Leptin and HPA axis activity in diabetic rats: Effects of adrenergic agonists

Type I Diabetes (T1D) is associated with reduced leptin levels and increased stress axis activity marked by elevations in norepinephrine (NE) levels in the paraventricular nucleus (PVN) of the hypothalamus. We hypothesized that leptin suppresses stress axis activity in T1D through central and peripheral mechanisms. In the first experiment, adult male Sprague Dawley rats were implanted with a cannula in the PVN and randomly divided into a non-diabetic group treated with vehicle (n = 6) and a diabetic group treated with streptozotocin (n = 13). Food intake and water intake was measured for 14 days. On the last day, a subset of diabetic rats were treated with 500 µg of leptin i.p. Rats were subjected to push-pull perfusion of the PVN and hourly blood sampling for 5 h. In the next experiment, diabetic rats were treated either with an alpha-2 adrenergic agonist, clonidine (CLON), or a beta adrenergic agonist isoproterenol (ISO), to reverse the effects of leptin. Rats were subjected to push pull perfusion and hourly blood sampling. In experiment 1, T1D increased food intake, water intake, NE release in the PVN and circulating CS levels. Leptin treatment decreased NE release modestly but produced a robust reduction in corticosterone (CS) levels. In experiment 2, CLON but not ISO was able to reverse the effect of leptin on NE levels in the PVN, however, both agonists were capable of blocking leptin's effects on circulating CS. These results suggest that in diabetic rats, the sensitivity of the hypothalamus to beta adrenergic agonists is altered, while the adrenals remain sensitive to both alpha and beta adrenergic agonists.