Strategies for Investigating G-Protein Modulation of Voltage-Gated Ca2+ Channels

Ghrelin Modulates Voltage-Gated Ca2+ Channels through Voltage-Dependent and Voltage-Independent Pathways in Rat Gastric Vagal Afferent Neurons

The orexigenic gut peptide ghrelin is an endogenous ligand for the growth hormone secretagogue receptor type 1a (GHSR1a). Systemic ghrelin administration has previously been shown to increase gastric motility and emptying. While these effects are known to be mediated by the vagus nerve, the cellular mechanism underlying these effects remains unclear. Therefore, the purpose of the present study was to investigate the signaling mechanism by which GHSR1a inhibits voltage-gated Ca2+ channels in isolated rat gastric vagal afferent neurons using whole-cell patch-clamp electrophysiology. The ghrelin pharmacological profile indicated that Ca2+ currents were inhibited with a log (Ic50) = -2.10 ± 0.44 and a maximal inhibition of 42.8 ± 5.0%. Exposure to the GHSR1a receptor antagonist (D-Lys3)-GHRP-6 reduced ghrelin-mediated Ca2+ channel inhibition (29.4 ± 16.7% vs. 1.9 ± 2.5%, n = 6, P = 0.0064). Interestingly, we observed that activation of GHSR1a inhibited Ca2+ currents through both voltage-dependent and voltage-independent pathways. We also treated the gastric neurons with either pertussis toxin (PTX) or YM-254890 to examine whether the Ca2+ current inhibition was mediated by the Gα i/o or Gα q/11 family of subunits. Treatment with both PTX (Ca2+ current inhibition = 15.7 ± 10.6%, n = 8, P = 0.0327) and YM-254890 (15.2 ± 11.9%, n = 8, P = 0.0269) blocked ghrelin's effects on Ca2+ currents, as compared with control neurons (34.3 ± 18.9%, n = 8). These results indicate GHSR1a can couple to both Gα i/o and Gα q/11 in gastric vagal afferent neurons. Overall, our findings suggest GHSR1a-mediated inhibition of Ca2+ currents occurs through two distinct pathways, offering necessary insights into the cellular mechanisms underlying ghrelin's regulation of gastric vagal afferents. SIGNIFICANCE STATEMENT: This study demonstrated that in gastric vagal afferent neurons, activation of GHSR1a by ghrelin inhibits voltage-gated Ca2+ channels through both voltage-dependent and voltage-independent signaling pathways. These results provide necessary insights into the cellular mechanism underlying ghrelin regulation of gastric vagal afferent activity, which may benefit future studies investigating ghrelin mimetics to treat gastric motility disorders.

Coexpressed δ-, μ-, and κ-Opioid Receptors Modulate Voltage-Gated Ca2+ Channels in Gastric-Projecting Vagal Afferent Neurons

Opioid analgesics are frequently associated with gastrointestinal side effects, including constipation, nausea, dysphagia, and reduced gastric motility. Though it has been shown that stimulation of opioid receptors expressed in enteric motor neurons contributes to opioid-induced constipation, it remains unclear whether activation of opioid receptors in gastric-projecting nodose ganglia neurons contributes to the reduction in gastric motility and emptying associated with opioid use. In the present study, whole-cell patch-clamp recordings were performed to determine the mechanism underlying opioid receptor-mediated modulation of Ca2+ currents in acutely isolated gastric vagal afferent neurons. Our results demonstrate that CaV2.2 channels provide the majority (71% ± 16%) of Ca2+ currents in gastric vagal afferent neurons. Furthermore, we found that application of oxycodone, U-50488, or deltorphin II on gastric nodose ganglia neurons inhibited Ca2+ currents through a voltage-dependent mechanism by coupling to the Gα i/o family of heterotrimeric G-proteins. Because previous studies have demonstrated that the nodose ganglia expresses low levels of δ-opioid receptors, we also determined the deltorphin II concentration-response relationship and assessed deltorphin-mediated Ca2+ current inhibition following exposure to the δ-opioid receptor antagonist ICI 174,864 (0.3 µM). The peak mean Ca2+ current inhibition following deltorphin II application was 47% ± 24% (EC50 = 302.6 nM), and exposure to ICI 174,864 blocked deltorphin II-mediated Ca2+ current inhibition (4% ± 4% versus 37% ± 20%). Together, our results suggest that analgesics targeting any opioid receptor subtype can modulate gastric vagal circuits. SIGNIFICANCE STATEMENT: This study demonstrated that in gastric nodose ganglia neurons, agonists targeting all three classical opioid receptor subtypes (μ, δ, and κ) inhibit voltage-gated Ca2+ channels in a voltage-dependent mechanism by coupling to Gαi/o. These findings suggest that analgesics targeting any opioid receptor subtype would modulate gastric vagal circuits responsible for regulating gastric reflexes.

G-Protein Modulation of Voltage-Gated Ca2+ Channels from Isolated Adult Rat Superior Cervical Ganglion Neurons

Sympathetic neurons isolated from adult rat superior cervical ganglia (SCG) are a well-established model to study G-protein modulation of voltage-gated Ca(2+) channels (VGCCs). SCG neurons can be easily dissociated and are amendable to heterologous expression of genes, including genetic tools to study G-protein signaling pathways, within a time frame to maintain good spatial voltage-clamp control of membrane potential during electrophysiological recordings (8-36 h postdissociation). This protocol focuses on examining G-protein modulation of VGCCs; however, the procedures and experimental setup for acute application of agonists can be applied to study modulation of other ion channels (e.g., M-current, G-protein-coupled inwardly rectifying K(+) channels). We also discuss some common sources of artifacts that can arise during acute drug application onto dissociated neurons, which can mislead interpretation of results.