Árpád Párducz

Gonadal hormones as promoters of structural synaptic plasticity: Cellular mechanisms

It is now obvious that the CNS is capable of undergoing a variety of plastic changes at all stages of development. Although the magnitude and distribution of these changes may be more dramatic in the immature animal, the adult brain retains a remarkable capacity for undergoing morphological and functional modifications. Throughout development, as well as in the postpubertal animal, gonadal steroids exert an important influence over the architecture of specific sex steroid-responsive areas, resulting in sexual dimorphisms at both morphological and physiological levels. We are only now beginning to gain insight into the mechanisms involved in gonadal steroid-induced synaptic changes. The number of synaptic inputs to specific neuronal populations is sexually dimorphic and this can be modulated by changes in the sex steroid environment. These modifications can be correlated with other morphological changes, such as glial cell activation, that are occurring simultaneously in the same anatomical area. Indeed, the close physical relationship between glial cells and neuronal synaptic contacts makes them an ideal candidate for participating in this process. Interestingly, not only can the morphology and immunoreactivity of glial cells be modulated by gonadal steroids, but a close negative correlation between the number of synapses and the amount of glial ensheathing of a neuron has been demonstrated, suggesting an active participation of these cells in this process. Glia have sex steroid receptors, are capable of producing and metabolizing steroids, and can produce other neuronal trophic factors in response to sex steroids. Hence, their role in gonadal steroid-induced synaptic plasticity is becoming more apparent. In addition, there is recent evidence that this process may involve certain cell surface molecules, such as the N-CAMs, since a specific isoform of this molecule, previously referred to as the embryonic form, is found in those areas of the brain which maintain the capacity to undergo synaptic remodelling. However, there is much work to be done in order to fully understand this phenomenon and before bringing it into a clinical setting in hopes of treating neurodegenerative diseases or injuries to the nervous system.

Steroid hormones and neurosteroids in normal and pathological aging of the nervous system

Without medical progress, dementing diseases such as Alzheimers disease will become one of the main causes of disability. Preventing or delaying them has thus become a real challenge for biomedical research. Steroids offer interesting therapeutical opportunities for promoting successful aging because of their pleiotropic effects in the nervous system: they regulate main neurotransmitter systems, promote the viability of neurons, play an important role in myelination and influence cognitive processes, in particular learning and memory. Preclinical research has provided evidence that the normally aging nervous system maintains some capacity for regeneration and that age-dependent changes in the nervous system and cognitive dysfunctions can be reversed to some extent by the administration of steroids. The aging nervous system also remains sensitive to the neuroprotective effects of steroids. In contrast to the large number of studies documenting beneficial effects of steroids on the nervous system in young and aged animals, the results from hormone replacement studies in the elderly are so far not conclusive. There is also little information concerning changes of steroid levels in the aging human brain. As steroids present in nervous tissues originate from the endocrine glands (steroid hormones) and from local synthesis (neurosteroids), changes in blood levels of steroids with age do not necessarily reflect changes in their brain levels. There is indeed strong evidence that neurosteroids are also synthesized in human brain and peripheral nerves. The development of a very sensitive and precise method for the analysis of steroids by gas chromatography/mass spectrometry (GC/MS) offers new possibilities for the study of neurosteroids. The concentrations of a range of neurosteroids have recently been measured in various brain regions of aged Alzheimers disease patients and aged non-demented controls by GC/MS, providing reference values. In Alzheimers patients, there was a general trend toward lower levels of neurosteroids in different brain regions, and neurosteroid levels were negatively correlated with two biochemical markers of Alzheimers disease, the phosphorylated tau protein and the beta-amyloid peptides. The metabolism of dehydroepiandrosterone has also been analyzed for the first time in the aging brain from Alzheimer patients and non-demented controls. The conversion of dehydroepiandrosterone to Delta5-androstene-3beta,17beta-diol and to 7alpha-OH-dehydroepiandrosterone occurred in frontal cortex, hippocampus, amygdala, cerebellum and striatum of both Alzheimers patients and controls. The formation of these metabolites within distinct brain regions negatively correlated with the density of beta-amyloid deposits.

Neurotoxin induced nerve cell degeneration: Possible involvement of calcium

Neurotoxin induced nerve cell degeneration has been studied in sensory ganglia of newborn and in the area postrema of adult rats following the administration of the selective sensory neurotoxin, capsaicin and the amino acid excitotoxin, glutamic acid, respectively. Light microscopic histochemical, autoradiographic, electroncytochemical and X-ray microanalytical studies revealed that degeneration of certain small-sized, type B primary sensory neurons, induced by capsaicin, was associated with a marked accumulation of calcium predominantly in mitochondria of the damaged ganglion cells. Similarly, monosodium glutamate treatment resulted in the appearance of calcium-containing electron-dense granules in mitochondria of degenerating area postrema neurons. In addition, after a combined administration of 45Ca2+ and capsaicin or monosodium glutamate, significantly higher levels of radioactivity have been detected by liquid scintillation spectroscopy in the Gasserian ganglia and the area postrema, respectively. It is concluded that an enhancement in intracellular calcium level may be intimately involved in the process of neuronal cell death and may represent a common basic mechanism responsible for the development of cellular events leading ultimately to the degeneration of nerve cells.

Remodeling of Hippocampal Spine Synapses in the Rat Learned Helplessness Model of Depression

BACKGROUND Although it has been postulated for many years that depression is associated with loss of synapses, primarily in the hippocampus, and that antidepressants facilitate synapse growth, we still lack ultrastructural evidence that changes in depressive behavior are indeed correlated with structural synaptic modifications. METHODS We analyzed hippocampal spine synapses of male rats (n=127) with electron microscopic stereology in association with performance in the learned helplessness paradigm. RESULTS Inescapable footshock (IES) caused an acute and persistent loss of spine synapses in each of CA1, CA3, and dentate gyrus, which was associated with a severe escape deficit in learned helplessness. On the other hand, IES elicited no significant synaptic alterations in motor cortex. A single injection of corticosterone reproduced both the hippocampal synaptic changes and the behavioral responses induced by IES. Treatment of IES-exposed animals for 6 days with desipramine reversed both the hippocampal spine synapse loss and the escape deficit in learned helplessness. We noted, however, that desipramine failed to restore the number of CA1 spine synapses to nonstressed levels, which was associated with a minor escape deficit compared with nonstressed control rats. Shorter, 1-day or 3-day desipramine treatments, however, had neither synaptic nor behavioral effects. CONCLUSIONS These results indicate that changes in depressive behavior are associated with remarkable remodeling of hippocampal spine synapses at the ultrastructural level. Because spine synapse loss contributes to hippocampal dysfunction, this cellular mechanism may be an important component in the neurobiology of stress-related disorders such as depression.

Estradiol induces plasticity of gabaergic synapses in the hypothalamus

The number of axosomatic synapses on arcuate neurons of the adult rat hypothalamus fluctuates following the sequence of increasing circulatory estradiol during the ovarian cycle. To determine whether estrogen is affecting GABAergic synaptic contacts we studied the number of GABA-immunoreactive axosomatic synapses in adult ovariectomized rats injected either with 17 beta estradiol (100 micrograms/100 g body weight) or with sesame oil vehicle. The number of immunoreactive axosomatic synapses was significantly reduced in estradiol-treated rats (77 +/- 8 vs 56 +/- 6 synapses per 1000 microns of perikaryal membrane in control and estradiol-treated rats, respectively) while the number of non-immunoreactive synapses was not significantly affected by the hormonal treatment (44 +/- 6 vs 35 +/- 5 synapses per 1000 microns of perikaryal membrane in control and estradiol-treated rats, respectively). Estradiol administration also resulted in a significant decrease in the percentage of perikaryal membrane covered by immunoreactive synapses. These results suggest that physiological levels of estradiol may induce a remodeling of GABAergic inhibitory inputs on arcuate neurons.

Synaptic remodeling induced by gonadal hormones: neuronal plasticity as a mediator of neuroendocrine and behavioral responses to steroids.

During recent decades, it has become a generally accepted view that structural neuroplasticity is remarkably involved in the functional adaptation of the CNS. Thus, cellular morphology in the brain is in continuous transition throughout the life span, as a response to environmental stimuli. The effects of the environment on neuroplasticity are mediated by, to some extent, the changing levels of circulating gonadal steroid hormones. Today, it is clear that the function of gonadal steroids in the brain extends beyond simply regulating reproductive and/or neuroendocrine events. In addition, or even more importantly, gonadal steroids participate in the shaping of the developing brain, while their actions during adult life are implicated in higher brain functions such as cognition, mood and memory. A large body of evidence indicates that gonadal steroid-induced functional changes are accompanied by alterations in neuron and synapse numbers, as well as in dendritic and synaptic morphology. These structural modifications are believed to serve as a morphological basis for changes in behavior and cellular activity. Due to their growing functional and clinical significance, the specificity, timeframe, as well as the molecular and cellular mechanisms of hormone-induced neuroplasticity have become the focus of many studies. In this review, we briefly summarize current knowledge and the most significant recent discoveries from our laboratories on estrogen- and dehydroepiandrosterone-induced synaptic remodeling in the hypothalamus and hippocampus, two important brain areas heavily involved in autonomic and cognitive operations, respectively.

Estrogen-induced hypothalamic synaptic plasticity and pituitary sensitization in the control of the estrogen-induced gonadotrophin surge

Proper gonadal function requires coordinated (feedback) interactions between the gonads, adenohypophysis, and brain: the gonads elaborate sex steroids (progestins, androgens, and estrogens) and proteins (inhibin-activin family) during gamete development. In both sexes, the brain-pituitary gonadotrophin-regulating interaction is coordinated by estradiol through its opposing actions on pituitary gonadotrophs (sensitization of the response to gonadotrophin-releasing hormone [GnRH]) versus hypothalamic neurons (inhibition of GnRH secretion). This dynamic tension between the gonadotrophs and the GnRH cells in the brain regulates the circulating gonadotrophins and is termed reciprocal/negative feedback. In females, reciprocal/negative feedback dominates ∼90% of the ovarian cycle. In a spectacular exception, the dynamic tension is broken during the surge of circulating estrogen that marks follicle and oocyte(s) maturation. The cause is an estradiol-induced disinhibition of the GnRH neurons that releases GnRH secretion to the highly sensitized pituitary gonadotrophs that in turn release the gonadotrophin surge (the estrogen-induced gonadotrophin surge [EIGS], also known as positive feedback). Studies during the past 4 decades have shown this disinhibition to result from estrogen-induced synaptic plasticity (EISP), including a reversible ∼ 50% loss in arcuate nucleus synapses. The disinhibited GnRH secretion occurs during maximal gonadotroph sensitization and results in the EIGS. Specific immunoneutralization of estradiol blocks the EISP and EIGS. The EISP is accompanied by increases in insulinlike growth factor 1, polysialylated neural cell adhesion molecule, and ezrin, 3 proteins that the authors believe are the links between estrogen-induced astroglial extension and the EISP that releases GnRH secretion at the moment of maximal sensitization of the pituitary gonadotrophs. The result is the paradoxical surge of gonadotrophins at the peak of ovarian estrogen secretion and the triggering of ovulation. This enhanced understanding of the mechanics of gonadotrophin control clarifies elements of the involved feedback loops and opens the way to a better understanding of the neurobiology of reproduction.

Effect of systemic nitroglycerin on CGRP and 5-HT afferents to rat caudal spinal trigeminal nucleus and its modulation by estrogen

Systemic administration of nitroglycerin, a nitric oxide donor, triggers in migraine patients a delayed attack of unknown mechanism. After puberty migraine is more prevalent in women. Attacks can be triggered by abrupt falls in plasma estrogen levels, which accounts in part for sexual dimorphism, but lacks an established neurobiological explanation. We studied the effect of nitroglycerin on the innervated area of calcitonin gene‐related peptide (CGRP) and serotonin‐immunoreactive afferents to the superficial laminae of the spinal portion of trigeminal nucleus caudalis, and its modulation by estrogen. In male rats, nitroglycerin produced after 4 h a significant decrease of the area innervated by CGRP‐immunoreactive afferents and an increase of that covered by serotonin‐immunoreactive fibres. These effects were not observed in the superficial laminae of thoracic dorsal horns. The effect of nitroglycerin was similar in ovariectomized females. In estradiol‐treated ovariectomized females the area in the spinal portion of trigeminal nucleus caudalis laminae I–II covered by CGRP‐immunoreactive fibres was lower and that of serotonin‐immunoreactive fibres was higher than in males and for both transmitters not significantly changed after nitroglycerin. The bouton size of CGRP profiles was smaller in estradiol‐treated ovariectomized females, whereas after nitroglycerin it decreased significantly but only in males and ovariectomized females. Nitroglycerin, i.e. nitric oxide, is thus able to differentially influence afferent fibres in the superficial laminae of rat spinal trigeminal nucleus caudalis. Estradiol modulates the basal expression of these transmitters and blocks the nitroglycerin effect. These data may contribute to understanding the mechanisms by which estrogens influence migraine severity and the triggering of attacks by nitric oxide.

The highly sialylated isoform of the neural cell adhesion molecule is required for estradiol-induced morphological synaptic plasticity in the adult arcuate nucleus

The large quantities of polysialic acid (PSA) characterizing highly sialylated isoform of the neural cell adhesion molecule (PSA‐NCAM), greatly reduce cell adhesion and render this particular cell surface adhesion molecule a likely candidate to intervene in dynamic neuronal phenomena, such as synaptic plasticity. The hypothalamic arcuate nucleus expresses high levels of PSA‐NCAM and maintains a high capacity for neuroplastic changes in the adult. Thus, in the arcuate nucleus of female rats, varying circulating levels of estrogen give rise to a reversible reduction in the number of axo‐somatic GABA synapses, together with a changing ensheathing of neuronal somata by astrocytes. To examine the role of PSA in such changes, we perturbed its expression, either by blockade with antibodies raised against this carbohydrate moiety (delivered intracerebroventricularly), or by its enzymatic cleavage after microinjection of endoneuraminidase N over the arcuate nucleus. Either procedure was performed in ovariectomized adult rats that received concurrent treatment with 17β‐estradiol. Morphological synaptic plasticity was analysed using the unbiased disector method to assess synaptic densities in ultrathin sections of the arcuate nucleus immunogold‐labelled for GABA. As expected, 17β‐estradiol induced a significant reduction in the number of GABAergic axo‐somatic synapses, a reduction which did not occur after infusion of anti‐PSA antibodies or in vivo enzymatic removal of PSA from NCAM. Taken together, our results provide strong evidence that the presence of large quantities of the PSA moiety on NCAM is a necessary prerequisite for estrogen‐induced phasic remodelling of synapses in the adult female arcuate nucleus.

Effects of Estradiol on Learned Helplessness and Associated Remodeling of Hippocampal Spine Synapses in Female Rats

BACKGROUND Despite the fact that women are twice as likely to develop depression as men, our understanding of depression neurobiology in female subjects is limited. We have recently reported in male rats that development of helpless behavior is associated with a severe loss of hippocampal spine synapses, which is reversed by treatment with the antidepressant desipramine. Considering that estradiol has a hippocampal synaptogenic effect similar to those of antidepressants, the presence of estradiol during the female reproductive life might influence behavioral and synaptic responses to stress and depression. METHODS With electron microscopic stereology, we analyzed hippocampal spine synapses in association with helpless behavior in ovariectomized female rats (n = 70), under different conditions of estradiol exposure. RESULTS Stress induced an acute and persistent loss of hippocampal spine synapses, whereas subchronic treatment with desipramine reversed the stress-induced synaptic loss. Estradiol supplementation given either before stress or before escape testing of nonstressed animals increased the number of hippocampal spine synapses. Correlation analysis demonstrated a statistically significant negative correlation between the severity of helpless behavior and hippocampal spine synapse numbers. CONCLUSIONS These findings suggest that hippocampal spine synapse remodeling might be a critical factor underlying learned helplessness and, possibly, the neurobiology of depression.