Adolescent Δ9-Tetrahydrocannabinol Exposure Selectively Impairs Working Memory but Not Several Other mPFC-Mediated Behaviors

Abstract

As the frequency of cannabis use by 14-16-year-olds increases, it becomes increasingly important to understand the effect of cannabis on the developing central nervous system. Using mice as a model system, we treated adolescent (28 day old) C57BL6/J mice of both sexes for 3 weeks with 3 mg/kg tetrahydrocannabinol (THC). Starting a week after the last treatment, several cognitive behaviors were analyzed. Mice treated with THC as adolescents acquired proficiency in a working memory task more slowly than vehicle-treated mice. Working memory recall in both sexes of THC-treated mice was also deficient during increasing cognitive load compared to vehicle-treated mice. Our adolescent THC treatment did not strongly affect social preference, anxiety behaviors, or decision-making behaviors on the elevated T maze task. In summary, under the conditions of this study, adolescent THC treatment of mice markedly affected the establishment, and persistence of working memory, while having little effect on decision-making, social preference or anxiety behaviors. This study provides further support that adolescent THC affects specific behavioral domains.

Keywords: adolescent; cognitive load; medial prefrontal cortex (mPFC); tetrahydrocannabinol; working memory.

Figure 1

Experimental paradigm. Timeline of experimental design. Adolescent animals received daily intraperitoneal injections of THC (3 mg/kg) 21-days from PND28-49. This was followed by food restriction for 1-week (PND50-56) before behavioral experiments commenced.

Figure 2

Decision making. Ratio of exploration times spent in different sections of the T-maze on the final day of habituation with both the high- and low-reward arms closed (low risk condition) for male (A) and female (B) mice. On the next day, mice were placed on the maze with the high-reward arm open, and the ratio of exploration times for male (C) and female (D) mice for each section were calculated. All data were analyzed by two-way ANOVA with Sidak's test and data are presented as (mean ± SEM). *P < 0.05; **P < 0.01; ****P < 0.001 compared between low and high reward arm. ^#P < 0.05 compared to corresponding vehicle group, n.s., P > 0.05 (N = 4–5 per group).

Figure 3

Working memory. Working memory is impaired in adolescent mice chronically treated with THC. Success rate (out of 10 trials) during delayed-alternating T-maze training for 8 days of males (A) and females (B). Following 8 days of training, increasingly delayed internal times were tested in 5 trials for male (C) and female (D) mice. All data were analyzed by two-way ANOVA with Sidak's test and data are presented as (mean ± SEM). *P < 0.05; **P < 0.01 compared between naïve and THC groups. ***P < 0.001; ****P < 0.0001 comparison over time or between vehicle and THC groups. ^#P < 0.05; ^##P < 0.01; ^###P < 0.001; ^####P < 0.0001 compared between vehicle and THC groups (N = 4–14 per group).

Figure 4

Social ability. Δ⁹-THC treatment did not affect social interactions. Even though male or female naïve, vehicle, and THC treated mice showed different bias in in the chamber paired with an empty wire cup or familiar mouse (A) or paired with a familiar mouse or a stranger mouse (B). Similarly, regardless of sex, three groups of treatment caused no difference in the time intending to the chamber (C) or the partner vs. stranger cup (D). Data in (A,B) were analyzed by two-way ANOVA with Sidak's test, and data in (C,D) by repeated-measures two-way ANOVA with Tukey's test. Data are presented as (mean ± SEM). *P < 0.05; ***P < 0.001 compared between partner and empty chamber or P compared between familiar and stranger chamber (N = 4–14 per group).

Figure 5

Elevated plus maze. Chronic adolescent Δ⁹-THC treatment doesn't affect mouse anxiety behaviors as measured on the elevated plus maze. Ratio of exploration time (A,B) and entries into (C,D) the open and closed arms, respectively, of the elevated plus maze as compared to vehicle treated animals. In contrast, naïve mice of both sexes showed less activity as measured by distance traveled, and male naïve mice showed fewer entries into the open arm, as compared to vehicle or Δ⁹-THC-treated mice (E). All data were analyzed by one-way ANOVA with Tukey's test, and data are presented as (mean ± SEM) *p < 0.05; ***p < 0.001 compared to naïve groups (N = 4–14 per group).