Aberrant projection induced by otocyst removal maintains normal tonotopic organization in the chick cochlear nucleus.

Nucleus magnocellularis (NM), a second-order nucleus in the chick auditory system, is topographically and tonotopically organized. The basilar papilla (cochlea) projects onto the ipsilateral NM via the auditory nerve. The anteromedial region of NM is innervated by the proximal end of the basilar papilla and responds to high-frequency sounds; more posterolateral regions receive input from more distal locations along the papilla and respond to progressively lower frequencies. NM projects exclusively to the third-order neurons of nucleus laminaris (NL). Otocyst removal prevents the formation of the ipsilateral cochlea and cochlear nerve and results in the development of an aberrant functional projection from the contralateral NM to the "deafferented" NM on the operated side of the brain (Jackson and Parks, 1988). In the present experiment, the otocyst was removed unilaterally and the tonotopic organization of the deafferented NM was physiologically mapped at 17-18 d of embryonic age (E17-E18). Quantitative analyses revealed that the frequency organization of the deafferented NM is almost identical to that in normal embryos. Progressively higher characteristic frequencies were recorded at successively more anterior and more medial locations in the nucleus, and the orientation of the tonotopic axis was indistinguishable from normal. Furthermore, the correlation between characteristic frequency and anatomical location is comparable in the deafferented (r = 0.91) and normal (r = 0.87) NM. The only noticeable discrepancy is that characteristic frequencies in NM on both sides of the brain of operated embryos are higher than the frequencies observed previously at comparable regions of the nucleus in unoperated controls.(ABSTRACT TRUNCATED AT 250 WORDS)     

Development of the time coding pathways in the auditory brainstem of the barn owl

The barn owl's head grows after hatching, causing interaural distances to more than double in the first 3 weeks posthatch. These changes expose the bird to a constantly increasing range of interaural time cues. We have used Golgi and ultrastructural techniques to analyze the development of the connections and cell types of the nucleus magnocellularis (NM) and the nucleus laminaris (NL) with reference to the growth of the head. The time coding circuit is formed but immature at the time of hatching. In the month posthatch, the auditory nerve projection to the NM matures, and appears adult-like by posthatch day (P)21. NM neurons show a late growth of permanent dendrites starting at P6. Over the first month, these dendrites change in length and number, depending upon rostrocaudal position, to establish the adult pattern in which high best frequency neurons have few or no dendrites. These changes are not complete by P21, when NM neurons still have more dendrites than in the adult owl. The neurons of NL have many short dendrites before hatching. Their number is greatly reduced by P6, and then does not change during later development. Like NM neurons, NL neurons and dendrites grow in the first month posthatch, and at P21, NL dendrites are longer than those in the adult owl. Thus, the auditory brainstem circuits grow in the first month after hatching, but are not yet mature at the time the head reaches its adult size. © 1996 Wiley-Liss, Inc.     

Development of GABA immunoreactivity in brainstem auditory nuclei of the chick: ontogeny of gradients in terminal staining

The development of gamma-aminobutyric acid-immunoreactivity (GABA-I) in nucleus magnocellularis (NM) and nucleus laminaris (NL) of the chick was studied by using an antiserum to GABA. In posthatch chicks, GABA-I is localized to small, round punctate structures in the neuropil and surrounding nerve cell bodies. Electron microscopic immunocytochemistry demonstrates that these puncta make synaptic contact with neuronal cell bodies in NM; thus, they are believed to be axon terminals. GABAergic terminals are distributed in a gradient of increasing density from the rostromedial to the caudolateral regions of NM. The distribution of GABA-I was studied during embryonic development. At embryonic days (E) 9-11, there is little GABA-I staining in either NM or NL. Around E12-14, a few fibers are immunopositive but no gradient is seen. More GABA-I structures are present at E14-15. They are reminiscent of axons with varicosities along their length, preterminal axonal thickenings and fiber plexuses. At E15, terminals become apparent circumscribing neuronal somata and are also discernible in the neuropil of both nuclei. In E16-17 embryos, terminals are the predominantly labeled GABA-I structures and they are uniformly distributed throughout NM. The density of GABAergic terminals increases in caudolateral regions of NM such that by E17-19, there is a gradient of increasing density of GABA-I terminals from the rostromedial to caudolateral regions of NM. The steepness of this gradient increases during development and is the greatest in posthatch (P) chicks. Cell bodies labeled with the GABA antiserum are located around the borders of both NM and NL and in the neuropil between these two nuclei. Occasionally, GABA-I neurons can be found within these auditory brainstem nuclei in both embryonic and posthatch chicks. Nucleus angularis (NA) contains some GABAergic cells. The appearance of GABA-I terminals around E15 is correlated in time with the formation of end-bulbs of Held on NM neurons. Thus, the ontogeny of presumed inhibitory inputs to chick auditory brainstem nuclei temporally correlates with, and could modulate the development of, excitatory auditory afferent structure and function.     

GABAergic neurons in brainstem auditory nuclei of the chick: distribution, morphology, and connectivity

The second- and third-order auditory nuclei in the brainstem of the chicken, nucleus magnocellularis (NM) and nucleus laminaris (NL), receive afferents that are immunoreactive to gamma-aminobutyric acid (GABA). In order to investigate the source(s) of these GABAergic afferents, we examined the distribution, morphology, and connectivity of GABAergic neurons in and adjacent to NM and NL in chicks from 7 days of incubation to 12 days posthatch. Immunocytochemical techniques revealed the presence of approximately 150 GABA-labeled neurons within the neuropil surrounding NM and NL on each side of the brainstem. Most of these neurons are located between NM and NL and along the lateral border of NM. GABAergic neurons are multipolar; their thick dendritic processes branch extensively and give rise to several thin, secondary processes. Frequently, the GABA-labeled processes arborize within NM or NL. The morphology of these non-NM/NL neurons was investigated further with Golgi impregnation and specific neuronal markers (antisera to microtubule-associated protein). Our observations suggest that a considerable portion of GABAergic input to NM and NL originates from local GABAergic neurons. In order to determine other possible sources of GABAergic input to NM and NL, we injected tracers unilaterally into NM/NL. A small number (20-30) of neurons were retrogradely labeled in the trapezoid body, almost exclusively ipsilaterally. No labeled cells were found in other regions of the brainstem, except for the contralateral NM. Unilateral injections of horseradish-peroxidase-labeled wheat germ agglutinin into the paraflocculus revealed only minor terminal labeling in the lateral region of NL bilaterally. The number and distribution of GABAergic terminals in NM and NL appeared normal after transection of the crossed dorsal cochlear tract.     

Pre‐target axon sorting in the avian auditory brainstem

Topographic organization of neurons is a hallmark of brain structure. The establishment of the connections between topographically organized brain regions has attracted much experimental attention, and it is widely accepted that molecular cues guide outgrowing axons to their targets in order to construct topographic maps. In a number of systems afferent axons are organized topographically along their trajectory as well, and it has been suggested that this pre‐target sorting contributes to map formation. Neurons in auditory regions of the brain are arranged according to their best frequency (BF), the sound frequency they respond to optimally. This BF changes predictably with position along the so‐called tonotopic axis. In the avian auditory brainstem, the tonotopic organization of the second‐ and third‐order auditory neurons in nucleus magnocellularis (NM) and nucleus laminaris (NL) has been well described. In this study we examine whether the decussating NM axons forming the crossed dorsal cochlear tract (XDCT) and innervating the contralateral NL are arranged in a systematic manner. We electroporated dye into cells in different frequency regions of NM to anterogradely label their axons in XDCT. The placement of dye in NM was compared to the location of labeled axons in XDCT. Our results show that NM axons in XDCT are organized in a precise tonotopic manner along the rostrocaudal axis, spanning the entire rostrocaudal extent of both the origin and target nuclei. We propose that in the avian auditory brainstem, this pretarget axon sorting contributes to tonotopic map formation in NL. J. Comp. Neurol. 521:2310–2320, 2013. © 2012 Wiley Periodicals, Inc.     

Organization and development of brain stem auditory nuclei of the chicken: organization of projections from n. magnocellularis to n. laminaris.

The tonotopic and topographic organization of the bilateral projection from second-order auditory neurons of nucleus magnocellularis (NM) to nucleus laminaris (NL) was examined in young chickens. In one group of birds, the NM axons which innvervate the contralateral NL were severed by cutting the crossed dorsal cochlear tract at the midline. Heavy terminal degeneration in NL was confined to the neuropil area immediately ventral to the perikaryl lamina. Very little degeneration was seen in the dorsal neuropil region. In a second series of animals, the charactertistic frequency (CF) of cells in an area of NM was first determined by microelectrode recording techniques and then a small electrolytic lesion was made through the recording electrode. Following survival periods of 24-48 hours, the distribution of projections from the lesioned area to the ipsilateral and contralateral NL was examined using the Fink-Heimer method. As previously described in the pigeon, projections from NM terminate densely in the neuropil region immediately dorsal to the ipsilateral NL cell bodies and ventral to the perikaryl layer on the contralateral side, providing each NL neuron with segregated binaural innervation. Lesions in any area of the NM produced degeneration confined to a limited caudo-rostral and medio-lateral portion of both laminar nuclei. To investigate this topographic relationship, the cuado-rostral extents of the lesion in NM and of the resulting degeneration in both NL were determined. Linear regression and correlation analyses then related these positional values to each other and to the CF found at the center of each lesion. All correlations were highly significant and ranged from 0.78 between the position of the lesion in NM and CF to 0.91 between the caudo-rostral position of degeneration in the NL ipsilateral and contralateral to the lesion. It is concluded that neurons in NM project in a very discrete topographic, tonotopic and symmetrical fashion to NL on both sides of the brain, contributing to the binaural response properties and tonotopic organization of neurons in NL. The results also suggest that the organization of projections from NM to NL could provide a mechanism for the differential transmission delay required by a "place" model of low-frequency sound localization.     

Formation of the avian nucleus magnocellularis from the auditory anlage

In the avian auditory system, the neural network for computing the localization of sound in space begins with bilateral innervation of nucleus laminaris (NL) by nucleus magnocellularis (NM) neurons. We used antibodies against the neural specific markers Hu C/D, neurofilament, and SV2 together with retrograde fluorescent dextran labeling from the contralateral hindbrain to identify NM neurons within the anlage and follow their development. NM neurons could be identified by retrograde labeling as early as embryonic day (E) 6. While the auditory anlage organized itself into NM and NL in a rostral-to-caudal fashion between E6 and E8, labeled NM neurons were visible throughout the extent of the anlage at E6. By observing the pattern of neuronal rearrangements together with the pattern of contralaterally projecting NM fibers, we could identify NL in the ventral anlage. Ipsilateral NM fibers contacted the developing NL at E8, well after NM collaterals had projected contralaterally. Furthermore, the formation of ipsilateral connections between NM and NL neurons appeared to coincide with the arrival of VIIIth nerve fibers in NM. By E10, immunoreactivity for SV2 was heavily concentrated in the dorsal and ventral neuropils of NL. Thus, extensive pathfinding and morphological rearrangement of central auditory nuclei occurs well before the arrival of cochlear afferents. Our results suggest that NM neurons may play a central role in formation of tonotopic connections in the auditory system.     

Expression of GABA(B) receptor in the avian auditory brainstem: ontogeny, afferent deprivation, and ultrastructure

Nucleus magnocellularis (NM), nucleus angularis (NA), and nucleus laminaris (NL), second- and third-order auditory neurons in the avian brainstem, receive GABAergic input primarily from the superior olivary nucleus (SON). Previous studies have demonstrated that both GABA(A) and GABA(B) receptors (GABA(B)Rs) influence physiological properties of NM neurons. We characterized the distribution of GABA(B)R expression in these nuclei during development and after deafferentation of the excitatory auditory nerve (nVIII) inputs. We used a polyclonal antibody raised against rat GABA(B)Rs in the auditory brainstem during developmental periods that are thought to precede and include synaptogenesis of GABAergic inputs. As early as embryonic day (E)14, dense labeling is observed in NA, NM, NL, and SON. At earlier ages immunoreactivity is present in somas as diffuse staining with few puncta. By E21, when the structure and function of the auditory nuclei are known to be mature, GABA(B) immunoreactivity is characterized by dense punctate labeling in NM, NL, and a subset of NA neurons, but label is sparse in the SON. Removal of the cochlea and nVIII neurons in posthatch chicks resulted in only a small decrease in immunoreactivity after survival times of 14 or 28 days, suggesting that a major proportion of GABA(B)Rs may be expressed postsynaptically or on GABAergic terminals. We confirmed this interpretation with immunogold TEM, where expression at postsynaptic membrane sites is clearly observed. The characterization of GABA(B)R distribution enriches our understanding of the full complement of inhibitory influences on central auditory processing in this well-studied neuronal circuit.     

Organization and development of brain stem auditory nuclei of the chicken: tonotopic organization of n. magnocellularis and n. laminaris.

Extracellular recordings of responses to tone-burst stimulation were used to determine the tonotopic organization of n. magnocellularis (NM) and n. laminaris (NL) in hatching chickens. NM cells show "primary-like" response patterns to ipsilateral stimulation, and are arranged in dorso-ventral isofrequency columns. Units responding to the highest frequency tones (about 4,100 Hz) are situated at the rostromedial pole of the medial division. Units with lower characteristic frequencies (CF's) are found at successively caudal and lateral sites, until extremely low CF's ( less than 500 Hz) are represented dorsoventrally in the daudolateral tail of the lateral division. No evidence was found of auditory input to the region which receives projections from the macula lagena. NL receives polarized, binaural, excitatory input. Units have similar CF's and thresholds to tones presented to either ear. The tonotopic organization in NL matches that found in NM--high CF's rostromedially and low CF's caudal and lateral. Quantitative procedures were developed for relating CF to the position of a unit within either nucleus. These analyses account for 79% and 89% of the frequency variance found within NM and NL, respectively, and predict the CF of a neuron by its position within each nucleus.     

Expression of EphB receptors and EphrinB ligands in the developing chick auditory brainstem

Nucleus magnocellularis (NM) in the avian auditory brainstem receives auditory input from nerve the VIIIth and projects bilaterally to nucleus laminaris (NL). This projection preserves binaural segregation in that ipsilateral NM projects to dorsal dendrites of NL and contralateral NM projects to ventral dendrites of NL. We have begun to examine the molecular signals that influence segregation of inputs onto discrete regions of NL cells. We previously showed that the Eph receptor, EphA4, is expressed selectively in the dorsal NL neuropil from embryonic day (E) 9 to E11, when NM axons grow into the NL neuropil. This asymmetric distribution suggests that EphA4 acts as a guidance molecule during binaural segregation. We report here on the developmental changes in the expression of two other Eph receptors, EphB2 and EphB5, and two ligands, ephrin-B1 and ephrin-B2, in the chick auditory brainstem. These proteins are expressed in the auditory nuclei during the maturation of the NM-NL projection. EphB2, EphB5, and ephrin-B1 are expressed in dorsal and ventral NL neuropil and at the midline of the brainstem at E10-E12. At this age, ephrin-B2, a ligand for EphB receptors and for EphA4, is expressed in NL cell bodies and NM-NL axons. The expression of these proteins diminishs in the posthatch ages examined. These results suggest that several members of the Eph family are involved in maturation of the nuclei and their projections. Moreover, ephrin-B2 in growing axons may interact with the asymmetrically expressed EphA4 during the establishment of binaural segregation.     

Interaural timing difference circuits in the auditory brainstem of the emu (Dromaius novaehollandiae)

In the auditory system, precise encoding of temporal information is critical for sound localization, a task with direct behavioral relevance. Interaural timing differences (ITDs) are computed using axonal delay lines and cellular coincidence detectors in nucleus laminaris (NL). We present morphological and physiological data on the timing circuits in the emu, Dromaius novaehollandiae, and compare these results with those from the barn owl (Tyto alba) and the domestic chick (Gallus gallus). Emu NL was composed of a compact monolayer of bitufted neurons whose two thick primary dendrites were oriented dorsoventrally. They showed a gradient in dendritic length along the presumed tonotopic axis. The NL and nucleus magnocellularis (NM) neurons were strongly immunoreactive for parvalbumin, a calcium-binding protein. Antibodies against synaptic vesicle protein 2 and glutamic acid decarboxlyase revealed that excitatory synapses terminated heavily on the dendritic tufts, while inhibitory terminals were distributed more uniformly. Physiological recordings from brainstem slices demonstrated contralateral delay lines from NM to NL. During whole-cell patch-clamp recordings, NM and NL neurons fired single spikes and were doubly rectifying. NL and NM neurons had input resistances of 30.0 +/- 19.9 Momega and 49.0 +/- 25.6 Momega, respectively, and membrane time constants of 12.8 +/- 3.8 ms and 3.9 +/- 0.2 ms. These results provide further support for the Jeffress model for sound localization in birds. The emu timing circuits showed the ancestral (plesiomorphic) pattern in their anatomy and physiology, while differences in dendritic structure compared to chick and owl may indicate specialization for encoding ITDs at low best frequencies.     

Tonotopic organization of the superior olivary nucleus in the chicken auditory brainstem

Topographic maps are salient features of neuronal organization in sensory systems. Inhibitory components of neuronal circuitry are often embedded within this organization, making them difficult to isolate experimentally. The auditory system provides opportunities to study the topographic organization of inhibitory long-range projection nuclei, such as the superior olivary nucleus (SON). We analyzed the topographic organization of response features of neurons in the SON of chickens. Quantitative methods were developed to assess and communicate this organization. These analyses led to three main conclusions: 1) sound frequency is linearly arranged from dorsal (low frequencies) to ventral (high frequencies) in SON; 2) this tonotopic organization is less precise than the organization of the excitatory nuclei in the chicken auditory brainstem; and 3) neurons with different response patterns to pure tone stimuli are interspersed throughout the SON and show similar tonotopic organizations. This work provides a predictive model to determine the optimal stimulus frequency for a neuron from its spatial location in the SON.     

Tonotopic Order in the Adult and Developing Auditory System of the Rat as Shown by c-fos Immunocytochemistry

Immediate éarly genes such as the proto-oncogene c- fos can be expressed in neurons following synaptic excitation by sensory stimulation. C- fos immunocytochemistry has subsequently been shown to be a very sensitive marking technique for neuronal activity. Here, antibodies against the c- fos protein product Fos were used to map the tonotopic organization in the auditory system of adult and developing rats. After stimulating adult rats with pure-tone pulses, bands of Fos-immunoreactive neurons revealed the frequency representation in seven brainstem nuclei: all three subdivisions of the cochlear nucleus, the lateral superior olive, the medial nucleus of the trapezoid body, the ventral nucleus of the trapezoid body, the rostral periolivary nucleus, the dorsal nucleus of the lateral lemniscus and the inferior colliculus. With the exception of the dorsal cochlear nucleus and the inferior colliculus, tonotopicity has not been previously demonstrated in the brainstem nuclei of the rat. During development two striking results were obtained. First, beginning at postnatal day 14 (i.e. ∼2 days after physiological hearing begins in rats), not only low but also high frequencies were able to induce strong Fos immunoreactivity, indicating that gradual recruitment of formerly unresponsive high-frequency sites does not occur in the rat. Second, a gradual age-related shift of the position of isofrequency bands was not seen in any of the nuclei, suggesting that changes in frequency-place code do not occur after 2 weeks postnatally. These results indicate that the rat's auditory brainstem nuclei achieve their adult-like tonotopic organization early on, implying a somewhat different developmental time course than is found in other mammalian species.     

Projections of the cochlear nuclei and nucleus laminaris to the inferior colliculus of the barn owl

The barn owl determines the directions from which sounds emanate by computing the interaural differences in the timing and intensity of sounds. These cues for sound localization are processed in independent channels originating at nucleus magnocellularis (NM) and nucleus angularis (NA), the cochlear nuclei. The cells of NM are specialized for encoding the phase of sounds in the ipsilateral ear. The cells of NA are specialized for encoding the intensity of sounds in the ipsilateral ear. NM projects solely, bilaterally, and tonotopically to nucleus laminaris (NL). NL and NA project to largely nonoverlapping zones in the central nucleus of the inferior colliculus (ICc), thus forming hodological subdivisions in which time and intensity information may be processed. The terminal field of NL occupies a discrete zone in the rostromedial portion of the contralateral ICc, which we have termed the "core" of ICc. The terminal field of NA surrounds the core of ICc and thus forms a "shell" around it. The projection from NL to the core conserves tonotopy. Low-frequency regions of NL project to the dorsal portions of the core whereas higher-frequency regions project to more ventral portions. This innervation pattern is consistent with earlier physiological studies of tonotopy. Physiological studies have also suggested that NL and the core of ICs contain a representation of the location of a sound source along the horizontal axis. Our data suggest that the projection from NL to the core preserves spatiotopy. Thus, the dorsal portion of NL on the left, which contains a representation of eccentric loci in the right hemifield, innervates the area of the right ICc core that represents eccentric right loci. The more ventral portion of the left NL, which represents loci close to the vertical meridian, innervates the more rostral portions of the right core, which also represents loci near the vertical meridian.     

Quantitative study of plasticity in the auditory nuclei of chick under conditions of prenatal sound attenuation and overstimulation with species specific and music sound stimuli

Morphological effects of prenatal sound attenuation and sound overstimulation by species specific and music sounds on the brainstem auditory nuclei of chick have been evaluated quantitatively. Changes in length, volume, neuron number, size of neuronal nuclei and glial numbers of second and third order auditory nuclei, n. magnocellularis (NM) and n. laminaris (NL), were determined from thionine-stained serial sections of control and experimental groups on posthatch day 1 using stereological methods. Significant increase in volume of both auditory nuclei attributable to increase in length of nucleus, number and size of neurons, number of glia as well as neuropil was observed in response to both species specific and music overstimulation given during the critical period of development. The enhanced development of auditory nuclei in response to enriched environment prenatally indicates a positive effect of activity on neurons which may have clinical implications in addition to providing explanation for preference to auditory cues in the postnatal life. Reduction in neuron number with a small increase in proportion of cell nuclei of large size as well as an increase in glial numbers was seen in both NM and NL of the prenatally sound attenuated chicks. The increase in size of some neuronal nuclei may probably be evidence of enhanced synthesis of proteins involved in cell death or an attempt at recovery. The dissociated response of neurons and glia under sound attenuated and auditory stimulated conditions suggests that they are independently regulated by activity-dependent signals with glia also being under influence of other signals for a role in removal of dead cell debris.     

Calretinin expression in the chick brainstem auditory nuclei develops and is maintained independently of cochlear nerve input

The expression of the calcium-binding protein calretinin (CR) in the chick brainstem auditory nuclei angularis (NA), laminaris (NL), and magnocelularis (NM) was studied during normal development and after deafening by surgical removal of the otocyst (embryonic precursor of the inner ear) or columella (middle ear ossicle). CR mRNA was localized by in situ hybridization by using a radiolabeled oligonucleotide chick CR probe. CR immunoreactivity (CR-IR) was localized on adjacent tissue sections. CR mRNA signal in the auditory nuclei was expressed at comparable levels at embryonic day (E)9 and E11 and increased thereafter to reach the highest levels in posthatch chicks. CR-IR neurons were apparent in NM and NA at E11 and in NL by E13, and CR-IR increased in all three auditory nuclei thereafter. Neither unilateral nor bilateral otocyst removal caused detectable changes in the intensity of CR mRNA expression or CR-IR in the auditory nuclei at any of the several ages examined. Similarly, columella removal at posthatching day 2 or 3 failed to significantly affect CR mRNA or CR-IR levels at 3 hours, 1 day, or 3–4 days survival times. We conclude that cochlear nerve input is not necessary for expression of either calretinin mRNA or protein and that the profound decrease in sound-evoked activity caused by columella removal does not affect the maintenance of CR expression after hatching. J. Comp. Neurol. 383:112–121, 1997. © 1997 Wiley-Liss, Inc.     

Timing and topography of nucleus magnocellularis innervation by the cochlear ganglion

This series of experiments examined the arrival and organization of cochlear nerve axons in the primary auditory brainstem nucleus, nucleus magnocellularis (NM), of the chick. DiI and DiD were injected into the cochlear nerve, cochlear ganglion, and basilar papilla (i.e., avian cochlea) in fixed tissue and labeled axons were studied in NM and its vicinity. Cochlear nerve axons first penetrate NM between stages 29 (E6) and 36 (E10). Axons penetrate NM in a middle-to-posterior-to-anterior developmental sequence; the anterior, high-frequency region of NM receives axons last. When cochlear nerve axons arrive in the NM, they are already organized in a topographic map related to the position of their cell bodies along the basilar papilla, foreshadowing the tonotopic mapping observed between NM and the basilar papilla later in development. Evidence of a topographic map was also observed in the other primary auditory brainstem nucleus, nucleus angularis. These results indicate that topographic mapping of position (and ultimately characteristic frequency) between the basilar papilla and NM is established as cochlear nerve axons arrive in the NM prior to the onset of synaptic activity. .     

Plasticity in the tonotopic organization of the medial geniculate body in adult cats following restricted unilateral cochlear lesions

To investigate subcortical contributions to cortical reorganization, the frequency organization of the ventral nucleus of the medial geniculate body (MGv) in six normal adult cats and in eight cats with restricted unilateral cochlear lesions was investigated using multiunit electrophysiological recording techniques. The tonotopic organization of MGv in the lesioned animals, with severe mid-to-high frequency hearing losses, was investigated 40-186 days following the lesioning procedure. Frequency maps were generated from neural responses to pure tone bursts presented separately to each ear under barbiturate anesthesia. Consideration of the frequency organization in normal animals, and of the apparently normal representation of the ipsilateral (unlesioned) cochlea in lesioned animals, allowed for a detailed specification of the extent of changes observed in MGv. In the lesioned animals it was found that, in the region of MGv in which mid-to-high frequencies are normally represented, there was an "expanded representation" of lesion-edge frequencies. Neuron clusters within these regions of enlarged representation that had "new" characteristic frequencies displayed response properties (latency, bandwidth) very similar to those in normal animals. Thresholds of these neurons were not consistent with the argument that the changes merely reflect the residue of prelesion responses, suggesting a dynamic process of reorganization. The tonotopic reorganization observed in MGv is similar to that seen in the primary auditory cortex and is more extensive than the reorganization found in the auditory midbrain, suggesting that the auditory thalamus plays an important role in cortical plasticity.     

Development of glutamate receptors in auditory neurons from long-term organotypic cultures of the embryonic chick hindbrain

We used long-range organotypic cultures of auditory nuclei in the chick hindbrain to test the development of glutamate receptor activity in auditory neurons growing in a tissue environment that includes early deprivation of peripheral glutamatergic input, subsequent to removal of the otocyst. Cultures started at embryonic day (E)5, and lasted from 6 h to 15 days. Neuronal migration, clustering and axonal extension from the nucleus magnocellularis (NM) to the nucleus laminaris (NL) partially resembled events in vivo . However, the distinctive laminar organization of the NL was not observed. Glutamate receptor (GluR) activity was tested with optical recordings of intracellular Ca²⁺ in the NM. α-Amino-3-hydroxy-5-methyl-4-isoxazoleproprionic acid (AMPA)/kainate receptors had Ca²⁺ responses with a time course similar to that in control slices. Peak amplitude, however, was significantly lower. N -methyl- d -aspartate (NMDA)-mediated Ca²⁺ responses were higher in 2-day cultures (E5 + 2d) than in E7 explant controls, returning later to control values. Metabotropic GluRs did not elicit Ca²⁺ responses at standard agonist doses. Blocking NMDA or AMPA/kainate receptors with specific antagonists for 10 days in culture did not limit neuronal survival. Blocking metabotropic GluRs resulted in complete neuronal loss. Thus, ionotropic GluRs are not required for NM neuronal survival. However, their activity during development is affected when neurons grow in an in vitro environment that includes prevention of arrival of peripheral glutamatergic input.     

Developmental regulation of EphA4 expression in the chick auditory brainstem

The avian auditory brainstem nuclei nucleus magnocellularis (NM) and nucleus laminaris (NL) display highly precise patterns of neuronal connectivity. NM projects tonotopically to the dorsal dendrites of ipsilateral NL neurons and to the ventral dendrites of contralateral NL neurons. The precision of this binaural segregation is evident at the earliest developmental stage at which connections can be observed. We have begun to examine the possibility that Eph receptor tyrosine kinase signaling is involved in establishing these spatially segregated connections. The expression of the EphA4 tyrosine kinase was examined at several developmental stages. EphA4 is expressed in rhombomere 5, which contains progenitors for both NM and NL. In this rhombomere, the labeling becomes striped during the time that precursor cells migrate to the auditory anlage. At the precise time when NM-NL projections are forming, EphA4 expression in NL is asymmetric, with markedly higher expression in the dorsal NL neuropil than in the ventral neuropil, suggesting a possible role in guiding growing axons to the appropriate region. At later embryonic ages EphA4 expression is symmetric around NL, and is absent in NM. As auditory function matures, EphA4 expression decreases so that by 4 days after hatch no EphA4 antibody labeling is evident in the auditory brainstem nuclei.