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Discussion: functional role of M1, M2, and M3 muscarinic receptors in overactive bladder
DISCUSSION: FUNCTIONAL ROLE OF M1, M2, AND M3 MUSCARINIC RECEPTORS IN OVERACTIVE BLADDER YASUHIKO IGAWA
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harmacologic treatment of overactive bladder should aim principally to decrease detrusor activity and increase functional bladder capacity during the filling phase. It should not, however, decrease detrusor contractility during the voiding phase. This profile of activity should improve the symptoms of incontinence and urinary frequency. Is there an antimuscarinic drug that meets these criteria? Unfortunately, bladder relaxants, including antimuscarinic drugs, generally inhibit detrusor activity not only during the filling phase but also during the voiding phase. It is therefore often difficult to achieve adequate bladder capacity without residual urine. Elderly people, in particular, show weak detrusor contractions during voiding, which often results in inefficient voiding and a significant amount of residual urine. As Dr. Chapple emphasized in his presentation, desirable antimuscarinic drugs for the treatment of overactive bladder should have bladder selectivity without undesirable actions on other organs, including the salivary glands, gastrointestinal tract, heart, and eye. Selectivity for facilitation of the storage function without any inhibitory activity on voiding function is also important. The muscarinic receptor family currently comprises five pharmacologically defined receptors, M1–M5.1,2 Immunoprecipitation studies have demonstrated the existence of both M2 and M3 receptors in the human detrusor membrane, the ratio being 3:1.3 Radioligand binding studies have shown a dominant (60% to 80%) M2 population and a smaller (20% to 40%) M3 population in human bladder.2,4 Despite the predominance of M2 receptors in the detrusor of several species, it is well documented that the pharmacologically defined M3 subtype mediates bladder contraction. Acetylcholine (ACh), released from postganglionic cholinFrom the Department of Urology, Shinshu University School of Medicine, Matsumoto, Japan Reprint requests: Yasuhiko Igawa, MD, PhD, Associate Professor, Department of Urology, Shinshu University School of Medicine, 3-1-1 Asahi, Matsumoto 390-8621, Japan © 2000, ELSEVIER SCIENCE INC. ALL RIGHTS RESERVED
ergic nerves, interacts with postjunctional M3 muscarinic receptors to cause direct detrusor contraction via stimulation of phospholipase C, thereby increasing phosphoinositide turnover. This raises the question: what is the functional role of the large population of M2 receptors in the detrusor membrane? During the filling phase, the bladder is inhibited by sympathetic stimulation, whereas parasympathetic stimulation is normally suppressed. Norepinephrine (NA), released from sympathetic nerves, increases bladder compliance through -adrenoceptor–mediated relaxation of the bladder by stimulating adenylate cyclase. The -adrenoceptors mediating relaxation of the human bladder have been defined as the 3 subtype.5,6 In rats, Hegde et al.7 demonstrated that M2 receptors can indirectly mediate bladder contraction, both in vitro and in vivo, by reversing -adrenoceptor–mediated relaxation. During the voiding phase, ACh is released from parasympathetic nerves. This activates postjunctional M2 receptors, and causes inhibition of the -adrenoceptor–mediated stimulation of adenylate cyclase. The result is reversal of the relaxed tone of the bladder, in concert with M3-receptor– mediated direct contraction of the bladder. This dual mechanism may provide more efficient and complete emptying of the bladder. Recently, such M2 muscarinic receptor–mediated inhibition of adenylate cyclase has also been demonstrated in cultured human detrusor cells.8 In addition, it has been proposed that sympathetic inhibitory stimulation of the bladder partially reduces the level of bladder wall tension transduced by the bladder wall mechanoreceptors, and thereby delays the time at which the micturition threshold is attained.9 Braverman et al.10 demonstrated an increase in M2 receptor density with no change in M3 receptor density in the hypertrophic bladders of rats following pelvic denervation or spinal cord injury. This increase in M2 receptor density was consistent with the change in affinity of the antagonists of inhibition of carbachol-induced contractions. The authors concluded that M2 receptors or a combinaUROLOGY 55 (5A): 47– 49, 2000 • 0090-4295/00/$20.00 PII S0090-4295(99)00493-8 47
FIGURE 1. Muscarinic receptors and -adrenoceptors regulate bladder tone and contraction in overactive bladder. AC ⫽ adenylate cyclase; Ach ⫽ acetylcholine; DG ⫽ diacylglycerol; IP3 ⫽ inositol trisphosphate; PIP ⫽ polyinositol trisphosphate; PLC⫽ phospholipase C; NE ⫽ norepinephrine; ROC ⫽ receptor-operated channel; SR ⫽ sarcoplasmic reticulum.
FIGURE 2. Pharmacotherapy for overactive bladder.
tion of M2 and M3 receptors may directly mediate detrusor contraction in these pathologic rats (Figure 1). Taken together, 3 agonists alone or combined with an M2 antagonist might be a promising choice of treatment for overactive bladder and facilitate a storage function as, theoretically, these 48
drugs do not directly inhibit voiding bladder contractions (Figure 2). Antimuscarinic drugs also act prejunctionally at the cholinergic nerve terminals. It has been shown that inhibitory M2/4 receptors and facilitatory M1 receptors that regulate ACh release exist at the UROLOGY 55 (Supplement 5A), May 2000
cholinergic nerve terminals in the rat11,12 and rabbit13 bladder. A mechanism involving such facilitatory prejunctional M1 receptors may contribute to the dysfunction observed in overactive bladder (Figure 1). In fact, it has been proposed that presynaptic muscarinic facilitatory mechanisms are up-regulated in the cholinergic nerve terminals in the bladder of chronic spinal cord–transected rats.14 Therefore, antagonists of M1 receptors may inhibit a certain type of overactive detrusor contraction (Figure 2). In conclusion, though the muscarinic receptors remain the main target in the pharmacologic treatment of overactive bladder, the complex functions of the muscarinic receptors in the bladder and elsewhere in the body make it difficult to determine the optimal profile of subtype selectivity of antimuscarinic drugs for overactive bladder. Both M2 and M3 receptors may be functionally operative in the bladder, though their relative contributions may be altered by disease and by aging. Prejunctional facilitatory M1 receptors may contribute to a certain type of pathologic condition. Finally, the effects of muscarinic receptor blockade in the central nervous system should not be ignored. REFERENCES 1. Caulfield MP, and Birdsall NJM: International Union of Pharmacology: XVII. Classification of muscarinic acetylcholine receptors. Pharmacol Rev 50: 279 –290, 1998. 2. Hegde SS, and Eglen RM: Muscarinic receptor subtypes modulating smooth muscle contractility in the urinary bladder. Life Sci 64: 419 – 428, 1999. 3. Wang P, Luthin GR, and Ruggieri MR: Muscarinic acetylcholine receptor subtypes mediating urinary bladder contractility and coupling to GTP binding proteins. J Pharmacol Exp Ther 273: 959 –966, 1995.
UROLOGY 55 (Supplement 5A), May 2000
4. Chiarini A, Budriesi R, Bolognesi NL, et al: In vitro characterization of tripitramine, a polymethylene tetraamine displaying high selectivity and affinity for muscarinic M2 receptors. Br J Pharmacol 114: 1507–1517, 1995. 5. Igawa Y, Yamazaki Y, Takeda H, et al: Possible 3-adrenoceptor-mediated relaxation of the human detrusor. Acta Physiol Scand 164: 117–118, 1998. 6. Igawa Y, Yamazaki Y, Takeda H, et al: Functional and molecular biological evidence for a possible 3-adrenoceptor in the human detrusor muscle. Br J Pharmacol 126: 819 – 825, 1999. 7. Hegde SS, Choppin A, Bonhaus D, et al: Functional role of M2 and M3 muscarinic receptors in the urinary bladder of rats in vitro and in vivo. Br J Pharmacol 120: 1409 –1418, 1997. 8. Daniels DV, Meloy TD, Loury DN, et al: Functional pharmacological characterization of the muscarinic cholinoceptor-mediated inhibition of adenylyl cyclase in primary cultured human bladder detrusor smooth muscle cells (abst). Life Sci 64: 590, 1999. 9. Vaughan CW, and Satchell PM: Urine storage mechanisms. Prog Neurobiol 46: 215–237, 1995. 10. Braverman A, Legos J, Young W, et al: M2 receptors in genito-urinary smooth muscle pathology. Life Sci 64: 429 – 436, 1999. 11. Somogyi GT, and de Groat WC: Evidence for inhibitory nicotinic and facilitatory muscarinic receptors in cholinergic nerve terminals of the rat urinary bladder. J Auton Nerv Syst 37: 89 –98, 1992. 12. Braverman AS, Kohn IJ, Luthin GR, et al: Prejunctional M1 facilitatory and M2 inhibitory muscarinic receptors mediate rat bladder contractility. Am J Physiol 274(2, Part 2): R517–R523, 1998. 13. Tobin G, and Sju¨gren C: In vivo and in vitro effects of muscarinic receptor antagonists on contractions and release of [3H]acetylcholine in the rabbit urinary bladder. Eur J Pharmacol 281: 1– 8, 1995. 14. Somogyi GT, Zernova GV, Yoshiyama M, et al: Frequency dependence of muscarinic facilitation of transmitter release in urinary bladder strips from neurally intact or chronic spinal cord transected rats. Br J Pharmacol 125: 241– 246, 1998.
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harmacologic treatment of overactive bladder should aim principally to decrease detrusor activity and increase functional bladder capacity during the filling phase. It should not, however, decrease detrusor contractility during the voiding phase. This profile of activity should improve the symptoms of incontinence and urinary frequency. Is there an antimuscarinic drug that meets these criteria? Unfortunately, bladder relaxants, including antimuscarinic drugs, generally inhibit detrusor activity not only during the filling phase but also during the voiding phase. It is therefore often difficult to achieve adequate bladder capacity without residual urine. Elderly people, in particular, show weak detrusor contractions during voiding, which often results in inefficient voiding and a significant amount of residual urine. As Dr. Chapple emphasized in his presentation, desirable antimuscarinic drugs for the treatment of overactive bladder should have bladder selectivity without undesirable actions on other organs, including the salivary glands, gastrointestinal tract, heart, and eye. Selectivity for facilitation of the storage function without any inhibitory activity on voiding function is also important. The muscarinic receptor family currently comprises five pharmacologically defined receptors, M1–M5.1,2 Immunoprecipitation studies have demonstrated the existence of both M2 and M3 receptors in the human detrusor membrane, the ratio being 3:1.3 Radioligand binding studies have shown a dominant (60% to 80%) M2 population and a smaller (20% to 40%) M3 population in human bladder.2,4 Despite the predominance of M2 receptors in the detrusor of several species, it is well documented that the pharmacologically defined M3 subtype mediates bladder contraction. Acetylcholine (ACh), released from postganglionic cholinFrom the Department of Urology, Shinshu University School of Medicine, Matsumoto, Japan Reprint requests: Yasuhiko Igawa, MD, PhD, Associate Professor, Department of Urology, Shinshu University School of Medicine, 3-1-1 Asahi, Matsumoto 390-8621, Japan © 2000, ELSEVIER SCIENCE INC. ALL RIGHTS RESERVED
ergic nerves, interacts with postjunctional M3 muscarinic receptors to cause direct detrusor contraction via stimulation of phospholipase C, thereby increasing phosphoinositide turnover. This raises the question: what is the functional role of the large population of M2 receptors in the detrusor membrane? During the filling phase, the bladder is inhibited by sympathetic stimulation, whereas parasympathetic stimulation is normally suppressed. Norepinephrine (NA), released from sympathetic nerves, increases bladder compliance through -adrenoceptor–mediated relaxation of the bladder by stimulating adenylate cyclase. The -adrenoceptors mediating relaxation of the human bladder have been defined as the 3 subtype.5,6 In rats, Hegde et al.7 demonstrated that M2 receptors can indirectly mediate bladder contraction, both in vitro and in vivo, by reversing -adrenoceptor–mediated relaxation. During the voiding phase, ACh is released from parasympathetic nerves. This activates postjunctional M2 receptors, and causes inhibition of the -adrenoceptor–mediated stimulation of adenylate cyclase. The result is reversal of the relaxed tone of the bladder, in concert with M3-receptor– mediated direct contraction of the bladder. This dual mechanism may provide more efficient and complete emptying of the bladder. Recently, such M2 muscarinic receptor–mediated inhibition of adenylate cyclase has also been demonstrated in cultured human detrusor cells.8 In addition, it has been proposed that sympathetic inhibitory stimulation of the bladder partially reduces the level of bladder wall tension transduced by the bladder wall mechanoreceptors, and thereby delays the time at which the micturition threshold is attained.9 Braverman et al.10 demonstrated an increase in M2 receptor density with no change in M3 receptor density in the hypertrophic bladders of rats following pelvic denervation or spinal cord injury. This increase in M2 receptor density was consistent with the change in affinity of the antagonists of inhibition of carbachol-induced contractions. The authors concluded that M2 receptors or a combinaUROLOGY 55 (5A): 47– 49, 2000 • 0090-4295/00/$20.00 PII S0090-4295(99)00493-8 47
FIGURE 1. Muscarinic receptors and -adrenoceptors regulate bladder tone and contraction in overactive bladder. AC ⫽ adenylate cyclase; Ach ⫽ acetylcholine; DG ⫽ diacylglycerol; IP3 ⫽ inositol trisphosphate; PIP ⫽ polyinositol trisphosphate; PLC⫽ phospholipase C; NE ⫽ norepinephrine; ROC ⫽ receptor-operated channel; SR ⫽ sarcoplasmic reticulum.
FIGURE 2. Pharmacotherapy for overactive bladder.
tion of M2 and M3 receptors may directly mediate detrusor contraction in these pathologic rats (Figure 1). Taken together, 3 agonists alone or combined with an M2 antagonist might be a promising choice of treatment for overactive bladder and facilitate a storage function as, theoretically, these 48
drugs do not directly inhibit voiding bladder contractions (Figure 2). Antimuscarinic drugs also act prejunctionally at the cholinergic nerve terminals. It has been shown that inhibitory M2/4 receptors and facilitatory M1 receptors that regulate ACh release exist at the UROLOGY 55 (Supplement 5A), May 2000
cholinergic nerve terminals in the rat11,12 and rabbit13 bladder. A mechanism involving such facilitatory prejunctional M1 receptors may contribute to the dysfunction observed in overactive bladder (Figure 1). In fact, it has been proposed that presynaptic muscarinic facilitatory mechanisms are up-regulated in the cholinergic nerve terminals in the bladder of chronic spinal cord–transected rats.14 Therefore, antagonists of M1 receptors may inhibit a certain type of overactive detrusor contraction (Figure 2). In conclusion, though the muscarinic receptors remain the main target in the pharmacologic treatment of overactive bladder, the complex functions of the muscarinic receptors in the bladder and elsewhere in the body make it difficult to determine the optimal profile of subtype selectivity of antimuscarinic drugs for overactive bladder. Both M2 and M3 receptors may be functionally operative in the bladder, though their relative contributions may be altered by disease and by aging. Prejunctional facilitatory M1 receptors may contribute to a certain type of pathologic condition. Finally, the effects of muscarinic receptor blockade in the central nervous system should not be ignored. REFERENCES 1. Caulfield MP, and Birdsall NJM: International Union of Pharmacology: XVII. Classification of muscarinic acetylcholine receptors. Pharmacol Rev 50: 279 –290, 1998. 2. Hegde SS, and Eglen RM: Muscarinic receptor subtypes modulating smooth muscle contractility in the urinary bladder. Life Sci 64: 419 – 428, 1999. 3. Wang P, Luthin GR, and Ruggieri MR: Muscarinic acetylcholine receptor subtypes mediating urinary bladder contractility and coupling to GTP binding proteins. J Pharmacol Exp Ther 273: 959 –966, 1995.
UROLOGY 55 (Supplement 5A), May 2000
4. Chiarini A, Budriesi R, Bolognesi NL, et al: In vitro characterization of tripitramine, a polymethylene tetraamine displaying high selectivity and affinity for muscarinic M2 receptors. Br J Pharmacol 114: 1507–1517, 1995. 5. Igawa Y, Yamazaki Y, Takeda H, et al: Possible 3-adrenoceptor-mediated relaxation of the human detrusor. Acta Physiol Scand 164: 117–118, 1998. 6. Igawa Y, Yamazaki Y, Takeda H, et al: Functional and molecular biological evidence for a possible 3-adrenoceptor in the human detrusor muscle. Br J Pharmacol 126: 819 – 825, 1999. 7. Hegde SS, Choppin A, Bonhaus D, et al: Functional role of M2 and M3 muscarinic receptors in the urinary bladder of rats in vitro and in vivo. Br J Pharmacol 120: 1409 –1418, 1997. 8. Daniels DV, Meloy TD, Loury DN, et al: Functional pharmacological characterization of the muscarinic cholinoceptor-mediated inhibition of adenylyl cyclase in primary cultured human bladder detrusor smooth muscle cells (abst). Life Sci 64: 590, 1999. 9. Vaughan CW, and Satchell PM: Urine storage mechanisms. Prog Neurobiol 46: 215–237, 1995. 10. Braverman A, Legos J, Young W, et al: M2 receptors in genito-urinary smooth muscle pathology. Life Sci 64: 429 – 436, 1999. 11. Somogyi GT, and de Groat WC: Evidence for inhibitory nicotinic and facilitatory muscarinic receptors in cholinergic nerve terminals of the rat urinary bladder. J Auton Nerv Syst 37: 89 –98, 1992. 12. Braverman AS, Kohn IJ, Luthin GR, et al: Prejunctional M1 facilitatory and M2 inhibitory muscarinic receptors mediate rat bladder contractility. Am J Physiol 274(2, Part 2): R517–R523, 1998. 13. Tobin G, and Sju¨gren C: In vivo and in vitro effects of muscarinic receptor antagonists on contractions and release of [3H]acetylcholine in the rabbit urinary bladder. Eur J Pharmacol 281: 1– 8, 1995. 14. Somogyi GT, Zernova GV, Yoshiyama M, et al: Frequency dependence of muscarinic facilitation of transmitter release in urinary bladder strips from neurally intact or chronic spinal cord transected rats. Br J Pharmacol 125: 241– 246, 1998.
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