Brain Mechanism May Be Key To Why Mating Is More Likely In The Spring By Emil Venere Three Hopkins scientists have discovered a key brain mechanism that may explain why some animals are much more likely to mate in spring than in winter. The researchers, studying a small mouse-like mammal called a prairie vole, have discovered that in the spring, compared with winter, the animal's brain produces much more of a protein that indicates activation of nerve cells essential for mating. The findings will be published in an upcoming issue of the international journal Brain Research. Scientists already knew that voles produce the protein when they are exposed to a specific chemical scent called a pheromone contained in the male's urine and other secretions. Unlike humans, most animals only have sex when hormones called androgens and estrogens are at high levels in the blood. These so-called sex hormones make mating behavior more likely. In prairie voles, females show no interest in mating with males unless they are exposed to the appropriate chemical signal from the males' urine. The chemical stimulus is converted in the brain into a signal that triggers the release of hormones in the female, leading to mating behavior, said Randy Nelson, an associate professor in the Department of Psychology. Hopkins researchers wanted to test their hypothesis that the animals might actually be more sensitive in springtime to the chemical signals controlling reproduction. "So we presented the female with urine at different times of the year, and then we looked at the effects on cells in the brain," said Gregory Ball, another associate professor of psychology who is involved in the research. "It sounds rather unpalatable but remember, most nonhuman animals go around sniffing each other. "We are kind of a strange mammal in that we don't use olfaction in the way that most of our relatives do." Past research had shown that females are much more responsive to the pheromone in spring, even if they are exposed to large amounts of the scent in winter. But the Hopkins team discovered that the female's brain actually produces more of the protein during lab-simulated springtime conditions, Dr. Ball said. When nerve cells in the brain are turned on, certain genes are activated. As a female smells a male's urine, the pheromone activates one type of gene, called c-fos, which "expresses" a protein known, appropriately, as fos. After a cascade of other events in the nervous and endocrine systems, the animal becomes willing to mate. It's called estrus, a point in the female sexual cycle similar to menstrual periods in humans. "We can see what neurons turn on from receptors in the nose, all the way up through the brain, and track, essentially, the neurocircuitry underlying sexual motivation," Dr. Nelson said. Like many animals, the voles do not mate during winter, even if the females are "primed artificially with estrogens," he said. Scientists believe one reason behind the low sexual motivation in winter is that the animal's brain physiology changes as the number of daylight hours increases and decreases. Additional research is testing that idea. Humans don't have anything like behavioral estrus. Although they experience major changes in hormones during ovulation, human sexual behavior and mating are not so profoundly regulated by hormones. "That is a very important thing that affects our social system, that we can engage in mating at any time in the cycle," Dr. Ball noted.
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