Modulation of [3H]muscimol binding by picrotoxin, pentobarbitone, and etomidate was investigated in rat cerebellar and cerebral cortical membranes. In cerebellum, at 37 degrees C in the presence of chloride ions (150 mM), picrotoxin and picrotoxinin decreased specific [3H]muscimol binding to 43 +/- 3% of control, with an EC50 of 1.2 +/- 0.1 microM. [3H]Muscimol saturation experiments in the presence and absence of picrotoxin indicated that the picrotoxin effect was primarily due to a loss of high-affinity muscimol sites with KD approximately equal to 10 nM. Pentobarbitone enhanced specific [3H]muscimol binding to 259 +/- 3% of control, with EC50 = 292 +/- 37 microM, and etomidate increased binding to 298 +/- 18%, with EC50 = 7.1 +/- 1.0 microM. The influence of temperature and chloride ion concentration on these effects was investigated by comparing experiments at 37 and 0 degrees C in the presence or absence of chloride at constant ionic strength. The results indicate that studies at 0 degrees C underestimate the coupling between GABA receptors and barbiturate sites and that they greatly overestimate the importance of chloride ions in this phenomenon. In cerebral cortical membranes (37 degrees C, 150 mM Cl-), the effect of picrotoxin was similar to that observed in cerebellum, whereas the effects of pentobarbitone and etomidate were greater, but occurred at higher concentrations.

The dihydropyridine binding sites associated with rat neocortical synaptosomes and microvessels were compared using an in vitro [3H]PN 200-110 [(+)-[methyl-3H]-isopropyl 4-(2,1,3-benzoxadiazol-4-yl)-1,4-dihydro-2,6-dimethyl-5- methoxycarbonylpyridine-3-carboxylate] binding assay. Saturation experiments yielded similar KD values (approximately 70 pM) and Bmax values (approximately 400 fmol/mg of protein) for the two membrane preparations. Interaction experiments with [3H]PN 200-110 and various calcium-modulating substances provided further evidence for the practically identical nature of the synaptosomal and microvascular dihydropyridine binding sites. These findings predict that lipophilic dihydropyridines, simultaneously occupying the two central binding sites, have the dual effect of altering neuronal function and local blood flow.