- Email: [email protected]
Adrenergic receptors in brain microvessels
[INS ~-~)ctober 1'.~83
422 gisms as well as between a given synergism and the environment can be simplified by selection of only essential parts of the detailed information about the current state of the synergisms and the enviromnent. Let us consider a simple example. Suppose a running cat comes across an obstacle it has to jurnp over. To do so without stopping, an additional excitation must be sent to limb extensors at the moment when the limb touches the ground, i.e. in the stance phase. From this example one can see that the co-operation between the locomotor synergism and that of jumping can be organized without detailed information about their activity, since only 'knowledge' of the phase of the locomotor cycle is necessary for triggering the nervous mechanisms controlling jumping. What are the possible mechanisms of the interaction between synergisms and between a synergism and the environment? Mechanisms controlling different motor acts and analysing the environment may be located in different parts of the nervous system (for instance, the locomotor synergism is located in the spinal cord, while the synergism of jumping over an obstacle seems, at least partly, to he located in the cerebral cortex). To organize the interaction between synergisms, each synergism could have its 'representation' ('receptors') in the domains of all other synergisms. But such a mode of interaction seems to be vulnerable from the evolutionary point of view, since development of synergisms and an increase in the number of synergisms and of their possible combinations would have made the system of the interaction overcomplicated. The problem of interaction could be solved much more easily if there was a special organ responsible for this function. We suggest that the cerebellum is such an organ. We would like to draw the reader's attention to the ~emarkable fact that the cerebellum receives information from all the motor centres and from the majority of receptors and, in its turn, sends the signals to all the motor centres. At the same time, the cerebellum is not necessary for any particular movement, i.e. it does not belong directly to any synergism. This fact will seem less surprising if one accepts the hypothesis that the cerebellum provides co-operation between the synergisms and adapts them to the environment. The cerebellum meets all the requirements for such a role because: (1) it receives detailed information about the state of motor synergisms and the environment; (2) out of this information it selects essential data concerning both the activity of motor synergisms and the state of the environment; and (3) it can regulate the transmission of signals from one part of the nervous system to another.
Acknowledgements The authors thank Dr E. V. Evans for his valuable comments on the manuscript. Reading list I Airman, 1. A., Bechterev, N. N., Rndionova, E A.. Shmigidina, G. N. and Syka, J. (1976) Exp. Brain Res. 26, 285-298 2 Arshavsky, Yu. I., Berkinblit, M. B., Fukson, O. l , Gelfand,, I. M. and Orlovsky, G. N. (1972) Brain Res. 43,272-275 3 Arshavsky, Yu. 1., Berkinblit, M. B., Fukson, O. 1., Gelfand, 1. M. and Orlovsky, G. N. (1972) Brain Res. 43, 276-279 4 Arshavsky, Yu. L, Gelfand, I. M., Orlovsky, G. N. and Pavlova, G. A. (1978)Brain Res. 151, 479-491 5 Arshavsky, Yu. 1., Gelfand, I. M., Orlovsky, G. N. and Pavlova, G. A. (1978)Brain Res. 151, 493--506 6 Arshavsky, Yu. 1., Gdfand, I. M., Orlovsky, G. N. and Pavlova, G. A. (1978)Brain Res. 159, 99--1 l0 7 Arshavsky, Yu. I., Orlovsky, G. N., Pavlova, G. A. and Perret, C. (1978) Brain Res. 159, 111-123 8 Berkinblit, M. B. Deliagina, T. G., Feldman, A. G., Gelfand, I. M. and Orlovsky G. N. (1978) J. Neurophysiol. 41, 1040-1057 9 Bemstein, N. (1967) The Coordination and Regulation of Movements, Pergamon, Oxford 10 Boytls, C. C. (1977)Soc. Neurosci. Abstr. 3, 55 11 Brooks, V. B. and Thach, W. T. (1981) in Handbook of Physiology: The Nervous System, Vol. I1, pp. 877-946, Williams and Wilkins, Baltimore
12 Deliagina, I. G., Feldman, A ~ , Geltand, j M and Orlovsky, G. N, {1975~ Brain Re,s !(~L 297-313 13 Dow, R. S, and Moruzzi, G. (i958) Fhe Physuflogy and Pathology of the Cerebellum, Uaiversity of Minnesota Press, Minneapolis 14 Grillner, S., Hongo, T. and t.und, S ( 1971 ) Erp. Brain Res 12,457--479 15 Hongo, T,, Jankowska. E. and Lundberg, A. (1969)Exp. Brqin Res. 7,344-364 16 Lundberg, A (1971) £~p Brain Re.s ~2, 317-330 17 Orlovsky, G. N. (1972)Brain R~. 40, 99-112 18 Orlovsky, G. N. and Shik, M. L. (1976) in Inter-
national Review of Physiology: Neurophysiology H (Porter, R., ed.), Vol. 10, pp. 281-317, University Park Press, Baltimore 19 Oscarsson, O. (1973) in Handbook of Sensory Physiology (tggo, A., ed.), Vol. I1, pp. 339--380~ Springer- Verlag, Berli~ 20 Robert~n, L. T. and Grimm, R J. (1975) Exp. Brain Res. 23,447-462 21 Shapovalov, A. 1. (1975) Rev. Physiol.~ Biochem. Pharmacol. 72, 1-54
Yu. L Arshavsky is a Senior Scientific Worker at the Laboratory of Intercellular Interaction, Institute of Problems of Information Transmission, Academy of Sciences, Moscow 101447, USSR. I. M. Gelfand is Chief and G. N Orlovsky is a Senior Scientific Worker at the Department of Mathematical Methods in Biology, Belozersky Interfaculty Laboratory, Moscow State University, Moscow 117234. USSR.
Admnergic in brain microvesNts James A. Nathanson The brain is isolated metabolically from the rest of the body by two major anatomical barrier systems. The first is. the blood--brain barrier, consisting of capillary endothelium and astrocytic processes, which separates the systemic vasculature from the extracellular fluid compartment of the brain. The second is the blood-CSF barrier, consisting of the epithelial cells of the choroid plexus, which separates the choroidal (part of the systemic) vasculature from the cerebrospinal fluid. Recent anatomical, biochemical and physiological studies raise the possibility that the cellular components of these barrier systems may be subject to adrenergic regulation. This article will focus in particular on possible hormonal and neurogenic regulation of the blood-brain barrier and the cerebral microvasculature. The reader is referred to a recent review by Nathanson for a discussion of possible adrenergic regulation of the choroid plexus 15.
~xapmmdwm~ vs. e~_~,~me.aym~ Much of the extensive literature concerning adrenergic regulation of the cerebral vasculature is derived from studies of the larger, so-called extraparenchymal, cerebral vessels which run along the surface and sulci of the brain within the plat membrane. (See the recent review by Edvinsson, Ref. 3.) Until recently, little was known about the pharmacology of the small, ~ n chymal cerebral microvessels which branch
1983. ElsevierSciencePublishersB.V . Amster~tam 0378- 5912/83/$01.(~3
from the larger vessels after they have entered the brain substance. (Most freqnently, the term cerebral microvessels refers to arterioles, venules and capillaries with diameters less than about 200/xm.) The microvessels are of importance in at least two ways: fu~st, under normal conditions the microvasculature contrilwates a considerable portion of the total cerebral resistance to blood flow; second, as mentioned above, the cerebral capillaries constitute part of the blood-brain barrier. Thus,
423
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an understanding of the regulation of the microvasculature has potential clinical implications to such areas as stroke, cerebral edema, and the delivery of drugs and nutrients across the blood-brain barrier. For a number of years the major influences on cerebral blood flow (CBF) and vascular permeability were thought to be almost entirely metabolic, related primarily to changes in hydrogen-ion concentration and associated alterations of CCh tension. (Such changes are physiologically advantageous, since increased metabolic work resulting in a greater release of acidic metabolites and CO2 - can bring about a compensatory vasodilatation and an increase in regional CBF.) It has recently become apparent that, in addition to metabolic changes, neurogenic and hormonal factors (amines, peptides, etc. ) may also play a role in modulating CBF and cerebral vascular permeability. Specifically, evidence supporting an adrenergic influence on the cerebral microvasculature has come through (1) anatomical studies of possible adrenergic innervation of cerebral microvessels, (2) biochemical studies of adrenergic receptors on isolated preparations of microvessels, and (3) physiological studies of adrenergic effects on cerebral vascular permeability.
Anatomical evidence for microvessel adrenergic innervation As reviewed recently by Edvinsson 3, anatomical studies in a number of species have long indicated that the major extraparenchymal cerebral arteries are surrounded by nerves from postganglionic sympathetic neurons originating in the paravertebral sympathetic ganglia. Recent evidence suggests that, in addition to the sympathetic chain, there may be a second major source of cerebrovascular adrenergic innervation. Histochemical studies have shown that the small intraparenchymal cerebral arterioles, and perhaps capillaries as well, are frequently associated, with nerve fibers which contain dopamine-/3hydroxylase (DBH) and which show catecholamine histofluorescence. The fact that these neurons persist following bilateral cervical ganglionectomy has led to the suggestion that the fibers arise centrally from brain-stem noradrenergic nuclei, such as the locus coeruleus. Electronmicroscope studies have shown boutons containing dense-core, presumably adrenergic, vesicles contiguous to intraparenchymal blood vessels in certain areas of the brain. In the hypothalamus, such adrenergic varicosities appear to make close contact to capillary endothelium and pericytes. Whether these boutons are associated anatomically with synaptic specializations on cerebral vessels is still
consist almost entirely of vessels 10-100 /~m in diameter and up to 1-2 mm long. Often, elaborate arborization of vessels is present and can be followed for three or four branch points. Enzymatic studies show that these microvessels are markedly enriched, relative to whole brain, in alkaline phosphatase, y-glutamyl transpeptidase, and various other known 'markers' for vascular tissue. Microvessels also contain very low concentrations of substance P and prostaglandin D2 relative to brain, although noradrenaline and related enzymes have been reported to be present in relatively high concentrations t2.2°. Light microscopy of living and stained microvessels shows that the capillaries consist almost entirely of endothelial cells and pericytes, and that the Biochemical studies of cerebral larger vessels consist of endothelial, muscle microvesseis and, occasionally, adventitial cells and Biochemical studies of the cerebral intraluminal blood elements. Although microvasculature received a tremendous glial and neuronal stains have been reported boost when methods were developed for not to stain microvessel preparations, isolating in quantity intact, metabolically recent electron-microscope observations active, intraparenchymal cerebral micro- show that these vessels sometimes have vessels2'4A°. The temptation to biochemi- adhering glial end-feet22. caily analyse this 'essence of blo 3 × l0 -4 M). sage through a narrow slit or orifice, such Isoproterenol-stimulated enzyme activity is as is present in a loose-fitting teflon-glass blocked by low concentrations of the homogenizer or through fine (150-300 fl-adrenergic antagonist propranoloi (K, = /xm) nylon-mesh screens. (Recently, selec- 2.4 × 10 9 M), but is blocked only by much tive enzymatic digestion of minced brain higher concentrations of c~-adrenergic or has also been used to release microvessels dopamine blockers. Stimulated activity is from cerebral tissue.) Separation of the ves- also inhibited by low concentrations of the sels released from the disrupted or digested fl-2 selective antagonist IPS 339 (K, = 4 x non-vascular tissue is then accomplished on 10 -9 M), but only by higher concentrations the basis of: (1) size, by graded sieving of the selective fl- 1 antagonists atenolol (K, through nylon mesh (the elongated, branch- = 6 × 10 6 M) or practolol (Ki = 1.8 × ing vessels are caught); (2) density, 10 -5 M). These agonist and antagonist through the use of sucrose, Percoll or albu- properties are quite similar to those demonmin gradients; or (3) adherence to glass strated by 132-adrenergic receptors and 132beads. Frequently a combination of stimulated adenylate cyclase present in methods is used to obtain a preparation other tissues and suggest, therefore, that the which, by light microscopy, appears to majority of adenylate-cyclase-associated
unclear, since in most cases postsynaptic densities have not been observed. This lack of observation does not of course rule out a regulatory role for adrenergic neurons on vascular receptors not associated with densities. In fact, on the basis of the histochemical demonstration of non-sympathetic DBHcontaining terminals near intraparenchymal blood vessels, Hartman, Zide and Udenfriend postulated over a decade ago that the centrally arising noradrenergic system may be involved in the microregulation of cerebral blood flow 8. This hypothesis, although not yet proven, has served as an attractive focus for a number of biochemical and physiological investigations in the past several years.
424 adrenergic receptors in cat cerebral microvessels are 132. Recent studies by other laboratories, using ligand-binding techniques, have suggested that porcine and rat cerebral microvessels also contain a predominance of /32-adrenergic receptors. These f'mdings are interesting since prior physiological studies of extraparenchymal pial vessels have shown that precontracted arteries demonstrate a 13~-adrenergic vasodilatory response. As yet, knowledge of the cellular di~ tribution of microvessel /3~-adrenergic receptors that have been detected by biochemical methods is incomplete. However, studies in our own laboratory have shown that vascular fractions which contain the smallest cerebral microvessels (mostly capillaries) demonstrate higher adrenergic sensitivity than fractions containing larger arterioles or pial vessels. Because of a decreasing proportion of muscle and adventitial cells in the smaller microvessels, it is possible that/32-adrenergic receptors may be enriched in cerebral vascular endothelial cells or pericytes. This possibility, which is supported indirectly by recent studies demonstrating the presence of /32adrenergic receptors on isolated aortic endothelial cells21, does not, of course, rule out the presence of/32 receptors on cerebrovascular muscle cells. It is also unclear whether astrocytic foot processes remaining attached to microvessels also contribute to the results seen in biochemical studies, although certain recent experiments characterizing adrenergic receptors in purified astrocytes have suggested a predominance of /31 receptors. As mentioned above, ligand-binding studies have also indicated the presence of ~2- and, less definitively, oa-adienergic receptors in microvessei preparations. As yet, however, there has been no detailed biochemical characterization of these ~e receptors.
Physiological evidence for adrenergic effects on cerebral vascular permeability The presence of adrenergic receptors in cerebral capillaries is consistent with the possibility of an adrenergic vasoregulatory mechanism in the cerebral microvasculature. More direct evidence for such a mechanism has come from recent studies of adrenergie effects on alterations in cerebral vascular permeability6,17-19. These studies use, as an experimental paradigm, the recent findings that cerebral capillaries appear to demonstrate an incomplete and variable permeability to such small molecules as water and ethanol. Raiehie, H',a'tman and colleagues have shown, in a small number of bilaterally sympathectomized monkeys, that electrical or chemical (with intracerebral carbachol) stimulation of the noradrenergic nucleus locus
TINS
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coeruleus results in an increase in the cere- Acknowledgements The studies from the author's laboratory were bral extraction of 1sO-labelled waterlL Simultaneous measurements, which indi- supported, in part, by NIH Grant NS 16356 and cate a decrease in cerebral blood flow, sug- by grants from the McKnight Foundation and the Pharmaceutical Manufacturers Association gest that the increased extraction is due to Foundation. an increase in capillary permeability. More recently, Preskorn, Hartman and Reading list colleagues have employed a double extrac1 Baca, G. M. and Palmer, G. C. (197q)Biochem. tion technique which utilizes freely permPharmacol. 28, 2847-2849 2 Brendel, K., Meezan, E. and Carlson, E. C. eable butanol and diffusion-limited tritiated (1974)Science 185,953--955 water to measure cerebral permeability and 3 Edvinsson, L. (19821 Trend~ Neuro&:i. 5, cerebral blood flow in the raPL They have 425-429 found that both the acute and chronic 4 Goldstein, G. W., Wolinsky, J. S, Csejtey, J. and administration of tricyclic antidepressants Diamond, I. (1975)J. Neurochem. 25,717-727 increases cerebral water extraction inde5 Gross, P. M., Teasdale, G. M., Angerson, W. J. pendently of cerebral blood flow. This and Harper, A. M. (1974) Brain Res. 210 6 Grubb, R. L., Raichle, M. E. and Eichling, J. O. effect is blocked by the adrenergic antagon(1978) Brain Res. 144,204-207 ist phenoxybenzamine, or by prior treat7 Harik, S. 1., Sharma, V. K., Wetherbee, J. R., ment with the catecholamine neurotoxin Warren, R. H. and Banerjee, S. P. (1981)J. 6-hydroxydopamine. Preskom and colCereb. Blood Flow Metab. l, 32%338 leagues have also found that the administra8 Hartman, B. K., Zide, D. and Udenfi'iend, S. tion of lithium or electroconvulsive shock, (19721 Proc. Natl Acad. Sci. USA 69, 2722-2726 treatments known (among their other 9 Herbst, T. J., Raichle, M. E. and Ferrendelfi, J. A. effects) to decrease central noradrenergic (19791 Science 204, 330-332 activity, blunts the increase in cerebral water permeability that normally occurs 10 Joo, F. and Karnushina, 1. (19731 C~tobios 8, 41-48 with increasing blood CO2 tension. 11 Kamushina, L L., Palncios, J. M., Barbm, G.. These findings in monkeys and rats supDux, E., Joo, F. and Schwartz, J. C. (19801J. ply some additional evidence for a possible Neurochem. 34, 1201-1208 adrenergic influence on cerebrovascular 12 Lai, F. M., Udenfriend, S. andSpeetor, S. (1975) Proc. Natl Acad. Sci. USA 72, 4622--4625 permeability. The site of adrenergic action on the vasculature cannot, of course, be 13 Nathanson, J. A. (1980)Life Sci. 26, 1793-1799 14 Nathanson, J. A. andGlaser, G. H. (1979)Nature determined from the physiological data (London) 278, 567-569 alone, and could be quite indirect (for 15 Nathanson, J. A. (1982) Trends PharmacoL Sci. example adrenergic alteration in the release 3,452-454 of some other vasoregulatory hormone). 16 Peroutka, S. J., Moskowitz, M. A., Reinhard, J. F. and Snyder, S. H. (19801 Science 208, However, in conjunction with the above 610-612 anatomical and biochemical data, the results of the physiological studies are more 17 Preskom. S. H.. Hartman, B. K., Raiehle. M. E. and Clark, H. B (1980) J. Pharmacol Exp. compelling. They also raise some interestTher. 213. 313--320 ing questions. For example, although the 18 Preskom. S. H.. Irwin, G. H.. Simpson, S , presence of noradrenergic terminals on the Friesen. D.. Rinne. J. and Jerkovieh. G. (19811 Science 213,469--471 brain side of microvessels suggests possible neurogenic regulation of the blood-brain 19 Raichle. M. E.. Hartrnan, B. K., Eichling, J. O. and Sharpe, L G. (19751Proc. Natl Acad. Sci. barrier, what might be the role of circulaUSA 72. 3726-3730 ting catecholamines in regulating barrier 20 Reinhard. J. F.. Leibmarm, J. E., Schlosberg, function? Because it appears unlikely that A. J. and Moskowitz. M. (1979) ~ 21)6, 85--87 such circulating amines can penetrate to the 21 Schafer. A. I..Gimbrone. M. A, and Handin, R. 1. parenchymai side of endothelial cells, it (19801 Biochem. Biophys. Res. Commun. 96, 1640-1647 used to be thought that these amines had little effect on cerebrovascular function. 22 White. F. P.. Button. G. R. and Norenberg. M. D. (1981 bJ. Neurochem. 36. 328-332 However, the probable presence of adrenergic receptors and adrenergic- * This may be analogous to the situation that has been sensitive adenylate cyclase on cerebral vas- shown to exist for histamine which is present in mast cular endothelium raises the possibility that cells on the lmmnchymal side of cerebtat vessels. circulating amines could alter endothelial When injected by the intr~_,nrotid route, histamine cell function without penetration. In this causes an increase in cerebral vascular ~ ' t y to regard, it is interesting that the majority of sucrose without an associated change in cerebrtd blood flow. Thishistaminereslmmeis of an 1t2 type, similar /3-adrenergic receptors found on cerebral to the characteristics of a histamine-sensitive adenylate capillaries appear to be of /3~ subtype, cyclase which has been found in cerebral microwhich has greater sensitivity to circulating VeSSelsT M . adrenaline than to noradrenaline. It is conceivable, therefore, that adrenergic influ- James A. Nathanson is at the Department o f ences on the microvasculature might come Neurology, Harvard Medical School Masfrom the luminal as well as the parenchymal sachusetts General Hospital, Boston. MA 02114. USA. sides of cerebral blood vessels*.
422 gisms as well as between a given synergism and the environment can be simplified by selection of only essential parts of the detailed information about the current state of the synergisms and the enviromnent. Let us consider a simple example. Suppose a running cat comes across an obstacle it has to jurnp over. To do so without stopping, an additional excitation must be sent to limb extensors at the moment when the limb touches the ground, i.e. in the stance phase. From this example one can see that the co-operation between the locomotor synergism and that of jumping can be organized without detailed information about their activity, since only 'knowledge' of the phase of the locomotor cycle is necessary for triggering the nervous mechanisms controlling jumping. What are the possible mechanisms of the interaction between synergisms and between a synergism and the environment? Mechanisms controlling different motor acts and analysing the environment may be located in different parts of the nervous system (for instance, the locomotor synergism is located in the spinal cord, while the synergism of jumping over an obstacle seems, at least partly, to he located in the cerebral cortex). To organize the interaction between synergisms, each synergism could have its 'representation' ('receptors') in the domains of all other synergisms. But such a mode of interaction seems to be vulnerable from the evolutionary point of view, since development of synergisms and an increase in the number of synergisms and of their possible combinations would have made the system of the interaction overcomplicated. The problem of interaction could be solved much more easily if there was a special organ responsible for this function. We suggest that the cerebellum is such an organ. We would like to draw the reader's attention to the ~emarkable fact that the cerebellum receives information from all the motor centres and from the majority of receptors and, in its turn, sends the signals to all the motor centres. At the same time, the cerebellum is not necessary for any particular movement, i.e. it does not belong directly to any synergism. This fact will seem less surprising if one accepts the hypothesis that the cerebellum provides co-operation between the synergisms and adapts them to the environment. The cerebellum meets all the requirements for such a role because: (1) it receives detailed information about the state of motor synergisms and the environment; (2) out of this information it selects essential data concerning both the activity of motor synergisms and the state of the environment; and (3) it can regulate the transmission of signals from one part of the nervous system to another.
Acknowledgements The authors thank Dr E. V. Evans for his valuable comments on the manuscript. Reading list I Airman, 1. A., Bechterev, N. N., Rndionova, E A.. Shmigidina, G. N. and Syka, J. (1976) Exp. Brain Res. 26, 285-298 2 Arshavsky, Yu. I., Berkinblit, M. B., Fukson, O. l , Gelfand,, I. M. and Orlovsky, G. N. (1972) Brain Res. 43,272-275 3 Arshavsky, Yu. 1., Berkinblit, M. B., Fukson, O. 1., Gelfand, 1. M. and Orlovsky, G. N. (1972) Brain Res. 43, 276-279 4 Arshavsky, Yu. L, Gelfand, I. M., Orlovsky, G. N. and Pavlova, G. A. (1978)Brain Res. 151, 479-491 5 Arshavsky, Yu. 1., Gelfand, I. M., Orlovsky, G. N. and Pavlova, G. A. (1978)Brain Res. 151, 493--506 6 Arshavsky, Yu. 1., Gdfand, I. M., Orlovsky, G. N. and Pavlova, G. A. (1978)Brain Res. 159, 99--1 l0 7 Arshavsky, Yu. I., Orlovsky, G. N., Pavlova, G. A. and Perret, C. (1978) Brain Res. 159, 111-123 8 Berkinblit, M. B. Deliagina, T. G., Feldman, A. G., Gelfand, I. M. and Orlovsky G. N. (1978) J. Neurophysiol. 41, 1040-1057 9 Bemstein, N. (1967) The Coordination and Regulation of Movements, Pergamon, Oxford 10 Boytls, C. C. (1977)Soc. Neurosci. Abstr. 3, 55 11 Brooks, V. B. and Thach, W. T. (1981) in Handbook of Physiology: The Nervous System, Vol. I1, pp. 877-946, Williams and Wilkins, Baltimore
12 Deliagina, I. G., Feldman, A ~ , Geltand, j M and Orlovsky, G. N, {1975~ Brain Re,s !(~L 297-313 13 Dow, R. S, and Moruzzi, G. (i958) Fhe Physuflogy and Pathology of the Cerebellum, Uaiversity of Minnesota Press, Minneapolis 14 Grillner, S., Hongo, T. and t.und, S ( 1971 ) Erp. Brain Res 12,457--479 15 Hongo, T,, Jankowska. E. and Lundberg, A. (1969)Exp. Brqin Res. 7,344-364 16 Lundberg, A (1971) £~p Brain Re.s ~2, 317-330 17 Orlovsky, G. N. (1972)Brain R~. 40, 99-112 18 Orlovsky, G. N. and Shik, M. L. (1976) in Inter-
national Review of Physiology: Neurophysiology H (Porter, R., ed.), Vol. 10, pp. 281-317, University Park Press, Baltimore 19 Oscarsson, O. (1973) in Handbook of Sensory Physiology (tggo, A., ed.), Vol. I1, pp. 339--380~ Springer- Verlag, Berli~ 20 Robert~n, L. T. and Grimm, R J. (1975) Exp. Brain Res. 23,447-462 21 Shapovalov, A. 1. (1975) Rev. Physiol.~ Biochem. Pharmacol. 72, 1-54
Yu. L Arshavsky is a Senior Scientific Worker at the Laboratory of Intercellular Interaction, Institute of Problems of Information Transmission, Academy of Sciences, Moscow 101447, USSR. I. M. Gelfand is Chief and G. N Orlovsky is a Senior Scientific Worker at the Department of Mathematical Methods in Biology, Belozersky Interfaculty Laboratory, Moscow State University, Moscow 117234. USSR.
Admnergic in brain microvesNts James A. Nathanson The brain is isolated metabolically from the rest of the body by two major anatomical barrier systems. The first is. the blood--brain barrier, consisting of capillary endothelium and astrocytic processes, which separates the systemic vasculature from the extracellular fluid compartment of the brain. The second is the blood-CSF barrier, consisting of the epithelial cells of the choroid plexus, which separates the choroidal (part of the systemic) vasculature from the cerebrospinal fluid. Recent anatomical, biochemical and physiological studies raise the possibility that the cellular components of these barrier systems may be subject to adrenergic regulation. This article will focus in particular on possible hormonal and neurogenic regulation of the blood-brain barrier and the cerebral microvasculature. The reader is referred to a recent review by Nathanson for a discussion of possible adrenergic regulation of the choroid plexus 15.
~xapmmdwm~ vs. e~_~,~me.aym~ Much of the extensive literature concerning adrenergic regulation of the cerebral vasculature is derived from studies of the larger, so-called extraparenchymal, cerebral vessels which run along the surface and sulci of the brain within the plat membrane. (See the recent review by Edvinsson, Ref. 3.) Until recently, little was known about the pharmacology of the small, ~ n chymal cerebral microvessels which branch
1983. ElsevierSciencePublishersB.V . Amster~tam 0378- 5912/83/$01.(~3
from the larger vessels after they have entered the brain substance. (Most freqnently, the term cerebral microvessels refers to arterioles, venules and capillaries with diameters less than about 200/xm.) The microvessels are of importance in at least two ways: fu~st, under normal conditions the microvasculature contrilwates a considerable portion of the total cerebral resistance to blood flow; second, as mentioned above, the cerebral capillaries constitute part of the blood-brain barrier. Thus,
423
TINS - October 1983
an understanding of the regulation of the microvasculature has potential clinical implications to such areas as stroke, cerebral edema, and the delivery of drugs and nutrients across the blood-brain barrier. For a number of years the major influences on cerebral blood flow (CBF) and vascular permeability were thought to be almost entirely metabolic, related primarily to changes in hydrogen-ion concentration and associated alterations of CCh tension. (Such changes are physiologically advantageous, since increased metabolic work resulting in a greater release of acidic metabolites and CO2 - can bring about a compensatory vasodilatation and an increase in regional CBF.) It has recently become apparent that, in addition to metabolic changes, neurogenic and hormonal factors (amines, peptides, etc. ) may also play a role in modulating CBF and cerebral vascular permeability. Specifically, evidence supporting an adrenergic influence on the cerebral microvasculature has come through (1) anatomical studies of possible adrenergic innervation of cerebral microvessels, (2) biochemical studies of adrenergic receptors on isolated preparations of microvessels, and (3) physiological studies of adrenergic effects on cerebral vascular permeability.
Anatomical evidence for microvessel adrenergic innervation As reviewed recently by Edvinsson 3, anatomical studies in a number of species have long indicated that the major extraparenchymal cerebral arteries are surrounded by nerves from postganglionic sympathetic neurons originating in the paravertebral sympathetic ganglia. Recent evidence suggests that, in addition to the sympathetic chain, there may be a second major source of cerebrovascular adrenergic innervation. Histochemical studies have shown that the small intraparenchymal cerebral arterioles, and perhaps capillaries as well, are frequently associated, with nerve fibers which contain dopamine-/3hydroxylase (DBH) and which show catecholamine histofluorescence. The fact that these neurons persist following bilateral cervical ganglionectomy has led to the suggestion that the fibers arise centrally from brain-stem noradrenergic nuclei, such as the locus coeruleus. Electronmicroscope studies have shown boutons containing dense-core, presumably adrenergic, vesicles contiguous to intraparenchymal blood vessels in certain areas of the brain. In the hypothalamus, such adrenergic varicosities appear to make close contact to capillary endothelium and pericytes. Whether these boutons are associated anatomically with synaptic specializations on cerebral vessels is still
consist almost entirely of vessels 10-100 /~m in diameter and up to 1-2 mm long. Often, elaborate arborization of vessels is present and can be followed for three or four branch points. Enzymatic studies show that these microvessels are markedly enriched, relative to whole brain, in alkaline phosphatase, y-glutamyl transpeptidase, and various other known 'markers' for vascular tissue. Microvessels also contain very low concentrations of substance P and prostaglandin D2 relative to brain, although noradrenaline and related enzymes have been reported to be present in relatively high concentrations t2.2°. Light microscopy of living and stained microvessels shows that the capillaries consist almost entirely of endothelial cells and pericytes, and that the Biochemical studies of cerebral larger vessels consist of endothelial, muscle microvesseis and, occasionally, adventitial cells and Biochemical studies of the cerebral intraluminal blood elements. Although microvasculature received a tremendous glial and neuronal stains have been reported boost when methods were developed for not to stain microvessel preparations, isolating in quantity intact, metabolically recent electron-microscope observations active, intraparenchymal cerebral micro- show that these vessels sometimes have vessels2'4A°. The temptation to biochemi- adhering glial end-feet22. caily analyse this 'essence of blo
unclear, since in most cases postsynaptic densities have not been observed. This lack of observation does not of course rule out a regulatory role for adrenergic neurons on vascular receptors not associated with densities. In fact, on the basis of the histochemical demonstration of non-sympathetic DBHcontaining terminals near intraparenchymal blood vessels, Hartman, Zide and Udenfriend postulated over a decade ago that the centrally arising noradrenergic system may be involved in the microregulation of cerebral blood flow 8. This hypothesis, although not yet proven, has served as an attractive focus for a number of biochemical and physiological investigations in the past several years.
424 adrenergic receptors in cat cerebral microvessels are 132. Recent studies by other laboratories, using ligand-binding techniques, have suggested that porcine and rat cerebral microvessels also contain a predominance of /32-adrenergic receptors. These f'mdings are interesting since prior physiological studies of extraparenchymal pial vessels have shown that precontracted arteries demonstrate a 13~-adrenergic vasodilatory response. As yet, knowledge of the cellular di~ tribution of microvessel /3~-adrenergic receptors that have been detected by biochemical methods is incomplete. However, studies in our own laboratory have shown that vascular fractions which contain the smallest cerebral microvessels (mostly capillaries) demonstrate higher adrenergic sensitivity than fractions containing larger arterioles or pial vessels. Because of a decreasing proportion of muscle and adventitial cells in the smaller microvessels, it is possible that/32-adrenergic receptors may be enriched in cerebral vascular endothelial cells or pericytes. This possibility, which is supported indirectly by recent studies demonstrating the presence of /32adrenergic receptors on isolated aortic endothelial cells21, does not, of course, rule out the presence of/32 receptors on cerebrovascular muscle cells. It is also unclear whether astrocytic foot processes remaining attached to microvessels also contribute to the results seen in biochemical studies, although certain recent experiments characterizing adrenergic receptors in purified astrocytes have suggested a predominance of /31 receptors. As mentioned above, ligand-binding studies have also indicated the presence of ~2- and, less definitively, oa-adienergic receptors in microvessei preparations. As yet, however, there has been no detailed biochemical characterization of these ~e receptors.
Physiological evidence for adrenergic effects on cerebral vascular permeability The presence of adrenergic receptors in cerebral capillaries is consistent with the possibility of an adrenergic vasoregulatory mechanism in the cerebral microvasculature. More direct evidence for such a mechanism has come from recent studies of adrenergie effects on alterations in cerebral vascular permeability6,17-19. These studies use, as an experimental paradigm, the recent findings that cerebral capillaries appear to demonstrate an incomplete and variable permeability to such small molecules as water and ethanol. Raiehie, H',a'tman and colleagues have shown, in a small number of bilaterally sympathectomized monkeys, that electrical or chemical (with intracerebral carbachol) stimulation of the noradrenergic nucleus locus
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coeruleus results in an increase in the cere- Acknowledgements The studies from the author's laboratory were bral extraction of 1sO-labelled waterlL Simultaneous measurements, which indi- supported, in part, by NIH Grant NS 16356 and cate a decrease in cerebral blood flow, sug- by grants from the McKnight Foundation and the Pharmaceutical Manufacturers Association gest that the increased extraction is due to Foundation. an increase in capillary permeability. More recently, Preskorn, Hartman and Reading list colleagues have employed a double extrac1 Baca, G. M. and Palmer, G. C. (197q)Biochem. tion technique which utilizes freely permPharmacol. 28, 2847-2849 2 Brendel, K., Meezan, E. and Carlson, E. C. eable butanol and diffusion-limited tritiated (1974)Science 185,953--955 water to measure cerebral permeability and 3 Edvinsson, L. (19821 Trend~ Neuro&:i. 5, cerebral blood flow in the raPL They have 425-429 found that both the acute and chronic 4 Goldstein, G. W., Wolinsky, J. S, Csejtey, J. and administration of tricyclic antidepressants Diamond, I. (1975)J. Neurochem. 25,717-727 increases cerebral water extraction inde5 Gross, P. M., Teasdale, G. M., Angerson, W. J. pendently of cerebral blood flow. This and Harper, A. M. (1974) Brain Res. 210 6 Grubb, R. L., Raichle, M. E. and Eichling, J. O. effect is blocked by the adrenergic antagon(1978) Brain Res. 144,204-207 ist phenoxybenzamine, or by prior treat7 Harik, S. 1., Sharma, V. K., Wetherbee, J. R., ment with the catecholamine neurotoxin Warren, R. H. and Banerjee, S. P. (1981)J. 6-hydroxydopamine. Preskom and colCereb. Blood Flow Metab. l, 32%338 leagues have also found that the administra8 Hartman, B. K., Zide, D. and Udenfi'iend, S. tion of lithium or electroconvulsive shock, (19721 Proc. Natl Acad. Sci. USA 69, 2722-2726 treatments known (among their other 9 Herbst, T. J., Raichle, M. E. and Ferrendelfi, J. A. effects) to decrease central noradrenergic (19791 Science 204, 330-332 activity, blunts the increase in cerebral water permeability that normally occurs 10 Joo, F. and Karnushina, 1. (19731 C~tobios 8, 41-48 with increasing blood CO2 tension. 11 Kamushina, L L., Palncios, J. M., Barbm, G.. These findings in monkeys and rats supDux, E., Joo, F. and Schwartz, J. C. (19801J. ply some additional evidence for a possible Neurochem. 34, 1201-1208 adrenergic influence on cerebrovascular 12 Lai, F. M., Udenfriend, S. andSpeetor, S. (1975) Proc. Natl Acad. Sci. USA 72, 4622--4625 permeability. The site of adrenergic action on the vasculature cannot, of course, be 13 Nathanson, J. A. (1980)Life Sci. 26, 1793-1799 14 Nathanson, J. A. andGlaser, G. H. (1979)Nature determined from the physiological data (London) 278, 567-569 alone, and could be quite indirect (for 15 Nathanson, J. A. (1982) Trends PharmacoL Sci. example adrenergic alteration in the release 3,452-454 of some other vasoregulatory hormone). 16 Peroutka, S. J., Moskowitz, M. A., Reinhard, J. F. and Snyder, S. H. (19801 Science 208, However, in conjunction with the above 610-612 anatomical and biochemical data, the results of the physiological studies are more 17 Preskom. S. H.. Hartman, B. K., Raiehle. M. E. and Clark, H. B (1980) J. Pharmacol Exp. compelling. They also raise some interestTher. 213. 313--320 ing questions. For example, although the 18 Preskom. S. H.. Irwin, G. H.. Simpson, S , presence of noradrenergic terminals on the Friesen. D.. Rinne. J. and Jerkovieh. G. (19811 Science 213,469--471 brain side of microvessels suggests possible neurogenic regulation of the blood-brain 19 Raichle. M. E.. Hartrnan, B. K., Eichling, J. O. and Sharpe, L G. (19751Proc. Natl Acad. Sci. barrier, what might be the role of circulaUSA 72. 3726-3730 ting catecholamines in regulating barrier 20 Reinhard. J. F.. Leibmarm, J. E., Schlosberg, function? Because it appears unlikely that A. J. and Moskowitz. M. (1979) ~ 21)6, 85--87 such circulating amines can penetrate to the 21 Schafer. A. I..Gimbrone. M. A, and Handin, R. 1. parenchymai side of endothelial cells, it (19801 Biochem. Biophys. Res. Commun. 96, 1640-1647 used to be thought that these amines had little effect on cerebrovascular function. 22 White. F. P.. Button. G. R. and Norenberg. M. D. (1981 bJ. Neurochem. 36. 328-332 However, the probable presence of adrenergic receptors and adrenergic- * This may be analogous to the situation that has been sensitive adenylate cyclase on cerebral vas- shown to exist for histamine which is present in mast cular endothelium raises the possibility that cells on the lmmnchymal side of cerebtat vessels. circulating amines could alter endothelial When injected by the intr~_,nrotid route, histamine cell function without penetration. In this causes an increase in cerebral vascular ~ ' t y to regard, it is interesting that the majority of sucrose without an associated change in cerebrtd blood flow. Thishistaminereslmmeis of an 1t2 type, similar /3-adrenergic receptors found on cerebral to the characteristics of a histamine-sensitive adenylate capillaries appear to be of /3~ subtype, cyclase which has been found in cerebral microwhich has greater sensitivity to circulating VeSSelsT M . adrenaline than to noradrenaline. It is conceivable, therefore, that adrenergic influ- James A. Nathanson is at the Department o f ences on the microvasculature might come Neurology, Harvard Medical School Masfrom the luminal as well as the parenchymal sachusetts General Hospital, Boston. MA 02114. USA. sides of cerebral blood vessels*.