Environmental modulation of behavioral tolerance in spinal rats

We previously reported that the antinociceptive effect of morphine on the thermal tail withdrawal reflex (tail flick, TF) was reduced in rats that had practiced the TF response prior to morphine administration. This phenomenon, termed behavioral tolerance, was observed even when rats were spinally transected following the TF tests. The present experiments were conducted to determine whether such retention of behavioral tolerance after spinal transection was dependent on prior experience with the TF procedure, or whether other nociceptive pretreatments would also reduce opiate antinociception on the TF test in spinal rats. Separate groups of rats were pretreated with either nociceptive thermal TF or Hot Plate (HP), or mechanical Tail Pinch (TP) stimuli, as well as non-nociceptive exposure to the experimental context )Habituation) or No Pretreatment. Approximately half of the rats were spinally transected after their respective pretreatment, while the other half were left intact. When Intact rats were subsequently tested on the TF at 30, 60, 120 and 180 min after morphine administration, only the TF pretreated group was tolerant, relative to the Non-pretreated control group. However, this was a transient effect, and was only significant at the first, 30-min test; there was no difference among the groups in the total duration of antinociception across all the time points. When spinal rats were tested after morphine administration, there was also an overall significant difference among the groups at 30 min. In this case, the differences were maintained across all time points.(ABSTRACT TRUNCATED AT 250 WORDS)