Effects of ageing and dehydroepiandrosterone administration on pro-opiomelanocortin mRNA expression in the anterior and intermediate lobes of the rat pituitary.
Résumé
There is still controversy about the influence of ageing on the activity of the hypothalamo-pituitary-adrenocortical (HPA) axis in the rat. The first objective of the present study was to evaluate the influence of ageing on the activity of the HPA axis by measuring pituitary proopiomelanocortin (POMC, the precursor of ACTH and alphaH) mRNA levels in the anterior and intermediate lobes in young (50-55-day old) and aged (18-month-old) rats of both sexes. The second goal of the study was to evaluate the effect of 2.5 day administration of dehydroepiandrosterone (DHEA), a steroid precursor which has been shown to improve some ageing-associated deficits. In the young male anterior pituitary, DHEA induced a 17.5% increase in POMC mRNA levels. In aged males, anterior pituitary POMC mRNA levels were 22% lower than those detected in young animals. DHEA treatment produced a 26% increase, then completely restoring mRNA levels when compared to those found in young vehicle-treated males. In the young female, DHEA did not induce any changes in anterior pituitary POMC mRNA. In aged females a 24% reduction in the hybridization signal was observed. This reduction was completely reversed by DHEA which induced a 45% increase over the levels observed in vehicle-treated aged animals. In the intermediate lobe, the results were very similar to those obtained in the anterior lobe, although the observed effects induced by ageing and DHEA were less striking. These results together with previous ones indicating an age-related decrease in corticotropin-releasing hormone (CRH) neuronal activity suggest that ageing is associated with a decrease in HPA axis activity. They also demonstrate that a short-term DHEA treatment can exert a beneficial influence by reversing the decrease in pituitary POMC mRNA expression which occurs as a consequence of ageing.