Uptake and release of [3H]dopamine by the median eminence: evidence for presynaptic dopaminergic receptors and for dopaminergic feedback inhibition.

The accumulation and release of [3H]dopamine by the median eminence in vitro was studied after treatments with different pharmacological agents, to determine whether such a procedure would be useful for measuring neuronal activity in the tuberoinfundibular dopaminergic system. The accumulation of [3H]dopamine was temperature, time, and sodium dependent, and reduced by unlabelled dopamine and by a potent dopamine uptake blocker, nomifensine. The outflow of tritium was studied after blocking the oxidative deamination of dopamine by nialamide. The outflow of tritium was elicited consistently by biphasic square wave electrical pulses and by high molarity potassium ions. The response to electrical stimulation was dependent largely on calcium and partially on sodium. The response to high molarity potassium ions was reduced in the absence of calcium ions. The response to electrical stimulation was increased by nomifensine and by a dopaminergic antagonist, haloperidol, and was reduced by dopamine and by a dopaminergic agonist, piribedil. The inhibitory action of dopamine was antagonized by haloperidol. These results indicate the existence of uptake and release mechanisms in the tuberoinfundibular dopamine neurons, and suggest that dopamine may inhibit its own release via dopaminergic receptors. This in vitro method may be useful for measuring dopamine uptake and release by tuberoinfundibular dopaminergic neurons.