Cyclic adenosine 3',5'-monophosphate mediates beta-receptor actions of noradrenaline in rat hippocampal pyramidal cells

A disinhibitory circuit mechanism explains a general principle of peak performance during mid-level arousal

Perceptual decision-making is highly dependent on the momentary arousal state of the brain, which fluctuates over time on a scale of hours, minutes, and even seconds. The textbook relationship between momentary arousal and task performance is captured by an inverted U-shape, as put forward in the Yerkes-Dodson law. This law suggests optimal performance at moderate levels of arousal and impaired performance at low or high arousal levels. However, despite its popularity, the evidence for this relationship in humans is mixed at best. Here, we use pupil-indexed arousal and performance data from various perceptual decision-making tasks to provide converging evidence for the inverted U-shaped relationship between spontaneous arousal fluctuations and performance across different decision types (discrimination, detection) and sensory modalities (visual, auditory). To further understand this relationship, we built a neurobiologically plausible mechanistic model and show that it is possible to reproduce our findings by incorporating two types of interneurons that are both modulated by an arousal signal. The model architecture produces two dynamical regimes under the influence of arousal: one regime in which performance increases with arousal and another regime in which performance decreases with arousal, together forming an inverted U-shaped arousal-performance relationship. We conclude that the inverted U-shaped arousal-performance relationship is a general and robust property of sensory processing. It might be brought about by the influence of arousal on two types of interneurons that together act as a disinhibitory pathway for the neural populations that encode the available sensory evidence used for the decision.

Sensitive genetically encoded sensors for population and subcellular imaging of cAMP in vivo

Cyclic adenosine monophosphate (cAMP) signaling integrates information from diverse G-protein-coupled receptors, such as neuromodulator receptors, to regulate pivotal biological processes in a cellular-specific and subcellular-specific manner. However, in vivo cellular-resolution imaging of cAMP dynamics remains challenging. Here, we screen existing genetically encoded cAMP sensors and further develop the best performer to derive three improved variants, called cAMPFIREs. Compared with their parental sensor, these sensors exhibit up to 10-fold increased sensitivity to cAMP and a cytosolic distribution. cAMPFIREs are compatible with both ratiometric and fluorescence lifetime imaging and can detect cAMP dynamics elicited by norepinephrine at physiologically relevant, nanomolar concentrations. Imaging of cAMPFIREs in awake mice reveals tonic levels of cAMP in cortical neurons that are associated with wakefulness, modulated by opioids, and differentially regulated across subcellular compartments. Furthermore, enforced locomotion elicits neuron-specific, bidirectional cAMP dynamics. cAMPFIREs also function in Drosophila. Overall, cAMPFIREs may have broad applicability for studying intracellular signaling in vivo.

Corticotropin Releasing Factor Mediates KCa3.1 Inhibition, Hyperexcitability, and Seizures in Acquired Epilepsy

Temporal lobe epilepsy (TLE), the most common focal seizure disorder in adults, can be instigated in experimental animals by convulsant-induced status epilepticus (SE). Principal hippocampal neurons from SE-experienced epileptic male rats (post-SE neurons) display markedly augmented spike output compared with neurons from nonepileptic animals (non-SE neurons). This enhanced firing results from a cAMP-dependent protein kinase A-mediated inhibition of KCa3.1, a subclass of Ca2+-gated K+ channels generating the slow afterhyperpolarizing Ca2+-gated K+ current (IsAHP). The inhibition of KCa3.1 in post-SE neurons leads to a marked reduction in amplitude of the IsAHP that evolves during repetitive firing, as well as in amplitude of the associated Ca2+-dependent component of the slow afterhyperpolarization potential (KCa-sAHP). Here we show that KCa3.1 inhibition in post-SE neurons is induced by corticotropin releasing factor (CRF) through its Type 1 receptor (CRF1R). Acute application of CRF1R antagonists restores KCa3.1 activity in post-SE neurons, normalizing KCa-sAHP/IsAHP amplitudes and neuronal spike output, without affecting these variables in non-SE neurons. Moreover, pharmacological antagonism of CRF1Rs in vivo reduces the frequency of spontaneous recurrent seizures in post-SE chronically epileptic rats. These findings may provide a new vista for treating TLE.SIGNIFICANCE STATEMENT Epilepsy, a common neurologic disorder, often develops following a brain insult. Identifying key cellular mechanisms underlying acquired epilepsy is critical for developing effective antiepileptic therapies. In an experimental model of acquired epilepsy, principal hippocampal neurons manifest hyperexcitability because of downregulation of KCa3.1, a subtype of Ca2+-gated K+ ion channels. We show that KCa3.1 downregulation is mediated by corticotropin releasing factor (CRF) acting through its Type 1 receptor (CRF1R). Congruently, acute application of selective CRF1R antagonists restores KCa3.1 channel activity, leading to normalization of neuronal excitability. In the same model, injection of a CRF1R antagonist to epileptic animals markedly decreases the frequency of electrographic seizures. Therefore, targeting CRF1Rs may provide a new strategy in the treatment of acquired epilepsy.

The Molecular Basis for the Calcium-Dependent Slow Afterhyperpolarization in CA1 Hippocampal Pyramidal Neurons

Neuronal signal transmission depends on the frequency, pattern, and timing of spike output, each of which are shaped by spike afterhyperpolarizations (AHPs). There are classically three post-spike AHPs of increasing duration categorized as fast, medium and slow AHPs that hyperpolarize a cell over a range of 10 ms to 30 s. Intensive early work on CA1 hippocampal pyramidal cells revealed that all three AHPs incorporate activation of calcium-gated potassium channels. The ionic basis for a fAHP was rapidly attributed to the actions of big conductance (BK) and the mAHP to small conductance (SK) or Kv7 potassium channels. In stark contrast, the ionic basis for a prominent slow AHP of up to 30 s duration remained an enigma for over 30 years. Recent advances in pharmacological, molecular, and imaging tools have uncovered the expression of a calcium-gated intermediate conductance potassium channel (IK, KCa3.1) in central neurons that proves to contribute to the slow AHP in CA1 hippocampal pyramidal cells. Together the data show that the sAHP arises in part from a core tripartite complex between Cav1.3 (L-type) calcium channels, ryanodine receptors, and IK channels at endoplasmic reticulum-plasma membrane junctions. Work on the sAHP in CA1 pyramidal neurons has again quickened pace, with identified contributions by both IK channels and the Na-K pump providing answers to several mysteries in the pharmacological properties of the sAHP.

PKA-RIIβ autophosphorylation modulates PKA activity and seizure phenotypes in mice

Temporal lobe epilepsy (TLE) is one of the most common and intractable neurological disorders in adults. Dysfunctional PKA signaling is causally linked to the TLE. However, the mechanism underlying PKA involves in epileptogenesis is still poorly understood. In the present study, we found the autophosphorylation level at serine 114 site (serine 112 site in mice) of PKA-RIIβ subunit was robustly decreased in the epileptic foci obtained from both surgical specimens of TLE patients and seizure model mice. The p-RIIβ level was negatively correlated with the activities of PKA. Notably, by using a P-site mutant that cannot be autophosphorylated and thus results in the released catalytic subunit to exert persistent phosphorylation, an increase in PKA activities through transduction with AAV-RIIβ-S112A in hippocampal DG granule cells decreased mIPSC frequency but not mEPSC, enhanced neuronal intrinsic excitability and seizure susceptibility. In contrast, a reduction of PKA activities by RIIβ knockout led to an increased mIPSC frequency, a reduction in neuronal excitability, and mice less prone to experimental seizure onset. Collectively, our data demonstrated that the autophosphorylation of RIIβ subunit plays a critical role in controlling neuronal and network excitabilities by regulating the activities of PKA, providing a potential therapeutic target for TLE.

The signaling proteins GPR158 and RGS7 modulate excitability of L2/3 pyramidal neurons and control A-type potassium channel in the prelimbic cortex

Stress profoundly affects physiological properties of neurons across brain circuits and thereby increases the risk for depression. However, the molecular and cellular mechanisms mediating these effects are poorly understood. In this study, we report that chronic physical restraint stress in mice decreases excitability specifically in layer 2/3 of pyramidal neurons within the prelimbic subarea of the prefrontal cortex (PFC) accompanied by the induction of depressive-like behavioral states. We found that a complex between G protein-coupled receptor (GPCR) 158 (GPR158) and regulator of G protein signaling 7 (RGS7), a regulatory GPCR signaling node recently discovered to be a key modulator of affective behaviors, plays a key role in controlling stress-induced changes in excitability in this neuronal population. Deletion of GPR158 or RGS7 enhanced excitability of layer 2/3 PFC neurons and prevented the impact of stress. Investigation of the underlying molecular mechanisms revealed that the A-type potassium channel Kv4.2 subunit is a molecular target of the GPR158-RGS7 complex. We further report that GPR158 physically associates with Kv4.2 channel and promotes its function by suppressing inhibitory modulation by cAMP-protein kinase A (PKA)-mediated phosphorylation. Taken together, our observations reveal a critical mechanism that adjusts neuronal excitability in L2/3 pyramidal neurons of the PFC and may thereby modulate the effects of stress on depression.

Visualizing Protein Kinase A Activity In Head-fixed Behaving Mice Using In Vivo Two-photon Fluorescence Lifetime Imaging Microscopy

Neuromodulation exerts powerful control over brain function. Dysfunction of neuromodulatory systems results in neurological and psychiatric disorders. Despite their importance, technologies for tracking neuromodulatory events with cellular resolution are just beginning to emerge. Neuromodulators, such as dopamine, norepinephrine, acetylcholine, and serotonin, trigger intracellular signaling events via their respective G protein-coupled receptors to modulate neuronal excitability, synaptic communications, and other neuronal functions, thereby regulating information processing in the neuronal network. The above mentioned neuromodulators converge onto the cAMP/protein kinase A (PKA) pathway. Therefore, in vivo PKA imaging with single-cell resolution was developed as a readout for neuromodulatory events in a manner analogous to calcium imaging for neuronal electrical activities. Herein, a method is presented to visualize PKA activity at the level of individual neurons in the cortex of head-fixed behaving mice. To do so, an improved A-kinase activity reporter (AKAR), called tAKARα, is used, which is based on Förster resonance energy transfer (FRET). This genetically-encoded PKA sensor is introduced into the motor cortex via in utero electroporation (IUE) of DNA plasmids, or stereotaxic injection of adeno-associated virus (AAV). FRET changes are imaged using two-photon fluorescence lifetime imaging microscopy (2pFLIM), which offers advantages over ratiometric FRET measurements for quantifying FRET signal in light-scattering brain tissue. To study PKA activities during enforced locomotion, tAKARα is imaged through a chronic cranial window above the cortex of awake, head-fixed mice, which run or rest on a speed-controlled motorized treadmill. This imaging approach will be applicable to many other brain regions to study corresponding behavior-induced PKA activities and to other FLIM-based sensors for in vivo imaging.

β3-Adrenergic receptor-dependent modulation of the medium afterhyperpolarization in rat hippocampal CA1 pyramidal neurons

Action potential firing in hippocampal pyramidal neurons is regulated by generation of an afterhyperpolarization (AHP). Three phases of AHP are recognized, with the fast AHP regulating action potential firing at the onset of a burst and the medium and slow AHPs supressing action potential firing over hundreds of milliseconds and seconds, respectively. Activation of β-adrenergic receptors suppresses the slow AHP by a protein kinase A-dependent pathway. However, little is known regarding modulation of the medium AHP. Application of the selective β-adrenergic receptor agonist isoproterenol suppressed both the medium and slow AHPs evoked in rat CA1 hippocampal pyramidal neurons recorded from slices maintained in organotypic culture. Suppression of the slow AHP was mimicked by intracellular application of cAMP, with the suppression of the medium AHP by isoproterenol still being evident in cAMP-dialyzed cells. Suppression of both the medium and slow AHPs was antagonized by the β-adrenergic receptor antagonist propranolol. The effect of isoproterenol to suppress the medium AHP was mimicked by two β₃-adrenergic receptor agonists, BRL37344 and SR58611A. The medium AHP was mediated by activation of small-conductance calcium-activated K⁺ channels and deactivation of H channels at the resting membrane potential. Suppression of the medium AHP by isoproterenol was reduced by pretreating cells with the H-channel blocker ZD7288. These data suggest that activation of β₃-adrenergic receptors inhibits H channels, which suppresses the medium AHP in CA1 hippocampal neurons by utilizing a pathway that is independent of a rise in intracellular cAMP. This finding highlights a potential new target in modulating H-channel activity and thereby neuronal excitability. NEW & NOTEWORTHY The noradrenergic input into the hippocampus is involved in modulating long-term synaptic plasticity and is implicated in learning and memory. We demonstrate that activation of functional β₃-adrenergic receptors suppresses the medium afterhyperpolarization in hippocampal pyramidal neurons. This finding provides an additional mechanism to increase action potential firing frequency, where neuronal excitability is likely to be crucial in cognition and memory.

A Highly Sensitive A-Kinase Activity Reporter for Imaging Neuromodulatory Events in Awake Mice

Neuromodulation imposes powerful control over brain function, and cAMP-dependent protein kinase (PKA) is a central downstream mediator of multiple neuromodulators. Although genetically encoded PKA sensors have been developed, single-cell imaging of PKA activity in living mice has not been established. Here, we used two-photon fluorescence lifetime imaging microscopy (2pFLIM) to visualize genetically encoded PKA sensors in response to the neuromodulators norepinephrine and dopamine. We screened available PKA sensors for 2pFLIM and further developed a variant (named tAKARα) with increased sensitivity and a broadened dynamic range. This sensor allowed detection of PKA activation by norepinephrine at physiologically relevant concentrations and kinetics, and by optogenetically released dopamine. In vivo longitudinal 2pFLIM imaging of tAKARα tracked bidirectional PKA activities in individual neurons in awake mice and revealed neuromodulatory PKA events that were associated with wakefulness, pharmacological manipulation, and locomotion. This new sensor combined with 2pFLIM will enable interrogation of neuromodulation-induced PKA signaling in awake animals. VIDEO ABSTRACT.

Does Global Astrocytic Calcium Signaling Participate in Awake Brain State Transitions and Neuronal Circuit Function?

We continuously need to adapt to changing conditions within our surrounding environment, and our brain needs to quickly shift between resting and working activity states in order to allow appropriate behaviors. These global state shifts are intimately linked to the brain-wide release of the neuromodulators, noradrenaline and acetylcholine. Astrocytes have emerged as a new player participating in the regulation of brain activity, and have recently been implicated in brain state shifts. Astrocytes display global Ca²⁺ signaling in response to activation of the noradrenergic system, but whether astrocytic Ca²⁺ signaling is causative or correlative for shifts in brain state and neural activity patterns is not known. Here we review the current available literature on astrocytic Ca²⁺ signaling in awake animals in order to explore the role of astrocytic signaling in brain state shifts. Furthermore, we look at the development and availability of innovative new methodological tools that are opening up for new ways of visualizing and perturbing astrocyte activity in awake behaving animals. With these new tools at hand, the field of astrocyte research will likely be able to elucidate the causal and mechanistic roles of astrocytes in complex behaviors within a very near future.

Hypocretin/Orexin Peptides Alter Spike Encoding by Serotonergic Dorsal Raphe Neurons through Two Distinct Mechanisms That Increase the Late Afterhyperpolarization

Orexins (hypocretins) are neuropeptides that regulate multiple homeostatic processes, including reward and arousal, in part by exciting serotonergic dorsal raphe neurons, the major source of forebrain serotonin. Here, using mouse brain slices, we found that, instead of simply depolarizing these neurons, orexin-A altered the spike encoding process by increasing the postspike afterhyperpolarization (AHP) via two distinct mechanisms. This orexin-enhanced AHP (oeAHP) was mediated by both OX1 and OX2 receptors, required Ca(2+) influx, reversed near EK, and decayed with two components, the faster of which resulted from enhanced SK channel activation, whereas the slower component decayed like a slow AHP (sAHP), but was not blocked by UCL2077, an antagonist of sAHPs in some neurons. Intracellular phospholipase C inhibition (U73122) blocked the entire oeAHP, but neither component was sensitive to PKC inhibition or altered PKA signaling, unlike classical sAHPs. The enhanced SK current did not depend on IP3-mediated Ca(2+) release but resulted from A-current inhibition and the resultant spike broadening, which increased Ca(2+) influx and Ca(2+)-induced-Ca(2+) release, whereas the slower component was insensitive to these factors. Functionally, the oeAHP slowed and stabilized orexin-induced firing compared with firing produced by a virtual orexin conductance lacking the oeAHP. The oeAHP also reduced steady-state firing rate and firing fidelity in response to stimulation, without affecting the initial rate or fidelity. Collectively, these findings reveal a new orexin action in serotonergic raphe neurons and suggest that, when orexin is released during arousal and reward, it enhances the spike encoding of phasic over tonic inputs, such as those related to sensory, motor, and reward events.

SIGNIFICANCE STATEMENT

Orexin peptides are known to excite neurons via slow postsynaptic depolarizations. Here we elucidate a significant new orexin action that increases and prolongs the postspike afterhyperpolarization (AHP) in 5-HT dorsal raphe neurons and other arousal-system neurons. Our mechanistic studies establish involvement of two distinct Ca(2+)-dependent AHP currents dependent on phospholipase C signaling but independent of IP3 or PKC. Our functional studies establish that this action preserves responsiveness to phasic inputs while attenuating responsiveness to tonic inputs. Thus, our findings bring new insight into the actions of an important neuropeptide and indicate that, in addition to producing excitation, orexins can tune postsynaptic excitability to better encode the phasic sensory, motor, and reward signals expected during aroused states.

IKCa channels are a critical determinant of the slow AHP in CA1 pyramidal neurons

Control over the frequency and pattern of neuronal spike discharge depends on Ca2+-gated K+ channels that reduce cell excitability by hyperpolarizing the membrane potential. The Ca2+-dependent slow afterhyperpolarization (sAHP) is one of the most prominent inhibitory responses in the brain, with sAHP amplitude linked to a host of circuit and behavioral functions, yet the channel that underlies the sAHP has defied identification for decades. Here, we show that intermediate-conductance Ca2+-dependent K+ (IKCa) channels underlie the sAHP generated by trains of synaptic input or postsynaptic stimuli in CA1 hippocampal pyramidal cells. These findings are significant in providing a molecular identity for the sAHP of central neurons that will identify pharmacological tools capable of potentially modifying the several behavioral or disease states associated with the sAHP.

β-Adrenergic receptor signaling and modulation of long-term potentiation in the mammalian hippocampus

Encoding new information in the brain requires changes in synaptic strength. Neuromodulatory transmitters can facilitate synaptic plasticity by modifying the actions and expression of specific signaling cascades, transmitter receptors and their associated signaling complexes, genes, and effector proteins. One critical neuromodulator in the mammalian brain is norepinephrine (NE), which regulates multiple brain functions such as attention, perception, arousal, sleep, learning, and memory. The mammalian hippocampus receives noradrenergic innervation and hippocampal neurons express β-adrenergic receptors, which are known to play important roles in gating the induction of long-lasting forms of synaptic potentiation. These forms of long-term potentiation (LTP) are believed to importantly contribute to long-term storage of spatial and contextual memories in the brain. In this review, we highlight the contributions of noradrenergic signaling in general and β-adrenergic receptors in particular, toward modulating hippocampal LTP. We focus on the roles of NE and β-adrenergic receptors in altering the efficacies of specific signaling molecules such as NMDA and AMPA receptors, protein phosphatases, and translation initiation factors. Also, the roles of β-adrenergic receptors in regulating synaptic "tagging" and "capture" of LTP within synaptic networks of the hippocampus are reviewed. Understanding the molecular and cellular bases of noradrenergic signaling will enrich our grasp of how the brain makes new, enduring memories, and may shed light on credible strategies for improving mental health through treatment of specific disorders linked to perturbed memory processing and dysfunctional noradrenergic synaptic transmission.

Disrupted in schizophrenia 1 modulates medial prefrontal cortex pyramidal neuron activity through cAMP regulation of transient receptor potential C and small-conductance K+ channels

BACKGROUND

Disrupted in schizophrenia 1 (DISC1) is a protein implicated in schizophrenia, bipolar disorder, major depressive disorder, and autism. To date, most of research examining DISC1 function has focused on its role in neurodevelopment, despite its presence throughout life. DISC1 also regulates cyclic adenosine monophosphate (cAMP) signaling by increasing type 4 phosphodiesterase catabolism of cAMP when cAMP concentrations are high. In this study, we tested the hypothesis that DISC1, through its regulation of cAMP, modulates I-SK and I-TRPC channel-mediated ionic currents that we have shown previously to regulate the activity of mature prefrontal cortical pyramidal neurons.

METHODS

We used patch-clamp recordings in prefrontal cortical slices from adult rats in which DISC1 function was reduced in vivo by short hairpin RNA viral knockdown or in vitro by dialysis of DISC1 antibodies.

RESULTS

We found that DISC1 disruption resulted in an increase of metabotropic glutamate receptor-induced intracellular calcium (Ca2+) waves, small-conductance K+ (SK)-mediated hyperpolarization and a decrease of transient receptor potential C (TRPC)-mediated sustained depolarization. Consistent with a role for DISC1 in regulation of cAMP signaling, forskolin-induced cAMP production also increased intracellular Ca2+ waves, I-SK and decreased I-TRPC. Lastly, inhibiting cAMP generation with guanfacine, an α2A-noradrenergic agonist, normalized the function of SK and TRPC channels.

CONCLUSIONS

Based on our findings, we propose that diminished DISC1 function, such as occurs in some mental disorders, can lead to the disruption of normal patterns of prefrontal cortex activity through the loss of cAMP regulation of metabotropic glutamate receptor-mediated intracellular Ca2+ waves, SK and TRPC channel activity.

Role of small conductance Ca²⁺-activated K⁺ channels in controlling CA1 pyramidal cell excitability

Small-conductance Ca(2+)-activated K(+) (SK or K(Ca)2) channels are widely expressed in the CNS. In several types of neurons, these channels were shown to become activated during repetitive firing, causing early spike frequency adaptation. In CA1 pyramidal cells, SK channels in dendritic spines were shown to regulate synaptic transmission. However, the presence of functional SK channels in the somata and their role in controlling the intrinsic firing of these neurons has been controversial. Using whole-cell voltage-clamp and current-clamp recordings in acute hippocampal slices and focal applications of irreversible and reversible SK channel blockers, we provide evidence that functional SK channels are expressed in the somata and proximal dendrites of adult rat CA1 pyramidal cells. Although these channels can generate a medium duration afterhyperpolarizing current, they play only an auxiliary role in controlling the intrinsic excitability of these neurons, secondary to the low voltage-activating, noninactivating K(V)7/M channels. As long as K(V)7/M channels are operative, activation of SK channels during repetitive firing does not notably affect the spike output of CA1 pyramidal cells. However, when K(V)7/M channel activity is compromised, SK channel activation significantly and uniquely reduces spike output of these neurons. Therefore, proximal SK channels provide a "second line of defense" against intrinsic hyperexcitability, which may play a role in multiple conditions in which K(V)7/M channels activity is compromised, such as hyposmolarity.

Adrenoreceptor modulation of oromotor pathways in the rat medulla

Regulation of feeding behavior involves the integration of multiple physiological and neurological pathways that control both nutrient-seeking and consummatory behaviors. The consummatory phase of ingestion includes stereotyped oromotor movements of the tongue and jaw that are controlled through brain stem pathways. These pathways encompass not only cranial nerve sensory and motor nuclei for processing feeding-related afferent signals and supplying the oromotor musculature but also reticular neurons for orchestrating ingestion and coordinating it with other behaviors that utilize the same musculature. Based on decerebrate studies, this circuit should be sensitive to satiety mechanisms mediated centrally by A2 noradrenergic neurons in the caudal nucleus of the solitary tract (cNST) that are potently activated during satiety. Because the first observable phase of satiety is inhibition of oromotor movements, we hypothesized that norepinephrine (NE) would act to inhibit prehypoglossal neurons in the medullary reticular formation. Using patch-clamp electrophysiology of retrogradely labeled prehypoglossal neurons and calcium imaging to test this hypothesis, we demonstrate that norepinephrine can influence both pre- and postsynaptic properties of reticular neurons through both α1- and α2-adrenoreceptors. The α1-adrenoreceptor agonist phenylephrine (PE) activated an inward current in the presence of TTX and increased the frequency of both inhibitory and excitatory miniature postsynaptic currents. The α2-adrenoreceptor agonist dexmedetomidine (DMT) inhibited cNST-evoked excitatory currents as well as spontaneous and miniature excitatory currents through presynaptic mechanisms. The diversity of adrenoreceptor modulation of these prehypoglossal neurons may reflect their role in a multifunctional circuit coordinating both ingestive and respiratory lingual function.

Neurobiological dissociation of retrieval and reconsolidation of cocaine-associated memory

Drug use is provoked by the presentation of drug-associated cues, even following long periods of abstinence. Disruption of these learned associations would therefore limit relapse susceptibility. Drug-associated memories are susceptible to long-term disruption during retrieval and shortly after, during memory reconsolidation. Recent evidence reveals that retrieval and reconsolidation are dependent on β-adrenergic receptor (β-AR) activation. Despite this, whether retrieval and reconsolidation are dependent on identical or distinct neural mechanisms is unknown. The prelimbic medial prefrontal cortex (PL-mPFC) and basolateral amygdala (BLA) have been implicated in the expression and reconsolidation of associative memories. Therefore, we investigated the necessity of β-AR activation within the PL-mPFC and BLA for cocaine-associated memory retrieval and reconsolidation in rats. Before or immediately after a cocaine-induced conditioned place preference (CPP) retrieval trial, β-AR antagonists were infused into the PL-mPFC or BLA, followed by daily testing. PL-mPFC infusions before, but not after, a CPP trial disrupted CPP memory retrieval and induced a persistent deficit in retrieval during subsequent trials. In contrast, BLA β-AR blockade had no effect on initial CPP memory retrieval, but prevented CPP expression during subsequent trials indicative of reconsolidation disruption. Our results reveal a distinct dissociation between the neural mechanisms required for cocaine-associated memory retrieval and reconsolidation. Using patch-clamp electrophysiology, we also show that application of a β-AR antagonist prevents norepinephrine-induced potentiation of PL-mPFC pyramidal cell and γ-aminobutyric-acid (GABA) interneuron excitability. Thus, targeted β-AR blockade could induce long-term deficits in drug-associated memory retrieval by reducing neuronal excitability, providing a novel method of preventing cue-elicited drug seeking and relapse.

Activity-dependent transport of the transcriptional coactivator CRTC1 from synapse to nucleus

Long-lasting changes in synaptic efficacy, such as those underlying long-term memory, require transcription. Activity-dependent transport of synaptically localized transcriptional regulators provides a direct means of coupling synaptic stimulation with changes in transcription. The CREB-regulated transcriptional coactivator (CRTC1), which is required for long-term hippocampal plasticity, binds CREB to potently promote transcription. We show that CRTC1 localizes to synapses in silenced hippocampal neurons but translocates to the nucleus in response to localized synaptic stimulation. Regulated nuclear translocation occurs only in excitatory neurons and requires calcium influx and calcineurin activation. CRTC1 is controlled in a dual fashion with activity regulating CRTC1 nuclear translocation and cAMP modulating its persistence in the nucleus. Neuronal activity triggers a complex change in CRTC1 phosphorylation, suggesting that CRTC1 may link specific types of stimuli to specific changes in gene expression. Together, our results indicate that synapse-to-nuclear transport of CRTC1 dynamically informs the nucleus about synaptic activity.

Neuromodulation of I(h) in layer II medial entorhinal cortex stellate cells: a voltage-clamp study

Stellate cells in layer II of medial entorhinal cortex (mEC) are endowed with a large hyperpolarization-activated cation current [h current (I(h))]. Recent work using in vivo recordings from awake behaving rodents demonstrate that I(h) plays a significant role in regulating the characteristic spatial periodicity of "grid cells" in mEC. A separate, yet related, line of research demonstrates that grid field spacing changes as a function of behavioral context. To understand the neural mechanism or mechanisms that could be underlying these changes in grid spacing, we have conducted voltage-clamp recordings of I(h) in layer II stellate cells. In particular, we have studied I(h) under the influence of several neuromodulators. The results demonstrate that I(h) amplitude can be both upregulated and downregulated through activation of distinct neuromodulators in mEC. Activation of muscarinic acetylcholine receptors produces a significant decrease in the I(h) tail current and a hyperpolarizing shift in the activation, whereas upregulation of cAMP through application of forskolin produces a significant increase in the I(h) amplitude and a depolarizing shift in I(h) activation curve. In addition, there was evidence of differential modulation of I(h) along the dorsal-ventral axis of mEC. Voltage-clamp protocols were also used to determine whether M current is present in stellate cells. In contrast to CA1 pyramidal neurons, which express M current, the data demonstrate that M current is not present in stellate cells. The results from this study provide key insights into a potential mechanism that could be underlying changes seen in grid field spacing during distinct behavioral contexts.

LIM domain only 4 (LMO4) regulates calcium-induced calcium release and synaptic plasticity in the hippocampus

The LIM domain only 4 (LMO4) transcription cofactor activates gene expression in neurons and regulates key aspects of network formation, but the mechanisms are poorly understood. Here, we show that LMO4 positively regulates ryanodine receptor type 2 (RyR2) expression, thereby suggesting that LMO4 regulates calcium-induced calcium release (CICR) in central neurons. We found that CICR modulation of the afterhyperpolarization in CA3 neurons from mice carrying a forebrain-specific deletion of LMO4 (LMO4 KO) was severely compromised but could be restored by single-cell overexpression of LMO4. In line with these findings, two-photon calcium imaging experiments showed that the potentiation of RyR-mediated calcium release from internal stores by caffeine was absent in LMO4 KO neurons. The overall facilitatory effect of CICR on glutamate release induced during trains of action potentials was likewise defective in LMO4 KO, confirming that CICR machinery is severely compromised in these neurons. Moreover, the magnitude of CA3-CA1 long-term potentiation was reduced in LMO4 KO mice, a defect that appears to be secondary to an overall reduced glutamate release probability. These cellular phenotypes in LMO4 KO mice were accompanied with deficits in hippocampus-dependent spatial learning as determined by the Morris water maze test. Thus, our results establish LMO4 as a key regulator of CICR in central neurons, providing a mechanism for LMO4 to modulate a wide range of neuronal functions and behavior.

Activation of protein kinase A and C prevents recovery from persistent depolarization produced by oxygen and glucose deprivation in rat hippocampal neurons

Intracellular recordings were made from rat hippocampal CA1 neurons in rat brain slice preparations to investigate whether cAMP-dependent protein kinase (PKA) and calcium/phospholipid-dependent protein kinase C (PKC) contribute to the membrane dysfunction induced by oxygen and glucose deprivation (OGD). Superfusion of oxygen- and glucose-deprived medium produced a rapid depolarization ∼5 min after the onset of the superfusion. When oxygen and glucose were reintroduced immediately after the rapid depolarization, the membrane depolarized further (persistent depolarization) and reached 0 mV after 5 min from the reintroduction. The pretreatment of the slice preparation with PKA inhibitors, H-89 and Rp-cAMPS, and an adenylate cyclase inhibitor, SQ 22, 536, significantly restored the membrane toward the preexposure potential level after the reintroduction of oxygen and glucose in a concentration-dependent manner. On the other hand, a phospholipase C inhibitor, U73122, a PKC inhibitor, GF109203X, and a nonselective protein kinase inhibitor, staurosporine, also significantly restored the membrane after the reintroduction. Moreover, an inositol-1,4,5-triphosphate receptor antagonist, 2-aminoethyl diphenylborinate, and calmodulin inhibitors, trifluoperazine and W-7, significantly restored the membrane after the reintroduction, while neither an α-subunit-selective antagonist for stimulatory G protein, NF449, a Ca(2+)/calmodulin-dependent kinase II inhibitor, KN-62, nor a myosin light chain kinase inhibitor, ML-7, significantly restored the membrane after the reintroduction. These results suggest that the activation of PKA and/or PKC prevents the recovery from the persistent depolarization produced by OGD. The Ca(2+)/calmodulin-stimulated adenylate cyclase may contribute to the activation of PKA.

Cyclic AMP signaling control of action potential firing rate and molecular circadian pacemaking in the suprachiasmatic nucleus

Circadian pacemaking in suprachiasmatic nucleus (SCN) neurons revolves around transcriptional/posttranslational feedback loops, driven by protein products of "clock" genes. These loops are synchronized and sustained by intercellular signaling, involving vasoactive intestinal peptide (VIP) via its VPAC2 receptor, which positively regulates cAMP synthesis. In turn, SCN cells communicate circadian time to the brain via a daily rhythm in electrophysiological activity. To investigate the mechanisms whereby VIP/VPAC2/cAMP signaling controls SCN molecular and electrical pacemaking, we combined bioluminescent imaging of circadian gene expression and whole-cell electrophysiology in organotypic SCN slices. As a potential direct target of cAMP, we focused on hyperpolarization-activated, cyclic nucleotide-gated (HCN) channels. Mutations of VIP-ergic signaling compromised the SCN molecular pacemaker, diminishing the amplitude and intercellular synchrony of circadian gene expression. These deficits were transiently reversed by elevation of cAMP. Similarly, cellular synchrony in electrical firing rates was lost in SCN slices lacking the VPAC2 receptor for VIP. Whole-cell current-clamp recordings in wild-type (WT) slices revealed voltage responses shaped by the conductance I(h), which is mediated by HCN channel activity. The influence of I(h) on voltage responses showed a modest peak in early circadian day, identifying HCN channels as a putative mediator of cAMP-dependent circadian effects on firing rate. I(h), however, was unaffected by loss of VIP-ergic signaling in VPAC2-null slices, and inhibition of cAMP synthesis had no discernible effect on I(h) but did suppress gene expression and SCN firing rates. Moreover, only sustained but not acute, pharmacological blockade of HCN channels reduced action potential (AP) firing. Thus, our evidence suggests that in the SCN, cAMP-mediated signaling is not a principal regulator of HCN channel function and that HCN is not a determinant of AP firing rate. VIP/cAMP-dependent signaling sustains the SCN molecular oscillator and action potential firing via mechanisms yet to be identified.

Neuroplasticity regulation by noradrenaline in mammalian brain

The neuromodulator noradrenaline (NA) is released in almost all brain areas in a highly diffused manner. Its action is slow, as it acts through G protein-coupled receptors, but its wide release in the brain makes NA a crucial regulator for various fundamental brain functions such as arousal, attention and memory processes [102]. To understand how NA acts in the brain to promote such diverse actions, it is necessary to dissect the cellular actions of NA at the level of single neurons as well as at the level of neuronal networks. In the present article, we will provide a compact review of the main literatures concerning the NA actions on neuroplasticity processes. Depending on which subtype of adrenoceptor is activated, NA differently affects intrinsic membrane properties of postsynaptic neurons and synaptic plasticity. For example, beta-adrenoceptor activation is mainly related to the potentiation of synaptic responses and learning and memory processes. alpha2-adrenoceptor activation may contribute to a high-order information processing such as executive function, but currently the direction of synaptic plasticity modification by alpha2-adrenoceptors has not been clearly determined. The activation of alpha1-adrenoceptors appears to mainly induce synaptic depression in the brain. But its physiological roles are still unclear: while its activation has been described as beneficial for cognitive functions, it may also exert detrimental effects in some brain structures such as the prefrontal cortex.

Effects of weak environmental magnetic fields on the spontaneous bioelectrical activity of snail neurons

We examined the effects of 50-Hz magnetic fields in the range of flux densities relevant to our current environmental exposures on action potential (AP), after-hyperpolarization potential (AHP) and neuronal excitability in neurons of land snails, Helix aspersa. It was shown that when the neurons were exposed to magnetic field at the various flux densities, marked changes in neuronal excitability, AP firing frequency and AHP amplitude were seen. These effects seemed to be related to the intensity, type (single and continuous or repeated and cumulative) and length of exposure (18 or 20 min). The extremely low-frequency (ELF) magnetic field exposures affect the excitability of F1 neuronal cells in a nonmonotonic manner, disrupting their normal characteristic and synchronized firing patterns by interfering with the cell membrane electrophysiological properties. Our results could explain one of the mechanisms and sites of action of ELF magnetic fields. A possible explanation of the inhibitory effects of magnetic fields could be a decrease in Ca(2+) influx through inhibition of voltage-gated Ca(2+) channels. The detailed mechanism of effect, however, needs to be further studied under voltage-clamp conditions.

D-Aspartic acid is a novel endogenous neurotransmitter

D-aspartic acid (D-Asp) is present in invertebrate and vertebrate neuroendocrine tissues, where it carries out important physiological functions and is implicated in nervous system development. We show here that D-Asp is a novel endogenous neurotransmitter in two distantly related animals, a mammal (Rattus norvegicus) and a mollusk (Loligo vulgaris). Our main findings demonstrate that D-Asp is present in high concentrations in the synaptic vesicles of axon terminals; synthesis for this amino acid occurs in neurons by conversion of L-Asp to D-Asp via D-aspartate racemase; depolarization of nerve endings with K(+) ions evokes an immediate release of D-Asp in a Ca(2+) dependent manner; specific receptors for D-Asp occur at the postsynaptic membrane, as demonstrated by binding assays and by the expansion of squid skin chromatophores; D-aspartate oxidase, the specific enzyme that oxidizes D-Asp, is present in the postsynaptic membranes; and stimulation of nerve endings with D-Asp triggers signal transduction by increasing the second messenger cAMP. Taken together, these data demonstrate that D-Asp fulfills all criteria necessary to be considered a novel endogenous neurotransmitter. Given its known role in neurogenesis, learning, and neuropathologies, our results have important implications for biomedical and clinical research.

The KCNQ5 potassium channel mediates a component of the afterhyperpolarization current in mouse hippocampus

Mutations in KCNQ2 and KCNQ3 voltage-gated potassium channels lead to neonatal epilepsy as a consequence of their key role in regulating neuronal excitability. Previous studies in the brain have focused primarily on these KCNQ family members, which contribute to M-currents and afterhyperpolarization conductances in multiple brain areas. In contrast, the function of KCNQ5 (Kv7.5), which also displays widespread expression in the brain, is entirely unknown. Here, we developed mice that carry a dominant negative mutation in the KCNQ5 pore to probe whether it has a similar function as other KCNQ channels. This mutation renders KCNQ5(dn)-containing homomeric and heteromeric channels nonfunctional. We find that Kcnq5(dn/dn) mice are viable and have normal brain morphology. Furthermore, expression and neuronal localization of KCNQ2 and KCNQ3 subunits are unchanged. However, in the CA3 area of hippocampus, a region that highly expresses KCNQ5 channels, the medium and slow afterhyperpolarization currents are significantly reduced. In contrast, neither current is affected in the CA1 area of the hippocampus, a region with low KCNQ5 expression. Our results demonstrate that KCNQ5 channels contribute to the afterhyperpolarization currents in hippocampus in a cell type-specific manner.

The fruit essential oil of Pimpinella anisum L. (Umblliferae) induces neuronal hyperexcitability in snail partly through attenuation of after-hyperpolarization

AIM OF THE STUDY

Many biological actions of Pimpinella anisum L. (Ainse), including antiepileptic activity have been demonstrated; however, there is no data concerning its precise cellular mechanisms of action. We determined whether the fruit essential oil of anise affect the bioelectrical activity of snail neurons in control condition or after pentylenetetrazol (PTZ) induced epileptic activity.

MATERIALS AND METHODS

Intracellular recordings were made under the current clamp condition and the effects of anise oil (0.01% or 0.05%) alone or in combination with PTZ were assessed on the firing pattern, action potential configuration and postspike potentials.

RESULTS

Anise oil changed the firing pattern from regular tonic discharge to irregular and then to bursting in intact cells or resulted in the robustness of the burst firing and the steepness of the paroxysmal shift induced by PTZ treatment. It also significantly increased the firing frequency and decreased both the after-hyperpolarization potential (AHP) following single action potential and the post-pulse AHP.

CONCLUSIONS

Likely candidate cellular mechanisms underlying the hyperexcitability produced by anise oil include enhancement of Ca(2+) channels activity or inhibition of voltage and/or Ca(2+) dependent K(+) channels activity underlying AHPs. These finding indicates that a certain caution is needed when Pimpinella anisum is used for treating patients suffering from epilepsy.

SK (KCa2) channels do not control somatic excitability in CA1 pyramidal neurons but can be activated by dendritic excitatory synapses and regulate their impact

Calcium-activated K(+) channels of the K(Ca)2 type (SK channels) are prominently expressed in the mammalian brain, including hippocampus. These channels are thought to underlie neuronal excitability control and have been implicated in plasticity, memory, and neural disease. Contrary to previous reports, we found that somatic spike-evoked medium afterhyperpolarizations (mAHPs) and corresponding excitability control were not caused by SK channels but mainly by Kv7/KCNQ/M channels in CA1 hippocampal pyramidal neurons. Thus apparently, these SK channels are hardly activated by somatic Na(+) spikes. To further test this conclusion, we used sharp electrode, whole cell, and perforated-patch recordings from rat CA1 pyramidal neurons. We found that SK channel blockers consistently failed to suppress mAHPs under a range of experimental conditions: mAHPs following single spikes or spike trains, at -60 or -80 mV, at 20-30 degrees C, in low or elevated extracellular [K(+)], or spike trains triggered by synaptic stimulation after blocking N-methyl-d-aspartic acid receptors (NMDARs). Nevertheless, we found that SK channels in these cells were readily activated by artificially enhanced Ca(2+) spikes, and an SK channel opener (1-ethyl-2-benzimidazolinone) enhanced somatic AHPs following Na(+) spikes, thus reducing excitability. In contrast to CA1 pyramidal cells, bursting pyramidal cells in the subiculum showed a Na(+) spike-evoked mAHP that was reduced by apamin, indicating cell-type-dependent differences in mAHP mechanisms. Testing for other SK channel functions in CA1, we found that field excitatory postsynaptic potentials mediated by NMDARs were enhanced by apamin, supporting the idea that dendritic SK channels are activated by NMDAR-dependent calcium influx. We conclude that SK channels in rat CA1 pyramidal cells can be activated by NMDAR-mediated synaptic input and cause feedback regulation of synaptic efficacy but are normally not appreciably activated by somatic Na(+) spikes in this cell type.

Individual and additive effects of neuromodulators on the slow components of afterhyperpolarization currents in layer V pyramidal cells of the rat medial prefrontal cortex

The effects of 5-hydroxytryptamine (5-HT), noradrenaline (NA), dopamine (DA) and the muscarinic receptor agonist carbachol (CCh) on the voltage step-induced outward currents underlying afterhyperpolarization (AHP), consisting of a medium (I(mAHP)) and slow (I(sAHP)) component, were investigated in layer V pyramidal cells of the rat medial prefrontal cortex (mPFC). Whole-cell voltage clamp recordings were performed in vitro to quantitatively measure I(mAHP) and I(sAHP) and to examine their functional link to spike-frequency adaptation in the presence of agonists. CCh, 5-HT and NA all reduced the I(sAHP) and the spike adaptation, and, in some cells, replaced the I(sAHP) by the slow inward currents (I(sADP)) underlying the slow afterdepolarization (sADP). DA, however, failed to increase the frequency despite its comparable inhibition of the I(sAHP) over a range of concentrations. In order to test the neuromodulator agonists to see if they have additive actions on the I(sAHP), the effects of co-application of two agonists that increased spike-frequency, 5-HT+NA, 5-HT+CCh and CCh+NA, all at the concentration 30 microM were examined. Specific combinations that included CCh showed additive effects on the slow afterpolarization currents, possibly via both inhibition of I(sAHP) and generation of I(sADP). These findings suggest that neuromodulators have differential effects on the link between the I(sAHP) modulation and spike-frequency adaptation, and that they could exert additive effects on the slow aftercurrents following a strong excitation and, therefore, regulate the repetitive firing properties of the output cells of the rat mPFC.

Enhanced neuronal excitability in rat CA1 pyramidal neurons following trace eyeblink conditioning acquisition is not due to alterations in I M

Previous work done by our laboratory has demonstrated a reduction of the post-burst afterhyperpolarization (AHP) and accommodation following trace eyeblink conditioning in rabbit CA1 pyramidal neurons. Our laboratory has also demonstrated a reduction in the AHP in rat CA1 pyramidal neurons following spatial learning. In the current study we have extended our findings in rabbits by showing a reduction in both the AHP and accommodation in F344 X BN rat CA1 pyramidal neurons following acquisition of trace eyeblink conditioning. A component current of the AHP, I(M), was evaluated with a specific blocker of this current, and showed no apparent contribution to the learning-related increase in neuronal excitability. Rather, a reduction in an isoproterenol-sensitive component of the AHP, presumably sI(AHP), was observed to underlie the learning-specific change.

Differential regulation of synaptic transmission by adrenergic agonists via protein kinase A and protein kinase C in layer V pyramidal neurons of rat cerebral cortex

Activation of alpha1- and beta-adrenoceptors modulates excitatory neural transmission in the cerebral cortex in opposite manners. Our in vitro optical imaging study using a voltage sensitive dye has revealed that an alpha1-adrenoceptor agonist, phenylephrine, suppresses the excitatory propagation evoked by stimulation of the white matter, whereas a beta-adrenoceptor agonist, isoproterenol, tends to potentiate the excitatory propagation especially in the deeper layers. The present study aimed to explore what kind of second messengers are involved in noradrenergic modulation of synaptic transmission by using intracellular recording in rat cerebrocortical slice preparation. Evoked excitatory postsynaptic potentials (eEPSPs) were recorded from regular spiking and bursting pyramidal neurons in layer V, which generate single and complex action potentials in response to a short (5 ms) depolarizing current pulse injection, respectively. Application of phenylephrine attenuated eEPSPs, and prazosin, an alpha1-adrenoceptor antagonist, precluded the phenylephrine-induced suppression of eEPSPs. The EPSPs suppression by phenylephrine was blocked by pre-application of a protein kinase C (PKC) inhibitor, chelerythrine, whereas a PKC activator, phorbol 12-myristate 13-acetate (phorbol ester), mimicked the effect of phenylephrine. On the other hand, application of isoproterenol enhanced eEPSPs, and propranolol, a beta-adrenoceptor antagonist, precluded the excitatory effect of isoproterenol on eEPSPs. The membrane permeant analog of cyclic-3',5'-AMP (cAMP), N6,2'-O-dibutyryl-AMP (db-cAMP), mimicked the facilitatory effect of isoproterenol. Isoproterenol-induced enhancement of eEPSPs was promoted by pre-application of 4-(3-butoxy-4-methoxybenzyl)-2-imidazolidinone (Ro 20-1724), a cAMP-specific phosphodiesterase inhibitor. A selective protein kinase A inhibitor, N-[2-(p-bromocinnamylamino)ethyl]-5-soquinolinesulfonamide (H-89), inhibited the excitatory effect by isoproterenol. There was no significant difference in the effects of adrenergic agonists/antagonists and protein kinase activators/inhibitors between regular spiking and bursting neurons in layer V. Thus, it is likely that the suppressive effect on eEPSPs by activation of alpha1-adrenoceptors was mediated by protein kinase C, and excitatory effect by activation of beta-adrenoceptors was mediated by cAMP/protein kinase A cascade in layer V pyramidal neurons.

Impaired hippocampal LTP in inbred mouse strains can be rescued by β-adrenergic receptor activation

Long-term potentiation (LTP), an activity-dependent enhancement of synaptic strength, and memory can be influenced by neuromodulatory transmitters such as norepinephrine (NE) and also by genetic background. beta-Adrenergic receptor activation can facilitate the expression of hippocampal CA1 LTP induced by weak stimulus patterns, but its influence on LTP induced by stronger stimulus patterns is unclear. We examined neural NE and dopamine (DA) levels, beta-adrenergic receptor expression and hippocampal LTP in genetically diverse inbred mouse strains. Brain tissue levels of NE were significantly lower in strains 129S1/SvImJ (129), BALB/cByJ (BALB) and C3H/HeJ (C3H) than in C57BL/6NCrlBR (B6). Western blot analysis showed that hippocampal beta(1)-adrenergic receptor expression was similar in strains B6, 129 and C3H, but was increased in BALB. LTP was induced in area CA1 of hippocampal slices by four trains of high-frequency stimulation (HFS) of the Schaeffer collaterals in the four inbred strains. Two hours after induction, LTP was significantly reduced in strains 129, BALB and C3H compared to B6, correlating with neural NE levels. We rescued hippocampal LTP in strains 129, BALB and C3H to levels seen in B6 by bath application of 1 microm isoproterenol, a beta-adrenergic receptor agonist, during HFS. Propranolol, a beta-adrenergic receptor antagonist, blocked this rescue in 129, BALB and C3H but did not affect LTP in strain B6. Thus, although this form of multitrain LTP does not rely on beta-adrenergic receptor activation, our data show that pharmacological activation of beta-adrenergic receptors during multiple trains of HFS can rescue CA1 LTP in genetically diverse strains with impaired LTP.

M-channels (Kv7/KCNQ channels) that regulate synaptic integration, excitability, and spike pattern of CA1 pyramidal cells are located in the perisomatic region

To understand how electrical signal processing in cortical pyramidal neurons is executed by ion channels, it is essential to know their subcellular distribution. M-channels (encoded by Kv7.2-Kv7.5/KCNQ2-KCNQ5 genes) have multiple important functions in neurons, including control of excitability, spike afterpotentials, adaptation, and theta resonance. Nevertheless, the subcellular distribution of these channels has remained elusive. To determine the M-channel distribution within CA1 pyramidal neurons, we combined whole-cell patch-clamp recording from the soma and apical dendrite with focal drug application, in rat hippocampal slices. Both a M-channel opener (retigabine [N-(2-amino-4-(4-fluorobenzylamino)-phenyl) carbamic acid ethyl ester]) and a blocker (XE991 [10,10-bis(4-pyridinylmethyl)-9(10H)-antracenone]) changed the somatic subthreshold voltage response but had no observable effect on local dendritic responses. Under conditions promoting dendritic Ca2+ spikes, local somatic but not dendritic application of M-channel blockers (linopirdine and XE991) enhanced the Ca2+ spikes. Simultaneous dendritic and somatic whole-cell recordings showed that the medium afterhyperpolarization after a burst of spikes underwent strong attenuation along the apical dendrite and was fully blocked by somatic XE991 application. Finally, by combining patch-clamp and extracellular recordings with computer simulations, we found that perisomatic M-channels reduce the summation of EPSPs. We conclude that functional M-channels appear to be concentrated in the perisomatic region of CA1 pyramidal neurons, with no detectable M-channel activity in the distal apical dendrites.

Autocorrelogram sorting: a novel method for evaluating negative-feedback regulation of spike firing

Spontaneously firing units were recorded extracellularly from the hippocampus of anesthetized rats, and autocorrelograms were conventionally constructed. These conventional autocorrelograms were sorted into frequency-specific autocorrelograms according to the instantaneous firing frequency of the spike at Deltat=0, which was calculated based on the interval between the spike at Deltat=0 and the preceding spike. In this fashion, we found that autocorrelogram values were negatively correlated with instantaneous firing frequency during the 4-12 ms post-spike time period. The negative correlation during the 4-6 ms post-spike period could not have been due to the refractory period or GABAergic inhibition, and thus represented a third type of feedback regulation of spike firing completed within single neurons. Application of acetylcholine significantly enhanced this feedback regulation. Our 'autocorrelogram sorting' method thus proved to be successful in detecting cholinergically enhanced feedback regulation of spike firing intrinsic to single neurons.

Influence of norepinephrine on somatosensory neuronal responses in the rat thalamus: a combined modeling and in vivo multi-channel, multi-neuron recording study

Norepinephrine released within primary sensory circuits from locus coeruleus afferent fibers can produce a spectrum of modulatory actions on spontaneous or sensory-evoked activity of individual neurons. Within the ventral posterior medial thalamus, membrane currents modulated by norepinephrine have been identified. However, the relationship between the cellular effects of norepinephrine and the impact of norepinephrine release on populations of neurons encoding sensory signals is still open to question. To address this lacuna in understanding the net impact of the noradrenergic system on sensory signal processing, a computational model of the rat trigeminal somatosensory thalamus was generated. The effects of independent manipulation of different cellular actions of norepinephrine on simulated afferent input to the computational model were then examined. The results of these simulations aided in the design of in vivo neural ensemble recording experiments where sensory-driven responses of thalamic neurons were measured before and during locus coeruleus activation in waking animals. Together the simulated and experimental results reveal several key insights regarding the regulation of neural network operation by norepinephrine including: 1) cell-specific modulatory actions of norepinephrine, 2) mechanisms of norepinephrine action that can improve the tuning of the network and increase the signal-to-noise ratio of cellular responses in order to enhance network representation of salient stimulus features and 3) identification of the dynamic range of thalamic neuron function through which norepinephrine operates.

Protein kinase signalling requirements for metabotropic action of kainate receptors in rat CA1 pyramidal neurones

Hippocampal pyramidal neurones display a Ca(2+)-dependent K(+) current responsible for the slow afterhyperpolarization (I(sAHP)), a prominent regulator of excitability. There is considerable transmitter convergence onto I(sAHP) but little information about the interplay between the kinase-based transduction mechanisms underlying transmitter action. We have added to existing information about the role of protein kinase C (PKC) in kainate receptor actions by demonstrating that direct postsynaptic activation of PKC with either 1-oleoyl-2-acethylsn-glycerol (OAG) or indolactam is sufficient to inhibit I(sAHP). The physiological correlate of this action - activation of PKC by kainate receptors - requires G alpha(i/o) proteins. The cAMP/PKA system is well documented to subserve the actions of monoamine transmitters. We have found an additional role for the cAMP/PKA system as a requirement for kainate receptor-mediated inhibition of I(sAHP). Inhibition of adenylyl cyclase with dideoxyadenosine or PKA with either H89 or RpcAMPs blocked kainate receptor-mediated actions but did not prevent the actions of direct PKC activation with either OAG or indolactam. We therefore propose that the PKA requirement is upstream from the actions of PKC. We additionally report a downstream link in the form of increased mitogen-activated protein (MAP) kinase activity, which may explain the long duration of metabotropic actions of kainate receptors on I(sAHP).

Clinically relevant doses of fluoxetine and reboxetine induce changes in the TrkB content of central excitatory synapses

We have studied the effect of low doses of two widely used antidepressants, fluoxetine (Flx) and reboxetine (Rbx), on excitatory synapses of rat brain cortex and hippocampus. After 15 days of Flx treatment (0.67 mg/kg/day), its plasma level was 20.7+/-5.6 ng/ml. Analysis of postsynaptic densities (PSDs) by immunoblotting revealed no changes in the glutamate receptor subunits GluR1, NR1, NR2A/B, mGluR1alpha nor in the neurotrophin receptor p75(NTR). However, the brain-derived neurotrophic factor (BDNF) receptor TrkB decreased by 42.8+/-6%, and remained decreased after 6 weeks of treatment. The BDNF and TrkB content in homogenates of cortex and hippocampus began to rise at 9 and 15 days, respectively, and remained high for up to 6 weeks. Similar results were obtained following chronic Rbx administration at 0.128 mg/kg/day. We propose that BDNF, whose synthesis is increased by antidepressants, and which is in part released at synaptic sites, binds to TrkB in PSDs, leading to the internalization of the BDNF-TrkB complex and, thus, to a decrease of TrkB in the PSDs. This was paralleled by greater levels of phosphorylated (ie activated) TrkB in the light membrane fraction, that contains signaling endosomes. The retrograde transport of endocyted BDNF/TrkB complexes from spines to cell bodies, where it activates the synthesis of more BDNF, is a protracted process, potentially requiring several cycles of TrkB/BDNF complex endocytosis and transport. This positive feedback mechanism may help explain the time-lag between drug administration and its therapeutic effect, that is, the antidepressant drug paradox.

Calcium-activated afterhyperpolarizations regulate synchronization and timing of epileptiform bursts in hippocampal CA3 pyramidal neurons

Calcium-activated potassium conductances regulate neuronal excitability, but their role in epileptogenesis remains elusive. We investigated in rat CA3 pyramidal neurons the contribution of the Ca(2+)-activated K(+)-mediated afterhyperpolarizations (AHPs) in the genesis and regulation of epileptiform activity induced in vitro by 4-aminopyridine (4-AP) in Mg(2+)-free Ringer. Recurring spike bursts terminated by prolonged AHPs were generated. Burst synchronization between CA3 pyramidal neurons in paired recordings typified this interictal-like activity. A downregulation of the medium afterhyperpolarization (mAHP) paralleled the emergence of the interictal-like activity. When the mAHP was reduced or enhanced by apamin and EBIO bursts induced by 4-AP were increased or blocked, respectively. Inhibition of the slow afterhyperpolarization (sAHP) with carbachol, t-ACPD, or isoproterenol increased bursting frequency and disrupted burst regularity and synchronization between pyramidal neuron pairs. In contrast, enhancing the sAHP by intracellular dialysis with KMeSO(4) reduced burst frequency. Block of GABA(A-B) inhibitions did not modify the abnormal activity. We describe novel cellular mechanisms where 1) the inhibition of the mAHP plays an essential role in the genesis and regulation of the bursting activity by reducing negative feedback, 2) the sAHP sets the interburst interval by decreasing excitability, and 3) bursting was synchronized by excitatory synaptic interactions that increased in advance and during bursts and decreased throughout the subsequent sAHP. These cellular mechanisms are active in the CA3 region, where epileptiform activity is initiated, and cooperatively regulate the timing of the synchronized rhythmic interictal-like network activity.

Kinetics of ion channel modulation by cAMP in rat hippocampal neurones

Ion channel regulation by cyclic AMP and protein kinase A is a major effector mechanism for monoamine transmitters and neuromodulators in the CNS. Surprisingly, there is little information about the speed and kinetic limits of cAMP-PKA-dependent excitability changes in the brain. To explore these questions, we used flash photolysis of caged-cAMP (DMNB-cAMP) to provide high temporal resolution. The resultant free cAMP concentration was calculated from separate experiments in which this technique was used, in excised patches, to activate cAMP-sensitive cyclic nucleotide-gated (CNG) channels expressed in Xenopus oocytes. In hippocampal pyramidal neurones we studied the modulation of a potassium current (slow AHP current, I(sAHP)) known to be targeted by multiple transmitter systems that use cAMP-PKA. Rapid cAMP elevation by flash photolyis of 200 microm DMNB-cAMP completely inhibited the K(+) current. The estimated yield (1.3-3%) suggests that photolysis of 200 microm caged precursor is sufficient for full PKA activation. By contrast, extended gradual photolysis of 200 microm DMNB-cAMP caused stable but only partial inhibition. The kinetics of rapid cAMP inhibition of the K(+) conductance (time constant 1.5-2 s) were mirrored by changes in firing patterns commencing within 500 ms of rapid cAMP elevation. Maximal increases in firing were short-lasting (< 60 s) and gave way to moderately enhanced levels of spiking. The results demonstrate how the fidelity of phasic monoamine signalling can be preserved by the cAMP-PKA pathway.

Enhancement of hippocampal pyramidal cell excitability by the novel selective slow-afterhyperpolarization channel blocker 3-(triphenylmethylaminomethyl)pyridine (UCL2077)

The slow afterhyperpolarization (sAHP) in hippocampal neurons has been implicated in learning and memory. However, its precise role in cell excitability and central nervous system function has not been explicitly tested for 2 reasons: 1) there are, at present, no selective inhibitors that effectively reduce the underlying current in vivo or in intact in vitro tissue preparations, and 2) although it is known that a small conductance K(+) channel that activates after a rise in [Ca(2+)](i) underlies the sAHP, the exact molecular identity remains unknown. We show that 3-(triphenylmethylaminomethyl)pyridine (UCL2077), a novel compound, suppressed the sAHP present in hippocampal neurons in culture (IC(50) = 0.5 microM) and in the slice preparation (IC(50) approximately 10 microM). UCL2077 was selective, having minimal effects on Ca(2+) channels, action potentials, input resistance and the medium afterhyperpolarization. UCL2077 also had little effect on heterologously expressed small conductance Ca(2+)-activated K(+) (SK) channels. Moreover, UCL2077 and apamin, a selective SK channel blocker, affected spike firing in hippocampal neurons in different ways. These results provide further evidence that SK channels are unlikely to underlie the sAHP. This study also demonstrates that UCL2077, the most potent, selective sAHP blocker described so far, is a useful pharmacological tool for exploring the role of sAHP channels in the regulation of cell excitability in intact tissue preparations and, potentially, in vivo.

Paraoxon suppresses Ca2+ spike and afterhyperpolarization in snail neurons: Relevance to the hyperexcitability induction

The effects of organophosphate (OP) paraoxon, active metabolite of parathion, were studied on the Ca(2+) and Ba(2+) spikes and on the excitability of the neuronal soma membranes of land snail (Caucasotachea atrolabiata). Paraoxon (0.3 muM) reversibly decreased the duration and amplitude of Ca(2+) and Ba(2+) spikes. It also reduced the duration and the amplitude of the afterhyperpolarization (AHP) that follows spikes, leading to a significant increase in the frequency of Ca(2+) spikes. Pretreatment with atropine and hexamethonium, selective blockers of muscarinic and nicotinic receptors, respectively, did not prevent the effects of paraoxon on Ca(2+) spikes. Intracellular injection of the calcium chelator BAPTA dramatically decreased the duration and amplitude of AHP and increased the duration and frequency of Ca(2+) spikes. In the presence of BAPTA, paraoxon decreased the duration of the Ca(2+) spikes without affecting their frequency. Apamin, a neurotoxin from bee venom, known to selectively block small conductance of calcium-activated potassium channels (SK), significantly decreased the duration and amplitude of the AHP, an effect that was associated with an increase in spike frequency. In the presence of apamin, bath application of paraoxon reduced the duration of Ca(2+) spike and AHP and increased the firing frequency of nerve cells. In summary, these data suggest that exposure to submicromolar concentration of paraoxon may directly affect membrane excitability. Suppression of Ca(2+) entry during the action potential would down regulate Ca(2+)-activated K(+) channels leading to a reduction of the AHP and an increase in cell firing.

Regulation of surface localization of the small conductance Ca2+-activated potassium channel, Sk2, through direct phosphorylation by cAMP-dependent protein kinase

Small conductance, Ca2+-activated voltage-independent potassium channels (SK channels) are widely expressed in diverse tissues; however, little is known about the molecular regulation of SK channel subunits. Direct alteration of ion channel subunits by kinases is a candidate mechanism for functional modulation of these channels. We find that activation of cyclic AMP-dependent protein kinase (PKA) with forskolin (50 microm) causes a dramatic decrease in surface localization of the SK2 channel subunit expressed in COS7 cells due to direct phosphorylation of the SK2 channel subunit. PKA phosphorylation studies using the intracellular domains of the SK2 channel subunit expressed as glutathione S-transferase fusion protein constructs showed that both the amino-terminal and carboxyl-terminal regions are PKA substrates in vitro. Mutational analysis identified a single PKA phosphorylation site within the amino-terminal of the SK2 subunit at serine 136. Mutagenesis and mass spectrometry studies identified four PKA phosphorylation sites: Ser465 (minor site) and three amino acid residues Ser568, Ser569, and Ser570 (major sites) within the carboxyl-terminal region. A mutated SK2 channel subunit, with the three contiguous serines mutated to alanines to block phosphorylation at these sites, shows no decrease in surface expression after PKA stimulation. Thus, our findings suggest that PKA phosphorylation of these three sites is necessary for PKA-mediated reorganization of SK2 surface expression.

Memory in astrocytes: a hypothesis

Background

Recent work has indicated an increasingly complex role for astrocytes in the central nervous system. Astrocytes are now known to exchange information with neurons at synaptic junctions and to alter the information processing capabilities of the neurons. As an extension of this trend a hypothesis was proposed that astrocytes function to store information. To explore this idea the ion channels in biological membranes were compared to models known as cellular automata. These comparisons were made to test the hypothesis that ion channels in the membranes of astrocytes form a dynamic information storage device.

Results

Two dimensional cellular automata were found to behave similarly to ion channels in a membrane when they function at the boundary between order and chaos. The length of time information is stored in this class of cellular automata is exponentially related to the number of units. Therefore the length of time biological ion channels store information was plotted versus the estimated number of ion channels in the tissue. This analysis indicates that there is an exponential relationship between memory and the number of ion channels. Extrapolation of this relationship to the estimated number of ion channels in the astrocytes of a human brain indicates that memory can be stored in this system for an entire life span. Interestingly, this information is not affixed to any physical structure, but is stored as an organization of the activity of the ion channels. Further analysis of two dimensional cellular automata also demonstrates that these systems have both associative and temporal memory capabilities.

Conclusion

It is concluded that astrocytes may serve as a dynamic information sink for neurons. The memory in the astrocytes is stored by organizing the activity of ion channels and is not associated with a physical location such as a synapse. In order for this form of memory to be of significant duration it is necessary that the ion channels in the astrocyte syncytium be electrically in contact with each other. This function may be served by astrocyte gap junctions and suggests that agents that selectively block these gap junctions should disrupt memory.

High-Frequency Stimulation Together with Adrenoceptor Activation Facilitates the Maintenance of Long-Term Potentiation at Visual Cortical Inhibitory Synapses

Long-term potentiation (LTP) at inhibitory synapses of rat visual cortex requires firing of presynaptic cells for maintenance, at least at a low frequency. We examined the roles of adrenoceptors in this LTP maintenance. Although high-frequency stimulation (HFS) failed to produce LTP in normal Ca2+ medium, it produced pathway-specific LTP with addition of noradrenaline to the medium soon after HFS. However, this LTP disappeared after washout of noradrenaline. HFS applied during noradrenaline application produced LTP persisting even after washout, indicating that HFS together with adrenoceptor activation makes the adrenergic facilitation enduring. After washout, LTP was produced further by HFS of the conditioned, but not the unconditioned, pathway by the first HFS. Pharmacological examination demonstrated that alpha2 and beta, but not alpha1, receptors facilitated LTP maintenance synergistically. Bath application, but not postsynaptic loading, of either the adenylyl cyclase activator forskolin or the protein kinase C (PKC) activator phorbol ester facilitated LTP maintenance. These results suggest that adrenergic facilitation of LTP maintenance is mediated by presynaptic adrenoceptors via a subfamily of adenylyl cyclases stimulated by Gsalpha, Gibetagamma, and PKC. Thus, it is likely that the activity of noradrenergic afferents takes part in the control of LTP duration at visual cortical inhibitory synapses.

Bidirectional modification of presynaptic neuronal excitability accompanying spike timing-dependent synaptic plasticity

Correlated pre- and postsynaptic activity that induces long-term potentiation is known to induce a persistent enhancement of the intrinsic excitability of the presynaptic neuron. Here we report that, associated with the induction of long-term depression in hippocampal cultures and in somatosensory cortical slices, there is also a persistent reduction in the excitability of the presynaptic neuron. This reduction requires postsynaptic Ca(2+) elevation and presynaptic PKA- and PKC-dependent modification of slow-inactivating K(+) channels. The bidirectional changes in neuronal excitability and synaptic efficacy exhibit identical requirements for the temporal order of pre- and postsynaptic activation but reflect two distinct aspects of activity-induced modification of neural circuits.

Adenylate Cyclase-Mediated Forms of Neuronal Plasticity in Hippocampal Area CA1 Are Reduced With Aging

Beta-adrenergic receptors and the cyclic AMP signaling pathway play an important role in neuronal plasticity and in learning and memory and are known to change with aging. We examined the effects of β-adrenergic stimulation paired with 5-Hz low frequency stimulation (LFS) of Schaffer collateral-commissural afferents on population spike amplitude in area CA1 of hippocampal slices from young (3 mo) and aged (22 mo) Fischer 344 rats. Application of the β-adrenergic agonist isoproterenol (1 μM) for 10 min followed immediately by 3 min LFS produced long-lasting potentiation in young hippocampi, but the magnitude of potentiation in aged rats was significantly attenuated and was not long-lasting. In slices prepared from young rats, long-term potentiation (LTP) induced by this protocol occludes subsequent attempts to produce conventional high frequency stimulation-induced LTP, and vice versa, suggesting that these two forms of potentiation share one or more molecular mechanisms. Age-related differences in response to LFS alone were not observed, but significant differences in response to β-adrenergic stimulation were apparent. Similarly, significant age-related differences in response to direct activation of adenylate cyclase with forskolin (10 μM) were observed. In both age groups, this enhancement produced by isoproterenol or forskolin is only transient, returning to baseline within 60 or 90 min, respectively. Taken together, these studies of adenylate cyclase-mediated forms of potentiation in area CA1 suggest that there is an age-related defect, either upstream or downstream of adenylate cyclase activation, in this important signaling system. Such changes may contribute to the compromised performance on memory tasks that is often observed with normal aging.

Beta1 adrenergic receptor-mediated enhancement of hippocampal CA3 network activity

Norepinephrine is an endogenous neurotransmitter distributed throughout the mammalian brain. In higher cortical structures such as the hippocampus, norepinephrine, via beta adrenergic receptor (AR) activation, has been shown to reinforce the cognitive processes of attention and memory. In this study, we investigated the effect of beta1AR activation on hippocampal cornu ammonis 3 (CA3) network activity. AR expression was first determined using immunocytochemistry with antibodies against beta1ARs, which were found to be exceptionally dense in hippocampal CA3 pyramidal neurons. CA3 network activity was then examined in vitro using field potential recordings in rat brain slices. The selective betaAR agonist isoproterenol caused an enhancement of hippocampal CA3 network activity, as measured by an increase in frequency of spontaneous burst discharges recorded in the CA3 region. In the presence of alphaAR blockade, concentration-response curves for isoproterenol, norepinephrine, and epinephrine suggested that a beta1AR was involved in this response, and the rank order of potency was isoproterenol > norepinephrine = epinephrine. Finally, equilibrium dissociation constants (pK(b)) of subtype-selective betaAR antagonists were functionally determined to characterize the AR subtype modulating hippocampal CA3 activity. The selective beta1AR antagonists atenolol and metoprolol blocked isoproterenol-induced enhancement, with apparent K(b) values of 85 +/- 36 and 3.9 +/- 1.7 nM, respectively. In contrast, the selective beta2AR antagonists ICI-118,551 and butoxamine inhibited isoproterenol-mediated enhancement with apparent low affinities (K(b) of 222 +/- 61 and 9268 +/- 512 nM, respectively). Together, this pharmacological profile of subtype-selective betaAR antagonists indicates that in this model, beta1AR activation is responsible for the enhanced hippocampal CA3 network activity initiated by isoproterenol.

Kv7/KCNQ/M and HCN/h, but not KCa2/SK channels, contribute to the somatic medium after-hyperpolarization and excitability control in CA1 hippocampal pyramidal cells

In hippocampal pyramidal cells, a single action potential (AP) or a burst of APs is followed by a medium afterhyperpolarization (mAHP, lasting approximately 0.1 s). The currents underlying the mAHP are considered to regulate excitability and cause early spike frequency adaptation, thus dampening the response to sustained excitatory input relative to responses to abrupt excitation. The mAHP was originally suggested to be primarily caused by M-channels (at depolarized potentials) and h-channels (at more negative potentials), but not SK channels. In recent reports, however, the mAHP was suggested to be generated mainly by SK channels or only by h-channels. We have now re-examined the mechanisms underlying the mAHP and early spike frequency adaptation in CA1 pyramidal cells by using sharp electrode and whole-cell recording in rat hippocampal slices. The specific M-channel blocker XE991 (10 microm) suppressed the mAHP following 1-5 APs evoked by current injection at -60 mV. XE991 also enhanced the excitability of the cell, i.e. increased the number of APs evoked by a constant depolarizing current pulse, reduced their rate of adaptation, enhanced the after depolarization and promoted bursting. Conversely, the M-channel opener retigabine reduced excitability. The h-channel blocker ZD7288 (4-ethylphenylamino-1,2-dimethyl-6-methylaminopyrimidinium chloride; 10 microm) fully suppressed the mAHP at -80 mV, but had little effect at -60 mV, whereas XE991 did not measurably affect the mAHP at -80 mV. Likewise, ZD7288 had little or no effect on excitability or adaptation during current pulses injected from -60 mV, but changed the initial discharge during depolarizing pulses injected from -80 mV. In contrast to previous reports, we found that blockade of Ca2+-activated K+ channels of the SK/KCa type by apamin (100-400 nm) failed to affect the mAHP or adaptation. A computational model of a CA1 pyramidal cell predicted that M- and h-channels will generate mAHPs in a voltage-dependent manner, as indicated by the experiments. We conclude that M- and h-channels generate the somatic mAHP in hippocampal pyramidal cells, with little or no net contribution from SK channels.