Cued fear memory generalization increases over time

Abstract

Fear memory is a highly stable and durable form of memory, even over vast (remote) time frames. Nevertheless, some elements of fear memory can be forgotten, resulting in generalization. The purpose of this study is to determine how cued fear memory generalizes over time and measure underlying patterns of cortico-amygdala synaptic plasticity. We established generalization gradients at recent (1-d) and remote (30-d) retention intervals following auditory cued fear conditioning in adult male C57BL/6 mice. Results revealed a flattening of the generalization gradient (increased generalization) that was dissociated from contextual fear generalization, indicating a specific influence of time on cued fear memory performance. This effect reversed after a brief exposure to the novel stimulus soon after learning. Measurements from cortico-amygdala imaging of the activity-regulated cytoskeletal () protein using immunohistochemistry after cued fear memory retrieval revealed a stable pattern of expression in the dorsolateral amygdala, but temporally dynamic expression in the cortex. Over time, increased fear memory generalization was associated with a reduction in expression in the agranular insular and infralimbic cortices while discrimination learning was associated with increased expression in the prelimbic cortex. These data identify the dorsolateral amygdala, medial prefrontal, and insular cortices as loci for synaptic plasticity underlying cued fear memory generalization over time.

Assaying Fear Memory Discrimination and Generalization: Methods and Concepts

“…While samples sizes should be determined a priori using a statistical power analysis (e.g., G*Power, Univeritat Dusseldorf), we have found that 12-15 animals/group produces reliable measures (Pollack et al, 2018 The stimulus protocol is designed by the experimenter. For example, a stimulus protocol we have used for training in mice is: The subject freely explores the chamber for 3 min prior to the presentation of a 20-s, 5-kHz, 72-75 dB CS that co-terminates with a 0.5-s, 0.5-mA US.…”

“…For instance, Pollack et al. () tested a set of tone frequencies (2 to 12 kHz) after training at 5 kHz in mice. Results revealed discrimination at 2, 3, and 12 kHz (recent) and generalization at 2 and 3 kHz (remote).…”

“…For instance, a given non-target stimulus may be presented at both a recent and remote time point following learning. This design was shown to lead to increased discrimination at a remote time point when the same test stimulus was presented at a recent time point following learning (Pollack et al, 2018).…”

“…However, cued fear generalization experiments have been conducted using tone stimuli at lower frequencies. For instance, Pollack et al (2018) tested a set of tone frequencies (2 to 12 kHz) after training at 5 kHz in mice. Results revealed discrimination at 2, 3, and 12 kHz (recent) and generalization at 2 and 3 kHz (remote).…”

“…Current Protocols in Neuroscience increase generalization (reduce discrimination) is the passage of time. The passage of time has been shown to magnify both cued (Pollack et al, 2018) and contextual fear (Lynch, Cullen, Jasnow, & Riccio, 2013;Poulos et al, 2016;Sauerhöfer et al, 2012;Wiltgen & Silva, 2007). Approximately 30 days between training and testing has been consistently shown to increase both cued and contextual fear memory generalization, although shorter retention intervals have been reported (Lynch et al, 2013).…”