The Brain as a Chemical Machine

Nicotinic receptors and neuronal communication

Jean-Pierre Changeux, Stuart J. Edelstein

• Publisher: Odile Jacob
• Year: 2005
• Print ISBN: 9782738128751
• eBook ISBN: 9782738179487
• Topic: Medizin
• Subtopic: Medizinische Theorie, Praxis & Referenz

The purpose of this book is to give a clear and straightforward account of the remarkable properties of the nicotinic receptor for acetylcholine, a membrane protein involved in chemical transduction in the nervous system that is also the target of a widely used drug, nicotine. This molecule also happens to be the first pharmacological receptor and ion channel ever to have been identified. Jean-Pierre Changeux has played a leading role with Stuart J. Edelstein in the investigation of nicotinic acetylcholine receptors and allosteric proteins. The aim of this book is not only to review the most recent experimental and theoretical breakthroughs in the study of the nicotinic receptor, but also to give the reader a sense of the intellectual excitement and adventure that accompanied the various stages of discovery. This richly illustrated volume furnishes an exceptional opportunity for scientists and students to follow the course of a major advance in our understanding of the molecular basis of brain functions. Jean-Pierre Changeux is honorary professor at the Collège de France and at the Institut Pasteur, a member of the French Academy of Sciences. In addition to L'Homme neuronal [Neuronal Man] he is the author of Raison et Plaisir and L'Homme de vérité. He is also co-author, with Alain Connes, of Matière à penser [Conversations on Mind, Matter, and Mathematics] and, with Paul Ricœur, of La Nature et la Règle [What Makes Us Think?]. All thought-provoking works. Stuart J. Edelstein is Professor of Biochemistry at the University of Geneva and a foreign associate member of the Academy of Sciences. "The nicotinic acetylcholine receptor has served for many decades as the prototype for neurotransmitter receptors. Acetylcholine was the first neurotransmitter shown to be involved in the fonction of the mammalian brain and its nicotinic receptor the first receptor to be characterized. Jean-Pierre Changeux is the indisputable pioneer in this field. This volume summarizes with great lucidity the history of a highly important topic in neuroscience." Paul Greengard, Nobel laureate in Medecine - The Rockefeller University "From the molecule to thought itself - an extraordinary journey! Changeux and Edelstein are uniquely qualified to relate this utterly fascinating story, whose philosophical implications are no less important than the scientific research underlying them." Jean-Marie Lehn, Nobel laureate in Chemistry - ISIS-Université Louis Pasteur, Strasbourg "The human brain is as much a chemical as an electrical network. Its intricacy and sophistication set it apart from any known technical device. The groundbreaking papers by Monod, Jacob, Wyman, and Changeux in the 1960s on chemical regulation and control were eye-opening for all us who were doing experimental research in ths field, and they have turned out to be crucial for understanding biological evolution and learning in a broad sense. Since then Changeux and Edelstein have achieved international fame for their work on nicotinic acetylcholine receptors, amply documented in this masterful account." Manfred Eigen, Nobel laureate in Chemistry - Max Planck Institute for Biophysical Chemistry, Göttingen "One hesitates to call this book a monograph, for despite its comprehensive treatment of a complex subject it is not meant solely for specialized readers. In concentrating on a single class of neuroreceptors, the nicotinic acetylcholine receptor, it seeks to draw out general principles which apply more widely. It will therefore be welcomed not only by serious workers and students in the field of neurobiology, but also by anyone interested in the broader field of neuroscience." Sir Aaron Klug OM FRS, Nobel laureate in Chemistry - University of Cambridge "Changeux and Edelstein have provided a concise yet highly comprehensive account of perhaps the prototypical neurotransmitter complex, the nicotinic acetylcholine receptor. The story of how the roles played by this signal transduction system in nicotine dependence, learning, memory, and the processes of cognition came to be unraveled is an exciting saga, both beautiful and profound. A lovely historico-scientific document." Floyd E. Bloom, Professor Emeritus - The Scripps Research Institute "Changeux and Edelstein describe a classically Cartesian process of scientific investigation that leads to a most non-Cartesian conclusion. Having elucidated the mechanisms of action and interaction by which the various elements that make up the nicotinic acetylcholine receptor operate throughout the nervous system, from neuromuscular junctions to the brain itself, the authors turn to the role of thse structures and mechanisms in supporting cognition and giving access to consciousness - thus parting ways with Descartes and the view that the mind is able somehow to exist independently of the body. A work of truly remarkable erudition and insight." Roger Guillemin, Nobel laureate in Medicine - Salk Institute for Biological Studies "This book is unlike any recent scientific book. It is more like a forty-year research meeting in one of the world's most creative neurobiology laboratories—an intellectual tour de fortcheat surveys the developmental trends and achievements of twentieth-century neuroscience in molecular, structural, and functional terms. The book therefore becomes an extraordinary educational saga, moving from Sir Henry Dale's pharmacology of nicotine to genetic diseases involving mutations of the cation channel function of nicotinic acetylcholine receptors. Research into these archetypal proteins has been carried out by pharmacologists, biochemists, molecular biologists, electrophysiologists, behavioral scientists, and geneticists, with Jean-Pierre Changeux and his coworkers participating in every aspect of this remarkable inquiry. Nicotinic acetylcholine receptors are the workhorse of the fast actions of the chemical signal acetylcholine, abundantly transmitted in both the peripheral and the central nervous system. Thanks to their variable sub- unit composition they come in many flavors, mediating control of voluntary muscles in the periphery and helping to regulate reward functions, cognition, and memory in the brain. This rich functionality leads the authors to describe models of neuromuscular junction development as well a global workspace model of cognitive function and its role in effortful learning. The nicotinic acetylcholine receptor was among the first ligand-gated ion channels to be sequenced and studied by patch-clamp methods. It has been the object of neurobiological research in England, France, Germany, Japan, and the United States, with contributions of equal weight being made by many teams of researchers over a number of decades, all carefully chronicled and explained by the authors. This book is to be highly recommended to young scientists who want to discover into how many fields a single protein molecule can take them—from snake venom action to myasthenia gravis, addiction, learning, and schiz- ophrenia—if they are willing patiently to learn new research techniques rather than specialize in a single method or instrument. To investigate the nicotinic acetylcholine receptor in all its aspects requires a Renaissance mind, and it is exactly this that Changeux and Edelstein have brought to bear on one of the most studied topics in neuroscience of the last century." TAMAS BARTFAI, Chair and Professor, Department of Neuropharmacology The Scripps Research Institute

Chapter 1

Historical Background

The word “neuroscience” is relatively new, referring as it does to a field of scientific inquiry that developed only recently. Until the late 1960s, investigation of the nervous system, and the brain in particular, dealt mainly with its anatomy (observed by means of light or electron microscopy), its physiology (using electrical recordings), and its chemistry and pharmacology. Psychology and animal behavior were still seen as belonging to the humanities rather than the physical sciences, despite the attempts of physiologists to establish causal relationships between brain anatomy and behavior. In 1971, the first annual meeting of the newly created Society for Neuroscience attracted 1,100 scientists. Was this event evidence of a major paradigm shift resulting from the formulation of a new body of explanatory hypotheses? Or was it simply the consequence of a development and strengthening of local modes of interaction between disciplines, with novel techniques being introduced from different domains of research, extending and combining concepts from distinct, apparently unrelated schools of thought? It is still too early to decide which of these interpretations is correct.

At nearly the same moment, convergence among a number of different scientific disciplines had led to the identification of the first neurotransmitter receptor. Four distinct research traditions — or what we may refer to as “cultures” — contributed to this achievement, and to modern work on receptors generally, focusing respectively on receptor pharmacology and chemical transmission at synapses; enzyme activity and stereochemical specificity of binding sites; electric potentials and ion channels; and allosteric transitions and elementary mechanisms of signal transduction. We begin by examining these four scientific cultures in historical perspective. In subsequent chapters we will consider the current understanding of acetylcholine receptors with respect to genes and their regulation, quaternary structures, agonist-binding sites, ion channels, allosteric regulation, genetic diseases, synaptic membrane assembly, and higher cognitive functions.

Receptor Pharmacology and Chemical Transmission

Early Investigations of Drug Action

The action of drugs on the human organism was first observed in prehistoric times, in connection with shamanic practices based on primitive experiments involving natural substances derived mainly from plants. The word “pharmacology” itself comes from the Greek pharmakon, signifying the magical action of a substance either in curing disease or causing death. The emergence of rational medicine with the Hippocratic school soon eliminated the term’s magical content, and it gradually came to designate a substance that modifies the condition of the organism, whether healthy or sick, in a definite manner. With the introduction of the notions of “active principle” and “active dose,” due to Paracelsus in the sixteenth century, and the insight that these substances were chemical entities having distinct structures, due to Lavoisier in the late eighteenth century, the modern idea of pharmacological agents was established. Our concern here is with the specific effects and mode of action of such agents, particularly certain compounds extracted from plants such as nicotine and curare.

The systematic experimental investigation of the effects of medicines and poisons made considerable progress in the mid-nineteenth century with the pioneering work of the French physiologist Claude Bernard. Several of his 1857 lectures at the Collège de France on toxic and medicinal substances were devoted to the physiological effects of curare (a substance used by native South Americans to poison the tips of arrows) and the target of its paralytic action. Bernard sought to understand its physiological effect by “localizing” the site of this action. Curare, he said, acts as a “chemical lancet” that “dissects the motor system,” by which he meant that curare blocks motor (but not sensory) activity. In particular, he demonstrated that curare does not alter muscle contraction, but instead affects the action of the motor nerve on the muscle. Surprisingly, however, he concluded that curare acted on the motor nerve ending and from there traveled through the central nervous system. His disciple Alfred Vulpian correctly reported in 1886 that curare does not act on the motor nerves themselves — more precisely, the ventral roots of the spinal cord — but causes “communication between nerve fibers and muscle fibers to be interrupted” (see discussion in Changeux, 1979).

Early Conceptions of “Receptors”

The theoretical notion of a pharmacological receptor as it is used today in the neurosciences can be traced to the work of the English physiologist John Newport Langley during the period 1905 – 1908, though a closely related concept had already emerged independently from studies in immunology carried out by the German bacteriologist Paul Ehrlich (1897). Inspired by the work of Claude Bernard (to whom he gave ample credit), Langley sought to localize and further specify the actions of curare and nicotine on neuromuscular preparations from fowl. Like curare, nicotine is native to the New World. It was first isolated by the French chemist Nicholas-Louis Vauquelin in 1809 from tobacco plants that had been introduced in Europe in the sixteenth century, in part owing to the interest of the French ambassador to Lisbon, Jean Nicot de Villemain. Langley showed that nicotine blocks transmission in a manner different from that of curare. Applied to neuromuscular preparations, it first causes a contraction and then produces a blocking effect. In modern terms, nicotine (like acetylcholine) behaves as an agonist, and the response desensitizes. Curare, on the other hand, does not cause contraction, but instead blocks the effect of nicotine; that is, it behaves as an antagonist of nicotine action.

It remained unclear from the work of Bernard and Vulpian, reviewed by Langley (1906) in his Croonian Lecture of that year, as well as from studies by Wilhelm Kühne in Germany, whether curare (and nicotine) acted on the neural or muscular side of the motor endplate. To resolve this issue, Langley (1905, 1906) denervated the frog gastrocnemius muscle. Several weeks later, despite degeneration of the nerve terminal, the muscle tissue still responded to nicotine and curare, “the only difference apparently observed being an increased response to small doses of the poison.” Since neither of these compounds prevented the contraction of the muscle, he concluded that “the muscle substance which combines with nicotine and curare is not identical with the substance which contracts. It is convenient to have a term for the especially excitable constituent, and I have called it the receptive substance. It receives the stimulus and by transmitting it causes contraction” (1906).

In an important further observation, Langley (1907) noted that “a small drop of nicotine placed near, but not touching a nerve ending is without effect, whilst placed on the nerve ending produces local contraction.” The receptive substance therefore appeared to be concentrated under the nerve terminal. Comparing the effect of nicotine on striated muscle with that of adrenaline on “unstriated muscle and glands” (Elliott, 1904, 1905), Langley concluded that “the different systems of efferent nerves would chiefly, at any rate, owe their difference to the different characters of the receptive substances of the cells with which they have become connected.” This physiological concept of a receptor, whose normal function is to receive “special substances secreted by the end of the nerve” — what we now call neurotransmitters — complemented the structural views of stereochemical specificity developed by Ehrlich and Fischer (see below). Even though Langley’s “receptor theory” was dismissed as overly speculative, even “unnecessary,” by some of the most distinguished pharmacologists of the period (Dale, 1943), it nonetheless provided a useful and productive interpretation of experimental data on drug action in both peripheral and central synapses. The distinction by Dale (1914) between the “muscarine action” of “certain esters and ethers of choline” upon smooth muscles (blocked by atropine) and the “nicotine action” of these esters and ethers upon striated voluntary muscles (blocked by curare) appears in retrospect to have been in agreement with Langley’s receptor theory, and indeed may have been influenced by it.

Discovery of the Synapse

Anatomical studies of brain tissue in the nineteenth century wrestled with the question whether its component cells form a reticular continuum or a discontinuous network. A turning point was reached in 1889 with the publication by the Spanish anatomist Santiago Ramón y Cajal of a paper entitled “Conexión general des los elementos nerviosos,” in which he defended the idea — developed and extended in his authoritative Textura del sistema nervioso del hombre y de los vertebrados (1899) — that nerve cells form independent units, or “neurons” (the term introduced by Heinrich Waldeyer in 1891), which are juxtaposed in “contiguity” rather than in “continuity” with one another. The neuron was supposed to mediate the propagation of electric signals — for which Cajal used characteristic arrows to specify the “marche des courants,” or movement of currents — throughout the neural network. According to his concept of dynamic polarization, the nerve impulses collected by the dendrite and the cell body propagate in a polarized manner toward the axon. However, Cajal’s neuron doctrine did not clearly specify the function, if any, of the “articulation” between nerve cells (today called the synapse) in the polarity of neuronal transmission; indeed, Cajal favored assigning responsibility for this process to the neurofibrillary network within the nerve cell. It was the English physiologist Sir Charles Sherrington who explicitly argued on behalf of the synapse’s role in this regard in 1897. Trained by Langley, with whom he had published an article in 1884 on the consequences of excising portions of the brain in dogs and monkeys, Sherrington met Cajal on a visit to Spain in 1885 and his subsequent experimental work on spinal reflexes owed much to Cajal’s influence. On the basis of in vivo experiments, Sherrington conceived of the synapse as a kind of intracellular “valve” that creates the polarity and threshold for transmission underlying what he called the “discontinuous chain of neurons.” More than two centuries earlier, in his Traité de l’homme (1648), Descartes had proposed nearly the same word — “valvule” — for structures that orient the unidirectional flow of “animal spirits” from the nerve to the muscle.

Confirmation of the status of the synapse as a morphological entity had to await the development of the electron microscope (Palade and Palay, 1954; Robertson, 1956). The images demonstrate that the juxtaposed cell membranes do not fuse, but are separated by a gap about 50 100 nm wide, with vesicles 30 – 60 nm in diameter visible on the side of the nerve ending and characteristic densities and folds (Couteaux, 1958) on the postsynaptic side (see Figure 1.1). Although the function of the synapse as a valve in the propagation of nerve signals had already largely been established by physiological data at the beginning of the century, controversy persisted until the 1950s with regard to the actual mechanism of synaptic transmission. Was it electrical or chemical?

A chemical basis for the propagation of signals in the nervous system had initially been suggested by the German physiologist Emil Du Bois-Reymond, and later was explicitly formulated by one of Langley’s students, Thomas R. Elliott (1904, 1905). The neurotransmitter synthesized and stored in the nerve ending is released upon the arrival of an electrical impulse; after diffusion in the synaptic cleft, it serves as a chemical relay that creates a converse chemoelectrical transduction at the level of the postsynaptic membrane. As a result of subsequent studies on the chemistry and pharmacology of neurotransmitters by Otto Loewi, Henry Dale, Ulf von Euler, and their students (see review in von Euler, 1981), the role of chemical transmission in the peripheral nervous system came to be accepted. With regard to activity in the brain itself, however, the electrical hypothesis continued to receive support from a number of distinguished scientists (among them Erlanger and Gasser, Lorente de Nó, and Eccles), until finally it was recognized that chemical and electrical synapses coexist in the nervous system (see review in Eccles, 1964).

FIGURE 1.1 Diagram of the motor endplate showing the junction between a motor nerve and a striated skeletal muscle fiber. The nerve ending is in light gray and the fringe under the nerve represents the folds of the muscle postsynaptic membrane. The gray nuclei with small nucleoli belong to the Schwann cells of the nerve; the larger paler ones with large nucleoli are specialized muscle nuclei known as “fundamental nuclei,” which play a critical role in the biosynthesis of the postsynaptic membrane. Mitochondria (black dots) also accumulate in the vicinity of these nuclei in the postsynaptic domain. (From Couteaux, 1978.)

Enzyme Αctivity and Stereochemical Specificity

The Forerunners of Modern Enzymology

A second research tradition, focusing on enzymes and related processes, played a crucial role in the history of receptors. Beginning in the nineteenth century, it developed alongside, and at times together with, the pharmacological tradition. In this context it should be recalled that Louis Pasteur’s early work as a chemist, establishing the relationship between the crystalline form of tartrate isomers and their action on polarized light, led him to the seminal observation that living organisms such as molds and bacteria selectively utilize — and therefore recognize — a particular isomer in an asymmetrical (or “stereospecific”) manner. The extension of this concept to the fermentation of organic substances subsequently gave rise to the concept of enzyme stereospecificity. Inquiring into the difference in taste between isomeric forms of asparagine, Pasteur had already postulated in 1886 that the “asymmetric active body which would play a role in the nervous impression, translated by a sugar taste in the one case, and a nearly insipid one in the other, would be nothing other, in my view, than the nervous matter itself, an asymmetric matter, as for all primordial substances of life” (see review by Debru, 1983).

The German chemist Emil Fischer (1894), in his extensive studies of the chemical synthesis of sugars, their isomeric forms and stereochemistry, and their selective transformations by yeast through respiration and fermentation, went further than this. He proposed that cells contain “chemically active agents” displaying a configuration complementary to that of the sugars on which they act, and concluded: “It is known that invertin and emulsin show some similarity to proteic substances and undoubtedly, like them, possess a molecule that is constructed in an asymmetric manner. Their limited action on glucoside may then be explained by the hypothesis that an association of molecules necessary to trigger the chemical process cannot take place without an analogous geometrical construction. To use a picture, I would like to say that enzyme and glucoside have to fit each other like a lock and key in order to exert a chemical effect on each other” (Fischer, 1894).

Several years later Ehrlich (1897) extended the stereochemical concepts applied by Fischer to fermentation to the immune reaction. Referring to the interaction of toxins with “antitoxic antibodies,” Ehrlich stated that the “capacity to bind the antibodies must be related to the existence of specific atomic groupings that belong to the toxic complex, display a maximal specific affinity for a given atomic grouping of the antitoxic complex, and easily insert themselves in it, like a key and a lock, to use Emil Fischer’s well-known analogy” (reprinted in Ehrlich, 1957).

In order to go beyond the qualitative notions of Fischer and Langley, quantitative analysis of experimental data was needed. Progress in this area could not be achieved without the development of a mathematical formalism. Interestingly, almost identical models were independently proposed for enzymes (Henri, 1903) and for drug action (Hill, 1910); later models utilized in pharmacology (Clark, 1926; Ariëns, 1954; Gaddum, 1957) were inspired by work on enzyme kinetics (Michaelis and Me...

Table of contents

• Couverture
• Titre
• Copyright
• Preface
• Chapter 1 - Historical Background
• Chapter 2 - Purification and Characterization of the Nicotinic Receptor
• Chapter 3 - Evolution and Diversification of Pentameric Receptor Channels
• Chapter 4 - Chemical Structure of the Agonist-Binding Site
• Chapter 5 - Identification and Properties of the Nicotinic Receptor Ion Channel
• Chapter 6 - Activation and Desensitization at the Structural Level
• Chapter 7 - Three-Dimensional Structure at the Amino Acid Level
• Chapter 8 - Inherited Pathologies of the Acetylcholine Receptor
• Chapter 9 - Genesis of the Postsynaptic Membrane by Targeted Receptor Gene Transcription at the Motor Endplate
• Chapter 10 - Supramolecular Assembly of the Postsynaptic Membrane
• Chapter 11 - Molecular Biology of Brain Nicotinic Receptors
• Chapter 12 - Nicotinic Receptors and Brain Functions
• Appendix
• Works cited
• Index