Sex differences in anterior commissure size in the rat

Transient Hypothyroidism During Lactation Arrests Myelination in the Anterior Commissure of Rats. A Magnetic Resonance Image and Electron Microscope Study

Thyroid hormone deficiency at early postnatal ages affects the cytoarchitecture and function of neocortical and telencephalic limbic areas, leading to impaired associative memory and in a wide spectrum of neurological and mental diseases. Neocortical areas project interhemispheric axons mostly through the corpus callosum and to a lesser extent through the anterior commissure (AC), while limbic areas mostly project through the AC and hippocampal commissures. Functional magnetic resonance data from children with late diagnosed congenital hypothyroidism and abnormal verbal memory processing, suggest altered ipsilateral and contralateral telencephalic connections. Gestational hypothyroidism affects AC development but the possible effect of transient and chronic postnatal hypothyroidism, as occurs in late diagnosed neonates with congenital hypothyroidism and in children growing up in iodine deficient areas, still remains unknown. We studied AC development using in vivo magnetic resonance imaging and electron microscopy in hypothyroid and control male rats. Four groups of methimazole (MMI) treated rats were studied. One group was MMI-treated from postnatal day (P) 0 to P21; some of these rats were also treated with L-thyroxine (T4) from P15 to P21, as a model for early transient hypothyroidism. Other rats were MMI-treated from P0 to P150 and from embryonic day (E) 10 to P170, as a chronic hypothyroidism group. The results were compared with age paired control rats. The normalized T2 signal using magnetic resonance image was higher in MMI-treated rats and correlated with the number and percentage of myelinated axons. Using electron microscopy, we observed decreased myelinated axon number and density in transient and chronic hypothyroid rats at P150, unmyelinated axon number increased slightly in chronic hypothyroid rats. In MMI-treated rats, the myelinated axon g-ratio and conduction velocity was similar to control rats, but with a decrease in conduction delays. These data show that early postnatal transient and chronic hypothyroidism alters AC maturation that may affect the transfer of information through the AC. The alterations cannot be recovered after delayed T4-treatment. Our data support the neurocognitive delay found in late T4-treated children with congenital hypothyroidism.

The size of non-hippocampal brain regions varies by season and sex in Richardson's ground squirrel

Sex- and season-specific modulation of hippocampal size and function is observed across multiple species, including rodents. Other non-hippocampal-dependent behaviors exhibit season and sex differences, and whether the associated brain regions exhibit similar variation with sex and season remains to be fully characterized. As such, we examined the brains of wild-caught Richardson's ground squirrels (RGS; Urocitellus richardsonii) for seasonal (breeding, non-breeding) and sex differences in the volumes of specific brain areas, including: total brain volume, corpus callosum (CC), anterior commissure (AC), medial prefrontal cortex (mPFC), total neocortex (NC), entorhinal cortex (EC), and superior colliculus (SC). Analyses of variance and covariance revealed significant interactions between season and sex for almost all areas studied, primarily resulting from females captured during the breeding season exhibiting larger volumes than females captured during the non-breeding season. This was observed for volumes of the AC, mPFC, NC, EC, and SC. Where simple main effects of season were observed for males (the NC and the SC), the volume advantage favoured males captured during the NBr season. Only two simple main effects of sex were observed: males captured in the non-breeding season had significantly larger total brain volume than females captured in the non-breeding season, and females captured during the breeding season had larger volumes of the mPFC and EC than males captured in the breeding season. These results indicate that females have more pronounced seasonal differences in brain and brain region sizes. The extent to which seasonal differences in brain region volumes vary with behaviour is unclear, but our data do suggest that seasonal plasticity is not limited to the hippocampus and that RGS is a useful mammalian species for understanding seasonal plasticity in an ecologically relevant context.

Subsite awareness in neuropathology evaluation of National Toxicology Program (NTP) studies: a review of select neuroanatomical structures with their functional significance in rodents

This review article is designed to serve as an introductory guide in neuroanatomy for toxicologic pathologists evaluating general toxicity studies. The article provides an overview of approximately 50 neuroanatomical subsites and their functional significance across 7 transverse sections of the brain. Also reviewed are 3 sections of the spinal cord, cranial and peripheral nerves (trigeminal and sciatic, respectively), and intestinal autonomic ganglia. The review is limited to the evaluation of hematoxylin and eosin-stained tissue sections, as light microscopic evaluation of these sections is an integral part of the first-tier toxicity screening of environmental chemicals, drugs, and other agents. Prominent neuroanatomical sites associated with major neurological disorders are noted. This guide, when used in conjunction with detailed neuroanatomic atlases, may aid in an understanding of the significance of functional neuroanatomy, thereby improving the characterization of neurotoxicity in general toxicity and safety evaluation studies.

The development of myelin in the brain of the juvenile rat

The development process of myelination varies between region and species. Fully myelinated fibers are required if mammalian neural circuits are to function normally. Histology samples at staggered time points throughout the study were examined at days 4, 5, 7, 8, 10, 14, 17, 24, 37, and 44. We suggest that the development of myelin in the juvenile rodent brain can be conveniently separated into 3 phases. Evaluation of myelin basic protein-stained sections of the areas of brain that contain the elements of the developing limbic system over the sensitive period from postnatal day (PND) 14 to 34 may provide an insight into possible toxicity that may lead to cognition and learning issues in adults. We will hope to develop this notion further in the future. The precise chronology of the development of the blood-brain barrier in rats has yet to be established; thus, there is potential for significant exposure of the juvenile brain to chemicals that do not cross the blood-brain barrier in the adult. Thus, it is suggested that evaluation of myelin development should probably be extended to all new chemical entities intended for pediatric use, and not just those that are intended for central nervous system use.

The morphological and developmental changes of the anterior commissure of male Wistar rats submitted to protein malnutrition in the postnatal period

The aim of this project was to study the effects of protein malnutrition in the anterior commissure of male albino Wistar rats, submitted to diets with different levels of protein to evaluate the possible morphological and developmental changes of the myelinated nervous fibers and glial cells of this important neural structure during the postnatal period until 120 days of age. In this study the animals (dams and pups) were divided into three groups: the control group which received a commercial diet from the laboratory (22% protein); the nourished group which received a diet of 16% of protein (casein); and the malnourished group which received a diet of 6% of protein (casein). Rat pups were fed by lactating females during the lactation period. After weaning the pups received the same diet until 120 days of age. The results obtained showed that the malnourished animals presented less body and brain growth and important morphological changes in the anterior commissure such as the retard in the myelination, damaged myelinic nervous fibers, and the increase of the interstitial space characterizing edema and vacuolization. The current study shows that the effects of protein malnutrition in the anterior commissure cause a bigger alteration in the posterior limb of the referred commissure, indicating that it is more vulnerable to malnutrition and more susceptible to damages of the anterior limb.

The control of sexual differentiation of the reproductive system and brain

This review summarizes current knowledge of the genetic and hormonal control of sexual differentiation of the reproductive system, brain and brain function. While the chromosomal regulation of sexual differentiation has been understood for over 60 years, the genes involved and their actions on the reproductive system and brain are still under investigation. In 1990, the predicted testicular determining factor was shown to be theSRYgene. However, this discovery has not been followed up by elucidation of the actions of SRY, which may either stimulate a cascade of downstream genes, or inhibit a suppressor gene. The number of other genes known to be involved in sexual differentiation is increasing and the way in which they may interact is discussed. The hormonal control of sexual differentiation is well-established in rodents, in which prenatal androgens masculinize the reproductive tract and perinatal oestradiol (derived from testosterone) masculinizes the brain. In humans, genetic mutations have revealed that it is probably prenatal testosterone that masculinizes both the reproductive system and the brain. Sexual differentiation of brain structures and the way in which steroids induce this differentiation, is an active research area. The multiplicity of steroid actions, which may be specific to individual cell types, demonstrates how a single hormonal regulator, e.g. oestradiol, can exert different and even opposite actions at different sites. This complexity is enhanced by the involvement of neurotransmitters as mediators of steroid hormone actions. In view of current environmental concerns, a brief summary of the effects of endocrine disruptors on sexual differentiation is presented.

Sex differences in the hippocampal dentate gyrus of the guinea-pig before puberty

The aim of the present research was to ascertain the presence of sex differences in the hippocampal dentate gyrus of the guinea-pig, a long-gestation rodent which gives birth to mature young and whose brain is at a more advanced stage of maturation at birth than that of the rat and mouse. The brains of neonatal (15-16 days old) and prepubescent (45-46 days old) male and female guinea pigs were Golgi-Cox stained. Granule cells were sampled from the upper (suprapyramidal) and lower (infrapyramidal) blade of the septal dentate gyrus and their dendritic tree and soma were measured. The analysis was conducted separately on granule cells with soma in the superficial (superficial granule cells) and deep (deep granule cells) half of the granule cell layer. Numerous sex differences were found in the upper blade of the dentate gyrus. Neonatal males had more dendritic branches than females in the innermost dendritic tree of both superficial and deep granule cells, but females had more branches over the middle/outer dendritic tree and a longer dendritic length. In prepubescent animals, the sex difference in the middle dendritic tree of the superficial granule cells changed direction, with males having more branches than females. In the deep granule cells, the sex differences were similar to those in neonatal animals. In both granule cell types, the dendritic length was similar in the two sexes. While no sex differences were found in dendritic spine density in neonatal animals, in prepubescent animals spine density was greater in females. In the lower blade the granule cells showed very few sex differences in both neonatal and prepubescent animals. This study shows wide dynamically changing sex differences in the granule cells located in the upper blade of the septal dentate gyrus, but almost no differences in the lower blade. These results demonstrate that sex differences are not ubiquitous in the dentate gyrus and suggest that the lower blade, unlike the upper blade, might be involved in non-sexually dimorphic behaviors.

Histological and ultrastructural characterization of interfascicular neurons in the rat anterior commissure

The histological, connectional, and ultrastructural characteristics of a peculiar neuron type in the rat anterior commissure (AC) are described. Since these cells are located among the axonal fascicles of the rostral and caudal parts of the AC, they are termed interfascicular neurons (IFN). In rapid-Golgi sections IFNs appeared in two forms: internuncial (i.e., short axon) and projection neurons (i.e., long axon). The axon of the internuncial neurons terminates upon neighboring IFNs. The projection neurons give rise to an axon which is either incorporated into commissural fibers or ramifies into 12-26 collaterals running laterally in opposite directions along commissural axons. Immunohistochemistry to microtubule-associated protein 2 combined with confocal microscopy showed that IFNs display short varicose dendrites which remain confined to the domain of the AC. The neuronal nature of IFNs was confirmed with the electron microscope on the basis of distinctive organelles and the presence of synaptic inputs. Small areas of neuropil surround some IFNs. These areas are composed of proximal dendrites, terminal axons, axo-shaft and axo-spinous synapses. Because IFNs with their afferents and efferents constitute sufficient elements to integrate neural inputs, it is proposed that they may be involved in processing nerve impulses proceeding from the bilateral cerebral structures connected by the AC.

Myelination in the splenium of the corpus callosum in adult male and female rats

Previous work reported increases in the number of myelinated axons in the splenium of the rat corpus callosum between 25 and 60 days of age. In the present study, we quantified the area occupied by myelinated axons using a light microscopic point counting technique at 60, 120 and 180 days. Myelinated axons increased across these ages (p=0.001). Thus, myelination of the rat corpus callosum persists well into adulthood.

Effects of gonadal hormones on the morphology of neurons from the medial amygdaloid nucleus of rats

The medial amygdala (MeA) has receptors for gonadal hormones and is a sexually dimorphic area in rats. The aims of the present work were (1) to look at sex differences and the effect of gonadal hormone withdrawal in males castrated as offspring or at adulthood on neuronal soma area in the anterior and posterior MeA and (2) to study the dendritic branching and the density of dendritic spines in neurons from the MeA of intact males and females. Animals were adult rats, for which the single-section Golgi method was used. Stellate and bitufted cells were found in the MeA. Comparing data among groups, no significant difference in cell body area was found. Dendrites divide sparingly and have very different lengths, and a statistical difference (p < 0.001, males higher than females) in the spine density in the anterior MeA, but not in the posterior MeA, was found. These results suggest that castration does not alter the somal area in males submitted to gonadectomy during the early postnatal period or at adulthood. In addition, the already described sex difference in this nucleus may be more related to the neuropil than the neuronal somal area, which may be relevant for the function of the MeA.

Sex and species differences in mouse and rat forebrain commissures depend on the method of adjusting for brain size

Sex differences in the forebrain commissures (corpus callosum, hippocampal commissure, and anterior commissure) were examined in B6D2F2 hybrid mice and Sprague-Dawley rats. Twenty-four male-female littermate pairs of mice were perfused at each of 21, 42 and 63 days of age and the midsagittal area of the commissures was measured from en bloc stained tissue. Twenty-two male-female littermate pairs of rats were examined at 110 days of age using the same methods. Male mice had larger bodies than females but no sex differences were found for mouse brain weight or commissure areas. In contrast, a significant sex difference was found for rat body, brain, corpus callosum and hippocampal commissure sizes. Four methods were used to adjust for differences in brain size (ratio, geometric, linear regression, and allometric). When the two species were analysed separately, neither mice nor rats showed significant sex differences in commissure areas relative to brain size if regression or allometric adjustments were made. Even when data from mice and rats were combined into one large group with a wide range of values, no species or sex differences were apparent after adjustments were made for brain size with either the regression or allometric methods. The use of ratios to adjust for differences in overall size is not recommended, especially because this method does not effectively remove the influence of brain size from commissure size; a substantial correlation is often present between the ratio and brain size.