Role of the sympathetic autonomic nervous system in hypoxic remodeling of the fetal cerebral vasculature

Abstract

Fetal hypoxia triggers compensatory angiogenesis and remodeling through mechanisms not fully elucidated. In response to hypoxia, hypoxia-inducible factor drives expression of cytokines that exert multiple effects on cerebral structures. Among these, the artery wall is composed of a heterogeneous cell mix and exhibits distinct patterns of cellular differentiation and reactivity. Governing these patterns are the vascular endothelium, smooth muscle (SM), adventitia, sympathetic perivascular nerves (SPN), and the parenchyma. Although an extensive literature details effects of nonneuronal factors on cerebral arteries, the trophic role of perivascular nerves remains unclear. Hypoxia increases sympathetic innervation with subsequent release of norepinephrine (NE), neuropeptide-Y (NPY), and adenosine triphosphate, which exert motor and trophic effects on cerebral arteries and influence dynamic transitions among SM phenotypes. Our data also suggest that the cerebrovasculature reacts very differently to hypoxia in fetuses and adults, and we hypothesize that these differences arise from age-related differences in arterial SM phenotype reactivity and proximity to trophic factors, particularly of neural origin. We provide an integration of recent literature focused on mechanisms by which SPN mediate hypoxic remodeling. Our recent findings suggest that trophic effects of SPN on cerebral arteries accelerate functional maturation through shifts in SM phenotype in an age-dependent manner.

Figure 1. The Continuum of Vascular Smooth Muscle Phenotypes

The medial layer of arteries consists of a highly heterogeneous mix of cells of diverse origins. Many but not all smooth muscle cells begin as adventitial fibroblasts. These fibroblasts initially differentiate into myofibroblasts and then into smooth muscle myocytes that migrate through the medial layer. Migratory myocytes can then transform into proliferative, synthetic and contractile smooth muscle in response to growth factor stimulation. These patterns of differentiation are not terminal, and can be reversed when certain growth and stress factors are introduced in the local environment. In this manner, the artery wall is highly dynamic and heterogeneous in terms of both its structural and functional characteristics.

Figure 2. Chronic hypoxia modulates contractile responses to transmural nerve stimulation in an age-dependent manner

Following 110 days of hypoxic acclimatization, reactivity to electrical nerve stimulation in ovine fetal cerebral arteries was significantly enhanced compared to normoxic controls. Conversely, hypoxic acclimatization modestly depressed contractile reactivity to nerve stimulation in adult cerebral arteries. Results are presented as mean ± SEM. For fetal normoxic (FN), fetal hypoxic (FH), and adult normoxic (SNC) groups, N=16. For the adult hypoxic group (SHC), N=21.

Figure 3. Dopamine βhydroxylaseand tyrosine hydroxylase staining demonstrate major developmental differences

Immunofluorescent staining for both dopamine β hydroxylase (DβH) and tyrosine hydroxylasein ovine middle cerebral arteries revealed distinct and well-developed neuronal terminals at the medial-adventitial border. In adult arteries, long axes of smooth muscle cell nuclei were oriented circumferentially and adventitial cells were relatively sparse. In contrast, within fetal arteries the neuronal terminals were much more diffuse with extensions well into the medial layer. In addition, adventitial cell density was much greater than in adult arteries and smooth muscle nuclei were more abundant but less organized in fetal arteries.

Figure 4. Chronic hypoxia enhances guanethidine-resistant contractions in fetal arteries

To test the possible involvement of non-adrenergic factors in modulation of arterial reactivity during hypoxic acclimatization, electrical nerve stimulation was applied before and after catecholamine depletion with guanethidine. The guanethidine-sensitive (GS) component was an index of the adrenergic contribution to arterial reactivity whereas the guanethidine resistant (GR) component was an index of the sympathetic release of a contractile and potentially trophic molecule other than NE. In adult arteries, hypoxia decreased only the GS component. Conversely, in fetal arteries hypoxia increased both the GR and GS components, suggesting that hypoxia preferentially enhanced release of a non-adrenergic transmitter from sympathetic perivascular nerves in fetal arteries. Results are presented as mean ± SEM.

Figure 5. Overview Schematic

Hypoxic acclimatization induces the release of trophic factors such as VEGF, which stimulates growth and expansion of the sympathetic perivascular innervation. These nerves, in turn release NE, NPY and ATP, all of which act through their respective receptors to promote contractile differentiation of smooth muscle. These phenotypic changes enhance contractility but depress stiffness, as shown by denervation experiments. Independent of the sympathetic nerves, hypoxia can increase wall thickness and stiffness while also depressing contractility. Owing to these opposing effects, the final influence of hypoxia on artery structure and function depends on the balance between nerve-dependent and nerve-independent mechanisms.