Faculty Bibliography

'Consolidation' has been used to describe distinct but related processes. In considering the implications of our recent findings on the lability of reactivated fear memories, we view consolidation and reconsolidation in terms of molecular events taking place within neurons as opposed to interactions between brain regions. Our findings open up a new dimension in the study of memory consolidation. We argue that consolidation is not a one-time event, but instead is reiterated with subsequent activation of the memories

Although much has been learned about the neurobiological mechanisms underlying Pavlovian fear conditioning at the systems and cellular levels, relatively little is known about the molecular mechanisms underlying fear memory consolidation. The present experiments evaluated the role of the extracellular signal-regulated kinase/mitogen-activated protein kinase (ERK/MAPK) signaling cascade in the amygdala during Pavlovian fear conditioning. We first show that ERK/MAPK is transiently activated-phosphorylated in the amygdala, specifically the lateral nucleus (LA), at 60 min, but not 15, 30, or 180 min, after conditioning, and that this activation is attributable to paired presentations of tone and shock rather than to nonassociative auditory stimulation, foot shock sensitization, or unpaired tone-shock presentations. We next show that infusions of U0126, an inhibitor of ERK/MAPK activation, aimed at the LA, dose-dependently impair long-term memory of Pavlovian fear conditioning but leaves short-term memory intact. Finally, we show that bath application of U0126 impairs long-term potentiation in the LA in vitro. Collectively, these results demonstrate that ERK/MAPK activation is necessary for both memory consolidation of Pavlovian fear conditioning and synaptic plasticity in the amygdala

Physiological studies suggest that afferents to the lateral nucleus of the amygdala (LA) from the auditory thalamus initiate feedforward inhibition [Li et al. (1996b)]. This model of neural processing requires that thalamic afferents synapse directly onto inhibitory interneurons. To determine whether such synaptic contacts occur, we combined anterograde tract tracing with interneuron immunocytochemistry. The anterograde tracer biotinylated dextran amine (BDA) was injected into the auditory thalamus. Inhibitory interneurons in the LA were identified using antibodies directed against gamma aminobutyric acid (GABA) or one of the calcium binding proteins (CBPs), parvalbumin (PARV), calbindin (CALB), or calretinin (CALR), since CBPs identify distinct populations of GABAergic cells within the amygdala. The distribution of GABAergic and CBP interneurons in each subregion of the LA was examined by light microscopy and the relationships between thalamo-amygdala terminals and interneurons were examined by confocal and electron microscopy. Immunoreactive cells were distributed in all three subdivisions of LA, except for CALR-ir neurons, which were sparse in the dorsal subregion and were found mainly in the ventromedial and ventrolateral subregions. Confocal microscopy revealed some thalamo-amygdala terminals in close proximity to LA interneurons, while electron microscopy showed that thalamo-amygdala terminals made direct synaptic contacts onto distal dendritic processes of inhibitory neurons. These data provide morphological evidence that thalamic afferents synapse directly onto inhibitory interneurons in LA, and are consistent with the possibility that inputs from the auditory thalamus initiate feedforward inhibition in LA. This architecture could play an important role in the suppression of background neural noise, thereby enhancing the response of LA cells to incoming auditory stimuli

Auditory information from the posterior thalamus reaches the lateral nucleus of the amygdala (LA) by way of two pathways: a direct thalamo-amygdala projection and a polysynaptic thalamo-cortico-amygdala projection. However, the quantitative extent of thalamic neurons that project to the LA or to the auditory association cortex (AAC) is not known. Furthermore, the extent and topographical distribution of bifurcating cells that project to both LA and AAC are also unknown. Therefore, separate tracers were injected into LA and either into all of AAC or within discrete regions of AAC, such as temporal areas TE3 or perirhinal cortex (PRh), and quantitative analyses were performed on labeling within the subregions of the auditory thalamus in rats. Following LA injections, retrogradely labeled cells were most numerous in the posterior intralaminar nucleus (PIN; 48.0% of all labeled thalamic cells), whereas labeled cells following injections of the entire AAC were most numerous in the dorsal division of the medial geniculate nucleus (MGd; 32.9% of all labeled thalamic cells). Following AAC injections localized to only TE3, the MGd again had the majority of labeled cells (35.9%), whereas following AAC injections localized to PRh, the PIN had the most labeled cells (32.8%). Double-labeled cells were found in all the thalamic regions studied and were most commonly observed in the PIN (43.7% of all double-labeled cells following injections into LA and throughout the AAC). The percentage of double-labeled cells as a proportion of either LA-projecting or AAC-projecting cells varied among the thalamic nuclei studied, ranging from 2.9% up to 42.4%. The topographic distribution of double-labeled cells in the thalamic nuclei resembled that of single-labeled cells following LA injections more than single-labeled cells following AAC injection. These findings suggest that double-labeled cells contribute substantially to many of the direct thalamo-amygdala and indirect thalamo-AAC-amygdala projections. Among other functions, these bifurcating cells may help regulate the processing of input to the LA arriving from these two pathways to allow for certain types of plasticity in the LA during fear conditioning

Although the lateral and basal nuclei of the amygdala are believed to be essential for the acquisition of Pavlovian fear conditioning, studies using post-training manipulations of the amygdala in the inhibitory avoidance learning paradigm have recently called this view into question. We used the GABA(A) agonist muscimol to functionally inactivate these nuclei immediately after single-trial Pavlovian fear conditioning or single-trial inhibitory avoidance learning. Immediate post-training infusions of muscimol had no effect on Pavlovian conditioning but produced a dose-dependent effect on inhibitory avoidance. However, pre-training infusions dose-dependently disrupted Pavlovian conditioning. These findings indicate that the amygdala plays an essential role in the acquisition of Pavlovian fear conditioning and contributes to the modulation of memory consolidation of inhibitory avoidance but not of Pavlovian fear conditioning

Previous studies have shown that long-term potentiation (LTP) can be induced in the lateral nucleus of the amygdala (LA) after stimulation of central auditory pathways and that auditory fear conditioning modifies neural activity in the LA in a manner similar to LTP. The present experiments examined whether intra-LA administration of inhibitors of protein synthesis or protein kinase A (PKA) activity, treatments that block LTP in hippocampus, interfere with memory consolidation of fear conditioning. In the first series of experiments, rats received a single conditioning trial followed immediately by intra-LA infusions of anisomycin (a protein synthesis inhibitor) or Rp-cAMPS (an inhibitor of PKA activity) and were tested 24 hr later. Results indicated that immediate post-training infusion of either drug dose-dependently impaired fear memory retention, whereas infusions 6 hr after conditioning had no effect. Additional experiments showed that anisomycin and Rp-cAMPS interfered with long-term memory (LTM), but not short-term memory (STM), of fear and that the effect on LTM was specific to memory consolidation processes rather than to deficits in sensory or performance processes. Findings suggest that the LA is essential for memory consolidation of auditory fear conditioning and that this process is PKA and protein-synthesis dependent

'New' memories are initially labile and sensitive to disruption before being consolidated into stable long-term memories. Much evidence indicates that this consolidation involves the synthesis of new proteins in neurons. The lateral and basal nuclei of the amygdala (LBA) are believed to be a site of memory storage in fear learning. Infusion of the protein synthesis inhibitor anisomycin into the LBA shortly after training prevents consolidation of fear memories. Here we show that consolidated fear memories, when reactivated during retrieval, return to a labile state in which infusion of anisomycin shortly after memory reactivation produces amnesia on later tests, regardless of whether reactivation was performed 1 or 14 days after conditioning. The same treatment with anisomycin, in the absence of memory reactivation, left memory intact. Consistent with a time-limited role for protein synthesis production in consolidation, delay of the infusion until six hours after memory reactivation produced no amnesia. Our data show that consolidated fear memories, when reactivated, return to a labile state that requires de novo protein synthesis for reconsolidation. These findings are not predicted by traditional theories of memory consolidation.