Opioid stimulation in the ventral tegmental area facilitates the onset of maternal behavior in rats

Abstract

This research investigated the effect of an increase or decrease in opioid activity in the ventral tegmental area (VTA) on the onset of maternal behavior in rats. In Experiment 1, the latency to show maternal behavior toward foster rat pups (sensitization latency) was determined in maternally naive female rats given either nothing or a unilateral intra-VTA injection of morphine sulfate (MS) (0.0, 0.01, 0.03, 0.1 or 0.3 J.Lg), on the first three days of a lO-day period' of constant exposure to pups. Rats treated with 0.03 J.Lg MS had significantly shorter sensitization latencies than did rats treated with 0,0 J.Lg MS, 0.01 J.Lg MS, or receiving no treatment (higher doses of morphine produced intermediate results). The facilitating effect of intra-VTA MS on the onset of maternal behavior was blocked by pretreatment with naltrexone hydrochloride and was found to have a specific site of action in the VTA (MS injections dorsal to the VTA were ineffective). In Experiment 2, sensitization latencies were determined in periparturitional rats given a bilateral intra-VTA injection of either the opioid antagonist naltrexone methobromide (quaternary naltrexone), its vehicle, a sham injection, or left untreated 40 min after delivery of the last pup. The mothers' own pups were removed at delivery; mothers were nonmaternal at the time of testing. Quaternary naltrexone treatment produced significantly slower sensitization to foster pups than did control conditions. Total activity and pup-directed activity did not differ significantly with treatment. The results demonstrate that increased opioid activity in the VTA facilitates the onset of maternal behavior in inexperienced nonpregnant female rats, and decreased opioid activity in the VTA disrupts the rapid onset of maternal behavior at parturition.

Ingestion of amniotic fluid enhances the facilitative effect of VTA morphine on the onset of maternal behavior in virgin rats

“…2 and 3). Overall, these findings replicated those of Thompson and Kristal (1996): a dose of 0.03 μg morphine, injected into the VTA facilitates the onset of maternal behavior, but a dose of 0.01 μg of morphine does not. The individual infusion groups were then compared across morphine doses.…”

“…Discussion Thompson and Kristal (1996) showed that VTA microinjection of 0.03 μg and 0.1 μg, but not of 0.01 μg morphine facilitates the onset of maternal behavior. The present study confirmed that 0.03 μg morphine, but not 0.01 μg morphine, injected into the VTA, shortens the latency to onset of maternal behavior in naïve virgins.…”

“…We predicted that ingested amniotic fluid would enhance the facilitative effect of VTA morphine on the onset of maternal behavior in virgin female rats. Specifically, we hypothesized that (a) the ingestion of amniotic fluid alone (in the absence of opioids) would not facilitate the onset of maternal behavior, (b) the injection of morphine into the VTA would facilitate the onset of maternal behavior (a confirmation of the findings of Thompson and Kristal [1996]), and (c) the ingestion of amniotic fluid would hasten the onset of maternal behavior above and beyond the facilitation produced by increased opioid activity in the VTA, i.e., that virgin rats treated with VTA morphine and that had ingested amniotic fluid would become maternal in fewer days than would those receiving VTA morphine alone, and that those receiving VTA morphine alone would become maternal in fewer days than would those not receiving VTA morphine.…”

“…Endogenous opioids have been shown to affect the rapid onset of maternal behavior at parturition in both a positive and negative fashion (Mann and Bridges, 1992;Mann et al, 1991;Rubin and Bridges, 1984;Thompson and Kristal, 1996). Central opioids can affect the onset and maintenance of maternal behavior.…”

“…Increased opioids in the medial preoptic area (MPOA) inhibit maternal responsiveness; the MPOA is an area widely accepted to be crucial for the mediation of maternal behavior (Mann and Bridges, 1992;Mann et al, 1991;Numan 2006;Rubin and Bridges, 1984). In contrast, morphine injected into the ventral tegmental area (VTA), which modulates motivation and reward, facilitates the onset of maternal behavior in virgin female rats and injection of the opioid antagonist naltrexone into the VTA disrupts the onset of maternal behavior in parturient female rats (Thompson and Kristal, 1996). Placentophagia (ingestion of afterbirth material) occurs in most nonhuman and non-aquatic mammalian species at parturition (Kristal, 1980(Kristal, , 1991(Kristal, , 1998.…”