Cholinergic excitation of mammalian hippocampal pyramidal cells

Muscarinic acetylcholine activity modulates cortical silent period, but not motor evoked potentials, during muscle contractions

This study used transcranial magnetic stimulation (TMS) to determine if muscarinic receptor blockade affects muscle responses during voluntary contractions. Motor evoked potentials (MEPs) were recorded from biceps brachii in 10 subjects (age: 23 ± 2) during 10%, 25%, 50%, 75%, and 100% maximal voluntary contractions (MVCs). Each contraction intensity was examined under non-fatigued and fatigued conditions. All measurements were obtained post-ingestion of 25 mg promethazine or placebo. MEP area and the duration of the TMS-evoked silent period (SP) were calculated for all contractions. No drug-related differences were detected for MEP area during non-fatigued or fatigued contractions. A main effect of drug was detected for the SP (p = 0.019) where promethazine increased SP duration by an average of 0.023 [Formula: see text] 0.015 s. This drug effect was only identified for the unfatigued contractions and not following the sustained fatiguing contractions (p = 0.105). The cholinergic system does not influence corticospinal excitability during voluntary muscle contractions, but instead affects neural circuits associated with the TMS-evoked SP. Given the prevalence of cholinergic properties in prescription and over-the-counter medications, the current study enhances our understanding of mechanisms that may contribute to motor side-effects.

Neural Burst Firing and Its Roles in Mental and Neurological Disorders

Neural firing patterns are critical for specific information coding and transmission, and abnormal firing is implicated in a series of neural pathologies. Recent studies have indicated that enhanced burst firing mediated by T-type voltage-gated calcium channels (T-VGCCs) in specific neuronal subtypes is involved in several mental or neurological disorders such as depression and epilepsy, while suppression of T-VGCCs relieve related symptoms. Burst firing consists of groups of relatively high-frequency spikes separated by quiescence. Neurons in a variety of brain areas, including the thalamus, hypothalamus, cortex, and hippocampus, display burst firing, but the ionic mechanisms that generating burst firing and the related physiological functions vary among regions. In this review, we summarize recent findings on the mechanisms underlying burst firing in various brain areas, as well as the roles of burst firing in several mental and neurological disorders. We also discuss the ion channels and receptors that may regulate burst firing directly or indirectly, with these molecules highlighted as potential intervention targets for the treatment of mental and neurological disorders.

Presynaptic Inhibitory Effects of Acetylcholine in the Hippocampus: A 40-Year Evolution of a Serendipitous Finding

Cholinergic regulation of hippocampal circuit activity has been an active area of neurophysiological research for decades. The prominent cholinergic innervation of intrinsic hippocampal circuitry, potent effects of cholinomimetic drugs, and behavioral responses to cholinergic modulation of hippocampal circuitry have driven investigators to discover diverse cellular actions of acetylcholine in distinct sites within hippocampal circuitry. Further research has illuminated how these actions organize circuit activity to optimize encoding of new information, promote consolidation, and coordinate this with recall of prior memories. The development of the hippocampal slice preparation was a major advance that accelerated knowledge of how hippocampal circuits functioned and how acetylcholine modulated these circuits. Using this preparation in the early 1980s, we made a serendipitous finding of a novel presynaptic inhibitory effect of acetylcholine on Schaffer collaterals, the projections from CA3 pyramidal neurons to dendrites of CA1 pyramidal cells. We characterized this effect at cellular and pharmacological levels, published the findings in the first volume of the Journal of Neuroscience, and proceeded to pursue other scientific directions. We were surprised and thrilled to see that, nearly 40 years later, the paper is still being cited and downloaded because the data became an integral piece of the foundation of the science of cholinergic regulation of hippocampal function in learning and memory. This Progressions article is a story of how single laboratory findings evolve through time to be confirmed, challenged, and reinterpreted by other laboratories to eventually become part of the basis of fundamental concepts related to important brain functions.

Noradrenergic Suppression of Persistent Firing in Hippocampal CA1 Pyramidal Cells through cAMP-PKA Pathway

Persistent firing is believed to be a cellular correlate of working memory. While the effects of noradrenaline (NA) on working memory have widely been described, its effect on the cellular mechanisms of persistent firing remains largely unknown. Using in vitro intracellular recordings, we demonstrate that persistent firing is supported by individual neurons in hippocampal CA1 pyramidal cells through cholinergic receptor activation, but is dramatically attenuated by NA. In contrast to the classical theory that recurrent synaptic excitation supports persistent firing, suppression of persistent firing by NA was independent of synaptic transmission, indicating that the mechanism is intrinsic to individual cells. In agreement with detrimental effects of cAMP on working memory, we demonstrate that the suppressive effect of NA was through cAMP-PKA pathway. In addition, activation of β1 and/or β3 adrenergic receptors, which increases cAMP levels, suppressed persistent firing. These results are in line with working memory decline observed during high levels of NA and cAMP, which are implicated in high stress, aging, and schizophrenia.

Scopolamine Impairs Spatial Information Recorded With "Miniscope" Calcium Imaging in Hippocampal Place Cells

The hippocampus and associated cholinergic inputs have important roles in spatial memory in rodents. Muscarinic acetylcholine receptors (mAChRs) are involved in the communication of cholinergic signals and regulate spatial memory. They have been found to impact the memory encoding process, but the effect on memory retrieval is controversial. Previous studies report that scopolamine (a non-selective antagonist of mAChR) induces cognitive deficits on animals, resulting in impaired memory encoding, but the effect on memory retrieval is less certain. We tested the effects of blocking mAChRs on hippocampal network activity and neural ensembles that had previously encoded spatial information. The activity of hundreds of neurons in mouse hippocampal CA1 was recorded using calcium imaging with a miniaturised fluorescent microscope and properties of place cells and neuronal ensemble behaviour in a linear track environment were observed. We found that the decoding accuracy and the stability of spatial representation revealed by hippocampal neural ensemble were significantly reduced after the administration of scopolamine. Several other parameters, including neural firing rate, total number of active neurons, place cell number and spatial information content were affected. Similar results were also observed in a simulated hippocampal network model. This study enhances the understanding of the function of mAChRs on spatial memory impairment.

Melamine Disrupts Acetylcholine-Mediated Neural Information Flow in the Hippocampal CA3-CA1 Pathway

Considering the cognitive and synaptic deficits following intragastric administration of melamine, the aim of the current investigation was to test whether the hippocampal oscillations were affected. The local field potential (LFP) was recorded in the hippocampal CA3–CA1 pathway of Wistar rats during a spatial-dependent Y-maze task. The general partial directed coherence (gPDC) method was used to assess the directionality of neural information flow (NIF) between the CA3 and CA1 regions. The levels of acetylcholine (ACh) and its esterolytic protease, acetylcholinesterase (AChE), were detected in the hippocampus (HPC) following the behavioral test. The values of phase synchronization between the CA3 and CA1 regions in delta, low theta, and high theta oscillations were reduced significantly in the melamine-treated group. Moreover, the coupling directional index and the strength of CA3 driving CA1 were critically decreased in the above three frequency bands as well. Meanwhile, a reduction in ACh expression and an enhancement in AChE activity were found in the HPC of melamine-treated rats. Intrahippocampal infusion with ACh could mitigate the weakened neural coupling and directional NIF in parallel with spatial learning improvements. However, infusion of scopolamine, an acetylcholine receptor antagonist, could block the mitigative effects of ACh treatment in melamine rats. These findings provide first evidence that ACh-mediated neuronal coupling and NIF in the CA3–CA1 pathway are involved in spatial learning deficits induced by chronic melamine exposure.

Involvement of TRPC4 and 5 Channels in Persistent Firing in Hippocampal CA1 Pyramidal Cells

Persistent neural activity has been observed in vivo during working memory tasks, and supports short-term (up to tens of seconds) retention of information. While synaptic and intrinsic cellular mechanisms of persistent firing have been proposed, underlying cellular mechanisms are not yet fully understood. In vitro experiments have shown that individual neurons in the hippocampus and other working memory related areas support persistent firing through intrinsic cellular mechanisms that involve the transient receptor potential canonical (TRPC) channels. Recent behavioral studies demonstrating the involvement of TRPC channels on working memory make the hypothesis that TRPC driven persistent firing supports working memory a very attractive one. However, this view has been challenged by recent findings that persistent firing in vitro is unchanged in TRPC knock out (KO) mice. To assess the involvement of TRPC channels further, we tested novel and highly specific TRPC channel blockers in cholinergically induced persistent firing in mice CA1 pyramidal cells for the first time. The application of the TRPC4 blocker ML204, TRPC5 blocker clemizole hydrochloride, and TRPC4 and 5 blocker Pico145, all significantly inhibited persistent firing. In addition, intracellular application of TRPC4 and TRPC5 antibodies significantly reduced persistent firing. Taken together these results indicate that TRPC4 and 5 channels support persistent firing in CA1 pyramidal neurons. Finally, we discuss possible scenarios causing these controversial observations on the role of TRPC channels in persistent firing.

Carbachol-Induced theta-like oscillations in the rodent brain limbic system: Underlying mechanisms and significance

Theta oscillations (4-12 Hz) represent one of the most prominent physiological oscillatory activity in the mammalian EEG. They are observed in several areas of the hippocampus and in parahippocampal structures. Theta oscillations play important roles in modulating synaptic plasticity during memory and learning; moreover, they are dependent on septal cholinergic inputs. Theta oscillations can be reproduced in vitro in several regions of the temporal lobe in the absence of the septum by employing the cholinergic agonist carbachol (CCh). Here, we review the mechanisms underlying CCh-induced theta oscillations. We address: (i) the ability of temporal lobe neuronal networks to oscillate independently at theta frequency during CCh treatment; (ii) the contribution of intrinsic ionic currents; (iii) the participation of principal cells and interneurons; and (iv) their pharmacological profiles. We also discuss the similarities between CCh-induced theta oscillations and physiological type II theta activity, as well as their roles in synaptic plasticity. Finally, we consider experimental evidence pointing to the contribution of spontaneous and CCh-induced theta activity to epileptiform synchronization.

Acetylcholine Facilitates a Depolarization-Induced Enhancement of Inhibition in Rat CA1 Pyramidal Neurons

Cholinergic mechanisms in the hippocampus regulate forms of synaptic plasticity linked with cognition and spatial navigation, but the underlying mechanisms remain largely unknown. Here, in rat hippocampal CA1 pyramidal cells under blockade of ionotropic glutamate receptors, we report that a single acetylcholine pulse and repeated depolarization activated a robust and enduring postsynaptic depolarization-induced enhancement of inhibition (DEI) that masked a presynaptic depolarization-induced suppression of inhibition (DSI). Increased cytosolic Ca2+ and M1-muscarinic receptor activation caused the rise in voltage-sensitive α5βγ2-containing γ-aminobutyric acid type-A receptors that generated DEI. In summary, this muscarinic-mediated activity-dependent plasticity rapidly transfers depolarization effects on inhibition from presynaptic suppression or DSI to postsynaptic enhancement or DEI, a change potentially relevant in behavior.

Modulation of Hippocampal Circuits by Muscarinic and Nicotinic Receptors

This article provides a review of the effects of activation of muscarinic and nicotinic receptors on the physiological properties of circuits in the hippocampal formation. Previous articles have described detailed computational hypotheses about the role of cholinergic neuromodulation in enhancing the dynamics for encoding in cortical structures and the role of reduced cholinergic modulation in allowing consolidation of previously encoded information. This article will focus on addressing the broad scope of different modulatory effects observed within hippocampal circuits, highlighting the heterogeneity of cholinergic modulation in terms of the physiological effects of activation of muscarinic and nicotinic receptors and the heterogeneity of effects on different subclasses of neurons.

IK1 channels do not contribute to the slow afterhyperpolarization in pyramidal neurons

In pyramidal neurons such as hippocampal area CA1 and basolateral amygdala, a slow afterhyperpolarization (sAHP) follows a burst of action potentials, which is a powerful regulator of neuronal excitability. The sAHP amplitude increases with aging and may underlie age related memory decline. The sAHP is due to a Ca²⁺-dependent, voltage-independent K⁺ conductance, the molecular identity of which has remained elusive until a recent report suggested the Ca²⁺-activated K⁺ channel, IK1 (KCNN4) as the sAHP channel in CA1 pyramidal neurons. The signature pharmacology of IK1, blockade by TRAM-34, was reported for the sAHP and underlying current. We have examined the sAHP and find no evidence that TRAM-34 affects either the current underling the sAHP or excitability of CA1 or basolateral amygdala pyramidal neurons. In addition, CA1 pyramidal neurons from IK1 null mice exhibit a characteristic sAHP current. Our results indicate that IK1 channels do not mediate the sAHP in pyramidal neurons.

DOI:http://dx.doi.org/10.7554/eLife.11206.001

Neurons carry signals in the form of electrical impulses called action potentials. These nerve impulses result from ions flowing through proteins called ion channels in the neuron’s membrane, and they determine how the neuron communicates with neighboring neurons. The number of action potentials a neuron can produce can vary over a wide range. In the brain, a particular kind of ion channel limits the number of action potentials that many neurons produce via a negative feedback mechanism. That is to say, nerve impulses activate this ion channel and the activated channel then makes the neuron less able to send further nerve impulses for a while.The activity of this ion channel increases with age and it may be responsible for some forms of age-related decline in cognitive abilities. However, the exact identity of the ion channel responsible was unclear.

Recent research has suggested the ion channel in question was a protein called IK1. This conclusion was largely based on how this ion channel responded to drugs in the laboratory. Wang, Materos-Aparico et al. sought to verify this conclusion and, in contrast with the previous reports, found that the IK1 ion channel did not respond to these drugs in the same way when it was in neurons in the brains of mice.

In further experiments, mice that had been engineered to lack the IK1 ion channel still showed the characteristic negative feedback that regulates the firing of action potentials. Thus, Wang, Materos-Aparico et al. found no evidence to support the previous conclusion, and instead conclude that the exact identity of this important ion channel in the brain has yet to be defined.

DOI:http://dx.doi.org/10.7554/eLife.11206.002

Induction of Anti-Hebbian LTP in CA1 Stratum Oriens Interneurons: Interactions between Group I Metabotropic Glutamate Receptors and M1 Muscarinic Receptors

An anti-Hebbian form of LTP is observed at excitatory synapses made with some hippocampal interneurons. LTP induction is facilitated when postsynaptic interneurons are hyperpolarized, presumably because Ca(2+) entry through Ca(2+)-permeable glutamate receptors is enhanced. The contribution of modulatory transmitters to anti-Hebbian LTP induction remains to be established. Activation of group I metabotropic receptors (mGluRs) is required for anti-Hebbian LTP induction in interneurons with cell bodies in the CA1 stratum oriens. This region receives a strong cholinergic innervation from the septum, and muscarinic acetylcholine receptors (mAChRs) share some signaling pathways and cooperate with mGluRs in the control of neuronal excitability.We therefore examined possible interactions between group I mGluRs and mAChRs in anti-Hebbian LTP at synapses which excite oriens interneurons in rat brain slices. We found that blockade of either group I mGluRs or M1 mAChRs prevented the induction of anti-Hebbian LTP by pairing presynaptic activity with postsynaptic hyperpolarization. Blocking either receptor also suppressed long-term effects of activation of the other G-protein coupled receptor on interneuron membrane potential. However, no crossed blockade was detected for mGluR or mAchR effects on interneuron after-burst potentials or on the frequency of miniature EPSPs. Paired recordings between pyramidal neurons and oriens interneurons were obtained to determine whether LTP could be induced without concurrent stimulation of cholinergic axons. Exogenous activation of mAChRs led to LTP, with changes in EPSP amplitude distributions consistent with a presynaptic locus of expression. LTP, however, required noninvasive presynaptic and postsynaptic recordings.

SIGNIFICANCE STATEMENT

In the hippocampus, a form of NMDA receptor-independent long-term potentiation (LTP) occurs at excitatory synapses made on some inhibitory neurons. This is preferentially induced when postsynaptic interneurons are hyperpolarized, depends on Ca(2+) entry through Ca(2+)-permeable AMPA receptors, and has been labeled anti-Hebbian LTP. Here we show that this form of LTP also depends on activation of both group I mGluR and M1 mAChRs. We demonstrate that these G-protein coupled receptors (GPCRs) interact, because the blockade of one receptor suppresses long-term effects of activation of the other GPCR on both LTP and interneuron membrane potential. This LTP was also detected in paired recordings, although only when both presynaptic and postsynaptic recordings did not perturb the intracellular medium. Changes in EPSP amplitude distributions in dual recordings were consistent with a presynaptic locus of expression.

Transition between encoding and consolidation/replay dynamics via cholinergic modulation of CAN current: A modeling study

Hippocampal place cells that are activated sequentially during active waking get reactivated in a temporally compressed (5-20 times) manner during slow-wave-sleep and quiet waking. The two-stage model of the hippocampus suggests that neural activity during awaking supports encoding function while temporally compressed reactivation (replay) supports consolidation. However, the mechanisms supporting different neural activity with different temporal scales during encoding and consolidation remain unclear. Based on the idea that acetylcholine modulates functional transition between encoding and consolidation, we tested whether the cholinergic modulation may adjust intrinsic network dynamics to support different temporal scales for these two modes of operation. Simulations demonstrate that cholinergic modulation of the calcium activated non-specific cationic (CAN) current and the synaptic transmission may be sufficient to switch the network dynamics between encoding and consolidation modes. When the CAN current is active and the synaptic transmission is suppressed, mimicking the high acetylcholine condition during active waking, a slow propagation of multiple spikes is evident. This activity resembles the firing pattern of place cells and time cells during active waking. On the other hand, when CAN current is suppressed and the synaptic transmission is intact, mimicking the low acetylcholine condition during slow-wave-sleep, a time compressed fast (∼10 times) activity propagation of the same set of cells is evident. This activity resembles the time compressed firing pattern of place cells during replay and pre-play, achieving a temporal compression factor in the range observed in vivo (5-20 times). These observations suggest that cholinergic system could adjust intrinsic network dynamics suitable for encoding and consolidation through the modulation of the CAN current and synaptic conductance in the hippocampus.

Persistent sodium current drives conditional pacemaking in CA1 pyramidal neurons under muscarinic stimulation

Hippocampal CA1 pyramidal neurons are normally quiescent but can fire spontaneously when stimulated by muscarinic agonists. In brain slice recordings from mouse CA1 pyramidal neurons, we examined the ionic basis of this activity using interleaved current-clamp and voltage-clamp experiments. Both in control and after muscarinic stimulation, the steady-state current-voltage curve was dominated by inward TTX-sensitive persistent sodium current (I(NaP)) that activated near -75 mV and increased steeply with depolarization. In control, total membrane current was net outward (hyperpolarizing) near -70 mV so that cells had a stable resting potential. Muscarinic stimulation activated a small nonselective cation current so that total membrane current near -70 mV shifted to become barely net inward (depolarizing). The small depolarization triggers regenerative activation of I(NaP), which then depolarizes the cell from -70 mV to spike threshold. We quantified the relative contributions of I(NaP), hyperpolarization-activated cation current (I(h)), and calcium current to pacemaking by using the cell's own firing as a voltage command along with specific blockers. TTX-sensitive sodium current was substantial throughout the entire interspike interval, increasing as the membrane potential approached threshold, while both Ih and calcium current were minimal. Thus, spontaneous activity is driven primarily by activation of I(NaP) in a positive feedback loop starting near -70 mV and providing increasing inward current to threshold. These results show that the pacemaking "engine" from I(NaP) is an inherent property of CA1 pyramidal neurons that can be engaged or disengaged by small shifts in net membrane current near -70 mV, as by muscarinic stimulation.

Free energy, precision and learning: the role of cholinergic neuromodulation

Acetylcholine (ACh) is a neuromodulatory transmitter implicated in perception and learning under uncertainty. This study combined computational simulations and pharmaco-electroencephalography in humans, to test a formulation of perceptual inference based upon the free energy principle. This formulation suggests that ACh enhances the precision of bottom-up synaptic transmission in cortical hierarchies by optimizing the gain of supragranular pyramidal cells. Simulations of a mismatch negativity paradigm predicted a rapid trial-by-trial suppression of evoked sensory prediction error (PE) responses that is attenuated by cholinergic neuromodulation. We confirmed this prediction empirically with a placebo-controlled study of cholinesterase inhibition. Furthermore, using dynamic causal modeling, we found that drug-induced differences in PE responses could be explained by gain modulation in supragranular pyramidal cells in primary sensory cortex. This suggests that ACh adaptively enhances sensory precision by boosting bottom-up signaling when stimuli are predictable, enabling the brain to respond optimally under different levels of environmental uncertainty.

Long-lasting intrinsic persistent firing in rat CA1 pyramidal cells: a possible mechanism for active maintenance of memory

The hippocampus is critical for memory tasks which require an active maintenance of memory for a short period of time; however, the underlying neural mechanisms remain unknown. Most theoretical and computational models, which date back to the classic proposals by Donald Hebb in , have been self-constrained by anatomy, as most models rely on the recurrent connectivity in region CA3 to support "reverberating activity" capable of memory maintenance. However, several physiological and behavioral studies have specifically implicated region CA1 in tasks which require an active maintenance of memory. Here, we demonstrate that despite limited recurrent connectivity, CA1 contains a robust cellular mechanism for active memory maintenance in the form of self-sustained persistent firing. Using in vitro whole-cell recordings, we demonstrate that brief stimulation (0.2-2 s) reliably elicits long-lasting (> 30 s) persistent firing that is supported by the calcium-activated non-selective cationic current. In contrast to more traditional ideas, these data suggest that the hippocampal region CA1 is capable of active maintenance of memory.

The calcium-activated slow AHP: cutting through the Gordian knot

The phenomenon known as the slow afterhyperpolarization (sAHP) was originally described more than 30 years ago in pyramidal cells as a slow, Ca(2+)-dependent afterpotential controlling spike frequency adaptation. Subsequent work showed that similar sAHPs were widely expressed in the brain and were mediated by a Ca(2+)-activated potassium current that was voltage-independent, insensitive to most potassium channel blockers, and strongly modulated by neurotransmitters. However, the molecular basis for this current has remained poorly understood. The sAHP was initially imagined to reflect the activation of a potassium channel directly gated by Ca(2+) but recent studies have begun to question this idea. The sAHP is distinct from the Ca(2+)-dependent fast and medium AHPs in that it appears to sense cytoplasmic [Ca(2+)](i) and recent evidence implicates proteins of the neuronal calcium sensor (NCS) family as diffusible cytoplasmic Ca(2+) sensors for the sAHP. Translocation of Ca(2+)-bound sensor to the plasma membrane would then be an intermediate step between Ca(2+) and the sAHP channels. Parallel studies strongly suggest that the sAHP current is carried by different potassium channel types depending on the cell type. Finally, the sAHP current is dependent on membrane PtdIns(4,5)P(2) and Ca(2+) appears to gate this current by increasing PtdIns(4,5)P(2) levels. Because membrane PtdIns(4,5)P(2) is essential for the activity of many potassium channels, these finding have led us to hypothesize that the sAHP reflects a transient Ca(2+)-induced increase in the local availability of PtdIns(4,5)P(2) which then activates a variety of potassium channels. If this view is correct, the sAHP current would not represent a unitary ionic current but the embodiment of a generalized potassium channel gating mechanism. This model can potentially explain the cardinal features of the sAHP, including its cellular heterogeneity, slow kinetics, dependence on cytoplasmic [Ca(2+)], high temperature-dependence, and modulation.

The Inflammatory Chemokine CXCL10 Modulates Synaptic Plasticity and Neuronal Activity in the Hippocampus

Chemokines, a family member of cytokines, have been shown to play a major role in central nervous system inflammation. Among other chemokines, CXCR3 and its ligand CXCL10 are involved in the pathophysiology of several neuroinflammatory conditions. Most of these conditions are also associated with an increased incidence of seizure or epilepsy. Using age-matched wild-type (WT), as well as CXCR3-receptor-deficient (CXCR3-KO) mice, the present study aimed to investigate the effect of the chemokine CXCL10 and its receptor CXCR3 on synaptic plasticity as well as neuronal activities in hippocampal brain slices. Using field potential and intracellular recordings, the effect of exogenous CXCL10 on tetanus-induced long-term potentiation (LTP) as well as the neuronal spike activity was evaluated in hippocampal CA1 area. Exogenous application of CXCL10 enhanced LTP in WT mice, whereas it exerted no significant effect on CXCR3-KO mice. During intracellular recordings of spontaneous spike activity, exogenous application of CXCL10 significantly enhanced the amplitude, duration, and after-hyperpolarization of action potentials in slices obtained from WT mice compared to CXCR3-KO mice. In addition, CXCR3-KO mice exhibited a lower GABAA-mediated excitation in hippocampal CA1 neurons compared to WT mice. These data show that the inflammatory chemokine CXCL10, probably via its receptor CXCR3, modulates neuronal activity and synaptic plasticity in the hippocampus. CXCL10 may be involved in seizures observed during neuroinflammatory diseases such as meningitis and encephalitis.

Synergistic actions of metabotropic acetylcholine and glutamate receptors on the excitability of hippocampal CA1 pyramidal neurons

A variety of neurotransmitters are responsible for regulating neural activity during different behavioral states. Unique responses to combinations of neurotransmitters provide a powerful mechanism by which neural networks could be differentially activated during a broad range of behaviors. Here, we show, using whole-cell recordings in rat hippocampal slices, that group I metabotropic glutamate receptors (mGluRs) and muscarinic acetylcholine receptors (mAChRs) synergistically increase the excitability of hippocampal CA1 pyramidal neurons by converting the post-burst afterhyperpolarization to an afterdepolarization via a rapidly reversible upregulation of Ca(v)2.3 R-type calcium channels. Coactivation of mAChRs and mGluRs also induced a long-lasting enhancement of the responses mediated by each receptor type. These results suggest that cooperative signaling via mAChRs and group I mGluRs could provide a mechanism by which cognitive processes may be modulated by conjoint activation of two separate neurotransmitter systems.

Cholinergic modulation of the CAN current may adjust neural dynamics for active memory maintenance, spatial navigation and time-compressed replay

Suppression of cholinergic receptors and inactivation of the septum impair short-term memory, and disrupt place cell and grid cell activity in the medial temporal lobe (MTL). Location-dependent hippocampal place cell firing during active waking, when the acetylcholine level is high, switches to time-compressed replay activity during quiet waking and slow-wave-sleep (SWS), when the acetylcholine level is low. However, it remains largely unknown how acetylcholine supports short-term memory, spatial navigation, and the functional switch to replay mode in the MTL. In this paper, we focus on the role of the calcium-activated non-specific cationic (CAN) current which is activated by acetylcholine. The CAN current is known to underlie persistent firing, which could serve as a memory trace in many neurons in the MTL. Here, we review the CAN current and discuss possible roles of the CAN current in short-term memory and spatial navigation. We further propose a novel theoretical model where the CAN current switches the hippocampal place cell activity between real-time and time-compressed sequential activity during encoding and consolidation, respectively.

M1 and M4 receptors modulate hippocampal pyramidal neurons

Acetylcholine (ACh), acting at muscarinic ACh receptors (mAChRs), modulates the excitability and synaptic connectivity of hippocampal pyramidal neurons. CA1 pyramidal neurons respond to transient ("phasic") mAChR activation with biphasic responses in which inhibition is followed by excitation, whereas prolonged ("tonic") mAChR activation increases CA1 neuron excitability. Both phasic and tonic mAChR activation excites pyramidal neurons in the CA3 region, yet ACh suppresses glutamate release at the CA3-to-CA1 synapse (the Schaffer-collateral pathway). Using mice genetically lacking specific mAChRs (mAChR knockout mice), we identified the mAChR subtypes responsible for cholinergic modulation of hippocampal pyramidal neuron excitability and synaptic transmission. Knockout of M1 receptors significantly reduced, or eliminated, most phasic and tonic cholinergic responses in CA1 and CA3 pyramidal neurons. On the other hand, in the absence of other G(q)-linked mAChRs (M3 and M5), M1 receptors proved sufficient for all postsynaptic cholinergic effects on CA1 and CA3 pyramidal neuron excitability. M3 receptors were able to participate in tonic depolarization of CA1 neurons, but otherwise contributed little to cholinergic responses. At the Schaffer-collateral synapse, bath application of the cholinergic agonist carbachol suppressed stratum radiatum-evoked excitatory postsynaptic potentials (EPSPs) in wild-type CA1 neurons and in CA1 neurons from mice lacking M1 or M2 receptors. However, Schaffer-collateral EPSPs were not significantly suppressed by carbachol in neurons lacking M4 receptors. We therefore conclude that M1 and M4 receptors are the major mAChR subtypes responsible for direct cholinergic modulation of the excitatory hippocampal circuit.

Blockade of different muscarinic receptor subtypes changes the equilibrium between excitation and inhibition in rat visual cortex

We have shown that cortical acetylcholine modulates the balance between excitation and inhibition evoked in layer 5 pyramidal neurons of rat visual cortex [Lucas-Meunier E, Monier C, Amar M, Baux G, Frégnac Y, Fossier P (2009) Cereb Cortex 19:2411-2427]. Our aim is now to establish a functional basis for the role of the different types of muscarinic receptors (MRs) on glutamate fibers and on GABAergic interneurons and to analyse their contribution to the modulation of excitation-inhibition balance in the rat visual cortex. To ascertain that there was a basis for our functional study, we first checked for the presence of the various MR subtypes by single cell RT-PCR and immunolabeling experiments. Then, recording the composite responses in layer 5 pyramidal neurons to layer 1-2 stimulation (which also recruits cholinergic fibers) in the presence of specific antagonists of the different types of MR allowed us to determine their modulatory role. We show that the specific blockade of the widely distributed M1R (with the mamba toxin, MT7) induced a significant increase in the excitatory conductance without modifying the inhibitory conductance, pointing to a localization of M1R on glutamatergic neurons where their activation would decrease the release of glutamate. From our functional results, M2/M4Rs appear to be located on glutamatergic neurons afferent to the recorded layer 5 pyramidal neuron and they decrease glutamate release. The extended distribution of M4Rs in the cortex compared to the restricted distribution of M2R (layers 3-5) is in favour of a major role as a modulator of M4R. The selective antagonist of M3Rs, 4-DAMP, decreased the inhibitory conductance, showing that activated M3Rs increase the release of GABA and thus are located on GABAergic interneurons. The activation of the different types of MRs located either on glutamatergic neurons or on GABAergic interneurons converges to reinforce the dominance of inhibitory inputs thus decreasing the excitability of layer 5 pyramidal neurons.

Enhanced spatial memory and hippocampal long-term potentiation in p75 neurotrophin receptor knockout mice

Previous reports have described increases in the size and number of cholinergic neurons in the basal forebrain in p75 neurotrophin receptor (p75(NTR)) knockout mice. In an earlier study, we also found improved spatial memory in these mice, raising the possibility that p75(NTR) regulates hippocampal function by its effects on the cholinergic basal forebrain. We therefore investigated hippocampal long-term potentiation in p75(NTR) knockout mice that shared the same genetic background as control 129/Sv mice. We also investigated heterozygous mice, carrying just one functional p75(NTR) allele. The p75(NTR) knockout mice had enhanced long-term potentiation in the Schafer collateral fiber synapses of the hippocampus. Heterozygous mice had an intermediate level, greater than controls but less than knockout mice. Hippocampal choline acetyltransferase activity was also markedly elevated in p75(NTR) knockout mice, with a smaller increase in heterozygous mice. In the Barnes maze, p75(NTR) knockout mice displayed markedly superior learning to controls, and this was evident over the three age brackets tested. At each age, the performance of heterozygous mice was intermediate to the other groups. In the open field test, p75(NTR) knockout mice exhibited greater stress-related behavioral responses, including freezing, than did control animals. There were no differences between the three groups in a test of olfactory function. The dose-dependent effects of p75(NTR) gene copy number on hippocampal plasticity and spatial memory indicate that p75(NTR) has profound effects on hippocampal function. Bearing in mind that p75(NTR) is very sparsely expressed in the adult hippocampus and has a potent effect on hippocampal choline acetyltransferase activity, the effects of p75(NTR) on hippocampal function are likely to be mediated indirectly, by its actions on basal forebrain cholinergic neurons.

Cholinergic terminals in the cat visual cortex: Ultrastructural basis for interaction with glutamate-immunoreactive neurons and other cells

Acetylcholine (ACh) is one of the transmitters utilized by extrathalamic afferents to modulate stimulus-driven neurotransmission and experience-dependent plasticity in the visual cortex. Since these processes also depend on the activation of glutamatergic receptors, cholinergic terminals may exert their effects via direct modulation of excitatory neurotransmission. The objective of this study was to determine whether the ultrastructural relationships between cholinergic terminals, glutamate-immunoreactive neurons, and other unlabeled cells support this idea. Sections from aldehyde-fixed visual cortex (area 17) of adult cats were immunolabled for the following molecules: (1) choline acetyltransferase (ChAT), the acetylcholine-synthesizing enzyme; (2) L-glutamate; or (3) ChAT simultaneously with L-glutamate by combining electron-microscopic immunogold and immunoperoxidase techniques. None of the cortical terminals were dually labeled, suggesting that (1) the labeling procedure was free of chemical or immunological cross reactions; and (2) glutamate immunoreactivity probably reflects the transmitter, and not metabolic, pool of L-glutamate. Comparisons between cholinergic and noncholinergic axons revealed that (1) ChAT-immunoreactive axons formed fewer identifiable synaptic contacts within single ultrathin sections (P < 0.01 using chi-square test); and (2) more of the cholinergic axons occurred directly opposed to other terminals (P < 0.0015 by chi-square test), including 21% of which resided directly across asymmetric, axo-spinous junctions. Dual labeling showed that a third of the synaptic targets for cholinergic terminals contained detectable levels of glutamate immunoreactivity. Some of the axo-spinous junctions juxtaposed to cholinergic axons also exhibited glutamate immunoreactivity presynaptically. These observations provide ultrastructural evidence for direct, cholinergic modulation of glutamatergic pyramidal neurons within the mammalian neocortex. Prevalence of juxtapositions between cholinergic terminals and axo-spinous synapses supports the following ideas: (1) ACh may modulate the release of noncholinergic transmitters, including Glu; (2) Glu may modulate ACh release; and (3) these processes may be concurrent with cholinergic modulation of glutamatergic synapses at postsynaptic sites.

Muscarinic cholinergic modulation of synaptic transmission and plasticity in rat hippocampus following chronic lead exposure

The cholinergic system is believed to be associated with learning and memory functions. Lead (Pb2+) is a well-known neurotoxic metal that causes irreversible damage to the central nervous system (CNS). To investigate whether Pb2+ interferes with cholinergic modulation, we examined the effects of carbachol (CCh), a muscarinic cholinergic agonist, on synaptic transmission and plasticity in the CA1 area of the hippocampus of developmentally Pb2+-exposed rats. The results showed that: (1) In both control and Pb2+-exposed rats, 0.1 microM CCh significantly enhanced tetanus-induced long-term potentiation (LTP), while 5 microM CCh induced a reversible depression of field excitatory postsynaptic potentials (fEPSPs). However, both the enhancement of LTP and depression of fEPSPs were significantly smaller in Pb2+-exposed rats than in controls, suggesting that the extent of the effect of CCh on the cholinergic system was depressed by Pb2+. (2) In Pb2+-exposed rats, the enhancement of LTP induced by 0.1 microM CCh was attenuated by pirenzepine, a M1AChR antagonist, but was not affected by methoctramine tetrahydrochloride (M-105), a M2/4AChR antagonist. The depression of fEPSPs induced by 5 microM CCh was reduced by either pirenzepine or M-105. (3) Furthermore, paired-pulse facilitation (PPF) was not affected by 0.1 microM CCh in control and Pb2+-exposed rats but was increased by 5 microM CCh in either group; the increase in PPF was less pronounced in Pb2+-treated when compared to control rats. These results suggested that cholinergic modulation could be impaired by Pb2+, and this kind of impairment might occur via different mAChR subtypes. Our study delineated the effects of Pb2+ on muscarinic modulation, and this might be one of the underlying mechanisms by which Pb2+ impairs learning and memory.

Repeated blockade of GABAA receptors in the medial septal region induces epileptiform activity in the hippocampus

Changes in the activity of putative interneurons of the stratum oriens of the hippocampus and hippocampal EEG after the delivery of the GABAA receptor antagonist bicuculline (1.0nmol/1microl) to the medial septal region were investigated in awake rabbits. The injection of bicuculline produced a sharp increase in the firing rate in 94.3% of hippocampal cells. The effect of bicuculline on the neuronal theta rhythmicity depended on the number of injections. The first five daily infusions decreased the theta activity in 85.7% of cells. On the fourth to fifth experimental days paroxysmal discharges and 8-15Hz oscillations were recorded in the hippocampal EEG. Six to seven further daily bicuculline injections following a brief diminution of theta activity produced a sharp augmentation of theta oscillations in 78.9% of cells and provoked seizures. Immediately before seizures, stabilization of theta bursts and an increase in burst frequency was usually observed in putative interneurons. During seizures, neuronal rhythmic activity was either disordered and then turned into seizure discharges or was inhibited, partially or completely. In the hippocampal EEG, the power of theta rhythm before seizures usually strongly increased compared with controls. Injection of the GABAA agonist muscimol (30nmol/1microl) 15min before bicuculline infusion prevented the development of seizures. These findings suggest that the interplay between septal neurons via GABAA receptors is critical in the tuning of septal output signals that insure generation of natural theta rhythm as well as adequate functioning of the hippocampus.

Multiple Gq-coupled receptors converge on a common protein synthesis-dependent long-term depression that is affected in fragile X syndrome mental retardation

Gq-coupled, M1 muscarinic acetylcholine receptors (mAChRs) facilitate hippocampal learning, memory, and synaptic plasticity. M1 mAChRs induce long-term synaptic depression (LTD), but little is known about the underlying mechanisms of mAChR-dependent LTD and its link to cognitive function. Here, we demonstrate that chemical activation of M1 mAChRs induces LTD in hippocampal area CA1, which relies on rapid protein synthesis, as well as the extracellular signal-regulated kinase and mammalian target of rapamycin translational activation pathways. Synaptic stimulation of M1 mAChRs, alone, or together with the Gq-coupled glutamate receptors (mGluRs), also results in protein synthesis-dependent LTD. New proteins maintain mAChR-dependent LTD through a persistent decrease in surface AMPA receptors. mAChRs stimulate translation of the RNA-binding protein, Fragile X mental retardation protein (FMRP) and FMRP target mRNAs. In mice without FMRP (Fmr1 knock-out), a model for human Fragile X syndrome mental retardation (FXS), both mGluR- and mAChR-dependent protein synthesis and LTD are affected. Our results reveal that multiple Gq-coupled receptors converge on a common protein synthesis-dependent LTD mechanism, which is aberrant in FXS. These findings suggest novel therapeutic strategies for FXS in the form of mAChR antagonists.

Phasic cholinergic signaling in the hippocampus: functional homology with the neocortex?

Acetylcholine (ACh) acts as a neurotransmitter in both the hippocampus and neocortex to facilitate learning, memory, and cognitive function. Here we show that transient muscarinic ACh receptor (mAChR) activation inhibits action potential generation in CA1, but not in CA3, pyramidal neurons via activation of an SK-type calcium-activated potassium conductance. Hyperpolarizing responses generated by focal ACh application near the somata of CA1 pyramidal neurons were blocked by atropine or the M1-like mAChR antagonist pirenzepine, but not by the M2-like mAChR antagonist methoctramine. Inhibitory cholinergic responses required intracellular calcium signaling, as evidenced by their sensitivity to depletion of internal calcium stores or internal calcium chelation. Cholinergic inhibition did not require GABAergic synaptic transmission, but was blocked by apamin, an SK channel antagonist. In contrast to inhibitory effects in CA1 neurons, ACh was primarily depolarizing, and enhanced action potential firing in CA3 pyramidal neurons. These results, when combined with recent data in neocortical neurons, suggest a functional homology in phasic cholinergic signaling in the hippocampus and neocortex whereby ACh preferentially inhibits those neurons in the lower cortical layers (CA1 and layer 5 neurons) that provide the majority of extracortical efferent projections.

MGluR-mediated calcium waves that invade the soma regulate firing in layer V medial prefrontal cortical pyramidal neurons

Factors that influence the activity of prefrontal cortex (PFC) pyramidal neurons are likely to play an important role in working memory function. One such factor may be the release of Ca2+ from intracellular stores. Here we investigate the hypothesis that metabotropic glutamate receptors (mGluRs)-mediated waves of internally released Ca2+ can regulate the intrinsic excitability and firing patterns of PFC pyramidal neurons. Synaptic or focal pharmacological activation of mGluRs triggered Ca2+ waves in the dendrites and somata of layer V medial PFC pyramidal neurons. These Ca2+ waves often evoked a transient SK-mediated hyperpolarization followed by a prolonged depolarization that respectively decreased and increased neuronal excitability. Generation of the hyperpolarization depended on whether the Ca2+ wave invaded or came near to the soma. The depolarization also depended on the extent of Ca2+ wave propagation. We tested factors that influence the propagation of Ca2+ waves into the soma. Stimulating more synapses, increasing inositol trisphosphate concentration near the soma, and priming with physiological trains of action potentials all enhanced the amplitude and likelihood of evoking somatic Ca2+ waves. These results suggest that mGluR-mediated Ca2+ waves may regulate firing patterns of PFC pyramidal neurons engaged by working memory, particularly under conditions that favor the propagation of Ca2+ waves into the soma.

Effects of Pb2+ on muscarinic modulation of glutamatergic synaptic transmission in rat hippocampal CA1 area

Lead (Pb(2+)) is a pollutant commonly found in the environment. It causes a wide variety of detrimental effects on developing central nervous system. However, the mechanisms of its neurotoxicity remained to be elucidated. In hippocampus, the muscarinic cholinergic system modulates certain forms of synaptic transmission and plasticity, and plays an important role in learning and memory. In this study, the effects of Pb(2+) on muscarinic modulation of glutamatergic synaptic transmission in hippocampal CA1 area were investigated using the conventional whole-cell patch-clamp technique in rat hippocampal slices. In the presence of nicotinic antagonist mecamylamine, carbachol (CCh), a cholinergic agonist, concentration-dependently inhibited glutamatergic excitatory postsynaptic currents (EPSCs), enhanced paired-pulse facilitation (PPF) and the response to 10-Hz pulse-trains. The analysis of the spontaneous excitatory postsynaptic currents (sEPSCs) showed the activation of muscarinic receptors by CCh decreased the frequency, amplitude and decay time of sEPSCs. The 10 microM Pb(2+) depressed the inhibition of EPSCs by CCh, reduced the CCh-induced enhancement of PPF and the response to 10-Hz pulse-trains, and also affected the modulation of sEPSCs by CCh. The results suggested that the activation of muscarinic acetylcholine (ACh) receptors in hippocampus could modulate glutamatergic synaptic transmission, while Pb(2+) exposure would lead to an alteration of muscarinic modulation, which might be involved in the Pb(2+)-induced impairment of synaptic transmission and plasticity during learning and memory.

Expression of muscarinic and dopaminergic receptors and monoamine levels frontal cortex of epileptic rats

Apart from stroke, epilepsy is the most common neurological disorder with 0.5% of prevalence. The present study was performed in order to determine the monoamine levels, (M(1)-like) muscarinic and (D(1)- and D(2)-like) dopaminergic receptor changes in frontal cortex of adult rats after pilocarpine-induced status epilepticus (SE). Male Wistar rats were treated with a single dose of pilocarpine (400 mg/kg, s.c.) and the control group received 0.9% saline (s.c.). Both groups were sacrificed 1 h after treatment. The frontal cortex was dissected for neurochemical assays. The results show a downregulation of 27% in M(1) muscarinic receptor density, but in the dissociation constant (K(d)) value remained unaltered. D(1) and D(2) dopaminergic receptor densities and their K(d) values remained unaltered. Monoamine and metabolites levels presented decreases of 44%, 27%, 30% and 42% in dopamine (DA), homovanilic acid (HVA), norepinephrine (NE) and 5-hydroxyindoleacetic acid (5-HIAA) contents, respectively. Moreover, in serotonin (5-HT) level remained unaltered and the 4-hydroxy-3-methoxy-phenylacetic acid (DOPAC) concentration was augmented by 34%. The results suggest that dopaminergic system in this area studied may not be directly involved in the seizures and status epilepticus, but different monoamines and metabolites can be modified in this cerebral area during seizure process. In conclusion, the neurochemical alterations that occur in frontal cortex of adult rats observed during the establishment of the status epilepticus induced by pilocarpine are decrease in M(1) receptor density concentration and a reduction in DA and NE levels.

Differential expression of TASK channels between horizontal interneurons and pyramidal cells of rat hippocampus

Among the electrophysiological properties differentiating stratum oriens horizontal interneurons from pyramidal neurons of the CA1 hippocampal subfield are the more depolarized resting potential and the higher input resistance; additionally, these interneurons are also less sensitive to ischemic damage than pyramidal cells. A differential expression of pH-sensitive leakage potassium channels (TASK) could contribute to all of these differences. To test this hypothesis, we studied the expression and properties of TASK channels in the two cell types. Electrophysiological recordings from acute slices showed that barium- and bupivacaine-sensitive TASK currents were detectable in pyramidal cells but not in interneurons and that extracellular acidification caused a much stronger depolarization in pyramidal cells than in interneurons. This pyramidal cell depolarization was paralleled by an increase of the input resistance, suggesting the blockade of a background conductance. Single-cell reverse transcription-PCR experiments showed that the expression profile of TASK channels differ between the two cell types and suggested that these channels mediate an important share of the leakage current of pyramidal cells. We suggest that the different expression of TASK channels in these cell types contribute to their electrophysiological differences and may result in cell-specific sensitivity to extracellular acidification in conditions such as epilepsy and ischemia.

Spatio-temporal cholinergic modulation in cultured networks of rat cortical neurons: spontaneous activity

Activation of the cholinergic innervation of the cortex has been implicated in sensory processing, learning, and memory. At the cellular level, acetylcholine both increases excitability and depresses synaptic transmission, and its effects on network firing are hard to predict. We studied the effects of carbachol, a cholinergic agonist, on network firing in cultures of rat cortical neurons, using electrode arrays to monitor the activity of large numbers of neurons simultaneously. These cultures show stable spontaneous synchronized burst firing which propagates through dense synaptic connections. Carbachol (10-50 microM), acting through muscarinic receptors, was found to induce a switch to asynchronous single-spike firing and to result in a loss of regularity and fragmentation of the burst structure. To obtain a quantitative measure of cholinergic actions on cortical networks, we applied a cluster Poisson-process model to sets of paralleled spike-trains in the presence and absence of carbachol. This revealed that the time series can be well-characterized by such a simple model, consistent with the observed 1/f(b)-like spectra (0.04<b<2.08). After applying higher concentrations of carbachol the property of the spectra shifted toward a Poisson-process (white) spectrum. These results indicate that cholinergic neurotransmitters have a strong and reliable desynchronizing action on cortical neural activity.

Galantamine increases excitability of CA1 hippocampal pyramidal neurons

Galantamine is a third generation cholinesterase inhibitor and an allosteric potentiating ligand of nicotinic acetylcholine receptors. It enhances learning in aging rabbits and alleviates cognitive deficits observed in patients with Alzheimer's disease. We examined galantamine's effect on CA1 neurons from hippocampal slices of young and aging rabbits using current-clamp, intracellular recording techniques. Galantamine (10-200 microM) dose-dependently reduced the postburst afterhyperpolarization and the spike-frequency accommodation of CA1 neurons from both young and aging animals. These reductions were partially, but significantly, reversed by the addition of the muscarinic receptor antagonist, atropine (1 microM), to the perfusate. In contrast, the nicotinic acetylcholine receptor antagonist, alpha-bungarotoxin (10 nM), had no effect; i.e. alpha-bungarotoxin did not reverse the afterhyperpolarization and accommodation reductions. The allosteric potentiating ligand effect was examined by stimulating the Schaffer collateral and measuring the excitatory postsynaptic potentials for 30 min during bath application of galantamine. Galantamine (200 microM) significantly enhanced the excitatory postsynaptic potential amplitude and area over time. These effects were blocked by 10 nM alpha-bungarotoxin, supporting a role for galantamine as an allosteric potentiating ligand. We did not observe a facilitation of the excitatory postsynaptic potentials with 1 microM galantamine. However, when the excitatory postsynaptic potential was pharmacologically isolated by adding 10 microM gabazine (GABA(A) receptor antagonist) to the perfusate, 1 microM galantamine potentiated the subthreshold excitatory postsynaptic potentials into action potentials. We propose that the learning enhancement observed in aging animals and the alleviation of cognitive deficits associated with Alzheimer's disease after galantamine treatment may in part be due to the enhanced function of both nicotinic and muscarinic excitatory transmission on hippocampal pyramidal neurons.

Pilocarpine-induced status epilepticus: Monoamine level, muscarinic and dopaminergic receptors alterations in striatum of young rats

Behavioural changes, muscarinic and dopaminergic receptors density and levels of monoamines were measured in striatum of rats after pilocarpine-induced status epilepticus (SE). Wistar rats at the age of 21 days were treated with pilocarpine (400mg/kg; subcutaneously) whilst the control group was treated with 0.9% saline (s.c.). Both groups were sacrificed 1h following the treatment. SE induced a muscarinic receptor downregulation of 64% in pilocarpine group. This effect was also observed to be 57% in D(1) and 32% in D(2). In the dissociation constant (K(d)) values in muscarinic and D(1) receptor no alterations were verified. On the other hand, the K(d) value for D(2) was observed to increase 41%. High performance liquid chromatography determinations showed 63, 35, 77 and 64% decreases in dopamine, 3-methoxy-phenylacetic acid, serotonin and 5-hydroxyindoleacetic acid contents, respectively. The homovanilic acid level was verified to increase 119%. The noradrenaline content was unaltered. A direct evidence of monoamine levels alterations can be verified during seizure activity and receptor density changes appear to occur in an accentuated way in immature brain during the estabilishment of SE induced by pilocarpine.

Gain modulation by serotonin in pyramidal neurones of the rat prefrontal cortex

Serotonin (5-HT) is widely implicated in brain functions and diseases. The vertebrate brain is extensively innervated by 5-HT fibres originating from the brain stem, and 5-HT axon terminals interact with other neurones in complex ways. The cellular mechanisms underlying 5-HT function in the brain are not well understood. The present study examined the effect of 5-HT on the responsiveness of neurones in the neocortex. Using patch-clamp recording in acute slices, we showed that 5-HT substantially increased the slope (gain) of the firing rate-current curve in layer 5 pyramidal neurones of the rat prefrontal cortex. The effect of 5-HT on gain is confined to the range of firing rate (0-10 Hz) that is known to be behaviourally relevant. 5-HT also changed current threshold for spike train generation, but this effect was inconsistent, and was independent of the effect on gain. The gain modulation by 5-HT was mediated by 5-HT2 receptors, and involved postsynaptic mechanisms. 5-HT2-mediated gain increase could not be attributed to changes in the membrane potential, the input resistance or the properties of action potentials, but was associated with a reduction of the afterhyperpolarization and an induction of the slow afterdepolarization. Blocking Ca2+ entry with Cd2+ increased the gain by itself and blocked 5-HT2- mediated gain increase. Buffering [Ca2+](i) with 25 mM EGTA also substantially reduced 5-HT2- mediated gain increase. Noradrenaline, which blocked the afterhyperpolarization, also induced a moderate increase in gain. Together, our results suggest that 5-HT may regulate the dynamics of cortical circuits through multiplicative scaling.

The effects on place cells of local scopolamine dialysis are mimicked by a mixture of two specific muscarinic antagonists

Using a dialysis probe near CA1 hippocampal recording electrodes, we infused nonspecific (scopolamine) and specific (methoctramine, pirenzepine) antagonists of muscarinic cholinergic transmission to determine their effects on the positional firing properties of place cells. Both low (0.5 mM) and high (2.0 or 3.0 mM) scopolamine significantly decreased in-field firing rate, increased the ratio of out-of-field to in-field rate, and reduced the smoothness of rate maps, while tending to increase out-of-field rate. Thus, local nonspecific muscarinic blockade mimicked the effects seen with intracerebroventricular application, suggesting that blockade of receptors local to the recorded cells plays an essential role. Unexpectedly, dialysis of scopolamine reduced locomotor activity, again duplicating the effects of intracerebroventricular administration. Most effects of methoctramine (1.0 mM), which blocks presynaptic m2 and m4 receptors, were initially strong but then diminished over hours. Methoctramine produced a significant increase only in out/in ratio and out-of-field rate, whereas it tended to increase in-field rate and monotonically decrease smoothness. Pirenzepine (3.0 mM), which blocks postsynaptic m1 receptors, produced a significant increase only in out/in ratio, whereas it tended to increase out-of-field rate and decrease in-field rate; all these effects were monotonic with respect to time. A mixture of methoctramine plus pirenzepine recapitulated the place-cell effects of scopolamine, although neither the mixture nor its separate components affected behavior. We conclude that the effects of scopolamine on place cells likely result from a combination of blockade of postsynaptic m1 receptors, leading to reduced excitability, with blockade of presynaptic m2 and m4 receptors, leading to increased out-of-field firing.

Regulation of epileptiform activity in hippocampus by nicotinic acetylcholine receptor activation

Nicotinic acetylcholine receptors (nAChRs) regulate neuronal excitability within the CNS. To assess the possible modulatory influence of nAChRs on epileptiform activity, a range of nAChR ligands were applied during experimentally induced epileptiform activity in rat hippocampal slices. Bath application of the potassium channel blocker 4-aminopyridine (4AP; 10-50 microM) resulted in the development of spontaneous epileptiform bursting activity in area CA3 that consisted of short duration (257+/-15 ms) field events occurring regularly at a frequency of 0.4+/-0.02 Hz. Subsequent co-application of the selective nAChR agonists 1,1-dimethyl-4-phenyl-piperazinium iodide (DMPP; 0.3-300 microM), choline (0.01-3mM) and lobeline (3-30 microM) produced sustained and concentration-dependent increases in burst frequency with maximal frequency potentiation of 37+/-5%, 27+/-5% and 24+/-11%, respectively. DMPP (10-30 microM; n=31) also potentiated epileptiform bursting induced by reducing GABA(A) receptor-mediated synaptic transmission using 20 microM bicuculline or enhancing NMDA receptor-mediated excitation by lowering extracellular Mg(2+). Irrespective of the epileptiform model studied all nAChR agonist induced frequency potentiation was reversed upon washout of the agonist or co-application of one of the selective nAChR antagonists dihydro-beta-erythroidine (10-30 microM), mecamylamine (50-200 microM) or alpha-bungarotoxin (100 nM). These results provide compelling evidence that activation of nAChRs exacerbate epileptiform activity in the rat hippocampus.

Evidence for the involvement of G-proteins in the generation of the slow poststimulus afterdepolarisation (sADP) induced by muscarinic receptor activation in rat olfactory cortical neurones in vitro

The involvement of G-proteins in generating the slow poststimulus afterdepolarising potential (sADP) induced by muscarinic receptor activation in immature (P10-20) rat olfactory cortical brain slice neurones was investigated under whole-cell patch clamp, using GTP-gamma-S (G-protein activator) or GDP-beta-S (G-protein blocker)-filled electrodes. In control experiments using K methylsulphate electrodes, cell resting potential (V(m)) and spike firing properties were unaffected over 10-15 min recording, although input resistance (R(N)) was slightly increased ( approximately 14%). Oxotremorine-M (OXO-M; 10 microM) produced a reversible slow depolarisation, an increase in R(N) ( approximately 90%) and induction of a slow poststimulus inward tail current (I(ADP)) (measured under voltage clamp at -60 mV) that was sustained during drug exposure (up to 15 min); the amplitude of slow inward rectifier (I(h)) currents activated from -50 mV were also apparently increased. By contrast, in GTP-gamma-S-loaded cells, R(N) was consistently decreased ( approximately 22%) and spike firing threshold (V(th)) was raised ( approximately 5 mV) after 10 min recording. In approximately 60% of loaded cells, a persistent muscarinic slow inward current and I(ADP) were induced by OXO-M; I(h) relaxation amplitude was also significantly decreased. The effects of GTP-gamma-S on R(N), V(th) and I(h) were partly counteracted by adding Ba(2+) (100 microM) to the bathing medium or mimicked by adding baclofen (GABA(B) receptor agonist; 100 microM) to normally-recorded cells. Intracellular GDP-beta-S (up to 30 min) had no effect on cell membrane properties or I(h), but irreversibly blocked the muscarinic slow inward current and I(ADP) induced by OXO-M. We conclude that both muscarinic responses require G-protein-linked transduction mechanisms for their generation.

Partial adenosine A(1) receptor agonists inhibit sarin-induced epileptiform activity in the hippocampal slice

Organophosphate poisoning can result in seizures and subsequent neuropathology. One possible therapeutic approach would be to employ adenosine A(1) receptor agonists, which have already been shown to have protective effects against organophosphate poisoning. Using an in vitro model of organophosphate-induced seizures, we have investigated the ability of several adenosine A(1) receptor agonists to inhibit epileptiform activity induced by the organophosphate sarin, in the CA1 stratum pyramidale of the guinea pig hippocampal slice. Application of the adenosine A(1) receptor agonist N(6)-cyclopentyladenosine (CPA) or the partial adenosine A(1) receptor agonists 2-deoxy-N(6)-cyclopentyladenosine (2-deoxy-CPA) and 8-butylamino-N(6)-cyclopentyladenosine (8-butylamino-CPA) abolished epileptiform activity in a concentration-related manner. The rank order of potency was CPA (IC(50) 4-5 nM) >2-deoxy-CPA (IC(50) 113-119 nM)=8-butylamino-CPA (IC(50) 90-115 nM). These data suggest that partial adenosine A(1) receptor agonists, which have fewer cardiovascular effects, should be further evaluated in vivo as potential treatments for organophosphate poisoning.

Pyridostigmine enhances glutamatergic transmission in hippocampal CA1 neurons

Pyridostigmine, a carbamate acetylcholinesterase (AChE) inhibitor, is routinely employed in the treatment of the autoimmune disease myasthenia gravis. Due to its positively charged ammonium group, under normal conditions pyridostigmine cannot cross the blood-brain barrier (BBB) and penetrate the brain. However, several studies have suggested that under conditions in which the BBB is disrupted, pyridostigmine enters the brain, changes cortical excitability, and leads to long-lasting alterations in gene expression. The aim of this study was to characterize the mechanisms underlying pyridostigmine-induced changes in the excitability of central neurons. Using whole cell intracellular recordings in hippocampal neurons we show that pyridostigmine decreases repetitive firing adaptation and increases the appearance of excitatory postsynaptic potentials. In voltage clamp recordings, both pyridostigmine and acetylcholine (ACh) increased the frequency but not the amplitude of excitatory postsynaptic currents. These effects were reversible upon the administration of the muscarinic receptor antagonist, atropine, and were not blocked by tetrodotoxin. We conclude that pyridostigmine, by increasing free ACh levels, causes muscarinic-dependent enhancement of excitatory transmission. This mechanism may explain central side effects previously attributed to this drug as well as the potency of AChE inhibitors, including nerve-gas agents and organophosphate pesticides, in the initiation of cortical synchronization, epileptic discharge, and excitotoxic damage.

Muscarinic blockade slows and degrades the location-specific firing of hippocampal pyramidal cells

The firing of rat hippocampal pyramidal cells is determined both by the animal's location and by the state of the hippocampal EEG. Because cholinergic transmission plays a role in EEG activity, we expected that its modification would alter place cell activity. We therefore investigated the effects on place cell activity of blocking muscarinic transmission with intracerebroventricular injections of scopolamine. Scopolamine reduced both the rate of place cell discharge inside firing fields and the spatial coherence of the fields; discharge outside of the fields also showed small increases. After injections, fields were shifted farther from their previous location than for saline controls, indicating reduced reproducibility after muscarinic blockade. Scopolamine increased the time rats were stationary, but changes in place cell activity persisted even after analysis was restricted to periods of walking, suggesting that the behavioral changes cannot account for the cell discharge changes. The scopolamine effects were dose dependent to an extent that varied between different measures. The firing rates of interneurons showed only a minor trend to decrease after scopolamine. Nevertheless, the spatial coherence of interneuron firing patterns was reduced, consistent with the recent demonstration that their positional firing is mediated by the location-specific firing of pyramids (Marshall et al., 2002). These results demonstrate that acetylcholine enhances positional firing patterns in the hippocampus. Muscarinic blockade weakens the positional firing of most place cells and therefore renders them less useful for precise representation of the environment. This effect may underlie the difficulties in spatial learning and problem solving caused by abnormalities of cholinergic transmission.

Selective muscarinic regulation of functional glutamatergic Schaffer collateral synapses in rat CA1 pyramidal neurons

Analysis of the cholinergic regulation of glutamatergic neurotransmission is an essential step in understanding the hippocampus because it can influence forms of synaptic plasticity that are thought to underlie learning and memory. We studied in vitro the cholinergic regulation of excitatory postsynaptic currents (EPSCs) evoked in rat CA1 pyramidal neurons by Schaffer collateral (SC) stimulation. Using "minimal" stimulation, which activates one or very few synapses, the cholinergic agonist carbamylcholine (CCh) increased the failure rate of functional more (36 %) than of silent synapses (7 %), without changes in the EPSC amplitude. These effects of CCh were insensitive to manipulations that increased the probability of release, such as paired pulse facilitation, increases in temperature and increases in the extracellular Ca(2+) : Mg(2+) ratio. Using "conventional" stimulation, which activates a large number of synapses, CCh inhibited more the pharmacologically isolated non-NMDA (86 %) than the NMDA (47 %) EPSC. The changes in failure rate, EPSC variance and the increased paired pulse facilitation that paralleled the inhibition imply that CCh decreased release probability. Muscarine had similar effects. The inhibition by both CCh and by muscarine was prevented by atropine. We conclude that CCh reduces the non-NMDA component of SC EPSCs by selectively inhibiting transmitter release at functional synapses via activation of muscarinic receptors. The results suggest that SCs have two types of terminals, one in functional synapses, selectively sensitive to regulation through activation of muscarinic receptors, and the other in silent synapses less sensitive to that regulation. The specific inhibition of functional synapses would favour activity-dependent plastic phenomena through NMDA receptors at silent synapses without the activation of non-NMDA receptors and functional synapses.

Muscarinic activation of a cation current and associated current noise in entorhinal-cortex layer-II neurons

The effects of muscarinic stimulation on the membrane potential and current of in situ rat entorhinal-cortex layer-II principal neurons were analyzed using the whole cell, patch-clamp technique. In current-clamp experiments, application of carbachol (CCh) induced a slowly developing, prolonged depolarization initially accompanied by a slight decrease or no significant change in input resistance. By contrast, in a later phase of the depolarization input resistance appeared consistently increased. To elucidate the ionic bases of these effects, voltage-clamp experiments were then carried out. In recordings performed in nearly physiological ionic conditions at the holding potential of -60 mV, CCh application promoted the slow development of an inward current deflection consistently associated with a prominent increase in current noise. Similarly to voltage responses to CCh, this inward-current induction was abolished by the muscarinic antagonist, atropine. Current-voltage relationships derived by applying ramp voltage protocols during the different phases of the CCh-induced inward-current deflection revealed the early induction of an inward current that manifested a linear current/voltage relationship in the subthreshold range and the longer-lasting block of an outward K(+) current. The latter current could be blocked by 1 mM extracellular Ba(2+), which allowed us to study the CCh-induced inward current (I(CCh)) in isolation. The extrapolated reversal potential of the isolated I(CCh) was approximately 0 mV and was not modified by complete substitution of intrapipette K(+) with Cs(+). Moreover, the extrapolated I(CCh) reversal shifted to approximately -20 mV on removal of 50% extracellular Na(+). These results are consistent with I(CCh) being a nonspecific cation current. Finally, noise analysis of I(CCh) returned an estimated conductance of the underlying channels of approximately 13.5 pS. We conclude that the depolarizing effect of muscarinic stimuli on entorhinal-cortex layer-II principal neurons depends on both the block of a K(+) conductance and the activation of a "noisy" nonspecific cation current. We suggest that the membrane current fluctuations brought about by I(CCh) channel noise may facilitate the "theta" oscillatory dynamics of these neurons and enhance firing reliability and synchronization.

Neuromodulation, theta rhythm and rat spatial navigation

Cholinergic and GABAergic innervation of the hippocampus plays an important role in human memory function and rat spatial navigation. Drugs which block acetylcholine receptors or enhance GABA receptor activation cause striking impairments in the encoding of new information. Lesions of the cholinergic innervation of the hippocampus reduce the amplitude of hippocampal theta rhythm and cause impairments in spatial navigation tasks, including the Morris water maze, eight-arm radial maze, spatial reversal and delayed alternation. Here, we review previous work on the role of cholinergic modulation in memory function, and we present a new model of the hippocampus and entorhinal cortex describing the interaction of these regions for goal-directed spatial navigation in behavioral tasks. These mechanisms require separate functional phases for: (1) encoding of pathways without interference from retrieval, and (2) retrieval of pathways for guiding selection of the next movement. We present analysis exploring how phasic changes in physiological variables during hippocampal theta rhythm could provide these different phases and enhance spatial navigation function.

Functional specificity of G alpha q and G alpha 11 in the cholinergic and glutamatergic modulation of potassium currents and excitability in hippocampal neurons

In hippocampal and other cortical neurons, action potentials are followed by a slow afterhyperpolarization (sAHP) generated by the activation of small-conductance Ca(2+)-activated K(+) channels and controlling spike frequency adaptation. The corresponding current, the apamin-insensitive sI(AHP), is a well known target of modulation by different neurotransmitters, including acetylcholine (via M(3) receptors) and glutamate (via metabotropic glutamate receptor 5, mGluR(5)), in CA1 pyramidal neurons. The actions of muscarinic and mGluR agonists on sI(AHP) involve the activation of pertussis toxin-insensitive G-proteins. However, the pharmacological tools available so far did not permit the identification of the specific G-protein subtypes transducing the effects of M(3) and mGluR(5) on sI(AHP). In the present study, we used mice deficient in the Galpha(q) and Galpha(11) genes to investigate the specific role of these G-protein alpha subunits in the cholinergic and glutamatergic modulation of sI(AHP) in CA1 neurons. In mice lacking Galpha(q), the effects of muscarinic and glutamatergic agonists on sI(AHP) were nearly abolished, whereas beta-adrenergic agonists acting via Galpha(s) were still fully effective. Modulation of sI(AHP) by any of these agonists was instead unchanged in mice lacking Galpha(11). The additional depolarizing effects of muscarinic and glutamatergic agonists on CA1 neurons were preserved in mice lacking Galpha(q) or Galpha(11). Thus, Galpha(q), but not Galpha(11), mediates specifically the action of cholinergic and glutamatergic agonists on sI(AHP), without affecting the modulation of other currents. These results provide to our knowledge one of the first examples of the functional specificity of Galpha(q) and Galpha(11) in central neurons.