For some people, the word “pheromones” conjures up images of potent, seductive, or even aphrodisiacal chemicals that fuel the mating process, but a new study in mice shows that these compounds may do far more, providing a wealth of information regarding social and reproductive status for decoding by a highly specialized sensory apparatus.

There are two major neural pathways involved in the detection and decoding of olfactory stimuli: the main olfactory bulb (MOB) and the accessory olfactory bulb (AOB). The two systems are similar on several levels, but the AOB pathway displays greater sensitivity and selectivity in its responses to sensory stimuli, and furthermore appears to be the primary mediator of response to pheromonal signals, in that disruption of AOB function dramatically influences sex recognition and mating behavior.

To assess fully AOB responses to pheromones, one must take brain activity readings from alert, behaving mice. To this end, Minmin Luo and her colleagues at Duke University Medical Center (Durham, NC) used a miniature microdrive, a specialized system for precisely navigating electrodes within the brain, to take measurements of the activity of individual neurons within the AOB in response to a test animal's inspection of mice of different sexes and genetic backgrounds (Science, 21 February).

Luo's group was excited by the high selectivity of responses that they observed. Many of the trials identified individual cells that displayed strong excitatory or inhibitory behavior only in response to very specific combinations of genetic strain and sex. One neuron responded only to males or females from the BALB/c strain, whereas another's activation depended on exposure to CBA mice. Some cells responded only to females from a particular strain, although the investigators saw no generically sex-specific neuronal responses. Luo's group even identified two neurons that were triggered only by exposure to CBA male castrates, but not to CBA females or normal males. Clean fake mice, on the other hand, triggered no AOB response even after prolonged sniffing.

The authors suggest that the AOB may be involved in assembling “pheromonal images” through the integrated input of a relatively small number of highly specialized neurons. The initial results are compelling, and Luo's group is currently planning further studies to assess the connections between AOB activity and animal behavior.

“We are very interested in studying the pheromonal memories formed within the pathway,” says Luo, “[as well as] recording from behaving mice to study the physiological mechanisms underlying pheromone-triggered social behavior.”