Na+-dependent gamma-aminobutyric acid (GABA) transport in the choroid plexus of rabbit

The betaine-GABA transporter (BGT1, slc6a12) is predominantly expressed in the liver and at lower levels in the kidneys and at the brain surface

The Na+- and Cl−-dependent GABA-betaine transporter (BGT1) has received attention mostly as a protector against osmolarity changes in the kidney and as a potential controller of the neurotransmitter GABA in the brain. Nevertheless, the cellular distribution of BGT1, and its physiological importance, is not fully understood. Here we have quantified mRNA levels using TaqMan real-time PCR, produced a number of BGT1 antibodies, and used these to study BGT1 distribution in mice. BGT1 (protein and mRNA) is predominantly expressed in the liver (sinusoidal hepatocyte plasma membranes) and not in the endothelium. BGT1 is also present in the renal medulla, where it localizes to the basolateral membranes of collecting ducts (particularly at the papilla tip) and the thick ascending limbs of Henle. There is some BGT1 in the leptomeninges, but brain parenchyma, brain blood vessels, ependymal cells, the renal cortex, and the intestine are virtually BGT1 deficient in 1- to 3-mo-old mice. Labeling specificity was assured by processing tissue from BGT1-deficient littermates in parallel as negative controls. Addition of 2.5% sodium chloride to the drinking water for 48 h induced a two- to threefold upregulation of BGT1, tonicity-responsive enhancer binding protein, and sodium- myo-inositol cotransporter 1 (slc5a3) in the renal medulla, but not in the brain and barely in the liver. BGT1-deficient and wild-type mice appeared to tolerate the salt treatment equally well, possibly because betaine is one of several osmolytes. In conclusion, this study suggests that BGT1 plays its main role in the liver, thereby complementing other betaine-transporting carrier proteins (e.g., slc6a20) that are predominantly expressed in the small intestine or kidney rather than the liver.

GABA transporters in the mammalian cerebral cortex: localization, development and pathological implications

The extracellular levels of gamma-aminobutyric acid (GABA), the main inhibitory neurotransmitter in the mammalian cerebral cortex, are regulated by specific high-affinity, Na+/Cl- dependent transporters. Four distinct genes encoding GABA transporters (GATs), named GAT-1, GAT-2, GAT-3, and BGT-1 have been identified using molecular cloning. Of these, GAT-1 and -3 are expressed in the cerebral cortex. Studies of the cortical distribution, cellular localization, ontogeny and relationships of GATs with GABA-releasing elements using a variety of light and electron microscopic immunocytochemical techniques have shown that: (i) a fraction of GATs is strategically placed to mediate GABA uptake at fast inhibitory synapses, terminating GABA's action and shaping inhibitory postsynaptic responses; (ii) another fraction may participate in functions such as the regulation of GABA's diffusion to neighboring synapses and of GABA levels in cerebrospinal fluid; (iii) GATs may play a role in the complex processes regulating cortical maturation; and (iv) GATs may contribute to the dysregulation of neuronal excitability that accompanies at least two major human diseases: epilepsy and ischemia.

Transport of 14C-gamma-aminobutyric acid into brain, cerebrospinal fluid and choroid plexus in neonatal and adult rats

In general blood to brain entry of amino acids is greater in the neonatal rats compared to the adults. gamma-Aminobutyric acid (GABA), a neurotransmitter amino acid, shows limited transport across the blood-brain barrier (BBB) in the adult rat. Characteristics of GABA entry into the immature rat brain is yet to be addressed. This investigation was set to study the entry of GABA into brain of the neonatal rat compared to the adult. Using the bilateral in situ brain perfusion technique, the entry of 14C-GABA into brain, cerebrospinal fluid (CSF) and lateral ventricles choroid plexuses was studied in the adult and neonatal rats. 14C-GABA uptake into neonatal rat brain after 20 min perfusion was 0.116+/-0.014 ml g(-1), approximately twice that of the adults (P<0.01). Half saturation constant, K(m), did not change with age (P>0.05), whereas maximal transport into the brain, V(max), was reduced from 0.152 to 0.068 nmol min(-1) g(-1) showing a significant reduction with age (P<0.05). In the neonate the entry of GABA into the CSF was dominant when compared to that into the brain, this could be due to a greater diffusional component, K(d), which was detected to be high in the neonate. In conclusion, the uptake of 14C-GABA into brain of the immature rats exceeded that in the adults which is thought to be due to both greater maximal transport and greater diffusion in the neonate compared to the adult.

gamma-Aminobutyric acid transporters in the cat periaqueductal gray: a light and electron microscopic immunocytochemical study

The gamma-aminobutyric acid (GABA) plasma membrane transporters (GATs) mediate GABA uptake into presynaptic axon terminals and glial processes, thus contributing to the regulation of the magnitude and duration of the action of GABA at the synaptic cleft. The aim of the present study was to investigate the expression of three high-affinity GABA transporters (GAT-1, GAT-2, and GAT-3) in the periaqueductal gray matter (PAG) of adult cats by using immunocytochemistry with affinity-purified antibodies. Light microscopic observations revealed GAT-1 immunoreactivity in punctate structures, particularly dense in the lateral portion of the dorsolateral PAG column. Weak GAT-2-immunopositive puncta were homogeneously distributed in the PAG. GAT-3 immunoreactivity was detected in each column of the PAG but was more intense in the dorsolateral PAG column and around the aqueduct. Electron microscopic studies showed GAT-1 immunoreactivity in distal astroglial processes, in unmyelinated and small myelinated axons, and in axon terminals making symmetric synapses on both PAG neurons and dendrites. GAT-2 immunoreactivity was present mostly in the form of patches of different sizes in the cytoplasm of neuronal elements like the perikarya and dendrites of PAG neurons, in myelinated and unmyelinated axons, and in the axon terminals forming both symmetric and asymmetric synapses. Labeling was also observed in nonneuronal elements. Astrocytic cell bodies and their distal processes as well as the ependymal cells lining the wall of the aqueduct showed patches of GAT-2 immunoreactivity. Electron microscopic observation revealed GAT-3 immunoreactivity exclusively in distal astrocytic processes adjacent to the somata of PAG neurons and in axon terminals making both symmetric and asymmetric synapses. The present results suggest that three types of termination systems of GABAergic transmission are present in the cat periaqueductal gray matter.

Mechanisms of 5-aminolevulinic acid uptake at the choroid plexus

5-Aminolevulinic acid (5-ALA) is a precursor of porphyrins and heme that has been implicated in the neuropsychiatric symptoms associated with porphyrias. It is also being used clinically to delineate malignant gliomas. The blood-CSF barrier may be an important interface for 5-ALA transport between blood and brain as in vivo studies have indicated 5-ALA is taken up by the choroid plexuses whereas the normal blood-brain barrier appears to be relatively impermeable. This study examines the mechanisms of 5-[(3)H]ALA uptake into isolated rat lateral ventricle choroid plexuses. Results suggest that there are two uptake mechanisms. The first was a Na(+)-independent uptake system that was pH dependent (being stimulated at low pH). Uptake was inhibited by the dipeptide Gly-Gly and by cefadroxil, an alpha-amino-containing cephalosporin. These properties are the same as the proton-dependent peptide transporters PEPT1 and PEPT2, which have recently been shown to transport 5-ALA in frog oocyte expression experiments. Choroid plexus uptake was not inhibited by captopril, a PEPT1 inhibitor, suggesting PEPT2-mediated uptake. The presence of PEPT2 and absence of PEPT1 in the choroid plexus were confirmed by western blotting. The second potential mechanism was both Na(+) and HCO(3)(-) dependent and appears to be an organic anion transporter, although it is possible that removal of Na(+) and HCO(3)(-) may indirectly affect PEPT2 by affecting intracellular pH. The presence of PEPT2 and a putative Na(+)/HCO(3)(-)-dependent organic anion transporter is important not only for an understanding of 5-ALA movement between blood and brain but also because these transporters may affect the distribution of a number of drugs between blood and CSF.

Choroid plexus in the central nervous system: biology and physiopathology

Choroid plexuses (CPs) are localized in the ventricular system of the brain and form one of the interfaces between the blood and the central nervous system (CNS). They are composed of a tight epithelium responsible for cerebrospinal fluid secretion, which encloses a loose connective core containing permeable capillaries and cells of the lymphoid lineage. In accordance with its peculiar localization between 2 circulating fluid compartments, the CP epithelium is involved in numerous exchange processes that either supply the brain with nutrients and hormones, or clear deleterious compounds and metabolites from the brain. Choroid plexuses also participate in neurohumoral brain modulation and neuroimmune interactions, thereby contributing greatly in maintaining brain homeostasis. Besides these physiological functions, the implication of choroid plexuses in pathological processes is increasingly documented. In this review, we focus on some of the novel aspects of CP functions in relation to brain development, transfer of neuro-humoral information, brain/immune system interactions, brain aging, and cerebral pharmaco-toxicology.

Autoradiographic distribution of radioactivity from (14)C-GABA in the mouse

We investigated the distribution of radioactivity from (14)C-labeled gamma-aminobutyric acid (GABA) in the mouse by in vivo autoradiography to clarify the tissues that show GABA uptake and/or GABA binding. Male mice were injected intravenously with (14)C-GABA in both the absence and presence of an excess of unlabeled GABA, baclofen and isoguvacine. Whole-body autoradiography of (3)H-baclofen, a GABA(B) receptor agonist was also performed. At short intervals after (14)C-GABA injection ( 3 and 6 minutes), very high radioactivity was detected in the kidney cortex, liver, pineal gland, hypophysis, median eminence of the hypothalamus, and cervical ganglion. The hyaline cartilage and glandular part of the stomach showed moderate radioactivity. In the presence of an excess amount of unlabeled GABA, radioactivity in most of tissues decreased significantly, but no significant difference in radioactivity was observed in the presence of baclofen and isoguvacine, agonists of GABA(A) and GABA(B) receptors, respectively. Autoradiography of (3)H-baclofen showed that the kidney had high level of radioactivity, whereas the activity in other tissues and organs was similar or lower than in the blood except for the content of the urinary bladder and the pancreas at 15 minutes after injection. These data indicate that radioactivity from incorporated (14)C-GABA into a variety of cells is much higher than that from bound (14)C-GABA to the receptor sites. Our results suggest that GABA can be quickly localized in many organs of the mouse body after 3 minutes following injection, and GABA may serve multiple functions in those organs.

Sodium and chloride-dependent high and low-affinity uptakes of GABA by brain capillary endothelial cells

The mechanisms of carrier-mediated transport of gamma-aminobutyric acid (GABA) at the blood-brain barrier (BBB) were examined by investigating [3H]GABA uptake by isolated bovine brain capillaries, monolayers of primary cultured brain capillary endothelial cells (BCECs) attached to plates or suspended BCECs. The uptake of [3H]GABA was concentration-dependent and saturable. Nonlinear regression analysis of the original data indicated the existence of two distinct high and low-affinity GABA transporters on isolated brain capillaries or suspended BCECs, with Km1, Km2, Vm1 and Vm2 equal to 25.3 microM, 485.2 microM, 3.6 and 8.4 nmol/5 min/mg protein, respectively, for the capillaries, and 21.3 microM, 322.0 microM, 6.1 and 15.7 nmol/5 min/mg protein, respectively, for the suspended BCECs. In contrast, a single low-affinity transporter was found for monolayers of BCECs attached to plates with Km and Vm equal to 338.7 microM and 18.8 nmol/5 min/mg protein, respectively. Subcellular location of the two distinct transporters on BCECs is discussed, suggesting that the low-affinity GABA transporter is probably localized to the luminal membrane of BCECs, and the high-affinity GABA transporter is probably localized to the antiluminal membrane. Low temperature (4 degreesC) and metabolic inhibitors markedly diminished both high and low-affinity uptakes of [3H]GABA by isolated brain capillaries. The substitution of Na+ with choline+, K+ or Li+ with the counter anion Cl- almost completely abolished both uptakes. Substitution of Cl- with Br-, I-, F- or NO3- in the presence of Na+ significantly reduced both uptakes to different extents. Alanine, leucine, phenylalanine, arginine, glutamate and pyruvate had no obvious effect on either uptake. Probenecid, amino-oxyacetic acid, beta-alanine, taurine, betaine, and nipecotic acid significantly reduced both uptakes. These data suggested that both the GABA transporters at the BBB were temperature, metabolic energy, Na+ and Cl--dependent, and may be specific and different from the known monocarboxylic acid, GABA and other amino acid transporters, which may play a role in the disposition of GABA in the brain.