The brain tryptophan hydroxylase activity in the sleep-like states in frog

The activity of the rate-limiting enzyme of serotonin biosynthesis, tryptophan hydroxylase, was determined in the brain stem in active awake frogs, and frogs in three sleep-like states: with plastic muscle tone (SLS-1), with rigid muscle tone (SLS-2), and with relaxed muscle tone (SLS-3). Significant decrease in the enzyme activity has been found in frogs in SLS-1 and SLS-2 compared to awake animals. The development in frogs a cataleptic-like immobility after treating the animals with rhythmic lighting was accompanied with a decrease in the brain tryptophan hydroxylase activity. These results provide strong evidence for the involvement of the brain serotonin in frogs in the control of evolutionary ancient sleep-like states, probably by the regulation of muscle tone.