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The involvement of progesterone (P) withdrawal during late pregnancy in the initiation of maternal behavior was examined in primigravid rats. Females that were ovariectomized (0), hysterectomized (H), or ovariectomized and hysterectomized (OH) at 1000 h on day 17 of pregnancy responded maternally to foster pups significantly faster than did sham-operated intact (I) controls. Approximately 50% of 0 and OH animals and 8% of both H and I females became maternal within 30 h after surgery. At 48 h after surgery 90–100% of O, H, and OH animals responded maternally to foster young, whereas only 8% of I pregnant females were maternal. Increases in the number of I animals that exhibited maternal behavior were observed 72 and 96 h after surgery, 1–2 days before parturition. Serum P concentrations were measured in a second set of 0, H, OH, and I animals bled at 1000 h on day 17, 1600 h on day 17, 1000 h on day 18, and/or 1000 h on day 19 of pregnancy or post-sperm smear (pss). Serum P levels in I females did not change significantly between 1000 h on day 17 and 1000 h on day 19 of pregnancy (x range 79.1 ± 8.6 to 58.1 ± 4.8 ng/ml). However, a dramatic decrease (≥50%) in serum P levels occurred within 6 h after surgery in O and OH animals, and serum P concentrations in these groups continued to decline over the ensuing 42 h. In H animals serum P concentrations 6 h after surgery (x = 81.0 ± 10.6 (SEM) ng/ml) did not differ from those of I controls (x = 76.3 ± 5.8 ng/ml h but had declined significantly (-40%) by 24 h (1000 h day 18 pss) after hysterectomy. A further drop in serum P concentrations was found in H animals 48 h after surgery. In a second series of experiments the rapid onset of maternal behavior induced by OH on day 17 of pregnancy was prevented by subcutaneous Silastic implants containing P. Serum P concentrations in separate groups of implanted animals were significantly elevated (X = 45.5 ± 14.4 ng/ml) when compared to P levels present in oil-implanted controls (x = 12.7 ± 2.2 ng/ml) bled 24 h after OH. P levels remained elevated in Pimplanted animals bled 96 h after OH. These data indicate that the decline in serum P concentrations after pregnancy termination stimulates the onset of maternal behavior in rats. Maternal behavior appears about 24-30 h after progesterone withdrawal commences. (Endocrinology102: 258, 1978)
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