Review
Copyright ©2010 Baishideng. All rights reserved.
World J Diabetes. Mar 15, 2010; 1(1): 12-18
Published online Mar 15, 2010. doi: 10.4239/wjd.v1.i1.12
Diabetic retinopathy: Role of inflammation and potential therapies for anti-inflammation
Gregory I Liou
Gregory I Liou, Department of Ophthalmology, Medical College of Georgia, GA 30912, United States
Author contributions: Liou GI contributed solely to this paper.
Correspondence to: Gregory I Liou, PhD, Professor, Department of Ophthalmology, Medical College of Georgia, 1120 15th Street, Augusta, GA 30912, United States. giliou@mcg.edu
Telephone: +1-706-7214599
Received: August 31, 2009
Revised: February 23, 2010
Accepted: March 2, 2010
Published online: March 15, 2010

Abstract

Diabetic retinopathy is a leading cause of blindness among working-age adults. Despite many years of research, treatment options for diabetic retinopathy remain limited and with adverse effects. Discovery of new molecular entities with adequate clinical activity for diabetic retinopathy remains one of the key research priorities in ophthalmology. This review is focused on the therapeutic effects of cannabidiol (CBD), a non-psychoactive native cannabinoid, as an emerging and novel therapeutic modality in ophthalmology based on systematic studies in animal models of inflammatory retinal diseases including diabetic retinopathy - a retinal disease associated with vascular-neuroinflammation. Special emphasis is placed on novel mechanisms which may shed light on the pharmacological activity associated with CBD preclinically. These include a self-defence system against inflammation and neurodegeneration mediated by inhibition of equilibrative nucleoside transporter and activation of adenosine receptor by treatment with CBD.

Key Words: Cannabidiol, Anti-inflammation, Diabetic retinopathy, Retinal microglia, Adenosine receptors, Equilibrative nucleoside transporters


Citation: Liou GI. Diabetic retinopathy: Role of inflammation and potential therapies for anti-inflammation. World J Diabetes 2010; 1(1): 12-18
INTRODUCTION

During the past decade, it has become clear that inflammation is a key feature in diabetes that leads to long-term complications in specific organs, in particular the eye and kidney. In the eye, the major complication is diabetic retinopathy, a leading cause of blindness in the Western world affecting three-fourths of diabetic patients within 15 years after onset of the disease[1,2]. Many diabetic patients are referred to an ophthalmologist for evaluation and treatment only after visual complications have already occurred. The recommended treatment for diabetic retinopathy has been laser photo-coagulation but the procedure also destroys neural tissues. Therefore, there is a great need for the development of new non-invasive therapies. These visual complications are most likely associated with oxidative stress and inflammation. Our research in diabetic retinopathy has focused on delineating the inflammatory and neurodegenerative processes involved. We have identified new non-invasive receptor-based therapies for mitigating the retinal damage associated with diabetes. This review is focused on the therapeutic effects of cannabidiol (CBD) on animal models of diabetic retinopathy. Special emphasis is placed on novel mechanisms described in recent studies of retinal models which help to explain some of the pharmacological effects observed with CBD.

DIABETIC RETINOPATHY (DR)

DR is a chronic ocular disorder that, if untreated, will lead to legal blindness. In the United States, over 20 million adults (or 10% of the total population) currently have diabetes. Of this group, over 12 000 patients will be diagnosed with new-onset blindness annually, making it one of the leading causes of legal blindness in Americans within the age group of 20-74[3]. Type I diabetics usually have high incidence of retinopathy although retinopathy occurs in almost all patients with diabetes for 20 years or more[1]. The earliest detectable signs of retinopathy are categorized as nonproliferative diabetic retinopathy (NPDR). NPDR is clinically subdivided into mild, moderate and severe categories. Loss of retinal pericytes and alterations in retinal blood flow are preclinical changes that are often non-detectable by physical exam[4,5]. Retinal venous dilation and microaneurysms are the first alterations detectable by ophthalmoscopy. Following these alterations, intraretinal hemorrhage and exudation may occur. These may then lead to macular edema, which, if untreated may lead to irreversible vision loss and blindness. As hyperglycemia persists, the disease progresses to moderate and severe NPDR which presents with hemorrhages and venous beading, suggesting decreased retinal circulation and dilated capillaries[6].

Proliferative diabetic retinopathy (PDR) is the next stage when proliferation of new blood vessels begins. Approximately 50 percent of patients with severe NPDR progress to PDR within one year[7]. This stage is characterized by the onset of ischemia-induced new vessel proliferation from the optic nerve head as well as in the retina. These new vessels are fragile and tend to bleed easily resulting in vitreous hemorrhage. If untreated, the neovascularization will undergo fibrosis and contraction leading to traction retinal detachments. Additional complications may include neovascular glaucoma due to sprouting of new vessels on the iris and in the trabecular meshwork of the anterior chamber[8].

DR is a vascular-neuroinflammatory disease

The early signs of diabetic retinopathy in experimental diabetic models include vascular inflammatory reactions due to oxidative stress, pro-inflammatory cytokines, and the consequent binding of leukocyte adhesion molecules CD18 and intercellular adhesion molecule 1 (ICAM-1)[9]. These reactions lead to breakdown of the blood-retinal barrier (BRB) function, vascular occlusion and tissue ischemia, which in turn leads to neuronal cell death[9-14]. However, diabetes could also directly affect metabolism within the neural retina leading to neuronal cell death. Whether diabetes affects vascular or neural retina first, both microglial and macroglial cells are activated[15]. The function of activated macroglia in transporting[16] and metabolizing glutamate may be impaired[17] (unpublished observations). This leads to glutamate accumulation[18-20]. Glutamate excitotoxicity occurs via activation of N-methyl-D-aspartic acid (NMDA) and non-NMDA receptors, to directly or indirectly induce calcium influx and the release of superoxides, leading to neuronal cell death[21]. This is followed by neuro-inflammation, during which activated microglial cells migrate toward dying neurons and release inflammatory cytokines to further exacerbate the damage[22]. These findings suggest that pharmacological interventions that reduce oxidative stress and inflammation might be effective neuroprotectants for diabetic retinopathy[20,23].

Microglia in DR

Normally quiescent microglia become activated during early diabetes[24-27]. Cytokines such as interleukin (IL)-1β, IL-6, γ-interferon, and tumor necrosis factor-α (TNF-α) have been shown to directly activate microglia[28,29]. Activated microglia release (or promote the release of) glutamate, reactive oxygen species (ROS), IL-1β, IL-3, IL-6, TNF-α, vascular endothelial growth factor (VEGF), lymphotoxin, matrix metalloproteinases (MMPs) and nitric oxide (NO)[15,30]. The cytokines IL-1β, IL-6, TNF-α, and lymphotoxin alter expression of vascular cell adhesion molecules to recruit lymphocytes and macrophages to injury sites[31]. Lymphotoxin, TNF-α, NO and ROS can directly kill cells[32,33]. VEGF, NO and MMPs can weaken the BRB, thus enhancing the infiltration of leukocytes into the retina. It remains unclear why diabetes would incite microglia activation in the retina but research on retinal microglia activation may provide substantial insights into the pathogenesis of DR[34]. Cultured microglia have been used extensively to study microglial behavior. Treatment of microglia or macrophage-like cells with advanced glycation end-products (AGE) or Amadori-albumin[35,36], high glucose[37] or with endotoxins such as lipopolysaccharide (LPS) has been used as a model to simulate inflammation[38-40].

ROLES OF ADENOSINE RECEPTORS (ARs) AND NUCLEOSIDE TRANSPORTERS IN INFLAMMATION

Adenosine, an endogenous purine nucleoside, has been proposed to modulate a variety of physiological responses by stimulating specific extracellular receptors[41-43]. ARs have been classified as A1, A2A, A2B, and A3 receptors[44]. Under stress and ischemia conditions, the local tissue concentrations of extracellular adenosine are increased due to the release of adenosine itself, or of AMP, which is metabolized extracellularly to adenosine. This increased adenosine can protect against excessive cellular damage via a negative feedback mechanism[45] (unpublished observations). Adenosine released at inflamed sites exhibits anti-inflammatory effects through A2AAR[46]. Sub-threshold doses of an inflammatory stimulus that caused minimal tissue damage in wild-type mice were sufficient to induce extensive tissue damage and more prolonged and higher levels of pro-inflammatory cytokines in knock-out mice that lacked the A2AAR (A2AAR -/- mice)[47]. A2AAR agonist treatment blocked the inflammation, functional and histological changes associated with diabetic nephropathy in wild-type diabetic mice, whereas it had no effect on the A2AAR -/- diabetic mice[48]. A2AAR, a Gs-protein-coupled receptor, can increase levels of immunosuppressive cAMP in microglia or other immune cells[49]. Stimulation of the A2AAR decreases leukocyte adhesion and blocks the associated release of oxygen free radicals[50]. Adenosine released can activate endothelial adenosine receptors, leading to increases in intracellular cAMP and resealing of the endothelial junctions thereby promoting vascular barrier function[51]. Moreover, A2AAR activation induces the synthesis and release of nerve growth factor thereby is neuroprotective[52].

Although adenosine and its agonists are protective in animal models of inflammation, their therapeutic application has been limited by systemic side effects such as hypotension, bradycardia, and sedation[53]. Moreover, adenosine usually disappears very rapidly in physiological or inflammatory conditions due to rapid reuptake and subsequent intracellular metabolism[54]. Endogenous adenosine levels at inflamed sites are reported to increase further because of the increased need for energy supplied by ATP, which is metabolized to AMP and adenosine ultimately[55]. In addition, the activity of 5’-nucleotidase, which metabolizes AMP to adenosine, is reported to increase in inflammatory conditions[56]. It is therefore assumed that prevention of adenosine uptake into the cells and its subsequent metabolism can selectively enhance extracellular concentrations of adenosine at inflamed sites, resulting in an anti-inflammatory effect[57]. Protective or ameliorating effects of adenosine uptake inhibitors in ischemic cardiac and cerebral injury, organ transplantation, seizures, thrombosis, insomnia, pain and inflammatory diseases have been reported[58]. Preclinical and clinical results indicate the possibility of therapeutic application of adenosine uptake inhibitors[58,59].

Adenosine reuptake and degradation

Adenosine disappears rapidly in physiological or inflammatory conditions due to rapid reuptake via nucleoside transporters (NTs) and subsequent intracellular metabolism[54]. There are two subtypes of NTs: Concentrative NTs which are dependent on the presence of extracellular sodium, and equilibrative NT (ENTs). In the microglial cells, the majority of adenosine transport is not affected by sodium removal suggesting ENTs are the primary transporters functioning in these cells[60]. ENTs are further classified into two subtypes on the basis of their sensitivities to inhibition by the drug S-(4-nitrobenzyl)-6-thioinosine [nitrobenzylmercaptopurine riboside (NBMPR)]. NBMPR-sensitive ENTs bind NBMPR with high affinity and have the functional designation equilibrative sensitive (ENT1). NBMPR-insensitive transporters are designated ENT2. Dipyridamole, an inhibitor for both ENT1 and ENT2[61], is used clinically as a coronary vasodilator and a platelet aggregation inhibitor[62,63]. Dipyridamole plus aspirin improves retinal vasculature patterns in experimental diabetes[64].

Role of ENT1 in adenosine function in diabetes

ENT1 plays an integral role in adenosine function in diabetes by regulating adenosine levels in the vicinity of adenosine receptors. It was reported that adenosine uptake by ENT1 in human aortic smooth muscle cells (HASMCs) was increased by hyperglycemia[65]. To provide insight into mechanisms by which ENT1 was modulated by hyperglycemia, kinetic studies of adenosine transport and [3H]NBMPR binding were performed[65]. The results show that Vmax (representing the number of ENT1) of adenosine transport in high glucose (HG)-treated HASMCs was increased without affecting Km (representing the affinity of ENT1). Similarly, Bmax (representing the number of ENT1) of the high-affinity [3H]NBMPR binding was increased without affecting Kd (representing the affinity of ENT1). Consistent with these observations, HG increased mRNA and protein expression of ENT1. Pathologically, the increase in ENT1 activity in diabetes may affect the availability of adenosine in the vicinity of adenosine receptors and, thus, alter vascular functions in diabetes. Pharmacological intervention of ENT1 activity may prove to be effective therapeutics in diabetes. Current studies are in progress to elucidate the effect of hyperglycemia on the function and expression of ENT1 in the retinal microglial and vascular endothelial cells.

CANNABINOIDS AND CANNABINOID RECEPTORS

The best-known cannabinoids from marijuana are (-)-Δ9-tetrahydrocannabinol (THC), cannabinol (CBN), and (-)-cannabidiol (CBD) (Figure 1)[66]. THC, but not CBN or CBD, is known to exert psychotropic effects[67,68]. Cannabinoids are also known to be therapeutic with properties of anti-inflammation[69,70] and anti-oxidation[71]. Cannabinoids produce their biological effects by acting through at least two receptors. Receptor CB1 (cloned) is responsible for psychoactivity and is expressed in the brain[72] and retinal neurons[73,74]. Receptor CB2 (cloned) is expressed in immune cells[75] and cerebral microglial cells[76], but also in the retina[77]. These receptors are coupled to Gi/o proteins to inhibit adenylyl cyclase activity and immediate early gene signaling pathway(s)[78]. Receptor CB1 is also coupled through Gi/o proteins to inhibit voltage-sensitive calcium channels[79] and activate potassium channels[80].

Figure 1
Figure 1 The best-known cannabinoids from marijuana. (-)-Δ9-tetrahydrocannabinol (THC), but not cannabinol or (-)-cannabidiol (CBD), is known to exert psychotropic effects.

CBD has very low affinity to either CB1 or CB2[81,82]. This low affinity of CBD for CB1 accounts for its inability to produce the subjective “high” and cognitive effects that are characteristic of marijuana and THC. CBD is very effective as a scavenger of ROS. The antioxidative effect of CBD is superior to α-tocopherol and ascorbate in vitro and in vivo[71] due to its ability to scavenge ROS and block NADPH oxidase[40]. CBD also has potent anti-inflammatory actions and have been shown to decrease inflammatory cytokines in arthritis[83] and in diabetes[12], prevent cerebral damage during ischemia[84] and to prevent cerebral infarction[85]. CBD is well tolerated when chronically administered to humans[86] and has been approved for the treatment of inflammation, pain and spasticity associated with multiple sclerosis in patients since 2005[87]. CBD attenuates high glucose-induced endothelial cell inflammatory response and barrier disruption in human coronary endothelial cells[88]. It also decreases the incidence of diabetes in non-obese diabetic mice[89] and is neuroprotective and BRB-preserving in streptozotocin-induced diabetes[12]. Most recently, CBD has been shown to decrease retinal inflammation by blocking ROS and TNF-α formation, p38 MAP kinase activation and microglial activation[40]. Current data in the effects of intraocularly introduced CBD in diabetic animal model are consistent with its anti-inflammatory activity (unpublished observations).

CBD enhances AR-mediated anti-inflammation

It has recently been shown that nanomolar concentrations of CBD or THC could inhibit uptake of adenosine by ENT1 in murine microglia, RAW264.7 macrophages[60] and in rat retinal microglia[39]. CBD synergistically enhances adenosine’s TNF-α suppression upon LPS treatment. Moreover, in vivo treatment with a low dose of CBD decreases TNF-α production in serum in the LPS-treated mice; this effect is reversed by treatment with an A2AAR antagonist and abolished in A2AAR -/- mice[60]. Similar results are observed in the rat retina[39]. These studies demonstrate that CBD has the ability to enhance adenosine signaling through inhibition of uptake and provide a non-cannabinoid receptor mechanism by which CBD can decrease endotoxin-induced inflammation. Current data suggest that CBD inhibits diabetes-induced retinal inflammation by the same mechanism (unpublished observations). A hypothetical pathway illustrating how CBD works to reduce retinal inflammation in diabetes is shown in Figure 2.

Figure 2
Figure 2 The hypothetical mechanism of anti-inflammation effect CBD in diabetic retinopathy. Diabetes causes release of adenosine and pro-inflammatory cytokines via superoxide formation and MAPK activation, leading to DR. Adenosine-initiated anti-inflammation via A2AAR-Gs-cAMP signaling is terminated rapidly due to adenosine reuptake by equilibrative nucleoside transporter (ENT) and subsequent metabolism. CBD blocks superoxide formation and inhibits adenosine reuptake via inhibiting ENT1, thereby activating A2AAR-Gs-cAMP signaling.
CONCLUSION

Recent evidence suggests that local inflammation plays a major role in the pathogenesis of diabetic retinopathy. The function of CBD as an antioxidant to block oxidative stress and as an inhibitor of adenosine reuptake to enhance a self-defense mechanism against retinal inflammation represents a novel therapeutic approach to the treatment of ophthalmic complications associated with diabetes. This study is important for the development of adenosine reuptake inhibitors as a potentially novel and effective therapy for diabetic retinopathy. However, the therapeutic values of these agents should be confirmed by clinical trials. Furthermore, depending on the difference in the genetic make-ups for the metabolism and pharmacological target of CBD, it may be important to consider CBD as a personalized medicine, i.e. adjusted dosages according to individual’s genetic make-ups, to offer significant advantages over traditional clinical approaches[90].

Footnotes

Supported in part by grants from American Diabetes Association and Knights Templar Educational Foundation (GIL)

Peer reviewer: Ugur Cavlak, Professor, Pamukkale Universitesi Fizik Tedavi ve Rehab, Yuksekokulu Kinikli Kampusu Yeni Rektorluk Binasi B Kati 20070 Denizli, Turkey

S- Editor Zhang HN L- Editor Roemmele A E- Editor Liu N

References
1.  Klein R, Klein BE, Moss SE, Davis MD, DeMets DL. The Wisconsin epidemiologic study of diabetic retinopathy. II. Prevalence and risk of diabetic retinopathy when age at diagnosis is less than 30 years. Arch Ophthalmol. 1984;102:520-526.  [PubMed]  [DOI]
2.  Sjølie AK, Stephenson J, Aldington S, Kohner E, Janka H, Stevens L, Fuller J. Retinopathy and vision loss in insulin-dependent diabetes in Europe. The EURODIAB IDDM Complications Study. Ophthalmology. 1997;104:252-260.  [PubMed]  [DOI]
3.   Available from: http://www.diabetes.org/diabetes-statistics/complications.jsp.  [PubMed]  [DOI]
4.  Cogan DG, Toussaint D, Kuwabara T. Retinal vascular patterns IV. Diabetic retinopathy. Arch Ophthalmol. 1961;66:366-378.  [PubMed]  [DOI]
5.  Bursell SE, Clermont AC, Kinsley BT, Simonson DC, Aiello LM, Wolpert HA. Retinal blood flow changes in patients with insulin-dependent diabetes mellitus and no diabetic retinopathy. Invest Ophthalmol Vis Sci. 1996;37:886-897.  [PubMed]  [DOI]
6.  Benson WE. Vascular disorders: diabetic retinopathy. In Ophthalmology Yanoff M, Duker JS, editors. Mosby: London Philadelphia. 1999;8. 20. 1.  [PubMed]  [DOI]
7.  Palmberg PF. Diabetic retinopathy. Diabetes. 1977;26:703-709.  [PubMed]  [DOI]
8.  Aiello LP, Gardner TW, King GL, Blankenship G, Cavallerano JD, Ferris FL 3rd, Klein R. Diabetic retinopathy. Diabetes Care. 1998;21:143-156.  [PubMed]  [DOI]
9.  Joussen AM, Poulaki V, Le ML, Koizumi K, Esser C, Janicki H, Schraermeyer U, Kociok N, Fauser S, Kirchhof B. A central role for inflammation in the pathogenesis of diabetic retinopathy. FASEB J. 2004;18:1450-1452.  [PubMed]  [DOI]
10.  Barber AJ, Lieth E, Khin SA, Antonetti DA, Buchanan AG, Gardner TW. Neural apoptosis in the retina during experimental and human diabetes. Early onset and effect of insulin. J Clin Invest. 1998;102:783-791.  [PubMed]  [DOI]
11.  El-Remessy AB, Behzadian MA, Abou-Mohamed G, Franklin T, Caldwell RW, Caldwell RB. Experimental diabetes causes breakdown of the blood-retina barrier by a mechanism involving tyrosine nitration and increases in expression of vascular endothelial growth factor and urokinase plasminogen activator receptor. Am J Pathol. 2003;162:1995-2004.  [PubMed]  [DOI]
12.  El-Remessy AB, Al-Shabrawey M, Khalifa Y, Tsai NT, Caldwell RB, Liou GI. Neuroprotective and blood-retinal barrier-preserving effects of cannabidiol in experimental diabetes. Am J Pathol. 2006;168:235-244.  [PubMed]  [DOI]
13.  Kern TS. Contributions of inflammatory processes to the development of the early stages of diabetic retinopathy. Exp Diabetes Res. 2007;1-14.  [PubMed]  [DOI]
14.  Ali TK, Matragoon S, Pillai BA, Liou GI, El-Remessy AB. Peroxynitrite mediates retinal neurodegeneration by inhibiting nerve growth factor survival signaling in experimental and human diabetes. Diabetes. 2008;57:889-898.  [PubMed]  [DOI]
15.  Langmann T. Microglia activationin retinal degeneration. J Leukoc Biol. 2007;81:1345-1351.  [PubMed]  [DOI]
16.  Li Q, Puro DG. Diabetes-induced dysfunction of the glutamate transporter in retinal Müller cells. Invest Ophthalmol Vis Sci. 2002;43:3109-3116.  [PubMed]  [DOI]
17.  Lieth E, LaNoue KF, Antonetti DA, Ratz M. Diabetes reduces glutamate oxidation and glutamine synthesis in the retina. The Penn State Retina Research Group. Exp Eye Res. 2000;70:723-730.  [PubMed]  [DOI]
18.  Ambati J, Chalam KV, Chawla DK, D'Angio CT, Guillet EG, Rose SJ, Vanderlinde RE, Ambati BK. Elevated gamma-aminobutyric acid, glutamate, and vascular endothelial growth factor levels in the vitreous of patients with proliferative diabetic retinopathy. Arch Ophthalmol. 1997;115:1161-1166.  [PubMed]  [DOI]
19.  Lieth E, Barber AJ, Xu B, Dice C, Ratz MJ, Tanase D, Strother JM. Glial reactivity and impaired glutamate metabolism in short-term experimental diabetic retinopathy. Penn State Retina Research Group. Diabetes. 1998;47:815-820.  [PubMed]  [DOI]
20.  Kowluru RA, Engerman RL, Case GL, Kern TS. Retinal glutamate in diabetes and effect of antioxidants. Neurochem Int. 2001;38:385-390.  [PubMed]  [DOI]
21.  Coyle JT, Puttfarcken P. Oxidative stress, glutamate, and neurodegenerative disorders. Science. 1993;262:689-695.  [PubMed]  [DOI]
22.  Kreutzberg GW. Microglia: a sensor for pathological events in the CNS. Trends Neurosci. 1996;19:312-318.  [PubMed]  [DOI]
23.  Madsen-Bouterse SA, Kowluru RA. Oxidative stress and diabetic retinopathy: pathophysiological mechanisms and treatment perspectives. Rev Endocr Metab Disord. 2008;9:315-327.  [PubMed]  [DOI]
24.  Rungger-Brändle E, Dosso AA, Leuenberger PM. Glial reactivity, an early feature of diabetic retinopathy. Invest Ophthalmol Vis Sci. 2000;41:1971-1980.  [PubMed]  [DOI]
25.  Zeng XX, Ng YK, Ling EA. Neuronal and microglial response in the retina of streptozotocin-induced diabetic rats. Vis Neurosci. 2000;17:463-471.  [PubMed]  [DOI]
26.  Barber AJ, Antonetti DA, Kern TS, Reiter CE, Soans RS, Krady JK, Levison SW, Gardner TW, Bronson SK. The Ins2Akita mouse as a model of early retinal complications in diabetes. Invest Ophthalmol Vis Sci. 2005;46:2210-2218.  [PubMed]  [DOI]
27.  Krady JK, Basu A, Allen CM, Xu Y, LaNoue KF, Gardner TW, Levison SW. Minocycline reduces proinflammatory cytokine expression, microglial activation, and caspase-3 activation in a rodent model of diabetic retinopathy. Diabetes. 2005;54:1559-1565.  [PubMed]  [DOI]
28.  Suzumura A, Sawada M, Yamamoto H, Marunouchi T. Effects of colony stimulating factors on isolated microglia in vitro. J Neuroimmunol. 1990;30:111-120.  [PubMed]  [DOI]
29.  Basu A, Krady JK, Enterline JR, Levison SW. Transforming growth factor beta1 prevents IL-1beta-induced microglial activation, whereas TNFalpha- and IL-6-stimulated activation are not antagonized. Glia. 2002;40:109-120.  [PubMed]  [DOI]
30.  Sayyah M, Javad-Pour M, Ghazi-Khansari M. The bacterial endotoxin lipopolysaccharide enhances seizure susceptibility in mice: involvement of proinflammatory factors: nitric oxide and prostaglandins. Neuroscience. 2003;122:1073-1080.  [PubMed]  [DOI]
31.  Adamis AP, D'Amato RJ. Shedding light on diabetic retinopathy. Ophthalmology. 1995;102:1127-1128.  [PubMed]  [DOI]
32.  Venters HD, Tang Q, Liu Q, VanHoy RW, Dantzer R, Kelley KW. A new mechanism of neurodegeneration: a proinflammatory cytokine inhibits receptor signaling by a survival peptide. Proc Natl Acad Sci USA. 1999;96:9879-9884.  [PubMed]  [DOI]
33.  Shen WH, Zhou JH, Broussard SR, Johnson RW, Dantzer R, Kelley KW. Tumor necrosis factor alpha inhibits insulin-like growth factor I-induced hematopoietic cell survival and proliferation. Endocrinology. 2004;145:3101-3105.  [PubMed]  [DOI]
34.  Gardner TW, Antonetti DA, Barber AJ, LaNoue KF, Levison SW. Diabetic retinopathy: more than meets the eye. Surv Ophthalmol. 2002;47 Suppl 2:S253-S262.  [PubMed]  [DOI]
35.  Wang AL, Yu AC, He QH, Zhu X, Tso MO. AGEs mediated expression and secretion of TNF alpha in rat retinal microglia. Exp Eye Res. 2007;84:905-913.  [PubMed]  [DOI]
36.  Schalkwijk CG. Comment on “AGEs mediated expression and secretion of TNF alpha in rat retinal microglia" by Dr Wang et al. Exp Eye Res. 2007;85:572-573; author reply 574.  [PubMed]  [DOI]
37.  Quan Y, Du J, Wang X. High glucose stimulates GRO secretion from rat microglia via ROS, PKC, and NF-kappaB pathways. J Neurosci Res. 2007;85:3150-3159.  [PubMed]  [DOI]
38.  Wang AL, Yu AC, Lau LT, Lee C, Wu le M, Zhu X, Tso MO. Minocycline inhibits LPS-induced retinal microglia activation. Neurochem Int. 2005;47:152-158.  [PubMed]  [DOI]
39.  Liou GI, Auchampach JA, Hillard CJ, Zhu G, Yousufzai B, Mian S, Khan S, Khalifa Y. Mediation of cannabidiol anti-inflammation in the retina by equilibrative nucleoside transporter and A2A adenosine receptor. Invest Ophthalmol Vis Sci. 2008;49:5526-5531.  [PubMed]  [DOI]
40.  El-Remessy AB, Tang Y, Zhu G, Matragoon S, Khalifa Y, Liu EK, Liu JY, Hanson E, Mian S, Fatteh N. Neuroprotective effects of cannabidiol in endotoxin-induced uveitis: critical role of p38 MAPK activation. Mol Vis. 2008;14:2190-2203.  [PubMed]  [DOI]
41.  Collis MG, Hourani SM. Adenosine receptor subtypes. Trends Pharmacol Sci. 1993;14:360-366.  [PubMed]  [DOI]
42.  Hask G, Cronstein BN. Adenosine: an endogenous regulator of innate immunity. Trends Immunol. 2004;25:33-39.  [PubMed]  [DOI]
43.  Hask G, Pacher P, Vizi ES, Illes P. Adenosine receptor signaling in the brain immune system. Trends Pharmacol Sci. 2005;26:511-516.  [PubMed]  [DOI]
44.  Fredholm BB, IJzerman AP, Jacobson KA, Klotz KN, Linden J. International Union of Pharmacology. XXV. Nomenclature and classification of adenosine receptors. Pharmacol Rev. 2001;53:527-552.  [PubMed]  [DOI]
45.  Ralevic V, Burnstock G. Receptors for purines and pyrimidines. Pharmacol Rev. 1998;50:413-492.  [PubMed]  [DOI]
46.  Bong GW, Rosengren S, Firestein GS. Spinal cord adenosine receptor stimulation in rats inhibits peripheral neutrophil accumulation. The role of N-methyl-D-aspartate receptors. J Clin Invest. 1996;98:2779-2785.  [PubMed]  [DOI]
47.  Ohta A, Sitkovsky M. Role of G-protein-coupled adenosine receptors in downregulation of inflammation and protection from tissue damage. Nature. 2001;414:916-920.  [PubMed]  [DOI]
48.  Awad AS, Huang L, Ye H, Duong ET, Bolton WK, Linden J, Okusa MD. Adenosine A2A receptor activation attenuates inflammation and injury in diabetic nephropathy. Am J Physiol Renal Physiol. 2006;290:F828-F837.  [PubMed]  [DOI]
49.  Cheng HC, Shih HM, Chern Y. Essential role of cAMP-response element-binding protein activation by A2A adenosine receptors in rescuing the nerve growth factor-induced neurite outgrowth impaired by blockage of the MAPK cascade. J Biol Chem. 2002;277:33930-33942.  [PubMed]  [DOI]
50.  Cronstein BN. Adenosine, an endogenous anti-inflammatory agent. J Appl Physiol. 1994;76:5-13.  [PubMed]  [DOI]
51.  Eltzschig HK, Weissmüller T, Mager A, Eckle T. Nucleotide metabolism and cell-cell interactions. Methods Mol Biol. 2006;341:73-87.  [PubMed]  [DOI]
52.  Heese K, Fiebich BL, Bauer J, Otten U. Nerve growth factor(NGF) expression in rat microglia is induced by adenosine A2a-receptors. Neurosci Lett. 1997;231:83-86.  [PubMed]  [DOI]
53.  Williams M. Challenges in developing P2 purinoceptor-based therapeutics. Ciba Found Symp. 1996;198:309-321.  [PubMed]  [DOI]
54.  Möser GH, Schrader J, Deussen A. Turnover of adenosine in plasma of human and dog blood. Am J Physiol. 1989;256:C799-C806.  [PubMed]  [DOI]
55.  Eigler A, Greten TF, Sinha B, Haslberger C, Sullivan GW, Endres S. Endogenous adenosinecurtails lipopolysaccharide-stimulated tumour necrosis factor synthesis. Scand J Immunol. 1997;45:132-139.  [PubMed]  [DOI]
56.  Johnson SM, Patel S, Bruckner FE, Collins DA. 5'-Nucleotidase as a marker of both general and local inflammation in rheumatoid arthritis patients. Rheumatology (Oxford). 1999;38:391-396.  [PubMed]  [DOI]
57.  Baldwin SA, Mackey JR, Cass CE, Young JD. Nucleoside transporters: molecular biology and implications for therapeutic development. Mol Med Today. 1999;5:216-224.  [PubMed]  [DOI]
58.  Noji T, Nan-ya K, Mizutani M, Katagiri C, Sano J, Takada C, Nishikawa S, Karasawa A, Kusaka H. KF24345, an adenosine uptake inhibitor, ameliorates the severity and mortality of lethal acute pancreatitis via endogenous adenosine in mice. Eur J Pharmacol. 2002;454:85-93.  [PubMed]  [DOI]
59.  Noji T, Nan-ya K, Mizutani M, Katagiri C, Sano J, Takada C, Nishikawa S, Karasawa A, Kusaka H. KF24345, an adenosine uptake inhibitor, ameliorates the severity and mortality of lethal acute pancreatitis via endogenous adenosine in mice. Eur J Pharmacol. 2002;454:85-93.  [PubMed]  [DOI]
60.  Carrier EJ, Auchampach JA, Hillard CJ. Inhibition of an equilibrative nucleoside transporter by cannabidiol: a mechanism of cannabinoid immunosuppression. Proc Natl Acad Sci USA. 2006;103:7895-7900.  [PubMed]  [DOI]
61.  Dunwiddie TV, Diao L. Regulation of extracellular adenosine in rat hippocampal slices is temperature dependent: role of adenosine transporters. Neuroscience. 2000;95:81-88.  [PubMed]  [DOI]
62.  Picano E, Michelassi C. Chronic oral dipyridamole as a ‘novel' antianginal drug: the collateral hypothesis. Cardiovasc Res. 1997;33:666-670.  [PubMed]  [DOI]
63.  De Schryver EL. Dipyridamole in stroke prevention: effect of dipyridamole on blood pressure. Stroke. 2003;34:2339-2342.  [PubMed]  [DOI]
64.  De la Cruz JP, Moreno A, Mu?oz M, Garca Campos JM, Snchez de la Cuesta F. Effect of aspirin plus dipyridamole on the retinal vascular pattern in experimental diabetes mellitus. J Pharmacol Exp Ther. 1997;280:454-459.  [PubMed]  [DOI]
65.  Leung GP, Man RY, Tse CM. D-Glucose upregulates adenosine transport in cultured human aortic smooth muscle cells. Am J Physiol Heart Circ Physiol. 2005;288:H2756-H2762.  [PubMed]  [DOI]
66.  Howlett AC, Barth F, Bonner TI, Cabral G, Casellas P, Devane WA, Felder CC, Herkenham M, Mackie K, Martin BR. International Union of Pharmacology. XXVII. Classification of cannabinoid receptors. Pharmacol Rev. 2002;54:161-202.  [PubMed]  [DOI]
67.  Belgrave BE, Bird KD, Chesher GB, Jackson DM, Lubbe KE, Starmer GA, Teo RK. The effect of cannabidiol, alone and in combination with ethanol, on human performance. Psychopharmacology (Berl). 1979;64:243-246.  [PubMed]  [DOI]
68.  Razdan RK. Structure-activity relationships in cannabinoids. Pharmacol Rev. 1986;38:75-149.  [PubMed]  [DOI]
69.  Kaminski NE. Regulation of the cAMP cascade, gene expression and immune function by cannabinoid receptors. J Neuroimmunol. 1998;83:124-132.  [PubMed]  [DOI]
70.  Buckley NE, McCoy KL, Mezey E, Bonner T, Zimmer A, Felder CC, Glass M, Zimmer A. Immunomodulation by cannabinoids is absent in mice deficient for the cannabinoid CB(2) receptor. Eur J Pharmacol. 2000;396:141-149.  [PubMed]  [DOI]
71.  Hampson AJ, Grimaldi M, Axelrod J, Wink D. Cannabidiol and (-)Delta9-tetrahydrocannabinol are neuroprotective antioxidants. Proc Natl Acad Sci USA. 1998;95:8268-8273.  [PubMed]  [DOI]
72.  Matsuda LA, Lolait SJ, Brownstein MJ, Young AC, Bonner TI. Structure of a cannabinoid receptor and functional expression of the cloned cDNA. Nature. 1990;346:561-564.  [PubMed]  [DOI]
73.  Straiker A, Stella N, Piomelli D, Mackie K, Karten HJ, Maguire G. Cannabinoid CB1 receptors and ligands in vertebrate retina: localization and function of an endogenous signaling system. Proc Natl Acad Sci USA. 1999;96:14565-14570.  [PubMed]  [DOI]
74.  Yazulla S, Studholme KM, McIntosh HH, Deutsch DG. Immunocytochemical localization of cannabinoid CB1 receptor and fatty acid amide hydrolase in rat retina. J Comp Neurol. 1999;415:80-90.  [PubMed]  [DOI]
75.  Munro S, Thomas KL, Abu-Shaar M. Molecular characterization of a peripheral receptor for cannabinoids. Nature. 1993;365:61-65.  [PubMed]  [DOI]
76.  Walter L, Franklin A, Witting A, Wade C, Xie Y, Kunos G, Mackie K, Stella N. Nonpsychotropic cannabinoid receptors regulate microglial cell migration. J Neurosci. 2003;23:1398-1405.  [PubMed]  [DOI]
77.  Lu Q, Straiker A, Lu Q, Maguire G. Expression of CB2 cannabinoid receptor mRNA in adult rat retina. Vis Neurosci. 2000;17:91-95.  [PubMed]  [DOI]
78.  Fowler CJ. Plant-derived, synthetic and endogenous cannabinoids as neuroprotective agents. Non-psychoactive cannabinoids, ‘entourage' compounds and inhibitors of N-acyl ethanolamine breakdown as therapeutic strategies to avoid pyschotropic effects. Brain Res Brain Res Rev. 2003;41:26-43.  [PubMed]  [DOI]
79.  Caulfield MP, Brown DA. Cannabinoid receptor agonists inhibit Ca current in NG108-15 neuroblastoma cells via a pertussis toxin-sensitive mechanism. Br J Pharmacol. 1992;106:231-232.  [PubMed]  [DOI]
80.  Deadwyler SA, Hampson RE, Bennett BA, Edwards TA, Mu J, Pacheco MA, Ward SJ, Childers SR. Cannabinoids modulate potassium current in cultured hippocampal neurons. Receptors Channels. 1993;1:121-134.  [PubMed]  [DOI]
81.  Felder CC, Joyce KE, Briley EM, Mansouri J, Mackie K, Blond O, Lai Y, Ma AL, Mitchell RL. Comparison of the pharmacology and signal transduction of the human cannabinoid CB1 and CB2 receptors. Mol Pharmacol. 1995;48:443-450.  [PubMed]  [DOI]
82.  Showalter VM, Compton DR, Martin BR, Abood ME. Evaluation of binding in a transfected cell line expressing a peripheral cannabinoid receptor (CB2): identification of cannabinoid receptor subtype selective ligands. J Pharmacol Exp Ther. 1996;278:989-999.  [PubMed]  [DOI]
83.  Malfait AM, Gallily R, Sumariwalla PF, Malik AS, Andreakos E, Mechoulam R, Feldmann M. The nonpsychoactive cannabis constituent cannabidiol is an oral anti-arthritic therapeutic in murine collagen-induced arthritis. Proc Natl Acad Sci USA. 2000;97:9561-9566.  [PubMed]  [DOI]
84.  Braida D, Pegorini S, Arcidiacono MV, Consalez GG, Croci L, Sala M. Post-ischemic treatment with cannabidiol prevents electroencephalographic flattening, hyperlocomotion and neuronal injury in gerbils. Neurosci Lett. 2003;346:61-64.  [PubMed]  [DOI]
85.  Mishima K, Hayakawa K, Abe K, Ikeda T, Egashira N, Iwasaki K, Fujiwara M. Cannabidiol prevents cerebral infarction via a serotonergic 5-hydroxytryptamine1A receptor-dependent mechanism. Stroke. 2005;36:1077-1082.  [PubMed]  [DOI]
86.  Cunha JM, Carlini EA, Pereira AE, Ramos OL, Pimentel C, Gagliardi R, Sanvito WL, Lander N, Mechoulam R. Chronic administration of cannabidiol to healthy volunteers and epileptic patients. Pharmacology. 1980;21:175-185.  [PubMed]  [DOI]
87.  Barnes MP. Sativex: clinical efficacy and tolerability in the treatment of symptoms of multiple sclerosis and neuropathic pain. Expert Opin Pharmacother. 2006;7:607-615.  [PubMed]  [DOI]
88.  Rajesh M, Mukhopadhyay P, Btkai S, Hask G, Liaudet L, Drel VR, Obrosova IG, Pacher P. Cannabidiol attenuates high glucose-induced endothelial cell inflammatory response and barrier disruption. Am J Physiol Heart Circ Physiol. 2007;293:H610-H619.  [PubMed]  [DOI]
89.  Weiss L, Zeira M, Reich S, Har-Noy M, Mechoulam R, Slavin S, Gallily R. Cannabidiol lowers incidence of diabetes in non-obese diabetic mice. Autoimmunity. 2006;39:143-151.  [PubMed]  [DOI]
90.  Liou GI, El-Remessy AB, Ibrahim AS, Caldwell RB, Khalifa YM, Gunes A, Nussbaum JJ. Cannabidiol as a Putative Novel Therapy for Diabetic Retinopathy: A Postulated Mechanism of Action as an Entry Point for Biomarker-Guided Clinical Development. Current Pharmacogenomics and Personalized Medicine. 2009;7:215-222.  [PubMed]  [DOI]