Time-dependent alterations in NOS1 immunoreactivity in feline pudendal motoneurons following retrograde axonal transport of diphtheria toxin

Neuronal nitric oxide synthase immunoreactivity (NOS1-ir) in sacral somatic motor neurons of normal adult cats was compared with NOS1-ir in cats survi ving 1 to 10 weeks after injection of the ADP-ribosylating protein diphther ia toxin (DTX) into one-half of the external anal sphincter. Levels of immu nostaining were measured by microdensitometry. In non-operated cats, 60% of motor neurons in the ventrolateral (VL) and Onuf's nucleus (ON) showed hig h levels of NOS1-ir with lower NOS1-ir in 40%. Intramuscular injection of D TX caused cytopathology in motoneurons in ON, but not in VL with onset at 1 week, and regression by 10 weeks. Immunocytochemistry and microdensitometr y disclosed an associated rise in levels of NOS1-ir in both the ipsilateral and contralateral ON at 1 week, which persisted up to 4 weeks, but reduced to normality by 10 weeks. Simultaneous neuronal swelling in ON precluded r aised staining intensity being an artifact of neuronal atrophy. Despite res triction of cytopathology to ON, motoneurons in VL also exhibited acute ele vation with subsequent normalisation of NOS1-ir over an identical time-cour se. Conclusions. Since DTX inhibits protein synthesis, (i) activation of NO S 1 in acute toxicity probably reflects raised intracellular calcium due to loss of calcium homeostasis; (ii) the bilateral response in ON may indicat e uptake of DTX by contralateral pudendal axons crossing the sphincter midl ine; and (iii) raised NOS1-ir in VL indicates a wider response in nuclei sy naptically coupled to ON. Recovery of neuronal morphology and normalisation of NOS1-ir in sublethal toxicity contrast with the protracted elevation of NOS1-ir reported by others following axonal lesions associated with neuron al death and muscle target deprivation. (C) 1999 Elsevier Science B.V. All rights reserved.