Infantile handling and the facilitation of discrimination and reversal learning

A meta-analytic study of the effects of early maternal separation on cognitive flexibility in rodent offspring

Adverse early life experiences, such as maternal separation, are associated with an increased risk for several mental health problems. Symptoms induced by maternal separation that mirror clinically relevant aspects of mental problems, such as cognitive inflexibility, open the possibility of testing putative therapeutics prior to clinical development. Although several animal (e.g., rodent) studies have evaluated the effects of early maternal separation on cognitive flexibility, no consistent conclusions have been drawn. To clarify this issue, in this study, a meta-analysis method was used to systematically explore the relationship between early maternal separation and cognitive flexibility in rodent offspring. Results indicate that early maternal separation could significantly impair cognitive flexibility in rodent offspring. Moderator analyses further showed that the relationship between early maternal separation and cognitive flexibility was not consistent in any case, but was moderated by variations in the experimental procedures, such as the deprivation levels, task characteristics, and rodent strains. These clarify the inconsistent effects of maternal separation on cognitive flexibility in rodents and help us better understand the association between early life adversity and cognitive development.

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Meta-analysis method was used to discuss the inconsistent effects of maternal separation on cognitive flexibility in rodent.

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Maternal separation was found to necessarily impair the cognitive flexibility in rodent.

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Variations in the experimental procedures moderated the relationship between maternal separation and cognitive flexibility.

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Further studies on environment-cognition associations in rodents should take experimental procedural factors into account.

Effects of an early life experience on rat brain cannabinoid receptors in adolescence and adulthood

Neonatal handling is an experimental model of early life experience associated with resilience in later life challenges, altering the ability of animals to respond to stress. The endocannabinoid system of the brain modulates the neuroendocrine and behavioral effects of stress, while this system is also capable of being modulated by stress exposure itself. The present study has addressed the question of whether neonatal handling in rats could affect cannabinoid receptors, in an age- and sex-dependent manner, using in situ hybridization and receptor binding techniques. Different effects of neonatal handling were observed in adolescent and adult brain on CB1 receptor mRNA and [3H]CP55,940 binding levels, which in some cases were sexually dimorphic. Neonatal handling interfered in the developmental trajectories of CB1 receptor mRNA levels in striatum and amygdaloid nuclei, as well as of [3H]CP55,940 binding levels in almost all regions studied. Adult handled rats showed reduced [3H]CP55,940 binding levels in the prefrontal cortex, striatum, nucleus accumbens and basolateral amygdala, while binding levels in prefrontal cortex of adolescent handled rats were increased. Finally, handling resulted in decreases in female [3H]CP55,940 binding levels in the striatum, nucleus accumbens, CA3 and DG of dorsal hippocampus and basolateral amygdala. Our results suggest that a brief and repeated maternal separation during the neonatal period induces changes on cannabinoid receptors differently manifested between adolescence and adulthood, male and female brain, which could be correlated to their stress response.

Infantile Handling and the Extinction of Responses Acquired under Food Reward

A comparison was made of the runway behaviour of rats which had been handled from Days 1–21 and their non-handled litter mates. The training began on Day 70 after the animals were habituated to a restricted food schedule for 10 days. The subjects were given six trials each day in the runway and were rewarded with a 0.045 g. Noyes pellet. After 10 days of rewarded training trials, subjects were given 6 extinction trials a day for 10 days. Results showed that handled rats ran faster than non-handled rats during acquisition and during the first 3 days of extinction. The extinction data suggested that the relationship between emotionality and effects of frustrative non-reward should be re-evaluated.

Neonatal handling decreases unconditioned anxiety, conditioned fear, and improves two-way avoidance acquisition: a study with the inbred Roman high (RHA-I)- and low-avoidance (RLA-I) rats of both sexes

The present study evaluated the long-lasting effects of neonatal handling (NH; administered during the first 21 days of life) on unlearned and learned anxiety-related responses in inbred Roman High- (RHA-I) and Low-avoidance (RLA-I) rats. To this aim, untreated and neonatally-handled RHA-I and RLA-I rats of both sexes were tested in the following tests/tasks: a novel object exploration (NOE) test, the elevated zero maze (ZM) test, a “baseline acoustic startle” (BAS) test, a “context-conditioned fear” (CCF) test and the acquisition of two-way active—shuttle box—avoidance (SHAV). RLA-I rats showed higher unconditioned (novel object exploration test -“NOE”-, elevated zero maze test -“ZM”-, BAS), and conditioned (CCF, SHAV) anxiety. NH increased exploration of the novel object in the NOE test as well as exploration of the open sections of the ZM test in both rat strains and sexes, although the effects were relatively more marked in the (high anxious) RLA-I strain and in females. NH did not affect BAS, but reduced CCF in both strains and sexes, and improved shuttle box avoidance acquisition especially in RLA-I (and particularly in females) and in female RHA-I rats. These are completely novel findings, which indicate that even some genetically-based anxiety/fear-related phenotypes can be significantly modulated by previous environmental experiences such as the NH manipulation.

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.

Infantile experience and play motivation

Fully-fledged affective systems in mature animals are in part the result of the impact of infantile experience on brain development. The present experimental series examines whether tactile stimulation in infancy (early handling) influences rough-and-tumble play (R&T) throughout the juvenile period, using a testing regime of 17 days divided into five parts where handled (H) and nonhandled (NH) Wistar rats are assessed daily. In Parts 1 and 2 (age range at the start: 30-33 days) the objective is to study the amount of R&T that the rats are capable of exhibiting under varying lengths of social deprivation. In Part 3 (37-40 days) the objective is to determine whether familiarity with the experimental situation has independent or interactive effects with early handling. In Part 4 (40-43 days) the objective is to obtain evidence of the suppressing effects of an unexpected contextual change. In Part 5 (56-59 days) the objective is to study whether the effects of early handling can still be present at an age when R&T has practically vanished in NH rats. Results show that early handling invigorates R&T affecting pins (i.e., the most rewarding component) at the expense of dorsal contacts by enhancing play motivation in a specific manner, and that it is able to dilate the inverted-U developmental curve of this behavior, thereby providing strong evidence for a direct effect on the neuropsychological systems for play motivation.

Long-term effects of neonatal handling on mu-opioid receptor levels in the brain of the offspring

Neonatal handling is an experimental paradigm of an early experience which permanently alters hypothalamic-pituitary-adrenal axis function resulting in increased ability to cope with stress, and decreased emotionality. In the present work we investigated the effect of neonatal handling on adult rat brain mu-opioid receptor levels, since the opioid system is known to play an important role in emotional processing, anxiety and stress responses. Neonatal handling resulted in increased levels of mu-opioid receptors in the basolateral and central amygdaloid nuclei, in the CA3 and CA4 hippocampal areas, in the ventral tegmental area, the nucleus accumbens and the prefrontal cortex. Handled animals of both sexes had lower anxiety as measured in the elevated plus maze. The increased mu receptor levels could participate in the molecular mechanisms underlying the well-documented decreased stress and anxiety responses of handled animals.

Early behavioral intervention, brain plasticity, and the prevention of autism spectrum disorder

Advances in the fields of cognitive and affective developmental neuroscience, developmental psychopathology, neurobiology, genetics, and applied behavior analysis have contributed to a more optimistic outcome for individuals with autism spectrum disorder (ASD). These advances have led to new methods for early detection and more effective treatments. For the first time, prevention of ASD is plausible. Prevention will entail detecting infants at risk before the full syndrome is present and implementing treatments designed to alter the course of early behavioral and brain development. This article describes a developmental model of risk, risk processes, symptom emergence, and adaptation in ASD that offers a framework for understanding early brain plasticity in ASD and its role in prevention of the disorder.

Effect of neonatal handling on adult rat spatial learning and memory following acute stress

Brief neonatal handling permanently alters hypothalamic-pituitary-adrenal axis function resulting in increased ability to cope with stress. Since stress is known to affect cognitive abilities, in the present study we investigated the effect of brief (15 min) handling on learning and memory in the Morris water maze, following exposure to an acute restraint stress either before training or recall. Exposure of non-handled rats to the acute stress prior to training resulted in quicker learning of the task, than in the absence of the stressor. When acute stress preceded acquisition, male handled rats showed an overall better learning performance, and both sexes of handled animals were less impaired in the subsequent memory trial, compared to the respective non-handled. In addition, the number of neurons immunoreactive for GR was higher in all areas of Ammon's horn of the handled rats during the recall. In contrast, the number of neurons immunoreactive for MR was higher in the CA1 and CA2 areas of the non-handled males. When the acute restraint stress was applied prior to the memory test, neonatal handling was not effective in preventing mnemonic impairment, as all animal groups showed a similar deficit in recall. In this case, no difference between handled and non-handled rats was observed in the number of GR positive neurons in the CA2 and CA3 hippocampal areas during the memory test. These results indicate that early experience interacts with sex and acute stress exposure in adulthood to affect performance in the water maze. Hippocampal corticosterone receptors may play a role in determining the final outcome.

Effects of neonatal handling on the basal forebrain cholinergic system of adult male and female rats

Neonatal handling is an early experience which results in improved function of the hypothalamic-pituitary-adrenal axis, increased adaptability and coping as a response to stress, as well as better cognitive abilities. In the present study, we investigated the effect of neonatal handling on the basal forebrain cholinergic system, since this system is known to play an important role in cognitive processes. We report that neonatal handling results in increased number of choline-acetyl transferase immunopositive cells in the septum/diagonal band, in both sexes, while no such effect was observed in the other cholinergic nuclei, such as the magnocellular preoptic nucleus and the nucleus basalis of Meynert. In addition, neonatal handling resulted in increased M1 and M2 muscarinic receptor binding sites in the cingulate and piriform cortex of both male and female rats. A handling-induced increase in M1 muscarinic receptor binding sites was also observed in the CA3 and CA4 (fields 3 and 4 of Ammon's horn) areas of the hippocampus. Furthermore, a handling-induced increase in acetylcholinesterase staining was found only in the hippocampus of females. Our results thus show that neonatal handling acts in a sexually dimorphic manner on one of the cholinergic parameters, and has a beneficial effect on BFCS function, which could be related to the more efficient and adaptive stress response and the superior cognitive abilities of handled animals.

Environmental complexity and central nervous system development and function

Environmental restriction or deprivation early in development can induce social, cognitive, affective, and motor abnormalities similar to those associated with autism. Conversely, rearing animals in larger, more complex environments results in enhanced brain structure and function, including increased brain weight, dendritic branching, neurogenesis, gene expression, and improved learning and memory. Moreover, in animal models of CNS insult (e.g., gene deletion), a more complex environment has attenuated or prevented the sequelae of the insult. Of relevance is the prevention of seizures and attenuation of their neuropathological sequelae as a consequence of exposure to a more complex environment. Relatively little attention, however, has been given to the issue of sensitive periods associated with such effects, the relative importance of social versus inanimate stimulation, or the unique contribution of exercise. Our studies have examined the effects of environmental complexity on the development of the restricted, repetitive behavior commonly observed in individuals with autism. In this model, a more complex environment substantially attenuates the development of the spontaneous and persistent stereotypies observed in deer mice reared in standard laboratory cages. Our findings support a sensitive period for such effects and suggest that early enrichment may have persistent neuroprotective effects after the animal is returned to a standard cage environment. Attenuation or prevention of repetitive behavior by environmental complexity was associated with increased neuronal metabolic activity, increased dendritic spine density, and elevated neurotrophin (BDNF) levels in brain regions that are part of cortical-basal ganglia circuitry. These effects were not observed in limbic areas such as the hippocampus.

Early-life handling stimulation and environmental enrichment: are some of their effects mediated by similar neural mechanisms?

Neonatal (early) handling (EH) and environmental enrichment (EE) of laboratory rodents have been the two most commonly used methods of providing supplementary environmental stimulation in order to study behavioral and neurobiological plasticity. A large body of research has been generated since the 1950s, unequivocally showing that both treatments induce profound and long-lasting behavioral and neural consequences while also inducing plastic brain effects and being "protective" against some age-related deficits. The present work is aimed at reviewing the main neurobehavioral effects of both manipulations, with the final purpose of comparing them and trying to find out to what extent the effects of both treatments may share (or not) possible neural mechanisms.

The effects of early handling on latent inhibition in male and female rats

Latent inhibition (LI) is a behavioral paradigm in which repeated exposure to stimuli not followed by meaningful consequences renders these stimuli ineffective for subsequent learning. The development of LI is considered to reflect learning not to attend to, ignore, or tune out irrelevant stimuli. The present study investigated the differences in the development of LI between handled and nonhandled males and females. Infantile handled (Days 1-22) and nonhandled, male and female Wistar rats were tested in maturity in the LI paradigm. The LI procedure consisted of two stages: pre-exposure, where animals received 60 presentations of the to-be-conditioned stimulus (tone) and test, where the animals acquired a two-way active avoidance response with the tone serving as the warning signal. Handled animals reached higher percentage of avoidance responses as compared with nonhandled animals. Latent inhibition was obtained in both the handled and the nonhandled females, but only the handled males showed the LI effect. Nonhandled males failed to develop LI. The results indicate that the effects of handling are evident in learning tasks that do not involve motivational-emotional variables, i.e., learning to ignore irrelevant stimuli; handling differentially affects males and females, with a much greater impact on males and the nonhandling procedure has significant deleterious consequences on adult behavior.