NOS mRNA in the paraventricular nucleus of young and old rats after immobilization stress.

Nitric oxide synthesizing (NOS) enzyme has been localized in distinct neural populations, including the hypothalamic paraventricular nucleus (PVN). Using in situ hybridization, we have investigated possible changes in NOS mRNA in the PVN of young and old rats after 2 h of immobilization stress. In old rats a 3-fold increase in NOS mRNA was observed in the PVN immediately after the stressor, whereas in young rats the increase reached significance when animals were killed 3 h after the end of the stress procedure. The present results suggest that nitric oxide in the PVN may be involved in the stress response. The differences observed in NOS mRNA between young and old rats could be related to an age-dependent different responsiveness to stressful stimuli.