The Use of Photoresponse Kinetics as a Molecular Thermometer

Abstract

 

Purpose

 

Heating of the retinal pigment epithelium (RPE) has been considered as a potential treatment for degenerative retinal diseases. During the treatment, the temperature of the RPE must be monitored continuously in order to reach the therapeutic effect while avoiding damaging the cells. The photoreceptors are located at the proximal side of the RPE so that the outer segments (OSs) are partially embedded in the RPE layer leading to the temperatures of OSs and RPE being nearly equal. The photoreceptor light response kinetics are set by the phototransduction machinery located in the OS. The kinetics depend on temperature and, at least in principle, could be used as a temperature indicator during the heat treatment. The objective of this work is to find out whether the temperature of the photoreceptors can be determined accurately enough from the leading edge of ERG responses in the temperature-range 37 - 42 °C and to define a parameter that best represents the temperature. This method could be used for the RPE temperature determination in animal models.

 

Methods

 

We recorded ERG flash responses from isolated mouse retinas (C57BL/6J) at three temperatures: 37, 39.5, and 42 °C. The retinas were perfused with Ringer’s solution containing DL-AP4 and BaCl₂ to isolate the photoreceptor response. Three fixed stimulus intensities were used to give three different responses: a linear range, a half-saturated and a saturated response. The temperature-dependence was determined for two parameters: the time and the slope at the steepest point of the leading edge (Fig. 1).

 

Results

 

The results from three retinas are given in Table 1. Relative changes of the parameter values were quite similar between the different response types. The relative shortening of the inflection point time was on average 20% per 2.5 °C. The relative increase of the slope at the inflection point was greater, on average 39% per 2.5 °C, but the variance between the results was also higher.

 

Conclusions

 

Temperature elevations of 2.5 and 5 °C have a considerable effect on the parameter values which implies that the temperature could be determined accurately enough with this method. The next phase of this study is to test the method on synaptically active retina and later in vivo.

 

 

Figure 1. The time and the slope at the steepest point (inflection point) of the leading edge of a photoresponse.

 

 

Table 1. The relative change of the parameter value (compared to 37 °C), mean ± stdev % (n).

 

Keywords: 510 electroretinography: non-clinical • 688 retina • 701 retinal pigment epithelium