BACKGROUND:Motoneurons from the Onuf’s nucleus of the spinal cord, which innervate the striated muscle of the pelvic floor, play an important role in erection, ejaculation, and urine control. Serotonin (5-hydroxytryptamine, 5-HT) regulates motoneuron activity from the Onuf’s nucleus of the spinal cord. However, few studies exist that describe 5-HT receptor distribution in the Onuf’s nucleus. In addition, the nature of the effects of 5-HT receptor on the innervating striated muscle of the pelvic floor is controversial. OBJECTIVE: To investigate the distribution of serotonin 5-HT2A and 5-HT7 receptors in motoneurons of Onuf’s nucleus in the spinal cord of male rats, and to analyze the relationship of 5-HT2A and 5-HT7 receptor to central modulation of urogenital function. DESIGN, TIME AND SETTING: The neural morphology experiment was performed at the Ultramicro-structure Laboratory of Reproductive Medicine, Basic Medical College, Chongqing Medical University, China from April to December 2007. MATERIALS: Ten adult, Sprague Dawley rats (eight males and two females) were randomly divided into gender control group (n = 4, 50% male and 50% female) and a retrograde tracing group (n = 6, 100% male) Recombinant pseudorabies virus (PRV-152) was provided by Professor LW Enquist from Princeton University, USA. Rabbit anti-5-HT2A and 5-HT7 receptor antibodies were purchased from Diasorin, France. METHODS: In the gender control group, the spinal L5-6 segments were harvested, sliced, and then incubate antibodies specific against 5-HT2A or 5-HT7 receptors for immunohistochemical staining. In the retrograde tracing group, PRV-152 was separately injected into the right ischiocavernosus (ischiocavernosus subgroup, n = 3) and the right external urethral sphincter (external urethral sphincter subgroup, n = 3). Four days after injection, L5-6 segments were harvested, sliced, and incubated with antibodies specific against 5-HT2A or 5-HT7 receptors for double-labeling immunofluorescence staining. MAIN OUTCOME MEASURES: Distribution analysis of 5-HT2A and 5-HT7 receptors in Onuf’s nucleus utilizing optical or laser confocal microscopy. RESULTS: 5-HT2A receptor immunoreactivity was revealed primarily in the medial region of the dorsolateral nucleus of Onuf’s nucleus. 5-HT7 receptor expression was observed in the lateral part of the dorso-lateral nucleus. 5-HT2A and 5-HT7 receptor expressions in the Onuf’s nucleus were significantly greater in male rats, compared to female rats. Double-labeling immunofluorescence demonstrated that 5-HT2A recepto were distributed primarily in the surrounding motoneurons innervating the ischiocavernosus, and 5-HT7 receptors were primarily expressed in motoneurons innervating the external urethral sphincter. CONCLUSION: Motoneurons innervating the ischiocavernosus and external urethral sphincter are located primarily in the medial and lateral region of the dorsolateral nucleus of L5-6 segments. The 5-HT2A receptor-innervating ischiocavernosus may be preferentially involved in the regulation of sexual reflex, and the 5-HT7 receptor-innervating external urethral sphincter may mainly join in regulating micturition reflex. BACKGROUND: Motoneurons from the Onuf’s nucleus of the spinal cord, which innervate the striated muscle of the pelvic floor, play an important role in erection, ejaculation, and urine control. Serotonin (5-hydroxytryptamine, 5- HT) regulates motoneuron activity from the Onuf’s nucleus of the spinal cord. However, few studies exist that describe 5-HT receptor distribution in the Onuf’s nucleus. In addition, the nature of the effects of 5-HT receptor on the innervating striated muscle of the pelvic floor is controversial. OBJECTIVE : To investigate the distribution of serotonin 5-HT2A and 5-HT7 receptors in motoneurons of Onuf’s nucleus in the spinal cord of male rats, and to analyze the relationship of 5-HT2A and 5-HT7 receptor to central modulation of urogenital function. DESIGN , TIME AND SETTING: The neural morphology experiment was performed at the Ultramicro-structure Laboratory of Reproductive Medicine, Basic Medical College, Chongqing Medical University, China from April to D MATERIALS: Ten adult, Sprague Dawley rats (eight males and two females) were randomly divided into gender control groups (n = 4, 50% male and 50% female) and a retrograde tracing group (n = 6, 100% male) Recombinant pseudorabies virus (PRV-152) was provided by Professor LW Enquist from Princeton University, USA. Rabbit anti-5-HT2A and 5-HT7 receptor antibodies were purchased from Diasorin, France. METHODS: In the gender control group, the spinal L5-6 segments were harvested, sliced, and then incubate antibodies specific against 5-HT2A or 5-HT7 receptors for immunohistochemical staining. In the retrograde tracing group, PRV-152 were separately injected into the right ischiocavernosus (ischiocavernosus subgroup, n = 3) and the right external urethral sphincter (external urethral sphincter subgroup, n = 3). Four days after injection, L5-6 segments were harvested, sliced, and incubated with antibodies specific against 5-HT2A or 5-HT7 receptors for double- labeling immunofluoresc ence staining. MAIN OUTCOME MEASURES: Distribution analysis of 5-HT2A and 5-HT7 receptors in Onuf’s nucleus utilizing optical or laser confocal microscopy. RESULTS: 5-HT2A receptor immunoreactivity was was primarily in the medial region of the dorsolateral nucleus of Onuf’s nucleus. 5 -HT7 receptor expression was observed in the lateral part of the dorso-lateral nucleus. 5-HT2A and 5-HT7 receptor expressions in the Onuf’s nucleus were significantly greater in male rats, compared to female rats. Double-labeling immunofluorescence demonstrated that 5- HT2A recepto were distributed in the surrounding motoneurons innervating the ischiocavernosus, and 5-HT7 receptors were were expressed in motoneurons innervating the external urethral sphincter. CONCLUSION: Motoneurons innervating the ischiocavernosus and external urethral sphincter are located in the medial and lateral region of the dorsolateral nucleus of L5-6 segments. The 5-HT2A receptor-innervating ischiocaver nosus may be preferentially involved in the regulation of sexual reflex, and the 5-HT7 receptor-innervating external urethral sphincter may mainly join in regulating micturition reflex.