The Effects of Electrical and Optical Stimulation of Midbrain Dopaminergic Neurons on Rat 50-kHz Ultrasonic Vocalizations

Divergent Acute and Enduring Changes in 50-kHz Ultrasonic Vocalizations in Rats Repeatedly Treated With Amphetamine and Dopaminergic Antagonists: New Insights on the Role of Dopamine in Calling Behavior

BACKGROUND

Rats emit 50-kHz ultrasonic vocalizations (USVs) in response to nonpharmacological and pharmacological stimuli, with addictive psychostimulants being the most effective drugs that elicit calling behavior in rats. Earlier investigations found that dopamine D1-like and D2-like receptors modulate the emission of 50-kHz USVs stimulated in rats by the acute administration of addictive psychostimulants. Conversely, information is lacking on how dopamine D1-like and D2-like receptors modulate calling behavior in rats that are repeatedly treated with addictive psychostimulants.

METHODS

We evaluated the emission of 50-kHz USVs in rats repeatedly treated (×5 on alternate days) with amphetamine (1 mg/kg, i.p.) either alone or together with (1) SCH 23390 (0.1-1 mg/kg, s.c.), a dopamine D1 receptor antagonist; (2) raclopride (0.3-1 mg/kg, s.c.), a selective dopamine D2 receptor antagonist; or (3) a combination of SCH 23390 and raclopride (0.1 + 0.3 mg/kg, s.c.). Calling behavior of rats was recorded following pharmacological treatment, as well as in response to the presentation of amphetamine-paired cues and to amphetamine challenge (both performed 7 days after treatment discontinuation).

RESULTS

Amphetamine-treated rats displayed a sensitized 50-kHz USV emission during repeated treatment, as well as marked calling behavior in response to amphetamine-paired cues and to amphetamine challenge. Antagonism of D1 or D2 receptors either significantly suppressed or attenuated the emission of 50-kHz USVs in amphetamine-treated rats, with a maximal effect after synergistic antagonism of both receptors.

CONCLUSIONS

These results shed further light on how dopamine transmission modulates the emission of 50-kHz USVs in rats treated with psychoactive drugs.

Lateral preoptic area glutamate neurons relay nociceptive information to the ventral tegmental area

The ventral tegmental area (VTA) has been proposed to play a role in pain, but the brain structures modulating VTA activity in response to nociceptive stimuli remain unclear. Here, we demonstrate that the lateral preoptic area (LPO) glutamate neurons relay nociceptive information to the VTA. These LPO glutamatergic neurons synapsing on VTA neurons respond to nociceptive stimulation and conditioned stimuli predicting nociceptive stimulation and also mediate aversion. In contrast, LPO GABA neurons synapsing in the VTA mediate reward. By ultrastructural quantitative synaptic analysis, ex vivo electrophysiology, and functional neuroanatomy we identify a complex circuitry between LPO glutamatergic and GABAergic neurons and VTA dopaminergic, GABAergic, and glutamatergic neurons. We conclude that LPO glutamatergic neurons play a causal role in the processing of nociceptive stimuli and in relaying information about nociceptive stimuli. The pathway from LPO glutamatergic neurons to the VTA represents an unpredicted interface between peripheral nociceptive information and the limbic system.

Increased self-triggered vocalizations in an epidermal growth factor-induced rat model for schizophrenia

Rats elicit two types of ultrasonic vocalizations (USVs), positive (30–80 kHz; high pitch) and negative (10–30 kHz; low pitch) voices. As patients with schizophrenia often exhibit soliloquy-like symptoms, we explored whether an animal model for schizophrenia is similarly characterized by such self-triggered vocalizations. We prepared the animal model by administering an inflammatory cytokine, epidermal growth factor (EGF), to rat neonates, which later develop behavioral and electroencephalographic deficits relevant to schizophrenia. EGF model rats and controls at young (8–10 weeks old) and mature (12–14 weeks old) adult stages were subjected to acclimation, female pairing, and vocalization sessions. In acclimation sessions, low pitch USVs at the mature adult stage were more frequent in EGF model rats than in controls. In the vocalization session, the occurrences of low pitch self-triggered USVs were higher in EGF model rats in both age groups, although this group difference was eliminated by their risperidone treatment. Unlike conventional negative USVs of rats, however, the present low pitch self-triggered USVs had short durations of 10–30 ms. These results suggest the potential that self-triggered vocalization might serve as a translatable pathological trait of schizophrenia to animal models.

Dopamine D2-Subtype Receptors Outside the Blood-Brain Barrier Mediate Enhancement of Mesolimbic Dopamine Release and Conditioned Place Preference by Intravenous Dopamine

Dopamine (DA) is a cell-signaling molecule that does not readily cross the blood-brain barrier. Despite this, peripherally administered DA enhances DA levels in the nucleus accumbens and alters DA-related behaviors. This study was designed to investigate whether DA subtype-2 receptors are involved in the enhancement of nucleus accumbens (NAc) DA levels elicited by intravenous DA administration. This was accomplished by using microdialysis in the NAc and extracellular single unit recordings of putative DA neurons in the ventral tegmental area (VTA). Additionally, the reinforcing properties of intravenous DA were investigated using a place conditioning paradigm and the effects of intravenous DA on ultrasonic vocalizations were assessed. Following administration of intravenous dopamine, the firing rate of putative DA neurons in the VTA displayed a biphasic response and DA levels in the nucleus accumbens were enhanced. Pretreatment with domperidone, a peripheral-only DA D2 receptor (D2R) antagonist, reduced intravenous DA mediated increases in VTA DA neuron activity and NAc DA levels. Pretreatment with phentolamine, a peripheral α-adrenergic receptor antagonist, did not alter the effects of IV DA on mesolimbic DA neurotransmission. These results provide evidence for peripheral D2R mediation of the effects of intravenous DA on mesolimbic DA signaling.

Novel, User-Friendly Experimental and Analysis Strategies for Fast Voltammetry: Next Generation FSCAV with Artificial Neural Networks

Fast-scan adsorption-controlled voltammetry (FSCAV) was recently derived from fast-scan cyclic voltammetry to estimate the absolute concentrations of neurotransmitters by using the innate adsorption properties of carbon fiber microelectrodes. This technique has improved our knowledge of serotonin dynamics in vivo. However, the analysis of FSCAV data is laborious and technically challenging. First, each electrode requires post-experimental in vitro calibration. Second, current analysis methods are semi-manual and time-consuming and require a steep learning curve. Finally, the calibration methods used do not adapt to nonlinear electrode responses. In this work, we provide freely accessible computational solutions to these issues. First, we design an artificial neural network (ANN) and train it with a large data set (calibrations from 140 electrodes by six different researchers) to achieve calibration-free estimations and improve predictive error. We discuss the power of the ANN to obtain a low predictive error without electrode-specific calibrations as a function of being able to predict the sensitivity of the electrode. We use the ANN to successfully predict the absolute serotonin concentrations of real in vivo data. Finally, we create a fast and user-friendly, fully automated analysis web platform to simplify and reduce the expertise required for the postanalysis of FSCAV signals.

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.

Diazepam attenuates the effects of cocaine on locomotion, 50-kHz ultrasonic vocalizations and phasic dopamine in the nucleus accumbens of rats

BACKGROUND AND PURPOSE

Currently, there is no effective drug to treat cocaine-use disorder, which affects millions of people worldwide. Benzodiazepines are potential therapeutic candidates, as microdialysis and voltammetry studies have shown that they can decrease dopamine concentrations in the nucleus accumbens of rodents and block the increase in dopamine levels and appetitive 50-kHz ultrasonic vocalizations (USVs) induced by amphetamine in rats.

EXPERIMENTAL APPROACH

Here, we tested whether administration of 2.5-mg·kg^-1 diazepam (i.p.) in adult male rats could block the effects of 20-mg·kg^-1 cocaine (i.p.) on electrically evoked phasic dopamine signals in the nucleus accumbens measured by fast-scan cyclic voltammetry, as well as 50-kHz USV and locomotor activity.

KEY RESULTS

Cocaine injection increased evoked dopamine signals up to threefold within 5 min, and the increase was significantly higher than baseline for at least 75 min. The injection of diazepam, 5 min after cocaine, attenuated the cocaine effect by nearly 50%, and this attenuation was maintained for at least 40 min. Behaviourally, cocaine increased the number of appetitive 50-kHz calls by about 12-fold. Diazepam significantly blocked this effect for the entire duration of the session. Also, cocaine-treated rats were more active than controls and diazepam significantly attenuated cocaine-induced locomotion, by up to 50%.

CONCLUSION AND IMPLICATIONS

These results suggest that the neurochemical and psychostimulant effects of cocaine can be mitigated by diazepam.

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.

Measuring mania-like elevated mood through amphetamine-induced 50-kHz ultrasonic vocalizations in rats

Rats emit 50-kHz ultrasonic vocalizations (USV) in appetitive situations, reflecting a positive affective state. Particularly high rates of 50-kHz USV are elicited by the psychostimulant d-amphetamine. Exaggerated 50-kHz USV emission evoked by d-amphetamine is modulated by dopamine, noradrenaline and 5-hydroxytyrptamine receptor ligands and inhibited by the mood stabilizer lithium, the gold standard anti-manic drug for treating bipolar disorder. This indicates that exaggerated 50-kHz USV emission can serve as a reliable and valid measure for assessing mania-like elevated mood in rats with sufficient translational power for gaining a better understanding of relevant pathophysiological mechanisms and the identification of new therapeutic targets. The improved capacity to study the effects of anti-manic pharmacological interventions on a broader range of behaviours by including exaggerated 50-kHz USV emission as preclinical outcome measure complementary to locomotor hyperactivity will refine rodent models for mania.

The neuroscience of positive emotions and affect: Implications for cultivating happiness and wellbeing

This review paper provides an integrative account regarding neurophysiological correlates of positive emotions and affect that cumulatively contribute to the scaffolding for happiness and wellbeing in humans and other animals. This paper reviews the associations among neurotransmitters, hormones, brain networks, and cognitive functions in the context of positive emotions and affect. Consideration of lifespan developmental perspectives are incorporated, and we also examine the impact of healthy social relationships and environmental contexts on the modulation of positive emotions and affect. The neurophysiological processes that implement positive emotions are dynamic and modifiable, and meditative practices as well as flow states that change patterns of brain function and ultimately support wellbeing are also discussed. This review is part of "The Human Affectome Project" (http://neuroqualia.org/background.php), and in order to advance a primary aim of the Human Affectome Project, we also reviewed relevant linguistic dimensions and terminology that characterizes positive emotions and wellbeing. These linguistic dimensions are discussed within the context of the neuroscience literature with the overarching goal of generating novel recommendations for advancing neuroscience research on positive emotions and wellbeing.

Spontaneous Ultrasonic Vocalization Transmission in Adult, Male Long-Evans Rats Is Age-Dependent and Sensitive to EtOH Modulation

Ultrasonic vocalizations (USVs) are well-established markers of motivational and emotional status. Recent work from our lab has provided novel evidence for a role of USVs in models of ethanol (EtOH) use. For instance, USV acoustic characteristics can be used to accurately discriminate between rats selectively bred for high EtOH intake (e.g., alcohol-preferring (P) and high-alcohol-drinking (HAD)) versus EtOH-avoiding (e.g., alcohol-non-preferring (NP) and low-alcohol-drinking (LAD)) strains, as well as differentiate between male and female rats. In the present study we sought to explore the effect of age and alcohol availability on spontaneously emitted 50–55 kHz frequency modulated (FM) and 22–28 kHz USVs in adult, male Long–Evans rats. With the hypothesis that age and alcohol experience influence spontaneous USV emissions, we examined USV data collected across a 24-week intermittent EtOH access experiment in male Long–Evans rats. USV counts and acoustic characteristic (i.e., mean frequency, duration, bandwidth and power) data revealed distinct age-dependent phenotypes in both 50–55 kHz FM and 22–28 kHz USV transmission patterns that were modulated by EtOH exposure. These results highlight the influence of age and EtOH experience on the unique emotional phenotypes of male Long–Evans rats.

AUTS2 Governs Cerebellar Development, Purkinje Cell Maturation, Motor Function and Social Communication

Autism susceptibility candidate 2 (AUTS2), a risk gene for autism spectrum disorders (ASDs), is implicated in telencephalon development. Because AUTS2 is also expressed in the cerebellum where defects have been linked to ASDs, we investigated AUTS2 functions in the cerebellum. AUTS2 is specifically localized in Purkinje cells (PCs) and Golgi cells during postnatal development. Auts2 conditional knockout (cKO) mice exhibited smaller and deformed cerebella containing immature-shaped PCs with reduced expression of Cacna1a. Auts2 cKO and knock-down experiments implicated AUTS2 participation in elimination and translocation of climbing fiber synapses and restriction of parallel fiber synapse numbers. Auts2 cKO mice exhibited behavioral impairments in motor learning and vocal communications. Because Cacna1a is known to regulate synapse development in PCs, it suggests that AUTS2 is required for PC maturation to elicit normal development of PC synapses and thus the impairment of AUTS2 may cause cerebellar dysfunction related to psychiatric illnesses such as ASDs.

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Loss of Auts2 leads to the reduction of cerebellar size

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AUTS2 promotes the dendritic maturation of Purkinje cells

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AUTS2 participates in PF and CF synapse development of Purkinje cells

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Auts2 cKO mice exhibit the impaired motor learning and vocal communications

Dopamine and Addiction

Addiction is commonly identified with habitual nonmedical self-administration of drugs. It is usually defined by characteristics of intoxication or by characteristics of withdrawal symptoms. Such addictions can also be defined in terms of the brain mechanisms they activate; most addictive drugs cause elevations in extracellular levels of the neurotransmitter dopamine. Animals unable to synthesize or use dopamine lack the conditioned reflexes discussed by Pavlov or the appetitive behavior discussed by Craig; they have only unconditioned consummatory reflexes. Burst discharges (phasic firing) of dopamine-containing neurons are necessary to establish long-term memories associating predictive stimuli with rewards and punishers. Independent discharges of dopamine neurons (tonic or pacemaker firing) determine the motivation to respond to such cues. As a result of habitual intake of addictive drugs, dopamine receptors expressed in the brain are decreased, thereby reducing interest in activities not already stamped in by habitual rewards.

Dopamine transients do not act as model-free prediction errors during associative learning

Dopamine neurons are proposed to signal the reward prediction error in model-free reinforcement learning algorithms. This term represents the unpredicted or 'excess' value of the rewarding event, value that is then added to the intrinsic value of any antecedent cues, contexts or events. To support this proposal, proponents cite evidence that artificially-induced dopamine transients cause lasting changes in behavior. Yet these studies do not generally assess learning under conditions where an endogenous prediction error would occur. Here, to address this, we conducted three experiments where we optogenetically activated dopamine neurons while rats were learning associative relationships, both with and without reward. In each experiment, the antecedent cues failed to acquire value and instead entered into associations with the later events, whether valueless cues or valued rewards. These results show that in learning situations appropriate for the appearance of a prediction error, dopamine transients support associative, rather than model-free, learning.

Differential rewarding effects of electrical stimulation of the lateral hypothalamus and parabrachial complex: Functional characterization and the relevance of opioid systems and dopamine

BACKGROUND

Since the discovery of rewarding intracranial self-stimulation by Olds and Milner, extensive data have been published on the biological basis of reward. Although participation of the mesolimbic dopaminergic system is well documented, its precise role has not been fully elucidated, and some authors have proposed the involvement of other neural systems in processing specific aspects of reinforced behaviour.

AIMS AND METHODS

We reviewed published data, including our own findings, on the rewarding effects induced by electrical stimulation of the lateral hypothalamus (LH) and of the external lateral parabrachial area (LPBe) - a brainstem region involved in processing the rewarding properties of natural and artificial substances - and compared its functional characteristics as observed in operant and non-operant behavioural procedures.

RESULTS

Brain circuits involved in the induction of preferences for stimuli associated with electrical stimulation of the LBPe appear to functionally and neurochemically differ from those activated by electrical stimulation of the LH.

INTERPRETATION

We discuss the possible involvement of the LPBe in processing emotional-affective aspects of the brain reward system.

The antagonistic relationship between aversive and appetitive emotional states in rats as studied by pharmacologically-induced ultrasonic vocalization from the nucleus accumbens and lateral septum

Rats can emit 22-kHz or 50-kHz ultrasonic vocalizations (USVs) in negative, as well as positive contexts which index their emotional state. 22-kHz USVs are emitted during aversive contexts and can be initiated by activation of the ascending cholinergic pathways originating from the laterodorsal tegmental nucleus or initiated pharmacologically by injection of cholinergic agonists into target areas of these pathways (medial cholinoceptive vocalization strip). Conversely, 50-kHz USVs are emitted during positive pro-social contexts and can be initiated by stimulation of ascending dopaminergic pathways originating from the ventral tegmental area or by injection of dopamine agonists into target areas of these pathways (nucleus accumbens shell). Recently, we have shown an inhibitory effect a positive emotional state has on the emission of carbachol-induced 22-kHz USVs from the anterior hypothalamic/medial preoptic area. However, this structure is a fragment of that cholinoceptive vocalization strip. We wanted to examine if we could observe similar effect when the aversive state is induced from the lateral septum, the most rostral division of the cholinoceptive vocalization strip. The results supported previous findings. First, microinjection of the dopamine agonist R-(-)-apomorphine into the nucleus accumbens shell resulted in increased emission of frequency modulated (FM) 50-kHz USVs that are regarded as signals expressing a positive emotional state in rats. Second, FM 50-kHz USVs and not flat (F) 50-kHz USVs were able to decrease 22-kHz USVs induced by microinjections of carbachol into the lateral septum. This research provides further support to the hypothesis that the initiation of a positive emotional state functionally antagonizes initiation of a negative emotional state in rats.

Mania-like elevated mood in rats: Enhanced 50-kHz ultrasonic vocalizations after sleep deprivation

Mania is characterized by elevated drive and mood but animal models of mania have often neglected elevated mood. Ultrasonic vocalizations (USV) of 50-kHz emitted by rats are thought to index the subject's positive affective state. Fifty-kHz USV emission is increased by amphetamine, an effect blocked by lithium administration. Sleep deprivation (SD) is an environmental model of mania and the present study evaluated SD effects on behavioral activity and USV emission, together with the impact of lithium treatment. Adult rats were submitted to 24h or 72h SD, and locomotor activity and USV emission were assessed. To test their sensitivity to a standard antimanic drug, these behavioral parameters were also evaluated after acute administration of lithium carbonate (25, 50 or 100 mg/kg, i.p.). Striatal monoamine content was measured post-mortem. SD (24h and 72h) led to increased locomotor activity, rearing behavior and 50-kHz USV emission, together with a change in the call profile characterized by an increase in the percentage of frequency-modulated 50-kHz USV, which may indicate the mania-like consequences of SD. Importantly, all SD effects were reverted by lithium administration. SD also led to a decrease in dopamine content in the ventral striatum, while increasing dopamine turnover. In conclusion, SD increased 50-kHz USV emission, an effect prevented by acute lithium administration. This suggests 50-kHz USV as a new marker for mania-like elevated mood, which shows construct validity (associated with increased dopaminergic tone), face validity (reflecting increased positive affect) and predictive validity (high sensitivity to lithium treatment).

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.

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

BACKGROUND

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

NEW METHOD

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

RESULTS

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

COMPARISON WITH EXISTING METHODS

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

CONCLUSIONS

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

Alcohol-naïve USVs distinguish male HAD-1 from LAD-1 rat strains

Ultrasonic vocalizations (USVs) are mediated through specific dopaminergic and cholinergic neural pathways and serve as real-time measures of positive and negative emotional status in rodents. Although most USV studies focus primarily on USV counts, each USV possesses a number of characteristics shown to reflect activity in the associated neurotransmitter system. In the present study, we recorded spontaneously emitted USVs from alcohol-naïve high alcohol drinking (HAD-1) and low alcohol drinking (LAD-1) rats. Using our recently developed WAAVES algorithm, we quantified four acoustic characteristics (mean frequency, duration, power, and bandwidth) from each 22-28 kHz and 50-55 kHz frequency-modulated (FM) USV. This rich USV representation allowed us to apply advanced statistical techniques to identify the USV acoustic characteristics that distinguished HAD-1 from LAD-1 rats. Linear mixed models (LMM) examined the predictability of each USV characteristic in isolation and linear discriminant analysis (LDA), and binomial logistic regression examined the predictability of linear combinations of the USV characteristics as a group. Results revealed significant differences in acoustic characteristics between HAD-1 and LAD-1 rats in both 22-28 kHz and 50-55 kHz FM USVs. In other words, these rats selectively bred for high- and low-alcohol consumption can be identified as HAD-1 or LAD-1 rats with high classification accuracy (approximately 92-100%) exclusively based on their emitted 22-28 kHz and 50-55 kHz FM USV acoustic characteristics. In addition, acoustic characteristics of 22-28 kHz and 50-55 kHz FM USVs emitted by alcohol-naïve HAD-1 and LAD-1 rats significantly correlate with their future alcohol consumption. Our current findings provide novel evidence that USV acoustic characteristics can be used to discriminate between alcohol-naïve HAD-1 and LAD-1 rats, and may serve as biomarkers in rodents with a predisposition for, or against, excessive alcohol intake.

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.

Comparing rewarding and reinforcing properties between 'bath salt' 3,4-methylenedioxypyrovalerone (MDPV) and cocaine using ultrasonic vocalizations in rats

Abuse of synthetic psychostimulants like synthetic cathinones has risen in recent years. 3,4-Methylenedioxypyrovalerone (MDPV) is one such synthetic cathinone that demonstrates a mechanism of action similar to cocaine. Compared to cocaine, MDPV is more potent at blocking dopamine and norepinephrine reuptake and is readily self-administered by rodents. The present study compared the rewarding and reinforcing properties of MDPV and cocaine using systemic injection dose-response and self-administration models. Fifty kilohertz ultrasonic vocalizations (USVs) were recorded as an index of positive affect throughout experiments. In Experiment 1, MDPV and cocaine dose-dependently elicited 50-kHz USVs upon systemic injection, but MDPV increased USVs at greater rates and with greater persistence relative to cocaine. In Experiment 2, latency to begin MDPV self-administration was shorter than latency to begin cocaine self-administration, and self-administered MDPV elicited greater and more persistent rates of 50-kHz USVs versus cocaine. MDPV-elicited 50-kHz USVs were sustained over the course of drug load-up whereas cocaine-elicited USVs waned following initial infusions. Notably, we observed a robust presence of context-elicited 50-kHz USVs from both MDPV and cocaine self-administering rats. Collectively, these data suggest that MDPV has powerfully rewarding and reinforcing effects relative to cocaine at one-tenth doses. Consistent with prior work, we additionally interpret these data in supporting that MDPV has significant abuse risk based on its potency and subjectively positive effects. Future studies will be needed to better refine therapeutic strategies targeted at reducing the rewarding effects of cathinone analogs in efforts to ultimately reduce abuse liability.

Stereoselective Differences between the Reinforcing and Motivational Effects of Cathinone-Derived 4-Methylmethcathinone (Mephedrone) In Self-Administering Rats

Mephedrone (4-methylmethcathinone (4-MMC)) (MEPH) is a new psychoactive substance (NPS) of the synthetic cathinone class. MEPH has a chiral center and exists as two enantiomers (R-,S-MEPH), yet stereospecific effects of MEPH have not been extensively investigated in preclinical assays. Because significant behavioral and neurochemical differences can exist between enantiomers, probing effects of stereochemistry on biological activity enables separation of adverse and therapeutic effects. Our prior work showed that R-MEPH, relative to S-MEPH, produced greater locomotor activation, place preference, and facilitation of brain reward thresholds in rodents. The present study sought to determine if MEPH enantiomers display stereospecific reward and reinforcement in rat self-administration assays. In Experiment 1, rats were trained to self-administer racemic MEPH (0.50 mg/kg/inf), and dose substitution effects of R-MEPH (0.50 mg/kg/inf) and S-MEPH (0.25, 0.50, 2.00 mg/kg/inf) were examined. In Experiment 2, separate rats were trained to self-administer R-MEPH (0.25, 0.50, 2.00 mg/kg/inf) or S-MEPH (0.25, 0.50, 2.00 mg/kg/inf) and were thereafter evaluated under progressive-ratio access conditions. Within this cohort, 50 kHz ultrasonic vocalizations (USVs) were recorded to measure potential differences in subjective positive affect associated with MEPH enantiomer self-administration. We identified enantiomer- and dose-dependent effects on infusions earned during self-administration following acquisition of racemic MEPH, with greatest infusions under low-effort, fixed-ratio 1 access conditions from low-dose S-MEPH self-administration. When taxed with progressive-ratio access conditions, rats trained to self-administer R-MEPH showed higher break points than those of rats trained to self-administer S-MEPH. Additionally, R-MEPH elicited greatest rates of 50 kHz USVs compared to S-MEPH. Taken together, these data suggest that the R-enantiomer of MEPH is primarily responsible for the rewarding, reinforcing, and motivational properties of racemic MEPH, which increases our understanding of stereospecific preferences pertaining to MEPH abuse.

Role of hypocretin/orexin receptor blockade on drug-taking and ultrasonic vocalizations (USVs) associated with low-effort self-administration of cathinone-derived 3,4-methylenedioxypyrovalerone (MDPV) in rats

RATIONALE

Synthetic psychostimulant abuse, including cathinone-derived 3,4-methylenedioxypyrovalerone (MDPV), continues to increase in many countries. Similar to cocaine but with greater potency, MDPV elicits a transient sympathomimetic response by blocking cellular uptake of dopamine (DA) and norepinephrine (NE)-administration in some users is reported as euphoria-inducing much like cocaine and amphetamine. Pharmacological agents that disrupt excitatory transmission onto midbrain DA-producing neurons, including hypothalamic hypocretin/orexin (hcrt/ox) receptor antagonists, present attractive targets to aide abstinence maintenance by reducing psychostimulant-associated reward and reinforcement.

OBJECTIVE

The present study sought to assess the degree to which suvorexant, a dual hcrt/ox receptor antagonist, influences drug-taking as well as ultrasonic vocalizations (USVs) associated with MDPV self-administration.

METHODS

Rats were trained to self-administer MDPV (~0.03 mg/kg/inf, 3-s) for 14 days under a fixed-ratio 1 schedule of reinforcement, and effects of suvorexant (0, 3, 10, 30 mg/kg, i.p.) on drug-taking was assessed. USVs were recorded during a 30-min pre-lever period as well as during 2-h of MDPV self-administration.

RESULTS

We observed that suvorexant modestly suppressed the number of MDPV infusions earned. Notably, we observed that suvorexant reduced 50-kHz USVs associated with pre- and post-lever time-points but did not noticeably alter call type profiles. Upon comparison of the two measures, we observed trending positive associations between suvorexant-induced changes in drug-taking and 50-kHz USVs.

CONCLUSIONS

Results from this exploratory study provide support for the following: (1) studying how suvorexant may provide benefit to humans with stimulant use disorders, (2) identifying a potential role for orexin transmission in cathinone abuse, and (3) further interrogating the potential utility of rat USVs to predict drug consumption in preclinical models of substance use disorders.

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.

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.

Automated acoustic analysis of 50-kHz ultrasonic vocalizations using template matching and contour analysis

Ultrasonic vocalizations are a useful tool for inferring affective states in the rat and have been incorporated in research paradigms modeling important human conditions. While the majority of studies report the quantity or rate of observed ultrasonic vocalizations, growing evidence suggests that critical data may be contained in the acoustic features of individual vocalizations. Thus, the goal of the present study was to develop and validate a method for measuring acoustic parameters of ultrasonic vocalizations that were collected using automatic template detection. Acoustic parameters derived using this method were found to be comparable to those collected using commercially available software.

Parallel role for the dopamine D1 receptor in gambling and amphetamine reinforcement in healthy volunteers

This study investigated the role of dopamine, and specifically the D1 receptor (D1R), in the reinforcing effects of a slot-machine game in healthy volunteers ( n=30). To compare gambling and drug effects, subjects received the prototypic psychostimulant drug d-amphetamine (AMPH; 20 mg) in a multi-session, placebo-controlled design. To isolate D1R, half the subjects were pretreated with the preferential D2 receptor antagonist haloperidol (HAL; 3 mg), and the other half with the mixed D1-D2 antagonist fluphenazine (FLU; 3 mg) before the game (Phase I) and AMPH (Phase II). HAL decreased and FLU increased the post-game desire to gamble and post-AMPH desire to take AMPH again, as well as amphetamine scale ratings on the Addiction Research Center Inventory after gambling and AMPH. The effects of the antagonists on desire to gamble and to take AMPH again were significantly intercorrelated. HAL increased and FLU decreased the salience of negative affective words on a rapid reading task after both reinforcers. HAL also decreased the salience of gambling words after AMPH. Both reinforcers increased diastolic blood pressure equally under antagonists and placebo. Results indicate that D1R plays a parallel role in the psychostimulant-like, incentive-motivational, and salience-enhancing effects of gambling and AMPH. Moderate D1R activation appears to optimize these effects in healthy subjects.