Characterization and subcellular localization of brain muscarinic receptors labelled in vivo by [3H]dexetimide

Dynamic interaction between sigma-1 receptor and Kv1.2 shapes neuronal and behavioral responses to cocaine

The sigma-1 receptor (Sig-1R), an endoplasmic reticulum (ER) chaperone protein, is an interorganelle signaling modulator that potentially plays a role in drug-seeking behaviors. However, the brain site of action and underlying cellular mechanisms remain unidentified. We found that cocaine exposure triggers a Sig-1R-dependent upregulation of D-type K(+) current in the nucleus accumbens (NAc) that results in neuronal hypoactivity and thereby enhances behavioral cocaine response. Combining ex vivo and in vitro studies, we demonstrated that this neuroadaptation is caused by a persistent protein-protein association between Sig-1Rs and Kv1.2 channels, a phenomenon that is associated to a redistribution of both proteins from intracellular compartments to the plasma membrane. In conclusion, the dynamic Sig-1R-Kv1.2 complex represents a mechanism that shapes neuronal and behavioral response to cocaine. Functional consequences of Sig-1R binding to K(+) channels may have implications for other chronic diseases where maladaptive intrinsic plasticity and Sig-1Rs are engaged.

Specific binding component of the "inactive" stereoisomer (S,S)-[125I] IQNB to rat brain muscarinic receptors in vivo

In vivo nonspecific binding can be estimated using the inactive stereoisomer of a receptor radioligand. However, the binding of the inactive stereoisomer may be partially specific. Specific binding of the inactive (S,S)-[125I]IQNB was estimated from the inhibition induced by a competing nonradioactive ligand. This technique differed from the usual approach, since it was used to study the inactive rather than the active stereoisomer. The results indicate that there is substantial specific binding for (S,S)-[125I]IQNB.

Pharmacological characterization and positron emission tomography evaluation of 4-[76Br]bromodexetimide and 4-[76Br]bromolevetimide for investigations of central muscarinic cholinergic receptors

4-[76Br]bromodexetimide and its inactive enantiomer 4-[76Br]bromolevetimide were prepared via electrophilic bromodesilylation using chloramine-T and no-carrier-added (NCA) [76Br]NH4. In vitro, Bmax measured on rat cortex membranes were 3.7 +/- 0.2 and < 0.07 pmol/mg protein for 4-[76Br]bromodexetimide and 4-[76Br]bromolevetimide, respectively. The kD of 4-[76Br]bromodexetimide was 1.9 +/- 0.3 nM. In vivo studies in rats showed specific uptake of 4-[76Br]bromodexetimide in cortex, striatum, thalamus and hippocampus. No specific uptake was observed with 4-[76Br]bromolevetimide. With [76Br]bromodexetimide, positron emission tomography (PET) studies in primates demonstrated a preferential accumulation of the radioactivity in the cortex and striatum which was displaced to the level of cerebellum by dexetimide. With 4-[76Br]bromolevetimide, the radioactivity concentrations in the cortex and striatum were similar to that of cerebellum.

Evaluation of a purification procedure for the muscarinic receptor for the purpose of quantitative receptor assays of anticholinergics. Part A: The membrane-bound receptor

The presented purification procedure for the muscarinic receptor from calf striatum includes the extraction of lipids with hexane in the first step and the removal of 39% of non-receptor proteins with 2 M NaCl in the second step. The simplicity of such an approach to the purification of the receptor warrants its use in the routine practice for quantitative purposes. The high affinity binding of tertiary 3H-dexetimide (3H-DEX) and quaternary 3H-N-methylscopolamine (3H-NMS) is preserved after the removal of irrelevant lipids and proteins from the P2-pellet. The overall yield of receptors--80%, when labelled with 3H-NMS, was satisfactory. Moreover, the final product, the NaCl-pellet, exerts a higher density of 3H-NMS binding sites per mg proteins by a factor of about 1.7. The overall yield of receptors and purification factor were lower, when measured with 3H-DEX. The total yield of 3H-DEX binding sites amounted to about 40% and the receptor density per mg protein decreased by a factor of 0.85. We did not succeed in the improvement of the ratio specific/non-specific binding, neither for 3H-DEX nor for 3H-NMS for the purified receptor preparations. The use of 3H-NMS is preferable to 3H-DEX in plasma sample assays because of a negligible effect of plasma on ligand binding when compared with 3H-DEX.

In vivo visualization of central muscarinic receptors using [11C]quinuclidinyl benzilate and positron emission tomography in baboons

The muscarinic antagonist, quinuclidinyl benzilate (QNB), labeled with carbon 11 was used as a radioligand to visualize in vivo by positron emission tomography (PET) the central muscarinic acetylcholine receptors (mAChR) in baboons (Papio papio). The binding characteristics of [11C]QNB showed its specific binding to central mAChR. [11C]QNB brain uptake was high in cerebral cortex and striatum, areas that are rich in mAChR, whereas it decreased rapidly in cerebellum, evidencing non-specific binding in this structure that is almost devoid of mAChR. These results are consistent with the known cerebral distribution of mAChR in primates. [11C]QNB specific cerebral binding was enhanced by pretreatment with methyl-QNB, a peripherally acting muscarinic antagonist. Specifically labeled binding sites alone were blocked by prior administration of dexetimide, a muscarinic antagonist. Specific radioactivity was driven out from mAChR-rich regions by atropine and dexetimide, drugs with high affinity for mAChR. This competition was stereospecific since only dexetimide, the pharmacologically active isomer of benzetimide, was able to compete with the radioligand on its binding sites. A relationship between the occupancy of [11C]QNB-labeled receptors by atropine or dexetimide and the concomitant induction of a pharmacological effect was also detected by simultaneous PET scanning and electroencephalographic recording. Since mAChR form an important part of choline receptors in the central nervous system, [11C]QNB appears to be a suitable radiotracer to monitor cerebral physiological or pathological phenomena linked to the cholinergic system in living subjects.

In vivo biodistribution of two [18F]-labelled muscarinic cholinergic receptor ligands: 2-[18F]- and 4-[18F]-fluorodexetimide

Two [18F]-labelled analogues of the potent muscarinic cholinergic receptor (m-AChR) antagonist, dexetimide, were evaluated as potential ligands for imaging m-AChR by positron emission tomography (PET). Intravenous administration of both 2-[18F]- or 4-[18F]-fluorodexetimide resulted in high brain uptake of radioactivity in mice. High binding levels were observed in m-AChR rich areas, such as cortex and striatum, with low levels in the receptor-poor cerebellum. Uptake of radioactivity was saturable and could be blocked by pre-administration of dexetimide or atropine. Drugs with different sites of action were ineffective at blocking receptor binding. The results indicate that both radiotracers are promising candidates for use in PET studies.

Haloperidol-sensitive (+)[3H]SKF-10,047 binding sites (sigma sites) exhibit a unique distribution in rat brain subcellular fractions

The distribution of haloperidol-sensitive (+)[3H]N-allylnormetazocine ((+)[3H]SKF-10,047) binding sites (sigma sites) in subcellular fractions of rat brain homogenates was extensively characterized. In synaptosomal fractions, enriched in choline acetyltransferase activity, sigma sites were present in lower concentrations than in whole brain homogenates. On the other hand, microsomal and myelin fractions were found to be enriched in sigma sites. A similar pattern of enrichment was seen for 5'-nucleotidase activity, a general plasma membrane marker. However, subsequent experiments in which microsomes were subfractionated on linear sucrose gradients led to the recovery of sigma sites over a significantly lower density range than 5'-nucleotidase activity or ATP-stimulated [3H]ouabain binding, an additional plasma membrane marker. In addition, previously reported distributions of a number of other subcellular markers, including those for endoplasmic reticulum, were found to contrast with the observed distribution of sigma sites. It is concluded that rat brain sigma sites are not concentrated at synaptic regions of plasma membrane. However, the possibility that sigma sites are localized to specialized areas of nonsynaptic plasma membrane cannot be excluded.

Direct measurement of muscarinic agents in the central nervous system of mice using ex vivo binding

Muscarinic agents produce a range of side effects including hypothermia and tremor. Although these responses can be used to estimate the in vivo activity of these muscarinic agents in the central nervous system (CNS), the approach is limited by compensatory feedback mechanisms and the difficulty of equating degree of receptor occupancy to effect. We have developed an ex vivo assay to measure the potency and penetration of muscarinic agents into the CNS. The muscarinic antagonists scopolamine and N-methylscopolamine dose dependently inhibited the ex vivo binding of [3H]oxotremorine-M to homogenates of mouse whole brain membranes. Following intraperitoneal administration these compounds had ED50 values of 2.6 and 26 mg/kg respectively, which were comparable to the doses which inhibited RS86 induced hypothermia in mice. Three muscarinic agonists RS86, pilocarpine and arecoline also demonstrated CNS activity in this assay with ED50 values of 11, 23 and 220 mg/kg. RS86 and pilocarpine additionally showed good penetration into the CNS with estimated values of 1.5 and 0.31% of the administered dose. These values were comparable with the ability of these compounds to induce a centrally mediated hypothermic response. These studies demonstrate a simple, quick and reliable biochemical means of assessing a muscarinic agent's potency and penetration within the CNS.

Synthesis of radiotracers for studying muscarinic cholinergic receptors in the living human brain using positron emission tomography: [11C]dexetimide and [11C]levetimide

Dexetimide (Fig. 1a), a potent muscarinic cholinergic receptor antagonist, and levetimide (Fig. 1b), its pharmacologically inactive enantiomer, were labeled with 11C for non-invasive in vivo studies of muscarinic cholinergic receptors in the human brain using positron emission tomography. The syntheses were completed in approximately 32 min using [alpha-11C]benzyl iodide as the precursor. The synthesis, purification, characterization and determination of specific activity are presented and discussed.

Muscarinic cholinergic receptor subtypes in normal human brain and Alzheimer's presenile dementia

Brain muscarinic M1 and M2 binding sites, as defined by their affinity for pirenzepine, were studied in 24 regions of 3 normal control brains. For each region, the binding of [3H]QNB, a non-subtype selective antagonist, was saturable with a mean KD of 0.117 +/- 0.066 nM. Computer-assisted analysis of the pirenzepine competition binding curves yielded the amount of high affinity (M1) and low affinity sites (M2) as well as their respective Ki values for this ligand. The neocortex contained a mixed population of 67% M1 and 33% M2 sites, without locoregional heterogeneity. The muscarinic receptors of the caudate nucleus, putamen, pallidum, hippocampus and nucleus amygdalis were predominantly of the M1 type as well. The cerebellum and the brain-stem are examples of regions containing over 90% M2 sites. White matter structures like the centrum ovale appeared to contain low concentrations of [3H]QNB binding sites, predominantly of the M1 subtype. The data for the frontal cortex were compared with data obtained in 3 patients who died from Alzheimer's disease with very early onset: the muscarinic receptor concentrations were somewhat lower but their M1/M2 ratio remained unchanged. GTP caused an appreciable rightward shift and steepening of the carbachol competition binding curve in M2 predominant regions such as the pons, whereas only a slight shift was observed in the cortex. In the presence of GTP, the alkylating reagent N-ethylmaleimide caused a 35-fold increase of the affinity for carbachol in M2 predominant regions. In contrast, regions where the receptors are predominantly of the M1 type, N-ethylmaleimide caused only a 5-fold increase in agonist affinity. These findings confirm our previously formulated hypothesis that the ability of N-ethylmaleimide to modulate the agonist affinity is an additional criterion for the characterization of M1 and M2-type receptors.

Different agonist binding properties of M1 and M2 muscarinic receptors in calf brain cortex membranes

The muscarinic antagonist 1-[benzilic 4,4'-3H]-quinuclidinyl benzilate [3H]-QNB) bound to a single class of non-cooperative sites in calf cerebral cortex membranes (KD = 0.29 nM and Bmax = 1.06 pM/mg protein). Computer-assisted analysis of the shallow pirenzepine/[3H]-QNB competition binding curves indicated that 68% of these sites were of the M1-subtype and the remaining 32% of the M2 subtype. Respective Ki-values for pirenzepine were 27 nM and 1.14 microM. Binding characteristics of the antagonist atropine and of the agonist carbachol for M2 were evaluated by performing competition binding with 0.5 nM [3H]-QNB in the presence of 2 microM pirenzepine. The binding characteristics for the M1 receptors were obtained indirectly by subtracting the curve for M2 from the total curve, or directly by competition binding with 0.3 nM [3H]-pirenzepine. Atropine competition curves were steep for M1 and M2 and were not affected by 1 mM GTP nor by 1 mM N-ethylmaleimide. The carbachol competition curve was shallow for M2. The steep curves for M1 indicate that this receptor subclass was only composed of low agonist affinity sites. GTP, which caused a rightward shift and a steepening of the carbachol competition curve for M2, did not affect the curves for M1. N-ethylmaleimide provoked a leftward shift and a steepening of the carbachol competition curve for M2 and abolished GTP modulation. A leftward shift was also observed for M1, but of a smaller magnitude (i.e. 3-4-fold for M1 compared to 17-fold for M2). These data suggest that, in calf brain cortex, M1 and M2 receptors show different susceptibility towards GTP and N-ethylmaleimide modulation.

Molecular distinction between calf heart and brain muscarine receptors: different N-ethylmaleimide modulation of agonist binding

Muscarinic acetylcholine receptors in calf heart and forebrain membranes were identified by binding of 1-[benzilic-4,4'-3H]quinuclidinyl benzilate ([3H]QNB). We were able to solubilise these receptors with a yield of 50% of the proteins by treatment of the membranes with digitonin. The existence of two or more receptor subclasses with different agonist affinity in the heart membranes was evidenced by the shallow carbachol/[3H]QNB competition binding curves. The receptors displayed only low agonist affinity, in the presence of GTP as well as after solubilisation. The alkylating reagent N-ethylmaleimide (NEM) caused a 70-fold increase in agonist affinity for the solubilised receptors whereas GTP was ineffective. A similar difference in affinity was observed for the membranes when agonist competition curves in the presence of NEM were compared to those in the presence of GTP. NEM caused only a 2- to 3-fold increase of the agonist affinity for solubilised brain cortex membranes. These data suggest that heart and brain muscarine receptors are structurally different.

Agonist mediated conformational changes of solubilized calf forebrain muscarinic acetylcholine receptors

Muscarinic receptors in calf forebrain membranes can be identified by the specific binding of the radiolabelled antagonist [3H]dexetimide. These receptors (2.8 pM/mg protein) comprise two non-interconvertible subpopulations with respectively high and low agonist affinity but with the same antagonist affinity. For all the agonists tested the low affinity sites represent 85 +/- 5% of the total receptor population. 0.5% Digitonin solubilized extracts contain 0.8 pM muscarinic receptor/mg protein. In contrast with the membranes, these extracts contain only sites with low agonist affinity. The alkylating reagent N-ethylmaleimide causes an increase of the acetylcholine affinity for the low affinity sites in membranes as well as for the solubilized sites. This effect is time dependent until a maximal 3-fold increase in affinity is attained. The rate of N-ethylmaleimide action is enhanced by the concomitant presence of agonists. In contrast, N-ethylmaleimide does not affect antagonist binding. This suggests that agonists mediate a conformational change of both the membrane bound low affinity muscarinic sites and of the solubilized sites, resulting in their increased susceptibility towards NEM alkylation.

In vitro and in vivo evidence for the existence of presynaptic muscarinic cholinergic receptors in the rat hippocampus

The intrahippocampal injection of kainic acid cleared 50% of muscarinic receptors and favored the detection of a further 20% loss in hippocampal presynaptic muscarinic receptors produced by electrolytic lesion of the medial septal nucleus as determined by Scatchard analysis of the saturation isotherms of [3H]dexetimide binding. In accordance, a decrease of about 20% in the in vivo accumulation of [3H]dexetimide in the hippocampus was found in animals lesioned in the medial septal nucleus. This effect occurred at both the dose of 5 micrograms/kg and at the saturating dose of 100 micrograms/kg of [3H]dexetimide. The results suggest that the loss was due to decreased receptor number rather than decreased receptor affinity.

Subcellular fractionation of striatum: sedimentation properties of dopaminergic synaptosomes

Linear sucrose density gradient centrifugation of a crude synaptosomal-mitochondrial preparation of rat striatum was performed at 82,500g for 7.5, 15 and 30 min and 1, 4 and 20 h. After centrifugation various marker enzyme activities were measured throughout the gradients, viz. tyrosine hydroxylase (TH) and DOPA decarboxylase (DD) as markers of dopaminergic synaptosomes, lactate dehydrogenase (LDH) as a general synaptosomal marker and monoamine oxidase (MAO) as a mitochondrial marker. At all centrifugation times the distribution patterns of TH and DD activity coincided almost perfectly. Notable differences were found between the sedimentation properties of these TH/DD-containing particles and LDH-containing particles: TH and DD were symmetrically distributed in the gradient much sooner than LDH, at all centrifugation times the top of the TH and DD curves was lying deeper in the gradient than the highest LDH activity, and TH and DD became enriched in the gradients to a much greater extent than LDH. It is concluded that rat striatal dopaminergic synaptosomes form a relatively homogeneous population of particles sedimenting faster into the gradients than the bulk of striatal synaptosomes does. This distinct sedimentation behaviour of the dopaminergic synaptosomes can be usefully applied for analytical purposes.

Analytical subcellular fractionation of rat cortex: resolution of serotonergic nerve endings and receptors

An analytical procedure for the subcellular fractionation of rat brain cortex is presented; it consists of a two-step procedure involving a differential centrifugation using the five-fraction scheme and an isopycnic centrifugation in continuous sucrose gradients. All fractions obtained were analyzed for their content of various constituents, such as receptor binding, uptake, and several marker enzymes. Special attention was paid to the subcellular distribution of the serotonin S2 receptors; they were mainly recovered in the microsomal P fraction, but a significant amount was also associated with the mitochondrial (M and L) fractions. After equilibration in density gradients, serotonin S2 receptors revealed two peaks, which were similarly affected after treatment with amitriptyline and/or yohimbine. There is no evidence to suggest that serotonin S2 receptors are associated with nerve endings containing the neurotransmitter serotonin. Although three main profiles, a microsomal, a mitochondrial, and a mixed one, clearly appear from the differential centrifugation, subgroups of these main profiles were also found. For instance, the microsomal distribution patterns of serotonin S2 receptors and 5'-nucleotidase are very similar, but differ from that of UDP-galactosyltransferase. Similarly, the mitochondrial profiles of cytochrome oxidase and 5-HT (serotonin) uptake are different. An analytical approach for brain fractionation, when performed with appropriate measurements (cytochrome oxidase, amine uptake, 5'-nucleotidase, and receptor binding), is rapid and clearly differentiates pre- and postsynaptic constituents.

Agonist-mediated conformational changes of muscarinic receptors in rat brain

Muscarinic acetylcholine receptors were identified in the microsomal P fraction of rat forebrain by the specific binding of the radiolabeled antagonist [3H]dexetimide. Binding occurred to a single class of noncooperative sites (3.25 mumol/mg protein) with an equilibrium dissociation constant of 1.1 nM. Agonist/[3H]-dexetimide competition binding experiments allowed the distinction between two major muscarinic receptor subpopulations, having respectively high affinity (20% of the total receptor population) and low affinity for agonists, but with the same affinity for antagonists. A 610-fold difference in affinity was calculated for carbamoyl-choline, the agonist extensively investigated in this study. The alkylating reagent N-ethylmaleimide did not affect the total receptor number, antagonist binding to the high-affinity and low-affinity sites, nor agonist binding to the high-affinity sites. The reagent, however, caused a net increase in agonist affinity for the low-affinity sites. This process was dependent on time and dose of N-ethylmaleimide, until a maximal increase in affinity (fourfold increase for carbamoylcholine) was attained. This suggests a quantal conversion of the low-affinity sites by the reagent into an alkylated form, which possesses a higher affinity for agonists but an unchanged affinity for antagonists. The rate of alkylation was enhanced by the presence of agonists but not of antagonists, which is indicative for the ability of agonists to mediate a conformational change of these sites. The close correlation between the N-ethylmaleimide-mediated increase in drug affinity for the low-affinity sites and the ability of the drugs to enhance alkylation of these sites by N-ethylmaleimide can be explained by the ability of (a) muscarinic drugs to interact with the low-affinity sites according to the Monod-Wyman-Changeux 'Plausible Model' and (b) N-ethylmaleimide to freeze these sites in the 'active' conformation by alkylation.