Selective breeding for infant vocal response: a role for postnatal maternal effects?

Variations in maternal behavior in rats selected for infant ultrasonic vocalization in isolation

Individual differences in maternal behavior in rodents are associated with altered physiology and behavior in offspring across their lifespan and across generations. Offspring of rat dams that engage in high frequencies of high-arched-back nursing and pup-licking (High-LG) show attenuated stress responses compared to those engaging in lower frequencies (Low-LG). Selective breeding also produces widespread alterations in physiology and behavior that are stable over generations. To examine processes underlying generational and developmental influences on anxiety in an animal model, we developed two lines of rats that emit either extremely high (High-USV) or low (Low-USV) rates of 45kHz ultrasonic vocalizations in isolation at postnatal day 10. Compared to the Low-USV line, High-USV rats display increased indices of anxiety- and depression-like behavior in adulthood. The current study assessed maternal behaviors as well as oxytocin and vasopressin receptor density in High-USV and Low-USV dams to determine if selective breeding had produced differences that paralleled those found in Low- and High-LG dams. We found that Low-USV dams engage in more high-arched nursing and pup-licking than High-USV dams. Differences in oxytocin and vasopressin receptor levels were not widespread throughout the brain, with line differences in the piriform cortex and nucleus accumbens. This research illustrates the potential interplay between genetically determined (USV line) and environmental (postnatal mother-infant interactions) factors in accounting for the phenotypes associated with maternal separation induced postnatal vocalizations.

Reduction in ultrasonic vocalizations in pups born to rapid eye movement sleep restricted mothers in rat model

The effects of rapid eye movement sleep restriction (REMSR) in rats during late pregnancy were studied on the ultrasonic vocalizations (USVs) made by the pups. USVs are distress calls inaudible to human ears. Rapid eye movement (REM) sleep was restricted in one group of pregnant rats for 22 hours, starting from gestational day 14 to 20, using standard single platform method. The USVs of male pups were recorded after a brief isolation from their mother for two minutes on alternate post-natal days, from day one till weaning. The USVs were recorded using microphones and were analysed qualitatively and quantitatively using SASPro software. Control pups produced maximum vocalization on post-natal days 9 to 11. In comparison, the pups born to REMSR mothers showed not only a reduction in vocalization but also a delay in peak call making days. The experimental group showed variations in the types and characteristics of call types, and alteration in temporal profile. The blunting of distress call making response in these pups indicates that maternal sleep plays a role in regulating the neural development involved in vocalizations and possibly in shaping the emotional behaviour in neonates. It is suggested that the reduced ultrasonic vocalizations can be utilized as a reliable early marker for affective state in rat pups. Such impaired vocalization responses could provide an important lead in understanding mother-child bonding for an optimal cognitive development during post-partum life. This is the first report showing a potential link between maternal REM sleep deprivation and the vocalization in neonates and infants.

Selective breeding for infant rat separation-induced ultrasonic vocalizations: developmental precursors of passive and active coping styles

Human depression and anxiety disorders show inherited biases across generations, as do antisocial disorders characterized by aggression. Each condition is preceded in children by behavioral inhibition or aggressive behavior, respectively, and both are characterized by separation anxiety disorders. In affected families, adults and children exhibit different forms of altered autonomic nervous system regulation and hypothalamic-pituitary-adrenal activity in response to stress. Because it is difficult to determine mechanisms accounting for these associations, animal studies are useful for studying the fundamental relationships between biological and behavioral traits. Pharmacologic and behavioral studies suggest that infant rat ultrasonic vocalizations (USV) are a measure of an early anxiety-like state related to separation anxiety. However, it was not known whether or not early ultrasound emissions in infant rats are markers for genetic risk for anxiety states later in life. To address these questions, we selectively bred two lines of rats based on high and low rates of USV to isolation at postnatal (P) 10 days of age. To our knowledge, ours is the only laboratory that has ever selectively bred on the basis of an infantile trait related to anxiety. The High and Low USV lines show two distinct sets of patterns of behavior, physiology and neurochemistry from infancy through adulthood. As adults High line rats demonstrate "anxious"/"depressed" phenotypes in behavior and autonomic nervous system (ANS) regulation to standard laboratory tests. In Lows, on the other hand, behavior and autonomic regulation are consistent with an "aggressive" phenotype. The High and Low USV lines are the first genetic animal models implicating long-term associations of contrasting "coping styles" with early attachment responses. They thus present a potentially powerful model for examining gene-environment interactions in the development of life-long affective regulation.

Selective breeding for an infant phenotype: rat pup ultrasonic vocalization (USV)

To examine processes underlying generational and developmental influences on anxiety, this laboratory produced two lines of (N:NIH strain) rats, selectively bred on the basis extreme rates of ultrasonic vocalization in 2 minutes of isolation at Postnatal Day 10. The research reviewed in this article focuses on: (1) establishment of the selectively bred lines; (2) defining infant behavioral and physiological phenotypes and (3) determining whether infantile USV phenotypes endure over development. The High and Low lines have diverged widely in their USV rates from each other and from the Random control line, which has maintained N:NIH strain rates overall from generation to generation. Beginning in the 11th generation, High USV pups have shown significantly higher frequencies of defecation and urination during isolation screening than the Low USV and random control line. Both lines show altered autonomic regulation of heart rates (HR) in response to stressors as juveniles and adults. These differences in HR responses in High and Low lines appear to be mediated by changes in the balance of sympathetic versus parasympathetic mechanisms. Other behavioral characteristics of the High line are consistent with an "anxious"/ "depressive" phenotype, such as vocalizations to touch in a novel environment, and performance in the Porsolt Swim, whereas Low line shows few differences in anxiety behavior. Future work will resolve the similarities and differences in the High and Low phenotypes and provide a developmental perspective to the growing body of information about affective regulation in humans and animals provided by selectively bred animal models.

Differences in affective behaviors and hippocampal allopregnanolone levels in adult rats of lines selectively bred for infantile vocalizations

Allopregnanolone, 3 alpha-hydroxy-5 alpha-pregnan-20-one (3 alpha,5 alpha-THP), a progesterone metabolite, is an endogenous neurosteroid mediating affective behaviors via its positive modulation of GABA(A) receptors. In order to better understand the role of this neurosteroid in individual differences in affective behavior, we used an animal model based on selective breeding for an infantile affective trait, ultrasonic vocalizations (USV). Adult male and female (in either proestrus or diestrus) rats that had been bred for low (low line) or high (high line) rates of USV after maternal separation were tested in a series of affective behavioral tests: open field, emergence, social interaction, defensive freezing, and the Porsolt forced swim task. Concentrations of allopregnanolone in combined hippocampus and amygdala tissue were then measured. low line subjects showed significantly lower anxiety and depression responses in the emergence, open field, and Porsolt forced swim tasks than did high line subjects. Proestrus females exhibited less affective behaviors than diestrus females or males. Allopregnanolone levels in hippocampus/amygdala were significantly higher in low line subjects compared to high line subjects, and in proestrus females compared to diestrus females and males. These data indicate that: (1) affective behaviors in lines selectively bred for an infantile anxiety trait exhibit selection persistence into adulthood; and (2) levels of allopregnanolone in the limbic system parallel selected disparities in affective behavior, suggesting a selection for alterations in the neurosteroid/GABA(A) receptor system in these lines.

Repeated neonatal pain influences maternal behavior, but not stress responsiveness in rat offspring

Early preterm neonates in the Neonatal Intensive Care Unit (NICU) are subjected to repeated painful procedures which could sensitize their responses to pain and potentiate neuroendocrine and behavioral responses to subsequent stressors in the long-term. In this study, we used the model of the neonatal rat to test the effects of repeated pain during the first 2 weeks of life on neuroendocrine responses (CRF, ACTH and corticosterone) to stressors varying in intensity and on maternal behavior in the postnatal period. To closely mimic the type of repeated painful stimulus experienced by preterm neonates (i.e., heelstick), neonatal rats aged day 2-14 were submitted daily to having their rear heels warmed to 34 degrees C and pricked (handled and pain, HP) or not (handled, H) with a needle. For the procedure, all pups were separated from their mothers for a total period of 15 min and reunited afterwards. Unhandled (UH) pups not subjected to daily maternal separation were used as controls. On days 6 and 12, litters from the HP and H groups were videotaped for 90 min upon return with the mother and maternal behavior was analyzed. Frequency of ultrasonic vocalizations (USV) were recorded during the procedure and upon return of pups with the mother. On day 15 and 20, rat pups from all groups were exposed to a 3-min ether vapor stressor or to an openfield for 10 min. Plasma ACTH and corticosterone concentrations were determined at 0, 5, 30, 60 and 120 min after stress onset. Our results show that repeated pain did not modify body weight of the pups, however, on day 6 of life, maternal pup grooming was increased significantly (P<0.05) in the HP group compared to the H group. Frequency of USV was not changed between H and HP rats either during the separation or after reunion with the mother. Plasma ACTH and corticosterone levels under basal or stimulated conditions were not different between UH, H and HP groups. However, the UH pups showed a tendency towards higher ACTH secretion after stress compared to H and HP groups. These results suggest that repeated pain during the first 2 weeks of life in the rat does not lead to significant changes in stress responsiveness in 2-week-old pups, but we suggest that changes in mother-pup interaction (increased grooming) might act as a buffer on the cumulative effect of pain on stress responsiveness.

Maternal behavior changes after immune challenge of neonates with developmental effects on adult social behavior

To examine whether maternal responsiveness during interactions with endotoxin-treated pups contributes to long-term effects on social development, neonatal mice were fostered on postnatal day 1 to dams from three selectively bred lines that differ in social behaviors. On day 5, neonates were administered saline or 0.5 mg/kg endotoxin (lipopolysaccharide, i.p.). Observations of undisturbed dams and litters on days 2, 4, 6, and 8 showed modest line differences in maternal behaviors. At the peak intensity of the transient illness induced by endotoxin (3 hr postinjection on day 5), dams increased licking and decreased time off-nest for endotoxin, but not saline-treated pups. As adults, fostered-reared males were observed in brief social interactions. Males exposed to endotoxin early in life showed changes in adult social behaviors that depended on foster dam line as well as individual differences in maternal responsiveness. Maternal responsiveness to stressed neonates can ameliorate the social-developmental effects of early illness.

Developmental effects of selective breeding for an infantile trait: the rat pup ultrasonic isolation call

This article describes how continued selection for divergent levels of the 10-day-old infant rat's ultrasonic vocal (USV) response to isolation affects the time course of development of that and other possible co-selected traits from 3 to 21 days postnatally. Since selective breeding for an infantile trait has not been reported before, we collected from colleagues a number of predicted outcomes that reflect the wide range of current opinion on the relationship between microevolutionary and developmental processes. After 15 generations of selective breeding, we found widely divergent USV responses between 10-day-old High USV line (300 USV/2 min) and low USV line (15 USV/2 min) pups. The developmental trajectory of USV responses at 3 and 7 days of age also was markedly altered in both these lines in comparison to the randomly bred controls, but was much less affected in 14-, 18-, or 21-day-old pups, contrary to all predictions. The development of other behavioral responses to isolation generally remained unaffected by the continued selection as did physical traits, measures of temperature regulation and classic developmental milestones. Only two traits showed evidence of co-selection: High line pups showed more urination/defecation in response to isolation from 10 days on, and more rapid ear canal opening at 10 days. These and other findings are presented and discussed in relation to the developmental and evolutionary concepts on which the different predictions were based.