mechanism of action of lsd

Sigma Xi, The Scientific Research Society

Mechanisms of Action of LSD: The study of serotonin-containing neurons in the brain mayprovide the key to understanding drug-induced hallucinations and their relationship todreams and psychosisAuthor(s): Barry L. Jacobs and Michael E. TrulsonSource: American Scientist, Vol. 67, No. 4 (July-August 1979), pp. 396-404Published by: Sigma Xi, The Scientific Research SocietyStable URL: http://www.jstor.org/stable/27849328 .Accessed: 29/01/2014 06:32

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Mechanisms of Action of LSD

The study of serotonin-containing neurons in the brain may provide the key to understanding drug induced hallucinations and their relationship to dreams and psychosis

Barry L. Jacobs Michael E. Trulson

"Last Friday, April 16,1943,1 was forced to stop my work in the laboratory in the middle of the afternoon and to go home, as I was seized by a peculiar restlessness associated with a sensation of mild dizzi ness. Having reached home, I lay down and sank in a kind of drunkenness which was not unpleasant and which was char acterized by extreme activity of imagi nation. As I lay in a dazed condition with my eyes closed (I experienced daylight as disagreeably bright) there surged upon me an uninterrupted stream of fantastic

images of extraordinary plasticity and vividness and accompanied by an intense, kaleidoscope-like play of colors. This condition gradually passed off after about two hours."

This description, from the journal of Albert Hofmann (1), a Swiss chemist working in Basel during World War II, heralds the dawn of the era of ex

perimental study of hallucinogenic drugs, for Hofmann's experience re sulted from his accidental ingestion of an unknown quantity of d-lysergic

Barry Jacobs, Associate Professor of Psychol ogy at Princeton University and a member of the interdepartmental graduate program in

neuroscience, received his doctorate at UCLA and was a postdoctoral fellow in the psychia try department at Stanford. In 1977-78 he was a visiting scientist at the Salk Institute. His research interests include the neural bases of complex mammalian behavior, animal models

of human psychopathology, psychoactive drugs, and sleep. Michael Trulson, who re ceived his doctorate in biopsychology from the

University of Iowa in 1974, has been at Princeton since then as a lecturer and director

of the neurochemistry laboratory in the psy chology department. He is currently an Alfred P. Sloan fellow. His research interests are

primarily in the area of brain neurochemistry and behavior. Address for Professor Jacobs:

Program in Neuroscience, Department of Psychology, Princeton University, Princeton, NJ 08544.

acid diethylamide (LSD). Three days later, in an attempt to verify that the episode was indeed attributable to the ingestion of LSD, Hofmann took what he thought would be a small quantity of the drug, 250 fig. As it turned out, this was approximately five times the dose necessary to pro duce intense hallucinations in an av erage adult male. His personal ac count of the drug's effects was re

markably accurate, since virtually everything he described has been confirmed by subsequent laboratory studies with large numbers of subjects under controlled conditions.

The effects of LSD can be divided into three general categories (2): so

matic symptoms?dizziness, weak ness, tremors, nausea, creeping or

tingling sensations on the skin, and blurred vision; perceptual symp toms?altered shapes and colors, vi sual hallucinations, synesthesia (a

mixing of senses, such as the trans formation of sounds into changes in visual perception), and a distorted time sense; affective and cognitive symptoms?large and rapid mood changes, difficulty in thinking, de personalization, and dreamlike feel ing. The small amount of LSD needed to produce these profound psycho logical effects makes it the most po tent psychoactive drug known, on a

microgram-for-microgram basis.

During the past fifteen years, re search in the field of neuroscience has

greatly elucidated the neurobiological mechanisms that mediate LSD's psychological effects. The primary purpose of this article is to describe the research that has led to our

present understanding of these mechanisms. Much of the discussion will be centered on serotonin, a brain

neurotransmitter that appears to play a key role in the action of LSD. More generally, we will consider brain se rotonin in the broad context of its role in mammalian behavior.

The discussion of the mechanism of action of LSD also illustrates the revolution that has occurred during the past two decades in our under standing of the action of a variety of psychoactive drugs, such as cocaine, amphetamine, and chlorpromazine (the most frequently prescribed an tipsychotic). Although these drugs act through a number of different

mechanisms, they share the common property of altering chemical neuro transmission in the brain. Therefore, our story logically begins with a brief review of chemical neurotransmis sion.

The human brain is comprised of tens of billions of nerve cells, or neurons. Neurons bring information to the brain concerning the current status of, or the occurrence of any change in, the body's internal milieu and the external world. Neurons also call into action many of the glands and mus cles of the body. By acting upon each other, in ways we are just beginning to understand, neurons provide the tremendous integrative capacity of the brain that underlies human

memory, thought, and emotion.

The action of neuron upon neuron is carried out by means of small amounts of chemicals released from the terminal endings of the neurons into the minute gaps, or synapses, between them (Fig. 1). These mole cules then cross the synapse and im

pinge upon the dendrites or the cell body of the receiving neuron and produce either excitation or inhibi

396 American Scientist, Volume 67


Figure 1. One neuron makes contact with an other not by actually touching but by com

municating across minute spaces, called sy napses, by means of chemical neurotransmit ters. The classical synapse is between the axon terminal of one neuron and the dendrites or cell body of another neuron. Fig. 2 shows details of synaptic neurotransmission.

tion. If the summated effect of the hundreds, or even thousands, of in puts that a neuron receives favors excitation and reaches a threshold level, the cell will "fire" and transmit an electrical impulse down its axon.

When the impulse reaches the ter minal portion of the axon and causes the release of some small proportion of the chemical neurotransmitter stored there, the entire cycle begins again.

LSD and serotonin: Historical overview For many years scientists had known of a blood-borne chemical that pro duced vasoconstriction (a serum factor that affected blood vessel tonus, hence the name serotonin) and of a substance present in the gut that caused intestinal motility. In the mid-twentieth century, serotonin, the single compound producing both these effects, was isolated and syn thesized, and its molecular structure elucidated as 5-hydroxytryptamine. Soon thereafter, in 1953-54, serotonin

was found to be present in the mam malian central nervous system (CNS) in significant quantities and to be concentrated in varying amounts in different regions of the brain. This led to the proposal that serotonin was a CNS neurotransmitter. In the twenty-five years since that time, an impressive body of evidence has been accumulated to indicate that seroto nin does indeed act as a central neu rotransmitter.

Structurally, the serotonin molecule is similar to a portion of the larger LSD molecule in that they both con tain an indole nucleus. Based on this structural similarity, Gaddum, working in England, and Wooley, in the United States, independently proposed that LSD might act to block the synaptic action of serotonin in the brain (3, 4). They buttressed their argument by demonstrating that LSD exerted a powerful blocking effect on serotonin's action in peripheral tissue studied in vitro. This hypothesis at tracted a great deal of attention, be cause, in addition to being considered a hallucinogenic drug, LSD was thought by many to be a prototypic psychotomimetic drug?i.e. one whose effects mimicked psychosis. Thus, the argument went, a key ele ment in the etiology of psychosis might be either decreased amounts of brain serotonin or the synthesis of an endogenous compound that anta gonized serotonin's action in the brain.

This ambitious hypothesis was soon shaken by the report that brom-LSD, which is LSD with a single bromine atom attached, although as effective as LSD in blocking serotonin's action on peripheral tissue, was devoid of potent psychic effects in humans (5). It was therefore unlikely that the ability of LSD to block the action of serotonin could account for its hal lucinogenic action, since this ability

was shared by a close analog of LSD that was nonhallucinogenic. It is worth noting, however, that both sides of this argument were based on studies of LSD's action on peripheral tissue. Because of the general inac cessibility of brain tissue, there had been very little direct test of the central effects of LSD. The remain der of this paper will be concerned with examining these latter studies, but because so many of them involve brain serotonin, we must first provide

postsynaptic neuron

Figure 2. An axon terminal of a serotonin containing neuron (presynaptic neuron) makes synaptic contact with one of its target neurons

(postsynaptic neuron). L-tryptophan, the amino acid precursor of serotonin, is brought to the neuron by the blood. Serotonin is syn thesized from tryptophan inside serotonergic nerve terminals and is stored in packets called vesicles. When an action potential invades the axon terminal, the vesicles release their con tents into the synaptic gap and bombard the postsynaptic neuron to produce either excita tion or inhibition. Serotonin is inactivated by being taken back up into the terminal, where it is catabolized by monoamine oxidase (MAO) to form 5-hydroxyindoleacetic acid (5 HIAA).

serotonin

Figure 3. The indole nucleus structure of the serotonin molecule is similar to that of several hallucinogenic drugs, including LSD (as shown in Fig. 6).

some background information about this neurotransmitter.

Basics of brain serotonin Serotonin is a small and relatively simple molecule that is synthesized from the essential amino acid tryp tophan. After a meal, tryptophan is transported, by the blood, from the gut to serotonin-containing neurons, where serotonin synthesis takes place (Figs. 2 and 3). The newly formed serotonin is stored in the axon ter

1979 July-August 397


minai in packets called vesicles. When an action potential invades the axon

terminal, the serotonin molecules are released into the synaptic gap and bombard the serotonin receptors on the postsynaptic, or target, neuron. Inactivation of serotonin's synaptic action is accomplished primarily through a process by which the mol ecules are taken back into the axon terminal. Serotonin is destroyed by the enzyme monoamine oxidase, and the resulting major catabolite of se rotonin, 5-hydroxyindoleacetic acid (5-HIAA), can be measured in brain tissue, urine, and cerebrospinal fluid.

In the mid-1960s, a seminal series of studies employing the newly devel

oped technique of fluorescence his

tochemistry detailed the localization of the cell bodies and axon terminals of the CNS neurons that utilize sero tonin as their transmitter (6, 7).

When brain tissue is treated with

para-formaldehyde gas and then bombarded with ultraviolet light, serotonin-containing neurons give off a yellow fluorescence (Fig. 4). This

technique revealed that CNS seroto nin was localized almost exclusively within neurons of the raphe nuclei (small clusters of cells on the midline of the brainstem) and their axon ter minals. The total number of these neurons, at least in the rat brain, where they have been studied most

intensively, is no more than 10,000-20,000. Thus, they represent a very small proportion of all CNS neurons.

The serotonin-containing neurons (or raphe neurons) have widely ramifying axons that are often sent out over

great distances. For example, those in the posterior portion of the brainstem send their axons down the length of the spinal cord, and those in the dor sal and median raphe nuclei of the anterior brainstem send their axons into various portions of the forebrain. In the forebrain, raphe neurons most

heavily innervate portions of the vi sual system and portions of the limbic system, a group of structures known to be important in emotional experi ence and expression (Fig. 5). Al

though most of these anatomical de tails have been worked out in the rat,% they have relevance to humans be cause the pattern of distribution of serotonin in the central nervous sys tem is fairly constant across a variety of mammalian species.

Figure 4. Serotonin-containing neurons are

made visible through the use of fluorescence

histochemistry. In this photomicrograph of the dorsal raphe nucleus in the midbrain of the rat, each oval yellow spot is the cell body of a sero

tonin-containing neuron; the dark areas at the bottom on both sides are large fiber tracts

(medial longitudinal fasciculi). The photomi crograph covers approximately 1 mm of brain tissue from top to bottom. (Photomicrograph courtesy of George Aghajanian, Yale Univer

sity.)

LSD and serotonin: Modern era The first report that LSD had a sig nificant effect on brain neuro transmission was published in 1961 by Daniel Freedman (8). He found that a single injection of LSD in creased the level of brain serotonin in the rat by 24 percent, while brom LSD, in even higher doses, failed to affect brain serotonin. Thus, although LSD and brom-LSD had similar ef fects on serotonin in the periphery, their central effects were significantly different. In an important extension of this study, Freedman and his col leagues reported that in addition to increases in brain serotonin, LSD produced significant decreases in the brain level of the major metabolite of serotonin, 5-HIAA (9). These findings led to the hypothesis that LSD might inactivate or depress the activity of serotonin neurons. The increase in brain serotonin was attributed to its accumulation within neurons that were no longer releasing it due to their inactivity.

Fortunately, because of the pioneer ing anatomical studies employing fluorescence histochemistry, this hypothesis could be directly exam ined by recording the electrical ac

tivity of serotonergic (i.e. serotonin containing) cell bodies localized in

tight clusters in the brain stem of the rat. In 1968, George Aghajanian in serted microelectrodes into the region of the dorsal raphe nucleus of anes thetized rats and, after obtaining a stable baseline sample of the cell's activity, he intravenously adminis tered a low dose of LSD (10). Nor

mally, the discharge of these neurons in anesthetized rats is slow (1-2 spikes/second) and regular. Following the injection of LSD, as hypothesized, these serotonergic cells displayed an

abrupt and complete cessation of ac

tivity. By contrast, the activity of

neighboring nonserotonergic neurons was either unaffected or slightly in creased by LSD. In a subsequent study, Aghajanian reported that brom-LSD, even in much higher doses, had a much smaller effect on

serotonergic neurons than LSD (11). Finally, through the use of microion

tophoresis, which permits the direct application of small amounts of sub stances to single neurons, Aghajanian demonstrated that LSD depressed the activity of serotonergic neurons

through an effect directly on the cell body and had little direct effect on

any of the other CNS neurons that were studied (12).

LSD's specific depression of raphe neuronal activity, in conjunction with the uniformly inhibitory synaptic action of serotonin in the forebrain, produces a disinhibition of raphe target, or postsynaptic, neurons. Be cause the densest aggregations of these target neurons are in areas of the brain that mediate processing of visual or emotive information, we have an obvious mechanism for ex

plaining the major affective, percep tual, and cognitive effects of LSD. Thus, it is hypothesized that LSD acts to depress the activity of seroto nin-containing neurons, which, through disinhibition, cause a release of activity of neurons in the visual system, the limbic system, and many other brain areas (13). This model does not preclude the possibility that LSD may also exert a direct action on other brain neurons, including sero tonin target cells.

The argument that a significant pro



portion of LSD's psychic effects can be attributed to its action on sero

tonergic neurons is buttressed by the fact that other hallucinogens which produce similar psychic effects, such as psilocin, N,N dimethyltryptamine (DMT), and 5-methoxy DMT (5

MeODMT), also have an indole nu cleus structure (Fig. 6) and depress raphe unit activity (11, 14). On the other hand, psychoactive drugs that elicit different psychic effects, such as

A-tetrahydrocannabinol (THC?the active component of marijuana) or

amphetamine, neither have the indole nucleus structure nor suppress the activity of serotonin-containing neurons. The few human experiments that bear on this issue also provide indirect support for the serotonin

hypothesis. For example, drugs that decrease brain serotonin levels pot entiate the effects of LSD in humans, and drugs that increase brain sero tonin levels decrease the effects.

It was against this backdrop of re search that we began our own studies of LSD and, more generally, the brain serotonin system. We felt that a number of important issues remained to be resolved. Since we are ulti

mately interested in behavior, would the effects of hallucinogenic drugs on

serotonergic neurons be the same in freely moving animals as they were in anesthetized or immobilized animals? On a microgram-for-microgram basis, why is LSD so much more potent than other hallucinogens? What role, if any, do other neurotransmitter systems play in the action of halluci nogenic drugs? What mediates the rapid and dramatic diminution in LSD's psychological effects that fol lows its repeated administration? Is the decrease in activity of brain se rotonin neurons that is produced by LSD approximated by any normal physiological condition?

An animal model for LSD Resolution of many of these issues necessitated examining the behav ioral effects of LSD. Since the use of human subjects in such experiments is precluded for ethical reasons, we turned to animal experiments. How ever, using animals in studies dealing with variables like hallucinations, which are based exclusively on self report, raises the unanswerable question of how to discern what a nonverbal organism is feeling, thinking, or perceiving. Another tack,

hippocampus

PP- P --

...MFB

amygdala

Figure 5. The axons of serotonin-containing neurons often project over great distances. Clusters of cell bodies (dots) of serotonin

containing neurons, both the dorsal and me dian raphe nuclei as well as the group of cells labeled B-9, are shown in cross section through the midbrain of the rat. Axons from these cell bodies ascend into the forebrain to make con

tact with structures such as the lateral genic ulate nucleus (LGNV, a portion of the visual

system) and the hippocampus and amygdala (portions of the limbic system). The medial forebrain bundle (MFB) is one of the major pathways connecting the midbrain raphe neurons with their forebrain target cells.

which obviates the need to impute an

underlying similarity of state, in volves using some aspect of animal behavior (or physiology) as a model of the human variable. Such models are founded on the assumption that their validity can be established through demonstrating that changes in the animal directly parallel changes in humans. The changes need not be homologous, or even analogous, but merely must covary systematically.

In order to qualify as an animal model for the actions of hallucinogenic drugs in humans, a particular behavior

would have to change specifically in response to this class of drug and no other, vary in frequency or magnitude in a dose-dependent manner, be elicited by drug doses within the human range, and closely parallel the

major parameters of the drug's effects in humans (e.g. development of tol erance).

We chose the cat as our experimental animal because of its vast behavioral repertoire and the ease with which any behavioral changes could be ob served. Although previous studies had examined the effects of LSD and related hallucinogens on behavior in a variety of species, few of them had explored a complete dose range or

closely examined the behavioral changes. Most important, none of them had reported an effect that was demonstrated to be specific to LSD. Accordingly, in our initial study, we administered LSD to cats and at tempted to compile a complete and detailed record of all major behavioral changes, with special attention to

Figure 6. The hallucinogenic drugs LSD, psi- have an indole nucleus structure similar to that

locin, and DMT (N,N-dimethyltryptamine) of the serotonin molecule (Fig. 3).



behaviors that might be evoked spe cifically by LSD.

We discovered that, in addition to increased frequency of head and body shakes, grooming, investigatory be havior, and hallucinatory-like re

sponses, two other behaviors, not

previously reported, were observed with high probability under LSD (15, 16). Limb flicks and abortive grooming increased in frequency in direct relation to the dose of LSD

(beginning from a baseline of essen

tially zero in saline-treated animals and progressively increasing in ani mals treated with 2.5, 10, 25, and 50

Mg/kg of body weight). Limb flicking is a species-specific behavior seen in normal cats almost exclusively in re

sponse to the presence of a foreign substance, such as water, on the paw. The paw is lifted and rapidly and

repetitively shaken or snapped out ward from the body (Fig. 7). In abor tive grooming, the cat orients to the body surface as if to groom but does not perform the consummatory grooming response (bite, lick, or

scratch) or performs it in midair. Apparently, as the cat begins to groom, it becomes distracted and never finishes the grooming sequence.

Whether the limb flicks, abortive grooming, head and body shakes, and the like actually represent halluci nations is irrelevant, since the model simply employs these measures as parallels to hallucinations in hu mans.

The specificity of these behavioral changes is indicated by the fact that they are never seen in response to single injections of many other classes of psychoactive drugs, such as THC, amphetamine, caffeine, atropine (an anticholinergic), and chlorphenira mine (an antihistaminic). Nor were they seen in response to brom-LSD, the nonhallucinogenic relative of LSD, or tryptamine, a nonhallucino genic indole nucleus compound. Most important, however, these behaviors

were elicited by hallucinogens that are structurally related to LSD (DMT, 5-MeODMT, and psilocin) and known to depress the activity of serotonin-containing neurons (17). They were also elicited by a drug that blocks serotonin's action on its target neurons and by a drug that decreases brain levels of serotonin by inhibiting its synthesis. Furthermore, when LSD is administered to cats previ ously treated with a serotonin-syn

thesis inhibitor, the behavioral effects of the two drugs are synergistic (18). Therefore, besides helping to estab lish the validity of the model, the data from these drug studies also buttress the serotonin hypothesis of halluci nogenic drug action.

In an attempt to extend the useful ness of this model to known actions of hallucinogens in humans, we ob served something surprising. If an adult human is given an effective dose of LSD (1 Mg/kg) on day 1 and then again on days 2,3, and 4, there will be a marked decrease in the effective ness of the drug in producing psychic change by the second day, and an al most complete loss of effectiveness by day 4 (19, 20). This effect is called tolerance. Paralleling these studies, we gave LSD to cats in an intermedi

Figure 7. When cats are given LSD, or other related hallucinogenic drugs, they display several behaviors that are seen exclusively in

response to these drugs. One of these is the limb flick, which is seen in normal cats only in

response to the presence of a foreign substance on the paw. The paw is raised from the ground and then rapidly shaken or flicked away from the body.

ate (10 Mg/kg) or a high (50 Mg/kg) dose on day 1 and then administered an additional 50 Mg/kg dose on day 2 (21). Much to our surprise, the single 10 Mg/kg pretreatment produced a nearly complete blockade of the be havioral effects of the 50 Mg/kg dose, and the single 50 Mg/kg pretreatment made the second 50 Mg/kg dose as

behaviorally ineffective as an injec tion of saline. In subsequent behav ioral studies we found that this tol erance to LSD, which was complete 24 hours after the initial injection, actually had begun to develop within two hours after the injection, at a time when the drug itself was still exerting its primary effect! We shall return to this finding later, when we discuss research on the mechanism mediating tolerance.

One of the most intriguing aspects of these studies was the fact that the

magnitude of the peak behavioral effect of LSD was significantly greater than that produced by psilo cin, DMT, 5-MeODMT, serotonin receptor blockade, or serotonin de pletion. For example, a 50 fig/kg dose of LSD produced an average of ap proximately 40 limb flicks per hour, whereas the other drugs, regardless of dose, typically produced 5-10 limb flicks per hour. Since all of these drugs, including LSD, were known to block serotonin neurotransmission, we reasoned that the magnitude of LSD's behavioral effect must be at tributable to some additional action. Therefore, we began to explore the possibility that, in addition to sero tonin, other neurotransmitters might be involved.

Studies in several other laboratories had indicated that LSD also acted to mimic the action of the neuro transmitter dopamine (22, 23). We confirmed this directly with electro physiological studies that examined the effect of LSD on the activity of dopaminergic neurons in the rat brain (24). Furthermore, using a simple behavioral model in the rat, we found that another very potent hallucino genic drug, DOM (2,5-dimethoxy-4 methylamphetamine), also had a

significant dopaminergic effect, whereas the indole nucleus halluci nogens psilocin, DMT, and 5 MeODMT were virtually devoid of dopaminergic action (25). Since a previous study had reported that DOM also depressed the activity of serotonin-containing neurons, we

hypothesized that DOM might be as behaviorally effective as LSD in our cat model. This was confirmed with behavioral studies, in which we found that DOM produced approximately 40 limb flicks per hour (17).

Thus, the most potent hallucinogenic drugs may be those that both inacti vate brain serotonin and mimic brain dopamine. Serotonin inactivation may be necessary and sufficient for hallucinogenesis (psilocin, DMT, and 5-MeODMT have serotonergic, but no dopaminergic, action), while the dopaminergic action may modulate the amplitude of the effect. This is supported by clinical evidence. When patients who are having "bad trips" on LSD are given antipsychotic drugs, which are potent dopamine-receptor blockers (e.g. chlorpromazine), they



typically report a diminution in the intensity of the experience but a continuation of the hallucinatory activity. The dopaminergic action of hallucinogenic drugs might also be relevant to the fact that they are fre quently considered to mimic psy chosis (26), since the preponderance of current biochemical evidence bearing on schizophrenia indicates an overactivity in the brain dopamine system. Thus, consideration of a

dopaminergic action of hallucinogenic drugs, in addition to their action on brain serotonin, seems to explain what previously appeared to be somewhat anomalous and disparate findings.

Serotonin neurons and behavior The next step in our research was a crucial one for us. We were interested in recording the activity of seroto nin-containing neurons in freely moving cats so that behavior could be studied concomitantly. Over the past ten years, in conjunction with Dennis

McGinty and Ronald Harper at UCLA, we have developed and re fined a technique first used by James Olds. Basically, it involves recording single neuron activity by means of bundles of insulated microwires that can be advanced through the brain in small steps by an attached mechani cal microdrive (Fig. 8). These elec trodes differ from classical metal

microelectrodes in that they are flexible and have much larger diam eters at the tips (32 vs. 0.1 to 1.0 ^ m). This method allows us to maintain recordings from single cells even fol lowing rather violent movements on the part of the cat (Fig. 9), and therefore allows us to study the ac tivity of the same neuron over long periods of time (often several days).

Prior to examining the effects of hal lucinogenic drugs on both behavior and the activity of serotonin-con taining neurons in the cat, we felt that it was important to provide a general context for these data by first char acterizing the spontaneous activity of these neurons across the sleep wakefulness-arousal continuum (27). During a quiet waking state, sero

tonergic neurons discharge with the slow, regular pattern that character izes the activity of serotonergic neu rons in anesthetized rats. However, this activity can be modulated in both directions, depending on the state of

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Figure 8. A special apparatus was devised to record the electrical activity of single neurons in awake, freely moving animals. A cat is placed in a large, soundproofed box that is electrically shielded. The animal's behavior can be con

tinuously monitored and recorded by videotape through a one-way mirror in one wall of the chamber. Various gross electrodes, such as

those for recording the EEG, as well as the microelectrodes, are attached to a standard connector and the entire assembly is secured to the skull with an acrylic. During an experi

ment, the cat is connected to various amplifiers and a polygraph machine by means of a flexible cable attached to the connector on the animal's head.

* * * * * * * * * *.*.* * *

*T *

Figure 9. An oscilloscope records the electrical

activity of a single serotonin-containing neuron in an awake, freely moving cat, such as the one

shown in Fig. 8. In this 20-second sample of

activity, each vertical line is a neuronal dis

charge.

the cat. The activity increases during periods when the cat becomes active, and briefly increases still further in

response to an arousing or alerting stimulus (e.g. a click or flash). On the other hand, the activity of these cells decreases as the cat becomes quies cent and drowsy (Fig. 10). Their ac

tivity decreases still further when the cat enters the first phase of sleep, and finally ceases during the next stage of

sleep (termed REM sleep because of the appearance of rapid eye move

ments). These data, in conjunction with other evidence, recently led us to propose that the oft-noted phe nomenological similarity of dreams (which occur most vividly in REM sleep) and drug-induced hallucina tions might be mediated, in part, by a common neurochemical event?

inactivation of central serotonergic neurotransmission (28).

With the basic characterization of the activity of these neurons completed, we turned to directly examining the behavioral effects of hallucinogenic drugs, while simultaneously recording the activity of serotonin-containing neurons. We will first describe our results with 5-MeODMT (29), be cause they were the most straight forward. This drug produced dose dependent decreases in the activity of serotonergic neurons and dose-de

pendent increases in specific behav iors (the limb flick response is the

most reliable and easiest to quantify). Furthermore, the onset, offset, and peak of the behavioral effects of 5 MeODMT were temporally corre



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45 m?n

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Figure 10. The activity of serotonin-containing neurons varies dramatically as a function of the state of the animal. This figure illustrates the

activity of the same serotonin-containing neuron across the complete sleep-waking arousal continuum. Each strip shows 60 sec onds of data, and each vertical line represents a cell discharge, or spike. The highest rate of

discharge is seen during active waking; the ac

tivity slows somewhat during quiet waking and as the animal becomes drowsy. A dramatic

slowing occurs as the animal enters sleep (slow-wave sleep-1 through slow-wave sleep-3), until the cell finally becomes silent during the REM phase of sleep, when vivid dreaming typically takes place.

Figure 11. An injection of LSD, in a dose of 50

Hg/kg of body weight, rapidly and dramatically affects the activity of serotonin-containing neurons. Within 15 minutes the cell's activity has significantly slowed, and it reaches its nadir

approximately 45-60 minutes after the injec tion. This cell's activity was maximally de creased by about 75% from the pre-drug base line. The cellular activity returns to normal in 4-6 hours.

lated with the onset, offset, and peak of the changes in neural activity.

By directly correlating behavioral changes with changes in the activity of serotonin-containing neurons, this

study provided perhaps the strongest direct support for the serotonin hy pothesis of hallucinogenic drug ac tion. One of the most interesting as

pects was the finding that significant behavioral changes were often asso ciated with small decreases in raphe unit activity (e.g. 15-20 percent). This indicated that rather subtle varia tions in the outputs of these neurons

might have profound behavioral ef fects.

When we used this approach to ex amine the behavioral and electro

physiological effects of LSD, the re sults were in general very similar to those seen with 5-MeODMT, but with two important differences. First, a high dose of LSD (50 Mg/kg) pro duced a depression of serotonergic neuronal activity that lasted for ap proximately 4 hours (Fig. 11), while the behavioral effects lasted for at least 6-8 hours. Second, when the 50 jug/kg dose was re-administered the next day, it produced little or no be havioral effect, but the neuronal

change was as large as that on the previous day (30, 31).

These two somewhat anomalous findings?i.e. behavior outlasting neuronal change and neuronal change without behavior?are probably in terrelated. It appears that even while

LSD is exerting its primary depres sant effect on serotonin neurons, it is also producing a change in some set of

postsynaptic neurons that will outlast the primary effect and continue to mediate the behavioral change. This is supported by the experiment de scribed above, in which we saw toler ance develop to a single dose of LSD while this dose was still exerting its primary behavioral effect.

General support of these notions about changes in postsynaptic re ceptors came from experiments using gross behavioral measures in rats, in which we observed that repeated ad ministration of LSD markedly re duces the sensitivity of serotonergic target neurons to LSD (32). We have also found that repeated adminis



tration of LSD decreases the number of postsynaptic binding sites for both serotonin and LSD and may affect the affinity of serotonin for its re ceptor sites (33). Repeated adminis tration of LSD might therefore result in a decreased capacity of LSD and/or serotonin to stimulate neurons post synaptic to serotonergic neurons. Thus, tolerance to LSD is not med iated by a change in responsivity or

sensitivity on the part of serotonergic neurons, but an important change seems to occur at the next neuron in the series. What is not clear at present, however, is the exact nature of the change in the postsynaptic neurons.

Why is the relationship between be havior and neuronal change so

straightforward with 5-MeODMT and apparently so complex with LSD? We do not yet have the answer to this at the cellular level, but we do know that no other hallucinogenic drug approaches LSD in its capacity to produce rapidly developing, long lasting, and dramatic tolerance. This remains one of the more intriguing unanswered questions regarding LSD's mechanism of action.

General implications Since animals, including humans, do not commonly ingest plants contain ing hallucinogenic compounds, the system of serotonin neurons must have evolved to subserve some adap tive function other than mediating hallucinations. A large behavioral literature indicates that serotonin

may play a general inhibitory role with respect to a variety of sensory motor processes. Blockade of seroto

nin neurotransmission, whether by destruction of serotonergic neurons, inhibition of its synthesis, or blockade of its receptors, consistently produces an animal that is hypersensitive to virtually all environmental stimuli and hyperactive in virtually all sit uations. Through a general inhibitory function, serotonin neurons may serve to modulate an organism's be havior and maintain it within nar

rowly specified limits.

It seems reasonable that some basic role would be served by neurons whose cell bodies are localized in the lower, more primitive, portions of the brain and whose axons reach widely and diffusely throughout the central nervous system. Consistent with this

concept is the fact that these neurons appear to be among the first to dif ferentiate during the development of the mammalian CNS. This is also supported by the fact that serotonin neurons discharge, in a wide variety of situations, with an almost clocklike regularity?e.g. in anesthetized rats

(34), in awake, freely moving cats (27), and even when examined in 400-yum thick slabs of rat brain tissue main tained in vitro (35).

The slowness and regularity of the

activity of these neurons probably denotes a tonic neuronal function, as

opposed to a rapid and variable ac

tivity, which would carry more in formation and subserve a more dy namic function. A group of farsighted neuroanatomists, who were among the first to study the raphe, specu lated that it is "probably a primitive part of the brainstem which shows relatively little differentiation during the phylogenetic ascent of the verte brates. Correspondingly, one would be inclined to ascribe to it relatively simple, but fundamental and impor tant tasks in the function of the brain" (36).

Our studies of serotonin neurons indicate that as overall level of motor

activity or arousal increases, so does the activity of these cells. Recipro cally, as the animal becomes quies cent and drowsy, the activity of these cells declines, possibly because this inhibitory control is no longer neces sitated. As the animal enters sleep, the cells fire still more slowly, and during REM sleep, a state in which tonic muscle activity is abolished, the cells stop firing. If we consider the action of hallucinogenic drugs in this context, we see a fully awake animal with a brain serotonin system func tioning as though the animal were

asleep. This may provide an impor tant insight into understanding hal lucinations and perhaps, more gen erally, other altered states of con sciousness. In a given behavioral sit uation, an altered state of conscious ness may occur when a key brain mechanism, such as the serotonin

system, functions in a manner that is

appropriate to a different behavioral situation.

We have tried to explain the behav ioral effects of LSD in terms of

changes in the activity of single brain cells. As with any complex behavioral process and any centrally acting drug,

a complete explanation will, of ne

cessity, involve a good deal more than the activity of one set of neurons. There are also many unanswered questions about the actions of LSD, such as the precise mechanism underlying tolerance. An important sign of health and vitality in any sci entific field is the ability to undergo change and revision. Neuroscience is at present one of the more vigorous fields of scientific investigation, and we therefore have no doubt that the story we have told will undergo sig nificant modification and exten sion.

References 1. A. Hofmann. 1968. Psychotomimetic

agents. In Drugs Affecting the Central Nervous System, ed. A. Burger, pp. 184 85. Marcel Dekker.

2. L. E. Hollister. 1968. Chemical Psychoses, p. 34. Charles C. Thomas.

3. J. H. Gaddum and K. A. Hameed. 1954.

Drugs which antagonize 5-hydroxytryp tamine. Brit. J. Pharm. 9:240-48.

4. D. W. Wooley and E. Shaw. 1954. A bio chemical and pharmacological suggestion about certain mental disorders. PNAS 40:228-31.

5. A. Cerletti and E. Rothlin. 1955. Role of

5-hydroxytryptamine in mental disease and its antagonism to lysergic acid deriv atives. Nature 176:785-86.

6. A. Dahlstrom and K. Fuxe. 1964. Evidence for the existence of monoamine-containing neurons in the central nervous system. I. Demonstration of monoamines in the cell bodies of brainstem neurons. Acta Physiol. Scand. 62:Suppl. 232,1-55.

7. K. Fuxe. 1965. Evidence for the existence of monoamine neurons in the central ner vous system. IV. The distribution of monoamine terminals in the central ner vous system. Acta Physiol. Scand. 64:

Suppl. 247, 41-85.

8. D. X. Freedman. 1961. Effects of LSD-25 on brain serotonin. J. Pharm. Exp. Ther. 134:160-66.

9. J. A. Rosecrans, R. A. Lovell, and D. X. Freedman. 1967. Effects of lysergic acid

diethylamide on the metabolism of brain

5-hydroxytryptamine. Biochem. Pharmac. 16:2011-21.

10. G. K. Aghajanian, W. E. Foote, and M. H. Sheard. 1968. Lysergic acid diethylamide: Sensitive neuronal units in the midbrain

raphe. Science 161:706-08.

11. G. K. Aghajanian, W. E. Foote, and M. H. Sheard. 1970. Action of psychotogenic drugs on midbrain raphe neurons. J. Pharm. Exp. Ther. 171:178-87.

12. H. J. Haigler and G. K. Aghajanian. 1974.

Lysergic acid diethylamide and serotonin: A comparison of effects on serotonergic neurons and neurons receiving a seroton

ergic input. J. Pharm. Exp. Ther. 188: 688-99.

13. G. K. Aghajanian, H. J. Haigler, and J. L.

Bennett. 1975. Amine receptors in the CNS III. 5-Hydroxytryptamine in brain. In



Handbook of Psychopharmacology, Vol.

6, ed. L. L. Iversen, S. D. Iversen, and S. H.

Snyder. Plenum Press.

14. S. S. Mosko and B. L. Jacobs. 1977. Elec

trophysiological evidence against negative neuronal feedback from the forebrain

controlling mid-brain raphe unit activity. Brain Res. 119:291-303.

15. B. L. Jacobs, M. E. Trulson, and W. C. Stern. 1976. An animal behavior model for

studying the actions of LSD and related

hallucinogens. Science 194:741-43.

16. B. L. Jacobs, M. E. Trulson, and W. C. Stern. 1977. Behavioral effects of LSD in the cat: Proposal of an animal behavior

model for studying the actions of halluci

nogenic drugs. Brain Res. 132:301-14.

17. B. L. Jacobs, M. E. Trulson, A. D. Stark, and G. R. Christoph. 1977. Comparative effects of hallucinogenic drugs on behavior of the cat. Commun. Psychopharm. 1: 243-54.

18. M. E. Trulson and B. L. Jacobs. 1976. LSD acts synergistically with serotonin deple tion: Evidence from behavioral studies in cats. Pharm. Biochem. Behau. 4:231-34.

19. L. S. Cholden, A. Kurland, and C. Savage. 1955. Clinical reactions and tolerance to LSD in chronic schizophrenia. J. Nerv.

Ment. Dis. 122:211-21.

20. H. Isbell, R. E. Belleville, H. F. Fraser, A.

Wikler, and C. R. Logan. 1956. Studies on

lysergic acid diethylamide (LSD-25). I. Effects in former morphine addicts and

development of tolerance during chronic intoxication. Arch. Neurol. Psychiat. 76: 468-78.

21. M. E. Trulson and B. L. Jacobs. 1977. Usefulness of an animal behavioral model in studying the duration of action of LSD and the onset and duration of tolerance to LSD in the cat. Brain Res. 132:315-26.

22. K. Von Hungen, S. Roberts, and D. F. Hill. 1974. LSD as an agonist and antagonist at central dopamine receptors. Nature 252: 588-89.

23. L. Pieri, M. Pieri, and W. Haefely. 1974. LSD as an agonist of dopamine receptors in the striatum. Nature 252:586-88.

24. G. R. Christoph, D. M. Kuhn, and B. L. Jacobs. 1977. Electrophysiological evi dence for a dopaminergic action of LSD:

Depression of unit activity in the sub stantia nigra of the rat. Life Sei. 21: 1585-96.

25. M. E. Trulson, A. D. Stark, and B. L. Ja cobs. 1977. Comparative effects of hallu

cinogenic drugs on rotational behavior in rats with unilateral 6-hydroxydopamine lesions. Eur. J. Pharm. 44:113-19.

26. M. B. Bowers. 1972. Acute psychosis in duced by psychotomimetic drug abuse. Arch. Gen. Psychiat. 27:437-40.

27. M. E. Trulson and B. L. Jacobs. 1979.

Raphe unit activity in freely moving cats: Correlation with level of behavioral arousal. Brain Res. 163:135-50. This con firmed and extended the results of a pre vious study by D. J. McGinty and R. M.

Harper, 1976, Dorsal raphe neurons: De pression of firing during sleep in cats, Brain Res. 101:569-75.

28. B. L. Jacobs. 1978. Dreams and halluci nations: A common neurochemical mech

anism mediating their phenomenological similarity. Neurosci. & Biobehav. Rev. 2: 59-69.

29. M. E. Trulson and B. L. Jacobs. 1979. Ef fects of 5-methoxy-N,N-dimethyltrypta

mine on behavior and raphe unit activity in freely-moving cats. Eur. J. Pharm. 54: 43-50.

30. M. E. Trulson and B. L. Jacobs. In press. Dissociations between the effects of LSD on behavior and raphe unit activity in

freely moving cats. Science.

31. M. E. Trulson, C. A. Ross, and B. L. Jacobs. 1977. Lack of tolerance to the depression of raphe unit activity by lysergic acid di

ethylamide. Neuropharm. 16:771-74.

32. M. E. Trulson, C. A. Ross, and B. L. Jacobs. 1976. Behavioral evidence for the stimu lation of CNS serotonin receptors by high doses of LSD. Psychopharm. Comm. 2: 149-64.

33. M. E. Trulson and B. L. Jacobs. In press. Alterations of serotonin and LSD receptor binding following repeated administration of LSD. Life Sei.

34. S. S. Mosko and B. L. Jacobs. 1974. Mid brain raphe neurons: Spontaneous activity and response to light. Physiol. Behau. 13:589-93.

35. S. S. Mosko and B. L. Jacobs. 1976. Re

cording of dorsal raphe unit activity in vitro. Neurosci. Lett. 2:195-200.

36. E. Taber, A. Brodai, and P. Walberg. 1961. The raphe nuclei of the brainstem in the cat. I. Normal topography and cyt?archi tecture and general discussion. J. Comp.

Neurol. 116:161-88.

"Whatever happened to elegant solutions?"



Article Contentsp. 396p. 397p. 398p. 399p. 400p. 401p. 402p. 403p. 404

Issue Table of ContentsAmerican Scientist, Vol. 67, No. 4 (July-August 1979), pp. 388-504Front MatterSigma Xi News [pp. 388-389, 503-504]Letters to the Editors [pp. 390, 392-395]Mechanisms of Action of LSD: The study of serotonin-containing neurons in the brain may provide the key to understanding drug-induced hallucinations and their relationship to dreams and psychosis [pp. 396-404]Climate and the Ocean: Measurements of changes in sea-surface temperature should permit us to forecast certain climatic changes several months ahead [pp. 405-416]Self-Awareness in Primates: The sense of identity distinguishes man from most but perhaps not all other forms of life [pp. 417-421]The Therapy of Diabetes: Insulin therapy does not always prevent the serious complications of diabetes; current research is directed at more closely reproducing the functioning of a healthy pancreas [pp. 422-431]The Feeding Mechanisms of Baleen Whales: Since Robert Sibbald first described baleen whales in 1692, we have come to distinguish three typesthe right whales, grazers on copepods; the finner whales, engulfers of krill and fish; and the gray whale, a forager of the sea bottom [pp. 432-440]Ancient Hydraulic Techniques in the Chiapas Highlands: Strategies used by the Maya in southeastern Mexico for efficient management of soil and water resources provide evidence of cultural change and population growth [pp. 441-449]Radiocarbon Dating with Accelerators: Direct detection of 14C promises to revolutionize radiocarbon dating [pp. 450-457]Nitrogen Fixation: Basic to Applied: The increasing cost of nitrogen fertilizerwhich is essential for high-yielding cerealshas triggered research into the mechanism of natural nitrogen fixation [pp. 458-466]The Scientists' BookshelfReview: untitled [pp. 467-467]Review: untitled [pp. 467-468]Review: untitled [pp. 468-469]Review: untitled [pp. 469-469]Review: untitled [pp. 469-470]Review: untitled [pp. 470-470]Physical SciencesReview: untitled [pp. 470-470]Review: untitled [pp. 470, 472]Review: untitled [pp. 472-472]Review: untitled [pp. 472-472]Review: untitled [pp. 472-472]Review: untitled [pp. 472-472]Review: untitled [pp. 472-473]Review: untitled [pp. 473-473]Review: untitled [pp. 473-473]Review: untitled [pp. 473-473]

Earth SciencesReview: untitled [pp. 473-474]Review: untitled [pp. 474-474]Review: untitled [pp. 474-474]Review: untitled [pp. 474-474]Review: untitled [pp. 474-475]Review: untitled [pp. 475-475]Review: untitled [pp. 475-476]Review: untitled [pp. 476-476]

Life SciencesReview: untitled [pp. 476-476]Review: untitled [pp. 476-477]Review: untitled [pp. 477-477]Review: untitled [pp. 477-477]Review: untitled [pp. 477-478]Review: untitled [pp. 478-478]Review: untitled [pp. 478-478]Review: untitled [pp. 478-478]Review: untitled [pp. 478-479]Review: untitled [pp. 479-479]Review: untitled [pp. 479-479]Review: untitled [pp. 479-479]Review: untitled [pp. 479-480]Review: untitled [pp. 480-480]Review: untitled [pp. 480-480]Review: untitled [pp. 480-480]Review: untitled [pp. 480-480]Review: untitled [pp. 480-481]Review: untitled [pp. 481-481]Review: untitled [pp. 481-481]Review: untitled [pp. 481-482]Review: untitled [pp. 482-482]Review: untitled [pp. 482-482]Review: untitled [pp. 482-483]Review: untitled [pp. 483-483]Review: untitled [pp. 483-483]Review: untitled [pp. 483-483]Erratum: Handbook of Physiology, Section 9: Reactions to Environmental Agents [pp. 483-483]Review: untitled [pp. 483-483]Review: untitled [pp. 483-483]Review: untitled [pp. 483-484]Review: untitled [pp. 484-484]Review: untitled [pp. 484-484]Review: untitled [pp. 484-484]Review: untitled [pp. 484-484]Review: untitled [pp. 485-485]

Behavioral SciencesReview: untitled [pp. 485-485]Review: untitled [pp. 485-485]Review: untitled [pp. 485-486]Review: untitled [pp. 486-486]Review: untitled [pp. 486-486]Review: untitled [pp. 486-487]Review: untitled [pp. 487-487]Review: untitled [pp. 487-487]Review: untitled [pp. 487-488]Review: untitled [pp. 488-488]Review: untitled [pp. 488-488]Review: untitled [pp. 488-489]

Mathematics and Computer ScienceReview: untitled [pp. 489-489]Review: untitled [pp. 489-489]Review: untitled [pp. 489-489]Review: untitled [pp. 489-490]Review: untitled [pp. 490-490]Review: untitled [pp. 490-490]Review: untitled [pp. 490-490]

Engineering and Applied SciencesReview: untitled [pp. 490-490]Review: untitled [pp. 490-491]Review: untitled [pp. 491-491]Review: untitled [pp. 491-491]Review: untitled [pp. 491-491]Review: untitled [pp. 491-492]Review: untitled [pp. 492-492]Review: untitled [pp. 492-492]

History and Philosophy of ScienceReview: untitled [pp. 493-493]Review: untitled [pp. 493-493]Review: untitled [pp. 493-493]Review: untitled [pp. 493-494]

Books Received for Review [pp. 494-502]Back Matter

mechanism of action of lsd

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