Differential noxious and motor tolerance of chronic delta opioid receptor agonists in rodents

In the present study, we asked whether multiple intrathecal injections of deltorphin II, a selective delta opioid receptor (DOPR) agonist, induced DOPR tolerance in three behavioral assays. Unilateral inflammation caused by complete Freund's adjuvant (CFA) injection into the rat or mouse hind paw (CFA model) induced thermal hyperalgesic response that was transiently and dose-dependently reduced by intrathecal administration of deltorphin II or morphine. In both rodent species, the effect of deltorphin II was not modified by a single prior administration of deltorphin II, suggesting an absence of acute tolerance in this paradigm. Repeated administration of intrathecal deltorphin II or s.c. SB-235863 (five consecutive injections over 60 h) also failed to impair the antihyperalgesic response to delta opioid receptor agonist, whereas repeated intrathecal or s.c. injections of morphine induced a significant decrease in the subsequent thermal antihyperalgesic response to morphine. In mice, deltorphin II also induced a rapid, transient motor incoordination/ataxia-like behavior as tested with the accelerating rotarod. In contrast to the antihyperalgesic responses, tolerance to the motoric effect of deltorphin II was evident in mice previously exposed to multiple intrathecal agonist injections, but not multiple saline administrations. Using the tail flick antinociceptive test, we found that DOPR-mediated analgesia was significantly reduced by repeated exposure to deltorphin II. Altogether, these observations suggest that repeated injections of DOPR agonists induce differential tolerance effects on antihyperalgesic, antinociceptive, and motor incoordination/ataxia-like behaviors related to DOPR activation by deltorphin II.