Tooth loss-associated neuroplasticity of mastication-related motor cortical neurons

Objectives: The cerebral cortex contains neurons that play a pivotal role in controlling rhythmic masticatory jaw movements. However, the population characteristics of individual cortical neuronal activity during mastication and the impact of tooth loss on these characteristics remain unclear. Thus, in this study, we aimed to determine the activity patterns of mastication-related motor cortical neurons elicited during mastication and examine the effects of tooth extraction on neuronal activity using two-photon Ca²⁺ imaging in head-restrained awake mice.

Methods: GCaMP6f-expressing adeno-associated virus serotype 1 was injected into the left motor cortex (centered 2 mm anterior and 2 mm lateral to the bregma) and electromyography (EMG) electrodes were implanted into the right masseter and digastric muscles of 6-8-week-old C57BL/6j mice. Three weeks after surgery, in vivo two-photon Ca²⁺ imaging of layer (L) 2/3 neurons and simultaneous EMG recordings were performed during the masticatory sequence.

Results: Mastication induced a remarkable increase in the power and frequency of Ca²⁺ responses and correlated with majority of the mastication-related motor cortical L2/3 neuronal activity. These mastication-related changes correlated with the activity of neurons with low baseline activity that occurred before mastication. Extraction of the right upper three molars caused clear neuroplastic changes in the mastication-induced Ca²⁺ activity of L2/3 neurons.

Conclusions: Our in vivo imaging study provides new insights into the neuronal basis of tooth loss-induced cortical neuroplasticity, and suggests a possible therapeutic approach for oral sensorimotor dysfunction.