Remote memories depend on coordinated activity in the hippocampus and frontal cortices, but the timeline of these interactions is debated. Astrocytes sense and modify neuronal activity, but their role in remote memory is scarcely explored. We expressed the Gi-coupled designer receptor hM4Di in CA1 astrocytes and discovered that astrocytic manipulation during learning specifically impaired remote, but not recent, memory recall and decreased activity in the anterior cingulate cortex (ACC) during retrieval. We revealed massive recruitment of ACC-projecting CA1 neurons during memory acquisition, which was accompanied by the activation of ACC neurons. Astrocytic Gi activation disrupted CA3 to CA1 communication in vivo and reduced the downstream response in the ACC. In behaving mice, it induced a projection-specific inhibition of CA1-to-ACC neurons during learning, which consequently prevented ACC recruitment. Finally, direct inhibition of CA1-to-ACC-projecting neurons spared recent and impaired remote memory. Our findings suggest that remote memory acquisition involves projection-specific functions of astrocytes in regulating CA1-to-ACC neuronal communication.
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We thank the entire Goshen Lab for their support. A.K. is supported by the JBC GOLD Scholarship. A.A. is supported by the Adams fellowship. This project has received funding from the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation program (grant agreement no. 803589 to I.G.). I.G. is also supported by the Israel Science Foundation (ISF grant no. 1815/18), the Israeli Centers of Research Excellence Program (center no. 1916/12) and Canada–Israel grants (CIHR–ISF, grant no. 2591/18). M.L. is a Sachs Family Lecturer in Brain Science and is supported by the ISF (ISF grant no. 1024/17) and the Einstein Foundation. We thank Y. Ziv, A. Citri, I. Slutsky and A. Doron for their critical reading of the manuscript.
The authors declare no competing interests.
Peer review information Nature Neuroscience thanks Sheena Josselyn, Andrea Volterra, and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.
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Extended Data Fig. 1 Prolonged Gi pathway activation in CA1 astrocytes reduces their calcium activity (Related to Fig. 1).
Following an injection of AAV8-GFAP::hM4Di-mCherry, hM4Di was expressed in 87% (491/552 cells from 4 mice) of CA1 astrocytes (a), with >96% specificity (491/507 cells, from 4 mice) (b). c, d, Minimal co-localization with the neuronal nuclear marker NeuN was detected (scale bar 50 µm; 0.9% expression in neurons, 7/766 cells). (e, g) Representative astrocytes expressing GCaMP6f (white) and hM4Di-mCherry (not visible, but see Fig. 1d) were exposed either to ACSF (e, f) or CNO (g, h). Representative ROIs, and their activity in corresponding colors are presented. Scale bars = 100 μm. CNO application triggered a decrease in baseline intracellular Ca2+ levels, and reduced the total size of Ca2+ events in these cells (see Fig. 1f,g). Data presented as mean ± standard error of the mean (SEM).
Extended Data Fig. 2 Astrocytic Gi activation in CA1 during learning had no effect on auditory-cued remote memory (Related to Fig. 1).
GFAP::hM4Di mice were injected with Saline (n = 7) or CNO (n = 6) 30 min before Fear Conditioning (FC) acquisition. CNO application before training had no effect on exploration of the conditioning cage (a), or on auditory-cued memory recall either 24 hr after acquisition (b) or 20 days after that (c) in a novel context, with both groups showing increased freezing during tone presentation (p < 0.001, p < 0.01, respectively). d, Bilateral double injection of AAV5-CaMKIIα::hM4Di-mCherry resulted in hM4Di-mCherry expression in CA1 Neurons only (top). Scale bar – 100 µm. The groups did not differ in the percent of hM4Di-expressing cells level of expression (Saline - 20.5%, CNO - 20.6%; bottom). CaMKIIα::hM4Di mice were injected with either Saline (n = 9) or CNO (n = 10) 30 min before FC acquisition. CNO application before training had no effect on exploration of the conditioning cage (e), or on auditory-cued memory recall either 24 hr after acquisition (f) or 20 days after that (g) in a novel context, with both groups showing increased freezing during tone presentation (p < 0.000001, p < 0.00001, respectively). (h) In a new group of GFAP::hM4Di mice, CNO administration (n = 12) only during the recall tests had no effect on either recent or remote memory, compared to Saline-injected controls (n = 12). In these mice, CNO administration during recall also had no effect on auditory cued memory either 24 hr after acquisition (i) or 20 days after that (j), compared to Saline-injected controls. When CNO was not administered during acquisition of the non-associative place recognition task, the GFAP::hM4Di mice (n = 6) from Fig. 1l showed equivalent performance to controls (n = 7; p < 0.01)(K). Example exploration traces and average Δ are shown (right). Data presented as mean ± SEM.
Extended Data Fig. 3 CNO application itself during learning had no effect on remote memory (Related to Fig. 1).
a, Bilateral double injection of AAV8-GFAP-eGFP resulted in eGFP expression in CA1 astrocytes only. Scale bar – left 300 µm, right 50 µm. Mice expressing eGFP in their CA1 astrocytes were injected with either Saline (n = 6) or CNO (n = 7) 30 min before fear conditioning acquisition. CNO administration before training to eGFP-expressing mice had no effect on baseline freezing or recent contextual memory recall one day later (b). Neither did CNO have any effect on remote memory 20 days later or 45 days after that (c). In the non-associative place recognition test, CNO application before a first visit to a new environment had no effect on remote memory 28 days later (s), reflected by a similar decrease (p < 0.0001) in the exploration between Saline injected (n = 6) and CNO-treated mice (n = 7) Example exploration traces and the average change (Δ) following treatment are shown on the right. Data presented as mean ± SEM.
Extended Data Fig. 4 CNO administration during acquisition reduces CA1 and ACC activity at the time of remote recall only in GFAP::hM4Di mice, and does not affect neuronal proliferation, differentiation, or survival (Related to Fig. 2).
a, Active neurons expressing cFos were quantified in the CA1, ACC, dentate gyrus (DG), retrosplenial cortex (RSC), and basolateral amygdala (BLA). GFAP::hM4Di mice from Fig. 2a, b that were injected with CNO (n = 6) before fear conditioning and showed impaired remote recall compared to Saline controls (n = 6), also demonstrated reduced number of cFos expressing neurons in CA1 and ACC (p < 0.05 for both) (b). No changes in cFos expression in the DG or RSC were observed in these mice, but the reduced fear was accompanied by a significant reduction in cFos expression in the BLA (p < 0.011) (c). GFAP::eGFP control mice were injected with CNO (n = 5) or Saline (n = 5) before fear conditioning, and then tested on the next day. No changes were observed in recent memory (d) or in the number of neurons active during recent recall in the CA1 or ACC (e). Other GFAP::eGFP mice were injected with CNO (n = 5) or Saline (n = 6) before fear conditioning, and then tested on the next day and again 21 days later. No changes were observed in recent or remote memory (f), or in the number of neurons active during remote recall in the CA1 or ACC (g). Representative images of GFAP::eGFP (green) and cFos (red in H,J green in I,K) following recent (h, i) or remote (j, k) recall in the CA1 (h, j) and ACC (i, k) are presented. l, GFAP::eGFP mice were injected with CNO or Saline together with BrdU before fear conditioning, and then tested on the next day. No changes were observed in stem cell proliferation (Brdu in white) (m) or in the number of young, Doublecortine (DCx)-positive neurons (white) (n). o, GFAP::eGFP mice were injected with CNO or Saline and BrdU before fear conditioning, and then tested 21 days later. No changes were observed in stem cell proliferation and differentiation (p) or in the number of young, DCx-positive neurons (q). Scale bars = 100μm for CA1, ACC and whole DG, 10 μm for zoomed-in cells. Data presented as mean ± SEM.
Extended Data Fig. 5 Gi pathway activation in CA1 astrocytes during memory acquisition does not affect the recruitment of the RSC and DG (Related to Fig. 3).
a, GFAP::hM4Di mice that were injected with CNO (n = 9) or Saline (n = 9) 30 minutes before fear conditioning showed similar immediate freezing following shock administration to Saline-injected controls. b, Active neurons expressing cFos were quantified in the in the CA1, basolateral amygdala (BLA), ACC, retrosplenial cortex (RSC) and dentate gyrus (DG) of GFAP::hM4Di mice that were injected with CNO (n = 9) or Saline (n = 9) 30 minutes before fear conditioning, or in home-caged mice (CNO n = 4, Saline n = 4). c, Representative images of hM4Di (red) and cFos (green) in the CA1 (C) and ACC (d) of home caged GFAP::hM4Di mice showing no effect of CNO administration on cFos levels. cFos-expressing astrocytes are observed below and above the CA1 pyramidal layer. Scale bars=100μm. e, Fear-conditioned GFAP::hM4Di mice showed increased cFos levels in the BLA compared to home-caged mice (p < 0.01), but CNO administration had no effect on either group. Fear-conditioning and CNO administration had no effect on cFos levels in the RSC and DG. Representative images of hM4Di (red) and cFos (green) in the BLA (f), RSC (g) and DG (h) are presented. (i) Double staining for cFos and GFAP showed a negligible (0.34%) percent of ACC astrocytes that express cFos. (j) An electrode dipped in DiI was placed in the ACC to record the response to CA1 activation. k, The location of the electrode in the ACC is shown in crimson, and no ChR2-eYFP positive axons (green) are observed in this region. All scale bars = 100μm. Data presented as mean ± SEM.
Extended Data Fig. 6 Gi pathway activation in CA1 astrocytes has no effect on cFos expression in home-caged mice (Related to Fig. 4).
a, b, Representative images of hM4Di in astrocytes (red), GFP in ACC-projecting CA1 neurons (green) and cFos (pink) in the CA1 of Saline- (A) or CNO- (B) injected home-caged mice are presented. No effect of CNO on cFos levels was observed. c, e, Representative images of hM4Di in astrocytes (red), GFP in NAc-projecting CA1 neurons (green) and cFos (pink) in the CA1 of Saline- (C) or CNO- (E) injected fear-conditioned mice are presented, showing no effect of the astrocytic manipulation on CA1→NAc neurons activity. The GFP-positive axons of these CA1 neurons are clearly observed in the NAc (d, f), with no apparent effect on cFos expression in this region. All scale bars=50μm.
Extended Data Fig. 7 Specific inhibition of CA1-to-ACC projection during learning had no effect on auditory-cued memory (Related to Fig. 5).
CA1→ACC-hM4Di mice were injected with Saline (n = 9) or CNO (n = 9) 30 min before FC acquisition. CNO application before training had no effect on exploration of the conditioning cage (a), or on auditory-cued memory recall either 24 hr after acquisition (b) or 20 days after that (c) in a novel context, with both groups showing increased freezing during tone presentation (p < 0.00001, p < 0.0001, respectively). Data presented as mean ± SEM.
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Kol, A., Adamsky, A., Groysman, M. et al. Astrocytes contribute to remote memory formation by modulating hippocampal–cortical communication during learning. Nat Neurosci 23, 1229–1239 (2020). https://doi.org/10.1038/s41593-020-0679-6
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