MECHANISMS OF ADAPTATION IN VERTEBRATE PHOTORECEPTORS

JAMES W CLACK, Purdue University

Abstract

Extracellular measurements of photoreceptor responses to flashed stimuli were used to examine 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 which vary with intensity of the background light. The stabilization of threshold ultimately occurs even if the background illumination concurrently bleaches as much as 50 percent of the rhodopsin in the receptors. Values of stabilized increment thresholds measured after varying extents of bleaching, when plotted against background intensity in log-log coordinates, tend to converge with increasing intensity of the background. These 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. Responses to test-flashes were recorded extracellularly, and intracellularly from dark- and light-adapted rod photoreceptors in the isolated retina of the skate, and toad, Bufo marinus, respectively. Properties of photoresponses were analyzed under each condition of adaptation when retinas were treated with 1 mM GTP and ATP, as well as certain of their hydrolyisis-resistant analogs. When light-adapted skate retinas were treated with (beta),(gamma)-methylene GTP (GMPPCP), guanylyl imidodiphosphate (GMPPNP), or (beta),(gamma)-methylene ATP (AMPPCP), there resulted a sustained decrease in threshold. Treatment with GTP or ATP resulted in transient threshold decreases. When applied to toad retinas that previously had been light-adapted, GMPPCP increased both the amplitude and duration of photoresponses. When similarly applied to either dark- or light-adapted toad retinas, neither GTP nor ATP had any effect on rod photoresponses. Treatment of dark-adapted retinas with GTP, ATP, GMPPCP, or AMPPCP did not alter response parameters in either of these preparations. These results are discussed in terms of nucleoside triphosphate-dependent mechanisms for rod adaptation.

Degree

Ph.D.

Subject Area

Biology

COinS