The effect of dopamine on neurohypophysial hormone release in vivo and from the rat neural lobe and hypothalamus in vitro.

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Abstract
The rat hypothalamus (containing the supraoptic nuclei, paraventricular nuclei, median eminence and proximal pituitary stalk) was incubated in vitro and shown to be capable of releasing the neurohypophysial hormones, oxytocin and arginine vasopressin at a steady basal rate about 1/20 that of the rat neural lobe superfused in vitro. The hypothalamus and neural lobe in vitro released both hormones in a similar arginine vasopressin/oxytocin ratio of about 1.2:1. When release was expressed relative to tissue hormone content, the hypothalamus released about 3.times. as much arginine vasopressin and 6.times. as much oxytocin as the neural lobe. Dopamine in a concentration range of 10-13-10-9 M caused graded increases in hormone release from the hypothalamus in vitro to a maximum 5-fold increase over preceding basal levels. The demonstration that apomorphine also stimulated hormone release whereas noradrenaline was relatively ineffective suggested that a specific dopamine receptor was involved. A separate cholinergic component in the release process was indicated by the finding that acetylcholine stimulated release to a maximum 5-fold increase in concentrations of 10-13-10-9 M. The fact that the isolated hypothalamus can be stimulated by dopamine and acetylcholine to release increased amount of oxytocin and arginine vasopressin raises the question of the origin and fate of the hormones released in this way. The possibility that they could be released into the hypophysial portal circulation from median eminence to affect the anterior lobe of the pituitary is discussed. In similar doses both dopamine and noradrenaline injected into the lateral cerebral ventricles of the brain of the anesthetized, hydrated, lactating rat caused the release of arginine vasopressin and oxytocin. Apomorphine released both hormones but at a higher dose level and to less effect than the catecholamines. The hormone release induced in vivo by dopamine could be prevented by the prior administration of haloperidol or phentolamine, and these antagonists were equally effective in blocking the hormone release due to noradrenaline. The involvement of a specific dopamine receptor was more clearly implicated by the use of pimozide, which completely inhibited the hormone release due to dopamine and apomorphine but not that due to noradrenaline. The release of neurohypophysial hormones perhaps is stimulated via a dopaminergic nervous pathway in addition to a cholinergic one. The possibility that the osmoreceptor mechanism for the release of antidiuretic hormone from the neural lobe of the pituitary may involve such a dopaminergic pathway is discussed.

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