Effects of long-term administration and withdrawal of tetrahydrocannabinols (delta 8-THC and delta 9-THC) on open-field behavior in rats

The behavior of female Wistar rats, injected daily with Δ⁸-THC (5.0 mg/kg), Δ⁹-THC (2.5 mg/kg) or vehicle was studied in an open-field test both during the two weeks' period of drug administration and after withdrawal. The behavior of rats allowed long-term acclimation to the laboratory (LTA), studied in Experiment 1, was compared with those of Experiment 2, which were subjected to short-term acclimation (STA). Throughout the injection period a depressant effect of both isomers of THC was seen on ambulation, rearing, grooming and latency. Length of acclimation period interacted with the effects of THC:s on ambulation, producing even an increased ambulation for Δ⁸-THC in the STA group. Minor indications of tolerance to Δ⁹-THC were observed in the LTA condition only. A kind of circling behavior was characteristic of both drug groups, its frequency declining with repeated injections. At drug withdrawal, most open-field measures slowly returned to control levels, whereas rate of grooming showed a manifold increase. Possible implications of the results for behavioral research with the THC:s are discussed.