Imaging Voltage Globally and in Isofrequency Lamina in Slices of Mouse Ventral Cochlear Nucleus

The cochlear nuclei (CNs) receive sensory information from the ear and perform fundamental computations before relaying this information to higher processing centers. These computations are performed by distinct types of neurons interconnected in circuits dedicated to the specialized roles of the auditory system. In the present study, we explored the use of voltage imaging to investigate CN circuitry. We tested two approaches based on fundamentally different voltage sensing technologies. Using a voltage-sensitive dye we recorded glutamate receptor-independent signals arising predominantly from axons. The mean conduction velocity of these fibers of 0.27 m/s was rapid but in range with other unmyelinated axons. We then used a genetically-encoded hybrid voltage sensor (hVOS) to image voltage from a specific population of neurons. Probe expression was controlled using Cre recombinase linked to c-fos activation. This activity-induced gene enabled targeting of neurons that are activated when a mouse hears a pure 15-kHz tone. In CN slices from these animals auditory nerve fiber stimulation elicited a glutamate receptor-dependent depolarization in hVOS probe-labeled neurons. These cells resided within a band corresponding to an isofrequency lamina, and responded with a high degree of synchrony. In contrast to the axonal origin of voltage-sensitive dye signals, hVOS signals represent predominantly postsynaptic responses. The introduction of voltage imaging to the CN creates the opportunity to investigate auditory processing circuitry in populations of neurons targeted on the basis of their genetic identity and their roles in sensory processing.

Local targets of T-stellate cells in the ventral cochlear nucleus

T-stellate cells in the ventral cochlear nucleus (VCN) are known to have local axon collaterals that terminate in the vicinity of their dendrites and cell bodies within the same isofrequency lamina in parallel with the auditory nerve fibers that innervate them. Excitatory synaptic connections between stellate cells within an isofrequency lamina are hypothesized to be involved in the nitric oxide-mediated upregulation of T-stellate responses to their auditory input. This could serve as a mechanism of variable gain control in the enhancement of responses to vowel spectral peaks. Previous studies have provided indirect evidence for these possible synaptic interconnections between T-stellate cells, but unequivocal identification has yet to be established. Here, we used retrograde neuronal tracing with adeno-associated viral vector or biotinylated dextran amine injected into the inferior colliculus (IC) to detect the postsynaptic target of T-stellate cells within the VCN. We show that backfilled T-stellate cell axons make monosynapatic connections on the labeled cell bodies and dendrites of other labeled T-stellate cells within an isofrequency lamina. Electron microscopy revealed that T-stellate terminals can also make synapses on structures not retrogradely labeled from the IC. Glycine antibodies combined with the viral labeling indicated that these nonbackfilled structures that the labeled T-stellate terminals were synapsing on are most likely the cell bodies and dendrites of two size categories of glycinergic VCN cells, whose sizes and relative numbers indicated they are the D- and L-stellate cells. These cells are known to provide inhibitory inputs back onto T-stellate cells. Our data indicate that, in addition to their auditory nerve input, T-stellate cells provide a second modulatable excitatory input to both inhibitory and excitatory cells in a VCN isofrequency lamina and may play a significant role in acoustic information processing.

Cellular Computations Underlying Detection of Gaps in Sounds and Lateralizing Sound Sources

In mammals, acoustic information arises in the cochlea and is transmitted to the ventral cochlear nuclei (VCN). Three groups of VCN neurons extract different features from the firing of auditory nerve fibers and convey that information along separate pathways through the brainstem. Two of these pathways process temporal information: octopus cells detect coincident firing among auditory nerve fibers and transmit signals along monaural pathways, and bushy cells sharpen the encoding of fine structure and feed binaural pathways. The ability of these cells to signal with temporal precision depends on a low-voltage-activated K⁺ conductance (g_KL) and a hyperpolarization-activated conductance (g_h). This 'tale of two conductances' traces gap detection and sound lateralization to their cellular and biophysical origins.

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⁺ (g_KL) and hyperpolarization-activated mixed cation conductances (g_h) mediate currents, I_KL and I_h, 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, g_KL and g_h contribute to the resting potential and shape responses to even small subthreshold synaptic currents. Resting g_KL and g_h 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 g_h and g_KL 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 g_KL and g_h. Third, I_KL 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 I_KL, and compensatory increased expression of an α-DTX-insensitive, tetraethylammonium (TEA)-sensitive I_KL. Fifth, I_h and I_KL 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 g_h and g_KL. The independence of resting potentials on g_KL and g_h shows that g_KL and g_h 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 g_KL and g_h.

Mutation of Npr2 leads to blurred tonotopic organization of central auditory circuits in mice

Tonotopy is a fundamental organizational feature of the auditory system. Sounds are encoded by the spatial and temporal patterns of electrical activity in spiral ganglion neurons (SGNs) and are transmitted via tonotopically ordered processes from the cochlea through the eighth nerve to the cochlear nuclei. Upon reaching the brainstem, SGN axons bifurcate in a stereotyped pattern, innervating target neurons in the anteroventral cochlear nucleus (aVCN) with one branch and in the posteroventral and dorsal cochlear nuclei (pVCN and DCN) with the other. Each branch is tonotopically organized, thereby distributing acoustic information systematically along multiple parallel pathways for processing in the brainstem. In mice with a mutation in the receptor guanylyl cyclase Npr2, this spatial organization is disrupted. Peripheral SGN processes appear normal, but central SGN processes fail to bifurcate and are disorganized as they exit the auditory nerve. Within the cochlear nuclei, the tonotopic organization of the SGN terminal arbors is blurred and the aVCN is underinnervated with a reduced convergence of SGN inputs onto target neurons. The tonotopy of circuitry within the cochlear nuclei is also degraded, as revealed by changes in the topographic mapping of tuberculoventral cell projections from DCN to VCN. Nonetheless, Npr2 mutant SGN axons are able to transmit acoustic information with normal sensitivity and timing, as revealed by auditory brainstem responses and electrophysiological recordings from VCN neurons. Although most features of signal transmission are normal, intermittent failures were observed in responses to trains of shocks, likely due to a failure in action potential conduction at branch points in Npr2 mutant afferent fibers. Our results show that Npr2 is necessary for the precise spatial organization typical of central auditory circuits, but that signals are still transmitted with normal timing, and that mutant mice can hear even with these deficits.

Synaptic transmission between end bulbs of Held and bushy cells in the cochlear nucleus of mice with a mutation in Otoferlin

Mice that carry a mutation in a calcium binding domain of Otoferlin, the putative calcium sensor at hair cell synapses, have normal distortion product otoacoustic emissions (DPOAEs), but auditory brain stem responses (ABRs) are absent. In mutant mice mechanotransduction is normal but transmission of acoustic information to the auditory pathway is blocked even before the onset of hearing. The cross-sectional area of the auditory nerve of mutant mice is reduced by 54%, and the volume of ventral cochlear nuclei is reduced by 46% relative to hearing control mice. While the tonotopic organization was not detectably changed in mutant mice, the axons to end bulbs of Held and the end bulbs themselves were smaller. In mutant mice bushy cells in the anteroventral cochlear nucleus (aVCN) have the electrophysiological hallmarks of control cells. Spontaneous miniature excitatory postsynaptic currents (EPSCs) occur with similar frequencies and have similar shapes in deaf as in hearing animals, but they are 24% larger in deaf mice. Bushy cells in deaf mutant mice are contacted by ∼2.6 auditory nerve fibers compared with ∼2.0 in hearing control mice. Furthermore, each fiber delivers more synaptic current, on average 4.8 nA compared with 3.4 nA, in deaf versus hearing control mice. The quantal content of evoked EPSCs is not different between mutant and control mice; the increase in synaptic current delivered in mutant mice is accounted for by the increased response to the size of the quanta. Although responses to shocks presented at long intervals are larger in mutant mice, they depress more rapidly than in hearing control mice.

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.

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.

The multiple functions of T stellate/multipolar/chopper cells in the ventral cochlear nucleus

Acoustic information is brought to the brain by auditory nerve fibers, all of which terminate in the cochlear nuclei, and is passed up the auditory pathway through the principal cells of the cochlear nuclei. A population of neurons variously known as T stellate, type I multipolar, planar multipolar, or chopper cells forms one of the major ascending auditory pathways through the brainstem. T Stellate cells are sharply tuned; as a population they encode the spectrum of sounds. In these neurons, phasic excitation from the auditory nerve is made more tonic by feedforward excitation, coactivation of inhibitory with excitatory inputs, relatively large excitatory currents through NMDA receptors, and relatively little synaptic depression. The mechanisms that make firing tonic also obscure the fine structure of sounds that is represented in the excitatory inputs from the auditory nerve and account for the characteristic chopping response patterns with which T stellate cells respond to tones. In contrast with other principal cells of the ventral cochlear nucleus (VCN), T stellate cells lack a low-voltage-activated potassium conductance and are therefore sensitive to small, steady, neuromodulating currents. The presence of cholinergic, serotonergic and noradrenergic receptors allows the excitability of these cells to be modulated by medial olivocochlear efferent neurons and by neuronal circuits associated with arousal. T Stellate cells deliver acoustic information to the ipsilateral dorsal cochlear nucleus (DCN), ventral nucleus of the trapezoid body (VNTB), periolivary regions around the lateral superior olivary nucleus (LSO), and to the contralateral ventral lemniscal nuclei (VNLL) and inferior colliculus (IC). It is likely that T stellate cells participate in feedback loops through both medial and lateral olivocochlear efferent neurons and they may be a source of ipsilateral excitation of the LSO.

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.

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.

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 approximately 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.

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.

Voltage-sensitive conductances of bushy cells of the Mammalian ventral cochlear nucleus

Bushy cells in the ventral cochlear nucleus convey firing of auditory nerve fibers to neurons in the superior olivary complex that compare the timing and intensity of sounds at the two ears and enable animals to localize sound sources in the horizontal plane. Three voltage-sensitive conductances allow bushy cells to convey acoustic information with submillisecond temporal precision. All bushy cells have a low-voltage-activated, alpha-dendrotoxin (alpha-DTX)-sensitive K(+) conductance (g(KL)) that was activated by depolarization past -70 mV, was half-activated at -39.0 +/- 1.7 (SE) mV, and inactivated approximately 60% over 5 s. Maximal g(KL) varied between 40 and 150 nS (mean: 80.8 +/- 16.7 nS). An alpha-DTX-insensitive, tetraethylammonium (TEA)-sensitive, K(+) conductance (g(KH)) was activated at voltages positive to -40 mV, was half-activated at -18.1 +/- 3.8 mV, and inactivated by 90% over 5 s. Maximal g(KH) varied between 35 and 80 nS (mean: 58.2 +/- 6.5 nS). A ZD7288-sensitive, mixed cation conductance (g(h)) was activated by hyperpolarization greater than -60 mV and half-activated at -83.1 +/- 1.1 mV. Maximum g(h) ranged between 14.5 and 56.6 nS (mean: 30.0 +/- 5.5 nS). 8-Br-cAMP shifted the voltage sensitivity of g(h) positively. Changes in temperature stably altered the steady-state magnitude of I(h). Both g(KL) and g(KH) contribute to repolarizing action potentials and to sharpening synaptic potentials. Those cells with the largest g(h) and the largest g(KL) fired least at the onset of a depolarization, required the fastest depolarizations to fire, and tended to be located nearest the nerve root.

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 alpha-dendrotoxin while T stellate cells were largely insensitive to alpha-dendrotoxin indicating that low-voltage-activated K+ conductances (gKL) are important determinants of the integration window.

Hyperpolarization-Activated Currents Regulate Excitability in Stellate Cells of the Mammalian Ventral Cochlear Nucleus

The differing biophysical properties of neurons the axons of which form the different pathways from the ventral cochlear nucleus (VCN) determine what acoustic information they can convey. T stellate cells, excitatory neurons the axons of which project locally and to the inferior colliculus, and D stellate cells, inhibitory neurons the axons of which project to the ipsi- and contralateral cochlear nuclei, fire tonically when they are depolarized, and, unlike other cell types in the VCN, their firing rates are sensitive to small changes in resting currents. In both types of neurons, the hyperpolarization-activated current (I(h)) reversed at -40 mV, was activated at voltages negative to -60 mV, and half-activated at approximately -88 mV; maximum hyperpolarization-activated conductances (g(h max)) were 19.1 +/- 2.3 nS in T and 30.3 +/- 2.6 nS in D stellate cells (means +/- SE). Activation and deactivation were slower in T than in D stellate cells. In both types of stellate cells, 50 microM 4(N-ethyl-N-phenylamino)1,2-dimethyl-6-(methylamino) pyridinium chloride (ZD7288) and 2 mM Cs(+) blocked a 6- to 10-fold greater conductance than the voltage-dependent g(h) determined from Boltzmann analyses at -62 mV. The voltage-insensitive, ZD7288-sensitive conductance was proportional to g(h max) and g(input). 8-Br-cAMP shifted the voltage dependence of I(h) in the depolarizing direction, increased the rate of activation, and slowed its deactivation in both T and D stellate cells. Reduction in temperature did not change the voltage dependence but reduced the maximal g(h) with a Q(10) of 1.3 and slowed the kinetics with a Q(10) of 3.3.

Temperature Affects Voltage-Sensitive Conductances Differentially in Octopus Cells of the Mammalian Cochlear Nucleus

Temperature is an important physiological variable the influence of which on macroscopic electrophysiological measurements in slices is not well documented. We show that each of three voltage-sensitive conductances of octopus cells of the mammalian ventral cochlear nucleus (VCN) is affected differently by changes in temperature. As expected, the kinetics of the currents were faster at higher than at lower temperature. Where they could be measured, time constants of activation, deactivation, and inactivation had Q10 values between 1.8 and 4.6. The magnitude of the peak conductances was differentially affected by temperature. While the peak magnitude of the high-voltage-activated K+ conductance, gKH, was unaffected by changes in temperature, the peak of the low-voltage-activated K+ conductance, gKL, was reduced by half when the temperature was lowered from 33 to 23°C ( Q10 = 2). Changing the temperature changed the kinetics and the magnitude of the hyperpolarization-activated mixed cation conductance, gh, but the changes in magnitude were transient. The voltage sensitivity of the three conductances was unaffected by temperature. The action of temperature on these conductances is reflected in the resting potentials and in the shapes of action potentials.

Cell-specific, spike timing–dependent plasticities in the dorsal cochlear nucleus

In the dorsal cochlear nucleus, long-term synaptic plasticity can be induced at the parallel fiber inputs that synapse onto both fusiform principal neurons and cartwheel feedforward inhibitory interneurons. Here we report that in mouse fusiform cells, spikes evoked 5 ms after parallel-fiber excitatory postsynaptic potentials (EPSPs) led to long-term potentiation (LTP), whereas spikes evoked 5 ms before EPSPs led to long-term depression (LTD) of the synapse. The EPSP-spike protocol led to LTD in cartwheel cells, but no synaptic changes resulted from the reverse sequence (spike-EPSP). Plasticity in fusiform and cartwheel cells therefore followed Hebbian and anti-Hebbian learning rules, respectively. Similarly, spikes generated by summing EPSPs from different groups of parallel fibers produced LTP in fusiform cells, and LTD in cartwheel cells. LTD could also be induced in glutamatergic inputs of cartwheel cells by pairing parallel-fiber EPSPs with depolarizing glycinergic PSPs from neighboring cartwheel cells. Thus, synaptic learning rules vary with the postsynaptic cell, and may require the interaction of different transmitter systems.

What's a cerebellar circuit doing in the auditory system?

The shapes of the head and ears of mammals are asymmetrical top-to-bottom and front-to-back. Reflections of sounds from these structures differ with the angle of incidence, producing cues for monaural sound localization in the spectra of the stimuli at the eardrum. Neurons in the dorsal cochlear nucleus (DCN) respond specifically to spectral cues and integrate them with somatosensory, vestibular and higher-level auditory information through parallel fiber inputs in a cerebellum-like circuit. Synapses between parallel fibers and their targets show long-term potentiation (LTP) and long-term depression (LTD), whereas those between auditory nerve fibers and their targets do not. This paper discusses the integration of acoustic and the proprioceptive information in terms of possible computational roles for the DCN.

Bidirectional synaptic plasticity in the cerebellum-like mammalian dorsal cochlear nucleus

The dorsal cochlear nucleus integrates acoustic with multimodal sensory inputs from widespread areas of the brain. Multimodal inputs are brought to spiny dendrites of fusiform and cartwheel cells in the molecular layer by parallel fibers through synapses that are subject to long-term potentiation and long-term depression. Acoustic cues are brought to smooth dendrites of fusiform cells in the deep layer by auditory nerve fibers through synapses that do not show plasticity. Plasticity requires Ca(2+)-induced Ca(2+) release; its sensitivity to antagonists of N-methyl-d-aspartate and metabotropic glutamate receptors differs in fusiform and cartwheel cells.

Octopus cells of the mammalian ventral cochlear nucleus sense the rate of depolarization

Whole cell patch recordings in slices show that the probability of firing of action potentials in octopus cells of the ventral cochlear nucleus depends on the dynamic properties of depolarization. Octopus cells fired only when the rate of rise of a depolarization exceeded a threshold value that varied between 5 and 15 mV/ms among cells. The threshold rate of rise was independent of whether depolarizations were evoked synaptically or by the intracellular injection of current. Previous work showed that octopus cells are contacted by many auditory nerve fibers, each providing less than 1-mV depolarization. Summation of synaptic input from multiple fibers is required for an octopus cell to reach threshold. In firing only when synaptic depolarization exceeds a threshold rate, octopus cells fire selectively when synaptic input is sufficiently large and synchronized for the small, brief unitary excitatory postsynaptic potentials (EPSPs) to sum to produce a rapidly rising depolarization. The sensitivity to rate of depolarization is governed by a low-threshold, alpha-dendrotoxin-sensitive potassium conductance (g(KL)). This conductance also shapes the peaks of action potentials, contributing to the precision in their timing. Firing in neighboring T stellate cells depends much less strongly on the rate of rise. They lack strong alpha-dendrotoxin-sensitive conductances. Octopus cells appear to be specialized to detect synchronization in the activation of groups of auditory nerve fibers, a common pattern in responses to natural sounds, and convey its occurrence with temporal precision.

Potassium currents in octopus cells of the mammalian cochlear nucleus

Octopus cells in the posteroventral cochlear nucleus (PVCN) of mammals are biophysically specialized to detect coincident firing in the population of auditory nerve fibers that provide their synaptic input and to convey its occurrence with temporal precision. The precision in the timing of action potentials depends on the low input resistance (approximately 6 MOmega) of octopus cells at the resting potential that makes voltage changes rapid (tau approximately 200 micros). It is the activation of voltage-dependent conductances that endows octopus cells with low input resistances and prevents repetitive firing in response to depolarization. These conductances have been examined under whole cell voltage clamp. The present study reveals the properties of two conductances that mediate currents whose reversal at or near the equilibrium potential for K(+) over a wide range of extracellular K(+) concentrations identifies them as K(+) currents. One rapidly inactivating conductance, g(KL), had a threshold of activation at -70 mV, rose steeply as a function of depolarization with half-maximal activation at -45 +/- 6 mV (mean +/- SD), and was fully activated at 0 mV. The low-threshold K(+) current (I(KL)) was largely blocked by alpha-dendrotoxin (alpha-DTX) and partially blocked by DTX-K and tityustoxin, indicating that this current was mediated through potassium channels of the Kv1 (also known as shaker or KCNA) family. The maximum low-threshold K(+) conductance (g(KL)) was large, 514 +/- 135 nS. Blocking I(KL) with alpha-DTX revealed a second K(+) current with a higher threshold (I(KH)) that was largely blocked by 20 mM tetraethylammonium (TEA). The more slowly inactivating conductance, g(KH), had a threshold for activation at -40 mV, reached half-maximal activation at -16 +/- 5 mV, and was fully activated at +30 mV. The maximum high-threshold conductance, g(KH), was on average 116 +/- 27 nS. The present experiments show that it is not the biophysical and pharmacological properties but the magnitude of the K(+) conductances that make octopus cells unusual. At the resting potential, -62 mV, g(KL) contributes approximately 42 nS to the resting conductance and mediates a resting K(+) current of 1 nA. The resting outward K(+) current is balanced by an inward current through the hyperpolarization-activated conductance, g(h), that has been described previously.

Correlation of AMPA receptor subunit composition with synaptic input in the mammalian cochlear nuclei

The composition of AMPA receptors in patches excised from somata and dendrites of six cell types in the mammalian cochlear nuclei was probed and compared electrophysiologically and pharmacologically with the rapid application of glutamate. Cells excited predominantly by auditory nerve fibers had AMPA receptors with exceptionally rapid gating (submillisecond deactivation and desensitization time constants). The nonlinear current-voltage relationship in the presence of spermine showed that few of these receptors had GluR2 subunits, and the insensitivity of desensitization to cyclothiazide indicated that they contained mostly flop splice variants. At synapses made by parallel fibers, AMPA receptors were slowly gating (time constants of deactivation and desensitization >1 msec) and contained higher levels of GluR2 and flip isoforms. However, receptors at auditory nerve synapses on cells that also receive parallel fiber input, the fusiform cells, had intermediate properties with respect to kinetics and contained GluR2 and flip isoforms. Given the diverse biophysical properties, patterns of innervation, patterns of electrical activity, and targets of each cell type in vivo, these data indicate that the kinetics and permeation properties of AMPA receptors are linked to factors associated with synaptic connectivity.

Cholinergic modulation of stellate cells in the mammalian ventral cochlear nucleus

The main source of excitation to the ventral cochlear nucleus (VCN) is from glutamatergic auditory nerve afferents, but the VCN is also innervated by two groups of cholinergic efferents from the ventral nucleus of the trapezoid body. One arises from collaterals of medial olivocochlear efferents, and the other arises from neurons that project solely to the VCN. This study examines the action of cholinergic inputs on stellate cells in the VCN. T stellate cells, which form one of the ascending auditory pathways to the inferior colliculus, and D stellate cells, which inhibit T stellate cells, are distinguished electrophysiologically. Whole-cell recordings from stellate cells in slices of the VCN of mice demonstrate that most T stellate cells are excited by cholinergic agonists through three types of receptors, whereas all D stellate cells tested were insensitive to cholinergic agonists. Nicotinic excitation in T stellate cells has two components. The faster component was blocked by alpha-bungarotoxin and methyllycaconitine, suggesting that receptors contained alpha7 subunits; the slower component was insensitive to both. Muscarinic receptors excite T stellate cells by blocking a voltage-insensitive, "leak" potassium conductance. Our results suggest that cholinergic efferent innervation enhances excitation by sounds of T stellate cells, opposing the inhibitory action of cholinergic innervation in the cochlea that is conveyed indirectly through the glutamatergic afferents. The inhibitory action of D stellate cells on their targets is probably not affected by cholinergic inputs. Excitation of T stellate cells by cholinergic efferents would be expected to enhance the encoding of spectral peaks in noise.

Role of biophysical specialization in cholinergic modulation in neurons of the ventral cochlear nuclei

In contacting arrays of different types of neurons whose axons have differing targets in the brain stem, the auditory pathway is subdivided into parallel ascending pathways, each of which carries a different type of information. Several distinct arrays of neurons in the ventral cochlear nuclei have anatomical and biophysical specializations which enable them to extract differing facets of acoustic information and to convey it up the auditory pathway. T stellate cells have higher input resistances and have lower firing thresholds than bushy or octopus cells, enabling their firing to be modulated by small currents. Cholinergic currents, driven by neurons in the ventral nucleus of the trapezoid body that are likely to include medial olivocochlear efferents, excite T stellate cells, but have subtle effects on the firing of bushy cells, and have no detectable influence on octopus cells and D stellate cells. We suggest that cholinergic excitation of T stellate cells contributes toward shifting their acoustic dynamic ranges and increasing the encoding of spectral peaks in noisy conditions and in awake animals.

Detection of synchrony in the activity of auditory nerve fibers by octopus cells of the mammalian cochlear nucleus

The anatomical and biophysical specializations of octopus cells allow them to detect the coincident firing of groups of auditory nerve fibers and to convey the precise timing of that coincidence to their targets. Octopus cells occupy a sharply defined region of the most caudal and dorsal part of the mammalian ventral cochlear nucleus. The dendrites of octopus cells cross the bundle of auditory nerve fibers just proximal to where the fibers leave the ventral and enter the dorsal cochlear nucleus, each octopus cell spanning about one-third of the tonotopic array. Octopus cells are excited by auditory nerve fibers through the activation of rapid, calcium-permeable, alpha-amino-3-hydroxy-5-methyl-4-isoxazole-propionate receptors. Synaptic responses are shaped by the unusual biophysical characteristics of octopus cells. Octopus cells have very low input resistances (about 7 M Omega), and short time constants (about 200 microsec) as a consequence of the activation at rest of a hyperpolarization-activated mixed-cation conductance and a low-threshold, depolarization-activated potassium conductance. The low input resistance causes rapid synaptic currents to generate rapid and small synaptic potentials. Summation of small synaptic potentials from many fibers is required to bring an octopus cell to threshold. Not only does the low input resistance make individual excitatory postsynaptic potentials brief so that they must be generated within 1 msec to sum but also the voltage-sensitive conductances of octopus cells prevent firing if the activation of auditory nerve inputs is not sufficiently synchronous and depolarization is not sufficiently rapid. In vivo in cats, octopus cells can fire rapidly and respond with exceptionally well-timed action potentials to periodic, broadband sounds such as clicks. Thus both the anatomical specializations and the biophysical specializations make octopus cells detectors of the coincident firing of their auditory nerve fiber inputs.

Hyperpolarization-activated, mixed-cation current (I(h)) in octopus cells of the mammalian cochlear nucleus

Octopus cells in the posteroventral cochlear nucleus of mammals detect the coincidence of synchronous firing in populations of auditory nerve fibers and convey the timing of that coincidence with great temporal precision. Earlier recordings in current clamp have shown that two conductances contribute to the low input resistance and therefore to the ability of octopus cells to encode timing precisely, a low-threshold K(+) conductance and a hyperpolarization-activated mixed-cation conductance, g(h). The present experiments describe the properties of g(h) in octopus cells as they are revealed under voltage clamp with whole-cell, patch recordings. The hyperpolarization-activated current, I(h), was blocked by extracellular Cs(+) (5 mM) and 4-(N-ethyl-N-phenylamino)-1,2-dimethyl-6-(methylamino) pyridinium chloride (50-100 nM) but not by extracellular Ba(2+) (2 mM). The reversal potential for I(h) in octopus cells under normal physiological conditions was -38 mV. Increasing the extracellular potassium concentration from 3 to 12 mM shifted the reversal potential to -26 mV; lowering extracellular sodium concentration from 138 to 10 mM shifted the reversal potential to -77 mV. These pharmacological and ion substitution experiments show that I(h) in octopus cells is a mixed-cation current that resembles I(h) in other neurons and in heart muscle cells. Under control conditions when cells were perfused intracellularly with ATP and GTP, I(h) had an activation threshold between about -35 to -40 mV and became fully activated at -110 mV. The maximum conductance associated with hyperpolarizing voltage steps to -112 mV ranged from 87 to 212 nS [150 +/- 30 (SD) nS, n = 36]. The voltage dependence of g(h) obtained from peak tail currents is fit by a Boltzmann function with a half-activation potential of -65 +/- 3 mV and a slope factor of 7. 7 +/- 0.7. This relationship reveals that g(h) was activated 41% at the mean resting potential of octopus cells, -62 mV, and that at rest I(h) contributes a steady inward current of between 0.9 and 2.1 nA. The voltage dependence of g(h) was unaffected by the extracellular application of dibutyryl cAMP but was shifted in hyperpolarizing direction, independent of the presence or absence of dibutyryl cAMP, by the removal of intracellular ATP and GTP.

Time course and permeation of synaptic AMPA receptors in cochlear nuclear neurons correlate with input

AMPA receptors mediate rapid glutamatergic synaptic transmission. In the mammalian cochlear nuclei, neurons receive excitatory input from either auditory nerve fibers, parallel fibers, or both fiber systems. The functional correlates of differences in the source of input were examined by recording AMPA receptor-mediated, miniature EPSCs (mEPSCs) in whole-cell voltage-clamp mode from identified neurons. Bushy, octopus, and T-stellate cells of the ventral cochlear nucleus (VCN) and tuberculoventral cells of the dorsal cochlear nucleus (DCN) receive most of their excitatory input from the auditory nerve; fusiform cells receive excitatory inputs from both the auditory nerve and parallel fibers; cartwheel cells receive excitatory input from parallel fibers alone. mEPSCs from bushy, octopus, T-stellate, and tuberculoventral cells had significantly faster decay time constants (0.35-0.40 msec) than did those from fusiform and cartwheel cells (1.32-1.79 msec). Some fusiform cells had two populations of mEPSCs with distinct time courses. mEPSCs in cells with auditory nerve input alone were inhibited by philanthotoxin, a blocker of calcium-permeable AMPA receptors, whereas mEPSCs in cells with parallel fiber input were not. Thus AMPA receptors postsynaptic to the auditory nerve differ from those postsynaptic to parallel fibers both in channel-gating kinetics and in their permeability to calcium. These results confirm the conclusion that synaptic AMPA receptors are specialized according to the source of input (Hunter et al., 1993; Rubio and Wenthold, 1997; Wang et al., 1998).

Role of intrinsic conductances underlying responses to transients in octopus cells of the cochlear nucleus

Recognition of acoustic patterns in natural sounds depends on the transmission of temporal information. Octopus cells of the mammalian ventral cochlear nucleus form a pathway that encodes the timing of firing of groups of auditory nerve fibers with exceptional precision. Whole-cell patch recordings from octopus cells were used to examine how the brevity and precision of firing are shaped by intrinsic conductances. Octopus cells responded to steps of current with small, rapid voltage changes. Input resistances and membrane time constants averaged 2.4 MOmega and 210 microseconds, respectively (n = 15). As a result of the low input resistances of octopus cells, action potential initiation required currents of at least 2 nA for their generation and never occurred repetitively. Backpropagated action potentials recorded at the soma were small (10-30 mV), brief (0.24-0.54 msec), and tetrodotoxin-sensitive. The low input resistance arose in part from an inwardly rectifying mixed cationic conductance blocked by cesium and potassium conductances blocked by 4-aminopyridine (4-AP). Conductances blocked by 4-AP also contributed to the repolarization of the action potentials and suppressed the generation of calcium spikes. In the face of the high membrane conductance of octopus cells, sodium and calcium conductances amplified depolarizations produced by intracellular current injection over a time course similar to that of EPSPs. We suggest that this transient amplification works in concert with the shunting influence of potassium and mixed cationic conductances to enhance the encoding of the onset of synchronous auditory nerve fiber activity.

THE ROLE OF TIMING IN THE BRAIN STEM AUDITORY NUCLEI OF VERTEBRATES

Vertebrate animals gain biologically important information from environmental sounds. Localization of sound sources enables animals to detect and respond appropriately to danger, and it allows predators to detect and localize prey. In many species, rapidly fluctuating sounds are also the basis of communication between conspecifics. This information is not provided directly by the output of the ear but requires processing of the temporal pattern of firing in the tonotopic array of auditory nerve fibers. The auditory nerve feeds information through several parallel ascending pathways. Anatomical and electrophysiological specializations for conveying precise timing, including calyceal synaptic terminals and matching axonal conduction times, are evident in several of the major ascending auditory pathways through the ventral cochlear nucleus and its nonmammalian homologues. One pathway that is shared by all higher vertebrates makes an ongoing comparison of interaural phase for the localization of sound in the azimuth. Another pathway is specifically associated with higher frequency hearing in mammals and is thought to make use of interaural intensity differences for localizing high-frequency sounds. Balancing excitation from one ear with inhibition from the other in rapidly fluctuating signals requires that the timing of these synaptic inputs be matched and constant for widely varying sound stimuli in this pathway. The monaural nuclei of the lateral lemniscus, whose roles are not understood (although they are ubiquitous in higher vertebrates), receive input from multiple pathways that encode timing with precision, some through calyceal endings.

Golgi cells in the superficial granule cell domain overlying the ventral cochlear nucleus: morphology and electrophysiology in slices

Golgi cells are poised to integrate multimodal influences by participating in circuits involving granule cells in the cochlear nuclei. To understand their physiological role, intracellular recordings were made from anatomically identified Golgi cells in slices of the cochlear nuclei from mice. Cell bodies, dendrites, and terminals for all seven labeled cells were restricted to the narrow plane of the superficial granule cell domain over the ventral cochlear nucleus. The axonal arborization was the most striking feature of all Golgi cells; a dense plexus of terminals covered an area 200-400 microm in diameter in the vicinity of the cell body and dendrites. Axonal beads often surrounded granule cell bodies, indicating that granule cells are probable targets. Cells had input resistances up to 130 M omega and fired regular, overshooting action potentials. Golgi cells probably receive auditory nerve input, because shocks to the cut end of the auditory nerve excited Golgi cells with excitatory postsynaptic potentials (EPSPs). The latency of EPSPs shortened to a minimum and the amplitude of EPSPs grew in several steps as the strength of shocks was increased. The minimum latency of EPSPs in Golgi cells was on average 1.3 milliseconds, 0.6 milliseconds longer than the minimum latencies of EPSPs in nearby octopus and T stellate cells. The long latency raises the possibility that Golgi cells receive input from slowly conducting, unmyelinated auditory nerve fibers. Golgi cells are also excited by interneurons with N-methyl-D-aspartate receptors, probably granule cells, because repetitive shocks and single shocks in the absence of extracellular Mg2+ evoked late EPSPs that were reversibly blocked by DL-2-amino-5-phosphono-valeric acid.

Synaptic inputs to stellate cells in the ventral cochlear nucleus

Auditory information is carried from the cochlear nuclei to the inferior colliculi through six parallel ascending pathways, one of which is through stellate cells of the ventral cochlear nuclei (VCN) through the trapezoid body. To characterize and identify the synaptic influences on T stellate cells, intracellular recordings were made from anatomically identified stellate cells in parasagittal slices of murine cochlear nuclei. Shocks to the auditory nerve consistently evoked five types of synaptic responses in T stellate cells, which reflect sources intrinsic to the cochlear nuclear complex. 1) Monosynaptic excitatory postsynaptic potentials (EPSPs) that were blocked by 6,7-dinitroquinoxaline-2,3-dione (DNQX), an antagonist of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors, probably reflected activation by auditory nerve fibers. Electrophysiological estimates indicate that about five auditory nerve fibers converge on one T stellate cell. 2) Disynaptic, glycinergic inhibitory postsynaptic potentials (IPSPs) arise through inhibitory interneurons in the VCN or in the dorsal cochlear nucleus (DCN). 3) Slow depolarizations, the source of which has not been identified, that lasted between 0.2 and 1 s and were blocked by -2-amino-5-phosphonovaleric acid (APV), the N-methyl-D-aspartate (NMDA) receptor antagonist. 4) Rapid, late glutamatergic EPSPs are polysynaptic and may arise from other T stellate cells. 5) Trains of late glycinergic IPSPs after single or repetitive shocks match the responses of D stellate cells, showing that D stellate cells are one source of glycinergic inhibition to T stellate cells. The source of late, polysynaptic EPSPs and IPSPs was assessed electrophysiologically and pharmacologically. Late synaptic responses in T stellate cells were enhanced by repetitive stimulation, indicating that the interneurons from which they arose should fire trains of action potentials in responses to trains of shocks. Late EPSPs and late IPSPs were blocked by APV and enhanced by the removal of Mg2+, indicating that the interneurons were driven at least in part through NMDA receptors. Bicuculline, a gamma-aminobutyric acid-A (GABAA) receptor antagonist, enhanced the late PSPs, indicating that GABAergic inhibition suppresses both the glycinergic interneurons responsible for the trains of IPSPs in T-stellate cells and the interneuron responsible for late EPSPs in T stellate cells. The glycinergic interneurons that mediate the series of IPSPs are intrinsic to the ventral cochlear nucleus because long series of IPSPs were recorded from T stellate cells in slices in which the DCN was removed. These experiments indicate that T stellate cells are a potential source of late EPSPs and that D stellate cells are a potential source for trains of late IPSPs.

Physiological identification of the targets of cartwheel cells in the dorsal cochlear nucleus

The integrative contribution of cartwheel cells of the dorsal cochlear nucleus (DCN) was assessed with intracellular recordings from anatomically identified cells. Recordings were made, in slices of the cochlear nuclei of mice, from 58 cartwheel cells, 22 fusiform cells, 3 giant cells, 5 tuberculoventral cells, and 1 cell that is either a superficial stellate or Golgi cell. Cartwheel cells can be distinguished electrophysiologically from other cells of the cochlear nuclei by their complex spikes, which comprised two to four rapid action potentials superimposed on a slower depolarization. The rapid action potentials were blocked by tetrodotoxin (n = 17) and were therefore mediated by voltage-sensitive sodium currents. The slow spikes were eliminated by the removal of calcium from the extracellular saline (n = 3) and thus were mediated by voltage-sensitive calcium currents. The spontaneous and evoked firing patterns of cartwheel cells were distinctive. Cartwheel cells usually fired single and complex spikes spontaneously at irregular intervals of between 100 ms and several seconds. Shocks to the DCN elicited firing that lasted tens to hundreds of milliseconds. With the use of these distinctive firing patterns, together with a pharmacological dissection of postsynaptic potentials (PSPs), possible targets of cartwheel cells were identified and the function of the connections was examined. Not only cartwheel and fusiform cells, but also giant cells, received patterns of synaptic input consistent with their having originated from cartwheel cells. These cell types responded to shocks of the DCN with variable trains of PSPs that lasted hundreds of milliseconds. PSPs within these trains appeared both singly and in bursts of two to four, and were blocked by 0.5 or 1 microM strychnine (n = 4 cartwheel, 4 fusiform, and 2 giant cells), indicating that cartwheel cells are likely to be glycinergic. In contrast with cartwheel cells, which are weakly excited by glycinergic input, glycinergic PSPs consistently inhibited fusiform and giant cells. Tuberculoventral cells and the putative superficial stellate cell received little or no spontaneous synaptic activity. Shocks to the DCN evoked synaptic activity that lasted approximately 5 ms. These cells therefore probably do not receive input from cartwheel cells. In addition, the brief firing of tuberculoventral cells and of the putative superficial stellate cell in response to shocks indicates that these cells are unlikely to contribute to the late, glycinergic synaptic potentials observed in cartwheel, fusiform, and giant cells.

Context-dependent synaptic action of glycinergic and GABAergic inputs in the dorsal cochlear nucleus

Cartwheel cells are prominent interneurons in the dorsal cochlear nucleus (DCN) that bear considerable homology to cerebellar Purkinje cells. They contact other cartwheel cells as well as fusiform cells, the principal cells of the DCN. In fusiform cells, the inhibition from cartwheel cells interacts with excitation mediated by granule cells and auditory nerve fibers, and shapes the output of the DCN in its ascent to the inferior colliculi. With intracellular recordings from anatomically identified cells in slices, synaptic inputs to fusiform and cartwheel cells were analyzed pharmacologically. Shocks to the auditory nerve and granule cell domains evoked glutamatergic, glycinergic, and GABA(A)ergic postsynaptic potentials (PSPs) in both cartwheel and fusiform cells. The temporal patterns of spontaneous and evoked glycinergic PSPs in fusiform and cartwheel cells were similar and mirrored the pattern of firing of cartwheel cells, probably reflecting the anatomical connections between these cell types and supporting the conclusion that cartwheel cells are glycinergic. In fusiform cells, glycinergic and GABA(A)ergic IPSPs evoked with shocks reversed at -68 mV on average. In marked contrast, glycinergic and GABA(A)ergic PSPs in cartwheel cells, as well as responses to exogenous application of 50-100 mM glycine or 100 microns muscimol, were depolarizing. Reversal potentials of PSPs and responses to glycine and muscimol were similar and averaged -52 mV. Glycinergic and GABA(A)ergic PSPs could elicit firing from cartwheel cells at their resting potentials, but could also reduce rapid firing during strong depolarizations. Thus, the action of glycinergic and GABA(A)ergic inputs on cartwheel cells depends on the electrophysiological context in which they occur.

Recordings from slices indicate that octopus cells of the cochlear nucleus detect coincident firing of auditory nerve fibers with temporal precision

Acoustic information in auditory nerve discharges is integrated in the cochlear nuclei, and ascends through several parallel pathways to higher centers. Octopus cells of the posteroventral cochlear nucleus form a pathway known to carry information in the timing of action potentials. Octopus cells have dendrites oriented to receive converging input from many auditory nerve fibers. In all 34 intracellular recordings from anatomically identified octopus cells in slices, shocks to the auditory nerve evoked brief, consistent, graded EPSPs. EPSPs were about 1 msec in duration. At all but the lowest shock strengths, the delays between shocks and the peaks of resultant EPSPs had SDs of 0.02 msec. Polysynaptic excitation, perhaps arising from the axon collaterals of octopus cells, was observed. No detectable glycinergic or GABAergic inhibition was evoked with shocks. The input resistances were low, around 10 M omega, voltage changes were rapid, with time constants of about 1 msec, and action potentials were small. The low input resistance resulted in part from a Cs(+)-sensitive conductance. In the presence of 10 or 15 mM extracellular Cs+ the time constants increased 20-fold in the hyperpolarizing voltage range. As several subthreshold inputs were required to produce suprathreshold responses, octopus cells detect the coincident firing of auditory nerve fibers. Under physiological conditions the low input resistance and resulting short time constant limit the time over which temporal summation of excitation from auditory nerve fibers can occur and thus provide temporal precision to electrical signaling.

Neuronal circuits associated with the output of the dorsal cochlear nucleus through fusiform cells

1. Intracellular recordings were made from 21 anatomically identified fusiform cells in the dorsal cochlear nucleus (DCN) of mice in slices. The aim of the experiments was to dissect the synaptic responses to shocks of the auditory nerve to correlate functional characteristics with the different classes of synaptic inputs. 2. When depolarized from rest (-57 +/- 5 mV) with current pulses, fusiform cells fired regular, overshooting action potentials that were followed by two undershoots. The frequency of firing increased with the strength of injected current by between 100 and 300 spikes/s/nA. The current-voltage relationship rectified between 10 and 15 mV below the resting potential. The slopes of current-voltage relationships of fusiform cells in the range between the resting potential and 10 mV hyperpolarization indicated an average input resistance of 86 +/- 37 M omega. 3. In each of the labeled fusiform cells frequent, spontaneous inhibitory postsynaptic potentials (IPSPs) were recorded singly or in bursts. Some, but not all, IPSPs were preceded by a slowly rising excitatory postsynaptic potential (EPSP). The temporal association of spontaneous EPSPs and IPSPs suggests that they are driven by a common source, possibly granule cells. 4. Shocks to the auditory nerve evoked synaptic responses consisting of early (1 to approximately 10 ms) and late (approximately 10 to 100 ms) components. 6,7-Dinitroquinoxaline-2,3-dione (DNQX) at 20 to 40 microM eliminated all detectable excitation and all late IPSPs. Late bursts of IPSPs, therefore, are mediated through a polysynaptic pathway that includes a DNQX-sensitive stage. Strong shocks to the nerve root elicited single monosynaptic IPSPs, indicating that inhibitory interneurons have processes close to the auditory nerve. Strychnine at 0.5 microM eliminated all detectable inhibition. 6. Cuts through the posteroventral cochlear nucleus (PVCN), which severed the descending branches of auditory nerve fibers, eliminated early EPSPs and IPSPs leaving late, slowly rising EPSPs and bursts of IPSPs in responses to shocks of the auditory nerve. Late, slowly rising EPSPs and bursts of IPSPs, as well as monosynaptic IPSPs, could also be evoked by stimulating the anteroventral cochlear nucleus (AVCN). 7. Focal applications of glutamate evoked excitation and inhibition from many parts of a slice, with patterns varying among cells, indicating that fusiform cells receive inputs through several groups of interneurons.(ABSTRACT TRUNCATED AT 400 WORDS)

In vitro modulation of somatic glycine-like immunoreactivity in presumed glycinergic neurons

Previous studies indicate that tuberculoventral and cartwheel cells in the dorsal cochlear nucleus as well as a group of stellate cells in the ventral cochlear nucleus are likely to be glycinergic. To test whether these neurons contain higher levels of free glycine than cells that are probably not glycinergic, immunocytochemical studies with antibodies against glycine conjugates were undertaken on slices of the murine cochlear nuclear complex. Present results show that the cell bodies of all three groups of neurons are immunolabeled. However, the somatic labeling of the tuberculoventral and cartwheel cells can be modulated by experimental conditions. In slices fixed immediately after cutting, many cell bodies in the deep layer of the dorsal cochlear nucleus (DCN), presumably tuberculoventral neurons, are labeled. As a slice is incubated in vitro, cell bodies in the deep layer of the DCN lose their glycine-like immunoreactivity. After 7 hours in vitro, labeled cells are absent in the deep DCN, but the immunoreactivity can be regained by electrically stimulating the auditory nerve for 20 minutes. The loss of immunoreactivity is prevented by electrical stimulation, by axotomy, and by inclusion of 0.8 microM tetrodotoxin, or 1 microM strychnine, or 50 microM colchicine or 50 microM beta-lumicolchicine in the bathing saline. Cartwheel cells retain their immunoreactivity during incubation in vitro without electrical stimulation, but lose it under two conditions. One is following a cut across the ventral cochlear nucleus (VCN) that severs most of their granule cell input, and the other is the inclusion of tetrodotoxin in the bathing saline. The labeling of cell bodies in the ventral cochlear nucleus and of puncta and processes is not changed by any of these experimental manipulations.

Cartwheel and superficial stellate cells of the dorsal cochlear nucleus of mice: intracellular recordings in slices

1. Intracellular recordings were made from identified cartwheel and stellate cells in the molecular and fusiform cell layers of the murine dorsal cochlear nucleus (DCN). The aim of the study was to identify and characterize their synaptic inputs and to learn how synaptic inputs and intrinsic electrical properties interact to generate firing patterns. 2. Eight cells labeled by the intracellular injection of biocytin were cartwheel cells. Their axon terminals extended from the deep part of the molecular layer through the fusiform cell layer. Their dendrites extended through the molecular layer and had spines. Both the dendritic and axonal arbors were small, having diameters of approximately 150 microns in the parasagittal plane. 3. When depolarized, cartwheel cells often fired bursts of rapid action potentials superimposed on a slow depolarization. The peaks of action potentials were usually overshooting. Individually occurring action potentials were followed by two afterhyperpolarizations, as in other cells of the DCN. During bursts, action potentials did not have two distinct repolarizing phases. 4. Excitatory postsynaptic potentials (EPSPs) were recorded from cartwheel cells spontaneously and after shocks to the nerve root or to the ventral cochlear nucleus (VCN). The EPSPs rose slowly. When they were suprathreshold they evoked action potentials singly or in bursts. EPSPs evoked by shocks to the nerve root or to the VCN had long latencies, the rise of EPSPs beginning between 5 and 10 ms after the shock. No inhibitory synaptic potentials, either spontaneous or driven with electrical stimulation, were detected in cells whose resting potentials were between -50 and -70 mV. 5. The locations from which excitatory input can be driven electrically are consistent with cartwheel cells receiving excitatory synaptic input from granule cells. 6. One labeled cell was a superficial stellate cell. It had smooth, straight dendrites that radiated parallel to the layers of the DCN; its axonal arbor was also planar and was restricted to the molecular layer. Both the dendritic and axonal arbors of this stellate cell were large, > 500 microns diam in the parasagittal plane. 7. The superficial stellate cell fired trains of action potentials at regular intervals that, like other cells of the DCN, were overshooting and were followed by double undershoots. 8. Shocks to the nerve root and to the surface of the VCN evoked EPSPs after 3.5 and 2 ms, respectively, in the superficial stellate cell. Chemical stimulation of the VCN also evoked excitation. No inhibitory synaptic input, spontaneous or driven, was detected.

Tuberculoventral cells of the dorsal cochlear nucleus of mice: intracellular recordings in slices

1. Intracellular recordings were made from six tuberculoventral cells (also called vertical or corn cells) whose identity was confirmed by labeling with biocytin, with the aim of understanding their projection patterns and how their synaptic inputs and their intrinsic electrical properties shape their responses to activation of auditory nerve fibers. 2. The cell bodies, dendrites, and local axonal terminals of all six tuberculoventral cells lay in narrow bands, 70-100 microns wide and parallel to the path of auditory nerve fibers, in the dorsal cochlear nucleus (DCN). Terminals in the ventral cochlear nucleus (VCN) also lay in narrow bands, parallel to the path of auditory nerve fibers in either or both the anteroventral and posteroventral cochlear nuclei. 3. When depolarized with current, tuberculoventral cells fired regularly. The peaks of action potentials were usually overshooting and were followed by two afterhyperpolarizations, resembling other cells of the DCN. Unlike some other cells in the DCN, however, neither of the afterhyperpolarizations resulted in an undershoot of the resting potential in the presence of depolarizing currents stronger than 0.3 nA. The second afterhyperpolarization was more variable than the first. 4. Shocks to the auditory nerve evoked monosynaptic as well as polysynaptic excitation and weak polysynaptic inhibition. These results show that, in addition to receiving excitation directly through auditory nerve fibers, tuberculoventral cells also receive excitation through interneurons. 5. To locate the interneurons in the cochlear nuclei whose activation affects a tuberculoventral cell monosynaptically or polysynaptically, glutamate was applied focally to various loci in the VCN while responses were recorded in a target tuberculoventral cell that was subsequently labeled. Excitation of this tuberculoventral cell arose from a rostrocaudal band in the VCN. 6. The results are consistent with tuberculoventral cells receiving excitatory synaptic input from auditory nerve fibers and from T stellate cells in the VCN. They could be inhibited by D stellate cells in the VCN.

Giant cells of the dorsal cochlear nucleus of mice: intracellular recordings in slices

1. In slices of the murine cochlear nuclear complex, intracellular recordings were made from five giant cells that were identified by intracellular labeling with biocytin. Giant cells form one of the two output pathways of the dorsal cochlear nucleus (DCN). Understanding how neuronal circuits and intrinsic electrical properties interact to control the firing of giant cells is a step toward understanding what acoustic information is conveyed through these cells. 2. Cell bodies of the labeled giant cells lay in the deep layer of the DCN. Dendrites, widespread both along the isofrequency axis and along the tonotopic axis, occupied mainly the deep layer, but some distal ends strayed into the molecular layer. Axons of giant cells were large, varying between 1 and 2 microns diam, and left through the dorsal acoustic stria. They were not observed to branch in the cochlear nuclei. 3. Giant cells fired large, overshooting action potentials that were followed by two afterhyperpolarizations. The first brought the membrane potential below rest, independent of the strength of injected current. The more variable second one produced either an undershoot or an inflection in the membrane potential between action potentials. 4. In each of the five labeled giant cells, shocks to the nerve root or to the anteroventral cochlear nucleus (AVCN) evoked a monosynaptic excitatory postsynaptic potential and two tandem inhibitory postsynaptic potentials (IPSPs) in the first 10 ms. Later IPSPs followed after latencies of between 10 and 50 ms. Monosynaptic excitation was usually cut short by the inhibition. 5. Strychnine, at 1 microM, blocked all IPSPs in the one giant cell tested, indicating that inhibitory input to this giant cell from circuits intrinsic to the cochlear nuclear complex was glycinergic. 6. The location of afferents was mapped for two giant cells. Both excitatory and inhibitory inputs to giant cells could be driven by the local application of glutamate to many loci in the AVCN and posteroventral cochlear nucleus, indicating that the ventral cochlear nucleus VCN contains interneurons that are monosynaptically or polysynaptically connected to giant cells. 7. An interpretation consistent with the results is that giant cells are excited by auditory nerve fibers and are inhibited by tuberculoventral cells. Giant cells may also be excited by granule or T stellate cells.

Tuberculoventral neurons project to the multipolar cell area but not to the octopus cell area of the posteroventral cochlear nucleus

Tuberculoventral neurons in the deep layer of the dorsal cochlear nucleus (DCN) provide frequency-specific inhibition to neurons in the anteroventral cochlear nucleus (AVCN) of the mouse (Wickesberg and Oertel, '88, '90). The present experiments examine the projection from the deep DCN to the posteroventral cochlear nucleus (PVCN). Horseradish peroxidase (HRP) injections into the PVCN reveal that the multipolar cell area, but not the octopus cell area, is innervated by neurons in the deep layer of the DCN. Injections into the multipolar cell area, in the rostral and ventral PVCN, labeled neurons across the entire rostrocaudal extent of the deep DCN. The labeled tuberculoventral neurons generally lay within the band of labeled auditory nerve terminals in the DCN. Injections of HRP into the octopus cell area, in the dorsal caudal PVCN, labeled almost no cells within the band of auditory nerve fiber terminals that were labeled by the same injection. The inhibition from tuberculoventral neurons onto ventral cochlear nucleus (VCN) neurons is likely to be mediated by glycine (Wickesberg and Oertel, '90). Slices of the cochlear nuclear complex were immunolabeled by an antibody against glycine conjugated with glutaraldehyde to bovine serum albumin (Wenthold et al., '87). Glycine-like immunoreactivity was found throughout the DCN, the AVCN and the multipolar cell area, but there was little labeling in the octopus cell area. This finding provides independent evidence that tuberculoventral neurons do not innervate the octopus cell area and indicates that the octopus cell area is anatomically and functionally distinct.

The role of intrinsic neuronal properties in the encoding of auditory information in the cochlear nuclei

It is now possible to relate the intrinsic electrical properties of particular cells in the cochlear nuclei of mammals with their biological function. In the layered dorsal cochlear nucleus, information concerning the location of a sound source seems to be contained in the spatial pattern of activation of a population of neurons. In the unlayered, ventral cochlear nucleus, however, neurons carry information in their temporal firing patterns. The voltage-sensitive conductances that make responses to synaptic current brief enable bushy cells to convey signals from the auditory nerve to the superior olivary complex with a temporal precision of at least 120 microseconds.

Delayed, frequency-specific inhibition in the cochlear nuclei of mice: a mechanism for monaural echo suppression

To understand how auditory information is processed in the cochlear nuclei, it is crucial to know what circuitry exists and how it functions. Previous anatomical experiments have shown that neurons in the deep layer of the dorsal cochlear nucleus (DCN) project topographically to the anteroventral cochlear nucleus (AVCN) (Wickesberg and Oertel, 1988). Because interneurons in the DCN and their targets in AVCN are excited by the same group of auditory nerve fibers, the projection is frequency-specific. Here we report that microinjections of glutamate in the DCN evoke trains of IPSPs in individual, impaled AVCN neurons in brain slices of the cochlear nuclear complex. Only injections along a rostrocaudal band in the DCN, matching the anatomical projection of tuberculoventral neurons, evoke IPSPs; elsewhere, there were no responses to the glutamate. The inhibition is blocked by 0.5 microM strychnine. Both bushy and stellate cells are targets of the inhibitory projection. Inhibition in the AVCN is delayed by an additional synaptic delay with respect to the excitation. Delayed, frequency-specific inhibition allows the first wavefront to be transmitted to higher auditory centers by bushy and stellate cells, while following inputs encoding signals of similar frequencies are attenuated at least for the duration of an IPSP. These findings are consistent with results from psychoacoustic experiments and suggest that this circuit provides a source of monaural echo suppression.

Morphology and physiology of cells in slice preparations of the posteroventral cochlear nucleus of mice

In an effort to understand what integrative tasks are performed in the cochlear nuclei, the present study was undertaken to describe neuronal circuits in the posteroventral cochlear nucleus (PVCN) anatomically and physiologically. The cochlear nuclear complex receives auditory information from the cochlea through the auditory nerve. Within the cochlear nuclei, signals travel along several parallel and interconnected pathways. From the cochlear nuclei, transformed versions of the signals are passed to higher auditory centers in the brainstem. We have recorded electrophysiological responses from cells that were subsequently visualized with horseradish peroxidase (HRP). Responses to shocks to the auditory nerve root and to intracellularly injected current pulses were recorded and correlated with morphology. Two types of stellate cells and octopus cells were distinguished. T stellate cells project out of the cochlear nuclei through the Trapezoid body; D stellate cells do not. The axons of D stellate cells extend Dorsalward to the dorsal cochlear nucleus (DCN) but have not been traced out of the nucleus. Both T and D stellate cells have terminal collaterals in the multipolar cell region of the PVCN and in the DCN. The endings of one T stellate cell formed a narrow band rostrocaudally in the fusiform cell layer of the DCN that resembled an isofrequency band. The endings of one D stellate cell lay closely apposed to multipolar cells in the deep layer of the DCN. The dendrites of T stellate cells are often aligned along the path of auditory nerve fibers and end in tufts, whereas those of D stellate cells extend radially in the plane of the lateral surface of the PVCN toward granule cell areas and branch sparingly. Octopus cells have dendrites oriented perpendicularly to the path of auditory nerve fibers. Their axons were cut medially in the slices; none had collateral branches. Both T and D stellate cells were monosynaptically excited to threshold by shocks to the nerve root, indicating that they could participate in local circuits that we measure physiologically. T stellate cells have action potentials that peak at about 0 mV and are followed by single undershoots. The D stellate cell that was best impaled fired overshooting action potentials that were followed by double undershoots. Octopus cells were monosynaptically excited to threshold by shocks to the auditory nerve.