Effects of phenobarbital and SKF 525A pretreatment, sex, liver injury, and vehicle on delta9-tetrahydrocannabinol toxicity

Preclinical studies on the reinforcing effects of cannabinoids. A tribute to the scientific research of Dr. Steve Goldberg

RATIONALE

The reinforcing effects of most abused drugs have been consistently demonstrated and studied in animal models, although those of marijuana were not, until the demonstration 15 years ago that delta-9-tetrahydrocannabinol (THC) could serve as a reinforcer in self-administration (SA) procedures in squirrel monkeys. Until then, those effects were inferred using indirect assessments.

OBJECTIVES

The aim of this manuscript is to review the primary preclinical procedures used to indirectly and directly infer reinforcing effects of cannabinoid drugs.

METHODS

Results will be reviewed from studies of cannabinoid discrimination, intracranial self-stimulation (ICSS), conditioned place preference (CPP), as well as change in levels of dopamine assessed in brain areas related to reinforcement, and finally from self-administration procedures. For each procedure, an evaluation will be made of the predictive validity in detecting the potential abuse liability of cannabinoids based on seminal papers, with the addition of selected reports from more recent years especially those from Dr. Goldberg's research group.

RESULTS AND CONCLUSIONS

ICSS and CPP do not provide consistent results for the assessment of potential for abuse of cannabinoids. However, drug discrimination and neurochemistry procedures appear to detect potential for abuse of cannabinoids, as well as several novel "designer cannabinoid drugs." Though after 15 years transfer of the self-administration model of marijuana abuse from squirrel monkeys to other species remains somewhat problematic, studies with the former species have substantially advanced the field, and several reports have been published with consistent self-administration of cannabinoid agonists in rodents.

Effects of delta 9-tetrahydrocannabinol, phenobarbital, and their combination on pregnancy and offspring in rats

delta 9-THC, sodium phenobarbital, or a combination of these drugs, were administered to pregnant mice on gestation days 6 to parturition. Levels of delta 9-THC in blood approximated levels attained in humans after smoking one to two marijuana cigarettes. Blood cannabinoid levels in mice were not significantly elevated by concurrent drug administration. Both drugs significantly reduced litter size and weight per pup at birth, but the combination of these drugs did not affect these outcomes to a significantly greater degree than either drug by itself. Phenobarbital but not delta 9-THC increased resorption rate.

Influence of fasting on the absorption and effects of delta9-tetrahydrocannabinol after oral administration in sesame oil

Tissue levels of 3H were higher 2 hr after oral administration of 3H-delta9-THC (10 mg/kg in sesame oil) to male Fischer rats in the morning compared with treatment in the afternoon. A corresponding reduction in potency was seen for the impairing effect of delta9-THC on performance of a conditioned avoidance response (CAR). The hypothesis that these effects were related to the interval between feeding (which normally occurs during the night in the nocturnal rat) and drug administration was supported when they were mimicked in overnight fasted and ad lib fed rats. Food deprivation decreased the rate of gastrointestinal absorption of 14C-delta9-THC in sesame oil. Peak plasma levels of 14C occurred 2-4 hr after administration in fed rats compared with 8 hr in fasted rats. When tested 2 hr after oral administration, delta9-THC caused significantly greater impairment of CAR performance in fed than fasted rats, whereas the opposite was found after 8 hr. Extraction and subsequent thin layer chromatography of plasma and brain from fed and fasted rats sacrificed 2 or 8 hr after oral administration of 10 mg/kg 14C-delta9-THC showed that brain levels of 11-hydroxy-delta9-THC rather than delta9-THC were correlated with the behavioral effect.

Acute sedative properties of SKF 525A in rats: implications for its use as a metabolism inhibitor in the study of psychoactive drugs

SKF 525A was found possess sedative properties in rats at doses of 25 and 50 mg per kg, when injected intraperitoneally. The behavioural effects of the drug were assessed in two ways. Firstly, by "Time Sampling Behavioural Categorisation" of explatory behaviour; and secondly by activity measurements obtained with an ultrasonic motion recorder. The results clearly demonstrate that SKF 525A has sedative properties in rats at doses which are conventionally used to inhibit metabolism of a wide range of drugs. The implications of these results for the use of SKF 5251 in the study of the actions of psychotropic drugs are discussed.