Characteristics of dopamine release from isolated nerve endings of the tuberoinfundibular neurones

In the present study the release of dopamine (DA) from a synaptosomal preparation obtained from the nerve endings of the tuberoinfundibular dopaminergic nerves has been analysed, utilizing a superfusion technique. The exposure of median eminence synaptosomes to increasing concentrations of K + ions produced a dose-dependent efflux of preaccumulated [ 3 H]DA. The removal of Ca 2+ ions from the superfusion medium completely prevented the high K + -induced release of [ 3 H]DA. Removal of Na + ions from the superfusion medium, a condition creating a favourable outward Na + gradient, produced a marked release of [ 3 H]DA. In order to establish whether this efflux of dopamine was a carrier-mediated process nomifensine, a dopamine transport inhibitor devoid of dopamine releasing effect in the median eminence, was added to the superfusion medium. Nomifensine largely counteracted the release of dopamine induced by the lack of Na + . d -Amphetamine produced a dose-related stimulation of [ 3 H]DA in synaptosomes from the median eminence. The possible existence of a direct control of dopamine release mediated by presynaptic receptors was tested by studying the effect of apomorphine on the efflux of preaccumulated [ 3 H]DA. Apomorphine was unable to modify both the spontaneous and the K + -evoked release of dopamine. The present results give support to the idea that dopamine released from the nerve endings of tuberoinfundibular neurones into the primary plexus acts as a neurotransmitter. The release of dopamine in these nerves does not seem to be regulated by autoreceptors.