beta-Endorphin: development of tolerance and its reversal by 5-hydroxytryptophan in cats.

Abstract
Tolerance to .beta.-endorphin developed acutely in cats if the administration of the peptide was repeated within the first 24 h. The tolerance was reversed immediately by systemic administration of the serotonin precursor, 5-hydroxytryptophan. 5-Hydroxytryptophan potentiates the analgesic effect of the subliminal dose of .beta.-endorphin.