Effects of Δ8-THC on dissociation of conditioned avoidance responding in tolerant and nontolerant rats

Factors influencing tolerance to the effects of delta9-THC on a conditioned avoidance response

Male, Fischer strain rats were resistant to the impairing effects of delta9--THC (15-60 mg/kg, IG) on performance of a conditioned pole-climb avoidance response (CAR) after daily subacute pretreatment for 4 or 6 days. A single administration of 20 mg/kg delta9--THC independent of the performance test did not attenuate the subsequent impairment caused by delta9--THC when tested 1-6 days later; however, administration 2 hr before each test attenuated the effect on subsequent tests given at intervals of 1-5 weeks. Similarly, subacute treatment with 20 mg/kg delta9--THC for 4 days independent of the performance test attenuated the impairment caused by delta9--THC during tests given to separate groups of rats 1 or 6, but not 14 days later. However, when the tests for tolerance were conducted repeatedly in the same rats, the attenuation appeared to persist for intervals up to 5 weeks. The results are discussed in terms of metabolic, functional and compensatory (behavioral) tolerance.

Tolerance to the effects of delta9-THC on shuttle-box performance and body temperature

Two groups of rats were trained in a shuttle-box and received delta9-tetrahydrocannabinol (delta9-THC), either before or after being tested. The drug-before group showed tolerance--within 3-6 sessions--to the response-inhibiting effect of THC. The drug-after animals appeared also to be tolerant when they received delta9-THC before being tested. It is concluded that the tolerance to this effect probably is not learned, but has a physiological base. This is corroborated by the finding that during the same study all the animals developed tolerance to the hypothermic effect of delta9-THC.

Role of experience in acquisition and loss of tolerance to the effect of delta-9-THC on spaced responding

Albino rats were given extensive training in spaced responding, using a DRL 30 sec schedule of food reinforcement (only lever presses more than 30 sec apart were reinforced). All rats then went 12 days without behavioral testing. During this period half the rats received daily intragastric doses of delta-9-tetrahydrocannabinol (THC) and the rest equal volumes of the THC vehicle. On day 13, some rats received THC 3 hr before behavioral testing while others received only vehicle. The former showed a sharp increase in lever press rate over baseline levels, but the vehicle control rats were unaffected. The rats with 12 prior THC doses were no less affected than those with no previous drug history. Continued testing resulted in recovery of baseline performance within 5 sessions, again with no effect of previous drug history. Similar results were obtained with doses of 4 mg/kg and 16 mg/kg, though the drug's effects were more pronounced at the higher dose. These results demonstrate that performance in the drug state can be a far more important determinant of tolerance than mere exposure to THC. Drug administration was then suspended for 1 week. Rats that had become tolerant to 4 mg/kg THC were then redivided into 3 new groups. One group received daily doses of vehicle and DRL sessions, a second received DRL sessions without vehicle, and 1 group received neither vehicle nor DRL sessions for this week. Subsequent DRL testing after THC administration showed that only the groups receiving DRL sessions in the intervening week lost their previously acquired tolerance. Experience thus appears to play an important role in loss of tolerance to THC as well as in acquisition of tolerance.