Tone-dependent vascular responses to astrocyte-derived signals

Abstract

A growing number of studies support an important contribution of astrocytes to neurovascular coupling, i.e., the phenomenon by which variations in neuronal activity trigger localized changes in blood flow that serve to match the metabolic demands of neurons. However, since both constriction and dilations have been observed in brain parenchymal arterioles upon astrocyte stimulation, the specific influences of these cells on the vasculature remain unclear. Using acute brain slices, we present evidence showing that the specific degree of constriction of rat cortical arterioles (vascular tone) is a key determinant of the magnitude and polarity of the diameter changes elicited by signals associated with neurovascular coupling. Thus elevation of extracellular K+ concentration, stimulation of metabotropic glutamate receptors (mGluR), or 11,12-epoxyeicosatrienoic acid application all elicited vascular responses that were affected by the particular resting arteriolar tone. Interestingly, the data suggest that the extent and/or polarity of the vascular responses are influenced by a delimited set point centered between 30 and 40% tone. In addition, we report that distinct, tone-dependent effects on arteriolar diameter occur upon stimulation of mGluR during inhibition of enzymes of the arachidonic acid pathway [i.e., phospholipase A2, cytochrome P-450 (CYP) omega-hydroxylase, CYP epoxygenase, and cycloxygenase-1]. Our findings may reconcile previous evidence in which direct astrocytic stimulation elicited either vasoconstrictions or vasodilations and also suggest the novel concept that, in addition to participating in functional hyperemia, astrocyte-derived signals play a role in adjusting vascular tone to a range where dilator responses are optimal.

Fig. 1.

Graded vasoconstriction induced by U-46619. Summary histogram of dose-dependent vasoconstriction induced by U-46619 in parenchymal arterioles in cortical brain slices. Results represent means ± SE. Six out of 56 vessels were exposed to two U-46619 concentrations.

Fig. 2.

Elevation of extracellular K⁺ concentration ([K⁺]) elicits tone-dependent vasodilation. A: changes in diameter measured in an arteriole exposed to elevated extracellular [K⁺] ([K⁺]_o) (10 mM). B: changes in vascular diameter plotted as a function of the initial diameter (relative to baseline) in 19 arterioles exposed to 10 mM [K⁺]_o. C: data from B are expressed as percent diameter (relative to baseline) before and after exposure to 10 mM [K⁺]_o, for each individual arteriole.

Fig. 3.

Glial activation elicits tone-dependent constrictions and dilations in brain parenchymal arterioles. A: plot of the changes in vascular diameter in response to the metabotropic glutamate receptor (mGluR) agonist trans-1-aminocyclopentane-1,3-dicarboxylic acid (t-ACPD) (100 μM) as a function of the initial diameter (relative to baseline) (n = 27). B: data from A are expressed to show the diameter of the arterioles after t-ACPD exposure (•) as a function of their initial diameter. Biphasic responses (○) represent secondary dilations in vessels that initially constricted in response to t-ACPD (denoted by dashed lines connecting the symbols). C: traces depicting the diameter changes measured for three arterioles as indicated by nos. in B. D: summary data of the responses elicited by t-ACPD (100 μM) on arterioles preconstricted with PGF_2α (5–35 μM). Results represent means ± SE.

Fig. 4.

Arachidonic acid metabolites are differentially involved in the constrictions and dilations induced by mGluR activation. Summary data showing the diameter changes in cortical arterioles induced by t-ACPD (100 μM) after incubation of brain slices with pharmacological blockers of the AA pathway. Vascular responses were grouped based on the initial degree of preconstriction. Results represent means ± SE. *P < 0.05 and **P < 0.01. Dunnett's test.

Fig. 5.

11,12-Epoxyeicosatrienoic acid (EET) induces tone-dependent constrictions and dilations in parenchymal cortical arterioles and increases astrocytic intracellular Ca²⁺ concentration ([Ca²⁺]_i). A: individual responses of 10 arterioles to 11,12-EET (100–500 nM). Constrictions (gray) followed by dilations (white) were observed in vessels with baseline preconstriction (black) values less than ∼30%, whereas dilatory responses occurred in more constricted vessels. B: representative example of the biphasic response (constriction followed by dilation) elicited by 11,12-EET (500 nM) in a cortical arteriole. C: changes in luminal diameter in an arteriole exposed to 11,12-EET (200 nM) in the absence or presence of TEA (1 mM). Note that 11,12-EET-induced vasodilation is blocked by TEA. D: example of the [Ca²⁺]_i transients elicited by focal application of 11,12-EET (1 μM) in perivascular glial endfeet. Dashed lines indicate vessels’ boundaries. ROI, region of interest. E: summary histogram of the changes in vascular diameter elicited by 11,12-EET in the absence (filled bars) or the presence of methyl arachidonyl fluorophosphonate (MAFP) (100 μM) or TEA (1 mM). MAFP data (and corresponding controls) were obtained from arterioles initially presenting a preconstriction <20% (n = 4); TEA data (and corresponding controls) were obtained from vessels initially displaying preconstriction >30% (n = 3). Results represent means ± SE. ***P < 0.001.

Fig. 6.

Working model. We propose that, under physiological conditions, a combination of intrinsic and extrinsic mechanisms maintains vascular tone at an optimal range, defined here as the “set point” (B). Following neuronal stimulation, released glutamate activates mGluR in astrocytes, resulting in an increase in [Ca²⁺]_i. The rise in [Ca²⁺]_i facilitates K⁺ movement in the perivascular space via large-conductance, Ca²⁺-dependent K⁺ channel (BK) activation at the endfoot, and it also initiates the metabolism of arachidonic acid (AA) via activation of phospholipase A₂ (PLA₂). Rapid vasodilation (C) is thus elicited by both K⁺ and vasoactive signals derived from AA metabolism, in particular cyclooxygenase (COX)-1 metabolites (e.g., PGE₂). We propose that, when vascular tone deviates from its set point, as it might occur under pathological or perhaps physiological conditions (A), astrocyte-derived signals will restore vascular tone to its set point. Although a combination of constrictor and dilator signals may be released by astrocytes, the resultant effect on vascular diameter will be determined by the sensitivity of the vessels to such signals, dictated in turn by their specific tone.