The supraoptic nucleus in hypothyroid and undernourished rats: an experimental morphometric study

The Effects of De-Whiskering and Congenital Hypothyroidism on The Development of Nitrergic Neurons in Rat Primary Somatosensory and Motor Cortices

Objective

The aim of the present study is to investigate the effects of chronic whisker deprivation on possible alterations to the development of nitrergic neurons in the whisker part of the somatosensory (wS1) and motor (wM1) cortices in offspring with congenital hypothyroidism (CH).

Materials and Methods

In the experimental study, CH was induced by adding propylthiouracil to the rats drinking water from embryonic day 16 to postnatal day (PND) 60. In whisker-deprived (WD) pups, all the whiskers were trimmed from PND 1 to 60. Nitrergic interneurons in the wS1/M1 cortices were detected by NADPH-diaphorase histochemistry staining technique in the control (Ctl), Ctl+WD, Hypo and Hypo+WD groups.

Results

In both wS1 and wM1 cortices the number of nitrergic neurons was significantly reduced in the Hypo and Hypo+WD groups compared to Ctl and Ctl+WD groups, respectively (P<0.05) while bilateral whisker deprivation had no remarkable effect. The mean soma diameter size of NADPH-d labeled neurons in the Ctl+WD and Hypo+WD groups was decreased compared to the Ctl and Hypo groups, respectively. A similar patterns of decreased NADPH-d labeled neurons in the wS1/M1 cortices occur in the processes of nitrergic neurons in both congenital hypothyroidism and whisker deprivation.

Conclusion

Our results suggest that both congenital hypothyroidism and whisker deprivation may disturb normal development of the wS1 and wM1 cortical circuits in which nitrergic neurons are involved.

Epigenetic regulation of estrogen-dependent memory

Hippocampal memory formation is highly regulated by post-translational histone modifications and DNA methylation. Accordingly, these epigenetic processes play a major role in the effects of modulatory factors, such as sex steroid hormones, on hippocampal memory. Our laboratory recently demonstrated that the ability of the potent estrogen 17β-estradiol (E2) to enhance hippocampal-dependent novel object recognition memory in ovariectomized female mice requires ERK-dependent histone H3 acetylation and DNA methylation in the dorsal hippocampus. Although these data provide valuable insight into the chromatin modifications that mediate the memory-enhancing effects of E2, epigenetic regulation of gene expression is enormously complex. Therefore, more research is needed to fully understand how E2 and other hormones employ epigenetic alterations to shape behavior. This review discusses the epigenetic alterations shown thus far to regulate hippocampal memory, briefly reviews the effects of E2 on hippocampal function, and describes in detail our work on epigenetic regulation of estrogenic memory enhancement.

Structural and functional alterations in the hippocampus due to hypothyroidism

Thyroid hormones (THs) exert a broad spectrum of effects on the central nervous system (CNS). Hypothyroidism, especially during CNS development, can lead to structural and functional changes (mostly resulting in mental retardation). The hippocampus is considered as one of the most important CNS structures, while the investigation and understanding of its direct and indirect interactions with the THs could provide crucial information on the neurobiological basis of the (frequently-faced in clinical practice) hypothyroidism-induced mental retardation and neurobehavioral dysfunction. THs-deficiency during the fetal and/or the neonatal period produces deleterious effects for neural growth and development (such as reduced synaptic connectivity, delayed myelination, disturbed neuronal migration, deranged axonal projections, decreased synaptogenesis and alterations in neurotransmitters' levels). On the other hand, the adult-onset thyroid dysfunction is usually associated with neurological and behavioural abnormalities. In both cases, genomic and proteomic changes seem to occur. The aim of this review is to provide an up-to-date synopsis of the available knowledge regarding the aforementioned alterations that take place in the hippocampus due to fetal-, neonatal- or adult-onset hypothyroidism.

Comparison of the number of vasopressin-producing hypothalamic neurons in rats and humans

The aim of this study was to assess the number and proportion of vasopressin-producing neurons in the hypothalamic magnocellular nuclei in rats and humans. Accurate and unbiased neuronal counts were estimated using the optical disector method. Arginine vasopressin-containing neurons were immunohistochemically visualized in formalin-fixed tissue sections. The magnocellular neurons were similar in size and morphology in both species. While the human hypothalamus contained significantly more vasopressin-containing neurons compared with the rat (36-fold increase), the proportion of vasopressin-containing neurons between species was similar. In both species, the majority of supraoptic neurons contained vasopressin, however the proportion of vasopressin-containing neurons in the human paraventricular nucleus was double that of the rat (nearly a 100-fold increase in number). These results suggest that the paraventricular nucleus contributes significantly to the release of vasopressin from the posterior pituitary in humans, whereas in rats vasopressin is mainly released by supraoptic neurons.

Effect of hypothyroidism on vasoactive intestinal polypeptide-immunoreactive neurons in forebrain-neurohypophysial nuclei of the rat brain

We have recently reported that hypothyroidism increases immunoreactive (IR)-vasoactive intestinal polypeptide (VIP) and VIP mRNA content in both parvocellular and magnocellular neurons of the rat, hypothalamic paraventricular nucleus (PVN). As VIP can stimulate vasopressin (AVP) secretion, we conducted an anatomical investigation to determine whether VIP-containing neurons in other regions of the brain that are involved with homeostatic mechanisms of water and salt conservation are also affected by hypothyroidism. The distribution and intensity of VIP immunostaining in neurons and fibers of the magnocellular-neurohypophysial system, including the hypothalamic PVN, supraoptic nucleus (SON) and accessory magnocellular cell groups, circumventricular subfornical organ (SFO), preoptic and anterior hypothalamus, midline thalamus, subthalamic zona incerta and posterior septal nuclei were studied using a highly sensitive immunocytochemical technique and unbiased neuronal counting methods, based on the optical dissector principle. Hypothyroidism increased the intensity of VIP immunostaining and/or the number/section, percentage and numerical density of IR-VIP neurons in the PVN, SON, nucleus circularis, periventricular preoptic nucleus of the hypothalamus and SFO. In addition, IR-VIP perikarya and/or fibers in the hypothalamic medial preoptic area and anterior periventricular nucleus, nucleus reuniens of the thalamus and dorsal fornix-triangular septal nucleus complex were also apparent in the hypothyroid animals while no immunostaining was seen in these areas in control animals. No quantitative and/or qualitative modifications in IR-VIP neurons and fibers were noted in the anterior hypothalamic area, suprachiasmatic nucleus, thalamic paraventricular nucles an subthalamic zona incerta between hypothyroid and control animals. These findings suggest an inverse relationship between thyroid hormone and VIP content and/or distribution of IR-VIP neurons in specific forebrain regions involved in the control of AVP release, extracellular fluid volume, thirst, blood pressure and anterior pituitary secretion. This raises the possibility that changes in fluid homeostasis and cardiovascular function occurring in hypothyroidism may be mediated, at least in part, by VIP-producing neurons in diverse regions of the brain.

Reorganization of mossy fiber synapses in male and female hypothyroid rats: a stereological study

Thyroid hormone deficiency has been reported to interfere with synaptogenesis, particularly in those regions of the brain where the neurons display a late and protracted histogenesis, although the extent of the synaptic alterations remains unknown. To provide detailed quantitative data on the effects of hypothyroidism upon synapses, a link of the hippocampal circuitry was selected: the contact between mossy fibers and dendritic excrescences of CA3 pyramidal cells (MF-CA3 synapses). Groups of six male and six female rats aged 30 and 180 days were analyzed separately after being treated as follows: (1) hypothyroid from day 0 until day 30 (30 day old hypothyroid group); (2) hypothyroid from day 0 until day 180 (180 day old hypothyroid group); (3) hypothyroid until day 30 and thenceforth maintained euthyroid (recovery group); and (4) and (5) 30 and 180 day old control groups, respectively. Timm staining, Golgi impregnation, and electron microscopy were employed to estimate the volume of the mossy fiber system, the number and size of mossy fiber boutons, and the number and related features of MF-CA3 synapses. The volume of the mossy fiber system and the number of synaptic boutons were reduced in all experimental groups. The total number of synapses was decreased in 30 day old hypothyroid rats, but did not differ among 180 day old animals. Postsynaptic densities were shorter in hypothyroid and recovery groups than in controls, although the reduction was not as marked in recovery rats as it was in hypothyroid animals. Structural alterations were noted in the pre- and postsynaptic compartments of MF-CA3 synapses of both 180 day old hypothyroid and recovery rats. These changes can be regarded as mechanisms of reorganization as they underlie the compensation for the hypothyroid-induced numerical reduction of synapses observed in 30 day old animals and enable a complete catch-up of their total number. However, synaptic reorganization was not fully achieved, as revealed by the reduction in the size of the synaptic sites in hypothyroid and recovery animals. Finally, we demonstrate that hypothyroidism did not interfere with the sex-related differences of MF-CA3 synapses described in normal rats.

Effects of chronic alcohol consumption and of dehydration on the supraoptic nucleus of adult male and female rats

Ethanol ingestion affects the hypothalamo-neurohypophysial system resulting in increased diuresis, dehydration and hyperosmolality. We studied the supraoptic nucleus, of the hypothalamus, in ethanol-treated rats, to determine if ethanol alone and/or the associated disturbances of water metabolism lead to structural alterations in a nucleus known to play a central role in fluid homeostasis. Groups of male and female rats were ethanol-treated until 12 and 18 months of age and compared with age-matched pair-fed controls. Twelve and 18-month-old control groups and 12-month-old water control groups (rats submitted to chronic dehydration) were also included in this study in an attempt to differentiate between the effects of undernutrition and dehydration/hyperosmolality, and the specific neurotoxic effects of ethanol. We estimated the volume of the supraoptic nucleus and the numerical density of its neurons and calculated the total number of supraoptic neurons. The volume of both supraoptic neurons and neuropil were also estimated. In immunostained material the ratio of vasopressin to oxytocin neurons and the cross-sectional areas of the two neuronal types were evaluated. There was marked neuronal loss in alcohol-treated rats, but the volume of the supraoptic nucleus was increased. The increase in the volume of the supraoptic nucleus correlated with and was due to increases in the volume was particularly marked for vasopressin neurons. No significant differences were found between controls and pair-fed controls in any of the parameters investigated. In water control rats, the volume of the supraoptic nucleus and of the supraoptic neurons and neuropil was also greater than in pair-fed controls. However, the variations found were not as marked as in ethanol-treated rats and there was no cell loss. These findings reveal, for the first time, that chronic ethanol consumption affects the morphology of supraoptic neurons and neuropil and, consequently, the structure of the entire supraoptic nucleus. Moreover, this study supports the view that ethanol has direct neurotoxic effects on supraoptic neurons because the alterations that occur are not mimicked in animals in which water metabolism alone is disturbed.

Effects of chronic alcohol consumption and withdrawal on the somatostatin-immunoreactive neurons of the rat hippocampal dentate hilus

Previous studies have demonstrated that the dentate granule and the CA3 pyramidal cells of the rat hippocampal formation are neuronal populations vulnerable to the toxic effects of ethanol. It also has been shown that the resulting alterations do not end after withdrawal from ethanol. As the neurons in the dentate hilus are heavily interconnected with the dentate granule cells, the authors decided to examine the fate of the hilar neurons after chronic alcohol consumption and withdrawal, inasmuch as the hilar somatostatin-immunoreactive (SS-I) neurons were found to be sensitive to cerebral ischemia and to seizures. The following groups of adult rats were studied: (1) alcohol-fed for 6 and 12 months; (2) alcohol-fed for 6 months and then switched to water for a further 6 months; (3) pair-fed controls; and (4) controls fed ad libitum. The authors determined the numerical density of hilar neurons and the number of its SS-I subpopulation. These were found to be significantly reduced in both the alcohol-fed and withdrawal groups when compared with the respective age-matched controls. The consequent loss of the integrative action of the hilar neurons, including the SS-Is, could explain some of the alcohol-related functional deficits as well as their persistence after withdrawal.

The supraoptic nucleus of the adult rat hypothalamus displays marked sexual dimorphism which is dependent on body weight

The neurons of the supraoptic nucleus in the rat hypothalamus are reported not to possess receptors for gonadal steroids and sexual dimorphism has not previously been described in this nucleus. We have analysed this nucleus in groups of Sprague-Dawley rats (six males or six females per group), one, two, six, 12 and 18 months after birth. Body and brain weights were recorded, the volume of the nucleus was determined from the right hemisphere and all other quantitative parameters were determined from the left nucleus. In addition, different groups of four male and four female rats aged two and 18 months were analysed after immunocytochemical staining to distinguish between vasopressin and oxytocin neurons. The total number of neurons was constant in all groups studied, despite which the volume of the supraoptic nucleus increased progressively with age in both males and females. The cross-sectional areas and volumes of supraoptic neurons also increased with age. The volume density of the neuropil remained constant in all groups and there was a progressive decrease with age in the numerical density of neurons. Immunocytochemistry revealed that the age-dependent increases in the size of the neurons involved primarily the vasopressin neurons. The age-related changes were much greater in males than in females, resulting in significant differences between the sexes at two, six, 12 and 18 months with respect to the volume of the supraoptic nucleus, the cross-sectional areas of neuronal somata and nuclei, and the volume of supraoptic neurons. Thus the supraoptic nucleus and its vasopressin neurons are larger in adult males than in age-matched females. Since we have also shown that body weight is very closely correlated with changes in the size of supraoptic neurons, and adult male rats are heavier than females of the same age, we suggest that these size changes reflect adaptation of the vasopressin neurons of the supraoptic nucleus to increasing functional demands associated with the regulation of water balance in bodies of increasing size.

Selective vulnerability of the hippocampal pyramidal neurons to hypothyroidism in male and female rats

Thyroid hormone deficiency has long been considered to affect profoundly such cognitive functions as learning and memory, which are known to depend on the structural integrity of the hippocampal formation. Since we previously found that the number of granule cells of the dentate gyrus is reduced in hypothyroid animals, we decided to extend our observations to the pyramidal cells of the hippocampus in order to gain further insight into the effects of hypothyroidism upon the other neuronal links of the hippocampal trisynaptic circuitry, inasmuch as CA1 neurons are known to be particularly vulnerable to aggressive agents. Groups of 6 male and 6 female rats aged 30 and 180 days were analysed separately after being treated as follows: (1) hypothyroid from day 0 until day 30 (30-day-old hypothyroid group); (2) respective 30-day-old control; (3) hypothyroid from day 0 until day 180 (180-day-old hypothyroid group); (4) hypothyroid until day 30 and thenceforth maintained euthyroid (recovery group); (5) hypothyroid since day 30 (adult hypothyroid group); and (6) respective 180-day-old control. The volume of the pyramidal cell layer of the CA1 and CA3 regions and the numerical density of the respective neurons were evaluated, thereby allowing us to estimate the total number of pyramidal cells in each hippocampal region. The areal density and the mean nuclear volume of CA1 and CA3 pyramidal cells were also estimated. In the CA3 region, we found that hypothyroidism, whatever its duration and time of onset, induces a reduction in the volume of the pyramidal cell layer and a parallel increase in the numerical density of its neurons, without interfering with the total number of pyramidal cells. Conversely, in the CA1 region, thyroid hormone deficiency started either neonatally or during maturity was found to lead to a decrease in the total number of pyramidal cells. Reductions ranging between 14.2 and 22.5% were found in 30 and 180-day-old hypothyroid groups. The reestablishment of a euthyroid state did not ameliorate the referred neuronal loss. The present results support the view that hypothyroidism induces small alterations in the structural organization of the hippocampal CA3 region, contrary to what happens in CA1 in which neuronal death occurs. Furthermore, the data presented herein demonstrate that the total number of CA1 pyramidal cells displays sexual dimorphism that is not affected by thyroid hormone manipulations.(ABSTRACT TRUNCATED AT 400 WORDS)

Effects of hypothyroidism upon the granular layer of the dentate gyrus in male and female adult rats: a morphometric study

The effects of hypothyroidism upon the structure of the central nervous system of adult rats are poorly understood in spite of evidence that the mature brain is vulnerable to this condition. Existing developmental studies show that the morphological changes induced by thyroid hormone deficiency are related to alterations in neurogenesis. We studied the granular layer of the dentate gyrus under different experimental conditions of hypothyroidism, because in rodents the neurogenesis of the granule cells continues during adulthood. The following groups of rats were analysed: 1) control; 2) hypothyroid from day 0 until day 180 (hypothyroid group); 3) hypothyroid until day 30 and henceforth maintained euthyroid (recovery group); and 4) hypothyroid since day 30 (adult hypothyroid group). Groups of 6 male rats and 6 female rats were analysed separately. The volume of the dentate gyrus granular layer and the numerical density of its neurons were evaluated, so we were able to estimate the total number of granule cells. Because in the experimental groups the volume of the granular layer and the numerical density of its neurons were reduced, the total number of granule cells was decreased. In the hypothyroid and recovery groups the alterations were identical and more striking than in the adult hypothyroid groups. The total number of granule cells displayed sexual differences in all groups studied except in the hypothyroid groups. The present results support the view that thyroid hormone deficiency interferes with the process of cell acquisition by reducing neuronal proliferation and that it also leads to increased cell death. These events underlie the irreversible morphological changes observed in the brain of hypothyroid rats, either during development or at maturity. The referred structural alterations are probably related to the functional deficits observed in this condition.