Identification of multiple call categories within the rich repertoire of adult rat 50-kHz ultrasonic vocalizations: effects of amphetamine and social context

Rationale 50-kHz ultrasonic vocalizations (USVs) emitted by adult rats are heterogeneous; they occur over a wide frequency range, show varying degrees of frequency modulation, and appear to differ in their behavioral significance. However, they have not been extensively categorized. Objectives The main objective of this study was to identify subtypes of 50-kHz USVs emitted by adult rats and to determine how amphetamine (AMPH) or social testing condition affects their relative and absolute production rate and acoustic characteristics. A second objective was to determine the extent of individual differences in call rate, call subtype profile, and acoustic parameters (i.e., duration, bandwidth, and mean peak frequency). Methods Adult male Long–Evans rats were administered systemic amphetamine (0.25–2 mg/kg, IP) and tested individually or with a cage mate for 20 min. Call categories were defined based on visual inspection of over 20,000 USV spectrograms. Surgical devocalization was performed on a subset of AMPH-tested rats in order to confirm the authenticity of call subtypes. Results Fourteen categories of 50-kHz USVs were recognized. Call subtypes were differentially affected by social context, AMPH dose, and time within session. In contrast, the acoustic characteristics of call subtypes were notably stable. Marked and stable inter-individual differences occurred with respect to overall 50-kHz call rate, acoustic parameters, and call profile. Conclusions The present findings, obtained under saline and amphetamine test conditions, provide the first detailed classification of adult rat 50-kHz USVs. Consideration of 50-kHz USV subtypes may advance our understanding of inter-rat communication and affective state.