Dynorphin neurotoxicity induced nitric oxide synthase expression in ventral horn cells of rat spinal cord

Nitric oxide (NO) mediation in the spinal cord injury induced by intrathecal (i.t.) dynorphin (Dyn) administration was studied with NADPH-diaphorase (Nd) histochemistry. Normally, there was rarely NO synthase (NOS) activity in spinal cord motoneurons, and Dyn A(1-17) 10 nmol, which produced only transient paralysis, did not induce Nd/NOS expression in ventral horn cells. After a paralyzing dose of i.t. Dyn A(1-17) 20 nmol, which definitely produced permanent paraplegia and neuronal death, Nd/NOS began to express in motoneurons at 30 min, increased in numbers and intensities at 2-4 h and persisted up to 8 h. Most of Nd/NOS motoneurons disappeared at 24 h coincident with the neuronal death. Quite a few intensively-stained Nd-positive small cells and swollen varicosities became visible only in rats with permanent paraplegia and neuronal death, beginning at 2 h, maximizing at 3-4 h and remaining up to 24 h. These results suggest that NOS expression was induced in the ventral horn of spinal cord, including small cells and varicosities as well as motoneurons closely correlated in time and degree with pathological changes in motoneurons caused by spinal Dyn neurotoxicity.