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BACKGROUND: The neuropeptide, substance P, has various bioactivities and is widely distributed in the central nervous system. Substance P participates in neural transmission in the spinal cord and plays an important role in regeneration and repair of nerve injury.OBJECTIVE: To investigate substance P mRNA expression in the anterior h of the spinal cord following brachial plexus injury.DESIGN, TIME AND SETTING: A molecular cell biology randomized controlled study was performed at the Department of Anatomy, Zhongshan Medical College, Sun Yat-sen University and the DaAn Gene Laboratory in May 2005.MATERIALS: A total of 29 adult male Sprague Dawley rats were randomly assigned to a control group (n=5) and an injury group (n = 24).METHODS: The injury group was divided into three subgroups. In subgroup A, the right seventh cervical vertebra (C7) anterior root was avulsed, and the residual nerve root at the distal end was removed. In subgroup B, the right C7 anterior root was avulsed, and the right C5 first thoracic vertebrae (TO posterior root was incised. Thus afferent pathways of the posterior root that connected with the anterior h motor neurons were blocked. In subgroup C, the right C7 anterior root was avulsed, and a right C5-6 hemisection was performed. Thus the descending fiber pathways of the cortex that connected with anterior h motor neurons were blocked. In the control group, the C5-T1 vertebral plate was opened, and then the skin was sutured.MAIN OUTCOME MEASURE: Substance P mRNA expression in the anterior h of the spinal cord was quantified using fluorescent quantitative reverse transcription-polymerase chain reaction.RESULTS: Substance P mRNA expression was low in the anterior h of the rat spinal cord in the control group. Substance P mRNA expression in the anterior h of the spinal cord was upregulated and was significantly higher in the injury group compared with the control group (P < 0.01 ). Substance P mRNA expression was highest in subgroup B.CONCLUSION: Brachial plexus anterior root avulsion is responsible for increased substance P expression in the anterior h of the rat spinal cord. Pathway disjunction in efferent fibers of the posterior root or cortex does not have an effect on substance P expression in the anterior h of the spinal cord.