SECTION IV. OTHER
IV. NEUROTRANSMISSION AND RECEPTORS A. General
In both the parasympathetic and sympathetic systems, the preganglionic neurotransmitter is acetylcholine, which affects nicotinic cholinergic receptors. The primary postganglionic parasympathetic neurotransmitter is also acetylcholine, which affects muscarinic cholinergic receptors, while the postganglionic sympathetic neurotransmitter is a catecholamine, norepinephrine, which affects the adrenergic receptors. Newer scientific data are supporting the existence of many other neurotransmitters and receptors responsible for lower urinary tract function. These include ATP, nitric oxide (NO), dopamine, serotonin, glutamine, gamma amino butyric acid (GABA), various neuropeptides, and prostanoids. Representative parasympathetic and sympathetic nerve terminals are depicted in Figures 2 and 3.
Figure 2
B. Cholinergic Mechanisms
The voiding phase of the micturition cycle is primarily controlled by the parasympathetic nervous system. Stimulation of the pelvic nerves produces a strong, sustained bladder contraction that leads to bladder emptying. Cholinergic receptors are ubiquitous throughout the bladder body but scarce at the region of the bladder neck and the ventral part of the urethra.
They are absent in an area called the superficial trigonal muscle. The human bladder has two muscarinic cholinergic receptor subtypes, M2 and M3. M2 receptors predominate immuno- histochemically (80% overall), but functional studies show that bladder contractions are mediated primarily by M3 receptors through hydrolysis of phosphoinositol and the resultant release of intracellular calcium (15,16). M1, M2, and M4 receptors are also present prejunctionally on nerve terminals in the bladder and are thought to play a modulating role through amplification (M1) and inhibition (M2 and M4) of acetylcholine release (17,18). Drugs with anticholinergic properties such as propantheline, dicyclomine, imipramine, and oxybutynin have been used for many years to suppress overactive detrusor contractions. The usefulness of these medicines, however, has been somewhat limited owing to their lack of specificity with regard to mucscarinic receptors. Muscarinic receptors are also present extensively in salivary glands, bowel, and the accommodation apparatus of the eye, with M3 receptors predominating in the salivary glands. As a result, the use of standard antimuscarinic drugs often leads to intolerable side effects such as dry mouth, constipation and visual disturbances.
Newer, more selective muscarinic antagonists are being developed. Tolterodine, a competitive antagonist that binds all receptor subtypes, was shown in clinical studies to be equal in efficacy to oxybutynin in reducing micturition frequency and urge incontinence episodes while having a lower incidence of dry mouth (19). The apparent bladder selectivity of these newer agents may be due to several factors. The action of these drugs on prejunctional muscarinic receptors may play a larger role than initially thought. In addition, heterogeneity of the M3 receptor population has been postulated. In fact, radioligand-binding studies showed tolterodine and oxybutynin to have similar affinities for M3 receptors in the bladder, but oxybutynin had an eightfold higher affinity for parotid gland M3 receptors. These types of Figure 3
sensitivity differences have also been detected with other M3-selective antagonists, darifenacin and zamifenacin (15,20).
Botulinum toxin, now commonly used for treatment of skeletal muscle spasticity, inhibits acetylcholine release from cholinergic nerve terminals. It has been used successfully in the treatment of detrusor-sphincter dyssynergia in spinal cord-injured men by injecting it into the external sphincter to lower outlet resistance (21). It is also showing promise as a treatment for bladder hyperreflexia and possibly overactive bladder. Injection of the toxin into detrusor muscle has been effective in suppressing contractions in these patient populations, and further studies are eagerly awaited (22).
Bethanechol chloride, a cholinergic agonist, has been used rather commonly to enhance voiding, and this seems theoretically sound. Although cholinergic agonists may raise baseline bladder pressure, the use of such agents does not appear to be clinically useful in promoting bladder emptying (23 – 25). There are several reasons for this. Cholinergic agonists appear to cause a reflex sympathetic urethral constriction, prohibiting coordinated voiding (26). Further- more, cholinergic activation leads to a feedback mechanism via the aforementioned pre- junctional M2 and M4 receptors, inhibiting further acetylcholine release. Finally, bethanechol is poorly absorbed from the gastrointestinal tract, necessitating subcutaneous administration or prohibitively high oral doses to achieve pharmacologic effect.
C. Adrenergic Mechanisms
The adrenergic receptors (aandb) have different distributions in the lower urinary tract. Thea receptors are distributed mainly in the urethra and bladder neck. They are further subclassified into a1 (postsynaptic) and a2 (presynaptic) receptors. Stimulation of a1 receptors regulates vasoconstriction and smooth muscle contraction, whereas stimulation ofa2 receptors inhibits the release of norepinephrine from nerve terminals through a negative feedback mechanism.
Several subtypes of thea1 receptor have been identified. Studies have shown thata1 receptors in the urethras of humans are of thea1a subtype (27,28). In addition, radioligand binding has suggested the majority of a receptors in female animals are a2, whereas a1 receptors pre- dominate in the male (29).
Phentolamine and phenoxybenzamine are both nonspecifica-adrenergic antagonists and are not routinely used in the treatment of voiding disorders. Prazosin, doxazosin, and terazosin are relatively selective antagonists ofa1 receptor sites and are commonly used in the treatment of outlet obstruction in males secondary to benign prostatic hyperplasia because of their relax- ing effect on prostatic smooth muscle. The use of these agents is somewhat limited by cardiovascular side effects owing to the presence ofa1 receptors throughout the vascular tree.
Tamsulosin, an a1a-selective antagonist, targets urethral smooth muscle with a decreased incidence of cardiovascular side effects such as postural hypotension. Although sometimes used to treat female bladder outlet obstruction, the efficacy of these drugs in this capacity has not been definitively established, chiefly owing to a lack of standardized criteria to define this entity in women. Any lack of efficacy may also be explained by the deficiency ofa1 receptors in the female urethra.a-Stimulating drugs such as phenylpropranolamine and ephedrine will increase the tone of urethra and bladder neck by stimulating smooth muscle contraction at these sites.
In fact, over-the-counter cold medications and decongestants are a common cause of urinary retention in elderly males. It is also for this reason that these drugs have been used in the pharmacologic treatment of stress incontinence in women (30).
There have been conflicting animal studies regarding the role ofareceptors in the spinal cord, with data to support both inhibitory and facilitative influences. Some studies have indicated an excitatory role for a1 receptors at both the end-organ level and in the spinal cord. These
effects include the release of NO, the enhancement of acetylcholine release (a1a), and direct excitatory effects on bladder smooth muscle (a1b/a1d) (31,32). This last postjunctional excitatory effect is hardly present in younger animals but becomes prominent in older animals, supporting the concept of neural plasticity and change of adrenergic receptor expression over time. In addition, the intrathecal administration of doxazosin (a1 antagonist) has been shown to suppress bladder hyperactivity and decrease the amplitude of bladder contractions in rats (33).
This effect was more pronounced in the setting of chronic bladder outlet obstruction, again suggesting the plastic nature of neural control of the diseased bladder. There is other evidence to support age-related changes in lower urinary tract adrenergic receptor expression, an intriguing concept that may hold promise for future research.
b-Adrenergic receptors in the urinary tract (b2) tend to cluster in the bladder body, as opposed to the bladder base and neck. They appear to modulate smooth-muscle relaxation;
bstimulants, e.g., terbutaline, cause bladder relaxation and may contribute to urinary retention when given in high doses for premature labor. Unfortunately, bagonists do not appear to be clinically useful in treating detrusor instability (DI) (34).
The role of the sympathetic nervous system in the lower urinary tract is a matter of dispute.
However, many authors advocate its major role in the lower urinary tract. The sympathetic nervous system acts primarily to facilitate the filling and/or storage phase of micturition and does so by three mechanisms: (a) increasing accommodation by stimulation of b-adrenergic receptors in the bladder body; (b) increasing outlet resistance by stimulation of the predominantlya-adrenergic receptors in the bladder base and proximal urethra and by causing an increase in activity of striated muscle of the pelvic floor (“guarding reflex”); and (c) inhibiting bladder contractility by means of a blocking effect on parasympathetic ganglionic transmission (35,36). Edvardsen postulated a spinal reflex in the cat—with afferents in the pelvic nerves and efferents in the hypogastric nerves—causing bladder relaxation during filling and therefore an increased volume threshold for micturition (37,38). Consistent with this hypothesis is the fact that in the catb-adrenergic blockade or surgical sympathectomy has been reported to increase bladder activity, decrease bladder capacity, and produce a shift to the left of the accommodation limb of the cystometric curve (39,40).
D. Nonadrenergic, Noncholinergic Mechanisms
The fact that not all bladder contractile activity can be blocked by atropine even with massive doses (the phenomenon of atropine resistance) has led to the postulation of nonadrenergic, noncholinergic (NANC) neurotransmitter system, which is responsible for part of the neurotransmission in the bladder. Experimental studies on bladder muscles have shown that the bladder contraction is biphasic. Only the contraction of the second phase can be blocked by atropine; not the first phase. ATP and other substances are responsible for the contraction of the first phase (41,42). More recently, numerous substances have been shown to play a role in regulation of the lower urinary tract. These substances acting as neurotransmitters or neuromodulators, include an extensive list, e.g., opioids, vasoactive intestinal polypeptide (VIP), serotonin, dopamine, glutamic acid, GABA, ATP, and prostaglandins (F2, E, E2). Many of these substances exhibit both inhibitory and facilitative influence on the micturition cycle at the spinal cord level and higher. These developments have significant potential implications for the future development of drugs affecting nervous control mechanisms in the urinary tract and elsewhere (43,44). However, the role of NANC mechanisms in the contractile activation of the human bladder is still disputed. In normal human detrusor, atropine was found to cause .95%
inhibition of electrically evoked contraction (45). In detrusor strips from patients with a diagnosis of unstable bladder or from patients with benign prostatic hyperplasia, atropine
resistance was found in up to 65% (46). These apparently conflicting data may be explained by differences in the tissues investigated. Ghoniem found a significant atropine-resistant component to the electrically induced detrusor contraction of meningomyelocele patients undergoing augmentation cystoplasty, which was absent in normal bladders of patients undergoing ureteral reimplantation (47). Most probably, normal human detrusor muscle exhibits little atropine resistance while abnormal detrusor exhibits high atropine resistance, making study of NANC mechanisms more attractive in disease states. There is much research being done in this regard, and these studies are eagerly awaited.
E. Purinergic Mechanisms
Purinergic receptors are classified as P1 and P2 based on their affinity for either adenosine or ATP respectively. ATP-sensitive P2 receptors can be subclassified into P2X and P2Y receptor families based on whether the mechanism is ion channel gated (P2X) or G-protein coupled (P2Y). The P2X family can be further subclassified into seven subtypes (P2X1, P2X2, etc.).
Levin suggested that purinergic stimulation initiates a bladder contraction (first phase) whereas cholinergic stimulation leads to sustained bladder emptying (48). Chancellor showed that ATP generated a more forceful smooth muscle contraction than a cholinergic agonist, a finding that was corroborated by Sneddon, who showed that purinergic mediated contraction is more forceful in neonates (49,50). Theobald demonstrated a greater rise in bladder pressure in cats treated with purinergic agonists versus cholinergic agonists (51). These findings have not been consistent, however, as others have shown decreased bladder emptying with purinergic agonists (52). Animal studies have implied the presence of multiple types of purinergic (P2X and P2Y) receptors in the bladder and that the response of detrusor muscle to purinergic stimulation is itself biphasic depending on the receptor stimulated (P2X, fast response; P2Y, slow, sustained response) (53). P2X1receptors have been shown to be dominant in rat detrusor and vascular smooth muscle (54). O’Reilly and coworkers studied P2X receptors and their role in idiopathic DI in human females. They found that P2X2receptors were increased and other P2X subtypes were decreased in women with idiopathic DI, again demonstrating the trend toward atropine resistance in abnormal bladders (55). These data hold promise as the search for novel approaches to the treatment of overactive bladder and other bladder disorders continues.
It is likely that purinergic mechanisms also play an excitatory role at higher sites, including parasympathetic ganglia and afferent nerve terminals in dorsal root ganglia. P2X3 receptors have been identified in neurons in dorsal root ganglia in addition to subepithelial afferent nerves plexuses in the bladder and ureteral wall (54,56). Intravesical administration of ATP activates bladder afferent fibers and desensitization of these afferents with suramin, a purinergic antagonist, decreased reflex bladder activity (57,58). Afferent activity induced by bladder distention was reduced in P2X3knockout mice (53). These data argue that purinergic mechanisms play a sensory role in the lower urinary tract as well and could provide potential targets for therapy of disorders such as sensory urgency and interstitial cystitis.
F. Dopaminergic Mechanisms
Central dopaminergic pathways appear to exhibit both inhibitory and excitatory influences on micturition, based on the site and receptor type stimulated. D1 or D1-like receptors mediate inhibition whereas D2 or D2-like receptors mediate excitatory reflexes. Activation of D1 receptors in the substantia nigra causes suppression of reflex bladder activity in cats (59).
Bladder hyperreflexia produced in monkeys through destruction of these pathways (inducing Parkinson-like motor symptoms) was also suppressed using a D1-like agonist in one study (60).
From a clinical standpoint, however, treatment in humans with standard anti-Parkinsonian medications does not appear to correlate well with improvement in bladder symptoms or urodynamic findings (61). These patients often require treatment with anticholinergics to control bladder hyperreflexia, underscoring the complex nature of voiding dysfunction.
The diversity of dopaminergic influence on micturition is demonstrated by the fact that stimulation of D2-like receptors in animals in both the pontine micturition center (PMC) and in the spinal cord can induce bladder hyperactivity (62,63). It is likely that central dopaminergic pathways play a significant role in micturition, but the translational value of current basic science knowledge to the clinical arena has yet to be realized.
G. Serotonergic Mechanisms
It is possible that serotonin (5HT) has an impact on neural control of the lower urinary tract at both the central and peripheral levels, although the degree of this impact is still largely unknown.
This uncertainty is a product of the multiple receptors that have been identified coupled with the lack of specific drugs with which to target them. There have been at least seven different 5HT receptors identified (5HT1, 5HT2, etc.). Nonetheless, immunohistochemical studies have identified 5HT-containing neurons in the pelvic ganglia. Similarly, 5HT-containing neurons in the raphe nucleus of the caudal brainstem project to the dorsal horn in addition to the autonomic and sphincter motor nuclei in the lumbosacral cord. In cats, activation of these 5HT neurons in the cord inhibits reflex bladder activity and decreases firing of sacral efferents to the bladder (62,64). Administration of 5HT antagonists in animals blocks these effects and causes a decreased functional bladder capacity indicating that descending serotonergic pathways cause tonic inhibition of the afferent limb of the micturition reflex (65). Of interest is the possible role of serotonergic pathways in enuresis or overactive bladder. Tricyclic antidepressants are often used in the treatment of nocturnal enuresis. The efficacy of these agents may be explained by decreased 5HT reuptake, increasing levels available for suppression of reflex detrusor activity. Is has also been shown that the incidence of overactive bladder and urge incontinence is greater in individuals with depression, a condition associated with low levels of 5HT.
Peripherally, 5HT has been shown to induce bladder contractions as well as facilitate acetylcholine release from nerve terminals in the bladder via activation of prejunctional receptors (66,67). Anatomically, the sympathetic autonomic nuclei and sphincter motor nuclei receive serotonergic input, and there is evidence to show that sphincter reflexes are facilitated by activation of 5HT receptors as in the case of duloxetine, a combined 5HT and norepinephrine reuptake inhibitor (68,69).
H. Glutaminergic Mechanisms
Glutamic acid or glutamate plays an important role as a facilatory transmitter in the central pathways controlling micturition. It is present in visceral afferents in the dorsal horn of the lumbosacral cord, spinal interneurons, and the descending pathway from the PMC to the sacral parasympathetic plexus (70,71). Glutamate appears to facilitate bladder function at all of these levels via either NMDA (N-methyl-D-aspartate) or AMPA (a-amino-3-hydroxy-5-methyl-4- isoxazoleproprionic acid) receptors. NDMA antagonists depress reflex bladder activity and sphincter electromyographic activity in anesthetized animals as well as animals with cord transection at the midthoracic level (72). This indicates that the spinal reflex pathways controlling micturition rely on glutaminergic transmitter mechanisms. Studies also indicate that differences in bladder and external sphincter sensitivity to glutaminergic suppression may be
explained by differing receptor expression at each site. In situ hybridization studies have revealed high messenger RNA for AMPA receptor subunits GluR-A and GluR-B in sacral parasympathetic preganglionic neurons, but not NR2 NMDA receptor subunits. Conversely, high levels of messenger RNA for all four AMPA receptor subunits (GluR-A thru D) as well as the NR1 NMDA subunit are expressed in the motorneurons of the EUS (53).
I. GABA Inhibitory Mechanisms
GABA is a well-known inhibitory transmitter in the central nervous system. It appears to influence micturition at both spinal and supraspinal sites via both GABA-A and GABA-B receptors. In animal studies, injection of a GABA-A agonist into the PMC suppressed reflex bladder activity and intrathecal administration of either GABA-A or GABA-B antagonists increased bladder capacity and decreased voiding pressure (73).