ACTIVATION OF PICROTOXIN-RESISTANT GABA RECEPTORS BY GABA AND RELATED-COMPOUNDS INDUCES MODULATION OF COCKROACH DORSAL PAIRED MEDIAN (DPM) NEURON FIRING

Activation of gamma-aminobutyric acid (GABA) receptors in insect dorsa l paired median (DPM) neurons induced two types of response which appe ared to be mediated by two different GABA receptor subtypes. When acti vated by bath application of GABA, one receptor subtype, insensitive t o picrotoxin (PTX), mediated a drastic reduction in the firing frequen cy, leading to a blockade of the spontaneous electrical activity. Thes e effects were accompanied by decreases in the amplitude and duration of the plateau action potential (AP) and the spike after-hyperpolariza tion (AHP). In most cases, a slight depolarization of the resting memb rane potential occurred. Bath application of the vertebrate GABA(B) re ceptor agonists 3-aminopropyl(methyl)phosphinic acid (SKF 97541) and 3 -aminopropylphosphinic acid (CGA 147823/CGP 27492) induced similar res ponses. Another GABA receptor subtype, less sensitive to GABA, mediate d a chloride dependent hyperpolarization that was suppressed by bath a pplication of PTX. The approximate locations of these two GABA recepto r subtypes were determined by local pressure microapplications of GABA and vertebrate GABAergic agonists. The PTX-sensitive receptors were l ocated predominantly on the surface of the ganglion where the apical p ole of the soma is situated, while the PTX-resistant receptors appeare d to be located deeper within the ganglion. These results reveal the e xistence of two GABA receptor subtypes on the DPM neurons and provide evidence for a functional role for PTX-resistant GABA receptors in the regulation of spontaneous firing. (C) 1997 Elsevier Science Ltd. All rights reserved.