Biomedical subjects

R H Helfert

Publications and source records attributed to R H Helfert.

23 records · Page 2Linked to original sources

PubMed1988

Ultrastructural characterization of gerbil olivocochlear neurons based on differential uptake of 3H-D-aspartic acid and a wheatgerm agglutinin-horseradish peroxidase conjugate from the cochlea.

Two populations of olivocochlear (OC) neurons have been identified in the gerbil brain stem on the basis of differential labeling patterns of 3H-D-aspartic acid (D-ASP) and wheatgerm agglutinin-horseradish peroxidase conjugate (WGA/HRP) from the cochlear perilymph. While both populations are capable of uptake and retrograde uptake of WGA/HRP, one population accumulates and retrogradely transports D-ASP (D-ASP OC neurons) and the other does not (non-D-ASP OC neurons). D-ASP OC neurons are found in or near the lateral superior olive, are small in size, and receive very few synaptic contacts. The vast majority of these synapses contain small, mildly pleomorphic vesicles with scattered dense core vesicles. Synapses with distinctly larger pleomorphic vesicles have also been observed. These neurons possess all of the features common to neurons of the lateral olivocochlear system. Non-D-ASP OC neurons are found primarily in the ventral nucleus of the trapezoid body, as well as in the area between the medial superior olive and the medial nucleus of the trapezoid body. These neurons are larger and receive greater numbers and types of synaptic contacts than those found on D-ASP OC neurons. The 2 most common synapses found on non-D-ASP OC neurons are axosomatic ones containing small, mildly pleomorphic vesicles and scattered dense core vesicles similar to those seen on the D-ASP OC neurons, and axodendritic synapses containing large, round vesicles. Much less frequently observed are synapses containing small, round vesicles or ones containing predominantly flat vesicles. The ultrastructural features of the non-D-ASP OC neurons correspond to those described for neurons of the medial olivocochlear system.

Animals↗

PubMed1987

Selective retrograde labeling of lateral olivocochlear neurons in the brainstem based on preferential uptake of 3H-D-aspartic acid in the cochlea.

We have previously shown that perfusion of the gerbil cochlea with probe concentrations of 3H-D-aspartic acid (D-ASP) results in immediate, selective labeling of 50-60% of the efferent terminals under the inner hair cells, presumably by high-affinity uptake. The present study was undertaken to determine the origin of these endings. Twenty-four hours after cochlear perfusion with D-ASP, labeled neurons were observed in the ipsilateral, and to a much lesser extent in the contralateral, lateral superior olivary nucleus (LSO). The cells were small, primarily fusiform, and showed fewer synaptic contacts than other LSO cells. Combined transport of D-ASP and horseradish peroxidase indicated that all olivocochlear neurons within the LSO that projected to the injected cochlea were labeled by D-ASP. Labeled fibers coursed dorsally from the LSO, joined contralateral fibers that had passed under the floor of the fourth ventricle, and entered the VIIIth nerve root at its ventromedial edge. Adjacent to the ventral cochlear nucleus (VCN), densely labeled collateral fibers crossed the nerve root to enter the VCN. Labeled fibers and terminals were prominent in the central VCN. Neither retrograde transport of D-ASP by medial olivocochlear and vestibular efferents nor anterograde transport by VIIIth nerve afferents was observed. The D-ASP-labeled cells and fibers are clearly lateral olivocochlear efferents. Retrograde transport of D-ASP thus allows the cells, axons, and collaterals of the lateral olivocochlear system to be studied, morphologically, in isolation from other cells that project to the cochlea. Since the olivocochlear neurons are almost certainly cholinergic, retrograde amino acid transport does not necessarily identify the primary neurotransmitter of a neuron. Rather, it indicates the presence of selective uptake by the processes of that neuron at the site of amino acid injection. Retrograde labeling appears to be markedly enhanced by the use of metabolically inert compounds such as d-isomer amino acids.

Animals↗

PubMed1987

Morphological features of five neuronal classes in the gerbil lateral superior olive.

Five morphologically distinct classes of neurons can be identified within the neuropil of the gerbil lateral superior olivary nucleus (LSO) by using a variety of histological techniques and electron microscopy. The physical features of these five classes resemble those found in the cat LSO and are identified, by using criteria and nomenclature established for the cat, as principal neurons, multiplanar neurons, marginal neurons, small neurons, and class 5 neurons. Principal cells compose approximately 75% of the total LSO neuronal population. They possess a discoid dendritic organization and are oriented rostrocaudally, perpendicular to the transverse curvatures of the LSO. Roughly 8% of the LSO population is composed of multiplanar neurons, whose dendritic fields are not restricted to any single plane of section. Both principal and multiplanar neurons share similar cytoplasmic features, and greater than 65% of their perikaryal surface is in contact with synaptic terminals. Small neurons compose approximately 11% of the LSO neurons, have the lowest percentage of their somal surface contacted by synaptic terminals (approximately 8%), and are found mostly in the middle/medial portions of the LSO. Marginal neurons, which compose approximately 6% of the LSO population, appear similar to principal neurons at the light microscopic level except that they are found along the contours of the LSO, oriented orthogonal to principal neurons. Approximately 28% of the somal surface of marginal neurons is in contact with synaptic terminals. The class 5 neuronal somata receive a similar number of axosomatic synaptic contacts as marginal neurons (approximately 31%) but are found well within the matrix of the LSO, aligned parallel to principal neurons. Class 5 neurons share the same light microscopic features as principal neurons and can be identified electron microscopically based only on the reduced percentage of somal surface occupied by synaptic terminals.

Animals↗

PubMed1986

Morphological evidence for the existence of multiple neuronal classes in the cat lateral superior olivary nucleus.

This study characterizes morphologically the neurons residing within the matrix of the cat lateral superior olive (LSO), excluding the hili and myelinated axon envelope. Several light microscopic techniques including Golgi impregnations, Nissl stains, and acetylcholinesterase histochemistry were used, as well as electron microscopy. Five distinct classes of neurons have been identified: principal neurons, multiplanar neurons, marginal neurons, small neurons, and class 5 neurons. These neuronal classes differ in regard to their size and shape, dendritic organization, perikaryal synaptic density, and their relative numbers. Principal neurons compose approximately three-quarters of the LSO neurons. They are multipolar and uniplanar in their dendritic arborization, radiating from the hili in rostrocaudal planes perpendicular to the curvatures of the LSO. In transverse sections the principal cell perikarya are fusiform and bipolar, with mean dimensions of 23 X 11 microns. More than 60% of the surface of these cells is contacted by synaptic terminals. Multiplanar neurons (averaging 23 X 19 microns) compose only 11% of the LSO neuronal population. Their dendritic arborization is not restricted to any particular plane, and their somal surface receives synaptic contacts similar, in number and type, to principal cells. Marginal neurons, although they are similar to principal neurons in shape and dendritic arborization, differ in that they are generally smaller (averaging 20 X 10.5 microns). They also possess fewer axosomatic synaptic contacts (approximately 33%), are oriented perpendicularly to principal neurons, are limited in distribution to the contours of the LSO immediately beneath the myelinated axon envelope, and constitute only 4% of the neuronal population. Small neurons (mean dimensions = 9 X 8 micron) compose 8% of the LSO neurons. They possess a multiplanar array of primary dendrites and have nuclei with multiple deep infoldings. Small neurons have the fewest axosomatic synaptic contacts of all classes of LSO neurons (approximately 10%). Additionally, there are neurons that are similar to principal neurons, but receive fewer axosomatic contacts (approximately 33%). These cells have been tentatively identified as class 5 neurons until more information on this type allows for the assignment of a more descriptive name. A number of acetylcholinesterase-positive neurons are also found within the LSO, whose relationship to the other classes of neurons is presently unresolved. Possible functions of the multiple neuronal types are discussed.

Acetylcholinesterase↗

PubMedDate not supplied

Central auditory aging: GABA changes in the inferior colliculus.

Age-related hearing loss (presbycusis) is a complex state that reflects pathologic changes along the entire auditory neuraxis. Loss of speech understanding, decreased ability to localize sounds, and a decreased ability to detect and extract signals in noise are characteristic problems encountered by the elderly. Central (neural) presbycusis frequently results in a dramatic loss in speech understanding without a parallel change in pure-tone thresholds. In spite of evidence that suggests these deficits cannot be fully explained by peripheral changes alone, few studies have examined the neurochemical basis of central auditory dysfunction in aging. Age-related alterations in neural circuits involved in the processing of acoustic information could reflect changes in the synthesis, degradation, uptake, release, and receptor sensitivity of neurotransmitters, perhaps secondary to cell loss and/or progressive deafferentation. A series of studies designed to test this hypothesis has examined aging in the central auditory system of the F344 rate. Age-related changes associated with GABA neurotransmitter function in an important auditory midbrain structure, the inferior colliculus, have been investigated. These studies found: (1) decreased numbers of GABA immunoreactive neurons; (2) decreased basal levels (concentrations) of GABA; (3) decreased GABA release; (4) decreased glutamic acid decarboxylase activity; (5) decreased GABAB receptor binding; (6) decreased numbers of presynaptic terminals; and (7) subtle GABAA receptor binding changes. Collectively, these age-related changes suggest altered GABA neurotransmitter function in the IC. Identification of specific neurotransmitter changes in structures important in speech processing could eventually lead to the development of pharmacotherapy for selective types of age-related hearing loss.

Aging↗