Abstract
A recent study by Kamberi, Mical & Porter (1971) demonstrated that injection of melatonin or serotonin into the third ventricle of male rats caused a release of prolactin and resultant increased plasma levels. The mechanism of this effect was presumed to be via an inhibition of prolactin-inhibitor factor (PIF) (Schally, Meites, Bowers & Ratner, 1964), although the possibility of stimulation of a postulated prolactin-releasing factor (Mishkinsky, Khazen & Sulman, 1968) was not discounted. As the pineal is the primary source of melatonin (Wurtman, Axelrod & Chu, 1963), and as serotonin is a precursor of melatonin, it was thought to be worthwhile to explore the effect of pinealectomy on pituitary and plasma levels of prolactin in the rat. Inbred male rats of the Wistar strain were pinealectomized according to the method of Hoffman & Reiter (1965), or sham-pinealectomized, at the age of 3 weeks (50 g) and 8 weeks (185 g).

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