Morphology of the octopus cell area of the cochlear nucleus in young and aging C57BL/6J and CBA/J mice

The influence of aging and age-related cochlear impairment on the ventral cochlear nucleus was evaluated by measuring morphological properties of the octopus cell area (OCA) in five age groups of inbred C57BL/6J and CBA/J mice (young adult to very old). The former strain demonstrates progressive cochlear sensorineural pathology and hearing loss during middle age; the latter has only modest sensorineural pathology late in life. Histological sections of the OCA were evaluated with serial sections and several strains for neurons, glia, and fibers, and Golgi impregnations were also used. Aging was associated with a decrease in volume of the OCA, a loss of neurons, slight decrease in neuron size, increased packing density of glial cells, and changes in dendrites ranging from minor to total loss of primary branches. The greatest changes occurred in extreme old age, beyond the median lifespan. Age-related changes were not exacerbated by sensorineural pathology in aging C57BL/6J mice. Individual octopus cells varied greatly in the extent of age-related abnormality.     

Morphometric study of the anteroventral cochlear nucleus of two mouse models of presbycusis

The dimensions and volume of the anterior ventral cochlear nucleus (AVCN), the density and number of AVCN neurons, and the size of neuronal somata nuclei (in Nissl-stained tissue) were determined in two mouse models of age-related hearing loss: the C57BL/6J strain, which undergoes progressive chronic sensorineural hearing loss with onset during young adulthood, and the CBA/J, which demonstrates only moderate hearing loss with onset late in life. Frontal and horizontal AVCN sections, as well as cochleas, were analyzed in 4 C57 age-groups (1, 7, 12, 19+ months) and in 3 CBA groups (1, 10, 22 months). Within each strain no significant changes in AVCN dimensions or volume occur with aging. In C57 mice, packing density and cell number decrease between 1 and 7 months, but remain stable thereafter, despite chronic severe hearing impairment. CBA mice show a reduction in AVCN cell number and packing density only during the second year of life. In aging C57 mice, the size of spherical and perhaps globular cells increases, whereas the size of multipolar cells tends to decrease slightly. In CBA mice, all three AVCN cell types tend to decrease in size with aging. The early cell loss and cell size increases in C57 mice are most consistent in the dorsal (high frequency) region of the AVCN. Likewise, loss of cochlear spiral ganglion cells is most pronounced in the base of the cochlea, which provides input to this region. The data indicate that aging is associated with rather different central effects, depending on AVCN cell type, cochleotopic organization, genotype, and/or the type of peripheral hearing loss involved. The C57 and CBA AVCNs also differ in several aspects irrespective of age. The volume of AVCN and number of AVCN neurons are significantly greater in C57 mice.     

Morphological changes in the anteroventral cochlear nucleus that accompany sensorineural hearing loss in DBA/2J and C57BL/6J mice

Morphological measurements were made on histological sections of the anteroventral cochlear nucleus (AVCN) in mice of the DBA/2J and C57BL/6J strains to determine the effects of sensorineural cochlear pathology on the number, packing density, and size of neurons and on AVCN volume. Both strains possess alleles that cause progressive cochlear pathology initially affecting the organ of Corti: in DBA mice, hearing loss is evident at 4 weeks of age and progresses rapidly; in C57 mice, hearing loss begins after 2 months of age and progresses more slowly. In both strains AVCN volume decreased, some loss of neurons occurred, and these changes paralleled the progression of peripheral hearing loss. Central changes were rapid in DBA mice, but the ultimate magnitude of the changes in 1-year-old mice did not differ between strains. Both strains differed from well-hearing CBA/J mice which exhibited no changes in the AVCN measures. The findings indicate that pathology of the organ of Corti in adult mice results in degenerative changes in the cochlear nucleus. The data also support earlier findings indicating that, if cochlear pathology does not begin prior to young adulthood, the age of onset and duration of sensorineural impairment have little effect on the ultimate magnitude of central effects.     

Glycine immunoreactivity and receptor binding in the cochlear nucleus of C57BL/6J and CBA/CaJ mice: Effects of cochlear impairment and aging

Glycinergic neurons in the cochlear nucleus (CN) of C57BL/6J (C57) and CBA/CaJ (CBA) mice were studied by using immunocytochemical and receptor-binding techniques. Adult C57 mice exhibit progressive cochlear pathology as they age, whereas aging CBA mice retain good hearing. In the CN of old C57 mice (18 months) with severe hearing loss, the number of glycine-immunoreactive neurons decreased significantly. The number (B_max) of strychnine-sensitive glycine receptors (GlyR) decreased significantly in the dorsal CN of old C57 mice. Significant effects were not observed in the CN of middle-aged C57 mice (with less-severe hearing loss) or in very old CBA mice (which do not exhibit severe hearing loss). The data suggest that the combination of severe hearing loss and old age results in deficits in one or more inhibitory glycinergic circuits in the CN. J. Comp. Neurol. 385:405–414, 1997. © 1997 Wiley-Liss, Inc.     

Plasticity of auditory cortex associated with sensorineural hearing loss in adult C57BL/6J mice

The representation of frequency was mapped in the primary auditory cortex (AI) of C57BL/6J (C57) mice during young adulthood (1.5–2 months) when hearing is optimal, and at 3, 6, and 12 months of age, a period during which progressive, high frequency, sensorineural hearing loss occurs in this strain. Maps were also obtained from CBA/CaJ mice which retain good hearing as they age. In AI of young adult C57 mice, and CBA mice, characteristic frequencies (CFs) of multiple-unit clusters were easily identified with extracellular recordings, and a general tonotopic organization was observed from dorsal (high frequency) to ventral and caudal (low frequency). In individual cases there appeared to be deviations from the above tonotopic organization, despite the fact that inbred mice are genetically invariant. As progressive loss of high frequency sensitivity ensued peripherally, a substantially increased representation of middle frequencies was observed in AI. There was no apparent change in the surface area of the auditory cortex despite the elimination of high frequencies, and virtually the entire auditory cortex became devoted to the middle frequencies (especially 10–13 kHz) for which sensitivity remained high. Similar age-related changes were not observed in normal-hearing CBA mice. These findings indicate that plasticity in the representation of frequency in AI is associated with high frequency hearing loss in C57 mice. © 1993 Wiley-Liss, Inc.     

Cellular correlates of progressive hearing loss in 129S6/SvEv mice

Several strains of mice hear well initially but show progressive sensorineural hearing loss. Affected cochlear cell types include all those known to be affected in human age-related hearing loss (ARHL), or presbycusis. Thus these mice have been offered as models of human ARHL. At present, however, few mouse ARHL models are sufficiently well described to serve as the basis for specific hypotheses about human ARHL. We examined 1-month-old and 15-month-old 129S6/SvEv (129S6) mice and compared them with BALB/cJ and CBA/J mice. Age-related elevation of compound action potential thresholds was interpreted in the light of endocochlear potentials and changes in hair cells, afferent neurons, fibrocytes in spiral limbus and ligament, and supporting cells within the organ of Corti. Aging in 129S6 mice was associated with high-frequency hearing loss. Four components of age-related cochlear degeneration emerged from quantitative analyses, including 1) basal loss of outer hair cells; 2) basal loss of type IV fibrocytes in the spiral ligament; 3) apical loss of fibrocytes in spiral limbus, and 4) anomalies of supporting cells in the cochlear base. Although neuronal loss was not consistently found, two mice showed loss of afferent dendrites and cell bodies in the cochlear apex without inner hair cell loss. Despite multifaceted degeneration, hearing loss in 129S6 mice appears to be best explained by degenerative changes in outer hair cells and in the organ of Corti, conforming to human sensory ARHL. Age-related changes in the apical spiral limbus may promote pathology of the medial organ of Corti and eventual loss of afferent neurons, with possible implications for human neural ARHL.     

Excitability of auditory neurons in the dorsal and ventral cochlear nuclei of and mice

Extracellular response properties were studied in neurons of the dorsal and ventral divisions of the cochlear nucleus (DCN and VCN, respectively) of (DBA) and (C57) mice. Mice of the former inbred strain show susceptibility to audiogenic seizures and have severe high frequency hearing loss when young; mice of the latter strain do not. Whereas afterdischarges had been readily observed in the inferior colliculus of DBA mice in a previous study, they were never observed in the cochlear nucleus. The incidence of nonmonotonic intensity functions, the slopes of intensity functions, and the incidence of inhibition in response areas indicated that inhibition was diminished in the DCN of DBA mice. However, in the VCN, these response properties did not differ between the two strains. There appeared to be an “amplification” of excitability (i.e., attenuation of inhibition) from VCN to DCN to inferior colliculus in DBA mice.     

The cytoarchitecture of the dorsal cochlear nucleus in the 3-month- and 26-month-old C57BL/6 mouse: A golgi impregnation study

The cytoarchitecture of the dorsal cochlear nucleus (DCN) was compared in 3- and 26-month-old C57BL/6 mice. The effects of genetically controlled progressive hearing loss present in the CNS in this mouse strain were analyzed with Nissl-stained and Golgi-impregnated material. The DCN was divided into the superficial molecular, an intermediate fusiform-granule, and the deep polymorphic layers. The molecular layer (ML) consisted of many fibers and a few small ovoid to spherical, fusiform, and granule cells. The fusiform-granule layer (FL) contained large fusiform and many granule cells. Most FL fusiform cells were oriented with their long axes perpendicular to the DCN surface and were present as small aggregations or individually. Cartwheel cells were adjacent to the FL fusiform cells. The deep polymorphic layer (PL) contained spherical, fusiform, granule, and multipolar neurons. The granule cells formed a dorsal cap of the DCN. From this cap, sheets of granule cells separated the DCN from the posterior ventral cochlear nucleus (PVCN) and from the brainstem. The internal organization, neuronal location, orientation, and morphology were similar in both age groups. The granule cells had four to five primary dendrites, varicosities, and few to no dendritic appendages. The FL fusiform cells displayed different dendritic morphology in the two ages. One or two elaborate primary ML apical dendrites in the 3-month-old mice were covered with spikelike dendritic spines. The basal one or two PL dendrites were less elaborate and had few dendrite spines. In contrast, FL fusiform neurons in 26-month-old mice had regular dendritic varicosities and fewer spines which were short and stumpy. Basal dendrites had varicosities and interruptions. Cartwheel neurons in 3-month-old mice had elaborate ML dendritic trees covered with dendritic spines. In 26-month-old mice the dendrites had many varicosities and fewer short blunted dendritic spines. Large multipolar neurons in older mice had thinner dendrities with more varicosities than were in the 3-mcnth group. In both age groups multipolar cells had few dendritic spines limited distally. Small and large spherical cells had two to five primary dendrites with varicosities, little higher-order branching, and spines. Fusiform cells had one or two primary dendrites, little secondary branching, and few to no spines. Minor degenerative changes were noted in spherical and fusiform cells in the two age groups. These included dendritic varicosities, interruptions, and some irregularities of somata surface. Degenerative changes present in the cochlea had significant effects on a limited population of DCN neurons. Finally, the neusronal morphology and architecture of the DCN in C57BL/6 mouse is similar to other mammalian species.     

Age-related changes in calbindin D-28k and calretinin immunoreactivity in the inferior colliculus of CBA/CaJ and C57Bl/6 mice

This study examines calbindin D-28k and calretinin immunoreactivity in the inferior colliculus (IC) of young and old mice of two strains. The CBA/CaJ mouse maintains good hearing until very late in life, whereas the C57Bl/6 strain exhibits severe sensorineural hearing loss at an early age. Young and old mice of both strains were selected with matching auditory brainstem response audiograms and gap detection thresholds. Brain sections were reacted with anti-calbindin D-28k (CB) and anti-calretinin (CR). Staining patterns were characterized and cell counts performed. CB immunoreactivity was high only in the nucleus of the commissure (NCO); counts revealed a 22.3% decrease in the number of CB+ cells in old CBA mice and a 25.1% decrease in old C57 mice. Calretinin immunoreactivity was high in the pericentral regions of the IC, but the central nucleus was devoid of CR+ cells. The dorsal cortex, lateral nucleus, and NCO showed increases of 42.3, 49.0, and 61%, respectively, in the number of CR+ cells, but only in the old CBA mice. No significant change was observed in the old C57 mice. Whereas decreases in CB immunoreactivity are common with age, this study is the first to report an age-related increase in CR immunoreactivity in the auditory system. The increase in CR+ cells is a possible compensatory adaptation to the decrease in CB+ cells. That the number of CR+ cells remains constant with age in C57 mice suggests this compensation may depend upon stimulus-driven activity, but this requires further study. J. Comp. Neurol. 386:92–110, 1997. © 1997 Wiley-Liss, Inc.     

Apical-to-basal gradients in age-related cochlear degeneration and their relationship to "primary" loss of cochlear neurons

The predominant conceptual framework for understanding human age-related hearing loss (ARHL, or presbycusis) holds that three different cochlear elements (organ of Corti, afferent neurons, and stria vascularis) can degenerate independently, and exert independent influences on hearing. Within this framework, temporal bones from subjects with ARHL may be classified as exemplifying sensory (referring to organ of Corti), "primary" neural (loss of afferent neurons without loss of their hair cell targets), strial, or mixed ARHL. While there is general agreement as to the types of cochlear cells most affected by aging, there is less agreement about how to classify ARHL, and whether contributions of particular structures to hearing loss can be isolated. The cochlear apex of humans and animals is particularly prone to apparent primary loss of neurons that may represent an aspect of neural ARHL. We recently reported that in 129S6/SvEv mice apical neuronal loss is often accompanied by abnormalities of spiral limbus, pillar cells, and Reissner's membrane (Ohlemiller and Gagnon [2004] J Comp Neurol 469:377-390). We proposed that the initial pathology occurs within limbus, leading to disruption of perilymphatic ion homeostasis, and eventual loss of neurons as one consequence. We have now examined this issue quantitatively in young and old mice of four different strains (129S6/SvEv, CBA/J, C57BL/6, and BALB/c). Abnormalities of apical spiral limbus were found to correlate only weakly with neuronal loss. Strong correlations were found between neuronal loss and abnormalities of both pillar cells and Reissner's membrane, however. Apical neuronal loss and apical-to-basal progression of pathology of limbus, pillar cells, and Reissner's membrane run counter to most reported age-related cochlear trends. Our findings suggest that these changes share a common triggering influence.     

Excitability of auditory neurons in the dorsal and ventral cochlear nuclei of DBA2 and C57BL6 mice

Extracellular response properties were studied in neurons of the dorsal and ventral divisions of the cochlear nucleus (DCN and VCN, respectively) of $\text{DBA}\text{2}$ (DBA) and $\text{C57BL}\text{6}$ (C57) mice. Mice of the former inbred strain show susceptibility to audiogenic seizures and have severe high frequency hearing loss when young; mice of the latter strain do not. Whereas afterdischarges had been readily observed in the inferior colliculus of DBA mice in a previous study, they were never observed in the cochlear nucleus. The incidence of nonmonotonic intensity functions, the slopes of intensity functions, and the incidence of inhibition in response areas indicated that inhibition was diminished in the DCN of DBA mice. However, in the VCN, these response properties did not differ between the two strains. There appeared to be an “amplification” of excitability (i.e., attenuation of inhibition) from VCN to DCN to inferior colliculus in DBA mice.     

Aging and presbycusis: effects on 2-deoxy-D-glucose uptake in the mouse auditory brain stem in quiet

Autoradiography was used to assess the incorporation of [2-14C]deoxy-D-glucose by the auditory brain stem of young and aging mice of the C57BL/6 strain (which demonstrates progressive chronic sensorineural hearing loss) and the CBA strain (which maintains good hearing until late in life). Animals were injected with labeled 2-deoxyglucose and placed in quiet for 45 min; brain stem sections were prepared for autoradiography. The amounts of 2-deoxyglucose incorporated into the anterior ventral cochlear nucleus (AVCN), inferior colliculus (IC), and trigeminal nerve (TN) were densitometrically analyzed. Within each subject, the densities of the three structures were statistically compared. In every mouse, inferior colliculus density was greater than that of the anterior ventral cochlear nucleus, which was greater than trigeminal nerve density. To compare subject groups, relative densities (inferior colliculus and anterior ventral cochlear nucleus re: trigeminal nerve) were used; no significant differences were found between groups. Thus, aging, with or without severe loss of hearing, is not associated with altered incorporation of 2-deoxyglucose (and presumably glucose) in quiet.     

Effects of aging and sensory loss on glial cells in mouse visual and auditory cortices

Normal aging is often accompanied by a progressive loss of receptor sensitivity in hearing and vision, whose consequences on cellular function in cortical sensory areas have remained largely unknown. By examining the primary auditory (A1) and visual (V1) cortices in two inbred strains of mice undergoing either age-related loss of audition (C57BL/6J) or vision (CBA/CaJ), we were able to describe cellular and subcellular changes that were associated with normal aging (occurring in A1 and V1 of both strains) or specifically with age-related sensory loss (only in A1 of C57BL/6J or V1 of CBA/CaJ), using immunocytochemical electron microscopy and light microscopy. While the changes were subtle in neurons, glial cells and especially microglia were transformed in aged animals. Microglia became more numerous and irregularly distributed, displayed more variable cell body and process morphologies, occupied smaller territories, and accumulated phagocytic inclusions that often displayed ultrastructural features of synaptic elements. Additionally, evidence of myelination defects were observed, and aged oligodendrocytes became more numerous and were more often encountered in contiguous pairs. Most of these effects were profoundly exacerbated by age-related sensory loss. Together, our results suggest that the age-related alteration of glial cells in sensory cortical areas can be accelerated by activity-driven central mechanisms that result from an age-related loss of peripheral sensitivity. In light of our observations, these age-related changes in sensory function should be considered when investigating cellular, cortical, and behavioral functions throughout the lifespan in these commonly used C57BL/6J and CBA/CaJ mouse models.     

Effects of prolonged exposure to an augmented acoustic environment on the auditory system of middle-aged C57BL/6J mice: cochlear and central histology and sex differences

Genetic progressive sensorineural hearing loss in mice of the C57BL/6J (B6) inbred strain begins at high frequencies during young adulthood and is severe by 12 months (middle age). Nightly treatment with an augmented acoustic environment (AAE)--12-hour periods of exposure to repetitive noise bursts of moderate intensity, begun at age 25 days--resulted in less severe hearing loss compared with control mice. Cochlear histopathological correlates of AAE treatment, assessed at 12-14 months of age, included lessened severity of progressive loss of outer hair cells in both sexes as well as small savings of spiral ganglion cells in females and inner hair cells in males. AAE effects on the number of surviving neurons (age 12-14 months) in the anterior ventral cochlear nucleus (AVCN) depended on sex. Compared with controls, the loss of AVCN neurons that typically accompanies the initial period of hearing loss (between 2 and 7 months of age) was not significantly affected by AAE treatment in females. In contrast, males treated with the AAE exhibited more severe loss of neurons in the dorsal and ventral extremes of the AVCN than male controls of the same age. AAE treatment begun at age 3-5 months resulted in significant but less severe loss of AVCN neurons in 1-year-old male mice.     

Auditory efferent feedback system deficits precede age-related hearing loss: contralateral suppression of otoacoustic emissions in mice

The C57BL/6J mouse has been a useful model of presbycusis, as it displays an accelerated age-related peripheral hearing loss. The medial olivocochlear efferent feedback (MOC) system plays a role in suppressing cochlear outer hair cell (OHC) responses, particularly for background noise. Neurons of the MOC system are located in the superior olivary complex, particularly in the dorsomedial periolivary nucleus (DMPO) and in the ventral nucleus of the trapezoid body (VNTB). We previously discovered that the function of the MOC system declines with age prior to OHC degeneration, as measured by contralateral suppression (CS) of distortion product otoacoustic emissions (DPOAEs) in humans and CBA mice. The present study aimed to determine the time course of age changes in MOC function in C57s. DPOAE amplitudes and CS of DPOAEs were collected for C57s from 6 to 40 weeks of age. MOC responses were observed at 6 weeks but were gone at middle (15-30 kHz) and high (30-45 kHz) frequencies by 8 weeks. Quantitative stereological analyses of Nissl sections revealed smaller neurons in the DMPO and VNTB of young adult C57s compared with CBAs. These findings suggest that reduced neuron size may underlie part of the noteworthy rapid decline of the C57 efferent system. In conclusion, the C57 mouse has MOC function at 6 weeks, but it declines quickly, preceding the progression of peripheral age-related sensitivity deficits and hearing loss in this mouse strain.     

Projections from the anterior ventral cochlear nucleus to the central nucleus of the inferior colliculus in young and aging C57BL/6 mice

Projections from the anterior ventral cochlear nucleus (AVCN) to the central nucleus of the inferior colliculus (ICC) were studied in young and aging C57BL/6 mice. The latter animals demonstrate progressive loss of hearing. Wheat germ agglutinin-horseradish peroxidase (WGA-HRP) was injected into the inferior colliculus and retrograde transport to the AVCN sections, quality of labelling, number of labelled neurons adjusted for injection size, or topographic organization of projections. Thus, despite progressing loss of auditory sensitivity, chronic profound hearing loss (oldest animals), and aging, projections from AVCN to ICC remain stable.     

Aging and presbycusis: Effects on 2-deoxy-d-glucose uptake in the mouse auditory brain stem in quiet

Autoradiography was used to assess the incorporation of [2-¹⁴C]deoxy-d-glucose by the auditory brain stem of young and aging mice of the strain (which demonstrates progressive chronic sensorineural hearing loss) and the CBA strain (which maintains good hearing until late in life). Animals were injected with labeled 2-deoxyglucose and placed in quiet for 45 min; brain stem sections were prepared for autoradiography. The amounts of 2-deoxyglucose incorporated into the anterior ventral cochlear nucleus (AVCN), inferior colliculus (IC), and trigeminal nerve (TN) were densitometrically analyzed. Within each subject, the densities of the three structures were statistically compared. In every mouse, inferior colliculus density was greater than that of the anterior ventral cochlear nucleus, which was greater than trigeminal nerve density. To compare subject groups, relative densities (inferior colliculus and anterior ventral cochlear nucleus re: trigeminal nerve) were used; no significant differences were found between groups. Thus, aging, with or without severe loss of hearing, is not associated with altered incorporation of 2-deoxyglucose (and presumably glucose) in quiet.     

Modulating calbindin and parvalbumin immunoreactivity in the cochlear nucleus by moderate noise exposure in mice.: A quantitative study on the dorsal and posteroventral cochlear nucleus

The number of calbindin D-28k and parvalbumin immunoreactive (IR) neurons were characterized on sections from the cochlear nucleus, dorsal cochlear nucleus (DCN) and posteroventral cochlear nucleus (PVCN) using two-dimensional quantification. After noise exposure (6–12 kHz, 2 h, at either 80 dB SPL or 103 dB SPL), the number of calbindin and parvalbumin immunoreactive neurons increased in CBA/CBA mice. Quantitative analysis of calbindin-IR in the PVCN did not show a statistically significant difference between any of the groups, whereas statistically significant differences in calbindin-IR were found in the DCN for the 103 dB and 80 dB group compared to the control group, and 103 dB compared to the 80 dB group, respectively. A statistically significant increase in the number of parvalbumin-IR neurons in the PVCN and the DCN was evident in the 103 dB and 80 dB group compared to the control group, and in the 103 dB compared to the 80 dB group. The data indicate that increasing sound stimulation causes a graded increase in the expression of calcium-binding protein immunoreactivity in the DCN and PVCN neurons and neuropil. This increase of protein expression is due to increased positive immunoreactivity in `silent' neurons. These findings implicate that these neurons have the possibility to react against trauma and display calbindin or parvalbumin as a rescue event. The ability to map sound-induced calcium-binding protein changes in auditory neurons may be useful in future studies designed for detecting early patterns of neurodegeneration and neuroprotection in the central auditory pathway.     

Two types of afferent terminals innervate cochlear inner hair cells in C57BL/6J mice

Afferent synapses on inner hair cells (IHC) transfer auditory information to the central nervous system (CNS). Despite the importance of these synapses for normal hearing, their response to cochlear disease and dysfunction is not well understood. The C57BL/6J mouse is a model for presbycusis and noise-induced hearing loss because of its age-related hearing loss and susceptibility to acoustic over-exposure. In this context, we sought to establish normal synaptic structure in order to better evaluate synaptic changes due to presbycusis and noise exposure. Ultrastructural analysis of IHCs and afferent terminals was performed in a normal hearing 3-month-old C57BL/6J mouse at cochlear sites corresponding to 8, 16 and 32 kHz using semi-serial sections. A stereologic survey of random sections was conducted of IHCs in 11 additional mice. Two morphologically distinct groups of afferent terminals were identified at all 3 frequency locations in 11 out of 12 animals. "Simple" endings demonstrated classic features of bouton terminals, whereas "folded" endings were larger in size and exhibited a novel morphologic feature that consisted of a fully internalized double membrane that partially divided the terminal into two compartments. In many cases, the double membrane was continuous with the outer terminal membrane as if produced by an invagination. We still must determine the generality of these observations with respect to other mouse strains.     

Glycine response in isolated dorsal cochlear nucleus of C57BL/6J mouse

Pharmacological properties of glycine (Gly)-induced Cl- current (ICl) in the dorsal cochlear nucleus (DCN) neurons acutely dissociated from C57BL/6J mouse were investigated in the whole-cell configuration of the patch-clamp technique. Gly-induced ICl increased in a sigmoidal manner with higher Gly concentrations. Strychnine blocked the Gly response competitively at low and non-competitively at high concentrations. Both glutamate (Glu) and N-methyl-D-aspartate (NMDA) responses were augmented by adding 10(-6) M Gly, at which concentration Gly did not induce any ICl. This facilitation was not affected by strychnine. Our results clearly show the existence of strychnine-sensitive and -insensitive glycine receptors in the DCN neurons.