Unsignaled morphine delivery does not disrupt the development of associative morphine tolerance in the rat

When morphine administration is paired with a distinctive context, tolerance to morphine's analgesic effects comes readily under the associative control of the drug-paired context. These associative tolerance effects are eliminated when a relatively short (i.e., 6 h) interdose interval (IDI) is used for conditioning. Contemporary models of learned tolerance explain the absence of learning at short IDIs by positing that residual morphine effects from a recent drug exposure disrupt the formation of drug-context associations. The present studies examined the impact of unsignaled morphine injections given 6 h prior to drug-context pairings on the development of associative tolerance. Analgesia was measured by the tail-flick method, and tolerance levels were assessed by dose-response curve methodology. Morphine preexposure had no detectable influence on the acquisition of associative tolerance when rats were tested immediately after conditioning, after a 30-day rest interval, or after a 30-day period of daily saline injections in their home-cage environment. These data suggest disruption of associative tolerance effects at short IDIs is not attributable to residual effects of morphine from the immediately preceding trial.