Various subsets of brain neurons express a hyperpolarization-activated inward current ( I h) that has been shown to be instrumental in pacing oscillatory activity at both a single-cell and a network level. A characteristic feature of the stellate cells (SCs) of entorhinal cortex (EC) layer II, those neurons giving rise to the main component of the perforant path input to the hippocampal formation, is their ability to generate persistent, Na+-dependent rhythmic subthreshold membrane potential oscillations, which are thought to be instrumental in implementing theta rhythmicity in the entorhinal-hippocampal network. The SCs also display a robust time-dependent inward rectification in the hyperpolarizing direction that may contribute to the generation of these oscillations. We performed whole cell recordings of SCs in in vitro slices to investigate the specific biophysical and pharmacological properties of the current underlying this inward rectification and to clarify its potential role in the genesis of the subthreshold oscillations. In voltage-clamp conditions, hyperpolarizing voltage steps evoked a slow, noninactivating inward current, which also deactivated slowly on depolarization. This current was identified as I h because it was resistant to extracellular Ba2+, sensitive to Cs+, completely and selectively abolished by ZD7288, and carried by both Na+and K+ ions. I h in the SCs had an activation threshold and reversal potential at approximately −45 and −20 mV, respectively. Its half-activation voltage was −77 mV. Importantly, bath perfusion with ZD7288, but not Ba2+, gradually and completely abolished the subthreshold oscillations, thus directly implicating I h in their generation. Using experimentally derived biophysical parameters for I h and the low-threshold persistent Na+ current ( I NaP) present in the SCs, a simplified model of these neurons was constructed and their subthreshold electroresponsiveness simulated. This indicated that the interplay between I NaP and I h can sustain persistent subthreshold oscillations in SCs. I NaP and I h operate in a “push-pull” fashion where the delay in the activation/deactivation of I h gives rise to the oscillatory process.

In most neurons, Na+ channels in the axon are complemented by others localized in the soma and dendrites to ensure spike back-propagation. However, cerebellar granule cells are neurons with simplified architecture in which the dendrites are short and unbranched and a single thin ascending axon travels toward the molecular layer before bifurcating into parallel fibers. Here we show that in cerebellar granule cells, Na+ channels are enriched in the axon, especially in the hillock, but almost absent from soma and dendrites. The impact of this channel distribution on neuronal electroresponsiveness was investigated by multi-compartmental modeling. Numerical simulations indicated that granule cells have a compact electrotonic structure allowing excitatory postsynaptic potentials to diffuse with little attenuation from dendrites to axon. The spike arose almost simultaneously along the whole axonal ascending branch and invaded the hillock the activation of which promoted spike back-propagation with marginal delay (<200 micros) and attenuation (<20 mV) into the somato-dendritic compartment. These properties allow granule cells to perform sub-millisecond coincidence detection of pre- and postsynaptic activity and to rapidly activate Purkinje cells contacted by the axonal ascending branch.

A multicompartmental biophysical model of entorhinal cortex layer II stellate cells was developed to analyze the ionic basis of physiological properties, such as subthreshold membrane potential oscillations, action potential clustering, and the medium afterhyperpolarization. In particular, the simulation illustrates the interaction of the persistent sodium current (I(Nap)) and the hyperpolarization activated inward current (Ih) in the generation of subthreshold membrane potential oscillations. The potential role of Ih in contributing to the medium hyperpolarization (mAHP) and rebound spiking was studied. The role of Ih and the slow calcium-activated potassium current Ikappa(AHP) in action potential clustering was also studied. Representations of Ih and I(Nap) were developed with parameters based on voltage-clamp data from whole-cell patch and single channel recordings of stellate cells (Dickson et al., J Neurophysiol 83:2562-2579, 2000; Magistretti and Alonso, J Gen Physiol 114:491-509, 1999; Magistretti et al., J Physiol 521:629-636, 1999a; J Neurosci 19:7334-7341, 1999b). These currents interacted to generate robust subthreshold membrane potentials with amplitude and frequency corresponding to data observed in the whole cell patch recordings. The model was also able to account for effects of pharmacological manipulations, including blockade of Ih with ZD7288, partial blockade with cesium, and the influence of barium on oscillations. In a model with a wider range of currents, the transition from oscillations to single spiking, to spike clustering, and finally tonic firing could be replicated. In agreement with experiment, blockade of calcium channels in the model strongly reduced clustering. In the voltage interval during which no data are available, the model predicts that the slow component of Ih does not follow the fast component down to very short time constants. The model also predicts that the fast component of Ih is responsible for the involvement in the generation of subthreshold oscillations, and the slow component dominates in the generation of spike clusters.

Mammalian cochlear inner hair cells (IHCs) are specialized to process developmental signals during immature stages and sound stimuli in adult animals. These signals are conveyed onto auditory afferent nerve fibres. Neurotransmitter release at IHC ribbon synapses is controlled by L-type CaV1.3 Ca2+ channels, the biophysics of which are still unknown in native mammalian cells. We have investigated the localization and elementary properties of Ca2+ channels in immature mouse IHCs under near-physiological recording conditions. CaV1.3 Ca2+ channels at the cell pre-synaptic site co-localize with about half of the total number of ribbons present in immature IHCs. These channels activated at about −70 mV, showed a relatively short first latency and weak inactivation, which would allow IHCs to generate and accurately encode spontaneous Ca2+ action potential activity characteristic of these immature cells. The CaV1.3 Ca2+ channels showed a very low open probability (about 0.15 at −20 mV: near the peak of an action potential). Comparison of elementary and macroscopic Ca2+ currents indicated that very few Ca2+ channels are associated with each docked vesicle at IHC ribbon synapses. Finally, we found that the open probability of Ca2+ channels, but not their opening time, was voltage dependent. This finding provides a possible correlation between presynaptic Ca2+ channel properties and the characteristic frequency/amplitude of EPSCs in auditory afferent fibres.

The functional and biophysical properties of a sustained, or “persistent,” Na+ current (I NaP) responsible for the generation of subthreshold oscillatory activity in entorhinal cortex layer-II principal neurons (the “stellate cells”) were investigated with whole-cell, patch-clamp experiments. Both acutely dissociated cells and slices derived from adult rat entorhinal cortex were used. I NaP , activated by either slow voltage ramps or long-lasting depolarizing pulses, was prominent in both isolated and, especially, in situ neurons. The analysis of the gating properties of the transient Na+ current (I NaT) in the same neurons revealed that the resulting time-independent “window” current (I NaTW) had both amplitude and voltage dependence not compatible with those of the observed I NaP , thus implying the existence of an alternative mechanism of persistent Na+-current generation. The tetrodotoxin-sensitive Na+ currents evoked by slow voltage ramps decreased in amplitude with decreasing ramp slopes, thus suggesting that a time-dependent inactivation was taking place during ramp depolarizations. When ramps were preceded by increasingly positive, long-lasting voltage prepulses, I NaP was progressively, and eventually completely, inactivated. The V1/2 of I NaP steady state inactivation was approximately −49 mV. The time dependence of the development of the inactivation was also studied by varying the duration of the inactivating prepulse: time constants ranging from ∼6.8 to ∼2.6 s, depending on the voltage level, were revealed. Moreover, the activation and inactivation properties of I NaP were such as to generate, within a relatively broad membrane-voltage range, a really persistent window current (I NaPW). Significantly, I NaPW was maximal at about the same voltage level at which subthreshold oscillations are expressed by the stellate cells. Indeed, at −50 mV, the I NaPW was shown to contribute to >80% of the persistent Na+ current that sustains the subthreshold oscillations, whereas only the remaining part can be attributed to a classical Hodgkin-Huxley I NaTW. Finally, the single-channel bases of I NaP slow inactivation and I NaPW generation were investigated in cell-attached experiments. Both phenomena were found to be underlain by repetitive, relatively prolonged late channel openings that appeared to undergo inactivation in a nearly irreversible manner at high depolarization levels (−10 mV), but not at more negative potentials (−40 mV).

Auditory information transfer to afferent neurons relies on precise triggering of neurotransmitter release at the inner hair cell (IHC) ribbon synapses by Ca2+ entry through CaV1.3 Ca2+ channels. Despite the crucial role of CaV1.3 Ca2+ channels in governing synaptic vesicle fusion, their elementary properties in adult mammals remain unknown. Using near-physiological recording conditions we investigated Ca2+ channel activity in adult gerbil IHCs. We found that Ca2+ channels are partially active at the IHC resting membrane potential (−60 mV). At −20 mV, the large majority (>70%) of Ca2+ channel first openings occurred with an estimated delay of about 50 μs in physiological conditions, with a mean open time of 0.5 ms. Similar to other ribbon synapses, Ca2+ channels in IHCs showed a low mean open probability (0.21 at −20 mV), but this increased significantly (up to 0.91) when Ca2+ channel activity switched to a bursting modality. We propose that IHC Ca2+ channels are sufficiently rapid to transmit fast signals of sound onset and support phase-locking. Short-latency Ca2+ channel opening coupled to multivesicular release would ensure precise and reliable signal transmission at the IHC ribbon synapse.

Cerebellar neurones show complex and differentiated mechanisms of action potential generation that have been proposed to depend on peculiar properties of their voltage-dependent Na + currents. In this study we analysed voltage-dependent Na + currents of rat cerebellar granule cells (GCs) by performing whole-cell, patch-clamp experiments in acute rat cerebellar slices. A transient Na + current (I NaT ) was always present and had the properties of a typical fast-activating/inactivating Na + current. In addition to I NaT , robust persistent (I NaP ) and resurgent (I NaR ) Na + currents were observed. I NaP peaked at ∼−40 mV, showed half-maximal activation at ∼−55 mV, and its maximal amplitude was about 1.5% of that of I NaT . I NaR was elicited by repolarizing pulses applied following step depolarizations able to activate/inactivate I NaT , and showed voltage-and time-dependent activation and voltage-dependent decay kinetics. The conductance underlying I NaR showed a bell-shaped voltage dependence, with peak at −35 mV. A significant correlation was found between GC I NaR and I NaT peak amplitudes; however, GCs expressing I NaT of similar size showed marked variability in terms of I NaR amplitude, and in a fraction of cells I NaR was undetectable. I NaT , I NaP and I NaR could be accounted for by a 13-state kinetic scheme comprising closed, open, inactivated and blocked states. Current-clamp experiments carried out to identify possible functional correlates of I NaP and/or I NaR revealed that in GCs single action potentials were followed by depolarizing afterpotentials (DAPs). In a majority of cells, DAPs showed properties consistent with I NaR playing a role in their generation. Computer modelling showed that I NaR promotes DAP generation and enhances high-frequency firing, whereas I NaP boosts near-threshold firing activity. Our findings suggest that special properties of voltage-dependent Na + currents provides GCs with mechanisms suitable for shaping activity patterns, with potentially important consequences for cerebellar information transfer and computation.

Layers II and V of the entorhinal cortex (EC) occupy a privileged anatomical position in the temporal lobe memory system that allows them to gate the main flow of information in and out of the hippocampus, respectively. In vivo studies have shown that layer II of the EC is a robust generator of theta as well as gamma activity. Theta may also be present in layer V, but the layer V network is particularly prone to genesis of short-lasting high-frequency oscillations ("ripples"). Interestingly, in vitro studies have shown that EC layers II and V, but not layer III, have the potential to act as independent pacemakers of population oscillatory activity. Moreover, it has also been shown that subgroups of principal neurons both within layers II and V, but not layer III, are endowed with autorhythmic properties. These are characterized by subthreshold oscillations where the depolarizing phase is driven by the activation of "persistent" Na + channels. We propose that the oscillatory properties of layer II and V neurons and local circuits are responsible for setting up the proper temporal dynamics for the coordination of the multiple sensory inputs that converge onto EC and thus help to generate sensory representations and memory encoding.