Potassium channels and neurovascular coupling

Learning on Jupiter, learning on the Moon: the dark side of the G-force. Effects of gravity changes on neurovascular unit and modulation of learning and memory

On earth, gravity vector conditions the development of all living beings by physically imposing an axis along which to build their organism. Thus, during their whole life, they have to fight against this force not only to maintain their architectural organization but also to coordinate the communication between organs and keep their physiology in a balanced steady-state. In space, astronauts show physiological, psychological, and cognitive deregulations, ranging from bone decalcification or decrease of musculature, to depressive-like disorders, and spatial disorientation. Nonetheless, they are confronted to a great amount of physical changes in their environment such as solar radiations, loss of light-dark cycle, lack of spatial landmarks, confinement, and obviously a dramatic decrease of gravity force. It is thus very hard to selectively discriminate the strict role of gravity level alterations on physiological, and particularly cerebral, dysfunction. To this purpose, it is important to design autonomous models and apparatuses for behavioral phenotyping utilizable under modified gravity environments. Our team actually aims at working on this area of research.

Inversion of neurovascular coupling by subarachnoid blood depends on large-conductance Ca2+-activated K+ (BK) channels

The cellular events that cause ischemic neurological damage following aneurysmal subarachnoid hemorrhage (SAH) have remained elusive. We report that subarachnoid blood profoundly impacts communication within the neurovascular unit-neurons, astrocytes, and arterioles-causing inversion of neurovascular coupling. Elevation of astrocytic endfoot Ca(2+) to ∼400 nM by neuronal stimulation or to ∼300 nM by Ca(2+) uncaging dilated parenchymal arterioles in control brain slices but caused vasoconstriction in post-SAH brain slices. Inhibition of K(+) efflux via astrocytic endfoot large-conductance Ca(2+)-activated K(+) (BK) channels prevented both neurally evoked vasodilation (control) and vasoconstriction (SAH). Consistent with the dual vasodilator/vasoconstrictor action of extracellular K(+) ([K(+)](o)), [K(+)](o) <10 mM dilated and [K(+)](o) >20 mM constricted isolated brain cortex parenchymal arterioles with or without SAH. Notably, elevation of external K(+) to 10 mM caused vasodilation in brain slices from control animals but caused a modest constriction in brain slices from SAH model rats; this latter effect was reversed by BK channel inhibition, which restored K(+)-induced dilations. Importantly, the amplitude of spontaneous astrocytic Ca(2+) oscillations was increased after SAH, with peak Ca(2+) reaching ∼490 nM. Our data support a model in which SAH increases the amplitude of spontaneous astrocytic Ca(2+) oscillations sufficiently to activate endfoot BK channels and elevate [K(+)](o) in the restricted perivascular space. Abnormally elevated basal [K(+)](o) combined with further K(+) efflux stimulated by neuronal activity elevates [K(+)](o) above the dilation/constriction threshold, switching the polarity of arteriolar responses to vasoconstriction. Inversion of neurovascular coupling may contribute to the decreased cerebral blood flow and development of neurological deficits that commonly follow SAH.

Hypocapnia induced vasoconstriction significantly inhibits the neurovascular coupling in humans

BACKGROUND/AIMS

Previous studies proved that vasodilation, caused by hypercapnia or acetazolamide, does not inhibit the visually evoked flow velocity changes in the posterior cerebral arteries. Our aim was to determine whether vasoconstriction induced by hypocapnia affects the neurovascular coupling.

METHODS

By using a visual cortex stimulation paradigm, visually evoked flow velocity changes were detected by transcranial Doppler sonography in both posterior cerebral arteries of fourteen young healthy adults. The control measurement was followed by the examination under hyperventilation. Visual-evoked-potentials were also recorded during the control and hyperventilation phases.

RESULTS

The breathing frequency increased from 16 ± 2 to 37 ± 3/min during hyperventilation, resulting in a decrease of the end-tidal CO(2) from 37 ± 3 to 25 ± 3 mm Hg and decrease of resting peak systolic flow velocity from 58 ± 11 to 48 ± 11 cm/s (p<0.01). To allow comparisons between volunteers, relative flow velocity was calculated in relation to baseline. Repeated measures analysis of variance revealed significant difference between the relative flow velocity time courses during hyper- and normoventilation (p<0.001). The maximum changes of visually evoked relative flow velocities were 26 ± 7% and 12 ± 5% during normoventilation and hyperventilation, respectively (p<0.01). Visual-evoked-potentials did not differ in the control and hyperventilation phases.

CONCLUSION

The significantly lower visually evoked flow velocity changes but preserved visual-evoked-potential during hyperventilation indicates that the hypocapnia induced vasoconstriction significantly inhibits the neuronal activity evoked flow response.

Purinergic mechanisms in gliovascular coupling

Regional elevations in cerebral blood flow (CBF) often occur in response to localized increases in cerebral neuronal activity. An ever expanding literature has linked this neurovascular coupling process to specific signaling pathways involving neuronal synapses, astrocytes and cerebral arteries and arterioles. Collectively, these structures are termed the "neurovascular unit" (NVU). Astrocytes are thought to be the cornerstone of the NVU. Thus, not only do astrocytes "detect" increased synaptic activity, they can transmit that information to proximal and remote astrocytic sites often through a Ca(2+)- and ATP-related signaling process. At the vascular end of the NVU, a Ca(2+)-dependent formation and release of vasodilators, or substances linked to vasodilation, can occur. The latter category includes ATP, which upon its appearance in the extracellular compartment, can be rapidly converted to the potent vasodilator, adenosine, via the action of ecto-nucleotidases. In the present review, we give consideration to experimental model-specific variations in purinergic influences on gliovascular signaling mechanisms, focusing on the cerebral cortex. In that discussion, we compare findings obtained using in vitro (rodent brain slice) models and multiple in vivo models (2-photon imaging; somatosensory stimulation-evoked cortical hyperemia; and sciatic nerve stimulation-evoked pial arteriolar dilation). Additional attention is given to the importance of upstream (remote) vasodilation; the key role played by extracellular ATP hydrolysis (via ecto-nucleotidases) in gliovascular coupling; and interactions among multiple signaling pathways.

Neurovascular coupling in cognitive impairment associated with diabetes mellitus

Although it is feared that diabetes-induced cognitive decline will become a major clinical problem worldwide in the future, the detailed pathological mechanism is not well known. Because patients with diabetes have various complications of vascular disease, with not only macrovascular but also microvascular disorders, vascular disorders in the brain are considered to be one of the mechanisms in diabetes-induced cognitive impairment. Indeed, disruption of the blood-brain barrier (BBB) has been observed in some diabetic patients and experimental diabetes models. Moreover, white matter lesions, part of the evidence of BBB dysfunction, are reported to be observed more frequently in patients with diabetes. Animal studies demonstrate that diabetes enhances BBB permeability through a decrease in the level of tight junction proteins and an increase in matrix metalloproteinase activity. However, there are several reports indicating that BBB disruption does not occur with diabetes. Therefore, the association of BBB breakdown with diabetes-induced cognitive impairment is not conclusive. Recently, neuronal diseases involving dementia have been induced experimentally through dysfunction of neurovascular coupling, which involves blood vessels, astrocytes and neutrons. Diabetes-induced cognitive decline may be induced via disruption of neurovascular coupling, with not only vascular disorder but also impairment of astrocytic trafficking. Here, the relation between vascular disorder and cognitive impairment in diabetes is discussed.

Cytochrome P450 eicosanoids and cerebral vascular function

The eicosanoids 20-hydroxyeicosatetraenoic acid (20-HETE) and epoxyeicosatrienoic acids (EETs), which are generated from the metabolism of arachidonic acid by cytochrome P450 (CYP) enzymes, possess a wide array of biological actions, including the regulation of blood flow to organs. 20-HETE and EETs are generated in various cell types in the brain and cerebral blood vessels, and contribute significantly to cerebral blood flow autoregulation and the coupling of regional brain blood flow to neuronal activity (neurovascular coupling). Investigations are beginning to unravel the molecular and cellular mechanisms by which these CYP eicosanoids regulate cerebral vascular function and the changes that occur in pathological states. Intriguingly, 20-HETE and the soluble epoxide hydrolase (sEH) enzyme that regulates EET levels have been explored as molecular therapeutic targets for cerebral vascular diseases. Inhibition of 20-HETE, or increasing EET levels by inhibiting the sEH enzyme, decreases cerebral damage following stroke. The improved outcome following cerebral ischaemia is a consequence of improving cerebral vascular structure or function and protecting neurons from cell death. Thus, the CYP eicosanoids are key regulators of cerebral vascular function and novel therapeutic targets for cardiovascular diseases and neurological disorders.

Neurovascular interaction and the pathophysiology of diabetic retinopathy

Diabetic retinopathy (DR) is the most severe of the several ocular complications of diabetes, and in the United States it is the leading cause of blindness among adults 20 to 74 years of age. Despite recent advances in our understanding of the pathogenesis of DR, there is a pressing need to develop novel therapeutic treatments that are both safe and efficacious. In the present paper, we identify a key mechanism involved in the development of the disease, namely, the interaction between neuronal and vascular activities. Numerous pathological conditions in the CNS have been linked to abnormalities in the relationship between these systems. We suggest that a similar situation arises in the diabetic retina, and we propose a logical strategy aimed at therapeutic intervention.

Vascular pathology and blood-brain barrier disruption in cognitive and psychiatric complications of type 2 diabetes mellitus

Vascular pathology is recognized as a principle insult in type 2 diabetes mellitus (T2DM). Co-morbidities such as structural brain abnormalities, cognitive, learning and memory deficits are also prevailing in T2DM patients. We previously suggested that microvascular pathologies involving blood-brain barrier (BBB) breakdown results in leakage of serum-derived components into the brain parenchyma, leading to neuronal dysfunction manifested as psychiatric illnesses. The current postulate focuses on the molecular mechanisms controlling BBB permeability in T2DM, as key contributors to the pathogenesis of mental disorders in patients. Revealing the mechanisms underlying BBB dysfunction and inflammatory response in T2DM and their role in metabolic disturbances, abnormal neurovascular coupling and neuronal plasticity, would contribute to the understanding of the mechanisms underlying psychopathologies in diabetic patients. Establishing this link would offer new targets for future therapeutic interventions.

Interactions between adenosine and K+ channel-related pathways in the coupling of somatosensory activation and pial arteriolar dilation

Multiple, perhaps interactive, mechanisms participate in the linkage between increased neural activity and cerebral vasodilation. In the present study, we assessed whether neural activation-related pial arteriolar dilation (PAD) involved interactions among adenosine (Ado) A(2) receptors (A(2)Rs), large-conductance Ca(2+)-operated K(+) (BK(Ca)) channels, and inward rectifier K(+) (K(ir)) channels. In rats with closed cranial windows, we monitored sciatic nerve stimulation (SNS)-induced PAD in the absence or presence of pharmacological blockade of A(2)Rs (ZM-241385), ecto-5'-nucleotidase (α,β-methylene-adenosine diphosphate), BK(Ca) channels (paxilline), and K(ir) channels (BaCl(2)). Individually, these interventions led to 53-66% reductions in SNS-induced PADs. Combined applications of these blockers led to little or no further repression of SNS-induced PADs, suggesting interactions among A(2)Rs and K(+) channels. In the absence of SNS, BaCl(2) blockade of K(ir) channels produced 52-80% reductions in Ado and NS-1619 (BK(Ca) channel activator)-induced PADs. In contrast, paxilline blockade of BK(Ca) channels was without effect on dilations elicited by KCl (K(ir) channel activator) and Ado suffusions, indicating that Ado- and NS-1619-associated PADs involved K(ir) channels. In addition, targeted ablation of the superficial glia limitans was associated with a selective 60-80% loss of NS-1619 responses, suggesting that the BK(Ca) channel participation (and paxilline sensitivity) derived largely from channels within the glia limitans. Additionally, blockade of either PKA or adenylyl cyclase caused markedly attenuated pial arteriolar responses to SNS and, in the absence of SNS, responses to Ado, KCl, and NS-1619. These findings suggested a key, possibly permissive, role for A(2)R-linked cAMP generation and PKA-induced K(+) channel phosphorylation in somatosensory activation-evoked PAD.

Vascular tone and neurovascular coupling: considerations toward an improved in vitro model

Neurovascular research has made significant strides toward understanding how the brain neurovascular unit accomplishes rapid and spatial increases in blood flow following neuronal activation. Among the experimental models used, the in vitro brain slice preparation provides unique information revealing the potential signals and cellular mechanisms involved in functional hyperemia. The most crucial limitation of this model, however, is the lack of intraluminal pressure and flow in the vessels being studied. Moreover, differences in basal vascular tone have led to varied interpretations regarding the polarity of vascular responses following neuron-to-glial stimulation. Given the complexity of astrocyte-induced neurovascular responses, we propose the use of a modified in vitro brain slice preparation, where intraluminal arteriolar pressure and flow are retained. Throughout this review, we discuss the advantages and disadvantages to be considered when using brain slices for neurovascular studies. Potential ways to overcome the current limitations are proposed.

Autonomic function and cerebral autoregulation in patients undergoing carotid endarterectomy

BACKGROUND

Carotid endarterectomy (CEA) is the first-line treatment in severe carotid stenosis to prevent stroke. Because of methodological limitations, the acute impact of CEA on baroreflex function and cerebral autoregulation is not well defined and was therefore investigated by applying a novel algorithm.

METHODS AND RESULTS

Systemic arterial blood pressure, ECG and respiration during metronomic breathing and Valsalva maneuver were continuously recorded in 18 patients with carotid stenosis before and after CEA, and in 10 healthy controls. Baroreflex sensitivity, frequency spectra of RR intervals and indices for cerebral autoregulation were evaluated by trigonometric regressive spectral analysis. Compared with the controls, patients had impaired baroreflex sensitivity. Baroreflex sensitivity and frequency spectra were not changed by CEA. Cerebral autoregulation of patients with carotid stenosis as calculated by phase shift was reduced compared with controls but it improved significantly after CEA. Improvement of cerebral autoregulation was independent of changes in cerebral blood flow velocity.

CONCLUSIONS

Baroreflex sensitivity and cerebral autoregulation are impaired in patients with carotid stenosis, conferring a high stroke risk. CEA improves cerebral autoregulation, but does not affect baroreflex sensitivity. For further risk reduction, interventional approaches targeting baroreflex function need to be considered.