Presynaptic alpha-adrenoceptors and opiate receptors: inhibition of [3H]noradrenaline release from rat cerebral cortex slices by different mechanisms.

The inhibitory effects of morphine and Cd2+ on electrically evoked [3H]noradrenaline release from superfused brain slices were unaffected when release was enhanced by increasing the pulse duration, while the inhibitory effect of noradrenaline and the enhancing effect of phentolamine were diminished. A similar enhancement of [3H]noradrenaline release by 4-aminopyridine reduced the modulatory effects of all drugs examined. Therefore there seem to be different mechanisms for the effect of presynaptic alpha-adrenoceptors and opiate receptors on the availability of Ca2+ for the stimulus-secretion coupling process in noradrenergic nerve terminals.