Impaired hippocampal glucoregulation in the cannabinoid CB1 receptor knockout mice as revealed by an optimized in vitro experimental approach

Potential Therapeutic Targets to Modulate the Endocannabinoid System in Alzheimer's Disease

Alzheimer's disease (AD), the most common neurodegenerative disease (NDD), is characterized by chronic neuronal cell death through progressive loss of cognitive function. Amyloid beta (Aβ) deposition, neuroinflammation, oxidative stress, and hyperphosphorylated tau proteins are considered the hallmarks of AD pathology. Different therapeutic approaches approved by the Food and Drug Administration can only target a single altered pathway instead of various mechanisms that are involved in AD pathology, resulting in limited symptomatic relief and almost no effect in slowing down the disease progression. Growing evidence on modulating the components of the endocannabinoid system (ECS) proclaimed their neuroprotective effects by reducing neurochemical alterations and preventing cellular dysfunction. Recent studies on AD mouse models have reported that the inhibitors of the fatty acid amide hydrolase (FAAH) and monoacylglycerol (MAGL), hydrolytic enzymes for N-arachidonoyl ethanolamine (AEA) and 2-arachidonoylglycerol (2-AG), respectively, might be promising candidates as therapeutical intervention. The FAAH and MAGL inhibitors alone or in combination seem to produce neuroprotection by reversing cognitive deficits along with Aβ-induced neuroinflammation, oxidative responses, and neuronal death, delaying AD progression. Their exact signaling mechanisms need to be elucidated for understanding the brain intrinsic repair mechanism. The aim of this review was to shed light on physiology and pathophysiology of AD and to summarize the experimental data on neuroprotective roles of FAAH and MAGL inhibitors. In this review, we have also included CB1R and CB2R modulators with their diverse roles to modulate ECS mediated responses such as anti-nociceptive, anxiolytic, and anti-inflammatory actions in AD. Future research would provide the directions in understanding the molecular mechanisms and development of new therapeutic interventions for the treatment of AD.

Transient gain of function of cannabinoid CB1 receptors in the control of frontocortical glucose consumption in a rat model of Type-1 diabetes

Here we aimed to unify some previous controversial reports on changes in both cannabinoid CB₁ receptor (CB₁R) expression and glucose metabolism in the forebrain of rodent models of diabetes. We determined how glucose metabolism and its modulation by CB₁R ligands evolve in the frontal cortex of young adult male Wistar rats, in the first 8 weeks of streptozotocin-induced type-1 diabetes (T1D). We report that frontocortical CB₁R protein density was biphasically altered in the first month of T1D, which was accompanied with a reduction of resting glucose uptake ex vivo in acute frontocortical slices that was normalized after eight weeks in T1D. This early reduction of glucose uptake in slices was also restored by ex vivo treatment with both the non-selective CB₁R agonists, WIN55212-2 (500 nM) and the CB₁R-selective agonist, ACEA (3 μM) while it was exacerbated by the CB₁R-selective antagonist, O-2050 (500 nM). These results suggest a gain-of-function for the cerebrocortical CB₁Rs in the control of glucose uptake in diabetes. Although insulin and IGF-1 receptor protein densities remained unaffected, phosphorylated GSKα and GSKβ levels showed different profiles 2 and 8 weeks after T1D induction in the frontal cortex. Altogether, the biphasic response in frontocortical CB₁R density within a month after T1D induction resolves previous controversial reports on forebrain CB₁R levels in T1D rodent models. Furthermore, this study also hints that cannabinoids may be useful to alleviate impaired glucoregulation in the diabetic cortex.

Chronic insulinopenia/hyperglycemia decreases cannabinoid CB1 receptor density and impairs glucose uptake in the mouse forebrain

Both endocannabinoids and insulin regulate peripheral and cerebral glucose homeostasis via convergent signaling pathways that are impacted by diabetes. Here we asked how glucose metabolism and important facets of insulin signaling are affected in the forebrain of cannabinoid CB₁ receptor knockout mice (CB₁R-KO) and their wild-type (WT) littermates, seven weeks after the induction of insulinopenia/hyperglycemia (diabetes) with intraperitoneal streptozotocin injection. Sham-injected animals served as control. Diabetes caused milder weight loss in the WT mice compared to the phenotypically ˜11% leaner CB₁R-KO, while hyperglycemia was similar. Resting [³H]deoxyglucose uptake was significantly reduced by ˜20% in acute ex vivo frontocortical and hippocampal slices obtained from both the sham-injected CB₁R-KO and the diabetic WT mice. Surprisingly, the third cohort, the diabetic CB₁R-KO showed no further impairment in glucose uptake, as compared to the sham-injected CB₁R-KO. Depolarization-induced [³H]deoxyglucose uptake was proportional to the respective resting values only in the cortex in all four cohorts. The dissipative metabolism of [¹⁴C]-U-glucose remained largely unaffected in all cohorts of animals. However, diabetes reduced cortical CB₁R density by ˜20%, as assessed by Western blotting. Albeit the changes in insulin signaling did not reflect the glucose uptake profile in each cohort, there were significant interactions between diabetes and genotype. In conclusion, a chronic decrease or lack of CB₁R expression reduces glucose uptake in the mouse brain. Additionally, diabetes failed to cause further impairment in cerebral glucose uptake in the CB₁R-KO. These suggest that diabetic encephalopathy may be in part associated with lower CB₁R expression.

Methamphetamine Induces Anhedonic-Like Behavior and Impairs Frontal Cortical Energetics in Mice

INTRODUCTION

We recently showed that a single high dose of methamphetamine (METH) induces a persistent frontal cortical monoamine depletion that is accompanied by helpless-like behavior in mice. However, brain metabolic alterations underlying both neurochemical and mood alterations remain unknown.

AIMS

Herein, we aimed at characterizing frontal cortical metabolic alterations associated with early negative mood behavior triggered by METH. Adult C57BL/6 mice were injected with METH (30 mg/kg, i.p.), and their frontal cortical metabolic status was characterized after probing their mood and anxiety-related phenotypes 3 days postinjection.

RESULTS

Methamphetamine induced depressive-like behavior, as indicated by the decreased grooming time in the splash test and by a transient decrease in sucrose preference. At this time, METH did not alter anxiety-like behavior or motor functions. Depolarization-induced glucose uptake was reduced in frontocortical slices from METH-treated mice compared to controls. Consistently, astrocytic glucose transporter (GluT1) density was lower in the METH group. A proton high rotation magic angle spinning (HRMAS) spectroscopic approach revealed that METH induced a significant decrease in N-acetyl aspartate (NAA) and glutamate levels, suggesting that METH decreased neuronal glutamatergic function in frontal cortex.

CONCLUSIONS

We report, for the first time, that a single METH injection triggers early self-care and hedonic deficits and impairs frontal cortical energetics in mice.

Hierarchical glucocorticoid-endocannabinoid interplay regulates the activation of the nucleus accumbens by insulin

Here we asked if insulin activation of the nucleus accumbens in vitro is reflected by an increase in (3)H-deoxyglucose ([(3)H]DG) uptake, thus subserving a new model to study molecular mechanisms of central insulin actions. Additionally, we investigated the dependence of this insulin effect on endocannabinoids and corticosteroids, two major culprits in insulin resistance. We found that in acute accumbal slices, insulin (3 and 300nM but not at 0.3nM) produced an increase in [(3)H]DG uptake. The synthetic cannabinoid agonist, WIN55212-2 (500nM) and the glucocorticoid dexamethasone (10μM), impaired insulin (300nM) action on [(3)H]DG uptake. The glucocorticoid receptor (GcR) antagonist, mifepristone (10μM) prevented dexamethasone from inhibiting insulin's action. Strikingly, this anti-insulin action of dexamethasone was also blocked by two CB1 cannabinoid receptor (CB1R) antagonists, O-2050 (500nM) and SR141716A (500nM), as well as by tetrahydrolipstatin (10μM), an inhibitor of diacylglycerol lipases-the enzymes responsible for the synthesis of the endocannabinoid, 2-arachidonoyl-glycerol (2-AG). On the other hand, the blockade of the post-synaptic 2-AG metabolizing enzymes, α,β-serine hydrolase domain 6/12 by WWL70 (1μM) also prevented the action of insulin, probably via increasing endogenous 2-AG tone. Additionally, an anti-insulin receptor (InsR) antibody immunoprecipitated CB1Rs from accumbal homogenates, indicating a physical complexing of CB1Rs with InsRs that supports their functional interaction. Altogether, insulin stimulates glucose uptake in the nucleus accumbens. Accumbal GcR activation triggers the synthesis of 2-AG that in turn binds to the known CB1R-InsR heteromer, thus impeding insulin signaling.

Stimulation of brain glucose uptake by cannabinoid CB2 receptors and its therapeutic potential in Alzheimer's disease

Cannabinoid CB2 receptors (CB2Rs) are emerging as important therapeutic targets in brain disorders that typically involve neurometabolic alterations. We here addressed the possible role of CB2Rs in the regulation of glucose uptake in the mouse brain. To that aim, we have undertaken 1) measurement of (3)H-deoxyglucose uptake in cultured cortical astrocytes and neurons and in acute hippocampal slices; 2) real-time visualization of fluorescently labeled deoxyglucose uptake in superfused hippocampal slices; and 3) in vivo PET imaging of cerebral (18)F-fluorodeoxyglucose uptake. We now show that both selective (JWH133 and GP1a) as well as non-selective (WIN55212-2) CB2R agonists, but not the CB1R-selective agonist, ACEA, stimulate glucose uptake, in a manner that is sensitive to the CB2R-selective antagonist, AM630. Glucose uptake is stimulated in astrocytes and neurons in culture, in acute hippocampal slices, in different brain areas of young adult male C57Bl/6j and CD-1 mice, as well as in middle-aged C57Bl/6j mice. Among the endocannabinoid metabolizing enzymes, the selective inhibition of COX-2, rather than that of FAAH, MAGL or α,βDH6/12, also stimulates the uptake of glucose in hippocampal slices of middle-aged mice, an effect that was again prevented by AM630. However, we found the levels of the endocannabinoid, anandamide reduced in the hippocampus of TgAPP-2576 mice (a model of β-amyloidosis), and likely as a consequence, COX-2 inhibition failed to stimulate glucose uptake in these mice. Together, these results reveal a novel general glucoregulatory role for CB2Rs in the brain, raising therapeutic interest in CB2R agonists as nootropic agents.

Adenosine A2B receptor activation stimulates glucose uptake in the mouse forebrain

ATP consumption during intense neuronal activity leads to peaks of both extracellular adenosine levels and increased glucose uptake in the brain. Here, we investigated the hypothesis that the activation of the low-affinity adenosine receptor, the A2B receptor (A(2B)R), promotes glucose uptake in neurons and astrocytes, thereby linking brain activity with energy metabolism. To this end, we mapped the spatiotemporal accumulation of the fluorescent-labelled deoxyglucose, 2-(N-(7-nitrobenz-2-oxa-1,3-diazol-4-yl)amino)-2-deoxyglucose (2-NBDG), in superfused acute hippocampal slices of C57Bl/6j mice. Bath application of the A(2B)R agonist BAY606583 (300 nM) triggered an immediate and stable (>10 min) increase of the velocity of 2-NBDG accumulation throughout hippocampal slices. This was abolished with the pretreatment with the selective A(2B)R antagonist, MRS1754 (200 nM), and was also absent in A(2B)R null-mutant mice. In mouse primary astrocytic or neuronal cultures, BAY606583 similarly increased (3)H-deoxyglucose uptake in the following 20 min incubation period, which was again abolished by a pretreatment with MRS1754. Finally, incubation of hippocampal, frontocortical, or striatal slices of C57Bl/6j mice at 37 °C, with either MRS1754 (200 nM) or adenosine deaminase (3 U/mL) significantly reduced glucose uptake. Furthermore, A(2B)R blockade diminished newly synthesized glycogen content and at least in the striatum, increased lactate release. In conclusion, we report here that A(2B)R activation is associated with an instant and tonic increase of glucose transport into neurons and astrocytes in the mouse brain. These prompt further investigations to evaluate the clinical potential of this novel glucoregulator mechanism.