Chronic stress sensitizes amphetamine-elicited 50-kHz calls in the rat: Dependence on positive affective phenotype and effects of long-term fluoxetine pretreatment

Acute anxiogenic effects of escitalopram are associated with mild alterations in D-amphetamine-induced behavior and social approach evoked by playback of 50-kHz ultrasonic vocalizations in rats

Rats communicate through auditory signals in the ultrasonic range, so-called ultrasonic vocalizations (USV). Short, high-frequency 50-kHz USV are associated with positive affective states and are emitted in appetitive situations, often rewarding social interactions, such as rough-and-tumble play and mating. Exaggerated levels of 50-kHz USV emission can be observed in response to psychostimulants, most notably d-amphetamine (AMPH). There is robust evidence suggesting that 50-kHz USV serve as affiliative signals and help to maintain or re-establish social proximity. A key neurotransmitter involved in behavioral regulation is serotonin (5-hydroxytryptamine, 5-HT). This includes both, the regulation of anxiety-related behavior and ultrasonic communication. Here, we show that acute treatment with the selective 5-HT reuptake inhibitor (SSRI) escitalopram (ESC) leads to increased anxiety-related behavior in the elevated plus maze and tested whether such acute anxiogenic effects of ESC result in alterations in ultrasonic communication in sender and/or receiver. To this aim, we conducted a dose-response study in male rats and assessed AMPH-induced hyperactivity and 50-kHz ultrasonic calling in the sender and social approach behavior evoked by playback of pro-social 50-kHz USV in the receiver. Acute ESC treatment affected both, sender and receiver. This was reflected in a lack of AMPH-induced changes in acoustic features of 50-kHz USV and absence of social exploratory behavior evoked by 50-kHz USV playback, respectively. Albeit the SSRI effects were relatively mild, this supports the notion that the 5-HT system is involved in the regulation of a key aspect of the social behavior repertoire of rodents, namely socio-affective communication through 50-kHz USV.

Behavioral analysis in laboratory rats: Challenges and usefulness of 50-kHz ultrasonic vocalizations

Many rodent species emit and detect vocalizations in the ultrasonic range. Rats use three classes of ultrasonic vocalizations depending on developmental stage, experience and the behavioral situation. Calls from one class emitted by juvenile and adult rats, the so-called 50-kHz calls, are typical for appetitive and social situations. This review provides a brief historical account on the introduction of 50-kHz calls in behavioral research followed by a survey of their scientific applications focusing on the last five years, where 50-kHz publications reached a climax. Then, specific methodological challenges will be addressed, like how to measure and report 50-kHz USV, the problem of assignment of acoustic signals to a specific sender in a social situation, and individual variability in call propensity. Finally, the intricacy of interpreting 50-kHz results will be discussed focusing on the most prevalent ones, namely as communicative signals and/or readouts of the sender's emotional status.

Effect of chronic variable stress on sensitization to amphetamine in high and low sucrose-consuming rats

BACKGROUND

Individual vulnerability to stress manifests in the interaction of innate properties and environment. There is a growing interest in the individual variability in vulnerability to stress and how it contributes to the development of psychiatric disorders. Intake of palatable substances is often measured in animal models. We have previously demonstrated that the consumption of sucrose solution is a stable trait in rats.

AIMS

The present study aimed to compare the sensitivity of rats with high vs low liquid sucrose consumption to chronic variable stress and the stress effect on behavioural sensitization to amphetamine.

METHODS

Male Wistar rats were subjected to a chronic stress regimen and subsequent repeated treatment with amphetamine (1 mg/kg, intraperitoneally). Fifty-kHz ultrasonic vocalizations, locomotor activity and stereotypies were measured.

RESULTS

In no-stress baseline conditions, the behavioural response to acute amphetamine was similar in rats with high vs low sucrose consumption. Prior chronic stress potentiated the effect of amphetamine only in rats with high sucrose consumption. Behavioural sensitization to repeated administration of amphetamine was observed in non-stressed rats with lower sucrose preference, but not in the respective stressed group that had increased monoamine turnover in the nucleus accumbens. In contrast, in rats with high sucrose preference the amphetamine sensitization effect was prevalent in stressed rats, but not in non-stressed animals.

INTERPRETATION

Chronic stress can change the psychostimulant effect but this depends on the inherent reward sensitivity of the animal. Trait-wise, sucrose intake reflects vulnerability to chronic stress and may interact with the development of addiction.

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'.