Transient and steady state electroretinal responses

Potentiation of 'on' bipolar cell flash responses by dim background light and cGMP in dogfish retinal slices

The high sensitivity of the vertebrate visual system results from amplification inherent in phototransduction in rods and from the amplification of rod signals on their synaptic transfer at the first synapse with 'on' bipolar cells. These cells possess a metabotropic glutamate receptor linked via a cGMP cascade to the control of cGMP-activated channels. In the study presented here, we show that very dim background light, isomerising only one rhodopsin in 1 out of 10 rods per second, potentiates 'on' bipolar cell responses to superimposed flashes. Responses to dim flashes, which were undetectable above the noise in the dark, were boosted above the increased noise level induced by the background. This potentiation could be reproduced by elevating cGMP, which increases with light, or by dialysing the cells with a non-hydrolysable cGMP analogue. Inhibition of tyrosine kinase activity also reproduced the effect and induced a speeding up of the rising phase of the flash response, similar to the action of dim background light. Conversely, inhibition of tyrosine phosphatase activity blocked the potentiation. These results suggest that cGMP promotes tyrosine-site dephosphorylation of 'on' bipolar cell cGMP-activated channels, resulting in a rise in the sensitivity to cGMP, as has recently been demonstrated for rod cGMP-activated channels. This constitutes a positive feedback mechanism such that as cGMP increases with light, the sensitivity of the channels to cGMP increases and boosts the signal above background noise. This mechanism would allow stochastic resonance to occur, facilitating single-photon detection when dark-adapted, and may therefore lead to improved discrimination.

Desensitization of skate photoreceptors by bleaching and background light

Through extracellular measurements of photoreceptor responses to flashed stimuli, we examined how the bleaching of rhodopsin affects increment receptor threshold in the isolated retina of the skate (Raja oscellata and R. erinacea). Both initially unbleached and previously bleached photoreceptors, when exposed to full-field luminous backgrounds of fixed intensity, attain approximately stable levels of increment threshold that vary with the intensity of the background light. Values of stabilized increment thresholds measured after various extents of bleaching (less than approximately 50%), when plotted against background intensity in log-log coordinates, tend to converge with increasing intensity of the background; this relationship of the increment threshold functions resembles that which Blakemore and Rushton (1965b) found to describe the transient effect of bleaching on psychophysical increment threshold for the human rod mechanism. Our data are consistent with the possibility that related photochemical processes govern the stabilized levels of receptor sensitivity exhibited by the isolated retina (a) during steady illumination and (b) long after substantial bleaching.