Co-existence in DUM neurones of two GluCl channels that differ in their picrotoxin sensitivity

Whole cell patch-clamp electrophysiology was used to study the effects of L-glutamate on dissociated cockroach (Periplaneta americana) dorsal unpaired median (DUM) neurones. Application of L-glutamate via pressure-ejection pipette resulted in a two-component hyperpolarization, consisting of an initial transient and a second, prolonged phase. Under voltage-clamp, using isotonic chloride in the saline and intrapipette solutions, two L-glutamate-gated inward currents were characterized. Their reversal potentials were close to the equilibrium potential for chloride ions. One component was selectively activated by ibotenate and was sensitive to picrotoxin (100 microM), BIDN (10 microM) and the phenylpyrazole insecticide fipronil (10 microM), known to be potent blockers of insect GABA-gated chloride channels. The second component was insensitive to picrotoxin (100 microM) and BIDN (10 microM). These findings demonstrated, for the first time, the co-existence of two pharmacologically distinct native insect neuronal L-glutamate-gated chloride channels.