Neuronal Allocation to a Hippocampal Engram - PubMed (original) (raw)

Neuronal Allocation to a Hippocampal Engram

Sungmo Park et al. Neuropsychopharmacology. 2016 Dec.

Abstract

The dentate gyrus (DG) is important for encoding contextual memories, but little is known about how a population of DG neurons comes to encode and support a particular memory. One possibility is that recruitment into an engram depends on a neuron's excitability. Here, we manipulated excitability by overexpressing CREB in a random population of DG neurons and examined whether this biased their recruitment to an engram supporting a contextual fear memory. To directly assess whether neurons overexpressing CREB at the time of training became critical components of the engram, we examined memory expression while the activity of these neurons was silenced. Chemogenetically (hM4Di, an inhibitory DREADD receptor) or optogenetically (iC++, a light-activated chloride channel) silencing the small number of CREB-overexpressing DG neurons attenuated memory expression, whereas silencing a similar number of random neurons not overexpressing CREB at the time of training did not. As post-encoding reactivation of the activity patterns present during initial experience is thought to be important in memory consolidation, we investigated whether post-training silencing of neurons allocated to an engram disrupted subsequent memory expression. We found that silencing neurons 5 min (but not 24 h) following training disrupted memory expression. Together these results indicate that the rules of neuronal allocation to an engram originally described in the lateral amygdala are followed in different brain regions including DG, and moreover, that disrupting the post-training activity pattern of these neurons prevents memory consolidation.

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Figures

Figure 1

Figure 1

CREB overexpression in DG neurons training preferentially biases their allocation to an engram supporting contextual fear memory (chemogenetic studies). (a) HSV vector microinjection produces strong localized infection of DG principal neurons. Example image from mouse brain 4 days post microinjection. (b) vCREB-hM4Di and hM4Di vectors infected a similar number of DG neurons. vhM4Di (_n_=15 sections from 5 mice), vCREB-hM4Di (_n_=15 sections from 5 mice). (c) DG neurons expressing hM4Di show endogenous levels of CREB protein, whereas DG neurons expressing vCREB-hM4Di show high levels of CREB. GFP (green, GFP, infected neuron), CREB (red, CREB protein expression). (d) Pre-test chemogenetic silencing of neurons that overexpressed CREB during training inhibits subsequent memory expression (CNO in mice expressing vCREB-hM4Di). Silencing a similar number of random neurons (expressing hM4Di without vCREB) failed to disrupt memory expression. These results indicate that the neurons overexpressing CREB are preferentially allocated to an engram. hM4Di and VEH (_n_=20); hM4Di and CNO (_n_=22); vCREB-hM4Di and VEH (_n_=23); vCREB-hM4Di and CNO (_n_=24). Data presented are mean±SEM. n.s., not statistically different, **p<0.01.

Figure 2

Figure 2

CREB overexpression in DG neurons preferentially biases their allocation to an engram supporting contextual fear memory. Optogenetically silencing their activity during a memory test selectively impairs memory expression. (a) Microinjection of vCREB-hM4Di produces strong localized transgene expression in DG principal neurons. (b) Blue light (BL+) silencing decreases freezing in mice with vCREB-iC++ vector but not in mice expressing iC++ vector alone, regardless of order of light presentation during test (BL+, BL−) (c). (b) vCREB-iC++ (_n_=14), iC++ (_n_=10), (c) vCREB-iC++ (_n_=9), iC++ (_n_=10). Data presented are mean±SEM. n.s., not statistically different, **p<0.01, ***p<0.001.

Figure 3

Figure 3

Post-training inhibition of DG neurons allocated to an engram disrupts subsequent contextual fear memory expression. (a) Silencing of neurons expressing vCREB-iC++ 5 min following training decreases subsequent freezing at test, even in the absence of blue light during the test. However, silencing neurons expressing iC++ alone (5 min post training) had no effect on subsequent memory test. vCREB-iC++ and post-training light (n =11), vCREB-iC++ and no light (_n_=5), iC++ and post-training light (_n_=6). (b) Silencing neurons expressing vCREB-iC++ 24 h following training does not impair freezing during the test in the absence of light. vCREB-iC++ and 24 h post-training light (_n_=10). Data presented are mean±SEM. n.s., not statistically different, *p<0.05, **p<0.01, ***p<0.001.

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