EFFECT OF TEMPERATURE ON VISUAL PROCESSES 



V. J. WuLFF, Department of Zoology, Syracuse 

 University, Syracuse, New York 



HIS PAPER will be limited to a discussion of the effect of temperature 

 upon the photochemical and electrical events which occur in photorecep- 

 tors or their parts. 



EFFECT OF TEMPERATURE ON BLEACHING OF RHODOPSIN 



One of the temperature-sensitive photochemical processes which has 

 received much study is the bleaching of rhodopsin following illumination. 

 Rhodopsin is the chief vertebrate visual pigment which mediates rod 

 function. It has long been supposed that the light-initiated bleaching of 

 this photosensitive pigment plays a role in the visual act. 



Hecht (4) was the first to publish quantitative data on the effect of 

 temperature on the extent of bleaching of extracted frog rhodopsin. His 

 data show that the extent of bleaching is the same at 5.2°C as it is at 2°C 

 and 36°C for a given exposure to light. The Qio of the bleaching process 

 was computed as equal to 1. This temperature independence of the extent 

 of bleaching of rhodopsin reported by Hecht confirmed some earlier ob- 

 servations of Klihne. It should be mentioned here that the bleaching proc- 

 ess was not the limiting process in Hecht's experiment. The methodology 

 Hecht employed was to expose rhodopsin solutions at different tempera- 

 tures to a constant light source for a fixed period of time. The solution 

 was then brought to room temperature in the dark for the density deter- 

 mination. Since the bleaching process proceeds in the dark, it went to com- 

 pletion and was not a limiting process. What the data of Hecht do show 

 is that the rate of excitation of rhodopsin is temperature-independent. 



In 1937, Lythgoe (6) first mentioned the occurrence of a color change 

 in illuminated rhodopsin that was cooled to ice temperature. The color 

 produced was orange and, since the orange color faded upon warming, even 

 in the dark, he called the orange state of rhodopsin 'transient orange'. 

 Since Lythgoe's original observation, blocking of the bleaching reaction by 

 low temperatures (i.e. dry ice temperatures) has been demonstrated by 

 Broda and Goodeve (1) and by Wald et al. (7) ; there can be no doubt 

 that the bleaching process is slowed appreciably, if not blocked entirely 

 by low temperatures. 



More recently Hagins (3) and Wulff et al. (9, 10) have obtained yet 

 additional evidence for the temperature dependence of the bleaching of 



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