Association of respiratory failure with inhibition of NaV1.6 in the phrenic nerve

As part of a drug discovery effort to identify potent inhibitors of NaV1.7 for the treatment of pain, we observed that inhibitors produced unexpected cardiovascular and respiratory effects in vivo. Specifically, inhibitors administered to rodents produced changes in cardiovascular parameters and respiratory cessation. We sought to determine the mechanism of the in vivo adverse effects by studying the selectivity of the compounds on NaV1.5, NaV1.4, and NaV1.6 in in vitro and ex vivo assays. Inhibitors lacking sufficient NaV1.7 selectivity over NaV1.6 were associated with respiratory cessation after in vivo administration to rodents. Effects on respiratory rate in rats were consistent with effects in an ex vivo hemisected rat diaphragm model and in vitro NaV1.6 potency. Furthermore, direct blockade of the phrenic nerve signaling was observed at exposures known to cause respiratory cessation in rats. Collectively, these results support a significant role for NaV1.6 in phrenic nerve signaling and respiratory function.

Translational Pharmacokinetic–Pharmacodynamic Modeling of NaV1.7 Inhibitor MK-2075 to Inform Human Efficacious Dose

MK-2075 is a small-molecule selective inhibitor of the NaV1.7 channel investigated for the treatment of postoperative pain. A translational strategy was developed for MK-2075 to quantitatively interrelate drug exposure, target modulation, and the desired pharmacological response in preclinical animal models for the purpose of human translation. Analgesics used as a standard of care in postoperative pain were evaluated in preclinical animal models of nociceptive behavior (mouse tail flick latency and rhesus thermode heat withdrawal) to determine the magnitude of pharmacodynamic (PD) response at plasma concentrations associated with efficacy in the clinic. MK-2075 was evaluated in those same animal models to determine the concentration of MK-2075 required to achieve the desired level of response. Translation of MK-2075 efficacious concentrations in preclinical animal models to a clinical PKPD target in humans was achieved by accounting for species differences in plasma protein binding and in vitro potency against the NaV1.7 channel. Estimates of human pharmacokinetic (PK) parameters were obtained from allometric scaling of a PK model from preclinical species and used to predict the dose required to achieve the clinical exposure. MK-2075 exposure–response in a preclinical target modulation assay (rhesus olfaction) was characterized using a computational PKPD model which included a biophase compartment to account for the observed hysteresis. Translation of this model to humans was accomplished by correcting for species differences in PK NaV1.7 potency, and plasma protein binding while assuming that the kinetics of distribution to the target site is the same between humans and rhesus monkeys. This enabled prediction of the level of target modulation anticipated to be achieved over the dosing interval at the projected clinical efficacious human dose. Integration of these efforts into the early development plan informed clinical study design and decision criteria.

Development of ProTx-II Analogues as Highly Selective Peptide Blockers of Nav1.7 for the Treatment of Pain

Inhibitor cystine knot peptides, derived from venom, have evolved to block ion channel function but are often toxic when dosed at pharmacologically relevant levels in vivo. The article describes the design of analogues of ProTx-II that safely display systemic in vivo blocking of Na_v1.7, resulting in a latency of response to thermal stimuli in rodents. The new designs achieve a better in vivo profile by improving ion channel selectivity and limiting the ability of the peptides to cause mast cell degranulation. The design rationale, structural modeling, in vitro profiles, and rat tail flick outcomes are disclosed and discussed.

Nav1.7 target modulation and efficacy can be measured in nonhuman primate assays

Humans with loss-of-function mutations in the Na_v1.7 channel gene (SCN9A) show profound insensitivity to pain, whereas those with gain-of-function mutations can have inherited pain syndromes. Therefore, inhibition of the Na_v1.7 channel with a small molecule has been considered a promising approach for the treatment of various human pain conditions. To date, clinical studies conducted using selective Na_v1.7 inhibitors have not provided analgesic efficacy sufficient to warrant further investment. Clinical studies to date used multiples of in vitro IC₅₀ values derived from electrophysiological studies to calculate anticipated human doses. To increase the chance of clinical success, we developed rhesus macaque models of action potential propagation, nociception, and olfaction, to measure Na_v1.7 target modulation in vivo. The potent and selective Na_v1.7 inhibitors SSCI-1 and SSCI-2 dose-dependently blocked C-fiber nociceptor conduction in microneurography studies and inhibited withdrawal responses to noxious heat in rhesus monkeys. Pharmacological Na_v1.7 inhibition also reduced odor-induced activation of the olfactory bulb (OB), measured by functional magnetic resonance imaging (fMRI) studies consistent with the anosmia reported in Na_v1.7 loss-of-function patients. These data demonstrate that it is possible to measure Na_v1.7 target modulation in rhesus macaques and determine the plasma concentration required to produce a predetermined level of inhibition. The calculated plasma concentration for preclinical efficacy could be used to guide human efficacious exposure estimates. Given the translatable nature of the assays used, it is anticipated that they can be also used in phase 1 clinical studies to measure target modulation and aid in the interpretation of phase 1 clinical data.

Pharmacological validation of a novel nonhuman primate measure of thermal responsivity with utility for predicting analgesic effects

Introduction

The development of novel analgesics to treat acute or chronic pain has been a challenge due to a lack of translatable measurements. Preclinical end points with improved translatability are necessary to more accurately inform clinical testing paradigms, which may help guide selection of viable drug candidates.

Methods

In this study, a nonhuman primate biomarker which is sensitive to standard analgesics at clinically relevant plasma concentrations, can differentiate analgesia from sedation and utilizes a protocol very similar to that which can be employed in human clinical studies is described. Specifically, acute heat stimuli were delivered to the volar forearm using a contact heat thermode in the same manner as the clinical setting.

Results

Clinically efficacious exposures of morphine, fentanyl, and tramadol produced robust analgesic effects, whereas doses of diazepam that produce sedation had no effect.

Conclusion

We propose that this assay has predictive utility that can help improve the probability of success for developing novel analgesics.

Development of a pharmacodynamic biomarker to measure target engagement from inhibition of the NGF-TrkA pathway

BACKGROUND

NGF signaling through TrkA triggers pathways involved in a wide range of biological effects. Clinical trials targeting either NGF or TrkA are ongoing to treat various diseases in the areas of oncology, neuroscience, and for pain, but there is no described measure of target engagement of TrkA in these studies.

NEW METHOD

We have developed custom ELISA assays to measure NGF-induced phosphorylation of TrkA specific for rodent and human receptors. Optimized tissue processing methods allow for detection in both the brain and in skin. In addition, TrkB and TrkC assays have been in established to evaluate selectivity against other neurotrophin receptors.

RESULTS

In a preclinical NGF-induced pain model, we show that pre-dosing with a TrkA inhibitor prevents phosphorylation of TrkA in the skin at a dose that is efficacious in reversal of thermal hypersensitivity. In addition, we show data in non-human primate and human skin supporting the potential use of this approach to enable translational target engagement. Comparison with existing methods: Existing methods involve animal models expressing TrkA tumors or injection of over-expressing TrkA recombinant cells into animals. Our method can measure target engagement in both normal and disease tissues in preclinical animal models and human skin.

CONCLUSIONS

We have developed methods to assess target engagement for drug programs aimed at disrupting NGF-induced TrkA signaling. This includes preclinical determination of selectivity against other neurotrophin receptors and estimation of functional peripheral restriction. Preliminary data supports this method can be translated into a clinical pharmacodynamic readout using human skin biopsies.

Maximizing diversity from a kinase screen: identification of novel and selective pan-Trk inhibitors for chronic pain

We have identified several series of small molecule inhibitors of TrkA with unique binding modes. The starting leads were chosen to maximize the structural and binding mode diversity derived from a high throughput screen of our internal compound collection. These leads were optimized for potency and selectivity employing a structure based drug design approach adhering to the principles of ligand efficiency to maximize binding affinity without overly relying on lipophilic interactions. This endeavor resulted in the identification of several small molecule pan-Trk inhibitor series that exhibit high selectivity for TrkA/B/C versus a diverse panel of kinases. We have also demonstrated efficacy in both inflammatory and neuropathic pain models upon oral dosing. Herein we describe the identification process, hit-to-lead progression, and binding profiles of these selective pan-Trk kinase inhibitors.

Decahydroquinoline amides as highly selective CB2 agonists: role of selectivity on in vivo efficacy in a rodent model of analgesia

A novel series of decahydroquinoline CB2 agonists is described. Optimization of the amide substituent led to improvements in CB2/CB1 selectivity as well as physical properties. Two key compounds were examined in the rat CFA model of acute inflammatory pain. A moderately selective CB2 agonist was active in this model. A CB2 agonist lacking functional CB1 activity was inactive in this model despite high in vivo exposure both peripherally and centrally.

HC-030031, a TRPA1 selective antagonist, attenuates inflammatory- and neuropathy-induced mechanical hypersensitivity

Background

Safe and effective treatment for chronic inflammatory and neuropathic pain remains a key unmet medical need for many patients. The recent discovery and description of the transient receptor potential family of receptors including TRPV1 and TRPA1 has provided a number of potential new therapeutic targets for treating chronic pain. Recent reports have suggested that TRPA1 may play an important role in acute formalin and CFA induced pain. The current study was designed to further explore the therapeutic potential of pharmacological TRPA1 antagonism to treat inflammatory and neuropathic pain.

Results

The in vitro potencies of HC-030031 versus cinnamaldehyde or allyl isothiocyanate (AITC or Mustard oil)-induced TRPA1 activation were 4.9 ± 0.1 and 7.5 ± 0.2 μM respectively (IC₅₀). These findings were similar to the previously reported IC₅₀ of 6.2 μM against AITC activation of TRPA1 [1]. In the rat, oral administration of HC-030031 reduced AITC-induced nocifensive behaviors at a dose of 100 mg/kg. Moreover, oral HC-030031 (100 mg/kg) significantly reversed mechanical hypersensitivity in the more chronic models of Complete Freunds Adjuvant (CFA)-induced inflammatory pain and the spinal nerve ligation model of neuropathic pain.

Conclusion

Using oral administration of the selective TRPA1 antagonist HC-030031, our results demonstrated that TRPA1 plays an important role in the mechanisms responsible for mechanical hypersensitivity observed in inflammatory and neuropathic pain models. These findings suggested that TRPA1 antagonism may be a suitable new approach for the development of a potent and selective therapeutic agent to treat both inflammatory and neuropathic pain.

Three-dimensional reconstruction of the axon arbor of a CA3 pyramidal cell recorded and filled in vivo

The three-dimensional intrahippocampal distribution of axon collaterals of an in vivo filled CA3c pyramidal cell was investigated. The neuron was filled with biocytin in an anesthetized rat and the collaterals were reconstructed with the aid of a NeuroLucida program from 48 coronal sections. The total length of the axon collaterals exceeded 0.5 m, with almost 40,000 synaptic boutons. The majority of the collaterals were present in the CA1 region (70.0%), whereas 27.6% constituted CA3 recurrent collaterals with the remaining minority of axons returning to the dentate gyrus. The axon arbor covered more than two thirds of the longitudinal axis of the hippocampus, and the terminals were randomly distributed both locally and distally from the soma. We suggest that the CA3 system can be conceptualized as a single-module, in which nearby and distant targets are contacted by the same probability (similar to a mathematically defined random graph). This arrangement, in combination with the parallel input granule cells and parallel output CA1 pyramidal cells, appears ideal for segregation and integration of information and memories.

Using extracellular action potential recordings to constrain compartmental models

We investigate the use of extracellular action potential (EAP) recordings for biophysically faithful compartmental models. We ask whether constraining a model to fit the EAP is superior to matching the intracellular action potential (IAP). In agreement with previous studies, we find that the IAP method under-constrains the parameters. As a result, significantly different sets of parameters can have virtually identical IAP's. In contrast, the EAP method results in a much tighter constraint. We find that the distinguishing characteristics of the waveform--but not its amplitude-resulting from the distribution of active conductances are fairly invariant to changes of electrode position and detailed cellular morphology. Based on these results, we conclude that EAP recordings are an excellent source of data for the purpose of constraining compartmental models.

Hilar mossy cells: functional identification and activity in vivo

Network oscillations are proposed to provide the framework for the ongoing neural computations of the brain. Thus, an important aspect of understanding the functional roles of various cell classes in the brain is to understand the relationship of cellular activity to the ongoing oscillations. While many studies have characterized the firing properties of cells in the hippocampal network including granule cells, pyramidal cells and interneurons, information about the activity of dentate mossy cells in the intact brain is scant. Here we review the currently available information and describe biophysical properties and network-related firing patterns of mossy cells in vivo. These new observations will assist in the extracellular identification of this unique cell type and help elucidate their functional role in behaving animals.

Integration and Segregation of Activity in Entorhinal-Hippocampal Subregions by Neocortical Slow Oscillations

Brain systems communicate by means of neuronal oscillations at multiple temporal and spatial scales. In anesthetized rats, we find that neocortical "slow" oscillation engages neurons in prefrontal, somatosensory, entorhinal, and subicular cortices into synchronous transitions between UP and DOWN states, with a corresponding bimodal distribution of their membrane potential. The membrane potential of hippocampal granule cells and CA3 and CA1 pyramidal cells lacked bimodality, yet it was influenced by the slow oscillation in a region-specific manner. Furthermore, in both anesthetized and naturally sleeping rats, the cortical UP states resulted in increased activity of dentate and most CA1 neurons, as well as the highest probability of ripple events. Yet, the CA3-CA1 network could self-organize into gamma bursts and occasional ripples during the DOWN state. Thus, neo/paleocortical and hippocampal networks periodically reset, self-organize, and temporally coordinate their cell assemblies via the slow oscillation.

Hippocampal CA3 pyramidal cells selectively innervate aspiny interneurons

The specific connectivity among principal cells and interneurons determines the flow of activity in neuronal networks. To elucidate the connections between hippocampal principal cells and various classes of interneurons, CA3 pyramidal cells were intracellularly labelled with biocytin in anaesthetized rats and the three-dimensional distribution of their axon collaterals was reconstructed. The sections were double-stained for substance P receptor (SPR)- or metabotropic glutamate receptor 1alpha (mGluR-1alpha)-immunoreactivity to investigate interneuron targets of the CA3 pyramidal cells. SPR-containing interneurons represent a large portion of the GABAergic population, including spiny and aspiny classes. Axon terminals of CA3 pyramidal cells contacted SPR-positive interneuron dendrites in the hilus and in all hippocampal strata in both CA3 and CA1 regions (7.16% of all boutons). The majority of axons formed single contacts (87.5%), but multiple contacts (up to six) on single target neurons were also found. CA3 pyramidal cell axon collaterals innervated several types of morphologically different aspiny SPR-positive interneurons. In contrast, spiny SPR-interneurons or mGluR-1alpha-positive interneurons in the hilus, CA3 and CA1 regions were rarely contacted by the filled pyramidal cells. These findings indicate a strong target selection of CA3 pyramidal cells favouring the activation of aspiny classes of interneurons.

On the Origin of the Extracellular Action Potential Waveform: A Modeling Study

Although extracellular unit recording is typically used for the detection of spike occurrences, it also has the theoretical ability to report about what are typically considered intracellular features of the action potential. We address this theoretical ability by developing a model system that captures features of experimentally recorded simultaneous intracellular and extracellular recordings of CA1 pyramidal neurons. We use the line source approximation method of Holt and Koch to model the extracellular action potential (EAP) voltage resulting from the spiking activity of individual neurons. We compare the simultaneous intracellular and extracellular recordings of CA1 pyramidal neurons recorded in vivo with model predictions for the same cells reconstructed and simulated with compartmental models. The model accurately reproduces both the waveform and the amplitude of the EAPs, although it was difficult to achieve simultaneous good matches on both the intracellular and extracellular waveforms. This suggests that accounting for the EAP waveform provides a considerable constraint on the overall model. The developed model explains how and why the waveform varies with electrode position relative to the recorded cell. Interestingly, each cell's dendritic morphology had very little impact on the EAP waveform. The model also demonstrates that the varied composition of ionic currents in different cells is reflected in the features of the EAP.

The Role of Amyloid-Beta Derived Diffusible Ligands (ADDLs) in Alzheimers Disease

The amyloid-beta (Abeta) cascade hypothesis of Alzheimer's disease (AD) has dominated research and subsequent therapeutic drug development for over two decades. Central to this hypothesis is the observation that Abeta is elevated in AD patients and that the disease is ultimately characterized by the central deposition of insoluble senile plaques. More recent evidence, however, suggests that the presence or absence of plaque is insufficient to fully account for the deleterious role of elevated Abeta in AD. Such studies support the basis for an alternate interpretation of the Abeta cascade hypothesis. Namely, that soluble oligomers of Abeta (i.e., ADDLs) accumulate and cause functional deficits prior to overt neuronal cell death or plaque deposition. Accordingly, the following review focuses on research describing the preparation and functional activity of ADDLs in vitro and in vivo. These studies provide the basis for an alternate, ADDL-based, view of the Abeta cascade hypothesis and accounts for the disconnect between plaque burden and cognitive deficits. Possible therapeutic approaches aimed at lowering ADDLs in AD patients are also considered.

Clozapine Potentiation ofN-Methyl-d-aspartate Receptor Currents in the Nucleus Accumbens: Role of NR2B and Protein Kinase A/Src Kinases

Clozapine is an atypical antipsychotic that has a unique clinical profile that distinguishes it from other typical and atypical antipsychotics. At present, the underlying mechanisms of action of clozapine are unclear. Recent studies in the field of schizophrenia suggest that compounds that potentiate N-methyl-d-aspartate (NMDA) receptor function in the appropriate brain regions might be an effective antipsychotic agent. One relevant region in which NMDA receptors play a key role in mediating neurotransmission is the nucleus accumbens. Therefore, we investigated the regulation of NMDA receptor currents and excitatory postsynaptic currents (EPSCs) by clozapine in nucleus accumbens neurons. Whole-cell patch-clamp recordings were performed in rat brain slices. We demonstrate that bath application of clozapine but not haloperidol or the selective 5-hydroxytryptamine 2A antagonist MDL100907 [(R)-(+)-alpha-(2,3-dimethoxyphenyl)-1-[2-(4-fluoro-phenyl)ethyl]-4-piperidine methanol] induces a robust potentiation of NMDA-evoked currents and of glutamatergic EPSCs and that this potentiation is dependent on dopamine release and postsynaptic activation of D1 receptors. Furthermore, the effect of clozapine is selective for NR2B subtype-containing NMDA receptors and is blocked by the selective Src family kinase inhibitor PP2 [4-amino-5-(4-chlorophenyl)-7-(t-butyl)pyrazolo[3,4-d]pyrimidine] and the protein kinase A-selective inhibitor N-[2-(p-bromocinnamylamino)ethyl]-5-isoquinolinesulfonamide but not by the protein kinase C-selective inhibitor bisindolylmaleimide I. This effect of clozapine in the nucleus accumbens might underlie the unique clinical profile of this atypical antipsychotic and provides a basis for novel treatment approaches.

Interneuron Diversity series: Circuit complexity and axon wiring economy of cortical interneurons

The performance of the brain is constrained by wiring length and maintenance costs. The apparently inverse relationship between number of neurons in the various interneuron classes and the spatial extent of their axon trees suggests a mathematically definable organization, reminiscent of 'small-world' or scale-free networks observed in other complex systems. The wiring-economy-based classification of cortical inhibitory interneurons is supported by the distinct physiological patterns of class members in the intact brain. The complex wiring of diverse interneuron classes could represent an economic solution for supporting global synchrony and oscillations at multiple timescales with minimum axon length.

Massively parallel recording of unit and local field potentials with silicon-based electrodes

Parallel recording of neuronal activity in the behaving animal is a prerequisite for our understanding of neuronal representation and storage of information. Here we describe the development of micro-machined silicon microelectrode arrays for unit and local field recordings. The two-dimensional probes with 96 or 64 recording sites provided high-density recording of unit and field activity with minimal tissue displacement or damage. The on-chip active circuit eliminated movement and other artifacts and greatly reduced the weight of the headgear. The precise geometry of the recording tips allowed for the estimation of the spatial location of the recorded neurons and for high-resolution estimation of extracellular current source density. Action potentials could be simultaneously recorded from the soma and dendrites of the same neurons. Silicon technology is a promising approach for high-density, high-resolution sampling of neuronal activity in both basic research and prosthetic devices.

Dopamine modulation of neuronal function in the monkey prefrontal cortex

We developed a brain slice preparation that allowed us to apply whole-cell recordings to examine the electrophysiological properties of identified synapses, neurons, and local circuits in the dorsolateral prefrontal cortex (DLPFC) of macaque monkeys. In this article, we summarize the results from some of our recent and current in vitro studies in the DLPFC with special emphasis on the modulatory effects of dopamine (DA) receptor activation on pyramidal and nonpyramidal cell function in superficial layers in DLPFC areas 46 and 9.

Single granule cells reliably discharge targets in the hippocampal CA3 network in vivo

Processing of neuronal information depends on interactions between the anatomical connectivity and cellular properties of single cells. We examined how these computational building blocks work together in the intact rat hippocampus. Single spikes in dentate granule cells, controlled intracellularly, generally failed to discharge either interneurons or CA3 pyramidal cells. In contrast, trains of spikes effectively discharged both CA3 cell types. Increasing the discharge rate of the granule cell increased the discharge probability of its target neuron and decreased the delay between the onset of a granule cell train and evoked firing in postsynaptic targets. Thus, we conclude that the granule cell to CA3 synapses are 'conditional detonators,' dependent on granule cell firing pattern. In addition, we suggest that information in single granule cells is converted into a temporal delay code in target CA3 pyramidal cells and interneurons. These data demonstrate how a neural circuit of the CNS may process information.

Spike train dynamics predicts theta-related phase precession in hippocampal pyramidal cells

According to the temporal coding hypothesis, neurons encode information by the exact timing of spikes. An example of temporal coding is the hippocampal phase precession phenomenon, in which the timing of pyramidal cell spikes relative to the theta rhythm shows a unidirectional forward precession during spatial behaviour. Here we show that phase precession occurs in both spatial and non-spatial behaviours. We found that spike phase correlated with instantaneous discharge rate, and processed unidirectionally at high rates, regardless of behaviour. The spatial phase precession phenomenon is therefore a manifestation of a more fundamental principle governing the timing of pyramidal cell discharge. We suggest that intrinsic properties of pyramidal cells have a key role in determining spike times, and that the interplay between the magnitude of dendritic excitation and rhythmic inhibition of the somatic region is responsible for the phase assignment of spikes.

Selective reduction by dopamine of excitatory synaptic inputs to pyramidal neurons in primate prefrontal cortex

We have employed in vitro physiological methods to investigate dopaminergic modulation of excitatory synaptic transmission in monkey prefrontal cortex (PFC) circuits. We show that combined activation of D1-like and D2-like dopamine receptors results in the reduction of extracellular stimulation-evoked isolated EPSCs in layer 3 pyramidal neurons. Using paired recordings from synaptically connected pyramidal neurons we have determined the basic properties of unitary synaptic connections between layer 3 pyramids in the primate PFC and, interestingly, we found that dopamine does not reduce synaptic transmission between nearby pairs of synaptically coupled PFC pyramidal neurons. This input specificity may be a critical aspect of the dopaminergic regulation of recurrent excitatory circuits in the PFC.

Revisiting the role of the hippocampal mossy fiber synapse

The mossy fiber pathway has long been considered to provide the major source of excitatory input to pyramidal cells of hippocampal area CA3. In this review we describe anatomical and physiological properties of this pathway that challenge this view. We argue that the mossy fiber pathway does not provide the main input to CA3 pyramidal cells, and that the short-term plasticity and amplitude variance of mossy fiber synapses may be more important features than their long-term plasticity or absolute input strength.

Hippocampal pyramidal cell-interneuron spike transmission is frequency dependent and responsible for place modulation of interneuron discharge

The interplay between principal cells and interneurons plays an important role in timing the activity of individual cells. We investigated the influence of single hippocampal CA1 pyramidal cells on putative interneurons. The activity of CA1 pyramidal cells was controlled intracellularly by current injection, and the activity of neighboring interneurons was recorded extracellularly in the urethane-anesthetized rat. Spike transmission probability between monosynaptically connected pyramidal cell-interneuron pairs was frequency dependent and highest between 5 and 25 Hz. In the awake animal, interneurons were found that had place-modulated firing rates, with place maps similar to their presynaptic pyramidal neuron. Thus, single pyramidal neurons can effectively determine the firing patterns of their interneuron targets.

Giant miniature EPSCs at the hippocampal mossy fiber to CA3 pyramidal cell synapse are monoquantal

The mechanisms generating giant miniature excitatory postsynaptic currents (mEPSCs) were investigated at the hippocampal mossy fiber (MF) to CA3 pyramidal cell synapse in vitro. These giant mEPSCs have peak amplitudes as large as 1,700 pA (13.6 nS) with a mean maximal mEPSC amplitude of 366 +/- 20 pA (mean +/- SD; 5 nS; n = 25 cells). This is compared with maximal mEPSC amplitudes of <100 pA typically observed at other cortical synapses. We tested the hypothesis that giant mEPSCs are due to synchronized release of multiple vesicles across the release sites of single MF boutons by directly inducing vesicular release using secretagogues. If giant mEPSCs result from simultaneous multivesicular release, then secretagogues should increase the frequency of small mEPSCs selectively. We found that hypertonic sucrose and spermine increased the frequency of both small and giant mEPSCs. The peptide toxin secretagogues alpha-latrotoxin and pardaxin failed to increase the frequency of giant mEPSCs, but the possible lack of tissue penetration of the toxins make these results equivocal. Because a multiquantal release mechanism is likely to be mediated by a spontaneous increase in presynaptic calcium concentration, a monoquantal mechanism is further supported by results that giant mEPSCs were not affected by manipulations of extracellular or intracellular calcium concentrations. In addition, reducing the temperature of the bath to 15 degrees C failed to desynchronize the rising phases of giant mEPSCs. Together these data suggest that the giant mEPSCs are generated via a monovesicular mechanism. Three-dimensional analysis through serial electron microscopy of the MF boutons revealed large clear vesicles (50 to 160 nm diam) docked presynaptically at the MF synapse in sufficient numbers to account for the amplitude and frequency of giant mEPSCs recorded electrophysiologically. It is concluded that release of the contents of a single large clear vesicle generates giant mEPSCs at the MF to CA3 pyramidal cell synapse.

Temporal interaction between single spikes and complex spike bursts in hippocampal pyramidal cells

Cortical pyramidal cells fire single spikes and complex spike bursts. However, neither the conditions necessary for triggering complex spikes, nor their computational function are well understood. CA1 pyramidal cell burst activity was examined in behaving rats. The fraction of bursts was not reliably higher in place field centers, but rather in places where discharge frequency was 6-7 Hz. Burst probability was lower and bursts were shorter after recent spiking activity than after prolonged periods of silence (100 ms-1 s). Burst initiation probability and burst length were correlated with extracellular spike amplitude and with intracellular action potential rising slope. We suggest that bursts may function as "conditional synchrony detectors," signaling strong afferent synchrony after neuronal silence, and that single spikes triggered by a weak input may suppress bursts evoked by a subsequent strong input.

Action potential threshold of hippocampal pyramidal cells in vivo is increased by recent spiking activity

Understanding the mechanisms that influence the initiation of action potentials in single neurons is an important step in determining the way information is processed by neural networks. Therefore, we have investigated the properties of action potential thresholds for hippocampal neurons using in vivo intracellular recording methods in Sprague-Dawley rats. The use of in vivo recording has the advantage of the presence of naturally occurring spatio-temporal patterns of synaptic activity which lead to action potential initiation. We have found there is a large variability in the threshold voltage (5.7+/-1.7 mV; n=22) of individual action potentials. We have identified two separate factors that contribute to this variation in threshold: (1) fast rates of membrane potential change prior to the action potential are associated with more hyperpolarized thresholds (increased excitability) and (2) the occurrence of other action potentials in the 1 s prior to any given action potential is associated with more depolarized thresholds (decreased excitability). We suggest that prior action potentials cause sodium channel inactivation that recovers with approximately a 1-s time constant and thus depresses action potential threshold during this period.

The apical shaft of CA1 pyramidal cells is under GABAergic interneuronal control

Dendrites of pyramidal cells perform complex amplification and integration (reviewed in Refs 5, 9, 12 and 20). The presence of a large proximal apical dendrite has been shown to have functional implications for neuronal firing patterns (13) and under a variety of experimental conditions, the largest increases in intracellular Ca2+ occur in the apical shaft.(4,8,15,16,19,21-23) An important step in understanding the functional role of the proximal apical dendrite is to describe the nature of synaptic input to this dendritic region. Using light and electron microscopic methods combined with in vivo labeling of rat hippocampal CA1 pyramidal cells, we examined the total number of GABAergic and non-GABAergic inputs converging onto the first 200microm of the apical trunk. The number of spines associated with excitatory terminals increased from <0.2 spines/microm adjacent to the soma to 5.5 spines/microm at 200microm from the soma, whereas the number of GABAergic, symmetric terminals decreased from 0.8/microm to 0.08/microm over the same anatomical region. GABAergic terminals were either parvalbumin-, cholecystokinin- or vasointestinal peptide-immunoreactive. These findings indicate that the apical dendritic trunk mainly receives synaptic input from GABAergic interneurons. GABAergic inhibition during network oscillation may serve to periodically isolate the dendritic compartments from the perisomatic action potential generating sites.

Dopamine increases excitability of pyramidal neurons in primate prefrontal cortex

Dopaminergic modulation of neuronal networks in the dorsolateral prefrontal cortex (PFC) is believed to play an important role in information processing during working memory tasks in both humans and nonhuman primates. To understand the basic cellular mechanisms that underlie these actions of dopamine (DA), we have investigated the influence of DA on the cellular properties of layer 3 pyramidal cells in area 46 of the macaque monkey PFC. Intracellular voltage recordings were obtained with sharp and whole cell patch-clamp electrodes in a PFC brain-slice preparation. All of the recorded neurons in layer 3 (n = 86) exhibited regular spiking firing properties consistent with those of pyramidal neurons. We found that DA had no significant effects on resting membrane potential or input resistance of these cells. However DA, at concentrations as low as 0.5 microM, increased the excitability of PFC cells in response to depolarizing current steps injected at the soma. Enhanced excitability was associated with a hyperpolarizing shift in action potential threshold and a decreased first interspike interval. These effects required activation of D1-like but not D2-like receptors since they were inhibited by the D1 receptor antagonist SCH23390 (3 microM) but not significantly altered by the D2 antagonist sulpiride (2.5 microM). These results show, for the first time, that DA modulates the activity of layer 3 pyramidal neurons in area 46 of monkey dorsolateral PFC in vitro. Furthermore the results suggest that, by means of these effects alone, DA modulation would generally enhance the response of PFC pyramidal neurons to excitatory currents that reach the action potential initiation site.

The multifarious hippocampal mossy fiber pathway: a review

The hippocampal mossy fiber pathway between the granule cells of the dentate gyrus and the pyramidal cells of area CA3 has been the target of numerous scientific studies. Initially, attention was focused on the mossy fiber to CA3 pyramidal cell synapse because it was suggested to be a model synapse for studying the basic properties of synaptic transmission in the CNS. However, the accumulated body of research suggests that the mossy fiber synapse is rather unique in that it has many distinct features not usually observed in cortical synapses. In this review, we have attempted to summarize the many unique features of this hippocampal pathway. We also have attempted to reconcile some discrepancies that exist in the literature concerning the pharmacology, physiology and plasticity of this pathway. In addition we also point out some of the experimental challenges that make electrophysiological study of this pathway so difficult.Finally, we suggest that understanding the functional role of the hippocampal mossy fiber pathway may lie in an appreciation of its variety of unique properties that make it a strong yet broadly modulated synaptic input to postsynaptic targets in the hilus of the dentate gyrus and area CA3 of the hippocampal formation.

Intracellular features predicted by extracellular recordings in the hippocampus in vivo

Multichannel tetrode array recording in awake behaving animals provides a powerful method to record the activity of large numbers of neurons. The power of this method could be extended if further information concerning the intracellular state of the neurons could be extracted from the extracellularly recorded signals. Toward this end, we have simultaneously recorded intracellular and extracellular signals from hippocampal CA1 pyramidal cells and interneurons in the anesthetized rat. We found that several intracellular parameters can be deduced from extracellular spike waveforms. The width of the intracellular action potential is defined precisely by distinct points on the extracellular spike. Amplitude changes of the intracellular action potential are reflected by changes in the amplitude of the initial negative phase of the extracellular spike, and these amplitude changes are dependent on the state of the network. In addition, intracellular recordings from dendrites with simultaneous extracellular recordings from the soma indicate that, on average, action potentials are initiated in the perisomatic region and propagate to the dendrites at 1.68 m/s. Finally we determined that a tetrode in hippocampal area CA1 theoretically should be able to record electrical signals from approximately 1, 000 neurons. Of these, 60-100 neurons should generate spikes of sufficient amplitude to be detectable from the noise and to allow for their separation using current spatial clustering methods. This theoretical maximum is in contrast to the approximately six units that are usually detected per tetrode. From this, we conclude that a large percentage of hippocampal CA1 pyramidal cells are silent in any given behavioral condition.

Accuracy of tetrode spike separation as determined by simultaneous intracellular and extracellular measurements

Simultaneous recording from large numbers of neurons is a prerequisite for understanding their cooperative behavior. Various recording techniques and spike separation methods are being used toward this goal. However, the error rates involved in spike separation have not yet been quantified. We studied the separation reliability of "tetrode" (4-wire electrode)-recorded spikes by monitoring simultaneously from the same cell intracellularly with a glass pipette and extracellularly with a tetrode. With manual spike sorting, we found a trade-off between Type I and Type II errors, with errors typically ranging from 0 to 30% depending on the amplitude and firing pattern of the cell, the similarity of the waveshapes of neighboring neurons, and the experience of the operator. Performance using only a single wire was markedly lower, indicating the advantages of multiple-site monitoring techniques over single-wire recordings. For tetrode recordings, error rates were increased by burst activity and during periods of cellular synchrony. The lowest possible separation error rates were estimated by a search for the best ellipsoidal cluster shape. Human operator performance was significantly below the estimated optimum. Investigation of error distributions indicated that suboptimal performance was caused by inability of the operators to mark cluster boundaries accurately in a high-dimensional feature space. We therefore hypothesized that automatic spike-sorting algorithms have the potential to significantly lower error rates. Implementation of a semi-automatic classification system confirms this suggestion, reducing errors close to the estimated optimum, in the range 0-8%.

Ultra-slow oscillation (0.025 Hz) triggers hippocampal afterdischarges in Wistar rats

Oscillations in neuronal networks are assumed to serve various physiological functions, from coordination of motor patterns to perceptual binding of sensory information. Here, we describe an ultra-slow oscillation (0.025 Hz) in the hippocampus. Extracellular and intracellular activity was recorded from the CA1 and subicular regions in rats of the Wistar and Sprague-Dawley strains, anesthetized with urethane. In a subgroup of Wistar rats (23%), spontaneous afterdischarges (4.7+/-1.6 s) occurred regularly at 40.8+/-15.7 s. The afterdischarge was initiated by a fast increase of population synchrony (100-250 Hz oscillation; "tonic" phase), followed by large-amplitude rhythmic waves and associated action potentials at gamma and beta frequency (15-50 Hz; "clonic" phase). The afterdischarges were bilaterally synchronous and terminated relatively abruptly without post-ictal depression. Single-pulse stimulation of the commissural input could trigger afterdischarges, but only at times when they were about to occur. Commissural stimulation evoked inhibitory postsynaptic potentials in pyramidal cells. However, when the stimulus triggered an afterdischarge, the inhibitory postsynaptic potential was absent and the cells remained depolarized during most of the afterdischarge. Afterdischarges were not observed in the Sprague-Dawley rats. Long-term analysis of interneuronal activity in intact, drug-free rats also revealed periodic excitability changes in the hippocampal network at 0.025 Hz. These findings indicate the presence of an ultra-slow oscillation in the hippocampal formation. The ultra-slow clock induced afterdischarges in susceptible animals. We hypothesize that a transient failure of GABAergic inhibition in a subset of Wistar rats is responsible for the emergence of epileptiform patterns.

Hebbian modification of a hippocampal population pattern in the rat

1. The study of the physiological role of long-term potentiation (LTP) is often hampered by the challenge of finding a physiological event that can be used to assess synaptic strength. We explored the possibility of utilising a naturally occurring event, the hippocampal sharp wave (SPW), for the assessment of synaptic strength and the induction of LTP in vivo. 2. We used two methods in which hippocampal cells were either recorded intracellularly or extracellularly in vivo. In both cases, a linear association between the magnitude of the SPW and cellular responsiveness was observed. 3. LTP was induced by depolarising cells during SPWs by either direct intracellular current injection or extracellular microstimulation adjacent to the cell body. Both of these approaches led to an increase in the slope of the linear association between SPWs and cellular responsiveness. 4. This change was achieved without a rise in overall cell excitability, implying that the synapses providing input to CA1 cells during sharp waves had undergone potentiation. 5. Our findings show that the Hebbian pairing of cellular activation with spontaneous, naturally occurring synaptic events is capable of inducing LTP.

Amplification of perforant-path EPSPs in CA3 pyramidal cells by LVA calcium and sodium channels

The perforant path forms a monosynaptic connection between the cells of layer II of the entorhinal cortex and the pyramidal cells in hippocampal area CA3. Although this projection is prominent anatomically, very little is known about the physiological properties of this input. The distal location of these synapses suggests that somatically recorded perforant-path excitatory postsynaptic potentials (EPSPs) may be influenced by the activation of voltage-dependent channels in CA3 cells. We observed that perforant-path EPSPs are reduced (by approximately 25%) by blockade of postsynaptic low-voltage-activated calcium and sodium channels, indicating that perforant-path EPSPs are amplified by the activation of these channels. These data suggest that the perforant path may represent an important and highly modifiable direct connection between the entorhinal cortex and area CA3.

Origin of the apparent asynchronous activity of hippocampal mossy fibers

Fiber volleys (FVs) from the stratum lucidum of rat hippocampal area CA3 were recorded extracellularly from in vitro slices in the presence of 10 mM kynurenic acid. In agreement with previous work, bulk stimulation of the dentate gyrus (DG) near the hilar border leads to an asynchronous FV. Transection of the stratum lucidum between the DG stimulation site and the CA3 recording site reduced or eliminated the early components of the asynchronous FV, indicating that they are of mossy fiber (MF) origin. In contrast, moving the stimulating electrode away from the hilus toward the hippocampal fissure reduced or eliminated the late components of the FV. Subsequently, we found that bulk stimulation on the DG/hilar border induces an antidromic population spike in CA3 pyramidal cells. Finally, the MFs and associational collaterals have different conduction velocities (0.51 and 0.37 m/s, respectively; temperature = 33 degrees C). From these data, we conclude that the late components of the asynchronous FV are due to antidromic activation of CA3 collaterals that have been shown to be present in the DG and hilus. A corollary of these findings is that bulk stimulation on the DG/hilar border can lead to at least two different monosynaptic inputs to CA3 pyramidal cells: the MFs and the antidromically activated associational collaterals. We suggest that when MF synaptic responses are being evoked with the use of bulk stimulation, stimulating electrodes should be placed in the outer molecular layer of the DG to prevent the activation of hilar-projecting associational collaterals. This procedure should be added to the previously proposed criteria for preventing polysynaptic contamination of the intracellularly recorded evoked MF synaptic response.

Large amplitude miniature excitatory postsynaptic currents in hippocampal CA3 pyramidal neurons are of mossy fiber origin

Neonatal (P0) gamma-irradiation was used to lesion selectively the mossy fiber (MF) synaptic input to CA3 pyramidal cells. This lesion caused a > 85% reduction in the MF input as determined by quantitative assessment of the number of dynorphin immunoreactive MF boutons. The gamma-irradiation lesion caused a reduction in the mean number of miniature excitatory postsynaptic currents (mEPSCs) recorded from CA3 pyramidal cells (2,292 vs. 1,429/3-min period; n = 10). The lesion also caused a reduction in the mean mEPSC peak amplitude from 19.1 +/- 0.45 to 14.6 +/- 0.49 pA (mean +/- SE; peak conductance 238.8 +/- 5.6 to 182.0 +/- 6.1 pS). Similarly, there was a reduction in the mean 10-85% rise time from 1.72 +/- 0.02 ms to 1.42 +/- 0.04 ms. The effects of the gamma-irradiation on both mEPSC amplitude and 10-85% rise time were significant at P < 0.002 and P < 0.005 (2-tailed Kolmogorov-Smirnov test). Based on the selectively of the gamma-irradiation, MF and non-MF mEPSC amplitude and 10-85% rise-time distributions were calculated. Both the amplitude and 10-85% rise-time distributions showed extensive overlap between the MF and non-MF mediated mEPSCs. The MF mEPSC distributions had a mean peak amplitude of 24.6 pA (307.5 pS) and a mean 10-85% rise time of 2.16 ms. THe non-MF mEPSC distributions had a mean peak amplitude of 12.2 pA (152.5 pS) and 10-85% rise time of 1.26 ms. The modes of the amplitude distributions were the same at 5 pA (62 pS). The MF and non-MF mEPSC amplitude and 10-85% rise-time distributions were significantly different at P << 0.001 (1-tailed, large sample Kolmogorov-Smirnov test). The data demonstrate that the removal of the MF synaptic input to CA3 pyramidal cells leads to the absence of the large amplitude mEPSCs that are present in control recordings.

Properties of LTP induction in the CA3 region of the primate hippocampus

Activity-dependent changes in synaptic strength, such as long-term potentiation (LTP), have been proposed to underlie memory storage in the brains of all mammals, including humans. However, most forms of synaptic plasticity, including LTP, are studied almost exclusively in rodents and related species. Thus, the hypothesis that LTP is important in human memory relies on the assumption that LTP is similar in the primate and rodent brains. We have begun to test this hypothesis by studying the properties and mechanisms of LTP induction in area CA3 of hippocampal slices from cynomolgus monkeys. We have found that LTP can be induced reliably at both mossy fiber-CA3 and collateral/associational-CA3 synapses in the primate brain, and that the properties of LTP induction at these synapses are similar to what we and others have observed in experiments using hippocampal slices from rodents. Also, we have investigated the role of opioids in mossy fiber synaptic transmission and LTP and have found no effect of the opioid antagonist naloxone nor the opioid agonist dynorphin on mossy fiber synaptic transmission or potentiation. These data suggest that LTP in the primate and rat brains has a similar induction mechanism and, thus, that the rodent is a useful animal model in which to study synaptic modification such as LTP.

Dendritic morphology and its effects on the amplitude and rise-time of synaptic signals in hippocampal CA3 pyramidal cells

Detailed anatomical analysis and compartmental modeling techniques were used to study the impact of CA3b pyramidal cell dendritic morphology and hippocampal anatomy on the amplitude and time course of dendritic synaptic signals. We have used computer-aided tracing methods to obtain accurate three-dimensional representations of 8 CA3b pyramidal cells. The average total dendritic length was 6,332 +/- 1,029 microns and 5,062 +/- 1,397 microns for the apical and basilar arbors, respectively. These cells also exhibited a rough symmetry in their maximal transverse and septotemporal extents (311 +/- 84 microns and 269 +/- 106 microns). From the calculated volume of influence (the volume of the neuropil from which the dendritic structures can receive input), it was found that these cells show a limited symmetry between their proximal apical and basilar dendrites (2.1 +/- 1.2 x 10(6) microns 3 and 3.5 +/- 1.1 x 10(6) microns 3, respectively). Based upon these data, we propose that the geometry of these cells can be approximated by a combination of two cones for the apical arbor and a single cone for the basilar arbor. The reconstructed cells were used to build compartmental models and investigate the extent to which the cellular anatomy determines the efficiency with which dendritic synaptic signals are transferred to the soma. We found that slow, long lasting signals show only approximately a 50% attenuation when they occur in the most distal apical dendrites. However, synaptic transients similar to those seen in fast glutamatergic transmission are transferred much less efficiently, showing up to a 95% attenuation. The relationship between the distance along the dendrites and the observed attenuation for a transient is described simply by single exponential functions with parameters of 195 and 147 microns for the apical and basilar arbors respectively. In contrast, there is no simple relation that describes how a transient is attenuated with respect to these cells' stratified inputs. This lack of a simple relationship arises from the radial orientation of the proximal apical and basilar dendrites. When combined, the anatomical and modeling data suggest that a CA3b cell can be approximated in three dimensions as the combination of three cones. The amplitude and time-course for a synaptic transient can then be predicted using two simple equations.

Aspartate and glutamate mediate excitatory synaptic transmission in area CA1 of the hippocampus

We examined whether L-aspartate (ASP) and L-glutamate (GLU) both function as endogenous neurotransmitters in area CA1 of the rat hippocampus. Radioligand displacement experiments using 3H-DL-alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (3H-AMPA) to label AMPA/kainate receptors and 3H-cis-4-phosphonomethyl-2-piperidine carboxylic acid (3H-CGS-19755) to label NMDA receptors confirmed that GLU (Ki approximately 500 nM) but not ASP (Ki > 1 mM) has high affinity for AMPA/kainate receptors whereas GLU (Ki approximately 250 nM) and ASP (Ki approximately 1.3 microM) both have high affinity for NMDA receptors. Elevating extracellular potassium concentration (50 mM, 1 min) evoked the calcium-dependent release of both ASP (approximately 50% increase) and GLU (approximately 200% increase) from hippocampal slices and from minislices of area CA1. Reducing extracellular glucose concentration (0.2 mM) reduced GLU release, enhanced ASP release, and reduced AMPA/kainate receptor-mediated responses more than NMDA receptor-mediated responses (to 7% and 34% of control, respectively). Fiber volleys, antidromic population spikes, membrane potential, input resistance, and ATP content all were not affected by glucose reduction. Unlike low glucose, the inhibitory neuromodulator adenosine (5 microM), which reduces ASP and GLU release to a similar extent, reduced AMPA/kainate and NMDA receptor-mediated population EPSPs similarly (to 11% and 12% of control, respectively). AMPA/kainate and NMDA receptor-mediated population EPSPs were also similarly reduced by 0.4 microM TTX (to 32% and 22% of control, respectively) and similarly enhanced by 10 microM 4-aminopyridine (to 206% and 248% of control, respectively). Finally, NMDA receptor-mediated EPSCs measured by whole-cell recording decayed faster in low glucose (73 msec vs 54 msec) but not in adenosine (73 msec vs 78 msec). Together, these results confirm that ASP and GLU are both involved in excitatory synaptic transmission at the Schaffer collateral-commissural terminals in area CA1 of the rat hippocampus.