Differential modulation of light-evoked on- and off-EPSCs by paired-pulse stimulation in salamander retinal ganglion cells.

Short-term plasticity of On- and Off-EPSPs, and its potential role in regulation of signal processing was studied in salamander retinal On-Off ganglion cells by whole-cell recording. Paired-pulse light stimulation resulted in a depression of On-, and an enhancement of Off-EPSCs. Recovery from depression and enhancement was exponential and complete by 20 s. Paired-pulse enhancement, but not depression, was abolished with increasing stimulus duration. Blockade of On-EPSC by L-2-amino-4-phosphonobutyrate (AP-4), an agonist at group III mGluRs, significantly increased Off-EPSCs evoked by short (<2 s) duration conditioning light stimuli, resulting in a reversal of the paired-pulse enhancement to depression. The acetylcholinesterase inhibitor eserine reduced Off-EPSC1 and increased the ratio of enhancement. An opposite effect was observed in the presence of the nACh receptor antagonist d-tubocurarine. AP-7, an antagonist of NMDA receptors attenuated the enhancement of Off-EPSCs. In current clamp mode paired-pulse stimulation resulted in a modulation of light evoked, as well as the depolarization-induced spike firing pattern of ganglion cells. The present study suggests that paired light stimulation differently modulates On and Off EPSPs, and the light-evoked spike firing pattern of On-Off ganglion cells.