Δ9-Tetrahydrocannabinol Vapor Exposure Produces Conditioned Place Preference in Male and Female Rats

THC Vapor Inhalation Attenuates Hyperalgesia in Rats Using a Chronic Inflammatory Pain Model

Humans use cannabinoid drugs to alleviate pain. As cannabis and cannabinoids are legalized in the United States for medicinal and recreational use, it has become critical to determine the potential utilities and harms of cannabinoid drugs in individuals living with chronic pain. Here, we tested the effects of repeated ∆9-tetrahydrocannabinol (THC) vapor inhalation on thermal nociception and mechanical sensitivity, in adult male and female Wistar rats using a chronic inflammatory pain model (ie, treated with complete Freund's adjuvant [CFA]). We report that repeated THC vapor inhalation rescues thermal hyperalgesia in males and females treated with CFA and also reduces mechanical hypersensitivity in CFA males but not females. Many of the antihyperalgesic effects of chronic THC vapor were still observable 24 hours after cessation of the last THC exposure. We also report plasma levels of THC and its major metabolites, some of which are cannabinoid type-1 receptor agonists, after the first and tenth days of THC vapor inhalation. Finally, we report that systemic administration of the cannabinoid type-1 receptor inverse agonist AM251 (1 mg/kg, I.P.) blocks the antihyperalgesic effects of THC vapor in males and females. These data provide a foundation for future work that will explore the cells and circuits underlying the antihyperalgesic effects of THC vapor inhalation in individuals with chronic inflammatory pain. PERSPECTIVE: Cannabinoids are thought to have potential utility in the treatment of chronic pain, but few animal studies have tested the effects of chronic THC or cannabis in animal models of chronic pain. We tested the effects of repeated THC vapor inhalation on chronic pain-related outcomes in male and female animals.

Sex Differences in Response-Contingent Cannabis Vapor Administration During Adolescence Mediate Enduring Effects on Behavioral Flexibility and Prefrontal Microglia Activation in Rats

Introduction: Cannabis is the most used illicit drug in the United States. With many states passing legislation to permit its recreational use, there is concern that cannabis use among adolescents could increase dramatically in the coming years. Historically, it has been difficult to model real-world cannabis use to investigate the causal relationship between cannabis use in adolescence and behavioral and neurobiological effects in adulthood. Materials and Methods: We used a response-contingent vapor administration model to investigate long-term effects of cannabis use during adolescence on the medial prefrontal cortex (mPFC) and mPFC-dependent behaviors in male and female rats. Results: Adolescent (35- to 55-day-old) female rats had significantly higher rates of responding for vaporized Δ9-tetrahydrocannabinol (THC)-dominant cannabis extract (CANTHC) compared with adolescent males. In adulthood (70-110 days old), female, but not male, CANTHC rats also took more trials to reach criterion and made more regressive errors in an automated attentional set-shifting task compared with vehicle rats, thereby indicating sex differences in behavioral flexibility impairments. Notably, sex-treatment interactions were not observed when rats of each sex were exposed to a noncontingent CANTHC vapor dosing regimen that approximated CANTHC vapor deliveries earned by females. No differences were observed in effort-based decision making in either sex. In the mPFC, female (but not male) CANTHC rats displayed more reactive microglia with no changes in myelin basic protein expression or dendritic spine density. Conclusion: Altogether, these data reveal important sex differences in rates of responding for CANTHC vapor in adolescence that may confer enduring alterations to mPFC structure and function and suggest that there may be subtle differences in the effects of response-contingent versus noncontingent cannabis exposure that should be systematically examined in future studies.

Δ9-tetrahydrocannabinol discrimination: Effects of route of administration in mice

Background

Route of administration is an important pharmacokinetic variable in development of translationally relevant preclinical models. Humans primarily administer cannabis through smoking, vaping, and edibles. In contrast, preclinical research has historically utilized injected Δ9-tetrahydrocannabinol (THC). The present study sought to examine how route of administration affected the potency and time course of THC's discriminative stimulus properties.

Methods

Adult female and male C57BL/6 mice were trained to discriminate intraperitoneal (i.p.) THC from vehicle in a drug discrimination procedure. After discrimination was acquired, a dose-effect curve was determined for i.p., oral (p.o.), subcutaneous (s.c.), and aerosolized THC. Subsequently, the time course of effects of each route of administration was determined.

Results

THC administered i.p., p.o., s.c., or via aerosolization fully substituted for i.p. THC. The potency of THC's psychoactive effects was similar for i.p., p.o., and s.c., except that THC was more potent when administered s.c. vs p.o. in females. All routes of administration had a similar potency in both sexes. The duration of THC's psychoactive effects was similar across i.p., s.c., and p.o. routes of administration, whereas aerosolized THC produced a faster onset and shorter duration of effects compared to the other routes.

Conclusion

THC administered via multiple routes of administration, including those commonly used in preclinical research (i.p. and s.c.) and more translationally relevant routes (aerosol and p.o.), produced THC-like discriminative stimulus effects in mice trained to discriminate i.p. THC. More precise predictions of THC's effects in humans may result from use of these translationally relevant routes of administration.