[Show abstract][Hide abstract] ABSTRACT: Sound information is encoded as a series of spikes of the auditory nerve fibers (ANFs), and then transmitted to the brainstem auditory nuclei. Features such as timing and level are extracted from ANFs activity and further processed as the interaural time difference (ITD) and the interaural level difference (ILD), respectively. These two interaural difference cues are used for sound source localization by behaving animals. Both cues depend on the head size of animals and are extremely small, requiring specialized neural properties in order to process these cues with precision. Moreover, the sound level and timing cues are not processed independently from one another. Neurons in the nucleus angularis (NA) are specialized for coding sound level information in birds and the ILD is processed in the posterior part of the dorsal lateral lemniscus nucleus (LLDp). Processing of ILD is affected by the phase difference of binaural sound. Temporal features of sound are encoded in the pathway starting in nucleus magnocellularis (NM), and ITD is processed in the nucleus laminaris (NL). In this pathway a variety of specializations are found in synapse morphology, neuronal excitability, distribution of ion channels and receptors along the tonotopic axis, which reduces spike timing fluctuation in the ANFs-NM synapse, and imparts precise and stable ITD processing to the NL. Moreover, the contrast of ITD processing in NL is enhanced over a wide range of sound level through the activity of GABAergic inhibitory systems from both the superior olivary nucleus (SON) and local inhibitory neurons that follow monosynaptic to NM activity.
Preview · Article · May 2014 · Frontiers in Neural Circuits
[Show abstract][Hide abstract] ABSTRACT: The axon initial segment (AIS) is the site of spike initiation in neurons. Previous studies revealed that spatial distribution of the AIS varies greatly among neurons to meet their specific needs. However, when and how this differentiation arises is unknown. Neurons in the avian nucleus laminaris (NL) are binaural coincidence detectors for sound localization and show differentiation in the distribution of the AIS, with shorter length and a more distal position from the soma with an increase in tuning frequency. We studied these characteristics of the AIS in NL neurons of the chicken during development and found that the AIS differentiates in its distribution after initial formation, and this is driven by activity-dependent and activity-independent mechanisms that differentially regulate distal and proximal boundaries of the AIS. Before hearing onset, the ankyrinG-positive AIS existed at a wide stretch of proximal axon regardless of tuning frequency, but Na(+) channels were only partially distributed within the AIS. Shortly after hearing onset, Na(+) channels accumulated along the entire AIS, which started shortening and relocating distally to a larger extent in neurons with higher tuning frequencies. Ablation of inner ears abolished the shortening of the AIS without affecting the position of its proximal boundary, indicating that both distal and proximal AIS boundaries are disassembled during development, and the former is dependent on afferent activity. Thus, interaction of these activity-dependent and activity-independent mechanisms determines the cell-specific distribution of the AIS in NL neurons and plays a critical role in establishing the function of sound localization circuit.
Preview · Article · Feb 2014 · The Journal of Neuroscience : The Official Journal of the Society for Neuroscience
[Show abstract][Hide abstract] ABSTRACT: Neurons in the nucleus laminaris (NL) of birds detect the coincidence of binaural excitatory inputs from the nucleus magnocellularis (NM) on both sides and process the interaural time differences (ITDs) for sound localization. Sustained inhibition from the superior olivary nucleus is known to control the gain of coincidence detection, which allows the sensitivity of NL neurons to ITD tolerate strong-intensity sound. Here, we found a phasic inhibition in chicken brain slices that follows the ipsilateral NM inputs after a short time delay, sharpens coincidence detection, and may enhance ITD sensitivity in low-frequency NL neurons. GABA-positive small neurons are distributed in and near the NL. These neurons generate IPSCs in NL neurons when photoactivated by a caged glutamate compound, suggesting that these GABAergic neurons are interneurons that mediate phasic inhibition. These IPSCs have fast decay kinetics that is attributable to the α1-subunit of the GABA receptor, the expression of which dominates in the low-frequency region of the NL. Model simulations demonstrate that phasic IPSCs narrow the time window of coincidence detection and increase the contrast of ITD-tuning during low-level, low-frequency excitatory input. Furthermore, cooperation of the phasic and sustained inhibitions effectively increases the contrast of ITD-tuning over a wide range of excitatory input levels. We propose that the complementary interaction between phasic and sustained inhibitions is the neural mechanism that regulates ITD sensitivity for low-frequency sound in the NL.
Preview · Article · Feb 2013 · The Journal of Neuroscience : The Official Journal of the Society for Neuroscience
[Show abstract][Hide abstract] ABSTRACT: Olfactory receptor neurons (ORNs), which undergo lifelong neurogenesis, have been studied extensively to understand how neurons form precise topographical networks. Neural projections from ORNs are principally guided by the genetic code, which directs projections from ORNs that express a specific odorant receptor to the corresponding glomerulus in the olfactory bulb. In addition, ORNs utilise spontaneous firing activity to establish and maintain the neural map. However, neither the process of generating this spontaneous activity nor its role as a guidance cue in the olfactory bulb is clearly understood. Utilising extracellular unit-recordings in mouse olfactory epithelium slices, we demonstrated that the hyperpolarisation-activated cyclic nucleotide-gated (HCN) channels in the somas of ORNs depolarise their membranes and boost their spontaneous firing rates by sensing basal cAMP levels; the odorant-sensitive cyclic nucleotide-gated (CNG) channels in cilia do not. The basal cAMP levels were maintained via the standing activation of β-adrenergic receptors. Using a tet-off system to over-express HCN4 channels resulted in the enhancement of spontaneous ORN activity and dramatically reduced both the size and number of glomeruli in the olfactory bulb. This phenotype was rescued by the administration of doxycycline. These findings suggest that cAMP plays different roles in cilia and soma and that basal cAMP levels in the soma are directly converted via HCN channels into a spontaneous firing frequency that acts as an intrinsic guidance cue for the formation of olfactory networks.
No preview · Article · Jan 2013 · The Journal of Physiology
[Show abstract][Hide abstract] ABSTRACT: Interaural time difference (ITD) is a major cue for localizing a sound source and is processed in the nucleus laminaris (NL) in birds. Coincidence detection (CD) is a crucial step for processing ITD and critically depends on the size and time course of EPSPs. Here, we investigated a role of metabotropic glutamate receptors (mGluRs) in the regulation of EPSP amplitude and CD in the NL of chicks. A nonspecific agonist of mGluRs (t-ACPD) reduced the amplitude and the extent of depression of EPSCs during a stimulus train, while the paired pulse ratio and the coefficient of variation of EPSC amplitude were increased. In contrast, the amplitudes of spontaneous EPSCs were not affected, but the frequency was reduced. Thus, the effects of t-ACPD were presynaptic and reduced the release of neurotransmitter from terminals in the NL. Expression of group-II mGluRs was graded along the tonotopic axis and was stronger towards the low frequency region in the NL. Both group-II (DCG-IV) and group-III (L-AP4) specific agonists reduced EPSC amplitude by presynaptic mechanisms, and the reduction was larger in the low frequency region; however, we could not find any effects of group-I specific agonists on EPSCs. The reduced EPSP amplitude in DCG-IV improved CD. A specific antagonist of group-II mGluRs (LY341495) increased the amplitude of both EPSCs and EPSPs and enhanced the depression during stimulus train, indicating constitutive activation of mGluRs in the NL. These observations indicate that mGluRs may work as autoreceptors and regulate EPSP size to improve CD in the NL.
Preview · Article · Oct 2012 · The Journal of Physiology
[Show abstract][Hide abstract] ABSTRACT: Offset neurons, which fire at the termination of sound, likely encode sound duration and serve to process temporal information. Offset neurons are found in most ascending auditory nuclei; however, the neural mechanisms that evoke offset responses are not well understood. In this study, we examined offset neural responses to tonal stimuli in the inferior colliculus (IC) in vivo with extracellular and intracellular recording techniques in mice. Based on peristimulus time histogram (PSTH) patterns, we classified extracellular offset responses into four types: Offset, Onset-Offset, Onset-Sustained-Offset and Inhibition-Offset types. Moreover, using in vivo whole-cell recording techniques, we found that offset responses were generated in most cells through the excitatory and inhibitory synaptic inputs. However, in a small number of cells, the offset responses were generated as a rebound to hyperpolarization during tonal stimulation. Many offset neurons fired robustly at a preferred duration of tonal stimulus, which corresponded with the timing of rich excitatory synaptic inputs. We concluded that most IC offset neurons encode the termination of the tone stimulus by responding to inherited ascending synaptic information, which is tuned to sound duration. The remainder generates offset spikes de novo through a post-inhibitory rebound mechanism.
Full-text · Article · May 2012 · Neuroscience Research
[Show abstract][Hide abstract] ABSTRACT: Patch-clamp is the best method for analyzing single-channel current activities today. The method is not good, however, at measuring the total number of channels expressed on the cell membrane that are responsible for the current of concern. In contrast, noise analysis has some advantage over the single-channel recording as a method to estimate the total number of ion channels. Also, noise analysis can obtain some channel information regarding size and kinetics even when individual channel events are so small to be detected directly as single-channel events. In this chapter the logic of noise analysis for stationary currents and nonstationary currents follows a description of several classic experiments conducted during the 1970s–1980s. Several technical issues frequently encountered during experiments are also addressed.
[Show abstract][Hide abstract] ABSTRACT: A glutamatergic end-bulb synapse in the avian nucleus magnocellularis relays temporal sound information from the auditory nerve. Here, we show that presynaptic Na(+)/K(+)-ATPase (NKA) activity at this synapse contributes to the maintenance of the readily releasable pool (RRP) of vesicles, thereby preserving synaptic strength. Whole-cell voltage clamp recordings were made from chick brainstem slices to examine the effects of NKA blocker dihydroouabain (DHO) on synaptic transmission. DHO suppressed the amplitude of EPSCs in a dose-dependent manner. This suppression was caused by a decrease in the number of neurotransmitter quanta released because DHO increased the coefficient of variation of EPSC amplitude and reduced the frequency but not the amplitude of miniature EPSCs. Cumulative plots of EPSC amplitude during a stimulus train revealed that DHO reduced the RRP size without affecting vesicular release probability. DHO did not affect [Ca(2+)](i)-dependent processes, such as the paired-pulse ratio or recovery time course from the paired-pulse depression, suggesting a minimal effect on Ca(2+) concentration in the presynaptic terminal. Using mathematical models of synaptic depression, we further demonstrated the contribution of RRP size to the synaptic strength during a high-frequency stimulus train to highlight the importance of presynaptic NKA in the auditory synapse.
Full-text · Article · Nov 2011 · Neuroscience Research
[Show abstract][Hide abstract] ABSTRACT: Interaural time differences (ITDs) are the primary cue animals, including humans, use to localize low-frequency sounds. In vertebrate auditory systems, dedicated ITD processing neural circuitry performs an exacting task, the discrimination of microsecond differences in stimulus arrival time at the two ears by coincidence-detecting neurons. These neurons modulate responses over their entire dynamic range to sounds differing in ITD by mere hundreds of microseconds. The well-understood function of this circuitry in birds has provided a fruitful system to investigate how inhibition contributes to neural computation at the synaptic, cellular, and systems level. Our recent studies in the chicken have made significant progress in bringing together many of these findings to provide a cohesive picture of inhibitory function.
Preview · Article · Apr 2011 · Journal of Neurophysiology
[Show abstract][Hide abstract] ABSTRACT: GABAergic modulation of activity in avian cochlear nucleus neurons has been studied extensively in vitro. However, how this modulation actually influences processing in vivo is not known. We investigated responses of chicken nucleus magnocellularis (NM) neurons to sound while pharmacologically manipulating the inhibitory input from the superior olivary nucleus (SON). SON receives excitatory inputs from nucleus angularis (NA) and nucleus laminaris (NL), and provides GABAergic inputs to NM, NA, NL, and putatively to the contralateral SON. Results from single-unit extracellular recordings from 2 to 4 weeks posthatch chickens show that firing rates of auditory nerve fibers increased monotonically with sound intensity, while that of NM neurons saturated or even decreased at moderate or loud sound levels. Blocking GABAergic input with local application of TTX into the SON induced an increase in firing rate of ipsilateral NM, while that of the contralateral NM decreased at high sound levels. Moreover, local application of bicuculline to NM also increased the firing rate of NM neurons at high sound levels, reduced phase locking, and broadened the frequency-tuning properties of NM neurons. Following application of DNQX, clear evidence of inhibition was observed. Furthermore, the inhibition was tuned to a broader frequency range than the excitatory response areas. We conclude that GABAergic inhibition from SON has at least three physiological influences on the activity of NM neurons: it regulates the firing activity of NM units in a sound-level-dependent manner; it improves phase selectivity; and it sharpens frequency tuning of NM neuronal responses.
Preview · Article · Sep 2010 · The Journal of Neuroscience : The Official Journal of the Society for Neuroscience
[Show abstract][Hide abstract] ABSTRACT: Deprivation of afferent inputs in neural circuits leads to diverse plastic changes in both pre- and postsynaptic elements that restore neural activity. The axon initial segment (AIS) is the site at which neural signals arise, and should be the most efficient site to regulate neural activity. However, none of the plasticity currently known involves the AIS. We report here that deprivation of auditory input in an avian brainstem auditory neuron leads to an increase in AIS length, thus augmenting the excitability of the neuron. The length of the AIS, defined by the distribution of voltage-gated Na(+) channels and the AIS anchoring protein, increased by 1.7 times in seven days after auditory input deprivation. This was accompanied by an increase in the whole-cell Na(+) current, membrane excitability and spontaneous firing. Our work demonstrates homeostatic regulation of the AIS, which may contribute to the maintenance of the auditory pathway after hearing loss. Furthermore, plasticity at the spike initiation site suggests a powerful pathway for refining neuronal computation in the face of strong sensory deprivation.