Hyperpolarization of fish retinal horizontal cells by kainate and quisqualate

Long-term light history modulates the light response kinetics of luminosity (L)-type horizontal cells in the roach retina

We have examined the effects of prolonged periods of darkness on the responses of luminosity-type horizontal cells (L-HCs) in the freshwater cyprinid, Rutilus rutilus. Two groups of retinae were compared, those recorded after 10 min dark adaptation (SA) and those recorded after 3 h dark adaptation (LA). The results suggest that long-term light history does not modify the general responsiveness of the L-HCs in this species. However, there are apparent changes in the receptive field of the cells and modifications to the kinetics of the light-evoked response. The kinetics changes involve both a delay in the onset of light response and a selective effect on the hyperpolarizing light-ON response. Thus the mean time constant (tau) for the SA cells was 32.4+/-2.39 ms (n=62), whilst that for the LA cells was 53.4+/-3.03 ms (n=61). These effects occur in the absence of changes in the relative spectral sensitivity or threshold sensitivity of the HCs. The results suggest that in some vertebrate retinae, prolonged darkness (light-history) may regulate long-term plasticity in the kinetics of the cone-HC pathway.

Effects of glutamic acid and related agents on horizontal cells in a marine teleost retina

Excitatory amino acids (EAAs) such as glutamic and aspartic acids, considered as the most likely neurotransmitters at the photoreceptor-horizontal cell synapse of teleost retinas, as well as agonists such as kainic acid and several of their antagonists, were applied to isolated and superfused retinas of the teleost Eugerres plumieri. Intracellular recordings from horizontal cells reveal that EAA receptors are of the kainate-quisqualate type. There is competitive inhibition between the agonist and antagonist agents used, and under their combined effect, the synapse under study remains operational, in a functional state, able to modulate the horizontal cell membrane potential upon retinal illumination.

Evidence for calcium/calmodulin dependence of spinule retraction in retinal horizontal cells

Horizontal cells of the carp retina alter their synaptic connections with cones during dark and light adaptation. At light onset, dendrites of horizontal cells, which are positioned laterally at the ribbon synapse, form "spinules," little processes with membrane densities. Spinules are retracted again during dark adaptation. Spinule retraction is also elicited upon glutamate application to the retina. In the present study, we address the question whether calcium/calmodulin-dependent pathways are involved in dark- and glutamate-evoked spinule retraction. Light-adapted retinas were isolated and subsequently dark adapted during incubation in media of different calcium concentrations. Spinule retraction was clearly blocked in low-calcium solutions (5 microM and 50 nM CaCl2). Incubation in medium containing cobalt chloride (2 mM) had the same effect. Both treatments blocked the glutamate-induced spinule retraction as well. These results indicate that spinule retraction is induced by a calcium influx into horizontal cells. To investigate whether calmodulin, the primary calcium receptor in eukaryotic cells, is present at the site of spinule formation, light- and dark-adapted retinas, embedded in LR White resin, were labelled with an antibody against calmodulin and gold-conjugated secondary antibodies. Horizontal cell dendrites at the ribbon synapse revealed strong calmodulin immunoreactivity, which was more than twice as high in light- as in dark-adapted retinas. The incubation of isolated retinas with the calmodulin antagonists W5 and W13 inhibited spinule retraction. In summary, these results suggest that spinule retraction may be regulated by calcium influx into horizontal cells and subsequent calcium/calmodulin-dependent pathways.

Localization and function of dopamine in the adult vertebrate retina

Dopamine (DA) has satisfied many of the criteria for being a major neurochemical in vertebrate retinae. It is synthesized in amacrine and/or interplexiform cells (depending on species) and released upon membrane depolarization in a calcium-dependent way. Strong evidence suggests that it is normally released within the retina during light adaptation, although flickering and not so much steady light stimuli have been found to be most effective in inducing endogenous dopamine release. DA action is not restricted to those neurones which appear to be in "direct" contact with pre-synaptic dopaminergic terminals. Neurones that are several microns away from such terminals can also be affected, presumably by short diffusion of the chemical. DA thus affects the activity of many cell types in the retina. In photoreceptors, it induces retinomotor movements, but inhibits disc shedding acting via D2 receptors, without significantly altering their electrophysiological responses. DA has two main effects upon horizontal cells: it uncouples their gap junctions and, independently, enhances the efficacy of their photoreceptor inputs, both effects involving D1 receptors. In the amphibian retina, where horizontal cells receive mixed rod and cone inputs, DA alters their balance in favour of the cone input, thus mimicking light adaptation. Light-evoked DA release also appears to be responsible for potentiating the horizontal cell-->cone negative feed-back pathway responsible for generation of multi-phasic, chromatic S-potentials. However, there is little information concerning action of DA upon bipolar and amacrine cells. DA effects upon ganglion cells have been investigated in mammalian (cat and rabbit) retinae. The results suggest that there are both synaptic and non-synaptic D1 and D2 receptors on all physiological types of ganglion cell tested. Although the available data cannot readily be integrated, the balance of evidence suggests that dopaminergic neurones are involved in the light/dark adaptation process in the mammalian retina. Studies of the DA system in vertebrate retinae have contributed greatly to our understanding of its role in vision as well as DA neurobiology generally in the central nervous system. For example, the effect of DA in uncoupling horizontal cells is one of the earliest demonstrations of the uncoupling of electrotonic junctions by a neurally released chemical. The many other, diverse actions of DA in the retina reviewed here are also likely to become model modes of neurochemical action in the nervous system.(ABSTRACT TRUNCATED AT 400 WORDS)

The interplexiform-horizontal cell system of the fish retina: effects of dopamine, light stimulation and time in the dark

Interplexiform cells contact cone horizontal cells in the fish retina and probably release dopamine at synaptic sites. The effects of dopamine, certain related compounds, and light and dark régimes were tested on the intracellularly recorded activity of horizontal cells in the superfused carp retina to elucidate the functional role of the interplexiform cell. Dopamine application onto retinae kept in the dark for 30-40 min increased the size of the responses of cone horizontal cells to small-spot stimuli but decreased response size to large- and full-field stimuli. Dopamine also altered the response waveform of these cells; the transient at response onset increased in size and the depolarizing afterpotential decreased in size. Haloperidol, a dopamine antagonist, blocked these effects of dopamine application. Forskolin, an adenylate cyclase activator, increased the size of the responses of the cells to small-spot stimuli. Superfusion of vasoactive intestinal peptide did not produce any effects on horizontal cells. The results indicate that dopamine produces multiple physiological effects on cone horizontal cells by activation of an intracellular enzyme system. We propose that some of these effects are probably related to an uncoupling of the gap junctions between horizontal cells, but that other effects are most likely not explained on this basis and reflect additional changes induced in the cells by dopamine. After prolonged periods of darkness (100-110 min), compared with short periods (30-40 min), L-type cone horizontal cells exhibited responses similar to those obtained during dopamine application. Dim flickering or continuous light backgrounds did not mimic the effects of dopamine. Although dopamine application onto retinae after short-term darkness produced dramatic effects on L-type cone horizontal cells, little or no effect was observed when dopamine was applied while the effects of a previous dopamine application were still present or after prolonged darkness. These results suggest that interplexiform cells may release dopamine after prolonged darkness and that interplexiform cells may regulate lateral inhibitory effects mediated by L-type cone horizontal cells as a function of time in the dark.

N-methyl-D-aspartate receptors coexist with kainate and quisqualate receptors on single isolated catfish horizontal cells

Horizontal cells enzymatically isolated from catfish retina were exposed to the putative neurotransmitters aspartate (Asp) or N-methyl-D-aspartate (NMDA). Under voltage clamp conditions, inward currents were recorded when the holding potential was more negative than zero and outward currents were recorded when the membrane potential was more positive than zero. The current voltage curve was highly non-linear in the range of membrane potential between -30 and -100 mV. This non-linearity was largely removed in zero magnesium solution. 2-Amino-phosphonovaleric acid selectively blocked Asp and NMDA responses. These response characteristics are consistent with the presence of NMDA receptors in these cells.