Intradermal Injection with Nerve Growth Factor: A Reproducible Model to Induce Experimental Allodynia and Hyperalgesia.

ABSTRACT

BACKGROUND: Nerve growth factor (NGF) plays a pivotal role in survival, growth, and differentiation of the nervous system. Increased levels of NGF have been reported in human pain disorders. Experimental injection of NGF in humans is known to evoke long-lasting mechanical sensitization and subsequent allodynia and hyperalgesia. METHODS: Reproducibility of intradermal injection of NGF was investigated. Twenty healthy male volunteers were included (mean age 24 years, range 19 to 31). The experiment consisted of 3 identical treatment periods with period 1 stimulating the right arm, period 2 the left arm, and period 3 stimulating the right arm again (period one and three were separated by at least 21 days). Pain intensity was assessed in response to several phasic stimuli in 3 adjacent sites of the volar forearm pressure; pinprick; brush; and heat before and after NGF injection. Additionally, areas of allodynia and secondary hyperalgesia were assessed. Rekindling with pressure was performed 1 hour and 24 hours after injection. Reproducibility was assessed with intraclass correlation coefficient (ICC 3,1). RESULTS: ICC values > 0.6 for all phasic stimuli and for the area of hyperalgesia. After NGF injection, pressure pain (P < 0.001) and heat pain (P < 0.01) sensitivity increased significantly. After rekindling, the area of hyperalgesia (von Frey 26 g) was significantly increased (P = 0.03) and sensitivity to pinprick was increased (P < 0.02). CONCLUSION: Intradermal NGF injection is capable of inducing reproducible allodynia and hyperalgesia, and the model is recommended for future experimental and pharmacological pain studies.