Exploring the role of locomotor sensitization in the circadian food entrainment pathway

ABHD6 Controls Amphetamine-Stimulated Hyperlocomotion: Involvement of CB1 Receptors

Introduction: Activation of cannabinoid 1 receptors (CB₁Rs) by endocannabinoids (eCBs) is controlled by both eCB production and eCB inactivation. Accordingly, inhibition of eCB hydrolyzing enzymes, monoacylglycerol lipase (MAGL) and α/β-hydrolase domain containing 6 (ABHD6), enhances eCB accumulation and CB₁R activation. It is known that inhibition of MAGL regulates select CB₁R-dependent behaviors in mice, including locomotor behaviors and their modulation by psychostimulants, but much less is known about the effect of inhibiting ABHD6 activity on such behaviors. Methods: We report a new mouse line that carries a genetic deletion of Abhd6 and evaluated its effect on spontaneous locomotion measured in a home cage monitoring system, motor coordination measured on a Rotarod, and amphetamine-stimulated hyperlocomotion and amphetamine sensitization (AS) measured in an open-field chamber. Results: ABHD6 knockout (KO) mice reached adulthood without exhibiting overt behavioral impairment, and we measured only mild reduction in spontaneous locomotion and motor coordination in adult ABHD6 KO mice compared to wild-type (WT) mice. Significantly, amphetamine-stimulated hyperlocomotion was enhanced by twofold in ABHD6 KO mice compared to WT mice and yet ABHD6 KO mice expressed AS to the same extent as WT mice. A twofold increase in amphetamine-stimulated hyperlocomotion was also measured in ABHD6 heterozygote mice and in WT mice treated with the ABHD6 inhibitor KT-182. It is known that amphetamine-stimulated hyperlocomotion is not affected by the CB₁R antagonist, SR141617, and we discovered that the enhanced amphetamine-stimulated hyperlocomotion resulting from ABHD6 inhibition is blocked by SR141617. Conclusions: Our study suggests that ABHD6 controls amphetamine-stimulated hyperlocomotion by a mechanistic switch to a CB₁R-dependent mechanism.

Control of exploration, motor coordination and amphetamine sensitization by cannabinoid CB1 receptors expressed in medium spiny neurons

Activation of cannabinoid 1 receptors (CB₁ R) modulates multiple behaviours, including exploration, motor coordination and response to psychostimulants. It is known that CB₁ R expressed by either excitatory or inhibitory neurons mediates different behavioural responses to CB₁ R activation, yet the involvement of CB₁ R expressed by medium spiny neurons (MSNs), the neuronal subpopulation that expresses the highest level of CB₁ R in the CNS, remains unknown. We report a new genetically modified mouse line that expresses functional CB₁ R in MSN on a CB₁ R knockout (KO) background (CB₁ R_(MSN) mice). The absence of cannabimimetic responses measured in CB₁ R KO mice was not rescued in CB₁ R_(MSN) mice, nor was decreased spontaneous locomotion, impaired instrumental behaviour or reduced amphetamine-triggered hyperlocomotion measured in CB₁ R KO mice. Significantly, reduced novel environment exploration of an open field and absence of amphetamine sensitization (AS) measured in CB₁ R KO mice were fully rescued in CB₁ R_(MSN) mice. Impaired motor coordination in CB₁ R KO mice measured on the Rotarod was partially rescued in CB₁ R_(MSN) mice. Thus, CB₁ R expressed by MSN control exploration, motor coordination, and AS. Our study demonstrates a new functional roles for cell specific CB₁ R expression and their causal link in the control of specific behaviors.

Overexpression of striatal D2 receptors reduces motivation thereby decreasing food anticipatory activity

Dopamine has been implicated in circadian timing underlying the food entrainable oscillator (FEO) circuitry and overexpression of the dopamine D2 receptor (D2R) in the striatum has been reported to reduce motivation to obtain food rewards in operant tasks. In the present study, we explored both of these mechanisms by examining food anticipatory activity (FAA) in dopamine D2 receptor-overexpressing (D2R-OE) mice under various durations of food availability. First, we noted that at baseline, there were no differences between D2R-OE mice and their littermates in activity level, food intake, and body weight or in circadian activity. Under conditions of very restricted food availability (4 or 6 hr), both genotypes displayed FAA. In contrast, under 8-hr food availability, control mice showed FAA, but D2R-OE mice did not. Normalization of D2R by administration of doxycycline, a tetracycline analogue, rescued FAA under 8-hr restricted food. We next tested for circadian regulation of FAA. When given ad libitum access to food, neither D2R-OE nor controls were active during the daytime. However, after an interval of food restriction, all mice showed elevated locomotor activity at the time of previous food availability in the day, indicating circadian timing of anticipatory activity. In summary, motivation is reduced in D2R-OE mice but circadian timing behavior is not affected. We conclude that an increase in striatal D2R reduces FAA by modulating motivation and not by acting on a clock mechanism.