C. Ranganath and G. Rainer, Neural mechanisms for detecting and remembering novel events, Nat. Rev. Neurosci, vol.4, pp.193-202, 2003.

M. T. Van-kesteren, D. J. Ruiter, G. Fernández, and R. N. Henson, How schema and novelty augment memory formation, Trends Neurosci, vol.35, pp.211-219, 2012.

J. E. Lisman and A. A. Grace, The hippocampal-VTA loop: controlling the entry of information into long-term memory, Neuron, vol.46, pp.703-713, 2005.

C. G. Mcnamara, Á. Tejero-cantero, S. Trouche, N. Campo-urriza, and D. Dupret, Dopaminergic neurons promote hippocampal reactivation and spatial memory persistence, Nat. Neurosci, vol.17, pp.1658-1660, 2014.

T. Takeuchi, Locus coeruleus and dopaminergic consolidation of everyday memory, Nature, vol.537, pp.357-362, 2016.

W. X. Pan and N. Mcnaughton, The supramammillary area: its organization, functions and relationship to the hippocampus, Prog. Neurobiol, vol.74, pp.127-166, 2004.

C. B. Saper and B. B. Lowell, The hypothalamus, Curr. Biol, vol.24, pp.1111-1116, 2014.

D. Wirtshafter, T. R. Stratford, and I. Shim, Placement in a novel environment induces fos-like immunoreactivity in supramammillary cells projecting to the hippocampus and midbrain, Brain Res, vol.789, pp.331-334, 1998.

M. Ito, T. Shirao, K. Doya, and Y. Sekino, Three-dimensional distribution of Fos-positive neurons in the supramammillary nucleus of the rat exposed to novel environment, Neurosci. Res, vol.64, pp.397-402, 2009.

Y. Kobayashi, Genetic dissection of medial habenula-interpeduncular nucleus pathway function in mice, Front. Behav. Neurosci, vol.7, p.17, 2013.

K. Franklin and G. Paxinos, The Mouse Brain in Stereotaxic Coordinates (Academic, 2007.

H. Hama, ScaleS: an optical clearing palette for biological imaging, Nat. Neurosci, vol.18, pp.1518-1529, 2015.

R. Soussi, N. Zhang, S. Tahtakran, C. R. Houser, and M. Esclapez, Heterogeneity of the supramammillary-hippocampal pathways: evidence for a unique GABAergic neurotransmitter phenotype and regional differences, Eur. J. Neurosci, vol.32, pp.771-785, 2010.

L. G. Reijmers, B. L. Perkins, N. Matsuo, and M. Mayford, Localization of a stable neural correlate of associative memory, Science, vol.317, pp.1230-1233, 2007.

N. P. Pedersen, Supramammillary glutamate neurons are a key node of the arousal system, Nat. Commun, vol.8, p.1405, 2017.

Y. Hashimotodani, F. Karube, Y. Yanagawa, F. Fujiyama, and M. Kano, Supramammillary nucleus afferents to the dentate gyrus co-release glutamate and GABA and potentiate granule cell output, Cell Rep, vol.25, pp.2704-2715, 2018.

N. X. Tritsch, A. J. Granger, and B. L. Sabatini, Mechanisms and functions of GABA co-release, Nat. Rev. Neurosci, vol.17, pp.139-145, 2016.

R. Boehringer, Chronic loss of CA2 transmission leads to hippocampal hyperexcitability, Neuron, vol.94, pp.642-655, 2017.
URL : https://hal.archives-ouvertes.fr/inserm-02769227

S. L. Resendez, Social stimuli induce activation of oxytocin neurons within the paraventricular nucleus of the hypothalamus to promote social behavior in male mice, J. Neurosci, vol.40, pp.2282-2295, 2020.

Z. Wu, A. E. Autry, J. F. Bergan, M. Watabe-uchida, and C. G. Dulac, Galanin neurons in the medial preoptic area govern parental behaviour, Nature, vol.509, pp.325-330, 2014.

B. A. Strange, M. P. Witter, E. S. Lein, and E. I. Moser, Functional organization of the hippocampal longitudinal axis, Nat. Rev. Neurosci, vol.15, pp.655-669, 2014.

J. K. Leutgeb, S. Leutgeb, M. B. Moser, and E. I. Moser, Pattern separation in the dentate gyrus and CA3 of the hippocampus, Science, vol.315, pp.961-966, 2007.

T. J. Mchugh, Dentate gyrus NMDA receptors mediate rapid pattern separation in the hippocampal network, Science, vol.317, pp.94-99, 2007.

M. E. Wintzer, R. Boehringer, D. Polygalov, and T. J. Mchugh, The hippocampal CA2 ensemble is sensitive to contextual change, J. Neurosci, vol.34, pp.3056-3066, 2014.

M. C. Chiang, A. J. Huang, M. E. Wintzer, T. Ohshima, and T. J. Mchugh, A role for CA3 in social recognition memory, Behav. Brain Res, vol.354, pp.22-30, 2018.

F. L. Hitti and S. A. Siegelbaum, The hippocampal CA2 region is essential for social memory, Nature, vol.508, pp.88-92, 2014.

R. A. Piskorowski, Age-dependent specific changes in area CA2 of the hippocampus and social memory deficit in a mouse model of the 22q11.2 deletion syndrome, Neuron, vol.89, pp.163-176, 2016.
URL : https://hal.archives-ouvertes.fr/inserm-02768786

G. M. Alexander, Social and novel contexts modify hippocampal CA2 representations of space, Nat. Commun, vol.7, p.10300, 2016.

A. S. Smith, S. K. Williams-avram, A. Cymerblit-sabba, J. Song, and W. S. Young, Targeted activation of the hippocampal CA2 area strongly enhances social memory, Mol. Psychiatry, vol.21, pp.1137-1144, 2016.

T. Meira, A hippocampal circuit linking dorsal CA2 to ventral CA1 critical for social memory dynamics, Nat. Commun, vol.9, p.4163, 2018.

, Sample traces (i) and time course of amplitudes ( j, IPSCs only, n = 22 GCs from 8 mice) of light-evoked EPSCs (black) and IPSCs (red) recorded in DG GCs before and after application of 10 ?M NBQX and 50 ?M APV (grey), and further application of 1 ?M SR95531 and 2 ?M CGP55845A (green)

. ****p-<-0, , vol.0001

*. Kolmogorov-smirnov-test and . ***p-<-0, Control sample traces are shown in black (EPSC) and red (IPSC), and traces following application of TTX and 4-AP are shown in grey. q, Time course of light-evoked EPSC and IPSC amplitudes the DG-projecting (e) and CA2-projecting (f) SuM neurons. Scale bars, 1 mm. g, h, Quantification of inputs from various brain regions to the DG-projecting (g, n = 4 mice) and CA2-projecting (h, n = 5 mice) SuM neurons. Both populations received extensive inputs from subcortical regions including the hypothalamus, brainstem, septum and nucleus accumbens. However, the inputs to the DG-projecting population were comparatively biased to brain regions in the reward and motor systems, such as the VTA, SI, AcbSh, LS and MS, whereas the CA2 projectors received proportionally greater inputs from neurons in socially engaged regions, Sample traces (n) and time course of amplitudes (o, IPSCs only, n = 7 from 4 mice) of light-evoked EPSCs (black) and IPSCs (red) , both SuM-DG and SuM-CA2 transmissions recruit a robust feed-forward inhibition. p, vol.0001

, MS, medial septal nucleus

S. Si and . Innominata,

;. Acbsh, Z. Zi, and . Incerta,

, LHA, lateral hypothalamic area

. Mpo,

, LPO, lateral preoptic area; PVH, paraventricular hypothalamic nucleus

, PH, posterior hypothalamic nucleus; PAG, periaqueductal grey

. Mrn and . Vta, Raphe: DR, dorsal raphe nucleus and MnR, median raphe nucleus. materials The novel Cre-expressing transgenic mouse (Csf2rb2-Cre) and plasmids used for custom virus generation are availble from the authors upon reasonable request

, Antibodies Antibodies used

. Elisa, . Ihc-frfl, . Icc, F. Ihc-fofr, . Cyt et al., -74816), clone C-15: PCP-4 is an affinity purified rabbit polyclonal antibody raised against a peptide mapping at the C-terminus of PCP-4 of human origin. PCP-4 is recommended for detection of PCP-4 of mouse, rat and human origin by Western Blotting (starting dilution 1:200, dilution range 1:100-1:1000), immunoprecipitation [1-2 ?g per 100-500 ?g of total protein (1 ml of cell lysate), immunofluorescence (starting dilution 1:50, dilution range 1:50-1:500 ), immunohistochemistry (including paraffin-embedded sections) (starting dilution 1:50, dilution range 1:50-1 :500) and solid phase ELISA (starting dilution 1:30, dilution range 1:30-1:3000). Anti-GFP antibody, Anti-cFos (Synaptic Systems, 226003): Polyclonal rabbit purified antibody, affinity purified with the immunogen. Albumin and azide were added for stabilization. Applications -WB: 1:250 to 1:500 (AP staining), ICC: 1 : 1000, IHC: 1:1000. Anti-PCP4 antibody, p.70007