Structure-activity relationships of imperialine derivatives and their anticholinergic activity

In order to check the structure-activity relationship and prepare more potent derivatives of imperialine with anticholinergic activity, imperialinol (2), 3 beta-acetoxyimperialine (3), 3 beta-propionoxyimperialine (4), and 3 beta-butyroxyimperialine (5) were prepared. Compounds 4 and 5 displayed better anticholinergic activity against muscarinic receptors of the heart and brain than imperialine (1). The decrease in activity in 2 showed the importance of the 6-keto functionality in imparting the anticholinergic activity.

Comparison of second-messenger responses to muscarinic receptor stimulation in M1-transfected A9 L cells

A9 L cells stable transfected with m1 muscarinic receptors were stimulated with the full agonist carbachol and with the partial agonist pilocarpine. The EC50 values and maximal activation from PI hydrolysis, calcium mobilization, membrane hyperpolarization, cell proliferation and arachidonic acid release were compared. Pilocarpine was approximately half as effective in eliciting PI hydrolysis, calcium mobilization and arachidonic acid release, but was almost as effective as carbachol in the other assays. These findings suggest that the intracellular signals leading to receptor-mediated cell proliferation and to membrane hyperpolarization are amplified, and that therefore these assays are not suitable for determining whether a compound is a partial agonist.

Modulation of adenylylcyclase by protein kinase C in human neurotumor SK-N-MC cells: evidence that the alpha isozyme mediates both potentiation and desensitization

Exposure of human SK-N-MC neurotumor cells to 4 beta-phorbol 12-myristate 13-acetate (PMA) increased isoproterenol stimulation of cyclic AMP levels by severalfold. This potentiation was blocked by inhibitors of protein kinase C (PKC) and did not occur in cells in which PKC had been down-regulated. PMA treatment also enhanced the stimulation by dopamine, cholera toxin, and forskolin. Thus, the effect of PMA on the adenylylcyclase system was postreceptor and involved either the guanine nucleotide binding regulatory (G) proteins or the cyclase itself. As PMA treatment did not impair the inhibition of isoproterenol stimulation by neuropeptide Y, an involvement of the inhibitory G protein Gi was unlikely. Cholate extracts of membranes from control and PMA-treated cells were equally effective in the reconstitution of adenylylcyclase activity in S49 cyc- membranes, which lack the stimulatory G protein subunit Gs alpha; thus, Gs did not appear to be the target of PMA action. Membranes from PMA-treated cells exhibited increased adenylylcyclase activity to all stimulators including Mn2+ and Mn2+ plus forskolin. In addition, activity was increased when control membranes were incubated with ATP and purified PKC from rat brain. This is consistent with a direct effect of PKC on the adenylylcyclase catalyst in SK-N-MC cells. PMA treatment also resulted in a shift to less sensitivity in the K(act) for isoproterenol but not for dopamine or CGP-12177 (a beta 3-adrenergic agonist) stimulation. Thus, the beta 1 but not the D1 or beta 3 receptors were being desensitized by PKC activation. Analysis of SK-N-MC cells by western blotting with antibodies against different PKC isozymes revealed that both the alpha and zeta isozymes were present in these cells. Whereas PKC-alpha was activated and translocated from cytosol to membrane by phorbol esters, the zeta isozyme was not. Thus, PKC-alpha, which has been implicated in desensitization in other cell lines, also appears to potentiate adenylylcyclase activity.

Muscarinic receptor-mediated increase in zeta-PKC expression in SK-N-SH human neuroblastoma cells

Anti-peptide antibodies specific for each protein kinase C (PKC) isozyme were used to screen SK-N-SH human neuroblastoma cells. These cells were found to express only alpha- and zeta-PKC. Stimulation of these cells with phorbol esters caused alpha- but not zeta-PKC to translocate from cytosolic to membrane fractions. Stimulation of these cells with carbachol, which releases inositol trisphosphate and diacylglycerol, caused a transient translocation of alpha-PKC but not of zeta-PKC. Carbachol did, however, cause a gradual increase in immunoreactive zeta-PKC which reached maximal values 10-20 min after stimulation. These results implicate zeta-PKC in a receptor-mediated signalling event.

3-alpha-Chlorimperialine: an M2-selective muscarinic receptor antagonist that penetrates into brain

Muscarinic M2 receptors have been found to be severely depleted in post-mortem brains of Alzheimer's patients. This loss of receptor may represent a useful diagnostic marker, if it could be quantitatively imaged with single-photon emission computed tomography (SPECT) or positron emission tomography (PET) imaging. In order to develop a radioligand with selectivity for muscarinic M2 receptors, we now report that 3-alpha-chlorimperialine is a potent M2 receptor antagonist with a Ki of 0.32 nM at M2 receptors, a 12-fold selectivity for M2 over M1 receptors, and a 5-fold selectivity for M2 over M4 receptors. Furthermore, 2% of the injected dose of 3-alpha-chlorimperialine per gram tissue penetrates into brain within 30 min, then washes out gradually. Taken together, these studies demonstrate that 3-alpha-chlorimperialine is a potent M2-selective muscarinic antagonist that penetrates into brain and may be a useful substrate for radioiodination and subsequent imaging of brain muscarinic M2 receptors.

Muscarinic receptor-mediated inhibition of mitogenesis via a protein kinase C-independent mechanism in M1-transfected A9 L cells

Previous studies have shown that activation of various PI-coupled receptors stimulates DNA synthesis in some cells, and inhibits DNA synthesis in others. In order to study this effect further, we measured carbachol-mediated [3H]thymidine incorporation in m1-transfected A9 L cells and in m1-transfected CHO cells, and found that carbachol profoundly inhibited DNA synthesis in both cell lines. This carbachol response was observed whether the cells were grown in the presence or in the absence of serum. The half-maximal value for carbachol-mediated inhibition of [3H]thymidine incorporation was 5.73 +/- 1.15 microM in m1-transfected A9 L cells. Pre-treatment of m1-transfected A9 L cells with the protein kinase C inhibitors staurosporine, sphingosine or phorbol 12-myristate 13-acetate-induced down-regulation did not prevent the carbachol inhibition of DNA synthesis, thereby demonstrating that this effect is not mediated via protein kinase C. Inhibition of [3H]thymidine incorporation was significant at 2 h, but not at 1 h of carbachol treatment. Recovery of [3H]thymidine incorporation following carbachol activation was also studied by blocking the receptor with atropine following carbachol stimulation. Following this treatment, cells needed at least 3 h to recover normal [3H]thymidine incorporation. Taken together, the kinetics of this response suggest that it is not mediated directly by a second messenger which typically has much more rapid kinetics.

A malignant melanoma imaging agent: synthesis, characterization, in vitro binding and biodistribution of iodine-125-(2-piperidinylaminoethyl)4-iodobenzamide

In order to develop improved radiopharmaceuticals for imaging malignant melanoma, we have synthesized and characterized 125I-and 131I-labeled (2-piperidinylaminoethyl)4-iodobenzamide (PAB). In vitro binding profiles of IPAB and N-(2-diethylaminoethyl)4-iodobenzamide (IDAB, a structurally related analog of IPAB) for a variety of neurotransmitter receptors suggested that both IPAB and IDAB possessed a high sigma-1 affinity and a low affinity for sigma-2 sites. In vitro homologous competition binding studies of [125I]PAB with human malignant melanoma cell A2058 showed that the tracer was bound to the cells with a high affinity (Ki = 6.0 nM) and that the binding was saturable. Biodistribution studies in nude mice implanted with human malignant melanoma xenografts showed good tumor uptake (3.87% ID/g at 1 hr, 2.91% ID/g at 6 hr and 1.02% ID/g at 24 hr) of [125I]PAB. High tumor-to-nontarget organ ratios were obtained at 24 hr postinjection. Tumor-to-blood, liver, muscle, lung, intestines, heart and brain ratios at 24 hr were 17.80, 3.88, 94.58, 14.29, 10.87, 37.07 and 90.01, respectively. Tumor imaging with [131I]PAB in a nude mice model xenografted with human malignant melanoma at 24 hr clearly delineated the tumor with very little activity in any other organ. These results demonstrate that sigma-1 receptors could be used as external markers for malignant melanoma.

Synthesis and structure-activity relationship of some 5-[[[(dialkylamino)alkyl]-1-piperidinyl]acetyl]-10,11-dihydro-5H- benzo[b,e][1,4]diazepin-11-ones as M2-selective antimuscarinics

A series of 5-[[[(dialkylamino)alkyl]-1-piperidinyl]acetyl]- 10,11-dihydro-5H-dibenzo[b,e][1,4]-diazepin-11-ones were prepared as potential M2-selective ligands. The compounds were evaluated for their affinity and selectivity for the muscarinic cholinergic receptor. The best M2-selective antimuscarinic agent studied is 5-[[4-[4-diethylamino)butyl]-1- piperidinyl]acetyl]-10,11-dihydro-5H-dibenzo[b,e][1,4]diazepin-11- one, which is approximately 10 times more potent at M2 receptors than previously known compounds such as 11-[[4-[4-(diethylamino)butyl]- 1-piperidinyl]acetyl]-5,11-dihydro-6H- pyrido[2,3-b][1,4]benzodiazepin-6-one (AQ-RA 741).

Use of ex vivo binding to measure the brain concentrations of putative radioligands

The development of radioligands capable of imaging brain receptors depends on, amongst other factors, the ability of such compounds to penetrate the blood-brain barrier. We describe an ex vivo binding technique for measuring the brain concentration of peripherally administered unlabeled compounds. This technique can be used early in the development of putative radioligands. The pharmacokinetics of brain penetration of three muscarinic antagonists are described: QNB, BrQNB and the 2-thienyl derivative of BrQNB and were found to compare favorably to previous studies using [3H]QNB. These studies demonstrate the effectiveness of ex vivo binding in assessing the brain concentration of peripherally administered unlabeled compounds.

A novel m2-selective muscarinic antagonist: binding characteristics and autoradiographic distribution in rat brain

Although several m2-selective muscarinic antagonists have been described, they are not particularly potent. Thus, the development of potent m2-selective compounds remains an important goal. We now report that a bio-isoster of AQ-RA 741 is both one order of magnitude more potent and slightly more selective than previously described compounds. DIBA, a di-benzo derivative of AQ-RA 741, in which the pyridine of the tricycle is replaced with a benzene ring, had Ki values of 4, 0.3, 11 and 2 nM at m1 through m4 receptors, respectively. These values were determined in competition studies with [3H]N-methylscopolamine ([3H]NMS) in membranes from transfected A9 L cells (m1 and m3), rat heart (m2) and NG108-15 cells (m4). AQ-RA 741 had Ki values of 34, 4, 86 and 15 nM at each of these receptors. The autoradiographic distribution of DIBA binding sites was determined by competition studies of [3H]NMS in rat brain. At low concentration, DIBA reduced [3H]NMS binding most significantly from superior colliculi, thalamus, hypothalamus, pontine nucleus, and interpeduncular nucleus, and not appreciably from caudate nucleus, cerebral cortical regions, or hippocampus, consistent with its binding to m2 receptors. These data indicate that DIBA is the most potent, m2-selective muscarinic antagonist yet described. DIBA should therefore become a useful probe in future studies of muscarinic function.

Calcium independence of phosphoinositide hydrolysis-induced increase in cyclic AMP accumulation in SK-N-SH human neuroblastoma cells

Previous work has shown that stimulation of muscarinic receptors in various cell lines increases intracellular cyclic AMP (cAMP) levels. This unusual response has been hypothesized to be mediated by stimulation of calcium/calmodulin-sensitive adenylate cyclase, secondary to inositol trisphosphate (IP3)-mediated calcium mobilization. To test this hypothesis, we stimulated muscarinic receptors in SK-N-SH human neuroblastoma cells while blocking the IP3-mediated rise in intracellular calcium concentration using two different methods. Loading cells with the intracellular calcium chelator 1,2-bis(o-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (BAPTA) abolished the carbachol-mediated intracellular calcium release without abolishing the carbachol-mediated increase in cAMP level. Similarly, in cells preexposed to carbachol, the agonist-induced change in intracellular calcium level was blocked, but the cAMP response was not. Thus, both of these methods failed to block the muscarinic receptor-mediated increase in cAMP level, thereby demonstrating that this cAMP level increase is not mediated by a detectable rise in intracellular calcium concentration.

Molecular probes for muscarinic receptors: derivatives of the M1-antagonist telenzepine

Functionalized congeners of the M1-selective muscarinic antagonist telenzepine (4,9-dihydro-3-methyl-4-[(4-methyl-1-piperazinyl)acetyl]-10H- thieno[3,4-b][1,5]benzodiazepin-10-one) were developed and found to bind to the receptor with affinities (Ki values) in approximately the nanomolar range. The derivatives contain a 10-aminodecyl group, which provides a nucleophilic functionality for further derivatization. The attachment of a spacer chain to the distal piperazinyl nitrogen was based on previous findings of enhanced affinity at muscarinic receptors in an analogous series of alkylamino derivatives of pirenzepine [J. Med. Chem. (1991) 34, 2133-2145]. The telenzepine derivatives contain prosthetic groups for radioiodination, protein cross-linking, photoaffinity labeling, and fluorescent labeling and biotin for avidin complexation. The affinity for muscarinic receptors in rat forebrain (mainly m1 subtype) was determined in competitive binding assays vs [3H]-N-methylscopolamine. A (p-aminophenyl)-acetyl derivative for photoaffinity labeling had a Ki value of 0.29 nM at forebrain muscarinic receptors (16-fold higher affinity than telenzepine). A biotin conjugate displayed a Ki value of 0.60 nM at m2-receptors and a 5-fold selectivity versus forebrain. The high affinity of these derivatives makes them suitable for the characterization of muscarinic receptors in pharmacological and spectroscopic studies, for peptide mapping, and for histochemical studies.

Muscarinic receptor subtype specificity of (N,N-dialkylamino)alkyl 2-cyclohexyl-2-phenylpropionates: cylexphenes (cyclohexyl-substituted aprophen analogues)

A series of aprophen [(N,N-diethylamino)ethyl 2,2-diphenylpropionate] analogues, called cylexphenes, were synthesized with alterations in (1) the chain length of the amine portion of the ester, (2) the alkyl groups on the amino alcohol, and (3) a cyclohexyl group replacing one of the phenyl rings. The antimuscarinic activities of these analogues were assessed in two pharmacological assays: the inhibition of acetylcholine-induced contraction of guinea pig ileum, and the blocking of carbachol-stimulated release of alpha-amylase from rat pancreatic acinar cells. These two tissues represent the M3(ileum) and M3(pancreas) muscarinic receptor subtypes. In addition, the analogues were also evaluated for their competitive inhibition of the binding of [3H]NMS to selected cell membranes, each containing only one of the m1, M2, m3, or M4 muscarinic receptor subtypes. The m1 and m3 receptors were stably transfected into A9 L cells. The replacement of one phenyl group of aprophen with a cyclohexyl group increased the selectivity of all the analogues for the pancreatic acinar muscarinic receptor subtype over the ileum subtype by more than 10-fold, with the (N,N-dimethylamino)propyl analogue exhibiting the greatest selectivity for the pancreas receptor subtype, over 30-fold. The cylexphenes also showed a decrease in potency in comparison to the parent compound when examined for the binding of [3H]NMS to the M2 subtype. In agreement with the pharmacological data obtained from the pancreas, the (N,N-dimethylamino)propyl cylexphene 3 demonstrated the greatest selectivity for the m3 subtype, and additionally showed a preference for the m1 and M4 receptor subtypes over the M2 receptor subtype in the binding assay. Thus, this compound showed a potent selectivity according to the pharmacological and binding assays between the muscarinic receptor subtypes of the pancreas and ileum. In both the pharmacological and binding assays, the potency of the analogues decreased markedly when the chain length and the bond distance between the carbonyl oxygen and protonated nitrogen were increased beyond three methylene groups. When the structures of these analogues were analyzed using a molecular modeling program, the bond distance between the carbonyl oxygen and protonated nitrogen was deduced to be more important for the antagonist activity than subtype specificity.

Differences in the functional responses of two cell lines each expressing Pi-hydrolysis-coupled muscarinic receptors

Fluorescent oxonol dyes were used to measure changes in the membrane potential of two different cell lines each expressing Pi-hydrolysis coupled muscarinic receptors. Both SK-N-SH human neuroblastoma cells and m1-transfected A9 L cells express muscarinic receptors which, when stimulated, elicit a large increase in intracellular calcium, and release of inositol phosphates. Despite the similarity in this second-messenger response, muscarinic stimulation resulted in a hyperpolarization in the transfected A9 L cells whereas a small depolarization was observed in the neuroblastoma cells. The carbachol-mediated hyperpolarization of the transfected A9 L cells could be mimicked by increasing intracellular calcium with the ionophore A23187, suggesting that it may be mediated by calcium-activated potassium channels. Exposure of SK-N-SH cells to A23187, on the other hand, had no effect on the membrane potential. These studies demonstrate that the activation of a second messenger system does not solely dictate the electrophysiological response of a cell, but that other factors such as the expression of ion-channels is critical in the determination of that response.

Penetration of adenosine antagonists into mouse brain as determined by ex vivo binding

The penetration of the adenosine antagonists 8-cyclopentyl-1,3-dimethylxanthine (CPT), 8-cyclopentyl-1,3-dipropylxanthine (CPX), 8-(p-sulfophenyltheophylline (8-PST), and 8-[4-[[[[(2-amino-ethyl)amino]carbonyl]methyl]oxy]phenyl]- 1,3-dipropylxanthine (XAC) into mouse brain was determined using ex vivo binding and locomotor studies. CPT and CPX (25 and 0.25 mg/kg, respectively) both penetrated into brain in substantial amounts: 49 and 17% of theoretical levels assuming free penetration throughout the body, 10 min after i.p. injection, respectively. Brain levels of CPT decreased rapidly, declining to undetectable levels by 30 min post-injection, whereas levels of CPX declined much more slowly. As expected, no detectable brain levels of 8-PST were found following i.p. injection of 50 mg/kg. XAC (20 mg/kg) penetrated into brain poorly: 1.6% after 10 min and 3.2% 20 min post-injection. The ability of CPT to stimulate locomotor activity paralleled the brain levels, i.e. it was similar to theophylline at short times and the effect rapidly diminished. These studies demonstrate the usefulness of ex vivo binding in determining CNS penetration of adenosine receptor ligands.

High affinity acylating antagonists for muscarinic receptors

The muscarinic antagonists pirenzepine and telenzepine were derivatized as alkylamino derivatives at a site on the molecules corresponding to a region of bulk tolerance in receptor binding. The distal primary amino groups were coupled to the cross-linking reagent meta-phenylene diisothiocyanate, resulting in two isothiocyanate derivatives that were found to inhibit muscarinic receptors irreversibly and in a dose-dependent fashion. Preincubation of rat forebrain membranes with an isothiocyanate derivative followed by radioligand binding using [3H]N-methylscopolamine diminished the Bmax value, but did not affect the Kd value. The receptor binding site was not restored upon repeated washing, indicating that irreversible inhibition had occurred. IC50 values for the irreversible inhibition at rat forebrain muscarinic receptors were 0.15 nM and 0.19 nM, for derivatives of pirenzepine and telenzepine, respectively. The isothiocyanate derivative of pirenzepine was non-selective as an irreversible muscarinic inhibitor, and the corresponding derivative prepared from telenzepine was 5-fold selective for forebrain (mainly m1) vs. heart (m2) muscarinic receptors.

Agents that stimulate phosphoinositide turnover also elevate cAMP in SK-N-SH human neuroblastoma cells

Stimulation of m1 and of m3 muscarinic receptors has previously been shown to increase intracellular cAMP levels in a variety of cells. Although the mechanism underlying this response is not fully understood, it has been hypothesized to be secondary to the IP3-mediated rise in intracellular calcium. In order to determine whether other means of elevating intracellular calcium also raise cAMP levels, we stimulated SK-N-SH human neuroblastoma cells with bradykinin or with maitotoxin. Both of these agents stimulated phospholipase C, stimulated inositol phosphate release and elevated cAMP levels, thus demonstrating that this cAMP response is not unique to muscarinic receptor stimulation.

Binding of radioiodinated SPECT ligands to transfected cell membranes expressing single muscarinic receptor subtypes

The equilibrium dissociation constant and the kinetic rate constants were determined for the binding of (R)-[3H]3-quinuclidinyl benzilate ([3H]QNB) and [125I]3-quinuclidinyl-4-iodobenzilate ((R,R)- and (R,S)-[125I]IQNB) to transfected cell membranes expressing one single muscarinic acetylcholine receptor (mAChR) subtype. The association and dissociation kinetics for the m2 subtype were more rapid than for the m1 and m3 subtypes. The differential kinetic properties may be useful for the single photon emission computed tomographic (SPECT) evaluation of regional mAChR subtype alterations in disease states.

Functionalized congener approach to muscarinic antagonists: analogues of pirenzepine

The M1-selective muscarinic receptor antagonist pirenzepine 6H-pyrido[2,3-b][1,4]benzodiazepin-6-one) was derivatized to explore points of attachment of functionalized side chains for the synthesis of receptor probes and ligands for affinity chromatography. The analogues prepared were evaluated in competitive binding assays versus [3H]-N-methylscopolamine at four muscarinic receptor subtypes (m1AChR-m4AChR) in membranes from rat heart tissue and transfected A9L cells. 9-(Hydroxymethyl)pirenzepine, 8-(methylthio)pirenzepine, and a series of 8-aminosulfonyl derivatives were synthesized. Several 5-substituted analogues of pirenzepine also were prepared. An alternate series of analogues substituted on the 4-position of the piperazine ring was prepared by reaction of 4-desmethylpirenzepine with various electrophiles. An N-chloroethyl analogue of pirenzepine was shown to form a reactive aziridine species in aqueous buffer yet failed to affinity label muscarinic receptors. Within a series of aminoalkyl analogues, the affinity increased as the length of the alkyl chain increased. Shorter chain analogues were generally much less potent than pirenzepine, and longer analogues (7-10 carbons) were roughly as potent as pirenzepine at m1 receptors, but were nonselective. Depending on the methylene chain length, acylation or alkyl substitution of the terminal amine also influenced the affinity at muscarinic receptors.

Muscarinic receptor binding and activation of second messengers by substituted N-methyl-N-[4-(1-azacycloalkyl)-2-butynyl]acetamides

A series of substituted azacycloalkyl analogues of the muscarinic agonist UH 5 (N-methyl-N-[4-(1-pyrrolidinyl)-2-butynyl]acetamide, 1a) were synthesized and evaluated pharmacologically. These compounds were developed as intermediates for further derivatization leading to functionalized congeners of 1a. The compounds were synthesized by using a Mannich-type condensation of N-acetyl-N-methylpropargylamine to various substituted saturated azaheterocycles. The compounds were screened at a single concentration in competitive binding assays in rat cerebral cortical membranes against either [3H]N-methylscopolamine (at 100 microM) or [3H]oxotremorine-M (at 1 microM) labels. Candidates were then selected for further evaluation of their effect on phosphoinositide (PI) turnover in membranes from A9L cells transfected with cDNA of either m1-muscarinic cholinergic receptors (m1AChRs) or m3AChRs. The analogues were also tested for the inhibition of adenylate cyclase in NG108-15 cells expressing m4AChRs. The azetidine analogue of 1a had a Ki value of 12 nM for the inhibition of [3H]oxotremorine-M binding in rat brain and had an agonist potency at m1-,m3-, and m4AChRs comparable to 1a. The substituted 5- and 6-member ring analogues generally had lower binding affinities and were less potent than 1a in stimulating PI turnover. Several compounds were moderately effective in inhibiting cyclic AMP production in NG108-15 cells.

Muscarinic receptor subtype selectivity of novel heterocyclic QNB analogues

In an effort at synthesizing centrally-active subtype-selective antimuscarinic agents, we derivatized QNB (quinuclidinyl benzilate), a potent muscarinic antagonist, by replacing one of the phenyl groups with less lipophilic heterocyclic moieties. The displacement of [3H]-N-methyl scopolamine binding by these novel compounds to membranes from cells expressing m1-m4 receptor subtypes was determined. Most of the novel 4-bromo-QNB analogues were potent and slightly selective for m1 receptors. The 2-thienyl derivative was the most potent, exhibiting a 2-fold greater potency than BrQNB at m1 receptors, and a 4-fold greater potency at m2 receptors. This compound was also considerably less lipophilic than BrQNB as determined from its retention time on C18 reverse phase HPLC. This compound may therefore be useful both for pharmacological studies and as a candidate for a radioiodinated SPECT imaging agent for ml muscarinic receptors in human brain.

Functionalized congener approach for the design of novel muscarinic agents. Synthesis and pharmacological evaluation of N-methyl-N-[4-(1-pyrrolidinyl)-2-butynyl] amides

A functionalized congener approach was used to design ligands for muscarinic cholinergic receptors (mAChRs). A series of omega-functionalized alkyl amides of N-methyl-4-(1-pyrrolidinyl)-2-butynamine (22) were prepared as functionalized analogues of UH 5 [N-methyl-N-[4-(1-pyrrolidinyl)-2-butynyl]acetamide], a muscarinic agonist related to oxotremorine. Intermediate 22 was coupled to a series of Boc-protected omega-amino acids, and the resulting amides were deprotected and acylated. Intermediate 22 was also acylated with succinic anhydride and derivatized. The synthetic intermediates and final compounds were evaluated in vitro for their effects on the turnover of phosphatidylinositides in SK-N-SH human neuroblastoma cells that express m3AChRs, and on the production of cyclic AMP in NG108-15 neuroblastoma x glioma cells that express only m4AChRs. The displacement of [3H]-N-methylscopolamine was also measured in membrane preparations from each of these cell lines. Conjugates of glycine and beta-alanine were agonists at m4AChRs, having little or no activity at m3AChRs. The potency in displacement of [3H]-N-methylscopolamine from both m3- and m4AChRs generally increased with increasing chain lengths of the omega-aminoalkyl congeners. The amides of 7-aminoheptanoic acid and 8-aminooctanoic acid, and their Boc-protected derivatives, had comparable affinities to UH 5 (Ki = 5.0 and 4.5 microM at m3AChRs and at m4AChRs, respectively) at both receptors but lacked any agonist effects.

Relationship between desensitization and sequestration of muscarinic cholinergic receptors in two neuronal cell lines

Agonist-induced sequestration and desensitization of muscarinic receptors was studied in two cell lines that each express differentially-coupled receptors. The NG108-15 glioma x neuroblastoma hybrid cells have muscarinic receptors coupled only to the inhibition of adenylate cyclase, whereas SK-N-SH human neuroblastoma cells have muscarinic receptors coupled only to the turnover of phosphoinositide. In both NG108-15 and in SK-N-SH cells, muscarinic agonists caused a rapid, temperature-dependent, loss of binding sites for [3H] N-methylscopolamine. In contrast, the density of binding sites for [3H] quinuclidinyl benzilate remained almost unchanged after treatment with carbachol. Since carbachol also caused a redistribution of muscarinic receptors from the plasma membrane to a lighter membrane fraction, the loss of binding sites for [3H] N-methyl-scopolamine was interpreted as being due to sequestration of receptors. In addition to agonist-induced sequestration, muscarinic agonists also caused desensitization of the receptor-mediated response in NG108-15 cells. In SK-N-SH cells, however, no such desensitization of the receptor-mediated response was found, despite the agonist-induced sequestration of receptors. These findings demonstrate that agonist-induced sequestration of receptors does not always lead to desensitization of receptors.

An agonist that is selective for adenylate cyclase-coupled muscarinic receptors

Compound BM5 [N-methyl-N(1-methyl-4-pyrrolidino-2-butynyl) acetamide] has previously been described as an agonist at postsynaptic muscarinic receptors and as an antagonist at presynaptic receptors. In the current work, we studied the ability of this compound to selectively stimulate phosphoinositide (PI) turnover in Chinese hamster ovary cells transfected with m1 muscarinic receptors and in SK-N-SH neuroblastoma cells that express only m3 receptors. We also studied the ability of this compound to stimulate adenylate cyclase inhibition in m2 muscarinic receptors from heart tissue and in m4 receptors expressed in NG108-15 cells. BM5 stimulated the two muscarinic receptor subtypes coupled to adenylate cyclase inhibition. In NG108-15 cells, 100 microM BM5 inhibited prostaglandin E1-stimulated cAMP formation by 36 +/- 1.5%, whereas 100 microM of the full agonist oxotremorine-M inhibited cAMP formation by 64.1 +/- 1.9%. The half-maximal concentration for BM5 inhibition of cAMP formation was 0.4 +/- 0.1 microM. In heart membranes, BM5 inhibited isoproterenol-stimulated adenylate cyclase by 24 +/- 2%, whereas oxotremorine inhibited this activity by 34 +/- 3%. In contrast to its activity at these receptor subtypes, BM5 did not stimulate the m1 or m3 receptor subtypes, which couple to PI turnover. In these latter two subtypes, BM5 inhibited oxotremorine-M-stimulated PI turnover with IC50 values of 10-20 microM. Therefore, BM5 is a partial agonist at adenylate cyclase-coupled muscarinic receptor subtypes and is a pure antagonist at PI turnover-coupled muscarinic receptor subtypes. These studies also suggest that, at least in some parts of the brain, postsynaptic muscarinic receptors are coupled to adenylate cyclase, whereas presynaptic muscarinic receptors are coupled to PI turnover.

Calcium influx mediated by nicotinic receptors and voltage sensitive calcium channels in SK-N-SH human neuroblastoma cells

When SK-N-SH human neuroblastoma cells were exposed to nicotine (NIC) or KCl they showed a dose-dependent transient increase (2- to 4-fold) in intracellular Ca2+ concentration ([Ca2+])i as detected by quin-2 fluorescence, with half maximal effects (EC50) observed at 13 microM and 26 mM, respectively. Tubocurarine and 1-isodihydrohistrionicotoxin potently blocked the NIC-evoked (IC50 congruent to 1 microM and 0.3 microM, respectively), but not the high [K+]o-evoked [Ca2+]i accumulation. The KCl-induced response was inhibited by verapamil and diltiazem (IC50 = 1.4 and 10.9 microM, respectively). Tetrodotoxin (3 microM) and tetraethylammonium (10 microM) had no effect on [Ca2+]i accumulation induced by either agent. Increases in [Ca2+]i could be evoked sequentially by NIC and KCl in the same cells suggesting independent mechanisms of Ca2+ entry. In a Ca2+-free medium, no response to either KCl or NIC was observed. However, when Ca2+ ions were restored, [Ca2+]i accumulation was enhanced to the same extent as cells suspended in a Ca2+-containing buffer. Long-term (18 hr) pretreatment of SK-N-SH cells with pertussis (100 ng/ml) or cholera toxins (10 nM) had no effect on NIC or KCl-induced [Ca2+]i accumulation. Together, these data demonstrate the presence of NIC receptors and voltage-sensitive Ca2+ channels on SK-N-SH neuroblastoma cells, through which [Ca2+]i may be modulated.

Pharmacological differences between muscarinic receptors coupled to phosphoinositide turnover and those coupled to adenylate cyclase inhibition

Pharmacological differences between muscarinic cholinergic receptors coupled to phosphoinositide turnover and those coupled to adenylate cyclase were studied. Stimulation of muscarinic receptors from SK-N-SH human neuroblastoma cells resulted in phosphoinositide hydrolysis, but not in inhibition of cAMP formation. As has been shown previously, stimulation of muscarinic receptors from NG108-15 neuroblastoma x glioma cells, on the other hand, resulted in inhibition of cAMP formation without any observable phosphoinositide hydrolysis. These two cell lines provide a useful model system in which to study differential coupling of muscarinic cholinergic receptors. Inhibition of [3H]N-methyl scopolamine [( 3H]NMS) binding and inhibition of carbachol-stimulated function by the antagonists pirenzepine, AF-DX 116, and 4-diphenylacetoxy-N-methylpiperidine methiodide (4-DAMP) were studied in this system. Pirenzepine inhibited [3H]NMS binding in both cell lines with low affinity (Ki of 130 and 160 nM in NG108-15 and SK-N-SH cells respectively), indicating that both cell lines express M2 receptors. None of the three antagonists studied exhibited any clear selectivity for the receptors in one cell line over those of the other. In contrast, several agonists including acetylcholine, bethanechol and carbachol exhibited pronounced selectivity. These agonists inhibited [3H]NMS binding to membranes from SK-N-SH cells with IC50 values that were 17-, 3- and 38-fold higher, respectively, than those of NG108-15 cells. This selectivity was still observed when whole cells rather than membranes were studied. These findings indicate that pharmacological differences between receptors coupled to phosphoinositide turnover and those coupled to cAMP inhibition can be detected with certain agonists, but not with the antagonists pirenzepine, AF-DX 116 or 4-DAMP.

Muscarinic receptor-mediated increase in cAMP levels in SK-N-SH human neuroblastoma cells

Stimulation of muscarinic cholinergic receptors in SK-N-SH human neuroblastoma cells resulted in a 1.5-4 fold increase in intracellular cAMP levels. This unusual response was sensitive to atropine and pirenzepine but insensitive to pertussis toxin. It was observable regardless of whether basal, PGE1- or forskolin-stimulated cAMP levels were measured. The half-maximal concentration for carbachol-stimulation of cAMP levels (6 microM) was similar to that for the previously determined carbachol-induced stimulation of phosphoinositide turnover in these cells, suggesting that the former is mediated by the latter. These data indicate that cross-talk between the phosphoinositide turnover system and the adenylate cyclase system results in increased cAMP levels in SK-N-SH cells in response to muscarinic receptor stimulation.

Muscarinic receptors in brain from four strains of inbred mice

The density of brain muscarinic receptors from four strains of inbred mice was determined. C57BL/6J mice had a significantly higher density of muscarinic receptors in the forebrain than Balb/cJ or C57BL/10J mice. In the midbrain, C57BL/6J mice also had the highest density of receptors and in the hindbrain, C57BL/6J and AKR/J mice had a two fold higher receptor density compared to the other two strains. These findings demonstrate that inbred strains of mice which exhibit a range of genetically-determined behaviors, have varying densities of muscarinic receptors.

Development of sodium channel protein during chemically induced differentiation of neuroblastoma cells

We have previously shown that the [3H]saxitoxin binding site of the sodium channel is expressed independently of the [125I]scorpion toxin binding site in chick muscle cultures and in rat brain. In the present work, we studied the development of the sodium channel protein during chemically induced differentiation of N1E-115 neuroblastoma cells, using [3H]saxitoxin binding, [125I]scorpion toxin binding, and 22Na uptake techniques. When grown in their normal culture medium, these cells are mostly undifferentiated, bind 90 +/- 10 fmol of [3H]saxitoxin/mg of protein and 112 +/- 14 fmol of [125I]scorpion toxin/mg protein, and, when stimulated with scorpion toxin and batrachotoxin, take up 70 +/- 5 nmol of 22Na/min/mg of protein. Cells treated with dimethyl sulfoxide (DMSO) or hexamethylene-bis-acetamide (HMBA) differentiate morphologically within 3 days. At this time, the [3H]saxitoxin binding, the [125I]scorpion toxin binding, and the 22Na uptake values are not very different from those of undifferentiated cells. With subsequent time in DMSO or HMBA, these values continue to increase, a result indicating that the main period of sodium channel expression occurs well after the cells have assumed the morphologically differentiated state. The data indicate that the expression of sodium channels and morphological differentiation are independently regulated neuronal properties, that the attainment of morphological differentiation is necessary but not in itself sufficient for full expression of the sodium channel proteins, and that, in contrast to the chick muscle cultures and rat brain, the [3H]saxitoxin site and [125I]scorpion toxin site appear to be coregulated in N1E-115 cells.

Loss of pirenzepine regional selectivity following solubilization and partial purification of the putative M1 and M2 muscarinic receptor subtypes

Pirenzepine inhibition of [3H]N-methylscopolamine ([3H]NMS) binding was studied in membranous, digitonin-solubilized, and partially purified muscarinic receptors from bovine cortex, an area of the brain rich in the putative M1 muscarinic receptor subtype, and from pons-medulla, an area rich in the putative M2 subtype. In accord with previous work, we found that pirenzepine bound to membranous receptors from cortex with an IC50 that was over one order of magnitude lower than to receptors from pons-medulla. After digitonin solubilization, however, this regional selectivity was significantly reduced. In receptors from pons-medulla, the IC50 for pirenzepine inhibition of [3H]NMS was reduced from 2.1 +/- 0.7 X 10(-6) M in membrane-bound receptors, to 4.3 +/- 0.3 X 10(-7) M after solubilization, whereas in receptors from cortex, the IC50 remained unchanged after solubilization. The solubilized receptors from both brain areas maintained their binding characteristics after partial purification over an ABT-Sepharose affinity column and a hydroxylapetite column. These findings raise the possibility that the different pirenzepine binding characteristics used to define M1 and M2 receptor subtypes are not inherent in the receptor protein itself, but may be due to coupling factors such as effector proteins, phospholipids or cytoskeletal proteins which could be associated with the membranous receptor and become dissociated from the receptor after solubilization.

Effects of verapamil on the binding characteristics of muscarinic receptor subtypes

The calcium channel antagonist verapamil inhibited [3H]N-methyl scopolamine ([3H]NMS ) binding to muscarinic receptors from both rat brain cortex, an area rich in M1 receptors, and from pons-medulla, an area rich in M2 receptors. Verapamil reduced the dissociation kinetics of [3H]NMS binding in cortical receptors but had no detectable effects on the dissociation kinetics of receptors from the pons-medulla. These data suggest that receptors from the cortex have an allosteric site which regulates the binding kinetics, whereas receptors from the pons-medulla lack this site.

Changes in the ratio of two separate toxin binding sites on the sodium channel protein during rat brain development

The development of two separate toxin binding sites from sodium channels of rat brain was studied. Whereas [3H]saxitoxin binding developed gradually, reaching adult levels by 4.5 weeks after birth, the binding of [125I]scorpion toxin increased rapidly for the first 10 days after birth, then declined. The stoichiometric ratio of [3H]saxitoxin binding site to [125I]scorpion toxin binding sites changed during development from 0.5 in 8 day old rats to 1.9 in adult rats, implying separate biological regulation for each of these sites.

Appearance of [3H]saxitoxin binding sites in developing rat brain

The binding of [3H]saxitoxin to a crude membrane fraction from rat brains of various ages was determined. We found that at two days before birth, the maximum number of binding sites is 100 fmol/mg of membrane protein. This value increases rapidly for the two weeks following birth, eventually reaching a plateau value of about 800 fmol of toxin sites per mg protein. Since published reports indicate that rat brain myelination occurs primarily between days 15 and 21, we conclude that the development of sodium channels, as measured by [3H]saxitoxin binding, precedes myelination.

Development of sodium channels during differentiation of chick skeletal muscle in culture. I. Binding studies

[125I]Scorpion toxin and [3H]saxitoxin, two neurotoxins that bind specifically to different sites on sodium channels, were used to monitor the development of sodium channels during synchronous differentiation of chick skeletal muscle in culture. [3H]Saxitoxin-binding sites are not detectable in myoblasts. They appear only after myoblasts fuse to form myotubes and increase gradually to reach a maximal value of 58 fmol/mg of protein by 8 days in culture. In contrast, [125I]scorpion toxin-binding sites are first detected during the period of cell fusion and increase after fusion to reach a maximum by 4 days in culture, well before either the maximal value of the [3H]saxitoxin binding or the appearance of fully developed sodium action potentials. Neither [3H]saxitoxin binding nor [125I] scorpion toxin binding affinity changes during development; they have values of 1.4 and 62 nM, respectively. These data, and data described in the companion paper (Baumgold, J., J. B. Parent, and I. Spector (1983) J. Neurosci. 3: 1004-1013), suggest that sodium channel development involves two sequential steps: incorporation into the cell membrane of an immature form of the sodium channel capable of binding scorpion toxin alone, followed by the acquisition of the ability to bind saxitoxin and thus to become a mature and functional channel.

Development of sodium channels during differentiation of chick skeletal muscle in culture. II. 22Na+ uptake and electrophysiological studies

The development of the functional properties of the sodium channel of chick skeletal muscle grown in culture was studied using 22Na uptake and electrophysiological techniques. In accord with the biochemical data in the preceding paper (Baumgold, J., J. B. Parent, and I. Spector (1983) J. Neurosci. 3: 995-1003), the functional manifestations of sodium channel expression are initiated shortly after cell fusion. At this stage, the myotubes have barely detectable sodium-dependent action potentials (Vmax = 1 to 9 V/sec) and exhibit a very small amount of batrachotoxin (BTX)-stimulated 22Na+ uptake. However, when these cultures are treated with scorpion toxin (ScTX), the amplitude and rate of rise of the sodium action potential increase dramatically (Vmax = 35 to 50 V/sec) and the (BTX)-stimulated 22Na+ uptake is much larger, suggesting that ScTX unmasks channels that are already present but nonfunctional in these immature myotubes. The two different rates of development of the biochemical properties of the sodium channel described in the preceding paper are also reflected in the two separate rates of development of its functional properties. In the absence of ScTX, the amplitude and Vmax of the action potential develop at a slow rate similar to that of the [3H]saxitoxin binding, eventually reaching a Vmax of 158 V/sec by day 10; the BTX-stimulated 22Na+ uptake also rises gradually, reaching 12 nmol of 22Na+/culture/min by day 7.5. In contrast, in the presence of ScTX, the Vmax of the Na+ action potential increases more rapidly, reaching 158 V/sec by day 5 and 220 V/sec by day 10. The BTX-stimulated 22Na+ uptake also increases more rapidly in the presence of ScTX. This rapid rate of development is very similar to that for [125I]ScTX binding. These findings and those in the preceding paper suggest the existence of two types of Na+ channels: an immature, nonfunctional channel capable of binding [125I]ScTX alone, and a mature, functional channel capable of binding both [125I]ScTX and [3H]saxitoxin. They further suggest that the insertion of the immature form of the channel protein into the cell membrane shortly after cell fusion is the first event in the expression of the sodium channel. During development, the sodium channel undergoes a structural change which renders it functional. The possibility that both the appearance of functional sodium channels during development and the rapid induction of functional channels by ScTX in immature myotubes reflect a post-translational modification or aggregation of immature nonfunctional channels is discussed.

Multiple-rate components of axonally transported proteins in the hypothalamo-neurohypophysial system of the rat

The transport of labeled proteins from the hypothalamus to the neurohypophysis following 35S-methionine injection into the rat supraoptic nucleus was studied using a unique approach adapted for the study of short-axon systems. Multiple-rate components to those found in other neuronal systems were demonstrated. Neurosecretory vesicle-containing proteins (e.g., neurophysins) were transported at fast rates (greater than 120 mm/day), whereas the cytoskeletal protein, actin, moved principally in the slow component of transport. Two-dimensional gel electrophoresis was used to analyze the diverse patterns of labeled proteins found in the various rate components of axonal transport in this system.

Membrane-associated cytoskeletal proteins in squid giant axons

Cytoskeletal proteins (e.g., tubulin, actin, and neurofilament proteins) in the squid giant axon are separable into KF-soluble and -insoluble forms. The KF-insoluble cytoskeletal components appear to constitute the major proteins in the subaxolemmal fibrous network on the inner surface of the axon. These cytoskeletal proteins and the subaxolemmal network are both highly soluble in KI solutions. Whereas giant axons tolerate prolonged perfusions in KF solutions with no loss of excitable properties, a relatively short perfusion with KI solution completely eliminates the excitability of the axon. The loss of this excitability correlates with the simultaneous dissolution of the subaxolemmal network of cytoskeletal proteins and the release of its proteins into the perfusate. These data support the hypothesis that cytoskeletal proteins associated with the inner surface of the axolemma are involved in the regulation of axonal excitability.

Calcium ion binding to lobster nerve membranes

The binding of 45Ca2+ to membrane material isolated from lobster walking leg nerves was studied using a rapid filtration technique. In solutions of high ionic strength (450 mM), the amount of 45Ca2+ bound to this membrane material was found to be highly dependent on the monovalent cation used in the incubating solution. The amount of 45Ca2+ bound was larger when the membranes were incubated in a KCl solution compared to when they were incubated in a NaCl solution. This difference was attributed to the ability of these closed membrane vesicles to accumulate Ca2+ into the vesicle when incubating in a KCl solution but not in a NaCl solution. This accumulation of Ca2+ was found to be independent of metabolic energy and depended primarily on the absence of Na+ from the incubation medium. At low ionic strength, the membranes formed open fragments and the amount of Ca2+ bound was no longer sensitive to the monovalent cation species in the incubation solution. The 45Ca2+ bound under these low ionic strength conditions was considered to be bound to anionic sites on the membranes.

Protein release from the internal surface of the squid giant axon membrane during excitation and potassium depolarization

The proteins in the perfusate collected from intracellularly perfused squid giant axons were analyzed after being labeled with radioactive 125-I-labeled Bolton-Hunter reagent. The rate of protein release into the perfusate was found to be increased by the following electrophysiological manipulations of the axons: (1) repetitive electrical stimulation at 60 Hz in axons perfused with normal potassium fluoride-containing solution or at 0.125 Hz in axons perfused with tetraethylammonium containing solution, (2) perfusion with 4-aminopyridine solution which induces spontaneous electrical activity in the axon, and (3) depolarization of the axon induced by raising the external potassium concentration. Sodium dodecyl sulfate polyacrylamide gel electrophoresis of the proteins released under these conditions yielded molecular weight profiles different from those of the extruded axoplasmic proteins. These observations indicate that there exists, in close association with the axonal membrane, aparticular group of proteins, the solubility of which is readily affected by changes in the state of the membrane.

An approach to the study of intracellular proteins related to the excitability of the squid giant axon

The technique for covalently labeling proteins with 125I-labelled Bolton-Hunter reagent was used to determine the quantities of proteins released from the axoplasmic side of the squid axon membrane. The reagent could be introduced into the interior of the axon by the technique of intracellular perfusion, the radioiodination reaction being carried out in situ. Alternatively, the reaction could be carried out in vitro, i.e., by mixing the reagent with samples of proteins dissolved in the intracellular perfusion fluid collected from the axon. This technique was found to be sensitive enough to permit analysis of a large number of protein samples collected from a single axon. By the method of sodium dodecyl sulfate polyacrylamide gel electrophoresis, it was found that proteins of approx. 56 000 daltons were released into the perfusate when a solution of potassium chloride or potassium bromide was introduced into the interior of an axon. Suppression of axonal excitability was associated with this release of proteins. The significance of these findings in relation to the structure and function of the axon is discussed.

Studies on the relationship of binding affinity to psychoactive and anticholingergic potency of a group of psychotomimetic glycolates

A study was undertaken of the possible relationship of the binding affinity of a series of anticholinergic psychotomimetic drugs to their psychopharmacological and anticholinergic effects. Binding was measured to brain homogenates and nerve endings using [3H]quinuclidinyl benzilate (QB), a highly potent psychotomimetic agent presumably affecting muscarinic sites in the brain. The two stereoisomers of QB were compared; and although the L-isomer had 15 times the binding affinity of D-isomer, the L-isomer had over 200 times the psychopharmacological potency of the D-form. When the relative binding affinity of a homologous series of glycolate esters was compared with their relative psychoacitve potency, the correlation was excellent; however, when compounds with heterocyclic amino rings (e.g., tropanol) other than quinuclidine and piperidine were considered, the correlation was poor. The correlation between binding affinity and antagonism to acetylcholine-induced contraction of ileum was more consistent. A study was undertaken on the effect of added lipids on QB binding to nerve endings, and it was found that phosphatidyl serine had a significant enhancing effect while gangliosides were inhibitory.