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

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