Decrease in litter weight gain and in progesterone secretion in lactating rats treated with antiserum to rat prolactin

In a previous paper (Tomogane, \l=O^\ta & Yokoyama, 1975), we demonstrated that prolactin was an important factor for stimulating the secretion of progesterone in lactating rats by using ergo-cornine, a specific inhibitor ofprolactin secretion. In the present paper, preliminary results (Tomogane et al., 1973) have been extended by neutralizing circulating prolactin with an antiserum to rat pro-lactin (anti-RPL). Methods Primiparous Wistar-Imamichi rats (250-300 g) were used. They were kept in a temperature-(24 \m=+-\2\s=deg\Cand light- (14 hr light:10 hr dark, lights on 05.00 hours) controlled room, and given free access to a commercial diet (CA-1, Nihon CLEA Ltd, Tokyo) and water. The day on which young were found between 09.00 and 12.00 hours was designated as Day 0 of lactation. Within 2 days of parturition the litter size was adjusted to 8,4 of each sex being kept whenever possible. Body weight and food consumption of the mother, and the gain in litter weight were recorded every morning(08.30-09.30 hours). Vaginal smears of the mothers were taken at the same time. The daily gain in litter weight was measured because it has been established that prolactin is indispensable for main¬ taining lactation in the rat (see Cowie & Tindal, 1971, for review). In Exp. I, animals were injected i.p. with either 1 ml anti-RPL or 1 ml normal rabbit serum(NRS) at 09.00 and 17.00 hours on Day 10 of lactation, and the litter gain and period of lactationa