L-Tryptophan's effects on brain chemistry and sleep in cats and rats: a review

In this review I shall discuss published and unpublished work from my laboratory dealing with L-tryptophan's effects on brain monoamines and sleep in cats and rats. From our work it appears that normal animals may not be suitable subjects for testing sleep-inducing effect of tryptophan since their slow-wave sleep (SWS) latency is relatively short. In polyphasic sleepers like cats, we did not observe tryptophan's hypnotic effect with any dosage used (10, 30 or 135 mg/kg). However, we found small, but statistically significant, sleep-inducing effect of tryptophan (30 mg/kg, IP) in normal rats. We have tried, therefore, to create insomniac cats with long sleep latencies by using methysergide, a serotonin receptor blocker. The results show that in insomniac cats hypnotic effect of tryptophan, a precursor to brain serotonin, was observed. It involved not only reduction of sleep latencies but also an increase in SWS. It seems likely that tryptophan's partial reversal of methysergide's effect in cats occurred via a dual mechanism of serotonergic activation and catecholaminergic deactivation, while its sleep-inducing effect in normal rats may have been due to the attenuation of the activity of brain catecholamines.