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. / Thesis / Doctor of Philosophy (PhD) / Obesity is a growing global pandemic caused by excessive energy intake over energy expenditure (EE). Some medications, such as certain selective serotonin (5-HT) reuptake inhibitor (SSRI) type antidepressants, also contribute to weight gain via reasons which are not fully understood. Currently available weight- loss medications decrease energy intake but do not affect EE. Recently, inhibiting the production of 5-HT outside the brain decreased weight gain in a model of obesity. Furthermore, this was associated with an improvement in the activity of a specialized type of adipose tissue, called brown adipose tissue (BAT). BAT is capable of expending energy in the form of thermogenesis and thus when active increases energy expenditure. We hypothesized that 5-HT impairs BAT activity and that blocking 5-HT activity may reduce weight gain in a model of antidepressant- induced weight gain. Herein, we investigated whether elevating 5-HT or increasing 5-HT downstream signaling modified BAT activity, which 5-HT receptor(s) is/are predominantly expressed in brown adipocytes (BAs), and what the effect on BAT would be if this/these receptors were eliminated. We found that in cell culture “supraphysiological” doses of 5-HT acutely impaired BA lipid mobilization and glucose metabolism; whereas, circulating concentrations of 5-HT impaired expression of select mitochondrial genes when serotonin transport was reduced. In mice, acute injections of high dose 5-HT attenuated BAT activity in response to an adrenergic stimulus. Acute treatment with an SSRI decreased EE and locomotor activity. Mice genetically lacking the serotonin transporter (the target of SSRIs) had increased weight gain (particularly fat mass), increased food intake and reduced locomotor activity, but improved BAT functional capacity. We subsequently identified that the predominantly expressed and active receptor in BAs was the 5- HT2A receptor. Genetically eliminating the 5-HT2A receptor in BAs prevented 5- HT’s reduction of a major mitochondrial gene expression regulator and improved BAT functional capacity in mice. Inhibiting 5-HT2A with a 5-HT2A brain impermeable antagonist, Xylamidine, increased BAT functional capacity in mice. Treating mice with Paroxetine (Paxil®), a SSRI known to increase weight gain, and Xylamidine did not attenuate Paroxetine-induced weight gain nor increase EE but did improve BAT functional capacity. In conclusion, we found that 1) chronic treatment with physiological levels of 5-HT impaired BAT functional capacity, 2) elimination/inhibition of adipocyte 5-HT2A improved BAT functional capacity in vivo and 3) inhibiting peripheral 5-HT2A alone did not attenuate Paroxetine- induced weight gain.
| Identifer | oai:union.ndltd.org:mcmaster.ca/oai:macsphere.mcmaster.ca:11375/25504 |
| Date | January 2020 |
| Creators | Green, Alexander E |
| Contributors | Steinberg, Gregory R, Medical Sciences (Cell Biology and Metabolism) |
| Source Sets | McMaster University |
| Language | English |
| Detected Language | English |
| Type | Thesis |
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