50-kHz Vocalizations Research Articles

The incentive value of males’ 50-kHz ultrasonic vocalizations for female rats (Rattus norvegicus).

Rats emit ultrasonic vocalizations (USVs) of 22 kHz and 50 kHz before, during, and after copulation. The 50-kHz vocalizations can be subdivided into flat and frequency-modulated (FM) trill calls. In this study, the role of 50-kHz USVs in sexual incentive motivation in female rats was examined. USVs were recorded from sexually active males during the precopulatory phase. In the first two experiments, "full 10-min song," flat-call, and FM-trill-call stimuli were used. In Experiment 1, a combination of complex and trill calls was used as the FM-trill call, whereas a mixture of multistep and upward ramp calls was used in Experiment 2. The auditory stimuli were played back to sexually receptive female rats in a sexual incentive-motivation test. For comparison, the odor of an intact male rat was also used as incentive stimulus. The flat-call stimulus did not induce approach behavior in any experiment, whereas the FM trill showed a short-lived effect in Experiment 2. The females approached the "full song" during the first minute of stimulus exposure in Experiment 1, but not in Experiment 2. When the entire 10-min test period was analyzed, the females in Experiment 1 did not approach the full song more than background noise, though they did so in Experiment 2. The olfactory stimulus, to the contrary, had a clear incentive value in both experiments. In a third experiment, a devocalized male, an intact vocalizing male and a female rat were used as incentive stimuli. The females did not approach a vocalizing male more than they approached a silent male, showing that USVs do not contribute to the male rat's incentive value. Overall, the results of the present series of experiments show that the male rat's USVs do not have any consistent incentive value for the sexually receptive female rat. This sharply contrasts the strong and reliable effect of male odor.

Phencyclidine decreases tickling-induced 50-kHz ultrasound vocalizations in juvenile rats

The objective of the present study was to examine the idea that the decrease in 50-kHz ultrasonic vocalizations elicited by tickling in juvenile rats following the administration of the psychotomimetic drug phencyclidine (PCP) may represent a valid model of the negative symptoms of schizophrenia. Fifty-kilohertz calls in rodents have been suggested to represent an archaic model of human laughter. Our results showed that daily tickling sessions produced a gradual increase in 50-kHz vocalizations, an effect that reached statistical significance from day 3. Administration of PCP (1 mg/kg, intraperitoneally) attenuated the 50-kHz calls induced by 4 consecutive days of tickling. The ability of several clinically effective or potential antipsychotics to reverse the effects of PCP was investigated. The 5-HT1A receptor partial agonist, buspirone (0.3 and 1 mg/kg, intraperitoneally), the dual D2/5-HT1A receptor ligand, SSR181507 (0.5-0.75 mg/kg, intraperitoneally), but not the atypical antipsychotic, aripiprazole (0.1-1 mg/kg, intraperitoneally), the 5-HT2A receptor antagonist, eplivanserin (0.3-3 mg/kg, intraperitoneally), and the GlyT1 inhibitor, SSR103800 (0.3-3 mg/kg, intraperitoneally) significantly attenuated the effects of PCP on 50-kHz calls. Importantly, in animals not treated with PCP, none of the drugs affected 50-kHz calls elicited by a first handling-tickling session, indicating that the action of buspirone and SSR181507 cannot be explained by an intrinsic effect. To investigate further the specificity of these drug effects, we tested the anxiolytic and antidepressant agents, diazepam (0.1-1 mg/kg, intraperitoneally) and fluoxetine (1-10 mg/kg, intraperitoneally), respectively, in this procedure. Neither drug affected tickling-induced 50-kHz calls in naive or PCP-treated rats. In conclusion, the results of the present study confirm that 50-kHz calls elicited by several tickling sessions in rats can be reduced by acute administration of PCP, and that this effect can be reversed by previous administration of compounds with 5-HT1A receptor agonist properties. As evidence for clinical efficacy of both agents on the negative symptoms of schizophrenia is weak or lacking, the current findings do not allow a definite conclusion to be drawn on the validity of this procedure as a model of this aspect of schizophrenia.

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.

Glucocorticoid Programing of the Mesopontine Cholinergic System

Stress perception, response, adaptation, and coping strategies are individually distinct, and the sequel of stress and/or glucocorticoids (GCs) is also distinct between subjects. In the last years, it has become clear that early life stress is a powerful modulator of neuroendocrine stress-responsive circuits, programing intrinsic susceptibility to stress, and potentiating the appearance of stress-related disorders such as depression, anxiety, and addiction. Herein we were interested in understanding how early life experiences reset the normal processing of negative stimuli, leading to emotional dysfunction. Animals prenatally exposed to GCs (in utero glucocorticoid exposure, iuGC) present hyperanxiety, increased fear behavior, and hyper-reactivity to negative stimuli. In parallel, we found a remarkable increase in the number of aversive 22 kHz ultrasonic vocalizations in response to an aversive cue. Considering the suggested role of the mesopontine tegmentum cholinergic pathway, arising from the laterodorsal tegmental nucleus (LDT) and pedunculopontine tegmental nucleus (PPT), in the initiation of 22 kHz vocalizations and hypothetically in the control of emotional arousal and tone, we decided to evaluate the condition of this circuit in iuGC animals. Notably, in a basal situation, iuGC animals present increased choline acetyltransferase (ChAT) expression in the LDT and PPT, but not in other cholinergic nuclei, namely in the nucleus basalis of Meynert. In addition, and in accordance with the amplified response to an adverse stimulus of iuGC animals, we found marked changes in the cholinergic activation pattern of LDT and PPT regions. Altogether, our results suggest a specific cholinergic pathway programing by prenatal GC, and hint that this may be of relevance in setting individual stress vulnerability threshold.

Behavioral changes after maternal separation are reversed by chronic constant light treatment

Background: Rats subjected to maternal separation display behavioral alterations (e.g. increased immobility in the forced swim test) and molecular changes (e.g. in growth factors and related signal transduction proteins). Light treatment has previously been shown to have antidepressant effects in rat models of depression, but has not been studied in a rodent model of maternal separation. Methods: This study focused on maternally separated rat pups. The aim of this study was to compare the effects of chronic constant light exposure during adolescence with the selective serotonin reuptake inhibitor (SSRI), escitalopram. Behavioral changes (exploratory activity in the open field and elevated plus maze, 22kHz ultrasonic vocalizations, immobility in the forced swim test) and molecular changes (brain-derived neurotrophic factor (BDNF), mitogen-activated protein kinase phosphatase-1 (MKP-1) in the ventral hippocampus, and mu-opioid receptors in the nucleus accumbens) were measured. Results: Animals that had been subjected to maternal separation displayed an increased number and duration of 22kHz vocalizations, increased immobility in the forced swim test, increased hippocampal BDNF, and decreased mu-opioid receptor levels in the nucleus accumbens in adulthood compared to controls. MKP-1 levels in the ventral hippocampus were not affected. After chronic light treatment, there was normalization of ultrasonic vocalizations, immobility on the forced swim test, and mu-opioid receptor levels in the nucleus accumbens. Chronic saline treatment reduced anxiety-like behavior and immobility in the forced swim test. Escitalopram did not have any significant effect in this rat model of depression. Conclusion: Chronic constant light treatment reversed a number of the behavioral and molecular effects of maternal separation. Light-induced up-regulation of mu-opioid receptors in the nucleus accumbens may play a key role in mediating such effects.

Effect of aging on ultrasonic vocalizations and laryngeal sensorimotor neurons in rats

While decline in vocal quality is prevalent in an aging population, the underlying neurobiological mechanisms contributing to age-related dysphonia are unknown and difficult to study in humans. Development of an animal model appears critical for investigating this issue. Using an established aging rat model, we evaluated if 50-kHz ultrasonic vocalizations in 10, 32-month-old (old) Fischer 344/Brown Norway rats differed from those in 10, 9-month-old (young adult) rats. The retrograde tracer, Cholera Toxin β, was injected to the thyroarytenoid muscle to determine if motoneuron loss in the nucleus ambiguus was associated with age. Results indicated that older rats had vocalizations with diminished acoustic complexity as demonstrated by reduced bandwidth, intensity, and peak frequency, and these changes were dependent on the type of 50-kHz vocalization. Simple calls of old rats had reduced bandwidth, peak frequency, and intensity while frequency-modulated calls of old rats had reduced bandwidth and intensity. Surprisingly, one call type, step calls, had increased duration in the aged rats. These findings reflect phonatory changes observed in older humans. We also found significant motoneuron loss in the nucleus ambiguus of aged rats, which suggests that motoneuron loss may be a contributing factor to decreased complexity and quality of ultrasonic vocalizations. These findings suggest that a rat ultrasonic phonation model may be useful for studying age-related changes in vocalization observed in humans.

Diversity of frequency-modulated 50-kHz vocalization response to intermittent amphetamine treatment in Sprague-Dawley rats

Diversity of frequency-modulated 50-kHz vocalization response to intermittent amphetamine treatment in Sprague-Dawley rats

Inter-individual diversity and intra-individual stability of amphetamine-induced sensitization of frequency-modulated 50-kHz vocalization in Sprague-Dawley rats.

RationalePropensity for drug dependence shows great diversity that is related to intrinsic neurobiological factors. This diversity is important both for the understanding of these traits and for the development of therapies.ObjectivesThe goals of the study were (1) to define, using ultrasonic vocalization characteristics, inter-individual differences in rats’ propensity for sensitization to amphetamine, (2) to test whether possible resistance to this effect could be overcome with repetitive treatment, and (3) to seek useful predictors of the propensity.MethodsRats were subject to tests meant to characterize their anxiety, pain sensitivity, and responses to novelty and natural rewards. Then they were subject to the so-called two-injection protocol of sensitization (using amphetamine) followed by 2 weeks of daily amphetamine treatment, 2-week withdrawal, and final amphetamine challenge. The development and outcome of sensitization were monitored by measuring 50-kHz vocalization.ResultsThe two-injection protocol yielded three patterns of changes in the frequency-modulated 50-kHz vocalization response to amphetamine. These patterns persisted after completion of the extended drug treatment. Rats with lower sensitivity to pain or with longer latency of their vocalization response to the first drug exposure showed an increased propensity for ultrasonic vocalization sensitization.ConclusionVulnerability to sensitization of frequency-modulated 50-kHz vocalization response of Sprague–Dawley rats to amphetamine, which supposedly reflects rats’ propensity for amphetamine dependence, shows large inter-individual diversity. Resistance to this effect, which is evident in a majority of the rats, cannot be overcome even with prolonged intermittent drug treatment under the conditions (novelty) that promote sensitization.

Quinpirole-induced 50 kHz ultrasonic vocalization in the rat: Role of D2 and D3 dopamine receptors

The goal of the present investigation was to study a full dose-response of quinpirole in production of species-specific 50 kHz ultrasonic vocalizations in rats, and to study involvement of D2 and D3 dopamine receptors in this response. Quinpirole, a D2/D3 dopamine agonist with high selectivity for D2 dopamine receptors, was injected into the shell of the nucleus accumbens. Quinpirole induced species-specific 50 kHz ultrasonic vocalizations at a wide range of doses as compared to saline. The dose-response study showed a triphasic effect of quinpirole and reached two comparable peak responses in the number of emitted vocalizations at 0.25 μg and 6 μg, respectively (a 24-fold dose difference). These two peaks were separated by a decreased phase. A medium dose range (0.5–1.0 μg) of quinpirole consistently depressed production of calls to the control level. Application of antagonists of D2 dopamine receptors (raclopride) and D3 dopamine receptors (U-99194A) before quinpirole revealed that quinpirole activates differentially the D2 and D3 dopamine receptors at different doses. The vocalization response induced by the low dose of quinpirole (0.25 μg) was antagonized by local pretreatment with the D3 receptor antagonist but not by the D2 receptor antagonist. On the other hand, the response induced by the high dose of quinpirole (6 μg) was antagonized by a similar local pretreatment with the D2 receptor antagonist but not by the D3 receptor antagonist. In conclusion, the results indicated that quinpirole can induce 50 kHz vocalizations after its direct intra-accumbens application in rats, and both D2 and D3 dopamine receptors are involved in the response. They play, however, different functional roles, as revealed by the triphasic effect of increasing doses of quinpirole.

Motor and locomotor responses to systemic amphetamine in three lines of selectively bred Long-Evans rats

The goal of the study was to measure spontaneous and amphetamine-induced motor and locomotor activity in three selectively bred lines of male Long-Evans rats. The number of 50kHz ultrasonic vocalizations (USVs) emitted in response to heterospecific play with human hand (“tickling”) had been measured daily in these lines of rats from 21 to 24days of age, as a criterion for dividing them into high vocalizing line, low vocalizing line, and random breeding and testing lines. This study sought to determine whether selection of rats based on their affective social-vocalizations also had effects on their locomotor performance and sensitivity to amphetamine. In this study adult animals from the 25th generation (with no further selection) were tested. The results showed that rats, which were selectively bred to emit high numbers of 50kHz vocalizations, also exhibited elevated levels of spontaneous locomotor activity. After systemic injection of d-amphetamine (1.5mg/kg), the level of motor and locomotor activity significantly increased further in all the lines as compared to saline controls. The horizontal and vertical activities and the distance covered by rats of the high line, both at the baseline and after amphetamine challenge, were significantly higher than those of the low line animals in absolute scores but not as proportion of relevant saline controls. Since appetitive 50kHz USVs and locomotor activity are both dependent on the activity of the dopamine system, it is concluded that selection of rats based on the expression of their positive emotional state is also selecting other features than vocalization, namely locomotor behavior. This may help explain why these animals are relatively resistant to depressogenic manipulations.

A cocaine cue is more preferred and evokes more frequency-modulated 50-kHz ultrasonic vocalizations in rats prone to attribute incentive salience to a food cue.

Individuals vary considerably in the extent to which they attribute incentive salience to food-associated cues. We asked whether individuals prone to attribute incentive salience to a food cue are also prone to attribute incentive properties to a stimulus associated with a drug of abuse-cocaine. We first identified those rats that attributed incentive salience to a food cue by quantifying the extent to which they came to approach and engage a food cue. We then used a conditioned place preference procedure to pair an injection of 10 mg/kg cocaine (i.p.) with one distinct floor texture (grid or holes) and saline with another. Following 8 days of conditioning, each rat was given a saline injection and placed into a chamber that had both floors present. We measured the time spent on each floor, and also 50-kHz ultrasonic vocalizations, which have been associated with positive affective states. Rats that vigorously engaged the food cue ("sign trackers") expressed a preference for the cocaine-paired floor compared to those that did not ("goal trackers"). In addition, sign trackers made substantially more frequency-modulated 50-kHz vocalizations when injected with cocaine and when later exposed to the cocaine cue. Rats prone to attribute incentive salience to a food cue are also prone to attribute incentive motivational properties to a tactile cue associated with cocaine. We suggest that individuals prone to attribute incentive salience to reward cues will have difficulty resisting them and, therefore, may be especially vulnerable to develop impulse control disorders, including addiction.

Activity of cholinergic neurons in the laterodorsal tegmental nucleus during emission of 22 kHz vocalization in rats

It has been postulated that the ascending cholinergic tegmental system is responsible for the initiation of the aversive emotional state with a concomitant alarm vocalization in the rat. It is assumed that the activity of cholinergic neurons of the laterodorsal tegmental nucleus (LDT) will cause release of acetylcholine in the target areas and will initiate the emission of 22 kHz vocalizations. The goal of the present study was to test the hypothesis that the cholinergic neurons of the LDT increase their activity during emission of 22 kHz alarm calls. Vocalizations were induced by an air puff or by intrahypothalamic-preoptic injection of carbachol. The activity of the LDT cholinergic neurons was studied by a double histochemical labelling for choline acetyltransferase, as a marker of cholinergic somata, and for c-Fos protein, as a marker of cells with heighten metabolic activity. Both air puff stimulation and intracerebral carbachol induced comparable 22 kHz alarm vocalizations. The activity of neurons in the LDT was significantly higher during prolonged emission of 22 kHz alarm calls induced by air puff or injection of carbachol than in the non-vocalizing or low-vocalizing controls. There were approximately two times more of all c-Fos-labelled cells in the LDT of vocalizing animals and 2.5 times more active cholinergic neurons during prolonged 22 kHz vocalization than in the control conditions without vocalization. However, the active cholinergic neurons constituted only a small proportion of all active LDT cells (2.3%). At the same time, there were no significant increases in the number of c-Fos-labelled cells in the neighbouring pedunculopontine nucleus (PPT). These findings lead to the conclusion that the neurons of the LDT, including cholinergic neurons, but not those of the PPT, significantly increased their activity during prolonged emission of alarm vocalizations, as evidenced by the c-Fos immunoreactivity.

Effects of intraaccumbens amphetamine on production of 50 kHz vocalizations in three lines of selectively bred Long-Evans rats

Effects of direct injections of amphetamine into the shell of the nucleus accumbens were studied in three lines of Long-Evans rats, two of which had been selected for low and high rates of 50 kHz calls in adolescence in response to a standard social stimulation, and compared to results from randomly selected rats. Injections of amphetamine into the medial shell of the nucleus accumbens significantly increased the number of 50 kHz vocalizations in the high line but not low line, as compared to the random controls. This response was shell specific and antagonized by raclopride. Rats of the high line emitted significantly more frequency-modulated calls, with broader bandwidth and higher mean peak frequency than rats of all other lines. It is concluded that the high line of Long-Evans rats represents animals prone to positively valenced emotional states dependent on endogenous shell dopamine, as compared to the low line animals. Low line rats were less vocal than high and random line rats and not significantly responsive to intraaccumbens amphetamine. Selection of rats on the basis of the number of emitted 50 kHz calls is a useful model for studying brain mechanisms of different emotional phenotypes. The results also indicate that accumbens shell dopamine responsivity may be critical in determining the positive or negative emotional phenotype of the organism.

Comparing the relative amount of testosterone required to restore sexual arousal, motivation, and performance in male rats

It is well established that male rat reproductive behaviors including sexual arousal, motivation, and performance are dependent on circulating levels of testosterone (T). The present study was designed to (1) compare the relative amount of T required to restore these different aspects of behavior in castrated rats, and (2) create an animal model for clinical populations with sexual impairments. Twenty-nine male Long–Evans rats were tested before and after castration for sexual performance (copulation), motivation (partner preference), and arousal (50 kHz ultrasonic vocalizations; measured together with scent marking). Sexual arousal was also inferred from copulation data. Rats were then assigned to one of four groups, and T was re-introduced via Silastic capsule implants varying in length and content: No T (empty capsules), Low T (2 mm capsules), Medium T (5 mm capsules), or High T (two 10 mm capsules). The highest dose was intended to restore physiological levels. Results indicate that High T is required for 50 kHz vocalizations, while Medium T was sufficient for the restoration of copulation, partner preference, and scent marking. These data suggest that sexual arousal may be most sensitive to reductions in testosterone. The role of T levels in measures of generalized and specific (sexual) arousal is discussed in the context of other reproductive behaviors. Furthermore, because the Low T group showed impairments across all behaviors during post-implant tests, we propose that these animals may provide a good animal model for studying clinical conditions marked by reduced motivation and arousal, including Hypoactive Sexual Desire Disorder.

Using bedding in a test environment critically affects 50-kHz ultrasonic vocalizations in laboratory rats

Rats utter distinct classes of ultrasonic vocalizations depending on their developmental stage, current state, and situational factors. One class, comprising the so-called 50-kHz calls, is typical for situations where rats are anticipating or actually experiencing rewarding stimuli, like being tickled by an experimenter, or when treated with drugs of abuse, such as the psychostimulant amphetamine. Furthermore, rats emit 50-kHz calls when exposed to a clean housing cage. Here, we show that such vocalization effects can depend on subtle details of the testing situation, namely the presence of fresh rodent bedding. Actually, we found that adult males vocalize more in bedded cages than in bare ones. Also, two experiments showed that adult rats emitted more 50-kHz calls when tickled on fresh bedding. Furthermore, ip amphetamine led to more 50-kHz vocalization in activity boxes containing such bedding as compared to bare ones. The analysis of psychomotor activation did not yield such group differences in case of locomotion and centre time, except for rearing duration in rats tested on bedding. Also, the temporal profile of vocalization did not parallel that of behavioural activation, since the effects on vocalization peaked and started to decline again before those of psychomotor activation. Therefore, 50-kHz calls are not a simple correlate of psychomotor activation. A final experiment with a choice procedure showed that rats prefer bedded conditions. Overall, we assume that bedded environments induce a positive affective state, which increases the likelihood of 50-kHz calling. Based on these findings, we recommend that contextual factors, like bedding, should receive more research attention, since they can apparently decrease the aversiveness of a testing situation. Also, we recommend to more routinely measure rat ultrasonic vocalization, especially when studying emotion and motivation, since this analysis can provide information about the subject's status, which may not be detected in its visible behaviour.

Ultrasonic communication in rats: Effects of morphine and naloxone on vocal and behavioral responses to playback of 50-kHz vocalizations

Rats emit ultrasonic vocalizations and it was hypothesized that these vocalizations have an important role in intra-specific communication. Recently, we demonstrated that playback of 50-kHz ultrasonic vocalizations can induce social approach, indicating that 50-kHz calls can serve to (re)establish or to maintain social contact. It is known that endogenous opioids are implicated in the regulation of social behavior, particularly in rough and tumble play. Here, we tested whether administration of opioid ligands can affect social approach in response to playback of 50-kHz calls in juvenile and adult rats. Rats were either treated with 1 mg/kg naloxone, 1 mg/kg morphine, or with saline vehicle. Administration of opioid ligands affected social approach at both ages. Specifically, in juvenile and adult rats, social approach displayed in response to playback of 50-kHz calls was reduced in case of naloxone treatment, but enhanced with morphine. Furthermore, juvenile rats treated with saline or morphine emitted a substantial amount of ultrasonic vocalizations in response to the playback of 50-kHz calls. Such ultrasonic calling was not seen in naloxone treated rats. Importantly, these drug-dependent differences were stimulus-specific, i.e. seen only in response to playback of 50-kHz calls and not in response to playback of background noise. The present finding that opioid ligands can affect social approach and ultrasonic vocalizations induced by playback of 50-kHz calls, indicates that an important feature of social interaction in rats, namely ultrasonic communication, is at least partially regulated by endogenous opioids.

Communication of Adult Rats by Ultrasonic Vocalization: Biological, Sociobiological, and Neuroscience Approaches

Rats have developed antipredator defensive adaptations to protect themselves from the large number of animals that prey on them. One such adaptation is the ability to communicate by ultrasonic vocalization, which decreases the likelihood of detection by a predator. Almost all rat vocalizations are inaudible to the human ear as well, so laboratory studies of ultrasonic vocalization require specialized electronic equipment. The most popular of these is the bat detector, which lowers ultrasonic frequency to a humanly audible frequency. Adult rats emit two types of ultrasonic calls: alarm (22 kHz) calls, in aversive and dangerous situations, and appetitive (50 kHz) calls, in appetitive or "friendly" (i.e., nonaggressive) behavioral situations. These two types of calls differ in all acoustic parameters, and their initiation depends on activity in different ascending tegmental pathways to the forebrain: 22 kHz calls require activity in the cholinergic system, and 50 kHz calls in the mesolimbic dopaminergic system. The release of acetylcholine in the diencephalon and forebrain is associated with the emission of 22 kHz vocalizations, and the release of dopamine in the nucleus accumbens with 50 kHz calls. Thus the two calls are reliable predictors of increased cholinergic or dopaminergic activity in the brain. The calls serve as indices of the rat's state, including its affective state, induced by activity of one or the other neurochemical system.

The effects of selective breeding for differential rates of 50‐kHz ultrasonic vocalizations on emotional behavior in rats

Fifty-kHz ultrasonic vocalizations have previously been shown to be positively correlated with reward and appetitive social behavior in rats, and to reflect a positive affective state. In this study, rats selectively bred for high and low rates of 50-kHz vocalizations as juveniles were tested as adults in a battery of behavioral tests for social/emotional behaviors. We found that animals selectively bred for high rates of 50-kHz vocalizations exhibited more crosses into the center area of the open field apparatus, were more likely to show a preference for a dilute sucrose solution (.8%) compared to tap water, and were less aggressive than randomly bred animals. Conversely, animals bred for low rates of 50-kHz calls produced more fecal boli during both open field testing and "tickling" stimulation, and made less contact with conspecifics in a social interaction test compared to randomly bred animals. We also observed that low line rats have elevated brain levels of cholecystokinin (CCK) in the cortex, which is consistent with literature showing that CCK content in the cortex is positively correlated with rates of aversive 22-kHz USVs. Conversely, high line animals had elevated levels of met-enkephalin in several brain regions, which is consistent with the role of endogenous-opioids in the generation 50-kHz USVs and positive affect. These results suggest that animals bred for high rates of 50-kHz may show a stress resilient phenotype, whereas low line rats may show a stress prone phenotype. As such these animals could provide novel insights into the neurobiology of emotion.

Clozapine and olanzapine exhibit an intrinsic anxiolytic property in two conditioned fear paradigms: Contrast with haloperidol and chlordiazepoxide

Psychotic fear and anxiety disturbances are seen at a relatively high frequency in patients with schizophrenia. Atypical anti-psychotics are believed to show superior efficacy in reducing these symptoms. However, clinical and preclinical evidence regarding their anxiolytic efficacy has been mixed. In this study, we evaluated the possible anxiolytic property of two atypicals clozapine and olanzapine and compared them with typical haloperidol and chlordiazepoxide (a prototype of sedative-anxiolytic drug) in two preclinical models of fear. In Experiment 1, we used a fear-induced passive avoidance and conditioned place aversion paradigm and examined the effects of clozapine (20 mg/kg, sc), haloperidol (0.05 mg/kg, sc) and chlordiazepoxide (10 mg/kg, ip). In Experiments 2 and 3, we used a two-way active avoidance conditioning paradigm and further compared the effects of clozapine (20 mg/kg, sc), haloperidol (0.05 mg/kg, sc), chlordiazepoxide (10 mg/kg, ip) and three doses of olanzapine (0.5, 1.0, and 2.0 mg/kg, sc). Results show that clozapine and chlordiazepoxide, but not haloperidol, significantly attenuated the shock conditioning-induced place aversion, decreased the amount of defecations and the number of the 22-kHz vocalizations. Clozapine also reduced the shock conditioning-induced hyperthermia. Similar to clozapine, olanzapine also significantly decreased the amount of defecations and reduced the shock conditioning-induced hyperthermia, but it did not inhibit the 22-kHz vocalizations. This study demonstrates that clozapine and olanzapine possess an intrinsic anxiolytic property, which is not attributable to its superior anti-“psychotic” effect or its favorable effects on motor functions or learning and memory processes. These findings also suggest that the combined use of passive avoidance and active avoidance conditioning models can be useful in better differentiating typical and atypical anti-psychotics as well as anxiolytics.

Effects of experience and context on 50-kHz vocalizations in rats

Rats can emit ultrasonic 50-kHz vocalizations which are generally assumed to reflect the animals' positive emotional state. However, some aspects question the reliability of 50-kHz calls as indicators of positive affective states. Firstly, rats also emit them in novel environments containing scents of other rats, or even while being victims of intra-species aggression. Secondly, huge inter-individual variability in call production can be observed. The present two studies were conducted to further determine factors other than reward, which may influence or even induce calling. Experiment A showed that 50-kHz calls were emitted in relatively high numbers during short isolation in test cages, and, to a lesser extent, also during testing in an open field and an elevated plus maze. Despite inter-individual variability, calling behavior was individually stable over days and occurred irrespective of whether rats were tested in a cage with or without familiar rat scents. These data indicate that 50-kHz calling is not necessarily a response to the presence of pleasurable or social stimuli. Additionally, it was observed that call emission during isolation is strongly affected by prior experience. Rats that had been trained repeatedly in an appetitive discrimination task emitted only few calls during short isolation in test cages, whereas naïve rats emitted high numbers of 50-kHz calls which decreased over time. The most likely explanation is that rats call in response to separation from the cage mate, as the first group was trained before the recordings, while the naïve rats were recorded immediately after separation. This explanation was supported by Experiment B, which showed that the rats that remained alone in the home cage also called at 50 kHz after separation from the cage mates. In both experiments, most of the 50-kHz calls were not frequency modulated, which lend support for the suggestion that this subtype has a social-coordinating function. The present findings urge sophisticated spectrographic analysis of ultrasonic vocalizations and caution when interpreting 50-kHz vocalizations, since specific subtypes of these calls can occur in contexts that are not necessarily pleasurable to rats, and are affected by prior experience and huge individual differences.