The effect of morphine on monoamine release and content in guinea-pig brain slices.

The effect of morphine on the efflux of (3H) monoamines as well as the endogenous monoamine contents in electrically stimulated brain slices was investigated. Only at a concentration as high as 30 μM did the drug reduce the tritium efflux and counteracted the monoamine depletion caused by prolonged electrical stimulation. This effect was antagonized by Naloxone 10 μM. Besides the good agreement between the two methods used to evaluate drug effects the discrepancy between morphine concentrations active on the neurosecretory process and those effective in the whole animal is stressed. The opioids may act in vivo either by modulating the firing rate of the monaminergic neurons or by affecting other related neuronal pools.