Dendritic Ca2+ transients evoked by action potentials in rat dorsal cochlear nucleus pyramidal and cartwheel neurons.
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Simultaneous fluorescence imaging and electrophysiologic recordings were used to investigate the Ca(2+) influx initiated by action potentials (APs) into dorsal cochlear nucleus (DCN) pyramidal cell (PC) and cartwheel cell (CWC) dendrites. Local application of Cd(2+) blocked Ca(2+) transients in PC and CWC dendrites, demonstrating that the Ca(2+) influx was initiated by dendritic Ca(2+) channels. In PCs, TTX eliminated the dendritic Ca(2+) transients when APs were completely blocked. However, the Ca(2+) influx could be partially recovered during an incomplete block of APs or when a large depolarization was substituted for the blocked APs. In CWCs, dendritic Ca(2+) transients evoked by individual APs, or simple spikes, were blocked by TTX and could be recovered during an incomplete block of APs or by a large depolarization. In contrast, dendritic Ca(2+) transients evoked by complex spikes, a burst of APs superimposed on a slow depolarization, were not blocked by TTX, despite eliminating the APs superimposed on the slow depolarization. These results suggest two different mechanisms for the retrograde activation of dendritic Ca(2+) channels: the first requires fast Na(+) channel-mediated APs or a large somatic depolarization, whereas the second is independent of Na(+) channel activation, requiring only the slow depolarization underlying complex spikes. (+info)
Computational diversity in the cochlear nucleus angularis of the barn owl.
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The cochlear nucleus angularis (NA) is widely assumed to form the starting point of a brain stem pathway for processing sound intensity in birds. Details of its function are unclear, however, and its evolutionary origin and relationship to the mammalian cochlear-nucleus complex are obscure. We have carried out extracellular single-unit recordings in the NA of ketamine-anesthetized barn owls. The aim was to re-evaluate the extent of heterogeneity in NA physiology because recent studies of cellular morphology had established several distinct types. Extensive characterization, using tuning curves, phase locking, peristimulus time histograms and rate-level functions for pure tones and noise, revealed five major response types. The most common one was a primary-like pattern that was distinguished from auditory-nerve fibers by showing lower vector strengths of phase locking and/or lower spontaneous rates. Two types of chopper responses were found (chopper-transient and a rare chopper-sustained), as well as onset units. Finally, we routinely encountered a complex response type with a pronounced inhibitory component, similar to the mammalian typeIV. Evidence is presented that this range of response types is representative for birds and that earlier conflicting reports may be due to methodological differences. All five response types defined were similar to well-known types in the mammalian cochlear nucleus. This suggests convergent evolution of neurons specialized for encoding different behaviorally relevant features of the auditory stimulus. It remains to be investigated whether the different response types correlate with morphological types and whether they establish different processing streams in the auditory brain stem of birds. (+info)
Activity-dependent regulation of the subcellular localization of neuronal calcium sensor-1 in the avian cochlear nucleus.
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Neurons in the avian cochlear nucleus, nucleus magnocellularis (NM), are highly sensitive to manipulations of afferent input, and removal of afferent activity through cochlear ablation results in the death of approximately 20-40% of ipsilateral NM neurons. The intracellular cascades that determine whether an individual NM neuron will die or survive are not fully understood. One early event observed in NM following deafferentation is a rapid rise in intracellular calcium concentration. In most cellular systems, the activity of calcium-binding proteins is believed to accommodate calcium influx. The calcium-binding protein, neuronal calcium sensor-1 (NCS-1), is an intracellular neuronal calcium sensor belonging to the EF-hand superfamily. NCS-1 has been implicated in calcium-dependent regulation of signaling cascades. To evaluate NCS-1 action in NM neurons, the localization of NCS-1 protein was examined. Double-label immunofluorescence experiments revealed that NCS-1 expression is evident in both the presynaptic nerve terminal and postsynaptic NM neuron. The postsynaptic expression of NCS-1 typically appears to be closely associated with the cell membrane. This close proximity of NCS-1 to the postsynaptic membrane could allow NCS-1 to function as a modulator of postsynaptic signaling events. Following deafferentation, NM neurons were more likely to show diffuse cytoplasmic NCS-1 labeling. This increase in the number of cells showing diffuse cytoplasmic labeling was observed 12 and 24 h following cochlea ablation, but was not observed 4 days following surgery. This activity-dependent regulation of NCS-1 subcellular localization suggests it may be associated with, or influenced by, processes important for the survival of NM neurons. (+info)
Differential expression of three distinct potassium currents in the ventral cochlear nucleus.
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In the ventral cochlear nucleus (VCN), neurons transform information from auditory nerve fibers into a set of parallel ascending pathways, each emphasizing different aspects of the acoustic environment. Previous studies have shown that VCN neurons differ in their intrinsic electrical properties, including the K+ currents they express. In this study, we examine these K+ currents in more detail using whole cell voltage-clamp techniques on isolated VCN cells from adult guinea pigs at 22 degrees C. Our results show a differential expression of three distinct K+ currents. Whereas some VCN cells express only a high-threshold delayed-rectifier-like current (IHT), others express IHT in combination with a fast inactivating current (IA) and/or a slow-inactivating low-threshold current (ILT). IHT, ILT, and IA, were partially blocked by 1 mM 4-aminopyridine. In contrast, only ILT was blocked by 10-100 nM dendrotoxin-I. A surprising finding was the wide range of levels of ILT, suggesting ILT is expressed as a continuum across cell types rather than modally in a particular cell type. IA, on the other hand, appears to be expressed only in cells that show little or no ILT, the Type I cells. Boltzmann analysis shows IHT activates with 164 +/- 12 (SE) nS peak conductance, -14.3 +/- 0.7 mV half-activation, and 7.0 +/- 0.5 mV slope factor. Similar analysis shows ILT activates with 171 +/- 22 nS peak conductance, -47.4 +/- 1.0 mV half-activation, and 5.8 +/- 0.3 mV slope factor. (+info)
Kinetic analyses of three distinct potassium conductances in ventral cochlear nucleus neurons.
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Neurons in the ventral cochlear nucleus (VCN) express three distinct K+ currents that differ in their voltage and time dependence, and in their inactivation behavior. In the present study, we quantitatively analyze the voltage-dependent kinetics of these three currents to gain further insight into how they regulate the discharge patterns of VCN neurons and to provide supporting data for the identification of their channel components. We find the transient A-type K+ current (IA) exhibits fourth-order activation kinetics (a4), and inactivates with one or two time constants. A second inactivation rate (leading to an a4bc kinetic description) is required to explain its recovery from inactivation. The dendrotoxin-sensitive low-threshold K+ current (ILT) also activates with fourth-order kinetics (w4) but shows slower, incomplete inactivation. The high-threshold K+ current (IHT) appears to consist of two kinetically distinct components (n2 + p). The first component activates approximately 10 mV positive to the second and has second-order kinetics. The second component activates with first-order kinetics. These two components also contribute to two kinetically distinct currents upon deactivation. The kinetic behavior of IHT was indistinguishable amongst cell types, suggesting the current is mediated by the same K+ channels amongst VCN neurons. Together these results provide a basis for more realistic modeling of VCN neurons, and provide clues regarding the molecular basis of the three K+ currents. (+info)
The roles potassium currents play in regulating the electrical activity of ventral cochlear nucleus neurons.
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Using kinetic data from three different K+ currents in acutely isolated neurons, a single electrical compartment representing the soma of a ventral cochlear nucleus (VCN) neuron was created. The K+ currents include a fast transient current (IA), a slow-inactivating low-threshold current (ILT), and a noninactivating high-threshold current (IHT). The model also includes a fast-inactivating Na+ current, a hyperpolarization-activated cation current (Ih), and 1-50 auditory nerve synapses. With this model, the role IA, ILT, and IHT play in shaping the discharge patterns of VCN cells is explored. Simulation results indicate that IHT mainly functions to repolarize the membrane during an action potential, and IA functions to modulate the rate of repetitive firing. ILT is found to be responsible for the phasic discharge pattern observed in Type II cells (bushy cells). However, by adjusting the strength of ILT, both phasic and regular discharge patterns are observed, demonstrating that a critical level of ILT is necessary to produce the Type II response. Simulated Type II cells have a significantly faster membrane time constant in comparison to Type I cells (stellate cells) and are therefore better suited to preserve temporal information in their auditory nerve inputs by acting as precise coincidence detectors and having a short refractory period. Finally, we demonstrate that modulation of Ih, which changes the resting membrane potential, is a more effective means of modulating the activation level of ILT than simply modulating ILT itself. This result may explain why ILT and Ih are often coexpressed throughout the nervous system. (+info)
Fast inhibition underlies the transmission of auditory information between cochlear nuclei.
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A direct commissural connection between cochlear nuclei provides a pathway by which binaural input can influence the processing of acoustic information through the ventral cochlear nucleus. Despite anatomical evidence to suggest the existence of such a pathway, its nature and behavior have not been investigated previously. This in vivo intracellular electrophysiological study provides direct evidence of monosynaptic (mean latency, 1.43 msec), inhibitory commissural input to T stellate cells. This inhibition is fast acting (duration, <10 msec), occurring with little synaptic delay ( approximately 0.3 msec). Electrical stimulation also revealed the initiation of antidromic responses in the onset chopper population, signifying D stellate neurons as a source of commissural inputs. Activation of the commissural connection was most evident in response to broadband stimuli. These results provide the first compelling evidence of a fast, monosynaptic commissural pathway arising from contralateral D stellate neurons providing broadband inhibitory input to T stellate cells. (+info)
Ultrastructural distribution of glycinergic and GABAergic neurons and axon terminals in the rat dorsal cochlear nucleus, with emphasis on granule cell areas.
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A knowledge of neurotransmitters in the neurons of the rat cochlear nuclear complex is of importance in understanding the function of auditory circuits. Using post-embedding ultrastructural immunogold labelling, the distribution of glycinergic and GABAergic neurons and axonal terminals has been studied in the molecular, fusiform and polymorphic layers of the rat dorsal cochlear nucleus (DCN). This technique is not limited by the penetration of antibodies into the nervous tissue as in pre-embedding methods, and allows a fine neurochemical mapping of the nervous tissue. Numerous glycinergic and GABAergic axon terminals contain pleomorphic and flat synaptic vesicles, and are present in all layers (1, 2, 3) of the dorsal cochlear nucleus. Glycine and GABA-negative large terminals (mossy fibres) are mainly seen in granule cell areas of layer 2 (fusiform layer). Mossy fibres contact the dendrites of GABA- and glycine-negative granule cells and of the few unipolar brush cells (excitatory neurons). The least common cells in the granule cell areas are GABAergic and glycinergic Golgi-stellate neurons. In unipolar brush cells, aggregations of vesicles seem to be the origin of their characteristic ringlet-bodies. Golgi-stellate cells send their inhibitory terminals to the dendrites of granule and unipolar brush cells, occasionally directly to mossy fibres. Small or (less frequently) large GABAergic terminals contact the soma or the main dendrite of unipolar brush cells. The circuit of a hypothetical functional unit of neurons in the DCN is proposed. The inputs from auditory tonotopic or non-auditory non-tonotopic mossy fibres eventually reach pyramidal cells through axons from the granule cells or unipolar brush cells. Pyramidal cells convey an excitatory signal from the DCN to higher mesencephalic nuclei for further elaboration of the acoustic signal. (+info)