Horseradish peroxidase: a tool for study of the neuroendocrine cell and other peptide-secreting cells

Murine MPDZ-linked hydrocephalus is caused by hyperpermeability of the choroid plexus

Though congenital hydrocephalus is heritable, it has been linked only to eight genes, one of which is MPDZ. Humans and mice that carry a truncated version of MPDZ incur severe hydrocephalus resulting in acute morbidity and lethality. We show by magnetic resonance imaging that contrast medium penetrates into the brain ventricles of mice carrying a Mpdz loss‐of‐function mutation, whereas none is detected in the ventricles of normal mice, implying that the permeability of the choroid plexus epithelial cell monolayer is abnormally high. Comparative proteomic analysis of the cerebrospinal fluid of normal and hydrocephalic mice revealed up to a 53‐fold increase in protein concentration, suggesting that transcytosis through the choroid plexus epithelial cells of Mpdz KO mice is substantially higher than in normal mice. These conclusions are supported by ultrastructural evidence, and by immunohistochemistry and cytology data. Our results provide a straightforward and concise explanation for the pathophysiology of Mpdz‐linked hydrocephalus.

Monitoring autophagy in Alzheimer's disease and related neurodegenerative diseases

This chapter describes detailed methods to monitor autophagy in neurodegenerative disorders, especially in Alzheimer's disease. Strategies to assess the competence of autophagy-related mechanisms in disease states ideally incorporate analyses of human disease and control tissues, which may include brain, fibroblasts, or other peripheral cells, in addition to animal and cell models of the neurodegenerative disease pathology and pathobiology. Cross-validation of pathophysiological mechanisms in the diseased tissues is always critical. Because of the cellular heterogeneity of the brain and the differential vulnerability of the neural cells in a given disease state, analyses focus on regional comparisons of affected and unaffected regions or cell populations within a particular brain region and include ultrastructural, immunological, and cell and molecular biological approaches.

Morphologic evidence that neurokinin B modulates gonadotropin-releasing hormone secretion via neurokinin 3 receptors in the rat median eminence

Recent studies suggest that arcuate neurokinin B (NKB) neurons play a role in the regulation of gonadotropin secretion, but there is little information on the relationship between these neurons and the hypothalamic reproductive axis. In the present study, dual-label fluorescent immunohistochemistry was used to visualize the relationship between gonadotropin-releasing hormone (GnRH) neurons and either proNKB or NK3 receptor (NK3R) immunoreactivity. Immunocytochemistry was also combined with i.p. injections of the fluorescent retrograde tracer aminostilbamidine to determine whether arcuate neuroendocrine neurons expressed either proNKB or NK3R. A dense interweaving and close apposition of GnRH and proNKB-immunoreactive (ir) fibers was observed within the rat median eminence, where GnRH axons expressed NK3R immunoreactivity. These data provide morphological evidence that NKB neurons could influence GnRH secretion via interaction with NK3R in the rat median eminence. Colocalization of GnRH and NK3R was also identified in fiber tracts converging within the organum vasculosum of the lamina terminalis. In contrast, only a small number (16%) of GnRH-ir somata exhibited NK3R staining. ProNKB and NK3R-ir somata were identified within the arcuate nucleus, but none of these neurons were labeled by aminostilbamidine. Thus, we found no evidence that arcuate NKB neurons project to the primary capillary plexus of the portal system. Arcuate neuroendocrine neurons, however, were surrounded and closely apposed by proNKB-ir puncta and fibers. These data suggest that NKB neurons could indirectly influence anterior pituitary function by inputs to arcuate neuroendocrine neurons, but through a receptor other than NK3R. Our results provide an anatomic framework for putative interactions between NKB neurons and the hypothalamic reproductive axis.

Ovarian steroids differentially modulate the gene expression of gonadotropin-releasing hormone neuronal subtypes in the ovariectomized cynomolgus monkey

In the present study, we compared the morphology and distribution of neurons expressing GnRH gene transcripts in the hypothalamus and forebrain of the cynomolgus monkey to that of the human. As in the human, three subtypes of GnRH neurons were identified. Type I GnRH neurons were small, oval cells with high levels of gene expression and were located within the basal hypothalamus. Type II GnRH neurons were small and sparsely labeled and were widely scattered in the hypothalamus, midline nuclei of the thalamus, and extended amygdala. Type III neurons displayed magnocellular morphology and intermediate labeling intensity and were located in the nucleus basalis of Meynert, caudate, and amygdala. In a second experiment, we determined the effect of estrogen or estrogen plus progesterone on the gene expression of GnRH neurons in the brains of young, ovariectomized cynomolgus monkeys. We report that hormone treatment resulted in a significant decrease in GnRH mRNA in type I neurons within the basal hypothalamus of ovariectomized monkeys. In contrast, there was no effect of hormone treatment on the gene expression of type III GnRH neurons in the nucleus basalis of Meynert. The present findings provide evidence that the increase in gene expression of type I GnRH neurons in postmenopausal women is secondary to the ovarian failure of menopause. The differential responses of type I and III GnRH neurons to hormone treatment provide additional evidence that distinct subpopulations of neurons expressing GnRH mRNA exist in the primate hypothalamus.

Blood to brain and brain to blood passage of native horseradish peroxidase, wheat germ agglutinin, and albumin: pharmacokinetic and morphological assessments

Native horseradish peroxidase (HRP) and the lectin wheat germ agglutinin (WGA) conjugated to HRP are protein probes represented in the blood-brain barrier (BBB) literature for elucidating morphological routes of passage between blood and brain. We report the application of established pharmacokinetic methods, e.g., multiple-time regression analysis and capillary depletion technique, to measure and compare bidirectional rates of passage between blood and brain for radioactive iodine-labeled HRP (I-HRP), WGA-HRP (I-WGA-HRP), and the serum protein albumin (I-ALB) following administration of the probes intravenously (i.v.) or by intracerebroventricular (i.c.v.) injection in mice. The pharmacokinetic data are supplemented with light and electron microscopic analyses of HRP and WGA-HRP delivered i.v. or by i.c.v. injection. The rates of bidirectional movement between blood and brain are the same for coinjected I-HRP and I-ALB. Blood-borne HRP, unlike WGA-HRP, has unimpeded access to the CNS extracellularly through sites deficient in a BBB, such as the circumventricular organs and subarachnoid space/pial surface. Nevertheless, blood-borne I-WGA-HRP enters the brain approximately 10 times more rapidly than I-HRP and I-ALB. Separation of blood vessels from the neocortical parenchyma confirms the entry of blood-borne I-WGA-HRP to the brain and sequestration of I-WGA-HRP by cerebral endothelial cells. Nearly half the I-WGA-HRP radioactivity associated with cortical vessels is judged to be subcellular. Light microscopic results suggest the extracellular pathways into the brain available to blood-borne native HRP do not represent predominant routes of entry for blood-borne WGA-HRP. Ultrastructural analysis further suggests WGA-HRP is likely to undergo adsorptive transcytosis through cerebral endothelia from blood to brain via specific subcellular compartments within the endothelium. Entry of blood-borne I-WGA-HRP, but not of I-ALB, is stimulated with coinjected unlabeled WGA-HRP, suggesting the latter may enhance the adsorptive endocytosis of blood-borne I-WGA-HRP. With i.c.v. coinjection of I-WGA-HRP and I-ALB, I-WGA-HRP exists the brain more slowly than I-ALB. The brain to blood passage of I-WGA-HRP is nil with inclusion of unlabeled WGA-HRP, which does not alter the exist of I-ALB. Adsorptive endocytosis of i.c.v. injected WGA-HRP appears restricted largely to cells lining the ventricular cavities, e.g., ependymal and choroid plexus epithelia. In summary, the data suggest that the bidirectional rates of passage between brain and blood for native HRP are comparable to those for albumin.

Morphology and function of capillary networks in subregions of the rat tuber cinereum

The differentiated cytology, cytochemistry, and functions within subdivisions of the tuber cinereum prompted this morphometric and physiological investigation of capillaries in the medium eminence and arcuate nucleus of albino rats. Morphometric studies established that the external zone of the median eminence had 3-5 times the number and surface area of true and sinusoidal capillaries than the internal or subependymal median eminence zones, or either of two subdivisions examined in the arcuate nucleus. Type-I true capillaries, around which Virchow-Robin spaces comprise 1% of arcuate tissue area, were situated proximally to the median eminence border. This finding is consistent with a premise that confluent pericapillary spaces enable infiltration of arcuate neurons by factors from capillary blood from the median eminence or Virchow-Robin spaces. Physiologically, the rate of penetration across the median eminence capillaries by blood-borne [14C]alpha-amino-isobutyric acid (a neutral amino acid used as a capillary permeability tracer) was 142 times greater than for capillaries in the distal arcuate nucleus within 12 s of tracer administration. A new finding was that the proximal arcuate nucleus had a permeability x surface area product of 69 microliters g-1 min-1, 34 times greater than that in more distal aspects of the tuber where blood-brain barrier properties exist. We also found that the microcirculatory transit time of a plasma space marker, [14C]sucrose, was considerably longer (1.2 s) in the median eminence and proximal arcuate nucleus than in the distal arcuate or ventromedial nucleus (0.4 s). By virtue of its high capillary permeability and extensive blood-tissue surface area, including the wide Virchow-Robin spaces, the median eminence external zone could be a gateway for flooding other tuberal compartments with blood-borne factors. This effect may be compounded by capillary bed specializations in the proximal arcuate nucleus where Type-I true capillaries, Type-III sinusoids, and pericapillary spaces are confluent with those in the median eminence. The results indicate that the proximal arcuate parenchyma could be exposed to circulating neuroactive substances on a moment-to-moment basis.

Blood-brain barrier alteration after microwave-induced hyperthermia is purely a thermal effect: I. Temperature and power measurements

The effect of microwave-induced hyperthermia on the blood-brain barrier was studied in 21 Sprague-Dawley rats. Under sodium pentobarbital anesthesia, animals were place in a stereotactic frame, and an interstitial microwave antenna operating at 2450 MHz was inserted in a bony groove drilled parallel to the sagittal suture. Some antennae were equipped with an external cooling jacket. Temperature measurements were made lateral to the antenna by fluoroptical thermometry, and power was calculated from the time-temperature profile. Five minutes prior to termination of microwave irradiation, horseradish peroxidase (1 mg/20 g body weight) was injected intravenously. Extravasation of horseradish peroxidase was observed in brain tissue heated above 44.3 degrees C for 30 minutes and at 42.5 degrees C for 60 minutes. Microwave irradiation failed to open the blood-brain barrier when brain temperatures were sustained below 40.3 degrees C by the cooling system. Extravasation of blood-borne peroxidase occurred at sites of maximal temperature elevation, even when these did not coincide with the site of maximum power density. The data suggest that microwave-induced hyperthermia is an effective means for opening the blood-brain barrier and that the mechanism is not related to the nonthermal effect of microwaves.

A small subpopulation of progesterone receptor-containing neurons in the guinea pig arcuate nucleus projects to the median eminence

In female guinea pigs, a combination of retrograde tracing and immunofluorescence for progesterone receptors (PR) was applied to determine if PR-immunoreactive (PR-IR) neurons in the arcuate nucleus (AR) send their axons directly to the median eminence (ME). Axonal projections to the ME were studied by different techniques using fluorescent dyes. From 31 adult animals, ovariectomized and primed by estradiol, small deposits of Lucifer Yellow (LY) were made on the cut surface of the ME, either by direct application of LY crystals or by iontophoresis. These techniques were carried out on excised mediobasal hypothalamus maintained in vitro and allowed visualization of AR perikarya projecting to the ME after dye diffusion in the severed axons. In another group of ten immature animals primed by estradiol, Granular Blue (GB) was injected in the jugular vein. Blood-borne GB was taken up in the ME by intact nerve endings and retrogradely transported to the perikarya of origin. PR-IR neurons and perikarya filled with LY or retrogradely labeled by GB were intermingled with each other throughout the rostrocaudal extent of the AR. Double-labeled cells, displaying PR immunoreactivity and dye labeling, were observed consistently, but their number was small. This result demonstrates that some AR neurons sending axonal projections to the ME are target cells for progesterone. As the majority of PR-IR neurons in the AR do not project to the ME, it is suggested that most PR-IR neurons present in this nucleus form local circuit projections or project to distant areas of the central nervous system.

Transcytosis of macromolecules through the blood-brain barrier: a cell biological perspective and critical appraisal

A critical appraisal is presented of nearly two decades of research publications and review articles advocating the bidirectional transcytosis of fluid-phase molecules, most notably native horseradish peroxidase (HRP), through the normal and experimentally modified blood-brain barrier (BBB). Extracellular routes circumventing the BBB in normal and pathological states and artifact introduced in histological preparation of CNS tissue exposed to blood-borne peroxidase are emphasized. The potential for transcytosis of macromolecules entering the nonfenestrated cerebral endothelium by the processes of non-specific fluid phase endocytosis (e.g., HRP), adsorptive endocytosis (e.g., lectins) and receptor-mediated endocytosis (e.g., ligands) is analyzed in the context of the cellular secretory process and the complimentary events of endocytosis and exocytosis at the luminal and abluminal plasma membranes. Available data suggest that the cerebral endothelium is polarized with regard to endocytosis and the internalization of cell surface membrane; hence, the transcytosis of specific macromolecules through the BBB may be vectorial. If these data are correct, the blood-brain barrier is not absolute, whereas its counterpart, the brain-blood barrier, may be.

Transcytosis of protein through the mammalian cerebral epithelium and endothelium. I. Choroid plexus and the blood-cerebrospinal fluid barrier

The potential for transcytosis (endocytosis----intracellular transport----exocytosis) of protein and membrane events associated with fluid phase and adsorptive endocytic processes within epithelia of the choroid plexus [blood-cerebrospinal fluid (CSF) barrier] were investigated in mice injected intravenously or into the lateral cerebral ventricle with native horseradish peroxidase (HRP) or the lectin wheatgerm agglutinin (WGA) conjugated to HRP. WGA binds to specific cell surface oligosaccharides and enters cells by the process of adsorptive endocytosis; native HRP is taken into cells non-specifically by fluid phase endocytosis. The lysosomal system of organelles and the endoplasmic reticulum, identified by enzyme cytochemical markers applied to choroid epithelia, were analysed for possible participation in transcytosis and compared to epithelial organelles harbouring the exogenous tracer proteins. Blood-borne native HRP was endocytosed readily by choroid epithelia whereas WGA-HRP was not, perhaps because WGA-HRP does not escape fenestrated endothelia as easily as native HRP. The blood-borne proteins incorporated within endocytic vesicles by choroid epithelia were directed to endosomes (prelysosomes) and secondary lysosomes (e.g. tubules, multivesicular/dense bodies) for eventual degradation and did not reach the apical/microvillus surface. Both CSF-borne native HRP and WGA-HRP entered choroid epithelia within endocytic vesicles derived from the microvillus border. Native HRP, ultimately sequestered within endosomes and secondary lysosomes, failed to undergo transcytosis through the epithelia into the basolateral clefts. Conversely, CSF-borne WGA-HRP was transported through the epithelia and released into the basolateral clefts within 10 min post-injection. The lectin conjugate labelled epithelial vesicles, endosomes, secondary lysosomes and, at 30 min post-injection, the transmost saccule of the Golgi complex which exhibits acid hydrolase activity. Tubular profiles, related either to the endosome apparatus or to the lysosomal system, and the endoplasmic reticulum did not appear involved in the transcytotic pathway. The data suggest that CSF-borne protein entering the choroid epithelium by adsorptive endocytosis can undergo rapid transcytosis through the cell. The results provide insight to transcytotic pathways utilizing vesicles, the endosomal apparatus, and the Golgi complex within the choroid epithelium for circumventing the blood-CSF barrier. Hypothesized membrane events and morphological associations among constitutents of the endomembrane system within the choroid epithelium are summarized diagrammatically.

Transcytotic pathway for blood-borne protein through the blood-brain barrier

The transcytosis of blood-borne protein through the blood-brain barrier, a consequence of recruitment of the Golgi complex within nonfenestrated cerebral endothelia, was identified in mice and rats injected intravenously with the lectin wheat germ agglutinin (WGA) conjugated to the enzymatic tracer horseradish peroxidase (HRP). WGA enters cells by adsorptive endocytosis after binding to specific cell surface oligosaccharides. Blood-borne WGA-HRP labeled the entire cerebrovascular tree from the luminal side 5 min after injection; pericytes, located on the abluminal surface of cerebral endothelia, sequestered the lectin conjugate 6 hr later. Endothelial organelles harboring WGA-HRP 3 hr after injection included the luminal plasmalemma, endocytic vesicles, endosomes (prelysosomes), secondary lysosomes, and the Golgi complex. The peroxidase reaction product labeled the abluminal surface of cerebral endothelia and occupied the perivascular clefts by 6 hr. Within 12 hr, organelles labeled with WGA-HRP in pericytes were identical to those observed in endothelia. Blood-borne native HRP, entering cells by bulk-phase endocytosis, was neither directed to the Golgi complex nor transferred across nonfenestrated cerebral endothelia. The results suggest that blood-borne molecules taken into the cerebral endothelium by adsorptive endocytosis and conveyed to the Golgi complex can, either by themselves or as vehicles for other molecules excluded from the brain, undergo transcytosis through the blood-brain barrier without compromising the integrity of the barrier.

Tubular profiles do not form transendothelial channels through the blood-brain barrier

The contribution of tubular profiles within the mammalian cerebral endothelium to the formation of transcellular channels was analysed following exposure of the endothelium to native horseradish peroxidase (HRP) dissolved in saline or dimethyl sulphoxide (DMSO) administered intravenously in mice. Within 5-15 min, but not at 30 min to 2 h postinjection, peroxidase-positive extravasations were evident within the parenchyma of the forebrain and brainstem of mice exposed and not exposed to DMSO. The extravasations may be associated with the rupture of interendothelial tight junctions at the level of arterioles as a consequence of the perfusion-fixation process. Ultrastructural inspection of endothelia within and away from areas of peroxidase extravasation revealed the following intraendothelial, peroxidase-positive organelles: presumptive endocytic vesicles, endosomes (a prelysosomal compartment), multivesicular and dense bodies, and tubular profiles. Statistical analysis of the concentration of HRP-labelled presumptive endocytic vesicles, which may coalesce to form tubules, within endothelia from mice injected intravenously with HRP-DMSO compared to mice receiving HRP-saline revealed no significant difference. HRP-positive tubular profiles were blunt-ended, variable in length and width, and appeared free in the cytoplasm or in continuity with dense bodies. Labelled tubules free in the cytoplasm were positioned parallel to the luminal and abluminal plasma membranes and were less frequently oblique or perpendicular to these membranes. Tubular profiles analysed in serial thin sections or with a goniometer tilt stage did not establish membrane continuities with the luminal and abluminal plasma membranes. Peroxidase-positive tubular profiles were similar morphologically to those exhibiting acid hydrolase activity but did not share morphological and enzyme cytochemical similarities with the endoplasmic reticulum that stained for glucose-6-phosphatase (G6Pase) activity. G6Pase-positive profiles of endoplasmic reticulum were not observed to contribute to a transendothelial canalicular network. Our results suggest that: (i) peroxidase-labelled tubules, acid hydrolase-positive tubules, and G6Pase-positive endoplasmic reticulum do not form transcellular channels through the cerebral endothelium; (ii) tubular profiles labelled with blood-borne HRP in the cerebral endothelium are associated with the endosome apparatus and/or the lysosomal system of organelles; and (iii) DMSO does not appear to alter the permeability of the blood-brain barrier to blood-borne protein.

Angioarchitecture of the CNS, pituitary gland, and intracerebral grafts revealed with peroxidase cytochemistry

Blood vessels of the fetal, neonatal, and adult subprimate and primate CNS, including circumventricular organs (e.g., median eminence, pituitary gland, etc.), and of solid CNS and nonneural (anterior pituitary gland) allografts placed within brains of adult mammalian hosts were visualized with peroxidase cytochemistry applied in three ways: to tissues from animals injected systemically with native horseradish peroxidase (HRP) or peroxidase conjugated to the lectin wheat germ agglutinin (WGA) prior to perfusion fixation; to tissues from animals infused with native HRP into the aorta subsequent to perfusion fixation; and to tissues from animals fixed by immersion and incubated for endogenous peroxidase activity in red cells retained within blood vessels. In neonatal and adult animals receiving native HRP intravascularly, non-fenestrated vessels contributing to a blood-brain barrier were outlined with HRP reaction product when tetramethylbenzidine (TMB) as opposed to diaminobenzidine (DAB) was used as the chromogen; fenestrated vessels of circumventricular organs were not discernible due to the density of extravascular reaction product. Fenestrated and non-fenestrated cerebral and extracerebral blood vessels exposed to bloodborne WGA-HRP were visible when incubated in TMB and DAB solutions. Native HRP infused into the aorta of fixed animals likewise labeled non- fenestrated vessels throughout the brain upon exposure to TMB or DAB but obscured fenestrated vessels of the circumventricular organs. Endogenous peroxidase activity of red cells, seen equally well with TMB and DAB, outlined blood vessels throughout the cerebral gray and white matter and all circumventricular organs in fetal, neonatal, and adult animals. Application of the three peroxidase cytochemical approaches to study the development or absence of a blood-brain barrier in intracerebral allografts demonstrated that the vascularization of day 16-19 fetal/1 day neonatal CNS allografts is not well defined prior to 7 days following intracerebral placement of the grafts. CNS allografts secured from donor sites expected to possess a blood-brain barrier exhibited blood vessels that were not leaky to HRP injected intravenously in the host. Fenestrated blood vessels associated with anterior pituitary allografts were evident prior to 3 days posttransplantation within the host brain and permitted blood-borne HRP in the host to enter the graft and surrounding host brain parenchyma.

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.

Wheat germ agglutinin binding sites in the adult mouse cerebellum: light and electron microscopic studies

The binding properties of derivatized wheat germ agglutinin (WGA) have been examined in fixed tissue sections from the adult mouse cerebellum and also in axonal tracing paradigms following cerebellar injections. The aim of these studies is to begin to distinguish the roles different binding sites may play in generating diverse biological activities which lead to neuronal uptake and axonal transport of lectins or glycoconjugates. Vibratome sections from aldehyde-fixed cerebellum were incubated in N-[acetyl-3H] WGA or WGA conjugated to horseradish peroxidase (WGA-HRP). Sections from this in vitro binding paradigm and those from cerebellar pressure injection cases using those tracers (in vivo binding paradigm) were processed for light microscopic autoradiography, histochemistry, and electron microscopy. Blocking experiments were also performed with various sugar haptens to confirm the binding specificity of these lectin preparations. Light microscopy of lectin binding patterns within the cerebellar cortex has revealed that both derivatized WGA preparations bind most intensely to the molecular layer. Within the deep cerebellar nuclei, binding is unique and produces a punctate delineation of cell bodies and dendrites. Electron microscopy revealed that these binding sites are associated with glial processes which abut the plasma membrane of deep nuclei cells. Cerebellar WGA-HRP injection sites contain labeled profiles involved in uptake and axonal transport of the labeled lectin (e.g., multivesicular and dense bodies) in addition to label associated with synapses, glia, undetermined components of the extracellular space, and neuronal plasma membranes. These sites are therefore presumed to possess a high affinity or capacity for binding derivatized WGA. Binding studies performed here thus reveal, for the first time, the existence of discrete glial sites that display an extraordinary attraction for lectins such as WGA. The roles such glial glycoconjugates play in diverse biological activities including neuronal uptake and transport of macromolecules need further study.

Cytology of brain stem neurons projecting to the caudal neurosecretory complex: an HRP-electron microscopic study

Brain stem projections to the neurons in the caudal neurosecretory complex (CNC) of Poecilia sphenops (molly) have been studied with HRP retrograde tracing. Using light microscopic procedures, HRP-labelled neurons were located in the reticular nucleus of the medulla (RMN) and in the vicinity of the nucleus of the medial longitudinal fascicle (NMLF). The present study was undertaken to examine the cytology of the brain stem neurons that project to the caudal neurosecretory complex using combined HRP-electron microscopic methods. Cells in the midbrain NMLF containing HRP-filled profiles were located bilaterally close to the midline and just beneath the ependyma. HRP reaction product was found additionally in the neurosecretory cells of the midbrain dorsal tegmental magnocellular nucleus (DTMN) located dorsal to the NMLF. Cells containing HRP-labelled profiles were also seen in the RMN and in neurons dorsal-lateral to the RMN. This latter group of neurons contained small dense core vesicles in addition to HRP labelled dense bodies.

Endocytic and exocytic pathways of the neuronal secretory process and trans-synaptic transfer of wheat germ agglutinin-horseradish peroxidase in vivo

The lectin wheat germ agglutinin (WGA) conjugated to horseradish peroxidase (HRP) was employed to study the endocytic and exocytic pathways of the secretory process in neurons and the potential for trans-synaptic transfer of molecules within the CNS. WGA-HRP binds to surface membrane oligosaccharides and enters cells by adsorptive endocytosis. The lectin conjugate was administered intranasally or into the cerebral ventricles of mice; postinjection survival times ranged from 5 minutes to 6 days. Due to binding of the lectin to ependymal cells subsequent to an intraventricular injection, only select populations of neurons (i.e., hippocampal formation; paraventricular nuclei; midbrain raphe; VI, X, XII motor nuclei; among others) were exposed extracellularly to WGA-HRP and became labeled by retrograde axoplasmic transport from axon terminals or by direct cell body/dendritic uptake. WGA-HRP delivered intranasally was endocytosed by first-order olfactory neurons and transported by anterograde axoplasmic flow to the terminal field within the glomerular layer of the main olfactory bulb; eventually perikarya of the mitral cell layer were labeled, presumably by anterograde trans-synaptic transfer of the lectin conjugate. In the variety of neurons analyzed ultrastructurally following exposure to WGA-HRP, the proposed sequence of intracellular pathways through which peroxidase reaction product was traced over time was: cell surface membrane----endocytic structures----endosomes (presecondary lysosomes)----transfer vesicles----transmost Golgi saccule----vesicles, vacuoles, and/or dense core granules. WGA-HRP also labeled vesicles and tubules that were channeled to and/or derived from spherical endosomes, dense bodies, and multivesicular bodies. The peroxidase-positive, membrane-delimited products of the trans Golgi saccule contributed to anterograde axonal transport vectors and accumulated within axon terminals. A second contribution to these vectors was provided by peroxidase-labeled tubules and dense bodies believed to represent components of the lysosomal compartment. Profiles of the axonal reticulum comparable to those that stained cytochemically for glucose-6-phosphatase activity, a marker for the endoplasmic reticulum, were not associated with the transport of WGA-HRP. Trans-synaptic transfer of WGA-HRP from primary olfactory neurons to postsynaptic cells in the olfactory bulb was reflected in peroxidase-positive endocytic vesicles, endosomes, dense bodies, and the trans Golgi saccule.(ABSTRACT TRUNCATED AT 400 WORDS)

The neuronal endoplasmic reticulum: its cytochemistry and contribution to the endomembrane system. II. Axons and terminals

The morphology and cytochemistry of the endoplasmic reticulum (ER) in axons and terminals of a number of different types of neurons in brains from mice were investigated ultrastructurally. The neurohypophysis received particular attention because the morphology and enzyme cytochemical activities of many of the preterminal swellings of hypothalamo-neurohypophysial axons are altered by chronic salt-stress. Membrane contrast and enzyme cytochemical staining techniques were employed to characterize the axonal reticulum and to determine if organelles representing the lysosomal system in the axon and the tubular profiles participating in the anterograde axonal transport of native horseradish peroxidase (HRP) are associated with the ER. Potential enzyme cytochemical markers for the axonal ER included glucose-6-phosphatase (G6Pase), thiamine pyrophosphatase, nucleoside diphosphatase, and acid hydroxylase activities. The anterograde transport of HRP was analyzed in undamaged hypothalamo-neurohypophysial neurons and in facial and hypoglossal motoneurons of mice receiving the protein in the lateral cerebral ventricle. The ER pervaded the axon and appeared as parallel, 20-40-nm-wide tubules interconnected by oblique anastomoses. Membrane thickness of the axonal reticulum measured 60-100 A, which is similar to that of the perikaryal ER. Enzyme cytochemical activities associated with the ER or lysosomes were not conspicuous in axons and terminals under normal conditions but became prominent in some axons and preterminal swellings manifesting an autophagic appearance within neurohypophyses from salt-stressed mice. Only G6Pase activity was a marker for the ER in these axons and preterminals. Many ER profiles in non-incubated sections and in G6Pase cytochemical preparations of salt-stressed neurohypophyses were wrapped around or interspersed among secretory granules, multilamellar bodies, and vacuoles that may represent forms of lysosomes involved in autophagy and crinophagy. Acid hydrolase activities were localized within the vacuoles as well as within 80-130-nm-wide, blunt-ended tubules in pituitary stalk axons; similar reactive tubules were confluent with large secondary lysosomes in neurosecretory cell bodies and may be derived from these lysosomes. Morphologically identical tubules transporting HRP in the anterograde direction were observed only in the salt-stressed hypothalamo-neurohypophysial neuron. The HRP-positive tubules very likely are affiliated with the lysosomal system.

Further studies of the secretory process in hypothalamo-neurohypophysial neurons: an analysis using immunocytochemistry, wheat germ agglutinin-peroxidase, and native peroxidase

The axonal endoplasmic reticulum (ER) and synaptic-like (micro)vesicles within axon terminals of the neurohypophysis and their contribution to the secretory process in hypothalamo-neurohypophysial neurons have been investigated cytochemically in normal mice and in mice given 2% salt water to drink for stimulation of hormone synthesis in and release from these neurons. Cytochemical techniques included the peroxidase-antiperoxidase (PAP) immunocytochemical method for localization of neurophysin, wheat germ agglutinin-horseradish peroxidase (WGA-HRP) as a tracer for the anterograde axonal transport of membrane from within the perikaryon, and blood-borne native horseradish peroxidase (HRP) as a tracer for internalized axon terminal membrane. The primary antiserum employed was directed against neurophysins I and II, the carrier proteins for the peptide hormones oxytocin and vasopressin, respectively. PAP reaction product was observed over neurosecretory granules but never over the endoplasmic reticulum, microvesicles or other organelles in axons and terminals of the neurohypophysis. WGA-HRP was delivered extracellularly to cell bodies of paraventricular neurons by cerebral ventriculocisternal perfusion. Internalized perikaryal surface membrane tagged with WGA-HRP was recycled through the innermost Golgi saccule (GERL) from which neurosecretory granules were formed. The anterograde axonal transport of membrane-bound WGA-HRP was manifested within the neurosecretory granules; WGA-HRP did not label the axonal reticulum or terminal microvesicles in the neurohypophysis. Blood-borne native HRP endocytosed into neurohypophysial terminals was associated with a plethora of microvesicles measuring 40-70 nm in diameter and vacuoles similar in size to the 100-300-nm-wide neurosecretory granules. The microvesicles contributed to the formation of numerous vacuoles. The internalization of axon terminal membrane as microvesicles incorporating HRP was quantitatively greater than vacuoles in both salt-stressed and control mice. The results suggest that in the hypothalamo-neurohypophysial system of the mouse the axonal ER and terminal microvesicles are not involved in the transport, storage, and exocytosis of neurosecretory material and perhaps other molecules processed through the innermost Golgi saccule. Nevertheless, a prominent population of the microvesicles within axon terminals of the neurohypophysis does participate in the secretory process. These vesicles are involved directly in the internalization of the terminal surface membrane subsequent to release of secretory granule content.(ABSTRACT TRUNCATED AT 400 WORDS)

Brain-blood barrier? Yes and no

Ventriculo-cisternal perfusion of horseradish peroxidase (HRP) in the mouse brain has demonstrated that a brain-blood barrier exists at the microvascular endothelium in brain parenchyma but not in the median eminence of the hypothalamus. The brain-blood barrier is similar to the blood-brain barrier in that: tight junctions prevent the movement of protein between endothelial cells, HRP taken into the endothelial cells is directed to lysosomal dense bodies, and, contrary to the literature, a vesicular transendothelial transport of HRP from brain to blood does not occur under normal conditions. The endocytosis of ventricular injected HRP from the abluminal side of the endothelium is demonstrably less than the endocytosis of intravenous injected HRP from the luminal side; hence, the cerebral endothelium expresses a degree of polarity regarding the internalization of its cell surface membrane and extracellular protein. The passage of cerebrospinal fluid-borne or blood-borne HRP between some ependymal cells of the median eminence is not precluded by tight junctions. These patent extracellular channels offer a direct pathway for the exchange of substances between cerebrospinal fluid in the third ventricle and fenestrated capillaries in the median eminence.