Immobilization stress was adopted as a prototype stress model for studying
the effects of stress on dopaminergic and noradrenergic neuronal activity in the rat
hypothalamus. Norepinephrine content was significantly increased after 4 hours of
immobilization stress. Also, the turnover rates of both dopamine and norepinephrine
were found to be significantly increased after the stress, providing the evidence of
increased synthesis rates of both neurotransmitters. These findings were consistent with
the increase in plasma corticosterone and catecholamine levels. The activities of tyrosine
hydroxylase and dopamine-j1-hydroxylase, the synthesizing enzymes for catecholamines,
were significantly increased after the stress, while that of monoamine oxidase,
the catabolizing enzyme, did not change to a significant degree at all. Kinetic
analysis of tyrosine hydroxylase, the rate-limiting step in catecholamine biosynthesis,
revealed that Vmax was significantly increased after the stress without significant
change of Km. These findings suggest that dopamine and norepinephrine may playa
significant role in mediating stress responses by increasing their neuronal activities
