Transient expression of acetylcholinesterase in the posterior ventral cochlear nucleus of rat brain.

In this report we partially characterize a pathway projecting to the posterior ventral cochlear nucleus (PVCN) of the rat brain that transiently expresses a high level of acetylcholinesterase (AChE). The AChE-positive axons form a network that envelops a discrete region of the PVCN that includes the octopus cell region and some cells rostral to it. AChE is first detectable by postnatal day 3 (P3), peaks in expression at about P7-10, and is barely detectable in our preparations by P15. We previously reported that neurons in the octopus cell region express high levels of alpha7 nAChR mRNA and alpha-bungarotoxin binding during the same time period. In light microscopic immunocytochemical studies using antibodies to the vesicular acetylcholine transporter (VAChT), we could not identify immunopositive boutons in the developing regions of the PVCN that express high levels of AChE-positive fibers despite distinct punctate labeling in other brain regions. Systematic electron microscopic examination of AChE histochemical staining throughout the PVCN revealed intense labeling of axons, but synaptic sites were devoid of reaction product. The source of the AChE-positive fibers is not known, but the fibers are not auditory nerve axons and probably not collaterals of the olivocochlear bundle.

Anatomy of olivocochlear neurons in the hamster studied with FluoroGold.

The golden hamster (Mesocricetus auratus) is often used in auditory research, but little is known about the anatomical organization of its olivocochlear (OC) neurons, the source of the efferent innervation of the organ of Corti. Accordingly, we labeled the OC neurons projecting to one cochlea by means of retrograde axonal transport of FluoroGold. In four animals, all labeled OC neurons were counted and digital images of the labeling were captured and analyzed morphometrically. In one case, a 3D computer reconstruction of the bilateral distribution of OC neurons was made. The largest group of OC neurons was comprised by small, intrinsic lateral OC neurons within the ipsilateral lateral superior olivary nucleus (LSO), almost all of which (97%) were located ipsilaterally. The second largest group consisted of medial OC neurons in the ventral nucleus of the trapezoid body, 75% of which were located contralaterally. The smallest group consisted of shell neurons surrounding the LSO, 80% of which projected ipsilaterally. These three types of neurons are generally similar in morphology and distribution to those previously described in the rat and the chinchilla. However, there were several unique findings, including the fact that the hamster possesses the smallest total number of OC neurons (mean 341) of any rodent yet studied.

Diversity of axonal ramifications belonging to single lateral and medial olivocochlear neurons.

A classification of olivocochlear (OC) neurons based on their location in either the lateral or medial region of the superior olivary complex provides a powerful tool for predicting their terminations beneath the inner (IHC) or outer hair cells (OHC) of the cochlea, respectively. Yet the morphology of axonal terminations belonging to single lateral OC (LOC) and medial OC (MOC) neurons, which can provide clues about the functional capabilities of individual efferent neurons, has been relatively unexplored. Following injections of biotinylated dextran amine into regions containing OC neurons in the rat, we reconstructed 19 LOC and 15 MOC axons in surface preparations of the cochlea. Confirming previous studies, LOC axons could be classified as either intrinsic or shell based on the length (short versus long) of their terminal ramifications beneath the IHC. However, intrinsic LOC axons were of two types, those that traveled to the organ of Corti without branching (simple intrinsic) and those that had three or more branches that converged on the same discrete patch of IHCs (converging intrinsic). Regarding shell neurons, we found that they may have as many as four intraganglionic branches that could innervate as much as 41% of cochlear length. Lastly, we found that MOC neurons were extremely diverse, not only in the number of their tunnel-crossing fibers (1-15), but also in both the number of boutons they formed (1-48) and in their basal-apical spans (1-45%). Analysis revealed that the number of tunnel-crossing fibers formed by a given axon was closely related to the total number of its terminal boutons, but not to its cochlear span. Analysis further suggested the existence of two distinct subtypes of MOC neurons on the basis of the number of tunnel-crossing fibers and boutons each possessed: the more common sparsely-branched and the quite rare highly-branched MOC neurons. In conclusion, the variety of axonal ramifications of individual LOC and MOC neurons has functional implications and raises the question of whether the various types of efferent neurons might be subject to selective control by ascending and descending central auditory and possibly non-auditory pathways.

Organization of olivocochlear neurons in the cat studied with the retrograde tracer cholera toxin-B.

We employed cholera toxin-B (CTB), an efficient retrograde tracer, to examine olivocochlear (OC) neurons in the cat. Our primary goals were (1). to determine whether the cat has two types of lateral OC (LOC) neurons as is found in certain rodents and (2). to document the morphology, number, and caudorostral distribution of OC neurons, bilaterally. Adult cats received injections of CTB through the round window of the left cochlea, and, after 3-6 days, the brains were sectioned transversely and CTB was revealed immunocytochemically in every section. In three cats, OC neurons were mapped, counted differentially according to cell group, and the numbers of each plotted bilaterally from caudal to rostral. In one cat, measurements were made on labeled LOC and medial OC (MOC) neurons. The results indicate that LOC neurons can be divided into two groups based on their proximity to the lateral superior olive (LSO): a more populous group of small neurons that have intimate contact with the LSO, designated marginal-LOC neurons, and a less populous, morphologically heterogeneous group, lying more distantly from the LSO, designated para-LOC neurons. Para-LOC neurons lying dorsal and rostral to LSO were significantly larger than marginal-LOC. We hypothesize that the cat marginal-LOC neurons and most probably the larger para-LOC neurons correspond to rodent intrinsic and shell LOC neurons, respectively, which have focal versus diffuse projections beneath the inner hair cells. Concerning MOC neurons, we confirm and extend previous observations on the clustering of these neurons near the rostral tip of the medial superior olivary nucleus and also show that MOC neurons differ in size according to cell group. Finally, we compare the present counts of OC neurons (mean total 1607, consisting of 1058 LOC neurons and 549 MOC neurons innervating one cochlea) and their proportional distribution ipsilaterally and contralaterally with those reported previously. Our estimate of the number of LOC neurons is somewhat higher than those previously obtained either by retrograde labeling with horseradish peroxidase or by counting unmyelinated axons in the olivocochlear bundle. In contrast, our estimate of the number of MOC neurons is very similar to those previously reported.