Serotonin Modulates Stellate Cell Excitability via 5-HT Receptors and HCN Channels in the Mouse Anteroventral Cochlear Nucleus

Serotonergic projections innervate both the dorsal and ventral cochlear nuclei; however, the electrophysiological consequences of serotonergic input in the ventral cochlear nucleus (VCN) remain incompletely understood. This study aimed to identify the serotonin receptor subtypes involved in serotonergic modulation of stellate cells in the mouse anteroventral cochlear nucleus (AVCN) and to determine the underlying ion channel mechanisms. Whole-cell patch-clamp recordings were performed in acute brain slices obtained from postnatal day 12–17 mice. Bath application of serotonin (25 µM) induced membrane depolarization (~5 mV) and increased action potential firing. Pharmacological experiments demonstrated that antagonists of 5-HT1A, 5-HT2A, and 5-HT2C receptors partially reversed the depolarization and reduced serotonin-induced inward currents, indicating that multiple receptor subtypes contribute to serotonergic excitation. Blockade of hyperpolarization-activated cyclic nucleotide-gated (HCN) channels with extracellular Cs+ suppressed approximately 95% of the serotonin-induced depolarization and inward current, implicating HCN channel-mediated Ih as a principal ionic mechanism. Serotonin significantly increased Ih amplitude. Analysis of steady-state activation revealed no statistically significant shift in V0.5; however, under near-resting membrane potential conditions, serotonin significantly reduced the slope factor of the activation curve, consistent with altered voltage sensitivity of Ih gating. Immunohistochemical analysis confirmed the presence of 5-HT1A, 5-HT2A, and 5-HT2C receptors in the AVCN. Together, these findings indicate that serotonergic excitation of AVCN stellate cells is mediated by coordinated activation of multiple 5-HT receptor subtypes and primarily involves modulation of HCN-dependent subthreshold membrane dynamics.