Heterogeneity of neuroendocrine stress responses in aging rat strains.

Hyperactivity of the hypothalamo-pituitary-adrenocortical (HPA) axis is linked with age-related decrements in cognition and neuronal survival. However, the nature and extent of age-related HPA axis deficits vary considerably across and indeed, within strains. The current study was designed to assess variance in HPA axis function using two rodent models commonly used in aging studies: Fischer 344 (F344) and F344/Brown-Norway F1 hybrid rats (F344/BN). We examined both basal and stress-induced ACTH and corticosterone (CORT) release in two stress contexts thought to differ in intensity: novel environment ('mild') and restraint ('intense'). Variability of the data was tested with a modification of the Brown-Forsythe test of homoscedasticity. The results indicated that F344 rats exhibit greater peak HPA responses. Furthermore, in most cases variability was increased in aged rats relative to young and middle-aged rats of the same strain, indicative of the emergence of individual differences in stress responsivity amongst older rats. The results suggest that these older rat strains may be useful models to further assess individual differences in neuroendocrine aging.