PubMed · 8218063
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G L Laffi, A B Safran. 1993. Persistent visual changes following hashish consumption.. https://doi.org/10.1136/bjo.77.9.601
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To evaluate the effects of light and darkness on pyroglutamyl peptidase I activity (pGluPI) and its left-right distribution, pGluPI was measured bilaterally in the retina and hypothalamus under selected light-dark schedules. Rats under a 12 h light-dark cycle were divided into four experimental groups. After the end of the 12 h dark period, the animals were kept two additional hours in darkness (group 1), or light (group 2). After the end of the 12 h light period, the animals were kept two additional hours in darkness (group 3), or light (group 4). Experiments were done in light or darkness depending on the 2 h period. In the retina, a previous 12 h light period led to higher values of enzyme activity than dark periods. Left-right predominance, however, depended on the previous 2 h period: the light period led to left predominance, whereas right predominance was found after the 2 h dark period. In the hypothalamus, a left predominance was found only in group 3. These results demonstrate that environmental light conditions influence pGluPI activity in the rat retina and hypothalamus.
Carboxypeptidase E (CPE, EC 3.4.17.10) is an essential enzyme in the post-translational processing of most neuroactive peptides. We have studied how the expression of CPE in the rat retina is modified in response to the light/dark cycle. Retinal CPE mRNA levels increase immediately after a change in lighting condition. After prolonged exposure to the dark or the light, however, CPE mRNA levels drop to comparable amounts. The increase CPE mRNA is most dramatic during the dark-to-light transition. Interestingly, during the same interval, CPE protein levels transiently decrease. This apparently contradictory change in mRNA and protein levels is attributable to a change in the spatial pattern of CPE expression in the retina in response to light and dark. Both in situ hybridization and indirect immunofluorescence studies indicate that CPE is expressed in photoreceptors in the dark. During light onset, CPE expression is rapidly induced in retinal ganglion cells, whereas expression in photoreceptors is reduced. This pattern is reversed when the animals are exposed to the dark. In contrast, CPE is apparently constitutively expressed in a subpopulation of cells in the inner nuclear layer in both the light and the dark. These changes in CPE expression are similar to light-induced changes in c-fos expression, suggesting that CPE may be a downstream target for Fos activation.
Nerve cell signals are different in form from the stimuli that evoke them and they exhibit complex spatio-temporal characteristics. This defines a neural coding problem which is addressed by two current theories: Multiple Meaning Theory holds that neural signals contain patterns that make statements about combinations of stimulus properties; the Task Dependence Hypothesis suggests that different features of identical neural signals mediate performance in different behavioral tasks. These coding issues were addressed by investigating the representation of sensory information in the distal nervous system after transduction of visual stimuli into bio-electric signals. The objects of study were light-evoked neural responses which had been intracellularly recorded from single retinula (photoreceptor) cells in Limulus lateral eyes. The efficacies with which sensory information was represented by various candidate neural codes were calculated using receiver operating characteristic (ROC) analyses to provide objective indices. The specific visual problem under investigation was discrimination between light flashes whose intensities differed by a very small amount. A wide range of light adaptation states and relative stimulus intensities were explored. Extremely stringent data quality standards were applied which restricted the investigation to cells whose potentials did not exhibit any statistically significant drift during the hours required for data collection. Seven cellular characterizations were simultaneously monitored to detect drift in a given cell's potentials; these characterizations included the value of the membrane potential and the values of six candidate codes. These codes were: the area under the light-evoked receptor potential (RP), the mean value of the RP, the peak height of the RP, the slope of the onset of the RP, the duration required for the RP to drop from its peak by a given amount, and the duration required for the RP to end. The results were: (1) Light adaptation increases efficacy. (2) Thus, light adaptation trades sensitivity for acuity (as characterized by ROC discriminations). (3) Increasing relative light flash intensity also increases efficacy. (4) The efficacies of the various codes are significantly different and fall in the following order: area > or = peak = mean > or = duration-end = slope = duration-drop. These findings further demonstrate that arbitrary characterizations of stimulus-response relationships are very likely to be incomplete. They particularly indicate that many commonly used and quite conventional neural analysis strategies may substantially under-estimate system performance.