Fine structure of the ependyma and intercellular junctions in the area postrema of the rat

Global Clock Coordination by the Brain Clock in the Suprachiasmatic Nucleus Through Relay and Amplification of Diffusible and Neural Signaling

The brain clock located in the suprachiasmatic nucleus (SCN) of the hypothalamus receives direct retinal input, thereby providing the entire body with an internal representation of external solar time. The pathways by which this small nucleus signals so broadly involve co-occurring nervous and diffusible output signals, although the latter are less understood. Portal pathways, such as the well-known pituitary portal pathway, provide a mechanism, whereby signals of neural origin can reach local, specialized targets without suffering dilution in the systemic blood supply. Newly discovered vascular pathways involve direct connections between each of the sensory circumventricular nuclei at its point of attachment to the brain. These nuclei line the brain's ventricles, and their leaky blood vessels and large perivascular spaces represent a route, whereby secretions from the SCN can be relayed and then amplified, providing a pathway to achieve global coordination of circadian clock signaling. This review provides a narrative that incorporates our understanding of SCN neural and diffusible output signals, with particular emphasis on the contribution of brain fluidic compartments and the fluids therein.

Targeting the Choroid Plexuses for Protein Drug Delivery

Delivery of therapeutic agents to the central nervous system is challenged by the barriers in place to regulate brain homeostasis. This is especially true for protein therapeutics. Targeting the barrier formed by the choroid plexuses at the interfaces of the systemic circulation and ventricular system may be a surrogate brain delivery strategy to circumvent the blood-brain barrier. Heterogenous cell populations located at the choroid plexuses provide diverse functions in regulating the exchange of material within the ventricular space. Receptor-mediated transcytosis may be a promising mechanism to deliver protein therapeutics across the tight junctions formed by choroid plexus epithelial cells. However, cerebrospinal fluid flow and other barriers formed by ependymal cells and perivascular spaces should also be considered for evaluation of protein therapeutic disposition. Various preclinical methods have been applied to delineate protein transport across the choroid plexuses, including imaging strategies, ventriculocisternal perfusions, and primary choroid plexus epithelial cell models. When used in combination with simultaneous measures of cerebrospinal fluid dynamics, they can yield important insight into pharmacokinetic properties within the brain. This review aims to provide an overview of the choroid plexuses and ventricular system to address their function as a barrier to pharmaceutical interventions and relevance for central nervous system drug delivery of protein therapeutics. Protein therapeutics targeting the ventricular system may provide new approaches in treating central nervous system diseases.

Neural stem cell phenotype of tanycyte-like ependymal cells in the circumventricular organs and central canal of adult mouse brain

Tanycyte is a subtype of ependymal cells which extend long radial processes to brain parenchyma. The present study showed that tanycyte-like ependymal cells in the organum vasculosum of the lamina terminalis, subfornical organ and central canal (CC) expressed neural stem cell (NSC) marker nestin, glial fibrillar acidic protein and sex determining region Y. Proliferation of these tanycyte-like ependymal cells was promoted by continuous intracerebroventricular infusion of fibroblast growth factor-2 and epidermal growth factor. Tanycytes-like ependymal cells in the CC are able to form self-renewing neurospheres and give rise mostly to new astrocytes and oligodendrocytes. Collagenase-induced small medullary hemorrhage increased proliferation of tanycyte-like ependymal cells in the CC. These results demonstrate that these tanycyte-like ependymal cells of the adult mouse brain are NSCs and suggest that they serve as a source for providing new neuronal lineage cells upon brain damage in the medulla oblongata.

Regional differences in the ependyma of the optic tectal ventricle of adult zebrafish with structures referring to brain hydrodynamics

Classical electron microscopic morphological studies provide detailed ultrastructural information, which may lend insights into cellular functions. As a follow-up to our morphological investigation of the adult zebrafish (Danio rerio) optic tectum, in this study, we have analyzed the ependymal structures lining the surfaces of the tectal ventricle: the torus, tegmental surface of the valvula cerebelli and the periventricular gray zone of the optic tectal cortex. We used toluidine blue stained plastic (semithin) sections for light microscopy and scanning electron microscopy. Our morphological findings of gated entrances and/or egresses indicate that, at least in the adult zebrafish brain, there may be a bidirectional direct flow communication between the ventricular cerebrospinal fluid and the parenchymal interstitial fluid.

Area Postrema: Fetal Maturation, Tumors, Vomiting Center, Growth, Role in Neuromyelitis Optica

INTRODUCTION

The area postrema in the caudal fourth ventricular floor is highly vascular without blood-brain or blood-cerebrospinal fluid barrier. In addition to its function as vomiting center, several others are part of the circumventricular organs for vasomotor/angiotensin II regulation, role in neuromyelitis optica related to aquaporin-4, and somatic growth and appetite regulation. Functions are immature at birth. The purpose was to demonstrate neuronal, synaptic, glial, or ependymal maturation in the area postrema of normal fetuses. We describe three area postrema tumors.

METHODS

Sections of caudal fourth ventricle of 12 normal human fetal brains at autopsy aged six to 40 weeks and three infants aged three to 18 months were examined. Immunocytochemical neuronal and glial markers were applied to paraffin sections. Two infants with area postrema tumors and another with neurocutaneous melanocytosis and pernicious vomiting also studied.

RESULTS

Area postrema neurons exhibited cytologic maturity and synaptic circuitry by 14 weeks'. Astrocytes coexpressed vimentin, glial fibrillary acidic protein, and S-100β protein. The ependyma is thin over area postrema, with fetal ependymocytic basal processes. A glial layer separates area postrema from medullary tegmentum. Melanocytes infiltrated area postrema in the toddler with pernicious vomiting; two children had primary area postrema pilocytic astrocytomas.

CONCLUSIONS

Although area postrema is cytologically mature by 14 weeks, growth increases and functions mature during postnatal months. We recommend neuroimaging for patients with unexplained vomiting and that area postrema neuropathology includes synaptophysin and microtubule-associated protein-2 in patients with suspected dysfunction.

Characterization of Proliferating Neural Progenitors after Spinal Cord Injury in Adult Zebrafish

Zebrafish can repair their injured brain and spinal cord after injury unlike adult mammalian central nervous system. Any injury to zebrafish spinal cord would lead to increased proliferation and neurogenesis. There are presences of proliferating progenitors from which both neuronal and glial loss can be reversed by appropriately generating new neurons and glia. We have demonstrated the presence of multiple progenitors, which are different types of proliferating populations like Sox2⁺ neural progenitor, A2B5⁺ astrocyte/ glial progenitor, NG2⁺ oligodendrocyte progenitor, radial glia and Schwann cell like progenitor. We analyzed the expression levels of two common markers of dedifferentiation like msx-b and vimentin during regeneration along with some of the pluripotency associated factors to explore the possible role of these two processes. Among the several key factors related to pluripotency, pou5f1 and sox2 are upregulated during regeneration and associated with activation of neural progenitor cells. Uncovering the molecular mechanism for endogenous regeneration of adult zebrafish spinal cord would give us more clues on important targets for future therapeutic approach in mammalian spinal cord repair and regeneration.

Tanycyte-like cells form a blood-cerebrospinal fluid barrier in the circumventricular organs of the mouse brain

Tanycytes are highly specialized ependymal cells that form a blood-cerebrospinal fluid (CSF) barrier at the level of the median eminence (ME), a circumventricular organ (CVO) located in the tuberal region of the hypothalamus. This ependymal layer harbors well-organized tight junctions, a hallmark of central nervous system barriers that is lacking in the fenestrated portal vessels of the ME. The displacement of barrier properties from the vascular to the ventricular side allows the diffusion of blood-borne molecules into the parenchyma of the ME while tanycyte tight junctions control their diffusion into the CSF, thus maintaining brain homeostasis. In the present work, we combined immunohistochemical and permeability studies to investigate the presence of tanycyte barriers along the ventricular walls of other brain CVOs. Our data indicate that, unlike cuboidal ependymal cells, ependymal cells bordering the CVOs possess long processes that project into the parenchyma of the CVOs to reach the fenestrated capillary network. Remarkably, these tanycyte-like cells display well-organized tight junctions around their cell bodies. Consistent with these observations, permeability studies show that this ependymal layer acts as a diffusion barrier. Together, our results suggest that tanycytes are a characteristic feature of all CVOs and yield potential new insights into their involvement in regulating the exchange between the blood, the brain, and the CSF within these "brain windows."

Neuromyelitis optica unique area postrema lesions: nausea, vomiting, and pathogenic implications

OBJECTIVE

To characterize the neuropathologic features of neuromyelitis optica (NMO) at the medullary floor of the fourth ventricle and area postrema. Aquaporin-4 (AQP4) autoimmunity targets this region, resulting in intractable nausea associated with vomiting or hiccups in NMO.

METHODS

This neuropathologic study was performed on archival brainstem tissue from 15 patients with NMO, 5 patients with multiple sclerosis (MS), and 8 neurologically normal subjects. Logistic regression was used to evaluate whether the presence of lesions at this level increased the odds of a patient with NMO having an episode of nausea/vomiting.

RESULTS

Six patients with NMO (40%), but no patients with MS or normal controls, exhibited unilateral or bilateral lesions involving the area postrema and the medullary floor of the fourth ventricle. These lesions were characterized by tissue rarefaction, blood vessel thickening, no obvious neuronal or axonal pathology, and preservation of myelin in the subependymal medullary tegmentum. AQP4 immunoreactivity was lost or markedly reduced in all 6 cases, with moderate to marked perivascular and parenchymal lymphocytic inflammatory infiltrates, prominent microglial activation, and in 3 cases, eosinophils. Complement deposition in astrocytes, macrophages, and/or perivascularly, and a prominent astroglial reaction were also present. The odds of nausea/vomiting being documented clinically was 16-fold greater in NMO cases with area postrema lesions (95% confidence interval 1.43-437, p = 0.02).

CONCLUSIONS

These neuropathologic findings suggest the area postrema may be a selective target of the disease process in NMO, and are compatible with clinical reports of nausea and vomiting preceding episodes of optic neuritis and transverse myelitis or being the heralding symptom of NMO.

A diffusion barrier between the area postrema and nucleus tractus solitarius

The blood-brain barrier (BBB) is a structural and functional barrier that prevents free exchange of circulating substances with the brain, where the endothelial cells of microvessels are joined by tight junctions. The circumventricular organs (CVOs), by contrast, lack tight junctions and exhibit more direct communication with the circulating blood and cerebrospinal fluid. Despite many outstanding morphological studies at the electron microscopic level, there remain misconceptions that the CVOs provide direct passage of blood-borne substances to the rest of the brain. This study will show the structure of the anatomical borders of the dorsal vagal complex in the brainstem. A distinct diffusion barrier between the area postrema (AP, a CVO) and the nucleus tractus solitarius (NTS) was illustrated by immunohistochemistry at both the light and electron microscopic levels. The border zone between the AP and NTS was underlined by a continuous monolayer of columnar cells that were immunopositive for both the tight junction protein zona occludin-1 and the astrocyte marker glial fibrillary acidic protein. This observation of a diffusion barrier between the AP and NTS resolves a long-standing dispute about whether the NTS is a structural extension of the AP with a leaky BBB.

Cytokine signals propagate through the brain

Interleukin-1 (IL-1) and tumor necrosis factor alpha (TNFalpha) are proinflammatory cytokines that are constitutively expressed in healthy, adult brain where they mediate normal neural functions such as sleep. They are neuromodulators expressed by and acting on neurons and glia. IL-1 and TNFalpha expression is upregulated in several important diseases/disorders. Upregulation of IL-1 and/or TNFalpha expression, elicited centrally or systemically, propagates through brain parenchyma following specific spatio-temporal patterns. We propose that cytokine signals propagate along neuronal projections and extracellular diffusion pathways by molecular cascades that need to be further elucidated. This elucidation is a prerequisite for better understanding of reciprocal interactions between nervous, endocrine and immune systems.

Tanycytic ependymoma

Tanycytic ependymoma is an uncommon fibrillar variant of ependymoma characterized by streams of piloid, or hair-like, cells having "ependymal" nuclei. True ependymal rosettes are absent, and perivascular rosettes are inconspicuous. Misinterpretation as schwannoma or astrocytoma is a diagnostic problem and well-documented cases are scarce. The purpose of this report is to document the ependymal features of the neoplasm and to increase awareness of the entity's existence. Biopsy tissues from three patients with tanycytic ependymoma were examined. All tumors consisted of sheets of spindle cells that were positive for glial fibrillary acidic and S-100 proteins. Ultrastructural examination showed characteristic ependymal features, including intracytoplasmic intermediate filaments, prominent intercellular junctions, numerous slender surface microvilli, and microvilli-lined lumina. Accurate recognition of the ependymal nature of this spindle neoplasm requires a high index of suspicion. Because the spindle cells are immunoreactive with antibodies to both glial fibrillary acidic and S-100 proteins, ultrastructural confirmation of ependymal features is necessary.

Histochemical evidence for abnormal copper distribution in the central nervous system of LEC mutant rat

After sulfide-silver staining if tissue sections treated with trichloroacetic acid, silver particles indicating the copper distribution were observed in several regions of normal rat brain, including the nucleus arcuatus, nucleus tractus solitarii, nucleus habenulae medialis, locus ceruleus, and cerebellum. However, copper in these regions disappeared or was greatly decreased in LEC mutant rats. The are postrema showed no detectable level of copper in normal rat brain, but exhibited an extremely high level of copper in LEC rats, with or without the onset of jaundice. The results suggest an abnormal copper metabolism in the central nervous system of LEC rat.

The connectivity of the area postrema in the ferret

The area postrema (AP) is the chemosensitive trigger zone for the emetic reflex. We have investigated the connectivity of the AP and adjacent solitary complex (SC) to identify possible sites of the motor emetic center. The AP and SC were infused with HRP or WGA-HRP in 30 ferrets that were perfused transcardially after 24-72 h. A block from the pons to upper cervical spinal cord, and one with hypothalamus and basal forebrain, was cut at 50 microns, reacted, and mounted. Data support the conclusion, at variance with those from other preparations, that in ferrets the AP has reciprocal connections only with the SC, which serves as a relay in both ascending and descending pathways between AP and higher levels of the neuraxis. Connectivity of the SC with brain stem and forebrain structures including the rostral ventrolateral medulla, parabrachial nuclei, paraventricular nucleus, and amygdala was demonstrated. At least in ferrets, our results suggest that the motor emetic center must be located within the SC. While this may not apply to all species, it is also possible that some reports of AP projections elsewhere were results of label within the SC. Alternatively, the somewhat different pattern of emesis in the ferret as compared to the dog (greater role for vagal inputs in response to radiation and cytotoxic drugs, lesser role for humoral inputs) may reflect differences in AP connectivity.

Metabolic fuels and reproduction in female mammals

A complete reproductive cycle of ovulation, conception, pregnancy, and lactation is one of the most energetically expensive activities that a female mammal can undertake. A reproductive attempt at a time when calories are not sufficiently available can result in a reduced return on the maternal energetic investment or even in the death of the mother and her offspring. Numerous physiological and behavioral mechanisms link reproduction and energy metabolism. Reproductive attempts may be interrupted or deferred when food is scarce or when other physiological processes, such as thermoregulation or fattening, make extraordinary energetic demands. Food deprivation suppresses both ovulation and estrous behavior. The neural mechanisms controlling pulsatile release of gonadotropin-releasing hormone (GnRH) and, consequently, luteinizing hormone secretion and ovarian function appear to respond to minute-to-minute changes in the availability of metabolic fuels. It is not clear whether GnRH-secreting neurons are able to detect the availability of metabolic fuels directly or whether this information is relayed from detectors elsewhere in the brain. Although pregnancy is less affected by fuel availability, both lactational performance and maternal behaviors are highly responsive to the energy supply. When a reproductive attempt is made, changes in hormone secretion have dramatic effects on the partitioning and utilization of metabolic fuels. During ovulatory cycles and pregnancy, the ovarian steroids, estradiol and progesterone, induce coordinated changes in the procurement, ingestion, metabolism, storage, and expenditure of metabolic fuels. Estradiol can act in the brain to alter regulatory behaviors, such as food intake and voluntary exercise, as well as adenohypophyseal and autonomic outputs. At the same time, ovarian hormones act on peripheral tissues such as adipose tissue, muscle, and liver to influence the metabolism, partitioning and storage of metabolic fuels. During lactation, the peptide hormones, prolactin and growth hormone, rather than estradiol and progesterone, are the principal hormones controlling partitioning and utilization of metabolic fuels. The interactions between metabolic fuels and reproduction are reciprocal, redundant, and ubiquitous; both behaviors and physiological processes play vital roles. Although there are species differences in the particular physiological and behavioral mechanisms mediating nutrition-reproduction interactions, two findings are consistent across species: 1) Reproductive physiology and behaviors are sensitive to the availability of oxidizable metabolic fuels. 2) When reproductive attempts are made, ovarian hormones play a major role in the changes in ingestion, partitioning, and utilization of metabolic fuels.

Early development of the human area postrema and subfornical organ

The first appearance and early development of two circumventricular organs, the area postrema (AP) and the subfornical organ (SFO), were investigated in human embryos and fetuses from the 4th to the 40th gestational weeks (GW). The AP appears very early in development, during the GW 10; its high vascularization can be seen from GW14, and differentiated neurons are observed from GW 16. The SFO is characterized by a late onset of development. It can first be distinguished at GW 17, but it does not attain cytological differentiation until the last weeks of gestation. It is suggested that the AP has important functions during fetal life, which are related to normal fetal weight and growth; in contrast the SFO, which is connected with drinking behavior and salt/water balance, seems to play a less essential role in early fetal life.

Interactions between meningeal cells and astrocytes in vivo and in vitro

At the interface between the meninges and the central nervous system there is a characteristic structure known as the glia limitans, consisting of many fine interdigitating astrocyte processes which contain both GFAP and vimentin, and a basal lamina. A similar structure is set up after brain injury where meningeal cells invade the lesion. We have experimentally put astrocytes and meningeal cells in contact with one another, both in vivo and in vitro, to see whether this results in the formation of a glia limitans. Cultured meningeal cells were injected into the hippocampus of adult rats, and from 1 to 12 weeks later brains were stained were stained for GFAP and vimentin. One week after injection there was a widespread astrocytic reaction stretching up to 2 mm from the injection, the cells being stained intensely for both GFAP and vimentin. Over the next 4-6 weeks this widespread reaction subsided, the only remaining vimentin stained astrocytes, apart from those at the normal glia limitans, being in contact with the injected meningeal cells, or with meningeal cells which had migrated into the injection needle track. In vitro a structure reminiscent of the glia limitans formed where patches of astrocytes abutted meningeal cells; the astrocytes formed a layer of fine interdigitating processes all running parallel to the interface between the two cell types, and there was heavy staining for laminin and fibronectin. We conclude that a glia limitans forms wherever astrocytes and meningeal cells come into contact.

Alcohol effects on the morphometric development of the subfornical organ and area postrema of the albino mouse

We have studied the development of the nuclear sizes of ependymocytes and neurons of two circumventricular organs of the male alcoholic mouse: the Subfornical Organ (SFO) and the Area Postrema (AP), comparing the results with a control group. The global volume of both centers was also studied. The results show that the SFO, a structure related to the control of fluid balance, responds to alcoholism with an increase of the global volume. This increase could be related to the variations of salt-water balance and/or blood pressure in chronic alcoholism. However, the size of cell nuclei in the SFO is not affected. In contrast, the AP responds to chronic alcoholism like other nervous centres, with a decrease of the nuclear size of its cells. The global volume of AP does not change.

Response to chronic insulin administration: effect of area postrema ablation

The effects of daily administration of protamine zinc insulin (PZI) on plasma insulin and glucose levels and on food intake and body weight of rats with lesions of the area postrema and adjacent caudal-medial portions of the nucleus of the solitary tract (APX rats) were examined. Prior to insulin treatment, APX rats weighted less and had lower plasma immunoreactive insulin (IRI) levels than nonlesioned controls but did not differ from controls in plasma glucose levels. Five daily injections of 5 U/kg PZI raised plasma IRI and lowered plasma glucose levels similarly for both lesioned and nonlesioned rats. When injected with increasing doses of PZI over a 30-day period, both lesioned and nonlesioned rats showed increases of food intake and rate of weight gain in response to 8 U/kg PZI. These data indicate that APX does not affect either physiological or behavioral responses to chronic peripheral insulin administration.

Allografts of CNS tissue possess a blood-brain barrier. I. Grafts of medial preoptic area in hypogonadal mice

This study represents the first part of a three-part investigation of blood vessels supplying CNS tissue transplanted within the brains of adult mammalian hosts. The results emphasize that blood vessels in solid CNS grafts contribute a blood-brain barrier to that of the host. Neurosecretory cells in basal forebrain grafts placed intraventricularly on the dorsal surface of the host median eminence, a neurosecretory site containing fenestrated blood vessels, do not stimulate similar blood vessels to inhabit the transplanted tissue. Solid grafts of the medial preoptic area containing neurons that synthesize and secrete gonadotropic hormone-releasing hormone (GnRH) were obtained from AKR mice and placed into the third cerebral ventricle of hypogonadal (HPG) mice genetically incapable of synthesizing GnRH. GnRH neurons in the allografts were confirmed immunohistochemically. Blood vessels supplying the host median eminence and the allograft at 10 days to 3 months post-transplantation were analyzed with peroxidase cytochemistry applied in three ways: to HPG mice injected systemically with native horseradish peroxidase; to HPG mice infused into the aorta with peroxidase subsequent to perfusion fixation; and to HPG mice brains fixed by immersion and incubated for endogenous peroxidase activity in red cells retained within blood vessels. The median eminence of the HPG mouse was innervated by GnRH neurons residing within the graft, and blood vessels traversing the median eminence-allograft interface were seen rarely. The allografts contained no fenestrated endothelia, and no extravasations of blood-borne HRP were related directly to leaky blood vessels supplying the grafted tissue. Endothelial cells throughout the CNS grafts were similar morphologically to blood-brain barrier endothelia; they were nonfenestrated, exhibited interendothelial tight junctional complexes and an endomembrane system of organelles, and they endocytosed blood-borne HRP that eventually was sequestered within dense body lysosomes. The results support the belief that blood vessels supplying CNS tissue transplanted to a host brain manifest endothelial characteristics identical to those of the tissue in normal life and to those of the host CNS.

Orthogonal arrays of particles in plasma membranes of Müller cells in the guinea pig retina

Plasma membranes of guinea pig Müller cells were examined with a freeze-fracture technique to see how orthogonal arrays are distributed in the avascular retina. Examination of the portion approximately intermediate between the optic disc and equator of the eyeball showed that all end-feet of Müller cells were provided with arrays. Orthogonal arrays were concentrated on vitreal end-foot membranes, i.e., membranes that were covered by the basal lamina and contacted the vitreous body, called vitreal membranes here. The arrays were rarely observed in the portions of end-feet that did not contact the vitreous body, called lateral membranes. The distribution density of arrays in the vitreal membranes was 122.5 +/- 45.3/microns2, which was over 10 times higher than that (9.6 +/- 9.6/microns2) in the lateral membranes. The arrays became numerous and extended in shape at the periphery of the vitreal membrane, characteristically aligned in rows at the border where vitreal met lateral membrane, but never intruded into the domain of lateral membrane. Some arrays were composed of loosely attached particles and/or rod-like profiles. Sometimes rod-like profiles, 9-13 nm wide and 20-50 nm long, called linear structures here, were isolated, and sometimes they appeared in rows. Ordinary intramembrane particles (IMPs) were significantly smaller and less numerous in vitreal than in lateral membranes. IMPs larger than 9 nm in diameter were significantly fewer in the vitreal membranes, which suggests that they have been consumed to form the arrays. Although the distribution of orthogonal arrays is similar to that of K+ channels (Newman: J. Neurosci., 7:2423-2432, 1987), we consider the array an unlikely candidate for the ion channel, because its subunit particles do not protrude onto either the inner or outer surface of the membrane (Gotow and Hashimoto: J. Neurocytol., 17:399-413, 1988). Judging from their unique alignment in rows where the membrane is bent and vitreal and lateral membranes meet, the arrays may contribute to some membrane stability, resisting the physical tension at the interface with mesenchymal tissue.

Deep-etch structure of astrocytes at the superficial glia limitans, with special emphasis on the internal and external organization of their plasma membranes

The cytoskeletal system in rat subpial astrocytes and the relationship between astrocytic plasma membrane and basal lamina or cytoplasmic components were examined with a quick-freeze deep-etch technique, mainly using chemically fixed tissues. Attention was focused on the way intramembrane particles (IMPs), particularly orthogonal arrays, are organized in the membranes and related to extramembrane components. The basal lamina was composed of a sheet-like network of strands (4-9 nm thick), some, which we have called 'trabecular' strands, extending through the lamina lucida to touch the astrocytic membrane at irregular intervals. The trabecular strands usually formed a bulbous structure where they touched the membrane, but sometimes appeared to intrude directly into the external lipid layer. The orthogonal arrays did not extend to the outer true surface, and no special structure was detectable in association with them. Small spherical protrusions (7-9 nm in diameter), related to neither the trabecular strands nor the arrays, were observed in the outer surface. Judging from their size and distribution, these are probably tops of tall globular IMPs. In the inner or cytoplasmic true surface, protrusions were relatively numerous; some were large, 15-20 nm in diameter, while others were small (8-10 nm). Some of the small protrusions were identified as transmembrane components. Although protrusions were more conspicuous in the inner than in the outer surface, none of them provided images related or similar to the orthogonal arrays. Some protrusions in the inner surface were connected with thin (4-5 nm) or thick (approximately 10 nm) filaments constituting the underlying network. The thin filaments were also anchored to the intermediate filaments which lay parallel with the astrocytic membranes. In the cytoplasm, the intermediate filaments were firmly packed to form bundles. Because the orthogonal arrays are probably embedded within the astrocytic membrane, they may not serve as a transmembrane channel but rather contribute to some stabilizing function for the membrane.

Central mechanisms for apomorphine-induced emesis in the dog

In order to investigate whether different receptor populations mediate emesis induced by intracerebroventricular (i.c.v.) and intravenous (i.v.) apomorphine, adult beagle dogs were tested with various doses of the drug with and without central and peripheral pretreatment with the dopamine antagonist sulpiride. The threshold dose of apomorphine to induce emesis by i.c.v. injections was 30-50 times lower than via the i.v. route, while the response latencies after i.c.v. administration were typically longer and the number of bouts of vomiting greater. I.v. pretreatment with sulpiride was more effective than i.c.v. pretreatment in blocking emesis induced by i.v. apomorphine, whereas both i.v. and i.c.v. sulpiride effectively blocked vomiting after i.c.v. apomorphine. Finally, in separate experiments, surgical interruption of blood flow in the region of the area postrema permanently abolished the emetic response to i.c.v. apomorphine, but only transiently disrupted emesis induced by i.v. apomorphine. These data suggest the possibility that i.v. and i.c.v. apomorphine-induced emesis may be mediated by separate dopamine receptors on the cerebrospinal fluid-side and blood-side of the area postrema.

Relationship between orthogonal arrays of particles and tight junctions as demonstrated in cells of the ventricular wall of the rat brain

Ependymal cells in the ventricular wall and in several circumventricular organs of the rat were compared by means of freeze-fracturing. In principle, tight junctions and orthogonal arrays of particles (OAP) do not coexist in the cells bordering the ventricular wall: (1) Ordinary ependymal cells of the rat possess OAP and are devoid of tight junctions. (2) Epithelial cells of the rat choroid plexus are connected by tight junctions; OAP are lacking here. In some cases, however, tight junctions and OAP coexist in the same cell. In the boundary zone between choroid plexus and ependyma of the rat, the density of OAP is very low, whereas the tight junctions are well developed. In the subfornical and the subcommissural organ (SCO) of the rat both structures are poorly developed; in the SCO they occur segregated in different membranous areas. An overview of the literature confirms that tight junctions and OAP mostly exclude each other. The possibility that in astrocytes and ependymal cells tight junctions may have been replaced by OAP during phylogeny is briefly discussed.

Avenues for entry of peripherally administered protein to the central nervous system in mouse, rat, and squirrel monkey

Pathways traversed by peripherally administered protein tracers for entry to the mammalian brain were investigated by light and electron microscopy. Native horseradish peroxidase (HRP) and wheat germ agglutinin (WGA) conjugated to peroxidase were administered intranasally, intravenously, or intraventricularly to mice; native HRP was delivered intranasally or intravenously to rats and squirrel monkeys. Unlike WGA-HRP, native HRP administered intranasally passed freely through intercellular junctions of the olfactory epithelia to reach the olfactory bulbs of the CNS extracellularly within 45-90 minutes in all species. The olfactory epithelium labeled with intravenously delivered HRP, which readily escaped vasculature supplying this epithelium. Blood-borne peroxidase also exited fenestrated vessels of the dura mater and circumventricular organs. This HRP in the mouse, but not in the other species, passed from the dura mater through patent intercellular junctions within the arachnoid mater; in time, peroxidase reaction product in the mouse brain was associated with the pial surface, the Virchow-Robin spaces of vessels penetrating the pial surface, perivascular clefts, and with phagocytic pericytes located on the abluminal surface of superficial and deep cerebral microvasculature. Blood-borne HRP was endocytosed avidly at the luminal face of the cerebral endothelium in all species. WGA-HRP and native HRP delivered intraventricularly to the mouse were not endocytosed appreciably at the abluminal surface of the endothelium; hence, the endocytosis of protein and internalization of cell surface membrane within the cerebral endothelium are vectorial. The low to non-existent endocytic activity and internalization of membrane from the abluminal endothelial surface suggests that vesicular transport through the cerebral endothelium from blood to brain and from brain to blood does not occur. The extracellular pathways through which probe molecules enter the mammalian brain offer potential routes of passage for blood-borne and air-borne toxic, carcinogenic, infectious, and neurotoxic agents and addictive drugs, and for the delivery of chemotherapeutic agents to combat CNS infections and deficiency states. Methodological considerations are discussed for the interpretation of data derived from application of peroxidase to study the blood-brain barrier.

Actions of angiotensin on area postrema of the rat

Previous studies have reported that rats drink more saline after area postrema has been removed. The results presented here indicate that prolonged administration of angiotensin II into area postrema of unrestrained rats at 4 pmol/h also caused them to drink more saline. They drank more when angiotensin was released in the anterolateral part of the organ than when it was released anteromedially. Diurnal variation of drinking was not disordered. Dose-response curves showed that rats lacking area postrema drank more saline in response to systemic angiotensin than sham operated animals. The very large 'spontaneous' consumption of saline by rats lacking area postrema was not diminished by saralasin, an angiotensin antagonist. It is concluded that area postrema is a site where systemic angiotensin can act to promote sodium consumption: and that although removing area postrema increases the sensitivity of the drinking response to systemic angiotensin, this enhanced sensitivity is not the cause of the increased sodium consumption.

Comparative aspects of the area postrema: fine-structural considerations help to determine its function

The area postrema is a circumventricular organ of the fourth ventricle of the mammalian brain. Although there are distinct gross anatomical differences in the appearance of this organ between "lower" mammals such as rodents and lagomorphs and "higher" mammals such as carnivores and primates, its fine structure is remarkably similar in all species studied. There are many suggestions in the literature for a specific function for this area of the brain, ranging from its being a chemoreceptive trigger zone for the emetic response to its being a regulatory nucleus for the sleep cycle. The present report describes some comparative studies on the ultrastructure of this organ. This information is discussed in relation to what is known about the neurochemistry of the area postrema and its connections with other brain regions and visceral structures. A suggestion is offered that our current knowledge of the area postrema is consistent with its performing many of its proposed functions in the context of a regulatory ("fine-tuning") center for many autonomic functions.

The central organization of the vagus nerve innervating the stomach of the rat

We employed the neural tracers cholera toxin-horseradish peroxidase and wheat germ agglutinin-horseradish peroxidase to examine the organization of the afferent and efferent connections of the stomach within the medulla oblongata of the rat. The major finding of this study is that gastric motoneurons of the dorsal motor nucleus (DMN) possess numerous dendrites penetrating discrete regions of the overlying nucleus of the solitary tract (NTS). In particular, dendritic labelling was present in areas of NTS which also received terminals of gastric vagal afferent fibers such as the subnucleus gelatinosus, nucleus commissuralis, and medial nucleus of NTS. This codistribution of afferent and efferent elements of the gastric vagus may provide loci for monosynaptic vagovagal interactions. A small number of dendrites of DMN neurons penetrated the ependyma of the fourth ventricle and a few others entered the ventral aspect of the area postrema, thus making possible the direct contact of preganglionic neurons with humoral input from the cerebrospinal fluid and/or the peripheral plasma. Nucleus ambiguus neurons projecting to the stomach predominantly innervate the forestomach. The dendrites of these cells, when labelled, were generally short, and extended beyond the compact cluster of ambiguus neurons in a ventrolateral direction, parallel to the fascicles of vagal efferent fibers traversing the medulla.

Tanycytes in the medial habenular nucleus of the rat

Tanycytes with foot processes contacting capillary basal membranes were identified in the rat medial habenular nucleus. They constitute a relatively small but constant population of cells among the conventional ependymal cells. In contrast to tanycytes in most circumventricular organs, habenular tanycytes possess cilia. Superimposed upon the cells are nerve fibers belonging to the serotonergic supraependymal axon plexus. Their ultrastructure differs in many respects from that of hypothalamic tanycytes.

The central neural connections of the area postrema of the rat

We applied the neuroanatomical tracers cholera toxin-horseradish peroxidase and wheat germ agglutinin-horseradish peroxidase to investigate the neural connections of the area postrema (AP) in the rat. We find that the AP projects to the nucleus of the solitary tract (NTS) and dorsal motor nucleus of the vagus bilaterally both rostral and caudal to obex; the nucleus ambiguus; the dorsal aspect of the spinal trigeminal tract and nucelus and the paratrigeminal nucleus; the region of the ventrolateral medullary catecholaminergic column; the cerebellar vermis; and a cluster of structures in the dorsolateral pons which prominently include a discrete set of subnuclei in the lateral parabrachial nucleus. The major central afferent input to the area postrema is provided by a group of neurons in the paraventricular and dorsomedial hypothalamic nuclei whose collective dendrites describe a horizontally oriented plexus which encircles the parvocellular nucleus of the hypothalamus bilaterally. In addition, the caudal NTS may project lightly to the AP. The lateral parabrachial nucleus provides a very light input as well. These connections, when considered in the context of the known vagal afferent input and reduced blood-brain barrier of AP, place this structure in a unique position to receive and modulate ascending interoceptive information and to influence autonomic outflow as well.

A serotonin-containing pathway from the area postrema to the parabrachial nucleus in the rat

A combined retrograde tracing-immunofluorescent technique was used to identify the relationships between the cellular population projecting to the parabrachial nucleus and the serotonin-immunoreactive cell population of the area postrema in rats. The retrograde fluorescent tracer True Blue was injected in the parabrachial region and 3 days later the animals were perfused. Serial cryostat sections were processed for serotonin immunofluorescence. Three different groups of labeled cells were identified in the area postrema. First, True Blue-positive cells (up to 250/section) that project to the parabrachial nucleus were observed distributed throughout the area postrema. Second, in the pargyline (a monoamine oxidase inhibitor)-treated animals a large number of serotonergic cells (up to 125/section) was observed distributed throughout the area postrema. There was a tendency to a heavier distribution of serotonin-immunoreactive cells in the dorsal two-thirds of the area postrema. Third, double-labelled cells were also seen. Twenty percent of the True Blue-labelled cells projecting to the parabrachial nucleus were serotonin-immunoreactive. Thirty nine percent of the serotonin-immunoreactive population was retrogradely labelled with True Blue. Thus a new serotonergic pathway from the area postrema to the parabrachial nucleus is described; this pathway may be important in the ascending transmission and modulation of chemical and visceral sensory input.

Discrete lesions of the area postrema abolish radiation-induced emesis in the dog

Studies carried out in several mammalian species during the 1950's led to the concept of a 'vomiting center' located in the dorsolateral reticular formation and a 'chemoreceptor trigger zone' (CTZ) within or near the area postrema (AP). This early work suggested that the AP was essential for vomiting induced by a variety of chemical emetics and by ionizing radiation. However, the lesion techniques used often produced significant damage to neural tissue underlying the AP, as well as to the AP itself, making localization of function very difficult. In the present study, electrolytic lesions confined to the AP abolished both radiation- and apomorphine-induced emesis in dogs. Thus, in addition to its postulated function in osmoreception and central cardiovascular regulation, the AP also appears to have a key role in vomiting initiated by chemical emetics and by ionizing irradiation.

Cytochemical characteristics of astrocytic plasma membranes specialized with numerous orthogonal arrays

Astrocytic membranes contacting the basal lamina are found to be less affected by filipin than subjacent lateral membranes. An abrupt change in density of lesions induced by filipin creates a border between subpial and lateral membranes at the glia limitans. This means that orthogonal array-crowded membranes may contain relatively less cholesterol than other astrocytic membrane domains. Another possible explanation for filipin resistance is also considered in relation to aggregated intramembrane particles of orthogonal arrays and/or membrane-associated filamentous elements including the basal lamina. The polygonal particle junction between astrocytic processes located just below the subpial membrane is strongly resistant to the action of filipin. Both membrane-associated enzymes, i.e. alkaline phosphatase (AlkPase) and Na+,K+-ATPase are commonly detected only in perivascular astrocytic membranes, and not in subpial membranes, suggesting a regional differentiation in function of astrocytic membranes. There are variations in the reactive deposits particularly of those for Na+,K+-ATPase. It is apparent that the distribution polarity of orthogonal arrays is not connected with that of either AlkPase or Na+,K+-ATPase. Judging from the relative resistance to filipin, however, astrocytes throughout the C.N.S., having domains specialized with orthogonal arrays, may possess a unique stabilizing mechanism for their own membranes contacting the basal lamina.

Area postrema/nucleus of the solitary tract ablations: analysis of the effects of hypophagia

The effect of hypophagia following lesions of the area postrema and caudal-medial aspect of the nucleus of the solitary tract AP/cmNTS) on body-weight, water intake and preference for palatable diets was examined. Following AP/cmNTS ablation, rats reduced pelleted-food intake to a degree which was sufficient to account for the weight loss and increased water:food ratios observed. Restricting food intakes of intact rats to levels taken by lesioned animals resulted in similar weight losses and increased water:food ratios. When offered both pelleted food and milk, lesioned rats took more calories as milk than did previously food-restricted intact rats. Thus, the hypophagia of AP/cmNTS lesioned rats does not account for their increased preference for milk diets. Lesioned rats ate less high-fat diet than did intact or sham-lesioned controls and did not increase their intakes when this diet was sweetened. At autopsy, retroperitoneal and epididymal fat-pad weights accounted for less of the total body weight of lesioned animals than controls suggesting that body-fat levels are reduced following AP/cmNTS ablation.

Filipin resistance in intermediate junction membranes of guinea pig ependyma: possible relationship to filamentous underlying

Plasma membranes in intermediate junctions of ependymal cells are found to show considerable resistance to the antibiotic filipin, suggesting low cholesterol in these membranes. Further, ependymal cells were treated with cytochalasin B (CB) infused into the cerebral ventricle in vivo, and then incubated with filipin. When treated with CB, intermediate junctions show a decrease in their underlying density, mainly composed of microfilaments, and their membranes are found to be more affected by filipin. This reduction of resistance to the antibiotic is clearly demonstrated by thin-section and freeze-fracture as well as quantitative analysis. Nonjunctional lateral membranes, however, show no significant difference in the degree of filipin effect whether treated with CB or not. Although biochemical data on lipid composition have not been available for the intermediate junction membranes, we bring forward a possibility that resistance to filipin in these membranes may come not from less cholesterol but from morphological membrane stability brought about by the filamentous underlying.

Intercellular junctions between specialized ependymal cells in the subcommissural organ of the rat

The permeability of intercellular junctions in specialized ependymal cells in the rat subcommissural organ (SCO) has been studied ultrastructurally by freeze-fracturing and tracer experiments with horseradish peroxidase (HRP). In addition to normal smooth membrane, areas which could be classified as a leaky tight junction are found within the ependymal junctional region. This consists of only one or two relatively continuous strands but with interruptions in the apical portion. Some strands are perpendicular to the apical membrane surface and often form hairpin-like bends in the basal portion of the junction. The junctional region also shows areas with no strands but only a rippled membrane structure which may be equivalent to very close appositions without fusion of adjacent ependymal cell membranes. The relative proportions of normal smooth membrane, strands and rippled structure in the junctional region is approximately 3:4:6 including two parts overlapping of the strands and rippled structure. Intraventricularly infused HRP passes through many junctions but is occasionally stopped, leaving unstained intercellular spaces of various lengths between membrane fusions of tight junctions. Even when it is stopped, the intercellular space below the junction is densely stained by the enzyme. Orthogonal arrays of intramembrane particles are found to be distributed on the basal and lateral cell membranes below the junctional region in the SCO ependyma.

Fine structural studies on ependymal paracellular and capillary transcellular permeability in the subcommissural organ of the guinea pig

Morphological investigations on the permeability of intercellular junctions between ependymal cells and between capillary endothelial cells in the subcommissural organ (SCO) of the guinea pig have been carried out using freeze-fracturing and tracer experiments with horseradish peroxidase (HRP). The ependymal junction reveals a moderately developed network of tight junctional strands surrounding the tall ependymal cell. The apical portion of this junctional network tends to form nearly complete strands, whereas the basal portion usually shows irregular, fragmented strands often arranged in hairpin-like structures. The passage of intraventricularly infused HRP is blocked, leaving unstained areas, at the level of membrane fusions. At the same time the lateral intercellular space below the junction is densely stained, probably due to invasion from the basal side through adjacent ordinary ependymal junctions. The SCO capillary endothelium shows a high distribution density of pinocytotic vesicles. Vesicular transport of intravascularly injected HRP is observed, but no HRP penetration occurs through the endothelial junction. The active participation of vesicles in tracer movement is shown in preparations fixed before administration of HRP. Extravasation of this tracer is indicated to some degree in the SCO capillary, but permeability here appears to be comparable to that of ordinary brain capillaries. Accordingly, the SCO ependymal tight junction seems to form an effective barrier not to blood plasma or similar materials but to apically secreted substances, preventing them from spreading back into SCO intercellular spaces.

Fine structure of ependymal cysts in and around the area postrema of the rat

Peculiar cells forming cysts were observed in the area postrema and sometimes also in the choroid plexus and the tela chorioidea near the area postrema, and were studied in detail by electron microscopy. The cytological features of the cyst cell and its junctional relationship to neighboring cells imply that cyst cells are derived from ependymal and choroid epithelial cells. The cyst cells usually contact directly the perivascular spaces of postremal, choroidal or pial capillaries, where the cytoplasm is often considerably attenuated. The cystic lumen is commonly filled with a flocculent material. The limiting membrane of the cystic lumen, which frequently bears cilia and microvilli, has the same thickness as the surface cell membrane. In many cases the cyst is surrounded by the cytoplasm of a single cell. In some cases, however, two cells participating in the formation of the cyst, although one is only a slender process and joined by a zonula occludens with the main cyst cell. Horseradish peroxidase (HRP) injected into the cerebrospinal fluid (CSF) space failed to enter the cystic lumen. A possible significance of the cyst in relation to the CSF and blood circulation was considered.