Foot shock analgesia. Lack of correlation with pituitary and plasma immunoreactive-beta-endorphin

Effects of inescapable foot shock on pain threshold and on levels of immunoreactive beta-endorphin (ir-beta EP) in anterior pituitary, neuro-intermediate lobe and plasma were determined by hot plate test and radioimmunoassay respectively. Whereas handling and conditioned stress failed to alter baseline pain threshold, 20 min inescapable foot shock produced modest and transient analgesia. Levels of plasma ir-beta EP and corticosterone were significantly and concurrently raised by all treatments and remained elevated 40 min after foot shock. Changes in plasma levels of ir-beta EP and corticosterone were found to parallel one another but did not correlate with changes in pain threshold. AP content of ir-beta EP was significantly reduced 60 min after the cessation of inescapable foot shock, and N-IL content from 20 min after. The findings suggest that (i) plasma levels of ir-beta EP and corticosterone do not reflect changes in pain threshold, and (ii) ir-beta EP levels in anterior pituitary, neurointermediate lobe and plasma probably are not causally related to foot shock analgesia.