Abstract
The hippocampus can be divided into distinct segments that make unique contributions to learning and memory. The dorsal segment supports cognitive processes like spatial learning and navigation while the ventral hippocampus regulates emotional behaviors related to fear, anxiety and reward. In the current study, we determined how pyramidal cells in ventral CA1 respond to spatial cues and aversive stimulation during a context fear conditioning task. We also examined the effects of high and low frequency stimulation of these neurons on defensive behavior. Similar to previous work in the dorsal hippocampus, we found that cells in ventral CA1 expressed high-levels of c-Fos in response to a novel spatial environment. Surprisingly, however, the number of activated neurons did not increase when the environment was paired with footshock. This was true even in the subpopulation of ventral CA1 pyramidal cells that send direct projections to the amygdala. When these cells were stimulated at high-frequencies (20 Hz) we observed feedforward inhibition of basal amygdala neurons and impaired expression of context fear. In contrast, low-frequency stimulation (4 Hz) did not inhibit principal cells in the basal amygdala and produced an increase in fear generalization. Similar results have been reported in dorsal CA1. Therefore, despite clear differences between the dorsal and ventral hippocampus, CA1 neurons in each segment appear to make similar contributions to context fear conditioning.
Highlights
The hippocampus can be divided into distinct segments that make unique contributions to learning and memory (Fanselow and Dong, 2010)
We compared changes in c-Fos expression after mice were exposed to a novel environment or underwent context fear conditioning
Compared to the control group, there was an increase in the number of c-Fos positive vCA1 neurons in mice that explored the novel environment or underwent context fear conditioning
Summary
The hippocampus can be divided into distinct segments that make unique contributions to learning and memory (Fanselow and Dong, 2010). The dorsal and ventral hippocampus share some important properties They have the same basic architecture and intrinsic organization (tri-synaptic loop) and neurons in both regions respond to spatial cues (e.g., place cells) (Ishizuka et al, 1990; Kjelstrup et al, 2008; Strange et al, 2014). Place cells in the VH are distinct; they have large, overlapping place fields that encode behaviorally-relevant contexts as opposed to precise spatial locations (Komorowski et al, 2013) Based on these findings, we hypothesized that dorsal (dCA1) and ventral (vCA1) CA1 neurons will respond to different stimuli during context fear conditioning. Similar results have been reported when dCA1 neurons are stimulated at low frequencies (Ryan et al, 2015) These data suggest that dorsal and ventral CA1 make similar contributions to context fear conditioning despite the functional differences between these regions
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