Posterior Pituitary Lobectomy Chronically Attenuates the Nocturnal Surge of Prolactin in Early Pregnancy

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Abstract
The posterior pituitary gland contains a potent PRL-releasing factor (PRF). The aim of this study was to determine whether this PRF was involved in the luteotropic PRL surges of early pregnancy. The posterior pituitary was removed from a group of rats at least 4 weeks before the experiment (LOBEX). LOBEX rats were tested for diabetes insipidus and checked postmortem for the absence of posterior lobes. Plasma samples were taken from these chronically LOBEX rats as well as from sham-operated and unoperated controls beginning at noon on day 3 of pregnancy and continuing for 3 days at times most likely to demonstrate the twice daily surges of PRL secretion. PRL in all groups was basal (less than 12 ng/ml) at noon and rose to a diurnal peak at 1800 h. While the unoperated and sham-operated controls then displayed a clear nocturnal surge of PRL, peaking at 300-400 ng/ml at 0300 h, LOBEX rats had a significantly attenuated peak of 102.7 +/- 17.7 ng/ml (P less than 0.001). On subsequent nights the nocturnal peaks were similarly attenuated in LOBEX rats, averaging 80.3 and 42.1 ng/ml on days 5 and 6, respectively. LOBEX rats all had normal sized litters, but lactation yields were depressed. These data support the hypothesis that the posterior pituitary secretes a potent PRL-releasing factor because its removal partially blocked the mating-induced nocturnal surge of PRL.

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