Therapeutic Potential of Metformin to Ameliorate Behavioral Symptoms of Autism

Abstract

Autism spectrum disorders (ASD) affect 1/39 boys and 1/144 girls and manifest in lifelong social interaction deficits and restrictive-repetitive traits of varying severity. Currently no treatments capable of both enhancing social interaction and curbing restrictive repetitive behaviors have been identified. In light of strikingly higher incidence of autism in males versus females, it is surprising that treatments for dysfunctions of steroid hormone metabolism have not been more extensively studied. Clinical evidence supports this approach, as in several studies prepubertal boys and adolescent girls with autism had more androgens and other steroids than age matched children without autism. The antidiabetic drug metformin was found to inhibit adrenergic steroid synthesis, particularly of androgens, through antagonism of melanocortin receptors. Metformin is already co-administered with antipsychotics to manage weight in some autism patients. The goal of this study was to establish in mice if an androgen inhibitor, metformin, can enhance social or reduce repetitive behavior with chronic use. BTBR T+Itpr3tf/J (BTBR) mice were used because this strain is a popular behavioral autism model with high testosterone, corticosterone and progesterone levels, and low serotonin (5-HT) transporter (SERT) density as compared to C57BL/6J (B6) mice, which were used as a strain control. Initially acute metformin treatment dose-dependently enhanced male BTBR mouse social interaction preference for a stranger mouse versus a novel object (p <0.05 different from control at 150 mg/kg dose, N= 9-11 mice), and had no adverse effects on B6 social behaviors. Metformin (150 mg/kg/d) was administered to 10 each adult male and female mice via drinking water for 28 days, yielding a steady state concentration in brain of 36 ± 12 ng/ml. A series of autism relevant behavior tests were performed to compare BTBR and B6 treatment responses. Three chamber social interaction preference tests revealed no effect of chronic metformin treatment on social preference in either strain by the measure of time spent in chambers. Yet BTBR male and female social dominance, measured by the tube test, increased slightly with chronic metformin treatment, but was still less than B6 mouse dominance, which was unaffected by chronic metformin treatment. Marble burying by mice over 30 min did not differ by strain, sex or metformin treatment. Cognitive flexibility in the water T-maze test, measured by number of errors made on day 1 of reversal learning, was unaffected by metformin treatment (F(1,100) = 0.02, p = 0.9). Males and females were pooled for this measure. [3H] histamine and [3H] metformin uptake were greater in hippocampal synaptosomes isolated from metformin treated male B6 mice than in untreated ones (N = 4). This finding may indicate chronic metformin treatment increases hippocampal expression of common transporters of these substrates such as organic cation transporters (OCTs), which also transport 5-HT and other monoamines. Overall, our findings indicate that metformin treatment can enhance social preference and dominance, and chronically it may enhance expression of OCTs.