OPIOID STIMULATION IN THE VENTRAL TEGMENTAL AREA FACILITATES THE ONSET OF MATERNAL-BEHAVIOR IN RATS

This research investigated the effect of an increase or decrease in op ioid activity in the ventral tegmental area (VTA) on the onset of mate rnal behavior in rats. In Experiment 1, the latency to show maternal b ehavior toward foster rat pups (sensitization latency) was determined in maternally naive female rats given either nothing or a unilateral i ntra-VTA injection of morphine sulfate (MS) (0.0, 0.01, 0.03, 0.1 or 0 .3 mu g), on the first three days of a 10-day period of constant expos ure to pups. Rats treated with 0.03 mu g MS had significantly shorter sensitization latencies than did rats treated with 0.0 mu g MS, 0.01 m u g MS, or receiving no treatment (higher doses of morphine produced i ntermediate results). The facilitating effect of intra-VTA MS on the o nset of maternal behavior was blocked by pretreatment with naltrexone hydrochloride and was found to have a specific site of action in the V TA (MS injections dorsal to the VTA were ineffective). In Experiment 2 , sensitization latencies were determined in periparturitional rats gi ven a bilateral intra-VTA injection of either the opioid antagonist na ltrexone methobromide (quaternary naltrexone), its vehicle, a sham inj ection, or left untreated 40 min after delivery of the last pup. The m others' own pups were removed at delivery; mothers were nonmaternal at the time of testing. Quaternary naltrexone treatment produced signifi cantly slower sensitization to foster pups than did control conditions . Total activity and pup-directed activity did not differ significantl y with treatment. The results demonstrate that increased opioid activi ty in the VTA facilitates the onset of maternal behavior in inexperien ced nonpregnant female rats, and decreased opioid activity in the VTA disrupts the rapid onset of maternal behavior at parturition.