Hyperpolarization-activated inward current in neurons of the rat's dorsal nucleus of the lateral lemniscus in vitro

Characterization in Dual Activation by Oxaliplatin, a Platinum-Based Chemotherapeutic Agent of Hyperpolarization-Activated Cation and Electroporation-Induced Currents

Oxaliplatin (OXAL) is regarded as a platinum-based anti-neoplastic agent. However, its perturbations on membrane ionic currents in neurons and neuroendocrine or endocrine cells are largely unclear, though peripheral neuropathy has been noted during its long-term administration. In this study, we investigated how the presence of OXAL and other related compounds can interact with two types of inward currents; namely, hyperpolarization-activated cation current (I_h) and membrane electroporation-induced current (I_MEP). OXAL increased the amplitude or activation rate constant of I_h in a concentration-dependent manner with effective EC₅₀ or K_D values of 3.2 or 6.4 μM, respectively, in pituitary GH₃ cells. The stimulation by this agent of I_h could be attenuated by subsequent addition of ivabradine, protopine, or dexmedetomidine. Cell exposure to OXAL (3 μM) resulted in an approximately 11 mV rightward shift in I_h activation along the voltage axis with minimal changes in the gating charge of the curve. The exposure to OXAL also effected an elevation in area of the voltage-dependent hysteresis elicited by long-lasting triangular ramp. Additionally, its application resulted in an increase in the amplitude of I_MEP elicited by large hyperpolarization in GH₃ cells with an EC₅₀ value of 1.3 μM. However, in the continued presence of OXAL, further addition of ivabradine, protopine, or dexmedetomidine always resulted in failure to attenuate the OXAL-induced increase of I_MEP amplitude effectively. Averaged current-voltage relation of membrane electroporation-induced current (I_MEP) was altered in the presence of OXAL. In pituitary R1220 cells, OXAL-stimulated I_h remained effective. In Rolf B1.T olfactory sensory neurons, this agent was also observed to increase I_MEP in a concentration-dependent manner. In light of the findings from this study, OXAL-mediated increases of I_h and I_MEP may coincide and then synergistically act to increase the amplitude of inward currents, raising the membrane excitability of electrically excitable cells, if similar in vivo findings occur.

Aldosterone-Sensing Neurons in the NTS Exhibit State-Dependent Pacemaker Activity and Drive Sodium Appetite via Synergy with Angiotensin II Signaling

Sodium deficiency increases angiotensin II (ATII) and aldosterone, which synergistically stimulate sodium retention and consumption. Recently, ATII-responsive neurons in the subfornical organ (SFO) and aldosterone-sensitive neurons in the nucleus of the solitary tract (NTS^HSD2 neurons) were shown to drive sodium appetite. Here we investigate the basis for NTS^HSD2 neuron activation, identify the circuit by which NTS^HSD2 neurons drive appetite, and uncover an interaction between the NTS^HSD2 circuit and ATII signaling. NTS^HSD2 neurons respond to sodium deficiency with spontaneous pacemaker-like activity-the consequence of "cardiac" HCN and Na_v1.5 channels. Remarkably, NTS^HSD2 neurons are necessary for sodium appetite, and with concurrent ATII signaling their activity is sufficient to produce rapid consumption. Importantly, NTS^HSD2 neurons stimulate appetite via projections to the vlBNST, which is also the effector site for ATII-responsive SFO neurons. The interaction between angiotensin signaling and NTS^HSD2 neurons provides a neuronal context for the long-standing "synergy hypothesis" of sodium appetite regulation.

I h and HCN channels in murine spiral ganglion neurons: tonotopic variation, local heterogeneity, and kinetic model

One of the major contributors to the response profile of neurons in the auditory pathways is the I h current. Its properties such as magnitude, activation, and kinetics not only vary among different types of neurons (Banks et al., J Neurophysiol 70:1420-1432, 1993; Fu et al., J Neurophysiol 78:2235-2245, 1997; Bal and Oertel, J Neurophysiol 84:806-817, 2000; Cao and Oertel, J Neurophysiol 94:821-832, 2005; Rodrigues and Oertel, J Neurophysiol 95:76-87, 2006; Yi et al., J Neurophysiol 103:2532-2543, 2010), but they also display notable diversity in a single population of spiral ganglion neurons (Mo and Davis, J Neurophysiol 78:3019-3027, 1997), the first neural element in the auditory periphery. In this study, we found from somatic recordings that part of the heterogeneity can be attributed to variation along the tonotopic axis because I h in the apical neurons have more positive half-activation voltage levels than basal neurons. Even within a single cochlear region, however, I h current properties are not uniform. To account for this heterogeneity, we provide immunocytochemical evidence for variance in the intracellular density of the hyperpolarization-activated cyclic nucleotide-gated channel α-subunit 1 (HCN1), which mediates I h current. We also observed different combinations of HCN1 and HCN4 α-subunits from cell to cell. Lastly, based on the physiological data, we performed kinetic analysis for the I h current and generated a mathematical model to better understand varied I h on spiral ganglion function. Regardless of whether I h currents are recorded at the nerve terminals (Yi et al., J Neurophysiol 103:2532-2543, 2010) or at the somata of spiral ganglion neurons, they have comparable mean half-activation voltage and induce similar resting membrane potential changes, and thus our model may also provide insights into the impact of I h on synaptic physiology.

Hyperpolarization-activated current (In) is reduced in hippocampal neurons from Gabra5-/- mice

Changes in the expression of γ-aminobutyric acid type A (GABA_A) receptors can either drive or mediate homeostatic alterations in neuronal excitability. A homeostatic relationship between α5 subunit-containing GABA_A (α5GABA_A) receptors that generate a tonic inhibitory conductance, and HCN channels that generate a hyperpolarization-activated cation current (I_h) was recently described for cortical neurons, where a reduction in I_h was accompanied by a reciprocal increase in the expression of α5GABA_A receptors resulting in the preservation of dendritosomatic synaptic function. Here, we report that in mice that lack the α5 subunit gene (Gabra5−/−), cultured embryonic hippocampal pyramidal neurons and ex vivo CA1 hippocampal neurons unexpectedly exhibited a decrease in I_h current density (by 40% and 28%, respectively), compared with neurons from wild-type (WT) mice. The resting membrane potential and membrane hyperpolarization induced by blockade of I_h with ZD-7288 were similar in cultured WT and Gabra5−/− neurons. In contrast, membrane hyperpolarization measured after a train of action potentials was lower in Gabra5−/− neurons than in WT neurons. Also, membrane impedance measured in response to low frequency stimulation was greater in cultured Gabra5−/− neurons. Finally, the expression of HCN1 protein that generates I_h was reduced by 41% in the hippocampus of Gabra5−/− mice. These data indicate that loss of a tonic GABAergic inhibitory conductance was followed by a compensatory reduction in I_h. The results further suggest that the maintenance of resting membrane potential is preferentially maintained in mature and immature hippocampal neurons through the homeostatic co-regulation of structurally and biophysically distinct cation and anion channels.

The hyperpolarization-activated non-specific cation current (In ) adjusts the membrane properties, excitability, and activity pattern of the giant cells in the rat dorsal cochlear nucleus

Giant cells of the cochlear nucleus are thought to integrate multimodal sensory inputs and participate in monaural sound source localization. Our aim was to explore the significance of a hyperpolarization-activated current in determining the activity of giant neurones in slices prepared from 10 to 14-day-old rats. When subjected to hyperpolarizing stimuli, giant cells produced a 4-(N-ethyl-N-phenylamino)-1,2-dimethyl-6-(methylamino) pyridinium chloride (ZD7288)-sensitive inward current with a reversal potential and half-activation voltage of -36 and -88 mV, respectively. Consequently, the current was identified as the hyperpolarization-activated non-specific cationic current (Ih ). At the resting membrane potential, 3.5% of the maximum Ih conductance was available. Immunohistochemistry experiments suggested that hyperpolarization-activated, cyclic nucleotide-gated, cation non-selective (HCN)1, HCN2, and HCN4 subunits contribute to the assembly of the functional channels. Inhibition of Ih hyperpolarized the membrane by 6 mV and impeded spontaneous firing. The frequencies of spontaneous inhibitory and excitatory postsynaptic currents reaching the giant cell bodies were reduced but no significant change was observed when evoked postsynaptic currents were recorded. Giant cells are affected by biphasic postsynaptic currents consisting of an excitatory and a subsequent inhibitory component. Inhibition of Ih reduced the frequency of these biphasic events by 65% and increased the decay time constants of the inhibitory component. We conclude that Ih adjusts the resting membrane potential, contributes to spontaneous action potential firing, and may participate in the dendritic integration of the synaptic inputs of the giant neurones. Because its amplitude was higher in young than in adult rats, Ih of the giant cells may be especially important during the postnatal maturation of the auditory system.

Parameters for a model of an oscillating neuronal network in the cochlear nucleus defined by genetic algorithms

Chopper neurons in the cochlear nucleus are characterized by intrinsic oscillations with short average interspike intervals (ISIs) and relative level independence of their response (Pfeiffer, Exp Brain Res 1:220-235, 1966; Blackburn and Sachs, J Neurophysiol 62:1303-1329, 1989), properties which are unattained by models of single chopper neurons (e.g., Rothman and Manis, J Neurophysiol 89:3070-3082, 2003a). In order to achieve short ISIs, we optimized the time constants of Rothman and Manis single neuron model with genetic algorithms. Some parameters in the optimization, such as the temperature and the capacity of the cell, turned out to be crucial for the required acceleration of their response. In order to achieve the relative level independence, we have simulated an interconnected network consisting of Rothman and Manis neurons. The results indicate that by stabilization of intrinsic oscillations, it is possible to simulate the physiologically observed level independence of ISIs. As previously reviewed and demonstrated (Bahmer and Langner, Biol Cybern 95:371-379, 2006a), chopper neurons show a preference for ISIs which are multiples of 0.4 ms. It was also demonstrated that the network consisting of two optimized Rothman and Manis neurons which activate each other with synaptic delays of 0.4 ms shows a preference for ISIs of 0.8 ms. Oscillations with various multiples of 0.4 ms as ISIs may be derived from neurons in a more complex network that is activated by simultaneous input of an onset neuron and several auditory nerve fibers.

Modification of membrane excitability of neurons in the rat's dorsal cortex of the inferior colliculus by preceding hyperpolarization

The inferior colliculus (IC) is among the largest nuclei in the central auditory system and is considered to be a major integration center in the auditory pathway. To understand how IC contributes to auditory processing, we investigated the effects of preceding hyperpolarization on membrane excitability and firing behavior of neurons located in the dorsal cortex of the inferior colliculus (ICD). We made whole-cell patch clamp recordings from ICD neurons (n=96) in rat brain slices. We classified ICD neurons into three types, i.e. sustained-regular, sustained-adapting and buildup, according to their responses to depolarizing current injection. Nearly 91% of the neurons had sustained firing throughout the period of current injection, showing either regular or adapting pattern. About 9% of the neurons exhibited a buildup pattern, in which sustained firing started after a long delay. Rebound depolarization and spikes after hyperpolarization were seen in 51.7% of the sustained neurons, but were not seen in buildup neurons. When depolarizing current was preceded by a hyperpolarizing current, various forms of the modification on membrane excitability were observed. For non-rebound neurons, the membrane excitability was either suppressed or unchanged after pre-hyperpolarization. The first spike latency lengthened in neurons whose firing changed to a buildup pattern, shortened in those whose firing changed to a pauser pattern, and remained unchanged in those whose discharge pattern remained sustained. For rebound neurons, the firing rate decreased in neurons whose firing pattern was changed to onset or pauser, increased in neurons whose firing was changed to adapting, or remained unchanged in neurons whose firing became irregular. The first spike latency was shortened in all the rebound cells. The results suggest that intrinsic membrane properties can play an important role in integration of excitatory and inhibitory inputs and thereby in determination of the output of ICD neurons.

Intrinsic membrane properties and synaptic response characteristics of neurons in the rat's external cortex of the inferior colliculus

The inferior colliculus (IC) can be divided into three anatomical subdivisions: the central nucleus (ICc), the dorsal cortex (ICd) and the external cortex (ICx). ICx receives its primary auditory inputs from ICc and auditory cerebral cortical areas, and non-auditory inputs from regions of motor and other sensory systems. This wide array of projections makes the ICx a distinct structure within the auditory brainstem. The purpose of the current study was to comprehensively characterize the neuronal population of ICx, by intrinsic and synaptic response properties. Visual whole-cell patch clamp recordings were taken from ICx neurons (N=129) from rats between postnatal days 8 to 12. Neurons displayed various types of firing patterns in response to current injection, including regular, adapting, pauser and bursting. The regular cells constitute the majority (66%), followed by adapting (18%), pauser (13%) and bursting cells (2%). In response to hyperpolarizing current injection, many neurons illustrated a pronounced sag in the membrane potential, representing a hyperpolarization-activated current (I(h)). Some neurons (25%) displayed a Ca(2+)-dependent rebound depolarization following negative current injection. In response to depolarizing current injection, 70% of ICx neurons displayed a Ca(2+)-mediated potential expressed as Ca(2+) spikes/humps, uncovered when Na(+) and K(+) currents were removed. Also, spikes displayed an undershoot which was in part mediated by Ca(2+). Stimulation of the ICc elicited graded synaptic responses, which displayed a combination of excitatory and/or inhibitory potentials, with excitation being predominant across firing patterns. Neurons displayed temporal summation in response to repetitive stimulation at 20 Hz and higher. The results indicate a relatively modest diversity in firing pattern and in intrinsic membrane properties, making this subnucleus distinct from its counterparts within the IC. The data show that ICx receives major excitatory input from ICc, supporting its role in integrating signals from brainstem and directing information to higher brain centers.

WITHDRAWN: Immunohistochemical localization of Ih channel HCN3 in the rat brain

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Sensitivity to Interaural Time Differences in the Dorsal Nucleus of the Lateral Lemniscus of the Unanesthetized Rabbit: Comparison With Other Structures

Interaural time differences, a cue for azimuthal sound location, are first encoded in the superior olivary complex (SOC), and this information is then conveyed to the dorsal nucleus of the lateral lemniscus (DNLL) and inferior colliculus (IC). The DNLL provides a strong inhibitory input to the IC and may serve to transform the coding of interaural time differences (ITDs) in the IC. Consistent with the projections from the SOC, the DNLL and IC had similar distributions of peak- and trough-type neurons, characteristic delays, and best ITDs. The ITD tuning widths of DNLL neurons were intermediate between those of the SOC and IC. Further sharpening is seen in the auditory thalamus, indicating that sharpening mechanisms are not restricted to the midbrain. The proportion of neurons that phase-locked to the tones delivered to each ear progressively decreased from the SOC to the auditory thalamus. The degree of phase-locking for a large majority of DNLL neurons was too weak to support their involvement in processing monaural inputs to generate a sensitivity to ITDs. The response rates of DNLL neurons were on average ∼60% greater than in the IC or SOC, indicating that the inhibitory input provided to the IC by the DNLL is robust.

Distribution of HCN1 and HCN2 in rat auditory brainstem nuclei

Auditory brainstem neurons that are involved in the precise analysis of the temporal pattern of sounds have ionic currents activated near the resting potential to shorten membrane time constants. One of these currents is the hyperpolarization-activated current (Ih). Molecular cloning of the channels underlying Ih revealed four different isoforms (HCN1-4). HCN1 and HCN2, which are widely distributed in the brain, differ in their activation kinetics, voltage dependence and sensitivity to cAMP. We determined the distribution of the HCN1 and HCN2 isoform in the auditory brainstem and midbrain of young rats (P20-30), using standard immunohistochemical techniques. HCN1 antibodies gave rise to punctate staining on the somatic and dendritic membrane. Strong HCN1 staining was present on octopus and bushy cells of the ventral cochlear nucleus, principal neurons of the lateral and medial superior olive, and neurons of the ventral nucleus of the lateral lemniscus. No HCN1 staining was observed in the dorsal cochlear nucleus and the medial nucleus of the trapezoid body (MNTB). In contrast, HCN2 staining was strongest in the MNTB and the dorsal nucleus of the lateral lemniscus. Strong HCN2 antibody labelling was also observed in bushy cells of the ventral cochlear nucleus. In the central nucleus of the inferior colliculus only a subpopulation of neurons showed HCN1 or HCN2 immunolabelling. This differential distribution of HCN1 and HCN2 channels is in agreement with the physiologically observed Ih currents in corresponding neuronal populations and might represent the basis for functional heterogeneity and diverse sensitivity to neuromodulators.

Immunohistochemical localization of Ih channel subunits, HCN1-4, in the rat brain

Hyperpolarization-activated cation currents (I(h)) contribute to various physiological properties and functions in the brain, including neuronal pacemaker activity, setting of resting membrane potential, and dendritic integration of synaptic input. Four subunits of the Hyperpolarization-activated and Cyclic-Nucleotide-gated nonselective cation channels (HCN1-4), which generate I(h), have been cloned recently. To better understand the functional diversity of I(h) in the brain, we examined precise immunohistochemical localization of four HCNs in the rat brain. Immunoreactivity for HCN1 showed predominantly cortical distribution, being intense in the neocortex, hippocampus, superior colliculus, and cerebellum, whereas those for HCN3 and HCN4 exhibited subcortical distribution mainly concentrated in the hypothalamus and thalamus, respectively. Immunoreactivity for HCN2 had a widespread distribution throughout the brain. Double immunofluorescence revealed colocalization of immunoreactivity for HCN1 and HCN2 in distal dendrites of pyramidal cells in the hippocampus and neocortex. At the electron microscopic level, immunogold particles for HCN1 and HCN2 had similar distribution patterns along plasma membrane of dendritic shafts in layer I of the neocortex and stratum lacunosum moleculare of the hippocampal CA1 area, suggesting that these subunits could form heteromeric channels. Our results further indicate that HCNs are localized not only in somato-dendritic compartments but also in axonal compartments of neurons. Immunoreactivity for HCNs often occurred in preterminal rather than terminal portions of axons and in specific populations of myelinated axons. We also found HCN2-immunopositive oligodendrocytes including perineuronal oligodendrocytes throughout the brain. These results support previous electrophysiological findings and further suggest unexpected roles of I(h) channels in the brain.

Hyperpolarization-activated current (Ih) in the inferior colliculus: distribution and contribution to temporal processing

Neurons in the inferior colliculus (IC) process acoustic information converging from inputs from almost all nuclei of the auditory brain stem. Despite its importance in auditory processing, little is known about the distribution of ion currents in IC neurons, namely the hyperpolarization-activated current Ih. This current, as shown in neurons of the auditory brain stem, contributes to the precise analysis of temporal information. Distribution and properties of the Ih current and its contribution to membrane properties and synaptic integration were examined by current- and voltage-clamp recordings obtained from IC neurons in acute slices of rats (P17-P19). Based on firing patterns to positive current injection, three basic response types were distinguished: onset, adapting, and sustained firing neurons. Onset and adapting cells showed an Ih-dependent depolarizing sag and had a more depolarized resting membrane potential and lower input resistance than sustained neurons. Ih amplitudes were largest in onset, medium in adapting, and small in sustained neurons. Ih activation kinetics was voltage dependent in all neurons and faster in onset and adapting compared with sustained neurons. Injecting trains of simulated synaptic currents into the neurons or evoking inhibitory postsynaptic potentials (IPSPs) by stimulating the lemniscal tract showed that Ih reduced temporal summation of excitatory and inhibitory potentials in onset but not in sustained neurons. Blocking Ih also abolished afterhyperpolarization and rebound spiking. These results suggest that, in a large proportion of IC cells, namely the onset and adapting neurons, Ih improves precise temporal processing and contributes to the temporal analysis of input patterns.

Hyperpolarization-activated (Ih) conductances affect brainstem auditory neuron excitability

Blockade of the hyperpolarization-activated cyclic-nucleotide-gated mixed-cationic conductance (I(h)) by ZD7288 markedly reduces excitability of neurons in the superior olivary complex (SOC), in vivo. Following pressure ejection application of 100 microM ZD7288, extracellular recorded single unit responses of 47/47 SOC neurons to monaural or binaural pure tone best frequency (BF) stimuli (30 dB above threshold) decreased by 49.7+/-19%, and background activity decreased by 56.3+/-18.1%. Pressure ejection of the vehicle did not affect excitability. The dose- and time-dependence of ZD7288 (10-100 microM) effects are consistent with specific blockade of I(h) currents. SOC neuron responses to pressure-ejected glutamate were also decreased following application of 100 microM ZD7288 by 76.7+/-28.0%, which suggests a predominant direct effect of ZD7288 on auditory cell excitability. The considerable variability in the magnitude of ZD7288 effects between cells was only partially accounted for by greater effects on neurons with BFs greater than 16 kHz. Therefore, I(h) channels significantly contribute to auditory brainstem neuron excitability, affecting their response level to acoustic stimuli. The variability in the ZD7288 reduction in excitability and its variation with the BF of units could be an indication of regulation and plasticity in neuronal encoding of sounds.

The roles potassium currents play in regulating the electrical activity of ventral cochlear nucleus neurons

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.

Membrane-based gating mechanism for auditory information in the mouse inferior colliculus

In order to analyse the intrinsic membrane-based mechanisms of neurons in the mouse inferior colliculus that are likely to contribute to the processing of acoustic signals, this study use whole cell patch clamp recordings in brain slices to characterize the dependence of depolarization evoked inward and outward currents on different prestimulus membrane potentials. Eighty-seven of one-hundred and one inferior colliculus neurons reacted during depolarizing voltage steps from a holding potential of -60 or -80 mV with a fast inactivating inward current followed by a slow inactivating outward current (type I neurons). Fourteen neurons showed outward currents but no inward currents during depolarizing voltage steps from a holding potential of -60 mV (type II neurons). However, these neurons reacted with TTX-sensitive fast inward currents, if the holding potential was set to -80 mV before the voltage steps occurred. The resting potential was not significantly different between type I (-64.3+/-3.5 mV) and type II (62.7+/-2.9 mV) neurons. If the neuronal behavior is the same in vivo, type II neurons must receive an inhibition which hyperpolarizes the membrane potential prior to the arrival of excitatory inputs to be able to generate action potentials. This finding suggests a further function for feedforward inhibition in the IC, namely to open a gate for transmission of excitatory information within a distinct time window. With this membrane based gating mechanism it is possible to detect time related information within an acoustic stimulus (e. g. coincidence) which is an essential task e. g. in the neuronal processing of speech.

Spectral determination of responses to species-specific calls in the dorsal nucleus of the lateral lemniscus

This study evaluated how neurons in the dorsal nucleus of the lateral lemniscus (DNLL) in Mexican free-tailed bats respond to both tone bursts and species-specific calls. Up to 20 calls were presented to each neuron, of which 18 were social communication and 2 were echolocation calls. We also measured excitatory response regions (ERRs): the range of tone burst frequencies that evoked discharges at a fixed intensity. Neurons were unselective for one or another call in that each neuron responded to any call so long as the call had energy that encroached on its ERR. Additionally, responses were evoked by the same set of calls, and with similar spike counts, when they were presented normally or reversed. By convolving activity in the ERRs with the spectrogram of each call, we showed that responses to tones accurately predicted discharge patterns evoked by species-specific calls. DNLL cells are remarkably homogeneous in that neurons having similar BFs responded to each of the species-specific calls with similar response profiles. The homogeneity was further illustrated by the ability to accurately predict the response profiles of a particular DNLL cell to species-specific calls from the ERR of another similarly tuned DNLL cell. Thus DNLL neurons tuned to the same or similar frequencies responded to species-specific calls with latencies and temporal discharge patterns that were so similar as to be virtually interchangeable. What this suggests is that DNLL responses evoked by complex sounds can be largely explained by a simple summation of the excitation in each neuron's ERR. Finally, superimposing the spectrograms of each call on the responses evoked by that call revealed that the DNLL population response re-creates both the spectral and the temporal features of each signal.

Modulation of a presynaptic hyperpolarization-activated cationic current (I(h)) at an excitatory synaptic terminal in the rat auditory brainstem

1. A hyperpolarization-activated non-specific cation current, I(h), was examined in bushy cell bodies and their giant presynaptic terminals (calyx of Held). Whole-cell patch clamp recordings were made using an in vitro brain slice preparation of the cochlear nucleus and the superior olivary complex. The aim was to characterise I(h) in identified cell bodies and synaptic terminals, to examine modulation by presynaptic cAMP and to test for modulatory effects of I(h) activation on synaptic transmission. 2. Presynaptic I(h) was activated by hyperpolarizing voltage-steps, with half-activation (V(1/2)) at -94 mV. Activation time constants were voltage dependent, showing an e-fold acceleration for hyperpolarizations of -32 mV (time constant of 78 ms at -130 mV). The reversal potential of I(h) was -29 mV. It was blocked by external perfusion of 1 mM CsCl but was unaffected by BaCl(2). 3. Application of internal cAMP shifted the activation curve to more positive potentials, giving a V(1/2) of -74 mV; hence around half of the current was activated at resting membrane potentials. This shift in half-activation was mimicked by external perfusion of a membrane-permeant analogue, 8-bromo-cAMP. 4. The bushy cell body I(h) showed similar properties to those of the synaptic terminal; V(1/2) was -94 mV and the reversal potential was -33 mV. Somatic I(h) was blocked by CsCl (1 mM) and was partially sensitive to BaCl(2). Somatic I(h) current density increased with postnatal age from 5 to 16 days old, suggesting that I(h) is functionally relevant during maturation of the auditory pathway. 5. The function of I(h) in regulating presynaptic excitability is subtle. I(h) had little influence on EPSC amplitude at the calyx of Held, but may be associated with propagation of the action potential at branch points. Presynaptic I(h) shares properties with both HCN1 and HCN2 recombinant channel subunits, in that it gates relatively rapidly and is modulated by internal cAMP.

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.

Hyperpolarization-activated inward potassium and calcium-sensitive chloride currents in beating pacemaker insect neurosecretory cells (dorsal unpaired median neurons)

Hyperpolarization-activated inward currents were studied in single adult cockroach Periplaneta americana pacemaker neurosecretory cells, identified as dorsal unpaired median neurons using the whole-cell patch-clamp technique. Under current clamp, injection of negative current produced a hyperpolarization of the cell membrane with a sag in the membrane potential toward the resting value. Under voltage clamp, the whole-cell current-voltage relationship exhibited an unexpected biphasic aspect. The global hyperpolarization-activated inward current could be dissociated by means of 4-acetamido-4'-isothiocyanostilbene-2,2'-disulfonic acid and tetraethylammonium chloride sensitivity, ionic selectivity, voltage dependence and activation threshold as inward potassium and calcium-sensitive chloride currents. The inward potassium current was activated around -80 mV. The reversal potential followed the potassium equilibrium potential when the extracellular potassium concentration was raised. This current was not dependent on the external sodium concentration and was sensitive to 10 mM tetraethylammonium chloride or 5 mM barium chloride. The hyperpolarization-activated inward calcium-sensitive chloride current was activated in a range of potential 20 mV more positive than the potassium current. The estimated reversal potential (-71 mV) was very close to the equilibrium potential for chloride ions ( 73 mV). Intracellularly applied 4-acetamido-4'-isothiocyanostilbene-2,2'-disulfonic acid, 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid and external application of 1 mM zinc chloride, calcium-free saline or high concentrations of intracellular 1,2-bis-(2-aminophenoxy)ethane-N,N,N',N'-tetra-acetate blocked the inward chloride current. Current-clamp experiments indicated that the inward potassium current accounted for inward rectification of dorsal unpaired median neurons. Our findings report, for the first time in pacemaker neurosecretory cells, the co-existence of two distinct hyperpolarization-activated inward currents which have specialized function in pacemaker activity.

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.

Synaptic mechanisms for coding timing in auditory neurons

Neurons in the cochlear ganglion and auditory brain stem nuclei preserve the relative timing of action potentials passed through sequential synaptic levels. To accomplish this task, these neurons have unique morphological and biophysical specializations in axons, dendrites, and nerve terminals. At the membrane level, these adaptations include low-threshold, voltage-gated potassium channels and unusually rapid-acting transmitter-gated channels, which govern how quickly and reliably action potential threshold is reached during a synaptic response. Some nerve terminals are remarkably large and release large amounts of excitatory neurotransmitter. The high output of transmitter at these terminals can lead to synaptic depression, which may itself be regulated by presynaptic transmitter receptors. The way in which these different cellular mechanisms are employed varies in different cell types and circuits and reflects refinements suited to different aspects of acoustic processing.

Features of ipsilaterally evoked inhibition in the dorsal nucleus of the lateral lemniscus

The dorsal nucleus of the lateral lemniscus (DNLL) is a binaural nucleus whose neurons are excited by stimulation of the contralateral ear and inhibited by stimulation of the ipsilateral ear. Here we report on several features of the ipsilaterally evoked inhibition in 95 DNLL neurons of the mustache bat. These features include its dependence on intensity, its tuning and the types of stimuli that are capable of evoking it. Inhibition was studied by evoking discharges with the iontophoretic application of glutamate, and then evaluating the strength and duration of the inhibition of the glutamate evoked background activity produced by stimulation of the ipsilateral ear. Excitatory responses were evoked by stimulation of the contralateral ear with best frequency (BF) tone bursts. Glutamate evoked discharges could be inhibited in all DNLL neurons and the inhibition often persisted for periods ranging from 10 to 50 ms beyond the duration of the tone burst that evoked it. The duration of the persistent inhibition increased with stimulus intensity. Stimulus duration had little influence on the duration of the persistent inhibition. Signals as short as 2 ms suppressed discharges for as long as 30 ms after the signal had ended. The frequency tuning of the total period of inhibition and the period of persistent inhibition were both closely matched to the tuning evoked by stimulation of the contralateral ear. Moreover, the effectiveness of complex signals for evoking persistent inhibition, such as brief FM sweeps and sinusoidally amplitude and frequency modulated signals, was comparable to that of tone bursts at the neuron's excitatory BF, so long as the complex signal contained frequencies at or around the neuron's excitatory BF. We also challenged DNLL cells with binaural paradigms. In one experiment, we presented a relatively long (40 ms) BF tone burst of fixed intensity to the contralateral ear, which evoked a sustained discharge, and a shorter, 10 ms signal of variable intensity to the ipsilateral ear. As the intensity of the 10 ms ipsilateral signal increased, it generated progressively longer periods of persistent inhibition and thus the discharges were suppressed for periods far longer than the 10 ms duration of the ipsilateral signal. With interaural time disparities, ipsilateral signals that led contralateral signals evoked a persistent inhibition that suppressed the responses to the trailing contralateral signals for periods of a least 15 ms. This suggests that an initial binaural sound that favors the ipsilateral ear should suppress the responses to trailing sounds that normally would be excitatory if they were presented alone. We hypothesize a circuit that generates the persistent inhibition and discuss how the results with binaural signals support that hypothesis.

Hyperpolarization-activated cationic currents (Ih) in neurones of the trigeminal mesencephalic nucleus of the rat

1. We studied the voltage-dependent current activated by membrane hyperpolarization in sensory proprioceptive trigeminal mesencephalic nucleus (MNV) neurones. 2. Membrane hyperpolarization (from -62 to -132 mV in 10 mV steps) activated slowly activating and non-inactivating inward currents. The hyperpolarization-activated currents could be described by activation curves with a half-maximal activation potential (V ) of -93 mV, slope (k) of 8.4 mV, and maximally activated currents (Imax) of around 1 nA. The reversal potential of the hyperpolarization-activated currents was -57 mV. 3. Extracellular Cs+ blocked hyperpolarization-activated currents rapidly and reversibly in a concentration-dependent manner with an IC50 of 100 microM and Hill slope of 0.8. ZD7288 (1 microM; 4-(N-ethyl-N-phenylamino)-1,2-dimethyl-6-(methylamino) pyridinium chloride), the compound developed as an inhibitor of the cardiac hyperpolarization-activated current (If), also blocked the hyperpolarization-activated currents in MNV neurones. Extracellular Ba2+ (1 mM) did not affect hyperpolarization-activated currents. We tested whether the hyperpolarization-activated currents contribute to the somatic membrane properties of MNV neurones by performing some experiments using current-clamp recording. In such experiments application of Cs+ (1 mM) produced no effect on neuronal resting membrane potentials. 4. During the course of our experiments we noticed that activating ATP-gated non-selective cation channels (P2X receptors) caused an inhibition of Ih associated with a V shift of 10 mV in the hyperpolarizing direction. This P2X receptor-mediated inhibition of Ih was blocked in recordings made with the rapid calcium chelator BAPTA (11 mM) in the pipette solution. 5. We conclude that the current activated by membrane hyperpolarization in MNV neurones is Ih on the basis of its similarity to Ih observed in other neuronal preparations. Activation of Ih can account for the anomalous time-dependent inward rectification that has previously been described in MNV neurones.