The Dopamine and Cannabinoid Interaction in the Modulation of Emotions and Cognition: Assessing the Role of Cannabinoid CB1 Receptor in Neurons Expressing Dopamine D1 Receptors

Abstract

Although cannabinoid CB1 receptors (CB1Rs) are densely expressed in neurons expressing dopamine D1 receptors (D1Rs), it is not fully understood to what extent they modulate emotional behaviors. We used conditional CB1R knock-out animals lacking CB1Rs in neurons expressing D1R (D1-CB1(-/-)) in order to answer this question. To elucidate the behavioral effects of CB1R deficiency in this specific neuronal subpopulation, we subjected D1-CB1(-/-) mice to a battery of behavioral tests which included exploration-based tests, depressive-like behavioral tests, social behavior, and fear-related memory paradigms. D1-CB1(-/-) did not show any difference in the exploration-based paradigms such as open field, elevated plus maze, or novel object investigation test, except for an increase in novelty-induced grooming. By contrast, they showed a mild anhedonia-like state as described by the slightly decreased preference for sweet solution, as compared to wild-type control group. This decrease, however, could be observed only during the first day of exposure, thus suggesting increased neophobia as an alternative explanation. Accordingly, mutant mice performed normally in the forced swim test, a procedure widely used for evaluating behavioral despair in rodents. However, weak- to moderate anxiety-like phenotypes were evident when D1-CB1(-/-) mice were tested for social behavior. Most strikingly, D1-CB1(-/-) mice exhibited significantly increased contextual and auditory-cued fear, with attenuated within session extinction, suggesting that a specific reduction of endocannabinoid signaling in neurons expressing dopamine D1Rs is able to affect acute fear adaptation. These results provided first direct evidence for a cross-talk between dopaminergic D1Rs and endocannabinoid system in terms of controlling negative affect.

Keywords: CB1R; D1R; anxiety; aversive memories; cannabinoids; dopamine; fear extinction; social behavior.

Figure 1

Assessment of general locomotor activity of D1–CB1^−/− mice in the open field (OF) test. Conditional D1–CB1^−/− mutant mice (KO) and their respective wild-type (WT) littermates were tested in an open field for 30 min. Values are mean ± SEM in terms of locomotor activity (A–D) rearing (E) and jumping episodes (F).

Figure 2

Anxiety-like behaviors of D1–CB1^−/− mice. Conditional D1–CB1^−/− mutant mice (KO) and their respective wild-type (WT) littermates were tested for 5 min in the elevated plus maze (upper panel) or in the light/dark box (lower panel). Data are presented as mean ± SEM regarding open arm entries and open arm time (A) the total arm entries (B), light compartment entries and light compartment time (C), and total compartment transitions (D).

Figure 3

Novel object investigation test. Conditional D1–CB1^−/− mutant mice (KO) and their respective wild-type (WT) littermates were exposed to two novel objects for 10 min. Data are presented as mean ± SEM regarding total investigation duration (A) and number of approaches (B).

Figure 4

Grooming behavior in D1–CB1^−/− mice. Conditional D1–CB1^−/− mutant mice (KO) and their respective wild-type (WT) littermates were tested for the grooming activity measure. Data are presented as mean ± SEM regarding latency to start grooming (A), number of grooming episodes (B) and total grooming duration (C). *p < 0.05 as compared to WT mice (unpaired t-test).

Figure 5

Depressive-like behaviors of D1–CB1^−/− mice. Conditional D1–CB1^−/− mutant mice (KO) and their respective wild-type (WT) littermates were tested in the sucrose consumption test (A) or in the forced swim test (FST) paradigm (B). Data are presented as mean ± SEM regarding percentage of sucrose consumption or mobility time expressed in seconds. *p < 0.05 as compared to WT mice (unpaired t-test).

Figure 6

Social behaviors in D1–CB1^−/− mice. Conditional D1–CB1^−/− mutant mice (KO) and their respective wild-type (WT) littermates were tested in the social interaction (A,B) or in the social investigation (C,D) test. Data are presented as mean ± SEM regarding time in interaction and number of interactions. E1: empty tube 1; E2: empty tube 2; F1: tube with female. *p < 0.05, **p < 0.01, ***p < 0.01 (Newman–Keuls post hoc test or unpaired t-test).

Figure 7

Fear memory in D1–CB1^−/− mice. Auditory-cued (Tone) and contextual (Context) fear memory assessed by freezing responses (mean ± SEM) of conditional D1–CB1^−/− mutant mice (KO) and their respective wild-type (WT) littermates in two independent sets of experiments (A/B, C/D). If not stated otherwise, freezing was averaged over the entire 180 s observation periods (A,C) or analyzed in 20 s intervals (B,D). *p < 0.05, **p < 0.01, ***p < 0.001 as compared to WT mice (unpaired t-test).