Pseudopregnancy in the Rat from Brief Treatment with Progesterone: Effect of Isolation

Abstract

THE fact that pseudopregnancy can be induced in the rat by administration of œstrogen has long been known¹. If given during œstrus, that is, on the day after ovulation, only a single moderately large injection is necessary^2,3. Alloiteau and Vignal⁴ have reported that one large dose of progesterone during œstrus would similarly induce pseudopregnancy in the majority of case The same results were obtained with 5 daily injections of 2 mg beginning with œstrus. They further reported⁵ that 4–5 daily injections of prolactin starting in œstrus initiated pseudopregnancies that were self-sustaining after the injections were stopped, presumably the combined result of the luteotrophic action of prolactin and the positive feed-back action of progesterone with respect to secretion of prolactin by the animals' own pituitaries. Rothchild and Schubert⁶ recently confirmed the effect of the single 10-mg dose of progesterone. While that work was in progress, the experiment was repeated in my colony and the results were again confirmed. However, as the number of trials increased, a lower frequency of pseudopregnancy became apparent in animals having 4-day cycles compared with those in which the cycles lasted 5 days.