Bicuculline-sensitive and insensitive effects of THIP on the binding of [3H]flunitrazepam

The gamma-aminobutyric acid (GABAA) receptor partial agonist THIP inhibits the binding of [3H]flunitrazepam to unwashed membranes from the forebrain of the rat at 0 degrees C in the absence of chloride ions, reducing the affinity and Bmax of benzodiazepine (BZ) receptors. The decrease in affinity of benzodiazepine receptors seems to be due to antagonism by THIP of the effects of endogenous GABA, present in the unwashed membrane preparation. When the binding with washed membranes was determined at 30 degrees C, in the presence of chloride ions, THIP like GABA enhanced the binding of [3H]flunitrazepam through an increase in the affinity of benzodiazepine receptors. The changes in the affinity of benzodiazepine receptors induced by THIP seem to be mediated by bicuculline-sensitive receptors. However, in the presence and absence of bicuculline, THIP reduced the total number of benzodiazepine binding sites, probably in a bicuculline insensitive manner.