Expression of Key Modulators of Serotonergic Neurotransmission after Chronic Metformin Treatment

Abstract

Despite metformin’s popularity as a treatment for metabolic and developmental disorders its mechanisms of action are still unknown. The goals of this project were to (1) assess the behavioral benefits of chronic metformin treatment alone for core autism symptoms, (2) assess if metformin acts on melanocortin 4 (MCR4) receptors in the brain to alter hormone levels in serum, and (3) to determine if metformin increases brain levels of peroxisome proliferator‐activated receptors α, δ, and γ (PPARs), which modulate insulin signaling throughout the body and increase in the liver with chronic metformin treatment. Since BTBR social behavior is enhanced by selective serotonin reuptake inhibitors (SSRIs) such as sertraline, and their chronic use reduces or otherwise alters receptor and transporter expression in the brain, we hypothesized that with chronic metformin treatment, expression of key regulators of serotonin neurotransmission such as the serotonin transporter (SERT) or organic cation transporters (OCTs) responsible for serotonin clearance would be reduced. Also, given prior evidence that chronic SSRI treatment increases serotonin levels in brain extracellular fluid, we hypothesized that expression of serotonin receptors such as 5‐HT2A among others would be reduced. This compensatory response occurs with chronic SSRI treatments. To explore these hypotheses BTBR T+ Itpr3tf/J (BTBR) and C57BL/6J (C57) mice were given chronic metformin treatment for six weeks, with behavior tests concluding in week 5, before euthanasia and blood and brain collection. With chronic metformin we found social interaction preference and social dominance were enhanced in male but not female BTBR mice. Blood serum was analyzed for hormonal changes, with C57 metformin treated mice seeing a reduction in corticosterone compared to controls. PPARδ was lower for metformin treated female C57 mice than control females, while PPARγ was found to be different between strain controls. Chronic metformin elevated blood glucose levels in female but not male mice. Male mice showed no changes in MCR 4 or SERT expression. Immunohistochemistry revealed an increase in OCT3 labeling in the brain of a BTBR male treated with metformin compared to a BTBR control male. The density of 5‐HT2A receptors was decreased in the anterior olfactory nucleus for BTBR mice following chronic metformin treatment. Overall, these findings indicate chronic metformin treatments that enhance social behaviors in male, but not female BTBR mice also have divergent effects on blood sugar among sexes. We did not find any evidence of chronic metformin treatment altering MCR4 or SERT but there was a decrease in 5‐HT2A expression. Effects on hormones were not present for BTBR mice but corticosterone was lower for C57 metformin treated males than controls, and metformin may increase OCT expression in BTBR males.