Stephen Maren

Neuronal signalling of fear memory

The learning and remembering of fearful events depends on the integrity of the amygdala, but how are fear memories represented in the activity of amygdala neurons? Here, we review recent electrophysiological studies indicating that neurons in the lateral amygdala encode aversive memories during the acquisition and extinction of Pavlovian fear conditioning. Studies that combine unit recording with brain lesions and pharmacological inactivation provide evidence that the lateral amygdala is a crucial locus of fear memory. Extinction of fear memory reduces associative plasticity in the lateral amygdala and involves the hippocampus and prefrontal cortex. Understanding the signalling of aversive memory by amygdala neurons opens new avenues for research into the neural systems that support fear behaviour.

Neurotoxic lesions of the dorsal hippocampus and Pavlovian fear conditioning in rats

Electrolytic lesions of the dorsal hippocampus (DH) produce deficits in both the acquisition and expression of conditional fear to contextual stimuli in rats. To assess whether damage to DH neurons is responsible for these deficits, we performed three experiments to examine the effects of neurotoxic N-methyl-D-aspartate (NMDA) lesions of the DH on the acquisition and expression of fear conditioning. Fear conditioning consisted of the delivery of signaled or unsignaled footshocks in a novel conditioning chamber and freezing served as the measure of conditional fear. In Experiment 1, posttraining DH lesions produced severe retrograde deficits in context fear when made either 1 or 28, but not 100, days following training. Pretraining DH lesions made 1 week before training did not affect contextual fear conditioning. Tone fear was impaired by DH lesions at all training-to-lesion intervals. In Experiment 2, posttraining (1 day), but not pretraining (1 week), DH lesions produced substantial deficits in context fear using an unsignaled shock procedure. In Experiment 3, pretraining electrolytic DH lesions produced modest deficits in context fear using the same signaled and unsignaled shock procedures used in Experiments 1 and 2, respectively. Electrolytic, but not neurotoxic, lesions also increased pre-shock locomotor activity. Collectively, this pattern of results reveals that neurons in the DH are not required for the acquisition of context fear, but have a critical and time-limited role in the expression of context fear. The normal acquisition and expression of context fear in rats with neurotoxic DH lesions made before training may be mediated by conditioning to unimodal cues in the context, a process that may rely less on the hippocampal memory system.

The contextual brain: implications for fear conditioning, extinction and psychopathology

Contexts surround and imbue meaning to events; they are essential for recollecting the past, interpreting the present and anticipating the future. Indeed, the brains capacity to contextualize information permits enormous cognitive and behavioural flexibility. Studies of Pavlovian fear conditioning and extinction in rodents and humans suggest that a neural circuit including the hippocampus, amygdala and medial prefrontal cortex is involved in the learning and memory processes that enable context-dependent behaviour. Dysfunction in this network may be involved in several forms of psychopathology, including post-traumatic stress disorder, schizophrenia and substance abuse disorders.

Contextual and temporal modulation of extinction : Behavioral and biological mechanisms

Extinction depends, at least partly, on new learning that is specific to the context in which it is learned. Several behavioral phenomena (renewal, reinstatement, spontaneous recovery, and rapid reacquisition) suggest the importance of context in extinction. The present article reviews research on the behavioral and neurobiological mechanisms of contextual influences on extinction learning and retrieval. Contexts appear to select or retrieve the current relationship of the conditional stimulus (CS) with the unconditional stimulus (US), and they are provided by physical background cues, interoceptive drug cues, emotions, recent trials, and the passage of time. The current article pays particular attention to the effects of recent trials and trial spacing. Control of fear extinction by physical context involves interactions between the dorsal hippocampus and the lateral nucleus of the amygdala. This interaction may be mediated by gamma-aminobutyric acid (GABA)-ergic and adrenergic mechanisms.

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.

The Amygdala and Fear Conditioning: Has the Nut Been Cracked?

The foregoing discussion reveals that our understanding of the basic neurobiological mechanisms of aversive learning has advanced considerably. However, there are still a number of issues that remain to be tackled. First, is the amygdala a storage site for conditional fear memories? In favor of this hypothesis, recent data indicate that the amygdala has a long-term role in expressing fear conditioning over time. We have recently found that selective neurotoxic lesions of the BLA, which spared the CE (a critical point given the role of the CE in fear performance), produce deficits in the expression of conditional fear when made up to 28 days after training (Maren et al. 1996xMaren, S, Aharonov, G, and Fanselow, M.S. Behav. Neurosci., in press. 1996; See all ReferencesMaren et al. 1996). Kim and Davis 1993xKim, M and Davis, M. Behav. Neurosci. 1993; 107: 1088–1092Crossref | PubMed | Scopus (40)See all ReferencesKim and Davis 1993 have reported a similar result with electrolytic CE lesions. Hence, these reports are consistent with the storage of aversive memories in the amygdala. In contrast, McGaugh and his colleagues have evidence that the amygdala has a temporary role in the consolidation of aversive memories (McGaugh 1989xMcGaugh, J.L. Annu. Rev. Neurosci. 1989; 12: 255–287Crossref | PubMedSee all ReferencesMcGaugh 1989). Therefore, additional work is required to determine under what conditions the amygdala has an enduring versus a temporary role in fear conditioning.Second, is the amygdala involved in learning, performance, or both? That is, the deficits in both the acquisition and long-term expression of fear conditioning produced by either CE or BLA lesions do not reveal a specific role for these amygdaloid nuclei in either the learning or performance of conditional fear. However, the identification of CS–US convergence in the LA (Romanski et al. 1993xRomanski, L.M, Clugnet, M.C, Bordi, F, and LeDoux, J.E. Behav. Neurosci. 1993; 107: 444–450Crossref | PubMed | Scopus (248)See all ReferencesRomanski et al. 1993), the rapid development of short-latency associative neuronal firing in the LA (Quirk et al. 1995xQuirk, G.J, Repa, C, and LeDoux, J.E. Neuron. 1995; 15: 1029–1039Abstract | Full Text PDF | PubMed | Scopus (483)See all ReferencesQuirk et al. 1995), the selective of effects of intra-amygdala NMDA receptor antagonists on the acquisition of conditional fear (Miserendino et al. 1990xMiserendino, M.J, Sananes, C.B, Melia, K.R, and Davis, M. Nature. 1990; 345: 716–718Crossref | PubMed | Scopus (558)See all ReferencesMiserendino et al. 1990), the discovery of NMDA receptor-dependent LTP in the BLA in vivo (Maren and Fanselow 1995xMaren, S and Fanselow, M.S. J. Neurosci. 1995; 15: 7548–7564PubMedSee all ReferencesMaren and Fanselow 1995), and the selective effects of damage to the human amygdala on fear CRs (Bechara et al. 1995xBechara, A, Tranel, D, Damasio, H, Adolphs, R, Rockland, C, and Damasio, A.R. Science. 1995; 269: 1115–1118Crossref | PubMedSee all ReferencesBechara et al. 1995) favor the amygdala as a learning structure. Nonetheless, these findings do not preclude a role of the amygdala in the performance of fear responses, and the effects of amygdala lesions on innate or unconditioned fear (e.g.,5xBlanchard, D.C and Blanchard, R.J. J. Comp. Physiol. Psychol. 1972; 81: 281–290Crossref | PubMed | Scopus (467)See all References, 1xAdolphs, R, Tranel, D, Damasio, H, and Damasio, A.R. J. Neurosci. 1995; 15: 5879–5891PubMedSee all References) seem consistent with this possibility. Perhaps the amygdala is required for both the learning and performance of conditional fear, functions that may be mediated by the BLA and CE, respectively.Third, does LTP in the BLA underlie the acquisition of Pavlovian fear conditioning and associative neuronal discharges in the amygdala? As discussed in this minireview, there are now a number of provocative findings that suggest a role for LTP in the amygdala in the acquisition of fear conditioning; it is tempting to speculate that LTP is also responsible for the development of conditional neuronal activity in the amygdala during learning. However, there are lessons to be learned from a close examination of other attempts to link synaptic plasticity mechanisms with learning, for example, efforts to link hippocampal LTP and spatial learning. As Barnes 1995xBarnes, C.A. Neuron. 1995; 15: 751–754Abstract | Full Text PDF | PubMed | Scopus (130)See all ReferencesBarnes 1995 has pointed out, what has frequently been taken as strong evidence for a role of hippocampal LTP in spatial learning has later been shown to have limited validity or has been explained as a spurious correlation. In retrospect, this work has demonstrated that the task of linking synaptic plasticity with learning is exceptionally difficult. Thus, caution must be exercised when making claims that LTP in the amygdala underlies fear conditioning. Indeed, considerably more work will be required to understand the extent to which LTP in the amygdala serves as a mechanism for Pavlovian fear conditioning.And, fourth, how do neuronal ensembles in the amygdala encode CSs, and how do these codes translate into learned behavior? Now that we have begun to identify neuronal correlates of aversive learning in the amygdala, important questions for future research are the nature of the ensemble firing patterns that encode CSs in the amygdala and the translation of these ensemble codes into the diversity of fear CRs observed following training. Single-unit recordings in LA have begun to reveal how pairs of neurons in the amygdala might encode CSs (Quirk et al. 1995xQuirk, G.J, Repa, C, and LeDoux, J.E. Neuron. 1995; 15: 1029–1039Abstract | Full Text PDF | PubMed | Scopus (483)See all ReferencesQuirk et al. 1995), but the translation of neuronal firing in the amygdala into behavioral CRs is a problem that has yet to be addressed. Clearly, further detailed physiological investigations of neuronal activity in the amygdala and interconnected structures are required to begin to answer these important questions.Despite these unresolved issues, however, there is general consensus that the neurons in the amygdala are necessary for the acquisition of Pavlovian fear conditioning. The recent and exciting findings discussed in this minireview bring us one step closer to understanding the basic neurobiological processes underlying this important form of behavioral plasticity. And although we have made considerable progress in understanding these mechanisms, there is still much to be done before we crack the brains almond.

N-methyl-D-aspartate receptors in the basolateral amygdala are required for both acquisition and expression of conditional fear in rats

Three experiments examined the effects of intra-amygdaloid infusions of an N-methyl-D-aspartate (NMDA) receptor antagonist, D,L-2-amino-5-phosphonovalerate (APV), on contextual fear conditioning in rats. In Experiment 1, APV infusion into the basolateral amygdala (BLA), before training, disrupted the acquisition of contextual fear. In Experiment 2, APV produced a disruption of both the acquisition and expression of contextual fear. This blockade of contextual fear was not state dependent, not due to a shift in footshock sensitivity, and not the result of increased motor activity in APV-treated rats. In Experiment 3, fear conditioning was not affected by a posttraining APV infusion into the BLA. These results indicate that NMDA receptors in the BLA are necessary for both the acquisition and expression of Pavlovian fear conditioning to contextual cues in rats.

Hippocampal Inactivation Disrupts the Acquisition and Contextual Encoding of Fear Extinction

In recent studies, inactivation of the dorsal hippocampus before the retrieval of extinguished fear memories disrupted the context-dependent expression of these memories. In the present experiments, we examined the role of the dorsal hippocampus in the acquisition of extinction. After pairing an auditory conditional stimulus (CS) with an aversive footshock [unconditional stimulus (US)], rats received an extinction session in which the CS was presented without the US. In experiment 1, infusion of muscimol, a GABAA receptor agonist, into the dorsal hippocampus before the extinction training session decreased the rate of extinction. Moreover, when later tested for fear to the extinguished CS, all rats that had received hippocampal inactivation before extinction training demonstrated renewed fear regardless of the context in which testing took place. This suggests a role for the dorsal hippocampus in both acquiring the extinction memory and encoding the CS–context relationship that yields the context dependence of extinction. In experiment 2, inactivation of the dorsal hippocampus before testing also disrupted the context dependence of fear to the extinguished CS. In experiment 3, quantitative autoradiography revealed the boundaries of muscimol diffusion after infusion into the dorsal hippocampus. Together, these results reveal that the dorsal hippocampus is involved in the acquisition, contextual encoding, and context-dependent retrieval of fear extinction. Learning and remembering when and where aversive events occur is essential for adaptive emotional regulation.

Sex differences in hippocampal long-term potentiation (LTP) and Pavlovian fear conditioning in rats: positive correlation between LTP and contextual learning

Three experiments investigated sex differences in hippocampal long-term potentiation (LTP) and Pavlovian fear conditioning in rats. Experiment 1 revealed a robust sex difference in the magnitude of LTP induced at perforant path synapses in the dentate gyrus of pentobarbital-anesthetized rats. This sex difference in LTP was evident in rats of 35 and 60 days of age and was not the result of pre-LTP sex differences in perforant path synaptic transmission; 20-day-old rats did not show LTP. An analysis of field potentials evoked during LTP induction revealed a sex difference in the magnitude of N-methyl-D-aspartate (NMDA) receptor activation that was highly correlated with the magnitude of LTP. Experiment 2 showed that males condition more fear, measured as freezing, to the contextual conditional stimuli (CSs) of a conditioning chamber compared to their female counterparts. This sex difference in conditional freezing was apparent with both low and high unconditional stimulus (US, footshock) intensities. Experiment 3 revealed that the enhanced fear conditioning in males was specific to contextual CSs, and consisted of a more rapid rate of conditioning. Together, these experiments reveal a positive correlation between the magnitude of hippocampal LTP and a form of learning that depends on the hippocampus. Furthermore, they suggest a neural basis for sex differences in hippocampus-dependent learning tasks.

Electrolytic Lesions of the Fimbria/Fornix, Dorsal Hippocampus, or Entorhinal Cortex Produce Anterograde Deficits in Contextual Fear Conditioning in Rats

Recent data indicate that dorsal hippocampal (DH) lesions disrupt Pavlovian fear conditioning to contextual cues in rats. In the present study, we examined the effects of electrolytic lesions of the fimbria/fornix (FX) or entorhinal cortex (EC), the primary afferent projection systems to the DH, on contextual fear conditioning in rats. Conditioning consisted of the delivery of unsignaled footshocks in a novel observation chamber, and freezing served as the measure of conditional fear. Electrolytic lesions of the FX, DH, or EC made 1 week before training produced anterograde impairments in both immediate postshock freezing on the conditioning day and freezing during the context extinction test 24 h following training. The deficits in conditional freezing produced by FX, DH, and EC lesions were not statistically different, although the deficits in rats with FX or EC lesions tended to be more severe than those in rats with DH lesions. In addition to producing deficits in conditional freezing, FX, DH, or EC lesions produced a pronounced locomotor hyperactivity. Within the lesion and sham groups, however, locomotor activity was not significantly correlated with conditional freezing. These results indicate that contextual fear deficits in rats with hippocampal formation damage are equivalent following either FX, DH, or EC lesions. The relationship of freezing deficits and locomotor hyperactivity in rats with hippocampal formation lesions is discussed.