THE EFFECT OF INSULIN-INDUCED HYPOGLYCEMIA ON GENE-EXPRESSION IN THE HYPOTHALAMIC-PITUITARY-ADRENAL AXIS OF THE RAT

This study has examined the effects of insulin-induced hypoglycemia on expression of the CRH, arginine vasopressin, and POMC genes and corresponding peptides in freely moving, unanesthetized, male Sprague-Dawley rats. Animals were infused with 150 mM NaCl for 3 days before the experimental day and were then administered insulin (4 U/kg) or saline iv. In one experiment animals were killed 0, 30, 60, or 90 min after insulin or saline, and RNA was isolated from anterior pituitary, cerebral cortex, and punches of the hypothalamic paraventricular and supraoptic nuclei. In a second experiment, animals were killed 90 min after insulin or saline treatment, and RNA was isolated from whole hypothalami. RNA was analyzed by Northern blot. Plasma glucose fell from 106 +/- 5 to 38 +/- 2 mg/dl after insulin administration and remained low for the duration of the experiment. Plasma levels of ACTH, corticosterone, and vasopressin were 10-, 6-, and 4-fold higher, respectively, in the insulin-treated vs. control animals (by analysis of variance, P < 0.0001 in all cases), while plasma CRH was unchanged. During hypoglycemia POMC mRNA levels were 1.8-fold higher in the insulin-treated group (by analysis of variance, P < 0.025). In contrast, paraventricular nucleus, whole hypothalamic, and parietal cortex CRH mRNA and vasopressin mRNA were unchanged. These data support previous studies which indicated that POMC gene expression is increased by hypoglycemia. However, we found no evidence for an increase in paraventricular nucleus or cerebral cortex CRH mRNA expression during hypoglycemia-associated stimulation of the hypothalamic-pituitary-adrenal axis, suggesting that another factor(s) may mediate the observed increase in POMC gene expression.