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The lateral nucleus of the amygdala (LA) is a site of convergence for auditory (conditioned stimulus) and foot-shock (unconditioned stimulus) inputs during fear conditioning. The auditory pathways to LA are well characterized, but less is known about the pathways through which foot shock is transmitted. Anatomical tracing and physiological recording studies suggest that the posterior intralaminar thalamic nucleus, which projects to LA, receives both auditory and somatosensory inputs. In the present study we examined the expression of the immediate-early gene c-fos in the LA in rats in response to foot-shock stimulation. We then determined the effects of posterior intralaminar thalamic lesions on foot-shock-induced c-Fos expression in the LA. Foot-shock stimulation led to an increase in the density of c-Fos-positive cells in all LA subnuclei in comparison to controls exposed to the conditioning box but not shocked. However, some differences among the dorsolateral, ventrolateral and ventromedial subnuclei were observed. The ventrolateral subnucleus showed a homogeneous activation throughout its antero-posterior extension. In contrast, only the rostral aspect of the ventromedial subnucleus and the central aspect of the dorsolateral subnucleus showed a significant increment in c-Fos expression. The density of c-Fos-labeled cells in all LA subnuclei was also increased in animals placed in the box in comparison to untreated animals. Unilateral electrolytic lesions of the posterior intralaminar thalamic nucleus and the medial division of the medial geniculate body reduced foot-shock-induced c-Fos activation in the LA ipsilateral to the lesion. The number of c-Fos labeled cells on the lesioned side was reduced to the levels observed in the animals exposed only to the box. These results indicate that the LA is involved in processing information about the foot-shock unconditioned stimulus and receives this kind of somatosensory information from the posterior intralaminar thalamic nucleus and the medial division of the medial geniculate body.

Using functional magnetic resonance imaging and an experimental paradigm of instructed fear, we observed a striking pattern of decreased activity in primary motor cortex with increased activity in dorsal basal ganglia during anticipation of aversive electrodermal stimulation in 42 healthy participants. We interpret this pattern of activity in motor neurocircuitry in response to cognitively-induced fear in relation to evolutionarily-conserved responses to threat that may be relevant to understanding normal and pathological fear in humans.

Learning and memory depend on signaling molecules that affect synaptic efficacy. The cytoskeleton has been implicated in regulating synaptic transmission but its role in learning and memory is poorly understood. Fear learning depends on plasticity in the lateral nucleus of the amygdala. We therefore examined whether the cytoskeletal-regulatory protein, myosin light chain kinase, might contribute to fear learning in the rat lateral amygdala. Microinjection of ML-7, a specific inhibitor of myosin light chain kinase, into the lateral nucleus of the amygdala before fear conditioning, but not immediately afterward, enhanced both short-term memory and long-term memory, suggesting that myosin light chain kinase is involved specifically in memory acquisition rather than in posttraining consolidation of memory. Myosin light chain kinase inhibitor had no effect on memory retrieval. Furthermore, ML-7 had no effect on behavior when the training stimuli were presented in a non-associative manner. Anatomical studies showed that myosin light chain kinase is present in cells throughout lateral nucleus of the amygdala and is localized to dendritic shafts and spines that are postsynaptic to the projections from the auditory thalamus to lateral nucleus of the amygdala, a pathway specifically implicated in fear learning. Inhibition of myosin light chain kinase enhanced long-term potentiation, a physiological model of learning, in the auditory thalamic pathway to the lateral nucleus of the amygdala. When ML-7 was applied without associative tetanic stimulation it had no effect on synaptic responses in lateral nucleus of the amygdala. Thus, myosin light chain kinase activity in lateral nucleus of the amygdala appears to normally suppress synaptic plasticity in the circuits underlying fear learning, suggesting that myosin light chain kinase may help prevent the acquisition of irrelevant fears. Impairment of this mechanism could contribute to pathological fear learning.

We sought to determine whether contextual fear conditioning, a hippocampal-dependent task, would affect neurogenesis in the dentate gyrus of the hippocampus, and if so, to identify which aspect of the training experience accounts for the change. The immediate shock deficit paradigm was used, together with bromodeoxyuridine immunohistochemistry, to isolate the contribution of different aspects of contextual fear conditioning to neurogenesis. Contextual fear learning caused a 33% decrease in the number of proliferating cells that was anatomically restricted to the dentate gyrus with no change in cell survival or differentiation. This attenuation was not related to exposure to the conditioned stimulus alone, the footshock unconditioned stimulus alone, or the expression of fear to the context after training. Instead, the effect of context conditioning on cell proliferation appears to be specifically due to the formation of an association between the context and shock during training, an amygdala dependent function.

Glucocorticoids, released in high concentrations from the adrenal cortex during stressful experiences, bind to glucocorticoid receptors in nuclear and peri-nuclear sites in neuronal somata. Their classically known mode of action is to induce gene promoter receptors to alter gene transcription. Nuclear glucocorticoid receptors are particularly dense in brain regions crucial for memory, including memory of stressful experiences, such as the hippocampus and amygdala. While it has been proposed that glucocorticoids may also act via membrane bound receptors, the existence of the latter remains controversial. Using electron microscopy, we found glucocorticoid receptors localized to non-genomic sites in rat lateral amygdala, glia processes, presynaptic terminals, neuronal dendrites, and dendritic spines including spine organelles and postsynaptic membrane densities. The lateral nucleus of the amygdala is a region specifically implicated in the formation of memories for stressful experiences. These newly observed glucocorticoid receptor immunoreactive sites were in addition to glucocorticoid receptor immunoreactive signals observed using electron and confocal microscopy in lateral amygdala principal neuron and GABA neuron soma and nuclei, cellular domains traditionally associated with glucocorticoid immunoreactivity. In lateral amygdala, glucocorticoid receptors are thus also localized to non-nuclear-membrane translocation sites, particularly dendritic spines, where they show an affinity for postsynaptic membrane densities, and may have a specialized role in modulating synaptic transmission plasticity related to fear and emotional memory.

The amygdala is critical for acquiring and expressing conditioned fear responses elicited by sensory stimuli that predict future punishment, but there is conflicting evidence about whether the amygdala is necessary for perceiving the aversive qualities of painful or noxious stimuli that inflict primary punishment. To investigate this question, rats were fear conditioned by pairing a sequence of auditory pips (the conditioned stimulus, or CS) with a brief train of shocks to one eyelid (the unconditioned stimulus, or US). Conditioned responding to the CS was assessed by measuring freezing responses during a test session conducted 24 h after training, and unconditioned responding to the US was assessed by measuring head movements evoked by the eyelid shocks during training. We found that pre-training electrolytic lesions of the amygdala's lateral (LA) nucleus blocked acquisition of conditioned freezing to the CS, and also significantly attenuated unconditioned head movements evoked by the US. Similarly, bilateral inactivation of the amygdala with the GABA-A agonist muscimol impaired acquisition of CS-evoked freezing, and also attenuated US-evoked responses during training. However, when amygdala synaptic plasticity was blocked by infusion of the NR2B receptor antagonist ifenprodil, acquisition of conditioned freezing was impaired but shock reactivity was unaffected. These findings indicate that neural activity within the amygdala is important for both predicting and perceiving the aversive qualities of noxious stimuli, and that synaptic plasticity within LA is the mechanism by which the CS becomes associated with the US during fear conditioning.

The bed nucleus of the stria terminalis (BNST) is believed to be a critical relay between the central nucleus of the amygdala (CE) and the paraventricular nucleus of the hypothalamus in the control of hypothalamic-pituitary-adrenal (HPA) responses elicited by conditioned fear stimuli. If correct, lesions of CE or BNST should block expression of HPA responses elicited by either a specific conditioned fear cue or a conditioned context. To test this, rats were subjected to cued (tone) or contextual classical fear conditioning. Two days later, electrolytic or sham lesions were placed in CE or BNST. After 5 days, the rats were tested for both behavioral (freezing) and neuroendocrine (corticosterone) responses to tone or contextual cues. CE lesions attenuated conditioned freezing and corticosterone responses to both tone and context. In contrast, BNST lesions attenuated these responses to contextual but not tone stimuli. These results suggest CE is indeed an essential output of the amygdala for the expression of conditioned fear responses, including HPA responses, regardless of the nature of the conditioned stimulus. However, because lesions of BNST only affected behavioral and endocrine responses to contextual stimuli, the results do not support the notion that BNST is critical for HPA responses elicited by conditioned fear stimuli in general. Instead, the BNST may be essential specifically for contextual conditioned fear responses, including both behavioral and HPA responses, by virtue of its connections with the hippocampus, a structure essential to contextual conditioning. The results are also not consistent with the hypothesis that BNST is only involved in unconditioned aspects of fear and anxiety.

Consolidation is a process through which labile memories are made persistent [Science 287 (2000) 248]; [Annu Rev Psychol 55 (2004) 51]. When retrieved, a consolidated memory is rendered labile again and undergoes reconsolidation [Learn Mem 7 (2000) 73]; [Trends Neurosci 26 (2003) 65]). Reconsolidation thus offers the opportunity to manipulate memory after it is formed, and may therefore provide a means of treating intrusive memories associated with post-traumatic stress disorder (PTSD). Reconsolidation is most usually studied using protein synthesis inhibitors, which is not practical in humans. However, the beta adrenergic receptor antagonist propranolol impairs consolidation of declarative memory in humans [Science 287 (2000) 248]; [Nature 371 (1994) 702] and consolidation and reconsolidation of inhibitory avoidance learning in rats [Brain Res 368 (1986) 125]; [J Neurosci 19 (1999) 6623]. Here, we show that systemic or intra-amygdala infused propranolol blocks reconsolidation but not consolidation. If the effects on reconsolidation are verified in humans, the results would suggest the possibility that propranolol after memory retrieval might be an effective way of treatment of intrusive memories in PTSD. That the systemic effects of propranolol on reconsolidation are achieved via an action in the amygdala is especially important in light of the fact that PTSD involves alterations in the amygdala [Arch Gen Psychiatry 53 (1996) 380].

Plasticity in the lateral nucleus of the amygdala is thought to be critical for the acquisition of Pavlovian fear conditioning. The pathways that transmit auditory conditioned stimulus information originate in auditory processing regions of the thalamus and cortex, but the pathways mediating transmission of unconditioned stimuli to the amygdala are poorly understood. Recent studies suggest that somatosensory (footshock) unconditioned stimulus information is also relayed in parallel to the lateral nucleus of the amygdala from the thalamus (the posterior intralaminar thalamic complex, PIT) and the cortex (parietal insular cortex). In the present study we reexamined this issue. Our results showed that bilateral electrolytic lesions of the PIT alone blocked fear conditioning, whereas bilateral excitotoxic PIT lesions had no effect. These electrolytic PIT lesions did not affect fear conditioning using a loud noise as unconditioned stimulus, defining the effects of PIT lesions as a disruption of somatosensory as opposed to auditory processing. Finally, we performed combined bilateral excitotoxic lesions of the PIT nuclei and electrolytic lesions of the parietal insular cortex. These, like excitotoxic lesions of PIT alone, had no effect on the acquisition of fear conditioning. Thus, somatosensory regions of the thalamus and cortex may well be important routes of unconditioned stimulus transmission to the amygdala in fear conditioning, but information about the unconditioned somatosensory stimulus is also transmitted from other sources that send fibers through, but do not form essential synapses in, the thalamus en route to the amygdala.

Previous studies suggest that the left and right amygdalae are interconnected in rodents. The origin and topography of these connections have, however, remained obscure. In the present study, we investigated the interamygdaloid projections originating in the different divisions of the basal and accessory basal nuclei of the rat amygdala by using the Phaseolus vulgaris leucoagglutinin anterograde tract-tracing technique. The basal nucleus gave rise to substantial interamygdaloid projections. However, the density of the projections depended on the location of Phaseolus vulgaris leucoagglutinin injection in the basal nucleus. The magnocellular and intermediate divisions projected heavily to the homonymous regions on the contralateral side, as well as to the nucleus of the lateral olfactory tract. The parvicellular division projected lightly to the homonymous region on the contralateral side, to the contralateral anterior amygdaloid area and to the medial division of the central nucleus. The contralateral projections originating in the accessory basal nucleus were light compared to those of the basal nucleus. These data indicate that interamygdaloid connections in the rat brain are extensive and topographically organized. Via these connections, one amygdala may rapidly activate the contralateral side. This may explain, for example, why the epileptic seizures in one amygdala spread contralaterally and cause the development of independent seizure activity in kindling model of temporal lobe epilepsy.

We have previously found that the coupling of changes in autonomic activity and emotional behavior to acoustic stimuli through classical fear conditioning survives bilateral ablation of auditory cortex but is disrupted by bilateral lesions of the medial geniculate nucleus or inferior colliculus in rats. Auditory fear conditioning thus appears to be mediated by the relay of acoustic input from the medial geniculate nucleus to subcortical rather than cortical targets. Since the medial geniculate nucleus projects, in addition to auditory cortex, to a striatal field, involving portions of the posterior neostriatum and underlying archistriatum (amygdala), we have sought to determine whether interruption of connections linking the medial geniculate nucleus to this subcortical field also disrupts conditioning. The conditioned emotional response model studied included the measurement of increases in mean arterial pressure and heart rate and the suppression of exploratory activity and drinking by the acoustic conditioned stimulus following delayed classical conditioning, where the footshock unconditioned stimulus appeared at the end of the conditioned stimulus. The peak increase in arterial pressure and the duration of activity and drinking suppression were greater in unoperated animals subjected to delayed conditioning than in pseudoconditioned controls, where the footshock was randomly rather than systematically related to the acoustic stimulus. Increases in heart rate, however, did not differ in conditioned and pseudoconditioned groups. While the arterial pressure and behavioral responses therefore reflect associative conditioning, the heart rate response does not. Rats were prepared with bilateral lesions of the medial geniculate nucleus, bilateral lesions of the striatal field or asymmetrical unilateral lesions destroying the medial geniculate nucleus on one side and the striatal field on the contralateral side. The latter preparation leaves one medial geniculate nucleus and one striatal field intact but disconnected and thus produces a selective auditory deafferentation of the intact striatal field. Control groups included animals with unilateral lesion of the medial geniculate nucleus, with unilateral lesion of the medial geniculate nucleus combined with lesion of the ipsilateral striatal field, unilateral lesion of the medial geniculate combined with lesion of the contralateral anterior neostriatum (a striatal area outside of the medial geniculate nucleus projection field).(ABSTRACT TRUNCATED AT 400 WORDS)