Dopamine's role in social modulation of infant isolation-induced vocalization: I. Reunion responses to the dam, but not littermates, are dopamine dependent

Corticotropin-Releasing Factor Receptor 1 in the Anterior Cingulate Cortex Mediates Maternal Absence-Induced Attenuation of Transport Response in Mouse Pups

A human infant initially shows non-selective sociality, and gradually develops selective attachment toward its caregiver, manifested as "separation anxiety." It was unclear whether such sophistication of attachment system occurs in non-human mammals. To seek a mouse model of separation anxiety, we utilized a primitive attachment behavior, the Transport Response, in that both human and mouse newborns immediately stop crying and stay immobile to cooperate with maternal carrying. We examined the mouse Transport Response in three social contexts: 30-min isolation in a novel environment, 30-min maternal absence experienced with littermates in the home cage, and the control home-cage condition with the mother and littermates. The pups after postnatal day (PND) 13 attenuated their Transport Response not only in complete isolation but also by maternal absence, and activated several brain areas including the periventricular nucleus of the hypothalamus, suggesting that 30-min maternal absence was perceived as a social stress by mouse pups after PND13. This attenuation of Transport Response by maternal absence was independent with plasma corticosterone, but was diminished by prior administration of a corticotropin-releasing factor receptor 1 (CRFR1) antagonist. Among 18 brain areas examined, only neurons in the anterior cingulate cortex (ACC) co-express c-fos mRNA and CRFR1 after maternal absence. Consistently, excitotoxic ACC lesions inhibited the maternal absence-induced attenuation of Transport Response. These data indicate that the expression of mouse Transport Response is influenced not only by social isolation but also by maternal absence even in their home cage with littermates after PND13, at least partly via CRF-CRFR1 signaling in the ACC.

Comparative studies of social buffering: A consideration of approaches, terminology, and pitfalls

KIYOKAWA, Y. and HENNESSY, M.B. Comparative studies of social buffering: A consideration of approaches, terminology, and pitfalls…NEUROSCI BIOBEHAV REV XXX-XXX, .- Over the past decades, there has been an increasing number of investigations of the impact of social variables on neural, endocrine, and immune outcomes. Among these are studies of "social buffering"-or the phenomenon by which affiliative social partners mitigate the response to stressors. Yet, as social buffering studies have become more commonplace, the variety of approaches taken, definitions employed, and divergent results obtained in different species can lead to confusion and miscommunication. The aim of the present paper, therefore, is to address terminology and approaches and to highlight potential pitfalls to the study of social buffering across nonhuman species. We review and categorize variables currently being employed in social buffering studies and provide an overview of responses measured, mediating sensory modalities and underlying mechanisms. It is our hope that the paper will be useful to those contemplating examination of social buffering in the context of their own research.

Social Odors: Alarm Pheromones and Social Buffering

In this chapter, I describe 2 types of olfactory communication in rats, which appear to arouse anxiety and relief, respectively. In alarm pheromonal communication, rats release 4-methylpentanal and hexanal from their perianal region when they are stressed. These molecules activate the anxiety circuit, including the bed nucleus of the stria terminalis, when 4-methylpentanal and hexanal are simultaneously detected by the vomeronasal system and the main olfactory system, respectively. Consequently, recipient rats show a variety of anxiety responses, depending on the threatening stimuli. In appeasing olfactory communication, non-stressed rats release an appeasing olfactory signal, which is detected by the main olfactory system of other rats. When detected, this olfactory signal suppresses activation of the basolateral complex of the amygdala and, as a result, ameliorates stress responses elicited by an auditory conditioned stimulus during social buffering phenomenon. Because social buffering appears to be based on affinity and attachment to accompanying animals, the appeasing olfactory signal may arouse relief in rats. A definition of social buffering is also proposed as we still have no set definition for the term social buffering yet.

Vocal coselection in rat pup ultrasonic vocalizations

Selective breeding and natural selection that select for one trait often bring along other correlated traits via coselection. Selective breeding for an infantile trait, high or low call rates of isolation‐induced ultrasonic vocalization of rat pups, also alters functions of some brain systems and emotional behaviors throughout life. We examined the effect of breeding for call rate on acoustic parameters that are of communicative significance. Selecting for higher call rate produced calls of significantly increased amplitude and bandwidth relative to a randomly bred line. Selecting for lower rate produced calls of decreased duration. These nonmorphological, functional trait changes demonstrate enhanced communicatory potential and energy expenditure for the High line and the opposite for the Low line. This demonstration of coselection in a communicatory system suggests an underlying heritable suite of linked acoustic vocalization characteristics that in noisy environments could enhance dam–pup communication and lead to selection of emotionality traits with beneficial responses to stress.

Effects of maternal deprivation and the duration of reunion time on rat pup ultrasonic vocalization responses to isolation: possible implications for human infant studies

In a paradigm that may serve as a translational model for maternal separation experiences of human infants in neonatal intensive care units, we examined how the duration of reunion with the dam influenced the phenomenon of maternal potentiation of ultrasonic vocalizations, in which isolated rat pups increase rates of vocalization following brief interactions with dams. We report that maternal potentiation in 12-13 day-old rats did not occur after reunions with their anesthetized dam that lasted longer than 15-min. However, after 18 hr maternal separation, isolated pups given reunions with their anesthetized dam increased vocalization rate even with reunions as long as 3 hr. Using a split-cage apparatus that prevented physical contact, the impact of 18 hr separations on maternal potentiation was partially offset by experiencing olfactory and/or auditory stimuli of the mother. These results suggest that maintaining partial maternal sensory exposure during prolonged maternal separation can reduce responses elicited by subsequent maternal separation.

Selective social buffering of behavioral and endocrine responses and Fos induction in the prelimbic cortex of infants exposed to a novel environment

In mammals, the presence of the mother can reduce or "buffer" stress responses of her young in threatening conditions. We compared the effect of the mother, a familiar littermate, and an unfamiliar adult male on three classes of response shown by guinea pig pups in a novel environment: short latency active behaviors, particularly vocalizing; slower developing passive behaviors that appear mediated by inflammatory mechanisms; and hypothalamic-pituitary-adrenal activity. We also examined Fos induction in the prelimbic cortex, a region hypothesized to mediate buffering effects. Only the mother significantly suppressed all classes of behavior. The greatest selectivity was observed for passive behavioral responses. Contrary to expectations, the adult male reduced plasma cortisol levels of pups as effectively as did the mother. The presence of the male also resulted in increased Fos induction in the prelimbic cortex and high levels of social interaction. Maternal buffering was not associated with prelimbic activity. These results confirm the ability of the mother to reduce active behavioral and HPA responses and suggest a specific maternal buffering effect on the later developing passive behavioral responses. The findings also demonstrate an unexpected ability of adult males to reduce HPA responses and raise the possibility that different social partners buffer HPA activity through different underlying processes.

Parental potentiation of vocalization as a marker for filial bonds in infant animals

Maternal and paternal potentiation of vocalization are two parts of a promising model of early life social bonds that has been and can be a useful tool in research. Most mammalian infants vocalize when isolated. Interactions with adult females just before isolation have been found to increase vocalizations in several species. Interactions with littermates and other social stimuli do not. In guinea pigs and pigs, the response is specific to the dam. In rats and octagon degus, an unrelated adult female from the colony is sufficient. The presence of an intact adult male in the test chamber with dam-reared pups evokes behavioral inhibition, a fear response. Previous exposure to the male in the home cage, biparental rearing, dramatically transforms the response of the pup. The pup treats the adult male as it does its dam, including potentiation of vocalization during a subsequent isolation. This article outlines the methods, advantages, and disadvantages of parental potentiation as a research tool, as well as a brief review of the evidence supporting its use as a marker for filial attachment. Future research directions are outlined.

Dopamine receptor D2 deficiency reduces mouse pup ultrasonic vocalizations and maternal responsiveness

Dopamine signalling facilitates motivated behaviours, and the D2 dopamine receptor (D2R) is important in mother-infant interactions. D2R antagonists disrupt maternal behaviour and, in isolated rat pups, reduce ultrasonic vocalizations (USVs) that promote maternal interaction. Here, we examined the effects of genetic D2R signalling deficiency on pup-dam interaction with Drd2 knockout (D2R KO) mice. Using heterozygous (HET) cross littermates, the effect of pup genotype on isolation-induced USVs was quantified. Independent of parental genotype, D2R-deficient pups emitted fewer USVs than wild type (WT) littermates in a gene dose-dependent manner. Using reciprocal D2R KO-WT crosses, we examined how parental genotype affects pup USVs. Heterozygous pups from D2R KO dams produced fewer USVs than HET pups from WT dams. Also, exposure to USV-emitting pups increased plasma prolactin levels in WT dams but not in D2R KO dams, and KO dams showed delayed pup retrieval and nest building. These findings indicate the importance of the interaction between pup and dam genotypes on behaviour and further support the role of D2R signalling in maternal care.

Requirement for the endocannabinoid system in social interaction impairment induced by coactivation of dopamine D1 and D2 receptors in the piriform cortex

The dopamine receptor family consists of D1-D5 receptors (D1R-D5R), and we explored the contributions of each dopamine receptor subtype in the piriform cortex (PirC) to social interaction impairment (SII). Rats received behavioral tests or electrophysiological recording of PirC neuronal activity after injection of the D1R/D5R agonist SKF38393, the D2R/D3R/D4R agonist quinpirole, or both, with or without pretreatment with dopamine receptor antagonists, D1R or D5R antisense oligonucleotides, the cannabinoid CB1 receptor antagonist AM281, or the endocannabinoid transporter inhibitor VDM11. Systemic injection of SKF38393 and quinpirole together, but not each one alone, induced SII and increased PirC firing rate, which were blocked by D1R or D2R antagonist. Intra-PirC microinfusion of SKF38393 and quinpirole together, but not each one alone, also induced SII, which was blocked by D1R antisense oligonucleotides or D2R antagonist but not by D3R or D4R antagonist or D5R antisense oligonucleotides. SII induced by intra-PirC SKF38393/quinpirole was blocked by AM281 and enhanced by VDM11, whereas neither AM281 nor VDM11 alone affected social interaction behavior. Coadministration of SKF38393 and quinpirole produced anxiolytic effects without significant effects on locomotor activity, olfaction, and acquisition of olfactory short-term memory. These findings suggest that SII induced by coactivation of PirC D1R and D2R requires the endocannabinoid system.

Rat pup social motivation: a critical component of early psychological development

Examining the role of the offspring in early social dynamics is especially difficult. Human developmental psychology has found infant behavior to be a vital part of the early environmental setting. In the rodent model, the different ways that a rodent neonate or pup can influence social dynamics are not well known. Typically, litters of neonates or pups offer complex social interactions dominated by behavior seemingly initiated and maintained by the primary caregiver (e.g., the dam). Despite this strong role for the caregiver, the young most likely influence the litter dynamics in many powerful ways including communication signals, discrimination abilities and early approach behavior. Nelson and Panksepp (1996) developed a preference task to examine early rodent pup social motivation. We have used the same task to examine how variations in maternal care or different environmental perturbations could alter the rat pup preferences for social-related stimuli. Rat pups receiving low levels of maternal licking and grooming were impaired in maternal odor cue learning and emitted lower levels of 22kHz ultrasounds compared to pups from the high licking and grooming cohort. Prenatal stress or early exposure to a toxicant (polychlorinated biphenyl) altered early social preferences in the rat pup in different ways indicating that diverse strategies are expressed and specific to the type of perturbation exposure. A greater focus on the offspring motivation following early 'stressors' will allow for more complete understanding of the dynamics in behavior during early social development.

Dopamine's role in social modulation of infant isolation-induced vocalization: I. Reunion responses to the dam, but not littermates, are dopamine dependent

Rat pups' vocalization during social separation and the cessation of vocalization upon social reunion (contact quieting) model early life affiliative relationships. The present study examined the roles of dopamine (DA) receptors in regulating contact quieting. Contact quieting to the dam, but not to littermates, was disrupted by either blockade or exogenous stimulation of DA D1-like receptors. The D2 antagonist raclopride also prevented the quieting effect of reunion with the dam and had a lesser effect on the quieting properties of littermates. In contrast, the D2 agonist quinpirole permitted or enhanced contact quieting. Combined systemic and local striatal administration of D2 ligands showed that stimulation of striatal D2 receptors can enhance, but is not necessary for, contact quieting to the dam. These results are consistent with the literature linking the neural mechanisms of affiliation and reinforcement. This is also the first demonstration that the neurochemical substrates of an infant comfort response to dams differ from a behaviorally similar response to siblings.

Dopamine's role in social modulation of infant isolation-induced vocalization: II. Maternally modulated infant separation responses are regulated by D1- and D2-family dopamine receptors

Mammalian infant behavior directed toward caregivers is critical to survival and may play a role in establishing social bonds. Most mammalian infants vocalize when isolated. Rat pups vocalize at a higher rate when isolated following an interaction with an adult female than after an interaction with littermates, a phenomenon termed maternal potentiation. We previously reported that the D2 receptor family agonist quinpirole disrupts maternal potentiation at a dose that does not alter vocalization rate following contact with littermates. Here we further examine the role of dopamine in maternal potentiation by testing effects of both D1 and D2 receptor family ligands, alone and in combination, on maternal potentiation. We tested the drugs' effects on isolation vocalization subsequent to littermate contact and then another isolation preceded by a brief "reunion" period of exposure either to the anesthetized dam or a handling-only "pickup" condition. D2 receptor stimulation blocked the increase in vocalizations following reunion with the dam. The D2 agonist effect in the dam-reunion condition was much larger than its small effect in the pickup condition, providing further evidence that D2 receptors exert a selective modulation of maternal potentiation. On the other hand, systemic administration of the D1 agonist SKF81297 reduced isolation vocalizations nonspecifically, across all the experimental conditions. Finally, the D1 and D2 receptor dual antagonist, alpha-flupenthixol, increased isolation vocalizations and disrupted potentiation, but at doses that also inhibited locomotion. We conclude that D2 receptor family activation has a more selective effect of disrupting maternal potentiation than D1 receptor family activation.