The endomorphins (EM1: Tyr-Pro-Trp-Phe-NH2, and EM2: Tyr-Pro-Phe-Phe-NH2) are recently discovered endogenous ligands for mu-opioid receptors (MORs) with role of neurotransmitters or neuromodulators in mammals. Cessation of their physiological action may be effected through rapid enzymatic degradation by the dipeptidyl-peptidase IV (DPPIV) found in the brain synaptic membranes. An in vitro superfusion system was utilized to investigate the actions of EM1, EM2 and specific DPPIV inhibitor diprotin A on the striatal release of dopamine (DA) induced by electrical stimulation in rats. The involvement of the different MORs (MOR1 and MOR2) in this process was studied by pretreatment with MOR antagonists beta-funaltrexamine (a MOR1 and MOR2 antagonist) and naloxonazine (a MOR1 antagonist). EM1 significantly increased the tritium-labelled dopamine DA release induced by electrical stimulation. EM2 was effective only when the slices were pretreated with diprotin A. beta-Funaltrexamine antagonized the stimulatory effects of both EM1 and EM2. The administration of naloxonazine did not appreciably influence the action of EM1, but blocked the action of EM2, at least when the slices were pretreated with diprotin A. These data suggest that both EM1 and EM2 increase DA release from the striatum and, though diprotin A does not affect the action of EM1, it inhibits the enzymatic degradation of EM2. The DA-stimulating action induced by EM1 seems to be mediated by MOR2, while that evoked by EM2 appears to be transmitted by MOR1.
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