Cerebral GABA release and GAD activity in protein- and tryptophan-restricted rats during development

Maternal protein restriction during the lactation period disrupts the ontogenetic development of behavioral traits in male Wistar rat offspring

Neonatal undernutrition in rats results in short- and long-term behavioral and hormonal alterations in the offspring. It is not clear, however, whether these effects are present since the original insult or if they develop at some specific age later in life. Here, we assessed the ontogenetic profile of behavioral parameters associated with anxiety, exploration and memory/learning of Wistar rat offspring that were subjected to protein malnutrition during lactation. Dams and respective litters were separated into two groups: (1) protein-restricted (PR), which received a hypoproteic chow (8% protein) from birth to weaning [postnatal day (PN) 21]; (2) control (C), which received normoproteic chow. Offspring's behaviors, corticosterone, catecholamines, T3 and T4 levels were assessed at PN21 (weaning), PN45 (adolescence), PN90 (young adulthood) or PN180 (adulthood). PR offspring showed an age-independent reduction in the levels of anxiety-like behaviors in the Elevated Plus Maze and better memory performance in the Radial Arm Water Maze. PR offspring showed peak exploratory activity in the Open Field earlier in life, at PN45, than C, which showed theirs at PN90. Corticosterone was reduced in PR offspring, particularly at young adulthood, while catecholamines were increased at weaning and adulthood. The current study shows that considerable age-dependent variations in the expression of the observed behaviors and hormonal levels exist from weaning to adulthood in rats, and that protein restriction during lactation has complex variable-dependent effects on the ontogenesis of the assessed parameters.

Physiological slowing and upregulation of inhibition in cortex are correlated with behavioral deficits in protein malnourished rats

Protein malnutrition during early development has been correlated with cognitive and learning disabilities in children, but the neuronal deficits caused by long-term protein deficiency are not well understood. We exposed rats from gestation up to adulthood to a protein-deficient (PD) diet, to emulate chronic protein malnutrition in humans. The offspring exhibited significantly impaired performance on the 'Gap-crossing' (GC) task after reaching maturity, a behavior that has been shown to depend on normal functioning of the somatosensory cortex. The physiological state of the somatosensory cortex was examined to determine neuronal correlates of the deficits in behavior. Extracellular multi-unit recording from layer 4 (L4) neurons that receive direct thalamocortical inputs and layers 2/3 (L2/3) neurons that are dominated by intracortical connections in the whisker-barrel cortex of PD rats exhibited significantly low spontaneous activity and depressed responses to whisker stimulation. L4 neurons were more severely affected than L2/3 neurons. The response onset was significantly delayed in L4 cells. The peak response latency of L4 and L2/3 neurons was delayed significantly. In L2/3 and L4 of the barrel cortex there was a substantial increase in GAD65 (112% over controls) and much smaller increase in NMDAR1 (12-20%), suggesting enhanced inhibition in the PD cortex. These results show that chronic protein deficiency negatively affects both thalamo-cortical and cortico-cortical transmission during somatosensory information processing. The findings support the interpretation that sustained protein deficiency interferes with features of cortical sensory processing that are likely to underlie the cognitive impairments reported in humans who have suffered from prolonged protein deficiency.

Early postnatal protein malnutrition causes resistance to the anxiolytic effects of diazepam as assessed by the fear-potentiated startle test

Given that protein malnutrition induces structural, neurochemical and functional changes in the CNS, the present study aimed to investigate the effects of different early periods of protein malnutrition on the behavior and reactivity to diazepam (DZ) in a model of anxiety: the fear-potentiated startle (FPS). Male Wistar rats (n = 110) from well-nourished (16 %-protein) or malnourished (6%-protein) litters were distributed in five different groups: W (well-nourished), M7 (malnourished for 7-days, since day 0), M14 (14-days), M21 (21-days) and M28 (28-days). The results obtained in FPS revealed that malnourished-animals acquired the startle response, irrespective of the time they were exposed to the diet. Besides, DZ reduced the startle amplitude in the noise-alone and light-noise trials. The data concerning the total freezing time showed that the expression of this response was affected by malnutrition and varied in accordance with the findings of previous studies in which malnutrition procedures was imposed for long periods (more than 50 days). Therefore, we suggest that early protein malnutrition: (a) did not produce deficits in the associative learning process of these animals in the FPS, and (b) decreased freezing time in the FPS and produce hyporeactivity to the effects of DZ in rats malnourished for 21 days or more, indicating alterations in the GABAergic neurotransmitter system.

Effects of brief stress exposure during early postnatal development in Balb/CByJ mice: II. Altered cortical morphology

Early life experience can significantly determine later mental health status and cognitive function. Neonatal stress, in particular, has been linked to the etiology of mental health disorders as divergent as mood disorder, schizophrenia, and autism. Our study uses a Balb/CByJ mouse model to test the hypothesis, that neonatal stress will alter development and subsequent environmental modulation of neocortex. Using a split litter design, we generated stressed mice (STR) and within litter controls (LMC) along with age-matched, untreated animals (AMC), to serve as across litter controls. Short, daily exposure to a psychosocial/physical stressor, during the first week of life, resulted by adulthood in significant changes in neocortical thickness and architecture, which were further modulated by exposure to behavioral testing. Surprisingly, cortical size in LMC mice was also affected. These observations were compared to the effects of environmental enrichment in the same mouse strain. Our data indicate that LMC and STR males share with environmentally enriched males, an increase in thickness in infra-granular cortical layers, while STR also display a stress selective decrease in supragranular layers, in response to behavioral training as adults.

Differential response to gepirone but not to chlordiazepoxide in malnourished rats subjected to learned helplessness

The learned helplessness (LH) paradigm is characterized by learning deficits resulting from inescapable events. The aims of the present study were to determine if protein-calorie malnutrition (PCM) alters learning deficits induced by LH and if the neurochemical changes induced by malnutrition alter the reactivity to treatment with GABA-ergic and serotonergic drugs during LH. Well-nourished (W) and PCM Wistar rats (61 days old) were exposed or not to inescapable shocks (IS) and treated with gepirone (GEP, 0.0-7.5 mg/kg, intraperitoneally, N = 128) or chlordiazepoxide (0.0-7.5 mg/kg, intraperitoneally, N = 128) 72 h later, 30 min before the test session (30 trials of escape learning). The results showed that rats exposed to IS had higher escape latency than non-exposed rats (12.6 +/- 2.2 vs 4.4 +/- 0.8 s) and that malnutrition increased learning impairment produced by LH. GEP increased the escape latency of W animals exposed or non-exposed to IS, but did not affect the response of PCM animals, while chlordiazepoxide reduced the escape deficit of both W and PCM rats. The data suggest that PCM animals were more sensitive to the impairment produced by LH and that PCM led to neurochemical changes in the serotonergic system, resulting in hyporeactivity to the anxiogenic effects of GEP in the LH paradigm.

Protein malnutrition differentially alters the number of glutamic acid decarboxylase-67 interneurons in dentate gyrus and CA1–3 subfields of the dorsal hippocampus

In 30- and 90-day-old rats, using immunohistochemistry for glutamic acid decarboxylase 67 (GAD-67), we have tested whether malnutrition during different periods of hippocampal development produces deleterious effects on the population of GABA neurons in the dentate gyrus (DG) and cornu Ammonis (CA1-3) of the dorsal hippocampus. Animals were under one of four nutritional conditions: well-nourished controls (Con), prenatal protein malnourished (PreM), postnatal protein malnourished (PostM), and chronic protein malnourished (ChroM). We found that the number of GAD-67-positive (GAD-67+) interneurons was higher in the DG than in the CA1-3 areas of both Con and malnourished groups. Regarding the DG, the number of GAD-67+ interneurons was increased in PreM and PostM and decreased in ChroM at 30 days. At 90 days of age the number of GAD-67+ interneurons was increased in PostM and ChroM and remained unchanged in PreM. With respect to CA1-3, the number of labeled interneurons was decreased in PostM and ChroM at 30 days of age, but no change was found in PreM. At 90 days no changes in the number of these interneurons were found in any of the groups. These observations suggest that 1) the cell death program starting point is delayed in DG GAD-67+ interneurons, and 2) protein malnutrition differentially affects GAD-67+ interneuron development throughout the dorsal hippocampus. Thus, these changes in the number of GAD-67+ interneurons may partly explain the alterations in modulation of dentate granule cell excitability, as well as in the emotional, motivational, and memory disturbances commonly observed in malnourished rats.

Role of maternal corticosterone in the development and maturation of the aminoacidergic systems of the rat brain

Previous studies have suggested an important role for maternal glucocorticoids in the development of the aminoacidergic systems of the rat brain. This study examines the effect of metyrapone (2-methyl-1,2-di-3-pyridyl-1-propanone), i.p.-administered to gestating mothers, on the maturation of the aminoacidergic systems of their offsprings' brains. gamma-Aminobutyric acid (GABA) and glutamate concentrations were determined in male and female offspring at postnatal days (PN) 23 and 90 in four brain areas: the hippocampus, hypothalamus, striatum and cortex. The activity of glutamic acid decarboxylase (GAD), the most important enzyme in the synthesis of GABA, was also analysed. The results show that a reduction in maternal corticosterone during gestation leads to a reduced GABAergic content in all brain areas studied at PN23; permanent organizational changes occurred in the cortex, striatum and hypothalamus. Maternal metyrapone treatment also affected the development of the glutamatergic systems, females being more affected than males at both PN23 and PN90 particularly in the hypothalamus and cortex. The metyrapone treatment produced no changes in GAD activity at PN23, but induced an important increase in this activity at PN90.

GAD67-positive puncta: contributors to learning-dependent plasticity in the barrel cortex of adult mice

We have previously shown that a classical aversive conditioning paradigm involving stimulation of a row of facial vibrissae (whiskers) in the mouse produced expansion of the cortical representation of the activated vibrissae ("trained row"). This was demonstrated by labeling with 2-deoxyglucose (2DG) in layer IV of the barrel cortex. We have also shown that functional reorganization of the S1 cortex is accompanied by increases in the density of small GABAergic cells, and in GAD67 mRNA in the hollows of barrels representing the "trained row". The aim of this study was to determine whether GAD67-positive puncta (boutons) are affected by learning. Unbiased optical disector counting was applied to sections from the mouse barrel cortex that had been immunostained using a polyclonal antibody against GAD67. Quantification of the numerical density of GAD67-positive boutons was performed for four groups of mice: those that had been given aversive conditioning, pseudoconditioned mice with random application of the unconditioned stimulus, mice that had received only whisker stimulation, and naive animals. This study is the first to demonstrate that learning-dependent modification of mature somatosensory cortex is associated with a 50% increase in GAD67-positive boutons in the hollows of "trained" barrels compared with those of control barrels. Sensory learning seems to mobilize the activity of the inhibitory transmission system in the cortical region where plastic changes were previously detected by 2DG labeling.

Gestational and postnatal malnutrition affects sensitivity of young rats to picrotoxin and quinolinic acid and uptake of GABA by cortical and hippocampal slices

It is widely known that a complex interaction between excitatory and inhibitory systems is required to support the adequate functioning of the brain and that significant alterations induced by early protein restriction are complex, involving many systems. Based on such assumptions, we investigated the effects of maternal protein restriction during pregnancy and lactation followed by offspring protein restriction on some GABAergic and glutamatergic parameters, which mediate inhibitory and excitatory transmission, respectively. The sensitivity of young malnourished rats to convulsant actions of the GABA(A) receptor antagonist picrotoxin (PCT; s.c.) and to N-methyl-d-aspartate (NMDA) receptor agonist quinolinic acid (QA; i.c.v) and also gamma-amino-n-butyric acid (GABA) and glutamate uptake by cortical and hippocampal slices were evaluated in P25 old rats. Early protein malnutrition induced higher sensitivity to picrotoxin, which could be associated with the observed higher GABA uptake by cortical, and hippocampal slices in malnourished rats. In contrast, we observed lower sensitivity to quinolinic acid in spite of unaltered glutamate uptake by the same cerebral structures. Picrotoxin enhanced GABA uptake in hippocampus in well- and malnourished rats; however, it did not affect cortical GABA uptake. Our data corroborate our previous report, showing that malnutrition depresses the glutamatergic activity, and point to altered modulation of GABAergic neurotransmission. Such findings allow us to speculate that malnutrition may affect the excitatory and inhibitory interaction.