Serotonin 2C receptors inhibit hypothalamic CRH neurons to suppress appetite

The serotonin 2C receptor (Htr2c) is one of the plausible targets for the development of appetite suppressants. Previous studies have demonstrated the complexity of neuronal circuitry underlying the appetite-suppressing effects of Htr2c stimulation. To develop a safe and effective anti-obesity medication targeting Htr2c, we need to better understand how Htr2c agonists suppress appetite. In this study, we focused on the effects of Htr2c agonists on corticotropin-releasing hormone (CRH) neurons to identify the contribution of humoral components to the suppression of fasting-induced food intake.

We used the Crh-ires-cre mice to fluorescently label CRH neurons for whole-cell patch-clamp recordings (Crh-ires-cre::tdTomato mice) and to delete Htr2c selectively in CRH neurons by breeding with Htr2c^flox/Y mice (Crh-ires-cre::Htr2c^flox/Y mice). We also injected Htr2c-targeting short hairpin RNA (shRNA) into the paraventricular nucleus of the hypothalamus (PVH) of Crh-ires-cre mice to knock down Htr2c selectively in CRH neurons within the PVH (CRH^PVH neurons). Using these model mice, we tested the effects of WAY161503, a selective Htr2c agonist, on CRH neuronal activity ex vivo as well as fasting-induced food intake and plasma corticosterone (CORT) levels in vivo.

WAY161503 inhibited the activity of CRH^PVH neurons. The appetite-suppressing effects of WAY161503 were significantly attenuated when Htr2c was deleted selectively in CRH^PVH neurons. On the other hand, WAY161503 promoted the reduction of plasma CORT levels during fasting-induced refeeding via Htr2c expressed by CRH^PVH neurons. Importantly, when mice were pretreated with RU486, a glucocorticoid receptor antagonist that blocks CORT action, WAY161503 suppressed food intake whether CRH^PVH neurons expressed functional Htr2c or not. Finally, we characterized the expression of single-minded 1 (Sim1) messenger RNA (mRNA), Crh mRNA, and Htr2c mRNA in PVH neurons, which may help to explain the effects of Htr2c stimulation on fasting-induced refeeding.

Our results demonstrate that Htr2c expression in the CRH^PVH neurons is necessary for the appetite-suppressing effects of WAY161503 during fasting-induced refeeding. Importantly, we found that WAY161503 suppresses the hypothalamic-pituitary-adrenal (HPA) axis and promotes the reduction of plasma CORT levels, thereby enabling the appetite-suppressing effects of Htr2c stimulation during fasting-induced refeeding. To our knowledge, this study is the first to highlight the necessity of coordination between neural and humoral pathways for the suppression of fasting-induced food intake by Htr2c agonists.