Influence of the glia limitans on pial arteriolar relaxation in the rat

Abstract

We examined whether damage to the glia limitans (GL), via exposure to the gliotoxin L-␣-aminoadipic acid (L-␣AAA), alters hypercapniainduced pial arteriolar dilation in vivo. Anesthetized female rats were prepared with closed cranial windows. Pial arteriolar diameters were measured using intravital microscopy. L-␣AAA (2 mM) was injected into the space under the cranial windows 24 h before the study, and injury to the GL was confirmed by light microscopy. L-␣AAA was associated with a reduction in pial arteriolar CO 2 reactivity to 40 -50% of the level seen in vehicle-treated controls, with no further reduction in the CO 2 response after nitric oxide (NO) synthase (NOS) inhibition via N -nitro-L-arginine (L-NNA). Subsequent blockade of prostanoid synthesis, via indomethacin (Indo), reduced CO2 reactivity to 10 -15% of normal. In vehicle-treated controls, L-NNA, followed by Indo, reduced the response to ϳ50% and then to 15-20% of the normocapnic value, respectively. On the other hand, L-␣AAA had no effect on vascular responses to the endothelium-dependent vasodilator acetylcholine or the NO donor SNAP and did not alter cortical somatosensory evoked responses. This indicates an absence of any direct L-␣AAA actions on pial arterioles or influence on neuronal transmission. Furthermore, L-␣AAA did not alter the vasodilation elicited by topical application of an acidic artificial cerebrospinal fluid solution, suggesting that the GL influences the pial arteriolar relaxation elicited by hypercapnic, but not local extracellular (EC), acidosis. That differences exist in the mechanisms mediating hypercapniaversus EC acidosis-induced pial arteriolar dilations was further exemplified by the finding that topical application of a neuronal NOS (nNOS)-selective blocker (ARR-17477) reduced the response to hypercapnia (by ϳ65%) but not the response to EC acidosis. Disruption of GL gap junctional communication, using an antisense oligodeoxynucleotide (ODN) connexin43 knockdown approach, was accompanied by a 33% lower CO 2 reactivity versus missense ODN-treated controls. These results suggest that the GL contribution to the hypercapnic vascular response appears to involve the NO-dependent component rather than the prostanoid-dependent component and may involve gap junctional communication. We speculate that the GL may act to facilitate the spread, to pial vessels, of hypercapnia-induced vasodilating signals arising in the comparatively few scattered nNOS neurons that lie well beneath the GL. connexin; gap junction; hypercapnia; L-␣-aminoadipic acid; nitric oxide; prostanoids BECAUSE OF THE INTIMATE ANATOMIC RELATIONSHIP between cerebral resistance vessels and astrocytes (19), it has been postulated that astrocytes play a vital role in the regulation of local perfusion in the brain (7). In fact, experimental evidence to that effect has been published (43). Small pial arterioles (Ͻ50 m), although exhibiting a relative paucity of direct neural connections (8, 33), nevertheless appear to be regulated by extravascular influences (23,38). F...

Contributions of astrocytes and CO to pial arteriolar dilation to glutamate in newborn pigs

“…This is in contrast to previous research using adult rats where, although producing similar histological evidence of glia limitans disruption, L-AAA reduced dilation to ADP ϳ50% with the other 50% apparently being endothelium/eNOS-dependent (55,56). The reason for this difference is not known but could relate to increased envelopment of pial vessels observed in piglets compared with adult rats where astrocyte processes are only found on the half of the vessel toward the brain (55, 56, present study).…”

“…Injury to the superficial cortical glia limitans under the cranial window was produced by treatment with the selective glia toxin L-2-␣-aminoadipic acid (L-AAA) (24,55,56). The high cellular specificity of L-AAA apparently results from the rapid uptake of the toxin by the cystine-glutamate antiporter expressed by glia and not other brain cells (16,47).…”

“…The precise mechanism of injury remains uncertain. The method we used was modified from one developed to produce removal of the influence of glia limitans on pial arteriolar responses in the adult rat (55,56). L-AAA (2 mM) was placed under the cranial window for 5 h and removed by flushing with aCSF.…”

Role of Astrocytes in Neurodegenerative Diseases

“…Nevertheless, much of the pial circulation is in contact with the glia-limitans, a de-facto extension of astrocytic processes (Kontos et al, 1971;Xu et al, 2004). This domain organization has been proposed as being the key linking element of the neuronal-(astrocyte)-vascular unit (Volterra & Meldolesi, 2005).…”

Regulation of cerebral vasculature in normal and ischemic brain

“…Pial arterioles and arteries lying free in the subarachnoid space are not covered (Jones, 1970). Nevertheless, much of the pial circulation is in contact with the glia-limitans, a de-facto extension of astrocytic processes (Kontos et al, 1971;Xu et al, 2004).…”