INHIBITION OF GLYCINE-INDUCED CURRENT BY MORPHINE IN NUCLEUS-TRACTUS-SOLITARII NEURONS OF GUINEA-PIGS

We studied the effect of morphine on the current induced by glycine in acutely dissociated nucleus tractus solitarii (NTS) neurones of guine a pigs, by use of the whole-cell patch clamp technique. Morphine inhib ited 30 mu M glycine-induced current (I-gly), without affecting the cu rrent caused by 30 mu M GABA. The effect of morphine was concentration -dependent, with a maximal effect at 1 mM, and reversible. The half-ma ximum inhibitory concentration of morphine was 30 mu M. The effect of morphine was not depressed by naloxone, an opioid antagonist. Furtherm ore, the effect was not substantially affected by methiothepin, a 5-HT 1 antagonist, ketanserin, a 5-HT2 antagonist and MDL-72222, a 5-HT3 an tagonist. Morphine at 30 mu M shifted the concentration-response curve for I-gly to the right without affecting the maximum value. The effec t of morphine on I-gly showed no use-dependence. The results indicate that morphine inhibits I-gly in the NTS neurones, and further suggest that morphine at the concentration used may act on the glycine recepto r-ionophore complex, but not on the Cl-channel of the complex. (C) 199 8 Prous Science. All rights reserved.