The objective of the present study was to determine the binding properties of muscarinic receptors in six brain regions in mature and old rats of both sexes by employing direct binding of [3H]-antagonist as well as of the labeled natural neurotransmitter, [3H]-acetylcholine [( 3H]-AcCh). In addition, age-related factors were evaluated in the modulation processes involved in agonist binding. The results indicate that as the rat ages the density of the muscarinic receptors is altered differently in the various brain regions: it is decreased in the cerebral cortex, hippocampus, striatum and olfactory bulb of both male and female rats, but is increased (58%) in the brain stem of senescent males while no significant change is observed for females. The use of the highly sensitive technique measuring direct binding of [3H]-AcCh facilitated the separate detection of age-related changes in the two classes (high- and low-affinity) of muscarinic agonist binding sites. In old female rats the density of high-affinity [3H]-AcCh binding sites was preserved in all tissues studied, indicating that the decreases in muscarinic receptor density observed with [3H]-antagonist represent a loss of low-affinity agonist binding sites. In contrast, [3H]-AcCh binding is decreased in the hypothalamus and increased in the brain stem of old male rats. These data imply sexual dimorphism of the aging process in central cholinergic mechanisms.

Acetylcholine↗

PubMedDate not supplied

Disopyramide and quinidine bind with inverse selectivity to muscarinic receptors in cardiac and extracardiac rat tissues.

We investigated the interactions of disopyramide and quinidine with the muscarinic receptor in tissue homogenates from rat atrium, ventricle, cortex, submandibular gland, and urinary bladder by means of competition binding experiments, using the tritium-labeled antagonist N-methyl-4-piperidyl benzilate. The drugs displayed heterogeneous characteristics of binding to the muscarinic receptors in the different tissues. The binding affinity of quinidine to the muscarinic receptor in atrial tissue was five to 10 times greater than in the other tissues studied, whereas the affinity of disopyramide to the muscarinic receptor in the heart was five times lower than in the other tissues. This inverse selectivity shown by the two drugs in their binding to cardiac and to noncardiac tissues may explain the extracardiac antimuscarinic side effects of treatment with disopyramide and their absence with quinidine.

Animals↗

PubMedDate not supplied

Competitive inhibition of acetylcholinesterase by bretylium: possible mechanism for its induction of norepinephrine release.

The antiarrhythmic drug bretylium tosylate competitively inhibits acetylcholinesterase activity. The Ki values for the inhibition of the purified enzyme (from electric eel), and acetylcholinesterase activity of crude rat ventricular and cortical homogenates were 6 X 10(-5), 3 X 10(-5), and 8 X 10(-5) M respectively. These values are close to the concentrations of the drug known to induce norepinephrine release from cardiac adrenergic presynaptic vesicles. It is suggested that inhibition of acetylcholinesterase activity by bretylium induces norepinephrine release through the effect of accumulated acetylcholine on nicotinic receptors in adrenergic nerve terminals.

Acetylcholinesterase↗

PubMedDate not supplied

Endothelins and sarafotoxins: effects on motility, binding properties and phosphoinositide hydrolysis during the estrous cycle of the rat uterus.

The effects of four peptides of the endothelin/sarafotoxin (ET/SRTX) family on the motility of the rat uterus were examined during the different stages of the estrous cycle. ET-1, ET-3, SRTX-b and SRTX-c showed similar effects on the contraction of the uterus: a slight increase in the maximum tension of the spontaneous rhythmic contractions, a suppression of the relaxation phase of these contractions and an increase in their rate. All three effects were concentration dependent. Of the four peptides, ET-1 and SRTX-b showed the highest potency and efficacy, suggesting that among the various peptides of this family so far studied, ET-1 and SRTX-b are the two full agonists. The rank order of susceptibility of the different stages was, in most cases: proestrus greater than estrus greater than metestrus. Freshly excised diestrus uteri showed no spontaneous contractions and did not respond to any of the peptides. The binding potency of ET-1 and SRTX-b to uterine membranes was similar at the various estrous stages, but their maximal binding decreased gradually from proestrus to diestrus. All four peptides induced phosphoinositide (PI) hydrolysis in uterine slices at all four different stages, with ET-1 and SRTX-b again being more potent than ET-3 or SRTX-c. The maximal PI hydrolysis correlated with the increased rate of the rhythmic contractions. It is suggested that the reaction of the rat uterus to the ET/SRTX peptides depends on its hormonal status and that ET may act in concert with steroid hormones in the modulation of the estrous cycle.

Animals↗

PubMedDate not supplied

Application of competition kinetics to investigate rat brain muscarinic receptors.

We have recently developed a method for obtaining kinetic parameters of unlabeled ligands that is based on analyzing the effect of their competition on the binding kinetics of a labeled ligand. We employed this method to investigate the kinetics of muscarinic agonist binding to rat brain medulla-pons homogenates. The agonists studied were acetylcholine, carbamylcholine and oxotremorine, with N-methyl-4-[3H]-piperidyl benzilate employed as the radiolabeled ligand. Our results suggest that the binding of muscarinic agonists to high-affinity sites is characterized by dissociation rate constants higher by two orders of magnitude than those of antagonists and by similar association rate constants. In contrast, the major differences between the kinetic binding parameters of agonists and antagonists to the low-affinity agonist binding sites are in the association rate constants, which were 2 to 5 orders of magnitude lower for agonists. The data demonstrate that isomerization of the muscarinic receptors following ligand binding is insignificant in the case of agonists, unlike that of antagonists. Moreover, as is shown in the medulla-pons preparation, agonist-induced interconversion between high- and low-affinity binding sites does not occur to an appreciable extent.

Acetylcholine↗

PubMedDate not supplied

Interaction of the antiarrhythmic drug amiodarone with the muscarinic receptor in rat heart and brain.

The possible interaction between the muscarinic receptor and the antiarrythmic drug amiodarone was studied physiologically in the guinea pig ileum, as well as by competition binding experiments in rat brain and cardiac tissues, using the highly specific tritiated muscarinic antagonist N-methyl-4-piperidyl benzilate. In these studies, amiodarone was found to affect both antagonist and agonist binding to the muscarinic receptor. The drug's inhibitory effect on the binding of antagonist to cerebral cortex muscarinic receptors was consistent with mutually exclusive binding of the compounds [KI = (1.0 +/- 0.2)10(-5) M]. On the other hand, in the brain stem and in cardiac tissues (atrium and ventricle) the inhibitory effect on the binding of muscarinic antagonist could not be fitted to a simple model of competitive inhibition. The possible mode of interaction is discussed. Compared with its activity in the cerebral cortex, amiodarone was a more potent inhibitor of muscarinic antagonist binding in the brain stem and in the atrium and ventricle of the heart [apparent KI values were (6.5 +/- 0.1)10(-6), (4.0 +/- 0.1)10(-6), and (4.0 +/- 0.1)10(-6) M, respectively]. In view of the KI values and the serum concentration of amiodarone observed therapeutically (10(-6) M), the effect of amiodarone on the muscarinic system may have clinical relevance. In both the brain stem and the cardiac preparations, amiodarone converted sites that bind agonist with high affinity into low-affinity sites. Agonist binding in the cerebral cortex was not affected.

Acetylcholine↗