[Tryptophan-5-hydroxylase activity in the rabbit brain synaptosomes at different times after the single administration of the opioid peptide Tyr-D-Ala-Gly-Phe-NH2].

The activity of the key enzyme of serotonin biosynthesis--tryptophan-5-hydroxylase (T-5-H) was investigated in the synaptosomes of the motor cortex and caudate nucleus of the rabbit brain 30 min or 5 days after single injection of opioid tetrapeptidamide Tyr-D-Ala-Gly-Phe-NH2 (TPA). TPA was injected subcutaneously at a dose of 500 micrograms/kg of rabbit body weight. T-5-H activity in caudate nucleus synaptosomes was two times higher than that of the motor cortex synaptosomes and accounted for 159.67 and 80.84 pmoles of formed 5-hydroxytryptophan/mg protein per hour. 30 min after single injection of TPA the enzyme activity in the synaptosomes of the motor cortex and caudate nucleus decreased by 64.0 and 43.0%, respectively. 5 days following single TPA injection T-5-H activity in the motor cortex synaptosomes increased by 68.4% and in caudate nucleus synaptosomes decreased by 35.6%. Thus, it was established that TPA displayed a pronounced effect on T-5-H activity. The delayed effect of opioid TPA on T-5-H activity was discovered which was manifested on day 5 after the single injection. Possible mechanisms of TPA effect on T-5-H are discussed.