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

Amphetamine-induced prolonged disturbances in tissue levels of dopamine and serotonin in the rat brain

BACKGROUND

A hallmark of psychostimulants is the persistence of neurobiological changes they produce. The difficulty in reversing long-time effects of psychostimulants use is why addiction therapy is so ineffective. This study aimed to look for such drug-induced changes that can be detected even after many weeks of abstinence.

METHODS

Rats were given 12 doses of amphetamine (Amph) at 1.5 mg/kg. The rewarding effect of Amph was assessed using ultrasonic vocalization. After 14 and 28 days of abstinence, tissue levels of dopamine (DA), serotonin (5-HT), and their metabolites were measured in the prefrontal cortex (PFC), nucleus accumbens (Acb), dorsomedial (CPuM), and dorsolateral (CPuL) striatum.

RESULTS

After 28 days of abstinence, DA levels were increased in the dorsal striatum while 5-HT levels were decreased in all brain regions studied. The opposite direction of changes in DA and 5-HT tissue levels observed in the dorsal striatum may be related to the changes in the emotional state during abstinence and may contribute to the incubation of craving and relapses. Tissue levels of 5-HT and DA showed intra- and inter-structural correlations, most pronounced after 14 days of abstinence. Most of them were absent in the control group (ctrl), which may indicate that their appearance was related to the changes induced by earlier Amph administration. We did not find any associations between reward sensitivity and the persistence of Amph-induced neurochemical disturbances.

CONCLUSIONS

Administration of 12 moderate doses of Amph causes prolonged changes in DA and 5-HT tissue levels. The direction and severity of the changes are dependent on the brain region and the neurotransmitter studied.

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.

Appetitive 50 kHz calls in a pavlovian conditioned approach task in Cacna1c haploinsufficient rats

We have previously shown that rats emit high-frequency 50 kHz ultrasonic vocalizations (USV) during sign- and goal-tracking in a common Pavlovian conditioned approach task. Such 50 kHz calls are probably related to positive affect and are associated with meso-limbic dopamine function. In humans, the CACNA1C gene, encoding for the α_1C subunit of the L-type voltage-gated calcium channel Ca_V1.2, is implicated in several mental disorders, including mood disorders associated with altered dopamine signaling. In the present study, we investigated sign- and goal-tracking behavior and the emission of 50 kHz USV in Cacna1c haploinsufficent rats in a task where food pellet delivery is signaled by an appearance of an otherwise inoperable lever. Over the course of this Pavlovian training, these rats not only increased their approach to the reward site, but also their rates of pressing the inoperable lever. During subsequent extinction tests, where reward delivery was omitted, extinction patterns differed between reward site (i.e. magazine entries) and lever, since magazine entries quickly declined whereas behavior towards the lever transiently increased. Based on established criteria to define sign- or goal-tracking individuals, no CACNA1C rat met a sign-tracking criterion, since around 42% of rats tested where goal-trackers and the other 58% fell into an intermediate range. Regarding USV, we found that the CACNA1C rats emitted 50 kHz calls with a clear subject-dependent pattern; also, most of them were of a flat subtype and occurred mainly during initial habituation phases without cues or rewards. Compared, to previously published wildtype controls, Cacna1c haploinsufficent rats displayed reduced numbers of appetitive 50 kHz calls. Moreover, similar to wildtype littermate controls, 50 kHz call emission in Cacna1c haploinsufficent rats was intra-individually stable over training days and was negatively associated with goal-tracking. Together, these findings provide evidence in support of 50 kHz calls as trait marker. The finding that Cacna1c haploinsufficent rats show reductions of 50 kHz calls accompanied with more goal-tracking, is consistent with the assumption of altered dopamine signaling in these rats, a finding which supports their applicability in models of mental disorders.

A Protocol for the Non-invasive Method of Ultrasound Separation During the Sociosexual Vocal-Non-contact Model in Rats

Ultrasonic vocalization (USV) is one of the measurable behavioral parameters of sociosexual interactions in rats. To precisely and accurately describe the neurobehavioral properties of USV and the potentially related specific emotional responsiveness of animals, we need to know which animals vocalize and what is their exact behavioral and physiological response. To this end, we modified the non-contact cage [non-contact erection model (NCE)] by adding a modification [vocalization-non-contact erection (VOC-NCE)] that makes it possible to assign emitted ultrasonic signals to a particular animal. Typically, the NCE cage consists of two compartments separated by perforated baffles. A male is placed in one section, and a receptive female is placed in the other section. This makes possible the accurate description of sexual parameters related to the cues controlled by the experimenter. In VOC-NCE, we completely separated the male USV from the female USV by three appropriately perforated baffles and located microphones combined with ultrasonic screening. We recorded emission in both typical bands, the so-called 22- and 50-kHz bands, with various subtypes, thus highlighting the utility of our protocol to investigate the sexual dimorphism of vocalization. Similar to the anticipatory model, we showed that emission can occur without acoustic feedback from concomitants during the VOC-NCE test. Therefore, we propose a relatively simple method for assigning individual vocalization. We discuss its usefulness and limitations in assessing vocal differentiation related to sexual parameters, adaptive changes during conditioning procedures, and further applications.

Anticipatory 50-kHz Precontact Ultrasonic Vocalizations and Sexual Motivation: Characteristic Pattern of Ultrasound Subtypes in an Individual Analyzed Profile

We verified the hypothesis of the existence of forms of individual-specific differences in the emission of anticipatory precontact vocalization (PVs) indicating individualization related to sexual experience and motivation in male rats. Long-Evans males were individually placed in a chamber and 50-kHz ultrasounds were recorded during 5-min periods. In experiment 1, PVs were recorded before the introduction of a female in four consecutive sessions during the acquisition of sexual experience. In experiment 2, PVs were analyzed in three groups of sexually experienced males: with the highest, moderate, and the lowest sexual motivation based on previous copulatory activity. In both experiments, the total number of ultrasounds, as well as 14 different specific subtypes, was measured. The ultrasound profiles for each male were created by analyzing the proportions of specific dominant subtypes of so-called 50-kHz calls. We decided that the dominant ultrasounds were those that represented more than 10% of the total recorded signals in a particular session. The number of PVs was positively correlated with the acquisition of sexual experience and previous copulatory efficiency (measured as the number of sessions with ejaculation). Furthermore, PVs showed domination of the frequency modulated signals (complex and composite) as well as flat and short with upward ramp ultrasounds with some individual differences, regardless of the level of sexual motivation. The results show a characteristic pattern of PVs and confirm the hypothesis that the number of PVs is a parameter reflecting the level of sexual motivation.

Increased emissions of 50-kHz ultrasonic vocalizations in hemiparkinsonian rats repeatedly treated with dopaminomimetic drugs: A potential preclinical model for studying the affective properties of dopamine replacement therapy in Parkinson's disease

Dopamine replacement therapy used in Parkinson's disease (PD) may induce alterations in the emotional state that can underlie the manifestation of iatrogenic psychiatric-like disturbances. The preclinical investigation of these disturbances is limited, also because few reliable paradigms are available to study the affective properties of dopaminomimetic drugs in parkinsonian animals. To provide a relevant experimental tool in this respect, we evaluated whether dopaminomimetic drugs modified the emission of 50-kHz ultrasonic vocalizations (USVs), a behavioral marker of positive affect, in rats bearing a unilateral lesion with 6-hydroxydopamine in the medial forebrain bundle. Apomorphine (2 or 4 mg/kg, i.p.), L-3,4-dihydroxyphenilalanine (L-DOPA, 6 or 12 mg/kg, i.p.), or pramipexole (2 or 4 mg/kg, i.p.) were administered in a test cage (× 5 administrations) on alternate days. Seven days after treatment discontinuation, rats were re-exposed to the test cage to measure conditioned calling behavior and thereafter received a drug challenge. Hemiparkinsonian rats treated with either apomorphine or L-DOPA, but not pramipexole, markedly vocalized during repeated treatment and after challenge, and showed conditioned calling behavior. Moreover, apomorphine, L-DOPA and pramipexole elicited different patterns of 50-kHz USV emissions and rotational behavior, indicating that calling behavior in hemiparkinsonian rats treated with dopaminomimetic drugs is not a byproduct of motor activation. Taken together, these results suggest that measuring 50-kHz USV emissions may be a relevant experimental tool for studying how dopaminomimetic drugs modify the affective state in parkinsonian rats, with possible implications for the preclinical investigation of iatrogenic psychiatric-like disturbances in PD.

Ultrasonic vocalizations and individual differences in rats performing a Pavlovian conditioned approach task

Rats emit distinct types of ultrasonic vocalizations (USV), including high-frequency 50-kHz USV, which occur in appetitive situations. Such 50-kHz USV are thought to reflect positive affective states, for example in case of reward anticipation, and are linked to dopamine signaling. The present study was conducted to investigate whether rats emit 50-kHz USV during a Pavlovian conditioned approach task and whether trait-like differences in 50-kHz USV emission are associated with sign- versus goal-tracking. We hypothesize that individuals engaging more with a cue predicting a food reward will also elicit more 50-kHz USV. In order to test this, we investigated 34 female rats and gauged USV while they underwent a Pavlovian conditioned approach training and extinction paradigm. For one, we found a high subject-dependent variability in the emission of 50-kHz calls. These were not largely affected by state differences, since these 50-kHz USV were observed throughout the task. During task progress and in most subjects, there was a rather complete shift toward goal-tracking, but subjects engaging more with the cue predicting a reward also emitted higher numbers of appetitive 50-kHz calls. This supports the hypothesis that sign-tracking is positively associated with the emission of 50-kHz USV. The high subject-dependent variability in the emission of 50-kHz calls warrants special attention in future appetitive studies.

Using anticipatory and drug-evoked appetitive ultrasonic vocalization for monitoring the rewarding effect of amphetamine in a rat model of drug self-administration

Measuring ultrasonic vocalizations (USVs) allows studying psychoactive drug use-related affective states in laboratory rats and may help understand changes underlying the progress of addictions. We aimed at finding an effective scheme for amphetamine self-administration training in rats, identifying factors affecting their anticipatory and drug-evoked, frequency-modulated 50-kHz USV responses, and verifying whether the rewarding action of amphetamine promotes current drug intake during the training. Therefore, we monitored amphetamine intake and anticipatory and drug-evoked USVs in two rat cohorts trained using two different training schemes. Then we retrospectively divided these cohorts into low-amphetamine and high-amphetamine intake subsets and analyzed their frequency-modulated 50-kHz USV responses accordingly. Anticipatory (i.e., drug-context-related) USVs as well as USVs induced by self-administration training-related non-pharmacological manipulations (tested in an additional rat group) showed surprisingly high call rates but faded spontaneously relatively quickly. Only the scheme employing short cycles of training sessions (two instead of six) and intermittent instead of continuous intra-session drug availability yielded long-lasting escalation of amphetamine intake in a sizable subset. This subset showed high initial amphetamine-evoked USV call rate, which suggests that a strong rewarding action of the drug early in the SA training favors intake escalation. A major decrease in the drug-evoked USVs during advanced training indicated the emergence of tolerance to the rewarding action in these rats, a phenomenon that is characteristic of addiction. Frequency-modulated 50-kHz rat USVs are a good index of the rewarding action of amphetamine at the absence of USVs induced by drug context and other training-related factors.

Individual behavioural predictors of amphetamine-induced emission of 50 kHz vocalization in rats

Measurement of ultrasonic vocalizations (USVs) produced by adult rats represents a highly useful index of emotional arousal. The associations found between 50 kHz USV production and a variety of behavioural and pharmacological protocols increasingly suggests they serve as a marker of positive motivational states. This study used a powerful within-subjects design to investigate the relationships among individual differences in approach to a sweet-food reward, predisposition to emit 50 kHz USVs spontaneously, and 50 kHz USVs emission following acute systemic administration of amphetamine. Both approach motivation and predisposition to call were found to not correlate with each other but did predict 50 kHz USV response to acute amphetamine. These two behavioural phenotypes appear to represent dissociable predictors of acute amphetamine-induced emission of 50 kHz USVs in a non-sensitization paradigm. In contrast to that, a measure of sucrose preference was not found to predict 50 kHz USV emission following amphetamine. Acute amphetamine was also found to increase average sound frequency of emitted USVs and selectively increase the proportion of Trill subtype 50 kHz USVs. Together, these data demonstrate that acute amphetamine-induced 50 kHz USVs in the adult rat represent more than just a univariate motivational state and may represent the product of dissociable subsystems of emotional behavior.

Differences in the dopaminergic reward system in rats that passively and actively behave in the Porsolt test

The aim of the study was to assess appetitive responses and central dopaminergic neurotransmission in passive and active rats divided according to their immobility time in the Porsolt swim test and exposed to restraint stress. Passive rats had more episodes of appetitive 50-kHz ultrasonic vocalization (USV) during rat encounter after social isolation and spent significantly more time in the amphetamine-associated context in conditioned place preference test, compared to active rats. Restraint stress decreased sucrose preference, but increased appetitive vocalization and reinforced the conditioned place preference only in passive animals that was associated with increased dopamine concentration in the amygdala. Restraint stress increased also the level of Cocaine- and Amphetamine Regulated Transcript (CART) peptide, a neuromodulator linked to dopamine neurotransmission, in the central nucleus of amygdala, while decreasing it the nucleus accumbens shell in passive rats. In the parvocellular region of paraventricular nucleus of the hypothalamus passive animals had a higher expression of CART compared to passive restraint rats and active control rats. The obtained results show that active and passive rats in the Porsolt test differ significantly in response to appetitive stimuli, which can be additionally changed under stress conditions. The underlying mechanisms are probably associated with differences in dopaminergic activity and CART signaling in reward system.

Inter-individual differences in serotonin and glutamate co-transmission reflect differentiation in context-induced conditioned 50-kHz USVs response after morphine withdrawal

A growing body of research provides compelling evidence that in rats 50-kHz USVs are a form of expression of positive emotions. Context-induced 50-kHz USVs emission is variable among rats, indicating individual differences in contextual response bound up with pharmacological reward. The aims of this study were to: extract the most important neurotransmitters related to context-induced conditioned 50-kHz USVs response; find biological basis of existing inter-individual differences in context-induced conditioned 50-kHz USVs response; create a model of all-to-all neurotransmitters correlations. The data collected here confirms that re-exposure to the context of morphine administration after the withdrawal period increases the level of 50-kHz USVs and this contextual response is associated with elevated serotonin concentrations in amygdala, hippocampus and mPFC and with increased Glu/Gln ratio in nucleus accumbens. The concentration of serotonin increases simultaneously in amygdala, nucleus accumbens and hippocampus. Moreover, 5-HT concentration in amygdala is bound up with glutamate level in this structure as well as in hippocampus. Furthermore, Glu/Gln ratio in nucleus accumbens has strong associations with Glu/Gln ratio simultaneously in VTA, amygdala, striatum and hippocampus. All-to-all-analysis indicate that concentration of glutamate in hippocampus is proportional to glutamate in VTA and GABA concentration in the hippocampus. We have also demonstrated that Glu/GABA ratio in VTA and amygdala was elevated after post withdrawal re-exposure to the pharmacological reward paired context. Presented analysis indicates a strong correlation between serotonergic and glutamatergic systems in context-induced conditioned response. The strength of this co-transmission correlates with the number of 50-kHz USVs emitted in response to morphine-paired context.

Mapping trait-like socio-affective phenotypes in rats through 50-kHz ultrasonic vocalizations

RATIONALE

Fifty-kilohertz ultrasonic vocalizations (USV) in rats are believed to express inter-individual differences in trait-like positive affective phenotypes. Emission of 50-kHz USV can be induced by amphetamine (AMPH) to model mania-like positive affect, raising the possibility that predispositions for high 50-kHz USV production confer susceptibility to mania-like states. Such 50-kHz USV presumably express the sender's motivation for social contact and elicit social approach behavior in receivers.

OBJECTIVES

We recently showed that AMPH-induced 50-kHz USV are paralleled by mania-like patterns of enhanced social approach behavior towards playback of 50-kHz USV. Here, we assessed whether these AMPH effects are dependent on trait-like inter-individual differences in 50-kHz USV production.

METHODS

To this aim, we subdivided juvenile rats into those emitting low (LC) and high (HC) rates of baseline 50-kHz USV and compared them across four AMPH dosage conditions: 0.0, 0.5, 1.0, and 2.5 mg/kg.

RESULTS

HC rats were considerably more susceptible to AMPH in inducing 50-kHz USV than LC rats, consistently across all examined doses. They further appeared to attribute more incentive salience to signals of rewarding social contact, as evidenced by enhanced social approach behavior towards 50-kHz USV playback, a response pattern also seen in LC rats after receiving AMPH treatment. HC but not LC rats emitted aversive 22-kHz USV following 50-kHz USV playback, indicating increased proneness to experience negative affective states if no actual social consequence followed the incentive signal.

CONCLUSION

Inter-individual differences in 50-kHz USV map onto a unique trait-like socio-affective phenotype associated with enhanced emotional reactivity towards social and non-social reward, possibly conferring risk to mania-like states.

Mapping trait-like socio-affective phenotypes in rats through 50-kHz ultrasonic vocalizations

RATIONALE

Fifty-kilohertz ultrasonic vocalizations (USV) in rats are believed to express inter-individual differences in trait-like positive affective phenotypes. Emission of 50-kHz USV can be induced by amphetamine (AMPH) to model mania-like positive affect, raising the possibility that predispositions for high 50-kHz USV production confer susceptibility to mania-like states. Such 50-kHz USV presumably express the sender's motivation for social contact and elicit social approach behavior in receivers.

OBJECTIVES

We recently showed that AMPH-induced 50-kHz USV are paralleled by mania-like patterns of enhanced social approach behavior towards playback of 50-kHz USV. Here, we assessed whether these AMPH effects are dependent on trait-like inter-individual differences in 50-kHz USV production.

METHODS

To this aim, we subdivided juvenile rats into those emitting low (LC) and high (HC) rates of baseline 50-kHz USV and compared them across four AMPH dosage conditions: 0.0, 0.5, 1.0, and 2.5 mg/kg.

RESULTS

HC rats were considerably more susceptible to AMPH in inducing 50-kHz USV than LC rats, consistently across all examined doses. They further appeared to attribute more incentive salience to signals of rewarding social contact, as evidenced by enhanced social approach behavior towards 50-kHz USV playback, a response pattern also seen in LC rats after receiving AMPH treatment. HC but not LC rats emitted aversive 22-kHz USV following 50-kHz USV playback, indicating increased proneness to experience negative affective states if no actual social consequence followed the incentive signal.

CONCLUSION

Inter-individual differences in 50-kHz USV map onto a unique trait-like socio-affective phenotype associated with enhanced emotional reactivity towards social and non-social reward, possibly conferring risk to mania-like states.

The amphetamine-associated context exerts a stronger motivational effect in low-anxiety rats than in high-anxiety rats

This study used the conditioned place preference test to explore the effects of subchronic amphetamine administration on drug-associated cues in rats with different emotional reactivity. We also examined the changes in markers of dopaminergic activity in brain regions in response to the amphetamine-paired context, after a withdrawal period preceded by subchronic amphetamine treatment. We used low-anxiety (LR) and high-anxiety (HR) rats, which are known to exhibit distinct levels of susceptibility to amphetamine. Compared to HR rats, LR rats spent significantly more time in the amphetamine-paired compartment after the withdrawal period preceded by subchronic amphetamine treatment. Compared to HR control rats, LR control rats showed higher expression of the D1 receptor in the nucleus accumbens core (NAC core) and basolateral amygdala and higher expression of the D2 receptor in the NAC core. After the amphetamine treatment and withdrawal period, the LR rats showed higher D1 receptor expression in the NAC core, an increased level of homovanilic acid (HVA) in the prefrontal cortex, the NAC and the central amygdala than HR rats, as well as lower D2 receptor expression in the NAC core and the amygdala than LR control rats. These results indicate that the differences in the activity of the dopaminergic mesolimbic system in the HR and LR rats are maintained and even enhanced after a multi-day break in the use of the drug, indicating the occurrence of sensitisation. These findings show that the innate reactivity of the limbic dopaminergic innervations, dependent on the level of emotional reactivity, may significantly and chronically modify the development and maintenance of sensitisation to amphetamine.

Effects of amphetamine on pro-social ultrasonic communication in juvenile rats: Implications for mania models

Communication is the act of information transfer between sender and receiver. In rats, vocal communication can be studied through ultrasonic vocalizations (USV). 50-kHz USV occur in appetitive situations, most notably juvenile play, likely expressing the sender׳s positive affective state. Such appetitive 50-kHz USV serve important pro-social communicative functions and elicit social exploratory and approach behavior in the receiver. Emission of 50-kHz USV can be induced pharmacologically by the administration of psychostimulant drugs, such as amphetamine. However, it is unknown whether amphetamine affects the pro-social communicative function of 50-kHz USV in the receiver. We therefore assessed dose-response effects of amphetamine (0.0mg/kg, 0.5mg/kg, 1.0mg/kg, 2.5mg/kg, 5.0mg/kg) on pro-social ultrasonic communication on both, sender and receiver, in juvenile rats. We found an inverted U-shaped effect of amphetamine on 50-kHz USV emission, with 50-kHz USV levels being strongly enhanced by moderate doses, yet less prominent effects were seen following the highest dose. Likewise, amphetamine exerted inverted U-shaped effects on social exploratory and approach behavior induced by playback of appetitive 50-kHz USV. Social approach was enhanced by moderate amphetamine doses, but completely abolished following the highest dose. Amphetamine further dose-dependently promoted the emission of 50-kHz USV following playback of appetitive 50-kHz USV, indicating more vigorous attempts to establish social proximity. Our results support an important role of dopamine in closing a perception-and-action-loop through linking mechanisms relevant for detection and production of social vocalizations. Moreover, our approach possibly provides a new means to study mania-like aberrant social interaction and communication in animal models for bipolar disorder.

Low-anxiety rats are more sensitive to amphetamine in comparison to high-anxiety rats

This study utilised the two injection protocol of sensitisation (TIPS) and the conditioned place preference test to validate and extend previous findings on the effects of amphetamine on positive reinforcement-related 50 kHz ultrasonic vocalisation (USV) in rats. We also examined changes in the expression of c-Fos and the NMDA receptor 2B (GluN2B) subunit, markers of neuronal activity and plasticity, in brain regions of rats in response to TIPS. We used low anxiety-responsive (LR) and high anxiety-responsive (HR) rats, which are known to exhibit different fear-conditioned response strengths, different susceptibilities to amphetamine in the TIPS procedure and different amphetamine-dependent 50 kHz USV responses. The LR rats, compared to the HR rats, not only vocalised much more intensely but also spent significantly more time in the amphetamine-paired compartment. After the second dose of amphetamine, the LR rats exhibited more c-Fos and GluN2B activation in layers II and III of the M1/M2 motor cortex area and prefrontal cortex (PRE, PRL, IL) and also presented with more GluN2B activation in the basal amygdala. These data reveal that HR and LR rats exhibit different levels of reactivity in the cortical-limbic pathway, which controls reward-related motivational processes. These findings contribute to the general hypothesis that heterogeneity in emotional processes is one of the causes of sensitisation to amphetamine and drug addiction.

Voluntary ethanol consumption changes anticipatory ultrasonic vocalizations but not novelty response

Novelty and sensation seeking (NSS) and affective disorders are correlated with earlier ethanol (ETOH) consumption, and sustained drinking into adulthood. Understanding the NSS response and affective response before and after voluntary ETOH consumption could elucidate important individual differences promoting sustained ETOH consumption. This study determined that NSS and affective response to rewarding stimulation-measured by ultrasonic vocalizations (USVs)-change after adolescent ETOH voluntary drinking. Rats were tested for their NSS response using the inescapable novelty test. Then rats were tested for their affective response to a natural reward and USVs were measured. The natural reward was experimenter-induced play behavior. Rats were exposed to ETOH for 8 weeks using an intermittent two bottle paradigm. After 8 weeks of voluntary consumption, rats were retested for their response to NSS and affective response to natural reward. Results indicate that voluntary ETOH consumption did not change the response to novelty. Control and ETOH exposed rats decreased their novelty response equally after ETOH consumption, suggesting the decrease was due to age. Importantly, voluntary ETOH consumption changed affective USVs. Compared to water-drinking control rats, ETOH-consuming rats elicited greater anticipatory trill USVs to a natural reward-associated context during a post-drinking probe test. Tickle-induced trill USVs did not change differently between ETOH and control rats. These results provide evidence that voluntary intermittent ETOH exposure increases the anticipation of reward and may represent a form of incentive salience. We postulate these diverging effects could be due to differences in incentive salience or reward processing. Together, these results suggest that voluntary ETOH consumption changes the affective response to conditioned and unconditioned natural rewards and offers a behavioral mechanism for studying affective reward processing after ETOH consumption.

Environmental and Pharmacological Modulation of Amphetamine- Induced 50-kHz Ultrasonic Vocalizations in Rats

Rats emit high-frequency 50-kHz ultrasonic vocalizations (USV) in appetitive situations like social interactions. Drugs of abuse are probably the most potent non-social elicitors of 50-kHz USV, possibly reflecting their euphorigenic properties. Psychostimulants induce the strongest elevation in 50-kHz USV emission, particularly amphetamine (AMPH), either when applied systemically or locally into the nucleus accumbens (Nacc). Emission of AMPH-induced 50-kHz USV depends on test context, such as the presence of conspecifics, and can be manipulated pharmacologically by targeting major neurotransmitter systems, including dopamine (DA), noradrenaline (NA), and serotonin (5-HT), but also protein kinase C (PKC) signaling. Several D₁ and D₂ receptor antagonists, as well as typical and atypical antipsychotics block the AMPH-induced elevation in 50-kHz USV. Inhibiting D₁ and D₂ receptors in the Nacc abolishes AMPH-induced 50-kHz USV, indicating a key role for this brain area. NA neurotransmission also regulates AMPH-induced 50-kHz USV emission given that α₁ receptor antagonists and α₂ receptor agonists exert attenuating effects. Supporting the involvement of the 5-HT system, AMPH-induced 50-kHz USV are attenuated by 5-HT_2C receptor activation, whereas 5-HT_2C receptor antagonism leads to the opposite effect. Finally, treatment with lithium, tamoxifen, and myricitrin was all found to result in a complete abolishment of the AMPH-induced increase in 50-kHz USV, suggesting the involvement of PKC signaling. Neurotransmitter systems involved in AMPH-induced 50-kHz USV emission only partially overlap with other AMPH-induced behaviors like hyperlocomotion. The validity of AMPH-induced 50-kHz USV as a preclinical model for neuropsychiatric disorders is discussed, particularly with relevance to altered drive and mood seen in bipolar disorder.

Ultrasonic Vocalizations as a Measure of Affect in Preclinical Models of Drug Abuse: A Review of Current Findings

The present review describes ways in which ultrasonic vocalizations (USVs) have been used in studies of substance abuse. Accordingly, studies are reviewed which demonstrate roles for affective processing in response to the presentation of drug-related cues, experimenter- and self-administered drug, drug withdrawal, and during tests of relapse/reinstatement. The review focuses on data collected from studies using cocaine and amphetamine, where a large body of evidence has been collected. Data suggest that USVs capture animals’ initial positive reactions to psychostimulant administration and are capable of identifying individual differences in affective responding. Moreover, USVs have been used to demonstrate that positive affect becomes sensitized to psychostimulants over acute exposure before eventually exhibiting signs of tolerance. In the drug-dependent animal, a mixture of USVs suggesting positive and negative affect is observed, illustrating mixed responses to psychostimulants. This mixture is predominantly characterized by an initial bout of positive affect followed by an opponent negative emotional state, mirroring affective responses observed in human addicts. During drug withdrawal, USVs demonstrate the presence of negative affective withdrawal symptoms. Finally, it has been shown that drug-paired cues produce a learned, positive anticipatory response during training, and that presentation of drug-paired cues following abstinence produces both positive affect and reinstatement behavior. Thus, USVs are a useful tool for obtaining an objective measurement of affective states in animal models of substance abuse and can increase the information extracted from drug administration studies. USVs enable detection of subtle differences in a behavioral response that might otherwise be missed using traditional measures.

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.

Novelty response and 50 kHz ultrasonic vocalizations: Differential prediction of locomotor and affective response to amphetamine in Sprague-Dawley rats

RATIONALE

Novelty and sensation seeking (NSS) predisposes humans and rats to experiment with psychostimulants. In animal models, different tests of NSS predict different phases of drug dependence. Ultrasonic vocalizations (USVs) are evoked by psychomotor stimulants and measure the affective/motivation response to stimuli, yet the role NSS has on USVs in response to amphetamine is not determined.

OBJECTIVES

The aim of the present study was to determine if individual differences in NSS and USVs can predict locomotor and USV response to amphetamine (0.0, 0.3, and 1.0 mg/kg) after acute and chronic exposure.

METHODS

Thirty male rats were tested for their response to novelty (IEN), choice to engage in novelty (NPP), and heterospecific play (H-USV). Rats were administered non-contingent amphetamine or saline for seven exposures, and USVs and locomotor activity were measured. After a 14-day rest, rats were administered a challenge dose of amphetamine.

RESULTS

Regression analyses indicated that amphetamine dose-dependently increased locomotor activity and the NPP test negatively predicted treatment-induced locomotor activity. The H-USV test predicted treatment-induced frequency-modulated (FM) USVs, but the strength of prediction depended on IEN response.

CONCLUSIONS

Results provide evidence that locomotor activity and FM USVs induced by amphetamine represent different behavioral responses. The prediction of amphetamine-induced FM USVs by the H-USV screen was changed by the novelty response, indicating that the affective value of amphetamine-measured by FM USVs-depends on novelty response. This provides evidence that higher novelty responders may develop a tolerance faster and may escalate intake faster.

Poor sensitization of 50-kHz vocalization response to amphetamine predicts rat susceptibility to self-administration of the drug

RATIONALE

Our previous studies showed promise for using sensitization of the frequency-modulated 50-kHz vocalization response to amphetamine (AMPH) as an index of rat vulnerability to AMPH addiction.

OBJECTIVE

This study aimed to test the utility of sensitizing frequency-modulated (FM) 50-kHz vocalization in the AMPH self-administration paradigm as well as the ability of N-acetylcysteine to prevent self-administration relapse.

METHODS

Rats were subjected to the so-called two-injection protocol of sensitization (TIPS) using AMPH and were categorized as low-sensitized callers (LCTIPS) or high-sensitized callers (HCTIPS) based on the individual outcomes. Then, they were given 44 sessions of AMPH self-administration followed by a 17-session N-acetylcysteine-aided extinction course and a single session of AMPH-primed self-administration reinstatement.

RESULTS

LCTIPS compared to HCTIPS rats showed no considerable difference in the FM 50-kHz vocalization rate during the self-administration training or extinction course, but they were considerably more likely to acquire AMPH self-administration and experience drug-induced reinstatement of this trait. Moreover, the LCTIPS rats were more likely than HCTIPS rats to have a markedly higher FM 50-kHz vocalization rate after AMPH reinstatement. N-acetylcysteine did not affect the course of self-administration extinction or the instrumental or FM 50-kHz vocalization responses to AMPH reinstatement.

CONCLUSIONS

There is no link between the FM 50-kHz vocalization and key characteristics of AMPH self-administration. Additionally, N-acetylcysteine does not help prevent AMPH self-administration relapse. However, there is a high predictive value for poor sensitization of the FM 50-kHz vocalization response to AMPH with respect to the acquisition and maintenance of self-administration of this psychostimulant.

κ-opioid receptor as a key mediator in the regulation of appetitive 50-kHz ultrasonic vocalizations

RATIONALE

Acute administration of high doses of morphine reduced 50-kHz ultrasonic vocalizations (USVs). Although morphine meets the classical criteria for inducing 50-kHz USVs (it causes place preference and induces dopamine release in nucleus accumbens), it also inhibits appetitive vocalizations.

OBJECTIVE

The aims of this study were to (i) study the pharmacological impact of κ-opioid (KOR) and μ-opioid receptor (MOR) ligands on the emission of 50-kHz USVs triggered by social interaction after long-term isolation and (ii) analyze the concentrations of the main neurotransmitters in reward-related structures (ventral tegmental area (VTA), nucleus accumbens (NAcc), and medial prefrontal cortex (mPFC)).

METHODS

In an attempt to define the effects of opioid-receptor activation on the reward system, we used a social interaction test (after 21 days isolation). HPLC analysis was used to determine the monoamine and amino acid concentrations in reward-related structures.

RESULTS

U-50488 (10.0 mg/kg), morphine (5.0 and 1.0 mg/kg), and naltrexone (5.0 mg/kg) decreased, and nor-BNI (10.0 mg/kg) increased 50-kHz USVs. Acute pretreatment with nor-BNI or naltrexone reduced the 50-kHz suppression induced via morphine. The biochemical data showed several variations between groups regarding dopamine concentrations, serotonin, and their metabolites; these data may suggest that the levels of emitted ultrasound in the 50-kHz band are inversely proportional to the 5-hydroxyindoleacetic acid (5-HIAA)/3-methoxytyramine (3-MT) ratio in the VTA.

CONCLUSIONS

These results indicate an important role for KOR in the regulation of 50-kHz USV emissions and suggest that KOR activation may be a key mediator in the regulation of reward responses. Changes in the balance between serotonin and dopamine concentrations in the VTA may be a key predictor for 50-kHz USV emission.

Critical involvement of 5-HT2C receptor function in amphetamine-induced 50-kHz ultrasonic vocalizations in rats

RATIONALE

Rats emit various distinct types of ultrasonic vocalizations (USV), with high-frequency 50-kHz USV typically occurring in appetitive situations being elicited by administering drugs of abuse, most notably amphetamine (AMPH), possibly reflecting drug wanting/craving and/or liking.

OBJECTIVES

Because 50-kHz USV emission is, at least in part, dopamine (DA) dependent and 5-HT2C agonists inhibit DA neurotransmission, we hypothesized that AMPH-induced 50-kHz USV can be attenuated by pretreatment with a 5-HT2C agonist.

METHODS

In experiments I and II, a dose-response curve for AMPH-induced 50-kHz USV was established, and the partial dependency of AMPH-induced 50-kHz USV on DA neurotransmission was validated by pretreatment with the D2-antagonist eticlopride. In experiment III, rats were pretreated with the 5-HT2C agonist CP 809,101 (0.0, 0.3, 1.0, 3.0, and 10 mg/kg), while in experiment IV, CP 809,101 (3.0 mg/kg), the 5-HT2C antagonist SB 242084 (1.0 mg/kg), or the combination of the two, was applied before AMPH administration (2.0 mg/kg). Finally, in experiment V, rats were treated with SB 242084 (0.0, 0.1, 0.3, and 1.0 mg/kg) only, i.e., in absence of AMPH.

RESULTS

The 5-HT2C agonist CP 809,101 dose-dependently blocked AMPH-induced 50-kHz USV, most notably trills, a call subtype that is considered to exclusively reflect a positive affective state, while the 5-HT2C antagonist SB 242084 induced opposite effects. Moreover, SB 242084 induced 50-kHz USV by its own.

CONCLUSIONS

5-HT2C receptors are critically involved in AMPH-induced 50-kHz USV, with 5-HT2C antagonism resulting in a stimulant-like effect. Attenuation of drug wanting/craving and/or liking by coadministration of a 5-HT2C agonist could be a translational pharmacodynamic biomarker.

Harmonic and frequency modulated ultrasonic vocalizations reveal differences in conditioned and unconditioned reward processing

Novelty and sensation seeking (NSS) and ultrasonic vocalizations (USVs) are both used as measures of individual differences in reward sensitivity in rodent models. High responders in the inescapable novelty screen have a greater response to low doses of amphetamine and acquire self-administration more rapidly, while the novelty place preference screen is positively correlated with compulsive drug seeking. These screens are uncorrelated and implicated in separate drug abuse models. 50 kHz USVs measure affective state in rats and are evoked by positive stimuli. NSS and USVs are each implicated in drug response, self-administration, and reveal differences in individual behavior, yet their relationship with each other is not understood. The present study screened rats for their response to novelty and measured USVs of all call types in response to heterospecific play to determine the relationships between these individual difference traits. Generally, we hypothesized that 50k Hz USVs would be positively correlated with the NPP screen, and that 22 kHz would be positively correlated with the IEN screen. Results indicate none of the screens were correlated indicating they are measuring different individual difference traits. However, examination of the subtypes of USVs indicated harmonic USVs and the novelty place preference were positively correlated. Harmonic 50 kHz USVs increased in response to reward associated context, suggesting animals conditioned to the heterospecific tickle arena and anticipated rewarding stimuli, while FM only increased in response to tickling. USV subtypes can be used to elucidate differences in attribution of incentive value across conditioned stimuli and receipt of rewarding stimuli. These data provide strong support that harmonic and FM USVs can be used to understand reward processing in addition to NSS.

Repeated amphetamine administration and long-term effects on 50-kHz ultrasonic vocalizations: possible relevance to the motivational and dopamine-stimulating properties of the drug

Ultrasonic vocalizations (USVs) of 50kHz are thought to indicate positive affective states in rats, and are increasingly being used to investigate the motivational properties of drugs of abuse. However, previous studies have observed that only dopaminergic psychostimulants of abuse, but not other addictive drugs, stimulate 50-kHz USVs immediately after their administration. This would suggest that 50-kHz USVs induced by addictive dopaminergic psychostimulants might reflect rewarding dopaminergic effect, rather than motivational effect. To elucidate this issue, our study compared the effects of the psychostimulant of abuse amphetamine and the dopamine receptor agonist apomorphine on 50-kHz USVs. Rats that received five drug administrations on alternate days in a novel test-cage were first re-exposed to the test-cage 7 days after treatment discontinuation to assess drug-conditioning, and then received a drug challenge. USVs were recorded throughout the experiments together with locomotor activity. To further clarify how amphetamine and apomorphine influenced 50-kHz USVs, rats were subdivided into "low" and "high" vocalizers, and time-dependence of drug effects was assessed. Amphetamine and apomorphine stimulated both 50-kHz USVs and locomotor activity, though they elicited dissimilar changes in these behaviors, depending on drug dose, rats׳ individual predisposition to vocalize, and time. Moreover, only amphetamine-treated rats displayed both sensitized 50-kHz USVs emission and conditioned vocalizations on test-cage re-exposure. These results indicate that the effects of amphetamine on 50-kHz USVs are not mimicked by a dopaminergic agonist with a low abuse potential, and may further support the usefulness of 50-kHz USVs in the study of the motivational properties of psychoactive drugs.

N-acetyl cysteine does not modify the sensitization of the rewarding effect of amphetamine as assessed with frequency-modulated 50-kHz vocalization in the rat

A satisfactory pharmacological cure for addictions to psychostimulants has not yet been developed. Because of the well-known role of changes in the corticoaccumbal and corticostriatal glutamatergic system(s) in drug seeking and relapses in psychostimulant addiction, much hope is presently linked to the use of agents that restore glutamate homeostasis. In this regard, one of the most promising agents is N-acetyl cysteine, which has been shown to reverse some changes in neuroplasticity associated with psychostimulant addiction/dependence. In this study, we used the enhancement of locomotor activity and the induction of frequency-modulated 50-kHz ultrasonic vocalization (FM 50-kHz USV) to test the possible stimulant properties of N-acetyl cysteine itself in various experimental settings (acute and subchronic administration in amphetamine-naïve and amphetamine-pretreated rats) and the capacity of N-acetyl cysteine to attenuate both the rewarding effects of amphetamine and the behavioral sensitization to this stimulant in rats showing considerable differences in their susceptibility to the FM 50-kHz USV sensitization. Our data showed no stimulant properties of N-acetyl cysteine and no acute effect of the drug on the rewarding properties of amphetamine. Moreover, no effect of N-acetyl cysteine on the pre-existing sensitization of the FM 50-kHz USV and locomotor activity responses to amphetamine were observed, independent of the susceptibility of the rats to the FM 50-kHz USV sensitization. Hence, N-acetyl cysteine seems to be ineffective at reversing the neurobiological changes underlying the sensitization of these responses to amphetamine in rats.

Sexual performance and precontact 50-kHz ultrasonic vocalizations in WAG/Rij rats: effects of opioid receptor treatment

WAG/Rij rats are genetically selected animals that model absence epilepsy in rats. Ultrasonic vocalizations and sexual behavior - both ethologically relevant markers of reward system functioning - are poorly described in this strain. The aim of our experiment was to investigate reward-dependent precontact 50-kHz vocalizations (PVs) and copulatory behavior as well as the effects of opioid receptor treatment on such behaviors in sexually experienced WAG/Rij males and rats from two control strains: Sprague-Dawley and Crl: Han Wistar. We analyzed the effects of the opioid receptor antagonist naltrexone (3 mg/kg) and the agonist morphine (1 mg/kg) administration. Additionally, we analyzed the initiation of copulation in sexually naïve males before drug treatment. A significantly lower number of sexually naïve WAG/Rij rats initiated copulation. Sexually experienced WAG/Rij males differed at the control session (after physiological saline treatment) compared with Sprague-Dawley rats: WAG/Rij rats displayed more 50-kHz precontact vocalizations and had longer mount and intromission latencies, longer ejaculation latency, longer postejaculatory latency to exploration, longer 22-kHz vocalization duration after ejaculation, and longer postejaculatory intromission latency. Compared with Crl: Han Wistar rats, WAG/Rij males displayed longer mount latency and shorter 22-kHz vocalization duration. Neither naltrexone nor morphine affected PVs in all groups. On the other hand, opioid receptor treatment differently influenced the number of intromissions required to achieve ejaculation and 22-kHz postejaculatory vocalization duration in WAG/Rij rats than in both control groups. This suggests functional differences in the opioid system in this strain. As a result of the number of males that initiated copulation as well as the number of intromissions to ejaculation and 22-kHz postejaculatory vocalizations which all depend on D1 receptor activation, we suggest that the proportion of opioid receptor to D1 receptors in WAG/Rij rats is different when compared with the control strains. The reward system of Wag/Rij rats with absence epilepsy is sensitive to social rewards (high level of precontact 50-kHz ultrasounds) although this strain displays a lower level of sexual motivation (longer mount latency) compared with other control strains. A lower number of sexually naïve rats initiating copulation and longer mount latency in sexually experienced males could suggest a moderate depressive-like syndrome in this strain of rats.

High-anxiety rats are less sensitive to the rewarding affects of amphetamine on 50kHz USV

This study assessed behaviour, as measured by 50kHz calls related to positive affect, in rats with different fear conditioned response strengths: low-anxiety rats (LR) and high-anxiety rats (HR), after amphetamine injection in a two-injection protocol (TIPS). The results showed that the first dose of amphetamine evoked similar behavioural effects in frequency-modulated (FM) 50kHz calls in the LR and HR groups. The second injection of amphetamine resulted in stronger FM 50kHz calls in LR compared with HR rats. The biochemical data ('ex vivo' analysis) showed that the LR rats had increased basal levels of dopamine in the amygdala, and increased homovanilic acid (HVA), dopamine's main metabolite, in the amygdala and prefrontal cortex compared with HR rats. The 'in vivo' analysis (microdialysis study) showed that the LR rats had increased HVA concentrations in the basolateral amygdala in response to an aversively conditioned context. Research has suggested that differences in dopaminergic system activity in the amygdala and prefrontal cortex may be one of the biological factors that underlie individual differences in response to fear stimuli, which may also affect the rewarding effects of amphetamine.

Rat ultrasonic vocalizations demonstrate that the motivation to contextually reinstate cocaine-seeking behavior does not necessarily involve a hedonic response

Human self-reports often indicate that changes in mood are a major contributor to drug relapse. Still, arguments have been made that instances of drug-seeking following abstinence in animal models (i.e. relapse/reinstatement) may be outside of hedonic control. Therefore, the present study utilized ultrasonic vocalizations in the rat in order to evaluate affect during cocaine self-administration and contextual reinstatement of cocaine-seeking in a pre-clinical model of drug relapse (abstinence-reinstatement model). Results show that while subjects effectively reinstated drug-seeking (lever pressing) following 30 days of abstinence, and spontaneously recovered/reinstated drug-seeking following 60 days of abstinence, ultrasonic vocalizations did not increase over baseline levels during either reinstatement session. These results are consistent with previous results from our laboratory and current theories of addiction suggesting that cues that are weakly associated with drug consumption can motivate drug-seeking behavior that is outside of hedonic processing.

Anticipatory 50 kHz ultrasonic vocalizations are associated with escalated alcohol intake in dependent rats

Rats emit 50kHz ultrasonic vocalizations (USVs) in situations of increased motivation, such as during the anticipation of palatable food or drugs of abuse. Whether the same holds true for the anticipation of alcohol intake remains unknown. Alcohol drinking in a nondependent state is thought to be mediated by its rewarding effects (positive reinforcement), whereas drinking in the dependent state is motivated by alcohol's stress-relieving effects (negative reinforcement). Here, we measured context-elicited 50kHz USVs in alcohol-dependent (alcohol vapor-exposed) and nondependent rats immediately before operant alcohol self-administration sessions. Dependent rats showed escalated levels of alcohol intake compared with nondependent rats. Overall, dependent and nondependent rats showed similar levels of anticipatory 50kHz USVs. However, the number of anticipatory USVs was positively correlated with alcohol intake in dependent rats but not nondependent rats. Additionally, dependent rats with higher alcohol intake displayed increased anticipatory 50kHz USVs compared with rats that had lower alcohol intake, whereas no difference was observed between rats with high and low alcohol intake in the nondependent group. Increased 50kHz USVs were specific for the anticipation of alcohol self-administration and did not generalize to a novel environment. These findings suggest that anticipatory 50kHz USVs may be an indicator of context-elicited negative reinforcement learning.

Ultrasonic vocalizations: evidence for an affective opponent process during cocaine self-administration

RATIONALE

Preclinical models of cocaine addiction in the rodent have shown that cocaine induces both positive and negative affective states. These observations have led to the notion that the initial positive/euphoric state induced by cocaine administration may be followed by an opposing, negative process. In the rodent, one method for inferring positive and negative affective states involves measuring their ultrasonic vocalizations (USVs). Previous USV recordings from our laboratory suggested that the transition between positive and negative affect might involve decaying or sub-satiety levels of self-administered cocaine.

OBJECTIVES

In order to explicitly test the role of cocaine levels on these affective states, the present study examined USVs when calculated body levels of cocaine were clamped (i.e., held at a constant level via experimenter-controlled infusions) at, below, or above subjects' self-determined drug satiety thresholds.

RESULTS

USVs indicated that (1) positive affect was predominantly observed during the drug loading period, but declined quickly to near zero during maintenance and exhibited little relation to calculated drug level, and (2) in contrast, negative affect was observed at sub-satiety cocaine levels, but was relatively absent when body levels of cocaine were clamped at or above subjects' satiety thresholds.

CONCLUSIONS

The results reinforce the opponent-process hypothesis of addiction and suggest that an understanding of the mechanisms underlying negative affect might serve to inform behavioral and pharmacological therapies.

Direct and long-lasting effects elicited by repeated drug administration on 50-kHz ultrasonic vocalizations are regulated differently: implications for the study of the affective properties of drugs of abuse

Several studies suggest that 50-kHz ultrasonic vocalizations (USVs) may indicate a positive affective state in rats, and these vocalizations are increasingly being used to investigate the properties of psychoactive drugs. Previous studies, however, have focused on dopaminergic psychostimulants and morphine, whereas little is known about how other drugs modulate 50-kHz USVs. To further elucidate the neuropharmacology of 50-kHz USVs, the present study characterized the direct and long-lasting effects of different drugs of abuse, by measuring the number of 50-kHz USVs and their 'trill' subtype emitted by adult male rats. Rats received repeated administrations of amphetamine (2 mg/kg, i.p.), 3,4-methylenedioxymethamphetamine (MDMA, 7.5 mg/kg, i.p.), morphine (7.5 mg/kg, s.c.), or nicotine (0.4 mg/kg, s.c.), on either consecutive or alternate days (five administrations in total) in a novel environment. Seven days later, rats were re-exposed to the drug-paired environment, subjected to USVs recording, and then challenged with the same drug. Finally, 7 d after the challenge, rats were repeatedly exposed to the drug-paired environment and vocalizations were measured. Amphetamine was the only drug to stimulate 50-kHz USVs and 'trill' subtype emission during administration and challenge. Conversely, all rats emitted 50-kHz USVs when re-exposed to the test cage, and this effect was most marked in morphine-treated rats, and less evident in nicotine-treated rats. This study demonstrates that the direct and long-lasting effects of drugs on 50-kHz USVs are regulated differently, providing a better understanding of the usefulness of these vocalizations in the study of psychoactive drugs.

The effect of chronic administration of corticosterone on anxiety- and depression-like behavior and the expression of GABA-A receptor alpha-2 subunits in brain structures of low- and high-anxiety rats

The aim of this study was to examine changes in rat emotional behavior and determine differences in the expression of GABA-A receptor alpha-2 subunits in brain structures of low- (LR) and high-anxiety (HR) rats after the repeated corticosterone administration. The animals were divided into LR and HR groups based on the duration of their conditioned freezing in a contextual fear test. Repeated daily administration of corticosterone (20 mg/kg) for 21 days decreased activity in a forced swim test, reduced body weight and decreased prefrontal cortex corticosterone concentration in both the LR and HR groups. These effects of corticosterone administration were stronger in the HR group in comparison with the appropriate control group, and compared to LR treated and LR control animals. Moreover, in the HR group, chronic corticosterone administration increased anxiety-like behavior in the open field and elevated plus maze tests. The behavioral effects in HR rats were accompanied by a decrease in alpha-2 subunit density in the medial prefrontal cortex (prelimbic cortex and frontal association cortex) and by an increase in the expression of alpha-2 subunits in the basolateral amygdala. These studies have shown that HR rats are more susceptible to anxiogenic and depressive effects of chronic corticosterone administration, which are associated with modification of GABA-A receptor function in the medial prefrontal cortex and basolateral amygdala. The current data may help to better understand the neurobiological mechanisms responsible for individual differences in changes in mood and emotions induced by repeated administration of high doses of glucocorticoids or by elevated levels of these hormones associated with chronic stress or affective pathology.

The covariation of acoustic features of infant cries and autonomic state

The evolution of the autonomic nervous system provides an organizing principle to interpret the adaptive significance of physiological systems in promoting social behavior and responding to social challenges. This phylogenetic shift in neural regulation of the autonomic nervous system in mammals has produced a neuroanatomically integrated social engagement system, including neural mechanisms that regulate both cardiac vagal tone and muscles involved in vocalization. Mammalian vocalizations are part of a conspecific social communication system, with several mammalian species modulating acoustic features of vocalizations to signal affective state. Prosody, defined by variations in rhythm and pitch, is a feature of mammalian vocalizations that communicate emotion and affective state. While the covariation between physiological state and the acoustic frequencies of vocalizations is neurophysiologically based, few studies have investigated the covariation between vocal prosody and autonomic state. In response to this paucity of scientific evidence, the current study explored the utility of vocal prosody as a sensitive index of autonomic activity in human infants during the Still Face challenge. Overall, significant correlations were observed between several acoustic features of the infant vocalizations and autonomic state, demonstrating an association between shorter heart period and reductions in heart period and respiratory sinus arrhythmia following the challenge with the dampening of the modulation of acoustic features (fundamental frequency, variance, 50% bandwidth, and duration) that are perceived as prosody.

Changes in the brain expression of alpha-2 subunits of the GABA-A receptor after chronic restraint stress in low- and high-anxiety rats

This study assessed the mechanisms underlying the behavioral differences between high- (HR) and low-anxiety (LR) rats selected for their behavior in the contextual fear test (i.e., the duration of the freezing response was used as a discriminating variable). Rats were subjected to chronic restraint stress (21 days, 3h daily). We found that in the HR group, chronic restraint stress decreased rat activity in the Porsolt test and reduced the concentration of corticosterone in the prefrontal cortex. The behavioral changes were accompanied by a lower expression of alpha-2 GABA-A receptor subunits in the secondary motor cortex (M2 area) and in the dentate gyrus of the hippocampus (DG) compared to LR restraint animals. Moreover, restraint stress increased the density of alpha-2 GABA-A subunits in the basolateral amygdala (BLA) in HR rats and decreased the expression of these subunits in the DG and M2 areas compared to the HR control group. The present results suggest that, in HR rats exposed to chronic restraint stress, the function of hippocampal and cortical GABAergic neurotransmission is attenuated and that this effect could have important influences on the functioning of the hypothalamic-pituitary-adrenal axis and on depressive symptoms.

The role of dopaminergic transmission through D1-like and D2-like receptors in amphetamine-induced rat ultrasonic vocalizations

RATIONALE

Systemic amphetamine (AMPH) administration increases the rate of 50-kHz ultrasonic vocalizations (USVs) in adult rats and preferentially enhances the 'trill' subtype; these effects of AMPH critically depend on noradrenergic transmission, but the possible contributions of dopamine are unclear.

OBJECTIVE

To assess the role of dopamine in 50-kHz USVs emitted drug-free and following systemic AMPH administration.

METHODS

Adult male Long-Evans rats pre-selected for high AMPH-induced calling rates were tested with AMPH (1 mg/kg, intraperitoneal (IP)) and saline following pretreatment with the following dopamine receptor antagonists: SCH 23390 (0.005-0.02 mg/kg, subcutaneous (SC)), SCH 39166 (0.03-0.3 mg/kg, SC), haloperidol (0.1, 0.2 mg/kg, IP), sulpiride (20-80 mg/kg, SC), raclopride (0.1-0.5 mg/kg, SC), clozapine (4 mg/kg, SC), risperidone (0.5 mg/kg, SC), and pimozide (1 mg/kg, IP). The dopamine and noradrenaline reuptake inhibitors (GBR 12909 and nisoxetine, respectively) were also tested, alone and in combination.

RESULTS

SCH 23390, SCH 39166, haloperidol, and raclopride dose-dependently inhibited vocalizations under AMPH and suppressed the proportion of trill calls. Sulpiride, however, had no discernable effect on call rate or profile, even at a high dose that reduced locomotor activity. Single doses of clozapine, risperidone, and pimozide all markedly decreased calling under saline and AMPH. Finally, GBR 12909 and nisoxetine failed to promote 50-kHz USVs detectably or alter the subtype profile, when tested alone or in combination.

CONCLUSIONS

The rate of 50-kHz USVs and the call subtype profile following systemic AMPH administration depends on dopaminergic neurotransmission through D1-like and D2-like receptors. However, inhibiting dopamine and/or noradrenaline reuptake appears insufficient to induce calling.

A rodent "self-report" measure of methamphetamine craving? Rat ultrasonic vocalizations during methamphetamine self-administration, extinction, and reinstatement

Rats emit ultrasonic vocalizations (USVs) in a variety of contexts, and it is increasingly clear that USVs reflect more complex information than mere positive and negative affect states. We sought to examine USVs in a common model of addiction and relapse, the self-administration/reinstatement paradigm, in order to gain insight into subjective states experienced by rats during various types of methamphetamine seeking. We measured three subtypes of "50kHz" USVs [flats, trills, and non-trill frequency modulated (FM) USVs], as well as long and short duration "22kHz" USVs, during self-administration and extinction training, and during reinstatement elicited by cues, a methamphetamine prime, cues+prime, or the pharmacological stressor yohimbine. During self-administration and extinction, rats emitted many flats and FMs, (and short duration "22kHz" USVs on day 1 of self-administration), but few trills. In contrast, methamphetamine priming injections potently enhanced FMs and trills, and trill production was correlated with the degree of methamphetamine+cue-elicited reinstatement. Cues alone yielded increases only in flat USVs during reinstatement, though a subset of rats displaying strong cue-induced reinstatement also emitted long duration, aversion-related "22kHz" USVs. Although yohimbine administration caused reinstatement, it did not induce "22kHz" USVs in methamphetamine-experienced or methamphetamine-naïve rats (unlike footshock stress, which did induce long duration "22kHz" USVs). These findings demonstrate heterogeneity of rat USVs emitted during different types of methamphetamine seeking, and highlight their potential usefulness for gaining insight into the subjective states of rats in rodent models of drug addiction and relapse.