Abstracts

Hilar mossy cells (MCs) are large glutamatergic neurons found within the dentate hilus and are known to receive at least two main inputs, mossy fiber inputs from the dentate granule cells and recurrent collateral and commissural inputs from the CA3 pyramidal cells and probably other mossy cells. Although many mossy cells are lost during the development of temporal lobe epilepsy, there is evidence that at least a portion remains. Therefore, we predict that there are two populations of mossy cells in the normal hilus.
mEPSCs were recorded from mossy cells in the hippocampal hilus from 250 [micro]M brain slices from young to adult rats using whole cell patch clamp recording techniques. Mossy cells were identified during the recordings by their large size and characteristic, slowly decaying EPSCs. In addition Lucifer Yellow fills were used to further identify these cells by their large spines and thorny excressences. Recordings were made at room temperature in artificial CSF containing TTX (1 uM) and bicuculline (30 uM). Data were acquired at 70kHz after being filtered at 10 kHz with a 4 Pole Bessel low pass filter. Data were further digitally filtered at 2-5 kHz for analysis.
We have recorded miniature excitatory postsynaptic currents (mEPSCs) from MCs and found two distinct populations of events based on their kinetics and quantal size. The most obvious distinguishing feature of these events is a distinct quantal size (approximately 9 pA for mossy fibers and 6 pA for CC inputs). Interestingly, mEPSCs comprise not a single quanta as would be expected, but may be made up of one (9 pA), two (18 pA), three (27 pA), or many multiples of the quantal size, as would be expected for evoked EPSCs. CC inputs, on the other hand were 6 pA, 12 pA, 18 pA,etc.
The events attributed to the mossy fibers also have slower kinetics (approximately 8-12 msec) than the more distally occurring collateral commissural events (4-8 msec). The two populations of events can be separated by blocking the mossy fiber events with DCG-IV (0.1 to 1 uM). In the presence of this Group II selective mGluR agonist, discrete quantal peaks are more obvious.
The proportion of mossy fiber and CC inputs varied between cells from approximately 30 to 70%
Mossy cells receive two distinct inputs in normal hippocampal slices. These inputs are readily distinguished by quantal size, and by rise and decay time constant kinetics. We hypothesize that our observation of mEPSCs of varying quantal size indicates that there is a varying probability of release or multiple receptor clusters resulting in the multiple quanta at individual synapses. If so, this is a novel finding that alters our understanding of synaptic transmission. Furthermore, the proportion of these two inputs varies from cell to cell and could underlie the variability in susceptibility to cell death between mossy cells.
[Supported by: NIH]