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.