The luteotropic stimuli necessary to transform the corpus luteum of the estrous cycle into a corpus luteum of pseudopregnancy on the morning of diestrus-2 (Day 2), as reflected by a dramatic divergence in progesterone secretion, were studied (Day 1 was taken as the first day of diestrus of pseudopregnancy). The requirement of prolactin (PRL) as a luteotropic stimulus was determined by inhibiting the diurnal and nocturnal PRL surges that occur immediately before and during the divergence in progesterone. Following cervical stimulation, 1 mg of 2-Br-α-ergocryptine (EC) was injected at 1100 and 2300 h on Day 1 (lights on 0600-1800 h), and the animals were decapitated at 2-4 h intervals from 1100 h on Day 1 to 1700 h on Day 2. In the control animals, the PRL surges on Day 1 and Day 2 were associated with an increase in progesterone secretion on Day 2. However, the regimen of EC treatment resulted in an inhibition of PRL surges, prolactin remaining at baseline values from 1100 h on Day 1 to 1700 h on Day 2. The inhibition of PRL secretion was associated with a fall in progesterone concentration to reach baseline values by 1700 h on Day 2. Furthermore, a group of animals similarly treated with EC returned to vaginal estrus 2 days later. LH concentrations did not differ in control and EC-treated animals. The effect of EC on corpus luteum function could be completely reversed by the simultaneous administration of PRL. In addition, if PRL was administered at 1100 h and 2300 h on diestrus-1 of the estrous cycle, in an attempt to mimic the surges of pseudopregnancy, regression of the corpora lutea did not occur. Progesterone levels increased to reach values comparable to those observed in pseudopregnancy on diestrus-2. The role of LH was studied by administering a dose of LH antiserum at 1100 and 2300 h on Day 1 of pseudopregnancy. This treatment failed to inhibit the increase in progesterone observed on Day 2. These results demonstrate that the surges of plasma PRL initiated by cervical stimulation are responsible for transforming a corpus luteum of the estrous cycle into a corpus luteum of pseudopregnancy, as reflected by an increase in progesterone secretion on Day 2. LH seems to have a minor role in maintaining corpus luteum function beyond that observed during the estrous cycle.
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