The formation, degradation, and function of cyclic nucleotides in the nervous system

Potassium dependent regulation of astrocyte water permeability is mediated by cAMP signaling

Astrocytes express potassium and water channels to support dynamic regulation of potassium homeostasis. Potassium kinetics can be modulated by aquaporin-4 (AQP4), the essential water channel for astrocyte water permeability regulation. We investigated whether extracellular potassium ([K(+)](o)) can regulate astrocyte water permeability and the mechanisms of such an effect. Studies were performed on rat primary astrocytes and a rat astrocyte cell line transfected with AQP4. We found that 10 mM [K(+)](o) caused an immediate, more than 40%, increase in astrocyte water permeability which was sustained in 5 min. The water channel AQP4 was a target for this regulation. Potassium induced a significant increase in intracellular cAMP as measured with a FRET based method and with enzyme immunoassay. We found that protein kinase A (PKA) could phosphorylate AQP4 in vitro. Further elevation of [K(+)](o) to 35 mM induced a global intracellular calcium response and a transient water permeability increase that was abolished in 5 min. When inwardly rectifying potassium (Kir)-channels were blocked, 10 mM [K(+)](o) also induced a calcium increase and the water permeability increase no longer persisted. In conclusion, we find that elevation of extracellular potassium regulates AQP4 and astrocyte water permeability via intracellular signaling involving cAMP. A prolonged increase of astrocyte water permeability is Kir-channel dependent and this response can be impeded by intracellular calcium signaling. Our results support the concept of coupling between AQP4 and potassium handling in astrocytes.

Electron microscopical localization of guanylate cyclase activity in the neocortex of the guinea pig

The localization of the guanylate cyclase (GC) activity has been established in the neocortex of adult guinea pigs by means of electron microscopical histochemistry [the DMSO-method of Fujimoto et al. (1981)]. Reaction product was deposited within a population of large- and medium-sized cortical neurons as well as in the cytoplasm of a part of the dendrites of variable size and in the cytoplasm and the nuclear membrane of a number of protoplasmic astrocytes. In the perikarya of the positive neurons, the reaction precipitate was mainly located within the cisterns of the rough endoplasmic reticulum and on the nuclear membrane. In the dendrites, the reaction product was usually distributed in close contact with microtubules, microfilaments, and beneath the postsynaptic membranes of a number of axodendritic synaptic contacts. The axons and all presynaptic boutons were negative. Thus, the localization of the GC could be determined as exclusively postsynaptic. The results obtained support the view for the probable participation of cyclic GMP in the cholinergic, glutaminergic or GABAergic, or peptidergic transmitter mechanisms in the central nervous system.

Noradrenaline modulates calcium channels in avian dorsal root ganglion cells through tight receptor-channel coupling

Averaged ensemble Ba currents were recorded from tissue cultured embryonic chick dorsal root ganglion (d.r.g.) cells using the cell-attached patch-clamp technique. Noradrenaline (NA) applied to extrapatch membrane had no clear consistent effect on drug-free patch currents. This finding supports a previous suggestion that second messengers may not be involved in NA-mediated decreases in Ca currents in sensory neurones (Forscher & Oxford, 1985). Cell-attached patch currents sometimes increased slowly after extrapatch application of NA, but were not reversibly decreased by drug treatment. Large patch currents were used to trigger cellular action potentials. NA reversibly decreased action potential duration as reflected in extracellularly recorded patch action currents. Simultaneously recorded inward patch currents were not affected. D.r.g. cell adenylate cyclase activity was assayed. NA did not affect intracellular cyclic AMP levels at concentrations which cause 30-70% decreases in gCa in dialysed cells (Forscher & Oxford, 1985). Treatment with forskolin (50 microM) or isoprenaline (10 microM) resulted in 60- and 2-fold increases respectively in adenylate cyclase activity over basal levels. These results suggest that NA decreases Ca currents by direct NA interactions with the Ca channel or a molecule tightly coupled to channel function in d.r.g. cells.

The occurrence of cyclic AMP in aging cultures of the fungus Aspergillus ornatus

The presence of endogenous cAMP was demonstrated in mycelial mats of Aspergillus ornatus Raper grown on cellulose xanthate membranes overlying a defined agar medium. Using a competitive binding assay cAMP was detected in 30 separate samples. Purification techniques increased the apparent cAMP concentration in excess of eight times the level observed in crude extracts. Cyclic AMP was only slightly detectable in extracts treated with phosphodiesterase. The concentration of endogenous cAMP as measured in purified extracts declined by approximately 60% in non-growing aging mycelial mats. Although there is an apparent age-related decline in endogenous cAMP levels and the activity of the tryptophan inducible enzyme, 2,3-dihydroxybenzoic acid carboxylyase (EC 4.1.1.46), (OPCA carboxylyase), the addition of 10(-3) M cAMP and/or 0.1% tryptophan to the culture medium did not increase OPCA carboxylyase activity in the fungus.

Comparison of proteins involved with cyclic AMP metabolism between synaptic membrane and postsynaptic density preparations isolated from canine cerebral cortex and cerebellum

Synaptic membrane and postsynaptic density (PSD) fractions isolated from canine cerebral cortex and cerebellum were assayed for the following proteins: adenylate cyclase and phosphodiesterase (PDE) activities against cyclic AMP and cyclic GMP, the regulatory subunit of the cyclic AMP-dependent protein kinase, and the substrate proteins for this kinase. The results were expressed on the basis of both the protein content of the fractions and the number of synapses in the synaptic membrane fractions. The number of synapses on a constant protein content basis was about three times higher in the cerebral cortex synaptic membrane fraction than in the comparable cerebellar fraction. Adenylate cyclase activity was from 3.4 to 5.6 times higher in the cerebral cortex membrane fraction than in the cerebellar membrane fraction based on protein content but only slightly higher based on synapse counts. PSD fractions had no adenylate cyclase activity. The cyclic AMP-PDE activity was from 17 to 27 times higher in the cerebral cortex membrane fraction than in the cerebellar membrane fraction based on protein content, and about five times higher based on synapse counts. By doing PDE histochemistry at the electron microscopy level it was found that all the cerebral cortex PSDs in the isolated fraction contained PDE activity, none being found associated with the broken-up material in the fraction. The amount of the regulatory subunit of the cyclic AMP-dependent protein kinase was about equal in the two fractions based on protein, but about one-third lower in cerebral cortex fraction than in cerebellar fractions. In the cerebral cortex membrane fraction the primary substrate for the cyclic AMP-dependent protein kinase is synapsin I, with much lower amounts in the cerebellar membrane fraction. The PSD fraction from the two sources also showed these differences in synapsin I content. In the cerebellar membrane fraction, the primary substrate for the enzyme is a approximately 245,000 Mr protein not found in the cerebral cortex membrane fraction. The findings that the turnover of cyclic AMP is much higher in cerebral cortex synapses than in cerebellar synapses, and that differences are found between the cerebral cortex and cerebellum with regard to the substrate proteins for the cyclic AMP-dependent protein kinase indicate a divergence in the effect of cyclic AMP between cerebral cortex and cerebellar synapses.

1-(4-Aminophenyl)isoquinoline derivatives. Potent inhibitors of calcium-independent and calcium-dependent phosphodiesterases from rat cerebral cortex

The effects of a series of 1-(4-aminophenyl)isoquinoline derivatives on the activity of calcium-independent and calcium-dependent phosphodiesterases purified from rat cerebral cortex were examined. Agents were approximately equipotent (IC50 values, 0.2 to 25 microM) in inhibiting the calcium-dependent hydrolysis of either cyclic AMP or cyclic GMP, while they were 6-35 times more effective as inhibitors of cyclic AMP hydrolysis when compared to cyclic GMP hydrolysis using the calcium-independent enzyme. The diastereomers of 3-(carbomethoxy)propenamido demonstrated a marked difference in specificity. The cis-isomer was very potent in inhibiting cyclic AMP or cyclic GMP hydrolysis by either enzyme (IC50 values, 0.2 to 8 microM) while the trans-isomer was only effective in inhibiting calcium-independent cyclic AMP hydrolysis (IC50 values, 2.5 microM). Kinetic analyses of the type of inhibition of the calcium-dependent enzyme revealed that the various agents were competitive inhibitors of cyclic GMP hydrolysis and noncompetitive inhibitors of cyclic AMP hydrolysis. A reverse pattern of inhibition by the isoquinoline derivatives was found using the calcium-independent phosphodiesterase, i.e. noncompetitive inhibition of cyclic GMP while competitive inhibition of cyclic AMP. Inhibition of phosphodiesterases by these agents was also manifest using intact brain slices prepared from rat cerebral cortex. Thus, the agents were found to potentiate forskolin-elicited accumulations of cyclic AMP by 100-700% and increased the half-time for the decline in cyclic AMP following forskolin stimulation from 3 to 6 min.

3H-noradrenaline release from rat neocortical slices in the absence of extracellular Ca2+ and its presynaptic alpha 2-adrenergic modulation. A study on the possible role of cyclic AMP

In Ca2+ -free EGTA-containing medium veratrine (3-25 microM) concentration-dependently enhanced the efflux of 3H-noradrenaline from (radiolabelled) rat neocortical slices. Clonidine (1 microM) inhibited and phentolamine (3 microM) enhanced veratrine-induced 3'-noradrenaline release and the modulatory effects were inversely related to the veratrine concentration used. Dibutyryl-cyclic AMP, 8-Bromo-cyclic AMP (10 microM--3 mM) and the adenylate cyclase activators NaF (2 mM) and forskolin (10 microM) enhanced 3H-noradrenaline release induced by 3 microM veratrine, but had no effect on spontaneous tritium efflux. In the presence of these drugs the modulatory effects of clonidine and phentolamine on 3H-noradrenaline release were reduced as expected from the enhanced efficacy of veratrine. In contrast to these drugs the selective cyclic AMP-phosphodiesterase inhibitor ZK 62771 reduced veratrine (3 microM)-induced 3H-noradrenaline release in Ca2+ -free medium. In the presence of 1.2 mM Ca2+, 3H-noradrenaline release induced by 13 mM K+ was also inhibited. However, when 3H-noradrenaline release was effected in the presence of tetrodotoxin (0.3 microM) or by electrical field-stimulation (1 Hz), ZK 62771 slightly but significantly enhanced the release. It is postulated that cyclic AMP is involved in the secretion process in central noradrenergic varicosities and that presynaptic alpha2-adrenoceptors upon activation inhibit the secretion process through an inhibition of a presynaptically located adenylate cyclase.

Effects of vasoactive intestinal polypeptide, monoamines, prostaglandins, and 2-chloroadenosine on adenylate cyclase in rat cerebral microvessels

Adenylate cyclase in microvessels isolated from rat cerebral cortex was stimulated by guanine nucleotides, catecholamines, prostaglandin E1, prostaglandin E2, and 2-chloroadenosine. Catecholamine stimulation was mediated by interaction with beta-adrenergic receptors. The order of relative potency was: isoproterenol greater than epinephrine greater than norepinephrine. Activation of microvessel adenylate cyclase by prostaglandins E1 and E2 as well as by 2-chloroadenosine was dose related. Twenty-two peptides were tested for possible effects on the microvessel adenylate cyclase. Only vasoactive intestinal polypeptide (VIP) was stimulatory. No inhibitory action was observed. Activation by VIP required guanosine triphosphate and was dose dependent from 10 nM to 1 microM (ED50 = 0.1 microM). At 30 degrees C, stimulation of adenylate cyclase by the peptide increased linearly with time for up to 15 min. The effect of VIP was not inhibited by phentolamine or propranolol, suggesting that its action was not elicited by interaction with alpha- or beta-adrenergic receptors. Activation achieved by VIP and isoproterenol, prostaglandin E1, or 2-chloroadenosine was the sum of the individual stimulations, suggesting that receptors for VIP were distinct from those for isoproterenol, prostaglandin E1, and 2-chloroadenosine.

Effects of cyclic AMP analogues and phosphodiesterase inhibitors on K+-induced [3H]noradrenaline release from rat brain slices and on its presynaptic alpha-adrenergic modulation

The possible role of cyclic AMP in the presynaptic alpha-adrenoceptor-mediated modulation of [3H]noradrenaline (NA) release induced by 13 mM K+ from superfused rat cerebral cortex slices was investigated. Both dibutyryl-cyclic AMP (db-cAMP) and 8-bromo-cyclic AMP (8-Br-cAMP) dose-dependently (10(-4) - 10(-2) M) enhanced K+-induced (3H]NA release, maximally to about 160% of control. In contrast, db-cAMP had no effect on calcium-induced [3H]NA release in the presence of the calcium ionophore A 23187. Surprisingly, the phosphodiesterase (PDE) inhibitors 3-isobutyl-1-methylxanthine (IBMX). 7-benzyl-IBMX, 4-(3-cyclopentyloxy-4-methoxyphenyl)-2-pyrrolidone (ZK 62771), and 4-(3-butoxy-4-methoxybenzyl)-2-imidazolidinone (Ro 20-1724) appeared to inhibit K+-induced [3H]NA release in a dose-dependent (10(-5) - 10(-3) M) manner. At a concentration of 10(-4) M, AK 62771 caused an inhibition of [3H]NA release by 30%, and this inhibitory effect was not affected by 10(-6) M phentolamine nor by 10(-3) M db-cAMP or 10(-4) M theophylline. Theophylline by itself enhanced [3H]NA release to about 135% of control. The inhibitor effect of the alpha-adrenoceptor agonist oxymetazoline (1 micro M) and the enhancing effect of the antagonist phentolamine (1 micro M) on [3H]NA release were significantly decreased in the presence of 10(-3) M db-cAMP or 8-Br-cAMP, whereas 10(-4) M ZK 62771 had no effect. In the presence of 10(-2) M NaF, a potent activator of adenylate cyclase, the inhibitory effect of oxymetazoline (1 micro M) on [3H]NA release was significantly decreased. The data obtained with the cyclic AMP analogues support the hypothesis that activation of presynaptic alpha-receptors modulating NA release results in an inhibition of a presynaptic adenylate cyclase. Possible causes for the anomalous effects of th PDE inhibitors are discussed.

Induction of female sexual behavior by GTP in ovariectomized estrogen primed rats

The effect of both intrahypothalamic and systemic administration of guanosine triphosphate (GTP) on lordosis behavior was studied in ovariectomized and ovariectomized-adrenalectomized, estrogen-primed rats (estradiol benzoate, 4 micrograms). This estrogen dose per se induced only weak or no lordosis behavior. Injection of GTP into the medial hypothalamic area (100 micrograms in 2.5 microliters) elicited lordosis behavior with relatively short latency in 6 out of 7 rats. Systemic administration of GTP in a dose range of 0.8 mg to 5.0 mg to ovariectomized estrogen-primed rats, stimulated intense lordosis behavior in all subjects. Weak lordosis responses were displayed within the first 12 hr after GTP injection, but as 48 hr all rats were highly estrous. Lordosis behavior remained for up to eight days, its duration being related to the dose of GTP administered. GTP (2 mg) induced lordosis behavior in ovariectomized, adrenalectomized estrogen-primed rats, thus excluding the participation of adrenal steroids in this effect. The results are interpreted in terms of the stimulation of adenyl cyclase-cAMP systems by GTP.

Effects of beta-adrenoceptor antagonists on the firing rate of noradrenergic neurones in the locus coeruleus of the rat

Acute i.v. administration of the non-selective beta-adrenoceptor antagonist dl-propranolol given in incremental doses (less than 40 mg/kg) did not affect the firing rate of locus coeruleus (LC) neurones in the rat, as revealed by single cell recording techniques. Furthermore, no effect was seen 4 h after a single i.p. dose of this beta-blocker (10 mg/kg). However, repeated treatment with dl-propranolol (1, 5, 10 or 20 mg/kg i.p., twice daily for 4 days) produced a significant, dose-dependent decrease of the average LC neuronal firing rate in comparison to controls. The dextro isomer of propranolol, which has negligible beta-blocking activity but the same local anaesthetic potency as the racemate, had no corresponding effect. The non-selective beta-adrenoceptor antagonist sotalol, which is one of the most hydrophilic beta-blockers, had much less inhibitory effect on LC neurones than dl-propranolol. The beta 1-selective antagonist metoprolol did not change the firing of noradrenergic neurones in the LC after similar treatment for 4 days. However, when the rats were subjected to oral treatment for 28 days, metoprolol was found to produce a slight inhibitory effect although much less than dl-propranolol. In view of these findings we propose a stimulatory and mainly beta 2-adrenoceptor-mediated control mechanism for the noradrenergic neurones in the LC. This mechanism seems to be characterized by a delayed responsiveness.

Function of calmodulin in postsynaptic densities. I. Presence of a calmodulin-activatable cyclic nucleotide phosphodiesterase activity

The postsynaptic density (PSD) fraction from canine cerebra cortex was found to contain an endogenous cyclic nucleotide-phosphodiesterase activity that was independent on Mn2+ and/or Mg2+ but not on Ca2+. Maximal activity was obtained at 1 micrometer Mn2+. This cyclic nucleotide phosphodiesterase activity was not decreased upon removal of the calmodulin from the PSD fraction, nor was it increased by the addition of calmodulin to a postsynaptic density fraction deficient in calmodulin. The enzymatic activity could be extracted by sonication, with the soluble enzyme having properties similar to those found in the native structure. Two peaks of cyclic nucleotide phosphodiesterase activities could be obtained after S-300 Sephacryl column chromatography of this soluble fraction: fraction I (excluded peak) and fraction II (215,000 mol wt). The fraction I activity preferred cyclic AMP over cyclic GMP and was not activated by calmodulin. The fraction II activity has an approximately fourfold lower Km for cyclic GMP over cyclic AMP. This fraction II activity was activatable by calmodulin, which increased the Vmax and decreased the Km in the case of both cyclic nucleotides. We conclude that two activities are present in the PSD, one activatable, and one not activatable, by calmodulin.

Increase of cGMP and accumulation of 45Ca2+ evoked by drugs acting on sodium or potassium channels

Cyclic guanosine monophosphate (cGMP) and 45Ca2+ accumulation were measured in mouse cerebellar slices after treatment with compounds known to affect ion channels in excitable membranes. Scorpion toxin and sea anemone toxin II raise cGMP and 45Ca2+ contents. Both toxins are known to keep the activated sodium channel open. 4-Aminopyridine and tetraethylammonium, which block potassium conductance, also increase cGMP and promote dose-dependently the 45Ca2+ accumulation. The effects of these 4 drugs but not the effects of depolarizing K+-concentrations on the accumulation of cGMP and 45Ca2+ are inhibited by tetrodotoxin. The cyclic GMP content of non-excitable cells, such as isolated hepatocytes from rat and guinea pig, is not affected in a comparable manner. We conclude that in excitable cells, sodium influx triggers an increase of intracellular free calcium and in that way a rise of cGMP. This effect is independent of the ion channel primarily affected. Conversely, the concentration of cGMP might serve as an indicator of intracellular free calcium.

Age-related changes of cyclic nucleotide levels in rat brain regions

Cyclic AMP and cyclic GMP levels were measured in seven brain areas of rats 4-30 months old. In several brain areas cyclic nucleotides were higher in 4-month-old rats than in rats 12 months old or older. On the other hand, in the hypothalamus cyclic GMP levels were decreased only in 30-month-old rats, a pattern of onset similar to that of senile deterioration.

Attachment of the synapse-specific phosphoprotein protein I to the synaptic membrane: a possible role of the collagenase-sensitive region of protein I

The purified synapse-specific phosphoprotein Protein I was previously shown to be degraded by a bacterial collagenase, through a series of intermediates, to a collagenase-resistant fragment of molecular weight about 48,000 containing a phosphorylated serine residue. In this study, a purified synaptic membrane fraction containing Protein I was treated with Cl. histolyticum collagenase; membrane-bound and membrane-free proteins were then phosphorylated using [gamma-32P]ATP and analyzed by SDS-polyacrylamide gel electrophoresis and autoradiography. It was observed that Protein I bound to the synaptic membrane was susceptible to the collagenase and degraded to fragments of molecular weights about 68,000, 62,000, and 48,000; the 68,000 fragment remained bound to the membrane whereas the 62,000 and 48,000 fragments were dissociated from the membrane. These observations suggest that the peptide moiety of mol. wt. 6000, present in the 68,000 fragment but absent from the 62,000 fragment, may play a crucial role in anchoring Protein I to the synaptic membrane.

Increased brain serotonergic and noradrenergic activity after repeated systemic administration of the beta-2 adrenoceptor agonist salbutamol, a putative antidepressant drug

Subchronic (5 mg/kg SC, twice daily for 14 days) but not acute administration of the beta-2-adrenoceptor agonist salbutamol to rats caused a significant increase in the accumulation of 5-hydroxytryptophan in the limbic forebrain, the corpus striatum and the cerebral cortex when measured during 30 min after inhibition of L-amino acid decarboxylase by NSD 1015 (100 mg/kg IP). Simultaneously assayed tryptophan concentrations in the same brain regions were not affected. These results indicate an increase in the in vivo rate of tryptophan hydroxylation in the brain, produced by subchronic salbutamol administration. The effect of salbutamol treatment on brain catecholamine(CA) utilization was estimated by studying the disappearance of CA in the brain after inhibition of tyrosine hydroxylase by alpha-methyltyrosine methyl ester (H 44/68), 250 mg/kg IP, 3.5 h before sacrifice. Subchronically but not acutely administered salbutamol caused both a significant increase in endogenous noradrenaline (NA) levels and an increase NA utilization. Dopamine levels and turnover were, however, not altered by either acute or subchronic treatment. The activation, probably centrally elicited, of brain NA and 5-hydroxytryptamine systems by the subchronic salbutamol regimen supports the concept of beta-adrenoceptor mediated regulation of brain monoamine systems, and could contribute to the clinically reported antidepressant activity of beta-2-adrenoceptor agonists.

Reduced brain monoamine synthesis by systemic treatment with terbutaline, a beta 2-receptor agonist

The effects of acutely (5 mg/kg s.c.) or subchronically (2.5 mg/kg s.c., twice daily, 4 days) administered terbutaline, a beta 2-receptor agonist, on the in vivo rate of tryptophan and tyrosine hydroxylation in various rat brain parts were studied. The accumulation of 5-hydroxytryptophan (5-HTP) during 30 min following treatment with NSD 1015, 100 mg/kg i.p., an inhibitor of L-aromatic amino acid decarboxylase, appeared reduced in several brain parts by the terbutaline treatments, the effect being significant in the limbic forebrain and the hemispheres after subchronic administration. This treatment also reduced the simultaneously measured accumulation of 3,4-dihydroxyphenylalanine (DOPA) in the same brain parts as well as in corpus striatum, where the effect was seen also after acutely administered terbutaline. The concentration of tryptophan in the various brain parts was not significantly affected by the terbutaline treatments and the tyrosine levels were only reduced in some brain parts (the hemispheres and the brain stem). The central effects obtained by terbutaline treatment may be mediated indirectly via peripheral inputs to e.g. the monoamine carrying neurons and/or via putative changes in cerebral blood flow.

Regulation of cyclic AMP formation in brain tissue by alpha-adrenergic receptors: requisite intermediacy of prostaglandins of the E series

The accumulations of cyclic AMP elicited by norepinephrine in slices of rat cerebral cortex or hypothalamus were markedly reduced after incubations with prostaglandin synthetase (8,11,14-eicosatrienoate, hydrogen-donor:oxygen oxidoreductase, EC 1.14.99.1) inhibitors such as indomethacin, aspirin, flufenamic acid, and acetoaminophen. Responses of cyclic AMP-generating systems to beta-adrenergic agonists or adenosine were unchanged by treatment with indomethacin and the reduction in the norepinephrine response appeared due primarily to a loss of the alpha-adrenergic component. The accumulation of cyclic AMP elicited by prostaglandin E2 [mean effective dose (EC50) 4 micro M] was increased by 2-fold by treatment with indomethacin. The alpha-adrenergic component of the norepinephrine response was fully restored by very low concentrations of prostaglandin E2 (EC50 20 nM). Prostaglandins of the F series had no effect on cyclic AMP generation under a variety of conditions. It appears that low levels of prostaglandins of the E series are required--perhaps by a calcium-dependent mechanism--for the expression of alpha-adrenergic receptor-mediated activation of cyclic AMP formation in brain tissue.

Distribution and properties of protein kinase and protein phosphatase activities in synaptosomal plasma membranes and synaptic junctions

Some characteristics of the protein kinase activity associated with a synaptosomal plasma membrane (synaptic membrane) fraction and a synaptic junction fraction have been compared. Autoradiography of the phosphorylated fractions separated on sodium dodecyl sulfate polyacrylamide gels showed that cyclic AMP stimulates the phosphorylation of five polypeptides in synaptic membranes, whereas no cyclic AMP dependency could be detected in synaptic junctions. Kinetic studies demonstrated that synaptic junctions contain a high Km and a low Km protein kinase activity while only the high Km activity could be detected in synaptic membranes. The intrinsic ATPase activity of synaptic membranes was shown to strongly interfere with measurements of protein kinase activity. Cyclic AMP binding experiments revealed a 2.6-fold enrichment of cyclic AMP binding capacity in synaptic junctions as compared to synaptic membranes. Protein phosphatase activity was not detected in synaptic junctions but was associated with synaptic membranes, where cyclic AMP was shown to either stimulate or inhibit the dephosphorylation of different polypeptides.

Investigations on the mechanism of cyclic guanosine monophosphate increase due to depolarizing agents as studied with sea anemone toxin II in mouse cerebellar slices

Sea anemone toxin II (ATX II) and MCD-peptide, like other depolarizing agents, raise the content of cGMP and to a lesser extent of cAMP in mouse cerebellar slices. Na+ influx and Ca2+ movement are involved in their mode of action, as indicated by the following observations: 1. The rise of cGMP due to ATX II, MCD-peptide and high potassium was diminished when Na+ had been replaced by Li+. 2. The effects of both toxins and veratridine, but not of high potassium stimulation were prevented by tetrodotoxin (TTX). 3. The cGMP accumulation due to both toxins was abolished in the absence of extracellular Ca2+. 4. The so-called Ca2+-antagonist (-)-D-600 blocked the increase of cGMP due to ATX II, MCD-peptide, veratridine and high potassium. 5. ATX II stimulated the 45Ca2+ uptake in mouse cerebellar slices which was prevented by TTX and (-)-D-600.