Potentiated amygdaloid auditory-evoked potentials and freezing behavior after fear conditioning in mice

Elucidation of cellular and molecular mechanisms underlying fear-related memory would greatly benefit from the possibility of combined behavioral and electrophysiological recordings in genetically modified mice. As a first step to this goal, we implanted adult C57BL/6J mice with recording electrodes aimed at the basolateral amygdaloid complex and trained them in an auditory fear conditioning paradigm. After conditioning, animals with paired tone and footshock presentation showed not only intensified freezing behavior lasting for 2 days, but also increases, lasting 4 days, in slope and amplitude of the most negative component of auditory-evoked potentials triggered by the conditioned stimulus. These effects could not be observed in animals with unpaired tone and footshock presentation. Thus, our data show that a long-lasting association of a former neutral tone with an aversive situation is accompanied by a long-lasting increase of auditory-evoked potentials in freely moving mice. However, extinction of the potentiation of auditory-evoked potentials and freezing behavior followed different time courses, thus making a direct relationship between these responses unlikely.     

Impaired recognition of fear in voices and reduced anxiety after unilateral temporal lobe resection

It has been reported that bilateral amygdala damage in humans compromises the recognition of fear and anger in nonverbal vocalizations (Scott et al., 1997). We addressed the possibility that unilateral temporal lobe damage might be sufficient to impair fear recognition in voices. For this purpose, we tested patients after left (n = 10) or right (n = 8) medial temporal lobe resection for the relief of intractable epilepsy using a set of nonverbal vocalizations (Belin, Fillion-Bilodeau, & Gosselin, 2008). To focus more narrowly on the role of amygdala subparts, we differentiated patients with complete amygdala damage vs. damage limited to the basolateral complex of the amygdala. The results confirmed for the first time that unilateral amygdala lesion including the basolateral complex can selectively impair recognition of fear and surprise expressed by voices, supporting the notion that the amygdala is a multimodal structure. Interestingly, this impairment was observed in patients with incomplete resection of the amygdala that spared the central nucleus and the corticomedial complex, suggesting that a resection of the basolateral complex is sufficient to affect fear recognition. Given that fear has often been considered as a precursor of anxiety, we also investigated the effect of such lesions on self-reported anxiety. The same patients appeared to be less anxious than control participants in their mood questionnaires. The association of impaired fear perception and decreased anxiety level is considered in the light of recent human and animal data, providing support for a neurobiological basis of mood changes in patients with unilateral temporal lobe damage.     

Dietary-induced obesity disrupts trace fear conditioning and decreases hippocampal reelin expression

Both obesity and over-consumption of palatable high fat/high sugar “cafeteria” diets in rats has been shown to induce cognitive deficits in executive function, attention and spatial memory. Adult male Sprague–Dawley rats were fed a diet that supplemented standard lab chow with a range of palatable foods eaten by people for 8 weeks, or regular lab chow. Memory was assessed using a trace fear conditioning procedure, whereby a conditioned stimulus (CS) is presented for 10 s and then 30 s after its termination a foot shock (US) is delivered. We assessed freezing to the CS (flashing light) in a neutral context, and freezing in the context associated with footshock.A dissociation was observed between levels of freezing in the context and to the CS associated with footshock. Cafeteria diet fed rats froze less than control chow fed rats in the context associated with footshock (P < 0.01), indicating that encoding of a hippocampus-dependent context representation was impaired in these rats. Conversely, cafeteria diet fed rats froze more (P < 0.05) to the CS than chow fed rats, suggesting that when hippocampal function was compromised the cue was the best predictor of footshock, as contextual information was not encoded.Dorsal hippocampal mRNA expression of inflammatory and neuroplasticity markers was analysed at the end of the experiment, 10 weeks of diet. Of these, mRNA expression of reelin, which is known to be important in long term potentiation and neuronal plasticity, was significantly reduced in cafeteria diet fed rats (P = 0.003). This implicates reductions in hippocampal plasticity in the contextual fear memory deficits seen in the cafeteria diet fed rats.     

Testosterone levels in healthy men are related to amygdala reactivity and memory performance

Testosterone is a steroid hormone thought to influence both emotional and cognitive functions. It is unknown, however, if testosterone also affects the interaction between these two domains, such as the emotional arousal-induced enhancement of memory. Healthy subjects (N=234) encoded pictures taken from the International Affective Picture System (IAPS) during functional magnetic resonance imaging (fMRI) and underwent a free recall test 10 min after memory encoding. We show that higher endogenous testosterone levels at encoding were associated with higher arousal ratings of neutral pictures in men. fMRI analysis revealed that higher testosterone levels were related to increased brain activation in the amygdala during encoding of neutral pictures. Moreover, endogenous testosterone levels were positively correlated with the number of freely recalled neutral pictures. No such relations were found in women. These findings point to a male-specific role for testosterone in enhancing memory by increasing the biological salience of incoming information.     

The amygdala and appraisal processes: stimulus and response complexity as an organizing factor

The amygdala has been implicated in a variety of functions, ranging from attention to memory to emotion. In theories about the amygdala's role in conditioned fear, the lateral amygdala (LA) is the primary, perhaps unique, interface for incoming conditioned sensory stimuli and the central nucleus is the major output station. Recent studies indicate, however, that amygdala output pathways may be dissociated as a function of the type of conditioned fear behavior. Based on behavioral, electrophysiological and anatomical evidence, the present discussion proposes a modification of the traditional model of input pathways to the amygdala such that the LA activation as a sensory interface is limited to relatively simple, unimodal conditioned stimulus features whereas the basal amygdaloid nucleus (B) may serve as an amygdaloid sensory interface for complex, configural conditioned stimulus information. We further argue that the partition of amygdalar nuclei according to a complexity dimension appears to correspond both for input and output pathways and thus constitutes a common organizing factor in the functional anatomy of the amygdala. The extensive intra-amygdala wiring is assumed to underlie the computations necessary to perform behavioral decisions of various levels of complexity. Collectively, these results endow the amygdala with a more sophisticated role in guiding motivation and behavior.     

Two-stage theory of conditioning: involvement of the cerebellum and the amygdala

Classical conditioning is thought to proceed through two successive stages: fast rate emotional conditioning followed by slower motor conditioning. To verify the involvement of the amygdala and the cerebellum in these two stages of learning, rats were subjected to paired tone-airpuff (CS-US) trials. Lick suppression to CS was used as an index of conditioned emotional response (emotional CRs) and head movement was used as an index of motor CRs. The results showed that the fast acquisition of emotional CRs was dependent on the integrity of the amygdala and the slow acquisition of motor CRs was dependent on the integrity of the cerebellar interpositus nucleus. Cerebellar lesions had no effect on the acquisition of the emotional CRs but prevented the extinction of the emotional CRs seen in intact rats after massive conditioning. These findings suggest that the amygdala and the cerebellum provide the neuronal substrates of the fast and slow conditioning systems, respectively, and that conditioning-related cerebellar output interacts with the amygdala-based emotional conditioning.     

Effects of dorsal striatum lesions in tone fear conditioning and contextual fear conditioning

It has been suggested that the striatum mediates hippocampus-independent memory tasks. Classical fear conditioning to a discrete stimulus such as a tone is not affected by hippocampal lesion, whereas contextual fear conditioning is an hippocampus dependent task. The purpose of the present study was to verify the effect of dorsal striatal lesions on tone and contextual fear conditioning. The lesioned rats were not impaired in contextual fear conditioning but in tone fear conditioning both electrolytically and neurotoxically lesioned animals showed less freezing compared with controls. The lesion effect was observed after a postoperative recovery period of 14 days but not after 2 months. The results support the hypothesis that the dorsal striatum is involved in hippocampus-independent memory tasks, but, in spite of this involvement, it does not seem to be a critical structure.     

Involvement of the cholinergic system in conditioning and perceptual memory

The cholinergic systems play a pivotal role in learning and memory, and have been the centre of attention when it comes to diseases containing cognitive deficits. It is therefore not surprising, that the cholinergic transmitter system has experienced detailed examination of its role in numerous behavioural situations not least with the perspective that cognition may be rescued with appropriate cholinergic ‘boosters’.Here we reviewed the literature on (i) cholinergic lesions, (ii) pharmacological intervention of muscarinic or nicotinic system, or (iii) genetic deletion of selective receptor subtypes with respect to sensory discrimination and conditioning procedures. We consider visual, auditory, olfactory and somatosensory processing first before discussing more complex tasks such as startle responses, latent inhibition, negative patterning, eye blink and fear conditioning, and passive avoidance paradigms.An overarching reoccurring theme is that lesions of the cholinergic projection neurones of the basal forebrain impact negatively on acquisition learning in these paradigms and blockade of muscarinic (and to a lesser extent nicotinic) receptors in the target structures produce similar behavioural deficits. While these pertain mainly to impairments in acquisition learning, some rare cases extend to memory consolidation. Such single case observations warranted replication and more in-depth studies. Intriguingly, receptor blockade or receptor gene knockout repeatedly produced contradictory results (for example in fear conditioning) and combined studies, in which genetically altered mice are pharmacological manipulated, are so far missing. However, they are desperately needed to clarify underlying reasons for these contradictions.Consistently, stimulation of either muscarinic (mainly M₁) or nicotinic (predominantly α7) receptors was beneficial for learning and memory formation across all paradigms supporting the notion that research into the development and mechanisms of novel and better cholinomimetics may prove useful in the treatment of neurodegenerative or psychiatric disorders with cognitive endophenotypes.     

Unconditioned responses and functional fear networks in human classical conditioning

Human imaging studies examining fear conditioning have mainly focused on the neural responses to conditioned cues. In contrast, the neural basis of the unconditioned response and the mechanisms by which fear modulates inter-regional functional coupling have received limited attention. We examined the neural responses to an unconditioned stimulus using a partial-reinforcement fear conditioning paradigm and functional MRI. The analysis focused on: (1) the effects of an unconditioned stimulus (an electric shock) that was either expected and actually delivered, or expected but not delivered, and (2) on how related brain activity changed across conditioning trials, and (3) how shock expectation influenced inter-regional coupling within the fear network. We found that: (1) the delivery of the shock engaged the red nucleus, amygdale, dorsal striatum, insula, somatosensory and cingulate cortices, (2) when the shock was expected but not delivered, only the red nucleus, the anterior insular and dorsal anterior cingulate cortices showed activity increases that were sustained across trials, and (3) psycho-physiological interaction analysis demonstrated that fear led to increased red nucleus coupling to insula but decreased hippocampus coupling to the red nucleus, thalamus and cerebellum. The hippocampus and the anterior insula may serve as hubs facilitating the switch between engagement of a defensive immediate fear network and a resting network.     

Impaired fear extinction in adolescent rodents: Behavioural and neural analyses

Despite adolescence being a developmental window of vulnerability, up until very recently there were surprisingly few studies on fear extinction during this period. Here we summarise the recent work in this area, focusing on the unique behavioural and neural characteristics of fear extinction in adolescent rodents, and humans where relevant. A prominent hypothesis posits that anxiety disorders peak during late childhood/adolescence due to the non-linear maturation of the fear inhibition neural circuitry. We discuss evidence that impaired extinction retention in adolescence is due to subregions of the medial prefrontal cortex and amygdala mediating fear inhibition being underactive while other subregions that mediate fear expression are overactive. We also review work on various interventions and surprising circumstances which enhance fear extinction in adolescence. This latter work revealed that the neural correlates of extinction in adolescence are different to that in younger and older animals even when extinction retention is not impaired. This growing body of work highlights that adolescence is a unique period of development for fear inhibition.     

Functional disconnections in the direct and indirect amygdala pathways for fear processing in schizophrenia

BACKGROUND: Schizophrenia patients show reduced neural activity, relative to controls, in the amygdala and its projection to the medial prefrontal cortex (MPFC) in response to fear perception. In this study we tested the hypothesis that schizophrenia is characterized by abnormal functional connectivity in the amygdala network underlying fear perception. METHODS: Functional MRI images were acquired from 14 schizophrenia patients and 14 matched healthy control subjects during an emotion perception task, in which fearful and neutral facial expression stimuli were presented pseudorandomly under nonconscious (using masking) and conscious conditions. Both subtraction and functional connectivity analyses were undertaken using a region of interest approach. RESULTS: In response to fearful facial expressions, schizophrenia patients displayed reduced amygdala activity, compared to controls, in both the conscious and nonconscious conditions. The amygdala displayed a reversal of the normal pattern of connectivity with the brainstem, visual cortex, and also with the dorsal and ventral divisions of the MPFC in the schizophrenia patients. CONCLUSIONS: The presence of functional disconnections in amygdala pathways suggests that schizophrenia patients have a failure in coordinating their automatic orienting to salient signals and the associated prefrontal monitoring of these signals.     

Long-term potentiation in the amygdala: a mechanism for emotional learning and memory

In the mammalian brain, LTP is an enduring form of synaptic plasticity that is posited to have a role in learning and memory. Compelling new evidence for this view derives from studies of LTP in the amygdala, a brain structure that is essential for simple forms of emotional learning and memory, such as Pavlovian fear conditioning in rats. More specifically, antagonists of the NMDA receptor block both amygdaloid LTP induction and fear conditioning, fear conditioning induces increases in amygdaloid synaptic transmission that resemble LTP, and genetic modifications that disrupt amygdaloid LTP eliminate fear conditioning. Collectively, these results provide the most-convincing evidence to date that LTP mediates learning and memory in mammals.     

Emotional memory in schizophrenia

Emotionally arousing scenes are better remembered than neutral ones. The biological basis of this emotional memory effect has been studied in lesion and neuro-imaging studies and depends upon an interaction between the amygdala and medial temporal lobe memory systems including the hippocampus. This study sought to investigate whether patients with schizophrenia had performance deficits on emotional memory tasks consistent with abnormal amygdala function. Patients with schizophrenia and matched control subjects were shown scenes with negative, positive and neutral emotional content. Subjects rated the slides according to how emotionally arousing they found them and then performed surprise memory tests at 10 min (recall) and 3 weeks (recall and recognition). Subjects with schizophrenia did not differ from control subjects in their ratings of the slides. However, patients showed a significant loss of the emotional enhancement of recognition memory for both negative and positive scenes. In addition, patients showed an overall deficit in recall memory, with a selective impairment in recall of the most arousing negative slides. These findings are consistent with the view that medial temporal lobe and in particular amygdala function is abnormal in schizophrenia.     

Neural state and trait bases of mood-incongruent memory formation and retrieval in first-episode major depression

Mood-congruent cognitive biases constitute critical factors for the vulnerability to depression and its maintenance. One important aspect is impaired memory for positive information during depression and after recovery. To elucidate its state (during depression only) and trait (during depression and recovery) related neural bases, we investigated medication free depressed, recovered, and healthy individuals with functional MRI while they memorized and recognized happy and neutral face stimuli. The imaging results revealed group differences in mood-incongruent successful memory encoding and retrieval activity already in the absence of significant memory performance differences. State effects were observed in the amygdala and posterior cingulate cortex. Whereas the amygdala was generally involved in memory formation, its activity predicted subsequent forgetting of neutral faces in depressed patients. Furthermore, the amygdala and posterior cingulate cortex were involved in memory retrieval of happy faces in depressed patients only. Trait effects were observed in the fusiform gyrus and prefrontal cortex. The fusiform gyrus was involved in memory formation and retrieval of happy faces in both patient groups, whereas it was involved in memory formation and retrieval of neutral faces in healthy individuals. Similar trait effects were observed during memory retrieval in the orbitofrontal cortex and left inferior frontal gyrus. Thus, while memory processing of positive information in the amygdala and posterior cingulate cortex is biased during depression only, memory processing in the fusiform gyrus and prefrontal cortex is biased also after recovery. These distinct neural mechanisms may respectively constitute symptom maintenance and cognitive vulnerability factors for depression.     

Behavioral and neuropsychological foundations of olfactory fear conditioning

Pavlovian fear conditioning procedures have been a fruitful means of exploring the neural substrates of associative learning. There is now substantial evidence suggesting that many aspects of conditioned fear depend critically upon the integrity of the amygdala and the perirhinal cortex. Recent studies in our laboratory examining the contributions of these areas to olfactory and contextual fear conditioning are reviewed; collectively the results of these studies suggest that the amygdala participates critically in the acquisition and expression of fear conditioned to both an olfactory conditioned stimulus (CS) and to the training context, while the perirhinal cortex contributes to olfactory, but not contextual, fear conditioning. Moreover, it appears that perirhinal cortex may play a prominent role in recognition of the CS following conditioning. These results are discussed in light of the extent to which they replicate and extend previous research examining the contributions of these areas to fear conditioned to auditory and visual CSs.     

Neural responses to dynamic expressions of fear in schizophrenia

Abnormalities in social functioning are a significant feature of schizophrenia. One critical aspect of these abnormalities is the difficulty these individuals have with the recognition of facial emotions, particularly negative expressions such as fear. The present work focuses on fear perception and its relationship to the paranoid symptoms of schizophrenia, specifically, how underlying limbic system structures (i.e. the amygdala) react when probed with dynamic fearful facial expressions. Seven paranoid and eight non-paranoid subjects (all males) with a diagnosis of schizophrenia took part in functional magnetic resonance imaging study (1.5T) examining neural responses to emerging fearful expressions contrasted with dissipating fearful expressions. Subjects viewed emerging and dissipating expressions while completing a gender discrimination task. Their brain activation was compared to that of 10 healthy male subjects. Increased hippocampal activation was seen in the non-paranoid group, while abnormalities in the bilateral amygdalae were observed only in the paranoid individuals. These patterns may represent trait-related hippocampal dysfunction, coupled with state (specifically paranoia) related amygdala abnormalities. The findings are discussed in light of models of paranoia in schizophrenia.     

Cerebral glucose utilization after aversive conditioning and during conditioned fear in the rat

Regional cerebral glucose utilization (rCMRglu) was studied in rats with and without previous aversive conditioning. Four groups of rats were studied. Two groups of rats were aversely conditioned by placing them in a shock chamber (conditioned stimulus) where they received random footshocks. The two remaining groups were placed in the shock chamber but not conditioned. Regional CMRglu and systemic parameters (heart rate, mean arterial blood pressure (MABP), blood gases and pH, plasma catecholamines, and plasma glucose) were measured in unconditioned and conditioned rats in the presence and in the absence of the conditioned stimulus. The changes in rCMRglu described below appeared to be global and not limited to specific regions. Results are as follows: (1) transferring unconditioned rats to the shock chamber had no significant effect on rCMRglu even though the systemic parameters indicated a stress response. It appears that stress capable of inducing changes in heart rate, MABP, and plasma catecholamines is not necessarily accompanied by increases in cerebral glucose utilization. (2) Conditioned rats not exposed to the shock chamber at the time rCMRglu was measured had decreased rates of rCMRglu compared to rats that were not conditioned. Except for plasma epinephrine, which increased after conditioning, systemic parameters were not affected. (3) Conditioned fear, elicited by transferring conditioned rats to the shock chamber, increased rCMRglu when compared to a control group that was conditioned to footshock using the same paradigm but not exposed to the shock chamber at the time rCMRglu was measured. The systemic parameters indicated a stress response in conditioned rats transferred to the shock chamber.(ABSTRACT TRUNCATED AT 250 WORDS)     

Glucocorticoids and the regulation of memory in health and disease

Over the last decades considerable evidence has accumulated indicating that glucocorticoids – stress hormones released from the adrenal cortex – are crucially involved in the regulation of memory. Specifically, glucocorticoids have been shown to enhance memory consolidation of emotionally arousing experiences, but impair memory retrieval and working memory during emotionally arousing test situations. Furthermore, growing evidence indicates that these different glucocorticoid effects all depend on emotional arousal-induced activation of noradrenergic transmission within the basolateral complex of the amygdala (BLA) and on interactions of the BLA with other brain regions, such as the hippocampus and neocortical regions. Here we review findings from both animal and human experiments and present an integrated perspective of how these opposite glucocorticoid effects might act together to serve adaptive processing of emotionally significant information. Furthermore, as intense emotional memories also play a crucial role in the pathogenesis and symptomatology of anxiety disorders, such as posttraumatic stress disorder (PTSD) or phobias, we discuss to what extent the basic findings on glucocorticoid effects on emotional memory might have implications for the understanding and treatment of these clinical conditions. In this context, we review data suggesting that the administration of glucocorticoids might ameliorate chronic anxiety by reducing retrieval of aversive memories and enhancing fear extinction.