The effects of opioid and benzodiazepine antagonists on dam-induced reductions in rat pup isolation distress

The neurochemistry of social reward during development: What have we learned from rodent models?

Social rewards are fundamental to survival and overall health. Several studies suggest that adequate social stimuli during early life are critical for developing appropriate socioemotional and cognitive skills, whereas adverse social experiences negatively affect the proper development of brain and behavior, by increasing the susceptibility to develop neuropsychiatric conditions. Therefore, a better understanding of the neural mechanisms underlying social interactions, and their rewarding components in particular, is an important challenge of current neuroscience research. In this context, preclinical research has a crucial role: Animal models allow to investigate the neurobiological aspects of social reward in order to shed light on possible neurochemical alterations causing aberrant social reward processing in neuropsychiatric diseases, and they allow to test the validity and safety of innovative therapeutic strategies. Here, we discuss preclinical research that has investigated the rewarding properties of two forms of social interaction that occur in different phases of the lifespan of mammals, that is, mother-infant interaction and social interactions with peers, by focusing on the main neurotransmitter systems mediating their rewarding components. Together, the research performed so far helped to elucidate the mechanisms of social reward and its psychobiological components throughout development, thus increasing our understanding of the neurobiological substrates sustaining social functioning in health conditions and social dysfunction in major psychiatric disorders.

Analysis of ultrasonic vocalizations emitted by infant rodents

Altricial rodent pups emit ultrasonic vocalizations (USVs), which are whistle-like sounds with frequencies between 30 and 90 kHz. These signals play an important communicative role in mother-offspring interaction because they elicit in the dam a prompt response as concerning care-giving behaviors. To investigate neurobehavioral development, the analysis of the number of USVs presents several advantages: (1) USVs are one of the few responses produced by very young rodents that can be quantitatively analyzed and elicited by quantifiable stimuli; (2) USV emission follows a clear ontogenetic profile from birth to the second to third week of life, thus allowing longitudinal analysis during very early post-natal ontogeny. The reported role played by several receptor agonists and antagonists in modulating the USV rate makes this measure highly informative in investigating the effects of toxicants and, more generally, psychoactive compounds on the development of selected brain systems.

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.

Difluoromethylornithine (DFMO) reduces deficits in isolation-induced ultrasonic vocalizations and balance following neonatal ethanol exposure in rats

Neonatal ethanol (EtOH) exposure is associated with central nervous system dysfunction and neurotoxicity in rats. Increases in polyamine levels have been implicated as one underlying mechanism for some of EtOH's effects on the developing brain. In this study we addressed whether the inhibition of polyamine biosynthesis by alpha-difluoromethylornithine (DFMO) could reduce behavioral deficits induced by early EtOH exposure. Male and female rat pups received ethanol (6 g/kg/day EtOH i.g.), or isocaloric maltose (control) from postnatal days (PND) 1-8. On PND 8, animals were injected with either saline or DFMO (500 mg/kg, s.c.) immediately following the final neonatal treatment. Subjects were tested for isolation-induced ultrasonic vocalizations (USV) on PND 16; spontaneous activity in an open field apparatus on PND 20 and 21; and balance on PND 31. Animals exposed to EtOH neonatally displayed an increased latency to the first USV and reduced frequencies of USV, hyperactivity and preference for the center of the open field and poorer balance relative to controls. DFMO minimized these deficits in latency to the first USV and balance. These data provide further support that polyamines play a role in some of the functional deficits associated with EtOH exposure during early development and that reducing polyamine activity can improve outcome.

Acquisition and expression of a socially mediated separation response

Separation and reunion responses have been used to investigate social relationships in many species, including humans. When isolated from their mothers and siblings, infant rats vocalize in the ultrasonic range. An isolated pup reduces its rate of vocalization when placed in contact with familiar stimuli, particularly social ones such as its dam or littermates. The isolated pup's vocalization is greatly increased if the pup has been in contact with its mother immediately before isolation, a phenomenon called maternal potentiation. Early experience can play a role in the acquisition of potentiation. If rat pups are reared by both dam and sire, or even reared by the dam in the presence of the sire's odor, the pups show potentiation to the sire instead of the fear-related behavioral inhibition. Littermates, home cage shavings, and other familiar stimuli from the rearing environment do not elicit increased vocalizations during a subsequent isolation. The neurobiological mechanisms by which the sire becomes capable of potentiating vocalization are unknown, but are hypothesized to depend on the processes underlying development of an odor preference. Expression of potentiation is hypothesized to be related to reward processes in part because dopamine activity plays a regulatory role. Activation of dopamine type 2 receptors in the nucleus accumbens blocks maternal potentiation without altering vocalization rate in an initial isolation. The modulation of isolation-induced vocalization by social interactions provides a paradigm for investigating the neurobehavioral mechanisms underlying acquisition and expression of early life social bonds.

Social buffering: relief from stress and anxiety

Communication is essential to members of a society not only for the expression of personal information, but also for the protection from environmental threats. Highly social mammals have a distinct characteristic: when conspecific animals are together, they show a better recovery from experiences of distress. This phenomenon, termed 'social buffering', has been found in rodents, birds, non-human primates and also in humans. This paper reviews classical findings on social buffering and focuses, in particular, on social buffering effects in relation to neuroendocrine stress responses. The social cues that transmit social buffering signals, the neural mechanisms of social buffering and a partner's efficacy with respect to social buffering are also detailed. Social contact appears to have a very positive influence on the psychological and the physiological aspects of social animals, including human beings. Research leading towards further understanding of the mechanisms of social buffering could provide alternative medical treatments based on the natural, individual characteristics of social animals, which could improve the quality of life.

Developmental changes in the behavioral and autonomic effects of kappa opioid receptor stimulation of the midbrain periaqueductal gray

Kappa opioid receptors stimulation with U50,488 is known to modulate behaviors during the early postnatal period, but the specific neuroanatomical locus of many of these effects is unexplored. In the present study, we infused U50,488 into the midbrain periaqueductal gray (PAG) and investigated the effects of this drug on behavior and heart rate of 1-, 2-, and 3-week-old rats. U50,488 increased activity most potently in 1- and 2-week-old subjects. Ultrasonic vocalization (USV) production was increased in 1-week-old subjects, but not in 2- or 3-week-old pups. Heart rate changes were similarly seen in younger aged subjects. At 1 week, U50,488 decreased heart rate, but at 2 weeks it increased heart rate. There was no effect of this drug on heart rate at 3 weeks. At 1 week, USVs were more potently elicited from dorsal than lateral PAG infusion sites. No other site-specific effects within the PAG were seen. The age-related decline in behavioral effects elicited by U50,488 is consistent with other published reports, and to the extent that kappa receptor activity mediates infant separation responses, implicates the PAG as a modulator of those responses.

Establishment of a preference by the newborn lamb for its mother: the role of opioids

Mother-young relationships in sheep are characterized by individual recognition and a rapidly developing exclusive bond. The authors examined the role of opioids in establishment of the lamb's preference for its mother. Newborn lambs received the opioid receptor antagonist naltrexone (0.0, 1.5, or 3.0 mg/kg i.p.), and lambs were tested at 24 hr and 48 hr of age. At 24 hr, controls spent significantly more time near the mother than near an alien ewe; no significant difference was obtained for the naltrexone-treated groups. The effect of naltrexone persisted at 48 hr. No other significant behavioral difference was observed. Results support the hypothesis that opioids mediate the establishment of mother preference and the view that positive affect associated with social attachment and maternal care may be modulated by opioids.

Emotion regulation and touch in infants: the role of cholecystokinin and opioids

Behavioral-pharmacological research in infant rats supports the role of cholecystokinin (CCK) and opioid peptides in mediating early learning of new associations with aspects of the nest and dam, such as maternal odor, milk, and contact. The current paper reviews research that examines the hypothesis that these neuropeptide systems are further involved in mediating emotion regulation in infants, thus playing a role in the emergence of stress-reactivity and other motivational systems. The beneficial effects of maternal proximity, handling, and touch on the development of emotion regulation have been demonstrated in both human and animal models. Interventions that promote tactile stimulation of the infant ("touch therapy") and infant-mother contact ("skin-to-skin contact" or "kangaroo care") have been shown to improve the infant's ability to self-regulate, and to moderate the effects of some risk factors. Theoretical perspectives and empirical findings regarding emotion regulation in infants are first discussed. This is followed by a review of work providing evidence in animal models (and suggestive evidence in humans) for the importance of CCK and opioid neuropeptides in affecting infant emotion regulation and the impact of touch-based interventions, in particular in the context of infant-mother attraction, contact, separation, and attachment.

Social, thermal, and temporal influences on isolation-induced and maternally potentiated ultrasonic vocalizations of rat pups

Sensory and temporal factors have been demonstrated to be involved in the regulation of isolation-induced ultrasonic vocalizations (USV) of young rats. Sensory cues include thermal, olfactory, and tactile modalities. Temporal factors include the time spent in isolation. The goal of the present research was to examine the interaction of these factors in both isolation-induced and maternally potentiated USV. Maternal potentiation of USV occurs when a brief interaction with the dam, even a passive (anesthetized) dam, elicits an augmented vocal response to a subsequent isolation, with rates of USV in rat pups well above those emitted in standard isolation tests. We found that passive maternal potentiation of USV did occur under all conditions tested. Neither a 30-min prior isolation nor high ambient temperature prevented an increase in USV rate over the rate of the original isolation. After 30-min isolation at warm temperatures when the rate of USV had fallen to zero, the pups increased vocalization in the presence of the dam as well as in the subsequent isolation. Temporal and thermal factors also interacted significantly in regulating the level of the USV emitted by the pups during the first isolation, in the presence of the anesthetized dam, and during the second isolation.

Ultrasonic vocalisation emitted by infant rodents: a tool for assessment of neurobehavioural development

Ultrasonic vocalisations (USVs) emitted by altricial rodent pups are whistle-like sounds with frequencies between 30 and 90 kHz. These signals play an important communicative role in mother-offspring interaction since they elicit in the dam a prompt response concerning caregiving behaviours. Both physical and social parameters modulate the USV emission in the infant rodent. Recently, a more detailed analysis of the ultrasonic vocalisation pattern, considering the spectrographic structure of sounds has allowed a deeper investigation of this behaviour. In order to investigate neurobehavioural development, the analysis of USVs presents several advantages, mainly: (i) USVs are one of the few responses produced by very young mice that can be quantitatively analysed and elicited by quantifiable stimuli; (ii) USV production follows a clear ontogenetic profile from birth to PND 14-15, thus allowing longitudinal neurobehavioural analysis during very early postnatal ontogeny. The study of this ethologically-ecologically relevant behaviour represent a valid model to evaluate possible alterations in the neurobehavioural development of perinatally treated or genetically modified infant rodents. Furthermore, the role played by several receptor agonists and antagonists in modulating USV rate makes this measure particularly important when investigating the effects of anxiogenic and anxiolytic compounds, and emotional behaviour in general.

Brain substrates of infant-mother attachment: contributions of opioids, oxytocin, and norepinephrine

The aim of this paper is to review recent work concerning the psychobiological substrates of social bonding, focusing on the literature attributed to opioids, oxytocin and norepinephrine in rats. Existing evidence and thinking about the biological foundations of attachment in young mammalian species and the neurobiology of several other affiliative behaviors including maternal behavior, sexual behavior and social memory is reviewed. We postulate the existence of social motivation circuitry which is common to all mammals and consistent across development. Oxytocin, vasopressin, endogenous opioids and catecholamines appear to participate in a wide variety of affiliative behaviors and are likely to be important components in this circuitry. It is proposed that these same neurochemical and neuroanatomical patterns will emerge as key substrates in the neurobiology of infant attachments to their caregivers.

A CCK(A)-receptor antagonist administered to the neonate alters mother-infant interactions in the rat

The importance of the infant's cholecystokinin (CCK) system for eliciting optimal maternal care was examined in 6-9-day-old Sprague-Dawley rats. After administration of either vehicle, CCK-8 (1 or 8 microg/kg) or devazepide (1 mg/kg; a selective CCK(A) receptor antagonist), pups were either individually isolated (Experiment 1) or individually reunited with their dam (Experiment 2) and the rats' behavior was observed. When isolated, pups that received devazepide displayed significantly more head-lifting and wall-climbing attempts than vehicle-treated controls, suggesting that endogenous CCK dampens activity. Devazepide-treated rats were found more frequently in proximity with their mothers when reunited with them, and they emitted more ultrasonic vocalizations compared to vehicle controls. Pups treated with 1 microg/kg CCK received less body licking than vehicle controls. In addition, dams hovered and crouched over devazepide-treated pups more than over pups treated with 1 microg/kg CCK. The results suggest that endogenous CCK has a calming, quieting effect in the neonatal pup and that this, in turn, results in less infant-mother attractivity and reduced levels of maternal care.

The effects of melatonin on isolation distress in chickens

Melatonin (0.1-1.0 mg/kg) reduced isolation-induced distress vocalizations (DVs) in young domestic chickens in a dose-dependent manner. This effect was unaffected by the administration of d-amphetamine (1.0 mg/kg) suggesting that melatonin's effects were not merely due to fatigue. The melatonin reduction in DVs was not naloxone reversible, indicating an action independent of the endogenous opioid system. However, chronic pretreatment with naltrexone facilitated the melatonin effect, suggesting a complex relationship between melatonin and the endogenous opioids in regulating distress vocalizations. Chickens exhibited a marked reduction in DVs when isolation chambers were darkened, suggesting endogenous, as well as exogenous, melatonin mediation of isolation distress; however; pinealectomy only partially reversed the darkness effect. Pinealectomized animals, like control animals, exhibited a reduction in DVs following melatonin treatment; however, the melatonin effect was shorter lasting. The implications that these results may have for socialization and emotional distress are discussed.

Behavioral and hormonal responses to stress in the newborn mouse: effects of maternal deprivation and chlordiazepoxide

These studies investigated behavioral and hormonal responses to stress in developing mice. Experiment 1 examined the effects of 24-hr maternal deprivation on corticosterone (CORT) secretion and ultrasonic vocalization (UVZ) rate in 4-, 8-, and 12-day-old mice. At these ages, exposure to a novel environment resulted in minimal changes in CORT secretion. Maternal deprivation increased pups' CORT secretion in an age-dependent fashion but did not affect their UVZ rate. The aim of experiment 2 was to test the effects of chlordiazepoxide (CDP), an anxiolytic compound, on CORT secretion and UVZ in both normally reared and in maternally deprived 8-day-old mice. CDP administration elevated CORT secretion in a dose-dependent fashion, producing larger CORT increases in deprived (DEP) animals. CDP affected UVZ only in nondeprived (NDEP) animals: UVZ rate was decreased by high CDP doses. Overall, these findings demonstrate that the infant mouse shows a period of stress hyporesponsiveness similar to the rat and that maternal presence contributes to inhibit adrenocortical activity. CDP administration, but not novelty exposure, increased CORT secretion in 8-day-old normally reared mice suggesting that during the stress hyporesponsive period, the HPA axis is capable of responding only to specific stimuli. Changes in HPA axis activity and UVZ rate resulting from maternal deprivation and/or CDP challenge do not seem to be directly related.

U50,488 increases ultrasonic vocalizations in 3-, 10-, and 18-day-old rat pups in isolation and the home cage

Separation-induced calling in the young of many species can be modulated by the opioid system. Morphine reduces ultrasonic vocalizations (USVs) produced by isolated rat pups, an effect blocked by naltrexone. Central administration of the mu and delta opiate receptor agonists DAMGO and DPDPE reduce USV; kappa receptor agonist U50,488 increases them. We now find that peripheral U50,488 not only boosted calling rates in isolated 3-, 10-, and 18-day-old rat pups, but also induced calling in pups of these ages tested in the home cage with their littermates, where USVs are seldom heard in nature. U50,488 lowered rectal temperature, although temperature loss and USV were not correlated within drug treatment groups.

Prenatal cocaine exposure increases the behavioral sensitivity of neonatal rat pups to ligands active at opiate receptors

Offspring of dams given 40 mg/kg cocaine HCl (C40) from gestational day 8-20 (E8-E20), pair-fed dams injected daily with saline (PF), nutritional control dams placed on a 40% cellulose based diet and injected with saline daily (NC), and untreated dams (LC) were examined. Offspring were given morphine (0.0, 0.1, or 0.5 mg/kg SC) on postnatal day 10-11 (P10-11) in Experiment 1, and isolation-induced ultrasonic vocalizations were measured. Planned comparisons indicated that both C40 and NC offspring exhibited a greater sensitivity to the morphine-related decrease in isolation-induced ultrasounds than LC controls. However, the presence of an anesthetized littermate suppressed isolation-induced ultrasounds equally across all groups, with all groups of offspring spending equal amounts of time in physical contact with the littermate. A tail-flick measure of analgesia indicated that PF animals were hyperalgesic relative to the other prenatal treatment groups; however, no differences in sensitivity to morphine were seen across the prenatal groups. In Experiment 2, animals were given the selective delta, [D-Pen2,D-Pen5]-enkephalin (DPDPE), and mu, [D-Ala2-NMe-Phe4Gly ol]-enkephalin (DAMGO) agonists ICV and ultrasonic vocalizations were recorded. Results indicated that both C40 and NC offspring were more sensitive to the low dose of DAMGO; however, because of the profound suppression of vocalizations seen at both doses of DPDPE, potential differences among the prenatal treatment groups in responsiveness to the delta agonist were difficult to detect.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of CRF on isolated Japanese quails selected for fearfulness and for sociality

The effects of CRF on the behavior and plasma corticosterone levels of isolated Japanese quail chicks were determined. Doses of 15, 30, and 45 micrograms/kg of CRF reduced the number of distress vocalizations emitted during isolation in chicks of a control line. The behavioral responses to a 15 micrograms/kg dose of CRF in lines selected either for high (HSR) or low (LSR) level of sociality or for low (STI) or high (LTI) fearfulness were compared. CRF suppressed vocalizations and increased the number of pecks at food. In LTI and STI chicks, CRF decreased the number of distress calls. In HSR chicks, CRF increased the number of pecks at food and suppressed vocalizations. No effect of CRF was found in LSR chicks. Chicks of the HSR, LSR, LTI, STI, and control lines were injected with 15 micrograms/kg of CRF or saline solution and the level of corticosterone measured following a 15-min isolation period. CRF or isolation alone slightly activated the HPA axis, whereas CRF plus isolation greatly increased corticosterone levels except in the LSR line. These differential responses of LSR chicks and chicks of the other lines, indicate a possible role of CRF in the regulation of social reinstatement behavior.

Endogenous opiates: 1990

This paper, an examination of works published during 1990, is thirteenth in a series of our annual reviews of the research involving the behavioral, nonanalgesic, effects of the endogenous opiate peptides. The specific topics this year include stress; tolerance and dependence, eating; drinking; gastrointestinal, renal, and hepatic functions; mental illness; learning, memory, and reward; cardiovascular responses; respiration and thermoregulation; seizures and other neurological disorders; electrical-related activity; locomotor activity; sex, pregnancy, development, and aging; immunological responses; and other behavior.

Differential effects of specific opioid receptor agonists on rat pup isolation calls

The possibility was investigated that specific opioid receptor types might selectively alter the production of isolation-induced ultrasonic vocalizations. Intracisternal injections of mu, delta and kappa opioid receptor agonists were administered to isolated 10-day-old rat pups. The mu receptor agonist [D-Ala2-NMe-Phe4-Gly-ol]-enkephalin (DAMGO) and delta receptor agonist [D-Pen2, D-Pen5]-enkephalin (DPDPE) both reduced the rate of isolation-induced ultrasonic calling in the absence of sedation. The kappa receptor agonist U50,488 had the opposite effect, significantly raising the rate of vocalization. Fourteen-day-old pups, with a larger delta receptor population, showed a greater sensitivity to DPDPE than was seen in the younger animals.

Isolation-induced vocalization in Wistar rat pups is not increased by naltrexone

Rat pups, when removed from dam and littermates and isolated in an unfamiliar milieu, emit a characteristic ultrasonic vocalization that can be quieted by the administration of a nonsedating dose of morphine. Wistar pups, aged 7, 10, 12, 14, or 16 days, were tested after receiving intraperitoneal injections of the opiate antagonist naltrexone (0.0, 0.5, 1.0, or 5.0 mg/kg). The rate of isolation-induced ultrasonic vocalizations was unaffected by naltrexone at any dose, and there were no significant naltrexone-related changes on other behavioral measures. The complexity of the opioid system is discussed, as it may be involved in the vocal reaction to isolation.