Neonatal cryoanesthesia affects the morphology of the visual cortex in the adult rat

Changes in sex differences in neuroanatomical structure and cognitive behavior across the life span

Sex differences occur in the structure and function of the rat cerebral cortex and hippocampus, which can change from the juvenile period through old age. Although the evidence is incomplete, it appears that in at least some portions of the cortex these differences develop due to the rise of ovarian hormones at puberty and are potentially not dependent on the perinatal rise in testosterone, which is essential for sexual differentiation of the hypothalamus and sexual behavior. During aging of female rats, the presence of continued ovarian hormone secretion after cessation of the estrous cycle also influences sex differences in neuroanatomical structure and cognitive behavior, resulting in nullification or reversal of sex differences seen in younger adults. Sex differences can be altered by experience in a stimulating environment during the juvenile/adolescent period, and sex differences in performance even can be affected by the parameters of a task. Thus, broad generalizations about differences such as "spatial ability" are to be avoided. It is clear that to understand how the brain produces behavior, sex and hormones have to be taken into account.

The effect of brief neonatal cryoanesthesia on physical development and adult cognitive function in mice

Deep hypothermia (cryoanesthesia) is often used as general anesthesia during surgery in neonatal rodents. Neonatal cryoanesthesia has been used recently to generate somatic brain transgenic (SBT) mouse models via intracerebral ventricular injection of rAAV vectors into both non-transgenic mice and numerous transgenic mouse models. Since, the evaluation of cognition is one of the main experimental endpoints in many of these studies, we examined the consequences of brief neonatal cryoanesthesia on the physical development and mnemonic function of adult mice. Two groups of 129FVBF1 pups from reciprocal breeding crosses underwent cryoanesthesia for 6 min (Cryo6) or 12 min (Cryo12), respectively, within the first hours (<12h) of postnatal life. A group of pups separated from the nest and kept in ambient temperature of 33 °C for 6 min served as a control. Our results revealed that lowering the temperature of pups to ~8 °C (Cryo6) or ~5 °C (Cryo12) did not affect their body weight at pre-weaning stage and in the adulthood. The evaluation of cognitive function in adult mice revealed strong and comparable to control spatial reference, and context and tone fear memories of neonatally cryoanesthetized mice. Also, the experimental and control groups had comparable brain weight at the end of the study. Our results demonstrate that neonatal cryoanesthesia, lasting up to 12 min, has no adverse effects on the body weight of mice during development, and on their cognition in the adulthood.

Independently outgrowing neurons and geometry-based synapse formation produce networks with realistic synaptic connectivity

Neuronal signal integration and information processing in cortical networks critically depend on the organization of synaptic connectivity. During development, neurons can form synaptic connections when their axonal and dendritic arborizations come within close proximity of each other. Although many signaling cues are thought to be involved in guiding neuronal extensions, the extent to which accidental appositions between axons and dendrites can already account for synaptic connectivity remains unclear. To investigate this, we generated a local network of cortical L2/3 neurons that grew out independently of each other and that were not guided by any extracellular cues. Synapses were formed when axonal and dendritic branches came by chance within a threshold distance of each other. Despite the absence of guidance cues, we found that the emerging synaptic connectivity showed a good agreement with available experimental data on spatial locations of synapses on dendrites and axons, number of synapses by which neurons are connected, connection probability between neurons, distance between connected neurons, and pattern of synaptic connectivity. The connectivity pattern had a small-world topology but was not scale free. Together, our results suggest that baseline synaptic connectivity in local cortical circuits may largely result from accidentally overlapping axonal and dendritic branches of independently outgrowing neurons.

Sexual differentiation of the adolescent rodent brain: hormonal influences and developmental mechanisms

This article is part of a Special Issue "Puberty and Adolescence". Sexual differentiation is the process by which the nervous system becomes structurally and functionally dissimilar in females and males. In mammals, this process has been thought to occur during prenatal and early postnatal development, when a transient increase in testosterone secretion masculinizes and defeminizes the developing male nervous system. Decades of research have led to the views that structural sexual dimorphisms created during perinatal development are passively maintained throughout life, and that ovarian hormones do not play an active role in feminization of the nervous system. Furthermore, perinatal testosterone was thought to determine sex differences in neuron number by regulating cell death and cell survival, and not by regulating cell proliferation. As investigations of neural development during adolescence became more prominent in the late 20th century and revealed the extent of brain remodeling during this time, each of these tenets has been challenged and modified. Here we review evidence from the animal literature that 1) the brain is further sexually differentiated during puberty and adolescence; 2) ovarian hormones play an active role in the feminization of the brain during puberty; and 3) hormonally modulated, sex-specific addition of new neurons and glial cells, as well as loss of neurons, contribute to sexual differentiation of hypothalamic, limbic, and cortical regions during adolescence. This architectural remodeling during the adolescent phase of sexual differentiation of the brain may underlie the known sex differences in vulnerability to addiction and psychiatric disorders that emerge during this developmental period.

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.

Therapeutic effects of complex rearing or bFGF after perinatal frontal lesions

We investigated the effects of an enriched environment and/or basic fibroblast growth factor (bFGF) on recovery from neonatal frontal injury in rats. Rats received medial frontal lesions, or sham surgery, on postnatal day (P) 2/3. In the first set of experiments (Experiments 1 and 2), rats were housed in enriched environments that consisted of a large enclosure with multiple objects (or standard housing) for 90 days beginning at weaning (P22) or in adulthood (P110). In Experiment 3, the rats either received 7 days of subcutaneous bFGF beginning on the day after surgery or bFGF plus enriched housing beginning at weaning. After the 90-day housing period, the animals were tested on a spatial navigation task and a skilled reaching task. Early lesions of the medial frontal cortex caused severe impairments in spatial learning but this deficit was markedly reduced with enriched housing, bFGF, or a combination of both, with the latter being most effective. The housing effects varied with age, however: the earlier the experience began, the better the outcome. Enriched housing increased dendritic length in cortical pyramidal neurons, an effect that was greater in the lesion than the control animals, and enriched housing reversed the lesion-induced decrease in spine density. Enriched environment increased the thickness of the cortical mantle in both lesion and controls whereas bFGF had no effect. Experience thus can affect functional and anatomical outcome after early brain injury but the effects vary with age at experience and may be facilitated by treatment with bFGF.

Response to neonatal anesthesia: effect of sex on anatomical and behavioral outcome

Numerous studies have documented the consequences of exposure to anesthesia in models of term and post-term infants, evaluating the incidence of cell loss, physiological alterations and cognitive dysfunction. However, surprisingly few studies have investigated the effect of anesthetic exposure on outcomes in newborn rodents, the developmental equivalent of premature human infants. This is critical given that one out of every eight babies born in the United States is premature, with an increased prevalence of surgical procedures required in these individuals. Also, no studies have investigated if the genetic sex of the individual influences the response to neonatal anesthesia. Using the newborn rat as the developmental equivalent of the premature human, we documented the effect of a single bout of exposure to either the inhalant isoflurane or the injectable barbiturate phenobarbital on hippocampal anatomy, hippocampal dependent behavioral performance and normal developmental endpoints in male and female rats. While both forms of anesthesia led to significant decrements in cognitive abilities, along with a significant reduction in volume and neuron number in the hippocampus in adulthood, the decrements were significantly greater in males than in females. Interestingly, the deleterious effects of anesthesia were manifest on developmental measures including surface righting and forelimb grasp, but were not evident on basic physiological parameters including body weight or suckling. These findings point to the hazardous effects of exposure to anesthesia on the developing CNS and the particular sensitivity of males to deficits.

Deficient social and play behavior in juvenile and adult rats after neonatal cortical lesion: effects of chronic pubertal cannabinoid treatment

The aim of the present study was to investigate the effects of neonatal excitotoxic lesions of the medial prefrontal cortex (mPFC) on social play, social behavior unrelated to play, and self-grooming in juvenile and adult rats. We additionally examined the behavioral effects of chronic pubertal treatment with the cannabinoid agonist WIN 55,212-2 (WIN) in order to test the hypothesis that early lesions render the brain vulnerable to cannabinoid intake in later life. Neonatal mPFC lesions and pubertal WIN treatment disrupted social play, social behavior, and self-grooming in juvenile and adult rats. Additionally, we observed more social play behaviors during light cycle in WIN-treated than in vehicle-treated rats. Notably, the combination of surgery and WIN treatment disrupted social behavior in lesioned and sham-lesioned rats. The present data indicate that the mPFC is important for adequate juvenile response selection in the context of social play and might be involved in the development of adult social and nonsocial behavior. Moreover, our data add further evidence for an involvement of the cannabinoid system in anxiety and social behavior. Additive effects of neonatal surgery-induced stress or cortical lesions in combination with pubertal cannabinoid administration are also shown. The disturbances of social and nonsocial behavior in rats are comparable to symptoms of early frontal cortex damage, as well as neurodevelopmental disorders in humans, such as schizophrenia and autism. Therefore, we propose the combination of neonatal cortical lesions with chronic cannabinoid administration during puberty as an animal model for studying neuronal mechanisms of impaired social functioning in neuropsychiatric disorders.

Sex-specific alterations of cerebral cortical cell size in rats exposed prenatally to dioxin

Sex-specific patterns of cerebral cortical lateralization have been documented consistently in both the human and animal brain. Male rats tend to exhibit pronounced right hemisphere dominance compared with females, while females typically exhibit more diffuse lateralization patterns and greater left hemisphere bias compared with males. Prenatal TCDD (2,3,7,8 tetrachlorodibenzo-p-dioxin) exposure produces demasculinization of male offspring sexual behavior, suggesting interference with sexual differentiation of the brain. In previous studies, a reversal of cortical thickness patterns in rats was shown after prenatal TCDD exposure on gestational day 8 (GD 8). The current study, based on the same brain sections, attempted to define changes in the number of cortical cells and cell size distributions in brains of offspring from TCDD-treated dams. Pregnant females were given a single oral dose of 0 or 180 ng kg(-1) TCDD on GD 8. Cell counts and sizes were determined in 3-month-old offspring. Areas 17 and 18a at bregma -3.8 were analysed using digitized, enhanced images of brain sections produced by a photomicroscope fitted with a high-resolution digital camera. Prenatal TCDD exposure altered the relative proportions of smaller and larger cell sizes in male, but not in female offspring. Both exposed males and females, however, exhibited a significant reversal of hemispheric dominance based on cell number. These findings demonstrate that prenatal exposure to TCDD alters the normal patterns of cortical cell asymmetry in a manner consistent with our previous data on thickness patterns.

Androgen receptor expression in the developing male and female rat visual and prefrontal cortex

Gonadal steroid hormones are known to influence the development of the cerebral cortex of mammals. Steroid hormone action involves hormone binding to cytoplasmic or nuclear receptors, followed by DNA binding and gene transcription. The goals of the present study were twofold: to determine whether androgen receptors are present during development in two known androgen sensitive regions of the rat cerebral cortex, the primary visual cortex (Oc1) and the anterior cingulate/frontal cortex (Cg1/Fr2); and to determine whether androgen receptor (AR) expression in these regions differs between developing males and females. We used immunocytochemistry to detect AR protein on postnatal days 0, 4, and 10, and in situ hybridization to detect AR mRNA on postnatal day 10 in male and female rats. The level of AR expression was specific to the cortical region, with higher AR immunoreactive cell density and more AR mRNA in Oc1 than in Cg1/Fr2. AR immunoreactive cell density increased with age in both regions. Finally, on postnatal day 10, males had a higher AR immunoreactive cell density and more AR mRNA in Oc1 than did females. Thus, the presence of ARs may allow androgens to directly influence the development the cerebral cortex.

Manipulation of gonadal hormones in neonatal rats alters the morphological response of cortical neurons to brain injury in adulthood

The authors examined the effects of sex and neonatal hormones on the response of pyramidal cells (Layer III, parietal cortex) to injury of the medial frontal cortex in the adult rat. At birth, males were gonadectomized (GDX) or sham-operated. Females were given testosterone (T) or oil injections. In adulthood, rats that had been left intact at birth were GDX, and they then received bilateral medial frontal cortex lesions or sham surgery. Rats not exposed to T at birth exhibited losses of dendritic arbor (males GDX at birth) or dendritic spine density (oil-treated females). Compensation after cortical injury is dependent on the rat's sex and history of exposure to gonadal steroids.

Neonatal sex steroids affect responses to Seoul virus infection in male but not female Norway rats

Previous studies illustrate that after inoculation with Seoul virus (i.e., the naturally occurring hantavirus found in Norway rats), adult male rats produce higher antibody responses, exhibit higher Th1 responses (i.e., IgG2a, IL-2, and IFN gamma), and shed virus longer than females, but these difference are not altered by manipulation of sex steroids in adulthood. To determine whether sex steroid hormones organize adult responses to hantavirus infection, at 2-4 days of age, male rats were gonadectomized and female rats were injected with testosterone. As adults, animals were inoculated with 10(4)pfu of Seoul virus. Neonatally gonadectomized males (NGM), control females (CF), and neonatal testosterone-treated females (NTF) had lower anti-Seoul virus IgG and IgG2a responses than control males (CM) 20, 30, and 40 days post-inoculation. Neonatal testosterone treatment had no effect on female antibody responses to infection. NGM, CF, and NTF shed virus in saliva and feces for a shorter duration than CM. There was no effect of neonatal hormone manipulation either on the percentage of animals with detectable virus or on the number of virus copies within each target organ. Genetic males, regardless of hormone manipulation, had higher virus replication in lung tissue than did genetic females. Neonatal sex steroids influence the sexual dimorphism in host immune function but do not modify virus replication in target tissues.

Ovarian hormones after postnatal day 20 reduce neuron number in the rat primary visual cortex

Previous work from our lab has documented a sex difference in neuron number in the binocular region of the adult rat primary visual cortex (Oc1B), with males having 19% more neurons than females. In the present study, the role of developmental steroid hormones in the formation of this difference was explored. Male and female rats underwent neonatal hormone manipulation (female + testosterone or dihydrotestosterone; male + flutamide) followed by gonadectomy on postnatal day 20. Animals that did not undergo hormone manipulation were either gonadectomized or sham operated at day 20. Neuron number was quantified in the monocular (Oc1M) and binocular (Oc1B) subfields of the adult rat primary visual cortex using the optical disector technique. As adults, day 20 gonadectomized females, as well as females + testosterone and females + dihydrotestosterone, had significantly more neurons than intact females. There was no difference in neuron number between postnatal day 20 gonadectomized males, males + flutamide, and intact males. Also, intact males had significantly more neurons than intact females in both in Oc1M and Oc1B. It appears that ovarian steroids after day 20 are the primary cause of the lower number of neurons in the primary visual cortex of the female rat.

Quantifying presynaptic terminals at the light microscope level in intact and deafferented central nervous structures

Quantification of presynaptic terminals often requires laborious techniques that involve tissue preparation for ultrastructural analysis. Modern preembedding immunohistochemical techniques provide a high morphological resolution at the light microscope level, thus allowing us to identify immunostained presynaptic boutons using specific antibodies. When absolute density of boutons (D(a)) is analysed for comparison between control and deafferented nervous tissue, quantification may be distorted due to tissue shrinkage that follows deafferentiation. The magnitude of this effect must be, therefore, estimated to correct quantitative data. Using the superior colliculus (SC) as a model, an easily applicable protocol to quantify the density of small size labelled particles in control and deafferented nervous tissue is described. This protocol was used to analyse the effect of neonatal and adult enucleation on the adult pattern of cholinergic input to the rat SC. Statistical treatment of data demonstrated that neonatal enucleation caused a drastic increase in bouton density in the visual collicular layers, stratum zonale (SZ) and stratum griseum superficiale (SGS). The same lesion carried out in adult animals caused an increase in the bouton density exclusively in the SZ.

Cryoanethesia on postnatal day 1, but not day 10, affects adult behavior and cortical morphology in rats

Hypothermia was used to induce anesthesia in infant rats on postnatal days 1 or 10. In adulthood measures of spatial learning and activity were taken before the brains were harvested for measurement of cortical thickness and dendritic arborization in layer III pyramidal cells in parietal cortex. Cryoanethesia on day 1, but not day 10, produced a small, but statistically significant, impairment in learning a spatial navigation task as well as a reduction in cortical thickness and dendritic arborization. This study confirms that cryoanesthesia is not a benign treatment in newborn pups but appears to be without effect in older animals. It is important that all studies using cryoanesthesia have sham control animals exposed to the same degree of hypothermia.

Androgens reduce cell death in the developing rat visual cortex

We have previously shown that males have more neurons than females in the primary visual cortex, and neonatal androgens play an important role in this difference. Also, we have found that females experience more cell death during development in this region than in males. Therefore, we hypothesized that the neonatal hormone environment directly influences the amount of cell death. In the present experiment, female rats were implanted with dihydrotestosterone (DHT) or estradiol at postnatal day 1. These animals, along with control males and females, were sacrificed on postnatal days 6, 11 and 25. Using unbiased stereology to quantify neuronal and pyknotic cell density, we observed that females implanted with DHT had a similar pattern and proportion of cells dying as control males. Additionally, developmental cell death in females implanted with estradiol was not significantly different than control females. Thus, neonatal androgens have an inhibitory effect on developmental cell death in the rat primary visual cortex.

Neonatal halothane anesthesia affects cortical morphology

Neonatal cryoanesthesia has recently been documented to affect morphology and behavior after a single exposure [Dev. Brain Res. 111 (1998) 89; Horm. Behav. 37 (2000) 169]. In the current experiment, we investigated the effect of one-time exposure to halothane inhalant anesthesia on neonatal rats of both sexes. Fifteen minutes of exposure on postnatal day one resulted in detectable changes in the volume of the visual cortex at 3 months. Thus, neonatal halothane alters neural development and its effects are observable in the adult rat.

Organizational and activational effects of gonadal steroid hormones on the EEG of male and female rats

To analyze organizational and activational effects of sex steroids on adult rat electroencephalographic activity (recorded at postnatal day 100), seven groups were included: males (48)-intact, neonatally or adult castrated; females (64)-intact, ovariectomized and exposed pre- or neonatally to testosterone propionate. In males, neonatal orchidectomy increased beta relative power, whereas both neonatal and adult castration reduced interparietal correlation. In females, prenatal testosterone administration produced higher theta absolute power; theta relative power was higher in all experimental groups, whereas beta1 and beta2 were decreased by prenatal and increased by neonatal virilization; prenatal virilization enhanced, while neonatal virilization and adult ovariectomy decreased interparietal correlation. These data indicate that females are more sensitive to early prenatal than to neonatal organizational effects of sex steroids, and some electroencephalographic features are feminized in castrated males and virilized in perinatally androgenized females.

Hippocampal anatomy and water maze performance are affected by neonatal cryoanesthesia in rats of both sexes

There is recent evidence that cryoanesthesia, commonly used during neonatal hormone manipulations (e.g., gonadectomy), has deleterious effects on the morphology of the splenium of the corpus callosum and primary visual cortex in adult rats of both sexes. (Nuñez and Juraska, 1998; Nuñez, Kim, and Juraska, 1998). In the present study, the effect of neonatal cryoanesthesia on the morphology of the hippocampus and dentate gyrus and on performance in the Morris water maze was investigated. Cold exposure for as brief as 30 min (5 degrees C) on Postnatal Day 1 resulted in a significant decrease in the volume of the hippocampus and in brain weight of adults. Performance on the water maze was also impaired in cold-exposed animals. This study indicates that not only morphology but also behavioral performance in adulthood are affected by neonatal cryoanesthesia.