Individual differences in frequency modulated 50-kHz ultrasonic vocalizations reflect but do not predict cocaine self-administration in rats.

Frequency-modulated (FM) 50-kHz ultrasonic vocalizations (USVs) in rats reflect a positive emotional state. The aim of this study was to evaluate whether anticipatory and cocaine-induced FM 50-kHz USVs can predict individual vulnerability to cocaine self-administration behavior. Twenty singly housed rats underwent behavioral testing (elevated plus maze, novel object recognition, and tickling-induced vocalizations) and amphetamine sensitization. The animals then participated in 22 self-administration (SA) sessions (2 days on / 1 day off), during which active nose pokes and FM 50-kHz USVs were recorded during both the anticipatory (cocaine-free) and cocaine phases. Despite pronounced individual differences in SA levels, none of the analyzed behavioral markers predicted SA acquisition. The FM 50-kHz USV response to the first dose of amphetamine showed a bimodal distribution; rats with a weak response exhibited significantly higher levels of cocaine SA. Due to substantial behavioral differences, further analyses were conducted in low-SA and high-SA groups. Anticipatory FM 50-kHz USVs did not correlate with SA intensity and were similarly high in both groups. During the cocaine phase, high-SA rats displayed significantly stronger vocal responses than low-SA rats. FM 50-kHz USVs reflect both contextual and drug-induced positive affect but are highly sensitive to housing conditions and experimental procedures. The response to the first amphetamine dose may serve as a predictive marker of vulnerability to cocaine SA. However, anticipatory FM 50-kHz USVs do not provide a reliable indicator of SA intensity. Accurate interpretation of FM 50-kHz USVs requires integration with behavioral and neurobiological markers to effectively identify addiction risk phenotypes.