Glutamate is the major excitatory neurotransmitter, found throughout the mammalian central nervous system (CNS), where it contributes to synaptic plasticity, learning, and memory, as well as pain and sensory transmission as well as neurodegenerative disease states (16). Glutamate acts on a wide range of receptors and these are subdivided into two main groups, ionotropic and metabotropic receptors. The ionotropic receptor (iGluR) group can be further divided into subcategories (16).

α-amino-3-hydroxy-5-methyl-4-isoxazoleproprionate (AMPA) receptors

Kainate receptors

N-methyl-D-aspartate (NMDA) receptors

Ionotropic glutamate receptors contain cation-specific ion channels (16). However, the metabotropic receptors (mGluRs) are G-protein linked receptors, coupled to GTP-binding proteins and therefore modulate intracellular cell signaling via certain messenger systems (16). There is an entire family of mGluR G-protein linked receptors (mGluR_1-8) that exert both inhibitory and excitatory actions on both pre- and postsynaptic sites (16). Interestingly, mGluR2/3 and 5 receptor subunits are expressed widely in the superficial dorsal horn and thus serve to modulate sensory transmission at the spinal level.

The focus of this section is the ionotropic glutamate receptors, because these have been studied in significantly more detail than the metabotropic receptors. The three main iGluR receptors are named according to their agonist specificities, although it is important to note that although kainate and AMPA receptors (non-NMDA receptors) are structurally and functionally different, their respective agonists share the same affinity for the other receptor. Molecular and expression cloning have identified the cDNA’s encoding subunits within each of the three ionotropic receptor (16).

AMPA: GluR 1-4, similar size, 68 to 73% sequence homology.

Kainate: GluR5-7, similar size, 75 to 80% sequence homology. KA1 and KA2 are larger than GluR5-7. KA1 and KA2 share 70% sequence homology.

NMDA: NR1 (eight splice variants reported) and NR2_A-D, low sequence homology (<18%), 40 to 50% between NR2 subunits. NR3A may be a modulatory subunit, important in development of synaptic elements (7,16).

The AMPA receptor is a major glutamate receptor, mediating fast excitatory transmission throughout the mammalian CNS (16). AMPA receptors are permeable to both Na⁺ and K⁺ ions, and relatively impermeable to Ca²⁺ ions. However, AMPA receptors, which express anything but the GluR2 subunits (i.e., GluR1, 3, and 4) are indeed permeable to Ca²⁺ ions. Studies have shown that AMPA receptors are located throughout the CNS, particularly the hippocampus and superficial cerebral cortex (16). Despite important roles in pain transmission, the AMPA receptors mediate much CNS excitatory transmission including tactile events and so are unlikely to be a feasible target.

The kainate receptor is widely distributed, although very little is known regarding its function compared to the AMPA receptor. Kainate receptor subunits (iGluR5-7) are distributed within lamina I-III of the spinal cord (35). DRG neurons are also believed to contain functional kainate receptors, strongly implicating expression of kainate receptors on primary afferent neurons in the superficial dorsal
horn (13). Indeed, it is believed that presynaptic kainate receptors serve to regulate sensory, particularly nociceptive, transmission in the superficial dorsal horn (13), and so may be a target for controlling afferent activity.

The NMDA receptor mediates excitatory neurotransmission in the entire nervous system (26). However the role of the NMDA receptor is extremely diverse and is implicated in neuronal plasticity, gene expression, as well as neuronal growth and survival within the CNS (26). The NMDA receptor has unique properties compared to the AMPA and kainate receptors. NMDA receptors are blocked by Mg²⁺ in a voltage-dependent manner, so at normal resting membrane potentials the NMDA receptor does not allow the passage of ions through its pore and so is non-functional (16). Interestingly, NR1-NR2A and NR1-NR2B subunits appear to be blocked by Mg²⁺ ions more efficiently than the other heteromeric complexes, which form the NMDA receptor (17).

NMDA receptor subunits form heteromeric receptor complexes consisting of four subunits (tetrameric) with large current response (16,26). It is therefore widely believed that NR1 NMDA receptor subunits are co-expressed with NR2 subunits, forming functional NMDA receptors. Combination of the NR1 subunit with different NR2 subunits (A-D) form a variety of functional NMDA receptor subtypes but with varying properties; it is thus clear that NR2 subunits are primarily modulatory subunits and differ somewhat in their functionality (16).

Interestingly, studies in rat brains have revealed that the NMDA receptor subunits are expressed in distinct areas of the CNS in varying quantities. NMDA receptors are largely distributed in the hippocampal CA1 region, as well as other areas of the brain (predominantly the forebrain). The NR1 subunit, consistent with these findings, is also found abundantly throughout the entire CNS. However, the regional distributions of the NR2 subunits are relatively distinct and include spinal cord (16,35).

Immunocytochemical studies in the rat have revealed that, within the dorsal horn of the spinal cord NMDAR2B, subunits are found predominantly in lamina I-III, as are NMDAR1 subunits (35). Other reports suggest that NR2D subunits can also be located in the superficial dorsal horn. Interestingly, NMDAR2A and NMDRAR2C subunits were not found in the dorsal horn, suggesting these receptors play a minimal role in nociceptive transmission at the spinal cord level (35). These points are pertinent to the issue of attempting to control the actions of a ubiquitous transmitter. The available drugs that act on the NMDA receptor are presently ketamine, which is effective in patients with difficult pains yet with undesirable side effects, and drugs such as dextromethorphan, which lack potency and have failed in some trials with facial pains. Subtypes of the receptor may allow selective actions on those receptors that are implicated more in pain than global forebrain function (5).

It is clear that both glutamate and the amino acid glycine are required to activate the NMDA receptor channel, and such findings suggest glycine is a co-agonist in the activation and function of NMDA receptor activity (34). Furthermore, the NMDA receptor structure also contains modulatory sites for polyamines, protons, redox agents, and Zn²⁺ suggesting that other agents may affect the activity of NMDA receptors, alongside glycine (26).

NMDA receptors, as mentioned, have a variety of distinct properties. One of the most intriguing properties is the wind-up effect consistently attributed to NMDA receptor channels, following repetitive, high-threshold stimuli of nociceptive neurons (8,11). Wind-up describes the sharp increase in response following repetitive constant stimuli and this effect is thought to underlie the mechanisms for acute pain transmission and central sensitization in the CNS (8,11). Central sensitization frequently arises following tissue injury or neuronal insult. Such damage often evokes increased excitability of dorsal horn neurons, increased receptive field sizes, and heightened response properties in nociceptive and sensory nerve fibers (8,11,25,27).

In electrophysiologic studies, wind-up can be evoked following repetitive electrical stimuli administered at C-fiber thresholds (8,11). Repetitive noxious stimuli prompts peptide released from presynaptic C-fiber afferents within the superficial laminae and this, in turn, results in the depolarization required to remove the voltage-dependent Mg²⁺ block of NMDA receptors. The recruitment of NMDA receptors in the neuronal response to repetitive noxious stimuli is also enhanced by nitric oxide production (8,11,25). Wind-up is thus the result of progressive increases in neuronal response properties, which are no longer proportional to the original evoked stimulus and which decay after minutes if the afferent barrage subsides. Electrophysiologic studies whereby the wind-up response is significantly blocked by NMDA receptor antagonists confirm such a role of NMDA receptors in the windup response and alterations after peripheral (27). Similar studies in the trigeminal complex have revealed that wind-up, which depends on NMDA receptors, can be observed and that c-fos (33) labeling of nociceptive neurons in nucleus caudalis can be modulated by ionotropic and metabotropic glutamate receptors (20). There is further convincing evidence that sagittal sinus activation activates NMDA-dependent mechanisms in nucleus caudalis (6).