Projection of the cochlear and lagenar nerves on the cochlear nuclei of the pigeon

Avian adeno-associated virus as an anterograde transsynaptic vector

BACKGROUND

Retrograde and anterograde transsynaptic viral vectors are useful tools for studying the input and output organization of neuronal circuitry, respectively. While retrograde transsynaptic viral vectors are widely used, viral vectors that show anterograde transsynaptic transduction are not common.

NEW METHOD

We chose recombinant avian adeno-associated virus (A3V) carrying the mCherry gene and injected it into the eyeball, cochlear duct, and midbrain auditory center of chickens. We observed different survival times to examine the virus transcellular transport and the resulting mCherry expression. To confirm the transcellular transduction mode, we co-injected A3V and cholera toxin B subunit.

RESULTS

Injecting A3V into the eyeball and cochlea labeled neurons in the visual and auditory pathways, respectively. Second-, and third-order labeling occurred approximately two and seven days, respectively, after injection into the midbrain. The distribution of labeled neurons strongly suggests that A3V transport is preferentially anterograde and transduces postsynaptic neurons.

COMPARISON WITH EXISTING METHOD(S)

A3V displays no extrasynaptic leakage and moderate speed of synapse passage, which is better than other viruses previously reported. Compared with AAV1&9, which have been shown to pass one synapse anterogradely, A3V passes several synapses in the anterograde direction.

CONCLUSIONS

A3V would be a good tool to study the topographic organization of projection axons and their target neurons.

Distinct Neural Properties in the Low-Frequency Region of the Chicken Cochlear Nucleus Magnocellularis

Topography in the avian cochlear nucleus magnocellularis (NM) is represented as gradually increasing characteristic frequency (CF) along the caudolateral-to-rostromedial axis. In this study, we characterized the organization and cell biophysics of the caudolateral NM (NMc) in chickens (Gallus gallus). Examination of cellular and dendritic architecture first revealed that NMc contains small neurons and extensive dendritic processes, in contrast to adendritic, large neurons located more rostromedially. Individual dye-filling study further demonstrated that NMc is divided into two subregions, with NMc2 neurons having larger and more complex dendritic fields than NMc1. Axonal tract tracing studies confirmed that NMc1 and NMc2 neurons receive afferent inputs from the auditory nerve and the superior olivary nucleus, similar to the adendritic NM. However, the auditory axons synapse with NMc neurons via small bouton-like terminals, unlike the large end bulb synapses on adendritic NM neurons. Immunocytochemistry demonstrated that most NMc2 neurons express cholecystokinin but not calretinin, distinct from NMc1 and adendritic NM neurons that are cholecystokinin negative and mostly calretinin positive. Finally, whole-cell current clamp recordings revealed that NMc neurons require significantly lower threshold current for action potential generation than adendritic NM neurons. Moreover, in contrast to adendritic NM neurons that generate a single-onset action potential, NMc neurons generate multiple action potentials to suprathreshold sustained depolarization. Taken together, our data indicate that NMc contains multiple neuron types that are structurally, connectively, molecularly, and physiologically different from traditionally defined NM neurons, emphasizing specialized neural properties for processing low-frequency sounds.

Distribution of Vesicular Glutamate Transporter 2 and Ionotropic Glutamate Receptors in the Auditory Ganglion and Cochlear Nuclei of Pigeons (Columba livia)

Glutamate is a principal excitatory neurotransmitter in the auditory system. Our previous studies revealed localization of glutamate receptor mRNAs in the pigeon cochlear nuclei, suggesting the existence of glutamatergic input from the auditory nerve to the brainstem. This study demonstrated localization of mRNAs for vesicular glutamate transporter 2 (vGluT2) and ionotropic glutamate receptors (AMPA, kainate and NMDA) in the auditory ganglion (AG) and cochlear nuclei (magnocellular, angular and laminar nuclei). VGluT2 mRNA was intensely expressed in AG and intensely or moderately in the cochlear nuclei. The AG and cochlear nuclei showed intense-to-moderate mRNA signals for GluA2, GluA3, GluA4, GluK4 and GluN1. These results suggest that the pigeon AG neurons receives glutamatergic input from hair cells and in turn projects to the magnocellular and angular nuclei. Glutamate may play a pivotal role in the excitatory synapse transmission in the peripheral auditory pathway of birds.

Heterogeneous calretinin expression in the avian cochlear nucleus angularis

Multiple calcium-binding proteins (CaBPs) are expressed at high levels and in complementary patterns in the auditory pathways of birds, mammals, and other vertebrates, but whether specific members of the CaBP family can be used to identify neuronal subpopulations is unclear. We used double immunofluorescence labeling of calretinin (CR) in combination with neuronal markers to investigate the distribution of CR-expressing neurons in brainstem sections of the cochlear nucleus in the chicken (Gallus gallus domesticus). While CR was homogeneously expressed in cochlear nucleus magnocellularis, CR expression was highly heterogeneous in cochlear nucleus angularis (NA), a nucleus with diverse cell types analogous in function to neurons in the mammalian ventral cochlear nucleus. To quantify the distribution of CR in the total NA cell population, we used antibodies against neuronal nuclear protein (NeuN), a postmitotic neuron-specific nuclear marker. In NA neurons, NeuN label was variably localized to the cell nucleus and the cytoplasm, and the intensity of NeuN immunoreactivity was inversely correlated with the intensity of CR immunoreactivity. The percentage of CR + neurons in NA increased from 31 % in embryonic (E)17/18 chicks, to 44 % around hatching (E21), to 51 % in postnatal day (P) 8 chicks. By P8, the distribution of CR + neurons was uniform, both rostrocaudal and in the tonotopic (dorsoventral) axis. Immunoreactivity for the voltage-gated potassium ion channel Kv1.1, used as a marker for physiological type, showed broad and heterogeneous postsynaptic expression in NA, but did not correlate with CR expression. These results suggest that CR may define a subpopulation of neurons within nucleus angularis.

Central projections of lagenar primary neurons in the chick

Perception of linear acceleration and head position is the function of the utricle and saccule in mammals. Nonmammalian vertebrates possess a third otolith endorgan, the macula lagena. Different functions have been ascribed to the lagena in arboreal birds, including hearing, equilibrium, homing behavior, and magnetoreception. However, no conclusive evidence on the function of the lagena in birds is currently available. The present study is aimed at providing a neuroanatomical substrate for the function of the lagena in the chicken as an example of terrestrial birds. The afferents from the lagena of chick embryos (E19) to the brainstem and cerebellum were investigated by the sensitive lipophilic tracer Neuro Vue Red in postfixed ears. The results revealed that all the main vestibular nuclei, including the tangential nucleus, received lagenar projections. No lagenar terminals were found in auditory centers, including the cochlear nuclei. In the cerebellum, the labeled terminals were found variably in all of the cerebellar nuclei. In the cerebellar cortex, the labeled fibers were found mostly in the uvula, with fewer afferents in the flocculus and paraflocculus. None was seen in the nodulus. The absence of lagenar afferent projections in auditory nuclei and the presence of a projection pattern in the vestibular nuclei and cerebellum similar to that of the utricle and saccule suggest that the primary role of the lagena in the chick lies in the processing of vestibular information related to linear acceleration and static head position.

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.

Astrocyte-secreted factors modulate a gradient of primary dendritic arbors in nucleus laminaris of the avian auditory brainstem

Neurons in nucleus laminaris (NL) receive binaural, tonotopically matched input from nucleus magnocelluaris (NM) onto bitufted dendrites that display a gradient of dendritic arbor size. These features improve computation of interaural time differences, which are used to determine the locations of sound sources. The dendritic gradient emerges following a period of significant reorganization at embryonic day 15 (E15), which coincides with the emergence of astrocytes that express glial fibrillary acidic protein (GFAP) in the auditory brainstem. The major changes include a loss of total dendritic length, a systematic loss of primary dendrites along the tonotopic axis, and lengthening of primary dendrites on caudolateral NL neurons. Here we have tested whether astrocyte-derived molecules contribute to these changes in dendritic morphology. We used an organotypic brainstem slice preparation to perform repeated imaging of individual dye-filled NL neurons to determine the effects of astrocyte-conditioned medium (ACM) on dendritic morphology. We found that treatment with ACM induced a decrease in the number of primary dendrites in a tonotopically graded manner similar to that observed during normal development. Our data introduce a new interaction between astrocytes and neurons in the auditory brainstem and suggest that these astrocytes influence multiple aspects of auditory brainstem maturation.

Estradiol-dependent modulation of serotonergic markers in auditory areas of a seasonally breeding songbird

Because no organism lives in an unchanging environment, sensory processes must remain plastic so that in any context, they emphasize the most relevant signals. As the behavioral relevance of sociosexual signals changes along with reproductive state, the perception of those signals is altered by reproductive hormones such as estradiol (E2). We showed previously that in white-throated sparrows, immediate early gene responses in the auditory pathway of females are selective for conspecific male song only when plasma E2 is elevated to breeding-typical levels. In this study, we looked for evidence that E2-dependent modulation of auditory responses is mediated by serotonergic systems. In female nonbreeding white-throated sparrows treated with E2, the density of fibers immunoreactive for serotonin transporter innervating the auditory midbrain and rostral auditory forebrain increased compared with controls. E2 treatment also increased the concentration of the serotonin metabolite 5-HIAA in the caudomedial mesopallium of the auditory forebrain. In a second experiment, females exposed to 30 min of conspecific male song had higher levels of 5-HIAA in the caudomedial nidopallium of the auditory forebrain than birds not exposed to song. Overall, we show that in this seasonal breeder, (a) serotonergic fibers innervate auditory areas; (b) the density of those fibers is higher in females with breeding-typical levels of E2 than in nonbreeding, untreated females; and (c) serotonin is released in the auditory forebrain within minutes in response to conspecific vocalizations. Our results are consistent with the hypothesis that E2 acts via serotonin systems to alter auditory processing.

Localization of cerebellin-2 in late embryonic chicken brain: implications for a role in synapse formation and for brain evolution

Cerebellin-1 (Cbln1), the most studied member of the cerebellin family of secreted proteins, is necessary for the formation and maintenance of parallel fiber-Purkinje cell synapses. However, the roles of the other Cblns have received little attention. We previously identified the chicken homolog of Cbln2 and examined its expression in dorsal root ganglia and spinal cord (Yang et al. [2010] J Comp Neurol 518:2818-2840). Interestingly, Cbln2 is expressed by mechanoreceptive and proprioceptive neurons and in regions of the spinal cord where those afferents terminate, as well as by preganglionic sympathetic neurons and their sympathetic ganglia targets. These findings suggest that Cbln2 may demonstrate a tendency to be expressed by synaptically connected neuronal populations. To further assess this possibility, we examined Cbln2 expression in chick brain. We indeed found that Cbln2 is frequently expressed by synaptically connected neurons, although there are exceptions, and we discuss the implications of these findings for Cbln2 function. Cbln2 expression tends to be more common in primary sensory neurons and in second-order sensory regions than it is in motor areas of the brain. Moreover, we found that the level of Cbln2 expression for many regions of the chicken brain is very similar to that of the mammalian homologs, consistent with the view that the expression patterns of molecules playing fundamental roles in processes such as neuronal communication are evolutionarily conserved. There are, however, large differences in the pattern of Cbln2 expression in avian as compared to mammalian telencephalon and in other regions that show the most divergence between the two lineages.

Connections of the auditory brainstem in a songbird, Taeniopygia guttata. I. Projections of nucleus angularis and nucleus laminaris to the auditory torus

Auditory information is important for social and reproductive behaviors in birds generally, but is crucial for oscine species (songbirds), in particular because in these species auditory feedback ensures the learning and accurate maintenance of song. While there is considerable information on the auditory projections through the forebrain of songbirds, there is no information available for projections through the brainstem. At the latter levels the prevalent model of auditory processing in birds derives from an auditory specialist, the barn owl, which uses time and intensity parameters to compute the location of sounds in space, but whether the auditory brainstem of songbirds is similarly functionally organized is unknown. To examine the songbird auditory brainstem we charted the projections of the cochlear nuclei angularis (NA) and magnocellularis (NM) and the third-order nucleus laminaris (NL) in zebra finches using standard tract-tracing techniques. As in other avian species, the projections of NM were found to be confined to NL, and NL and NA provided the ascending projections. Here we report on differential projections of NA and NL to the torus semicircularis, known in birds as nucleus mesencephalicus lateralis, pars dorsalis (MLd), and in mammals as the central nucleus of the inferior colliculus (ICc). Unlike the case in nonsongbirds, the projections of NA and NL to MLd in the zebra finch showed substantial overlap, in agreement with the projections of the cochlear nuclei to the ICc in mammals. This organization could suggest that the "what" of auditory stimuli is as important as "where."

Three subdivisions of the auditory midbrain in chicks (Gallus gallus) identified by their afferent and commissural projections

The auditory midbrain is a site of convergence of multiple auditory channels from the brainstem. In birds, two separate ascending channels have been identified, through which time and intensity information is sent to the nucleus mesencephalicus lateralis, pars dorsalis (MLd), the homologue of the central nucleus of the mammalian inferior colliculus. Using in vivo anterograde and retrograde tracing techniques, the current study provides two lines of anatomical evidence supporting the presence of a third ascending channel to the chick MLd. First, three non-overlapping zones of the MLd receive inputs from three distinct cell groups in the caudodorsal brainstem. The projections from the nucleus angularis (NA) and nucleus laminaris (NL) are predominantly contralateral and may correspond to the time and intensity channels. A rostromedial portion of the MLd receives bilateral projections mainly from the regio intermedius, an interposed region of cells lying at a caudal level between the NL and NA, as well as scattered neurons embedded in the 8th nerve tract, and probably a very ventral region of the NA. Second, the bilateral zones of the MLd on two sides of the brain are reciprocally connected and do not interact with other zones of the MLd via commissural connections. In contrast, the NL-recipient zone projects contralaterally upon the NA-recipient zone. The structural separation of the third pathway from the NA and NL projections suggests a third information-processing channel, in parallel with the time and intensity channels. Neurons in the third channel appear to process very low frequency information including infrasound, probably utilizing different mechanisms than that underlying higher frequency processing.

Development of the delay lines in the nucleus laminaris of the chicken embryo revealed by optical imaging

One strategy in localizing a sound source in the azimuthal plane is the comparison of arrival times of sound stimuli at the two ears. The processing of interaural time differences (ITDs) in the auditory brainstem was suggested by the Jeffress model in 1948. In chicks, binaural neurons in the nucleus laminaris (NL) receive input from both ipsilateral and contralateral nucleus magnocellularis (NM) neurons, with the axons of the latter acting as delay lines. A given neuron in the NL responds maximally to coinciding input from both NM neurons. To achieve maximum resolution of sound localization in the NL, the conduction velocity along these delay lines must be precisely tuned. Here, we examined the development of this velocity between embryonic days (E)12 and E18. Our optical imaging approach visualizes the contralateral delay lines along almost the complete NL of the chicken embryo. Optical imaging with the voltage-sensitive dye RH 795 showed no significant differences in the velocity between E12 and E15, but a significant increase from E15 to E18, at both 21 degrees C and 35 degrees C. Surprisingly, at 21 degrees C the conduction velocity in the dorso-lateral part of the NL was significantly higher compared to the situation in the ventro-medial part. The observed development in contralateral conduction velocity may be due to a developmental increase in myelination of the NM axons. Indeed, antibody staining against myelin-associated glycoprotein (alpha-MAG) showed no myelination of the NM axon branches within the NL at E12 and E15. On the other hand, a clear alpha-MAG immunoreactivity occurred at E18. Our results therefore describe the developmental physiological properties of the delay line in the chicken embryo.

Fusion as an evolutionary principle of the vertebrate labyrinth

OBJECTIVES

This study was undertaken to demonstrate changes in the innervation of vestibular and auditory sense organs with the evolutionary ascent of the vertebrate labyrinth.

METHODS

Dissected labyrinths and their nerve supply prepared by the Sudan black B technique of Rasmussen were examined and photographed with a Canon A100 camera interfaced with a Zeiss operating microscope.

RESULTS

In lizards and alligators, the utricular sense organ is represented by 2 small maculae, each with a separate nerve branching off the ampullary nerves to the anterior and lateral canal cristae. These 2 maculae fuse into a bilobed macula with ascent in frogs and pigeons, eventually becoming a single large macula with its nerve supply from the superior vestibular division in guinea pigs, cats, lions, monkeys, and humans. Along with these changes, there is a fusion of the lateral and anterior canal ampullary nerves and of the bifurcating branches of the vertical canal ampullary nerves. The saccular macula is single, but receives a dual innervation from the superior division (anterior ramus) and the inferior division (posterior ramus) of the eighth nerve in alligators, pigeons, guinea pigs, cats, lions, monkeys, and humans. The main innervation is usually from the inferior division; however, saccular innervation is from the inferior division in lizards and from the superior division in frogs. The auditory sense organ is represented by a curved tube with a low-frequency receptor (lagena) at its distal end in lizards, alligators, and pigeons. In mammals, in which there is a coiled cochlea with a variable number of turns, low frequencies are recorded at the apical turn. This configuration may represent fusion of the lagena into the apical end of the auditory sense organ.

CONCLUSIONS

Fusion of sense organs and of their nerve supply appears to be an evolutionary principle in the vertebrate labyrinth.

Fate map of the chick embryo neural tube

Fate-map studies have provided important information in relation to the regional topology of brain areas in different vertebrate species. Moreover, these studies have demonstrated that the distribution of presumptive territories in neural plate and neural tube are highly conserved in vertebrates. The aim of this review is to re-examine and correlate the distribution of presumptive neuroepithelial domains in the chick neural tube with molecular information and discuss recent data. First, we review descriptive fate map studies of neural plate in different vertebrate species that have been studied using diverse fate-mapping methods. Then, we summarize the available data on the localization of neuroepithelial progenitors for the brain subregions in the chick neural tube at stage HH10-11, the most used stage for experimental embryology. This analysis is mainly focused on experimental fate mapping results using quail-chick chimeras.

Development of Functional Synaptic Connections in the Auditory System Visualized With Optical Recording: Afferent-Evoked Activity Is Present From Early Stages

A comprehensive survey of auditory network formation was performed in the brain stem of the chicken embryo using voltage-sensitive dye recording. Intact medulla/brain stem preparations with the auditory branch of the eighth nerve attached were dissected from 5.5- to 8-day chicken embryos, and responses evoked by nerve stimulation were recorded optically. In the medulla of 7- and 8-day embryos, we identified four response areas, corresponding to ipsilateral Nucleus magnocellularis (NM) and Nucleus angularis (NA), which receive the auditory afferents, and ipsi- and contralateral Nucleus laminaris (NL), which receive projections from NM. The optical responses consisted of a fast spikelike signal followed by a long-lasting slow signal, which reflected the sodium-dependent action potential and glutamatergic excitatory postsynaptic potential (EPSP), respectively. In NM, NA, and NL, the EPSP-related slow optical signals were detected from some 6-day and all 7- and 8-day preparations, indicating that functional synaptic connectivity in these nuclei arises by the 7-day stage. In the pons of 7- and 8-day embryos, we identified two additional response areas, which evidently correspond to ipsi- and contralateral Nucleus lemnisci lateralis (NLL), the higher-order nuclei of the auditory pathway. Furthermore, we detected optical responses from the contralateral cerebellum, which possibly correspond to transient projections observed only during embryogenesis. The present study demonstrates that functional auditory circuits are established in the chicken embryo at stages earlier than previously reported. We discuss the possible role of afferent-evoked activity with reference to auditory neural network formation.

Assembly of the Brainstem Cochlear Nuclear Complex Is Revealed by Intersectional and Subtractive Genetic Fate Maps

The cochlear nuclear complex (CN) is the entry point for central auditory processing. Although constituent neurons have been studied physiologically, their embryological origins and molecular profiles remain obscure. Applying intersectional and subtractive genetic fate mapping approaches, we show that this complex develops modularly from genetically separable progenitor populations arrayed as rostrocaudal microdomains within and outside the hindbrain (lower) rhombic lip (LRL). The dorsal CN subdivision, structurally and topographically similar to the cerebellum, arises from microdomains unexpectedly caudal and noncontiguous to cerebellar primordium; ventral CN subdivisions arise from more rostral LRL. Magnocellular regions receive contributions from LRL and coaxial non-lip progenitors; contrastingly, ensheathing granule cells derive principally from LRL. Also LRL-derived and molecularly similar to CN granule cells are precerebellar mossy fiber neurons; surprisingly, these ostensibly intertwined populations have separable origins and adjacent but segregated migratory streams. Together, these findings provide new platforms for investigating the development and evolution of auditory and cerebellar systems.

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.

Avian superior olivary nucleus provides divergent inhibitory input to parallel auditory pathways

The avian auditory brainstem displays parallel processing, a fundamental feature of vertebrate sensory systems. Nuclei specialized for temporal processing are largely separate from those processing other aspects of sound. One possible exception to this parallel organization is the inhibitory input provided by the superior olivary nucleus (SON) to nucleus angularis (NA), nucleus magnocellularis (NM), and nucleus laminaris (NL) and contralateral SON (SONc). We sought to determine whether single SON neurons project to multiple targets or separate neuronal populations project independently to individual target nuclei. We introduced two different fluorescent tracer molecules into pairs of target nuclei and quantified the extent to which retrogradely labeled SON neurons were double labeled. A large proportion of double-labeled SON somata were observed in all cases in which injections were made into any pair of ipsilateral targets (NA and NM, NA and NL, or NM and NL), suggesting that many individual SON neurons project to multiple targets. In contrast, when injections involved the SONc and any or all of the ipsilateral targets, double labeling was rare, suggesting that contralateral and ipsilateral targets are innervated by distinct populations of SON neurons arising largely from regionally segregated areas of SON. Therefore, at the earliest stages of auditory processing, there is interaction between pathways specialized to process temporal cues and those that process other acoustic features. We present a conceptual model that incorporates these results and suggest that SON circuitry, in part, functions to offset interaural intensity differences in interaural time difference processing.

Distribution of the limbic system-associated membrane protein (LAMP) in pigeon forebrain and midbrain

The limbic system-associated membrane protein (LAMP) is an adhesion molecule involved in specifying regional identity during development, and it is enriched in the neuropil of limbic brain regions in mammals but also found in some somatic structures. Although originally identified in rat, LAMP is present in diverse species, including avians. In this study, we used immunolabeling with a monoclonal antibody against rat LAMP to examine the distribution of LAMP in pigeon forebrain and midbrain. LAMP immunolabeling was prominent in many telencephalic regions previously noted as limbic in birds. These regions include the hippocampal complex, the medial nidopallium, and the ventromedial arcopallium. Subpallial targets of these pallial regions were also enriched in LAMP, such as the medial-most medial striatum. Whereas some telencephalic areas that have not been regarded as limbic were also LAMP-rich (e.g., the hyperpallium intercalatum and densocellulare of the Wulst, the mesopallium, and the intrapeduncular nucleus), most nonlimbic telencephalic areas were LAMP-poor (e.g., field L, the lateral nidopallium, and somatic basal ganglia). Similarly, in the diencephalon and midbrain, prominent LAMP labeling was observed in such limbic areas as the dorsomedial thalamus, the hypothalamus, the ventral tegmental area, and the central midbrain gray, as well as in a few nonlimbic areas such as nucleus rotundus, the shell of the nucleus pretectalis, the superficial tectum, and the parvocellular isthmic nucleus. Thus, as in mammals, LAMP in birds appears to be enriched in most known forebrain and midbrain limbic structures but is present as well in some somatic structures.

Synaptic physiology in the cochlear nucleus angularis of the chick

Nucleus angularis (NA), one of the two cochlear nuclei in birds, is important for processing sound intensity for localization and most likely has role in sound recognition and other auditory tasks. Because the synaptic properties of auditory nerve inputs to the cochlear nuclei are fundamental to the transformation of auditory information, we studied the properties of these synapses onto NA neurons using whole cell patch-clamp recordings from auditory brain stem slices from embryonic chickens (E16-E20). We measured spontaneous excitatory postsynaptic currents (EPSCs), and evoked EPSCs and excitatory postsynaptic potentials (EPSPs) by using extracellular stimulation of the auditory nerve. These excitatory EPSCs were mediated by AMPA and N-methyl-D-aspartate (NMDA) receptors. The spontaneous EPSCs mediated by AMPA receptors had submillisecond decay kinetics (556 micros at E19), comparable with those of other auditory brain stem areas. The spontaneous EPSCs increased in amplitude and became faster with developmental age. Evoked EPSC and EPSP amplitudes were graded with stimulus intensity. The average amplitude of the EPSC evoked by minimal stimulation was twice as large as the average spontaneous EPSC amplitude (approximately 110 vs. approximately 55 pA), suggesting that single fibers make multiple contacts onto each postsynaptic NA neuron. Because of their small size, minimal EPSPs were subthreshold, and we estimate at least three to five inputs were required to reach threshold. In contrast to the fast EPSCs, EPSPs in NA had a decay time constant of approximately 12.5 ms, which was heavily influenced by the membrane time constant. Thus NA neurons spatially and temporally integrate auditory information arriving from multiple auditory nerve afferents.

Primary innervation of the avian and mammalian cochlear nucleus

The auditory nerve of birds and mammals exhibits differences and similarities, but given the millions of years since the two classes diverged from a common ancestor, the similarities are much more impressive than the differences. The avian nerve is simpler than that of mammals, but share many fundamental features including principles of development, structure, and physiological properties. Moreover, the available evidence shows that the human auditory nerve follows this same general organizational plan. Equally impressive are reports that homologous genes in worms, flies, and mice exert the same heredity influences in man. The clear implication is that animal studies will produce knowledge that has a direct bearing on the human condition.

Computational diversity in the cochlear nucleus angularis of the barn owl

The cochlear nucleus angularis (NA) is widely assumed to form the starting point of a brain stem pathway for processing sound intensity in birds. Details of its function are unclear, however, and its evolutionary origin and relationship to the mammalian cochlear-nucleus complex are obscure. We have carried out extracellular single-unit recordings in the NA of ketamine-anesthetized barn owls. The aim was to re-evaluate the extent of heterogeneity in NA physiology because recent studies of cellular morphology had established several distinct types. Extensive characterization, using tuning curves, phase locking, peristimulus time histograms and rate-level functions for pure tones and noise, revealed five major response types. The most common one was a primary-like pattern that was distinguished from auditory-nerve fibers by showing lower vector strengths of phase locking and/or lower spontaneous rates. Two types of chopper responses were found (chopper-transient and a rare chopper-sustained), as well as onset units. Finally, we routinely encountered a complex response type with a pronounced inhibitory component, similar to the mammalian typeIV. Evidence is presented that this range of response types is representative for birds and that earlier conflicting reports may be due to methodological differences. All five response types defined were similar to well-known types in the mammalian cochlear nucleus. This suggests convergent evolution of neurons specialized for encoding different behaviorally relevant features of the auditory stimulus. It remains to be investigated whether the different response types correlate with morphological types and whether they establish different processing streams in the auditory brain stem of birds.

Developmental changes in membrane excitability and morphology of neurons in the nucleus angularis of the chicken

In order to understand how sound intensity information is extracted and processed in the auditory nuclei, we investigated the neuronal excitability in the nucleus angularis (NA) of the chicken (P0-5) and the chicken embryo (E16-21). In embryos, neurons fired basically in three patterns in response to current injections: the onset pattern (19 %), the tonic pattern (52 %) and the pause pattern (29 %). After hatching, neurons fired either in the tonic pattern (83 %) or in the onset pattern (17 %). In both pre- and post-hatch periods, multiple firing neurons (tonic and pause) increased the maximum rate of rise of the action potential 2.6-fold, the fall 3.9-fold, and the maximum firing frequency 4-fold, and shifted the threshold potential to be more negative. After hatching, the firing frequency of tonic neurons reached a maximum at about 650 Hz. Application of TEA (1 mM) reduced the firing frequency, broadened action potentials and reduced the maximum rate of fall, but the threshold current was not changed. Dendrotoxin-I (DTX, 100 nM) reduced the threshold current. Application of DTX induced the onset neuron to fire repetitively. Branching patterns of auditory nerve fibres (ANFs) in NA were visualized by labelling with 1,1'-dioctadecyl-3,3,3',3'-tetramethylindocarbocyanine perchlorate (Di-I) placed within the cochlea. Di-I placed near the apex of the cochlea labelled the ventral part of the NA, and Di-I placed in the base labelled the dorso-lateral part. Tonic neurons labelled with biocytin extended dendrites in parallel with the projection of ANFs in the nucleus after hatching. ANF activity of a limited range of characteristic sound frequencies is thought to be extracted by tonic neurons and encoded into firing frequencies proportional to the strength of the input.

Effect of prenatal auditory enrichment on developmental expression of synaptophysin and syntaxin 1 in chick brainstem auditory nuclei

Neural activity plays an important role in shaping the developing brain. We have determined the consequence of increased auditory stimulation on the developmental profile of synaptic proteins, synaptophysin and syntaxin 1, in the chick brainstem auditory nuclei, nucleus magnocellularis and nucleus laminaris, by immunohistochemistry and western blotting techniques. The chick embryos were provided with patterned sounds of species-specific calls or musical notes of a sitar, a stringed instrument, in a graded manner from embryonic day 10 (E10) through hatching, for 15 min every hour. During normal synaptogenesis of nucleus magnocellularis and nucleus laminaris, synaptophysin immunoreactivity increased significantly from E8 to E20, in parallel with synapse formation, and reduced at hatching. The embryos receiving species-specific sound stimuli exhibited a similar pattern with higher levels of immunoreactivity, though the difference between the study groups was not statistically significant. The music stimulated embryos showed an earlier peak at E16, followed by a gradual decline until hatching. In all three groups studied, syntaxin immunoreactivity showed a surge at E12, followed by a decline at E16 and subsequent stabilization. The stimulated groups continually expressed higher amounts of syntaxin immunoreactivity. The results suggest that prenatal sound stimulation enhances the normal pattern of synaptic protein expression in these auditory nuclei.

Intrinsic neuronal properties of the chick nucleus angularis

In vitro whole cell recording revealed intrinsic firing properties and single-cell morphology in the cochlear nucleus angularis (NA) of the chick. We classified three major classes of neurons: one-spike, damped, and tonic. A delayed inward rectifying current was observed in all classes during hyperpolarization injections. One-spike neurons responded with a single spike to depolarizing current injection and had small (stubby) radiate dendritic trees. Damped neurons responded with only a few spikes at the onset of positive current injection. More positive current inputs led to a damped response. Damped cell dendrites had a planar orientation parallel to the isofrequency axis in NA. Tonic cells produced trains of action potentials in response to a depolarizing current injection. Three variations of the tonic type had multipolar morphology, with dendrites oriented either radially (I and III) or perpendicular to the tonotopic axis (II; vertical). Tonics I and III differed in the shape of their action potential undershoot. Thus NA is both physiologically and morphologically heterogeneous.

The cytoarchitecture of the nucleus angularis of the barn owl (Tyto alba)

The cochlear nucleus angularis (NA) of the barn owl (Tyto alba) was analyzed using Golgi, Nissl, and tract tracing techniques. NA forms a column of cells in the dorsolateral brainstem that partly overlaps with, and is rostral and lateral to, the cochlear nucleus magnocellularis (NM). Highest best frequencies are mapped in lateral NA (NAl), intermediate in medial NA (NAm), and lowest in the foot region (NAf). Cell density followed the tonotopic axis and decreased with decreasing best frequency. NA contained four major cell classes: planar, radiate, vertical, and stubby. Planar and radiate classes were further subdivided into bipolar and multipolar types according to their number of primary dendrites. Planar neurons were confined to an isofrequency band, whereas radiate neurons had dendrites that could extend across an isofrequency band. Vertical cells had long dendrites oriented perpendicularly to isofrequency bands. Stubby cells were the most numerous and were confined to an isofrequency band because of their short dendrites. Neurons in each of these four classes projected to the inferior colliculus and dorsal nucleus of the lateral lemniscus.

A quantitative study of cochlear afferent axons in birds

This paper is a comparative study of auditory-nerve morphology in birds. The chicken (Gallus gallus), the emu (Dromaius novaehollandiae) and the starling (Sturnus vulgaris) were chosen as unspecialised birds that have already been used in auditory research. The data are discussed in comparison to a similar earlier study on the barn owl, a bird with highly specialised hearing, in an attempt to separate general avian patterns from species specialisations. Average numbers of afferent fibres from 8775 (starling) to 12¿ omitted¿406 (chicken) were counted, excluding fibres to the lagenar macula. The number of fibres representing different frequency ranges showed broad maxima in the chicken and emu, corresponding to hearing ranges of best sensitivity and/or particular behavioural relevance. Mean axon diameters were around 2 microm in the chicken and starling, and around 3 microm in the emu. Virtually all auditory afferents were myelinated. The mean thickness of the myelin sheaths was between 0.33 microm (starling) and 0.4 microm (emu). There was a consistent pattern in the diameters of axons deriving from different regions. Axons from very basal, i.e. highest-frequency, parts of the basilar papilla were always the smallest. In the emu and the chicken, axons from the middle papillar regions were, in addition, larger than axons innervating apical regions.

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.

Electrical response properties of avian lagena type II hair cells: a model system for vestibular filtering

Data presented represent the first electrical recordings from avian lagena type II hair cells. The perforated-patch variant of the whole cell recording technique was used to investigate how the macroscopic currents shaped the voltage response of the hair cells. Voltage-clamp data separated cells into two broad classes on the basis of differences in activation rates, rates and degree of inactivation, and pharmacological sensitivity. Current-clamp recordings revealed low-quality membrane voltage oscillations (Qc < 1) during pulse current injections. Oscillation frequency correlated with activation rate of the macroscopic currents. The quality of membrane oscillations (Qc) varied linearly with frequency for cells with little inactivation. For cells with rapid inactivation, no relationship was found between Qc and frequency. Rapid inactivation may serve to extend the bandwidth of vestibular hair cells. The frequency measured from voltage responses to pulsed currents may reflect the corner frequency of the cell. The filtering properties of avian lagena hair cells are like those found in all other vestibular end organs, suggesting that the electrical membrane properties of these cells are not responsible for specializing them to a particular stimulus modality.

Brainstem connections of the macula lagenae in the chicken

The macula lagenae, an otolithic hair-cell organ with probable vestibular function, lies close to the apical end of the avian auditory hair-cell epithelium, the papilla basilaris. In an earlier study in the pigeon in which lesioning techniques were used, Boord and Rasmussen ([1963] J. Comp. Neurol. 120:463-473) reported finding a projection of lagenar fibers to parts of the cochlear nuclei (nucleus magnocellularis and nucleus angularis). Subsequent to this report, it has been generally assumed that at least part of the cochlear nuclei has a vestibular or a combined vestibular-auditory function. In this study, we labeled fibers innervating the macula lagenae of the chicken by using a lipophilic fluorescent tracer. The analysis of Vibratome sections of the brainstem with epifluorescence illumination showed no projection to the cochlear nuclei. In cases in which the apical part of the papilla basilaris was contaminated with tracer, however, we found labeling of the cochlear nuclei in the same areas as described with the lesioning technique in the pigeon. Our results thus imply that there is no processing of information from the macula lagenae in the cochlear nucleus of the chicken. In addition, we studied the origin of the few labeled efferent neurons in the brainstem. The location of all somata encountered was restricted to an area medial to the nucleus facialis dorsalis and corresponded to the dorsal efferent cell group, from which efferents to other vestibular organs also originate.

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.

Voltage-gated ionic currents and their roles in timing coding in auditory neurons of the nucleus magnocellularis of the chick

Avian cochlear neurons of the nucleus magnocellularis (NMC) are known to encode temporal information of sound. The neuron generated only a single action potential at a stable timing even though suprathreshold currents of long duration (> 100 ms) was injected. The threshold for the action potential was -42 mV. In voltage-clamp experiments, a TTX-sensitive Na current was activated at membrane potentials more positive than -50 mV. A low voltage activated (LVA) Ca current and a high voltage activated (HVA) Ca current were observed. The LVA Ca current was activated from -65 mV and showed a voltage dependent inactivation. The HVA Ca current was activated from -40 mV and did not show any inactivation. The LVA Ca current and the HVA Ca current were sensitive to Ni2+ (0.1 mM) and Nifedipine (10-20 mM), respectively. NMC neurons showed a TEA-sensitive K current and a 4-AP-sensitive K current. With 4-AP (0.5 mM) in a bathing medium, the threshold of action potential was decreased to -49 mV and the timing of action potential generation showed a wider distribution than that of control. Ni2+ (0.1 mM) reversed effects of 4-AP on the threshold and the variability of action potential onsets. It is concluded that a 4-AP-sensitive current counteracts the LVA Ca current that facilitates Na spike generation, and sets a threshold to a higher level for generating a single action potential at a precise timing following synaptic inputs from the auditory nerve.

Differential central projections of vestibular afferents in pigeons

The question of whether a differential distribution of vestibular afferent information to central nuclear neurons is present in pigeons was studied using neural tracer compounds. Discrete tracing of afferent fibers innervating the individual semicircular canal and otolith organs was produced by sectioning individual branches of the vestibular nerve that innervate the different receptor organs and applying crystals of horseradish peroxidase, or a horseradish peroxidase/cholera toxin mixture, or a biocytin compound for neuronal uptake and transport. Afferent fibers and their terminal distributions within the brainstem and cerebellum were visualized subsequently. Discrete areas in the pigeon central nervous system that receive primary vestibular input include the superior, dorsal lateral, ventral lateral, medial, descending, and tangential vestibular nuclei; the A and B groups; the intermediate, medial, and lateral cerebellar nuclei; and the nodulus, the uvula, and the paraflocculus. Generally, the vertical canal afferents projected heavily to medial regions in the superior and descending vestibular nuclei as well as the A group. Vertical canal projections to the medial and lateral vestibular nuclei were observed but were less prominent. Horizontal canal projections to the superior and descending vestibular nuclei were much more centrally located than those of the vertical canals. A more substantial projection to the medial and lateral vestibular nuclei was seen with horizontal canal afferents compared to vertical canal fibers. Afferents innervating the utricle and saccule terminated generally in the lateral regions of all vestibular nuclei in areas that were separate from the projections of the semicircular canals. In addition, utricular fibers projected to regions in the vestibular nuclei that overlapped with the horizontal semicircular canal terminal fields, whereas saccular afferents projected to regions that received vertical canal fiber terminations. Lagenar afferents projected throughout the cochlear nuclei, to the dorsolateral regions of the cerebellar nuclei, and to lateral regions of the superior, lateral, medial, and descending vestibular nuclei.

Neuronal architecture of the dorsal nucleus (cochlear nucleus) of the frog, Rana pipiens pipiens

The neuronal architecture of the dorsal nucleus of the Northern leopard frog (Rana pipiens pipiens), which is a homolog of the cochlear nucleus of mammals and birds, was investigated. Our study showed that the frog dorsal nucleus contains a number of morphologically distinct cell types that are discernible in terms of the cellular architecture as derived from Nissl-stained material and in terms of the dendritic profile as revealed by horseradish peroxidase-filled single neurons. These cell types are bushy cells, bipolar (or fusiform) cells, octopus cells, stellate cells, giant cells, radiate (or round) cells, and a variety of small cells. The different cell types occupy different regions of the nucleus. Therefore, our results suggest that the dorsal nucleus should no longer be considered to be a uniform nucleus containing a homogeneous population of neurons. Homologies of these cell types with those described in other vertebrate species, including mammals, are proposed.

Convergence of somatosensory and auditory projections in the avian torus semicircularis, including the central auditory nucleus

Projections of dorsal column, spinal, and cochlear nuclei upon the central nucleus of the torus semicircularis (otherwise known as nucleus mesencephalicus lateralis, pars dorsalis, or MLd) and upon other toral nuclei were investigated in pigeon by anterograde and retrograde tracing and electrophysiological methods. The anatomical results showed that caudal regions of the dorsal column nuclei and medial lamina V of the upper four cervical spinal segments have extensive projections upon the contralateral central auditory nucleus and upon other nuclei of the torus, in particular the core portion of the preisthmic superficial area of Puelles et al. (L. Puelles, C. Rrobles, M. Martiez-de-la-Torre, and S. Martinez, 1994, J. Comp. Neurol. 340:98-125). The projections of nucleus angularis were found to terminate throughout most of the contralateral central nucleus except the dorsomedial portion at rostral levels, where the majority of the projections of nucleus laminaris were concentrated. Nucleus angularis (and to a lesser extent nucleus laminaris) was also found to have substantial projections to certain noncentral toral nuclei, in particular to the caudomedial shell nucleus of Puelles et al. (1994). As shown positively with both Nissl and cytochrome oxidase staining and negatively with substance P labeling, this nucleus is a medial extension of more caudal regions of the central nucleus, and it is suggested that it should be included as part of the auditory midbrain. The electrophysiological results confirmed the anatomical findings by showing that evoked potentials and multiunit activity can be recorded throughout the central and noncentral toral nuclei by using electrical stimulation of the radial nerve and auditory click stimuli. The core portion of the preisthmic superficial area, however, can be regarded as a distinct somatosensory nucleus of the midbrain. It is concluded that there is substantial convergence of somatosensory and auditory inputs within both central auditory and noncentral nuclei of the torus semicircularis in pigeon.

Morphological fate of rhombomeres in quail/chick chimeras: a segmental analysis of hindbrain nuclei

Quail rhombomeres two to six (r2-r6) were individually grafted homotopically into the hindbrain of chick embryos at 2 days of incubation. Nine to 10 days after the operation the chimeric embryos were fixed and processed for parallel cytoarchitectural and immunocytochemical study (with an anti-quail antibody) in order to map the anatomical fate of the grafted tissue. Emphasis was placed on conventionally identified and distinct neuronal populations composing the sensory and motor longitudinal columns. Grafted rhombomeres consistently developed as complete transverse slices of the chimeric hindbrain. Interrhombomeric cell migration was either sparse or restricted to specific nuclei. The cranial nerve motor nuclei showed rhombomeric origins consistent with the patterns described in early embryos. Unexpectedly, alar r2 was found to form the auricular part of the cerebellum. As regards the cochlear nuclei, we found that nucleus angularis derives from r3 to r6, nucleus laminaris from r5 to r6, nucleus magnocellularis from r6 to r7 and nucleus olivaris superior from r5. The nuclei of the lateral lemniscus originated between r1 and r3. We also delimited the respective rhombomeric subdivisions of the sensory vestibular and trigeminal columns, both of which extend from r1 caudalwards throughout the hindbrain. There were consistently some interrhombomeric neuronal migrations inside the vestibular column, some motor nuclei and the reticular formation, involving only one rhombomere length. The pontine nuclei, which extended from r1 to r7, showed neuronal migrations that crossed several rhombomeres. On the whole, these results represent the first anatomical analysis of the mature avian hindbrain in terms of rhombomere-derived domains.

Neuronal organization of the cochlear nuclei in alligator lizards: a light and electron microscopic investigation

The organization of neurons and fibers in the cochlear nuclei of the alligator lizard (Gerrhonotus multicarinatus) was examined with light and electron microscopy. In this species, much is known about the anatomy and physiology of the inner ear including the cochlear nerve, but little is known about the synaptic connections of cochlear fibers on second-order neurons. These data will help to develop general principles addressing the cellular organization of the vertebrate auditory system. Subdivisions of the cochlear nuclei were defined on the basis of their histologic appearance and neuronal composition. Neuron classes were proposed from their light microscopic and ultrastructural features. Nucleus magnocellularis medialis consists of a homogeneous population of neurons called "lesser ovoid" cells. Nucleus magnocellularis lateralis consists of "greater ovoid" and "small" cells. Nucleus angularis lateralis consists of "spindle" cells. Lastly, nucleus angularis medialis contains a population of large neurons called "duckhead" and "multipolar" cells, and a population of smaller neurons called "bulb" and "agranular" cells. These neuron populations are differentially innervated by tectorial and free-standing cochlear fibers that are associated with separate frequency ranges. All neuronal populations except agranular cells were observed to receive synaptic input from cochlear nerve fibers. In nucleus magnocellularis medialis and nucleus angularis medialis, primary afferents form both chemical and electrical synapses with resident neurons. These observations imply that acoustic information is synaptically processed in fundamentally distinct ways in the cochlear nuclei of alligator lizards and distributed along separate neural circuits. Thus, the characteristic structural and functional dichotomy of the alligator lizard inner ear is extended to central auditory pathways by way of cochlear nerve projections.

A depolarizing inhibitory response to GABA in brainstem auditory neurons of the chick

Neurons in the brainstem auditory nuclei, n. magnocellularis and n. laminaris, of the chick are contacted by terminals containing the inhibitory neurotransmitter gamma-aminobutyric acid (GABA). In this report we describe the physiological response of these neurons to GABA using an in vitro slice preparation. In brainstem auditory neurons, GABA produced a depolarization of up to 20 mV and an associated decrease in input resistance. This depolarization was inhibitory; action potentials generated by orthodromic synaptic drive, antidromic stimulation and intracellular current injection were prevented by GABA application. The GABA response still occurred when synaptic transmission was prevented by perfusing the slice with a medium containing low Ca2+ and high Mg2+ concentrations. Thus, the effects of GABA were directly on the postsynaptic neuron and not via an interneuron. Whole-cell voltage clamp of neurons revealed that the reversal potential of the inward current was approximately -45 mV, suggesting that the channel responsible for this response is not selective for Cl- or K+. Pharmacological analyses suggest that this GABA receptor has properties distinct from those typical of either GABAa or GABAb receptors. Although a similar response was observed with the GABAa agonist, muscimol, it was not blocked by the GABAa antagonist, bicuculline. The response was not evoked by the GABAb agonist, baclofen, and was not blocked by the GABAb antagonist phaclofen. This unusual depolarizing response is not a common feature of all brainstem neurons. Neurons located in the neighboring medial vestibular nucleus show a more traditional response to GABA application. At resting potential, these neurons show a hyperpolarizing or biphasic response associated with a decrease in input resistance and inhibition of their spontaneous activity. GABA-induced responses in the medial vestibular nucleus are blocked by bicuculline. These results suggest that an unusual form of the GABA receptor is present in the brainstem auditory system of the chick. It is possible that this form of GABA receptor provides an efficient mechanism for inhibiting the relatively powerful EPSPs received by brainstem auditory neurons, or it may play a trophic role in the afferent regulation of neuronal integrity in this system.

Efferent neurons to the macular lagena in the embryonic chick

The lipophilic dye, DiI, was placed into the macula lagena of paraformaldehyde-fixed embryonic chicks. Retrogradely labeled cells were found bilaterally in the pontine reticular formation (RF) between the dorsal facial nucleus and the abducens nerve root. This location is similar to that of the dorsomedial group of efferent cells that project to the basilar papilla. No lagenar efferent neurons, however, were found near the superior olivary nucleus where the ventrolateral group of cochlear efferents is located. Whether efferent neurons in the pontine RF send collaterals to both the basilar papilla and to the macula lagena has yet to be determined.

The effect of acoustic overexposure on the tonotopic organization of the nucleus magnocellularis

We assessed the effect a sound-induced cochlear lesion had on the tonotopic organization of the nucleus magnocellularis (NM) immediately after acoustic overexposure and following a twelve day recovery period. The acoustic overexposure was a 0.9 kHz tone at 120 dB sound pressure level (SPL) for 48 h. Initially after the acoustic overexposure, the tonotopic organization of the NM was statistically different from that of age-matched controls. Specifically, it appeared that the center frequencies of units in the frequency region of the NM associated with the acoustic overexposure had higher center frequencies than their control counterparts. Following a twelve day recovery period, when threshold sensitivity and frequency selectivity were operating normally, the tonotopic organization of the NM was not statistically different from age-matched controls. We suggest that the sound-induced changes in the tonotopic organization of the NM reflect peripheral damage in the basilar papilla. It has been well documented that similar exposure paradigms produce a loss of short hair cells and a degeneration of the tectorial membrane in the region of the basilar membrane associated with the overexposure. We postulate that the loss of these structures alters the micromechanics and tuning of the basilar membrane which is reflected in the observed changes in NM tonotopy. Following the recovery period, when those structures destroyed by the overexposure had regenerated and basilar membrane micromechanics were operating normally, the tonotopic organization of the NM returned to normal.