Guanine nucleotides preferentially inhibit binding of antagonists (male) and agonists (female) to muscarinic receptors of rat adenohypophysis.
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Animals↗
Biomedical subjects
S Avissar
Publications and source records attributed to S Avissar.
Temperature dependence of agonist binding to muscarinic receptors in rat hypothalamic regions.
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Animals↗
Molecular regulation of receptors: interaction of beta-estradiol and progesterone with the muscarinic system.
The effects of various substrates on the binding of agonists to muscarinic receptors were studied in the rat hypothalamus and adenohypophysis by competition experiments using the highly specific tritiated muscarinic antagonist N-methyl-4-piperidyl benzilate. It was found that agonist binding properties were affected only by the steroid sex hormones (beta-estradiol and progesterone), both of which resulted in a decrease in the proportion of high-affinity binding sites and a decrease in the dissociation constant. This suggests a link between the muscarinic system and the mechanism by which these steroids exert their gonadotropin-releasing effect on the adenohypophysis. We propose a model to depict the putative relationship between the muscarinic system and other receptor systems, including that which controls the steroid sex hormones.
Animals↗
Studies on muscarinic receptors in mouse and rat hypothalamus: a comparison of sex and cyclical differences.
The ligand binding properties of muscarinic receptors present in whole hypothalamus as well as in its three regions (preoptic area, median and posterior regions) were studied in male rats and mice, as well as in female rats during various stages of the estrous cycle, using the tritiated antagonist N-methyl-4-piperidyl benzilate. Kinetic and equilibrium analysis of antagonist binding as well as competition experiments with agonists, were used to probe the nature of the binding sites and possible differences between specific muscarinic sites in the various preparations. We could detect differences in agonist binding parameters between male and female rats in the preoptic area. In female rats binding of agonist to high affinity state is characterized by a lower affinity (27 nM) than in males (3.7 nM). Secondly, the population of agonist high affinity sites at the proestrous stage is much higher than that at other stages of cycle (66 vs. 38%). In addition, the binding properties of muscarinic receptors following intracisternal 6-hydroxydopamine lesion were investigated. This treatment resulted in a decrease of about one sixth of the total muscarinic receptors in the hypothalamus. The data are best interpreted as a degeneration of existing presynaptic muscarinic receptors located on catecholamine terminals in the hypothalamus. The pre- and postsynaptic nature of the muscarinic receptors, their localization in view of their binding properties, and their possible physiological role are discussed.
Animals↗
Biochemical characterization and sex dimorphism of muscarinic receptors in rat adenohypophysis.
The ligand binding properties of muscarinic receptors present in rat adenohypophysis were studied using the tritiated antagonists N-methyl-4-piperidyl benzilate, 3-quinuclidinyl benzilate, and methylscopolamine. Equilibrium analysis of antagonists binding as well as competition experiments with several agonists were used to probe the nature of binding sites. The nature of antagonist binding in the adenohypophysis points to heterogeneity of binding sites, in contrast to other brain regions, in which homogeneous populations of muscarinic receptors were detected. Some of the binding properties, e.g. affinity constants, population of high affinity agonist binding sites, etc., clearly indicated differences between male and female rats as well as differences in female rats at various stages of estrous cycle. The nature of these receptors and their possible physiological role are discussed.
Animals↗
On the regulation of acetylcholine release: a study utilizing Torpedo synaptosomes and synaptic vesicles.
1. Addition of ATP to isolated highly purified Torpedo synaptic vesicles results in 45Ca2+ uptake. 2. Ca2+ dependent ACh release from Torpedo synaptosomes is accompanied by the phosphorylation of a specific protein with an apparent subunit molecular weight of about 100,000 (band alpha). 3. Activation of the presynaptic muscarinic receptors by an agonist inhibits Ca2+-dependent ACh release from Torpedo synaptosomes. This process seems to be mediated through an interference with the phosphorylation of the band alpha protein, and not by blocking the voltage-dependent presynaptic Ca2+ channel.
Acetylcholine↗
Ca2+-dependent protein phosphorylation of purely cholinergic Torpedo synaptosomes.
Preincubation of intact, purely cholinergic Torpedo synaptosomes with [32P]Pi results in the incorporation of 32P into about 10 specific proteins. Depolarizing the Torpedo synaptosomes by a high K+ buffer or treatment with the Ca2+ ionophore A23187 result in Ca2+ uptake, in acetylcholine (ACh) release, and in a marked increase of 32P incorporation into a specific protein band with an apparent subunit molecular weight of 100,000 (band alpha). The kinetics of synaptosomal 45Ca2+ uptake, of 32P incorporation into band alpha, and of ACh release is similar and reach maximal values about 45 s after the synaptosomes have been treated. Sr2+ and Ba2+ can replace Ca2+ in evoking both K+ depolarization-dependent ACh release and 32P incorporation into band alpha. The effectiveness of these ions (Ca2+ greater than Sr2+ greater than Ba2+) is similar in both cases. The data presented suggest that Ca2+ accumulation by Torpedo synaptosomes leads to an increase in the phosphorylation of a specific protein and to ACh release. This phosphoprotein may be involved in the regulation of presynaptic processes which underly ACh release.
Acetylcholine↗
Mechanism of acetylcholine release: possible involvement of presynaptic muscarinic receptors in regulation of acetylcholine release and protein phosphorylation.
Acetylcholine (AcCho) release from purely cholinergic Torpedo synaptosomes was evoked by K+ depolarization in the presence of Ca2+. Activation of muscarinic receptors, present in the synaptosomal fraction, by the agonist oxotremorine resulted in the inhibition of AcCho liberation. This inhibition was abolished by the muscarinic antagonist atropine, which by itself has no effect. These findings suggest that the muscarinic receptor, present in the electric organ of Torpedo is presynaptic and that its physiological function is to regulate AcCho release by negative feedback. The mechanism of presynaptic muscarinic inhibition was investigated by examining the effect of muscarinic ligands on synaptosomal 45Ca2+ uptake and on the level of phosphorylation of specific synaptosomal proteins. Ca2+-dependent K+ depolarization-induced synaptosomal AcCho release was accompanied by 45Ca2+ uptake and by a marked increase in the phosphorylation of a specific synaptosomal protein (band alpha) of approximately 100,000 daltons. Activation of the muscarinic receptor by the agonist oxotremorine had no detectable effect on synaptosomal 45Ca2+ uptake but resulted in the concomitant inhibition of AcCho release and of phosphorylation of band alpha. The muscarinic antagonist atropine abolished the inhibitory effect of oxotremorine both on AcCho liberation and on phosphorylation of band alpha. These findings suggest that phosphorylation of band alpha may be involved in regulation of the presynaptic processes that underly AcCho release and that activation of the muscarinic receptor by agonists may inhibit AcCho release by blocking the phosphorylation of band alpha.
Acetylcholine↗
Functional and quantitative measures of receptor-coupled G proteins in human mononuclear leukocytes: no change with age.
Aging has been associated with alterations in signal transduction for a number of neurotransmitter receptors in human tissues. Heterotrimeric G proteins play a pivotal role in postreceptor information transduction, by coupling a variety of hormone and neurotransmitter receptors to several intracellular effector functions. Developmental and age-related changes in the abundance of specific G alpha subunits have been shown in the human brain. In the present study, functional and quantitative measures of G proteins were conducted in human mononuclear leukocytes obtained from 19 healthy subjects of increasing age. Gs protein function, assessed through cholera toxin-sensitive beta-adrenergic and dopaminergic agonists induced increases in 3H-Gpp(NH)p binding capacities to membranes of mononuclear leukocytes, and Gi protein function, assessed through pertussis toxin-sensitive muscarinic agonist induced increase in guanine nucleotide binding capacity, were found to be unaltered by increasing age. Immunobloting analyses with specific polyclonal antibodies against G alpha s, G alpha i, and G alpha q subunit proteins in mononuclear leukocyte membranes obtained from the same subjects showed that the quantities of these proteins in mononuclear leukocytes were as well independent of age. Insofar as age-related alterations in cellular information transduction mechanisms in peripheral tissues are important from the etiological, diagnostic, and pharmacological aspects of age-related disorders, it is important to know that both the coupling of receptors to G proteins, the function of these proteins, and their abundance in human peripheral mononuclear leukocytes stays unaltered by the aging process.
Adult↗