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Chang H-Y, Cheng C-L, Chen J-JJ, de Groat WC. Serotonergic drugs and spinal cord transections indicate that different spinal circuits are involved in external urethral sphincter activity in rats. Am J Physiol Renal Physiol 292: F1044 -F1053, 2007. First published October 17, 2006; doi:10.1152/ajprenal.00175.2006.-Lower urinary tract function is regulated by spinal and supraspinal reflexes that coordinate the activity of the urinary bladder and external urethral sphincter (EUS). Two types of EUS activity (tonic and bursting) have been identified in rats. This study in urethane-anesthetized female rats used cystometry, EUS electromyography, spinal cord transection (SCT) at different segmental levels, and analysis of the effects of 5-HT1A receptor agonist (8-OH-DPAT) and antagonist (WAY100635) drugs to examine the origin of tonic and bursting EUS activity. EUS activity was elicited by bladder distension or electrical stimulation of afferent axons in the pelvic nerve (pelvic-EUS reflex). Tonic activity evoked by bladder distension was detected in spinal cord-intact rats and after acute and chronic T8 -9 or L3-4 SCT but was abolished after L6 -S1 SCT. Bursting activity was abolished by all types of SCT except chronic T8 -9 transection. 8-OH-DPAT enhanced tonic activity, and WAY100635 reversed the effect of 8-OH-DPAT. The pelvic-EUS reflex consisted of an early response (ER) and late response (LR) when the bladder was distended in spinal cord-intact rats. ER remained after acute or chronic T8 -9 and L3-4 SCT, but was absent after L6 -S1 SCT. LR occurred only in chronic T8 -9 SCT rats where it was enhanced or unmasked by 8-OH-DPAT. The results indicate that spinal serotonergic mechanisms facilitate tonic and bursting EUS activity. The circuitry for generating different patterns of EUS activity appears to be located in different segments of the spinal cord: tonic activity at L6 -S1 and bursting activity between T8 -9 and L3-4. bladder; electromyography; 5-HT1A receptor; pelvic nerve; bursting THE STORAGE AND RELEASE OF urine are dependent on the coordinated activity of the urinary bladder smooth muscle and the external urethral sphincter (EUS) striated muscle in the lower urinary tract (LUT). This coordination is mediated by neural mechanisms in the brain and spinal cord that are stimulated by afferent input from the bladder. In normal rats, the EUS exhibits tonic activity before the onset of voiding and bursting activity during voiding. It is believed that bursting activity represents rhythmic contractions and relaxations of the EUS that are necessary for efficient bladder emptying (20). A detailed analysis (4) of the EUS bursting pattern has shown that it consists of silent (urethral opening) and active (urethral closing) periods, averaging 104 and 67 ms in duration, respectively. Efficient voiding depends on the duration and number of urethral openings during voiding. Suppression of EUS bursting activity with neuromuscular blocking agents decreases voiding efficiency (31).Initial studies in deeply anesthetized rats ...

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