In the present study we examined whether the descending facilitation from the rostral ventromedial medulla (RVM) is required for the enhancement of formalin-evoked nocifensive behavior following repeated forced swim stress. Rats were subjected to forced or sham swim stress for 3 days. Withdrawal latency to noxious thermal stimuli and mechanical withdrawal threshold to von Frey filaments did not change significantly in both groups at 24 h after the last stress session. The forced swim stress showed significantly enhanced nocifensive behavior to the subcutaneous administration of formalin at 2 days after the last stress session (1330.1 ± 62.8 s), compared to the sham swim (1076 ± 102.4 s, p < 0.05) and naive groups (825.9 ± 83.2 s, p < 0.01). The destruction of the RVM with ibotenic acid led to prevent the enhancement of formalin-evoked nocifensive behavior in the forced swim group. These findings suggest that the descending facilitation from the RVM may be involved in the enhancement of formalin-evoked nocifensive behavior following the forced swim stress.
- Descending system