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

Abstract

Astrocytes can act as intermediaries between neurons and cerebral arterioles to regulate vascular tone in response to neuronal activity. Release of glutamate from presynaptic neurons increases blood flow to match metabolic demands. CO is a gasotransmitter that can be related to neural function and blood flow regulation in the brain. The present study addresses the hypothesis that glutamatergic stimulation promotes perivascular astrocyte CO production and pial arteriolar dilation in the newborn brain. Experiments used anesthetized newborn pigs with closed cranial windows, piglet astrocytes, and cerebrovascular endothelial cells in primary culture and immunocytochemical visualization of astrocytic markers. Pial arterioles and arteries of newborn pigs are ensheathed by astrocytes visualized by glial fibrillary acidic protein staining. Treatment (2 h) of astrocytes in culture with L-2-␣-aminoadipic acid (L-AAA), followed by 14 h in toxin free medium, dose-dependently increased cell detachment, suggesting injury. Conversely, 16 h of continuous exposure to L-AAA caused no decrease in endothelial cell attachment. In vivo, topical L-AAA (2 mM, 5 h) disrupted the cortical glia limitans histologically. Such treatment also eliminated pial arteriolar dilation to the astrocytedependent dilator ADP and to glutamate but not to isoproterenol or CO. Glutamate stimulated CO production by the brain surface that also was abolished following L-AAA. In contrast, tetrodotoxin blocked dilation to N-methyl-D-aspartate but not to glutamate, isoproterenol, or CO or the glutamate-induced increase in CO. The concurrent loss of CO production and pial arteriolar dilation to glutamate following astrocyte injury suggests astrocytes may employ CO as a gasotransmitter for glutamatergic cerebrovascular dilation. glia limitans; cerebrovascular circulation; gasotransmitter; glia toxin IN THE CEREBROVASCULAR circulation, signals to vascular smooth muscle can come from endothelium, nerves, astrocytes, or pericytes, which interact to form a neurovascular unit (15). L-Glutamic acid (glutamate) is the principal excitatory neurotransmitter in the central nervous system (38). It is a dilator of cerebral arterioles in vivo (8, 12), although direct effects of glutamate on the vascular smooth muscle are uncertain. Release of glutamate from presynaptic neurons dilates cerebral arterioles, causing blood flow to increase to match metabolic demands (37).Astrocytes are the most abundant cell type in the higher mammalian brain. They function as intermediaries between neurons and cerebral arterioles in regulation of cerebral vascular tone in response to neuronal activity, thereby adjusting cerebral blood flow to metabolism (15,25). Whether astrocytes are involved in dilation in response to glutamate released at excitatory synapses and the nature of astrocyte-derived vasodilator(s) employed remain uncertain.The gas CO is produced physiologically by catabolism of heme to CO, iron, and biliverdin (36). This reaction is catalyzed by heme oxygenase (HO) with oxidation of...

Glutamate-induced calcium signals stimulate CO production in piglet astrocytes

“…Our previous studies in piglets demonstrated that glutamate stimulates astrocytic CO production in primary-cultured astrocytes and in vivo (24,26,31). We also demonstrated that astrocytes release CO and astrocyte-derived CO causes glutamate-induced pial arteriolar dilation in vivo (24), supporting a regulatory role for astrocytic CO in neurovascular coupling in the newborn brain.…”

“…Functional hyperemia has been investigated extensively, and many astrocyte-derived vasoactive mediators, including ADP (40, 41), ATP (34, 36), epoxyeicosatrienoic acids (1-3), K ϩ (11, 13, 33), and prostacyclin (42), appear to be involved, depending on experimental models. We have shown that carbon monoxide (CO) is another critical regulator of cerebrovascular tone in newborn piglets (19,24,26,31,39).CO dilates newborn pig pial arterioles (22,25). CO causes vasodilation by stimulating Ca 2ϩ sparks and large-conductance Ca 2ϩ -activated K ϩ (BK Ca ) channels and by increasing effective coupling between Ca 2ϩ sparks and BK Ca channels in arterial smooth muscle cells (17,18,25,38).…”

Enteral Supplements of a Carbon Monoxide Donor CORM-A1 Protect against Cerebrovascular Dysfunction Caused by Neonatal Seizures

“…2,15,17,19,35 Pial arteriolar dilation to heme, the HO substrate, also involves both endothelial and astrocyte components that express HO-2 and produce vasodilator CO upon stimulation. 2,4,14,17,35 During the delayed postictal period, we observed a dramatic loss of cerebral vascular dilation to astrocyte-dependent vasodilator ADP, and endothelium-astrocyte-dependent stimuli, including glutamate, quisqualate, and heme. These findings, for the first time, show that neonatal seizures cause astrocyte injury leading to loss of astrocyte regulation of cerebral blood flow.…”

“…Pial arteriolar dilation to ADP requires the presence of intact and functionally active glia limitans astrocytes. 14,15 Postictal cerebral vascular responses to ADP (10 − 4 mol/L) were greatly reduced compared with the intact control group (Figure 2).…”

“…[7][8][9] Astrocytes are key cell components of the cerebral blood flow regulation. [11][12][13][14][15] However, until now, the functional consequences of neonatal seizures on the astrocyte component of the neurovascular unit remain unknown. We also wanted to investigate whether the therapeutically preferable enteral route of CORM-A1 administration can deliver gaseous CO to the brain to provide long-term cerebrovascular protection.…”