Characterization of binding sites for [(3)H]Gaba in Drosophila melanogaster CNS membranes

Drosophila melanogaster membranes prepared from cephalic ganglia contain two apparently independent populations of saturable binding sites for [(3)H]GABA: the higher-affinity sites (K(d) = 23 nM; B(max = 0.97 pmol/mg)) are already labeled at 0 degrees C, while a lower-affinity population of binding sites (K(d = 258 nM); B(max = 8.51 pmol/mg)) can be additionally identified at 23 degrees C and higher temperatures. GABA and then muscimol, together with ?-aminovalerate, are very efficient displacers of the specific binding while the A-specific agonist isoguvacine, the A-specific antagonist bicuculline methiodide and the B-specific agonist baclofen have low to negligible effect. [(3)H]GABA binding is not modulated by allosteric effectors characteristic of the vertebrate A-site, except for an anomalous stimulation by DMCM. Both [(3)H]flunitrazepam and [(3)H]TBOB give "specific" binding values under different experimental conditions but, after kinetic analysis, the binding appears to be non-saturable in both cases. We conclude that there are marked differences between the Drosophila GABA receptor and the vertebrate GABA(A) receptor complex, although our results do not exclude the existence of an insect receptor complex, associated to a Cl(?) channel, with different structural and pharmacological properties.