Effects of GABAB receptor agonists and antagonists on glycemia regulation in mice

Abstract

γ-Aminobutyric acid (GABA) inhibits insulin secretion through GABAB receptors in pancreatic β-cells. We investigated whether GABAB receptors participated in the regulation of glucose homeostasis in vivo. BALB/c mice acutely pre-injected with the GABAB receptor agonist baclofen (7.5 mg/kg, i.p.) presented glucose intolerance and diminished insulin secretion during a glucose tolerance test (GTT, 2 g/kg body weight, i.p.). The GABAB receptor antagonist 2-hydroxysaclofen (15 mg/kg, i.p.) improved the GTT and reversed the baclofen effect. Also a slight increase in insulin secretion was observed with 2-hydroxysaclofen. In incubated islets 1.10−5 M baclofen inhibited 20 mM glucose-induced insulin secretion and this effect was reversed by coincubation with 1.10−5 M 2-hydroxysaclofen. In chronically-treated animals (18 days) both the receptor agonist (5 mg/kg/day i.p.) and the receptor antagonist (10 mg/kg/day i.p.) induced impaired GTTs; the receptor antagonist, but not the agonist, also induced a decrease in insulin secretion. No alterations in insulin tolerance tests, body weight and food intake were observed with the treatments. In addition glucagon, insulin-like growth factor I, prolactin, corticosterone and growth hormone, other hormones involved in glucose metabolism regulation, were not affected by chronic baclofen or 2-hydroxysaclofen. In islets obtained from chronically injected animals with baclofen, 2-hydroxysaclofen or saline (as above), GABAB2 mRNA expression was not altered. Results demonstrate that GABAB receptors are involved in the regulation of glucose homeostasis in vivo. Treatment with receptor agonists or antagonists, given acutely or chronically, altered glucose homeostasis and insulin secretion alerting to the need to evaluate glucose metabolism during the clinical use of these drugs.