The ammalian ear contains two types of auditory receptors, inner and outer hair cells, that lie in dose proximity to each other within the sensory epithelium of the cochlea. In adult mammals, these two classes of auditory hair cells are innervated by separate populations ofafferent neurons that differ strikingly in their cellular morphology and their pattern of arborization within the cochlea. At present, it Is unclear when or how these disinctive patterns of cochlear innervation emerge and become segregated during development. In the present study, an in vitro horseradish peroxidase labeling method was used to examine the formation of individual auditory neuron arbors at the same location within the apex of the developing gerbil cochlea. At birth, most cochlear neurons displayed peripheral arbors that embraced both inner and outer hair cell receptors. During the next 6 days, however, the arbors of individual cochlear afferents become confined to either the inner or outer hair cell zone, and thus there is a complete segregation of afferent innervation. This neural segregation occurs principally through the withdrawal of inappropriate connectimns to the outer hair cell ystem and is completed well before hearing commences.A central goal of neurobiology is to determine how developing neurons choose their specific synaptic partners. An excellent system for examining this issue is the mammalian auditory endorgan, the cochlea, since its pattern of innervation is among the most precise of any sensory or motor system. The adult cochlea contains two types of auditory receptors, termed inner (IHCs) and outer (OHCs) hair cells, which receive separate and highly distinctive patterns of afferent innervation. Ninety percent of all cochlear afferents project to individual IHCs via thick unbranched radial afferents, each of which terminates in a single synapse. The OHCs, on the other hand, receive a more diffuse innervation from a relatively few thin spiral afferents, each of which projects for hundreds of micrometers along the length of cochlea before branching to contact from 5 to 50 receptorsUltrastructural studies have provided evidence for substantial ontogenic changes in the form, number, and spatial arrangement of synapses between mammalian auditory receptors and cochlear nerve fibers (9-16). To understand the process by which these synaptic patterns are adjusted during development, however, it is necessary to examine the early morphological changes that occur in the formation of indi- In the present investigation, I have employed an in vitro horseradish peroxidase (HRP) method that permits labeling of small populations of auditory nerve fibers at any location within the developing cochlea. The Mongolian gerbil was selected for study since this species displays a prolonged period of postnatal cochlear maturation. In addition, the gerbil cochlea, like that of the mouse (26), lengthens <10% after birth, from an average of 10.8 mm in the newborn (unpublished data) to 12.1 mm in the adult (27), which greatly reduces ...

Central auditory pathways were traced in Japanese carp, Cyprinus carpio, using electrophysiological mapping and HRP tract-tracing methods. Multiunit recordings made from the carp torus semicircularis, the major midbrain area for processing octavolateralis information, revealed a mediolateral segregation of auditory and lateral line sensory modalities. Iontophoretic injections of HRP were made into the medial torus to trace afferent and efferent projections of the carp auditory midbrain. Following unilateral HRP injections into the medial torus, retrogradely labeled neurons were observed within six nuclei of the carp medulla. Two octaval nuclei, the anterior octavus nucleus and descending octavus nucleus, contained HRP-filled neurons. Labeled neurons were also observed within the ipsilateral superior olive, scattered among fibers of both lateral lemnisci, and bilaterally within the medullary reticular formation. In addition, bilateral retrograde cell labeling was found within a group of Purkinje-like cells located adjacent to the IVth ventricle, just rostral to the level of the VIIIth nerve. Few labeled neurons were found within the nucleus medialis, a principal target for lateral line afferents within the medulla. At midbrain levels, retrogradely labeled neurons were observed within the contralateral torus semicircularis and the ipsilateral optic tectum. Three forebrain nuclei project to the carp auditory midbrain. Within the diencephalon, descending projections originate from the anterior tuberal nucleus, bilaterally, and from the ipsilateral central posterior thalamic nucleus. The ipsilateral caudal telencephalon also projects to the carp auditory midbrain via large multipolar neurons within area dorsalis pars centralis. Anterograde labeling of fibers and terminals revealed efferent projections of the carp auditory midbrain to the following targets: the ipsilateral superior olive, the ipsilateral medullary reticular formation, the deep layers of the optic tectum, the contralateral torus semicircularis, the anterior tuberal nucleus, and the central posterior thalamic nucleus. These results, together with recent studies of lateral line pathways in teleosts (Finger, '80, '82a), demonstrate that central auditory and lateral line pathways are anatomically distinct in the carp, at least from medullary to diencephalic levels. Furthermore, there are striking similarities in the organization of the central auditory pathways of the carp and those of amphibians and land vertebrates.

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Horseradish peroxidase injections into dorsomedial and dorsolateral regions of the goldfish (Carassius auratus) telencephalon demonstrate, by retrograde cell labeling, that the teleost telencephalon receives a pattern of projections from the thalamus remarkably similar to those of land vertebrates. The evidence provides support for a homology between the dorsomedial region and the corpus striatum of land vertebrates and a homology between two dorsolateral regions and the dorsal and medial pallium of land vertebrates.