Regional redistribution of β-endorphin in the rat brain: The effect of stress

• 1.

  1. After an acute mild swim (20°, 3 min), levels of β-endorphin-like immunoreactivity decreased in the anterior pituitary to 48% of controls and in the intermediate-posterior pituitary to 62%. In the brain, of the 14 regions examined, four showed remarkable increases: central n. of amygdala to 167%, n. interstitialis striae terminalis to 174%, n. raphe dorsalis to 170%, and n. paraventricularis to 136%.

• 2.

  2. An acute severe swim (4°, 3 min) caused a decrease of the opioid to 66% and 93% of controls in the anterior and intermediate-posterior pituitary, respectively, and an increase in central n. of amygdala to 227%, in n. interstitialis striae terminalis to 173%, in n. raphe magnus to 148%, in n. paraventricularis to 155%, and in periaqueductal gray (PAG) to 193%.

• 3.

  3. Mild swimmings applied chronically for 21 days resulted in return of the opioid levels nearly to control values as habituation developed.

• 4.

  4. After chronic swimmings of gradually increased severity during the 21 day period (water temperature gradually decreased from 20° to 4° and the time of swimming gradually increased from 3 to 9 min), no habituation occurred but further increases in β-endorphin-like immunoreactivity were observed.

• 5.

  5. The present results show that stress of swimming affects pituitary and brain levels of β-endorphin and that in brain, the most affected are areas of midbrain and limbic structures. These effects are reversed by repeated exposure to mild swimming conditions, but not when the conditions of swimming gradually increase in severity.