Neurons in the cochlear nuclei controlling the tensor tympani muscle in the rat: A study using pseudorabies virus

Abstract

The middle ear muscle reflex has been implicated in modulation of auditory input and protection of the inner ear from acoustic trauma. However, the identification of neurons in the cochlear nuclei participating in this reflex has not been fully elucidated. In the present study, we injected the retrograde transynaptic tracer pseudorabies virus into single tensor tympani (TT) muscles, and identified transynaptically labeled cochlear nucleus neurons at multiple survival times. Motoneurons controlling TT were located ventral to the ipsilateral motor trigeminal nucleus and extended rostrally towards the medial aspect of the lateral lemniscus. Transynaptically-labeled neurons were observed bilaterally in the dorsal and dorso-medial parts of ventral cochlear nuclei as early as 48 h after virus injection, and had morphological features of radiate multipolar cells. After ≥ 69 h, labeled cells of different types were observed in all cochlear nuclei. At those times, labeling was also detected bilaterally in the medial nucleus of the trapezoid body and periolivary cell groups in the superior olivary complex. Based on the temporal course of viral replication, our data strongly suggest the presence of a direct projection of neurons from the ventral cochlear nuclei bilaterally to the TT motoneuron pool in rats. The influence of neurons in the cochlear nuclei upon TT activity through direct and indirect pathways may account for multifunctional roles of this muscle in auditory functions. Keywordsdirect and indirect acoustic reflex pathways; middle ear muscle; transynaptic transport

Auditory System

“…Furthermore, the small cell cap is emerging as a nexus of auditory brainstem regulatory pathways. Neurons from the cap area are components of circuits that control the middle ear reflex (Billig et al, 2007) and cochlear output (Ye et al, 2000).…”

Commissural Neurons in the Rat Ventral Cochlear Nucleus

“…The small cell cap is emerging as a nexus of auditory brainstem regulatory pathways. Neurons that reside there are components of circuits that control the middle ear reflex (Billig et al 2007) and cochlear output (Ye et al 2000). The small cell cap is the preferred target of the collateral projections of medial olivocochlear neurons (Liberman and Brown 1986;Brown et al 1988;Benson et al 1996), whose principal role is to modulate cochlear gain via their innervation of outer hair cells.…”

Identification of Inputs to Olivocochlear Neurons Using Transneuronal Labeling with Pseudorabies Virus (PRV)

“…1A) and labeled profiles ("blebs," Fig. 1B) that may signify cells in the late stages of cytopathic changes from PRV infection (Billig et al 2007). MOC somata major and minor axis diameters (average 25.4 × 15.4 μm, SD 5.4 × 3.3 μm, for 51 neurons) were significantly larger than LOC somata from within the LSO (average 15.9×10.5 μm, SD 3.1× 2.6 μm, for 28 neurons).…”

“…One factor in low counts may be that PRV may kill some OC neurons (e.g., Fig. 1), but this factor requires long postinoculation intervals (O'Donnell et al 1997;Card 2001;Billig et al 2007). Horvath et al report a 2:1 contra/ipsi ratio of MOC neurons all of which were located in VNTB.…”

“…For tensor tympani injections (Billig et al 2007), there is early labeling of large neurons with long, radiating dendrites (although longer survival times yield a wider variety of neurons). These radiate multipolar cells, some of which have been reported to project to the opposite CN (Doucet and Ryugo 2006;Brown et al 2013b), were not labeled in the present studies.…”