REDUCING EXTRACELLULAR CL- SUPPRESSES DIHYDROPYRIDINE-SENSITIVE CA2+ CURRENTS AND SYNAPTIC TRANSMISSION IN AMPHIBIAN PHOTORECEPTORS

A reduction in extracellular chloride suppresses light-evoked currents of second-order retinal neurons (bipolar and horizontal cells) by red ucing release of glutamate from photoreceptors. The underlying mechani sms responsible for this action of reduced extracellular Cl- were stud ied with a combination of electrophysiological recordings from single neurons in a retinal slice preparation and image analyses of intracell ular Ca2+ (Fura-2) and pH bis-(2-carboxyethyl)-5-(and-6)-carboxyfluore scein, acetoxymethyl ester] in dissociated photoreceptors. The results show that reducing extracellular Cl- suppresses a dihydropyridine (DH P)-sensitive Ca2+ current (I-Ca) in photoreceptors. It is proposed tha t suppression of I-Ca results in suppression of photoreceptor neurotra nsmission. The suppressive effect of low Cl- on I-Ca is not due to ant agonism by the substituting anion nor is it mediated by changes in ext racellular or intracellular pH. We conclude that normal extracellular levels of Cl- are important for maintenance of the voltage-gated Ca2channels that support neurotransmission from photoreceptors. Several i deas are presented about the mechanisms by which Cl- supports photorec eptor neurotransmission and the possibility that modulations of Cl- mi ght play a physiological role in the regulation of Ca2+ channels in ph otoreceptors and, hence, photoreceptor function.