Cholinergic input from the ventral nucleus of the trapezoid body to cochlear root neurons in rats.

Brain stem pathways are essential for the modulation of the acoustic startle reflex by sounds; nevertheless, the neural circuits that convey fast auditory information to the primary acoustic startle circuit are still unclear. In the rat, cochlear root neurons (CRNs) comprise the first component of the primary acoustic startle circuit and are critical in the initiation and full expression of the acoustic startle reflex. To determine whether CRNs receive auditory descending inputs, we developed tract-tracing studies combined with immunohistochemistry, electron microscopy, morphometry, and confocal microscopy. Either FluoroGold or biotinylated dextran amine (BDA) injections in CRNs showed retrogradely labeled neurons in the ventral nucleus of the trapezoid body (VNTB). We verified the projection to CRNs by injecting BDA into the VNTB. Our results showed that neurons from VNTB project bilaterally and directly to CRNs, giving off numerous endings onto cell bodies and preferentially dendrites of CRNs. Electron microscopy analysis of labeled VNTB terminals demonstrated that they made multiple symmetric synapses and contained small round vesicles. Colocalization of the vesicular acetylcholine transporter and fluorescein dextran after injection in the VNTB indicated that these terminals use acetylcholine as neurotransmitter. We also revealed that the inferior colliculus, an important nucleus mediating the auditory prepulse inhibition, projects to VNTB neurons that innervate CRNs. Our data show a novel and short descending auditory pathway from the VNTB to the first nucleus of the primary acoustic startle circuit that might play an important role in the auditory prepulse inhibition of the startle reflex elicited by sounds.