Development and heterogeneity of rat 22 kHz vocalizations

Development of vocal responses to air-puff was analyzed in rats from postnatal day 2-65 with a special attention to first adult-type 22 kHz calls. Neonates and infants vocalized with stereotypic distress calls to cold air of the puff. Juveniles responded to air-puff with short, primitive 22 kHz calls, and adult rats produced species-typical 22 kHz vocalizations of at least three subtypes. It is hypothesized that the category of emitted vocalizations was dependent on the stage of the limbic system development. Emission of primitive 22 kHz calls by juveniles corresponds to activity of an ancient part of the limbic system. Based on the analysis of a large population of 22 kHz calls, it is proposed that there are three subtypes of 22 kHz aversive vocalizations in adult rats. (1) Short 22 kHz vocalizations (20-300 ms) are serving as warning calls or expressing frustration, uncertainty, and novelty testing. Duration of these calls is shortened at higher level of aversive arousal. (2) Medium duration, long 22 kHz vocalizations (300-1000 ms) are serving expression of alarm, anxiety, or potential danger. Duration of these calls is also shortened at higher level of aversive arousal. (3) Extra long duration 22 kHz vocalizations appear to stimuli defined as annoyers and is expressing annoyance, irritation, and exasperation. Duration of these calls is prolonged at higher level of aversive arousal and can reach the longest durations for 22 kHz vocalizations. All three subtypes of 22 kHz calls are associated with certain form of negative emotional state as uncertainty, discomfort, displeasure, and aversion.