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.