Novel spinal pathways identified by neuronal c-Fos expression after urethrogenital reflex activation in female guinea pigs

• Pudendal sensory nerve-spinal pathways activated by the urethrogenital reflex were examined.
• c-Fos immunoreactivity in reflexly activated neurons in spinal cord was visualized and analyzed.
• The reflex caused sharp increase in c-Fos-expressing neurons in more spinal areas at S2 than at L3.
• L4 spinal transection caused further increase in these neurons in some observed areas at S2 and L3.
• There are ascending and descending spinal inhibitory circuits at sacral and lumbar spinal levels.

Pudendal nerve-spinal pathways are involved in urethrogenital sensation, pain and sexual activity. However, details of these pathways and their modulation are unclear. We examined spinal pathways activated by the urethrogenital reflex (UGR) and visualized by c-Fos immunoreactivity in reflexly activated neurons within spinal cord. In anesthetized female guinea pigs, a balloon was inserted into the urethra and inflated with short-repeat or long-continuous distension to activate the UGR. A second balloon recorded reflex contractions of the vagina and uterus. Two control groups had either no balloon or a vaginal balloon (VB) only. Ninety minutes after UGR activation, c-Fos immunoreactivity in L3 and S2 spinal segments was examined. Reflex activated c-Fos immunoreactivity also was investigated in some animals with acute spinal transections at either L4 or T12 levels. There was no significant difference in spinal c-Fos expression between the control groups. Short-repeat distension reliably induced a UGR and a two- to threefold increase in c-Fos-expressing neurons throughout dorsal, intermediate and lateral spinal gray matter at S2 and about twofold increase in superficial dorsal horn at L3. T12 transection had little effect on c-Fos expression at either spinal level. However, after L4 transection, UGR generation was associated with a four- to sixfold increase in c-Fos-expressing neurons in lateral horn (LH) and central canal areas at S2, and but only 20–30% increase at L3. Thus, UGR activates preganglionic neurons projecting to pelvic viscera in both sacral and lumbar spinal cord. The reflex also must activate ascending and descending spinal inhibitory circuits that suppress c-Fos-expression in neurons at both sacral and lumbar spinal levels.