Thalamic projections from the cerebellar nuclei receiving proprioceptive signals from the jaw muscles in rats

The neural mechanisms involved in transmission of proprioceptive information from muscles are not fully understood. Proprioceptive signals from jaw-closing muscle spindles (jcms) are relayed to the supratrigeminal nucleus (Su5), and, then, from the Su5 to the thalamus including the oval paracentral nucleus (OPC) via the trigeminal lemniscal pathway and to the jcms-IntDL and jcms-MedDL (the dorsolateral hump of interposed cerebellar nucleus [IntDL] and the dorsolateral protuberance of medial cerebellar nucleus [MedDL], which were electrophysiologically identified to receive jcms-proprioception). In this study, we investigated the cerebellothalamic projections from the jcms-IntDL and jcms-MedDL in rats by injecting an anterograde tracer into these two subnuclei. The thalamic projection sites from the jcms-IntDL and jcms-MedDL appeared bilaterally with a contralateral predominance and broadly overlapped, but with considerable differences. Namely, the jcms-IntDL principally projected to the ventrolateral (VL), ventromedial (VM), ventral anterior, paracentral (PC) thalamic nuclei, and OPC, whereas the jcms-MedDL mainly targeted the parafascicular and centrolateral thalamic nuclei as well as the VL, VM, and PC. Overall, these thalamic projections from the jcms-MedDL appeared stronger than those from the jcms-IntDL. In the OPC and VM, the size of axonal boutons from the jcms-IntDL was larger than that from the jcms-MedDL. We also investigated the cerebellar nucleus neurons projecting to the OPC by using retrograde tracer injections into the OPC, and confirmed that the cerebellothalamic and lemniscal pathways, which possibly transport the same proprioceptive information, converge on the OPC. Our findings demonstrate characteristics of thalamic projections from the cerebellar nuclei conveying muscle proprioceptive information.