Abstract
The prerequisites for responsible cannabis use are at the heart of current inquiries into cannabis decriminalization by policy makers as well as academic and nonacademic stakeholders at a global scale. Δ(9)-tetrahydrocannabinol (Δ(9)-THC), the prime psychoactive compound of the cannabis sativa, as well as cannabimimetics that resemble the pharmacological properties and psychological effects of Δ(9)-THC, lend themselves handsomely to the preclinical scrutiny of reward-related behavior because they carry marked translational value. Although a functional dichotomy of the psychological effects of Δ(9)-THC (rewarding versus aversive) has been abundantly reported in place conditioning (PC) paradigms, and might be best attributed to a dose-dependence of Δ(9)-THC, most PC studies with Δ(9)-THC feature no significant effects at all. Therefore, after decades of rigorous research, it still remains undetermined whether Δ(9)-THC generally exerts rewarding or aversive effects in rodents. Here, we set out to extrapolate the commonly alleged dose-dependence of the rewarding and aversive effects of Δ(9)-THC from the existing literature, at the behavioral pharmacological level of analysis. Specifically, our meta-analysis investigated: (i) the alleged bidirectional effects and dose-dependence of Δ(9)-THC in the PC test; (ii) methodological inconsistencies between PC studies; and (iii) other pharmacological studies on cannabinoids (i.e., dopamine release, anxiety, stress, conditioned taste aversion, catalepsy) to substantiate the validity of PC findings. Our findings suggest that: (i) Δ(9)-THC dose-dependently generates rewarding (1 mg/kg) and aversive (5 mg/kg) effects in PC; (ii) an inconsistent use of priming injections hampers a clear establishment of the rewarding effects of Δ(9)-THC in PC tests and might explain the seemingly contradictory plethora of nonsignificant THC studies in the PC test; and (iii) other pharmacological studies on Δ(9)-THC substantiate the dose-dependent biphasic effects of Δ(9)-THC in PC. A standardized experimental design would advance evidence-based practice in future PC studies with Δ(9)-THC and facilitate the pointed establishment of rewarding and aversive effects of the substance.