Modulation of excitability by alpha-dendrotoxin-sensitive potassium channels in neocortical pyramidal neurons

Axonal sodium channel distribution shapes the depolarized action potential threshold of dentate granule neurons

Intrinsic excitability is a key feature dictating neuronal response to synaptic input. Here we investigate the recent observation that dentate granule neurons exhibit a more depolarized voltage threshold for action potential initiation than CA3 pyramidal neurons. We find no evidence that tonic GABA currents, leak or voltage-gated potassium conductances, or the expression of sodium channel isoform differences can explain this depolarized threshold. Axonal initial segment voltage-gated sodium channels, which are dominated by the Na(V)1.6 isoform in both cell types, distribute more proximally and exhibit lower overall density in granule neurons than in CA3 neurons. To test possible contributions of sodium channel distributions to voltage threshold and to test whether morphological differences participate, we performed simulations of dentate granule neurons and of CA3 pyramidal neurons. These simulations revealed that cell morphology and sodium channel distribution combine to yield the characteristic granule neuron action potential upswing and voltage threshold. Proximal axon sodium channel distribution strongly contributes to the higher voltage threshold of dentate granule neurons for two reasons. First, action potential initiation closer to the somatodendritic current sink causes the threshold of the initiating axon compartment to rise. Second, the proximity of the action potential initiation site to the recording site causes somatic recordings to more faithfully reflect the depolarized threshold of the axon than in cells like CA3 neurons, with distally initiating action potentials. Our results suggest that the proximal location of axon sodium channels in dentate granule neurons contributes to the intrinsic excitability differences between DG and CA3 neurons and may participate in the low-pass filtering function of dentate granule neurons.

Characterization of voltage-gated K+ currents contributing to subthreshold membrane potential oscillations in hippocampal CA1 interneurons

CA1 inhibitory interneurons at the stratum lacunosum-moleculare and radiatum junction (LM/RAD-INs) display subthreshold membrane potential oscillations (MPOs) involving voltage-dependent Na(+) and A-type K(+) currents. LM/RAD-INs also express other voltage-gated K(+) currents, although their properties and role in MPOs remain unclear. Here, we characterized these voltage-gated K(+) currents and investigated their role in MPOs. Using outside-out patch recordings from LM/RAD-IN somata, we distinguished four voltage-gated K(+) currents based on their pharmacology and activation/inactivation properties: a fast delayed rectifier current (I(Kfast)), a slow delayed rectifier current (I(Kslow)), a rapidly inactivating A-type current (I(A)), and a slowly inactivating current (I(D)). Their relative contribution to the total K(+) current was I(A) > I(Kfast) > I(Kslow) = I(D). The presence of I(D) and the relative contributions of K(+) currents in LM/RAD-INs are different from those of other CA1 interneurons, suggesting the presence of differential complement of K(+) currents in subgroups of interneurons. We next determined whether these K(+) currents were sufficient for MPO generation using a single-compartment model of LM/RAD-INs. The model captured the subthreshold voltage dependence of MPOs. Moreover, all K(+) currents were active at subthreshold potentials but I(D), I(A), and the persistent sodium current (I(NaP)) were most active near threshold. Using impedance analysis, we found that I(A) and I(NaP) contribute to MPO generation by modulating peak spectral frequency during MPOs and governing the voltage range over which MPOs occur. Our findings uncover a differential expression of a complement of K(+) channels that underlies intrinsic rhythmic activity in inhibitory interneurons.

Control of submillisecond synaptic timing in binaural coincidence detectors by K(v)1 channels

Neurons in the medial superior olive process sound-localization cues via binaural coincidence detection, in which excitatory synaptic inputs from each ear are segregated onto different branches of a bipolar dendritic structure and summed at the soma and axon with submillisecond time resolution. Although synaptic timing and dynamics critically shape this computation, synaptic interactions with intrinsic ion channels have received less attention. Using paired somatic and dendritic patch-clamp recordings in gerbil brainstem slices together with compartmental modeling, we found that activation of K(v)1 channels by dendritic excitatory postsynaptic potentials (EPSPs) accelerated membrane repolarization in a voltage-dependent manner and actively improved the time resolution of synaptic integration. We found that a somatically biased gradient of K(v)1 channels underlies the degree of compensation for passive cable filtering during propagation of EPSPs in dendrites. Thus, both the spatial distribution and properties of K(v)1 channels are important for preserving binaural synaptic timing.

Electrogenic tuning of the axon initial segment

Action potentials (APs) provide the primary means of rapid information transfer in the nervous system. Where exactly these signals are initiated in neurons has been a basic question in neurobiology and the subject of extensive study. Converging lines of evidence indicate that APs are initiated in a discrete and highly specialized portion of the axon-the axon initial segment (AIS). The authors review key aspects of the organization and function of the AIS and focus on recent work that has provided important insights into its electrical signaling properties. In addition to its main role in AP initiation, the new findings suggest that the AIS is also a site of complex AP modulation by specific types of ion channels localized to this axonal domain.

Participation of Kv1 channels in control of membrane excitability and burst generation in mesencephalic V neurons

The function and biophysical properties of low threshold Kv1 current in control of membrane resonance, subthreshold oscillations, and bursting in mesencephalic V neurons (Mes V) were examined in rat brain stem slices (P8-P12) using whole cell current and voltage patch-clamp methods. alpha-dendrotoxin application, a toxin with high specificity for Kv1.1, 1.2, and 1.6 channels, showed the presence of a low-threshold K(+) current that activated rapidly around -50 mV and was relatively noninactivating over a 1-s period and had a V(1/2)max of -36.2 mV. Other toxins, specific for individual channels containing either Kv 1.1, 1.2, or 1.3 alpha-subunits, were applied individually, or in combination, and showed that Kv1 channels are heteromeric, composed of combinations of subunits. In current-clamp mode, toxin application transformed the high-frequency resonant properties of the membrane into a low-pass filter and concomitantly reduced the frequency of the subthreshold membrane oscillations. During this period, rhythmical bursting was transformed into low-frequency tonic discharge. Interestingly, in a subset of neurons that did not show bursting, low doses of alpha-dendrotoxin (alpha-DTX) sufficient to block 50% of the low threshold Kv1 channels induced bursting and increased the resonant peak impedance and subthreshold oscillations, which was replicated with computer simulation. This suggests that a critical balance between inward and outward currents is necessary for bursting. This was replicated with computer simulation. Single cell RT-PCR and immunohistochemical methods confirmed the presence of Kv1.1, 1.2, and 1.6 alpha-subunits in Mes V neurons. These data indicate that low threshold Kv1 channels are responsible for membrane resonance, contribute to subthreshold oscillations, and are critical for burst generation.

Action potential initiation and propagation: upstream influences on neurotransmission

Axonal action potentials initiate the cycle of synaptic communication that is key to our understanding of nervous system functioning. The field has accumulated vast knowledge of the signature action potential waveform, firing patterns, and underlying channel properties of many cell types, but in most cases this information comes from somatic intracellular/whole-cell recordings, which necessarily measure a mixture of the currents compartmentalized in the soma, dendrites, and axon. Because the axon in many neuron types appears to be the site of lowest threshold for action potential initiation, the channel constellation in the axon is of particular interest. However, the axon is more experimentally inaccessible than the soma or dendrites. Recent studies have developed and applied single-fiber extracellular recording, direct intracellular recording, and optical recording techniques from axons toward understanding the behavior of the axonal action potential. We are starting to understand better how specific channels and other cellular properties shape action potential threshold, waveform, and timing: key elements contributing to downstream transmitter release. From this increased scrutiny emerges a theme of axons with more computational power than in traditional conceptualizations.

Spiral ganglion neurones: an overview of morphology, firing behaviour, ionic channels and function

The spiral ganglion cells provide the afferent innervation of the hair cells of the organ of Corti. Ninety-five percent of these cells (termed type I spiral ganglion neurones) are in synaptic contact with the inner hair cells, whereas about 5% of them are type II cells, which are responsible for the sensory innervation of the outer hair cells. To understand the function of the spiral ganglion neurones, it is important to explore their membrane properties, understand their activity patterns and describe the variety of ionic channels determining their behaviour. In this review, a brief description is given of the various experimental methods that allow the investigation of the spiral ganglion cells, followed by the discussion of their action potential firing patterns and ionic conductances. The presence, distribution and significance of the K(+) currents of the spiral ganglion cells are specifically addressed, along with the introduction of the putative subunit compositions of the relevant voltage-gated K(+) channels.

Ionic channel function in action potential generation: current perspective

Over 50 years ago, Hodgkin and Huxley laid down the foundations of our current understanding of ionic channels. An impressive progress has been made during the following years that culminated in the revelation of the details of potassium channel structure. Nevertheless, even today, we cannot separate well currents recorded in central mammalian neurons. Many modern concepts about the function of sodium and potassium currents are based on experiments performed in nonmammalian cells. The recent recognition of the fast delayed rectifier current indicates that we need to reevaluate the biophysical role of sodium and potassium currents. This review will consider high quality voltage clamp data obtained from the soma of central mammalian neurons in the view of our current knowledge about proteins forming ionic channels. Fast sodium currents and three types of outward potassium currents, the delayed rectifier, the subthreshold A-type, and the D-type potassium currents, are discussed here. An updated current classification with biophysical role of each current subtype is provided. This review shows that details of kinetics of both sodium and outward potassium currents differ significantly from the classical descriptions and these differences may be of functional significance.

Differential effects of isoflurane on A-type and delayed rectifier K channels in rat substantia nigra

The authors previously demonstrated that isoflurane, a widely used volatile anesthetic, induced depolarization and increased the frequency of spontaneous action potentials in principal dopamine neurons in rat substantia nigra pars compacta. We studied the effects of isoflurane on voltage-dependent K channels to clarify the mechanisms of the increase in excitability in these neurons. Voltage-clamp whole-cell recordings were made in rat midbrain slices. We recorded the outward membrane currents in response to depolarizing voltage steps from -120 mV and -25 mV and isolated the transient outward current mediated through A-type K channels by subtraction. Isoflurane at clinically relevant concentrations accelerated the decay of the A-type K current and delayed the recovery from inactivation without changing the steady-state inactivation curves. Isoflurane did not affect the non-inactivating outward current. Addition of 4-aminopyridine partially occluded the excitatory effects of isoflurane in current-clamp recordings. These results demonstrate that isoflurane accelerated the inactivation and delayed the recovery from inactivation of A-type K channels in principal neurons in rat substantia nigra pars compacta without affecting delayed rectifier K channels. These effects may contribute in part to excitation of these neurons and the isoflurane-induced increases in dopamine release reported in vitro and in vivo.

Layer V neurons in mouse cortex projecting to different targets have distinct physiological properties

Layer V pyramidal neurons are anatomically and physiologically heterogeneous and project to multiple intracortical and subcortical targets. However, because most physiological studies of layer V pyramidal neurons have been carried out on unidentified cells, we know little about how anatomical and physiological properties relate to subcortical projection site. Here we combine neuroanatomical tract tracing with whole cell recordings in mouse somatosensory cortex to test whether neurons with the same projection target form discrete subpopulations and whether they have stereotyped physiological properties. Our findings indicate that corticothalamic and -trigeminal neurons are two largely nonoverlapping subpopulations, whereas callosal and corticostriatal neurons overlap extensively. The morphology as well as the intrinsic membrane and firing properties of corticothalamic and corticotrigeminal neurons differ from those of callosal and corticostriatal neurons. In addition, we find that each class of projection neuron exhibits a unique compliment of hyperpolarizing and depolarizing afterpotentials that further suggests that cortical neurons with different subcortical targets are distinct from one another.

Differential regulation of action potential firing in adult murine thalamocortical neurons by Kv3.2, Kv1, and SK potassium and N-type calcium channels

Sensory signals of widely differing dynamic range and intensity are transformed into a common firing rate code by thalamocortical neurons. While a great deal is known about the ionic currents, far less is known about the specific channel subtypes regulating thalamic firing rates. We hypothesized that different K(+) and Ca(2+) channel subtypes control different stimulus-response curve properties. To define the channels, we measured firing rate while pharmacologically or genetically modulating specific channel subtypes. Inhibiting Kv3.2 K(+) channels strongly suppressed maximum firing rate by impairing membrane potential repolarization, while playing no role in the firing response to threshold stimuli. By contrast, inhibiting Kv1 channels with alpha-dendrotoxin or maurotoxin strongly increased firing rates to threshold stimuli by reducing the membrane potential where action potentials fire (V(th)). Inhibiting SK Ca(2+)-activated K(+) channels with apamin robustly increased gain (slope of the stimulus-response curve) and maximum firing rate, with minimum effects on threshold responses. Inhibiting N-type Ca(2+) channels with omega-conotoxin GVIA or omega-conotoxin MVIIC partially mimicked apamin, while inhibiting L-type and P/Q-type Ca(2+) channels had small or no effects. EPSC-like current injections closely mimicked the results from tonic currents. Our results show that Kv3.2, Kv1, SK potassium and N-type calcium channels strongly regulate thalamic relay neuron sensory transmission and that each channel subtype controls a different stimulus-response curve property. Differential regulation of threshold, gain and maximum firing rate may help vary the stimulus-response properties across and within thalamic nuclei, normalize responses to diverse sensory inputs, and underlie sensory perception disorders.

Axon Initial Segment Kv1 Channels Control Axonal Action Potential Waveform and Synaptic Efficacy

Action potentials are binary signals that transmit information via their rate and temporal pattern. In this context, the axon is thought of as a transmission line, devoid of a role in neuronal computation. Here, we show a highly localized role of axonal Kv1 potassium channels in shaping the action potential waveform in the axon initial segment (AIS) of layer 5 pyramidal neurons independent of the soma. Cell-attached recordings revealed a 10-fold increase in Kv1 channel density over the first 50 microm of the AIS. Inactivation of AIS and proximal axonal Kv1 channels, as occurs during slow subthreshold somatodendritic depolarizations, led to a distance-dependent broadening of axonal action potentials, as well as an increase in synaptic strength at proximal axonal terminals. Thus, Kv1 channels are strategically positioned to integrate slow subthreshold signals, providing control of the presynaptic action potential waveform and synaptic coupling in local cortical circuits.

Seizures and reduced life span in mice lacking the potassium channel subunit Kv1.2, but hypoexcitability and enlarged Kv1 currents in auditory neurons

Genes Kcna1 and Kcna2 code for the voltage-dependent potassium channel subunits Kv1.1 and Kv1.2, which are coexpressed in large axons and commonly present within the same tetramers. Both contribute to the low-voltage-activated potassium current I Kv1, which powerfully limits excitability and facilitates temporally precise transmission of information, e.g., in auditory neurons of the medial nucleus of the trapezoid body (MNTB). Kcna1-null mice lacking Kv1.1 exhibited seizure susceptibility and hyperexcitability in axons and MNTB neurons, which also had reduced I Kv1. To explore whether a lack of Kv1.2 would cause a similar phenotype, we created and characterized Kcna2-null mice (-/-). The -/- mice exhibited increased seizure susceptibility compared with their +/+ and +/- littermates, as early as P14. The mRNA for Kv1.1 and Kv1.2 increased strongly in +/+ brain stems between P7 and P14, suggesting the increasing importance of these subunits for limiting excitability. Surprisingly, MNTB neurons in brain stem slices from -/- and +/- mice were hypoexcitable despite their Kcna2 deficit, and voltage-clamped -/- MNTB neurons had enlarged I Kv1. This contrasts strikingly with the Kcna1-null MNTB phenotype. Toxin block experiments on MNTB neurons suggested Kv1.2 was present in every +/+ Kv1 channel, about 60% of +/- Kv1 channels, and no -/- Kv1 channels. Kv1 channels lacking Kv1.2 activated at abnormally negative potentials, which may explain why MNTB neurons with larger proportions of such channels had larger I Kv1. If channel voltage dependence is determined by how many Kv1.2 subunits each contains, neurons might be able to fine-tune their excitability by adjusting the Kv1.1:Kv1.2 balance rather than altering Kv1 channel density.

Selective control of cortical axonal spikes by a slowly inactivating K+ current

Neurons are flexible electrophysiological entities in which the distribution and properties of ionic channels control their behaviors. Through simultaneous somatic and axonal whole-cell recording of layer 5 pyramidal cells, we demonstrate a remarkable differential expression of slowly inactivating K(+) currents. Depolarizing the axon, but not the soma, rapidly activated a low-threshold, slowly inactivating, outward current that was potently blocked by low doses of 4-aminopyridine, alpha-dendrotoxin, and rTityustoxin-K alpha. Block of this slowly inactivating current caused a large increase in spike duration in the axon but only a small increase in the soma and could result in distal axons generating repetitive discharge in response to local current injection. Importantly, this current was also responsible for slow changes in the axonal spike duration that are observed after somatic membrane potential change. These data indicate that low-threshold, slowly inactivating K(+) currents, containing Kv1.2 alpha subunits, play a key role in the flexible properties of intracortical axons and may contribute significantly to intracortical processing.

The action potential in mammalian central neurons

The action potential of the squid giant axon is formed by just two voltage-dependent conductances in the cell membrane, yet mammalian central neurons typically express more than a dozen different types of voltage-dependent ion channels. This rich repertoire of channels allows neurons to encode information by generating action potentials with a wide range of shapes, frequencies and patterns. Recent work offers an increasingly detailed understanding of how the expression of particular channel types underlies the remarkably diverse firing behaviour of various types of neurons.

Effect of interleukin-1beta on I(A) and I(K) currents in cultured murine trigeminal ganglion neurons

To investigate the effect of interleukin-1beta (IL-1beta) on I(A) and I(K) currents in cultured murine trigeminal ganglion (TG) neurons, whole-cell patch clamp technique was used to record the I(A) and I(K) currents before and after 20 ng/mL I(L)-1beta perfusion. Our results showed that 20 ng/mL IL-1beta inhibited I(A) currents (18.3 +/- 10.7)% (n=6, P<0.05). I(L)-1beta at 20 ng/mL had no effect on G-V curve of I(A) but moved the H-infinity curve V0.5 from -36.6+/-6.1 mV to -42.4+/-5.2 mV (n=5, P<0.01). However, 20 ng/mL IL-1beta had effect on neither the amplitude nor the G-V curve of I(K). IL-1beta was found to selectively inhibit I(A) current in TG neurons and the effect may contribute to hyperalgesia under various inflammatory conditions.

Area-specific resonance of excitatory networks in neocortex: control by outward currents

During disinhibition or low [Mg++](o) buffer, 7-14 Hz ( approximately 10 Hz) oscillations are generated by excitatory networks of interconnected pyramidal cells in motor (agranular) cortex but are absent in barrel (granular) cortex. Here we studied if the inability of barrel cortex to produce approximately 10 Hz oscillations during these conditions is because barrel cortex networks lack the necessary cellular mechanisms or, alternatively, because those mechanisms are inhibited by outward currents. The results show that blockers of slowly inactivating voltage-dependent K+ currents unmask approximately 10 Hz oscillations in barrel cortex, and this occurs in unison with the unmasking of intrinsic inward Ca++ currents that are kept suppressed by the outward currents. Moreover, the approximately 10 Hz oscillations unmasked in barrel cortex occur independently in upper and lower layers indicating that the approximately 10 Hz oscillation mechanisms are kept suppressed in multiple networks. The results reveal that the propensity of distinct excitatory networks of neocortex to generate epileptiform oscillatory activities is controlled by outward currents.

Functional Roles of Kv1 Channels in Neocortical Pyramidal Neurons

Pyramidal neurons from layers II/III of somatosensory and motor cortex express multiple Kv1 α-subunits and a current sensitive to block by α-dendrotoxin (α-DTX). We examined functional roles of native Kv1 channels in these cells using current-clamp recordings in brain slices and current- and voltage-clamp recordings in dissociated cells. α-DTX caused a significant negative shift in voltage threshold for action potentials (APs) and reduced rheobase. Correspondingly, a ramp-voltage protocol revealed that the α-DTX–sensitive current activated at subthreshold voltages. AP width at threshold increased with successive APs during repetitive firing. The steady-state threshold width for a given firing rate was similar in control and α-DTX, despite an initially broader AP in α-DTX. AP voltage threshold increased similarly during a train of spikes under control conditions and in the presence of α-DTX. α-DTX had no effect on input resistance or resting membrane potential and modest effects on the amplitude or width of a single AP. Accordingly, experiments using AP waveforms (APWs) as voltage protocols revealed that α-DTX–sensitive current peaked late during the AP repolarization phase. Application of α-DTX increased the rate of firing to intracellular current injection and increased gain (multiplicative effects), but did not alter spike-frequency adaptation. Consistent with these findings, voltage-clamp experiments revealed that the proportion of outward current sensitive to α-DTX was highest during the interval between two APWs, reflecting slow deactivation kinetics at −50 mV. Finally, α-DTX did not alter the selectivity of pyramidal neurons for DC versus time-varying stimuli.

Dendritic D-type potassium currents inhibit the spike afterdepolarization in rat hippocampal CA1 pyramidal neurons

In CA1 pyramidal neurons, burst firing is correlated with hippocampally dependent behaviours and modulation of synaptic strength. One of the mechanisms underlying burst firing in these cells is the afterdepolarization (ADP) that follows each action potential. Previous work has shown that the ADP results from the interaction of several depolarizing and hyperpolarizing conductances located in the soma and the dendrites. By using patch-clamp recordings from acute rat hippocampal slices we show that D-type potassium current modulates the size of the ADP and the bursting of CA1 pyramidal neurons. Sensitivity to alpha-dendrotoxin suggests that Kv1-containing potassium channels mediate this current. Dual somato-dendritic recording, outside-out dendritic recordings, and focal application of dendrotoxin together indicate that the channels mediating this current are located in the apical dendrites. Thus, our data present evidence for a dendritic segregation of Kv1-like channels in CA1 pyramidal neurons and identify a novel action for these channels, showing that they inhibit action potential bursting by restricting the size of the ADP.

Dopamine D1/5 receptor-mediated long-term potentiation of intrinsic excitability in rat prefrontal cortical neurons: Ca2+-dependent intracellular signaling

Prefrontal cortex (PFC) dopamine D1/5 receptors modulate long- and short-term neuronal plasticity that may contribute to cognitive functions. Synergistic to synaptic strength modulation, direct postsynaptic D1/5 receptor activation also modulates voltage-dependent ionic currents that regulate spike firing, thus altering the neuronal input-output relationships in a process called long-term potentiation of intrinsic excitability (LTP-IE). Here, the intracellular signals that mediate this D1/5 receptor-dependent LTP-IE were determined using whole cell current-clamp recordings in layer V/VI rat pyramidal neurons from PFC slices. After blockade of all major amino acid receptors (V(hold) = -65 mV) brief tetanic stimulation (20 Hz) of local afferents or application of the D1 agonist SKF81297 (0.2-50 microM) induced LTP-IE, as shown by a prolonged (>40 min) increase in depolarizing pulse-evoked spike firing. Pretreatment with the D1/5 antagonist SCH23390 (1 microM) blocked both the tetani- and D1/5 agonist-induced LTP-IE, suggesting a D1/5 receptor-mediated mechanism. The SKF81297-induced LTP-IE was significantly attenuated by Cd(2+), [Ca(2+)](i) chelation, by inhibition of phospholipase C, protein kinase-C, and Ca(2+)/calmodulin kinase-II, but not by inhibition of adenylate cyclase, protein kinase-A, MAP kinase, or L-type Ca(2+) channels. Thus this form of D1/5 receptor-mediated LTP-IE relied on Ca(2+) influx via non-L-type Ca(2+) channels, activation of PLC, intracellular Ca(2+) elevation, activation of Ca(2+)-dependent CaMKII, and PKC to mediate modulation of voltage-dependent ion channel(s). This D1/5 receptor-mediated modulation by PKC coexists with the previously described PKA-dependent modulation of K(+) and Ca(2+) currents to dynamically regulate overall excitability of PFC neurons.

Resonance (approximately 10 Hz) of excitatory networks in motor cortex: effects of voltage-dependent ion channel blockers

The motor cortex generates synchronous network oscillations at frequencies between 7 and 14 Hz during disinhibition or low [Mg2+]o buffers, but the underlying mechanisms are poorly understood. These oscillations, termed here approximately 10 Hz oscillations, are generated by a purely excitatory network of interconnected pyramidal cells because they are robust in the absence of GABAergic transmission. It is likely that specific voltage-dependent currents expressed in those cells contribute to the generation of approximately 10 Hz oscillations. We tested the effects of different drugs known to suppress certain voltage-dependent currents. The results revealed that drugs that suppress the low-threshold calcium current and the hyperpolarization-activated cation current are not critically involved in the generation of approximately 10 Hz oscillations. Interestingly, drugs known to suppress the persistent sodium current abolished approximately 10 Hz oscillations. Furthermore, blockers of K+ channels had significant effects on the oscillations. In particular, blockers of the M-current abolished the oscillations. Also, blockers of both non-inactivating and slowly inactivating voltage-dependent K+ currents abolished approximately 10 Hz oscillations. The results indicate that specific voltage-dependent non-inactivating K+ currents, such as the M-current, and persistent sodium currents are critically involved in generating approximately 10 Hz oscillations of excitatory motor cortex networks.

Oscillatory and Intrinsic Membrane Properties of Guinea Pig Nucleus Prepositus Hypoglossi Neurons In Vitro

Numerous models of the oculomotor neuronal integrator located in the prepositus hypoglossi nucleus (PHN) involve both highly tuned recurrent networks and intrinsic neuronal properties; however, there is little experimental evidence for the relative role of these two mechanisms. The experiments reported here show that all PHN neurons (PHNn) show marked phasic behavior, which is highly oscillatory in ∼25% of the population. The behavior of this subset of PHNn, referred to as type D PHNn, is clearly different from that of the medial vestibular nucleus neurons, which transmit the bulk of head velocity-related sensory vestibular inputs without integrating them. We have investigated the firing and biophysical properties of PHNn and developed data-based realistic neuronal models to quantitatively illustrate that their active conductances can produce the oscillatory behavior. Although some individual type D PHNn are able to show some features of mathematical integration, the lack of robustness of this behavior strongly suggests that additional network interactions, likely involving all types of PHNn, are essential for the neuronal integrator. Furthermore, the relationship between the impulse activity and membrane potential of type D PHNn is highly nonlinear and frequency-dependent, even for relatively small-amplitude responses. These results suggest that some of the synaptic input to type D PHNn is likely to evoke oscillatory responses that will be nonlinearly amplified as the spike discharge rate increases. It would appear that the PHNn have specific intrinsic properties that, in conjunction with network interconnections, enhance the persistent neural activity needed for their function.

Recording, analysis, and function of dendritic voltage-gated channels

Ever since the publication of the Hamill et al. [Hamill et al., Pflügers Arch, 391:85-100, 1981] paper and the following increase in popularity of acute brain slice preparations, there has been a large increase in the volume of publications investigating voltage-gated channels in the central nervous system using the patch-clamp technique. In the preceding decade, investigations of voltage-gated channels have moved out of the somatic region into dendrites providing much needed information about dendritic voltage-gated channels. In this study, we review some aspects related to the investigation of voltage-gated ion channels in dendrites: recording, analysis, and function.

Galanin receptor-1 knockout mice exhibit spontaneous epilepsy, abnormal EEGs and altered inhibition in the hippocampus

Galanin is a widely-distributed neuropeptide that acts as an endogenous anticonvulsant. We have recently generated a galanin receptor type 1 knockout mouse (Galr1(-/-)) that develops spontaneous seizures. Our aim here was to characterize the seizures by making electroencephalogram (EEG) recordings from this animal, and also to elucidate the cellular basis of its epileptic phenotype by studying the neurophysiology of CA1 pyramidal neurons in acute hippocampal slices. EEGs showed that major seizures had a partial onset with secondary generalization, and that paroxysms of spike-and-slow waves occurred and were associated with hypoactivity. The interictal EEG was also abnormal, with a marked excess of spike-and-slow waves. Slice experiments showed that resting potential, input resistance, intrinsic excitability, paired-pulse facilitation of excitatory and inhibitory postsynaptic currents (EPSCs and IPSCs), stimulus--response plots for EPSCs, and several properties of spontaneous miniature EPSCs and IPSCs were all unchanged in the mutant mouse compared with wildtype. However, the frequency of miniature IPSCs was significantly reduced in the mutants. These results suggest that impaired synaptic inhibition in the hippocampus may contribute to the local onset of seizures in the Galr1(-/-) mouse.

Independent roles of calcium and voltage-dependent potassium currents in controlling spike frequency adaptation in lateral amygdala pyramidal neurons

The calcium-dependent afterhyperpolarization (AHP) that follows trains of action potentials is responsible for controlling action potential firing patterns in many neuronal cell types. We have previously shown that the slow AHP contributes to spike frequency adaptation in pyramidal neurons in the rat lateral amygdala. In addition, a dendritic voltage-gated potassium current mediated by Kv1.2-containing channels also suppresses action potential firing in these neurons. In this paper we show that this voltage-gated potassium current and the slow AHP act together to control spike frequency adaptation in lateral amygdala pyramidal neurons. The two currents have similar effects on action potential number when firing is evoked either by depolarizing current injections or by synaptic stimulation. However, they differ in their control of firing frequency, with the voltage-gated potassium current but not the slow AHP determining the initial frequency of action potential firing. This dual mechanism of controlling firing patterns is unique to lateral amygdala neurons and is likely to contribute to the very low levels of firing seen in lateral amygdala neurons in vivo.

Expression and biophysical properties of Kv1 channels in supragranular neocortical pyramidal neurones

Potassium channels are extremely diverse regulators of neuronal excitability. As part of an investigation into how this molecular diversity is utilized by neurones, we examined the expression and biophysical properties of native Kv1 channels in layer II/III pyramidal neurones from somatosensory and motor cortex. Single-cell RT-PCR, immunocytochemistry, and whole cell recordings with specific peptide toxins revealed that individual pyramidal cells express multiple Kv1 alpha-subunits. The most abundant subunit mRNAs were Kv1.1 > 1.2 > 1.4 > 1.3. All of these subunits were localized to somatodendritic as well as axonal cell compartments. These data suggest variability in the subunit complexion of Kv1 channels in these cells. The alpha-dendrotoxin (alpha-DTX)-sensitive current activated more rapidly and at more negative potentials than the alpha-DTX-insensitive current, was first observed at voltages near action potential threshold, and was relatively insensitive to holding potential. The alpha-DTX-sensitive current comprised about 10% of outward current at steady-state, in response to steps from -70 mV. From -50 mV, this percentage increased to approximately 20%. All cells expressed an alpha-DTX-sensitive current with slow inactivation kinetics. In some cells a transient component was also present. Deactivation kinetics were voltage dependent, such that deactivation was slow at potentials traversed by interspike intervals during repetitive firing. Because of its kinetics and voltage dependence, the alpha-DTX-sensitive current should be most important at physiological resting potentials and in response to brief stimuli. Kv1 channels should also be important at voltages near threshold and corresponding to interspike intervals.

Kv1 channels selectively prevent dendritic hyperexcitability in rat Purkinje cells

Purkinje cells, the sole output of the cerebellar cortex, encode the timing signals required for motor coordination in their firing rate and activity pattern. Dendrites of Purkinje cells express a high density of P/Q-type voltage-gated calcium channels and fire dendritic calcium spikes. Here we show that dendritic subthreshold Kv1.2 subunit-containing Kv1 potassium channels prevent generation of random spontaneous calcium spikes. With Kv1 channels blocked, dendritic calcium spikes drive bursts of somatic sodium spikes and prevent the cell from faithfully encoding motor timing signals. The selective dendritic function of Kv1 channels in Purkinje cells allows them to effectively suppress dendritic hyperexcitability without hindering the generation of somatic action potentials. Further, we show that Kv1 channels also contribute to dendritic integration of parallel fibre synaptic input. Kv1 channels are often targeted to soma and axon and the data presented support a major dendritic function for these channels.

Serotonin mediates learning-induced potentiation of excitability

Sensitization potentiates excitability in an interneuron, the S-cell, that is critical for this form of learning in the whole-body shortening reflex of the medicinal leech. Serotonin (5-HT) also increases S-cell excitability, and serotonergic modulation is known to be critical for sensitization of whole-body shortening, suggesting that 5-HT mediates learning-induced enhancement of S-cell excitability. In this paper, the role of 5-HT in mediating sensitization-induced potentiation of S-cell excitability was examined. Potentiation of S-cell excitability by 5-HT was blocked by the 5-HT receptor antagonist methysergide and by intracellular injection of the G-protein inhibitor GDP-beta-S, indicating that a metabotropic 5-HT receptor was involved. Bath application of Rp-cAMP, an inhibitor of protein kinase A (PKA), blocked 5-HT-induced potentiation of excitability, whereas db-cAMP, a cAMP analogue that activates PKA, mimicked the potentiating effects of 5-HT on the S-cell. During sensitization of the shortening reflex in semi-intact preparations, methysergide and Rp-cAMP prevented learning-induced potentiation of S-cell excitability, as well as the increase in S-cell activity that normally occurs during sensitization. Furthermore, sensitization-induced increases in the shortening reflex did not occur in preparations treated with methysergide or Rp-cAMP. These results demonstrate that sensitization-induced enhancement of S-cell excitability is mediated by 5-HT and suggests that these changes may contribute to this form of learning.

Constraining compartmental models using multiple voltage recordings and genetic algorithms

Compartmental models with many nonlinearly and nonhomogeneous distributions of voltage-gated conductances are routinely used to investigate the physiology of complex neurons. However, the number of loosely constrained parameters makes manually constructing the desired model a daunting if not impossible task. Recently, progress has been made using automated parameter search methods, such as genetic algorithms (GAs). However, these methods have been applied to somatically recorded action potentials using relatively simple target functions. Using a genetic minimization algorithm and a reduced compartmental model based on a previously published model of layer 5 neocortical pyramidal neurons we compared the efficacy of five cost functions (based on the waveform of the membrane potential, the interspike interval, trajectory density, and their combinations) to constrain the model. When the model was constrained using somatic recordings only, a combined cost function was found to be the most effective. This combined cost function was then applied to investigate the contribution of dendritic and axonal recordings to the ability of the GA to constrain the model. The more recording locations from the dendrite and the axon that were added to the data set the better was the genetic minimization algorithm able to constrain the compartmental model. Based on these simulations we propose an experimental scheme that, in combination with a genetic minimization algorithm, may be used to constrain compartmental models of neurons.

Serotonin modulates axo-axonal coupling between neurons critical for learning in the leech

S cells form a chain of electrically coupled neurons that extends the length of the leech CNS and plays a critical role in sensitization during whole-body shortening. This process requires serotonin, which acts in part by altering the pattern of activity in the S-cell network. Serotonin-containing axons and varicosities were observed in Faivre's nerve where the S-to-S-cell electrical synapses are located. To determine whether serotonin modulates these synapses, S-cell action-potential (AP) propagation was studied in a two-ganglion chain containing one electrical synapse. Suction electrodes were placed on the cut ends of the connectives to stimulate one S cell while recording the other, coupled S cell's APs. A third electrode, placed en passant, recorded the APs near the electrical synapse before they propagated through it. Low concentrations of the gap junction inhibitor octanol increased AP latency across the two-ganglion chain, and this effect was localized to the region of axon containing the electrical synapse. At higher concentrations, APs failed to propagate across the synapse. Serotonin also increased AP latency across the electrical synapse, suggesting that serotonin reduced coupling between S cells. This effect was independent of the direction of propagation and increased with the number of electrical synapses in progressively longer chains. Furthermore, serotonin modulated instantaneous AP frequency when APs were initiated in separate S cells and in a computational model of S-cell activity after mechanosensory input. Thus serotonergic modulation of S-cell electrical synapses may contribute to changes in the pattern of activity in the S-cell network.

Stichodactyla helianthus peptide, a pharmacological tool for studying Kv3.2 channels

Voltage-gated potassium (Kv) channels regulate many physiological functions and represent important therapeutic targets in the treatment of several clinical disorders. Although some of these channels have been well-characterized, the study of others, such as Kv3 channels, has been hindered because of limited pharmacological tools. The current study was initiated to identify potent blockers of the Kv3.2 channel. Chinese hamster ovary (CHO)-K1 cells stably expressing human Kv3.2b (CHO-K1.hKv3.2b) were established and characterized. Stichodactyla helianthus peptide (ShK), isolated from S. helianthus venom and a known high-affinity blocker of Kv1.1 and Kv1.3 channels, was found to potently inhibit 86Rb+ efflux from CHO-K1.hKv3.2b (IC50 approximately 0.6 nM). In electrophysiological recordings of Kv3.2b channels expressed in Xenopus laevis oocytes or in planar patch-clamp studies, ShK inhibited hKv3.2b channels with IC50 values of approximately 0.3 and 6 nM, respectively. Despite the presence of Kv3.2 protein in human pancreatic beta cells, ShK has no effect on the Kv current of these cells, suggesting that it is unlikely that homotetrameric Kv3.2 channels contribute significantly to the delayed rectifier current of insulin-secreting cells. In mouse cortical GABAergic fast-spiking interneurons, however, application of ShK produced effects consistent with the blockade of Kv3 channels (i.e., an increase in action potential half-width, a decrease in the amplitude of the action potential after hyperpolarization, and a decrease in maximal firing frequency in response to depolarizing current injections). Taken together, these results indicate that ShK is a potent inhibitor of Kv3.2 channels and may serve as a useful pharmacological probe for studying these channels in native preparations.

Calcium signal-dependent plasticity of neuronal excitability developed postnatally

Neuronal plasticity and its development were investigated at pyramidal neurons in the cortical slices of rats. The threshold and probability of firing spikes were measured by using whole-cell recording to assess neuronal excitability. Postsynaptic high frequency activity (HFA) at the pyramidal neurons, evoked by 20 trains (250-ms interval) of five depolarization-pulses (1 ms) at 100 Hz, persistently lowered the threshold and increased the probability of firing spikes. After long-term enhancement of neuronal excitability by HFA was stable, another HFA induced further enhancement. Infusing 1 mM 1,2-bis(2-aminophenoxy)-ethane-N, N,N',N'-tetraacetic acid or 100 microM CaMKII(281-301) into the recording neurons prevented HFA-induced long-term enhancement of neuronal excitability. The infusion of 40 microM calcineurin autoinhibitory peptide enhanced neuronal excitability, which occluded HFA effect. HFA-induced long-term enhancement of intrinsic excitability expressed at most pyramidal neurons after postnatal day (PND) 14, but not at those before PND 9. Our results show a new type of neuronal plasticity induced by physiological activity at cortical neurons, which requires calcium-dependent protein phosphorylation and develops during postnatal period. An upregulation of intrinsic excitability at cortical neurons facilitates their activity and broadens signal codes; consequently, their computational ability is upgraded.

Dopamine modulates excitability of basolateral amygdala neurons in vitro

The amygdala plays a role in affective behaviors, which are modulated by the dopamine (DA) innervation of the basolateral amygdala complex (BLA). Although in vivo studies indicate that activation of DA receptors alters BLA neuronal activity, it is unclear whether DA exerts direct effects on BLA neurons or whether it acts via indirect effects on BLA afferents. Using whole cell patch-clamp recordings in rat brain slices, we investigated the site and mechanisms through which DA regulates the excitability of BLA neurons. Dopamine enhanced the excitability of BLA projection neurons in response to somatic current injections via a postsynaptic effect. Dopamine D1 receptor activation increased excitability and evoked firing, whereas D2 receptor activation increased input resistance. Current- and voltage-clamp experiments in projection neurons showed that D1 receptor activation enhanced excitability by modulating a 4-aminopyridine- and alpha-dendrotoxin-sensitive, slowly inactivating K+ current. Furthermore, DA and D1 receptor activation increased evoked firing in fast-spiking BLA interneurons. Consistent with a postsynaptic modulation of interneuron excitability, DA also increased the frequency of spontaneous inhibitory postsynaptic currents recorded in projection neurons without changing release of GABA. These data demonstrate that DA exerts direct effects on BLA projection neurons and indirect actions via modulation of interneurons that may work in concert to enhance the neuronal response to large, suprathreshold inputs, while suppressing weaker inputs.

Contrast enhancement: a physiological effect of striatal dopamine?

Dopamine functions as an important neuromodulator in the dorsal striatum and ventral striatum/nucleus accumbens. Evidence is accumulating for the idea that striatal neurons compete with each other for control over the animal's motor resources, and that dopamine plays an important modulatory role that allows a particular subset of neurons, encoding a specific behavior, to predominate in this competition. One means by which dopamine could facilitate selection among competing neurons is to enhance the contrast between stronger and weaker excitations (or to increase the "signal to noise ratio" among neurons, where the firing of the most excited neurons is assumed to transmit signal and the firing of the least excited to transmit noise). Here, we review the electrophysiological evidence for this hypothesis and discuss potential cellular mechanisms by which dopamine-mediated contrast enhancement could occur.

Long-Term Potentiation of Intrinsic Excitability in LV Visual Cortical Neurons

Neuronal excitability has a large impact on network behavior, and plasticity in intrinsic excitability could serve as an important information storage mechanism. Here we ask whether postsynaptic excitability of layer V pyramidal neurons from primary visual cortex can be rapidly regulated by activity. Whole cell current-clamp recordings were obtained from visual cortical slices, and intrinsic excitability was measured by recording the firing response to small depolarizing test pulses. Inducing neurons to fire at high-frequency (30–40 Hz) in bursts for 5 min in the presence of synaptic blockers increased the firing rate evoked by the test pulse. This long-term potentiation of intrinsic excitability (LTP-IE) lasted for as long as we held the recording (>60 min). LTP-IE was accompanied by a leftward shift in the entire frequency versus current ( F-I) curve and a decrease in threshold current and voltage. Passive neuronal properties were unaffected by the induction protocol, indicating that LTP-IE occurred through modification in voltage-gated conductances. Reducing extracellular calcium during the induction protocol, or buffering intracellular calcium with bis-( o-aminophenoxy)- N,N,N′,N′-tetraacetic acid, prevented LTP-IE. Finally, blocking protein kinase A (PKA) activation prevented, whereas pharmacological activation of PKA both mimicked and occluded, LTP-IE. This suggests that LTP-IE occurs through postsynaptic calcium influx and subsequent activation of PKA. Activity-dependent plasticity in intrinsic excitability could greatly expand the computational power of individual neurons.

Properties of single voltage-dependent K+ channels in dendrites of CA1 pyramidal neurones of rat hippocampus

Voltage-dependent K(+) channels in the apical dendrites of CA1 pyramidal neurones play important roles in regulating dendritic excitability, synaptic integration, and synaptic plasticity. Using cell-attached, voltage-clamp recordings, we found a large variability in the waveforms of macroscopic K(+) currents in the dendrites. With single-channel analysis, however, we were able to identify four types of voltage-dependent K(+) channels and we categorized them as belonging to delayed-rectifier, M-, D-, or A-type K(+) channels previously described from whole-cell recordings. Delayed-rectifier-type K(+) channels had a single-channel conductance of 19 +/- 0.5 pS, and made up the majority of the sustained K(+) current uniformly distributed along the apical dendrites. The M-type K(+) channels had a single-channel conductance of 11 +/- 0.8 pS, did not inactivate with prolonged membrane depolarization, deactivated with slow kinetics (time constant 100 +/- 6 ms at -40 mV), and were inhibited by bath-applied muscarinic agonist carbachol (10 microm). The D-type K(+) channels had a single-channel conductance of around 18 pS, and inactivated with a time constant of 98 +/- 4 ms at +54 mV. The A-type K(+) channels had a single-channel conductance of 6 +/- 0.6 pS, inactivated with a time constant of 23 +/- 2 ms at +54 mV, and contributed to the majority of the transient K(+) current previously described. These results suggest both functional and molecular complexity for K(+) channels in dendrites of CA1 pyramidal neurones.

Presynaptic K+ channels: electrifying regulators of synaptic terminal excitability

Potassium channels are crucial regulators of neuronal excitability, setting resting membrane potentials and firing thresholds, repolarizing action potentials and limiting excitability. Although most of our understanding of K+ channels is based on somatic recordings, there is good evidence that these channels are present in synaptic terminals. In recent years the improved access to presynaptic compartments afforded by direct recording techniques has indicated diverse roles for native K+ channels, from suppression of aberrant firing to action potential repolarization and activity-dependent modulation of synaptic activity. This article reviews the growing evidence for multiple roles and discrete localization of distinct K+ channels at presynaptic terminals.

Kv1.2-containing K+ channels regulate subthreshold excitability of striatal medium spiny neurons

A slowly inactivating, low-threshold K(+) current has been implicated in the regulation of state transitions and repetitive activity in striatal medium spiny neurons. However, the molecular identity of the channels underlying this current and their biophysical properties remain to be clearly determined. Because previous work had suggested this current arose from Kv1 family channels, high-affinity toxins for this family were tested for their ability to block whole cell K(+) currents activated by depolarization of acutely isolated neurons. alpha-Dendrotoxin, which blocks channels containing Kv1.1, Kv1.2, or Kv1.6 subunits, decreased currents evoked by depolarization. Three other Kv1 family toxins that lack a high affinity for Kv1.2 subunits, r-agitoxin-2, dendrotoxin-K, and r-margatoxin, failed to significantly reduce currents, implicating channels with Kv1.2 subunits. RT-PCR results confirmed the expression of Kv1.2 mRNA in identified medium spiny neurons. Currents attributable to Kv1.2 channels activated rapidly, inactivated slowly, and recovered from inactivation slowly. In the subthreshold range (ca. -60 mV), these currents accounted for as much as 50% of the depolarization-activated K(+) current. Moreover, their rapid activation and relatively slow deactivation suggested that they contribute to spike afterpotentials regulating repetitive discharge. This inference was confirmed in current-clamp recordings from medium spiny neurons in the slice preparation where Kv1.2 blockade reduced first-spike latency and increased discharge frequency evoked from hyperpolarized membrane potentials resembling the "down-state" found in vivo. These studies establish a clear functional role for somato-dendritic Kv1.2 channels in the regulation of state transitions and repetitive discharge in striatal medium spiny neurons.

Developmental change in expression and subcellular localization of two shaker-related potassium channel proteins (Kv1.1 and Kv1.2) in the chick tangential vestibular nucleus

The chick tangential nucleus is a major avian vestibular nucleus whose principal cells participate in two vestibular reflexes. Intracellular recordings have shown that the principal cells acquire their mature firing pattern gradually during development. At embryonic day 16 (E16), most principal cells fire a single spike, whereas shortly after hatching (H) the vast majority fire repetitively on depolarization. The transition in firing pattern was likely due in part to a downregulation of a low-threshold, sustained, dendrotoxin-sensitive (DTX) potassium current, I(DS). Since the DTX-sensitive potassium channel subunits Kv1.1 and Kv1.2 generate sustained currents, in the present study we applied fluorescence immunocytochemistry and confocal microscopy to characterize their developmental expression at E16, H1, and H9. At E16, both Kv1.1 and Kv1.2 staining were confined to the principal cell bodies. Immunolabeling decreased significantly for both proteins at H1, and more so by H9. Double-labeling with a monoclonal antibody against microtubule-associated protein 2 (MAP2) in hatchlings showed that some Kv1.1 remained as clusters within the cell body, at the base of the dendrites, and in the axon initial segment. In hatchlings, Kv1.2 staining decreased in the cell bodies and simultaneously appeared in the neuropil, colocalized with biocytin-labeled primary vestibular fibers and vestibular "spoon" terminals. Also, double-labeling with synaptotagmin showed that Kv1.2 colocalized with many nonvestibular terminals surrounding the principal cell bodies. These results identified developmental decreases in the staining of these two potassium channel protein subunits and changes in their subcellular localization corresponding to the downregulation of I(DS) defined electrophysiologically around hatching. Accordingly, both of these protein subunits could be involved in regulating excitability of the principal cells.

Cooperative activation of dopamine D1 and D2 receptors increases spike firing of nucleus accumbens neurons via G-protein betagamma subunits

Dopamine in the nucleus accumbens modulates both motivational and addictive behaviors. Dopamine D1 and D2 receptors are generally considered to exert opposite effects at the cellular level, but many behavioral studies find an apparent cooperative effect of D1 and D2 receptors in the nucleus accumbens. Here, we show that a dopamine-induced enhancement of spike firing in nucleus accumbens neurons in brain slices required both D1 and D2 receptors. One intracellular mechanism that might underlie cooperativity of D1 and D2 receptors is activation of specific subtypes of adenylyl cyclases by G-protein betagamma subunits (Gbetagamma) released from the Gi/o-linked D2 receptor in combination with Galpha(s)-like subunits from the D1 receptor. In this regard, dopaminergic enhancement of spike firing was prevented by inhibitors of protein kinase A or Gbetagamma. Furthermore, intracellular perfusion with Gbetagamma enabled D1 receptor activation but not D2 receptor activation to enhance spike firing. Finally, our data suggest that these pathways may increase spike firing by inhibition of a slow A-type potassium current. These results provide evidence for a novel cellular mechanism through which cooperative action of D1 and D2 receptors in the nucleus accumbens could mediate dopamine-dependent behaviors.

Dopamine D1-class receptors selectively modulate a slowly inactivating potassium current in rat medial prefrontal cortex pyramidal neurons

The dopamine (DA) innervation of medial prefrontal cortex (mPFC) regulates cognitive activity in a complex manner. Alterations of DA function, particularly via the DA D1 receptor class (D1R), are implicated in both schizophrenia and drug addiction, yet the precise roles of DA in modulating mPFC excitability remain unclear. We focused on DA modulation of voltage-gated K(+) current (VGKC) in acutely dissociated rat mPFC pyramidal neurons. We defined three components of the whole-cell VGKC according to biophysical and pharmacological properties. The A-type current (I(A)), with rapid activation and inactivation kinetics, was completely inactivated by prolonged holding of the membrane potential at -40 mV and was sensitive to the K(+) channel blocker 4-aminopyridine (4-AP) but not tetraethylammonium (TEA) or dendrotoxin (DTX). The slowly inactivating K(+) current (I(D)), with rapid activation but relatively slow inactivation, was the major contributor to VGKC and was completely inactivated at -40 mV and sensitive to TEA and DTX but less so to 4-AP. The very slowly inactivating K(+) current (I(K)) was elicited by command steps to more depolarized potentials from a prolonged holding potential of -40 mV and was sensitive to all three blockers. Stimulation of DA D2 receptors failed to alter any component of whole-cell VGKC. Stimulation of DA D1Rs selectively suppressed I(D), an effect mimicked by the adenylyl cyclase activator forskolin, the active cAMP analog Sp-cAMP, and the protein phosphatase inhibitor okadaic acid. Inhibition of protein kinase A (PKA) with either PKI or Rp-cAMP abolished D1R modulation. Thus, the DA D1R/cAMP/PKA signaling pathway mediates modulation of I(D) by DA in rat mPFC pyramidal neurons.

Kv3 potassium conductance is necessary and kinetically optimized for high-frequency action potential generation in hippocampal interneurons

Kv3 channels are thought to be essential for the fast-spiking (FS) phenotype in GABAergic interneurons, but how these channels confer the ability to generate action potentials (APs) at high frequency is unknown. To address this question, we developed a fast dynamic-clamp system (approximately 50 kHz) that allowed us to add a Kv3 model conductance to CA1 oriens alveus (OA) interneurons in hippocampal slices. Selective pharmacological block of Kv3 channels by 0.3 mm 4-aminopyridine or 1 mm tetraethylammonium ions led to a marked broadening of APs during trains of short stimuli and a reduction in AP frequency during 1 sec stimuli. The addition of artificial Kv3 conductance restored the original AP pattern. Subtraction of Kv3 conductance by dynamic clamp mimicked the effects of the blockers. Application of artificial Kv3 conductance also led to FS in OA interneurons after complete K+ channel block and even induced FS in hippocampal pyramidal neurons in the absence of blockers. Adding artificial Kv3 conductance with altered deactivation kinetics revealed a nonmonotonic relationship between mean AP frequency and deactivation rate, with a maximum slightly above the original value. Insertion of artificial Kv3 conductance with either lowered activation threshold or inactivation also led to a reduction in the mean AP frequency. However, the mechanisms were distinct. Shifting the activation threshold induced adaptation, whereas adding inactivation caused frequency-dependent AP broadening. In conclusion, Kv3 channels are necessary for the FS phenotype of OA interneurons, and several of their gating properties appear to be optimized for high-frequency repetitive activity.

Dendrotoxin-sensitive K(+) currents contribute to accommodation in murine spiral ganglion neurons

We have previously identified two broad electrophysiological classes of spiral ganglion neuron that differ in their rate of accommodation (Mo & Davis, 1997a). In order to understand the underlying ionic basis of these characteristic firing patterns, we used alpha-dendrotoxin (alpha-DTX) to eliminate the contribution of a class of voltage-gated K(+) channels and assessed its effects on a variety of electrophysiological properties by using the whole-cell configuration of the patch-clamp technique. Exposure to alpha-DTX caused neurons that initially displayed rapid accommodation to fire continuously during 240 ms depolarizing test pulses within a restricted voltage range. We found a non-monotonic relationship between number of action potentials fired and membrane potential in the presence of alpha-DTX that peaked at voltages between -40 to -10 mV and declined at more depolarized and hyperpolarized test potentials. The alpha-DTX-sensitive current had two components that activated in different voltage ranges. Analysis of recordings made from acutely isolated neurons gave estimated half-maximal activation voltages of -63 and 12 mV for the two components. Because alpha-DTX blocks the Kv1.1, Kv1.2 and Kv1.6 subunits, we examined the action of the Kv1.1-selective blocker dendrotoxin K (DTX-K). We found that this antagonist reproduced the effects of alpha-DTX on neuronal firing, and that the DTX-K-sensitive current also had two separate components. These data suggest that the transformation from a rapidly adapting to a slowly adapting firing pattern was mediated by the low voltage-activated component of DTX-sensitive current with a potential contribution from the high voltage-activated component at more depolarized potentials. In addition, the effects of DTX-K indicate that Kv1.1 subunits are important constituents of the underlying voltage-gated potassium channels.

Gating, modulation and subunit composition of voltage-gated K(+) channels in dendritic inhibitory interneurones of rat hippocampus

GABAergic interneurones are diverse in their morphological and functional properties. Perisomatic inhibitory cells show fast spiking during sustained current injection, whereas dendritic inhibitory cells fire action potentials with lower frequency. We examined functional and molecular properties of K(+) channels in interneurones with horizontal dendrites in stratum oriens-alveus (OA) of the hippocampal CA1 region, which mainly comprise somatostatin-positive dendritic inhibitory cells. Voltage-gated K(+) currents in nucleated patches isolated from OA interneurones consisted of three major components: a fast delayed rectifier K(+) current component that was highly sensitive to external 4-aminopyridine (4-AP) and tetraethylammonium (TEA) (half-maximal inhibitory concentrations < 0.1 mM for both blockers), a slow delayed rectifier K(+) current component that was sensitive to high concentrations of TEA, but insensitive to 4-AP, and a rapidly inactivating A-type K(+) current component that was blocked by high concentrations of 4-AP, but resistant to TEA. The relative contributions of these components to the macroscopic K(+) current were estimated as 57 +/- 5, 25 +/- 6, and 19 +/- 2 %, respectively. Dendrotoxin, a selective blocker of Kv1 channels had only minimal effects on K(+) currents in nucleated patches. Coapplication of the membrane-permeant cAMP analogue 8-(4-chlorophenylthio)-adenosine 3':5'-cyclic monophosphate (cpt-cAMP) and the phosphodiesterase blocker isobutyl-methylxanthine (IBMX) resulted in a selective inhibition of the fast delayed rectifier K(+) current component. This inhibition was absent in the presence of the protein kinase A (PKA) inhibitor H-89, implying the involvement of PKA-mediated phosphorylation. Single-cell reverse transcription-polymerase chain reaction (RT-PCR) analysis revealed a high abundance of Kv3.2 mRNA in OA interneurones, whereas the expression level of Kv3.1 mRNA was markedly lower. Similarly, RT-PCR analysis showed a high abundance of Kv4.3 mRNA, whereas Kv4.2 mRNA was undetectable. This suggests that the fast delayed rectifier K(+) current and the A-type K(+) current component are mediated predominantly by homomeric Kv3.2 and Kv4.3 channels. Selective modulation of Kv3.2 channels in OA interneurones by cAMP is likely to be an important factor regulating the activity of dendritic inhibitory cells in principal neurone-interneurone microcircuits.

The role of Kv1.2-containing potassium channels in serotonin-induced glutamate release from thalamocortical terminals in rat frontal cortex

Serotonin 5-HT(2A) receptors have been implicated in psychiatric illness and the psychotomimetic effects of hallucinogens. In brain slices, focal stimulation of 5-HT(2A) receptors in rat prefrontal cortex results in dramatically increased glutamate release onto layer V pyramidal neurons, as measured by an increase in "spontaneous" (nonelectrically evoked) EPSCs. This glutamate release is blocked by tetrodotoxin (TTX) and is thought to involve local spiking in thalamocortical axon terminals; however, the detailed mechanism has remained unclear. Here, we investigate parallels in EPSCs induced by either serotonin or the potassium channel blockers 4-aminopyridine (4-AP) or alpha-dendrotoxin (DTX). DTX, a selective blocker of Kv1.1-, Kv1.2-, and Kv1.6-containing potassium channels, has been shown to release glutamate in cortical synaptosomes, presumably by inhibiting a subthreshold-activated, slowly inactivating potassium conductance. By comparing DTX with other potassium channel blockers, we found that the ability to induce EPSCs in cortical pyramidal neurons depends on affinity for Kv1.2 subunits. DTX-induced EPSCs are similar to 5-HT-induced EPSCs in terms of sensitivity to TTX and omega-agatoxin-IVA (a blocker of P-type calcium channels) and laminar selectivity. The involvement of thalamocortical terminals in DTX-induced EPSCs was confirmed by suppression of these EPSCs by micro-opiates and thalamic lesions. More directly, DTX-induced EPSCs substantially occlude those induced by 5-HT, suggesting a common mechanism of action. No occlusion by DTX was seen when EPSCs were induced by a nicotinic mechanism. These results indicate that blockade of Kv1.2-containing potassium channels is part of the mechanism underlying 5-HT-induced glutamate release from thalamocortical terminals.