Adolescent Cannabis exposure differentially affects heroin reinforcement and accumbens dopamine transmission in Lewis and Fischer 344 rats

Background Vulnerability to drug addiction depends on acquired as well as genetic factors (Swendsen & Le Moal 2010). Among acquired factors is previous exposure to other drugs of abuse. Thus, exposure to Cannabis has been suggested to predispose to heroin abuse and dependence (Gateway Hypothesis) (Kandel et al. 2006). Previous studies have failed to provide evidence for increase in heroin reinforcement after cannabinoid pre-exposure (Solinas et al, 2004). However, Cannabis use often begins in adolescence, a critical phase of brain development. Indeed, Ellgren et al (2007) did report an increased taking of iv heroin after adolescent THC exposure and suggested a reduction of heroin reward. Genetic risk factors also play a major role in drug addiction. Given these premises, we thought to mimick more closely the human condition by introducing genetic background as a variable. Therefore we studied the influence of adolescent cannabinoid exposure on heroin reward and reinforcement and on in vivo dopamine stimulation by heroin in two inbred rat strains differentially vulnerable to drugs of abuse, the addiction prone Lewis and the addiction resistant Fisher344 strain. Methods Male Fischer344 and Lewis inbred rats aged 6 weeks (38-42 postnatal day, mid adolescence, Charles River ,Calco, Italy) were utilized. One group received for 3 days, twice daily, increasing doses of THC (2, 4 and 8 mg/kg i.p.) while the other received vehicle (3 ml/kg i.p.). All experiments were performed at least 30 days after last THC exposure. Microdialysis of DA was performed according to Cadoni et al (2009) in rats implanted with probes in the n.accumbens (NAc) shell and core. CPP was performed in a two compartment apparatus according to Fenu et al ( 2006). On day 1 (pre-conditioning), unconditioned preference was recorded for 15 min.. On days 2 to 7 (conditioning) rats received heroin (0.5 mg/kg s.c.) or saline on alternate days and were confined to a given compartment for 30 min. Testing of place preference was performed for 15 min. On the extinction phase, rats were re-exposed to the apparatus for 6 times. For reinstatement, heroin (0.5 mg/kg) was given in the home cage and after 40 min rats were tested for place preference. After 21 days rats were retested for preference three times. Behavior was scored as time (sec) spent performing each behavioral item. The items recorded for extinction were: jumping, sniffing, gnawing, locomotion and rearing; for reinstatement: stupor, licking, sniffing, gnawing and locomotion. Fixed ratio (FR) heroin iv SA was performed according to Lecca et al in 1h daily sessions (except for weekends). Following acquisition on FR1, SA was switched to FR3 and then to FR5. After 29 FR sessions, progressive ratio (PR 3-4) was introduced for 13 sessions of 4 hours each, followed by 8 extinction sessions. Finally, reinstatement was performed by presentation of drug-cues and, next day, by a priming injection of heroin (0.5 mg/kg sc). Cumulative active and inactive nose-pokes per session, heroin intake and breaking points per session were recorded and analyzed by ANOVA and post-hoc test. Results THC (1.0 mg/kg i.p.) stimulated shell DA in Lewis but not in Fischer 344 rats. Adolescent THC exposure potentiated DA stimulant effects of heroin (0.5 mg/kg sc) in the shell and core of Lewis and only in the core of Fischer344 rats. Control Lewis rats developed stronger CPP to heroin and resistance to extinction compared with Fischer344 strain. In Lewis rats, THC exposure did not increase heroin CPP but potentiated the effect of heroin priming. In Fischer344 rats, THC exposure increased heroin CPP and made it resistant to extinction. Lewis rats showed marked seeking behavior during extinction and hedonic reactions on heroin priming. Fisher344 rats