Facilitatory effect of pituitary adenylate cyclase activating polypeptide on micturition in normal, conscious rats

The Influence of an Adrenergic Antagonist Guanethidine (GUA) on the Distribution Pattern and Chemical Coding of Dorsal Root Ganglia (DRG) Neurons Supplying the Porcine Urinary Bladder

Although guanethidine (GUA) was used in the past as a drug to suppress hyperactivity of the sympathetic nerve fibers, there are no available data concerning the possible action of this substance on the sensory component of the peripheral nervous system supplying the urinary bladder. Thus, the present study was aimed at disclosing the influence of intravesically instilled GUA on the distribution, relative frequency, and chemical coding of dorsal root ganglion neurons associated with the porcine urinary bladder. The investigated sensory neurons were visualized with a retrograde tracing method using Fast Blue (FB), while their chemical profile was disclosed with single-labeling immunohistochemistry using antibodies against substance P (SP), calcitonin gene-related peptide (CGRP), pituitary adenylate cyclase activating polypeptide (PACAP), galanin (GAL), neuronal nitric oxide synthase (nNOS), somatostatin (SOM), and calbindin (CB). After GUA treatment, a slight decrease in the number of FB⁺ neurons containing SP was observed when compared with untreated animals (34.6 ± 6.5% vs. 45.6 ± 1.3%), while the number of retrogradely traced cells immunolabeled for GAL, nNOS, and CB distinctly increased (12.3 ± 1.0% vs. 7.4 ± 0.6%, 11.9 ± 0.6% vs. 5.4 ± 0.5% and 8.6 ± 0.5% vs. 2.7 ± 0.4%, respectively). However, administration of GUA did not change the number of FB⁺ neurons containing CGRP, PACAP, or SOM. The present study provides evidence that GUA significantly modifies the sensory innervation of the porcine urinary bladder wall and thus may be considered a potential tool for studying the plasticity of this subdivision of the bladder innervation.

Botulinum toxin type A induces changes in the chemical coding of substance P-immunoreactive dorsal root ganglia sensory neurons supplying the porcine urinary bladder

Botulinum toxin (BTX) is a potent neurotoxin which blocks acetylcholine release from nerve terminals, and therefore leads to cessation of somatic motor and/or parasympathetic transmission. Recently it has been found that BTX also interferes with sensory transmission, thus, the present study was aimed at investigating the neurochemical characterization of substance P-immunoreactive (SP-IR) bladder-projecting sensory neurons (BPSN) after the toxin treatment. Investigated neurons were visualized with retrograde tracing method and their chemical profile was disclosed with double-labelling immunohistochemistry using antibodies against SP, calcitonin gene-related peptide (CGRP), pituitary adenylate cyclase activating polypeptide (PACAP), neuronal nitric oxide synthase (nNOS), galanin (GAL), calbindin (CB), and somatostatin (SOM). In the control group (n = 6), 45% of the total population of BPSN were SP-IR. Nearly half of these neurons co-expressed PACAP or CGRP (45% and 35%, respectively), while co-localization of SP with GAL, nNOS, SOM or CB was found less frequently (3.7%, 1.8%, 1.2%, and 0.7%, respectively). In BTX-treated pigs (n = 6), toxin-injections caused a decrease in the number of SP-IR cells containing CGRP, SOM or CB (16.2%, 0.5%, and 0%, respectively) and a distinct increase in these nerve cells immunopositive to GAL (27.2%). The present study demonstrates that BTX significantly modifies the chemical phenotypes of SP-IR BPSN.

Autonomic consequences of spinal cord injury

Spinal cord injury (SCI) results not only in motor and sensory deficits but also in autonomic dysfunctions. The disruption of connections between higher brain centers and the spinal cord, or the impaired autonomic nervous system itself, manifests a broad range of autonomic abnormalities. This includes compromised cardiovascular, respiratory, urinary, gastrointestinal, thermoregulatory, and sexual activities. These disabilities evoke potentially life-threatening symptoms that severely interfere with the daily living of those with SCI. In particular, high thoracic or cervical SCI often causes disordered hemodynamics due to deregulated sympathetic outflow. Episodic hypertension associated with autonomic dysreflexia develops as a result of massive sympathetic discharge often triggered by unpleasant visceral or sensory stimuli below the injury level. In the pelvic floor, bladder and urethral dysfunctions are classified according to upper motor neuron versus lower motor neuron injuries; this is dependent on the level of lesion. Most impairments of the lower urinary tract manifest in two interrelated complications: bladder storage and emptying. Inadequate or excessive detrusor and sphincter functions as well as detrusor-sphincter dyssynergia are examples of micturition abnormalities stemming from SCI. Gastrointestinal motility disorders in spinal cord injured-individuals are comprised of gastric dilation, delayed gastric emptying, and diminished propulsive transit along the entire gastrointestinal tract. As a critical consequence of SCI, neurogenic bowel dysfunction exhibits constipation and/or incontinence. Thus, it is essential to recognize neural mechanisms and pathophysiology underlying various complications of autonomic dysfunctions after SCI. This overview provides both vital information for better understanding these disorders and guides to pursue novel therapeutic approaches to alleviate secondary complications.

Neural control of the lower urinary tract

This article summarizes anatomical, neurophysiological, pharmacological, and brain imaging studies in humans and animals that have provided insights into the neural circuitry and neurotransmitter mechanisms controlling the lower urinary tract. The functions of the lower urinary tract to store and periodically eliminate urine are regulated by a complex neural control system in the brain, spinal cord, and peripheral autonomic ganglia that coordinates the activity of smooth and striated muscles of the bladder and urethral outlet. The neural control of micturition is organized as a hierarchical system in which spinal storage mechanisms are in turn regulated by circuitry in the rostral brain stem that initiates reflex voiding. Input from the forebrain triggers voluntary voiding by modulating the brain stem circuitry. Many neural circuits controlling the lower urinary tract exhibit switch-like patterns of activity that turn on and off in an all-or-none manner. The major component of the micturition switching circuit is a spinobulbospinal parasympathetic reflex pathway that has essential connections in the periaqueductal gray and pontine micturition center. A computer model of this circuit that mimics the switching functions of the bladder and urethra at the onset of micturition is described. Micturition occurs involuntarily in infants and young children until the age of 3 to 5 years, after which it is regulated voluntarily. Diseases or injuries of the nervous system in adults can cause the re-emergence of involuntary micturition, leading to urinary incontinence. Neuroplasticity underlying these developmental and pathological changes in voiding function is discussed.

Tetrodotoxin induced changes in the chemical coding of dorsal root ganglion neurons supplying the porcine urinary bladder

Tetrodotoxin (TTX) mode of action is based on a blocking of fast sodium channels in nerve cell membrane what, in turn, abolishes the propagation of the action potential along the nerve fibers. TTX is currently used in experimental therapies focused on neoplastic or neurogenic pain, however, as for now there is no data concerning the influence of TTX on dorsal root ganglion (DRG) sensory neurons function. Thus, the present study was aimed at characterization of neurochemical coding of porcine sensory bladder-projecting cells after bladder instillation with TTX. Retrograde tracer Fast Blue (FB) was injected into the urinary bladder wall of six juvenile female pigs and three weeks later bladder instillation with TTX (12 microg per animal) was carried out in all animals. A week later, DRGs of interest were harvested from all animals and the neurochemical characterization of FB+ neurons was performed using routine double-immunofluorescence labeling technique on 10-microm-thick cryostat sections. In TTX-treated animals the number of FB+ cells containing galanin (GAL), nitric oxide synthase (NOS), somatostatin (SOM) and calbindin (CB) was 2.5%, 2%, 0.25% and 0.2%, respectively and that of pituitary adenylate cyclase-activating polypeptide (PACAP)-immunoreactive (IR) cells was 43%. These data when compared with previous reports, demonstrated that TTX profoundly changed the chemical coding of porcine bladder-projecting sensory neurons thus implicating that it may be used in case of hypoactivity of afferent part of reflex arc responsible for transmission of sensory information from the urinary bladder.

The influence of resiniferatoxin on the chemical coding of neurons in dorsal root ganglia supplying the urinary bladder in the female pig

Although resiniferatoxin (RTX) becomes more often used in experimental therapies of sensory system disorders, so far there is no data concerning the influence of RTX on the chemical coding of neurons in dorsal root ganglia (DRG) supplying the urinary bladder in the pig, an animal species considered as a reliable animal model for investigation dealing with human lower urinary tract disorders. Retrograde tracer Fast Blue (FB) was injected into the wall of the right half of the urinary bladder in six juvenile female pigs, and three weeks later, bladder instillation of RTX (500 nmol per animal) was carried out in all the animals. After a week, DRGs were harvested from all the pigs and the neurochemical characterization of FB+ neurons was performed using routine single-immunofluorescence labeling technique on 10-microm-thick cryostat sections. RTX instillation resulted in a distinct decrease in the numbers of FB+ cells containing calcitonin gene-related peptide (CGRP), nitric oxide synthase (NOS), somatostatin (SOM) and calbindin (CB) when compared with those found in the healthy animals (18% vs. 36%, 1% vs. 6%, 0.8% vs. 4% and 0.5% vs. 3%, respectively), and an increase in the number of pituitary adenylate cyclase-activating polypeptide (PACAP)- and galanin (GAL)-immunoreactive (IR) nerve cells (51% vs. 26% and 47% vs. 6.5%). The results obtained suggest that RTX could be taken into consideration when the neuroactive agents are planned to be used in experimental therapies of selected neurogenic bladder illnesses.

Effects of pituitary adenylate cyclase activating polypeptide in the urinary system, with special emphasis on its protective effects in the kidney

Pituitary adenylate cyclase activating polypeptide (PACAP) is a widespread neuropeptide with diverse effects in the nervous system and peripheral organs. One of the most well-studied effects of PACAP is its cytoprotective action, against different harmful stimuli in a wide variety of cells and tissues. PACAP occurs in the urinary system, from the kidney to the lower urinary tract. The present review focuses on the nephroprotective effects of PACAP and summarizes data obtained regarding the protective effects of PACAP in different models of kidney pathologies. In vitro data show that PACAP protects tubular cells against oxidative stress, myeloma light chain, cisplatin, cyclosporine-A and hypoxia. In vivo data provide evidence for its protective effects in ischemia/reperfusion, cisplatin, cyclosporine-A, myeloma kidney injury, diabetic nephropathy and gentamicin-induced kidney damage. Results accumulated on the renoprotective effects of PACAP suggest that PACAP is an emerging candidate for treatment of human kidney pathologies.

Conantokin G-induced changes in the chemical coding of dorsal root ganglion neurons supplying the porcine urinary bladder

Conantokin G (CTG), isolated from the venom of the marine cone snail Conus geographus, is an antagonist of N-methyl-d-aspartate receptors (NMDARs), the activation of which, especially those located on the central afferent terminals and dorsal horn neurons, leads to hypersensitivity and pain. Thus, CTG blocking of NMDARs, has an antinociceptive effect, particularly in the case of neurogenic pain treatment. As many urinary bladder disorders are caused by hyperactivity of sensory bladder innervation, it seems useful to estimate the influence of CTG on the plasticity of sensory neurons supplying the organ. Retrograde tracer Fast Blue (FB) was injected into the urinary bladder wall of six juvenile female pigs. Three weeks later, intramural bladder injections of CTG (120 μg per animal) were carried out in all animals. After a week, dorsal root ganglia of interest were harvested from all animals and neurochemical characterization of FB+ neurons was performed using a routine double-immunofluorescence labeling technique on 10-μm-thick cryostat sections. CTG injections led to a significant decrease in the number of FB+ neurons containing substance P (SP), pituitary adenylate cyclase activating polypeptide (PACAP), somatostatin (SOM), calbindin (CB) and nitric oxide synthase (NOS) when compared with healthy animals (20% vs. 45%, 13% vs. 26%, 1.3% vs. 3%, 1.2 vs. 4% and 0.9% vs. 6% respectively) and to an increase in the number of cells immunolabelled for galanin (GAL, 39% vs. 6.5%). These data demonstrated that CTG changed the chemical coding of bladder sensory neurons, thus indicating that CTG could eventually be used in the therapy of selected neurogenic bladder illnesses.

Plasticity in reflex pathways to the lower urinary tract following spinal cord injury

The lower urinary tract has two main functions, storage and periodic expulsion of urine, that are regulated by a complex neural control system in the brain and lumbosacral spinal cord. This neural system coordinates the activity of two functional units in the lower urinary tract: (1) a reservoir (the urinary bladder) and (2) an outlet (consisting of bladder neck, urethra and striated muscles of the external urethra sphincter). During urine storage the outlet is closed and the bladder is quiescent to maintain a low intravesical pressure. During micturition the outlet relaxes and the bladder contracts to promote efficient release of urine. This reciprocal relationship between bladder and outlet is generated by reflex circuits some of which are under voluntary control. Experimental studies in animals indicate that the micturition reflex is mediated by a spinobulbospinal pathway passing through a coordination center (the pontine micturition center) located in the rostral brainstem. This reflex pathway is in turn modulated by higher centers in the cerebral cortex that are involved in the voluntary control of micturition. Spinal cord injury at cervical or thoracic levels disrupts voluntary control of voiding as well as the normal reflex pathways that coordinate bladder and sphincter function. Following spinal cord injury the bladder is initially areflexic but then becomes hyperreflexic due to the emergence of a spinal micturition reflex pathway. However the bladder does not empty efficiently because coordination between the bladder and urethral outlet is lost. Studies in animals indicate that dysfunction of the lower urinary tract after spinal cord injury is dependent in part on plasticity of bladder afferent pathways as well as reorganization of synaptic connections in the spinal cord. Reflex plasticity is associated with changes in the properties of ion channels and electrical excitability of afferent neurons and appears to be mediated in part by neurotrophic factors released in the spinal cord and/or the peripheral target organs.

Neuropeptides in lower urinary tract function

Numerous neuropeptide/receptor systems including vasoactive intestinal polypeptide, pituitary adenylate cyclase-activating polypeptide, calcitonin gene-related peptide, substance P, neurokinin A, bradykinin, and endothelin-1 are expressed in the lower urinary tract (LUT) in both neural and nonneural (e.g., urothelium) components. LUT neuropeptide immunoreactivity is present in afferent and autonomic efferent neurons innervating the bladder and urethra and in the urothelium of the urinary bladder. Neuropeptides have tissue-specific distributions and functions in the LUT and exhibit neuroplastic changes in expression and function with LUT dysfunction following neural injury, inflammation, and disease. LUT dysfunction with abnormal voiding, including urinary urgency, increased voiding frequency, nocturia, urinary incontinence, and pain, may reflect a change in the balance of neuropeptides in bladder reflex pathways. LUT neuropeptide/receptor systems may represent potential targets for therapeutic intervention.

Neural control of the lower urinary tract: peripheral and spinal mechanisms

This review deals with individual components regulating the neural control of the urinary bladder. This article will focus on factors and processes involved in the two modes of operation of the bladder: storage and elimination. Topics included in this review include: (1) The urothelium and its roles in sensor and transducer functions including interactions with other cell types within the bladder wall ("sensory web"), (2) The location and properties of bladder afferents including factors involved in regulating afferent sensitization, (3) The neural control of the pelvic floor muscle and pharmacology of urethral and anal sphincters (focusing on monoamine pathways), (4) Efferent pathways to the urinary bladder, and (5) Abnormalities in bladder function including mechanisms underlying comorbid disorders associated with bladder pain syndrome and incontinence.

Changes in afferent activity after spinal cord injury

AIMS

To summarize the changes that occur in the properties of bladder afferent neurons following spinal cord injury.

METHODS

Literature review of anatomical, immunohistochemical, and pharmacologic studies of normal and dysfunctional bladder afferent pathways.

RESULTS

Studies in animals indicate that the micturition reflex is mediated by a spinobulbospinal pathway passing through coordination centers (periaqueductal gray and pontine micturition center) located in the rostral brain stem. This reflex pathway, which is activated by small myelinated (Adelta) bladder afferent nerves, is in turn modulated by higher centers in the cerebral cortex involved in the voluntary control of micturition. Spinal cord injury at cervical or thoracic levels disrupts voluntary voiding, as well as the normal reflex pathways that coordinate bladder and sphincter function. Following spinal cord injury, the bladder is initially areflexic but then becomes hyperreflexic due to the emergence of a spinal micturition reflex pathway. The recovery of bladder function after spinal cord injury is dependent in part on the plasticity of bladder afferent pathways and the unmasking of reflexes triggered by unmyelinated, capsaicin-sensitive, C-fiber bladder afferent neurons. Plasticity is associated with morphologic, chemical, and electrical changes in bladder afferent neurons and appears to be mediated in part by neurotrophic factors released in the spinal cord and the peripheral target organs.

CONCLUSIONS

Spinal cord injury at sites remote from the lumbosacral spinal cord can indirectly influence properties of bladder afferent neurons by altering the function and chemical environment in the bladder or the spinal cord.

Pituitary adenylate cyclase-activating polypeptide and its receptors: 20 years after the discovery

Pituitary adenylate cyclase-activating polypeptide (PACAP) is a 38-amino acid C-terminally alpha-amidated peptide that was first isolated 20 years ago from an ovine hypothalamic extract on the basis of its ability to stimulate cAMP formation in anterior pituitary cells (Miyata et al., 1989. PACAP belongs to the vasoactive intestinal polypeptide (VIP)-secretin-growth hormone-releasing hormone-glucagon superfamily. The sequence of PACAP has been remarkably well conserved during evolution from protochordates to mammals, suggesting that PACAP is involved in the regulation of important biological functions. PACAP is widely distributed in the brain and peripheral organs, notably in the endocrine pancreas, gonads, respiratory and urogenital tracts. Characterization of the PACAP precursor has revealed the existence of a PACAP-related peptide, the activity of which remains unknown. Two types of PACAP binding sites have been characterized: type I binding sites exhibit a high affinity for PACAP and a much lower affinity for VIP, whereas type II binding sites have similar affinity for PACAP and VIP. Molecular cloning of PACAP receptors has shown the existence of three distinct receptor subtypes: the PACAP-specific PAC1-R, which is coupled to several transduction systems, and the PACAP/VIP-indifferent VPAC1-R and VPAC2-R, which are primarily coupled to adenylyl cyclase. PAC1-Rs are particularly abundant in the brain, the pituitary and the adrenal gland, whereas VPAC receptors are expressed mainly in lung, liver, and testis. The development of transgenic animal models and specific PACAP receptor ligands has strongly contributed to deciphering the various actions of PACAP. Consistent with the wide distribution of PACAP and its receptors, the peptide has now been shown to exert a large array of pharmacological effects and biological functions. The present report reviews the current knowledge concerning the pleiotropic actions of PACAP and discusses its possible use for future therapeutic applications.

Afferent nerve regulation of bladder function in health and disease

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.

The role of vasoactive intestinal polypeptide and pituitary adenylate cyclase-activating polypeptide in the neural pathways controlling the lower urinary tract

Vasoactive intestinal polypeptide (VIP) and pituitary adenylate cyclase-activating polypeptide (PACAP) are expressed in the neural pathways regulating the lower urinary tract. VIP-immunoreactivity (IR) is present in afferent and autonomic efferent neurons innervating the bladder and urethra, whereas PACAP-IR is present primarily in afferent neurons. Exogenously applied VIP relaxes bladder and urethral smooth muscle and excites parasympathetic neurons in bladder ganglia. PACAP relaxes bladder and urethral smooth muscle in some species (pig) but excites the smooth muscle in other species (mouse). Intrathecal administration of VIP in cats with an intact spinal cord suppresses reflex bladder activity, but intrathecal administration of VIP or PACAP in rats enhances bladder activity and suppresses urethral sphincter activity. PACAP has presynaptic facilitatory effects and direct excitatory effects on lumbosacral parasympathetic preganglionic neurons. Chronic spinal cord transection produces an expansion of VIP-IR (cats) and PACAP-IR (rats) in primary afferent axons in the lumbosacral spinal cord and unmasks spinal excitatory effects of VIP on bladder reflexes in cats. Intrathecal administration of PACAP6-38, a PAC1 receptor antagonist, reduces bladder hyperactivity in chronic spinal-cord-injured rats. These observations raise the possibility that VIP or PACAP have a role in the control of normal or abnormal voiding.

Effects of intrathecal administration of pituitary adenylate cyclase activating polypeptide on lower urinary tract functions in rats with intact or transected spinal cords

Effects of intrathecally administered pituitary adenylate cyclase activating polypeptide-38 (PACAP-38, 0.1-30 microg) on lower urinary tract function were examined in unanesthetized, decerebrate rats with an intact spinal cord and after chronic spinal cord transection (SCT). PACAP-38 was also studied in rats with intact or bilaterally transected hypogastric nerves (HGNs), to determine if sympathetic pathways to the bladder influenced responses. In SCT rats with intact HGNs under isovolumetric conditions, 30 mug of PACAP-38 but not lower doses (0.1-10 microg) increased (mean 194%) bladder contraction amplitude (BCA). In SCT rats with sectioned HGNs, 10 microg and 30 microg of PACAP-38 increased BCA by 62% and 195%, respectively. On the other hand, during continuous infusion cystometrograms (CMGs) in SCT rats with intact or sectioned HGNs, PACAP-38 (10 microg and 30 microg) markedly reduced or completely suppressed BCA (60% and 90%, respectively) and reduced external urethral sphincter (EUS) EMG activity (58% and 91%, respectively). During CMGs in spinal cord intact rats, with intact HGNs PACAP-38 30 microg increased BCA (26%) but after HGN section PACAP-38 10 microg and 30 microg increased BCA by 21% and 35%. These results suggest that after SCT, PACAP-38 activates spinal circuitry to facilitate the parasympathetic outflow to the urinary bladder and that the elimination of sympathetic pathways enhances this effect. The decrease in BCA by PACAP-38 during CMGs in SCT rats is most reasonably attributed to a reduction in urethral outlet resistance due to suppression of excitatory EUS reflexes.

Spinal ERK activation contributes to the regulation of bladder function in spinal cord injured rats

The extracellular signal-regulated kinase 1 and 2 (ERK) pathway, regulated by phosphorylation on specific amino acids, is emerging as an important signaling cascade in neurones, transducing sensory input into cellular responses. In the mammalian nervous system, the ERK pathway has been found to mediate plasticity events. Particularly, in the spinal cord, ERK play an important role in allodynia and hyperalgesia. Recently, it was demonstrated that ERK activation is upregulated in the spinal cord of rats with chronic bladder inflammation and contributes to bladder overactivity. Thus, in this study we sought to assess the involvement of ERK in micturition reflexes associated to spinal cord injury (SCI) in the rat. Bladder function in chronic SCI rats was altered compared to spinal intact rats. PhosphoERK levels were upregulated in the L6 spinal cord segment, particularly after saline infusion for 2 h. The increase in spinal ERK phosphorylation was specifically restricted to L6 spinal segment. No variation in the levels of total ERK protein was observed. Intrathecal administration of PD98059, a specific inhibitor of ERK phosphorylation, reduced the frequency and amplitude of bladder contractions in SCI animals but not in spinal intact ones. Overall, our results demonstrate increased activation of the ERK pathway in the spinal cord from SCI rats, restricted to spinal segments that receive sensory input arising from the bladder. Since the use of PD98059 reduced the frequency and amplitude of bladder contractions, ERK inhibitors may provide a new therapeutic approach to the treatment of bladder overactivity after spinal injuries.

Role for pituitary adenylate cyclase activating polypeptide in cystitis-induced plasticity of micturition reflexes

Pituitary adenylate cyclase activating polypeptide (PACAP) peptides are expressed and regulated in sensory afferents of the micturition pathway. Although these studies have implicated PACAP in bladder control, the physiological significance of these observations has not been firmly established. To clarify these issues, the roles of PACAP and PACAP signaling in micturition and cystitis were examined in receptor characterization and physiological assays. PACAP receptors were identified in various tissues of the micturition pathway, including bladder detrusor smooth muscle and urothelium. Bladder smooth muscle expressed heterogeneously PAC(1)null, PAC(1)HOP1, and VPAC(2) receptors; the urothelium was more restricted in expressing preferentially the PAC(1) receptor subtype only. Immunocytochemical studies for PAC(1) receptors were consistent with these tissue distributions. Furthermore, the addition of 50-100 nM PACAP27 or PACAP38 to isolated bladder strips elicited transient contractions and sustained increases in the amplitude of spontaneous phasic contractions. Treatment of the bladder strips with tetrodotoxin (1 muM) did not alter the spontaneous phasic contractions suggesting direct PACAP effects on bladder smooth muscle. PACAP also increased the amplitude of nerve-evoked contractions. By contrast, vasoactive intestinal polypeptide had no direct effects on bladder smooth muscle. In a rat cyclophosphamide (CYP)-induced cystitis paradigm, intrathecal or intravesical administration of PAC(1) receptor antagonist, PACAP6-38, reduced cystitis-induced bladder overactivity. In summary, these studies support roles for PACAP in micturition and suggest that inflammation-induced plasticity in PACAP expression in peripheral and central micturition pathways contribute to bladder dysfunction with cystitis.

Neurochemical plasticity and the role of neurotrophic factors in bladder reflex pathways after spinal cord injury

Transection of the spinal cord that interrupts the spinobulbospinal micturition reflex pathway, abolishes voluntary voiding and initially produces an areflexic bladder with complete urinary retention. However, depending upon the species, reflex bladder activity slowly recovers over the course of weeks or months. In chronic spinal animals, reflex mechanisms in the lumbosacral spinal cord are capable of duplicating many of the functions performed by reflex pathways in animals with an intact spinal cord and can induce bladder hyperreflexia. However, the bladder does not empty efficiently due to a loss of bladder-sphincter coordination (bladder-sphincter dyssynergia). In contrast to normal animals in which the sphincter relaxes during voiding, animals with a spinal cord injury exhibit sphincter contractions during voiding, an increase in urethral outlet resistance, urinary retention, bladder hyperreflexia, bladder overdistension, and an increase in bladder afferent cell size. Changes in electrophysiological or neurochemical properties of bladder afferent cells in the dorsal root ganglia and of spinal pathways could contribute to the emergence of the spinal micturition reflex, bladder hyperreflexia and changes in the pharmacologic responses of reflex pathways in the lumbosacral spinal cord after spinal cord injury. Urinary bladder hyperreflexia after spinal cord injury may reflect a change in the balance of neuroactive compounds in bladder reflex pathways. This review will detail: (1) changes in the neurochemical phenotype of bladder afferent neurons and of spinal neurons mediating micturition reflexes after spinal cord injury, with an emphasis on three neuroactive compounds, neuronal nitric oxide synthase (nNOS), galanin, and pituitary adenylate cyclase activating polypeptide (PACAP); (2) possible functional consequences on bladder reflexes of changes in spinal cord neurochemistry after spinal cord injury, and (3) the potential role of neurotrophic factors expressed in the urinary bladder or spinal cord after spinal cord injury in mediating these neurochemical changes.

Mechanisms underlying the recovery of lower urinary tract function following spinal cord injury

The lower urinary tract has two main functions, the storage and periodic expulsion of urine, which are regulated by a complex neural control system in the brain and lumbosacral spinal cord. This neural system coordinates the activity of two functional units in the lower urinary tract: (1) a reservoir (the urinary bladder) and (2) an outlet (consisting of bladder neck, urethra and striated muscles of the pelvic floor). During urine storage the outlet is closed and the bladder is quiescent, thereby maintaining a low intravesical pressure over a wide range of bladder volumes. During micturition the outlet relaxes and the bladder contracts to promote the release of urine. This reciprocal relationship between bladder and outlet is generated by visceral reflex circuits, some of which are under voluntary control. Experimental studies in animals indicate that the micturition reflex is mediated by a spinobulbospinal pathway passing through a coordination center (the pontine micturition center) located in the rostral brainstem. This reflex pathway is in turn modulated by higher centers in the cerebral cortex that are presumably involved in the voluntary control of micturition. Spinal cord injury at cervical or thoracic levels disrupts voluntary control of voiding as well as the normal reflex pathways that coordinate bladder and sphincter functions. Following spinal cord injury, the bladder is initially areflexic but then becomes hyperreflexic due to the emergence of a spinal micturition reflex pathway. Studies in animals indicate that the recovery of bladder function after spinal cord injury is dependent in part on plasticity of bladder afferent pathways and the unmasking of reflexes triggered by capsaicin-sensitive C-fiber bladder afferent neurons. The plasticity is associated with changes in the properties of ion channels and electrical excitability of afferent neurons, and appears to be mediated in part by neurotrophic factors released in the spinal cord and the peripheral target organs.

PACAP 38 is involved in the non-adrenergic non-cholinergic inhibitory neurotransmission in the pig urinary bladder neck

AIMS

To investigate the role played by pituitary adenylate cyclase activating polypeptide 38 (PACAP 38) in the non-adrenergic non-cholinergic (NANC) neurotransmission of the pig urinary bladder neck.

METHODS

Urothelium-denuded bladder neck strips were dissected and mounted in organ baths containing a physiological saline solution (PSS) at 37 degrees C and gassed with 5% CO(2) and 95% O(2), for isometric force recording. The relaxations to transmural nerve stimulation (EFS) or PACAP 38 were performed on strips precontracted with 1 microM phenylephrine (PhE). EFS experiments were carried out in the absence and the presence of guanethidine (10 microM), atropine (0.1 microM), and N(G)-nitro-L-arginine (L-NOARG, 100 microM), to block noradrenergic neurotransmission, muscarinic receptors, and nitric oxide (NO) synthase, respectively.

RESULTS

EFS (2-16 Hz, 1 ms duration, 20 sec trains, 75 mA current output) evoked frequency-dependent relaxations which were reduced by the VIP/PACAP receptor antagonist PACAP (6-38) (3 microM), and by the neurotoxin of the capsaicin-sensitive primary afferents capsaicin (10 microM), and abolished by the neuronal voltage-activated Na(+) channel blocker tetrodotoxin (TTX, 1 microM). The vasoactive intestinal peptide (VIP) receptor antagonist [Lys(1), Pro(2,5), Arg(3,4), Tyr(6)]-VIP (3 microM) failed to modify the EFS-induced relaxations. PACAP 38 (1 nM-1 microM) induced concentration-dependent relaxations which were reduced by PACAP (6-38), TTX and by the neuronal voltage-gated Ca(2+) channel inhibitor omega-conotoxin GVIA (omega-CgTX, 1 microM).

CONCLUSIONS

The results suggest that PACAP 38, mainly released from capsaicin-sensitive primary afferents, is involved in the NANC inhibitory neurotransmission of the pig urinary bladder neck, producing relaxation through neuronal and muscle VIP/PACAP receptor activation.

Changes in pituitary adenylate cyclase activating polypeptide expression in urinary bladder pathways after spinal cord injury

These studies examined changes in the pituitary adenylate cyclase activating polypeptide (PACAP) expression in micturition reflex pathways after spinal cord injury (SCI) of various durations. In spinal-intact animals, PACAP immunoreactivity (IR) was expressed in fibers in the superficial dorsal horn in all segmental levels examined (L1, L2, L4-S1). Bladder-afferent cells (35-45%) in the dorsal root ganglia (DRG; L1, L2, L6, S1) from spinal-intact animals also exhibited PACAP-IR. After SCI (6 weeks), PACAP-IR was dramatically increased in spinal segments and DRG (L1, L2, L6, S1) involved in micturition reflexes. The density of PACAP-IR was increased in the superficial laminae (I-II) of the L1, L2, L6, and S1 spinal segments. No changes in PACAP-IR were observed in the L4-L5 segments. Staining was also dramatically increased in a fiber bundle extending ventrally from Lissauer's tract (LT) in lamina I along the lateral edge of the dorsal horn to the sacral parasympathetic nucleus (SPN) in the L6-S1 spinal segments (lateral collateral pathway of Lissauer, LCP). After SCI (range 48 h to 6 weeks), PACAP-IR in cells in the L1, L2, L6, and S1 DRG significantly (P < or = 0.001) increased and the percentage of bladder-afferent cells expressing PACAP-IR also significantly (P < or = 0.001) increased (70-92%). No changes were observed in the L4-L5 DRG. PACAP-IR was reduced throughout the urothelium and detrusor smooth muscle whole mounts after SCI. These studies demonstrate changes in PACAP expression in micturition reflex pathways after SCI that may contribute to urinary bladder dysfunction or reemergence of primitive voiding reflexes after SCI.

Inhibitory control of the urinary bladder in the neonatal rat in vitro spinal cord-bladder preparation

Urinary bladder activity of the neonatal rat is tonically inhibited by neural input from the spinal cord passing through axons in the pelvic nerve. The present study was undertaken to examine the organization of this inhibitory mechanism using in vitro spinal cord-bladder preparations of neonatal rats in which the lumbosacral dorsal roots (DRs) or ventral roots (VRs) were transected. Isovolumetric bladder contractions occurring spontaneously or induced by electrical stimulation of the bladder wall (ES-BW) were measured. In DR transected (DRT) preparations, removal of the spinal cord significantly enhanced (50-59%) the amplitude of spontaneous and ES-BW-evoked bladder contractions; whereas in VR transected (VRT) preparations removal of the spinal cord produced only a small enhancement (6.7-12%). However, in VRT preparations, electrical stimulation of the dorsal roots reduced the amplitude of spontaneous contractions, an effect blocked by a nicotinic ganglionic blocking agent, hexamethonium. In DRT preparations, MK-801 enhanced the amplitude of spontaneous and ES-BW-evoked contractions. These results demonstrate that bladder activity of the neonatal rat is tonically inhibited by input from the lumbosacral spinal cord via parasympathetic pathways in the pelvic nerve. The inhibitory outflow is not dependent upon afferent input to the cord but is facilitated by NMDA glutamatergic transmission in the spinal cord. Antidromic activation of afferent axons also appears to induce inhibition in the bladder via a mechanism involving nicotinic cholinergic receptors. These findings suggest that spinal and peripheral inhibitory mechanisms may play an important role in controlling voiding in the neonatal rat.

Tachykinins as modulators of the micturition reflex in the central and peripheral nervous system

In the normal urinary bladder, tachykinins (TKs) are expressed in a population of bladder nociceptors that is sensitive to the excitatory and desensitizing effects of capsaicin (i.e., capsaicin-sensitive primary afferent neurons (CSPANs)). Several endobiotics or xenobiotics excite CSPANs and release TKs and other mediators at both the peripheral and spinal cord level. The peripheral release of TKs determines a set of responses (known as neurogenic inflammation) that includes vasodilatation, plasma protein extravasation, smooth muscle contraction and stimulation of afferent nerves. Following chronic inflammation, both immune cells and capsaicin-resistant sensory neurons can de novo express TKs: whether these pools of TKs are releasable and contribute to inflammatory processes is presently unsettled. At the spinal cord level, the release of TKs contributes in determining an altered pattern of vesicourethral reflexes in response to nociceptive stimulation of the bladder by conveying: (a) the afferent transmission to supraspinal sites, and (b) descending or sensory inputs to the sacral parasympathetic nucleus (SPN). Recent evidence also attribute a synergetic role of TKs in the supraspinal modulation of the sensory arm of the micturition reflex. The overall available information suggests that TK receptor antagonists may affect bladder motility/reflexes which occur during different pathological states, while having little influence on the normal motor bladder function.

Pharmacology of the lower urinary tract

The functions of the lower urinary tract, to store and periodically release urine, are dependent on the activity of smooth and striated muscles in the urinary bladder, urethra, and external urethral sphincter. This activity is in turn controlled by neural circuits in the brain, spinal cord, and peripheral ganglia. Various neurotransmitters, including acetylcholine, norepinephrine, dopamine, serotonin, excitatory and inhibitory amino acids, adenosine triphosphate, nitric oxide, and neuropeptides, have been implicated in the neural regulation of the lower urinary tract. Injuries or diseases of the nervous system, as well as drugs and disorders of the peripheral organs, can produce voiding dysfunctions such as urinary frequency, urgency, and incontinence or inefficient voiding and urinary retention. This chapter will review recent advances in our understanding of the pathophysiology of voiding disorders and the targets for drug therapy.

Effects of pituitary adenylate cyclase activating polypeptide on lumbosacral preganglionic neurons in the neonatal rat spinal cord

The effects of PACAP-38 on phasic and tonic preganglionic neurons (PGN) in L6 and S1 spinal cord slices from neonatal rats (5--11 days old) were studied using the whole-cell patch clamp technique. PGN were identified by retrograde axonal transport of a fluorescent dye (Fast Blue, 5 microl of 4% solution) injected into the intraperitoneal space 3--7 days prior to the study. Bath application of pituitary adenylate cyclase activating polypeptide (PACAP) (20 nM) increased the frequency of spontaneous excitatory postsynaptic potentials (EPSPs) and spontaneous firing in both types of PGN. PACAP markedly increased the number (200--800%) and frequency of action potentials elicited by depolarizing current pulses in phasic PGN, but had a smaller effect on tonic PGN. PACAP decreased the threshold for action potential generation by approximately 25% in both types of neurons (e.g. -34.0+/-1.5 to -38.4+/-1.7 mV from a holding potential of -50 mV in phasic PGN, P<0.005). PACAP did not affect the duration of the action potential. The amplitude of the spike after hyperpolarization was not changed but the duration was significantly reduced by PACAP from 204.4+/-12.2 to 106.2+/-8.1 ms in tonic but not in phasic PGN. PACAP suppressed a transient outward current that was also suppressed by 4-aminopyridine (0.5 mM). These results coupled with the immunohistochemical identification of a dense collection of PACAP fibers in the region of the PGN, raises the possibility that PACAP may function as an excitatory transmitter in lumbosacral parasympathetic reflex pathways in the neonatal rat.

Tissue distribution of PACAP27 and -38 in oligochaeta: PACAP27 is the predominant form in the nervous system of Lumbricus polyphemus

The levels of pituitary adenylate cyclase-activating polypeptide (PACAP)27 and -38 were measured in the nervous, intestinal, excretory, and reproductive systems of Lumbricus polyphemus by radioimmunoassay. Although both PACAP27 and -38 were significantly detectable in all of the examined tissues, the distribution of the peptides was very heterogeneous. Their highest concentrations were found in the cerebral ganglia and the ventral cord, followed by the alimentary tract and the nephridial system, respectively. Moreover, the reproductive system also contained a substantial amount of PACAP. The dominant form of the peptide discovered in the majority of tissues was PACAP27. Interestingly, about 10 times more PACAP27 than PACAP38 was found, with the latter representing only a fraction of PACAP-like immunoreactivity in the tissues of Lumbricus polyphemus.

Pituitary adenylate cyclase activating polypeptide immunoreactivity in capsaicin-sensitive nerve fibres supplying the rat urinary tract

Pituitary adenylate cyclase activating peptide is a new member of the vasoactive intestinal polypeptide family of peptides which is present in the brain as well as neuronal elements of a number of peripheral organs. Pituitary adenylate cyclase activating peptide occurs in two forms, pituitary adenylate cyclase activating peptide-38 and the C-terminally truncated 27 amino acid form, pituitary adenylate cyclase activating peptide-27, both derived from the same precursor which in addition gives rise to a structurally-related peptide, pituitary adenylate cyclase activating peptide-related peptide. Using specific radioimmunoassays for pituitary adenylate cyclase activating peptide-38, pituitary adenylate cyclase activating peptide-27 and pituitary adenylate cyclase activating peptide-related peptide we found that all three pituitary adenylate cyclase activating peptide-precursor-derived peptides were present in tissue extracts from the ureter, the urinary bladder and the urethra. Pituitary adenylate cyclase activating peptide-38 was the dominating peptide with the highest concentration in the ureter. When extracts from the urinary bladder were fractionated by reverse phase high pressure liquid chromatography immunoreactive components corresponding to synthetic pituitary adenylate cyclase activating peptide-38, pituitary adenylate cyclase activating peptide-27 and pituitary adenylate cyclase activating peptide-related peptide were identified with the respective antisera. By immunohistochemistry, using a specific monoclonal mouse anti-pituitary adenylate cyclase activating peptide antibody, pituitary adenylate cyclase activating peptide-immunoreactivity was shown to have a widespread distribution in the rat urinary tract, localized exclusively to nerve fibres. No immunoreactive neuronal cell bodies were observed in any of the tissues. Pituitary adenylate cyclase activating peptide was shown to be located in varicose nerve fibres associated with blood vessels and smooth muscle. The majority of pituitary adenylate cyclase activating peptide-positive nerve fibres and bundles were, however, present in subepithelial plexuses from which delicate varicose nerve fibres entered the urothelium. Double immunostaining for pituitary adenylate cyclase activating peptide and a marker for sensory neurons, calcitonin-gene related peptide, disclosed that the two peptides were almost completely co-localized while the co-existence between pituitary adenylate cyclase activating peptide and the structurally related peptide vasoactive intestinal polypeptide, was scarce. Neonatal capsaicin-treatment caused a marked reduction in the concentration of immunoreactive pituitary adenylate cyclase activating peptide in all regions of the rat urinary tract, being most prominent in the ureter. By immunohistochemistry it was shown that the sensory neurotoxin caused a reduction in the number and intensity of pituitary adenylate cyclase activating peptide-immunoreactive nerve fibres in all organs of the urinary tract which was most prominent in the epithelial and subepithelial layers. Identical changes were observed for the calcitonin-gene related peptide-containing nerve fibres, while vasoactive intestinal polypeptide-positive nerve fibres were unaffected by capsaicin-treatment. In conclusion pituitary adenylate cyclase activating peptide is present in the rat urinary tract mainly in the form of pituitary adenylate cyclase activating peptide-38. Immunoreactive nerve fibres were associated with the epithelium, blood vessels and smooth musculature. Pituitary adenylate cyclase activating peptide was almost completely co-localized with calcitonin-gene related peptide and by neonatal capsaicin treatment the two peptides were identically affected. The findings suggest that pituitary adenylate cyclase activating peptide is a sensory neurotransmitter in the rat urinary tract.

Intrathecal pituitary adenylate cyclase activating polypeptide facilitates the spinal nociceptive flexor reflex in the rat

We have examined the effects of intrathecal (i.t.) pituitary adenylate cyclase activating polypeptide on the spinal nociceptive flexor reflex in decerebrate, spinalized, unanaesthetized rats. The flexor reflex was elicited by electrical stimulation applied subcutaneously to the sural nerve innervation area and recorded as electromyogram activity from ipsilateral hamstring muscles. Pituitary adenylate cyclase activating polypeptide(l-27) was administered over a wide dose range (10 ng to 10 mu g) and elicited a dose-dependent facilitation of the flexor reflex and did not depress the reflex at any dose. Furthermore, pituitary adenylate cyclase activating polypeptide did not inhibit the facilitation of the flexor reflex induced by repetitive stimulation of C-fibres. It is concluded that pituitary adenylate cyclase activating polypeptide had an excitatory effect on spinal cord function which may indicate a role for this peptide in nociceptive transmission and modulation. Moreover, in contrast to previous studies, we found no evidence suggesting that pituitary adenylate cyclase activating polypeptide exerts antinociceptive action at spinal level.