Bi-directional modulation of bed nucleus of stria terminalis neurons by 5-HT: molecular expression and functional properties of excitatory 5-HT receptor subtypes

Activation of neurons in the anterolateral bed nucleus of the stria terminalis (BNSTALG) plays an important role in mediating the behavioral response to stressful and anxiogenic stimuli. Application of 5-HT elicits complex postsynaptic responses in BNSTALG neurons, which includes (1) membrane hyperpolarization (5-HTHyp), (2) hyperpolarization followed by depolarization (5-HTHyp-Dep), (3) depolarization (5-HTDep) or (4) no response (5-HTNR). We have shown that the inhibitory response is mediated by activation of postsynaptic 5-HT1A receptors. Here, we used a combination of in vitro whole-cell patch-clamp recording and single cell reverse transcriptase polymerase chain reaction (RT-PCR) to determine the pharmacological properties and molecular profile of 5-HT receptor subtypes mediating the excitatory response to 5-HT in BNSTALG neurons. We show that the depolarizing component of both the 5-HTHyp/Dep and the 5-HTDep response was mediated by activation of 5-HT2A, 5-HT2C and/or 5-HT7 receptors. Single cell RT-PCR data revealed that 5-HT7 receptors (46%) and 5-HT1A receptors (41%) are the most prevalent receptor subtypes expressed in BNSTALG neurons. Moreover, 5-HT receptor subtypes are differentially expressed in type I-III BNSTALG neurons. Hence, 5-HT2C receptors are almost exclusively expressed by type III neurons, whereas 5-HT7 receptors are expressed by type I and II neurons, but not type III neurons. Conversely, 5-HT2A receptors are found predominantly in type II neurons. Finally, bi-directional modulation of individual neurons occurs only in type I and II neurons. Significantly the distribution of 5-HT receptor subtypes in BNSTALG neurons predicted the observed expression pattern of 5-HT responses determined pharmacologically. Together, these results suggest that 5-HT can differentially modulate the excitability of type I-III neurons, and further suggest that bi-directional modulation of BNSTALG neurons occurs primarily through an interplay between 5-HT1A and 5-HT7 receptors. Hence, modulation of 5-HT7 receptor activity in the BNSTALG may offer a novel avenue for the design of anxiolytic medications.