Difference in escaping electric footshock by genetic mouse lines selectively bred for divergent levels of swim-induced analgesia.

Inbred mouse lines selectively bred for divergent levels of swim induced analgesia were differed in the ability to escape electric footshock. Mice displaying a high analgesia on a hot plate, after swimming for 3 min at 20 degrees centigrade terminated electric current applied at ascending intensity to the grid floor at a higher value compared to the low analgesia line. The difference was particularly pronounced after swim stress, when mice of the former line manifested a serious escape deficit by failing to terminate electric current elikiting apparently aversive phenomena, such as vocalization, runing and jumping. This escape deficit was reserved by naloxone, an opioid antagonist. Results are interpreted in terms of an assumed amnesic effect of endogenous peptides released under the conditions of swim stress. Such interpretation is justified by the data indicating a greater opioid involvement in the swim analgesia in the high analgesia line, compared to low analgesia mice, in which non-opioid mechanisms prevail.