Depolarisation-dependent protein phosphorylation in rat cortical synaptosomes is inhibited by fluphenazine at a step after calcium entry

Phosphorylation of synapsin I and dynamin in rat forebrain synaptosomes: modulation by sigma (sigma) ligands

The effects of sigma (sigma) ligands on protein phosphorylation were examined in crude, rat forebrain synaptosomes. Synaptosomes were prelabelled with 32P(i) and incubated with the sigma ligands 1,3-di-o-tolylguanidine (DTG), (+)pentazocine and (-)pentazocine (3, 10, 30, 100, 300 microM), or haloperidol, reduced haloperidol, and (+)SKF 10,047 (100 microM). Aliquots were then incubated for 10 s in control (5 mM K+) or depolarising buffer (41 mM K+). All the sigma ligands increased basal phosphorylation of synapsin Ib and other proteins including dynamin, and inhibited the depolarisation-dependent increase in phosphorylation of synapsin Ib in synaptosomes. The effects of these ligands are not directly on protein kinases or protein phosphatases. This indicates that the sigma ligands are mediating their effects via interaction with sigma binding sites, and suggest, for the first time, that protein phosphorylation may be one mechanism through which sigma ligands produce their biological effects.

Activation of striatal tyrosine hydroxylase by neurocatin, a neuroregulator from mammalian brain

Neurocatin, a neuroregulatory factor isolated from mammalian brain, is a powerful affector of dopamine synthesis in striatal rat synaptosomes. Incubation of intact synaptosomes with neurocatin caused an increase in the rate of dopamine synthesis measured by accumulation of DOPA. The increase is rapid (within two minutes) and dependent on the concentration of added neurocatin. The stimulatory effect of neurocatin on dopamine synthesis occurred only in intact synaptosomes and was almost completely abolished by lysis of the synaptosomes with Triton X-100 or sonification prior to neurocatin addition. The kinetic parameters of tyrosine hydroxylase were measured in lysates prepared from synaptosomes preincubated with neurocatin. These showed that with increasing neurocatin concentration there was an increase in Vmax with no significant change in KM for the pteridine cofactor, compared to control. Activation of tyrosine hydroxylase by neurocatin is at least partially caused by a receptor mediated increase in phosphorylation of the enzyme. Protein kinase C and protein kinase II may be involved in this process.

Fluoxetine-induced inhibition of synaptosomal [3H]5-HT release: possible Ca(2+)-channel inhibition

Fluoxetine, a selective 5-HT uptake inhibitor, inhibited 15 mM K(+)-induced [3H]5-HT release from rat spinal cord and cortical synaptosomes at concentrations greater than 0.5 uM. This effect reflected a property shared by another selective 5-HT uptake inhibitor paroxetine but not by less selective uptake inhibitors such as amitriptyline, desipramine, imipramine or nortriptyline. Inhibition of release by fluoxetine was inversely related to both the concentration of K+ used to depolarize the synaptosomes and the concentration of external Ca2+. Experiments aimed at determining a mechanism of action revealed that fluoxetine did not inhibit voltage-independent release of [3H]5-HT release induced by the Ca(2+)-ionophore A 23187 or Ca(2+)-independent release induced by fenfluramine. Moreover the 5-HT autoreceptor antagonist methiothepin did not reverse the inhibitory actions of fluoxetine on K(+)-induced release. Further studies examined the effects of fluoxetine on voltage-dependent Ca2+ channels and Ca2+ entry. Whereas fluoxetine and paroxetine inhibited binding of [3H]nitrendipine to the dihydropyridine-sensitive L-type Ca2+ channel, the less selective uptake inhibitors did not alter binding. The dihydropyridine antagonist nimodipine partially blocked fluoxetine-induced inhibition of release. Moreover enhanced K(+)-stimulated release due to the dihydropyridine agonist Bay K 8644 was reversed by fluoxetine. Fluoxetine also inhibited the K(+)-induced increase in intracellular free Ca2+ in fura-2 loaded synaptosomes. These data are consistent with the suggestion that fluoxetine inhibits K(+)-induced [3H]5-HT release by antagonizing voltage-dependent Ca2+ entry into nerve terminals.

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.

4-Aminopyridine stimulates B-50 (GAP43) phosphorylation and [3H]noradrenaline release in rat hippocampal slices

In situ phosphorylation of the presynaptic protein kinase C substrate B-50 was investigated in rat hippocampal slices incubated with the convulsant drug 4-aminopyridine (4-AP). Phosphorylation of B-50 was significantly enhanced 1 min after the addition of 4-AP (100 microM). This increase by 4-AP was concentration dependent (estimated EC50 30-50 microM). Concomitant with the changes in B-50 phosphorylation, 4-AP also dose-dependently stimulated [3H]noradrenaline [( 3H]NA) release from the slices. 4-AP stimulated [3H]NA release within 5 min to seven times the control level. The B-50 phosphorylation induced by 4-AP remained elevated after removal of the convulsant, this is contrast to B-50 phosphorylation induced by depolarization with K+. A similar persistent increase was observed for [3H]NA release after a 5-min incubation period with 4-AP. These results give more insight into the molecular mechanisms underlying 4-AP-induced epileptogenesis and provide further evidence for the correlation between B-50 phosphorylation and neurotransmitter release in the hippocampal slice.

Protein kinase C and dopamine release--I. Measurement by thiophosphorylation

To examine the hypothesis that protein kinase C (PKC) plays a role in the release of dopamine (DA) in the nigrostriatal pathway, a new thiophosphorylation procedure was developed to monitor PKC activity. In this method, tissues were incubated with adenosine 5'-[gamma-thio35S]triphosphate, and the transfer of the gamma-thiophosphoryl group to histones or endogenous substrate proteins was measured. The thiophosphorylation showed a marked dependency on both calcium and lipids, and the endogenous substrate proteins being thiophosphorylated were similar to those reported as being specific substrates of PKC using [32P]ATP. Furthermore, the thiophosphorylation activity measured in the presence of calcium and lipids did not reflect cAMP-dependent or calmodulin-dependent protein kinase activities. Besides providing an accurate measure of PKC activity, thiophosphorylation has the advantage that it measures a phosphorylating activity that is independent of phosphatase activity because the thiophosphorylated substrates are resistant to the action of phosphatases.

Altered protein phosphorylation in intact rat cortical synaptosomes after in vivo administration of fluphenazine

Phenothiazines such as fluphenazine are able to inhibit calcium-stimulated protein kinases in vitro in both lysed and intact synaptosomes. In this study protein phosphorylation was assayed in intact synaptosomes isolated from the cerebral cortex of rats treated chronically (21 days, 10 mg/kg, i.p.) or acutely (1 hr, 10 mg/kg, i.p.) with fluphenazine. When intact synaptosomes from chronically treated animals were prelabeled with 32Pi, there were two effects on protein phosphorylation: an increase in the basal labeling of many phosphoproteins and a decrease in depolarization-evoked protein phosphorylation. Acute injections had even more pronounced effects, but the direction and nature of the effects were the same. No effects on K+-stimulated calcium entry or on protein phosphatase activity were detected. When lysed synaptosomes from chronically treated animals were labeled in the presence of [gamma-32P]ATP, a small decrease in calmodulin-dependent and cAMP-dependent protein phosphorylation was observed. The results suggest that two different in vivo mechanisms may underlie these effects, and these are discussed. We proposed that intact synaptosomes may be a good model in which to study the in vivo mechanisms of the action of fluphenazine since they appear to retain at least some effects of the drug after subcellular fractionation.

Involvement of pentylenetetrazole in synapsin I phosphorylation associated with calcium influx in synaptosomes from rat cerebral cortex

To determine precisely how pentylenetetrazole (PTZ) is involved in the biochemical processes at the presynaptic nerve terminal, the effect of PTZ, under various conditions, on the phosphorylation of synapsin I (previously called protein I) was investigated, using 32Pi in synaptosomes from rat cerebral cortex. PTZ markedly stimulated the incorporation of 32P into this protein as determined by sodium dodecyl sulfate (SDS)-polyacrylamide gel electrophoresis and autoradiography, but it failed to stimulate protein phosphorylation in Ca2+-free medium containing ethylene glycol bis-(beta-aminoethylether)-N',N'-tetraacetic acid (EGTA). Moreover, the PTZ-stimulated synapsin I phosphorylation was reversed by addition of EGTA sufficient to chelate all external free Ca2+. PTZ also stimulated synaptosomal accumulation of Ca2+. The PTZ-stimulatory effects of both synapsin I phosphorylation and synaptosomal accumulation of Ca2+ were inhibited markedly by tetrodotoxin as well as by cobalt chloride and lanthanum chloride. The calmodulin antagonists N-(6-aminohexyl)-5-chloro-1-naphthalenesulfonamide (W-7, strongly) and N-(6-aminohexyl)-1-naphthalenesulfonamide (W-5, weakly) reduced the PTZ-stimulatory effect on synapsin I phosphorylation by about 75 and 15%, respectively, whereas these antagonists had essentially no effect on PTZ-stimulated synaptosomal accumulation of Ca2+. These results suggest that PTZ causes the influx of Ca2+ into the presynaptic nerve terminal secondary to the elevated Na+ and is consequently involved in the synapsin I phosphorylation step, facilitating the Ca2+/calmodulin-mediated presynaptic event leading to seizure discharge.

Regulation of the phosphorylation and dephosphorylation of a 96,000 dalton phosphoprotein (P96) in intact synaptosomes

When intact rat brain synaptosomes are depolarized there is a significant increase in the phosphorylation of many proteins, and a rapid dephosphorylation of a 96,000 dalton protein termed P96. The mechanisms governing dephosphorylation are shown to be distinct from the mechanisms leading to increased phosphorylation of proteins such as synapsin I. Depolarization-dependent P96 dephosphorylation was found to be rapid (preceding the phosphorylation of synapsin I) and fully reversible, and required both depolarization and calcium entry. The phosphorylation of P96 was specifically increased by fluphenazine and by the calcium channel agonist (BAY K 8644) and antagonist (verapamil) by unknown mechanisms. Phosphorylation was also increased in the presence of dibutyryl cAMP indicating some role for cAMP-dependent protein kinase in P96 labeling. Preliminary evidence also raises the possibility of a role for protein kinase C. The characteristics of this unique synaptosomal protein suggest that it may play an important role in nerve terminal function.

Dephosphorylation of synaptosomal proteins P96 and P139 is regulated by both depolarization and calcium, but not by a rise in cytosolic calcium alone

Depolarization of intact synaptosomes activates calcium channels, leads to an influx of calcium, and increases the phosphorylation of several neuronal proteins. In contrast, there are two synaptosomal phosphoproteins labeled in intact synaptosomes with 32Pi, termed P96 and P139, which appear to be dephosphorylated following depolarization. Within intact synaptosomes P96 was found in the cytosol whereas P139 was present largely in membrane fractions. Depolarization-stimulated dephosphorylation was fully reversible and continued for up to five cycles of depolarization/repolarization, suggesting a physiological role for the phenomenon. The basal phosphorylation of these proteins was at least partly regulated by cyclic AMP, since dibutyryl cyclic AMP produced small but significant increases in P96 and P139 labeling, even in the presence of fluphenazine at concentrations that inhibited calcium-stimulated protein kinases. Depolarization-dependent dephosphorylation was independent of a rise in intracellular calcium, since agents such as guanidine and low concentrations of A23187, which increase intracellular calcium without activating the calcium channel, did not initiate P96 or P139 dephosphorylation. These agents did sustain increases in the phosphorylation of a number of other proteins including synapsin I and protein III. The results suggest that the phosphorylation of these two synaptosomal proteins is intimately linked to the membrane potential and that their dephosphorylation is dependent on both the mechanism of calcium entry and calcium itself, rather than simply on a rise in intracellular free calcium.

Phosphorylation and dephosphorylation of chromaffin cell proteins in response to stimulation

Stimulation of bovine chromaffin cell in culture changed (increased or decreased) the phosphorylation state of several proteins as examined by 32P incorporation. Enhanced phosphorylation of 22 protein bands as well as increased dephosphorylation of a 20.4 kilodaltons protein band was observed when extracts of cultured chromaffin cells stimulated by either acetylcholine or high K+ were subjected to mono-dimensional gel electrophoresis. For several protein bands, the degree of phosphorylation was larger in cells stimulated by acetylcholine than in those challenged by a depolarizing concentration of K+. The most affected phosphoproteins have apparent molecular weights of 14,800, 29,000, 33,000, 57,000 (tubulin subunit), 63,000 (tyrosine hydroxylase subunit) and 94,000. The presence of a low extracellular calcium concentration (0.5 mM Ca2+ plus 15 mM Mg2+) in the incubation medium inhibited (38-100%) the acetylcholine-evoked increases in protein phosphorylation observed previously for 18 protein bands. Trifluoperazine at the concentration required for 50% inhibition of acetylcholine-induced catecholamine release decreases (33-100%) the stimulation-induced phosphorylation in all polypeptides, with the exception of the 14.8 kilodaltons and the dephosphorylated 20.4 kilodaltons components which were not affected. Two-dimensional gel electrophoresis analysis revealed that exposure of chromaffin cells to acetylcholine produced two types of effect on protein phosphorylation: activation of protein kinase activities affecting about 30 polypeptides; activation of protein phosphatase activities resulting in the dephosphorylation of about 40 polypeptides, most of them appearing as minor phosphoproteins, with the exception of the alpha-subunit of pyruvate dehydrogenase and the 20.4 kilodaltons polypeptide. On the basis of their molecular properties (molecular weight and pI) and their abundance in chromaffin cells, the 80 kilodaltons phosphoprotein which focused at pI 4.8 and the 117.5 kilodaltons phosphoprotein which focused at pI 5.0 were identified as chromogranins A and B, respectively. The relationship between acetylcholine-induced protein phosphorylation (or dephosphorylation) and catecholamine secretion was also investigated. The time course of protein phosphorylation (or dephosphorylation) paralleled or preceded [3H]noradrenaline release for 16 phosphoproteins.(ABSTRACT TRUNCATED AT 400 WORDS)

Single ion channel activity in peptidergic nerve terminals of the isolated rat neurohypophysis related to stimulation of neural stalk axons

Scanning electron micrographs revealed that the cut face of the sagittally bisected rat neural lobe was characterized by fine fibres bearing multiple rounded swellings (0.5-5 micron in diameter) presumed to be axonal swellings and peptidergic nerve terminals. Patch-clamp electrodes sealed (seal resistance greater than 10(10) omega) on to the cell membrane in such regions revealed (36 out of 43 patches) opening of channels conducting inward current following stimulation of the neural stalk at stimulus frequencies between 5 and 20 Hz. Channel opening did not occur immediately on stimulation (delay 5-15 s) and persisted 1-25 s after stimulation of the axons ceased, implying the possible existence of a depolarization-evoked intracellular messenger. Addition of TTX (5-20 microM) to the bath to block spike propagation abolished stalk stimulation-evoked channel opening but TTX in the patch electrode did not prevent opening. Opening was, however, prevented by addition of 4 mM Co2+ or 4 mM Mn2+. The slope conductance of this channel in medium with 10 mM Ca2+ was approximately 7 pS which was increased to 19 pS in medium containing 95 mM Ba2+. A second inward channel with a slope conductance of 56 pS in medium with 10 mM Ca2+ was also characterized and outward channels were noted in four patches. The electrical properties of nerve terminals are difficult to study because of their small size. The demonstration that ion channel activity can be recorded from mammalian peptidergic nerve terminals thus offers wide scope for future studies on the relation between stimulus and secretion.

Calcium channel agonists and antagonists regulate protein phosphorylation in intact synaptosomes

Protein phosphorylation in intact synaptosomes is highly sensitive to alterations in calcium fluxes and was used to probe the possible mechanism of action of the calcium channel agonist BAY K 8644 and antagonists verapamil and nifedipine. These agents (at 1 microM) all increased the basal phosphorylation of a specific set of 4 synaptosomal phosphoproteins termed P139, P124, P96 and P60, but did not alter depolarization-dependent protein phosphorylation. The increases could not be explained by a direct stimulation of protein kinases and appears unrelated to the known effects of these drugs on K+-stimulated neurotransmitter release. This finding may reveal a possible new mechanism of action for drugs which interact with calcium channels.

Depolarization-dependent protein phosphorylation in rat cortical synaptosomes: characterization of active protein kinases by phosphopeptide analysis of substrates

Depolarization of synaptosomes is known to cause a calcium-dependent increase in the phosphorylation of a number of proteins. It was the aim of this study to determine which protein kinases are activated on depolarization by analyzing the incorporation of 32Pi into synaptosomal phosphoproteins and phosphopeptides. The following well-characterized phosphoproteins were chosen for study: phosphoprotein "87K," synapsin Ia and Ib, phosphoproteins IIIa and IIIb, the catalytic subunits of calmodulin kinase II, and the B-50 protein. Each was initially identified as a phosphoprotein in lysed synaptosomes after incubation with [gamma-32P]ATP. Mobility on two-dimensional polyacrylamide gels and phosphorylation by specific protein kinases were the primary criteria used for identification. A technique was developed that allowed simultaneous analysis of the phosphopeptides derived from all of these proteins. Phosphopeptides were characterized in lysed synaptosomes after activating cyclic AMP-, calmodulin-, and phospholipid-stimulated protein kinases in the presence of [gamma-32P]ATP. Phosphoproteins labelled in intact synaptosomes after incubation with 32Pi were then compared with those seen after ATP-labelling of lysed synaptosomes. As expected from previous work, phosphoprotein "87K," and synapsin Ia and Ib were labelled, but for the first time, phosphoproteins IIIa, IIIb, and the B-50 protein were identified as being labelled in intact synaptosomes; the calmodulin kinase II subunits were hardly phosphorylated. From a comparison of the phosphopeptide profiles it was found that cyclic AMP-, calmodulin-, and phospholipid-stimulated protein kinases are all active in intact synaptosomes and their activity is dependent on extrasynaptosomal calcium. The activation of cyclic AMP-stimulated protein kinases in intact synaptosomes was confirmed by the addition of dibutyryl cyclic AMP and theophylline which specifically increased the labelling of phosphopeptides in synapsin Ia and Ib and in phosphoproteins IIIa and IIIb. On depolarization of intact synaptosomes, a number of phosphopeptides showed increased labelling and the pattern suggested that cyclic AMP-, calmodulin-, and phospholipid-stimulated protein kinases were all activated. No new peptides were phosphorylated, suggesting that depolarization simply increased the activity of already active protein kinases and that there was no depolarization-specific increase in protein phosphorylation.

A rapid method for isolation of synaptosomes on Percoll gradients

A new rapid method for fractionation of crude synaptosomes (postmitochondrial pellet, P2) on a discontinuous 4-step Percoll gradient is described. The homogeneity and integrity of the 5 major subcellular fractions were determined by analysis of the distribution of protein, lactate dehydrogenase, cytochrome oxidase, pyruvate dehydrogenase, synapsin I (a synaptic vesicle marker) and the myelin basic proteins. The biochemical results were substantiated by quantitative electron microscopy. Fractions 3, 4 and 5 were enriched in synaptosomes and contained 19.7, 40.6 and 19.5% of the intact, identifiable synaptosomes in P2, respectively. Fraction 1 was enriched in membranous material, fraction 2 in myelin and fraction 5 in extrasynaptosomal mitochondria. The synaptosomes in fractions 3, 4 and 5 differed in their size, and their content of mitochondria, synapsin I and neurotransmitters. These results suggest that partial separation of different pools of synaptosomes has been achieved. The synaptosomes in fractions 3, 4 and 5 are viable, as they take up calcium, phosphate and noradrenaline; they are metabolically normal as judged by their ability to perform protein phosphorylation and they respond normally to depolarization by increasing calcium uptake, protein phosphorylation and neurotransmitter release. The synaptosomes in fraction 4 are relatively homogeneous and appear to be free of contamination from lysed synaptosomes and synaptic plasma membranes. This constitutes a major advantage of the Percoll method over traditional procedures which involve centrifugation to equilibrium. We have therefore confirmed (J. Neurochem., 43 (1984) 1114-1123) the advantages of Percoll use over traditional procedures, while further reducing the time taken, and extended our analysis to show that the present procedure provides a fractionation of synaptosomes into different pools of viable synaptosomes.

Antipsychotic drugs depress acetylcholine release in the Torpedo electric organ, a purely cholinergic system

We used the electric organ of Torpedo, a modified neuromuscular system, to investigate the direct effects of antipsychotic drugs on cholinergic transmission. All the antipsychotic drugs tested inhibited transmission by decreasing the amount of ACh released by nerve impulses. Their potency for this action was as follows: trifluoperazine less than clozapine less than thiethylperazine less than droperidol less than haloperidol less than chlorpromazine = beta-flupentixol less than alpha-flupentixol. Depression of ACh release by antipsychotics was poorly reversible, and was not mediated by dopamine receptors in this system since neither dopamine nor apomorphine had any effect on transmission. Antipsychotics did not act through presynaptic cholinergic receptors since the effect was not antagonized by atropine or quinuclidinyl benzilate. Trifluoperazine had no effect on the total ACh content of the tissue, on the compartmentation of ACh inside and outside synaptic vesicles, or on the rate of ACh turnover or the accumulation of 45Ca observed after repetitive stimulation. We conclude that antipsychotic drugs depress the neurally evoked release of ACh by acting directly on the releasing mechanism.

Depolarisation-dependent protein phosphorylation and dephosphorylation in rat cortical synaptosomes is modulated by calcium

The effect of calcium on protein phosphorylation was investigated using intact synaptosomes isolated from rat cerebral cortex and prelabelled with 32Pi. For nondepolarised synaptosomes a group of calcium-sensitive phosphoproteins were maximally labelled in the presence of 0.1 mM calcium. The phosphorylation of these proteins was slightly decreased in the presence of strontium and absent in the presence of barium, consistent with the decreased ability of these cations to activate calcium-stimulated protein kinases. Addition of calcium alone to synaptosomes prelabelled in its absence increased phosphorylation of a number of proteins. On depolarisation in the presence of calcium certain of the calcium-sensitive phosphoproteins were further increased in labelling above nondepolarised levels. These increases were maximal and most sustained after prelabelling at 0.1 mM calcium. On prolonged depolarisation at this calcium concentration a slow decrease in labelling was observed for most phosphoproteins, whereas a greater rate and extent of decrease occurred at higher calcium concentrations. At 2.5 mM calcium a rapid and then a subsequent slow dephosphorylation was observed, indicating two distinct phases of dephosphorylation. Of all the phosphoproteins normally stimulated by depolarisation, only phosphoprotein 59 did not exhibit the rapid phase of dephosphorylation at high calcium concentrations. Replacing calcium with strontium markedly decreased the extent of change observed on depolarisation whereas barium decreased phosphorylation changes even further. Taken together these data suggest that an influx of calcium into synaptosomes initially activates protein phosphorylation, but as the levels of intrasynaptosomal calcium rise protein dephosphorylation predominates. Other phosphoproteins were dephosphorylated immediately on depolarisation in the presence of calcium. The fine control of protein phosphorylation levels exerted by calcium supports the idea that the synaptosomal phosphoproteins could play a role in modulating events such as neurotransmitter release in the nerve terminal.