Integration of sensory and cortical information in the brainstem during mastication in mice

Mastication is a vital function that relies on precise synchronization among multiple brainstem regions, known as being part of a central pattern generator (CPG). Movements can be triggered either by stimulating a sensory-motor region called the cortical masticatory area (CMA), well-documented in various species but not yet formally identified in mice, or by stimulating the oro-facial sensory primary afferents which fibers form the trigeminal tract (Vtr). However, its unclear whether these different inputs activate distinct components of the CPG or converge on the same. This study aims at mapping brainstem areas activated by cortical and sensory inputs using immunohistochemistry against the cellular activity marker, c-Fos and Ca2+-imaging, respectively. Optogenetic stimulation of the cortical masticatory area (CMA) in awake, head-fixed mice reliably induced rhythmic jaw movements (RJMs) and increased c-Fos expression in multiple brainstem regions, with strongest activation in the peritrigeminal area (PeriV) and parvocellular reticular formation (PCRt) ventral to the trigeminal motor nucleus (NVmt). In contrast, in vitro electrical stimulation of trigeminal sensory afferents (Vtr) predominantly activated neurons and astrocytes in the main sensory nucleus (NVsnpr), the dorsal area of PeriV, adjacent to it, and PCRt. The areas containing the highest numbers of activated cells differed clearly according to the type of inputs and overlapped only in the PCRt, ventral and slightly medial to the trigeminal motor nucleus and the most dorso-medial part of PeriV. These findings demonstrate that cortical and sensory inputs take part in distinct components of the brainstem masticatory circuitry, with PCRt emerging as a point of convergence and provide new insights into the components of the CPG of mastication.