BACKGROUND: Substance P participates in pain transmission and modulation, suggesting a close association with migraine headaches. The clinical application of magnesium has been effective in treating migraines, and the action mechanisms underlying migraines correlate with substance P expression.OBJECTIVE: To analyze different magnesium doses on behavior and substance P mRNA expression in the midbrain of a rat migraine model, and to determine the action pathway of migraine treatment using magnesium.DESIGN, TIME AND SETTING: A completely randomized, controlled, animal experiment was performed at the Experimental Animal Center and Central Laboratory in the Second Hospital of Jilin University between 2007 and 2008.MATERIALS: Magnesium sulfate (25%) was supplied by Tianjin Pharmaceutical Jiaozuo, China. Nitroglycerin was provided by Shanxi Kangbao Biological, China. Substance P primer sequence was synthesized by TaKaRa Biotechnology (Dalian), China. METHODS: A total of 36 healthy, adult, Wistar rats were randomly assigned to 6 groups: control, migraine, low- and high-dose magnesium sulfate treated, and low- and high-dose magnesium sulfate control, with 6 rats in each group. Migraines were induced by subcutaneous injection of 10 mg/kg nitroglycerin in the migraine and low- and high-dose magnesium sulfate treated groups, and 2 mL/kg physiological saline was administered to rats in the control and low- and high-dose of magnesium sulfate control groups. Five minutes following administration, rats in low-dose groups were intraperitoneally injected with 100 mg/kg magnesium sulfate, while those in high-dose groups were injected with 300 mg/kg magnesium sulfate. No interventions were administered to the control and migraine groups.MAIN OUTCOME MEASURES: At 2 hours after nitroglycerin injection, substance P mRNA expression in the rat midbrain was measured by real-time quantitative polymerase chain reaction. At 60-90 minutes after nitroglycerin injection, behavioral changes of pain were analyzed in the experimental rats.RESULTS: The migraine group exhibited significantly lower levels of substance P mRNA expression compared with the control group (P < 0.05). Following magnesium sulfate injection, substance P mRNA expression increased, compared with the migraine and control groups (P < 0.05). In the low- and high-dose magnesium sulfate treated groups, pain behavior was remarkably ameliorated, compared with the migraine group (P < 0.05), particularly with the high-dose injection (P < 0.05).CONCLUSION: Magnesium relieved pain behaviors in a rat migraine model in a dose-dependent manner, and the therapeutic effect was achieved in conjunction with increased substance P expression in the midbrain.