D-aspartate potentiates the effects of both L-aspartate and L-glutamate on carp horizontal cells

Effects of glutamic acid and related agents on horizontal cells in a marine teleost retina

Excitatory amino acids (EAAs) such as glutamic and aspartic acids, considered as the most likely neurotransmitters at the photoreceptor-horizontal cell synapse of teleost retinas, as well as agonists such as kainic acid and several of their antagonists, were applied to isolated and superfused retinas of the teleost Eugerres plumieri. Intracellular recordings from horizontal cells reveal that EAA receptors are of the kainate-quisqualate type. There is competitive inhibition between the agonist and antagonist agents used, and under their combined effect, the synapse under study remains operational, in a functional state, able to modulate the horizontal cell membrane potential upon retinal illumination.

A glutamate-elicited chloride current with transporter-like properties in rod photoreceptors of the tiger salamander

Glutamate, when puffed near the synaptic terminals, elicits a current in rod photoreceptors. The current is strongly dependent upon both the intracellular and extracellular chloride concentration: its reversal potential follows the predicted Nernst potential for a chloride permeable channel. The glutamate-elicited current also requires the presence of extracellular sodium. This glutamate-elicited current is pharmacologically like a glutamate transporter: it is elicited, in order of efficacy, by L-glutamate, L-aspartate, L-cysteate, D-aspartate, and D-glutamate, all shown to activate glutamate transport in other systems. Furthermore, it is reduced by the glutamate transport antagonists dihydrokainate (DHKA) and D,L-threo-3-hydroxyaspartate (THA). THA, when applied alone, elicits a current similar to that elicited by glutamate. The current cannot be activated by the glutamate receptor agonists kainate, quisqualate, NMDA and APB, nor can it be blocked by the glutamate receptor antagonists CNQX and APV. Thus, the current does not appear to be mediated by a conventional glutamate receptor. Taken together, the ionic dependence and pharmacology of this current suggest that it is generated by glutamate transporter coupled to a chloride channel.

Non-NMDA type excitatory amino acid receptors mediate rod input to horizontal cells in the isolated rat retina

The actions of excitatory amino acid agonists and antagonists on rod driven horizontal cells were studied in the isolated retina of the rat. Horizontal cells were depolarised by L-glutamate, kainate and quisqualate but not by N-methyl-D-aspartate (NMDA). The broad band excitatory amino acid antagonist, kynurenate and the non-NMDA antagonist, 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX), hyperpolarised horizontal cells, blocking the light responses and also the effects of agonists. In contrast, the competitive NMDA antagonists, 3-[(+/-)-2-carboxypiperazin-4-yl]-propyl-1-phosphonate (CPP) and 2-amino-5-phosphono-pentanoate (AP5) were without effect. Thus, rat horizontal cells possess excitatory amino acid receptors of the non-NMDA type and these mediate their rod driven inputs.