Comparative investigation of retinal responses to brief light stimuli: 2-amino-4-phosphonobutyrate studies--I. Frog retina, Rana ridibunda

ON-OFF Interactions in the Retina: Role of Glycine and GABA

In the vertebrate retina, visual signals are segregated into parallel ON and OFF pathways, which provide information for light increments and decrements. The segregation is first evident at the level of the ON and OFF bipolar cells and it apparently remains as signals propagate to higher brain visual centers. A fundamental question in visual neuroscience is how these two parallel pathways function: are they independent from each other or do they interact somehow? In the latter case, what kinds of mechanisms are involved and what are the consequences from this cross-talk? This review summarizes current knowledge about the types of interactions between the ON and OFF channels in nonmammalian and mammalian retina. Data concerning the ON-OFF interactions in distal retina revealed by recording of single bipolar cell activity and electroretinographic ON (b-wave) and OFF (d-wave) responses are presented. Special emphasis is put on the ON-OFF interactions in proximal retina and their dependence on the state of light adaptation in mammalian retina. The involvement of the GABAergic and glycinergic systems in the ON-OFF crosstalk is also discussed.

Non-NMDA receptors in frog retina: an immunocytochemical study

Glutamate is one of the main neurotransmitters in the retina. Its effects are mediated by a large number of ionotropic and metabotropic membrane receptors. The distribution of ionotropic AMPA receptor subunits GluR1-4, kainate receptor subunits GluR5-7 and KA2, delta receptors 1-2, as well as the metabotropic receptor mGluR6 were studied in the frog retina. Indirect immunofluorescence was used to localize the different receptor subunits. Results showed that all subunits, with the exception of GluR1 and GluR5, are widely distributed in the retina. They are mainly located in both plexiform layers: the outer (OPL) and the inner one (IPL), where punctate labelling, a sign of synaptic localization, is observed. The metabotropic receptor mGluR6 is localised only in the OPL. The AMPA receptor subunit GluR4 is localised on the glial Müller cells of the retina. The vast majority of the subunits possess specific patterns of labelling that indicate that they are involved with different retinal functions. The significance of the AMPA receptors and involvement of glia in modulation of synaptic transmission are discussed.

Participation of the GABAergic system in the action of 2-amino-4-phosphonobutyrate on the OFF responses of frog retinal ganglion cells

Perfusion of dark adapted frog eyecups with the ON pathway blocker 2-amino-4-phosphonobutyrate (APB) not only abolished the ganglion cells (GCs)' ON responses and the ERG b-wave, but it markedly potentiated the OFF responses of all ON-OFF and phasic OFF GCs and the d-wave amplitude of a simultaneously recorded ERG as well. The blockade of GABA(A) and GABA(C) receptors by picrotoxin eliminated this potentiating effect in 24 out of 41 GCs, although in the rest of the cells it did not produce any change in the APB effect. On the other hand, the d-wave potentiation was preserved during the GABAergic blockade in all experiments. Our results indicate that GABAergic transmission is involved in the inhibition exerted by the ON upon the OFF channel in part of the ON-OFF and phasic OFF GCs in the frog retina. The tonic OFF GCs probably do not receive an inhibitory input from the ON channel, because their light responses were not altered either by APB alone or by APB during blockade of GABA(A) and GABA(C) receptors.