SYNAPTIC CURRENT KINETICS IN A SOLELY AMPA-RECEPTOR-OPERATED GLUTAMATERGIC SYNAPSE FORMED BY RAT RETINAL GANGLION NEURONS

1. Postnatal rat retinal ganglion cells (RGCs) can be maintained and i dentified in dissociated long-term culture. After 4-7 days in vitro th ey form glutamatergic synapses with other RGCs or putative amacrine ce lls. Here we intended to characterize the postsynaptic features of the se in vitro synapses. 2. Pair patch-clamp recordings in the whole cell configuration were performed to study the properties of synaptic glut amate receptors. Immunohistochemically and physiologically identified RGCs were activated by short depolarizing voltage steps. This elicited glutamatergic excitatory postsynaptic currents (EPSCs) in coupled neu rons. At room temperature, evoked EPSCs (eEPSCs) had latencies between 3 and 7 ms and amplitudes between 36.4 and 792.6 pA. 3. Postsynaptic neurons were electrotonically compact and therefore well suited for an alysis of fast synaptic events. All cells were responsive to exogenous glutamate, lpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (A MPA) and N-methyl-D-aspartate (NMDA). The current-voltage relationship s of AMPA-activated currents were linear, whereas NMDA-induced whole c ell currents displayed the typical characteristics including a negativ e slope conductance in the presence of Mg2+. In contrast to AMPA-activ ated currents, NMDA-activated currents had the usual slow onset and de cay. 4. RGCs obviously failed to generate NMDA-receptor-mediated EPSCs , because all postsynaptic cells lacked a slow current component even in the absence of added Mg2+ and in the presence of glycine. Retinal e EPSCs were completely blocked by 6,7-dinitroquinoxaline-2,3-dione (DNQ X). 5. eEPSCs as well as spontaneous EPSCs (sEPSCs) were characterized by a very rapid time course. In eEPSCs, 20-80% rise times and time co nstants of decay (tau(D)S) were on average 0.64 and 1.96 ms, respectiv ely. sEPSCs were extremely fast, with average rise times of 0.34 ms an d tau(D)S Of 1.20 ms. The latter numbers closely correspond to the val ues obtained for DNQX-sensitive miniature EPSC (mEPSC) in postnatal da y 5 rat RGCs in situ. 6. To clarify whether the decay of fast AMPA-rec eptor-mediated EPSCs of retinal neurons was determined by the onset of glutamate receptor desensitization, we compared the decay of sEPSCs w ith the decay of the glutamate response of excised outside-out membran e patches. Glutamate-activated currents were elicited by a rapid super fusion device (time constant of rise = 0.7 ms). The response to 1 mM o f glutamate decayed 2 to 4 times more slowly than the sEPSCs. 7. These results suggest that desensitization did not limit the rate of decay of purely AMPA-mediated EPSCs in response to ganglion cell activation. Very likely, the time course of EPSCs reflects channel deactivation, as in some other types of glutamatergic synapses studied so far.