Decreased voluntary activity and amygdala levels of serotonin and dopamine in ovariectomized rats

Estrogen is involved in numerous activities in the brain, such as learning, memory, fear, anxiety and mood. However, little is known about the pathways involved in the effects of estrogen in the brain. Therefore, to improve our understanding of the effects of estrogen, we examined the effects of ovariectomy (OVX)-a model of estrogen deficiency and menopause-on psychiatric functions, including voluntary activity. Female Wistar rats underwent OVX or sham operation. Voluntary momentum and circadian activity were monitored at 2 and 6 weeks, respectively. Rats also underwent microdialysis of the amygdala to determine serotonin (5-HT) and dopamine levels. Although the circadian rhythm was unchanged at 2 weeks, voluntary activity at 6 weeks was significantly lower in OVX rats than that in sham rats. This was due to significantly reduced voluntary activity in the 12-h dark phase, while no significant difference was detected in the 12-h light phase. Both 5-HT and dopamine levels in the amygdala were significantly lower in OVX rats than those in sham rats at 6 weeks after the procedure. In conclusion, these results indicate that estrogen is an important mediator of voluntary activity in rats, particularly during the dark phase. These effects of estrogen appear to involve reduced 5-HT and dopamine release in the amygdala. Further studies are needed to determine whether estrogen (and its deficiency) influences the synthesis, vesicular packaging, release, re-uptake and degradation of these transmitters.