Brain Research, 295 (1984) 179-183 179 Elsevier BRE 20029 Short Communications On the sensitivity of H1 horizontal cells of the carp retina to glutamate, aspartate and their agonists MICHAEL ARIEL, ERIC M. LASATER, STUART C. MANGEL and JOHN E. DOWLING Department of Cellular and Developmental Biology, The Biological Laboratories, Harvard University, Cambridge, MA 02138 (U.S.A.) (Accepted October 18th, 1983) Key words: excitatory amino acids -- photoreceptor transmitter -- horizontal cells -- retina Threshold concentrations for u-glutamate, L-aspartate and their agonists, kainate, quisqualate and N-methyl-D.e-aspartate,were determined for horizontal cells of the intact carp retina and horizontal cells that were enzymatically isolated and maintained in culture. Our results indicate that uptake or other mechanisms decrease the apparent sensitivityof these cells in the intact retina to e-glutamate by 25-200 times. Our findings also suggest that e-glutamate is more likely a photoreceptor transmitter than u-aspartate. It has been known for a decade that the acidic ami- no acids, L-aspartate and L-glutamate, depolarize retinal horizontal cells, suggesting that one or both of these substances could serve as a photoreceptor transmitter4,8,2t,zL An objection to this view is that relatively high concentrations (1-25 mM) of these agents are required to depolarize horizontal cells, in- dicating perhaps, that the effects of these amino acids are non-specific11,14, 23. However, virtually all experi- ments so far carried out have employed pieces of in- tact retina in which there are powerful uptake mech- anisms for excitatory amino acids 3,9A5,19,26. A recent report found that D-aspartate, thought to saturate these uptake mechanisms, potentiated the effects of L-glutamate on horizontal cells by up to 15 times 12, suggesting that uptake mechanisms may alter signifi- cantly the concentrations of acidic amino acids reach- ing postsynaptic sites. We tested this hypothesis in two ways: (1) by employing glutamate and aspartate analogues that are not actively taken up by retinal tis- sue and (2) by testing the sensitivity of isolated hori- zontal cells to the acidic amino acids and their analog- ues. The sensitivity of horizontal cells in the intact ret- ina and of the isolated horizontal cells to L-gluta- mate, L-aspartate and their analogues, quisqualate, kainate and N-methyl-D,L-aspartate, was deter- mined in the following way. In dark-adapted retinas, cone horizontal cells of the luminosity (H1) type were penetrated with micropipettes and the concen- tration of drug required to evoke a reliable cellular depolarization (3-5 mV) or reduction in their light response was determined. Isolated horizontal cells, maintained 2-5 days in culture, were likewise pen- etrated with micropipettes, and the concentration of drug that elicited a threshold depolarization (5-10 mV) of the cell measured. (Isolated horizontal cells in culture are easily recognized because of their size and characteristic shape16,27, and they appear to be mainly of the HI type because approximately 75% of them take up exogenously applied GABA17,20.) Drugs were administered to the receptor surface of the intact retina either by superfusion or by atomiza- tion. The latter method is analogous to iontophore- sis, a common method of drug application elsewhere in the brain. Agents were delivered to the isolated cells by means of triple barrel micropipettes brought to within 100/~m of the cells. Details of the experi- Correspondence: M. Ariel, The Biological Laboratories, Harvard University, Cambridge, MA 02138, U,S.A. 0006-8993/84/$03.00 © 1984 Elsevier Science Publishers B.V.