The involvement of endogenous opiate systems in learned helplessness and stress-induced analgesia.

The participation of endogenous opiate systems in the induction and expression of learned helplessness (LH) and stress-induced analgesia (SIA) was investigated in rats exposed to escapable and inescapable footshock. Following an initial footshock, analgesia was observed only in those animals that could not control their stress exposure and this SIA was prevented by the administration of naloxone. Analgesia was no longer evident in this inescapable group after 48 h. However, exposure to a shuttlebox escape task at this time reinstated the SIA but did not produce SIA in animals previously exposed to escapable footshock. Shuttlebox escape deficits indicative of LH were also exhibited by animals that received an inescapable footshock stress 48 h prior to testing. The analgesia and LH observed in the inescapable group were not affected by the administration of naloxone (3 mg/kg, IP) 10 min before shuttlebox exposure but were prevented when the same dose of naloxone was given before the initial stress. Thus, endogenous opiates clearly participate in the initial induction of LH and SIA and, although the degree of endogenous opiate involvement in the subsequent expression of these behaviors is unclear, it seems evident that their expression can occur in the presence of opiate antagonism and may therefore require the participation of additional transmitter systems.