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

Fetal heart rate variability responsiveness to maternal stress, non-invasively detected from maternal transabdominal ECG

PURPOSE

Prenatal stress (PS) during pregnancy affects in utero- and postnatal child brain-development. Key systems affected are the hypothalamic-pituitary-adrenal axis and the autonomic nervous system (ANS). Maternal- and fetal ANS activity can be gauged non-invasively from transabdominal electrocardiogram (taECG). We propose a novel approach to assess couplings between maternal (mHR) and fetal heart rate (fHR) as a new biomarker for PS based on bivariate phase-rectified signal averaging (BPRSA). We hypothesized that PS exerts lasting impact on fHR.

METHODS

Prospective case-control study matched for maternal age, parity, and gestational age during the third trimester using the Cohen Perceived Stress Scale (PSS-10) questionnaire with PSS-10 over or equal 19 classified as stress group (SG). Women with PSS-10 < 19 served as control group (CG). Fetal electrocardiograms were recorded by a taECG. Coupling between mHR and fHR was analyzed by BPRSA resulting in fetal stress index (FSI). Maternal hair cortisol, a memory of chronic stress exposure for 2-3 months, was measured at birth.

RESULTS

538/1500 pregnant women returned the questionnaire, 55/538 (10.2%) mother-child pairs formed SG and were matched with 55/449 (12.2%) consecutive patients as CG. Maternal hair cortisol was 86.6 (48.0-169.2) versus 53.0 (34.4-105.9) pg/mg (p = 0.029). At 36 + 5 weeks, FSI was significantly higher in fetuses of stressed mothers when compared to controls [0.43 (0.18-0.85) versus 0.00 (- 0.49-0.18), p < 0.001].

CONCLUSION

Prenatal maternal stress affects the coupling between maternal and fetal heart rate detectable non-invasively a month prior to birth. Lasting effects on neurodevelopment of affected offspring should be studied.

TRIAL REGISTRATION

Clinical trial registration: NCT03389178.

Expression of muscarinic acetylcholine receptors and choline acetyltransferase enzyme in cultured antennal sensory neurons and non-neural cells of the developing moth Manduca sexta

Antennal sensory neurons of Manduca sexta emerge from epidermal cells that also give rise to sheath cells surrounding the peripheral parts of the neurons and to glial cells that enwrap the sensory axons in the antennal nerve. Reciprocal interactions between sensory neurons and glial cells are believed to aid in axon growth and guidance, but the exact nature of these interactions is not known. We investigated the possibility of cholinergic interactions in this process by locating muscarinic acetylcholine receptors (mAChRs) and choline acetyltransferase (ChAT) enzyme in cultured antennal sensory neurons and non-neural cells. ChAT and mAChRs were present in the sensory neurons from the first day in culture. Therefore, the sensory neurons are probably cholinergic, as previously suggested, but they may also be controlled by ACh. In 7-day-old cultures a subgroup of small non-neural cells with processes expressed ChAT activity, and in 14-day-old cultures non-neural cells that formed lamellipodia and scaffoldlike structures on the culture substrate were labeled with ChAT antibody. mAChR activity was detected in similar non-neural cells but only in areas surrounding the nuclei. In addition, mAChRs were found in flat lamellipodia and filopodia forming cells that were present in 1-day-old cultures and grew in size during the 2 week investigation period. These findings suggest muscarinic cholinergic interactions between the neural and non-neural cells during the development of Manduca antenna.

Allosteric interactions at muscarinic cholinoceptors

1. An allosteric interaction occurs when the binding of a ligand to its site on a receptor is able to modify the binding of another ligand to a topographically distinct site on the same receptor and vice versa. The muscarinic cholinoceptors represent the best-studied examples of allosteric phenomena among the G-protein-coupled receptor superfamily. 2. The simplest model describing allosteric interactions at muscarinic cholinoceptors is the ternary complex model, which allows for a three-way interaction between the receptor, a classical (orthosteric) ligand and an allosteric modulator. The interaction may be quantified using the dissociation constant of each ligand for its respective binding site on the free receptor and the 'co-operativity factor' alpha. This latter term is the ratio of affinities of a ligand for the occupied versus the unoccupied receptor and is a measure of the magnitude of the cooperativity between two concomitantly bound ligands. 3. Identification of allosteric phenomena requires the utilization of both radioligand binding and functional approaches. Manifestations of allosterism include: (i) a limited ability to influence radioligand binding as the concentration of the latter is increased; (ii) alterations in the dissociation rate of orthosteric ligands; (iii) curvilinear Schild regressions; and (iv) nonadditivity of agonist/orthosteric antagonist/allosteric modulator combination concentration ratios. 4. Allosteric modulators of muscarinic cholinoceptors represent a diverse range of compounds. Some of the most studied agents include gallamine, alcuronium and the bis-ammonium compounds, C7/3'-phth and W84. Alcuronium has proven a most useful pharmacological tool, as it has been shown to display both positive and negative co-operativity, depending on the receptor subtype and orthosteric ligand involved in the interaction. 5. Evidence has accumulated pointing to the existence of a common allosteric binding site on the muscarinic cholinoceptors, located close to the orthosteric site, but at a more extracellular level. However, the possibility of more than one accessory binding site on various receptor subtypes cannot be excluded. 6. Allosteric modulators offer a number of potential therapeutic advantages, including a ceiling level to their effects and the possibility of 'absolute selectivity' of action, based on the degree of co-operativity rather than the affinity of the modulator for any one receptor subtype.

Drugs and receptors. An overview of the current state of knowledge

This paper reviews the theoretical concepts and methods utilised with isolated tissues to characterise drugs and drug receptors. Specifically the impact, on the in vitro measurement of agonist affinity and relative efficacy, of the idea that receptors bind to transduction proteins in the lipid bilayer of the cell membrane is discussed. The effects of ternary complex formation of agonist-receptor equilibria raise theoretical objections to the measurement of agonist receptor equilibrium dissociation constants. Possible 'promiscuity' of receptors with respect to the G-proteins with which they can interact makes classification of receptors by agonists suspect. The use of Schild analysis for the measurement of antagonist affinity and subsequent classification of receptors is considered in the light of recent data showing that estimates calculated with this method are heterogeneous. Resultant analysis for the detection of allosteric effects is also discussed. Lastly, the impact of molecular biology on the drug and drug receptor classification process is considered, as well as the effects of pathological processes on drug action at the receptor level.

Challenges for receptor theory as a tool for drug and drug receptor classification

Increased understanding in the field of receptor pharmacology, born of the sophisticated techniques now available to us, has confounded rather than simplified the problem of receptor classification. The International Union of Pharmacology (IUPHAR) is currently sponsoring a Receptor Nomenclature Committee whose aims are to recommend a rational system of classification, a formidable task given the complexity and volume of data in the literature (agonist/antagonist potencies, coupling mechanisms, primary structures, etc.) that will need to be incorporated. The Committee's chairman, Terry Kenakin, outlines here the limitations of classical receptor theory for drug receptor classification and suggests that any functional classification system must take into account not only affinity and intrinsic efficacy but also, at the very least, parameters relating to the transducing properties of receptors. If this is not done, then receptor classification data obtained from studies with agonists and antagonists may be different and lead to confusion.

Characterization of agonist and antagonist binding to muscarinic cholinergic receptors solubilized from rat cerebral cortex

The muscarinic acetylcholine receptor was solubilized from rat brain cortex by the zwitter-ionic detergent, 3-[3-cholamidopropyl)dimethylamino)-1-propane sulfonate (CHAPS). The supernatant, after centrifugation at 100,000 x g, was shown to contain molecules with binding sites for both 3H-pirenzepine (3H-PZ) and 3H-(-) quinuclidinyl benzilate (3H-QNB). Maximum binding values for 3H-PZ and 3H-QNB binding to solubilized receptors were approximately 176 +/- 24 pmol/g and 370 +/- 53 pmol/g of protein, respectively. The Kd values for 3H-PZ and 3H-QNB binding to solubilized receptors were 27 +/- 6.3 nM and 0.17 +/- 0.03 nM, respectively. The rank order of potencies of muscarinic drugs, in terms of their ability to inhibit binding of either 3H-PZ or 3H-QNB, was atropine greater than pirenzepine greater than oxotremorine greater than carabachol. Pirenzepine inhibited 3H-QNB binding with a Hill coefficient of 0.77, but inhibited 3H-PZ with a Hill coefficient of 0.94. Hill coefficients for agonists were less than 1. These findings indicate that muscarinic receptors solubilized from rat brain cortex retain their abilities to interact selectively with muscarinic receptor agonists and antagonists.

Muscarinic activity of McN-A-343 and its value in muscarinic receptor classification

The affinity and potency of McN-A-343 (4-(m-chlorophenyl-carbamoyloxy) -2-butynyltrimethylammonium chloride) has been assessed at a range of M1 and M2 muscarinic receptors. McN-A-343 was shown to act as a full agonist at M2 receptors present in the guinea-pig isolated taenia caeci (-log EC50 = 5.14). McN-A-343 exhibited no agonist action in the guinea-pig ileum, atria, bladder or trachea. McN-A-343 was not selective in terms of affinity since its dissociation constants at M1 and M2 binding sites in the rat cerebral cortex and myocardium respectively, were very similar (cortical pPKi = 5.05; myocardial pKi = 5.22). The selectivity previously reported for the compound may be due to differences in intrinsic efficacy and/or tissue receptor reserve. Based on differential antagonist affinities, the muscarinic receptor profile of the taenia caeci, trachea and bladder was similar to that observed in the ileum, but dissimiliar to that observed in the atria.

Structural microheterogeneity of the muscarinic cholinergic receptor is not related to functional diversity identified by differences in affinity for pirenzepine

The muscarinic receptor for acetylcholine shows a diversity in ligand binding properties and effector mechanisms which have suggested the existence of two subtypes (M1 and M2), to which the selective antagonist pirenzepine binds with markedly different affinities. The receptor from rat brain, covalently labelled with the alkylating antagonist tritiated propylbenzilylcholine mustard, displays a structural microheterogeneity on electrophoresis, covering the region of apparent molecular weight 66,000-76,000, with dominant components at 68,000 and 73,000. Selective inhibition by pirenzepine of labelling of the M1 receptor with tritiated mustard has been analysed on fluorographs of sodium dodecyl sulphate-polyacrylamide gels and shown to cause a uniform reduction in radioactive labelling of the broad receptor peak, rather than selectively inhibiting either the high- or low-molecular-weight regions of the band. It is further shown that although this receptor microheterogeneity is found for each of four brain regions studied, it is not found for the heart receptor, which gives a discrete labelled band of apparent molecular weight 72,000. It is therefore suggested that the structural microheterogeneity is the result of tissue-specific, posttranslational modification of the molecule, such as glycosylation, and is not directly related to the functional diversity of the receptor.

Competitive interaction of pirenzepine with rat brain muscarinic acetylcholine receptors

In the present work, we studied the details of the interaction of the nonclassical muscarinic receptor antagonist pirenzepine with [3H]quinuclidinyl benzilate binding sites in rat brain homogenates. Pirenzepine showed biphasic competition curves with a Hill coefficient lower than unity, and these curves were better described according to a two-site receptor model. The affinities and the relative preponderance of these sites were constant at different ligand concentrations, in accordance with a competitive type of interaction. Similarly, pirenzepine did not influence the rate of dissociation of the [3H]quinuclidinyl benzilate-receptor complex, even at relatively high concentrations. However, although low concentrations of pirenzepine decreased the affinity of [3H]quinuclidinyl benzilate for the receptor without affecting the density of the binding sites, higher concentrations of the antagonist decreased the receptor number in a reversible fashion. Schild plots of these data indicated an apparent deviation from simple competition in this experimental design, an observation which can be attributed to the selectivity of pirenzepine for different receptor subtypes. Furthermore, pirenzepine, at concentrations high enough to saturate both its high- and low-affinity sites protected [3H]quinuclidinyl benzilate binding sites in the brain against irreversible alkylation by propylbenzilylcholine mustard. Therefore, our data support a competitive nature of interaction of pirenzepine with rat brain muscarinic receptors.

Muscarinic acetylcholine receptor: thermodynamic analysis of the interaction of agonists and antagonists

The thermodynamic parameters of the interaction of agonists and antagonists with heart and brain muscarinic receptors were determined. The binding of quinuclidinyl [3H]benzilate and the inhibition of quinuclidinyl benzilate (QNB) binding by agonists and antagonists were examined at temperatures between 2 degrees C and 27 degrees C. The density of specific binding sites and the relative proportions of high- and low-affinity binding components of drugs were unaffected by the temperature changes. The binding of atropine was entropy driven in brain and heart membranes. In contrast, net values of these thermodynamic parameters for QNB binding and for the high-affinity binding component of pirenzepine to brain membranes were decreased with the enhancement of the temperature. The low-affinity binding component of the agonists carbachol, oxotremorine and pilocarpine was enthalpy driven. Their high-affinity binding component was entropy driven at 2 degrees C and became enthalpy driven when the incubation temperature was increased. The guanine nucleotide Gpp[NH]p partly prevented the temperature-dependent decrease of net entropy and enthalpy values. Considering that the net changes of thermodynamic parameters are relevant of the interactions between the ligand, the receptor protein and the adjoining membranous molecules, a three-state conformational model is proposed for the muscarinic receptor protein. The receptor selectivity is reappreciated owing to these three states of the receptor protein and the different components of the muscarinic receptor complexes.

Muscarinic receptor subtypes in bovine adrenal medulla

Muscarinic receptors in bovine adrenal medullary microsomes were characterized by radioligand binding assay, using l-[3H]quinuclidinyl benzilate (QNB), a muscarinic antagonist. Specific [3H]QNB binding to microsomes was rapid, reversible, saturable and of high affinity. Saturation experiments revealed a single class of binding sites for the radioligand with a maximum number of binding sites and an apparent dissociation constant of 162.6 fmoles/mg protein and 40.3 pM respectively. According to computer-assisted nonlinear regression analysis, however, drug/[3H]QNB competition curves indicated the presence of at least two affinity sites for muscarinic agonists (acetylcholine, carbamylcholine, oxotremorine), with a high (K1) and a low (K2) affinity (e.g. K1 = 664.8 nM and K2 = 36.5 microM for acetylcholine). The two affinity sites for acetylcholine showed only minimal regulation by magnesium and guanosine 5'-triphosphate. Furthermore, the presence of two affinity sites was suggested for the antagonists pirenzepine and gallamine, but not for atropine and pilocarpine. The K1 and K2 values for pirenzepine were 23.7 and 429 nM, respectively, with 54.5% of total sites having a high affinity. These results indicate that at least two distinct subtypes of muscarinic receptors exist in the bovine adrenal medulla and that they are distinguished by their relative binding affinity for muscarinic agonists and antagonists. The receptors are predominantly composed of the affinity state termed M1, as described for the receptors of sympathetic ganglia.

Dicyclomine, benzhexol and oxybutynine distinguish between subclasses of muscarinic binding sites

The interactions of various unlabelled antimuscarinic drugs with the muscarinic receptors in the cerebral cortex, heart and urinary bladder were studied by a receptor binding technique, using (-)[3H]QNB as radioligand. In contrast to the other drugs examined, dicyclomine, benzhexol, oxybutynine and pirenzepine were bound with a significantly higher affinity in the cortex than in the heart and bladder. Furthermore, not only pirenzepine, but also dicyclomine and benzhexol were capable of distinguishing between two populations of muscarinic binding sites in the cortex. The low affinity sites for these drugs in the cortex were characterised by dissociation constants which were similar to those determined in the heart and the bladder, respectively. It was concluded that dicyclomine and benzhexol, like pirenzepine, are selective antagonists at the putative M1-receptor. Oxybutynine exhibited the same affinity profile but the tissue selectivity of this drug was less pronounced.

The binding of pirenzepine to digitonin-solubilized muscarinic acetylcholine receptors from the rat myocardium

The binding of pirenzepine to digitonin-solubilized rat myocardial muscarinic acetylcholine receptors has been examined at 4 degrees C. Solubilization produced only small changes in the binding of N-methylscopolamine and atropine. In contrast to the low affinity binding of pirenzepine found to be present in in the membranes, high affinity binding was detected in the soluble preparation. In both preparations, pirenzepine binding was complex. High affinity pirenzepine binding (KD approximately 3 X 10(-8)M) to the soluble myocardial receptors could be monitored directly using [3H]-pirenzepine. [3H]-pirenzepine-labelled soluble myocardial receptors have a sedimentation coefficient of 11.1 s. This indicates that [3H]-pirenzepine binds predominantly to the uncoupled form of the receptor. However, [3H]-pirenzepine-agonist competition experiments indicated that the high affinity pirenzepine binding sites are capable of coupling with a guanosine 5'-triphosphate (GTP)-binding protein. Pirenzepine affinities for the soluble myocardial receptors were unaffected by their state of association with the GTP-binding proteins found in the heart. The equilibrium binding properties of the soluble cortical and myocardial receptors were very similar. However, the binding kinetics of the myocardial receptor were much slower. It appears that the membrane environment can affect the affinity of pirenzepine for the rat myocardial muscarinic receptor. Removal of the constraint by solubilization allows the expression of high affinity pirenzepine binding.

Solubilization and characterization of high and low affinity pirenzepine binding sites from rat cerebral cortex

An apparently monomeric form of the digitonin-solubilized muscarinic acetylcholine receptor from the rat cerebral cortex retains a high affinity of 7 X 10(7) M-1 for pirenzepine. Muscarinic receptor binding sites in the rat cerebral cortex with a low affinity for pirenzepine are solubilized with relatively little change in affinity. The ability of pirenzepine to distinguish between subtypes of muscarinic binding site in the cerebral cortex is manifest in both the membrane-bound and soluble state.

The interaction of McN-A-343 with pirenzepine and other selective muscarine receptor antagonists at a prejunctional muscarine receptor

The effect of several muscarine receptor antagonists on responses to carbachol (CCh) and McN-A-343 (McN) were compared in the perfused rabbit ear artery preparation stimulated via noradrenergic nerves at 3 Hz in the presence of cocaine (10 microM) and yohimbine (1 microM). The slope of the dose-response curve to McN was significantly less (P less than 0.05) than that for CCh although both agonists produced up to 100% inhibition of responses to nervous stimulation. All the antagonists investigated produced parallel shifts of the dose-response curve to the agonists and atropine, fenipramide or stercuronium gave a similar pA2 value with either agonist. Pirenzepine was a competitive antagonist when CCh was used, as judged by a slope of 0.96 +/- 0.10 for the Arunlakshana-Schild (A-S) plot (pKB 6.2). Displacement of 3H-(-)QNB binding by pirenzepine gave a pKI value of 6.0 which was not significantly different to the pKB value. When McN was used as the agonist, the dose-ratios obtained with pirenzepine (0.5 microM) were significantly different (P less than 0.01) to those with CCh as agonist and the slope of the A-S plot over the concentration range of 0.1 to 3 microM was significantly less than 1.0 (P less than 0.01), indicating that the inhibition was not a simple competitive interaction. It is suggested that the interaction of McN and pirenzepine may involve an allosteric mechanism.

Solubilization and characterization of muscarinic receptors from bovine brain

This study compared the capacity of different detergents to solubilize the muscarinic cholinergic receptor (mAChR) from bovine brain, evaluated various procedures for the measurement of [3H]-L-quinuclidinyl benzilate [( 3H]-L-QNB) binding to solubilized receptors, and examined some physical and chemical characteristics of the soluble material. An active form of the mAChR was solubilized using digitonin (1%), Triton X-100 (0.5%), and a digitonin-cholate mixture (1%, 0.1%). Values of maximal binding (Bmax) were 2.01, 0.47, and 0.68 pmoles/mg protein, respectively. Comparison of equilibrium dialysis, charcoal adsorption, and polyethylene glycol precipitation indicated that these methods differ in their estimation of Bmax. A decrease in [3H]-L-QNB binding to digitonin solubilized receptors occurred upon dilution or incubation at 37 degrees. The half-life at 37 degrees C was 25 min., but was increased by glycerol. Antagonist binding to digitonin solubilized receptors was saturable and of high affinity. Agonist binding had Hill coefficients less than 1 and was increased by micromolar concentrations of cupric ions.