Chronic Adolescent Δ9-Tetrahydrocannabinol Treatment of Male Mice Leads to Long-Term Cognitive and Behavioral Dysfunction, Which Are Prevented by Concurrent Cannabidiol Treatment

Abstract

Introduction: The high prevalence of adolescent cannabis use, the association between this use and later psychiatric disease, and increased access to high-potency cannabis highlight the need for a better understanding of the long-term effects of adolescent cannabis use on cognitive and behavioral outcomes. Furthermore, increasing Δ⁹-tetrahydrocannabinol (THC) in high-potency cannabis is accompanied by a decrease in cannabidiol (CBD), thus an understanding of the interactions between CBD and THC in the neurodevelopmental effects of THC is also important. The current study examined the immediate and long-term behavioral consequences of THC, CBD, and their combination in a mouse model of adolescent cannabis use. Materials and Methods: Male CD1 mice received daily injections of THC (3 mg/kg), CBD (3 mg/kg), CBD+THC (3 mg/kg each), vehicle, or remained undisturbed in their home cage (no handling/injections), either during adolescence (postnatal day [PND] 28-48) or during early adulthood (PND 69-89). Animals were then evaluated with a battery of behavioral tests 1 day after drug treatment, and again after 42 drug-free days. The tests included the following: open field (day 1), novel object recognition (NOR; day 2), marble burying (day 3), elevated plus maze (EPM; day 4), and Nestlet shredding (day 5). Results: Chronic administration of THC during adolescence led to immediate and long-term impairments in object recognition/working memory, as measured by the NOR task. In contrast, adult administration of THC caused immediate, but not long term, impairment of object/working memory. Adolescent chronic exposure to THC increased repetitive and compulsive-like behaviors, as measured by the Nestlet shredding task. Chronic administration of THC, either during adolescence or during adulthood, led to a delayed increase in anxiety as measured by the EPM. All THC-induced behavioral abnormalities were prevented by the coadministration of CBD+THC, whereas CBD alone did not influence behavioral outcomes. Conclusion: These data suggest that chronic exposure to THC during adolescence leads to some of the behavioral abnormalities common in schizophrenia. Interestingly, CBD appeared to antagonize all THC-induced behavioral abnormalities. These findings support the hypothesis that adolescent THC use can impart long-term behavioral deficits; however, cotreatment with CBD prevents these deficits.

Keywords: cannabis; neurodevelopment; object recognition; schizophrenia; working memory.

FIG. 1.

Injection and behavioral testing schedule. (Top graph) Adolescent-treated animals: no treatment control (n=11), vehicle (n=9), THC (n=10), CBD (n=11), CBD+THC (n=11). Behavioral testing during (A) early adulthood and (B) after a 42-day drug-free delay. (Bottom graph) Adult-treated animals: no treatment control (n=9), vehicle (n=10), THC (n=12), CBD (n=8), CBD+THC (n=12). Behavioral testing during (C) early adulthood and (D) after a 42-day drug-free delay. Panels A–D in Figures 2–7 will use this same convention where panel A represents adolescently treated animals tested immediately after 3 weeks of treatment, etc. CBD, cannabidiol; THC, tetrahydrocannabinol

FIG. 2.

Adolescent THC impairs NOR performance. Object recognition was scored using a discrimination index (see Materials and Methods section). Adolescent treated and (A) immediately tested, (B) delay tested. Adult treated and (C) immediately tested, (D) delay tested. ***p<0.001 and *p<0.05. NOR, novel object recognition.

FIG. 3.

Similar exploration times in the NOR test among all treatment groups. Exploration time was the time the mouse spent engaged with either object (see Materials and Methods section for details). Adolescent treated and (A) immediately tested, (B) delay tested. Adult treated and (C) immediately tested, (D) delay tested. *p<0.05.

FIG. 4.

Adolescent THC increases Nestlet shredding. The amount of Nestlet shred was determined by measuring unshred Nestlet 75 min after placing the Nestlet in with the mouse. Adolescent treated and (A) immediately tested, (B) delay tested. Adult treated and (C) immediately tested, (D) delay tested. ***p<0.001, **p<0.01, *p<0.05.

FIG. 5.

Adolescent THC transiently enhances marble burying. Buried marbles were those that were at least three-fourth covered with bedding. Adolescent treated and (A) immediately tested, (B) delay tested. Adult treated and (C) immediately tested, (D) delay tested. ***p<0.001, **p<0.01, *p<0.05.

FIG. 6.

Adult THC consistently decreases EPM open arm entries and adolescent THC decreases open arm entries after a delay. A movement was considered an entry into the maze arm if head and shoulders entered the arm. Adolescent treated and (A) immediately tested, (B) delay tested. Adult treated and (C) immediately tested, (D) delay tested. ***p<0.001, **p<0.01, *p<0.05. EPM, elevated plus maze.

FIG. 7.

Adolescent and adult THC decrease EPM open arm duration after a THC-free delay. Mice were considered to be within an arm until their entire body had exited the arm. Adolescent treated and (A) immediately tested, (B) delay tested. Adult treated and (C) immediately tested, (D) delay tested. ***p<0.001, **p<0.01, *p<0.05.

FIG. 8.

Combining THC with CBD mildly increases THC plasma levels. THC levels were determined 24 h after 21 days of daily THC injections. Plasma was collected and THC levels were analyzed as described in Materials and Methods section. Concurrent CBD treatment increased plasma THC levels by ∼30%. * p<0.027.