Human Placental Lactogen Infusions into the Medial Preoptic Area Stimulate Maternal Behavior in Steroid-Primed, Nulliparous Female Rats

Abstract

The effects of central administration of human placental lactogen (hPL) on the onset of maternal behavior were measured in steroid-primed, adult ovariectomized, nulliparous rats. Rats were fitted with bilateral cannulas directed at the medial preoptic area (MPOA) and gonadectomized 1 week before being implanted sc with progesterone (P)-filled Silastic implants (treatment Day 1). On Day 11 P capsules were removed, and each female was implanted sc with a single estradiol (E2) capsule. On Days 11 to 13 animals were infused bilaterally with 40 ng of hPL/infusion or given 0.4 μl vehicle. Subjects were given hormone or vehicle 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 daily with bromocriptine (2 mg/kg) to suppress endogenous PRL secretion from Day 11 to the completion of testing. The results showed that central infusions of hPL stimulated a fast onset of maternal behavior relative to controls. Latencies to display specific components as well as complete maternal behavior 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 between 7.8 and 8.0 AP resulted in the fastest rates of onset of maternal behavior. These findings demonstrate that central infusion of a heterologous placental lactogen, hPL, is capable of stimulating maternal responsiveness in female rats and suggest to us that the central stimulation of maternal behavior may normally be brought about by exposure to placental lactogens as well as prolactin.