Neurons in the chicken nucleus laminaris (NL), the third-order auditory nucleus involved in azimuth sound localization, receive bilaterally segregated (ipsilateral vs. contralateral) glutamatergic excitation from the cochlear nucleus magnocellularis and GABAergic inhibition from the ipsilateral superior olivary nucleus. Here, I investigate the voltage-gated calcium channels (VGCCs) that trigger the excitatory and the inhibitory transmission in the NL. Whole-cell recordings were performed in acute brainstem slices. The excitatory transmission was predominantly mediated by N-type VGCCs, as the specific N-type blocker ω-Conotoxin-GVIA (1-2.5 μM) inhibited excitatory postsynaptic currents (EPSCs) by ∼90%. Blockers for P/Q- and L-type VGCCs produced no inhibition, and blockade of R-type VGCCs produced a small inhibition. In individual cells, the effect of each VGCC blocker on the EPSC elicited by activation of the ipsilateral input was the same as that on the EPSC elicited by activation of the contralateral input, and the two EPSCs had similar kinetics, suggesting physiological symmetry between the two glutamatergic inputs to single NL neurons. The inhibitory transmission in NL neurons was almost exclusively mediated by N-type VGCCs, as ω-Conotoxin-GVIA (1 μM) produced a ∼90% reduction of inhibitory postsynaptic currents, whereas blockers for other VGCCs produced no inhibition. In conclusion, N-type VGCCs play a dominant role in triggering both the excitatory and the inhibitory transmission in the NL, and the presynaptic VGCCs that mediate the two bilaterally segregated glutamatergic inputs to individual NL neurons are identical. These features may play a role in optimizing coincidence detection in NL neurons.
voltage-gated calcium channel; excitatory postsynaptic current; inhibitory postsynaptic current; coincidence detection; sound localization
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 from 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.
auditory brainstem; axonal projection; γ-aminobutyric acid (GABA); interaural time difference (ITD); tonotopic organization
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.
astrocytes; inhibitory synapses; auditory brainstem; nucleus laminaris; synaptogenesis; superior olivary nucleus
Afferent input regulates neuronal dendritic patterning locally and globally through distinct mechanisms. To begin to understand these mechanisms, we differentially manipulate afferent input in vivo and assess effects on dendritic patterning of individual neurons in chicken nucleus laminaris (NL). Dendrites of NL neurons segregate into dorsal and ventral domains, receiving excitatory input from the ipsilateral and contralateral ears, respectively, via nucleus magnocellularis (NM). Blocking action potentials from one ear, by either cochlea removal or temporary treatment with tetrodotoxin (TTX), leads to rapid and significant retraction of affected NL dendrites (dorsal ipsilaterally and ventral contralaterally) within 8h as compared to the other dendrites of the same neurons. The degree of retraction is comparable to that induced by direct deafferentation resulting from transection of NM axons. Importantly, when inner ear activity is allowed to recover from TTX treatments, retracted NL dendrites regrow to their normal length within 48h. The retraction and growth involve elimination of terminal branches and addition of new branches. Examination of changes in NL dendrites at 96h following unilateral cochlear removal, a manipulation that induces cell loss in NM and persistent blockage of afferent excitatory action potentials, reveals a significant correlation between cell death in the ipsilateral NM and the degree of dendritic retraction in NL. These results demonstrate that presynaptic action potentials rapidly and reversibly regulate dendritic patterning of postsynaptic neurons in a compartment specific manner, while long-term dendritic maintenance may be regulated in a way that is correlated with the presence of silent presynaptic appositions.
Neurons of the avian cochlear nucleus magnocellularis (NM) receive glutamatergic inputs from the spiral ganglion cells via the auditory nerve and feedback GABAergic inputs primarily from the superior olivary nucleus. We investigated regulation of Ca2+ signaling in NM neurons with ratiometric Ca2+ imaging in chicken brain slices. Application of exogenous glutamate or GABA increased the intracellular Ca2+ concentration ([Ca2+]i) in NM neurons. Interestingly, GABA-induced Ca2+ responses persisted into neuronal maturation, in both standard and energy substrate enriched artificial cerebrospinal fluid. More importantly, we found that electrical stimulation applied to the glutamatergic and GABAergic afferent fibers innervating the NM was able to elicit transient [Ca2+]i increases in NM neurons, and the amplitude of the Ca2+ responses increased with increasing frequency and duration of the electrical stimulation. Antagonists for ionotropic glutamate receptors significantly blocked these [Ca2+]i increases, whereas blocking GABAA receptors did not affect the Ca2+ responses, suggesting that synaptically released glutamate but not GABA induced the Ca2+ signaling in vitro. Furthermore, activation of GABAA receptors with exogenous agonists inhibited synaptic activity-induced [Ca2+]i increases in NM neurons, suggesting a role of GABAA receptors in the regulation of Ca2+ homeostasis in the avian cochlear nucleus neurons.
auditory; Ca2+ imaging; glutamate receptor; GABA receptor; neuromodulation
The brainstem auditory pathway is obligatory for all aural information. Brainstem auditory neurons must encode the level and timing of sounds, as well as their time-dependent spectral properties, the fine structure and envelope, which are essential for sound discrimination. This study focused on envelope coding in the two cochlear nuclei of the barn owl, nucleus angularis (NA) and nucleus magnocellularis (NM). NA and NM receive input from bifurcating auditory nerve fibers and initiate processing pathways specialized in encoding interaural time (ITD) and level (ILD) differences, respectively. We found that NA neurons, though unable to accurately encode stimulus phase, lock more strongly to the stimulus envelope than NM units. The spectrotemporal receptive fields (STRFs) of NA neurons exhibit a pre-excitatory suppressive field. Using multilinear regression analysis and computational modeling, we show that this feature of STRFs can account for enhanced across-trial response reliability, by locking spikes to the stimulus envelope. Our findings indicate a dichotomy in envelope coding between the time and intensity processing pathways as early as at the level of the cochlear nuclei. This allows the ILD processing pathway to encode envelope information with greater fidelity than the ITD processing pathway. Furthermore, we demonstrate that the properties of the neurons’ STRFs can be quantitatively related to spike timing reliability.
Nucleus angularis; STRF; spectrotemporal tuning; cochlear nuclei; barn owl; response reliability
Interaural time difference (ITD), or the difference in timing of a sound wave arriving at the two ears, is a fundamental cue for sound localization. A wide variety of animals have specialized neural circuits dedicated to the computation of ITDs. In the avian auditory brainstem, ITDs are encoded as the spike rates in the coincidence detector neurons of the nucleus laminaris (NL). NL neurons compare the binaural phase-locked inputs from the axons of ipsi- and contralateral nucleus magnocellularis (NM) neurons. Intracellular recordings from the barn owl's NL in vivo showed that tonal stimuli induce oscillations in the membrane potential. Since this oscillatory potential resembled the stimulus sound waveform, it was named the sound analog potential (Funabiki et al., 2011). Previous modeling studies suggested that a convergence of phase-locked spikes from NM leads to an oscillatory membrane potential in NL, but how presynaptic, synaptic, and postsynaptic factors affect the formation of the sound analog potential remains to be investigated. In the accompanying paper, we derive analytical relations between these parameters and the signal and noise components of the oscillation. In this paper, we focus on the effects of the number of presynaptic NM fibers, the mean firing rate of these fibers, their average degree of phase-locking, and the synaptic time scale. Theoretical analyses and numerical simulations show that, provided the total synaptic input is kept constant, changes in the number and spike rate of NM fibers alter the ITD-independent noise whereas the degree of phase-locking is linearly converted to the ITD-dependent signal component of the sound analog potential. The synaptic time constant affects the signal more prominently than the noise, making faster synaptic input more suitable for effective ITD computation.
phase-locking; sound localization; auditory brainstem; periodic signals; oscillation; owl
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.
Auditory system; Sound localization; Nucleus magnocellularis; Nucleus laminaris; Coincidence detection; Interaural canal; GABA
A wide variety of neurons encode temporal information via phase-locked spikes. In the avian auditory brainstem, neurons in the cochlear nucleus magnocellularis (NM) send phase-locked synaptic inputs to coincidence detector neurons in the nucleus laminaris (NL) that mediate sound localization. Previous modeling studies suggested that converging phase-locked synaptic inputs may give rise to a periodic oscillation in the membrane potential of their target neuron. Recent physiological recordings in vivo revealed that owl NL neurons changed their spike rates almost linearly with the amplitude of this oscillatory potential. The oscillatory potential was termed the sound analog potential, because of its resemblance to the waveform of the stimulus tone. The amplitude of the sound analog potential recorded in NL varied systematically with the interaural time difference (ITD), which is one of the most important cues for sound localization. In order to investigate the mechanisms underlying ITD computation in the NM-NL circuit, we provide detailed theoretical descriptions of how phase-locked inputs form oscillating membrane potentials. We derive analytical expressions that relate presynaptic, synaptic, and postsynaptic factors to the signal and noise components of the oscillation in both the synaptic conductance and the membrane potential. Numerical simulations demonstrate the validity of the theoretical formulations for the entire frequency ranges tested (1–8 kHz) and potential effects of higher harmonics on NL neurons with low best frequencies (<2 kHz).
phase-locking; sound localization; auditory brainstem; periodic signals; oscillation; owl
The auditory systems of birds and mammals use timing information from each ear to detect interaural time difference (ITD). To determine whether the Jeffress-type algorithms that underlie sensitivity to ITD in birds are an evolutionarily stable strategy, we recorded from the auditory nuclei of crocodilians, who are the sister group to the birds. In alligators, precisely timed spikes in the first-order nucleus magnocellularis (NM) encode the timing of sounds, and NM neurons project to neurons in the nucleus laminaris (NL) that detect interaural time differences. In vivo recordings from NL neurons show that the arrival time of phase-locked spikes differs between the ipsilateral and contralateral inputs. When this disparity is nullified by their best ITD, the neurons respond maximally. Thus NL neurons act as coincidence detectors. A biologically detailed model of NL with alligator parameters discriminated ITDs up to 1 kHz. The range of best ITDs represented in NL was much larger than in birds, however, and extended from 0 to 1000 μs contralateral, with a median ITD of 450 μs. Thus, crocodilians and birds employ similar algorithms for ITD detection, although crocodilians have larger heads.
While adaptation is widely thought to facilitate neural coding, the form of adaptation should depend on how the signals are encoded. Monaural neurons early in the interaural time difference (ITD) pathway encode the phase of sound input using spike timing rather than firing rate. Such neurons in chicken nucleus magnocellularis (NM) adapt to ongoing stimuli by increasing firing rate and decreasing spike timing precision. We measured NM neuron responses while adapting them to simulated physiological input, and used these responses to construct inputs to binaural coincidence detector neurons in nucleus laminaris (NL). Adaptation of spike timing in NM reduced ITD sensitivity in NL, demonstrating the dominant role of timing in the short-term plasticity as well as the immediate response of this sound localization circuit.
Neurons of the cochlear nuclei are anatomically and physiologically specialized to optimally encode temporal and spectral information about sound stimuli, in part for binaural auditory processing. The avian cochlear nucleus magnocellularis (NM) integrates excitatory eighth nerve inputs and depolarizing GABAergic inhibition such that temporal fidelity is enhanced across the synapse. The biophysical mechanisms of this depolarizing inhibition, and its role in temporal processing, are not fully understood. We used whole-cell electro-physiology and computational modeling to examine how subthreshold excitatory inputs are integrated and how depolarizing IPSPs affect spike thresholds and synaptic integration by chick NM neurons. We found that both depolarizing inhibition and subthreshold excitatory inputs cause voltage threshold accommodation, nonlinear temporal summation, and shunting. Inhibition caused such large changes in threshold that subthreshold excitatory inputs were followed by a refractory period. We hypothesize that these large shifts in threshold eliminate spikes to asynchronous inputs, providing a mechanism for the enhanced temporal fidelity seen across the eighth nerve/cochlear nucleus synapse. Thus, depolarizing inhibition and threshold shifting hone the temporal response properties of this system so as to enhance the temporal fidelity that is essential for auditory perception.
Understanding binaural perception requires detailed analyses of the neural circuitry responsible for the computation of interaural time differences (ITDs). In the avian brainstem, this circuit consists of internal axonal delay lines innervating an array of coincidence detector neurons that encode external ITDs. Nucleus magnocellularis (NM) neurons project to the dorsal dendritic field of the ipsilateral nucleus laminaris (NL) and to the ventral field of the contralateral NL. Contralateral-projecting axons form a delay line system along a band of NL neurons. Binaural acoustic signals in the form of phase-locked action potentials from NM cells arrive at NL and establish a topographic map of sound source location along the azimuth. These pathways are assumed to represent a circuit similar to the Jeffress model of sound localization, establishing a place code along an isofrequency contour of NL. Three-dimensional measurements of axon lengths reveal major discrepancies with the current model; the temporal offset based on conduction length alone makes encoding of physiological ITDs impossible. However, axon diameter and distances between Nodes of Ranvier also influence signal propagation times along an axon. Our measurements of these parameters reveal that diameter and internode distance can compensate for the temporal offset inferred from axon lengths alone. Together with other recent studies these unexpected results should inspire new thinking on the cellular biology, evolution and plasticity of the circuitry underlying low frequency sound localization in both birds and mammals.
Sound; Localization; Auditory; Brainstem; Axon; Conduction; Velocity
Calcium signaling plays a role in synaptic regulation of dendritic structure, usually on the time scale of hours or days. Here we use immunocytochemistry to examine changes in expression of the plasma membrane calcium ATPase type 2 (PMCA2), a high-affinity calcium efflux protein, in the chick nucleus laminaris (NL) following manipulations of synaptic inputs. Dendrites of NL neurons segregate into dorsal and ventral domains, receiving excitatory input from the ipsilateral and contralateral ears, respectively, via nucleus magnocellularis (NM). Deprivation of the contralateral projection from NM to NL leads to rapid retraction of ventral, but not the dorsal, dendrites of NL neurons. Immunocytochemistry revealed symmetric distribution of PMCA2 in two neuropil regions of normally innervated NL. Electron microscopy confirmed that PMCA2 localizes in both NM terminals and NL dendrites. As early as 30 minutes following transection of the contralateral projection from NM to NL or unilateral cochlea removal, significant decreases in PMCA2 immunoreactivity were seen in the deprived neuropil of NL compared to the other neuropil which continued to receive normal input. The rapid decrease correlated with reductions in the immunoreactivity for the microtubule-associated protein 2, which affects cytoskeleton stabilization. These results suggest that PMCA2 is regulated independently in ventral and dorsal NL dendrites and/or their inputs from NM in a way that is correlated with presynaptic activity. This provides a potential mechanism by which deprivation can change calcium transport that, in turn, may be important for rapid, compartment-specific dendritic remodeling.
calcium homeostasis; afferent regulation; dendritic remodeling; activity-dependency; nucleus laminaris
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.”
cochlear nuclei; central nucleus of inferior colliculus; MLd; zebra finch; avian
The lateral superior olive (LSO) is believed to encode differences in sound level at the two ears, a cue for azimuthal sound location. Most high-frequency-sensitive LSO neurons are binaural, receiving inputs from both ears. An inhibitory input from the contralateral ear, via the medial nucleus of the trapezoid body (MNTB), and excitatory input from the ipsilateral ear enable level differences to be encoded. However, the classical descriptions of low-frequency-sensitive neurons report primarily monaural cells with no contralateral inhibition. Anatomical and physiological evidence, however, shows that low-frequency LSO neurons receive low-frequency inhibitory input from ipsilateral MNTB, which in turn receives excitatory input from the contralateral cochlear nucleus and low-frequency excitatory input from the ipsilateral cochlear nucleus. Therefore, these neurons would be expected to be binaural with contralateral inhibition. Here, we re-examined binaural interaction in low-frequency (less than ~3 kHz) LSO neurons and phase locking in the MNTB. Phase locking to low-frequency tones in MNTB and ipsilaterally driven LSO neurons with frequency sensitivities < 1.2 kHz was enhanced relative to the auditory nerve. Moreover, most low-frequency LSO neurons exhibited contralateral inhibition: ipsilaterally driven responses were suppressed by raising the level of the contralateral stimulus; most neurons were sensitive to interaural time delays in pure tone and noise stimuli such that inhibition was nearly maximal when the stimuli were presented to the ears in-phase. The data demonstrate that low-frequency LSO neurons of cat are not monaural and can exhibit contralateral inhibition like their high-frequency counterparts.
lateral superior olive; medial nucleus of the trapezoid body; interaural time delay; interaural level difference; sound localization; phase locking
The nature of the synaptic connection from the auditory nerve onto the cochlear nucleus neurons has a profound impact on how sound information is transmitted. Short-term synaptic plasticity, by dynamically modulating synaptic strength, filters information contained in the firing patterns. In the sound-localization circuits of the brain stem, the synapses of the timing pathway are characterized by strong short-term depression. We investigated the short-term synaptic plasticity of the inputs to the bird’s cochlear nucleus angularis (NA), which encodes intensity information, by using chick embryonic brain slices and trains of electrical stimulation. These excitatory inputs expressed a mixture of short-term facilitation and depression, unlike those in the timing nuclei that only depressed. Facilitation and depression at NA synapses were balanced such that postsynaptic response amplitude was often maintained throughout the train at high firing rates (>100 Hz). The steady-state input rate relationship of the balanced synapses linearly conveyed rate information and therefore transmits intensity information encoded as a rate code in the nerve. A quantitative model of synaptic transmission could account for the plasticity by including facilitation of release (with a time constant of ~40 ms), and a two-step recovery from depression (with one slow time constant of ~8 s, and one fast time constant of ~20 ms). A simulation using the model fit to NA synapses and auditory nerve spike trains from recordings in vivo confirmed that these synapses can convey intensity information contained in natural train inputs.
Identification of shared features between avian and mammalian auditory brainstem circuits has provided much insight into the mechanisms underlying early auditory processing. However, previous studies have highlighted an apparent difference in inhibitory systems; synaptic inhibition is thought to be slow and GABAergic in birds, but to have fast kinetics and be predominantly glycinergic in mammals. Using patch-clamp recordings in chick brainstem slices, we found this distinction is not exclusively true. Consistent with previous work, inhibitory postsynaptic currents (IPSCs) in nucleus magnocellularis (NM) were slow and mediated by GABAA receptors. However, IPSCs in nucleus laminaris (NL) and a subset of neurons in nucleus angularis (NA) had rapid time courses two to three-fold faster than those in NM. Further, we found IPSCs in NA were mediated by both glycine and GABAA receptors, demonstrating for the first time a role for fast glycinergic transmission in the avian auditory brainstem. Although NM, NL and NA have unique roles in auditory processing, the majority of inhibitory input to each nucleus arises from the same source, ipsilateral superior olivary nucleus (SON). Our results demonstrate remarkable diversity of inhibitory transmission among the avian brainstem nuclei and suggest differential glycine and GABAA receptor activity tailors inhibition to the specific functional roles of NM, NL, and NA despite common SON input. We additionally observed that glycinergic/GABAergic activity in NA was usually depolarizing and could elicit spiking activity in NA neurons. Because NA projects to SON, these excitatory effects may influence the recruitment of inhibitory activity in the brainstem nuclei.
Auditory; GABA; Glycine; Patch Clamp; Inhibition; Synapse
Medial olivocochlear (MOC) neurons originate in the superior olivary complex and project to the cochlea, where they act to reduce the effects of noise masking and protect the cochlea from damage. MOC neurons respond to sound via a reflex pathway, however, in this pathway the cochlear nucleus cell type that provides input to MOC neurons is not known. We investigated whether multipolar cells of the ventral cochlear nucleus have projections to MOC neurons, by labeling them with injections into the dorsal cochlear nucleus. The projections of one type of labeled multipolar cell, planar neurons, were traced into the ventral nucleus of the trapezoid body, where they were observed terminating on MOC neurons (labeled in some cases by a second cochlear injection of Fluorogold). These terminations formed what appear to be excitatory synapses, i.e. containing small, round vesicles and prominent postsynaptic densities. These data suggest that cochlear nucleus planar multipolar neurons drive the MOC neuron’s response to sound.
synapse; reflex; superior olivary complex; descending system
Inhibitory transmission is critical to sensory and motor processing and is believed to play a role in experience-dependent plasticity. The main inhibitory neurotransmitter in vertebrates, GABA, has been implicated in both sensory and motor aspects of vocalization in songbirds. To understand the role of GABAergic mechanisms in vocal communication, GABAergic elements must be characterized fully. Hence, we investigated GABA immunohistochemistry in the zebra finch brain, emphasizing auditory areas and song control nuclei. Several nuclei of the ascending auditory pathway showed a moderate to high density of GABAergic neurons including the cochlear nuclei, nucleus laminaris, superior olivary nucleus, mesencephalic nucleus lateralis pars dorsalis, and nucleus ovoidalis. Telencephalic auditory areas, including field L subfields L1, L2a and L3, as well as the caudomedial nidopallium (NCM) and mesopallium (CMM), contained GABAergic cells at particularly high densities. Considerable GABA labeling was also seen in the shelf area of caudodorsal nidopallium, and the cup area in the arcopallium, as well as in area X, the lateral magnocellular nucleus of the anterior nidopallium, the robust nucleus of the arcopallium and nidopallial nucleus HVC. GABAergic cells were typically small, most likely local inhibitory interneurons, although large GABA-positive cells that were sparsely distributed were also identified. GABA-positive neurites and puncta were identified in most nuclei of the ascending auditory pathway and in song control nuclei. Our data are in accordance with a prominent role of GABAergic mechanisms in regulating the neural circuits involved in song perceptual processing, motor production, and vocal learning in songbirds.
GAD; Avian; NCM; Songbird; Plasticity; HVC
The magnocellular neurons of the hypothalamic supraoptic nucleus (SON) are a major source of both systemic and central release of the neurohypophyseal peptides, oxytocin (OXT) and arginine–vasopressin (AVP). Both OXT and AVP are released from the somatodendritic compartment of magnocellular neurons and act within the SON to modulate the electrophysiological function of these cells. Cannabinoids (CBs) affect hormonal output and the SON may represent a neural substrate through which CBs exert specific physiological and behavioural effects. Dynamic modulation of synaptic inputs is a fundamental mechanism through which neuronal output is controlled. Dendritically released OXT acts on autoreceptors to generate endocannabinoids (eCBs) which modify both excitatory and inhibitory inputs to OXT neurons through actions on presynaptic CB receptors. As such, OXT and eCBs cooperate to shape the electrophysiological properties of magnocellular OXT neurons, regulating the physiological function of this nucleus. Further study of eCB signalling in the SON, including its interaction with AVP neurons, promises to extend our understanding of the synaptic regulation of SON physiological function.
PMID: 18655878 CAMSID: cams2631
hypothalamus; oxytocin; magnocellular neurons; retrograde messengers
The mammalian oculomotor nucleus receives a strong γ-aminobutyric acid (GABA)ergic synaptic input, whereas such projections have rarely been reported in fish. In order to determine whether this synaptic organization is preserved across vertebrates, we investigated the GABAergic projections to the oculomotor nucleus in the goldfish by combining retrograde transport of biotin dextran amine, injected into the antidromically identified oculomotor nucleus, and GABA immunohistochemistry. The main source of GABAergic afferents to the oculomotor nucleus was the ipsilateral anterior octaval nucleus, with only a few, if any, GABAergic neurons being located in the contralateral tangential and descending nuclei of the octaval column. In mammals there is a nearly GABAergic inhibitory inputs; thus, the vestibulooculomotor GABAergic circuitry follows a plan that appears to be shared throughout the vertebrate phylogeny. The second major source of GABAergic projections was the rhombencephalic reticular formation, primarily from the medial area but, to a lesser extent, from the inferior area. A few GABAergic oculomotor projecting neurons were also observed in the ipsilateral nucleus of the medial longitudinal fasciculus. The GABAergic projections from neurons located in both the reticular formation surrounding the abducens nucleus and the nucleus of the medial reticular formation have primarily been related to the control of saccadic eye movements. Finally, all retrogradely labeled internuclear neurons of the abducens nucleus, and neurons in the cerebellum (close to the caudal lobe), were negative for GABA. These data suggest that the vestibuloocular and saccadic inhibitory GABAergic systems appear early in vertebrate phylogeny to modulate the firing properties of the oculomotor nucleus motoneurons.
oculomotor system; vestibuloocular reflex; saccadic eye movements; GABA immunohistochemistry; fish
In the intermediate nucleus of the lateral lemniscus (INLL), some neurons display a form of spectral integration in which excitatory responses to sounds at their best frequency are inhibited by sounds within a frequency band at least one octave lower. Previous work showed that this response property depends on low-frequency-tuned glycinergic input. To identify all sources of inputs to these INLL neurons, and in particular the low-frequency glycinergic input, we combined retrograde tracing with immunohistochemistry for the neurotransmitter glycine. We deposited a retrograde tracer at recording sites displaying either high best frequencies (>75 kHz) in conjunction with combination-sensitive inhibition, or at sites displaying low best frequencies (23–30 kHz). Most retrogradely labeled cells were located in the ipsilateral medial nucleus of the trapezoid body (MNTB) and contralateral anteroventral cochlear nucleus. Consistent labeling, but in fewer numbers, was observed in the ipsilateral lateral nucleus of the trapezoid body (LNTB), contralateral posteroventral cochlear nucleus, and a few other brainstem nuclei. When tracer deposits were combined with glycine immunohistochemistry, most double-labeled cells were observed in the ipsilateral MNTB (84%), with fewer in LNTB (13%). After tracer deposits at combination-sensitive recording sites, a striking result was that MNTB labeling occurred in both medial and lateral regions. This labeling appeared to overlap the MNTB labeling that resulted from tracer deposits in low-frequency recording sites of INLL. These findings suggest that MNTB is the most likely source of low-frequency glycinergic input to INLL neurons with high best frequencies and combination-sensitive inhibition. This work establishes an anatomical basis for frequency integration in the auditory brainstem.
combination sensitivity; spectral integration; glycine; mustached bat; INLL; Pteronotus parnellii
In the auditory system, precise encoding of temporal information is critical for sound localization, a task with direct behavioral relevance. Interaural timing differences 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 MΩ and 49.0 ± 25.6 MΩ, 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; nucleus laminaris; nucleus magnocellularis; dendrite; coincidence detection; sound localization
Competition between presynaptic inputs has been suggested to shape dendritic form. This hypothesis can be directly tested on bitufted, auditory neurons in chicken nucleus laminaris (NL). Each NL neuron contains two relatively symmetrical dendritic arbors; the dorsal dendrite receives excitatory glutamatergic input from the ipsilateral ear and the ventral dendrites receive corresponding input from the contralateral ear. To assess the effect of relative synaptic strength on NL dendrites, we used single cell electroporation, electrophysiology and live, two-photon laser scanning microscopy to manipulate both the amount and the balance of synaptic input to the two matching sets of dendrites. With simultaneous activation, both sets of dendrites changed together, either growing or retracting over the imaging period. In contrast, stimulation of only one set of dendrites (either dorsal or ventral) resulted in the unstimulated dendrites losing total dendritic branch length, while the stimulated dendrites exhibited a tendency to grow. In this system, balanced input leads to balanced changes in the two sets of dendrites while imbalanced input results in differential changes. Time-lapse imaging revealed that NL dendrites respond to differential stimulation by first decreasing the size of their unstimulated dendrites, and then increasing the size of their stimulated dendrites. This result suggests that the relative activity of presynaptic neurons dynamically controls dendritic structure in NL, and that dendritic real estate can rapidly be shifted from inactive inputs to active inputs.
dendrites; activity; competition; auditory; time-lapse imaging