Highway to hell or magic smoke? The dose-dependence of Δ9-THC in place conditioning paradigms

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. Delta(9)-tetrahydrocannabinol (Delta(9)-THC), the prime psychoactive compound of the cannabis sativa, as well as cannabimimetics that resemble the pharmacological properties and psychological effects of Delta(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 Delta(9)-THC (rewarding versus aversive) has been abundantly reported in place conditioning (PC) paradigms, and might be best attributed to a dose-dependence of Delta(9)-THC, most PC studies with Delta(9)-THC feature no significant effects at all. Therefore, after decades of rigorous research, it still remains undetermined whether Delta(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 Delta(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 Delta(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) Delta(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 Delta(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 Delta(9)-THC substantiate the dose-dependent biphasic effects of Delta(9)-THC in PC. A standardized experimental design would advance evidence-based practice in future PC studies with Delta(9)-THC and facilitate the pointed establishment of rewarding and aversive effects of the substance.