By David N. Leff
The breathalyzer test to which a cop subjects a person suspected of driving drunk has one built-in flaw. By measuring ethanol levels in the blood, it makes the assumption that everyone metabolizes alcohol at the same rate. In fact, how an individual holds his (or her) liquor depends, at least in part, on genetic factors.
A more discriminating test than breathing into a balloon is reported in today's Science, dated Oct. 25, 1997. It bears the title: "Fyn-kinase as a determinant of ethanol sensitivity: Relation to NMDA-receptor function." The article's authors are a team of Japanese neurobiologists, behavioral geneticists and psychologists at the University of Tokyo and several separate research institutes.
Instead of testing human subjects, they probed the responses of mice injected peritoneally with graduated doses of alcohol, or placebo saline solution. Instead of breath and blood, they measured the animals' hypnotic reactions to ethanol by analyzing changes in their behavior, correlated with alterations in their brain cells.
Their knockout mice came in two varieties: one lacking both parental genes for the enzyme Fyn-tyrosine kinase — i.e., homozygous; the other making do with only one inherited fyn gene — heterozygous. The latter produced respectable levels of the Fyn enzyme; the former, none at all.
Once dosed with drug or placebo, each inebriated mouse was laid on its back in a V-shaped trough, then stop-watched to see how long it took to sober up enough to turn over and stand up on its feet.
"At all doses tested," the Science article reported, "the duration of the loss of righting reflex after ethanol administration was significantly longer for homozygous, Fyn-deficient mice than for heterozygous controls."
Normal animals, with functioning Fyn-TK genes, took from three to 40 minutes to get over their drunken stupor, depending on how much alcohol they'd received. Homozygous knockouts, lacking the enzyme, averaged twice as long as normals at each dose. Mice that never touched the stuff stood up immediately.
The Fyn-minus knockout mice proved hypersensitive to the effects of booze.
Everyone knows that alcohol goes to the head, but just where in the brain — and how it works there — are matters of conjecture and investigation.
The Japanese co-authors removed their animals' brains at staggered time intervals after alcohol or saline dosing and examined the hippocampus, a region in which the fyn gene is richly expressed. They were looking in vivo to confirm recent in vitro evidence reported elsewhere that ethanol enhances tyrosine phosphorylation.
Phosphorylation — adding a phosphate group to a protein — is the commonest way by which the body regulates protein function. Fyn employs phosphorylation to enhance the action of certain receptor proteins in the brain, notably the N-methyl-D-aspartate receptor (NMDAR) in the hippocampus. This NMDAR is a complex of many proteins in a neuron's membrane, which picks up specific chemical messages from nearby cells, especially governing behavioral effects.
Five years ago, neurophysiologist Eric Kandel, a Howard Hughes investigator at Columbia University's College of Physicians & Surgeons, in New York, first showed that animals lacking Fyn have a deficiency in behavior. "They have a poor memory for spatial tasks," Kandel told BioWorld Today. "We showed that that memory deficit was associated with a disturbance in LTP" (long-term potentiation, a long-lasting change in nerve-cell excitability thought to underlie memory).
One of Kandel's associates, he observed, was the first to show that Fyn phosphorylates a subunit of that NMDAR.
In their hippocampal cells, the Japanese co-authors detected enhanced phosphorylation of one specific NMDAR subunit, e2, in their heterozygous mice, five minutes after ethanol or saline dosage, but not in Fyn-minus knockouts. This effect apparently reflected the neurons' recovery from suppression by the alcohol.
Acute Ethanol Tolerance Defined
They suggested that "enhancement of tyrosine phosphorylation of NMDAR subunit e2 is a basis of the acute tolerance observed." Kandel defines acute ethanol tolerance as "how much alcohol you can consume the first time without being knocked bezonko."
Commenting on this Japanese finding, he said: "It's been known for some time that alcohol abuse and drug abuse are forms of memory. That is, you become dependent on something; it's a persistent change in neuronal activity. This seems to be a very nice example of that, in the case of alcoholism, in which the same substrate that is important in certain forms of memory storage is recruited by alcohol abuse."
The Science paper starts off: "Ethanol is among the most widely abused drugs in the world, yet the neural mechanisms responsible for [alcohol] intoxication and dependence are largely unknown."
On this score, an editorial accompanying the Science article quoted pharmacologist Adron Harris, at the University of Colorado Health Sciences Center, in Denver: "Initial insensitivity [to alcohol] seems to be a strong predictor of alcoholism later in life." And because too little Fyn kinase increases alcohol sensitivity, it could be that too much of the enzyme may lead to lowered sensitivity."
If so, Harris pointed out, the Fyn kinse gene may be a target for researchers searching for genes that might predispose people to alcoholism.
Anent which Kandel observed, "Any gene that is identified is a potential drug target of interest. But this is just early days [in that research]." *