M.N. Kruse

NEURAL CONTROL OF URETHRAL OUTLET ACTIVITY IN VIVO: ROLE OF NITRIC OXIDE

The present study investigated the role of nitric oxide (NO) in the reflex changes in urethral outlet activity during micturition. Isovolumetric bladder contractions, urethral pressure and external urethral sphincter electromyogram (EUS EMG) activity were recorded independently in urethane-anesthetized rats. During reflex bladder contractions, the urethra exhibited reflex responses characterized by an initial decrease in urethral pressure in conjunction with a rise in bladder pressure. This was followed by a period of high frequency oscillations (HFOs) associated with maximal urethral relaxation and burst type EUS EMG activity. Administration of N-nitro-L-arginine (L-NOARG) 10 mg./kg. intravenously, a nitric oxide synthase inhibitor, reversibly decreased the magnitude (62%, p < 0.05) and duration (40%, p < 0.05) of reflex urethral relaxation (N = 7). In 4 additional experiments, L-NOARG (10 to 15 mg./kg. intravenously) completely eliminated reflex urethral relaxation during micturition, and this effect was reversed in all animals by the administration of L-arginine (100 to 150 mg./kg. intravenously). Administration of N-nitro-D-arginine (D-NOARG) (10 to 30 mg./kg. intravenously) had no effect on reflex urethral relaxation. Neuromuscular blockade (vecuronium bromide 5 mg./kg. intravenously) reversibly decreased resting urethral pressure and eliminated the HFOs. The urethral smooth muscle relaxation that remained after neuromuscular blockade was eliminated following administration of L-NOARG (10 mg./kg. intravenously) in 2 of 3 animals. These results suggest that reflex urethral responses during micturition involve changes in both smooth and striated muscle activity, and that the predominant neurotransmitter mechanisms that mediate reflex urethral smooth muscle relaxation involve NO.

Pontine control of the urinary bladder and external urethral sphincter in the rat

Neurons in the rostral pontine tegmentum are known to have an important role in controlling micturition. The present experiments used urethane anesthetized rats to examine the effects of electrical stimulation at various sites in the pons on bladder and external urethral sphincter activity and on the volume threshold for inducing micturition. Stimulation with short trains of pulses (50 Hz, 1-3 s trains, 1-15 V) in the laterodorsal tegmental nucleus (LDT), the periaqueductal grey (PAG) or the lateral parabrachial nucleus (L-PBN) elicited contractions of a partially filled, quiescent bladder. However stimulation during a bladder contraction aborted the contraction indicating that these areas have inhibitory as well as excitatory effects. Continuous stimulation (50 Hz) in the PAG or L-PBN during a cystometrogram decreased bladder capacity (mean decrease 36%). Conversely, continuous stimulation in the pontine reticular formation (in or near the dorsal subcoeruleus nucleus and medial parabrachial nucleus) increased bladder capacity (mean increase 50%). Stimulation at pontine sites (LDT, PAG and L-PBN) which elicited bladder contractions also elicited an increase in external urethral sphincter activity. A similar increase in urethral sphincter activity occurred during reflex micturition induced by bladder distension. These data suggest that bladder capacity and the coordination of bladder and external urethral functions are controlled by various neuronal populations in the rostral pons of the rat.

Influence of spinal cord injury on the morphology of bladder afferent and efferent neurons

Severe micturition dysfunction can occur following spinal cord injury (SCI) due to abnormal contractions of the urethral sphincter during bladder contractions (bladder/sphincter dyssynergia). This causes urinary retention, bladder overdistension, and increases the workload of the bladder leading to hypertrophy of the bladder muscle. Bladder hypertrophy induced by urethral outlet ligation in rats is accompanied by enlargement of both the afferent and efferent neurons innervating the bladder. The primary aim of this study was to test whether SCI-induced bladder hypertrophy produces a similar enlargement of bladder afferent neurons in the dorsal root ganglia (DRG) or efferent neurons in the major pelvic ganglia (MPG). Following SCI in female Wistar rats, there was a four-fold increase in bladder weight. The mean cross-sectional area of bladder DRG cell profiles increased approx. 50% after SCI; however, the mean area of MPG cell profiles did not change significantly. Urinary diversion (disconnecting the ureters from the bladder) prevented both the bladder hypertrophy and the DRG cell hypertrophy after SCI, suggesting that bladder hypertrophy drives DRG cell enlargement. On the other hand, since the size of MPG cells did not change significantly after SCI, bladder hypertrophy does not mandate MPG cell enlargement. However, preliminary results indicate that the mean cross-sectional area of MPG cells did increase (2-3 times) in SCI rats when the neural input to the MPG was eliminated by transecting the pelvic and hypogastric nerves; this suggests that the lack of change in size of MPG cells after SCI may be due to an inhibitory influence from the spinal cord.

Spinal pathways mediate coordinated bladder/urethral sphincter activity during reflex micturition in decerebrate and spinalized neonatal rats

Coordination between the urinary bladder and the external urethral sphincter is necessary for normal voiding. However, it is uncertain whether the spinal cord or brainstem generates this coordination. Bladder and urethral sphincter activity were examined during reflex voiding induced by perineal stimulation or bladder distension in decerebrate non-spinalized and spinalized 15 to 26-day-old neonatal rats. Perineal stimulation induced voiding and coordinated bladder/sphincter activity in both types of rats, indicating that spinal pathways can generate coordinated voiding behavior. The discoordination observed during voiding induced by bladder distension in spinalized pups may be due to the loss of descending pathways or to the emergence of detrimental spinal reflexes.

Properties of the descending limb of the spinobulbospinal micturition reflex pathway in the cat

The micturition reflex is thought to be mediated by a spinobulbospinal reflex pathway passing through the rostral pons. This study examined the properties of the descending limb of the reflex pathway by monitoring the responses of the lower urinary tract to stimulation of the pons in the decerebrate cat. Electrical stimulation (300 microseconds pulses at 50 Hz intratrain frequencies, 300-500 ms trains, 0.5-15 V) in the region of the locus coeruleus (P 0.5-3.1/L 2-4/H to -2.75) was used to activate the descending excitatory pathway to the sacral parasympathetic nucleus. Low intensity stimulation induced small amplitude, short duration (14 +/- 11 cm H2O, 10 +/- 3 s) bladder contractions in a partially full bladder, whereas higher intensity stimulation induced large amplitude, long duration (69 +/- 29 cm H2O, 70 +/- 44 s) contractions which were similar to distension-induced reflex micturition contractions. The evoked bladder contractions coincided with a reduction in external urethral sphincter (EUS) EMG activity. Following bilateral L7-S3 dorsal root transection, electrical stimulation of the pons still elicited the small amplitude bladder contractions, but the larger amplitude, long duration micturition contractions were abolished. During these small evoked bladder contractions, a suppression of EUS activity still occurred following deafferentation, indicating a pontine mediated bladder/EUS synergy. It is concluded that the pons can initiate bladder contractions and coordinated bladder-sphincter activity, but that afferent feedback (via the dorsal roots) is needed to maintain the large amplitude micturition contractions.

Effect of urinary diversion on the recovery of micturition reflexes after spinal cord injury in the rat.

Patients with suprasacral spinal cord injury usually exhibit severe lower urinary tract dysfunction, which is generally attributed to loss of supraspinal input to the spinal micturition centers. However, some of the dysfunction may also arise secondary to bladder overdistension during the initial period of bladder areflexia. This study evaluated the consequences of bladder overdistension by performing urinary diversion in spinalized (T8-T10) rats. Bladder function was evaluated in urethane-anesthetized control and spinalized animals approximately 24 days after diversion. Chronically spinalized diverted and nondiverted rats exhibited similar micturition dysfunction: bladder/sphincter dyssynergia, incomplete voiding and ineffective (nonvoiding) bladder contractions. These data indicate that neither the condition of the bladder (such as chronic overdistension or bladder hypertrophy) nor afferent input from the bladder to the spinal cord dictates the development of reflex micturition and micturition dysfunction after spinal cord injury, suggesting that the dysfunction is intrinsic to spinal micturition reflex pathways.

Consequences of spinal cord injury during the neonatal period on micturition reflexes in the rat.

This study examined the changes in micturition reflexes following spinal cord transection in neonatal rats to determine: (1) whether injury to the immature nervous system allows greater recovery of function than injury in adult animals and (2) whether the management of the lower urinary tract during the initial period following spinal injury influences the subsequent recovery of function. In one experiment, bladder-to-bladder reflexes in decerebrate neonatal rats (Day 15-Day 26) were tested 5-11 days after T8-T10 spinalization. While there was no difference in the amplitude and duration of reflex bladder contractions or bladder capacity between these pups and their nonspinalized controls, the spinalized pups exhibited incomplete voiding and an uncoordinated urethral sphincter (bladder/sphincter dyssynergia). It is concluded that the dyssynergia is inherent to the spinal micturition reflexes and is not due to an initial period of bladder areflexia and overdistension since in both the control and spinalized neonates micturition is initiated by a somatobladder reflex triggered when the mother licks the perineum. A second experiment tested whether neonatal spinal cord injury led to improved bladder function in adulthood. Postnatal Day 1 rat pups were spinalized at T8-T10 and returned to their mothers for the remainder of the neonatal period, and their bladder reflexes were tested 4-6 months later under urethane anesthesia. These rats showed the same lower urinary tract dysfunctions (bladder/sphincter dyssynergia, high residual volumes, decreased percentage voided volumes, and large-amplitude, long-duration bladder contractions) as adult rats that were spinalized as adults.(ABSTRACT TRUNCATED AT 250 WORDS)

Differences in fluorogold and wheat germ agglutinin-horseradish peroxidase labelling of bladder afferent neurons

Rat urinary bladder afferent neurons were significantly smaller (34%) when labelled with Fluorogold (FG) than when labelled with wheat germ agglutinin-horseradish peroxidase (WGA-HRP). This study showed that this difference was due to an artifact of tissue processing (ethanol dehydration) and was not due to uptake and transport of the two tracers by two different subpopulations of bladder afferents.