Ependyma of the goat. Part IV. Ependymal lining of the median eminence in goats under physiological and experimental conditions

The sexual differentiation of the ME ependymal lining surface structure has been proved in goats. Suitable hormonal preparations applied to females during anestrus induced on the median eminence the same picture as that during estrus. Only after gonadectomy in both the females and males there were dense macrophages-like SEC on the ME protruding from the deeper layers of the eminence. Their potential function is discussed.

Estrogen receptor-immunoreactive glia, endothelia, and ependyma in guinea pig preoptic area and median eminence: electron microscopy

The presence of estrogen receptors (ERs) in nonneural cells in brain, including glia, ependyma, and endothelia, has not previously been documented with electron microscopy. This study employed immunocytochemistry to investigate whether ER immunoreactivity (ER-ir) is present in glial, ependymal, or endothelial cells in the medial preoptic area (POA) and median eminence (ME) in the brain of gonadally intact female guinea pigs. Tissue sections through these regions were immunostained with monoclonal antibody H222 for ER localization using 3,3',5,5'-tetramethylbenzidine (TMB) as the chromogen. ER-ir cells were identified ultrastructurally by the presence of distinct spicule-like TMB crystals in nuclei. While neurons constituted the clear majority of ER-immunopositive cells, labeled astrocytes, ependyma, and endothelia were also present. Distinct intranuclear TMB crystals were present in astrocytes at the anterior pole of the POA within the preventricular periventricular nucleus, anterior compact subnucleus of the medial preoptic nucleus (MPNa), and organum vasculosum of the lamina terminalis, indicating ER-ir. In the MPNa, cell counts performed at the ultrastructural level revealed that 9.6% (15 of 156) of the astrocytes were ER-ir. To further explore the relationship of ERs with astrocytes, ER/glial fibrillary acidic protein (GFAP) double labeling experiments were performed using TMB and diaminobenzidine tetrahydrochloride for ER and GFAP localization, respectively. These studies verified the presence of ERs in astrocytes at the anterior pole of the POA and demonstrated the presence of ERs in GFAP-ir cells in the ME. Cell counts at the ME showed that 23 of 50 (46%) GFAP-ir cells were ER-ir. ER-ir was also present in scattered ependymal cells lining the third ventricle at the POA and overlying the ME. Typically, approximately four to eight ER-ir ependymal cells were present around the perimeter of the third ventricle, although occasionally small aggregations of greater numbers of labeled cells were observed. Both common ependyma and cells morphologically identified as tanycytes were ER-ir. Some endothelial cells and vascular smooth muscle cells also contained ERs. While approximately 11% of the vessels were lined by ER-ir cells in sections through the MPNa and preventricular periventricular nucleus, approximately 15% of the vessels were labeled in the organum vasculosum of the lamina terminalis. In the ME a greater percentage (59%) of the vessels contained ER-ir endothelial cells. Collectively, these results indicate that in addition to regulating the activity of neurons, estrogen may affect brain function through effects exerted on astrocytes, ependymal cells, and endothelial cells.

Effect of paraformaldehyde fixation on localization and characterization of insulin-like growth factor-I (IGF-I) receptors in the rat brain

In order to design an approach for localizing IGF-I receptors within the intact CNS, the effect of paraformaldehyde (PAF) fixation on receptor binding was examined. Cryostat sections of rat brains, which were perfused with 0 to 10% PAF, were incubated in 125I-IGF-I and assayed for binding under equilibrium conditions. Binding was quantified from 10 brain regions, involving laminae and nuclei from median eminence, thalamus, hippocampus, choroid plexus, pyriform cortex, and cerebral cortex, by computer densitometry of film autoradiographs. The specific binding, saturation curves, Bmax and Ka, ligand specificity, and binding reversibility of IGF-I binding sites were not significantly affected by 1% or 2% PAF. However, 4% PAF elevated IGF-I receptor total binding, nonspecific binding, and Ka, and decreased Bmax, presumably by increasing the number of tissue-receptor interconnections. Only nonspecific 125I-IGF-I binding persisted when 10% PAF was used. These results indicate that tissue perfused with 2% PAF can be used for localizing IGF-I receptors by autoradiographic binding methods.

Ultrastructural localization of galanin immunoreactivity in the rat median eminence

The aim of this study was to examine, by use of a pre-embedding immunoperoxidase technique, the ultrastructural localization of galanin immunoreactivity in the external layer of the rat median eminence. Galanin immunoreactivity was only observed in axonal profiles. Immunoreactive fibers were found in contact with the following non-immunoreactive structures: (1) axonal profiles that contain dense granular vesicles and clear vesicles, (2) axonal profiles that contain predominantly clear vesicles, (3) glial cell bodies, and (4) processes of tanycytes. Labeled terminals were also observed in the proximity of the perivascular space of the portal vessels. The results suggest possible interactions between galanin-immunoreactive terminals and other terminals containing peptide and/or other transmitters in the external layer of the median eminence.

Light and electron microscopic immunocytochemical evidence that delta sleep-inducing peptide and gonadotropin-releasing hormone are coexpressed in the same nerve structures in the guinea pig median eminence

The relationships between delta sleep-inducing peptide (DSIP) and GnRH immunoreactivity within the guinea pig median eminence are investigated by light and electron microscopic immunocytochemistry. Indirect immunofluorescence and elution-restaining experiments show that at the light microscopic level the distribution patterns of DSIP and GnRH immunoreactivity are indistinguishable. This suggested the possible coexistence of both immunoreactivities within the same fibers and neurosecretory endings. At the electron microscopic level, a preembedding double-immunolabeling technique using both indirect immunoperoxidase and immunogold methods, clearly indicate that DSIP and GnRH immunoreactivity are frequently colocalized within single secretory granules. In addition DSIP/GnRH immunoreactive nerve endings were also observed often in close proximity to tanycyte elements. Taken together, the present results provide for the first time ultrastructural evidence for the presence of DSIP immunoreactivity and demonstrate that DSIP and GnRH immunoreactivities may be coexpressed within the same neuronal elements in the median eminence.

Myelinated Herring bodies in the median eminence of the cat

An electron-microscopic study was carried out on the median eminence of cats during post-natal development. From the moment of birth (observations performed 12 hours later) Herring bodies were seen in the fibrillary layer of the median eminence. At 45 days after birth, myelinated nerve fibres could be observed, some of them containing neurosecretory granules. The number of myelinated fibres in the median eminence increased with age and at 90 days some Herring bodies appeared surrounded by myelin sheaths; these mainly contained neurosecretory granules and a few mitochondria.

Ultrastructural demonstration of neurohaemal contacts in the internal zone of the median eminence of the Mongolian gerbil (Meriones unguiculatus): correlation with synaptophysin immunohistochemistry

Electron microscopy of the median eminence (ME) of the Mongolian gerbil (Meriones unguiculatus) revealed that, unlike most other mammalian species, abundant neurohaemal contacts were present not only in the external zone (EZ), but also in the internal zone (IZ) up to the subependymal layer. In the IZ, nerve terminals with dense core vesicles and/or small clear vesicles abutted on the outer basal lamina of the perivascular space of portal capillaries, alternating with tanycyte processes. In addition to these neurohaemal contacts, several layers of vesicle-filled varicosities surrounded the portal vasculature. An analysis of serial thin sections showed that the latter varicosities could also reach the perivascular basal lamina or contact it through small extensions in other planes of section. Apparently at least some of the nerve terminals making neurohaemal contacts were en passant in nature. A correlative investigation of synaptophysin (a major integral membrane protein of small synaptic vesicles) immunoreactivity at the light microscopical level demonstrated a conspicuously dense immunostaining around portal capillaries in both EZ and IZ of the proximal and distal ME (neural stalk). Since this perivascular accumulation of immunoreactivity coincides precisely with the ultrastructural accumulation of vesicle-filled axons which establish numerous neurohaemal contacts, it is concluded that this pattern of synaptophysin immunostaining indicates sites of neurohaemal contacts at the light microscopical level. During postnatal development, the perivascular concentration of synaptophysin immunoreactivity in the IZ appeared concomitantly with the early postnatal expansion of long portal capillary loops into the IZ.(ABSTRACT TRUNCATED AT 250 WORDS)

Systematic presence of GABA-immunoreactivity in the tubero-infundibular and tubero-hypophyseal dopaminergic axonal systems: an ultrastructural immunogold study on several mammals

Immunoreactivities for tyrosine hydroxylase (TH), gamma-aminobutyric acid (GABA) and, in some cases, glutamic acid decarboxylase (GAD) were detected by light and electron microscopy in axons projecting into the median eminence and pituitary gland of various mammals (rats, mice, guinea pigs, cats, rabbits and hares). Light microscope immunoperoxidase reactions were performed on adjacent semithin sections of plastic-embedded samples. In the median eminence external zone, the distributions of the TH- and GAD- or GABA-immunoreactive endings were very similar in the anterior and lateral areas, while medially the GABA-labelled endings predominated. Comparable distribution patterns were found in the various species examined. In the pituitary gland, the distributions of GABA- and TH-immunoreactivities were superimposable in the intermediate lobes of all species examined, except in the rabbit and hare in which both types of innervation were lacking. For electron microscopy, the immunogold procedure was applied to sections of lowicryl-embedded samples; simultaneous detection of GABA- and TH-immunoreactivities was enabled by recto-verso double labelling with gold particles of distinct diameters. In the median eminence, GABA-immunoreactivity occurred systematically in the TH-positive endings, while distinct GABA-positive/TH-negative axons were also detected. In the intermediate lobe, the colocalization of TH- and GABA-immunoreactivities was a constant feature of the axons innervating the melanotrophic cells in all the species examined, except in the Leporidae. The functional significance of this colocalization remains to be determined.

Colocalization of delta sleep inducing peptide and luteinizing hormone releasing hormone in neurosecretory vesicles in rat median eminence

The colocalization of immunoreactivities similar to delta sleep inducing peptide (DSIP) and luteinizing hormone releasing hormone (LH-RH) was investigated by light and electron microscopic immunocytochemistry in the rat median eminence. At the light microscopic level, DSIP and LH-RH immunostained fibers, and varicosities exhibited a similar distribution pattern throughout the median eminence. Immunoreactive axons were mainly found in the lateral part of the external layer. Using an elution-restaining technique, the coexistence of LH-RH and DSIP immunoreactivities was observed in most labelled axons. To determine the intracellular localization of DSIP and LH-RH, we used double immunocytochemical labelling with species-specific antibodies and secondary antibodies conjugated to different sizes of gold particles. The two peptides were found colocalized in single axons. Immunoreactive terminals frequently showed direct membrane apposition with tanycyte processes but rare contacts with portal capillaries. No staining was observed in tanycytes and ependymal or glial elements. Moreover, we could demonstrate that LH-RH and DSIP (or a closely related molecular form) are contained not only in the same axons, but also in the same approximately 100-nm dense-core vesicles, suggesting cosecretion of these peptides.

Characterization of melatonin binding sites in the harderian gland and median eminence of the rat

The characterization of specific melatonin binding sites in the Harderian gland (HG) and median eminence (ME) of the rat was studied using [125I]melatonin. Binding of melatonin to membrane crude preparations of both tissues was dependent on time and temperature. Thus, maximal binding was obtained at 37 degrees C after 30-60 min incubation. Binding was also dependent on protein concentration (up to 1.5 mg/ml). The specific binding of [125I]melatonin was saturable, exhibiting only one class of binding sites in both tissues. The dissociation constants (Kd) were 170 and 190 pM for ME and HG, respectively. The concentration of the binding sites in ME was 8 fmol/mg protein, and in the HG 4 fmol/mg protein. In competition studies, binding of [125I]melatonin to ME or HG was inhibited by increasing concentration of native melatonin; 50% inhibition was observed at about 702 and 422 nM for ME and HG, respectively. Additionally, the [125I]melatonin binding to the crude membranes was not affected by the addition of different drugs such as norepinephrine, isoproterenol, phenylephrine, propranolol, or prazosin. The results confirm the presence of melatonin binding sites in median eminence and show, for the first time, the existence of melatonin binding sites in the Harderian gland.

[The ependymal lining of the median eminence in goats]

Ependymal epithelium of eminentia mediana in goats, examined under a light microscope, was found to be formed by beta 2 tanycytes, basal processes reaching the surface of zona externa and bifurcating in zona interna. The thickest is zona interna, consisting of dense fibres of different arrangement. A scanning electrone microscope revealed that the structure of the ependymal surface of eminentia mediana in goats depends on the phase of sexual activity of a given individual. In female and male animals the findings are almost identical (ependymoresorption is prevailing) in the period of quiescence (anoestrus, out of the season, April); an analogical situation was observed in females and males in the period of mating (hyperependymosecretion, October, November). The ependymal surface of eminentia mediana in female animals has also a typical structure in the preparatory pro-estrous stage of sexual cycle, when the results were almost identical with those in a hermaphrodite (secretion and resorption are almost equilibrated). The occurrence of supraependymal cells on the surface of eminentia mediana is scarce. These cells are very similar to nerve cells as to their external morphological structure. The presence of supraependymal fibres on the surface of the bottom of the IIIrd cerebral ventricle can be explained only partly by the finding of supraependymal cells. The investigations revealed that the structure of the surface of the ependymal epithelium of eminentia mediana in goats is markedly differentiated according to the sex; this confirms the participation of tanycytes in the control of the reproductive cycle.

Topographical relationships between catecholamine- and neuropeptide-containing fibers in the median eminence of the newt, Triturus alpestris. An ultrastructural immunocytochemical study

Dopaminergic and peptidergic nerve fibers were simultaneously demonstrated with a double-labeling technique at the ultrastructural level. The first antibody, raised against tyrosine hydroxylase, was applied during the preembedding phase and visualized with the peroxidase method. The second antibody, raised against one of the peptides met-enkephalin, somatostatin or gonadotropin-releasing hormone (GnRH), was applied to the ultrathin sections and visualized with gold-labeled goat anti-rabbit IgG. The fibers of both categories were present in the zona externa of the median eminence, frequently contacting the basal lamina of the portal vessels. In addition, topographical relationships between different types of nerve fibers were observed in the perivascular areas, although there were no morphological signs of synaptic specializations. Using serial sections, it could be established that one GnRH-fiber contacted both a dopaminergic fiber and a fiber immunoreactive for met-enkephalin. The observations support earlier physiological data concerning the regulation of the hypothalamo-hypophyseal axis, with special emphasis on the release of neurohormones in the median eminence of the newt.

Changes in the kinetics of [3H]dopamine release from median eminence and striatal synaptosomes during aging

The release of preaccumulated tritium-labeled dopamine [( 3H]DA) was examined in isolated nerve terminals (synaptosomes) prepared from the median eminence (ME) and corpus striatum (CS) of young (2-3 months), middle-aged (11-12 months), and old (19-21 months) male rats. Fractional release of [3H]DA was measured over 1- to 10-sec time intervals under basal (5 mM K+) and depolarizing (75 mM K+) conditions in the presence of calcium. No differences in the rate of basal efflux between the age groups were observed in either ME or CS preparations. Fast-phase evoked [3H]DA release (0-1 sec) from CS synaptosomes was unchanged from young to middle-aged, but was decreased in old preparations. These data demonstrate that the nigrostriatal nerve terminal has a diminished ability to respond fully to depolarizing stimuli in advanced age. Mean serum PRL levels in old rats were 2.3-fold greater than those in both young and middle-aged rats, while serum LH levels were decreased 2.0-fold in middle-aged and old compared with those in young rats. The fact that LH levels were already decreased in middle-aged rats while PRL levels had not yet increased suggests that decreased gonadotropin titers in old rats do not result from the coincident hyperprolactinemia. In ME synaptosomes, depolarization-induced [3H]DA release was decreased at all time points in middle-aged preparations compared to that in young preparations. The reduced fractional release from the middle-aged ME synaptosomes was due to a depressed rate of release during the initial second of depolarization. Evoked release from ME terminals of old rats was comparable to that measured in the young group. Thus, there occurred an age-related biphasic change in the initial rate of evoked DA release from ME synaptosomes. Diminished response of ME dopaminergic terminals to depolarizing stimuli during middle age may be important in the later development of hyperprolactinemia in aging male rats. The increased PRL available for feedback on the tuberoinfundlbular dopaminergic neurons may, in turn, be associated with the apparent recovery of evoked [3H]DA release from ME synaptosomes of old rats.

[Ultrastructural features of somatostatin-containing neurons of the hypothalamus of rats with protein insufficiency]

The electron microscopic investigation was performed to analyze somatostatin-contained nerve terminals in the median eminence of 21 days old malnourished rats' hypothalamus. In nerve terminals of malnourished animals in compared with controls ones there was found the increased density of granular vesicles (11.62 +/- 0.40 and 8.56 +/- 0.39 in 1 micron2, respectively) and decreased density of electron lucent vesicles with 120-160 nm diameter (1.66 +/- 0.18 and 3.43 +/- 0.26 in 1 micro2, respectively). The revealed increase in density of granular vesicles in axon terminals with positive immunohistochemical reaction to somatostatin in malnourished rats was explained by slow somatostatin release.

The glial architecture of the median eminence of the Mongolian gerbil (Meriones unguiculatus); a study of glial fibrillary acidic protein (GFAP) immunoreactivity in semithin sections

The glial architecture of the median eminence (ME) of the Mongolian gerbil (Meriones unguiculatus) was studied immunohistochemically. For this purpose, semithin sections of the proximal ME were processed according to the PAP technique using antibodies directed against glial fibrillary acidic protein (GFAP). Various glial cells were stained. Their distribution, the arrangement and morphology of their processes, and the spatial relations with adjacent tissue components could be examined in detail. Most of the immunoreactive cells were identified as either tanycytes (present throughout the internal zone, but preferentially located in the ependymal and subependymal layer), or as tanycyte-like cells (present throughout the external zone, but preferentially situated in the reticular layer). The processes of both cell types established numerous contacts with capillaries of the primary portal plexus in the external zone. Moreover, many projections of tanycyte processes to capillaries of the internal zone were revealed, most notably in the subependymal layer. Peculiar uni- and bipolar cells could be detected in the fibre layer of the internal zone, the processes of which were oriented parallel to the course of the axons of the hypothalamo-neurohypophyseal system. It was demonstrated that the methodology used to study the glial cells of the ME was also well applicable to the neural lobe. This technique, therefore, provides a valuable tool for the precise visualization of the majority of glial cells in the whole neurohypophysis of the gerbil. Thus, by sequential immunostaining of serial semithin sections investigations concerning the presence of multiple substances within single neurohypophyseal glial cells become possible.

Stress selectively activates the vasopressin-containing subset of corticotropin-releasing hormone neurons

A functional ultrastructural assay was used to determine the response of corticotropin releasing hormone (CRH) neurosecretory cells to short-term stress. Depletion of neurosecretory vesicles from axonal swellings in the external zone of the rat median eminence was used as a measure of functional activity. One hour of immobilization or 5 h of insulin-induced hypoglycemia caused marked depletion of vesicles from the vasopressin (VP)-containing CRH axons, but had no effect on the VP-deficient subpopulation of CRH axons. Injection of colchicine (100 micrograms) into the lateral ventricle also resulted in selective depletion of vesicles from the VP-containing subpopulation over the course of 5 h, with no depletion from the VP-deficient axons. By 24 h after injection of 100 micrograms colchicine, however, both the VP-containing and the VP-deficient axons were severely depleted of neurosecretory vesicles. These data demonstrate for the first time that the CRH neurosecretory system contains functionally distinct components, and that the VP secreting component may specifically mediate the ACTH response to short-term stress.

Nerve endings from rat brain tissue release copper upon depolarization. A possible role in regulating neuronal excitability

Membrane vesicles from rat cerebral cortex were prepared and the functional response of the GABAA receptor was followed by monitoring GABA-activated influx of the radiotracer 36Cl- ion. CuCl2 decreased GABA-activated 36Cl- influx into synaptosomal membrane vesicles. The effect of Cu2+ was concentration dependent (5-500 microM CuCl2) and occurred with saturating (1 mM) as well as low (30 microM) GABA concentrations. A similar inhibition of the responses to muscimol (30 microM) was also observed with 50 microM CuCl2. In addition, release of copper from cortical synaptosomes and median eminence was followed by atomic absorption technique. An increased release of copper into the extracellular space was observed upon depolarization with 50 mM K+. A minimal concentration of copper was estimated to be 100 microM in the synaptic cleft. These findings suggest that copper may play a role in regulating neuronal excitability.

Topographic relations between tyrosine hydroxylase- and luteinizing hormone-releasing hormone-immunoreactive fibers in the median eminence of adult rats

Employing electron microscopic double immunolabeling, we determined a close apposition of tyrosine hydroxylase (TH) and luteinizing hormone-releasing hormone (LHRH) nerve fibers in the rat median eminence (ME). These axo-axonic contacts occurred frequently in the internal and palisade zones, i.e. at the level of the fiber preterminals. In the superficial area of the ME, major TH fibers abutted on the basal lamina and some were projected into the pericapillary space of the portal vessels. Conversely, LHRH fibers were arrested by the endfeet of tanycytes in reaching the basal lamina.

Functional and morphological changes in mediobasal hypothalamus of streptozocin-induced diabetic rats. In vitro study of LHRH release

To investigate the role of the mediobasal hypothalamus (MBH) in diabetic gonadal axis disorders, the MBHs of adult male streptozocin-induced diabetic (STZ-D) rats were examined after incubation in basal conditions or in K+-enriched medium and compared with those of controls. Diabetes lasted 1 mo. Both luteinizing-hormone-releasing hormone (LHRH) release and MBH morphology were studied. After incubation in basal conditions, the LHRH release was unchanged. By light microscopy, the dilated-axon cross sections were more numerous (P less than .01) in the basal arcuate nucleus and in the median eminence. By electron microscopy, the ratio of exocytoses to neurosecretory granules observed in the median eminence axon cross sections was smaller (P less than .05). The total LHRH immunoreactivity, the number of labeled axons, and the amount of positive material in the axons were reduced (P less than .05). After incubation in K+-enriched medium, the LHRH release was markedly reduced (P less than .01). The number and area of dilated-axon cross sections, possibly because of the relation between exocytosis and physiological dilation, were less augmented (P less than .01). Whereas the number of exocytoses and the ratio of exocytoses to neurosecretory granules were not decreased, the total LHRH immunoreactivity and the number of labeled axons were reduced (P less than .05). The releasable LHRH pool therefore seems to be exhausted in control MBH because of long-term stimulation and reduced in the MBH of STZ-D rats because of diabetes. In conclusion, STZ-D causes functional and anatomical MBH lesions that should be pathogenetically relevant for the disorders of the gonadal axis documented in this animal model.

Immunocytochemical and physiological evidence of a synapse between dopamine- and luteinizing hormone releasing hormone-containing neurons in the ewe median eminence

Immunocytochemical labeling revealed that the arcuate nucleus (ARN) of the ewe's hypothalamus contains numerous tyrosine hydroxylase (TH)-positive neurons, that appear to lack dopamine-beta-hydroxylase (DBH)-like immunoreactivity. Axons of these presumed dopaminergic neurons converge in the median eminence (ME) with Luteinizing Hormone Releasing Hormone (LHRH)-containing axons originating mostly from neurons situated in the medial preoptic area. Electron microscopic double labeling revealed synaptic contacts between TH-positive presynaptic profiles and LHRH-containing postsynaptic elements. Samples of ME, ARN, paraarcuate and lateral hypothalamus were dissected and incubated to assess LHRH release and tissue content. Only ME-LHRH release was significantly reduced in the presence of dopamine (DA). All other regions released equal amounts with and without DA. Thus, a presynaptic dopaminergic inhibition of LHRH-containing axons at the level of the ME might contribute to the regulation of LHRH release into the portal vessels.

Correlative scanning-immunoelectromicroscopic analysis of neuropeptide localization and neuronal plasticity in the endocrine hypothalamus

Thirty-two Sprague-Dawley rats were divided into four groups, eight rats per group. Animals were hypophysectomized with removal of both the pars distalis and the neural lobe of the neurohypophysis. Groups of eight rats were euthanized 1, 2, 4 and 8 weeks following hypophysectomy and prepared for routine scanning electron microscopy (SEM) and correlative immunoelectron microscopy employing antisera against arginine vasopressin (AVP). Eight normal rats served as controls. In experimental rats that survived one to eight weeks posthypophysectomy, remarkable neuroanatomical alterations were notable in the median eminence and adjacent third cerebral ventricular lumen. In contrast to normal control rats, large numbers of neurites were observed with SEM to insinuate from the lateral recess into the cerebral ventricular lumen and as early as one week following hypophysectomy they overgrew the apical surfaces of ependymal cells that constitute the lining of the cerebral ventricle. Immunoelectron microscopy revealed that a significant proportion of these neurites were magnocellular in origin in that they harbored AVP-positive neurosecretory vesicles. In addition to large numbers of invading magnocellular neurites, neuronal perikayria with apparent axosomatic synapses were observed to emerge upon the thick feltwork of invading axons, the latter of which appeared to freely terminate within the ventricular lumen. AVP-positive axon profiles were, in addition, seen to terminate upon the basal lamina of portal perivascular spaces in the zona externa of the median eminence. These data are consistent with the idea that following hypophysectomy (to include high stalk section of the neurohypophyseal system), that there is rapid, and dynamic sprouting and regrowth of AVP-positive axons into the adjacent third cerebral ventricular lumen and to the contact zone of the median eminence as well. This phenomenon may represent a compensatory physiological response to injury of the neurohypophyseal system characterized by a highly plastic neuroanatomical reorganization of magnocellular elements which appear to utilize the CSF of the third cerebral ventricle as a functional terminus for the neurocisternal secretion of AVP which ultimately enters the systemic circulation.

[Immunohistochemical research on the insulin-sensitive elements of the hypothalamic median eminence]

The indirect peroxidase-labeled antibody method was used to localize insulin receptors in the median eminence of rat hypothalamus. For experiments the rabbit antisera against synthetic peptides correspond to the deduced amino acid sequences of alpha- and beta-subunits of human placenta insulin receptor used as the first sera. The antisera were produced by prof. N. Yanaihara. Results have shown that after intraventricular injection of 1 U insulin the electron dense particles of DAB reaction were distributed on the surface membrane of tanycytes and axon terminals in the median eminence of hypothalamus. The functional properties of revealed receptors corresponded to IGF-1 receptors. Results support the hypothesis of central regulatory action of insulin in brain by its interaction with central IGF-1 receptors on the membrane of neuroependymal elements in the median eminence of hypothalamus.

Ultrastructure and synaptic organization of luteinizing hormone-releasing hormone (LHRH) neurons in the anestrous ewe

Electron microscopic immunocytochemistry was employed to examine the ultrastructure of luteinizing hormone-releasing hormone (LHRH) neurons and their projections to the median eminence in the sheep brain. LHRH perikarya in the preoptic area of anestrous ewes are less innervated than nonimmunoreactive cells in the same sections, but still receive numerous synaptic inputs, primarily onto distal dendrites and small somatic protuberances. Axon terminals synapsing upon LHRH cells contain a combination of clear spherical vesicles and larger dense-core vesicles. Interestingly, LHRH cell bodies and dendrites are almost entirely surrounded by glial processes. These processes intervene between immunoreactive elements that at a light microscopic level appear to be in contact with each other. Thus no evidence was obtained at the ultrastructural level for contacts among adjacent LHRH cells or dendrites in the preoptic area. Synaptic inputs onto LHRH cell bodies and dendrites appear to penetrate this glial sheath. In contrast to the absence of contacts among LHRH cells in the preoptic area, individual LHRH terminals in the median eminence are often clustered in direct plasma membrane contact. Comparisons between animals of differing reproductive status are needed to determine whether alterations in synaptic inputs, glial ensheathment, or LHRH-LHRH appositions, may underlie seasonal changes in the activity of LHRH neurons.

Selective effects of hyperprolactinemia on in vitro dopamine release from median eminence synaptosomes

Prolactin is thought to exert an autoregulatory, negative feedback effect on its own secretion via stimulation of the tuberoinfundibular dopaminergic (TIDA) neurons. To investigate possible mechanisms involved in this feedback, the effects of experimentally induced hyperprolactinemia on the release of 3H-dopamine (3H-DA) were studied in nerve terminals (synaptosomes) isolated from rat median eminence (ME), the TIDA neuronal projection field. Synaptosomes were prepared from adult male rats treated with ovine prolactin (oPRL) or the vehicle for 48 hr. Synaptosomes were incubated in 0.1 microM 3H-DA at 30 degrees C until steady-state conditions were achieved, and then release of the preaccumulated transmitter was measured over 1-20 sec time intervals under basal and depolarizing conditions. Release of 3H-DA elicited by depolarization of the terminals was significantly greater in ME synaptosomes prepared from oPRL-treated animals as compared with preparations from controls. This effect of the hyperprolactinemia appeared to be specific to the TIDA neurons since oPRL treatment did not result in increased evoked release of 3H-DA from terminals prepared from the mesolimbic, tuberohypophyseal, or nigrostriatal dopaminergic neurons. Basal efflux in all preparations was not changed from controls. The increased evoked release in oPRL-treated ME occurred when depolarization was induced either with high external [KCl] or veratridine. The enhanced 3H-DA efflux was evident during depolarization over a wide range of external calcium concentrations (0.01-3.0 mM), in the presence of 20 nM Ni2+ to block Ca2+ influx through voltage-gated channels, or when all external Ca2+ had been chelated, indicating that this effect of oPRL involves DA released through a mechanism independent of external calcium.

Annular gap junctions in cuboidal ependymocytes of the median eminence of the rat after intraventricular administration of methysergide and thyrotropin releasing hormone

The annular gap junctions in the cuboidal ependymocytes of the median eminence of rats were studied after intraventricular administration (3rd ventricle) of methysergide and thyrotropin releasing hormone. Isolated annular gap junctions were found in the ependymocytes of normal animals. In experimental rats an increase was observed in the number of annular gap junctions at 5 min after administration of the drugs; the increase was more apparent at 10 min postadministration. Likewise, the ependymocytes showed a clear development of the Golgi complex and an increase in the number of vesicles close to the lateral membranes. Accordingly, the increase observed in annular gap junctions could be interpreted as a sign of cellular activity indicative of a greater exchange of substances through the ependymocytes.