GABAA receptors in auditory brainstem nuclei of the chick during development and after cochlea removal

Expression patterns of chloride transporters in the auditory brainstem of developing chicken

GABAergic transmission changes from depolarization to hyperpolarization in most vertebrate brain regions during development. By contrast, in the auditory brainstem of chicken a depolarizing effect of GABA persists after hatching. Since auditory brainstem neurons that receive GABAergic input have a Cl^- reversal potential above resting membrane potential, a specifically tuned activity of Cl^- transporters is likely. We here present a developmental study of the expression patterns of several members of the SLC12 family (NKCC1, NKCC2, KCC1, KCC2, KCC4, CCC6, CCC9) and of AE3 at developmental ages E7, E10, E12, E15, E17, and P1 with quantitative RT-PCR. NKCC2 and CCC9 were not detected in auditory brainstem (positive control: kidney). KCC1, CCC6 and AE3 were expressed, but not regulated, while NKCC1, KCC2 and KCC4 were regulated. The expression of the latter transporters increased, with KCC2 exhibiting the strongest expression at all time points. Biochemical analysis of the protein expression of NKCC1, KCC2 and KCC4 corroborated the findings on the mRNA level. All three transporters showed a localization at the outer rim of the cells, with NKCC1 and KCC2 expressed in neurons, and KCC4 predominantly in glia. The comparison of the published chloride reversal potential and expression of transporter proteins suggest strong differences in the efficiency of the three transporters. Further, the strong KCC2 expression could reflect a role in the structural development of auditory brainstem synapses that might lead to changes in the physiological properties.

Interplay between low threshold voltage-gated K(+) channels and synaptic inhibition in neurons of the chicken nucleus laminaris along its frequency axis

Central auditory neurons that localize sound in horizontal space have specialized intrinsic and synaptic cellular mechanisms to tightly control the threshold and timing for action potential generation. However, the critical interplay between intrinsic voltage-gated conductances and extrinsic synaptic conductances in determining neuronal output are not well understood. In chicken, neurons in the nucleus laminaris (NL) encode sound location using interaural time difference (ITD) as a cue. Along the tonotopic axis of NL, there exist robust differences among low, middle, and high frequency (LF, MF, and HF, respectively) neurons in a variety of neuronal properties such as low threshold voltage-gated K⁺ (LTK) channels and depolarizing inhibition. This establishes NL as an ideal model to examine the interactions between LTK currents and synaptic inhibition across the tonotopic axis. Using whole-cell patch clamp recordings prepared from chicken embryos (E17–E18), we found that LTK currents were larger in MF and HF neurons than in LF neurons. Kinetic analysis revealed that LTK currents in MF neurons activated at lower voltages than in LF and HF neurons, whereas the inactivation of the currents was similar across the tonotopic axis. Surprisingly, blockade of LTK currents using dendrotoxin-I (DTX) tended to broaden the duration and increase the amplitude of the depolarizing inhibitory postsynaptic potentials (IPSPs) in NL neurons without dependence on coding frequency regions. Analyses of the effects of DTX on inhibitory postsynaptic currents led us to interpret this unexpected observation as a result of primarily postsynaptic effects of LTK currents on MF and HF neurons, and combined presynaptic and postsynaptic effects in LF neurons. Furthermore, DTX transferred subthreshold IPSPs to spikes. Taken together, the results suggest a critical role for LTK currents in regulating inhibitory synaptic strength in ITD-coding neurons at various frequencies.

Slowly emerging glycinergic transmission enhances inhibition in the sound localization pathway of the avian auditory system

Localization of low-frequency acoustic stimuli is processed in dedicated neural pathways where coincidence-detecting neurons compare the arrival time of sound stimuli at the two ears, or interaural time disparity (ITD). ITDs occur in the submillisecond range, and vertebrates have evolved specialized excitatory and inhibitory circuitry to compute these differences. Glycinergic inhibition is a computationally significant and prominent component of the mammalian ITD pathway. However, evidence for glycinergic transmission is limited in birds, where GABAergic inhibition has been thought to be the dominant or exclusive inhibitory transmitter. Indeed, previous work showed that GABA antagonists completely eliminate inhibition in avian nuclei specialized for processing temporal features of sound, nucleus magnocellularis (NM) and nucleus laminaris (NL). However, more recent work shows that glycine is coexpressed with GABA in synaptic terminals apposed to neurons in both nuclei (Coleman WL, Fischl MJ, Weimann SR, Burger RM. J Neurophysiol 105: 2405-2420, 2011; Kuo SP, Bradley LA, Trussell LO. J Neurosci 29: 9625-9634, 2009). Here we show complementary evidence of functional glycine receptor (GlyR) expression in NM and NL. Additionally, we show that glycinergic input can be evoked under particular stimulus conditions. Stimulation at high but physiologically relevant rates evokes a slowly emerging glycinergic response in NM and NL that builds over the course of the stimulus. Glycinergic response magnitude was stimulus rate dependent, representing 18% and 7% of the total inhibitory current in NM and NL, respectively, at the end of the 50-pulse, 200-Hz stimulus. Finally, we show that the glycinergic component is functionally relevant, as its elimination reduced inhibition of discharges evoked by current injection into NM neurons.

Transgenic quail as a model for research in the avian nervous system: a comparative study of the auditory brainstem

Research performed on transgenic animals has led to numerous advances in biological research. However, using traditional retroviral methods to generate transgenic avian research models has proved problematic. As a result, experiments aimed at genetic manipulations on birds have remained difficult for this popular research tool. Recently, lentiviral methods have allowed the production of transgenic birds, including a transgenic Japanese quail (Coturnix coturnix japonica) line showing neuronal specificity and stable expression of enhanced green fluorescent protein (eGFP) across generations (termed here GFP quail). To test whether the GFP quail may serve as a viable alternative to the popular chicken model system, with the additional benefit of genetic manipulation, we compared the development, organization, structure, and function of a specific neuronal circuit in chicken (Gallus gallus domesticus) with that of the GFP quail. This study focuses on a well-defined avian brain region, the principal nuclei of the sound localization circuit in the auditory brainstem, nucleus magnocellularis (NM), and nucleus laminaris (NL). Our results demonstrate that structural and functional properties of NM and NL neurons in the GFP quail, as well as their dynamic properties in response to changes in the environment, are nearly identical to those in chickens. These similarities demonstrate that the GFP quail, as well as other transgenic quail lines, can serve as an attractive avian model system, with the advantage of being able to build on the wealth of information already available from the chicken.

Astrocyte-secreted factors modulate the developmental distribution of inhibitory synapses in nucleus laminaris of the avian auditory brainstem

Nucleus laminaris (NL) neurons in the avian auditory brainstem are coincidence detectors necessary for the computation of interaural time differences used in sound localization. In addition to their excitatory inputs from nucleus magnocellularis, NL neurons receive inhibitory inputs from the superior olivary nucleus (SON) that greatly improve coincidence detection in mature animals. The mechanisms that establish mature distributions of inhibitory inputs to NL are not known. We used the vesicular GABA transporter (VGAT) as a marker for inhibitory presynaptic terminals to study the development of inhibitory inputs to NL between embryonic day 9 (E9) and E17. VGAT immunofluorescent puncta were first seen sparsely in NL at E9. The density of VGAT puncta increased with development, first within the ventral NL neuropil region and subsequently throughout both the ventral and dorsal dendritic neuropil, with significantly fewer terminals in the cell body region. A large increase in density occurred between E13–15 and E16–17, at a developmental stage when astrocytes that express glial fibrillary acidic protein (GFAP) become mature. We cultured E13 brainstem slices together with astrocyte-conditioned medium (ACM) obtained from E16 brainstems and found that ACM, but not control medium, increased the density of VGAT puncta. This increase was similar to that observed during normal development. Astrocyte-secreted factors interact with the terminal ends of SON axons to increase the number of GABAergic terminals. These data suggest that factors secreted from GFAP-positive astrocytes promote maturation of inhibitory pathways in the auditory brainstem.

Chloride cotransporters, chloride homeostasis, and synaptic inhibition in the developing auditory system

The role of glycine and GABA as inhibitory neurotransmitters in the adult vertebrate nervous system has been well characterized in a variety of model systems, including the auditory, which is particularly well suited for analyzing inhibitory neurotransmission. However, a full understanding of glycinergic and GABAergic transmission requires profound knowledge of how the precise organization of such synapses emerges. Likewise, the role of glycinergic and GABAergic signaling during development, including the dynamic changes in regulation of cytosolic chloride via chloride cotransporters, needs to be thoroughly understood. Recent literature has elucidated the developmental expression of many of the molecular components that comprise the inhibitory synaptic phenotype. An equally important focus of research has revealed the critical role of glycinergic and GABAergic signaling in sculpting different developmental aspects in the auditory system. This review examines the current literature detailing the expression patterns and function (chapter 1), as well as the regulation and pharmacology of chloride cotransporters (chapter 2). Of particular importance is the ontogeny of glycinergic and GABAergic transmission (chapter 3). The review also surveys the recent work on the signaling role of these two major inhibitory neurotransmitters in the developing auditory system (chapter 4) and concludes with an overview of areas for further research (chapter 5).

Topography and morphology of the inhibitory projection from superior olivary nucleus to nucleus laminaris in chickens (Gallus gallus)

The avian nucleus laminaris (NL) is involved in computation of interaural time differences (ITDs) that encode the azimuthal position of a sound source. Neurons in NL are bipolar, with dorsal and ventral dendritic arbors receiving input from separate ears. NL neurons act as coincidence detectors that respond maximally when input from each ear arrives at the two dendritic arbors simultaneously. Computational and physiological studies demonstrated that the sensitivity of NL neurons to coincident inputs is modulated by an inhibitory feedback circuit via the superior olivary nucleus (SON). To understand the mechanism of this modulation, the topography of the projection from SON to NL was mapped, and the morphology of the axon terminals of SON neurons in NL was examined in chickens (Gallus gallus). In vivo injection of AlexaFluor 568 dextran amine into SON demonstrated a coarse topographic projection from SON to NL. Retrogradely labeled neurons in NL were located within the zone of anterogradely labeled terminals, suggesting a reciprocal projection between SON to NL. In vivo extracellular physiological recording further demonstrated that this topography is consistent with tonotopic maps in SON and NL. In addition, three-dimensional reconstruction of single SON axon branches within NL revealed that individual SON neurons innervate a large area of NL and terminate on both dorsal and ventral dendritic arbors of NL neurons. The organization of the projection from SON to NL supports its proposed functions of controlling the overall activity level of NL and enhancing the specificity of frequency mapping and ITD detection.

GABAergic and glycinergic inhibition modulate monaural auditory response properties in the avian superior olivary nucleus

The superior olivary nucleus (SON) is the primary source of inhibition in the avian auditory brainstem. While much is known about the role of inhibition at the SON's target nuclei, little is known about how the SON itself processes auditory information or how inhibition modulates these properties. Additionally, the synaptic physiology of inhibitory inputs within the SON has not been described. We investigated these questions using in vivo and in vitro electrophysiological techniques in combination with immunohistochemistry in the chicken, an organism for which the auditory brainstem has otherwise been well characterized. We provide a thorough characterization of monaural response properties in the SON and the influence of inhibitory input in shaping these features. We found that the SON contains a heterogeneous mixture of response patterns to acoustic stimulation and that in most neurons these responses are modulated by both GABAergic and glycinergic inhibitory inputs. Interestingly, many SON neurons tuned to low frequencies have robust phase-locking capability and the precision of this phase locking is enhanced by inhibitory inputs. On the synaptic level, we found that evoked and spontaneous inhibitory postsynaptic currents (IPSCs) within the SON are also mediated by both GABAergic and glycinergic inhibition in all neurons tested. Analysis of spontaneous IPSCs suggests that most SON cells receive a mixture of both purely GABAergic terminals, as well as terminals from which GABA and glycine are coreleased. Evidence for glycinergic signaling within the SON is a novel result that has important implications for understanding inhibitory function in the auditory brainstem.

Inhibitory synaptogenesis in the rat anteroventral cochlear nucleus

Spherical cells in the anteroventral division of the cochlear nucleus, which relay excitatory inputs from the auditory nerve, also receive both GABAergic and glycinergic inhibitory synapses. Inhibition mediated by GABA and glycine fulfils essential roles in the processing abilities of these and other auditory neurons. However, the developmental program leading to a mature complement of GABAergic and glycinergic synapses and microcircuits is largely unknown. Because of their relatively simple geometry, spherical cells provide an excellent model for unraveling basic developmental patterns of inhibitory synaptogenesis. Using a combination of high resolution immunocytochemical methods, we report that, in the rat, synapses containing GABA or glycine are deployed on spherical cell bodies over a time period extending well beyond hearing onset. Such postnatal developmental recruitment of inhibitory endings is progressive, although there are two distinct leaps in their numbers. The first occurs by the end of the first postnatal week, prior to hearing onset, and the second, during the third postnatal week, after hearing onset. This pattern suggests that adjustments in inhibition could be driven by acoustic experience. While GABAergic and glycinergic endings are maturing and growing in number and size, their neurotransmitter content also appears to be developmentally regulated. Quantitative ultrastructural immunocytochemistry with colloidal gold suggests that GABA and glycine accumulation in synaptic endings follows a staggered pattern, with labeling stabilizing at adult levels by postnatal day 21. This may account for adjustments in synaptic efficacy and strength.

The analysis of interaural time differences in the chick brain stem

The brain stem auditory system of the chick has proven to be a useful model system for analyzing how the brain encodes temporal information. This paper reviews some of the work on a circuit in the brain stem that compares the timing of information coming from the two ears to determine the location of a sound source. The contralateral projection from the cochlear nucleus, nucleus magnocellularis (NM), to nucleus laminaris (NL) forms a delay line as it proceeds from medial to lateral across NL. NL neurons function like coincidence detectors in that they respond maximally when input from the two ears arrive simultaneously. This arrangement may allow NL to code sound space by the relative level of activity across the nucleus. The head anatomy of the chick allows for enhancement of the functional interaural time differences. Comparing the functional interaural time differences to the length of the neural delay line suggests that each NL can encode approximately one hemifield of sound space. Finally it is suggested that inhibitory input into the NM-NL circuit may provide a means to dynamically adjust the gain of the circuit to allow accurate coding of sound location despite changes in overall sound intensity.

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.

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.

Manipulation of inhibition in the owl's nucleus laminaris and its effects on optic tectum neurons

Differences in arrival time and intensity (or level) of sound between the ears serve as cues for localization of sound in many animals. Barn owls use interaural time difference (ITD) and interaural level difference (ILD) for localization in azimuth and elevation, respectively. The owl's brain processes these two cues in separate pathways. The nucleus laminaris is the first site that detects ITDs by methods of delay lines and coincidence detection. The nucleus ventralis lemnisci lateralis, pars posterior is the first site of processing ILDs. The two pathways merge in the inferior colliculus to give rise to sensitivity to combinations of ITD and ILD. This selectivity is relayed to the optic tectum where neurons are sensitive to both visual and auditory stimuli. The present paper reports the results of manipulating inhibition in the nucleus laminaris and its effects on the optic tectum neurons. Injection of GABA or muscimol (a GABA(A) receptor agonist) in the nucleus laminaris reduces the responses of its neurons to ITD. This finding proves that GABA(A) receptor-mediated inhibition acts on the nucleus laminaris neurons. The same treatment did not affect the neurons of the nucleus ventralis lemnisci lateralis, pars posterior, whereas it reduced the response of the optic tectum neurons to ITD-ILD pairs. We conclude that although the two pathways are independent, the process of combining ITD and ILD creates a new relationship in which the output of the neuron varies with the amplitude of either input. This conclusion is consistent with the recent finding that the combination sensitivity is due to a multiplication of ITD and ILD inputs.

Optical approaches to embryonic development of neural functions in the brainstem

The ontogenetic approach to physiological events is a useful strategy for understanding the functional organization/architecture of the vertebrate brainstem. However, conventional electrophysiological techniques are difficult or impossible to employ in the early embryonic central nervous system. Optical techniques using voltage-sensitive dyes have made it possible to monitor neural activities from multiple regions of living systems, and have proven to be a useful tool for analyzing the embryogenetic expression of brainstem neural function. This review describes recent progress in optical studies made on embryonic chick and rat brainstems. Several technical issues concerning optical recording from the embryonic brainstem preparations are discussed, and characteristics of the optical signals evoked by cranial nerve stimulation or occurring spontaneously are described. Special attention is paid to the chronological analyses of embryogenetic expression of brainstem function and to the spatial patterning of the functional organization/architecture of the brainstem nuclei. In addition, optical analyses of glutamate, GABA, and glycine receptor functions during embryogenesis are described in detail for the chick nucleus tractus solitarius. This review also discusses intrinsic optical signals associated with neuronal depolarization. Some emphases are also placed on the physiological properties of embryonic brainstem neurons, which may be of interest from the viewpoint of developmental neurobiology.

Effect of GABA on the processing of interaural time differences in nucleus laminaris neurons in the chick

Neurons in the avian nucleus laminaris (NL) are the first to receive binaural information and are presumed to play a role in encoding interaural time differences (ITD). NL not only receives excitatory projections from the ipsi- and contralateral nucleus magnocellularis, but also receives inhibitory (GABAergic) input. This study investigates how GABA (gamma-aminobutyric acid) influences ITD coding in NL. Intracellular responses of chick NL neurons were studied in a brain slice preparation. Both excitatory inputs to NL were electrically activated and the delay between trains of bilateral stimuli (simulated-interaural time difference [s-ITD]) was varied. The resulting s-ITD functions were recorded in the presence of 0-75 microM GABA. The discharge rate of NL neurons varied with s-ITD. Cells responded maximally using s-ITDs at which the peak of the ipsi- and contralateral excitatory postsynaptic potentials occurred simultaneously (favourable s-ITD). At unfavourable s-ITDs, the discharge rates usually fell below unilateral levels. GABA had contrary effects on the s-ITD functions depending on the drug concentration. A low GABA dose enhanced excitability at favourable s-ITD, but not at unfavourable s-ITDs. In contrast, higher GABA concentrations diminished excitability. Moderate GABA concentrations had no consistent effect. These results suggest that the GABAergic input to NL will either increase or decrease the excitability of the NL neuron depending on the degree to which this GABAergic input is activated. A gain control hypothesis is presented in which the GABAergic input makes ITD processing in NL independent of the stimulus intensity by adjusting the excitability of NL neurons.

Differences in expression of GABAA receptor subunits, but not benzodiazepine binding, in the chick brainstem auditory system

Neurons in nucleus magnocellularis (NM) and nucleus laminaris (NL) of the chick brainstem auditory system show an unusual physiological response to GABA. Examination of these nuclei using in situ hybridization for GABAA receptor subunits showed a differential expression of the gamma 2 and alpha 1 subunits. The gamma 2 subunit was found in both NM and NL, but the alpha 1 subunit was found in NL only. Like NL, other areas of the tissue section that showed labeling with the gamma 2 probe, such as the medial vestibular nucleus (VeM) and granule cells of the cerebellum (CB), also labeled with the alpha 1 probe. Thus, given that NM labeled with the gamma 2 probe, the absence of the alpha 1 subunit was unusual in this tissue. This difference in subunit composition suggests that there may also be a difference in GABA receptor function in NM compared to these neighboring areas. One feature of the GABAA receptor believed to be related to the presence of gamma 2 and alpha 1 subunits is specific pharmacological properties of the benzodiazepine modulatory site. It has been proposed that the alpha 1 subunit is necessary for producing a GABAA receptor with a benzodiazepine site that has Type I binding characteristics. The present experiments challenge this hypothesis. Based on the differential presence of the alpha 1 subunit, one would expect that GABA receptors in NM would show different benzodiazepine binding properties than NL, VeM, and CB. However, displacement of 3H-flunitrazepam binding using CL 218,872, which differentiates between the Type I and Type II receptors, showed no difference between these areas. Additionally, the relatively high affinity for CL 218,872 suggests that even NM contains Type I receptors.

Transneuronal regulation of ribosomes after blockade of ionotropic excitatory amino acid receptors

Elimination of auditory nerve activity results in death and atrophy of neurons in the cochlear nucleus, nucleus magnocellularis (NM), of the chick. One early event believed to lead to cell death and atrophy is the disruption of ribosomes in the NM neuron. A useful assay for visualizing these ribosomal changes is immunolabeling with the antibody Y10B, which recognizes ribosomal RNA. Activity-dependent changes in Y10B labeling have been observed both in vivo, after unilateral cochlea removal and in vitro after unilateral auditory nerve stimulation. Although it is clear that activity is crucial for maintaining ribosomal integrity, the identity of the important transynaptic signal(s) is not known. It is possible that this trophic signal is glutamate, the neurotransmitter release from the auditory nerve. The present study investigates the role of ionotropic glutamate receptors in the activity-dependent regulation of ribosomes, as measured by the Y10B immunoreactivity. Brain slices containing the auditory nerve and NM on both sides were obtained from hatchling chicks. The auditory nerve on one side of the slice was stimulated for 1 h. The slice was then processed for Y10B immunoreactivity. As expected, greater Y10B immunolabeling was observed on the stimulated side of the slice. Unexpectedly, however, this immunolabeling difference was still observed after blocking NMDA receptors (50 microM DL-APV), non-NMDA receptors (20 microM CNQX), or blocking both ionotropic receptor subtypes (APV and CNQX). This was true even though CNQX eliminated driven postsynaptic potentials. These data suggest that ionotropic glutamate receptors are not necessary for the activity-dependent regulation of ribosomes in NM neurons.

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.

Activity-dependent regulation of a ribosomal RNA epitope in the chick cochlear nucleus

Elimination of auditory nerve activity results in rapid metabolic changes, cell atrophy, and cell death in nucleus magnocellularis (NM), the cochlear nucleus of the chick. The transneuronal signals involved in the activity-dependent regulation of NM neurons are not well understood. One of the most rapid transneuronal effects is alteration in protein synthesis by NM neurons. Previous studies using an in vitro preparation of the brain stem auditory system suggested that up-regulation of protein synthesis in NM neurons requires the action of some trophic substance released by active auditory nerve fibers. Here, similar results were obtained when measuring changes in immunoreactivity using a monoclonal antibody (Y10B) that recognizes ribosomal RNA. This immunolabeling assay has advantages over the global protein synthesis assay in that it is not sensitive to possible changes in specific activity of the precursor pool or possible differences in the uptake of the labeled amino acids. Unilateral stimulation of the auditory nerve for 1 h resulted in greater immunolabeling of NM neurons on the stimulated side of the slice. This is consistent with previous in vivo results after unilateral deafferentation. Blockade of synaptic transmission by maintaining the slice in a low-Ca2+/high Mg2+ medium prevented the stimulation-induced difference in immunolabeling. Electrical stimulation of the postsynaptic NM neurons alone (antidromic stimulation, via electrical stimulation of NM neuron axons) did not result in greater immunolabeling. Rather, antidromically stimulated neurons tended to show lighter labeling. Thus, the transneuronal regulation of ribosomes in NM neurons appears to require some substance released from the active auditory nerve.(ABSTRACT TRUNCATED AT 250 WORDS)

Choline acetyltransferase-immunoreactive cochlear efferent neurons in the chick auditory brainstem

Cholinergic neurons in the chick auditory brainstem were studied with the aid of an antiserum to choline acetyltransferase (ChAT), the biosynthetic enzyme for acetylcholine. ChAT-immunoreactive (ChAT-I) neurons were found in a ventrolateral and a dorsomedial cell group. The ventrolateral group is a rostrocaudally directed column of cells that surround the superior olive (SO), are ventromedial to the ventral facial nucleus (VIIv), and are lateral to the nucleus pontis lateralis (PL) as far rostrally as the nucleus subceruleus ventralis. Cells in the dorsomedial group were found in the pontine reticular formation medial to the dorsal facial nucleus and lateral to the abducens nerve root. Occasionally, small ChAT-I cells were found in the crossed dorsal cochlear tract and in the medial vestibular nucleus near the dorsal border of the caudal nucleus magnocellularis (NM). No ChAT-I neurons or fibers were observed in NM, nucleus angularis, nucleus laminaris, in the nuclei of the lateral lemniscus, or in the nucleus mesencephalicus lateralis pars dorsalis. To determine which cholinergic neurons project to the cochlea, a double-labeling technique was used combining ChAT-I and the retrograde transport of biotinylated dextran amine (BDA) from the inner ear. Double-labeled cells were found bilaterally in both the ventrolateral and dorsomedial cell groups, with the exception of large ChAT-I cells dorsal to the SO, which do not appear to project to the cochlea. Cholinergic cells that project to the cochlea were classified into three morphological groups: multipolar, elongate, and round-to-oval. Both the ventrolateral and the dorsomedial cell groups appear to have a mixture of these different cell types. The average somal area of cholinergic cochlear efferents was 246 microns 2. Only about 70% of the cochlear efferent neurons, however, are cholinergic.