ABSTRACT
Adult neurogenesis occurs in the hippocampus and the olfactory bulb of the mammalian CNS. Recent studies have demonstrated that newborn granule cells of the adult hippocampus are postsynaptic targets of excitatory and inhibitory neurons, but evidence of synapse formation by the axons of these cells is still lacking. By combining retroviral expression of green fluorescent protein in adult-born neurons of the mouse dentate gyrus with immuno-electron microscopy, we found output synapses that were formed by labeled terminals on appropriate target cells in the CA3 area and the hilus. Furthermore, retroviral expression of channelrhodopsin-2 allowed us to light-stimulate newborn granule cells and identify postsynaptic target neurons by whole-cell recordings in acute slices. Our structural and functional evidence indicates that axons of adult-born granule cells establish synapses with hilar interneurons, mossy cells and CA3 pyramidal cells and release glutamate as their main neurotransmitter.
Subject(s)
Dentate Gyrus/cytology , Neurons/physiology , Synapses/physiology , Synaptic Transmission/physiology , Action Potentials/drug effects , Action Potentials/physiology , Action Potentials/radiation effects , Animals , Carrier Proteins/biosynthesis , Carrier Proteins/genetics , Dentate Gyrus/metabolism , Dentate Gyrus/virology , Female , GABA Antagonists/pharmacology , Gene Transfer Techniques , Genes, Reporter , Glutamic Acid/metabolism , Humans , Interneurons/physiology , Interneurons/ultrastructure , Light , Luminescent Proteins/biosynthesis , Luminescent Proteins/genetics , Mice , Mice, Inbred C57BL , Moloney murine leukemia virus/genetics , Mossy Fibers, Hippocampal/metabolism , Mossy Fibers, Hippocampal/ultrastructure , Mossy Fibers, Hippocampal/virology , Neurons/drug effects , Neurons/radiation effects , Organ Culture Techniques , Patch-Clamp Techniques , Photic Stimulation , Presynaptic Terminals/physiology , Presynaptic Terminals/ultrastructure , Synapses/genetics , Synapses/ultrastructure , Synaptic Transmission/drug effects , Synaptic Transmission/radiation effectsABSTRACT
PURPOSE: Epileptic seizures are frequently seen after viral encephalitis. Herpes simplex virus type 1 (HSV-1) encephalitis is the most common cause of acquired epilepsy in humans. However, little information is available about the neuropathogenesis of HSV-1-associated seizures. We have developed an in vitro HSV-1-infected organotypic hippocampal slice culture to elucidate the underlying mechanisms of HSV-1-associated acute seizure activity. METHODS: Hippocampal slice cultures were prepared from postnatal day 10 to 12 rat pups. Wild-type HSV-1 strain RE (1 x 10(5) PFU) was applied to cultures at 14 days in vitro. The excitability of CA3 pyramidal cells and hippocampal network properties were measured with electrophysiological recordings. Hematoxylin-eosin (H&E) and Timm stains were used. RESULTS: HSV-1 infection induces epileptiform activity, neuron loss, and subsequently a dramatic increase of mossy fiber sprouting in the supragranular area. With intracellular recordings, surviving CA3 pyramidal cells exhibited a more depolarizing resting membrane potential concomitant with an increase in membrane input resistance and had a lower threshold to generate synchronized bursts and a decrease in the amplitude of afterhyperpolarization than did controls. When the antiherpes agent acyclovir was applied with a delay of 1 or 24 h after HSV-1 infection, a dramatic inhibition of HSV-1 replication and protection of the neuron loss were observed. CONCLUSIONS: These results suggest that a direct change in the excitability of the hippocampal CA3 neuronal network and HSV-1-induced neuron loss resulting in subsequent mossy fiber reorganization may play an important role in the generation of epileptiform activity.
