The molecular structure of botulinum neurotoxins contains three functionally distinct domains: binding, translocating, and catalytic. As discussed in Chap.​ 1, the first two domains are included in the heavy chain (HC) of the toxin, whereas the light chain (LC, 50 KD) catalyzes and inactivates the SNARE proteins and prevents the release of neurotransmitters from the presynaptic vesicles. The HC is a 100 KD protein and has two terminals, N and C. Through its C terminal, the heavy chain binds to the synaptic membrane receptors (ganglioside, SV2). Following binding, HC translocates the toxin molecule through the synaptic membrane to the cell’s interior. The light chain is a zinc-endopeptidase protein which is bound to the HC by a disulfide bond. Once inside, the light chain is detached from the HC and acts upon the synaptic proteins (SNARE) to block their function (vesicular membrane fusion and transmitter release).

The function of the various domains of the toxin varies between different BoNT serotypes. For instance, the binding domain of one toxin may show strong affinity for one cell receptor and weak affinity for another.

Botulinum neurotoxin chimeras are genetically engineered molecules with combined domains from different toxins in order to improve the overall function of the toxins. Usually, a chimera is stronger than either of the two parent toxins. In recent years, the use of such chimeras in animal models has been able to induce less or more paralytic toxin effect and longer duration of toxins’ action or more specifically target certain cells (neuron or non-neuron). Pertaining to pain treatment, there are toxin chimeras which target specifically the sensory neurons.

The efficacy of BoNT-A in the treatment of chronic migraine has been attributed, at least in part, to the inhibition of the release of calcitonin gene-related peptide, a potent pro-inflammatory pain mediator (Cernuda-Morollón et al. 2014) which is also implicated in the burning pain resulting from exposure to capsaicin (the chemical contained in hot pepper). However, neither BoNT-A nor BoNT-E by itself alleviates or prevents the neuropathic pain caused by exposure to this agent. Capsaicin exerts its effect by activating the transient receptor potential vanilloid receptor type 1 (TRPV1), expressed abundantly on the surface of sensory neurons, dorsal root ganglia (DRG) (Nagy et al. 2014). Activation of TRPV1 is essential for exocytosis of CGRP and requires an intact SNAP25 function. BoNT-E is more potent than BoNT-A and acts faster than BoNT-A on SNAP25, but it has a shorter duration of action. It has been postulated that failure of BoNT-E in alleviating capsaicin-induced pain may be related to the paucity of specific binding receptors for this toxin on the surface of sensory cells. It has been hypothesized that an A/E toxin chimera may be effective against capsaicin-induced neuropathic pain using the powerful binding action of the type A toxin. A BoNT-A/E chimera has been engineered in which the HC domain of BoNT-A binds the toxin to sensory neuron’s surface and by making a channel in the cell membrane translocates the E-protease into the synapse. This chimera effectively blocks the release of CGRP from TRPV1 receptors in response to capsaicin exposure in cell cultures of sensory neurons (Meng et al. 2009). Additional studies have shown that A/E chimera also prevents the emergence of capsaicin-induced pain in animals as judged by alleviation of the behavioral manifestations of pain after peripheral exposure. Clinical trials with A/E chimera in human with pain disorders are currently being conducted.