Disruption of medial septum and diagonal bands of Broca cholinergic projections to the ventral hippocampus disrupt auditory fear memory

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Abstract

In classical fear conditioning, a neutral conditioned stimulus (CS) is paired with an aversive unconditioned stimulus (US), which leads to a fear memory. If the CS is repeatedly presented without the US after fear conditioning, the formation of an extinction memory occurs, which inhibits fear memory expression. A previous study has demonstrated that selective cholinergic lesions in the medial septum and vertical limb of the diagonal bands of Broca (MS/vDBB) prior to fear and extinction learning disrupt contextual fear memory discrimination and acquisition of extinction memory. MS/vDBB cholinergic neurons project to a number of substrates that are critical for fear and extinction memory. However, it is currently unknown which of these efferent projections are critical for contextual fear memory discrimination and extinction memory. To address this, we induced cholinergic lesions in efferent targets of MS/vDBB cholinergic neurons. These included the dorsal hippocampus (dHipp), ventral hippocampus (vHipp), medial prefrontal cortex (mPFC), and in the mPFC and dHipp combined. None of these lesion groups exhibited deficits in contextual fear memory discrimination or extinction memory. However, vHipp cholinergic lesions disrupted auditory fear memory. Because MS/vDBB cholinergic neurons are the sole source of acetylcholine in the vHipp, these results suggest that MS/vDBB cholinergic input to the vHipp is critical for auditory fear memory. Taken together with previous findings, the results of this study suggest that MS/vDBB cholinergic neurons are critical for fear and extinction memory, though further research is needed to elucidate the role of MS/vDBB cholinergic neurons in these types of emotional memory.

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