BDNF contributes to the neonatal incision-induced facilitation of spinal long-term potentiation and the exacerbation of incisional pain in adult rats

Neonatal surgical injury exacerbates spinal microglial reactivity, modifies spinal synaptic function, leading to exaggerated pain hypersensitivity after adult repeated incision. Whether and how the alteration in microglial reactivity and synaptic plasticity are functionally related remain unclear. Previously, we and others have documented that spinal brain-derived neurotrophic factor (BDNF), secreted from microglia, contributes to long-term potentiation (LTP) in adult rodents with neuropathic pain. Here, we demonstrated that the mRNA and protein expression of spinal BDNF are significantly upregulated in adult rats subjected to neonatal incision and adult repeated incision (nIN-IN). Neonatal incision facilitates spinal LTP induced by BDNF or high frequency electrical stimulation after adult incision, including a decreased induction threshold and an increased magnitude of LTP. Coincidently, inhibition of spinal BDNF abrogates the LTP facilitation, alleviates the mechanical allodynia and thermal hyperalgesia in nIN-IN rats. By contrast, spinal application of exogenous BDNF in the adult rats with a single neonatal incision mimics the LTP facilitation and pain hypersensitivity, which have been found in nIN-IN rats. Exogenous BDNF-induced exacerbation of pain hypersensitivity could be blocked by BDNF inhibitor. In addition, blockade of microglial reactivity by intrathecal application of minocycline attenuates the elevation of BDNF and the LTP facilitation, and also, alleviates pain hypersensitivity in nIN-IN rats. In conclusion, spinal BDNF, at least partly derived from microglia, contributes to the neonatal incision-induced facilitation of spinal LTP and to the exacerbation of incisional pain in adult rats. Thus, spinal BDNF may combine the changes of microglial reactivity and synaptic plasticity in nIN-IN rats.