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The long-term impact of prenatal methylmercury (MeHg) exposure on the stress response during active learning was investigated. Pregnant rats were gavage fed MeHg (8 mg/kg) on gestational day 15. Ninety-day-old rats born to both MeHg- and saline-treated dams were subjected to an active avoidance test. The active avoidance-experienced rats (AAERs) with prenatal exposure to MeHg showed significant impairment in learning ability and exhibited higher levels of corticosterone than the untreated AAERs. The present findings suggest that the abnormal increase in plasma corticosterone levels could contribute to the poor performance of MeHg-treated AAERs in this learning task.

We examined the effects of chronic morphine treatment and withdrawal on the expression of metabotropic glutamate (mGlu)1, mGlu5, and mGlu2/3 receptors in the nucleus accumbens and caudate putamen. Rats received a 14-day morphine treatment (escalating doses from 10 to 140 mg/kg). Receptor density was evaluated after chronic treatment and after 1, 3, and 14 days of withdrawal. mGlu1 and mGlu5 expression in the nucleus accumbens and in the caudate putamen was not affected by any of the experimental manipulations. In contrast, mGlu2/3 receptors in the nucleus accumbens, but not in the caudate putamen, increased at day 1, 3, and 14 of withdrawal. We suggest that an increased expression of mGlu2/3 receptors in the nucleus accumbens might contribute to the symptoms of morphine withdrawal.

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