HUMAN PLACENTAL-LACTOGEN INFUSIONS INTO THE MEDIAL PREOPTIC AREA STIMULATE MATERNAL-BEHAVIOR IN STEROID-PRIMED, NULLIPAROUS FEMALE RATS

The effects of central administration of human placental lactogen (hPL ) on the onset of maternal behavior were measured in steroid-primed, a dult ovariectomized, nulliparous rats. Rats were fitted with bilateral cannulas directed at the medial preoptic area (MPOA) and gonadectomiz ed 1 week before being implanted sc with progesterone (P)-filled Silas tic implants (treatment Day 1). On Day 11 P capsules were removed, and each female was implanted sc with a single estradiol (E(2)) capsule. On Days 11 to 13 animals were infused bilaterally with 40 ng of hPL/in fusion or given 0.4 mu l vehicle. Subjects were given hormone or vehic le infusions five times during this period, twice each on Days 11 and 12 (1000 and 1600 hr) and once on Day 13 (1000 hr). Behavioral testing began on day 12 after the 1000 hr infusions and continued daily for 6 days. All females were injected sc twice dairy with bromocriptine (2 mg/kg) to suppress endogenous PRL, secretion from Day 11 to the comple tion of testing. The results showed that central infusions of hPL stim ulated a fast onset of maternal behavior relative to controls. Latenci es to display specific components as well as complete maternal behavio r toward foster young were about 1 day in the hPL-treated group and 4 days in vehicle-infused controls. Infusions of hPL into the MPOA betwe en 7.8 and 8.0 AP resulted in the fastest rates of onset of maternal b ehavior. These findings demonstrate that central infusion of a heterol ogous placental lactogen, hPL, is capable of stimulating maternal resp onsiveness in female rats and suggest to us that the central stimulati on of maternal behavior may normally be brought about by exposure to p lacental lactogens as well as prolactin. (C) 1995 Academic Press, Inc.