By David N. Leff

"Feelings of lust make good evolutionary sense," observed endocrinologist Donald Pfaff. "After all," he added, "if sex were not enjoyable, we might fail to do it."

This fact of life obtains equally for mice and men - as well as women. It facilitates Pfaff's in vivo research into the sexual mores of mammals. He heads the Rockefeller University's Laboratory of Neurobiology and Behavior, and is senior author of a paper in the latest Proceedings of the National Academy of Sciences (PNAS), dated Dec. 19, 2000. Its lead author is behavioral neuroscientist Sonoko Ogawa.

Their article in PNAS bears the title: "Abolition of male sexual behaviors in mice lacking estrogen receptors a and b ( abERKO)."

"Mice and humans have very similar molecular structure in their estrogen receptors," Ogawa told BioWorld Today, "and testosterone is an almost identical steroid hormone between humans and mice. Testosterone," she explained, "is the male hormone, but it can also be converted to estrogen - and affecting the estrogen receptor, it has a behavioral effect. So experimenting with these genetically engineered mice, we were able to study the mechanism by which testosterone works to control behavior.

"In this paper," Ogawa said, "we report that the estrogen receptors are a lot more important than people normally think. For the first time we could look at the behavioral effects in mice lacking both types of estrogen receptor, alpha and beta. We found that these two different kinds of behavior are regulated in different ways. Sex drive and aggression usually show a high correlation, but not necessarily in this case."

Their current work began five or six years ago, Ogawa recounted, when research endocrinologist Kenneth Korach, program director of environmental diseases and medicine at NIH's National Institute of Environmental Health Sciences in Research Triangle Park, N.C., knocked out the alpha estrogen receptor gene from mice. He is co-senior author of the PNAS paper.

Two KO Genes Bigger Than Sum Of Parts

"At that time," Ogawa recalled, "we didn't know about the existence of a beta estrogen receptor. Then three or four years ago, that second receptor, beta, was cloned. This was a big surprise to all of us because for a long time we believed there was only one estrogen receptor. Then Dr. Korach made a beta KO mouse, and we studied its behavior, too - which we published last year. So naturally, we wanted to see, if we knocked out both of those genes, what would happen. The answer makes the point of our present paper."

When she introduced highly raunchy females into the home cages of males deprived of both alpha and beta estrogen receptor genes, their sexual behaviors were completely disrupted. These double-KO animals showed zero levels of penile intromissions - penetration - and sperm ejaculation, as well as simple mounting behavior. In contrast, KO males lacking only the beta receptor gene retained all sexual behavior components intact.

Divested of both alpha and beta genes, the double-KO mice lost not only all their masculine libidinous oomph, but their macho aggressiveness along with it. That is, they lunged, bit, boxed, and chased intruding males less - similar to the pugnacious alpha-KO animals. Half of them even tried to take their bullying out on females, even though they were indifferent. Beta knockouts, on the other hand, got more aggressive.

Aside from these behavioral manifestations, the alpha-beta double-KO mice were similar to the alpha-minus males, including infertility, reduced numbers and motility of sperm, yet normal gonads. "Grossly anatomically," Ogawa pointed out, "they have testes and all these organs, but sperm count is very low. Also, double-knockout females are not fertile, either. They don't show any estrous cycle - corresponding to the menstrual cycle in humans. They probably do not copulate. But even in males, the alpha estrogen receptor KO has a big effect on fertility.

"These findings," their paper summed up, "suggest that at least one of the estrogen receptors is required for the expression of simple mounting in male mice."

Prevalence Of Human Alpha Mutation: One

Korach told BioWorld Today how he and a colleague discovered the only known human patient with a mutation in the alpha-estrogen receptor gene:

"About five years ago," he recalled, "in collaboration with pediatric endocrinologist Eric Smith at the University of Cincinnati Medical Center, we identified the patient. This 26-year-old man presented to an orthopedist with knock-knees - genu valgam in medical parlance. A whole-body X-ray," Korach continued, "revealed he had an osteoporotic skeleton, with a bone density of about a 70-year-old woman. He did not respond to estrogen treatment, comparable to what our male alpha-KO mice showed. The physicians were able surgically to straighten out his knees, since when he has been pretty well lost to follow-up. We identified the estrogen receptor mutation," Korach went on. "It caused the protein to be truncated so the gene did not express a functional protein."

As for putative clinical applications of these hormonal findings, Korach pointed out, "There are mouse studies suggesting that there could be some cognitive deficits in the estrogen-receptor alpha knockout that have not been reported in Ogawa's data. These are called the complex eight-arm-maze tests where the animal is put in the middle of a maze with eight arms. These are baited and the animal learns how quickly it can find the food. That's where the alpha KOs appeared to show brain deficits. Ogawa presented this as a poster at the last neuroscience meeting.

"In the biotech-pharmaceutical field," Korach observed, "knowing that in fact the biological action is in fact ligand-activated, that brings out the potentials of development of drugs specifically for treatment of this type of behavioral modification. If one can then use this as a stepping stone to identifying genes as modulators of some of this response, triggered by estrogen, its receptor or its pathway, it helps in mapping the brain. I think these are all areas that would have obvious benefits to science, but also could be beneficial for commercialization regarding the development of gene screening for drug discovery, and so forth."