Plasma prolactin in the rat during suckling without prior separation from pups

The relation between suckling and plasma prolactin (Prl) was studied in the rat, without prior separation of the dam from its pups. When the pups were replaced by a hungry foster litter, upon renewed suckling plasma Prl showed episodic increases and decreases in individual rats. When, subsequent to litter removal, similar rats were injected with perphenazine, a significant increase of plasma Prl was observed. This indicates that a decline of plasma Prl during suckling was not caused by exhaustion of Prl stores in the pituitary. In 22 individual rats blood was sampled every other minute while observations were made on nursing behaviour of the dams. During apparent suckling, increases as well as decreases of plasma Prl occurred. However, in most cases suckling did not affect plasma Prl, i.e. it remained stable at a high or a relatively low level. On the other hand, a considerable rise of plasma Prl was frequently observed when a dam was away from the nest. The data indicate that in the physiological situation Prl secretion from the pituitary is not directly related suckling activity, though episodes of suckling are essential to maintain a high Prl secretory capacity of the pituitary gland.