The Effects of Elevated Serotonin (5-HT) Signaling on Brown Adipose Tissue

Abstract

Inhibiting peripheral serotonin (5-HT) synthesis has been shown to prevent the development of diet-induced obesity, glucose intolerance, insulin resistance and hepatic lipid deposition and to increase brown adipose tissue (BAT) thermogenic capacity. This thesis investigated 1) what effects 5-HT has on brown adipocytes (BAs), 2) if 5-HT and/or selective serotonin reuptake inhibitors (SSRIs) impair BAT function, 3) if 5-HT directly inhibits BA via a receptor-mediated mechanism, 4) which 5-HT receptor is predominantly expressed in BAT, 5) if 5-HT receptor antagonism improves BAT function and 6) if 5-HT receptor inhibition reduces SSRI induced weight gain. In murine BAs, 5-HT at concentrations ≥100 μM acutely reduced lipolysis, lipid accumulation and glycolytic flux but did not impair oxygen consumption; whereas 10 nM 5-HT reduced Ucp1 promoter activity via an extracellular receptor-mediated mechanism. Acute injection of 5-HT or the selective serotonin reuptake inhibitor (SSRI) Paroxetine decreased BA T thermogenic capacity and energy expenditure (EE), respectively. Mice lacking the serotonin transporter gene (Slc6a4-/- mice) had increased adiposity, decreased locomotor activity and increased food intake. However, male Slc6a4-/- mice had increased BAT thermogenic capacity, in contrast to the reduced EE expenditure following acute administration of Paroxetine. Using, RNA-Seq analysis and measurements of canonical 5-HT receptor second messengers (i.e. Ca2+ and cAMP transients), 5-HT2A was identified as the highest expressed 5-HT receptor in murine and human BAs and the only detected active 5-HT receptor in murine BAs. Genetic elimination of 5-HT2A prevented 5-HT induced increases in Ca2+ transient peaks and decreases in Ppargc1a mRNA expression in cultured BAs. In vivo ablation of 5-HT2A in adipose tissue increased BAT thermogenic capacity. Furthermore, in silico analyses predicted that pharmacological inhibition of 5- HT2A would induce a thermogenic program. In vitro, 5-HT2A receptor antagonists eliminated 5-HT induced Ca2+ transients and in vivo, a single injection of a peripherally-restricted 5-HT2A antagonist (Xylamidine) prevented 5-HT-induced impairments in BAT-mediated EE. Chronic administration of Xylamidine to chow- fed mice for 5-weeks improved BA T thermogenesis. Co-administration of Xylamidine with Paroxetine, however, did not attenuate Paroxetine-induced weight gain but did improve BAT functional capacity Therefore, 5-HT2A antagonism improves BAT thermogenic capacity but does not increase EE. This represents a novel therapeutic approach for increasing thermogenic capacity that may be used in conjunction with BAT activating strategies to increase EE and attenuate obesity.