Elsevier

Experimental Neurology

Volume 263, January 2015, Pages 285-292
Experimental Neurology

Regular Article
Novel connection between newborn granule neurons and the hippocampal CA2 field

https://doi.org/10.1016/j.expneurol.2014.10.021Get rights and content

Highlights

  • Newborn neurons form synaptic contacts with CA2 pyramidal neurons.

  • Newborn neurons participate in the “alternative trisynaptic circuit”.

  • This connection can be modulated by both pathological and neuroprotective stimuli.

Abstract

Newborn neurons are continuously added to the hippocampal dentate gyrus (DG) throughout life. Mature and immature granule neurons are believed to send their axonal projections exclusively to the hippocampal CA3 field. However, recent data point to an alternative trisynaptic circuit, involving a direct axonal projection from mature granule neurons to the CA2 field. Whether this circuit takes place only in mature granule neurons or, on the contrary, whether immature granule neurons also contribute to this novel connection is unknown. We used various retroviral vectors to show that immature granule neurons send axonal processes to and establish synaptic contacts with CA2 pyramidal neurons and that axonal growth follows a similar time course to that described for CA3 innervation. In addition, we provide experimental evidence demonstrating that the pathway connecting newborn granule neurons and the CA2 field can be modulated by physiological and deleterious stimuli.

Introduction

Newborn neurons are continuously added to the subgranular zone (SGZ) of the hippocampal dentate gyrus (DG) throughout life. Progenitor cells asymmetrically divide and give rise to transiently amplifying neuronal precursors. Immature neuroblasts actively proliferate and differentiate into mature granule neurons, going through various stages of maturation. During the differentiation of newborn neurons their dendritic tree complexity increases, and axons are projected towards the CA3 region (Zhao et al., 2006), establishing functional synapses with CA3 pyramidal neurons (Toni et al., 2008). At the end of this maturation process, newborn neurons are fully integrated in the classic trisynaptic circuit and are functionally indistinguishable from surrounding mature granule neurons (Zhao et al., 2006). However, during immaturity, newborn neurons are characterized by a lower activation threshold (Schmidt-Hieber et al., 2004), and adult hippocampal neurogenesis (AHN) is involved in hippocampal-dependent learning, being crucial for pattern separation (Clelland et al., 2009, Nakashiba et al., 2012). AHN is a tightly regulated process modulated by numerous external stimuli.

It has been widely demonstrated that the classic hippocampal trisynaptic circuit (Enthorrinal Cortex ((EC))-DG-CA3-CA1) is one of the most important networks involved in memory and learning processes. However, recent data point to the existence of an alternative trisynaptic circuit (Kohara et al., 2014) that involves the connection between mature granule neurons and CA2 pyramidal neurons. This alternative circuit is crucial for contextual and social memory (Hitti and Siegelbaum, 2014, Wintzer et al., 2014). However, whether newborn granule neurons contribute to this connection remains unknown. Here we analyzed the potential contribution and time course of the connections between newborn granule neurons and the hippocampal CA2 region. To this end, we used various retroviral vectors (encoding for either GFP or Synaptophysin-GFP), and specific markers for the axons of newborn neurons (3R-Tau) and for the CA2 field (RGS14 and PCP4) (Antonio et al., 2014, Evans et al., 2014). In addition, we show how both deleterious and neuroprotective stimuli modulate these connections.

Section snippets

Animals

Six-week-old female C57BL/6JRcc mice were obtained from Harlan Laboratories. Animals were housed in a specific pathogen-free colony facility, in accordance with European Community Guidelines (directive 86/609/EEC), and handled following European and local animal care protocols. Animal experiments received the approval of the CBMSO's Ethics Committee. Five mice per experimental condition and time point were used in all the experiments.

Physical exercise protocol

Animals were habituated to the treadmill apparatus for 15 min

Results

Here we sought to determine whether there are direct connections between newborn granule and CA2 pyramidal neurons. For this purpose, we injected two types of retrovirus in order to analyze the time course, anatomical distribution, and morphology of the mossy fiber terminals (MFTs) of newborn granule neurons in the CA2 field.

Discussion

In the classical view of the hippocampal trisynaptic circuit, information from the EC is transmitted sequentially to the DG, CA3 and then to CA1, thus forming the main efferent information pathway of the hippocampus (van Strien et al., 2009). Recently, a novel alternative trisynaptic circuit involving the CA2 region has been described (Kohara et al., 2014). The CA2 region is characterized by large pyramidal neurons lacking thorny excrescences on their apical dendrites. A widely accepted

Conflicts of interest

The authors declare no conflict of interest.

Acknowledgments

This study was funded by grants from the Spanish Ministry of Health (SAF 2006-02424, BFU-2008-03980, BFU-2010-21507), the Comunidad de Madrid (SAL/0202/2006), the Centro de Investigación Biomédica en Red sobre Enfermedades Neurodegenerativas (CIBERNED, ISCIII) (CB401), and the Fundación R. Areces.

The authors thank E. García for the help in producing retroviral vectors; N. de la Torre for the help with manuscript edition; F.H. Gage for providing the plasmids used for the production of

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