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The Journal of Neurophysiology Vol. 82 No. 4 October 1999, pp. 1883-1894
Copyright ©1999 by the American Physiological Society
Department of Pharmacology and Cancer Biology and Department of Neurobiology, Duke University Medical Center, Durham, North Carolina 27710
Molnár, Péter and
J. Victor Nadler.
Mossy Fiber-Granule Cell Synapses in the Normal and Epileptic
Rat Dentate Gyrus Studied With Minimal Laser Photostimulation. J. Neurophysiol. 82: 1883-1894, 1999. Dentate granule cells become synaptically interconnected in the
hippocampus of persons with temporal lobe epilepsy, forming a recurrent
mossy fiber pathway. This pathway may contribute to the development and
propagation of seizures. The physiology of mossy fiber-granule cell
synapses is difficult to characterize unambiguously, because electrical
stimulation may activate other pathways and because there is a low
probability of granule cell interconnection. These problems were
addressed by the use of scanning laser photostimulation in slices of
the caudal hippocampal formation. Glutamate was released from a caged
precursor with highly focused ultraviolet light to evoke action
potentials in a small population of granule cells. Excitatory synaptic
currents were recorded in the presence of bicuculline. Minimal laser
photostimulation evoked an apparently unitary excitatory postsynaptic
current (EPSC) in 61% of granule cells from rats that had experienced
pilocarpine-induced status epilepticus followed by recurrent mossy
fiber growth. An EPSC was also evoked in 13-16% of granule cells from
the control groups. EPSCs from status epilepticus and control groups
had similar peak amplitudes (~30 pA), 20-80% rise times (~1.2
ms), decay time constants (~10 ms), and half-widths (~8 ms). The
mean failure rate was high (~70%) in both groups, and in both groups
activation of N-methyl-D-aspartate receptors
contributed a small component to the EPSC. The strong similarity
between responses from the status epilepticus and control groups
suggests that they resulted from activation of a similar synaptic
population. No EPSC was recorded when the laser beam was focused in the
dentate hilus, suggesting that indirect activation of hilar mossy cells
contributed little, if at all, to these results. Recurrent mossy fiber
growth increases the density of mossy fiber-granule cell synapses in the caudal dentate gyrus by perhaps sixfold, but the new synapses appear to operate very similarly to preexisting mossy fiber-granule cell synapses.
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