Short Communication Single-channel Recordings from Insect Neuronal Gaba-activated Chloride Channels

GABA-receptor-operated chloride ion channels are widely distributed on membranes of insect nervous tissue (for a review, see Sattelle, 1990). For example, in the cockroach Periplaneta americana radioligand binding (Lummis and Sattelle, 1986), autoradiography (Lummis and Sattelle, 1985) and GABA-activated ^Cl flux (Wafford et al. 1987) studies have demonstrated the presence of GABAoperated chloride channels throughout the nervous system. Electrophysiological studies initiated by R. M. Pitman, J-. J. Callec, R. J. Walker and colleagues have demonstrated the presence of many GABA-sensitive neurones in the cockroach central nervous system (Pitman and Kerkut, 1970; Walker et al. 1971; Callec, 1974). Recent studies on identified neurones (Wafford and Sattelle, 1986; Sattelle et al. 1988) and cultured neurones (Beadle and Lees, 1986; Neumann et al. 1987; Shimahara et al. 1987) have enhanced our understanding of the pharmacology of cockroach GABA receptors. However, few studies have yielded data on the unitary conductance properties of insect neuronal GABA-operated chloride channels. Shimahara et al. (1987) have used noise analysis to estimate that the conductance of chloride channels activated by GABA (5.0xlO~moll~) on cultured embryonic cockroach (Periplaneta americana) brain neurones is 18.6pS, with a mean open time of 11.8 ms. Attempts to obtain recordings of insect GABAactivated chloride channels using the patch-clamp technique have proved difficult, and this has been attributed to non-uniformity of the distribution of GABA receptors on embryonic cells (Pichon and Beadle, 1988). No such data are available for adult neurones, and the present study provides an initial description of GABA-operated chloride channels of dissociated neurones from thoracic ganglia of the adult cockroach (Periplaneta americana). Metathoracic (T3) ganglia were isolated from adult male cockroaches (Periplaneta americana) and neurones dissociated according to a modified version of the technique of Pinnock and Sattelle (1987). T3 ganglia were desheathed and