Drug addiction is a chronic disorder resulted from complex interaction of genetic, environmental, and developmental factors. Epigenetic mechanisms play an important role in the development and maintenance of addiction and also memory formation in the brain. We have examined passive avoidance memory and morphine conditioned place preference (CPP) in the offspring of male and/or female rats with a history of adulthood morphine consumption. Adult male and female animals received chronic oral morphine for 21days and then were maintained drug free for 10days. After that, they were let to mate with either an abstinent or control rat. Male offspring's memory was evaluated by step through test. Besides, rewarding effects of morphine were checked with CCP paradigm. Offspring of abstinent animals showed significant memory impairment compared to the control group which was more prominent in the offspring of abstinent females. Conditioning results showed that administration of a high dose of morphine (10mg/kg) that could significantly induce CPP in control rats, was not able to induce similar results in the offspring of morphine abstinent parents; and CPP was much more prominent when it was induced in the offspring of morphine exposed females compared to the progeny of morphine exposed males. It is concluded that parental morphine consumption in adulthood even before mating has destructive effects on memory state of the male offspring and also leads to tolerance to the rewarding effects of morphine. These effects are greater when the morphine consumer parent is the female one.

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Addiction to drugs, including opioids is the result of an interplay between environmental and genetic factors. It has been shown that the progeny of addict people is at higher risk for drug addiction. However, the mechanisms of such trans-generational effects of drugs are not so clear. Here we have evaluated the effects of parental morphine consumption on anxiety, morphine preference, and mRNA expression of dopamine receptors in F1 and F2 male offspring. Morphine was chronically administered to adult male and female Wistar rats followed by 14-day abstinence before mating. Morphine preference and anxiety-like behavior in the offspring were measured by two-bottle-choice paradigm and elevated-plus maze, respectively. Real-time PCR was used to measure the mRNA expression level of dopamine receptors in the striatum, nucleus accumbens, prefrontal cortex, and hippocampus of F1 animals. The results indicated that F1 but not the F2 male progeny of morphine-exposed parents had a greater preference for morphine, and more anxiety-like behavior compared to the offspring of saline-treated parents. In F1 male progeny of morphine-treated parents, D1 and D5 dopamine receptors were significantly increased in the prefrontal cortex and nucleus accumbens. D5 and D2 receptors were decreased in the hippocampus. D4 dopamine receptor was augmented in striatum and hippocampus and decreased in the prefrontal cortex. Adulthood exposure to chronic morphine in male and female rats before conception leads to higher morphine preference and increased anxiety in F1 but not F2 male progeny. Alterations of dopamine receptor expression in the reward system may be one mechanism responsible for observed changes in F1 offspring.

Opioid addiction is one of the most crucial issues in the world. Opioid abuse by parents makes children more prone to many psychological disorders such as drug addiction. Therefore, this study was carried out to examine the effect of morphine exposure 10 days before gestation on morphine and methamphetamine preference in male offspring. Adult Wistar rats (male and female) received morphine orally for 21 days and were drug free for 10 days. Thereafter, they were allowed to mate with either a morphine-abstinent or drug-naive rat. The male offspring were tested for morphine and methamphetamine preference with a three-bottle choice test. Moreover, the rewarding effects of morphine and methamphetamine were evaluated using a conditioned place preference test. To determine the mechanisms underlying these changes, monoamine oxidase-B (MAO-B) level was measured in the nucleus accumbens (NAC). Offspring of morphine-abstinent mothers and offspring of both-abstinent parents were found to consume morphine more than those of other groups, but in the case of methamphetamine, there were no differences. In addition, the offspring of morphine-abstinent parent(s) did not condition with a high dose of morphine in the conditioned place preference test. Administration of methamphetamine induced conditioning at different doses in controls and offspring of one or two morphine-abstinent parent(s), and there were no effects of parental morphine exposure on the dose of methamphetamine that was required for conditioning. Moreover, the level of MAO-B was increased in the NAC of offspring of morphine-abstinent parents as compared with the control group. These results demonstrate that offspring of a morphine-abstinent mother and a drug-naive father and offspring of two morphine-abstinent parents were more susceptible to opioid but not methamphetamine addiction. Moreover, parental morphine consumption did not have any effect on the reinforcing effect of methamphetamine in their offspring but induced morphine tolerance in the offspring. Although the level of MAO-B was elevated in the NAC, this did not correlate with the methamphetamine preference in offspring.