Quantitative measurement of protein kinase C immunoreactivity in rod bipolar cells of the goldfish retina

PKC{alpha} is essential for the proper activation and termination of rod bipolar cell response

PURPOSE

Protein kinase (PKC)-α is abundant in retinal bipolar cells. This study was performed to explore its role in visual processing.

METHODS

PKCα-knockout (Prkca(-/-)) mice and control animals were examined by using electroretinography (ERG), light microscopy, and immunocytochemistry.

RESULTS

The Prkca(-/-) mice showed no signs of retinal degeneration up to 12 months of age, but ERG measurements indicated a decelerated increase in the ascending limb of the scotopic (rod-sensitive) b-wave as well as a delayed return to baseline. These results suggest that PKCα is an important modulator that affects bipolar cell signal transduction and termination. Confocal microscopy of retinal sections showed that PKCα co-localized with calbindin, which indicates a PKCα localization in close proximity to the horizontal cell terminals. In addition, the implicit time of the ERG c-wave originating from the retinal pigment epithelium (RPE) and the recovery of photoreceptors from bleaching conditions were substantially faster in the knockout mice than in the wild-type control animals.

CONCLUSIONS

These results suggest that PKCα is a modulator of rod-bipolar cell function by accelerating glutamate-driven signal transduction and termination. This modulation is of importance in the switch between scotopic and photopic vision. Furthermore, PKCα seems to play a role in RPE function.

Cation Cl- cotransporters in the dendrites of goldfish bipolar cells

Bipolar cells receive an input from gamma-aminobutyric acid ergic horizontal cells in distal retinas. gamma-Aminobutyric acid can induce either an inhibitory or an excitatory Cl(-) response based on the intracellular Cl(-) concentration in the bipolar dendrites. Two cation Cl(-) cotransporters, Na-K-2Cl(-) and K-2Cl(-), differently control local Cl(-) levels in neurons by uptaking or extracting Cl(-), respectively. By using immunocytochemical techniques, we found that Na-K-2Cl(-) and K-2Cl(-) were preferentially distributed in On-bipolar and Off-bipolar dendrites in goldfish retinas. Na-K-2Cl(-) was colocalized with protein kinase C in On-bipolar dendrites, whereas K-2Cl(-) was located in Off-bipolar dendrites, fully overlapping with ionotropic glutamate receptor GluR4. Possibly, an internal Cl(-) level is higher in the On-bipolar dendrites than in the Off-bipolar dendrites. gamma-Aminobutyric acid inputs in the distal retina might excite On-bipolar cells, but inhibit Off-bipolar cells.

Distribution of phosphorylated protein kinase C alpha in goldfish retinal bipolar synaptic terminals: control by state of adaptation and pharmacological treatment

Protein kinase C (PKC) is a signalling enzyme critically involved in many aspects of synaptic plasticity. In cyprinid retinae, the PKC alpha isoform is localized in a subpopulation of depolarizing bipolar cells that show adaptation-related morphological changes of their axon terminals. We have studied the subcellular localization of phosphorylated PKC alpha (pPKC alpha) in retinae under various conditions by immunohistochemistry with a phosphospecific antibody. In dark-adapted retinae, pPKC alpha immunoreactivity is weak in the cytoplasm of synaptic terminals, labelling being predominantly associated with the membrane compartment. In light-adapted cells, immunoreactivity is diffusely distributed throughout the terminal. Western blot analysis has revealed a reduction of pPKC alpha immunoreactivity in cytosolic fractions of homogenized dark-adapted retinae compared with light-adapted retinae. Pharmacological experiments with the isoform-specific PKC blocker Goe6976 have shown that inhibition of the enzyme influences immunolabelling for pPKC alpha, mimicking the effects of light on the subcellular distribution of immunoreactivity. Our findings suggest that the state of adaptation modifies the subcellular localization of a signalling molecule (PKC alpha) at the ribbon-type synaptic complex. We propose that changes in the subcellular distribution of PKC alpha immunoreactivity might be one component regulating the strength of the signal transfer of the bipolar cell terminal.

A comparative study of protein kinase C-like immunoreactive cells in the retina

The present study is a morphological and quantitative analysis of protein kinase C-like immunoreactive (PKC-L ir) bipolar cells in the retinas of five different vertebrate species (chicken, tench, zebrafish, goldfish and rat). The morphology of PKC-L-ir bipolar cell axon terminals in fish differs significantly from those of chicken and rat retinas. Fish have bulky terminals whereas chicken and rat have their terminals in the form of small knob-shaped branches. In tench and goldfish, PKC-L-ir bipolar cells gradually decrease in size from the medial (i.e., in tench: mean +/- SD soma area of 30.09 +/- 5.98 microm2) to the peripheral (i.e., in tench: 19.93 +/- 1.73 microm2) retinal regions. This is not observed in chicken, rat or zebrafish where there is more homogeneity in s oma and axon terminal sizes between different retinal regions. Except in chicken, cell density increases from the central (i.e., in tench: mean +/- SD 1795.88 +/- 242.35 cells/mm2) to the peripheral (i.e., in tench: 4295.41 +/- 279.23 cells/mm2) retina. This study provides data that show relevant differences in the PKC-L-ir bipolar morphology and density among birds, fish and mammals. Moreover, these structural variations could mean not only differences in the cellular physiology, but also in the patterns of development and maintenance of the retina in each species.

Localisation of the GABA(C) receptors at the axon terminal of the rod bipolar cells of the mouse retina

In the vertebrate retina, the rod bipolar cells make reciprocal synapses with amacrine cells at the axon terminal. Amacrine cells may perform a fine control of the transmitter release from rod bipolar cells by means of GABAergic synapses acting on different types of GABA receptors. To clarify this possibility GABA-induced currents were recorded by the patch-clamp whole cell method in rod bipolar cells enzymatically dissociated from the mouse retina. All cells tested showed a desensitising chloride-sensitive GABA-induced current. When GABA 30 microM was applied in presence of 100 microM biccuculine, a blocker of the GABA(A) receptors, a slow-desensitising component of the current still remains. This current was blocked when GABA 30 microM was applied in presence of 100 microM 3-aminopropylphosphonic acid, an antagonist of the GABA(C) receptors. The current mediated by GABA(C) receptors showed an EC50 of less that 5 microM; the ionic current through the GABA(A) receptor showed an EC50 of ca. 30 microM. Two pieces of evidence demonstrated that the GABA(C)-mediated current was localised at the axon terminal of rod bipolar cells: (1) cells lacking the axon terminal only showed the biccuculine-sensitive GABA-induced current; and (2) after mechanical section of the axon terminal, bipolar cells lost the slow-desensitising component of the GABA-induced current. We conclude that the rod bipolar cells express two types of ionotropic GABA receptors, and that the high sensitive GABA(C) receptors are mainly localised at the level of the axon terminal and therefore may contribute to the modulation of the transmitter release from the rod bipolar cell.