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. 2013 Mar 15;339(6125):1290-5.
doi: 10.1126/science.1229534.

A neural circuit for memory specificity and generalization

Wei Xu  1 , Thomas C Südhof
Affiliations

A neural circuit for memory specificity and generalization

Wei Xu et al. Science. .

Abstract

Increased fear memory generalization is associated with posttraumatic stress disorder, but the circuit mechanisms that regulate memory specificity remain unclear. Here, we define a neural circuit-composed of the medial prefrontal cortex, the nucleus reuniens (NR), and the hippocampus-that controls fear memory generalization. Inactivation of prefrontal inputs into the NR or direct silencing of NR projections enhanced fear memory generalization, whereas constitutive activation of NR neurons decreased memory generalization. Direct optogenetic activation of phasic and tonic action-potential firing of NR neurons during memory acquisition enhanced or reduced memory generalization, respectively. We propose that the NR determines the specificity and generalization of memory attributes for a particular context by processing information from the medial prefrontal cortex en route to the hippocampus.

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Figures

Fig. 1
Fig. 1. Distinct mPFC neurons project to different synaptic targets
A, Design of SynaptoTag AAV used for tracing synaptic connections. The synapsin promoter in the AAV drives bicistronic expression of soluble mCherry and a presynaptic EGFP-synaptobrevin-2 fusion protein (EGFP-Syb2). B, SynaptoTag AAV mapping of mPFC projections. Representative low-resolution (left and middle panels) and high-resolution images (right panels) illustrate synaptic targets for mPFC neurons. Red mCherry-labeling marks axonal fibers, while green EGFP-labeling marks synapses projecting from the mPFC (yellow = coincident red and green labeling; abbreviations: ACC, anterior cingulate cortex; BLA, basolateral nucleus of the amygdala; IL, infralimbic cortex; PL, prelimbic cortex; for complete sections, see Fig. S1). C, Retrograde labeling of mPFC neurons after injection of Alexa Fluor-488 and -594 labeled cholera toxin-B (CTB-488 and CTB-594) into the N. reuniens (N.re.; green) and the mediodorsal thalamic nucleus (MD; red), respectively. Low-power micrographs (left panels) show injection areas, while high-power images (right panels) depict the three major mPFC regions (ACC, anterior cingulate cortex; PL, prelimbic cortex; IL, infralimbic cortex). Most traced neurons were dominated by the presence of one fluorophore (for additional mPFC projections, see Fig. S2).
Fig. 2
Fig. 2. mPFC projection to the N. reuniens controls memory specificity
A, Design of AAVs used for inactivating synaptic transmission in subsets of projection neurons with specific synaptic targets. Double-floxed inverted TetTox-AAV (2xFlx-TetTox AAV) encodes bicistronic expression of EGFP for visualizing infected neurons and of TetTox for blocking synaptic transmission. The coding region of the double-floxed inverted TetTox-AAV is not translated until cre-recombinase flips the inverted coding region into the correct orientation. WGA-cre AAV mediates bicistronic expression of mCherry and WGA-cre. When this AAV infects a neuron, WGA-cre is trans-neuronally transferred to connected neurons, whereas mCherry is only expressed in the infected neuron. B, Coronal brain section of a mouse that was injected with 2xFlx-TetTox AAV in the mPFC, and with WGA-cre AAV in the dorsomedial striatum. The green EGFP fluorescence in the mPFC indicates that trans-synaptically transported WGA-cre activated expression of TetTox and EGFP in the mPFC. For high-magnification images, additional examples, and quantification of the trans-synaptic transport efficiency, see Figs. S3–S5. C, Experimental protocol for analyzing the behavioral effects of selective TetTox expression in mPFC neurons that project to specific targets. 2xFIx-TetTox AAV was stereotactically injected into the mouse mPFC, and WGA-cre AAV was injected into the striatum, mediodorsal thalamic nucleus, N. reuniens, or mPFC (control = no WGA-cre AAV injection). Mice were tested four weeks later for contextual fear conditioning (context test), fear conditioning in an altered context to measure memory precision, and cued fear conditioning (tone test). For additional information, see Fig. S6. D, Fear conditioning measured with the experimental strategy described in C in multiple independent experiments (numbers in bars = number of mice analyzed). The discrimination index was calculated as the difference between the percentage freezing in the training context and the altered context, divided by the sum of the two percentages. Data are means ± SEMs; statistical significance (* P<0.05; ** P<0.01) was assessed by (i) two-way mixed-model ANOVA with Bonferroni’s post-hoc test comparing the freezing levels, or (ii) one-way ANOVA followed by Turkey’s post-hoc test for the discrimination index.
Fig. 3
Fig. 3. N. reuniens bidirectionally controls fear memory generalization
A, Representative coronal brain section showing local expression of EGFP (green) after stereotactic injection into the N. reuniens of lentiviruses encoding EGFP and TetTox or the neuroligin-2 knockdown (NL2 KD). B, Schema of the effects of TetTox expression or of the neuroligin-2 knockdown (NL2 KD) on the activity of neurons in the N. reuniens. The neuroligin-2 knockdown decreases inhibition of N. reuniens neurons, thereby activating these neurons, whereas TetTox blocks synaptic outputs from N. reuniens neurons. C, Effect of neuroligin-2 knockdown on the frequency of spontaneous inhibitory miniature synaptic events (mIPSCs), recorded in acute N. reuniens slices from mice that were injected with neuroligin-2 knockdown lentivirus (number of neurons/mice analyzed is shown in the bars). D, Experimental protocol for testing fear memory after TetTox expression or neuroligin-2 knockdown in the N. reuniens. E, Bi-directional changes in fear memory generalization by neuronal silencing with TetTox or neuronal activation with the neuroligin-2 knockdown. Mice injected with lentivirus expressing only EGFP were used as control (mouse numbers are indicated in bars). F and G, Same as C and D, except that mice were injected with control or TetTox virus after fear conditioning training. H and I, Effect of fear conditioning training and of TetTox expression or neuroligin-2 knockdown in the N. reuniens on the activity levels of neurons in different target brain regions. Control, TetTox, or the neuroligin-2 knockdown (NL2 KD) lentiviruses were injected into the N. reuniens of adult mice. Mice were subjected to fear conditioning training (‘+training’) or received no training (‘naïve’) and sacrificed 90 min after training. Brain sections were stained for c-Fos (red) to measure neuronal activation and for NeuN to label all neuronal nuclei (blue). Panel H depicts representative images of the hippocampal CA1 region, and panel I quantification of c-Fos expression in the indicated brain regions (n= 12–18 brain sections from 4 mice in each group; for additional data, see Figs. S11 and S12). Data shown are means ± SEMs. Statistical significance (* P<0.05; ** P<0.01; *** P<0.001) was assessed by 2-tailed Student’s t-test (C and G), two-way ANOVA followed by Bonferroni’s post-hoc test (E, comparing freezing levels, and I), or one-way ANOVA followed by Turkey’s post-hoc test (discrimination index in E).
Fig. 4
Fig. 4. Firing pattern of N. reuniens neurons dictates memory generalization
A, Coronal brain section illustrating expression of ChIEF-tdTomato (red fluorescent channelrhodopsin) in the N. reuniens (top), and high-magnification micrograph showing ChIEF-tdTomato expressing N. reuniens neurons and their axonal fibers (bottom). B, Experimental protocol for testing the effect of different optogenetic stimulation patterns of N. reuniens neurons on fear conditioning behavior, with the stimulation patterns illustrated below the time diagram. N. reuniens neurons were stimulated throughout the 6-min training period either by a 4 Hz tonic stimulation or a 30 Hz phasic stimulation, administered for 0.5 s every 5 seconds; stimulus light pulses were 15 ms. C, Tonic and phasic optogenetic stimulation produced opposite effects on fear memory generalization. Control mice also expressed channelrhodopsin and contained an implanted optical fiber, but were not stimulated. Data shown are means ± SEMs; numbers in bars indicate number of mice analyzed. Statistical significance (* P<0.05; ** P<0.01) was assessed by two-way mixed-model ANOVA followed by Bonferroni’s post-hoc test comparing the freezing levels, or by one-way ANOVA followed by Turkey’s post-hoc test for the discrimination index.
Fig. 5
Fig. 5. Model for the mechanism of N. reuniens’ control of memory generalization
A, Schematic diagram of the synaptic interactions between the mPFC, N. reuniens, and hippocampus in controlling memory generalization. B, Illustration of the modular composition of memory features. We posit that memories differentially incorporate a composite of specific attributes. The more prominent a feature is, the more likely it is included in memory, as illustrated here with a baseball containing additional features besides ‘ballness’. We propose that N. reuniens neurons control memory generalization by regulating the number of features that are incorporated into a memory. For a more detailed discussion, see Fig. S13.
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References

    1. Mahan AL, Ressler KJ. Trends Neurosci. 2012 Jan;35:24. - PMC - PubMed
    1. Gotlib IH, Joormann J. Annu Rev Clin Psychol. 2010;6:285. - PMC - PubMed
    1. Watson JB, Rayner R. Am Psychol. 2000 Mar;55:313. - PubMed
    1. Frankland PW, Cestari V, Filipkowski RK, McDonald RJ, Silva AJ. Behav Neurosci. 1998 Aug;112:863. - PubMed
    1. Freeman FG, Kramarcy NR, Lee J. Physiol Behav. 1973 Aug;11:273. - PubMed

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