A role for protein kinase C in the electrically evoked release of [3H] gamma-aminobutyric acid in rabbit caudate nucleus

Modulation of GABA release by second messenger substances and NO in mouse brain stem slices under normal and ischemic conditions

GABA is the inhibitory neurotransmitter in most brain stem nuclei. The properties of release of preloaded [(3)H]GABA were now investigated with slices from the mouse brain stem under normal and ischemic (oxygen and glucose deprivation) conditions, using a superfusion system. The ischemic GABA release increased about fourfold in comparison with normal conditions. The tyrosine kinase inhibitor genistein had no effect on GABA release, while the phospholipase inhibitor quinacrine reduced both the basal and K(+)-evoked release in normoxia and ischemia. The activator of protein kinase C (PKC) 4beta-phorbol 12-myristate 13-acetate had no effects on the releases, whereas the PKC inhibitor chelerythrine reduced the basal release in ischemia. When the cyclic guanosine monophosphate (cGMP) levels were increased by superfusion with zaprinast and other phosphodiesterase inhibitors, GABA release was reduced under normal conditions. The NO donors S-nitroso-N-acetylpenicillamine (SNAP) and hydroxylamine (HA) enhanced the basal and K(+)-stimulated release by acting directly on presynaptic terminals. Under ischemic conditions GABA release was enhanced when cGMP levels were increased by zaprinast. This effect was confirmed by inhibition of the release by the guanylate cyclase inhibitor 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one (ODQ). The NO-producing agents SNAP, HA, and sodium nitroprusside potentiated GABA release in ischemia. These effects were reduced by the NO synthase inhibitor N(G)-nitro-L: -arginine, but not by ODQ. The results show that particularly NO and cGMP regulate both normal and ischemic GABA release in the brain stem. Their effects are however complex.

Neurokinin-1 receptors in the rat nucleus tractus solitarius: pre- and postsynaptic modulation of glutamate and GABA release

Neurokinins such as substance P and neurokinin A have long been thought to act as neurotransmitters or modulators in the nucleus tractus solitarius. However, the role and location of the receptors for these peptides have remained unclear. We examined the consequences of activation of the neurokinin-1 (NK1) receptor subtype in the rat nucleus tractus solitarius using whole-cell patch clamp recordings in brain slices. Application of delta-Ala-Phe-Phe-Pro-MeLeu-D-Pro[spiro-gamma-lactam]-Leu-Trp-NH2 (a specific NK1 agonist) or neurokinin A resulted in depolarization, evident as a slow inward current, mediated by direct postsynaptic NK1 receptor activation. The effect was conserved in the presence of tetrodotoxin, and protein kinase C-dependent since it was blocked by 2-[1-(3-dimethylaminopropyl)indol-3-yl]-3-(indol-3-yl)maleimide, a specific protein kinase C inhibitor. In addition, an increase in the frequency and amplitude of spontaneous excitatory postsynaptic currents was observed, reflecting increased glutamate release induced by NK1 receptor activation. This effect was abolished by tetrodotoxin, suggesting that it resulted from increased firing in afferent neurons, subsequent to somatodendritic excitation via NK1 receptors. Furthermore, spontaneous inhibitory postsynaptic currents were increased in frequency and amplitude showing that GABA release was promoted by NK1 receptor activation. However, amplitude of miniature inhibitory postsynaptic currents was unaltered by NK1 receptor activation, but the increase in frequency persisted. These findings suggest that NK1 receptors are located on presynaptic terminals as well as at somatodendritic sites of GABAergic neurons. The increase in GABA release was also shown to be protein kinase C-dependent. The data presented here show NK1 receptors in the rat nucleus tractus solitarius are present both excitatory and inhibitory neurons. Activation of these receptors can result in increases in release of both GABA and glutamate, suggesting a crucial modulatory role for NK1 receptors in the rat nucleus tractus solitarius.

Protein kinase C modulates field-evoked transmitter release from cultured rat cerebellar granule cells via a dendrotoxin-sensitive K+ channel

The role of protein kinase C (PKC) in the control of neurotransmitter release from cultured rat cerebellar granule cells was investigated. Release of preloaded [3H]-D-aspartate which is incorporated into synaptic vesicles in this preparation was evoked by electrical field stimulation or elevated KCl. PKC activation by phorbol esters resulted in a large facilitation of field-evoked Ca(2+)-dependent [3H]-D-aspartate release and a lesser enhancement of KCl-stimulated release. Inhibition of PKC by Ro 31-8220 or staurosporine virtually abolished field-evoked release but had no effect on KCl-evoked release. Field-evoked, but not KCl-evoked, synaptic vesicle exocytosis monitored by the fluorescent vesicle probe FM2-10 was inhibited by staurosporine. PKC was not directly modulating neurite Ca2+ channels coupled to release, as Ro 31-8220 did not inhibit these channels. Activation or inhibition of PKC modulated field-evoked plasma membrane depolarization, but had no effect on KCl-evoked depolarization, consistent with a regulation of Na+ or K+ channels activated by field stimulation. No modulation of field-evoked neurite Na+ influx was seen using phorbol esters. Phorbol ester-induced facilitation of field-evoked [3H]-D-aspartate release and neurite Ca2+ entry was non-additive with that produced by the specific K+ channel antagonist dendrotoxin-1, suggesting that PKC modulates transmitter release from field-stimulated cerebellar granule cells by inhibiting a dendrotoxin-1-sensitive K+ channel.

Protein kinase C: a physiological mediator of enhanced transmitter output

Protein kinase C (PKC), activated by either diacylglycerol and/or arachidonic acid, through the activation of presynaptic receptors or nerve or nerve depolarization is involved is involved in the enhancement of transmitter release from many neural types. This facilities is most likely mediated by the phosphorylation of proteins involved in vesicle dynamics although a role for ion channels cannot be ruled out. PKC is not fundamental to the release process but rather has a modulatory role of PKC is to help maintain transmitter output during prolonged or elevated levels of activation and this seems to parallel suggestions that PKC is involved in the movement of reserve pools of vesicles into release-study sites. presynaptic facilitatory actions mediated by PKC are also involved in integrated modulatory functions such as long term potentiation, again where it elevates or maintains transmitter output. Although studies have tried to identify specific roles for various PKC isoforms, the actions of phorbol esters in elevators transmitter release do not fit with known potencies on individual isoforms and lit suggests that PKC may be located at an intraneuronal location which is difficult to access for lipophilic phorbol esters and further work is required in this area.

Localization of subspecies of protein kinase C in the mammalian central nervous system

Activation of protein kinase C (PKC) is regulated by dual second messengers; diacylglycerol (DG) produced by receptor mediated hydrolysis of phosphatidylinositol and Ca2+ which is released by inositol 1,4,5-triphosphate (IP3) from intracellular stores in the endoplasmic reticulum. In the mammalian central nervous system, available evidence suggests that PKC plays a prominent role in the processing of neuronal signals and in the short-term or long-term modulation of synaptic transmission. This enzyme is a member of a family consisting of at least eight subspecies, alpha, beta I, beta II, gamma, delta, epsilon, zeta and eta. The homologous structure of each subspecies makes difficult resolution of the enzymological properties of the enzyme. The distinct functional roles of PKC subspecies in mammalian tissues have been elucidated by defining the localization of each subspecies. We identified alpha-, beta I-, beta II- and gamma-PKC subspecies in the rat brain by in situ hybridization and by light and electron microscopic immunohistochemistry, using antibodies specific for each subspecies. Most immunoreactions of the alpha, beta I, beta II and gamma subspecies were evident in neurons and there were few, if any, in glial cells. In this article, we summarize known cellular and subcellular localizations of PKC subspecies in mammalian CNS and some aspects of current studies in neuronal functions regulated by this enzyme are discussed.

Cytoskeleton and molecular mechanisms in neurotransmitter release by neurosecretory cells

The process of exocytosis is a fascinating interplay between secretory vesicles and cellular components. Secretory vesicles are true organelles which not only store and protect neurotransmitters from inactivation but also provide the cell with efficient carriers of material for export. Different types of secretory vesicles are described and their membrane components compared. Associations of several cytoplasmic proteins and cytoskeletal components with secretory vesicles and the importance of such associations in the mechanism of secretion are discussed. A description of possible sites of action for Ca2+ as well as possible roles for calmodulin, G-proteins and protein kinase C in secretion are also presented. Important aspects of the cytoskeleton of neurosecretory cells are discussed. The cytoskeleton undergoes dynamic changes as a result of cell stimulation. These changes (i.e. actin filament disassembly) which are a prelude to exocytosis, play a central role in secretion. Moreover, advanced electrophysiological techniques which allow the study of secretory vesicle-plasma membrane fusion in real-time resolution and at the level of the single secretory vesicle, have also provided a better understanding of the secretory process.

Phorbol ester enhances excitatory amino acid-induced dopamine release from mesencephalic cell cultures

The hypothesis that protein kinase C activation can modulate excitatory amino acid-induced dopamine release was tested by investigating effects of phorbol esters, direct activators of protein kinase C, on dopamine release stimulated by N-methyl-D-aspartate (NMDA) and non-NMDA sub-types of excitatory amino acid agonists in fetal rat mesencephalic cell cultures. The phorbol ester, 12-O-tetradecanoyl phorbol-13-acetate (TPA), enhanced dopamine release evoked by NMDA, kainate, quisqualate and by K+ depolarization. Release in the presence of NMDA and TPA was completely abolished by the NMDA antagonist, MK-801. TPA enhancement of NMDA-stimulated dopamine release was likely due to protein kinase C activation by the phorbol ester since (1) the NMDA response was enhanced by nanomolar concentrations of TPA, (2) two phorbol esters capable of activating protein kinase C enhanced the NMDA response while an inactive phorbol ester did not, (3) staurosporine, a potent protein kinase C inhibitor, blocked TPA enhancement of the NMDA response. TPA enhancement of NMDA-stimulated dopamine release was not blocked by H8, an inhibitor with high affinity for cyclic nucleotide dependent kinases, while forskolin, a direct activator of adenylate cyclase, had no effect on NMDA-stimulated release, indicating a lack of involvement of cAMP-dependent kinase in the TPA effect. TPA enhanced NMDA-stimulated release both in the presence and absence of Mg2+, indicating that TPA enhancement was not due to reversal of a Mg2+ blockade of the NMDA receptor.(ABSTRACT TRUNCATED AT 250 WORDS)

Pineal and retinal protein kinase C isoenzymes: cooperative activation by calcium and zinc metallothionein

Protein kinase C and its family of multiple subspecies play pivotal roles in cell-surface mediated signal transduction. For example, in the process of synthesizing melatonin, the activation of alpha 1-adrenergic receptor sites in the pineal gland causes translocation of protein kinase C, which in turn enhances the beta-adrenergic-activated accumulation of both cyclic AMP and cyclic GMP. In the retina, protein kinase C phosphorylates rhodopsin and hence is involved in visual transduction. The activation of protein kinase C depends on the presence of phospholipid and Ca++. In this communication, we report that the bovine pineal gland and retina possess unique protein kinase C isoenzymes that are distinct from those seen in the rat brain. Furthermore, in retinoblastoma cells in culture, protein kinase C is stimulated by a cooperative interaction between calcium and zinc. Moreover, the subcellular regions of retina that exhibit the highest activity of protein kinase C also possess the highest concentration of zinc. In view of the fact that the bovine pineal gland and retina continually synthesize metallothionein and other low molecular weight zinc binding proteins, we propose that zinc and metallothionein participate in signal transduction in the retina and pineal gland. The action of metallothionein, a zinc binding protein, in activating protein kinase C is opposite to that of calcium binding protein, which is a potent inhibitor of protein kinase C.

Adenosine A1-receptor-mediated inhibition of evoked acetylcholine release in the rat hippocampus does not depend on protein kinase C

The possible involvement of protein kinase C and/or a lipoxygenase product in the mechanism by which adenosine inhibits release of [3H]acetylcholine evoked by electrical pulses from [3H]choline-labelled hippocampal slices was examined. For comparison, the muscarinic autoreceptors were examined using carbachol. The order of potency of adenosine analogues (CHA = R-PIA greater than NECA much greater than CGS 21680, CV 1808) indicates that the adenosine receptor responsible is of the A1 subtype. Adenosine (10 microM) and R-PIA (0.1 microM) were virtually equiactive as inhibitors and were antagonized to an equal extent by 8-CPT with a potency (IC50 approximately 25 nM) which is also compatible with A1-receptor mediation. The effects of carbachol and of R-PIA were not antagonized by the lipoxygenase inhibitor NDGA (10 or 50 microM). Stimulation of protein kinase C by the phorbol ester 4 beta-phorbol 12,13-dibutyrate caused a concentration-dependent increase in stimulation-evoked 3H overflow, but did not antagonize the presynaptic inhibitory effect of R-PIA or carbachol (0.01-1 microM). Staurosporine (0.1 microM), which inhibited the stimulating effect of phorbol dibutyrate, did not alter the effects of carbachol or R-PIA. The presynaptic effects of phorbol dibutyrate, R-PIA and adenosine were reduced by pretreatment with N-ethylmaleimide (100 microM for 10 min), which inactivates G-proteins. The evoked transmitter release was unaffected by nifedipine (1 microM) in the presence and in the absence of phorbol dibutyrate. These results indicate that adenosine, by acting at presynaptic A1-receptors, reduces transmitter release by a mechanism that involves neither an NDGA-sensitive lipoxygenase nor protein kinase C. The results also indicate that the enhancement of transmitter release by phorbol esters is due to protein kinase C activation and that a G-protein may be involved in the effect but a dihydropyridine-sensitive L-type Ca2+ channel probably is not.

Depolarization-induced phosphorylation of the protein kinase C substrate B-50 (GAP-43) in rat cortical synaptosomes

We studied the molecular events underlying K(+)-induced phosphorylation of the neuron-specific protein kinase C substrate B-50. Rat cortical synaptosomes were prelabelled with 32P-labelled orthophosphate. B-50 phosphorylation was measured by an immunoprecipitation assay. In this system, various phorbol esters, as well as a synthetic diacylglycerol derivative, enhance B-50 phosphorylation. K+ depolarization induces a transient enhancement of B-50 phosphorylation, which is totally dependent on extracellular Ca2+. Also, the application of the Ca2+ ionophore A23187 induces B-50 phosphorylation, but the magnitude and kinetics of A23187-induced B-50 phosphorylation differ from those induced by depolarization. The protein kinase inhibitors 1-(5-isoquinolinylsulfonyl)-2-methylpiperazine (H-7), N-(6-aminohexyl)-5-chloro-1-naphthalenesulfonamide (W-7), and staurosporine antagonize K(+)- as well as PDB-induced B-50 phosphorylation, whereas trifluoperazine and calmidazolium are ineffective under both conditions. We suggest that elevation of the intracellular Ca2+ level after depolarization is a trigger for activation of protein kinase C, which subsequently phosphorylates its substrate B-50. This sequence of events could be of importance for the mechanism of depolarization-induced transmitter release.