Mechanism of the effects of hypothalamic deafferentation on prolactin secretion in the rat

Male rats with complete hypothalamic deafferentation had consistently lower serums prolactin concentrations than controls when the blood samples were obtained under other anesthesia. However, when rats were decapitated, both groups had similar low prolactin levels. Posterolateral deafferentation was as effective as complete deafferentation in preventing the stress-induced prolactin release, whereas anterior frontal deafferentation had only a small effect, L-Dopa (100 mg/kg body wt, ip) decreased prolactin titers in both control and deafferented animals, whereas reserpine (1 mg/kg body wt, ip) had the opposite effect. Since both drugs inhibited prolactin release from pituitaries in vitro, the decrease of prolactin levels following L-dopa in vivo might have been caused not only by stimulation of PIF release but also at least partly by the direct effect of the drug on the pituitary. However, the increase of serum prolactin following reserpine was in all probability caused by inhibition of PIF secretion. Electrolytic lesions in the median eminence of deafferented rats caused an elevation of serum prolactin which was more marked in female than in male rats. On the contrary, deafferentation in the females affected prolactin levels less than in males. It is concluded that hypothalamic deafferentation prevents ether-induced release of prolactin and that the "low" levels of the deafferented animals are probably due to a tonic release of prolactin-inhibiting factor (PIF) from the isolated island. It is though that this continuous release of PIF might be maintained by persisting autonomous activity of the adrenergic, presumably dopaminergic, neurons contained in the isolated island.