Characterization of high affinity binding of [ 3H]propyl bicyclic phosphate to house fly head extracts

Abstract

The head of the house fly, Musca domestica L., was found to contain saturable components of specific binding of 4- n-propyl[2,3- 3H]-2,6,7-trioxa-1-phosphabicyclo[2.2.2]octane 1-oxide ([ 3H]Pr-BP). A ratio of specific to total binding of radioreceptor assays was quite favorable, being 0.89 under the standard conditions. The apparent dissociation constant and maximal binding capacity were estimated to be about 4 n M and about 30 fmol/mg of protein, respectively, although association, dissociation, and saturation analyses suggested the presence of two or more populations of binding sites. Specific [ 3H]Pr-BP binding showed a marked negative temperature coefficient and was little affected by pH in incubation media. Anions which pass through chloride channels attenuated specific [ 3H]Pr-BP binding whereas impermeable anions enhanced it. Specific binding was selectively inhibited by insecticidal bicyclic phosphorus esters. Various neuroactive chemicals such as GABA agonists, GABA antagonists, cyclodiene insecticides, and benzodiazepines had little effect on specific binding. There was a correlation between GABA content and the density of specific Pr-BP binding sites in each part of the house fly body. In many respects, however, characteristics of the current binding site were different from those of GABA receptor-coupled sites already characterized with [ 3H]Pr-BP and the [ 35S] t-butyl thiono analog in the rat brain. Bicyclic phosphorus esters may act on site(s) apart from the GABA neurotransmission system in the house fly.