Input-output properties of neurons generated in the adult hippocampus
The
dentate gyrus of the mammalian hippocampus generate neurons throughout
life. Developing granule cells (GCs) require 6-8 weeks to reach
functional maturity. It has recently been proposed that developing
(immature) GCs might play a critical role in information processing.
However, whether young GCs can integrate afferent signals to produce an
output remains unknown, and the output of new GCs has never been
characterized. We have studied input-output conversion in developing
GCs of the adult mouse hippocampus. Glutamatergic inputs onto young
neurons are very weak compared to those of mature cells, yet
stimulation of cortical excitatory axons elicits a similar spiking
probability in neurons at either developmental stage. Young neurons are
highly efficient in transducing ion fluxes into membrane depolarization
due to their high input resistance, which decreases substantially in
mature neurons due to the delayed expression of a potassium
conductance, with a consequent reduction in intrinsic excitability.
To
study neurotransmitter release, new GCs were transduced with
light-activated cation channel Channelrhodopsin 2 (ChR2) and a
fluorescent reporter gene. Blue light elicited spikes in GCs expressing
ChR2. Postsynaptic responses were then assessed in randomly selected
hilar interneurons. About 10% of >110 neurons reliably displayed
postsynaptic currents (PSCs) in response to light pulses delivered to
fluorescent GCs in the slice. PSCs were blocked by glutamate receptor
antagonists, indicating that adult-born GCs make functional synaptic
contacts and release glutamate as their main neurotransmitter.
Therefore, newly generated CGs can integrate, process and convey
information onto target cells in the adult dentate gyrus.