A native picrotoxin-resistant GABA-gated chloride channel receptor subtypein cockroach neurons

Among insect GABA receptors, the GABA-gated chloride channel subtype is ins ensitive to bicuculline and has been thought to be composed of two populati ons because of differences in chloride conductance increase, GABA and picro toxin (PTX) sensitivity. To characterize this possible diversity in GABA-ga ted chloride channels, electropharmacological experiments were performed on giant interneuron synaptic GABA receptors and on somatic GABA receptors of dorsal unpaired median (DUM) neuron and fast coral depressor (D-f) motoneu ron of the cockroach Periplaneta americana (L). Electrophysiological assays performed at cercal-afferent giant interneuron synapses demonstrated that a biphasic increase in membrane conductance, in response to long-lasting (3 0s) neuropilar microapplication of GABA, could be explained by the existenc e of two GABA-operated chloride channel receptor subtypes. The low stable m embrane conductance increase, representing less than 30% of the maximum rea ched during the early transient phase, was not desensitized quickly. It was reproduced by neuropilar microapplication of cis-4-aminocrotonic acid (CAC A) and, in contrast to the fast phase, was not antagonized by bath applicat ion of 10(-5)M PTX. Long-lasting (3 min) pneumatic pressure application of GABA on the cell body of motoneuron D-f evoked a fast transient hyperpolari zation followed by a slower phase of further hyperpolarization. PTX (10(-5) M) blocked the fast transient phase and revealed a slow stable hyperpolariz ation. PTX (10(-4)M) blocked the major part of the remaining GABA response. The slow hyperpolarization was reproduced by application of CACA. Similar effects of GABA and CACA were recorded on DUM neuron cell bodies. All of th ese observations are consistent with the possible existence of two GABA-gat ed chloride channel subtypes in the insect CNS. (C) 1999 Society of Chemica l Industry.