Differential effects of periodic maternal separation on adult stress coping in a rat model of extremes in trait anxiety.

We studied interactions of genetic and environmental factors shaping adult emotionality and stress coping, and tested the hypothesis that repeated periodic maternal deprivation (PMD) exerts differential effects on adult behavioral and neuroendocrine stress responsiveness in dependence on the genetic predisposition to either hyper- or hypo-anxiety. Exposure of male Wistar rats bidirectionally bred for either high (HAB) or low (LAB) anxiety-related behavior to PMD between postnatal days 2 and 15 resulted in a behavioral approximation of the selected lines. This was reflected by test-dependent signs of reduced anxiety-related behavior in adult HAB rats and of enhanced levels of anxiety in LAB rats compared with their corresponding unstressed controls. In addition to behavioral parameters, differential effects of PMD were also seen with respect to the responsiveness of the hypothalamo-pituitary-adrenocortical axis to acute stressor exposure (novel environment) in adulthood. The corticotrophin (ACTH) and corticosterone hyper-responses seen in control rats of the HAB line compared with those of the LAB line became attenuated in PMD-HAB rats, whereas PMD did not significantly alter neuroendocrine responses in LAB rats. Thus, as a result of PMD, both ACTH and corticosterone responses became indistinguishable between HAB and LAB rats. Although HAB dams spent more time on the nest with the litter compared with LAB dams during the first 5 days postpartum, licking and grooming behavior did not differ between the lines prior to separation, and was found to be increased to the same extent in both HAB and LAB dams during the first hour immediately after reunion with the pups. In contrast to early life stress, exposure of adult HAB and LAB rats to a 10-day unpredictable stress schedule failed to alter their emotional measures. The mitigating effect of PMD on both behavioral and neuroendocrine parameters in rats representing extremes in trait anxiety might reflect an evolutionary benefit as the genetic variability among individuals of a species is sustained while allowing adequate responses to potentially dangerous stimuli in adulthood dependent on early life conditions.