Daily variations in GABA receptor function in Syrian hamster cerebral cortex.

The existence of diurnal changes in postsynaptic expression of gamma-aminobutyric acid (GABA) type A receptors was assessed in cerebral cortex of Syrian hamsters by measuring [3H]GABA binding and the influx of 36Cl- in synaptoneurosomes. A diurnal variation in dissociation constant of [3H]GABA binding to cerebral cortex membranes, and the absence of diurnal differences in maximal number of sites, were found. When the nycthemeral changes in muscimol-stimulated 36Cl- uptake by cortical synaptoneurosomes were assessed, a maximum occurred late at night (i.e. 0400 h). At 1600 h, micromolar concentrations of flunitrazepam potentiated significantly the influx of chloride induced by muscimol, while at 0400 h flunitrazepam did not exert any significant effect on 36Cl- uptake. The results indicate that postsynaptic type A GABAergic activity peaked at nocturnal hours in the cerebral cortex of Syrian hamsters.