Characterization of subtypes of gamma-aminobutyric acid receptors in an Ascaris muscle preparation by binding assay and binding of PF1022A, a new anthelmintic, on the receptors

We examined the effect of PF1022A, one of the gabergic anthelmintics newly developed in Japan, on gamma-aminobutyric acid (GABA) receptors using a radioligand binding technique in isolated membrane preparations of the nematode Ascaris suum. Membrane protein was prepared from the homogenate of somatic muscle cells after ultracentrifugation. In addition to the basic binding of [2,3-3H-(N)]-GABA, the radioligand [methyl-3H]-bicuculline is used to identify the GABAA receptor, whereas [butyl-4-3H]-baclofen is employed for GABAB receptor sites. The dissociation constants (Kd values) and the maximal numbers of binding sites (Bmax values) from Scatchard plotting for GABA receptors are close to those obtained in mammalian brain. PF1022A displaced in a concentration-dependent way the binding of [2,3-3H(N)]-GABA and [methyl-3H]-bicuculline as did other specific gabergic agents. In addition, PF1022A decreased the binding of [butyl-4-3H]-baclofen at a higher concentration, although this binding did not represent GABAB sites. In a comparison of the inhibition constants (Ki values) of PF1022A with those of other agents, it is conclusive that PF1022A bound with GABA receptors. A direct effect of PF1022A on GABA receptors can thus be postulated.