Modulation of principalis nucleus neuronal excitability in trigeminal nerve-injured rats

Objectives: The gracile nucleus (GN) and principal sensory trigeminal nucleus (PrV) are known as the important innocuous sensory relay nuclei in the paw and orofacial regions, respectively. We have reported that the GN neurons changes in their response properties following sciatic nerve injury. It is highly possible that PrV neurons change in their response characteristics following trigeminal nerve injury as observed in GN neurons. In order to evaluate whether the trigeminal nerve injury affects the excitability of PrV neurons, nocifensive behavior and single neuronal activity were precisely analyzed in rats with infraorbital nerve (ION) injury. Methods: SD rats were anesthetized with pentobarbital sodium (50mg/kg, i.p.) and chronic constriction nerve injury (CCI) was applied to the ION with 4.0 chromic guts. Escape threshold to mechanical stimulation of the whisker pad region and PrV neuronal activities were precisely analyzed in the ION-CCI rats at 7-14 days after ION surgery. Results: Escape threshold to mechanical stimulation of the whisker pad region in ION-CCI rats (3.6±0.8 g, mean±se, n=6) were significantly lower than those in sham rats (37.3±11.3, n=4). Background activities of PrV neurons in ION-CCI rats (9.5±1.5 Hz, n=16) were significantly higher than those of sham rats (0.2±0.2 Hz, n=6). The evoked responses of PrV neurons to innocuous mechanical stimulation (1g) (15.7±1.1 spikes/s, peak freq. – BG-activity, n=16) was significantly higher in ION-CCI rats than those of sham rats (3.5±1.1 spikes/s, n=6). Conclusion: The present data that the PrV neurons increase in their excitability following ION-CCI suggest that hyperexcitability of PrV neurons is involved in sensory abnormalities in the orofacial regions induced by trigeminal nerve injury.