Shock controllability and opioid substrates of escape performance and nociception: differential effects of peripherally and centrally acting naltrexone. 
Rats exposed to inescapable shock exhibited analgesia and a significant impairment of shock-escape learning in a shuttle box situation 24 hr later.  In contrast, rats exposed to escapable shock or to no shock displayed neither effect.  Naltrexone (10 mg/kg) significantly reduced the analgesia and completely eliminated the escape deficit in inescapably shocked rats but induced hyperalgesia, coupled with a marked deterioration of escape performance, in escapably shocked and nonshocked rats.  The same dose of quaternary naltrexone, which has low ability to cross the blood-brain barrier, had no effect on either the antinociception or the escape deficit produced by inescapable shock, although it also induced escape impairment and hyperalgesia in rats preexposed to escapable shock or to no shock.  A second experiment demonstrated that both the escape interference and the antinociceptive consequences of prior inescapable shock could be reduced partially by a much lower dose (1 mg/kg) of naltrexone but 50 times this amount of quaternary naltrexone was still without effect.  These results imply that the consequences of exposure to inescapable shock are mediated by activation of central opioid processes whereas naltrexone-induced effects in escapably shocked and nonshocked animals may be peripherally mediated.  The relevance of these findings to the possible role of nociception in escape performance is discussed.