Playback of Alarm and Appetitive Calls Differentially Impacts Vocal, Heart-Rate, and Motor Response in Rats

Differential neural decoding of alarm and avoidance information from vocal alarm calls in humans

Many animals and humans use scream calls to signal imminent threats, which typically evoke alarm and escape responses in recipients. Compared to animals, human scream calls are more diversified with varying levels of alarm signaling. Certain low-alarm human screams thus might not elicit full neural alarm cascades and threat avoidance actions. Here we investigated the neural circuits for decoding alarm and avoid-approach information from positive and negative scream calls in humans. Alarm and avoid-approach decisions showed minimal neural overlap, with alarm decisions recruiting neural cascades of sensory-affective decoding for action preparation, whereas avoid-approach judgments recruited neural systems for spatio-affective decoding for social decision-making. Furthermore, decision patterns revealed both an alarm quotient and an avoid quotient for risk arbitration, which linked the likelihood of an urgent response (alarm, avoid) to a slowing of choosing a potential safe option in response to scream calls. While the avoid quotient positively predicted neural activity in a broad cortico-limbic network, the alarm quotient predicted neural suppression with increasing alarm levels, especially in the amygdala as part of a presumed limbic alarm system. This points to a critical involvement of the amygdala at neural levels of choice arbitrations rather than in threat evaluations signaled by screams.

22 and 50 kHz rat ultrasonic vocalization playback reveals sex differences in behavior and cFos in brain regions associated with affective processing

Adult rats communicate using ultrasonic vocalization (USV) frequencies indicating negative (22 kHz) or positive (50 kHz) affective states. Playback of USVs can serve as an ethologically translational method to study affective processing in response to socially communicated states. However, few studies have examined behavioral and neural effects of USV playback in both male and female rats. Here, adult male and female Sprague-Dawley rats experienced a 20-min open field test (OFT) with either silence, 22 kHz, or 50 kHz recorded USV playback. Center exploration and locomotor activity were analyzed to characterize sex differences in playback effects. Results suggest that females display greater sensitivity to frequency-specific effects of USV playback in this paradigm compared to males. 50 kHz USV playback evoked an immediate increase in center exploration and locomotor activity in females, indicating the appetitive nature of 50 kHz USVs. Initially, 22 kHz playback inhibited center exploration in the OFT compared to 50 kHz. However, females exhibited a switch in behavioral strategy in response to 22 kHz following prolonged playback. Following OFT, neural activity was quantified via the immediate early gene cFos. Results from cFos quantification showed sex- and region-specific differences in neural recruitment in areas of the brain associated with affective processing, including the prefrontal cortex, amygdala, bed nucleus of the stria terminalis, and nucleus accumbens. Taken together, this work provides a normative baseline for understanding how sex influences behavioral and neural responses to USV playback, which can be leveraged to study anxiety, communication, and affect in an ethologically relevant assay.

Male rats emit aversive 44-kHz ultrasonic vocalizations during prolonged Pavlovian fear conditioning

Rats are believed to communicate their emotional state by emitting two distinct types of ultrasonic vocalizations. The first is long '22-kHz' vocalizations (>300 ms, <32-kHz) with constant frequency, signaling aversive states, and the second is short '50-kHz' calls (<150 ms, >32 kHz), often frequency-modulated, in appetitive situations. Here, we describe aversive vocalizations emitted at a higher pitch by male Wistar and spontaneously hypertensive rats (SHR) in an intensified aversive state - prolonged fear conditioning. These calls, which we named '44-kHz' vocalizations, are long (>150 ms), generally at a constant frequency (usually within 35-50-kHz range) and have an overall spectrographic image similar to 22-kHz calls. Some 44-kHz vocalizations are comprised of both 22-kHz-like and 44-kHz-like elements. Furthermore, two separate clustering methods confirmed that these 44-kHz calls can be separated from other vocalizations. We observed 44-kHz calls to be associated with freezing behavior during fear conditioning training, during which they constituted up to 19.4% of all calls and most of them appeared next to each other forming uniform groups of vocalizations (bouts). We also show that some of rats' responses to the playback of 44-kHz calls were more akin to that of aversive calls, for example, heart rate changes, whereas other responses were at an intermediate level between aversive and appetitive calls. Our results suggest that rats have a wider vocal repertoire than previously believed, and current definitions of major call types may require reevaluation. We hope that future investigations of 44-kHz calls in rat models of human diseases will contribute to expanding our understanding and therapeutic strategies related to human psychiatric conditions.

How can ethology inform the neuroscience of fear, aggression and dominance?

The study of behaviour is dominated by two approaches. On the one hand, ethologists aim to understand how behaviour promotes adaptation to natural contexts. On the other, neuroscientists aim to understand the molecular, cellular, circuit and psychological origins of behaviour. These two complementary approaches must be combined to arrive at a full understanding of behaviour in its natural setting. However, methodological limitations have restricted most neuroscientific research to the study of how discrete sensory stimuli elicit simple behavioural responses under controlled laboratory conditions that are only distantly related to those encountered in real life. Fortunately, the recent advent of neural monitoring and manipulation tools adapted for use in freely behaving animals has enabled neuroscientists to incorporate naturalistic behaviours into their studies and to begin to consider ethological questions. Here, we examine the promises and pitfalls of this trend by describing how investigations of rodent fear, aggression and dominance behaviours are changing to take advantage of an ethological appreciation of behaviour. We lay out current impediments to this approach and propose a framework for the evolution of the field that will allow us to take maximal advantage of an ethological approach to neuroscience and to increase its relevance for understanding human behaviour.

Behavioural response of female Lewis rats toward 31-kHz ultrasonic calls

Rodent ultrasonic vocalisations can be used to assess social behaviour and have attracted increasing attention. Rats emit 50-kHz and 22-kHz calls during appetitive and aversive states, respectively. These calls induce behavioural and neural responses in the receiver by transmitting the internal states of the rats, thus serving communicative functions. Recently, we discovered that female Lewis rats emit 31-kHz calls under social isolation and inequality conditions; however, the biological significance of 31-kHz calls remains unknown. In the present study, we conducted three playback experiments to examine the behavioural effects of 31-kHz calls. In the first experiment, Lewis female rats were exposed to four types of sound: 22-kHz, 50-kHz, 31-kHz calls, and environmental noise. As a result, rats stayed significantly longer in the area with a sound-producing speaker, regardless of the sound type, than in the silent speaker area. The duration spent around the sound-producing speaker was particularly extended during the 50-kHz or 31-kHz call playback, compared to the environmental noise or 22-kHz call playback. In the second experiment, rats were exposed to refined versions of sound stimuli that were synthesised to preserve prominent frequency components while removing background noise from original calls. Rats significantly preferred to stay around the speaker for the synthesised 50-kHz and 31-kHz sounds, but not for the synthesised 22-kHz sound. However, in the third experiment, additional 31-kHz sound synthesised from calls emitted by a different rat did not elicit a significant preference for the source side. These results suggest that the rats paid attention to the 31-kHz call, although it is plausible that acoustic variability in the 31-kHz USV may affect their approach behaviour.

Rats that learn to vocalize for food reward emit longer and louder appetitive calls and fewer short aversive calls

Rats are social animals that use ultrasonic vocalizations (USV) in their intraspecific communication. Several types of USV have been previously described, e.g., appetitive 50-kHz USV and aversive short 22-kHz USV. It is not fully understood which aspects of the USV repertoire play important functions during rat ultrasonic exchange. Here, we investigated features of USV emitted by rats trained in operant conditioning, is a form of associative learning between behavior and its consequences, to reinforce the production/emission of 50-kHz USV. Twenty percent of the trained rats learned to vocalize to receive a reward according to an arbitrarily set criterion, i.e., reaching the maximum number of proper responses by the end of each of the last three USV-training sessions, as well as according to a set of measurements independent from the criterion (e.g., shortening of training sessions). Over the training days, these rats also exhibited: an increasing percentage of rewarded 50-kHz calls, lengthening and amplitude-increasing of 50-kHz calls, and decreasing number of short 22-kHz calls. As a result, the potentially learning rats, when compared to non-learning rats, displayed shorter training sessions and different USV structure, i.e. higher call rates, more rewarded 50-kHz calls, longer and louder 50-kHz calls and fewer short 22-kHz calls. Finally, we reviewed the current literature knowledge regarding different lengths of 50-kHz calls in different behavioral contexts, the potential function of short 22-kHz calls as well as speculate that USV may not easily become an operant response due to their primary biological role, i.e., communication of emotional state between conspecifics.

Playback of rat 22-kHz ultrasonic vocalizations as a translational assay of negative affective states: An analysis of evoked behavior and brain activity

The subjective nature of human emotions makes them uniquely challenging to investigate in preclinical models. While behavioral assays in rodents aim to evaluate affect (i.e., anxiety, hypervigilance), they often lack ethological validity. Playback of negatively valenced 22-kHz ultrasonic vocalizations (USVs) in rats shows promise as a translational tool to investigate affective processing. Much like how human facial expressions can communicate internal states, rats emit 22-kHz USVs that similarly convey negative affective states to conspecifics indicating possible threat. 22-kHz USV playback elicits avoidance and hypervigilant behaviors, and recruit brain regions comparable to those seen in human brains evoked by viewing fearful faces. Indeed, 22-kHz playback alters neural activity in brain regions associated with negative valence systems (i.e., amygdala, bed nucleus of the stria terminalis, periaqueductal gray) alongside increases in behaviors typically associated with anxiety. Here, we present evidence from the literature that supports leveraging 22-kHz USV playback in rat preclinical models to obtain clinically relevant and translational findings to identify the neural underpinnings of affective processing and neuropathological dysfunction.

Spontaneously hypertensive rats manifest deficits in emotional response to 22-kHz and 50-kHz ultrasonic playback

Many symptoms used routinely for human psychiatric diagnosis cannot be directly observed in animals which cannot describe their internal states. However, the ultrasonic vocalizations (USV) rodents use to communicate their emotional states can be measured. USV have therefore become a particularly useful tool in brain disease models. Spontaneously hypertensive rats (SHR) are considered an animal model of attention deficit hyperactivity disorder (ADHD) and schizophrenia. However, the specifics of SHR's behavior have not been fully described and there is very little data on their USV. Recently, we developed a communication model, in which Wistar rats are exposed to pre-recorded playbacks of aversive (22-kHz) or appetitive (50-kHz) USV, and their vocal responses depend on the extent of prior fear conditioning (0, 1, 6 or 10 shocks). Here, we investigated SHR's behavior and heart rate (HR) in our communication model, in comparison to Wistar rats employed as controls. In general, SHR emitted typical USV categories, however, they contained more short 22-kHz and less 50-kHz USV overall. Moreover, fewer SHR, in comparison with Wistar rats, emitted long 22-kHz USV after fear conditioning. SHR did not show a 50-kHz playback-induced HR increase, while they showed a profound 22-kHz playback-induced HR decrease. Finally, the number of previously delivered conditioning shocks appeared to have no effect on the investigated vocal, locomotor and HR responses of SHR. The phenomena observed in SHR are potentially attributable to deficits in emotional perception and processing. A lower number of 50-kHz USV emitted by SHR may reflect observations of speech impairments in human patients and further supports the usefulness of SHR to model ADHD and schizophrenia.

Response Calls Evoked by Playback of Natural 50-kHz Ultrasonic Vocalizations in Rats

Rats are highly social animals known to communicate with ultrasonic vocalizations (USV) of different frequencies. Calls around 50 kHz are thought to represent a positive affective state, whereas calls around 22 kHz are believed to serve as alarm or distress calls. During playback of natural 50-kHz USV, rats show a reliable and strong social approach response toward the sound source. While this response has been studied in great detail in numerous publications, little is known about the emission of USV in response to natural 50-kHz USV playback. To close this gap, we capitalized on three data sets previously obtained and analyzed USV evoked by natural 50-kHz USV playback in male juvenile rats. We compared different rat stocks, namely Wistar (WI) and Sprague-Dawley (SD) and investigated the pharmacological treatment with the dopaminergic D2 receptor antagonist haloperidol. These response calls were found to vary broadly inter-individually in numbers, mean peak frequencies, durations and frequency modulations. Despite the large variability, the results showed no major differences between experimental conditions regarding call likelihood or call parameters, representing a robust phenomenon. However, most response calls had clearly lower frequencies and were longer than typical 50-kHz calls, i.e., around 30 kHz and lasting generally around 0.3 s. These calls resemble aversive 22-kHz USV of adult rats but were of higher frequencies and shorter durations. Moreover, blockade of dopamine D2 receptors did not substantially affect the emission of response calls suggesting that they are not dependent on the D2 receptor function. Taken together, this study provides a detailed analysis of response calls toward playback of 50-kHz USV in juvenile WI and SD rats. This includes calls representing 50-kHz USV, but mostly calls with lower frequencies that are not clearly categorizable within the so far known two main groups of USV in adult rats. We discuss the possible functions of these response calls addressing their communicative functions like contact or appeasing calls, and whether they may reflect a state of frustration. In future studies, response calls might also serve as a new read-out in rat models for neuropsychiatric disorders, where acoustic communication is impaired, such as autism spectrum disorder.

Increased Vocalization of Rats in Response to Ultrasonic Playback as a Sign of Hypervigilance Following Fear Conditioning

We investigated the effects of prior stress on rats' responses to 50-kHz (appetitive) and 22-kHz (aversive) ultrasonic playback. Rats were treated with 0, 1, 6 or 10 shocks (1 s, 1.0 mA each) and were exposed to playbacks the following day. Previous findings were confirmed: (i) rats moved faster during 50-kHz playback and slowed down after 22-kHz playback; (ii) they all approached the speaker, which was more pronounced during and following 50-kHz playback than 22-kHz playback; (iii) 50-kHz playback caused heart rate (HR) increase; 22-kHz playback caused HR decrease; (iv) the rats vocalized more often during and following 50-kHz playback than 22-kHz playback. The previous shock affected the rats such that singly-shocked rats showed lower HR throughout the experiment and a smaller HR response to 50-kHz playback compared to controls and other shocked groups. Interestingly, all pre-shocked rats showed higher locomotor activity during 50-kHz playback and a more significant decrease in activity following 22-kHz playback; they vocalized more often, their ultrasonic vocalizations (USV) were longer and at a higher frequency than those of the control animals. These last two observations could point to hypervigilance, a symptom of post-traumatic stress disorder (PTSD) in human patients. Increased vocalization may be a valuable measure of hypervigilance used for PTSD modeling.

Biological Functions of Rat Ultrasonic Vocalizations, Arousal Mechanisms, and Call Initiation

This review summarizes all reported and suspected functions of ultrasonic vocalizations in infant and adult rats. The review leads to the conclusion that all types of ultrasonic vocalizations subserving all functions are vocal expressions of emotional arousal initiated by the activity of the reticular core of the brainstem. The emotional arousal is dichotomic in nature and is initiated by two opposite-in-function ascending reticular systems that are separate from the cognitive reticular activating system. The mesolimbic cholinergic system initiates the aversive state of anxiety with concomitant emission of 22 kHz calls, while the mesolimbic dopaminergic system initiates the appetitive state of hedonia with concomitant emission of 50 kHz vocalizations. These two mutually exclusive arousal systems prepare the animal for two different behavioral outcomes. The transition from broadband infant isolation calls to the well-structured adult types of vocalizations is explained, and the social importance of adult rat vocal communication is emphasized. The association of 22 kHz and 50 kHz vocalizations with aversive and appetitive states, respectively, was utilized in numerous quantitatively measured preclinical models of physiological, psychological, neurological, neuropsychiatric, and neurodevelopmental investigations. The present review should help in understanding and the interpretation of these models in biomedical research.

Limited generalizability, pharmacological modulation, and state-dependency of habituation towards pro-social 50-kHz calls in rats

Communication constitutes a fundamental component of mammalian social behavior. Rats are highly social animals and emit 50-kHz ultrasonic vocalizations (USV), which function as social contact calls. Playback of 50-kHz USV leads to strong and immediate social approach responses in receiver rats, but this response is weak or even absent during repeated 50-kHz USV playback. Given the important role of 50-kHz USV in initiating social contact and coordinating social interactions, the occurrence of habituation is highly unexpected. It is not clear why a social signal characterized by significant incentive salience loses its power to change the behavior of the receiver so rapidly. Here, we show that the habituation phenomenon displayed by rats in response to repeated playback of 50-kHz USV (1) is characterized by limited generalizability because it is present in Wistar but not Sprague-Dawley rats, (2) can be overcome by amphetamine treatment, and (3) depends on the subject’s internal state.

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Rats display social approach in response to playback of pro-social 50-kHz calls

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Repeated playback leads to habituation with limited generalizability

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Habituation can be overcome by amphetamine treatment

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Habituation depends on the subject’s internal state