L-aspartate: evidence for a role in cone photoreceptor synaptic transmission in the carp retina.

Abstract

A number of putative neurotransmitter substances and their antagonists were applied to the carp retina while intracellular recordings from L-type cone horizontal cells were made. Of all the substances tested, L-aspartate was found to be the most potent agent in depolarizing these horizontal cells in dark-adapted, partially light-adapted, and Co2+-treated retinas. Furthermore, DL-a-aminoadipate, an L-aspartate antagonist, blocked the effects of both the endogenous photoreceptor transmitter and exogenously applied L-aspartate on the horizontal cells. The results suggest that L-aspartate and the natural transmitter interact with the same population of postsynaptic receptors in the horizontal cell membrane.In darkness, the vertebrate photoreceptor is partially depolarized by a sodium current that flows into the outer segment of the cell (1). This depolarization appears to result in a continuous release of neurotransmitter from the receptor onto second-order neurons (2). Light, by supressing the sodium current, hyperpolarizes the photoreceptor and depresses the release of transmitter (3, 4). The light response of second-order neurons (the horizontal and bipolar cells) is thus caused by a decrease of neurotransmitter secretion from the receptor terminal.Horizontal cells and one subclass of bipolar cell [the hyperpolarizing bipolar cell (HBC)] hyperpolarize in response to light; another subclass of bipolar cell [the depolarizing bipolar cell (DBC)] depolarizes in light. It is reasonable to assume, therefore, that, if the photoreceptor transmitter acts directly on these elements, it will depolarize horizontal cells and the HBC and hyperpolarize the DBC. Blockade of transmitter release from the receptor terminals, on the other hand, should mimic the effects of light on these cell types (5-7).

Responses of solitary retinal horizontal cells from Carassius auratus to L‐glutamate and related amino acids.

“…It is surprising that L-aspartate produced no change in the membrane potentials of solitary horizontal cells, since L-aspartate seems to depolarize horizontal cells in situ (Murakami et al 1972;Wu & Dowling, 1978;Ishida & Fain, 1981). Horizontal cells dissociated from carp retinas have also been found not to be depolarized by L-aspartate (Lasater & Dowling, 1982).…”

Antagonism of Excitatory Amino Acid‐induced Responses and of Synaptic Excitation in the Isolated Spinal Cord of the Frog

“…The possibility that the transmitter operating at these synapses is L-aspartate, as discussed in our earlier papers, is supported by the finding that DL-a-aminoadipate blocks both L-aspartate-induced responses and the synaptic activation of carp retinal neurones. These cells are far less sensitive to L-glutamate than to L-aspartate (Wu & Dowling, 1978). Nevertheless, none of the endogenous acidic amino acids that have actions sensitive to NMDA antagonists, including several sulphur-containing amino acids in addition to L-glutamate and L-aspartate, can be ruled out in the case of the frog spinal cord.…”

L-glutamate: a neurotransmitter candidate for cone photoreceptors in the monkey retina

“…Recent studies indicate that among the known transmitter candidates, L-aspartate (LAsp), L-glutamate (L-Glu), or both might function as transmitter for at least some photoreceptor cells. It has been known for some time that exogenously applied L-ASP or L-Glu causes depolarization of horizontal cells and off-center bipolars, while the on-center bipolars are hyperpolarized by these amino acids (Cervetto and MacNichol, 1972;Dowling and Ripps, 1972; Ishida and Fain, 198 1;Kondo and Toyoda, 1980;Murakami et al, 1972Murakami et al, , 1975 NegishiandDrujan, 1979; Shiellset al, 1981; Slaughter and Miller, 198 1;Wu and Dowling, 1978). Although these observations suggest that L-ASP or L-Glu is the photoreceptor transmitter, the results have been questioned, since relatively high (mM) concentrations of the amino acids were required to elicit significant responses.…”