Light-Induced Changes in the Incorporation of [14C]Q9rotic Acid and Uridine into the Ribonucleic Acid and Uridine Nucleotides of Rat Brain Cortex

Visual stimulation has been shown to affect the visual parts of the nervous system both structurally (Fifkova, 1968; Valverde, 1971) and biochemically (Singh & Talwar, 1967). The first exposure of rats to light has been shown to result both in a transient increase in the incorporation of labelled lysine into neuronal protein in the visual cortex (Rose, 1967,1972; Sinha &Rose, 1972) and in marked histological changes (Cragg, 1967). We have studied the metabolism of RNA and uridine nucleotides in the cortex of littermate rats exposed to light for the first time. However, in contrast with the previous studies cited above (where the rats were kept in the dark until 7 weeks after birth) weexposed rats to light 1 day after weaning, i.e. 22 days after birth. Previously we have shown that rats injected with 1 pCi of [6-14C]orotic acid in the dark and then exposed to light for 2h had a significantly higher incorporation of label into visual-cortex RNA than that of rats that were replaced in the dark for 2h after injection (Dewar & Winterburn, 1973). The present study was designed to investigate whether this efect was transient or not, and whether it was a reflexion of an alteration in the rate of uridine phosphorylation. Litters of ‘Piebald Virol Glaxo’ rats were maintained with their dams in a well-ventilated light-proof box until 22 days after birth. The dams were removed after 21 days. Rats in the experimental group were removed from the light-proof box and exposed communally to a light-source consisting of a 60W bulb held 0.30m (1 ft) above thecage for periods ranging from 0 to 14h. They were then injected intraventricularly with 1 pCi of [6-14C]orotic acid, exposed to light for a further 1 h and then killed. Dark control animals were injected intraventricularly with 1 pCi of [6-14C]orotic acid in the dark and killed 1 h later. To minimize any variation between litters, animals from the same litter were exposed to light for varying lengths of time-as opposed to complete litters being exposed to light for one period. In addition, dark control rats were killed at the same time as the experimental rats to minimize any contribution of circadian rhythms to the final result. The visual cortex and frontal cortex were removed and the RNA was extracted, determined and its radioactivity determined as previously described (Dewar & Winterburn, 1973). The incorporation data were expressed initially as a ‘relative specific radioactivity’ (d.p.m./mg of RNA per d.p.m. in acid-soluble fraction/mg wet wt. of tissue) but to minimize differences between litters and experiments the relative specificradioactivity values have been standardized by dividing each value by the mean of the dark control values for each experiment. Similar standardizing procedures have been utilized and discussed in previous studies on the effects of functional activity on RNA and protein metabolism in the central nervous system (Rose, 1967; Rose et al., 1970; Dewar et al., 1972). The first exposure of newly weaned rats to light appears to result in a transient increase in the incorporation of [14C]orotic acid into the RNA of the visual cortex (Fig. 1). Exposure for a total of 1 -2h results in an increase of over 20 % relative to the dark control. This is significant at theP<0.001 level. After 3-4h the incorporation rate reverts to the dark control value and remains the same thereafter. The rate of labelling of the frontal-cortex RNA also shows a small transient increase after exposure of the rats to light. This increase was not statistically significant and was only seen after 1 h. After 15h there was a depression in the labelling rate of the frontal-cortex RNA but this was not statistically significant.