Postweaning Development of Negative Feedback in the Pituitary-Adrenal System of the Rat

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Abstract
In response to ether or electric shock, plasma corticosterone concentrations in weanling and adult rats rose to equivalent levels by 15 min, but then diverged, with the weanlings showing a later peak and a slower return to resting levels. An interpretation of this effect, in terms of an immature feedback mechanism in weanling rats, was supported by an experiment in which pretreatment with peripherally injected dexamethasone completely blocked a plasma corticosterone increase, in response to an ether and blood sampling stress in adults, but only partially reduced this response in weanlings. The failure of basal hypothalamic implants of corticoids to differentiate between ages in their ability to inhibit this stress response supported the suggestion of extra-hypothalamic inhibitory systems as the loci of the deficit in weanlings.

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