Transneuronal tracing of vestibulo-trigeminal pathways innervating the masseter muscle in the rat

Abstract

Previous studies reported that the activity of trigeminal motoneurons innervating masseter muscles is modulated by vestibular inputs. We performed the present study to provide an anatomical substrate for these physiological observations. The transynaptic retrograde tracer pseudorabies virus-Bartha (PRV-BA) was injected into multiple sites of the lower third of the superficial layer of the masseter muscle in rats, a subset of which underwent a sympathectomy prior to virus injections, and the animals were euthanized 24-120 hrs later. Labeled masseteric motoneurons were first found in the ipsilateral trigeminal motor nucleus following a 24-hr post inoculation period; subsequent to 72-hr survival times, the number of infected motoneurons increased, and at ≥ 96-hrs many of these cells showed signs of cytopathic changes. Following 72-hr survival times, a few transynaptically-labeled neurons appeared bilaterally in the medial vestibular nucleus (MVe) and the caudal prepositus hypoglossi (PH) and in the ipsilateral spinal vestibular nucleus (SpVe). At survival times of 96-120 hrs, labeled neurons were consistently observed bilaterally in all vestibular nuclei (VN), although the highest concentration of infected cells was located in the caudal part of the MVe, the SpVe, and the caudal portion of PH. The distribution and density of labeling in the VN and PH were similar in sympathectomized and non-sympathectomized rats. These anatomical data provide the first direct evidence that neurons in the VN and PH project bilaterally to populations of motoneurons innervating the lower third of the superficial layer of the masseter muscle. The MVe, PH, and SpVe appear to play a predominant integrative role in producing vestibulo-trigeminal responses.

Sources of the motor and somatic sensory innervation of the trapezius muscle in the rat

“…In this paper, describing trans-neuronal tracing of the diaphragm, the presence of perikarya infected with pseudorabies virus was found within many nuclei; namely, pontine and medullary respiratory groups, the medial and lateral medullary reticular formation, the region immediately ventral to the spinal trigeminal nucleus, raphe pallidus and obscurus and the vestibular nuclei. Another paper describing trans-neuronal tracing done with the same tracer injected into the masseter muscle in the rat reported the presence of the traced neurons in all, ipsi-and contra-lateral vestibular nuclei (Giaconi et al, 2006), which is in accordance with our observations. Such a configuration can be explained in two ways.…”

A monosynaptic pathway links the vestibular nuclei and masseter muscle motoneurons in rats

“…Vestibular inputs might also reach masseter motoneurons through relays in Sp5, since this nucleus contained anterogradely-labeled axons following the injection of BDA into the MVePC or PH (see Table 2) and is known to provide efferents to Mo5 (Westberg and Olsson 1991;Inoue et al 2002). These data are consistent with findings that neurons in the pontomedullary reticular formation and Sp5 are infected at short survival times following the injection of pseudorabies virus into the masseter muscle (Giaconi et al 2006). …”

“…1B shows photomicrographs of a subset of the injection sites. Furthermore, b-CT-labeled motoneurons were observed in the lateral and ventral portions of the ipsilateral Mo5 in every rat; these portions of the nucleus have been demonstrated to contain masseter motoneurons (Mizuno et al 1975;Lynch 1985;Giaconi et al 2006). …”

“…In addition, a prior anatomical study using the transneuronal transport of pseudorabies virus revealed that a neural pathway links the caudal vestibular nuclei and PH with masseter motoneurons (Giaconi et al 2006). The present study extended these findings by showing that at least some of the neural connections between the vestibular nucleus complex and Mo5 are monosynaptic.…”

“…A recent study that employed pseudorabies virus to transneuronally trace the polysynaptic pathways providing inputs to jaw musculature demonstrated that neurons concentrated in the dorsomedial part of the parvicellular division of medial vestibular nucleus (MVePC) and the caudal prepositus hypoglossi (PH) project bilaterally to motoneurons innervating the lower third of the masseter muscle (Giaconi et al 2006). A limitation of this experiment was that it was not possible to ascertain whether the projections of MVePC and PH to masseter motoneurons were direct or multisynaptic.…”