Laura Schweitzer

Glutamate neurotoxicity in the developing rat cochlea: physiological and morphological approaches

The neurotoxic effects of exogenous glutamate were studied in the rat cochlea. Glutamate-treated rats (4 g/kg/day i.p., postnatal days 2-9) exhibited electrophysiologically-measured elevations in high frequency thresholds usually associated with hair cell loss in the basal region of the cochlea. While surface preparations of the organ of Corti revealed no loss of hair cells, there was a dramatic and selective reduction of neurons in the basal, high frequency-related portion of the spiral ganglion. This sensitivity of developing spiral ganglion cells to the neurotoxicity of glutamate is consistent with the hypothesis that glutamate or a structurally related substance is a neurotransmitter at afferent synapses of cochlear hair cells.

Anatomical correlates of the passive properties underlying the developmental shift in the frequency map of the mammalian cochlea

As the cochlea develops, the cells in the basal cochlea become sensitive to progressively higher frequencies. To identify features of cochlear morphology that may underlie the place code shift, measurements of infant and adult gerbil cochleas were made at both the light and electron microscopic levels. The measurements included areas of the cochlear duct, basilar membrane, and organ of Corti, height and width of the basilar membrane, thickness of the tympanic cover layer, thickness of the upper and lower basilar membrane fiber bands, and optical density of the basilar membrane. The results indicated that basilar membrane dimensions do not change as the place code shifts and that regions that code for the roughly the same frequency (e.g., approximately 11.2 kHz) at different ages can have basilar membranes of very different dimensions. In contrast, the size of the organ of Corti and the thickness of fiber bands inside the basilar membrane do change in ways consistent with the shift in the frequency map.

Development of gerbil outer hair cells after the onset of cochlear function: An ultrastructural study

It has recently been proposed that elements which contribute to active cochlear processes develop at the same time (between postnatal day (PND) 12 and 21) as the shift of the place code in the developing gerbil cochlea. Since outer hair cells (OHCs) have been implicated in these processes, we have hypothesized that developing OHCs will exhibit changes in anatomical features that contribute to cochlear maturation. Our results demonstrate that the ultrastructural characteristics of OHCs change after the onset of hearing (PND 12), during the time that cochlear nonlinearities are being established (PND 12-21). Differences are primarily associated with the distribution of cytoplasmic organelles. The subsurface cisternae (SSC), which are thought to be related to the mechanical support of the outer hair cell, to cell motility, and therefore to cochlear mechanics, are present at PND 10 but remain immature, with cisternal layers added during the preweanling period. In immature OHCs, more mitochondria are centrally-located than in mature OHCs. During development mitochondria come to form a continuous row near the innermost leaflet of the SSC. These ultrastructural features undergo rapid change during the maturation of peripheral auditory function.

Structure and function of gustatory neurons in the nucleus of the solitary tract: II. Relationships between neuronal morphology and physiology

This study employed intracellular recording and labeling techniques to examine potential relationships between the physiology and morphology of brainstem gustatory neurons. When we considered the neuronal response to the four “prototypic” tastants, we were able to demonstrate a positive correlation between breadth of responsiveness and the number of dendritic branch points. An analysis of the response to eight tastants also revealed an association between dendritic spine density and the breadth of responsiveness, with more narrowly tuned neurons exhibiting more spines. Interestingly, a neurons “best response” was a relatively poor predictor of neuronal morphology. When we focused on those neurons that responded to only one tastant, however, a number of potentially important relationships became apparent. We found that the cells that only responded to quinine were smaller than the neurons that only responded to NaCl, HCl, or sucrose. The HCl‐only neurons, however, were more widespread in the rostrocaudal dimension than the neurons that only responded to NaCl. A number of additional structure‐function relationships were identified when we examined the neuronal response to selected tastants. We found that neurons that responded to sucrose but not quinine, as well as neurons that responded to quinine but not sucrose, were more widespread in the mediolateral dimension than neurons that responded to both sucrose and quinine. We also discovered that the neurons that responded to NaCl, but not to NH4Cl or KCl, were larger than neurons that responded to all three salts. We believe that these results support the hypothesis that there are relationships between the structure and function of gustatory neurons in the nucleus of the solitary tract, with the data highlighting the importance of three themes: 1) the relationship between dendritic specializations and tuning, 2) the relationship between dendritic arbor orientation and response properties, and 3) the potential importance of stimulus‐specific neurons.

Structure and function of gustatory neurons in the nucleus of the solitary tract. IV. The morphology and synaptology of GABA-immunoreactive terminals

In the visual, auditory and somatosensory systems, insight into the synaptic arrangements of specific types of neurons has proven useful in understanding how sensory processing within that system occurs. The neurotransmitter GABA is present in the nucleus of the solitary tract and based on the fact that the vast majority of cells respond to GABA, its agonists and antagonists, and that over 45% of synaptic terminals in the rostral subdivision of the nucleus of the solitary tract are GABA-immunoreactive, GABA is thought to play an important role in gustatory processing. The following study was carried out to establish the distribution of GABA-immunoreactive terminals within the nucleus of the solitary tract. Specifically, the distribution on to physiologically-identified gustatory neurons was determined using post-embedding electron immuno-histochemistry. GABA-immunoreactive terminals synapse with gustatory neuronal somata and all portions of their dendrites, but non-GABAergic terminals synapse only with distal dendrites of the gustatory cells and on to correspondingly small unidentified dendritic profiles in the neuropil. There is a differential distribution of two subtypes of GABA-immunoreactive terminals on to proximal and distal portions of the gustatory neurons as well. Finally, a model for the synaptic arrangements involving gustatory and GABAergic neurons is proposed.

Cell types in the rostral nucleus of the solitary tract

The rostral subdivision of the nucleus of the solitary tract (rNST) is not laminated or otherwise organized into clearly segregated cell types. Although a variety of experimental approaches have yielded a wealth of information, the definition of cell types in this nucleus has been difficult, as reflected in the sometimes contradictory literature on morphological cell typing. The present review discusses how rNST neurons have been classified in the past and adds to the evidence that distinct neuron types exist in this nucleus. Consistencies in the literature, as well as inconsistencies among studies, are discussed. Furthermore, we have included a summary of our own results that help provide additional data relevant to cell typing. The definition of cell types in other central nervous system nuclei has helped our understanding of the organization of these nuclei and our understanding of the relationships between the morphology and function of neurons. It is hoped that this synthesis of the extant literature will facilitate the many ongoing efforts to correlate neuronal morphology and physiology in the gustatory system.

Morphology of HRP-labelled cochlear nerve axons in the dorsal cochlear nucleus of the developing hamster

To study the development of the central terminal arbors of the cochlear nerve fibers in the dorsal cochlear nucleus, horseradish peroxidase-labelled axons in young and adult hamsters were analyzed morphometrically. Brainstem slices with whole cochlear nuclei were maintained in a slice chamber and the cochlear nerve root was injected with a mixture of wheat germ agglutinin-horseradish peroxidase, horseradish peroxidase and poly-L-ornithine. The poly-L-ornithine was added to keep the injection site small; small injections resulted in only a few axons being labelled and permitted reconstruction of individual fibers. Axons underwent an initial period of ingrowth that was completed prior to the onset of hearing (postnatal day 16). After this time the morphology and area of influence of the axons remained unchanged but the nucleus continued to increase in size. Since no additional cochlear nerve axons grow into the nucleus during this period of nuclear growth, the existing axons necessarily become more widely spaced as development proceeds. These anatomical changes may contribute to the progressive narrowing of auditory cell tuning curves.

Actin distribution along the lateral wall of gerbil outer hair cells

Outer hair cells can contract parallel to their long axis, and it has been hypothesized that actin may play a role in this contraction. In this study, actin distribution was examined in the gerbil organ of Corti using postembedment immunoelectron microscopy. In addition to regions typically labelled by actin antibodies and observed by epifluorescence--the cuticular plate, stereocilia, and supporting cell processes--these procedures preserved the ultrastructure of the cell and allowed us to demonstrate actin reactivity along the lateral wall of the outer hair cells between the subsurface cisterns and the plasma membrane. This region is the location of structures (pillars and cortical cytoskeleton) though to be associated with contraction of the outer hair cells.

Details of the central projections of the cochlear nerve in the hamster revealed by the fluorescent tracer DiI

A new fluorescent tracer, DiI, may be used postmortem in perfused animals making placement in small structures such as the cochlea much less difficult. We have mapped cochlear nerve terminations in the cochlear nucleus with DiI and, using three-dimensional reconstructions, have demonstrated the topography and geometry of the cochlear input. By placing DiI in two regions of the cochlea, we have demonstrated that terminations from the cochlea form stacked sheets of inputs. If enough closely situated ganglion cells were labeled in the cochlea, a well-delineated stripe of label could be found in each section of the cochlear nucleus up to the superficial granule cell region. Here a small extension of label, separate from the stripe may represent the medial olivocochlear axons. When fewer axons were labelled in the cochlear nerve, the sheets were absent and discontinuous patches of label were found. These patches apposed rostrocaudally to form bands of label that run orthogonal to the tonotopic organization.

Changes in GABA-immunoreactivity during development of the rostral subdivision of the nucleus of the solitary tract

GABA plays an important role in the processing of gustatory information in the rostral nucleus of the solitary tract. The following study used post-embedment immunohistochemistry in the rat brainstem to localize GABA at both the light and electron microscopic levels to characterize the developmental distribution of GABA and synaptogenesis of GABA-immunoreactive terminals in the rostral nucleus of the solitary tract. During the first postnatal week, GABA is present in the rostral nucleus of the solitary tract, but less of it is synaptic than any time later in development. Of the few synaptic terminals present at postnatal day 1, less than 20% are GABA-immunoreactive. This proportion more than doubles to reach adult levels by postnatal day 10. By weaning (postnatal day 20), GABA-immunoreactive cells are found in nearly the same density as in the adult. Development continues after weaning and is characterized by a disproportionate loss of non-GABA-containing cells. Finally, one previously identified subtype of GABA-immunoreactive terminal matures very late during the postweaning phase of development. The study provides the first analysis of the development of GABA-related circuitry in the rostral nucleus of the solitary tract using anatomical methods. These data provide the background with which to view the emerging physiology of developing taste neurons.