Phenotype Offers New Perception on Cocaine

[coin]

The Scientist 16[2]:16, Jan. 21, 2002

Phenotype Offers New Perception on Cocaine

Researchers say glutamate is more essential to addiction than dopamine

By Tom Hollon

In cocaine research, dopamine and glutamate make a brilliant star and

supporting player, respectively. One takes center stage, the anointed

crowd-pleaser; the other, though a leading actor in other productions, is so

overshadowed that admiration of its performance is relegated to an acquired

taste. A quick PubMed search recently disclosed their perceived importance:

3,628 abstracts on cocaine and dopamine, 178 for cocaine and glutamate.

Courtesy of Fran^ßois Conquet

Fran^ßois Conquet

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Now, however, perceptions may shift'Äînot that dopamine descends from the

firmament, but that glutamate will sparkle as brightly. Recent knockout mouse

evidence1 from researchers led by Fran^ßois Conquet, CEO of Addex

Pharmaceuticals in Geneva, Switzerland, reveals that glutamate's role in

cocaine dependence is even more central than dopamine's.

The case for dopamine's centrality remains airtight. Cocaine binds the

dopamine transporter, blocking reuptake of dopamine into presynaptic neurons.

Blockade increases dopamine concentration in synapses, an event responsible

for cocaine's pleasurable effects and suggested as key to developing drug

dependence. But although loss of the transporter and dopamine receptors in

knockout mice may alter behavior toward cocaine, always the drug remains

addictive. When the dopamine transporter is lost, for instance, mice may

still become cocaine dependent through cocaine's ability to bind the

serotonin transporter. This is not necessarily surprising, observes Peter

Kalivas, of the Medical University of South Carolina in Charleston, who is a

leading investigator of the glutamate-cocaine relationship. "The ability of

an organism to predict rewarding stimuli in the environment is absolutely

critical to survival," says Kalivas, "so there probably is some redundancy."

Contrast this redundancy to what Conquet finds when metabotropic glutamate

receptor mGluR5 disappears: Without mGluR5, mice turn up noses and whiskers

to cocaine, even though their dopaminergic systems respond to cocaine as

usual. "These are the first knockout mice completely unresponsive to this

powerfully addictive drug," says Conquet, who engineered the knockout mice

when he was at GlaxoSmithKline in Lausanne, Switzerland. From this phenotype

emerges a new picture of dopamine and glutamate: Sustaining cocaine-seeking

behavior requires both neurotransmitters, while only glutamate is

indispensable for cocaine dependence.

The Consolation Prize

Glutamate, the main excitatory neurotransmitter, is associated with learning

and memory. Its receptors divide into ionotropic and metabotropic forms

important to this function. "Learning occurs in part from changes in both

ionotropic and metabotropic signals," Kalivas explains. "You adjust both in

order to change the way cells communicate." Ionotropic receptors are also

called ligandgated ion channels.Ligand binding opens the channel so ions can

pass through the cell membrane. Generally these are ion channels first,

receptors second, controlling very quick changes in membrane current. In

comparison, metabotropic glutamate receptors bring slower changes; largely

they modulate signals from other neurotransmitters, acting through second

messenger systems. They belong to the seven-transmembrane, G-protein linked

superfamily of receptors. Conquet studies metabotropic receptors mGluR1 and

mGluR5, which act through the phospholipase C signaling pathway.

Conquet's discovery of mGluR5's role in cocaine addiction originates in his

second-place finish in a race to make mGluR5 knockout mice. Conquet was at

the time head of Glaxo's experimental pathology unit, where his job was to

make knockout mice deficient in various neuronal receptors. He lost to John

Roder, of the Hospital for Sick Children, in Toronto. By showing that mGluR5

mutant mice perform poorly in the Morris water maze test and in fear-related

learning, Roder implicated mGluR5 in spatial learning and memory.2 Roder's

experiments suggest that mGluR5 is involved in long-term potentiation (LTP)

within the hippocampus. Scooped, Conquet had to ask himself if Roder's

description of the phenotype was complete. A possibility Roder might have

missed, Conquet decided, was how the mice would react to cocaine.

The possibility of a connection between mGluR5 and cocaine appealed to

Conquet's sense of drug dependence as a form of learned behavior. He knew

that cocaine increases glutamate concentration in the nucleus accumbens, a

brain region associated with cocaine dependence and stimulation of locomotor

activity, and the location for the natural reward circuitry for food, water,

mating and maternal behavior. Kalivas and his associates have demonstrated

that mGluR5 receptors are down regulated following chronic cocaine

administration.3

Conquet turned for help to his colleague Christian Chiamulera, who works on

sychiatric drugs in a Glaxo laboratory in Verona, Italy. Willing to take a

flyer on a wild idea, but wanting to avoid weeks of work with nothing to show

for it, Chiamulera suggested a quick-and-dirty observation of cocaine as a

psychostimulant: Inject cocaine into the bellies of the knockouts, then look

for hyperactivity.

When Conquet watched the first injections, mmediately he worried something

was wrong. Instead of frenzied exploration if their surroundings, the mGluR5

knockouts lolled about as if nothing had happened. They verified in fact that

the mice had received walloping doses. To their excitement, wild type mice on

cocaine behaved as expected'Äîno sleepwalkers or indolent beachcombers here.

These creatures were ready to jitterbug 'til dawn at Mardi Gras. Conquet and

Chiamulera were ready to join them. Chiamulera would now follow up with more

elaborate experiments. For a test that approximates cocaine addiction in

humans, Conquet brought in Mark Epping-Jordan, another Glaxo scientist, to do

cocaine self-administration experiments.

Chiamulera confirmed his initial results. Wild type mice responded to cocaine

in a dose-dependent manner: The higher the dose, the more hyperactive they

became. Knockouts remained unperturbed regardless of dose. Abolishing mGluR5

abolishes cocaine-induced hyperactivity.

Epping-Jordan began the self-administration experiments by first training

knockout and wild type mice to press a lever in order to get food. Both

groups learned lever pressing equally well. Then he substituted intravenous

cocaine for food and watched what happened. Wild type mice responded

enthusiastically to the new menu, and would press for cocaine a dozen or more

times an hour. MGluR5 mutants ignored cocaine at every dose; within a few

sessions they would learn levers no longer produced food and stop pressing.

It was possible that the connection between cocaine dependence and mGluR5 was

indirect, that loss of mGluR5 during development altered molecules even

closer to control of dependence. Conquet's group examined the issue by asking

if 2-methyl-6-(phenylethynyl)-pyridine (MPEP), a selective mGluR5 antagonist,

reduced cocaine self-administration in normal mice. It did'ÄîIn dose-dependent

fashion, MPEP decreased demand for cocaine by up to 50%. The link, then, is

direct and essential. "Somehow, glutamate transmission at mGluR5 is critical

for the animal to recognize the rewarding effects of cocaine," says Kalivas.

"The surprising thing is that it must be a secondary effect, because cocaine

does not act directly on glutamate transmission. There is no binding by

cocaine directly to any protein that has to do with glutamate transmission."

Leaving Natural Reward Along

Loss of mGluR5 apparently leaves the dopaminergic system intact. Using

microdialysis to measure dopamine in freely moving mice, the researchers

found dopamine concentrations in the nucleus accumbens were the same for

mutant and normal mice, with or without cocaine. Levels of D1 and D2-class

dopamine receptors and dopamine transporters were also normal.Most striking

is that reward systems strongly influenced by dopamine'Äînourishment, mating,

and nursing'Äîwere also unaffected by loss of mGluR5. Conquet emphasizes that

no other knockout has behaved this way: "This is the first time a mammal has

been found insensitive to cocaine while its other reward-based systems remain

normal."

He continues, "Dopamine receptor knockouts fail to curb cocaine dependence

because mGluR5 is still working. They just affect general dopaminergic

activity," and with considerable "collateral damage". Experiments with

dopamine receptor agonists also indicate that dopamine does not lie at the

center of cocaine dependence: "People have shown that you can never induce

dependence from scratch with dopamine agonists. But you can maintain the

process with these compounds once dependence is ongoing, probably after mGluR5 has turned the system on."

Kalivas now distinguishes dopamine and glutamate by their short and long term

effects. "The acute rewarding properties that keep people coming back to the

drug are mediated by dopamine," he says. "The 'Once an addict, always an

addict' kinds of folklore that really make an addict are probably long-term

changes in glutamate transmission." In retrospect, this isn't surprising:

"All of neuroscience has been pointing to glutamate transmission as the

critical player in the brain's ability to adapt to the environment." Cocaine

addiction is one such adaptation.

From Scientist to Entrepreneur

Conquet founded Addex only a few weeks ago, departing Glaxo for better

opportunities to continue his work. Following Glaxo's merger with SmithKline,

drugs against cocaine addiction seemed better markets for smaller companies.

Glaxo's larger size demands larger markets if the pharma giant is to sustain

itself. For a small firm like Addex, a new mGluR5 antagonist could be quite

profitable. Why develop a new drug when MPEP exists? Because MPEP dissolves

very poorly and barely crosses the blood-brain barrier, Conquet

explains.Conquet does not know if mGluR5 plays a role with ethanol and

nicotine addiction. Self-administration experiments have not been done.

Partly he hasn't had time, since he's busy starting Addex. Partly the mice

haven't had time, since other drugs of abuse, especially alcohol, require

longer training periods. Whether mGluR5 influences other so-called

addictions, is a question left for the distant future.

If Addex does find a good mGluR5 antagonist, therapeutic possibilities may

extend well beyond helping snorters and crackheads stay clean. Glutamate may

be implicated in numerous psychiatric disorders, according to Kalivas. mGluR5

inhibitors have been suggested as possible treatments for Alzheimer Disease,

Parkinsonian akinesia, muscle rigidity, stroke, anxiety, and inflammatory

pain. But as always, Kalivas reminds, once a good drug candidate is in hand,

only running the clinical experiments will tell for sure.

Tom Hollon ([email protected]) is a freelance writer in Rockville, Md.

References

1. C. Chiamulera et al., "Reinforcing and locomotor stimulant effects of

cocaine are absent in mGluR5 null mutant mice," Nature Neuroscience, 4:873-4,

September 2001.

2. Z. Jia et al, "Gene targeting reveals a role for the glutamate receptors

mGluR5 and GluR2 in learning and memory," Physiology and Behavior,

73:793-802, August 2001.

3. C.J. Swanson et al., "Repeated cocaine administration attenuates group I

metabotropic glutamate receptor-mediated glutamate release and behavioral

activation: a potential role for Homer," Journal of Neuroscience, 21:9043-52,

Nov. 15, 2001.

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The Scientist 16[2]:16, Jan. 21, 2002

© Copyright 2002, The Scientist, Inc. All rights reserved.

[rkundalini]

Ponderings of the curious:

1) Does this have anything to do with the addictive potential of GHB?

2) Does this have any implications for the use of MSG (monosodium glutamate) in "food"?

[t st tantra]

l-glutamine willcounteract some effects of grass.at about 1gm u may not be able 2 get off at all.

t s t.