Tracking the fear memory engram: discrete populations of neurons within amygdala, hypothalamus, and lateral septum are specifically activated by auditory fear conditioning

Abstract

Memory formation is thought to occur via enhanced synaptic connectivity between populations of neurons in the brain. However, it has been difficult to localize and identify the neurons that are directly involved in the formation of any specific memory. We have previously used fos-tau-lacZ (FTL) transgenic mice to identify discrete populations of neurons in amygdala and hypothalamus, which were specifically activated by fear conditioning to a context. Here we have examined neuronal activation due to fear conditioning to a more specific auditory cue. Discrete populations of learning-specific neurons were identified in only a small number of locations in the brain, including those previously found to be activated in amygdala and hypothalamus by context fear conditioning. These populations, each containing only a relatively small number of neurons, may be directly involved in fear learning and memory.

Neurons Specifically Activated by Fear Learning in Lateral Amygdala Display Increased Synaptic Strength

“…The Gaussian fit to the highest frequency accounted for 25% of cells in the fear learning mice but only 4% in home-caged mice. These empirical observations are consistent with the idea that only a subpopulation of neurons in LAvl exhibit altered properties following fear learning (Repa et al, 2001; Sehgal et al, 2014), and with previous findings that fear learning activates a specific subpopulation of neurons in LAvl (Wilson and Murphy, 2009; Trogrlic et al, 2011; Butler et al, 2015). We next used an approach that allowed the electrophysiological recording of neurons specifically activated by fear conditioning.…”

“…3 C , D ). The pattern of FTL + neurons we observed in conditioned mice is consistent with the pattern of FTL activity described in previous studies (Wilson and Murphy, 2009; Trogrlic et al, 2011; Butler et al, 2015). …”

“…1 A , B ; pre-shock freezing M = 0.66, SD = 0.82; test freezing M = 29.5, SD = 12; t (5) = 6, p = 0.0018; pre-shock moving M = 32, SD = 13; test moving M = 5.4, SD = 2.7; t (5) = 6.3, p = 0.0015). A small subset of neurons within the LAvl has been previously identified to be specifically activated by fear conditioning (Wilson and Murphy, 2009; Trogrlic et al, 2011; Butler et al, 2015). Whole-cell recordings were made from neurons in this LAvl subnucleus region from fear-conditioned and home cage control mice as marked in Figure 1 C .…”

“…These findings are consistent with a change in synaptic properties of neurons within the LAvl after fear conditioning. However, based on previous reports identifying activated neurons histochemically, only a subset of neurons within the LAvl are expected to have altered properties (Wilson and Murphy, 2009; Trogrlic et al, 2011; Butler et al, 2015). To explore this further, we applied a Gaussian peak-fitting algorithm to the average sPSC amplitude of each cell recorded from either fear-conditioned or home-caged mice (Fig.…”

“…The immediate early gene c-fos has been extensively used as a marker of neuronal activation (Knapska et al, 2007) and previous studies have employed c-fos regulated expression in the transgenic fos - tau-LacZ ( FTL ) mouse line to identify functionally activated circuitry in the brain (Murphy et al, 2007; Wilson and Murphy, 2009). Using FTL mice, a number of discrete, anatomically defined populations of neurons in different parts of the brain were identified as being specifically activated by fear learning (Wilson and Murphy 2009; Trogrlic et al, 2011; Butler et al, 2015). In particular, a population of learning-activated FTL + neurons was found within a spatially restricted region of the ventrolateral nucleus of the LA (LAvl).…”