Muscarinic agonists cause calcium influx and calcium mobilization in forebrain neurons in vitro

Purinergic and muscarinic receptor activation activates a common calcium entry pathway in rat neocortical neurons and glial cells

The nature of metabotropic purinergic and muscarinic receptor-mediated increases in intracellular calcium in primary rat neocortical neurons and glial cells has been investigated using fluorescence imaging techniques. Bath-application of ATP and muscarine (10 microM) elicited a characteristic increase in intracellular calcium in both neurons and glial cells. The profile of this response consisted of an initial transient increase followed by a sustained elevation (the plateau phase) which was dependent on extracellular calcium. Examination of the pharmacological basis of the purinergic receptor-mediated calcium response using 10 microM 2-methyl-thio ATP (MeS-ATP) and UTP revealed that P(2Y) receptor activation underlies this response. The calcium influx pathway responsible for the sustained calcium response was inhibited by metal ions. In both cell types La(3+) and Zn(2+) (100 microM) effectively inhibited the plateau phase of the response, whilst 100 microM Ni(2+) had little or no effect. In conclusion, P(2Y) purinergic and muscarinic receptor activation evoke a sustained increase in intracellular calcium in neocortical neurons and glial cells. This response has similar characteristics to that we have previously described following mGlu(5) activation. We propose that in these cell types stimulation of metabotropic receptors coupled to phosphoinositide turnover activates a common calcium entry pathway that is distinct from voltage-gated calcium channels and resembles store-operated calcium entry.

Muscarinic receptors mediate enhancement of spontaneous GABA release in the chick brain

The functional role of muscarinic acetylcholine receptors in the lateral spiriform nucleus was studied in chick brain slices. Whole-cell patch-clamp recordings of neurons in the lateral spiriform nucleus revealed that carbachol enhanced GABAergic spontaneous inhibitory postsynaptic currents. The duration of the response to carbachol was significantly reduced after blockade of muscarinic receptors with atropine. In the presence of the nicotinic receptor antagonist dihydro-beta-erythroidine, carbachol produced a delayed but prolonged enhancement of spontaneous GABAergic inhibitory postsynaptic currents that was completely blocked by atropine. Muscarine also enhanced the frequency of spontaneous GABAergic inhibitory postsynaptic currents in a dose-dependent manner, but had no effect on inhibitory postsynaptic current amplitude. While 4-diphenylacetoxy-N-(2-chloroethyl)-piperidine hydrochloride, a M3 antagonist, completely blocked muscarine's effect, telenzepine, a M1 antagonist, and tropicamide, a M4 antagonist, only partially decreased the response to muscarine. Pirenzepine, a M1 antagonist, and methoctramine, a M2 antagonist, potentiated muscarine's enhancement of spontaneous GABAergic inhibitory postsynaptic currents. Muscarine's action was blocked by tetrodotoxin, cadmium chloride and omega-conotoxin GVIA, but was not affected by dihydro-beta-erythroidine, 6-cyano-7-nitroquinoxaline-2,3-dione, D(-)-2-amino-5-phosphonopentanoic acid, naloxone or fluphenazine. These results demonstrate that activation of both muscarinic and nicotinic acetylcholine receptors can enhance GABAergic inhibitory postsynaptic currents in the lateral spiriform nucleus. The muscarinic response has a slower onset but lasts longer than the nicotinic effect. The M3 receptor subtype is predominantly involved in enhancing spontaneous GABAergic inhibitory postsynaptic currents. These M3 receptors must be located some distance from GABA release sites, since activation of voltage-dependent sodium channels, and consequent activation of N-type voltage-dependent calcium channels, is required to trigger enhanced GABA release following activation of muscarinic receptors.

Calcium dependence of depolarization-induced suppression of inhibition in rat hippocampal CA1 pyramidal neurons

1. We made whole-cell recordings from CA1 pyramidal cells in the rat hippocampal slice preparation to study the calcium (Ca2+) dependence of depolarization-induced suppression of inhibition (DSI). DSI is a retrograde signalling process in which voltage-dependent Ca2+ influx into a pyramidal cell leads to a transient decrease in the release of GABA from interneurons. 2. To investigate the Ca2+ dependence of DSI without altering extracellular divalent cations, we varied the type and amount of Ca2+ chelator (EGTA or BAPTA) in the recording pipette (keeping the chelator : Ca2+ ratio constant). Evoked inhibitory postsynaptic currents (IPSCs) were induced in the presence of antagonists of ionotropic glutamate receptors. DSI was induced by depolarizing voltage steps, lasting from 0.025 to 5 s, to 0 mV. 3. DSI was directly dependent on the duration of the voltage step used to induce it, from threshold up to a maximal value of IPSC suppression, whether EGTA or BAPTA was used, and whether their concentrations were 0.1, 0.5 or 2 mM. For instance, a voltage step lasting 1.37 s produced half-maximal DSI with 2 mM BAPTA, but with 0. 1 mM BAPTA, half-maximal DSI was achieved with a step lasting 0.186 s. Peak DSI was the same in all cases, and DSI was blocked with either 10 mM EGTA or BAPTA in the pipette. Bath application of carbachol could overcome the block of DSI by 10 mM EGTA but not by 10 mM BAPTA. 4. We calculated that a voltage step lasting approximately 100 ms would be necessary to activate half-maximal DSI in the absence of exogenous Ca2+ buffers. 5. Log-log plots of calculated total Ca2+ influx, estimated from time integrals of Ca2+ currents, versus DSI yielded a straight line with a slope of approximately 1, and increasing extracellular [Ca2+] from 2.5 to 5 mM did not change the slope. 6. The time course of decay of DSI was well described by an exponential function with a time constant of approximately 20 s and was not affected by changes in either concentration or type of Ca2+ buffer. 7. The data suggest that, in its Ca2+ dependence, DSI more closely resembles the slow release of neuropeptides and hormones than it does the process of fast release of many neurotransmitters.

A characterization of muscarinic receptor-mediated intracellular Ca2+ mobilization in cultured rat hippocampal neurones

1. The properties of muscarinic receptor-mediated Ca2+ mobilization were investigated in hippocampal cultures using fluorescent imaging techniques. 2. Somatic responses to carbachol (1-10 microM) were observed in 21 % of neurones under control conditions (5.4 mM K+, 1. 8 mM Ca2+, 0.5-1 microM tetrodotoxin). Smaller responses were observed in Ca2+-free medium. 3. In cells where responses to carbachol were absent under control conditions, responses were often observed following depolarization with high extracellular K+ (16. 2-25 mM). These responses decreased in magnitude with time after the depolarizing episode. Mobilization of Ca2+ from stores using caffeine (50 mM) exhibited similar properties. 4. Carbachol responses were greatly facilitated in the presence of moderate elevations in extracellular K+ or Ca2+ levels (2- or 3-fold, respectively). These conditions were usually, but not always, associated with a small increase in cytosolic Ca2+ levels (< 50 nM). 5. Muscarinic responses in 10.8 mM K+ were inhibited by 80-95 % in the presence of the L-type voltage-gated Ca2+ channel antagonists nitrendipine (2-5 microM) or nifedipine (10 microM). Depletion of intracellular Ca2+ stores with thapsigargin (2-10 microM) blocked responses. 6. Oscillatory Ca2+ mobilizing responses were observed in some cells. Their expression was facilitated by moderate cytosolic Ca2+ elevations and by increasing the duration of carbachol exposure. 7. Ca2+ mobilizing responses were also observed in dendritic regions. These were smaller than somatic responses, but had faster decay kinetics. 8. In conclusion, muscarinic receptor-mediated Ca2+ mobilization in cultured hippocampal neurones shows a strong Ca2+ dependence. Moderate intracellular Ca2+ rises greatly facilitate muscarinic responses and uncover, in some cells, oscillatory Ca2+ mobilization. These effects appear to reflect the loading state of intracellular Ca2+ stores.

Differences in the stimulation of the phosphoinositide cycle by amine neurotransmitters in cultured rat forebrain neurones and astrocytes

In this study, we compared the stimulation by carbachol (CCh), noradrenaline (NA), and histamine (HA) of phosphoinositide hydrolysis in rat forebrain neuronal and glial cultures. When Ca2+ was omitted from the stimulation buffer (low microM extracellular Ca2+), amine-induced [3H]inositol phosphate accumulation was reduced to a higher extent in astrocytes (70-80% for CCh and NA and 100% for HA) than in neurones (around 50-60% for all the amines). Furthermore, guanosine 5'-[gamma-thio]trisphosphate (GTP[S]) stimulation of phosphoinositidase C (PIC) in membranes was 5-fold higher in neurones than in astrocytes. These results indicate differences in the mechanism of PIC stimulation in the two cell types. After 30 min stimulation in the presence of 10 mM Li+, a higher accumulation of [3H]inositol 4-monophosphate and [3H]inositol 1,4-bisphosphate than of [3H]inositol 1/3-monophosphate occurred for all agonists in neurones, whereas the opposite was observed in astrocytes. Moreover, in these cells stimulation for 5 min in the absence of Li+ produced a 2-3-fold accumulation of all metabolites of the 3-kinase pathway of inositol-1,4,5-trisphosphate metabolism but not of those of the 5-phosphatase pathway. Thus, regardless of the amine receptor stimulated, the 3-kinase route appeared to prevail in astrocytes and the 5-phosphatase pathway in neurones. The histamine response in neurones differed from that of the other agonists in that it rapidly declined. Taken together these results indicate that the heterogeneity in amine stimulation of the phosphoinositide cycle previously observed in brain slices could arise to a great extent from the cellular diversity of this preparation and be related to the differential contribution of the amine receptors located in neurones and astrocytes.

Muscarinic activation of a voltage-dependent cation nonselective current in rat association cortex

The ionic mechanism underlying the acetylcholine-induced depolarization of layer V pyramidal neurons of rat prefrontal cortex was examined using whole-cell recording in in vitro rat brain slices. Consistent with previous results, pressure application of acetylcholine to layer V pyramidal neurons elicited a strong depolarization. Pharmacological analysis of this response indicated that it was mediated by the stimulation of muscarinic receptors as it was mimicked by muscarinic agonists, but not by nicotine, and was blocked by atropine. The inward current responsible for the depolarization resulted from the activation of a voltage-dependent, cation nonselective current. Thus, the amplitude of the current was critically dependent on extracellular sodium concentration but not on extracellular potassium or chloride concentration. Examination of the I-V relationship for the muscarinic current using voltage clamp revealed that the current reversed near -15 mV and exhibited a strong voltage dependence, turning off rapidly in the subthreshold range. The voltage dependence of the current led to the appearance of a current associated with a conductance decrease when examined using steady-state voltage- or current-clamp measurements. This might have led to earlier misidentification of this response as mediated by a decrease in potassium conductance. These results question the traditional interpretation that muscarinic depolarization in cortex is mediated by a decrease in potassium conductance. They indicate that the fundamental mechanism responsible for muscarinic depolarization in prefrontal cortex involves the activation of a voltage-dependent, cation nonselective current. This current might represent a previously unsuspected mechanism capable of mediating slow depolarization in the central nervous system.

gamma-Aminobutyric acid increases intracellular Ca2+ concentration in cultured cortical neurons: role of Cl- transport

The effect of gamma-aminobutyric acid (GABA) on intracellular Ca2+ concentration ([Ca2+]i) in cultured prenatal rat cortical neurons was investigated using fluorescence imaging. GABA or muscimol, but not baclofen, increased [Ca2+]i in a dose-dependent manner. The GABAA receptor antagonists, bicuculline and picrotoxin, inhibited the GABA response. Furosemide, an inhibitor of the Na+/K+/2Cl- cotransporter, inhibited the GABA response in a noncompetitive manner. Ethacrynic acid, an inhibitor of an ATP-dependent Cl- pump, also inhibited the GABA-induced increased in [Ca2+]i. These results suggest a role for Cl- transport processes in the GABA response. The coapplication of GABA and high K+ led to a non-additive increase in the GABA response. The GABA response was also inhibited by nifedipine, a voltage-gated Ca2+ channel blocker, and abolished by the absence of extracellular Ca2+. Results indicate that the GABA response shares a common pathway of Ca2+ movement with the high K(+)-induced response. These observations suggest that the stimulation with GABA results in Ca2+ influx through voltage-gated Ca2+ channels, and that these effects are dependent on Cl- transport systems.

Ethanol inhibits muscarinic receptor-stimulated phosphoinositide metabolism and calcium mobilization in rat primary cortical cultures

In recent years, it has been hypothesized that muscarinic receptor-stimulated phosphoinositide (PI) metabolism may represent a relevant target for the developmental neurotoxicity of ethanol. Age-, brain region-, and receptor-specific inhibitory effects of ethanol on this system have been found, both in vitro and after in vivo administration. As a direct consequence of this action, alterations of calcium homeostasis would be expected, through alterations of inositol trisphosphate formation, which mediates intracellular calcium mobilization. In the present study, the effects of ethanol (50-500 mM) on carbachol-stimulated PI metabolism and free intracellular calcium levels were investigated in rat primary cortical cultures, by measuring release of inositol phosphates and utilizing the two calcium probes fluo-3 and indo-1 on an ACAS (Adherent Cell Analysis and Sorting) Laser Cytometer. Ethanol exerted a concentration-dependent inhibition of carbachol-stimulated PI metabolism. In addition, ethanol's inhibitory effect paralleled the temporal development of the muscarinic receptor signal transduction system, with the strongest inhibition (25-50%) occurring when maximal stimulation by carbachol occurs (days 5-7). Ethanol also exerted a concentration-dependent decrease in free intracellular calcium levels following carbachol stimulation. Both initial calcium spike amplitude, seen in all responsive cells, as well as the total number of cells responding to carbachol, were decreased by ethanol. The inhibitory effects of ethanol seemed dependent upon preincubation time, in that a longer preincubation (30 min) with the lowest dose (50 mM), showed almost the same decrease in responding cell number and reduction in spike amplitude in responding cells, as a shorter incubation (10 min) with the highest ethanol dose (500 mM). The specificity of the response to carbachol was demonstrated by blocking the response with 10 microM atropine. Moreover, experiments with carbachol in calcium-free buffer with 1 mM EGTA indicated that the initial calcium spike was due to intracellular calcium mobilization from intracellular stores. Since calcium is believed to play important roles in cell proliferation and differentiation, these results support the hypothesis that this intracellular signal-transduction pathway may be a target for ethanol, contributing to its developmental neurotoxicity.

Increase in [Ca2+]i by CCh in adult rat sympathetic neurons are not dependent on intracellular Ca2+ pools

We have examined the effects of the muscarinic agonists, carbachol (CCh) and oxotremorine (Oxo), on the intracellular free Ca2+ concentration ([Ca2+]i) in acutely dissociated sympathetic neurons from adult rats using fura 2-based microfluorometry. The drugs increased [Ca2+]i by 86 +/- 7 and 38 +/- 10 nM for CCh and Oxo, respectively (both 10 microM). Basal [Ca2+]i was 52 +/- 3 nM. Depletion of the caffeine-sensitive Ca2+ store or blockade of the Ca(2+)-adenosinetriphosphatase with thapsigargin did not alter the effect of either agonist on the rise in [Ca2+]i. On the other hand, the omission of Ca2+ from the perfusion solution or the use of TA-3090, a Ca2+ channel antagonist, blocked the effects of CCh and Oxo. In whole cell current-clamp recordings, the muscarinic agonists elicited a depolarization and action potential firing, which probably explained the rise in [Ca2+]i observed with microfluorimetric recording. In addition to their direct effects on the [Ca2+]i, muscarinic agonists also reduced the rise in [Ca2+]i induced by a nicotinic agonist. This inhibitory effect, observed in 68% of cells that responded to the nicotinic agonist, was blocked by atropine and pertussis toxin, whereas the muscarinic agonist-induced increase in [Ca2+]i was blocked by atropine but was pertussis toxin insensitive. These results suggest that at least two muscarinic receptors are present on sympathetic neurons and that they mediate opposite effect on the fluctuation of [Ca2+]i.

Muscarinic receptor stimulation increases the spontaneous [3H]GABA release in the rat substantia nigra through muscarinic receptors localized on striatonigral terminals

The effect of muscarinic agonists on the spontaneous release of [3H]GABA was investigated in vitro on rat substantia nigra slices. Acetylcholine (5 x 10(-5) M) in the presence of eserine (5 x 10(-5) M) induced a 12.3% increase of the spontaneous release of [3H]GABA. Similarly, carbachol (5 x 10(-4) M) enhanced by 9% the release of [3H]GABA. This effect was Ca(2+)-dependent, it was abolished in the presence of 0.4 mM Ca2+ and enhanced from 9 to 17% when Ca(2+)-concentration of the superfusion medium was increased from 1.3 to 2.4 mM. The carbachol effect was mediated by muscarinic receptors since it was abolished by atropine (2 x 10(-6) M). The pharmacologically M2 muscarinic receptor subtypes seems to be involved as the carbachol-induced effect was abolished by AF-DX384MS (10(-6) M), an M2 antagonist and was only partially reversed by pirenzepine (10(-5) and 10(-4) M), an M1 antagonist which at these doses also block the M2 receptors. The absence of effect of SCH23390 (10(-6) M) a D1 antagonist as well as the lack of effect of CNQX (10(-5) M) and dizocilpine maleate (10(-6) M), two glutamate antagonists, on the carbachol-induced effect indicated that neither dopamine (through D1 receptors) nor glutamate (through ionotropic receptors) were involved in the response. In addition, the persistence of the carbachol-induced effect in the presence of tetrodotoxin (2 x 10(-7) M) suggests a direct muscarinic-mediated modulation of [3H]GABA. The localization of muscarinic receptors on striatonigral fibres was confirmed by autoradiographic studies showing a decrease of [3H]pirenzepine binding in the substantia nigra after a unilateral striatal lesion induced by kainic acid injection. This latter result provides evidence of the presence of M1 receptors on striatonigral terminals as the concentration of [3H]pirenzepine used (10 nM) is M1-selective. These results indicate a cholinergic modulation of GABA release in the rat substantia nigra mediated by muscarinic receptors localized on striatonigral terminals. The involvement of the m4 muscarinic receptor subtype that have a M1/M2 pharmacology is discussed.

Non-steroidal anti-inflammatory drugs and spinal nociceptive processing

Non-steroidal anti-inflammatory drugs have a direct action on spinal nociceptive processing in vivo with a relative order of potency which correlates with their capacity as inhibitors of cyclooxygenase activity. However, recent clinical surveys and new in vivo evidence strongly suggest that for some of these agents, centrally mediated analgesia may also be achieved by additional mechanisms, which are independent of prostaglandin synthesis inhibition. In this review we explore the likelihood for such mechanisms following an extensive survey of existing data. The implications of these mechanisms are discussed in the light of our current understanding of spinal nociceptive processing.

Increase of c-fos and ras oncoproteins in the denervated neuropil of the rat dentate gyrus

When the entorhinal cortical input to the rat dentate gyrus is destroyed, the process of sprouting and synaptogenesis begins within the denervated dendritic laminae. The present study used immunohistochemical methods to determine whether there was an increase in the oncoproteins c-fos and ras within the denervated neuropil of the dentate gyrus during this period of terminal growth and synapse formation. Animals were prepared for immunolabeling one, three, six and 30 days after unilateral lesion of the entorhinal cortex. Rats were perfused with paraformaldehyde fixative and brain sections were incubated with antibodies to either c-fos or ras oncoprotein. Qualitative light microscopic analysis showed a marked increase in both c-fos and ras proteins over the denervated zone at three days postlesion when compared to both the intact contralateral control and the naive control. At one- and six-day postlesion intervals there was also an increase in labeling over the denervated neuropil with each oncoprotein; however, the intensity of label was reduced relative to that of the three-day time interval. No increase in labeling over the denervated zone was visible for either antibody at 30 days postlesion. The high level of both c-fos and ras labeling in the denervated molecular layer was confirmed with Western blot analysis of dissected molecular layers from lesioned and contralateral control hippocampi. Controls for antibody and method specificity showed that the labeling was specific for c-fos and ras proteins. The high level of c-fos labeling over the denervated molecular layer was uniform with scattered punctate sites of reaction product interspersed in the neuropil. Glial cell bodies in the neuropil contained the highest levels of c-fos oncoprotein. The granule cell nuclei showed an apparent reduction in the level of c-fos labeling at one, three and six days postlesion when compared with the nuclear staining of naive control cases. At 30 days postlesion, high levels of labeling over the denervated zone were not visible and c-fos localization had returned to the typical predominant nuclear sites seen in controls. Ras oncoprotein localization was diffuse in the cell processes of the molecular layer, with intermittent glial labeling within the denervated zone. No cell nuclei labeling was observed with antibodies to ras protein. These results show that both c-fos and ras oncoproteins are increased within the denervated neuropil of the dentate gyrus during sprouting and synapse formation.(ABSTRACT TRUNCATED AT 400 WORDS)

Acetylcholine enhances NMDA-evoked calcium rise in hippocampal neurons

The effects of acetylcholine (ACh) on changes in [Ca]i produced by the glutamate agonist N-methyl-D-aspartate (NMDA) were measured in cultured rat hippocampal neurons loaded with the fluorescent calcium indicator Fluo-3 in a confocal laser scanning microscope. NMDA produced a dose-dependent reversible rise in [Ca]i. ACh had a smaller and less consistent effect on [Ca]i but could cause a marked enhancement of the reactivity of neurons to NMDA. This effect was reversed by the presence of the muscarinic antagonist atropine. AMPA, another glutamate agonist which causes a rise in [Ca]i by activating voltage gated calcium influx was less affected by ACh. Caffeine, which releases calcium from intracellular stores also enhanced reactivity of these neurons to NMDA. It is suggested that ACh can enhance reactivity to NMDA by releasing calcium from internal stores.

LAN-1: a human neuroblastoma cell line with M1 and M3 muscarinic receptor subtypes coupled to intracellular Ca2+ elevation and lacking Ca2+ channels activated by membrane depolarization

The LAN-1 clone, a cell line derived from a human neuroblastoma, possesses muscarinic receptors. The stimulation of these receptors with increasing concentrations of carbachol (CCh; 1-1,000 microM) caused a dose-dependent increase of the intracellular free Ca2+ concentration ([Ca2+]i). This increase was characterized by an early peak phase (10 s) and a late plateau phase. The removal of extracellular Ca2+ reduced the magnitude of the peak phase to approximately 70% but completely abolished the plateau phase. The muscarinic-activated Ca2+ channel was gadolinium (Gd3+) blockade and nimodipine and omega-conotoxin insensitive. In addition, membrane depolarization did not cause any increase in [Ca2+]i. The CCh-induced [Ca2+]i elevation was concentration-dependently inhibited by pirenzepine and 4-diphenylacetoxy-N-methylpiperidine methiodide, two rather selective antagonists of M1 and M3 muscarinic receptor subtypes, respectively, whereas methoctramine, an M2 antagonist, was ineffective. The coupling of M1 and M3 receptor activation with [Ca2+]i elevation does not seem to be mediated by a pertussis toxin-sensitive guanine nucleotide-binding protein or by the diacylglycerol-protein kinase C system. The mobilization of [Ca2+]i elicited by M1 and M3 muscarinic receptor stimulation seems to be dependent on an inositol trisphosphate-sensitive intracellular store. In addition, ryanodine did not prevent CCh-induced [Ca2+]i mobilization, and, finally, LAN-1 cells appear to lack caffeine-sensitive Ca2+ stores, because the methylxanthine was unable to elicit intracellular Ca2+ mobilization, under basal conditions, after a subthreshold concentration of CCh (0.3 microM), or after thapsigargin.

Potentiation of N-methyl-D-aspartate-stimulated dopamine release from rat brain slices by aluminum fluoride and carbachol

N-Methyl-D-aspartate (NMDA) stimulated the release of endogenous dopamine from striatal slices prepared from adult Sprague-Dawley rats. A mixture of sodium fluoride and aluminum chloride (AlF4-) added to the slices significantly potentiated the NMDA-stimulated release of dopamine in a concentration- and time-dependent manner. The AlF4- mixture had no effect on the nonstimulated basal efflux of dopamine, and no increases in NMDA-stimulated release were observed when NaF was replaced with NaCl. Similarly, AlCl3 or a mixture of NaCl and AlCl3 had no effect on NMDA-stimulated release. The AlF(4-)-induced increase in NMDA-stimulated dopamine release was totally blocked by magnesium or the selective NMDA glycine antagonist 7-chlorokynurenic acid. Striatal slices depolarized with KCl (15 mM) also released dopamine and this release was similarly potentiated by AlF4-. However, KCl-stimulated dopamine release from striatal synaptosomes was not potentiated by concentrations of AlF4- that greatly increased release from striatal slices. NMDA did not stimulate the release of dopamine from striatal synaptosomes in the absence or presence of aluminum fluoride. Modulators of adenylate cyclase (forskolin) and protein kinase C (phorbol esters) did not enhance NMDA-stimulated dopamine release. The protein kinase C inhibitor H-7 also did not reduce the potentiating effects of AlF4-. The mixed cholinergic agonist carbachol and the calcium ionophore A23187 mimicked the AlF4- effect although the increase in NMDA-stimulated dopamine release produced by these agents was less than that seen with AlF4-.(ABSTRACT TRUNCATED AT 250 WORDS)

Confocal microscopic imaging of [Ca2+]i in cultured rat hippocampal neurons following exposure to N-methyl-D-aspartate

1. The confocal laser scanning microscope (CLSM) was used in conjunction with the calcium indicator dye Fluo-3 to record changes in free intracellular calcium concentration ([Ca2+]i) in cultured hippocampal neurons in response to superfusion of N-methyl-D-aspartate (NMDA). 2. NMDA caused a rapid rise in [Ca2+]i in all parts of the neuron. The rise in [Ca2+]i was dependent on activation of an NMDA receptor, was enhanced by the removal of Mg2+ and addition of glycine to the superfusion medium, and was dependent on normal [Ca2+]o. 3. The rise of [Ca2+]i was seen first near the membrane. A wave of elevated [Ca2+]i moved centripetally at a rate of 117 microns/s. 4. Dantrolene pre-incubation caused a significant reduction in the efficacy of the NMDA-induced rise in [Ca2+]i, indicating that at least part of the rise is caused by intracellular release of calcium. 5. The replacement of calcium by barium caused a reduction in the response to NMDA, but a significant response was still present in these cells, supporting the assumption that NMDA causes release of calcium from intracellular stores. 6. The removal of sodium from the superfusion medium prolonged the [Ca2+]i rise in response to NMDA indicating that the Na-Ca antiporter is instrumental in reducing [Ca2+]i. 7. These studies demonstrate the multiplicity of regulating mechanisms of [Ca2+]i following activation of NMDA receptors.

Ethanol and voltage- or receptor-mediated increases in cytosolic Ca2+ in brain cells

Dissociated brain cells were isolated from newborn rat pups and loaded with fura-2. Different mechanisms for stimulating increased free intracellular Ca2+ concentrations [( Ca2+]i) were examined in the absence and presence of ethanol. KCl, carbachol, and kainate concentration-dependently increased [Ca2+]i. Quisqualate also elevated [Ca2+]i but did not produce clear concentration-dependent increases. KCl, carbachol, and quisqualate responses reached peak levels within 10-30 s and then desensitized within 90 s. However, kainate-stimulated increases in [Ca2+]i plateaued and did not decline after 90 s. Of these different [Ca2+]i-mediated processes, only 60 mM KCl stimulation was significantly inhibited by 100 mM ethanol, while lower KCl concentrations were not affected. Carbachol-induced release of intracellular Ca2+ and activation of non-NMDA (i.e., kainate, quisqualate) excitatory amino acid receptor-operated cation channels were also not significantly inhibited by 100 mM ethanol. Thus, in acutely dissociated brain cells from newborn rats, only Ca2+ influx via voltage- and, as reported previously, NMDA-operated Ca2+ channels were sensitive to ethanol inhibition.

Oxidized glutathione modulates N-methyl-D-aspartate- and depolarization-induced increases in intracellular Ca2+ in cultured rat forebrain neurons

We have investigated the interaction of reduced and oxidized glutathione (GSSG) with intracellular Ca2+ increases produced by N-methyl-D-aspartate (NMDA), kainate and KCl in primary cultures of forebrain neurons derived from fetal rats. Responses to NMDA, applied with glycine, were inhibited by GSSG (10 mM), but were unaffected by reduced glutathione and L-cysteine. Inhibition by GSSG was still apparent after cells were oxidized by 5,5'-dithio-bis-2-nitrobenzoic acid, and this effect showed spontaneous but only partial reversal. This suggests that modulation of the redox site on the NMDA receptor could not account for all of the effects produced by GSSG. However, the observation that complete recovery from GSSG treatment required exposure of cells to dithiothreitol suggests that oxidation of the redox site contributes to the action of GSSG. GSSG also inhibited responses produced by 50 mM KCl but not those produced by 50 microM kainate. The effects of GSSG on KCl responses were fully and rapidly reversible. These results suggest that high concentrations of GSSG may modulate NMDA receptors, and that some of the actions of GSSG may be mediated by the redox site on the receptor complex.

Quisqualate and carbachol-induced increases in intrasynaptosomal free calcium are mediated by different products of phospholipid hydrolysis

The mechanisms by which quisqualate and carbachol increase intrasynaptosomal free calcium ([Ca2+]i) were studied in rat cortical synaptosomes. Quisqualate (0.01-100 microM) and carbachol (100-1000 microM) increased [Ca2+]i in Fura-2 acetoxymethyl ester (Fura-2 AM)-loaded synaptosomes. The resting level of [Ca2+]i was 118 nM. The maximum increase (55%) was produced by 10 microM quisqualate which had an EC50 of 0.2 microM. The maximum increase (28%) elicited by carbachol occurred at 1000 microM and the EC50 was 30 microM. The stimulatory effects of quisqualate on [Ca2+]i were blocked by heparin (100 I.U.) but not by staurosporine (1 microM), nifedipine (1 microM) or omega-conotoxin fraction GVIA (omega-CgTx) (0.5 microM). On the other hand, the effects of carbachol on [Ca2+]i were abolished by staurosporine, nifedipine or omega-CgTx but not by heparin. Carbachol (100 microM) also significantly increased 45Ca accumulation into either resting or K+ (30 mM)-depolarised synaptosomes and these effects were inhibited by staurosporine and nifedipine. Quisqualate (10 microM) had no effect on 45Ca accumulation under resting or depolarised conditions. When quisqualate and carbachol were used in combination, there were apparently additive effects on [Ca2+]i but not on 45Ca accumulation. It is concluded that carbachol increases [Ca2+]i by facilitating Ca2+ entry through L-type Ca2+ channels via a 1,2-diacylglycerol (DAG)-protein kinase C (PKC)-dependent pathway while quisqualate mobilizes Ca2+ from inositol 1,4,5-trisphosphate (IP3)-sensitive stores.

Role of calcium in regulation of phosphoinositide signaling pathway

Using primary neuronal cultures we have examined the role of extracellular Ca2+ in a receptor-regulated phosphoinositide turnover. We report that receptor (glutamic acid and acetylcholine)-activated phosphoinositide turnover requires the presence of extracellular Ca2+ (EC50 = 21.1 microM). The requirement for Ca2+ appears to be at an intracellular level and is highly selective for Ca2+. We also found that several inorganic and organic Ca2+ channel blockers, including La3+ and verapamil, inhibit phosphoinositide turnover. However, the pharmacological profile of these agents in this regard was distinct from their actions at the voltage-sensitive Ca2+ channels. To explain the above requirement for extracellular Ca2+ in agonist-stimulated phosphoinositide turnover and its sensitivity to Ca(2+)-channel blockers, we propose a hypothetical model suggesting that Ca2+, following IP-3-mediated mobilization, exerts a facilitatory action on the activity of receptor-phospholipase C complex. We further propose that in the absence of extracellular Ca2+ or in the presence of certain Ca(2+)-channel blockers, refilling of calciosomes is ineffectual or inhibited, causing its depletion and subsequent inactivation of agonist-stimulated phosphoinositide turnover.

Differential effects of elevated calcium ion concentrations on inositol phospholipid responses in mouse and rat cerebral cortical slices

Inositol phospholipid turnover in cerebral cortical slices from mouse and rat was assessed using a [3H]inositol pre-labelling technique followed by anion exchange chromatography to isolate [3H]inositol phosphates ([3H]InsP chi). In both mouse and rat cerebral cortical slices, elevating the CaCl2 concentration of the Krebs medium from 1.3 to 4 mM did not significantly enhance the accumulation of [3H]InsP chi in the absence of any stimulus, or in the presence of glutamate (3 mM), depolarizing concentrations of KCl (25 mM), 5-hydroxytryptamine (0.3 mM), the calcium ionophore A23187 (33 microM) or carbachol (1 mM). However, the accumulations of [3H]InsP chi induced by histamine (1 mM) or noradrenaline (0.1 mM) were significantly increased by between 95 and 178% in cerebral cortical slices from both species by the elevation of extracellular calcium. Analysis of the individual inositol phosphates revealed that elevated ambient calcium enhanced the histamine-generated accumulations of [3H]InsP2, [3H]InsP3 and [3H]InsP4 by up to two-fold, while only the [3H]InsP3 response to carbachol was significantly increased. Under the same conditions, histamine, but not carbachol, selectively increased the accumulation of [3H]PtdInsP2 by up to 50%. The [3H]InsP chi responses to histamine and noradrenaline in combination with the calcium ionophore A23187 were greater-than-additive, inferring an enhancement of the receptor response by raised intracellular calcium. However, the combination of A23187 with glutamate or KCl resulted in significantly less-than-additive [3H]InsP chi responses. The [3H]InsP chi response to carbachol or 5-hydroxytryptamine was not significantly altered in the presence of A23187. Taken together, these results indicate heterogeneity between the mechanisms of inositol phospholipid turnover induced by these various stimuli in mammalian cerebral cortical slices.

Reduction of NMDA receptors with dithiothreitol increases [3H]-MK-801 binding and NMDA-induced Ca2+ fluxes

1. We have investigated the modulation of N-methyl-D-aspartate (NMDA) receptor activation by the sulphydryl redox reagents dithiothreitol (DTT) and 5,5-dithio-bis-2-nitrobenzoic acid (DTNB). 2. Increases in [3H]-MK-801 binding produced by glutamate, glycine and spermidine were enhanced by DTT (2mM) and diminished by DTNB (0.5 mM). 3. The inhibition of [3H]-MK-801 binding by CGS 19755 and 7-chlorokynurenate was not altered by 2 mM DTT. However, the potency of the competitive polyamine antagonist, arcaine, was decreased by DTT. 4. NMDA-induced Ca2+ fluxes into primary cultures of rat forebrain neurones were enhanced by DTT in a DTNB-reversible fashion. In addition to augmenting the magnitude of NMDA-induced increase in intracellular free Ca2+, 10 mM DTT also prolonged the duration of the Ca2+ signal. However, DTT had no effect on the increase in Ca2+ produced by depolarizing neurones with 50 mM KCl. 5. These studies show that the reduction of disulphide bonds on the NMDA receptor complex by DTT increases activation. The precise site of these groups remains unclear but they are unlikely to form an integral part of the glutamate, glycine or polyamine binding domains. The enhancement of the activation of the NMDA receptor by DTT is associated with increased Ca2+ fluxes. The possible pathophysiological consequences of receptor reduction are discussed.

Stimulation of muscarinic acetylcholine receptors increases synaptosomal free calcium concentration by protein kinase-dependent opening of L-type calcium channels

In synaptosomes prepared from rat cerebral cortex, free cytosolic calcium concentration ([Ca2+]i) was measured using the fluorescent dye fura-2. Incubation of fura-2-loaded synaptosomes with carbachol increased [Ca2+]i in a dose-dependent manner (1-1,000 microM), with a maximum response of 22 +/- 2% at approximately 100 microM and an EC50 (calculated concentration producing 50% of the maximum response) of 30 microM. The effect of carbachol (100 microM) on [Ca2+]i was antagonised by atropine, but not by hexamethonium (10 microM). The calculated concentration of atropine needed for 50% inhibition (IC50) was 260 nM. The rise in [Ca2+]i produced by carbachol was reduced in the absence of extrasynaptosomal Ca2+ and effectively blocked by the L-type calcium channel blocker nifedipine (with an IC50 of 29 nM). The response to carbachol was reduced if the synaptosomes were preincubated with the protein kinase inhibitors H7 [1-(5-isoquinolinylsulfonyl)-2- methylpiperazine] (from 17% in the solvent control to 4%) and staurosporine (from 20% in the solvent control to 3%). These results show that stimulation of muscarinic acetylcholine receptors in synaptosomes increases [Ca2+]i by protein kinase-dependent activation of 1,4-dihydropyridine-sensitive calcium channels.

Regulation of calcium concentrations in synaptosomes: alpha 2-adrenoceptors reduce free Ca2+ by closure of N-type Ca2+ channels

The relationship between intrasynaptosomal total (CaT) and free ([Ca2+]i) calcium and 45Ca accumulation was studied under physiological and K(+)-depolarised conditions in rat cortical synaptosomes. Under physiological conditions, CaT (10.7 mM) was approximately 10,000 times higher than [Ca2+]i (118 nM), showing that there is a large reservoir of sequestered calcium in synaptosomes. 45Ca accumulation was rapid (initial rate, 3.4 nmol/mg protein/min), substantial (7 nmol/mg protein in 2 min), and depolarisation dependent, and reached equilibrium after 5 min. At equilibrium, only 10% of CaT was freely exchangeable. This pool was much larger than the free Ca2+ pool. CaT, [Ca2+]i, and 45Ca accumulations were directly related to the Ca2+ concentration in the buffer, suggesting that [Ca2+]i is not highly conserved but is maintained by simple equilibria between the various pools. Clonidine reduced 45Ca accumulation in a time- and dose-dependent manner. Maximum inhibition (40% at 100 microM) occurred at 2 min and the IC50 was 80 nM. The reduction caused by clonidine (1 microM) reached equilibrium after 5 min, but this equilibrium value was lower than in controls, suggesting that clonidine changes the exchangeable Ca2+ pool size. The effects of clonidine (1 microM) on [Ca2+]i (26% reduction) and on 45Ca accumulation (24% reduction) were most apparent under physiological conditions. However, while it was not dependent on depolarisation, it did not occur in physiological buffer containing low K+ concentration (0.1-1 mM). The inhibitory effect of clonidine on 45Ca accumulation is receptor mediated as it was antagonised by idazoxan (1 microM).(ABSTRACT TRUNCATED AT 250 WORDS)