Neonatal maternal separation alters reward-related ultrasonic vocalizations in rat dams

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.

Maternal separation: Does it hold the potential to model consequences of postpartum depression?

The postpartum period is a sensitive time where women are especially vulnerable to develop postpartum depression (PPD), with 10%-15% of women affected. This review investigates whether the maternal separation (MS) paradigm in rodents holds the potential to help to understand mothers suffering from PPD. MS is a well-established stress model to investigate effects on infants, whereas effects on the dam are often overlooked. The database PubMed was searched for studies investigating effects of daily MS within the first weeks after parturition on dams in rats and mice and compared to findings in PPD mothers. MS was categorized as brief MS (5-45 min) with or without handling of pups and long MS (3-4 h and longer). MS alters maternal care, depressive-like behavior, anxiety, and aggression; leads to alterations in neuronal gene expression; and affects hormone and neurotransmitter levels similar to observations in PPD patients. Even though there are disparities between human and rodent mothers, with some results differing in directionality, as well as the reason for separation (self-induced in PPD, externally induced in MS), the overall effects found on neurobiological, hormonal, and behavioral levels mostly coincide. Thus, the MS paradigm can add relevant knowledge to existing PPD animal models, further advancing the study of PPD.

Maternal separation in rats induces neurobiological and behavioral changes on the maternal side

The time after parturition is a sensitive period for mothers where they are prone to develop psychopathological symptoms. Studies investigating dams after separation from their pups (maternal separation, MS) showed that MS induces alterations similar to postpartum depression. This study aims to give further details on affected behavior and neurobiology of dams after MS. MS in rats from postnatal day 2-20 over four hours daily was performed. Upon reunion, maternal behavior, and ultrasonic vocalization (USV) of dams were measured. On the day of weaning, dams were tested for anxiety-like behavior in the elevated-plus-maze and marble burying test. Then Morc1 mRNA in the medial prefrontal cortex and Nr3c1 encoding the glucocorticoid receptor mRNA in the hippocampus were measured using real-time PCR to examine possible neurobiological correlates in psychopathology and social behavior. GABA and glutamate serum levels were analyzed by high-performance liquid chromatography as peripheral markers for stress-induced psychopathology. MS in dams increased maternal care towards pups even though both groups show high levels of maternal behavior even in late lactation. Furthermore, the emission of 50-kHz and 22-kHz USVs increased significantly. No differences in anxiety-like behavior were detected. MS further reduced Morc1 but not Nr3c1 expression. Serum GABA but not glutamate levels were significantly increased in separated dams. This study reinforces the benefit of investigating dams after MS for studying postpartum stress. Subclinical markers mainly connected to depression, namely Morc1 and GABA, proved to be useful allowing for earlier detection of symptoms of critical postpartum stress.

Characteristics of pup ultrasonic vocalizations and parental behavior responses in midday gerbils (Meriones meridianus)

Ultrasonic vocalizations (USVs) play an important role in parent-infant interactions during the neonatal period of rodents because of the pups' need to be suckled and protected by their parents. However, studies on the effects of USVs on parental care are focused on maternal behavior, and little data on paternal care are available, with especially few investigations in midday gerbils. In this study, we examined the effects of early social deprivation (ED) on offspring due to USVs and parental behavior responses in midday gerbils (Meriones meridianus). The results indicated that the number of USVs in ED gerbils increased on postnatal day nine (40 calls/minute), whereas the number of gerbils in the parental care (PC) group remained high for up to 2 weeks (46.6 calls/minute), before decreasing. The number of USVs and duration of single syllables from postnatal day 3 to 21 in ED pups were significantly lower than those in PC pups. ED increased maternal licking, grooming behavior, and nest-building, but decreased huddling and exploring behavior. Similarly, ED increased paternal behavior, like nest-building, but reduced exploring and self-grooming. Overall, gerbil pups displayed high levels of USVs and various syllable types before weaning. ED significantly reduced the number of USVs and syllable types, but increased maternal licking, grooming, and paternal nest-building behavior.

Effects of the phencyclidine model of schizophrenia and nicotine on total and categorized ultrasonic vocalizations in rats

Patients with schizophrenia smoke cigarettes at a higher rate than the general population. We hypothesized that a factor in this comorbidity is sensitivity to the reinforcing and reinforcement-enhancement effects of nicotine. Phencyclidine (PCP) was used to model behavioral changes resembling negative symptoms of schizophrenia in rats. Ultrasonic vocalizations (USVs) in rats have been used to measure emotional states, with 50 kHz USVs indicating positive states and 22 kHz USVs indicating negative states. Total and categorized numbers of 22 and 50 kHz USVs and USVs during a visual stimulus (e.g. a potential measure of reinforcement-enhancement) were examined in rats following injection of PCP (2.0 mg/kg) and/or nicotine (0.2 or 0.4 mg/kg) daily for 7 days. PCP was then discontinued and all rats received nicotine (0.2 and 0.4 mg/kg) and PCP (2.0 mg/kg) on three challenge days. PCP acutely decreased 50 kHz vocalizations, whereas repeated nicotine potentiated rates of vocalizations, with similar patterns during light presentations. Rats in the PCP and nicotine combination groups made more 50 kHz vocalizations compared with rats in the control groups on challenge days. We conclude that PCP may produce a reward deficit, which is shown by decreased 50 kHz USVs, and behaviors post-PCP exposure may best model the comorbidity between schizophrenia and nicotine.

Identification of multiple call categories within the rich repertoire of adult rat 50-kHz ultrasonic vocalizations: effects of amphetamine and social context

RATIONALE

50-kHz ultrasonic vocalizations (USVs) emitted by adult rats are heterogeneous; they occur over a wide frequency range, show varying degrees of frequency modulation, and appear to differ in their behavioral significance. However, they have not been extensively categorized.

OBJECTIVES

The main objective of this study was to identify subtypes of 50-kHz USVs emitted by adult rats and to determine how amphetamine (AMPH) or social testing condition affects their relative and absolute production rate and acoustic characteristics. A second objective was to determine the extent of individual differences in call rate, call subtype profile, and acoustic parameters (i.e., duration, bandwidth, and mean peak frequency).

METHODS

Adult male Long-Evans rats were administered systemic amphetamine (0.25-2 mg/kg, IP) and tested individually or with a cage mate for 20 min. Call categories were defined based on visual inspection of over 20,000 USV spectrograms. Surgical devocalization was performed on a subset of AMPH-tested rats in order to confirm the authenticity of call subtypes.

RESULTS

Fourteen categories of 50-kHz USVs were recognized. Call subtypes were differentially affected by social context, AMPH dose, and time within session. In contrast, the acoustic characteristics of call subtypes were notably stable. Marked and stable inter-individual differences occurred with respect to overall 50-kHz call rate, acoustic parameters, and call profile.

CONCLUSIONS

The present findings, obtained under saline and amphetamine test conditions, provide the first detailed classification of adult rat 50-kHz USVs. Consideration of 50-kHz USV subtypes may advance our understanding of inter-rat communication and affective state.