Effect of the entorhinal cortex on diurnal ACTH and corticosterone release in rats

In our previous study, a lesion in the entorhinal cortex was found to significantly attenuate the elevation of adrenocorticotropic hormone (ACTH) in plasma during immobilization stress. The aim of the present study was to investigate whether the entorhinal cortex exerts a modulatory effect on circadian ACTH and corticosterone release.

Ibotenic acid (15 microg/microl) was stereotaxically bilaterally injected into the entorhinal cortex of rats. Two weeks after the injections, ACTH and corticosterone levels in plasma were measured at 0800 h, 1300 h and 1800 h.

Compared with sham-operated control rats, rats with entorhinal cortex lesions produced by ibotenic acid showed either significantly elevated plasma ACTH or plasma corticosterone levels at 0800 h, but no difference at 1300 h or 1800 h.

The results of the present study indicate that the entorhinal cortex plays a certain role in the regulation by the central nervous system of the circadian rhythm of the hypothalamic-pituitary-adrenal (HPA) axis.