title: Concomitant activation of two types of glutamate receptor mediates excitation of salamander retinal ganglion cells.
abstract: 1. Cells in the ganglion cell layer of salamander retinal slices were voltage clamped using patch pipettes. Light elicited transient excitatory postsynaptic currents (EPSCs) in on-off ganglion cells and sustained EPSCs in on ganglion cells. Light-evoked inhibitory postsynaptic currents in these cells could be blocked by 100 microM-bicuculline methobromide and 500 nM-strychnine. 2. In the presence of external Cd2+, at a concentration that blocked light-evoked synaptic inputs, N-methyl-D-aspartate (NMDA) and the non-NMDA-receptor agonists, quisqualate and kainate, gated conductances in both on-off and on ganglion cells. The current-voltage (I-V) curve for the conductance elicited by NMDA had a negative slope between -40 and -70 mV and a reversal potential near 0 mV. The I-V curves for the non-NMDA-receptor-mediated conductances were nearly linear and also had reversal potentials near 0 mV. 3. I-V curves were measured at an early time point near the peak of transient EPSCs and at a later time point during the decay phase of the responses. The late I-V curve had a negative slope below -40 mV. The early I-V curve had a positive slope over the entire voltage range but the slope was greater at positive than at negative potentials. The evoked current reversed near 0 mV at both time points. 4. The region of negative slope of the late I-V curve was eliminated when Mg2+ was removed from the external saline. A slowly decaying component of transient EPSCs was eliminated in 20 microM-DL-2-amino-7-phosphonoheptanoate (AP7), an NMDA-receptor antagonist. 5. Application of 1 microM-6-cyano-7-nitroquinoxaline-2,3-dione (CNQX), a non-NMDA-receptor antagonist at this concentration, blocked a fast component of transient EPSCs. 6. Our results demonstrate that the synaptic inputs to on-off ganglion cells have two components: a slower NMDA-receptor-mediated component having a time-to-peak of 110 +/- 45 ms and an e-fold decay time of 209 +/- 35 ms at -31 mV (mean +/- S.D., n = 5), and a faster non-NMDA-receptor-mediated component having a time-to-peak of 28 +/- 10 ms and an e-fold decay time of 43 +/- 20 ms at -31 mV (n = 8). 7. A similar analysis of sustained EPSCs of on ganglion cells showed that these currents resulted from sustained activation of both NMDA and non-NMDA receptors.
authors.affiliation: Department of Ophthalmology, University of California, San Francisco 94143.
authors.forename: S
authors.initials: S
authors.lastname: Mittman
authors.forename: W R
authors.initials: WR
authors.lastname: Taylor
authors.forename: D R
authors.initials: DR
authors.lastname: Copenhagen
mesh_terms: D000557:Ambystoma; D000818:Animals; D004594:Electrophysiology; D005971:Glutamates / Q000502:physiology*; D010775:Photic Stimulation; D017470:Receptors, Glutamate; D016194:Receptors, N-Methyl-D-Aspartate / Q000502:physiology*; D017981:Receptors, Neurotransmitter / Q000502:physiology*; D012165:Retinal Ganglion Cells / Q000502:physiology*
chemical_list: D005971:Glutamates; D017470:Receptors, Glutamate; D016194:Receptors, N-Methyl-D-Aspartate; D017981:Receptors, Neurotransmitter