Descending projections from the inferior colliculus to medial olivocochlear efferents: Mice with normal hearing, early onset hearing loss, and congenital deafness.

Auditory efferent neurons reside in the brain and innervate the sensory hair cells of the cochlea to modulate incoming acoustic signals. Two groups of efferents have been described in mouse and this report will focus on the medial olivocochlear (MOC) system. Electrophysiological data suggest the MOC efferents function in selective listening by differentially attenuating auditory nerve fiber activity in quiet and noisy conditions. Because speech understanding in noise is impaired in age-related hearing loss, we asked whether pathologic changes in input to MOC neurons from higher centers could be involved. The present study investigated the anatomical nature of descending projections from the inferior colliculus (IC) to MOCs in 3-month old mice with normal hearing, and in 6-month old mice with normal hearing (CBA/CaH), early onset progressive hearing loss (DBA/2), and congenital deafness (homozygous Shaker-2). Anterograde tracers were injected into the IC and retrograde tracers into the cochlea. Electron microscopic analysis of double-labelled tissue confirmed direct synaptic contact from the IC onto MOCs in all cohorts. These labelled terminals are indicative of excitatory neurotransmission because they contain round synaptic vesicles, exhibit asymmetric membrane specializations, and are co-labelled with antibodies against VGlut2, a glutamate transporter. 3D reconstructions of the terminal fields indicate that in normal hearing mice, descending projections from the IC are arranged tonotopically with low frequencies projecting laterally and progressively higher frequencies projecting more medially. Along the mediolateral axis, the projections of DBA/2 mice with acquired high frequency hearing loss were shifted medially towards expected higher frequency projecting regions. Shaker-2 mice with congenital deafness had a much broader spatial projection, revealing abnormalities in the topography of connections. These data suggest that loss in precision of IC directed MOC activation could contribute to impaired signal detection in noise.

Adenomatous Polyposis Coli Protein Deletion in Efferent Olivocochlear Neurons Perturbs Afferent Synaptic Maturation and Reduces the Dynamic Range of Hearing.

Normal hearing requires proper differentiation of afferent ribbon synapses between inner hair cells (IHCs) and spiral ganglion neurons (SGNs) that carry acoustic information to the brain. Within individual IHCs, presynaptic ribbons show a size gradient with larger ribbons on the modiolar face and smaller ribbons on the pillar face. This structural gradient is associated with a gradient of spontaneous rates and threshold sensitivity, which is essential for a wide dynamic range of hearing. Despite their importance for hearing, mechanisms that direct ribbon differentiation are poorly defined. We recently identified adenomatous polyposis coli protein (APC) as a key regulator of interneuronal synapse maturation. Here, we show that APC is required for ribbon size heterogeneity and normal cochlear function. Compared with wild-type littermates, APC conditional knock-out (cKO) mice exhibit decreased auditory brainstem responses. The IHC ribbon size gradient is also perturbed. Whereas the normal-developing IHCs display ribbon size gradients before hearing onset, ribbon sizes are aberrant in APC cKOs from neonatal ages on. Reporter expression studies show that the CaMKII-Cre used to delete the floxed APC gene is present in efferent olivocochlear (OC) neurons, not IHCs or SGNs. APC loss led to increased volumes and numbers of OC inhibitory dopaminergic boutons on neonatal SGN fibers. Our findings identify APC in efferent OC neurons as essential for regulating ribbon heterogeneity, dopaminergic terminal differentiation, and cochlear sensitivity. This APC effect on auditory epithelial cell synapses resembles interneuronal and nerve-muscle synapses, thereby defining a global role for APC in synaptic maturation in diverse cell types. SIGNIFICANCE STATEMENT: This study identifies novel molecules and cellular interactions that are essential for the proper maturation of afferent ribbon synapses in sensory cells of the inner ear, and for normal hearing.

Further observations on cerebellar climbing fibers. A study by means of light microscopy, confocal laser scanning microscopy and scanning and transmission electron microscopy

The intracortical pathways of climbing fibers were traced in several vertebrate cerebella using light microscopy, confocal laser scanning microscopy, scanning and transmission electron microscopy. They were identified as fine fibers up to 1(micron thick, with a characteristic crossing-over bifurcation pattern. Climbing fiber collaterals were tridimensionally visualized forming thin climbing fiber glomeruli in the granular layer. Confocal laser scanning microscopy revealed three types of collateral processes at the interface between granular and Purkinje cell layers. Scanning electron microscopy showed climbing fiber retrograde collaterals in the molecular layer. Asymmetric synaptic contacts of climbing fibers with Purkinje dendritic spines and stellate neuron dendrites were characterized by transmission electron microscopy. Correlative microscopy allowed us to obtain the basic three-dimensional morphological features of climbing fibers in several vertebrates and to show with more accuracy a higher degree of lateral collateralization of these fibers within the cerebellar cortex. The correlative microscopy approach provides new views in the cerebellar cortex information processing.

Morphological relationships of peptidergic and noradrenergic nerve terminals to olivocochlear neurones in the rat.

In the rat, the outer hair cells in the cochlea receive direct synaptic input from neurones in the ventral nucleus of the trapezoid body. These so-called medial olivocochlear neurones exert an inhibitory influence on the cochlear neural output. Electrophysiological in vitro studies suggest that the activity of medial olivocochlear neurones may be affected by a variety of neuropeptides as well as noradrenaline, but anatomical confirmation of direct synaptic input is still lacking. We have investigated, at the light microscopical level, the morphological relationships between terminals containing noradrenaline, substance P, cholecystokinin and leu-enkephalin, and medial olivocochlear neurones in the rat. A retrograde tracer was injected into the cochlea to label medial olivocochlear neurones and a double labelling immunocytochemical method was used to visualise the retrograde tracer as well as the neurotransmitters within each brain section. Light microscopical analysis revealed nerve endings containing substance P, cholecystokinin and leu-enkephalin in close apposition to the dendrites of medial olivocochlear neurones, and nerve endings containing dopamine-beta-hydroxylase, a marker for noradrenaline, in close contact with the somata as well as dendrites of medial olivocochlear neurones. Although the technique cannot prove the existence of functional synaptic contacts, the results are broadly consistent with electrophysiological data and suggest a direct input to medial olivocochlear neurones from substance P, cholecystokinin, leu-enkephalin and noradrenaline-containing neural pathways. Differences in the densities and spatial distribution of the various neuropharmacological inputs suggest differences in the relative strengths and possible roles of these diverse inputs to the olivocochlear system.

In vitro induction of microcyst-like structures in the superior olivary complex.

To investigate the etiology of hole formation in the gerbil and rat central auditory system, organotypic cultures were grown in control and veratridine-containing media. The latter condition is known to increase neuronal activity. Tissue was obtained at postnatal day 6 and grown for 6-9 days in vitro, a period prior to the formation of holes in vivo. In both rats and gerbils, veratridine led to the appearance of large numbers of holes, and these were phenotypically similar to those found in vivo. These results support the idea that hole formation is an activity-dependent phenomenon, and suggest that it is not restricted to the mature gerbil auditory system.

[Morphology of glial cells in the lower olivary complex of the pigeon (Columba livia). II. Oligodendrocytes]. / Morfoloiia na glialnite kletki v dolniia olivaren kompleks na gulub (Columba livia). II. Oligodendrotsiti.

The morphologic peculiarities of oligodendrocytes in the lower olivary complex (LOC) of a pigeon were examined at light and electron microscopy level. The analysis on the basis of the applied histological techniques (staining with cresyl-violet and silver impregnation according to Goldgi-Rio Hortega) showed that oligodendrocytes were small rounded cells with nuclei stained more darkly than those of astrocytes. Oligodendrocytic arborizations were thin, slightly ramified and small in number. The predominant number of oligodendrocytes were of median type in LOC of a pigeon, observed by the electron microscope. The nuclei of oligodendrocytes were dense, seen by the electron microscope, in comparison with those of the astrocytes. The cytoplasm contained abundance of short and median cysternae of granulated endoplasmic reticulum, microtubules, but there were no microfilaments. Gap junctions contacts of the type astro-oligodendrocytes were found on the surface of cellular membrane. Oligodendrocytes were located in the perineuronal position, connected with myelinated nervous fibers or were situated freely among the elements of the neuropila. The functional significance of these glial cells and their probable role in neuronal metabolism is discussed on the basis of the comparative analysis of the morphologic characteristics of oligodendrocytes in LOC of a pigeon and those of mamals. The present work is a part of our observations on the structure of LOC in phylogenic aspect.

Congenital neuronal ceroid lipofuscinosis. A case report.

A case of congenital neuronal ceroid lipofuscinosis, a 9-day-old male child of Pakistani parents, is reported. Only 5 congenital cases have been described previously. The cerebral and cerebellar cortex showed extensive nerve-cell loss. Granular material with histochemical characteristics of ceroid-lipofuscin was deposited in neurons, macrophages and glial cells throughout the brain. Similar material was found in macrophages in the lymphoid system and in certain other organs. Ultrastructurally, the material was identical with that described in the infantile type of ceroid lipofuscinosis. It is pointed out that the metabolic exchange between the blood of the mother and the foetus is not sufficient to prevent the intra-uterine progression of the disease.

[Morphological aspects (optical and ultrastructural) of hypertrophy of the olivary bulb]. / Aspects morphologiques (optiques et ultrastructuraux de l'hypertrophie de l'olive bulbaire

The present paper is an anatomical study of 3 cases which evolved with hypertrophy of the inferior olives: a case of zoster encephalitis, a multifocal leucoencephalopathy and a post-traumatic encephalopathy. In all three cases a relationship was demonstrated between the dentate nucleus and dento-olivary pathway - a relation of topical order in some instances - and the hypertrophic lesion of the inferior olive. Examination in the electron microscope revealed in the hypertrophic olivary cell not only dystrophic lesions but also progressive intracellular phenomena, such as maintenance of the Nissl bodies, multiplication of the mitochondria and especially marked proliferation of the neuro-filaments. This shows that, at least in some of the evolutive phases, hypertrophy of the inferior olives is based upon a real hypertrophy of the olivary cell. In addition, lesions of the glial system of the hypertrophic inferior olives were noted with the presence of osmiophil degeneration whose morphology clearly differed from that of Rosenthal's fibres.