Lack of effects of melatonin administration and pinealectomy on the milk ejection response in the rat.

The effects of melatonin administration and pinealectomy on the milk ejection response were studied in the rat. The amounts of milk obtained by a litter during 1 hr suckling and during an additional 30 min suckling after 1 I.U. of oxytocin injection, following 7 or 8 hr separation from the mother on the 10th or 16th day of lactation, were considered as the naturally ejected milk and the residual milk and used as the index of the milk ejection response. Melatonin in doses of 0.8 or 1.6 mg was injected subcutaneously into the mother 2 hr before the suckling commenced. The amounts of the naturally ejected milk and the residual milk were not significantly influenced by these treatments compared with saline injected controls. The pinealectomized and shamoperated female rats were bred and the postpartum lactation was studied. No significant difference was observed between the 2 groups in the litter growth for 12 days from the 4th day of lactation, the total feed consumption of the mother during this period and also the milk ejection response determined on day 16 of lactation. Thus, the administration of melatonin and pinealectomy did not exert any significant effect on the milk ejection response under these experimental conditions. Also, pinealectomy did not affect the incidence of the vaginal estrous smear and the length of gestation period.