Brain region-dependent effects of dexamethasone on counterregulatory responses to hypoglycemia in conscious rats

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Abstract
The aim of this study was to determine whether activation of central type II glucocorticoid receptors can blunt autonomic nervous system counterregulatory responses to subsequent hypoglycemia. Sixty conscious unrestrained Sprague-Dawley rats were studied during 2-day experiments. Day 1 consisted of either two episodes of clamped 2-h hyperinsulinemic (30 pmol·kg−1·min−1) hypoglycemia (2.8 ± 0.1 mM; n = 12), hyperinsulinemic euglycemia (6.2 ± 0.1 mM; n = 12), hyperinsulinemic euglycemia plus simultaneous lateral cerebroventricular infusion of saline (24 μl/h; n = 8), or hyperinsulinemic euglycemia plus either lateral cerebral ventricular infusion ( n = 8; LV-DEX group), fourth cerebral ventricular ( n = 10; 4V-DEX group), or peripheral ( n = 10; P-DEX group) infusion of dexamethasone (5 μg/h), a specific type II glucocorticoid receptor analog. For all groups, day 2 consisted of a 2-h hyperinsulinemic (30 pmol·kg−1·min−1) or hypoglycemic (2.9 ± 0.2 mM) clamp. The hypoglycemic group had blunted epinephrine, glucagon, and endogenous glucose production in response to subsequent hypoglycemia. Consequently, the glucose infusion rate to maintain the glucose levels was significantly greater in this group vs. all other groups. The LV-DEX group did not have blunted counterregulatory responses to subsequent hypoglycemia, but the P-DEX and 4V-DEX groups had significantly lower epinephrine and norepinephrine responses to hypoglycemia compared with all other groups. In summary, peripheral and fourth cerebral ventricular but not lateral cerebral ventricular infusion of dexamethasone led to significant blunting of autonomic counterregulatory responses to subsequent hypoglycemia. These data suggest that prior activation of type II glucocorticoid receptors within the hindbrain plays a major role in blunting autonomic nervous system counterregulatory responses to subsequent hypoglycemia in the conscious rat.