Effect of Nonsuckling Interval and Duration of Suckling on the Suckling-Induced Fall in Pituitary Prolactin Concentration in the Rat

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Abstract
Two and 30 min. of suckling by 6 pups following 8 hr. of nonsuckling on postpartum day 7 induced equivalent falls in pituitary prolactin concentration in the lactating rat. Neither 2 nor 30 min. of suckling by 6 pups induced a fall in the pituitary prolactin concentration if the mothers were previously nonsuckled for 16 hr. Increasing the number of pups to 9 at the time of suckling likewise was ineffective. Milk ejection, however, was normal in all rats after 16 hr. of nonnursing. Normal reductions in pituitary prolactin concentration in response to 30 min. of suckling by 6 pups occurred providing either 1 or 2 short periods of suckling stimulation (5 or 30 min.) were provided to the mammary glands during the first 8 hr. of the 16-hr, isolation period. These data suggest that in the rat the merchanism involved in prolactin release in response to suckling operates efficiently only if the sucking stimulus is applied periodically and at short intervals.