To explore the changes of depressive-like behavior, pain threshold and the expression of 5-HT and 5-HT2A receptor mRNA and BDNF mRNA in hippocampus of rats with inflammatory pain and the intervention of duloxetine, and study the relationship between nerve material change in hippocampus and inflammatory pain.

Fifty-four adult male SD rats were divided into A, B and C group by random number table, 18 rats in each group.Processing of each groups: group A (plantar injection of saline solution), B group (inflammatory pain model), C group(inflammatory pain model+ duloxetine by intragastric administration). The inflammatory pain models were established by subcutaneously injecting mixture of CFA and saline into the left plantar of the rats.Duloxetine [10 mg/(kg·d)] was delivered to rats by intragastric administration for 2 weeks.The alterations of 50% paw withdrawal threshold (PWT) were measured before and 2 weeks after CFA injection with Von Frey Fibres.The depression-like behaviors tested with sucrose preference test.While the levels of 5-HT2A receptor mRNA and BDNF mRNA in the hippocampus were detected by RT-PCR and the expression of 5-HT in the hippocampus was detected by ELISA.

Before treatment, the 50% PWT of the three groups were [(52.60±1.60)g], [(52.93±1.95)g], [(51.47±2.59)g], the difference had no statistical significance (F=0.81, P=0.46). Two weeks later, 50%PWT [(41.97±3.88)g] and sugar consumption rate [(80.12±2.91)%] of the group B lower than those of the group A [(49.30±3.20)g, (87.33±5.22)%], the differences had statistical significance (LSD-t=2.46, 3.21; P=0.03, 0.01). Compared with the group B, the 50%PWT [(49.83±7.39)g] and sugar consumption rate [(85.52±3.13)%] in the group C were higher [LSD-t=2.64, 2.40; P=0.02, 0.03]. Compared with A group, 5-HT of B group [(1.69±0.26)ng/mg; (2.27±0.30)ng/mg] in hippocampus was decreased (LSD-t=2.38, P=0.03), the expressions of 5-HT2A receptor mRNA of the two groups (0.62±0.28; 1.20±0.30) in the hippocampus had no statistical significance (LSD-t=1.57, P=0.14), the expression of BDNF mRNA of group B (0.68±0.29; 1.58±0.60) was lower (LSD-t=2.46, P=0.03). Compared with group B, the 5-HT of group C [(2.82±0.62)ng/mg] in hippocampus was increased (LSD-t=4.61, P=0.00), the expressions of 5-HT2A receptor mRNA (1.70±1.04) and BDNF mRNA (1.75±0.88) of group C in hippocampus were higher (LSD-t=2.91, 2.91; P=0.01, 0.01).

The pain threshold decrease and depression-like behavior appears, and the expression of 5-HT, 5-HT2A receptor mRNA and BDNF mRNA in hippocampus decrease in the rat with chronic inflammatory pain.Duloxetine can improve pain threshold and depression-like behavior of the rat with chronic inflammatory pain by increasing the expression of 5-HT, 5-HT2A receptor mRNA and the BDNF mRNA in hippocampus.