To examine persisting effects of depolarizing rises in extracellular potassium concentration ([K+](o)) on synapses, we depolarized cells to simulate ischemia-like rises in [K+](o). Elevated [K+](o) for 1-16 hr severely depressed glutamate signaling, while mildly depressing GABA transmission. The glutamate-specific changes were plastic over several hours and involved a decrease in the size of the pool of releasable vesicles. Rather than a reduction of the number of vesicles per release site, the change involved functional elimination of release sites. This change was clearly dissociable from a second effect, depressed probability of transmitter release, which was common to both glutamate and GABA transmission. Thus, while other recent evidence links alteration of the releasable pool size with changes in p(r), our results suggest the two can be independently manipulated. Selective depression of glutamate release may provide an adaptive mechanism by which neurons limit excitotoxicity.

Although neurosteroids have rapid effects on GABA A receptors, study of steroid actions at GABA receptors has been hampered by a lack of pharmacological antagonists. In this study, we report the synthesis and characterization of a steroid analog, (3␣,5␣)-17-phenylandrost-16-en-3-ol (17PA), that selectively antagonized neurosteroid potentiation of GABA responses. We examined 17PA using the ␣1␤2␥2 subunit combination expressed in Xenopus laevis oocytes. 17PA had little or no effect on baseline GABA responses but antagonized both the response augmentation and the direct gating of GABA receptors by 5␣-reduced potentiating steroids. The effect was selective for 5␣-reduced potentiating steroids; 5␤-reduced potentiators were only weakly affected. Likewise, 17PA did not affect barbiturate and benzodiazepine potentiation. 17PA acted primarily by shifting the concentration response for steroid potentiation to the right, suggesting the possibility of a competitive component to the antagonism. 17PA also antagonized 5␣-reduced steroid potentiation and gating in hippocampal neurons and inhibited anesthetic actions in X. laevis tadpoles. Analogous to benzodiazepine site antagonists, the development of neurosteroid antagonists may help clarify the role of GABA-potentiating neurosteroids in health and disease.

The in vivo effects of administration of the synthetic, functional biomimetic cation [Cr(3)O(O(2)CCH(2)CH(3))(6)(H(2)O)(3)](+) to healthy and type I and type II diabetic model rats are described. In contrast to current chromium-containing nutrition supplements, which only serve as sources of absorbable chromium, the trinuclear cation has been shown in in vitro assays to interact with the insulin receptor, activating its kinase activity, presumably by trapping the receptor in its active conformation. Thus, treatment of rats with the trinuclear cation would be expected to result in changes in lipid and carbohydrate metabolism related to insulin action. After 24 weeks of intravenous administration (0-20 micro g Cr/kg body mass), the cation results in a concentration-dependent lowering of levels of fasting blood plasma LDL cholesterol, total cholesterol, triglycerides, and insulin and of 2-h plasma insulin and glucose levels after a glucose challenge; these results confirm a previous 12-week study examining the effect of the synthetic cation on healthy rats and are in stark contrast to those of administration of other forms of Cr(III) to rats, which have no effect on these parameters. The cation has little, if any, effect on rats with STZ-induced diabetes (a type I diabetes model). However, Zucker obese rats (a model of the early stages of type II diabetes) after 24 weeks of supplementation (20 micro g/kg) have lower fasting plasma total, HDL, and LDL cholesterol, triglycerides, and insulin levels and lower 2-h plasma insulin levels. The lowering of plasma insulin concentrations with little effect on glucose concentrations suggests that the supplement increases insulin sensitivity.

Depolarization promotes neuronal survival through moderate increases in Ca(2+) influx, but the effects of survival-promoting depolarization (vs conventional trophic support) on neuronal signaling are poorly characterized. We found that chronic, survival-promoting depolarization, but not conventional trophic support, selectively decreased the somatic Ca(2+) current density in hippocampal and cerebellar granule neurons. Depolarization rearing depressed multiple classes of high-voltage activated Ca(2+) current. Consistent with the idea that these changes also affected synaptic Ca(2+) channels, chronic depolarization presynaptically depressed hippocampal neurotransmission. Six days of depolarization rearing completely abolished glutamate transmission but altered GABA transmission in a manner consistent with the alterations of Ca(2+) current. The continued survival of depolarization-reared neurons was extremely sensitive to the re-establishment of basal culture conditions and was correlated with the effects on intracellular Ca(2+) concentration. Thus, compared with cells reared on conventional trophic factors, depolarization evokes homeostatic changes in Ca(2+) influx and signaling that render neurons vulnerable to cell death on activity reduction.

Fetal alcohol exposure results in cognitive and neurobehavioral deficits, but the effects of modifying genetic loci on the severity of these sequelas have not been well characterized. Although the cAMP signaling pathway has been shown to be an important modulator of ethanol sensitivity in adult mice, its potential role in modulating ethanol-induced neurodegeneration has not been examined. Adenylyl cyclases (ACs) 1 and 8 produce cAMP in response to intracellular calcium elevation and modulate several aspects of neuronal function, including ethanol sensitivity. AC1 and AC8 are expressed widely throughout the brain of neonatal mice, and genetic deletion of both AC1 and AC8 in double-knock-out (DKO) mice enhances ethanol-induced neurodegeneration in the brains of neonatal mice. In addition, ethanol treatment induces significantly greater levels of caspase-3 activation in the brains of DKO mice compared with wild-type (WT) mice, reflecting higher numbers of apoptotic neurons. Administration of the NMDA receptor antagonist MK801 [(+)-5-methyl-10,11-dihydro-5H-dibenzo [a,d] cyclohepten-5,10-imine hydrogen maleate] or the GABA(A) receptor potentiator phenobarbital, which mimics components of the effects of ethanol on neurons, results in significantly greater neurodegeneration in the brains of neonatal DKO mice than WT mice. Furthermore, loss of a single calcium-stimulated AC isoform potentiates neurodegeneration after administration of ethanol, MK801, or phenobarbital. In contrast, the levels of physiological cell death, death after hypoxia/ischemia, and excitotoxic cell death are not increased in the brains of DKO mice. Thus, AC1 and AC8 are critical modulators of neurodegeneration induced by activity blockade in the neonatal brain and represent genetic loci that may potentially modify the severity of fetal alcohol syndrome.