The involvement of beta-endorphin in swim-induced antinociception in female mice.

A 30 sec swim in water at 30 degrees C reduced the writhing response produced in the female mice by i.p. acetic acid. Peripherally administered, naloxone and beta-endorphin1-27 antagonized this swim-induced antinociception. However, i.c.v. administration of these compounds had minimal effects on this phenomenon. beta-Endorphin1-27 was effective in antagonizing the antinociceptive effect of beta-endorphin, but not that of morphine. Furthermore, the doses of naloxone required to antagonize the swim-induced antinociception were similar to those required to antagonize beta-endorphin which produced the same degree of antinociceptive response, but were much higher than those required to block the effect of morphine. These results suggest the involvement of beta-endorphin in swim-induced antinociception in female mice and its interaction with some peripheral opioid receptor(s) other than the mu-receptor.