Evidence for a Slowly Inactivating K+ Current in Prefrontal Cortical Cells

Comparative Effects of Domain-Specific Human Monoclonal Antibodies Against LGI1 on Neuronal Excitability

BACKGROUND AND OBJECTIVES

Autoantibodies to leucine-rich glioma inactivated protein 1 (LGI1) cause an autoimmune limbic encephalitis with frequent focal seizures and anterograde memory dysfunction. LGI1 is a neuronal secreted linker protein with 2 functional domains: the leucine-rich repeat (LRR) and epitempin (EPTP) regions. LGI1 autoantibodies are known to interfere with presynaptic function and neuronal excitability; however, their epitope-specific mechanisms are incompletely understood.

METHODS

We used patient-derived monoclonal autoantibodies (mAbs), which target either LRR or EPTP domains of LGI1 to investigate long-term antibody-induced alteration of neuronal function. LRR- and EPTP-specific effects were evaluated by patch-clamp recordings in cultured hippocampal neurons and compared with biophysical neuron modeling. Kv1.1 channel clustering at the axon initial segment (AIS) was quantified by immunocytochemistry and structured illumination microscopy techniques.

RESULTS

Both EPTP and LRR domain-specific mAbs decreased the latency of first somatic action potential firing. However, only the LRR-specific mAbs increased the number of action potential firing together with enhanced initial instantaneous frequency and promoted spike-frequency adaptation, which were less pronounced after the EPTP mAb. This also led to an effective reduction in the slope of ramp-like depolarization in the subthreshold response, suggesting Kv1 channel dysfunction. A biophysical model of a hippocampal neuron corroborated experimental results and suggests that an isolated reduction of the conductance of Kv1-mediated K+ currents largely accounts for the antibody-induced alterations in the initial firing phase and spike-frequency adaptation. Furthermore, Kv1.1 channel density was spatially redistributed from the distal toward the proximal site of AIS under LRR mAb treatment and, to a lesser extant, under EPTP mAb.

DISCUSSION

These findings indicate an epitope-specific pathophysiology of LGI1 autoantibodies. The pronounced neuronal hyperexcitability and SFA together with dropped slope of ramp-like depolarization after LRR-targeted interference suggest disruption of LGI1-dependent clustering of K+ channel complexes. Moreover, considering the effective triggering of action potentials at the distal AIS, the altered spatial distribution of Kv1.1 channel density may contribute to these effects through impairing neuronal control of action potential initiation and synaptic integration.

Modulatory effects of inhibition on persistent activity in a cortical microcircuit model

Neocortical network activity is generated through a dynamic balance between excitation, provided by pyramidal neurons, and inhibition, provided by interneurons. Imbalance of the excitation/inhibition ratio has been identified in several neuropsychiatric diseases, such as schizophrenia, autism and epilepsy, which also present with other cognitive deficits and symptoms associated with prefrontal cortical (PFC) dysfunction. We undertook a computational approach to study how changes in the excitation/inhibition balance in a PFC microcircuit model affect the properties of persistent activity, considered the cellular correlate of working memory function in PFC. To this end, we constructed a PFC microcircuit, consisting of pyramidal neuron models and all three different interneuron types: fast-spiking (FS), regular-spiking (RS), and irregular-spiking (IS) interneurons. Persistent activity was induced in the microcircuit model with a stimulus to the proximal apical dendrites of the pyramidal neuron models, and its properties were analyzed, such as the induction profile, the interspike intervals (ISIs) and neuronal synchronicity. Our simulations showed that (a) the induction but not the firing frequency or neuronal synchronicity is modulated by changes in the NMDA-to-AMPA ratio on FS interneuron model, (b) removing or decreasing the FS model input to the pyramidal neuron models greatly limited the biophysical modulation of persistent activity induction, decreased the ISIs and neuronal synchronicity during persistent activity, (c) the induction and firing properties could not be altered by the addition of other inhibitory inputs to the soma (from RS or IS models), and (d) the synchronicity change could be reversed by the addition of other inhibitory inputs to the soma, but beyond the levels of the control network. Thus, generic somatic inhibition acts as a pacemaker of persistent activity and FS specific inhibition modulates the output of the pacemaker.

An Approximation to the Adaptive Exponential Integrate-and-Fire Neuron Model Allows Fast and Predictive Fitting to Physiological Data

For large-scale network simulations, it is often desirable to have computationally tractable, yet in a defined sense still physiologically valid neuron models. In particular, these models should be able to reproduce physiological measurements, ideally in a predictive sense, and under different input regimes in which neurons may operate in vivo. Here we present an approach to parameter estimation for a simple spiking neuron model mainly based on standard f-I curves obtained from in vitro recordings. Such recordings are routinely obtained in standard protocols and assess a neuron's response under a wide range of mean-input currents. Our fitting procedure makes use of closed-form expressions for the firing rate derived from an approximation to the adaptive exponential integrate-and-fire (AdEx) model. The resulting fitting process is simple and about two orders of magnitude faster compared to methods based on numerical integration of the differential equations. We probe this method on different cell types recorded from rodent prefrontal cortex. After fitting to the f-I current-clamp data, the model cells are tested on completely different sets of recordings obtained by fluctuating ("in vivo-like") input currents. For a wide range of different input regimes, cell types, and cortical layers, the model could predict spike times on these test traces quite accurately within the bounds of physiological reliability, although no information from these distinct test sets was used for model fitting. Further analyses delineated some of the empirical factors constraining model fitting and the model's generalization performance. An even simpler adaptive LIF neuron was also examined in this context. Hence, we have developed a "high-throughput" model fitting procedure which is simple and fast, with good prediction performance, and which relies only on firing rate information and standard physiological data widely and easily available.

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.

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.

The principal features and mechanisms of dopamine modulation in the prefrontal cortex

Mesocortical [corrected] dopamine (DA) inputs to the prefrontal cortex (PFC) play a critical role in normal cognitive process and neuropsychiatic pathologies. This DA input regulates aspects of working memory function, planning and attention, and its dysfunctions may underlie positive and negative symptoms and cognitive deficits associated with schizophrenia. Despite intense research, there is still a lack of clear understanding of the basic principles of actions of DA in the PFC. In recent years, there has been considerable efforts by many groups to understand the cellular mechanisms of DA modulation of PFC neurons. However, the results of these efforts often lead to contradictions and controversies. One principal feature of DA that is agreed by most researchers is that DA is a neuromodulator and is clearly not an excitatory or inhibitory neurotransmitter. The present article aims to identify certain principles of DA mechanisms by drawing on published, as well as unpublished data from PFC and other CNS sites to shed light on aspects of DA neuromodulation and address some of the existing controversies. Eighteen key features about DA modulation have been identified. These points directly impact on the end result of DA neuromodulation, and in some cases explain why DA does not yield identical effects under all experimental conditions. It will become apparent that DA's actions in PFC are subtle and depend on a variety of factors that can no longer be ignored. Some of these key factors include distinct bell-shaped dose-response profiles of postsynaptic DA effects, different postsynaptic responses that are contingent on the duration of DA receptor stimulation, prolonged duration effects, bidirectional effects following activation of D1 and D2 classes of receptors and membrane potential state and history dependence of subsequent DA actions. It is hoped that these factors will be borne in mind in future research and as a result a more consistent picture of DA neuromodulation in the PFC will emerge. Based on these factors, a theory is proposed for DA's action in PFC. This theory suggests that DA acts to expand or contract the breadth of information held in working memory buffers in PFC networks.

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.

Corticostriatal plasticity: life after the depression

Experience-dependent changes in corticostriatal transmission efficacy are likely to support the role of the striatum in reinforcement-based motor learning. Whereas long-term depression at glutamatergic corticostriatal synapses has long been regarded as the normal form of striatal plasticity, recent work provides evidence that use-dependent potentiation can naturally occur at these connections through an increase in both synaptic efficacy and postsynaptic intrinsic excitability. By decreasing the weight of cortical inputs required to fire striatal output neurons, short-term and long-term potentiation at corticostriatal connections can jointly participate in the formation of sensorimotor links by which specific context-dependent patterns of cortical activity can engage selected motor programs.

Low-Probability Transmission of Neocortical and Entorhinal Impulses Through the Perirhinal Cortex

One model of episodic memory posits that during slow-wave sleep (SWS), the synchronized discharges of hippocampal neurons in relation to sharp waves “replay” activity patterns that occurred during the waking state, facilitating synaptic plasticity in the neocortex. Although evidence of replay was found in the hippocampus in relation to sharp waves, it was never shown that this activity reached the neocortex. Instead, it was assumed that the rhinal cortices faithfully transmit information from the hippocampus to the neocortex and reciprocally. Here, we tested this idea using 3 different approaches. 1) Stimulating electrodes were inserted in the entorhinal cortex and temporal neocortex and evoked unit responses were recorded in between them, in the intervening rhinal cortices. In these conditions, impulse transfer occurred with an extremely low probability, in both directions. 2) To rule out the possibility that this unreliable transmission resulted from the artificial nature of electrical stimuli, crosscorrelation analyses of spontaneous neocortical, perirhinal, and entorhinal firing were performed in unanesthetized animals during the states of waking and SWS. Again, little evidence of propagation could be obtained in either state. 3) To test the idea that propagation occurs only when large groups of neurons are activated within a narrow time window, we computed perievent histograms of neocortical, perirhinal, and entorhinal neuronal discharges around large-amplitude sharp waves. However, these synchronized entorhinal discharges also failed to propagate across the perirhinal cortex. These findings suggest that the rhinal cortices are more than a relay between the neocortex and hippocampus, but rather a gate whose properties remain to be identified.

Mechanisms of dopamine activation of fast-spiking interneurons that exert inhibition in rat prefrontal cortex

Prefrontal cortical dopamine (DA) modulates pyramidal cell excitability directly and indirectly by way of its actions on local circuit GABAergic interneurons. DA modulation of interneuronal functions is implicated in the computational properties of prefrontal networks during cognitive processes and in schizophrenia. Morphologically and electrophysiologically distinct classes of putative GABAergic interneurons are found in layers II-V of rat prefrontal cortex. Our whole cell patch-clamp study shows that DA induced a direct, TTX-insensitive, reversible membrane depolarization, and increased the excitability of fast-spiking (FS) interneurons. The DA-induced membrane depolarization was reduced significantly by D1/D5 receptor antagonist SCH 23390, but not by the D2 receptor antagonist (-)sulpiride, D4 receptor antagonists U101958 or L-745870, alpha1-adrenoreceptor antagonist prazosin, or serotoninergic receptor antagonist mianserin. The D1/5 agonists SKF81297 or dihydrexidine, but not D2 agonist quinpirole, also induced a prolonged membrane depolarization. Voltage-clamp analyses of the voltage-dependence of DA-sensitive currents, and the effects of changing [K(+)](O) on reversal potentials of DA responses, revealed that DA suppressed a Cs(+)-sensitive inward rectifier K(+) current and a resting leak K(+) current. D1/D5, but not D2 agonists mimicked the suppressive effects of DA on the leak current, but the DA effects on the inward rectifier K(+) current were not mimicked by either agonist. In a subgroup of FS interneurons, the slowly inactivating membrane outward rectification evoked by depolarizing voltage steps was also attenuated by DA. Collectively, these data showed that DA depolarizes FS interneurons by suppressing a voltage-independent 'leak' K(+) current (via D1/D5 receptor mechanism) and an inwardly rectifying K(+) current (via unknown DA mechanisms). Additional suppression of a slowly inactivating K(+) current led to increase in repetitive firing in response to depolarizing inputs. This D1-induced increase in interneuron excitability enhances GABAergic transmission to PFC pyramidal neurons and could represent a mechanism via which DA suppresses persistent firing of pyramidal neurons in vivo.

A model of prefrontal cortex dopaminergic modulation during the delayed alternation task

Working memory performance is modulated by the level of dopamine (DA) D1 receptors stimulation in the prefrontal cortex (PFC). This modulation is exerted at different time scales. Injection of D1 agonists/antagonists exerts a long-lasting influence (several minutes or hours) on PFC pyramidal neurons. In contrast, during performance of a cognitive task, the duration of the postsynaptic effect of phasic DA release is short lasting. The functional relationships of these two time scales of DA modulation remain poorly understood. Here we propose a model that combines these two time scales of DA modulation on a prefrontal neural network. The model links the cellular and behavioral levels during performance of the delayed alternation task. The network, which represents the activity of deep-layer pyramidal neurons with intrinsic neuronal properties, exhibits two stable states of activity that can be switched on and off by excitatory inputs from long-distance cortical areas arriving in superficial layers. These stable states allow PFC neurons to maintain representations during the delay period. The role of an increase of DA receptors stimulation is to restrict inputs arriving on the prefrontal network. The model explains how the level of working memory performance follows an inverted U-shape with an increased stimulation of DA D1 receptors. The model predicts that (1) D1 receptor agonists increase perseverations, (2) D1 antagonists increase distractability, and (3) the duration of the postsynaptic effect of phasic DA release in the PFC is adjusted to the delay period of the task. These results show how the precise duration of the postsynaptic effect of phasic DA release influences behavioral performance during a simple cognitive task.

Dopamine controls fundamental cognitive operations of multi-target spatial working memory

This study addresses computationally how the prefrontal cortical circuit performs operations of multiple items in spatial working memory. The basic idea is that dopamine controls the circuit dynamics for the operations by changing the ratio of the NMDA-channel transmission to the AMPA-channel transmission. There is evidence that this ratio is a function of dopamine D1 receptor activation. The simulation shows that the model circuit performs several different operations of multi-target spatial working memory depending on this ratio. When the ratio is low, 'replacement' occurs from the previously loaded target to a new one. In intermediate levels of the ratio, a new target is 'added' to the previously loaded target, resulting in the coexistence of more than one target. For higher ratios, the circuit 'rejects' other succeedingly received target stimuli. This study suggests four important issues: First, the cortical circuit can perform operations of multi-target spatial working memory. Second, the circuit can switch the modes of the operations by changing the NMDA-to-AMPA ratio. Third, dopamine would have major roles in the operations of multi-target spatial working memory. Fourth, the intracortical inhibition (especially of the cross-directional) plays an important role in regulating the competition between targets.

An integrative theory of the phasic and tonic modes of dopamine modulation in the prefrontal cortex

This paper presents a model of both tonic and phasic dopamine (DA) effects on maintenance of working memory representations in the prefrontal cortex (PFC). The central hypothesis is that DA modulates the efficacy of inputs to prefrontal pyramidal neurons to prevent interferences for active maintenance. Phasic DA release, due to DA neurons discharges, acts at a short time-scale (a few seconds), while the tonic mode of DA release, independent of DA neurons firing, acts at a long time-scale (a few minutes). The overall effect of DA modulation is modeled as a threshold restricting incoming inputs arriving on PFC neurons. Phasic DA release temporary increases this threshold while tonic DA release progressively increases the basal level of this threshold. Thus, unlike the previous gating theory of phasic DA release, proposing that it facilitates incoming inputs at the time of their arrival, the effect of phasic DA release is supposed to restrict incoming inputs during a period of time after DA neuron discharges. The model links the cellular and behavioral levels during performance of a working memory task. It allows us to understand why a critical range of DA D1 receptors stimulation is required for optimal working memory performance and how D1 receptor agonists (respectively antagonists) increase perseverations (respectively distractability). Finally, the model leads to several testable predictions, including that the PFC regulates DA neurons firing rate to adapt to the delay of the task and that increase in tonic DA release may either improve or decrease performance, depending on the level of DA receptors stimulation at the beginning of the task.

Intrinsic properties of rat striatal output neurones and time-dependent facilitation of cortical inputs in vivo

In vivo intracellular recordings were performed from striatal output neurones (SONs) (n = 34) to test the role of their intrinsic membrane properties in the temporal integration of excitatory cortical synaptic inputs.

In a first series of experiments, intracellular injection of a test depolarising current pulse was preceded by a 200 ms suprathreshold prepulse, the two pulses having the same intensity. An increase in intrinsic excitability was observed as a decrease (55 ± 21 ms, n = 13) (mean ± s.d.) in latency to the first action potential of the test response compared to the prepulse response. This value decayed exponentially as a function of the time interval between the current pulses (τ= 364 ± 37 ms, n = 5). The voltage response of SONs was not modified by a prepulse that induced a membrane depolarisation < −62 mV.

The effect of the suprathreshold prepulse was tested on monosynaptic cortically evoked excitatory postsynaptic potentials (EPSPs). The ability to induce suprathreshold EPSPs was markedly increased by the prior depolarisation (n = 11 cells). This facilitation decayed progressively as a function of the time intervals between prepulses and cortical stimuli. The potentiation was not observed on small EPSPs reaching a peak potential < −65 mV (n = 3).

We conclude that SONs can optimise cortical information transfer by modifying their intrinsic excitability as a function of their past activation. It is likely that this time-dependent facilitation results, at least in part, from the kinetics of a striatal slowly inactivating potassium current available around −60 mV that recovers slowly from inactivation.

Role of a striatal slowly inactivating potassium current in short-term facilitation of corticostriatal inputs: a computer simulation study

Striatal output neurons (SONs) integrate glutamatergic synaptic inputs originating from the cerebral cortex. In vivo electrophysiological data have shown that a prior depolarization of SONs induced a short-term (</=1 sec) increase in their membrane excitability, which facilitated the ability of corticostriatal synaptic potentials to induce firing. Here we propose, using a computational model of SONs, that the use-dependent, short-term increase in the responsiveness of SONs mainly results from the slow kinetics of a voltage-dependent, slowly inactivating potassium A-current. This mechanism confers on SONs a form of intrinsic short-term memory that optimizes the synaptic input-output relationship as a function of their past activation.

A model study of cellular short-term memory produced by slowly inactivating potassium conductances

We analyzed the cellular short-term memory effects induced by a slowly inactivating potassium (Ks) conductance using a biophysical model of a neuron. We first described latency-to-first-spike and temporal changes in firing frequency as a function of parameters of the model, injected current and prior history of the neuron (deinactivation level) under current clamp. This provided a complete set of properties describing the Ks conductance in a neuron. We then showed that the action of the Ks conductance is not generally appropriate for controlling latency-to-first-spike under random synaptic stimulation. However, reliable latencies were found when neuronal population computation was used. Ks inactivation was found to control the rate of convergence to steady-state discharge behavior and to allow frequency to increase at variable rates in sets of synaptically connected neurons. These results suggest that inactivation of the Ks conductance can have a reliable influence on the behavior of neuronal populations under real physiological conditions.

Dopamine-mediated stabilization of delay-period activity in a network model of prefrontal cortex

The prefrontal cortex (PFC) is critically involved in working memory, which underlies memory-guided, goal-directed behavior. During working-memory tasks, PFC neurons exhibit sustained elevated activity, which may reflect the active holding of goal-related information or the preparation of forthcoming actions. Dopamine via the D1 receptor strongly modulates both this sustained (delay-period) activity and behavioral performance in working-memory tasks. However, the function of dopamine during delay-period activity and the underlying neural mechanisms are only poorly understood. Recently we proposed that dopamine might stabilize active neural representations in PFC circuits during tasks involving working memory and render them robust against interfering stimuli and noise. To further test this idea and to examine the dopamine-modulated ionic currents that could give rise to increased stability of neural representations, we developed a network model of the PFC consisting of multicompartment neurons equipped with Hodgkin-Huxley-like channel kinetics that could reproduce in vitro whole cell and in vivo recordings from PFC neurons. Dopaminergic effects on intrinsic ionic and synaptic conductances were implemented in the model based on in vitro data. Simulated dopamine strongly enhanced high, delay-type activity but not low, spontaneous activity in the model network. Furthermore the strength of an afferent stimulation needed to disrupt delay-type activity increased with the magnitude of the dopamine-induced shifts in network parameters, making the currently active representation much more stable. Stability could be increased by dopamine-induced enhancements of the persistent Na(+) and N-methyl-D-aspartate (NMDA) conductances. Stability also was enhanced by a reduction in AMPA conductances. The increase in GABA(A) conductances that occurs after stimulation of dopaminergic D1 receptors was necessary in this context to prevent uncontrolled, spontaneous switches into high-activity states (i.e., spontaneous activation of task-irrelevant representations). In conclusion, the dopamine-induced changes in the biophysical properties of intrinsic ionic and synaptic conductances conjointly acted to highly increase stability of activated representations in PFC networks and at the same time retain control over network behavior and thus preserve its ability to adequately respond to task-related stimuli. Predictions of the model can be tested in vivo by locally applying specific D1 receptor, NMDA, or GABA(A) antagonists while recording from PFC neurons in delayed reaction-type tasks with interfering stimuli.

Physiological properties of central medial and central lateral amygdala neurons

Mounting evidence implicates the central (CE) nucleus of the amygdala in the mediation of classically conditioned fear responses. However, little data are available regarding the intrinsic membrane properties of CE amygdala neurons. Here, we characterized the physiological properties of CE medial (CE(M)) and CE lateral (CE(L)) amygdala neurons using whole cell recordings in brain slices maintained in vitro. Several classes of CE neurons were distinguished on the basis of their physiological properties. Most CE(M) cells (95%), here termed "late-firing neurons," displayed a marked voltage- and time-dependent outward rectification in the depolarizing direction. This phenomenon was associated with a conspicuous delay between the onset of depolarizing current pulses and the first action potential. During this delay, the membrane potential (V(m)) depolarized slowly, the steepness of this depolarizing ramp increasing as the prepulse V(m) was hyperpolarized from -60 to -90 mV. Low extracellular concentrations of 4-aminopyridine (30 microM) reversibly abolished the outward rectification and the delay to firing. Late-firing CE(M) neurons displayed a continuum of repetitive firing properties with cells generating single spikes at one pole and high-frequency (> or =90 Hz) spike bursts at the other. In contrast, only 56% of CE(L) cells displayed the late-firing behavior prevalent among CE(M) neurons. Moreover, these CE(L) neurons only generated single spikes in response to membrane depolarization. A second major class of CE(L) cells (38%) lacked the characteristic delay to firing observed in CE(M) cells, generated single spikes in response to membrane depolarization, and displayed various degrees of inward rectification in the hyperpolarizing direction. In both regions of the CE nucleus, two additional cell types were encountered infrequently (< or =6% of our samples). One type of neurons, termed "low-threshold bursting cells" had a behavior reminiscent of thalamocortical neurons. The second type of cells, called "fast-spiking cells," generated brief action potentials at high rates with little spike frequency adaptation in response to depolarizing current pulses. These findings indicate that the CE nucleus contains several types of neurons endowed with distinct physiological properties. Moreover, these various cell types are not distributed uniformly in the medial and lateral sector of the CE nucleus. This heterogeneity parallels anatomic data indicating that these subnuclei are part of different circuits.

Modulation of a slowly inactivating potassium current, I(D), by metabotropic glutamate receptor activation in cultured hippocampal pyramidal neurons

I(D) is a slowly inactivating 4-aminopyridine (4-AP)-sensitive potassium current of hippocampal pyramidal neurons and other CNS neurons. Although I(D) exerts multifaceted influence on CNS excitability, whether I(D) is subject to modulation by neurotransmitters or neurohormones has not been clear. We report here that one prominent effect of metabotropic glutamate receptor (mGluR) activation by short (3 min) exposure to 1S, 3R-1-aminocyclopentane-1,3-dicarboxylic acid (1S,3R-ACPD) (100 microM) is suppression of I(D) by acceleration of its inactivation. I(D) was identified as a target of mGluR-mediated modulation because inactivation of a component of outward current sensitive to 100-200 microM 4-AP was accelerated by 1S,3R-ACPD, and because 4-AP occluded any further actions of 1S,3R-ACPD. Enhancement of I(D) inactivation was induced by the group I-preferring agonist RS-3, 5-dihydroxyphenylglycine (3,5-DHPG) and the group II-preferring agonist 2S,2'R,3'R)-2-(2',3'dicarboxycyclopropyl)-glycine (DCG-IV), but not by the group III-preferring agonist L(+)-2-amino-4-phosphonobutyric acid (L-AP4); it was blocked by the broadly acting mGluR antagonist S-alpha-methyl-4-carboxyphenylglycine (S-MCPG). Furthermore, inactivation of I(D) was enhanced by inclusion of GTPgammaS in the internal solution and blocked by inclusion of GDPbetaS. Metabotropic GluR-induced suppression of I(D) was manifest in three aspects of excitability previously linked to I(D) by their sensitivity to 4-AP: reduction in input conductance and enhanced excitability at voltages just positive to the resting potential, reduced delay to action potential firing during depolarizing current injections, and delayed action potential repolarization. We suggest that mGluR-induced suppression of I(D) could contribute to enhancement of hippocampal neuron excitability and synaptic connections.

Effect of antipsychotic drugs and selective dopaminergic antagonists on dopamine-induced facilitatory activity in prelimbic cortical pyramidal neurons. An in vitro study

Intracellular recordings were obtained from 119 pyramidal neurons localized in prelimbic cortex, five in the dorsal cingulate cortex, one in the infralimbic cortex, one in the border of prelimbic and cingulate cortex and two in the border of prelimbic and infralimbic cortex. The passive membrane properties of these pyramidal neurons (i.e. resting membrane potential, input membrane resistance, shape of the tetrodotoxin-sensitive action potentials, spike frequency adaptation with a prominent postspike afterhyperpolarization, tetrodotoxin-sensitive inward rectification in the depolarizing direction and the absence of bursting) suggested that they resembled regular spiking or intrinsically bursting pyramidal neurons. Bath application of dopamine (EC50 of 1.8 microM) produced a reversible facilitatory effect on all 119 pyramidal neurons localized in the middle layer of the prelimbic cortex. No consistent change in membrane potential was detected during the application of dopamine. No effect of dopamine was noted on the nine pyramidal neurons that were not localized in the prelimbic cortex. The facilitatory effect of dopamine in prelimbic cortex was concentration dependently antagonized by haloperidol, risperidone, quetiapine, clozapine and by the selective D4 dopaminergic receptor antagonist L-745,870, but not by the selective D2/D3 dopaminergic receptor antagonist (-)-sulpiride. (+)-SCH 23390, which is a selective D1/D5 dopamine receptor antagonist, produced, similarly to dopamine, a facilitatory effect per se, and an additive effect when co-administered with dopamine. These results provide evidence that dopamine has a facilitatory effect specifically on pyramidal neurons localized in the middle layer of prelimbic cortex. Antipsychotic drugs and L-745,870 block this effect of dopamine.

Reduction in potassium currents in identified cutaneous afferent dorsal root ganglion neurons after axotomy

Potassium currents have an important role in modulating neuronal excitability. We have investigated the effects of axotomy on three voltage-activated K(+) currents, one sustained and two transient, in cutaneous afferent dorsal root ganglion (DRG) neurons. Fourteen to 21 days after axotomy, L(4) and L(5) DRG neurons were acutely dissociated and were studied 2-8 h after plating. Whole cell patch-clamp recordings were obtained from identified cutaneous afferent neurons (46-50 microm diam); K(+) currents were isolated by blocking Na(+) and Ca(2+) currents with appropriate ion replacement and channel blockers. Separation of the current components was achieved on the basis of sensitivity to dendrotoxin or 4-aminopyridine and by the response to variation in conditioning voltage. Both control and injured neurons displayed qualitatively similar complex K(+) currents composed of distinct kinetic and pharmacological components. Three distinct K(+) current components, a sustained (I(K)) and two transient (I(A) and I(D)), were identified in variable proportions. However, total peak current was reduced by 52% in the axotomized cells when compared with control cells. Two current components were reduced after ligation, I(A) by 60%, I(K) by over 65%, compared with control cells. I(D) appeared unaffected after acute ligation. These results indicate a large reduction in overall K(+) current, resulting from reductions in I(K) and I(A), on large cutaneous afferent neurons after nerve ligation and have implications for excitability changes of injured primary afferent neurons.

A model of visuospatial working memory in prefrontal cortex: recurrent network and cellular bistability

We report a computer simulation of the visuospatial delayed-response experiments of Funahashi et al. (1989), using a firing-rate model that combines intrinsic cellular bistability with the recurrent local network architecture of the neocortex. In our model, the visuospatial working memory is stored in the form of a continuum of network activity profiles that coexist with a spontaneous activity state. These neuronal firing patterns provide a population code for the cue position in a graded manner. We show that neuronal persistent activity and tuning curves of delay-period activity (memory fields) can be generated by an excitatory feedback circuit and recurrent synaptic inhibition. However, if the memory fields are constructed solely by network mechanisms, noise may induce a random drift over time in the encoded cue position, so that the working memory storage becomes unreliable. Furthermore, a "distraction" stimulus presented during the delay period produces a systematic shift in the encoded cue position. We found that the working memory performance can be rendered robust against noise and distraction stimuli if single neurons are endowed with cellular bistability (presumably due to intrinsic ion channel mechanisms) that is conditional and realized only with sustained synaptic inputs from the recurrent network. We discuss how cellular bistability at the single cell level may be detected by analysis of spike trains recorded during delay-period activity and how local modulation of intrinsic cell properties and/or synaptic transmission can alter the memory fields of individual neurons in the prefrontal cortex.

Activity-dependent regulation of 'on' and 'off' responses in cat visual cortical receptive fields

1. A supervised learning procedure was applied to individual cat area 17 neurons to test the possible role of neuronal co-activity in controlling the plasticity of the spatial 'on-off' organization of visual cortical receptive fields (RFs). 2. Differential pairing between visual input evoked in a fixed position of the RF and preset levels of postsynaptic firing (imposed iontophoretically) were used alternately to boost the 'on' (or 'off') response to a 'high' level of firing (S+ pairing), and to reduce the opponent response (respectively 'off' or 'on') in the same position to a 'low' level (S- pairing). This associative procedure was repeated 50-100 times at a low temporal frequency (0.1-0.15 s-1). 3. Long-lasting modifications of the ratio of 'on-off' responses, measured in the paired position or integrated across the whole RF, were found in 44 % of the conditioned neurons (17/39), and in most cases this favoured the S+ paired characteristic. The amplitude change was on average half of that imposed during pairing. Comparable proportions of modified cells were obtained in 'simple' (13/27) and 'complex' (4/12) RFs, both in adult cats (4/11) and in kittens within the critical period (13/28). 4. The spatial selectivity of the pairing effects was studied by pseudorandomly stimulating both paired and spatially distinct unpaired positions within the RF. Most modifications were observed in the paired position (for 88 % of successful pairings). 5. In some cells (n = 13), a fixed delay pairing procedure was applied, in which the temporal phase of the onset of the current pulse was shifted by a few hundred milliseconds from the presentation or offset of the visual stimulus. Consecutive effects were observed in 4/13 cells, which retained the temporal pattern of activity imposed during pairing for 5-40 min. They were expressed in the paired region only. 6. The demonstration of long-lasting adaptive changes in the ratio of 'on' and 'off' responses, expressed in localized subregions of the RF, leads us to suggest that simple and complex RF organizations might be two stable functional states derived from a common connectivity scheme.

Morphologically identified cutaneous afferent DRG neurons express three different potassium currents in varying proportions

Outward K+ currents were recorded using a whole cell patch-clamp configuration, from acutely dissociated adult rat cutaneous afferent dorsal root ganglion (DRG) neurons (L4 and L5) identified by retrograde labeling with Fluoro-gold. Recordings were obtained 16-24 h after dissociation from cells between 39 and 49 mm in diameter with minimal processes. These cells represent medium-sized DRG neurons relative to the entire population, but are large cutaneous afferent neurons giving rise to myelinated axons. Voltage-activated K+ currents were recorded routinely during 300-ms depolarizing test pulses increasing in 10-mV steps from -40 to +50 mV; the currents were preceded by a 500-ms conditioning prepulse of either -120 or -40 mV. Coexpression of at least three components of K+ current was revealed. Separation of these components was achieved on the basis of sensitivities to the K+ channel blockers, 4-aminopyridine (4-AP) and dendrotoxin (DTx), and by the current responses to variation in conditioning voltage. Changing extracellular K+ concentration from 3 to 40 mM resulted in a shift to the right of the I-V curve commensurate with K+ being the principal charge carrier. Presentation of 100 mM 4-AP revealed a rapidly activating K+ current sensitive to low concentrations of 4-AP. High concentrations of 4-AP (6 mM) extinguished all inactivating current, leaving almost pure sustained current (IK). On the basis of the relative distribution of K+ currents neurons could be separated into three distinct categories: fast inactivating current (IA), slow inactivating current (ID), and sustained current (IK); only IA and IK; and slow inactivating current and IK. However, IK was always the dominant outward current component. These results indicate that considerable variation in K+ currents is present not only in the entire population of DRG neurons, as previously reported, but even within a restricted size and functional group (large cutaneous afferent neurons).

Biophysical characterization and functional consequences of a slowly inactivating potassium current in neostriatal neurons

Neostriatal spiny projection neurons can display a pronounced delay in their transition to action potential discharge that is mediated by a slowly developing ramp depolarization. The possible contribution of a slowly inactivating A-type K+ current (IAs) to this delayed excitation was investigated by studying the biophysical and functional properties of IAs using whole cell voltage- and current-clamp recording from acutely isolated neostriatal neurons. Isolation of IAs from other voltage-gated, calcium-independent K+ currents was achieved through selective blockade of IAs with low concentrations (10 microM) of the benzazepine derivative, 6-chloro-7,8-dihydroxy-3-allyl- 1-phenyl-2,3,4,5-tetra-hydro-1H-3-benzazepine (APB; SKF82958) and subsequent current subtraction. Examination of the voltage dependence of activation showed that IAs began to flow at approximately -60 mV in response to depolarization. The voltage dependence of inactivation revealed that approximately 50% of IAs channels were available at the normal resting potential (-80 mV) of these cells, but that only 20% of the channels were available at membrane potentials corresponding to spike threshold (about -40 mV). At these depolarized membrane potentials, the rate of activation was moderately rapid (tau approximately 60 ms), whereas the rate of inactivation was slow (tau approximately 1.5 s). The time course of removal of inactivation of IAs at -80 mV also was relatively slow (tau approximately 1.0 s). The subthreshold availability of IAs combined with its rapid activation and slow inactivation rates suggested that this current should be capable of dampening the onset of prolonged depolarizing responses, but over time its efficacy should diminish, slowly permitting the membrane to depolarize toward spike threshold. Voltage recording experiments confirmed this hypothesis by demonstrating that application of APB at a concentration (10 microM) that selectively blocks IAs substantially decreased the latency to discharge and increased the frequency of firing of neostriatal neurons. The properties of IAs suggest that it should play a critical role in placing the voltage limits on the recurring episodes of subthreshold depolarization which are characteristic of spiny neurons recorded in vivo. However, the voltage dependence and recovery kinetics of inactivation of IAs predict that its effectiveness will vary exponentially with the level and duration of hyperpolarization which precedes depolarizing episodes. Thus long periods of hyperpolarization should increase the availability of IAs and dampen succeeding depolarizations; whereas brief epochs of hyperpolarization should not sufficiently remove inactivation of IAs, thereby reducing its ability to limit subsequent depolarizing responses.

Role of voltage-gated K+ currents in mediating the regular-spiking phenotype of callosal-projecting rat visual cortical neurons

Role of voltage-gated K+ currents in mediating the regular-spiking phenotype of callosal-projecting rat visual cortical neurons. J. Neurophysiol. 78: 2321-2335, 1997. Whole cell current- and voltage-clamp recordings were combined to examine action potential waveforms, repetitive firing patterns, and the functional roles of voltage-gated K+ currents (IA, ID, and IK) in identified callosal-projecting (CP) neurons from postnatal (day 7-13) rat primary visual cortex. Brief (1 ms) depolarizing current injections evoke single action potentials in CP neurons with mean +/- SD (n = 60) durations at 50 and 90% repolarization of 1.9 +/- 0.5 and 5.5 +/- 2.0 ms, respectively; action potential durations in individual cells are correlated inversely with peak outward current density. During prolonged threshold depolarizing current injections, CP neurons fire repetitively, and two distinct, noninterconverting "regular-spiking" firing patterns are evident: weakly adapting CP cells fire continuously, whereas strongly adapting CP cells cease firing during maintained depolarizing current injections. Action potential repolarization is faster and afterhyperpolarizations are more pronounced in strongly than in weakly adapting CP cells. In addition, input resistances are lower and plateau K+ current densities are higher in strongly than in weakly adapting CP cells. Functional studies reveal that blockade of ID reduces the latency to firing an action potential, and increases action potential durations at 50 and 90% repolarization. Blockade of ID also increases firing rates in weakly adapting cells and results in continuous firing of strongly adapting cells. After applications of millimolar concentrations of 4-aminopyridine to suppress IA (as well as block ID), action potential durations at 50 and 90% repolarization are further increased, and firing rates are accelerated over those observed when only ID is blocked. Using VClamp/CClamp and the voltage-clamp data in the preceding paper, mathematical descriptions of IA, ID, and IK are generated and a model of the electrophysiological properties of rat visual cortical CP neurons is developed. The model is used to simulate the firing properties of strongly adapting and weakly adapting CP cells and to explore the functional roles of IA, ID, and IK in shaping the waveforms of individual action potentials and controlling the repetitive firing properties of these cells.

Three kinetically distinct Ca2+-independent depolarization-activated K+ currents in callosal-projecting rat visual cortical neurons

Three kinetically distinct Ca2+-independent depolarization-activated K+ currents in callosal-projecting rat visual cortical neurons. J. Neurophysiol. 78: 2309-2320, 1997. Whole cell, Ca2+-independent, depolarization-activated K+ currents were characterized in identified callosal-projecting (CP) neurons isolated from postnatal day 7-16 rat primary visual cortex. CP neurons were identified in vitro after in vivo retrograde labeling with fluorescently tagged latex microbeads. During brief (160-ms) depolarizing voltage steps to potentials between -50 and +60 mV, outward K+ currents in these cells activate rapidly and inactivate to varying degrees. Three distinct K+ currents were separated based on differential sensitivity to 4-aminopyridine (4-AP); these are referred to here as IA, ID, and IK, because their properties are similar (but not identical) K+ currents termed IA, ID, and IK in other cells. The current sensitive to high (>/=100 mu M) concentrations of 4-AP (IA) activates and inactivates rapidly; the current blocked completely by low (</=50 mu M) 4-AP (ID) activates rapidly and inactivates slowly. A slowly activating, slowly inactivating current (IK) remains in the presence of 5 mM 4-AP. IA, ID, and IK also were separated and characterized in experiments that did not rely on the use of 4-AP. All CP cells express all three K+ current types, although the relative densities of IA, ID, and IK vary among cells. The experiments here also have revealed that IA, ID, and IK display similar voltage dependences of activation and steady state inactivation, whereas the kinetic properties of the currents are distinct. At +30 mV, for example, mean +/- SD activation taus are 0. 83 +/- 0.24 ms for IA, 1.74 +/- 0.49 ms for ID, and 14.7 +/- 4.0 ms for IK. Mean +/- SD inactivation taus for IA and ID are 26 +/- 7 ms and 569 +/- 143 ms, respectively. Inactivation of IK is biexponential with mean +/- SD inactivation time constants of 475 +/- 232 ms and 3,128 +/- 1,328 ms; approximately 20% of the 4-AP-insensitive current is noninactivating. For all three components, activation is voltage dependent, increasing with increasing depolarization, whereas inactivation is voltage independent. Both IA and IK recover rapidly from steady state inactivation with mean +/- SD recovery time constants of 38 +/- 7 ms and 79 +/- 26 ms, respectively; ID recovers an order of magnitude more slowly (588 +/- 274 ms). The properties of IA, ID, and IK in CP neurons are compared with those of similar currents described previously in other mammalian central neurons and, in the accompanying paper, the roles of these conductances in regulating the firing properties of CP neurons are explored.

Calcium-dependent, slowly inactivating potassium currents in cultured neurons of rat neocortex

Slowly inactivating outward currents were examined in neurons from rat anterior cortex dissociated at postnatal day 1 and recorded after 7-48 days in vitro by the use of whole-cell patch-clamp technique, in the presence of 0.5-0.8 microM tetrodotoxin (TTX). 50 microM carbachol and 1-5 mM CsCl2. Experiments were often carried out in the additional presence of 1-5 mM CsCl2, which blocks the anomalous, inwardly rectifying IQ, the fast Ca(2+)-dependent K+ current (IC), and 50 microM carbachol, which depresses the IM current. These currents were evoked by depolarizing steps to -40 +/- 5 mV from a conditioning hyperpolarization to -110 +/- 10 mV. Their sensitivity to elevation from 2.5 to 12.5 mM in extracellular K+ concentration, together with their sensitivity to 5-15 mM tetraethylammonium, suggests that they are mainly carried by K+ ions. Their activation and inactivation curves show that the threshold for activation is -65 mV, that their inactivation is achieved at -75 mV and that potentials more negative than -120 mV are needed to abolish it. The time-dependence of de-inactivation gives a maximal current amplitude for conditioning hyperpolarizations of 2 s and is best described by a monoexponential function with a time constant of 0.7 s. Slow transient K+ currents were depressed by low doses of 4-aminopyridine (30-100 microM), which indicates the occurrence of an ID-type component in the recorded K+ currents. No slowly declining K+ current was expressed when a recording solution containing 10 mM 1,2-bis (2-aminophenoxy)ethane-N,N,N'-N'-tetraacetic acid (BAPTA), instead of 1-5 mM BAPTA, was used. When recorded without Ca2+ chelator in the pipette, slowly declining K+ currents were blocked by bath-applied 40-50 microM BAPTA-aminoethoxy, revealing a large-amplitude, rapidly inactivating outward current. This residual component is insensitive to 50 microM 4-aminopyridine and may include a current more related to the IA-type. Our data provide evidence that, in cultured cortical neurons from rat, the expression of an ID-like K+ current is highly dependent on internal Ca2+ concentration.

Whole-cell analysis of NGK2 (mKv3.1a) K+ channels stably expressed in mouse fibroblast cells

NGK2 (mKv3.1a) K+ channel cDNA was introduced into mouse B82 fibroblast cells to express in a mammalian system. The NGK2 current in the stably transformed fibroblast cells exhibited a high threshold for activation and slow decay with two components. The data suggest that the NGK2 channel may contribute to slowly inactivating K+ currents observed in excitable and inexcitable cells.

Outward potassium currents activated by depolarization in rat globus pallidus

Voltage-dependent potassium currents play a key role in shaping the firing pattern of central neurons. Their pharmacological and physiological identification is rather important in the structures which are involved in the filtering of input/output messages. In this regard, globus pallidus external segment (GPe) is indicated as a crucial station in the well-known indirect pathway of the basal ganglia. Among the potassium conductances which have been indicated to condition the firing behavior and the neuronal integrative properties in many central neurons, we analysed the depolarization-activated ones by means of patch-clamp recordings in the whole-cell configuration. Two main families of calcium-independent outward potassium currents are activated by depolarization in GPe neurons acutely isolated from the adult rat. From depolarized holding potentials (-50/-45 mV), a slowly-activating, sustained current is evoked; it manifests very little inactivation and it is available at rather depolarized potentials (-30 mV/-20 mV). This current is relatively resistant to 4-aminopyridine (4-AP) but it is blocked by tetraethilammonium ions (TEA) and consequently it resembles delayed rectifier current (Ik). From negative holding potentials (-80/-100 mV), on the other hand, A-like conductances are activated. Together with a fast-inactivating transient current, another component is observed in a significant proportion of recordings (45%). This current shows half-inactivation voltage around -90 mV, peculiar sensitivity to micromolar doses of 4-AP and a slow rate of recovery from inactivation. The presence and the modulation of these A-like currents may be a very critical aspect in the membrane physiology of pallidal neurons.

Neural network models of cortical functions based on the computational properties of the cerebral cortex

We describe a biologically plausible modelling framework based on the architectural and processing characteristics of the cerebral cortex. Its key feature is a multicellular processing unit (cortical column) reflecting the modular nature of cortical organization and function. In this framework, we describe a neural network model organization and function. In this framework, we describe a neural network model of the neuronal circuits of the cerebral cortex that learn different functions associated with different parts of the cortex: 1) visual integration for invariant pattern recognition, performed by a cooperation between temporal and parietal areas; 2) visual-to-motor transformation for 3D arm reaching movements, performed by parietal and motor areas; and 3) temporal integration and storage of sensorimotor programs, performed by networks linking the prefrontal cortex to associative sensory and motor areas. The architecture of the network is inspired from the features of the architecture of cortical pathways involved in these functions. We propose two rules which describe neural processing and plasticity in the network. The first rule (adaptive tuning if gating) is an analog of operant conditioning and permits to learn to anticipate an action. The second rule (adaptive timing) is based on a bistable state of activity and permits to learn temporally separate events forming a behavioral sequence.