Figure 4.

Effects of morphine on thermal sensitivity. Morphine resulted in a decreased aversion toward the hot side (left and right panels) in a dose-dependent manner, indicated by an increase in time spent on the hot side during these testing sessions relative to saline administration. There was no effect of morphine on thermal sensitivity when assessed in the cold vs neutral condition (center panel). There was an overall age effect in the “hot vs neutral” and “cold vs neutral” testing conditions (left two panels), with aged rats spending more time on the “neutral” side compared with adult rats. Brackets in the left and center panels indicate a significant age difference; * indicates a statistically significant difference from saline; and + indicates a statistically significant difference from 0.56 mg/kg morphine.