Mother rats bar-press for pups: effects of lesions of the mpoa and limbic sites on maternal behavior and operant responding for pup-reinforcement.

This series of studies explored the operant response rates for pup-reinforcement of female Sprague Dawley rats that were either postpartum or cycling and sustained lesions of the medial preoptic area (mpoa), the lateral amygdala, the nucleus accumbens, or sham lesions. The last experiment tested the effects on operant responding of preventing direct access to pups in mpoa and sham-lesioned postpartum mothers. All animals were trained prior to mating on an FR-1 bar-press schedule to criterion (50 presses in 30 min) for a food (Froot Loops) reward in an operant chamber. At the end of pregnancy animals that were to be tested postpartum were provided in their home cages with six newborn foster pups; mother-litter interactions were observed on the last 3 days of pregnancy and throughout the postpartum period. On each of these same days after a period of separation from pups, females were tested in the operant box for delivery of rat pups. With each bar-press response, a rat pup rather than a Fruit Loop was delivered down a gentle shoot into the hopper. Non-postpartum, but maternal, multiparous animals who were showing estrous cycles were tested using the same procedures. The first and second studies showed that animals (both postpartum and as cycling multiparous animals) with mpoa lesions exhibited a significant reduction in bar-press rate for pup reinforcement in the operant box. In postpartum animals, amygdala lesions also produced a bar-press deficit, whereas nucleus accumbens lesions did not. All lesioned groups showed deficits in maternal responding in the home cage and deficits in retrieval in the operant box. These results indicate that systems associated with the mpoa mediate both the stereotypical maternal behaviors and pup-reinforcement. In contrast, the expression of home cage maternal behavior is dependent on the integrity of both the amygdala and nucleus accumbens, whereas operant responding need not be. These results indicate a dissociation of mechanisms mediating expression of the species-typical maternal behavior and pup-reinforcement.