EXPLORING THE ROLE OF THE PREMOTOR TRIGEMINAL COMPLEX IN MASTICATORY FUNCTION

Masticatory movements are part of the more complex behavior of feeding. They are generated by a brainstem network, known as a Central Pattern Generator (CPG), but it is assumed that their coordination with other movements during feeding occurs elsewhere.
The boundaries of the CPG are known, but its organisation is less understood. Here we mapped, within these boundaries, areas activated during cortically evoked mastication, using c-fos immunostaining. Optogenetic stimulation of the cortical masticatory area (CMA) evoked rhythmic jaw movements (RJMs), and increased activity in the trigeminal main sensory nucleus (NVsnpr), and in regions medial (JuxtV) and ventral (PCRt) to the trigeminal motor nucleus.
Optogenetic stimulation of JuxtV and PCRt in head-fixed awake animals produced some uncoordinated jaw movements. In contrast, stimulation of NVsnpr induced robust RJMs, and some hand-to-mouth movements resembling those occurring during natural feeding and food manipulation, raising the intriguing possibility that complex behaviors may be partly encoded at the CPG level.
We have previously shown that NVsnpr neurons shift their firing pattern from tonic to rhythmic bursting when the extracellular Ca²⁺ concentration is decreased by release of S100β, an astrocytic Ca²⁺-binding protein. Here, we show that S100β levels increase in NVsnpr during CMA-induced RJMs, while injection of an anti-S100β antibody delays these movements. A selective Nav1.6 blocker (4,9-anhydroTTX ), acting on the conductance modulated by S100β, also greatly reduced the frequency and amplitude of cortically induced RJMs.
These results will help resolve the composition of the masticatory CPG and establish the role of astrocytes in rhythmogenesis.