[Delayed effect of the tetrapeptide tyr-D-ala-gly-phe-NH2 on the serotonin content of the synaptosomes of the rabbit brain].

The content of serotonin in synaptosomes of the motor cortex and caudate nucleus of the rabbit brain was studied 5 days after a single injection of the opiate tetrapeptide--Tyr-D-Ala-Gly-Phe-NH2 amide (TPA). TPA was injected subcutaneously in a dose of 500 micrograms/kg bw rabbit. The content of serotonin in synaptosomes of the motor cortex of control rabbits was significantly less than in synaptosomes of the caudate nucleus, being equal to 5.82 and 45.79 micrograms/mg protein, respectively. Five days following TPA single injection the content of serotonin in synaptosomes of the caudate nucleus decreased by 51.7%. The changes in the serotonin content in motor cortex synaptosomes were insignificant. Thus, the postponed effect of opiate TPA on the serotoninergic brain system was discovered, which became manifest on day 5 after a single injection. A possible mechanism of the TPA action on the serotoninergic system is discussed. A possible role of the postponed effects of neuropeptides in short-term memory is suggested.