CB1 receptors are also expressed throughout regions implicated in reward and addiction, including the central and basolateral amygdala, prefrontal cortex, hippocampus, dorsolateral striatum, ventral tegmental area, and, to a lesser extent, the nucleus accumbens . These regions are considered key parts of the ‘social brain,’ based on imaging and network studies . The regional distribution of the aforementioned endocannabinoid-synthesizing and -degradative enzymes is largely similar to the picture of CB1 depicted here . But it is worth noting that the anatomical distribution of these enzymes has been studied less extensively than that of CB1 receptors. Human PET imaging studies have also validated the distribution of CB1 receptors in, for example, schizophrenia and addiction . In addition to less documentation, the value of enzyme expression as a proxy for signaling is questionable , because enzyme activities can be modified post-translationally, biochemical synthetic routes may be multiple or unclear, particularly for anandamide, and expression reflects basal rather than stimulated states, indoor grow rack which is problematic for the on-demand nature of endocannabinoid mobilization. Initial animal studies of endocannabinoid signaling in social behavior focused at the level of CB1 receptors. Synthetic cannabinoid agonists tend to decrease direct interactions.
Why, however, remains unclear because of the diverse actions of cannabinoids, such as in cognition and stress reactivity. Like marijuana, the broad actions of an agonist expose social effects to modification by a host of factors related to context. One example is the complex effects on aggression – a component of the social repertoire. Consistent with human studies that found that marijuana decreases hostility , studies in experimental animals have found that cannabinoid agonists decrease aggression . But this may not be necessarily prosocial, as agonists can actually increase defensive posturing . Other studies have reported an opposite effect – that agonists actually enhance aggression under certain stressful conditions, in conjunction with flight acts . Consistently, cannabinoid agonists can be anxiogenic . Thus, the state of stress may play an important role in modifying the response to a cannabinoid. Indeed, Haller and colleagues observed that CB1 -/- mice exhibited less direct social interactions in an unfamiliar environment, but not in a home-cage environment. Regional manipulation of CB1 may also lead to a different outcome than whole-brain manipulation. Over expression of CB1 in the medial prefrontal cortex reduced interactions and increased withdrawal . This effect was concomitant with altered emotional reactivity and decreased cognitive flexibility . Together with the previously described human marijuana studies, these results support a role for CB1 in social behavior, but suggest complex interactions between factors such as the affected neural circuits of cognition, stress, and reward, which additionally can be modified by context.
The picture offered by anandamide seems to be, in general, less complex and more prosocial than that provided by cannabinoid receptors. Mice lacking the hydrolytic enzyme of anandamide, fatty acid amide hydrolase , which show elevated levels of anandamide, exhibit increased direct social interactions . In a model of phencyclidine-induced social withdrawal, which is related to schizophrenia, the FAAH inhibitor URB597 improved the social impairment . Trezza and colleagues have investigated the differential roles of CB1 agonists and anandamide in social play in juvenile rats, a characteristic behavior that peaks in juveniles but wanes in adulthood . Juvenile peers engage in ‘rough-and-tumble’ play, including pouncing and pinning. Trezza et al. found that a synthetic cannabinoid agonist decreases social play, whereas the FAAH inhibitor URB597 increases social play . Furthermore, play increases levels of anandamide in the nucleus accumbens and amygdala . These studies suggest that anandamide may be largely prosocial, in contrast to cannabinoid agonists. However, like studies on CB1, the various neural faculties involved across these studies, from social cognition to reward, raise further questions into how anandamide affects social behavior. In addition, no study to date has reported a clear role for 2- AG in social behavior. In a separate line of research, developmental effects of cannabinoids also support their role in social behavior. Developmental exposure to cannabinoid agents has been found to influence the expression of social behavior later in adulthood.
Treatment with the cannabinoid agonist, WIN55,212-2 over 25 days in adolescent rats impaired social interactions after the animals developed into adults . The authors also conducted lesion experiments to conclude that the medial prefrontal cortex is crucial for the proper development of social functioning and likely involved in this impairment. This type of persistent impairment after adolescent cannabinoid exposure has been confirmed by several other studies . An open question, however, is whether this exposure affects the endocannabinoid system itself or interacts with other systems to lead to long-term impairment. While these are not mutually exclusive, support for the first possibility would also indicate an important role for endocannabinoid signaling in social development. Indeed, it is particularly striking that the expression of CB1 receptors throughout regions of the social brain reaches a peak during adolescence, and decreases into adulthood . A recent study aimed at probing the significance of this increase used a mutagenesis-induced gain of-function rat model. The study found that over expressing CB1 receptors prolonged the adolescent behavioral repertoire, including increased impulsivity and social play, into adulthood . That is, the adult mutants’ behavioral profile looked like that of regular adolescent rats. Although the model may have confounders, and endocannabinoid signaling in these mutants requires investigation, the study raises the hypothesis that CB1 receptors are crucial for the proper development of social behavior. They suggest that rather than modulating the development of other social brain systems, endocannabinoid signaling itself mediates the social transition between adolescence and adulthood. Furthermore, these studies highlight the unappreciated, long-term risks of marijuana intake during brain development.The studies briefly outlined above, covering from marijuana in humans to components of the endocannabinoid system in rodents, suggest that endocannabinoid signaling in various domains of the social brain plays a role in the development and acute expression of social behavior. In order to establish such a role, however, several knowledge gaps must be addressed: the functions served by endocannabinoid signaling in specific facets of sociality must be isolated, such as cognition, reward, or anxiety; in this line of thinking, the afferent mechanisms for the recruitment of endocannabinoid signaling must be identified, perhaps within a circuit-based framework to define social behavior; it must be determined whether impairment in endocannabinoid signaling might be responsible for cardinal social impairments in neuropsychiatric conditions.Human studies have shown that marijuana heightens the saliency of social interactions , enhances interpersonal communication , and decreases hostile feelings within small social groups . The neural mechanisms underlying these prosocial effects are unclear but are likely to involve activation of CB1 cannabinoid receptors, the main molecular target of marijuana in the human brain . Consistent with this idea, CB1 receptors are highly expressed in associational cortical regions of the frontal lobe and subcortical structures that underpin human social-emotional functioning . Moreover, the receptors and their endogenous lipid-derived ligands – anandamide and 2-arachidonoyl-sn-glycerol – have been implicated in the control of social play and social anxiety , two crucial aspects of the social experience. Another essential facet of social behavior, the adaptive reinforcement of interactions among members of a group , requires the oxytocin-dependent induction of long-term synaptic plasticity at excitatory synapses of the nucleus accumbens , a key region in the brain reward circuit.
Because the endocannabinoid system regulates the reinforcement of various natural stimuli as well as NAc neurotransmission , indoor farming equipment in the present study we tested the hypothesis that this signaling complex might cooperate with oxytocin to control social reward.As a first test of the role of anandamide in social reward, we isolated juvenile, group-reared mice for 24 hours and then either returned them to their group or left them in isolation for 3 additional hours . We then removed and snap-froze their brains, collected micropunches from various regions of interest and measured endocannabinoid content by liquid chromatography-mass spectrometry. In contrast to conventional neurotransmitters, which are sequestered in storage vesicles and released by neurosecretion, the endocannabinoids are produced on-demand from membrane phospholipid precursors . For this reason, the tissue levels of these lipid substances provide an accurate estimate of mobilization during signaling activity . We found that anandamide levels were substantially elevated in NAc and ventral hippocampus of mice that had been returned to their group, when compared to animals left in isolation . By contrast, no such changes were seen in the amygdala, dorsal striatum, ventral midbrain or other structures included in our survey . Moreover, socialization did not change the levels of 2-AG , an endocannabinoid substance whose roles are often distinct from anandamide’s ; or the levels of oleoylethanolamide , a bioactive lipid that is structurally related to anandamide but acts through a distinct receptor mechanism . The results suggest that social contact stimulates anandamide mobilization in NAc and vHC, two regions of the mouse brainthat are involved in the control of motivated behavior. Importantly, one of these regions, the NAc, has been recently implicated in the regulation of social reward by the hypothalamic neuropeptide oxytocin . Interrupting the activity of fatty acid amide hydrolase , the main anandamide-degrading enzyme in the brain, enhances the biological actions of this endocannabinoid neurotransmitter . Therefore, to explore the function of socially driven anandamide mobilization, we tested juvenile mice lacking the faah gene in a conditioned place preference task that specifically evaluates social reward .Compared to their wild-type littermates, faah-/- mice displayed substantially higher levels of social CPP . This phenotypic difference was specific to social context, because CPP for high-fat food or cocaine was unchanged , and was abolished by administration of the CB1 antagonist AM251, which decreased place preference in both faah-/- and wild-type mice . Furthermore, the two genotypes showed similar performance levels in the three-chambered social approach task, which measures direct social approach behavior rather than social reward . Confirming FAAH’s role and ruling out developmental compensation as a possible confounder, we noted that single systemic injections of the FAAH inhibitor URB597 during the socialization phase of the test replicated the prosocial phenotype seen in faah-/- mice in a dose-dependent manner . Furthermore, as seen with FAAH deletion, blocking FAAH with URB597 did not change performance in the social approach task . Noteworthy, when the sCPP protocol was modified so that mice underwent isolation conditioning first, along with URB597 treatment, the subjects developed normal preference to the social context , suggesting that enhancing anandamide signaling via FAAH inhibition increases social reward rather than decrease isolation aversion. This effect may require the induction of long-term depression at excitatory synapses of the NAc, which receives direct oxytocinergic input from the paraventricular nucleus of the hypothalamus . We asked therefore whether socializationinduced oxytocin neurotransmission might regulate anandamide signaling in the NAc, and obtained three sets of results that supported this idea. First, systemic administration of the brain permeant oxytocin receptor antagonist, L-368,899, abolished the rises in anandamide levels elicited in NAc by social contact . Conversely, intracerebroventricular infusionof the OTR agonist WAY-267464 elevated such levels in the absence of social contact and in an OTR-dependent manner . Lastly, and similar to social stimulation, site-selective chemogenetic activation of oxytocin-secreting neurons in the PVN, which we previously showed to increase oxytocin transmission , increased anandamide mobilization in the NAc. Administration of clozapine-N-oxide in mice engineered to express CNO receptors exclusively in oxytocinergic neurons of the PVN, strongly elevated anandamide content in the NAc, while having no effect on 2-AG . The same interventions produced a similar, but not identical set of responses in the vHC. Intracerebroventricular administration of the OTR agonist WAY-267464 increased anandamide levels , but oxytocinergic neuron activation with CNO produced only a trend toward increased anandamide mobilization, which was not statistically significant . Moreover, OTR blockade increased, rather than decreased, anandamide levels in the vHC . These results suggest that oxytocin transmission tightly controls anandamide mobilization in the NAc, but not the vHC, where its effects appear to be complex and possibly indirect.The present study provides evidence that supports an obligatory role for anandamide in social reward. We show that social contact stimulates anandamide mobilization in a brain region, the NAc, that is crucially involved in the reinforcing properties of natural stimuli , including social reward ; this effect is prevented by blockade of the OTR and is mimicked by pharmacological or chemogenetic activation of this receptor; and heightened anandamide signaling enhances social reward and occludes the prosocialeffects of oxytocin.