Hypoglycemia unawareness and counterregulatory failure are serious and life-threatening drawbacks to the use of intensive insulin therapy to control type 1 diabetes. Adrenomedullary epinephrine secretion is one of the most rapidly effective and readily preserved defenses against hypoglycemia. Elucidating neural pathways underlying the regulation of counterregulatory responses, and of epinephrine in particular, will aid in designing therapies to combat hypoglycemia unawareness. We have recently found that reduced brain histaminergic activity correlates with impaired counterregulation, and that inhibiting histamine synthesis decreases epinephrine responses to hypoglycemia. As part of an overall goal to identify neural mechanisms for hypoglycemia unawareness, we hypothesize that brain histamine neurons are involved in hypoglycemia-induced secretion of epinephrine, and possibly other counterregulatory hormones. We further hypothesize that manipulation of central histamine levels will aid in identifying additional neural pathways controlling these responses. To address our hypotheses, we will: (1) use specific histamine antagonists and measure counterregulatory hormones to test if hypoglycemia-induced epinephrine secretion will be selectively (a) inhibited by brain-penetrant histamine H1- or H2- receptor antagonists, (b) unaffected by nonpenetrant antagonists, and (c) enhanced by increasing central histamine with the H3 autoreceptor antagonist, thioperamide. We will also (2) integrate hypoglycemic clamp techniques in unrestrained, chronically cannulated mice to test (a) the impact of streptozotocin (STZ)-diabetes on the progression of counterregulatory failure and (b) the hypothesis that thioperamide-evoked increases in central histamine will reverse counterregulatory deficits induced by recurrent hypoglycemia in STZ mice. Finally (3), using brains of mice from Aims 1 and 2, we will combine in situ hybridization analysis of neuronal c-fos gene expression with histochemical analysis (histidine decarboxylase mRNA) or histamine antagonist administration, to test the hypotheses that (a) defined neural pathways, including hypothalamic histamine neurons, exhibit altered activity after recurrent hypoglycemia, and (b) experimental alterations in central histaminergic tone will reveal additional neuron populations relevant to counterregulation. These studies will provide novel information on the neuropharmacology of glucose control that may reveal therapeutic strategies to prevent hypoglycemia unawareness.
