The influence of social environmental condition on the production of stress-induced 22 kHz calls in adult male Wistar rats

Stress-Induced Ultrasonic Vocalization in Laboratory Rats and Mice: A Scoping Review

Introduction: Ultrasonic vocalization (USV) can indicate affective states-including psychosocial stress-in mice and rats. However, stress-induced USV changes could be confounded by laboratory experimental variables such as the type of behavioral stress paradigm, the elicitation method, rodent strain, etc. We sought to provide a review of the current literature to delineate how psychosocial stress-altered rodent USVs may be affected by factors of age, sex, strain, species, elicitation paradigm, and stressor. Methods: We used PubMed, Scopus (Elsevier), PsycINFO (EBSCO), and the following Web of Science (Clarivate) databases: Biological Abstracts, CAB Abstracts, Science Citation Index-Expanded, and Emerging Sources Citation Index. The studies identified by our search strategy were independently screened by two authors with the following inclusion criteria: peer-reviewed, in English, reported original data, and described USV in response to stress in rats or mice. The data extracted included USV acoustic parameters (mean peak frequency and mean amplitude (loudness)), details of the stress and USV elicitation paradigms, rodent species, age, and sex variables. Results: The following screening of 5309 titles/abstracts and 687 full-text articles revealed 148 articles. Footshock (20%), cold exposure (14%), and maternal separation (23.5%) were the most commonly used stress paradigms (duration and type of stressor varied across studies), with the total number of USV calls being the most commonly reported acoustic outcome. In rats, 121 articles described stress-altered USVs, while 25 studies reported the same in mice, and two reported multiple rodent species (rats and mice, alongside other rodent species such as gerbils). With respect to stress-altered USV changes with age, mice and rats increase USV rates after birth, with a peak around 6 to 10 days, and decrease USVs until weanling age. Of the five studies that reported sex-related differences in stress-induced USVs, females had an increased number of calls and lower average peak frequency in response to stress when compared to males. Only two to four studies reported strain-related differences in stress-induced vocalizations in rats and mice, respectively. Conclusions: The data from this review lay the groundwork for better understanding rodent USVs in response to psychosocial stress with effects of elicitation paradigm, stressor, age, and sex.

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.

The bed nucleus of the stria terminalis in threat detection: task choice and rodent experience

Behavioural reactivity to potential threat is used to experimentally refine models of anxiety symptoms in rodents. We present a short review of the literature tying the most commonly used tasks to model anxiety symptoms to functional recruitment of bed nucleus of the stria terminalis circuits (BNST). Using a review of studies that investigated the role of the BNST in anxiety-like behaviour in rodents, we flag the certain challenges for the field. These stem from inconsistent methods of reporting the neuroanatomical BNST subregions and the interpretations of specific behaviour across a wide variety of tasks as 'anxiety-like'. Finally, to assist in interpretation of the findings, we discuss the potential interactions between typically used 'anxiety' tasks of innate behaviour that are potentially modulated by the social and individual experience of the animal.

Male pheromones and their reception by females are co-adapted to affect mating success in two subspecies of brown rats

Pheromonal communication plays a key role in the sociosexual behavior of rodents. The coadaptation between pheromones and chemosensory systems has been well illustrated in insects but poorly investigated in rodents and other mammals. We aimed to investigate whether coadaptation between male pheromones and female reception might have occurred in brown rats Rattus norvegicus. We recently reported that major urinary protein (MUP) pheromones are associated with male mating success in a brown rat subspecies, R. n. humiliatus (Rnh). Here, we discovered that MUPs were less polymorphic and occurred at much lower concentrations in males of a parapatric subspecies, R. n. caraco (Rnc), than in Rnh males, and found no association between pheromones and paternity success. Moreover, the observation of Rnc males that experienced chronic dyadic encounters and established dominance–submission relationships revealed that the dominant males achieved greater mating success than the subordinate males, but their MUP levels did not differ by social status. These findings suggest that male mating success in Rnc rats is related to social rank rather than to pheromone levels and that low concentration of MUPs might not be a reliable signal for mate choice in Rnc rats, which is different from the findings obtained in Rnh rats. In addition, compared with Rnh females, Rnc females exhibited reduced expression of pheromone receptor genes, and a lower number of vomeronasal receptor neurons were activated by MUP pheromones, which imply that the female chemosensory reception of pheromones might be structurally and functionally coadapted with male pheromone signals in brown rats.

Fear Extinction and Predictive Trait-Like Inter-Individual Differences in Rats Lacking the Serotonin Transporter

Anxiety disorders are associated with a failure to sufficiently extinguish fear memories. The serotonergic system (5-hydroxytryptamine, 5-HT) with the 5-HT transporter (5-HTT, SERT) is strongly implicated in the regulation of anxiety and fear. In the present study, we examined the effects of SERT deficiency on fear extinction in a differential fear conditioning paradigm in male and female rats. Fear-related behavior displayed during acquisition, extinction, and recovery, was measured through quantification of immobility and alarm 22-kHz ultrasonic vocalizations (USV). Trait-like inter-individual differences in novelty-seeking, anxiety-related behavior, habituation learning, cognitive performance, and pain sensitivity were examined for their predictive value in forecasting fear extinction. Our results show that SERT deficiency strongly affected the emission of 22-kHz USV during differential fear conditioning. During acquisition, extinction, and recovery, SERT deficiency consistently led to a reduction in 22-kHz USV emission. While SERT deficiency did not affect immobility during acquisition, genotype differences started to emerge during extinction, and during recovery rats lacking SERT showed higher levels of immobility than wildtype littermate controls. Recovery was reflected in increased levels of immobility but not 22-kHz USV emission. Prominent sex differences were evident. Among several measures for trait-like inter-individual differences, anxiety-related behavior had the best predictive quality.

Emission of 22 kHz vocalizations in rats as an evolutionary equivalent of human crying: Relationship to depression

There is no clear relationship between crying and depression based on human neuropsychiatric observations. This situation originates from lack of suitable animal models of human crying. In the present article, an attempt will be made to answer the question whether emission of rat aversive vocalizations (22 kHz calls) may be regarded as an evolutionary equivalent of adult human crying. Using this comparison, the symptom of crying in depressed human patients will be reanalyzed. Numerous features and characteristics of rat 22 kHz aversive vocalizations and human crying vocalizations are equivalent. Comparing evolutionary, biological, physiological, neurophysiological, social, pharmacological, and pathological aspects have shown vast majority of common features. It is concluded that emission of rat 22 kHz vocalizations may be treated as an evolutionary vocal homolog of human crying, although emission of 22 kHz calls is not exactly the same phenomenon because of significant differences in cognitive processes between these species. It is further concluded that rat 22 kHz vocalizations and human crying vocalizations are both expressing anxiety and not depression. Analysis of the relationship between anxiety and depression reported in clinical studies supports this conclusion regardless of the nature and extent of comorbidity between these pathological states.

Ultrasonic vocalizations as a tool in studying emotional states in rodent models of social behavior and brain disease

Rodents emit ultrasonic vocalizations (USVs) to communicate the presence of positive or negative emotional states and to coordinate social interactions. On this basis, USVs are increasingly being used as a behavioral readout in rodent studies of affect, motivation and social behavior. Notably, several investigations have demonstrated that rodents emit USVs when tested in experimental paradigms that are used in preclinical studies of psychiatric and neurological diseases. Moreover, it has been shown that calling behavior may be influenced by genetic and/or environmental factors (i.e., stress), early rearing conditions that have been implicated in brain disease, as well as psychoactive drugs. Hence, measuring USV emissions has emerged as a useful tool in studying the mechanisms that underlie the emotional disturbances featuring certain brain diseases, as well as in the development of suited pharmacological therapies. This review provides an overview of the behavioral significance of USV emissions and describes the contexts that promote calling behavior in rats and mice. Moreover, the review summarizes the current evidence concerning the use of USVs as a marker of affect in rat and mouse models of sociability, psychiatric diseases and neurological diseases, and discusses the strengths and current limitations of using USVs as a behavioral readout in rodent studies of emotional behavior. This article is part of the Special Issue entitled 'The neuropharmacology of social behavior: from bench to bedside'.

Assessment of housing density, space allocation and social hierarchy of laboratory rats on behavioural measures of welfare

Minimum space allowances for laboratory rats are legislated based on weight and stocking rates, with the understanding that increased housing density encourages crowding stress. However, there is little evidence for these recommendations, especially when considering positive welfare outcomes. This study consisted of two experiments which investigated the effects of housing density (rats per cage), space allocation (surface area per rat) and social rank (dominance hierarchy) on the ability to perform simple behavioural tests. Male Sprague Dawley (SD) rats (n = 64) were allocated to either high-density (n = 8) or low-density (n = 8) cages. The second experiment investigated the effects of surface area. SD rats (n = 40) were housed in dyads in either the large (n = 10) or small (n = 10) cage. In both experiments, animals were tested on a judgment bias paradigm, with their responses to an ambiguous stimulus being ascribed as optimistic or pessimistic. Animals were also tested on open-field, novel-object recognition and social-interaction tests. Recordings were taken from 1700-2100h daily for rat observation and social rank establishment. Dominant animals responded with significantly more optimistic decisions compared to subordinates for both the housing density (p<0.001) and space allocation (p = 0.0015) experiment. Dominant animals responded with increased social affiliative behaviours in the social-interaction test, and spent more time in the centre of the open-field test for both experiments. No significance was detected between housing density or space allocation treatments. These findings suggest that social rank is a significantly greater modifier of affective state than either housing density or space allocation. This finding has not yet been reported and suggests that future drafts of housing guidelines should consider animal social status in addition to floor space requirements.

Acoustic Communication in Rats: Effects of Social Experiences on Ultrasonic Vocalizations as Socio-affective Signals

Ultrasonic vocalizations (USV) serve important communicative functions as socio-affective signals in rats. In aversive situations, such as inter-male aggression and predator exposure, 22-kHz USV are emitted. They likely function as appeasement signals during fighting and/or as alarm calls to warn conspecifics. In appetitive situations, 50-kHz USV are uttered, most notably during social interactions, such as rough-and-tumble play and mating. It is believed that they fulfill an affiliative function as social contact calls. Social experiences or their lack, such as social isolation, can have profound impact on the emission of 22- and 50-kHz USV by the sender in later life, albeit direction and strength of observed effects vary, with time point of occurrence and duration being critical determinants. Little, however, is known about how social experiences affect the behavioral responses evoked by 22- and 50-kHz USV in the recipient. By means of our 50-kHz USV radial maze playback paradigm, we recently showed that the behavioral response elicited in the recipient is affected by post-weaning social isolation. Rats exposed to four weeks of isolation during the rough-and-tumble play period did not display social approach behavior toward 50-kHz USV but some signs of social avoidance. We further found that physical environmental enrichment providing minimal opportunities for social interactions has similar detrimental effects. Together, this indicates that social experiences can affect socio-affective communication in rodents, both at the level of sender and recipient. Deficits seen following post-weaning social isolation or physical environmental enrichment might be useful to model aspects of neurodevelopmental disorders characterized by social and communication deficits, such as autism and schizophrenia.

Relationship between 22-kHz calls and testosterone in male rats

Ultrasonic calls in rats induced by the presence of a predator, referred to as "22-kHz calls," are mainly emitted by socially dominant male rats. Testosterone levels are closely related to social dominance in male rats. In the present study, we investigated the relationship between the emission of stress-induced 22-kHz calls and circulating testosterone levels in male rats, using a combination of surgery (castration or sham operation) and chronic steroid administration (testosterone or cholesterol) to modify circulating testosterone levels. We also assessed the effects of androgen and/or estrogen receptor antagonists on the emission of 22-kHz calls in male rats. An air puff stimulus, known to reliably induce 22-kHz calls in rats, was used as a stressor. Castrated rats with cholesterol implants exhibited significantly fewer 22-kHz calls than rats that had received a sham operation and cholesterol implants, and there was no significant difference between castrated rats with testosterone implants and rats that had received a sham operation and cholesterol implants. Only male rats pretreated with a binary mixture of androgen and estrogen antagonists exhibited significantly fewer 22-kHz calls than controls. These results show that testosterone in male rats has a positive effect on the emission of stress-induced 22-kHz calls, and the calls may be regulated by the activation of both androgen and estrogen receptors.

Rats selectively bred for low levels of play-induced 50 kHz vocalizations as a model for autism spectrum disorders: a role for NMDA receptors

Early childhood autism is characterized by deficits in social approach and play behaviors, socio-emotional relatedness, and communication/speech abnormalities, as well as repetitive behaviors. These core neuropsychological features of autism can be modeled in laboratory rats, and the results may be useful for drug discovery and therapeutic development. We review data that show that rats selectively bred for low rates of play-related pro-social ultrasonic vocalizations (USVs) can be used to model social deficit symptoms of autism. Low-line animals engage in less social contact time with conspecifics, show lower rates of play induced pro-social USVs, and show an increased proportion of non-frequency modulated (i.e. monotonous) ultrasonic vocalizations compared to non-selectively bred random-line animals. Gene expression patterns in the low-line animals show significant enrichment in autism-associated genes, and the NMDA receptor family was identified as a significant hub. Treatment of low-line animals with the NMDAR functional glycine site partial agonist, GLYX-13, rescued the deficits in play-induced pro-social 50-kHz USVs and reduced monotonous USVs. Since the NMDA receptor has been implicated in the genesis of autistic symptoms, it is possible that GLYX-13 may be of therapeutic value in the treatment of autism.

The effect of post-weaning individual housing on 50-kHz calls emitted from male rats to sexually receptive female rats

Post-weaning individual housing induces significant alterations in the social behaviors of laboratory rats. Adult rats use ultrasonic vocalizations as a primary method to communicate social affective states. In this study, we examined the effect of prolonged post-weaning individual housing on the emission of male ultrasonic vocalizations referred to as "50-kHz calls", which are positively correlated with positive and social affective states in adult male rats. We used an anesthetized sexually receptive female rat to induce 50-kHz calls in adult males. Male subjects that were housed individually emitted fewer 50-kHz calls (both frequency modulated and flat 50-kHz calls) in adulthood compared to subjects housed socially after weaning. Our results showed that prolonged post-weaning individual housing of male rats reduced the male's ability to recognize social and reward cues from female rats and/or to emit sufficient and appropriate 50-kHz calls to female rats. These effects may be associated with developmental dysfunction induced by prolonged post-weaning individual housing.

The critical point at which post-weaning individual housing conditions affect the emission of 22-kHz calls in male rats

It is known that long-term post-weaning individual housing significantly reduces emissions of 22-kHz calls in male rats. In this study, we assessed post-weaning successive changes in 22-kHz calls emitted by male rats under two different types of post-weaning housing conditions (individually and socially). In addition, we evaluated the critical point at which a significant reduction in 22-kHz calls could be observed in male rats housed individually after weaning. Significantly fewer 22-kHz calls were emitted by individually housed rats compared to socially housed rats at 16 weeks of age, indicating that 13 weeks after weaning may be a critical point for the reduction of 22-kHz calls caused by post-weaning individual housing.

On the relationships between ultrasonic calling and anxiety-related behavior in rats

In the present review, the phenomenon of ultrasonic vocalization in rats will be outlined, including the three classes of vocalizations, namely 40-kHz calls of pups, and 22- and 50-kHz calls of juvenile and adult rats, their general relevance to behavioral neuroscience, and their special relevance to research on anxiety, fear, and defense mechanisms. Here, the emphasis will be placed on 40- and 22-kHz calls, since they are typical for various situations with aversive properties. Among other topics, we will discuss whether such behavioral signals can index a certain affective state, and how these signals can be used in social neuroscience, especially with respect to communication. Furthermore, we will address the phenomenon of inter-individual variability in ultrasonic calling and what we currently know about the mechanisms, which may determine such variability. Finally, we will address the current knowledge on the neural and pharmacological mechanisms underlying 22-kHz ultrasonic vocalization, which show a substantial overlap with mechanisms known from other research on fear and anxiety, such as those involving the periaqueductal gray or the amygdala.

Close relationship between the frequency of 22-kHz calls and vocal tract length in male rats

It is known that the size of the components of the sound production apparatus in mammals may affect the acoustic structure of vocalizations. Therefore, some acoustic variables such as voice frequency may change with age in association with body size and body weight increases. However, whether this relationship also applies to ultrasonic vocalizations emitted by laboratory rats has not been investigated. Thus, in the present study, we first recorded changes in three acoustic variables (mean frequency, duration, and bandwidth) of air puff-induced 22-kHz calls in male rats during their growth period and assessed the relationship between these changes and body weight gain (Experiment 1). Then we directly recorded several body size measures including components of the sound production apparatus in 6- and 12-week-old male rats and examined the correlation between these values and the acoustic variables of 22-kHz calls (Experiment 2). In Experiment 1, the mean frequency of 22-kHz calls in male rats during the growth period showed negative correlations with body weight gain, while the duration of 22-kHz calls showed positive correlations. In Experiment 2, only a close negative correlation between the mean frequency of 22-kHz calls and vocal tract length in male rats was found. These results suggest that the age-related decrease in the mean frequency of 22-kHz calls may be ascribed to anatomical elongation of the vocal tract length in association with the growth of male rats. These acoustic differences could inform the receivers about the age of the signaler.

The cardiovascular and endocrine responses to voluntary and forced diving in trained and untrained rats

The mammalian diving response, consisting of apnea, bradycardia, and increased total peripheral resistance, can be modified by conscious awareness, fear, and anticipation. We wondered whether swim and dive training in rats would 1) affect the magnitude of the cardiovascular responses during voluntary and forced diving, and 2) whether this training would reduce or eliminate any stress due to diving. Results indicate Sprague-Dawley rats have a substantial diving response. Immediately upon submersion, heart rate (HR) decreased by 78%, from 453 +/- 12 to 101 +/- 8 beats per minute (bpm), and mean arterial pressure (MAP) decreased 25%, from 143 +/- 1 to 107 +/- 5 mmHg. Approximately 4.5 s after submergence, MAP had increased to a maximum 174 +/- 3 mmHg. Blood corticosterone levels indicate trained rats find diving no more stressful than being held by a human, while untrained rats find swimming and diving very stressful. Forced diving is stressful to both trained and untrained rats. The magnitude of bradycardia was similar during both voluntary and forced diving, while the increase in MAP was greater during forced diving. The diving response of laboratory rats, therefore, appears to be dissimilar from that of other animals, as most birds and mammals show intensification of diving bradycardia during forced diving compared with voluntary diving. Rats may exhibit an accentuated antagonism between the parasympathetic and sympathetic branches of the autonomic nervous system, such that in the autonomic control of HR, parasympathetic activity overpowers sympathetic activity. Additionally, laboratory rats may lack the ability to modify the degree of parasympathetic outflow to the heart during an intense cardiorespiratory response (i.e., the diving response).

The effects of buspirone and diazepam on aversive context- and social isolation-induced ultrasonic vocalisation

Rats emit two types of high-frequency vocalisations (aversive and appetitive calls) in different behavioural situations. The aims of this paper were to examine an animal model of appetitive behaviour (as an element of social interaction) and to study the effects of selected psychotropic drugs on appetitively evoked ultrasonic vocalisation (USVs) and aversive context-evoked USVs. Specifically, we analysed the impact of the encounter of pairs of adult rats after long-term isolation on ultrasound vocalisation. It was found that isolation of the adult rats significantly enhanced the appetitive ultrasound vocalisations (50-kHz) during encounters between pairs of rats. In the pharmacological part of the study, we found that diazepam (1.0 mg kg(-1)) significantly increased isolation-induced appetitive USVs (50 kHz) and decreased aversive context-evoked USVs (22-kHz). Buspiron (3.0 mg kg(-1)) decreased the aversive context-evoked USVs and had no effect on isolation-induced appetitive USVs. These data indicate that long-term, isolation-induced 50-kHz USVs in adult rats represent a new behavioural parameter under control of the central GABAergic system, which can be used to study the effects of anxiolytic drugs.