Postsynaptic membrane domains in the molecular layer of the cerebellum: a correlation between presynaptic inputs and postsynaptic plasma membrane organization

Neuroplastic changes in the hypothalamic arcuate nucleus: the estradiol effect is accompanied by increased exoendocytotic activity of neuronal membranes

1. In the rat hypothalamic arcuate nucleus, estradiol induces coordinated changes in the number of axosomatic synapses, the amount of glial ensheathing, and the ultrastructure of the membrane of neuronal somas. In the present study we used conventional electron microscopy and freeze-fracture to examine cellular mechanisms responsible for the estradiol-induced changes at the membrane level. 2. In freeze-fracture replicas taken 10-60 min and 24 hr after injection of 17 beta-estradiol to adult ovariectomized females, it was found that there was a rapid increase in the number of exoendocytotic images that reached a plateau by 30 min. 3. In thin sections from animals injected 24 hr earlier we demonstrated a significant increase in coated vesicles in the periphery of the neurons and coated pits in the perikaryal membranes and decreased axosomatic synapses. 4. We conclude that these morphological alterations are signaling estrogen-induced transport and/or turnover of perikaryal membrane constituents and extracellular components which may affect interneuronal and neuroglial interactions.

A new organotypic culture method to study the actions of steroid hormones on the nervous system

A new organotypic culture method for growing slices of nervous system tissue, based on the use of hyaluronic acid as a growth supporting milieu, is described. This method allows cultures derived from either fetuses or newborns to grow and develop with markedly reduced amounts of added serum. Organotypic cultures from fetal rat hypothalamus were exposed to 17 beta estradiol and compared to control cultures exposed to the ethanol vehicle. When exposed to estradiol, cultures showed an outgrowth of thick nerve fibers that was accompanied by an elevation in the number of microtubules present in the neuronal processes, an increment in the number of synapses, and an increased morphological differentiation of synaptic terminals. Freeze-fracture analysis of neuronal membranes from estradiol-treated cultures revealed a significant increase in the number of exoendocytotic images and a decrease in the number of intramembranous particles. Estradiol's effects parallel those found in in vivo studies, indicating that hyaluronic acid-based organotypic cultures represent an appropriate model to study hormonal influences on the developing nervous system.

The role of estradiol and progesterone in phased synaptic remodelling of the rat arcuate nucleus

During the estrous cycle there is a phasic synaptic remodelling in the hypothalamic arcuate nucleus, consisting in a loss and regain of axo-somatic synapses during the 48 h period between the morning of proestrus and the morning of metestrus. Synaptic changes are accompanied by cyclic modifications in the number of intramembrane particles in the plasma membrane of arcuate neuronal somas. To test the effect of the ovarian steroids on arcuate axo-somatic synapses we treated castrated females either with oil vehicle, 17 beta-estradiol, progesterone, or a combination of estradiol and progesterone, and observed them for 48 h. The number of axo-somatic synaptic profiles showed a 33% fall by 24 h after estradiol treatment and returned to control levels by 48 h. The effect of estradiol on axo-somatic synapses was accompanied by a marked and reversible modification of the number of intramembrane particles in the plasma membrane of arcuate neuronal somas. Progesterone alone did not affect the number of axo-somatic synaptic profiles nor the number of intramembrane particles, but when administered together with estradiol, blocked the effects of estradiol on neuronal membrane and synapses.

Cycloheximide mimics effects of oestradiol that are linked to synaptic plasticity of hypothalamic neurons

The synaptic connectivity of the rat arcuate nucleus, a hypothalamic area rich in oestradiol receptors, is rapidly affected by physiological modifications of hormonal levels. A rise of oestradiol in plasma elicits a coordinated neuronal-glial response that begins with a rapid fall in the number of small (< 10 nm) intramembrane particles and a rapid increase in the number of large (> 10 nm) intramembrane particles in neuronal membranes, followed by a modification in the branching of astrocytic processes and finally results in decreased number of axo-somatic synapses and increased glial wrapping of the neuronal somas. In the course of a series of studies aimed to test possible non-genomic effects of oestradiol on neuronal membranes we analyzed the effect of the systemic administration of the protein synthesis inhibitor cycloheximide on the ultrastructure of arcuate neurons and granule cells of the cerebellar cortex, an area of the brain with low levels of estrogen receptors. Cycloheximide resulted in a significant inhibition of protein synthesis in hypothalmus and cerebellum of ovariectomized rats. Under these circumstances, the number of small intramembrane particles was reduced in hypothalamic and cerebellar neuronal membranes while the number of large intramembrane particles showed a decrease in cerebellar membranes and a transient increase in arcuate neuronal somas. Furthermore, cycloheximide resulted in an increased glial wrapping of arcuate neuronal somas but not of cerebellar granule cells. The ensheathing of arcuate neurons by glial was associated with a 41% decrease in the number of axo-somatic synapses. These results indicate that the protein synthesis inhibitor cycloheximide may elicit the integrated neuronal-glial response that is associated with the hormonally induced remodelling of synaptic contacts on arcuate neurons.

Freeze-fracture immunocytochemical study of the expression of native and recombinant GABAA receptors

To assess the density and distribution of native and recombinant GABAA receptors we used label-fracture and fracture-flip technologies combined with immunocytochemistry using monoclonal and polyclonal Abs directed against the extracellular domain of the GABAA receptor protein located in the freeze-fracture replicas. In cortical neurons there is a high density of GABAA receptors on both soma and dendrites with some areas were the density of receptors is higher, but there are no well defined clusters. In cerebellar granule cells most of the receptors are distributed in round clusters both in neurites and soma. In astroglial cells the receptor density is lower than in neurons and only occasionally they appear in clusters. In cells transfected with cDNAs encoding for various molecular forms of GABAA receptor subunits, the receptor density is moderate when cDNAs for alpha, beta and gamma subunits are cotransfected; however, on cells cotransfected with cDNAs for beta and gamma subunits the receptor density is significantly lower. Recombinant receptors appear randomly distributed and occasionally they aggregate in small groups.

Embryonic development of synapses on spiking local interneurones in locust

The development of synapses on an identified population of spiking local interneurones in the thoracic ganglia of embryonic locusts was examined by means of intracellular horseradish peroxidase injection and electron microscopy. In adult locusts, spiking local interneurones of the midline group receive direct inputs onto a ventral field of branches from leg mechanosensory afferents and in turn make output synapses, mainly from a dorsal field of branches, directly upon leg motor neurones, nonspiking local interneurones, and intersegmental interneurones. The aim of this study is to examine the development of these connections. These interneurones are born relatively late in embryogenesis and are not identifiable until approximately 55% of development. At this time (55-60%) only simple filopodial contacts or punctate contacts are evident between the stained interneurones and other neurones. By 65-70% embryogenesis, vesicles are found adjacent to regions where apposed membranes are symmetrically thickened with amorphous electron-dense material. These symmetrical contacts lack distinct presynaptic bar-shaped densities and therefore, are not considered to be synapses. At this stage, the interneurones do not produce action potentials upon intracellular injection of depolarising current. Morphologically identifiable synapses, with vesicles, a presynaptic bar, and relatively little postsynaptic density, are first evident at 70-75%, coincident with the time of arrival of the majority of leg mechanosensory afferents into the central nervous system. At this stage, action potentials and synaptic potentials are also recorded for the first time. The midline spiking interneurones thus become electrically excitable when synapses are first recognisable, at approximately 70% embryogenesis. Most of the synapses found on the interneurones are outputs. The ratio of outputs to inputs on ventral branches is 7.5:1 which contrasts markedly to the adult ratio of 1:2. By 85-90%, output synapses still predominate on the ventral branches, but the ratio of outputs to inputs is reduced to almost 2:1. Dorsal branches have predominantly output synapses throughout embryogenesis. The ratio of dorsal outputs to inputs at 85-90% is 8.5:1 which compares with the adult ratio of 6.5:1. At this stage, action potentials and synaptic activity are always recorded.

Changes of rat striatal neuronal membrane morphology and steroid content during the estrous cycle

It is well documented that sex steroids affect striatal dopamine systems. However, the mechanism(s) of these hormonal effects in the striatum is still not well understood. We now report that gonadal steroid hormones during the estrous cycle affect the morphology and steroid hormone content of the rat striatum. Rats displaying at least two consecutive estrous cycles were included in this study as well as a group of female rats ovariectomized two weeks before being killed. The striatum was dissected from one half of each brain and used for morphological studies. From the other half of each brain, the striatum was dissected and steroid hormone concentrations in striatum and the remainder of the brain were determined. Tissues and serum concentrations of 17 beta-estradiol, progesterone and prolactin were measured by specific radioimmunoassays. Serum 17 beta-estradiol and prolactin concentrations peaked in proestrus, while progesterone was high in diestrus and proestrus. 17 beta-Estradiol levels were higher in the striatum than in the rest of the brain; both were also shown to fluctuate during the estrous cycle and with a pattern similar to that observed in serum. Progesterone serum levels showed a similar pattern of changes during the estrous cycle to progesterone concentrations in the striatum and the rest of the brain. The ultrastructure of the striatal dendritic membranes was studied by freeze-fracture. A significant difference in the content of intramembranous particles in dendritic shafts, which are mainly contacted by dopaminergic synapses, was found during the estrous cycle. The numerical density of large (greater than 10 nm) intramembranous particles was increased in diestrus I and II and in the afternoon of proestrus compared to estrus, the morning of proestrus and ovariectomized rats. In contrast, the numerical density of small (less than 10 nm) intramembranous particles was decreased in cycling animals compared to ovariectomized rats and fell in the afternoon of proestrus and then progressively increased in the following days to peak in the morning of proestrus. A negative correlation between steroid concentrations and small intramembranous particle density was observed, while the correlation was positive for large particles. No changes were observed in the membranes of dendritic spines, the main postsynaptic target for cortical afferents. In summary, this is the first report that concentrations of 17 beta-estradiol and progesterone in the striatum fluctuate during the estrous cycle. This is associated with estrous cycle-dependent changes of intramembranous particle density of striatal dendritic membranes. Our data therefore indicate that the striatum is a brain region hormonally modulated under physiological conditions.

Estrogen-like effects of the mammary carcinogen 7,12-dimethylbenz(alpha)anthracene on hypothalamic neuronal membranes

Previous studies have shown that in Sprague-Dawley female rats, but not in Wistar females, the mammary carcinogen dimethylbenz(alpha)anthracene (DMBA) results in extended preovulatory prolactin and estradiol surges, associated with inhibition of preovulatory gonadotropin surges, and in the induction of mammary tumors. Because earlier studies of similar endocrine states have shown this to be linked to hypothalamic arcuate nucleus neuronal membrane organization, in this study freeze-fracture methodology was used to determine whether DMBA may affect the ultrastructure of the neuronal membrane in the arcuate nucleus. The effects of estradiol valerate and DMBA were studied on 55- to 60-day-old cycling females, in Sprague-Dawley and Wistar rats, 8 weeks after the treatment. DMBA alone (15 mg/rat by gastric intubation) resulted in a significant decrease in the numerical density of intramembrane protein particles (IMP) in Sprague-Dawley rats but not in Wistar rats. The SC injection of estradiol valerate (1 mg/rat) resulted in a significant decrease of IMP numbers in both strains of rats. Although the subcutaneous injection of DMBA alone (1 mg/rat) did not affect IMP numerical density in either strain, the same potentiated the effect of estradiol valerate (1 mg/rat) on IMP's in Sprague-Dawley but not in Wistar females. These results indicate that DMBA affects the organization of neuronal plasma membrane in the hypothalamus of Sprague-Dawley rats. Wistar females are insensitive to both the endocrine and neuronal membrane effects of DMBA. Estradiol affected neuronal membranes in both strains and potentiated DMBA's effect. It appears that the estrogen-sensitive mechanism of DMBA activation may be lacking in Wistar rats.

Freeze-fracture organization of chromatin and cytoplasm in neurons and astroglia of rat cerebellar cortex

The cytology of the cell nucleus and cytoplasm of neurons and astroglia of the rat cerebellar cortex has been investigated by freeze-fracture electron microscopy. The main differential characteristics in the cytoplasm of the several cell types of the cerebellar cortex were: (1) the organization of endoplasmic reticulum elements, including special configurations of lamellar bodies and hypolemmal complexes, (2) the polarity, extension and arrangement of Golgi cisterns and associated tubulovesicular elements; (3) the connection pattern among different membrane-bounded cellular compartments; and (4) the architecture of endomembranes (i.e. presence of pits and fenestrations). In the nucleus, the main differential features were the the three-dimensional view of the nuclear envelope, the distribution of nuclear pores and the aggregation pattern of chromatin, visualized as clusters of nuclear particles in cross-fractures. The quantitative analysis of chromatin revealed four peaks of nuclear particle sizes (8, 12, 17 and 21 nm) that may correspond to variable degrees of coiling of the polynucleosomal chain in the chromatin fibre. Significant differences were observed in the proportion, numerical density and size distribution of aggregated nuclear particles in heterochromatin domains among the different cell types of the cerebellar cortex. The percentage of nuclear particles in aggregates varied from 10% in Purkinje cells to 64% in granule cells. Astrocytes and Bergmann glia showed intermediate values (about 40%). The percentage of nuclear particles in aggregates showed a significant (P less than 0.05) negative linear correlation with the nuclear volume, the number of pores per unit nuclear volume and the total number of pores per nucleus. In granule cells and astroglia, heterochromatin domains had a greater percentage of large nuclear particles (greater than 10 nm) than did euchromatin domains, whereas in interneurons, Purkinje and Golgi cells heterochromatin and euchromatin showed a similar proportion of large particles. Nuclear particles in euchromatin exhibited a similar pattern of distribution in all cerebellar cells.

Loss of sexual dimorphism in rat arcuate nucleus neuronal membranes with reproductive aging

Arcuate neurons of the rat hypothalamus have a sexual dimorphic membrane phenotype: quantitative analysis of freeze-fracture replicas has revealed that a population of intramembrane protein particles (IMP) of small size (less than 10 nm) is enriched in the plasma membrane of perikarya and dendritic shafts of cycling females compared to males, whereas a population of large IMPs (greater than 10 nm) is enriched in the membrane of dendritic shafts of males. This different membrane organization is associated with a sex dimorphic synaptic connectivity. To determine whether sex differences in neuronal membrane are affected by reproductive senescence, IMPs were assessed in freeze-fracture replicas of arcuate neuronal plasma membranes of male and female Sprague-Dawley rats aged 3, 15, and 18 months. Three-month-old cycling females were studied on the morning of estrus. Senescent females were in constant estrus (15 months old) or in constant diestrus (18 months old). Young females had more IMPs with diameters under 10 nm in the inner and outer leaflets of the plasma membrane of the perikarya and dendritic shafts compared to males of the same age. In addition, young males showed an increased number of large (greater than 10 nm) IMPs in the outer membrane leaflet of dendritic shafts. No sex differences were detected in the membrane of dendritic spines. In senescent females the number of small IMPs was decreased in the perikarya and dendritic shafts compared to young females while the number of large particles was increased in the outer leaflet of the membrane of dendritic shafts, reaching values similar to those observed in males. IMP counts were not modified with aging in males and in dendritic spines of females. These results indicate that reproductive aging in female rats is associated with a remodeling of neuronal plasma membranes in arcuate neurons.

Estradiol increases the number of nuclear pores in the arcuate neurons of the rat hypothalamus

Freeze-fracture replicas of hypothalamic arcuate neurons and of Purkinje and granule cells of the cerebellar cortex from adult female rats were assessed in order to test the possible influence of estradiol on nuclear pores. Rats were ovariectomized and injected either with estradiol or with vehicle. An additional group of rats in proestrus was also studied. Pore diameter was not affected by ovariectomy or estrogen treatment. In arcuate neurons, the number of nuclear pores per nuclear membrane area, the total number of pores per nucleus, and the percentage of nuclear pores arranged in clusters were decreased by ovariectomy and increased within 30 minutes after estradiol administration to ovariectomized rats. The effect of estradiol on nuclear pores was sustained for several days; the number of pores and the percentage of pores in clusters reverted to control values by 1 month after the hormonal treatment. None of the above mentioned changes was observed in Purkinje and granule cells of the cerebellar cortex. These results indicate that estradiol may modulate the number and distribution of nuclear pores in arcuate neurons and suggest that the modification of the ultrastructure of the nuclear envelope may be one of the first effects of gonadal steroids on target cells.

Ultrastructural alterations in plasma membranes from drug-resistant P388 murine leukemia cells

Freeze-fracture studies of daunomycin-sensitive and daunomycin-resistant P388 cell lines, reveal a significant increase in the numerical density of intramembrane particles at both, the protoplasmic and the exoplasmic leaflets of the plasma membrane from the drug-resistant cells. Such change in plasma membrane architecture is not accompanied by overexpression of P-glycoproteins. Furthermore, drug-sensitive cells exhibited an increased number of exo-endocytotic images when compared to drug-resistant cells. Our observations suggest that there are global changes in the structural organization of the plasma membrane, which are related to the acquisition of the cellular drug-resistant phenotype.

African green monkeys have sexually dimorphic and estrogen-sensitive hypothalamic neuronal membranes

Previous studies have shown sex differences in intramembrane particle content in the arcuate neurons of the rat hypothalamus. In this study, freeze-fracture replicas were prepared from the infundibular hypothalamus of adult African green monkeys (Cercopithecus aethiops) in order to determine whether primates also have sexual dimorphism in neuronal membranes. Intramembrane particles (IMP) were quantitatively assessed in the perikaryal plasma membranes of infundibular neurons. Four groups of monkeys were studied: intact males, intact females, ovariectomized females injected with 20 mg of estradiol valerate over 10 days and ovariectomized females injected with vehicle (castor oil). Membranes from females showed an increased numerical density of IMPs when compared to males. Ovariectomy of females did not affect IMP content, while estrogen administration resulted in a significant decrease in IMP numerical density to reach male values. These findings indicate a sex difference in neuronal membranes in the hypothalamus of monkeys and suggest that as in rodents, neuronal plasma membrane organization in higher primates may be modulated by gonadal steroids.

Sexual differentiation of synaptic connectivity and neuronal plasma membrane in the arcuate nucleus of the rat hypothalamus

Plasma membranes of the hypothalamic arcuate neurons of the rat show a sexually dimorphic phenotype: the numerical density of intramembrane protein particles is greater in females. Male and female Sprague-Dawley rats, 10, 20 and 100 days old, were studied in order to determine whether sexual differentiation of the neuronal plasma membrane in the soma of arcuate neurons is associated with the establishment of sex differences in the pattern of axo-somatic synaptic contacts. Axo-somatic synapses were counted in thin sections of the arcuate nucleus and intramembrane particles were assessed in freeze-fracture replicas of the neuronal membrane. The number of synapses per length of perikaryal membrane increased from day 10 to day 20 in both sexes, reaching by 20 days values similar to those found on day 100. A sex difference in the number of synapses was observed only in 20-day-old and 100-day-old rats: neurons from females showed a greater number of presynaptic inputs than males (P less than 0.05). This sex difference was abolished by administration of testosterone propionate to 5-day-old females. Quantitative evaluation of freeze-fracture replicas of the arcuate neuronal perikarya revealed sex differences in the numerical density of intramembrane particles at all time points studied: neurons from females contained significantly more particles in their plasma membranes than neurons from males or androgenized females of the same age (P less than 0.001). These results indicate that sexual differentiation of the plasma membrane in neuronal somas precedes the establishment of sex differences in axo-somatic synapses. The results are compatible with a possible role of neuronal membranes in the sexual differentiation of synaptic connectivity.

Distribution of nuclear pores and chromatin organization in neurons and glial cells of the rat cerebellar cortex

Nuclear pores were assessed on freeze-fracture replicas from different neuronal and glial cell types of the rat cerebellar cortex. Nuclear diameter and perimeter were measured on semithin sections, and nuclear surface area and volume were calculated from these data. The proportion of inner nuclear membrane in apposition to condensed chromatin was measured on thin sections. The values of nuclear pore numerical density (number/micron2) were as follows (mean +/- S.D.): Purkinje cells, 22 +/- 3; Golgi cells 17 +/- 3; granule cells, 6 +/- 4; stellate and basket cells, 6 +/- 1; protoplasmic astrocytes, 11 +/- 1; Bergmann glia, 10 +/- 1; oligodendrocytes, 6 +/- 1. The total number of nuclear pores per nucleus varied from 18,451 +/- 2,336 (Purkinje cells) to 621 +/- 394 (granule cells) among neurons, and from 1,782 +/- 162 (protoplasmic astrocytes) to 402 +/- 67 (oligodendrocytes) among glial cells. The number of nuclear pores per unit nuclear volume (number/micron3), a parameter related to nucleocytoplasmic transport capacity, varied from 15 +/- 2 in Purkinje cells to 6 +/- 4 in granule cells. The proportion of nuclear membrane free of condensed chromatin was significantly (P less than 0.01) correlated to pore numerical density and total number of pores per nucleus. Some nuclear pores were associated in clusters of two or more pores. The amount of pore clustering was measured by counting the proportion of pores associated in clusters. This proportion varied among the different cell types from 82% in Purkinje cells to 44% in stellate and basket cells. The amount of pore clustering showed a positive linear correlation to pore numerical density and pore number per nucleus. However, the proportion of pores in clusters was not significantly correlated with the amount of condensed chromatin applied against the inner nuclear membrane.

Estradiol induces rapid remodelling of plasma membranes in developing rat cerebrocortical neurons in culture

Exo-endocytotic images and intramembrane particles were quantitatively assessed in freeze-fracture replicas from the plasma membrane of dispersed fetal rat cortical neurons (day 16 gestation) grown for 24 days in culture. The addition of 10(-10) M 17 beta-estradiol to the culture medium resulted in a significant increase in the numerical density of exo-endocytotic images within 1 min. A further increase of the number of exo-endocytotic images associated to a significant decrease in the number of intramembrane particles was observed in cells exposed for 10 min to 17 beta-estradiol. Similar results were observed when the cells were exposed to 17 beta-estradiol for 17 days. No effects on exo-endocytotic images and intramembrane particles were observed when 17 alpha-estradiol was added, instead of 17 beta-estradiol, to the cultures. These results indicate that physiological levels of 17 beta-estradiol can have rapid effects upon the ultrastructure of the neuronal membrane of developing cerebrocortical neurons.

Sex differences in plasma membrane concanavalin A binding in the rat arcuate neurons

Previous studies have shown that synaptic connections and organization of neuronal membranes are sexually dimorphic in the arcuate nucleus of developing and adult rats. These sex differences can be abolished by the perinatal androgenization of females. In this study the label-fracture method of Pinto da Silva and Kan was used in order to determine whether membrane sex differences are related to the glycoconjugates in neuronal plasma membranes. Six Sprague-Dawley female rats treated with testosterone on the day of birth, six control females injected with vehicle and six intact males were studied when they were 100 days old. The arcuate nucleus was dissected and incubated for 2 hours in a solution of 0.25 mg/ml concanavalin A, washed in buffer and incubated for 3 hours in a suspension of horseradish peroxidase-coated colloidal gold. Then, freeze-fracture replicas of the arcuate nucleus were prepared. Colloidal gold labeling was observed to be codistributed with intramembrane particles in the outer membrane face of the neuronal perikaryal plasma membrane. The numerical density of small (less than 10 nm) intramembrane particles and colloidal gold particles was significantly greater in control female membranes when compared to males or to androgenized females. The labeling was significantly reduced when the arcuate nucleus was incubated with concanavalin A in presence of 0.5 M methyl-alpha-manopyranoside. These findings indicate a sex difference in the density and distribution of glycoconjugates and intramembranous particles in the neuronal plasma membrane that is dependent on the perinatal levels of sex steroids and is concordant with, and could be the cause of, sex differences in the pattern of synaptic contacts.

Momentary alteration of the postsynaptic membrane during transmission of a single nerve impulse

Transmission of a nerve impulse at neuromuscular and other synapses is an extremely brief event. By using rapid-freezing and cryofracture techniques in the electric organ of Torpedo, synaptic transmission was found to be accompanied by significant changes affecting the postsynaptic membrane for a few milliseconds. In the replicas, the protoplasmic leaflet of this membrane was seen to contain intramembrane particles (IMPs) of two different forms, globular and elongated. Globular IMPs had a mean diameter of 8.8 nm; they were the most frequently found (80% in unstimulated specimens). Elongated IMPs had a major diameter of 17.9 nm, about twice that of globular IMPs. Transmission of a single nerve impulse was accompanied by a marked decrease in the number of globular IMPs and by an increase in the number of elongated IMPs, as if there were a coalescence of two adjacent round particles to form an elongated one. These changes started soon after the electrical stimulus and lasted for approximately 3 ms. IMPs in the postsynaptic protoplasmic face are thought to correspond to a certain proportion of nicotinic acetylcholine receptors that were extracted with this leaflet during the fracture process. The phenomenon described here reflects an abrupt change in the membrane, probably linked to activation of the acetylcholine nicotinic receptors.

Synaptic remodeling in the rat arcuate nucleus during the estrous cycle

Adult female rats showing regular vaginal cycles were studied in order to determine the number of axosomatic synapses in thin sections of the arcuate nucleus. The number of synapses per length of perikaryal membrane was significantly decreased in estrus, compared to other days of the estrous cycle (P less than 0.05). The reduction in the number of synapses in estrus was accompanied by a decrease in the percentage of the average length of perikaryal membrane covered by presynaptic terminals and by an increase in the percentage of membrane in close apposition of glial processes. Since the average perikaryal perimeter was not significantly changed during the estrous cycle, these results indicate a net decrease in the number of arcuate nucleus axosomatic synapses between proestrus and estrus, with a reinnervation of arcuate neurons between estrus and metestrus. These results suggest that there is a physiological synaptic turnover in the arcuate nucleus of the rat during the estrous cycle.

Fine structure of synaptogenesis in the vertebrate central nervous system

This article reviews studies of the formation of synaptic junctions in the vertebrate central nervous system. It is focused on electron microscopic investigations of synaptogenesis, although insights from other disciplines are interwoven where appropriate, as are findings from developing peripheral and invertebrate nervous systems. The first part of the review is concerned with the morphological maturation of synapses as described from both qualitative and quantitative perspectives. Next, epigenetic influences on synaptogenesis are examined, and later in the article the concept of epigenesis is integrated with that of hierarchy. It is suggested that the formation of synaptic junctions may take place as an ordered progression of epigenetically modulated events wherein each level of cellular affinity becomes subordinate to the one that follows. The ultimate determination of whether a synapse is maintained, modified or dissolved would be made by the changing molecular fabric of its junctional membranes. In closing, a hypothetical model of synaptogenesis is proposed, and an hierarchial order of events is associated with a speculative synaptogenic sequence. Key elements of this hypothesis are 1) epigenetic factors that facilitate generally appropriate interactions between neurites; 2) independent expression of surface specializations that contain sufficient information for establishing threshold recognition between interacting neurites; 3) exchange of molecular information that biases the course of subsequent junctional differentiation and ultimately results in 4) the stabilization of synaptic junctions into functional connectivity patterns.

Sexual differentiation of the neuronal plasma membrane: neonatal levels of sex steroids modulate the number of exo-endocytotic images in the developing rat arcuate neurons

Exo-endocytotic images and intramembrane protein particles (IMP) were quantitatively assessed in freeze-fracture replicas from the plasma membrane of arcuate neurons of rats aged 0 (newborns), 10, 20 and 100 days postpartum. Membranes contained significantly (P less than 0.02) more IMPs in females than in males. Exo-endocytotic images were increased in newborn and 10-day-old males when compared to adult males or to developing females (48 +/- 6 vs 6 +/- 1 images/100 micron 2 in 10-day-old male and female rats, respectively). Androgenization of females with a single injection of testosterone propionate on the day of birth resulted in an increased number of exo-endocytotic images in developing animals (75 +/- 9 images/100 micron 2, 10-day-old rats) and in the abolishment of the sex differences in the number of IMPs.

Neuronal membrane remodelling during the oestrus cycle: a freeze-fracture study in the arcuate nucleus of the rat hypothalamus

Freeze-fracture methodology was used to study the organization of the neuronal plasma membrane in the rat arcuate nucleus, an oestrogen sensitive area of the hypothalamus. The quantitative evaluation of freeze-fracture replicas of the perikarya, dendritic shafts and dendritic spines revealed that the numerical density of intramembranous particles varied during the ovarian cycle. The number of small (less than 10 nm) particles was decreased, and the number of large (greater than 10 nm) particles was increased, in the P-face of the perikaryal plasma membranes in prooestrus and oestrus when compared to dioestrus. This change was associated with a significant increase in the number of exo-endocytotic images in the perikaryal plasma membrane in prooestrus. Changes in intramembranous particles during the ovarian cycle were not detected in arcuate nucleus dendritic shafts and dendritic spines.

Estrogen-induced synaptic remodelling in adult rat brain is accompanied by the reorganization of neuronal membranes

Adult cycling female rats were injected with estradiol valerate (2 mg/100 g body wt.), a treatment which has previously been shown to result in synaptic remodelling in the arcuate nucleus and constant vaginal estrus. During the 32 weeks following estrogen treatment, arcuate nucleus neuronal plasma membranes were quantitatively assessed for intramembrane particle (IMP) number and size using freeze-fracture techniques. Neuronal membranes from untreated cycling females, females injected with oil and untreated males were also studied. Untreated rats had dimorphic sexual phenotypes in membrane organization; female rats had more IMP than males, mainly due to greater numbers of small (less than 10 nm) particles. These sex differences were observed in perikarya and dendritic shafts, but not in dendritic spines. Following estrogen treatment the density of IMP in membranes from females decreased. The IMP changes were found only in neuronal perikarya and dendritic shafts, not in dendritic spines, and were mainly due to a massive decrease in the number of small (less than 10 nm) IMP which was only partially offset by an increase in the number of large (greater than or equal to 10 nm) IMP. Thus, by 32 weeks after estradiol valerate treatment, the number and size of IMP in neuronal membranes from females were not different from those seen in normal males. These results strengthen the idea that estradiol may affect the turnover of certain neuronal membrane components in sex-steroid sensitive areas of the brain.

Nuclear pores in rat hypothalamic arcuate neurons: sex differences and changes during the oestrous cycle

The numerical density of nuclear pores was assessed on freeze-fracture replicas of hypothalamic arcuate neurons from adult male and female rats. In females the nuclear pore density fluctuated during the oestrous cycle and was higher in oestrus than in pro-oestrus, metoestrus and dioestrus. Nuclear pore density in males and in metoestrus and dioestrus females was similar. The nuclear pore density in male rats was significantly less than that in oestrus and pro-oestrus females. The variation of the number of pores per unit nuclear volume and the total number of pores per nucleus was similar to the variation of the numerical density of nuclear pores. These results provide morphological evidence of cyclic changes in neuronal nucleocytoplasmic traffic during the ovarian cycle.

Rapid effects of gonadal steroids upon hypothalamic neuronal membrane ultrastructure

Freeze-fracture methodology was used to study rat hypothalamic arcuate nucleus (AN) neuronal plasma membrane organization following in vitro perfusion of brain slices with 17-beta-estradiol (17 beta E2) or other test compounds. Physiological levels (10(-10) M) of 17 beta E2 caused an increase in neuronal membrane exo-endocytotic pits within 1 min of perfusion. The increased density of pits was dose related, sustained at a constant rate during 10 min of perfusion, reverted to control values after perfusion with estradiol-free medium for 1 h, and was accompanied by an increased uptake of horseradish peroxidase by the arcuate nucleus in brain slices. The 17 beta E2-induced increase in exo-endocytotic pit density was blocked by tamoxifen (10(-8) M). Cholesterol (10(-10) M), 17-alpha-estradiol (10(-6) M) or dihydrotestosterone (10(-6) M) had no effect on exo-endocytotic pit density. Testosterone had about 50% the potency of 17 beta E2 in increasing exo-endocytotic pit density. These results indicate that physiological levels of 17 beta E2 can have rapid effects upon arcuate nucleus neuronal membrane ultrastructure.

Brief occurrence of a population of presynaptic intramembrane particles coincides with transmission of a nerve impulse

Small pieces of Torpedo electric organ were cryofixed at 1-ms time intervals in a liquid medium at -190 degrees C before, during, and after the passage of a single nerve impulse. In contrast to studies using this or other preparations, these experiments were done without 4-aminopyridine or other drugs that potentiate transmitter release. Freeze-fracture replicas were made from the most superficial layers of the tissue, where the rate of cooling was rapid enough to retain ultrastructure in the absence of chemical fixation. We found that the transmission of an impulse was accompanied by the momentary appearance of a population of large intramembrane particles in both the protoplasmic (P) and the external (E) leaflets of the presynaptic plasma membrane. The change was very brief, appearing soon after the stimulus artifact. It lasted for 2-3 ms. Large pits denoting vesicle openings at the presynaptic membrane were found in a small proportion of nerve terminals; their number did not increase during transmission of the nerve impulse. Reducing the temperature from 16 to 5 degrees C slowed the time course of both the electrophysiological response and the change in intramembrane particles. The number of large particles did not increase when stimulation was applied in a low-Ca medium, a condition where the nerve terminals were still depolarized by the action potential but did not release the neurotransmitter. From these and other observations, we conclude that this transient change of intramembrane particles is closely linked to the mechanism of acetylcholine release at the nerve-electroplaque junction.

Freeze-fracture characterization of proteolipid protein and basic protein of central nervous system myelin incorporated in liposomes

Proteolipid protein (PLP) and basic protein (BP) of central nervous system myelin were purified from calf brain white matter and incorporated in liposomes of L-dimyristoyl-alpha-phosphatidylcholine (DML) or in liposomes formed with an extract of natural lipids from myelin. Freeze-fracture replicas of the liposomes were prepared to study the number and size of intramembrane protein particles (IMP) in the fracture faces of the lipid bilayer. Globular and elongated IMP were observed in the freeze-fracture liposome membranes after incorporation of proteolipid protein. Globular IMP were the most frequently found (91-96% of the total IMP), and some of them showed a tiny black spot or pit on the top, suggesting the presence of hydrophilic channels in these particles. Globular and elongated IMP were also observed in the fractured membranes when basic protein was incorporated in liposomes. Again, globular IMP were the most frequent (92-95%) but no spots were present on the top. In addition, both globular and elongated IMP generated by basic protein were significantly larger than IMP generated by PLP. The proportion, size and form of globular and elongated particles generated by PLP and BP were unaffected by the amount of protein incorporated in liposomes (0.13-0.75 protein/lipid, w/w) nor by the type of lipid matrix used (DML or myelin natural lipid mixture). Intramembrane particles were absent from membranes of liposomes of pure lipid.

Harmaline-induced changes in plasma membrane of Purkinje cells: a trans-synaptic effect mediated by climbing fibers

Fixed cerebellar cortex slices of adult male albino rats injected with saline, were exposed to the cholesterol probe filipin and freeze-fractured. Under these conditions, the plasma membrane of Purkinje cell dendrites was labeled with only a limited number of typical, 25-30 nm diameter filipin-sterol complexes. In contrast, in rats injected with harmaline, a drug that induces a rhythmic activation of climbing fibers, dendritic membranes showed an increased number of filipin-sterol complexes. This increased filipin labeling of Purkinje cell dendrites was not observed, however, when harmaline was injected into rats after destruction of climbing fibers. These results suggest that climbing fiber activation may induce a reorganization of the lipid matrix in the plasma membrane of Purkinje cells.

Insulin modulates neuronal plasma membrane development in human fetal spinal cord neurons in culture

The number of intramembrane protein particles (IMP) in the protoplasmic face of the perikaryal plasma membrane was evaluated in neurons from 9 week-old and 12 week-old human fetal spinal cord in culture. An increased number of IMP was observed in membranes from 12 week-old fetal neurons when compared to membranes from 9 week-old fetal neurons. The addition of insulin (100-2500 microU/ml) to the culture media resulted in a significantly increased number of IMP in neuronal membranes. Incubation with glucose (1.5-6 mg/ml) did not modify the number of IMP and glucose did not potentiate the effect of insulin when both glucose (3 mg/ml) and insulin (500 microU/ml) were added to the culture media. The results suggest that insulin may modulate the development of neuronal membranes and that this effect is not mediated by an increased glucose utilization.

Trans-synaptic modulation of Purkinje cell plasma membrane organization by climbing fiber axonal flow

Destruction of the inferior olivary nucleus in the rat by thermocoagulation results in the degeneration of climbing fibers in the cerebellar cortex. Under these conditions the plasma membrane of Purkinje cells was selectively affected at the level of the spines on large dendrites (LD spines), the postsynaptic targets for climbing fibers. The number of small (less than 10 nm) non-junctional intramembrane particles (IMP) was significantly decreased in E-faces during the first 3 days following the lesion (47% of control IMP values). Later, the number of IMPs progressively recovered to reach 67% of the control IMP values 1 month following the lesion. IMP numbers in the P-face and in the postsynaptic junctional aggregates of the E-face were unaffected by destruction of climbing fibers. Following injection of colchicine into the inferior olivary nucleus, a treatment that inhibits axonal transport in climbing fibers, a selective decrease in IMP numbers on the E-faces of LD spines was also found. The maximal decrease was found during the first 10 days after injection (48% of control values), and then the number of IMPs gradually increased to reach control values by 90 days post-injection. In the case of colchicine treatment also, the reduction in the number of IMPs affected selectively a class of non-junctional small IMPs, less than 10 nm in diameter. These data show that there is a similar selective modification in the membrane structure of Purkinje cells, not only following destruction of climbing fibers by thermocoagulation, but also following inhibition of axonal flow in climbing fibers by colchicine injection.(ABSTRACT TRUNCATED AT 250 WORDS)

Differential distribution of intermembranous particles in the plasmalemma of the migrating cerebellar granule cells

The size and distribution of intramembraneous particles in 5 different plasmalemma domains of migrating cerebellar granule cells was studied by the freeze-fracture method. The quantitative analysis in 5-20-day-old rats reveals that the membrane of the leading tip of the descending process, the soma and vertical process that are associated with Bergmann glial fibers, have a significantly higher density of small (less than 11 nm diameter) intramembraneous protein particles than the horizontal processes and their growth cones which grow along previously generated parallel fibers. This regional difference in density of membrane associated particles may be related to the selective outgrowth of the two classes of granule cell neurites along glial and neuronal surfaces.