Changes in plasma progestin, prolactin, LH and FSH at luteal activation with phenobarbital anesthesia in the rat.

Progesterone (P), 20 alpha-hydroxypregn-4-en-3-one (20 alpha-OHP), prolactin (PRL), LH and FSH in peripheral blood plasma were measured during the early stages of phenobarbital-induced pseudopregnancy (PHB-PSP; Furudate, 1975) and the changes in these hormone levels were compared with those during the 5-day estrous cycle and early stages of ordinary pseudopregnancy (PSP) induced by cervical stimulation. During the cycle, surging releases of LH, FSH and PRL occurred in the afternoon of the first day of vaginal estrus (E1) and levels of these hormones were low during the other stages. Two large rises in P were observed; one in the afternoon of day E1 and the other during 2 days of vaginal diestrus (D1 and D2). 20 alpha-OHP showed daily diurnal rises sometime between 1600 and 2300 hr. A phenobarbital (PHB) injection (100 mg/kg, sc) at 1900 hr on D1 initiated daily PRL surges, which were similar to those observed following cervical stimulation though of less magnitude, without altering the levels of LH and FSH significantly. Continued elevation of P and lowering of 20 alpha-OHP levels were observed as during the ordinary PSP. Continued vaginal diestrus of PHB-PSP was blocked by simultaneous injections of bromoergocryptine (CB-154; 1 mg, sc) and PHB. In addition, the PSP-like vaginal diestrus was evident in all the rats receiving PRL administration twice daily for 3 consecutive days, staring at 2100 hr on D1. Therefore, PRL surges induced by PHB administration will be solely responsible for the luteal functionalization in PHB-PSP rats.