Improved antioxidant defense in the ventral tegmental area increases pain tolerance in male rats.

The ventral tegmental area (VTA) is a brain region recently shown to interfere with pain perception. According to previous studies, improvement of antioxidant defense mechanisms reduces pain. The aim of this study was to show that lack of social interaction influences oxidative stress in the VTA and that this results in increased pain tolerance. In this study, 21 male Sprague-Dawley rats were randomly divided into 2 groups: paired and socially isolated (SI). After one week of acclimatization, the SI rats were isolated for 14 days and the other rats were paired for the same period. On day 15, pain tolerance was assessed through the tail flick test, and two days later the rats were investigated for anxiety in an elevated plus maze (EPM). Indices of oxidative stress (levels of malondialdehyde, glutathione, nitrite/nitrate and catalase activity in the VTA) were then studied in the animals. Malondialdehyde in the VTA was found to be reduced in the SI rats compared with the paired rats. Furthermore, catalase activity, glutathione and nitrite/nitrate levels in the VTA were increased in SI compared with paired rats. Pain tolerance on the tail flick test was increased in SI rats compared with paired rats, while anxiety, as assessed by EPM, was reduced in the SI rats compared with the paired rats. Improvement of antioxidant defense and reduction of oxidative stress in the VTA increases pain tolerance and reduces anxiety. In this sense, environmental conditions play an important role in pain control.