Héctor González-Pardo

Effects of maternal separation, early handling, and standard facility rearing on orienting and impulsive behavior of adolescent rats

Effects of maternal separation in rats have been extensively investigated, but no studies have examined its effects in rat adolescence. We examined the effects of neonatal infant-mother separation (MS) for 6h/day and early handling (EH) for 10 days during the first 2 weeks of life by comparing MS and EH groups to standard facility reared (SFR) controls. At adolescence, the animals were evaluated in a novel and familiar open-field, the light-dark box, and the sucrose consumption test. Behavioral indices included orienting behavior (rearing frequency and duration), impulsive behavior (movement velocity and risk taking by entering the center of the open field or the light compartment of the light-dark box), hyperactivity (ambulatory distance and stereotypic movement), and reward-seeking behavior (sucrose drinking time). The prolonged MS during the first 2 weeks of life resulted in decreased orienting behavior and increased impulsive behavior in adolescence. Measures of ambulatory and stereotypic movements showed that MS rats were hyperactive in the novel environment whereas EH rats were less active overall. The impulsive/hyperactive phenotype produced by this MS protocol may provide a useful animal model to investigate the neurological basis for the similar behavioral phenotype found in attention deficit/hyperactivity disorder.

Brain differences in newborn rats predisposed to helpless and depressive behavior

Inborn brain differences in metabolic capacity were mapped in congenitally helpless rats, a genetically selected strain predisposed to show helpless and depressive behavior. There are a number of brain regions showing abnormal metabolism in adult congenitally helpless rats. Some of these alterations may be innate while others may be due to environmental factors, such as maternal care and postnatal stress. To identify which brain structures show innate differences, brains of newborn rats from congenitally helpless and non-helpless strains were compared using cytochrome oxidase histochemistry, an endogenous marker of regional metabolic capacity. A smaller subset of regions affected in adults showed significantly less metabolic activity in the newborn brains, including paraventricular hypothalamus, habenula, hippocampus, subiculum, lateral septal nucleus, anterior cingulate cortex, infralimbic cortex, and medial orbitofrontal cortex. A covariance analysis further revealed a striking reduction of functional connectivity in the congenitally helpless brain, including a complete decoupling of limbic forebrain regions from midbrain/diencephalic regions. This pattern of brain metabolism suggests that helplessness vulnerability is linked to altered functioning of limbic networks that are key to controlling the hypothalamic-pituitary-adrenal axis. This implies that vulnerable animals have innate deficits in brain systems that would normally allow them to cope with stress, predisposing them in this manner to more readily develop helpless and depressive behaviors.

Spatial learning of the water maze: progression of brain circuits mapped with cytochrome oxidase histochemistry.

The progression of brain circuits involved in spatial learning tasks is still a matter of debate. In addition, the participation of individual regions at different stages of spatial learning remains a controversial issue. In order to address these questions, we used quantitative cytochrome oxidase histochemistry as a metabolic brain mapping method applied to rats (Rattus norvegicus) trained in a water maze for 1, 3 or 5 days of training. Sustained changes throughout training were found in the lateral septal nucleus and anteroventral thalamic nucleus. As compared to naïve or habituation groups, rats with 1 day of training in the spatial learning task showed involvement of the lateral mammillary nucleus, basolateral amygdala and anterodorsal thalamic nucleus. By 5 days of training, there were mean changes in the hippocampal CA3 field and the prefrontal cortex. The regions involved and their pattern of network interactions changed progressively over days of training. At 1-day there was an open serial network of pairwise correlations. At 3-days there was a more closed reciprocal network of intercorrelations. At 5-days there were three separate parallel networks. In addition, brain-behavior correlations showed that CA1 and CA3 hippocampal fields together with the parietal cortex are related to the mastery of the spatial learning task. The present study extends previous findings on the progressive contribution of neural networks to spatial learning.

Sex-related differences in spatial learning during the early postnatal development of the rat

Some authors have reported that male rats younger than 21 days old are unable to perform spatial learning correctly because they have still not developed the ability to use extra-maze cues. In experiment 1, we analyzed spatial learning in 14-, 21-, 30- and 42-day-old rats using the Morris water maze (MWM). According to our results, a good performance was observed in 30-day-old male rats whereas this was not observed in female rats until they were 42 days old. In experiment 2 we studied the role of sex hormones in this kind of learning using the MWM and 30-day-old rats (castrated male rats and female rats treated with testosterone propionate (TP) after birth). The latter group, the male control group and the castrated males all solved the task correctly. The objective of experiment 3 was to determine possible differences between the sexes in the use of taxon strategies in the T water maze. To summarize, sexual dimorphism was only observed in spatial learning during development.

Changes in brain oxidative metabolism induced by water maze training.

Although the hippocampus has been shown to be essential for spatial memory, the contribution of associated brain regions is not well established. Wistar rats were trained to find a hidden escape platform in the water maze during eight days. Following training, the oxidative metabolism in different brain regions was evaluated using cytochrome oxidase histochemistry. Metabolic activations were found in the prelimbic cortex, cornu ammonis (CA) 1 subfield of the dorsal hippocampus and the anterior thalamic nuclei, relative to yoked swim controls and naïve rats. In addition, many cross-correlations in brain metabolism were observed among the latter regions. These results support the implication of a hippocampal-prefrontal-thalamic system to spatial memory in rats.

Acute effects of 17β-estradiol and genistein on insulin sensitivity and spatial memory in aged ovariectomized female rats

Aging is characterized by decline in metabolic function and insulin resistance, and both seem to be in the basis of neurodegenerative diseases and cognitive dysfunction. Estrogens prevent age-related changes, and phytoestrogens influence learning and memory. Our hypothesis was that estradiol and genistein, using rapid-action mechanisms, are able to modify insulin sensitivity, process of learning, and spatial memory. Young and aged ovariectomized rats received acute treatment with estradiol or genistein. Aged animals were more insulin-resistant than young. In each age, estradiol and genistein-treated animals were less insulin-resistant than the others, except in the case of young animals treated with high doses of genistein. In aged rats, no differences between groups were found in spatial memory test, showing a poor performance in the water maze task. However, young females treated with estradiol or high doses of genistein performed well in spatial memory task like the control group. Only rats treated with high doses of genistein showed an optimal spatial memory similar to the control group. Conversely, acute treatment with high doses of phytoestrogens improved spatial memory consolidation only in young rats, supporting the critical period hypothesis for the beneficial effects of estrogens on memory. Therefore, genistein treatment seems to be suitable treatment in aged rats in order to prevent insulin resistance but not memory decline associated with aging. Acute genistein treatment is not effective to restore insulin resistance associated to the early loss of ovarian function, although it can be useful to improve memory deficits in this condition.

Influence of gonadal steroids on the glial fibrillary acidic protein‐immunoreactive astrocyte population in young rat hippocampus

It is known that expression of glial fibrillary acidic protein (GFAP) as an astrocyte‐specific marker can be regulated by levels of circulating gonadal steroids during postnatal development. In addition, astrocytes play an important role in the physiology of the hippocampus, a brain region considered sexually dimorphic at the neuronal level in rodents. To evaluate the contribution of glial cells to gender‐related differences in the hippocampus, we estimated the number of GFAP‐immunoreactive (GFAP‐IR) astrocytes in the hippocampus (CA1 and CA3 areas, dorsal and ventral regions) of male and female rats aged 30 days. Groups of 30‐day‐old masculinized females (TP‐females; injected with testosterone propionate at birth) and feminized males (FLU‐males, castrated and treated with flutamide, an androgen receptor antagonist) were included to assess the effects of gonadal hormones on these hippocampal astrocytes. Using the optical fractionator method, the total number of GFAP‐IR cells found in CA1 and CA3 areas was significantly higher in males compared to that in age‐matched females. This numerical pattern was reversed in TP‐females and FLU‐males in both hippocampal areas. In addition, more GFAP‐IR cells were found in dorsal hippocampus than in the ventral region in the CA1 area from all experimental groups, whereas this result was found in the CA3 area from males and TP‐females. Our results suggest an essential contribution of gonadal hormones to gender differences found in the astrocyte population of the rat hippocampus during development.

Involvement of the mammillary bodies in spatial working memory revealed by cytochrome oxidase activity

In view of the inconclusive findings relating the nuclei of the mammillary bodies (MB) with spatial memory, we evaluated the oxidative metabolic activity of the medial and lateral nuclei of the mammillary bodies (MB) after training young rats (30 days) of both sexes in the Morris water maze. Different groups were trained in spatial working (WM) or reference memory (RM) tasks, respectively. The corresponding naïve groups swam for the same amount of time as the trained groups but without the escape platform. Control groups were added that had not been manipulated in any way. No sex-related differences were detected in the working memory task although males exhibited better reference memory than females. Cytochrome oxidase (CO) activity, an endogenous metabolic marker for neuronal activity, was measured in all the groups. CO activity increased significantly in both MB nuclei of male and female rats only in the spatial working memory group. In addition, high CO activity in the lateral nucleus of the MB was linearly correlated with lower escape latencies in both sexes after training in the working memory task. No CO activity changes were found in the basolateral amygdala (BL) in any of the experimental groups. This nucleus was used as a control brain region because of its participation in emotional behavior. The results suggest a specific role of the MB nuclei in spatial working memory in both sexes.

Adolescent female rats are more resistant than males to the effects of early stress on prefrontal cortex and impulsive behavior

We tested the hypothesis that adolescent Sprague-Dawley females may be more resistant than males to display impulsive behavior and lower prefrontal cortex thickness after mother-infant separation (MS). Starting at postnatal day 2 (P2), the MS group was separated 6 hr/day and the early handled (EH) group 15 min/day for 10 days, and another group was standard facility reared (SFR). Subjects were examined for novel open-field activity (P28), light-dark apparatus (P29), familiar open-field (P30) and frontal cortical thickness. This protocol resulted in impulsive behavior in MS rats relative to EH and SFR, but this effect was less pronounced in females than males. MS affected the two sexes differently in terms of decreased prefrontal cortex dorsoventral thickness, with this effect being significant in males but not females. Neuroanatomical and behavioral documentation that adolescent females are more resistant than males to ADHD-like effects of maternal separation have not been previously reported.

Epigenetics and its implications for Psychology

BACKGROUND Epigenetics is changing the widely accepted linear conception of genome function by explaining how environmental and psychological factors regulate the activity of our genome without involving changes in the DNA sequence. Research has identified epigenetic mechanisms mediating between environmental and psychological factors that contribute to normal and abnormal behavioral development. METHOD the emerging field of epigenetics as related to psychology is reviewed. RESULTS the relationship between genes and behavior is reconsidered in terms of epigenetic mechanisms acting after birth and not only prenatally, as traditionally held. Behavioral epigenetics shows that our behavior could have long-term effects on the regulation of the genome function. In addition, epigenetic mechanisms would be related to psychopathology, as in the case of schizophrenia. In the latter case, it would be especially relevant to consider epigenetic factors such as life adversities (trauma, disorganized attachment, etc.) as related to its clinical manifestations, rather than genetic factors. Moreover, epigenetics implies overcoming classical dualist dichotomies such as nature-nurture, genotype-phenotype or pathogenesis-pathoplasty. CONCLUSIONS In general, it can be stated that behavior and environment will finally take on a leading role in human development through epigenetic mechanisms.