B.B. Brodie

A concept for a role of serotonin and norepinephrine as chemical mediators in the brain.

It is not the purpose of this paper to make a survey of the work suggesting that serotonin is a central neurohumoral agent, since much of this is being discussed by other contributors to this monograph. Rather, we should like to offer a concept that implicates serotinon and norepinephrine as chemical mediators of mutally antagonistic centers in the brain. This thesis, which is admittedly oversimplified, endeavors to explain the actions of the tranquilizing agents reserpine and chlorpromazine and the hallucinogenic agents LSD and mescaline in terms of interactions with serotonin and norepinephrine in the central nervous system. As presented here this concept may begin to trace the outlines of a coherent picture, but one, we are sure, that will bear only a faint resemblance to the picture that will ultimately emerge. The concept is only a working hypothesis, but i t has already proved useful in linking a number of unrelated observations and in suggesting certain experiments that might test it. It is fascinating to learn how the discovery of serotonin, a substance that appears to have no part in the general metabolism of cells, has proved to be of such significance to the pharmacologist, the biochemist, the neurophysiologist and, possibly, to the psychiatrist. When the vasoconstrictive material in blood platelets was finally isolated and identified as 5-hydroxytryptaminel1, i t was soon proved identical with the enteramine that Erspamer3 had extracted years before from the gastrointestinal tracts of the vertebrates and from other organs in the invertebrates. A number of notions concerning its role in the body were considered, most of which took into account its profound contractive action on smooth muscle. Among these possibilities were the control of vascular tone and, therefore, of systemic blood pressure; control of gastrointestinal motility; regulation of water excretion by affecting kidney arterioles; and regulation of hemostatic action by affecting blood vessels after release from platel e t ~ . ~ . Several of these suggestions can be more or less summarily dealt with a t once. Since serotonin is normally present in the body almost entirely in a bound and therefore presumably inactive form, i t is doubtful that there is enough of the free form circulating in plasma to affect the vascular tone either of the body as a whole or of the kidneys in particular. Results from our laboratory indicate that it is not involved in any obvious way in hemostasis, since animals and humans whose platelets have been depleted of serotonin by the administration of reserpine show no change in bleeding or clotting time.6 It is possible that serotonin controls some aspects of gastrointestinal motility, although no direct evidence is as yet forthcoming in this connection. The clue pointing to a role for serotonin in the functioning of the central

Species, strain and sex differences in metabolism of hexobarbitone, amidopyrine, antipyrine and aniline☆

Abstract Species and strain differences in response to hexobarbitone and presumably to antipyrine, amidopyrine and other drugs can be expressed largely in terms of the activities of the drug-transforming enzymes in microsomes. However, variation in the inherent sensitivity of the central nervous system may also be a factor. The sex difference in the rat in response to hexobarbitone and presumably to other drugs is also a reflection of the activity of the enzyme system in liver microsomes. Sex hormones have a role in regulating sex difference in drug metabolism. The guinea pig and mouse show no sex difference in the metabolism of hexobarbitone. Implications of these findings to drug action and to the development of new drugs are discussed.

Some physico-chemical factors in drug action.

THERE are two facets to the problem of the biochemical and physical aspects of drug action: the influence of drugs on the body and the influence of the body on drugs. Although the present article is concerned mainly with the latter a few thoughts on the influence of drugs on biochemical mechanisms may not be out of place. This aspect of drug action is truly an enigma, which attempts to solve have yielded little success thus far. The mechanism of action of drugs is explainable at a gross physiological level only. This permits us to say, for example, that dibenamine blocks the action of noradrenaline at hypothetical receptor sites ; or that quinidine acts by slowing the conduction time or by increasing the refractory period of heart muscle. But the question of their precise biochemical mechanisms is unanswered*. The orthodox approach, repeated faithfully with almost every new therapeutic agent of importance, has been to determine its effect on one known enzyme system after another on the assumption that the response to the drug is caused by an interference with a known biochemical process. The almost constant failure to relate the effect of a drug on an enzyme system in vitro with its response in the body arouses suspicion that the present approach may be aimed at the wrong target. Physiologists have also experienced little success in connecting known biochemical events with the specialised functions of organs-the beating of the heart, the response of a nerve cell, or the gastric secretion of acid are good examples. Perhaps the present failure to relate biochemical reactions to physiological function or to drug action is an inevitable consequence of thinking in terms of the “universality” of tissue catalysts, itself a useful concept in biochemical speculation, but one which may have been carried too far. This idea implies that functional differences between organs like brain and kidney arise mainly from differences in the organisation and control of the same tissue catalysts. This is like saying that nature plays all her combinations with a pack of only fifty-two cards and that the difference between various kinds of cells is a reflection merely of the difference in the way the cards are dealt. It seems more plausible to explain the specialised functions of the brain and kidney in terms of biochemical reactions that have an unique role in each organ and that these are unlikely to be present in the unicellular organism. The

Compulsive Sexual Activity Induced by p-Chlorophenylalanine in Normal and Pinealectomized Male Rats

p-Chlorophenylalanine depletes brain serotonin and induces longlasting sexual excitation in male rats. The effect of p-chlorophenylalanine is potentiated by pargyline. Administration of 5-hydroxytryptophan to rats treated with p-chlorophenylalanine plus pargyline blocks the sexual excitation. p-Chlorophenylalanine also elicits sexual excitation in pinealectomized rats; this effect is not mediated by the lack of indole hormones in the pineal but may be the consequence of depletion of 5-hydroxytryptophan in the brain and the resulting imbalance between 5-hydroxytryptophan and catecholamine activity in the central nervous system.

PATHWAYS OF DRUG METABOLISM

THE action of a drug would probably last a lifetime if the body did not have ways of limiting its duration. In this lecture I shall discuss the nature of a number of these mechanisms. At one time the kidney was considered the most important organ in enabling the body to dispose of drugs. But it is becoming more and more evident that the kidney usually excretes only a small proportion of a drug in an unchanged form and the bulk as inactive derivatives. Of course there are notable exceptions: tolazoline, an adrenergic blocking agent, is excreted almost entirely unchanged ; a number of sulphonamides are only in part metabolised ; and a considerable fraction of penicillin is found unchanged in urine. But, by and large, the action of most drugs is terminated by their biotransformation. Although the biotransformation of a vast number of drugs in the whole organism has been studied, we have known very little, until recently, of the nature of enzymatic mechanisms concerned in these processes of “detoxication.” The fate of drugs in the body has interested our laboratory for a number of reasons. At one time we considered it possible that biotransformation mechanisms might explain how certain drugs exert their pharmacological action; that is, drugs in being metabolised might become enmeshed in mechanisms involved in the normal economy of the body and thus interfere with normal function. As we shall see later, this viewpoint is difficult to entertain since most drugs are not measurably metabolised in the organ where they act. Another reason for our interest in drug metabolism was the possibility that this might be a backdoor approach to general biochemistry, with the drug being used as a bait to induce unknown enzymes to disclose themselves. Finally, it is thought that a detailed knowledge of enzymes involved in drug “detoxication” might help the medicinal chemist to develop compounds of either high or low stability in the body, whichever would be more desirable in gaining a desired therapeutic result.

A Spectrophotofluorometric Study of Compounds of Biological Interest

Abstract An experimental spectrophotofluorometer and two commercial instruments of essentially the same design have been applied to a broad survey of the fluorescence characteristics of organic metabolites in solution. Useful ultraviolet or visible fluorescence was found to be exhibited by a large number of the light-absorbing compounds of biochemical significance, many of which had not previously been reported as fluorescing in solution. For each fluorophor, the wavelengths of maximum activation and fluorescence, the optimum pH for the development of fluorescence intensity, and an arbitrary measure of the ultimate sensitivity are presented. Some practical aspects of the application of spectrophotofluorometry to qualitative and quantitative analysis are discussed.

Role of sodium, potassium, quabain and reserpine in uptake, storage and metabolism of biogenic amines in synaptosomes

Abstract PREVIOUS reports have shown that after homogenization of rat brain in isotonic sucrose the apparently intact nerve endings are pinched off and can be separated by centrifugation in a discontinuous sucrose gradient (1,2). These so-called synaptosomes contain synaptic vesicles which store acetylcholine, norepinephrine (NE) and serotonin (5HT). However, the synaptosomes appeared to be functionally different from nerve endings in the intact brain in that at 37°C they rapidly lose their content of NE and 5HT (3). This laboratory has previously reported that the storage of NE in slices of rat heart is greatly enhanced by Na+ and impaired by K+ (4). In subsequent studies it was shown that Na+ is essential for the system that stores NE in sympathetic nerve endings and that this process is competitively inhibited by K+ (5,6). Other workers have also postulated that Na+ is required for the uptake of NE by sympathetic nerve endings (7,8). The present paper shows that Na+ is essential for the process that stores and accumulates NE and 5HT and that in synaptosomes the action of Na+ is inhibited by K+. Moreover, evidence is presented which suggests that biogenic amines in synaptosomes are synthesized from endogenous precursors.

Relationship between the lipid solubility of drugs and their oxidation by liver microsomes.

Abstract The oxidative dealkylation of foreign N-alkylamines by rabbit liver microsomes appears to be limited to compounds which are lipid soluble, as shown by high chloroform to water partition coefficients at physiologic pH. Since the microsomal hydroxylation of aromatic compounds also appears to be limited to lipid soluble substances, it is suggested that an intracellular fat-like boundary separates normally occurring polar substances from the highly non-specific microsomal enzymes. The dealkylation of foreign alkylamines is catalysed by at least two enzyme systems in liver microsomes.

Norepinephrine Depletion as a Possible Mechanism of Action of Guanethidine a New Hypotensive Agent

Summary Guanethidine (Su-5864), a new hypotensive agent, depletes the norepinephrine level in the heart of rabbits and cats. It is suggested that guanethidine lowers blood pressure by producing chemical sympathectomy through depletion of norepinephrine from peripheral nerve endings.

Bromobenzene Metabolism and Hepatic Necrosis

Treatment of rats with Phenobarbital stimulates the metabolism of bromobenzene and potentiates the hepatic necrosis elicited by the hydrocarbon. In contrast, administration of SKF 525-A or of piperony