The effects of prolonged superfusions with acidic amino acids and their agonists on field potentials and horizontal cell photoresponses in the turtle retina

Abstract

The effects of prolonged superfusions with acidic amino acids and their agonists, kainic acid (KA) and N-methyl-D-aspartate (NMDA), on horizontal cells, and extracellular field potentials were studied in the turtle everted eyecup preparation using simultaneous intracellular and extracellular recordings. In a fresh preparation initial superfusions with each of the above agents usually induced a large (up to 60 mV) transient negative extracellular field potential recorded adjacent to horizontal cells, followed by a sustained negative potential of lesser amplitude (up to 10 mV). The amplitude of the sustained potential did not vary with subsequent superfusions, whereas that of the transient phase was reduced. KA and NMDA were much more potent (at least 300 times) in evoking these field potentials than either acidic amino acid. The horizontal cell transmembrane potential was monitored as the difference between the intra- and extracellular potentials. Superfusion with KA and NMDA produced a triphasic time course of the drug effect consisting of an initial depolarization with reduced photoresponses, a rehyperpolarization of the membrane accompanied by a growth of the light responses followed by a gradual depolarization and loss of photoresponses. Superfusion with the acidic amino acids usually produced a biphasic response that resembled qualitatively the first two phases of the response to KA and NMDA. This biphasic response was occasionally followed by a gradual depolarization and loss of the light response. The kinetics of the transient component of the field potential and the rapid reduction and regrowth of the photoresponses recorded during superfusion with these agents suggests an initial action of these drugs, which is of a nonsynaptic origin and which may be an expression of a drug-induced spreading depression. The kinetics of the photoresponses recorded during superfusion with KA, L-aspartate, and L-glutamate were similar but differed from those recorded during superfusion with NMDA. The difference in the effects of NMDA and KA on photoresponse kinetics suggests that two types of acidic amino acid receptors may be present in the outer plexiform layer of the turtle retina.