it seems he was discouraged by the lack of a favorable response from his seniors, langley in particular, and a few years later he left physiological research. the ″birth certificate″ of chemical neurotransmission the breakthrough for chemical neurotransmission came when, in 1921, otto loewi in graz demonstrated the ″humorale übertragbarkeit der herznervenwirkung″ in amphibians. vagusstoff transmitted inhibition from the vagus nerves, and acceleransstoff transmitted stimulation from the sympathetic nerves to the heart. loewi took some years to commit himself with respect to the nature of the stoffe, but in 1926 he was sure that vagusstoff was acetylcholine, writing in 1936 ″i no longer hesitate to identify the sympathicusstoff with adrenaline.″ he was correct in the latter statement. in most amphibian organs including the heart, the concentration of adrenaline far exceeds that of noradrenaline, and adrenaline is indeed the main transmitter. in mammals, however, difficulties arose. in a comprehensive structure-activity study of adrenaline-like compounds, dale and the chemist george barger in 1910 found that elliott's hypothesis assumed a stricter parallelism between the effects of sympathetic nerve impulses and adrenaline than actually existed. for example, sympathetic impulses shared with adrenaline contractile effects in the trigone but not relaxant effects in the fundus of the cat's urinary bladder. in this respect, ″amino-ethanol-catechol″ – noradrenaline – mimicked sympathetic nerves more closely than adrenaline did. the harvard medical school physiologist walter bradford cannon, who had popularized the idea of a sympatho-adrenal system preparing the body for fight and flight, and his colleague arturo rosenblueth developed an elaborate but ″queer″ theory of two sympathins, sympathin e (excitatory) and sympathin i (inhibitory). the belgian pharmacologist zénon bacq as well as canadian and us-american pharmacologists between 1934 and 1938 suggested that noradrenaline might be the – or at least one – postganglionic sympathetic transmitter. however, nothing definite was brought to light till after the war. in the meantime, dale created a terminology that has since imprinted the thinking of neuroscientists: nerve cells should be named after their transmitter, i.e. cholinergic if the transmitter was ″a substance like acetylcholine", and adrenergic if it was ″some substance like adrenaline″. in 1936, the year when loewi accepted adrenaline as the (amphibian) sympathetic transmitter, dale and loewi received the nobel prize in physiology or medicine ″for their discoveries relating to chemical transmission of nerve impulses″. formation and destruction in a review of earlier work on catecholamine biosynthesis, german-british biochemist hermann blaschko (1900–1993) wrote: "our modern knowledge of the biosynthetic pathway for the catecholamines begins in 1939, with the publication of a paper by peter holtz and his colleagues: they described the presence in the guinea-pig kidneys of an enzyme that they called dopa decarboxylase, because it catalyzed the formation of dopamine and carbon dioxide from the amino acid l-dopa." the paper by peter holtz (1902–1970) and his coworkers referred to in that quote originated from the institute of pharmacology in rostock. already in that same year, both blaschko at cambridge and holtz in rostock predicted the entire sequence tyrosine → l-dopa → oxytyramine = dopamine → noradrenaline → adrenaline. edith bülbring, who also had fled national socialist racism in 1933, demonstrated methylation of noradrenaline to adrenaline in adrenal tissue in oxford in 1949, and julius axelrod detected phenylethanolamine n-methyltransferase in bethesda, maryland in 1962. the two remaining enzymes, tyrosine hydroxylase and dopamine β-hydroxylase, were also characterized around 1960. even before contributing to the formation pathway, blaschko had discovered a destruction mechanism. an enzyme tyramine oxidase described in 1928 also oxidized dopamine, noradrenaline and adrenaline. it was later named monoamine oxidase. this seemed to clarify the fate of the catecholamines in the body, but in 1956 blaschko suggested that, because the oxidation was slow, “other mechanisms of inactivation … will be found to play an important part. here is a gap in our knowledge which remains to be filled.” within a year, axelrod narrowed the gap by showing that dopamine, noradrenaline and adrenaline were o-methylated by catechol-o-methyl transferase. to fill the gap completely, however, the role of membranes had to be appreciated ((see below)). noradrenaline thanks to holtz and blaschko it was clear that animals synthesized noradrenaline. what was needed to attribute a transmitter role to it was proof of its presence in tissues at effective concentrations and not only as a short-lived intermediate. on april 16, 1945, ulf von euler of karolinska institute in stockholm, who had already discovered or co-discovered substance p and prostaglandins, submitted to nature the first of a series of papers that gave this proof. after many bioassays and chemical assays on organ extracts he concluded that mammalian sympathetically innervated tissues as well as, in small amounts, the brain, but not the nerve-free placenta, contained noradrenaline and that noradrenaline was the sympathy of cannon and rosenblueth, the ″physiological transmitter of adrenergic nerve action in mammals″. overflow of noradrenaline into the venous blood of the cat's spleen upon sympathetic nerve stimulation two years later bore out the conclusion. in amphibian hearts, on the other hand, the transmitter role of adrenaline was confirmed. the war prevented peter holtz and his group in rostock from being recognized side by side with von euler as discoverers of the second catecholamine transmitter noradrenaline. their approach was different. they sought for catecholamines in human urine and found a blood pressure-increasing material urosympathin that they identified as a mixture of dopamine, noradrenaline and adrenaline. “as to the origin of urosympathin we would like to suggest the following. dopamine in urine is the fraction that was not consumed for the synthesis of sympathin e and i. … sympathin e and i, i.e. noradrenaline and adrenaline, are liberated in the region of the sympathetic nerve endings when these are excited.” the manuscript was received by springer-verlag in leipzig on october 8, 1944. on october 15, the printing office in braunschweig was destroyed by an airstrike. publication was delayed to volume 204, 1947, of naunyn-schmiedebergs archiv für pharmakologie und experimentelle pathologie. peter holtz later used to cite the paper as ″holtz et al. 1944/47″ or ″holtz, credner and kroneberg 1944/47″. remembering his and barger's structure-activity analysis of 1910, dale wrote in 1953: “doubtless i ought to have seen that nor-adrenaline might be the main transmitter – that elliott’s theory might be right in principle and faulty only in this detail. … it is easy, of course, to be wise in the light of facts recently discovered; lacking them i failed to jump to the truth, and i can hardly claim credit for having crawled so near and then stopped short of it.” brain stem nerve cell bodies with noradrenaline and (inset) serotonin the next step led to the central nervous system. it was taken by marthe vogt, a refugee from germany who at that time worked with john henry gaddum in the institute of pharmacology of the university of edinburgh. ″the presence of noradrenaline and adrenaline in the brain has been demonstrated by von euler (1946) and holtz (1950). these substances were supposed, undoubtedly correctly, to occur in the cerebral vasomotor <= vasoconstrictor> nerves. the present work is concerned with the question whether these sympathomimetic amines, besides their role as transmitters at vasomotor endings, play a part in the function of the central nervous tissue itself. in this paper, these amines will be referred to as sympathin, since they were found invariably to occur together, with noradrenaline representing the major component, as is characteristic for the transmitter of the peripheral sympathetic system.″ vogt created a detailed map of noradrenaline in the dog brain. its uneven distribution, not reflecting the distribution of vasomotor nerves, and its persistence after removal of the superior cervical ganglia made it ″tempting to assign to the cerebral sympathin a transmitter role like that which we assign to the sympathin found in the sympathetic ganglia and their postganglionic fibers.″ her assignment was confirmed, the finishing touch being the visualization of the noradrenaline as well as adrenaline and ((see below)) dopamine pathways in the central nervous system by annica dahlström and kjell fuxe with the formaldehyde fluorescence method developed by nils-åke hillarp (1916–1965) and bengt falck (born 1927) in sweden and by immunochemistry techniques. dopamine as noradrenaline is an intermediate on the path to adrenaline, dopamine is on the path to noradrenaline (and hence adrenaline.) in 1957 dopamine was identified in the human brain by researcher katharine montagu. in 1958/59 arvid carlsson and his group in the pharmacology department of the university of lund, including the medical students åke bertler and evald rosengren, not only found dopamine in the brain, but also – like noradrenaline in marthe vogt's exemplary study – in uneven distribution, quite different from the distribution of noradrenaline. this argued for a function beyond an intermediate. the concentration was highest in the corpus striatum, which contained only traces of noradrenaline. carlsson's group had previously found that reserpine, which was known to cause a parkinsonism syndrome, depleted dopamine (as well as noradrenaline and serotonin) from the brain. they concluded that ″dopamine is concerned with the function of the corpus striatum and thus with the control of motor function″. thus for the first time the reserpine-induced parkinsonism in laboratory animals and, by implication, parkinson's disease in humans was related to depletion of striatal dopamine. a year later oleh hornykiewicz, who had been introduced to dopamine by blaschko and was carrying out a color reaction on extracts of human corpus striatum in the pharmacological institute of the university of vienna, saw the brain dopamine deficiency in parkinson's disease ″with <his> own naked eye: instead of the pink color given by the comparatively high concentrations of dopamine in the control samples, the reaction vials containing the extracts of the parkinson's disease striatum showed hardly a tinge of pink discoloration″. in 1970, von euler and axelrod were two of three winners of the nobel prize in physiology or medicine, “for their discoveries concerning the humoral transmitters in the nerve terminals and the mechanism for their storage, release and inactivation”, and in 2000 carlsson was one of three winners who got the prize “for their discoveries concerning signal transduction in the nervous system”. membrane passage membranes play a twofold role for catecholamines: catecholamines must pass through membranes and deliver their chemical message at membrane receptors. catecholamines are synthesized inside cells and sequestered in intracellular vesicles. this was first shown by blaschko and arnold welch (1908–2003) in oxford and by hillarp and his group in lund for the adrenal medulla and later for sympathetic nerves and the brain. in addition the vesicles contained adenosine triphosphate (atp), with a molar noradrenaline:atp ratio in sympathetic nerve vesicles of 5.2:1 as determined by hans-joachim schümann (1919–1998) and horst grobecker (born 1934) in peter holtz′ group at the goethe university frankfurt. blaschko and welch wondered how the catecholamines got out when nervous impulses reached the cells. exocytosis was not among the possibilities they considered. it required the analogy of the ″quantal″ release of acetylcholine at the neuromuscular junction shown by bernard katz, third winner of the 1970 nobel prize in physiology or medicine; the demonstration of the co-release with catecholamines of other vesicle constituents such as atp and dopamine β-hydroxylase; and the unquestionable electron microscopic images of vesicles fusing with the cell membrane – to establish exocytosis. acetylcholine, once released, is degraded in the extracellular space by acetylcholinesterase, which faces that space. in the case the catecholamines, however, the enzymes of degradation monoamine oxidase and catechol-o-methyl transferase, like the enzymes of synthesis, are intracellular. not metabolism but uptake through cell membranes therefore is the primary means of their clearance from the extracellular space. the mechanisms were deciphered beginning in 1959. axelrod's group in bethesda wished to clarify the in vivo fate of catecholamines using radioactively labelled catecholamines of high specific activity, which had just become available. 3h-adrenaline and 3h-noradrenaline given intravenously to cats were partly o-methylated, but another part was taken up in the tissues and stored unchanged. erich muscholl (born 1926) in mainz, who had worked with marthe vogt in edinburgh, wished to know how cocaine sensitized tissues to catecholamines – a fundamental mechanism of action of cocaine discovered by otto loewi and alfred fröhlich in 1910 in vienna. intravenous noradrenaline was taken up into the heart and spleen of rats, and cocaine prevented the uptake, ″thus increasing the amount of noradrenaline available for combination with the adrenergic receptors″. the uptake