Site-specific habituation of insulin-induced hypoglycemic induction of Fos immunoreactivity in glucocorticoid receptor: immunopositive neurons in the male rat brain

Abstract

Current studies show that type II glucocorticoid receptor (GR) stimulation during recurring insulin-induced hypoglycemia (RIIH) results in diminished hypoglycemic activation of neurons in discrete CNS metabolic structures, namely the lateral hypothalamic area (LHA), hypothalamic paraventricular (PVH) and dorsomedial (DMH) nuclei, and nucleus of the solitary tract (NTS). The present work utilized immunofluorescence histochemistry to evaluate the reactivity of GR-expressing neurons in characterized hypothalamic, thalamic, and hindbrain metabolic structures to glucoprivation, and to determine if antecedent hypoglycemic stimulation of central GR decreases Fos protein expression by these neurons. Groups of adult male rats were injected subcutaneously with one or four doses of the intermediate-acting insulin, Humulin NPH, on as many days, while controls received diluent only. Rats injected with four doses of insulin were pretreated by intracerebroventricular administration of the selective GR antagonist, CP-475222, or vehicle alone prior to insulin doses 1-3. All animals were sacrificed by trancardial perfusion 2 h after injections on day four of the study. Mean numbers of GR-immunoreactive (-ir) neurons did not differ between groups injected with diluent versus one dose of insulin in each structure evaluated, but were significantly elevated above baseline on the fourth day of RIIH in the LHA and DMH, but not the PVH, VMH, ARC, thalamic paraventricular (PVT), or NTS. Counts of GR-ir-positive neurons in each site were similar between groups treated with CP-475222 or vehicle icv during RIIH. While mean numbers of GR-plus Fos-ir neurons in the PVH, DMH, LHA, and NTS, but not the PVT were significantly elevated after one dose of NPH, this increase was abolished in each site by RIIH. Pharmacological antagonism of central GR during antecedent hypoglycemia prevented RIIH-associated habituation of Fos colabeling of GR-expressing neurons in the PVH, DMH, and LHA. These data show that RIIH increases nuclear immunolabeling for GR in discrete CNS metabolic structures, evidence that recurring metabolic stress may amplify receptor-mediated genomic regulatory function in local neurons. The results also demonstrate that GR-containing neurons in the LHA, DMH, PVH, and NTS react to hypoglycemia by induction of the Fos stimulus-transcription cascade, and that precedent stimulation of central GR is critical for RIIH-associated habituation of this functional response in the former three sites.