The effect of 4-aminopyridine on micturition reflex in normal or capsaicin-desensitized rats

Neurogenic Causes of Detrusor Underactivity

Detrusor underactivity (DU) is a poorly understood dysfunction of the lower urinary tract which arises from multiple etiologies. Symptoms of DU are non-specific, and a pressure-flow urodynamic study is necessary to differentiate DU from other conditions such as overactive bladder (OAB) or bladder outlet obstruction (BOO). The prevalence of DU ranges from 10-48%, and DU is most prevalent in elderly males. The pathophysiology underlying DU can be from both neurogenic and non-neurogenic causes. In this article, we review the neurogenic causes of detrusor underactivity, including diabetic bladder dysfunction, spinal cord injury, multiple sclerosis, Parkinson's disease, cerebrovascular accident, traumatic brain injury, and Fowler's syndrome. As knowledge about the underlying causes of DU advances, there have been several potential therapeutic approaches proposed to help those who suffer from this condition.

Urodynamic evaluation and electrical and pharmacologic neurostimulation. The rat model

We introduce a rat model that allows simultaneous or independent recording of bladder and sphincteric activity. Via a polyethylene tube inserted into the bladder dome, bladder pressure is measured in response to constant intravesical saline perfusion. The electrical activity of the intra-abdominal urethra (a well-defined striated muscular tube which, in the rat, constitutes the external urethral sphincter) is recorded simultaneously with an electromyography needle electrode. Thus, precise statements can be made about detrusor/sphincter interrelationships. Changes in urodynamic parameters with the anesthetics urethane, methoxyflurane (Metofane), and thiobutabarbital sodium (Inactin) were investigated. High-frequency oscillations in intraluminal bladder pressure could be demonstrated during micturition cycles only in rats anesthetized with urethane or Metofane. As this high-frequency activity is generated by the striated muscle of the intra-abdominal urethra, the external sphincter of the rat is the force behind urine expulsion. The anesthetic Inactin combined with a low intravesical perfusion rate attenuated spontaneous bladder and sphincteric activity and abolished micturition cycles. This rat model can provide accurate and reproducible measurements of urodynamic changes in response to electrical stimulation of the pelvic and pudendal nerves and pharmacologic stimulation with neuropeptides at the lumbosacral spinal cord level. We recommend using this model with urethane or Metofane for physiologic studies of micturition and with Inactin for meticulous neuropharmacologic and electrostimulatory evaluation of urodynamic parameters.

Acrylamide-induced visceral neuropathy: evidence for the involvement of capsaicin-sensitive nerves of the rat urinary bladder

The mechanisms underlying the severe urinary retention induced by acrylamide intoxication were studied in detail in the rat. Subcutaneous treatment with acrylamide monomer (50 mg/kg daily for 10 days) almost completely impaired the micturition reflex, resulting in urinary retention. In fact, the ability to eliminate an oral water load was virtually abolished, while bladder filling with saline (transvesical cystometrogram) failed to activate reflex micturition. Instead, a picture of overflow incontinence resulted in urethane-anaesthetized rats, which was not reversed by intravenous administration of 4-aminopyridine. The nerve-mediated contractile response to field stimulation (0.1-20 Hz, 0.5 ms, 60 V) of the isolated bladder was unaffected, thus suggesting the integrity of bladder efferent innervation, and no evidence was found from in vitro experiments that the myogenic contractility of the bladder was depressed by acrylamide treatment. Conversely, the sensory nerve-mediated response to capsaicin was abolished and sensory nerve fibres of the bladder were selectively depleted of their content of substance P- and calcitonin gene-related peptide immunoreactivity following acrylamide treatment. In fact, concentrations of the same neuropeptides in other organs, including the adjoining ureters, were unaffected. As to the urethral segment, including the striated sphincter, the D-tubocurarine (0.2 mM)-sensitive urethral response to electrical stimulation (0.1 Hz, 0.1 ms, 20 V) was significantly reduced in acrylamide-treated animals. At the same level, neurofilament protein immunostaining revealed striking accumulations of neurofilament protein-like material in motor end-plates, thus indicating that neuromuscular junctions of the urethral striated sphincter were severely affected. Thus, the afferent arm of the micturition reflex was shown to be severely deranged by acrylamide intoxication, especially in its capsaicin-sensitive component. Since twitch-like contractions of the urethral striated sphincter are probably involved in promoting bladder voiding, a decreased efficiency of this mechanism could participate in the picture of urinary retention induced by acrylamide.

Involvement of capsaicin-sensitive nerves of the rat urinary bladder in acrylamide neuropathy

Acrylamide monomer is neurotoxic in man and experimental animals, producing a sensorimotor distal axonopathy. In spite of remarkable effect of acrylamide on micturition, resulting in urine retention in both man and rat, bladder autonomic innervation has been little studied. This study focused on the effect of acrylamide on capsaicin-sensitive nerves of the rat bladder, because of the role played by these fibres in regulating the afferent arm of reflex micturition.

Effect of urethane anesthesia on the micturition reflex in capsaicin-treated rats

Cystometry was performed on rats with a chronically implanted intravesical catheter (2 days before cystometry) before and after induction of urethane-anesthesia. Four groups of animals were investigated: group A, vehicle-treated; group B, capsaicin-treated as adults (50 mg/kg s.c.) 4 days before cystometry; group C, capsaicin-treated as adults (125 mg/kg s.c.) 4 days before cystometry; and group D, capsaicin-treated as newborns (50 mg/kg) 2 months before. Rats of group D had a markedly enlarged bladder. Before induction of urethane anesthesia, the bladder capacity of rats of groups A, B and C was not significantly different from each other, while that of rats of group D was larger than controls. After induction of anesthesia bladder capacity in groups B and C was significantly increased as compared to group A and micturition was abolished in group D (overflow incontinence). Amplitude of micturition contraction was significantly reduced in all groups after induction of anesthesia but was unaffected by capsaicin pretreatment. It is concluded that the facilitatory action of capsaicin-sensitive nerves on micturition threshold is more evident in anesthetized than awake rats and that capsaicin-resistant bladder afferents are more sensitive to the depressant action of urethane than the capsaicin-sensitive ones.

Motor and inflammatory effect of hyperosmolar solutions on the rat urinary bladder in relation to capsaicin-sensitive sensory nerves

1. Intravesical administration of hyperosmolar NaCl or urea solutions produced a concentration-dependent stimulatory action on the micturition reflex in urethane-anesthetized rats. This effect was not modified in rats pretreated with capsaicin as adults (50 mg/kg s.c. 4 days before). 2. Hyperosmolar NaCl also produced Evans blue leakage (plasma extravasation) in the rat bladder. This effect was greatly reduced by extrinsic bladder denervation and in rats desensitized to capsaicin as newborns but not as adults. 3. Cumulative addition of NaCl produced a concentration-dependent increase in tone and biphasic effects on neurogenic contractions of the rat isolated bladder. These effects were not modified by in vitro capsaicin desensitization. 4. These findings do not support the idea that true osmoreceptors are present in the rat urinary bladder. The neurogenic component of the inflammatory response to hyperosmolar NaCl could involve activation of a subpopulation of bladder sensory fibers susceptible to the neurotoxic action of capsaicin in the early postnatal period only.

Topical versus systemic capsaicin desensitization: specific and unspecific effects as indicated by modification or reflex micturition in rats

The aim of this study was to determine the acute and delayed effect of topical application of high concentrations of capsaicin on the rat urinary bladder on micturition reflex and compare the effects of "topical" bladder desensitization with those produced by systemic (subcutaneous administration) capsaicin desensitization. On acute application, capsaicin (1-3%) produced a transient bladder contraction, not observed in capsaicin-pretreated rats. After a transient increase in excitability of the micturition reflex, topical capsaicin suppressed micturition and overflow incontinence ensued which was reverted by intravenous injection of 4-aminopyridine. Topical capsaicin also abolished reflex micturition in rats which had been systemically treated with capsaicin as adults (50 mg/kg, 7 days before) and reduced significantly the neurogenic bladder contractions produced by intravenous dimethylphenylpiperazinium or neurokinin A, while the direct (myogenic) response to neurokinin A was unaffected. In rats whose bladder was pre-exposed to 1-3% topical capsaicin (7 days before) the micturition reflex was affected in a manner which is qualitatively and quantitatively similar to that observed in rats treated with capsaicin as adults, e.g. increase in bladder capacity with no change in voiding efficiency. Topical capsaicin desensitization of the rat urinary bladder was shown to produce a selective impairment of bladder sensory nerves without any sign of desensitization in other areas of the body using both functional (hot plate, wiping, plasma extravasation) and neurochemical (determination of substance P-like immunoreactivity) assays. Systemically administered capsaicin (7 days before) had little effect on reflex micturition at 12.5 mg/kg but the change in bladder capacity produced at a dose of 25 mg/kg was comparable with that produced at 350 mg/kg. These findings provide evidence that selective desensitization of peripheral terminals of capsaicin-sensitive nerves of the rat urinary bladder inactivates their sensory and "efferent" function in a manner similar to that observed after systemic capsaicin desensitization in adult rats. The functional deficit of reflex micturition produced in this way can be overcome by increasing the stimulus to void. By contrast, neonatal capsaicin desensitization produced a long lasting abolition of reflex micturition. These data are in keeping with the hypothesis that adult versus neonatal capsaicin desensitization may be used as a tool to distinguish between two sets of sensory nerves in the rat urinary bladder.(ABSTRACT TRUNCATED AT 400 WORDS)