Cholecystokinin levels in the rat brain during the estrous cycle

Cholecystokinin (CCK) is widely distributed in the brain, and its expression has been shown to be regulated by estrogen. In the present study we used radioimmunoassay to monitor CCK levels in rat brain during a normal estrous cycle. Compared to di-estrous and estrous, CCK-like immunoreactivity was significantly reduced in cingulate and frontal cortex, hippocampus, striatum and hypothalamus during pro-estrous, that is the phase with the highest plasma estradiol levels. These results provide further evidence that circulating steroid hormones in the female rat can influence expression of a brain peptide, in this case CCK, and primarily in the limbic system, which is interesting in the context that CCK has been associated with anxiety and depression in both animals and humans.