A VTA to Basal Amygdala Dopamine Projection Contributes to Signal Salient Somatosensory Events during Fear Learning

The amygdala is a brain area critical for the formation of fear memories. However, the nature of the teaching signal(s) that drive plasticity in the amygdala are still under debate. Here, we use optogenetic methods to investigate the contribution of ventral tegmental area (VTA) dopamine neurons to auditory-cued fear learning in male mice. Using anterograde and retrograde labeling, we found that a sparse and relatively evenly distributed population of VTA neurons projects to the basal amygdala (BA). In vivo optrode recordings in behaving mice showed that many VTA neurons, among them putative dopamine neurons, are excited by footshocks, and acquire a response to auditory stimuli during fear learning. Combined cfos imaging and retrograde labeling in dopamine transporter (DAT) Cre mice revealed that a large majority of BA projectors (>95%) are dopamine neurons, and that BA projectors become activated by the tone-footshock pairing of fear learning protocols. Finally, silencing VTA dopamine neurons, or their axon terminals in the BA during the footshock, reduced the strength of fear memory as tested 1 d later, whereas silencing the VTA-central amygdala (CeA) projection had no effect. Thus, VTA dopamine neurons projecting to the BA contribute to fear memory formation, by coding for the saliency of the footshock event and by signaling such events to the basal amygdala.

The dopaminergic VTA - basal amygdala projection contributes to signal aversive state in fear memory

Wei Tang

Dopamine released from midbrain neurons is an important neuromodulatory signal during associative learning in many forebrain circuits. One model of associative learning is auditory cued fear learning, during which an innocuous sensory percept like a tone (called conditioned stimulus, CS), acquires an aversive meaning after pairing with an aversive event, like a mild footshock (called unconditioned stimulus, US). The amygdala plays an important role in fear learning in mammals. Nevertheless, our knowledge of the synaptic- or neuromodulatory input(s) which carry information about the US to the amygdala is still limited. Here, I have investigated the hypothesis that dopamine, released from midbrain dopaminergic neurons in the ventral tegmental area (VTA) of mice, might signal aversive sensory events to the amygdala during fear learning. Using viral anterograde tracing approaches in transgenic mice expressing Cre-recombinase under the control of the dopamine transporter promoter (DAT-Cre), we found a dopaminergic projection from VTA to the basal amygdala (BA) and to the central amygdala (CeA). In addition, retrograde tracing approaches showed that tyrosine hydroxylase positive (TH+) dopamine neurons targeting these two areas might be differentially distributed in the VTA. To investigate a role of these projections during fear learning, we silenced the electrical activity of VTA dopamine neurons, or else their projection fibers to the BA or the CeA, by using Cre-dependent expression of a light-driven proton pump Archaerhodopsin (Arch). Silencing VTA neurons specifically during the footshock presentation (US) led to a moderate reduction of fear memory tested one day following the initial US - CS pairing. Silencing the dopaminergic fibers bilaterally in the BA (but not in the CeA) during footshock presentation had a somewhat larger effect on fear memory retrieval, and similarly reduced contextual fear memory. Furthermore, we made recordings of the spiking activity of DAT+ - as well as of non-identified VTA neurons in-vivo in behaving mice using optrodes. We found that about half of all units strongly respond to footshock stimulation (US), and show increasing responses to tones (CS) during the US - CS pairing. To investigate the possible cellular functions of dopamine in the BA during fear learning, we performed in-vitro recordings of genetically identified parvalbumin (PV)-positive interneurons and BA principal neurons that project to the CeA. This showed that bath application of dopamine or of a D1- receptor agonist enhances the evoked firing rate of CeA projectors in the BA. Finally, bath application of dopamine reduces afferent excitatory inputs onto PV-positive interneurons in the BA and in the LA. Taken together, by using complementary optogenetic, electrophysiological and anatomical approaches, this PhD thesis suggests that VTA dopamine neurons facilitate associative learning in the BA by providing a neuromodulatory signal to the BA during the time of the footshock. Furthermore, the VTA might also provide dopamine to the amygdala during fear memory retrieval. Thus, this work contributes to an understanding of the functional role of the dopaminergic VTA to BA projection in auditory-cued fear learning.

EPFL2018

The HDAC inhibitor CI-994 acts as a molecular memory aid by facilitating synaptic and intracellular communication after learning

Allison Marie Burns, Johannes Gräff, Jose Vicente Sanchez Mut, Giulia Santoni

Long-term memory formation relies on synaptic plasticity, neuronal activity-dependent gene transcription, and epigenetic modifications. Multiple studies have shown that HDAC inhibitor (HDACi) treatments can enhance individual aspects of these processes and thereby act as putative cognitive enhancers. However, their mode of action is not fully understood. In particular, it is unclear how systemic application of HDACis, which are devoid of substrate specificity, can target pathways that promote memory formation. In this study, we explore the electrophysiological, transcriptional, and epigenetic responses that are induced by CI-994, a class I HDACi, combined with contextual fear conditioning (CFC) in mice. We show that CI-994–mediated improvement of memory formation is accompanied by enhanced long-term potentiation in the hippocampus, a brain region recruited by CFC, but not in the striatum, a brain region not primarily implicated in fear learning. Furthermore, using a combination of bulk and single-cell RNA-sequencing, we find that, when paired with CFC, HDACi treatment engages synaptic plasticity-promoting gene expression more strongly in the hippocampus, specifically in the dentate gyrus (DG). Finally, using chromatin immunoprecipitation-sequencing (ChIP-seq) of DG neurons, we show that the combined action of HDACi application and conditioning is required to elicit enhancer histone acetylation in pathways that underlie improved memory performance. Together, these results indicate that systemic HDACi administration amplifies brain region-specific processes that are naturally induced by learning.

2022

Amygdala upregulation of NCAM polysialylation induced by auditory fear conditioning is not required for memory formation, but plays a role in fear extinction

Kamila Markram, Maria del Carmen Sandi Perez

There is much interest to understand the mechanisms leading to the establishment, maintenance, and extinction of fear memories. The amygdala has been critically involved in the processing of fear memories and a number of molecular changes have been implicated in this brain region in relation to fear learning. Although neural cell adhesion molecules (NCAMs) have been hypothesized to play a role, information available about their contribution to fear memories is scarce. We investigate here whether polysialylated NCAM (PSA-NCAM) contributes to auditory fear conditioning in the amygdala. First, PSA-NCAM expression was evaluated in different amygdala nuclei after auditory fear conditioning at two different shock intensities. Results showed that PSA-NCAM expression was increased 24 h post-training only in animals subjected to the highest shock intensity (1mA). Second, PSA-NCAM was cleaved in the basolateral amygdaloid complex through micro-infusions of the enzyme endoneuraminidase N, and the consequences of such treatment were investigated on the acquisition, consolidation, remote memory expression, and extinction of conditioned fear memories. Intra-amygdaloid cleavage of PSA-NCAM did not affect acquisition, consolidation or expression of remote fear memories. However, intra-amygdaloid PSA-NCAM cleavage enhanced fear extinction processes. These results suggest that upregulation of PSA-NCAM is a correlate of fear conditioning that is not necessary for the establishment of fear memory in the amygdala, but participates in mechanisms precluding fear extinction. These findings point out PSA-NCAM as a potential target for the treatment of psychopathologies that involve impairment in fear extinction.

2007

The dopaminergic VTA - basal amygdala projection contributes to signal aversive state in fear memory

Wei Tang

EPFL2018