Independent roles of calcium and voltage-dependent potassium currents in controlling spike frequency adaptation in lateral amygdala pyramidal neurons

Short-term neuronal and synaptic plasticity act in synergy for deviance detection in spiking networks

Sensory areas of cortex respond more strongly to infrequent stimuli when these violate previously established regularities, a phenomenon known as deviance detection (DD). Previous modeling work has mainly attempted to explain DD on the basis of synaptic plasticity. However, a large fraction of cortical neurons also exhibit firing rate adaptation, an underexplored potential mechanism. Here, we investigate DD in a spiking neuronal network model with two types of short-term plasticity, fast synaptic short-term depression (STD) and slower threshold adaptation (TA). We probe the model with an oddball stimulation paradigm and assess DD by evaluating the network responses. We find that TA is sufficient to elicit DD. It achieves this by habituating neurons near the stimulation site that respond earliest to the frequently presented standard stimulus (local fatigue), which diminishes the response and promotes the recovery (global fatigue) of the wider network. Further, we find a synergy effect between STD and TA, where they interact with each other to achieve greater DD than the sum of their individual effects. We show that this synergy is caused by the local fatigue added by STD, which inhibits the global response to the frequently presented stimulus, allowing greater recovery of TA-mediated global fatigue and making the network more responsive to the deviant stimulus. Finally, we show that the magnitude of DD strongly depends on the timescale of stimulation. We conclude that highly predictable information can be encoded in strong local fatigue, which allows greater global recovery and subsequent heightened sensitivity for DD.

Elucidating the role of hypoxia/reoxygenation in hippocampus-dependent memory impairment: do SK channels play role?

Professionals and mountaineers often face the problem of reperfusion injury due to re-oxygenation, upon their return to sea-level after sojourn at high altitude. Small conductance calcium-activated potassium channels (SK channels) have a role in regulating hippocampal synaptic plasticity. However, the role of SK channels under hypoxia-reoxygenation (H/R) is unknown. The present study hypothesized that SK channels play a significant role in H/R induced cognitive dysfunction. Sprague-Dawley rats were exposed to simulated HH (25,000 ft) continuously for 7 days followed by reoxygenation periods 3, 6, 24, 48, 72 and 120 h. It was observed that H/R exposure caused impairment in spatial memory as indicated by increased latency (p < 0.001) and pathlength (p < 0.001). The SK1 channel expression increased upon HH exposure (102.89 ± 7.055), which abrogated upon reoxygenation. HH exposure results in an increase in SK2 (CA3, 297.67 ± 6.69) and SK3 (CA1, 246 ± 5.13) channels which continued to increase gradually upon reoxygenation. The number of pyknotic cells (24 ± 2.03) (p < 0.01) and the expression of caspase-3 increased with HH exposure, which continued in the reoxygenation group (177.795 ± 1.264). Similar pattern was observed in lipid peroxidation (p < 0.001), LDH activity (p < 0.001) and ROS production (p < 0.001). A positive correlation of memory, cell death and oxidative stress indicates that H/R exposure increases oxidative stress coupled with SK channel expression, which may play a role in H/R-induced cognitive decline and neurodegeneration.

L-type calcium channel contributions to intrinsic excitability and synaptic activity during basolateral amygdala postnatal development

The amygdala contributes toward emotional processes such as fear, anxiety, and social cognition. Furthermore, evidence suggests that increased excitability of basolateral amygdala (BLA) principal neurons underlie certain neuropsychiatric disorders. Gain-of-function mutations in neuronal L-type calcium channels (LTCCs) are linked to neurodevelopmental diseases, including autism spectrum disorders (ASDs). While LTCCs are expressed throughout the BLA, direct evidence for increased LTCC activity affecting BLA excitability and potentially contributing to disease pathophysiology is lacking. In this study, we utilized a pharmacological approach to examine the contributions of LTCCs to BLA principal cell excitability and synaptic activity at immature (postnatal day 7, P7) and juvenile (P21) developmental stages. Acute upregulation of LTCC activity in brain slices by application of the agonist (S)-Bay K 8644 resulted in increased intrinsic excitability properties including firing frequency response, plateau potential, and spike-frequency adaptation selectively in P7 neurons. Contrastingly, for P21 neurons, the main effect of (S)-Bay K 8644 was to enhance burst firing. (S)-Bay K 8644 increased spontaneous inhibitory synaptic currents at both P7 and P21 but did not affect evoked synaptic currents at either stage. (S)-Bay K 8644 did not alter P7 spontaneous excitatory synaptic currents, although it increased current amplitude in P21 neurons. Overall, the results provide support for the notion that alteration of LTCC activity at specific periods of early brain development may lead to functional alterations to neuronal network activity and subsequently contribute to underlying mechanisms of amygdala-related neurological disorders.NEW & NOTEWORTHY The role of L-type calcium channels (LTCCs) in regulating neuronal electrophysiological properties during development remains unclear. We show that in basolateral amygdala principal neurons, an increase of LTCC activity alters both neuronal excitability and synaptic activity. The results also provide evidence for the distinct contributions of LTCCs at different stages of neurodevelopment and shed insight into our understanding of LTCC dysfunction in amygdala-related neurological disorders.

Tune it in: mechanisms and computational significance of neuron-autonomous plasticity

The activity of a neural network is a result of synaptic signals that convey the communication between neurons and neuron-based intrinsic currents that determine the neuron’s input-output transfer function. Ample studies have demonstrated that cell-based excitability, and in particular intrinsic excitability, is modulated by learning and that these modifications play a key role in learning-related behavioral changes. The field of cell-based plasticity is largely growing, and it entails numerous experimental findings that demonstrate a large diversity of currents that are affected by learning. The diverse effect of learning on the neuron’s excitability emphasizes the need for a framework under which cell-based plasticity can be categorized to enable the assessment of the computational roles of the intrinsic modifications. We divide the domain of cell-based plasticity into three main categories, where the first category entails the currents that mediate the passive properties and single-spike generation, the second category entails the currents that mediate spike frequency adaptation, and the third category entails a novel learning-induced mechanism where all excitatory and inhibitory synapses double their strength. Curiously, this elementary division enables a natural categorization of the computational roles of these learning-induced plasticities. The computational roles are diverse and include modification of the neuronal mode of action, such as bursting, prolonged, and fast responsive; attention-like effect where the signal detection is improved; transfer of the network into an active state; biasing the competition for memory allocation; and transforming an environmental cue into a dominant cue and enabling a quicker formation of new memories.

The Ca2+-activated chloride channel anoctamin-2 mediates spike-frequency adaptation and regulates sensory transmission in thalamocortical neurons

Neuronal firing patterns, which are crucial for determining the nature of encoded information, have been widely studied; however, the molecular identity and cellular mechanisms of spike-frequency adaptation are still not fully understood. Here we show that spike-frequency adaptation in thalamocortical (TC) neurons is mediated by the Ca²⁺-activated Cl⁻ channel (CACC) anoctamin-2 (ANO2). Knockdown of ANO2 in TC neurons results in significantly reduced spike-frequency adaptation along with increased tonic spiking. Moreover, thalamus-specific knockdown of ANO2 increases visceral pain responses. These results indicate that ANO2 contributes to reductions in spike generation in highly activated TC neurons and thereby restricts persistent information transmission.

Spike-frequency adaptation in thalamocortical (TC) neurons is important for sensory transmission though the underlying mechanisms are not fully understood. Here, the authors identify a role for the calcium-activated chloride channel, ANO2, in mediating TC spiking adaptations and visceral pain response.

Delayed effects of corticosterone on slow after-hyperpolarization potentials in mouse hippocampal versus prefrontal cortical pyramidal neurons

The rodent stress hormone corticosterone changes neuronal activity in a slow and persistent manner through transcriptional regulation. In the rat dorsal hippocampus, corticosterone enhances the amplitude of calcium-dependent potassium currents that cause a lingering slow after-hyperpolarization (sAHP) at the end of depolarizing events. In this study we compared the putative region-dependency of the delayed effects of corticosterone (approximately 5 hrs after treatment) on sAHP as well as other active and passive properties of layer 2/3 pyramidal neurons from three prefrontal areas, i.e. the lateral orbitofrontal, prelimbic and infralimbic cortex, with the hippocampus of adult mice. In agreement with previous studies, corticosterone increased sAHP amplitude in the dorsal hippocampus with depolarizing steps of increasing amplitude. However, in the lateral orbitofrontal, prelimbic and infralimbic cortices we did not observe any modifications of sAHP amplitude after corticosterone treatment. Properties of single action potentials or % ratio of the last spike interval with respect to the first spike interval, an indicator of accommodation in an action potential train, were not significantly affected by corticosterone in all brain regions examined. Lastly, corticosterone treatment did not induce any lasting changes in passive membrane properties of hippocampal or cortical neurons. Overall, the data indicate that corticosterone slowly and very persistently increases the sAHP amplitude in hippocampal pyramidal neurons, while this is not the case in the cortical regions examined. This implies that changes in excitability across brain regions reached by corticosterone may vary over a prolonged period of time after stress.

Learning enhances intrinsic excitability in a subset of lateral amygdala neurons

Learning-induced modulation of neuronal intrinsic excitability is a metaplasticity mechanism that can impact the acquisition of new memories. Although the amygdala is important for emotional learning and other behaviors, including fear and anxiety, whether learning alters intrinsic excitability within the amygdala has received very little attention. Fear conditioning was combined with intracellular recordings to investigate the effects of learning on the intrinsic excitability of lateral amygdala (LA) neurons. To assess time-dependent changes, brain slices were prepared either immediately or 24-h post-conditioning. Fear conditioning significantly enhanced excitability of LA neurons, as evidenced by both decreased afterhyperpolarization (AHP) and increased neuronal firing. These changes were time-dependent such that reduced AHPs were evident at both time points whereas increased neuronal firing was only observed at the later (24-h) time point. Moreover, these changes occurred within a subset (32%) of LA neurons. Previous work also demonstrated that learning-related changes in synaptic plasticity are also evident in less than one-third of amygdala neurons, suggesting that the neurons undergoing intrinsic plasticity may be critical for fear memory. These data may be clinically relevant as enhanced LA excitability following fear learning could influence future amygdala-dependent behaviors.

Diversity of layer 5 projection neurons in the mouse motor cortex

In the primary motor cortex (M1), layer 5 projection neurons signal directly to distant motor structures to drive movement. Despite their pivotal position and acknowledged diversity these neurons are traditionally separated into broad commissural and corticofugal types, and until now no attempt has been made at resolving the basis for their diversity. We therefore probed the electrophysiological and morphological properties of retrogradely labeled M1 corticospinal (CSp), corticothalamic (CTh), and commissural projecting corticostriatal (CStr) and corticocortical (CC) neurons. An unsupervised cluster analysis established at least four phenotypes with additional differences between lumbar and cervical projecting CSp neurons. Distinguishing parameters included the action potential (AP) waveform, firing behavior, the hyperpolarisation-activated sag potential, sublayer position, and soma and dendrite size. CTh neurons differed from CSp neurons in showing spike frequency acceleration and a greater sag potential. CStr neurons had the lowest AP amplitude and maximum rise rate of all neurons. Temperature influenced spike train behavior in corticofugal neurons. At 26°C CTh neurons fired bursts of APs more often than CSp neurons, but at 36°C both groups fired regular APs. Our findings provide reliable phenotypic fingerprints to identify distinct M1 projection neuron classes as a tool to understand their unique contributions to motor function.

Reduction of an afterhyperpolarization current increases excitability in striatal cholinergic interneurons in rat parkinsonism

Striatal cholinergic interneurons show tonic spiking activity in the intact and sliced brain, which stems from intrinsic mechanisms. Because of it, they are also known as "tonically active neurons" (TANs). Another hallmark of TAN electrophysiology is a pause response to appetitive and aversive events and to environmental cues that have predicted these events during learning. Notably, the pause response is lost after the degeneration of dopaminergic neurons in animal models of Parkinson's disease. Moreover, Parkinson's disease patients are in a hypercholinergic state and find some clinical benefit in anticholinergic drugs. Current theories propose that excitatory thalamic inputs conveying information about salient sensory stimuli trigger an intrinsic hyperpolarizing response in the striatal cholinergic interneurons. Moreover, it has been postulated that the loss of the pause response in Parkinson's disease is related to a diminution of I(sAHP), a slow outward current that mediates an afterhyperpolarization following a train of action potentials. Here we report that I(sAHP) induces a marked spike-frequency adaptation in adult rat striatal cholinergic interneurons, inducing an abrupt end of firing during sustained excitation. Chronic loss of dopaminergic neurons markedly reduces I(sAHP) and spike-frequency adaptation in cholinergic interneurons, allowing them to fire continuously and at higher rates during sustained excitation. These findings provide a plausible explanation for the hypercholinergic state in Parkinson's disease. Moreover, a reduction of I(sAHP) may alter synchronization of cholinergic interneurons with afferent inputs, thus contributing to the loss of the pause response in Parkinson's disease.

Deletion of the mouse homolog of KCNAB2, a gene linked to monosomy 1p36, results in associative memory impairments and amygdala hyperexcitability

Ablation of the distal end of the short arm of chromosome 1 [1p36 deletion syndrome (1p36DS)] is one of the most commonly occurring terminal deletion syndromes in humans, occurring in ∼1 in 5000 newborns. Subjects with 1p36DS manifest a wide range of clinical features including growth delay, congenital heart defects, and craniofacial dysmorphism. In addition, individuals with 1p36DS often exhibit some form of neurological abnormality and are typically cognitively impaired. Although there is significant variability with regard to the extent of the deletion, several genes have been mapped to region 1p36 that are known to regulate neuronal function. One such gene--KCNAB2--encodes the potassium channel auxiliary subunit Kvβ2, which has been previously shown to modulate voltage-gated potassium currents in heterologous expression systems. Here, we present experiments characterizing mice in which the ortholog of KCNAB2 was deleted. We find that deletion of Kcnab2 in mice leads to deficits in associative learning and memory. In addition, using whole-cell current-clamp, we find that deletion of Kcnab2 leads to a reduction in the slow afterhyperpolarization following a burst of action potentials and a concomitant increase in neuronal excitability in projection neurons in the lateral nucleus of the amygdala. Our results suggest that loss of Kvβ2 likely contributes to the cognitive and neurological impairments observed in 1p36DS patients.

Chronic stress causes amygdala hyperexcitability in rodents

BACKGROUND

Chronic stress is a major health concern, often leading to depression, anxiety, or when severe enough, posttraumatic stress disorder. While many studies demonstrate that the amygdala is hyperresponsive in patients with these disorders, the cellular neurophysiological effects of chronic stress on the systems that underlie psychiatric disorders, such as the amygdala, are relatively unknown.

METHODS

In this study, we examined the effects of chronic stress on the activity and excitability of amygdala neurons in vivo in rats. We used in vivo intracellular recordings from single neurons of the lateral amygdala (LAT) to measure neuronal properties and determine the cellular mechanism for the effects of chronic stress on LAT neurons.

RESULTS

We found a mechanism for the effects of chronic stress on amygdala activity, specifically that chronic stress increased excitability of LAT pyramidal neurons recorded in vivo. This hyperexcitability was caused by a reduction of a regulatory influence during action potential firing, facilitating LAT neuronal activity. The effects of stress on excitability were occluded by agents that block calcium-activated potassium channels and reversed by pharmacological enhancement of calcium-activated potassium channels.

CONCLUSIONS

These data demonstrate a specific channelopathy that occurs in the amygdala after chronic stress. This enhanced excitability of amygdala neurons after chronic stress may explain the observed hyperresponsiveness of the amygdala in patients with posttraumatic stress disorder and may facilitate the emergence of depression or anxiety in other patients.

Emotional enhancement of memory: how norepinephrine enables synaptic plasticity

Changes in synaptic strength are believed to underlie learning and memory. We explore the idea that norepinephrine is an essential modulator of memory through its ability to regulate synaptic mechanisms. Emotional arousal leads to activation of the locus coeruleus with the subsequent release of norepineprine in the brain, resulting in the enhancement of memory. Norepinephrine activates both pre- and post-synaptic adrenergic receptors at central synapses with different functional outcomes, depending on the expression pattern of these receptors in specific neural circuitries underlying distinct behavioral processes. We review the evidence for noradrenergic modulation of synaptic plasticity with consideration of how this may contribute to the mechanisms of learning and memory.

Pathophysiology of locus ceruleus neurons in a mouse model of Rett syndrome

Rett syndrome (RTT) is a neurodevelopmental disorder caused by loss-of-function mutations in the Methyl-CpG-binding protein-2 (MECP2) gene and is characterized by derangements in cognition, behavior, motor control, respiration and autonomic homeostasis, as well as seizures. Deficits in norepinephrine (NE) are thought to contribute to RTT pathogenesis, but little is known about how MeCP2 regulates function of noradrenergic neurons. We therefore characterized morphological, electrical, and neurochemical properties of neurons in the locus ceruleus (LC), the major source of noradrenergic innervation to the central neuraxis, in Mecp2 mutant mice. We found that MeCP2 null LC neurons are electrically hyperexcitable, smaller in size, and express less of the NE-synthesizing enzyme tyrosine hydroxylase (TH) compared with wild-type neurons. Increased excitability of mutant neurons is associated with reductions in passive membrane conductance and the amplitude of the slow afterhyperpolarization. Studies in Mecp2 heterozygotes, which are mosaic for the null allele, demonstrated that electrical hyperexcitability and reduced neuronal size are cell-autonomous consequences of MeCP2 loss, whereas reduced TH expression appears to reflect both cell-autonomous and non-autonomous influences. Finally, we found reduced levels of TH and norepinephrine in cingulate cortex, a forebrain target of the LC. Thus, genetic loss of MeCP2 results in a somewhat paradoxical LC neuron phenotype, characterized by both electrical hyperexcitability and reduced indices of noradrenergic function. Given the importance of the LC in modulating activity in brainstem and forebrain networks, we hypothesize that dysregulation of LC function in the absence of MeCP2 plays a key role in the pathophysiology of RTT.

Impaired long-term potentiation and enhanced neuronal excitability in the amygdala of Ca(V)1.3 knockout mice

Previously, we demonstrated that mice in which the gene for the L-type voltage-gated calcium channel Ca(V)1.3 is deleted (Ca(V)1.3 knockout mice) exhibit an impaired ability to consolidate contextually-conditioned fear. Given that this form of Pavlovian fear conditioning is critically dependent on the basolateral complex of the amygdala (BLA), we were interested in the mechanisms by which Ca(V)1.3 contributes to BLA neurophysiology. In the present study, we used in vitro amygdala slices prepared from Ca(V)1.3 knockout mice and wild-type littermates to explore the role of Ca(V)1.3 in long-term potentiation (LTP) and intrinsic neuronal excitability in the BLA. We found that LTP in the lateral nucleus (LA) of the BLA, induced by high-frequency stimulation of the external capsule, was significantly reduced in Ca(V)1.3 knockout mice. Additionally, we found that BLA principal neurons from Ca(V)1.3 knockout mice were hyperexcitable, exhibiting significant increases in firing rates and decreased interspike intervals in response to prolonged somatic depolarization. This aberrant increase in neuronal excitability appears to be at least in part due to a concomitant reduction in the slow component of the post-burst afterhyperpolarization. Together, these results demonstrate altered neuronal function in the BLA of Ca(V)1.3 knockout mice which may account for the impaired ability of these mice to consolidate contextually-conditioned fear.

A biologically realistic network model of acquisition and extinction of conditioned fear associations in lateral amygdala neurons

The basolateral amygdala plays an important role in the acquisition and expression of both fear conditioning and fear extinction. To understand how a single structure could encode these "opposite" memories, we developed a biophysical network model of the lateral amygdala (LA) neurons during auditory fear conditioning and extinction. Membrane channel properties were selected to match waveforms and firing properties of pyramidal cells and interneurons in LA, from published in vitro studies. Hebbian plasticity was implemented in excitatory AMPA and inhibitory GABA(A) receptor-mediated synapses to model learning. The occurrence of synaptic potentiation versus depression was determined by intracellular calcium levels, according to the calcium control hypothesis. The model was able to replicate conditioning- and extinction-induced changes in tone responses of LA neurons in behaving rats. Our main finding is that LA activity during both acquisition and extinction can be controlled by a balance between pyramidal cell and interneuron activations. Extinction training depressed conditioned synapses and also potentiated local interneurons, thereby inhibiting the responses of pyramidal cells to auditory input. Both long-term depression and potentiation of inhibition were required to initiate and maintain extinction. The model provides insights into the sites of plasticity in conditioning and extinction, the mechanism of spontaneous recovery, and the role of amygdala NMDA receptors in extinction learning.

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.

Competition between calcium-activated K+ channels determines cholinergic action on firing properties of basolateral amygdala projection neurons

Acetylcholine (ACh) is an important modulator of learning, memory, and synaptic plasticity in the basolateral amygdala (BLA) and other brain regions. Activation of muscarinic acetylcholine receptors (mAChRs) suppresses a variety of potassium currents, including sI(AHP), the calcium-activated potassium conductance primarily responsible for the slow afterhyperpolarization (AHP) that follows a train of action potentials. Muscarinic stimulation also produces inositol 1,4,5-trisphosphate (IP(3)), releasing calcium from intracellular stores. Here, we show using whole-cell patch-clamp recordings and high-speed fluorescence imaging that focal application of mAChR agonists evokes large rises in cytosolic calcium in the soma and proximal dendrites in rat BLA projection neurons that are often associated with activation of an outward current that hyperpolarizes the cell. This hyperpolarization results from activation of small conductance calcium-activated potassium (SK) channels, secondary to the release of calcium from intracellular stores. Unlike bath application of cholinergic agonists, which always suppressed the AHP, focal application of ACh often evoked a paradoxical enhancement of the AHP and spike-frequency adaptation. This enhancement was correlated with amplification of the action potential-evoked calcium response and resulted from the activation of SK channels. When SK channels were blocked, cholinergic stimulation always reduced the AHP and spike-frequency adaptation. Conversely, suppression of the sI(AHP) by the beta-adrenoreceptor agonist, isoprenaline, potentiated the cholinergic enhancement of the AHP. These results suggest that competition between cholinergic suppression of the sI(AHP) and cholinergic activation of the SK channels shapes the AHP and spike-frequency adaptation.

Functions of SK channels in central neurons

1. SK channels are small-conductance calcium-activated potassium channels that are widely expressed in neurons. The traditional view of the functional role of SK channels is in mediating one component of the after-hyperpolarization that follows action potentials. Calcium influx via voltage-gated calcium channels active during action potentials opens SK channels and the resultant hyperpolarization lowers the firing frequency of action potentials in many neurons. 2. Recent advances have shown that, in addition to controlling action potential firing frequency, SK channels are also important in regulating dendritic excitability, synaptic transmission and synaptic plasticity. 3. In accordance with their role in modulating synaptic plasticity, SK channels are also important in regulating several learning and memory tasks and may also play a role in a number of neurological disorders. 4. The present review discusses recent findings on the role of SK channels in central neurons.

Norepinephrine enables the induction of associative long-term potentiation at thalamo-amygdala synapses

Emotional arousal, linked to a surge of norepinephrine (NE) in the amygdala, leads to creation of stronger and longer-lasting memories. However, little is known about the synaptic mechanisms of such modulatory NE influences. Long-term potentiation (LTP) in auditory inputs to the lateral nucleus of the amygdala was recently linked to the acquisition of fear memory. Therefore we explored whether LTP induction at thalamo-amygdala projections, conveying the acoustic conditioned stimulus information to the amygdala during fear conditioning, is under adrenergic control. Using whole-cell recordings from amygdala slices, we show that NE suppresses GABAergic inhibition of projection neurons in the lateral amygdala and enables the induction of LTP at thalamo-amygdala synapses under conditions of intact GABA(A) receptor-mediated inhibition. Our data indicate that the NE effects on the efficacy of inhibition could result from a decrease in excitability of local circuit interneurons, without direct effects of NE on release machinery of the GABA-containing vesicles or the size of single-quanta postsynaptic GABA(A) receptor-mediated responses. Thus, adrenergic modulation of local interneurons may contribute to the formation of fear memory by gating LTP in the conditioned stimulus pathways.

Glucocorticoids enhance the excitability of principal basolateral amygdala neurons

A large body of pharmaco-behavioral data implicates the basolateral nucleus of the amygdala (BLA) in the facilitation of memory consolidation by emotions. Overall, this evidence suggests that stress hormones released during emotional arousal increase the activity of BLA neurons. In turn, this increased BLA activity would facilitate synaptic plasticity elsewhere in the brain, to which the BLA projects. However, the direct effects of glucocorticoids on BLA neurons are incompletely understood. In the present study, we examined the direct effects of corticosterone (CORT) on principal neurons of the rat BLA in vitro using whole-cell patch-clamp recordings. We found that application of a stress level of CORT for 20 min caused significant changes in the passive properties and responsiveness of BLA cells measured 1-2 h later. Indeed, CORT application produced a depolarization of the resting potential, an increase in input resistance, and a dramatic decrease in spike-frequency adaptation. In addition, GABA(A) IPSPs evoked by stimulation of the external capsule were significantly reduced by CORT application. This effect of CORT was not attributable to a reduction in the amount of GABA released because GABA(B) IPSPs were unchanged and the resistance drop associated with GABA(A) IPSPs was not altered. Rather, we found that this effect of CORT resulted from a positive shift of the GABA(A) reversal potential. Overall, these results suggest that, in agreement with previous behavioral findings, glucocorticoids enhance the excitability of principal BLA cells by increasing their intrinsic excitability and decreasing the impact of GABA(A) IPSPs.