Infantile stimulation induces brain lateralization in rats

Morphological alterations in the congenital blind based on the analysis of cortical thickness and surface area

To explore the morphological aspects of the functional reorganization of the blind's visual cortex, we analyzed the regional cortical thickness and cortical surface area in the congenitally blind subjects (CB) compared to the late-onset blind (LB) and sighted controls (SC). Cortical thickness was calculated from high-resolution T1-weighted magnetic resonance images of 21 young CB (blind from birth, mean age=27.1 yr), 12 LB, and 35 young SC. Analysis of covariance of cortical layer thickness with global thickness, age, and gender as covariates was done node-by-node on the entire cortical surface. Further analysis of mean thickness and surface area was performed for 33 automatically parceled cortical regions. Voxel-based morphometry was also conducted to compare results with cortical thickness and surface area. We found increased cortical thickness in the regions involved in vision and eye movement, such as the pericalcarine sulcus, cingulate cortex, and right frontal eye field, but cortical thinning in the left somatosensory cortex and right auditory cortex of CB compared to SC. CB had significantly reduced surface extent in the primary and associated visual areas, which explains volumetric atrophies in the visual cortex of CB despite increased cortical thickness. Conversely, LB tended to have cortical thinning in the primary visual cortex with a slight or no significant reduction in the surface extent. These morphological alterations in CB may indicate cortical reorganization at the visual cortex in connection with other sensory cortices.

Consequences of different housing conditions on brain morphology in laying hens

The aim of this study was to analyze the impact of physical and social stress on the avian forebrain morphology. Therefore, we used laying hens kept in different housing systems from puberty (approximately 16 weeks old) until the age of 48 weeks: battery cages, small littered ground pen, and free range system. Cell body sizes and catecholaminergic and serotonergic innervation patterns were investigated in brain areas expected to be sensitive to differences in environmental stimulation: hippocampal substructures and the nidopallium caudolaterale (NCL), a functional analogue of the prefrontal cortex. Our analysis shows both structures differing in the affected morphological parameters. Compared to battery cage hens, hens in the free range system developed larger cells in the dorsomedial hippocampus. Only these animals exhibited an asymmetry in the tyrosine hydroxylase density with more fibres in the left dorsomedial hippocampus. We assume that the higher spatial complexity of the free range system is the driving force of these changes. In contrast, in the NCL the housing systems affected only the serotonergic innervation pattern with highest fibre densities in free range hens. Moreover hens of the free range system displayed the worst plumage condition, which most likely is caused by feather pecking causing an altered serotonergic innervation pattern. Considering the remarkable differences between the three housing conditions, their effects on hippocampal structures and the NCL were surprisingly mild. This observation suggests that the adult brain of laying hens displays limited sensitivity to differences in social and physical environment induced post-puberty, which warrants further studies.

Hippocampal asymmetry in exploratory behavior to vasoactive intestinal polypeptide

The effects of vasoactive intestinal polypeptide (VIP) microinjected uni- or bilaterally into the CA1 hippocampal area of male Wistar rats at a dose of 10, 50 and 100 ng on exploratory behavior were examined. VIP microinjected bilaterally at a high dose (100 ng) significantly decreased the horizontal movements, while at low doses (10 and 50 ng) had no effect on the exploratory activity. Microinjections of VIP into the left hippocampal CA1 area at doses 50 and 100 ng suppressed the exploratory activity, while right-side VIP administration at a dose 100 ng significantly increased horizontal movements compared to the respective controls. Vertical activity was stimulated only by VIP administered into the right hippocampal CA1 area at the three doses used. Neither bilateral nor left injections of VIP induced changes in the vertical movements. The main finding was the presence of hippocampal asymmetry in exploratory behavior to unilateral microinjections of VIP depending on the dose and the microinjected hemisphere.

An epigenetic induction of a right-shift in hippocampal asymmetry: selectivity for short- and long-term potentiation but not post-tetanic potentiation

In humans, it is well established that major psychological functions are asymmetrically represented between the left and right cerebral cortices. The developmental origin of such functional lateralization remains unknown. Using the rat as a model system, we examined whether exposing neonates briefly to a novel environment can differentially affect synaptic plasticity in the left and right hippocampi during adulthood. During the first 3 weeks of life, one half of the pups from a litter spent 3 min daily away from their familiar home environment (Novel) while their littermates remained in that familiar environment (Home). At adulthood (7-months old), post-tetanic potentiation (PTP) of excitatory post-synaptic potentials (EPSPs), a very short-lasting form of plasticity, was greater among the Novel than the Home rats in both left and right hippocampi. In contrast, the novelty-induced increases in short- and long-term potentiation (STP, LTP), two relatively longer-lasting forms of plasticity, were found only in the right hippocampus. These findings demonstrate that a phase-selective asymmetry in hippocampal synaptic plasticity can be induced epigenetically by seemingly small systematic differences in early life environment. The selectivity of this asymmetry for the longer-lasting forms of synaptic plasticity suggests that the observed asymmetry in plasticity may contribute specifically to an asymmetric learning process which, in turn, may contribute to a functional asymmetry in the neocortex.

Hippocampal asymmetry in serotonergic modulation of learning and memory in rats

The modulation of learning and memory after left or right microinjections of the selective 5-HT1A receptor agonist 8-OH-DPAT and of the 5-HT1A receptor antagonist NAN190 into the hippocampal CA1 area of male Wistar rats was studied. Microinjections of 8-OH-DPAT (1 microg) into the right or left CA1 hippocampal area produced a significant decrease in the number of avoidances in a shuttle box. The impairing effect of 8-OH-DPAT was more pronounced when injected into the right hippocampus compared to the left one. Microinjections of NAN190 (1 microg) into the right or left CA1 hippocampal area produced a significant increase in the number of avoidances in a shuttle box. Right microinjections of NAN190 increased the number of avoidances more strongly than compared to left injections. These effects on learning and memory were more pronounced after injection of either of the serotonergic agents into the right CA1 hippocampal area compared to the left. The stronger memory-modulating effect after injection of 8-OH-DPAT or NAN190 into the right CA1 hippocampal area suggests a rightward bias in the rat.

Evolution of hemispheric specialisation of antagonistic systems of management of the body's energy resources

Excellent and rich reviews of lateralised behaviour in animals have recently been published indexing renewed interest in biological theorising about hemispheric specialisation and yielding rich theory. The present review proposes a new account of the evolution of hemispheric specialisation, a primitive system of "management of the body's energy resources". This model is distinct from traditionally evoked cognitive science categories such as verbal/spatial, analytic/holistic, etc., or the current dominant neuroethological model proposing that the key is approach/avoidance behaviour. Specifically, I show that autonomic, immune, psychomotor, motivational, perceptual, and memory systems are similarly and coherently specialised in the brain hemispheres in rodents and man. This energy resource management model, extended to human neuropsychology, is termed here the "psychic tonus" model of hemispheric specialisation.

Lateralization of olfaction in the honeybee Apis mellifera

Lateralization of function is a well-known phenomenon in humans. The two hemispheres of the human brain are functionally specialized such that certain cognitive skills, such as language or musical ability, conspecific recognition, and even emotional responses, are mediated by one hemisphere more than the other [1, 2]. Studies over the past 30 years suggest that lateralization occurs in other vertebrate species as well [3-11]. In general, lateralization is observed in different sensory modalities in humans as well as vertebrates, and there are interesting parallels (reviewed in [12]). However, little is known about functional asymmetry in invertebrates [13, 14] and there is only one investigation in insects [15]. Here we show, for the first time, that the honeybee Apis mellifera displays a clear laterality in responding to learned odors. By training honeybees on two different versions of the well-known proboscis extension reflex (PER) paradigm [16, 17], we demonstrate that bees respond to odors better when they are trained through their right antenna. To our knowledge, this is the first demonstration of asymmetrical learning performance in an insect.

Neonatal novelty exposure ameliorates anoxia-induced hyperactivity in the open field

We investigated in an animal model of neonatal anoxia whether effects of oxygen deprivation on emotional reactivity can be reversed by neonatal novelty exposure, a behavioral method, involving daily 3min away from the home cage for the first 3 weeks of life. Male neonates were exposed to either 100% N2 gas (Anoxia) or room air (Control) for 25min on postnatal day 1. Within each of the two treatment conditions, one-half of the neonates were further individually exposed to relatively novel non-home cages for 3min daily during postnatal days 2-21 (Novel: NAnoxia=20; NControl=16), while the other half remained in the home cage (Home: NAnoxia=19; NControl=19). Emotional reactivity to an open field was evaluated on postnatal day 25 during four 20-s trials. Among home rats, temporal patterns of open-field activity across multiple trials and initial-trial activity significantly differed between the Anoxia and Control rats. In contrast, these differences were eliminated among the Novel rats. These results show that neonatal novelty exposure, an early-stimulation method that has recently been shown to enhance spatial and social memory, adaptive control of stress response, and hippocampal synaptic plasticity, can also eliminate neonatal anoxia-induced changes in emotional reactivity. These findings suggest that brief and repeated, but mild, changes in the postnatal environment may serve to counteract some of the aversive effects induced by neonatal trauma associated with oxygen deprivation.

Hemisphere-specific effects on serotonin but not dopamine innervation in the nucleus accumbens of gerbils caused by isolated rearing and a single early methamphetamine challenge

The aim of this study was twofold: We examined whether serotonin (5-HT) and dopamine (DA) innervations of the nucleus accumbens are lateralised and whether the environment or the combination with an early pharmacological impact might interfere with the postnatal maturation of the monoaminergic innervation. Male gerbils were assigned to either enriched rearing (ER) or isolated rearing (IR). Animals from both rearing conditions additionally received a single dose of either methamphetamine [MA (50 mg/kg ip)] or saline on postnatal day 14. DA and 5-HT fibres of the adult animals (postnatal day 90-110) were immunocytochemically stained and fibre densities were quantified in nucleus accumbens core and shell of both the left and right hemisphere. Our data demonstrate that the DA and 5-HT innervation is not lateralised in saline-treated animals of both rearing conditions. IR increases the DA fibre density in both hemispheres of saline controls, whereas an additional MA treatment reverses this effect. In both ER and IR groups, MA provokes an excessive 5-HT fibre in growth of only the right hemisphere. The combination of IR with MA induces right-side asymmetries of the 5-HT fibre density in both the core and shell. From the data obtained, we conclude that the maturation of the monoaminergic innervation of the nucleus accumbens is vulnerable to postnatal stimuli. The subtle "innervation imbalance" observed in our studies is consistent with previously reported effects in other brain regions of this animal model and may be causative for behavioural disturbances.

Developmentally stable sex-dependent modulation of turning asymmetry by neonatal novelty exposure

In rats, early life stimulation can enhance learning and memory and induce parallel changes in brain asymmetry. Despite persistent interest in human brain asymmetry, relatively little is known in animal models about developmental stability of early-experience effects on asymmetry and how early-experience may affect males and females differently in asymmetry measures across developmental stages. We exposed male and female neonatal rats to a novel cage for 3min per day during the first 3 weeks of life and measured spontaneous turning behavior at juvenility (7 weeks of age) and adulthood (7 months of age). We found that (1) the effects of such neonatal novelty exposure on turning bias are developmentally stable, and (2) neonatal novelty exposure differentially modulates turning bias in males and females. We briefly discuss implications of these findings in terms of the role of brain asymmetry in modulating cognitive and emotional development.

Lateralisation in birds of prey: adaptive and phylogenetic considerations

Lateralisation, the different use of one or other side or appendix of the body, is basically determined by brain asymmetry which, in turn, is likely to be due to adaptive reasons. Several studies have been carried out on birds in general. However, birds of prey in particular, although they are very good candidates, have not been investigated from the sensory lateralisation point of view. In fact, many species scan for prey while perched and capture terrestrial prey with the feet, having at the same time the obvious necessity to keep their balance. This paper, therefore, investigates the existence of some sort of lateralisation in several species of both Falconiformes and Strigiformes temporarily in captivity. Attention is given to: (a) the direction of body rotation when perceiving a sound stimulus from behind the body and (b) the use of the feet when grasping a terrestrial prey. Lateralisation was found to be clearly present in both types of tests, although with some difference in its expression. In fact, almost every species tested rotated its body anti-clockwise, i.e. to the left, both in the first test and in repeated tests, with no noticeable difference between Falconiformes and Strigiformes. Also prey grasping showed a preferential use of one foot. Falconiformes preferred clearly to grasp the prey with one foot only in both the first test and in subsequent ones. Strigiformes, on the other hand, preferred using both feet, although a not insignificant proportion of individuals used one foot. Only the little owl seemed to have the tendency to prefer to use the right foot only, in a similar manner to Falconiformes. In fact, this bird is the most "diurnal" owl species among those tested, suggesting that lateralisation in footedness might be affected by adaptive constraints more than by phylogenetic similarities. Lateralisation, therefore, seems to be very widespread among birds of prey. Preferential use of the right foot also appears to be a general habit, and this is probably connected to the use of left hemisphere when manipulating food items.

Differential effects of somatostatin on exploratory behavior after unilateral injections in to rat neostriatum

The effects of somatostatin (SRIF) microinjected unilaterally (left or right) at a dose of 10, 50, and 100 ng into the neostriatum of male Wistar rats on exploratory behavior were studied. Unilateral injections of SRIF suppressed dose-related the exploratory activity as decreased the number of horizontal and vertical movements compared to the respective controls. The effect was more pronounced when SRIF was microinjected into the right neostriatum as compared to the left neostriatum. These findings suggest some asymmetric effects of SRIF, depending on the dose and the microinjected hemisphere.

Modification of social memory, hypothalamic-pituitary-adrenal axis, and brain asymmetry by neonatal novelty exposure

Although corticosterone (a stress hormone) is known to influence social behavior and memory processes, little has been explored concerning its modulatory role in social recognition. In rats, social recognition memory for conspecifics typically lasts <2 hr when evaluated using a habituation paradigm. Using neonatal novelty exposure, a brief and transient early life stimulation method known to produce long-lasting changes in the hypothalamic-pituitary-adrenal axis, we found that social recognition memory was prolonged to at least 24 hr during adulthood. This prolonged social memory was paralleled by a reduction in the basal blood concentration of corticosterone. The same neonatal stimulation also resulted in a functional asymmetry expressed as a greater right-turn preference in a novel environment. Rats that preferred to turn right showed better social recognition memory. These inter-related changes in basal blood corticosterone concentration, turning asymmetry, and social recognition memory suggest that stress hormones and brain asymmetry are likely candidates for modulating social memory. Furthermore, given that neonatal stimulation has been shown to improve learning and memory performance primarily under aversive learning situations, the neonatal novelty exposure-induced enhancement in social recognition broadens the impact of early life stimulation to include the social domain.

The neurobiological consequences of early stress and childhood maltreatment

Early severe stress and maltreatment produces a cascade of neurobiological events that have the potential to cause enduring changes in brain development. These changes occur on multiple levels, from neurohumoral (especially the hypothalamic-pituitary-adrenal [HPA] axis) to structural and functional. The major structural consequences of early stress include reduced size of the mid-portions of the corpus callosum and attenuated development of the left neocortex, hippocampus, and amygdala. Major functional consequences include increased electrical irritability in limbic structures and reduced functional activity of the cerebellar vermis. There are also gender differences in vulnerability and functional consequences. The neurobiological sequelae of early stress and maltreatment may play a significant role in the emergence of psychiatric disorders during development.

Neonatal novelty exposure, dynamics of brain asymmetry, and social recognition memory

Brief and transient early-life stimulation via neonatal handling and neonatal novelty exposure can lead to differential changes within the right and left brains. In rats, these lateralized changes have been demonstrated behaviorally, neuroanatomically, and neurophysiologically. Recently, we found that neonatal novelty exposure can prolong the duration of social recognition memory from less than 2 hr to at least 24 hr among male rats reared in social isolation and that this enhancement is associated with an initial right-turn preference in a novel testing cage. In contrast to stable forms of asymmetry, such as handedness, we show that this turning asymmetry is dynamic-decreasing as the animal adjusts to the novel testing environment over a 2-day period. This change in turning asymmetry was found only among animals that experienced neonatal novelty exposure during the first 3 weeks of their lives. Furthermore, individual differences in short-term social recognition memory for a conspecific can be predicted by this change in functional asymmetry.

Neonatal exposure to novelty enhances long-term potentiation in CA1 of the rat hippocampus

Exposing rats to an enriched environment over an extended period of time has been shown to enhance hippocampal long-term potentiation (LTP). Whether such prolonged exposure to environmental manipulation is necessary for LTP enhancement and whether the environmentally induced enhancement can persist long after the cessation of the environmental manipulation remain unknown. Using a novelty exposure procedure modified from the method of neonatal handling, we exposed neonatal rats to a non-home environment for 3 min/day during the first 3 weeks of life. We examined the LTP of both population spikes and excitatory postsynaptic potentials (EPSPs), in vitro, in the CA1 of the hippocampus during adulthood (7-8 and 13-14 months of age). We found that both the LTP of population spikes and the LTP of EPSPs were enhanced among animals who experienced neonatal novelty exposure. These results demonstrate that effective environmental enhancement of LTP can be achieved by as brief and as transient a manipulation as a 3-min/day exposure over the first 3 weeks of life. The resulting enhancement can outlast the environmental manipulation by at least 1 year.

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.

Lateralisation of predator avoidance responses in three species of toads

Lateralisation of responses to presentation of a simulated predator was investigated in three species of toads: two European species (the common toad, Bufo bufo, and the green toad, Bufo viridis) and one species introduced to Australia from South America, the cane toad Bufo marinus . First a simulated snake was presented moving rapidly towards the toad in the frontal field of vision and the toad's escape responses, including jumps to the right and to the left, were recorded. No significant bias in left or right side jumping was apparent in this test. Next the simulated snake was presented in the left or right lateral field of vision in random order. Escape and defensive responses were elicited more strongly, in all three species, when the stimulus was on the toad's left side compared to its right side. Reaction times scored in the experiments with B. marinus, alone, did not differ from left to right. There were, however, species differences in the types of escape responses with respect to the laterality: B. viridis and B. marinus showed similar patterns of more sideways jumps with left presentation and more frontal jumps with right presentation. Sideways jumps were not lateralised in B. bufo, but this species showed more frontal jumps when the presentation was on the left side. These findings suggest that the selective involvement of structures located in the right side of the brain (left monocular visual field) in emotional responses (particularly fear responses) could be a phylogenetic ancient trait.

Early life environment modulates 'handedness' in rats

Right handedness is one of the most prominent markers of human functional brain asymmetry. Deviation from this norm appears to be associated with certain developmental disorders. While many studies have dealt with the genetic contribution to the determination of handedness, few have examined whether environmental factors that are subtler than forced hand switching can modulate the development of handedness. In this study, we exposed rats to a novel environment for 3 min daily during their first 3 weeks of life and found that their paw preferences during both infancy and adulthood showed a leftward shift compared with the controls. This result suggests that 'handedness' can be modified by rather subtle early environmental manipulation. Since exposure to a novel environment does not involve a direct asymmetric activation of the sensory--motor system underlying paw-use, mechanisms beyond this paw-specific system must exist to mediate the observed modulation of 'handedness'.

Neonatal novelty exposure modulates hippocampal volumetric asymmetry in the rat

Early life environmental manipulations have been shown to affect hippocampal-dependent learning, hippocampal volume and cerebral lateralization. In this study, we investigated the effects of neonatal stimulation on hippocampal volumetric asymmetry. Long-Evans hooded rats were exposed to a novel non-home environment 3 min daily for the first 3 weeks of life. Histological measures of the left and right hippocampus were made at 8 months of age. We found that neonatal novelty exposure resulted in a long-lasting change in hippocampal volumetric asymmetry. Specifically, this brief and transient early life stimulation increased the right hippocampal volumetric dominance at mid-adulthood.

Neonatal exposure to novel environment enhances hippocampal-dependent memory function during infancy and adulthood

Early life experience affects behavior and brain mechanisms. Handling rats during the first three weeks in life can slow age-related cognitive decline (as measured by a hippocampal-dependent spatial learning task) and reduce age-related hippocampal neuron loss. It is not clear, however, whether this early environmental influence on learning is selective for old age or is more general, affecting cognitive development during infancy and young adulthood as well. We briefly exposed neonatal rats to a novel non-home environment for 3 min daily during the first three weeks of life (as a component of the handling method). We found that this brief early environmental manipulation resulted in enhanced hippocampal-dependent learning immediately after weaning and that this learning enhancement persisted into adulthood. These results suggest that subtle early life events can affect cognitive development in all developmental stages and that changes in neural mechanisms other than neuron number are likely to mediate the learning enhancement at multiple developmental stages.

Asymmetrical hatching behaviors influence the development of postnatal laterality in domestic chicks (Gallus gallus)

Lateralized motor behaviors have been reported in some avian species. For instance, footedness has been reported in parrots and domestic chicks, and turning biases have been reported in such species as quail and domestic chicks. This study examined the effects of asymmetrical hatching behaviors on the development of lateralized turning bias and footedness in domestic chicks. Asymmetrical hatching behaviors are counter-clockwise full body turns that many precocial birds make to escape the egg. To study the role of such coordinated prenatal motor behaviors in the development of lateralization, hatching behaviors were systematically disrupted following pipping. Subjects were subsequently tested on two measures of laterality: footedness and turning bias. Results indicated a significant reduction in individual and group lateralization for both measures. These findings suggest that the hatching behaviors found in domestic chicks serve to induce the development of strong motor biases at both the individual and population level.

Lateralized learning and memory effects of angiotensin II microinjected into the rat CA1 hippocampal area

The effects of angiotensin II (ANG II) microinjected unilaterally (left or right) and bilaterally (left and right) at a dose of 0.5 microg (0.5 nmol) into the CA1 hippocampal area of male Sprague Dowley rats on learning and memory (shuttle box) were studied. Bilateral microinjections of ANG II improved learning, i.e. increased the number of avoidances during the two training days as compared to the respective controls microinjected with saline. ANG II facilitated learning and memory, especially when microinjected into the left CA1 hippocampal area as compared to the respective controls microinjected with saline. Left-side microinjection of ANG II increased the number of avoidances on the first and second training day as compared to the right-side microinjection of ANG II. These findings suggest asymmetric effects of ANG II on cognitive processes in hippocampus.

Possible evolutionary origins of cognitive brain lateralization

Despite the substantial literature on the functional architecture of the asymmetries of the human brain, which has been accumulating for more than 130 years since Dax and Broca's early reports, the biological foundations of cerebral asymmetries are still poorly understood. Recent advances in comparative cognitive neurosciences have made available new animal models that have started to provide unexpected insights into the evolutionary origins and neuronal mechanisms of cerebral asymmetries. Animal model-systems, particularly those provided by the avian brain, highlight the interrelations of genetic, hormonal and environmental events to produce neural and behavioural asymmetries. Novel evidences showing that functional and structural lateralization of the brain is widespread among vertebrates (including fish, reptiles and amphibians) have accumulated rapidly. Perceptual asymmetries, in particular, seem to be ubiquitous in everyday behaviour of most species of animals with laterally placed eyes; in organisms with wider binocular overlap (e.g., amphibians), they appear to be retained for initial detection of stimuli in the extreme lateral fields. We speculate that adjustment of head position and eye movements may play a similar role in mammals with frontal vision as does the choice for right or left lateral visual fields in animals with laterally placed eyes. A first attempt to trace back the origins of brain asymmetry to early vertebrates is presented, based on the hypothesis that functional incompatibility between the logical demands associated with very basic cognitive functions is central to the phenomenon of cerebral lateralization.

Differential behavioral effects of angiotensin II microinjected unilaterally into the CA1 hippocampal area

The behavioral responses of rats to unilateral microinjections of angiotensin II (ATII) into the left or right CA1 hippocampal area were studied. Unilateral (left or right) injections of ATII at a dose of 0.5 microg decreased locomotor activity but, at a dose of 1.0 microg, ATII increased it compared to the respective controls. The effect was more pronounced when ATII was microinjected into the left CAI hippocampal area. The elevated plus-maze experiments showed that ATII microinjections into the right CA1 hippocampal area at a dose of 0.5 microg decreased the ratio of the number of entries into the open arms to the total number of entries (into the open and closed arms). These findings suggest some asymmetric effects of ATII, depending on the dose, the behavioral test and the microinjected hemisphere.

Sex differences in EEG in adult gonadectomized rats before and after hormonal treatment

EEG activity was recorded from the left and right parietal cortex in adult male and female Wistar rats that were gonadectomized (GNX) after puberty during 2 days without and 3 days with hormonal treatment (either testosterone propionate, 5 alpha-DHT or vehicle in males and progesterone, estradiol benzoate or vehicle in females). In contrast to EEG characteristics reported for intact rats, GNX abolished right over left parietal activation in both sexes and, sex differences in EEG interhemispheric correlation and in theta and delta relative power in the right parietal; additionally GNX males showed higher absolute power than females. Hormonal treatment reestablished interparietal asymmetry in both sexes and a lack of sex differences in absolute power, however, it was not enough to reestablish sex differences in delta and theta proportion in the right parietal nor in interhemispheric correlation. Differential effects were obtained with testosterone propionate and 5 alpha-DHT in males suggesting that activational effects of testosterone on EEG are probably exerted through testosterone or its aromatized metabolites. The results of our study indicate that the activational effects of gonadal steroids after puberty are necessary for maintaining sex differences in the EEG of the adult rat.

The Effects of Early Environment on the Development of Functional Laterality in Morris Maze Performance

The current study examined the contribution of early postnatal experience to the functional lateralisation of spatial ability in the male rat. Litters were handled (H) or non-handled (NH) during the first 20 days of life, and three males from each litter were tested in the Morris water maze in adulthood. Two subjects from each litter were monocularly tested, one with the right eye patched and the other with the left eye patched. A third subject in each litter was tested with both eyes open. Handling interacted with Eye Patch for time and distance measures. Performance of Right- and Left-Patched rats was equivalent for the NH group, but the H subjects tested with the Right Patch outperformed H subjects with the Left Patch. These results confirm studies that found spatial navigation performance to be lateralised to the right hemisphere in male rats. The data indicate that functional asymmetry of spatial navigation behaviour in the adult male rat is sensitive to environmental influences during early development.

Lateralization of rotational behavior in developing and adult hamsters

Rotational asymmetries were studied in developing and adult hamsters, and compared to verify if early lateralization is a predictor of the animals' later performance. Animals were divided into two groups: group I (GI) was tested from P46 (P1 = day of birth) to P62, and group II (GII) was tested daily from P2 to P60. They were placed in a cylindric arena for 5 min under video recording, and their 90 degree right and left displacements were counted and normalized. Since adult animals of both groups did not differ significantly in the distribution of asymmetries, the data were pooled together: 38.8% were non-lateralized (NL), 39.4% were right-rotators (RR), and 21.8% were left-rotators (LR). No significant difference was discerned between males and females. Distribution of asymmetries in GII animals between P2 and P10 showed a predominance of lateralized (64.7% RR and 21.5% LR) over NL animals (13.8%). The proportion of pups that maintained their classification into adulthood was only 46%, and the kappa coherence coefficient for these data was only 0.09. We conclude that: (1) most adults are lateralized, RR being more frequent; (2) the proportion of lateralized adults is not significantly altered by early testing; (3) there is no significant difference between males and females; (4) most developing hamsters are lateralized, right-rotators being more frequent; and (5) the animals' early classification is not a good predictor of their preference as adults.

Cellular, morphometric, ontogenetic and connectional substrates of anatomical asymmetry

Although anatomical cerebral asymmetry appears in all animals that have been examined, its link to functional lateralization is not clear. In an attempt to further elucidate this relationship between structure and function, we have compared, in rats and humans, brains that have asymmetric architectonic areas to those that are symmetric. We have found that (1) asymmetry is the result of the production of a small side rather than the production of a large side; (2) architectonic asymmetry is the result of changes in the total numbers of neurons rather than cell-packing density; (3) events occurring early in corticogenesis--specifically during the period of progenitor cell proliferation and/or death--are important for the formation of asymmetric cortical areas; and (4) symmetric brains have relatively greater numbers of callosal fibers and more patches of termination than their asymmetric counterparts. These results, taken together, suggest that if anatomic asymmetry underlies functional lateralization, it may have more to do with the different organization of symmetric and asymmetric brains, rather than simply which hemisphere (or brain region) is larger.

Lateralized effect of unilateral somatosensory cortex contusion on behavior and cortical reorganization

Previous studies have shown that rats recover function after unilateral somatosensory cortex lesions, possibly by transfer of information processing to other brain areas not normally involved in those functions. In the present study, adult rats underwent unilateral contusions of the somatosensory cortex with ablation of the barrel receptor field. Behavioral testing with modified beam-walking and sensory neglect tasks demonstrated persistent somatosensory deficits in rats with left contusions but no apparent deficits in right injured animals. After 2 months, the [14C]2-deoxyglucose (2-DG) method was used to show the metabolic activity produced by unilateral stimulation of the facial vibrissae. In left injured animals, cortical metabolic activity rostral and caudal to the injury site was depressed both under basal conditions and during right vibrissal stimulation. On the other hand, comparison of the pattern of [14C]2-DG uptake in the intact, right cortex revealed changes in the pattern of glucose utilization associated with left injury combined with right vibrissal stimulation. Pattern changes were quantified by measuring the area in which glucose utilization was within the highest 25% of this range (high activity area; HAA). Right vibrissal stimulation in left injured rats caused an expansion of this HAA in the intact occipital/temporal cortex. Also, in the intact somatosensory cortex of left injured rats, there was an enlarged HAA whether or not vibrissal stimulation was performed. Thus, a combination of depressed peri-injury metabolic activity and aberrant activity in remote brain areas occurs following unilateral somatosensory cortex injury. It remains to be shown whether these factors ameliorate or contribute to persistent behavioral deficits.

Chick imprinting performance and susceptibility to acute stress associated to flunitrazepam receptor increase

One-day-old chicks were selected on their performance in imprinting behaviour and termed high-imprinted (H-I), partially imprinted (P-I) and low-imprinted (L-I) chicks. Then, H-I and L-I chicks were submitted to acute handling stress and [3H]flunitrazepam receptor-binding was performed on synaptosomal membranes from forebrain at various times after handling. The receptor number significantly increased in L-I but not in H-I chicks at 30 min after handling while the affinity remained unchanged at all times. In addition, when the three selected groups were maintained to reach 15 days of age and then they were submitted to acute swimming stress, the degree of receptor increase was also inversely related to the degree of imprinting performance. The receptor increase associated to swimming stress was higher in the left hemisphere, suggesting an interhemispheric asymmetry of stress effects. The results suggest that more-imprinted chicks are less susceptible than less-imprinted chicks to acute stress associated to central benzodiazepine receptor increase, probably due to differences in the degree of endogenous emotionality.

Environmental enrichment reverses the detrimental action of early inconsistent stimulation and increases the beneficial effects of postnatal handling on shuttlebox learning in adult rats

Certain types of environmental stimulation administered during critical periods of neural development can enduringly modify adult behavior. The present experiments show that postnatal handling of Sprague-Dawley rats (administered from postnatal days 1 to 22) and/or living in an enriched environment (EE; from weaning until the age of 100 days) clearly improved the ability to learn a two-way active avoidance task in adulthood. In addition the results demonstrated that postnatal inconsistent stimulation (from postnatal days 1 to 22) impaired avoidance acquisition in the same task. This detrimental effect of inconsistent stimulation was reversed by EE. Our findings provide evidence that different types of early experience can influence learning abilities in distinct directions and with different strengths.

Hippocampal asymmetry in the behavioral responses to the 5-HT1A receptor agonist 8-OH-DPAT

The present study examined the behavioral responses of rats to unilateral and bilateral injections of the selective serotonin 1A (5-HT1A)-receptor agonist 8-hydroxydipropylaminotetralin hydrobromide (8-OH-DPAT) 1 microgram into the hippocampal CA1 area of male Wistar rats. 8-OH-DPAT increased locomotor activity, which was most pronounced with injections into the left hippocampus. The agonist impaired learning and memory (shuttle-box), especially when injected into the right hippocampus. The elevated plus-maze experiments showed that neither left nor right nor bilateral hippocampal injections of 8-OH-DPAT produced any anxiogenic effect. However, with Vogel's conflict test, right injections of 8-OH-DPAT produced anxiety. The present study has revealed hippocampal asymmetry in the behavioral responses to the 5-HT1A-receptor agonist 8-OH-DPAT.

The effects of corticosteroids on lateral bias in female rats

In Experiment 1 female rats were given one trial per day for 8 days in a T-maze, and their initial direction of choice (left/right) was noted. Vaginal smears were also obtained daily. After this some animals were adrenalectomized and given Ringer's solution; others were adrenalectomized and given hydrocortisone replacement; a third group was sham adrenalectomized, and a fourth group was an intact control. A week after surgery the animals were again tested for 8 days in the T-maze and vaginal smears were obtained. In Experiment 2 rats were subjected to the same surgical treatments as described above and were then tested for 8 days in the T-maze. In Expt. 1 there was no direction bias among the four groups prior to surgery. However, after surgery the Adrenalectomy+Ringer's group showed a significant increase in their rightward choices in the T-maze. This was also found in Expt. 2. Both adrenalectomized groups in Expt. 1 had a significant reduction in the duration of the estrus phase of their cycle. We conclude that corticosterone can affect lateral preference in a T-maze through a mechanism independent of the hormonal changes involved in the estrus cycle.

Postnatal handling, perinatal flumazenil, and adult behavior of the Roman rat lines

The effect of infantile handling stimulation and/or perinatal flumazenil (Ro 15-1788; a benzodiazepine receptor antagonist; 3.7 mg/kg/day) administration on exploratory and emotional-related behavior was investigated using adult females from the Roman high- and low-avoidance (RHA/Verh and RLA/Verh) lines. When rats (6 months old) were exposed to a hexagonal tunnel maze including an illuminated central arena, it was found that RHA/Verh rats were more active, explored more maze area, showed more outward preference, and more frequently entered the illuminated center than RLA/Verh rats. In addition, postnatal stimulation decreased emotional-related behavior in both lines of rats, as expressed by increased entry into, and time spent in, the central arena. Perinatal flumazenil treatment decreased entry into the maze central arena in both rat lines but this effect was counteracted by postnatal (handling) stimulation. Thus, the present study extends to adult RHA/Verh and RLA/Verh rats the positive long-lasting effects of postnatal handling and shows postnatal handling x flumazenil interactions in some behavioral parameters related to the pattern of exploration and exploratory efficiency.

Biological substrates of anatomic asymmetry

Asymmetric cortical areas differ in volume and in the number of neurons. There are also differences between asymmetric and symmetric areas. As asymmetry increases, the total area of the region decreases, suggesting that when a brain is symmetric, it is the result of two large sides rather than two small sides. Also, these volume differences are caused by changes in the number of cells, not changes in cell-packing density. The ontogenetic basis for this difference in cell numbers likely relates to events that occur quite early in corticogenesis before final mitosis of proliferative units, but definitive proof is lacking. Finally, the pattern and degree of callosal connections differ between symmetric and asymmetric brains, with differential axonal pruning being implicated as the likely mechanism.

The effects of early experience on callosal development and functional lateralization in pigmented BALB/c mice

A proportion of animals of the BALB/c inbred mouse strain have an unusually small (sometimes absent) midsagittal area of the corpus callosum (CC). In this study, we used a large sample of both males and females (total n = 198) from a pigmented congenic BALB/c line to investigate the relations among preweaning handling, area of CC, and direction and degree of lateralization as measured by Collins' paw preference task. Twenty litters were handled daily from the day after birth until day 25 (weaning) according to Denenberg's procedure and 18 litters were left undisturbed until weaning. All animals were tested for degree and direction of paw preference in a modification of Collins' apparatus at about 60 days and measures taken on CNS structures at 100 days of age. There were no handling or sex effects on degree or direction of paw preference or on the extent of CC defects, but for animals in the normal range (CC > or = 0.7 mm2), those which had been handled had significantly smaller callosa. No significant differences were detected between right and left hemisphere weights, and these measures did not appear to be related to the behavioural measures. There was no significant correlation between CC area and degree of paw preference nor was there any relationship between total agenesis and degree of handedness. This last result is particularly interesting in light of recent evidence that ILn/J mice, all acallosal, are exclusively ambilateral.

Early stimulation effects on novelty-induced behavior in two psychogenetically-selected rat lines with divergent emotionality profiles

The present study shows that postnatal handling (H: consisting of removing the pups from the nest twice daily and placing them individually in plastic cages lined with paper towel for a period of 10 min, between postnatal days 1 and 21) and/or environmental enrichment (E: for a period of 6 months) of Roman high- and low-avoidance (RHA/Verh and RLA/Verh) rats induced long-lasting decreases in emotional reactivity (i.e. reduced defecations in the open field, OF, and hole-board, HB, tests) as well as increases in exploratory behavior (i.e. head-dipping) in a manner dependent upon the rat line (there were 'line x H' and 'line x E' interactions). It is reported for the first time that RHA/Verh rats show more head-dipping behavior than RLA/Verh rats, and that the environmental treatments can increase head-dipping of RLA/Verh animals to the level shown by RHA/Verh rats.

Differential effects of early stimulation and/or perinatal flumazenil treatment in young Roman low- and high-avoidance rats

The effect of infantile handling-stimulation and/or perinatal flumazenil (3.7 mg/kg/day) administration on exploratory and emotional-related behavior was investigated using Roman high- and low-avoidance (RHA/Verh and RLA/Verh) rats. Postnatal handling increased exploration in 30-day-old rats of both psychogenetically selected lines when they were exposed to a hexagonal tunnel maze including an illuminated central arena. Likewise, postnatal stimulation decreased emotional reactivity in both lines of rats, as expressed by increased entry into the central arena, decreased defecation and vocalization frequency, but these effects were more pronounced in the RLA/Verh line. There were interactions between perinatal flumazenil treatment and rat line, indicating that flumazenil enhanced entry into the maze central arena in handled-RLA/Verh rats, whereas a tendency toward the opposite effect was observed in drug-treated and handled-RHA/Verh animals. Thus, the present study emphasizes that the effects of environmental manipulations are partly dependent upon genetic factors, and that pharmacological effects also depend on both genetic and environmentally-induced predisposition.

Infantile (handling) stimulation and behavior in young Roman high- and low-avoidance rats

The effect of infantile handling stimulation on exploratory and emotional behavior of Roman high- and low-avoidance (RHA/Verh and RLA/Verh) weanling rats was investigated. Postnatally handled and nonhandled, 4-week-old males and females from both psychogenetically selected lines were exposed to a hexagonal tunnel maze, including an illuminated central arena. Postnatal handling increased exploratory behavior and decreased emotional reactivity as expressed by increased entries into the central arena and a reduction in defecations in both lines of rats. These effects were more pronounced in the RLA/Verh rats. In agreement with earlier studies using nonselected adult rats, the females of both lines (especially those from the RHA/Verh line) were more sensitive than males to the positive influences of early stimulation.

Childhood experience, interpersonal development, and reproductive strategy: and evolutionary theory of socialization

The concept of "reproductive strategy" drawn from the field of behavioral ecology is applied to the study of childhood experience and interpersonal development in order to develop an evolutionary theory of socialization. The theory is presented in terms of 2 divergent development pathways considered to promote reproductive success in the contexts in which they have arisen. One is characterized, in childhood, by a stressful rearing environment and the development of insecure attachments to parents and subsequent behavior problems; in adolescence by early pubertal development and precocious sexuality; and, in adulthood, by unstable pair bonds and limited investment in child rearing, whereas the other is characterized by the opposite. The relation between this theory and prevailing theories of socialization, specifically, attachment, social-learning, and discrete-emotions theory, is considered and research consistent with our evolutionary theory is reviewed. Finally, directions for future research are discussed.

Beneficial effects of infantile stimulation on coping (avoidance) behavior in rats are prevented by perinatal blockade of benzodiazepine receptors with Ro 15-1788

The present study shows that postnatal 'consistent' handling (CH; which consisted of removing the pups from the nest and placing them individually in plastic cages lined with paper towel for a period of 15 min daily between 1 and 22 postnatal days) of rats had long-lasting improving effects on coping with a stressful task (i.e. enhancement on the early acquisition of two-way active avoidance), but such effects were completely prevented when CH treatment was combined with chronic perinatal Ro 15-1788 (7 mg/kg/day, between prenatal day 19 and postnatal day 22) administration (i.e. blockade of benzodiazepine receptor (BZR)). A long-lasting decremental effect was also observed in the same task in rats which received postnatal 'inconsistent' handling (INCH; in which stimulation of pups was changed every day between 1 and 22 postnatal days), without being affected by the concomitant perinatal Ro 15-1788 treatment. These results suggest that intact ontogeny of BZRs is necessary to obtain the enduring positive effects of CH on emotional behavior (i.e. early acquisition of two-way active avoidance).

Sensorimotor performance and rotation correlate to lesion size in right but not left hemisphere brain infarcts in the spontaneously hypertensive rat

In order to correlate behavioural deficits to lesion size and to reveal possible functional asymmetries in the rat brain, locomotor activity, rotation and sensorimotor integration to touch were studied in spontaneously hypertensive rats (SHR) subjected to right or left middle cerebral artery occlusion. Control and infarcted rats showed no difference in locomotor activity. Infarcted rats tended to rotate towards the side of the lesion. A large sensorimotor deficit was found contralateral to the infarcted hemisphere. The absolute values of the side-biases for the rotation and sensorimotor tests were of the same degree irrespective of lesion side. Whereas the left hemisphere lesion size did not correlate to the behavioural outcome, the size of the right hemisphere lesion was highly correlated to the total sensorimotor deficit. Furthermore, the sensorimotor deficit of specific body parts was found to correlate to the damage of certain brain regions in a rostrocaudal fashion, reminiscent of a somatotopical organization. The extent of ipsilateral rotation correlated to brain tissue loss at the level of the posterior caudate-putamen. The present results indicate an asymmetrical organization for brain functions involved in the performance of the rotation and sensorimotor tests.