Response of Rat Brain Indoles and Motor Activity to Short Light‐dark Cycles¹

Since the vigilance states of the rat can be largely controlled by one hour light‐one hour dark cycles, we investigated the effect of this photoperiod on the rat brain indoles and motor activity. Groups of 9 rats were killed 15‐20 min or 45‐50 min after the onset of the 1‐hour light or 1‐hour dark period. Serotonin (5HT) and 5‐hydroxyindoleacetic acid were determined fluorometrically in cortex, hypothalamus and brain stem. Tryptophan was determined fluorometrically in serum, cortex and hypothalamus. Cortical tryptophan was highest at the end of the dark period, whereas cortical 5‐HT and 5‐hydroxyindoleacetic. acid were highest at the beginning of the light period. Motor activity was high during darkness and low during light. When the biochemical results were compared to the motor activity records of the individual animals, the decrease in motor activity at the onset of light correlated significantly with the cortical 5‐hydroxyin‐ doleacetic acid/5‐HT ratio in the animals killed at the beginning of the light period. The results indicate a rapid response of the cortical indoles to the onset of light and support the hypothesis that the induction of sleep is related to the brain indole metabolism.