Differences in endothelium-dependent cerebral dilation by bradykinin and acetylcholine

Recent Advancements in Nanodiamond Mediated Brain Targeted Drug Delivery and Bioimaging of Brain Ailments: A Holistic Review

BACKGROUND

The brain is a vital and composite organ. By nature, the innate make-up of the brain is such that in anatomical parlance, it is highly protected by the "Blood-Brain Barrier", which is a nexus of capillary endothelial cells, basement membrane, neuroglial membrane and glialpodocytes. The same barrier, which protects and isolates the interstitial fluid of the brain from capillary circulation, also restricts the therapeutic intervention. Many standing pharmaceutical formulations are ineffective in the treatment of inimical brain ailments because of the inability of the API to surpass and subsist inside the Blood Brain Barrier.

OBJECTIVE

This is an integrated review that emphasizes on the recent advancements in brain-targeted drug delivery utilizing nanodiamonds (NDs) as a carrier of therapeutic agents. NDs are a novel nanoparticulate drug delivery system, having carbon moieties as their building blocks and their surface tenability is remarkable. These neoteric carbon-based carriers have exceptional, mechanical, electrical, chemical, optical, and biological properties, which can be further rationally modified and augmented.

CONCLUSION

NDs could be the next"revolution "in the field of nanoscience for the treatment of neurodegenerative disorders, brain tumors, and other pernicious brain ailments. What sets them apart from other nanocarriers is their versatile properties like diverse size range and surface modification potential, which makes them efficient enough to move across certain biological barriers and offer a plethora of brain targeting and bioimaging abilities. Lay Summary: The blood-brain barrier (BBB) poses a major hurdle in the way of treating many serious brain ailments. A range of nanoparticle based drug delivering systems have been formulated, including solid lipid nanoparticles, liposomes, dendrimers, nanogels, polymeric NPs, metallic NPs (gold, platinum, andironoxide) and diamondoids (carbonnanotubes). Despite this development, only a few of these formulations have shown the ability to cross the BBB. Nanodiamonds, because of their small size, shape, and surface characteristics, have a potential in moving beyond the diverse and intricate BBB, and offer a plethora of brain targeting capabilities.

Cerebral Small Vessel Disease: Targeting Oxidative Stress as a Novel Therapeutic Strategy?

Cerebral small vessel disease (SVD) is a major contributor to stroke, and a leading cause of cognitive impairment and dementia. Despite the devastating effects of cerebral SVD, the pathogenesis of cerebral SVD is still not completely understood. Moreover, there are no specific pharmacological strategies for its prevention or treatment. Cerebral SVD is characterized by marked functional and structural abnormalities of the cerebral microcirculation. The clinical manifestations of these pathological changes include lacunar infarcts, white matter hyperintensities, and cerebral microbleeds. The main purpose of this review is to discuss evidence implicating oxidative stress in the arteriopathy of both non-amyloid and amyloid (cerebral amyloid angiopathy) forms of cerebral SVD and its most important risk factors (hypertension and aging), as well as its contribution to cerebral SVD-related brain injury and cognitive impairment. We also highlight current evidence of the involvement of the NADPH oxidases in the development of oxidative stress, enzymes that are a major source of reactive oxygen species in the cerebral vasculature. Lastly, we discuss potential pharmacological strategies for oxidative stress in cerebral SVD, including some of the historical and emerging NADPH oxidase inhibitors.

The effects of obesity on the cerebral vasculature

The incidence of obesity in the population is increasing at an alarming rate, with this comes an increased risk of insulin resistance (IR). Obesity and IR increase an individual's risk of having a stroke and they have been linked to several forms of dementia. Stroke and dementia are associated with, or exacerbated by, reduced cerebral blood flow, which has recently been described in obese patients. In this review we will discuss the effects of obesity on cerebral artery function and structure. Regarding their function, we will focus on the endothelium and nitric oxide (NO) dependent dilation. NO dependent dilation is impaired in cerebral arteries from obese rats, and the majority of evidence suggests this is a result of increased oxidative stress. We will also describe the limited studies showing that inward cerebral artery remodeling occurs in models of obesity, and that the remodeling is associated with an increase in the damage caused by cerebral ischemia. We will also discuss some of the more paradoxical findings associated with stroke and obesity, including the evidence that obesity is a positive factor for stroke survival. Finally we will discuss the evidence that links these changes in vascular structure and function to cognitive decline and dementia.

Chronic renal failure alters endothelial function in cerebral circulation in mice

We examined structure, composition, and endothelial function in cerebral arterioles after 4 wk of chronic renal failure (CRF) in a well-defined murine model (C57BL/6J and apolipoprotein E knockout female mice). We also determined quantitative expression of endothelial nitric oxide synthase (eNOS), phosphorylated eNOS (on serine 1177 and threonine 495), and caveolin-1; quantitative expression of markers of vascular inflammation or oxidative stress [Rock-1, Rock-2, VCAM-1, and peroxisome proliferator-activated receptor-γ (PPARγ)]; and the plasma concentration of l-arginine and asymmetric dimethylarginine (ADMA). Our hypothesis was that endothelial function would be impaired in cerebral arterioles during CRF following either a decrease in NO production (through alteration of eNOS expression or regulation) or an increase in NO degradation (due to oxidative stress or vascular inflammation). Endothelium-dependent relaxation was impaired during CRF, but endothelium-independent relaxation was not. CRF had no effect on cerebral arteriolar structure and composition. Quantitative expressions of eNOS, eNOS phosphorylated on serine 1177, caveolin-1, Rock-1, Rock-2, and VCAM-1 were similar in CRF and non-CRF mice. In contrast, quantitative expression of PPARγ (which exercises a protective role on blood vessels) was significantly lower in CRF mice, whereas quantitative expression of eNOS phosphorylated on the threonine 495 (the inactive form of eNOS) was significantly higher. Lastly, the plasma concentration of ADMA (a uremic toxin and an endogenous inhibitor of eNOS) was elevated and plasma concentration of l-arginine was low in CRF. In conclusion, endothelial function is impaired in a mouse model of early stage CRF. These alterations may be related (at least in part) to a decrease in NO production.

Endogenous adipose-derived factors diminish coronary endothelial function via inhibition of nitric oxide synthase

Adipocytokines may be the molecular link between obesity and vascular disease. However, the effects of these factors on coronary vascular function have not been discerned. Accordingly, the goal of this investigation was to delineate the mechanisms by which endogenous adipose-derived factors affect coronary vascular endothelial function. Both isolated canine coronary arteries and coronary blood flow in anesthetized dogs were studied with and without exposure to adipose tissue. Infusion of adipose-conditioned buffer directly into the coronary circulation did not change baseline hemodynamics; however, endothelial-dependent vasodilation to bradykinin was impaired both in vitro and in vivo. Coronary vasodilation to sodium nitroprusside was unaltered by adipose tissue. Oxygen radical formation did not cause the impairment because quantified dihydroethidium staining was decreased by adipose tissue and neither a superoxide dismutase mimetic nor catalase improved endothelial function. Inhibition of nitric oxide (NO) synthase with L-NAME diminished bradykinin-mediated relaxations and eliminated the subsequent vascular effects of adipose tissue. In vitro measurement of NO demonstrated that adipose tissue exposure quickly lowered baseline NO and abolished bradykinin-induced NO production. The results indicate that adipose tissue releases factor(s) that selectively impair endothelial-dependent dilation via inhibition of NO synthase-mediated NO production.

Oxidant and redox signaling in vascular oxygen sensing mechanisms: basic concepts, current controversies, and potential importance of cytosolic NADPH

Vascular smooth muscle (VSM) derived from pulmonary arteries generally contract to hypoxia, whereas VSM from systemic arteries usually relax, indicating the presence of basic oxygen-sensing mechanisms in VSM that are adapted to the environment from which they are derived. This review considers how fundamental processes associated with the generation of reactive oxygen species (ROS) by oxidase enzymes, the metabolic control of cytosolic NADH, NADPH and glutathione redox systems, and mitochondrial function interact with signaling systems regulating vascular force in a manner that is potentially adapted to be involved in Po2 sensing. Evidence for opposing hypotheses of hypoxia, either decreasing or increasing mitochondrial ROS, is considered together with the Po2 dependence of ROS production by Nox oxidases as sensors potentially contributing to hypoxic pulmonary vasoconstriction. Processes through which ROS and NAD(P)H redox changes potentially control interactive signaling systems, including soluble guanylate cyclase, potassium channels, and intracellular calcium are discussed together with the data supporting their regulation by redox in responses to hypoxia. Evidence for hypothesized potential differences between systemic and pulmonary arteries originating from properties of mitochondrial ROS generation and the redox sensitivity of potassium channels is compared with a new hypothesis in which differences in the control of cytosolic NADPH redox by the pentose phosphate pathway results in increased NADPH and Nox oxidase-derived ROS in pulmonary arteries, whereas lower levels of glucose-6-phosphate dehydrogenase in coronary arteries may permit hypoxia to activate a vasodilator mechanism controlled by oxidation of cytosolic NADPH.

Hydrogen peroxide acts as an EDHF in the piglet pial vasculature in response to bradykinin

We investigated the mechanism of EDHF-mediated dilation to bradykinin (BK) in piglet pial arteries. Topically applied BK (3 micromol/l) induced vasodilation (62 +/- 12%) after the administration of N(omega)-nitro-L-arginine methyl ester (L-NAME) and indomethacin, which was inhibited by endothelial impairment or by the BK(2) receptor antagonist HOE-140 (0.3 micromol/l). Western blotting showed the presence of BK(2) receptors in brain cortex and pial vascular tissue samples. The cytochrome P-450 antagonist miconazole (20 micromol/l) and the lipoxygenase inhibitors baicalein (10 micromol/l) and cinnamyl-3,4-dyhydroxy-alpha-cyanocinnamate (1 micromol/l) failed to reduce the BK-induced dilation. However, the H(2)O(2) scavenger catalase (400 U/ml) abolished the response (from 54 +/- 11 to 0 +/- 2 microm; P < 0.01). The ATP-dependent K(+) (K(ATP)) channel inhibitor glibenclamide (10 micromol/l) had a similar effect as well (from 54 +/- 11 to 16 +/- 5 microm; P < 0.05). Coapplication of the Ca(2+)-dependent K(+) channel inhibitors charybdotoxin (0.1 micromol/l) and apamin (0.5 micromol/l) failed to reduce the response. We conclude that H(2)O(2) mediates the non-nitric oxide-, non-prostanoid-dependent vasorelaxation to BK in the piglet pial vasculature. The response is mediated via BK(2) receptors and the opening of K(ATP) channels.

Compensatory role of NO in cerebral circulation of piglets chronically treated with indomethacin

We hypothesize that inhibitory effects exist between prostanoids and nitric oxide (NO) in their contributions to cerebral circulation. Piglets (1-4 days old) were divided into three chronically treated (6-8 days) groups: control piglets, piglets treated with indomethacin (75 mg/day), and piglets treated with N(omega)-nitro-L-arginine methyl ester (L-NAME, 100 mg x kg(-1) x day(-1)). Pial arterioles dilated in response to hypercapnia similarly among the three groups (41 +/- 4, 40 +/- 6, and 45 +/- 11%). Cerebrospinal fluid cAMP increased in control piglets, while cGMP increased in indomethacin-treated piglets. L-NAME, but not 7-nitroindazole, inhibited the response to hypercapnia only in indomethacin-treated piglets (40 +/- 6 vs. 17 +/- 5%). Topical sodium nitroprusside or iloprost restored dilation in response to hypercapnia. Similar results were obtained when the dilator was bradykinin. Pial arterioles of control and L-NAME-treated piglets constricted in response to ACh (-24 +/- 3%). However, those of indomethacin-treated piglets dilated in response to ACh (15 +/- 2%). This dilation was inhibited by L-NAME. NO synthase activity, but not endothelial NO synthase expression, increased after chronic indomethacin treatment. These data suggest that chronic inhibition of cyclooxygenase can increase the contribution of NO to cerebrovascular circulatory control in piglets.

Effects of NADH and NADPH on superoxide levels and cerebral vascular tone

Reactive oxygen species are important modulators of cerebral vascular tone. Recent evidence, mainly from the aorta, suggests that NAD(P)H oxidase is a major source of vascular superoxide. The goal of the present study was to examine the effects of NADH and NADPH that are commonly used to stimulate NAD(P)H oxidase activity, on superoxide levels and cerebral vascular tone. Basilar arteries and cerebral arterioles from normal rabbits were studied in vitro using isolated tissue baths and in vivo using a cranial window, respectively. In the basilar artery, NADH produced a biphasic response; low concentrations (0.1-10 microM NADH) produced marked relaxation, whereas higher concentrations (30-100 microM NADH) produced contraction. Responses to NADH were significantly (P < 0.05) inhibited in the presence of 4,5-dihydroxy-1,3-benzene-disulfonic acid (Tiron; a scavenger of superoxide, 10 mM). In contrast, NADPH (10-100 microM) produced moderate contraction of the basilar artery, which was inhibited in the presence of Tiron. In vivo, NADH produced Tiron-sensitive dilatation of cerebral arterioles. NADH and NADPH dose dependently increased superoxide levels in the basilar artery, as detected by lucigenin (5 microM)-enhanced chemiluminescence, but increases in superoxide were significantly greater for NADPH than NADH. These increases in superoxide were markedly reduced in the presence of polyethylene glycol-superoxide dismutase (300 U/ml) or diphenylene iodonium [0.1 mM, an inhibitor of flavin-containing enzymes, including NAD(P)H oxidase] but were not affected by indomethacin, N(G)-nitro-L-arginine, or allopurinol. These data suggest that NADH- and NADPH-induced changes in cerebral vascular tone are mediated by superoxide, produced by a flavin-containing enzyme, most likely NAD(P)H oxidase, but not xanthine oxidase or nitric oxide synthase.

Endothelial NO and prostanoid involvement in newborn and juvenile pig pial arteriolar vasomotor responses

Specific cerebrovascular dilatory responses in newborn piglets are entirely prostanoid dependent, but require both nitric oxide (NO) and prostanoids in juveniles. We examined endothelial dependency and mechanisms of NO- and prostanoid-mediated cerebrovascular responses in anesthetized newborn and juvenile pigs implanted with closed cranial windows. Light/dye endothelial injury inhibited newborn and juvenile hypercapnic and bradykinin (BK) responses and inhibited dilation to acetylcholine in juveniles. Iloprost and NO act permissively in restoring light/dye inhibited newborn and juvenile responses, respectively. Differences in sensitivity to iloprost and sodium nitroprusside were not observed. Juvenile (not newborn) hypercapnic and BK cerebrovascular responses were sensitive to soluble guanylyl cyclase inhibition. Pial arteriolar diameter and cortical production of prostacyclin, cAMP, and cGMP in response to BK were measured under control conditions, after treatment with indomethacin and/or N(omega)-nitro-L-arginine methyl ester (L-NAME). Indomethacin inhibited BK responses in newborns. Juvenile responses were inhibited by L-NAME, and mildly by indomethacin. Cortical 6-keto-PGF(1 alpha), cAMP, and cGMP increased in response to BK in both age groups. Newborn cerebrovascular responses are largely NO independent, but NO becomes more important with maturation.

COX-2-dependent delayed dilatation of cerebral arterioles in response to bradykinin

Bradykinin (BK) is released in the brain during injury and inflammation. Activation of endothelial BK receptors produces acute dilatation of cerebral arterioles that is mediated by reactive oxygen species (ROS). ROS can also modulate gene expression, including expression of the inducible isoform of cyclooxygenase (COX-2). We hypothesized that exposure of the brain to BK would produce acute dilatation, which would be followed by a delayed dilatation mediated by COX-2. To test this hypothesis in anesthetized rats, BK was placed twice in cranial windows for 7 min, after which the windows were flushed to remove residual BK. The two BK exposures were separated by 30 min. Each BK exposure produced acute dilatation of cerebral arterioles, after which diameter rapidly returned to baseline. Over the subsequent 4.5 h after the second BK exposure, arterioles dilated 48 +/- 8%. Treatment of the cranial window with NS-398, a selective COX-2 inhibitor, or dexamethasone, significantly attenuated the delayed dilatation. Aminoguanidine, a selective inhibitor of inducible nitric oxide synthase, did not alter the delayed dilatation. Cotreatment of cranial windows with BK, superoxide dismutase, and catalase also prevented the delayed dilatation. In separate experiments, exposure of the cortical surface to BK upregulated leptomeningeal expression of COX-2 mRNA. Our results suggest that acute, time-limited exposure of the brain to BK produces delayed dilatation of cerebral arterioles dependent on expression and activity of COX-2.

Acute effects of bradykinin on cerebral microvascular permeability in the anaesthetized rat

1. The permeability response to acutely applied bradykinin and [des-Arg9]-bradykinin on single cerebral venular capillaries has been investigated using the low molecular mass fluorescent dyes Lucifer Yellow and Sulforhodamine B with the single vessel occlusion technique. 2. When bradykinin was applied repeatedly for up to 2 h, the permeability increase was small and reversible for concentrations that ranged from 5 nM to 50 microM. 3. The logEC50 of the permeability response to bradykinin was -5.3 +/- 0.15 (logM; mean +/- s.e.m.). This was reduced to -6.37 +/- 0.24 with the angiotensin-converting enzyme inhibitor captopril, to -6.33 +/- 0.19 with the neutral endopeptidase inhibitor phosphoramidon and to -7.3 +/- 0.20 with captopril and phosphoramidon combined. 4. The permeability response to bradykinin was blocked by the bradykinin B2 receptor antagonist HOE 140, by inhibition of the Ca2+-independent phospholipase A2, by the scavenging of free radicals, or by inhibition of both cyclo-oxygenase and lipoxygenase in combination. Block of Ca2+ entry channels with SKF 96365 had no effect on the response. 5. Application of [des-Arg9]-bradykinin also increased permeability over the concentration range 5 nM to 50 microM, with a logEC50 of -5.6 +/- 0. 37. This response was not affected by free radical scavenging, but was completely blocked by the histamine H2 receptor blocker cimetidine. 6. These results imply that the acute permeability response to bradykinin is mediated via the release of arachidonic acid, which is acted on by cyclo-oxygenase and lipoxygenase resulting in the formation of free radicals, and that the response to [des-Arg9]-bradykinin is mediated via histamine.

Reinforcement of arteriolar myogenic activity by endogenous ANG II: susceptibility to dietary salt

The purpose of this study was to determine whether endogenous ANG II augments arteriolar myogenic behavior in striated muscle. Because circulating ANG II is decreased during high salt intake, we also investigated whether dietary salt could alter any influence of ANG II on myogenic behavior. Normotensive rats fed low-salt (0.45%, LS) or high-salt (7%, HS) diets were enclosed in a ventilated box with the spinotrapezius muscle exteriorized for intravital microscopy. Dietary salt did not affect resting arteriolar diameters. Microvascular pressure elevation by box pressurization caused greater arteriolar constriction in LS rats (up to 12 microm) than in HS rats (up to 4 microm). The ANG II-receptor antagonists saralasin and losartan attenuated myogenic responsiveness in LS rats but not HS rats. The bradykinin-receptor antagonist HOE-140 had no effect on myogenic responsiveness in LS rats but augmented myogenic responsiveness in HS rats. HOE-140 with the angiotensin-converting enzyme inhibitor captopril attenuated myogenic responsiveness to a greater extent in LS rats than in HS rats. We conclude that endogenous ANG II normally reinforces arteriolar myogenic behavior in striated muscle and that attenuated myogenic behavior associated with high salt intake is due to decreased circulating ANG II and increased local kinin levels.

Effects of local reduction in pressure on endothelium-dependent responses of cerebral arterioles

BACKGROUND AND PURPOSE

The goal of this study was to examine effects of local reductions in intravascular pressure and dP/dt on endothelium-dependent responses of cerebral arterioles in normotensive Wistar-Kyoto rats (WKY) and stroke-prone spontaneously hypertensive rats (SHRSP).

METHODS

WKY and SHRSP underwent clipping of one carotid artery at 1 month of age. At 6 months of age, responses of pial arterioles were examined in vivo in anesthetized rats. Bilateral craniotomies were performed to expose pial arterioles in the sham-operated and clipped cerebral hemispheres. Pressure (servo-null) was measured in sham-operated and clipped pial arterioles, and arteriolar diameter was measured before and during suffusion with bradykinin, A23187, and nitroprusside.

RESULTS

Carotid clipping normalized pial arteriolar pulse pressure but not mean pressure in SHRSP. Responses of sham-operated pial arterioles to bradykinin and A23187 were less in SHRSP than in WKY. Responses of sham-operated pial arterioles to nitroprusside were greater in SHRSP than in WKY. Carotid clipping in SHRSP normalized responses of pial arterioles to bradykinin but not A23187 and had no effect on responses to nitroprusside.

CONCLUSIONS

These findings suggest that elevated intravascular pressure per se may contribute to impairment of endothelium-dependent relaxation to at least some agonists in cerebral arterioles during chronic hypertension. Furthermore, the findings lead us to speculate that arteriolar pulse pressure may play a more important role than mean pressure in development of impaired endothelium dilatation during chronic hypertension.

Nitric oxide and the cerebral circulation

BACKGROUND

Nitric oxide (NO) is a potent vasodilator that was initially described as the mediator of endothelium-dependent relaxation (endothelium-derived relaxing factor, EDRF). It is now known that NO is produced by a variety of other cell types.

SUMMARY OF REVIEW

Endothelium produces NO (EDRF) under basal conditions and in response to a variety of vasoactive stimuli in large cerebral arteries and the cerebral microcirculation. Endothelium-dependent relaxation is impaired in the presence of several pathophysiological conditions. This impairment may contribute to cerebral ischemia or stroke. Activation of glutamate receptors appears to be a major stimulus for production of NO by neurons. Neuronally derived NO may mediate local increases in cerebral blood flow during increases in cerebral metabolism. NO synthase-containing neurons also innervate large cerebral arteries and cerebral arterioles on the brain surface. Activation of parasympathetic fibers that innervate cerebral vessels produces NO-dependent increases in cerebral blood flow. Increases in cerebral blood flow during hypercapnia also appear to be dependent on production of NO. Astrocytes may release some NO constitutively, but astrocytes and microglia can release relatively large quantities of NO after induction of NO synthase in response to endotoxin or some cytokines. Expression of inducible NO synthase, perhaps in response to local production of cytokines, may exert cytotoxic effects in brain during or after ischemia.

CONCLUSIONS

Because endothelium, neurons, and glia can all produce NO in response to some stimuli, the influence of NO on the cerebral circulation appears to be very important. Under normal conditions, constitutively produced NO influences basal cerebral vascular tone and mediates vascular responses to a diverse group of stimuli. The inducible form of NO synthase produces much greater amounts of NO that may be an important mediator of cytotoxicity in brain.

Angiotensin degradation products mediate endothelium-dependent dilation of rabbit brain arterioles

This study demonstrates that the hexapeptide angiotensin II-(3-8) and L-arginine, generated through enzymatic degradation of angiotensin, mediate endothelium-dependent dilation in rabbit brain arterioles. Topical application of angiotensin II (10(-5) M) on the brain surface of anesthetized rabbits caused 21.6 +/- 4.5% (mean +/- SEM) cerebral arteriolar dilation. The cyclooxygenase inhibitor indomethacin did not change this dilation. The natural degradation product of angiotensin II in the brain, angiotensin III, also induced vasodilation at concentrations of 10(-7) to 10(-5) M. The dilation to angiotensin II and angiotensin III was eliminated in the presence of 10(-5) M methylene blue, a known inhibitor of endothelium-dependent vasodilation. Amastatin, an aminopeptidase inhibitor and blocker of enzymatic angiotensin degradation, also inhibited the response to angiotensin II and angiotensin III. The angiotensin fragment angiotensin II-(3-8), which lacks the amino-terminal L-arginine residue of angiotensin III, did not elicit an arteriolar response. When angiotensin II-(3-8) was topically applied subsequent to L-arginine, a 21.2 +/- 2.9% vasodilation was observed. L-Arginine itself induced only moderate vasodilation with a maximum of 4.0 +/- 0.9% at 10(-5) M L-arginine. The dilating response to angiotensin II-(3-8) after L-arginine was inhibited by methylene blue. It was not affected by amastatin. It is concluded that degradation products of angiotensin, rather than angiotensin II itself, induce endothelium-dependent dilation in rabbit brain arterioles without involvement of cyclooxygenase products.(ABSTRACT TRUNCATED AT 250 WORDS)