A study published in the open-access journal PLoS ONE reports that during sexual interactions with female mice, male mice emit high-frequency sounds. Researcher Haoran Wang (University of Toronto) and colleagues found that the vocalizations are linked to approach behaviors and genes that are in charge of positive emotions.

It has long been known in the scientific community that emotions and other personality traits and disorders are correlated across family members. However, it is not easy to find out which genes are crucial in controlling emotions. Since humans and mice have similar numbers of genes, mice are useful in studying human genetic behavior. One specific advantage of mice is that their genes can be added or deleted, and researchers have analyzing mouse behaviors to establish associations between genes with complex behaviors.

The researchers from the University of Toronto and Northwestern University in Chicago used the number and density of different male and female vocalizations to index the emotional responses of mice. Inaudible to humans, these high-frequency calls are measured using special microphones and computer programs. Males often made simple whistles or modulated calls before mounting the females (in the courtship phase). However, the vocalizations shifted to more complex “chirp-like” calls after mounting. As intensity increased during this phase, the sounds increased in number and became more complex.

When single genes related to the neurotransmitters dopamine and acetylcholine were deleted, the researchers found a significant reduction of male calls. Both of these neurotransmitters are important for emotional expression in humans and rats. Specifically, deleting the M5 and M2 acetylcholine receptor genes reduced the number of male vocalizations. Additionally, the duration, frequency, and bandwidth of the calls changed after M2 and M5 receptor deletion.

Contrarily, mutations in the M4 muscarinic receptor and D2 dopamine receptor were not associated with a change in the number of vocalizations. Mutations in the D2 dopamine receptor, however, were found to change the length of calls.

Wang notes that, “This work has supported the theory, proposed by Jeffrey Burgdorf and Jaak Pansepp, that 50-kHz calls reflect positive affect in rats, and extended that theory to higher-frequency mouse calls.”

The investigators also noted a role for dopamine by finding that the street drug amphetamine activates the brain dopamine system and induces a “chirp-like” call at very low doses. However, male mice that did not have the M5 receptor gene – important for activating dopamine neurons – did not manifest the chirp.

Wang and colleagues conclude by positing that their results offer a new way to screen for emotion genes and to develop drugs for controlling emotions, drug abuse and mood disorders.

Ultrasonic Vocalizations Induced by Sex and Amphetamine in M2, M4, M5 Muscarinic and D2 Dopamine Receptor Knockout Mice
Wang H, Liang S, Burgdorf J, Wess J, Yeomans J.
PLoS ONE (2008). 3(4): e1893.
doi:10.1371/journal.pone.0001893
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Written by: Peter M Crosta