NK1 receptor activation by geniohyoid primary afferents modulates parasympathetic postganglionic neuronal excitability in the rat

Previous studies have indicated that the geniohyoid (GH) muscle receives innervation via both the hypoglossal nerve (CNXII) and the ansa cervicalis. Our recent studies revealed that the efferent root that contributes to the ansa cervicalis is a parasympathetic pathway and contains postganglionic cell bodies. Afferent axons from the GH muscle also travel via the ansa cervicalis, and afferent cell bodies are located in spinal ganglia. The present study attempts to locate the central terminations of these afferents. From the peripheral cut end of the ansa cervicalis, we recorded afferent discharges that coincided with inspiration and these were elicited by stretch of the GH muscle. After cutting CNXII proximal to its union with the ansa cervicalis, we applied horseradish peroxidase to the branch of CNXII that innervates the GH muscle. This procedure labeled cells ipsilaterally in the C2 spinal ganglia but not in the brainstem or upper spinal cord. Substance P-reactive terminals in the peripheral CNXII trunk were in apparent contact with vasoactive intestinal peptide-reactive cell bodies. Addition of the NK1 receptor agonist SPNK1 excited parasympathetic postganglionic neurons and the specific NK1 receptor antagonist GR82334 blocked these effects in vitro. These results suggest that GH primary afferents synapse on parasympathetic postganglionic neurons in the CNXII trunk and that activation of SPNK1 receptors modulates activity in these neurons.