Ilsa R. Schwartz

Differential labeling of sensory cell and neural populations in the organ of Corti following amino acid incubations

Label localization was compared by light and electron microscopic autoradiography in the organ of Corti of the gerbil, following in vivo incubation with one of eight 3H-labeled amino acids. Following incubation with GABA, efferent endings underneath the outer hair cells were heavily labeled, as were all tunnel crossing fibers, many small fibers in the inner spiral bundle and some efferents beneath the inner hair cells. This pattern was absent following incubations with the GABA analog muscimol. Following L- and especially D-aspartic acid incubations, efferent endings under the inner hair cells and small fibers of the inner spiral bundle were preferentially labeled. It is concluded that these differential labeling patterns after GABA and aspartic acid incubations reflect two populations of efferent fibers in the organ of Corti: one terminating primarily upon the outer hair cells and one underneath the inner hair cells. Cochlear hair cells showed the highest level of labeling following incubation with alanine. Inner hair cells were more heavily labeled than outer hair cells following incubations with alanine and glycine. Hair cell labeling was comparable to that of supporting cells for most amino acids. However, for alanine and glycine the labeling of inner hair cells, and for L-aspartic and glutamic acid the labeling of all hair cells, was higher than that of structural cells. Labeling of hair cells was substantially lower than that of structural cells following taurine incubations.

The differential distribution of label following uptake of 3H-labeled amino acids in the dorsal cochlear nucleus of the cat. An autoradiographic study.

Abstract Light and electron microscopic autoradiography demonstrated a differential distribution of label in the dorsal cochlear nucleus (DCN) of the cat following the incubation of adjacent fresh brain slices with micromolar amounts of different tritiated amino acids in oxygenated Ringers bicarbonate solutions. After incubation with GABA, glutamic acid, or glycine, label was found primarily over synaptic terminals. It was most heavily concentrated in the outer molecular layer after GABA uptake, over the inner molecular layer after glutamic acid, and more evenly spread over the molecular layer, fusiform cell layer, and deep DCN after glycine. Labeling with aspartic acid was unimpressive. There was diffuse labeling over all of the DCN after incubation with taurine and alanine and, to a lesser extent, with aspartic acid. After incubation with taurine a few cells in the molecular layer were labeled; following alanine more and different cells (probably glia) were labeled. The distribution of labeled synaptic terminals can be correlated with the differential distribution of different classes of axonal input to the DCN.

Preferential glutamine uptake by cochlear hair cells: implications for the afferent cochlear transmitter

The cochlear uptake of amino acids which are putative neurotransmitters, or closely-related compounds, was examined autoradiographically in the gerbil. Hair cells showed no preferential uptake of most compounds tested. However, preferential accumulation of glutamine by cochlear hair cells was striking. Vestibular hair cells showed no affinity for this amino acid. Glutamine uptake by cochlear hair cells may play an important role in afferent synaptic transmission, by providing transmitter precursor and/or by clearing the synaptic cleft.

Central projections of primary vestibular fibers in the bullfrog: I. The vestibular nuclei†

The central projections of the vestibular end organs in the bullfrog Rana calesbeiana were analyzed by using horseradish peroxidase labeling of the primary vestibular afferents. Separate extracellular injections were made of Lhe anterior branch, the posterior branch, the ampullary nerve of each of the three semicircular canals, and the branch to the saccule.

Central projections of primary vestibular fibers in the bullfrog. II. Nerve branches from individual receptors

The fibers from the nerves innervating each of the three semicircular canals and the saccule were labeled by injecting horseradish peroxidase extracellularly into these nerves. The projections into the various veslibular nuclei of each receptor were studied in transverse sections of the brain stem throughout the vestibular nuclear area. All five vestibular nuclei receive primary afferents throughout their areas. There are differences in the projection patterns of the canals. In the superior and ventral vestibular nuclei, the location of the projections depends on the crista injected. The anterior canal projects ventrally, the horizontal canal centrally, and the posterior canal more dorsally. Each canal, however, sends fibers to all areas, with overlap of fibers from the different cristae. The cerebellar nucleus receives uniform innervation from the three canals. The medial vestibular nucleus in the rostral and caudal areas receives only thin fibers from each canal, with considerable overlap. The descending nucleus in the rostral and caudal areas receives innervation from the cristae, also with considerable overlap, but with greater intensity in the ventral part of the caudal portion of the nucleus. Each crista sends fibers to the cerebellar granular layer and to the base of the cerebellar Purkinje cell layer. These fibers also innervate the reticular formation below the entry zone of the eighth nerve. The saccule innervates both the dorsal (acoustic) and the ventral nuclei, the latter in the most dorsal position. The innervation of the utricle could be ascertained only in the middle section of the descending and the medial nuclei, an area which does not receive significant innervation from the cristae. Primary afferent fibers course in the vestibular tract, forming a longitudinal bundle lateral to the vestibular nuclei. In the bundle the larger fibers are medially situated.

Consumption of a High-Galactose Diet Induces Diabetic-Like Changes in the Inner Ear

Diabetes mellitus is a disease that affects multiple organ systems. In our laboratory it has been shown that there is a significant loss of outer hair cells in genetically diabetic rats. Galactosemia can also produce diabetic-like changes. This study was performed to demonstrate whether these changes also occur in the cochlea. Three groups of Sprague-Dawley rats were used and fed either a control diet, a 50% galactose diet, or a 50% galactose diet with the addition of an aldose reductase inhibitor. After 6 months the animals were killed, and the cochleas were removed, fixed, and stained. Diabetes-induced damage was assessed by counting the hair cells and calculating the neuroganglion cell density. The histopathologic changes induced by galactose were manifested as outer hair cell loss and a decrease in neuroganglion cell density. Control animals had the least amount of hair cell loss and the greatest neuroganglion cell density of all three groups. Galactose-only animals demonstrated the most pronounced changes in both hair cell loss and neuroganglion cell degeneration; however, only changes of neuroganglion cell density in the basal turn were significant. The addition of an aldose reductase inhibitor provided inconclusive results in both hair cell determination and neuroganglion cell density; however, generally the inhibitor partially prevented the damage produced by galactose. These results suggest that a high-galactose diet can induce diabetic-like changes in the cochlea.

The Effect of Surfactant on Eustachian Tube Function in a Gerbil Model of Otitis Media with Effusion

The relationship of eustachian tube surfactant and otitis media with effusion on eustachian tube opening pressure was studied in a gerbil model. Injection of killed Streptococcus pneumoniae bacteria created a serous effusion that increased eustachian tube opening pressure. The introduction of exogenous surfactant to this system resulted in a dramatic decrease in eustachian tube opening pressure in both normal ears and those with effusion. Identifying means to increase surfactant in the eustachian tube could be beneficial in reducing persistent otitis media with effusion.

Preferential amino acid uptake identifies Type II spiral ganglion neurons in the gerbil

The localization of 3H-labeled amino acids was compared by light and electron microscopic autoradiography in the spiral ganglion of the gerbil, following in vivo intracochlear incubations. Following incubations with taurine, a population of heavily labeled neurons could be distinguished from lightly labeled neurons. The heavily labeled population comprised 5-6% of spiral ganglion neurons, and included the least myelinated cells. Ultrastructurally, the heavily labeled neurons were characterized by a loosely coiled Schwann cell sheath covering the cell body, and the presence of abundant cytoplasmic microfilaments. The unlabeled cells showed a typical perikaryal myelin sheath and cytoplasm rich in rough endoplasmic reticulum. It is concluded that the spiral ganglion of the gerbil contains both Type I and Type II neurons, and that Type II neurons preferentially incorporate the amino acid taurine. Type II neurons are therefore biochemically as well as morphologically distinct from Type I neurons.

Axonal Organization in the Cat Medial Superior Olivary Nucleus

Publisher Summary This chapter provides an overview of axonal organization in the cat medial superior olivary nucleus. Axons of elongate cells in the medial nucleus of the trapezoid body (MNTB) have a similar brush-like arborization pattern in the dorsomedial periolivary nucleus. Studies have confirmed that the horizontally oriented candelabra like spread of axons of anterior ventral cochlear nucleus spherical cells and the interrelationships of branches of these axons with more than one adjacent, horizontally oriented sheet of cells. These studies also identified four additional Golgi patterns: one of intrinsic collaterals of axons of medial superior olive (MSO) neurons, two of axons of periolivary neurons, and one belonging to axons of unknown origin. Rostrocaudally running axons ramifying in the MSO fiber zones are observed in both neonatal and adult cats. From their location, they probably make contacts primarily on dendrites or on rostrocaudally elongated cell bodies that have a similar orientation and location. The arborization pattern that has been traced to central cell band cells involves branching within a restricted area within the central band that might be associated with one or more somata and proximal dendrites. Further studies have demonstrated that an efficient mechanism for accumulating label after glycogen incubations in a specific population of synaptic terminals in both the MSO and lateral superior olivary nucleus (LSO). The morphological characteristics of these terminals are consistent with the features known to characterize endings of cochlear nucleus projections.

Nipecotic acid: preferential accumulation in the cochlea by GABA uptake systems and selective retrograde transport to brainstem.

[3H]Nipecotic acid was shown to be preferentially accumulated by the same cochlear structures which selectively accumulate [3H]gamma-aminobutyric acid ([3H]-GABA), including the terminals of a subset of olivocochlear neurons. With both amino acids, olivocochlear fibers selectively transported label in a retrograde direction, from cochlea to brainstem. However, only [3H]nipecotic acid produced dense labeling, and labeling of cell bodies in the superior olive, presumably because it is metabolized very slowly. Nipecotic acid appears to provide a selective retrograde tracer, specific to neurons whose terminals exhibit preferential GABA uptake.