Functional electron microscopy of the hypothalamic median eminence

Characteristics of blood-brain barrier heterogeneity between brain regions revealed by profiling vascular and perivascular cells

The blood-brain barrier (BBB) protects the brain and maintains neuronal homeostasis. BBB properties can vary between brain regions to support regional functions, yet how BBB heterogeneity occurs is poorly understood. Here, we used single-cell and spatial transcriptomics to compare the mouse median eminence, one of the circumventricular organs that has naturally leaky blood vessels, with the cortex. We identified hundreds of molecular differences in endothelial cells (ECs) and perivascular cells, including astrocytes, pericytes and fibroblasts. Using electron microscopy and an aqueous-based tissue-clearing method, we revealed distinct anatomical specializations and interaction patterns of ECs and perivascular cells in these regions. Finally, we identified candidate regionally enriched EC-perivascular cell ligand-receptor pairs. Our results indicate that both molecular specializations in ECs and unique EC-perivascular cell interactions contribute to BBB functional heterogeneity. This platform can be used to investigate BBB heterogeneity in other regions and may facilitate the development of central nervous system region-specific therapeutics.

Insulin synthesized in the paraventricular nucleus of the hypothalamus regulates pituitary growth hormone production

Evidence has mounted that insulin can be synthesized in various brain regions, including the hypothalamus. However, the distribution and functions of insulin-expressing cells in the hypothalamus remain elusive. Herein, we show that in the mouse hypothalamus, the perikarya of insulin-positive neurons are located in the paraventricular nucleus (PVN) and their axons project to the median eminence; these findings define parvocellular neurosecretory PVN insulin neurons. Contrary to corticotropin-releasing hormone expression, insulin expression in the PVN was inhibited by restraint stress (RS) in both adult and young mice. Acute RS–induced inhibition of PVN insulin expression in adult mice decreased both pituitary growth hormone (Gh) mRNA level and serum GH concentration, which were attenuated by overexpression of PVN insulin. Notably, PVN insulin knockdown or chronic RS in young mice hindered normal growth via the downregulation of GH gene expression and secretion, whereas PVN insulin overexpression in young mice prevented chronic RS–induced growth retardation by elevating GH production. Our results suggest that in both normal and stressful conditions, insulin synthesized in the parvocellular PVN neurons plays an important role in the regulation of pituitary GH production and body length, unveiling a physiological function of brain-derived insulin.

Insulin produced in the paraventricular nucleus regulates body length by modulating pituitary growth hormone expression and secretion under both normal and stress conditions.

Tanycytes and the Control of Thyrotropin-Releasing Hormone Flux Into Portal Capillaries

Central and peripheral mechanisms that modulate energy intake, partition and expenditure determine energy homeostasis. Thyroid hormones (TH) regulate energy expenditure through the control of basal metabolic rate and thermogenesis; they also modulate food intake. TH concentrations are regulated by the hypothalamus-pituitary-thyroid (HPT) axis, and by transport and metabolism in blood and target tissues. In mammals, hypophysiotropic thyrotropin-releasing hormone (TRH) neurons of the paraventricular nucleus of the hypothalamus integrate energy-related information. They project to the external zone of the median eminence (ME), a brain circumventricular organ rich in neuron terminal varicosities and buttons, tanycytes, other glial cells and capillaries. These capillary vessels form a portal system that links the base of the hypothalamus with the anterior pituitary. Tanycytes of the medio-basal hypothalamus express a repertoire of proteins involved in transport, sensing, and metabolism of TH; among them is type 2 deiodinase, a source of 3,3',5-triiodo-L-thyronine necessary for negative feedback on TRH neurons. Tanycytes subtypes are distinguished by position and phenotype. The end-feet of β2-tanycytes intermingle with TRH varicosities and terminals in the external layer of the ME and terminate close to the ME capillaries. Besides type 2 deiodinase, β2-tanycytes express the TRH-degrading ectoenzyme (TRH-DE); this enzyme likely controls the amount of TRH entering portal vessels. TRH-DE is rapidly upregulated by TH, contributing to TH negative feedback on HPT axis. Alterations in energy balance also regulate the expression and activity of TRH-DE in the ME, making β2-tanycytes a hub for energy-related regulation of HPT axis activity. β2-tanycytes also express TRH-R1, which mediates positive effects of TRH on TRH-DE activity and the size of β2-tanycyte end-feet contacts with the basal lamina adjacent to ME capillaries. These end-feet associations with ME capillaries, and TRH-DE activity, appear to coordinately control HPT axis activity. Thus, down-stream of neuronal control of TRH release by action potentials arrival in the external layer of the median eminence, imbricated intercellular processes may coordinate the flux of TRH into the portal capillaries. In conclusion, β2-tanycytes appear as a critical cellular element for the somatic and post-secretory control of TRH flux into portal vessels, and HPT axis regulation in mammals.

Tanycytes: A rich morphological history to underpin future molecular and physiological investigations

Tanycytes are located at the base of the brain and retain characteristics from their developmental origins, such as radial glial cells, throughout their life span. With transport mechanisms and modulation of tight junction proteins, tanycytes form a bridge connecting the cerebrospinal fluid with the external limiting basement membrane. They also retain the powers of self-renewal and can differentiate to generate neurones and glia. Similar to radial glia, they are a heterogeneous family with distinct phenotypes. Although the four subtypes so far distinguished display distinct characteristics, further research is likely to reveal new subtypes. In this review, we have re-visited the work of the pioneers in the field, revealing forgotten work that is waiting to inspire new research with today's cutting-edge technologies. We have conducted a systematic ultrastructural study of α-tanycytes that resulted in a wealth of new information, generating numerous questions for future study. We also consider median eminence pituicytes, a closely-related cell type to tanycytes, and attempt to relate pituicyte fine morphology to molecular and functional mechanism. Our rationale was that future research should be guided by a better understanding of the early pioneering work in the field, which may currently be overlooked when interpreting newer data or designing new investigations.

Ultrastructure of Hypothalamic Neurons and of the Median Eminence

Our light, and electron microscopic (EM) findings within the hypothalamic supraoptic (SO) and paraventricular (PV) nuclei of the normal female rabbit are in agreement with those reported earlier by other investigators for the same nuclei of the dog and rat. The neurons of these nuclei are the hypothalamic synthesis sites of the neurohypophyseal hormones.

With the exception of the arcuate nucleus, none of the hypothalamic nuclei associated with the control of adenohypohpyseal function have been studied extensively with the electron microscope. On the basis of our EM findings within the female rabbit hypothalamus, all neurons observed within the preoptic (PO) and suprachiasmatic (SCH) nuclei of the non-mated control animal were morphologically identical to the conventional neuron as described by Peters, Palay and Webster (1970). However, following coitus, castration and laparotomy, many neurons of these nuclei showed subcellular changes that have been repeatedly associated with enhanced protein synthesis. These large ‘neurosecretory’ neurons were usually located near capillaries and characterized by their well developed Rough endoplasmic reticulum (RER) and Golgi profiles, dense populations of mitochondria and lysosomes and by the presence of a homogeneous population of densecore vesicles (DCV) showing a peak distribution of 120-140 nm. Since similar neurons were not observed within the PO and SCH of the normal control rabbit it is suggested that we were observing functional states of the same type of neuron and that these ultrastructural changes occur in response to endocrine manipulation.

Two types of neurons described as ‘pale’ and ‘dark’ were observed within the arcuate nucleus of both the control and experimental female rabbit. Ultrastructurally, these neuron types were identical to those described by other investigators for the rat. It has been suggested that the ‘pale’ and ‘dark’ neurons of this hypothalamic nucleus represent functional states of the same type of cell. However, increases in the ratio of ‘dark’ to ‘pale’ neurons as observed within the arcuate nucleus of the rat following castration, were not seen in the rabbit. Similar findings were also not evident within the arcuate nucleus of the female rabbit following coitus.

As far as could be determined, all neurons of the ventromedial (VMN) nuclei of both the control and experimental rabbit were morphologically identical to the smaller, conventional type neuron. Certainly, ultrastructural changes similar to those observed within the PO and SCH nuclei of the female rabbit following coitus, castration or laparotomy, were never observed.

The basic zonation and subcellular organization of the female rabbit Median Eminence (ME) is similar to that described for other mammalian species. Our EM findings within the external layer of the rabbit ME, however, are not entirely in agreement with the earlier study of Duffy and Menefeef 1965). These investigators reported only one population of DCV within the axon terminals of the rabbit ME external layer. We feel that we have ultrastructural evidence for the presence of at least two distinct populations of DCV within this layer of the rabbit ME. Furthermore, since these vesicle populations occurred within separate axon profiles and terminals, differences in their content and origin are suggested.

Certainly, the relationship between releasing factors (RF) and the various populations of DCV observed within the external layer of the mammalian ME is not well established. The smaller (90 nm - 100 nm) DCV we have observed probably contain the catecholamines, while those of larger (120 nm - 140 nm) diameters may well represent the carriers of the RF associated with gonadotropic activity. The latter view is based primarily on our finding or numerous ‘vesicle ghosts’ within the axon terminals abutting the perivascular space (PVS) of portal capillaries of rabbits sacrificed at 10 minutes post-coitus. The mean diameters of 137±14 nm obtained for these ghosts strongly supports the suggested depletion of only the larger of the two DCV populations. Similar changes were not apparent within the axon terminals containing homogenous populations of only the smaller DCV.

Unquestionably, the precise hypothalamic synthesis sites for the RF associated with control of adenohypophyseal function, continues to provoke comment. From the results obtained from countless studies that have employed a variety of neuroendocrinilogical techniques, two main hypothalamic centers of RF synthesis have been suggested: a) the medial basal hypothalamus (MBH) or hypophysiotropic area (HTA) and b) the anterior hypothalamus. The ultrastructural studies carried out to date within this laboratoiy are in favour of the latter for the following reasons:

1) — the presence of large DCV and ‘vesicle ghosts’ within the external layer of the rabbit ME with diameters similar to those of the large (120-150 nm) DCV synthesized within the PO and SCH nuclei of the same animal in response to coitus, castration and laparotomy.

2) — the absence of evidence for the storage of these large DCV within the somata of PO and SCH nuclei, suggesting their immediate transport toward the ME.

3) — the absence of any ultrastructural changes within neuron somata of the rabbit arcuate nuclei which might reflect enhanced neurosecretory activity in response to coitus and/or castration.

These ultrastructural findings within the rabbit hypothalamus may, therefore, provide the first evidence of a morphological nature for the actual release of RF from their ME storage sites, as well as their synthesis within certain neurons of the anterior hypothalamus.

Scanning and Transmission Electron Microscopy of the Ependymal Lining of the Third Ventricle

In its simplest form, the ependyma of the third ventricle consists of a single layer of cuboidal cells. Although these typical mural cells constitute the greater part of the lining of the ventricle, a specialized variety of ependymal cell (the tanycyte) can also be distinguished within circumscribed areas of the ventricular wall. Although such cells are found scattered throughout the dorsoventral extent of the third ventricle, they are particularly numerous along the ventrolateral walls and floor. The regional variation in the surface morphology of the ventricle walls as evident with the scanning electron microscope is consistent with this pattern of tanycyte distribution. Ultrastructural studies have established that the tanycyte is a fundamentally distinct cell with a long basal process extending into the subjacent neuropil and frequently directed toward a capillary wall. This unique morphology conforms closely to its three-dimensional appearance as demonstrated with the scanning electron microscope. The significance of ependymal tanycytes particularly of the third ventricle derives largely from the connections they establish between the ventricular lumen and vasculature of the median eminence. This intriguing structural relationship has led to the suggestion that ependymal cells and cerebrospinal fluid of the third ventricle may be involved in the regulation of adenohypophysial activity. Evidence indicating the functional involvement of specialized ependymal cells in the neuroendocrine control of pituitary activity is reviewed.

Diversity of the hypothalamo-neurohypophysial system and its hormonal genes

The hypothalamic neurosecretory cells (NSCs) which produce and release neurohypophysial hormones are involved in controls of diverse physiological phenomena including homeostatic controls of unconscious functions and reproduction. The far and wide distribution of neurosecretory processes in the discrete brain loci and the neurohypophysis is appropriate for coordination of neural and endocrine events that are required for the functions of NSCs. The presence of dye couplings and intimate contacts among NSCs supports harmonious production and release of hormone to maintain the plasma level within a certain range which is adequate for a particular physiological condition. Neurosecretory cells integrate diverse input signals from internal and external sources that define this particular physiological condition, although reactions of NSCs vary among different species, and among different cell types. An input signal to NSC is received by specific receptors and transduced as unique intracellular signals, important for the various functions of neurohypophysial hormones. Orchestration of multiple intracellular signaling systems, activities of which are individually modulated by input signals, determines the rates of synthesis and release of hormone through regulation of gene expression. The first step of gene expression, i.e., transcription, is amenable for diverse reaction of NSCs, because the 5' upstream regions of genes encoding neurohypophysial hormones are highly variable.

The design of barriers in the hypothalamus allows the median eminence and the arcuate nucleus to enjoy private milieus: the former opens to the portal blood and the latter to the cerebrospinal fluid

The blood-brain barrier (BBB) is a single uninterrupted barrier that in the brain capillaries is located at the endothelial cells and in the circumventricular organs, such as the choroid plexuses (CP) and median eminence (ME), is displaced to specialized ependymal cells. How do hypothalamic hormones reach the portal circulation without making the BBB leaky? The ME milieu is open to the portal vessels, while it is closed to the cerebrospinal fluid (CSF) and to the arcuate nucleus. The cell body and most of the axons of neurons projecting to the ME are localized in areas protected by the BBB, while the axon terminals are localized in the BBB-free area of the ME. This design implies a complex organization of the intercellular space of the median basal hypothalamus. The privacy of the ME milieu implies that those neurons projecting to this area would not be under the influence of compounds leaking from the portal capillaries, unless receptors for such compounds are located at the axon terminal. Amazingly, the arcuate nucleus also has its private milieu that is closed to all adjacent neural structures and open to the infundibular recess. The absence of multiciliated cells in this recess should result in a slow CSF flow at this level. This whole arrangement should facilitate the arrival of CSF signal to the arcuate nucleus. This review will show how peripheral hormones can reach hypothalamic targets without making the BBB leaky.

Expression of vesicular glutamate transporter-2 in gonadotrope and thyrotrope cells of the rat pituitary. Regulation by estrogen and thyroid hormone status

Immunocytochemical studies of the rat adenohypophysis identified a cell population that exhibits immunoreactivity for type-2 vesicular glutamate transporter (VGLUT2), a marker for glutamatergic neuronal phenotype. The in situ hybridization detection of VGLUT2 mRNA expression in adenohypophysial cells verified that VGLUT2 immunoreactivity is due to local synthesis of authentic VGLUT2. Dual-immunofluorescent studies of the hypophyses from male rats showed the presence of VGLUT2 in high percentages of LH (93.3 +/- 1.3%)-, FSH (44.7 +/- 3.9%)-, and TSH (70.0 +/- 5.6%)-immunoreactive cells and its much lower incidence in cells of the prolactin, GH, and ACTH phenotypes. Quantitative in situ hybridization studies have established that the administration of a single dose of 17-beta-estradiol (20 microg/kg; sc) to ovariectomized rats significantly elevated VGLUT2 mRNA in the adenohypophysis 16 h postinjection. Thyroid hormone dependence of VGLUT2 expression was addressed by the comparison of hybridization signals in animal models of hypo- and hyperthyroidism to those in euthyroid controls. Although hyperthyroidism had no effect on VGLUT2 mRNA, hypothyroidism increased adenohypophysial VGLUT2 mRNA levels. This coincided with a decreased ratio of VGLUT2-immunoreactive TSH cells, regarded as a sign of enhanced secretion. The presence of the glutamate marker VGLUT2 in gonadotrope and thyrotrope cells, and its up-regulation by estrogen or hypothyroidism, address the possibility that endocrine cells of the adenohypophysis may cosecrete glutamate with peptide hormones in an estrogen- and thyroid status-regulated manner. The exact roles of endogenous glutamate observed primarily in gonadotropes and thyrotropes, including its putative involvement in autocrine/paracrine regulatory mechanisms, will require clarification.

Ferutinin stimulates nitric oxide synthase activity in median eminence of the rat

Several species of Ferula genus have been used in folk medicine in digestive disorders, rheumatism, headache, arthritis, and as tranquilizers, antispasmodic and aphrodisiac. From the dry and powdered roots of Ferula hermonis Boiss was extracted the oxygenated sesquiterpene 1,5-trans-daucane type: ferutinine (1). The structure of (1) was established by spectroscopic methods as: IR, (1)H RMN, (13)C RMN, COSY, HMBC, HMQC, NOESY, EIMS, and CIMS. The possible signaling pathway of ferutinin (1) in nervous tissue in vitro was assessed and the results showed that this compound is able to increase nitric oxide synthase activity and inositol monophosphate accumulation (49%, each) in the median eminence of the rat brain, suggesting that compound (1) is associated to the activation of phosphoinositide breakdown and nitric oxide production (NO), the last is a gaseous intercellular messenger known to play a broad role in human biology from homeostasis to pathology.

Developmental changes in the expression of ATP7A during a critical period in postnatal neurodevelopment

ATP7A is a P-type ATPase that transports copper from cytosol into the secretory pathway for loading onto cuproproteins or efflux. Mutations in Atp7a cause Menkes disease, a copper-deficiency disorder fatal in the postnatal period due to severe neurodegeneration. Early postnatal copper injections are known to diminish degenerative changes in some human patients and mice bearing mutations in Atp7a. In situ hybridization studies previously demonstrated that ATP7A transcripts are expressed widely in the brain. ATP7A-specific antibody was used to study the neurodevelopmental expression and localization of ATP7A protein in the mouse brain. Based on immunoblot analyses, ATP7A expression is most abundant in the early postnatal period, reaching peak levels at P4 in neocortex and cerebellum. In the developing and adult brain, ATP7A levels are greatest in the choroid plexus/ependymal cells of the lateral and third ventricles. ATP7A expression decreases in most neuronal subpopulations from birth to adulthood. In contrast, ATP7A expression increases in CA2 hippocampal pyramidal and cerebellar Purkinje neurons. ATP7A is expressed in a subset of astrocytes, microglia, oligodendrocytes, tanycytes and endothelial cells. ATP7A is largely localized to the trans-Golgi network, adopting the cell-specific and developmentally-regulated morphology of this organelle. The presence of ATP7A in the axons of postnatal, but not adult, optic nerve suggests stage-specific roles for this enzyme. In sum, the precisely-regulated neurodevelopmental expression of ATP7A correlates well with the limited therapeutic window for effective treatment of Menkes disease.

Cellular composition and cytoarchitecture of the adult human subventricular zone: a niche of neural stem cells

The lateral wall of the lateral ventricle in the human brain contains neural stem cells throughout adult life. We conducted a cytoarchitectural and ultrastructural study in complete postmortem brains (n = 7) and in postmortem (n = 42) and intraoperative tissue (n = 43) samples of the lateral walls of the human lateral ventricles. With varying thickness and cell densities, four layers were observed throughout the lateral ventricular wall: a monolayer of ependymal cells (Layer I), a hypocellular gap (Layer II), a ribbon of cells (Layer III) composed of astrocytes, and a transitional zone (Layer IV) into the brain parenchyma. Unlike rodents and nonhuman primates, adult human glial fibrillary acidic protein (GFAP)+ subventricular zone (SVZ) astrocytes are separated from the ependyma by the hypocellular gap. Some astrocytes as well as a few GFAP-cells in Layer II in the SVZ of the anterior horn and the body of the lateral ventricle appear to proliferate based on proliferating cell nuclear antigen (PCNA) and Ki67 staining. However, compared to rodents, the adult human SVZ appears to be devoid of chain migration or large numbers of newly formed young neurons. It was only in the anterior SVZ that we found examples of elongated Tuj1+ cells with migratory morphology. We provide ultrastructural criteria to identify the different cells types in the human SVZ including three distinct types of astrocytes and a group of displaced ependymal cells between Layers II and III. Ultrastructural analysis of this layer revealed a remarkable network of astrocytic and ependymal processes. This work provides a basic description of the organization of the adult human SVZ.

Historical contributions of research on birds to behavioral neuroendocrinology

The contributions from bird research to behavioral neuroendocrinology are enormous and wide ranging. I have selected examples that illustrate how through the twentieth century to the present day, birds as experimental models continue to play a major role. Investigations on birds were pivotal in the beginnings of endocrinology, and have laid the foundations for present day developments in neurobiology, physiological ecology and evolutionary biology of neuroendocrine systems. The extensive literature on avian behavior also provides a unique and broad base for the field in general and future progress on a broad front can be expected. Molecular genetics in avian systems is advanced providing us with phylogenetic perspective allowing the comparisons of different avian populations that provide valuable models for integrative research.

Expression of brain-derived neurotrophic factor and its receptors in the median eminence cells with sensitivity to stress

The median eminence (ME) is considered as the final common pathway connecting the nervous and endocrine systems. In this neurohemal structure, dynamic interactions among nerve terminals, tanycytes, and astrocytes determine through plastic processes the neurohormones access to the portal blood. Because brain-derived neurotrophic factor (BDNF) is involved in plastic changes, we investigated its presence and that of its receptors (TrkB) in the different cellular types described in the ME. Using in situ hybridization and immunohistochemical techniques, we demonstrated that BDNF immunoreactivity was essentially located in the astrocytes and to a lesser extent in tanycytes. By contrast, BDNF was not detected in nerve terminals reaching the external layer of the ME. TrkB antibodies recognizing the extracellular receptor domain labeled all of these different cell types, suggesting an autocrine or paracrine action of BDNF at this level. More selective antibodies showed that TrkB.FL immunostaining was found in tanycytes and nerve endings, whereas TrkB.T1 immunostaining was localized in all cellular types. Immobilization stress increased BDNF mRNA and BDNF immunoreactivity patterns and induced biphasic BDNF release from the ME, as analyzed by push-pull perfusion. In addition, we observed that 60-min stress intensified BDNF immunoreactivity in the internal layer and also its colocalization with glial fibrillary acidic protein. Stress also accentuated BDNF immunostaining in the perivascular space in elements that were not labeled with antibodies recognizing fibroblast or endothelial cells. These data disclosed a novel location of BDNF and its receptors in the ME, which are presumably involved in dynamic processes such as hormone release.

Distribution of the high-affinity choline transporter in the central nervous system of the rat

In cholinergic nerve terminals, Na(+)- and Cl(-)-dependent, hemicholinium-3-sensitive, high-affinity choline uptake is thought to be the rate-limiting step in acetylcholine synthesis. The high-affinity choline transporter cDNA responsible for the activity was recently cloned. Here we report production of a highly specific antibody to the high-affinity choline transporter and distribution of the protein in the CNS of the rat. The antibody stained almost all known cholinergic neurons and their terminal fields. High-affinity choline transporter-immunoreactive cell bodies were demonstrated in the olfactory tubercle, basal forebrain complex, striatum, mesopontine complex, medial habenula, cranial nerve motor nuclei, and ventral horn and intermediate zone of the spinal cord. Noticeably, high densities of high-affinity choline transporter-positive axonal fibers and puncta were encountered in many brain regions such as cerebral cortex, hippocampus, amygdala, striatum, several thalamic nuclei, and brainstem. Transection of the hypoglossal nerve resulted in a loss of high-affinity choline transporter immunoreactivity in neurons within the ipsilateral hypoglossal motor nucleus, which paralleled a loss of immunoreactivity to choline acetyltransferase. The antibody also stained brain sections from human and mouse, suggesting cross-reactivity. These results confirm that the high-affinity choline transporter is uniquely expressed in cholinergic neurons and is efficiently transported to axon terminals. The antibody will be useful to investigate possible changes in cholinergic cell bodies and axon terminals in human and rodents under various pathological conditions.

Glial and neuronal localization of ionotropic glutamate receptor subunit-immunoreactivities in the median eminence of female rats: GluR2/3 and GluR6/7 colocalize with vimentin, not with glial fibrillary acidic protein (GFAP)

Female rat median eminence was immunostained with anti-NR1, GluR1, GluR2/3, GluR6/7, or KA2. GluR2/3- and GluR6/7-immunoreactivities were detected in cells lining the basal portion of the third ventricle. To identify these cells as tanycytes, the median eminence was dual-immunostained with glutamate receptors and glial cytoskeletal marker proteins, such as vimentin or glial fibrillary acidic protein (GFAP). Both GluR2/3 and GluR6/7 were shown to colocalize with vimentin, not with GFAP. These results suggest the potential role for tanycytes in conducting glutamate signaling.

Excitatory amino acids act on the median eminence nerve terminals to induce gonadotropin-releasing hormone release in female rats

The present study is designed to examine the terminal regulation of gonadotropin-releasing hormone (GnRH) release by excitatory amino acids in the median eminence of ovariectomized (OVX) rats. In in vitro experiments, median eminence tissues were superfused in the medium containing glutamate or excitatory amino acid agonists, such as N-methyl-d,l-aspartate or kainate. These drugs induced a Ca2+-dependent GnRH release from median eminence fragments. The agonists also stimulated GnRH release from superfused synaptosome prepared from the median eminence tissues in a Ca2+-dependent manner. In the immunocytochemical study, immunoreactivity for glutamate or its ionotropic receptor subtypes, such as NR1, GluR1, GluR2/3, GluR6/7, and KA2, was examined in the median eminence of OVX rats under electron microscopy. Immunoreactivities for glutamate or its receptor subtypes were observed on the nerve terminals, most of which were located in close proximity to the other nerve terminals without forming synaptic contacts. In addition, quite a few synaptic contacts which were immunopositive for GluR1, GluR2/3, KA2, or glutamate were found in this area. The present results indicate that excitatory amino acids stimulate GnRH release by acting at the nerve terminals of the median eminence in a Ca2+-dependent manner in the absence of gonadal steroid. The effect of excitatory amino acids in this area might be mediated by glutamate receptors mainly in nonsynaptic fashion, such as by volume transmission.

Glial and neuronal localization of neuronal nitric oxide synthase immunoreactivity in the median eminence of female rats

Localization of neuronal nitric oxide synthase-immunoreactivity (nNOS-IR) in the median eminence of female rats (n=4) was examined by electron microscopy to explore the possibility that nitric oxide is involved in the terminal regulation of neurosecretory peptides such as GnRH. Under light microscopy, a dense distribution of nNOS-IR was observed in this region. Electronmicroscopically, nNOS-IR was found in glial elements and nerve terminals containing dense-core vesicles. We also found a few nNOS-immunopositive synapses, in which intense immunoreactivity was found on the postsynaptic density and mitochondrial membrane. The localization of nNOS-IR in nerve terminals and glial elements in the median eminence might indicate that nNOS plays a role in regulating the release of neurosecretory peptide.

Ependymal development, proliferation, and functions: a review

A survey of the literature shows that proliferation of ependyma occurs largely during the embryonic and early postnatal periods of development in most species. Differentiation of these cells proceeds along particular regional and temporal gradients as does the expression of various cytoskeletal (vimentin, cytokeratins, glial fibrillary acidic protein) and secretory proteins (S-100). Turnover declines significantly postnatally, and only low levels of residual activity persist into adulthood under normal conditions. Although the reported response of ependyma to injury is somewhat equivocal, only limited regenerative capacity appears to exist and to varying degrees in different regions of the neuraxis. Proliferation has been most often observed in response to spinal cord injury. Indeed, the ependyma plays a significant role in the initiation and maintenance of the regenerative processes in the spinal cord of inframammalian vertebrates. In the human, however, ependyma appears never to regenerate at any age nor re-express cytoskeletal proteins characteristic of immature cells. The functions of ependyma including tanycytes, a specialized form of ependymal cell that persists into adulthood within circumscribed regions of the nervous system, are still largely speculative. Fetal unlike mature ependyma is believed to be secretory and is believed to play a role in neurogenesis, neuronal differentiation/axonal guidance, transport, and support. In the adult brain, mature ependyma is not merely an inert lining but may regulate the transport of ions, small molecules, and water between the cerebrospinal fluid and neuropil and serve an important barrier function that protects neural tissue from potentially harmful substances by mechanisms that are still incompletely understood.

Localization of two cholinergic markers, choline acetyltransferase and vesicular acetylcholine transporter in the central nervous system of the rat: in situ hybridization histochemistry and immunohistochemistry

Choline acetyltransferase (ChAT) and vesicular acetylcholine transporter (VAChT) are proteins that are required for cholinergic neurotransmission. Present knowledge concerning the organization of cholinergic structures has been derived primarily from immunohistochemistry for ChAT. In the present study, we investigated the distribution of mRNAs and the corresponding proteins for ChAT and VAChT by in situ hybridization histochemistry and immunohistochemistry. The patterns of distribution of perikarya containing ChAT mRNA. ChAT protein, VAChT mRNA and VAChT protein were similar in most regions, and co-localization in the same neuron of mRNAs for ChAT and VAChT, that of ChAT mRNA and ChAT protein, and that of VAChT mRNA and VAChT protein were demonstrated. However, in the cerebral cortex and hypothalamus, ChAT-immunoreactive perikarya were present, but they did not contain mRNAs for ChAT and VAChT, and VAChT protein. On the other hand, in the cerebellum, Purkinje cell bodies contained VAChT mRNA and VAChT protein, but they did not contain either ChAT mRNA or ChAT protein. Axon bundles were clearly revealed by immunohistochemistry for ChAT, but they were not detected by that for VAChT. Both ChAT and VAChT antibodies revealed preterminal axons and terminal-like structures. In the forebrain, they were present in the olfactory bulb, nucleus of the lateral olfactory tract, olfactory tubercle, lateral septal nucleus, amygdala, hippocampus, neocortex, caudate-putamen, thalamus and median eminence of the hypothalamus. In the brainstem, they were localized in the superior colliculus, interpeduncular nucleus and some cranial nerve motor nuclei, and further in the ventral horn of the spinal cord. These results indicate strongly that ChAT and VAChT are expressed in most of the cholinergic neurons, and that immunohistochemistry for VAChT is as useful to detect cholinergic terminal fields as that for ChAT.

Adenosine deaminase in rodent median eminence: detection by antibody to the mouse enzyme and co-localization with adenosine deaminase-complexing protein (CD26)

Adenosine deaminase in the hypothalamic tuberomammillary nucleus and median eminence of rat and mouse brains was investigated with two different antibodies generated against the enzyme derived from either calf or mouse. Both antibodies labelled neurons in the tuberomammillary nucleus and, as determined in rat, they immunolabelled the same neurons. In the median eminence, immunopositive fibres and terminals were detected with anti-mouse adenosine deaminase in both rat and mouse, while no such staining was seen in either species with antibody against the calf enzyme. These fibres were most concentrated in the external median eminence, had a more restricted distribution than those containing either galanin or tyrosine hydroxylase and only partially overlapped with oxytocin-positive fibres. By electron microscopy, adenosine deaminase was found in terminals containing both small, clear vesicles with diameters of 35 to 45 nm and large dense-core vesicles with diameters of 100 to 140 nm. Preadsorption of antibodies with purified enzyme derived from the species against which they were directed eliminated all staining in rat, while antibody adsorptions across species were less effective. Preadsorption of anti-mouse adenosine deaminase antibody with the mouse deaminase led to increased labelling in mouse median eminence, suggesting an interaction between tissue components and antibody-linked enzyme. Tests for the presence of adenosine deaminase-complexing protein (CD26) with an antibody against this protein gave positive labelling in the median eminence of both species and this labelling was co-distributed with that seen for adenosine deaminase. These results confirm the expression of adenosine deaminase in restricted populations of neurons in rodent brain as revealed with a novel antibody, suggest the presence of a distinct form or localization of the enzyme in the median eminence, and raise the possibility that it contributes, perhaps along with CD26, to purinergic regulation of hormone secretion in this structure.

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.

Monoamine innervation of the organum vasculosum laminae terminalis (OVLT): a high resolution radioautographic study in the rat

The monoamine innervation of the organum vasculosum laminae terminalis (OVLT) was examined in the adult rat by light and electron microscope radioautography after intraventricular administration of tritiated serotonin [( 3H]5-HT) or dopamine [( 3H]DA). Radioautographic and biochemical controls after 5,7-dihydroxytryptamine or 6-hydroxydopamine lesioning established the respective serotonin (5-HT) and catecholamine (CA) identities of the axonal varicosities labeled under the conditions of the present experiments. For descriptive purposes, the OVLT was subdivided in three parts: two parenchymal zones, one juxtaventricular, the other juxtavascular, and the vascular core. Almost 10% of all axonal varicosities in the OVLT were found to be labeled with [3H]5-HT. This 5-HT innervation was most prominent in the rostrocaudal and ventrodorsal portions of the juxtaventricular zone and the dorsal aspect of the juxtavascular zone; there was none in the vascular core. [3H]DA-labeled varicosities were much less abundant and yet more numerous than earlier histofluorescent and immunohistochemical studies would have predicted. They predominated in the juxtavascular zone, where a majority presumably had a dopamine (DA) rather than a noradrenaline identity. Some were also found in the vascular core, where they most likely corresponded to peripheral autonomic noradrenaline endings. In the juxtaventricular zone of the OVLT, a significant proportion of the [3H]5-HT-labeled varicosity profiles could be observed to form axodendritic synapses, but in the juxtavascular zone no 5-HT or any [3H]DA-labeled ones were ever seen in synaptic junction. In the juxtavascular zone, the 5-HT and the presumed DA endings established close relationships with neurosecretory axons, and with astrocytic or tanycytic processes on which they occasionally formed "synaptoid contacts." A few endings of either type were also seen to about directly on the outer basement membrane of the perivascular space. It therefore appears probable that in OVLT monoamines influence neural and nonneural elements. At a proximal level of regulation (juxtaventricular zone), 5-HT could act both synaptically and nonsynaptically as an interneuronal transmitter or modulator. In contrast, distally (juxtavascular zone), both DA and 5-HT could be released as neurohormones in addition to modulating neurosecretion. 5-HT and DA varicosities in the OVLT could also behave as sensors for circulating factors that do not cross the blood-brain barrier.

Contractile effects of perivascularly applied vasopressin on the pial artery of the cat brain

1. The effects of perivascularly applied vasopressin on the diameter of pial arteries (control 298 +/- 14 S.E. micron) of the brain were examined after chronic implantation of a cranial window in fifteen anaesthetized cats. 2. Application of vasopressin resulted in a dose-dependent contraction. The threshold concentration for contraction was 3 X 10(-10) M, the half-maximal effective concentration (ED50) (1.6 +/- 0.2) X 10(-9) M, and the maximum reduction in artery diameter 37 +/- 2%. 3. The contraction was powerfully inhibited by perivascular application of a 10(-7) M solution of the vasopressin antagonist, [1-(beta-mercapto-beta,beta-cyclopentamethylenepropionic acid),2-(O-methyl)tyrosine]arginine vasopressin. 4. Perivascular application of noradrenaline induced a dose-dependent contraction of the pial artery. The ED50 was (8.9 +/- 2.5) X 10(-7) M, and the maximum reduction in artery diameter was 33 +/- 2%. 5. Such noradrenaline-induced contraction was not modified at all in the presence of a subthreshold dose (2 X 10(-10) M) of vasopressin (P greater than 0.05, for the over-all difference in size of the contraction, ED50 and maximum contraction). 6. In another experimental setting it was also found that neither the subthreshold nor a suprathreshold (10(-9) M) dose of vasopressin modified the contraction induced by 10(-6) M-noradrenaline (P greater than 0.05, compared to the contraction in the absence of vasopressin). 7. Thus a powerful and sensitive contractile response of the pial arteries to perivascularly applied vasopressin was demonstrated. However, the modifying effect of vasopressin on the contraction induced by perivascularly applied noradrenaline was minimal.

Axovascular relationships in developing median eminence of perinatal rats with special reference to luteinizing hormone-releasing hormone projections

Topographical relationships of neurosecretory axons with the capillaries of the primary portal plexus were studied in the median eminence of rats from the 14th fetal till the 9th postnatal day by means of electron microscopy combined with morphometric analysis. Special attention was given to the light and electron microscopic immunocytochemical examination of luteinizing hormone-releasing hormone projections to the median eminence. Neurosecretory axons possessing secretory granules and clear microvesicles were first observed in the median eminence at the 14th fetal day. However, all of them were situated at a distance from the primary portal plexus. By the 20th fetal day, neurosecretory axons reached the external basal lamina of the primary portal plexus giving rise to so-called axovascular contacts. Some axons even penetrated into the perivascular space, apparently facilitating the neurohormone delivery into the hypophysial portal circulation. From that time on, both the number of the axons abutting on the external basal lamina and the entire area of axovascular contacts increased gradually. As to luteinizing hormone-releasing hormone axons, they grew into the median eminence from the 18th fetal day concentrating in older fetuses and neonates either over the primary portal plexus, or around the infundibular recess of the 3rd ventricle. After birth, the concentration and distribution of luteinizing hormone-releasing hormone axons within the median eminence became similar to those of adults. Luteinizing hormone-releasing hormone axons were found to arise from the neurons of septopreoptic area including the diagonal band of Broca. These data suggest the onset of neurohormone release in the median eminence from the 14th fetal day followed by the establishment of the hypothalamic control over the pituitary functions during the perinatal period in rats.

Spinal tanycytes in the adult rat: a correlative Golgi gold-toning study

In Golgi impregnated transverse sections through the cervical spinal cord of the 7-12-week-old adult rat, numerous tanycytes were observed radiating from the ependyma into the gray matter that surrounds the central canal. The tail processes of these tanycytes terminated as foot processes in association with blood vessels. Spinal tanycytes were classified into ependymal (E) and subependymal (S) types on the basis of the shape and position of the soma. The soma of the E tanycyte was shaped as a column and was entirely located within the ependyma. In contrast, the soma of the S tanycyte was flask shaped, with the widest portion of the flask located subependymally and the elongated portion extending through the ependyma ultimately reaching the luminal surface. Selected Golgi impregnated sections were gold toned and deimpregnated for direct correlative analysis at the ultrastructural level. Gold-toned tanycytes contained the fine clusters of gold particles underlying the plasma membrane of the cell body and coarse clusters of gold particles throughout the tail and foot processes. The apical surface of tanycytes was characterized by numerous microvilli and large cytoplasmic protrusions that evaginated from the apical surface into the lumen of the central canal. At the luminal surface, adjacent tanycytes were joined laterally by junctional complexes with punctate tight junctions and zonulae adhaerentes associated with fibrils and microtubules. In contrast, gap junctions, hemidesmosomes, and puncta adhaerentia were found between adjacent tail processes of tanycytes. The foot processes interdigitated with one another and abutted the basal lamina around the perivascular space of blood vessels. The basal lamina was continuous around the lateral walls of foot processes and filled the spaces between membranous infoldings of the lateral walls. These basal membrane labyrinths were continuous with the basal lamina of the blood vessel and may provide an extensive surface relation between the perivascular space and the neighboring extracellular compartment. The findings of the present study support the contention that tanycytes may modify the composition of substances moving between the perivascular and extracellular spaces.

Neurohemal contact in the internal zone of the rabbit median eminence

The concept of neurosecretion as the mechanism by which neural control of adenohypophyseal function is accomplished was based on the observation that long capillary loops penetrate deeply into the supraopticohypophyseal tract as it passes through the median eminence internal zone. However, neural contact upon these capillary loops has not been demonstrated in the mammalian median eminence. The present transmission electron microscopic investigation of the rabbit median eminence demonstrates neurohemal contact in the median eminence internal zone. Axons containing small lucent vesicles 53.3 +/- 3.28 nm in diameter (mean +/- SEM) or small lucent and large granular vesicles with a mean diameter of 122.4 (+/- 3.28 nm) in their terminals make neurohemal contact with capillary loops in the internal zone and form a cuff about them. These terminals resemble terminals found in the external zone. Intravenous injection of the false neurotransmitter 5-hydroxydopamine (5-OH-DA) renders small lucent vesicles granular in both the external and internal zone. The effect of 5-OH-DA injection is abolished by concurrent reserpine administration. Whereas large granular vesicles in many terminals become lucent after reserpine administration, in others they remained electron dense. Viewed in the light of previous studies our findings suggest that the internal plexus arises from the external plexus and invaginates the neuropil carrying connective tissue and parvicellular axon terminals of aminergic and peptidergic systems from the external zone into the internal zone, that some elements making neurohemal contact with long capillary loops are terminals of the noradrenergic reticular infundibular tract arising outside the hypothalamus in the brainstem, and that long capillary loops form a system of repeating microvascular modules which markedly increase the surface available for neurohemal contact.

Light- and electron-microscopic immunocytochemistry of glutamic acid decarboxylase (GAD) in the basal hypothalamus: morphological evidence for neuroendocrine gamma-aminobutyrate (GABA)

GABAergic cells and axon terminals were localized in the basal hypothalamus of differnet species (rat, mouse and cat), by means of an immunocytochemical approach using a specific and well-characterized antiserum to the GABA biosynthetic enzyme, glutamate decarboxylase. Light-microscopic visualization was performed with an indirect immunofluorescence method and electron-microscopic observations were made on material with pre-embedding staining and use of the peroxidase-antiperoxidase procedure. At the light-microscopic level, a dense immunofluorescent plexus was observed over both the medial and lateral parts of the external layer of the median eminence. The labelling extended from the rostal part of the median eminence up to the pituitary stalk. Over the subependymal and internal layers only a few immunoreactive dots were visible, except around the blood vessels where they appeared more concentrated. Immunoreactive varicosities could be found following the outlines of the capillary loops and lining tanycyte processes, especially in the median eminance midportion. At the electron-microscopic level, the immunolabelling was exclusively found over neuronal profiles in the median eminence. The latter represented a small fraction of the total number of varicosities visible on the same section. Labelled profiles typically contained numerous small clear synaptic vesicles and only a few or no dense-core vesicles. In the subependymal and internal layers, rare labelled endings were found close to ependymal cells or among transversally cut fibers, respectively. In the palisadic zone, elongated positive boutons were visible intermingled with bundles of unlabelled axons and glial or ependymal processes. In the neurohemal contact zone, immunoreactive endings were observed among unlabelled neurosecretory endings in close vicinity to fenestrated capillary perivascular space. Small moderately intense immunofluorescent varicosities were observed all over the hypothalamus. The density of the glutamate decarboxylase-positive network was higher than in most diencephalic regions. Intraventricular or topical injection of colchicine allowed the visualization of small lightly immunoreactive cells in the diffusion area of colchicine. In the arcuate nucleus labelled axonal endings containing small pleomorphic synaptic vesicles and sometimes a few dense-core vesicles were observed at the electron-microscopic level. Typical synaptic junctions were commonly found between positive endings and unlabelled perikarya, or more frequently, unlabelled dendrites. These findings show that glutamate decarboxylase-containing endings are localized ed in several strategic sites for potential GABAergic neuroendocrine regulations. The GABAergic endings found among neurosecretory endings in the neurohemal contact zone may provide the morphological support for the release of gamma-aminobutyrate into the portal blood flow as an hypothalamic hypophysiotropic hormone.

The hypothalamo-hypophysial system of hypophysectomized rats. II. Structure and ultrastructure of the median eminence

The median eminence (ME) of hypophysectomized rats was studied by means of light and electron microscopy. Paraldehyde-fuchsin (PAF)-positive material is seen in the external zone (EZ) of the ME 2--5 days after the operation. Its amount gradually increases especially in the caudal part of the ME during the following few days. Some PAF-positive fibers make contact with the subependymally located blood capillaries. In the most caudal region of the recessus infundibuli they penetrate into the third ventricle. PAF-positive material decreases markedly from the ME of rats two months after hypophysectomy and exposure to a 1% salt load. Fibers of types A1, A2 and B containing granules of 120--220 nm, 100--150 nm and 80--100 nm in diameter, respectively, are seen in the EZ of the ME in hypophysectomized rats, although almost exclusively A2- and B-type structures make contact with the primary portal capillaries in intact animals. All types of neurosecretory fibers establish contact with the subependymal nonfenestrated blood capillaries and penetrate the recessus infundibuli. Some neurosecretory terminals of different types make direct contact with the glandular cells of the pars tuberalis or are separated from them by a thin basal lamina. It is assumed that mainly neurosecretory fibers of types A2 and B are permanently connected with the primary portal capillaries in the EZ of the ME in intact mammals, while the overwhelming majority of fibers of A1-type shows ingrowth during the course of postoperative reparation. The possible physiological significance of the described changes is discussed.

The ventricular surface of the monkey mediobasal hypothalamus. A scanning electron microscopic study

The ventricular lining of the infundibular recess of monkey brains of both sexes was investigated with SEM. Based on the distribution patterns of surface profiles regional differences in the tanycytic ependyma are described. Variations in the number of surface profiles were observed in females during the ovarian cycle. These variations may be an indication not only of absorptive and/or secretory activities, but may also be related to cell membrane redistribution and membrane turnover in dependence of endocrine factors. In the males such variations were not registered. An essental sexual dimorphism does not seem to exist in the tanycytic ependyma of the monkey, only the reaction pattern is different. Supraependymal macrophages (SEC) were more numerous in the ovulating females than in the males or menstruating females. In addition to these SEC some monkeys had a second SEC type which was arranged in sheets. These are interpreted to be glioses. A variable number of axons can also be found in the lower regions of the infundibular recess. Their morphology suggests the presence of several types of axons, but they do not appear to constitute an organized network. No significant sexual differences were registered in relation to the axons.

Hypothalamo-hypophysial function following the lesion of tanycytes in the median eminence of the rat

The transport of hormones from the cerebrospinal fluid to the adenohypophysis by the tanycytes of the median eminence was examined in male rats. Electron microscopy revealed that all ependymal cells including the tanycytes disappear or degenerate in rats subjected to electric cauterization of the ependymal layer lining the third ventricle. However, the granular axons in the palisade layer of the median eminence remain intact. In rats subjected to electric lesion, no significant change was found in either the serum-LH level or in the weight of the adenohypophysis, testes, adrenal and thyroid glands. It is concluded that the tanycytes do not participate in the hypothalamic regulation of hypophysial function.

Autoradiography of GABA in the rat hypothalamic median eminence

Light microscopy autoradiographs of the rat hypothalamic median eminence were prepared after injection of high specific activity tritiated GABA and GABA structural analogs. Following intracardiac injection of labeled GABA with short (15 min) survival time, a dense accumulation of silver grains was observed over the external layer of the median eminence. The silver grains appeared much less numerous and randomly scattered over the internal layer. No conspicuously labeled cells could be detected in the median eminence. A similar pattern of labeling was observed after 10 min in vitro incubation of the median eminence with a low concentration (2.5 x 10(-7) M) of labeled GABA. Clusters of silver grains were also visible over the external layer following intraventricular injection of labeled GABA. In this latter case, however, other sites of labeling were revealed over the internal and ependymal layers. The dense labeling over the external layer with tritiated GABA was partially reduced by a simultaneous intracardiac injection of a 50-fold excess of non-radioactive cis-aminocyclohexane carboxylic acid--a reported preferential substrate for GABA neuronal uptake--but it was not displaced by a 2000-fold excess of non-radioactive beta-alanine--a reported specific substrate for GABA glial uptake. Intracardiac injection of triated beta-alanine led to a faint and even labeling over the entire median eminence with no preferential accumulation of silver grains over the various layers. Following intraventricular injection of labeled beta-alanine the tanycytes and their processes as well as numerous glial cells appeared heavily labeled. These results suggested that there exist cell elements in the external layer of the hypothalamic median eminence which are capable of accumulating exogenous GABA according to its neuronal uptake characteristics. Although the exact nature of these cells is not readily apparent at this stage of our investigations, these findings led us to speculate that there might be a subpopulation of GABAergic nerve endings in the vicinity of the primary plexus capillaries.

The hypothalamo-hypophysial system of the lamprey, Lampetra fluviatilis L. II. The proximal neurosecretory contact region

The proximal neurosecretory contact region (PNCR) of the lamprey, a homologue of the median eminence of tetrapods, was studied by light, fluorescence and electron microscopy. Paraldehyde fuchsin-positive neurosecretory fibers are seen mainly in the central part of the rostral subdivision of the PNCR. The Falck-Hillarp technique reveals a weak, mainly diffuse yellow-green fluorescence in the PNCR. The ultrastructure of the tanycyte layer of the PNCR is very similar to that in the neurohypophysis of the same species, although the funnel-shaped protrusions of the third ventricle in the rostral part of the PNCR are more frequent than in the neurophypophysis. Peptidergic A1 and A2 neurosecretory fibers are characterized by neurosecretory granules of 120-200 nm and 100-150 nm in diameter, respectively. Monoaminergic B type fibers contain granules 80-100 nm in diameter. Neurosecretory terminals and the vascular endfeet of tanycytes make contact with the basement membrane of the avascular connective tissue layer separating the PNCR from the hypophysial pars distalis. It is suggested that both peptide and monoamine neurohormones diffuse through the thick connective tissue septa into the underlying blood vessels which supply the pars distalis and thus affect the function of its glandular cells.

Changes in the distribution of non-neuronal elements in rat median eminence and in anterior pituitary hormone secretion after activation of tuberoinfundibular dopamine neurones by brain stimulation or nicotine

In order to investigate the posssibility of acute functional changes in non-neuronal elements (mainly tanycytes) of the median eminence, the proportion of portal capillary surface covered by such elements was measured by quantitative electron microscopy in ovariectomized, oestrogen-progesterone-pretreated rats. In some of these animals, the functional state of the tuberoinfundibular dopamine (DA) neurones was assessed by histochemical microfluorimetry. Serum concentrations of luteinizing hormone (LH), growth hormone (GH) and prolactin were determined by radioimmunoassay. Two different types of treatment, i.e. systemic administration of nicotine (1 mg/kg, s.c.) or electrical stimulation in the medial amygdaloid nucleus, markedly reduced the percentage of capillary surface covered by non-neuronal profiles within 20 and 15 min, respectively. At the same time, the tuberoinfundibular DA system responded by an increase in cellular fluorescence intensity, reflecting neuronal activation. Medial preoptic stimulation had basically the same effect but with more variability in the change in capillary coverage by tanycytes. The action of nicotine was prevented by pretreatment with the DA receptor blocking agent, pimozide (5 mg/kg), which indicates (1) that a dopaminergic mechanism was involved in the nicotine effect and (2) that the tanycyte response was elicited by DA released from nerve terminals acting at some receptor site. Nicotine also lowered serum levels of GH and prolactin. Pimozide antagonized only the effect on prolactin. While the reaction of DA neurones and capillary coverage by tanycytes were correlated with each other in individual rats, no statistically significant correlation was observed between tanycyte response and hormone levels, so that no conclusions can as yet be drawn as to the neuroendocrine significance of the tanycyte reaction. These results indicate that rapid changes in the proportion or portal capillary surface covered by non-neuronal profiles can be elicited by stimulation of extrahypothalamic brain areas or by activation of cholinergic mechanisms. The tanycyte response appears to be mediated at least in part by the tuberoinfundibular DA neurones.

The effect of parenteral glutamate treatment on the localization of neurotransmitters in the mediobasal hypothalamus

The localization of cholinergic, aminergic and amino acid-ergic neurones in the mediobasal hypothalamus has been studied in normal rat brain and in brains where neurones in nucleus arcuatus were destroyed by repeated administration of 2 mg/g body weight monosodium glutamate to newborn animals. In normal animals acetylcholinesterase staining, choline acetyltransferase and aromatic L-amino acid decarboxylase were concentrated in the median eminence and the arcuate nucleus. Glutamate decarboxylase was concentrated at the boundary between the ventromedial and the arcuate nuclei, with lower activity in the arcuate nucleus and very low activity in the median eminence. Nucleus arcuatus contained an intermediate level of high affinity glutamate uptake. In the lesioned animals, there were significant decreases in choline acetyltransferase, acetylcholinesterase staining and glutamate decarboxylase in the median eminence, whereas choline acetyltransferase activity and acetylcholinesterase staining, but not glutamate decarboxylase activity, were decreased in nucleus arcuatus. Aromatic L-amino acid decarboxylase was unchanged in all regions studied. The high affinity uptakes of glutamate, dopamine and noradrenaline, and the endogenous amino acid levels were also unchanged in the treated animals. The results indicate the existence of acetylcholine- and GABA-containing elements in the tuberoinfundibular tract. They further indicate that the dopamine cells in the arcuate nucleus are less sensitive to the toxic effect of glutamate than other cell types, possibly because they contain less glutamate receptors.

The rhombencephalic recess in the rat. A light and electron microscopic study

The rhombencephalic recess, an ependymal organ, has been studied for the first time by light- and electron microscopy. It is situated mediosagitally on the floor of the rhomboid fossa at the level of the colliculus facialis. The recess and the superimposed tissue are built up by tanycytes, their apices being connected by tight junctions. HRP, injected into the c.s.f., does not penetrate into the intercellular clefts of the recess area. The recess area reveals a certain autonomy regarding its supply with arteries and capillaries. A blood-brain barrier exists, but shows slight leakage in circumscribed areas as a result of intense transendothelial vesicular transport. The organization of the recess area is compared with that of other ependymal organs, especially circumventricular organs.