By David N. Leff
Remember angel dust?
A generation ago, this psychoactive street drug was the poor man's (and boy's) narcotic fix, before crack cocaine took over the market.
Angel dust is phencyclidine, used originally as an anesthetic in veterinary medicine. It acts by blocking the ion channels that trigger glutamate, one of the brain's two main neurotransmitters.
Glutamate is an excitatory neurotransmitter; its receptor accounts for angel dust's behavioral effects — from hallucinations to extreme violence. Its psychoactive opposite is benzodiazepam, better known as valium.
Valium's target is a receptor of the brain's other key neurotransmitter, GABA (gamma-aminobutyric acid.)
Most synapses in the central nervous system use either GABA or L-glutamate as neurotransmitters to control neuronal inhibition and excitation.
GABA does its work via two quite different receptors, GABAA and GABAB.
GABAA is the target not only of valium but of many other psychoactive drugs on the market. Its receptor, at the downstream end of nerve synapses, is involved in high-speed signal transduction between nerve cells.
"Neurons in the brain receive excitatory inputs and inhibitory inputs," explained molecular biologist Bernhard Bettler. "If a neuron gets a lot of excitatory inputs, it's going to fire an action potential. An inhibitory input makes it less likely to fire. That's why valium makes you sleepy." Bettler heads a research laboratory in the therapeutic nervous system area of Novartis Pharma AG, of Basel, Switzerland.
The GABAB receptor is structurally something else. It can sit on both ends of synapses, and modulates signal transduction via slower pathways inside the cell.
Scientists cloned the GABAA receptor a decade ago, but until today, the existence of the GABAB receptor was only postulated; its molecule had not been isolated. Back in 1972, an anti-spastic drug named baclofen was introduced to treat the muscle spasms of multiple sclerosis episodes and to alleviate spinal cord injury. Chemically, baclofen is an analogue of GABA, but doesn't react to the GABAA receptor. So the hunt has been on ever since, for a presumed GABAB protein.
Today's issue of Nature, dated March 20, 1997, reports: "Expression cloning of GABAB receptors uncovers similarity to metabotropic glutamate receptors." The paper's authors are Bettler and his laboratory team at Novartis.
Hunt Now On For GABAB Subtypes, Drugs
"Being in a pharmaceutical company," Bettler said, "we are now mostly interested to clone additional subtypes of GABAB receptors. We don't know whether they exist," he added, "but judging from similar glutamate receptors, where there are eight subtypes, we would expect GABAB receptors to be similar."
He and his co-authors are now trying to clone these presumed subtypes, "and design specific drugs for each of them. We would like to build up a high-throughput screening system, so we can screen thousands of compounds and receptor subtypes to find specific drugs. Then we will see how these compounds behave, whether they show therapeutic benefit in animal models of disease."
As for which entities, the Nature paper cites in tentative terms schizophrenia, epilepsy — especially absent epilepsy — and also cognition enhancement and depression treatment.
Describing how he found and cloned GABAB, Bettler observed, "This receptor was first characterized in 1980 or '81. It proved very difficult to purify the protein for it, simply because there were no high-affinity drugs that would bind to it. What we did," he continued, "being in a pharmaceutical company, was to design some really high-affinity compounds, in the nanomolar range, which we managed to radiolabel."
He and his team then created a cDNA library from rat cerebral cortex, cloned that in an expression vector, and transfected monkey COS cells with pools of cDNAs. These they then incubated with radioactive ligand, and checked for cells that would bind it. "That took us about a year, processing about 60,00 cDNA clones a week," Bettler concluded, "until we found the GABAB receptor."
Its gene sequence is 4.4 kilobases long; its protein consists of 960 amino acids.
Cellular neurophysiologist David Brown, who heads pharmacology at Cambridge University, wrote an editorial in Nature accompanying the Novartis paper.
It was Brown's co-author, Norman Bowery, who discovered years ago that baclofen did not interact with GABAA, leading to the suspected existence of GABAB.
Now, Brown told BioWorld Today, "If past experience is anything to go on, there will be more variants of this B-receptor found, present in different locations of the brain — some pre-synaptic, some post-synaptic. And I suppose if that's true," he added, "then it means that the pharmaceutical industry can begin to design drugs that selectively inhibit one or another of these receptors.
"Clearly A Market — But It's Unclear'
"Ideally," Brown added, "it would be nice to have a good antagonist [receptor blocker] I think, because this would increase the release of the inhibitory transmitter itself, by antagonizing the depression of release. And I suppose that would be useful as an anti-epileptic."
He continued: "There are also suggestions in the literature that if you inhibit the receptor, you may be able to improve memory. But this is a long way away. It will be provable only when you've got some new drugs."
"From the biotech point of view," he observed, "there is clearly a market point here, but I don't think the market is very clear yet, because I don't think we know enough about what the receptor does. The other approach, that can be used now, probably, is to produce knockouts, now that we know the gene, so we can define more clearly what the GABAB receptor is doing."
Brown himself "will probably do some tricks with the genes, such as putting the sequence into nerve cells that don't have it, and try to find out what happens," he said. "Or try and delete it from nerve cells that do have it, to see what difference that makes.
"We did a little work on it in the past," Brown concluded, "but not for the past ten years, because there wasn't anything new to work on. Now there is." *