ABSTRACT
Objective To investigate the changes in the expression of nerve growth factor (NGF) and brain-derived neurotrophic (BDNF) in the dorsal root ganglion (DRG) and spinal dorsal horn following peripheral inflammatory hyperalgesia induced by formalin injected into the plantar region of the right hindpaw in rats. Methods Twenty-four adult SD rats weighing 250-400 g were studied. Peripheral inflammatory hyperalgesia was induced by an intraplantar injection of 5% formalin into right hindpaw. The animals were randomly divided into 4 groups (n=6 in each group), group A:control; group B:2 h after formalin injection; group C:24 h after formalin injection and group D: 48 h after formalin injection. Pain response was recorded. The expression of NGF and BDNF in the inflamed skin, DRG and spinal dorsal horn was detected using immuno-histochemistry technique and image analysis method. Results There were significant bi-phasic nociceptive responses induced by intraplantar injection of formalin in rats. The expression of NGF started to increase 2 h after injection, in the inflamed skin, ipsilateral DRG and dorsal horn neurons and reached the peak level at 24 h after injection. The expression of BDNF in ipsilateral DRG and dorsal horn neurons was unchanged 2 h after injection and started to increase at 24 h after injection. There was a positive correlateion between the expression of NGF and BDNF in ipsilateral DRG neurons.Conclusion NGF is not only an important transmitter in the peripheral mechanicsm of inflammatory pain but also involved in central sensitization through facilitating expression of BDNF in dorsal horn neurons.