Effects of denial of reward through maternal contact in the neonatal period on adult hypothalamic-pituitary-adrenal axis function in the rat

Maternal neglect alters reward-anticipatory behavior, social status stability, and reward circuit activation in adult male rats

Introduction

Adverse early life experiences affect neuronal growth and maturation of reward circuits that modify behavior under reward predicting conditions. Previous studies demonstrate that rats undergoing denial of expected reward in the form of maternal contact (DER-animal model of maternal neglect) during early post-natal life developed anhedonia, aggressive play-fight behaviors and aberrant prefrontal cortex structure and neurochemistry. Although many studies revealed social deficiency following early-life stress most reports focus on individual animal tasks. Thus, attention needs to be given on the social effects during group tasks in animals afflicted by early life adversity.

Methods

To investigate the potential impact of the DER experience on the manifestation of behavioral responses induced by natural rewards, we evaluated: 1) naïve adult male sexual preference and performance, and 2) anticipatory behavior during a group 2-phase food anticipation learning task composed of a context-dependent and a cue-dependent learning period.

Results

DER rats efficiently spent time in the vicinity of and initiated sexual intercourse with receptive females suggesting an intact sexual reward motivation and consummation. Interestingly, during the context-dependent phase of food anticipation training DER rats displayed a modified exploratory activity and lower overall reward-context association. Moreover, during the cue-dependent phase DER rats displayed a mild deficit in context-reward association while increased cue-dependent locomotion. Additionally, DER rats displayed unstable food access priority following food presentation. These abnormal behaviours were accompanied by overactivation of the ventral prefrontal cortex and nucleus accumbens, as assessed by pCREB levels.

Conclusions/discussion

Collectively, these data show that the neonatal DER experience resulted in adulthood in altered activation of the reward circuitry, interfered with the normal formation of context-reward associations, and disrupted normal reward access hierarchy formation. These findings provide additional evidence to the deleterious effects of early life adversity on reward system, social hierarchy formation, and brain function.

Effects of neonatal rearing by different types of foster mother on the distribution of corticotropin-releasing factor neurons in the central amygdaloid nucleus in rats

The mother-child relationship of newborns plays an essential role in the development of the central nervous system, and an inadequate relationship, such as mother-child separation, can cause deficits of mental function in adulthood. However, insufficient research has examined the effects of foster mothers. We assigned some neonatal rats to one of two foster mothers: one that was lactating and feeding her first litter (FL group) and one that had one previous experience of childbirth and feeding but no current litter (FE group). Other pups were raised by their own mother (OM group) or subjected to maternal separation (MS group). Pups were placed with the foster mother (FL and FE groups) or separated from their mother (MS group) for 3 h/day on postnatal days 1-20. At age 6 weeks, each group was divided into two subgroups, one with 30 min of acute restraint stress loading (FL-R, FE-R, OM-R, and MS-R) and one without it (FL, FE, OM, and MS). Then, we compared the density of corticotropin-releasing factor-immunoreactive (CRF-ir) neurons in the central amygdaloid nucleus (CeA). The density of CRF-ir neurons in the CeA was significantly lower in the FL-R and MS-R subgroups than in the FL and MS subgroups, respectively. The results suggest that differences in care received during the neonatal period affect maturation of CRF neurons in the CeA and may have negative effects on the synthesis and release of CRF.

Social touch during development: Long-term effects on brain and behavior

In this paper, our goal is to explore what is known about the role of social touch during development. We first address the neural substrates of social touch and the role of tactile experience in neural development. We discuss natural variation in early exposure to social touch, followed by a discussion on experimental manipulations of social touch during development and "natural experiments", such as early institutionalization. We then consider the role of other developmental and experiential variables that predict social touch in adults. Throughout, we propose and consider new theoretical models of the role of social touch during development on later behavior and neurobiology.

Effects of a Neonatal Experience Involving Reward Through Maternal Contact on the Noradrenergic System of the Rat Prefrontal Cortex

The noradrenergic system plays an important role in prefrontal cortex (PFC) function. Since early life experiences play a crucial role in programming brain function, we investigated the effects of a neonatal experience involving reward through maternal contact on the noradrenergic system of the rat PFC. Rat pups were exposed during Postnatal days (PNDs) 10-13, to a T-maze in which contact with the mother was used as a reward (RER). RER males had higher norepinephrine levels in the PFC both on PND 13 and in adulthood. The RER experience resulted in adulthood in increased levels of the active demethylase GADD45b, hypomethylation of the β1 adrenergic receptor (ADRB1) gene promoter, and consequent enhanced expression of its mRNA in the PFC. In addition, protein and binding levels of the ADRB1, as well as those of its downstream effector phosphorylated cAMP response element-binding protein were elevated in RER males. The higher activity of the PFC noradrenergic system of the RER males was reflected in their superior performance in the olfactory discrimination and the contextual fear extinction, 2 PFC noradrenergic system-dependent behavioral tasks.

A novel model of early experiences involving neonatal learning of a T-maze using maternal contact as a reward or its denial as an event of mild emotional adversity

We developed a novel animal model of early life experiences in which rat pups are trained during postnatal days (PND) 10-13 in a T-maze with maternal contact as a reward (RER group) or its denial (DER group) as a mildly aversive event. Both groups of animals learn the T-maze, albeit the RER do so more efficiently. Training results in activation of the basal ganglia in the RER and of the hippocampus and prefrontal cortex in the DER. Moreover, on PND10 DER training leads to increased corticosterone levels and activation of the amygdala. In adulthood, male DER animals show better mnemonic abilities in the Morris water maze while the RER exhibit enhanced fear memory. Furthermore, DER animals have a hypofunctioning serotonergic system and express depressive-like behavior and increased aggression. However, they have increased hippocampal glucocorticoid receptors, indicative of efficient hypothalamic-pituitary-adrenal axis function, and an adaptive pattern of stress-induced corticosterone response. The DER experience with its relatively negative emotional valence results in a complex behavioral phenotype, which cannot be considered simply as adaptive or maladaptive.

Effects of an Early Experience Involving Training in a T-Maze Under either Denial or Receipt of Expected Reward through Maternal Contact

The mother is the most salient stimulus for the developing pups and a number of early experience models employ manipulation of the mother-infant interaction. We have developed a new model which in addition to changes in maternal behavior includes a learning component on the part of the pups. More specifically, pups were trained in a T-maze and either received (RER rats) or were denied (DER) the reward of maternal contact, during postnatal days 10-13. Pups of both experimental groups learn the T-maze, but the RER do so more efficiently utilizing a procedural-type of learning and memory with activation of the dorsal basal ganglia. On the other hand, the DER experience leads to activation of the hippocampus, prefrontal cortex, and amygdala in the pups. In adulthood, male DER animals exhibit better mnemonic abilities in the Morris water maze and higher activation of the hippocampus, while they have decreased brain serotonergic activity, exhibit a depressive-like phenotype and proactive aggressive behavior in the resident-intruder test. While male RER animals assume a reactive coping style in this test, and showed increased freezing during both contextual and cued memory recall following fear conditioning.