The effects of microiontophoretically applied morphine and transmitter substances in rats during chronic treatment and after withdrawl from morphine.

The effects of microiontophoretically applied acetylcholine, noradrenaline, 5-hydroxytryptamine and morphine were studied on single brain stem neurones of rats during chronic morphine pretreatment and 24 h after its withdrawal. No significant changes were observed in the initial spontaneous neuronal firing rate or in the qualitative or quantitative effects of acetylcholine, noradrenaline or 5-hydroxytryptamine. However, in chronically treated animals there was a significant decrease in the number of neurones excited by morphine or showing tachyphylaxis to morphine on repeated microiontophoretic applications.We suggest that some of the cellular central nervous system changes which occur during chronic morphine treatment are reflected by the decrease in sensitivity of neurones to morphine excitation.