Activation of adenosine A₂A receptors suppresses the emission of pro-social and drug-stimulated 50-kHz ultrasonic vocalizations in rats: possible relevance to reward and motivation

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.

Positive maternal affect during mother-litter interaction is reduced in new mother rats exhibiting a depression-like phenotype

The experience of positive affect during new motherhood is considered essential for a healthy mother-infant relationship, with life-long consequences for both mother and child. Affective availability and contingent responsiveness are often compromised in mothers experiencing postpartum depression, yet how maternal affect impacts parenting is not fully understood. In this study, we used the Wistar-Kyoto (WKY) rat model of depression and ultrasonic vocalizations to examine the relationship between maternal affect and parenting. We examined the affective and behavioral response of WKY and control new mother rats during social interactions with their offspring. Our results show that WKY mothers displayed altered USV signaling accompanying substantial disturbances in their maternal caregiving. In addition, WKY mothers failed to adjust vocal frequency in coordination with offspring proximity and interaction compared to control mothers. A follow up experiment demonstrated that the administration of the adenosine A2A receptor antagonist MSX-3 ameliorated both maternal behavioral deficits and low positive affect in WKY mothers. Together, our results highlight the importance of maternal positive affect in the dyad relationship and suggest a role for the striatopallidal pathway in the affective processing of parenting.

Quantification of brainstem norepinephrine relative to vocal impairment and anxiety in the Pink1-/- rat model of Parkinson disease

Vocal communication impairment and anxiety are co-occurring and interacting signs of Parkinson Disease (PD) that are common, poorly understood, and under-treated. Both vocal communication and anxiety are influenced by the noradrenergic system. In light of this shared neural substrate and considering that noradrenergic dysfunction is a defining characteristic of PD, tandem investigation of vocal impairment and anxiety in PD relative to noradrenergic mechanisms is likely to yield insights into the underlying disease-specific causes of these impairments. In order to address this gap in knowledge, we assessed vocal impairment and anxiety behavior relative to brainstem noradrenergic markers in a genetic rat model of early-onset PD (Pink1-/-) and wild type controls (WT). We hypothesized that 1) brainstem noradrenergic markers would be disrupted in Pink1-/-, and 2) brainstem noradrenergic markers would be associated with vocal acoustic changes and anxiety level. Rats underwent testing of ultrasonic vocalization and anxiety (elevated plus maze) at 4, 8, and 12 months of age. At 12 months, brainstem norepinephrine markers were quantified with immunohistochemistry. Results demonstrated that vocal impairment and anxiety were increased in Pink1-/- rats, and increased anxiety was associated with greater vocal deficit in this model of PD. Further, brainstem noradrenergic markers including TH and α1 adrenoreceptor immunoreactivity in the locus coeruleus, and β1 adrenoreceptor immunoreactivity in vagal nuclei differed by genotype, and were associated with vocalization and anxiety behavior. These findings demonstrate statistically significant relationships among vocal impairment, anxiety, and brainstem norepinephrine in the Pink1-/- rat model of PD.

Association between Novel Object Recognition/Spontaneous Alternation Behavior and Emission of Ultrasonic Vocalizations in Rats: Possible Relevance to the Study of Memory

Rats emit ultrasonic vocalizations (USVs) in situations with emotional valence, and USVs have also been proposed as a marker for memories conditioned to those situations. This study investigated whether USV emissions can predict and/or be associated with the behavior of rats in tests that evaluate unconditioned memory. To this end, rats were subjected to “tickling”, a procedure of heterospecific play that has emotional valence and elicits the emission of USVs, and afterwards evaluated in the novel object recognition test (NOR) and in the single trial continuous spontaneous alternation behavior (SAB) test in a Y maze. The number of 22-kHz USVs (aversive) and 50-kHz USVs (appetitive) emitted in response to tickling and during NOR and SAB tests were scored, and the correlations among them and with rats’ behavior evaluated. Rats emitted 50-kHz USVs, but not 22-kHz USVs, during the NOR and SAB tests, and such calling behavior was not linked with the behavioral readouts indicative of memory function in either test. However, rats that prevalently emitted 22-kHz USVs in response to tickling displayed an impaired NOR performance. These findings suggest that measuring the emission of USVs could be of interest in studies of unconditioned memory, at least with regard to 22-kHz USVs.

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.

Extracellular Dopamine Levels in Nucleus Accumbens after Chronic Stress in Rats with Persistently High vs. Low 50-kHz Ultrasonic Vocalization Response

Fifty-kHz ultrasonic vocalizations (USVs) in response to an imitation of rough-and-tumble play ('tickling') have been associated with positive affective states and rewarding experience in the rat. This USV response can be used as a measure of inter-individual differences in positive affect. We have previously shown that rats with persistently low positive affectivity are more vulnerable to the effects of chronic variable stress (CVS). To examine whether these differential responses are associated with dopaminergic neurotransmission in the nucleus accumbens (NAc), juvenile male Wistar rats were categorized as of high or low positive affectivity (HC and LC, respectively), and after reaching adulthood, extracellular dopamine (DA) levels in the NAc shell were measured using in vivo microdialysis after three weeks of CVS. Baseline levels of DA were compared as well as the response to K⁺-induced depolarization and the effect of glial glutamate transporter EAAT2 inhibition by 4 mM l-trans-pyrrolidine-2,4-dicarboxylate (PDC). DA baseline levels were higher in control LC-rats, and stress significantly lowered the DA content in LC-rats. An interaction of stress and affectivity appeared in response to depolarization where stress increased the DA output in HC-rats whereas it decreased it in LC-rats. These results show that NAc-shell DA is differentially regulated in response to stress in animals with high and low positive affect.

Influence of dopamine transmission in the medial prefrontal cortex and dorsal striatum on the emission of 50-kHz ultrasonic vocalizations in rats treated with amphetamine: Effects on drug-stimulated and conditioned calls

Rat ultrasonic vocalizations (USVs) of 50 kHz are increasingly being evaluated as a behavioral marker of the affective properties of drugs. Studies in amphetamine-treated rats have shown that activation of dopamine transmission in the nucleus accumbens (NAc) initiates the emission of 50-kHz USVs, but little is known on how dopamine transmission in other brain regions modulates the effects of drugs on calling behavior. To clarify this issue, we evaluated 50-kHz USV emissions in rats subjected to dopaminergic denervation of either the medial prefrontal cortex (mPFC) or the dorsal striatum (DS) and treated with amphetamine. Rats received amphetamine (1 mg/kg, i.p. × 5) on alternate days in a test cage; 7 days later, they were re-exposed to the test cage, to measure calling behavior that may reflect drug conditioning, and then challenged with amphetamine (1 mg/kg, i.p.). The numbers of total and categorized 50-kHz USVs emitted were evaluated, along with immunofluorescence for Zif-268 in the NAc. Dopamine-denervated and sham-operated rats displayed comparable patterns of calling behavior during amphetamine treatment and after amphetamine challenge. Conversely, rats that were dopamine-denervated in the mPFC, but not DS, emitted low numbers of 50-kHz USVs on test cage re-exposure. Finally, dopamine-denervated rats displayed a less marked increase in Zif-268-positive neurons in the NAc shell after amphetamine challenge, compared with sham-operated rats. These results may be relevant to identify the neuronal circuits that modulate 50-kHz USV emissions in rats treated with amphetamine, as well as the interplay between calling behavior and affective properties of drugs.

Ultrasonic vocalizations as a tool in studying emotional states in rodent models of social behavior and brain disease

Rodents emit ultrasonic vocalizations (USVs) to communicate the presence of positive or negative emotional states and to coordinate social interactions. On this basis, USVs are increasingly being used as a behavioral readout in rodent studies of affect, motivation and social behavior. Notably, several investigations have demonstrated that rodents emit USVs when tested in experimental paradigms that are used in preclinical studies of psychiatric and neurological diseases. Moreover, it has been shown that calling behavior may be influenced by genetic and/or environmental factors (i.e., stress), early rearing conditions that have been implicated in brain disease, as well as psychoactive drugs. Hence, measuring USV emissions has emerged as a useful tool in studying the mechanisms that underlie the emotional disturbances featuring certain brain diseases, as well as in the development of suited pharmacological therapies. This review provides an overview of the behavioral significance of USV emissions and describes the contexts that promote calling behavior in rats and mice. Moreover, the review summarizes the current evidence concerning the use of USVs as a marker of affect in rat and mouse models of sociability, psychiatric diseases and neurological diseases, and discusses the strengths and current limitations of using USVs as a behavioral readout in rodent studies of emotional behavior. This article is part of the Special Issue entitled 'The neuropharmacology of social behavior: from bench to bedside'.

Effects of anxiogenic drugs on the emission of 22- and 50-kHz ultrasonic vocalizations in adult rats

RATIONALE

Adult rat 22-kHz vocalizations are often associated with alarm or distress, whereas a subset of 50-kHz calls is preferentially emitted in response to amphetamine and other rewarding stimuli. Whether any 50-kHz calls reflect anxiety is unknown.

OBJECTIVE

To determine the effects of anxiogenic drugs on 50-kHz call rate and call subtype profile, in comparison with D-amphetamine.

METHODS

Adult male rats received systemic amphetamine (1 mg/kg) three times several days before testing. Ultrasonic vocalizations were then recorded after acute intraperitoneal injection of amphetamine or one of five anxiogenic drugs: yohimbine (2.5 mg/kg), N-methyl-β-carboline-3-carboxamide (FG 7142, 5 mg/kg), pentylenetetrazol (PTZ, 20 mg/kg), m-chlorophenylpiperazine (mCPP, 1 mg/kg), caffeine (25 mg/kg), or vehicle.

RESULTS

The duration of immobility was increased by FG 7142, PTZ, and mCPP; this measure was unchanged by yohimbine and reduced by the locomotor stimulant drugs amphetamine and caffeine. Conversely, the 50-kHz call rate was reduced by FG 7142, PTZ and mCPP, and increased by caffeine and amphetamine. Overall, the most common 50-kHz call subtypes were flat, trill, step-up, and complex. Consistent with previous reports, amphetamine increased the relative prevalence of trill calls while reducing the relative prevalence of flat calls. Yohimbine and caffeine reduced flat call prevalence, whereas mCPP reduced trill call prevalence. No other shifts in the call profile were observed, and no anxiogenic drug induced 22-kHz calls.

CONCLUSION

Anxiogenic drugs, as a class, did not uniformly alter the 50-kHz call rate or subtype profile. Amphetamine-induced effects on 50-kHz call rate and profile do not reflect anxiety.

Noradrenergic receptor modulation influences the acoustic parameters of pro-social rat ultrasonic vocalizations

Rats produce high rates of ultrasonic vocalizations (USVs) in social situations; these vocalizations are influenced by multiple neurotransmitter systems. Norepinephrine (NE) plays a significant role in vocalization biology; however, the contribution of NE to normal, prosocial vocal control has not been well established in the rat. To address this, we used NE adrenoceptor agonists (Cirazoline, Clonidine) and antagonists (Prozasin, Atipamezole, Propranolol) to quantify the contribution of specific alpha-1, alpha-2, and beta NE receptors to USV parameters in male Long Evans rats during seminaturalistic calling. We found that multiple USV acoustic variables (intensity, bandwidth, duration, peak frequency, and call profile) are modified by alterations in NE signaling. Very generally, agents that increased NE neurotransmission (Atipamezole) or activated alpha-1 receptors (Cirazoline), led to an increase in intensity and duration, respectively. Agents that decreased NE neurotransmission (Clonidine) or blocked alpha-1 receptors (Prazosin) reduced call rate, intensity, and bandwidth. However, the beta-receptor antagonist, Propranolol, was associated with increased call rate, duration, and intensity. Limb motor behaviors were largely unaffected by any drug, with the exception of Clonidine. Higher doses of Clonidine significantly reduced gross motor, grooming, and feeding behavior. These results confirm the involvement of NE transmission in vocal control in the rat, and suggest that this USV model is useful for studying the neuropharmacology of behavioral measures that may have implications for disease states, such as Parkinson's disease. (PsycINFO Database Record

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.

Emission of categorized 50-kHz ultrasonic vocalizations in rats repeatedly treated with amphetamine or apomorphine: Possible relevance to drug-induced modifications in the emotional state

The emission of 50-kHz ultrasonic vocalizations (USVs) is increasingly emerging as a potential behavioral marker of the subjective effects that psychoactive drugs elicit in rats. However, multiple categories of 50-kHz USVs have been identified, which are thought to possess different behavioral significance. Besides, limited information is available on how psychoactive drugs affect the emission of categorized 50-kHz USVs. To further elucidate this issue, we evaluated the numbers of multiple categories of 50-kHz USVs emitted by rats repeatedly treated with amphetamine (1 or 2 mg/kg, i.p.) or apomorphine (2 or 4 mg/kg, i.p.), two drugs that elicit similar and dissimilar subjective effects. Amphetamine- and apomorphine-treated rats emitted patterns of categorized 50-kHz USVs that varied according to the drug administered, drug dose, and number of drug administrations. Nevertheless, the numbers of several categories of 50-kHz USVs were positively correlated with the number of total calls emitted (i.e., the sum of categorized 50-kHz USVs). Moreover, a marked interindividual variability in the emission of categorized 50-kHz USVs was observed. Taken together, the present results may be relevant to further elucidating the interplay between calling of the 50-kHz USVs group and psychopharmacological profile of drugs.

Endogenous Opiates and Behavior: 2016

This paper is the thirty-ninth consecutive installment of the annual review of research concerning the endogenous opioid system. It summarizes papers published during 2016 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides, opioid receptors, opioid agonists and opioid antagonists. The particular topics that continue to be covered include the molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors related to behavior, and the roles of these opioid peptides and receptors in pain and analgesia, stress and social status, tolerance and dependence, learning and memory, eating and drinking, drug abuse and alcohol, sexual activity and hormones, pregnancy, development and endocrinology, mental illness and mood, seizures and neurologic disorders, electrical-related activity and neurophysiology, general activity and locomotion, gastrointestinal, renal and hepatic functions, cardiovascular responses, respiration and thermoregulation, and immunological responses.

Role of adenosine A2A receptors in motor control: relevance to Parkinson's disease and dyskinesia

Adenosine is an endogenous purine nucleoside that regulates several physiological functions, at the central and peripheral levels. Besides, adenosine has emerged as a major player in the regulation of motor behavior. In fact, adenosine receptors of the A_2A subtype are highly enriched in the caudate-putamen, which is richly innervated by dopamine. Moreover, several studies in experimental animals have consistently demonstrated that the pharmacological antagonism of A_2A receptors has a facilitatory influence on motor behavior. Taken together, these findings have envisaged A_2A receptors as a promising target for symptomatic therapies aimed at ameliorating motor deficits. Accordingly, A_2A receptor antagonists have been extensively studied as new agents for the treatment of Parkinson's disease (PD), the epitome of motor disorders. In this review, we provide an overview of the effects that adenosine A_2A receptor antagonists elicit in rodent and primate experimental models of PD, with regard to the counteraction of motor deficits as well as to manifestation of dyskinesia and motor fluctuations. Moreover, we briefly present the results of clinical trials of A_2A receptor antagonists in PD patients experiencing motor fluctuations, with particular regard to dyskinesia. Finally, we discuss the interaction between A_2A receptor antagonists and serotonin receptor agonists, since combined administration of these drugs has recently emerged as a new potential therapeutic strategy in the treatment of dyskinesia.

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.

Modulation of Rat 50-kHz Ultrasonic Vocalizations by Glucocorticoid Signaling: Possible Relevance to Reward and Motivation

BACKGROUND

Rats emit 50-kHz ultrasonic vocalizations (USVs) to communicate positive emotional states, and these USVs are increasingly being investigated in preclinical studies on reward and motivation. Although it is the activation of dopamine receptors that initiates the emission of 50-kHz USVs, non-dopaminergic mechanisms may modulate calling in the 50 kHz frequency band. To further elucidate these mechanisms, the present study investigated whether the pharmacological manipulation of glucocorticoid signaling influenced calling.

METHODS

Rats were administered corticosterone (1-5 mg/kg, s.c.), the glucocorticoid receptor antagonist mifepristone (40 or 100 mg/kg, s.c.), or the corticosterone synthesis inhibitor metyrapone (50 or 100 mg/kg, i.p.). The effects of these drugs on calling initiation and on calling recorded during nonaggressive social contacts or after the administration of amphetamine (0.25 or 1 mg/kg, i.p.) were then evaluated.

RESULTS

Corticosterone failed to initiate the emission of 50-kHz USVs and did not influence pro-social and amphetamine-stimulated calling. Similarly, mifepristone and metyrapone did not initiate calling. However, metyrapone suppressed pro-social calling and calling stimulated by a moderate dose (1 mg/kg, i.p.) of amphetamine. Conversely, mifepristone attenuated calling stimulated by a low (0.25 mg/kg, i.p.), but not moderate (1 mg/kg, i.p.), dose of amphetamine and had no influence on pro-social calling.

CONCLUSIONS

The present results demonstrate that glucocorticoid signaling modulates calling in the 50 kHz frequency band only in certain conditions and suggest that mechanisms different from the inhibition of corticosterone synthesis may participate in the suppression of calling by metyrapone.

Effects of repeated morphine on ultrasonic vocalizations in adult rats: increased 50-kHz call rate and altered subtype profile

RATIONALE

Adult rat 50-kHz vocalizations have been proposed to indicate a positive affective state, putatively revealed by a predominance of trill calls over flat calls. However, short-term exposure to non-sedative doses of the euphorigen morphine suppresses calling, with no discernible shift in trill or flat call prevalence.

OBJECTIVES

This study aimed to determine whether morphine acutely increases 50-kHz call rates or alters the relative prevalence of trill or flat calls, after long-term morphine exposure or acute pharmacological pretreatment.

METHODS

Experiment 1 comprised 10 once-daily tests, alternating between saline and morphine, 1 mg/kg SC, followed by dose-response testing (0, 0.3, 1, and 3 mg/kg). Experiment 2 was similar but included additional testing with morphine in combination with the antinausea drug ondansetron or the peripheral opioid antagonist methylnaltrexone. In experiment 3, morphine was again combined with ondansetron or methylnaltrexone but in rats that were initially drug naïve.

RESULTS

In animals that were initially drug naïve, morphine tended to suppress calling and did not alter the 50-kHz call subtype profile. In morphine-experienced rats, morphine acutely increased the 50-kHz call rate and promoted trills over flat calls; short calls were also inhibited. Neither ondansetron nor methylnaltrexone detectably altered any effect of morphine on calling, nor did these two drugs affect 50-kHz calling when given alone.

CONCLUSIONS

With chronic exposure, morphine acutely enhances 50-kHz calling and differentially promotes trill calls, mainly at the expense of flat calls. These effects appear consistent with a positive affect interpretation of 50-kHz vocalizations.