Scientific Understanding of Consciousness
Consciousness as an Emergent Property of Thalamocortical Activity

Amygdala Neuron Populations for Fear and Reward

 

Nature  520, 675–678 (30 April 2015)

A circuit mechanism for differentiating positive and negative associations

Praneeth Namburi, et.al

Neuroscience Graduate Program, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA

Department of Genetics, Harvard Medical School, 77 Avenue Louis Pasteur, NRB 356, Boston, Massachusetts 02115, USA

Undergraduate Program in Neuroscience, Wellesley College, Wellesley, Massachusetts 02481, USA

Undergraduate Program in Neuroscience, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA

Master’s Program in Biomedical Sciences, University of Amsterdam, Amsterdam 1098 XH, The Netherlands

McGovern Institute for Brain Research, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA

[paraphrase]

The ability to differentiate stimuli predicting positive or negative outcomes is critical for survival, and perturbations of emotional processing underlie many psychiatric disease states. Synaptic plasticity in the basolateral amygdala complex (BLA) mediates the acquisition of associative memories, both positive and negative. Different populations of BLA neurons may encode fearful or rewarding associations, but the identifying features of these populations and the synaptic mechanisms of differentiating positive and negative emotional valence have remained unknown. Here we show that BLA neurons projecting to the nucleus accumbens (NAc projectors) or the centromedial amygdala (CeM projectors) undergo opposing synaptic changes following fear or reward conditioning. We find that photostimulation of NAc projectors supports positive reinforcement while photostimulation of CeM projectors mediates negative reinforcement. Photoinhibition of CeM projectors impairs fear conditioning and enhances reward conditioning. We characterize these functionally distinct neuronal populations by comparing their electrophysiological, morphological and genetic features. Overall, we provide a mechanistic explanation for the representation of positive and negative associations within the amygdala.

The BLA, including lateral and basal nuclei of the amygdala, receives sensory information from multiple modalities, and encodes motivationally relevant stimuli. Partially non-overlapping populations of BLA neurons encode cues associated with appetitive or aversive outcomes. The acquisition of the association between a neutral stimulus and an aversive outcome such as a foot shock has been shown to induce long term potentiation (LTP) of synapses onto lateral amygdala neurons, mediated by postsynaptic increases in α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor (AMPAR)-mediated currents in a N-methyl-d-aspartate receptor (NMDAR)-dependent manner. Similarly, increases in glutamatergic synaptic strength of inputs to BLA neurons are necessary for the formation of a stimulus–reward association. Yet the similarity in neural encoding and synaptic changes induced by learning a positive or negative association and the contrasting nature of the ensuing outputs (reward-seeking or fear-related behaviours) presents an ostensible paradox: how is it possible that potentiation of synapses onto neurons in the BLA can underlie learned associations that lead to such different behavioural responses?

One hypothesis is that BLA neurons project to many downstream regions, including the canonical circuits for reward and fear and the neurons that project to different targets undergo distinct synaptic changes with positive or negative associative learning. For example, BLA projections to the NAc have been implicated in reward-related behaviours, while BLA projections to the CeM have been linked to the expression of conditioned fear. However, the unique synaptic changes onto projection-identified BLA neurons have never been explored.

The BLA, including lateral and basal nuclei of the amygdala, receives sensory information from multiple modalities, and encodes motivationally relevant stimuli. Partially non-overlapping populations of BLA neurons encode cues associated with appetitive or aversive outcomes. The acquisition of the association between a neutral stimulus and an aversive outcome such as a foot shock has been shown to induce long term potentiation (LTP) of synapses onto lateral amygdala neurons, mediated by postsynaptic increases in α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor (AMPAR)-mediated currents in a N-methyl-d-aspartate receptor (NMDAR)-dependent manner. Similarly, increases in glutamatergic synaptic strength of inputs to BLA neurons are necessary for the formation of a stimulus–reward association. Yet the similarity in neural encoding and synaptic changes induced by learning a positive or negative association and the contrasting nature of the ensuing outputs (reward-seeking or fear-related behaviours) presents an ostensible paradox: how is it possible that potentiation of synapses onto neurons in the BLA can underlie learned associations that lead to such different behavioural responses?

One hypothesis is that BLA neurons project to many downstream regions, including the canonical circuits for reward and fear, and the neurons that project to different targets undergo distinct synaptic changes with positive or negative associative learning. For example, BLA projections to the NAc have been implicated in reward-related behaviours, while BLA projections to the CeM have been linked to the expression of conditioned fear. However, the unique synaptic changes onto projection-identified BLA neurons have never been explored.

To investigate this, we selected the NAc and CeM as candidate target regions and examined the synaptic changes onto either NAc-projecting BLA neurons (NAc projectors) or CeM-projecting BLA neurons (CeM projectors) following fear conditioning or reward conditioning. To identify the projection target of BLA neurons, we injected retrogradely travelling fluorescent beads (retrobeads) into either the NAc or CeM to label BLA neurons projecting axon terminals to these regions. After retrobead migration upstream to BLA cell bodies, we trained mice in fear or reward conditioning paradigms wherein a tone was paired with either a foot shock or sucrose delivery. Mice in reward conditioning groups were food restricted 1 day before the conditioning session to increase motivation to seek sucrose. AMPAR/NMDAR ratio, a proxy for glutamatergic synaptic strength, increases after either fear or reward conditioning in the BLA. We used matched experimental parameters across groups in an acute slice preparation stimulating axons arriving via the internal capsule and performing whole-cell patch-clamp recordings in retrobead-identified NAc projectors and CeM projectors, which we observed to be topographically intermingled.

We found that in NAc projectors, fear conditioning decreased the AMPAR/NMDAR ratio relative to controls exposed to the same number of tones and shocks, but where the tones and shocks were unpaired. Conversely, following the acquisition of the association between a tone and sucrose delivery, synapses on NAc projectors showed an increase in AMPAR/NMDAR ratio relative to unpaired controls that were also food restricted and received the same number of tones and volume of sucrose. Importantly, we also included naive and food-restricted naive groups, as food restriction itself could alter AMPAR/NMDAR ratio.

In contrast, synapses on CeM projectors from the paired group showed an increase in AMPAR/NMDAR ratio following fear conditioning, relative to unpaired controls. Following reward conditioning, CeM projectors from mice that learned the tone–sucrose association showed a decrease in AMPAR/NMDAR ratio relative to unpaired controls. In addition to AMPAR/NMDAR ratios, we also examined paired-pulse ratios, and did not detect any differences between groups, suggesting a postsynaptic mechanism of plasticity.

These results support a model wherein NAc and CeM projectors undergo opposing changes in synaptic strength following fear and reward learning, such that relative synaptic strengths onto CeM projectors increase following fear conditioning and decrease following reward learning.

NAc and CeM projectors are populations of BLA neurons that undergo opposing synaptic changes following fear or reward conditioning, and optogenetic manipulation of NAc and CeM projectors reveals causal relationships with valence-specific behaviours. Further, we have identified distinguishing electrophysiological, morphological and gene expression characteristics that facilitate further investigation. Our study suggests that the indelible nature of valence encoding observed in amygdala neurons is mediated by connectivity, and the topographical intermingling of these populations may serve to facilitate interaction. In conclusion, the BLA is a site of divergence for circuits mediating positive and negative emotional or motivational valence.

[end of paraphrase]

 

Return to — Amygdala