Nicotinic and muscarinic agonists, phorbol esters, and agents which raise cyclic AMP levels phosphorylate distinct groups of proteins in the superior cervical ganglion

Histochemical detection of age- and injury-related changes in signal transduction in the superior cervical ganglion

The superior cervical ganglion (SCG) is thought to be a good model for correlation studies of morphology, function and metabolism of neurons. The SCG has a relatively simple organization, it can be easily manipulated in situ, and it maintains synaptic transmission and a high metabolic rate during in vitro incubations. The histology and structure of SCG neurons have been characterized in detail, and physiologic stimuli, injury and aging have all been found to induce changes in the SCG morphology. During the last decade, research in the field of signal transduction has greatly expanded. Several signal transduction pathways have been identified that participate in the regulation of neurotransmitter synthesis, gene expression, neuronal excitability and growth factor responses of sympathetic neurons. We have been interested in using the SCG to study some of the second and third messengers involved in converting external stimuli received by sympathetic neurons into cellular short- and long-term events. Using immunohistochemistry, we have investigated protein kinase C-subtypes and the immediate early gene product Fos in the SCG, and characterized some of the changes induced by injury and aging in these messenger molecules. We will review the results and discuss the advantages and disadvantages of using histological methods in the study of signal transduction in sympathetic neurons.

Long-term depression of a sympathetic ganglionic response to opioids by prolonged synaptic activity and by phorbol esters

We studied the effect of the adenylate cyclase activator forskolin, of protein kinase C-activating phorbol esters and of prolonged preganglionic input activation on the inhibitory response of the perfused superior cervical ganglion of the cat to exogenous met-enkephalin (Met-ENK). Met-ENK inhibited, in a concentration-dependent manner, the postganglionic compound action potential evoked by cervical sympathetic trunk stimulation. The inhibition was reversible, was blocked by naloxone as well as by pertussis toxin and showed no homologous desensitization in the concentration range 0.01-10 microM. Pretreatment of the ganglion with 4 beta-phorbol 12,13-dibutyrate or 4 beta-phorbol 12,13-diacetate depressed the Met-ENK response for several hours, while pretreatment with forskolin had no effect. This action of phorbol esters was prevented by the protein kinase inhibitor H-7 but not by the calmodulin antagonist W-7 or the protein kinase A inhibitor HA 1004 and was calcium-dependent. Recovery of the response from the depression produced by phorbol esters was not affected by a protein synthesis inhibitor. A 40 Hz 20 min stimulus train to the cervical sympathetic trunk mimicked the effect of phorbol esters, depressing for several hours the inhibition produced by Met-ENK. Stimulus trains of duration shorter than 5 min or frequency lower than 5 Hz were ineffective. This effect of prolonged preganglionic stimulation occurred even when the stimulus train was delivered during complete block of nicotinic and muscarinic ganglionic transmission but was lost when the stimulus train was delivered during perfusion with calcium-free Krebs. The protein kinase inhibitor H-7 prevented the depression of the Met-ENK response by the train, while W-7 and HA 1004 had no effect. These findings suggest that, in the superior cervical ganglion of the cat, a kinase, activated by phorbol esters and inhibited by H-7, exerts a long-term control of the ganglion cell responsiveness to opiate receptor activation. A similar mechanism can be synaptically activated by a non-cholinergic transmitter, released by the preganglionic axons during prolonged, high frequency, activity.

Protein phosphorylation in intact superior cervical ganglion during regeneration

The incorporation of radioactive phosphate into proteins of both normal and regenerating superior cervical ganglion nerve of the rat is reported. Incorporation studies carried out by in vitro and in vivo methods are compared. In the in vitro method, excised intact ganglia or their homogenates were incubated in the presence of inorganic phosphate or ATP, respectively, under various conditions. Proteins were analyzed by gel electrophoresis followed by autoradiography, in which quantitative but not qualitative differences between regenerating and control cases were apparent. In the in vivo procedure, inorganic phosphate was injected into the living animal 4 h before removal of ganglia. At least fivefold more proteins became labeled in vivo than in vitro, whereas no similarity in the pattern of labeling between the two methods was observed. For example, the most heavily labeled protein in the in vivo method, tentatively identified as microtubule-associated protein-2, was not detected on autoradiograms of proteins labeled by the in vitro method. In this latter method, an 85-kDa species and growth-associated protein-43 were always labeled, and the extent of their phosphorylation was enhanced by the additional presence of phosphatidylserine and Ca2+, a result indicating that these labeled species are substrates of protein kinase C. The in vitro conditions also led to the labeling of proteins identified as alpha- and beta-tubulin. Comparison of the methods suggests that removal of the ganglion interferes with the function of protein phosphorylation systems and that this effect involves elements of the cytoskeleton.

Effects of isolation and high helium pressure on the nucleolus of sympathetic neurons in the rat superior cervical ganglion

In prokaryotes, unicellular eukaryotes and cell-free systems, pressure is known to exert an inhibitory effect on protein synthesis and RNA metabolism, the mechanism(s) of which remain to be investigated in detail. The purpose of the present in vitro study was to compare ultrastructural and quantitative changes of the nucleolus, which is the site of ribosome biogenesis, in sympathetic neurons of rat superior cervical ganglia (SCG) maintained for 2, 3 and 5 h in NCTC 109 medium and subjected to pressure or not. In control SCG (left) the nucleolus greatly increased in volume (+ 33%) 2 h after excision, in comparison with SCG fixed immediately. This overall enlargement was found to reflect a marked increase in all nucleolar components (from 16 to 87%). After 5 h, volumes of nucleolus, fibrillar centers and vacuolar component returned to control values, whereas dense fibrillar and granular components remained affected. Such early and transient changes are regarded as reflecting basic metabolic changes associated with increased nucleolar RNA that should be of primary concern to experiments using SCG transplanted in culture media. Compression under helium up to 180 atmospheric pressure for 1 h of right SCG maintained for 2 h in culture medium, was shown to induce, on the contrary, a marked decrease in nucleolar volume (-39%) and in volumes of all nucleolar components (from -36 to -51%). When they were kept at constant high pressure for 1 and 3 h a progressive recovery of volumes of nucleoli and nucleolar components was observed. Consequently, compression was shown to exert opposite effects to those of isolation of SCG. Present data are interpreted as an inhibitory effect of pressure on ribosome biogenesis. Such observations on a vertebrate neuron might open a new field in the search for cellular mechanisms underlying the effects of pressure on living organisms and especially on the nervous system.

Novel recognition site for L-quisqualate sensitizes neurons to depolarization by L-2-amino-4-phosphonobutanoate (L-AP4)

Brief exposure of rat hippocampal slices to L-quisqualate sensitizes pyramidal neurons to depolarization by L-2-amino-4-phosphonobutanoate (L-AP4). We report here experiments designed to clarify the duration, pharmacology, mechanism, and pathway specificity of this 'QUIS-effect'. The quisqualate-induced sensitization to L-AP4 decreases only 3-fold over a 4 h period. No compound besides quisqualate has been found to induce the QUIS-effect, including quisqualate analogues, potent excitatory amino acid agonists, L-glutamate, L-aspartate, and compounds known to stimulate second messenger systems in hippocampal slices. Of 43 compounds assayed here, only 5 are able to block the induction of the QUIS-effect. Although these blockers are also potent ligands at a chloride-dependent glutamate uptake site, the marked difference in rank ordering of compounds for QUIS-effect blockade and uptake site potency suggests that the QUIS-effect is not induced through this uptake site. The QUIS-effect can be induced in the CA1 region, the medial perforant path, and the lateral olfactory tract of the rat, and in the guinea pig CA1. It cannot be induced in the L-AP4-sensitive rat lateral perforant path (LPP), suggesting that the receptors for L-AP4 in the LPP may be distinct from those that are sensitized by quisqualate in the other pathways.

Phosphorylation of elongation factor 2 in the rat superior cervical ganglion

Elongation factor 2 (EF-2) and its associated kinase, Ca2+/calmodulin-dependent protein kinase III, have recently been identified as a major protein phosphorylation system in mammalian tissues. We have measured the phosphorylation of EF-2 in 32P-labeled superior cervical ganglia. Phosphorylation of EF-2 was increased by preganglionic stimulation or by treatment of the ganglion with dimethylphenylpiperazinium or veratridine. Increases in EF-2 phosphorylation presumably reflect the activation of Ca2+/calmodulin-dependent protein kinase III by nicotinic stimulation and depolarization. The phosphorylation of EF-2 may help to coordinate neuronal protein synthesis with neuronal activity.

Vasopressin stimulates the phosphorylation of an 83,000 Mr protein in the superior cervical ganglion

1. 32P-Labeled proteins from the superior cervical ganglion of the rat were separated by two-dimensional gel electrophoresis and visualized by autoradiography. 2. The most heavily labeled phosphoprotein in the ganglion had a relative molecular weight of 83,000 and a pI of 4.5. Phosphorylation of this protein was increased by phorbol 12,13-dibutyrate, an activator of the Ca2+/phospholipid-dependent protein kinase, protein kinase C. This protein appears to be similar or identical to a specific protein kinase C substrate that has been described in other tissues (Blackshear, P. J., et al., J. Biol. Chem. 261:1459-1469, 1986). 3. Phosphorylation of this protein was also increased by treatment of the ganglion with phospholipase C (Bacillus cereus) but was not increased by 8-bromo-cyclic AMP or by nicotinic agonists. Vasopressin increased the hydrolysis of inositol-containing phospholipids in the ganglion and also increased the labeling of the 83,000 Mr protein. Thus, vasopressin appears to activate protein kinase C in the ganglion. 4. Muscarine, which also increased phospholipid metabolism in the ganglion, did not increase the phosphorylation of the 83,000 Mr protein. Muscarine and vasopressin stimulate phospholipid metabolism in different structures within the ganglion (Horwitz, J., et al., J. Pharmacol. Exp. Ther. 237:312-317, 1986). Muscarine may increase phospholipid metabolism in structures that do not contain significant amounts of the 83,000 Mr protein.

Preganglionic stimulation increases the phosphorylation of tyrosine hydroxylase in the superior cervical ganglion by both cAMP-dependent and Ca2+-dependent protein kinases

Electrical stimulation of the preganglionic cervical sympathetic trunk increases the phosphorylation of tyrosine hydroxylase in the superior cervical ganglion of the rat by a nicotinic mechanism and by a noncholinergic mechanism. We have measured the incorporation of [32P]Pi into specific tryptic phosphopeptides in tyrosine hydroxylase in order to identify the protein kinases that phosphorylate this enzyme in electrically stimulated ganglia. 32P-labeled tyrosine hydroxylase was isolated from the ganglion by immunoprecipitation and polyacrylamide gel electrophoresis and was subjected to tryptic hydrolysis. Seven tryptic peptides were resolved from these hydrolysates by two-dimensional thin-layer electrophoresis and chromatography. Preganglionic stimulation (20 Hz, 5 min) increased the incorporation of 32P into four of these peptides. In the presence of cholinergic antagonists, however, electrical stimulation increased the labeling of only one phosphopeptide. From a comparison of the effects of preganglionic stimulation with the effects of agonists that activate specific protein kinases, we conclude that electrical stimulation increases the phosphorylation of tyrosine hydroxylase by both a cAMP-dependent protein kinase and a Ca2+/calmodulin-dependent protein kinase. The nicotinic component of preganglionic stimulation appears to be mediated by a Ca2+/calmodulin-dependent protein kinase, while the noncholinergic component appears to be mediated by cAMP-dependent protein kinase. Although protein kinase C can phosphorylate tyrosine hydroxylase, this kinase does not appear to participate in the stimulation-induced phosphorylation of tyrosine hydroxylase in the superior cervical ganglion.