Picrotoxin and convulsant binding sites in mammalian brain

Abstract

( 3H)α-Dihydropicrotoxinin (DHP) binds to membrane sites in mammalian brain which appear to be related to the convulsant action of picrotoxin at the level of membrane Cl −channels which are at least partially regulated by the inhibitory neurotransmitter GABA. DHP binding to rat cerebral cortex (assayed by centrifugation) showed a single class of sites ( K d = 2 μM) and B max of 4 pmol/mg protein, and was competitively inhibited by depressant barbiturates (1–10μM), excitatory barbiturates (10–100 nM), and cage convulsants, e.g. bicyclophosphates (1–10 μM), but not by GABA or related compounds. DHP binding was stereospecifically and potently inhibited by pyrethroid insecticides (10–1000 nM), by depressant benzodiazepines (0.5–50 μM), and excitatory benzodiazepines (e.g. RO5–3663) at <0.1μM, and also by 0.2 mM hypoxanthine and cytosine, but not nicotinamide, and by aqueous extracts of brain (40 μg protein, 10 5x g supernatant) at 0° without preincubation. These or related substances may be endogenous ligands for the receptor sites which bind picrotoxin and other convulsants. Anticonvulsants may act at the same site (Cl −channels?) in the opposite manner.