By David N. Leff

Today, marijuana is known, and illicitly used, for its mood of euphoria. But how it works in the human brain remains largely hidden in a black box, the subject of much neuropsychopharmacological research.

This week, two major scientific weeklies, Nature and Science, report separate but related discoveries in the workings of cannabinoid molecules at the cellular and molecular levels. Coincidentally, both articles are by the same author, neuropharmacologist Daniele Piomelli, at the Neurosciences Institute on the campus of Scripps Research Institute, in La Jolla, Calif.

His paper in today's Nature, dated Aug. 21, 1997, reports discovery of "a second endogenous cannabinoid that modulates long-term potentiation."

And in tomorrow's Science, dated Aug. 22, 1997, Piomelli reports the "Functional role of high-affinity anandamide transport, as revealed by selective inhibition."

Both findings, he told BioWorld Today, have biochemical implications for marijuana-based therapeutic agents.

Tetrahydrocannabinol's receptor (THC), he recalled, was discovered a decade ago. About five years later, "a group in Jerusalem discovered an endogenous ligand to the THC receptor, called anandamide." Since then, Piomelli added, "We have learned much more about anandamide's biochemistry, but when it comes to its physiological roles, we are still pretty much in the dark."

Recently, Piomelli and his team showed that anandamide is produced in neurons and released from neurons during electrophysiological activity. They then faced the question of how the substance could be inactivated after its release.

The human brain has solved this off-switch machinery for controlling such major neurotransmitters as dopamine, epinephrine and serotonin. So the La Jolla scientists decided to track the trajectory of free anandamide from outside its neurons to the cells' interior—taking a leaf from the neurotransmitters' transport—and synthesizing analogues to anandamide that blocked this transport.

"We set up a collaboration," Piomelli recounted, "with the laboratory of Alex Makriyannis, at the University of Connecticut, in Storrs, Conn.

"One of the analogues that Alex and his colleagues synthesized," Piomelli recounted, "a compound called aM404, turned out to be very promising. It inhibited quite efficiently anandamide transport into neurons and astrocytes [major glial cells in the brain], but without any effect on THC receptors."

Mice Mimic Drug's Real-Life Activity In Human Brain

So far, so good, but only in vitro. "We had the drug," Piomelli observed, "so we could ask the question what the transporter was doing in real life. Does it really participate in selectively terminating the actions of anandamide?"

By injecting laboratory mice with anandamide or aM404 alone, or both together, "we found that aM404 dramatically potentiated the effects of anandamide, whereas by itself it had no direct effect."

Piomelli continued: "The conclusion of these experiments seems to us very straightforward: Once anandamide is produced in the neurons, and released outside them, it goes and binds to cannabinoid receptors. Then, despite the fact that chemically it has nothing to do with dopamine or epinephrine or serotonin, it's disposed of in much the same way as those neurotransmitters are. That is, anandamide enters its neurons and astrocytes, which break it down to inactive fragments."

Piomelli added: "That's the experimental value. But there's also a therapeutic value, because there's big talk these days about the medicinal uses of marijuana. It's a very political discussion. The problem for me," he observed, "is that as a pharmacologist, I find it hard to believe that in 1997, people would want to take a whole plant and smoke it as a means of providing medicinal entities.

"In 1997," he went on, "it seems we ought to be able to do something better than that. One reason we haven't is because we don't understand the cannabinoid system very well in the first place.

"But the favorable medicinal properties of marijuana are obvious," he pointed out. "It is obviously a very strong analgesic, a very strong anti-emetic, a very strong anti-inflammatory compound, and it produces appetite stimulation. All of these effects would obviously be beneficial to cancer patients suffering the side effects of chemotherapy."

He continued: "I think only someone not blinded by political passion would not be able to see this. On the other hand, only someone blinded by the opposite political passion would not see that if you smoke marijuana or hashish, you cannot drive a car or operate a machine of any sort, because your motor coordination and memory have been impaired.

"So what one wants to do is dissociate side effects from therapeutic effects. One wedge to this thicket would be to try to block anandamide inactivation and transport." Along these lines, he cited "a classical example in the pharmaceutical literature: If one were to give serotonin receptor agonists, one would get a lot of side effects. But when you give serotonin uptake inhibitors, such as Prozac, the effects are much more restricted.

"So this is pretty much the experimental value of aM404, as well as the possible therapeutic value of drugs that might be produced from it."

Discovery Of Second Cannabis Receptor Ligand

Piomelli's report in today's Nature, he observed, "is complementary in a way to our anandamide study in Science."

"It stands on its own feet because it's the demonstration that the brain contains a second endogenous cannabinoid, which chemically we called 2AG. It is present in about 175-fold greater amounts than anandamide."

He and his co-authors found that there is at least one region in the brain, the hippocampus, in which electrophysiological stimulation yielded sizable increases in 2AG formation, without any change in anandamide.

"The hippocampus," Piomelli pointed out, serves a very important role in the formation of short-term memories, the transfer of information from temporary to short-term, and then eventually long-term memory.

"It is also widely accepted now that one cellular phenomenon plays a role in short-term memory. and this phenomenon is called long-term potentiation. It's basically a strengthening, of synaptic efficiency.

"Possibly the therapeutic interest—and where I also see the complementarity between this Nature report and the anandamide transport study in Science," Piomelli observed, "is that in the hippocampus we find 2AG. This raises one important point: THC, marijuana, provokes a very drastic loss in short-term memory, particularly when high doses are provided. So our results would suggest that THC acts by interfering with the normal forgetting processes, in which endocannabinoids are involved.

"When we consider," he added, "that the hippocampus is only, or mainly, where 2AG but not anandamide is produced, this suggests to us that we can try to design medicinal agents that act either on one or the other compound."

Piomelli concluded: "My lab does biochemistry and pharmacology because we really believe that you have to do both together. We use drugs to understand biochemical systems, and vice versa." *