Rainer Fried
The Creighton University Omaha, Nebraska 68131
Introduction to Neurochemistry
Neurochemistry is a relatively new branch of biochemistry; its scope has been discussed in "Preview of Neurochemistry," [J. CHEM.EDUC.,45, 181 (196S)l. I t is a rapidly expanding field, in which much information is already available. I n the present section, some of the topics of neurochemistry will be presented. The choice and treatment of subjects discussed will be colored by the personal interests of the author; topics not discussed a t length are as significant as those presented in greater detail. Historical Development
Modern neurochemistry started with the work of Thudichum, a German scientist working in England during the second half of the 19th century. Some of the landmarks of the development of neurochemistry are mentioned in Table 1. Table 1.
Summary Development of Neurochemistry
Year
Event
1865-1882 1921 1928 1955 1956 1956 1958
Thudichum Loewi. "VsrmsStoff" First ~rain5hemistryDepartment First Neurochemistry Book ( I ) Journal of Neuroehemislry First Brain chemistry Congress Neurochemistry Division of the American Academy of Neurology International Society for Neurochemistry
1967
The first research institute dealing with brain and nervous system biochemistry was established in 1928 a t the Kaiser Wilhelm Institute for Psychiatry, and mas headed initially by I. Page. At present, t,he number of specialized institutes is considerable; new institutes are being organized, and many biochemistry departments contain neurochemistry divisions; neurochemistry laboratories may also be integrated with neurology and psychiatry departments. A specialized scientific journal, Journal ofNeuroehenzistry, has reached Volume 1.5, and contains over a thousand pages. A maoualof neurochemistry has been published inits second EDITOR'S NOTE: A brief "Preview of Neurochemistry" hy Dr. Fried appeared in the Research Summaries ,for Teachers column in the Awllarchissue of THIS JOURN.+L 145, 181 (1968)l. This article expands the discussion of the various mess of neurochemical research. The bibliography varies from the usual style for this article: titles of journal articles are given to aid the reader who is interested but, unfamiliar with the literature of neurochemistry.
322 1 Journal of Chemical Education
edition in 1962 (1). It contains over a thousand pages, and lists several thousand references; even now it is outdated in many areas. Neurochemistry is the topic of many international symposia and special congresses. The development of neurochemistry has been ell described in interesting essays (2-4). Even after exhaustive research of the chemical composition of the brain, the brain was considered an organ devoid of metabolic activity for many years. As late as 1957, Page (3) considered neurochemistry to be "a field of endeavor that is not yet fully formed." Great progress has been made since then. One may hopefully expect that therapy for many mental diseases as well as presently intractable neurological diseases will become available. Through neurochemistry, a better understanding of the mechanism of nerve action will be obtained, and the mysteries of the mind will come ever closer to being understood. I n the following section, emphasis will be given to metabolic aspects of neurochemistry, which differ significantly from those of other organs. Special Biochemistry of the Nervous System
Perhaps no other organ shows as close and intricate a correlation among physiology, pharmacology, and biochemistry as does the brain. Here all these sciences are essentially interwoven in order to carry out the specific functions of the nervous system. Keeping this in mind, the present review will focus mainly on biochemical asnects. and will concentrate on those fields which are'correlated with brain function. In addition to neurophysiology, anatomical features of the nervous system play an important role in relation to neurochemistry. While in other organs, e.g., liver, all portions present the same biochemical pattern, this is not true for the nervous system. One finds differeut concentrations of metabolites, as well as differeut e:izymic activity. The following features have special significance. Heterogeneous Cell Population. Neurous aud glial cells, which in turn are subdivided into different types of gliae, show differences in metabolism. Regzonal Biochemistry. Biochemical differences are manifest in different parts of the nervous system (white matter and grey matter), as well as in the central and peripheral neurons. Anatomical regions of the brain show great differences in concentration of metabolites and activity of enzymes; differences are even found in consecutive layers of the same anatomical region, for example, cerebral cortex. Blood-Brain Barrier. By means of this harrier, the brain and nervous system are chemically isolated from other portions of the body, and from the blood, and are
therefore not in equilibrium with the blood and with the metabolism occurring in other sites. Myelzn. A special protective shield, myelin, covers great portions of the nervous system, and is essential for the conduction of the nerve impulse within the nerve fiber. Synapses and Neuro-Muscular Junction. These regions carry out transmission of the nervous impulse from one nerve into another, and from nerve into muscle. The transmission of the nervous impulse involves a set of specific chemical reactions, many of which are unique to these regions. Axons. Nerve cells can have very great length, and parts of them are far removed from their nucleus. The transport of biochemicals within one single nerve fiber, from the nucleus to the remote regions, involves special problems. Some of these special features will he discussed below. Composition o f the Nervous System
Although other scientists contributed t o the early phases of neurochemistry, modern neurochemistry derives from the pioneer work of Thudichum, a physician of great versatility and broad range of interest, who discovered and isolated many of the components of the nervous system, and coined many of the names which are still used today (such as lecithin, cephalin, etc.). It is remarkable that the analytical findings reported by Thudichum compare well with the currently accepted values obtained with much better instrumentation and methodology -. than were available to him (5, 6).
With few exceptions, the metabolism of the nervous system, as well as its composition, is similar to that of other organs. I n Table 2, the composition of skeletal muscle and brain are compared. I t can be seen that Table 2.
Composition of Brain and Muscle"
Com~onent Water Inorganic salts Soluble organic Carbohydrate Protein Lipids Simple fats Cholesterol Phosphatides Cerebrosides
Skeletal Muscle (%)
Whole Brain (%)
75
77-78 1 2 1 8
1
3-5 1 18-20 1 1 2 1
1 2-3 5-6 2
the protein level of brain is about half of that found in muscle, while the fat concentration is about three times as high. Lipids are found in higher amount in white matter of the brain than in grey matter, constituting 56 and 32% of the dry weight, respectively. The brain has a high content of proteo-lipids (which are soluble in chloroform-methanol) and of globulins, 7%-hile albumin and collagen concentrations are low (Table 3). The fat composition of nervous tissue is remarkable for its high content of phospholipids, and of cholesterol, while the concentration of simple triglycerides is much lover than in other organs.
Table 3.
Composition of Mammalion Brain"
Maer~mole~ules Pmtein ~rotedipids Globulins Albumins Collagen
% 8 4 0.5
0.2
%
Lipids Total lipids
a
choksteroi
Triglyeeride Cerebroside Sulfatides Oang1iosides
14 3.3 0. I 4
0.8 0.1
Glyoogen RNA DNA
Neurohumaml Compounds
.~
~
~
%
~,
K A N F E J. ~ , N., in "Encyclopedia of Biochemistry." 1967 ( I 8 ) .
Of special interest are the neurohumoral compounds (Table 3), which are correlated with transmission of the nerve impulse, and have important higher function, including the regulation of behavior. It should be noted that y-aminobutyric acid, which had been proposed as inhibitory transmitter substance, is present in much higher concentration than the other neurohumoral agents. Among free amino acids and amides, by far the highest concentration is that of glutamic acid and glutamine, which are present in 0.01 M solution. The second highest amino acid derivative is acetylaspartate, the function of which is unknown. The nervous system also shows a high concentration of cystathionine, a compound especially abundant in patients with neuroblastoma (Fig. 1). A protein designated S.O -0 C. u Fn,, I OH 100 has recently been isoHC-NH. HC-NH2 HC-N-F-CHI lated from the brain; it is I H O 7 CH2 I CH2 found in no other organs c * c L:zH but is present in nervous I I $ , ct",, WH.I tissue of all species investiam800g u t a m ac ~ a c e t ~ l gated. It is characterized Y.mtynac ( g l u t a m d aspartr ac by i t s high c o n t e n t of H H ~ H *H ~ H o glutamic acid, a n d by ~ - C - C - C - S - C - $ - C : , , ~ Cflathbnme . its acidity. I t s function HO ... A". ' .. .< yet known Figure 1. Aminoacidsofthebroin. is Glutamic acid h a s also been isolated from the brain in small peptides. Attempts have been made to correlate glutamic acid with intelligence and learning capacity, but these concepts could not be established firmly. N L
Neurons a n d Gliae
A unique feature of the nervous system is the cluster of glial cells (10-12) closely surrounding the neurons. Electron microscope pictures have demonstrated that the glial cells are packed around the neurons, and take the place of interstitial fluid and extracellular space found surrounding cells of other organs. Neurons are much larger than glial cells, but the number of gliae is about ten times as high as the number of neurons. Very sophisticated methods and instrumentation, developed by H y d h and his collaborators, made it possible to isolate individual neurons and small clusters of gliae. These methods have been further developed by Giacobini, Lowry, and others, and have contributed significantly to the understanding of brain metabolism. Volume 45, Number 5, M a y 1968
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323
Glial cells were originally considered to be devoid of n~etaholicactivity, and their function seemed to he limited to binding neuronal cells together. The name "glial cell" ("glue" cells) was coined to describe this role. This coucept has proved to be incorrect. I t is now known that glial cells have high metabolic activity. H y d h proposed the interesting suggestion that neuronal and glial metabolism tend to balance each other, and activities which would rise in neurons would decrease simultaneously in glial cells, and vice versa. This may be a significant component of the mechanism of metabolic control in the nervous system. It is interesting to note that most of the tumors of the nervous system originate in glial cells, while neuronal tumors are much rarer. One important enzyme difference concerns the cholinesterase systems, which will be discussed further below: "true" cholinesterase is a component of neuronal cells, while "pseudocholinesterase" is mainly found in gliae. Regional Biochemistry (13)
It has become clear that profound concentration differences of enzymes, metabolites, and neurohumoral agents are found in different areas of the brain, even in closely adjacent ones. A consequence of this is that when preparations from whole brains are used for experiments, the results represent average values. I n order to correlate biochemical concentrations and events with biological and psychic functions, it is necessary to work with well defined anatomical portions of the brain. The concentration of a given metabolite or neurohumoral agent can vary several hundred fold in different regions of the brain. The highest concentration in a given region does not necessarily indicate that the metabolite is of special significance for this brain region; indeed, in some cases, a compound of particular importance within a defined brain region is found in concentrations far removed from t,he extreme values. For example, the concentration of dopamine ranges from a high value of 3.1-8.0 @g/g in the caudate nucleus to a low of 0.01-0.07 pg/g in the thal* mus, while its concentration in the substantia nigra, where it is specially important for normal brain function, is 0.40 pg/g (l4).' A representative distribution of enzymes in five brain regions is shown in Table 4. Table 4.
Regional Distribution of Enzymes in Pigeon Brain and Liver*
Anstomioal Region
CHTP-D
MA0
898
0.26
2258 1958 880 366
0.26
16.5 20.8
C ~ A C ~ AChE
Telenoephalon Disnoe~halonplus optic lobes Cerebellum Medulls oblongata plus pons Liver
2.26 8.54 0.17 6.80 ...
0.03
0.10
0.57
8.2 19.4 44.5
pmola/g/hr;
" All npeoific enryme activities expressed as n = 6 for all hasays except AChE (n = 7). & C h A o = oholine acetylase: AChE = acstyloholinesterass: 5-HTP-D = hydrox~try~tophe.n decaiboaylase: M A 0 = monhmine APnLsoN, el el., J . Ncurochem., 11,341
(1964).
oaidase.
Such a variation is typical, and can be shown to exist for other enzymes and brain regions.
factors which would impair its normal functioning. The existence of the blood-brain barrier mas discovered and originally studied by Ehrlich, the discoverer of Salvarsan, who was also a pioneer in immunochemistry and chemotherapy. Ehrlich found that n-hen dyes are injected into a living animal, they spread uniformly throughout the body, but the brain and nervous system remain unstained. This difference in staining does not occur when dyes are injected after death. Thus, the blood-brain barrier is essentially related to the processes of life. Its function is to buffer the composition of the brain against possible noxious influences which can stem from poisons in the blood. This protection is active not only against poisons, but also against imbalances of compounds normally present in the body. Injection of amino acids causes profound changes in the level of free amino acids in the liver, but affects the concentration of these compounds but slightly in the brain. The nature of the blood-brain barrier and its mechanism of action is not yet clear, although it is intensively studied. It is likewise not yet possible to define unequivocally the rules by which a compound either transverses the blood-brain barrier, or is blocked by it. Generally, charged molecules cannot penetrate the barrier from the blood stream, while passage is facilitated for nonpolar molecules. Fat-solubility is a factor which enhances penetration into the brain. I t is noteworthy that glucose and oxygen can penetrate the brain freely. The blood-brain barrier has so-called vectorial properties; that is, there is a difference in the direction in which metabolites and foreign compounds can penetrate it. Substances, which will not pass into the brain from the blood stream, will be found in the blood and distributed throughout the body, after being injected into the brain directly. I t should be mentioned that barriers also exist within the brain; and there is no free diffusion of metabolites from a given brain region into all others. The blood-brain barrier is impaired in certain abnormal states. Thus, lack of oxygen eliminates the blood-brain barrier effect, but it is reestablished when oxygen is again present in required concentrations. Certain diseases, for example, brain tumors, cause a disappearance or loss of efficiencyof the blood-brain barrier. The penetration of certain compounds containing radioactive isotopes is used as a diagnostic test for brain tumors. The radioactive compounds do not enter the brain under normal conditions, but can be found inside the brain when brain tumors are present. The blood-brain barrier is not developed at birth, and requires several months (the duration depends on the species) till it is fully formed. Thus very young animals lack a protective factor which is present in older ones; young animals are therefore more prone to intoxication. A practical implication of great signilicauce is that drugs, which would not cause harm to the pregnant mother may cause considerable damage in the embryo or nursing infant. One must add here that the existence of the blood-brain harrier is not the only
Blood-Brain Barrier (15)
The brain is mechanically well protected by the skull, the scalp, and the cerebro-spinal fluid. I t is equally important to protect the brain against harmful chemical 324
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Journal of Chemical Education
'Values reported in the Literature for the same compound extend over a rather wide concentration range; this is in part due to technical differences related to their analysis.
factor which plays a role in and varies between embryos and young animals on the one hand, and fully developed ones on the other. A number of enzymes necessary for detoxication, especially the microsomal NAD-linked cytocbrome reductases, are likewise late in developing. Transmission of the Nerve Impulse (16)
The physiological aspects of the transmission of the nerve impulse has been amply reviewed (17) and will not be discussed here. Different mechanisms are involved for conduction of the electrical impulse along a nerve fiber, and for crossing from one nerve into another (at synaptic junctions) or from nerve into other tissues (e.g., neuromuscular junction). The migration of the impulse is accompanied by exchange of Na+ and I ~ L E RN. , E., "Chemical Coding of Behavior in the Brain."Sczenee. , , 148.328-338 11965). , ~~~, SCHILDKRAUT, J. J., AND KETY,S. S., "Biogenie Amines and Emotion,"Science, 156,2130 (1967). BENNETT, E. L., DIAMOND, M. C.,KRECH,D., AND ROSENzaErG, RI. R., "Chemical and AnatomicalPlasticity of the Brain," Science, 146, 610-619 (1964). WOOLF,L. I., "Inherited Metabolic Disorders: Errors of Phenvlalanine and Tvrosine Metabolism." Ado. Clin. he my, 6,97-230 (1963j. SEEGMILLER, J. E., ROSENBLOOM, F. M., AND KELLEY, W. N., "Enzyme Defect Associated with Sex-Linked Human Neurological Disorder and Excessive Purine Synthesis," Science, 155,1682-1684 (1967). D. X., "Biochemical AsGIARM-4~, N. J., A N D FREEDMAN, pects of the Actions of Psychotomimetic Drugs," Pharmacol. Rev., 17, 1-25 (1965). CL.Ietaholic Diseases of the Nervous System," in " D i s e ~ ~ e of s Metabolism," (Editor: G. G. Ilunmn), W. B. Saunders, New York, 5t,h Edition, 1964, nn. 140.5-30. S o u n ~ ~T. s ,L., "Biochemistry of Mental Disease," Harper and Row, New York, 1962. JORDAN, W. K., aspect,^ of Brain Metabolism Important in Cerebral Palsy," Cerebral Pals?, Review, 14, 3-10 (19.53). SCHEINBERG. L. C.. AND KOREY.S. R.. "Multi~leSclerosis." Ann. Rev. Med., 13,411430 (1962): AND ACHESON, E. D., "MultiLUMSDEN, C. E., MCALPINE, ple Sclerosis, a Reappraisal," Livingston, Edinburgh, 1965. SOURKES, T. L., A N D POIRIER,L. J., "Neurochemicd B~ase.5 of Tremor and Other Disorders of Movement," Can. Med. Assoc. J., 94, 53-60 (1966). ~~~
I
I
~ \ ~ A C I N T OF.S HC., , "Formation, Storage and Release of Aretvleholine at Nerve Endina." Can. J . Btoehem. ~ h & l . , 37, 343-356 (19.59). MACINTOSH. F. C., "Synthesis m d St,orageof Acetylcholine in Ne~.vousTissue," Can. J . Riochem. Physiol., 41, 23.552371 (1963). "Symposium on the Function of Acetylcholine as a Synaptic Transmitter," Can. J . Rioehem. Ph?,siol., 41, 2553,?fix - - .- ilQfi?> - ..-- ,. EHRENPREIS, S., i'Acet~ylcholine and Nerve Activity," Xature, 201, 887-893, (1964). G. B., "A New General Concept of theNeurohumKOELLE, oral Functions of Acetylcholine and Aeetylrholinestersse," J. PRarm. (London), 14, 63-90 (1!16'2). WHIPPI.E,H. E., (Editor), "llesea~.chin Uemyelinating Diseases,'' Ann. A'. Y. Acad. Sci., 122, 1-570 (1965). L. A,, N O ~ O N W., T., TERRY,R. T., "The PrepAI-TILIO, nrat,ion and Some Propertic? of Purified Myelin from the Central Nervous System," J . Neurochem., 11, 17-27 (1964). VANDENHEUYEL, F. A,, " S t m ~ t u r dStudies of Biological Uembranes; the Structure of Myelin," Ann. N. Y. Aead. St?... 122.. 67-76 (1965). FRIEDE,R. L., "Topographic Brain Chemistry," Academic P~.esx,New York, 1966, seep. 413. ~'HI~T.\KE V.RP., , AND GR-IY,E. C., "The Synapse: Biology and Morphnlogy," Bril. Med. Bul., 18, 223-228 (19621. . . ECCLES,J. C., "The Physiology of Synapses," New York, Academia Prc9siis,1964. H HIT TAKER, V. P., MICHAEI~SON, I . A., A N D KIRKLAND, R. J., "The Separation of Synaptic Vesicles from Nerve Ending Particles (Synaptosomes)," Biochem. J., 90, 293303 (1864). R., "Praet,ieal Neuro~ I C I L W X I11., N , A N D RODNIGHT, chemistry," L M e , Brown Ca., Boslon, 1962. SALGANIKOFF, L., AND DERO~ERTIS, E., "Sub~ellula~ Distribution of the Enzymes of the Glutamic Acid, G1ut.amine, and r-Amino-Butyric Acid Cycler; in Hat Brain," J . Neuroehem., 12,287311 (1963). BRODIE.B. B.. I N D COSTA. , E.., "Some Current Views on Bmin Monamines," Ps~,chopharmacologyService Center Bull., 2, #5,l-25 (1962). BRADLEY, P. B., (Edilov), "Pharmacdogy of the Central Nervous System," Brit. Med. Bull., 21, 1-96 (1965). ACHESON, G. H., (Editor), "Second Symposium on Cat* rholamines," Pharm. Reus. 18, Part 1, 1-804 (1966). HORNYKIEWICZ, O., "Dopamine (3-Hydroxytyramine) and Brain Formstion."Pharm. Rev.. 18.925-964 11966). GLOWINSKI, J., " ~ e t a b o l i s mof ~ore$nephrine in the Central Nervous System,"Phawn. Rev., 18, 1201-1238 (1966). GARR.~~IN S.,I ,AND VALZELLI,L., "fierotinin," Elsevier Publishing Co., Amsterdam, 1965.
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Volume
45, Number 5, May 1968 / 335
