Brain arousal heightens sexual activity in male mice

Donald W. Pfaff, head of the Laboratory of Neurobiology and Behavior, and his colleagues, including first author Zachary M. Weil, a postdoc in the lab, are conducting an ongoing large-scale selective breeding project to produce mice that have high or low levels of generalized arousal, the fundamental force of the central nervous system that gives rise to all mammalian behaviors. In only six generations, the researchers observed that mice with high levels of generalized arousal were highly excitable, mounted females quickly but awkwardly and finished very rapidly.

"These male mice genetically selected for elevated brain arousal are a little bit like teenage boys," says Pfaff, who led the research. "They are very excited by the opposite sex but they don't know exactly what to do and so they rush around and mount the animal in all kinds of inappropriate positions, like from the side, which of course does not work. It takes them a long time to get to an intromission, an actual insertion. But then once they get that first intromission, they ejaculate rapidly."

The researchers further sorted the mice by their own generalized arousal score versus their parents' and observed their anxious-like behavior. Compared to mice whose parents scored low on brain arousal, those with high-scoring parents ventured less often into open or lit spaces, where they could be snatched by a predator; they also reacted more to neutral odors, suggesting that their sensory system was on high alert. However, when it came to test differences in mice that actually scored high or low on generalized arousal, the researchers didn't find any.

The finding may explain how the nervous energy of high-scoring males can lead to successful yet awkward sexual exploits that in the end are evolutionarily advantageous. "The same twitchiness that leads to this crazy, sexual excitability, I believe, also leads to the anxiety," says Pfaff. "In evolutionary terms, a high level of generalized arousal is probably highly beneficial."

Problems in the controls over arousal mechanisms may explain a number of diseases, including depression and attention deficit/hyperactivity disorder, Pfaff explains. Depression, for instance, often manifests as an inability to initiate motivated behaviors like getting out of bed or doing simple household chores. On the other hand, ADHD may arise from an overactive arousal mechanism that needs to be tamped down. "This research tries to get to the bottom of what activates behavior," says Pfaff. "It tries to untangle what puts it into gear."