Calcium-Dependent Chloride Currents in Vertebrate Central Neurons

Abstract

Anion-selective ion channels activated by inhibitory neurotransmitters such as γ-aminobutyric acid (GABA) and glycine occur in the soma-dendritic membrane of a majority of CNS neurons (see Chapters 7–9). Until recently, these were the only well-characterized anion-selective channels in vertebrate neurons and there was no evidence for any additional types of anion channel, although the existence of a leakage conductance permeable to Cl⁻ was considered to be probable. Some new work, on a variety of preparations ranging from amphibian egg cells to mammalian CNS neurons in tissue culture, has changed this picture with the discovery of a conductance mechanism that is selectively permeable to anions and activated by a rise in Ca ²⁺_i activity. The study of this conductance mechanism is in its infancy and, although some sophisticated biophysical techniques have been applied to the problem, the small conductances of these ion channels, our lack of selective pharmacological probes, and the added complexity of studying processes linked to changes in Ca ²⁺_i activity, have prevented any real understanding of the physiological functions of Ca²⁺-dependent Cl⁻ channels in nerve cells. Similar studies in exocrine cells that secrete electrolyte solutions suggest that Ca²⁺-activated Cl⁻ fluxes may participate in the production of tears in the lacrimal gland (Marty et al., 1984).