Δ 9 -tetrahydrocannabinol (THC) is the intoxicating constituent of cannabis and is responsible for the drug's reinforcing effects. Retrospective human studies suggest that cannabis use during adolescence is linked to long-term negative psychological outcomes, but in such studies it is difficult to distinguish the effects of THC from those of coexisting factors. Therefore, translationally relevant animal models are required to properly investigate THC effects in adolescents. However, though the relevance of these studies depends upon human-relevant dosing, surprisingly little is known about THC pharmacology and its effects on behavior and brain activity in adolescent rodents-especially in females. Here, we conducted a systematic investigation of THC pharmacokinetics, metabolism and distribution in blood and brain, and of THC effects upon behavior and neural activity in adolescent Long Evans rats of both sexes. We administered THC during an early-middle adolescent window (postnatal days 27-45) in which the brain may be particularly sensitive to developmental perturbation by THC. We determined the pharmacokinetic profile of THC and its main first-pass metabolites (11-hydroxy-THC and 11-nor-9-carboxy-THC) in blood and brain following acute injection (0.5 or 5 mg/kg, intraperitoneal). We also evaluated THC effects on behavioral assays of anxiety, locomotion, and place conditioning, as well as c-Fos expression in 14 brain regions. Confirming previous work, we find marked sex differences in THC metabolism, including a female-specific elevation in the bioactive metabolite 11-hydroxy-THC. Furthermore, we find dose-dependent and sexdependent effects on behavior, neural activity, and functional connectivity across multiple nodes of brain stress and reward networks. Our findings are relevant for interpreting results of rat adolescent THC exposure studies, and may lend new insights into how THC impacts the brain in a sex-dependent manner.

We investigated the pharmacokinetic properties of D 9 -tetrahydrocannabinol (D 9 -THC), the main psychoactive constituent of cannabis, in adolescent and adult male mice. The drug was administered at logarithmically ascending doses (0.5, 1.6, and 5 mg/kg, i.p.) to pubertal adolescent (37-day-old) and adult (70day-old) mice. D 9 -THC and its first-pass metabolites-11hydroxy-D 9 -THC and 11-nor-9-carboxy-D 9 -THC (11-COOH-THC)-were quantified in plasma, brain, and white adipose tissue (WAT) using a validated isotope-dilution liquid chromatography/tandem mass spectrometry assay. D 9 -THC (5 mg/kg) reached 50% higher circulating concentration in adolescent mice than in adult mice. A similar age-dependent difference was observed in WAT. Conversely, 40%-60% lower brain concentrations and brain-to-plasma ratios for D 9 -THC and 50%-70% higher brain concentrations for D 9 -THC metabolites were measured in adolescent animals relative to adult animals. Liver microsomes from adolescent mice converted D 9 -THC into 11-COOH-THC twice as fast as adult microsomes. Moreover, the brains of adolescent mice contained higher mRNA levels of the multidrug transporter breast cancer resistance protein, which may extrude D 9 -THC from the brain, and higher mRNA levels of claudin-5, a protein that contributes to blood-brain barrier integrity. Finally, administration of D 9 -THC (5 mg/kg) reduced spontaneous locomotor activity in adult, but not adolescent, animals. The results reveal the existence of multiple differences in the distribution and metabolism of D 9 -THC between adolescent and adult male mice, which might influence the pharmacological response to the drug. SIGNIFICANCE STATEMENTAnimal studies suggest that adolescent exposure to D 9 -tetrahydrocannabinol (D 9 -THC), the intoxicating constituent of cannabis, causes persistent changes in brain function. These studies generally overlook the impact that age-dependent changes in the distribution and metabolism of the drug might exert on its pharmacological effects. This report provides a comparative analysis of the pharmacokinetic properties of D 9 -THC in adolescent and adult male mice and outlines multiple functionally significant dissimilarities in the distribution and metabolism of D 9 -THC between these two age groups.