α-Conotoxin Vc1.1 inhibits human dorsal root ganglion neuroexcitability and mouse colonic nociception via GABA B receptors.

Objective: α-Conotoxin Vc1.1 is a small disulfide-bonded peptide from the venom of the marine cone snail Conus victoriae . Vc1.1 has antinociceptive actions in animal models of neuropathic pain, but its applicability to inhibiting human dorsal root ganglion (DRG) neuroexcitability and reducing chronic visceral pain (CVP) is unknown.
Design: We determined the inhibitory actions of Vc1.1 on human DRG neurons and on mouse colonic sensory afferents in healthy and chronic visceral hypersensitivity (CVH) states. In mice, visceral nociception was assessed by neuronal activation within the spinal cord in response to noxious colorectal distension (CRD). Quantitative-reverse-transcription-PCR, single-cell-reverse-transcription-PCR and immunohistochemistry determined γ-aminobutyric acid receptor B (GABA _B R) and voltage-gated calcium channel (Ca _V 2.2, Ca _V 2.3) expression in human and mouse DRG neurons.
Results: Vc1.1 reduced the excitability of human DRG neurons, whereas a synthetic Vc1.1 analogue that is inactive at GABA _B R did not. Human DRG neurons expressed GABA _B R and its downstream effector channels Ca _V 2.2 and Ca _V 2.3. Mouse colonic DRG neurons exhibited high GABA _B R, Ca _V 2.2 and Ca _V 2.3 expression, with upregulation of the Ca _V 2.2 exon-37a variant during CVH. Vc1.1 inhibited mouse colonic afferents ex vivo and nociceptive signalling of noxious CRD into the spinal cord in vivo, with greatest efficacy observed during CVH. A selective GABA _B R antagonist prevented Vc1.1-induced inhibition, whereas blocking both Ca _V 2.2 and Ca _V 2.3 caused inhibition comparable with Vc1.1 alone.
Conclusions: Vc1.1-mediated activation of GABA _B R is a novel mechanism for reducing the excitability of human DRG neurons. Vc1.1-induced activation of GABA _B R on the peripheral endings of colonic afferents reduces nociceptive signalling. The enhanced antinociceptive actions of Vc1.1 during CVH suggest it is a novel candidate for the treatment for CVP.
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