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We have studied the effects of long-term social isolation of male Wistar rats, after early weaning (16 days), on the activity of the hypothalamo-pituitary-adrenal (HPA) axis. In addition to studying basal HPA activity, the response of the HPA axis to 15 min of immobilization stress was examined. Plasma corticosterone concentrations were measured, and the relative weights of adrenal glands, thymus, and testes were obtained, the latter to check whether gonadal function was affected by the isolation paradigm. Moreover, we carried out a quantitative immunohistochemical study of pituitary ACTH and its hypothalamic secretagogues: CRF, arginine vasopressin (AVP), and oxytocin (OT), both at the level of the synthesizing cell bodies in the hypothalamic paraventricular nucleus and of the releasing fibers in the median eminence (ME). Body weight and daily consumption of food and water were not altered, but social isolation caused a reduction in plasma corticosterone levels, both under basal and stress-stimulated conditions; this was correlated with an increased thymus weight, without affecting adrenal or testicular weights. The immunohistochemical study revealed that isolation caused a smaller increase in the number of ACTH-immunoreactive cells in the pars distalis of the anterior pituitary after exposure to restraint stress, as compared with control animals. This result indicates that fewer corticotrophs were activated by restraint stress in isolated animals, such cells being smaller and exhibiting a smaller ACTH-immunoreactive area than in control animals. Isolated animals also showed an increase in the content of CRF-ir fibers in the ME and a smaller decrease in the neuropeptide immunoreactivity after stress than that observed in control animals. This result could indicate a reduced release of CRF into the portal vasculature in response to acute stress and may partially explain the reduced activation of corticotrophs observed in the pituitary of isolated animals. However, no changes were found in the content of CRF, AVP, or OT within the paraventricular nucleus, nor of the AVP or OT content in the ME. The results of this study show that long-term social isolation after early weaning caused a hypofunction of the HPA axis in the adult rat. This hypofunction was particularly evident after exposure to an acute stressor, suggesting a desensitization of this axis to stressful stimuli.

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