Positive effects of allosteric modulators on the binding properties and the function of muscarinic acetylcholine receptors

Data are reviewed indicating that allosteric modulators can enhance the affinities of muscarinic receptors for their antagonists and agonists, that the enhancement of the affinity for agonists is relevant functionally, and that the allosterically induced conformational change also affects the interaction between the receptors and the G proteins.

M1, M3 and M5 muscarinic receptors stimulate mitogen-activated protein kinase

We report here that the M1, M3 and M5 muscarinic acetylcholine receptor subtypes that have been shown to couple to phosphoinositide hydrolysis also activate the mitogen-activated protein kinase (MAPK). Pharmacological characterization as well as mechanistic details of the activation pathway are presented. Carbachol-induced MAPK activation was time- and concentration-dependent at all subtypes. Pharmacological characterization of the MAPK response revealed that McN-A-343 was a partial agonist at the M1 and M3 subtypes, and that pilocarpine was a partial agonist at the M3 and M5 receptors. Carbachol-mediated MAPK activation at these receptor subtypes was pertussis toxin and wortmannin insensitive. By contrast, both agents significantly inhibited carbachol-induced MAPK activation by the M2 muscarinic receptor subtype. Furthermore, two independent single point mutations in the M1 receptor attenuated carbachol-induced activation of MAPK. Activation of MAPK at the M1, M3 and M5 muscarinic receptor subtypes was not dependent on intracellular or extracellular Ca2+, but was partially dependent upon protein kinase C. These data suggest that activation of MAPK by M1, M3 and M5 muscarinic receptors involves protein kinase C-dependent and independent pathways.

Activation of neuronal nitric oxide synthase by M2 muscarinic receptors associated with a small increase in intracellular calcium

We investigated the coupling of the M2 muscarinic acetylcholine receptors expressed in Chinese hamster ovary cells to activation of neuronal nitric oxide (NO) synthase. Stimulation of guanylate cyclase activity in detector neuroblastoma cells was used as an indirect measure of the generation of NO in Chinese hamster ovary cells. The muscarinic agonist carbachol induced marked time- and concentration-dependent enhancement of the activity of NO synthase. Activation of neuronal NO synthase by M2 muscarinic receptors was associated with a small increase in the concentration of intracellular Ca2+. These data suggest the presence of alternate mechanisms of activation of neuronal NO synthase which might be operative in the absence of large changes in the concentration of cellular Ca2+. These findings help to understand the mechanisms of activation of NO synthase.

Differential coupling of m1, m3, and m5 muscarinic receptors to activation of neuronal nitric oxide synthase

The selectivity of coupling of m1, m3, and m5 muscarinic receptors to activation of the neuronal type of nitric oxide synthase was investigated. Stimulation with the agonist carbachol of all three receptor subtypes expressed in Chinese hamster ovary cells resulted in a rapid and transient activation of the enzyme, as measured by stimulation of guanylate cyclase in reporter neuroblastoma cells. Carbachol was more potent and efficacious at m5 receptors than at the other two receptor subtypes. Stimulation of all three muscarinic receptors resulted in an increased concentration of intracellular calcium, with a time course that preceded activation of nitric oxide synthase. At each receptor subtype, there was a close relationship between the magnitude of the maximal calcium response and that of enzyme activation.

Activation of muscarinic acetylcholine receptors via their allosteric binding sites

Ligands that bind to the allosteric-binding sites on muscarinic acetylcholine receptors alter the conformation of the classical-binding sites of these receptors and either diminish or increase their affinity for muscarinic agonists and classical antagonists. It is not known whether the resulting conformational change also affects the interaction between the receptors and the G proteins. We have now found that the muscarinic receptor allosteric modulators alcuronium, gallamine, and strychnine (acting in the absence of an agonist) alter the synthesis of cAMP in Chinese hamster ovary (CHO) cells expressing the M2 or the M4 subtype of muscarinic receptors in the same direction as the agonist carbachol. In addition, most of their effects on the production of inositol phosphates in CHO cells expressing the M1 or the M3 muscarinic receptor subtypes are also similar to (although much weaker than) those of carbachol. The agonist-like effects of the allosteric modulators are not observed in CHO cells that have not been transfected with the gene for any of the subtypes of muscarinic receptors. The effects of alcuronium on the formation of cAMP and inositol phosphates are not prevented by the classical muscarinic antagonist quinuclidinyl benzilate. These observations demonstrate for the first time that the G protein-mediated functional responses of muscarinic receptors can be evoked not only from their classical, but also from their allosteric, binding sites. This represents a new mechanism of receptor activation.

Intricate regulation of nitric oxide synthesis in neurons

There is little doubt that nitric oxide (NO) is one of the most important second messengers yet to be discovered, particularly in relation to its diverse roles in the regulation of neuronal function. As expected, synthesis of such a multifunctional molecule has to be under very tight control. For example, there is evidence that the rate of production of NO in neurons is regulated by several second messengers and their related protein kinases. NO by itself is also able to elicit negative feedback on the activity NO synthase (NOS) to attenuate its own rate of synthesis. Furthermore, NO modulates the release of neurotransmitters and alters the sensitivity of receptors that are coupled to stimulation of its synthesis. In healthy neurons, all of these intricate mechanisms are expected to cross-talk in harmony to result in the generation of optimal amounts of NO.

Selective enhancement of antagonist ligand binding at muscarinic M2 receptors by heparin due to receptor uncoupling

The selectivity of heparin in inducing potentiation of binding of antagonist ligands to muscarinic receptors was investigated at the five known subtypes of muscarinic receptors. The effects of heparin on binding of [3H]N-methylscopolamine at equilibrium was studied in Chinese hamster ovary (CHO) cells which express each of the individual muscarinic receptor subtypes and in membranes prepared from these cells. Heparin markedly increased equilibrium binding of subsaturating concentrations of the ligand only in membranes of CHO cells which express muscarinic M2 receptors. These effects of heparin were qualitatively similar to those obtained in heart membranes. In contrast, heparin did not influence ligand binding to muscarinic M2 receptors in intact cells. The positive cooperative effects of heparin at muscarinic receptors were abolished following treatment of cells with pertussis toxin. The latter treatment by itself resulted in a significant increase in [3H]N-methylscopolamine binding. Taken together with previous reports of heparin-induced uncoupling of receptors and G-proteins, these data suggest that the effects of heparin on ligand binding to muscarinic M2 receptors might be due to disruption of receptor-G-protein interactions which results in enhancement of binding of antagonist ligands to the receptor.

Constitutive activity of the M1-M4 subtypes of muscarinic receptors in transfected CHO cells and of muscarinic receptors in the heart cells revealed by negative antagonists

We investigated whether muscarinic receptors of the M1-M4 receptor subtypes are constitutively active. We have found that the synthesis of cyclic AMP was enhanced by the muscarinic antagonists atropine and N-methylscopolamine (NMS) in Chinese hamster ovary (CHO) cells stably transfected with human m2 and m4 muscarinic receptor genes and in rat cardiomyocytes expressing the M2 receptor subtype, and that the production of inositol phosphates was inhibited by atropine and NMS in CHO cells stably transfected with human m1 and m3 and with rat m1 muscarinic receptor genes. The muscarinic antagonists quinuclidinyl benzilate and AF-DX 116 had no effect in some cases and acted like atropine and NMS in others. We conclude that the M1-M4 subtypes of muscarinic receptors are constitutively active in the CHO cell lines expressing them and in cardiomyocytes and that atropine and NMS act as negative antagonists on these receptor subtypes by stabilizing them in the inactive conformation.

Positive and negative allosteric interactions on muscarinic receptors

It was previously found that alcuronium increases the binding of [3H]methyl-N-scopolamine to cardiac muscarinic receptors by a positive allosteric action while its effect on the binding of [3H]quinuclidinyl benzilate is negative. The, features of the antagonist's molecule which decide whether its allosteric interaction with alcuronium is positive or negative are not known. In the present work, it was found that alcuronium has a positive allosteric effect also on the binding of [3H]atropine and [3H]methyl-N-piperidinyl benzilate to muscarinic receptors in rat heart atria and that its effect on the binding of [3H]methyl-N-quinuclidinyl benzilate is negative. A comparison of the five radiolabelled antagonists that have been investigated so far indicates that the type of allosteric interaction (positive or negative) is not determined by the presence or absence of the quaternary nitrogen or of the benzilyl moiety in the molecule of the antagonist. Apparently, features of the N-bearing moiety of muscarinic antagonists other than the presence of a charge on nitrogen play a key role in the determination of the type of interaction.

Investigation of the role of an amino acid triplet repeat in differentiating drug-receptor interaction at m1 and m2 muscarinic receptors

The first putative extracellular domains of both m1 and m2 muscarinic receptors contain a triplet of amino acid residues consisting of leucine (L), tyrosine (Y), and threonine (T). This triplet is repeated as LYTLYT in m2 receptors. However, it is repeated in a transposed fashion (LYTTYL) in the sequence of m1 receptors. In this work we employed site-directed mutagenesis to investigate the possible significance of this unique sequence diversity in determining the distinct differential drug-receptor interaction at the two receptor subtypes. Mutation of the LYTTYL sequence of m1 receptors to the corresponding m2 receptor LYTLYT sequence, however, did not significantly change the binding affinity of the agonist carbachol or the affinity of the majority of a series of receptor antagonists which are able to discriminate between wild-type m1 and m2 receptors. The reverse mutation at the m2 receptor also did not modify agonist affinity, but altered affinity of several receptor subtype-selective antagonists. The magnitude of affinity changes, however, was small, and the direction of these changes was opposite to what would be expected if the m2 receptor LYTLYT seqence were important for determining the binding profile of m2-receptor-selective antagonists. Our data suggest that the LYTTYL-LYTLYT sequence differences between ml and m2 muscarinic receptors are not important for determining receptor pharmacology.

Expression of endogenous muscarinic acetylcholine receptors in Chinese hamster ovary cells

Chinese hamster ovary (CHO) cells are commonly used for expression of the genes of cloned neurotransmitter receptors to study their pharmacology and coupling to signal transduction pathways. It is usually assumed that host cells do not endogenously express the specific receptor under consideration. We demonstrate in this report that CHO cells contain endogenous functional muscarinic acetylcholine receptors which, in some circumstances, might complicate interpretation of data related to the properties of exogenously expressed receptors.

Tachykinins alter inositol phosphate formation, but not cyclic AMP levels, in primary cultures of neonatal rat spinal neurons through activation of neurokinin receptors

The naturally occurring tachykinins, substance P, neurokinin A and neurokinin B, induce the formation of inositol phosphates or cAMP in a variety of tissues but their effects on neurons have not been resolved. We used primary cultures of neonatal rat spinal cord to determine whether neurokinin receptors mediate changes in these second messengers in spinal neurons. We found that substance P, neurokinin A and neurokinin B induced the formation of inositol phosphates in a concentration-dependent manner with similar potencies (EC50S: 3.6, 5.7 and 21.3 nM, respectively), but at concentrations tested (0.1-1.0 microM) these peptides had no effect on cAMP levels. All three tachykinins induced the formation of inositol phosphates predominately by activation of neurokinin1 receptors. CP-96,345 and WIN 51,708, neurokinin1 receptor antagonists, attenuated the response to substance P, neurokinin A and neurokinin B. GR 103,537, a neurokinin2 receptor antagonist, had no effect on the responses induced by any of the tachykinins. Furthermore, the selective neurokinin1 receptor agonist, GR-73632, induced the formation of inositol phosphates in a concentration-dependent manner, whereas the selective neurokinin2 receptor agonist, GR-64349, generated inositol phosphates only at the highest concentration tested (10 microM). Senktide, a neurokinin3 receptor agonist, did not induce the formation of inositol phosphates at any of the concentrations tested (0.01-10 microM). Inositol phosphate formation appeared to be due to a direct effect of the tachykinins on neuronal neurokinin1 receptors. These results suggest that biological responses in spinal neurons following activation of neurokinin1 receptors are mediated mainly by the hydrolysis of phosphoinositol 4,5-bisphosphate to form inositol 1,4,5-trisphosphate and diacylglycerol. It remains to be determined which of these second messengers mediates the increased neuronal excitability and depolarization that occurs in response to substance P.

Subtype selectivity of the positive allosteric action of alcuronium at cloned M1-M5 muscarinic acetylcholine receptors

The neuromuscular blocking drug alcuronium was found earlier to increase the affinity of muscarinic receptors for methyl-N-scopolamine (NMS). This effect could be observed in some but not in other tissues. Subtype selectivity of the positive allosteric action of alcuronium was now investigated in radioligand binding experiments in Chinese hamster ovary (CHO) cells stably transfected with the genes encoding the M1-M5 subtypes of muscarinic receptors. Alcuronium had a particularly high affinity for the M2 receptor subtype (Kd = 0.6 microM) and its affinity for muscarinic receptor subtypes diminished in the order M2 > M4 = M3 > M1 > M5. Alcuronium allosterically increased the binding of (3H)NMS to membranes containing receptors of the M2 (cooperativity factor alpha = 0.38) and M4 subtypes (alpha = 0.72) and inhibited the binding of (3H)NMS to membranes containing receptors of the M1, M3 and M5 subtypes (alpha = 3.35-4.35). The positive effects of alcuronium could also be observed in experiments with (3H)NMS binding to intact whole cells, indicating that the positive allosteric action of alcuronium occurs by alcuronium binding to receptor domains that are accessible from the extracellular space. Alcuronium diminished the affinity for (3H)quinuclidinyl benzilate [(3H)QNB] at all five subtypes of muscarinic receptors and slowed down the dissociation of both (3H)NMS and (3H)QNB; its decelerating effect on radioligand dissociation was most pronounced at the M2 receptor subtype. Differences between the effects of alcuronium on individual muscarinic receptor subtypes are apparently responsible for differences between the allosteric effects of alcuronium on muscarinic receptors in various tissues that had been described previously.

Anomalous increase in nitric oxide synthase activity by certain nitric oxide-generating compounds in intact neuronal cells

It has been shown that nitric oxide (NO) regulates NO synthase (NOS) activity through negative feedback in cytosolic enzyme preparations in various cell types. We compared the effects of the NO-generating compounds S-nitroso-N-acetylpenicillamine (SNAP), 3-morpholinosydnonimine (SIN-1), and sodium nitroprusside (SNP) on NOS activity in intact neuroblastoma N1E-115 cells and in the cytosol obtained from the same cells. Enzyme activity was measured by the conversion of L-[3H]arginine into L-[3H]citrulline. At concentrations that elicit almost complete inhibition of NOS activity in cytosolic enzyme preparations of these cells, SIN-1 and SNP did not cause significant attenuation of enzyme activity measured at 45 min in intact cells. It is surprising that SIN-1 and SNP markedly stimulated L-[3H]citrulline formation in a time- and concentration-dependent manner when cells were incubated with the compounds for > 1.5 h. Neither inhibitory nor stimulatory effects of SNAP on NOS were observed in intact N1E-115 cells. This is in contrast to the inhibitory effects of SNAP in cytosolic preparations of the enzyme. The increased NOS activity by SIN-1 or SNP in intact cells was dependent on the presence of extracellular Ca2+, suggesting that it might be due to increased Ca2+ influx. On the other hand, measurements of the activity of lactate dehydrogenase showed that there was no generalized increase in cell permeability in response to SIN-1 or SNP. There was no agreement in the rank order of potencies of these compounds in activating guanylate cyclase and in affecting NOS activity, both in broken-cell preparations and in intact cells.(ABSTRACT TRUNCATED AT 250 WORDS)

Cardioselective derivatives of 2,2-diphenyl-2-ethylthioacetate do not discriminate between m2 and m3 muscarinic receptors expressed in CHO cells

Characteristics of interaction of two derivatives of 2,2-diphenyl-2-ethylthioacetate with muscarinic receptors were studied in Chinese hamster ovary (CHO) cells stably transfected with the genes of human m2 and m3 muscarinic receptors. Data from radioligand-receptor binding assays and measurements of m2 receptor-inhibited cyclic AMP formation and m3 receptor-stimulated phosphoinositide (PI) hydrolysis showed that this new series of muscarinic receptor antagonists exhibited a middle range of affinities in binding to muscarinic receptors (Ki = 0.2-0.7 mumol/l), without being able to discriminate between m2 and m3 receptors. They completely displaced [3H]N-methylscopolamine ([3H]NMS) binding at equilibrium and inhibited receptor-mediated increase in PI turnover in m3 CHO cells and decrease in cyclic AMP synthesis in m2 CHO cells in an apparent competitive manner. However, higher concentrations of the compounds (> 10 mumol/l) decelerated the kinetics of atropine-induced dissociation of [3H]NMS at m2 and m3 receptors, indicating an allosteric interaction. Collectively, our results demonstrate that these derivatives of 2,2-diphenyl-2-ethylthioacetate display a mixed mechanism of interaction with muscarinic receptors, being competitive at low concentrations and allosteric at higher concentrations. In contrast to previous reports of a significantly higher affinity at cardiac M2 as compared to ileal M3 receptor, these compounds do not exhibit such selectivity when the two receptor subtypes are expressed in the same type of cells.

Regulation of neuronal nitric oxide synthase by histone, protamine, and myelin basic protein

We examined the effects of endogenous basic proteins rich in the amino acid L-arginine on neuronal NO synthase activity by monitoring cyclic GMP formation in intact neuron-like neuroblastoma N1E-115 cells. Histone, protamine and myelin basic protein significantly stimulated cyclic GMP formation, both in a time- and concentration-dependent manner. These effects were blocked by hemoglobin and NO synthase inhibitors. Removal of the extracellular/intracellular Ca2+ gradient by a Ca2+ chelator completely abolished the cyclic GMP responses elicited by histone and protamine, suggesting that influx of extracellular Ca2+ might be involved in their activation of NO synthase. The effects of myelin basic protein on cyclic GMP formation, however, appeared to be due to Ca2+ release from intracellular stores. In cytosolic preparations of rat cerebellum, these basic proteins inhibited the metabolism of L-arginine into L-citrulline by NO synthase. We conclude from our findings that endogenous basic proteins might be involved in the regulation of neuronal NO synthase activity. Their effects on the enzyme could be either stimulatory or inhibitory, depending on whether the basic proteins exert their effects extracellularly or intracellularly, respectively.

Marked induction of calcium-independent nitric oxide synthase activity after focal cerebral ischemia

We studied the effect of focal cerebral ischemia on inducible (iNOS) and constitutive (cNOS) nitric oxide synthase enzymatic activities in the affected brain. The middle cerebral artery (MCA) was occluded in spontaneously hypertensive rats. Animals were killed 1, 2, 4, and 7 days later. cNOS and iNOS enzymatic activities were determined in the infarcted cortex using the assay of Bredt and Snyder. cNOS was assayed in the presence of calcium, whereas iNOS was assayed in the absence of calcium and in the presence of tetrahydrobiopterin. The validity of the iNOS assay was verified in rats treated with bacterial lipopolysaccharide. In these animals, the magnitude of the induction of iNOS enzymatic activity in lung, spleen, and brain paralleled the expression of iNOS mRNA, assessed by reverse-transcription polymerase chain reaction. After MCA occlusion, calcium-dependent (cNOS) activity was markedly reduced only in lesioned cerebral cortex at days 1-7 (p < 0.001; analysis of variance and Tukey's test). In contrast to cNOS, calcium-independent (iNOS) activity was induced substantially in the infarct (p < 0.005) but not in the contralateral intact cortex (p > 0.05). iNOS activity peaked at day 2 and was not different from baseline at day 7 (p > 0.05). No NADPH diaphorase-positive neurons were observed in the area of the lesion at days 1-7. Macrophages appeared at day 2 and invaded the infarcted tissue by day 7. At this time, numerous glial fibrillary acidic protein-positive astrocytes were observed within the lesion. The results suggest that the decline in calcium-dependent (cNOS) activity reflects loss of NOS neurons within the lesion.(ABSTRACT TRUNCATED AT 250 WORDS)

Polyamines inhibit nitric oxide synthase in rat cerebellum

The polyamines spermine, spermidine and putrescine share some basic structural features with L-arginine, the substrate of nitric oxide (NO) synthase. The effects of the polyamines on neuronal NO synthase activity were studied in cytosolic preparations of rat cerebellum and cultured cerebellar granule neurons. Spermine, spermidine and putrescine all inhibited the conversion of [3H]L-arginine to [3H]L-citrulline by NO synthase, with the following rank order of potency: spermine > spermidine > putrescine. These inhibitory effects of the polyamines on [3H]L-citrulline formation were also observed in intact cultured cerebellar granule neurons upon stimulation of N-methyl-D-aspartate (NMDA) receptors. Evidence was obtained, however, that endogenous polyamines are not involved in regulation of NMDA-stimulated NO synthase activity. Thus, the observed inhibitory effects of exogenous polyamines might not reflect a physiological role in modulating NO generation in neurons.

Prolonged inhibition of brain nitric oxide synthase by short-term systemic administration of nitro-L-arginine methyl ester

We studied the dose-response characteristics and the temporal profile of inhibition of brain nitric oxide (NO) synthase (NOS) elicited by i.v. administration of the NOS inhibitor nitro-L-arginine methyl ester (L-NAME). L-NAME was administered i.v. in awake rats equipped with a venous cannula. L-NAME was injected in cumulative doses of 5, 10, 20 and 40 mg/kg and rats were sacrificed 30 min after the last dose. NOS catalytic activity was assayed in forebrain cytosol as the conversion of [3H]L-arginine into [3H]L-citrulline. L-NAME attenuated brain NOS activity in a dose-dependent manner but enzyme activity could not be inhibited by more than approximately 50%. After a single 20 mg/kg injection of L-NAME the inhibition of brain NOS activity was time dependent and reached a stable level at 2 hrs (52% of vehicle). Inhibition after a single injection was still present at 96 hrs, albeit to a lower magnitude. We conclude that intravenous administration of L-NAME in rats at concentrations commonly used in physiological experiments leads to a dose and time-dependent but partial inhibition of brain NOS catalytic activity. The finding that the inhibition persists for several days after a single administration is consistent with the hypothesis that nitro-L-arginine, the active principle of L-NAME, binds to NOS irreversibly.

An arginine residue conserved in most G protein-coupled receptors is essential for the function of the m1 muscarinic receptor

An exceptionally conserved sequence that is shared among most G protein-coupled neurotransmitter receptors is an aspartate-arginine-tyrosine triplet that is located at the amino terminus of the putative second cytoplasmic domain, where the arginine residue is invariant. Using the m1 subtype of muscarinic acetylcholine receptors as an example, we induced a point mutation of the arginine residue at position 123 into asparagine. This mutation resulted in a precipitous decrease in the coupling of m1 receptors to activation of phosphoinositide hydrolysis, in spite of the expression of the wild-type and mutant receptors at similar concentrations in Chinese hamster ovary cells. There were no significant effects on antagonist or partial agonist binding. In marked contrast, whereas binding of the full agonist carbachol to wild-type receptors exhibited high and low affinity components, this agonist bound to a single low affinity state in asparagine-123 mutant cells. Furthermore, agonist-induced enhancement of the specific binding of guanosine-5'-O-(3-[35S] thio)triphosphate was not observed in membranes of cells expressing the mutant receptor. A similar mutation in the m2 muscarinic receptor resulted in a significant but smaller decrease in its coupling to inhibition of cAMP formation. On the other hand, a point mutation of tyrosine-124 in the m1 receptor sequence produced less marked changes in agonist-induced phosphoinositide hydrolysis and no effects on agonist or antagonist binding to the receptor. Taken together, our data demonstrate for the first time that this highly conserved arginine residue plays an important role in coupling of muscarinic receptors to signal transduction mechanisms.

Inhibition of neuronal nitric oxide synthase by antipsychotic drugs

There is rapidly accumulating evidence that generation of nitric oxide (NO) through a Ca2+ and calmodulin-dependent pathway plays various important roles in the central nervous system. In the present study, effects of several antipsychotics on the activity of NO synthase were investigated in rat cerebellum and neuroblastoma N1E-115 cells, due to the known ability of these agents to inhibit calmodulin. In cytosolic preparations of rat cerebellum, the antipsychotic drugs inhibited the conversion of [3H]L-arginine into [3H]L-citrulline by NO synthase in a concentration-dependent manner. This inhibition was noncompetitive in nature, and it exhibited an excellent correlation with blockade of calmodulin activity. Furthermore, these drugs attenuated cyclic GMP formation induced by a calcium ionophore in N1E-115 cells, a response which takes place as a consequence of NO generation. Taken together, our data demonstrate that antipsychotic drugs inhibit NO formation in vitro. It is unlikely, however, that these actions might contribute to their therapeutic and/or side effects, since they take place at relatively high concentrations.

Efficient coupling of m5 muscarinic acetylcholine receptors to activation of nitric oxide synthase

The coupling of m5 muscarinic acetylcholine receptors to the generation and release of nitric oxide (NO) was investigated. Chinese hamster ovary cells, which stably express m5 receptors, were transiently transfected with the gene encoding neuronal NO synthase and used as a model system. Increased generation of NO upon stimulation of cells by muscarinic agonists was detected by an increase in cyclic GMP in admixed mouse neuroblastoma N1E-115 cells or more directly by measuring the conversion of L-arginine into L-citrulline. Carbachol increased cyclic GMP formation in the mixture of cells in a time- and concentration-dependent manner, with a half-maximal response occurring in the nanomolar range. This response was significantly attenuated by scavengers of NO or inhibitors of NO synthase. This high potency of carbachol was also observed in measurements of L-citrulline formation. A series of muscarinic agonists were as efficacious as carbachol in stimulating NO synthase, whereas McN-A-343 and pilocarpine were partial agonists in this regard. Evidence for an exceptionally high efficiency of coupling of m5 receptors to this response and its possible implication in the interaction between cholinergic and dopaminergic neurotransmission is discussed.

An artifact associated with using trypan blue exclusion to measure effects of amyloid beta on neuron viability

It is important to apply an appropriate test for determining cell viability, in order to properly evaluate the role of the amyloid beta protein in neuronal degeneration in Alzheimer's disease. In the current paper, we present evidence that the putative neurotoxic fragment 25-35 of amyloid beta causes loss of trypan blue exclusion in differentiated mouse neuroblastoma N1E-115 cells which suggests a potential neurotoxic effect. Surprisingly, no parallel changes in apparent cell viability were observed when fluorescein diacetate staining or release of lactate dehydrogenase were measured. Positive staining with trypan blue was also induced by incubating cell membranes prepared from N1E-115 cells or rat hippocampus with amyloid beta 25-35. Our results indicate that amyloid beta might induce trypan blue adsorption on the cell membrane. Therefore, caution should be taken when trypan blue exclusion is used in studies of the potential neurotoxicity of amyloid beta peptides.

Allosteric interaction of dynorphin and myelin basic protein with muscarinic receptors

Interaction of the basic peptides dynorphin A and myelin basic protein with muscarinic receptors was investigated in rat heart and cerebral cortex using radioligand receptor binding assays. Results showed that these peptides inhibit the binding of the muscarinic ligand [3H]N-methylscopolamine at equilibrium and alter the kinetics of ligand dissociation in an allosteric fashion. The number of basic amino acid residues in the composition of dynorphin A is important in eliciting its allosteric interactions. Our data suggest that endogenous basic peptides play a role in regulating the conformation of muscarinic receptors.

Role of intercellular and intracellular communication by nitric oxide in coupling of muscarinic receptors to activation of guanylate cyclase in neuronal cells

Muscarinic receptor-mediated cyclic GMP formation and release of nitric oxide (NO) (or a precursor thereof) were compared in mouse neuroblastoma N1E-115 cells. [3H]Cyclic GMP was assayed in cells prelabeled with [3H]guanine. Release of NO upon the addition of muscarinic agonists to unlabeled neuroblastoma cells (NO donor cells) was quantitated indirectly by its ability to increase the [3H]cyclic GMP level in labeled cells whose muscarinic receptors were inactivated by irreversible alkylation (NO detector cells). Carbachol increased NO release in a concentration-dependent manner, with half-maximal stimulation at 173 microM (compared to 96 microM for direct activation of cyclic GMP formation). The maximal effect of carbachol in stimulating release of NO when measured indirectly was lower than that in elevating [3H]cyclic GMP directly in donor cells. Hemoglobin was more effective in blocking the actions of released NO than in attenuating direct stimulation of [3H]cyclic GMP synthesis. There was a good correlation between the ability of a series of muscarinic agonists to release NO or to activate [3H]cyclic GMP formation directly, and the potency of pirenzepine in inhibiting the two responses. Furthermore, there was a similar magnitude of desensitization of both responses by prolonged receptor activation or stimulation of protein kinase C. NO release was also regulated in relation to the cellular growth phase. A model is proposed in which a fraction of NO generated upon receptor activation does not diffuse extracellularly and stimulates cyclic GMP synthesis within the same cell where it is formed (locally acting NO). The remainder of NO that is extruded extracellularly might travel to neighboring cells (neurotransmitter NO) or might be taken back into the cells of origin (homing NO).