partial agonist via Gao subunits, with agonist efficacy much greater than THC at Gai, and slightly greater than THC at Gao. AEA and THC can antagonize each other; this in part is due to cross-tolerance. Falenski et al. demonstrated that subchronic administration of THC in FAAH2/2 knockout mice caused greater tolerance to THC than did subchronic administration of THC in wildtype mice. Thus elevated levels of AEA in FAAH2/2 knockouts produced additive effects with THC. Vann et al. trained rats to discriminate THC; trained rats injected with PMSF, which inhibits FAAH, showed 2.7-fold greater discrimination than rats injected with vehicle. In other words, inhibiting AEA degradation led to an increase in the potency of THC. Further, THC was more potent at producing antinociception, decreasing spontaneous activity, and increasing ring immobility when co-administered with PMSF as compared to vehicle. In summary, the effects of THC upon the eCB system oscillate between potentiation and suppression, depending on acute versus chronic dosage. The dividing line between “acute”and “chronic”is a gray zone, and likely differs amongst individuals. Suplita et al. summarized the situation: they studied “stress antinociception,”where rodents become less responsive to painful stimuli following exposure to an environmental stressor. Stress antinociception is mediated, in part, by the coordinated release of 2-AG and AEA. Acute administration of THC potentiated eCBmediated stress antinociception. The converse was also true: animals exposed acutely to foot shock, which elicits eCB-mediated stress antinociception, became sensitized to the effects of THC. Chronic administration of THC predictably dampened stress antinociception. The converse was not true: chronic exposure to foot shock failed to dampen antinociception induced by either WIN-55,212-2 or by further footshocks. The potential synergy between THC and the eCB system is analogous to the potential synergy between AEA and 2-AG: Rodent studies that combined FAAH and MAGL inhibitors indicated that AEA and 2-AG may activate CB1 receptors in different parts of the central nervous system. Each causes unique Systematic Review of eCB Modulation behavioral effects, and when both are enhanced, new effects emerge. Long and colleagues showed that AEA and 2-AG independently dampen pain sensation, but together their effects are dramatically enhanced. Cannabis is more than THC. Adding CBD to THC in mice enhanced CB1 expression in hippocampus and hypothalamus. CBD increased hippocampal cell survival and neurogenesis, whereas THC had the opposite effect; the CBD response was absent in CB12/2 knockout mice. CBD inhibited the cellular uptake of AEA and its breakdown by FAAH. A separate systematic review regarding the effects of CBD on THC is currently underway. Several other non-THC cannabinoids interact with enzymes of the eCB system. For example, cannabidivarin and cannabidiolic acid are moderately potent inhibitors of PubMed ID:http://www.ncbi.nlm.nih.gov/pubmed/19630872 DAGLa, and cannabigerol and cannabichromene are relatively potent inhibitors of anandamide cellular uptake. Interestingly, cannabis extracts enriched in cannabinoids, such as THC-acid BDS and CBD-BDS, were more potent than the corresponding pure PubMed ID:http://www.ncbi.nlm.nih.gov/pubmed/19631704 compounds at inhibiting MAGL 
and AEA cellular uptake. activation by agonists, CB1 and CB2 desensitize and downregulate. A desensitized receptor drives less receptor-mediated signal transduction, and develops cross-tolerance to all agonists–eCBs and Chebulinic acid price phytocannabino
