Estrous cycle lengthening by pentobarbital in the rat: prolactin involvement

Pentobarbital lengthens the estrous cycle when injected on diestrus 1 afternoon in the 4-day cyclic female rat. The present study was undertaken to elucidate the mechanisms involved. Bromocriptine (2.5 mg/kg sc) given 30 min before pentobarbital (30 mg/kg ip) prevented the effects of pentobarbital on both cycle lengthening and progesterone levels in diestrus 2. Serum prolactin concentrations rose 10 min after pentobarbital injection, peaked at 30 min, and returned to control values after 60 min. Our results suggest that pentobarbital-induced cycle lengthening is due to a high progesterone production on diestrus 2 morning related to the luteotropic action of prolactin.