Imipramine Ameliorates Pain-related Negative Emotion via Induction of Brain-derived Neurotrophic Factor

Depression-like behavior is often complicated by chronic pain. Antidepressants including imipramine (IMI) are widely used to treat chronic pain, but the mechanisms are not fully understood. Brain-derived neurotrophic factor (BDNF) is a neuromodulator that reduces depression by regulating synaptic transmission. We aimed to characterize the antidepressant effects of IMI without analgesia based on BDNF (trkB)-mediated signaling and gene expression in chronic pain. A chronic constriction injury (CCI) model was constructed in Sprague-Dawley (SD) rats. IMI (5 mg/kg, i.p.) was administered from day 10 after CCI. The pain response was assessed using the paw withdrawal latency (PWL) and depression was judged from the immobility time in a forced swim test. Anti-BDNF antibody, K252a, or 5,7-dihydroxytryptamine (5,7-DHT) were used to examine the antidepressant effects of imipramine. Changes in pERK1/2 (immunohistochemistry), 5-HT and BDNF (ELISA), and BDNF mRNA (RTPCR) were measured in the anterior cingulate cortex (ACC), rostral ventromedial medulla (RVM), and spinal cord. After CCI, rats showed decreased PWL and increased immobility time. A low dose of IMI reduced the immobility time without having analgesic effects. This antidepressant effect was reversed by anti-BDNF antibody, K252a, and 5,7-DHT. IMI reduced excessive activation of pERK1/2 associated with decreased pCREB and BDNF mRNA, and these changes were reversed by 5,7-DHT. These results show that IMI reduces pain-related negative emotion without influencing pain and that this effect is diminished by denervation of 5-HT neurons and by anti-BDNF treatment. IMI also normalizes derangement of ERK/CREB coupling, which leads to induction of BDNF. This suggests a possible interaction between 5-HT and BDNF.