INTRODUCTIONDrugs of abuse are chemically divergent with distinct primary targets of action. Repeated intake of any drug of abuse, however, results in many common features of drug dependence. Activation of the mesocorticolimbic dopamine system has been implicated in the positive reinforcing (reward) effects of drugs of abuse, [1][2][3][4] although recent studies demonstrated a dopamine-independent rewarding effect of morphine. 5,6) The mesocorticolimbic dopamine reward system originates in the midbrain ventral tegmental area (VTA) and projects to the nucleus accumbens (NAc), prefrontal cortex and other limbic areas. The VTA-NAc dopamine system also plays a role in the discriminative stimulus effects of drugs. 7,8) Once drug dependence has developed, it can be a life-long condition under which individuals show compulsive drug taking, intense cravings, and vulnerability to relapse even after years of abstinence. This is a major clinical problem in treating patients suffering from drug dependence. Basic and clinical studies suggest that aberrant synaptic plasticity in the prefrontal glutamatergic system innervating the NAc is related to compulsive drug seeking. 9) Brain imaging studies in addicted subjects revealed that craving is associated with the activation of the prefrontal cortex.9,10) To study the molecular mechanism of relapse, an animal model for relapse of drugseeking behavior has been established in mice by using a reinstatement procedure involving the intravenous self-administration of drugs. 11,12) Regarding the long-lasting abnormalities in drug dependence, repeated exposure to amphetamine (AMPH) in rats produces a long-lasting increase in the length of dendrites, and the number of branched spines, 13) and interferes with the ability of experience in a complex environment to increase dendritic arborization and spine density in the NAc and prefrontal cortex.14) We have demonstrated that repeated methamphetamine (METH) treatment results in a long-lasting impairment of novelty-induced extracellular signal-regulated kinase 1/2 activation in the prefrontal cortex in mice.
15)These structural and functional abnormalities may contribute to long-term behavioral consequences of drug abuse including drug dependence, psychosis, and cognitive impairment. Changes in transcription factors may result in long-term changes in gene expression, thereby contributing to neuronal adaptations associated with drug dependence.16) For instance, a prolonged induction of DFosB, which can activate transcription at AP-1 sites, is a common response to many classes of addictive drugs. [17][18][19] Chronic drug exposure induces the expression of various neurotrophins, cytokines, and proteinases in the brain, which plays a crucial role in the development and relapse of drug dependence. [20][21][22] These endogenous modulators of drug dependence are functionally classified into two groups, pro-addictive and anti-addictive factors.20) The former, including basic fibroblast growth factor (bFGF), 23,24) brain-derived neurotrophic factor (...