Electrophysiological Evidence That Dentate Hilar Mossy Cells are Excitatory
and Innervate Both Granule Cells and Interneurons.
Scharfman, Helen E.
Neurology Research Center and Departments of Pharmacology and Neurology,
Helen Hayes Hospital Columbia University, College of Physicians and Surgeons,
West Haverstraw, NY 10993-1195 New York, NY 10032.
APStracts 2:0082N, 1995.
SUMMARY AND CONCLUSIONS
1. The hypothesis that dentate hilar "mossy" cells are excitatory was
tested by simultaneous intracellular recording in rat hippocampal slices.
Mossy cells were recorded simultaneous to their potential targets, granule
cells and interneurons. The GABA A receptor antagonist bicuculline was used in
most experiments to block the normally strong inhibitory inputs to granule
cells that could mask excitatory effects of mossy cells. Some cells were
recorded with electrodes containing the marker Neurobiotin so that their
identity could be confirmed morphologically. 2. A mossy cell action potential
was immediately followed by a brief depolarization in a granule cell in 20 of
1316 pairs (1.5%) that were recorded in the presence of bicuculline. The mean
amplitude of depolarizations was 1.99 ¯+ 0.24 mV ( ¯+ standard error of the
mean) when the postsynaptic membrane potential was -55 to -65 mV.
Depolarizations could trigger an action potential if the granule cell was
depolarized from its resting potential so that its membrane potential was -50
to -60 mV. These data suggest that mossy cells excite granule cells
monosynaptically. 3. Monosynaptic excitation of an interneuron by a mossy cell
was recorded in 4 of 47 (8.5%) simultaneously-recorded mossy cells and
interneurons, also in the presence of bicuculline. The mean interneuron
depolarization was 1.64 ¯+ 0.29 mV when the interneuron membrane potential was
approximately -60 mV. When an interneuron was at its resting potential (-52 to
-63 mV), action potentials were often triggered by the depolarizations. 4.
Without bicuculline present, mossy cells had no apparent monosynaptic effects
on granule cells, as has been previously reported (Scharfman et al., 1990).
However, effects that appeared to be polysynaptic were observed in 5 of 92
pairs (5.4%). Specifically, a small, brief hyperpolarization occurred in
granule cells 2.5-7.3 msec after the peak of a mossy cell action potential.
Given the results indicating that mossy cells excite interneurons, and the
long latency to onset of the hyperpolarization, one possible explanation for
the hyperpolarization is that mossy cells excited interneurons that inhibited
granule cells. 5. The results suggest that mossy cells are excitatory neurons.
In addition, mossy cells appear to innervate both granule cells and
interneurons which are located within several hundred microns of the mossy
cell soma. The only detectable effect on granule cells in this area under
normal conditions appears to be disynaptic and inhibitory. However, when GABA
A receptor-mediated inhibition is blocked, monosynaptic excitation of granule
cells by mossy cells can be detected.
Received 20 December 1994; accepted in final form 9 March 1995.
APS Manuscript Number J796-4.
Article publication pending J. Neurophysiol.
ISSN 1080-4757 Copyright 1995 The American Physiological Society.
Published in APStracts on 19 April 1995.