Effects of porcine hypothalamic extract on prolactin release in the rat

The effect of crude extract of porcine stalk-medium eminence (SME) on release of prolactin, measured by radioimmunoassay, has been studied in the rat. In male rats, intravenous administration of 3-4 SME equivalents caused an acute increase in serum prolactin levels. This prolactin-releasing factor (PRF) effect was observed in rats anesthetized with either pentobarbital or ether, and was not dependent upon estrogen pretreatment of the animals. Prolactin release was not seen with doses of 5 or 50 mug of synthetic thyrotropin-releasing hormone (TRH). The presence of porcine SME extract enhanced the release of prolactin by male rat pituitary halves incubated for 2 h in Medium 199. This effect was observed whether or not pituitary donors were pretreated with estrogen. In contrast to the observations in male animals, administration of 3 porcine SME equivalents to lactating rats separated from their litters for 4 h did not elevate serum prolactin. Failure to observe PRF activity could not be accounted for by the anesthetic employed. In vitro, porcine SME extract did not stimulate release of prolactin by pituitaries from lactating rats. Intraperitoneal injection of extract of 3 porcine SME equivalents inhibited the increase in serum prolactin which occurs when lactating rats are reunited with their litters for 30 min after 4 h of separation. These results are consistent with the concept that suckling-induced prolactin release is accomplished by cessation of discharge of a prolactin release-inhibiting factor (PIF). Furthermore, since effects of PRF were observed only in male rats, it is also concluded that the physiological state of animals treated with hypothalamic extracts is an important factor in determining responsiveness to either PIF or PRF.