Afferent nerve regulation of bladder function in health and disease

Abstract

The afferent innervation of the urinary bladder consists primarily of small myelinated (Adelta) and unmyelinated (C-fiber) axons that respond to chemical and mechanical stimuli. Immunochemical studies indicate that bladder afferent neurons synthesize several putative neurotransmitters, including neuropeptides, glutamic acid, aspartic acid, and nitric oxide. The afferent neurons also express various types of receptors and ion channels, including transient receptor potential channels, purinergic, muscarinic, endothelin, neurotrophic factor, and estrogen receptors. Patch-clamp recordings in dissociated bladder afferent neurons and recordings of bladder afferent nerve activity have revealed that activation of many of these receptors enhances neuronal excitability. Afferent nerves can respond to chemicals present in urine as well as chemicals released in the bladder wall from nerves, smooth muscle, inflammatory cells, and epithelial cells lining the bladder lumen. Pathological conditions alter the chemical and electrical properties of bladder afferent pathways, leading to urinary urgency, increased voiding frequency, nocturia, urinary incontinence, and pain. Neurotrophic factors have been implicated in the pathophysiological mechanisms underlying the sensitization of bladder afferent nerves. Neurotoxins such as capsaicin, resiniferatoxin, and botulinum neurotoxin that target sensory nerves are useful in treating disorders of the lower urinary tract.

Fig. 1

Sympathetic, parasympathetic, and somatic innervation of the urogenital tract of the male cat. Sympathetic preganglionic pathways emerge from the lumbar spinal cord and pass to the sympathetic chain ganglia and then via the inferior splanchnic nerves (ISN) to the inferior mesenteric ganglia (IMG). Preganglionic and postganglionic sympathetic axons then travel in the hypogastric nerve to the pelvic plexus and the urogenital organs. Parasympathetic preganglionic axons which originate in the sacral spinal cord pass in the pelvic nerve to ganglion cells in the pelvic plexus and to distal ganglia in the organs. Sacral somatic pathways are contained in the pudendal nerve, which provides an innervation to the penis, the ischiocavernosus (IC), bulbocavernosus (BC), and external urethral sphincter (EUS) muscles. The pudendal and pelvic nerves also receive postganglionic axons from the caudal sympathetic chain ganglia. These three sets of nerves contain afferent axons from the lumbosacral dorsal root ganglia. U ureter, PG prostate gland, VD vas deferens

Fig. 2

(a) Experimental methods for performing patch-clamp recordings on bladder afferent neurons obtained from rats with chronic cystitis. Chronic cystitis was induced by intraperitoneal injection of cyclophosphamide. Fluorescent dye (fast blue) injected into the bladder wall was transported via Aδ- and C-fiber bladder afferent axons to neurons in the dorsal root ganglia (DRG). L6 and S1 DRG were dissected and dissociated into single neurons by enzymatic methods. Wholecell patch-clamp recordings were then performed on fast blue-labeled bladder afferent neurons that were identified with a fluorescence microscope. (b) Characteristics of a bladder afferent neuron (24-µm diameter, C-fiber afferent neuron, top record) exhibiting tetrodotoxin (TTX)-resistant action potentials and a bladder afferent neuron (33-µm diameter, Aδ-fiber afferent neuron, bottom record) exhibiting TTX-sensitive action potentials. The left panels are voltage responses and action potentials evoked by 30-ms depolarizing current pulses injected through the patch pipette in current-clamp conditions. Asterisks with dashed lines indicate the thresholds for spike activation. The second panels on the left side show the effects of TTX application (1 µM) on action potentials. The third panels from the left show firing patterns during membrane depolarization (700-ms duration). The panels on the right show the responses to extracellular application of capsaicin (1 µM) in voltage-clamp conditions. Note that he TTX-resistant bladder afferent neuron (a) exhibited phasic firing (i.e., one to two spikes during prolonged membrane depolarization) and an inward current in response to capsaicin, while the TTX-sensitive afferent neuron exhibited tonic firing (i.e., repetitive firing during membrane depolarization) and no response to capsaicin

Fig. 3

(a) Summary of the events involved in chronic inflammation of the bladder and hyperexcitability of C-fiber bladder afferent neurons. The events that occur following chronic bladder inflammation (1) are indicated by sequential numbers (2–7). DRG dorsal root ganglia, 5-HT serotonin, PGE prostaglandin E, NGF nerve growth factor. (b) Primary afferent pathways to the L6 spinal cord of the rat project to the dorsal commissure (DCM), the superficial dorsal horn (DH), and the sacral parasympathetic nucleus (SPN), which contains parasympathetic preganglionic neurons. The afferent nerves consist of myelinated Aδ axons, which respond to bladder distension and contraction, and unmyelinated C-fiber axons, which respond to noxious stimuli. (c) Spinal neurons that express c-fos following the activation of bladder afferents by a noxious stimulus (acetic acid) to the bladder are located in the same regions of the L6 spinal segment that receive afferent input

Fig. 4

Receptors present in the urothelium (left side) and in sensory nerve endings in the bladder mucosa (center) and putative chemical mediators that are released by the urothelium, nerves, or smooth muscle (right side) that can modulate the excitability of sensory nerves. Urothelial cells and sensory nerves express common receptors (P2X, TRPV1, and TRPM8). Distension of the bladder activates stretch receptors and triggers the release of urothelial transmitters such as ATP, acetylcholine, and nitric oxide that may interact with adjacent nerves. Receptors in afferent nerves or the urothelium can respond to changes in pH, osmolality, high K⁺ concentration, chemicals in the urine, or inflammatory mediators released in the bladder wall. Neuropeptides (neurokinin A) released from sensory nerves in response to distension or chemical stimulation can act on neurokinin-2 autoreceptors to sensitize the mechanosensitive nerve endings. The smooth muscle can generate force which may influence some mucosal endings. Nerve growth factor released from muscle or urothelium can exert an acute and chronic influence on the excitability of sensory nerves via an action on TrkA receptors. ACh acetylcholine, MAChR muscarinic acetylcholine receptor, TRPV1 transient receptor potential vanilloid receptor 1 that are sensitive to capsaicin, TRPM8 menthol/cold receptor, NO nitric oxide, Trk-A tropomyosin-related kinase A receptor

Fig. 5

Organization of the parasympathetic excitatory reflex pathway to the detrusor muscle. The scheme is based on electrophysiological studies in cats. In animals with an intact spinal cord, micturition is initiated by a supraspinal reflex pathway passing through a center in the brainstem. The pathway is triggered by myelinated afferents (Aδ-fibers), which are connected to the tension receptors in the bladder wall. Injury to the spinal cord above the sacral segments interrupts the connections between the brain and spinal autonomic centers and initially blocks micturition. However, over a period of several weeks following cord injury, a spinal reflex mechanism emerges, which is triggered by unmyelinated vesical afferents (C-fibers); the A-fiber afferent inputs are ineffective. The C-fiber reflex pathway is usually weak or undetectable in animals with an intact nervous system. Stimulation of the C-fiber bladder afferents by instillation of ice-water into the bladder (cold stimulation) activates voiding responses in patients with spinal cord injury. Capsaicin (20–30 mg, subcutaneously) blocks the C-fiber reflex in chronic spinal cats, but does not block micturition reflexes in intact cats. Intravesical capsaicin also suppresses detrusor hyperreflexia and cold-evoked reflexes in patients with neurogenic bladder dysfunction. Glutamate is the main neurotransmitter released by Ad and C afferent fibers at synapses in the spinal cord; C-fiber afferents additionally release neuropeptides such as substance P (SP) or vasoactive intestinal polypeptide (VIP) as neurotransmitters. In animals with an intact spinal cord, noxious stimulation can activate C-fiber afferents, which leads to a facilitation of the micturition reflex pathway

Fig. 6

Neural circuits controlling continence and micturition. (a) Urine storage reflexes. During the storage of urine, distension of the bladder produces low-level afferent firing in the pelvic nerve, which in turn stimulates (1) the sympathetic outflow to the bladder outlet (base and urethra) and (2) pudendal outflow to the external urethral sphincter. These responses occur by spinal reflex pathways and represent guarding reflexes, which promote continence. Sympathetic firing also inhibits detrusor muscle and modulates transmission in bladder ganglia. A region in the rostral pons (the pontine storage center) increases external urethral sphincter activity. (b) Voiding reflexes. During elimination of urine, intense bladder afferent firing activates spinobulbospinal reflex pathways passing through the pontine micturition center, which stimulate the parasympathetic outflow to the bladder and internal sphincter smooth muscle and inhibit the sympathetic and pudendal outflow to the urethral outlet. Ascending afferent input from the spinal cord may pass through relay neurons in the periaqueductal gray (PAG) before reaching the pontine micturition center