Amine-induced responses of pial and penetrating cerebral arteries: evidence for heterogeneous responses

Modulatory effects of noradrenergic and serotonergic signaling pathway on neurovascular coupling

Dynamic changes in astrocyte Ca2+ are recognized as contributors to functional hyperemia, a critical response to increased neuronal activity mediated by a process known as neurovascular coupling (NVC). Although the critical role of glutamatergic signaling in this process has been extensively investigated, the impact of behavioral state, and the release of behavior-associated neurotransmitters, such as norepinephrine and serotonin, on astrocyte Ca2+ dynamics and functional hyperemia have received less attention. We used two-photon imaging of the barrel cortex in awake mice to examine the role of noradrenergic and serotonergic projections in NVC. We found that both neurotransmitters facilitated sensory stimulation-induced increases in astrocyte Ca2+. Interestingly, while ablation of serotonergic neurons reduced sensory stimulation-induced functional hyperemia, ablation of noradrenergic neurons caused both attenuation and potentiation of functional hyperemia. Our study demonstrates that norepinephrine and serotonin are involved in modulating sensory stimulation-induced astrocyte Ca2+ elevations and identifies their differential effects in regulating functional hyperemia.

Heterogeneity of Sensory-Induced Astrocytic Ca2+ Dynamics During Functional Hyperemia

Astrocytic Ca²⁺ fluctuations associated with functional hyperemia have typically been measured from large cellular compartments such as the soma, the whole arbor and the endfoot. The most prominent Ca²⁺ event is a large magnitude, delayed signal that follows vasodilation. However, previous work has provided little information about the spatio-temporal properties of such Ca²⁺ transients or their heterogeneity. Here, using an awake, in vivo two-photon fluorescence-imaging model, we performed detailed profiling of delayed astrocytic Ca²⁺ signals across astrocytes or within individual astrocyte compartments using small regions of interest next to penetrating arterioles and capillaries along with vasomotor responses to vibrissae stimulation. We demonstrated that while a 5-s air puff that stimulates all whiskers predominantly generated reproducible functional hyperemia in the presence or absence of astrocytic Ca²⁺ changes, whisker stimulation inconsistently produced astrocytic Ca²⁺ responses. More importantly, these Ca²⁺ responses were heterogeneous among subcellular structures of the astrocyte and across different astrocytes that resided within the same field of view. Furthermore, we found that whisker stimulation induced discrete Ca²⁺ “hot spots” that spread regionally within the endfoot. These data reveal that astrocytic Ca²⁺ dynamics associated with the microvasculature are more complex than previously thought, and highlight the importance of considering the heterogeneity of astrocytic Ca²⁺ activity to fully understanding neurovascular coupling.

Optical imaging and modulation of neurovascular responses

The cerebral microvasculature consists of pial vascular networks, parenchymal descending arterioles, ascending venules and parenchymal capillaries. This vascular compartmentalization is vital to precisely deliver blood to balance continuously varying neural demands in multiple brain regions. Optical imaging techniques have facilitated the investigation of dynamic spatial and temporal properties of microvascular functions in real time. Their combination with transgenic animal models encoding specific genetic targets have further strengthened the importance of optical methods for neurovascular research by allowing for the modulation and monitoring of neuro vascular function. Image analysis methods with three-dimensional reconstruction are also helping to understand the complexity of microscopic observations. Here, we review the compartmentalized cerebral microvascular responses to global perturbations as well as regional changes in response to neural activity to highlight the differences in vascular action sites. In addition, microvascular responses elicited by optical modulation of different cell-type targets are summarized with emphasis on variable spatiotemporal dynamics of microvascular responses. Finally, long-term changes in microvascular compartmentalization are discussed to help understand potential relationships between CBF disturbances and the development of neurodegenerative diseases and cognitive decline.

Beta1-adrenergic receptor-mediated dilation of rat cerebral artery requires Shaker-type KV1 channels on PSD95 scaffold

Postsynaptic density-95 (PSD95) is a scaffolding protein in cerebral vascular smooth muscle cells (cVSMCs), which binds to Shaker-type K(+) (KV1) channels and facilitates channel opening through phosphorylation by protein kinase A. β1-Adrenergic receptors (β1ARs) also have a binding motif for PSD95. Functional association of β1AR with KV1 channels through PSD95 may represent a novel vasodilator complex in cerebral arteries (CA). We explored whether a β1AR-PSD95-KV1 complex is a determinant of rat CA dilation. RT-PCR and western blots revealed expression of β1AR in CA. Isoproterenol induced a concentration-dependent dilation of isolated, pressurized rat CA that was blocked by the β1AR blocker CGP20712. Cranial window imaging of middle cerebral arterioles in situ showed isoproterenol- and norepinephrine-induced dilation that was blunted by β1AR blockade. Isoproterenol-induced hyperpolarization of cVSMCs in pressurized CA was blocked by CGP20712. Confocal images of cVSMCs immunostained with antibodies against β1AR and PSD95 indicated strong colocalization, and PSD95 co-immunoprecipitated with β1AR in CA lysate. Blockade of KV1 channels, β1AR or disruption of PSD95-KV1 interaction produced similar blunting of isoproterenol-induced dilation in pressurized CA. These findings suggest that PSD95 mediates a vasodilator complex with β1AR and KV1 channels in cVSMCs. This complex may be critical for proper vasodilation in rat CA.

White Matter Disease and an Incomplete Circle of Willis

Introduction

White matter disease occurs as a consequence of small vessel disease; however, hypoperfusion may also play a role. We investigated whether patients with less cerebral vessel anastomosis may develop more white matter disease.

Methods

Magnetic resonance imaging (1.5t) with intracranial magnetic resonance angiography data was collected on a convenience sample between July 2008 and January 2009. All patients were independently assessed for circle of Willis variants by two researchers and categorized into two groups: those with a complete circle of Willis and those with an incomplete circle of Willis (absent vessels). The complete group was sub-divided into a classical group (entirely normal circle of Willis) and a hypoplastic group (hypoplasia but no absent vessels). White matter disease assessment was conducted for these groups, by two researchers blind to magnetic resonance angiography findings, on all patients over 50 years old.

Results

The circle of Willis was characterized in 163 patients, while 90 (>50 years) underwent white matter disease assessment. The kappa inter-rater reliability between both circle of Willis assessors and between both white matter disease assessors was 0·57 and 0·63, respectively. The prevalence of circle of Willis variants strongly correlated with the seminal paper by Riggs and Rupp. Independent of age and gender, those with an incomplete circle of Willis ( n = 68) exhibited 58% more white matter disease than those with a complete circle of Willis ( n = 22) (white matter disease score 6·52 vs. 4·11, respectively, P = 0·03). Patients with absent anterior vessels exhibited more frontal white matter disease than those with intact anterior vessels (3·7 vs. 1·72, P < 0·001). Patients with absent posterior vessels exhibited more occipital white matter disease than those with intact posterior vessels (2·52 vs. 1·34, P = 0·014).

Conclusion

These data suggest that congenital absence of anastomotic capacity correlates with incident white matter disease, thus alluding to a hypoperfusion mechanism in the development of white matter disease.

Evidence for a predominant intrinsic sympathetic control of cerebral blood flow alterations in an animal model of cerebral arteriovenous malformation

In terms of neurogenic cerebral blood flow (CBF) control, the activity of the sympathetic nervous system (SNS) has a regulating effect. The impact of a manipulation of both the peripheral (via the perivascular sympathetic net) and central components (via the intracortical noradrenergic terminals originating from the locus coeruleus) on CBF-and especially on hyperperfusion syndromes-is unclear. To test the specific patterns following such alterations, cortical oxygen saturation (rSO2), regional CBF (rCBF), and cortical interstitial norepinephrine (NE) concentrations were measured. Twelve weeks after either the creation of an extracranial AV fistula or sham operation, 80 male Sprague-Dawley rats underwent one of the following procedures: (1) no SNS manipulation, (2) peripheral SNS inhibition via bilateral sympathectomy, (3) central SNS inhibition via the neurotoxin DSP-4, or (4) complete SNS inhibition. Norepinephrine concentrations were lowest after complete inhibition (NE [nmol]: pre, 1.8 ± 1.2; post, 2.4 ± 1.8) and highest following peripheral inhibition (NE [nmol]: pre, 3.6 ± 1.9; post, 6.6 ± 4.4). Following fistula occlusion, rCBF (laser Doppler unit [LDU]) and rSO2 (%SO2) increases were highest after complete inhibition (pre: 204 ± 14 LDU, 34 ± 3%SO2; post: 228 ± 18 LDU, 39 ± 3%SO2) and lowest after peripheral inhibition (pre: 221 ± 18 LDU, 41 ± 2%SO2; post: 226 ± 14 LDU, 47 ± 2%SO2). Thus, a complete inhibition down-regulates SNS activity and provokes a cortical hyperperfusion condition. With this, the hitherto unknown predominant role of the intrinsic component could be demonstrated for the first time in vivo.

Association between cerebral microbleeds and prior primary intracerebral hemorrhage in ischemic stroke patients

OBJECTIVE

To investigate association between cerebral microbleeds (CMB) and prior intracerebral hemorrhage (ICH) on MRI and topographic correlation of the two types of lesions.

PATIENTS AND METHODS

Two hundred and sixty consecutive patients (67.0+/-11.1 years) with ischemic stroke were included. CMB and prior ICH were assessed on T2-gradient-echo MRI. The presence and number of CMB as predictors for prior ICH were examined. Topographic correlations between CMB and ICH lesions in patients with prior ICH in the infratentorial, basal ganglionic/thalamic and cortico-subcortical regions were tested.

RESULTS

CMB were observed in 113 (43.5%) patients and a total of 50 prior primary ICH lesions were observed in 39 (15.0%) patients. Among the ICH lesions, 39 (78%) were asymptomatic. Presence of CMB (odds ratio 2.53, p=0.015) and number of CMB (odds ratio 1.11, p<0.001) were independent determinants for prior ICH. Topographic correlation between CMB and ICH was significant in the basal ganglionic/thalamic region (p=0.017), but not in the infratentorial (p=0.548) or cortico-subcortical regions (p=0.389).

CONCLUSION

CMB were associated with prior ICH on MRI of patients with ischemic stroke. CMB in the basal ganglion or thalamus was associated with prior ICH in the same region.

Perivascular innervation of penetrating brain parenchymal arterioles

We investigated the functional heterogeneity of cerebral pial arteries that are extrinsically innervated versus penetrating brain parenchymal arterioles (PA) that are intrinsically innervated by comparing myogenic activity and reactivity to neurotransmitter. Pial middle cerebral arteries (MCA, n = 6) and PA (n = 6) that branched off the MCA and penetrated the brain tissue were isolated from male Wistar rats and studied in vitro under pressurized conditions for reactivity to serotonin (5-hydroxytryptamine, 5-HT), noradrenaline (NA), and indolactam-V (IL-V), a protein kinase C (PKC) agonist. In a separate group of vessels from the same locations (n = 12), perivascular nerve density was determined after staining for protein gene product 9.5 (PGP 9.5). PAs were significantly smaller than MCAs, and possessed greater myogenic tone at all pressures studied. MCAs reacted to both 5-HT and NA with concentration-dependent contraction, however, PA had little to no response to either neurotransmitter. The percent constriction to 5-HT and NA for MCA versus PA at the maximum concentration was: 31 +/- 6% versus 1.0 +/- 1.0% and 13 +/- 5% versus 2.6 +/- 1.8% (P < 0.01). However, both types of vessels contracted with similar reactivity to PKC activation with IL-V (41 +/- 4% versus 37 +/- 7%, ns). Perivascular nerve density correlated with reactivity such that MCAs were densely innervated with varicose fibers within the adventitia; however, PA had very few or no adventitial fibers. The differential response to neurotransmitter suggests that there is significant heterogeneity in the cerebral circulation. It appears that in PA, the dominant vasoconstricting stimulus is intrinsic myogenic tone and that the role of neurotransmitter and intrinsic innervation is beyond that of controlling CBF.

Comparison of dopamine and norepinephrine after traumatic brain injury and hypoxic-hypotensive insult

After severe brain trauma, blood-brain barrier disruption and alteration of cerebral arteriolar vasoreactive properties may modify the cerebral response to catecholamines. Therefore, the goal of the present study was to compare the effects of dopamine and norepinephrine in a model of brain injury that consisted of a weight-drop model of injury complicated by a 15-min hypoxic-hypotensive insult (HH). Sprague-Dawley rats (n = 7 in each group) received, after brain injury, an infusion of either norepinephrine (TNE group) or dopamine (TDA group) in order to increase cerebral perfusion pressure (CPP) above 70 mm Hg. In addition, a control group (C group, no trauma) and a trauma group (T group, brain injury, no catecholamine infusion) were studied. Mean arterial pressure (MAP), intracranial pressure (ICP, intraparenchymal fiberoptic device), and local cerebral blood flow (LCBF, extradural laser-Doppler fiber) were measured throughout the protocol. In T group, brain injury and HH induced a decrease in CPP (by an increase of ICP and a decrease of MAP), and a decrease of LCBF. Both norepinephrine and dopamine failed to increase CPP, and ICP was significantly higher in TNE and TDA groups than in T group. Interestingly, norepinephrine was not able to alleviate the decrease in MAP. Neither norepinephrine or dopamine could induce an increase of MAP. LCBF decreased similarly in T, TNE and TDA groups. In conclusion, norepinephrine and dopamine are not able to restore values of CPP above 70 mm Hg in a model of severe brain trauma. Furthermore, their systemic vasopressor properties are altered.

Innervation of cerebral arteries by nerves containing 5-hydroxytryptamine and noradrenaline

Noradrenaline (NA)-containing nerves, mainly originating in the sympathetic superior cervical ganglia, supply large and small cerebral arteries. In large cerebral arteries, nerves containing serotonin (5-hydroxytryptamine, 5-HT) may represent neuronal uptake of circulating 5-HT by sympathetic nerves. 5-HT-containing nerves supplying small pial vessels probably have a central origin in the dorsal raphe nucleus. In most species, NA is a weak vasoconstrictor (alpha 1- or alpha 2-adrenoceptors), while 5-HT is a potent vasoconstrictor (5-HT2 or 5-HT1-like receptors) of large cerebral arteries. In contrast, both NA and 5-HT tend to cause vasodilatation in small pial vessels and arterioles. Adrenergic and serotonergic transmission can be modulated by pH, a range of putative neurotransmitters and neuromodulators, and by the endothelium. Sumatriptan, a 5-HT1-like receptor agonist, has been shown to be effective in the treatment of migraine. Changes in NA- or 5-HT-containing nerves and/or in the responses of cerebral vessels to NA and 5-HT have been observed in a variety of vascular disorders, including cerebral vasospasm following subarachnoid haemorrhage, hypertension, and atherosclerosis.

Endothelial dysfunction in cerebral vessels following carotid artery infusion of phorbol ester in rabbits: the role of polymorphonuclear leukocytes

The effect of 4 beta-phorbol-12 beta-myristate-13 alpha-acetate (PMA) on endothelium-dependent and endothelium-independent vasoconstriction and vasodilation was studied in isolated segments of rabbit middle cerebral artery (MCA). Concentration-dependent responses of the left and right MCA to the constrictors KCl, noradrenaline, uridine 5'-triphosphate, serotonin, and histamine, as well as to the dilators acetylcholine, bradykinin, sodium nitroprusside, and calcium ionophore (A23187), were compared in control animals and after PMA injection into the left common carotid artery. In the control animals there was no significant difference in the responses of the left and right MCA to either the constrictors or the dilators studied. After PMA injection the endothelium-dependent relaxation in response to acetylcholine, bradykinin, and A23187 was reduced in the left MCA (PMA-injected side), whereas the effect of the endothelium-independent dilator sodium nitroprusside remained unchanged. Simultaneously greater contractile responses of the left MCA to serotonin and histamine were obtained. Neither infusion of L-arginine in vivo before the PMA injection nor incubation of the isolated MCA segments with L-arginine affected this difference in MCA reactivity. Platelet depletion did not change the PMA-induced reduction in the endothelium-dependent relaxation, whereas after leukocyte depletion this reduction practically disappeared. These results suggest that the PMA-induced brain microembolia causes acute endothelial dysfunction, which is possibly mediated by intravascular activation of leukocytes and is independent of nitric oxide synthesis from L-arginine. This phenomenon might play an important role in cerebral angiospastic disorders after intravascular activation of leukocytes in cerebral ischemia and reperfusion.

Role of endothelium in the response of the vein wall to magnesium withdrawal

Complete absence of magnesium has a two-fold effect on the arterial tone: direct smooth muscle contraction and relaxation via endothelium-derived relaxing factor (EDRF) release. In the present study performed on a systemic vein we investigated (1) which of these effects dominates following reduction of magnesium concentration from 1.2 mM to 0.8 and 0.4 mM and (2) whether the vessel segments asymmetrically respond when the magnesium concentration is reduced on either the intra- or extraluminal side. The effects of reducing magnesium concentration on both the isometric tension of isolated ring preparations and the diameter of isolated, perfused and superfused feline femoral veins were investigated. In noradrenaline-precontracted rings, rapid decreases in the extracellular magnesium concentration from 1.2 mM to 0.8 and 0.4 mM caused relaxation, whereas total omission of magnesium returned the tone to the level of the initial tone induced by noradrenaline. Both in the presence of haemoglobin (5 x 10(-6) M), and in vessels without endothelium, lowering the magnesium concentration caused a dose-dependent elevation of the noradrenaline-induced tone. In perfused and superfused noradrenaline-contracted vein segments, each reduction of extraluminal magnesium concentration caused contraction of the vessels, regardless of whether the endothelium was intact or not. A decrease in intraluminal magnesium concentration did not alter the diameter of the vessel when the endothelium was intact, but caused contraction when the endothelium was disrupted. The results of the present study demonstrate that both the reduction of magnesium concentration or its complete absence cause an EDRF-mediated relaxation and a directly mediated smooth muscle contraction in the femoral vein of the cat.(ABSTRACT TRUNCATED AT 250 WORDS)

Cerebrovascular and functional consequences of 5-HT1A receptor activation

Cerebral glucose utilization and blood flow were measured in rats using 2-deoxy-D-[14C]glucose and [14C]iodoantipyrine quantitative autoradiography, respectively, following treatment with the 5-HT1A receptor agonist 8-hydroxy-2-(di-n-propylamino) tetralin (8-OH-DPAT). In control and 8-OH-DPAT-treated animals blood flow and glucose use were similarly correlated, but the ratio was increased following 8-OH-DPAT treatment. Since 5-HT1A receptor activation is known to reduce neuronal 5-HT release, these results are consistent with a vasoconstrictor role for endogenous serotonin.