Chronic neuroinflammation implicates protracted activation of both microglial and astroglial cells, with the consequent sustained release of pro-inflammatory molecules 
, that act in an autocrine way to self-perpetuate reactive gliosis and in a paracrine way to kill neighboring neurons, thus expanding the neuropathological damage. Once started, neuroinflammation promotes neuronal death, powering a vicious cycle responsible for the progression of the pathology 
. The possibility of interfering with this detrimental cycle by molecules which can reduce reactive gliosis has been proposed as a novel rationale to develop drugs able to blunt neuronal damage and consequently slow the course of AD.
Results from the present study prove the selective involvement of PPARγ in the anti-inflammatory and neuroprotective effects of CBD here observed either in vitro and in vivo. In addition, CBD significantly promoted neurogenesis in Aβ injured rat hippocampi, much expanding its already wide spectrum of beneficial actions exerted in AD models, a non negligible effect, due to its capability to activate PPARγ.
CBD was already reported to exert a marked anti-inflammatory effect through the A2A and 5HT1A receptors 
, as well as to improve brain function 
. In addition, it has been already demonstrated that CBD markedly downregulate reactive gliosis by reducing pro-inflammatory molecules and cytokine release that strongly occurs in Aβ neurotoxicity. This activity was linked to its ability to act as a potent inhibitor of NFκB activation induced by Aβ challenge 
. The present findings, confirming the formerly obtained results and extending our knowledge about CBD pharmacology, indicate that a selective PPARγ activation occurs upstream to CBD-mediated NFκB inhibition. Such activation appears to be responsible for a large plethora of CBD effects. Indeed, the interaction of CBD at the PPARγ site results in a profound inhibition of reactive gliosis as showed by the reduction of both GFAP and S100B protein expression together with a marked decline of pro-inflammatory molecules and cytokine release observed in Aβ challenged astrocytes.
The in vitro findings were replicated in in vivo experiments, since once again CBD treatment resulted in a profound inhibition of astrocytic activation surrounding CA1 area Aβ-injected and induced a rescue of CA1 neuronal viability in comparison to control.
Along this line, results from hippocampal homogenates fully matched with in vitro
data previously obtained from primary cultured astrocytes. Also in this case, CBD through the activation of PPARγ provoked a marked reduction of NO, TNFα, and IL-1β release in association with a parallel decline of GFAP, S100B, and iNOS protein expression. The observation that in the hippocampus the selective activation of PPARγ caused a decrease in p50 and p65 protein expression, further reinforces the importance of the sequence of events PPARγ activation/NFκB inhibition as responsible for the CBD anti-inflammatory effect. The PPARγ mediated inhibition of S100B induced by CBD represents a crucial step in interrupting self-perpetuation of the reactive gliosis cycle. Indeed, the over-release of this astroglial-derived neurotrophin actively exacerbates the pro-inflammatory cytokine loop fuelled by Aβ stimulation, massively accelerates amyloidogenicity by promoting cleavage of APP to Aβ and induces tau protein hyperphosphorylation, by disrupting the Wnt pathway 
Notably, both in vivo and in vitro, a selective involvement of PPARγ at the base of the neuroprotective and anti-gliotic effect of CBD appears peremptory, since a complete loss of any beneficial pharmacological activity of this phytocannabinoid occurs when it was co-administered with the PPARγ antagonist GW9662.
Neuroprotective effects exerted by PPARγ agonists in neuropathological conditions, including Aβ induced neuroinflammation and neurodegeneration have been largely described in the past years 
. Besides microglial cells, emerging data indicate astrocytes and neurons as fundamental cell type targets for the beneficial role of PPARγ ligands 
. Astrocytes represent the most abundant glial cells type in the CNS. Once these cells undergo reactive activation, they produce cytokines and other molecules involved in inflammatory response, which are thought to significantly contribute to expand brain damage. Interestingly astrocytes express the highest level of PPARγ in the CNS 
, and accumulating evidence over the last decade indicate that PPARγ agonists may finely regulate their detrimental functions during a protracted activation exerting a profound anti-inflammatory and neuroprotective effects 
Such well-defined and highly effective beneficial activity due to PPARγ activation has recently demonstrated to functionally inactivate NFκB promoters inducing a massive down regulation of the activation of this transcription factor. Traditional PPARγ agonists like thiazolidinediones were reported to inhibit the overproduction of NO, IL-6, and TNF-α as well as the increased expression of the inducible enzymes iNOS and COX2 induced in LPS-stimulated astrocytic and microglial cultures 
. Taken together all these findings fully match those concerning the anti-inflammatory and neuroprotective activities exhibited by CBD in this study.
AD neuroinflammation is a highly important feature that has been considered responsible for the progressive worsening of the disease. Given the tremendous complexity of AD, however, it appears to be oversimplified reasoning to consider only reactive astrogliosis and neuronal degeneration as the unique factors at the basis of the functional decline provoked by the course of the illness.
Neuropathological investigation has provided evidence that in AD brains progressive neuronal loss is not accompanied by a new neuronal replacement while alterations in neurogenesis have been reported to occur 
. Both aspects are considered to be important in contributing to the long-term development of disease. Therefore the disease symptoms could partly be due to the impaired replacement of new hippocampal neurons from endogenous neuronal stem cells 
, which is believed to promote learning and memory. Although there were some controversies over whether neurogenesis is increased or reduced in the pathogenesis of AD, later studies have confirmed deficient maturation of new neurons in AD patients 
. As a result, an approach to enhance neurogenesis and/or maturation should be considered potential therapies for AD.
Moreover, activation of PPARγ has been reported to promote neurogenesis and agonists at these receptors regulate neuronal stem cell proliferation and differentiation as well 
. In addition PPARγ activation promotes neurite outgrowth in mature neurons, significantly contributing to a proper neuronal connectivity in neuronal networks 
. According to these observations, results from the current investigation demonstrate that CBD-mediated activation of PPARγ is associated with a significant neurogenic activity in the granule cell layer of the hippocampal DG. Results indicate that CBD markedly counteracts the massive reduction of neurogenesis in the DG caused by Aβ exposure towards control animals and this effect is consequential, as expected, to a selective PPARγ involvement.
It has been recently asserted that CBD could promote adult hippocampal neurogenesis by activating CB1 receptors 
. Such an assumption was supported by the observation that the CBD neurogenic effect was lost in CB1 knock-out mice, suggesting that the pro-neurogenic action of this phytocannabinoid was clearly dependent on the interaction at the CB1 receptor, which shows a wide expression over the entire DG, including the neuronal precursor cells. However, the observation that Δ9-tetrahydrocannabinol, a preferential agonist at CB1 receptor site, failed in the same investigation to affect hippocampal neurogenesis, tempers the notion that CB1 receptor activation and CBD neurogenetic properties are linked in a straightforward way. Since CBD exhibits negligible affinity to CB1 receptors and the majority of actions appears to be cannabinoid receptor independent, it is possible to assume that the CBD effect on neurogenesis could only involve CB1 receptor sites indirectly.
In conclusion, results of the present research demonstrate that CBD may exert protective functions through a PPARγ dependent activation, which leads to a reduction in reactive gliosis and consequently in neurodegeneration. Moreover, in the current experimental conditions this phytocannabinoid appears to stimulate neurogenesis since it increases DCX immunopositive cell proliferation rate in rat DG.
Innovative therapeutic approaches which could significantly improve AD course require new molecules that will be able to have an impact on different pathological pathways, which converge at the progressive neurological decline. CBD has shown a capability to profoundly reduce reactive astrogliosis and to guarantee both direct and indirect neuronal protection in Aβ induced neuroinflammation/neurodegeration. So far, the lack of understanding of the precise molecular mechanism involved in CBD pharmacological actions, has had limited interest and has puzzled investigators. Currently, findings of the present study throw some light on the issue, and frame CBD as a new PPARγ activator.