Molecular mechanisms of fear learning and memory

Abstract

Pavlovian fear conditioning is a particularly useful behavioral paradigm for exploring the molecular mechanisms of learning and memory because a well-defined response to a specific environmental stimulus is produced through associative learning processes. Synaptic plasticity in the lateral nucleus of the amygdala (LA) underlies this form of associative learning. Here, we summarize the molecular mechanisms that contribute to this synaptic plasticity in the context of auditory fear conditioning, the form of fear conditioning best understood at the molecular level. We discuss the neurotransmitter systems and signaling cascades that contribute to three phases of auditory fear conditioning: acquisition, consolidation, and reconsolidation. These studies suggest that multiple intracellular signaling pathways, including those triggered by activation of Hebbian processes and neuromodulatory receptors, interact to produce neural plasticity in the LA and behavioral fear conditioning. Collectively, this body of research illustrates the power of fear conditioning as a model system for characterizing the mechanisms of learning and memory in mammals and potentially for understanding fear-related disorders, such as PTSD and phobias.

FIGURE 1. Auditory Fear Conditioning in Rats

In a typical auditory fear conditioning procedure, rats are habituated to the conditioning chamber but given no stimuli. During the conditioning session the electric shock unconditioned stimulus (US) is paired with the auditory conditioned stimulus (CS) several times (usually 1–5). The effects of conditioning are then assessed in a test session during which the conditioned stimulus is presented alone. Most studies measure “freezing” behavior, which is an innate defensive response elicited by the conditioned stimulus after conditioning. An unpaired control group in which the conditioned stimulus and unconditioned stimulus are presented in a non-overlapping manner is often used. The conditioned stimulus elicits little or no freezing prior to conditioning in either the paired or unpaired group (not shown). Both the paired and unpaired group freeze during the training session due to the shock presentation. In the test session, the paired group exhibits considerably more conditioned stimulus-elicited freezing than the unpaired. Differences between the paired and unpaired group reflect the association that is learned as a result of conditioned-unconditioned stimuli pairing.

FIGURE 2. Fear Conditioning Circuit

Convergence of the auditory conditioned stimulus and nociceptive unconditioned stimulus in the amygdala is essential for fear conditioning. Convergence of conditioned and unconditioned stimuli occurs in lateral nucleus of the amygdala (LA), especially in the dorsal subnucleus (LAd), leading to synaptic plasticity in LA. Plasticity may also occur in the central nucleus (CE) and in the auditory thalamus. LA connects with CE directly and indirectly by way of connections in the basal (B), accessory basal (AB), and intercalated (ic) regions. CE connects with hypothalamic and brainstem areas that control the expression of conditioned fear responses, including freezing, autonomic (ANS) and hormonal responses. CeL, lateral nucleus of CE; CeM, medial nucleus of CE; PAG, periaqueductal gray; LH, lateral hypothalamus; PVN, paraventricular nucleus of the hypothalamus.

FIGURE 3. Working Model of Molecular Processes in the LA Mediating Acquisition and Consolidation of Fear Memories

All dotted lines denote hypothetical pathways. Molecules and processes in green are known to be involved in the acquisition of fear conditioning. Molecules and process in black are known to be involved specifically in the consolidation or maintenance of fear conditioning. Purple labels denote molecules or elements whose role is not established for fear conditioning, but are part of an established intracellular signaling pathway. Abbreviations: AC, adenyl cyclase; AKAP, A-kinase anchoring protein; Arc, activity-regulated cytoskeletal-associated protein; β-AR, Beta Adrenergic Receptor; BDNF, Brain Derived Neurotrophic Factor; Ca²⁺, Calcium; CaMKII, Ca²⁺/Calmodulin (Cam) Dependent Protein Kinase II; CREB, cAMP Response Element (CRE) binding protein; EGR-1, early growth response gene 1; GluA1, Glutamate AMPA Receptor Subunit 1; GluA2/3, Glutamate AMPA Receptor Subunit 2 and 3 heteromer; IP3, inositol 1,4,5-triphosphate; MAPK, Mitogen Activated Protein Kinase; mGluR, Metabotropic Glutamate Receptor; mTOR, Mammalian Target of Rapamycin; NF-kB, nuclear factor kappa-light-chain-enhancer of activated B cells NMDA-R, N-methyl-d-aspartate Glutamate Receptor; NO, Nitric Oxide; NOS, Nitric Oxide Synthase; NSF, N-ethylmaleimide-sensitive factor; PI3-K, phosphatidylinositol-3 kinase; PKA, Protein Kinase A; PKC, Protein Kinase C; PKG, cGMP-dependent protein kinase; PKMζ, protein kinase M ζ; RNA, Ribonucleic Acid; TrkB, tyrosine kinase B; VGCC, Voltage Gated Calcium Channel

FIGURE 4. Working model of molecular mechanisms mediating fear memory reconsolidation

All lines are hypothetical. Molecules and processes in green are known to be involved in the initiation of reconsolidation. Molecules and process in black are known to be involved in reconsolidation of fear conditioning. Purple labels denote molecules or elements whose role is not established for fear conditioning, but are part of an established intracellular signaling pathway. AC, adenyl cyclase; AKAP, A-kinase anchoring protein; Arc, activity-regulated cytoskeletal-associated protein; β-AR, Beta Adrenergic Receptor; BDNF, brain derived neurotrophic factor; Ca²⁺, Calcium; CREB, cAMP Response Element (CRE) binding protein; Egr-1, Early Growth Response Protein 1; MAPK, Mitogen Activated Protein Kinase; mTOR Mammalian Target of Rapamycin; NMDA-R, N-methyl-d-aspartate Glutamate Receptor; Npas4, Neuronal PAS domain protein 4; RNA, Ribonucleic Acid.