These results suggest that MGL-Tg mice show similar levels of social interaction and social interest as WT mice, regardless of the familiarity or the reciprocity of the social stimulus. Therefore, in contrast to high-fat food, the impaired CPP of MGL-Tg mice to social interactions may represent a problem of consolidation rather than reward processing.It has been postulated that drugs of abuse ‘hijack’ natural reward circuits . We therefore tested MGL-Tg mice in cocaine CPP to see if the drug might recruit 2-AG signaling. In a standard CPP box, we conditioned mice to either saline or cocaine in four 30-min sessions, alternating over a total of 8 days . Both WT and MGL-Tg mice developed a strong post-conditioning preference for the cocaine chamber . This result suggests that the conditioned effects of cocaine do not depend on 2-AG signaling. Furthermore, it indicates that MGL-Tg mice are cognitively able to perform CPP in general.In particular, we have recently shown that social contact increases anandamide mobilization in the NAc, and that this response plays an important role in social reward . If 2-AG is also involved in social-reward processes, as our previous results suggest, social stimulation should increase 2-AG levels. We isolated juvenile C57Bl6J mice for 24 h, and then returned half to their cage-mates while keeping the other half isolated for 6 h . We collected and snap-froze their brains, took micro-punches of tissue, and measured lipid content using liquid chromatography-mass spectrometry . We found that, compared to mice that remained isolated, socially stimulated mice showed an 83% increase in 2-AG levels in the nucleus accumbens and a 40% increase in the ventral hippocampus , cannabis vertical farming but did not show a change in 2-AG levels in the medial prefrontal cortex . This suggests that social interactions cause amobilization of 2-AG signaling.
Levels of the endocannabinoid anandamide, in contrast, were unchanged in the NAc after this 6-h social stimulation . It is important to note, however, that we previously observed an opposite pattern for a 3-h social stimulation, which elicited an increase in levels of anandamide but not 2-AG . It is possible that social contact elevates 2-AG levels due to an increase in general activity. Therefore, we compared 24-h-isolated animals to control animals that were similarly handled but remained social . In this comparison, isolation had no effect on levels of 2-AG in the NAc, but decreased 2-AG by 55% in the vHC and by 50% in the mPFC . This profile of 2-AG change was not the opposite of that elicited by social stimulation – that is, NAc 2-AG increased during stimulation but did not change during isolation, and mPFC did not change during stimulation but decreased during isolation. In contrast, the pattern of 2-AG levels was opposite in the vHC – 2-AG increased during stimulation and decreased during isolation. The magnitudes of these opposing effects in the vHC were relatively comparable . Isolation also did not alter levels of anandamide in the NAc . We interpret these results to mean that social stimulation induces 2-AG signaling in the NAc that is distinct from what is active during baseline social activity, i.e. a ‘social 2-AG tone.’ Other contributions in the mPFC and vHC remain to be determined. Nevertheless, because the NAc is a key region for brain reward, the results suggest that 2-AG likely contributes to the reward of social interactions. These results support the previous finding that MGL-Tg are deficient in social CPP.It has been postulated that the signaling system underlying reward for social interactions overlaps with those involved in the control of other natural rewards .
The available literature emphasizes the important role of the endocannabinoid system in motivated behaviors , raising the theoretical possibility of an endocannabinoid substrate that is common to different natural rewards. Our study provides evidence in support of this possibility through complementary behavioral analysis of a unique model of selective 2-AG reduction with biochemical analysis of 2-AG mobilization in brains regions important for reward. Our main findings are that 2-AG reduction impairs CPP for social interactions, and that social stimulation increases 2-AG levels in the NAc. These results clearly identify 2-AG as a reward signal for social interaction. Additionally, we found that 2-AG reduction impairs CPP to high-fat food. Collectively, our results suggest that 2-AG is involved in regulating the incentive salience of two essential aspects of behavior, feeding and social contact. A limited number of studies have addressed the involvement of endocannabinoid signaling in the control of social behavior. Genetic CB1 deletion and administration of CB1 agonists alter social interactions. The effect of CB1 agonists can be bidirectional, depending on dosage and thus likely the competing circuits involved . Furthermore, increasing anandamide activity via genetic deletion or inhibition of its hydrolytic enzyme, FAAH, increases direct social interactions in mice and social play in rats . Recently, we demonstrated that anandamide-mediated endocannabinoid signaling is important in the control of social reward . Social contact mobilizes anandamide in an oxytocin receptor-dependent manner. Consistently, chemogenetic activation of oxytocin neurons in the hypothalamus increases anandamide mobilization in the NAc. Pharmacological and genetic enhancement of anandamide activity increases social CPP and offsets the effects of oxytocin blockade.
These results suggest that a cooperative oxytocin-driven anandamide signal regulates social reward . Thus, the distinct effects of CB1 agonists and FAAH inhibitors on social behavior present a complex picture with two knowledge gaps: whether 2-AG-mediated activation of CB1 receptors also participates in social behaviors and if so, the type of interaction and associated neural circuits that are regulated. The present study addresses these gaps by exploiting a new transgenic mouse to reduce endogenous 2-AG signaling without overt compensation, using CPP to specifically model social reward rather than interaction in general, and measuring socially mobilized 2-AG levels in brain areas that are part of the reward circuit. At the same time, our study raises an important question for future studies. The temporal profiles of socially stimulated 2-AG versus anandamide are distinct. In the NAc, a 3-h social stimulation increases the levels of anandamide, but not 2-AG , whereas a 6-h stimulation elevates levels of 2-AG, but not anandamide . Distinct temporal profiles of endocannabinoid mobilization have been demonstrated in other situations – for example, in response to drugs of abuse as well as in stress-induced analgesia mediated by the periaqueductal grey – and thus are likely to be functionally meaningful. One plausible explanation is that anandamide is involved in the initial saliency of a social encounter whereas 2-AG is involved in consolidating information from prolonged social contact. This hypothesis is supported by two pieces of evidence. First, oxytocin is thought of as a saliency signal that is more proximal to the initial processing of social sensory information . Oxytocin tightly drives anandamide formation in the NAc, whereas 2-AG production is driven by prolonged social contact but not oxytocin . Secondly, anandamide is more specific to social reward whereas 2-AG is more generalizable to other rewards, as elevating anandamide increases CPP for social interactions but not for high-fat food or cocaine , whereas reducing 2-AG activity reduces CPP to both social interactions and high-fat food . An extensive literature also supports an important role of CB1 signaling in food reward. CB1 receptor antagonism and genetic deletion suppress not only food intake, but also CPP and self-administration . In contrast, information is sparse regarding the individual endocannabinoid transmitters that might be involved. Available studies have found that systemic administration of exogenous anandamide increases food intake , and 2-AG administration into the NAc increases feeding . Exogenous administration, however, does not simulate physiological conditions. The intake phenotype may also equivocate the metabolic and motivational aspects of endocannabinoid signaling. Our study addresses these concerns by specifically manipulating 2-AG signaling and using a model of CPP to represent the rewarding value of high-fat food. A lack of CPP encompasses different impairments in aspects of reward signaling, such as the processing of initial sensory cues, integration and recruitment of limbic regions, and the consolidation of the memory for the reward. In order to specify the component in which 2-AG may play a larger role, cannabis drying rack we also measured the intake of high-fat food and the social interactions of MGL-Tg mice. We found that MGL-Tg mice show less intake of high-fat food but appear normal in direct, reciprocal social interactions as well as social approach. In light of the lack of CPP for both high-fat and social stimuli in MGL-Tg mice, these results may point to a dichotomous role for 2-AG – perhaps in the processing of high-fat reward versus the consolidation of social reward. These speculations are consistent with the aforementioned feeding literature and prolonged action of 2-AG, as compared to anandamide, after social stimulation. In line with this thinking, the phenotype of a lack of CPP across high-fat and social stimuli, but present CPP for cocaine, must be taken to represent a qualitative rather than quantitative difference. That is, reducing 2-AG impairs different processes for the reward signaling of these stimuli, rather than different amounts of the same process. Perhaps because cocaine bypasses 2-AG-regulated signaling and is more dopamine-dependent, CPP develops regardless of 2-AG signaling.
Nevertheless, whether 2-AG influences dopamine in MGL-Tg mice and whether dopamine is the ultimate effector of all these rewards remains to be determined. Indeed, the magnitude of cocaine CPP as a function of dosage does not vary in a graded manner, but develops in an all-or-none fashion, starting at the minimum dose range that we used . Additional investigation into these possibilities is needed. While MGL-Tg mice represent a technological advance to address the aforementioned knowledge gaps, phenotypic results also come with potential weaknesses. First, forebrain 2-AG reduction may alter general cognition so that mice are unable to perform the CPP task. However, our finding that MGL-Tg mice develop normal cocaine CPP argues against this possibility. Second, improper development or socialization may render MGL-Tg abnormal. But this is unlikely because MGL over expression is controlled by the CAMKII promoter, which lacks developmental activity, and MGL-Tg mice do not show overt abnormalities in tests of general health . Normal levels of social interaction and social approach also speak to proper socialization. It remains true that our results do not completely exclude these possibilities. More nuanced forms of cognition or social interaction may not be detected in our tests. In conclusion, our study identifies a common endocannabinoid substrate – 2-AG – in the regulation of two natural rewards, feeding and social contact. This identification provides a basis to motivate further investigation of the circuitry and physiology regulated by 2-AG signaling in natural reward, the dichotomous roles of anandamide and 2-AG in different social contexts, as well as how such signaling may be exploited in addiction.A core feature across autism spectrum disorder is impairment in social functioning. People with ASD restrict themselves to repetitive behaviors and show deficits in social reciprocity and communication1 . The underlying basis for social impairment in ASD is unknown and no pharmacological treatment is available. One theory – the social motivation theory – posits that the psychopathology of ASD is rooted in a decreased desire to be social3. The neural substrates of normal social behavior are only now beginning to emerge . Perhaps the best account so far has come from the study of oxytocin. This neuropeptide is crucial in many aspects of social behavior, including affiliation and reward. Investigations are ongoing into the contributions of the oxytocin system to ASD and oxytocin-based therapies for ASD8 . Recent reports suggest that early treatment with oxytocin may be useful for improving social behavior in animal models as well as in human patients10. Nevertheless, identifying the key neural systems underlying social behavior and understanding how they interact with oxytocin remains an enormous challenge5 . One candidate is the endogenous cannabinoid system, a modulatory neurotransmitter system that may play an important role in social behavior. This signaling complex consists of lipid-derived messengers called ‘endocannabinoids’ whose actions in the brain are mainly, albeit not exclusively, mediated through CB1 cannabinoid receptors. A series of enzymes catalyze endocannabinoid synthesis and degradation to control the activity of these substances. Fatty acid amide hydrolase catalyzes the intracellular hydrolysis of the endocannabinoid anandamide. In an effort aimed at probing anandamide function in social behavior, we found that genetic removal of FAAH in mice increases direct social interactions, while Trezza et al. noted that pharmacological FAAH inhibition promotes social play in juvenilerats. More recently, we identified a signaling mechanism by which oxytocin drives anandamide mediated signaling at CB1 receptors to control the rewarding properties of social interactions.