Experimental evidence suggests that the transport of endocannabinoids might work bi-directionally. Accordingly, it is possible that pharmacological blockade of the latter affects not only the re-uptake, but also the release of endocannabinoids, thus preventing them from stimulating CB1 receptors. We used biochemical, pharmacological, and behavioral approaches to investigate the effects of the transporter inhibitor OMDM-2 on social interaction, a behavioral assay that requires activation of CB1 receptors. The underlying mechanisms of OMDM-2 were compared with those of the Fatty Acid Amide Hydrolase (FAAH) inhibitor URB597.
Systemic administration of OMDM-2 reduced social interaction, but in contrast to URB597-induced social deficit, this effect was not reversed by the TRPV1 antagonist capsazepine. The CB1 antagonist AM251, which did not affect URB597-induced social withdrawal, exacerbated OMDM-2 effect. In addition, the potent CB1 agonist CP55,940 reversed OMDM-2-, but not URB597-, induced social withdrawal. Blockade of CB1 receptor by AM251 reduced social interaction and the cholecystokinin CCK2 antagonist LY225910 reversed this effect. Similarly, OMDM-2-induced social withdrawal was reversed by LY225910, whereas URB597 effect was not. Elevation of endocannabinoid levels by URB597 or JZL184, an inhibitor of 2-AG degradation, failed to reverse OMDM-2-induced social withdrawal, and did not show additive effects on cannabinoid measurements when co-administered with OMDM-2.
Taken together, these findings indicate that OMDM-2 impaired social interaction in a manner that is consistent with reduced activation of presynaptic CB1 receptors. As cannabinoid reuptake inhibitors may impair endocannabinoid release, caution should be taken when using these drugs to enhance endocannabinoid tone in vivo.