Neuronal nitric oxide synthase immunoreactivity (NOS1-ir) in sacral motor neurons of normal adult cats was compared with that in cats surviving 1–10 wk after unilateral transection and ligation of the pudendal nerve. Levels of immunostaining were measured by microdensitometry. In nonoperated cats 60% of motor neurons in the ventrolateral nucleus (VL) and Onuf's nucleus (ON) showed high levels of NOS1-ir with lower NOS1-ir in 40%. Following axotomy, motor neurons in ON on both sides of the cord showed an acute rise in mean level of NOS1-ir at 1 wk, with a further increase at 2 wk. Mean levels of NOS1-ir in the ipsilateral and contralateral ON remained elevated at 10 wk after axotomy. Elevation of NOS1-ir occurred in the VL with a similar time-course to that in ON, implying a wider response in motor nuclei synaptically coupled to ON. Measurements of neuronal size in ON and VL revealed an increase in neuronal size in ON but not VL, indicating increased NOS1-ir in ON was not an artifact of neuronal atrophy. The proportion of motor neurons in ON and VL possessing higher levels of NOS1-ir increased from 60% in controls to 100% at 2–3 wk postaxotomy. The proportion slightly declined by 8 wk due to re-emergence of motor neurons exhibiting low NOS1-ir, but remained greater than normal at 10 wk in both nuclei. Based on evidence from related analyses of synaptology, we argue that acute axotomy induced alterations in presynaptic complement which increased overall Ca2+ influx and thereby stimulated NOS1-ir.

Nitric oxide synthase immunoreactivity (bNOS-ir) was examined in the sacral spinal cord of the cat, macaque monkey and human using an antibody to the c-terminal region of neuronal NOS. In S2 of all 3 species NOS-ir was identified in both dorsal and ventral horns. In cat, monkey and human, bNOS-r occurred in sensory neurons of superficial laminae and the base of the dorsal horn, in autonomic neurons around the central canal and in the intermediolateral sacral spinal nucleus. In all 3 species, a large proportion of somatic motor nuclei in the ventromedial (VM), ventrolateral (VL) nuclei, and Onuf's nucleus (ON) showed high bNOS-ir, while others exhibited markedly lower immunoreactivity. Validatory experiments showed separate cellular localisation for bNOS, inducible NOS (iNOS), and endothelial NOS (eNOS) with only bNOS being localised to neuronal perikarya. Comparative morphometric analyses of the relative proportions and diameters of motor neurons in the VL, VM and ON exhibiting high and low levels of bNOS-ir revealed statistically significant differences in proportions in individual nuclei, and differences in size were generally not statistically significant. Finally, a comparison between cat sacral and thoracic spinal cord showed bNOS-ir in motor neurons of S2 was subject to less animal and rostrocaudal segment variation than in T10.