William D. Steers

Mechanisms underlying the recovery of urinary bladder function following spinal cord injury

Micturition in cats and rats with an intact neuraxis is dependent upon a spinobulbospinal reflex activated by A delta bladder afferents. This report describes changes in micturition reflexes 2 h to 14 weeks following spinal cord transection at the lower thoracic level. In acute spinal cats micturition reflexes were blocked, however, several weeks after transection, a long latency (180-200 ms) spinal reflex could be activated by C-fiber bladder afferents. This reflex was blocked by capsaicin in doses (20-30 mg/kg, s.c.) that did not affect micturition reflexes in intact cats. Micturition reflexes were unmasked in acute spinal and facilitated in chronic spinal cats by naloxone, an opioid antagonist. Spinal neurons and axons containing opioid peptides were more prominent below the level of transection in chronic spinal cats. VIP, a putative neurotransmitter in C-fiber bladder afferents, inhibited micturition reflexes when injected intrathecally (2-10 micrograms) in intact cats but facilitated micturition reflexes in spinal cats (doses 0.1-1 micrograms, i.t.). VIP-containing C-fiber afferent projections to lamina I of the sacral spinal cord expanded in spinal cats. Thus VIP afferents may have an important role in the recovery of bladder reflexes after spinal injury. Paraplegic animals also exhibit bladder-sphincter dyssynergia, which causes functional outlet obstruction. Studies in rats have revealed that outlet obstruction induced by partial urethral ligation facilitates spinal micturition reflex pathways and causes an expansion of HRP-labelled bladder afferent projections in the spinal cord. These findings raise the possibility that the alterations in central reflex connections in paraplegic animals may be induced in part by changes in peripheral afferent input secondary to outlet obstruction.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of bladder outlet obstruction on micturition reflex pathways in the rat.

The effect of outlet obstruction on neural pathways to the bladder was examined six weeks after urethral ligation in rats. Ligated rats exhibited a seven-fold increase in bladder weight, a six-fold increase in volume, and significant increases (p less than 0.001) in micturition pressure, residual volume and compliance measured under urethane anesthesia. Unanesthetized ligated rats in comparison to control rats voided smaller volumes of urine more frequently (0.7 vs 1.1 voids/hr., p less than 0.001). Electrophysiological examination of vesical afferent and efferent peripheral neural pathways in the pelvic and hypogastric nerves and sympathetic chain did not reveal any differences in thresholds, axonal conduction velocities or ganglionic transmission. A significant difference (p less than 0.05) was noted in the percentage of obstructed rats (100%) exhibiting a short latency spinal reflex as compared to controls (35%). The long latency supraspinal micturition reflex was similar in both groups. These data suggest that obstruction in rats is accompanied by some degree of neural plasticity resulting in a more prominent spinal reflex that could contribute to the development of the unstable bladder following obstruction in humans and unanesthetized animals.

Excitatory and inhibitory influences on bladder activity elicited by electrical stimulation in the pontine micturition center in the rat

Electrical stimulation at various sites in the dorsal pontine tegmentum in urethane anesthetized rats modulated activity of the urinary bladder as well as efferent firing on bladder postganglionic nerves. Electrical stimulation (0.2 ms 50 Hz, 5-20 V or 30-150 microA, 2-5 s train duration) using a microelectrode (tip diameter, 10-20 microns) in an excitatory area located rostral and medial to the locus coeruleus evoked short latency (less than 2 s) large amplitude (greater than 20 cm H2O) bladder contractions and increased firing on the bladder postganglionic nerves. Stimulation at sites adjacent to the excitatory area inhibited bladder postganglionic nerve firing and bladder activity. Inhibitory responses were evident as either a decrease in intravesical pressure, an increased interval between bladder contractions, or an interruption or elimination of bladder contractions. The threshold intensity for excitation using a large electrode (2-4 V) was slightly higher than that for inhibition (1.5-2 V). The optimum sites for evoking bladder contractions were located in and close to the laterodorsal tegmental nucleus (LDT) and in the periaqueductal gray just dorsal or dorsolateral to the LDT. The extent of the area that induced bladder contractions was 0.5-1.2 mm in diameter in each rat when a microelectrode was employed for electrical stimulation. Electrical stimulation in the optimum site for evoking bladder contractions induced relatively little striated muscle activity and produced no short-latency blood pressure changes. The longer latency blood pressure changes associated with a spontaneous bladder contraction were still present following a stimulation of the dorsolateral pons. These data are consistent with the view that neurons in the dorsal pontine tegmentum play an important role in the regulation of urine storage as well as urine release.

Morphological plasticity in efferent pathways to the urinary bladder of the rat following urethral obstruction

Partial urethral ligation in female Wistar rats produces changes in the neural control of the lower urinary tract including bladder hyperactivity and facilitation of a spinal micturition reflex pathway. To gain insight into the mechanisms underlying these changes, axonal tracing studies were conducted to examine the postganglionic efferent limb of the micturition reflex pathway which originates in the major pelvic ganglion (MPG). Forty microliters of the tracer Fluoro-Gold (4%) were injected into the right side of the bladder in urethral-obstructed (n = 10) and control (n = 4) rats 6 weeks after urethral ligation or sham surgery. As a control Fast blue (40 microliters, 5%) was injected into the colon to label neurons in the MPG innervating the intestine. Obstructed rats exhibited a 6-fold increase (p less than 0.001) in bladder weight (0.848 gm) compared to controls (0.148 gm). A significant increase (p less than 0.001) in the size of labeled bladder postganglionic neurons in the MPG was noted in obstructed rats (576.4 microns 2, n = 4) as compared to controls (299.6 microns 2). However, labeled, colon postganglionic neurons in the MPG in obstructed (312.9 microns 2) rats were not enlarged compared to controls (359.4 microns 2). Neuronal hypertrophy was not associated with a change in the number of labeled MPG neurons in control and obstructed groups.(ABSTRACT TRUNCATED AT 250 WORDS)

Electrophysiological analysis of the ascending and descending components of the micturition reflex pathway in the rat

Electrophysiological techniques were used to examine the organization of the spinobulbospinal micturition reflex pathway in the rat. Electrical stimulation of afferent axons in the pelvic nerve evoked a long latency (136 +/- 41 ms) response on bladder postganglionic nerves, whereas stimulation in the dorsal pontine tegmentum elicited shorter latency firing (72 +/- 25 ms) on these nerves. Transection of the pelvic nerve eliminated these responses. Firing on the bladder postganglionic nerves was evoked by stimulation in a relatively limited area of the pons within and close to the laterodorsal tegmental nucleus (LDT) and adjacent ventral periaqueductal gray. Stimulation at sites ventral to this excitatory area inhibited at latencies of 107 +/- 11 ms the asynchronous firing on the bladder postganglionic nerves elicited by bladder distension. Electrical stimulation of afferents in the pelvic nerve evoked short latency (13 +/- 3 ms) negative field potentials in the dorsal part of the periaqueductal gray as well as long latency (42 +/- 7 ms) field potentials in and adjacent to the LDT. The responses were not altered by neuromuscular blockade. Similar responses were elicited by stimulation of afferent axons in the bladder nerves. The sum of the latencies of the ascending and descending pathways between the LDT and the pelvic nerve (i.e. 72 ms plus 42 ms = 114 ms) is comparable although somewhat shorter (22 ms) than the latency of the entire micturition reflex. These results provide further evidence that the micturition reflex in the rat is mediated by a spinobulbospinal pathway which passes through the dorsal pontine tegmentum, and that neurons in the periaqueductal gray as well as the LDT may play as important role in the regulation of the micturition.

Effects of streptozotocin-induced diabetes on bladder function in the rat.

Streptozotocin-induced (STZ, 60 mg./kg. I.V.) diabetic male rats underwent cystometrographic (CMG) and electrophysiologic evaluations under urethane anesthesia (1.2 gm./kg.) to determine whether the neural pathways to the urinary bladder are altered in these animals. Diabetic rats (n = 6) in comparison to controls (n = 8) had significantly greater micturition volumes (3.0 +/- 0.8 ml. vs. 0.7 +/- 0.4 ml., p less than 0.001), bladder compliances (0.51 +/- 0.15 cm. H2O/ml. vs. 0.12 +/- 0.09 cm. H2O/ml., p less than 0.001) and bladder weights (225.2 +/- 21.4 mg. vs. 112.2 +/- 18.0 mg., p less than 0.01). No differences were noted in: 1) the thresholds or conduction velocities of axons in the bladder postganglionic nerves, 2) transmission in the major ganglion or 3) the latencies for firing in the supraspinal parasympathetic reflex pathway to the bladder. However, the supraspinal reflex, which was facilitated by bladder distension in 38% of control rats, was not facilitated in any diabetic rats. Another apparent difference in diabetic rats was the absence of spinal reflex response which was noted in 38% of control animals. This study confirmed that CMG changes in STZ-induced diabetic rats are similar to those observed clinically in patients with diabetic autonomic neuropathy, and in addition raise the possibility that these changes are produced by a defect in autonomic reflexes. It is also clear that alterations of the micturition reflex pathway in diabetic rats are distinct from those associated with another type of enlarged, abnormal bladder induced by bladder outlet obstruction.