Metabotropic induction of persistent activity in layers II/III of anterior cingulate cortex

Cell-type-specific cholinergic control of granular retrosplenial cortex with implications for angular velocity coding across brain states

Cholinergic receptor activation enables the persistent firing of cortical pyramidal neurons, providing a key cellular basis for theories of spatial navigation involving working memory, path integration, and head direction encoding. The granular retrosplenial cortex (RSG) is important for spatially-guided behaviors, but how acetylcholine impacts RSG neurons is unknown. Here, we show that a transcriptomically, morphologically, and biophysically distinct RSG cell-type - the low-rheobase (LR) neuron - has a very distinct expression profile of cholinergic muscarinic receptors compared to all other neighboring excitatory neuronal subtypes. LR neurons do not fire persistently in response to cholinergic agonists, in stark contrast to all other principal neuronal subtypes examined within the RSG and across midline cortex. This lack of persistence allows LR neuron models to rapidly compute angular head velocity (AHV), independent of cholinergic changes seen during navigation. Thus, LR neurons can consistently compute AHV across brain states, highlighting the specialized RSG neural codes supporting navigation.

Cholinergic neuromodulation of prefrontal attractor dynamics controls performance in spatial working memory

The behavioral and neural effects of the endogenous release of acetylcholine following stimulation of the Nucleus Basalis of Meynert (NB) have been recently examined (Qi et al. 2021). Counterintuitively, NB stimulation enhanced behavioral performance while broadening neural tuning in the prefrontal cortex (PFC). The mechanism by which a weaker mnemonic neural code could lead to better performance remains unclear. Here, we show that increased neural excitability in a simple continuous bump attractor model can induce broader neural tuning and decrease bump diffusion, provided neural rates are saturated. Increased memory precision in the model overrides memory accuracy, improving overall task performance. Moreover, we show that bump attractor dynamics can account for the nonuniform impact of neuromodulation on distractibility, depending on distractor distance from the target. Finally, we delve into the conditions under which bump attractor tuning and diffusion balance in biologically plausible heterogeneous network models. In these discrete bump attractor networks, we show that reducing spatial correlations or enhancing excitatory transmission can improve memory precision. Altogether, we provide a mechanistic understanding of how cholinergic neuromodulation controls spatial working memory through perturbed attractor dynamics in PFC.

Dysregulated neuromodulation in the anterior cingulate cortex in chronic pain

Chronic pain is a significant global socioeconomic burden with limited long-term treatment options. The intractable nature of chronic pain stems from two primary factors: the multifaceted nature of pain itself and an insufficient understanding of the diverse physiological mechanisms that underlie its initiation and maintenance, in both the peripheral and central nervous systems. The development of novel non-opioidergic analgesic approaches is contingent on our ability to normalize the dysregulated nociceptive pathways involved in pathological pain processing. The anterior cingulate cortex (ACC) stands out due to its involvement in top-down modulation of pain perception, its abnormal activity in chronic pain conditions, and its contribution to cognitive functions frequently impaired in chronic pain states. Here, we review the roles of the monoamines dopamine (DA), norepinephrine (NE), serotonin (5-HT), and other neuromodulators in controlling the activity of the ACC and how chronic pain alters their signaling in ACC circuits to promote pathological hyperexcitability. Additionally, we discuss the potential of targeting these monoaminergic pathways as a therapeutic strategy for treating the cognitive and affective symptoms associated with chronic pain.

The genie in the bottle-magnified calcium signaling in dorsolateral prefrontal cortex

Neurons in the association cortices are particularly vulnerable in cognitive disorders such as schizophrenia and Alzheimer's disease, while those in primary visual cortex remain relatively resilient. This review proposes that the special molecular mechanisms needed for higher cognitive operations confer vulnerability to dysfunction, atrophy, and neurodegeneration when regulation is lost due to genetic and/or environmental insults. Accumulating data suggest that higher cortical circuits rely on magnified levels of calcium (from NMDAR, calcium channels, and/or internal release from the smooth endoplasmic reticulum) near the postsynaptic density to promote the persistent firing needed to maintain, manipulate, and store information without "bottom-up" sensory stimulation. For example, dendritic spines in the primate dorsolateral prefrontal cortex (dlPFC) express the molecular machinery for feedforward, cAMP-PKA-calcium signaling. PKA can drive internal calcium release and promote calcium flow through NMDAR and calcium channels, while in turn, calcium activates adenylyl cyclases to produce more cAMP-PKA signaling. Excessive levels of cAMP-calcium signaling can have a number of detrimental effects: for example, opening nearby K+ channels to weaken synaptic efficacy and reduce neuronal firing, and over a longer timeframe, driving calcium overload of mitochondria to induce inflammation and dendritic atrophy. Thus, calcium-cAMP signaling must be tightly regulated, e.g., by agents that catabolize cAMP or inhibit its production (PDE4, mGluR3), and by proteins that bind calcium in the cytosol (calbindin). Many genetic or inflammatory insults early in life weaken the regulation of calcium-cAMP signaling and are associated with increased risk of schizophrenia (e.g., GRM3). Age-related loss of regulatory proteins which result in elevated calcium-cAMP signaling over a long lifespan can additionally drive tau phosphorylation, amyloid pathology, and neurodegeneration, especially when protective calcium binding proteins are lost from the cytosol. Thus, the "genie" we need for our remarkable cognitive abilities may make us vulnerable to cognitive disorders when we lose essential regulation.

Effects of blocking mGluR5 on primate dorsolateral prefrontal cortical neuronal firing and working memory performance

RATIONALE

Metabotropic glutamate type 5 receptor (mGluR5) antagonists are under development for treating cognitive disorders such as Fragile X syndrome and Alzheimer's disease, largely based on success in mouse models, where post-synaptic mGluR5 stimulation weakens synaptic functions in hippocampus. However, human trials of mGluR5 antagonists have yet to be successful. This may be due in part to the differing effects of mGluR5 in hippocampus vs. prefrontal cortex, as mGluR5 are primarily post-synaptic in rodent hippocampus, but are both pre- and post-synaptic in the dorsolateral prefrontal cortical (dlPFC) circuits known to subserve working memory.

OBJECTIVES AND METHODS

The current study examined the effects of the selective mGluR5 negative allosteric modulator, MTEP (3-((2-Methyl-1,3-thiazol-4-yl)ethynyl)pyridine hydrochloride), on neuronal firing and working memory performance in aging rhesus monkeys with naturally occurring impairments in neuronal firing and cognitive performance.

RESULTS

We found that iontophoresis of MTEP directly onto dlPFC "Delay cells" had an inverted U dose-response, where low doses tended to enhance task-related firing, but higher doses suppressed neuronal firing. Similar effects were seen on cognitive performance following systemic MTEP administration (0.0001-0.1 mg/kg), with MTEP producing erratic dose-response curves. In the subset of monkeys (50%) that showed replicable improvement with MTEP, co-administration with the mGluR5 PAM, CDPPB (3-Cyano-N-(1,3-diphenyl-1H-pyrazol-5-yl)benzamide), blocked MTEP beneficial effects, consistent with mGluR5 actions.

CONCLUSIONS

The mixed effects of MTEP on cognitive performance may arise from opposing actions at pre- vs. post-synaptic mGluR5 in dlPFC. These data from monkeys suggest that future clinical trials should include low doses, and identification of potential subgroup responders.

The Unexplored Territory of Neural Models: Potential Guides for Exploring the Function of Metabotropic Neuromodulation

The space of possible neural models is enormous and under-explored. Single cell computational neuroscience models account for a range of dynamical properties of membrane potential, but typically do not address network function. In contrast, most models focused on network function address the dimensions of excitatory weight matrices and firing thresholds without addressing the complexities of metabotropic receptor effects on intrinsic properties. There are many under-explored dimensions of neural parameter space, and the field needs a framework for representing what has been explored and what has not. Possible frameworks include maps of parameter spaces, or efforts to categorize the fundamental elements and molecules of neural circuit function. Here we review dimensions that are under-explored in network models that include the metabotropic modulation of synaptic plasticity and presynaptic inhibition, spike frequency adaptation due to calcium-dependent potassium currents, and afterdepolarization due to calcium-sensitive non-specific cation currents and hyperpolarization activated cation currents. Neuroscience research should more effectively explore possible functional models incorporating under-explored dimensions of neural function.

Random Recurrent Networks Near Criticality Capture the Broadband Power Distribution of Human ECoG Dynamics

Brain electric field potentials are dominated by an arrhythmic broadband signal, but the underlying mechanism is poorly understood. Here we propose that broadband power spectra characterize recurrent neural networks of nodes (neurons or clusters of neurons), endowed with an effective balance between excitation and inhibition tuned to keep the network on the edge of dynamical instability. These networks show a fast mode reflecting local dynamics and a slow mode emerging from distributed recurrent connections. Together, the 2 modes produce power spectra similar to those observed in human intracranial EEG (i.e., electrocorticography, ECoG) recordings. Moreover, such networks convert spatial input correlations across nodes into temporal autocorrelation of network activity. Consequently, increased independence between nodes reduces low-frequency power, which may explain changes observed during behavioral tasks. Lastly, varying network coupling causes activity changes that resemble those observed in human ECoG across different arousal states. The model links macroscopic features of empirical ECoG power to a parsimonious underlying network structure, and suggests mechanisms for changes observed across behavioral and arousal states. This work provides a computational framework to generate and test hypotheses about cellular and network mechanisms underlying whole brain electrical dynamics, their variations across states, and potential alterations in brain diseases.

Muscarinic acetylcholine receptor-dependent persistent activity of layer 5 intrinsic-bursting and regular-spiking neurons in primary auditory cortex

Cholinergic signaling coupled to sensory-driven neuronal depolarization is essential for modulating lasting changes in deep-layer neural excitability and experience-dependent plasticity in the primary auditory cortex. However, the underlying cellular mechanism(s) associated with coincident cholinergic receptor activation and neuronal depolarization of deep-layer cortical neurons remains unknown. Using in vitro whole cell patch-clamp recordings targeted to neurons (n = 151) in isolated brain slices containing the primary auditory cortex (AI), we investigated the effects of cholinergic receptor activation and neuronal depolarization on the electrophysiological properties of AI layer 5 intrinsic-bursting and regular-spiking neurons. Bath application of carbachol (5 µM; cholinergic receptor agonist) paired with suprathreshold intracellular depolarization led to persistent activity in these neurons. Persistent activity may involve similar cellular mechanisms and be generated intrinsically in both intrinsic-bursting and regular-spiking neurons given that it 1) persisted under the blockade of ionotropic glutamatergic (kynurenic acid, 2 mM) and GABAergic receptors (picrotoxin, 100 µM), 2) was fully blocked by both atropine (10 µM; nonselective muscarinic antagonist) and flufenamic acid [100 µM; nonspecific Ca²⁺-sensitive cationic channel (CAN) blocker], and 3) was sensitive to the voltage-gated Ca²⁺ channel blocker nifedipine (50 µM) and Ca²⁺-free artificial cerebrospinal fluid. Together, our results support a model through which coincident activation of AI layer 5 neuron muscarinic receptors and suprathreshold activation can lead to sustained changes in layer 5 excitability, providing new insight into the possible role of a calcium-CAN-dependent cholinergic mechanism of AI cortical plasticity. These findings also indicate that distinct streams of auditory processing in layer 5 intrinsic-bursting and regular-spiking neurons may run in parallel during learning-induced auditory plasticity.NEW & NOTEWORTHY Cholinergic signaling coupled to sensory-driven neuronal depolarization is essential for modulating lasting changes in experience-dependent plasticity in the primary auditory cortex. Cholinergic activation together with cellular depolarization can lead to persistent activity in both intrinsic-bursting and regular-spiking layer 5 pyramidal neurons. A similar mechanism involving muscarinic acetylcholine receptor, voltage-gated Ca²⁺ channel, and possible Ca²⁺-sensitive nonspecific cationic channel activation provides new insight into our understanding of the cellular mechanisms that govern learning-induced auditory cortical and subcortical plasticity.

Cellular, Synaptic and Network Effects of Acetylcholine in the Neocortex

The neocortex is densely innervated by basal forebrain (BF) cholinergic neurons. Long-range axons of cholinergic neurons regulate higher-order cognitive function and dysfunction in the neocortex by releasing acetylcholine (ACh). ACh release dynamically reconfigures neocortical microcircuitry through differential spatiotemporal actions on cell-types and their synaptic connections. At the cellular level, ACh release controls neuronal excitability and firing rate, by hyperpolarizing or depolarizing target neurons. At the synaptic level, ACh impacts transmission dynamics not only by altering the presynaptic probability of release, but also the magnitude of the postsynaptic response. Despite the crucial role of ACh release in physiology and pathophysiology, a comprehensive understanding of the way it regulates the activity of diverse neocortical cell-types and synaptic connections has remained elusive. This review aims to summarize the state-of-the-art anatomical and physiological data to develop a functional map of the cellular, synaptic and microcircuit effects of ACh in the neocortex of rodents and non-human primates, and to serve as a quantitative reference for those intending to build data-driven computational models on the role of ACh in governing brain states.

Muscarinic and Nicotinic Modulation of Neocortical Layer 6A Synaptic Microcircuits Is Cooperative and Cell-Specific

Acetylcholine (ACh) is known to regulate cortical activity during different behavioral states, for example, wakefulness and attention. Here we show a differential expression of muscarinic ACh receptors (mAChRs) and nicotinic ACh receptors (nAChRs) in different layer 6A (L6A) pyramidal cell (PC) types of somatosensory cortex. At low concentrations, ACh induced a persistent hyperpolarization in corticocortical (CC) but a depolarization in corticothalamic (CT) L6A PCs via M ₄ and M₁ mAChRs, respectively. At ~ 1 mM, ACh depolarized exclusively CT PCs via α₄β₂ subunit-containing nAChRs without affecting CC PCs. Miniature EPSC frequency in CC PCs was decreased by ACh but increased in CT PCs. In synaptic connections with a presynaptic CC PC, glutamate release was suppressed via M₄ mAChR activation but enhanced by nAChRs via α₄β₂ nAChRs when the presynaptic neuron was a CT PC. Thus, in L6A, the interaction of mAChRs and nAChRs results in an altered excitability and synaptic release, effectively strengthening CT output while weakening CC synaptic signaling.

Involvement of extrasynaptic glutamate in physiological and pathophysiological changes of neuronal excitability

Glutamate is the most abundant neurotransmitter of the central nervous system, as the majority of neurons use glutamate as neurotransmitter. It is also well known that this neurotransmitter is not restricted to synaptic clefts, but found in the extrasynaptic regions as ambient glutamate. Extrasynaptic glutamate originates from spillover of synaptic release, as well as from astrocytes and microglia. Its concentration is magnitudes lower than in the synaptic cleft, but receptors responding to it have higher affinity for it. Extrasynaptic glutamate receptors can be found in neuronal somatodendritic location, on astroglia, oligodendrocytes or microglia. Activation of them leads to changes of neuronal excitability with different amplitude and kinetics. Extrasynaptic glutamate is taken up by neurons and astrocytes mostly via EAAT transporters, and astrocytes, in turn metabolize it to glutamine. Extrasynaptic glutamate is involved in several physiological phenomena of the central nervous system. It regulates neuronal excitability and synaptic strength by involving astroglia; contributing to learning and memory formation, neurosecretory and neuromodulatory mechanisms, as well as sleep homeostasis.The extrasynaptic glutamatergic system is affected in several brain pathologies related to excitotoxicity, neurodegeneration or neuroinflammation. Being present in dementias, neurodegenerative and neuropsychiatric diseases or tumor invasion in a seemingly uniform way, the system possibly provides a common component of their pathogenesis. Although parts of the system are extensively discussed by several recent reviews, in this review I attempt to summarize physiological actions of the extrasynaptic glutamate on neuronal excitability and provide a brief insight to its pathology for basic understanding of the topic.

Conditional Bistability, a Generic Cellular Mnemonic Mechanism for Robust and Flexible Working Memory Computations

Persistent neural activity, the substrate of working memory, is thought to emerge from synaptic reverberation within recurrent networks. However, reverberation models do not robustly explain the fundamental dynamics of persistent activity, including high-spiking irregularity, large intertrial variability, and state transitions. While cellular bistability may contribute to persistent activity, its rigidity appears incompatible with persistent activity labile characteristics. Here, we unravel in a cellular model a form of spike-mediated conditional bistability that is robust and generic. and provides a rich repertoire of mnemonic computations. Under asynchronous synaptic inputs of the awakened state, conditional bistability generates spiking/bursting episodes, accounting for the irregularity, variability, and state transitions characterizing persistent activity. This mechanism has likely been overlooked because of the subthreshold input it requires, and we predict how to assess it experimentally. Our results suggest a reexamination of the role of intrinsic properties in the collective network dynamics responsible for flexible working memory.SIGNIFICANCE STATEMENT This study unravels a novel form of intrinsic neuronal property: conditional bistability. We show that, thanks to its conditional character, conditional bistability favors the emergence of flexible and robust forms of persistent activity in PFC neural networks, in opposition to previously studied classical forms of absolute bistability. Specifically, we demonstrate for the first time that conditional bistability (1) is a generic biophysical spike-dependent mechanism of layer V pyramidal neurons in the PFC and that (2) it accounts for essential neurodynamical features for the organization and flexibility of PFC persistent activity (the large irregularity and intertrial variability of the discharge and its organization under discrete stable states), which remain unexplained in a robust fashion by current models.

Do TRPC channels support working memory? Comparing modulations of TRPC channels and working memory through G-protein coupled receptors and neuromodulators

Working memory is a crucial ability we use in daily life. However, the cellular mechanisms supporting working memory still remain largely unclear. A key component of working memory is persistent neural firing which is believed to serve short-term (hundreds of milliseconds up to tens of seconds) maintenance of necessary information. In this review, we will focus on the role of transient receptor potential canonical (TRPC) channels as a mechanism underlying persistent firing. Many years of in vitro work have been suggesting a crucial role of TRPC channels in working memory and temporal association tasks. If TRPC channels are indeed a central mechanism for working memory, manipulations which impair or facilitate working memory should have a similar effect on TRPC channel modulation. However, modulations of working memory and TRPC channels were never systematically compared, and it remains unanswered whether TRPC channels indeed contribute to working memory in vivo or not. In this article, we review the effects of G-protein coupled receptors (GPCR) and neuromodulators, including acetylcholine, noradrenalin, serotonin and dopamine, on working memory and TRPC channels. Based on comparisons, we argue that GPCR and downstream signaling pathways that activate TRPC, generally support working memory, while those that suppress TRPC channels impair it. However, depending on the channel types, areas, and systems tested, this is not the case in all studies. Further work to clarify involvement of specific TRPC channels in working memory tasks and how they are affected by neuromodulators is still necessary in the future.

Nonlinear Relationship Between Spike-Dependent Calcium Influx and TRPC Channel Activation Enables Robust Persistent Spiking in Neurons of the Anterior Cingulate Cortex

Continuation of spiking after a stimulus ends (i.e. persistent spiking) is thought to support working memory. Muscarinic receptor activation enables persistent spiking among synaptically isolated pyramidal neurons in anterior cingulate cortex (ACC), but a detailed characterization of that spiking is lacking and the underlying mechanisms remain unclear. Here, we show that the rate of persistent spiking in ACC neurons is insensitive to the intensity and number of triggers, but can be modulated by injected current, and that persistent spiking can resume after several seconds of hyperpolarization-imposed quiescence. Using electrophysiology and calcium imaging in brain slices from male rats, we determined that canonical transient receptor potential (TRPC) channels are necessary for persistent spiking and that TRPC-activating calcium enters in a spike-dependent manner via voltage-gated calcium channels. Constrained by these biophysical details, we built a computational model that reproduced the observed pattern of persistent spiking. Nonlinear dynamical analysis of that model revealed that TRPC channels become fully activated by the small rise in intracellular calcium caused by evoked spikes. Calcium continues to rise during persistent spiking, but because TRPC channel activation saturates, firing rate stabilizes. By calcium rising higher than required for maximal TRPC channel activation, TRPC channels are able to remain active during periods of hyperpolarization-imposed quiescence (until calcium drops below saturating levels) such that persistent spiking can resume when hyperpolarization is discontinued. Our results thus reveal that the robust intrinsic bistability exhibited by ACC neurons emerges from the nonlinear positive feedback relationship between spike-dependent calcium influx and TRPC channel activation.SIGNIFICANCE STATEMENT Neurons use action potentials, or spikes, to encode information. Some neurons can store information for short periods (seconds to minutes) by continuing to spike after a stimulus ends, thus enabling working memory. This so-called "persistent" spiking occurs in many brain areas and has been linked to activation of canonical transient receptor potential (TRPC) channels. However, TRPC activation alone is insufficient to explain many aspects of persistent spiking such as resumption of spiking after periods of imposed quiescence. Using experiments and simulations, we show that calcium influx caused by spiking is necessary and sufficient to activate TRPC channels and that the ensuing positive feedback interaction between intracellular calcium and TRPC channel activation can account for many hitherto unexplained aspects of persistent spiking.

Cell-Specific Cholinergic Modulation of Excitability of Layer 5B Principal Neurons in Mouse Auditory Cortex

The neuromodulator acetylcholine (ACh) is crucial for several cognitive functions, such as perception, attention, and learning and memory. Whereas, in most cases, the cellular circuits or the specific neurons via which ACh exerts its cognitive effects remain unknown, it is known that auditory cortex (AC) neurons projecting from layer 5B (L5B) to the inferior colliculus, corticocollicular neurons, are required for cholinergic-mediated relearning of sound localization after occlusion of one ear. Therefore, elucidation of the effects of ACh on the excitability of corticocollicular neurons will bridge the cell-specific and cognitive properties of ACh. Because AC L5B contains another class of neurons that project to the contralateral cortex, corticocallosal neurons, to identify the cell-specific mechanisms that enable corticocollicular neurons to participate in sound localization relearning, we investigated the effects of ACh release on both L5B corticocallosal and corticocollicular neurons. Using in vitro electrophysiology and optogenetics in mouse brain slices, we found that ACh generated nicotinic ACh receptor (nAChR)-mediated depolarizing potentials and muscarinic ACh receptor (mAChR)-mediated hyperpolarizing potentials in AC L5B corticocallosal neurons. In corticocollicular neurons, ACh release also generated nAChR-mediated depolarizing potentials. However, in contrast to the mAChR-mediated hyperpolarizing potentials in corticocallosal neurons, ACh generated prolonged mAChR-mediated depolarizing potentials in corticocollicular neurons. These prolonged depolarizing potentials generated persistent firing in corticocollicular neurons, whereas corticocallosal neurons lacking mAChR-mediated depolarizing potentials did not show persistent firing. We propose that ACh-mediated persistent firing in corticocollicular neurons may represent a critical mechanism required for learning-induced plasticity in AC.

SIGNIFICANCE STATEMENT

Acetylcholine (ACh) is crucial for cognitive functions. Whereas in most cases the cellular circuits or the specific neurons via which ACh exerts its cognitive effects remain unknown, it is known that auditory cortex (AC) corticocollicular neurons projecting from layer 5B to the inferior colliculus are required for cholinergic-mediated relearning of sound localization after occlusion of one ear. Therefore, elucidation of the effects of ACh on the excitability of corticocollicular neurons will bridge the cell-specific and cognitive properties of ACh. Our results suggest that cell-specific ACh-mediated persistent firing in corticocollicular neurons may represent a critical mechanism required for learning-induced plasticity in AC. Moreover, our results provide synaptic mechanisms via which ACh may mediate its effects on AC receptive fields.

Cav 1.3 channels play a crucial role in the formation of paroxysmal depolarization shifts in cultured hippocampal neurons

OBJECTIVE

An increase of neuronal Ca_v 1.3 L-type calcium channels (LTCCs) has been observed in various animal models of epilepsy. However, LTCC inhibitors failed in clinical trials of epileptic treatment. There is compelling evidence that paroxysmal depolarization shifts (PDSs) involve Ca²⁺ influx through LTCCs. PDSs represent a hallmark of epileptiform activity. In recent years, a probable epileptogenic role for PDSs has been proposed. However, the implication of the two neuronal LTCC isoforms, Ca_v 1.2 and Ca_v 1.3, in PDSs remained unknown. Moreover, Ca²⁺ -dependent nonspecific cation (CAN) channels have also been suspected to contribute to PDSs. Nevertheless, direct experimental support of an important role of CAN channel activation in PDS formation is still lacking.

METHODS

Primary neuronal networks derived from dissociated hippocampal neurons were generated from mice expressing a dihydropyridine-insensitive Ca_v 1.2 mutant (Ca_v 1.2DHP^-/- mice) or from Ca_v 1.3^-/- knockout mice. To investigate the role of Ca_v 1.2 and Ca_v 1.3, perforated patch-clamp recordings were made of epileptiform activity, which was elicited using either bicuculline or caffeine. LTCC activity was modulated using the dihydropyridines Bay K 8644 (agonist) and isradipine (antagonist).

RESULTS

Distinct PDS could be elicited upon LTCC potentiation in Ca_v 1.2DHP^-/- neurons but not in Ca_v 1.3^-/- neurons. In contrast, when bicuculline led to long-lasting, seizure-like discharge events rather than PDS, these were prolonged in Ca_v 1.3^-/- neurons but not in Ca_v 1.2DHP^-/- neurons. Because only the Ca_v 1.2 isoform is functionally coupled to CAN channels in primary hippocampal networks, PDS formation does not require CAN channel activity.

SIGNIFICANCE

Our data suggest that the LTCC requirement of PDS relates primarily to Ca_v 1.3 channels rather than to Ca_v 1.2 channels and CAN channels in hippocampal neurons. Hence, Ca_v 1.3 may represent a new therapeutic target for suppression of PDS development. The proposed epileptogenic role of PDSs may allow for a prophylactic rather than the unsuccessful seizure suppressing application of LTCC inhibitors.

A unifying hypothesis for M1 muscarinic receptor signalling in pyramidal neurons

KEY POINTS

Phasic release of acetylcholine (ACh) in the neocortex facilitates attentional processes. Acting at a single metabotropic receptor subtype, ACh exerts two opposing actions in cortical pyramidal neurons: transient inhibition and longer-lasting excitation. Cholinergic inhibitory responses depend on calcium release from intracellular calcium stores, and run down rapidly at resting membrane potentials when calcium stores become depleted. We demonstrate that cholinergic excitation promotes calcium entry at subthreshold membrane potentials to rapidly refill calcium stores, thereby maintaining the fidelity of inhibitory cholinergic signalling. We propose a 'unifying hypothesis' for M1 receptor signalling whereby inhibitory and excitatory responses to ACh in pyramidal neurons represent complementary mechanisms governing rapid calcium cycling between the endoplasmic reticulum, the cytosol and the extracellular space.

ABSTRACT

G_q -coupled M1-type muscarinic acetylcholine (ACh) receptors (mAChRs) mediate two distinct electrophysiological responses in cortical pyramidal neurons: transient inhibition driven by calcium-dependent small conductance potassium ('SK') channels, and longer-lasting and voltage-dependent excitation involving non-specific cation channels. Here we examine the interaction of these two cholinergic responses with respect to their contributions to intracellular calcium dynamics, testing the 'unifying hypothesis' that rundown of inhibitory SK responses at resting membrane potentials (RMPs) reflects depletion of intracellular calcium stores, while mAChR-driven excitation acts to refill those stores by promoting voltage-dependent entry of extracellular calcium. We report that fidelity of cholinergic SK responses requires the continued presence of extracellular calcium. Inhibitory responses that diminished after repetitive ACh application at RMPs were immediately rescued by pairing mAChR stimulation with subthreshold depolarization (∼10 mV from RMPs) initiated with variable delay (up to 500 ms) after ACh application, but not by subthreshold depolarization preceding mAChR stimulation. Further, rescued SK responses were time-locked to ACh application, rather than to the timing of subsequent depolarizing steps, suggesting that cholinergic signal transduction itself is not voltage-sensitive, but that depolarization facilitates rapid cycling of extracellular calcium through the endoplasmic reticulum to activate SK channels. Consistent with this prediction, rescue of SK responses by subthreshold depolarization required the presence of extracellular calcium. Our results demonstrate that, in addition to gating calcium release from intracellular stores, mAChR activation facilitates voltage-dependent refilling of calcium stores, thereby maintaining the ongoing fidelity of SK-mediated inhibition in response to phasic release of ACh.

L-type calcium channels and MAP kinase contribute to thyrotropin-releasing hormone-induced depolarization in thalamic paraventricular nucleus neurons

In rat paraventricular thalamic nucleus (PVT) neurons, activation of thyrotropin-releasing hormone (TRH) receptors enhances neuronal excitability via concurrent decrease in a G protein-coupled inwardly rectifying K (GIRK)-like conductance and opening of a cannabinoid receptor-sensitive transient receptor potential canonical (TRPC)-like conductance. Here, we investigated the calcium (Ca(2+)) contribution to the components of this TRH-induced response. TRH-induced membrane depolarization was reduced in the presence of intracellular BAPTA, also in media containing nominally zero [Ca(2+)]o, suggesting a critical role for both intracellular Ca(2+) release and Ca(2+) influx. TRH-induced inward current was unchanged by T-type Ca(2+) channel blockade, but was decreased by blockade of high-voltage-activated Ca(2+) channels (HVACCs). Both the pharmacologically isolated GIRK-like and the TRPC-like components of the TRH-induced response were decreased by nifedipine and increased by BayK8644, implying Ca(2+) influx via L-type Ca(2+) channels. Only the TRPC-like conductance was reduced by either thapsigargin or dantrolene, suggesting a role for ryanodine receptors and Ca(2+)-induced Ca(2+) release in this component of the TRH-induced response. In pituitary and other cell lines, TRH stimulates MAPK. In PVT neurons, only the GIRK-like component of the TRH-induced current was selectively decreased in the presence of PD98059, a MAPK inhibitor. Collectively, the data imply that TRH-induced depolarization and inward current in PVT neurons involve both a dependency on extracellular Ca(2+) influx via opening of L-type Ca(2+) channels, a sensitivity of a TRPC-like component to intracellular Ca(2+) release via ryanodine channels, and a modulation by MAPK of a GIRK-like conductance component.

Opposing presynaptic roles of BDNF and ProBDNF in the regulation of persistent activity in the entorhinal cortex

Background

Sustained, persistent firing (PF) of cortical pyramidal neurons following a short depolarization is a crucial cellular mechanism required for spatial and working memory. Pyramidal neurons in the superficial and deep layers of the medial and lateral entorhinal cortex (EC) display this property of prolonged firing activity. Here, we focused on the regulation of this activity in EC neurons by mature brain derived neurotrophic factor (BDNF) and its precursor proBDNF.

Results

Using patch clamp electrophysiology in acute mouse cortical slices, we observed that BDNF facilitates cholinergic PF in pyramidal neurons in layer V of the medial EC. Inhibition of TrkB with K252a blocks the potentiating effect of BDNF whereas inhibition of p75NTR with function-blocking antibodies does not. By recording spontaneous excitatory post-synaptic currents (sEPSC), we find that BDNF acts pre-synaptically via TrkB to increase glutamate release whereas proBDNF acting via p75NTR acts to reduce it. MPEP abolished the facilitating effect of BDNF on PF, demonstrating that the metabotropic glutamate receptor mGluR5 plays a critical role in the BDNF effect. In contrast, paired pulse ratio and EPSC measurements indicated that proBDNF, via presynaptic p75NTR, is a negative regulator of glutamate release in the EC.

Conclusions

Taken together, our findings demonstrate that the BDNF/TrkB pathway facilitates persistent activity whereas the proBDNF/p75NTR pathway inhibits this mnemonic property of entorhinal pyramidal neurons.

Peripheral Neuropathy Induces HCN Channel Dysfunction in Pyramidal Neurons of the Medial Prefrontal Cortex

Neuropathic pain is a debilitating condition for which the development of effective treatments has been limited by an incomplete understanding of its molecular basis. The cationic current Ih mediated by hyperpolarization-activated cyclic nucleotide-gated (HCN) channels plays an important role in pain by facilitating ectopic firing and hyperexcitability in DRG neurons, however little is known regarding the role of Ih in supraspinal pain pathways. The medial prefrontal cortex (mPFC), which is reported to be involved in the affective aspects of pain, exhibits high HCN channel expression. Using the spared nerve injury (SNI) model of neuropathic pain in Long-Evans rats and patch-clamp recordings in layer II/III pyramidal neurons of the contralateral mPFC, we observed a hyperpolarizing shift in the voltage-dependent activation of Ih in SNI neurons, whereas maximal Ih remained unchanged. Accordingly, SNI mPFC pyramidal neurons exhibited increased input resistance and excitability, as well as facilitated glutamatergic mGluR5-mediated persistent firing, compared with sham neurons. Moreover, intracellular application of bromo-cAMP abolished the hyperpolarizing shift in the voltage-dependent activation of Ih observed in SNI neurons, whereas protein kinase A (PKA) inhibition further promoted this shift in both SNI and sham neurons. Behaviorally, acute HCN channel blockade by local injection of ZD7288 in the mPFC of SNI rats induced a decrease in cold allodynia. These findings suggest that changes in the cAMP/PKA axis in mPFC neurons underlie alterations to HCN channel function, which can influence descending inhibition of pain pathways in neuropathic conditions. Significance statement: Recent studies investigating the role of the medial prefrontal cortex (mPFC) in neuropathic pain have led to an increased awareness of how affective and cognitive factors can influence pain perception. It is therefore imperative that we advance our understanding of the involvement of supraspinal pain pathways. Our electrophysiological and behavioral results support an important role for hyperpolarization-activated cyclic nucleotide-gated channels and the cAMP/protein kinase A signaling axis in promoting hyperexcitability and persistent firing in pyramidal neurons of the mPFC in neuropathic animals. These findings offer novel insights, with potential therapeutic implications, into pathophysiological mechanisms underlying the abnormal contribution of layer II/III prefrontal pyramidal neurons to chronic pain states.

Examination of Single Nucleotide Polymorphisms (SNPs) in Transient Receptor Potential (TRP) Ion Channels in Chronic Fatigue Syndrome Patients

Background

The transient receptor potential (TRP) superfamily in humans comprises 27 cation channels with permeability to monovalent and divalent cations. These channels are widely expressed within humans on cells and tissues and have significant sensory and regulatory roles on most physiological functions. Chronic fatigue syndrome (CFS) is an unexplained disorder with multiple physiological impairments.

OBJECTIVES

The purpose of this study was to determine the role of TRPs in CFS.

Methods

The study comprised 115 CFS patients (age = 48.68 ± 1.06 years) and 90 nonfatigued controls (age = 46.48 ± 1.22 years). CFS patients were defined according to the 1994 Center for Disease Prevention and Control criteria for CFS. A total of 240 single nucleotide polymorphisms (SNPs) for 21 mammalian TRP ion channel genes ( TRPA1, TRPC1, TRPC2, TRPC3, TRPC4, TRPC6, TRPC7, TRPM1, TRPM2, TRPM3, TRPM4, TRPM5, TRPM6, TRPM7, TRPM8, TRPV1, TRPV2, TRPV3, TRPV4, TRPV5, and TRPV6) were examined via the Agena Biosciences iPLEX Gold assay. Statistical analysis was performed using the PLINK analysis software.

Results

Thirteen SNPs were significantly associated with CFS patients compared with the controls. Nine of these SNPs were associated with TRPM3 (rs12682832; P < 0.003, rs11142508; P < 0.004, rs1160742; P < 0.08, rs4454352; P < 0.013, rs1328153; P < 0.013, rs3763619; P < 0.014, rs7865858; P ≤ 0.021, rs1504401; P ≤ 0041, rs10115622; P ≤ 0.050), while the remainder were associated with TRPA1 (rs2383844; P ≤ 0.040, rs4738202; P ≤ 0.018) and TRPC4 (rs6650469; P ≤ 0.016, rs655207; P ≤ 0.018).

Conclusion

The data from this pilot study suggest an association between TRP ion channels, predominantly TRPM3 and CFS. This and other TRPs identified may contribute to the etiology and pathomechanism of CFS.

Cooperativity between remote sites of ectopic spiking allows afterdischarge to be initiated and maintained at different locations

Many symptoms of nerve damage arise from ectopic spiking caused by hyperexcitability. Ectopic spiking can originate at the site of axonal damage and elsewhere within affected neurons. This raises the question of whether localized damage elicits cell-wide changes in excitability and/or if localized changes in excitability can drive abnormal spiking at remote locations. Computer modeling revealed an example of the latter involving afterdischarge (AD)--stimulus-evoked spiking that outlasts stimulation. We found that AD originating in a hyperexcitable region of axon could shift to the soma where it was maintained. This repositioning of ectopic spike initiation was independent of distance between the two sites but relied on the rate and number of ectopic spikes originating from the first site. Nonlinear dynamical analysis of a reduced model demonstrated that properties which rendered the axonal site prone to initiating AD discouraged it from maintaining AD, whereas the soma had the inverse properties thus enabling the two sites to interact cooperatively. A first phase of AD originating in the axon could, by providing sufficient drive to trigger somatic AD, give way to a second phase of AD originating in the soma such that spiking continued when axonal AD failed. Ectopic spikes originating from the soma during phase 2 AD propagated successfully through the defunct site of axonal spike initiation. This novel mechanism whereby ectopic spiking at one site facilitates ectopic spiking at another site is likely to contribute to the chronification of hyperexcitability in conditions such as neuropathic pain.

A robust in vivo-like persistent firing supported by a hybrid of intracellular and synaptic mechanisms

Persistent firing is believed to support short-term information retention in the brain. Established hypotheses make use of the recurrent synaptic connectivity to support persistent firing. However, this mechanism is known to suffer from a lack of robustness. On the other hand, persistent firing can be supported by an intrinsic cellular mechanism in multiple brain areas. However, the consequences of having both the intrinsic and the synaptic mechanisms (a hybrid model) on persistent firing remain largely unknown. The goal of this study is to investigate whether a hybrid neural network model with these two mechanisms has advantages over a conventional recurrent network based model. Our computer simulations were based on in vitro recordings obtained from hippocampal CA3 pyramidal cells under cholinergic receptor activation. Calcium activated non-specific cationic (CAN) current supported persistent firing in the Hodgkin-Huxley style cellular models. Our results suggest that the hybrid model supports persistent firing within a physiological frequency range over a wide range of different parameters, eliminating parameter sensitivity issues generally recognized in network based persistent firing. In addition, persistent firing in the hybrid model is substantially more robust against distracting inputs, can coexist with theta frequency oscillations, and supports pattern completion.

Acetylcholine excites neocortical pyramidal neurons via nicotinic receptors

The neuromodulator acetylcholine (ACh) shapes neocortical function during sensory perception, motor control, arousal, attention, learning, and memory. Here we investigate the mechanisms by which ACh affects neocortical pyramidal neurons in adult mice. Stimulation of cholinergic axons activated muscarinic and nicotinic ACh receptors on pyramidal neurons in all cortical layers and in multiple cortical areas. Nicotinic receptor activation evoked short-latency, depolarizing postsynaptic potentials (PSPs) in many pyramidal neurons. Nicotinic receptor-mediated PSPs promoted spiking of pyramidal neurons. The duration of the increase in spiking was membrane potential dependent, with nicotinic receptor activation triggering persistent spiking lasting many seconds in neurons close to threshold. Persistent spiking was blocked by intracellular BAPTA, indicating that nicotinic ACh receptor activation evoked persistent spiking via a long-lasting calcium-activated depolarizing current. We compared nicotinic PSPs in primary motor cortex (M1), prefrontal cortex (PFC), and visual cortex. The laminar pattern of nicotinic excitation was not uniform but was broadly similar across areas, with stronger modulation in deep than superficial layers. Superimposed on this broad pattern were local differences, with nicotinic PSPs being particularly large and common in layer 5 of M1 but not layer 5 of PFC or primary visual cortex (V1). Hence, in addition to modulating the excitability of pyramidal neurons in all layers via muscarinic receptors, synaptically released ACh preferentially increases the activity of deep-layer neocortical pyramidal neurons via nicotinic receptors, thereby adding laminar selectivity to the widespread enhancement of excitability mediated by muscarinic ACh receptors.

Oral administration of the p38α MAPK inhibitor, UR13870, inhibits affective pain behavior after spinal cord injury

The p38α mitogenous activated protein kinase (MAPK) cell signaling pathway is a key mechanism of microglia activation and has been studied as a target for neuropathic pain. The effect of UR13870, a p38α MAPK inhibitor, on microglia expression in the anterior cingulate cortex (ACC) and spinal dorsal horn was addressed after T9 contusion spinal cord injury (SCI) in the rat, in addition to behavioral testing of pain-related aversion and anxiety. Administration of intravenous UR13870 (1mg/kg i.v.) and pregabalin (30 mg/kg i.v.) reduced place escape avoidance paradigm (PEAP) but did not affect open-field anxiety behavior 42 days after SCI. PEAP behavior was also reduced in animals administered daily with oral UR13870 (10mg/kg p.o.) and preserved spinal tissue 28 days after SCI. Although UR13870 (10mg/kg p.o.) failed to reduce OX-42 and glial fibrillar acid protein immunoreactivity within the spinal dorsal horn, a reduction toward the control level was observed close to the SCI site. In the anterior cingulate cortex (ACC), a significant increase in OX-42 immunoreactivity was identified after SCI. UR13870 (10mg/kg p.o.) treatment significantly reduced OX-42, metabotropic glutamate type 5 receptor (mGluR5), and NMDA (N-methyl-d-aspartate) 2B subunit receptor (NR2B) expression in the ACC after SCI. To conclude, oral treatment with a p38α MAPK inhibitor reduces the affective behavioral component of pain after SCI in association with a reduction of microglia and specific glutamate receptors within the ACC. Nevertheless the role of neuroinflammatory processes within the vicinity of the SCI site in the development of affective neuropathic pain cannot be excluded.

Early stress prevents the potentiation of muscarinic excitation by calcium release in adult prefrontal cortex

BACKGROUND

The experience of early stress contributes to the etiology of several psychiatric disorders and can lead to lasting deficits in working memory and attention. These executive functions require activation of the prefrontal cortex (PFC) by muscarinic M1 acetylcholine (ACh) receptors. Such Gαq-protein coupled receptors trigger the release of calcium (Ca(2+)) from internal stores and elicit prolonged neuronal excitation.

METHODS

In brain slices of rat PFC, we employed multiphoton imaging simultaneously with whole-cell electrophysiological recordings to examine potential interactions between ACh-induced Ca(2+) release and excitatory currents in adulthood, across postnatal development, and following the early stress of repeated maternal separation, a rodent model for depression. We also investigated developmental changes in related genes in these groups.

RESULTS

Acetylcholine-induced Ca(2+) release potentiates ACh-elicited excitatory currents. In the healthy PFC, this potentiation of muscarinic excitation emerges in young adulthood, when executive function typically reaches maturity. However, the developmental consolidation of muscarinic ACh signaling is abolished in adults with a history of early stress, where ACh responses retain an adolescent phenotype. In prefrontal cortex, these rats show a disruption in the expression of multiple developmentally regulated genes associated with Gαq and Ca(2+) signaling. Pharmacologic and ionic manipulations reveal that the enhancement of muscarinic excitation in the healthy adult PFC arises via the electrogenic process of sodium/Ca(2+) exchange.

CONCLUSIONS

This work illustrates a long-lasting disruption in ACh-mediated cortical excitation following early stress and raises the possibility that such cellular mechanisms may disrupt the maturation of executive function.

Spatiotemporal dissociation of brain activity underlying subjective awareness, objective performance and confidence

Despite intense recent research, the neural correlates of conscious visual perception remain elusive. The most established paradigm for studying brain mechanisms underlying conscious perception is to keep the physical sensory inputs constant and identify brain activities that correlate with the changing content of conscious awareness. However, such a contrast based on conscious content alone would not only reveal brain activities directly contributing to conscious perception, but also include brain activities that precede or follow it. To address this issue, we devised a paradigm whereby we collected, trial-by-trial, measures of objective performance, subjective awareness, and the confidence level of subjective awareness. Using magnetoencephalography recordings in healthy human volunteers, we dissociated brain activities underlying these different cognitive phenomena. Our results provide strong evidence that widely distributed slow cortical potentials (SCPs) correlate with subjective awareness, even after the effects of objective performance and confidence were both removed. The SCP correlate of conscious perception manifests strongly in its waveform, phase, and power. In contrast, objective performance and confidence were both contributed by relatively transient brain activity. These results shed new light on the brain mechanisms of conscious, unconscious, and metacognitive processing.

Scale-free brain activity: past, present, and future

Brain activity observed at many spatiotemporal scales exhibits a 1/f-like power spectrum, including neuronal membrane potentials, neural field potentials, noninvasive electroencephalography (EEG), magnetoencephalography (MEG), and functional magnetic resonance imaging (fMRI) signals. A 1/f-like power spectrum is indicative of arrhythmic brain activity that does not contain a predominant temporal scale (hence, 'scale-free'). This characteristic of scale-free brain activity distinguishes it from brain oscillations. Although scale-free brain activity and brain oscillations coexist, our understanding of the former remains limited. Recent research has shed light on the spatiotemporal organization, functional significance, and potential generative mechanisms of scale-free brain activity, as well as its developmental and clinical relevance. A deeper understanding of this prevalent brain signal should provide new insights into, and analytical tools for, cognitive neuroscience.

Spatiotemporal specificity in cholinergic control of neocortical function

Cholinergic actions are critical for normal cortical cognitive functions. The release of acetylcholine (ACh) in neocortex and the impact of this neuromodulator on cortical computations exhibit remarkable spatiotemporal precision, as required for the regulation of behavioral processes underlying attention and learning. We discuss how the organization of the cholinergic projections to the cortex and their release properties might contribute to this specificity. We also review recent studies suggesting that the modulatory influences of ACh on the properties of cortical neurons can have the necessary temporal dynamic range, emphasizing evidence of powerful interneuron subtype-specific effects. We discuss areas that require further investigation and point to technical advances in molecular and genetic manipulations that promise to make headway in understanding the neural bases of cholinergic modulation of cortical cognitive operations.

Cholinergic modulation of the medial prefrontal cortex: the role of nicotinic receptors in attention and regulation of neuronal activity

Acetylcholine (ACh) release in the medial prefrontal cortex (mPFC) is crucial for normal cognitive performance. Despite the fact that many have studied how ACh affects neuronal processing in the mPFC and thereby influences attention behavior, there is still a lot unknown about how this occurs. Here we will review the evidence that cholinergic modulation of the mPFC plays a role in attention and we will summarize the current knowledge about the role between ACh receptors (AChRs) and behavior and how ACh receptor activation changes processing in the cortical microcircuitry. Recent evidence implicates fast phasic release of ACh in cue detection and attention. This review will focus mainly on the fast ionotropic nicotinic receptors and less on the metabotropic muscarinic receptors. Finally, we will review limitations of the existing studies and address how innovative technologies might push the field forward in order to gain understanding into the relation between ACh, neuronal activity and behavior.

Norepinephrine drives persistent activity in prefrontal cortex via synergistic α1 and α2 adrenoceptors

Optimal norepinephrine levels in the prefrontal cortex (PFC) increase delay-related firing and enhance working memory, whereas stress-related or pathologically high levels of norepinephrine are believed to inhibit working memory via α1 adrenoceptors. However, it has been shown that activation of Gq-coupled and phospholipase C-linked receptors can induce persistent firing, a cellular correlate of working memory, in cortical pyramidal neurons. Therefore, despite its importance in stress and cognition, the exact role of norepinephrine in modulating PFC activity remains elusive. Using electrophysiology and optogenetics, we report here that norepinephrine induces persistent firing in pyramidal neurons of the PFC independent of recurrent fast synaptic excitation. This persistent excitatory effect involves presynaptic α1 adrenoceptors facilitating glutamate release and subsequent activation of postsynaptic mGluR5 receptors, and is enhanced by postsynaptic α2 adrenoceptors inhibiting HCN channel activity. Activation of α2 adrenoceptors or inhibition of HCN channels also enhances cholinergic persistent responses in pyramidal neurons, providing a mechanism of crosstalk between noradrenergic and cholinergic inputs. The present study describes a novel cellular basis for the noradrenergic control of cortical information processing and supports a synergistic combination of intrinsic and network mechanisms for the expression of mnemonic properties in pyramidal neurons.

Nanomolar ouabain augments Ca2+ signalling in rat hippocampal neurones and glia

Linkage of certain neurological diseases to Na(+) pump mutations and some mood disorders to altered Na(+) pump function has renewed interest in brain Na(+) pumps. We tested nanomolar ouabain on Ca(2+) signalling (fura-2) in rat hippocampal neurone-astrocyte co-cultures. The neurones and astrocytes express Na(+) pumps with a high-ouabain-affinity catalytic subunit (α3 and α2, respectively); both also express pumps with a ouabain-resistant α1 subunit. Neurones and astrocytes were identified by immunocytochemistry and by stimulation; 3-4 μM L-glutamate (Glu) and 3 μM carbachol (CCh) evoked rapid Ca(2+) transients only in neurones, and small, delayed transients in some astrocytes, whereas 0.5-1 μM ATP evoked Ca(2+) transients only in astrocytes. Both cell types responded to 5-10 μM Glu or ATP. The signals evoked by 3-4 μM Glu in neurones were markedly inhibited by 3-10 μm MPEP (blocks metabotropic glutamate receptor mGluR5) and 10 μm LY341495 (non-selective mGluR blocker), but not by 80 μm AP5 (NMDA receptor blocker) or by selective block of mGluR1 or mGluR2. Pre-incubation (0.5-10 min) with 1-10 nm ouabain (EC50 < 1 nm) augmented Glu- and CCh-evoked signals in neurones. This augmentation was abolished by a blocker of the Na(+)-Ca(2+) exchanger, SEA0400 (300 nm). Ouabain (3 nm) pre-incubation also augmented 10 μM cyclopiazonic acid plus 10 mm caffeine-evoked release of Ca(2+) from the neuronal endoplasmic reticulum (ER). The implication is that nanomolar ouabain inhibits α3 Na(+) pumps, increases (local) intracellular Na(+), and promotes Na(+)-Ca(2+) exchanger-mediated Ca(2+) gain and increased storage in the adjacent ER. Ouabain (3 nm) also increased ER Ca(2+) release and enhanced 0.5 μM ATP-evoked transients in astrocytes; these effects were mediated by α2 Na(+) pumps. Thus, nanomolar ouabain may strongly influence synaptic transmission in the brain as a result of its actions on the high-ouabain-affinity Na(+) pumps in both neurones and astrocytes. The significance of these effects is heightened by the evidence that ouabain is endogenous in mammals.

Muscarinic receptors in amygdala control trace fear conditioning

Intelligent behavior requires transient memory, which entails the ability to retain information over short time periods. A newly-emerging hypothesis posits that endogenous persistent firing (EPF) is the neurophysiological foundation for aspects or types of transient memory. EPF is enabled by the activation of muscarinic acetylcholine receptors (mAChRs) and is triggered by suprathreshold stimulation. EPF occurs in several brain regions, including the lateral amygdala (LA). The present study examined the role of amygdalar mAChRs in trace fear conditioning, a paradigm that requires transient memory. If mAChR-dependent EPF selectively supports transient memory, then blocking amygdalar mAChRs should impair trace conditioning, while sparing delay and context conditioning, which presumably do not rely upon transient memory. To test the EPF hypothesis, LA was bilaterally infused, prior to trace or delay conditioning, with either a mAChR antagonist (scopolamine) or saline. Computerized video analysis quantified the amount of freezing elicited by the cue and by the training context. Scopolamine infusion profoundly reduced freezing in the trace conditioning group but had no significant effect on delay or context conditioning. This pattern of results was uniquely anticipated by the EPF hypothesis. The present findings are discussed in terms of a systems-level theory of how EPF in LA and several other brain regions might help support trace fear conditioning.

Dual functions of perirhinal cortex in fear conditioning

The present review examines the role of perirhinal cortex (PRC) in Pavlovian fear conditioning. The focus is on rats, partly because so much is known, behaviorally and neurobiologically, about fear conditioning in these animals. In addition, the neuroanatomy and neurophysiology of rat PRC have been described in considerable detail at the cellular and systems levels. The evidence suggests that PRC can serve at least two types of mnemonic functions in Pavlovian fear conditioning. The first function, termed "stimulus unitization," refers to the ability to treat two or more separate items or stimulus elements as a single entity. Supporting evidence for this perceptual function comes from studies of context conditioning as well as delay conditioning to discontinuous auditory cues. In a delay paradigm, the conditional stimulus (CS) and unconditional stimulus (US) overlap temporally and co-terminate. The second PRC function entails a type of "transient memory." Supporting evidence comes from studies of trace cue conditioning, where there is a temporal gap or trace interval between the CS offset and the US onset. For learning to occur, there must be a transient CS representation during the trace interval. We advance a novel neurophysiological mechanism for this transient representation. These two hypothesized functions of PRC are consistent with inferences based on non-aversive forms of learning.

Impairment of long-term depression in the anterior cingulate cortex of mice with bone cancer pain

The anterior cingulate cortex (ACC) has been shown to play an important role in pain-related perception and chronic pain. However, little is known about the molecular mechanisms involved. To address this issue, we analyzed excitatory synaptic transmission and long-term synaptic plasticity in layer II/III pyramidal neurons within the rostral ACC (rACC) from mice with bone cancer pain induced by intra-tibia implantation of osteolytic fibrosarcoma cells. Ex vivo whole-cell patch-clamp recordings from rACC neurons showed no significant alterations in presynaptic glutamate release probability and postsynaptic α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor-mediated synaptic responses in mice with bone cancer pain. However, mechanical allodynia occurred in conjunction with decreased N-methyl-d-aspartate (NMDA)/AMPA ratio of synaptic currents elicited in bilateral rACC neurons. In addition, the induction of NMDA receptor-dependent long-term depression (LTD) at rACC synapses was impaired in rACC neurons of tumor-bearing mice. Western blot analysis revealed a significant decrease in the levels of NR1, NR2A, and NR2B subunits of NMDA receptors in the rACC under bone cancer pain condition. No significant changes in overall mRNA levels for any of the NMDA receptor subunits or calpain activity were observed in the rACC of tumor-bearing mice. These results indicate that tumor-induced injury or remodeling of primary afferent sensory nerve fibers that innervate the tumor-bearing bone may cause a persistent decrease in NMDA receptor expression in rACC neurons, resulting in a loss of LTD induction, thereby leading to long-term alterations of rACC activity and creating exaggerated pain behaviors.

Effects of acetylcholine on neuronal properties in entorhinal cortex

The entorhinal cortex (EC) receives prominent cholinergic innervation from the medial septum and the vertical limb of the diagonal band of Broca (MSDB). To understand how cholinergic neurotransmission can modulate behavior, research has been directed toward identification of the specific cellular mechanisms in EC that can be modulated through cholinergic activity. This review focuses on intrinsic cellular properties of neurons in EC that may underlie functions such as working memory, spatial processing, and episodic memory. In particular, the study of stellate cells (SCs) in medial entorhinal has resulted in discovery of correlations between physiological properties of these neurons and properties of the unique spatial representation that is demonstrated through unit recordings of neurons in medial entorhinal cortex (mEC) from awake-behaving animals. A separate line of investigation has demonstrated persistent firing behavior among neurons in EC that is enhanced by cholinergic activity and could underlie working memory. There is also evidence that acetylcholine plays a role in modulation of synaptic transmission that could also enhance mnemonic function in EC. Finally, the local circuits of EC demonstrate a variety of interneuron physiology, which is also subject to cholinergic modulation. Together these effects alter the dynamics of EC to underlie the functional role of acetylcholine in memory.

Calcium-activated sustained firing responses distinguish accessory from main olfactory bulb mitral cells

Many mammals rely on pheromones for mediating social interactions. Recent studies indicate that both the main olfactory system (MOS) and accessory olfactory system (AOS) detect and process pheromonal stimuli, yet the functional difference between these two chemosensory systems remains unclear. We hypothesized that the main functional distinction between the MOS and AOS is the type of sensory information processing performed by each system. Here we compared the electrophysiological responses of mitral cells recorded from the accessory olfactory bulb (AOB) and main olfactory bulb (MOB) in acute mouse brain slices to various stimuli and found them markedly different. The response of MOB mitral cells to brief (0.1 ms, 1-100 V) stimulation of their sensory afferents remained transient regardless of stimulus strength, whereas sufficiently strong stimuli evoked sustained firing in AOB mitral cells lasting up to several minutes. Using EPSC-like current injections (10-100 pA, 10 ms rise time constant, 5 s decay time constant) in the presence of various synaptic blockers (picrotoxin, CGP55845, APV, DNQX, E4CPG, and MSPG), we demonstrated that this difference is attributable to distinct intrinsic properties of the two neuronal populations. The AOB sustained responses were found to be mediated by calcium-activated nonselective cationic current induced by transient intense firing. This current was found to be at least partially mediated by TRPM4 channels activated by calcium influx. We hypothesize that the sustained activity of the AOS induces a new sensory state in the animal, reflecting its social context.

Acetylcholine-evoked afterdischarge in Aplysia bag cell neurons

A brief synaptic input to the bag cell neurons of Aplysia evokes a lengthy afterdischarge and the secretion of peptide hormones that trigger ovulation. The input transmitter is unknown, although prior work has shown that afterdischarges are prevented by strychnine. Because molluscan excitatory cholinergic synapses are blocked by strychnine, we tested the hypothesis that acetylcholine acts on an ionotropic receptor to initiate the afterdischarge. In cultured bag cell neurons, acetylcholine induced a short burst of action potentials followed by either return to near baseline or, like a true afterdischarge, transition to continuous firing. The current underlying the acetylcholine-induced depolarization was dose dependent, associated with increased membrane conductance, and sensitive to the nicotinic antagonists hexamethonium, mecamylamine, and α-conotoxin ImI. Whereas nicotine, choline, carbachol, and glycine did not mimic acetylcholine, tetramethylammonium did produce a similar current. Consistent with an ionotropic receptor, the response was not altered by intracellular dialysis with the G protein blocker guanosine 5′-(β-thio)diphosphate. Recording from the intact bag cell neuron cluster showed acetylcholine to evoke prominent depolarization, which often led to extended bursting, but only in the presence of the acetylcholinesterase inhibitor neostigmine. Extracellular recording confirmed that exogenous acetylcholine caused genuine afterdischarges, which, as per those generated synaptically, rendered the cluster refractory to further stimulation. Finally, treatment with a combination of mecamylamine and α-conotoxin ImI blocked synaptically induced afterdischarges in the intact bag cell neuron cluster. Acetylcholine appears to elicit the afterdischarge through an ionotropic receptor. This represents an expedient means for transient stimulation to elicit prolonged firing in the absence of ongoing synaptic input.

Ensembles of human MTL neurons "jump back in time" in response to a repeated stimulus

Episodic memory, which depends critically on the integrity of the medial temporal lobe (MTL), has been described as "mental time travel" in which the rememberer "jumps back in time." The neural mechanism underlying this ability remains elusive. Mathematical and computational models of performance in episodic memory tasks provide a specific hypothesis regarding the computation that supports such a jump back in time. The models suggest that a representation of temporal context, a representation that changes gradually over macroscopic periods of time, is the cue for episodic recall. According to these models, a jump back in time corresponds to a stimulus recovering a prior state of temporal context. In vivo single-neuron recordings were taken from the human MTL while epilepsy patients distinguished novel from repeated images in a continuous recognition memory task. The firing pattern of the ensemble of MTL neurons showed robust temporal autocorrelation over macroscopic periods of time during performance of the memory task. The gradually-changing part of the ensemble state was causally affected by the visual stimulus being presented. Critically, repetition of a stimulus caused the ensemble to elicit a pattern of activity that resembled the pattern of activity present before the initial presentation of the stimulus. These findings confirm a direct prediction of this class of temporal context models and may be a signature of the mechanism that underlies the experience of episodic memory as mental time travel.

Altered neocortical rhythmic activity states in Fmr1 KO mice are due to enhanced mGluR5 signaling and involve changes in excitatory circuitry

Despite the pronounced neurological deficits associated with mental retardation and autism, the degree to which neocortical circuit function is altered remains unknown. Here, we study changes in neocortical network function in the form of persistent activity states in the mouse model of fragile X syndrome--the Fmr1 knock-out (KO). Persistent activity states, or UP states, in the neocortex underlie the slow oscillation which occurs predominantly during slow-wave sleep, but may also play a role during awake states. We show that spontaneously occurring UP states in the primary somatosensory cortex are 38-67% longer in Fmr1 KO slices. In vivo, UP states reoccur with a clear rhythmic component consistent with that of the slow oscillation and are similarly longer in the Fmr1 KO. Changes in neocortical excitatory circuitry likely play the major role in this alteration as supported by three findings: (1) longer UP states occur in slices of isolated neocortex, (2) pharmacologically isolated excitatory circuits in Fmr1 KO neocortical slices display prolonged bursting states, and (3) selective deletion of Fmr1 in cortical excitatory neurons is sufficient to cause prolonged UP states whereas deletion in inhibitory neurons has no effect. Excess signaling mediated by the group 1 glutamate metabotropic receptor, mGluR5, contributes to the longer UP states. Genetic reduction or pharmacological blockade of mGluR5 rescues the prolonged UP state phenotype. Our results reveal an alteration in network function in a mouse model of intellectual disability and autism which may impact both slow-wave sleep and information processing during waking states.

Muscarinic receptor activation enables persistent firing in pyramidal neurons from superficial layers of dorsal perirhinal cortex

Persistent-firing neurons in the entorhinal cortex (EC) and the lateral nucleus of the amygdala (LA) continue to discharge long after the termination of the original, spike-initiating current. An emerging theory proposes that endogenous persistent firing helps support a transient memory system. This study demonstrated that persistent-firing neurons are also prevalent in rat perirhinal cortex (PR), which lies immediately adjacent to and is reciprocally connected with EC and LA. Several characteristics of persistent-firing neurons in PR were similar to those previously reported in LA and EC. Persistent firing in PR was enabled by the application of carbachol, a nonselective cholinergic agonist, and it was induced by injecting a suprathreshold current or by stimulating suprathreshold excitatory synaptic inputs to the neuron. Once induced, persistent firing lasted for seconds to minutes. Persistent firing could always be terminated by a sufficiently large and prolonged hyperpolarizing current; it was prevented by antagonists of muscarinic cholinergic receptors (mAChRs); and it was blocked by flufenamic acid. The latter has been suggested to inhibit a Ca(2+) -activated nonspecific cation conductance (G(CAN) ) that normally furnishes the sustained depolarization during persistent firing. In many PR neurons, the discharge rate during persistent firing was a graded function of depolarizing and/or hyperpolarizing inputs. Persistent firing was not prevented by blocking fast excitatory and inhibitory synaptic transmission, demonstrating that it can be generated endogenously. We suggest that persistent-firing neurons in PR, EC, LA, and certain other brain regions may cooperate in support of a transient-memory system.

Cholecystokinin facilitates neuronal excitability in the entorhinal cortex via activation of TRPC-like channels

Cholecystokinin (CCK) is one of the most abundant neuropeptides in the brain, where it interacts with two G protein-coupled receptors (CCK-1 and CCK-2). Activation of both CCK receptors increases the activity of PLC, resulting in increases in intracellular calcium ion (Ca(2+)) release and activation of PKC. Whereas high density of CCK receptors has been detected in the superficial layers of the entorhinal cortex (EC), the functions of CCK in this brain region have not been determined. Here, we studied the effects of CCK on neuronal excitability of layer III pyramidal neurons in the EC. Our results showed that CCK remarkably increased the firing frequency of action potentials (APs). The effects of CCK on neuronal excitability were mediated via activation of CCK-2 receptors and required the functions of G proteins and PLC. However, CCK-mediated facilitation of neuronal excitability was independent of inositol trisphosphate receptors and PKC. CCK facilitated neuronal excitability by activating a cationic channel to generate membrane depolarization. The effects of CCK were suppressed by the generic, nonselective cationic channel blockers, 2-aminoethyldiphenyl borate and flufenamic acid, but potentiated by gadolinium ion and lanthanum ion at 100 μM. Depletion of extracellular Ca(2+) also counteracted CCK-induced increases in AC firing frequency. Moreover, CCK-induced enhancement of neuronal excitability was inhibited significantly by intracellular application of the antibody to transient receptor potential channel 5 (TRPC5), suggesting the involvement of TRPC5 channels. Our results provide a cellular and molecular mechanism to help explain the functions of CCK in vivo.

Projection-specific neuromodulation of medial prefrontal cortex neurons

Mnemonic persistent activity in the prefrontal cortex (PFC) constitutes the neural basis of working memory. To understand how neuromodulators contribute to the generation of persistent activity, it is necessary to identify the intrinsic properties of the layer V pyramidal neurons that transfer this information to downstream networks. Here we show that the somatic dynamic and integrative properties of layer V pyramidal neurons in the rat medial PFC depend on whether they project subcortically to the pons [corticopontine (CPn)] or to the contralateral cortex [commissural (COM)]. CPn neurons display low temporal summation and accelerate in firing frequency when depolarized, whereas COM neurons have high temporal summation and display spike frequency accommodation. In response to dynamic stimuli, COM neurons act as low-pass filters, whereas CPn neurons act as bandpass filters, resonating in the theta frequency range (3-6 Hz). The disparate subthreshold properties of COM and CPn neurons can be accounted for by differences in the hyperpolarization-activated cyclic nucleotide gated cation h-current. Interestingly, neuromodulators hypothesized to enhance mnemonic persistent activity affect COM and CPn neurons distinctly. Adrenergic modulation shifts the dynamic properties of CPn but not COM neurons and increases the excitability of CPn neurons significantly more than COM neurons. In response to cholinergic modulation, CPn neurons were much more likely to display activity-dependent intrinsic persistent firing than COM neurons. Together, these data suggest that the two categories of projection neurons may subserve separate functions in PFC and may be engaged differently during working memory processes.