Muscarinic regulation of self-grooming behavior and ultrasonic vocalizations in the context of open-field habituation in rats

Biperiden reverses the increase in 50-kHz ultrasonic vocalizations but not the increase in locomotor activity induced by cocaine

Cocaine use disorder (CUD) is a worldwide public health problem, associated with severe psychosocial and economic impacts. Currently, no FDA-approved treatment is available for CUD. However, an emerging body of evidence from clinical and preclinical studies suggests that biperiden, an M1 muscarinic receptor antagonist, presents potential therapeutic use for CUD. These studies have suggested that biperiden may reduce the reinforcing effects of cocaine. It is well established that rodents emit 50-kHz ultrasonic vocalizations (USV) in response to natural rewards and stimulant drugs, including cocaine. Nonetheless, the effects of biperiden on the cocaine-induced increase of 50-kHz USV remains unknown. Here, we hypothesized that biperiden could antagonize the acute effects of cocaine administration on rat 50-kHz USV. To test this hypothesis, adult male Wistar rats were divided into four experimental groups: saline, 5 mg/kg biperiden, 10 mg/kg cocaine, and biperiden/cocaine (5 and 10 mg/kg, i.p., respectively). USV and locomotor activity were recorded in baseline and test sessions. As expected, cocaine administration significantly increased the number of 50-kHz USV. Biperiden administration effectively antagonized the increase in 50-kHz USV induced by cocaine. Cocaine administration also increased the emission of trill and mixed 50 kHz USV subtypes and this effect was antagonized by biperiden. Additionally, we showed that biperiden did not affect the cocaine-induced increase in locomotor activity, although biperiden administration per se increased locomotor activity. In conclusion, our findings indicate that administering biperiden acutely reduces the positive affective effects of cocaine, as demonstrated by its ability to inhibit the increase in 50-kHz USV.

Distinct acute stressors exert an antagonistic effect on complex grooming during novelty habituation in rats

Grooming is a common readout in multiple rat models of neuropsychiatric diseases. It is usually associated with distress and negative emotionality, but also with emotional de-arousal after stress. These seemingly conflicting interpretations may result from specific grooming sequences appearing at different arousal levels and during distinct phases of the stress response. To further explore this hypothesis, we analyzed how distinct stressors affect grooming syntaxis and kinetics. To that end, we explored the independent and interacting effects of foot shocks, corticosterone (CORT), and novelty on exploratory activity, grooming, and ultrasonic vocalizations (USVs) in an open-field test (OF). Wistar rats were intraperitoneally injected either with vehicle or CORT, placed in a chamber where half of them were foot-shocked and then assessed in the OF. The next day, animals were re-exposed to the shock chamber and then tested in the OF without receiving any treatment. On day 1, foot shocks and -to a less extent CORT- increased freezing and inhibited rearing in the chamber, but only foot shocks increased distress USVs. In the OF, both treatments suppressed complex grooming, with foot-shocks also inhibiting exploration and CORT marginally reducing rearing. On day 2, foot-shocked rats showed conditioned fear when re-exposed to the chamber. When tested in the OF, foot-shocked and CORT-treated animals still showed low levels of complex grooming, with the former group also showing increased distress USVs. In this study, all different stressors inhibited complex grooming, suggesting an inverse association between these grooming subtypes and negative emotionality.

Behavioral analysis in laboratory rats: Challenges and usefulness of 50-kHz ultrasonic vocalizations

Many rodent species emit and detect vocalizations in the ultrasonic range. Rats use three classes of ultrasonic vocalizations depending on developmental stage, experience and the behavioral situation. Calls from one class emitted by juvenile and adult rats, the so-called 50-kHz calls, are typical for appetitive and social situations. This review provides a brief historical account on the introduction of 50-kHz calls in behavioral research followed by a survey of their scientific applications focusing on the last five years, where 50-kHz publications reached a climax. Then, specific methodological challenges will be addressed, like how to measure and report 50-kHz USV, the problem of assignment of acoustic signals to a specific sender in a social situation, and individual variability in call propensity. Finally, the intricacy of interpreting 50-kHz results will be discussed focusing on the most prevalent ones, namely as communicative signals and/or readouts of the sender's emotional status.