Octopus Cells Research Articles

Perinatal nicotine exposure impairs the maturation of glutamatergic inputs in the auditory brainstem

Chronic perinatal nicotine exposure causes abnormal auditory brainstem responses and auditory processing deficits in children and animal models. The effect of perinatal nicotine exposure on synaptic maturation in the auditory brainstem was investigated in granule cells in the ventral nucleus of the lateral lemniscus, which receive a single calyx-like input from the cochlear nucleus. Perinatal nicotine exposure caused a massive reduction in the amplitude of the excitatory input current. This caused a profound decrease in the number and temporal precision of spikes in these neurons. Perinatal nicotine exposure delayed the developmental downregulation of functional nicotinic acetylcholine receptors on these neurons. Maternal smoking causes chronic nicotine exposure during early development and results in auditory processing deficits including delayed speech development and learning difficulties. Using a mouse model of chronic, perinatal nicotine exposure we explored to what extent synaptic inputs to granule cells in the ventral nucleus of the lateral lemniscus are affected by developmental nicotine treatment. These neurons receive one large calyx-like input from octopus cells in the cochlear nucleus and play a role in sound pattern analysis, including speech sounds. In addition, they exhibit high levels of α7 nicotinic acetylcholine receptors, especially during early development. Our whole-cell patch-clamp experiments show that perinatal nicotine exposure causes a profound reduction in synaptic input amplitude. In contrast, the number of inputs innervating each neuron and synaptic release properties of this calyx-like synapse remained unaltered. Spike number and spiking precision in response to synaptic stimulation were greatly diminished, especially for later stimuli during a stimulus train. Moreover, chronic nicotine exposure delayed the developmental downregulation of functional nicotinic acetylcholine receptors on these neurons, indicating a direct action of nicotine in this brain area. This presumably direct effect of perinatal nicotine exposure on synaptic maturation in the auditory brainstem might be one of the underlying causes for auditory processing difficulties in children of heavy smoking mothers.

Genetic perturbations suggest a role of the resting potential in regulating the expression of the ion channels of the KCNA and HCN families in octopus cells of the ventral cochlear nucleus

Low-voltage-activated K+ (gKL) and hyperpolarization-activated mixed cation conductances (gh) mediate currents, IKL and Ih, through channels of the Kv1 (KCNA) and HCN families respectively and give auditory neurons the temporal precision required for signaling information about the onset, fine structure, and time of arrival of sounds. Being partially activated at rest, gKL and gh contribute to the resting potential and shape responses to even small subthreshold synaptic currents. Resting gKL and gh also affect the coupling of somatic depolarization with the generation of action potentials. To learn how these important conductances are regulated we have investigated how genetic perturbations affect their expression in octopus cells of the ventral cochlear nucleus (VCN). We report five new findings: First, the magnitude of gh and gKL varied over more than two-fold between wild type strains of mice. Second, average resting potentials are not different in different strains of mice even in the face of large differences in average gKL and gh. Third, IKL has two components, one being α-dendrotoxin (α-DTX)-sensitive and partially inactivating and the other being α-DTX-insensitive, tetraethylammonium (TEA)-sensitive, and non-inactivating. Fourth, the loss of Kv1.1 results in diminution of the α-DTX-sensitive IKL, and compensatory increased expression of an α-DTX-insensitive, tetraethylammonium (TEA)-sensitive IKL. Fifth, Ih and IKL are balanced at the resting potential in all wild type and mutant octopus cells even when resting potentials vary in individual cells over nearly 10 mV, indicating that the resting potential influences the expression of gh and gKL. The independence of resting potentials on gKL and gh shows that gKL and gh do not, over days or weeks, determine the resting potential but rather that the resting potential plays a role in regulating the magnitude of either or both gKL and gh.

Modulating central gain in tinnitus: changes in nitric oxide synthase in the ventral cochlear nucleus.

A significant challenge in tinnitus research lies in explaining how acoustic insult leads to tinnitus in some individuals, but not others. One possibility is genetic variability in the expression and function of neuromodulators – components of neural signaling that alter the balance of excitation and inhibition in neural circuits. An example is nitric oxide (NO) – a free radical and potent neuromodulator in the mammalian brain – that regulates plasticity via both pre-synaptic and postsynaptic mechanisms. Changes in NO have previously been implicated in tinnitus generation, specifically in the ventral cochlear nucleus (VCN). Here, we examined nitric oxide synthase (NOS) – the enzyme responsible for NO production – in the guinea pig VCN following acoustic trauma. NOS was present in most cell types – including spherical and globular bushy cells, small, medium, and large multipolar cells, and octopus cells – spanning the entire extent of the VCN. The staining pattern was symmetrical in control animals. Unilateral acoustic over-exposure (AOE) resulted in marked asymmetries between ipsilateral and contralateral sides of the VCN in terms of the distribution of NOS across the cochlear nuclei in animals with behavioral evidence of tinnitus: fewer NOS-positive cells and a reduced level of NOS staining was present across the whole extent of the contralateral VCN, relative to the ipsilateral VCN. The asymmetric pattern of NOS-containing cells was observed as early as 1 day after AOE and was also present in some animals at 3, 7, and 21 days after AOE. However, it was not until 8 weeks after AOE, when tinnitus had developed, that asymmetries were significant overall, compared with control animals. Asymmetrical NOS expression was not correlated with shifts in the threshold hearing levels. Variability in NOS expression between animals may represent one underlying difference that can be linked to whether or not tinnitus develops after noise exposure.

Onset-inhibition in the auditory brainstem: a potential mechanism for signal enhancement of speech-like sounds

A prominent feature in the ascending auditory pathway is that of broadband onset inhibition, and there are many hypotheses regarding its functional role [1-4]. Here, we examine the possibility that it acts to suppress broadband onset splatter, the inharmonic noise that occurs at the beginning of sharp-onset sounds such as the glottal pulses in speech. We use experimental data from a circuit in the mammalian auditory brainstem formed by octopus cells in the posteroventral cochlear nucleus (PVCN), and their thick-axon projections to the ventral nucleus of the lateral lemniscus (VNLL) ending in calyx synapses. Octopus cells fire action potentials only in response to broadband auditory events, and they reliably stimulate cells located in the with a short latency. These cells, in turn, provide inhibitory connections to other cells (here referred to as the VNLL cells), which also receive excitatory input from cochlear nucleus (CN) cells [5]. We used a leaky integrate-and-fire (LIF) neuron model with experimentally constrained parameters to recreate the behavior of the circuit. The input to the model was provided by an existing auditory periphery model. A metric, the carrier clarity quotient (CCQ) was devised to quantify the signal strength of the carrier when modulated by a saw-tooth temporal envelope. This envelope is similar to that of glottal pulses in speech. It was observed that, at a certain relative delay between the excitatory and inhibitory input to the cells, the CCQ associate with a population of cells exceeded that of a comparable population of auditory nerve fibres (Figure ​(Figure11). Figure 1 The carrier clarity quotients for a modeled population of primary-like cells and Cell-C cells as a function of the relative delay between excitation and inhibition in the Cell-C population. Three trials are plotted.

Synaptic integration in dendrites: exceptional need for speed

Some neurons in the mammalian auditory system are able to detect and report the coincident firing of inputs with remarkable temporal precision. A strong, low-voltage-activated potassium conductance (g(KL)) at the cell body and dendrites gives these neurons sensitivity to the rate of depolarization by EPSPs, allowing neurons to assess the coincidence of the rising slopes of unitary EPSPs. Two groups of neurons in the brain stem, octopus cells in the posteroventral cochlear nucleus and principal cells of the medial superior olive (MSO), extract acoustic information by assessing coincident firing of their inputs over a submillisecond timescale and convey that information at rates of up to 1000 spikes s(-1). Octopus cells detect the coincident activation of groups of auditory nerve fibres by broadband transient sounds, compensating for the travelling wave delay by dendritic filtering, while MSO neurons detect coincident activation of similarly tuned neurons from each of the two ears through separate dendritic tufts. Each makes use of filtering that is introduced by the spatial distribution of inputs on dendrites.

Generating Synchrony from the Asynchronous: Compensation for Cochlear Traveling Wave Delays by the Dendrites of Individual Brainstem Neurons

Broadband transient sounds, such as clicks and consonants, activate a traveling wave in the cochlea. This wave evokes firing in auditory nerve fibers that are tuned to high frequencies several milliseconds earlier than in fibers tuned to low frequencies. Despite this substantial traveling wave delay, octopus cells in the brainstem receive broadband input and respond to clicks with submillisecond temporal precision. The dendrites of octopus cells lie perpendicular to the tonotopically organized array of auditory nerve fibers, placing the earliest arriving inputs most distally and the latest arriving closest to the soma. Here, we test the hypothesis that the topographic arrangement of synaptic inputs on dendrites of octopus cells allows octopus cells to compensate the traveling wave delay. We show that in mice the full cochlear traveling wave delay is 1.6 ms. Because the dendrites of each octopus cell spread across approximately one-third of the tonotopic axis, a click evokes a soma-directed sweep of synaptic input lasting 0.5 ms in individual octopus cells. Morphologically and biophysically realistic, computational models of octopus cells show that soma-directed sweeps with durations matching in vivo measurements result in the largest and sharpest somatic EPSPs. A low input resistance and activation of a low-voltage-activated potassium conductance that are characteristic of octopus cells are important determinants of sweep sensitivity. We conclude that octopus cells have dendritic morphologies and biophysics tailored to accomplish the precise encoding of broadband transient sounds.

An investigation of dendritic delay in octopus cells of the mammalian cochlear nucleus

Octopus cells, located in the mammalian auditory brainstem, receive their excitatory synaptic input exclusively from auditory nerve fibers (ANFs). They respond with accurately timed spikes but are broadly tuned for sound frequency. Since the representation of information in the auditory nerve is well understood, it is possible to pose a number of questions about the relationship between the intrinsic electrophysiology, dendritic morphology, synaptic connectivity, and the ultimate functional role of octopus cells in the brainstem. This study employed a multi-compartmental Hodgkin-Huxley model to determine whether dendritic delay in octopus cells improves synaptic input coincidence detection in octopus cells by compensating for the cochlear traveling wave delay. The propagation time of post-synaptic potentials from synapse to soma was investigated. We found that the total dendritic delay was approximately 0.275 ms. It was observed that low-threshold potassium channels in the dendrites reduce the amplitude dependence of the dendritic delay of post-synaptic potentials. As our hypothesis predicted, the model was most sensitive to acoustic onset events, such as the glottal pulses in speech when the synaptic inputs were arranged such that the model's dendritic delay compensated for the cochlear traveling wave delay across the ANFs. The range of sound frequency input from ANFs was also investigated. The results suggested that input to octopus cells is dominated by high frequency ANFs.

The magnitudes of hyperpolarization-activated and low-voltage-activated potassium currents co-vary in neurons of the ventral cochlear nucleus

In the ventral cochlear nucleus (VCN), neurons have hyperpolarization-activated conductances, which in some cells are enormous, that contribute to the ability of neurons to convey acoustic information in the timing of their firing by decreasing the input resistance and speeding-up voltage changes. Comparisons of the electrophysiological properties of neurons in the VCN of mutant mice that lack the hyperpolarization-activated cyclic nucleotide-gated channel α subunit 1 (HCN1(-/-)) (Nolan et al. 2003) with wild-type controls (HCN1(+/+)) and with outbred ICR mice reveal that octopus, T stellate, and bushy cells maintain their electrophysiological distinctions in all strains. Hyperpolarization-activated (I(h)) currents were smaller and slower, input resistances were higher, and membrane time constants were longer in HCN1(-/-) than in HCN1(+/+) in octopus, bushy, and T stellate cells. There were significant differences in the average magnitudes of I(h), input resistances, and time constants between HCN1(+/+) and ICR mice, but the resting potentials did not differ between strains. I(h) is opposed by a low-voltage-activated potassium (I(KL)) current in bushy and octopus cells, whose magnitudes varied widely between neuronal types and between strains. The magnitudes of I(h) and I(KL) were correlated across neuronal types and across mouse strains. Furthermore, these currents balanced one another at the resting potential in individual cells. The magnitude of I(h) and I(KL) is linked in bushy and octopus cells and varies not only between HCN1(-/-) and HCN1(+/+) but also between "wild-type" strains of mice, raising the question to what extent the wild-type strains reflect normal mice.

Auditory nerve fibers excite targets through synapses that vary in convergence, strength, and short-term plasticity.

Auditory nerve fibers are the major source of excitation to the three groups of principal cells of the ventral cochlear nucleus (VCN), bushy, T stellate, and octopus cells. Shock-evoked excitatory postsynaptic currents (eEPSCs) in slices from mice showed systematic differences between groups of principal cells, indicating that target cells contribute to determining pre- and postsynaptic properties of synapses from spiral ganglion cells. Bushy cells likely to be small spherical bushy cells receive no more than three, most often two, excitatory inputs; those likely to be globular bushy cells receive at least four, most likely five, inputs. T stellate cells receive 6.5 inputs. Octopus cells receive >60 inputs. The N-methyl-d-aspartate (NMDA) components of eEPSCs were largest in T stellate, smaller in bushy, and smallest in octopus cells, and they were larger in neurons from younger than older mice. The average AMPA conductance of a unitary input is 22 ± 15 nS in both groups of bushy cells, <1.5 nS in octopus cells, and 4.6 ± 3 nS in T stellate cells. Sensitivity to philanthotoxin (PhTX) and rectification in the intracellular presence of spermine indicate that AMPA receptors that mediate eEPSCs in T stellate cells contain more GluR2 subunits than those in bushy and octopus cells. The AMPA components of eEPSCs were briefer in bushy (0.5 ms half-width) than in T stellate and octopus cells (0.8-0.9 ms half-width). Widening of eEPSCs in the presence of cyclothiazide (CTZ) indicates that desensitization shortens eEPSCs. CTZ-insensitive synaptic depression of the AMPA components was greater in bushy and octopus than in T stellate cells.

Inhibitory projections from the ventral nucleus of the lateral lemniscus and superior paraolivary nucleus create directional selectivity of frequency modulations in the inferior colliculus: A comparison of bats with other mammals

This review considers four auditory brainstem nuclear groups and shows how studies of both bats and other mammals have provided insights into their response properties and the impact of their convergence in the inferior colliculus (IC). The four groups are octopus cells in the cochlear nucleus, their connections with the ventral nucleus of the lateral lemniscus (VNLL) and the superior paraolivary nucleus (SPON), and the connections of the VNLL and SPON with the IC. The theme is that the response properties of neurons in the SPON and VNLL map closely onto the synaptic response features of a unique subpopulation of cells in the IC of bats whose inputs are dominated by inhibition. We propose that the convergence of VNLL and SPON inputs generates the tuning of these IC cells, their unique temporal responses to tones, and their directional selectivities for frequency modulated (FM) sweeps. Other IC neurons form directional properties in other ways, showing that selective response properties are formed in multiple ways. In the final section we discuss why multiple formations of common response properties could amplify differences in population activity patterns evoked by signals that have similar spectrotemporal features.

Electrophysiological properties of ventral cochlear nucleus neurons of the dog

Neurons in the cochlear nucleus (CN) have distinct anatomical and biophysical specializations and extract various facets of auditory information which are transmitted to the higher auditory centres. The aim of the present study was to determine if the principal neurons (stellate, bushy and octopus cells) of the ventral cochlear nucleus (VCN) in 2-week-old dog brain slices share common electrophysiological properties with the principal neurons of mouse VCN. Stellate cells ( n = 21, of which three were anatomically identified), fired large, regular trains of action potentials in response to depolarizing current pulses. Input resistance and membrane time constant were 176 ± 35.9 MΩ ( n = 21) and 8.8 ± 1.4 ms ( n = 21), respectively. Bushy cells, ( n = 6, of which three were anatomically identified) responded with a single action potential at the onset of depolarizing current steps and showed large hyperpolarizing voltage changes that sag back toward rest to hyperpolarizing current pulses. Input resistance and membrane time constant were 120.4 ± 56.1 MΩ ( n = 5) and 7.6 ± 2.3 ms ( n = 5), respectively. Octopus cells ( n = 17, of which seven were anatomically identified) fired a single action potential at the start of a depolarizing current step and exhibited a pronounced depolarizing sag of the membrane potential towards the resting value to hyperpolarizing current steps. Input resistance and membrane time constant were 17.58 ± 1.3 MΩ ( n = 15) and 1.34 ± 0.13 ms ( n = 15), respectively. While stellate cells did not have a threshold rate of depolarization (d V/d t thresh), bushy and octopus had a d V/d t thresh of 5.06 ± 1.04 mV/ms ( n = 4) and 10.6 ± 2.0 mV/ms ( n = 6), respectively. In octopus cells, the single action potential was abolished by tetrodotoxin (TTX). An alpha-dendrotoxin (α-DTX)-sensitive, low-voltage-activated potassium conductance ( g KL) together with a ZD7288-sensitive, mixed-cation conductance ( g h) were responsible for the low input resistance, and as a consequence for the brief time constant of the octopus cells. We conclude that the principal neurons of the dog VCN are, as in mouse and cat, distinguishable on the basis of whole-cell patch-clamp recordings.

Neuronal degeneration of the auditory nervous center in the mouse model of cochlear ablation

To study the long-term effect of cochlear ablation on cochlear nucleus (CN). A total of 40 BALB/c adult mice were used in the present study. Mice were divided into experimental group and control group at random. In experimental group, the cochlea was destroyed with stainless steel needle under the anatomy microscopy. Mice were tested for ABR thresholds at second day and 4 months after operation to assess hearing sensitivity. Animals were allowed to survive for four months. Histological sections of the cochlear nucleus were evaluated with serial sections stained with Nissl staining and silver staining alternatively. Morphological change in anteroventral cochlear nucleus (AVCN), posteroventral cochlear nucleus (PVCN), dorsal cochlear nucleus (DCN) and octopus cell area (OCA) was observed evaluated four month later after the cochlear ablation. In the experimental group, after operation, early components of ABR couldn't be recognized. The volume of CN was decreased dramatically and the cells were less comparing to the control group. The volume of AVCN after cochlear ablation showed a decline to 22% less than the control group, and the number of neurons also dropped by 25%. In PVCN, the decreased volume by 40% with significant neuronal loss of 47% was observed four month later after the cochlear ablation. And in DCN, the volume shrinking to 24% and the loss of neurons reached to 39%. The most significant change was observed in OCA (octopus cell area), with the neuron numbers, the area and the volume of OCA decreased by 60%, 19%, and 47% respectively. There was a statistically significant difference (P<0.05, Mann-Whitney U test) in the morphological changes between the two groups. Neuronal degeneration was observed in cochlear nucleus after four months cochlear ablation.

Electrophysiological Properties of Octopus Neurons of the Cat Cochlear Nucleus: an In Vitro Study

Electrophysiological studies from mice in vitro have suggested that octopus cells of the mammalian ventral cochlear nucleus (VCN) are anatomically and biophysically specialized for detecting the coincident firing of a population of auditory nerve fibers. Recordings from cats in vivo have shown that octopus cells fire rapidly and with exceptional temporal precision as they convey the timing of that coincidence to higher auditory centers. The current study addresses the question whether the biophysical properties of octopus cells that have until now been examined only in mice, are shared by octopus cells in cats. Whole-cell patch-clamp recordings confirm that octopus cells in brain slices from kittens share the anatomical and biophysical features of octopus cells in mice. As in mice, octopus cells in kittens have large cell bodies and thick dendrites that extend in one direction. Voltage changes produced by depolarizing and hyperpolarizing current injection were small and rapid. Input resistances and membrane time constants in octopus cells of 16-day-old kittens were 15.8 +/- 1.5 MOmega (n = 16) and 1.28 +/- 0.3 ms (n = 16), respectively. Octopus cells fired only a single action potential at the onset of a depolarizing current pulse; suprathreshold stimuli were greater than 1.8 nA. A tetrodotoxin (TTX)-sensitive sodium conductance (gNa) was responsible for the generation of the action potentials. Octopus cells displayed outward rectification that lasted for the duration of the depolarizing pulses. Hyperpolarizations produced by the injection of current exhibited a depolarizing sag of the membrane potential toward the resting value. A 4-aminopyridine (4-AP) and alpha-dendrotoxin (alpha-DTX)-sensitive, low-voltage-activated potassium conductance (gKL) and a ZD7288-sensitive, mixed-cation conductance (gh) were partially activated at rest, giving the octopus cells low input resistances and, as a consequence, brief time constants. In 7-day-old kittens, action potentials were taller and broader, input resistances higher, and both inward and outward rectification was weaker than in 16-day-old kittens. Also as in mice, stellate cells of the VCN fired trains of action potentials with constant interspike intervals when they were depolarized (n = 10) and bushy cells of the VCN fired only a single action potential at the onset of depolarizations (n = 6). In conclusion, the similarity of octopus cells in mice and kittens suggests that the anatomical and biophysical specializations that allow octopus cells to detect and convey synchronous firing among auditory nerve fibers are common to all mammals.

Connections and synaptic function in the posteroventral cochlear nucleus of deaf jerker mice

Mutations in the gene that encodes espins can cause deafness and vestibular disorders; mice that are homozygous for the autosomal recessive jerker mutation in the espin gene never hear. Extracellular injections of biocytin into the anteroventral cochlear nucleus (AVCN) revealed that although the cochlear nuclei are smaller in je/je mice, the topography in its innervation resembles that in wild-type mice. Auditory nerve fibers innervate narrow, topographically organized, "isofrequency" bands in deaf animals over the ages examined, P18-P70. The projection of tuberculoventral cells was topographic in je/je as in wild-type mice. Terminals of auditory nerve fibers in the multipolar cell area included both large and small endings, whereas in the octopus cell area they were exclusively small boutons in je/je as in wild-type mice, but end bulbs near the nerve root of je/je animals were smaller than in hearing animals. In whole-cell recordings from targets of auditory nerve fibers, octopus and T stellate cells, miniature excitatory postsynaptic currents (mEPSCs) had similar shapes as in +/+, indicating that the properties of AMPA receptors were not affected by the mutation. In je/je animals the frequency of spontaneous mEPSCs was elevated, and synaptic depression in responses to trains of shocks delivered at between 100 and 333 Hz was greater than in wild-type mice, indicating that the probability of neurotransmitter release was increased. The frequency of spontaneous mEPSCs and extent of synaptic depression were greater in octopus than in T stellate cells, in both wild-type and in je/je mice.

Espin actin‐cytoskeletal proteins are in rat type I spiral ganglion neurons and include splice‐isoforms with a functional nuclear localization signal

The espins are Ca(2+)-resistant actin-bundling proteins that are enriched in hair cell stereocilia and sensory cell microvilli. Here, we report a novel localization of espins to a large proportion of rat type I spiral ganglion neurons (SGNs) and their projections to the cochlear nucleus (CN). Moreover, we show that a fraction of these espins is in the nucleus of SGNs owing to the presence of splice-isoforms that contain a functional nuclear localization signal (NLS). Espin antibody labeled approximately 83% of type I SGNs, and the labeling intensity increased dramatically during early postnatal development. Type II SGNs and vestibular ganglion neurons were unlabeled. In the CN, espin-positive auditory nerve fibers showed a projection pattern typical of type I SGNs, with intense labeling in the nerve root region and posteroventral CN (PVCN). The anteroventral CN (AVCN) showed moderate labeling, whereas the dorsal CN showed weak labeling that was restricted to the deep layer. Espin-positive synaptic terminals were enriched around nerve root neurons and octopus cells in the PVCN and were also found on globular bushy cells and multipolar neurons in the PVCN and AVCN. SGNs expressed multiple espin transcripts and proteins, including splice-isoforms that contain a nonapeptide, which is rich in positively charged amino acids and creates a bipartite NLS. The nonapeptide was necessary to target espin isoforms to the nucleus and was sufficient to target an unrelated protein to the nucleus when joined with the upstream di-arginine-containing octapeptide. The presence of cytoplasmic and nuclear espins in SGNs suggests additional roles for espins in auditory neuroscience.

In the ventral cochlear nucleus Kv1.1 and subunits of HCN1 are colocalized at surfaces of neurons that have low-voltage-activated and hyperpolarization-activated conductances

Principal cells of the ventral cochlear nucleus (VCN) differ in the magnitudes of low-voltage-activated potassium (gKL) and hyperpolarization-activated (gh) conductances that determine the time course of signaling. Octopus cells in mice have large gKL (500 nS) and gh (150 nS), bushy cells have smaller gKL (80 nS) and gh (30 nS), and T stellate cells have little gKL and a small gh (20 nS). gKL Arises through potassium channels of which ∼60% contain Kv1.1 (potassium channels in the shaker or KCNA family) subunits; gh arises through channels that include hyperpolarization and cyclic nucleotide gated (HCN) 1 subunits. The surfaces of cell bodies and dendrites of octopus cells in the dorsocaudal pole, and of similar cells along the ventrolateral edge of the PVCN, were brightly labeled by an antibody against HCN1 that was colocalized with labeling for Kv1.1. More anteriorly neurons with little surface labeling were intermingled among cell bodies and dendrites with surface labeling for both proteins, likely corresponding to T stellate and bushy cells. The membrane-associated labeling patterns for Kv1.1 and HCN1 were consistent with what is known about the distribution and the electrophysiological properties of the principal cells of the VCN. The cytoplasm of large cells and axonal paranodes contained immunofluorescent labeling for only Kv1.1.

Neural Timing in Highly Convergent Systems

In order to study how the convergence of many variable-response neurons on a single target can sharpen timing information, we investigate the limit as the number of input neurons and the number of incoming spikes required to fire the target both get large with the ratio fixed. We prove that the standard deviation of the firing time of the target cell goes to zero in this limit, and we derive the asymptotic forms of the density and the standard deviation near the limit. We use the theorems to understand the behavior of octopus cells in the mammalian cochlear nucleus.

Characterization of cochlear nucleus principal cells of Meriones unguiculatus and Monodelphis domestica by use of calcium-binding protein immunolabeling

Antibodies directed against calcium-binding proteins (CaBPs) parvalbumin, calbindin-D28k and calretinin were used as neuronal markers to identify and characterize different principal cell types in the mammalian cochlear nucleus. For this purpose, double immunofluorescence labeling and the combination of CaBP-labeling with pan-neuronal markers were applied to analyze the CaBPs distribution in neurons of the cochlear nucleus (CN) of the Mongolian gerbil ( Meriones unguiculatus) and the gray short-tailed opossum ( Monodelphis domestica). Despite of the fact, that these two mammalian species are not closely related, principal cell types in the CN of the two species showed many corresponding morphological features and similarities in immunolabeling of the CaBPs. Parvalbumin seems not to be suited as a differential neuronal marker in the CN since it is expressed by almost all neurons. In contrast, calbindin and calretinin were more restricted to specific cell types and showed a mostly complementary labeling pattern. As one of the most interesting findings, calbindin and calretinin were predominantly found in subpopulations of globular bushy cells and octopus cells in the ventral CN. Such a neuron-specific CaBP-expression in subpopulations of morphologically defined cell types argues for a more refined classification of CN cell types in Meriones and Monodelphis. Additionally, other cell types (cartwheel cells, unipolar brush cells, fusiform cells) were marked with calbindin or calretinin as well. Calretinin staining was predominantly observed in auditory nerve fibers and their endings including endbulbs of Held in Meriones. Spherical bushy cells showed a different calretinin-immunolabeling in Meriones and Monodelphis. This species-specific difference may be related to adaptive differences in auditory function.

Voltage-activated Calcium Currents in Octopus Cells of the Mouse Cochlear Nucleus

Octopus cells, neurons in the most posterior and dorsal part of the mammalian ventral cochlear nucleus, convey the timing of synchronous firing of auditory nerve fibers to targets in the contralateral superior paraolivary nucleus and ventral nucleus of the lateral lemniscus. The low input resistances and short time constants at rest that arise from the partial activation of a large, low-voltage-activated K(+) conductance (g(KL)) and a large mixed-cation, hyperpolarization-activated conductance (g(h)) enable octopus cells to detect coincident firing of auditory nerve fibers with exceptional temporal precision. Octopus cells fire conventional, Na(+) action potentials but a voltage-sensitive Ca(2+) conductance was also detected. In this study, we explore the nature of that calcium conductance under voltage-clamp. Currents, carried by Ca(2+) or Ba(2+) and blocked by 0.4 mM Cd(2+), were activated by depolarizations positive to -50 mV and peaked at -23 mV. At -23 mV they reached 1.1 +/- 0.1 nA in the presence of 5 mM Ca(2+) and 1.6 +/- 0.1 nA in 5 mM Ba(2+). Ten micromolar BAY K 8644, an agonist of high-voltage-activated L-type channels, enhanced I(Ba) by 63 +/- 11% (n = 8) and 150 microM nifedipine, an antagonist of L-type channels, reduced the I(Ba) by 65 +/- 5% (n = 5). Meanwhile, 0.5 microM omega-Agatoxin IVA, an antagonist of P/Q-type channels, or 1 microM omega-conotoxin GVIA, an antagonist of N-type channels, suppressed I(Ba) by 15 +/- 4% (n = 5) and 9 +/- 4% (n = 5), respectively. On average 16% of the current remained in the presence of the cocktail of blockers, indicative of the presence of R-type channels. Together these experiments show that octopus cells have a depolarization-sensitive g(Ca) that is largely formed from L-type Ca(2+) channels and that P/Q-, N-, and R-type channels are expressed at lower levels in octopus cells.

Rate thresholds determine the precision of temporal integration in principal cells of the ventral cochlear nucleus

The three types of principal cells of the ventral cochlear nucleus (VCN), bushy, octopus, and T stellate, differ in the detection of coincidence among synaptic inputs. To explore the role of the action-potential-generation mechanism in the detection of coincident inputs, we examined responses to depolarizing currents that increased at varying rates. To fire an action potential, bushy cells, likely of the globular subtype, had to be depolarized faster than 4.8 ± 2.8 mV/ms, octopus cells faster than 9.5 ± 3.6 mV/ms, and T stellate cells fired irrespective of the rate of depolarization. The threshold rate of depolarization permitted definition of a time window over which depolarization could contribute to generating action potentials. This integration window differed between cell types. It was 5.3 ± 1.8 ms for bushy cells and 1.4 ± 0.3 ms for octopus cells. T Stellate cells fired action potentials in response to even slow depolarizations, showing that their integration window was unlimited so that temporal summation in these cells is limited by the time course of synaptic potentials. The rate of depolarization threshold in octopus and bushy cells was decreased by α-dendrotoxin while T stellate cells were largely insensitive to α-dendrotoxin indicating that low-voltage-activated K + conductances ( g KL) are important determinants of the integration window.