EFFECTS OF PROGESTERONE ON PROLACTIN, HYPOTHALAMIC BETA-ENDORPHIN, HYPOTHALAMIC SUBSTANCE-P, AND MIDBRAIN SEROTONIN IN GUINEA-PIGS

Unlike rats, but similar to primates, guinea pigs exhibit prolonged fu nction of the corpus luteum and elevated progesterone secretion after ovulation. The gonadotropins, estrogen (E) and progesterone (P) have b een examined throughout the guinea pig estrous cycle. However, neither prolactin secretion nor its regulation by steroid hormones has been c haracterized, perhaps due to the lack of a specific radioimmunoassay. beta-Endorphin (BE), substance P (SP), and serotonin (5-HT) increase p rolactin secretion in rats and monkeys. BE and SP neurons in guinea pi gs and 5-HT neurons in monkeys contain progestin receptors which could mediate neuroendocrine effects of steroid hormones. Therefore, the ef fects off and P on prolactin, BE, SP, and 5-HT and its metabolite 5-HI AA were examined in guinea pigs which were ovariectomized, E treated ( 28 days), and E + P treated(14 days E + 14 days E + P). The rat NB2 ly mphoma cell line was used as a bioassay for serum prolactin. BE and SP levels were measured by radioimmunoassay in four hypothalamic areas: the preoptic region (POA), the mediobasal hypothalamus (MBH), the dors omedial hypothalamus (DMH), and the mamillary bodies (MB). 5-HT and 5- HIAA were measured in the midbrain raphe area by high-pressure liquid chromatography. E alone had little effect on serum prolactin levels, b ut E + P significantly increased prolactin as compared with ovariectom ized controls. The BE levels increased with E treatment and remained e levated with E + P treatment in MBH and POA. The BE content was stimul ated in DMH and MB by E + P treatment and not with E alone. The SP con tent in MBH, DMH, and MB increased in E-treated guinea pigs. However, SP then decreased upon addition of P to the E regimen. The 5-HT and 5- HIAA levels were similar in ovariectomized and E-treated groups. Addit ion of P to the E treatment significantly increased 5-HT and 5-HIAA le vels relative to the E only treatment. These studies show that P incre ases prolactin secretion in E-primed guinea pigs in a manner similar t o primates. 5-HT and 5-HIAA increased with P addition to an E regimen, but not with E alone. BE and SP both increase with E treatment, but B E remains elevated upon addition of P, whereas SP is suppressed by P. Thus, the steroid regulation of BE and 5-HT, but not SP, is consistent with a role in P-induced prolactin secretion in guinea pigs.