Dean A. HaycockSpecial To BioWorld Today
This experimental technique is almost Biblical in its approach: If agene offends thee (or at least interests thee), pluck it out. Then doyour experiment.
Suspect a specific neurotransmitter receptor of playing a role in theregulation of alcohol consumption and perhaps, alcoholism? Justraise a strain of mice in which the gene of interest (or offense) hasbeen deleted. Then compare the consumption of alcohol and itseffects in the "knockout" strain with those in a genetically identical(except for the missing gene, of course) wild strain.
The authors of a recent report in the September 1996 Nature Geneticsdid that and found a potentially important clue for the role ofserotonin _ and a specific serotonin receptor _ in alcoholismvulnerability, said David Goldman, of the National Institute onAlcohol Abuse and Alcoholism.
Several lines of evidence suggest that serotonin (or 5-hydroxytryptamine, 5-HT) plays a potentially significant roll inalcohol drinking behavior. Some human alcoholics are reported tohave less 5-HT activity in their central nervous systems. Andsensitivity and tolerance toward alcohol in lab rats and mice appearsto be influenced by the integrity of 5-HT transmitter systems in thebrain.
John Crabbe, of the Department of Veterans Affairs Medical Centerand the Oregon Health Sciences University, in Portland, Ore., and hisco-authors, used mice lacking the gene for the 5-HT 1B receptor. The5-HT 1B receptor is one of 15 or so serotonin receptor subtypes.(Serotonin receptors are divided into a few families with manysubtypes).
The researchers compared the 5-HT 1B knockout mice with the wild-type, 5-HT 1B gene-spared strain. They found that the knockout micedrank twice as much alcohol as water when 5, 10 and 20 percentalcohol solutions were available (that's 10, 20 and 40 proof by theway).
Furthermore, the mice lacking the 5-HT 1B gene held their liquorbetter in one standard test than did mice with the gene. They wereable to walk about on a floor made of wire mesh without slippingthrough the mesh to activate a sensor below. The mutant mice alsotended to develop tolerance to alcohol more slowly than their 5-HT1B-endowed peers.
No Mice Were Spared From The Pains Of Withdrawal
The two strains of mice, however, did not differ in tests of alcoholwithdrawal.
The studies strongly suggest that the 5-HT 1B receptor plays a role inethanol consumption and that changes in serotonin function in thebrain can affect responses to alcohol affecting ataxia withoutaffecting dependence.
A link between the 5-HT 1B receptor subtype and alcohol drinking isespecially intriguing because this receptor, located on presynapticnerve terminals, is believed to regulate the release of serotonin andother neurotransmitters.
"The absence of 5-HT 1B receptors may show up in some criticalbrain area as an effect that is run through dopamine or GABA orNMDA or pretty much anything else. It is going to be complicated tofollow," Crabbe said.
Researchers know, of course, that developing a strain of knockoutmice is not as clean or esthetically pleasing an approach as it mightappear at first. These mice, after all, develop without the gene theirfellow mice have. A missing gene during important or critical stagesof development could lead to significant or subtle changes in themutant mice.
"It is important to recognize that it is a mice model and the deletionof the receptor in the mouse is a fairly extreme genetic alteration,"Goldman warned.
He added that the gene deletion could increase alcohol consumptionin mice through mechanisms that are different from those in humans."However, I think [the study] is illustrative of one of the methods thatis available for getting clues for the relationship between a singlegene and behavior," Goldman said.
Goldman's lab has looked at the 5-HT 1B receptor in humans and, sofar, has not found significant genetic variation that would beassociated with alcoholism.
"That is not to say there would not be an involvement of this receptorin human alcoholism. [The research] certainly shows that alterationsin a single gene can affect a complicated behavior, which alcoholconsumption probably is," Goldman said.
While author Rene Hen, who developed the mutant mouse strain, andcolleagues, have looked for compensatory changes in other 5-HTsubtypes, they haven't found any, Crabbe told BioWorld Today.There are, however, "some fairly complicated differences in releaseof 5-HT in the various brain areas that are just starting to getcharacterized."
On a behavioral level, the knockout mice demonstrate an interestingdifference compared with wild-type controls. They are moreaggressive.
"There is a subtype of human alcoholics in whom alcohol releasesaggression. Not all, but some. And that is also true in rodent models.We are gearing up to see if these mice differ in response to alcohol interms of aggression," Crabbe said.
Other experiments will seek to identify the exact location of the 5-HT1B receptors responsible for the differences seen in the mutant mice.
"They are reasonably widely distributed in the brain," Crabbe noted,"and it is not likely that alcohol simply affects all areas of the brainequally to produce this drinking effect."
Hen, Crabbe and their collaborators are developing another strain ofmice which can be called "conditional" knockouts. These mice growup with the normal complement of 5-HT 1B receptors but can betreated once adulthood is reached to eliminate the function of thereceptor. That approach will allow the researchers to get around thequestions of whether or not the brains of the mutant mice developedcompensatory ways of dealing with the loss of one receptor subtype.
Another line of planned experimentation will use aggregationchimeras. These are mice which express the 5-HT 1B gene in somebrain regions but not in others. Different mice are expected to havedifferent distributions of the receptor.
The rodents will be tested for alcohol preference and the brains of themost extreme drinkers and the least extreme drinkers will beexamined for differences in receptor number and localization in anattempt to establish structure-function correlations.
Crabbe said he believes this work, in the long term, could lead topotential therapy for alcohol abuse.
"There has been some success, a couple of trials, reporting thatspecific serotonin re-uptake inhibitor drugs [of which Prozac is thegeneric commonly known one] have been effective in treating somealcoholics. So it could be that with this more precise knowledge ofthis receptor subtype involvement, a more well-focusedpharmacotherapy could be developed," he told BioWorld Today. n
(c) 1997 American Health Consultants. All rights reserved.