Picrotoxinin-sensitive binding of a convulsant
4'-ethynyl-4-n[2,3-3H2]propyl-bicycloorthobenzoate ([³H]EBOB) to
gamma-aminobutyric acid type A (GABA_A) receptors was characterized in
rat cerebrocortical and cerebellar membranes. The non-penetrating
organic anions, furosemide and niflumate, in spite of their structural
similarities, exerted differential effects on [³H]EBOB binding.
Furosemide, a loop diuretic and a specific antagonist of a cerebellar
GABA_A receptor population, and GABA decreased the inhibitory potencies
of each other in the cerebellum, while enhanced them in the cortex. The
inhibitory potencies of niflumate, an anti-inflammatory and a chloride
channel blocker. and GABA were enhanced by each other both in the
cerebellum and cortex. Removal of chloride ions did not modify the
effects of furosemide on [³H]EBOB binding. Furosemide antagonized the
inhibition of cerebellar [³H]EBOB binding by a low pentobarbital
concentration (0.1 mM), but enhanced the inhibition by a high
concentration (0.5 mM). The results indicate that [³H]EBOB binding can
be used to detect the known pharmacological features of the cerebellar
granule cell-specific 16 subunit-containing GABA_A receptors. The data
extends the properties of furosemide antagonism of this receptor subtype
to chloride insensitivity and interactions with barbiturate sites.