Previous stress increases in vivo biogenic amine response to swim stress

In vivo microdialysis was used to determine biogenic amines in medial prefrontal cortex of rats exposed to eight minutes of swim stress on two consecutive days. On the first day of stress, norepinephrine (NE) efflux increased by 183% over baseline after stress, while dopamine (DA) and serotonin (5-HT) remained stable throughout. On the second day of stress, a robust increase was observed in all 3 neurotransmitters measured, with (NE), (DA), and (5-HT) increasing by 310%, 441% and 496% respectively, and remaining elevated for an hour or more after stress. This suggests that the first exposure to swim stress, while not causing dramatic changes in biogenic amine release, may sensitize biogenic amines in medial prefrontal cortex to subsequent swim stress. Our results also serve as preliminary data concerning the neurochemical changes which might underlie the forced swimming model of "behavioral despair".