Morphine and methionine-enkephalin: different effects on spontaneous and evoked neuronal firing in the mesencephalic reticular formation of the rat.

The technique of microiontophoresis was used to determine the effects of morphine and methionine-enkephalin (met-enkephalin) on spontaneous neuronal firing and on firing evoked by a nociceptive stimulus (evoked firing). Morphine and metenkephalin produced one of the three following patterns of effects on single units in the mesencephalic reticular formation: 1) morphine but not met-enkephalin blocked evoked firing; 2) met-enkephalin but not morphine blocked evoked firing; and 3) both morphine and met-enkephalin blocked evoked firing. For neurons exhibiting each of these three patterns, the mean T100, a modification of the T50 which is analogous to a dose-response curve, correlated with the effects of the drugs on evoked firing. There appear to be differences in cell size and location which are associated with different effects of the drugs. The difference in the effects of the drugs on evoked firing cannot be explained by differences in transport number, diffusion or degradation of the drugs, nor by different locations of the drug ejection barrels. Naloxone, administered intravenously or micriontophoretically, antagonized the drug-induced blockade. The effects of morphine and met-enkephalin on spontaneous firing did not correlate with their effect on evoked firing. Furthermore, in the majority of cases, the effects of morphine and met-enkephalin on spontaneous firing were not the same. These data indicate that there may be more than one type of opiate receptor in the mesencephalic reticular formation.