5-Hydroxytryptamine2C receptors on pudendal motoneurons innervating the external anal sphincter

The time course of serotonin 2C receptor expression after spinal transection of rats: an immunohistochemical study

In the spinal cord serotonin (5-HT) systems modulate the spinal network via various 5-HT receptors. Serotonin 2A receptor and serotonin 2C receptor (5-HT2A and 2C receptors) are likely the most important 5-HT receptors for enhancing the motoneuron excitability by facilitating the persistent inward current (PIC), and thus play an important role for the pathogenesis of spasticity after spinal cord injury. In conjunction with our 5-HT2A receptor study, using a same sacral spinal transection rat model we have in this study examined 5-HT2C receptor immunoreactivity (5-HT2CR-IR) changes at seven different time intervals after spinal injury. We found that 5-HT2CR-IR was widely distributed in different regions of the spinal gray matter and was predominantly located in the neuronal somata and their dendrites although it seemed also present in axonal fibers in the superficial dorsal horn. 5-HT2CR-IR in different regions of the spinal gray matter was seen to be increased at 14days after transection (with an average ∼1.3-fold higher than in sham-operated group) but did not reach a significant level until at 21days (∼1.4-fold). The increase sustained thereafter and a plateau level was reached at 45days (∼1.7-fold higher), a value similar as that at 60days. When 5-HT2CR-IR analysis was confined to the ventral horn motoneuron somata (including a proportion of proximal dendrites) a significant increase was not detected until 45days post-operation. 5-HT2CR upregulation in the spinal gray matter is confirmed with Western blot in the rats 60days post-operation. The time course of 5-HT2CR upregulation in the spinal gray matter and motoneurons was positively correlated with the development of tail spasticity (clinical scores). This indicates that 5-HT2CR is probably an important factor underlying this pathophysiological development by increasing the excitability of both motoneurons and interneurons.

5-Hydroxytryptamine 5HT2C receptors form a protein complex with N-methyl-D-aspartate GluN2A subunits and activate phosphorylation of Src protein to modulate motoneuronal depolarization

N-Methyl-D-aspartate (NMDA)-gated ion channels are known to play a critical role in motoneuron depolarization, but the molecular mechanisms modulating NMDA activation in the spinal cord are not well understood. This study demonstrates that activated 5HT2C receptors enhance NMDA depolarizations recorded electrophysiologically from motoneurons. Pharmacological studies indicate involvement of Src tyrosine kinase mediates 5HT2C facilitation of NMDA. RT-PCR analysis revealed edited forms of 5HT2C were present in mammalian spinal cord, indicating the availability of G-protein-independent isoforms. Spinal cord neurons treated with the 5HT2C agonist MK 212 showed increased Src(Tyr-416) phosphorylation in a dose-dependent manner thus verifying that Src is activated after treatment. In addition, 5HT2C antagonists and tyrosine kinase inhibitors blocked 5HT2C-mediated Src(Tyr-416) phosphorylation and also enhanced NMDA-induced motoneuron depolarization. Co-immunoprecipitation of synaptosomal fractions showed that GluN2A, 5HT2C receptors, and Src tyrosine kinase form protein associations in synaptosomes. Moreover, immunohistochemical analysis demonstrated GluN2A and 5HT2C receptors co-localize on the processes of spinal neurons. These findings reveal that a distinct multiprotein complex links 5-hydroxytryptamine-activated intracellular signaling events with NMDA-mediated functional activity.

Neural control of the female urethral and anal rhabdosphincters and pelvic floor muscles

The urethral rhabdosphincter and pelvic floor muscles are important in maintenance of urinary continence and in preventing descent of pelvic organs [i.e., pelvic organ prolapse (POP)]. Despite its clinical importance and complexity, a comprehensive review of neural control of the rhabdosphincter and pelvic floor muscles is lacking. The present review places historical and recent basic science findings on neural control into the context of functional anatomy of the pelvic muscles and their coordination with visceral function and correlates basic science findings with clinical findings when possible. This review briefly describes the striated muscles of the pelvis and then provides details on the peripheral innervation and, in particular, the contributions of the pudendal and levator ani nerves to the function of the various pelvic muscles. The locations and unique phenotypic characteristics of rhabdosphincter motor neurons located in Onuf's nucleus, and levator ani motor neurons located diffusely in the sacral ventral horn, are provided along with the locations and phenotypes of primary afferent neurons that convey sensory information from these muscles. Spinal and supraspinal pathways mediating excitatory and inhibitory inputs to the motor neurons are described; the relative contributions of the nerves to urethral function and their involvement in POP and incontinence are discussed. Finally, a detailed summary of the neurochemical anatomy of Onuf's nucleus and the pharmacological control of the rhabdosphincter are provided.

Central representation of the inferior rectal nerve of the rat

PURPOSE

Obstetric injury to the pudendal nerve contributes significantly to fecal incontinence. The inferior rectal nerve, a terminal branch of the motor division of the pudendal nerve, innervates the external anal sphincter. Animal models have been developed to establish the scientific basis of sacral neuromodulation. The aims of this study were to determine the spinal location of inferior rectal nerve motoneurons projecting to the external anal sphincter and whether the inferior rectal nerve carries sensory fibers.

METHODS

Ten female virgin Wistar rats were used; 7 underwent bilateral inferior rectal nerve section and application of the neuronal tracer fluorogold. Five days later dorsal root ganglia L5 to S2 and the lumbosacral spinal cord were harvested and stained for activating transcription factor 3, a molecular marker of nerve injury. Three animals were used to confirm the specificity of activating transcription factor 3 nuclear labeling as a marker of axotomy.

RESULTS

Fluorogold-labeled motoneurons were found exclusively at L6 in the dorsomedial sections of Onuf's nuclei (left and right), which contained 30 +/- 9 motoneurons with a median diameter of 28.3 microm (24.4-31.0) (25th-75th centiles). Double-labeled sensory neurons (fluorogold-labeled cytoplasm and activating transcription factor 3-labeled nuclei) were found in dorsal root ganglia L6 to S2 and were smaller in diameter (20.5 microm (17.8-26.7); median (25th-75th centiles)) than motoneurons (P < .0,001).

CONCLUSIONS

The external anal sphincter receives both motor and sensory innervation from the inferior rectal nerve. Activating transcription factor 3 nuclear staining may prove useful for quantifying the degree of nerve injury in animal models of fecal incontinence.