Journal of Neuroscience, Vol 15, 6189-6199, Copyright © 1995 by Society for Neuroscience
Desensitizing glutamate receptors shape excitatory synaptic inputs to tiger salamander retinal ganglion cells
PD Lukasiewicz, JE Lawrence and TL Valentino
Department of Ophthalmology and Visual Sciences, Washington University School of Medicine, St. Louis, Missouri 63110-1093, USA.
AMPA/kainate (KA) receptors mediate a component of ganglion cell excitatory
postsynaptic currents (EPSCs). We investigated whether desensitization at
these receptors contribute to the shape of transient EPSCs in ON-OFF
ganglion cells. Whole-cell, voltage-clamp recordings were made from
ganglion cells in the retinal slice or in isolation. EPSCs were evoked by
either stimulating the slice with light or puffing K+ at the outer
plexiform layer (OPL). The AMPA/KA receptor-mediated component of the EPSCs
was isolated by including NMDA receptor antagonists in the bath. Strychnine
and picrotoxin blocked inhibitory inputs. In isolated ganglion cells,
cyclothiazide (10 microM), which blocks desensitization in non-NMDA
receptors, enhanced both the amplitude and the duration of currents evoked
by puffs of AMPA or glutamate. EPSCs evoked by K(+)-puffs in the OPL were
also enhanced by cyclothiazide (30 microM). When AMPA/KA receptors were
blocked with NBQX (10 microM), no enhancement of the EPSCs by cyclothiazide
was observed, indicating that cyclothiazide did not act presynaptically.
Cyclothiazide also enhanced the amplitude and duration of both the ON and
OFF light-evoked (L-) EPSCs recorded in ON-OFF ganglion cells.
Current-voltage relationships showed the enhancement was not voltage
dependent. When control and enhanced responses where normalized, it was
observed that the rate of desensitization of both the ON and OFF L- EPSCs
was decreased by cyclothiazide. Cyclothiazide selectively enhanced the
AMPA/KA receptor-mediated component of ganglion cells EPSCs, suggesting
that desensitization of AMPA/KA receptors shape transient L-EPSCs.