Intracellular cAMP Signaling Pathway via G_s Protein-Coupled Receptor Activation in Rat Primary Cultured Trigeminal Ganglion Cells

G protein-coupled receptors in trigeminal ganglion (TG) neurons are often associated with sensory mechanisms, including nociception. We have previously reported the expression of P2Y₁₂ receptors, which are G_i protein-coupled receptors, in TG cells. Activating P2Y₁₂ receptors decreased the intracellular free Ca²⁺ concentration ([Ca²⁺]_i). This indicated that intracellular adenosine 3',5'-cyclic monophosphate (cAMP) levels can mediate Ca²⁺ signaling in TG cells. Here, we report more extensive-expression patterns of G_s protein-coupled receptors in primary cultured TG neurons isolated from 7-day-old newborn Wistar rats and further examine the roles of these receptors in cAMP signaling using the BacMam sensor in these neurons. To identify TG neurons, we also measured [Ca²⁺]_i using fura-2 in TG cells and measured intracellular cAMP levels. TG neurons were positive for Gα_s protein-coupled receptors, beta-2 adrenergic (β₂), calcitonin gene-related peptide (CGRP), adenosine A_2A (A_2A), dopamine 1 (D1), prostaglandin I₂ (IP), and 5-hydroxytriptamine 4 (5-HT₄) receptor. Application of forskolin (FSK), an activator of adenylyl cyclase, transiently increased intracellular cAMP levels in TG neurons. The application of a phosphodiesterase inhibitor augmented the FSK-elicited intracellular cAMP level increase. These increases were significantly suppressed by the application of SQ22536, an adenylyl cyclase inhibitor, in TG neurons. Application of agonists for β₂, CGRP, A_2A, D1-like, IP, and 5-HT₄ receptors increased intracellular cAMP levels. These increases were SQ22536-sensitive. These results suggested that TG neurons express β₂, CGRP, A_2A, D1, IP, and 5-HT₄ receptors, and the activations of these Gα_s protein-coupled receptors increase intracellular cAMP levels by activating adenylyl cyclase.