Effects of light reversal on the circadian pattern of motor activity and voltammetric signals recorded in rat forebrain.

To investigate the functional relationships between the circadian changes in rat motor activity and changes in the extracellular concentration of ascorbic acid and homovanillic acid (HVA) monitored in the striatum and nucleus accumbens, reversal of the light/dark cycle was used to disturb the pattern of motor activity. Microcomputer-controlled linear sweep voltammetry with carbon-paste electrodes was used to continuously monitor circadian changes in the ascorbate signal and the HVA signal simultaneously in nucleus accumbens and striatum over a 13 day period in unrestrained rats; total motor activity for each animal was also recorded. Regression analyses were carried out on each day's data to investigate the relationships between motor activity and the two voltammetric signals. During days 1-5, the lighting was on normal 12/12 light/dark cycle and high correlations were observed. Reversal of the light/dark cycle on day 6 caused an immediate change in the pattern of motor activity and electrochemical signals; by days 7-8 after light reversal the relationships between lighting, ascorbate, HVA and motor activity were reestablished under the new lighting conditions. During the intervening period, however, there was a complete breakdown in some of the correlations. The findings are discussed in the light of the hypothesis that changes in brain extracellular ascorbate reflect changes in the release of excitatory amino acids, and in terms of a recent model of the role, in the control of motor activity, for cortical and mesencephalic inputs to forebrain subcortical regions.