THE BRAIN'S OWN MARIJUANA

Research into Natural Chemicals That Mimic Marijuana's Effects in The
Brain Could Help to Explain--and Suggest Treatments For--Pain,
Anxiety, Eating Disorders, Phobias and Other Conditions

Marijuana is a drug with a mixed history.

Mention it to one person, and it will conjure images of potheads lost
in a spaced-out stupor. To another, it may represent relaxation, a
slowing down of modern madness. To yet another, marijuana means hope
for cancer patients suffering from the debilitating nausea of
chemotherapy, or it is the promise of relief from chronic pain. The
drug is all these things and more, for its history is a long one,
spanning millennia and continents. It is also something everyone is
familiar with, whether they know it or not. Everyone grows a form of
the drug, regardless of their political leanings or recreational
proclivities. That is because the brain makes its own marijuana,
natural compounds called endocannabinoids (after the plant's formal
name, Cannabis sativa).

The study of endocannabinoids in recent years has led to exciting
discoveries. By examining these substances, researchers have exposed
an entirely new signaling system in the brain: a way that nerve cells
communicate that no one anticipated even 15 years ago. Fully
understanding this signaling system could have far-reaching
implications. The details appear to hold a key to devising treatments
for anxiety, pain, nausea, obesity, brain injury and many other
medical problems.

Ultimately such treatments could be tailored precisely so that they
would not initiate the unwanted side effects produced by marijuana
itself.

A Checkered Past

Marijuana and its various alter egos, such as bhang and hashish, are
among the most widely used psychoactive drugs in the world.

How the plant has been used varies by culture.

The ancient Chinese knew of marijuana's pain-relieving and
mind-altering effects, yet it was not widely employed for its
psychoactive properties; instead it was cultivated as hemp for the
manufacture of rope and fabric.

Likewise, the ancient Greeks and Romans used hemp to make rope and
sails.

In some other places, however, marijuana's intoxicating properties
became important.

In India, for example, the plant was incorporated into religious
rituals.

During the Middle Ages, its use was common in Arab lands; in
15th-century Iraq it was used to treat epilepsy; in Egypt it was
primarily consumed as an inebriant.

After Napoleon's occupation of Egypt, Europeans began using the drug
as an intoxicant. During the slave trade, it was transported from
Africa to Mexico, the Caribbean and South America.

Marijuana gained a following in the U.S. only relatively recently.
During the second half of the 19th century and the beginning of the
20th, cannabis was freely available without a prescription for a wide
range of ailments, including migraine and ulcers.

Immigrants from Mexico introduced it as a recreational drug to New
Orleans and other large cities, where it became popular among jazz
musicians.

By the 1930s it had fallen into disrepute, and an intense lobbying
campaign demonized "reefer madness." In 1937 the U.S. Congress,
against the advice of the American Medical Association, passed the
Marijuana Tax Act, effectively banning use of the drug by making it
expensive and difficult to obtain.

Ever since, marijuana has remained one of the most controversial drugs
in American society.

Despite efforts to change its status, it remains federally classified
as a Schedule 1 drug, along with heroin and LSD, considered dangerous
and without utility.

Millions of people smoke or ingest marijuana for its intoxicating
effects, which are subjective and often described as resembling an
alcoholic "high." It is estimated that approximately 30 percent of the
U.S. population older than 12 have tried marijuana, but only about 5
percent are current users.

Large doses cause hallucinations in some individuals but simply
trigger sleep in others.

The weed impairs short-term memory and cognition and adversely affects
motor coordination, although these setbacks seem to be reversible once
the drug has been purged from the body. Smoking marijuana also poses
health risks that resemble those of smoking tobacco.

On the other hand, the drug has clear medicinal benefits.

Marijuana alleviates pain and anxiety.

It can prevent the death of injured neurons. It suppresses vomiting
and enhances appetite--useful features for patients suffering the
severe weight loss that can result from chemotherapy.

Finding the Responsible Agent

Figuring out how the drug exerts these myriad effects has taken a long
time. In 1964, after nearly a century of work by many individuals,
Raphael Mechoulam of the Hebrew University in Jerusalem identified
delta-9-tetrahydrocannabinol (THC) as the compound that accounts for
virtually all the pharmacological activity of marijuana. The next step
was to identify the receptor or receptors to which THC was binding.

Receptors are small proteins embedded in the membranes of all cells,
including neurons, and when specific molecules bind to them--fitting
like one puzzle piece into another--changes in the cell occur.

Some receptors have water-filled pores or channels that permit
chemical ions to pass into or out of the cell. These kinds of
receptors work by changing the relative voltage inside and outside the
cell. Other receptors are not channels but are coupled to specialized
proteins called G-proteins. These G-protein-coupled receptors
represent a large family that set in motion a variety of biochemical
signaling cascades within cells, often resulting in changes in ion
channels.

In 1988 Allyn C. Howlett and her colleagues at St. Louis University
attached a radioactive tag to a chemical derivative of THC and watched
where the compound went in rats' brains.

They discovered that it attached itself to what came to be called the
cannabinoid receptor, also known as CB1. Based on this finding and on
work by Miles Herkenham of the National Institutes of Health, Lisa
Matsuda, also at the NIH, cloned the CB1 receptor.

The importance of CB1 in the action of THC was proved when two
researchers working independently--Catherine Ledent of the Free
University of Brussels and Andreas Zimmer of the Laboratory of
Molecular Neurobiology at the University of Bonn--bred mice that
lacked this receptor.

Both investigators found that THC had virtually no effect when
administered to such a mouse: the compound had nowhere to bind and
hence could not trigger any activity. (Another cannabinoid receptor,
CB2, was later discovered; it operates only outside the brain and
spinal cord and is involved with the immune system.)

INDIAN FAKIRS prepare bhang and ganja in this painting from the mid-
1700s. The history of marijuana extends far back in history, with
written records on its medical use appearing in ancient Chinese and
Egyptian texts.

Discovery in the 1960s of its active component, THC, eventually led to
identification of the brain's own "marijuana." As researchers
continued to study CB1, they learned that it was one of the most
abundant G-protein coupled receptors in the brain.

It has its highest densities in the cerebral cortex, hippocampus,
hypothalamus, cerebellum, basal ganglia, brain stem, spinal cord and
amygdala. This distribution explains marijuana's diverse effects.

Its psychoactive power comes from its action in the cerebral cortex.
Memory impairment is rooted in the hippocampus, a structure essential
for memory formation.

The drug causes motor dysfunction by acting on movement control
centers of the brain.

In the brain stem and spinal cord, it brings about the reduction of
pain; the brain stem also controls the vomiting reflex.

The hypothalamus is involved in appetite, the amygdala in emotional
responses.

Marijuana clearly does so much because it acts everywhere.

Over time, details about CB1's neuronal location emerged as well.
Elegant studies by Tamas F. Freund of the Institute of Experimental
Medicine at the Hungarian Academy of Sciences in Budapest and Kenneth
P. Mackie of the University of Washington revealed that the
cannabinoid receptor occurred only on certain neurons and in very
specific positions on those neurons.

It was densely packed on neurons that released GABA
(gamma-aminobutyric acid), which is the brain's main inhibitory
neurotransmitter (it tells recipient neurons to stop firing). CB1 also
sat near the synapse, the contact point between two neurons. This
placement suggested that the cannabinoid receptor was somehow involved
with signal transmission across GABA-using synapses. But why would the
brain's signaling system include a receptor for something produced by
a plant?

The Lesson of Opium

The same question had arisen in the 1970s about morphine, a compound
isolated from the poppy and found to bind to so-called opiate
receptors in the brain.

Scientists finally discovered that people make their own opioids--the
enkephalins and endorphins. Morphine simply hijacks the receptors for
the brain's opioids.

It seemed likely that something similar was happening with THC and the
cannabinoid receptor.

In 1992, 28 years after he identified THC, Mechoulam discovered a
small fatty acid produced in the brain that binds to CB1 and that
mimics all the activities of marijuana.

He named it anandamide, after the Sanskrit word ananda, "bliss."
Subsequently, Daniele Piomelli and Nephi Stella of the University of
California at Irvine discovered that another lipid, 2-arachidonoyl
glycerol (2-AG), is even more abundant in certain brain regions than
anandamide is. Together the two compounds are considered the major
endogenous cannabinoids, or endocannabinoids. (Recently investigators
have identified what look like other endogenous cannabinoids, but
their roles are uncertain.) The two cannabinoid receptors clearly
evolved along with endocannabinoids as part of natural cellular
communication systems.

Marijuana happens to resemble the endocannabinoids enough to activate
cannabinoid receptors.

Conventional neurotransmitters are water-soluble and are stored in
high concentrations in little packets, or vesicles, as they wait to be
released by a neuron.

When a neuron fires, sending an electrical signal down its axon to its
tips (presynaptic terminals), neurotransmitters released from vesicles
cross a tiny intercellular space (the synaptic cleft) to receptors on
the surface of a recipient, or postsynaptic, neuron.

In contrast, endocannabinoids are fats and are not stored but rather
are rapidly synthesized from components of the cell membrane.

They are then released from places all over the cells when levels of
calcium rise inside the neuron or when certain G-protein-coupled
receptors are activated.

As unconventional neurotransmitters, canna-bin-oids presented a
mystery, and for several years no one could figure out what role they
played in the brain.

Then, in the early 1990s, the answer emerged in a somewhat roundabout
fashion.

Scientists (including one of us, Alger, and his colleague at the
University of Maryland School of Medicine, Thomas A. Pitler) found
something unusual when studying pyramidal neurons, the principal cells
of the hippocampus. After calcium concentrations inside the cells rose
for a short time, incoming inhibitory signals in the form of GABA
arriving from other neurons declined.

At the same time, Alain Marty, now at the Laboratory of Brain
Physiology at the Rene Descartes University in Paris, and his
colleagues saw the same action in nerve cells from the cerebellum.
These were unexpected observations, because they suggested that
receiving cells were somehow affecting transmitting cells and, as far
as anyone knew, signals in mature brains flowed across synapses in one
way only: from the presynaptic cell to the postsynaptic one.

A New Signaling System

It seemed possible that a new kind of neuronal communication had been
discovered, and so researchers set out to understand this phenomenon.
They dubbed the new activity DSI, for depolarization-induced
suppression of inhibition. For DSI to have occurred, some unknown
messenger must have traveled from the postsynaptic cell to the
presynaptic GABA-releasing one and somehow shut off the
neurotransmitter's release.

Such backward, or "retrograde," signaling was known to occur only
during the development of the nervous system.

If it were also involved in interactions among adult neurons, that
would be an intriguing finding--a sign that perhaps other processes in
the brain involved retrograde transmission as well. Retrograde
signaling might facilitate types of neuronal information processing
that were difficult or impossible to accomplish with conventional
synaptic transmission. Therefore, it was important to learn the
properties of the retrograde signal.

Yet its identity remained elusive.

Over the years, countless molecules were proposed.

None of them worked as predicted.

Then, in 2001, one of us (Nicoll) and his colleague at the University
of California at San Francisco, Rachel I. Wilson--and at the same
time, but independently, a group led by Masanobu Kano of Kanazawa
University in Japan--reported that an endocannabinoid, probably 2-AG,
perfectly fit the criteria for the unknown messenger.

Both groups found that a drug blocking cannabinoid receptors on
presynaptic cells prevents DSI and, conversely, that drugs activating
CB1 mimic DSI. They soon showed, as did others, that mice lacking
cannabinoid receptors are incapable of generating DSI. The fact that
the receptors are located on the presynaptic terminals of GABA neurons
now made perfect sense.

The receptors were poised to detect and respond to endocannabinoids
released from the membranes of nearby postsynaptic cells.

Over time, DSI proved to be an important aspect of brain activity.
Temporarily dampening inhibition enhances a form of learning called
long-term potentiation--the process by which information is stored
through the strengthening of synapses.

Such storage and information transfer often involves small groups of
neurons rather than large neuronal populations, and endocannabinoids
are well suited to acting on these small assemblages. As fat-soluble
molecules, they do not diffuse over great distances in the watery
extracellular environment of the brain.

Avid uptake and degradation mechanisms help to ensure that they act in
a confined space for a limited period.

Thus, DSI, which is a short-lived local effect, enables individual
neurons to disconnect briefly from their neighbors and encode
information.

A host of other findings filled in additional gaps in understanding
about the cellular function of endocannabinoids. Researchers showed
that when these neurotransmitters lock onto CB1 they can in some cases
block presynaptic cells from releasing excitatory neurotransmitters.
As Wade G. Regehr of Harvard University and Anatol C. Kreitzer, now at
Stanford University, found in the cerebellum, endocannabinoids located
on excitatory nerve terminals aid in the regulation of the massive
numbers of synapses involved in coordinated motor control and sensory
integration. This involvement explains, in part, the slight motor
dysfunction and altered sensory perceptions often associated with
smoking marijuana.

Recent discoveries have also begun to precisely link the neuronal
effects of endocannabinoids to their behavioral and physiological
effects. Scientists investigating the basis of anxiety commonly begin
by training rodents to associate a particular signal with something
that frightens them. They often administer a brief mild shock to the
feet at the same time that they generate a sound.

After a while the animal will freeze in anticipation of the shock if
it hears the sound. If the sound is repeatedly played without the
shock, however, the animal stops being afraid when it hears the
sound--that is, it unlearns the fear conditioning, a process called
extinction. In 2003 Giovanni Marsicano of the Max Planck Institute of
Psychiatry in Munich and his co-workers showed that mice lacking
normal CB1 readily learn to fear the shock-related sound, but in
contrast to animals with intact CB1, they fail to lose their fear of
the sound when it stops being coupled with the shock.

The results indicate that endocannabinoids are important in
extinguishing the bad feelings and pain triggered by reminders of past
experiences. The discoveries raise the possibility that abnormally low
numbers of cannabinoid receptors or the faulty release of endogenous
cannabinoids are involved in post-traumatic stress syndrome, phobias
and certain forms of chronic pain. This suggestion fits with the fact
that some people smoke marijuana to decrease their anxiety. It is also
conceivable, though far from proved, that chemical mimics of these
natural substances could allow us to put the past behind us when
signals that we have learned to associate with certain dangers no
longer have meaning in the real world.

Devising New Therapies

The repertoire of the brain's own marijuana has not been fully
revealed, but the insights about endocannabinoids have begun helping
researchers design therapies to harness the medicinal properties of
the plant.

Several synthetic THC analogues are already commercially available,
such as nabilone and dronabinol. They combat the nausea brought on by
chemotherapy; dronabinol also stimulates appetite in AIDS patients.

Other cannabinoids relieve pain in myriad illnesses and
disorders.

In addition, a CB1 antagonist--a compound that blocks the receptor and
renders it impotent--has worked in some clinical trials to treat obesity.

But though promising, these drugs all have multiple effects because
they are not specific to the region that needs to be targeted.

Instead they go everywhere, causing such adverse reactions as
dizziness, sleepiness, problems of concentration and thinking
abnormalities.

One way around these problems is to enhance the role of the body's own
endocannabinoids. If this strategy is successful, endocannabinoids
could be called forth only under the circumstances and in the
locations in which they are needed, thus avoiding the risks associated
with widespread and indiscriminant activation of cannabinoid receptors.

To do this, Piomelli and his colleagues are developing drugs that
prevent the endocannabinoid anandamide from being degraded after it is
released from cells.

Because it is no longer broken down quickly, its anxiety-relieving
effects last longer.

Anandamide seems to be the most abundant endocannabinoid in some brain
regions, whereas 2-AG dominates in others.

A better understanding of the chemical pathways that produce each
endocannabinoid could lead to drugs that would affect only one or the
other. In addition, we know that endocannabinoids are not produced
when neurons fire just once but only when they fire five or even 10
times in a row. Drugs could be developed that would alter the firing
rate and hence endocannabinoid release.

A precedent for this idea is the class of anticonvulsant agents that
suppress the neuronal hyperactivity underlying epileptic seizures but
do not affect normal activity.

Finally, indirect approaches could target processes that themselves
regulate endocannabinoids. Dopamine is well known as the
neurotransmitter lost in Parkinson's disease, but it is also a key
player in the brain's reward systems.

Many pleasurable or addictive drugs, including nicotine and morphine,
produce their effects in part by causing dopamine to be released in
several brain centers.

It turns out that dopamine can cause the release of endocannabinoids,
and various research teams have found that two other
neurotransmitters, glutamate and acetylcholine, also initiate
endocannabinoid synthesis and release.

Indeed, endocannabinoids may be a source of effects previously
attributed solely to these neurotransmitters. Rather than targeting
the endocannabinoid system directly, drugs could be designed to affect
the conventional neurotransmitters. Regional differences in
neurotransmitter systems could be exploited to ensure that
endocannabinoids would be released only where they were needed and in
appropriate amounts.

In a remarkable way, the effects of marijuana have led to the still
unfolding story of the endocannabinoids. The receptor CB1 seems to be
present in all vertebrate species, suggesting that systems employing
the brain's own marijuana have been in existence for about 500 million
years.

During that time, endocannabinoids have been adapted to serve
numerous, often subtle, functions.

We have learned that they
do not affect the development of fear, but the forgetting of fear;
they do not alter the ability to eat, but the desirability of the
food, and so on. Their presence in parts of the brain associated with
complex motor behavior, cognition, learning and memory implies that
much remains to be discovered about the uses to which evolution has
put these interesting messengers.