Ca2+-dependent and Ca2+-independent nitric oxide synthase (NOS) activity, and neuronal and inducible NOS immunoreactivity (nNOS-IR and iNOS-IR), were investigated in the rabbit lower lumbar spinal cord after i) sciatic nerve transection and survival of experimental animals for 2 weeks, ii) treatment of animals with N-nitro-L-arginine (NNLA), an inhibitor of nNOS dosed at 20 mg/b.w. for 12 days, and iii) after treatment of animals with the inducible NOS (iNOS) inhibitor, aminoguanidine, dosed at 100 mg/b.w. for 4 and 12 days. Our attention was focused on the dorsal part of L4-L6 segments receiving sensory inputs from the sciatic nerve, and on the ventral part consisting of sciatic nerve motor neurons. Sciatic nerve transection increased Ca2+-dependent NOS activity and the density of nNOS in the dorsal part of the spinal cord on the ipsilateral side. NNLA treatment effectively reduced nNOS-IR in both the dorsal horn and the dorsal column, and decreased Ca2+-dependent NOS activity in the lower lumbar segments. Immunocytochemical analysis disclosed the up-regulation of iNOS immunoreactive staining after peripheral axotomy in α-motoneurons. The changes in iNOS expression and Ca2+-independent NOS activity were not significantly corrected by aminoguanidine treatment for 4 days. Long-lasting iNOS inhibition decreased Ca2+-independent NOS activity, but caused motor neuron degeneration and mediated small necrotic foci in the ventrolateral portion of the ventral horn. The results of the present study provide evidence that constitutive NOS inhibition by NNLA is more effective than specific long-lasting inhibition of iNOS by aminoguanidine treatment.

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