GABA_B receptor-mediated modulation of sensory neuron excitability: roles of Ca_V2.2, G-protein-coupled inwardly rectifying potassium (GIRK) channels, and hyperpolarisation-activated cyclic nucleotide-gated (HCN) channels in human and mouse nociception

Chronic visceral pain is a key symptom of irritable bowel syndrome. Modulation of voltage-gated calcium and potassium channels by G protein-coupled receptors plays a key role in dampening nociceptive transmission. Both baclofen and the analgesic peptide α-conotoxin Vc1.1 activate GABA_B receptors (GABA_BR), resulting in inhibition of Ca_V2.2 and Ca_V2.3 calcium channels to reduce colonic nociception. Recent studies have also shown that GABA_BR activation potentiates G-protein-coupled inwardly rectifying potassium (GIRK)-1/2 channels in mammalian sensory afferent neurons. In this study, we investigated the expression of these ion channel targets in rodent and human dorsal root ganglion (DRG) neurons, including those innervating the colon. We examined how Ca_V2.2 and GIRK channel antagonists, as well as a GIRK channel activator, influence the passive and active electrical properties of adult mouse DRG neurons. We also assessed the effects of α-conotoxin Vc1.1 on neuronal excitability in the presence of the selective Ca_V2.2 antagonist ω-conotoxin CVIE and the GIRK channel activator ML297. We further evaluated the impact of the GIRK channel antagonist tertiapin-Q on excitability in mouse colonic DRGs and afferents and explored the role of hyperpolarization-activated cyclic nucleotide-gated (HCN) channels. Our findings demonstrate that both Ca_V2.2 inhibition and GIRK channel potentiation reduce excitability in mouse DRGs, likely mediating the antinociceptive effects of Vc1.1 and baclofen observed in vivo. However, GIRK channel potentiation appears to play only a limited role in modulating excitability in colon-innervating DRGs and colonic afferents. These findings suggest that neurons innervating different body regions use distinct mechanisms to regulate excitability and nociceptive signalling.