Localization of catechol-O-methyltransferase in the leptomeninges, choroid plexus and ciliary epithelium: implications for the separation of central and peripheral catechols

Brain Plasma Membrane Monoamine Transporter in Health and Disease

Precise control of monoamine neurotransmitter levels in the central nervous system (CNS) is crucial for proper brain function. Dysfunctional monoamine signaling is associated with several neuropsychiatric and neurodegenerative disorders. The plasma membrane monoamine transporter (PMAT) is a new polyspecific organic cation transporter encoded by the SLC29A4 gene. Capable of transporting monoamine neurotransmitters with low affinity and high capacity, PMAT represents a major uptake₂ transporter in the brain. Broadly expressed in multiple brain regions, PMAT can complement the high-affinity, low-capacity monoamine uptake mediated by uptake₁ transporters, the serotonin, dopamine, and norepinephrine transporters (SERT, DAT, and NET, respectively). This chapter provides an overview of the molecular and functional characteristics of PMAT together with its regional and cell-type specific expression in the mammalian brain. The physiological functions of PMAT in brain monoamine homeostasis are evaluated in light of its unique transport kinetics and brain location, and in comparison with uptake₁ and other uptake₂ transporters (e.g., OCT3) along with corroborating experimental evidences. Lastly, the possibility of PMAT's involvement in brain pathophysiological processes, such as autism, depression, and Parkinson's disease, is discussed in the context of disease pathology and potential link to aberrant monoamine pathways.

The plasma membrane monoamine transporter (PMAT): Structure, function, and role in organic cation disposition

Plasma membrane monoamine transporter (PMAT) is a new polyspecific organic cation transporter that transports a variety of biogenic amines and xenobiotic cations. Highly expressed in the brain, PMAT represents a major uptake₂ transporter for monoamine neurotransmitters. At the blood-cerebrospinal fluid (CSF) barrier, PMAT is the principal organic cation transporter for removing neurotoxins and drugs from the CSF. Here I summarize our latest understanding of PMAT and its roles in monoamine uptake and xenobiotic disposition.

Catechol-O-methyltransferase (COMT) protein expression and activity after dopaminergic and noradrenergic lesions of the rat brain

The occurrence of catechol-O-methyltransferase (COMT) in presynaptic neurons remains controversial. This study utilized dopaminergic and noradrenergic toxins to assess the presence of COMT in the presynaptic neurons originating from the substantia nigra, ventral tegmental area or locus coeruleus. Destruction of dopaminergic and noradrenergic neurons was assessed by measuring the dopamine and noradrenaline content in the projection areas of these neurons. Additionally, COMT protein expression and activity were examined in several projection areas to determine whether there are any changes in COMT values. Colocalization studies were done to identify COMT-containing postsynaptic neurons. Despite successful lesioning of dopaminergic and noradrenergic neurons, no changes in COMT protein expression or activity could be noted. These results strongly suggest that COMT is not present in presynaptic dopaminergic and noradrenergic neurons. There was a high colocalization of COMT with the GABAergic marker of short neurons both in the striatum and cortex but only a weak, if any, with the cholinergic marker in the cortex.

Impaired monoamine and organic cation uptake in choroid plexus in mice with targeted disruption of the plasma membrane monoamine transporter (Slc29a4) gene

The choroid plexus (CP) forms the blood-cerebrospinal fluid (CSF) barrier and protects the brain from circulating metabolites, drugs, and toxins. The plasma membrane monoamine transporter (PMAT, SLC29A4) is a new polyspecific organic cation transporter that transports a wide variety of organic cations including biogenic amines, cationic drugs, and neurotoxins. PMAT is known to be expressed in the CP, but its specific role in CP transport of organic cations has not been clearly defined. Here we showed that PMAT transcript is highly expressed in human and mouse CPs, whereas transcripts of other functionally related transporters are minimally expressed in the CPs. Immunofluorescence staining further revealed that PMAT protein is localized to the apical (CSF-facing) membrane of the CP epithelium, consistent with a role of transporting organic cations from the CSF into CP epithelial cells. To further evaluate the role of PMAT in the CP, mice with targeted deletion of the Slc29a4 gene were generated and validated. Although Pmat(-/-) mice showed no overt abnormalities, the uptake of monoamines and the neurotoxin 1-methyl-4-phenylpyridinium was significantly reduced in CP tissues isolated from the knock-out mice. Together, our data demonstrated that PMAT is a major transporter for CP uptake of bioactive amines and xenobiotic cations. By removing its substrates from the CSF, PMAT may play an important role in protecting the brain from cationic neurotoxins and other potentially toxic organic cations.

Distribution of catechol-O-methyltransferase (COMT) proteins and enzymatic activities in wild-type and soluble COMT deficient mice

Catechol-O-methyltransferase (COMT) has both soluble (S-COMT) and membrane-bound (MB-COMT) isoforms. A specific COMT antibody was used in immunohistochemical and confocal co-localization studies to explore the distribution of COMT in general in normal mice and MB-COMT in particular, in an S-COMT deficient mouse line. In the peripheral tissues, high COMT protein and activity levels were observed in liver and kidney, whereas in the brain, COMT expression and activity were much lower. MB-COMT was widely distributed throughout all tissues, and overall, the MB-COMT distribution mimicked the distribution of S-COMT. MB-COMT displayed some preference for brain tissue, notably in the hippocampus. MB-COMT related enzymatic activity was also pronounced in the cerebral cortical areas and hypothalamus. MB-COMT, like S-COMT, was found to be an intracellular enzyme but it was not associated with plasma membranes in the brain. Both COMT forms were abundantly found in microglial cells and intestinal macrophages, but also in astroglial cells. COMT was also present in some neuronal cells, like pyramidal neurons, cerebellar Purkinje and granular cells and striatal spiny neurons, but not in major long projection neurons. Finally, it seemed that nuclear COMT is not visible in S-COMT deficient mice.

Distribution and functions of catechol-O-methyltransferase proteins: do recent findings change the picture?

Old and new results show that both catechol-O-methyltransferase (COMT) forms are found in all mouse tissues, demonstrating that COMT is a ubiquitous enzyme. Some novel findings are obvious when considering differences between old and new distribution data. In addition to the brain, membrane-bound form of COMT (MB-COMT) is found also in most peripheral mouse tissues at about equal amounts as soluble form of COMT (S-COMT), suggesting that their functions do not need to be very different. There are large differences between the species in the relative distribution of S-COMT and MB-COMT. According to the new data, it is evident that even in the animal tissues MB-COMT is not associated with the plasma membranes but with intracellular membranes, and that S-COMT resides not only in the cytoplasm but even in the nucleus.

Microscopic morphology and histology of the human meninges

The meninges comprise the dura mater and the leptomeninges (arachnoid and pia mater). Dura forms an outer endosteal layer related to the bones of the skull and spine and an inner layer closely applied to the arachnoid mater. Leptomeninges have multiple functions and anatomical relationships. The outer parietal layer of arachnoid is impermeable to CSF due to tight intercellular junctions; elsewhere leptomeningeal cells form demosomes and gap junctions. Trabeculae of leptomeninges compartmentalize the subarachnoid space and join the pia to arachnoid mater. In bacterial meningitis leptomeningeal cells secrete cytokines. Pia mater is reflected from the surface of the brain and spinal cord onto arteries and veins, thus separating the subarachnoid space from the brain and cord. A sheath of leptomeninges accompanies arteries into the brain and is related to the pathways for the drainage of interstitial fluid that play a role in inflammatory responses in the brain and appear to be blocked by amyloid-beta in Alzheimer's disease. Specialised leptomeningeal cells in the stroma of the choroid plexus form collagen whorls that become calcified with age. Leptomeningeal cells also form channels in the core and apical cap of arachnoid granulations for the drainage of CSF into venous sinuses. In the spine, leptomeninges form highly perforated intermediate sheets of arachnoid and delicate ligaments that compartmentalize the subarachnoid space; dentate ligaments anchor subpial collagen to the dura mater and stabilize the spinal cord. Despite the multiple anatomical arrangements and physiological functions, leptomeningeal cells retain many histological features that are similar from site to site.

Catechol O-methyltransferase mRNA expression in human and rat brain: evidence for a role in cortical neuronal function

Catechol O-methyltransferase (COMT) is involved in the inactivation of catecholamines, including the neurotransmitter dopamine. A Val(108/158) Met functional polymorphism of the COMT gene has been shown to affect working memory-associated frontal lobe function in humans. In the present study, in situ hybridization histochemistry was employed to determine the mRNA expression profile of COMT in the human prefrontal cortex, striatum and midbrain and in the rat forebrain. In both species, COMT mRNA signals were observed in large pyramidal and smaller neurons in all cortical layers of the prefrontal cortex as well as in medium and large neurons in the striatum. Levels of COMT mRNA were obviously higher in neurons than in glia. The striatum, which receives a dense dopaminergic input, expressed lower levels of COMT mRNA as compared with the prefrontal cortex. Consistent with previous protein expression data, COMT mRNA was abundant in ependymal cells lining the cerebral ventricles. In the midbrain, COMT mRNA was detected in dopaminergic neurons in both species, albeit at low levels. In the rat forebrain, dense labeling was also detected in choroid plexus and hippocampal dentate gyrus and Ammon's horn neurons. Contrary to expectations that COMT would be expressed predominantly in non-neuronal cells, the present study shows that neurons are the main cell populations expressing COMT mRNA in the prefrontal cortex and striatum. Combined with previous data about protein localization, the present results suggest that the membrane-bound isoform of COMT having a high affinity for dopamine is expressed at neuronal dendritic processes in human cortex, consistent with functional evidence that it plays an important role in dopaminergic neurotransmission.

Neuronal and non-neuronal catechol-O-methyltransferase in primary cultures of rat brain cells

Previous biochemical and histochemical studies have suggested that catechol-O-methyltransferase (COMT) is a predominantly glial enzyme in the brain. The aim of this work was to study its localization and molecular forms in primary cultures, where cell types can be easily distinguished with specific markers, COMT immunoreactivity was studied in primary astrocytic cultures from newborn rat cerebral cortex, and in neuronal cultures from rat brain from 18-day-old rat embryos using antisera against rat recombinant COMT made in guinea pig. Double-staining studies with specific cell markers to distinguish astrocytes, neurons and oligodendrocytes were performed. COMT immunoreactivity colocalized with a specific oligodendrocyte marker galactocerebroside in cells displaying oligodendrocyte morphology, flat cells displaying type-1 astrocyte morphology and glial fibrillary acidic protein, in branched cells displaying type-2 astrocyte morphology and in cell bodies of neurons, the processes of which displayed neurofilament immunoreactivity. Western blots detected both soluble 24 kDa and membrane-bound 28-kDa COMT proteins in neuronal and astrocyte cultures. The results suggest that COMT is synthesized by cultured astrocytes, oligodendrocytes and neurons.

Glial alpha 2-receptors probably inhibit the high-affinity uptake of noradrenaline into astrocytes in the rat brain in vivo

The effect of alpha 2-receptor blockage on the extraneuronal turnover of noradrenaline (NA) has been studied in the intact rat brain. Tropolone and yohimbine, along with reserpine or desmethylimipramine, were given 30 min after intracerebroventricular injection of [7-3H]NA, i.e. after the tracer had been stored or inactivated. Tropolone given alone did not change the fractions of 3H-activity recovered as [3H]NA from hypothalamus, septum, striatum and pons-medulla, but in the presence of yohimbine improved the [3H]NA recovery in all areas except pons-medulla. The maximum effect was seen in the hypothalamus of reserpine-treated rats. Since the alpha 2-autoreceptors were blocked, the increased [3H]NA recovery does not reflect a down-regulated neuronal NA turnover. Instead it seems to show that a fraction greater than normal of neuronally released NA had been taken up into astrocytes and remained unmetabolized if catechol-O-methyltransferase was inactive. It is assumed that yohimbine enabled the protective tropolone effect by blocking astrocytic alpha 2-receptors that otherwise, either by itself or by antagonizing beta-receptor-induced hyperpolarization or cAMP formation, had impaired parameters that stimulate the high-affinity NA Uptake 1 of astrocytes (e.g. membrane potential, Na+,K(+)-ATPase) or control the gap junction permeability in the glial syncytium.

Catechol-O-methyltransferase (COMT) in rat brain: immunoelectron microscopic study with an antiserum against rat recombinant COMT protein

Localization of catechol-O-methyltransferase (COMT) in rat cerebral cortex, neostriatum and cerebellar cortex was studied with preembedding immunoelectron microscopy using a specific antiserum raised against rat recombinant COMT protein. In all areas, immunoreactivity was found both in astrocytes and in neuronal processes. Reaction product was seen in the cytoplasm and in association with tubular structures of dendritic processes. Immunoreactivity was also located postsynaptically in dendritic spines and associated with the postsynaptic membrane. Strong immunoreaction was also seen in the cytoplasm of ependymal cells lining the ventricles, and in tanycytes in median eminence. The results suggest that postsynaptic dendritic spines and astrocytic processes may be the sites of catecholamine inactivation by COMT in rat brain.

Pathways of fluid drainage from the brain--morphological aspects and immunological significance in rat and man

There is firm physiological evidence for the lymphatic drainage of interstitial fluid and cerebrospinal fluid from the brains of rats, rabbits and cats. The object of this review, is to describe firstly the morphological aspects of lymphatic drainage pathways from the rat brain and secondly, to explore through scanning and transmission electron microscope techniques, the possibility of similar lymphatic drainage pathways in man. Interstitial and oedema fluid spreads diffusely through the white matter in the rat and appears to drain into the ventricular cerebrospinal fluid. In grey matter, however, tracers pass along perivascular spaces to the surface of the brain and into the cerebrospinal fluid. Paravascular compartments in the subarachnoid space follow the course of major arterial branches to the circle of Willis and thence along the ethmoidal arteries to the cribriform plate of the ethmoid bone. Particulate tracers, such as Indian ink, enter channels in the arachnoid beneath the olfactory bulbs and connect directly with nasal lymphatics through channels which pass through holes in the cribriform plate. Proteins and other solutes may also drain along other cranial nerves. Thus, there is a bulk flow pathway for interstitial and cerebrospinal fluid from the rat brain into cervical lymphatics. In man, it is probable that diffuse interstitial drainage of fluid from the white matter occurs in a similar way to that in the rat. Furthermore, the anatomical pathways exist by which bulk drainage of fluid could occur along perivascular spaces from the grey matter into perivascular spaces of the leptomeningeal arteries and thence into the cerebrospinal fluid (CSF).(ABSTRACT TRUNCATED AT 250 WORDS)

Electrophoretic analysis of low and high activity forms of catechol-O-methyltransferase in human erythrocytes

Analysis of the catechol-O-methyltransferase (COMT) enzyme in human RBC lysates from 15 samples exhibiting inherited variations in level of activity and thermal stability was performed. Electrophoretic blotting and immune fixation was carried out following sodium dodecyl sulfate polyacrylamide gel electrophoresis or isoelectric focusing of lysate protein. These techniques did not reveal a major structural alteration of the protein that could account for the observed variation in enzyme activity or thermal stability. Future studies utilizing molecular genetic techniques should make it possible to determine the basis for inherited variations in human RBC COMT activity and thermal stability.

Barrier functions of the leptomeninges: a study of normal meninges and meningiomas in tissue culture

The anatomical arrangement of the pia mater suggests that it may act as a regulatory interface between cerebrospinal fluid and the surface of the brain and between arterioles within the brain and the surrounding neural tissue. However, the functional aspects of such a barrier are difficult to evaluate in vivo. In the present study, the enzymic content and endocytotic capacities of normal leptomeningeal cells in situ and meningioma cells in confluent tissue culture are examined in relation to barrier functions of meningeal cells. Growth of cells in culture was obtained from human fetal and newborn rat leptomeninges and from 9/13 meningiomas. But, in only two meningiomas were the cultured cells characterized as meningeal in origin by using the strict criteria of desmosomes identified by immunocytochemistry or by electron microscopy. These two tumours had high (8-8.7%) Ki-67 labelling indices. Glutamine synthetase activity is present in normal meninges and in meningioma cells in culture; this enzyme together with catechol-O-methyltransferase could play a role in limiting the diffusion of neurotransmitters into brain tissue. A steady rate of endocytosis of carbon particles and fluorescent latex beads, 0.2-1 microns in diameter, was observed in cultured meningioma cells. Such endocytosis was inhibited by cytochalasin B indicating the active participation of intracellular microfilaments. Similar endocytosis has been observed in normal leptomeninges in vivo. The results of this study suggest that meningioma cells in culture reflect the barrier functions of the pia mater and may be used as a model to further investigate the functions of the pia mater.

Concentrations of tyrosine, L-dihydroxyphenylalanine, dopamine, and 3-O-methyldopa in the cerebrospinal fluid of Parkinson's disease

We determined the concentrations of tyrosine, L-3,4-dihydroxyphenylalanine (DOPA), dopamine (DA), and 3-O-methyldopa (3-OMD) in the cerebrospinal fluid (CSF) of parkinsonian patients and elevated potential interactions between the substances. We found a significant increase in tyrosine, and a significant decrease in DOPA, DA, and 3-OMD. We also found that for a given concentration of DOPA, DA and 3-OMD were proportional. In addition, the ratio of DA to 3-OMD was significantly shifted in favor of DA in parkinsonian patients.

Cloning and characterization of human placental catechol-O-methyltransferase cDNA

Catechol-O-methyltransferase (COMT) cDNA clones were isolated from a human placental cDNA library using synthetic oligonucleotides as probes. All four positive clones isolated contained an open reading frame, which potentially coded for a 24.4-kD polypeptide, presumably corresponding to the cytoplasmic form of the COMT (S-COMT). In addition to the S-COMT sequences, two of the clones carried extensions in the 5' end, which potentially coded for a 50-amino-acid peptide extending the S-COMT reading frame. This sequence contained a stretch of signal sequence-like hydrophobic amino acids in its amino terminus. The deduced human COMT polypeptide had 80% similarity with the previously characterized rat COMT. Expression of one of the cDNA clones in human K-562 cells resulted in cell clones with 3- to 10-fold increased COMT activity. Cell-free translation of transcripts synthesized in vitro from one of the short cDNAs yielded a 26-kD product, similar in size to human S-COMT. Translation of transcripts from one of the long cDNAs gave 30-kD and 26-kD polypeptides, suggesting translation initiation from two different AUG initiation codons. The 30-kD protein, but not the 25-kD protein, associated with microsomal membranes in translation lysates. A potential polyadenylation signal AATTAA was detected in the 3' ends of two of the clones 265 nucleotides downstream from the COMT translation termination codon. RNA blotting on human placental RNA revealed a 1.5-kb-long COMT-specific transcript. DNA analysis suggested that human, as well as rat, canine and monkey cells have one gene for COMT.

Isolation of the mRNA encoding rat liver catechol-O-methyltransferase

A highly specific, well characterized rabbit antiserum to purified rat liver catechol-O-methyltransferase (COMT; EC 2.1.1.6) and the procedure of polysome immunoadsorption have been used to isolate a messenger RNA which encodes a single polypeptide when translated in vitro. Western blotting and immune fixation have shown multiple active forms of the enzyme to exist; a major, soluble one with MW of 23,000 and pI of 5.2 and another, membrane-bound one with MW of 26,000 and a pI of 6.2 (1). When translated in vitro, the purified message synthesizes a protein of molecular weight (MW) 23,000 and pI 5.2, values in agreement with those for purified enzyme reported by other investigators (2,3). Only the soluble form is seen after in vitro translation; the other immunoreactive proteins possibly arise due to post-translational modifications which do not occur in the lysate; or perhaps another mRNA exists. Cloning of the COMT cDNA will resolve this issue and should be feasible in light of our data indicating that the mRNA isolated here represents 0.46% of total rat liver polyadenylated message.

Ultrastructure of the meninges at the site of penetration of veins through the dura mater, with particular reference to Pacchionian granulations. Investigations in the rat and two species of New-World monkeys (Cebus apella, Callitrix jacchus)

At the sites where a vein penetrates through the dura mater, two aspects deserve particular attention: (i) The delineation of the perivascular cleft, a space belonging to the interstitial cerebrospinal fluid (CSF) compartment, toward the interior hemal milieu of the dura mater. (ii) The relationship between the perivascular arachnoid layer and the subdural neurothelium at the point of vascular penetration. These problems were investigated in the rat and in two species of New-World monkeys (Cebus apella, Callitrix jacchus). Concerning the first aspect, tight appositions of meningeal cells to the vessel wall, the basal lamina of which is widened and enriched with microfibrils, prevent communication between the interstitial CSF in the perivascular cleft and the hemal milieu in the dura mater. With reference to the second aspect, the perivascular arachnoid cells are transformed into neurothelial cells at the point where they become exposed to the hemal milieu of the dura mater and subsequently continuous with the subdural neurothelium. Leptomeningeal protrusions encompassing outer CSF space can penetrate into the dura mater. These protrusions may expand and branch repeatedly, forming along the wall of the dural sinus Pacchionian granulations. At these sites, however, the structural integrity of the sinus wall and the Pacchionian granulation is not lost. Numerous vesiculations not only in the sinus and vascular walls, but also in the cellular arrays of the Pacchionian granulations or paravascular leptomeningeal protrusions indicate mechanisms of transcellular fluid transport. Moreover, the texture of the leptomeningeal protrusions favors an additional function of these structures as a "volume" buffer.

Kinetics and inhibition studies of catechol O-methyltransferase from the yeast Candida tropicalis

The Kms for esculetin and S-adenosyl-L-methionine for catechol O-methyltransferase from the yeast Candida tropicalis were 6.2 and 40 microM, respectively. S-Adenosyl-L-homocysteine was a very potent competitive inhibitor with respect to S-adenosyl-L-methionine, with a Ki of 6.9 microM. Of the catechol-related inhibitors, purpurogallin, with a Ki of 0.07 microM, showed the greatest inhibitory effect. Sulfhydryl group-blocking reagents, such as thiol-oxidizing 2-iodosobenzoic acid and mercaptide-forming p-chloromercuribenzoic acid, provided evidence for sulfhydryl groups in the active site of the enzyme. Yeast catechol O-methyltransferase is a metal-dependent enzyme and requires Mg2+ for full activity. Zn2+ and Mn2+ but not Ca2+ were able to substitute for Mg2+. Mn2+ showed optimal enzyme activation at concentrations 50- to 100-fold lower than those of Mg2+.

Epithelium modulates the reactivity of ovalbumin-sensitized guinea-pig airway smooth muscle

Mechanical removal of the airway epithelium alters the in vitro reactivity of airway smooth muscle. The modulation of reactivity may involve the release of inhibitory and excitatory factors from epithelial cells. Guinea pigs sensitized with ovalbumin have been used as an animal model of airway hyperreactivity. We evaluated the influence of the epithelium on the reactivity of in vitro tracheal smooth muscle from control and ovalbumin-sensitized guinea pigs, and the extent to which the presence of the epithelium affects the contractile response to in vitro challenge with ovalbumin. In both control and ovalbumin-sensitized tissues, epithelium removal increased the sensitivity of the preparations to histamine, methacholine and isoproterenol to a similar extent, i.e., 2- to 2.5-fold. Epithelium removal resulted in an 8.1-fold increase in sensitivity to ovalbumin in sensitized tissues. The epithelium appears not only to modulate the reactivity of the tissues to bronchoactive agents, but it also influences the magnitude of the contractile response following antigen challenge.

Neurochemical coupled actions of transmitters in the microvasculature of the brain

The discovery that monoamine nerves end on the central microvessels of the choroid plexus, pia-arachnoid and parenchyma has prompted an intense investigation as to their physiological and neuropathological roles. The source of the monoamine fibers to the pial vessels and choroid plexus was shown to be the superior cervical ganglion. Ganglionic stimulation causes vasoconstriction or vasodilation of pial vessels, an event depending upon the functional ratio of alpha to beta adrenergic receptors. Moreover, stimulation of the superior cervical ganglion evokes an inhibition of cerebrospinal fluid formation in choroid plexus. The locus coeruleus is the site of adrenergic nerve supply to the parenchymal capillaries and stimulation of this nucleus increases capillary permeability to small molecules and water. Neurotransmitter receptors (adrenergic, histamine, adenosine, dopamine, prostacyclin, prostaglandins and specific amino acids or neuropeptides) have been identified on microvessels and in many instances these transmitter actions are coupled to cyclic AMP synthesis. Moreover, cyclic AMP has been shown to increase the rate of capillary endothelial pinocytosis and produce brain edema. In small vessels containing smooth muscle cells cyclic AMP production improves cerebral blood flow via an initiation of vasodilatory processes. The presence of receptors for serotonin and acetylcholine have likewise been demonstrated to occur on cerebral microvessels. Limited information is available as to the receptor coupled actions of these two transmitters, but cholinergic mechanisms may act to restrict catecholamine-induced formation of cyclic AMP. Altered sensitivity of microvessels to neurotransmitters has been demonstrated following conditions of stroke, hypertension, aging, diabetes and X-irradiation.

Selective destruction of meningeal cells by 6-hydroxydopamine: a tool to study meningeal-neuroepithelial interaction in brain development

The effects of 6-hydroxydopamine (6-OHDA) on meningeal cells were studied at different ages in conjunction with blockade of the neuronal uptake 1 for catecholamines using nomifensine (NMF) and of the extraneuronal uptake 2 using normetanephrine (NMN). Our results show that maximal numbers of meningeal cells over the cerebellum of the newborn rat are destroyed by a threshold dose of 6-OHDA of 25 micrograms. The morphological characteristics which mark the time course of degeneration of meningeal cells were used to assess the effects of 6-OHDA given in conjunction with either NMF or NMN to differentiate between neuronal (uptake 1) and extraneuronal (uptake 2) effects of 6-OHDA. Uptake of 6-OHDA into meningeal cells and their subsequent degeneration was prevented by pretreatment with NMN but not with NMF. This finding indicates that meningeal cells have uptake 2 capacity but no uptake 1 membrane pump and explains both their uptake of 6-OHDA and their extreme sensitivity to the toxic effects of this drug. Application of this pharmacological regimen using NMF and NMN in conjunction with 6-OHDA thus allows selective destruction of meningeal cells which may be used experimentally to study the contribution of meningeal cells to brain development.

Soluble and particulate forms of rat catechol-O-methyltransferase distinguished by gel electrophoresis and immune fixation

Catechol-O-methyltransferase (COMT) was visualized in homogenates and subcellular fractions of rat tissues, including liver and brain, by gel electrophoresis, electrophoretic transfer of proteins to nitrocellulose (Western blotting), and immune fixation with antiserum to highly purified soluble rat liver COMT. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) of all tissue homogenates examined revealed three major immune-specific proteins with apparent molecular weights 23,000, 26,000, and 66,000 (23K, 26K and 66K). Centrifugation of homogenates at 100,000 X g for 60 min resulted in the enrichment of the 26K species protein in the pellet whereas the 23K and 66K proteins were the predominant forms in the supernatant. The 66K protein appeared in variable amounts depending on the tissue being examined and the length of transfer of protein and is assumed to be an "aggregate" of the smaller form(s). The 26K protein was essentially the only immunoreactive species seen in a purified preparation of rat liver outer mitochondrial membrane. Isoelectric focusing (IEF) under denaturing conditions and two-dimensional gel electrophoresis of brain and liver fractions showed that the 23K protein was resolved into three bands of pI 5.1, 5.2, and 5.3, whereas the 26K protein had a pI of 6.2. Analysis of COMT activity in slices from nondenaturing IEF gels indicated that the pI 5.1-5.3 species are biologically active; the pI 6.2 species could not be detected under these conditions. COMT activity was demonstrated, however, in outer mitochondrial membranes from rat liver, which contain predominantly the 26K, pI 6.2 immunoreactive species. The major form of COMT in all rat tissues examined is "soluble" with an apparent Mr of 23K and a pI of 5.2. The nature of the modifications giving rise to pI 5.1 and 5.3 forms of this enzyme are not clear, nor is the relationship between the 23K and 26K forms. Further studies are needed to elucidate the relationship of immunoreactive forms of COMT to each other, their intracellular location, and their functional significance.

Regioselective O-methylation of tetrahydropapaveroline and tetrahydroxyberbine in vivo in rat brain

Enzymatic O-methylation is a primary pathway for the metabolism of catecholamines in mammals and of isoquinoline alkaloids in plants. This report describes the differential O-methylation patterns of the racemates and enantiomers of two catecholamine-derived alkaloids, tetrahydropapaveroline (THP) and 2,3,10,11-tetrahydroxyberbine (THB), in the brain of the rat. One hour after intracerebroventricular administration of a specific isomeric form of each alkaloid, the O-methylated metabolites were isolated from the rat brain and subsequently quantified using high performance liquid chromatography. The isomeric form of THP or THB which was administered markedly influenced the pattern of O-methylation. The racemate and R-(+)-enantiomer of THP were mono-O-methylated predominantly at the 7 and 3' positions, while the S-(-)-enantiomer of THP was mono-O-methylated to an essentially equal degree at the 6, 7 and 3' positions. Minimal mono-O-methylation at the 4' position was detectable only with the racemate and (-)-enantiomer of THP. The racemate and enantiomers of THB were mono-O-methylated predominantly at the 2 and 11 positions and to a lesser extent at the 3 and 10 positions. Although minimal with the R-(+)-enantiomer, the 3 and the 10-O-methylation pathways were enhanced significantly with the S-(-)-enantiomer of THB. These results demonstrate that both enantiomers of THP and THB are O-methylated in vivo in rat brain and that the chiral centers of these alkaloids influence the position of O-methylation, thereby dictating the relative amounts of specific products formed.

Distribution of tritium label in the neonate rat brain following intracisternal or subcutaneous administration of [3H]6-OHDA. An autoradiographic study

The present report describes the distribution of tritium label after injection of newborn rats with [3H]6-hydroxydopamine ([3H]6-OHDA). The animals were injected either intracisternally (i.c.) or subcutaneously (s.c.), with or without pretreatment with nomifensine, which blocks the high-affinity uptake of both noradrenaline (NA) and dopamine (DA), and sacrificed at intervals from 40 min to 24 h post-injection (p.i.). In i.c. injected animals, tritium label is demonstrable as early as 40 min p.i. in neurons of all known NA and DA cell groups. In NA neurons, it is taken up into cell body, dendrites, preterminal and terminal axons. The intensity of neuronal labeling is highest within the first 4 h p.i. and decreases in most neurons with longer postinjection intervals. A significant proportion of both NA and DA neurons degenerate beginning 6 h p.i., the majority show morphological signs of the axon reaction 24 h p.i. Uptake of [3H]6-OHDA into serotonergic and non-catecholaminergic neurons is not demonstrable. [3H]6-OHDA is accumulated by the following extraneuronal cells of the CNS: ependymal cells, epithelial cells of the choroid plexus, subependymal macrophages, smooth muscle cells in the wall of large intraparenchymal blood vessels, meningeal cells and glial cells. The time course of accumulation and disappearance of the label varies among these extraneuronal elements. The meningeal cells show the highest labeling intensity and degenerate within 24 h p.i. After pretreatment of the animals with nomifensine, the uptake of [3H]6-OHDA into NA and DA neurons is totally blocked; by contrast uptake of the labeled drug into extraneuronal cells is not prevented. These findings show that [3H]6-OHDA is not only accumulated by neurons possessing the high-affinity uptake for NA or DA, but by numerous other, extraneuronal cells which also participate in the metabolism of catecholamines.

Immunohistochemical localization of catechol-O-methyltransferase in circumventricular organs of the rat: potential variations in the blood-brain barrier to native catechols

Catechol-O-methyltransferase (COMT) was localized in the organum vasculosum of the lamina terminals, subfornical organ, subcommissural organ and area postrema of rat brain using an indirect immunofluorescence technique. COMT immunofluorescence was apparent in neuroglia within the organum vasculosum and was most intense in the ependyma between this structure and the optic recess of the third ventricle. In both the subfornical organ and the area postrema, COMT was localized in a neuroglial network, but was noticeably absent in the ependymal layer. COMT immunofluorescence in the ependyma of the subcommissural organ was continuous with the more intense immunofluorescence of the cuboidal ependyma of the third ventricle. Each of the circumventricular organs studied, with the exception of the subcommissural organ, lies outside the blood-brain barrier. However, the unique pattern of COMT immunofluorescence in the area postrema and the subfornical organ suggests that these two structures, of all circumventricular organs, are most likely to permit the entry of peripherally circulating catechols to the cerebrospinal fluid.

Experimental studies on cerebellar foliation. I. A qualitative morphological analysis of cerebellar fissuration defects after neonatal treatment with 6-OHDA in the rat

The present report describes the natural history of defective cerebellar fissuration in the rat after neonatal treatment with 6-hydroxydopamine (6-OHDA). Within 24 hours after an intracisternal (IC) injection of 100 micrograms 6-OHDA cerebellar pial fibroblasts degenerated almost completely and were phagocytosed b macrophages within 2-5 days postinjection (dpi) leaving the cerebellar surface denuded of pia. Bergmann glia end feet at first exhibited morphological signs of gliosis and later formed new sprouts that penetrated the basal lamina and grew into the subarachnoid space covering regenerating pial fibroblasts and also invading ectopic colonies of external granular layer (EGL) cells. Breaches in the basal lamina appeared after the pial fibroblast had been destroyed and were confined to areas where Bergmann glia end feet were absent and where EGL cells were opposed to the basal lamina. EGL cells escaped through these fractures into the subarachnoid space in the fissures, where they proliferated to form large colonies of granule and stellate cells. In those fissures in which EGL ectopia featured, opposing folia fused and fissures were lost. These findings suggest that pial fibroblasts and the basal lamina have an important role in maintaining lobular partition during development of the cerebellum, in establishing cerebellar fissures, and in consolidating Bergmann glia-EGL cell relationships as a prerequisite for orderly migration of EGL cells.