Why The Same Smell Makes Males Aggressive And Females Amorous

A pheromone secreted from the tear glands of male mice triggers different signals in male and female brains.

AsianScientist (Jun. 29, 2017) – In mice, a single pheromone enhances sexual behaviors in females while promoting aggression in males. These findings, published in Neuron, show how specific chemical signals can activate distinct neural circuits and generate divergent behavioral responses.

In most animals, the sense of smell plays a critical role in controlling instinctive behaviors. For instance, chemical signals from a partner, competitor or predator elicit specific behavior in mice, namely mating, aggression and defensive behaviors, respectively.

Ever since a pheromone secreted by a female moth that attracts the opposite sex was identified in 1959, scientists have pinned down numerous chemicals that affect behavior in a wide variety of animal species, from insects to mammals to humans. Despite their growing database of known pheromones, scientists knew little about how the brain actually converts certain sensory input into appropriate behavioral output, especially in mammals.

“It is widely known that some chemicals, especially odors, can impact an animal’s instinctive behaviors even on first contact,” said Kazushige Touhara, a professor at the University of Tokyo’s Graduate School of Agricultural and Life Sciences, who supervised the study. “We assumed there was a neural mechanism in the brain that correctly connects important sensory information to appropriate behavioral centers in the brain.”

In this study, the research group used a male pheromone called ESP1 that has been shown to enhance sexual behaviors in female mice, while promoting aggression in males. Unlike other pheromones, which tend to be composed of a complex web of substances, ESP1 is a single chemical that is detected by a sole corresponding receptor, making it comparatively easy to track.

The group employed various viral tracing methods to visualize the neural circuit downstream of the ESP1 receptor/ They also mapped the path of neurons that conveys ESP1 signals in the brain. Using this method, researchers found that the information of ESP1 was routed differently in males and females by neurons in a region of the brain called the amygdala.

The researchers also found that activation of ESP1-responding neurons in the region of the brain called the hypothalamus enhanced sexual behavior in female mice, even in the absence of actual ESP1. In contrast, activation of neurons that responded to snake skin, a predator cue that elicits defensive behaviors, in the same brain area showed no change in sexual behaviors.

“This finding suggests that there are two different types of neurons, ESP1 and predator neurons, and only the former controls sexual behaviors in female mice,” explained Touhara.

A similar discovery in fruit flies, reported in an earlier independent study, showed that a particular sex pheromone enhances female sexual behaviors and male aggression via separate neural circuits between the sexes, suggests that a sexually distinct circuit may be a universal strategy for converting male pheromone information into appropriate behavioral output.

Further understanding the neural basis underlying the control of female sexual behaviors could also provide insights into the origin of sexual dysfunctions, the researchers said.

The article can be found at: Ishii et al. (2017) A Labeled-Line Neural Circuit for Pheromone-Mediated Sexual Behaviors in Mice.


Source: University of Tokyo; Photo: Kazushige Touhara.
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