Increased GABAergic tone in the ventromedial hypothalamus contributes to suppression of counterregulatory responses after antecedent hypoglycemia.

OBJECTIVE: We have previously demonstrated that modulation of gamma-aminobutyric acid (GABA) inhibitory tone in the ventromedial hypothalamus (VMH), an important glucose-sensing region in the brain, modulates the magnitude of glucagon and sympathoadrenal responses to hypoglycemia. In the current study, we examined whether increased VMH GABAergic tone may contribute to suppression of counterregulatory responses after recurrent hypoglycemia. RESEARCH DESIGN AND METHODS: To test this hypothesis, we quantified expression of the GABA synthetic enzyme, glutamic acid decarboxylase (GAD), in the VMH of control and recurrently hypoglycemic rats. Subsequently, we used microdialysis and microinjection techniques to assess changes in VMH GABA levels and the effects of GABA(A) receptor blockade on counterregulatory responses to a standardized hypoglycemic stimulus. RESULTS: Quantitative RT-PCR and immunoblots in recurrently hypoglycemic animals revealed that GAD(65) mRNA and protein were increased 33 and 580%, respectively. Basal VMH GABA concentrations were more than threefold higher in recurrently hypoglycemic animals. Furthermore, whereas VMH GABA levels decreased in both control and recurrently hypoglycemic animals with the onset of hypoglycemia, the fall was not significant in recurrently hypoglycemic rats. During hypoglycemia, recurrently hypoglycemic rats exhibited a 49-63% reduction in glucagon and epinephrine release. These changes were reversed by delivery of a GABA(A) receptor antagonist to the VMH. CONCLUSIONS: Our data suggest that recurrent hypoglycemia increases GABAergic inhibitory tone in the VMH and that this, in turn, suppresses glucagon and sympathoadrenal responses to subsequent bouts of acute hypoglycemia. Thus, hypoglycemia-associated autonomic failure may be due in part to a relative excess of the inhibitory neurotransmitter, GABA, within the VMH.

Type 1 corticotropin-releasing factor receptors in the ventromedial hypothalamus promote hypoglycemia-induced hormonal counterregulation.

Type 2 corticotropin-releasing factor (CRF) receptors (CRFR2) within the ventromedial hypothalamus (VMH), a key glucose-sensing region, play a major role in regulating the hormonal counterregulatory responses (CRRs) to acute hypoglycemia. The VMH expresses both subtypes of CRF receptors, CRFR1 and CRFR2. The objective of this study was to examine the role of the CRFR1 receptor in the VMH in the regulation of the CRR to acute hypoglycemia. To compare the hormonal CRR to hypoglycemia, awake and unrestrained Sprague-Dawley rats were bilaterally microinjected to the VMH with either 1) aECF, 2) CRF (1 pmol/side), 3) CRFR1 antagonist Antalarmin (500 pmol/side), or 4) CRF + Antalarmin prior to undergoing a hyperinsulinemic hypoglycemic (2.8 mM) clamp. A second series of studies also incorporated an infusion of [(3)H]glucose to allow the calculation of glucose dynamics. In addition the effect of CRFR1 antagonism in the paraventricular nucleus (PVN) was studied. Activation of VMH CRFR1 increased, whereas inhibition of CRFR1 suppressed hypoglycemia-induced CRRs. Inhibition of VMH CRFR1 also increased peripheral glucose utilization and reduced endogenous glucose production during hypoglycemia, whereas VMH CRF reduced peripheral glucose utilization. In contrast CRFR1 inhibition in the PVN blunted corticosterone but not epinephrine or glucagon CRR to hypoglycemia. In contrast to CRFR2 activation, CRFR1 activation within the VMH amplifies CRRs to acute hypoglycemia. The balance between these two opposing CRFRs in this key glucose-sensing region may play an important role in determining the magnitude of CRRs to acute hypoglycemia.

Cancer chemotherapy by deoxynucleotide depletion and E2F-1 elevation.

We propose that the lethality of commonly used anticancer drugs, e.g., methotrexate and cis-platinum are due, at least in part, to an increase of the E2F-1-mediated apoptotic cascade. The drugs directly or indirectly decrease deoxynucleoside triphosphates. The E2F family acts to provide control of S phase by transcribing genes required for deoxynucleoside triphosphate and DNA synthesis. Thus, a mechanism for control of E2F-1 is essential, a signal safeguarding against aberrant or uncontrolled cell proliferation. We have proposed a feedback control by NTPs that down-regulates E2F-1. Here, we provide evidence in support of this hypothesis.