Current aspects of the development of the blood-brain barrier

Development of an Intact Blood-Brain Barrier in Brain Tissue Transplants is Dependent on the Site of Transplantation

Transplantation of fetal septal forebrain tissue was performed to the anterior chamber of the eye, or intracranially to the rostral hippocampal formation in rats, to evaluate the impact of transplantation site on the development of an intact blood–brain barrier (BBB). The tissue was studied at 1, 2, 3, and 4 wk following transplantation by means of intravenous injection of Trypan blue, which is a vital stain not normally penetrating the BBB, as well as with an antibody specifically directed against the rat BBB, SMI71. In the intraocular septal transplants, there was a significant leakage of Trypan blue 1 wk postgrafting, associated with a few laminin-immunoreactive blood vessels that did not contain any SMI71-immunoreactivity. However, at 2 wk postgrafting, the intraocular grats exhibited an extensive plexus of thin-walled blood vessels expressing SMI71 immunoreactivity and no Trypan blue leakage. Thus, it appeared that a BBB had developed to some degree by 2 wk postgrafting in oculo. In the intracranial grafts, on the other hand, Trypan blue leakage could be seen as long as 3 wk postgrafting, and a dense plexus of blood vessels with SMI71 immunoreactivity was first seen at 4 wk postgrafting. Thus, the development of Trypan blue impermeability was delayed with 1 to 2 wk in the intracranial versus the intraocular grafts. Control experiments using psychological stress in adult rats as a means to transiently disrupt the BBB revealed that an increase in Trypan blue leakage correlated well with the disappearance of SMI71 immunoreactivity. Taken together, these studies demonstrate that the site of transplantation can influence the development of an intact BBB in neural tissue grafts.

Neurosteroids: Can a 2alpha,3alpha-epoxy ring make up for the 3alpha-hydroxyl group?

Seven steroid epoxides were prepared from 5α-pregn-2-en-20-one and 5α-pregn-3-en-20-one and their side-chain derivatives. All compounds were tested in vitro for binding to γ-aminobutyric acid (GABAA) receptor, some of them also in vivo for anticonvulsant action. 2α,3α-Epoxy-5α-pregnan-20-one inhibited the TBPS binding to the GABAA receptor and showed a moderate anticonvulsant action in immature rats. In contrast, its 3α,4α-isomer was inactive. More polar epoxide derivatives, modified at the side chain were less active or inactive. Noteworthy, diol 20, the product of trans-diaxial opening of the 2α,3α-epoxide 4, was not able to inhibit the TBPS binding, showing that the activity of the epoxide is due to the compound itself and not to its hydrolytic product. The 3α-hydroxyl group is known to be essential for the GABAA receptor binding. Despite the shortness of in vivo effects which are probably due to metabolic inactivation of the products prepared, our results show that the 2α,3α-epoxy ring is another structural pattern with ability to bind the GABAAR.

The rights and wrongs of blood-brain barrier permeability studies: a walk through 100 years of history

Careful examination of relevant literature shows that many of the most cherished concepts of the blood-brain barrier are incorrect. These include an almost mythological belief in its immaturity that is unfortunately often equated with absence or at least leakiness in the embryo and fetus. The original concept of a blood-brain barrier is often attributed to Ehrlich; however, he did not accept that permeability of cerebral vessels was different from other organs. Goldmann is often credited with the first experiments showing dye (trypan blue) exclusion from the brain when injected systemically, but not when injected directly into it. Rarely cited are earlier experiments of Bouffard and of Franke who showed methylene blue and trypan red stained all tissues except the brain. The term “blood-brain barrier” “Blut-Hirnschranke” is often attributed to Lewandowsky, but it does not appear in his papers. The first person to use this term seems to be Stern in the early 1920s. Studies in embryos by Stern and colleagues, Weed and Wislocki showed results similar to those in adult animals. These were well-conducted experiments made a century ago, thus the persistence of a belief in barrier immaturity is puzzling. As discussed in this review, evidence for this belief, is of poor experimental quality, often misinterpreted and often not properly cited. The functional state of blood-brain barrier mechanisms in the fetus is an important biological phenomenon with implications for normal brain development. It is also important for clinicians to have proper evidence on which to advise pregnant women who may need to take medications for serious medical conditions. Beliefs in immaturity of the blood-brain barrier have held the field back for decades. Their history illustrates the importance of taking account of all the evidence and assessing its quality, rather than selecting papers that supports a preconceived notion or intuitive belief. This review attempts to right the wrongs. Based on careful translation of original papers, some published a century ago, as well as providing discussion of studies claiming to show barrier immaturity, we hope that readers will have evidence on which to base their own conclusions.

Intracranial dynamics in pre-term infants and neonates: implications for physiotherapists

The control of cerebral circulation and intracranial dynamics differs markedly in the pre-term and full term neonate from that in the adult. Immaturity can combine with several clinical conditions and iatrogenic factors to predispose the neonate to cerebral lesions, which may subsequently increase morbidity. As physiotherapists play an increasingly important role in neonatology, it is important to appreciate the immaturity of the nervous system and to recognise the risk factors for such conditions as peri-intraventricular haemorrhage and periventricular leucomalacia. This paper describes intracranial dynamics in the pre-term and full term infant and illustrates how these factors may interact with clinical conditions to cause cerebral lesions. Studies which examine the effect of respiratory physiotherapy on intracranial dynamics are reviewed and suggestions made for further research.

Native and Complexed IGF-1: Biodistribution and Pharmacokinetics in Infantile Neuronal Ceroid Lipofuscinosis

Infantile neuronal ceroid lipofuscinosis (INCL) is a severe neurodegenerative disorder of childhood characterized by selective death of cortical neurons. Insulin-like growth factor 1 (IGF-1) is important in embryonic development and is considered as a potential therapeutic agent for several disorders of peripheral and central nervous systems. In circulation IGF-1 is mainly bound to its carrier protein IGFBP-3. As a therapeutic agent IGF-1 has shown to be more active as free than complexed form. However, this may cause side effects during the prolonged treatment. In addition to IGFBP-3 the bioavailability of IGF-1 can be modulated by using mesoporous silicon nanoparticles (NPs) which are optimal carriers for sustained release of unstable peptide hormones like IGF-1. In this study we compared biodistribution, pharmacokinetics, and bioavailability of radiolabeled free IGF-1, IGF-1/IGFBP-3, and IGF-1/NP complexes in a Cln1-/- knockout mouse model. IGF-1/NP was mainly accumulated in liver and spleen in all studied time points, whereas minor and more constant amounts were measured in other organs compared to free IGF-1 or IGF-1/IGFBP-3. Also concentration of IGF-1/NP in blood was relatively high and stable during studied time points suggesting continuous release of IGF-1 from the particles.

Phenobarbital blocks the lung edema induced by centrally injected tityustoxin in adult Wistar rats

The aim of this study was to evaluate the ability of phenobarbital to block the lung edema observed after intracerebroventricular (icv) injections of tityustoxin (TsTX), a toxic fraction of the Tityus serrulatus venom. We injected 1.74 microg icv (1.0 microl) of TsTX in Wistar rats pre-treated with 0.1 ml intramuscular injections of sterile saline or phenobarbital (60 or 170 mg/kg body weight). After the experiments the lungs were harvested and the pulmonary index (PI = lung/body weight x 100) calculated. The animals pre-treated with saline developed severe lung edema (PI = 1.8 +/- 0.2) after TsTX icv injection whereas those that received 170 mg/kg of phenobarbital presented no lung edema (PI = 0.71 +/- 0.02). Our results suggest that the lung edema induced by TsTX is of neurogenic nature and that 170 mg/kg of phenobarbital blocks TsTX induced lung edema.

The effects of ACTH and adrenocorticosteroids on seizure susceptibility in 15-day-old male rats

Infantile spasms are generalized convulsive seizures seen in the first year of life. They respond poorly to conventional anticonvulsants, but are often controlled by adrenocorticotropic hormone (ACTH) therapy. Other childhood seizures are also responsive to ACTH. The present study tested the effects of ACTH and related adrenocorticosteroids in prepubertal, 15-day-old rats. Compounds were tested against minimal (scMET) and maximal (MMT) pentylenetetrazol seizures, maximal electroconvulsive shock (MES) seizures, and hippocampal kindled seizures. ACTH had no significant anticonvulsant effects against any type of seizure. Several of the adrenocorticoid hormones, however, had strong anticonvulsant effects. Deoxycorticosterone (DOC) and progesterone (P4) both significantly suppressed scMET, MMT, and MES seizures 15 min after s.c. injection. DOC and P4 also shortened hippocampal discharge duration in the kindling model, and DOC, but not P4, suppressed the kindled convulsion. Aldosterone and corticosterone were effective against scMET seizures, and aldosterone was effective against MMT seizures. Dexamethasone and dihydroepiandrosterone had no anticonvulsant activity. These findings indicate that the adrenal steroid precursors, DOC and P4, have a broad spectrum of anticonvulsant activity in animal seizure models. They may play a role in mediating the anticonvulsant effects of ACTH in human infants.

Convulsant action of systemically administered glutamate and bicuculline methiodide in immature rats

Developmental changes of transport of drugs into the brain play an important role in ontogenetic neuropharmacology. Two convulsant drugs with different mechanisms of action (glutamate and bicuculline methiodide) were chosen to demonstrate these changes in developing rats. High dose of glutamate (4 g/kg i.p.) induced both minimal (predominantly clonic) and generalized tonic-clonic seizures in rat pups 7, 12, and 18 days old. In contrast, seizures were only exceptionally observed in 25 and 90 days old animals. Bicuculline methiodide was administered in a dose of 2 or 20 mg/kg i.p. The first sign of bicuculline methiodide action in all age groups was represented by automatisms, a symptomatology never seen after bicuculline hydrochloride administration. Minimal seizures were induced in 12-day-old and in a few 18-day-old and adult rats. Generalized seizures were common after the higher dose of bicuculline methiodide in 7- and 12-day-old rat pups, seldom in 18-day-old ones and never seen in 25-day-old and adult animals. Both glutamate and bicuculline methiodide enter the brain in immature rats but the mechanisms are probably different - glutamate is transported actively through the blood-brain barrier whereas no similar system is known for bicuculline methiodide.

Immunohistochemical and ultrastructural characterization of cortical plate microvasculature in the human fetus telencephalon

The blood-brain barrier (BBB) differentiation was investigated by immunohistochemistry and electron microscopy in the radial microvasculature of the telencephalon cortical plate (CP) of 12- and 18-week human fetuses. The BBB-specific glucose transporter isoform 1 (GLUT1) is expressed in both stages, with a main localization on the ablumenal and lateral plasma membranes of the endothelial cells. The endothelial cells are welded by short junctions with fusion points of the plasma membranes at 12 weeks and by extensive tight junctions at 18 weeks. The basal lamina is discontinuous beneath the endothelium-pericyte layer at 12 weeks and splits into two continuous layers circumscribing the pericytes in the later stage. The expression of laminin, a basal lamina glycoprotein, is continuous already at 12 weeks. The CP microvessels are tightly surrounded by processes of glial cells. Immunodetection of the cytoskeletal filament proteins, vimentin (VIM), and glial fibrillary acidic protein (GFAP), demonstrates that at 12 weeks the perivascular glial processes are mostly represented by VIM-stained fibers of the radial glia. At 18 weeks, GFAP-stained radial glia fibers, processes of VIM-stained astroblasts, and GFAP-positive astrocytes also build the perivascular envelopes. The results indicate that the vessel differentiation is already under way in the human CP at the midgestational age and entails the establishment of some barrier devices. The early relationship between perivascular glia coverage formation and endothelial barrier maturation suggests that also immature astroglial cells are involved in the setting up of the BBB.

Retrograde transynaptic pseudorabies virus infection of central autonomic circuits in neonatal rats

Pseudorabies virus (PRV) is widely used to map synaptically-linked neural circuits in adult animals. The present study sought to determine whether PRV has similar utility in neonatal rats, and whether central PRV infection in neonates elicits astrocytic and microglia/macrophage responses similar to those that contribute to specific transynaptic neuronal infection in adult rats. Retrograde transneuronal infection of autonomic circuits was examined 24-64 h after injection of an attenuated strain of PRV (PRV-Bartha) into the ventral stomach wall of 1-day-old rats. Brain and spinal cord sections were processed for immunocytochemical detection of PRV. Alternate sections were processed for immunolocalization of glial fibrillary acidic protein (GFAP) to identify fibrous astrocytes, or for an antigen associated with the complement C3bi receptor (OX42) to identify microglia. As in adult rats, the number and distribution of infected CNS neurons in neonatal rats increased progressively with advancing post-inoculation survival. Infected CNS neurons initially were restricted to the thoracic intermediolateral cell column and the dorsal motor nucleus of the vagus. Longer survival times led to retrograde transynaptic infection of additional neurons in the thoracic spinal cord, nucleus of the solitary tract, ventrolateral medulla, and caudal raphe nuclei. At the longest post-inoculation intervals, infected neurons also were observed in the area postrema and in certain autonomic-related regions of the rostral brainstem, hypothalamus, and amygdala. Quantitative analysis of immunolabeling in the dorsal vagal complex demonstrated that regions containing neurons at early stages of viral infection displayed increased astrocytic GFAP immunostaining; conversely, areas containing neurons at later stages of infection were characterized by a significant loss of GFAP staining and a parallel increase of OX42 microglia/macrophage immunolabeling. We conclude that PRV is effectively transported through synaptically-linked CNS circuits in neonatal rats, and that spatiotemporally-ordered responses by non-neuronal cells may contribute to the synaptic specificity of transneuronal viral transport.

Convulsant action of D,L-homocysteic acid and its stereoisomers in immature rats

PURPOSE

We wished to characterize the convulsant effect of homocysteic acid (HCA) in developing rats.

METHODS

Seizures were induced in 7-, 12-, 18-, and 25-day-old rats by intraperitoneal (i.p.) administration of D,L-HCA and in 12-day-old rats by i.p. injection of L- and D-stereoisomers of HCA. The animals were observed for 30 min after injection. The incidence, latencies, pattern of motor seizures, and all behavioral phenomena were noted. Fifty percent convulsant dose (CD50) values were calculated by probit analysis. Electrocorticograms (ECoG) were recorded after injection.

RESULTS

HCA did not elicit minimal clonic seizures whereas generalized tonic-clonic seizures (GTCS) occurred in all the age groups studied. Flexion (emprosthotonic) convulsions occurred to postnatal day 18. ECoG recordings exhibited delta activity in younger pups and sharp graphoelements in older pups, but electroclinical correlation was poor. Young animals were more sensitive to the convulsant effect of D,L-HCA. In addition, D-HCA was significantly more effective than L-HCA in inducing both flexion and generalized seizures.

CONCLUSIONS

Our data clearly indicate that seizures induced by HCA differ from those evoked by homocysteine. There are no qualitative differences in the motor pattern of seizures induced by the two stereoisomers of HCA, but marked differences were apparent in the very first signs of their action. These differences might be due to interaction with different glutamate receptor subtypes.

Seizures induced by homocysteine in rats during ontogenesis

We studied the convulsant action of homocysteine in 211 immature and adult Wistar albino rats. Homocysteine elicited minimal, predominantly clonic, and major generalized tonic-clonic seizures at six different developmental stages, from 7 days to adulthood. Nevertheless, some age-dependent differences in the seizure pattern were apparent. Minimal seizures in immature rats lasted < or = 20 min, thus representing an epileptic status, whereas in adult animals these seizures were much shorter, lasting only < or = 40 s. In addition, flexion seizures were observed in 7- and 12-day-old rats, only rarely in 15- and 18-day-old animals, and never in the 25-day-old and adult rats. ECoG recordings demonstrated a nearly isoelectric pattern during homocysteine-induced seizures in 7- and 12-day-old rat pups. In older rats, spikes or sharp waves were recorded, but precise electroclinical correlations were poor. The greater sensitivity of younger animals to kainic acid (KA) and N-methyl-D-aspartate (NMDA), as reported previously, was not evident in the case of homocysteine-induced seizures. This observation, together with a different behavioral pattern, suggests that homocysteine cannot be considered a simple agonist of the kainate or NMDA type of excitatory amino acid receptors. The exact mechanism of the convulsant action of homocysteine, both during development and in adulthood, remains to be clarified.

Cell adhesion molecules acting between C6 glioma and endothelial cells

The interactions between tumor cells and endothelium play a key role in the process of tumor growth, local invasion, and distant metastasis. In the present study, we examined the adhesion of C6 glioma cells to bovine endothelial cell (EC) monolayers and defined the cell adhesion molecules acting between these cells. Pretreatment of the EC monolayer with cytokines, tumor necrosis factor (TNF)-alpha, interleukin (IL)-1 beta, and interferon (INF)-gamma, significantly increased the adhesion of C6 glioma cells to the EC monolayer. The effect lasted more than 24 hours and was protein-synthesis dependent. The adhesion of C6 glioma cells to TNF-activated ECs was blocked by the monoclonal antibody to the intercellular adhesion molecule-1 (ICAM-1) or beta 2 integrin, whereas that of melanoma cells was not. These findings provide evidence that ICAM-1 and beta 2 integrin function as inducible cell surface molecules that can support the adhesion of C6 glioma cells to ECs, and may contribute to the characteristic growth of glial tumors in vivo.

Distribution of extravasated serum protein after cryoinjury in neonatal and adult rat brains

The sequelae of cryoinjury to unilateral cerebral cortex were compared in neonatal and adult rats. In neonatal rats, immunostaining for autologous albumin disclosed a wide spread of extravasated albumin in both hemispheres on day 1 and rapid clearance from the tissue by day 7, whereas in adults rats, the distribution of albumin had progressively increased by day 7 and was then restricted to the injury site by day 14. Horseradish peroxidase tracing revealed a leakage of serum proteins by day 3 in neonates and by day 7 in adults. The rapid clearance of serum proteins from the neonatal brain tissue appeared to be promoted by vimentin-positive radial glia in the subpial and periventricular regions. A possible causal relationship between the rapid clearance of serum proteins and unique outcome of the cryoinjury in the neonatal brain is discussed.

Experimental models of epilepsy in young animals

Seizures occur more frequently early in life. Some of these early seizures may eventually become epilepsy. Others are reactive seizures due to excessive environmental stimuli that, in any other age group, might not have elicited a similar response. To understand the developmental aspects of seizures and epilepsy in humans, it is important to study these processes in animals of equivalent ages. In this paper, we describe several animal models of developmental seizures, including their electroclinical manifestations and their validity in respect to human epileptic syndromes. There are several factors that may account for the increased seizure susceptibility of the immature brain, including the delayed development of effective systems or synaptic networks that are involved in the suppression of seizures. A better insight of the basic pathophysiology of seizures as a function of age in animal models will lead to the development of new therapeutic approaches for the treatment of age-specific epileptic disorders in humans.

Aminophylline exhibits convulsant action in rats during ontogenesis

Aminophylline-induced seizures were studied in 166 male albino rats in five age groups--7, 12, 18, 25 and 90 days old. Aminophylline injected in doses from 150-350 mg/kg i.p. elicited both minimal, clonic and major, i.e. generalized tonic-clonic seizures during the 60-min observation period. The pattern of minimal seizures did not change during development; major seizures exhibited changes in proportion to their three phases--running, tonic and clonic phases. Dependence on the dose of aminophylline was observed in the incidence of major seizures as well as in shortening of latencies of both types of seizures. More marked convulsant effects of aminophylline in 7-, 12- and 18-day-old rat pups than in older animals might be due to pharmacokinetic as well as pharmacodynamic factors.

Accumulation of theophylline, theobromine and paraxanthine in the fetal rat brain following a single oral dose of caffeine

This paper describes the disposition of caffeine and its metabolites, theophylline, theobromine and paraxanthine in the 20-day fetal and adult brains following a single maternal dose of 5 or 25 mg/kg caffeine. Brains and plasma were collected 5 and 30 min, and 1, 3, 8 and 24 h after dosing. It was found that fetal and adult caffeine AUC (area under the concentration-time curve) values did not differ between the brain and plasma at either dose. Caffeine's primary metabolites theophylline, theobromine and paraxanthine did, however, accumulate in the fetal brain at both doses resulting in a 3-fold increase in brain metabolite exposure compared to fetal circulatory levels. In contrast to the fetus, metabolite AUC values after a dose of 25 mg/kg were found to be lower in the brains of adults compared with plasma. This suggests that caffeine's primary metabolites might be selectively excluded from the adult brain. In conclusion we have shown that, unlike the adult, the fetal rat brain accumulates theophylline, theobromine and paraxanthine when exposed to caffeine doses comparable to those attainable by normal human consumption. Since many aspects of caffeine metabolism are similar in the rat and human, we suggested that particular attention should be paid to the consumption of caffeine during pregnancy.

Drug transport across the blood--brain barrier. I. Anatomical and physiological aspects

This review describes various aspects of the transport of drugs across the blood-brain barrier and comprises three parts. In this first part, the anatomical and physiological aspects of blood-brain transport are discussed. It appears that the blood-brain barrier has an anatomical basis at the endothelium of the capillary wall. This endothelium is characterized by the presence of very tight junctions. As a result, the transport by passive diffusion of drugs with a low lipophilicity, is restricted. For certain classes of closely related relatively hydrophilic compounds, however, the presence of specialized carrier systems has been demonstrated which may facilitate transport. Also evidence is presently available, that the permeability of the blood-brain barrier may be under active regulatory control. It is expected that improved knowledge of the anatomical and physiological aspects of the blood-brain barrier and its regulation will provide a scientific basis for the development of strategies to improve the transport of drugs into the central nervous system.

Cytochemistry of 5'-nucleotidase in the superior cervical ganglion of cat and guinea pig

The localization of 5'-nucleotidase, an adenosine-producing ectoenzyme, was studied by a cytochemical method in the superior cervical ganglion of the adult cat and guinea pig. The following subcellular sites of enzymatic activity were detected: (1) the surface of Schwann and satellite cells including the extracellular space between these cells and neuronal profiles; (2) the plasmalemma and pinocytotic vesicles of capillary endothelial cells; and (3) the synaptic clefts between cholinergic preganglionic axon terminals and sympathetic neurons. The simultaneous presence of 5'-nucleotidase at both glial elements and synapses within the adult peripheral nervous system (PNS) constitutes a novel distribution pattern for this enzyme which does not apply to the mature central nervous system (CNS), but which is rather typical for the developing CNS. These distributions of 5'-nucleotidase activity may reflect specific cellular requirements for nucleosides involved in parenchymal metabolism, in vascular transport processes and, possibly, in synaptic plasticity.

Brain macrophages and microglia respond differently to lesions of the developing and adult visual system

Traumatic injury in the brain usually results in rapid degeneration of neuronal elements and a response by peripherally derived macrophages (brain macrophages, BMOs) and resident microglia. One intriguing result of lesions performed in the developing brain as compared to lesions of the mature brain is the faster resolution of the cellular debris and the absence of significant scarring. The purpose of this study was to examine the response of BMOs to induced cell death distant to the lesion site and to investigate possible differences in the responding phagocytic populations (BMOs versus microglia) following lesions in neonates and adults. Ablation of the visual cortex at birth results in very rapid retrograde degeneration and removal of neurons of the dorsal lateral geniculate nucleus (dLGN) within a few days. Lesions to the visual cortex of adult rats also induce neurons within the dLGN to die, but these cells do so over a much more protracted time course. Utilizing differences in morphology and immunocytochemical staining with the monoclonal antibodies ED1 and OX-42 to distinguish between BMOs and microglia, we found that in the developing CNS, BMOs are signalled rapidly and specifically to the location of induced cell death. Microglia are not involved in this response. As might be expected, the temporal response in the adult is much more protracted. In contrast to the developing brain, microglia and not macrophages are the predominant responding cell class after the adult lesion. The data suggest that these are distinct populations of phagocytic cells that respond to brain damage during development and in the adult, which may be critical in modulating the resolution and growth response after injury.

Differential immunochemical markers reveal the normal distribution of brain macrophages and microglia in the developing rat brain

Brain macrophages and microglia play important roles in central nervous system (CNS) development, especially during regressive events in which particular neuronal and glial constituents are eliminated. The purpose of this study is to provide a complete map of brain macrophage and microglia distribution in all regions of the neuraxis from birth to sexual maturity. We have utilized morphology and immunostaining with the specific antibodies OX-42 and ED1 to distinguish between brain macrophages and microglia. Brain macrophages are large, round cells, 10-15 microns in diameter, with few or no cytoplasmic processes; these cells are ED1- and OX-42-immunopositive. Microglia have small cell bodies with numerous, ramified cytoplasmic processes. These cells are OX-42-positive, and ED1-negative. We found a specific pattern of distribution of brain macrophages, targeting specific cortical and subcortical areas transiently, including developing fiber tracts. These cells disappeared completely by the third postnatal week. In contrast, OX-42-positive microglia exhibited a gradual increase in number and were distributed uniformly throughout gray matter and within white matter tracts. These cells remain in the adult CNS, constituting the resident microglia population. We suggest that these two distinct phagocytic cell populations perform unique functions in the developing brain, including remodeling of restricted CNS areas by brain macrophages that is part of a normal morphological process.

Stereoselective transport of baclofen across the blood-brain barrier in rats as determined by the unit impulse response methodology

The blood-brain barrier transport characteristics of racemic baclofen and the separate R- and S-enantiomers have been determined in vivo in rats by using the unit impulse response methodology. Transport rate was determined as blood-brain barrier clearance, the volume of plasma per unit time cleared of baclofen by transport across the blood-brain barrier. Plasma elimination kinetics and CSF elimination kinetics did not differ among racemic baclofen and the R- and S-enantiomers. Transport of each compound could be described by a linear V(t) curve, suggesting the absence of saturable transport processes in the concentration range studied. However, for R-baclofen the blood-brain barrier clearance (4.7 +/- 1.0 microliters/min, mean +/- SE; n = 6) and cumulative transported amount (0.085 +/- 0.007%; n = 6) were significantly higher than these values for the S-enantiomer (1.1 +/- 0.3 microliters/min, 0.031 +/- 0.005%; n = 6) and racemic baclofen (1.0 +/- 0.1 microliters/min, 0.036 +/- 0.003%; n = 6). These findings indicate that there is stereoselective transport of baclofen across the blood-brain barrier.

Persistence of fluoro-gold following degeneration of labeled motoneurons is due to phagocytosis by microglia and macrophages

When the neural tracer Fluoro-Gold is used to retrogradely label a population of axotomized neurons, cellular labeling can persist in the axotomized nucleus even when Nissl staining indicates that the injured neurons have degenerated. In order to determine the identity of the labeled cells that remain, this study combines retrograde transport of Fluoro-Gold with immunocytochemical methods for identification of specific non-neuronal cell types following peripheral axotomy and Fluoro-Gold labeling of motoneurons in the dorsal motor nucleus of the vagus in neonatal and adult rats. Fourteen days following cervical vagotomy in neonatal rats, Nissl staining revealed a virtually complete loss of vagal motoneurons. Fourteen days after cervical vagotomy in adult rats, vagal motoneuronal loss was not yet extensive but chromatolysis had clearly begun. Injection of Fluoro-Gold into the vagus nerve just prior to the vagotomy led to Fluoro-Gold labeling of remaining vagal motoneurons. In addition, many other small, brightly labeled cells were present in the lesioned vagal nuclei of all rats. Immunofluorescent identification of astrocytes with anti-glial fibrillary acidic protein and microglia and macrophages with OX42 (anti-C3bi complement receptor) and ED1 (anti-monocyte/macrophage cytoplasmic antigen) demonstrated that the small, bright Fluoro-Gold-labeled cells were non-neuronal, non-astrocytic phagocytes, including microglia. These results indicate that phagocytic microglia and other macrophages sequester Fluoro-Gold in the axotomized dorsal motor nucleus of the vagus of neonatal and adult rats, leading to persistence of fluorescent cellular labeling following the loss of retrogradely labeled axotomized neurons.

Astrocytes induce neural microvascular endothelial cells to form capillary-like structures in vitro

Astrocytes maintain a unique association with the central nervous system microvasculature and are thought to play a role in neural microvessel formation and differentiation. We investigated the influence of astroglial cells on neural microvascular endothelial differentiation in vitro. Using an astroglial-endothelial coculture system, rat brain astrocytes and C6 cells of astroglial lineage are shown to induce bovine retinal microvascular endothelial (BRE) cells to form capillary-like structures. Light microscopic evidence for endothelial reorganization began within 48 hours and was complete 72-96 hours following the addition of BRE cells to 1-day-old astroglial cultures. The extent of BRE reorganization was quantitated by computer-assisted analysis and shown to be dependent upon the density of both the BRE and C6 cells within the cocultures. Coculture conditions in which BRE cells were separated from C6 cells by porous membranes failed to generate this endothelial cell change. Likewise, C6-conditioned media and C6-endothelial coculture conditioned media did not induce BRE cell reorganization. Extracellular laminin within the C6-endothelial cocultures, identified by indirect immunofluorescence, was concentrated at the endothelial-astroglial interface of capillary-like structures consistent with incipient basement membrane formation. Astroglial cells accumulated adjacent to capillary-like structures suggesting the presence of bidirectional influences between the reorganized endothelial cells and astroglia. This is the first demonstration of astroglial induction of angiogenesis in vitro and these findings support a functional role for perivascular astrocytes in the vascularization of neural tissue such as retina and brain.

Sequential appearance of anionic domains in the developing blood-brain barrier

The distribution of anionic sites in the walls of mouse brain micro-blood vessels (MBVs) during development and maturation of the blood-brain barrier (BBB) was studied by electron microscopy. Cationic colloidal gold (CCG) and Lowicryl K4M-embedded brain samples obtained from mouse fetuses (13th and 19th days) and from 1-, 5-, 12- and 24-day-old and adult mice were used. The labeling of anionic sites with CCG was more intense on the abluminal than on the luminal front of the endothelial cells (ECs) in fetuses and in newborn mice. Only a few anionic sites appear on the luminal front of the ECs of proliferating blood vessels invading the neural tissue in 13-day-old fetuses. They become slightly, although steadily, more abundant during further stages of development, and their number rapidly increases between the 12th and 24th day of life at which time they attain the density typical for mature animals. The maturation of the basement membrane (BM), which occurs during the myelinization period (12th-24th day of life), also coincides with an increasing concentration of anionic sites. These observations suggest that the gradual appearance of anionic sites on both fronts of the endothelium, as well as in the developing and maturing BM, represents one of the mechanisms responsible for differentiation of cerebral microvasculature into BBB-type MBVs.