Studies on neuroendocrinological mechanisms of memory in the Hatano rats

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Hatano high avoidance (HAA) and low avoidance (LAA) strains rats have been originally selected and bred from Sprague-Dawley rats for their shuttle-box task. The present study focused on the activity of the hypothalamo-pituitary-adrenocortical axis (HPA) of HAA and LAA rats in response to restraint stress. The restraint stress induced an elevation of plasma concentrations of ACTH, prolactin, corticosterone and progesterone, and in adrenal concentrations of coricosterone and progesterone. Peak levels of plasma ACTH during stress condition were significantly higher in HAA rats than LAA rats, while peak levels of prolactin were significantly lower in HAA rats than LAA rats. ACTH and prolactin syntheses in anterior pituitary glands were significantly higher in HAA rat than LAA rats during stress. The plasma concectrations of corticosterone, as shown bv percentage of initial values, during stress were significantly higher in HAA rats than LAA rats. Adrenal concentrations of corticosterone and progesterone increased during stress in both strains, although no significant strain deference was observed between HAA and LAA. These results indicate that the activities of HPA axis to acute restraint stress are higher in HAA rats than LAA rats, whereas prolactin secretory activity is vice versa. These endocrine differences in response to stress may be involved in regulation of memory.