Nevertheless, opioid neuropeptides have been shown to play an important role during perinatal neurodevelopment (Zagon et al., 1994,Wang et al., 2003) and further studies are needed to fully explore the opioid system during adolescent development. In contrast to the strong developmental changes apparent during adolescence, THC effects were discrete on both the opioid and cannabinoid signaling systems. stages during an intermittent THC paradigm (1.5 mg/kg i.p. every third day) from postnatal days (PNDs) 2849. Rat brains were examined 24 hours after injection at PND 29 (early adolescence), PND 38 (mid adolescence) and PND 50 (late adolescence) and analyzed for endocannabinoids (anandamide and 2-arachidonoylglycerol), Met-enkephalin, cannabinoid CB1receptors and opioid receptors (OR) in the NAc, caudate-putamen and prefrontal cortex (PFC). Of the markers analyzed, the endocannabinoid levels experienced the most strong alterations throughout adolescence and were specific to the PFC and NAc. Normal correlations between anandamide and 2-arachidonoylglycerol concentrations in the NAc (positive) and PFC (unfavorable) were reversed by THC. Other significant THC-induced effects were confined to the NAc increased anandamide, decreased Met-enkephalin GR 144053 trihydrochloride and decreased ORs. These findings emphasize the dynamic nature of the mesocorticolimbic endocannabinoid system during adolescence and the selective mesocorticolimbic disturbance as a consequence of adolescent cannabis exposure. Keywords:anandamide, 2-arachidonoylglycerol, mu opioid receptor, enkephalin, ontogeny, nucleus accumbens == Introduction == It is now well accepted that adolescence is an important period of active neural development (Rice and Barone, 2000,Charmandari et al., 2003) that may be particularly vulnerable to external insults and internal (e.g., hormonal and emotional) stress. Of growing concern is the fact that initiation of drug experimentation normally begins during adolescence (SAMHSA, 2007). Several clinical GR 144053 trihydrochloride studies have linked repeated early cannabis exposure with the development of schizophrenia (Arseneault et al., 2002,van Os et al., 2002,Fergusson et al., 2003,Green et al., 2004,Veen et al., 2004) and an increased risk of other illicit drug use (Yamaguchi and Kandel, 1984,Fergusson and Horwood, 2000,Lynskey et al., 2003,Agrawal et al., 2004). While epidemiological studies have tried to evaluate underlying factors that may predispose individuals to use cannabis as well as other illicit drugs, e.g. genetic predisposition, peer-pressure, drug availability and risk-taking behavior (e.g.Hall and Lynskey, 2005), experimental animal studies have helped to provide insights into direct neurobiological alterations in the brain induced by cannabis. These alterations may contribute to the disturbance of incentive neural pathways that influence the progression to future drug abuse. We recently observed in an adolescent rat model (Ellgren et al., 2007) that animals exposed to delta-9-tetrahydrocannabinol (THC; the main psychoactive ingredient of cannabis) at an early age resulted in higher intravenous heroin self-administration in adulthood. Moreover, molecular studies revealed specific alterations in the enkephalin opioid system within brain areas implicated in reward-related behavior, e.g. increased expression of proenkephalin (PENK) mRNA in the nucleus accumbens (NAc) shell and increased opioid receptor (OR) activity in the ventral tegmental area. The time course of THC effects on the opioid system as well on the endogenous cannabinoids is however unknown. The aim of the present study was to explore potential neurochemical alterations during adolescent brain development in association with THC exposure that could account for the opioid reward-related disturbances observed previously in adult rats with adolescent THC use. To date, most developmental studies of the cannabinoid (Berrendero et al., 1999,Fernandez-Ruiz et al., 2000,Perez-Rosado et al., 2000,Ade and Lovinger, 2007) and opioid (Xia and Haddad, 1991,Brana et al., 1995,Georges et al., 1998) systems have focused on the embryonic and early postnatal stages. As such, another important aim of the current investigation was to assess the ontogeny of the cannabinoid and opioid systems in reward-related brain areas in the adolescent rat brain. The levels of cannabinoid receptor type 1 (CB1), mu opioid receptor (OR), endocannabinoids (anandamide and 2-arachidonoylglycerol; 2-AG) and Met-enkephalin were analyzed in subregions of the striatum NAc shell, NAc core, and caudate-putamen of THC or vehicle exposed rats at ages corresponding to early (postnatal day; PND; 29), mid (PND 38) and late (PND 50) adolescence. Given the importance of the prefrontal cortex (PFC) in cognitive function and its protracted development until late adolescence/early adulthood, this cortical area was also examined == Methods == == Animals == Male Long-Evan rats (21 days old) were obtained from M&B Taconic, New York, USA. They were group-housed in a temperature-controlled environment on a reversed 12-h light/dark cycle (lights off at 11 a.m.) with ad libitum access to food and water. The rats were allowed to acclimate in their new environment GR 144053 trihydrochloride and were handled daily for one week before the start of the IL17RC antibody experiment. All animal experiments were performed in accordance with the guidelines of The Swedish National Board for Laboratory Animals under a protocol approved by the Ethical Committee of Northern Stockholm, Sweden. == Drugs == THC (10 mg/ml in ethanol solution;.