Glutamatergic neurotransmission in the first retinal synapse mediated by metabotropic receptors--electroretinographic study.

ABSTRACT

AIM: The aim of this study was to better characterize glutamatergic synaptic transmission between photoreceptors and retinal ON-bipolar cells. For this purpose we investigated the effect of L-serine orthophosphate (L-SOP) (an agonist of the group III metabotropic receptors) on the mass (electroretinographic) response of the ON-bipolar cells, which was obtained by isolated stimulation of the rods or the cones. MATERIALS AND METHODS: Isolated frog eyecup preparations (Rana ridibunda) were subjected to perfusion with L-SOP solution in various concentrations--10, 30, 100, 300, 500 and 1000 micromol/l. The effects of the agonist on the electroretinographic ON-response (b-wave) at isolated stimulation of the rods (adaptation to the dark, light stimuli of low intensity) or the cones (bright background illumination, stimuli of high intensity) were followed up. RESULTS: As a result of the effect of L-SOP we observed a dose-dependent decrease in amplitude and increase in peak latency of the electroretinographic (ERG) b-wave. This effect was better expressed in the responses which were mediated by the rods and even in some of the experiments the b-wave disappeared completely. As an additional effect we observed an increase of the OFF-response (the d-wave), which did not depend on the photoreceptor input. CONCLUSIONS: The mass response of the ON-bipolar cells (the b-wave in the ERG) elicited by isolated stimulation of the two kinds of photoreceptors (rods and cones) shows different sensitivity to the group III metabotropic receptor agonist L-serine-ortho-phosphate. This is indicative of a difference in the glutamatergic transmission in the synapses between the two types of photoreceptors and the retinal ON-bipolar cells.