Induction of the intermediate lobe pro-opiomelanocortin system with chronic swim stress and beta-adrenergic modulation of this induction.

Swimming at 25-30 degrees C for 30 min stimulates release of beta-endorphin from both the anterior and intermediate lobe of the pituitary in rats. Measurement of N-acetyl beta-endorphin-immunoreactivity (IR), which is specific for intermediate lobe secretion, indicates a 2- to 3-fold increase in N-acetyl beta-endorphin IR in plasma following this challenge. When swim is repeated on a daily basis, there is an increase in the amount of N-acetyl beta-endorphin IR released with repeated swim over time. As well as increased response to the swim challenge, these animals demonstrate an increase in the resting plasma levels of N-acetyl beta-endorphin IR and an increase in the intermediate lobe content of N-acetyl beta-endorphin IR. Molecular sieving of plasma from rats which were swum repeatedly demonstrates that this N-acetyl beta-endorphin IR consists of both larger molecular weight N-acetyl beta-endorphin IR, e.g. N-acetyl beta-endorphin1-31 and C-terminally shortened forms, e.g. N-acetyl beta-endorphin1-27. Administration of propranolol (3 mg/kg), a beta-adrenergic antagonist, 30 min before the onset of swim is able to block the intermediate lobe release of N-acetyl beta-endorphin IR with acute swim challenge. However, repeated administration of propranolol in conjunction with repeated swim is not able to block the swim stress-induced increase in plasma N-acetyl beta-endorphin IR or the increase in N-acetyl beta-endorphin IR content of the intermediate lobe. This is not due to decreased sensitivity to propranolol with repeated administration since in rats given chronic propranolol treatment an acute dose of propranolol is still able to block swim stress-induced release of N-acetyl beta-endorphin IR. Similarly, it is not due to a decreased efficacy of this dose of propranolol in rats which were swum chronically.