By David N. Leff
One concern that people with Parkinson's disease, schizophrenia or cocaine addiction have in common is a brain chemical called dopamine.
Dopamine is one of the brain's most powerful and versatile neurotransmitters, which means it allows electrical signals to traffic between nerve cells. When a person lacks the brain cells that manufacture dopamine, he or she is in trouble.
When that trouble is Parkinson's disease (PD), the sufferer loses fine control over the motor neurons that let muscles — especially in hands, arms and legs — do their thing.
In schizophrenia, cognition suffers from a lack of dopamine and its accessory proteins.
As for drug abuse, observed neuroscientist Allen Fienberg, at The Rockefeller University, in New York, "If you take cocaine or speed [amphetamine], you get a very large increase in the level of dopamine in the striatal region of the brain. So that's another human disease entity."
In its cascade of interacting neuronal effects on the brain, one of the first dominoes that dopamine pushes over is DARRP-32 — short for the jawbreaking biochemical name, 3',5'-monophosphate-regulated phosphoprotein (molecular weight 32 kiloDaltons).
The second domino is an enzyme, protein phosphate-1 (PP-1), Fienberg told BioWorld Today, "that handles the phosphorylation of many ion channels in the brain. These are vital for controlling the excitability of its neurons," he pointed out. "So it's regulating a very important regulator.
"Phosphorylation," he explained, "changes the way a protein functions, either enhancing it or lessening it, depending on where it is. Phosphorylation is not a predictable thing; you have to figure it out for each individual case."
Fienberg is first author of a paper on the subject in today's Science, dated Aug. 7, 1998. Its title: "DARRP-32: Regulator of the efficacy of dopaminergic neurotransmission."
The article's senior author is biochemist and neuroscientist Paul Greengard, in whose Laboratory of Molecular and Cellular Neuroscience at Rockefeller, Fienberg is a research associate.
A number of years ago the lab undertook a series of experiments designed to elucidate new targets for dopamine action. "We characterized a number of different proteins, among them DARRP-32," Fienberg recalled, "whose activity was changed in response to dopamine activation."
Knockouts Catch Up
During those years, Greengard and his colleagues subjected the neurotransmitter and its protein dominoes to extensive biochemical and electrochemical testing in vitro, but not behaviorally in a whole animal. Then in vivo biotechnology caught up with them.
"The reason we undertook the project we've just reported in Science," Fienberg pointed out, "was that it came along at the same time as the process of gene targeting, which allows you to create mice that lack particular proteins. It allowed us to generate knockout mice without a functioning gene for DARRP-32.
"The only way to test the behavioral effect of dopamine," he observed, "is to make an animal that doesn't have it. We looked at the acute biochemical response of our mutant mice to cocaine, and also at behavioral experiments."
Into their DARRP-32-minus mice and into normal control animals they injected raclopride, an antischizophrenic medication. This drug causes catalepsy in experimental animals; they freeze into a motionless state.
"The catalepsy assay is a well-known target for testing candidate schizophrenia medication," Fienberg observed. "We can't interview the mice, so we can't look at the same parameters, but that is the action in our mutant animals — drugs that hit the D-2 dopamine receptor."
Before getting the drug, both normal and mutant animals averaged 17 seconds of immobility. But on low-dose raclopride, wild-type mice doubled or tripled this baseline behavior. Knockouts barely responded.
And on the highest dose, the normals stayed frozen at the 900 percent level for two minutes or more; mutants much less.
"In general," Fienberg summarized, "all of the responses to dopamine, or drugs that act in the dopamine system, were diminished in the mutant mouse, the mouse that lacked DARRP-32. That tells me this is a very important protein in the action of dopamine inside the cell."
Drug Discovery 'In Year Or Two'
He made the point that "most of the studies that looked at the action of dopamine in knockout mice have focused on its surface receptors. And those reports received a lot of press.
"Ours is one of the few papers," Fienberg continued, "that have looked, in a knockout manner, at proteins inside of the neurons, this being an intracellular signal protein."
He pointed out that the most prevalent present therapy for Parkinson's disease is levodopa, which converts to dopamine. "You give them dopamine; it works for a while, and then it doesn't work. Nobody understands why.
"So understanding the events in the dopamine receiving cell," Fienberg went on, "how to interpret the striatal neurons' signal, may give us additional therapeutic targets.
"Because we've identified the DARRP-32 inhibition of protein phosphatase-1 as being a required action of dopamine, it suggests that this interaction is a new area for the development of drugs relating to those dopamine-deficiency disease states," he said, adding, "We haven't pursued it yet but we intend to.
"In a year or two we would screen for drugs that mimic the action of DARRP-32 inhibitng PP-1, in vitro, in a small-molecule library."
Whether such a drug-discovery effort would focus first on PD, schizophrenia or addiction is not yet clear, Fienberg said. "I couldn't guess which disease would have the best application for it. Dopamine," he concluded, "is misregulated in all those disorders; nobody knows just how." *