Quantitative autoradiography of [3H]ouabain binding sites in rat brain

Unaltered cerebral Na+,K+-ATPase activity after hypoxic/ischemic injury in piglets

We examined whether hypoxic/ischemic (H/I) stress decreased the cerebral Na(+),K(+)-ATPase enzyme activity (NEA) of newborn pigs. The effects of global ischemia (10 min), asphyxia (10 min), and incomplete forebrain ischemia (45 min) were analyzed in ten different brain regions. The lengths of the reperfusion periods varied between 15 min and 3 h. NEA was determined as the ouabain-sensitive fraction of the total ATPase activity of the sample. Marked regional differences in NEA were observed in all experimental groups, whereas NEA was not significantly affected in any of the brain structures investigated. The present results suggest that damaged brain Na(+),K(+)-ATPase may not be the cause of the neuronal-vascular impairment following H/I stress.

Regional distributions of hippocampal Na+,K(+)-ATPase, cytochrome oxidase, and total protein in temporal lobe epilepsy

Na+,K(+)-ATPase (the sodium pump) is a ubiquitous enzyme that consumes ATP to maintain an adequate neuronal transmembrane electrical potential necessary for brain function and to dissipate ionic transients. Reductions in sodium pump function augment the sensitivity of neurons to glutamate, increasing excitability and neuronal damage in vitro. Temporal lobe epilepsy (TLE) is one disease characterized by hyperexcitability and marked hippocampal neuronal losses that could depend in part, on impaired sodium pump capacity secondary to changes in sodium pump levels and/or insufficient ATP supply. To assess whether abnormalities in the sodium pump occur in this disease, we used [3H]ouabain to determine the density of Na+,K(+)-ATPase for each anatomic region of hippocampus by in vitro autoradiography. Tissues were surgically obtained from epileptic patients with hippocampal sclerosis and compared with specimens from patients with seizures originating from temporal lobe tumors and autopsy controls. Changes in cellular population arising from neuronal losses or gliosis were assessed by protein densities derived from quantitative computerized densitometry of Coomassie-stained tissue sections. We estimated regional differences in capacity for ATP generation by determining cytochrome c oxidase (CO) activity. Principal neurons of hippocampus exhibit high levels of sodium pump enzyme. Both epilepsy groups exhibited slight but significant increases in sodium pump density/unit mass of protein in the dentate molecular layer, CA2, and subiculum as compared with autopsy controls. Greater hilar sodium pump density was also observed in sclerotic hippocampi. In contrast, CO activity was reduced in both epilepsy types throughout hippocampus. Results suggest that although sodium pump protein in surviving neurons appears to be upregulated in epilepsy, sodium pump capacity may be limited by the reduced levels of CO activity. Functional reduction in sodium pump capacity may be an important factor in hyperexcitability and neuronal death.

Subchronic methamphetamine treatment enchances ouabain-induced striatal dopamine efflux in vivo

The effect of manipulation of the Na+ gradient between the intracellular and extracellular media on striatal dopamine (DA) efflux under steady-state conditions after subchronic methamphetamine (MAP) treatment was investigated. Rats were injected with 4 mg/kg MAP or saline (i.p.), once daily for 14 days. Seven days after the last injection, ouabain (10(-4) M), a selective inhibitor of the Na+,K(+)-ATPase, was infused locally through a semi-permeable probe in the striatum. Ouabain induced a significantly greater (P less than 0.01) increase of the DA concentrations in the striatal perfusate in the subchronic MAP than the control group. The levels of 3,4-dihydroxyphenylacetic acid (DOPAC) (P less than 0.05) and 5-hydroxyindoleacetic acid (5-HIAA) (P less than 0.05) were significantly higher in the subchronic MAP than in the control group. Reserpine pretreatment (5 mg/kg, i.p.) did not affect the enhanced ouabain-induced DA efflux (P less than 0.01) in the subchronic MAP group, and the levels of DOPAC (P less than 0.01), 5-HIAA (P less than 0.01) and HVA (P less than 0.01) were also significantly higher in the subchronic MAP than in the control group. In contrast, alpha-methyl-p-tyrosine (250 mg/kg, i.p.) pretreatment abolished the ouabain-induced efflux of DA, DOPAC and HVA, but not 5-HIAA, in both groups. Specific striatal [3H]ouabain binding and striatal Na+,K(+)-ATPase activity in the subchronic MAP and control groups did not differ significantly. These results suggest that subchronic MAP treatment facilitates the efflux of newly synthesized DA, which is induced by the ouabain-induced decrease of the Na+ gradient between intracellular and extracellular media.(ABSTRACT TRUNCATED AT 250 WORDS)

Inhibition of sodium-potassium-ATPase: a potentially ubiquitous mechanism contributing to central nervous system neuropathology

Direct and indirect evidence suggests that Na+/K(+)-ATPase activity is reduced or insufficient to maintain ionic balances during and immediately after episodes of ischemia, hypoglycemia, epilepsy, and after administration of excitotoxins (glutamate agonists). Recent results show that inhibition of this enzyme results in neuronal death, and thus a hypothesis is proposed that a reduction and/or inhibition of this enzyme contributes to producing the central neuropathy found in the above disorders, and identifies potential mechanisms involved. While the extent of inhibition of Na+/K(+)-ATPase during ischemia, hypoglycemia and epilepsy may be insufficient to cause neuronal death by itself, unless the inhibition is severe and prolonged, there are a number of interactions which can lead to a potentiation of the neurotoxic actions of glutamate, a prime candidate for causing part of the damage following trauma. Presynaptically, inhibition of the Na+/K(+)-ATPase destroys the sodium gradient which drives the uptake of acidic amino acids and a number of other neurotransmitters. This results in both a block of reuptake and a stimulation of the release not only of glutamate but also of other neurotransmitters which modulate the neurotoxicity of glutamate. An exocytotic release of glutamate can also occur as inhibition of the enzyme causes depolarization of the membrane, but exocytosis is only possible when ATP levels are sufficiently high. Postsynaptically, the depolarization could alleviate the magnesium block of NMDA receptors, a major mechanism for glutamate-induced neurotoxicity, while massive depolarization results in seizure activity. With less severe inhibition, the retention of sodium results in osmotic swelling and possible cellular lysis. A build-up of intracellular calcium also occurs via voltage-gated calcium channels following depolarization and as a consequence of a failure of the sodium-calcium exchange system, maintained by the sodium gradient.

Cytoarchitectural relationships between [3H]ouabain binding and mRNA for isoforms of the sodium pump catalytic subunit in rat brain

We examined the cell type-specific expression of the alpha 1, alpha 2, and alpha 3 subunits of the sodium pump in rat brain using in situ hybridization and [3H]ouabain autoradiography. These techniques allowed us to colocalize mRNA and functional alpha 2/alpha 3 pumps on adjacent sections. The perikarya of many neurons possessed high levels of alpha 1 and/or alpha 3 transcripts, while alpha 2 mRNA appeared to be present in only a few neuronal types. [3H]Ouabain binding in general paralleled the distribution of alpha 3 mRNA-positive neurons. The regional variation of alpha 1 and alpha 3 transcripts was complex and varied. Large neurons of the olfactory bulb and piriform cortex expressed high levels of alpha 3 transcripts, but low levels of alpha 1 mRNA. In frontal cortex, neurons of layers II-III were enriched in alpha 1 mRNA, while those in layer V exhibited high levels of alpha 3 transcripts. In the hippocampus, principal neurons expressed all three alpha subunit mRNAs. CA subfield pyramidal neurons exhibited a high alpha 3/alpha 1 ratio, while dentate granule cells and hilar pyramidal neurons expressed approximately equal levels of alpha 1 and alpha 3. In the cerebellum, Purkinje and Golgi cells were rich in alpha 3 mRNA, while the granule cells appeared to express only alpha 1 transcripts. The distribution of functional sodium pump protein, as localized by [3H]ouabain binding, was highest in the neuropil of the hippocampus and cerebral cortex, and lowest over perikarya and white matter. [3H]ouabain did not bind to alpha 1 pump units, as confirmed by the complete absence of labeling over the choroid plexus, a tissue expressing only alpha 1 mRNA. In the cerebellum, regions of dense [3H]ouabain binding were localized to the granule cell layer, the inner third of the molecular layer in the basket region, and the deep cerebellar nuclei. Surprisingly, the dense neuropil in the outer 2/3 of the molecular layer lacked high [3H]ouabain binding. Thus, functional alpha 3 sodium pump units appear distributed to the axon terminals and not to apical dendrites of Purkinje, Golgi and basket cells. A similar pattern of increased [3H]ouabain binding in axonal but not dendritic fields of alpha 3-enriched neurons was present in the cerebral cortex and the hippocampus. Considering that many alpha 3-enriched neurons are of the Golgi I type with long axons, the alpha 3 isoform may be preferentially directed into axons to function in presynaptic membranes.

Ultrastructural localization of Na+/K(+)-ATPase in rodent olfactory epithelium

The olfactory epithelium is comprised of bipolar sensory neurons, sustentacular cells, and basal cells. The sensory neurons have apical knobs and cilia, which project into the olfactory mucus toward the nasal lumen, and represent presumptive sites of odorant binding. Ionic currents, measured across this epithelium in both the resting and odorant-stimulated states, are known to be sustained, at least in part, by active transport of sodium. Information identifying the cellular sites of ion transport in olfactory sensory epithelium will therefore aid in elucidating the ionic mechanisms associated with olfactory transduction. The membrane-bound enzyme Na+/K(+)-ATPase mediates active ion transport in many other cells and tissues. We have consequently employed the cytochemical technique reported by Ernst (J. Histochem. Cytochem., 20 (1972) 23-38, 1322) to identify possible sites of elevated Na+/K(+)-ATPase activity in olfactory epithelium. This procedure detects inorganic phosphate (Pi) released from an artificial substrate (nitrophenyl phosphate) by enzyme catalytic activity. In the presence of strontium ion. Pi is precipitated near regions of high enzymatic activity, then converted to a product visible in the electron microscope. Parallel control preparations were incubated in media (1) supplemented with the specific Na+/K(+)-ATPase inhibitor ouabain (to abolish formation of specific reaction product); (2) with substrate deleted (to demonstrate possible non-specific binding of Sr2+ and/or Pb2+); or (3) with the necessary cofactor K+ deleted. In tissues incubated for demonstration of Na+/K(+)-ATPase activity, reaction product was associated with apical knobs, cilia and dendrites of olfactory receptor neurons at the apical surface. In the more proximal region of the epithelium, reaction product was associated with cell bodies and axons of the sensory neurons, and with the lateral membranes of sustentacular cells. Reaction product was deposited intracellularly, compatible with the known mechanism of the Na+/K(+)-ATPase enzymatic reaction. In control specimens incubated with ouabain, with substrate deleted, or with K+ deleted, only a small quantity of non-specific precipitate was observed. These results are discussed with reference to the various sodium currents implicated in olfactory transduction and transepithelial transport.

Effect of Na+, K(+)-ATPase modifiers on high-affinity ouabain binding determined by quantitative autoradiography

There is growing evidence on the existence of endogenous ouabain-like factors that modulate Na+, K(+)-ATPase activity. In this laboratory, two soluble subfractions (peaks I and II) were previously separated from rat cerebral cortex, which had opposite effects on Na+, K(+)-ATPase activity. Peak I stimulated and peak II inhibited the enzyme (Rodríguez de Lores Arnaiz and Antonelli de Gómez de Lima, Neurochem Res 11:933-947, 1986). The same effects are now reported for K(+)-p-nitrophenyphosphatase activity. Localization of high-affinity ouabain binding in rat brain was done by quantitative autoradiography using a microcomputer digital imaging system. Peak I did not modify, whereas peak II blocked ouabain binding in areas 3-4 of cerebral cortex, dentate gyrus, stria terminalis, thalamic nuclei, and basal ganglia. Similar results were obtained when ouabain binding was determined in rabbit cerebral cortex and by a conventional filtration assay in nerve ending membranes obtained from rat cerebral cortex. These results favour the idea that the factor present in peak II fraction might behave as an ouabain-like substance.

Changes of salt intake and of (Na+K)-ATPase activity in brain after high dose treatment with deoxycorticosterone

Mineralocorticoids (MC) have a dual effect on salt intake: in adrenalectomized rats, they reduce previously elevated salt intake; and in intact rats a high MC dose increases salt intake. We have studied the activity of (Na+K)-ATPase and [3H]ouabain binding in rats treated with deoxycorticosterone (DOC) in doses that elicited a salt appetite. Brains were removed from control and treated animals, and 20 different areas were punched out from brain slices cut every 300 microns. DOC treatment significantly reduced (Na+K)-ATPase activity in the lateral hypothalamic area, anterior amygdaloid and lateral amygdaloid nuclei, while increasing it in the periventricular gray matter; changes in other regions were not significant. Binding of [3H]ouabain was not modified by DOC treatment. In parallel experiments, we determined MC receptors in adrenalectomized rats. Binding of [3H]aldosterone was preferentially found in hippocampus, followed by lateral septum, anterior, posterior and lateral amygdaloid areas, with lower levels in other regions. However, there was no correlation between [3H]aldosterone binding and (Na+K)-ATPase activity in brain punches from either control or DOC-treated rats. Further experiments are needed to ascertain if (Na+K)-ATPase changes in discrete areas of the brain containing moderate levels of mineralocorticoid receptors, are related to the behavioral effects of DOC.

Localization and characterization of binding sites with high affinity for [3H]ouabain in cerebral cortex of rabbit brain using quantitative autoradiography

[3H]Ouabain binding was studied in sections of rabbit somatosensory cortex by quantitative autoradiography and in rabbit brain microsomal membranes using a conventional filtration assay. KD values of 8-12 nM for specific high-affinity binding of [3H]ouabain were found by both methods. High-affinity binding was not uniformly distributed in somatosensory cortex and was localized predominantly to laminae 1, 3, and 4. [3H]Ouabain binding in tissue sections was stimulated by the ligands Mg2+/Pi or Mg2+/ATP/Na+ and was inhibited by K+ (IC50 = 0.7-0.9 mM), N-ethylmaleimide, 5,5'-dithiobis(2-nitrobenzoic acid), and erythrosin B. We conclude that [3H]ouabain is reversibly and specifically bound with high affinity in rabbit brain tissue sections under conditions that favor phosphorylation of Na+,K+-ATPase. Quantitative autoradiography is a powerful tool for assessing the affinity and number of specific ouabain binding sites in brain tissue.

Autoradiographic visualization and characterization of [3H]ouabain binding to the Na+,K+-ATPase of rat brain and pineal

Ouabain binds to the catalytic subunit of Na+,K+-ATPase and specific [3H]ouabain binding can be used as a measure of the number of active enzyme molecules present in a given tissue. Specific [3H]ouabain binding can be demonstrated in frozen, cryostat sections from rat brain and pineal and these sites have the characteristics of Na+,K+-ATPase. Incubations carried out in the absence of ATP or the presence of excess unlabeled ouabain reduces specific binding by greater than or equal to 98%. The addition of K+ or omission of Mg2+ also result in a decrease in specific binding. Strophanthidin, digoxin and digoxigenin displace [3H]ouabain binding with IC50 values of 0.73, 0.48 and 1.4 microM, respectively. Scatchard analyses of specific [3H]ouabain binding in brain sections shows a single class of non-interacting binding sites with an apparent affinity (Kd) of 339 nM and a maximal binding capacity (Bmax) of 34.9 pmol/mg protein. [3H]Ouabain binding is unevenly distributed throughout the brain with the olfactory nuclei, superior colliculus, dentate gyrus, pontine nuclei and pineal gland having a relatively high density of binding sites. The outer layers (1-3) of the cerebral cortex show more labeling than the inner layers (4-6) and most other brain areas have intermediate levels of [3H]ouabain binding sites, whereas white matter has virtually no specific binding. Computer-assisted densitometry was used to measure changes in specific [3H]ouabain binding after kainic acid injection into the caudate nucleus. An initial increase in [3H]ouabain binding was observed at 1 and 24 h after lesioning and a decrease in [3H]ouabain binding was evident by 9 days after lesioning.(ABSTRACT TRUNCATED AT 250 WORDS)

Ouabain-induced changes in extracellular aspartate, glutamate and GABA levels in the rabbit olfactory bulb in vivo

The effect of ouabain on extracellular amino acid levels was investigated in the rabbit olfactory bulb using brain dialysis. Extracellular field potentials, elicited by stimulation of the lateral olfactory tract (LOT), were recorded simultaneously. Ouabain (100 microM) induced a rapid increase in extracellular aspartate, glutamate and gamma-aminobutyric acid. LOT-evoked potentials changed concomitantly, suggesting a neuronal depolarization.

Axonal transport of Na+,K+-ATPase identified as a ouabain binding site in rat sciatic nerve

Na+,K+-ATPase levels were measured in different segments of rat sciatic nerves by in vitro binding of [3H]ouabain. Binding sites were found to accumulate on both sides of a ligature tied on the sciatic nerve, indicating an anterograde and retrograde axoplasmic transport of Na+,K+-ATPase. Accumulation of Na+,K+-ATPase at the ligature was time dependent and appeared to occur through fast axoplasmic transport mechanisms. This accumulation on both sides of the ligature was also visualized by autoradiographic studies in longitudinal section of sciatic nerves using [3H]ouabain.