additional adrenergic action of amphetamines, mixed-acting xenobiotics, MAOIs, and tyramine is to raise synaptic NE concentrations by competing for uptake, thereby compounding their indirect or direct actions.

MAOIs

Monoamine oxidase inhibitors (MAOIs) are transported by NET into the neuron, where in which they act through several mechanisms (Table 13–4).^3,181 Inhibition of MAO, their main pharmacologic effect, results in increased cytoplasmic concentrations of NE and some indirect release of neurotransmitter into the synapse. As a minor effect they also displace NE from vesicles by raising pH in a manner similar to amphetamines. These actions explain the initial sympathomimetic findings following MAOI overdose and also account for occasional and unpredictable adrenergic crises that occur despite dietary compliance.

Nonspecific MAOIs inhibit both enzymes of MAO, preventing intestinal and hepatic degradation of bioactive amines. A person taking such an MAOI who then is exposed to indirect-acting sympathomimetics (eg, tyramine in cheese, ephedrine, DA, amphetamines) has a much larger cytoplasmic concentration of NE to transport into the synapse and, therefore, develops central and peripheral sympathomimetic findings. Monoamine oxidase inhibitors (MAOIs) specific for the MAO-B enzyme are less likely to predispose to food or drug interactions by maintaining significant hepatic and intestinal MAO activity. Furthermore, reversible MAO-A specific inhibitors are also less likely to provoke this reaction because their reversibility allows competition of exogenous amines with the inhibitor, resulting in its displacement from the enzyme and normal metabolism of the bioactive amines.³⁰³ Isoenzyme specificity is lost as the dose of the MAOI is increased. In fact, selegiline, currently marketed as a selective MAO-B inhibitor, partially inhibits MAO-A activity at therapeutic doses. Rasagiline is a newer MAO-B inhibitor that has 100-fold greater affinity for MAO-B than MAO-A, but still loses specificity at supratherapeutic doses.^96,109 Monoamine oxidase inhibitors (MAOI) enzyme specificity is of lesser importance when indirect-acting xenobiotics are administered parenterally (eg, intravenous DA or amphetamines). Linezolid is an antibiotic that displays nonspecific MAO inhibition,²⁰¹ while methylene blue mainly inhibits MAO-A.²⁵³

Occasionally, patients suffering from refractory depression respond to a combination of MAOIs and tricyclic antidepressants. This combination therapy is usually unaccompanied by excessive adrenergic activity because the inhibition of the membrane uptake transporter by the tricyclic antidepressant attenuates excessive reverse transport of elevated cytoplasmic NE concentrations produced by MAOIs.

COMT Inhibitors

Inhibitors of COMT are administered in the treatment of Parkinson disease to prevent the catabolism of concomitantly administered L-dopa. Entacapone only acts peripherally, whereas tolcapone also crosses the blood–brain barrier where it also prevents DA degradation as well.

α₂-Adrenergic Receptor Antagonists

Yohimbine produces selective competitive antagonism of α₂-adrenergic receptors to produce a mixed clinical picture. Peripheral postsynaptic α₂ blockade produces vasodilation. Blockade of presynaptic α₂-adrenergic receptors on cholinergic nerve endings (Fig. 13–4) enhances ACh release, occasionally producing bronchospasm¹⁴⁶ and contributing to diaphoresis. Similar presynaptic actions on peripheral noradrenergic nerves enhance catecholamine release (Fig. 13–5). Antagonism of central α₂-adrenergic receptors in the locus ceruleus results in CNS stimulation, whereas blockade of postsynaptic α₂-adrenergic receptors in the nucleus tractus solitarius may enhance sympathetic output (Fig. 13–6). The final result includes hypertension, tachycardia, anxiety, fear, agitation, mania, mydriasis, diaphoresis, and bronchospasm.¹⁵⁸ Mirtazapine, an antidepressant, antagnoizes central α₂-adrenergic receptors, which increases serotonin and NE release in the CNS.

Sympatholytics

Direct Antagonists

Direct α-adrenergic receptor and β-adrenergic receptor antagonists are noted in Table 13–5. After overdose, β-adrenergic receptor selectivity is less significant. Some β-adrenergic receptor antagonists also are partial agonists.

Xenobiotics That Prevent Norepinephrine Release

Xenobiotics that block the vesicle uptake transporter prevent the movement of NE into vesicles and deplete the nerve ending of this neurotransmitter, preventing NE release after depolarization. Reserpine and ketanserin inhibit both VMAT1 and VMAT2, whereas tetrabenazine only inhibits VMAT2. Like guanethidine, reserpine causes transient NE release with the initial dose or early in overdose. β-Adrenergic receptor antagonists block presynaptic β₂-adrenergic receptors to limit catecholamine release from nerve endings, although most of the antihypertensive action of β-adrenergic antagonists at therapeutic doses results from inhibition of renin release in the kidney.

α₂-Adrenergic Receptor Agonists

Numerous imidazoline derivatives (eg, clonidine) and structurally similar xenobiotics used as centrally acting antihypertensives or long-acting topical vasoconstrictors act by activating α₂-adrenergic receptors. These xenobiotics also display various affinities for what are termed imidazoline-binding sites, apart from α₂-adrenergic receptors. Although imidazoline-binding sites are proposed and subdivided into I₁, I₂, and I₃,⁷³ and endogenous ligands for these binding sites are identified (eg, agmatine, imidazole acetic acid ribotide, harmane, other β-carbolines),^25,102,237 it must be recognized that the molecular structures of imidazoline-binding sites have not been determined, nor have signaling pathways been characterized. As a result, major professional societies such as the International Union of Pharmacology Committee on Receptor Nomenclature and Drug Classification have not adopted the term imidazoline receptor.¹⁵⁴ The role of imidazoline-binding sites in mediating responses to xenobiotics remains controversial and ill-defined.

The ventromedial (depressor) and the rostral-ventrolateral (pressor) areas of the ventrolateral medulla (VLM) are responsible for the central regulation of cardiovascular tone and blood pressure. They receive afferent fibers from the carotid and aortic baroreceptors, which form the tractus solitarius via the nucleus tractus solitarius (NTS).¹³²

Ingestions of xenobiotics that activate α₂-adrenergic receptors (Table 13–5) produce a mixed clinical picture. Peripheral postsynaptic α₂-adrenergic receptor stimulation produces vasoconstriction, pallor, and hypertension, often with reflex bradycardia (Fig. 13–5). Peripheral presynaptic α₂-adrenergic receptor stimulation prevents NE release (Fig. 13–5), reducing sympathetic activation. Central α₂-adrenergic receptor stimulation in the locus ceruleus accounts for CNS and respiratory depression (Fig. 13–6). Recently acquired knowledge about α₂-adrenergic receptor subtypes and their effects on non-adrenergic neurons makes analysis of their hypotensive and sedative effects complex. Recent animal evidence suggests that sedation is not entirely mediated by α₂-adrenergic receptor activation on noradrenergic neurons in the locus ceruleus.⁹³ Stimulation of postsynaptic α_2A-adrenergic receptors in the NTS in the VLM is thought to inhibit sympathetic output and enhance parasympathetic tone, explaining hypotension with bradycardia (Fig. 13–6).¹³²

Dopamine-β-Hydroxylase Inhibition

Inhibition of dopamine-β-hydroxylase, a copper-containing enzyme (Fig. 13–5), prevents the conversion of DA to NE, resulting in less NE release and less α- and β-adrenergic receptor stimulation. Disulfiram and diethyldithiocarbamate, copper chelators, produce such inhibition.⁷⁵ Because NE release mediates most of the ability of DA to cause vasoconstriction, NE is the vasopressor of choice in a hypotensive patient taking disulfiram. Monoamine oxidase inhibitors and α-methyldopa also inhibit dopamine-β-hydroxylase, although this is not their main mechanism of action.¹⁸¹

Dopamine is relatively contraindicated in hypotensive patients who have overdosed on MAOIs. First, DA acts indirectly, and its administration might produce excessive adrenergic activity and exaggerated rises in blood pressure. Second, even if an adrenergic storm does not occur, most of the α-mediated vasoconstriction of dopamine is secondary to NE release. In the presence of MAOIs, NE synthesis may be impaired from concomitant dopamine-β-hydroxylase inhibition, and DA might not reliably raise blood pressure if cytoplasmic and vesicular stores are depleted. In the presence of impaired NE release or α-adrenergic receptor blockade from any cause, unopposed dopamine-induced vasodilation from action on peripheral DA and β-adrenergic receptors paradoxically lowers blood pressure further. Norepinephrine and EPI are used to support blood pressure relatively safely in patients taking MAOIs because they have little or no indirect action and are metabolized by COMT when given intravenously.

Because DA is the direct precursor of NE, noradrenergic vesicles contain DA. The release of NE from peripheral sympathetic nerves, therefore, always results in release of some DA (Fig. 13–5), as does the release of NE and EPI from the adrenal gland, explaining most of DA in blood. In peripheral tissues, activation of DA receptors causes vasodilation of renal, mesenteric, and coronary vascular beds. Dopamine also stimulates β-adrenergic receptors and, at high doses, directly stimulates α-adrenergic receptors. When DA is administered intravenously, most vasoconstriction is caused by dopamine-induced NE release.

Dopamine accounts for about one-half of all catecholamines in the brain and is present in greater quantities than NE or 5-HT. By contrast to the diffuse projections of noradrenergic neurons, dopaminergic neurons and receptors are highly organized and concentrated in several areas, especially in the basal ganglia and limbic system.^144,254

Excessive dopaminergic activity in the striatum and/or other areas from any cause (eg, increased release, impaired uptake, increased receptor sensitivity) produces acute choreoathetosis¹³⁶ and acute Gilles de la Tourette syndrome, with tics, spitting, and cursing. Excessive dopaminergic activity in the limbic system and frontal cortex, and perhaps in other areas, produces paranoia and psychosis. Dopaminergic activity in the nucleus accumbens is thought responsible for much of the drug craving and addictive behavior in patients abusing sympathomimetics, opioids, alcohol, and nicotine. Diminished dopaminergic tone (eg, impaired release, receptor blockade) in the striatum produces various extrapyramidal disorders such as acute dystonias and parkinsonism.^235,269,287

Synthesis, Release, and Reuptake

The steps of DA synthesis and vesicle storage are the same as those for NE, except that DA is not converted to NE after transport into vesicles (Fig. 13–7). Dopamine is removed from the synapse via reuptake by DAT, the membrane-bound DA transporter. Dopamine transporter and NET exhibit 66% homology in their amino acid sequences. Like NET, DAT is not completely specific for DA and transports amphetamines and other structurally similar sympathomimetics. Cytoplasmic DA has a fate similar to NE. It is pumped back into vesicles by VMAT2 (brain) and VMAT1 (neuroendocrine tissue, adrenal glands) or degraded by MAO and COMT.

Dopamine Receptors

All DA receptors are coupled to G proteins and are divided into 2 main groups. Dopamine D₁-like receptors (D₁ and D₅) are expressed as various subtypes and are linked to G_s to stimulate adenylate cyclase and to raise cAMP concentrations.²⁵⁷ Dopamine (D₁) receptors are found in the nuclei of the basal ganglia and the cerebral cortex, and D₅ receptors are concentrated in the hippocampus and hypothalamus. Dopamine is 5 to 10 times more potent at D₅ receptors than it is at D₁ receptors.

Dopamine (D₂)-like receptors (D₂, D₃, D₄) most commonly are linked to G_i/o to produce several actions, including inhibition of adenylate cyclase and the lowering of cAMP concentrations. Again, numerous subtypes of receptors exist (eg, D_2s, D_2L). Dopamine (D₂) receptors are concentrated in the basal ganglia and limbic system. Some D₂ receptors also reside on presynaptic membranes, where their activation limits neurotransmitter release, including the peripheral release of NE (Figs. 13–5 and 13–7). Dopamine (D₃) receptors are concentrated in the hypothalamic and limbic nuclei, whereas D₄ receptors are concentrated in the frontal cortex and limbic nuclei (rather than basal ganglia nuclei). Most agonists bind to the D₃ receptors with higher affinity than to D₂ receptors, whereas most antagonists bind preferentially to D₂ receptors.^144,254 Most agonists and antagonists express a lower affinity for D₄ receptors than they express for D₂ receptors; a notable exception is clozapine.

Xenobiotics

Table 13–6 provides examples of xenobiotics that affect dopaminergic neurotransmission.

Dopamine Agonism

Indirect-Acting and Mixed-Acting Xenobiotics

Most indirect-acting and mixed-acting sympathomimetics cause DA release. The mechanism of action is similar to that causing NE release. These xenobiotics diffuse into the neuron or undergo uptake by DAT before being transported into vesicles by VMAT2 where they buffer protons and displace DA into the cytoplasm for reverse transport by DAT into the synapse. Benztropine, diphenhydramine, trihexyphenidyl, and orphenadrine also cause DA release, perhaps contributing to their abuse potential, which is noted below.¹⁹³ Excessive dopaminergic activity following therapeutic doses or overdoses of decongestants (eg, pseudoephedrine), amphetamines, methylphenidate, and pemoline can produce acute choreoathetosis and the Tourette Syndrome.^39,165 Ingestion of excessive doses of L-dopa (which is converted to DA) produces similar symptoms.

Direct Agonists

Bromocriptine is an ergot derivative that directly activates DA receptors (mainly D₂-like). Toxic effects include those described above for indirect-acting xenobiotics. Apomorphine directly activates D₂ receptors. Such action at the chemoreceptor trigger zone (CTZ) produces vomiting, whereas agonism in the basal ganglia explains the use of apomorphine in the treatment of Parkinson disease. Fenoldopam is a D₁ receptor agonist that is used as a vasodilator in the treatment of hypertensive emergencies.

Dopamine (D₁-) and D₂-like receptor activation is the predominant mediator of locomotor effects from DA agonists. Activation of either D₁– or D₂-like receptors produces antiparkinsonian effects.^106,254 Cabergoline, ropinirole, and pramipexole are D₂-like receptor agonists used to treat Parkinson disease.^19,73,99

Reuptake Inhibition

Xenobiotics inhibiting DAT prevent DA reuptake and include cocaine, amphetamines, methylphenidate, and probably amantadine. Increased dopaminergic activity from cocaine toxicity produces choreoathetosis and Gilles de la Tourette syndrome. In general, antidepressants are not strong DA reuptake blockers. However, bupropion is more active in this regard.²⁴⁷

As noted earlier, much of the craving and addiction produced by sympathomimetics probably results from excessive dopaminergic activity in the mesolimbic system.²⁶⁹ Interestingly, the anticholinergics benztropine, diphenhydramine, trihexyphenidyl, and orphenadrine are also DA reuptake inhibitors, possibly explaining their abuse potential.^193,265 In fact, benztropine is one of the most potent DA reuptake inhibitors known. Amantadine, an antiparkinsonian drug that causes DA release and some inhibition of DA reuptake (as well as being anticholinergic), is also abused.

Increased Receptor Sensitivity

Several xenobiotics are thought to increase sensitivity of DA receptors, resulting in choreoathetosis, even with therapeutic doses such as is the case of phenytoin. Evidence exists that increased DA receptor sensitivity contributes to movement disorders resulting from amphetamines.⁴⁴ Tardive dyskinesia (discussed below) results, at least in part, from increased DA receptor sensitivity and occurs following chronic administration of D₂ receptor antagonists.

MAO Inhibition

Monoamine oxidase inhibitors (MAOIs) inhibit the breakdown of cytoplasmic DA. Some of the food and drug interactions with MAOIs result from excessive release of DA from nerve endings.

COMT Inhibition

Substrates of COMT include dopa, DA, NE, EPI, and their hydroxylated metabolites. Catechol-o-methyltransferase inhibitor (COMT) inhibitors (eg, entacapone, tolcapone) are given with levodopa to patients with Parkinson disease to prevent peripheral degradation of levodopa to 3-O-methyldopa. This allows more levodopa to traverse the blood–brain barrier and to be converted to DA by neuronal dopa decarboxylase. Tolcapone also inhibits COMT in the brain to prevent degradation of dopamine.¹²⁵

Dopamine Antagonism

Direct Receptor Blockade

Blockade of DA receptors is the specific aim of many therapeutics. The antipsychotic actions of butyrophenones, phenothiazines, and atypical antipsychotics mainly correlate with their ability to block D₂ receptors. Many phenothiazines block both D₁-like and D₂-like receptors, whereas haloperidol mainly blocks D₂-like receptors. Unfortunately, antipsychotics and metoclopramide also block DA receptors in the striatum, producing various extrapyramidal symptoms, including acute parkinsonism and dystonias.

Atypical antipsychotics produce fewer extrapyramidal effects and are thought to carry less risk of producing tardive dyskinesia.²⁴¹ The relative affinity of an antipsychotic for 5-HT_2A receptors over D₂ receptors has predictive value for atypical antipsychotics, with a lower risk of extrapyramidal symptoms.²²⁸ These include clozapine, olanzapine, quetiapine, risperidone, and ziprasidone.

The ratio of muscarinic (M₁) blockade to D₂-receptor blockade is also important in limiting extrapyramidal symptoms. Antipsychotics exhibiting strong antimuscarinic effects (eg, olanzapine, clozapine, thioridazine) are also less likely to induce extrapyramidal symptoms.²⁴¹

Buspirone, an anxiolytic, antagonizes D₂ receptors, which explains occasional extrapyramidal reactions. Various cyclic antidepressants, especially amoxapine, block D₂ receptors to some extent.

The chronic use of dopamine receptor antagonists causes upregulation of DA receptors. The continued use of or, especially, withdrawal of DA receptor antagonists (antipsychotics, metoclopramide, and occasionally antidepressants) results in excessive dopaminergic activity and tardive dyskinesia, characterized by choreiform movements typical of excessive dopaminergic influence in the striatum.

The blockade of DA receptors by numerous xenobiotics, including butyrophenones, phenothiazines, and metoclopramide, produces a poorly understood disorder called neuroleptic malignant syndrome. Neuroleptic malignant syndrome also rarely follows acute withdrawal of DA agonists (eg, stopping L-dopa, bromocriptine, or tolcapone). Neuroleptic malignant syndrome is characterized, in part, by mental status changes, autonomic instability, rigidity, and hyperthermia.

Indirect Antagonism

Reserpine and tetrabenazine inhibit VMAT to prevent transport of DA into storage vesicles and deplete nerve endings of DA. In fact, reserpine was used as an antipsychotic before the introduction of phenothiazines. 1-Methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP), a meperidine analog, undergoes activation by MAO-B, including that in glial cells, to a metabolite, 1-methyl-4-phenylpyridinium (MPP⁺), that undergoes uptake by and causes death of dopaminergic neurons. Both MAOIs and inhibitors of DA transporters prevent MPTP-induced destruction of dopaminergic neurons.

Serotonin (5-hydroxytryptamine, 5-HT) is an indole alkylamine found throughout nature (animals, plants, venoms). The serotonergic system is extremely diverse, with 14 subtypes of receptors organized into 7 classes. All of the receptor classes are coupled to G proteins except for 5-HT₃, which is a ligand-gated ion channel.

In the CNS, several hundred thousand serotonergic neurons lie in or in juxtaposition to numerous midline nuclei in the brain stem (raphe nuclei), from which they project to virtually all areas of the brain and spinal cord. Serotonin is involved with mood, emotion, learning, memory, personality, affect, appetite, aggression, motor function, temperature regulation, sexual activity, pain perception, sleep induction, and other basic functions. Serotonin is not essential for any of these processes, but modulates their quality and extent. A number of psychiatric disorders, including depression, anxiety, obsessive-compulsive disorder, dementia, schizophrenia, and eating disorders, are linked to altered serotonin function. Consequently, modification of serotonergic neurotransmission is an integral part of the treatment plan for most of these conditions.²³⁴ Serotonin is also the precursor for the pineal hormone, melatonin.