A quantitative study of the ultrastructural changes in the hypothalamo-neurohypophysial system during and after experimentally induced hypersecretion

Sexually Dimorphic Neuropeptide B Neurons in Medaka Exhibit Activated Cellular Phenotypes Dependent on Estrogen

Brain and behavior of teleosts are highly sexually plastic throughout life, yet the underlying neural mechanisms are largely unknown. On examining brain morphology in the teleost medaka (Oryzias latipes), we identified distinctively large neurons in the magnocellular preoptic nucleus that occurred much more abundantly in females than in males. Examination of sex-reversed medaka showed that the sexually dimorphic abundance of these neurons is dependent on gonadal phenotype, but independent of sex chromosome complement. Most of these neurons in females, but none in males, produced neuropeptide B (Npb), whose expression is known to be estrogen-dependent and associated with female sexual receptivity. In phenotypic analysis, the female-specific Npb neurons had a large euchromatic nucleus with an abundant cytoplasm containing plentiful rough endoplasmic reticulum, exhibited increased overall transcriptional activity, and typically displayed a spontaneous regular firing pattern. These phenotypes, which are probably indicative of cellular activation, were attenuated by ovariectomy and restored by estrogen replacement. Furthermore, the population of Npb-expressing neurons emerged in adult males treated with estrogen, not through frequently occurring neurogenesis in the adult teleost brain, but through the activation of preexisting, quiescent male counterpart neurons. Collectively, our results demonstrate that the morphological, transcriptional, and electrophysiological phenotypes of sexually dimorphic preoptic Npb neurons are highly dependent on estrogen and can be switched between female and male patterns. These properties of the preoptic Npb neurons presumably underpin the neural mechanism for sexual differentiation and plasticity of brain and behavior in teleosts.

Neuronal activity and axonal sprouting differentially regulate CNTF and CNTF receptor complex in the rat supraoptic nucleus

We demonstrated previously that the hypothalamic supraoptic nucleus (SON) undergoes a robust axonal sprouting response following unilateral transection of the hypothalamo-neurohypophysial tract. Concomitant with this response is an increase in ciliary neurotrophic factor (CNTF) and CNTF receptor alpha (CNTFRα) expression in the contralateral non-uninjured SON from which the axonal outgrowth occurs. While these findings suggest that CNTF may act as a growth factor in support of neuronal plasticity in the SON, it remained to be determined if the observed increase in neurotrophin expression was related to the sprouting response per se or more generally to the increased neurosecretory activity associated with the post-lesion response. Therefore we used immunocytochemistry and Western blot analysis to examine the expression of CNTF and the components of the CNTF receptor complex in sprouting versus osmotically-stimulated SON. Western blot analysis revealed a significant increase in CNTF, CNTFRα, and gp130, but not LIFRß, protein levels in the sprouting SON at 10days post lesion in the absence of neuronal loss. In contrast, osmotic stimulation of neurosecretory activity in the absence of injury resulted in a significant decrease in CNTF protein levels with no change in CNTFRα, gp130, or LIFRß protein levels. Immunocytochemical analysis further demonstrated gp130 localization on magnocellular neurons and astrocytes while the LIFRß receptor was found only on astrocytes in the SON. These results are consistent with the hypothesis that increased CNTF and CNTFR complex in the sprouting, metabolically active SON are related directly to the sprouting response and not the increase in neurosecretory activity.

[Structural plasticity of the adult central nervous system: insights from the neuroendocrine hypothalamus]

Accumulating evidence renders the dogma obsolete according to which the structural organization of the brain would remain essentially stable in adulthood, changing only in response to a need for compensatory processes during increasing age and degeneration. It has indeed become clear from investigations on various models that the adult nervous system can adapt to physiological demands by altering reversibly its synaptic circuits. This potential for structural and functional modifications results not only from the plastic properties of neurons but also from the inherent capacity of the glial cellular components to undergo remodeling as well. This is currently known for astrocytes, the major glial cells in brain which are well-recognized as dynamic partners in the mechanisms of synaptic transmission, and for the tanycytes and pituicytes which contribute to the regulation of neurosecretory processes in neurohemal regions of the hypothalamus. Studies on the neuroendocrine hypothalamus, whose role is central in homeostatic regulations, have gained good insights into the spectacular neuronal-glial rearrangements that may subserve functional plasticity in the adult brain. Following pioneering works on the morphological reorganizations taking place in the hypothalamo-neurohypophyseal system under certain physiological conditions such as dehydration and lactation, studies on the gonadotropic system that orchestrates reproductive functions have re-emphasized the dynamic interplay between neurons and glia in brain structural plasticity processes. This review summarizes the major contributions provided by these researches in the field and also addresses the question of the morphological rearrangements that occur on a 24-h basis in the central component of the circadian clock responsible for the temporal aspects of endocrine regulations. Taken together, the reviewed data highlight the close cooperation between neurons and glia in developing strategies for functional adaptation of the brain to the changing conditions of the internal and external environment.

Vesicle degradation in dendrites of magnocellular neurones of the rat supraoptic nucleus

The magnocellular neurones of the supraoptic nucleus (SON) and paraventricular nucleus release neuropeptide from their axon terminals and also from their dendrites. In the axon terminals, swellings known as Herring bodies are responsible for the degradation of aged, unreleased large dense-cored vesicles (LDCVs) by lysosomes. Dendrites of magnocellular neurones also contain a large number of LDCVs but specialised areas of vesicle degradation have yet to be discovered. Using immunofluorescence labelling for lysosomes in vasopressin-enhanced green fluorescent protein (vasopressin-eGFP) transgenic rats, we found that lysosomes are preferentially located in the centre of the dendrites where there was a high density of vasopressin-eGFP expression. These data suggest that there are local "hot spots", but not specific compartments for vesicle degradation in magnocellular dendrites.

Multivesicular bodies in neurons: distribution, protein content, and trafficking functions

Multivesicular bodies (MVBs) are intracellular endosomal organelles characterized by multiple internal vesicles that are enclosed within a single outer membrane. MVBs were initially regarded as purely prelysosomal structures along the degradative endosomal pathway of internalized proteins. MVBs are now known to be involved in numerous endocytic and trafficking functions, including protein sorting, recycling, transport, storage, and release. This review of neuronal MVBs summarizes their research history, morphology, distribution, accumulation of cargo and constitutive proteins, transport, and theories of functions of MVBs in neurons and glia. Due to their complex morphologies, neurons have expanded trafficking and signaling needs, beyond those of "geometrically simpler" cells, but it is not known whether neuronal MVBs perform additional transport and signaling functions. This review examines the concept of compartment-specific MVB functions in endosomal protein trafficking and signaling within synapses, axons, dendrites and cell bodies. We critically evaluate reports of the accumulation of neuronal MVBs based on evidence of stress-induced MVB formation. Furthermore, we discuss potential functions of neuronal and glial MVBs in development, in dystrophic neuritic syndromes, injury, disease, and aging. MVBs may play a role in Alzheimer's, Huntington's, and Niemann-Pick diseases, some types of frontotemporal dementia, prion and virus trafficking, as well as in adaptive responses of neurons to trauma and toxin or drug exposure. Functions of MVBs in neurons have been much neglected, and major gaps in knowledge currently exist. Developing truly MVB-specific markers would help to elucidate the roles of neuronal MVBs in intra- and intercellular signaling of normal and diseased neurons.

Morphology and metamorphosis of the peptidergic Va neurons and the median nerve system of the fruit fly, Drosophila melanogaster

Metamorphosis is a fundamental developmental process and has been intensively studied for various neuron types of Drosophila melanogaster. However, detailed accounts of the fate of identified peptidergic neurons are rare. We have performed a detailed study of the larval morphology and pupal remodelling of identified peptidergic neurons, the CAPA-expressing Va neurons of D. melanogaster. In the larva, Va neurons innervate abdominal median and transverse nerves that are typically associated with perisympathetic organs (PSOs), major neurohaemal release sites in insects. Since median and transverse nerves are lacking in the adult, Va neurites have to undergo substantial remodelling during metamorphosis. We have examined the hitherto uncharacterised gross morphology of the thoracic PSOs and the abdominal median and transverse nerves by scanning electron microscopy and found that the complete reduction of these structures during metamorphosis starts around pupal stage P7 and is completed at P9. Concomitantly, neurite pruning of the Va neurons begins at P6 and is preceded by the high expression of the ecdysone receptor (EcR) subtype B1 in late L3 larvae and the first pupal stages. New neuritic outgrowth mainly occurs from P7-P9 and coincides with the expression of EcR-A, indicating that the remodelling of the Va neurons is under ecdysteroid control. Immunogold-labelling has located the CAPA peptides to large translucent vesicles, which are released from the transverse nerves, as suggested by fusion profiles. Hence, the transverse nerves may serve a neurohaemal function in D. melanogaster.

Activity-dependent modulation of neurotransmitter innervation to vasopressin neurons of the supraoptic nucleus

Confocal microscopy was used to assess activity-dependent neuroplasticity in neurotransmitter innervation of vasopressin immunoreactive magnocellular neurons in the supraoptic nucleus (SON). Vesicular glutamate transporter 2, glutamic acid decarboxylase, and dopamine beta-hydroxylase (DBH) synaptic boutons were visualized in apposition to vasopressin neurons in the SON. A decrease in DBH synaptic boutons per cell was seen upon salt loading, indicating diminished noradrenergic/adrenergic innervation. Loss of DBH appositions to vasopressin neurons was associated with a general loss of DBH immunoreactivity in the SON. In contrast, the number of vesicular glutamate transporter 2 synaptic boutons per neuron increased with salt loading, consistent with increased glutamatergic drive of magnocellular SON neurons. Salt loading also caused an increase in the total number of glutamic acid decarboxylase synaptic boutons on vasopressinergic neurons, suggesting enhanced inhibitory innervation as well. These studies indicate that synaptic plasticity compensates for increased secretory demand and may indeed underlie increased secretion, perhaps via neurotransmitter-specific, activity-related changes in synaptic contacts on vasopressinergic magnocellular neurons in the SON.

Expression of tyrosine hydroxylase and vasopressin in magnocellular neurons of salt-loaded aged rats

Tyrosine hydroxylase (TH) is expressed in catecholaminergic neurons. However, under certain conditions it is also ectopically expressed in magnocellular neurons of the hypothalamus. To test the hypothesis that this expression of TH is related to the cellular activation of these neurons and/or to the vasopressin (VP) expression, we studied the expression of both TH and VP in control and salt-loaded aged rats. Our results demonstrate that aged rats show a marked TH expression in VP cells which is further increased by osmotic stimulation in the absence of increase in VP synthesis in the supraoptic nucleus. The presence of TH-immunopositive dendritic swellings in the ventral part of this nucleus reveals the high state of plasticity of these neurons. Furthermore, the lack of several actors of catecholamine biosynthesis in these neurons suggests a different role for TH. This study further demonstrates an ectopic expression of TH in hypothalamic neurons of aged rats and a TH expression linked to the activation of VP neurons but unrelated to VP synthesis.

Space flight affects magnocellular supraoptic neurons of young prepuberal rats: transient and permanent effects

Effects of microgravity on postural control and volume of extracellular fluids as well as stress associated with space flight may affect the function of hypothalamic neurosecretory neurons. Since environmental modifications in young animals may result in permanent alterations in neuroendocrine function, the present study was designed to determine the effect of a space flight on oxytocinergic and vasopressinergic magnocellular hypothalamic neurons of prepuberal rats. Fifteen-day-old Sprague-Dawley female rats were flown aboard the Space Shuttle Columbia (STS-90, Neurolab mission, experiment 150) for 16 days. Age-matched litters remained on the ground in cages similar to those of the flight animals. Six animals from each group were killed on the day of landing and eight animals from each group were maintained under standard vivarium conditions and killed 18 weeks after landing. Several signs of enhanced transcriptional and biosynthetic activity were observed in magnocellular supraoptic neurons of flight animals on the day of landing compared to control animals. These include increased c-Fos expression, larger nucleoli and cytoplasm, and higher volume occupied in the neuronal perikaryon by mitochondriae, endoplasmic reticulum, Golgi apparatus, lysosomes and cytoplasmic inclusions known as nematosomes. In contrast, the volume occupied by neurosecretory vesicles in the supraoptic neuronal perikarya was significantly decreased in flight rats. This decrease was associated with a significant decrease in oxytocin and vasopressin immunoreactive levels, suggestive of an increased hormonal release. Vasopressin levels, cytoplasmic volume and c-Fos expression returned to control levels by 18 weeks after landing. These reversible effects were probably associated to osmotic stimuli resulting from modifications in the volume and distribution of extracellular fluids and plasma during flight and landing. However, oxytocin levels were still reduced at 18 weeks after landing in flight animals compared to controls. This indicates that space flight during prepuberal age may induce irreversible modifications in the regulation of oxytocinergic neurons, which in turn may result in permanent endocrine and behavioral impairments.

Central peptidergic neurons are hyperactive during collateral sprouting and inhibition of activity suppresses sprouting

Little is known regarding the effect of chronic changes in neuronal activity on the extent of collateral sprouting by identified CNS neurons. We have investigated the relationship between activity and sprouting in oxytocin (OT) and vasopressin (VP) neurons of the hypothalamic magnocellular neurosecretory system (MNS). Uninjured MNS neurons undergo a robust collateral-sprouting response that restores the axon population of the neural lobe (NL) after a lesion of the contralateral MNS (). Simultaneously, lesioned rats develop chronic urinary hyperosmolality indicative of heightened neurosecretory activity. We therefore tested the hypothesis that sprouting MNS neurons are hyperactive by measuring changes in cell and nuclear diameters, OT and VP mRNA pools, and axonal cytochrome oxidase activity (COX). Each of these measures was significantly elevated during the period of most rapid axonal growth between 1 and 4 weeks after the lesion, confirming that both OT and VP neurons are hyperactive while undergoing collateral sprouting. In a second study the hypothesis that chronic inhibition of neuronal activity would interfere with the sprouting response was tested. Chronic hyponatremia (CH) was induced 3 d before the hypothalamic lesion and sustained for 4 weeks to suppress neurosecretory activity. CH abolished the lesion-induced increases in OT and VP mRNA pools and virtually eliminated measurable COX activity in MNS terminals. Counts of the total number of axon profiles in the NL revealed that CH also prevented axonal sprouting from occurring. These results are consistent with the hypothesis that increased neuronal activity is required for denervation-induced collateral sprouting to occur in the MNS.

Stereological study of the ultrastructural changes induced by chronic alcohol consumption and dehydration in the supraoptic nucleus of the rat hypothalamus

We have previously shown that prolonged alcohol ingestion leads to neuronal loss in the supraoptic nucleus of the rat and that the surviving neurons, mainly the vasopressinergic ones, display marked increase in volume. In an attempt to establish correlates for the volumetric alterations we have studied the organelles of supraoptic nucleus neurons in three groups of rats--ethanol-fed, pair-fed, and dehydrated, in all cases treated from 2 to 12 months of age. The volume and surface area of the rough endoplasmic reticulum and Golgi apparatus, and the volume of nucleoli and neurosecretory granules were estimated on the basis of the respective volume and surface densities. The volumes and surface areas of all quantified organelles were increased in both alcohol-fed and dehydrated animals, although the increases were greater in the former group. Changes in the organelles studied are commonly regarded as reliable indicators of the neurosecretory activity of magnocellular neurons. Thus, our results suggest that under conditions of chronic alcohol exposure, the synthesizing activity of the surviving supra-optic neurons is augmented to compensate for the alcohol-induced neuronal loss and/or as a consequence of the alcohol-induced hyperosmolality. Changes in the transport and release of the neurosecretory material cannot, however, be ruled out as an additional cause of neuronal enlargement.

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.

Ultrastructural evidence of sexual dimorphism in supraoptic neurons: a morphometric study

We have recently shown that in spite of the absence of receptors for gonadal steroids in the supraoptic nucleus (SON) of the rat hypothalamus, the volume of the nucleus and the size of its neurons are larger in males than in females, and that these differences between male and female rats are correlated with body weight and dependent on the vasopressinergic neurons. As supraoptic neurons and their organelles enlarge when they are engaged in active peptidergic secretion we have carried out a morphometric ultrastructural analysis to determine if cell structures involved in the synthesis and storage of neurosecretory material also display weight-dependent sex dimorphism. Groups of six male and six female rats aged 30, 60 and 180 days were used. Nucleoli, rough endoplasmic reticulum and neurosecretory granules were analysed and we estimated their volume or surface densities and the total volume of nucleoli and rough endoplasmic reticulum, and total surface area of rough endoplasmic reticulum. We found that, with the exception of neurosecretory granules, the densities of the organelles did not differ among the groups studied, but total values were higher in males. These differences were found to be weight-dependent. Since the organelles studied are regarded as reliable indicators of the neurosecretory activity of supraoptic neurons, our data fully support the view that the weight-dependent sexual dimorphism observed in this nucleus reflects greater synthetic activity of its vasopressinergic neurons associated with the need to maintain water balance in larger bodies.

Morphometric and neurosecretory changes in supraoptic neurons after D-amphetamine treatment

Several morphological and immunochemical characteristics of the neurosecretory neurons of the supraoptic nucleus (SON) have been studied of rats treated for 1 month with D-amphetamine sulfate (AMP) (8 mg/kg weight, daily). An increase of SON volume (11%) has been observed as a consequence of the growth of the dorsoventral axis. Neurosecretory neurons increased their nucleolar area (11.4%), their nuclear area (8.3%), and their cytoplasmatic area (18.3%). Vasopressin immunoreaction did not show any differences between treated and control animals, but oxytocin immunostaining displayed an important increase (23.7%) in the neuronal cytoplasm of the treated rats. The SON hypertrophy of the AMP-treated rats corresponded to the hypertrophy/hyperfunction of its oxytocinergic neurons, and could be considered as a new mechanism of the action of the AMP. The results are discussed in relation to the plastic features of the SON and its central (neuronal) and peripheral (hormonal) function.

Organization of nucleoli and nuclear bodies in osmotically stimulated supraoptic neurons of the rat

This study has analyzed variations in the number of nucleoli and nuclear bodies, as well as in their ultrastructural and cytochemical organization, after the osmotically induced activation of supraoptic nucleus (SON) neurons of the rat. The number of nucleoli and nuclear bodies and also the nucleolar size were determined on smear preparations of previously block-impregnated SON. The mean number of nucleoli per cell was 1.35 +/- 0.6 (mean +/- SDM) in control rats. No significant variations in this value were registered either in dehydrated or rehydrated rats. The mean nucleolar volume and the total nucleolar volume per cell showed a significant increase in dehydrated rats with respect to the controls, whereas these two parameters tended to return to control values in rats rehydrated after dehydration. The mean number of nuclear bodies per cell increased significantly from 0.56 +/- 0.50 (mean +/- SDM) in control rats to 1.54 +/- 1.1 after 6 days of dehydration. By electron microscopy, SON neurons displayed a reticulated nucleolar configuration. After the osmotically induced neuronal activation, there was an increase in the proportion of the total nucleolar area occupied by the granular component, and also a reduction in the mean fibrillar-center area. The most characteristic nucleolar features in rehydrated rats were the tendency for the granular component to be segregated and the occurrence of intranucleolar vacuoles. Ultrastructural cytochemistry with a specific silver method revealed a selective silver reaction on the coiled threads of the nuclear bodies--identified as "coiled bodies"--and on the nucleolar fibrillar components in all animal groups studied. Since nucleoli play a major role in ribosome biogenesis, a relationship between these nucleolar changes and the level of cellular activity of SON neurons is proposed. Furthermore, the response of nuclear "coiled bodies" to neuronal activation suggests their participation in the processing and transport of rRNA precursors.

Compensatory sprouting of uninjured magnocellular neurosecretory axons in the rat neural lobe following unilateral hypothalamic lesion

Axonal sprouting of intact neurons of the magnocellular neurosecretory system was investigated using a unilateral hypothalamic knife cut of the hypothalamoneurohypophysial tract to partially denervate the rat neural lobe (NL). Densitometric, morphometric, ultrastructural, and metabolic measures were utilized to demonstrate the compensatory response to denervation in this system. Densitometric analysis revealed a transient reduction in the intensity of vasopressin staining in the NL at 10 days postsurgery (PS) with a subsequent recovery by 20 days PS. There was a comparable initial reduction in the cross-sectional area of the NL followed by a more gradual recovery to normal by 90 days PS. Ultrastructural investigation revealed a reduction in total axon number in the NL at 10 days PS similar to the declines in vasopressin immunoreactivity and size of the NL. A subsequent partial recovery of axon number occurred, paralleling the return to normal NL size between 30 and 90 days PS. Hypertrophy of both somata and cell nuclei of magnocellular neurons in the supraoptic and paraventricular hypothalamic nuclei contralateral to the lesion was also apparent during this period. Daily measurements of urine osmolality revealed an initial transient hypoosmolality followed by a chronic hyperosmolality which persisted throughout the 90 day postsurgical period. There was a concomitant chronic decrease in both daily drinking and urine excretion volumes which began immediately following surgery. These results suggest that intact, contralateral magnocellular vasopressinergic efferents undergo compensatory sprouting as a result of partial denervation of the NL in the absence of a functional deficit in vasopressin.

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

The supraoptic nuclei of both male and female 30-day-old rats rendered hypothyroid by daily subcutaneous injection of propylthiouracil were studied and the results were compared with age- and sex-matched rats fed ad libitum (control rats) and with undernourished rats. Morphometric methods were used to evaluate the volume of the supraoptic nucleus and the areal and numerical densities of its neurons. These parameters allowed us to estimate the total number of neurons of this nucleus. In addition, the mean cross-sectioned area and mean nuclear diameter of the same neurons were also evaluated. The volume of the supraoptic nucleus was reduced both in hypothyroid and undernourished animals when compared with normal controls. The areal and numerical densities of neurons from the former groups were increased and the volume density of the neuropil was reduced. As a consequence, the total number of neurons was found to be identical in all groups studied. Furthermore, the mean nuclear diameters and cross-sectioned areas of the supraoptic neurons were similar in all groups. The volumetric differences and the increased packing found were always more marked for hypothyroid than for undernourished rats. Differences were not detected between male and female groups. The present results support the view that the increased cell packing in hypothyroid animals depends upon a reduction in the neuropil of the nucleus, as has been described under similar conditions in other central nervous system areas displaying identical patterns of neurogenesis. In addition, it was found that the effects of undernourishment cannot be discriminated from those dependent on neonatal hypothyroidism.

Multiple synapse formation: a possible compensatory mechanism for increased cell size in rat supraoptic nucleus

Abstract Magnocellular neuroendocrine cells of the rat supraoptic nucleus undergo numerous morphological changes during chronic dehydration of the animal. These changes include increases in cell size, the percentage of neuronal membrane in direct apposition and formation of new multiple synapses (i.e. terminals which form more than one synapse with adjacent somata and/or dendrites). Previous studies of multiple synapse formation in the supraoptic nucleus did not include adjustments for changes in somatic size, thus the full extent of synapse formation relative to increases in cell size was not known. That multiple synapse formation is a compensatory mechanism for increased somatic size was investigated by comparing animals that were either chronically dehydrated (by drinking 2% saline) or were well hydrated. Using morphometric techniques the percentage of somatic membrane contacted by single and multiple synapses was obtained. Estimates of somatic surface area were used together with stereologically derived estimates of the number of synapses per mum(2) to calculate the number of single and multiple axo-somatic synapses per neuron. Measures of soma-somatic and soma-dendritic membrane apposition and glial coverage were made to confirm earlier estimates of these parameters. Somatic surface area increased by more than 70% in dehydrated animals. Percentage of somatic membrane contacted by single synapses was lower (by 41%) and that contacted by multiple synapses was higher (by 100%) in dehydrated as compared to control animals. The number of single synapses was not different between the two groups but dehydrated animals had more multiple synapses per soma (23) than controls (6). With dehydration, there was an eight-fold increase in the percentage of somatic membrane contacted by adjacent somatic/dendritic membrane. The surface area per somain such direct apposition was fifteen-fold higher in dehydrates compared to controls. Glial processes covered the same proportion of somatic membrane in the two groups, while the actual area covered by glial processes per soma was higher in dehydrated animals. These results are consistent with the hypothesis that multiple synapse formation at least partially compensates for probable relative decreases in synaptic efficacy as synaptic density decreases when cells increase in size due to dehydration. This altered synaptic input as well as the increased direct apposition between adjacent neurons probably contribute to their enhanced activation during periods of increased hormone demand.

Stimulus-related changes in the dendrites of magnocellular neurones

A Golgi-Cox study was undertaken to determine whether enhanced electrical activity was associated with any morphological changes in the dendrites of the magnocellular neurones in the hypothalamic supraoptic nucleus. Brattleboro rats, animals dehydrated by administration of 2% sodium chloride solution instead of drinking water and animals given 1% sodium chloride solution and deoxycortone to induce vasopressin-dependent hypertension were compared with controls. In each of the stimulated groups, the cell bodies were hypertrophied implying that the stimuli were effective. Dendritic span (the area of a triangle drawn round, and containing the entire Golgi-stained dendritic tree) was significantly increased (p less than 0.01) in Brattleboro rats but was decreased by sodium chloride-induced dehydration (p less than 0.01). Deoxycortone treatment reversed the reduction induced by dehydration. Hippocampal cells showed no significant differences. Thus, the cells of the magnocellular system rapidly alter their morphology when stimulated but the changes are more complex than a simple hypertrophy associated with enhanced activity.

In situ hybridization analysis of osmotic stimulus-induced changes in ribosomal RNA in rat supraoptic nucleus

A quantitative in situ hybridization analysis was used to investigate changes in levels of ribosomal RNA (rRNA) in neurons of the supraoptic nucleus (SON) of rats stimulated osmotically by giving 2% NaCl as drinking solution for 0 (control rats), 1, 4, and 14 days. The quantitation was autoradiographically accomplished by in situ hybridization with a nick-translated tritiated ribosomal DNA probe and with the use of computer-based image analysis system. The mean number of grains per neuron in the ventral SON was significantly increased: 1.8-fold for 1 day, 2.9-fold for 4 days, and 1.7-fold for 14 days of salt loading, whereas the mean number of grains per neuron in the dorsal SON was increased 1.3-fold for 1 day, 2.5-fold for 4 days, and 1.7-fold for 14 days. Kolmogorov-Smirnov analysis of frequency histograms of grains per neuron indicated that the amount of rRNA in neurons in the ventral and dorsal SON was significantly increased by osmotic stimulation. These increases were accompanied by increases in cell size. The subcellular location of hybridizable rRNA in magnocellular neurons was altered by osmotic stimulation. Following 1-14 days of salt-drinking, rRNAs appeared to be more unevenly distributed throughout the cytoplasm. These findings are consistent with the notion that hyperosmotic stimulation has a substantial effect on the expression of rRNA genes in neurons of both the ventral and dorsal SON.

Lesions of the tissue surrounding the preoptic recess (AV3V region) affect neurosecretory cells in the paraventricular nuclei in the rat

Lesions of the tissue surrounding the preoptic recess (AV3V region) have severe effects on body fluid homeostasis; these include acute adipsia and failure of the antidiuretic response. Because neurosecretory cells in supraoptic nuclei comprise the major source of antidiuretic hormone (ADH) in this species, we have previously observed the fine structure of supraoptic nuclei in rats with AV3V lesions. Paraventricular nuclei are the other major source of ADH in rats. Therefore, in this investigation we compared the fine structure of paraventricular nuclei in rats which had received AV3V lesions 3 days earlier with that of control rats which had received sham lesions and either had drinking water available or had water withheld for 3 days. Degenerating axons and axon terminals were present in paraventricular nuclei of lesioned rats. The degenerating terminals were in axodendritic and less often in axosomatic synapses. Morphometric evaluation revealed that neurosecretory cells did respond to the dehydrated state of the adipsic-lesioned animals, but the response was significantly attenuated compared to that which occurred in sham-lesioned rats deprived of water for 3 days. It appears that AV3V lesions damage afferent connections and impair the response of neurosecretory cells to dehydration in paraventricular as well as supraoptic nuclei. However, in paraventricular nuclei the response is not completely prevented by AV3V lesions during the adipsic period as was observed in supraoptic nuclei. The presence of a response in paraventricular nuclei may be at least partially stimulated by reduced body fluid volume. Information from volume receptors would be carried from the medulla to paraventricular nuclei by ascending pathways which are not affected by AV3V lesions.

The activity of the hypothalamo-neurohypophysis during rehydration following water deprivation in the gerbil (Meriones unguiculatus) and the laboratory rat (Rattus norvegicus)

Water deprivation in gerbils and rats for 5 and 3 days respectively resulted in the same degree of dehydration of the animals and similar depletion of the neurohypophyseal vasopressin stores. Following access to drinking water, the vasopressin stores were replenished in the gerbil and rat in 2 and 6 days respectively. It was suggested that the quicker restoration of the store in the gerbil was due to the greater ability to synthesize vasopressin and to continuing high activity of the neurosecretory cells until the stores were replete.

Reversible fine structural changes in the supraoptic nucleus of the rat following intraventricular administration of colchicine

A single low dose (3.5 micrograms or 7.0 micrograms) of colchicine injected intraventricularly into normally hydrated or dehydrated (7 or 4 days) and, subsequently, rehydrated (3 hours) rats caused a number of characteristic changes within the perikarya of the supraoptic nucleus. The temporary slow-down of axonal transport of neurosecretory granulated vesicles (NGVs) and their continued synthesis led to a perikaryal accumulation of NGVs, and changes in their electron-density are considered to be indicative of their continued maturation. In some neurons, the biosynthetic pathway appears to be interrupted or temporarily impaired as evidenced by the accumulation of material of varying electron-density and of granules within the cisternae of the rough ER. An increased number of varying types of lysosomes and phagolysosomes is indicative for the disposal of NGVs within the perikaryon. Most of them are, however, removed through resumption of axonal transport which leads to the reestablishment of pre-experimental fine structural characteristics within approximately 9 days. In the osmotically stressed groups, the fine structural changes are more pronounced but equally reversible.

Altered CNS neuroanatomical organization of spontaneously hypertensive (SHR) rats

Compared to Wistar-Kyoto (WKY) normotensive control rats, spontaneously hypertensive (SHR) rats have significantly reduced brain weights (-10.6%) and brain volumes (-11.8%). Computerized morphometric analysis of soma cross-sectional areas of single neurons in 12 selected hypothalamic regions revealed significant differences between SHR and WKY animals. Neurons from the periventricular, medial and lateral preoptic nuclei and ventromedial hypothalamus show significantly increased soma cross-sectional areas in SHR animals when compared to normotensive controls. Cells located in the two circumventricular organs, organ vasculosum lamina terminalis (OVLT) and subfornical organ (SFO), also showed significantly greater cross-sectional areas in the SHR. In contrast, neurons in the paraventricular and arcuate nuclei and dorsomedial hypothalamus were significantly smaller in spontaneously hypertensive rats when compared to normotensive controls. Only neurons in supraoptic nucleus, lateral and anterior hypothalamus have equivalent cross-sectional areas in WKY and SHR animals. Differences also exist in the number of cells in certain nuclei in SHR animals. Cell densities in periventricular preoptic nucleus, paraventricular nucleus, arcuate nucleus, ventromedial and anterior hypothalamus, organ vasculosum lamina terminalis and subfornical organ were reduced in SHR animals compared to WKY controls. Because of decreased brain weight and volume along with observed morphometric differences in individual neuronal soma size and cell densities, it is suggested that the SHR brain differs significantly from normotensive control rats. The differences may underlie some of the abnormalities in cardiovascular and endocrine regulation associated with neurogenic hypertension.

Ultrastructural effects of anteroventral third ventricle lesions on supraoptic nuclei and neural lobes of rats

Small lesions of the tissue surrounding the anterior ventral third ventricle (AV3V) cause adipsia, but there is no compensatory antidiuretic response. Therefore, the fine structure of the supraoptic nucleus and neural lobe, the major sites of synthesis and release of antidiuretic hormone (ADH), were compared in rats rendered adipsic by AV3V lesions 3 days earlier, rats deprived of water for 3 days and rats drinking normally. In sham-lesioned rats which were deprived of water, neuronal somas in the supraoptic nucleus show signs of stimulated secretory activity. However, the neuronal somas of supraoptic nuclei of rats which did not drink because they were made adipsic by AV3V lesions resemble those of normally hydrated controls. Neural lobes of water deprived animals contain a sharply reduced number of neurosecretory granulated vesicles and reduced apposition of glial processes with the perivascular connective tissue compared to those of normally hydrated rats. In contrast, neural lobes of rats with AV3V lesions contain large accumulations of densely packed neurosecretory vesicles, as well as abundant dense bodies and multilamellar bodies which may be evidence of increased crinophagy, and they have increased interposition of glial processes between axon endings and the perivascular connective tissue. In rats with AV3V lesions the severe dehydration due to adipsia was unable to stimulate release of ADH. The accumulation of neurosecretory vesicles in the neural lobe indicates that transport of ADH to the neural lobe was not impaired in this time period, but that exocytosis of ADH-containing neurosecretory vesicles in the neural lobe was blocked.

The influence of age on the response of the supraoptic nucleus of the hypothalamo-neurohypophyseal system I. Ultrastructural aspects

The influence of age on the response of the supraoptic nucleus (SON) of the hypothalamo-neurohypophyseal system to physiological stress has been studied by means of the electron microscope. An osmotic load was applied to male C57BL/Icrfat mice at 6 and 28 months of age and the resulting changes in the ultrastructure of SON cells in response to this stress analysed. In the young animals the differences in morphology observed between the SON cells from control and from those of salt-loaded mice were very similar to those seen in salt-stressed rats at a similar age. Qualitative differences were noted in several sub-cellular components of old, salt-stressed mice. The mitochondria showed evidence of ultrastructural damage in cells from the old, stressed animals. The Golgi system and the rough endoplasmic reticulum both showed heterogeneity in structure when compared with cells from young salt-stressed mice. In addition, there was a decrease in the lipofuscin content of old, salt-stressed mice. These changes are discussed with respect to the overall efficiency of cellular activity in old, physiologically stressed animals.

Ultrastructure of neurons in the paraventricular nucleus of normal, dehydrated and rehydrated rats

Previous reports have shown that the percentage of neuronal somatic membrane in soma-somatic apposition (without intervening glia) increased with brief periods of dehydration (4--24 hr) and decreased with rehydration in the rat supraoptic and circularis nuclei. In the present study, the percentage of somal membrane in soma-somatic appositions was found to increase in the primarily vasopressin-containing lateral portion of the rat paraventricular nucleus with twelve hours of dehydration. Further evidence for altered cellular function in this nucleus was a decrease in the number of smaller dense core vesicles (< 2600A) per unit cytoplasmic area during initial dehydration (4--12 hr). No changes were detected, however, in the number of larger dense core vesicles (> 4000 A) or lysosomes (> 4000 A) per unit cytoplasm. Intranuclear membrane-bound vacuoles were found primarily in hydrated and rehydrated animals. No reliable changes were seen in the dilation of granular endoplasmic reticulum. Cilia were found in the neuropil and were occasionally traced to magnocellular somata. Differences in the patterns of morphological responses among the magnocellular hypothalamic nuclei suggest specializations in their roles, and further support a functional significance of neuronal membrane appositions.

The influence of age on the hypothalamo-neurohypophyseal system of the mouse: a quantitative ultrastructural analysis of the supraoptic nucleus

A quantitative study of various morphological parameters in endocrine cells of the neuroendocrine region of the laboratory mouse was carried out. The supraoptic nucleus of the hypothalamo-neurohypophyseal system synthesises the hormones vasopressin and, to a lesser extent, oxytocin, and transports them to the posterior pituitary. Female C57BL/Icrfat mice at 8 and 26 months of age, free of macroscopic pathology, were sampled when in a physiologically defined resting state. No significant differences in the volume fractions of most cell and subcellular components could be detected at the two ages studied; however, significant increases in the volume fractions of hormone-containing granules and lipofuscin (aging pigment) were demonstrated in older animals. These observations are similar to those made on rat endocrine pancreas also in the resting state. The notion that these hormone-secreting cells are "protected" to some extent from the aging process, and may have some of the qualities of "pace-maker" cells, is discussed.

Three-dimensional organization of the endoplasmic reticulum in supraoptic neurons of the rat. A structural functional correlation

Double impregnation staining of tissue was used to study on thick sections the three-dimensional configuration of the peripheral endoplasmic reticulum in neurosecretory neurons of the supraoptic nucleus in control and water-deprived rats. According to the spatial organization of the endoplasmic reticulum, two types of neurons are described in this report: Type I neurons which predominated in control rats (70%) exhibited characteristically large lamellar structures connected to loosely anastomosed tubules. In type II neurons, which prevailed in water-deprived rats (85%) the endoplasmic reticulum had the appearance of a highly-developed network of interconnected tubules, with no lamellar structures. Double impregnation staining combined with high resolution radioautography after [3H]leucine administration showed that the tubular configuration of the endoplasmic reticulum was the main active site of protein synthesis by contrast with the lamellar components, whose activity seemed poor. In terms of protein synthesis, the three-dimensional configuration of the peripheral endoplasmic reticulum of the supraoptic neurons appeared therefore to be closely connected with their functional state.

Altered neuroanatomical organization in the central nervous system of the genetically obese (ob/ob) mouse

Genetically obese (C57BL/6J ob/ob) mice have significantly reduced brain weights (-14.6%) and cortical brain volumes (-7.9%) compared to lean control mice (C57BL/6J +/+). Morphometric analyses of soma cross-sectional areas of individual neurons in select brain region also reveals significant alterations in the ob/ob mouse. Neurons from 8 out of 9 brain regions, including the ventromedial hypothalamic nucleus, show significantly decreased soma cross-sectional areas in ob/ob mice compared to controls. Only lateral hypothalamic area neurons have equivalent soma cross-sectional areas for these two mouse strains. The decreased brain weight and volume coupled with the observed morphometric changes in individual neuronal soma size suggest that the ob/ob mouse brain differs considerably from that of controls. These differences may underlie some of the endocrine abnormalities seen in this genetic obesity syndrome.

Nucleolar proliferation and cell size changes in rat supraoptic neurons following osmotic and volemic challenges

Subcutaneous injections of isotonic saline induced nucleolar proliferation in supraoptic neurons in animals sacrificed approximately 5 min postinjection. The magnitude of this proliferation was sustained 4 and 8 hr postinjection. Polyethylene glycol (PG) injections depleted blood volume 4 and 8 hr after the injection, but the percentage of SON cells with multiple nucleoli in these animals was not different from saline-injected controls. The anterior (SOa) portion of the SON in rats given 2% NaCl to drink instead of water for three days contained more cells with multiple nucleoli than controls. This effect was enhanced after five days ingestion, and accompanied by a similar response in the tuberal portion of SON (SOt). Rehydration for ten days after three days of 2% NaCl intake brought the percentage of cells with multiple nucleoli down to control levels. Cell area in SON cells paralleled nucleolar responses during dehydration and rehydration. The results demonstrate the sensitivity of nucleolar proliferation in SON to environmental changes ranging from osmotic to neurogenic stress.

Sequence of ultrastructural changes induced by activation in the posterior neurosecretory cells in the brain of Rhodnius prolixus with special reference to the role of lysosomes

The posterior neurosecretory cell (PNC) group in the brain of Rhodnius prolixus is composed of five ultrastructurally identical cells. The PNC were examined in the unfed fifth instar and at seven stages (from 15 min to 14 days) after activation was initiated by feeding. Each stage examined revealed successive changes in morphology which can be related to the synthesis, maturation, storage and transport of neurosecretory material. It is suggested, in particular, that the lysosomal system (dense bodies and multivesicular bodies) may play a role in the maturation of the secretory granules.

Ultrastructural comparisons of neurons of supraoptic and circularis nuclei in normal and dehydrated rats

A quantitative ultrastructural investigation was undertaken to compare the nucleus circularis (NC) and supraoptic nucleus (SON) of the rat both under normal and water-deprived conditions. NC was found to have dramatically more of its cells and membrane surface involved in direct soma-somatic contact than the SON. Water deprivation, even for one day, brought about a significant increase in both percentage of cells and membrane surface in contact in both nuclei, apparently by the retraction of fine glial processes from between the somata. The normal NC was made up of only one ultrastructurally identifiable cell type. The normal NC had no cells showing expanded endoplasmic reticulum, although these were seen following 5 days (but not 1 day) of water deprivation. The normal SON did have 4.4% of its cells showing expanded endoplasmic reticulum. This percentage significantly increased following water deprivation. The vesicle population per area of cytoplasm was very similar between the two normal nuclei. One day of water deprivation brought about a significant increase in less than 800 A vesicles in NC but not the SON. Five days of water deprivation resulted in a significant decrease in the lysosomal population per unit area in both nuclei. Vesicle changes have been discussed in relation to the volume changes in the cells.