In the brain hydrolysis of AG by MGL is
In the brain, hydrolysis of 2-AG by MGL is the primary source of AA for conversion to PGs by cyclooxygenase (Nomura et al., 2011). PGE2 production is required for IL-1β-evoked synapse loss (Mishra et al., 2012). Here, we tested whether decreased PG levels contributed to the synapse protective effects of JZL184. Activation of EP1-2Rs was necessary for gp120-induced synapse loss and JZL184 blocked gp120-induced PGE2 production. Thus, it is likely that JZL184 suppression of PGE2 production contributed to the protective effect. However, because AM630 completely blocked the synapse protection afforded by JZL184, we conclude that the reduction in PG levels produced by MGL inhibition was not the primary mechanism in this model. JZL184 did not suppress the basal level of PGE2 present in unstimulated cultures, suggesting that sufficient PGE2 from a source other than MGL was available to enable gp120-induced synapse loss. Furthermore, IL-1β potentiated NMDA receptors independent of EP1-2R activation, suggesting separate IL-1β actions on PG production and NMDA SB505124 signaling. A partial attenuation of gp120-evoked IL-1β production with the EP1-2R antagonist can be reconciled with its complete block of gp120-induced synapse loss by considering the PG regulation of synapse loss at two steps in the pathway activated by gp120. Activation of EP1-2Rs facilitates glial production of IL-1β and stimulates glutamate release resulting in biochemical potentiation of NMDARs and their direct activation, respectively (Mishra et al., 2012). Thus, we cannot rule out a contribution resulting from decreased PGE2 synthesis. The CB2R- and EP1-2R-depedence of JZL184 inhibition of IL-1β production were both partial effects, consistent with the idea that the inhibition of MGL has dual actions via CB2 receptor signaling and decreased PGE2 production. HIV-1 gp120 evoked synapse loss via a multi-step process (Fig. 7) involving multiple cell types. The primary effects of JZL184 appear to be on glia where activation of CB2R or reduced PGE2-dependent activation of EP1-2Rs inhibits the release of IL-1β. In a previous study we found that IL-1β induced PGE2 production was required for synapse loss, presumably via an EP1-2R mediated increase in presynaptic glutamate release (Mishra et al., 2012). If we consider a role for presynaptic glutamate release in gp120-induced synapse loss, then we might expect a CB1R-mediated component to JZL184-mediated synapse protection because 2-AG mediates a robust inhibition of excitatory synaptic transmission (Roloff et al., 2010, Straiker et al., 2009). However, neither rimonabant nor LY320135 affected JZL184-mediated protection, suggesting that the elevation in 2-AG might be localized to a microdomain with preferential access to microglia independent of presynaptic terminals. Cannabinoid receptor agonists have beneficial effects in models of HAND (Avraham et al., 2014, Kim et al., 2011, Purohit et al., 2014). However, drugs that directly activate cannabinoid receptors might cause receptor desensitization during long-term treatment, diminishing the neuroprotective efficacy of the endocannabinoid system. Agonists with actions on CB1 receptors or those with low selectivity may have abuse liability. Alternatively, inhibition of MGL only potentiates endogenously produced 2-AG so that receptor activation is dependent on a stimulus. High doses of JZL184 produced extensive CB1 receptor activation (Long et al., 2009). However, prolonged, low-dose JZL184 treatment elicited an anti-inflammatory effect without producing CB1 receptor tolerance or cannabinoid dependence (Kinsey et al., 2013). Furthermore, because brain AA production is primarily dependent on MGL, in contrast to the gut, drugs that inhibit MGL can reduce brain PG levels without the gastrointestinal side effects produced by nonsteroidal anti-inflammatory drugs (Scheiman, 2016). Thus, drugs that inhibit MGL show promise for reducing neuroinflammation in HAND.