The γ-aminobutyrate/benzodiazepine receptor from pig brain. Enhancement of γ-aminobutyrate-receptor binding by the anaesthetic propanidid

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Abstract
The binding of [3H]muscimol, a .gamma.-aminobutyrate (GABA) receptor agonist, to a membrane preparation from pig cerebral cortex was enhanced by the anaesthetic propanidid in a concentration-dependent manner. At 0.degree. C, binding was stimulated to 220% of control values, with 50% stimulation at 60 .mu.M-propanidid. At 37.degree. C, propanidid caused a more powerful stimulation of [3H]muscimol binding (340% of control values). Propanidid (1 mM) exerted little effect on the affinity of muscimol binding (KD approx. 10 nM), but increased the apparent number of high-affinity binding sites in the membrane by 2-fold. Enhancement of [3H]muscimol binding was observed only in the presence of Cl- ions, half-maximal activation being achieved at approx. 40 mM-Cl-. Picrotoxinin inhibited the stimulation of [3H]muscimol binding by propanidid with an IC50 (concentration causing 50% inhibition) value of approx. 25 .mu.M. The enhancement of [3H]muscimol binding by propanidid was not additive with the enhancement produced by secobarbital. Phenobarbital inhibited the effect of propanidid and secobarbital. The GABA receptor was solubilized with Triton X-100 or with Chaps {3-[(3-cholamidopropyl)dimethylammonio]propanesulphonate}. Propanidid and secobarbital did not stimulate the binding of [3H]muscimol after solubilization with Triton X-100. However, the receptor could be solubilized by 5 mM-Chaps with retention of the stimulatory effects of propanidid and secobarbital. Unlike barbiturates, propanidid did not stimulate the binding of [3H]flunitrazepam to membranes. It is suggested that the ability to modulate the [3H]muscimol site of the GABA-receptor complex may be a common and perhaps functional characteristic of general anaesthetics.