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Aim: To compare the inhibitory effects of 3 opioid receptor agonists, (D-Ala2, NMe-Phe4, Gly-ol)-enkephalin (DAGO), (D-Pen2,5)-enkephalin (D-PEN), and trans-(+/-)-3, 4-dichloro-N-methyl-N-[2-(1-pyrrolidinyl) cyclohexyl]-benzeneacetamide methanesulfonate (U-50488H) in different concentrations on synaptic transmission., Methods: The excitatory postsynaptic potentials (EPSP) in slice preparation of nucleus accumbens of rats were recorded using electric stimulation of the olfactory tubercle area and intracellular micropipettes filled with potassium acetate (3 mol.L-1)., Results: Superfusion of DAGO, D-PEN, and U-50488H (1 mumol.L-1) reduced the amplitude of EPSP and the inhibitory effect on EPSP were reversed by superfusing naloxone (Nal, 1 mumol.L-1), in which the DAGO-induced reduction of synaptic transmission was the most effective. The depolarizing responses to microiontophoretic injection of glutamate were reduced by superfusing DAGO in 19 neurons of slice preparation of nucleus accumbens., Conclusion: The inhibitory effects of DAGO, D-PEN, and U-50488H on EPSP were in a concentration-dependent manner, and the mechanism of opioid agonists (at least DAGO) reducing EPSP was related to a decrease of postsynaptic transmission mediated by glutamate.

In this study, electrical stimulation and microinjection in nucleus accumbens in urethane-anesthetized rats were conducted to observe the effect of electrical stimulation of nucleus accumbens on blood pressure and heart rate. The following results were observed: (1) Electrical stimulation of the nucleus accumbens of rat resulted in significant hypotension and bradycardia. (2) Kainic acid microinjected into the nucleus accumbens, could abolish the effects mentioned above. (3) Naloxone administered to the nucleus accumbens could block the cardiovascular inhibitory effect evoked by electrical stimulation of nucleus accumbens. The intra-accumbens injection of mu-receptor agonist, DAGO could also elicit hypotension and bradycardia to an extent comparable to that of the effect due to electrical stimulation, whereas kappa-receptor agonist, U-50 had no such an effect. (4) When the cardiovascular inhibitory effect elicited by electrical stimulation of the nucleus accumbens was observed, the discharge activity in locus ceruleus was also decreased. (5) Bilateral vagotomy could abolish the change in heart rate elicited by the electrical stimulation of nucleus accumbens, but not the hypotension. It is suggested that the mu-opioid receptors of neurons in nucleus accumbens are involved in cardiovascular activity.

The effect of exogenous opioid peptides on progesterone production by incubated rat luteal cells was studied. beta-endorphin (beta-EP) stimulated progesterone production in a dose-dependant manner (10(-8)-10(-6) mol/L); dynorphin exhibited a stimulatory effect only at 10(-6) mol/L, while Met-enkephalin had no substantial effect at dose from 10(-10)-10(-6) mol/L. mu-opioid receptor agonists DAGO and morphine also stimulated progesterone production. The stimulatory actions of beta-EP, DAGO and morphine were reversed completely by naloxone. On account of the fact that the beta-EP level in rat plasma is lower than that in the ovary, it seemed that beta-EP may be an intra-ovarian luteotrophic factor and be involved in the regulation of progesterone production. This action of beta-EP may be mediated by mu-opioid subtype receptors.