By David N. Leff
One of the hottest political battles raging in Washington this year is over the 2000 census. Given the unknown number of individuals who somehow fail to be counted close up and personally, should statistical averaging fill in those blanks?
Those missing people include a small but unreported number who died, or had to have liver transplants, from eating poisonous mushrooms. Many of the victims were immigrants to the U.S. from countries where picking wild mushrooms is a tradition. So last April, the North American Mycological Association propagated a bright red poster in 10 languages, reading: "WARNING! Picking and eating wild mushrooms can kill you!"
Among the deadly fungi, which sport names such as Destroying Angel and Death Cap, is a red-and-white specimen, Amanita muscaria. It pops up on fields and pastures during the summer months. That Amanita is known more colloquially as fly agaric, because at one time its toxic alkaloid was sold as a fly poison.
Within 15 to 30 minutes of ingesting an Amanita mushroom, the unwitting consumer starts to salivate, sweat and lacrimate profusely. Then comes violent vomiting and diarrhea. A pregnant woman may lactate. Blurred vision, irregular pulse, difficulty breathing and cardiac depression may also ensue. Victims normally recover within 24 hours, but severe cases may result in death from respiratory failure.
"The name muscarinic," said molecular pharmacologist J|rgen Wess, "comes from the alkaloid muscarine. This subfamily of acetylcholine receptors can be selectively activated by muscarine, which is not found outside the mushroom world. The other subclass, nicotinic receptors, are activated by nicotine."
Like so many plant and fungal toxins, the muscarinic receptors - which come in five subtypes - are under scrutiny by biotech and pharma laboratories, scoping their therapeutic potential. (See BioWorld Today, July 28, 1999, p. 2.)
Can PD Receptor Caper Play In Alzheimer's, Too?
"Of major current interest," observed Wess, "is the use of muscarinic agents as potential drug therapy for Alzheimer's disease (AD). There's a cholinergic theory of dementia in AD," he explained. "It's been known for more than 10 years that a group of cholinergic neurons in the basal forebrain degenerates and dies in AD patients. Those cells project into the hippocampus and cerebral cortex, brain areas of memory and learning.
"People discovered that acetylcholine was missing," Wess went on, "but their receptors are still there. So the idea was - and still is - that by stimulating these receptors with muscarinic agonists selected for these receptors, you may be able to overcome the cognitive deficits in AD."
Wess is chief of the Molecular Signaling Section in the Laboratory of Bioorganic Chemistry at the National Institute of Diabetes and Digestive and Kidney Diseases (NIDDKD) in Bethesda, Md.
"There's a precedent for that AD theory in Parkinson's disease [PD]," he pointed out, "where you have dopamine neurons that die in the brain's substantia nigra. Physicians treat PD quite effectively with drugs that stimulate dopamine receptors at the site where dopamine is normally released in the striatum. So based on that PD precedent," Wess went on, "people have hoped that similar replacement strategy might work in AD too."
Wess made the added point, "Muscarinic antagonists are frequently used also to treat PD, after the primary therapy with dopa, which metabolizes into dopamine. That's usually quite effective initially, but after five or 10 years diminishes. So in those later stages," he continued, "the dopa is frequently combined with muscarinic antagonists that penetrate the brain and are frequently effective to some extent. But these cause more side effects than dopa - mainly confusion, some cognitive impairment, dry mouth, blurred eyesight, because you paralyze some of the muscles that control vision."
Window On Possible Drug Development Ploy
"If one could pinpoint the striatal receptor to which these muscarinic antagonists exert their beneficial effects in PD," he pointed out, "one might be able to develop drugs that are selective for the specific muscarinic receptor subtype. Then one would be able to get rid of other side effects."
Wess is senior author of a paper in today's Proceedings of the National Academy of Sciences (PNAS), dated Aug. 31, 1999, which reports "Enhancement of D1 dopamine receptor-mediated locomotor stimulation in M4 muscarinic acetylcholine receptor knockout mice." Of the article's 10 co-authors, five are from NIDDKD, and five from the Lilly Research Laboratories of Eli Lilly & Co. in Indianapolis.
Wess recounted how this collaboration on the muscarinic receptor's M4 subtype came about:
"I approached the Lilly guys because they had a long history in muscarinic research. I said, 'Hey, I have constructed mice with their muscarinic receptor's M4 subtype knocked out, and would like to have them analyzed quickly.' So Lilly did most of the pharmacological testing. We generated the mice, performed the molecular analysis, and made the antibodies to detect the receptors.
"Conspicuously," Wess continued, "the M4 receptor is highly abundant in the striatum, which is located in the basal forebrain, and controls movement coordination in the body - locomotion. So, too, are several of the dopamine receptor subtypes, D1 and D2, which are colocalized in striatal projection neurons.
"The problem was that our M4 receptor-lacking KO mice looked pretty good, very healthy and normal. So I said, 'We really have to look at this locomotion stuff.' Which was where Lilly came into the play. They have 15 or 20 of these locomotion boxes that test movement in mice, so the experiments could be done very quickly at Indianapolis."
Following those animal tests, as described in their PNAS paper, Wess and his co-authors "are now, together with the Lilly labs, looking at neurotransmitters in the striatum, and how far the loss of the M4 receptor disturbs the balance of dopamine and acetylcholine in that brain region, because," he pointed out, "this is apparently the underlying evil in Parkinson's disease. You have an imbalance between acetylcholine and dopamine - which is mostly lacking in PD. We want to see what actually happens if we give muscarinic antagonists or dopamine agonists. How do we affect the neurotransmitter balance in the striatum?"