Inositol phospholipid hydrolysis in rat cerebral cortical slices: I. Receptor characterisation

Stimulation of inositol phospholipid breakdown in rat cortical and hippocampal miniprisms by noradrenaline, 5-hydroxytryptamine and carbachol: some methodological aspects

After incubation of miniprisms from rat cerebral cortex or hippocampus with 3H-myo-inositol, labelling of the phospholipid ("Lipid") and inositol phosphate ("InsP") fractions was found. Inositol phospholipid hydrolysis ("PI breakdown") was stimulated by noradrenaline, 5-hydroxytryptamine and carbachol. Expressing data as InsP/(Lipid + InsP) was found to be a superior measure of the rate of PI breakdown compared with the more commonly used InsP d.p.m. unit, since the former was found to be independent of the volume of the miniprism aliquot used and the degree of labelling of inositol phospholipids. The PI breakdown responses to noradrenaline, 5-hydroxytryptamine and particularly carbachol were found to be enhanced by increasing the assay [K+] from 5.88 mM to 18.2 mM. Storage of hippocampal samples at -70 degrees by the "slow freeze-fast thaw" method of Hardy et al. (1983) resulted in a decreased degree of labelling of the Lipid and InsP fractions and a loss of the PI response to noradrenaline when assayed at a [K+] of 5.88 mM, but a reasonable response was seen in these samples at an assay [K+] of 18.2 mM. The temperature of the Krebs-Henseleit buffer used in the preparation of the miniprisms was found to be important for the PI breakdown response.

Putative role of inositol phospholipid metabolism in neurons

Inositol phospholipids play a crucial role in the intracellular signal transduction in most cell types. Activation of an enzyme called phospholipase C or PIP2-phosphodiesterase (PIP2-PDE) leads to the production of two second messenger molecules, diacylglycerol (DG) and inositol 1,4,5-triphosphate (IP3). DG activates a kinase called protein kinase C, whereas IP3 mediates the release of Ca2+ from intracellular storage sites. The measurement of IP3 and its degradation products, inositol diphosphate (IP2) and inositol monophosphate (IP1) provides a way of assessing the extent to which this complex system has been activated. In the central nervous system (CNS) most of the studies on the neurotransmitter stimulated formation of inositol phosphates (IPs) have been performed on brain slices, a mixture of mainly neurons and glial cells. The recent development of pure neuronal cultures provides a means of determining which of these responses were of neuronal origin. The purpose of this review is to summarize the results obtained in neurons in primary culture together with a brief appraisal of the possible function of this second messenger system in neurons.

Effects of cerebral ischemia on [3H]inositol lipids and [3H]inositol phosphates of gerbil brain and subcellular fractions

Intracerebral injection of [3H]inositol into gerbil brain resulted in labeling of phosphoinositides and inositol-phosphates in various subcellular membrane fractions. Phosphatidylinositol (PI) comprised greater than 90% of the radioactivity of inositol lipids. However, the level of labeled poly-PI (with respect to PI) was higher in synaptosomes than in other membrane fractions. Ischemia induced in gerbils by ligation of the common carotid arteries resulted in a 30% decrease in labeled poly-PI in brain homogenates and this decrease was largely attributed to the poly-PI in synaptosomes (50% decrease). Among the inositol phosphates, the ischemia induction resulted in a decrease in labeling of inositol triphosphate (63%) and inositol biphosphate (38%), but labeling of inositol phosphate (IP) was increased by 59%. The results suggested a rapid turnover of the inositol phosphates in the gerbil brain. In general, changes in inositol lipids and inositol phosphates due to ischemia were attenuated after pretreatment with lithium (3 meq/kg) injected intraperitoneally 5 h prior to ligation. Surprisingly, lithium treatment alone did not cause an increase in IP labeling in the gerbil brain.

In vivo 31P NMR spectroscopy of agonist-stimulated phosphatidylinositol metabolism in cat brain

As a result of the agonist stimulation of muscarinic receptors, myo-inositol-1-phosphate accumulates in the presence of millimolar lithium concentrations. This accumulation of myo-inositol-1-phosphate can be detected by in vivo 31P NMR spectroscopy. Since myo-inositol-1-phosphate is a breakdown product of phosphatidylinositol, this may provide a means of noninvasively monitoring phosphatidylinositol metabolism in vivo.

Magnesium ions inhibit the stimulation of inositol phospholipid hydrolysis by endogenous excitatory amino acids in primary cultures of cerebellar granule cells

Omission of Mg2+ from the incubation buffer results in a six- to eightfold increase in [3H]inositol-1-phosphate ([3H]Ins-1-P) accumulation in primary cultures of cerebellar granule cells at 7-9 days in vitro. This increase is reversed by low concentrations of 2-amino-5-phosphono-valerate (APV), a result indicating that the absence of Mg2+ facilitates the activation of a specific receptor by the endogenous excitatory amino acids (presumably L-glutamate and L-aspartate) released from the granule cells. The absence of Mg2+ also potentiates the action of exogenously applied N-methyl-D-aspartate (NMDA), L-glutamate, L-aspartate, and kainate. In contrast, the action of quisqualate is virtually unaffected by Mg2+ and is resistant to APV inhibition. Addition of the depolarizing agent veratridine enhances the accumulation of [3H]Ins-1-P also in Mg2+-containing buffer. The action of veratridine is antagonized by APV, a result suggesting that, under depolarized conditions, the NMDA receptor can be activated by the endogenously released excitatory amino acids, despite the presence of Mg2+. Accordingly, in the presence of Mg2+, veratridine potentiates the action of exogenously applied NMDA but does not facilitate the action of quisqualate.

Inositol phospholipids are probably not the source of arachidonic acid for eicosanoid synthesis in astrocytes

In astrocyte-enriched cultures of the rat cerebral cortex the Ca2+ ionophore A23187 provoked the breakdown of inositol phospholipids, the liberation of arachidonic acid and the release of prostaglandins E2, F2 alpha, I2 and thromboxane A2. However, agonists for receptors also coupled to inositol phospholipid metabolism in these cells failed to produce an increase in the release of both arachidonic acid and eicosanoids. Results suggest that the A23187-stimulated release of arachidonic acid and eicosanoids is caused by a phospholipase A2-mediated attack on lipids other than the inositol phospholipids. Moreover, receptors linked to inositol lipid turnover are not involved in the control of eicosanoid release from astrocytes.

Agonist-stimulation of cerebral phosphoinositide turnover following long-term treatment with antidepressants

Receptor-mediated stimulation of the formation of inositol phosphates (IP) in cerebral tissue may serve as a useful tool for studying long-term changes in the function of serotonin-2 (5-HT2), alpha-1-adrenergic (al), and muscarinic-cholinergic (musc) receptors. In this study we have evaluated the effects of chronic treatment with various antidepressants on receptor-mediated formation of IP in rat brain. Imipramine (IMI: 10 mg/kg/day; 14 days), Bupropion (BUPR: 40 mg/kg/day; 14 days), Lithium (Li: 0.5% in diet; 7 days) and electroshock treatment (EST: 20-30 mA/day; 7 days) were investigated. Cross-chopped slices of cerebral cortex from control and treated rats were prelabelled with myo-3H-inositol in HEPES buffer containing 11.1 mM LiCl. Accumulation of IP was measured in the presence and absence of serotonin (5-HT, 10 uM), norepinepherine (NE, 5 uM), and carbamylcholine (CCH, 100 uM). Values for agonist-stimulated IP formation in control rats were: 5-HT = 123 +/- 5%; NE = 268 +/- 16%; CCh = 205 +/- 21% of the basal level. The IP response to 5-HT was significantly lower following BUPR and higher following EST. Responses to NE and CCH were significantly lower following BUPR treatment but were not affected by the other antidepressant treatments. These observations are consistent with results of receptor-binding studies indicating up-regulation of 5-HT2 receptors by EST but are not consistent with studies showing down-regulation of 5-HT2 receptors by IMI and a lack of effect on 5-HT2 receptors by BUPR. Our results are not supportive of the notion, based mainly on [3H]prazosin binding studies, that al receptors are up-regulated by EST as well as by different antidepressant drugs.

Evidence for spare alpha 1-adrenoceptors for the accumulation of inositol phosphates in smooth muscle

The accumulation of inositol phosphates (IP) in smooth muscle from rat vas deferens and caudal artery was maximally increased 3- to 4-fold in response to exposure of the tissues to 100 microM noradrenaline. Clonidine (up to 3 mM) was a partial agonist. Pretreatment of the tissues with the irreversible alpha-adrenoceptor antagonist phenoxybenzamine (0.3-10 microM) shifted the noradrenaline concentration-response curve to the right before depressing the maximum. The maximum of the clonidine concentration-response curve was depressed without significant change in the EC50 by the same treatment. These data, which are most easily interpreted as demonstrating the presence of a receptor reserve for IP accumulation, are discussed.

Stimulation of phosphoinositide hydrolysis by serotonin in C6 glioma cells

5-Hydroxytryptamine (serotonin or 5-HT) stimulated the incorporation of 32Pi into phosphatidylinositol (PI) but not into polyphosphoinositides in C6 glioma cells with an EC50 of 1.2 X 10(-7) M. The phosphoinositide response was blocked by the 5-HT2 antagonists ketanserin and spiperone but inhibited only partly by methysergide and mianserin. Atropine, prazosin, and yohimbine did not block the response, whereas fluphenazine and haloperidol did so partially but also inhibited basal incorporation by approximately 30%. The 5-HT1A agonist 8-hydroxy-2(di-n-propylamino)tetralin did not cause stimulation. Incubation with 5-HT (1 microM) for 1 h increased the incorporation of [2-3H]myoinositol into all phosphoinositides but not into inositol phosphates (IPs). Li+ alone at 10 mM increased labeling in inositol bisphosphate (IP2) and trisphosphate (IP3), whereas labeling in IP and phosphoinositides remained unaltered. Addition of 5-HT had no effect on this increase. Mn2+ at 1 mM enhanced labeling in PI, PI-4-phosphate, lyso-PI, glycerophosphoinositol, and IP, but the presence of 5-HT again did not cause further stimulation. 5-HT also stimulated the release of IPs in cells prelabeled with [2-3H]myo-inositol, incubated with LiCl (10 mM) and inositol (10 mM), and then exposed to 5-HT (1 microM). Radioactivity in IP2 and IP3 was very low, was stimulated approximately 50% as early as 30 s, and remained elevated for at least 20 min. Radioactivity in IP was at least 10 times as high as in IP3 but was increased only from 3 min on with a peak at 20 min, when the elevation was approximately 40 times that in IP3.(ABSTRACT TRUNCATED AT 250 WORDS)

Central serotonin receptors: effector systems, physiological roles and regulation

Radioligand binding studies have revealed four distinct serotonin (5HT) binding sites in rat brain that are thought to function as 5HT receptors. These include the 5HT-1a, 5HT-1b, 5HT-1c, and 5HT-2 binding sites. Studies have shown that the 5HT-2 binding site mediates a number of effects of 5HT agonists and serves as a 5HT receptor in neuronal and non-neuronal tissues. The 5HT-2 site employs phosphoinositide hydrolysis for signal transduction. The 5HT-1c binding site is also a functional receptor that is linked to phosphoinositide hydrolysis. However, the physiological role of the 5HT-1c receptor is not yet known. Lack of appropriate pharmacological tools for probing the 5HT-1a and 5HT-1b binding sites has made it difficult to definitively determine whether these binding sites are coupled to biochemical effector systems or mediate any of the physiological responses to 5HT agonists. However, there is some evidence that the 5HT-1a site is coupled to adenylate cyclase, and a number of functional roles for the 5HT-1a and 5HT-1b sites have been proposed.

Potassium depolarisation markedly enhances muscarinic receptor stimulated inositol tetrakisphosphate accumulation in rat cerebral cortical slices

Rat cerebral cortical slices labelled with [3H]-inositol were incubated with the muscarinic agonist carbachol in media containing normal 5.9 mM or elevated 24 mM K+ ions. Over the first few minutes both carbachol and elevated K+ stimulated the production of [3H]-inositol phosphates. The very rapid formation of [3H]-inositol tetrakis, tris and bisphosphate was followed by accumulation of [3H]-inositol monophosphate. However, elevated K+ resulted in a relatively larger stimulation of [3H]-inositol bisphosphate than muscarinic receptor stimulation. When carbachol effects were examined in media containing elevated K+, production of [3H]-inositol trisphosphate was apparently additive whereas the mono and bisphosphate displayed somewhat synergistic responses after 1-2 minutes. In contrast, [3H]-inositol tetrakisphosphate production was greatly enhanced and marked synergy was observed between the K+ and carbachol responses. The production of the tetrakisphosphate under these conditions was dependent on extracellular Ca2+ and a stimulatory effect of this divalent ion on the 3-kinase is discussed.

Astrocyte glutamate receptor activation promotes inositol phospholipid turnover and calcium flux

Astrocyte-enriched cultures prepared from the neonatal rat cortex were prelabelled with either [3H]myoinositol or 45Ca2+ and then exposed to various excitatory amino acids. This resulted in an increase in both the breakdown of membrane inositol phospholipids and Ca2+ flux with the following rank order of efficacy: quisqualate greater than or equal to glutamate (Glu) greater than kainate much greater than N-methyl-D-aspartate. Experiments performed with the Ca2+ ionophore A23187 and in the absence of medium Ca2+ suggested that Glu-evoked 45Ca2+ efflux was primarily the result of an increased influx of extracellular Ca2+. However, Glu-stimulated inositol lipid metabolism was found to be only partially dependent on extracellular Ca2+. The quisqualate-preferring receptor antagonist gamma-glutamylaminomethylsulphonic acid was found to be effective in reversing both Glu-evoked inositol lipid breakdown and Ca2+ flux. The results presented are suggestive of some form of interaction between Glu receptors coupled to inositol lipid turnover and Ca2+ channel opening in astrocytes.

Interaction of choline with muscarine receptor-stimulated phosphoinositide metabolism in the rat brain

Previous receptor binding studies had shown that choline can interact with low potency with muscarine cholinoceptors. In the present study we have investigated whether choline is capable of functionally activating muscarine receptors by investigating its ability in stimulating the hydrolysis of phosphoinositides, a response believed to be coupled in brain to the M1 subtype of muscarine receptors. The results indicated that choline was only a very weak inducer of inositol phosphates (InsPs) accumulation in rat cerebral cortex slices as compared with acetylcholine or charbachol. Maximal increase of InsPs accumulation, at a choline concentration of 10 mM, was only 39 +/- 7%, as compared with the 4- to 6-fold stimulation induced by the other compounds. This effect of choline was not modified by physostigmine nor by the uptake inhibitor hemicholinium-3. At high concentrations, however, choline antagonized the stimulatory effect of acetylcholine and carbachol, suggesting that it might act as a partial agonist at this subtype of muscarine receptors, similar to what has been observed with oxotremorine. Choline had no effect on noradrenaline-stimulated InsPs accumulation.

Regional variation in the characteristics of histamine H1-agonist mediated breakdown of inositol phospholipids in guinea-pig brain

The position of dose-response curves for histamine-induced accumulation of [3H]-inositol 1-phosphate ([3H]-IP1) in lithium-treated slices of guinea-pig brain prelabelled with [3H]-inositol differed significantly between cerebellum (EC50 5.1 +/- 1.0 microM) and cerebral cortex (EC50 16.3 +/- 0.7 microM). The Hill coefficients of the curves, 1.33 +/- 0.28 and 1.24 +/- 0.03, respectively, did not differ significantly. 2-Methylhistamine, N alpha,N alpha-dimethylhistamine and betahistine were partial agonists in both cerebellum and cerebral cortex, but all produced a greater percentage of the maximum response to histamine in cerebellar slices. In hippocampal slices the response of the partial agonists was intermediate between that in cerebellum and that in cerebral cortex. The four agonists produced an appreciable accumulation of [3H]-inositol 1-phosphate in cerebellar slices even in the absence of Li+ ion. The EC50 and Hill coefficients characterizing the dose-response curves for the four agonists were the same whether 10 mM LiCl was present or not. The affinity constant for mepyramine inhibition of the histamine-induced response was similar in cerebellum, 4.2 +/- 0.6 X 10(8) M-1, and cerebral cortex, 4.6 +/- 1.0 X 10(8) M-1. Curves of mepyramine inhibition of the responses to a fixed concentration of histamine gave no indication of any second component in the response to histamine in either cerebellum or cerebral cortex. The parameters of histamine inhibition of [3H]-mepyramine binding were similar in homogenates of guinea-pig cerebellum and cerebral cortex. These results indicate that H1-agonist-induced accumulation of IP1 may not be as directly related to agonist-receptor interaction as simple reaction schemes suggest.

Effect of ethanol on receptor-stimulated phosphatidic acid and polyphosphoinositide metabolism in mouse brain

The effects of chronic ethanol consumption as well as the effects of ethanol added in vitro on phosphoinositide metabolism were determined in mouse forebrain. [32P] incorporation into synaptosomal phosphatidic acid (PhA) was stimulated through both M1 muscarinic cholinergic and alpha 1 adrenergic receptor activation. Similarly, [3H]inositol 1-PO4 accumulation in brain slices was stimulated through these same receptors, but could also be stimulated by histamine1 receptor activation. In mice made physically dependent to ethanol, the magnitude of receptor-mediated [32P] incorporation in PhA did not differ from that of control animals. However, ethanol (100mM) added in vitro to synaptosomes from control mice significantly inhibited the carbamylcholine stimulated PhA response, but had no effect on the response to norepinephrine. Carbamylcholine stimulated [32P] incorporation into PhA, however, was no longer significantly inhibited by the addition of 100mM ethanol to synaptosomes from physically dependent-tolerant animals indicating that a cellular tolerance had developed. In contrast, receptor mediated [3H]inositol 1-PO4 accumulation in brain slices was not significantly affected by either chronic ethanol treatment or the in vitro addition of ethanol as high as 200mM. It is concluded that the muscarinic cholinergic stimulation of [32P] incorporation into PhA, but not [3H]inositol 1-PO4 accumulation is relatively more sensitive to the direct effects of ethanol than are the other receptor mediated phospholipid responses examined in the present investigation and that this sensitivity is lost in animals made behaviorally tolerant and physically dependent to ethanol.

Phorbol esters enhance neurotransmitter-stimulated cyclic AMP production in rat brain slices

The effect of phorbol esters on cyclic AMP production in rat CNS tissue was examined. Using a prelabeling technique for measuring cyclic AMP accumulation in brain slices, it was found that phorbol 12-myristate, 13-acetate (PMA) enhanced the cyclic AMP response to forskolin and a variety of neurotransmitter receptor stimulants while having no effect on second messenger accumulation itself. A short (15-min) preincubation period with PMA was required to obtain maximal enhancement, whereas the augmentation was lessened by prolonged exposure (3 h) to the phorbol. The response to PMA was concentration dependent (EC50 = 1 microM) and regionally selective, being most apparent in forebrain, and was not influenced by removal of extracellular calcium or by inhibition of phosphodiesterase or phospholipase A2. Only those phorbols known to stimulate protein kinase C augmented the accumulation of cyclic AMP. Moreover, the membrane substrates phosphorylated by endogenous C kinase and by a partially purified preparation of this enzyme were similar. The results suggest that phorbol esters, by activating protein kinase C, modify the cyclic AMP response to brain neurotransmitter receptor stimulation in brain by influencing a component of the adenylate cyclase system beyond the transmitter recognition site.

Trace amines (ethylamine, octopamine, and tryptamine) stimulate inositol phospholipid hydrolysis in rat cerebral cortex slices

Ethylamine, octopamine, tryptamine, and carbachol stimulate inositol phosphate accumulation in a dose-dependent way in rat cortical slices. Tyramine at 100 microM has no effect. The major inositol phosphate that is accumulated following stimulation is the monophosphate. The effect of carbachol is blocked by atropine but not by cyproheptadine, phenoxybenzamine, haloperidol or propranolol. None of the antagonists tried, including atropine, had an effect on the stimulation caused by ethylamine, octopamine or tryptamine.

Cyclic GMP formation and inositol phosphate accumulation do not share common origins in rat brain slices

Cyclic GMP formation and inositol phospholipid hydrolysis were studied in rat brain slices to determine if the two processes have common origins. Muscarinic cholinergic stimulation enhanced [3H]inositol phosphate ([ 3H]IP) accumulation from slices prelabelled with [3H]inositol but did not affect cyclic GMP formation in the cortex, striatum, or cerebellum. An elevated level of extracellular K+ stimulated accumulation of both cyclic GMP and [3H]IP in cortex slices. The former, but not the latter, was reduced by lipoxygenase and phospholipase A2 inhibition. Calcium channel activation enhanced and blockade reduced K+-stimulated [3H]IP formation without affecting the cyclic GMP level, and there were differences in the Ca2+ requirements for the two responses. Thus, there is no support for the concept that guanylate cyclase activation inevitably accompanies inositol phospholipid breakdown, and the evidence presented demonstrates that K+ stimulation promotes cyclic GMP and [3H]IP accumulation by different transducing pathways.

Phorbol esters alter muscarinic receptor binding and inhibit polyphosphoinositide breakdown in human neuroblastoma (SH-SY5Y) cells

Many recent reports have indicated that the effect of the phorbol ester tumor promoters is mediated through the Ca2+/phospholipid dependent protein kinase C. We have investigated the effect of two biologically active phorbol esters, 4 beta-phorbol 12 beta-myristate 13 alpha-acetate (PMA) and 4 beta-phorbol 12 beta,13 alpha-didecanoate (beta PDD) on muscarinic agonist binding and receptor-stimulated phosphoinositide breakdown in cultured human neuroblastoma (SH-SY5Y) cells. Preincubation of these cells with phorbol esters significantly reduced the carbachol-stimulated breakdown of inositol phospholipids and caused a decrease of agonist affinity for [3H](-)methyl quinuclidinyl benzilate ([3H](-)MQNB) binding without affecting the affinity of antagonist to the muscarinic receptor. The nontumor promoting 4 alpha-phorbol 12 beta,12 alpha-didecanoate (alpha PDD) was ineffective in our studies. These results suggest that the activation of protein kinase C may play an important role in regulating the muscarinic receptor system.

Monoamine receptor sensitivity changes following chronic administration of MDL 72394, a site-directed inhibitor of monoamine oxidase

(E)-beta-Fluoromethylene-m-tyrosine (MDL 72394) is not per se an inhibitor of monoamine oxidase (MAO) but is a substrate of aromatic L-amino acid decarboxylase (AADC) which liberates the potent MAO inhibitor (E)-beta-fluoromethylene-m-tyramine (MDL 72392). When co-administered to animals with the peripherally selective AADC inhibitor, carbidopa, MDL 72394 inhibited MAO selectively in the brain. Chronic (14 days plus 3 days withdrawal) administration of 0.5 mg/kg per day p.o. MDL 72394, 0.1 mg/kg per day p.o. MDL 72394 combined with 10 mg/kg per day p.o. carbidopa or 50 mg/kg per day p.o. pargyline produced equivalent inhibition of rat brain MAO and decreased the binding of [3H]clonidine and [3H]RX 781094 to the alpha 2-adrenoceptor and of [3H]dihydroalprenolol to the beta-adrenoceptor without changing binding of [3H]prazosin to the alpha 1-adrenoceptor. The locomotor depressant effect of clonidine was attenuated without attenuation of the hypotensive effect in rats treated chronically with the MAO inhibitors. Neither the sensitivity of the alpha 2-autoreceptor nor of the alpha 2-heteroreceptor was decreased in brain slices. However, the sensitivity of adenylate cyclase to activation by both noradrenaline and isoprenaline was significantly reduced. The number of 5-HT2 and 5-HT1A binding sites was decreased: the 5-HT1B binding sites remained unchanged. The effect of chronic MAO inhibitor treatment on 5-HT1A receptors was associated with a decrease in the behavioural response to 8-hydroxy-2-(di-n-propylamino)tetralin and the decrease in 5-HT2 binding was related to a small reduction in the sensitivity of the inositol phosphate system to stimulation by 5-HT. The lack of effect of chronic MAO treatment on the 5-HT autoreceptor measured in cortical slices corresponded to a lack of effect on the 5-HT1B binding site except that chronic administration of pargyline produced a small but significant decrease in 5-HT autoreceptor sensitivity. Overall, the data show that chronic administration of MDL 72394 has a profile of effects on central monoamine receptor binding and function similar to that seen following chronic administration of a number of clinically effective antidepressants.

Characterization of alpha 1-adrenoceptors which increase cyclic AMP accumulation in rat cerebral cortex

The pharmacological properties of the alpha-adrenoceptors which increase cyclic AMP accumulation were studied in slices of rat cerebral cortex after inactivation of beta-adrenoceptors with bromoacetylalprenololmenthane. Norepinephrine increased basal cyclic AMP accumulation 2-fold, and potentiated the effect of adenosine 5-fold. The Ki and EC50 values for antagonists and agonists for both the basal and potentiated responses were generally similar to those for alpha 1-adrenoceptor-stimulated inositol phosphate accumulation in the same preparation. However, significant differences in the potencies of all agonists, and the antagonists phentolamine and BE2254 were observed between the basal and potentiated cyclic AMP responses. The differences in agonist potencies did not appear to be due to the existence of a receptor reserve. Norepinephrine, epinephrine, alpha-methylnorepinephrine and 6-fluoronorepinephrine were full agonists, while methoxamine and phenylephrine were partial agonists in both systems. The results suggest that norepinephrine increases cyclic AMP accumulation in rat cerebral cortex through alpha 1-adrenoceptors similar to those increasing phosphatidylinositol metabolism in the same tissue.

Stereospecific regulation of [3H]inositol monophosphate accumulation by calcium channel drugs from all three main chemical classes

Depolarization of [3H]inositol-prelabelled rat cortical slices through the elevation of extracellular K+ levels leads to increased accumulation of [3H]inositol phosphates. In the presence of 18 mM K+, Ca2+ channel activators selectively stimulated the formation of [3H]inositol monophosphate ([3H]IP1) whereas Ca2+ channel blockers were inhibitory. Blockade of the Na+ channel by 1 microM tetrodotoxin had no effect but chelation of extracellular Ca2+ abolished the response. The enantiomers of the benzoxadiazol 1,4-dihydropyridine 202-791 showed opposite stereospecific regulation of [3H]IP1 formation: (+)-(S)-202-791 stimulated (252%; ED50: 88 nM), whereas (-)-(R)-202-791 inhibited (65% inhibition, ED50: 602 nM). The (-) enantiomer of Bay K 8644 was a potent [3H]IP1 stimulator (258%; ED50: 82 nM). While (+)-Bay K 8644 was inactive in the presence of 18 mM K+, it completely inhibited the (-)-Bay K 8644-induced stimulation with a Ki of 103 nM. Representatives of the other two main classes of Ca2+ channel blockers (phenylalkylamines and benzothiazepines) inhibited K+ depolarization-induced and (-)-Bay K 8644 enhanced [3H]IP1 formation in a dose-dependent, stereospecific manner. The results show that Ca2+ channel blockers are efficient modulators of depolarization-induced and Ca2+ channel activator-induced [3H]inositol monophosphate formation in brain, and demonstrate the functional coupling of three distinct drug receptor sites on neuronal Ca2+ channels.

Raising the ambient potassium ion concentration enhances carbachol stimulated phosphoinositide hydrolysis in rat brain hippocampal and cerebral cortical miniprisms

The influence of the ambient potassium ion concentration ([K+]) upon agonist stimulated hydrolysis of phosphoinositides (PI) has been studied in isolated miniprisms of rat hippocampus and cerebral cortex. When the external [K+] was raised from 6 to 18 mmol/l, there was little or no increase in the hydrolysis of PI in the absence of agonist, however, carbachol (100 mumol/l) stimulated hydrolysis was greatly enhanced in both brain regions studied. Thus, carbachol stimulated the hydrolysis of PI to 146% and 386% of control levels at potassium concentrations of 5.88 and 18.2 mmol/l, respectively, in the rat hippocampus. A similar enhancement of muscarine (100 mumol/l) stimulation was observed in cortical miniprisms with 18 mmol/l [K+]. A further enhancement was seen at higher ambient [K+], although basal hydrolysis of PI was then also increased. The carbachol-stimulated hydrolysis of PI found at both 6 and raised [K+] was prevented by atropine (1 and 10 mumol/l) and tetraethylammonium (20 mmol/l), but not by 10 mmol/l Mg2+. Pirenzepine (50 nmol/l) also reduced this response. The ions Cs+ and Rb+ (but not Li+ or Tris+) produced a similar enhancement of the carbachol stimulation to that found with K+. At a buffer [K+] of 6 mmol/l, noradrenaline (100 mumol/l) produced a 2-fold increase in the hydrolysis of PI whereas 5-hydroxytryptamine (100 mumol/l) and histamine (500 mumol/l) had little or no effect. However, histamine and 5-hydroxytryptamine did stimulate the hydrolysis of PI when [K+] was increased. Miniprism ATP content was not changed by a rise in [K+] to 18 mmol/l. The significance of these results is discussed in terms of the postsynaptic cellular events following cholinergic stimulation.

Depolarization increases inositolphosphate production in a particulate preparation from rat brain

We have studied the accumulation of inositol phosphates (InsP) due to depolarization. A particulate preparation of rat brain was introduced to rule out transmitter activated mechanisms and to allow free access for drugs of high molecular weights. Potassium depolarization doubled InsP within a few minutes. InsP accumulation depended on time and K+ concentration, and was affected neither by tetrodotoxin nor by atropine. Radioactive metabolites co-eluted with inositol mono-phosphate and inositol bis-phosphate, whereas only minor amounts appeared with inositol tris-phosphate. The content in phosphatidylinositols was decreased. No evidence was found for the involvement of a neurotransmitter. Sea anemone toxin II (around 1 mumol/l), which keeps the Na+-channels open, promoted the InsP accumulation in an atropine-resistant manner. Tetrodotoxin prevented it when given before, and inhibited it when given after initiation by sea anemone toxin II. Moreover the K+ channel blockers 4-aminopyridine, dendrotoxin and tetraethylammonium all caused InsP accumulation. Palytoxin was by far the most potent promoter of InsP accumulation with a detection limit below 10 pmol/l, and displayed a unique bell-shaped concentration-effect correlation. Ouabain (3 mumol/l and above) also elicited the InsP accumulation. The response to carbachol was not only inhibited completely by atropine, but also partially (more than 50%) by tetrodotoxin, which indicates the involvement of voltage-dependent sodium channels in the receptor-triggered InsP accumulation. Thus independent of the causative agent, depolarization promotes an InsP accumulation. We conclude that degradation of phosphatidylinositols is mediated not only by receptor occupation but also by a positive shift in membrane voltage.

An examination of the involvement of phospholipases A2 and C in the alpha-adrenergic and gamma-aminobutyric acid receptor modulation of cyclic AMP accumulation in rat brain slices

Experiments were undertaken to define the role of two calcium-associated enzyme systems in modulating transmitter-stimulated production of cyclic nucleotides in rat brain. Cyclic AMP (cAMP) accumulation was examined in cerebral cortical slices using a prelabeling technique. The enhancement of isoproterenol-stimulated cAMP production by alpha-adrenergic and gamma-aminobutyric acid-B (GABAB) agonists was reduced by exposing the tissue to EGTA, a chelator of divalent cations, or quinacrine, a nonselective inhibitor of phospholipase A2. Likewise, chronic (2 weeks) administration of corticosterone decreased the alpha-adrenergic and GABAB receptor modulation of second messenger production. Neither cyclooxygenase nor lipoxygenase inhibitors selectively influenced the facilitating response of alpha-adrenergic and GABAB agonists. Other experiments revealed that although norepinephrine and 6-fluoronorepinephrine stimulated inositol phosphate (IP) production in cerebral cortical slices with potencies equal to those displayed in the cyclic nucleotide assay, selective alpha 1-adrenergic agonists were less efficacious on IP formation and were without effect in the cAMP assay. Conversely, a selective alpha 2-adrenergic receptor agonist facilitated the cAMP response to a beta-adrenergic agonist without affecting IP formation. The rank orders of potency of a series of alpha-adrenergic antagonists suggest that IP accumulation is mediated solely by alpha 1-adrenergic receptors, whereas the augmentation of cAMP accumulation is regulated by a mixed population of alpha-adrenergic sites. The results suggest that the alpha-adrenergic and GABAB receptor-mediated enhancement of isoproterenol-stimulated cAMP formation appears to be more closely associated with phospholipase A2 than phospholipase C and may be mediated by arachidonate or some other fatty acid.

Adrenergic and cholinergic stimulation of arachidonate and phosphatidate metabolism in cultured astroglial cells

Primary cultures of neonatal rat brain polygonal astroglia, or cells of the C62B glioma line, were incubated with [1-14C]arachidonic acid (AA) in culture for 18 hr. In both culture systems, more than 80% of the added [1-14C]AA was taken up into cellular glycerolipids; less than 1% of the radioactivity in the cells was present in an unesterified form. When prelabeled C62B cells were stimulated with acetylcholine (ACh), there was a rapid accumulation of arachidonyl-phosphatidic acid (PA) accompanied by a liberation of [1-14C]AA. A variety of other neurotransmitters failed to activate this response in C62B glioma cells. In contrast to the agonist specificity of the response in C62B glioma cells, primary astroglia generated PA and liberated [1-14C]AA in response to several neurotransmitters (i.e., ACh, norepinephrine, glutamate, and histamine) Treatment of astroglia with a combination of norepinephrine, ACh, and histamine resulted in a greater production of PA and free [1-14C]AA than did treatment with any one of these neurotransmitters alone. The results suggest that cultures of astroglia can respond to several different neurotransmitters with specific changes in AA and PA metabolism. Thus, a variety of neurotransmitters initiate cascades of lipid metabolism which may be of physiological significance in glial function.

Serotonin increases the production of inositol phosphates and mobilises calcium via the 5-HT2 receptor in A7r5 smooth muscle cells

Serotonin (5-HT) induces inositol phosphate production and the efflux of 45Ca2+ in a smooth muscle cell line (A7r5) derived from rat aorta. These effects were pharmacologically characterised and compared to data obtained in radioligand binding studies performed with the 5-HT2 ligand [3H]ketanserin in rat brain cortex membranes. 5-HT causes in increase in the levels of inositol trisphosphate (InsP3), inositol bisphosphate (InsP2) and inositol phosphate (InsP1). InsP3 production was rapid and transient whereas InsP1 accumulated in a time and concentration dependent manner. The 5-HT stimulated InsP1 accumulation (pEC50 = 6.48) was potently and competitively inhibited by the 5-HT2 specific antagonists, pirenperone and ketanserin, whereas antagonists of other 5-HT receptors were active only at high concentrations. There was a significant correlation between inhibition of 5-HT stimulated InsP1 accumulation and 5-HT2 binding (r = 0.98, P = 0.0035). 5-HT stimulated the efflux of 45Ca2+ from preloaded cells with a pEC50 of 7.59. The rank order of potency for agonist induced Ca2+ efflux, 5-HT greater than alpha-methyl-5-HT greater than 1-methyl-5-HT greater than RU 24969 (5-methoxy-3[1,2,3,6,-tetrahydropyridin-4-yl]-1-H indole) greater than 8-OH-DPAT (8-hydroxy-2-(di-n-propylamino)-tetralin) greater than 8-OH-DPAT (8-hydroxy-2-(di-n-propylamino)-tetralin) greater than 5-CT (5-carboxamidotryptamine) is typical for a 5-HT2 receptor mediated event. The effect of 5-HT was competitively blocked by ketanserin (pA2 = 8.22).(ABSTRACT TRUNCATED AT 250 WORDS)

Autonomous control of phosphatidylinositol turnover by histamine and acetylcholine receptors in the N1E-115 neuron-like cell line

Histamine was found to stimulate the turnover of phosphatidylinositol (PI) in cultures of neuron-like NE-115 cells. Turnover was measured by increased production of [3H]inositol phosphates (breakdown) and by accelerated incorporation of 32P into PI (resynthesis). Data were consistent with hydrolysis of polyphosphoinositides being the initial event in receptor-stimulated PI turnover. This response to histamine desensitized within 10 min. Receptor systems for histamine and acetylcholine were tested for possible interactions: PI turnover in response to dual stimulation was approximately equal to the sum of the individual responses while prior desensitization of the acetylcholine receptor system had no effect on subsequent stimulation of the histamine receptor system. These results are consistent with the hypothesis that components of acetylcholine and histamine receptor systems responsible for PI turnover are autonomously organized and regulated. and regulated.

Regulation of phosphatidylinositol turnover in brain synaptoneurosomes: stimulatory effects of agents that enhance influx of sodium ions

Norepinephrine and carbamoylcholine stimulate accumulation of [3H]inositol phosphates from [3H]inositol-labeled guinea pig cerebral cortical synaptoneurosomes through interaction with alpha 1-adrenergic and muscarinic receptors, respectively. In addition to such agonist, a variety of natural products that affect voltage-dependent sodium channels can markedly stimulate accumulation of [3H]inositol phosphates. These include alkaloids that activate sodium channels, such as batrachotoxin, veratridine, and aconitine; peptide toxins that alter activation or slow inactivation of sodium channels, such as various scorpion toxins from Leiurus, Centruroides, and Tityus species; and agents that cause repetitive firing of sodium channel-dependent action potentials, such as pyrethroids and pumiliotoxin B. Ouabain, and agent that will increase accumulation of internal sodium by inhibition of Na+, K+-ATPase, also stimulates formation of [3H]inositol phosphates, as does monensin, a sodium ionophore. Tetrodotoxin and saxitoxin, specific blockers of voltage-dependent sodium channels, prevent or reduce the stimulatory effects of sodium channel agents and ouabain on phosphatidylinositol turnover, while having lesser or no effect, respectively, on receptor-mediated or monensin-mediated stimulation. Removal of extracellular sodium ions markedly reduces stimulatory effects of sodium channel agents, while removal of extracellular calcium ions with EGTA blocks both receptor-mediated and sodium channel agent-mediated phosphatidylinositol turnover. The results provide evidence for a hitherto unsuspected messenger role for sodium ions in excitable tissue, whereby neuronal activity and the resultant influx of sodium will cause activation of phospholipase systems involved in hydrolysis of phosphatidylinositols, thereby generating two second messengers, the inositol phosphates, which mobilize calcium from internal stores, and the diacylglycerols, which activate protein kinase C.

Noradrenaline-stimulated inositol phospholipid breakdown in rat dorsal lateral geniculate nucleus neurones

Noradrenaline-stimulated inositol phospholipid breakdown in matched vibratome sections through the rat dorsal lateral geniculate nucleus (dLGN). The response was measured as a large accumulation of [3H]inositol labelled inositol monophosphate and was mediated via activation of alpha 1-adrenergic receptors. Accumulation of [3H]inositol phosphates was reduced in kainic acid-lesioned animals, indicating that this response occurred within dLGN neurones and not afferent terminals. The results implicate inositol phospholipid breakdown as part of the mechanism of noradrenergic neurotransmission within the dLGN.

Phosphoinositide hydrolysis mediated by histamine H1-receptors in rat brain cortex

Histamine stimulated the accumulation of [3H]inositol 1-phosphate in the presence of lithium in [3H]inositol-prelabelled slices from rat brain cortex in a concentration-dependent manner, with an EC50 value of 94.7 microM. High concentrations of antagonists of histamine H2 receptors, muscarinic receptors, alpha 1-adrenoceptors and serotonin receptors did not inhibit the effect. The histamine H1-receptor antagonists mepyramine, triprolidine, promethazine, d-chlorpheniramine and the tricyclic antidepressant doxepin inhibited the response with Ki values corresponding to an interaction with histamine H1-receptors. The EC50 for the response was about three times lower than the Ki value (approximately 300 microM) for the inhibition by histamine of [3H]mepyramine binding to membranes from rat brain cortex. Partial inactivation of H1-receptors with the alkylating antagonist phenoxybenzamine resulted in similar reductions in [3H]mepyramine binding sites and in the maximal histamine-induced [3H]inositol 1-phosphate accumulation, without affecting the KD for the radioligand or the EC50 for the response. The apparent dissociation constant for histamine calculated from these experiments (KA = 92.2 microM) was not different from the EC50 value. The present results indicate that histamine-stimulated phosphoinositide hydrolysis in rat brain cortex is mediated by H1-receptors and that no receptor reserve is present.

H1-histaminergic activation stimulates inositol-1-phosphate accumulation in chromaffin cells

Adrenal medullary chromaffin cells maintained in vitro were prelabeled with [3H]inositol and the accumulation of [3H]inositol-1-phosphate, was determined following stimulation with a variety of pharmacological agents. Carbachol, bradykinin, and histamine produced significantly greater accumulation of [3H] inositol-1-phosphate over basal levels, with histamine producing the greatest effect. H1-histamine receptor antagonists, mepyramine, pyrilamine, tripelennamine and clemastine were all able to reduce or completely block the histamine response. The two specific H2-histamine receptor antagonists, cimetidine and ranitidine, had no effect on this response. Histamine dose-response characteristics in the presence of mepyramine and clemastine suggest the H1 antagonism to be competitive in nature.

Accumulations of inositol phosphates and cyclic AMP in brain slices: synergistic interactions of histamine and 2-chloroadenosine

2-Chloroadenosine, 5'-N-ethylcarboxamidoadenosine, N6-cyclohexyladenosine and other adenosine analogs enhance histamine-elicited, but not norepinephrine- or carbamylcholine-elicited accumulations of inositol phosphates in [3H]inositol-labeled guinea-pig cerebral cortical slices. The adenosine analogs alone have no effect on accumulations of inositol phosphates. The effect of 2-chloroadenosine is blocked by the adenosine receptor antagonists theophylline and 1,3-dialkyl-8-p-sulfophenylxanthines. The rank order of activity of the six adenosine analogs with respect to augmentation of histamine-elicited accumulation of inositol phosphates in guinea-pig cerebral cortical slices is different from the rank order at an A2-adenosine receptor that mediates synergistic accumulations of cyclic AMP by adenosine analogs and histamine in guinea-pig cerebral cortical slices.

Stimulation by noradrenaline of inositol phospholipid breakdown in the rat hippocampus: effect of the ambient potassium concentration

The stimulation by noradrenaline (NA) of inositol phospholipid (PI) breakdown has been studied using rat hippocampal miniprisms. Pretreatment with the monoamine oxidase inhibitor, pargyline, potentiated the stimulation produced by NA. On the other hand, pargyline pretreatment did not affect the stimulation of PI breakdown by the alpha 1-adrenoceptor agonist phenylephrine. NA- and phenylephrine-stimulated PI breakdown were enhanced by increasing the ambient potassium concentration in the assay from 5.88 to 18.2 mM. This enhancement did not, in the case of NA, change either the EC50 value for this agonist (2-3 microM) or the pA2 value for the competitive antagonism of the stimulation by the alpha 1-antagonist prazosin (pA2 value 9.2). Time-courses of the NA-stimulated PI turnover in different brain regions indicated that the rate of stimulation was in the order frontal cortex greater than hypothalamus greater than or equal to hippocampus much greater than cerebellum.

Serine-O-phosphate, an endogenous metabolite, inhibits the stimulation of inositol phospholipid hydrolysis elicited by ibotenic acid in rat hippocampal slices

Serine-O-phosphate (PS) inhibits the accumulation of 3H-inositolmonophosphate elicited by ibotenic acid in rat hippocampal slices incubated in the presence of 7 mM Li+. This inhibition is concentration- dependent and stereoselective, L-PS being 5 fold more potent than D-PS. Among different structural analogues of PS, only L-serine weakly antagonizes the action of ibotenic acid, whereas phosphorylcholine, phosphorylethanolamine and phosphothreonine are inactive even at high concentrations. These results are consistent with the hypothesis that L-PS may act as an endogenous regulator of excitatory amino acid receptor function.

Receptor-mediated inositol phospholipid hydrolysis in astrocytes

Astrocyte-enriched cultures of the neonatal rat cortex were incubated for 24 h with [3H]inositol to prelabel the membrane inositol phospholipids. Exposure of the cultures to either noradrenaline or carbachol in the presence of Li+ produced a time- and dose-dependent accumulation of intracellular [3H]inositol phosphates. The separation of the individual inositol phosphates formed in response to receptor stimulation revealed that the major 3H-metabolite accumulated under these conditions was inositol monophosphate but that at least some of this was due to the initial formation of inositol trisphosphate. The use of selective receptor antagonists showed that noradrenaline- and carbachol-induced [3H]inositol phosphate accumulation was the result of the activation of alpha 1-adrenoceptors and muscarinic acetylcholine (probably of the M1 subtype) receptors respectively. Agonist-evoked [3H]inositol phosphate accumulation were found to be additive but the simultaneous addition of agonists and the Ca2+ ionophore A23187, which also promoted inositol phospholipid hydrolysis, was not. Agonist-induced [3H]inositol phosphate accumulation was only partially dependent on extracellular Ca2+, whilst that elicited by A23187 was entirely Ca2+-dependent. The results suggest that alpha 1-adrenoceptors and muscarinic acetylcholine receptors in these cultures are present either on the same cells and linked to separate inositol lipid pools or associated with different subpopulations of astrocytes in these cultures. Moreover, inositol lipids other than phosphatidylinositol 4,5-bisphosphate may be hydrolysed in response to agonist stimulation.

Biochemical characterization of serotonin stimulated phosphoinositide turnover

Serotonin (5HT) stimulates phosphoinositide turnover in a number of tissues, but it is not known whether this effect is due to activation of a 5HT receptor which is coupled to phosphoinositide hydrolysis or if the effect is secondary to 5HT stimulated arachidonate metabolism or to the release of another neurotransmitter. In the present study we show that neither indomethacin nor BW 755C inhibits 5HT stimulated phosphoinositide hydrolysis in rat cerebral cortex, suggesting that neither cyclooxygenase nor lipoxygenase activity is required for the response to 5HT. Proteinase inhibitors do not potentiate the response to 5HT, suggesting that 5HT's effect is not due to stimulation of release of a peptide neurotransmitter. Tetrodotoxin does not inhibit the effect of 5HT and 5HT's effect is additive with that of KCl and veratrine. These data suggest that 5HT stimulated phosphoinositide hydrolysis is not dependent upon release of another neurotransmitter.