Evidence that the dopaminergic prolactin-inhibiting factor mechanism regulates only the depletion-transformation phase and not the release phase of prolactin secretion during suckling in the rat.

We have compared the effectiveness of stalk-median eminence (SME) extracts, dopamine, and bromocriptine (CB-154) in inhibiting the release of PRL into the plasma during suckling when injected before depletion with that when the injections were given after the pituitary stores of PRL had been depleted and transformed by 10 min of suckling. We found that a single injection of either neutralized acidic extracts of SME at a dose of one or two SME equivalents per rat or 625 ng dopamine effectively prevented both the depletion of PRL within the pituitary and the subsequent rise in plasma PRL when given 1-3 min before the onset of suckling. Likewise, a single injection of bromocriptine (0.5 mg) given 20-30 min before the onset of suckling totally blocked pituitary depletion and the subsequent release of PRL into the circulation. However, the PRL concentration rose normally in the plasma in response to suckling when either SME extract or bromocriptine was given after PRL depletion had first been effected by prior short term (10 min) suckling. These data indicate that the depletion-transformation phase but not the release phase (into the circulation) of PRL secretion is under inhibitory control by a dopaminergic PRL-inhibiting factor system.