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The Journal of Neurophysiology Vol. 83 No. 2 February 2000, pp. 693-704
Copyright ©2000 by the American Physiological Society
Departments of Neurology and Anatomy and The Neuroscience Training Program, University of Wisconsin, Madison, Wisconsin 53792
Lynch, Michael and
Thomas Sutula.
Recurrent Excitatory Connectivity in the Dentate Gyrus of Kindled
and Kainic Acid-Treated Rats. J. Neurophysiol. 83: 693-704, 2000. Repeated seizures induce mossy fiber
axon sprouting, which reorganizes synaptic connectivity in the dentate
gyrus. To examine the possibility that sprouted mossy fiber axons may
form recurrent excitatory circuits, connectivity between granule cells
in the dentate gyrus was examined in transverse hippocampal slices from normal rats and epileptic rats that experienced seizures induced by
kindling and kainic acid. The experiments were designed to functionally
assess seizure-induced development of recurrent circuitry by exploiting
information available about the time course of seizure-induced synaptic
reorganization in the kindling model and detailed anatomic characterization of sprouted fibers in the kainic acid model. When
recurrent inhibitory circuits were blocked by the GABAA
receptor antagonist bicuculline, focal application of glutamate
microdrops at locations in the granule cell layer remote from the
recorded granule cell evoked trains of excitatory postsynaptic
potentials (EPSPs) and population burst discharges in epileptic rats,
which were never observed in slices from normal rats. The EPSPs and burst discharges were blocked by bath application of 1 µM
tetrodotoxin and were therefore dependent on network-driven synaptic
events. Excitatory connections were detected between blades of the
dentate gyrus in hippocampal slices from rats that experienced kainic acid-induced status epilepticus. Trains of EPSPs and burst discharges were also evoked in granule cells from kindled rats obtained after
1
wk of kindled seizures, but were not evoked in slices examined 24 h after a single afterdischarge, before the development of sprouting.
Excitatory connectivity between blades of the dentate gyrus was also
assessed in slices deafferented by transection of the perforant path,
and bathed in artificial cerebrospinal fluid (ACSF) containing
bicuculline to block GABAA receptor-dependent recurrent
inhibitory circuits and 10 mM [Ca2+]o to
suppress polysynaptic activity. Low-intensity electrical stimulation of
the infrapyramidal blade under these conditions failed to evoke a
response in suprapyramidal granule cells from normal rats
(n = 15), but in slices from epileptic rats evoked an EPSP at a short latency (2.59 ± 0.36 ms) in 5 of 18 suprapyramidal granule cells. The results are consistent with formation
of monosynaptic excitatory connections between blades of the dentate
gyrus. Recurrent excitatory circuits developed in the dentate gyrus of
epileptic rats in a time course that corresponded to the development of mossy fiber sprouting and demonstrated patterns of functional connectivity corresponding to anatomic features of the sprouted mossy
fiber pathway.
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