Heat hyperalgesia induced by endoneurial nerve growth factor and the expression of substance P in primary sensory neurons.

Endoneurial nerve growth factor (30 ng) produced significant heat hyperalgesia in rats on postinjection days 3 and 5. The percentage of neuron profiles expressing the sensory neuropeptide substance P in the dorsal root ganglion (DRG), and the density and distribution of substance P immunoreactivity at the DRG and the dorsal horn remained essentially unchanged throughout the 10 days of study. NGF increased pain scores in the second phase of the formalin test on postinjection day 3, but not on days 5 and 10. Our results indicate that the observed heat hyperalgesia is not dependent on NGF-induced changes in SP content and release from primary sensory neurons.