[Morphine blocked the exitatory amino acid mediated membrane current in ambigual motoneurons of the rat]

Excitatory postsynaptic potentials (EPSPs) and responses of neurons in the compact formation of the neucleus ambiguus (AMBc) to pressure-ejected or bath-applied test substances were recorded intracellularly from sagittal slices of Sprague-Dawley rat medulla containing subnucleus centralis of solitary complex (NTSc), AMBc and solitarioambigual pathway. In 5 cells, recorded spontaneous EPSPs could be blocked by morphine (3-5 pmol) to AMBc. Electrical stimulation of NTSc evoked EPSPs in AMBc neurons, the amplitude of which were decreased to 71.1 +/- 6.2% (P < 0.001) with the addition to morphine. The morphine effect could be abolished by bath-applied naloxone (50 nmol/L). The amplitude of membrane depolarization induced by pressure-ejected N-methyl-D-aspartate (NMDA, 0.5-1.0 pmol), acetylcholine (3 pmol) and quisqualate (0.1-0.5 pmol) to NTSc could also be decreased by bath-applied morphine (10 mumol/L) respectively to 38.1 +/- 5.7% (P < 0.001), 32.8 +/- 5.5% (P < 0.01) and 29.6 +/- 7.1% (P < 0.05). The above results suggest that morphine is capable of block by NMDA, ACh and non-NMDA receptors, increaseing the M-current and decreasing the permeability of Na+ and Ca2+.