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A method to quantify happy emotion
Tomoyuki Kuwaki

It is empirically known that positive emotion has good influence on bodily health and may be useful in preventive medicine. However, mechanism of this phenomenon is scarcely understood because of paucity of quantification method of positive emotion in experimental animals. We hypothesized that measuring cataplexy would be a candidate. Cataplexy is one of the characteristics of narcolepsy (one of the sleep disorders) and is triggered mainly by happy emotions such as laughter in humans and palatable food in mice. To further evaluate mice’s cataplexy, we examined courtship behavior in orexin neuron-ablated mice (ORX-AB), one of the animal models of narcolepsy/cataplexy. Wild-type female mice were placed into the home cage of male ORX-AB and cataplexy-like behavior was observed along with ultrasonic vocalizations (USVs), also known as the “love song”. ORX-AB with a female encounter showed cataplexy-like behavior both during the dark and light periods, whereas ORX-AB with chocolate predominantly showed it during the dark period. During the light period observation, more than 85% of cataplexy-like bouts were preceded by USVs. A strong positive correlation was observed between the number of USVs and cataplexy-like bouts. Cataplexy-like behavior in narcoleptic mice is a good behavioral measure to study the brain mechanisms behind positive emotion because they can be induced by different kinds of positive stimuli, including chocolate and female courtship.

Sexual excitation induces courtship ultrasonic vocalizations and cataplexy-like behavior in orexin neuron-ablated male mice. Kuwaki T. & Kanno K. (2021) Communications Biology 4: 165 (https://www.nature.com/articles/s42003-021-01696-z), DOI: 10.1038/s42003-021-01696-z
Example movies of USV calls and cataplexy behavior are available at the supplemental information of the journal.

a: Schematic explanation of the experiment. Behavior of an ORX-AB male mouse was videotaped together with recordings of the ultrasound vocalization with a female or a male encounter in his home cage for 4h. Recorded ultrasound waves were analyzed offline to detect syllables (USV calls) indicate blue line in the bottom figure. b: The number of USV calls was greater with female than with male encounters. c: The number of cataplexy-like bouts classified according to the sex of the mouse encountered and whether it is preceded by USVs within 1 min before the onset of cataplexy-like behavior. With female encounters, more than 85% of cataplexy-like bouts were preceded by USVs (left-most column). d: Correlation analysis between the number of calls during the 1 min preceding cataplexy and the number of USV-associated cataplexies-like behavior. A strong association was observed.

Department of Physiology, Graduate School of Medical and Dental Sciences, Kagoshima University, Japan