Muscarinic modulation of conductances underlying the afterhyperpolarization in neurons of the rat basolateral amygdala

Functional neuroanatomy of basal forebrain projections to the basolateral amygdala: Transmitters, receptors, and neuronal subpopulations

The projections of the basal forebrain (BF) to the hippocampus and neocortex have been extensively studied and shown to be important for higher cognitive functions, including attention, learning, and memory. Much less is known about the BF projections to the basolateral nuclear complex of the amygdala (BNC), although the cholinergic innervation of this region by the BF is actually far more robust than that of cortical areas. This review will focus on light and electron microscopic tract-tracing and immunohistochemical (IHC) studies, many of which were published in the last decade, that have analyzed the relationship of BF inputs and their receptors to specific neuronal subtypes in the BNC in order to better understand the anatomical substrates of BF-BNC circuitry. The results indicate that BF inputs to the BNC mainly target the basolateral nucleus of the BNC (BL) and arise from cholinergic, GABAergic, and perhaps glutamatergic BF neurons. Cholinergic inputs mainly target dendrites and spines of pyramidal neurons (PNs) that express muscarinic receptors (MRs). MRs are also expressed by cholinergic axons, as well as cortical and thalamic axons that synapse with PN dendrites and spines. BF GABAergic axons to the BL also express MRs and mainly target BL interneurons that contain parvalbumin. It is suggested that BF-BL circuitry could be very important for generating rhythmic oscillations known to be critical for emotional learning. BF cholinergic inputs to the BNC might also contribute to memory formation by activating M1 receptors located on PN dendritic shafts and spines that also express NMDA receptors.

Neuronal localization of m1 muscarinic receptor immunoreactivity in the monkey basolateral amygdala

The basolateral nuclear complex (BNC) of the amygdala plays an important role in the generation of emotional/motivational behavior and the consolidation of emotional memories. Activation of M1 cholinergic receptors (M1Rs) in the BNC is critical for memory consolidation. Previous receptor binding studies in the monkey amygdala demonstrated that the BNC has a high density of M1Rs, but did not have sufficient resolution to identify which neurons in the BNC expressed them. This was accomplished in the present immunohistochemical investigation using an antibody for the m1 receptor (m1R). Analysis of m1Rs in the monkey BNC using immunoperoxidase techniques revealed that their expression was very dense in the BNC, and suggested that virtually all of the pyramidal projection neurons (PNs) in all of the BNC nuclei were m1R-immunoreactive (m1R+). This was confirmed with dual-labeling immunofluorescence using staining for calcium/calmodulin-dependent protein kinase II (CaMK) as a marker for BNC PNs. However, additional dual-labeling studies indicated that one-third of inhibitory interneurons (INs) expressing glutamic acid decarboxylase (GAD) were also m1R+. Moreover, the finding that 60% of parvalbumin (PV) immunoreactive neurons were m1R+ indicated that this IN subpopulation was the main GAD+ subpopulation exhibiting m1R expression. The cholinergic innervation of the amygdala is greatly reduced in Alzheimer's disease and there is currently considerable interest in developing selective M1R positive allosteric modulators (PAMs) to treat the symptoms. The results of the present study indicate that M1Rs in both PNs and INs in the primate BNC would be targeted by M1R PAMs.

Maturation of amygdala inputs regulate shifts in social and fear behaviors: A substrate for developmental effects of stress

Stress can negatively impact brain function and behaviors across the lifespan. However, stressors during adolescence have particularly harmful effects on brain maturation, and on fear and social behaviors that extend beyond adolescence. Throughout development, social behaviors are refined and the ability to suppress fear increases, both of which are dependent on amygdala activity. We review rodent literature focusing on developmental changes in social and fear behaviors, cortico-amygdala circuits underlying these changes, and how this circuitry is altered by stress. We first describe changes in fear and social behaviors from adolescence to adulthood and parallel developmental changes in cortico-amygdala circuitry. We propose a framework in which maturation of cortical inputs to the amygdala promote changes in social drive and fear regulation, and the particularly damaging effects of stress during adolescence may occur through lasting changes in this circuit. This framework may explain why anxiety and social pathologies commonly co-occur, adolescents are especially vulnerable to stressors impacting social and fear behaviors, and predisposed towards psychiatric disorders related to abnormal cortico-amygdala circuits.

Chronic Ethanol Exposure Potentiates Cholinergic Neurotransmission in the Basolateral Amygdala

Chronic intermittent ethanol (CIE) exposure dysregulates glutamatergic and GABAergic neurotransmission, facilitating basolateral amygdala (BLA) pyramidal neuron hyperexcitability and the expression of anxiety during withdrawal. It is unknown whether ethanol-induced alterations in nucleus basalis magnocellularis (NBM) cholinergic projections to the BLA mediate anxiety-related behaviors through direct modulation of GABA and glutamate afferents. Following 10 days of CIE exposure and 24 h of withdrawal, we recorded GABAergic and glutamatergic synaptic responses in BLA pyramidal neurons with electrophysiology, assessed total protein expression of cholinergic markers, and quantified acetylcholine and choline concentrations using a colorimetric assay. We measured α₇ nicotinic acetylcholine receptor (nAChR) dependent modulation of presynaptic function at distinct inputs in AIR- and CIE-exposed BLA coronal slices as a functional read-out of cholinergic neurotransmission. CIE/withdrawal upregulates the endogenous activity of α₇ nAChRs, facilitating release at both GABAergic' local' interneuron and glutamatergic synaptic responses to stria terminalis (ST) stimulation, with no effect at GABAergic lateral paracapsular cells (LPCs). CIE caused a three-fold increase in BLA acetylcholine concentration, with no changes in α₇ nAChR or cholinergic marker expression. These data illustrate that α₇ nAChR-dependent changes in presynaptic function serve as a proxy for CIE-dependent alterations in synaptic acetylcholine levels. Thus, cholinergic projections appear to mediate CIE-induced alterations at GABA/glutamate inputs.

Physiological Roles and Therapeutic Potential of Ca2+ Activated Potassium Channels in the Nervous System

Within the potassium ion channel family, calcium activated potassium (K_Ca) channels are unique in their ability to couple intracellular Ca²⁺ signals to membrane potential variations. K_Ca channels are diversely distributed throughout the central nervous system and play fundamental roles ranging from regulating neuronal excitability to controlling neurotransmitter release. The physiological versatility of K_Ca channels is enhanced by alternative splicing and co-assembly with auxiliary subunits, leading to fundamental differences in distribution, subunit composition and pharmacological profiles. Thus, understanding specific K_Ca channels’ mechanisms in neuronal function is challenging. Based on their single channel conductance, K_Ca channels are divided into three subtypes: small (SK, 4–14 pS), intermediate (IK, 32–39 pS) and big potassium (BK, 200–300 pS) channels. This review describes the biophysical characteristics of these K_Ca channels, as well as their physiological roles and pathological implications. In addition, we also discuss the current pharmacological strategies and challenges to target K_Ca channels for the treatment of various neurological and psychiatric disorders.

Muscarinic receptor subtype distribution in the central nervous system and relevance to aging and Alzheimer's disease

Muscarinic acetylcholine receptors (mAChRs) are G proteincoupled receptors (GPCRs) that mediate the metabotropic actions of acetylcholine (ACh). There are five subtypes of mAChR, M₁ - M₅, which are expressed throughout the central nervous system (CNS) on numerous cell types and represent promising treatment targets for a number of different diseases, disorders, and conditions of the CNS. Although the present review will focus on Alzheimer's disease (AD) and amnestic mild cognitive impairment (aMCI), a number of conditions such as Parkinson's disease (PD), schizophrenia, and others represent significant unmet medical needs for which selective muscarinic agents could offer therapeutic benefits. Numerous advances have been made regarding mAChR localization through the use of subtype-selective antibodies and radioligand binding studies and these efforts have helped propel a number of mAChR therapeutics into clinical trials. However, much of what we know about mAChR localization in the healthy and diseased brain has come from studies employing radioligand binding with relatively modest selectivity. The development of subtype-selective small molecule radioligands suitable for in vitro and in vivo use, as well as robust, commercially-available antibodies remains a critical need for the field. Additionally, novel genetic tools should be developed and leveraged to help move the field increasingly towards a systems-level understanding of mAChR subtype action. Finally, functional, proteomic, and genetic data from ongoing human studies hold great promise for optimizing the design and interpretation of studies examining receptor levels by enabling patient stratification. This article is part of the Special Issue entitled 'Neuropharmacology on Muscarinic Receptors'.

Input-specific contributions to valence processing in the amygdala

Reward and punishment are often thought of as opposing processes: rewards and the environmental cues that predict them elicit approach and consummatory behaviors, while punishments drive aversion and avoidance behaviors. This framework suggests that there may be segregated brain circuits for these valenced behaviors. The basolateral amygdala (BLA) is one brain region that contributes to both types of motivated behavior. Individual neurons in the BLA can favor positive over negative valence, or vice versa, but these neurons are intermingled, showing no anatomical segregation. The amygdala receives inputs from many brain areas and current theories posit that encoding of positive versus negative valence by BLA neurons is determined by the wiring of each neuron. Specifically, many projections from other brain areas that respond to positive and negative valence stimuli and predictive cues project strongly to the BLA and likely contribute to valence processing within the BLA. Here we review three of these areas, the basal forebrain, the dorsal raphe nucleus and the ventral tegmental area, and discuss how these may promote encoding of positive and negative valence within the BLA.

Localization of the M2 muscarinic cholinergic receptor in dendrites, cholinergic terminals, and noncholinergic terminals in the rat basolateral amygdala: An ultrastructural analysis

Activation of M2 muscarinic receptors (M2Rs) in the rat anterior basolateral nucleus (BLa) is critical for the consolidation of memories of emotionally arousing events. The present investigation used immunocytochemistry at the electron microscopic level to determine which structures in the BLa express M2Rs. In addition, dual localization of M2R and the vesicular acetylcholine transporter protein (VAChT), a marker for cholinergic axons, was performed to determine whether M2R is an autoreceptor in cholinergic axons innervating the BLa. M2R immunoreactivity (M2R-ir) was absent from the perikarya of pyramidal neurons, with the exception of the Golgi complex, but was dense in the proximal dendrites and axon initial segments emanating from these neurons. Most perikarya of nonpyramidal neurons were also M2R-negative. About 95% of dendritic shafts and 60% of dendritic spines were M2 immunoreactive (M2R(+) ). Some M2R(+) dendrites had spines, suggesting that they belonged to pyramidal cells, whereas others had morphological features typical of nonpyramidal neurons. M2R-ir was also seen in axon terminals, most of which formed asymmetrical synapses. The main targets of M2R(+) terminals forming asymmetrical (putative excitatory) synapses were dendritic spines, most of which were M2R(+) . The main targets of M2R(+) terminals forming symmetrical (putative inhibitory or neuromodulatory) synapses were unlabeled perikarya and M2R(+) dendritic shafts. M2R-ir was also seen in VAChT(+) cholinergic terminals, indicating a possible autoreceptor role. These findings suggest that M2R-mediated mechanisms in the BLa are very complex, involving postsynaptic effects in dendrites as well as regulating release of glutamate, γ-aminobutyric acid, and acetylcholine from presynaptic axon terminals. J. Comp. Neurol. 524:2400-2417, 2016. © 2016 Wiley Periodicals, Inc.

Impact of basal forebrain cholinergic inputs on basolateral amygdala neurons

In addition to innervating the cerebral cortex, basal forebrain cholinergic (BFc) neurons send a dense projection to the basolateral nucleus of the amygdala (BLA). In this study, we investigated the effect of near physiological acetylcholine release on BLA neurons using optogenetic tools and in vitro patch-clamp recordings. Adult transgenic mice expressing cre-recombinase under the choline acetyltransferase promoter were used to selectively transduce BFc neurons with channelrhodopsin-2 and a reporter through the injection of an adeno-associated virus. Light-induced stimulation of BFc axons produced different effects depending on the BLA cell type. In late-firing interneurons, BFc inputs elicited fast nicotinic EPSPs. In contrast, no response could be detected in fast-spiking interneurons. In principal BLA neurons, two different effects were elicited depending on their activity level. When principal BLA neurons were quiescent or made to fire at low rates by depolarizing current injection, light-induced activation of BFc axons elicited muscarinic IPSPs. In contrast, with stronger depolarizing currents, eliciting firing above ∼ 6-8 Hz, these muscarinic IPSPs lost their efficacy because stimulation of BFc inputs prolonged current-evoked afterdepolarizations. All the effects observed in principal neurons were dependent on muscarinic receptors type 1, engaging different intracellular mechanisms in a state-dependent manner. Overall, our results suggest that acetylcholine enhances the signal-to-noise ratio in principal BLA neurons. Moreover, the cholinergic engagement of afterdepolarizations may contribute to the formation of stimulus associations during fear-conditioning tasks where the timing of conditioned and unconditioned stimuli is not optimal for the induction of synaptic plasticity.

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.

M1-muscarinic receptors promote fear memory consolidation via phospholipase C and the M-current

Neuromodulators released during and after a fearful experience promote the consolidation of long-term memory for that experience. Because overconsolidation may contribute to the recurrent and intrusive memories of post-traumatic stress disorder, neuromodulatory receptors provide a potential pharmacological target for prevention. Stimulation of muscarinic receptors promotes memory consolidation in several conditioning paradigms, an effect primarily associated with the M1 receptor (M1R). However, neither inhibiting nor genetically disrupting M1R impairs the consolidation of cued fear memory. Using the M1R agonist cevimeline and antagonist telenzepine, as well as M1R knock-out mice, we show here that M1R, along with β2-adrenergic (β2AR) and D5-dopaminergic (D5R) receptors, regulates the consolidation of cued fear memory by redundantly activating phospholipase C (PLC) in the basolateral amygdala (BLA). We also demonstrate that fear memory consolidation in the BLA is mediated in part by neuromodulatory inhibition of the M-current, which is conducted by KCNQ channels and is known to be inhibited by muscarinic receptors. Manipulating the M-current by administering the KCNQ channel blocker XE991 or the KCNQ channel opener retigabine reverses the effects on consolidation caused by manipulating β2AR, D5R, M1R, and PLC. Finally, we show that cAMP and protein kinase A (cAMP/PKA) signaling relevant to this stage of consolidation is upstream of these neuromodulators and PLC, suggesting an important presynaptic role for cAMP/PKA in consolidation. These results support the idea that neuromodulatory regulation of ion channel activity and neuronal excitability is a critical mechanism for promoting consolidation well after acquisition has occurred.

Muscarinic cholinergic receptor M1 in the rat basolateral amygdala: ultrastructural localization and synaptic relationships to cholinergic axons

Muscarinic neurotransmission in the anterior basolateral amygdalar nucleus (BLa) mediated by the M1 receptor (M1R) is critical for memory consolidation. Although knowledge of the subcellular localization of M1R in the BLa would contribute to an understanding of cholinergic mechanisms involved in mnemonic function, there have been no ultrastructural studies of this receptor in the BLa. In the present investigation, immunocytochemistry at the electron microscopic level was used to determine which structures in the BLa express M1R. The innervation of these structures by cholinergic axons expressing the vesicular acetylcholine transporter (VAChT) was also studied. All perikarya of pyramidal neurons were labeled, and about 90% of dendritic shafts and 60% of dendritic spines were M1R+. Some dendrites had spines suggesting that they belonged to pyramidal cells, whereas others had morphological features typical of interneurons. M1R immunoreactivity (M1R-ir) was also seen in axon terminals, most of which formed asymmetrical synapses. The main targets of M1R+ terminals forming asymmetrical synapses were dendritic spines, most of which were M1R+. The main targets of M1R+ terminals forming symmetrical synapses were M1R+ perikarya and dendritic shafts. About three-quarters of VAChT+ cholinergic terminals formed synapses; the main postsynaptic targets were M1R+ dendritic shafts and spines. In some cases M1R-ir was seen near the postsynaptic membrane of these processes, but in other cases it was found outside of the active zone of VAChT+ synapses. These findings suggest that M1R mechanisms in the BLa are complex, involving postsynaptic effects as well as regulating release of neurotransmitters from presynaptic terminals.

Pharmacological enhancement of calcium-activated potassium channel function reduces the effects of repeated stress on fear memory

Repeated stress impacts emotion, and can induce mood and anxiety disorders. These disorders are characterized by imbalance of emotional responses. The amygdala is fundamental in expression of emotion, and is hyperactive in many patients with mood or anxiety disorders. Stress also leads to hyperactivity of the amygdala in humans. In rodent studies, repeated stress causes hyperactivity of the amygdala, and increases fear conditioning behavior that is mediated by the basolateral amygdala (BLA). Calcium-activated potassium (K(Ca)) channels regulate BLA neuronal activity, and evidence suggests reduced small conductance K(Ca) (SK) channel function in male rats exposed to repeated stress. Pharmacological enhancement of SK channels reverses the BLA neuronal hyperexcitability caused by repeated stress. However, it is not known if pharmacological targeting of SK channels can repair the effects of repeated stress on amygdala-dependent behaviors. The purpose of this study was to test whether enhancement of SK channel function reverses the effects of repeated restraint on BLA-dependent auditory fear conditioning. We found that repeated restraint stress increased the expression of cued conditioned fear in male rats. However, 1-Ethyl-2-benzimidazolinone (1-EBIO, 1 or 10 mg/kg) or CyPPA (5 mg/kg) administered 30 min prior to testing of fear expression brought conditioned freezing to control levels, with little impact on fear expression in control handled rats. These results demonstrate that enhancement of SK channel function can reduce the abnormalities of BLA-dependent fear memory caused by repeated stress. Furthermore, this indicates that pharmacological targeting of SK channels may provide a novel target for alleviation of psychiatric symptoms associated with amygdala hyperactivity.

Cholinergic innervation of pyramidal cells and parvalbumin-immunoreactive interneurons in the rat basolateral amygdala

The basolateral nucleus of the amygdala receives an extremely dense cholinergic innervation from the basal forebrain that is critical for memory consolidation. Although previous electron microscopic studies determined some of the postsynaptic targets of cholinergic afferents, the majority of postsynaptic structures were dendritic shafts whose neurons of origin were not identified. To make this determination, the present study analyzed the cholinergic innervation of the anterior subdivision of the basolateral amygdalar nucleus (BLa) of the rat using electron microscopic dual-labeling immunocytochemistry. The vesicular acetylcholine transporter (VAChT) was used as a marker for cholinergic terminals; calcium/calmodulin-dependent protein kinase II (CaMK) was used as a marker for pyramidal cells, the principal neurons of the BLa; and parvalbumin (PV) was used as a marker for the predominant interneuronal subpopulation in this nucleus. VAChT(+) terminals were visualized by using diaminobenzidine as a chromogen, whereas CAMK(+) or PV(+) neurons were visualized with Vector very intense purple (VIP) as a chromogen. Quantitative analyses revealed that the great majority of dendritic shafts receiving cholinergic inputs were CAMK(+) , indicating that they were of pyramidal cell origin. In fact, 89% of the postsynaptic targets of cholinergic terminals in the BLa were pyramidal cells, including perikarya (3%), dendritic shafts (47%), and dendritic spines (39%). PV(+) structures, including perikarya and dendrites, constituted 7% of the postsynaptic targets of cholinergic axon terminals. The cholinergic innervation of both pyramidal cells and PV(+) interneurons may constitute an anatomical substrate for the generation of oscillatory activity involved in memory consolidation by the BLa.

Location and function of the slow afterhyperpolarization channels in the basolateral amygdala

The basolateral amygdala (BLA) assigns emotional significance to sensory stimuli. This association results in a change in the output (action potentials) of BLA projection neurons in response to the stimulus. Neuronal output is controlled by the intrinsic excitability of the neuron. A major determinant of intrinsic excitability in these neurons is the slow afterhyperpolarization (sAHP) that follows action potential (AP) trains and produces spike-frequency adaptation. The sAHP is mediated by a slow calcium-activated potassium current (sI(AHP)), but little is known about the channels that underlie this current. Here, using whole-cell patch-clamp recordings and high-speed calcium imaging from rat BLA projection neurons, we examined the location and function of these channels. We determined the location of the sI(AHP) by applying a hyperpolarizing voltage step during the sI(AHP) and measuring the time needed for the current to adapt to the new command potential, a function of its electrotonic distance from the somatic recording electrode. Channel location was also probed by focally uncaging calcium using a UV laser. Both methodologies indicated that, in BLA neurons, the sI(AHP) is primarily located in the dendritic tree. EPSPs recorded at the soma were smaller, decayed faster, and showed less summation during the sAHP. Adrenergic stimulation and buffering calcium reduced the sAHP and the attenuation of the EPSP during the sAHP. The sAHP also modulated the AP in the dendrite, reducing the calcium response evoked by a single AP. Thus, in addition to mediating spike-frequency adaptation, the sI(AHP) modulates communication between the soma and the dendrite.

Neuronal localization of m1 muscarinic receptor immunoreactivity in the rat basolateral amygdala

Muscarinic cholinergic neurotransmission in the basolateral nuclear complex (BLC) of the amygdala is critical for memory consolidation in emotional/motivational learning tasks. Although knowledge of the localization of muscarinic receptor subtypes in the BLC would contribute to an understanding of the actions of acetylcholine in mnemonic function, previous receptor binding and in situ hybridization studies lacked the resolution necessary to identify which neurons in the BLC express different receptor subtypes. In the present study immunohistochemistry was used to study the neuronal localization of the m1 receptor. The intensity of m1 immunoreactivity varied in different nuclei of the amygdala, and was most robust in the BLC, and in the adjacent posterolateral cortical nucleus. The density and morphology of labeled neurons in the BLC suggested that the m1+ neuronal population included pyramidal cells, the principal neurons in this amygdalar region. In addition, there was dense punctate m1 immunoreactivity in the neuropil of the BLC. Dual labeling immunofluorescence studies of the BLC using antibodies to cell type specific markers were performed to more definitively determine the phenotype of m1-positive (m1+) neurons. An antibody to calcium/calmodulin protein kinase II (CaMK) was used to label pyramidal cells, whereas an antibody to glutamic acid decarboxylase was used to label interneurons. Virtually all of the intensely labeled m1+ neurons of the BLC were CaMK+ pyramidal cells. These data suggest that the ability of M1 receptor antagonists to impair memory consolidation in the BLC is mainly due to blockade of cholinergic influences on the activity of pyramidal neurons.

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.

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.

Muscarinic control of graded persistent activity in lateral amygdala neurons

The cholinergic system is crucially involved in several cognitive processes including attention, learning and memory. Muscarinic actions have profound effects on the intrinsic firing pattern of neurons. In principal neurons of the entorhinal cortex (EC), muscarinic receptors activate an intrinsic cation current that causes multiple self-sustained spiking activity, which represents a potential mechanism for transiently sustaining information about novel items. The amygdala appears to be important for experience-dependent learning by emotional arousal, and cholinergic muscarinic influences are essential for the amygdala-mediated modulation of memory. Here we show that principal neurons from the lateral nucleus of the amygdala (LA) can generate intrinsic graded persistent activity that is similar to EC layer V cells. This firing behavior is linked to muscarinic activation of a calcium-sensitive non-specific cation current and can be mimicked by stimulation of cholinergic afferents that originate from the nucleus basalis of Meynert (n. M). Moreover, we demonstrate that the projections from the n. M. are essential and sufficient for the control and modulation of graded firing activity in LA neurons. We found that activation of these cholinergic afferents (i) is required to maintain and to increase firing rates in a graded manner, and (ii) is sufficient for the graded increases of stable discharge rates even without an associated up-regulation of Ca2+. The induction of persistent activity was blocked by flufenamic acid or 2-APB and remained intact after Ca2+-store depletion with thapsigargin. The internal ability of LA neurons to generate graded persistent activity could be essential for amygdala-mediated memory operations.

Distribution of IP3-mediated calcium responses and their role in nuclear signalling in rat basolateral amygdala neurons

Metabotropic receptor activation is important for learning, memory and synaptic plasticity in the amygdala and other brain regions. Synaptic stimulation of metabotropic receptors in basolateral amygdala (BLA) projection neurons evokes a focal rise in free Ca(2+) in the dendrites that propagate as waves into the soma and nucleus. These Ca(2+) waves initiate in the proximal dendrites and show limited propagation centrifugally away from the soma. In other cell types, Ca(2+) waves have been shown to be mediated by either metabotropic glutamate receptor (mGluR) or muscarinic receptor (mAChR) activation. Here we show that mGluRs and mAChRs act cooperatively to release Ca(2+) from inositol 1,4,5-trisphosphate (IP(3))-sensitive intracellular Ca(2+) stores. Whereas action potentials (APs) alone were relatively ineffective in raising nuclear Ca(2+), their pairing with metabotropic receptor activation evoked an IP(3)-receptor-mediated Ca(2+)-induced Ca(2+) release, raising nuclear Ca(2+) into the micromolar range. Metabotropic-receptor-mediated Ca(2+)-store release was highly compartmentalized. When coupled with metabotropic receptor stimulation, large robust Ca(2+) rises and AP-induced amplification were observed in the soma, nucleus and sparsely spiny dendritic segments with metabotropic stimulation. In contrast, no significant amplification of the Ca(2+) transient was detected in spine-dense high-order dendritic segments. Ca(2+) rises evoked by photolytic uncaging of IP(3) showed the same distribution, suggesting that IP(3)-sensitive Ca(2+) stores are preferentially located in the soma and proximal dendrites. This distribution of metabotropic-mediated store release suggests that the neuromodulatory role of metabotropic receptor stimulation in BLA-dependent learning may result from enhanced nuclear signalling.

Ca2+-activated K+ channels: molecular determinants and function of the SK family

Ca(2+)-activated K(+) (K(Ca)) channels of small (SK) and intermediate (IK) conductance are present in a wide range of excitable and non-excitable cells. On activation by low concentrations of Ca(2+), they open, which results in hyperpolarization of the membrane potential and changes in cellular excitability. K(Ca)-channel activation also counteracts further increases in intracellular Ca(2+), thereby regulating the concentration of this ubiquitous intracellular messenger in space and time. K(Ca) channels have various functions, including the regulation of neuronal firing properties, blood flow and cell proliferation. The cloning of SK and IK channels has prompted investigations into their gating, pharmacology and organization into calcium-signalling domains, and has provided a framework that can be used to correlate molecularly identified K(Ca) channels with their native currents.

Facilitation of memory for extinction of drug-induced conditioned reward: role of amygdala and acetylcholine

These experiments examined the effects of posttrial peripheral and intra-amygdala injections of the cholinergic muscarinic receptor agonist oxotremorine on memory consolidation underlying extinction of amphetamine conditioned place preference (CPP) behavior. Male Long-Evans rats were initially trained and tested for an amphetamine (2 mg/kg) CPP. Rats were subsequently given limited extinction training, followed by immediate posttrial peripheral or intrabasolateral amygdala injections of oxotremorine. A second CPP test was then administered, and the amount of time spent in the previously amphetamine-paired and saline-paired apparatus compartments was recorded. Peripheral (0.07 or 0.01 mg/kg) or intra-amygdala (10 etag/0.5 microL) postextinction trial injections of oxotremorine facilitated CPP extinction. Oxotremorine injections that were delayed 2 h posttrial training did not enhance CPP extinction, indicating a time-dependent effect of the drug on memory consolidation processes. The findings indicate that memory consolidation for extinction of approach behavior to environmental stimuli previously paired with drug reward can be facilitated by posttrial peripheral or intrabasolateral amygdala administration of a cholinergic agonist.

Matching molecules to function: neuronal Ca2+-activated K+ channels and afterhyperpolarizations

Potassium channels regulate the membrane excitability of neurons, play a major role in shaping action potentials, determining firing patterns and regulating neurotransmitter release, and thus significantly contribute to neuronal signal encoding and integration. This review focuses on the molecular and cellular basis for the specific function of small-conductance calcium-activated potassium channels (SK channels) in the nervous system. SK channels are activated by an intracellular increase of free calcium during action potentials. They mediate currents that modulate the firing frequency of neurons. Three SK channel subunits have been cloned and form channels, which are voltage-insensitive, activated by submicromolar intracellular calcium concentrations, and are blocked, with different affinities, by a number of toxins and organic compounds. Different neurons in the central and peripheral nervous system express distinct subsets of SK channel subunits. Recent progress has been made in relating cloned SK channels to their native counterparts. These findings argue in favour of regulatory mechanisms conferring to native SK channels with specific subunit compositions distinct and specific functional profiles in different neurons.

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

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

17beta-Estradiol reduces excitatory postsynaptic potential (EPSP) amplitude in rat basolateral amygdala neurons

We examined the actions of estrogen on excitatory synaptic transmission in the basolateral amygdala (BLA), a brain region involved in learning, emotions, and the effects of stress. Intracellular recordings of monosynaptic excitatory postsynaptic potentials (EPSPs) were obtained from BLA neurons in a slice preparation. Bath application of 17beta-estradiol (2 micro M) reduced EPSP amplitude by an average of 77%. This reduction was readily reversed by washing with control saline and was not mimicked by the inactive isomer 17 alpha-estradiol. Other passive and active properties of BLA neurons were unaffected by 17beta-estradiol. The observed EPSP reduction is in sharp contrast to the potentiation of EPSPs by estrogen observed in other brain regions.

Posttraining intra-basolateral amygdala scopolamine impairs food- and amphetamine-induced conditioned place preferences

The present study investigated the role of cholinergic muscarinic receptor function within the basolateral amygdala memory in the consolidation of conditioned place preference (CPP) memory. Adult male Long-Evans rats were confined to treatment- or nontreatment-paired compartments for 30 min on 4 alternating days. After training, rats received intrabasolateral amygdala infusions of scopolamine (2.5 microg or 5.0 microg/0.5 microl) or saline. The rats were then given a 20-min test session, and the time spent in each of the compartments was recorded. Immediate posttraining (but not delayed 2 hr) scopolamine (5.0 microg) blocked acquisition of food- and amphetamine-induced CPPs. The findings indicate a time-dependent role for basolateral amygdala muscarinic receptors in memory consolidation underlying CPPs for natural and drug rewards.

Physiological role of calcium-activated potassium currents in the rat lateral amygdala

Principal neurons in the lateral nucleus of the amygdala (LA) exhibit a continuum of firing properties in response to prolonged current injections ranging from those that accommodate fully to those that fire repetitively. In most cells, trains of action potentials are followed by a slow afterhyperpolarization (AHP) lasting several seconds. Reducing calcium influx either by lowering concentrations of extracellular calcium or by applying nickel abolished the AHP, confirming it is mediated by calcium influx. Blockade of large conductance calcium-activated potassium channel (BK) channels with paxilline, iberiotoxin, or TEA revealed that BK channels are involved in action potential repolarization but only make a small contribution to the fast AHP that follows action potentials. The fast AHP was, however, markedly reduced by low concentrations of 4-aminopyridine and alpha-dendrotoxin, indicating the involvement of voltage-gated potassium channels in the fast AHP. The medium AHP was blocked by apamin and UCL1848, indicating it was mediated by small conductance calcium-activated potassium channel (SK) channels. Blockade of these channels had no effect on instantaneous firing. However, enhancement of the SK-mediated current by 1-ethyl-2-benzimidazolinone or paxilline increased the early interspike interval, showing that under physiological conditions activation of SK channels is insufficient to control firing frequency. The slow AHP, mediated by non-SK BK channels, was apamin-insensitive but was modulated by carbachol and noradrenaline. Tetanic stimulation of cholinergic afferents to the LA depressed the slow AHP and led to an increase in firing. These results show that BK, SK, and non-BK SK-mediated calcium-activated potassium currents are present in principal LA neurons and play distinct physiological roles.

Metrifonate decreases sI(AHP) in CA1 pyramidal neurons in vitro

Metrifonate, a cholinesterase inhibitor, has been shown to enhance learning in aging rabbits and rats, and to alleviate the cognitive deficits observed in Alzheimer's disease patients. We have previously determined that bath application of metrifonate reduces the spike frequency adaptation and postburst afterhyperpolarization (AHP) in rabbit CA1 pyramidal neurons in vitro using sharp electrode current-clamp recording. The postburst AHP and accommodation observed in current clamp are the result of four slow outward potassium currents (sI(AHP), I(AHP), I(M), and I(C)) and the hyperpolarization activated mixed cation current, I(h). We recorded from visually identified CA1 hippocampal pyramidal neurons in vitro using whole cell voltage-clamp technique to better isolate and characterize which component currents of the AHP are affected by metrifonate. We observed an age-related enhancement of the slow component of the AHP tail current (sI(AHP)), but not of the fast decaying component of the AHP tail current (I(AHP), I(M), and I(C)). Bath perfusion of metrifonate reduced sI(AHP) at concentrations that cause a reduction of the AHP and accommodation in current-clamp recordings, with no apparent reduction of I(AHP), I(M), and I(C). The functional consequences of metrifonate administration are apparently mediated solely through modulation of the sI(AHP).

Glucocorticoid enhancement of memory consolidation in the rat is blocked by muscarinic receptor antagonism in the basolateral amygdala

Glucocorticoid-induced memory enhancement is known to depend on beta-adrenoceptor activation in the basolateral amygdala (BLA). Additionally, inactivation of muscarinic cholinergic receptors in the rat amygdala blocks memory enhancement induced by concurrent beta-adrenergic activation. Together, these findings suggest that glucocorticoid-induced modulation of memory consolidation requires cholinergic as well as adrenergic activation in the BLA. Two experiments investigated this issue. The first experiment examined whether blockade of muscarinic cholinergic receptors in the BLA with atropine alters the memory-enhancing effects of the systemically administered glucocorticoid dexamethasone. Dexamethasone (0.3, 1.0 or 3.0 mg/kg, s.c.) administered to rats immediately after inhibitory avoidance training produced dose-dependent enhancement of 48-h retention. Concurrent bilateral infusions of the muscarinic cholinergic antagonist atropine (0.5 microg in 0.2 microL per side) into the BLA blocked the memory enhancement. The second experiment investigated whether the BLA is a locus of interaction between glucocorticoid and muscarinic activation. The specific glucocorticoid receptor (GR or type II) agonist RU 28362 (1.0, 3.0 or 10 ng) was infused into the BLA either alone or together with atropine immediately after training. The GR agonist produced dose-dependent memory enhancement and atropine blocked the memory enhancement. These findings indicate that muscarinic cholinergic activation within the BLA is critical for enabling glucocorticoid enhancement of memory consolidation and that enhancement of memory induced by GR activation in the BLA requires cholinergic activation within the BLA.

Differential distribution of three Ca(2+)-activated K(+) channel subunits, SK1, SK2, and SK3, in the adult rat central nervous system

Ca(2+)-activated, voltage-independent K(+) channels are present in most neurons and mediate the afterhyperpolarizations (AHPs) following action potentials. They present distinct physiological and pharmacological properties and play an important role in controlling neuronal firing frequency and spike frequency adaptation. We used in situ hybridization to characterize the distribution patterns of the three cloned SK channel subunits (SK1-3), the prime candidates likely to underlie Ca(2+)-dependent AHPs in the central nervous system. We found high levels of expression in regions presenting prominent AHP currents, such as, for example, neocortex and CA1-3 layers of the hippocampus (SK1 and SK2), reticularis thalami (SK1 and SK2), supraoptic nucleus (SK3), and inferior olivary nucleus (SK2 and SK3). Our results reveal the functional role of SK channels with defined subunit compositions in some neurons and open the way to the identification of the molecular determinants of AHP currents in many brain regions.

Basolateral amygdala is involved in modulating consolidation of memory for classical fear conditioning

Previous findings indicate that the basolateral amygdala complex of nuclei (BLC) is involved in modulating (i.e., enhancing or impairing) memory consolidation for aversive training such as inhibitory avoidance. The present study examined whether the BLC also modulates the consolidation of memory for classical fear conditioning in which a specific context is paired with footshock. Adult male rats with bilateral cannulae targeting the BLC were allowed, first, to habituate in a Y maze that had differently shaped and textured arms. On the next day the rats were placed in one maze arm (shock arm), and they received four unsignaled footshocks. In Experiment 1, immediately after the training some rats received BLC inactivation with lidocaine (10 microgram/0.2 microliter per side), and control rats received buffered saline. In Experiment 2, rats received immediate post-training intra-BLC infusions of the muscarinic receptor agonist oxotremorine (10 ng/0.2 microliter per side) or saline. On a 24 hr retention test each rat was placed in a "safe" arm of the maze and allowed to access all maze arms. Lidocaine-treated rats had impaired memory for the classical fear conditioning when they were compared with the saline-treated controls: they spent less time freezing, entered the shock arm more readily and more often, and spent more time in it. In contrast, oxotremorine-treated rats had a stronger memory for the context-footshock association as assessed by all measures of memory. Thus, post-training treatments affecting BLC function modulate memory for Pavlovian contextual fear conditioning in a manner similar to that found with other types of training.

Mechanisms and functional significance of a slow inhibitory potential in neurons of the lateral amygdala

A slow inhibitory potential (sIP) elicited upon synaptic activation in spiny, pyramidal-like cells with properties indicative of projection neurons was investigated in slices of the rat and guinea-pig lateral amygdala in vitro. The sIP succeeded the triphasic sequence of excitatory and fast/slow inhibitory postsynaptic potentials mediated via glutamate and GABA(A/B) receptors, respectively, was readily evoked upon repetitive stimulation of the external capsule and appeared to terminate epileptiform burst discharges during pharmacologically reduced GABAergic influence. The sIP reversed close to the Cl- equilibrium potential, but was not affected by altered transmembrane Cl- gradients and not abolished by antagonists to ligand-gated Cl- channels. Intracellular injection of QX 314 and resulting blockade of sodium spikes had no effect, whereas the Ca2+ chelator BAPTA blocked the sIP concomitantly with slow hyperpolarizing afterpotentials following intrinsically generated spike firing, thereby indicating the contribution of Ca2+-dependent mechanisms secondary to synaptic activation. During action of BAPTA and QX 314, an N-methyl-D-aspartate (NMDA) receptor-mediated potential was unmasked, which contributed to the sIP. The Ca2+-dependent mechanisms of the sIP involved a membrane K+ conductance, as was indicated by the dependence on the K+ gradient and the shift of the reversal potential towards the K+ equilibrium potential during blocked NMDA receptors. During the presence of GABA receptor antagonists, reduction of the Ca2+-activated K+ conductance through injection of BAPTA or application of dopamine induced a gradual shift of interictal-like single bursts of spikes towards the generation of re-occurring ictal-like activity. It is concluded that pyramidal-like projection cells in the AL can generate a sIP upon synaptic activation, which reflects the combined activation of an NMDA receptor-mediated cation current and a K+ current that is secondary to the rise in intracellular Ca2+ concentration resulting from the preceding depolarizing response. The sIP may play an important role in controlling excitatory activity in the amygdala, particularly in preventing the transformation of interictal-like activity towards recurrent epileptic discharges during periods of decreased GABAergic influence.

Ionic mechanisms of intrinsic oscillations in neurons of the basolateral amygdaloid complex

Ionic mechanisms underlying low-threshold (LTO) and high-threshold (HTO) oscillations occurring in a class of spiny neurons within the basolateral amygdaloid complex (see companion paper) were investigated in slice preparations of the guinea pig amygdala in vitro. LTOs were abolished through local application of tetrodotoxin (TTX, 10-20 microM) or a decrease in the extracellular sodium concentration ([Na+]o) from 153 to 26 mM, whereas HTOs were more readily elicited under these conditions. The effects of TTX and low [Na+]o were accompanied by a hyperpolarizing shift of the membrane potential by 3 +/- 1 mV and a decrease in apparent input resistance by 14 +/- 11 MOmega. LTOs were not observed during intracellular recording with QX 314 (50 microM) or Cs-acetate (2 M) containing micropipettes. At membrane potentials associated with LTO generation, voltage responses to sinusoidal current input with changing frequency between 0 and 10 Hz were characterized by a peak in the response (resonance) at 2.4 +/- 1 Hz, largely corresponding to the frequency range of the LTOs. Resonance behavior was evident as a peak in the impedance amplitude plot (ZA-plot) and a maximum in the fast Fourier transformation (FFT). Resonance and LTOs were concomitantly reduced by TTX and barium (Ba2+;2-10 mM) and were preserved during action of extracellular cesium (Cs+; 10-30 mM) or tetraethylammonium chloride (TEA; 20-50 mM), although the peak in the frequency domain tended to shift to lower values in TEA. Application of carbachol (50-200 microM) significantly reduced or blocked LTOs, whereas 4-aminopyridine (4-AP; 10 mM), iberiotoxin (Ibtx, 10 microM), and apamin (20 microM) had no effect. Slow depolarizing/repolarizing current ramps (12.5-125 pA/s) evoked HTOs as rhythmic deflections in membrane potential at either phase of the current ramp. Substitution of extracellular calcium (Ca2+) by magnesium and addition of cobalt chloride (2-4 mM) blocked HTOs but had no measurable effect on the propensity of the cells to produce LTOs. HTOs were abolished within approximately 10 min after impalement of the cells with a bis-(2-aminophenoxy)-N,N,N', N'-tetraacetic acid (BAPTA; 200 mM)-containing micropipette. Intracellular Cs+, extracellular Ba2+ (2-10 mM), or extracellular TEA (20-50 mM) induced an increase in amplitude of the rhythmic discharges and an increasingly slowed time course of repolarization at successive oscillatory events, until a steady depolarization was reached at -20 to -10 mV. Application of Ibtx (10 microM) reversibly abolished rhythmic activity during the repolarizing phase of the current ramp, whereas charybdotoxin (2-10 microM) and apamin (20 microM) had no effect. Changes in the chloride (Cl-) equilibrium potential by approximately +30 mV through intracellular recording with a KNO3 (3 M)-containing micropipette or lowering [Cl-]o from 128 to 4 mM, or blockade of Cl- conductances through niflumic acid (100 microM), did not significantly effect LTOs or HTOs. The generation of repetitive spike patterns on membrane depolarization was substantially influenced through removal of extracellular Ca2+ and associated blockade of HTOs, in that the initial high frequent discharge was abolished, frequency adaptation toward slow-rhythmic firing was delayed, and firing occurred at a more irregular pattern during strong depolarizing stimuli. It is concluded that a TTX-sensitive Na+ conductance and the M current contribute to generation of the LTOs, although their exact role in rhythmogenesisremains to be determined. HTOs seem to largely depend on a functional coupling between high-voltage-activated Ca2+ conductances, a Ca2+-activated K+ current presumably carried through BKCa channels, and additional voltage-dependent K+ conductances. In functional terms, the HTOs are important in determining spike frequency adaptation toward a slow-rhythmic firing pattern during maintained depolarizing influence.

Two types of intrinsic oscillations in neurons of the lateral and basolateral nuclei of the amygdala

Intracellular recordings in the guinea pig and cat basolateral amygdaloid (BL) complex maintained as slices in vitro revealed that a subpopulation of neurons (79%) in the lateral (AL) and basolateral (ABl) nuclei generated two types of slow oscillations of the membrane potential upon steady depolarization from resting potential. The cells were of a stellate or pyramidal-like shape and possessed spiny dendrites and an axon leaving the local synaptic environment, and thus presumably represented projection neurons. Similar oscillatory activity was observed in projection neurons of the cat AL nucleus recorded in vivo. Oscillatory activity with a low threshold of activation (low-threshold oscillation, LTO) appeared as rhythmic deflections (amplitudes, 2-6 mV) of the membrane potential positive to -60 mV. Fast Fourier transformation (FFT) demonstrated a range of frequencies of LTOs between 0.5 and 9 Hz, with >80% occurring at 1-3.5 Hz and an average at 2.3 +/- 1.1 Hz. LTOs were more regular after pharmacological blockade of synaptic transmission and were blocked by tetrodotoxin (TTX). Blockade of LTOs and Na+ spikes revealed a second type of oscillatory activity (high-threshold oscillation, HTO) at depolarizations beyond -40 mV, which was capable of triggering high-threshold spikes. HTOs ranged between 1 and 7.5 Hz, with >80% occurring at 2-6 Hz and an average at 5.8 +/- 1.1 Hz. HTOs vanished at a steady membrane polarization positive to -20 mV. Current versus voltage relations obtained under voltage-clamp conditions revealed two regions of negative slope conductance at -55 to -40 mV and at around -30 mV, which largely overlapped with the voltage ranges of LTOs and HTOs. TTX abolished the first region of negative slope conductance (-55 to -40 mV) and did not significantly influence the second region of negative slope conductance. Neuronal responses to maintained depolarizing current pulses consisted of an initial high-frequency discharge (up to 100 Hz), the frequency of which depended on the amplitude of the depolarizing current pulse, followed by a progressive decline ("adaptation") toward a slow-rhythmic firing pattern. The decay in firing frequency followed a double-exponential function, with time constants averaging 57 +/- 28 ms and 3.29 +/- 1.85 s, and approached steady-state frequencies at 6.3 +/- 2.9 Hz (n = 17). Slow-rhythmic firing remained at this frequency over a wide range of membrane polarization between approximately -50 and -20 mV, although individual electrogenic events changed from Na+ spikes and underlying LTOs to high-threshold spikes and underlying HTOs. Rhythmic regular firing was only interrupted at an intermediate range of membrane polarization by the occurrence of spike doublets. In conclusion, the integrative behavior of a class of neurons in the BL complex appears to be largely shaped by the slow-oscillatory properties of the membrane. While LTOs are likely to synchronize synaptic signals near firing threshold, HTOs are a major determinant for the slow steady-state firing patterns during maintained depolarizing influence. These intrinsic oscillatory mechanisms, in turn, can be assumed to promote population activity at this particular frequency, which ranges well within that of the limbic theta (Theta) rhythm and the delta (delta) waves in the electroencephalogram during slow-wave sleep.

Vasopressin receptor subtypes differentially modulate calcium-activated potassium currents in the horizontal limb of the diagonal band of Broca

The actions of vasopressin on acutely dissociated neurons within the rat horizontal limb of the diagonal band of Broca were examined using the whole-cell patch-clamp technique. Vasopressin elicited two distinct responses in 45 of 62 neurons. In one group of cells, 300 nM vasopressin decreased voltage-activated outward currents (26/45 cells) whereas in a second group, vasopressin increased outward currents (19/45 cells). The vasopressin-mediated decrease in outward currents was blocked by 1 microM Manning compound, a V1 receptor antagonist, suggesting that this response was mediated via V1 receptors. In contrast, the vasopressin-induced increase in outward current was blocked by 1 microM d(CH2)5)1,D-Ile2,Ile4,Arg8,Ala9, a V2 receptor antagonist, indicating that V2 receptor activation underlies this second response. When cells were perfused with 0 Ca2+/50 microM Cd2+, application of vasopressin did not cause any change in voltage-activated outward currents, suggesting that vasopressin modulates a calcium-dependent conductance. In the presence of 25 nM charybdotoxin, an Ic channel antagonist, vasopressin application did not influence outward currents, indicating that vasopressin modulates Ic. Currents through voltage-gated calcium channels which are responsible for activation of Ic were unaffected by vasopressin, suggesting a direct effect of vasopressin on Ic channels. These observations indicate a differential modulation of Ic channels by vasopressin via V1 and V2 receptors in the horizontal limb of the diagonal band of Broca. Our data also demonstrate the ionic mechanisms whereby vasopressin may act at V1 for V2 receptors to influence the excitability of the horizontal limb of the diagonal band of Broca neurons.

Removal of spike frequency adaptation via neuromodulation intrinsic to the Tritonia escape swim central pattern generator

For the mollusc Tritonia diomedea to generate its escape swim motor pattern, interneuron C2, a crucial member of the central pattern generator (CPG) for this rhythmic behavior, must fire repetitive bursts of action potentials. Yet, before swimming, repeated depolarizing current pulses injected into C2 at periods similar those in the swim motor program are incapable of mimicking the firing rate attained by C2 on each cycle of a swim motor program. This resting level of C2 inexcitability is attributable to its own inherent spike frequency adaptation (SFA). Clearly, this property must be altered for the swim behavior to occur. The pathway for initiation of the swimming behavior involves activation of the serotonergic dorsal swim interneurons (DSIs), which are also intrinsic members of the swim CPG. Physiologically appropriate DSI stimulation transiently decreases C2 SFA, allowing C2 to fire at higher rates even when repeatedly depolarized at short intervals. The increased C2 excitability caused by DSI stimulation is mimicked and occluded by serotonin application. Furthermore, the change in excitability is not caused by the depolarization associated with DSI stimulation or serotonin application but is correlated with a decrease in C2 spike afterhyperpolarization. This suggests that the DSIs use serotonin to evoke a neuromodulatory action on a conductance in C2 that regulates its firing rate. This modulatory action of one CPG neuron on another is likely to play a role in configuring the swim circuit into its rhythmic pattern-generating mode and maintaining it in that state.

Properties of a Ca2+-activated K+ conductance in acutely isolated pyramidal-like neurons from the rat basolateral amygdaloid complex

A calcium (Ca2+)-activated potassium (K+) conductance was studied in large pyramidal-like neurons acutely dissociated from the rat basolateral amygdaloid complex. Neurons were immunoreactive to anti-alpha(913-926), a sequence-directed antibody directed against the pore-forming alpha-subunit of the BK(Ca) channel, also termed slo. Whole cell current-voltage (I-V) relationships obtained on application of slow (46.7 mV/s) voltage ramps from -110 to +100 mV were N shaped positive to -30 mV. Maximal current activation occurred at +9.8 +/- 2.7 (SE) mV, with a mean current density of 404.8 +/- 25.0 pA/pF. Substitution of extracellular Ca2+ with manganese (Mn2+), or with magnesium (Mg2+) and addition of 5 mM ethyleneglycol-bis (beta-aminoethylether)-N,N,N',N'-tetraacetic acid, abolished the N-shaped I-V relationship with a reduction in maximal outward current to 15.3 +/- 2.3% of the control value. The Ca2+-sensitive K+ current component, as revealed by voltage step protocols, activated at depolarizations positive to -30 mV with a slow time course (time constant 430.7 +/- 78.6 ms). The current was reduced by 80.4 +/- 4.6% through 1 mM tetraethyammonium chloride and by 66.8 +/- 3.4% through 100 nM iberiotoxin, whereas apamin up to 1 microM had no effect. It is concluded that pyramidal-like neurons of the basolateral amygdaloid complex possess BK(Ca) channels and the corresponding macroscopic Ca2+-sensitive K+ conductance, activation of which will substantially contribute to the Ca2+-dependent regulation of electrogenic behavior in these neurons.

Metabotropic glutamate receptor subtypes mediating slow inward tail current (IADP) induction and inhibition of synaptic transmission in olfactory cortical neurones

1. The pharmacological features of the pre- and postsynaptic metabotropic glutamate receptors (mGluRs) present in the guinea-pig olfactory cortex, were examined in brain slices in vitro by use of a conventional intracellular current clamp/voltage clamp recording technique. 2. Bath-application of trans-aminocyclopentane-1,3-dicarboxylic acid (trans-ACPD) (50 microM) produced a sustained membrane depolarization, increase in cell excitability and induction of a post-stimulus inward (after depolarizing) tail current (IADP) (measured under 'hybrid' voltage clamp) similar to those evoked by the muscarinic receptor agonist oxotremorine-M (OXO-M, 2 microM). 3. L-Glutamate (0.25 1 mM. in the presence of 20 microM 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) and 100 microM-DL-amino-5-phosphono valeric acid (DL-APV)) or the broad spectrum mGluR agonists 1S,3R-aminocyclopentane-1,3-dicarboxylic acid (1S,3R-ACPD, 10 microM), 1S,3S-ACPD (50 microM), ibotenate (Ibo; 25 microM. in the presence of 100 microM DL-APV), the selective mGluR I agonists (S)-3,5-dihydroxyphenylglycine ((S)-3,5-DHPG, 10 microM), (S)-3-hydroxyphenylglycine ((S)-3HPG, 50 microM), or quisqualate (10 microM, in the presence of 20 microM CNQX), but not the mGluR II agonist 2S,1'S,2'S-2-(2'-carboxycyclopropyl)-glycine (L-CCG1,1 microM) or mGluR III agonist L(+)-2-amino-4-phosphonobutyric acid (L-AP4, 1 mM), were all effective in producing membrane depolarization and inducing a post-stimulus IADP. Unexpectedly, the proposed mGluR II-selective agonist (2S,1'R,2'R,3'R)-2-(2',3'-dicarboxycyclopropyl)-glycine (DCG-IV, 10 microM, in the presence of 100 microM DL-APV) was also active. 4. The excitatory effects induced by 10 microM 1S,3R-ACPD were reversibly antagonized by the mGluR I/II antagonist (1)-alpha-methyl-4-carboxyphenylglycine ((+)-MCPG, 0.5 1 mM), as well as the selective mGluR I antagonists (S)-4-carboxyphenylglycine ((S)-4CPG) and (S)-4-carboxy-3-hydroxyphenyl glycine ((S)-4C3HPG) (both at 1 mM), but not the nonselective mGluR antagonist L(+)-2-amino-3-phosphonopropionic acid (L-AP3, 1 mM) or the selective mGluR III antagonist (S)-alpha-methyl-L-AP4 (MAP4, 1 mM). 5. The excitatory postsynaptic potentials (e.p.s.ps), induced by single focal stimulation of cortical excitatory fibre tracts, were markedly reduced by 1S,3R-ACPD or L-AP4 (both at 10 microM), and by the selective mGluR II agonists (mGluR 1 antagonists) (S)-4CPG or (S)-4C3HPG (both at 1 mM) but not (S)-3,5-DHPG or (S)-3HPG (both at 100 microM). 6. The inhibitory effects of 1S-3R-ACPD, but not L-AP4, were reversibly blocked by (+)-MCPG (1 mM), whereas those produced by L-AP4, but not 1S,3R-ACPD, were blocked by the selective mGluR III antagonist MAP4 (1 mM). 7. It is concluded that a group I mGluR is most likely involved in mediating excitatory postsynaptic effects, whereas two distinct mGluRs (e.g. group II and III) might serve as presynaptic inhibitory autoreceptors in the guinea-pig olfactory cortex.

Synaptic transmission and plasticity in the amygdala. An emerging physiology of fear conditioning circuits

Numerous studies in both rats and humans indicate the importance of the amygdala in the acquisition and expression of learned fear. The identification of the amygdala as an essential neural substrate for fear conditioning has permitted neurophysiological examinations of synaptic processes in the amygdala that may mediate fear conditioning. One candidate cellular mechanism for fear conditioning is long-term potentiation (LTP), an enduring increase in synaptic transmission induced by high-frequency stimulation of excitatory afferents. At present, the mechanisms underlying the induction and expression of amygdaloid LTP are only beginning to be understood, and probably involve both the N-methyl-D-aspartate (NMDA) and alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA) subclasses of glutamate receptors. This article will examine recent studies of synaptic transmission and plasticity in the amygdala in an effort to understand the relationships of these processes to aversive learning and memory.

Metabotropic glutamate receptor agonist ACPD inhibits some, but not all, muscarinic-sensitive K+ conductances in basolateral amygdaloid neurons

Muscarinic agonists produce membrane depolarization and losses of spike frequency accommodation and the slow afterhyperpolarization (AHP) when applied to neurons of the basolateral amygdala (BLA). Underlying these changes are the muscarinic-induced inhibitions of several K+ conductances, including the voltage-activated M-current (IM), a slowly decaying Ca(2+)-activated current (IAHP), a voltage-insensitive leak current (ILeak), and the hyperpolarization-activated inward rectifier current (IIR). Similar depolarizations and losses of the slow AHP have been observed in other neuronal cell types following stimulation of metabotropic glutamate receptors. Therefore, we tested the effects of the metabotropic glutamate receptor agonist, 1-aminocyclopentane-1s,3R-dicarboxylic acid (ACPD), on pyramidal neurons impaled with a single microelectrode for current- and voltage-clamp recordings in a brain slice preparation of the rat BLA. Application of ACPD (20 or 100 microM) to BLA neurons inhibited IM and IAHP, resulting in membrane depolarization and reductions in the amplitude and duration of the slow AHP. However, ACPD did not inhibit the muscarinic-sensitive current IIR, nor was ILeak blocked in the majority of neurons examined. These findings suggest the possibility that muscarinic cholinergic and metabotropic glutamatergic receptor agonists may activate separate intracellular transduction pathways which have convergent inhibitory effects onto IM and IAHP in BLA pyramidal neurons.