REGULATION OF PITUITARY BETA-ENDORPHIN SECRETION IN AGING RATS - IN-VITRO RESPONSIVENESS TO DOPAMINE

The intermediate lobe of the mammalian pituitary is highly responsive to dopamine inhibition of beta-endorphin secretion. In this study, the ability of aged (12 months) intermediate lobes to respond to dopamine was compared to that of young (6 weeks) tissue, using a short-term in vitro incubation of isolated rat neurointermediate lobes, with measur ement of peptide release by radioimmunoassay. Tissue from the aged rat s released greater amounts of beta-endorphin peptide than amounts meas ured from young tissue at all time periods studied. The aged lobes wer e also found to be significantly more sensitive to dopamine than young glands, as measured by percent change of each group compared to respe ctive baseline release. In comparison, incubation of tissue from young animals in which the intermediate lobe had been acutely denervated by treating rats with injections of the catecholamine neurotoxin, B-hydr oxydopamine, did not differ in responsiveness to dopamine as compared to tissue from control rats. The observations suggest that aging inter mediate lobe, while being hypersecretory, is supersensitive to dopamin e, perhaps as the result of gradually reduced innervation.