Effect of axotomy and 17ß-estradiol on P2X7 receptor expression pattern in the hypoglossal nucleus of ovariectomized mice.

The objective of the study was to examine whether axotomy and 17ß-estradiol affects P2X7 receptor expression and distribution in the hypoglossal nucleus. The left hypoglossal nerve of ovariectomized mice was cut and animals received a single injection of 17ß-estradiol (25 µg/100g b.w. in 20% (2-hydroxypropyl)-ß-cyclodextrin) or vehicle one hour after axotomy. Mice were sacrificed on day 4 following surgery. The area fraction of P2X7 receptor immunoreactive structures and of CD11b immunolabeled microglia, P2X7 protein concentration, and the immunoreactivity pattern of estrogen receptor (ER) alpha/beta were analyzed on both sides of the hypoglossal nucleus. Following axotomy the area fraction of P2X7 immunoreactive neurons showed a decreasing tendency, while the area fraction of P2X7 immunolabeled microglia increased significantly on the axotomized side compared with the control side in mice injected with vehicle. In animals treated with 17ß-estradiol the decrease in area fraction of neural and the increase in area fraction of microglial P2X7 immunostaining on the axotomized side were significantly enhanced compared with animals injected with vehicle. The P2X7 immunoreactivity pattern on the control side of the nucleus remained unchanged after 17ß-estradiol injection. Semi-quantitative Western blots revealed no significant difference in P2X7 protein concentration comparing the axotomized side with the control side in either experimental group. The CD11b immunoreactive microglia area fraction increased significantly following axotomy, but was not affected by 17ß-estradiol. Neither ER alpha, nor beta colocalized with CD11b. Our results suggest that axotomy induces cell-type specific changes in P2X7 receptor expression, which may be directly regulated by 17ß-estradiol through ER alpha or beta in neurons, but not in activated microglia.

17ß-estradiol attenuates injury-induced microglia activation in the oculomotor nucleus.

Recent studies provide increasing data indicating the prominent role of estrogens in protecting the nervous system against the noxious consequences of nerve injury. It is also clear that in the process of nerve injury and recovery not only the neurons, but the glial cells are also involved and they are important components of the protective mechanisms. In the present article the effect of 17ß-estradiol on injury-induced microglia activation was studied in an animal model. Peripheral axotomy of the oculomotor neurons was achieved by the removal of the right eyeball including the extraocular muscles of ovariectomized adult mice. The time course and the extent of microglia activation was followed by the unbiased morphometric analysis of CD11b immunoreactive structures within the oculomotor nucleus. The first sign of microglia activation appeared after 24 h following injury, the maximal effect was found on the fourth day. In ovariectomized females hormone treatment (daily injection of 17ß-estradiol, 5 µg/100 g b.w.) decreased significantly the microglia reaction at postoperative day 4. Our results show that microglia response to nerve injury is affected by estradiol, that is these cells may mediate some of the hormonal effects and may contribute to protective mechanisms resulting in the structural and functional recovery of the nervous system.

17beta-Estradiol-induced remodeling of GABAergic axo-somatic synapses on estrogen receptor expressing neurons in the anteroventral periventricular nucleus of adult female rats.

Experimental data demonstrate that the nervous system is widely influenced by sex hormones and the brain is continuously shaped by the changing hormone milieu throughout the whole life. Earlier we demonstrated that on the effect of estradiol there is a cyclic synaptic remodeling, i.e. a transient decrease in the number of GABAergic axo-somatic synapses in the arcuate nucleus. By using preembedding estrogen receptor and postembedding GABA immunostaining, in the present paper we studied the specificity of this effect and we found that in the anteroventral periventricular nucleus (AvPv) of adult female rats 17beta-estradiol treatment does not affect all synapses and neurons. In contrast to the arcuate nucleus, hormonal treatment induces a significant increase of inhibitory axo-somatic synapses in the AvPv and we found selectivity at the level of the postsynaptic neurons, as well. We analyzed the hormone-induced synaptic remodeling in estrogen receptor alpha and beta immunoreactive and non-labeled cells and the change in synapse number was observed only in neurons which express estrogen beta receptor.

Oestrogen-modulated increase of calmodulin-dependent protein kinase II (CamKII) in rat spinal trigeminal nucleus after systemic nitroglycerin.

Migraine can be triggered by systemic administration of the nitric oxide (NO) donor nitroglycerin (NTG) and by abrupt falls in plasma oestradiol. Calmodulin-dependent protein kinase II (CamKII) present in superficial dorsal horns is thought to play a role in sensitization of central nociceptors, a phenomen present in migraineurs. We therefore examined in rats the expression of CamKII in the caudal trigeminal nucleus (TNC) after subcutaneous NTG (10 mg/kg) and its modulation by oestrogen. In male rats and in ovariectomized females, after 4 h NTG increased significantly CamKII expression in the superficial layers of TNC, but not in the upper thoracic spinal cord. NTG had no effect on CamKII expression in oestradiol-treated ovariectomized animals. Thus NTG, i.e. NO, selectively enhances CamKII in the rat TNC and oestradiol blocks this effect. These data may help to understand the mechanisms by which NO triggers migraine attacks and oestrogens influence migraine severity.

Fluctuation of synapse density in the arcuate nucleus during the estrous cycle.

The hypothalamic arcuate nucleus integrates different hormonal and neural signals to control neuroendocrine events, feeding, energy balance and reproduction. Previous studies have shown that in adult female rats the arcuate nucleus undergoes a cyclic fluctuation in the number of axo-somatic synapses during the estrous cycle, in parallel to the variation of ovarian hormone levels in plasma. In the present study we have used an unbiased stereological analysis in conjunction with postembedding immunocytochemistry to assess whether the synaptic remodeling during the estrous cycle in rats is specific for certain types of synapses. Our findings indicate that there is a significant decrease in the number of GABAergic axo-somatic synapses on proestrus afternoon and estrus day compared with other days of the estrous cycle. This decrease in GABAergic synapses is accompanied by an increase in the number of dendritic spine synapses. The synaptic density appears to cycle back to proestrus morning values on metestrus day. In contrast, the number of synapses on dendritic shafts does not change during the cycle. These results indicate that a rapid and selective synaptic turnover of arcuate synapses occurs in physiological circumstances.

Estrogen-induced region specific decrease in the density of 5-bromo-2-deoxyuridine-labeled cells in the olfactory bulb of adult female rats.

Effects of chronic estrogen treatment on the survival rate of newly integrated interneurons were studied in the olfactory bulb of adult (250-300 g) female rats. Ovariectomized rats received 17-beta estradiol dissolved in sesame oil (i.p., 100 microg/100 g body weight [b.w.]) during six consecutive days, and on day 6 they were also injected with the mitotic marker 5-bromo-2-deoxyuridine (BrdU, i.p., 50 mg/kg b.w.) in every 2 hours during 8 hours. After 21 days of survival animals were killed and the density of BrdU-immunoreactive cells was analyzed in the granule cell and glomerular layer both in the main and accessory olfactory bulb. A significant decrease was found in the density of BrdU-labeled cells in both layers examined in the accessory olfactory bulb of ovariectomized and estradiol-treated rats when compared with those of ovariectomized and vehicle-treated animals. In the main olfactory bulb, in contrast, no difference was observed in the density of BrdU-immunoreactive cells in either of the two layers. Our results suggest that cells destined to the glomerular and granule cell layers react in the same way to chronic estrogen treatment, and the effect of estradiol is region specific, at least, within the olfactory bulb. 17-Beta estradiol reduces the density of newly generated cells in the accessory olfactory bulb, an area involved in the perception of pheromones, thus having a role in regulating sexual behavior, while the rate of integration and survival of newly born cells in the first relay station of the main olfactory pathway, i.e. the main olfactory bulb, remains unchanged.

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.

Hormonal enhancement of neuronal firing is linked to structural remodelling of excitatory and inhibitory synapses.

The ovarian hormone estradiol induces morphological changes in the number of synaptic inputs in specific neuronal populations. However, the functional significance of these changes is still unclear. In this study, the effect of estradiol on the number of anatomically identified synaptic inputs has been assessed in the hypothalamic arcuate nucleus. The number of axo-somatic, axodendritic and spine synapses was evaluated using unbiased stereological methods and a parallel electrophysiological study was performed to assess whether synaptic anatomical remodelling has a functional consequence on the activity of the affected neurons. Estradiol administration to ovariectomized rats induced a decrease in the number of inhibitory synaptic inputs, an increase in the number of excitatory synapses and an enhancement of the frequency of neuronal firing. These results indicate that oestrogen modifications in firing frequency in arcuate neurons are temporally linked to anatomical modifications in the numerical balance of inhibitory and excitatory synaptic inputs.

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 17 beta-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, 17 beta-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.

Estrogen effects on arcuate neurons in rat. An in situ electrophysiological study.

In acute experiments, the effects of i.p. 17beta-estradiol on the activity of arcuate neurons were studied in ovariectomized rats. 17Beta-estradiol (100 microg/100g, i.p.) increased the spontaneous activity of the observed arcuate neurons with a latency of 20-25 min. In some neurons spontaneous activity could be influenced by stimulation of the olfactory and somatosensory systems. Activation of the trigeminal system significantly increased the spontaneous activity of the studied units, while stimulation of the accessory olfactory bulb decreased it, both with and without 17beta-estradiol treatment. It is suggested that the 20-25 min latency of the 17beta-estradiol effect is based not so much on membrane as on genomic mechanisms. This suggestion is supported by immunocytochemical studies: 17beta-estradiol treatment significantly decreased the number of GABA-positive axo-somatic synapses in the arcuate nucleus.

Modulation by thyroid hormones of the development of external plexiform layer in the rat olfactory bulb.

Width of the external plexiform layer in olfactory bulbs and mean area of mitral and granule cell dendritic and glial processes were measured of normal, hypo- and hyperthyroid rat pups at the age of 24 days. Hypothyroidism was induced by treating the rats with a reversible goitrogen 6-n-propyl-2-thiouracil dissolved in their drinking water, while the hyperthyroid group was given water containing thyroxine. The 6-n-propyl-2-thiouracil treatment was begun on gestational day 18 and on the day of birth. Thyroxine treatment started on the day of birth. Both treatments were continued till the day of sacrifice. A significant decrease in the width of the external plexiform layer of the olfactory bulb in the prenatally 6-n-propyl-2-thiouracil treated group and a significant increase in the width of the external plexiform layer of the hyperthyroid group was shown by the Student's paired t-test. The areas of neuronal and glial processes were measured at electron microscopic level by using an IBAS image analysing system. A significant decrease was found by the Kruskal-Wallis test and Dunn's range test in the mean area of (1) mitral cell dendrites in the prenatal 6-n-propyl-2-thiouracil treated group, (2) granule cell dendrites in both the postnatally 6-n-propyl-2-thiouracil treated and in the hyperthyroid groups and (3) glial processes in the thyroxine treated group comparing to controls.

Fast regeneration of the olfactory nerve in goldfish: fine structure and behaviour.

Normal olfaction-guided behaviour in goldfish returns surprisingly fast after bilateral transection of the olfactory nerve (ON). In order to find correlation between morphological changes and behaviour we performed parallel studies on the behaviour of the fish and the ultrastructure of the olfactory bulb in axotomised fish. Signs of degeneration were most prominent between the 4th and 6th day post-operatively. First appearance of regenerated ON fibres and terminals was noted on the 10th post-operative day. ON terminals became frequent again two weeks after the operation. This supports earlier suppositions that morphological regeneration plays a crucial role in the fast and complete restoration of olfaction-guided behaviour.