Modulation of trigeminal primary afferent activity in rats with chronic constriction nerve injury of the infraorbital nerve

In order to clarify the involvement of primary afferent neurons in neuropathic pain in the trigeminal region, a model involving chronic constriction nerve injury of the infraorbital nerve (ION-CCI) was developed in rats. Mechano-allodynia was observed 3 days after ION-CCI and lasted more than 14 days. At 3, 5, 7, and 14 days after ION-CCI, single unit activities were recorded from the ION. Aβ-, Aδ-, and C-units were identified on the basis of their conduction velocity Aδ-units were frequently encountered at the late period after ION-CCI. The highest spontaneous activity was recorded 3 days after ION-CCI during the observation period, and it progressively decreased after that. Mechanically evoked responses of Aδ-units were also highest 3 days after ION-CCI, and gradually decreased thereafter. Patch-clamp recording was performed on middle-to-large sized trigeminal ganglion (TRG) neurons. Patch-clamp recording analysis revealed that I_k (sustained) and I_A (transient) currents in rats with ION-CCI were significantly smaller than those in naïve rats. The hyperpolarization -activated current (I_h) was significantly larger in TRG neurons of rats with ION-CCI compared to that of naïve rats. These results suggest that I_h, I_k and I_A currents in Aδ - TRG neurons are involved in mechano - allodynia in the trigeminal region following peripheral nerve injury.