Guinea Pig Dorsal Cochlear Nucleus Research Articles

Inhibitory interneurons in a brainstem circuit adjust their inhibitory motifs to process multimodal input.

Inhibitory-interneuron networks, consisting of multiple forms of circuit motifs including reciprocal (inhibitory interneurons inhibiting other interneurons) and feedforward (inhibitory interneurons inhibiting principal neurons) connections, are crucial in processing sensory information. The present study applies a statistical method to in vivo multichannel spike trains of dorsal cochlear nucleus neurons to disentangle reciprocal and feedforward-inhibitory motifs. After inducing input-specific plasticity, reciprocal and feedforward inhibition are found to be differentially regulated, and the combined effect synergistically modulates circuit output. The findings highlight the interplay among different circuit motifs as a key element in neural computation. Inhibitory interneurons play an essential role in neural computations by utilizing a combination of reciprocal (interneurons inhibiting each other) and feedforward (interneuron inhibiting the principal neuron) inhibition to process information. To disentangle the interplay between the two inhibitory-circuit motifs and understand their effects on the circuit output, in vivo recordings were made from the guinea pig dorsal cochlear nucleus, a cerebellar-like brainstem circuit. Spikes from inhibitory interneurons (cartwheel cell) and principal output neurons (fusiform cell) were compared before and after manipulating their common multimodal input. Using a statistical model based on the Cox method of modulated renewal process of spike train influence, reciprocal- and feedforward-inhibition motifs were quantified. In response to altered multimodal input, reciprocal inhibition was strengthened while feedforward inhibition was weakened, and the two motifs combined to modulate fusiform cell output and acoustic-driven responses. These findings reveal the cartwheel cell's role in auditory and multimodal processing, as well as illustrated the balance between different inhibitory-circuit motifs as a key element in neural computation.

Bimodal stimulus timing-dependent plasticity in primary auditory cortex is altered after noise exposure with and without tinnitus.

Central auditory circuits are influenced by the somatosensory system, a relationship that may underlie tinnitus generation. In the guinea pig dorsal cochlear nucleus (DCN), pairing spinal trigeminal nucleus (Sp5) stimulation with tones at specific intervals and orders facilitated or suppressed subsequent tone-evoked neural responses, reflecting spike timing-dependent plasticity (STDP). Furthermore, after noise-induced tinnitus, bimodal responses in DCN were shifted from Hebbian to anti-Hebbian timing rules with less discrete temporal windows, suggesting a role for bimodal plasticity in tinnitus. Here, we aimed to determine if multisensory STDP principles like those in DCN also exist in primary auditory cortex (A1), and whether they change following noise-induced tinnitus. Tone-evoked and spontaneous neural responses were recorded before and 15 min after bimodal stimulation in which the intervals and orders of auditory-somatosensory stimuli were randomized. Tone-evoked and spontaneous firing rates were influenced by the interval and order of the bimodal stimuli, and in sham-controls Hebbian-like timing rules predominated as was seen in DCN. In noise-exposed animals with and without tinnitus, timing rules shifted away from those found in sham-controls to more anti-Hebbian rules. Only those animals with evidence of tinnitus showed increased spontaneous firing rates, a purported neurophysiological correlate of tinnitus in A1. Together, these findings suggest that bimodal plasticity is also evident in A1 following noise damage and may have implications for tinnitus generation and therapeutic intervention across the central auditory circuit.

Stimulus-Timing Dependent Multisensory Plasticity in the Guinea Pig Dorsal Cochlear Nucleus

Multisensory neurons in the dorsal cochlear nucleus (DCN) show long-lasting enhancement or suppression of sound-evoked responses when stimulated with combined somatosensory-auditory stimulation. By varying the intervals between sound and somatosensory stimuli we show for the first time in vivo that DCN bimodal responses are influenced by stimulus-timing dependent plasticity. The timing rules and time courses of the observed stimulus-timing dependent plasticity closely mimic those of spike-timing dependent plasticity that have been demonstrated in vitro at parallel-fiber synapses onto DCN principal cells. Furthermore, the degree of inhibition in a neuron influences whether that neuron has Hebbian or anti-Hebbian timing rules. As demonstrated in other cerebellar-like circuits, anti-Hebbian timing rules reflect adaptive filtering, which in the DCN would result in suppression of sound-evoked responses that are predicted by activation of somatosensory inputs, leading to the suppression of body-generated signals such as self-vocalization.

Immunocytochemistry of glycine in small neurons of the granule cell areas of the guinea pig dorsal cochlear nucleus: a post-embedding ultrastructural study.

The axon terminals of the acoustic nerve contact different part of the cochlear nucleus including granule cell areas. Little is known of the cell composition and neural circuits of granule cell areas present in the fusiform and upper polymorphic layers of the dorsal cochlear nucleus in the guinea pig. The present ultrastructural immunocytochemical study exploits the technique of post-embedding immunogold and silver intensification to reveal the characteristics of small neurons in granule cell areas. Few neurons (Golgi-stellate cells) use glycine as inhibitory neurotransmitter which is present in symmetric synaptic boutons with pleomorphic and flat vesicles. In contrast, most neurons (granule and unipolar brush cells) are not glycine-positive, and presumably not excitatory. Most of the large axons (mossy fibres) in granule areas are probably excitatory (glycine-negative and storing round synaptic vesicles) and contact unipolar brush cells forming large synapses or granule cell dendrites by small synapses. A few large glycinergic boutons (inhibitory) also contact unipolar brush cells. The excitatory circuit of mossy fibre-unipolar brush and granule cells may be inhibited by the glycinergic terminals from the few glycinergic cells (Golgi-stellate neurons) present within the granule cell areas. The latter are not contacted by large mossy-like glycine terminals.

Voltage-gated Ca2+ conductances in acutely isolated guinea pig dorsal cochlear nucleus neurons.

Although it is known that voltage-gated Ca2+ conductances (VGCCs) contribute to the responses of dorsal cochlear nucleus (DCN) neurons, little is known about the properties of VGCCs in the DCN. In this study, the whole cell voltage-clamp technique was used to examine the pharmacology and voltage dependence of VGCCs in unidentified DCN neurons acutely isolated from guinea pig brain stem. The majority of cells responded to depolarization with sustained inward currents that were enhanced when Ca2+ was replaced by Ba2+, were blocked partially by Ni2+ (100 microM), and were blocked almost completely by Cd2+ (50 microM). Experiments using nifedipine (10 microM), omegaAga IVA (100 nM) and omegaCTX GVIA (500 nM) demonstrated that a variety of VGCC subtypes contributed to the Ba2+ current in most cells, including the L, N, and P/Q types and antagonist-insensitive R type. Although a large depolarization from rest was required to activate VGCCs in DCN neurons, VGCC activation was rapid at depolarized levels, having time constants <1 ms at 22 degrees C. No fast low-threshold inactivation was observed, and a slow high-threshold inactivation was observed at voltages more positive than -20 mV, indicating that Ba2+ currents were carried by high-voltage activated VGCCs. The VGCC subtypes contributing to the overall Ba2+ current had similar voltage-dependent properties, with the exception of the antagonist-insensitive R-type component, which had a slower activation and a more pronounced inactivation than the other components. These data suggest that a variety of VGCCs is present in DCN neurons, and these conductances generate a rapid Ca2+ influx in response to depolarizing stimuli.

Physiology and morphology of complex spiking neurons in the guinea pig dorsal cochlear nucleus.

Intracellular recordings from the dorsal cochlear nucleus have identified cells with both simple and complex action potential waveforms. We investigated the hypothesis that cartwheel cells are a specific cell type that generates complex action potentials, based on their analogous anatomical, developmental, and biochemical similarities to cerebellar Purkinje cells, which are known to discharge complex action potentials. Intracellular recordings were made from a brain slice preparation of the guinea pig dorsal cochlear nucleus. A subpopulation of cells discharged a series of two or three action potentials riding on a slow depolarization as an all-or-none event; this discharge pattern is called a complex spike or burst. These cells also exhibited anodal break bursts, anomalous rectification, subthreshold inward rectification, and frequent inhibitory postsynaptic potentials (IPSPs). Seven complex-spiking cells were stained with intracellular dyes and subsequently identified as cartwheel neurons. In contrast, six identified simple-spiking cells recorded in concurrent experiments were pyramidal cells. The cartwheel cell bodies reside in the lower part of layer 1 and the upper part of layer 2 of the nucleus. The cells are characterized by spiny dendrites penetrating the molecular layer, a lack of basal dendritic processes, and an axonal plexus invading layers 2 and 3, and the inner regions of layer 1. The cartwheel cell axons made putative synaptic contacts at the light microscopic level with pyramidal cells and small cells, including stellate cells, granule cells, and other cartwheel cells in layers 1 and 2. The axonal plexus of individual cartwheel cells suggests that they can inhibit cells receiving input from either the same or adjacent parallel fibers and that this inhibition is distributed along the isofrequency contours of the nucleus.

Distribution and targets of the cartwheel cell axon in the dorsal cochlear nucleus of the guinea pig

This investigation attempted to determine the mode of distribution and synaptic targets of the cartwheel cell axon in the guinea pig dorsal cochlear nucleus (DCoN). Antiserum against PEP-19, a putative calcium-binding neuropeptide, was employed at the light and electron microscopic levels. We show that in the hind-brain of the guinea pig, cerebellar Purkinje cells and DCoN cartwheel cells are the most densely immunoreactive neurons. The PEP-19 immunoreaction product is localized to all neuronal compartments of these cells. Primary targets of cartwheel cell axons are the DCoN pyramidal cells, the large efferent neurons of layer 2. These neurons receive numerous immunoreactive synaptic boutons on their cell bodies and apical and basal dendritic arbors. A PEP-19-immunoreactive axonal plexus, largely formed by cartwheel cell axons, highlights layer 3, co-extensively with the basal arbors of pyramidal cells. This plexus is oriented predominantly in the transstrial plane of the DCoN, in parallel with the sheet-like basal dendritic arbor of pyramidal neurons and with the isofrequency bands of primary cochlear nerve fibers. PEP-19-positive boutons contain pleomorphic synaptic vesicles and form symmetric synaptic junctions, indicative of inhibitory innervation. In addition, immunoreactive boutons, similar to those synapsing on pyramidal neurons, were observed on the cell bodies and main dendritic trunks of cartwheel neurons, indicating a system of recurrent collaterals. Furthermore, a small number of PEP-19-positive axons of unknown origin reach the caudal rim of the posteroventral cochlear nucleus. Within the territory of distribution of the cartwheel cell axon are the dendrites of at least two other types of DCoN neuron, the vertical cells of Lorente de Nó and the giant cells. These neurons may represent additional targets of the cartwheel cell axon, but this remains to be ascertained with specific methods. Our data demonstrate that the cartwheel neurons modulate the activity of pyramidal neurons and, therefore, play a key role in shaping the output of the DCoN superficial layers.

A spectrotemporal analysis of DCN single unit responses to wideband noise in guinea pig

Spectrotemporal receptive fields (STRFs) were estimated for chopper and pauser units recorded in guinea pig dorsal cochlear nucleus (DCN). Sixteen wideband, periodic noise stimuli, represented as time-frequency surfaces of energy density, were crosscorre-lated in time with the unit's corresponding period histograms to determine if specific energy patterns tended to precede spike occurrence. The STRFs obtained were unique to the DCN, as compared to the ventral cochlear nucleus (VCN) [Clopton and Backoff. 1991, Hear. Res. 52, 329–344] in their degree of temporal and spectral complexity. Certain unit response types, classified from their peristimulus-time histograms (PSTHs) to tonebursts, were associated with distinctive patterns in the STRFs. All STRFs had at least one region of elevated energy density (peak region) closely preceding spike occurrence, which may reflect a short-pathway, primary excitatory input (or inputs) to the neuron. In addition, some units displayed low-energy regions (troughs) with greater temporal precedences on their STRFs, particularly when higher stimulus intensities were used. This analysis approach appears to have potential for investigating functional neural connectivity and predicting responses to novel complex stimuli, although specific implementations of the technique impose limitations on the interpretation of results.

Membrane properties and discharge characteristics of guinea pig dorsal cochlear nucleus neurons studied in vitro

Intracellular recordings were made from neurons of the guinea pig dorsal cochlear nucleus in an in vitro brain slice preparation. The membrane properties of the cells were studied, and the membrane potentials were manipulated by current injection to determine how intrinsic conductances might alter the cell discharge patterns. Eleven cells were marked with Lucifer yellow. Ten of these cells were identified as the large pyramidal cells of layer 2 of this nucleus, and 1 cell was identified as a "vertical" cell in layer 3. Two kinds of action potentials were observed: simple spikes and complex spikes. This report discusses only cells with simple spikes. Simple spiking cells (60/72 recorded cells; all stained cells were simple spiking cells) discharged in a regular fashion with depolarization, and had linear frequency-current relationships up to 2 nA with a mean slope of 116 Hz/nA. The discharge rate was approximately constant throughout the current pulse. Responses of simple spiking cells to depolarizing current steps superimposed on a steady-state membrane hyperpolarization were studied. When the membrane has been held hyperpolarized, small current pulses produce a long-latency regular train of action potentials. Larger current pulses superimposed on membrane hyperpolarization can produce a short-latency action potential followed by a long silent interval (i.e., a long first interspike interval), and finally a regular train of spikes. It is concluded that the membrane conductances of DCN pyramidal cells are capable of generating at least 3 discharge patterns (regular firing, long first spike latency, and long first interspike interval) depending on the state of the membrane potential prior to a depolarizing current step. These responses are similar to the "chopper," "buildup," and "pauser" discharge patterns reported for these cells in vivo in response to tone bursts. The modulation of the intrinsic membrane conductances by membrane polarization and the possible contribution of these conductances to the generation of DCN discharge patterns provide new insights into the mechanisms underlying the responses of DCN cells to acoustic stimuli.

Responses to parallel fiber stimulation in the guinea pig dorsal cochlear nucleus in vitro.

1. Parallel fibers of the guinea pig dorsal cochlear nucleus (DCN) were electrically stimulated at the pial surface of the nucleus in a brain-slice preparation. Extracellular field potentials produced by the parallel fibers and postsynaptic cells, and the response of single units were identified and characterized. Responses were compared with those reported for stimulation of parallel fibers in the cerebellum and to those seen with electrical stimulation of the auditory nerve. 2. Stimulation of the DCN parallel fibers generates a consistent set of extracellular field potentials. In layer 1 of the DCN, a short-latency triphasic wave (P1(1)-N1(1)-P2(1)) is followed by a slower negative wave (N2(1)). The onset phase of the N2(1) often exhibits a small positive notch (P2a1). In layer 2, an initial triphasic wave (P1(2)-N1(2)-P2(2)) is followed by a short-latency negative wave (N2(2)) and a slower positive wave (P3(2)). The N1(2) is approximately coincident with the N1(1), whereas the P3(2) is coincident with N2(1). The falling phase of the P3(2) is sometimes interrupted by a brief negative deflection (N3(2)). These field potentials are similar, but not identical to those reported for parallel fiber stimulation in the cerebellum in vivo (15). These responses differ substantially from those produced in the DCN by electrical stimulation of the auditory nerve (50). 3. Low-calcium solutions and pharmacologic manipulations were used to separate pre- and postsynaptic response components in the field potential records. When the slice is bathed in a low-calcium solution the P2a1, N2(1), N2(2), P3(2), and the brief late deflections are abolished. However, the P1(1)-N1(1)-P2(1) and P1(2)-N1(2)-P2(2) remain unaffected. A similar separation of pre- and postsynaptic components can be achieved with 100 microM adenosine or 0.5 mM kynurenic acid. It is concluded that the P1(1)-M1(1)-P2(1) wave is the compound action potential of the unmyelinated parallel fibers, whereas the longer-latency field potential components are generated postsynaptically. 4. The conduction velocity of the parallel fiber volley was measured to be 0.30 m/s at the pial surface, in a line approximately parallel to the strial axis of the nucleus. Mapping experiments reveal that the spread of the P1(1)-N1(1)-P2(1) is greatest along the strial axis, and more limited in the orthogonal direction. 5. Single units were recorded in layer 2. At a distance of 500-700 microns from the stimulating electrode, the latencies of single-unit discharges fall between 2.5 and 4 ms, at the time of the N2(2).(ABSTRACT TRUNCATED AT 400 WORDS)

Intracellular recordings from pyramidal cells of the guinea pig dorsal cochlear nucleus: [formula omitted], Depts. of Otolaryngology-Head and Neck Surgery and Neurosci. and The Center for Hearing Sciences, The Johns Hopkins Univ. Sch. of Med., Baltimore, MD 21205 U.S.A.

Intracellular recordings from pyramidal cells of the guinea pig dorsal cochlear nucleus: [formula omitted], Depts. of Otolaryngology-Head and Neck Surgery and Neurosci. and The Center for Hearing Sciences, The Johns Hopkins Univ. Sch. of Med., Baltimore, MD 21205 U.S.A.