Botulinum neurotoxin B (BoNT-B) mediates proteolytic cleavage of VAMP I/II (synaptobrevins I/II) in the family of soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) proteins. Such cleavage of SNARE proteins inhibits vesicle-membrane fusion at the synaptic terminal and inhibits neurotransmitter release. The aim of this study is to investigate the effects of intrathecal (IT) delivery of BoNT-B on neurotransmitter release from spinal primary afferent C-fibers and the effects on nociception in mouse. In the formalin mouse model, intraplantar (IPLT) formalin induces substance-P (SP) release from primary afferent C-fibers. SP binding to neurokinin-1 receptors (NK1-R) on dorsal horn neurons resulting in receptor internalization can be visualized using immunohistochemistry and confocal microscopy. Mice that received IT BoNT-B prior to IPLT formalin showed reduction in NK1-R internalization and spinal C-Fos protein activation. In contrast, intrathecal co-injection of BoNT-B and anti-BoNTB antibody showed prevention of these effects on neurotransmitter release. Intrathecal BoNT-B pretreatment also attenuated phase 2, and not phase 1, of formalin flinching behavior. To investigate effects of IT BoNT-B on neuropathic pain, mice received spinal nerve ligation (SNL) of the left L5 spinal nerve. SNL induced tactile allodynia 14 days post ligation upon stimulation using von Frey filaments. Mice that received IT BoNT-B showed increased paw withdrawal threshold, compared to vehicle animals, for the next 15 days. The data suggest that IT BoNT-B blocks neurotransmitter release from spinal primary afferent C-fibers in a facilitated state, attenuates formalin-induced nociceptive response, and attenuates spinal nerve injury- induced neuropathic pain, in the absence of motor impairment
