Two types of glutamate receptors differentially excite amacrine cells in the tiger salamander retina.
1. Excitatory inputs to amacrine cells in the salamander retinal slice preparation were examined using whole-cell patch pipette voltage-clamp techniques. In strychnine (500 nM) and bicuculline (100 microM), two types of amacrine cell were easily distinguished by their light-evoked excitatory responses: transient and sustained. 2. In transient amacrine cells the current-voltage (I-V) relation for the peak light-evoked current was non-linear with a negative slope region between -50 and -70 mV. Responses reversed near +10 mV and were prolonged at more positive holding potentials. 3. In DL-2-amino-phosphonoheptanoate (AP7, 30 microM), a selective N-methyl-D-aspartate (NMDA) receptor antagonist, both the negatively sloped region of the light I-V relation and the prolongation of the response at positive potentials were eliminated. In 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX, 2 microM), a selective non-NMDA receptor antagonist, light-evoked currents at the most hyperpolarized holding potentials were eliminated. At potentials positive to -85 mV the light-evoked currents lacked a fast onset. The light I-V relation in CNQX had a negative slope region between -35 and -80 mV. 4. With synaptic transmission blocked, kainate evoked responses in transient cells with a resultant I-V relation that was nearly linear, whereas glutamate and NMDA elicited responses with non-linear I-V relations. 5. Light-evoked currents in sustained amacrine cells had a nearly linear I-V relation and reversed near +10 mV. AP7 at a concentration of 30 microM did not affect the light-evoked currents in sustained cells, but 2 microM-CNQX eliminated all light-evoked currents in these cells. 6. With synaptic transmission blocked, sustained amacrine cells responded only to glutamate and kainate, not NMDA. The resultant I-V relations were linear. 7. We conclude that the light-evoked responses of transient amacrine cells are mediated by concomitant activation of both non-NMDA and NMDA receptors whereas the responses of sustained amacrine cells are mediated only by non-NMDA receptors. Furthermore, these data provide supportive evidence that the primary light-evoked excitatory neurotransmitter activating amacrine cells is glutamate.
Dixon
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DB
Department of Ophthalmology, University of California, San Francisco 94143-0730.
Copenhagen
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DR
D015763:2-Amino-5-phosphonovalerate / Q000031:analogs & derivatives*; D018750:6-Cyano-7-nitroquinoxaline-2,3-dione; D000596:Amino Acids / Q000494:pharmacology; D000818:Animals; D000927:Anticonvulsants / Q000494:pharmacology; D005074:Evoked Potentials, Visual / Q000187:drug effects / Q000502:physiology*; D010775:Photic Stimulation; D011810:Quinoxalines / Q000494:pharmacology; D016194:Receptors, N-Methyl-D-Aspartate / Q000187:drug effects / Q000502:physiology*; D012160:Retina / Q000187:drug effects / Q000502:physiology*; D014562:Caudata
D000596:Amino Acids; D000927:Anticonvulsants; D011810:Quinoxalines; D016194:Receptors, N-Methyl-D-Aspartate; C056723:FG 9041; D018750:6-Cyano-7-nitroquinoxaline-2,3-dione; D015763:2-Amino-5-phosphonovalerate; C031231:2-amino-7-phosphonoheptanoic acid