Bacteria-induced or bacterial product-induced preterm parturition in mice and rabbits is preceded by a significant fall in serum progesterone concentrations

Bacterial products are thought to induce labor by stimulating the production of pro-inflammatory cytokines and prostaglandins in gestational tissues, leading to the onset of preterm parturition. Progesterone withdrawal is a prerequisite of parturition in many species. Yet a role for progesterone in the mechanisms responsible for preterm parturition, in the setting of infection, is unclear. The current studies were conducted to determine if a fall in serum progesterone concentrations occurs before the onset of bacterial product-induced preterm parturition in animals. Accordingly, pregnant mice at day 15 (70% gestation) were injected i.p. with Escherichia coli lipopolysaccharide (LPS; 50 μg/mouse) and timed-pregnant rabbits were inoculated transcervically with a suspension of E. coli to cause an ascending intrauterine infection. Control animals in both groups received equal volumes of sterile phosphate-buffered saline (PBS) solution. Blood specimens were collected at regular intervals and serum progesterone levels were determined by RIA. Within 14 h of LPS administration, mice delivered their pups. The median concentrations of serum progesterone were significantly lower at 1 h, 4 h, 10 h, and at the onset of preterm parturition (11-12 h) after LPS injection, compared to that in animals given PBS. Similarly, E. coli-inoculated rabbits delivered 1-2 days posttranscervical inoculation and demonstrated 60% decrease in serum progesterone within 12-24 h of inoculation compared to those given PBS. Parturition in both control groups occurred at term, following typical progesterone withdrawal. It is concluded that LPS administration to pregnant mice and ascending intrauterine infection in pregnant rabbits is associated with a dramatic fall in serum progesterone concentrations prior to the onset of parturition.