INHIBITORY EFFECTS OF 3 OPIOID RECEPTOR A GONISTS ON SYNAPTIC TRANSMISSION

AIM: To compare the inhibitory effects of 3 opioid receptor agonists, (D-Ala(2), NMe-Phe(4), Gly-ol)-enkephalin (DAGO), (D-Pen(2,5))-enkepha lin (D-PEN), and -N-[2-(1-pyrrolidinyl)cyclohexyl]-benzeneacetamide me thanesulfonate (U-50488H) in different concentrations on synaptic tran smission. METHODS: The excitatory postsynaptic potentials (EPSP) in sl ice preparation of nucleus accumbens of rats were recorded using elect ric stimulation of the olfactory tubercle area and intracellular micro pipettes filled with potassium acetate (3 mol . L(-1)). RESULTS: Super fusion of DAGO, D-PEN, and U-50488H (1 mu mol . L(-1)) reduced the amp litude of EPSP and the inhibitory effect on EPSP were reversed by supe rfusing naloxone (Nal, 1 mu mol . L(-1)), in which the DAGO-induced re duction of synaptic transmission was the most effective. The depolariz ing responses to microiontophoretic injection of glutamate were reduce d by superfusing DAGO in 19 neurons of slice preparation of nucleus ac cumbens. CONCLUSION: The inhibitory effects of DAGO, D-PEN, and U-5048 8H on EPSP were in a concentration-dependent manner, and the mechanism of opioid agonists (at least DAGO) reducing EPSP was related to a dec rease of postsynaptic transmission mediated by glutamate.