Trigeminal Nerve Transection-Induced Neuroplastic Changes in the Somatosensory and Insular Cortices in a Rat Ectopic Pain Model.

The primary sensory cortex processes competitive sensory inputs. Ablation of these competitive inputs induces neuroplastic changes in local cortical circuits. However, information concerning cortical plasticity induced by a disturbance of competitive nociceptive inputs is limited. Nociceptive information from the maxillary and mandibular molar pulps converges at the border between the ventral secondary somatosensory cortex (S2) and insular oral region (IOR); therefore, S2/IOR is a suitable target for examining the cortical changes induced by a disturbance of noxious inputs, which often causes neuropathic pain and allodynia. We focused on the plastic changes in S2/IOR excitation in a model of rats subjected to inferior alveolar nerve transection (IANX). Our optical imaging using a voltage-sensitive dye (VSD) revealed that the maxillary molar pulp stimulation-induced excitatory propagation was expanded one to two weeks after IANX at the macroscopic level. At the cellular level, based on Ca2+ imaging using two-photon microscopy, the amplitude of the Ca2+ responses and the number of responding neurons in S2/IOR increased in both excitatory and inhibitory neurons. The in vitro laser scanning photostimulation (LSPS) revealed that Layer II/III pyramidal and GABAergic fast-spiking neurons in S2/IOR received larger excitatory inputs from Layer IV in the IANX models, which supports the findings obtained by the macroscopic and microscopic optical imaging. Furthermore, the inhibitory postsynaptic inputs to the pyramidal neurons were decreased in the IANX models, suggesting suppression of inhibitory synaptic transmission onto excitatory neurons. These results suggest that IANX induces plastic changes in S2/IOR by changing the local excitatory and inhibitory circuits.