α ₂ and β Adrenoceptors in the Mediobasal Hypothalamus and α₂ Adrenoceptors in the Preoptic‐Anterior Hypothalamus Stimulate Prolactin Secretion in the Conscious Male Rat

Plasma prolactin concentrations were measured in unanaesthetized male rats before and after stereotaxic microinjection of adrenergic agents into the mediobasal and preoptic‐anterior hypothalamus. In the mediobasal hypothalamus injection of the α₂ agonist clonidine produced a dose‐dependent increase in prolactin secretion over the dose range 0.1 to 10 nmoles, the stimulation due to 1 nmole being blocked by idazoxan (α₂ antagonist). Stimulation of prolactin release was also caused by isoprenaline (β agonist) and was significantly reduced by the β antagonist propranolol. The β₂ agonist salbutamol was also effective in stimulating prolactin secretion. However, the adrenergic agonists, noradrenaline (mixed α and β), phenylephrine (α₁) and tyramine (sympathomimetic) failed to affect prolactin secretion. In the preoptic‐anterior hypothalamus clonidine caused a dose‐dependent increase in prolactin secretion over the dose range 0.001 to 10 nmoles, the stimulation due to 0.1 nmole being abolished by idazoxan. While prolactin levels were significantly elevated by noradrenaline and tyramine, phenylephrine was ineffective. We conclude that the activation of α₂ and β₂ adrenoceptors in the mediobasal hypothalamus and of α₂ adrenoceptors in the preoptic‐anterior hypothalamus, on or near prolactin‐regulating neurons, results in increased prolactin secretion. An α₁ inhibitory action in the mediobasal hypothalamus has however not been ruled out. Adrenergic inputs in the preoptic‐anterior hypothalamus appear to exert a predominant facilitatory effect on prolactin secretion.