We specifically hypothesized that the introduction of a female mouse into a male ORX-AB mouse’s home cage would induce positive excitation and therefore induce cataplexy. In order to expand on these findings, we decided to characterize the positive emotion-related nature of cataplexy through experiments utilizing orexin neuron-ablated (ORX-AB) mice, an animal model of narcolepsy/cataplexy 14. Cataplexy is mainly observed during the dark period, the active period in nocturnal mice, but the reason is still unknown 7, 9, 13, 14. Studies have also shown that the amygdala is a key brain structure involved in the occurrence of cataplexy 6, 11, 12.
Deficiency of the neuropeptide orexin in the hypothalamus is one of the causes of this symptom 7 and activation of the NAc and the prefrontal cortex is thought to be an upstream trigger 9, 10. Apart from the positive emotional context, there are many studies pertaining to cataplexy as a symptom of narcolepsy 6, 7, 8, 9, 10, 11, 12, 13, 14. Cataplexy is unique in that head fixation is not necessary to observe associated behaviors and possible triggers are not restricted to food. It is thought to be a good behavioral measure of positive emotion because it is triggered by laughter in humans 4, playing in dogs 5, and palatable food in mice 6. Previous studies have shown successful estimation of positive emotion in mice through counting the number of tongue protrusions in response to the presentation of honey 2 and measurement of facial expressions in head-fixed mice with the assistance of artificial intelligence 3.Ĭataplexy is a form of muscle atonia seen in both human narcolepsy patients and animal narcolepsy models. In experimental animals, it is necessary to estimate their emotional state through behavioral observation because verbal communication is not possible. However, more detailed information on the neurotransmitters and neuronal pathways involved in the interaction between emotions and bodily health is currently unavailable. Brain imaging studies in humans 1 predicted the possible involvement of several brain structures in happiness, such as the nucleus accumbens (NAc), ventral pallidum, and anterior cingulate cortex. Although their benefits are well known, the underlying brain mechanisms are not well understood. Happiness and positive emotion are key for both mental and physical well-being. 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.
A strong positive correlation was observed between the number of USVs and cataplexy-like bouts. During the light period observation, more than 85% of cataplexy-like bouts were preceded by USVs. 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. 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”. 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. Cataplexy is triggered by laughter in humans and palatable food in mice.
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