GABA receptors on the cell-body membrane of an identified insect motor neuron

The pharmacology of a γ-aminobutyric acid (GABA) receptor on the cell body of an identified motor neuron of the cockroach (Periplaneta americana) was investigated by current-clamp and voltage-clamp methods. Iontophoretic application of GABA increased membrane conductance to chloride ions, and prolonged application resulted in desensitization. Hill coefficients, determined from dose-response data, indicated that binding of at least two GABA molecules was required tok activate the chloride channel. Differences between vertebrate GABA(A) receptors and insect neuronal GABA receptors were detected. For the GABA receptor of motor neuron D(f), the following rank order of potency was observed: isoguvancine > muscimol ≥ GABA > 3-aminopropanesulphonic acid. The GABA(B) receptor agonists (bicuculline, pitrazepin, RU5135 and picrotoxin), only picrotoxin (10-7 M) produced a potent, reversible block of the response to GABA of motor neuron D(f). Both picrotoxinin and picrotin also blocked GABA-induced currents. Bicuculline hydrochloride (10-4 M) and bicuculline methiodide (10-4 M) were both ineffective when applied at resting membrane potential (-65 mV), although at hyperpolarized levels partial block of GABA-induced current was sometimes observed. Pitrazepin (10-4 M) caused a partial, voltage-independent block of GABA-induced current. The steroid derivative RU5135 was inactive at 10-5 M. In contrast to the potent competitive blockade of vertebrate GABA(A) receptors by bicuculline, pitrazepin and RU5135, none of the weak antagonism caused by these drugs on the insect GABA receptor was competitive. Flunitrazepam (10-6 M) potentiated GABA responses, providing evidence for a benzodiazepine site on an insect GABA-receptor-chloride-channel complex.