Abstract
The dentate gyrus is a neurogenic zone where neurons continue to be born throughout life, mature and integrate into the local circuitry. In adults, this generation of new neurons is thought to contribute to learning and memory formation. As newborn neurons mature, they undergo a developmental sequence in which different stages of development are marked by expression of different proteins. Doublecortin (DCX) is an early marker that is expressed in immature granule cells that are beginning migration and dendritic growth but is turned off before neurons reach maturity. In the present study, we use a mouse strain in which enhanced green fluorescent protein (EGFP) is expressed under the control of the DCX promoter. We show that these neurons have high input resistances and some cells can discharge trains of action potentials. In mature granule cells, action potentials are followed by a slow afterhyperpolarization that is absent in EGFP-positive neurons. EGFP-positive neurons had a lower spine density than mature neurons and stimulation of either the medial or lateral perforant pathway activated dual component glutamatergic synapses that had both AMPA and NMDA receptors. NMDA receptors present at these synapses had slow kinetics and were blocked by ifenprodil, indicative of high GluN2B subunit content. These results show that EGFP-positive neurons in the DCX-EGFP mice are functionally immature both in their firing properties and excitatory synapses.
Highlights
The mammalian hippocampal dentate gyrus is part of the limbic system and plays an essential role in learning and memory formation
As different developmental stages can be differentiated by the expression of unique sets of proteins [4], we first verified the status of the enhanced green fluorescent protein (EGFP)-expressing neurons by testing for expression of DCX, glial fibrillary acidic protein (GFAP), PSA-NCAM, calretinin and calbindin (Fig. 1)
Consistent with the fact that DCX expression marks cells that develop into neurons, we found no overlap between GFAP and EGFP expression (Fig. 1A)
Summary
The mammalian hippocampal dentate gyrus is part of the limbic system and plays an essential role in learning and memory formation. Unlike in most other brain regions, the generation and development of new neurons in the dentate gyrus continues beyond the normal developmental period persisting at a slower rate throughout adult life in both rodents and humans [2,3]. This neurogenesis occurs in the sub granular zone (SGZ) between the granule cell layer and the hilus [4]. It is very useful to be able to identify newborn neurons directly, prior to experimentation
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