Impact of recurring intermediate insulin-induced hypoglycemia on hypothalamic paraventricular corticotropin-releasing hormone, oxytocin, vasopressin and glucokinase gene profiles: role of type II glucocorticoid receptors.

Activation of central type II glucocorticoid receptors (GR) during neutral protamine Hagedorn insulin (NPH) administration exacerbates recurring hypoglycemia. The hypothalamic paraventricular nucleus (PVN) integrates metabolic sensory input, controls autonomic and neuroendocrine motor outflow, and is characterized by abundant GR expression. The present studies investigated the hypothesis that PVN GR mediate intensification of hypoglycemia by serial NPH dosing, and that PVN glucokinase (GCK) and glucoregulatory neuropeptide genes acclimate to this treatment paradigm through GR-dependent mechanisms. Groups of adult male rats were injected subcutaneously with one or four doses of NPH, on as many days, while controls received vehicle. Bilateral administration of the selective GR antagonist, CP-472555, into the PVN prior to the first three NPH injections prevented amplification of hypoglycemia in response to the final insulin dose, while intra-PVN delivery of the GR agonist, dexamethasone, to euglycemic rats did not modify ensuing NPH-induced hypoglycemia. Quantitative real-time RT-PCR analysis of microdissected PVN tissue revealed that GCK, corticotropin-releasing hormone (CRH), oxytocin (OT), and vasopressin (VP) mRNA levels were unchanged in response to acute NPH, and baseline gene profiles measured 24 h after antecedent injections were similar to vehicle controls. In contrast, serial dosing with NPH elevated CRH and GCK, diminished OT, but did not alter VP gene transcripts. Intracerebroventricular CP-472555 delivery in conjunction with antecedent NPH dosing prevented transcriptional habituation of GCK and OT genes, but did not modify CRH or VP mRNA profiles. The present data show that activation of PVN GR during antecedent intermediate insulin-induced hypoglycemia is required for exacerbation of recurring hypoglycemia, and receptor stimulation in the absence of hypoglycemia and/or its sequelae does not intensify the effects of subsequent NPH administration. The results also provide evidence for acclimation of PVN CRH, GCK, and OT gene profiles to serial NPH dosing, and demonstrate that GR may be involved in GCK and OT transcriptional adaptation to ongoing intermediate insulin-induced hypoglycemia.