Reciprocal changes in the firing probability of lateral and central medial amygdala neurons

Amygdala Intercalated Cells: Gate Keepers and Conveyors of Internal State to the Circuits of Emotion

Generating adaptive behavioral responses to emotionally salient stimuli requires evaluation of complex associations between multiple sensations, the surrounding context, and current internal state. Neural circuits within the amygdala parse this emotional information, undergo synaptic plasticity to reflect learned associations, and evoke appropriate responses through their projections to the brain regions orchestrating these behaviors. Information flow within the amygdala is regulated by the intercalated cells (ITCs), which are densely packed clusters of GABAergic neurons that encircle the basolateral amygdala (BLA) and provide contextually relevant feedforward inhibition of amygdala nuclei, including the central and BLA. Emerging studies have begun to delineate the unique contribution of each ITC cluster and establish ITCs as key loci of plasticity in emotional learning. In this review, we summarize the known connectivity and function of individual ITC clusters and explore how different neuromodulators conveying internal state act via ITC gates to shape emotionally motivated behavior. We propose that the behavioral state-dependent function of ITCs, their unique genetic profile, and rich expression of neuromodulator receptors make them potential therapeutic targets for disorders, such as anxiety, schizophrenia spectrum, and addiction.

Two functions of the primate amygdala in social gaze

Appropriate gaze interaction is essential for primate social life. Prior studies have suggested the involvement of the amygdala in processing eye cues but its role in gaze behavior during live social exchanges remains unknown. We recorded the activity of neurons in the amygdala of two monkeys as they engaged in spontaneous visual interactions. We showed that monkeys adjust their oculomotor behavior and actively seek to interact with each other through mutual gaze. During fixations on the eye region, some amygdala neurons responded with short latency and more strongly to mutual than non-reciprocal gaze (averted gaze). Other neurons responded with long latency and were more strongly modulated by active, self-terminated mutual gaze fixations than by passively terminated ones. These results suggest that the amygdala not only participates to the evaluation of eye contact, but also plays a role in the timing of fixations which is crucial for adaptive social interactions through gaze.

Long-Lasting Effects of Prenatal Ethanol Exposure on Fear Learning and Development of the Amygdala

Prenatal ethanol exposure (PrEE) produces developmental abnormalities in brain and behavior that often persist into adulthood. We have previously reported abnormal cortical gene expression, disorganized neural circuitry along with deficits in sensorimotor function and anxiety in our CD-1 murine model of fetal alcohol spectrum disorders, or FASD (El Shawa et al., 2013; Abbott et al., 2016). We have proposed that these phenotypes may underlie learning, memory, and behavioral deficits in humans with FASD. Here, we evaluate the impact of PrEE on fear memory learning, recall and amygdala development at two adult timepoints. PrEE alters learning and memory of aversive stimuli; specifically, PrEE mice, fear conditioned at postnatal day (P) 50, showed deficits in fear acquisition and memory retrieval when tested at P52 and later at P70–P72. Interestingly, this deficit in fear acquisition observed during young adulthood was not present when PrEE mice were conditioned later, at P80. These mice displayed similar levels of fear expression as controls when tested on fear memory recall. To test whether PrEE alters development of brain circuitry associated with fear conditioning and fear memory recall, we histologically examined subdivisions of the amygdala in PrEE and control mice and found long-term effects of PrEE on fear memory circuitry. Thus, results from this study will provide insight on the neurobiological and behavioral effects of PrEE and provide new information on developmental trajectories of brain dysfunction in people prenatally exposed to ethanol.

Effect of electrical stimulation of the infralimbic and prelimbic cortices on anxiolytic-like behavior of rats during the elevated plus-maze test, with particular reference to multiunit recording of the behavior-associated neural activity

Fear and anxiety affect the activities of daily living and require concerted management, such as coping strategies, to preserve quality of life. The infralimbic (IL) and prelimbic (PL) medial prefrontal cortices have been implicated in the regulation of fear- and anxiety-like behavior, but their roles in overcoming fear- and anxiety-like behavior remain unknown. We investigated the anxiolytic-like effects of electrical stimulation of the IL and PL cortices in rats during the elevated plus-maze test. IL stimulation led to a significantly higher percentage of time spent and entries in the open arms, whereas PL stimulation did not have any significant behavioral effects. Subsequently, we recorded multiunit activity from the IL and PL cortices in rats using a wireless telemetry device, to determine whether activation of the IL occurs when rats enter the open arms in the elevated plus-maze test. The firing rate of IL neurons increased 1-3 s prior to entry from the closed arm to the open arm, whereas there were no corresponding changes in the firing rate of PL neurons. Taken together, the present findings suggest that the IL plays a key role in exerting active action to overcome anxiety-like behavior.

Deep brain stimulation in the central nucleus of the amygdala decreases 'wanting' and 'liking' of food rewards

We investigated the potential of deep brain stimulation (DBS) in the central nucleus of the amygdala (CeA) in rats to modulate functional reward mechanisms. The CeA is the major output of the amygdala with direct connections to the hypothalamus and gustatory brainstem, and indirect connections with the nucleus accumbens. Further, the CeA has been shown to be involved in learning, emotional integration, reward processing, and regulation of feeding. We hypothesized that DBS, which is used to treat movement disorders and other brain dysfunctions, might block reward motivation. In rats performing a lever-pressing task to obtain sugar pellet rewards, we stimulated the CeA and control structures, and compared stimulation parameters. During CeA stimulation, animals stopped working for rewards and rejected freely available rewards. Taste reactivity testing during DBS exposed aversive reactions to normally liked sucrose tastes and even more aversive taste reactions to normally disliked quinine tastes. Interestingly, given the opportunity, animals implanted in the CeA would self-stimulate with 500 ms trains of stimulation at the same frequency and current parameters as continuous stimulation that would stop reward acquisition. Neural recordings during DBS showed that CeA neurons were still active and uncovered inhibitory-excitatory patterns after each stimulus pulse indicating possible entrainment of the neural firing with DBS. In summary, DBS modulation of CeA may effectively usurp normal neural activity patterns to create an 'information lesion' that not only decreased motivational 'wanting' of food rewards, but also blocked 'liking' of rewards.

Modulation of instrumental responding by a conditioned threat stimulus requires lateral and central amygdala

Two studies explored the role of the amygdala in response modulation by an aversive conditioned stimulus (CS) in rats. Experiment 1 investigated the role of amygdala circuitry in conditioned suppression using a paradigm in which licking for sucrose was inhibited by a tone CS that had been previously paired with footshock. Electrolytic lesions of the lateral amygdala (LA) impaired suppression relative to sham-operated animals, and produced the same pattern of results when applied to central amygdala. In addition, disconnection of the lateral and central amygdala, by unilateral lesion of each on opposite sides of the brain, also impaired suppression relative to control subjects that received lesions of both areas on the same side. In each case, lesions were placed following Pavlovian conditioning and instrumental training, but before testing. This procedure produced within-subjects measures of the effects of lesion on freezing and between-group comparisons for the effects on suppression. Experiment 2 extended this analysis to a task where an aversive CS suppressed shuttling responses that had been previously food reinforced and also found effects of bilateral lesions of the central amygdala in a pre-post design. Together, these studies demonstrate that connections between the lateral and central amygdala constitute a serial circuit involved in processing aversive Pavlovian stimuli, and add to a growing body of findings implicating central amygdala in the modulation of instrumental behavior.

Effects of repeated stress on excitatory drive of basal amygdala neurons in vivo

Chronic stress leads to heightened affective behaviors, and can precipitate the emergence of depression and anxiety. These disorders are associated with increased amygdala activity. In animal models, chronic stress leads to increased amygdala-dependent behaviors, as well as hyperactivity of amygdala neurons. However, it is not known whether increased excitatory synaptic drive after chronic stress contributes to hyperactivity of basolateral amygdala (BLA; comprised of basal, lateral, and accessory basal nuclei) neurons. This study tested whether repeated stress causes an increase in excitatory drive of basal amygdala (BA) neurons in vivo, and whether this is correlated with an increase in the number of dendritic spines and a shift in dendritic distribution. Using in vivo intracellular recordings, this study found that repeated restraint stress caused an increase in the frequency of spontaneous excitatory synaptic events in vivo, which correlated with the number of dendritic spines in reconstructed neurons. Furthermore, parallel changes in the kinetics of the synaptic events and the distribution of spines indicated a more prominent functional contribution of synaptic inputs from across the dendritic tree. The shift in spine distribution across the dendritic tree was further confirmed with the examination of Golgi-stained tissue. This abnormal physiological drive of BA neurons after repeated stress may contribute to heightened affective responses after chronic stress. A reduction in the impact of excitatory drive in the BA may therefore be a potential treatment for the harmful effects of chronic stress in psychiatric disorders.

Single-unit activity in the medial prefrontal cortex during immediate and delayed extinction of fear in rats

Delivering extinction trials minutes after fear conditioning yields only a short-term fear suppression that fully recovers the following day. Because extinction has been reported to increase CS-evoked spike firing and spontaneous bursting in the infralimbic (IL) division of the medial prefrontal cortex (mPFC), we explored the possibility that this immediate extinction deficit is related to altered mPFC function. Single-units were simultaneously recorded in rats from neurons in IL and the prelimbic (PrL) division of the mPFC during an extinction session conducted 10 minutes (immediate) or 24 hours (delayed) after auditory fear conditioning. In contrast to previous reports, IL neurons exhibited CS-evoked responses early in extinction training in both immediate and delayed conditions and these responses decreased in magnitude over the course of extinction training. During the retention test, CS-evoked firing in IL was significantly greater in animals that failed to acquire extinction. Spontaneous bursting during the extinction and test sessions was also different in the immediate and delayed groups. There were no group differences in PrL activity during extinction or retention testing. Alterations in both spontaneous and CS-evoked neuronal activity in the IL may contribute to the immediate extinction deficit.

Selective participation of the bed nucleus of the stria terminalis and CRF in sustained anxiety-like versus phasic fear-like responses

The medial division of the central nucleus of the amygdala (CeA(M)) and the lateral division of the bed nucleus of the stria terminalis (BNST(L)) are closely related. Both receive projections from the basolateral amygdala (BLA) and both project to brain areas that mediate fear-influenced behaviors. In contrast to CeA(M) however, initial attempts to implicate the BNST in conditioned fear responses were largely unsuccessful. More recent studies have shown that the BNST does participate in some types of anxiety and stress responses. Here, we review evidence suggesting that the CeA(M) and BNST(L) are functionally complementary, with CeA(M) mediating short- but not long-duration threat responses (i.e., phasic fear) and BNST(L) mediating long- but not short-duration responses (sustained fear or 'anxiety'). We also review findings implicating the stress-related peptide corticotropin-releasing factor (CRF) in sustained but not phasic threat responses, and attempt to integrate these findings into a neural circuit model which accounts for these and related observations.

Differential effects of the CRF-R1 antagonist GSK876008 on fear-potentiated, light- and CRF-enhanced startle suggest preferential involvement in sustained vs phasic threat responses

The amplitude of the acoustic startle response is increased when elicited in the presence of brief cues that predict shock (fear-potentiated startle) and also when elicited during sustained exposure to bright light (light-enhanced startle). Although both effects are thought to reflect fear or anxiety, their neuroanatomical substrates differ. Although fear-potentiated startle is disrupted by reversible inactivation of the central nucleus of the amygdala (CeA) but not the closely related bed nucleus of the stria terminalis (BNST), light-enhanced startle is disrupted by BNST inactivation but not by CeA inactivation. Intraventricular infusions of corticotropin-releasing factor (CRF) also increase startle (CRF-enhanced startle) and this effect is mediated by CRF receptors within the BNST, with no involvement of the CeA. Together, these observations suggest that CeA- and BNST-dependent fear and anxiety may be differentially sensitive to CRF receptor blockade. We tested this by orally administering the novel, potent, and selective CRF-R1 antagonist GSK876008 to rats before CRF-enhanced, light-enhanced, or fear-potentiated startle testing. GSK876008 disrupted CRF-enhanced startle with a linear dose-response curve, and light-enhanced startle with a U-shaped dose-response curve, but did not disrupt fear-potentiated startle to a visual stimulus at any dose tested, and even augmented the response in some animals. GSK876008 also disrupted shock-related 'baseline' startle increases, which may have reflected context conditioning (shown elsewhere to also be BNST-dependent). Overall, these results suggest that short-duration CeA-dependent threat responses can be pharmacologically dissociated from longer duration BNST-dependent responses in terms of their sensitivity to CRF1 receptor antagonists.

Roles of the endocannabinoid system in learning and memory

The endocannabinoid system (ECS) plays a central role in the regulation of learning and memory processes. The fine-tuned regulation of neural transmission by the system is likely to be the mechanism underlying this important function. In this chapter, we review the data in the literature showing the direct involvement of the physiological activation of cannabinoid receptors in the modulation of different forms of learning and memory. When possible, we also address the likely mechanisms of this involvement. Finally, given the apparent special role of the ECS in the extinction of fear, we propose a reasonable model to assess how neuronal networks could be influenced by the endocannabinoids in these processes. Overall, the data reviewed indicate that, despite the enormous progress of recent years, much is still to be done to fully elucidate the mechanisms of the ECS influence on learning and memory processes.

Functional connectivity of the human amygdala using resting state fMRI

The amygdala is composed of structurally and functionally distinct nuclei that contribute to the processing of emotion through interactions with other subcortical and cortical structures. While these circuits have been studied extensively in animals, human neuroimaging investigations of amygdala-based networks have typically considered the amygdala as a single structure, which likely masks contributions of individual amygdala subdivisions. The present study uses resting state functional magnetic resonance imaging (fMRI) to test whether distinct functional connectivity patterns, like those observed in animal studies, can be detected across three amygdala subdivisions: laterobasal, centromedial, and superficial. In a sample of 65 healthy adults, voxelwise regression analyses demonstrated positively-predicted ventral and negatively-predicted dorsal networks associated with the total amygdala, consistent with previous animal and human studies. Investigation of individual amygdala subdivisions revealed distinct differences in connectivity patterns within the amygdala and throughout the brain. Spontaneous activity in the laterobasal subdivision predicted activity in temporal and frontal regions, while activity in the centromedial nuclei predicted activity primarily in striatum. Activity in the superficial subdivision positively predicted activity throughout the limbic lobe. These findings suggest that resting state fMRI can be used to investigate human amygdala networks at a greater level of detail than previously appreciated, allowing for the further advancement of translational models.

Attenuation of cue-induced heroin-seeking behavior by cannabinoid CB1 antagonist infusions into the nucleus accumbens core and prefrontal cortex, but not basolateral amygdala

As with other drugs of abuse, heroin use is characterized by a high incidence of relapse following detoxification that can be triggered by exposure to conditioned stimuli previously associated with drug availability. Recent findings suggest that cannabinoid CB(1) receptors modulate the motivational properties of heroin-conditioned stimuli that induce relapse behavior. However, the neural substrates through which CB(1) receptors modulate cue-induced heroin seeking have not been elucidated. In this study, we evaluated alterations in cue-induced reinstatement of heroin-seeking behavior produced by infusions of the CB(1) receptor antagonist SR 141716A (0, 0.3 and 3 microg per side) delivered into the prefrontal cortex (PFC), nucleus accumbens (NAC), and basolateral amygdala (BLA) of rats. Results show that following extinction of operant behavior the presentation of a discriminative stimulus conditioned to heroin availability reinstated nonreinforced lever pressing to levels comparable to preextinction levels. Intra-PFC SR 141716A dose-dependently reduced cue-induced reinstatement of heroin seeking, with a significant reduction following the 3 microg per side dose. In the NAC, both SR 141716A doses induced a significant reduction in cue-induced reinstatement, with the highest dose completely blocking the effect of the cue. In contrast, intra-BLA SR 141716A did not alter cue-induced reinstatement of responding while systemic administration of this antagonist (3 mg/kg, i.p.) significantly blocked cue-induced reinstatement in all three-placement groups (BLA, PFC, and NAC). These findings provide new insights into the neural mechanisms through which CB(1) receptors modulate the motivational properties of heroin-associated cues inducing relapse.

N150 in amygdalar ERPs in the rat: Is there modulation by anticipatory fear?

The hypothesis was tested whether the amygdalar N150 of rats, a slow, negative component in the event-related potential from the lateral amygdala, is sensitive to a state of anxious anticipation. A conditioning procedure was applied in which a series of six auditory stimuli was followed by a shock when presented alone, but not when the auditory stimuli were preceded by a visual stimulus. Heart rate recordings confirmed that the auditory stimulus train induced a state of increasing anticipatory fear and that this condition was modulated by the visual stimulus. During behavioral training, a N150 appeared in the amygdalar event-related potential evoked by the auditory stimuli, replicating previous findings. However, the amplitude of the N150 was not affected by whether or not the visual stimulus had been presented before. These results failed to support the idea that the N150 is related to the expectancy of an aversive event. An alternative interpretation, emphasizing the increase in arousal and attention that is inherent to aversive learning, is discussed.

Response properties of human amygdala subregions: evidence based on functional MRI combined with probabilistic anatomical maps

The human amygdala is thought to play a pivotal role in the processing of emotionally significant sensory information. The major subdivisions of the human amygdala—the laterobasal group (LB), the superficial group (SF), and the centromedial group (CM)—have been anatomically delineated, but the functional response properties of these amygdala subregions in humans are still unclear. We combined functional MRI with cyto-architectonically defined probabilistic maps to analyze the response characteristics of amygdala subregions in subjects presented with auditory stimuli. We found positive auditory stimulation-related signal changes predominantly in probabilistically defined LB, and negative responses predominantly in SF and CM. In the left amygdala, mean response magnitude in the core area of LB with 90–100% assignment probability was significantly larger than in the core areas of SF and CM. These differences were observed for pleasant and unpleasant stimuli. Our findings reveal that the probabilistically defined anatomical subregions of the human amygdala show distinctive fMRI response patterns. The stronger auditory responses in LB as compared with SF and CM may reflect a predominance of auditory inputs to human LB, similar to many animal species in which the majority of sensory, including auditory, afferents project to this subdivision of the amygdala. Our study indicates that the intrinsic functional differentiation of the human amygdala may be probed using fMRI combined with probabilistic anatomical maps.

Neural correlates of a ‘pessimistic’ attitude when anticipating events of unknown emotional valence

Since we do not know what future holds for us, we prepare for expected emotional events in order to deal with a pleasant or threatening environment. From an evolutionary perspective, it makes sense to be particularly prepared for the worst-case scenario. We were interested to evaluate whether this assumption is reflected in the central nervous information processing associated with expecting visual stimuli of unknown emotional valence. While being scanned with functional magnetic resonance imaging, healthy subjects were cued to expect and then perceive visual stimuli with a known emotional valence as pleasant, unpleasant, and neutral, as well as stimuli of unknown valence that could have been either pleasant or unpleasant. While anticipating pictures of unknown valence, the activity of emotion processing brain areas was similar to activity associated with expecting unpleasant pictures, but there were no areas in which the activity was similar to the activity when expecting pleasant pictures. The activity of the revealed regions, including bilateral insula, right inferior frontal gyrus, medial thalamus, and red nucleus, further correlated with the individual ratings of mood: the worse the mood, the higher the activity. These areas are supposedly involved in a network for internal adaptation and preparation processes in order to act according to potential or certain unpleasant events. Their activity appears to reflect a 'pessimistic' bias by anticipating the events of unknown valence to be unpleasant.

Mice lacking alpha1,3-fucosyltransferase IX demonstrate disappearance of Lewis x structure in brain and increased anxiety-like behaviors

The 3-fucosyl-N-acetyllactosamine [Lewis x (Le(x)), CD15, SSEA-1] carbohydrate structure is expressed on several glycolipids, glycoproteins, and proteoglycans of the nervous system and has been implicated in cell-cell recognition, neurite outgrowth, and neuronal migration during development. To characterize the functional role of Le(x) carbohydrate structure in vivo, we have generated mutant mice that lack alpha1,3-fucosyltransferase IX (Fut9(-/-)). Fut9(-/-) mice were unable to synthesize the Le(x) structure carried on glycoproteins and glycolipids in embryonic and adult brain. However, no obvious pathological differences between wild-type and Fut9(-/-) mice were found in brain. In behavioral tests, Fut9(-/-) mice exhibited increased anxiety-like responses in dark-light preference and in elevated plus maze tests. Immunohistochemical analysis showed that the number of calbindin-positive neurons was decreased in the basolateral amygdala in Fut9(-/-) mice. These observations indicated that the carbohydrates synthesized by Fut9 play critical roles in functional regulations of interneurons in the amygdalar subdivisions and suggested a role for the Le(x) structure in some aspects of emotional behavior in mice.

Distribution of serotonin transporter labeled fibers in amygdaloid subregions: implications for mood disorders

BACKGROUND

The serotonin transporter 5-HTT mediates responses to serotonin reuptake inhibitors (SSRIs), a mainstay treatment in mood disorders. The amygdala, a key emotional processing center, has functional abnormalities in mood disorders, which resolve following successful SSRI treatment. To better understand the effects of SSRIs in mood disorders, we examined the distribution of 5-HTT labeled fibers relative to specific nuclear groups in the amygdala.

METHODS

Immunocytochemical techniques were used to chart 5-HTT labeled fibers in the amygdala in coronal sections through the brain of six adult Macaques. Nissl staining was used to define nuclear groups in the amygdala.

RESULTS

The serotonin transporter 5-HTT is distributed heterogeneously in the primate amygdala, with the lateral subdivision of the central nucleus, intercalated cell islands, amygdalohippocampal area, and the paralaminar nucleus showing the heaviest concentrations.

CONCLUSIONS

5HTT-labeled fibers are very densely concentrated in output regions of the amygdala. High concentrations of 5-HTT-positive fibers in the central nucleus indicate that tight regulation of serotonin is critical in modulating fear responses mediated by this nucleus. High concentrations of 5-HTT-labeled fibers in the intercalated islands and parvicellular basal nucleus/paralaminar nucleus, which contain immature -appearing neurons, suggest a potential trophic role for serotonin in these subregions.

Fear extinction in rats: Implications for human brain imaging and anxiety disorders

Fear extinction is the decrease in conditioned fear responses that normally occurs when a conditioned stimulus (CS) is repeatedly presented in the absence of the aversive unconditioned stimulus (US). Extinction does not erase the initial CS-US association, but is thought to form a new memory. After extinction training, extinction memory competes with conditioning memory for control of fear expression. Deficits in fear extinction are thought to contribute to post-traumatic stress disorder (PTSD). Herein, we review studies performed in rats showing that the medial prefrontal cortex plays a critical role in the retention and expression of extinction memory. We also review human studies indicating that prefrontal areas homologous to those critical for extinction in rats are structurally and functionally deficient in patients with PTSD. We then discuss how findings from rat studies may allow us to: (1) develop new fear extinction paradigms in humans, (2) make specific predictions as to the location of extinction-related areas in humans, and (3) improve current extinction-based behavioral therapies for anxiety disorders.

Opposing influence of basolateral amygdala and footshock stimulation on neurons of the central amygdala

BACKGROUND

The basolateral complex (BLA) and the central nucleus of the amygdala (CeA) are believed to mediate the expression of affective responses. After affective learning, conditioned stimulus-related information is thought to be conveyed from the BLA to the CeA; the medial CeA (Cem), in turn, projects to hypothalamic and brainstem structures involved with induction of affective responses. Although the conditioned stimulus and unconditioned stimulus both evoke affective responses, the precise response often differs. It is unknown whether this difference is represented by distinct activity patterns of single Cem neurons. Furthermore, the nature of the interaction between the BLA and Cem is unknown.

METHODS

Using in vivo extracellular and intracellular recordings, we examined how the BLA affects the Cem and compared this with effects induced by footshock (unconditioned stimulus) in the same neurons.

RESULTS

Our results demonstrate that, contrary to conventional views, BLA stimulation primarily inhibits Cem neurons by a polysynaptic circuit, and show that single Cem neurons respond to both BLA input and footshock in an opposite manner.

CONCLUSIONS

These results demonstrate the predominantly inhibitory nature of the BLA-Cem interaction. These data further demonstrate the distinct cellular events that might lead to differential modulation of conditioned and unconditioned affective responses.

Localization of Id2 mRNA in the adult mouse brain

Id proteins are negative regulators of basic helix-loop-helix transcription factors and are involved in cellular differentiation and proliferation. Four members of the Id gene family exhibit closely related but distinct expression patterns in various mammalian organs of not only embryos but also adults. Among them, Id2 is known to be expressed in Purkinje cells and neurons in the cortical layers of the adult mouse brain, suggesting that Id2 is involved in some neural functions in the adult. To get insight into the role of Id2 in the nervous system, we investigated the localization of Id2 mRNA-expressing cells in the adult mouse brain in detail by in situ hybridization with the radiolabeled antisense probe and compared it with the localization of other Id gene family members. The results indicated that Id2 mRNA is detected in more varied brain regions than previously reported. These regions include the amygdaloid complex, caudate putamen, globus pallidus, substantia nigra pars reticulata, suprachiasmatic nucleus, and the anterior part of the subventricular zone. These results suggest the possibility that Id2 plays a role in the neural activity and cognitive functions. On the other hand, Id1 was barely detectable. Although moderate or low expression of Id3 was observed diffusely, high expression was observed in some specific regions including the molecular layer of the dentate gyrus and the external capsule. Id4 mRNA was detected in the regions such as the caudate putamen and the lateral amygdaloid nucleus. Thus, the expression pattern of Id2 is distinct from those of other Id gene family members.

Cellular processes in the amygdala: gates to emotional memory?

The amygdala is considered a core structure of the so-called limbic system and has been implicated in a variety of functions, including emotional interpretation of sensory information, emotional arousal, emotional memory, fear and anxiety, and related clinical disorders. Despite the clinical and functional importance of the amygdala, it is only recently that some general principles of intra-amygdaloid mechanisms of signal processing that are relevant for fear behavior and memory have emerged from behavioral, anatomical, electrophysiological, and neurochemical studies performed in the amygdala of various mammalian species in vivo, in situ and in vitro.

Chronic cold stress alters prefrontal cortical modulation of amygdala neuronal activity in rats

BACKGROUND

Recent studies suggest that long-term exposure to stress can sensitize animals to subsequent novel or acute stressors. Stressors affect amygdala activity, and the prefrontal cortex has been implicated in the regulation of responses to stress. Little is known, however, about how the physiology of amygdala neurons is altered by chronic stressors or the role of the prefrontal cortex in these changes.

METHODS

We used in vivo extracellular recordings from neurons in the rat central and basolateral amygdala nuclei to examine the effects of chronic stress on the basal firing and responses of amygdala neurons to a novel stressor. Additionally, prefrontal cortical afferents were severed to examine its role in the modulation of the response to stressors.

RESULTS

Chronic exposure to cold enhanced the sensitivity of central amygdala neurons to footshock. A portion of this may be due to enhanced basolateral amygdala output. Furthermore, prefrontal cortical regulation of this response is weakened by chronic stress.

CONCLUSIONS

The physiology of the amygdala is altered by chronic stress. Furthermore, the prefrontal cortical regulation of these responses may be weakened after chronic stress. This is a potential biological substrate for abnormal affect upon chronic stress and its effect on affective disorders.

Mu-opioid-mediated inhibition of glutamate synaptic transmission in rat central amygdala neurons

The central nucleus of the amygdala (CeA) plays an important role both in stimulus-reward learning for the reinforcing effects of drugs of abuse and in environmental condition-induced analgesia. Both of these two CeA functions involve the opioid system within the CeA. However, the pharmacological profiles of its opioid receptor system have not been fully studied and the synaptic actions of opioid receptors in the CeA are largely unknown. In this study with whole-cell voltage-clamp recordings in brain slices in vitro, we examined actions of opioid agonists on glutamate-mediated excitatory postsynaptic currents (EPSCs) in CeA neurons. Opioid peptide methionine-enkephalin (ME; 10 microM) produced a significant inhibition (38%) in the amplitude of evoked EPSCs, an action mimicked by the mu-opioid receptor agonist [D-Ala(2),N-MePhe(4),Gly-ol(5)]-enkephalin (DAMGO; 1 microM, 44%). Both effects of ME and DAMGO were abolished by the mu receptor antagonist CTAP (1 microM), suggesting a mu receptor-mediated effect. Neither delta-opioid receptor agonist [D-Pen(2),D-Pen(5)]-enkephalin (1 microM) nor kappa-opioid receptor agonist U69593 (300 nM) had any effect on the glutamate EPSC. ME significantly increased the paired-pulse ratio of the evoked EPSCs and decreased the frequency of miniature EPSCs without altering the amplitude of miniature EPSCs. Furthermore, the mu-opioid inhibition of the EPSC was blocked by 4-aminopyridine (4AP; 100 microM), a voltage-dependent potassium channel blocker, and by phospholipase A(2) inhibitors AACOCF(3) (10 microM) and quinacrine (10 microM). These results indicate that only the mu-opioid receptor is functionally present on presynaptic glutamatergic terminals in normal CeA neurons, and its activation reduces the probability of glutamate release through a signaling pathway involving phospholipase A(2) and the presynaptic, 4AP-sensitive potassium channel. This study provides evidence for the presynaptic regulation of glutamate synaptic transmission by mu-opioid receptors in CeA neurons.

Activation of nicotinic acetylcholine receptors increases the frequency of spontaneous GABAergic IPSCs in rat basolateral amygdala neurons

The basolateral amygdala (BLA) is a critical component of the amygdaloid circuit, which is thought to be involved in fear conditioned responses. Using whole cell patch-clamp recording, we found that activation of nicotinic acetylcholine receptors (nAChRs) leads to an action potential-dependent increase in the frequency of spontaneous GABAergic currents in principal neurons in the BLA. These spontaneous GABAergic currents were abolished by a low-Ca2+/high-Mg2+ bathing solution, suggesting that they are spontaneous inhibitory postsynaptic currents (sIPSCs). Blockade of ionotropic glutamate receptors did not prevent this increased frequency of sIPSCs nor did blockade of alpha7 nAChRs. Among the nAChR agonists tested, cystisine was more effective at increasing the frequency of the sIPSCs than nicotine or 1,1-dimethyl-4-phenyl piperazinium iodide, consistent with a major contribution of beta4 nAChR subunits. The nicotinic antagonist, dihydro-beta-erythroidine, was less effective than d-tubocurarine in blocking the increased sIPSC frequency induced by ACh, suggesting that alpha4-containing nAChR subunits do not play a major role in the ACh-induced increased sIPSC frequency. Although alpha2/3/4/7 and beta2/4 nAChR subunits were found in the BLA by RT-PCR, the agonist and antagonist profiles suggest that the ACh-induced increase in sIPSC frequency involves predominantly alpha3beta4-containing nAChR subunits. Consistent with this, alpha-conotoxin-AuIB, a nAChR antagonist selective for the alpha3beta4 subunit combination, inhibited the ACh-induced increase in the frequency of sIPSCs. The observations suggest that nicotinic activation increases the frequency of sIPSCs in the BLA by acting mainly on alpha3beta4-containing nicotinic receptors on GABAergic neurons and may play an important role in the modulation of synaptic transmission in the amygdala.

Inhibitory avoidance learning altered ensemble activity of amygdaloid neurons in rats

In this study, we examined single-unit activity in the amygdala before and after a rat had acquired an inhibitory avoidance task. Long-Evans rats with microwires chronically implanted into the central nucleus (CeA) or basolateral complex (BLC) of the amygdala were acclimatized to the apparatus of a step-through inhibitory avoidance task for three sessions. On the fourth session, rats in the experimental group received an inescapable footshock (3 mA, 1 s) as they stepped from the lit side into the dark side of the task apparatus, whereas rats in the control group received the same amount of shock on a different apparatus. All rats were tested for retention in the task apparatus 1 day after shock training. The experimental rats showed better retention than the controls as they stayed longer in the lit side. Ensemble unit activities were recorded in the amygdala nuclei from the indwelling wire bundles during the acclimation and test sessions. The data collected from well-isolated amygdala units showed that neuronal discharge habituated from the first to the third acclimation session. In the test session, the experimental group, but not the control group, showed elevated firing rates in the CeA or BLC neurons located on either side of the brain. These findings provide the first piece of evidence showing that learning of an inhibitory avoidance task leads to an increase in amygdala neuronal discharges during a retention test.

Lasting increases in basolateral amygdala activity after emotional arousal: implications for facilitated consolidation of emotional memories

Manipulations that reduce or enhance the activity of basolateral amygdala (BLA) neurons in the minutes to hours after training have been shown to respectively impair or facilitate retention on the inhibitory avoidance task. Although this suggests that BLA activity is altered after emotional arousal, such changes have not been directly demonstrated. To test this, we devised a feline analog of the inhibitory avoidance task and recorded BLA unit activity before and after a single inescapable footshock. Single-unit recordings revealed that the firing rate of many BLA neurons gradually increased after the footshock, peaking 30-50 min post-shock and then subsiding to baseline levels 2 h later. During this period of increased activity, the discharges of simultaneously recorded BLA cells were more synchronized than before the shock. Although it was known that pairing innocuous (conditioned stimulus, CS) and noxious stimuli modifies the responsiveness of BLA neurons to the CS, our results constitute the first demonstration that emotional arousal produces lasting increases in the spontaneous firing rates of BLA neurons. We propose that these changes in BLA activity may promote Hebbian interactions between coincident but spatially distributed activity patterns in BLA targets, facilitating the consolidation of emotional memories.

A gene for neuronal plasticity in the mammalian brain: Zif268/Egr-1/NGFI-A/Krox-24/TIS8/ZENK?

Zif268 is a transcription regulatory protein, the product of an immediate early gene. Zif268 was originally described as inducible in cell cultures; however, it was later shown to be activated by a variety of stimuli, including ongoing synaptic activity in the adult brain. Recently, mice with experimentally mutated zif268 gene have been obtained and employed in neurobiological research. In this review we present a critical overview of Zif268 expression patterns in the naive brain and following neuronal stimulation as well as functional data with Zif268 mutants. In conclusion, we suggest that Zif268 expression and function should be considered in a context of neuronal activity that is tightly linked to neuronal plasticity.

Synaptic properties and postsynaptic opioid effects in rat central amygdala neurons

An important output of amygdaloid nuclei, the central nucleus of the amygdala (CeA) not only mediates negative emotional behaviors, but also participates in the stimulus-reward learning and expression of motivational aspects of many drugs of abuse, and links environmentally stressful conditions such as fear to endogenous pain-inhibiting mechanisms. The endogenous opioid system in the CeA is crucial for both reward behaviors and environmental stress-induced analgesia. In this study using whole-cell voltage-clamp recordings, we investigated synaptic inputs and the postsynaptic effects of opioid agonists in CeA neurons. We found that synaptic inputs evoked within the CeA were mediated by both glutamate and GABA, but those evoked from the basolateral amygdala were primarily glutamatergic. Based on membrane properties, three types of cells were characterized. Type A neurons had no spike accommodation while type B neurons displayed characteristic accommodating response. Type A neurons were further classified as either A1 or A2, based on differences in resting membrane potential and the amplitude of after-hyperpolarizing potential. micro-Opioid receptor agonists hyperpolarized a subpopulation of CeA neurons, of which the vast majority was type A1. This micro agonist-induced hyperpolarization was mediated by the opening of inwardly rectifying potassium channels. In contrast, the kappa-opioid receptor agonist hyperpolarized only type B neurons. These results illustrate three types of CeA neurons with distinctive membrane properties and differential responses to opioid agonists. They may represent functionally distinct CeA cell groups for the integration and execution of CeA outputs in the aforementioned CeA functions.

Auditory-conditioned-fear-dependent c-Fos expression is altered in the emotion-related brain structures of Fyn-deficient mice

Fyn-tyrosine-kinase-deficient mice exhibit increased fearfulness. To elucidate the neural mechanisms of their emotional defects, we compared fyn(-/-) and fyn(+/-) mice by behavioral analysis of conditioned fear and by functional neuroanatomical analysis of the distribution of highly responsive neurons associated with conditioned fear. The mice were exposed to the auditory conditioned stimulus paired with electric shock as the unconditioned stimulus. After the fear conditioning, auditory stimulus-induced freezing behavior was enhanced in fyn(-/-) mice. When the occurrence of c-Fos-immunoreactive neurons in the brain of fear-conditioned mice was examined following exposure to the auditory stimulus, a significant increase in immunoreactive neurons was found in the amygdala, hypothalamus, and midbrain of both genotypes. The occurrence of conditioned-fear-dependent c-Fos-immunoreactive neurons was enhanced in the central, medial, cortical, and basomedial amygdaloid subdivisions, the hypothalamic nuclei, and the midbrain periaqueductal gray of the fyn(-/-) mice in comparison with the fyn(+/-) mice. However, remarkably, the occurrence of conditioned-fear-dependent c-Fos-immunoreactive neurons was very low in the basolateral and lateral amygdaloid subdivisions of the fyn(-/-) mice, in striking contrast to a significant increase in c-Fos-immunoreactive neurons in these subdivisions in the fyn(+/-) mice. These findings suggest that the increased excitability of the specific amygdaloid subdivisions including the central nucleus, and of the projection targets such as the hypothalamus and midbrain in fyn(-/-) mice, is directly related to the enhanced fear response, and that the decreased excitability in the basolateral and lateral amygdaloid subdivisions is involved in the defective control of the neural circuit for emotional expression in this mutant.

Emotional perseveration: an update on prefrontal-amygdala interactions in fear extinction

Fear extinction refers to the ability to adapt as situations change by learning to suppress a previously learned fear. This process involves a gradual reduction in the capacity of a fear-conditioned stimulus to elicit fear by presenting the conditioned stimulus repeatedly on its own. Fear extinction is context-dependent and is generally considered to involve the establishment of inhibitory control of the prefrontal cortex over amygdala-based fear processes. In this paper, we review research progress on the neural basis of fear extinction with a focus on the role of the amygdala and the prefrontal cortex. We evaluate two competing hypotheses for how the medial prefrontal cortex inhibits amygdala output. In addition, we present new findings showing that lesions of the basal amygdala do not affect fear extinction. Based on this result, we propose an updated model for integrating hippocampal-based contextual information with prefrontal-amygdala circuitry.

Low-Probability Transmission of Neocortical and Entorhinal Impulses Through the Perirhinal Cortex

One model of episodic memory posits that during slow-wave sleep (SWS), the synchronized discharges of hippocampal neurons in relation to sharp waves “replay” activity patterns that occurred during the waking state, facilitating synaptic plasticity in the neocortex. Although evidence of replay was found in the hippocampus in relation to sharp waves, it was never shown that this activity reached the neocortex. Instead, it was assumed that the rhinal cortices faithfully transmit information from the hippocampus to the neocortex and reciprocally. Here, we tested this idea using 3 different approaches. 1) Stimulating electrodes were inserted in the entorhinal cortex and temporal neocortex and evoked unit responses were recorded in between them, in the intervening rhinal cortices. In these conditions, impulse transfer occurred with an extremely low probability, in both directions. 2) To rule out the possibility that this unreliable transmission resulted from the artificial nature of electrical stimuli, crosscorrelation analyses of spontaneous neocortical, perirhinal, and entorhinal firing were performed in unanesthetized animals during the states of waking and SWS. Again, little evidence of propagation could be obtained in either state. 3) To test the idea that propagation occurs only when large groups of neurons are activated within a narrow time window, we computed perievent histograms of neocortical, perirhinal, and entorhinal neuronal discharges around large-amplitude sharp waves. However, these synchronized entorhinal discharges also failed to propagate across the perirhinal cortex. These findings suggest that the rhinal cortices are more than a relay between the neocortex and hippocampus, but rather a gate whose properties remain to be identified.

Interhemispheric involvement of the anterior cortical nuclei of the amygdala in rewarding brain stimulation

The amygdaloid complex is one of the structures thought to modulate brain stimulation reward (BSR) elicited from the median forebrain bundle (MFB). Previous metabolic and behavioral data from our laboratory point to the amygdaloid cortical nuclei as key to this process. In this study, thresholds for rewarding stimulation of the MFB were determined for 42 days, 21 days following an electrolytic lesion to amygdaloid nuclei ipsilateral to the stimulation electrode, and 21 days following one applied to the contralateral amygdala. A subset of animals showed post-lesion changes in MFB frequency thresholds that were maintained if not augmented after the second lesion. These ranged from 26% to 150% compared to baseline values, among the largest ever reported to our knowledge. Interestingly, damage to anterior sites within the cortical nuclei was the most effective in producing modifications to the rewarding value of the stimulation. Equally singular was the finding that contralateral lesions tended to alter thresholds more than ipsilateral ones, confirming our earlier finding of interhemispheric connectivity in amygdaloid modulation of MFB reward signals. This interpretation was substantiated by tracking long-term metabolic activity in the amygdala using cytochrome oxidase histochemistry. The density of reaction product at damaged amygdala sites was negatively correlated (r=-0.90) with the increases in thresholds obtained at contralateral MFB loci. Together with the fact that such large lesion effects are seldom obtained, our metabolic results point to the existence of a relationship between these nuclei and reward signals generated at the MFB. Moreover, our data suggest that this communication takes place interhemispherically.

Genes and mechanisms in the amygdala involved in the formation of fear memory

With a combined molecular, electrophysiological, and behavioral approach we have sought to correlate conditioned fear behavior with electrophysiological activities in the lateral amygdala and hippocampal formation in mice and rats and to determine the potential contribution of effector genes that are expressed in the basolateral amygdaloid complex during the late phase of pavlovian fear conditioning. Our data indicate that resonant/oscillatory electrical activity in projection neurons of the lateral amygdala provide an important cellular element of coherent theta activity in amygdalohippocampal pathways, which may represent a nondiscriminating neural correlate of conditioned fear. Correlated activity seems to contribute to the formation of synaptic plasticity in these networks, such as input-specific long-term depression of thalamoamygdaloid signals and consolidation of long-term potentiation in the dentate gyrus. Moreover, associative fear conditioning results in selective gene expression in the basolateral amygdaloid complex, involving molecular factors of structural reorganization and signal transduction, particularly GABA function, supporting the view that the amygdala is a site of neural plasticity and information storage during formation of fear memory.

Contextual inhibitory gating of impulse traffic in the intra-amygdaloid network

New data on the organization of the intra-amygdaloid circuit is reviewed, beginning with the basolateral (BL) complex, the main input station of the amygdala for sensory afferents, and concluding with the central (CE) nucleus, an important source of projections to brain-stem structures mediating fear responses. The BL complex is endowed with a highly divergent system of intrinsic glutamatergic connections. Yet, BL projection cells have unusually low firing rates. This apparent contradiction is explained by the presence of powerful inhibitory pressures in the BL amygdala: (1) interneurons that generate large-amplitude inhibitory synaptic potentials and (2) projection cells that express a Ca(2+)-dependent K(+) current that can be activated by subthreshold synaptic inputs. Likewise, excitatory projections from the BL amygdala to the CE nucleus are controlled by clusters of GABAergic neurons, termed the intercalated (ITC) cell masses. In response to BL inputs, ITC cells generate feedforward inhibition in CE neurons. However, ITC neurons exhibit properties that allow them to modify the amount of inhibition they generate depending on the distribution of BL activity in space and time. Indeed, ITC cell masses can inhibit each other via lateromedial connections. Moreover, they express an unusual K(+) conductance that modifies their response to BL inputs depending on their recent firing history. Thus, inhibitory mechanisms of the amygdala allow for flexible, context-dependent gating of BL impulses to the CE nucleus.

The endogenous cannabinoid system controls extinction of aversive memories

Acquisition and storage of aversive memories is one of the basic principles of central nervous systems throughout the animal kingdom. In the absence of reinforcement, the resulting behavioural response will gradually diminish to be finally extinct. Despite the importance of extinction, its cellular mechanisms are largely unknown. The cannabinoid receptor 1 (CB1) and endocannabinoids are present in memory-related brain areas and modulate memory. Here we show that the endogenous cannabinoid system has a central function in extinction of aversive memories. CB1-deficient mice showed strongly impaired short-term and long-term extinction in auditory fear-conditioning tests, with unaffected memory acquisition and consolidation. Treatment of wild-type mice with the CB1 antagonist SR141716A mimicked the phenotype of CB1-deficient mice, revealing that CB1 is required at the moment of memory extinction. Consistently, tone presentation during extinction trials resulted in elevated levels of endocannabinoids in the basolateral amygdala complex, a region known to control extinction of aversive memories. In the basolateral amygdala, endocannabinoids and CB1 were crucially involved in long-term depression of GABA (gamma-aminobutyric acid)-mediated inhibitory currents. We propose that endocannabinoids facilitate extinction of aversive memories through their selective inhibitory effects on local inhibitory networks in the amygdala.

Uniform range of conduction times from the lateral amygdala to distributed perirhinal sites

Much data indicate that the perirhinal (PRH) cortex plays a critical role in declarative memory and that the amygdala facilitates this process under emotionally arousing conditions. However, assuming that the amygdala does so by promoting Hebbian interactions in the PRH cortex is hard to reconcile with the fact that variable distances separate amygdala neurons from their PRH projection sites. Indeed, to achieve a synchronized activation of distributed PRH sites, amygdala axons should display a uniform range of conduction times, irrespective of distance to target. To determine if amygdala axons meet this condition, we measured the antidromic response latencies of lateral amygdala (LA) neurons to electrical stimuli delivered at various rostrocaudal levels of the PRH cortex in cats anesthetized with isoflurane. Although large variations in antidromic response latencies were observed, they were unrelated to the distance between the PRH stimulation sites and LA neurons. To determine whether this result was an artifact due to current spread, two control experiments were performed. First, we examined the antidromic response latency of intrinsic PRH neurons. Although we used the same methods as in the first experiment, the antidromic response latency of PRH neurons to electrical stimuli applied in the PRH cortex increased linearly with the distance between the stimulating and recording sites. Second, we measured the antidromic response latency of PRH neurons projecting to the LA. In this pathway, we also found a statistically significant correlation between conduction times and distance to target. Thus these results support the intriguing possibility that the conduction velocity and/or trajectory of LA axons are adjusted to compensate for variations in distance between the LA and distinct rostrocaudal PRH sites. We hypothesize that because of their uniform range of conduction times to the PRH cortex, LA neurons can generate short time windows of depolarization facilitating Hebbian associations between coincident, but spatially distributed, activity patterns in the PRH cortex. In this context, the temporal scatter of conduction times in the LA to PRH pathway is conceived as a mechanism used to lengthen the period of depolarization to compensate for conduction delays within intrinsic PRH pathways. In part, this mechanism might explain how the amygdala promotes memory storage in emotionally arousing conditions.

Parallels between cerebellum- and amygdala-dependent conditioning

Recent evidence from cerebellum-dependent motor learning and amygdala-dependent fear conditioning indicates that, despite being mediated by different brain systems, these forms of learning might use a similar sequence of events to form new memories. In each case, learning seems to induce changes in two different groups of neurons. Changes in the first class of cells are induced very rapidly during the initial stages of learning, whereas changes in the second class of cells develop more slowly and are resistant to extinction. So, anatomically distinct cell populations might contribute differentially to the initial encoding and the long-term storage of memory in these two systems.

Distribution of CB1 cannabinoid receptors in the amygdala and their role in the control of GABAergic transmission

Cannabinoids are the most popular illicit drugs used for recreational purposes worldwide. However, the neurobiological substrate of their mood-altering capacity has not been elucidated so far. Here we report that CB1 cannabinoid receptors are expressed at high levels in certain amygdala nuclei, especially in the lateral and basal nuclei, but are absent in other nuclei (e.g., in the central nucleus and in the medial nucleus). Expression of the CB1 protein was restricted to a distinct subpopulation of GABAergic interneurons corresponding to large cholecystokinin-positive cells. Detailed electron microscopic investigation revealed that CB1 receptors are located presynaptically on cholecystokinin-positive axon terminals, which establish symmetrical GABAergic synapses with their postsynaptic targets. The physiological consequence of this particular anatomical localization was investigated by whole-cell patch-clamp recordings in principal cells of the lateral and basal nuclei. CB1 receptor agonists WIN 55,212-2 and CP 55,940 reduced the amplitude of GABA(A) receptor-mediated evoked and spontaneous IPSCs, whereas the action potential-independent miniature IPSCs were not significantly affected. In contrast, CB1 receptor agonists were ineffective in changing the amplitude of IPSCs in the rat central nucleus and in the basal nucleus of CB1 knock-out mice. These results suggest that cannabinoids target specific elements in neuronal networks of given amygdala nuclei, where they presynaptically modulate GABAergic synaptic transmission. We propose that these anatomical and physiological features, characteristic of CB1 receptors in several forebrain regions, represent the neuronal substrate for endocannabinoids involved in retrograde synaptic signaling and may explain some of the emotionally relevant behavioral effects of cannabinoid exposure.

Lesions in the central nucleus of the amygdala: discriminative avoidance learning, discriminative approach learning, and cingulothalamic training-induced neuronal activity

The amygdala is critically involved in discriminative avoidance learning. Large lesions of the amygdala block discriminative avoidance learning and abolish cingulothalamic training-induced neuronal activity. These results indicated that amygdalar processing is critical for cingulothalamic plasticity. The larger lesions did not allow differentiation of the specific functioning of various amygdalar nuclei. Anatomical analysis showed that damage in the central (CE) nucleus of the amygdala was correlated with the severity of the behavioral deficit. The present study was carried out to determine whether smaller lesions, centered in the CE nucleus, would impair discriminative avoidance learning and block cingulothalamic plasticity. In addition, the possible role of the CE nucleus in appetitively motivated discriminative approach learning was examined for the first time. New Zealand White rabbits with CE nuclear lesions were first trained in the discriminative approach task. After attaining asymptotic performance, discriminative avoidance training sessions were alternated with continuing approach training sessions, one session each day. The rabbits with lesions were severely impaired in avoidance learning but showed no impairment of approach learning. Surprisingly, the attenuating effects of the lesions on cingulothalamic training-induced neuronal activity were more prevalent during approach learning than during avoidance learning. These results indicated that avoidance learning can be impaired by lesions centered in the CE nucleus that leave cingulothalamic plasticity largely intact and that the CE nucleus is involved in extra-cingulothalamic learning processes.

Auditory perception of laughing and crying activates human amygdala regardless of attentional state

Adequate behavioral responses to socially relevant stimuli are often impaired after lesions of the amygdala. Such lesions concern especially the recognition of facial and sometimes of vocal expression of emotions. Using low-noise functional magnetic resonance imaging (fMRI), we investigated in which way the amygdala, auditory cortex and insula are involved in the processing of affective nonverbal vocalizations (Laughing and Crying) in healthy humans. The same samples of male and female Laughing and Crying were presented in different experimental conditions: Simply listening to the stimuli, self-induction of the corresponding emotions while listening, and detection of artificial pitch shifts in the same stimuli. All conditions activated the amygdala similarly and bilaterally, whereby the amount of activation was larger in the right amygdala. The auditory cortex was more strongly activated by Laughing than by Crying with a slight right-hemisphere advantage for Laughing, both likely due to acoustic stimulus features. The insula was bilaterally activated in all conditions. The mean signal intensity change with stimulation was much larger in the amygdala than in auditory cortex and insula. The amygdala results seem to be in accordance with the right-hemisphere hypothesis of emotion processing which may not be applicable as strongly to the level of auditory cortex or insula.

N-Methyl-D-aspartate receptor antagonism blocks contextual fear conditioning and differentially regulates early growth response-1 messenger RNA expression in the amygdala: implications for a functional amygdaloid circuit of fear

N-Methyl-D-aspartate receptors in the amygdala are known to be crucial for the learning of conditioned fear, although the molecular cascades that N-methyl-D-aspartate receptors regulate are not well understood. Recent experiments from our laboratory have shown that messenger RNA expression of the immediate-early messenger gene, early growth response gene 1, increases in the lateral nucleus of the amygdala following contextual fear conditioning. However, the regulation of the increase in early growth response gene 1 expression is not known. To determine if N-methyl-D-aspartate receptors regulate both fear conditioning and the increase in early growth response gene 1 expression in the lateral nucleus of the amygdala, rats were infused i.c.v. with 2.5microg of the N-methyl-D-aspartate antagonist, DL-2-amino-5-phosphonovalerate. Most rats were killed 30min following one-trial contextual fear conditioning and their brains were processed for in situ hybridization detection of early growth response gene 1 messenger RNA expression. The remainder of the rats was tested for retention of fear conditioning 24h later. In DL-2-amino-5-phosphonovalerate-treated rats, post-shock freezing remained intact, whereas fear-conditioned freezing during the retention test was abolished. Image analysis of early growth response gene 1 messenger RNA revealed that DL-2-amino-5-phosphonovalerate blocked the fear-conditioning-associated increase in early growth response gene 1 expression in the lateral nucleus of the amygdala. In addition, DL-2-amino-5-phosphonovalerate significantly increased early growth response gene 1 expression in the central nucleus of the amygdala. The results reveal differential regulation of early growth response gene 1 messenger RNA in the amygdala by N-methyl-D-aspartate receptors and argue for a functional role of early growth response gene 1 in the formation of long-term memory for contextual fear. Furthermore, the results indicate a functional neuroanatomical circuit within the amygdala that includes dampening of excitatory and activation of inhibitory processes in distinct amygdala nuclei, resulting in the reduction of fear conditioning.

Parallel neural responses in amygdala subregions and sensory cortex during implicit fear conditioning

We used event-related fMRI to measure neural activity in volunteer subjects during acquisition of an implicit association between a visual conditioned stimulus (CS+) (angry face) and an auditory unconditioned stimulus (UCS) (aversive, loud noise). Three distinct functional regions were identified within left amygdala: a UCS (noise)-related lateral region, a CS+-related ventral region, and a dorsal region where CS+-related responses changed progressively across the learning session. Differential neural responses to the visual CS+ were also evoked in extrastriate and auditory cortices. Our results indicate that learning an association between biologically salient stimuli of different sensory modalities involves parallel changes of neural activity in segregated amygdala subregions and unimodal sensory cortices.

Activation of the left amygdala to a cognitive representation of fear

We examined the neural substrates involved when subjects encountered an event linked verbally, but not experientially, to an aversive outcome. This instructed fear task models a primary way humans learn about the emotional nature of events. Subjects were told that one stimulus (threat) represents an aversive event (a shock may be given), whereas another (safe) represents safety (no shock will be given). Using functional magnetic resonance imaging (fMRI), activation of the left amygdala was observed in response to threat versus safe conditions, which correlated with the expression of the fear response as measured by skin conductance. Additional activation observed in the insular cortex is proposed to be involved in conveying a cortical representation of fear to the amygdala. These results suggest that the neural substrates that support conditioned fear across species have a similar but somewhat different role in more abstract representations of fear in humans.

Slow and fast (gamma) neuronal oscillations in the perirhinal cortex and lateral amygdala

Most lesion studies emphasize the distinct contributions of the amygdala and perirhinal cortex to memory. Yet, the presence of strong reciprocal excitatory projections between these two structures suggests that they are functionally coupled. To gain some insight into this issue, the present study examined whether the close anatomical ties existing between perirhinal and lateral amygdala (LA) neurons are expressed in their spontaneous activity. To this end, multiple simultaneous recordings of single unit discharges and local field potentials were performed in the LA and perirhinal cortex in ketamine-xylazine anesthetized cats. The perirhinal cortex and LA exhibited a similar pattern of spontaneous activity. Recordings at both sites were dominated by a slow focal oscillation at 1 Hz onto which was superimposed a faster rhythm (approximately 30 Hz) whose amplitude fluctuated cyclically. Computing crosscorrelograms between focal waves recorded simultaneously in the perirhinal cortex and LA revealed a close relationship between their spontaneous activity. Even when recording sites were separated by as much as 8 mm, the slow focal oscillation remained highly correlated (r > or = 0.7). In contrast, the correlation between fast oscillations was usually lower (r approximately 0.3). Perievent histograms of neuronal discharges revealed that the firing probability of most LA and perirhinal neurons increased during the depth-negative component of the slow oscillation. In addition, respectively, 47 and 64% of LA and perirhinal neurons exhibited a significant modulation of firing probability in relation to the fast oscillations. Finally, crosscorrelating unit discharges simultaneously recorded in the LA and perirhinal cortex confirmed the presence of phase-related oscillatory events in both structures. In summary, our results suggest that the interconnections existing between the perirhinal cortex and LA can support the genesis of coherent neuronal activities at various frequencies. These results imply that cooperative interactions must be taking place between these structures.

Glycogen phosphorylase reactivity in the amygdala and bed nucleus of the stria terminalis

The present study examines the reactivity of the glial metabolic enzyme, glycogen phosphorylase, within the amygdala and bed nucleus of the stria terminalis. Reactivity for phosphorylase a, the active form of glycogen phosphorylase, was higher in all parts of the medial amygdaloid nucleus, in the medial division of the central amygdaloid nucleus, in the anterior amygdaloid area and in the bed nucleus of stria terminalis than in all parts of the lateral amygdaloid nucleus, the anterior cortical amygdaloid nucleus, the posteromedial and posterolateral cortical amygdaloid nuclei, the intercalated nucleus of the amygdala, main part and the intercalated nuclei. A greater degree of phosphorylase a reactivity was also observed in the basolateral amygdaloid nucleus, anterior and posterior parts, and in the basomedial amygdaloid nucleus, anterior part, while other parts of these nuclei were less reactive. Reactivity attributed to total glycogen phosphorylase enzyme, phosphorylase a+phosphorylase b activated by AMP, was higher and homogeneous across the amygdala. Phosphorylase a patterns are likely to reflect differences in the contribution of glycogenolysis to the metabolic support of cells in the amygdala and bed nucleus of the stria terminalis. Possible relationships to local neuronal activity and to differences in glycogenolytic neuromodulatory input are discussed.

A statistical theory of long-term potentiation and depression

The synaptic phenomena of long-term potentiation (LTP) and long-term depression (LTD) have been intensively studied for over twenty-five years. Although many diverse aspects of these forms of plasticity have been observed, no single theory has offered a unifying explanation for them. Here, a statistical "bin" model is proposed to account for a variety of features observed in LTP and LTD experiments performed with field potentials in mammalian cortical slices. It is hypothesized that long-term synaptic changes will be induced when statistically unlikely conjunctions of pre- and postsynaptic activity occur. This hypothesis implies that finite changes in synaptic strength will be proportional to information transmitted by conjunctions and that excitatory synapses will obey a Hebbian rule (Hebb, 1949). Using only one set of constants, the bin model offers an explanation as to why synaptic strength decreases in a decelerating manner during LTD induction (Mulkey & Malenka, 1992); why the induction protocols for LTP and LTD are asymmetric (Dudek & Bear, 1992; Mulkey & Malenka, 1992); why stimulation over a range of frequencies produces a frequency-response curve similar to that proposed by the BCM theory (Bienenstock, Cooper, & Munro, 1982; Dudek & Bear, 1992); and why this curve would shift as postsynaptic activity is changed (Kirkwood, Rioult, & Bear, 1996). In addition, the bin model offers an alternative to the BCM theory by predicting that changes in postsynaptic activity will produce vertical shifts in the curve rather than merely horizontal shifts.

5-HT(1A) receptor-mediated inhibition and 5-HT(2) as well as 5-HT(3) receptor-mediated excitation in different subdivisions of the rat amygdala

The techniques of extracellular single cell recording and microiontophoresis were used to study the effects of serotonin (5-HT) and of 5-HT(1A), 5-HT(2A/2C) and 5-HT(3) receptor agonists on the spontaneous activity of amygdaloid neurons in rats anesthetized with urethane. The background discharge rate was modified by 5-HT as well as by 5-HT agonists in about two-thirds of neurons tested in different nuclei of the amygdaloid complex. Whereas the 5-HT(2) and 5-HT(3) agonists significantly increased the neuronal discharge rate in nearly all subdivisions of the amygdala, the 5-HT(1A) agonist significantly inhibited the firing rate. Co-administration of bicuculline and 5-HT receptor agonists prevented the 8-OH-DPAT-induced increases in the firing rate in most cases tested, as well as the inhibitory effects of DOI or 2-methyl-5HT. Therefore, GABAergic interneurons seem to be involved in the mediation of serotonergic effects. The action of 5-HT agonists on the neuronal discharge rate was blocked by different receptor-specific antagonists. The results support the hypothesis that 5-HT exerts control throughout the amygdala by acting at least on 5-HT(1A), 5-HT(2A/2C) and 5-HT(3) receptors seemingly located both on projection and interneurons.