GABA-Gated CI⁻ Currents and Their Regulation by Intracellular Free Ca^{2 +}

Abstract

Intracellular Ca^{2 +} is well known to play a central role in the transduction of extra-cellular signals (Nishizuka, 1984) and in the control and modulation of fundamental cellular functions. In addition to well-established functions such as Ca^{2 +} -mediated transmitter release from nerve terminals (Katz and Miledi, 1967), the liberation of hormones from neuroendocrine cells (Thorn et al., 1978), and muscular contraction, several types of voltage-dependent ionic channels, including Ca^{2 +} channels themselves, have been found to be highly sensitive to changes in intracellular free Ca^{2 +} concentration ([Ca^{2 +}]_i) (Colquhoun et al., 1981; Eckert et al., 1981; Bader et al., 1982; Maruyama and Petersen, 1982; Yellen, 1982; Ashcroft and Stanfield, 1984; Marty et al., 1984; Owen et al., 1984; Bregestovski et al., 1986; see also Mayer et al., this volume). Other prominent examples are the Ca^{2 +} -activated K⁺ conductance in molluscan neurons (Meech and Standen, 1975; Thompson, 1977; Aldrich et al., 1979) and the Ca^{2 +} -activated anion conductance in mouse spinal cord neurons (Owen et al., 1984) and rat sensory neurons (Mayer, 1985). However, evidence increasingly suggests that not only electrically operated but also chemically gated neuronal membrane conductances are affected by such transient changes in [Ca^{2 +}]_i (Morita et al., 1979; Miledi, 1980; Chemeris et al., 1982). An increase in [Ca^{2 +}]_i not only facilitates the onset of desensitization to the nicotinic cation response of acetylcholine in the muscle endplate (Miledi, 1980) but also inhibits the nicotinic Cl⁻ response in Aplysia neurons (Chemeris et al., 1982) as well as the nicotinic cation response in frog sympathetic ganglion cells (Morita et al., 1979). [Ca^{2 +}]_i also behaves as a second messenger for many biologically active substances (Rasmussen and Barrett, 1984). In this chapter we report a powerful suppressive action of C^{2 +}]_i on the γ-aminobutyric acid (GABA)gated Cl⁻ current in isolated bullfrog sensory neurons. We also report evidence for a saturable intracellular binding site for Ca^{2 +} and other divalent cations which mediates this suppressive effect. We recorded macroscopic and single-channel membrane currents from internally perfused sensory neurons of bullfrog dorsal root ganglia using “concentration-clamp” (Akaike et al., 1986, 1987) and patch-clamp (Hamill et al., 1981) techniques.