Characterization of 5-hydroxytryptamine-induced depolarizations in rat isolated vagus nerve

An additional component of the depolarization induced by 5-hydroxytryptamine (5-HT) in the rat isolated vagus nerve has recently been attributed to activation of 5-HT₄ receptors. To confirm and extend this finding, extracellular recordings of D.C. potentials were made using the ‘grease-gap’ technique during continuous superfusion of the isolated nerve. Beginning at 1 nM, 5-HT induced small depolarizations that displayed a slow onset. At concentrations 1 μM, large depolarizations with rapid onset were elicited. In the presence of the 5-HT₃ receptor antagonists, granisetron or ondansetron, 5-HT responses were diminished and exhibited an increased latency to peak. These small, slow depolarizations were not reduced by 5-HT₁ or 5-HT₂ receptor antagonists, but were potently inhibited by the 5-HT₄ receptor antagonist GR 113808 (pA₂ = 9.3), and mimicked by 5-methoxytryptamine (pEC₅₀ = 5.3). 5-HT₄-mediated responses were larger at 37°C than at 31°C, but also showed marked diminution with repeated 5-HT applications at concentrations greater than 1 μM. Conversely, 5-HT₃ receptor responses were potentiated at lower temperatures (31°C). Consistent with the reported positive coupling of 5-HT₄ receptors to adenylyl cyclase, forskolin and 8-Br-cAMP produced slowly developing depolarizations which were qualitatively similar to 5-HT₄ receptor activation. Pre-depolarization of nerves with 10 μM forskolin or 300 μM 8-Br-cAMP diminished the effect of 5-HT₄ receptors. This study has confirmed the presence of 5-HT₄ receptors on the vagus nerve of the rat and defined some conditions that optimize their pharmacological isolation. The rat isolated vagus nerve constitutes a simple and robust preparation for studying 5-HT₄ receptors in the peripheral nervous system.