Failure to produce a non-opioid foot shock-induced antinociception in rats.

Brief continuous foot shock reportedly produces a naloxone-insensitive and thus non-opioid form of antinociception. In the present study, current intensity and duration of foot shock were varied: lower current intensities (0.5 or 1 mA) failed to produce a significant increase in tail flick (TF) latency, while current intensities of 3 mA and 6 mA applied for 2 or 3 min produced significant and long-lasting inhibition of the nociceptive TF reflex. Naloxone pretreatment attenuated significantly the antinociception developed at 3 mA but failed to affect that produced at 6 mA. It was noted, however, that higher current intensities damage the tail and the antinociceptive efficacy of footshock was reevaluated under conditions when the tail of the animal was not allowed to contact the electrified grid during foot shock. A significant short-lasting antinociception was produced only at the 6 mA current intensity. This antinociception could be attenuated by naloxone pretreatment, developed tolerance over time (8 days) and exhibited cross-tolerance with morphine, thus characterizing it as opioid in nature. These results raise the question to what extent damage to the tail contributes to the non-opioid foot shock-induced antinociception assessed using the nociceptive TF reflex.