Ultracytochemical distribution of ouabain-sensitive, K+-dependent, p-nitrophenylphosphatase in the synaptic layers of goldfish retina

Stimulation of sodium pump restores membrane potential to neurons excited by glutamate in zebrafish distal retina

Glutamate either depolarizes or hyperpolarizes retinal neurons. Those are the initial and primary effects. Using a voltage probe (oxonol, DiBaC4 (5)) to study dissociated zebrafish retinal neurons, we find a secondary, longer-term effect: a post-excitatory restoration of membrane potential, termed after-hyperpolarization (AHP). AHP occurs only in neurons that are depolarized by glutamate and typically peaks about 5 min after glutamate application. AHP is seen in dissociated horizontal cells (HCs) and hyperpolarizing, or OFF type, bipolar cells (HBCs). These cells commonly respond with only an AHP component. AHP never occurs in depolarizing, or ON type, bipolar cells (DBCs), which are cell types hyperpolarized by glutamate. AHP is blocked by 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX). It is evoked by kainate, AMPA and the AMPA-selective agonist (S)-5-fluorowillardiine, but not by NMDA, D-aspartate, the kainate-selective agonist SYM 2081 or by DL-2-amino-4-phosphonobutyric acid (DL-AP4). Cells with exclusively AHP responses are tonically depolarized. Resting potentials can be restored by nifedipine, suggesting a tonic, depolarizing action of L-type Ca2+ channels. However AHP is not blocked by nifedipine and is insensitive to [Cl-]o. AHP is blocked by Li+o substitution for Na+o and by ouabain. A mechanism is proposed in which Na+ entering through ionotropic AMPA channels stimulates Na+,K+-ATPase, which, by electrogenic action, restores membrane potential, generating the AHP response. Patterns of ATPase immunoreactivity support localization in the outer plexiform layer (OPL) as cone pedicles, HCs and BCs were positively labelled. Labelling was weaker in the inner plexiform layer (IPL) than in nuclear layers, though two IPL bands of immunoreactive BC terminals could be discerned, one in sublamina a and the other in sublamina b. Persistent stimulation of distal retina by photoreceptor glutamate may induce increased expression and activity of Na+,K+-ATPase, with a consequent impact on distal glutamate responses.

"Lens mitochondria" in the retinal cones of the tree-shrew Tupaia belangeri

In all mammals, the mitochondria of the cones of the retina are concentrated in the inner segment. Uniquely in tree-shrews (Tupaia, Scandentia, Mammalia), a "megamitochondrion" exhibiting highly specialized systems of densely packed cristae and a very electron dense matrix, is located apically in the inner segment. The ellipsoid is a solid body containing several megamitochondria and, towards its base, a large number of smaller mitochondria. The refractive index of isolated, but not oriented, inner segments of Tupaia belangeri is higher (XA = 1.405) than in any other mammal studied so far. The consistent geometrical pattern of the multilamellar crista-matrix systems, oriented longitudinally towards the outer segment, suggests an additional optical function of the megamitochondria.

Developmental changes of K(+)-dependent para-nitrophenylphosphatase (Na(+)-K(+)-ATPase) distribution in the synaptic regions in the cerebral cortex of rats

Using Mayahara's method, the distribution of K(+)-dependent p-nitrophenylphosphatase activity was examined electron microscopically in the synaptic regions of the cerebral cortex of 10, 15 and 60-day-old Wistar rats. The enzyme achieved gradually its characteristic localization and uniform distribution. The main developmental changes were associated with the establishment of the postsynaptic density's activity. The controls with ouabain revealed activity only on the postsynaptic densities.

Synaptic organization of regenerated retina in the goldfish

In the adult goldfish, any manipulation that significantly depletes retinal neurons stimulates neurogenesis and the regeneration of nearly normal retina. We sought to determine the extent to which the regenerated neurons formed normal synaptic connections. We used qualitative and quantitative electron microscopy to compare the organization of the synaptic layers in regenerated and normal retinas. In eight eyes, a small patch of retina was surgically excised, stimulating regeneration of new retina in its place. Animals were killed 16-20 weeks after surgery. Qualitative comparisons of the synaptic architecture of photoreceptor terminals in the outer plexiform layer and quantitative comparisons of the synaptic organization in the inner plexiform layer were made between the patch of regenerated retina and an adjacent intact site. In the regenerated outer plexiform layer, cone pedicles and rod spherules were not arranged as regularly as normal, but they formed normal-appearing synaptic contacts. In the regenerated inner plexiform layer, with one exception, the quantitative descriptors of the synaptic organization in the normal and regenerate were not significantly different: The planimetric and numerical densities of the synapses, number of synapses/inner retinal neuron, and, with the exception of the bipolar terminals in the inner plexiform layer, and synapse depth profiles were similar. These data suggest that 1) relatively normal synaptic connections are recreated during regeneration, 2) the cellular mechanisms that guide synaptogenesis during development act during retinal regeneration, and 3) the physiological response properties of regenerated neurons should be comparable to that found in the normal retina.

Na+, K(+)-ATPase of the photoreceptor: selective expression of alpha 3 and beta 2 isoforms

In photoreceptors, Na+, K(+)-ATPase maintains the ion gradients which power the dark current that sustains the response to light. The enzyme is composed of at least two polypeptides: alpha (the catalytic subunit) and beta. Three different isoforms of the alpha subunit and two isoforms of the beta subunit have been identified in rat. In some tissues, the isoenzymes have been shown to be differentially expressed during development or in response to varying physiological conditions. RNAs prepared from isolated photoreceptors and from whole retina were analyzed on blots that were hybridized with cDNA probes for the alpha 1, alpha 2, alpha 3, beta 1 and beta 2 isoforms. The predominant alpha and beta subunit mRNAs present in the photoreceptor preparation were those encoding the alpha 3 and beta 2 isoforms, accounting for 85% of the total alpha signal and 79% of the total beta signal, respectively. Proportions of each mRNA were similar in retina, but very different from those observed in two control tissues, brain and kidney. To confirm that the alpha-subunit mRNA species detected were translated, membranes prepared from isolated photoreceptors and whole retina were examined by immunoblotting. The antibodies detected a pattern of alpha isoform distribution in these tissues and in kidney and brain controls that agreed remarkably well with the pattern of mRNA expression in the same tissues. Moreover, the alpha 3 isoform was detectable in the inner segment plasma membrane of the photoreceptor by electron microscopic immunocytochemistry. These results indicate that alpha 3, and beta 2 are the predominant isoforms of Na+, K(+)-ATPase expressed in photoreceptors and retina.

Complex Expression Patterns for Na+,K+-ATPase Isoforms in Retina and Optic Nerve

Three genetically distinct isozymes of the catalytic subunit of the Na,K-ATPase have been detected and have been designated alpha1, alpha2, and alpha3. To determine whether their expression is restricted to identifiable neurons and glia, specific monoclonal antibodies were used for immunofluorescent localization in the rat retina and optic nerve. The patterns of staining were markedly different, suggesting differences in cellular localization. Photoreceptor inner segments and optic nerve fibers expressed predominantly alpha3. Müller glia in the retina and astrocytes in the optic nerve expressed alpha1 and alpha2. Isolated, dissociated bipolar, horizontal, and Müller cells expressed different isozymes separately or in combination. The complexity of staining of neurons and their axons and dendrites suggested that Na,K-ATPase isozyme expression is not stereotyped, but is tailored to the ion transport needs of individual cell types, and targeted to specified membrane domains.

Na+,K+-adenosine triphosphatase polarity in retinal photoreceptors: a role for cytoskeletal attachments

We have used isolated embryonic photoreceptor cells as a model system with which to examine the mechanisms responsible for the development and maintenance of asymmetric Na+,K+-ATPase (ATPase) distribution. Photoreceptor precursors, which appear round and process free at culture onset, develop structural and molecular properties similar to those of photoreceptor cells in vivo. ATPase, recognized by an anti-ATPase antibody, is distributed over the entire surface of round photoreceptor precursors. As the cells develop, ATPase becomes progressively concentrated in the inner segment (where it is found in cells of the intact retina). This phenomenon occurs in cells developing in the absence of intercellular contacts. The development of ATPase polarity correlates with a decrease in the fraction of ATPase molecules that are mobile in the membrane (as determined by fluorescence photobleaching recovery), as well as with an increase in the fraction of ATPase that remains associated with the cells after detergent extraction. The magnitudes of the mobile ATPase fractions agree well with those of the detergent-extractable fractions in both the immature and developed photoreceptors. The distribution of alpha spectrin and ATPase-immunoreactive materials appeared qualitatively similar, and quantitative image analysis showed similar gradients of spectrin and Na+,K+-ATPase immunofluorescence along the long axis of elongated photoreceptors. Moreover, detergent extractability of alpha spectrin and the ATPase showed similar modifications in response to changes in pH or KCl concentration. ATPase detergent-extractable and mobile fractions were not changed in cultures treated with cytoskeletal inhibitors such as nocodazole. These data are consistent with a role for an asymmetrically distributed, spectrin-containing subcortical cytoskeleton in the preferential accumulation of Na+,K+-ATPase in the photoreceptor inner segment.