Possible mechanisms for the effects of botulinum toxin on pain.

The therapeutic effects of botulinum toxin are principally, if not exclusively, derived from an alteration in the release of acetylcholine (ACh) at pre-synaptic neurons. The rationale for how these effects could be beneficial in conditions characterized by excessive muscle contraction is clear, but the hypotheses regarding botulinum toxin-induced effects on pain are highly speculative. We explore five possible mechanisms by which botulinum toxin could directly or indirectly alter pain, including: 1) changes in the sensitivity and response patterns of group III and IV muscle nociceptors, 2) diminished activity in the gamma-motor neurons and consequent changes in muscle spindle afferents, 3) alterations in cholinergic control of vascular and autonomic functions, including neurogenic inflammation, 4) induced neuroplastic changes in the processing of afferent somatosensory activity at multiple levels of the neuroaxis, and 5) direct non-cholinergic effects on pain afferents.