Central acoustic tract in an echolocating bat: an extralemniscal auditory pathway to the thalamus

Calbindin-like immunoreactivity in the central auditory system of the mustached bat, Pteronotus parnelli

With the aid of a polyclonal antibody specific for Calbindin D-28k, we studied the distribution of this calcium-binding protein in the central auditory system of the mustached bat, Pteronotus parnelli. Components of the cochlear nucleus (CN) that were calbindin-positive (cabp(+] included the root of the auditory nerve, multipolar and globular bushy cells in the anteroventral CN, multipolar and octopus cells in the posteroventral CN, and small and medium-size cells in the dorsal CN. Not stained were spherical bushy cells of the anteroventral CN and pyramidal/fusiform cells in the dorsal CN. In the superior olivary complex, labeled cells were found in the lateral and medial nuclei of the trapezoid body, the ventral and ventromedial periolivary nuclei, and the anterolateral periolivary nucleus. No cellular labeling was seen in the lateral superior olive. In the medial superior olive, only marginal cells were cabp(+). Labeled fibers could be seen surrounding the gosts of unlabeled cells in both the latter nuclei. Most cells in the intermediate nucleus and the columnar division of the ventral nucleus of the lateral lemniscus were cabp(+). However, the dorsal nucleus was cabp(-). A group of cabp(+) cells was also seen in the paralemniscal zone. The inferior colliculus had a relatively low density of cabp(+) cells. Labeled cells were more common in the caudal half of the central nucleus, and in the external nucleus and dorsal cortex. In the auditory thalamus, nearly every cell in the medial geniculate body was cabp(+), but those in the suprageniculate nucleus and in the posterior group did not stain. Small cells in the intermediate layer and giant cells in the deep layers of the superior colliculus were densely cabp(+). In the pons, cabp(+) cells and neuropil could be seen in the medial and lateral pontine nuclei (pontine gray). In conclusion, calbindin-like immunoreactivity was found in most of the brainstem auditory system, as well as in regions associated with acoustic orientation or control of vocalization. However, except for a minority of cells of the medial superior olive, it is conspicuously absent from the nuclei receiving binaural input below the level of the inferior colliculus.

Auditory projections from the cochlear nucleus to pontine and mesencephalic reticular nuclei in the rat

We investigated projections from the cochlear nucleus in the rat using the anterograde tracer Phaseolus vulgaris-leucoagglutinin. We focused on nuclei in the brainstem which are not considered to be part of the classical auditory pathway. In addition to labeling in auditory nuclei, we found presumed terminal fibers in 4 pontine and mesencephalic areas: (1) the pontine nucleus (PN), which receives bilateral projections from the antero- and posteroventral cochlear nuclei; (2) the ventrolateral tegmental nucleus (VLTg), which receives a contralateral projection from the rostral portion of the anteroventral cochlear nucleus; (3) the caudal pontine reticular nucleus (PnC), which receives bilateral input originating predominantly in the dorsal cochlear nucleus; and (4) the lateral paragigantocellular nucleus (LPGi), which receives projections from all subdivisions of the cochlear nuclei. In the VLTg and PnC, anterogradely labeled varicose axons were often found in close apposition to the primary dendrites and somata of large reticular neurons. Injections of the retrograde fluorescent tracer Fluoro-Gold into the VLTg demonstrated that the neurons of origin are mainly located contralaterally in the rostral anteroventral cochlear nucleus and in the cochlear root nucleus. The relevance of these auditory projections for short-latency audio-motor behaviors and acoustically elicited autonomic responses is discussed.