Pharmacology of an insect central nervous system [(3)H]l-glutamate binding site

Using membrane extracts from the nerve cord of the cockroach Periplaneta americana, we have characterized a putative [(3)H]l-glutamate receptor binding site. Binding was measured in a Tris-citrate buffer (pH 7.4), in the absence of Na(+), K(?), Cl(?) and Ca(2+). A dissociation constant (K(D)) of 0.83 +/- 0.35 ?M (mean +/- SD, n = 5) and a binding capacity (B(max)) of 20.5 +/- 10.8 pmol (mean +/- SD, n = 5) of [(3)H]l-glutamate bound per mg membrane protein were obtained from saturation experiments. Affinities of a range of glutamate receptor ligands reveal a pharmacological profile that is not readily assigned to the existing vertebrate classes of central nervous system (CNS) glutamate receptors. l-Glutamate and ibotenate were the most effective ligands tested. Quisqualate, l-aspartate, l-cysteate and l-homocysteate suppressed by 55-65% the binding of [(3)H]l-glutamate suggesting the possible existence of glutamate receptor subtypes in insect CNS. No evidence was obtained for the presence of an insect NMDA (N- methyl- d -aspartate)-sensitive receptor.