Postjunctional modulation by muscarinic M2 receptors of responses to electrical field stimulation of rat detrusor muscle preparations

Acetylcholinesterase Inhibitors and Drugs Acting on Muscarinic Receptors- Potential Crosstalk of Cholinergic Mechanisms During Pharmacological Treatment

BACKGROUND

Pharmaceuticals with targets in the cholinergic transmission have been used for decades and are still fundamental treatments in many diseases and conditions today. Both the transmission and the effects of the somatomotoric and the parasympathetic nervous systems may be targeted by such treatments. Irrespective of the knowledge that the effects of neuronal signalling in the nervous systems may include a number of different receptor subtypes of both the nicotinic and the muscarinic receptors, this complexity is generally overlooked when assessing the mechanisms of action of pharmaceuticals.

METHODS

We have search of bibliographic databases for peer-reviewed research literature focused on the cholinergic system. Also, we have taken advantage of our expertise in this field to deduce the conclusions of this study.

RESULTS

Presently, the life cycle of acetylcholine, muscarinic receptors and their effects are reviewed in the major organ systems of the body. Neuronal and non-neuronal sources of acetylcholine are elucidated. Examples of pharmaceuticals, in particular cholinesterase inhibitors, affecting these systems are discussed. The review focuses on salivary glands, the respiratory tract and the lower urinary tract, since the complexity of the interplay of different muscarinic receptor subtypes is of significance for physiological, pharmacological and toxicological effects in these organs.

CONCLUSION

Most pharmaceuticals targeting muscarinic receptors are employed at such large doses that no selectivity can be expected. However, some differences in the adverse effect profile of muscarinic antagonists may still be explained by the variation of expression of muscarinic receptor subtypes in different organs. However, a complex pattern of interactions between muscarinic receptor subtypes occurs and needs to be considered when searching for selective pharmaceuticals. In the development of new entities for the treatment of for instance pesticide intoxication, the muscarinic receptor selectivity needs to be considered. Reactivators generally have a muscarinic M2 receptor acting profile. Such a blockade may engrave the situation since it may enlarge the effect of the muscarinic M3 receptor effect. This may explain why respiratory arrest is the major cause for deaths by esterase blocking.

Octodon degus, a new model to study the agonist and plexus-induced response in the urinary bladder

Urinary bladder function consists in the storage and controlled voiding of urine. Translational studies require animal models that match human characteristics, such as Octodon degus, a diurnal rodent. This study aims to characterize the contractility of the detrusor muscle and the morphology and code of the vesical plexus from O. degus. Body temperature was measured by an intra-abdominal sensor, the contractility of detrusor strips was evaluated by isometric tension recording, and the vesical plexus was studied by electrical field stimulation (EFS) and immunofluorescence. The animals showed a diurnal chronotype as judged from core temperature. The myogenic contractile response of the detrusor muscle to increasing doses of KCl reached its maximum (31.04 mN/mm²) at 60 mM. In the case of cumulative dose-response of bethanecol, the maximum response (37.42 mN/mm²) was reached at 3.2 × 10^-4 M. The response to ATP was clearly smaller (3.8 mN/mm²). The pharmacological dissection of the EFS-induced contraction identified ACh and sensory fibers as the main contributors to this response. The neurons of the vesical plexus were located mainly in the trigone area, grouped in big and small ganglia. Out of them, 48.1 % of the neurons were nitrergic and 62.7 % cholinergic. Our results show functional and morphological similarities between the urinary bladder of O. degus and that of humans.

In vitro functional interactions of acetylcholine esterase inhibitors and muscarinic receptor antagonists in the urinary bladder of the rat

Obidoxime, a weak acetylcholine-esterase (AChE) inhibitor, exerts muscarinic receptor antagonism with a significant muscarinic M2 receptor selective profile. The current examinations aimed to determine the functional significance of muscarinic M2 receptors in the state of AChE inhibition, elucidating muscarinic M2 and M3 receptor interaction. In the in vitro examinations, methacholine evoked concentration-dependent bladder contractile and atrial frequency inhibitory responses. Although atropine abolished both, methoctramine (1 μmol/L) only affected the cholinergic response in the atrial preparations. However, in the presence of methoctramine, physostigmine, an AChE inhibitor, increased the basal tension of the bladder strip preparations (+68%), as well as the contractile responses to low concentrations of methacholine (< 5 μmol/L; +90-290%). In contrast to physostigmine, obidoxime alone raised the basal tension (+58%) and the responses to low concentrations of methacholine (< 5 μmol/L; +80-450%). Physostigmine concentration-dependently increased methacholine-evoked responses, similarly to obidoxime at low concentrations. However, at large concentrations (> 5 μmol/L), obidoxime, because of its unselective muscarinic receptor antagonism, inhibited the methacholine bladder responses. In conclusion, the current results show that muscarinic M2 receptors inhibit muscarinic M3 receptor-evoked contractile responses to low concentrations of acetylcholine in the synaptic cleft. The muscarinic M2 and M3 receptor crosstalk could be a counteracting mechanism in the treatment of AChE inhibition when using reactivators, such as obidoxime.

The M2-muscarinic receptor inhibits the development of streptozotocin-induced neuropathy in mouse urinary bladder

We investigate the role of M(2)-muscarinic receptors in maintaining neurogenic bladder contraction during hyperglycemia. Mice were injected with a single dose of streptozotocin (125 mg/kg), and neurogenic contraction of urinary bladder from wild type and M(2)-muscarinic receptor knockout (M(2) KO) mice was measured at 8 to 24 weeks after treatment. In wild-type bladder lacking urothelium, the summation of the cholinergic (64%) and purinergic (56%) components of the electrical-field-stimulated response exceeded 100%, indicating a reserve capacity. Although the cholinergic component was slightly less in the M(2) KO mouse, the total electrical-field-stimulated contraction was the same as wild type. The cholinergic and purinergic components of contraction in wild-type bladder were minimally affected by streptozotocin treatment. In M(2) KO bladder, streptozotocin treatment reduced both the cholinergic (after 8-9 and 20-24 weeks) and purinergic (after 20-24 weeks only) components. The loss of function was approximately 50 to 70%. Similar results were observed in bladder with intact urothelium. M(2) KO bladder was more sensitive to the relaxant effect of isoproterenol compared with wild type, and this difference significantly increased at the early and late time points after streptozotocin treatment. In the presence of urothelium, however, this difference in isoproterenol sensitivity was smaller with streptozotocin treatment, but this trend reversed over time. Our results show that M(2) receptors oppose urinary bladder distension in wild-type bladder and inhibit streptozotocin-induced neuropathy.

Expressions of uroplakins in the mouse urinary bladder with cyclophosphamide-induced cystitis

Even though uroplakins (UPs) are believed to serve a strong protective barrier against toxic materials, cyclophosphamide (CP) causes extensive cystitis. We investigated the expression of UPs in the urothelium in CP induced mouse cystitis. A total of 27 ICR female mice received a single intraperitoneal injection of 200 mg CP/kg. Nine CP-treated mice and 6 controls were sequentially killed at 12, 24, and 72 hr post injection. Extensive cystitis and an increased vesical weight were seen. These all peaked within 12 hr post injection and they tended to decrease thereafter. The level of all the UPs mRNA, the protein expressions of UP II and III on immunoblotting study, and the expression of UP III on immunolocalization study were maximally suppressed within 12 hr; this partially recovered at 24 hr, and this completely recovered at 72 hr post CP injection. In conclusion, CP reduced the expression of UPs. The reduction of the UPs mRNA and protein was time dependent, and this peaked within 12 hr after CP injection. However, the damage was rapidly repaired within 24 hr. This study demonstrates a dynamic process, an extensive reduction and rapid recovery, for the UPs expression of the mouse urinary bladder after CP injection.

In vitro characterization of parasympathetic and sympathetic responses in cyclophosphamide-induced cystitis in the rat

In cyclophosphamide-induced cystitis in the rat, detrusor function is impaired and the expression and effects of muscarinic receptors altered. Whether or not the neuronal transmission may be affected by cystitis was presently investigated. Responses of urinary strip preparations from control and cyclophosphamide-pretreated rats to electrical field stimulation and to agonists were assessed in the absence and presence of muscarinic, adrenergic and purinergic receptor antagonists. Generally, atropine reduced contractions, but in contrast to controls, it also reduced responses to low electrical field stimulation intensity (1-5 Hz) in inflamed preparations. In both types, purinoceptor desensitization with alpha,beta-methylene adenosine-5'-triphosphate (alpha,beta-meATP) caused further reductions at low frequencies (<10 Hz). The muscarinic receptor antagonists atropine, 4-diphenylacetoxy-N-methylpiperidine (4-DAMP) ('M(1)/M(3)/M(5)-selective'), methoctramine ('M(2)-selective') and pirenzepine ('M(1)-selective') antagonized the tonic component of the electrical field stimulation-evoked contractile response more potently than the phasic component. 4-DAMP inhibited the tonic contractions in controls more potently than methoctramine and pirenzepine. In inflamed preparations, the muscarinic receptor antagonism on the phasic component of the electrical field stimulation-evoked contraction was decreased and the pirenzepine and 4-DAMP antagonism on the tonic component was much less efficient than in controls. In contrast to controls, methoctramine increased -- instead of decreased -- the tonic responses at high frequencies. While contractions to carbachol and ATP were the same in inflamed and in control strips when related to a reference potassium response, isoprenaline-induced relaxations were smaller in inflamed strips. Thus, in cystitis substantial changes of the efferent functional responses occur. While postjunctional beta-adrenoceptor-mediated relaxations are reduced, effects by prejunctional inhibitory muscarinic receptors may be increased.