Rats selectively bred for low levels of 50 kHz ultrasonic vocalizations exhibit alterations in early social motivation

Cortical circuits modulate mouse social vocalizations

Vocalizations provide a means of communication with high fidelity and information rate for many species. Diencephalon and brainstem neural circuits have been shown to control mouse vocal production; however, the role of cortical circuits in this process is debatable. Using electrical and optogenetic stimulation, we identified a cortical region in the anterior cingulate cortex in which stimulation elicits ultrasonic vocalizations. Moreover, fiber photometry showed an increase in Ca2+ dynamics preceding vocal initiation, whereas optogenetic suppression in this cortical area caused mice to emit fewer vocalizations. Last, electrophysiological recordings indicated a differential increase in neural activity in response to female social exposure dependent on vocal output. Together, these results indicate that the cortex is a key node in the neuronal circuits controlling vocal behavior in mice.

Stable long-term individual differences in 50-kHz vocalization rate and call subtype prevalence in adult male rats: Comparisons with sucrose preference

Sucrose preference (SP) is a widely used measure of anhedonia in rat models of depression, yet depressed patients do not reliably show an analogous deficit. As an alternative affect-related measure, adult rat ultrasonic vocalizations (USVs) are attracting interest, but it is unclear whether SP and USVs provide independent measures. Here, we have assessed whether SP and USV emission are correlated in the absence of a depressogenic procedure. To this end, 24 male Long-Evans rats were tested daily for 24 days, with alternating SP tests and USV recordings; after a 3-month hiatus, USV emission was re-evaluated for 6 more days. SP was measured in simultaneous two-bottle choice tests, and USVs were recorded in an open field. The main measures were: SP, 50-kHz call rate, and relative prevalence of trill and flat call subtypes. These measures showed temporally-stable individual differences across the initial 24-day testing period, and at the 3-month USV follow-up tests. Correlational analysis revealed no significant relationships between SP and the three main USV measures. Rats differed consistently, not only in their 50-kHz call rates but also in their 50-kHz call profiles (i.e., the relative prevalence of 14 call subtypes); most rats preferentially emitted either trill or flat calls. Several inter-call subtype associations were detected, including a strong negative relationship between the relative prevalence of flat and trill calls. The 50-kHz call rate was correlated with the relative prevalence of only one call subtype (short calls, negative correlation), but was positively correlated with absolute emission rates for almost all subtypes. In conclusion, adult rats exhibited temporally-stable individual differences over weeks (SP) or months (USVs) of testing. This trait-like stability helped to reveal a lack of relationship between SP and the USV-related variables under study, suggesting that these measures may capture different constructs of possible relevance to animal models of depression.

Prefrontal Circuits guiding Social Preference: Implications in Autism Spectrum Disorder

Although Autism Spectrum Disorder (ASD) is increasing in diagnostic prevalence, treatment options are inadequate largely due to limited understanding of ASD's underlying neural mechanisms. Contributing to difficulties in treatment development is the vast heterogeneity of ASD, from physiological causes to clinical presentations. Recent studies suggest that distinct genetic and neurological alterations may converge onto similar underlying neural circuits. Therefore, an improved understanding of neural circuit-level dysfunction in ASD may be a more productive path to developing broader treatments that are effective across a greater spectrum of ASD. Given the social preference behavioral deficits commonly seen in ASD, dysfunction in circuits mediating social preference may contribute to the atypical development of social cognition. We discuss some of the animal models used to study ASD and examine the function and effects of dysregulation of the social preference circuits, notably the medial prefrontal cortex-amygdala and the medial prefrontal cortex-nucleus accumbens circuits, in these animal models. Using the common circuits underlying similar behavioral disruptions of social preference behaviors as an example, we highlight the importance of identifying disruption in convergent circuits to improve the translational success of animal model research for ASD treatment development.

Ultrasonic Vocalizations Emission across Development in Rats: Coordination with Respiration and Impact on Brain Neural Dynamics

Rats communicate using ultrasonic vocalizations (USV) throughout their life when confronted with emotionally stimulating situations, either negative or positive. The context of USV emission and the psychoacoustic characteristics of the vocalizations change greatly between infancy and adulthood. Importantly, the production of USV is tightly coordinated with respiration, and respiratory rhythm is known to influence brain activity and cognitive functions. This review goes through the acoustic characteristics and mechanisms of production of USV both in infant and adult rats and emphasizes the tight relationships that exist between USV emission and respiration throughout the rat's development. It further describes how USV emission and respiration collectively affect brain oscillatory activities. We discuss the possible association of USV emission with emotional memory processes and point out several avenues of research on USV that are currently overlooked and could fill gaps in our knowledge.

Early Life Maternal Separation and Maternal Behaviour Modulate Acoustic Characteristics of Rat Pup Ultrasonic Vocalizations

Early separation of preterm infants from their mothers has adverse, long-term neurodevelopmental consequences. We investigated the effects of daily maternal separation (MS) of rat pups from postnatal days 2-10 (PND2-10) on neurobehavioural responses to brief isolation at PND12 compared with pups receiving controlled handling without MS. Ultrasonic vocalizations (USV) were measured at PND12 during two, 3-minute isolations occurring immediately before and after a 3-minute maternal reunion. There were no significant differences in acoustic characteristics between MS and control animals in the first isolation. However, in the second isolation, MS pups produced a greater proportion of high (~60 kHz) vs low (~40 kHz) frequency calls. During this isolation, control pups made longer and louder low frequency calls compared to the first isolation, whereas MS pups did the opposite. Maternal behaviour of control and MS mothers modulated pup acoustic characteristics in opposite directions; higher maternal care was associated with more low frequency calls in control pups but more high frequency calls in MS pups. We hypothesize that MS results in USV emission patterns reflective of a greater stress response to isolation. This translational model can be used to identify mechanisms and interventions that may be exploited to overcome the negative, long-term effects of MS.

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'.

A possible social relative reward effect: Influences of outcome inequity between rats during operant responding

Social interactions/situations have dramatic influences on motivation. Creating animal models examining these influences promotes a better understanding of the psychological and biological underpinnings of social motivation. Rodents are sensitive to social history/experience during associative conditioning and food-sharing tasks. Would reward-oriented operant behavior be sensitive to social influences by showing a negative contrast-like effect when another organism obtains a greater value outcome? We used a side-by-side arrangement of operant response chambers wherein one animal obtained consistently high reward signaled by a discrete cue. The neighboring, experimental rat experienced different combinations of high and low reward trial sequences. Control conditions included distraction from a conspecific in the neighboring chamber (rat distractor) or cue/food dispenser operating without a conspecific (program distractor) in addition to testing subjects alone. Results support an influence of the other animal actively performing the task on the experimental subject's behavior. Primarily, responding was significantly slower for the low reward trials while the neighboring rat was receiving the higher magnitude reward. The lever-press and not food-cup retrieval latency was significantly slower during exposure to a conspecific neighbor performing the operant task. The effect was not obtained in all session sequences and was more pronounced using longer series of consecutive low reward trials. The slowing effect was also obtained with the program-distractor experience in a different trial sequence. These findings suggest a social-induced negative incentive contrast effect in rats possibly mediated by an outcome inequity process that could have key similarities to complex situational-affective effects on motivation involving frustration or jealously.

Rat 50-kHz ultrasonic vocalizations as a tool in studying neurochemical mechanisms that regulate positive emotional states

BACKGROUND

Adolescent and adult rats emit 50-kHz ultrasonic vocalizations (USVs) to communicate the appetitive arousal and the presence of positive emotional states to conspecifics.

NEW METHOD

Based on its communicative function, emission of 50-kHz USVs is increasingly being evaluated in preclinical studies of affective behavior, motivation and social behavior.

RESULTS

Emission of 50-kHz USVs is initiated by the activation of dopamine receptors in the shell subregion of the nucleus accumbens. However, several lines of evidence show that non-dopaminergic receptors may influence the numbers of 50-kHz USVs that are emitted, as well as the acoustic parameters of calls.

COMPARISON WITH EXISTING METHODS

Emission of 50-kHz USVs is a non-invasive method that may be used to study reward and motivation without the need for extensive training and complex animal manipulations. Moreover, emission of 50-kHz USVs can be used alone or combined with other well-standardized behavioral paradigms (e.g., conditioned place preference, self-administration).

CONCLUSIONS

This review summarizes the current evidence concerning molecular mechanisms that regulate the emission of 50-kHz USVs. Moreover, the review discusses the usefulness of 50-kHz USVs as an experimental tool to investigate how different neurotransmitter systems regulate the manifestations of positive emotional states, and also use of this tool in preclinical modeling of psychiatric diseases.

Understanding pup affective state through ethologically significant ultrasonic vocalization frequency

Throughout life, rats emit ultrasonic vocalizations (USV) when confronted with an aversive situation. However, the conditions classically used to elicit USV vary greatly with the animal's age (isolation from the dam in infancy, versus nociceptive stimulation in adults). The present study is the first to characterize USV responses to the same aversive event throughout development. Specifically, infant, juvenile and adult rats were presented with mild foot-shocks and their USV frequency, duration, and relationship with respiration and behavior were compared. In juvenile and adult rats, a single class of USV is observed with an age-dependent main frequency and duration (30 kHz/400 ms in juveniles, 22 kHz/900 ms in adults). In contrast, infant rat USV were split into two classes with specific relationships with respiration and behavior: 40 kHz/300 ms and 66 kHz/21 ms. Next, we questioned if these infant USV were also emitted in a more naturalistic context by exposing pups to interactions with the mother treating them roughly. This treatment enhanced 40-kHz USV while leaving 66-kHz USV unchanged suggesting that the use of USV goes far beyond a signal studied in terms of amount of emission, and can inform us about some aspects of the infant's affective state.

Social Reward and Empathy as Proximal Contributions to Altruism: The Camaraderie Effect

Natural selection favors individuals to act in their own interests, implying that wild animals experience a competitive psychology. Animals in the wild also express helping behaviors, presumably at their own expense and suggestive of a more compassionate psychology. This apparent paradox can be partially explained by ultimate mechanisms that include kin selection, reciprocity, and multilevel selection, yet some theorists argue such ultimate explanations may not be sufficient and that an additional "stake in others" is necessary for altruism's evolution. We suggest this stake is the "camaraderie effect," a by-product of two highly adaptive psychological experiences: social motivation and empathy. Rodents can derive pleasure from access to others and this appetite for social rewards motivates individuals to live together, a valuable psychology when group living is adaptive. Rodents can also experience empathy, the generation of an affective state more appropriate to the situation of another compared to one's own. Empathy is not a compassionate feeling but it has useful predictive value. For instance, empathy allows an individual to feel an unperceived danger from social cues. Empathy of another's stance toward one's self would predict either social acceptance or ostracism and amplify one's physiological sensitivity to social isolation, including impaired immune responses and delayed wound healing. By contrast, altruistic behaviors would promote well-being in others and feelings of camaraderie from others, thereby improving one's own physiological well-being. Together, these affective states engender a stake in others necessary for the expression of altruistic behavior.

The Effects of Perinatal Oxycodone Exposure on Behavioral Outcome in a Rodent Model

Opiate addiction is now a major public health problem. Perinatal insults and exposure to opiates such as morphine in utero are well known to affect development of the hypothalamic-pituitary-adrenal axis of the offspring adversely and are associated with a higher risk of developing neurobehavioral problems. Oxycodone is now one of the most frequently abused pain killers during pregnancy; however, limited data are available regarding whether and how perinatal oxycodone exposure (POE) alters neurobehavioral outcomes of the offspring. We demonstrated that exposure to 0.5 mg/kg/day oxycodone in utero was associated with hyperactivity in adult rats in an open field. No significant effects of POE were detected on isolation-induced ultrasonic vocalizations in the early postnatal period or on learning and memory in the water maze in adult offspring. Our findings are consistent with hyperactivity problems identified in children exposed to opiates in utero.

Chronic variable stress prevents amphetamine-elicited 50-kHz calls in rats with low positive affectivity

The relationship between stress response and positive affective states is thought to be bidirectional: whilst stress can lead to a blunted hedonic response, positive affect reduces the negative effects of stress. We have previously shown that persistently high positive affectivity as measured by 50-kHz ultrasonic vocalizations (USVs) is protective against chronic variable stress (CVS). The present study examined the effect of CVS on 50-kHz USVs elicited by amphetamine administration, simultaneously considering the stable inter-individual differences in positive affectivity. Forty juvenile male Wistar rats were categorised as of high (HC) or low (LC) positive affectivity based on their 50-kHz USV response to imitation of rough-and-tumble play ('tickling'). As adults, the rats were subjected to four weeks of CVS, after which D-amphetamine was administered in five daily doses followed by a challenge dose (all 1mg/kg IP) nine days later. CVS reduced sucrose preference in LC-rats only. After CVS, amphetamine-elicited 50-kHz USVs were significantly reduced in LC-rats, the effect of stress in HC-rats being smaller and less consistent. In previously stressed and amphetamine-treated LC-rats, locomotor response to amphetamine was attenuated. In stressed LC-rats, DOPAC levels and dopamine turnover were increased in striatum after amphetamine treatment, and dopamine D1 receptor levels were upregulated in nucleus accumbens. LC-rats had lower isoleucine levels in frontal cortex. These results show that stress-related changes in response to amphetamine are dependent on inter-individual differences in positive affectivity both at neurochemical and behavioural levels, and further support the notion of higher vulnerability of animals with low positive affect.

Vocal coselection in rat pup ultrasonic vocalizations

Selective breeding and natural selection that select for one trait often bring along other correlated traits via coselection. Selective breeding for an infantile trait, high or low call rates of isolation‐induced ultrasonic vocalization of rat pups, also alters functions of some brain systems and emotional behaviors throughout life. We examined the effect of breeding for call rate on acoustic parameters that are of communicative significance. Selecting for higher call rate produced calls of significantly increased amplitude and bandwidth relative to a randomly bred line. Selecting for lower rate produced calls of decreased duration. These nonmorphological, functional trait changes demonstrate enhanced communicatory potential and energy expenditure for the High line and the opposite for the Low line. This demonstration of coselection in a communicatory system suggests an underlying heritable suite of linked acoustic vocalization characteristics that in noisy environments could enhance dam–pup communication and lead to selection of emotionality traits with beneficial responses to stress.

Attenuation of social interaction-associated ultrasonic vocalizations and spatial working memory performance in rats exposed to chronic unpredictable stress

Exposure to unpredictable chronic mild stress (CUS) is a commonly used protocol in rats that is reported to evoke antidepressant-reversible behaviors such as loss of preference for a sweetened water solution which is taken as an analog of the anhedonia seen in major depression. However, the induction of anhedonic-like behavior by chronic mild stress, gauged by an animal's preference for sucrose solution, is not fully reproducible and consistent across laboratories. In this study, we compared a widely used behavioral marker of anhedonia - the sucrose preference test, with another phenotypic marker of emotional valence, social interaction-associated ultrasonic vocalizations as well as a marker of an anxiety-like phenotype, novelty-suppressed feeding, and cognitive performance in the eight arm radial maze task in adult male Sprague-Dawley rats. Chronic four-week exposure to unpredictable mild stressors resulted in 1) attenuation of social interaction-associated ultrasonic vocalizations 2) attenuation of spatial memory performance on the radial arm maze 3) attenuation of body weight gain and 4) increased latency to feed in a novelty-suppressed feeding task. However, chronic exposure to CUS did not result in any significant change in sucrose preference at one-week and three-week intervals. Our results argue for the utility of ultrasonic vocalizations in a social interaction context as a comparable alternative or adjunct to the sucrose preference test in determining the efficacy of CUS to generate an anhedonic-like phenotypic state.

Prenatal Stress Alters the Development of Socioemotional Behavior and Amygdala Neuron Excitability in Rats

Prenatal stress (PS) is a risk factor for neurodevelopmental disorders with diverse ages of onset and socioemotional symptoms. Some PS-linked disorders involve characteristic social deficits, such as autism spectrum disorders and schizophrenia, but PS also promotes anxiety disorders. We propose the diversity of symptoms following PS arises from perturbations to early brain development. To this end, we characterized the effects of PS on the developmental trajectory of physiology of the amygdala, a late-developing center for socioemotional control. We found that PS dampened socioemotional behavior and reduced amygdala neuron excitability in offspring during infancy (at postnatal days (P)10, 14, 17 and 21), preadolescence (day 28), and adulthood (day 60). PS offspring in infancy produced fewer isolation-induced vocalizations and in adulthood exhibited less anxiety-like behavior and deficits in social interaction. PS neurons had a more hyperpolarized resting membrane potential from infancy to adulthood and produced fewer action potentials. Moreover, adult amygdala neurons from PS animals expressed larger action potential afterhyperpolarizations and H-current relative to controls, further limiting excitability. Our results suggest that PS can suppress socioemotional behavior throughout development and produce age-specific alterations to amygdala physiology.

Endogenous vasopressin, innate anxiety, and the emission of pro-social 50-kHz ultrasonic vocalizations during social play behavior in juvenile rats

Although the involvement of the neuropeptide arginine vasopressin (AVP) in rodent social interaction is already extensively characterized, little is known about its role in social communication. Rats communicate in the ultrasonic range by means of ultrasonic vocalizations (USV). Depending on developmental stage and affective state, rats emit various distinct types of USV, with appetitive 50-kHz USV being induced by positive social interactions, like juvenile social play, probably serving an affiliative communicative function, namely to (re)establish or induce social proximity. In rats and mice selectively bred for low (LAB) and high (HAB) anxiety-related behavior, the emission of isolation-induced distress USV during maternal deprivation as pups correlates with innate high levels of hypothalamic AVP availability. Moreover, male LAB and HAB rats express deficits in social approach towards conspecifics, together with high and/or abnormal forms of aggression when confronted with harmless opponents, possibly due to a lack of social communication skills. The aim of this study was therefore (1) to investigate and characterize social play behavior and concomitant pro-social 50-kHz USV emission in male and female, juvenile LAB and HAB rats and to compare them to non-selected Wistar (NAB) rats; and (2) to link these findings pharmacologically to the central AVP system via applying an AVP 1a receptor (V1aR) antagonist (0.75 μg; Manning compound) or synthetic AVP (1 ng) into the lateral ventricle of male juvenile NAB rats. Our results show that reduced social play behavior in highly anxious male and female, juvenile HAB rats is accompanied by low amounts of pro-social 50-kHz USV, as compared to respective LAB and NAB rats, possibly reflecting a lack of positive affective states in expectation of or following social interactions in these individuals. Secondly, although synthetic AVP did not alter social play behavior and pro-social 50-kHz USV, we demonstrated for the first time that a blockade of the central AVP system not only reduces juvenile social play behavior, but at the same time pro-social 50-kHz USV emission rates, indicating an involvement of the social neuropeptide in regulating affiliative communication in rodents.

Sex differences in the influence of social context, salient social stimulation and amphetamine on ultrasonic vocalizations in prairie voles

Prairie voles (Microtus ochrogaster) are a socially monogamous rodent species and their cooperative behaviors require extensive communication between conspecifics. Rodents use ultrasonic vocalizations (USVs) to communicate and because a prairie vole breeder pair must engage in extensive cooperation for successful reproduction, auditory communication may be critical for this species. Therefore, we sought to characterize USVs in adult male and female prairie voles, and to determine how these calls are influenced by social context, salient social stimuli and the psychostimulant drug of abuse amphetamine (AMPH). Here, we characterize prairie vole USVs by showing the range of frequencies of prairie vole USVs, the proportion of various call types, how these call types compare between males and females, and how they are influenced by social stimulation and AMPH. AMPH caused a robust increase in the number of USVs in both males and females and there was a dramatic sex difference in the complexity of call structures of AMPH-induced USVs, with males emitting more elaborate calls. Moreover, we show that novel (i.e. salient) social cues evoked differential increases in USVs across sex, with males showing a much more robust increase in USV production, both with respect to the frequency and complexity of USV production. Exposure to an estrous female in particular caused an extraordinary increase in USVs in male subjects. These data suggest that USVs may be a useful measure of social motivation in this species, including how social behaviors can be impacted by drugs of abuse.

Rat Ultrasonic Vocalizations and Behavioral Neuropharmacology: From the Screening of Drugs to the Study of Disease

Several lines of evidence indicate that rats emit ultrasonic vocalizations (USVs) in response to a wide range of stimuli that are capable of producing either euphoric (positive) or dysphoric (negative) emotional states. On these bases, recordings of USVs are extensively used in preclinical studies of affect, motivation, and social behavior. Rat USVs are sensitive to the effects of certain classes of psychoactive drugs, suggesting that emission of rat USVs can have relevance not only to neurobiology, but also to neuropharmacology and psychopharmacology. This review summarizes three types of rat USVs, namely 40-kHz USVs emitted by pups, 22-kHz USVs and 50-kHz USVs emitted by young and adult animals, and relevance of these vocalizations to neuropharmacological studies. Attention will be focused on the issues of how rat USVs can be used to evaluate the pharmacological properties of different classes of drugs, and how rat USVs can be combined with other behavioral models used in neuropharmacology. The strengths and limitations of experimental paradigms based on the evaluation of rat USVs will also be discussed.

Emotion and relative reward processing: an investigation on instrumental successive negative contrast and ultrasonic vocalizations in the rat

Incentive contrast effects include changes in behavioral responses after a reward upshift (positive contrast) or downshift (negative contrast). Proposed influences on these behavioral changes are emotional state reactions after experiencing or anticipating a change in reward outcome. Rat ultrasonic vocalizations have been shown to be indicators of emotional state during behavior and anticipatory periods. The objective of the present study was to monitor rodent ultrasounds during incentive contrast using a classical runway procedure called instrumental successive negative contrast. The procedure is one that has been used often to examine incentive relativity because of its reliability in measuring negative contrast effects. Rats were trained to run in the alleyway to receive a high (12 pellets) or low magnitude (1 pellet) outcome. The high magnitude was then shifted to the low and running speeds in the alleyway for the reward and USV emission were compared. Replicating previous work, a negative contrast effect was observed with postshift running speeds significantly slower in the shifted group compared to the unshifted group. USVs did not follow the same pattern with an apparent lack of significant differences between the groups following the reward downshift. We also tested another group of animals using a visual predictive cue in the same runway test. When visual cues predicted high or low magnitude outcome, no incentive contrast was found for the running speeds following an outcome downshift, but a weak contrast effect was observed for the USV emission. These results demonstrate a separation between USVs and behavioral indicators of incentive contrast suggesting that concomitant shifts in negative affect may not be necessary for anticipatory relative reward processes.

Glix 13, a new drug acting on glutamatergic pathways in children and animal models of autism spectrum disorders

Recently standardized diagnostic instruments have been developed in diagnostic and therapeutic procedures for Autism Spectrumv Disorders (ASD). According to the DSM-5 criteria, individuals with ASD must show symptoms from early childhood. These symptoms are communication deficits and restricted, repetitive patterns of behaviour. It was recently described by Bioinformatic analysis that 99 modified genes were associated with human autism. Gene expression patterns in the low-line animals show significant enrichment in autism-associated genes and the NMDA receptor gene family was identified among these. Using ultrasonic vocalizations, it was demonstrated that genetic variation has a direct impact on the expression of social interactions. It has been proposed that specific alleles interact with a social reward process in the adolescent mouse modifying their social interaction and their approach toward each other. In this review we report that the monoclonal antibody-derived tetrapeptide GLYX-13 was found to act as an N-methyl-D-aspartate receptor modulator and possesses the ability to readily cross the blood brain barrier. Treatment with the NMDAR glycine site partial agonist GLYX-13 rescued the deficit in the animal model. Thus, the NMDA receptor has been shown to play a functional role in autism, and GLYX-13 shows promise for the treatment of autism in autistic children.

Differential ultrasonic indices of separation distress in the presence and absence of maternal cues in infant rats bred for high and low positive social affect

OBJECTIVE

Preclinical models of human mood disorders commonly focus on the study of negative affectivity, without comparably stressing the role of positive affects and their ability to promote resilient coping styles. We evaluated the role of background constitutional affect of rats by studying the separation and reunion responses of infants from low and high positive affect genetic lines (i.e., differentially selected for High and Low 50 kHz ultrasonic vocalisations (USVs).

METHODS

Infants from Low and High 50 kHz USV breeding lines were isolated from mothers and exposed to either social (familiar or unfamiliar bedding) or neutral (clean bedding) odour cues between two short isolation periods, and tested in homeothermic and hypothermic ambient temperatures. Negative affect was estimated by monitoring separation distress calls (35-45 kHz USVs).

RESULTS

Low Line pups called at higher rates than High Line, and their rates were stable regardless of odour cue. In contrast, High Line pups increased vocalisations during the second compared with the first isolation periods and during exposure to both familiar and unfamiliar odour cues, but not to neutral odour. Furthermore, the greatest increase in USV emission was seen in the second isolation period following exposure to the unfamiliar odour. However, both lines showed comparable elevated distress USVs to the thermal stressor.

CONCLUSION

High Line animals, selected for a positive affective phenotype (50 kHz USVs), exhibited reduced separation anxiety responses in infancy, making this a promising animal model for the role of constitutional affective states in emotional responsivity and potential resilience against emotional disorders.

Dopaminergic modulation of affective and social deficits induced by prenatal glucocorticoid exposure

Prenatal stress or exposure to elevated levels of glucocorticoids (GCs) can impair specific neurobehavioral circuits leading to alterations in emotional processes later in life. In turn, emotional deficits may interfere with the quality and degree of social interaction. Here, by using a comprehensive behavioral approach in combination with the measurement of ultrasonic vocalizations, we show that in utero GC (iuGC)-exposed animals present increased immobility in the forced swimming test, pronounced anhedonic behavior (both anticipatory and consummatory), and an impairment in social interaction at different life stages. Importantly, we also found that social behavioral expression is highly dependent on the affective status of the partner. A profound reduction in mesolimbic dopaminergic transmission was found in iuGC animals, suggesting a key role for dopamine (DA) in the etiology of the observed behavioral deficits. Confirming this idea, we present evidence that a simple pharmacological approach-acute L-3,4-dihydroxyphenylacetic acid (L-DOPA) oral administration, is able to normalize DA levels in iuGC animals, with a concomitant amelioration of several dimensions of the emotional and social behaviors. Interestingly, L-DOPA effects in control individuals were not so straightforward; suggesting that both hypo- and hyperdopaminergia are detrimental in the context of such complex behaviors.

Ultrasonic vocalizations, predictability and sensorimotor gating in the rat

Prepulse inhibition (PPI) is a measure of sensorimotor gating in diverse groups of animals including humans. Emotional states can influence PPI in humans both in typical subjects and in individuals with mental illness. Little is known about emotional regulation during PPI in rodents. We used ultrasonic vocalization recording to monitor emotional states in rats during PPI testing. We altered the predictability of the PPI trials to examine any alterations in gating and emotional regulation. We also examined PPI in animals selectively bred for high or low levels of 50kHz USV emission. Rats emitted high levels of 22kHz calls consistently throughout the PPI session. USVs were sensitive to prepulses during the PPI session similar to startle. USV rate was sensitive to predictability among the different levels tested and across repeated experiences. Startle and inhibition of startle were not affected by predictability in a similar manner. No significant differences for PPI or startle were found related to the different levels of predictability; however, there was a reduction in USV signals and an enhancement of PPI after repeated exposure. Animals selectively bred to emit high levels of USVs emitted significantly higher levels of USVs during the PPI session and a reduced ASR compared to the low and random selective lines. Overall, the results support the idea that PPI tests in rodents induce high levels of negative affect and that manipulating emotional styles of the animals alters the negative impact of the gating session as well as the intensity of the startle response.

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.

New theoretical and experimental approaches on maternal motivation in mammals

Maternal behavior is expressed in different modalities, physiological conditions, and contexts. It is the result of a highly motivated brain, that allows the female to flexibily adapt her caring activities to different situations and social demands. To understand how mothers coordinate maternal and other motivated behaviors we discuss the limitations of current theoretical approaches to study maternal motivation (e.g. distinction between appetitive and consummatory behaviors), and propose a different approach (i.e. motorically active vs. passive motivations) and a distinction between maternal motivated state and maternal motivated behaviors. We review the evidence supporting dopamine mediation of maternal motivation and describe how different phases of the dopaminergic response - basal, tonic, and phasic release in the nucleus accumbens - relate to increased salience, invigorating behavior, and behavioral switching. The existing and new experimental paradigms to investigate maternal motivation, and its coexpression and coordination with other social or non-social motivations are also analyzed. An example of how specificity of motivational systems (e.g. maternal and sexual behavior at postpartum estrus) could be processed at the neural level is also provided. This revision offers new theoretical and experimental approaches to address the fundamental question of how mothers flexibly adapt and coordinate the different components of maternal behavior with other motivated behaviors, also critical for the survival of the species.

Flexibility and adaptation of the neural substrate that supports maternal behavior in mammals

Maternal behavior is species-specific and expressed under different physiological conditions, and contexts. It is the result of neural processes that support different forms (e.g. postpartum, cycling sensitized and spontaneous maternal behavior) and modalities of mother-offspring interaction (e.g. maternal interaction with altricial/precocious young; selective/non-selective bond). To understand how the brain adapts to and regulates maternal behavior in different species, and physiological and social conditions we propose new neural models to explain different forms of maternal expression (e.g. sensitized and spontaneous maternal behavior) and the behavioral changes that occur across the postpartum period. We emphasize the changing role of the medial preoptic area in the neural circuitry that supports maternal behavior and the cortical regulation and adjustment of ongoing behavioral performance. Finally, we discuss how our accumulated knowledge about the psychobiology of mothering in animal models supports the validity of animal studies to guide our understanding of human mothering and to improve human welfare and health.

Rat pup social motivation: a critical component of early psychological development

Examining the role of the offspring in early social dynamics is especially difficult. Human developmental psychology has found infant behavior to be a vital part of the early environmental setting. In the rodent model, the different ways that a rodent neonate or pup can influence social dynamics are not well known. Typically, litters of neonates or pups offer complex social interactions dominated by behavior seemingly initiated and maintained by the primary caregiver (e.g., the dam). Despite this strong role for the caregiver, the young most likely influence the litter dynamics in many powerful ways including communication signals, discrimination abilities and early approach behavior. Nelson and Panksepp (1996) developed a preference task to examine early rodent pup social motivation. We have used the same task to examine how variations in maternal care or different environmental perturbations could alter the rat pup preferences for social-related stimuli. Rat pups receiving low levels of maternal licking and grooming were impaired in maternal odor cue learning and emitted lower levels of 22kHz ultrasounds compared to pups from the high licking and grooming cohort. Prenatal stress or early exposure to a toxicant (polychlorinated biphenyl) altered early social preferences in the rat pup in different ways indicating that diverse strategies are expressed and specific to the type of perturbation exposure. A greater focus on the offspring motivation following early 'stressors' will allow for more complete understanding of the dynamics in behavior during early social development.

Perinatal exposure to polychlorinated biphenyls alters social behaviors in rats

Perinatal exposure to polychlorinated biphenyls (PCBs) leads to significant alterations of neural and hormonal systems. These alterations have been shown to impair motor and sensory development. Less is known about the influence of PCB exposure on developing emotional and motivational systems involved in social interactions and social learning. The present study examined the impact of perinatal PCB exposure (mixture of congeners 47 and 77) on social recognition in juvenile animals, conspecific-directed investigation in adults and on neural and hormonal systems involved in social functions. We used a standard habituation-dishabituation paradigm to evaluate juvenile recognition and a social port paradigm to monitor adult social investigation. Areal measures of the periventricular nucleus (PVN) of the hypothalamus were obtained to provide correlations with related hormone and brain systems. PCB exposed rats were significantly impaired in social recognition as indicated by persistent conspecific-directed exploration by juvenile animals regardless of social experience. As adults, PCB exposure led to a dampening of the isolation-induced enhancement of social investigation. There was not a concomitant alteration of social investigation in pair-housed PCB exposed animals at this stage of development. Interestingly, PVN area was significantly decreased in juvenile animals exposed to PCB during the perinatal period. Shifts in hypothalamic regulation of hormones involved in social behavior and stress could be involved in the behavioral changes observed. Overall, the results suggest that PCB exposure impairs context or experience-dependent modulation of social approach and investigation. These types of social-context deficits are similar to behavioral deficits observed in social disorders such as autism and other pervasive developmental disorders.

The effects of prenatal stress on motivation in the rat pup

Exposure to prenatal stress (PNS) has been shown to induce a set of psychological and behavioral changes in developing offspring. We used the rodent model to investigate whether PNS produces changes in the ability of the pup to express social motivation. We used a set of behavioral tasks including monitoring ultrasonic vocalizations after isolation, a conditioned place preference, and a novel and familiar odor approach test. Pregnant Long-Evans rats were exposed to an unpredictable, variable stressor twice daily during the third week of gestation. Isolation vocalizations were assessed on postnatal day (PND) 10. Pup affinity for the dam was evaluated on PND 15. Typically, pups display a selective preference for an odor-paired environment only after the odor has been associated with the dam. This previous association produces a positive conditioned stimulus (CS). Normally, pups exposed to a neutral CS (odor paired with cotton balls) do not form this place preference. Results indicate that PNS exposed pups had significantly increased distress vocalizations and an equal preference for the positive and neutral conditioned stimuli. This type of alteration in forming early preferences could be detrimental because of decreases in the specificity of social learning and an impaired responsiveness in social relationships.

Repeated cocaine exposure induces sensitization of ultrasonic vocalization in rats

Ultrasonic vocalization at 55kHz (55kHz-USVs) by rodents has been proposed to be a behavioral manifestation of affectively positive incentive motivation. To examine the extent to which 55kHz-USV emissions correlate with cocaine-induced locomotor activity, we measured cocaine-induced 55kHz-USVs and their relationship to cocaine-induced locomotor sensitization in rats. We demonstrate that similar to locomotor responses, 55kHz-USVs are also sensitized by exposure to cocaine. Furthermore, we show that the magnitude of cocaine-induced 55kHz-USV sensitization is positively correlated with that of locomotor sensitization. Moreover, we demonstrate that rats selectively bred for high rates of 55kHz-USVs exhibit higher levels of cocaine-induced 55kHz-USV sensitization than animals selectively bred for low levels of 55kHz USVs. These results suggest that the neural circuits underlying 55kHz-USV, which may directly reflect affective experience/motivation, can be sensitized by cocaine in a way that resembles locomotor sensitization.