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

Abstract

The pharmacology of a $\gamma $-aminobutyric acid (GABA) receptor on the cell body of an identified motor neuron of the cockroach (Periplaneta americana) was investigated by current-clamp and voltage-clamp methods. Iontophoretic application of GABA increased membrane conductance to chloride ions, and prolonged application resulted in desensitization. Hill coefficients, determined from dose--response data, indicated that binding of at least two GABA molecules was required to activate the chloride channel. Differences between vertebrate GABA$_{\text{A}}$ receptors and insect neuronal GABA receptors were detected. For the GABA receptor of motor neuron D$_{\text{f}}$, the following rank order of potency was observed: isoguvacine > muscimol $\geq $ GABA > 3-aminopropanesulphonic acid. The GABA$_{\text{B}}$ receptor agonist baclofen was inactive. Of the potent vertebrate GABA receptor antagonists (bicuculline, pitrazepin, RU5135 and picrotoxin), only picrotoxin (10$^{-7}$ M) produced a potent, reversible block of the response to GABA of motor neuron D$_{\text{f}}$. 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$_{\text{A}}$ 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.