Changes in the responses of prolactin and corticosterone secretion to suckling at various stages of prolonged lactation in intact and ovariectomized rats.

Abstract

Ovariectomy postpones the onset of the decline in the milk production which occurs at late or prolonged stage of lactation in intact rats. To determine whether this effect of ovariectomy on milk production is mediated by prolactin or corticosterone secretion, the effect of ovariectomy on secretory responses of prolactin and corticosterone to suckling by pups was examined at various stages of normal and prolonged lactation. Lactating rats were ovariectomized on day 1 (day 0= the day of parturition) and litter size was adjusted to 8 on day 2. Pups were replaced with those aged 6-8 days at every 8th day from day 16 onwards. Mothers were allowed to suckle for 30 min after 4-hr separation from pups, and were bled for 2hr at 10-30 min intervals from the onset of suckling on days 8, 16, 24, 32 and 40. Plasma prolactin and corticosterone concentrations elevated in response to suckling on days 8 and 16 both in sham-operated and ovariectomized rats. The response of these hormones almost disappeared on day 24 onward. Ovariectomy did not affect the secretory response of prolactin and corticosterone to suckling from days 16 to 40 except that the level of prolactin response was lowered on day 8. These results suggest that neither prolactin nor corticosterone mediates the effect of ovariectomy on the decrease in the milk production which occurs around the end of normal lactation.