Gq/5-HT _2c receptor signals activate a local GABAergic inhibitory feedback circuit to modulate serotonergic firing and anxiety in mice

Abstract

Serotonin 2c receptors (5-HT 2c -Rs) are drug targets for certain mental disorders, including schizophrenia, depression, and anxiety. 5-HT 2c -Rs are expressed throughout the brain, making it difficult to link behavioral changes to circuit specific receptor expression. Various 5-HT-Rs, including 5-HT 2c -Rs, are found in the dorsal raphe nucleus (DRN); however, the function of 5-HT 2c -Rs and their influence on the serotonergic signals mediating mood disorders remain unclear. To investigate the role of 5-HT 2c -Rs in the DRN in mice, we developed a melanopsin-based optogenetic probe for activation of Gq signals in cellular domains, where 5-HT 2c -Rs are localized. Our results demonstrate that precise temporal control of Gq signals in 5-HT 2c -R domains in GABAergic neurons upstream of 5-HT neurons provides negative feedback regulation of serotonergic firing to modulate anxiety-like behavior in mice.S erotonin (5-hydroxytryptamine, or 5-HT) is an important modulator of anxiety circuits (1). The diverse effects of serotonin are mediated through various 5-HT receptors (5-HT-Rs), including 5-HT 1-7 -Rs (2). Recent pharmacologic and genetic studies have highlighted an important role of 5-HT 2c -Rs in anxiety disorders; however, the interpretation of physiological and behavioral data remains difficult owing to a lack of selective pharmacologic ligands (3).5-HT 2c -Rs are expressed in various cell types and brain regions of the anxiety circuit, including the amygdala and the dorsal raphe nucleus (DRN), a midbrain region containing high concentrations of 5-HT neurons. It has been suggested that 5-HT 2c -Rs are expressed in GABAergic neurons, and that 5-HT 2c -R activation may contribute to an inhibitory feedback control of 5-HT cell firing (4). The functional and behavioral consequences of such a possible inhibitory feedback mechanism for 5-HT firing have not yet been investigated, however.Unfortunately, current techniques for identifying the functions of 5-HT 2c -Rs in vertebrate brains are of limited value. For example, agonists and antagonists of 5-HT 2c -Rs are often unspecific, and their action is not restricted to a specific cell type. Complete and conditional knockouts of the receptor gene have limited control of developmental and compensation effects by other G-protein-coupled receptors (GPCRs), and none of the current techniques allows for the physiological control of the 5-HT 2c -R activation on a millisecond to second time scale.To overcome the limitations of pharmacologic and genetic approaches, we have developed a new light-activated GPCR based on vertebrate melanopsin (vMo). Both 5-HT 2c -Rs and vMo couple to the Gq signaling pathway (5, 6). To investigate the functional consequence of Gq signal activation in the cell types and cellular structures where 5-HT 2c -Rs are located, we virally expressed vMo carrying the C terminus (CT) of the 5-HT 2c -R in GABAergic neurons in the DRN. We found that light activation of vMo-CT 5-HT2c decreases the firing of 5-HT neurons and modulates anxiety behaviors in mice. O...

Activity of Raphé Serotonergic Neurons Controls Emotional Behaviors

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Ethanol induced adaptations in 5-HT2c receptor signaling in the bed nucleus of the stria terminalis: Implications for anxiety during ethanol withdrawal

“…These opposing effects of 5HT2c-R antagonism on these two aspects of anxiety may indicate that they are governed by divergent neural circuits. To illustrate this point, a recent study found that optogenetic Gq/5HT2c-R activation of GABAergic interneurons in the DR 5HT2c-Rs actually relieve anxiety in the open field (Spoida et al, 2014), while another study by Breese and colleagues found that a 5HT2c-R inverse agonist (SB 243,213) microinjected into the DR had no effect on CIE induced anxiety in the social interaction test (Overstreet et al, 2006). However, intra-amygdalar injections of SB 243,213 did relieve CIE induced deficits in social interaction, suggesting that 5HT2c-R signaling in the amygdala can modulate the social aspects of anxiety.…”

Model Organisms in G Protein–Coupled Receptor Research

“…This technique allows experimenters to use fiber optic or wireless light-emitting diode technology (Yizhar et al, 2011;Kim et al, 2013) to activate GPCR signaling within selected cell types in the mammalian brain of awake behaving animals. Elegant extensions of this approach have also been used in modifications of vertebrate rhodopsin with components of the 5HT1a or 5HT2c serotonin receptor for examining neural circuits and signaling in anxiety behavior Spoida et al, 2014). Recent work has also shown that chimeric opsin-wild-type receptors can be used to optically mimic opioid receptor signaling (Siuda et al, 2015) (Fig.…”