Sumatriptan elicits both constriction and dilation in human and bovine brain intracortical arterioles.

1. Little is known about serotonin (5-HT) receptors present on brain microvessels that are innervated by brainstem serotonergic neurons. Using 5-HT, sumatriptan and subtype selective 5-HT(1) receptor agonists and/or the 5-HT(1) receptor antagonist GR127935, we characterized the 5-HT receptors involved in regulating microvascular tone of pressurized intracortical arterioles (approximately 40--50 microm) isolated from human and bovine cerebral cortex. The role of nitric oxide (NO) on these responses was assessed with the N(omega)-nitro-L-arginine (L-NNA, 10(-5) M), an inhibitor of NO synthesis. Bovine pial arteries were studied for comparative purposes. 2. At spontaneous tone, 5-HT induced a dose-dependent constriction of human and bovine microarteries (respective pD(2) values of 7.3+/-0.2 and 6.9+/-0.1); a response potently inhibited by GR127935 (pIC(50) value of 8.5+/-0.1) in bovine microvessels. 3. In both species, the 5-HT(1) receptor agonist sumatriptan induced a biphasic response consisting of a small but significant dilation at low concentrations (1 and/or 10 nM) followed by a constriction at higher doses (pD(2) for contraction of 6.9+/-0.1 and 6.6+/-0.2 in human and bovine vessels, respectively). Pre-incubation with L-NNA abolished the sumatriptan-induced dilation and significantly shifted the dose-response of the constriction curve to the left. In contrast, the selective 5-HT(1D) (PNU-109291) and 5-HT(1F) (LY344864) receptor agonists were devoid of any vasomotor effect. 4. In bovine pial vessels, 5-HT and sumatriptan elicited potent constrictions (respective pD(2) of 7.2+/-0.1 and 6.6+/-0.1), a weak dilation being occasionally observed at low sumatriptan concentrations. 5. A significant negative correlation was observed between pial and intracortical vessels diameter and the extent of the dilatory response to 10(-9) M sumatriptan. Together, these results indicate that sumatriptan, most likely via activation of distinctly localized microvascular 5-HT(1B) receptors, can induce a constriction and/or a dilation which is sensitive to inhibition of NO synthesis and dependent on the size and, possibly, the existing tone of the vessels.

Muscarinic--but not nicotinic--acetylcholine receptors mediate a nitric oxide-dependent dilation in brain cortical arterioles: a possible role for the M5 receptor subtype.

Increases in cortical cerebral blood flow are induced by stimulation of basal forebrain cholinergic neurons. This response is mediated in part by nitric oxide (NO) and reportedly involves both nicotinic and muscarinic receptors, some of which are possibly located in the vessel wall. In the present study, the vasomotor response(s) elicited by acetylcholine (ACh) on isolated and pressurized bovine and/or human intracortical penetrating arterioles were investigated, and pharmacological characterization of the receptor involved in this response was carried out. Acetylcholine (10(-11) to 10(-4) mol/L) dose dependently dilated bovine and human intracortical arterioles at spontaneous tone (respective pD2 values of 6.4+/-0.3 and 7.2+/-0.3 and E(Amax) of 65.0+/-26.8 and 43.2+/-30.1% of the maximal dilation obtained with papaverine) and bovine arterioles after preconstriction with serotonin (pD2 = 6.3+/-0.1, E(Amax) = 80.0+/-17.9% of induced tone). In contrast, nicotine (10(-8) to 10(-4) mol/L) failed to induce any vasomotor response in bovine vessels whether at spontaneous or at pharmacologically induced tone. Application of the nitric oxide synthase (NOS) inhibitor Nomega-nitro-L-arginine (L-NNA; 10(-5) mol/L) elicited a gradual constriction (approximately 20%) of the arterioles, indicating the presence of constitutive NO release in these vessels. Nomega-Nitro-L-argigine (10(-5) to 10(-4) mol/L) also significantly blocked the dilation induced by ACh. The muscarinic ACh receptor (mAChR) antagonists pirenzepine, 4-DAMP, and AF-DX 384 dose dependently inhibited the dilatation induced by ACh (10(-5) mol/L) with the following rank order of potency: 4-DAMP (pIC50 = 9.2+/-0.3) >> pirenzepine (pIC50 = 6.7+/-0.4) > AF-DX 384 (pIC50 = 5.9+/-0.2). These results suggest that ACh can induce a potent, dose-dependent, and NO-mediated dilation of bovine and/or human intracortical arterioles via interaction with an mAChR that best corresponds to the M5 subtype.

Functional acetylcholine muscarinic receptor subtypes in human brain microcirculation: identification and cellular localization.

Acetylcholine is an important regulator of local cerebral blood flow. There is, however, limited information available on the possible sites of action of this neurotransmitter on brain intraparenchymal microvessels. In this study, a combination of molecular and functional approaches was used to identify which of the five muscarinic acetylcholine receptors (mAChR) are present in human brain microvessels and their intimately associated astroglial cells. Microvessel and capillary fractions isolated from human cerebral cortex were found by reverse transcriptase-polymerase chain reaction to express m2, m3, and, occasionally, m1 and m5 receptor subtypes. To localize these receptors to a specific cellular compartment of the vessel wall, cultures of human brain microvascular endothelial and smooth muscle cells were used, together with cultured human brain astrocytes. Endothelial cells invariably expressed m2 and m5 receptors, and occasionally the m1 receptor; smooth muscle cells exhibited messages for all except the m4 mAChR subtypes, whereas messages for all five muscarinic receptors were identified in astrocytes. In all three cell types studied, acetylcholine induced a pirenzepine-sensitive increase (62% to 176%, P<0.05 to 0.01) in inositol trisphosphate, suggesting functional coupling of m1, m3, or m5 mAChR to a phospholipase C signaling cascade. Similarly, coupling of m2 or m4 mAChR to adenylate cyclase inhibition in endothelial cells and astrocytes, but not in smooth muscle cells, was demonstrated by the ability of carbachol to significantly reduce (44% to 50%, P<0.05 to 0.01) the forskolin-stimulated increase in cAMP levels. This effect was reversed by the mAChR antagonist AFDX 384. The results indicate that microvessels are able to respond to neurally released acetylcholine and that mAChR, distributed in different vascular and astroglial compartments, could regulate cortical perfusion and, possibly, blood-brain barrier permeability, functions that could become jeopardized in neurodegenerative disorders such as Alzheimer's disease.

Expression of neuropeptide Y receptors mRNA and protein in human brain vessels and cerebromicrovascular cells in culture.

Neuropeptide Y (NPY) has been suggested as an important regulator of CBF. However, except for the presence of Y1 receptors in large cerebral arteries, little is known about its possible sites of action on brain vessels. In this study, we sought to identify the NPY receptors present in the human cerebrovascular bed. Specific Y1 receptor binding sites, localized on the smooth muscle of human pial vessels and potently competed by NPY, polypeptide YY (PYY), and the selective Y1 receptor antagonist BIBP 3226, were identified by quantitative radioautography of the Y1 radioligand [125I]-[Leu31, Pro34]-PYY. In contrast, no specific binding of the Y2-([125I]-PYY3-36) and Y4/Y5-(125I-human pancreatic polypeptide [hPP]) radioligands could be detected. By in situ hybridization, expression of Y1 receptor mRNA was restricted to the smooth muscle layer of pial vessels, whereas no specific signals were detected for either Y2, Y4, or Y5 receptors. Similarly, using reverse transcriptase-polymerase chain reaction (RT-PCR), mRNA for Y1 but not Y2, Y4, or Y5 receptors was consistently detected in isolated human pial vessels, intracortical microvessels, and capillaries. In human brain microvascular cells in culture, PCR products for the Y1 receptors were exclusively found in the smooth muscle cells. In cultures of human brain astrocytes, a cell type that associates intimately with brain microvessels, PCR products for Y1, Y2, and Y4 but not Y5 receptors were identified. Finally, NPY significantly inhibited the forskolin-induced cAMP production in smooth muscle but not in endothelial cell cultures. We conclude that smooth muscle Y1 receptors are the primary if not exclusive NPY receptors associated with human brain extraparenchymal and intraparenchymal blood vessels, where they most likely mediate cerebral vasoconstriction.