Developmental effects of in vivo and in vitro inhibition of nitric oxide synthase in neurons

The diffusible chemical messenger nitric oxide (NO) is involved in neuronal plasticity and it is, therefore, supposed to play a role in brain developm ent. A shortage of NO during the critical period of brain maturation may th eoretically have long-lasting consequences on the organization of the adult brain. We have performed in neonatal rats a chronic inhibition of the enzy me responsible for NO production, nitric oxide synthase (NOS), from postnat al day 3 to postnatal day 23, through administration of the competitive ant agonist N-nitro-L-arginine methylester (L-NAME). The calcium-dependent cata lytic activity resulted almost completely inhibited throughout the period o f treatment and it took more than 4 days after its suspension to get a full recovery. The expression of the neuronal isoform of the enzyme (nNOS), rev ealed by immunoblotting, was unchanged during the treatment and after it. T he histochemical reaction for NADPH diaphorase was reduced at the end of th e treatment and recovered in concomitance with the recovery of the catalyti c NOS activity. No gross structural alterations were detected in brain morp hology. The levels of three neurotransmitter-related and one astrocytic mar ker were unchanged in the cerebellum, hippocampus and cortex of 60-day-old rats which had been neonatally treated. A similar lack of significant effec ts on neurochemical brain maturation was also noticed in a parallel series of experiments, in which a short pulse of NOS inhibition was performed at a critical prenatal time of brain development, from gestational day 14 to ge stational day 19. In vitro, chronic exposure of cerebellar granule cells to L-NAME (500 mu M) resulted in slight decrease of surviving neurons after 8 days in culture and in better resistance to the challenge of stressful cul ture conditions. The present results suggest that the basic plan of brain o rganization can be achieved despite an almost complete NOS inhibition durin g the maturation period. In vitro, NOS inhibition may bring to more pronoun ced consequences on neuronal viability and function. (C) 1999 Published by Elsevier Science B.V. All rights reserved.