Normobaric and hyperbaric gas therapies in neurodegeneration: toward a multimodal protocol for early intervention.

Ultrasonic vocalizations (USVs) are high-frequency calls produced by rats known to correlate with emotional and motivational states, yet their diversity and functional relevance remain underexplored. In this study, we identified a novel 40 kHz inverted-U USV subtype associated with chocolate consumption. These calls were closely tied to feeding behavior and were selectively suppressed by an opioid receptor antagonist, suggesting modulation by the brain's reward system. Using machine learning–assisted classification, we established a robust method to detect these vocalizations. Our findings highlight the potential of specific USVs as noninvasive, real-time behavioral indicators of internal states, offering new tools for studying affect, reward, and communication in animal models. To understand the neurobiological mechanisms that underlie positive emotions, it is essential to identify behavioral and physiological responses to rewarding external stimuli, which in turn enables objective and quantitative assessment of such emotional states. Food consumption is accompanied by positive emotions in both animals and humans, particularly when the food is highly palatable, such as sweet-tasting foods indicating high nutritional value. These positive emotional experiences are closely associated with learning and motivation, particularly when stimuli reinforce future behaviors (). In animal studies, food palatability and associated positive emotions are frequently evaluated using preference tests, such as food choice paradigms comparing intake between two or more options (; ). However, preference tests are indirect measures: they assess palatability and emotional valence only through differences in cumulative food intake over time. To better understand the affective component of palatable food consumption, it is important to capture the immediate behavioral and physiological responses that occur at the moment of feeding. A commonly used method for real-time assessment of positive emotions is the taste reactivity test, in which animals receive passive intraoral infusions of sweet solutions (e.g., sucrose) via implanted oral cannulas and their facial and bodily responses are analyzed (). In this test, specific ingestive responses are defined as hedonic “liking.” However, this method is invasive and requires physical restraint for cannula connection. Moreover, it is limited to liquid taste stimuli and cannot assess emotional responses to solid foods. Therefore, more versatile, noninvasive, real-time measures of positive emotional states during food consumption in animals are needed to further investigate the underlying neural mechanisms.