Muscarinic cholinergic receptors in human foetal brain: characterization and ontogeny of [3H]quinuclidinyl benzilate binding sites in frontal cortex

Twenty-two frontal cortices from normal human foetal brains of gestational ages ranging from 16 to 40 weeks and five postnatal brains ranging from 5 to 50 years were analysed for the ontogeny of muscarinic receptors using [3H]quinuclidinyl benzilate (QNB) as the ligand. QNB binding sites were shown to be stable up to 4 1/2 months of storage at -70 degrees C. QNB binding was characterized in frontal cortices of 28-week-old foetal brains as muscarinic receptors by the following criteria: (1) it was localised mainly in particulate fraction; (2) binding was saturable at a concentration of 1.5 nM; (3) the cholinergic antagonists atropine and scopolamine competed for the binding, with IC50 values of 1 and 0.8 nM, respectively. The agonists oxotremorine, carbachol, and pilocarpine gave IC50 values of 1, 15 and 18 microM, respectively. Nicotinic receptor ligands and noncholinergic drugs could not compete for the binding. Bimolecular association and dissociation rate constants for the reversible binding are 6.23 X 10(8) M-1 X min-1 and 2.0 X 10(-2) X min-1, respectively. The equilibrium dissociation constant is 33 pM. The KD obtained by saturation binding data is 103 pM. Ontogeny of muscarinic receptors showed three distinct phases: In phase I, they appear between 16 and 18 weeks [average concentration 109 fmol/mg protein of total particulate fraction (TPF)] and slowly increase up to 20 weeks (average concentration 147 fmol/mg protein TPF). Phase II is a lag period between 20 and 24 weeks at which time receptor concentration does not change perceptibly (average concentration (67 fmol/mg protein TPF).(ABSTRACT TRUNCATED AT 250 WORDS)

Computer-assisted image analysis to quantify regional and specific receptor ligand binding: upregulation of [3H]QNB and [3H]WB4101 binding in denervated hippocampus

Quantitative regional analysis of receptor autoradiographs using the Nikon Magiscan image analysis system permits resolution of regional variations in specific binding in non-homogeneous CNS structures, such as the hippocampus. Cholinergic denervation, produced by fimbrial transections, elicits a 24% increase in atropine-displaceable [3H]QNB binding in whole coronal sections of the hippocampal formation, which is greatest in the dorsal subiculum, CA3 and dentate gyrus. This lesion also elicits a 69% increase in lower affinity [3H]WB4101 binding which is displaceable by phentolamine, but not by prazosin. This represents a sum of increases and decreases in binding in several subregions. Taken together, these findings serve to emphasize the need for normalized regional evaluation of subtracted images which have been calibrated, and linearized or transformed, to reveal binding specific to a single site.

Receptor binding profile of quinupramine, a new tricyclic antidepressant

The receptor binding profile, composed of the Ki-values measured in eight different receptor binding models using rat brain membranes, is reported for the new tricyclic antidepressant quinupramine, 10,11-dihydro-5-(3-quinuclidinyl)-5H-dibenz[b, f]azepine, and three reference compounds with a tertiaryamine side chain. Quinupramine was found to possess high affinity for muscarinic cholinergic and histamine H1 receptor binding sites in rat brain, whereas its affinity for imipramine binding sites was only one seventieth that of imipramine. Receptor binding profiles of the reference compounds were almost similar to that of quinupramine, except in the case of imipramine binding sites.

Choline acetyltransferase activity and [3H]quinuclidinylbenzilate binding in brains of scrapie-infected hamsters

Choline acetyltransferase (ChAT) activity and [3H]quinuclidinylbenzilate binding were studied in the brain of scrapie-infected hamsters and sham inoculated controls. Although scrapie-infected hamsters showed no reduction of ChAT activity compared to the controls, they showed a decrease in the affinity and maximum number of post-synaptic muscarinic receptors. Scrapie virus thus alters the cholinergic system at the post-synaptic rather than at the pre-synaptic level.

[3H]Pirenzepine and [3H]quinuclidinyl benzilate binding to brain muscarinic cholinergic receptors. Differences in measured receptor density are not explained by differences in receptor isomerization

Muscarinic receptor densities were measured in membranes prepared from rat cerebral cortex using [3H]pirenzepine and [3H]quinuclidinyl benzilate. Isotherms of equilibrium binding data modeled to a single apparent binding site for both ligands. However, as has been reported previously, [3H]pirenzepine labeled only a small fraction of the binding sites that were labeled by [3H]quinuclidinyl benzilate. This observation has been used to support the hypothesis that subtypes of muscarinic receptors exist. Several investigators have previously suggested that antagonist binding to muscarinic receptors involves an isomerization of the receptor-antagonist complex, and it is only the isomerized form of the receptor that is identified by radioligand binding studies. To examine the possibility that the difference in the density of binding sites identified by [3H]pirenzepine and [3H]quinuclidinyl benzilate is due to differences in the degree of isomerization of the receptor associated with the binding of each ligand, the kinetics of the binding of [3H]pirenzepine and [3H]quinuclidinyl benzilate to membranes prepared from rat cerebral cortex were examined. The pseudo-first-order rate constant of association for both ligands showed a nonlinear (hyperbolic) dependence on ligand concentration. These results suggested that a rapidly equilibrating initial binding step was followed by a more slowly equilibrating isomerization of the initially formed ligand-receptor complex. The kinetic data were computer-modeled to obtain estimates of the equilibrium constants for both reaction steps. The equilibrium constants for the isomerization step were 0.1 and 0.004 for [3H]pirenzepine and [3H]quinuclidinyl benzilate, respectively. Our measurements, in agreement with others, suggested that only the fraction of receptors which isomerized were measurable using filtration binding assays. Although essentially all (99.6%) of the [3H]quinuclidinyl benzilate binding sites appeared to isomerize, only 90% of the [3H]pirenzepine binding sites isomerized, and thus only 90% were measured in our assay. It therefore appears that differences in receptor isomerization can partially, but not wholly, account for the differences between [3H]pirenzepine and [3H]quinuclidinyl benzilate binding in rat cerebral cortex.

Kinetics of in vivo binding of antagonist to muscarinic cholinergic receptor in the human heart studied by positron emission tomography

Positron Emission Tomography (PET) was used to analyse in vivo antagonist binding to human myocardial muscarinic cholinergic receptor. The methiodide salt of the muscarinic antagonist, quinuclidinyl benzilate (MQNB), was labeled with the positron emitter, Carbon-11, and injected intravenously to 8 normal subjects. 11C-MQNB concentration was determined in vivo in the ventricular septum from 40 cross-sectional images acquired at the same transverse level over a period of 70 minutes. In 4 subjects, various amounts of unlabeled atropine were rapidly injected at 20 minutes to study whether atropine competitively inhibited MQNB. The kinetics of binding of 11C-MQNB were not the same in vivo and in vitro. The apparent dissociation rate of 11C-MQNB in vivo was much slower (by 1 to 2 orders of magnitude) than that observed in vitro with 3H-QNB. After atropine injection, 11C-MQNB dissociated from its binding sites at a rate that apparently depended on the amount of atropine present. 11C-MQNB kinetics were analysed with a mathematical model which assumes the existence of a boundary layer containing free ligand in the vicinity of the binding sites. The dissociation rate of the radioligand depends on the probability of its rebinding to a free receptor site.

Light microscopic autoradiographic localization of [3H]pirenzepine and [3H](−)quinuclidinyl benzilate binding in human stellate ganglia

We have utilized the LKB Ultrofilm method of autoradiography to anatomically localize putative M1 and M2 muscarinic receptor subtypes in human stellate ganglia. Ten micron sections were labeled in vitro with either 1 nM of the classical antagonist [3H](-)quinuclidinyl benzilate ([3H](-)QNB) or 20 nM of the non-classical antagonist [3H]pirenzepine ([3H]PZ), using 1 microM atropine sulfate to define non-specific binding for both ligands. Our results indicate that [3H](-)QNB and [3H]PZ binding sites are distributed within the principal ganglion cells and nerve bundles.

[125I] 3-quinuclidinyl 4-iodobenzilate: a high affinity, high specific activity radioligand for the M1 and M2-acetylcholine receptors

We have prepared a radioiodinated ligand which binds with high affinity to the muscarinic acetylcholine receptor (m-AChR). A derivative of 3-quinuclidinyl benzilate, [125I] labeled (R) 1-aza-bicyclo(2.2.2)oct-3-yl (R,S)-alpha-hydroxy-alpha-(4-[125I]iodophenyl)phenyl acetate (4- IQNB ) exhibits an affinity for the m-AChR from corpus striatum higher than that of (R) [3H] QNB. Additionally, [125I] 4- IQNB exhibits receptor selectivity for the M1 receptor since the affinity for the receptor from dog and rat heart is lower than that using dog or rat corpus striatum.

Differentiation between ligand trapping into intact cells and binding on muscarinic receptors

Binding properties of [3H] dexetimide , L-quinuclidinyl[phenyl-4-3H] benzilate and [3H]methylscopolamine were compared with intact 108 CC 15 cells and membrane preparations of those. The ability of the three ligands to label specifically muscarinic receptors on membrane fractions was quite similar. By contrast, when performed with intact cells, [3H] dexetimide and L-quinuclidinyl [phenyl-4-3H]benzilate revealed higher nonspecific binding which was prevented by methylamine, suggesting a trapping of the ligands within the cells presumably in the lysosomes. To the contrary, such nonspecific 'binding' or trapping was not detectable when [3H]methylscopolamine was used as ligand, a fact which makes this ligand particularly appropriate for labelling cell surface muscarinic receptors. It is concluded that more caution is needed in binding studies when performed with intact cells; indeed, besides specific binding on receptor sites, [3H]ligand can be entrapped within the cell and can even sometimes give the illusion of specific binding. The use of lysosomal agents which do not interfere with specific receptors on membrane preparations should allow one, in most cases, to discard the possibility of a trapping phenomenon in intact cells.

Simultaneous assay of muscarinic and beta-adrenergic receptors using a double isotope technique

Muscarinic receptor and beta-adrenergic receptor binding were measured simultaneously in a membrane fraction of bovine tracheal smooth muscle using [3H]-L-quinuclidinyl benzilate and [125I]-(-) iodocyanopindolol. The binding characteristics, affinity and receptor density, obtained in the double receptor assay and in the control experiments were the same within experimental error. Moreover, there appears to be neither a significant influence of an excess of d,1-propranolol on [3H]-L-quinuclidinyl benzilate binding nor a significant influence of an excess of 1-quinuclidinyl benzilate on [125I]-(-)iodocyanopindolol binding. The method is advantageous where both receptors have to be assayed and where limited amounts of biological material, like in biopsy specimen, are available.

Quantitative autoradiography of muscarinic cholinergic receptors in the rat brain

Using the in vitro autoradiographic technique with tritium-sensitive LKB sheet film and the liquid scintillation counting method, the distribution and the binding parameters of the muscarinic cholinergic receptors (MChR) were determined in various discrete regions of the rat brain. The results obtained in the present study were as follows: (1) Specific binding of [3H]QNB to the slide-mounted tissue sections increased slowly when incubated at room temperature; saturation occurred 2 h after incubation. Only 23% of [3H]QNB bound to the tissue section was dissociated 5 h after the addition of 20 microM atropine to the medium. These findings were very different from those obtained in the study using the tissue homogenates. (2) The regional distribution of MChR was determined using both autoradiographic and liquid scintillation counting methods. The distribution of MChR was heterogeneous, with highest densities in the striatum and nucleus accumbens and lowest in the globus pallidus, nucleus interpeduncularis and nucleus septi. Moreover, MChR were unevenly distributed within the subfields of each region. (3) In saturation binding studies using the slide-mounted tissue sections of 20 micron thickness the (Kd)app-values were similar but not exactly identical in 5 discrete regions, i.e. the striatum, somatosensory cortex, hippocampus (the subiculum + CA1 field), nucleus accumbens and gyrus dentatus, determined in the present study. The (Kd)app-value of each region was about 700 pM which was about 20 times higher than that obtained in the study using the tissue homogenates. However (Kd)app-values obtained with 5 and 10 micron tissue sections were approximately 3-fold lower.

In vivo competition studies with analogues of 3-quinuclidinyl benzilate

Among ligands that bind to the alpha- and beta-adrenoceptors and to the muscarinic acetylcholine receptor (m-AChR), those that bind to the latter have the best properties for external detection of receptor sites by gamma-camera imaging. To develop the optimal radiotracer, nonradioactive analogues of 3-quinuclidinyl benzilate (I) were tested in in vivo displacement studies with (-)-[3H]I to determine their ability to compete with (-)-[3H]I for the muscarinic acetylcholine receptor. There is a linear correlation between the ability to compete with (-)-[3H]I for the m-AChR and the affinity constant of the analogue as determined by in vitro assay, suggesting that the test is a valid indicator of in vivo distribution. One radioiodinated analogue, 3-quinuclidinyl p- iodobenzilate , bound to m-AChR in the heart and brain of rats.

Comparison of [3H]pirenzepine and [3H]quinuclidinylbenzilate binding to muscarinic cholinergic receptors in rat brain

The properties of [3H]quinuclidinylbenzilate ( [3H]QNB) binding and [3H]pirenzepine ( [3H]PZ) binding to various regions of rat brain were compared. [3H]PZ appeared to bind with high affinity to a single site, with a Kd value of approximately 15 nM in the cerebral cortex. The rank order of potencies of muscarinic drugs to inhibit binding of either [3H]QNB or [3H]PZ was QNB greater than atropine = scopolamine greater than pirenzepine greater than oxotremorine greater than bethanechol. Muscarinic antagonists (except PZ) inhibited both [3H]PZ and [3H]QNB binding with Hill coefficients of approximately 1. PZ inhibited [3H]QNB binding in cortex with a Hill coefficient of 0.7, but inhibited [3H]PZ binding with a Hill coefficient of 1.0. Hill coefficients for agonists were less than 1. The density of [3H]PZ binding sites was approximately half the density of [3H]QNB binding sites in cortex, striatum and hippocampus. In pons-medulla and cerebellum, the densities of [3H]PZ binding sites were 20 and 0%, respectively, relative to the densities of [3H]QNB binding sites. When unlabeled PZ was used to compete for [3H]QNB binding, the relative number of high-affinity PZ binding sites in cortex, pons and cerebellum agreed with the relative number of [3H]PZ binding sites in those regions. The binding of [3H]PZ and [3H]QNB was nonadditive in cortex. GTP inhibited high-affinity oxotremorine binding, but not PZ binding. Together, these data suggest that [3H]PZ binds to a subset of [3H]QNB binding sites. Whether this subset reflects the existence of subtypes of muscarinic receptors or is a consequence of coupling to another membrane protein remains to be seen.

The differential loss of [3H]pirenzepine vs [3H](−)Quinuclidinylbenzilate binding to soluble rat brain muscarinic receptors indicates that pirenzepine binds to an allosteric state of the muscarinic receptor

[3H]Pirenzepine [( 3H]PZ) and [3H] (-)Quinuclidinylbenzilate [( 3H] (-)QNB) specific binding to soluble rat brain muscarinic cholinergic receptors was assessed as a function of time subsequent to receptor solubilization. The soluble brain muscarinic receptor is stable at 4 degrees C when assayed by [3H] (-)QNB binding (t 1/2 = 80 hrs). In contrast the pirenzepine state of the receptor decays rapidly (t 1/2 = 3.0 hrs). Prior occupation of the receptor with [3H] (-)QNB or [3H]PZ increases the receptor stability by two to five fold (t 1/2 QNB greater than 1,000 hrs; t 1/2 PZ = 6.5 hrs). These data indicate that pirenzepine binds to an allosteric state of the muscarinic receptor and that caution should be employed in the assignment of receptor subtypes based solely upon the binding of ligands which recognize unique conformational states.

The characteristics of I-125 4-IQNB and H-3 QNB in vivo and in vitro

The accumulation of (R)-(H-3)-3-quinuclidinyl benzilate (H-3 QNB) and (R,S)-1-azabicyclo(2.2.2)oct-3-yl (R,S)-alpha-hydroxy-(4-[I-125]iodophenyl) benzeneacetate (I-125 4- IQNB ) in heart, caudate/putamen, and cerebellum of rats was determined at intervals from 15 min to 4 hr after injection. The behavior of the two radiotracers in the heart is consistent with in vitro results with respect to affinities and specificities. In the brain, however, the compounds differ in tissue selectivity. At high specific activity, neither compound provides localization that is consistent with the concentration of receptor in the tissues. The results of this study do not indicate quantification of receptor concentration by means of single external images.

External imaging of cerebral muscarinic acetylcholine receptors

A radioiodinated ligand that binds to muscarinic acetylcholine receptors was shown to distribute in the brain by a receptor-mediated process. With single-photon-emission imaging techniques, radioactivity was detected in the cerebrum but not in the cerebellum, whereas with a flow-limited radiotracer, radioactivity was detected in cerebrum and cerebellum. Single-photon-emission computed tomography showed good definition of the caudate putamen and cortex in man.

Effects of lithium on [3H](-)quinuclidinyl benzilate [( 3H](-)QNB) binding to rat brain muscarinic cholinergic receptors

Addition of lithium in vitro inhibited the binding of [3H](-)QNB to muscarinic cholinergic receptors of homogenates of tissue prepared from the striatum, cortex, and hippocampus of rat brain. Chronic in vivo exposure of rats to lithium in their food produced serum levels of lithium comparable to therapeutic levels. After in vivo exposure, the tissue homogenates prepared from these rats had an apparent decrease in receptor density in the three brain areas. However, if the tissue homogenates were washed twice, before addition to the assays, no differences in binding were detectable. Similar effects on unwashed and washed tissue homogenates can be demonstrated after in vivo exposure to lithium. Therefore, the apparent decrease in binding after in vivo lithium treatment is probably due to lithium retained in the tissue. No permanent alterations in the muscarinic receptor characteristics were measurable after the removal of the lithium. Nevertheless, in vivo interactions at muscarinic receptors may be important under conditions when therapeutic levels of lithium are present.

Muscarinic receptors on intact, cultured neurons. Characterization by [3H]quinuclidinylbenzilate binding

[3H]Quinuclidinylbenzilate (QNB) was used to identify muscarinic cholinergic receptors on intact, cultured neurons from fetal rat brains. Scatchard analysis revealed a single binding site with a dissociation constant Kd = approximately or equal to 170 pM. The rank order of potency of cholinergic drugs to displace [3H]QNB from intact neurons was similar to that observed using isolated membranes of brain homogenates. No difference in the rank order was observed with cultures of neurons from different brain regions which vary in their neuronal composition.

Contraction and [3H]QNB binding in collagenase isolated fundic smooth muscle cells

Smooth muscle cells from the guinea pig gastric fundus were isolated by successive collagenase digestions. Tritiated quinuclidinyl benzilate [( 3H]QNB) was used to study the binding characteristics of the muscarinic cholinergic receptors on these cells. Each cell bound 8.3 X 10(-19) mol of QNB, and a concentration of QNB of 0.19 nM was required to label one-half of the binding sites. This suggests a concentration of about 500,000 muscarinic cholinergic receptors per smooth muscle cell. The muscarinic cholinergic receptor antagonists atropine and scopolamine inhibited QNB binding with a 50% inhibiting concentration (IC50) in the nanomolar range, whereas the agonists acetylcholine (ACh), oxotremorine, and carbamylcholine had IC50S in the micromolar range. Hill coefficients (nH) for antagonists approached unity, but agonists displayed fractional nH. Exposure of cells to cholinergic muscarinic agonists resulted in dose-dependent decreases in cell length. The concentration of agonist required to induce half-maximal contractions (ED50) was 8.3 X 10(-12) M for ACh and 6.3 X 10(-13) M for oxotremorine. Atropine (10(-9) M) decreased the sensitivity to ACh, increasing the ED50 for ACh-induced contractions to 1.2 X 10(-10) M. These results suggest the existence of muscarinic receptor heterogeneity for cholinergic agonists but not for antagonists.

Differences in affinities of muscarinic acetylcholine receptor antagonists for brain and heart receptors

The affinities of atropine, scopolamine, 3-quinuclidinyl benzilate and twelve analogues of 3-quinuclidinyl benzilate were determined for the muscarinic acetylcholine receptor (m-AChR) using membrane preparations from caudate/putamen. The affinity constants thus obtained were compared with affinities previously reported for the m-AChR obtained from ventricular muscle. The affinities differed significantly for six of the compounds, the largest difference being 16-fold. Neither solubilization nor variation of physiologically significant salts led to a significant change in the affinity of that compound. These results are interpreted as supporting the subclassification of the muscarinic acetylcholine receptor.