By David N. Leff

Editor's note: Science Scan is a roundup of recently published biotechnology-relevant research.

Some laboratory rats and mice are condemned to serve as surrogates for life-threatening diseases. Other, perhaps more fortunate animals, are called upon to model addictive drugs of abuse, such as cocaine, nicotine or alcohol.

As described in the November 1999 issue of Nature Neuroscience, two dozen mice were treated to an open bar, and invited to drink as much or as little ethanol as they desired. The paper reporting this experiment, authored by neurologists at the University of California, San Francisco, bears the title: "Supersensitivity to allosteric GABA_A receptor modulators and alcohol in mice lacking PKCe. "

The scientists' motivation in tempting these rodents into alcoholism was seeking to solve long-standing puzzles over the highly variable effects of GABA_A - gamma-aminobutyrate type A - receptors on brain function, particularly on people under the influence.

GABA_A is a key neurotransmitter in the central nervous system, used by synapses to control neuronal inhibition and excitation. Valium (diazepam) is only one of several psychoactive drugs that rely on GABA receptors. These come in two persuasions, A and B, but only A is well enough understood to be of current use on the market.

A's receptor molecule consists of five radially arranged subunits. They comprise the segments of an ion channel, located in the middle. The GABA_A receptor manages many of ethanol's actions - but that's only half of this story line. The other half involves an enzyme called protein-kinase C-epsilon (PKCe), which is presumed to phosphorylate (add a phosphate group to) the GABA_A receptor. (See BioWorld Today, July 10, 1998, p. 1.)

Of those 24 mice bellying up to the researchers' bar, 12 were knockout (KO) mutants lacking the PKCe gene. Both cohorts confronted two self-serving bottles - one, pure water, the other spiked with ethanol. Initially, the alcohol content was 2 percent, somewhat less than beer. Then at four-day intervals it went up, first to 4 percent (in the beer range), then 6 percent (ale), finally 10 percent and 14 percent (the levels of wine).

During this carefully optional binge, both teetotaling and tippling mice were tested for behaviors comparable to those of ethanol in humans. These included measurement of restless locomotion in their cages, and the ability - or inability - of a mouse laid on its back to right itself on all fours within 30 seconds.

Behaviorally, the KO mutants, lacking the enzymic influence of PKCe on their GABA_A receptors, were largely abstinent. They said "no" to ethanol far more than did their full-blooded littermates, egged on by their GABA-energizing enzyme, to seek the psychic rewards of the drug.

The paper concludes that PKCe inhibitors - besides treating alcohol addiction in humans - "could provide a useful, non-sedating alternative for enhancing GABA_A receptor functions to treat disorders such as anxiety and epilepsy."

Not Every Knockout Animal Guarantees A Knockout, Cautions Swiss Prion Scientist

Elsewhere on the KO front, a "Perspective" commentary in the current Science, dated Oct. 29, 1999, posits a caveat to the validity of knockout-animal experiments. Its title is "PrP's Double Causes Trouble." The author, Swiss molecular biologist Charles Weissmann, is at the British Medical Research Council Prion Unit/Neurogenetics in London.

Weissmann buttresses his cautionary observation on an article in the Oct. 1, 1999, issue of the Journal of Molecular Biology, titled, "Ataxia in prion protein (PrP)-deficient mice is associated with upregulation of the novel PrP-like protein doppel." It proposes that "the linked expression of the Prnp and Prnd genes may play a previously unrecognized role in the pathogenesis of prion diseases or other illnesses." But Weissmann uses this paper to warn that "disrupting a gene may entail far more than the phenotype that meets the eye."

Rodents, Move Over: Make Room For Pets, Farm Animals, et al. In Genomic Research

Still further to the role of research's furry friends is the datum that the Jackson Labs, in Bar Harbor, Maine, lists more than 1,000 spontaneous mouse mutations in its genome database. Of these, 128 have been genomically characterized, and 58 (45 percent) carry homologous gene mutations discovered in humans with an associated genetic disease.

A paper in Science dated Oct. 15, 1999, cites these numbers. It's titled, "The promise of comparative genomics in mammals," and observes that "our ability to map and sequence human and mouse genes is outpacing attempts to discern their function." It makes the added point that "monogenic diseases, which were the great successes of the early years of the Human Gene Project, represent a simplification of reality. Most phenotypes are both polygenic and multifactorial."

But beyond the ubiquitous rats, mice and non-human primates of research, the paper points to two other categories of mammal with growing potential for genomic contributions. One consists of "companion animals" - i.e., pet cats and dogs; the other of domestic agricultural animals - cattle, sheep, pigs and horses. "Some 364 genetic diseases have been described in dogs," the Science paper points out, "and over 200 in cats. Cancer registries exist for both."

Down on the farm, the article cites several examples of human diseases first found in domestic animals:

¿ The muscular hypertrophy trait (double muscling) was mapped to bovine chromosome 2.

¿ A gene deletion in Belgian Blue cattle tracked the human myostatin trait.

¿ In sheep hereditary chondrodysplasia mapped to ovine chromosome 6, found homologous segments on the short arm of human chromosome 4 - and mouse chromosome 5.15.

¿ A porcine gene carries alleles that confer a stress syndrome analogous to malignant hypothermia in humans.

"Comparative genetic assessment," the paper concludes, "expands the utility of these gene maps in gene discovery, in functional genomics and in tracking the evolutionary forces that sculpted the genome organization of modern mammalian species."