Dopamine release via protein kinase C activation in the fish retina

Two Types of Cl Transporters Contribute to the Regulation of Intracellular Cl Concentrations in ON- and OFF-type Bipolar Cells in the Mouse Retina

In the retina, ON- and OFF-type bipolar cells are classified by subtype-specific center responses, which are attributed to differences in glutamate receptor subtypes. However, the mechanisms by which ON- and OFF-type bipolar cells generate subtype-specific surround responses remain unclear. One hypothesis for surround responses is that intracellular Cl concentrations ([Cl-]_i) are set at different levels to achieve opposite polarities for GABA responses in ON- and OFF-type bipolar cells. Although this hypothesis is supported by previous findings obtained from rod (ON-) type bipolar cells, there is currently no information on OFF-type bipolar cells. In the present study, we examined the distribution and function of the Cl transporters, the Na-K-Cl co-transporter (NKCC1) and K-Cl co-transporter (KCC2), in rod (ON-) and OFF-type bipolar cells using immunohistochemical, in situ hybridization, and electrophysiological methods. Rod (ON-) and OFF-type bipolar cells both expressed NKCC1 and KCC2. However, the functional contribution of NKCC1 and KCC2 to the regulation of [Cl^-]_i differed between rod (ON-) and OFF-type bipolar cells. Strong NKCC1 activity increased [Cl^-]_i in rod (ON-) type bipolar cells, while that of KCC2 decreased [Cl^-]_i in OFF-type bipolar cells. We also confirmed the presence of a [Cl^-]_i gradient between dendrites and axon terminals in rod (ON-type) bipolar cells. Thus, the subtype-specific control of [Cl^-]_i is achieved by the activity of NKCC1 relative to that of KCC2 and appears to influence the polarity of surround responses.

Activation of Protein Kinase C Modulates Light Responses in Horizontal Cells of the Turtle Retina

The effect of phorbol esters on the light-evoked responses of horizontal cells were studied in the turtle eyecup preparation. Phorbol esters caused a reduction in receptive field size and a significant decrease in the amplitude of responses to annular and full-field illumination; however, they caused only minor changes in responses to small spots in the receptive field centre. The dark membrane potential was not affected. The results suggest that phorbol esters may affect both coupling resistance and membrane resistance in horizontal cells. The effects of phorbol esters were blocked by the protein kinase C inhibitor staurosporine, and inactive phorbol ester had no effect, making it very likely that the phorbol ester effects were mediated through activation of protein kinase C. The above effects of the phorbol esters were considerably reduced by the dopamine antagonists haloperidol and fluphenazine, suggesting that they were in part mediated by release of dopamine.

Signal transduction mechanisms involved in ischemic preconditioning in the rat retina in vivo

Ischemic preconditioning (IPC) protects the rat retina against the injury that ordinarily follows severe ischemia. We showed previously that release of adenosine and de novo protein synthesis were required for IPC protection. The mechanisms of IPC were studied in the rat retina by examining the signal transduction mediators responsible, in particular, those theorized to be downstream of adenosine receptors. In addition, we examined the hypothesis that nitric oxide and hydroxyl radicals were involved in the IPC protective phenomenon. Retinal ischemia was produced for 60 min in ketamine/xylazine-anesthetized Sprague-Dawley rats, and recovery was measured using electroretinography. We tested the effects on the protective effect of IPC resulting from antagonism of protein kinase C, potassium ATP channels, nitric oxide synthase, or hydroxyl radicals. The effects of the inhibition of de novo protein synthesis or of protein kinase C, and blockade of potassium ATP channels on the mimicking of IPC by adenosine receptor agonists was examined.IPC protection was strongly attenuated by inhibition of protein kinase C and by blockade of potassium ATP channels, but unaffected by the inhibition of hydroxyl radicals. Blockade of nitric oxide synthase produced a trend toward enhancement of IPC protection. Mimicking of IPC protection by adenosine receptor agonists was inhibited by blockade of protein synthesis or of protein kinase C, as well as by potassium ATP channel antagonism. These results demonstrate that protein kinase C and potassium ATP channels are mediators of the protective effect produced by IPC. In addition, the results show that stimulation of adenosine receptor subtypes A1 and A2a is responsible for IPC protection via downstream stimulation of protein kinase C, the opening of potassium ATP channels, and de novo protein synthesis.

Blockade of neurotensin-induced motor activity by inhibition of protein kinase

The administration of neurotensin into the ventral tegmental area stimulates dopamine neurons and locomotor activity. Furthermore, when neurotensin is microinjected daily into the ventral tegmental area the motor stimulant response increases. The role of protein kinases in the motor stimulant effect of neurotensin was evaluated by coadministration of the protein kinase inhibitors H8 and H7 into the ventral tegmental area with neurotensin. It was found that the acute motor stimulant effect of neurotensin was abolished in a dose-dependent fashion by H8 coadministration. Neurotensin-induced activity was also blocked by H7. However, acute motor stimulation following microinjection of the mu opioid, Tyr-d-Ala-Gly-MePhe-Gly(ol) or the potassium channel antagonist apamin into the ventral tegmental area was not affected by coadministration with H8. The behavioral sensitization produced by daily neurotensin microinjection into the ventral tegmental area was also prevented by the coadministration of H8. These data indicate that the motor stimulation produced by acute and repeated neurotensin microinjection into the ventral tegmental area is dependent upon activation of protein kinase(s). Furthermore, Tyr-d-Ala-Gly-MePhe-Gly(ol) and apamine elicit locomotion independently of protein kinase(s).

cAMP-mediated second messenger mechanisms are involved in spinule formation in teleost cone horizontal cells

A number of light adaptive changes of teleost horizontal cells are mediated by dopamine D1 receptors coupled positively with the cAMP second messenger system. Spinules, finger-like extensions from horizontal cell dendrites directed towards the cone pedicle cytoplasm, are formed in response to a stimulation of D1 receptors. We studied the second messenger mechanism associated with this process using isolated dark-adapted cyprinid retinae. Increasing intracellular cAMP concentrations by adding a membrane permeable analogue, or by stimulating the adenylate cyclase and simultaneously blocking the degradation of cAMP, resulted in a significant increase of spinule numbers in spite of the absence of light. In contradistinction to using isolated retinae for pharmacological experiments, injection of drugs into the vitreous had inconsistent or negative results.

Serotonergic signalling between thyroid cells: protein kinase C and 5-HT2 receptors in the secretion and action of serotonin

Parafollicular (PF) cells of the thyroid gland are neural crest derivatives, which costore the neurotransmitter, 5-hydroxytryptamine (5-HT) with calcitonin. PF cells are located adjacent to follicular (F) cells within the basement membrane of thyroid follicles. It has been proposed that 5-HT serves an intercellular signalling function in the thyroid and that F cells are its target. This proposal was tested by using cell lines derived from PF (medullary thyroid carcinoma [MTC]) and F (FRTL-5) cells to study the mechanisms that mediate the secretion and action of 5-HT. Secretion of 5-HT by MTC cells was evoked by thyroid stimulating hormone, thyrotropin (TSH), elevated extracellular calcium (increases [Ca2+]e), or by agents that increase intracellular cAMP (increases [cAMP]i). When protein kinase C (PKC) was down-regulated by prolonged treatment of MTC cells with phorbol 12-myristate 13-acetate (PMA), or PKC was inhibited by staurosporin, the TSH- or PMA-evoked secretion of 5-HT was blocked; however, interference with PKC function did not affect 5-HT secretion evoked by increases [Ca2+]e or increases [cAMP]i. In the putative targets, FRTL-5 cells, 5-HT increased the turnover of phosphoinositides (PI), cytosolic calcium (increases [Ca2+]i), increases [cAMP]i, and biphasically modified the effect of TSH on cAMP. All of these 5-HT effects were inhibited by 5-HT2 receptor antagonists (spiperone and ketanserin) and by pertussis toxin (PTx), suggesting that the actions of 5-HT are mediated by 5-HT2 receptors, which are coupled to a G protein. This suggestion was supported by the following additional observations: FRTL-5 membranes bound the 5-HT2 agonist, [125I]2,5-dimethoxy-4-iodophenylisopropylamine ([125I]-DOI), and anti-idiotypic antibodies, which recognize 5-HT2 receptors. [125I]-DOI binding was inhibited by guanosine-5'-O-(3-thiotriphosphate) (GTP-gamma-S) and the antibodies were displaced by spiperone. Data are consistent with the hypothesis that 5-HT serves as a PF to F cell messenger.

Protein kinase C (alpha and beta) immunoreactivity in rabbit and rat retina: effect of phorbol esters and transmitter agonists on immunoreactivity and the translocation of the enzyme from cytosolic to membrane compartments

Using a monoclonal antibody against protein kinase C (PKC) that recognises the isoenzymes alpha, beta I, and beta II, positive immunoreactivity was observed throughout the cytoplasm of bipolar cells in both rat and rabbit retinas. PKC immunoreactivity was also associated with the outer segment of photoreceptors in the rabbit retina and presumed amacrine cells in the rat retina. The PKC immunoreactivity in the retina was unaffected in content or localisation in rats kept in continuous dark or light conditions over a period of 6 days. The localisation of PKC immunoreactivity in retinas was similar in 6-day-old, 16 day-old, or adult rabbits. However, the content of PKC was lowest at the youngest stage and highest in the adult rabbit retinas. Of the two active phorbol esters studied, only phorbol 12,13-dibutyrate (PDbut) at a concentration of 1 microM caused the PKC immunoreactivity in rabbit retina bipolar cells to be "transported" from the perikarya towards the axonal terminal processes. Biochemical analyses showed that most of the cytosolic PKC was translocated to the membrane compartment following such treatment. The other phorbol ester, phorbol 12-myristate 13-acetate, even at a concentration of 10 microM did not cause a similar transport of PKC immunoreactivity in the bipolar cells, although a partial translocation of the enzyme could be followed biochemically. Both the translocation and transport of PKC by PDbut could be reversed by simply incubating the retinas in physiological solution for 60 min. The "transport" and translocation processes were not obviously affected by the transport inhibitor colchicine or by known PKC inhibitor such as staurosporine, H-7, sphingosine, or polymyxin B. In addition, agonists known to stimulate inositol phosphates in the retina, viz., carbachol, noradrenaline, and quisqualate, or 4-aminopyridine did not cause a translocation or "transport" of PKC as observed for the phorbol esters.

Inhibition by taurine of the phosphorylation of specific synaptosomal proteins in the rat cortex: effects of taurine on the stimulation of calcium uptake in mitochondria and inhibition of phosphoinositide turnover

It has been previously observed that taurine inhibits PKC-activated phosphorylation of specific proteins including a approximately 20k Mr protein in rat cortical synaptosomes. In the present study, the mechanism of the above effects of taurine were investigated. In an intrasynaptosomal cytosol fraction obtained by subcellular fractionation, taurine did not have inhibitory effects on protein phosphorylation. However, taurine did inhibit the phosphorylation of the approximately 20k Mr protein in a reconstituted preparation containing intrasynaptosomal cytosol and mitochondria. Experiments measuring calcium uptake demonstrated that taurine increased the accumulation of 45Ca2+ in the mitochondrial fraction in incubation systems both in the absence and presence of added ATP. In addition, taurine inhibited the accumulation of 32P-labeled phosphatidic acid in synaptosomes indicative of a reduction in the levels of diacylglycerol. These results suggest that taurine may inhibit specific protein phosphorylation both by reducing cytosolic calcium levels and by inhibiting the turnover of phosphoinositides. These effects of taurine on the signal transduction cascade involving PKC and phosphoinositide metabolism indicate a potential biological role for taurine in the nervous system.

The role of protein kinase C and its neuronal substrates dephosphin, B-50, and MARCKS in neurotransmitter release

This article focuses on the role of protein phosphorylation, especially that mediated by protein kinase C (PKC), in neurotransmitter release. In the first part of the article, the evidence linking PKC activation to neurotransmitter release is evaluated. Neurotransmitter release can be elicited in at least two manners that may involve distinct mechanisms: Evoked release is stimulated by calcium influx following chemical or electrical depolarization, whereas enhanced release is stimulated by direct application of phorbol ester or fatty acid activators of PKC. A markedly distinct sensitivity of the two pathways to PKC inhibitors or to PKC downregulation suggests that only enhanced release is directly PKC-mediated. In the second part of the article, a framework is provided for understanding the complex and apparently contrasting effects of PKC inhibitors. A model is proposed whereby the site of interaction of a PKC inhibitor with the enzyme dictates the apparent potency of the inhibitor, since the multiple activators also interact with these distinct sites on the enzyme. Appropriate PKC inhibitors can now be selected on the basis of both the PKC activator used and the site of inhibitor interaction with PKC. In the third part of the article, the known nerve terminal substrates of PKC are examined. Only four have been identified, tyrosine hydroxylase, MARCKS, B-50, and dephosphin, and the latter two may be associated with neurotransmitter release. Phosphorylation of the first three of these proteins by PKC accompanies release. B-50 may be associated with evoked release since antibodies delivered into permeabilized synaptosomes block evoked, but not enhanced release. Dephosphin and its PKC phosphorylation may also be associated with evoked release, but in a unique manner. Dephosphin is a phosphoprotein concentrated in nerve terminals, which, upon stimulation of release, is rapidly dephosphorylated by a calcium-stimulated phosphatase (possibly calcineurin [CN]). Upon termination of the rise in intracellular calcium, dephosphin is phosphorylated by PKC. A priming model of neurotransmitter release is proposed where PKC-mediated phosphorylation of such a protein is an obligatory step that primes the release apparatus, in preparation for a calcium influx signal. Protein dephosphorylation may therefore be as important as protein phosphorylation in neurotransmitter release.

Taurine inhibits protein kinase C-catalyzed phosphorylation of specific proteins in a rat cortical P2 fraction

We previously reported that taurine inhibits the phosphorylation of specific proteins in a P2 synaptosomal fraction prepared from the rat cortex. In the present study, the regulation of the phosphorylation of an approximately 20K Mr protein whose phosphorylation is inhibited by taurine was further investigated. The phosphorylation of the approximately 20K Mr protein in a hypo-osmotically shocked P2 fraction from rat cortex was dependent on the free Ca2+ in the reaction medium. Depolarization induced by 30 mM K+ stimulated the phosphorylation of the approximately 20K Mr protein in an intact synaptosomal P2 preparation by 30-fold. This stimulation was inhibited 35% by taurine, whereas guanidinoethanesulfonic acid, a taurine analogue, did not have any effect, thereby indicating the specificity of taurine. Addition of phorbol 12-myristate 13-acetate, a phorbol ester, together with phosphatidylserine, stimulated the phosphorylation of the approximately 20K Mr protein in the hypo-osmotically shocked P2 synaptosomal fraction by fivefold, whereas cyclic AMP, cyclic GMP, and calmodulin did not have any effect on the phosphorylation of this particular protein. Phorbol 12-myristate 13-acetate-stimulated phosphorylation of the approximately 20K Mr protein is blocked 30% by taurine. Taurine also inhibited phorbol 12-myristate 13-acetate-activated phosphorylation of two other proteins that were similar in molecular weight and isoelectric point to the approximately 20K Mr protein on two-dimensional gels. These results suggest that taurine modulates the phosphorylation of specific proteins regulated by the signal transduction system in the brain. Thus, taurine may modulate neuroactivity by inhibiting the phosphorylation of specific proteins involved in regulatory function.

Protein kinase C mediates transient spinule-type neurite outgrowth in the retina during light adaptation

Light and dark adaptation of the teleost retina is accompanied by a remarkable morphological rearrangement of the synaptic connections between photoreceptors and second-order neurons: during light adaptation, numerous new neurites, the so-called spinules, arise from the terminal dendrites of horizontal cells invaginating the cone pedicle, and during dark adaptation, these spinules are retracted. The formation of these spinules is paralleled by the appearance of color opponency in horizontal and ganglion cells, which led to the suggestion that these spinules are the site of the inhibitory synapses in the negative feedback loop between cones and horizontal cells. The formation of the spinules in the light and their disappearance in darkness have a time course of minutes and are modulated by the neurotransmitters dopamine and glutamate, respectively. Neurotransmitters can modulate neuronal processing through a variety of second messengers that activate protein kinases, resulting most commonly in protein phosphorylation. Herein we report that activation of protein kinase C by phorbol esters promotes the formation of new horizontal-cell spinules in animals kept in the dark. Partial inhibition of protein kinase C activation with sphingosines prevents the formation of new spinules during light adaptation but does not affect established spinules. The spinule-forming effect of phorbol esters is not mediated by dopaminergic neurons, since the effect is also seen in retinas depleted of dopaminergic neurons. Phorbol esters also initiate the formation of spinules in synaptically isolated horizontal cells, demonstrating that they have a direct action on these cells. In addition, isolated horizontal cells have substrate proteins that are phosphorylated in a protein kinase C-dependent manner.