Effects of neuronal activity on inositol phospholipid metabolism in the rat autonomic nervous system

Long-term potentiation of nicotinic synaptic transmission in rat superior cervical ganglia produced by phorbol ester and tetanic stimulation

The long-term potentiation of nicotinic synaptic transmission induced by both active phorbol ester (4beta-phorbol-12,13-dibutyrate, PdBu) and tetanic trains of preganglionic stimulation was studied in single neurons of the superior cervical ganglion (SCG) of the rat using intracellular recording techniques. PdBu significantly increased the mean amplitude of both the unitary evoked fast excitatory postsynaptic potentials (EPSPs) and the fast excitatory postsynaptic currents (EPSCs) to 17.0+/-3.3 mV (control 8.4+/-1.9 mV, n=5) and 2.8+/-0.4 nA (control 0.8+/-0.1 nA, n=10), respectively. There was no significant change in either the resting membrane potential, input resistance, or the threshold for the initiation of an action potential. The response to exogenously applied acetylcholine (ACh) was also not changed following exposure to PdBu. In low-calcium, high-magnesium solutions, PdBu significantly increased the quantal content of EPSPs approximately threefold from a control of 0.9+/-0.2 (n=5) to 2.6+/-0.6 (n=5). The quantal content of EPSCs was also increased to 1.3+/-0.2 (control 0.5+/-0.1, n=10). PdBu increased the frequency of miniature EPSPs (mEPSPs) to 196+/-47% (n=6) of control, while the amplitude, rise time, rate of rise, and decay of mEPSPs were not significantly changed. Tetanic stimulation significantly increased the amplitude of the unitary synaptic EPSPs and EPSCs without significantly changing the resting membrane potential, input resistance, threshold for initiation of an action potential, or the response to exogenously applied ACh. Tetanic stimulation significantly increased quantal content of EPSPs and EPSCs threefold. The results obtained with tetanically induced LTP are similar to the results obtained with phorbol ester-induced LTP in these ganglion neurons. These results suggest that both tetanically induced and phorbol ester-induced LTP, in the rat, share similar mechanisms which involve, at least in part, activation of PKC-dependent mechanisms to increase quantal release from sympathetic preganglionic axon terminals.

Patch cramming reveals the mechanism of long-term suppression of cyclic nucleotides in intact neurons

To understand cyclic nucleotide dynamics in intact cells, we used the patch-cramming method with cyclic nucleotide-gated channels as real-time biosensors for cGMP. In neuroblastoma and sympathetic neurons, both muscarinic agonists and nitric oxide (NO) rapidly elevate cGMP. However, muscarinic agonists also elicit a long-term (2 hr) suppression (LTS) of subsequent cGMP responses. Muscarinic agonists elevate cGMP by triggering Ca2+ mobilization, which activates NO synthase to produce NO, leading to the activation of soluble guanylate cyclase (sGC). Here we examine the mechanism of LTS. Experiments using direct intracellular cGMP injection demonstrate that enhancement of phosphodiesterase (PDE) activity, rather than depression of sGC activity, is responsible for LTS. Biochemical measurements show that both cGMP and cAMP content is suppressed, consistent with the involvement of a nonselective PDE. Application of pharmacological agents that alter Ca2+ mobilization from intracellular stores and experiments involving injection of the Ca2+ chelator BAPTA show that Ca2+ mobilization is necessary and sufficient for LTS induction but also show that LTS maintenance is Ca2+-independent. Protein phosphatase injection reverses LTS, and specific inhibitors of Ca2+/calmodulin kinase II (CaMKII) prevent induction and inhibit maintenance. The switch between the Ca2+ dependence of LTS induction to the Ca2+ independence of LTS maintenance is consistent with CaMKII autophosphorylation, similar to proposed mechanisms of hippocampal long-term potentiation. Because the molecular machinery underlying LTS is common to many cells, LTS may be a widespread mechanism for long-term silencing of cyclic nucleotide signaling.

Real-time patch-cram detection of intracellular cGMP reveals long-term suppression of responses to NO and muscarinic agonists

Cyclic GMP (cGMP) is a crucial intracellular messenger in neuronal, muscle, and endocrine cells. The intracellular concentration of cGMP is regulated by various neurotransmitters, including acetylcholine (ACh) and nitric oxide (NO). While much is known about the biochemical steps leading to cGMP synthesis, little is known about cGMP kinetics in intact cells. Here, we use "patch-cramming," in which an excised, inside-out membrane patch containing cyclic nucleotide-gated ion channels is used as a biosensor, to obtain the first real-time measurements of cGMP in intact cells. Patch-cramming experiments on neuroblastoma cells show that both muscarinic agonists and NO rapidly elevate cGMP. NO elicits cGMP responses repeatedly without decrement, whereas responses to muscarinic agonists exhibit a profound and prolonged desensitization. Remarkably, muscarinic agonists also cause long-term (>30 min) suppression (LTS) of cGMP responses elicited by NO. Biochemical measurements reveal that rat sympathetic neurons also exhibit LTS of cGMP, suggesting that LTS is a widespread mechanism that may contribute to synaptic plasticity.

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.

Plasticity of cholinoreceptors of neurons of the common snail after effects on inositol-1,4,5-triphosphate- and CA(2+)-dependant mobilization of stored CA2+ and the level of phosphatidic acid

The influences on the depth of extinction of the inward current induced by acetylcholine (the ACh-current) of a number of compounds affecting the mobilization of stored Ca2+ and the intracellular level of Ca(2+)-mobilizing second messengers, namely, inositol-1,4,5-trisphosphate (IP3), inositol hexakisphosphate, TMB-8 (an inhibitor of (IP3)-dependent Ca2+ mobilization), tetracaine (an inhibitor of Ca(2+)-dependant mobilization of Ca2+), as well as phospholipase D, which leads to the formation of phosphatidic acid through the hydrolysis of phosphatidylcholine, were investigated in identified RPa3 and LPa3 neurons of the common snail using the two-electrode voltage clamp technique for the recording of the potential on the membrane. The participation of IP3, of IP3-dependant, and Ca(2+)-dependant mobilized intracellular Ca2+, as well as phosphatidic acid in the regulation of the plasticity of the cholinoreceptors of the neurons was demonstrated.

Synergistic regulation of a neuronal chloride current by intracellular calcium and muscarinic receptor activation: a role for protein kinase C

Using perforated patch recordings in combination with intracellular Ca2+ ([Ca2+]i) fluorescence measurements, we have identified a delayed Ca(2+)-dependent Cl- current in a mammalian sympathetic ganglion cell. This Cl- current is induced by the synergistic action of Ca2+ and diacylglycerol (DAG) and is blocked by inhibitors of protein kinase C. As a result, the current can be induced by acetylcholine through the conjoint activation of nicotinic receptors (to produce a rise in [Ca2+]i) and muscarinic receptors (to generate DAG). This demonstrates an unusual form of synergism between the two effects of a single transmitter mediated via separate receptors operating within a time scale that could be of physiological significance.

Evidence that nitric oxide mediates the cyclic GMP response to synaptic activity in the rat superior cervical ganglion

Preganglionic nerve stimulation in the rat superior cervical ganglion (SCG) caused an increase in guanosine 3':5'-cyclic monophosphate (cyclic GMP) in a Ca(2+)-dependent manner. This increase was inhibited by oxyhaemoglobin, and blocked stereoselectively by an inhibitor of nitric oxide synthase, NG-nitro-L-arginine. Thus, nitric oxide or a similar substance appears to mediate the neuronal cyclic GMP response to synaptic activity in the rat SCG.

Effects of muscarinic agonists and depolarizing agents on inositol monophosphate accumulation in the rabbit vagus nerve

The effects of muscarinic agonists and depolarizing agents on inositol phospholipid hydrolysis in the rabbit vagus nerve were assessed by the measurement of [3H]inositol monophosphate production in nerves that had been preincubated with [3H]inositol. After 1 h of drug action, carbachol, oxotremorine, and arecoline increased the inositol monophosphate accumulation, though the maximal increase induced by these agonists differed. Addition of the muscarinic antagonists atropine or pirenzepine shifted the carbachol dose-response curves to the right, without decreasing the carbachol maximal stimulatory effects. The KB for pirenzepine was 35 nM, which is characteristic of muscarinic high-affinity binding sites coupled to phosphoinositide turnover and often associated with the M1 receptor subtype. On the other hand, agents known to depolarize or to increase the intracellular Ca2+ concentration, e.g., elevated extracellular K+, ouabain, Ca2+, and the Ca2+ ionophore A23187, also increased inositol monophosphate accumulation. These effects were not mediated by the release of acetylcholine, as suggested by the fact that they could not be potentiated by the addition of physostigmine nor inhibited by the addition of atropine. The Ca(2+)-channel antagonist Cd2+, also known to inhibit the Na+/Ca2+ exchanger, was able to block the effects of K+ and ouabain, but did not alter those of carbachol. These results suggest that depolarizing agents increase inositol monophosphate accumulation in part through elevation of the intracellular Ca2+ concentration and that muscarinic receptors coupled to phosphoinositide turnover are present along the trunk of the rabbit vagus nerve.

A role for protein kinase C in long term potentiation of nicotinic transmission in the superior cervical ganglion of the rat

The superior cervical ganglion of rats was perfused with Ringer solution containing hexamethonium to produce a steady, partial, nicotinic block. The compound action potential (CAP) evoked by supramaximal single shock stimulation of the cervical sympathetic trunk (CST) was recorded from the internal carotid nerve. Bolus injection of the protein kinase C (PKC) activators 4 beta-phorbol-12,13-dibutyrate (PDBu) or 4 beta-phorbol-12,13-diacetate (PDAc) produced a marked, prolonged, dose-dependent potentiation of the CAP amplitude (e.g. 90% decay 2 h). A non-PKC activating phorbol ester (PE), 4 alpha-phorbol-12,13-didecanoate, produced no potentiation. The PE-induced potentiation was antagonized by the PKC inhibitor H-7. In addition, after 1 h exposure to PDBu (3 microM) and recovery from the potentiation (e.g. 2-4 h), a second exposure to PDBu or PDAc produced no potentiation. A 5 s 40 Hz supramaximal train to the CST produced a long lasting potentiation of the CAP (long-term potentiation, LTP) as described previously. However, a similar train did not evoke LTP after perfusion for 1 h with PDBu. The train-evoked LTP was depressed by the PKC inhibitor H-7 at a concentration which antagonized the PE-evoked potentiation. These data suggest that (i) PKC activation potentiates nicotinic transmission, and (ii) a component of the train-evoked LTP is mediated by PKC.

Potentiation of nicotinic transmission in the rat superior cervical sympathetic ganglion: effects of cyclic GMP and nitric oxide generators

The efficacy of nicotinic transmission in the rat superior cervical ganglion in vitro (24-26 degrees C) was estimated by extracellular recording of the postganglionic compound action potential response to stimulation of the preganglionic nerve at a slow rate (one shock every 60 s). Atropine (2 microM) was included to block muscarinic transmission, and hexamethonium (200-250 microM) was used to produce a submaximal response sensitive to potentiation and inhibition of nicotinic transmission. Upon exposure to 1-100 microM 8-bromo-guanosine 3',5'-cyclic monophosphate (8-Br-cGMP), nicotinic transmission was potentiated by 6 +/- 1% (n = 4) to 89 +/- 5% (n = 5) in a dose-dependent manner. 8-Bromo-adenosine 3',5'-cyclic monophosphate (8-Br-cAMP, 10-100 microM) also potentiated nicotinic transmission (3.8 +/- 0.3% (n = 3) to 43 +/- 4% (n = 3)). However, 8-Br-cGMP was at least 2-fold more effective than 8-Br-cAMP. Sodium nitroprusside (0.1 microM to 1 mM) and sodium azide (0.1-100 microM) were used to stimulate the formation of endogenous cGMP52. Nicotinic transmission was potentiated by these substances also. The response was increased by 3.4 +/- 0.7% (n = 4) to 32 +/- 2% (n = 5) upon exposure to 0.1-100 microM sodium nitroprusside, and by 5.5 +/- 0.9% (n = 3) to 18 +/- 4% (n = 4) upon exposure to 0.1-100 microM sodium azide. Ferricyanide ion (10-100 microM) appeared to be ineffective, as would be expected if the effect of nitroprusside was due to the nitric oxide rather than the cyanide or ferric moieties.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of membrane depolarization by high K+ on carbachol-stimulated phosphoinositides hydrolysis in guinea pig cerebral cortical slices

Stimulation of phosphoinositide hydrolysis by carbachol was studied in slices of guinea pig cerebral cortex under normal conditions (4.7 mM K+) and depolarization conditions with high K+ (42 mM K+). Slices were labeled with [myo-3H]-inositol, and the effects of carbachol and high K+ on the formation of inositol-bisphosphates (IP2) and inositol-trisphosphates (IP3) were determined. Carbachol (10 mM) caused only 140% stimulation of the formations of IP2 and IP3 over the control value in normal Krebs Ringer Buffer (KRB), but about 200% stimulation in high K+ medium. Dose-response curves for the effect of carbachol on the formations of IP2 and IP3 showed that high K+ medium selectively decreased the ED50 value of carbachol for IP2 formation about 3-fold. A Ca++ channel blocker, verapamil, inhibited the synergistic effect of carbachol and high K+ on IP2 formation, and a decrease in extracellular Ca++ also inhibited IP2 formation induced by high K+, but these treatments had little, if any, effect on IP3 formation. The possibility that IP2 may be directly generated by hydrolysis of phosphatidylinositol 4-monophosphate (PIP) as well as from hydrolysis of IP3 was discussed.

Carbachol and bradykinin increase the production of diacylglycerol from sources other than inositol-containing phospholipids in PC12 cells

Both carbachol and bradykinin increased diacylglycerol formation in PC12 pheochromocytoma cells. The effect of carbachol was apparent only in cells that had been treated with nerve growth factor. Incubation of the cells in Ca2(+)-free medium attenuated carbachol-stimulated diacylglycerol formation but did not reduce the response to bradykinin. Pretreatment of the cells with pertussis toxin did not affect either carbachol- or bradykinin-stimulated diacylglycerol formation; therefore, the inhibitory guanine nucleotide Gi probably does not mediate this response. The time course of carbachol-stimulated diacylglycerol accumulation did not coincide with the time course of inositol 1,4,5-trisphosphate (IP3) production. IP3 was elevated at the earliest time measured, 15 s, and then slowly declined so that by 5 min IP3 levels were only 50% of maximal. Diacylglycerol levels, in contrast, were not elevated for the first 2 min and then peaked at 5 min. These data indicate that hydrolysis of phosphatidylinositol 4,5-bisphosphate was not the major source of the diacylglycerol peak at 5 min. To investigate the source of diacylglycerol, I examined the fatty acid composition of the diacylglycerol by prelabeling the cells with [3H]palmitic acid and [14C]stearic acid. The 14C/3H ratio in diacylglycerol should reflect the phospholipid(s) from which it is derived. The 14C/3H ratio of the increment in diacylglycerol produced by carbachol and bradykinin was intermediate between the 14C/3H ratios of phosphatidylcholine and phosphatidylinositol. The 14C/3H ratio in triacylglycerol was similar to that of phosphatidylcholine. These data indicate that carbachol and bradykinin stimulate the formation of diacylglycerol from sources other than inositol-containing phospholipids; phosphatidylcholine and triacylglycerol are two possible sources of this diacylglycerol.

Dual effects of K+ depolarisation on inositol polyphosphate production in rat cerebral cortex

Depolarisation of [3H]inositol-prelabelled slices of rat cerebral cortex with elevated extracellular K+ induced a rapid and marked increase in inositol polyphosphate accumulation. Addition of the muscarinic antagonist atropine (10 microM) markedly inhibited the K+-induced accumulation of inositol tetrakisphosphate (InsP4), with only a slight reduction in stimulated inositol bis- and trisphosphate levels. Inhibitory effects on InsP4 were noted at the earliest time period measured (30 s) and suggested the involvement of released endogenous acetylcholine in part of the response. The atropine-insensitive component of depolarisation did not appear to be secondary to release of noradrenaline, histamine, or 5-hydroxytryptamine, because addition of prazosin, mepyramine, or ketanserin was without effect on the K+ response. Furthermore, secretion of a neuropeptide that could stimulate phosphoinositide hydrolysis was unlikely, because the peptidase inhibitor bacitracin was also without effect. The results suggest that endogenous acetylcholine can stimulate phosphoinositide metabolism by interacting with muscarinic receptors and that this is particularly evident on InsP4 accumulation. Atropine-insensitive responses may be secondary to Ca2+ entry via voltage-sensitive channels.

Phosphoinositide hydrolysis induced by depolarization and sodium channel activation in mouse cerebrocortical slices

Carbachol, a muscarinic receptor agonist and the sodium channel-activating agents, scorpion venom, veratridine, batrachotoxin and aconitine, were shown to stimulate the formation of [3H]inositol phosphates in [3H]inositol-labelled miniprisms, obtained from the cerebral cortex of the mouse. The inositol response to the Na+ channel-activating agents was inhibited by the sodium channel blocker tetrodotoxin (TTX), while the response induced by carbachol was partially resistant to TTX. The response to scorpion venom and the TTX-insensitive portion of the response to carbachol was additive, indicating different mechanisms. The presence of high potassium (K+) induced hydrolysis of inositide in a TTX-insensitive manner and was not additive with that resulting from sodium channel activators, thus indicating a common mechanism. The addition of large concentrations of magnesium to block the release of acetylcholine, did not inhibit the inositol response to high K+ or to veratridine. Calcium channel blockers such as nickel or cobalt, or the dihydropyridine calcium (Ca2+) channel activator BAY K 8644 and the calcium channel blocker nifedipine, nimodipine or PN-200 110 had little effect. Monensin, a sodium ionophore, stimulated the turnover of phosphatidylinositol at non-depolarizing concentrations and the omission of Na+ ions inhibited the response to sodium channel agents and to high K+. Thus, membrane potential and gradients of K+, Na+ and Ca2+ are all important factors determining the final effect on the turnover of phosphatidylinositol. The data are consistent with a model in which all these factors impinge on the Na+/Ca2+ exchanger regulating internal Ca2+ that, in turn, activates phospholipase C.

Inositol phospholipid metabolism during and following synaptic activation: role of adenosine

The metabolic pathway of inositol phospholipids represents a series of synthetic and hydrolytic reactions with inositol as a by-product. Hence, the rate of [3H]inositol release from prelabeled phospholipids can be used as a reflection of activity of this pathway. In the frog sympathetic ganglion prelabeled with [3H]inositol, we studied the effect of synaptic activity (orthodromic stimulation) on release of 3H-label into the medium. This release was interpreted as [3H]inositol release. The value was low at rest and increased significantly by 32% during orthodromic stimulation (20 Hz for 5 min). However, on cessation of the stimulation, [3H]inositol release increased rapidly by 148% and remained elevated for at least 45 min. This increase in [3H]inositol release during and after the stimulation period was reduced by suffusion of the ganglia with adenosine. We hypothesized that synaptic activation releases a long-lasting stimulatory agonist and a short-lasting inhibitory (adenosine) agonist or agonists affecting [3H]inositol release. To demonstrate the presence of a stimulatory agonist, two sympathetic ganglia were used. One was prelabeled with [3H]inositol, and the other was not. The two ganglia were placed together in a 5-microliter droplet of Ringer's solution containing atropine. Orthodromic stimuli applied to the nonlabeled ganglion elicited release of [3H]inositol from the nonstimulated ganglion. To test whether the adenosine formed during orthodromic stimulation inhibits [3H]inositol release, we destroyed endogenous adenosine by suffusion of the ganglia with adenosine deaminase during the stimulation period. We found that adenosine deaminase induced large increases in [3H]inositol release during the stimulation period, in contrast to an increase seen only during the poststimulation period when adenosine deaminase was omitted. Because [3H]inositol release is assumed to parallel changes in content of inositol phosphates, we anticipated no changes of the levels of these compounds during orthodromic stimulation. However, measurements showed that levels of inositol phosphates and inositol phospholipids were all elevated except for phosphatidylinositol 4-phosphate. On termination of the stimulus, they remained elevated, with a further increase in levels of inositol trisphosphate and phosphatidylinositol 4-phosphate. We conclude that endogenous adenosine inhibits [3H]inositol release, possibly by modulating several of the steps of the inositol phospholipid pathway.(ABSTRACT TRUNCATED AT 250 WORDS)

Inositol uptake and metabolism in molluscan neuronal tissue

The uptake of myo-[3H]inositol into neurones from Lymnaea stagnalis has been demonstrated to be a sodium-dependent process, saturable with a Km of approximately 50 microM and shown to be linear with time for at least 120 min. The rate of transport of myo-inositol into the cell appears to influence directly its incorporation into neuronal lipids. Using anion-exchange high-performance liquid chromatography, we have demonstrated a high rate of breakdown of phosphatidylinositol 4,5-bisphosphate in Lymnaea nerve under basal conditions. Stimulation with carbamylcholine enhanced production of inositol 1-phosphate, inositol bisphosphate, inositol 1,4,5-trisphosphate, and inositol 1,3,4-trisphosphate. Formation of inositol tetrakisphosphate was not detected. Electrical stimulation also caused an increased formation of inositol phosphates. These results provide evidence for an active myo-inositol transport system in molluscan neurones and suggest that the hydrolysis of inositol lipids may play a role as an intracellular signalling system in this tissue.

Formation of second messengers in response to activation of ion channels in excitable cells

1. Depolarization of excitable cells of the central nervous system results in the formation of the second messengers cyclic AMP, cyclic GMP, inositol phosphates, and diacylglycerides. 2. Depolarization-evoked accumulation of cyclic AMP in brain preparations can be accounted for mainly by the release of adenosine, which subsequently interacts with stimulatory adenosine receptor linked to adenylate cyclase. 3. Depolarization-evoked formation of cyclic GMP in brain preparations is linked to activation of voltage-dependent calcium channels, presumably leading to activation of guanylate cyclase by calcium ions. 4. In brain slices depolarization-evoked stimulation of phosphoinositide breakdown and subsequent formation of inositol phosphates and diacylglycerides are linked to activation of voltage-dependent calcium channels, which are sensitive to dihydropyridines, presumably leading to activation of phospholipase(s) C by calcium ions. 5. In the synaptoneurosome preparation depolarization-evoked stimulation of phosphoinositide breakdown does not involve activation of dihydropyridine-sensitive calcium channels and, instead, appears to be regulated primarily by the intracellular concentration of sodium ions. Thus, agents that induce increases in intracellular sodium--such as toxins that open or delay inactivation of voltage-dependent sodium channels; ouabain, an inhibitor of Na+/K+ ATPase that transports sodium outward and a sodium ionophore--all stimulate phosphoinositide breakdown. Mechanistically, increases in intracellular sodium either might directly affect phospholipase(s) C or might lead to influx of calcium ions through Na+/Ca2+ transporters. 6. Depolarization-evoked stimulation of cyclic AMP formation and phosphoinositide breakdown can exhibit potentiative interactions with responses to receptor agonists, thereby providing mechanisms for modulation of receptor responses by neuronal activity. 7. Since all these second messengers can induce phosphorylation of ion channels through the activation of specific kinases, it is proposed that depolarization-evoked formation of second messengers represents a putative feedback mechanism to regulate ion fluxes in excitable cells.

Protein kinase C activators mimic the M2-muscarinic receptor-mediated effects on the action potential in isolated sympathetic neurons of rabbits

Protein kinase C activators 1,2-oleoylacetylglycerol (OAG, 0.5-50 microM), a synthetic diacylglycerol analog, and phorbol-12,13-dibutyrate (Pb(Bu)2, 0.016-1.6 microM) depressed the calcium (Ca)-dependent components of action potentials in isolated superior cervical ganglion cells of rabbits. Similar depressions were elicited when the M2-muscarinic receptors were activated. This muscarinic modification of the action potential was obscured after the perfusion with protein kinase C inhibitor 1-(5-isoquinolinesulfonyl)-2-methylpiperazine (H-7, 50 microM). It seems that protein kinase C is an intermediator between the M2-muscarinic receptors and the Ca channels regulating the firing rate of the postganglionic cells.

In vivo electrical stimulation of the sympathetic nerve of the eye increases inositol phosphate production and prostaglandin release in the rabbit iris muscle

The effects of in vivo electrical stimulation of the sympathetic nerve of the eye on phosphatidylinositol 4,5-bisphosphate (PIP2) hydrolysis in rabbit iris and release of arachidonate and prostaglandin (PG) E2 into aqueous humor were investigated. myo-[3H]Inositol or [1-14C]arachidonate was injected intracamerally into each eye 3 h before electrical stimulation of one of the sympathetic trunks. Tissue phosphoinositides were determined by TLC, and 3H-labeled inositol phosphates were analyzed by either ion-exchange chromatography or HPLC. The aqueous humor was analyzed for 14C-labeled arachidonate and PGE2 by radiochromatography and for unlabeled PGE2 by radioimmunoassay. The results obtained from this study can be summarized as follows: (a) The rates of in vivo incorporation of myo-[3H]inositol into phosphoinositides and accumulation of 3H-labeled inositol phosphates in the iris muscle increased with time and then leveled off between 3 and 5 h. (b) Distribution of 3H radioactivity in inositol phosphates, as determined by HPLC, showed that of the total radioactivity in inositol phosphates, 53.6% was recovered in myo-inositol 1-phosphate, 36% in myo-inositol bisphosphate, 0.95% in myo-inositol 1,3,4-trisphosphate (1,3,4-IP3), and 2.6% in 1,4,5-IP3. (c) Electrical stimulation of the sympathetic nerve resulted in a significant loss of 3H radioactivity from PIP2 and a concomitant increase of that in IP3, an observation indicating that PIP2 is the physiological substrate for alpha 1-adrenergic receptors in this tissue. (d) Release of IP3 and liberation of arachidonate for PGE2 synthesis are dependent on the duration of stimulation and the intensity (voltage) of stimulus.(ABSTRACT TRUNCATED AT 250 WORDS)

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.

Cholinergic stimulation of inositol phosphate formation in bovine adrenal chromaffin cells: distinct nicotinic and muscarinic mechanisms

The ability of cholinergic agonists to activate phospholipase C in bovine adrenal chromaffin cells was examined by assaying the production of inositol phosphates in cells prelabeled with [3H]inositol. We found that both nicotinic and muscarinic agonists increased the accumulation of [3H]inositol phosphates (mainly inositol monophosphate) and that the effects mediated by the two types of receptors were independent of each other. The production of inositol phosphates by nicotinic stimulation required extracellular Ca2+ and was maximal at 0.2 mM Ca2+. Increasing extracellular Ca2+ from 0.22 to 2.2 mM increased the sensitivity of inositol phosphates formation to stimulation by submaximal concentrations of 1,1-dimethyl-4-phenyl-piperazinium iodide (DMPP) but did not enhance the response to muscarine. Elevated K+ also stimulated Ca2+-dependent [3H]inositol phosphate production, presumably by a non-receptor-mediated mechanism. The Ca2+ channel antagonists D600 and nifedipine inhibited the effects of DMPP and elevated K+ to a greater extent than that of muscarine. Ca2+ (0.3-10 microM) directly stimulated the release of inositol phosphates from digitonin-permeabilized cells that had been prelabeled with [3H]inositol. Thus, cholinergic stimulation of bovine adrenal chromaffin cells results in the activation of phospholipase C by distinct muscarinic and nicotinic mechanisms. Nicotinic receptor stimulation and elevated K+ probably increased the accumulation of inositol phosphates through Ca2+ influx and a rise in cytosolic Ca2+. Because Ba2+ caused catecholamine secretion but did not enhance the formation of inositol phosphates, phospholipase C activation is not required for exocytosis. However, diglyceride and myo-inositol 1,4,5-trisphosphate produced during cholinergic stimulation of chromaffin cells may modulate secretion and other cellular processes by activating protein kinase C and/or releasing Ca2+ from intracellular stores.

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.

Vasopressin-induced turnover of phosphatidylinositol in the sensory nervous system of the rat

Vasopressin and oxytocin immunoreactivity (AVP-IR, OT-IR) have been detected in the trigeminal and dorsal root ganglia (TG, DRG) of the rat. We have investigated whether AVP or OT have any neurotransmitter role in these tissues by measuring the effects of the peptides on levels of intracellular second messengers. AVP and OT at concentrations up to 3 x 10(-6) M have no effect on the accumulation of cAMP. However, in tissue prelabelled with 3H-inositol, and in the presence of 10 mM Li+, AVP and OT cause an increase in the accumulation of inositol phosphates (IP), in a dose-dependent manner. AVP causes a maximum stimulation of 1.7 fold of control in TG, (p less than 0.01) and of 2.5 fold in DRG (p less than 0.01) at a concentration of 3 x 10(-7) M. OT causes a maximum stimulation of 1.3 fold of control in TG, (p less than 0.01), and of 1.75 fold of control in DRG, at a concentration of 3 x 10(-6) M. The stimulation of IP turnover by AVP in both tissues is inhibited by the specific V1-antagonist, (CH2)5Tyr(Me)AVP, at a concentration of 2 x 10(-5) M. The V2-agonist, DDAVP, has no effect on IP accumulation in either tissue at concentrations up to 3 x 10(-6) M. The response to exogenous AVP is still present in ganglia incubated in media without added CaCl2. We conclude that the rat TG and DRG contain receptors for AVP, and that these receptors have characteristics associated with the V1 subtype.

Effects of phorbol dibutyrate on M currents and M current inhibition in bullfrog sympathetic neurons

1. Effects of bath-applied phorbol dibutyrate (PDBu) on M currents (IM) and on the inhibition of IM by muscarine and luteinizing hormone-releasing hormone (LHRH) were recorded in voltage-clamped bullfrog lumbar sympathetic ganglion cells. 2. PDBu (0.1-30 microM) produced a slowly developing, irreversible and partial (less than or equal to 60%) inhibition of IM. This effect was not replicated by 4-alpha-phorbol or by vehicle. 3. After treatment with PDBu, residual IM showed a reduced sensitivity to inhibition by muscarine or LHRH but not by Ba2+. The reduced response to muscarine appeared to result from a 10-fold shift in the concentration dependence for inhibition. 4. PDBu did not clearly reproduce the ability of muscarine to inhibit the slow, Ca-activated K current IAHP or to increase the leak conductance at hyperpolarized potentials. The latter effect of muscarine was enhanced, rather than inhibited, by PDBu. 5. IM and IAHP were not inhibited by 1 mM dibutyryl cyclic AMP or by 20 microM forskolin. 6. It is concluded that activation of protein kinase C, but not protein kinase A, partly replicates the effect of muscarine on frog sympathetic neurons.

Depolarisation-evoked release of acetylcholine can mediate phosphoinositide hydrolysis in slices of rat cerebral cortex

Depolarisation of [3H]inositol prelabelled slices of cerebral cortex of the rat, with elevated extracellular K+ or the alkaloid veratrine, induced a marked accumulation of [3H]inositol monophosphate in the presence of 5 mM Li+. The effects of these stimuli were concentration-related with maximal responses obtained at 30 mM K+ and 30 microM veratrine. Larger concentrations produced submaximal responses but also markedly suppressed the incorporation of [3H]inositol into phospholipid. The responses to K+ or veratrine were not sensitive to atropine, prazosin, mepyramine, ketanserin or the peptidase inhibitor bacitracin. However, in the presence of the cholinesterase inhibitor physostigmine, the responses to these stimuli were greatly enhanced and this could be blocked by atropine. Both veratrine and K+ markedly stimulate release of endogenous acetylcholine from the slices. Release appears to be linear with time over the 45 min period of continuous stimulation. Reduction of extracellular calcium severely suppressed both the release of acetylcholine and the atropine-sensitive component of the phosphoinositide response to K+. The results suggest that endogenous acetylcholine can stimulate phosphoinositide metabolism by interacting with muscarinic receptors. The atropine-insensitive component, at least in part, represents entry of Ca2+ through voltage-sensitive channels and perhaps a direct effect on phosphoinositide metabolism.

Phorbol 12,13-dibutyrate increases tyrosine hydroxylase activity in the superior cervical ganglion of the rat

Phorbol 12,13-dibutyrate (PDBu) increased the production of 3,4-dihydroxyphenylalanine (DOPA) in the superior cervical ganglion of the rat. This effect occurred without a detectable lag and persisted for at least 90 min of incubation. The action of PDBu was half-maximal at a concentration of approximately 0.1 microM; at high concentrations, PDBu produced about a twofold increase in DOPA accumulation. PDBu increased DOPA production in decentralized ganglia and in ganglia incubated in a Ca2+-free medium. The action of PDBu was additive with the actions of dimethylphenylpiperazinium, muscarine, and 8-Br-cyclic AMP, all of which also increase DOPA accumulation, and was not inhibited by the cholinergic antagonists hexamethonium (3 mM) and atropine (6 microM). Finally, PDBu did not increase the content of cyclic AMP in the ganglion. Thus, the action of PDBu does not appear to be mediated by the release of neurotransmitters from preganglionic nerve terminals, by the stimulation of cholinergic receptors in the ganglion, or by an increase in ganglionic cyclic AMP. PDBu also increased the incorporation of 32Pi into tyrosine hydroxylase. PDBu activates protein kinase C, which in turn may phosphorylate tyrosine hydroxylase and increase the rate of DOPA synthesis in the ganglion.