The aim of this study was to assess whether the age-related increase in microglial activation and the associated decrease in LTP were attenuated by chronic administration of the FAAH inhibitor, URB597. The data show that URB597 increased AEA, OEA, and PEA and that this was accompanied by a URB597-associated attenuation of the age-related neuroinflammatory changes and also the age-related impairment in LTP.
Increased expression of MHCII, CD11b, CD68, and CD40, which are commonly-used markers of microglial activation, were observed in hippocampus and also the cortex of aged, compared with young rats. This concurs with previously-reported findings which demonstrated that these, and other markers of microglial activation, were increased with age [24, 31, 32]. One of the most significant findings of this study is that these changes were attenuated in hippocampal and cortical tissue prepared from aged rats which received URB597. Cannabinboids are known to modulate certain aspects of microglial function in vitro; for instance the phytocannabinoid THC and the non-hydrolyzable analogue of anandamide, methanandamide, decreased LPS-induced cytokine production from rat cortical glial cells [21, 33], while AEA and 2-AG, as well as a number of synthetic cannabinoids, inhibited the LPS-induced release of TNFα  and the generation of nitrites  from cultured glial cells. At least in some studies [20, 33], the actions of the cannabinoids were not CB receptor mediated. There are other reports of a similar modulatory effect of synthetic cannabinoids on microglial activation in vitro including their ability to attenuate the ATP-induced increase in intracellular calcium concentration  and the neurotoxicity induced by Aβ-treated microglia . Similarly the LPS-induced release of TNFα and IL-1β from cultured astrocytes was attenuated by both anandamide and the anandamide uptake inhibitor, UCM707 . In addition to these effects in vitro, it has been shown that the increase in microglial activation induced by the central administration of LPS to rats for 21 days  or by daily intracerebroventricular injection of Aβ25-35 for 7 days  was attenuated by subcutaneously- or centrally-administered WIN55,212-2, respectively.
While a number of cells produce inflammatory cytokines, activated microglia are considered to be a primary source of cytokines such as IL-1β, IL-6, and TNFα in the brain. The present data indicate that the age-related increase in markers of microglial activation are accompanied by an increase in these cytokines confirming earlier reports of a similar parallel . The increase in cytokine production was markedly reduced in hippocampal tissue prepared from aged rats which received URB597 providing evidence of an anti-inflammatory effect of the FAAH inhibitor. URB597 treatment has been shown to decrease LPS-induced PGE2 production in cultured microglia though it did not attenuate the increases in COX2 and iNOS . Intra-peritoneal injections of URB597 have also been shown to reduce LPS induced increases in IL-1β in the hypothalamus in Sprague-Dawley rats . The synthetic cannabinoid, dexanabinol, which facilitated recovery and decreased cell death, reduced hippocampal expression of TNFα and IL-1β in the hippocampus after traumatic brain injury . Perhaps in contrast with this is the report that the CB2 agonist JWH-133, which decreased infarct volume following middle cerebral artery occlusion, did not attenuate the increase in TNF or IL-1β in ischaemic brain tissue .
In the past few years, it has become increasingly clear that neuroinflammation negatively impacts on neuronal plasticity  and specifically that LTP is decreased when microglial activation and/or inflammatory cytokine production is increased in hippocampus [24, 36, 41]. The present findings provide support for this inverse correlation, specifically demonstrating that LTP was decreased in dentate gyrus of aged rats. Significantly, the age-related deficit in LTP was attenuated by treatment with URB597 in parallel with its ability to decrease the expression of several markers of microglial activation and the production of inflammatory cytokines in the hippocampus. These changes concur with the findings of previous studies which indicated that when the age-related increase in microglial activation is attenuated, for example with minocycline , the anti-inflammatory cytokine IL-4 , the polyunsaturated fatty acids EPA and DPA , the cholesterol-lowering HMGCoA reductase, atorvastatin , and the PPARγ activator, rosiglitazone , then the ability of aged rats to sustain LTP is improved.
A facilitatory effect of cannabinoids on other forms of synaptic plasticity has also been reported. Thus the synthetic cannabinoid, WIN55,212-2, attenuates the impaired spatial learning observed in rats which received Aβ25-35 intracerebroventricularly for 7 days . This effect was coupled with changes in neuronal markers calbindin and α-tubulin in tissue prepared from the frontal cortex of mice. Similarly intraperitoneal administration of WIN55,212-2 or cannabidiol for 3 weeks attenuated the cognitive impairment induced by a single injection of Aβ, although the CB2 agonists, 1,1-dimethylbutyl-1-deoxy-Δ9-tetrahydrocannabinol [JWH-133] and 4-[4-(1,1-dimethylheptyl)-2,6-dimethoxyphenyl]-6,6-dimethyl-bicyclo[3.1.1]hept-2-ene-2-methanol [HU-308] did not ; in this case the Aβ-induced increase in IL-6 was attenuated by WIN55,212-2 or cannabidiol prompting the authors to conclude that the effect of the cannabinoids resulted from modulation of glial activation. The correlation between glial activation and spatial learning is not absolute since it has been reported that while the increase in microglial activation induced by the central administration of LPS for 21 days was attenuated by WIN55,212-2, treatment with WIN55,212-2 exacerbated the deficit in spatial learning . However the same group reported that when aged rats were treated with WIN-55,212-2, performance in a spatial learning task improved and this was correlated with a decrease in the number of activated microglia in CA3 but not dentate gyrus .
The effect of URB597 in the present study can be attributed to its ability to increase the concentration of endocannabinoids in the brain. The data indicate that the 28-day URB597 treatment regime used here increases concentrations of AEA, as well as two other fatty acid ethanolamides, PEA and OEA in the brain. The anti-inflammatory effects of AEA have been well documented both in vitro and in vivo[20, 43–46] and both PEA and OEA possess anti-inflammatory properties [47, 48]. While PEA appears to lack CB1 receptor binding activity, it interacts with the CB2 receptor which probably mediates its analgesic and anti-inflammatory effects [48–50]. In contrast, OEA may not interact with either CB1 or CB2 receptors, but rather engage one of the recently-described G protein-coupled orphan receptors . It is possible that any of these endocannabinoids/N-acylethanolamines, that are increased following URB597 treatment, may contribute to the anti-inflammatory effects described in the present study.
One of the challenges in neuroscience is to identify the age-related changes in the brain which present the most significant risks for development of neurodegenerative diseases and to reduce these changes. In addition to the findings presented here, a good deal of evidence suggests that neuroinflammation, probably functionally linked with microglial activation, is one such change. We demonstrate that increasing endocannabinoid tone provides a mechanism by which the age-related microglial activation and deficit in synaptic plasticity can be attenuated.