Neuronal ensembles in amygdala, hippocampus, and prefrontal cortex track differential components of contextual fear

Abstract

Although the circuit mediating contextual fear conditioning has been extensively described, the precise contribution that specific anatomical nodes make to behavior has not been fully elucidated. To clarify the roles of the dorsal hippocampus (DH), basolateral amygdala (BLA), and medial prefrontal cortex (mPFC) in contextual fear conditioning, activity within these regions was mapped using cellular compartment analysis of temporal activity using fluorescence in situ hybridization (catFISH) for Arc mRNA. Rats were delay-fear conditioned or immediately shocked (controls) and thereafter reexposed to the shocked context to test for fear memory recall. Subsequent catFISH analyses revealed that in the DH, cells were preferentially reactivated during the context test, regardless of whether animals had been fear conditioned or immediately shocked, suggesting that the DH encodes spatial information specifically, rather then the emotional valence of an environment. In direct contrast, neuronal ensembles in the BLA were only reactivated at test if animals had been fear conditioned, suggesting that the amygdala specifically tracks the emotional properties of a context. Interestingly, Arc expression in the mPFC was consistent with both amygdala- and hippocampus-like patterns, supporting a role for the mPFC in both fear and contextual processing. Collectively, these data provide crucial insight into the region-specific behavior of neuronal ensembles during contextual fear conditioning and demonstrate a dissociable role for the hippocampus and amygdala in processing the contextual and emotional properties of a fear memory.

Keywords: Arc; amygdala; catFISH; contextual fear; hippocampus; prefrontal cortex.

Figure 1.

A, Behavioral design. Rats were fear conditioned or immediately shocked (Event 1) and 20 min later were tested for context fear (Event 2). B, Time course of Arc mRNA transcription and translocation from the nucleus of a cell to the cytoplasm. C, Mean (±SEM) percentage freezing during context test (Event 2). Delay-conditioned rats show significant freezing to the context compared with immediately shocked rats displaying the immediate shock deficit. **p < 0.01.

Figure 2.

Mean (±SEM) percentage total neurons (sytox; green) that were positive for Arc mRNA (red) in a given cellular compartment, and representative fluorescent images. A, CA1 region of DH. Animals showed a significant increase in double-labeled cells (gray) in the DH, regardless of whether or not the environment was fear inducing (Immediate vs Delay). B, Cells in the BLA were only double labeled if animals were afraid (Delay group). C, Arc expression in PL cortex revealed overlap in neuronal ensembles involved in both events, which was further enhanced in fear-conditioned rats. D, Levels of Arc expression in the IL cortex showed no compartmental differences. ***p < 0.001, **p < 0.01, *p < 0.05.

Figure 3.

Mean (±SEM) percentage total neurons that were positive for Arc mRNA in a given cellular compartment of dorsal CA1. Rats were exposed to the context for 5 min, 30 or 5 min before being killed. Arc expression in the cytoplasm of rats exposed 30 min prior was significantly greater than that seen in the nucleus or in both compartments. ***p < 0.001.