Light adaptation of the late receptor potential in the cat retina

Activation of Ca2+--calmodulin kinase II induces desensitization by background light in dogfish retinal 'on' bipolar cells

Retinal 'on' bipolar cells possess a metabotropic glutamate receptor (mGluR6) linked to the control of a G-protein and cGMP-activated channels which functions to generate high synaptic amplification of rod signals under dark-adapted conditions. Desensitization of 'on' bipolar cells is initiated by a rise in Ca2+ during background light too weak to adapt rod photoreceptors. Desensitization could also be elicited by raising intracellular Ca2+ above 1 microM. In order to investigate the mechanism of desensitization, whole-cell current responses to brief flashes and to steps of light were obtained from voltage-clamped 'on' bipolar cells in dark-adapted dogfish retinal slices. The inclusion of Ca2+-calmodulin kinase II (CaMKII) inhibitor peptides in the patch pipette solutions not only blocked desensitization of 'on' bipolar cells by dim background light and by 50 microM Ca2+, but also increased their flash sensitivity. The substrate of phosphorylation by CaMKII is the 'on' bipolar cell cGMP-activated channels. Desensitization probably results from a reduction in their sensitivity to cGMP and a voltage-dependent decrease in their conductance. A role for protein kinase C (PKC) in this process was excluded since activating PKC independently of Ca2+ with the phorbol ester PMA failed to induce desensitization of 'on' bipolar cells.

A rise in intracellular Ca2+ underlies light adaptation in dogfish retinal 'on' bipolar cells

1. This investigation was to determine the extent to which retinal 'on' bipolar cells contribute to the adaptive changes that occur with light, which enable the rod visual system to operate over a wide range of ambient light intensities, and to elucidate the underlying adaptive mechanism. 2. Whole-cell voltage clamp recordings were obtained from bipolar cells in dark-adapted dogfish retinal slices. Current responses to brief flashes and steps of light were analysed. 'On' bipolar cell inward current light responses are mediated by a metabotropic glutamate receptor linked to the control of a cGMP cascade, with cGMP opening cation channels. Outward current responses to light of 'off' bipolar cells are mediated by the closure of ionotropic glutamate receptor channels. 3. When Ca2+ buffer was omitted from the patch pipette solution, 'on' bipolar cells rapidly desensitized to steps of light as dim as one rhodopsin molecule bleached per rod per second (1 Rh* s-1), whereas 'off' bipolar cells did not desensitize. Responses of 'on' bipolar cells to flashes in the presence of dim backgrounds recovered after a delay, but with diminished sensitivity, i.e. the cells adapted. 4. With the Ca2+ chelator BAPTA in the patch pipette solution, step responses of 'on' bipolar cells were sustained and flash responses following steps showed rapid recovery. Buffering Ca2+ in the patch pipette solution to 1 microM prevented desensitization, whereas 50 microM free Ca2+ reduced the 'on' bipolar cell flash responses, suppressed inward dark current and decreased input conductance. 5. We conclude that a major component of adaptation of the visual system is due to a reduction in gain at the rod-'on' bipolar cell synapse as a result of Ca2+ loading of the dendrites when their cGMP-gated cation channels open with light.

Weber and noise adaptation in the retina of the toad Bufo marinus

Responses to flashes and steps of light were recorded intracellularly from rods and horizontal cells, and extracellularly from ganglion cells, in toad eyecups which were either dark adapted or exposed to various levels of background light. The average background intensities needed to depress the dark-adapted flash sensitivity by half in the three cell types, determined under identical conditions, were 0.9 Rh*s-1 (rods), 0.8 Rh*s-1 (horizontal cells), and 0.17 Rh*s-1 (ganglion cells), where Rh* denotes one isomerization per rod. Thus, there is a range (approximately 0.7 log units) of weak backgrounds where the sensitivity (response amplitude/Rh*) of rods is not significantly affected, but where that of ganglion cells (1/threshold) is substantially reduced, which implies that the gain of the transmission from rods to the ganglion cell output is decreased. In this range, the ganglion cell threshold rises approximately as the square root of background intensity (i.e. in proportion to the quantal noise from the background), while the maintained rate of discharge stays constant. The threshold response of the cell will then signal light deviations (from a mean level) of constant statistical significance. We propose that this type of ganglion cell desensitization under dim backgrounds is due to a post-receptoral gain control driven by quantal fluctuations, and term it noise adaptation in contrast to the Weber adaptation (desensitization proportional to the mean background intensity) of rods, horizontal cells, and ganglion cells at higher background intensities.