Multiple functional defects in peripheral autonomic organs in mice lacking muscarinic acetylcholine receptor gene for the M3 subtype

Muscarinic receptors mediating contraction of female mouse urinary bladder: effects of oestrogen

Muscarinic receptors mediating contraction of bladder detrusor muscle from female mice were examined. Mice were untreated (A) or treated with oestradiol cypionate (200 microg/kg) 24 h (B) or 96 h (C) before experimentation, or were pregnant (day 17) (D). Saturation radioligand binding experiments using [(3)H]quinuclidinyl benzilate ([(3)H] QNB) indicated similar muscarinic receptor densities and affinities in bladders from groups A and B. Neither oestrogen treatment nor pregnancy altered pD(2) estimates for methacholine. Maximum responses to methacholine and high-K(+) physiological salt solution (KPSS) were significantly greater (P<0.05) in tissues from groups C and D than in A and B. Potencies of other muscarinic receptor agonists were similar in groups A and B with an order of acetylcholine plus physostigmine (10 microM) approximately methacholine plus physostigmine (10 microM)>methacholine approximately acetylcholine>bethanechol. Antagonist pK(B) estimates were similar in bladders from groups A and B with a rank order of: atropine>/=4-diphenyl acetoxy-N-methyl piperidine methiodide>parafluorohexahydrosiladifenidol approximately pirenzepine>himbacine, implicating muscarinic M(1) and/or M(5) as well as muscarinic M(3) receptors in mediating methacholine-induced bladder contraction.

Effects of muscarinic receptor type 3 knockout on mouse islet secretory responses

The impact of muscarinic type 3 receptor knockout (M3KO) on the cholinergic regulation of insulin secretion and phospholipase C (PLC) activation was determined. Islets isolated from control, wild-type mice or heterozygotes responded with comparable insulin secretory responses to 15 mM glucose. This response was markedly amplified by the inclusion of 10 microM carbachol. While 15 mM glucose-induced release remained similar to wild-type and heterozygote responses in M3KO mice, the stimulatory impact of carbachol was abolished. Stimulation with 15 mM glucose plus 50 microM carbachol increased fractional efflux rates of myo-[2-3H]inositol from control wild-type and heterozygote islets but not from M3KO islets. Fed plasma insulin levels of M3KO mice were reduced 68% when compared to values obtained from combined wild-type and heterozygote animals. These studies support the conclusion that the M3 receptor in islets is coupled to PLC activation and insulin secretion and that cholinergic stimulation of the islets may play an important role in the regulation of plasma insulin levels.

Loss of vagally mediated bradycardia and bronchoconstriction in mice lacking M2 or M3 muscarinic acetylcholine receptors

The presence of multiple muscarinic acetylcholine receptor (mAChR) subtypes in the heart and lung, combined with the lack of mAChR subtype-selective ligands, have complicated the task of identifying the mAChR subtypes mediating cardiac slowing (bradycardia) and airway narrowing (bronchoconstriction) due to vagal innervation. To determine which of the five mAChRs are responsible for the cholinergic control of heart rate and airway caliber in vivo, we performed experiments on mutant mice lacking the two prime candidates for such control, the M2 or M3 mAChR. Here, we report that in vivo, bradycardia caused by vagal stimulation or administration of the muscarinic agonist methacholine (MCh) was abolished in mice lacking functional M2 mAChRs (M2-/- mice). In contrast, heart rate responses remained unchanged in M3 receptor-deficient mice (M3-/- mice). The reduced hypotensive response of M3-/- mice to MCh suggests M3 mAChRs contribute to peripheral vasodilation. The M2-/- mice showed significantly enhanced in vivo bronchoconstrictor responses to vagal stimulation or MCh administration. In contrast, bronchoconstrictor responses were totally abolished in M3-/- mice. Because altered cardiac or pulmonary vagal tone is involved in a number of pathophysiological conditions, including cardiac arrhythmias, chronic obstructive pulmonary disease and asthma, these results should be of considerable therapeutic relevance.

Ventilatory pattern and chemosensitivity in M1 and M3 muscarinic receptor knockout mice

Acetylcholine (ACh) acting through muscarinic receptors is thought to be involved in the control of breathing, notably in central and peripheral chemosensory afferents and in regulations related to sleep-wake states. By using whole-body plethysmography, we compared baseline breathing at rest and ventilatory responses to acute exposure (5 min) to moderate hypoxia (10% O(2)) and hypercapnia (3 and 5% CO(2)) in mice lacking either the M(1) or the M(3) muscarinic receptor, and in wild-type matched controls. M(1) knockout mice showed normal minute ventilation (V(E)) but elevated tidal volume (V(T)) at rest, and normal chemosensory ventilatory responses to hypoxia and hypercapnia. M(3) knockout mice had elevated V(E) and V(T) at rest, a reduced V(T) response slope to hypercapnia, and blunted V(E) and frequency responses to hypoxia. The results suggest that M(1) and M(3) muscarinic receptors play significant roles in the regulation of tidal volume at rest and that the afferent pathway originating from peripheral chemoreceptors involves M(3) receptors.

Contractile role of M2 and M3 muscarinic receptors in gastrointestinal, airway and urinary bladder smooth muscle

Both M(2) and M(3) muscarinic receptors are expressed in smooth muscle and influence contraction through distinct signaling pathways. M(3) receptors interact with G(q) to trigger phosphoinositide hydrolysis, Ca(2+) mobilization and a direct contractile response. In contrast, M(2) receptors interact with G(i) and G(o) to inhibit adenylyl cyclase and Ca(2+)-activated K(+) channels and to potentiate a Ca(2+)-dependent, nonselective cation conductance. Ultimately, these mechanisms lead to the prediction that the influence of the M(2) receptor on contraction should be conditional upon mobilization of Ca(2+) by another receptor such as the M(3). Mathematical modeling studies of these mechanisms show that the competitive antagonism of a muscarinic response mediated through activation of both M(2) and M(3) receptors should resemble the profile of the directly acting receptor (i.e., the M(3)) and not that of the conditionally acting receptor (i.e., the M(2)). Using a combination of pharmacological and genetic approaches, we have identified two mechanisms for the M(2) receptor in contraction: 1) a high potency inhibition of the relaxation elicited by agents that increase cytosolic cAMP and 2) a low potency potentiation of contractions elicited by the M(3) receptor. The latter mechanism may be involved in muscarinic agonist-mediated heterologous desensitization of smooth muscle, which requires activation of both M(2) and M(3) receptors.

Comparison of in vitro selectivity profiles of solifenacin succinate (YM905) and current antimuscarinic drugs in bladder and salivary glands: a Ca2+ mobilization study in monkey cells

We investigated the effects of the new muscarinic receptor antagonist solifenacin succinate [YM905; (+)-(1S,3'R)-quinuclidin-3'-yl 1-phenyl-1,2,3,4-tetrahydroisoquinoline-2-carboxylate monosuccinate] and the current antimuscarinic drugs for the treatment of overactive bladder (oxybutynin, tolterodine and darifenacin) on intracellular Ca(2+) mobilization in response to M(3) muscarinic receptor activation in bladder smooth muscle and submandibular gland cells isolated from Cynomolgus monkeys. Solifenacin concentration-dependently inhibited carbachol-induced Ca(2+) mobilization, with affinity constant values (pKi) of 8.5 +/- 0.053 in bladder smooth muscle cells and 8.2 +/- 0.051 in submandibular gland cells (n = 5). The pKi value of solifenacin was almost equivalent to the values of oxybutynin, tolterodine and darifenacin in bladder smooth muscle cells (8.7, 8.5 and 8.4, respectively), while being lower than those in submandibular gland cells (9.0, 8.7 and 8.8, respectively). The bladder-selectivity index (Ki ratio: submandibular gland/bladder) for solifenacin (2.1) was statistically higher, moreover, than those for oxybutynin, tolterodine and darifenacin (0.51, 0.65 and 0.46, respectively). These findings consequently indicate solifenacin's unique profile in terms of its selectivity for bladder smooth muscle cells over salivary gland cells in non-human primates, relative to oxybutynin, tolterodine and darifenacin. Solifenacin may, therefore, confer a promising therapeutic advantage for reducing adverse effects, such as dry mouth, exhibited by current antimuscarinic therapy for overactive bladder.

Antimuscarinics for treatment of overactive bladder

For many years, antimuscarinic drugs have been the first-line pharmacological treatment for urgency, frequency, and urge incontinence, all symptoms of the disorder termed overactive bladder. Antimuscarinic treatment is not always effective and is associated with side-effects that limit its clinical use. The clinical significance of the effects of antimuscarinic drugs has been questioned lately. In this review, the rationale for the use of these drugs in the management of overactive bladder is re-examined and the results of treatment are discussed. I conclude that these drugs are the only treatment with undisputed effectiveness in the treatment of overactive bladder. They may not be the perfect treatment for all patients with this disorder, but their value for individual patients should not be underestimated. Further clinical trials with improvement in quality of life as the primary endpoint are needed and may give a fair reflection of the clinical value of antimuscarinic drugs.

Development of ovine fetal ileal motility: role of muscarinic receptor subtypes

OBJECTIVE

In the preterm human fetus, immaturity of gastrointestinal (GI) motility contributes to impairment of oral feeding and an increased risk of necrotizing enterocolitis. In view of the limited knowledge of fetal GI motility development, and the primary role of the muscarinic system in adult GI motility, we examined the development of GI muscarinic receptor subtypes associated with ileal motility.

STUDY DESIGN

Ovine term fetal, newborn, and pregnant adult ileal longitudinal muscle contractile responses to muscarinic agonists (bethanechol) and muscarinic nonspecific (atropine) and subtype specific-antagonists (M1-M4) were examined in organ baths. Immunohistochemical analysis of ileal muscle muscarinic receptor subtypes was correlated with contractile responses.

RESULTS

Bethanechol induced a concentration-dependent ileal contraction at all 3 age groups. Adult ileal maximal tension was 2-fold higher than that of the fetus and newborn, while 50% effective concentration (EC(50)) was similar at all ages. Atropine (10(-6)mol/L) inhibited contractility in fetal (67%+/-7%), newborn (82%+/-5%), and adult (97%+/-2%) in an age-dependent manner. The M3 antagonist exhibited robust inhibition at all age groups while the M2 antagonist demonstrated enhanced inhibition in the fetus. Immunohistochemical analysis indicated coexpression of subtype receptors in fetal, newborn, and adult ileal smooth muscle with increasing expression with advancing age.

CONCLUSION

These results demonstrate a specific developmental pattern of muscarinic receptor subtype expression. Knowledge and/or alterations of GI motility regulation may aid in the treatment of the preterm fetus or newborn.

Impaired gastric secretion and lack of trophic responses to hypergastrinemia in M3 muscarinic receptor knockout mice

BACKGROUND & AIMS

The physiologic significance of the M(3) muscarinic receptor is unclear due to an absence of specific ligand. In the present study, M(3) receptor knockout (KO) mice were used to elucidate the role of M(3) receptors in gastric acid secretion and gastric mucosal integrity.

METHODS

M(3) KO versus wild-type mice aged 1 month to 2 years were included. Gastric acid secretion was assessed by both direct intragastric pH measurement and pylorus ligation. Serum gastrin and gastric mucosal histamine levels were determined by radioimmunoassay and enzyme-linked immunosorbent assay, respectively. Morphologic analysis was performed by both immunohistochemistry and transmission electron microscopy.

RESULTS

Fasted M(3) KO mice exhibited higher intragastric pH, lower acid output after pylorus ligation, a lower proportion of active parietal cells, and higher serum gastrin levels than fasted wild-type mice. Acid secretion in response to carbachol, histamine, gastrin 17, and 2-deoxy-D-glucose was impaired in the mutant mice. Although carbachol was still able to cause approximately 30% acid output in M(3) KO mice, the acid secretion was inhibited by pirenzepine or famotidine. Despite remarkable hypergastrinemia in M(3) KO mice, there were no trophic responses in the oxyntic mucosa with respect to the mucosal thickness, proliferation rate, and numbers of parietal and enterochromaffin-like cells. Cholecystokinin type 2 receptor antagonist YM022 was without the effect in M(3) KO mice.

CONCLUSIONS

The present study shows that M(3) receptors are essential for basal acid secretion, a fully acid secretory response to histamine and gastrin, and the trophic responses of oxyntic mucosa to gastrin.

Acylation-stimulating protein (ASP)/complement C3adesArg deficiency results in increased energy expenditure in mice

Acylation-stimulating protein (ASP) acts as a paracrine signal to increase triglyceride synthesis in adipocytes. In mice, C3 (the precursor to ASP) knock-out (KO) results in ASP deficiency and leads to reduced body fat and leptin levels yet they are hyperphagic. In the present study, we investigated the mechanism for this energy repartitioning. Compared with wild-type (WT) mice, male and female C3(-/-) ASP-deficient mice had elevated oxygen consumption (VO2) in both the active (dark) and resting (light) phases of the diurnal cycle: +8.9% males (p < 0.05) +9.4% females (p < 0.05). Increased physical activity (movement) was observed during the dark phase in female but not in male KO animals. Female WT mice moved 16.9 +/- 2.4 m whereas KO mice moved 30.1 +/- 5.4 m, over 12 h, +78.4%, p < 0.05). In contrast, there was no difference in physical activity in male mice, but a repartitioning of dietary fat following intragastric fat administration was noted. This was reflected by increased fatty acid oxidation in liver and muscle in KO mice, with increased UCP2 (inguinal fat) and UCP3 (muscle) mRNA expression (p = 0.005 and 0.036, respectively). Fatty acid uptake into brown adipose tissue (BAT) and white adipose tissue (WAT) was reduced as reflected by a decrease in the fatty acid incorporation into lipids (BAT -68%, WAT -29%. The decrease of FA incorporation was normalized by intraperitoneal administration of ASP at the time of oral fat administration. These results suggest that ASP deficiency results in energy repartitioning through different mechanisms in male and female mice.

Muscarinic Agonist-Mediated Heterologous Desensitization in Isolated Ileum Requires Activation of Both Muscarinic M2 and M3 Receptors

We investigated the subtypes of the muscarinic receptor mediating short-term heterologous desensitization in the isolated ileum. Treatment of the ileum from C57BL/6 mice with acetylcholine (30 microM) for 20 min caused a subsequent decrease in contractile sensitivity to both prostaglandin F2alpha (PGF2alpha) and the muscarinic agonist, oxotremorine-M. This subsensitivity was characterized by 7- and 3-fold increases in the EC50 values of the agonists, respectively, with no significant effect on the maximal response. The subsensitivity to PGF2alpha was prevented in both M2 and M3 muscarinic receptor knockout mice. Similarly, the subsensitivity to oxotremorine-M was prevented in M2 knockout mice. Acetylcholine-mediated desensitization of histamine-induced contractions in the guinea pig ileum was inhibited by both M2- and M3-selective muscarinic antagonists with high potency, although careful analysis of the data suggested behavior more consistent with an M2 antagonistic profile. Modeling studies showed that the competitive antagonism of response contingent upon activation of two receptor subtypes should exhibit a pharmacological profile similar to that of the least sensitive signaling pathway. Our results demonstrate that muscarinic agonist-mediated short-term heterologous desensitization of intestinal smooth muscle is contingent upon activation of both M2 and M3 muscarinic receptors and that activation of either receptor by itself is insufficient to cause desensitization.

Signal transduction underlying carbachol-induced contraction of rat urinary bladder. I. Phospholipases and Ca2+ sources

We have reexamined the muscarinic receptor subtype mediating carbachol-induced contraction of rat urinary bladder and investigated the role of phospholipase (PL)C, D, and A2 and of intra- and extracellular Ca2+ sources in this effect. Based on the nonsubtype-selective tolterodine, the highly M2 receptor-selective (R)-4-[2-[3-(4-methoxy-benzoylamino)-benzyl]-piperidin-1-ylmethyl]-piperidine-1-carboxylic acid amide (Ro-320-6206), and the highly M3 receptor-selective darifenacin and 3-(1-carbamoyl-1,1-diphenylmethyl)-1-(4-methoxyphenylethyl)pyrrolidine (APP), contraction occurs via M3 receptors. Carbachol stimulated inositol phosphate formation in rat bladder slices, and this was abolished by the phospholipase C inhibitor 1-(6-[([17beta]-3-methoxyestra-1,3,5[10]-trien-17-yl)-amino]hexyl)-1H-pyrrole-2,5-dione (U 73,122; 10 microM). Nevertheless, U 73,122 (1-10 microM) did not significantly affect carbachol-stimulated bladder contraction. Carbachol had only little effect on PLD activity in bladder slices, but the PLD inhibitor butan-1-ol, relative to its negative control butan-2-ol (0.3% each), caused detectable inhibition of carbachol-induced bladder contraction. The cytosolic PLA2 inhibitor arachidonyltrifluoromethyl ketone weakly inhibited carbachol-induced contraction at a concentration of 300 microM, but the cyclooxygenase inhibitor indomethacin (1-10 microM) remained without effect. The Ca2+ entry blocker nifedipine (10-100 nM) almost completely inhibited carbachol-induced bladder contraction. In contrast, 1-[beta-[3-(4-methoxyphenyl)propoxy]-4-methoxyphenethyl]-1H-imidazole HCl (SKF 96,365; 10 microM), an inhibitor of store-operated Ca2+ channels, caused little inhibition. We conclude that carbachol-induced contraction of rat bladder largely depends on Ca2+ entry through nifedipine-sensitive channels and, perhaps, PLD, PLA2, and store-operated Ca2+ channels, whereas cyclooxygenase and, surprisingly, also PLC are not involved to a relevant extent.

Muscarinic mechanisms of antipsychotic atypicality

The interactions of the atypical antipsychotic drugs (APD) clozapine, olanzapine, risperidone, quetiapine and ziprasidone with muscarinic receptors were reviewed. Only clozapine and olanzapine have marked affinity for muscarinic receptors in radioligand binding studies; however, the affinity of these compounds is considerably lower than classical muscarinic antagonists. Although functional assays in cell lines transfected with muscarinic receptors suggest that olanzapine and clozapine have weak partial agonist activity at muscarinic receptors, particularly M4 receptors, studies in vitro and in vivo indicate that the compounds function as antagonists. In animal studies and in humans, clozapine has pronounced antimuscarinic effects whereas olanzapine has weak antimuscarinic effects. However, olanzapine significantly occupies central muscarinic receptors in humans. Overall, the role of muscarinic receptors in the antipsychotic effects of clozapine and olanzapine is controversial and complex.

Muscarinic subtypes profile modulation within a series of new antagonists, bridged bicyclic derivatives of 2,2-diphenyl-[1,3]-dioxolan-4-ylmethyl-dimethylamine

A set of new muscarinic antagonists, bridged bicyclic derivatives of 2,2-diphenyl-[1,3]-dioxolan-4-ylmethyl-dimethylamine (1), was synthesized and tested to evaluate their affinity and selectivity for M(1), M(2), M(3) and M(4) receptor subtypes. The conformational constraint of 1 in a bicyclic structure, and the variation in distance and stereochemistry of the active functions allowed us to modulate the selectivity of interaction with the M(1)-M(3) receptor subtypes. The most interesting compound was (cis,trans)-2-(2,2-diphenylethyl)-5-methyl-tetrahydro-[1,3]dioxolo[4,5-c]pyrrole oxalate (6), which is equipotent with Pirenzepine on rabbit vas deferens (M(1)-putative) but shows a better selectivity profile.

Alimentary tract innervation deficits and dysfunction in mice lacking GDNF family receptor alpha2

Subsets of parasympathetic and enteric neurons require neurturin signaling via glial cell line-derived neurotrophic factor family receptor alpha2 (GFRalpha2) for development and target innervation. Why GFRalpha2-deficient (Gfra2-/-) mice grow poorly has remained unclear. Here, we analyzed several factors that could contribute to the growth retardation. Neurturin mRNA was localized in the gut circular muscle. GFRalpha2 protein was expressed in most substance P-containing myenteric neurons, in most intrapancreatic neurons, and in surrounding glial cells. In the Gfra2-/- mice, density of substance P-containing myenteric ganglion cells and nerve bundles in the myenteric ganglion cell layer was significantly reduced, and transit of test material through small intestine was 25% slower compared to wild-type mice. Importantly, the knockout mice had approximately 80% fewer intrapancreatic neurons, severely impaired cholinergic innervation of the exocrine but not the endocrine pancreas, and increased fecal fat content. Vagally mediated stimulation of pancreatic secretion by 2-deoxy-glucose in vivo was virtually abolished. Retarded growth of the Gfra2-/- mice was accompanied by reduced fat mass and elevated basal metabolic rate. Moreover, the knockout mice drank more water than wild-type controls, and wet-mash feeding resulted in partial growth rescue. Taken together, the results suggest that the growth retardation in mice lacking GFRalpha2 is largely due to impaired salivary and pancreatic secretion and intestinal dysmotility.

Novel insights into muscarinic acetylcholine receptor function using gene targeting technology

Muscarinic acetylcholine receptors (mAChRs) modulate the activity of an extraordinarily large number of physiological functions. Individual members of the mAChR family (M(1)-M(5)) are expressed in a complex, overlapping fashion in most tissues and cell types. However, the identification of the precise physiological roles of individual mAChR subtypes remains a challenging task because, with the exception of a few snake toxins, mAChR ligands that can activate or inhibit specific mAChR subtypes with a high degree of selectivity are not yet available. Knowledge of the specific roles of mAChR subtypes is of considerable interest for the development of novel, clinically useful mAChR ligands. In this article, recent studies of mutant mouse strains developed, using gene targeting techniques, to be deficient in one of the three G(q)-coupled mAChR subtypes (M(1), M(3) and M(5)) are discussed. These investigations have led to many important new insights into the physiological roles of these receptor subtypes.

Postsynaptic M1 and M3 receptors are responsible for the muscarinic enhancement of retrograde endocannabinoid signalling in the hippocampus

The cholinergic system is crucial for higher brain functions including learning and memory. These functions are mediated primarily by muscarinic acetylcholine receptors (mAChRs) that consist of five subtypes (M(1)-M(5)). A recent study suggested a novel role of acetylcholine as a potent enhancer of endocannabinoid signalling that acts retrogradely from postsynaptic to presynaptic neurons. In the present study, we further investigated the mechanisms of this cholinergic effect on endocannabinoid signalling. We made paired whole-cell recordings from cultured hippocampal neurons, and monitored inhibitory postsynaptic currents (IPSCs). The postsynaptic depolarization induced a transient suppression of IPSCs (DSI), a phenomenon known to involve retrograde signalling by endocannabinoids. The cholinergic agonist carbachol (CCh) markedly enhanced DSI at 0.01-0.3 microM without changing the presynaptic cannabinoid sensitivity. The facilitating effect of CCh on DSI was mimicked by the muscarinic agonist oxotremorine-M, whereas it was eliminated by the muscarinic antagonist atropine. It was also blocked by a non-hydrolizable analogue of GDP (GDP-beta-S) that was applied intracellularly to postsynaptic neurons. The muscarinic enhancement of DSI persisted to a substantial degree in the neurons prepared from M1-knockout and M3-knockout mice, but was virtually eliminated in the neurons from M1/M3-compound-knockout mice. CCh still enhanced DSI significantly under the blockade of postsynatpic K(+) conductance, and did not significantly influence the depolarization-induced Ca(2+) transients. These results indicate that the activation of postsynaptic M1 and M3 receptors facilitates the depolarization-induced release of endocannabinoids.

Drug effects on salivary glands: dry mouth

OBJECTIVE

To identify drugs associated with the complaint of dry mouth.

MATERIALS AND METHODS

MEDLINE was searched for papers 1980-2002 using keywords, oral, mouth, salivary, drugs, dry mouth and xerostomia, and relevant secondary references were hand-searched.

RESULTS

Evidence was forthcoming for a number of xerogenic drugs, especially antimuscarinic agents, some sympathomimetic agents, and agents affecting serotonin and noradrenaline uptake, as well as a miscellany of other drugs such as appetite suppressants, protease inhibitors and cytokines.

CONCLUSION

Dry mouth has a variety of possible causes but drugs--especially those with anticholinergic activity against the M3 muscarinic receptor--are the most common cause of reduced salivation.

Loss of anti-cataleptic effect of scopolamine in mice lacking muscarinic acetylcholine receptor subtype 4

Motor dysfunction associated with dyskinesia can be caused by imbalance between dopaminergic and cholinergic actions. Antimuscarinic agents are used to treat extrapyramidal symptoms in Parkinson's disease and extrapyramidal side effects of antipsychotics. These therapeutic effects are mediated by blockade of the striatal muscarinic receptors, which comprise five distinct subtypes (M(1-5)). To evaluate the role of muscarinic M(4) receptors, we have generated mutant mice lacking this subtype (muscarinic M(4) receptor-knockout mice) and analyzed their cataleptic responses induced by haloperidol (an animal model of extrapyramidal side effects). While the muscarinic M(4) receptor-knockout mice developed the cataleptic response normally, systemic administration of scopolamine could not suppress the cataleptic response. These results suggest that acute, but not chronic, blockade of muscarinic M(4) receptors plays important roles in the therapeutic effects of antimuscarinic agents.

Gender comparison of muscarinic receptor expression and function in rat and human urinary bladder: differential regulation of M2 and M3 receptors?

Since symptoms of bladder dysfunction occur more frequently in women than in men and since muscarinic receptors are the physiologically most important system to mediate bladder contraction, we have compared the number, subtype distribution and function of muscarinic receptors in bladders from male and female rats. Muscarinic receptor function was also assessed in bladder strips from male and female human bladder. Male and female rats expressed a similar number of muscarinic receptors (144+/-5 vs. 140+/-6 fmol/mg protein in saturation radioligand binding). While competition binding curves for the moderately M(2)-selective methoctramine were not consistently better fitted by a two-site model, most competition curves for the M(3)-selective darifenacin were biphasic and yielded 29+/-10% and 31+/-7% high affinity sites (corresponding to M(3) receptors) in male and females, respectively. Immunoreactivity of alpha-subunits of the G-proteins G(q/11), G(i1/2), G(i3) and G(s) did not significantly differ between both genders. The muscarinic receptor agonist carbachol similarly stimulated inositol phosphate accumulation in bladder slices from male and female rats with calculated maximum responses of 69+/-17 and 77+/-18% over basal and pEC(50) values of 4.90+/-0.45 and 4.40+/-0.46, respectively. While darifenacin inhibited carbachol-stimulated inositol phosphate formation approximately 100-fold more potently than methoctramine, each antagonist was similarly potent in both genders. Carbachol concentration-dependently contracted bladder strips with a pEC(50) of 5.66+/-0.05 and 5.72+/-0.06 and maximum effects of 4.3+/-0.1 and 4.2+/-0.2 mN/mg wet weight in male and female rats, respectively. The contractile effect of carbachol was concentration-dependently antagonised by the non-selective atropine (1-30 nM), the M(1)-selective pirenzepine (1-30 M), the M(2)-selective methoctramine (1-10 microM) and the M(3)-selective darifenacin (10-100 nM), with the latter exhibiting a partly unsurmountable antagonism. The overall potency of all four antagonists suggested that contraction was mediated predominantly if not exclusively by M(3) receptors with no appreciable differences between both male and female rats. Similarly, the maximum effects (4.4+/-0.6 vs. 4.4+/-2.4 mN/mg) and pEC(50) (6.07+/-0.05 vs. 6.32+/-0.14) of carbachol did not differ between genders in bladder samples from 25 consecutive patients. We conclude that number und function of muscarinic receptors and the relative roles of their M(2) and M(3) subtypes do not differ between urinary bladders of male and female rats; at least with regard to overall muscarinic responsiveness this situation appears to be similar in humans.

Regulation of muscarinic acetylcholine receptor signaling

Multiple mechanisms regulate the signaling of the five members of the family of the guanine nucleotide binding protein (G protein)-coupled muscarinic acetylcholine (ACh) receptors (mAChRs). Following activation by classical or allosteric agonists, mAChRs can be phosphorylated by a variety of receptor kinases and second messenger-regulated kinases. The phosphorylated mAChR subtypes can interact with beta-arrestin and presumably other adaptor proteins as well. As a result, the various mAChR signaling pathways may be differentially altered, leading to short-term or long-term desensitization of a particular signaling pathway, receptor-mediated activation of the mitogen-activated protein kinase pathway downstream of mAChR phosphorylation, as well as long-term potentiation of mAChR-mediated phospholipase C stimulation. Agonist activation of mAChRs may also induce receptor internalization and down-regulation, which proceed in a highly regulated manner, depending on receptor subtype and cell type. In this review, our current understanding of the complex regulatory processes that underlie signaling of mAChR is summarized.

Overactive bladder--pharmacological aspects

The micturition reflex can be initiated by contraction or distension of detrusor smooth muscle cells, or by signals from the urothelium. It has been shown that bladder distension causes release of ATP from the urothelium, and that ATP can activate P2X3 receptors on suburothelial afferent nerve terminals to evoke a neural discharge. However, most probably the activation of afferent fibres during bladder filling involves not only ATP, but a cascade of inhibitory and stimulatory transmitters/mediators. These mechanisms may be targets for future drugs. Both in the normal and functionally disturbed bladder, muscarinic receptor stimulation produces the main part of detrusor contraction, but evidence is accumulating that in disease states, such as neurogenic bladders, outflow obstruction, idiopathic detrusor instability, interstitial cystitis, and also in the ageing bladder, a non-cholinergic activation via purinergic receptors may occur. If this component of activation is responsible not only for part of the bladder contractions, but also for the symptoms of the overactive bladder, it should be considered an important target for therapeutic interventions. Drags blocking different P2X receptor subtypes, or counteracting bladder contraction via other mechanisms, e.g. beta3-adrenoceptor stimulation, may be developed for treatment of the overactive bladder.

Evidence for cross-talk between M2 and M3 muscarinic acetylcholine receptors in the regulation of second messenger and extracellular signal-regulated kinase signalling pathways in Chinese hamster ovary cells

1. We have examined possible mechanisms of cross-talk between the G(q/11)-linked M(3) muscarinic acetylcholine (mACh) receptor and the G(i/o)-linked M(2) mACh receptor by stable receptor coexpression in Chinese hamster ovary (CHO) cells. A number of second messenger (cyclic AMP, Ins(1,4,5)P(3)) and mitogen-activated protein kinase (ERK and JNK) responses stimulated by the mACh receptor agonist methacholine were examined in CHO-m2m3 cells and compared to those stimulated in CHO-m2 and CHO-m3 cell-lines, expressing comparable levels of M(2) or M(3) mACh receptors. 2. Based on comparisons between cell-lines and pertussis toxin (PTx) pretreatment to eliminate receptor-G(i/o) coupling, evidence was obtained for (i) an M(2) mACh receptor-mediated contribution to the predominantly M(3) mACh receptor-mediated Ins(1,4,5)P(3) response and (ii) a facilitation of the inhibitory effect of M(2) mACh receptor on forskolin-stimulated cyclic AMP accumulation by M(3) mACh receptor coactivation at low agonist concentrations (MCh 10(-9)-10(-6) M). 3. The most profound cross-talk effects were observed with respect to ERK activation. Thus, while MCh stimulated ERK activation in both CHO-m2 and CHO-m3 cells (pEC(50) values: 5.64+/-0.09 and 5.57+/-0.16, respectively), the concentration-effect relation was approx 50-fold left-shifted in CHO-m2m3 cells (pEC(50): 7.17+/-0.07). In addition, the ERK response was greater and more sustained in CHO-m2m3 cells. In contrast, only minor differences were seen in the time-courses and concentration-dependencies of JNK activation in CHO-m3 and CHO-m2m3 cells. 4. Costimulation of endogenous P2Y(2) purinoceptors also caused an approx 10-fold left-shift in the MCh-stimulated ERK response in CHO-m2 cells, suggesting that the G(q/11)/G(i/o) interaction to affect ERK activation is not specific to muscarinic receptors. 5. PTx pretreatment of cells had unexpected effects on ERK activation by MCh in both CHO-m2m3 and CHO-m3 cells. Thus, in CHO-m3 cells PTx pretreatment caused a marked left-shift in the MCh concentration-effect curve, while in PTx-treated CHO-m2m3 cells the maximal responsiveness was decreased, but the potency of MCh was only slightly affected. 6. The data presented here strongly suggest that cross-talk between M(2) and M(3) mACh receptors occurs at the level of both second messenger and ERK regulation. Further, these data provide novel insights into the involvement of G(i/o) proteins in both positive and negative modulation of ERK responses evoked by G protein-coupled receptors.

Pharmacological control of gastric acid secretion for the treatment of acid-related peptic disease: past, present, and future

Pharmacological agents, such as histamine H(2) receptor antagonists and acid pump inhibitors, are now the most frequently used treatment for such acid-related diseases as gastroduodenal ulcers and reflux esophagitis. Based on increased understanding of the precise mechanisms of gastric acid secretion at the level of receptors, enzymes, and cytoplasmic signal transduction systems, further possibilities exist for the development of effective antisecretory pharmacotherapy. Gastrin CCK(2) receptor antagonists and locally active agents appear to represent promising therapies for the future. Development of gene targeting techniques has allowed production of genetically engineered transgenic and knockout mice. Such genetic technology has increased the investigative power for pharmacotherapy for not only antisecretory agents, but also treatment of mucosal diseases, such as atrophy, hyperplasia, and cancer. Elucidation of the origin of gastric parietal cells also represents an interesting investigative target that should allow a better understanding of not only acid-related diseases, but also the evolution of the stomach as an acid-secreting organ.

Role of specific muscarinic receptor subtypes in cholinergic parasympathomimetic responses, in vivo phosphoinositide hydrolysis, and pilocarpine-induced seizure activity

Muscarinic agonist-induced parasympathomimetic effects, in vivo phosphoinositide hydrolysis and seizures were evaluated in wild-type and muscarinic M1-M5 receptor knockout mice. The muscarinic agonist oxotremorine induced marked hypothermia in all the knockout mice, but the hypothermia was reduced in M2 and to a lesser extent in M3 knockout mice. Oxotremorine-induced tremor was abolished only in the M2 knockout mice. Muscarinic agonist-induced salivation was reduced to the greatest extent in M3 knockout mice, to a lesser degree in M1 and M4 knockout mice, and was not altered in M2 and M5 knockout mice. Pupil diameter under basal conditions was increased only in the M3 knockout mice. Pilocarpine-induced increases in in vivo phosphoinositide hydrolysis were completely absent in hippocampus and cortex of M1 knockout mice, but in vivo phosphoinositide hydrolysis was unaltered in the M2-M5 knockout mice. A high dose of pilocarpine (300 mg/kg) caused seizures and lethality in wild-type and M2-M5 knockout mice, but produced neither effect in the M1 knockout mice. These data demonstrate a major role for M2 and M3 muscarinic receptor subtypes in mediating parasympathomimetic effects. Muscarinic M1 receptors activate phosphoinositide hydrolysis in cortex and hippocampus of mice, consistent with the role of M1 receptors in cognition. Muscarinic M1 receptors appear to be the only muscarinic receptor subtype mediating seizures.

Increased relaxant action of forskolin and isoproterenol against muscarinic agonist-induced contractions in smooth muscle from M2 receptor knockout mice

The ability of forskolin and isoproterenol to inhibit the contractile action of the muscarinic agonist, oxotremorine-M, was investigated in smooth muscle from wild-type and M(2) muscarinic receptor knockout mice. Forskolin (5.0 micro M) caused a significant reduction in the contractile activity of oxotremorine-M in ileum, trachea, and urinary bladder from both wild-type and M(2) muscarinic receptor knockout mice. This reduction in contractile activity was characterized by decreases in potency or maximal response, but not always both. Similar results were obtained with isoproterenol (1.0 micro M). The relaxant effects of forskolin in ileum, trachea, and urinary bladder from M(2) receptor knockout mice were approximately 3- to 9-fold greater than those observed in the same tissues from wild-type mice. Similar results were obtained with isoproterenol in ileum and urinary bladder, although the differences between wild-type and M(2) receptor knockout tissues were less than those observed with forskolin. In contrast, there was no significant difference between the relaxant effect of isoproterenol in trachea from wild-type and M(2) receptor knockout mice. In contrast to the results observed with oxotremorine-M as the contractile agent, forskolin and isoproterenol did not exhibit greater relaxant activity against KCl-induced contractions in M(2) receptor knockout mice compared with wild-type mice. These results suggest that a component of the contractile response to muscarinic agonists in smooth muscle involves an M(2) muscarinic receptor-mediated inhibition of the relaxant effects of agents that increase cAMP levels.

Use of M1-M5 muscarinic receptor knockout mice as novel tools to delineate the physiological roles of the muscarinic cholinergic system

In this review we report recent findings on the physiological role of the five known muscarinic acetylcholine receptors (mAChRs) as shown by gene targeting technology. Using knockout mice for each mAChRs subtype, the role of mAChRs subtypes in a number of physiological functions was confirmed and new activities were discovered. The M1 mAChRs modulate neurotransmitter signaling in cortex and hippocampus. The M3 mAChRs are involved in exocrine gland secretion, smooth muscle contractility, pupil dilation, food intake, and weight gain. The role of the M5 mAChRs involves modulation of central dopamine function and the tone of cerebral blood vessels. mAChRs of the M2 subtype mediate muscarinic agonist-induced bradycardia, tremor, hypothermia, and autoinhibition of release in several brain regions. M4 mAChRs modulate dopamine activity in motor tracts and act as inhibitory autoreceptors in striatum. Thus, as elucidated by gene targeting technology, mAChRs have widespread and manifold functions in the periphery and brain.

Regulation of muscarinic acetylcholine receptor function in acetylcholinesterase knockout mice

Acetylcholinesterase (AChE) hydrolyzes acetylcholine to terminate cholinergic neurotransmission. Overstimulation of cholinergic receptors by excess acetylcholine is known to be lethal. However, AChE knockout mice live to adulthood, although they have weak muscles, do not eat solid food, and die early from seizures. We wanted to know what compensatory factors allowed these mice to survive. We had previously shown that their butyrylcholinesterase activity was normal and had not increased. In this report, we tested the hypothesis that AChE-/- mice adapted to the absence of AChE by downregulating cholinergic receptors. Receptor downregulation is expected to reduce sensitivity to agonists and to increase sensitivity to antagonists. Physiological response to the muscarinic agonists, oxotremorine (OXO) and pilocarpine, showed that AChE-/- mice were resistant to OXO-induced hypothermia, tremor, salivation, and analgesia, and to pilocarpine-induced seizures. AChE+/- mice had an intermediate response. The muscarinic receptor binding sites measured with [3H]quinuclinyl benzilate, as well as the protein levels of M1, M2, and M4 receptors measured with specific antibodies on Western blots, were reduced to be approximately 50% in AChE-/- brain. However, mRNA levels for muscarinic receptors were unchanged. These results indicate that one adaptation to the absence of AChE is downregulation of muscarinic receptors, thus reducing response to cholinergic stimulation.

Mice lacking M2 and M3 muscarinic acetylcholine receptors are devoid of cholinergic smooth muscle contractions but still viable

Cholinergic agents elicit prominent smooth muscle contractions via stimulation of muscarinic receptors that comprise five distinct subtypes (M1-M5). Although such contractions are important for autonomic organs, the role of each subtype has not been characterized precisely because of the poor selectivity of the currently available muscarinic ligands. Here, we generated a mutant mouse line (M2-/-M3-/- mice) lacking M2 and M3 receptors that are implicated in such cholinergic contractions. The relative contributions of M2 and M3 receptors in vitro was approximately 5 and 95% for the detrusor muscle contraction and approximately 25 and 75% for the ileal longitudinal muscle contraction, respectively. Thus, M1, M4, or M5 receptors do not seem to play a role in such contractions. Despite the complete lack of cholinergic contractions in vitro, M2-/-M3-/- mice were viable, fertile, and free of apparent intestinal complications. The urinary bladder was distended only in males, which excludes a major contribution by cholinergic mechanisms to the urination in females. Thus, cholinergic mechanisms are dispensable in gastrointestinal motility and female urination. After 10 Hz electrical field stimulation, noncholinergic inputs were found to be increased in the ileum of M2-/-M3-/- females, which may account for the lack of apparent functional deficits. Interestingly, the M2-/-M3-/- mice had smaller ocular pupils than M3-deficient mice. The results suggest a novel role of M2 in the pupillary dilation, contrary to the well known cholinergic constriction. These results collectively suggest that an additional mechanism operates in the control of pupillary constriction-dilatation.

Antimuscarinics and the overactive detrusor--which is the main mechanism of action?

Contraction of the bladder, voluntary or involuntary, involves stimulation of the muscarinic receptors on the detrusor by acetylcholine, released from activated cholinergic nerves. Antimuscarinics are the drugs of choice for treatment of detrusor overactivity and the overactive bladder (OAB) syndrome. However, antimuscarinics at clinically recommended doses have little effect on voiding contractions, and may act mainly during the bladder storage phase, during which there is normally no parasympathetic outflow from the spinal cord. Supporting this, antimuscarinics have been shown to reduce bladder tone during storage, and to increase cystometric bladder capacity. A basal release of acetylcholine from non-neuronal (urothelial) as well as neuronal sources has been demonstrated in isolated human detrusor muscle. It is suggested that this release, which is increased by stretching the muscle and in the aging bladder, contributes to detrusor overactivity and OAB by eventually increasing bladder afferent activity during storage.

Knockouts model the 100 best-selling drugs--will they model the next 100?

The biopharmaceutical industry is currently faced with a tremendous number of potential drug targets identified through the sequencing of the human genome. The challenge ahead is to delineate those targets with the greatest value for therapeutic intervention. Here, we critically evaluate mouse-knockout technology for target discovery and validation. A retrospective evaluation of the knockout phenotypes for the targets of the 100 best-selling drugs indicates that these phenotypes correlate well with known drug efficacy, illuminating a productive path forward for discovering future drug targets. Prospective mining of the druggable genome is being catalysed by large-scale mouse knockout programs combined with phenotypic screens focused on identifying targets that modulate mammalian physiology in a therapeutically relevant manner.

Mice lacking M2 and M3 muscarinic acetylcholine receptors are devoid of cholinergic smooth muscle contractions but still viable

Cholinergic agents elicit prominent smooth muscle contractions via stimulation of muscarinic receptors that comprise five distinct subtypes (M1-M5). Although such contractions are important for autonomic organs, the role of each subtype has not been characterized precisely because of the poor selectivity of the currently available muscarinic ligands. Here, we generated a mutant mouse line (M2-/-M3-/- mice) lacking M2 and M3 receptors that are implicated in such cholinergic contractions. The relative contributions of M2 and M3 receptors in vitro was approximately 5 and 95% for the detrusor muscle contraction and approximately 25 and 75% for the ileal longitudinal muscle contraction, respectively. Thus, M1, M4, or M5 receptors do not seem to play a role in such contractions. Despite the complete lack of cholinergic contractions in vitro, M2-/-M3-/- mice were viable, fertile, and free of apparent intestinal complications. The urinary bladder was distended only in males, which excludes a major contribution by cholinergic mechanisms to the urination in females. Thus, cholinergic mechanisms are dispensable in gastrointestinal motility and female urination. After 10 Hz electrical field stimulation, noncholinergic inputs were found to be increased in the ileum of M2-/-M3-/- females, which may account for the lack of apparent functional deficits. Interestingly, the M2-/-M3-/- mice had smaller ocular pupils than M3-deficient mice. The results suggest a novel role of M2 in the pupillary dilation, contrary to the well known cholinergic constriction. These results collectively suggest that an additional mechanism operates in the control of pupillary constriction-dilatation.

Radiation hybrid mapping of five muscarinic acetylcholine receptor subtype genes in Rattus norvegicus

Acetylcholine is the main neurotransmitter of the vestibular efferent system and a wide variety of muscarinic and nicotinic acetylcholine receptors are expressed in the vestibular periphery. The role of these receptors and in particular the role of muscarinic acetylcholine receptors in the physiology of the vestibular neuroepithelium is not understood. Congenic and consomic rats are a convenient way to investigate the involvement of candidate genes in the manifestation of defined traits. To use congenic or consomic rats to elucidate the roles of these receptors in vestibular physiology or pathology the chromosomal location of the genes encoding these receptors has to be determined. Using radiation hybrid (RH) mapping and a rat RH map server (www.rgd.mcw.edu/RHMAP SERVER/), we determined the chromosomal locations of the muscarinic acetylcholine receptor genes in the rat (Rattus norvegicus). The m1-m5 muscarinic subtypes mapped to the following chromosomes: Chrm1, chromosome 1; Chrm2, chromosome 4; Chrm3, chromosome 17; Chrm4, chromosome 3; and Chrm5, chromosome 3. With the chromosomal location for each of these muscarinic subtypes known, it is now possible to develop congenic and consomic strains of rats that can be used to study the functions of each of these subtypes.

Metabolic lessons from genetically lean mice

Different types of lean mice have been produced by genetic manipulation. Leanness can result from deficiency of stored energy or a lack of adipocytes to store the lipid. Mice lacking functional adipocytes are usually insulin resistant and have fatty livers, and elevated circulating triglyceride levels. Insulin resistance may result from the lack of adipocyte hormones (such as leptin) and increased metabolite (such as triglyceride) levels in nonadipose tissue. Mice with depleted adipocyte triglyceride levels typically are insulin sensitive and have normal or low liver and circulating triglycerides. Mechanisms to produce depleted adipocytes include increased energy expenditure by peripheral tissues, peripheral mechanisms to decrease food intake, and altered central regulation of these processes.

Current and future pharmacological treatment for overactive bladder

PURPOSE

Urinary incontinence and overactive bladder are important and common conditions that have received little general medical attention. We reviewed the magnitude and impact of these conditions, and discuss pharmacotherapy as well as new drugs under investigation.

MATERIALS AND METHODS

The main emphasis of this review is pharmacological therapy for the bladder. We discuss currently available agents, drugs under development and pharmacological targets that would be suitable targets for treating overactive bladder. Drugs such as duloxetine that target not bladder smooth muscle, but rather central nervous system control of the micturition reflex are undergoing clinical trials. We also discuss intravesical therapy and alternative drug delivery methods, such as intravesical capsaicin and botulinum toxin, with special emphasis on approaches to modulate bladder afferent nerve function for preventing overactive bladder.

RESULTS

There are many advantages to advanced drug delivery systems, including long-term therapeutic efficacy, decreased side effects and improved patient compliance. Future speculation such as gene therapy holds great promise for overactive bladder because it is possible to access all genitourinary organs via endoscopy and other minimally invasive techniques that are ideally suited for gene therapy.

CONCLUSIONS

Traditional anticholinergic therapies are limited in their effectiveness. There is great hope for future research regarding voiding dysfunction and urinary incontinence through a focus on afferent nerve intervention for preventing overactive bladder.

Muscarinic receptor subtypes and management of the overactive bladder

Anticholinergic agents are the most widely used therapy for urge incontinence despite exerting adverse effects, such as constipation, tachycardia, and dry mouth, that limit their use. These adverse effects result from a lack of selectivity for the bladder over other organs. Although M2-muscarinic receptors are the predominant cholinoreceptor present in urinary bladder, the smaller population of M3-receptors appears to be the most functionally important and mediates direct contraction of the detrusor muscle. M2-receptors modulate detrusor contraction by several mechanisms and may contribute more to contraction of the bladder in pathologic states, such as bladder denervation or spinal cord injury. Prejunctional inhibitory M2-receptors or M4-receptors and prejunctional facilitatory M1-muscarinic receptors in the bladder have also been reported, but their relevance to the clinical effectiveness of muscarinic antagonists is unknown. In clinical studies, tolterodine, a nonselective muscarinic antagonist, has been reported to be equally effective to oxybutynin but to induce less dry mouth. Controlled-release and intravesical, intravaginal, and rectal administrations of oxybutynin have all been reported to cause fewer adverse effects. Conversely, darifenacin, a new M3-selective antagonist, has been reported to have selectivity for the bladder over the salivary gland in vivo. Whether M3-selective or nonselective muscarinic antagonists will be the most clinically effective for the overactive bladder-preserving the best balance between efficacy and tolerability-has yet to be established, and comparative clinical trials between compounds, such as darifenacin (M3 selective) and tolterodine (nonselective) will be required.

Evaluation of muscarinic agonist-induced analgesia in muscarinic acetylcholine receptor knockout mice

Centrally active muscarinic agonists display pronounced analgesic effects. Identification of the specific muscarinic acetylcholine receptor (mAChR) subtype(s) mediating this activity is of considerable therapeutic interest. To examine the roles of the M(2) and M(4) receptor subtypes, the two G(i)/G(o)-coupled mAChRs, in mediating agonist-dependent antinociception, we generated a mutant mouse line deficient in both M(2) and M(4) mAChRs [M(2)/M(4) double-knockout (KO) mice]. In wild-type mice, systemic, intrathecal, or intracerebroventricular administration of centrally active muscarinic agonists resulted in robust analgesic effects, indicating that muscarinic analgesia can be mediated by both spinal and supraspinal mechanisms. Strikingly, muscarinic agonist-induced antinociception was totally abolished in M(2)/M(4) double-KO mice, independent of the route of application. The nonselective muscarinic agonist oxotremorine showed reduced analgesic potency in M(2) receptor single-KO mice, but retained full analgesic activity in M(4) receptor single-KO mice. In contrast, two novel muscarinic agonists chemically derived from epibatidine, CMI-936 and CMI-1145, displayed reduced analgesic activity in both M(2) and M(4) receptor single-KO mice, independent of the route of application. Radioligand binding studies indicated that the two CMI compounds, in contrast to oxotremorine, showed >6-fold higher affinity for M(4) than for M(2) receptors, providing a molecular basis for the observed differences in agonist activity profiles. These data provide unambiguous evidence that muscarinic analgesia is exclusively mediated by a combination of M(2) and M(4) mAChRs at both spinal and supraspinal sites. These findings should be of considerable relevance for the development of receptor subtype-selective muscarinic agonists as novel analgesic drugs.

Pharmacologic perspective on the physiology of the lower urinary tract

Myogenic activity, distention of the detrusor, and signals from the urothelium may initiate voiding. In the bladder, afferent nerves have been identified not only in the detrusor, but also suburothelially, where they form a plexus that lies immediately beneath the epithelial lining. Extracellular adenosine triphosphate (ATP) has been found to mediate excitation of small-diameter sensory neurons via P2X3 receptors, and it has been shown that bladder distention causes release of ATP from the urothelium. In turn, ATP can activate P2X3 receptors on suburothelial afferent nerve terminals to evoke a neural discharge. However, most probably, not only ATP but also a cascade of inhibitory and stimulatory transmitters and mediators are involved in the transduction mechanisms underlying the activation of afferent fibers during bladder filling. These mechanisms may be targets for future drugs. The central nervous control of micturition involves many transmitter systems, which may be suitable targets for pharmacologic intervention. gamma-Aminobutyric acid, dopamine, enkephalin, serotonin, and noradrenaline receptors and mechanisms are known to influence micturition, and potentially, drugs that affect these systems could be developed for clinical use. However, a selective action on the lower urinary tract may be difficult to obtain. Most drugs currently used for treatment of detrusor overactivity have a peripheral site of action, mainly the efferent (cholinergic) neurotransmission and/or the detrusor muscle itself. In the normal bladder, muscarinic receptor stimulation produces the main part of detrusor contraction, but evidence is accumulating that in disease states, such as neurogenic bladders, outflow obstruction, idiopathic detrusor instability, and interstitial cystitis, as well as in the aging bladder, a noncholinergic activation via purinergic receptors may occur. If this component of activation is responsible not only for part of the bladder contractions, but also for the symptoms of the overactive bladder, it should be considered an important target for therapeutic interventions.

Muscarinic receptors in isolated urinary bladder smooth muscle from different mouse strains

1. The pharmacological characteristics of muscarinic receptors in male and female mouse urinary bladder smooth muscle from different strains (C57Bl/6, 129/SvJ and hybrid backcross N1F2) were studied. 2. (+)-Cis-dioxolane, oxotremorine-M, acetylcholine, carbachol and pilocarpine induced concentration-dependent contractions of the urinary bladder smooth muscle (range for pEC(50)=6.4-6.6, 6.2-6.7, 6.2-6.4, 5.4-6.0 and 0.0-5.1, T(max)=1.9-4.7 g, 1.3-3.4 g, 1.0-3.0 g, 1.4-2.4 and 0.0-0.3 g, respectively, n=4-6 depending on the gender and the strain). In females, these contractions were competitively antagonized by a range of muscarinic receptor antagonists (pK(B) value range, depending on the strain): atropine (8.0-8.9), pirenzepine (6.1-6.4), 4-DAMP (7.6-8.4), methoctramine (5.6-6.1), p-F-HHSiD (7.5-7.7), zamifenacin (7.7-8.4) and darifenacin (8.2-8.7). 3. In recontraction studies, in which the muscarinic M(3) receptor population was decreased, and conditions optimized to study M(2) receptor activation, methoctramine exhibited an affinity estimate consistent with muscarinic M(3) receptors (pK(B)=6.26+/-0.08, pA(2)=6.31+/-0.07; pK(B)=6.09+/-0.22, pA(2)=6.08+/-0.01 for female inbred strain 129/SvJ and hybrid backcross N1F2, respectively) or intermediate between the one expected for this compound at M(2) and M(3) receptors, (pK(B)=6.66+/-0.08, pA(2)=7.00+/-0.27 for female inbred strain C57BL/6). 4. These data study suggest that muscarinic M(3) receptors are the predominant, if not the exclusive, subtype mediating contractile responses to muscarinic agonists in female mouse urinary bladder smooth muscle, with strain differences.

Role of the cholinergic muscarinic system in bipolar disorder and related mechanism of action of antipsychotic agents

The evidence for the involvement of cholinergic muscarinic receptors in mania and depression is reviewed. Small pilot trials with cholinesterase inhibitors and muscarinic agonists suggest that stimulation of muscarinic receptors may produce an antimanic effect, possibly by activation of muscarinic M(4) receptors. It is concluded that it is not likely that currently used mood stabilizers, such as lithium, valproic acid and carbamazepine, work directly through muscarinic receptor mechanisms. Furthermore, the evidence indicates that antipsychotic agents used for mania are working through the common mechanism of antagonism of dopamine D(2) receptors, and interactions with muscarinic receptors do not play a key role. Finally, it is hypothesized that olanzapine has robust antimanic activity, due to blockade of dopamine D(2) receptors and antagonism of other monoaminergic receptors. Olanzapine may normalize mood due to antidepressant-like activities, such as 5-HT(2A) receptor antagonism and increasing cortical norepinephrine and dopamine.

Multiple muscarinic acetylcholine receptor subtypes modulate striatal dopamine release, as studied with M1-M5 muscarinic receptor knock-out mice

A proper balance between striatal muscarinic cholinergic and dopaminergic neurotransmission is required for coordinated locomotor control. Activation of striatal muscarinic acetylcholine receptors (mAChRs) is known to modulate striatal dopamine release. To identify the mAChR subtype(s) involved in this activity, we used genetically altered mice that lacked functional M1-M5 mAChRs [knock-out (KO) mice]. In superfused striatal slices from wild-type mice, the non-subtype-selective muscarinic agonist oxotremorine led to concentration-dependent increases in potassium-stimulated [3H]dopamine release (by up to 60%). The lack of M1 or M2 receptors had no significant effect on the magnitude of these responses. Strikingly, oxotremorine-mediated potentiation of stimulated striatal [3H]dopamine release was abolished in M4 receptor KO mice, significantly increased in M3 receptor-deficient mice, and significantly reduced (but not abolished) in M5 receptor KO mice. Additional release studies performed in the presence of tetrodotoxin suggested that the dopamine release-stimulating M4 receptors are probably located on neuronal cell bodies, but that the release-facilitating M5 and the release-inhibiting M3 receptors are likely to be located on nerve terminals. Studies with the GABA(A) receptor blocker bicuculline methochloride suggested that M3 and M4 receptors mediate their dopamine release-modulatory effects via facilitation or inhibition, respectively, of striatal GABA release. These results provide unambiguous evidence that multiple mAChR subtypes are involved in the regulation of striatal dopamine release. These findings should contribute to a better understanding of the important functional roles that the muscarinic cholinergic system plays in striatal function.

Physiology of female micturition

Micturition is a dynamic physiologic process consisting of alternating storage and expulsion phases and is accomplished by complex interactions among innervation, smooth muscle, connective tissue, urothelium and supportive structures. Although our current understanding of the anatomy and physiology of the lower urinary tract is far from complete, intensive research over the last decade has dramatically improved our appreciation of the neural, biomechanical, biochemical, and morphologic properties of the bladder and urethra, as well as the hormonal influences and unique pelvic and perineal anatomy of women. Continued research related to the physiology of female micturition promises to offer new insights into the complex bladder-urethral interactions and to provide a basis for developing better management strategies for a variety of voiding dysfunctions in women.

Possible involvement of M5 muscarinic receptor in the enhancing actions of the novel gastroprokinetic agent Z-338 on nifedipine-sensitive voltage-dependent Ca2+ currents in guinea pig stomach

We investigated the effects of the novel gastroprokinetic agent Z-338 (N-(N-N'-diisopropylaminoethyl)-[2-(2-hydroxy-4,5-dimethoxybenzoylamino)-1,3-thiazole-4-yl] carboxyamide monohydrochloride trihydrate) on L-type voltage-dependent Ca2+ currents (ICa) in guinea pig gastric myocytes by using the whole-cell patch clamp technique. Bath-applied acetylcholine (ACh) produced biphasic effects on ICa, i.e., enhancement (1-100 nM) and inhibition (1-100 microM), both of which were abolished by pretreatment with atropine (10 microM) or intracellular perfusion of GDPbetaS (500 microM). Z-338 (> or = 1 nM, ED50: 120 nM) mimicked the enhancing effects of ACh, but did not inhibit ICa. The effects of Z-338 and ACh were non-additive and blocked by atropine and GDPbetaS, but not by pertussis toxin (PTX) pretreatment (500 ng/ml). ACh (> or = 1 microM) induced slow inward currents via activation of the muscarinic receptor/PTX-sensitive G-protein pathway, but Z-338 was devoid of these effects. Neither pirenzepine (1 microM), AF-DX116 (1 microM), nor oxybutynin (100 nM) could prevent Z-338 (1 microM) and ACh (10 nM) from enhancing ICa, whilst 4-DAMP (100 nM) blocked the effects of Z-338 and ACh. Bath-application of protein kinase C (PKC) activator PDBu (phorbol-12,13-dibutyrate) (250 nM) enhanced ICa, and conversely, pipette inclusion of PKC inhibitor peptide (150 microM) abolished the effects of ACh and Z-338 on ICa. These results collectively suggest that although contribution of the M3 receptor is not excluded, the major actions of Z-338 on gastric myocytes are potentiation of ICa through activation of M5-like receptor.