Large Amplitude Miniature Excitatory Postsynaptic Currents in

Comments

Transcription

Large Amplitude Miniature Excitatory Postsynaptic Currents in
Large Amplitude Miniature Excitatory Postsynaptic Currents in
Hippocampal CA3 Pyramidal Neurons Are of Mossy Fiber Origin
DARRELL A. HENZE, 1 J. PATRICK CARD, 1 GERMAN BARRIONUEVO, 1 AND YEZEKIEL BEN-ARI 2
1
Department of Neuroscience and Center for the Neural Basis of Cognition, University of Pittsburgh, Pittsburgh,
Pennsylvania 15260; and 2 Institut National de la Santé et de la Recherche Médicale, Unité 29, 75014, Paris, France
INTRODUCTION
One useful tool in investigations of the basic mechanisms
of neurotransmission is the study of the spontaneous miniature excitatory postsynaptic currents (mEPSCs). mEPSCs
are signals produced by the sodium action potential independent spontaneous release of neurotransmitter from the presynaptic terminal. The properties of mEPSCs, in particular
frequency of occurrence and relative amplitude, offer a
means to study the otherwise inaccessible presynaptic terminals within the CNS (Malgaroli and Tsien 1992; Manabe et
al. 1992). However, the major drawback in the use of
mEPSCs to study the presynaptic element is the difficulty
in identifying the synaptic origin of an individual synaptic
current. This is particularly problematic when studying complex neurons of the CNS that receive anatomically and physiologically heterogeneous populations of synapses such as is
the case for CA3 pyramidal neurons (Amaral and Witter
1989; Blackstad 1956; Blackstad et al. 1970; Hjorth-Simonsen 1973; Hjorth-Simonsen and Jeune 1972).
CA3 pyramidal cells receive a major glutamatergic synaptic input from the granule cells of the dentate gyrus via the
mossy fibers (MFs) (Neuman et al. 1988). The MFs make
synaptic contacts on to the proximal dendrites of the CA3
pyramidal cells and, therefore, are believed to play a critical
role in modulating activity in these cells. Numerous studies
already have demonstrated that the MF to CA3 pyramidal
cell synapse has unique features that distinguish it from other
cortical synapses. For example, the MF synaptic terminal
has a large presynaptic bouton (3–10 mm diam) with small
finger-like extensions (Amaral 1979; Blackstad and Kjaerheim 1961; Hamlyn 1962) that completely envelopes a large,
complex, postsynaptic ‘‘thorn.’’ Ultrastructural studies have
revealed that a single presynaptic MF bouton has °35 separate active sites (Chicurel and Harris 1992). The MF presynaptic terminal also has a distinctive complement of neurotransmitters with the opioid dynorphin being colocalized
with glutamate (Gall 1988). Finally, the presynaptic terminal of MFs is unique in that it can express at least two
distinctive forms of plasticity: a prolonged posttetanic potentiation (Langdon et al. 1995) and a N-methyl-D-aspartate
(NMDA) receptor independent form of long-term potentiation (Castillo et al. 1994; Johnston et al. 1992; Katsuki et
al. 1991; Langdon et al. 1995; Urban and Barrionuevo 1996;
Xiang et al. 1994; Zalutsky and Nicoll 1990).
Coincident with the presence of MF synapses, mEPSCs
recorded from CA3 pyramidal cells are often an order of
magnitude larger in amplitude than those observed from
other cells that are similar anatomically and physiologically
(e.g., CA1 pyramidal cells) (compare Jonas et al. 1993 with
Malgaroli and Tsien 1992 or Manabe et al. 1992). To determine whether the MF synapses are the source of the large
mEPSCs recorded from CA3 pyramidal cells, in the present
study, we have taken advantage of a hippocampal slice preparation in which the granule cells and their axons, the MFs,
are lesioned selectively (Dessi et al. 1991; Repressa et al.
1991). This selective lesion of the MFs was accomplished
by focal g-irradiation of neonatal rats; this has been shown to
remove the majority ( ú85%) of MFs from the hippocampal
formation. The selectivity of the g-irradiation induced lesion
is supported by a number of studies: CA3 dendritic arbors
and spine density are not affected (Gaiarsa et al. 1992; Repressa et al. 1991), the number of NMDA binding sites in
area CA3 is not affected (Dessi et al. 1991), and bicuculline
0022-3077/97 $5.00 Copyright q 1997 The American Physiological Society
/ 9k0e$$mr09 J448-6
09-02-97 13:43:39
neupa
LP-Neurophys
1075
Downloaded from http://jn.physiology.org/ by 10.220.32.247 on August 12, 2017
Henze, Darrell A., J. Patrick Card, German Barrionuevo, and
Yezekiel Ben-Ari. Large amplitude miniature excitatory postsynaptic currents in hippocampal CA3 pyramidal neurons are of mossy
fiber origin. J. Neurophysiol. 77: 1075–1086, 1997. Neonatal (P0)
g-irradiation was used to lesion selectively the mossy fiber (MF)
synaptic input to CA3 pyramidal cells. This lesion caused a ú85%
reduction in the MF input as determined by quantitative assessment
of the number of dynorphin immunoreactive MF boutons. The
g-irradiation lesion caused a reduction in the mean number of
miniature excitatory postsynaptic currents (mEPSCs) recorded
from CA3 pyramidal cells (2,292 vs. 1,429/3-min period; n Å
10). The lesion also caused a reduction in the mean mEPSC peak
amplitude from 19.1 { 0.45 to 14.6 { 0.49 pA (mean { SE; peak
conductance 238.8 { 5.6 to 182.0 { 6.1 pS). Similarly, there was
a reduction in the mean 10–85% rise time from 1.72 { 0.02 ms
to 1.42 { 0.04 ms. The effects of the g-irradiation on both mEPSC
amplitude and 10–85% rise time were significant at P õ 0.002
and P õ 0.005 (2-tailed Kolmogorov-Smirnov test). Based on the
selectivity of the g-irradiation, MF and non-MF mEPSC amplitude
and 10–85% rise-time distributions were calculated. Both the amplitude and 10–85% rise-time distributions showed extensive overlap between the MF and non-MF mediated mEPSCs. The MF
mEPSC distributions had a mean peak amplitude of 24.6 pA (307.5
pS) and a mean 10–85% rise time of 2.16 ms. The non-MF mEPSC
distributions had a mean peak amplitude of 12.2 pA (152.5 pS)
and 10–85% rise time of 1.26 ms. The modes of the amplitude
distributions were the same at 5 pA (62 pS). The MF and nonMF mEPSC amplitude and 10–85% rise-time distributions were
significantly different at P ! 0.001 (1-tailed, large sample Kolmogorov-Smirnov test). The data demonstrate that the removal of the
MF synaptic input to CA3 pyramidal cells leads to the absence of
the large amplitude mEPSCs that are present in control recordings.
1076
D. A. HENZE, J. P. CARD, G. BARRIONUEVO, AND Y. BEN-ARI
induces ‘‘normal’’ epileptic bursting (Gaiarsa et al. 1994).
Using this approach, we have determined that the presence
of the large mEPSCs is correlated with the presence of a
MF synapse onto CA3 pyramidal cells.
METHODS
Irradiation
Newborn Wistar rats (postnatal day P0) were irradiated at 6 Gy
(600 rad) by a cobalt bomb (Centre d’études nuclèaires, Fontenayau-Roses) using parameters previously shown to induce pronounced degeneration of granule cells in the dentate gyrus (Dessi
et al. 1991; Repressa et al. 1991). The g-rays were collimated to
irradiate only one side of the brain. The nonirradiated contralateral
side was used for control recordings.
The extent of g-irradiation-induced cell
and fiber loss in the hippocampus was characterized in six animals
that were irradiated unilaterally as neonates (see above) and prepared for morphological analysis. These animals were anesthetized
and killed by transcardiac infusion of buffered aldehyde fixative
28 days after g-irradiation exposure. The perfusion procedure included an initial infusion of phosphate buffered saline (PBS; 0.1
M, pH 7.4) into the ascending aorta followed by 200–300 ml of
paraformaldehyde-lysine-periodate fixative (McLean and Nakane
1974). The brain then was removed, postfixed for 2 h, washed in
PBS, and cryoprotected in 20% phosphate buffered sucrose for
Ç72 h, all at 47C. The forebrain was sectioned (35 mm/section)
in the coronal plane through the rostrocaudal extent of the hippocampus using a freezing microtome. Sections were collected serially in six wells of phosphate buffer such that each well contained
a one-in-six series of sections through the hippocampus. One well
of tissue was mounted immediately on gelatin-coated slides, stained
with cresyl violet, dehydrated, cleared, and coverslipped using standard procedures. This tissue was used to determine the relative
amount of cell loss in the dentate gyrus of the irradiated hippocampus. Remaining tissue was transferred to an ethylene glycol-based
cryopreservant (Watson et al. 1986) and stored at 0207C to preserve antigenicity for subsequent immunohistochemical processing.
The extent of MF degeneration in the stratum lucidum of the girradiated hippocampus was evaluated by immunohistochemical
localization of dynorphin immunoreactivity. Previous studies have
demonstrated that MF boutons in the rat hippocampus contain high
levels of dynorphin and that this peptide is a reliable marker of
the MF projection pathway (Gall 1988). A series of sections from
each of the six brains was washed thoroughly at room temperature
in multiple changes of PBS to remove the cryopreservant, and the
tissue then was processed for immunohistochemical localization
of dynorphin using a rabbit polyclonal antiserum generated against
dynorphin 1-8 (Peninsula Laboratories). In the first stage of this
procedure, the tissue was incubated in primary antiserum diluted
to a final concentration of 1:2,000 with PBS, normal donkey serum
(1%), and Triton X-100 (0.3%). The incubation was conducted
at 47C for 24–48 h. Antigen localization was visualized with the
avidin-biotin modification of the immunoperoxidase procedure
(Hsu et al. 1981) using an affinity-purified donkey anti-rabbit secondary antibody (Jackson ImmunoResearch) and Vectastain Elite
reagents (Vector Laboratories). Further details regarding application of this procedure in our laboratory have been published (Card
and Enquist 1994). Processed tissue was mounted on gelatin coated
slides, dehydrated, cleared, and coverslipped.
TISSUE PROCESSING.
/ 9k0e$$mr09 J448-6
Tissue analysis
Cresyl violet stained preparations were used to determine the
qualitative extent of cell loss in the g-irradiated hippocampus, and
dynorphin immunoreactivity was used to evaluate the degree of
MF loss in the afflicted stratum lucidum. In each case, the dentate
gyrus ipsilateral and contralateral to the irradiated side of the brain
were compared within each animal. As noted above, dynorphin
immunoreactivity in the stratum lucidum was used as the measure
of MF bouton loss in the irradiated hippocampus. Subjective determinations of the extent of the lesion were made by comparing the
density of staining on the control and lesion sides. Quantitative
determinations of the magnitude of MF bouton loss on the lesioned
side also were conducted using the following procedure. Three
comparable 100 mm2 segments of stratum lucidum were identified
in the control and irradiated sides of the brain, midway through
the rostro-caudal extent of the hippocampus. The relative position
of these three regions are illustrated in the schematic diagram
shown in Fig. 1. Varicose boutons in each of these regions displaying dynorphin immunoreactivity were mapped using a drawing
tube and a 140 objective on a Nikon Optiphot microscope. The
number of boutons were counted in each area and quantitative
comparisons of counts of lesion and control sides were made to
determine the degree of MF bouton loss in the irradiated hippocampus. No effort was made to quantitate the degree of cell loss within
the g-irradiated dentate gyrus.
In vitro slice preparation
Slices were prepared from the g-irradiated rats 27 to 40 days
old. Briefly, animals were anesthetized with equithesin and their
chest cavities were opened rapidly. The rats then were transcardially perfused with a cold oxygenated sucrose solution with
the following composition (in mM): 229 sucrose, 1.9 KCl, 1.2
Na2PO4 , 25 NaHCO3 , 10 glucose, 10 N-2-hydroxyethylpiperazineN *-2-ethanesulfonic acid (HEPES), and 6.0 MgCl2 . Before the
transcardial perfusion, the sucrose solution was bubbled with 95%
O2-5% CO2 to adjust the pH to 7.4 at 47C. After 1 min of transcardial perfusion, the rats were decapitated and the hippocampi
dissected from the brain. Each hippocampus then was sliced into
400-mm-thick slices perpendicular to its long axis using a vibratome. This sucrose perfusion technique originally was described
as a method to improve the viability of facial motoneurons in in
vitro slices taken from rat brain stem (Aghajanian and Rasmussen
1989). We found that the sucrose perfusion also improves the
viability neurons of area CA3 in hippocampal slices.
After sectioning, slices were maintained at room temperature
(19–237C) in an artificial cerebrospinal fluid (ACSF) consisting
of (in mM) 126 NaCl, 3.5 KCl, 1.2 Na2PO4 , 11 glucose, 25
NaHCO3 , 3.0 MgCl2 , and 3.0 CaCl2 . The ACSF was bubbled with
95% O2-5% CO2 (pH to 7.4). Individual slices were transferred as
09-02-97 13:43:39
neupa
LP-Neurophys
Downloaded from http://jn.physiology.org/ by 10.220.32.247 on August 12, 2017
Lesion characterization
FIG . 1. Areas examined to quantify dynorphin labeling of mossy fibers
(MFs). A schematic diagram of the transverse hippocampal slice illustrating
3 regions used to quantify the effect of g-irradiation on dynorphin labeled
MF boutons.
MOSSY FIBER
needed (between 2 and 12 h after slice preparation) to a submerged
chamber where they were constantly superfused with oxygenated
ACSF at 337C.
1077
sis of all mEPSCs included ‘‘smoothing’’ the raw data and its
subsequent derivatives using boxcar averaging with an 11-point
window. The detection algorithm was developed and run using
Igor Pro (Wavemetrics) on an accelerated Macintosh IIci. This
algorithm is similar to a published automated detection algorithm
for detecting miniature inhibitory postsynaptic potentials (Ankri et
al. 1994). All values given in the text are means { SE, unless
otherwise indicated. The accuracy of the mEPSC detection algorithm was evaluated by comparing the automated results with results collected manually. We considered three types of error: 1)
the number of false positives. The false positives were determined
for a single 60-s period where the algorithm found 1,341 events.
Of the detected 1,341 events, only 7 of those events (0.52%) could
be rejected as events not having an mEPSC-like appearance. 2)
The number of overlapping doublets that were not resolved. The
overlapping doublets for the same 60-s period was 57. The lack
of detection of these events can be explained by the fact that these
doublets, although inflected on the rising phase, did not reverse
direction and therefore did not have a 0 crossing in the first derivative. 3) The number of false negatives. There were numerous small
events that had an mEPSC-like appearance that were not detected
by the algorithm. These undetected events were always very small
( õ5 pA) and, by definition, had rising slopes õ5 pA/ms. Although
many of these small mEPSCs are recognizable by eye, it is impossible to establish a set of criteria to identify all of them because
they recede into the noisy baseline and therefore this error is not
quantifiable.
RESULTS
Electrophysiological recording
Spontaneous mEPSCs were recorded using the ‘‘blind’’ whole
cell recording technique (Blanton et al. 1989). Recordings for both
control and g-irradiated groups were from CA3 pyramidal cells
(verified in some cases by filling cells with biocytin) in the same
area used to quantify the amount of dynorphin immunoreactivity
(see Fig. 1). The mEPSCs were recorded using a voltage-clamp
amplifier, filtered at 3 kHz, digitized at 10 kHz, and then stored on
computer disk in 3-min epochs. The recordings also were collected
simultaneously to DAT tapes, which provided a continuous record
of the experiment. The analysis presented is based on one 3-min
epoch per cell selected for having a relatively stable baseline and
stable series resistance. Whole cell pipettes were filled with a solution consisting of (in mM) 50 CsF, 80 CsCl, 10 HEPES, and 1.0
bis-(o-aminophenoxy)-N,N,N *,N *-tetraacetic acidrK4 with a pH
of 7.0 and final osmolarity of 270 mOsm. Pipettes had a resistance
of 3–6 MV. Membrane potentials were voltage clamped at 080
mV. Data were rejected if the access resistance was ú20 MV or
a significant change ( ú15%) in whole cell series resistance was
observed. All recordings were carried out in the presence of 1 mM
tetrodotoxin and 10 mM bicuculline methiodide (Sigma).
Automated mEPSC detection algorithm
The detection and measurement of the mEPSCs was accomplished using an automated routine to permit the rapid analysis of
long recording epochs. The algorithm first calculated the first and
third derivatives of the raw data trace with respect to time. Then
mEPSCs were detected by scanning for peaks in the first derivative
which corresponds to the rising slopes of the mEPSCs. For all
recordings, an mEPSC was detected if its rising slope ú5 pA/ms.
After the detection of a mEPSC, the mEPSC peak was determined
from the first 0 crossing after the peak in the first derivative. The
foot of the mEPSC was determined from the 0 crossing in the third
derivative preceding the peak in the first derivative (Fig. 2). The
amplitude and 10–85% rise time were determined using the
mEPSC foot and peak detected by the search algorithm. The analy-
/ 9k0e$$mr09 J448-6
Anatomic effects of g-irradiation lesion
Clear changes in the cellular organization of the hippocampus were apparent in cresyl violet-stained coronal sections passing through the same portion of the rostrocaudal
axis that was selected for electrophysiological analysis in
other animals. As previously reported (Dessi et al. 1991;
Gaiarsa et al. 1992, 1994; Repressa et al. 1991), the most
obvious alterations were a profound cell loss in the dentate
gyrus and a decreased packing density of pyramidal cells in
the CA1 subfield of each g-irradiated hippocampus. These
changes were quite consistent between animals and were
distinguished easily when comparing the g-irradiated and
control hippocampus within the same animal (Fig. 3, A and
B). Below, we focus on the g-irradiation induced cell loss
in the dentate gyrus and the resultant alterations of the MF
projection to stratum lucidum.
g-Irradiated tissue exhibited extensive cell loss in the
granule cell layer of the dentate gyrus and a corresponding
decrease in the thickness of the molecular cell layer, changes
that were not present in the contralateral, nonirradiated tissue
from the same animals. The cell loss was apparent in both
the supra- and infrapyramidal blades of the granule cell layer
and occurred throughout the rostrocaudal axis of hippocampus. The characteristic pattern and magnitude of cell loss in
the dentate gyrus of all experimental animals is apparent in
comparing Fig. 3, C and D. These photomicrographs of
cresyl violet-stained preparations from a single animal illustrate the extensive cell loss in the irradiated hippocampus
(Fig. 3D) compared with the nonirradiated control hippocampus (Fig. 3C). In contrast, the immediately adjacent
polymorphic neurons of the hilus and the pyramidal cells of
area CA3 exhibited no apparent g-irradiation-induced
changes in either cell morphology, number, or distribution,
09-02-97 13:43:39
neupa
LP-Neurophys
Downloaded from http://jn.physiology.org/ by 10.220.32.247 on August 12, 2017
FIG . 2. Method for detecting miniature excitatory postsynaptic currents
(mEPSCs). Schematic illustration of method used to automate detection
and quantification of spontaneous mEPSCs. Raw data are smoothed using
boxcar averaging. First and 3rd derivatives then are found, each also being
boxcar averaged. Peaks in 1st derivative exceeding 5 pA/ms indicate an
event has occurred (1). After detection of an mEPSC, its amplitude and rise
time is determined as follows: peak of mEPSC is determined by scanning the
1st derivative forward for a 0 crossing (2). Foot of mEPSC is determined
by scanning backward in the 3rd derivative for a 0 crossing (3). Amplitude
change between points 2 and 3 represents mEPSC amplitude; 10–85% rise
times also are calculated using points 2 and 3 as 0 and 100%.
MEPSCS
1078
D. A. HENZE, J. P. CARD, G. BARRIONUEVO, AND Y. BEN-ARI
a finding consistent with previous reports (Dessi et al. 1991;
Gaiarsa et al. 1992, 1994; Repressa et al. 1991).
g-Irradiation-induced changes in the number of dynorphin immunoreactive MF boutons in the s. lucidum were
dramatic and consistent across animals. In the nonirradiated control hippocampus, a broadband of dynorphin immunoreactive varicose fibers filled the stratum lucidum
adjacent to the CA3 pyramidal neurons ( Fig. 4, A and
C ) . Scattered individual immunoreactive fibers also were
observed within the stratum pyramidale. This was in
marked contrast to the decrease in immunoreactive varicosities in comparable regions of the contralateral, g-irradiated hippocampus ( Fig. 4, B and D ) . The immunoreactive boutons in the g-irradiated hippocampus were reduced substantially throughout the full extent of CA3.
Comparative counts of the number of immunoreactive varicosities within each of three 100-mm2 areas ( see Fig. 1 ) of
the stratum lucidum in control and irradiated hippocampus
confirmed that there was a substantial decrease of these
profiles on the irradiated side. These counts revealed that
there was an Ç84% decrease in the number of immunoreactive varicosities irrespective of whether the data were
analyzed in individual 100 mm2 bins or grouped together
( Table 1 ) .
/ 9k0e$$mr09 J448-6
Electrophysiological effects of g-irradiation lesion
The present analysis compares recordings from cells in
10 slices from control and 10 slices from g-irradiated animals. All recordings were collected under voltage-clamp
conditions with the holding potential set to 080 mV. Application of 10 mM 6-cyano-7-nitroquinoxaline-2,3-dione completely abolished all observed mEPSCs, indicating that they
are mediated by glutamatergic a-amino-3-hydroxy-5methyl-4-isoxazolepropionic acid (AMPA) receptor activation (data not shown). Figure 5 illustrates the effect of girradiation on the overall frequency of mEPSCs recorded
from two selected CA3 cells. When the results from the 10
recordings from each condition are considered together, the
mean frequency of mEPSCs decreased from Ç12.5 to 7.8
Hz. This decrease was not statistically significant for P õ
0.05 (1-tailed, Student’s t-test, P Å 0.082) primarily due to
the large variation in mEPSC frequency between recordings
(standard deviations were 7.5 and 7.0 Hz for control and
g-irradiated tissue, respectively). The number of mEPSCs
detected at all amplitudes and rise times were reduced by
the g-irradiation treatment, however, larger amplitude (Fig.
6A) and slower rising (Fig. 6B) mEPSCs showed a greater
decrease. The mean amplitude decreased from 19.1 { 0.45
09-02-97 13:43:39
neupa
LP-Neurophys
Downloaded from http://jn.physiology.org/ by 10.220.32.247 on August 12, 2017
FIG . 3. Effect of g-irradiation on dentate gyrus and stratum lucidum of hippocampus. Morphology of control ( A and C)
and irradiated (B and D) hippocampus are illustrated in cresyl violet stained coronal sections. The same, which received
unilateral neonatal irradiation, was the source for all of photomicrographs. Comparison of A and B reveals profound girradiation-induced cell loss in dentate gyrus and a splaying of the CA1 pyramidal cell layer (small open block arrows in
B). The extent of granule cell loss and the accompanying reduction in thickness of the molecular layer (demarcated by bars
with arrowheads) resulting from irradiation is more readily apparent in comparing C and D. Scale bars: 440 mm in B for A
and B; 160 mm in D for C and D.
MOSSY FIBER
MEPSCS
1079
pA in control to 14.6 { 0.49 pA (P õ 0.05, 1-tailed Student’s t-test) in g-irradiated tissue (corresponding to mean
peak conductances of 238.8 { 5.6 pS and 182 { 6.1 pS,
TABLE
respectively). The median peak amplitude also was decreased in irradiated records, going from 13.9 to 11.2 pA
(173.8 to 140 pS). The distributions of mEPSC amplitudes
1.
Control
Irradiated
% Control
Rat
1
2
3
Total
1
2
3
Total
1
2
3
Total
4329
4330
4331
4332
4333
4334
287
268
411
424
379
368
298
289
352
307
502
399
271
282
275
335
454
472
856
839
1,038
1,066
1,335
1,239
17
49
114
51
62
67
31
51
108
63
30
77
31
50
85
59
33
57
79
150
307
173
125
201
0.059
0.183
0.277
0.120
0.164
0.182
0.104
0.176
0.307
0.205
0.060
0.193
0.114
0.177
0.309
0.176
0.073
0.121
0.092
0.179
0.296
0.162
0.094
0.162
0.164
0.174
0.162
0.164
Average
/ 9k0e$$mr09 J448-6
09-02-97 13:43:39
neupa
LP-Neurophys
Downloaded from http://jn.physiology.org/ by 10.220.32.247 on August 12, 2017
FIG . 4. Effect of g-irradiation on dynorphin labeled MF boutons. Dynorphin immunoreactivity in MFs of control ( A and
C) and irradiated (B and D) tissue is illustrated. The extent of dynorphin immunoreactive profiles in coronal sections through
control and irradiated hippocampus are shown in A and B, respectively. C and D are higher magnification views of dynorphinimmunoreactive fibers in stratum lucidum of control and irradiated tissue. A thick band of dynorphin immunoreactivity
adjacent to proximal portions of apical dendrites of CA3 pyramidal cells defines stratum lucidum in control tissue ( A and
C). In contrast, irradiation-induced degeneration of dentate granule cells causes a pronounced reduction in both density and
thickness of stratum lucidum, as defined by dynorphin immunoreactive fibers (B and D). The densest concentration of
dynorphin immunoreactive fibers in control and irradiated tissue is defined by regions between arrowheads on right-hand
border of C and D. Scattered immunoreactive boutons outside of this zone in irradiated animal are indicated by open block
arrows in D. Scale bars: 440 mm in B for A and B; 14 mm in D for C and D.
1080
D. A. HENZE, J. P. CARD, G. BARRIONUEVO, AND Y. BEN-ARI
are not normally distributed as reflected by the high skew
values of 4.3 and 5.5 for control and g-irradiated tissue,
respectively. The mean distribution of mEPSC amplitudes
from g-irradiated animals was significantly different from
the mean distribution from control animals (P õ 0.002, 2tailed Kolmogorov-Smirnov test, 1 pA bins). It can be seen
from both the standard and cumulative probability histograms that the relative differences between the amplitudes
of mEPSCs from control and g-irradiated tissue are greatest
for larger amplitudes. With respect to the 10–85% rise-time
distributions, the mean 10 – 85% rise time decreased from
1.72 { 0.02 ms in control to 1.42 { 0.04 ms ( P õ 0.05,
1-tailed Student’s t-test ) in g-irradiated tissue. g-Irradiation also decreased the median 10 – 85% rise time from 1.44
to 1.16 ms. The distributions of mEPSC 10 – 85% rise time
also are not normally distributed as reflected by the high
skew values of 2.3 and 2.5 for control and g-irradiated
tissue, respectively. The mean distribution of mEPSC 10 –
85% rise times from g-irradiated animals was significantly
different from the mean distribution from control animals
( P õ 0.005, 2-tailed Kolmogorov-Smirnov test, 0.1-ms
bins ) . It can be seen from both the standard and cumulative
probability histograms that the relative differences between
10 – 85% rise times of mEPSCs from control and g-irradiated tissue differ the most for longer rise times. Figure 7
illustrates the amplitude and rise-time distributions for
those mEPSCs that were absent from recordings from girradiated tissue. These distributions were obtained by simply subtracting the respective irradiated histogram from
the respective control histogram. To the extent that the girradiation-induced lesion is selective for MFs, these histograms nominally represent the distributions for mEPSCs
/ 9k0e$$mr09 J448-6
arising from MF synapses removed by the lesion ( see DISFigure 8 illustrates the general appearance of the
large and / or slow mEPSCs that are reduced greatly or absent from g-irradiated tissue. The examples shown in Fig.
8 are also representative of the few large mEPSCs that
were observed in g-irradiated tissue.
CUSSION ) .
DISCUSSION
Previous work has shown that destruction of the granule
cells by postnatal g-irradiation selectively reduces the MF
input to area CA3 (Dessi et al. 1991; Gaiarsa et al. 1992,
1994; Repressa et al. 1991). Using this preparation, we have
demonstrated that the large amplitude and/or slow rising
mEPSCs recorded from CA3 pyramidal cells are correlated
with the presence of normal, intact MFs.
Specificity of the g-irradiation-induced lesion
In the brain of the postnatal rat, there are only a few
populations of cells that have not finished their final division
and therefore are sensitive to g-irradiation. These cells include the granule cells of the olfactory bulb, the granule
cells of the cerebellum, and the granule cells of the dentate
gyrus (Bayer and Altman 1975b). Focal application of the
radiation permits the specific lesioning of the granule cells
of the dentate gyrus (Bayer and Altman 1975a). Other studies have shown that within the g-irradiated zone, only the
dentate gyrus granule cells and their MF input to area CA3
pyramidal cells are affected, whereas all other ionotropic
synaptic inputs, both excitatory and inhibitory, remain unchanged (Dessi et al. 1991; Gaiarsa et al. 1994). The best
09-02-97 13:43:39
neupa
LP-Neurophys
Downloaded from http://jn.physiology.org/ by 10.220.32.247 on August 12, 2017
FIG . 5. Effect of g-irradiation on mEPSCs recorded from hippocampal CA3 cells. Examples of raw data collected from
control and g-irradiated slices. A, top: 3 min of a voltage-clamp (VH Å 080 mV) whole cell recording from a control CA3
pyramidal cell (1 mM tetrdotoxin and 10 mM bicuculline present). A, bottom: similar recording from an g-irradiated CA3
pyramidal cell. B: an example of raw data from a control recording at a higher gain. Synaptic currents were abolished by
application of 10 mM 6-cyano-7-nitroquinoxaline-2,3-dione (data not shown).
MOSSY FIBER
MEPSCS
1081
evidence that sprouting of associational/commissural synapses is not significant is the observation that there is no
change in NMDA receptor density (as measured by immunohistochemistry) in the stratum oriens (Dessi et al. 1991).
Other reports also suggest that non-MF synapses do not
sprout as reflected by the loss of all spinelike structures on
the CA3 pyramidal cell in the stratum lucidum (Gaiarsa et
al. 1992; Repressa et al. 1991). It is important to point out
that a small amount of sprouting of glutamatergic inputs
from the associational collaterals does not affect our conclu-
/ 9k0e$$mr09 J448-6
sion that large and/or slow mEPSCs arise from the MF
synapses. Sprouting of associational synapses would only
serve to counter the observed decrease in the frequency of
mEPSCs and minimize the apparent effect of the g-irradiation lesion. With respect to the MFs, the 84% reduction in
the number of dynorphin positive boutons in the stratum
lucidum is exactly what is predicted given the reduction in
granule cells, and can be taken as a strong indication that
there is no MF sprouting. Although it is possible that the girradiation treatment may affect the ability of residual MFs
09-02-97 13:43:39
neupa
LP-Neurophys
Downloaded from http://jn.physiology.org/ by 10.220.32.247 on August 12, 2017
FIG . 6. Effect of g-irradiation on observed mEPSC amplitude and rise-time distributions. Average histogram of mEPSC
amplitudes (A) and 10–85% rise times ( B) recorded from control and g-irradiated tissue (n Å 10/group; {SE). Note girradiation affects all amplitudes and rise times. Both histograms are truncated for presentation purposes as illustrated by
broken axes. For amplitude, single events measuring °400 pA were recorded (an average of 16.9 mEPSCs with amplitudes
ú100 pA in control and 5.4 mEPSCs in g-irradiated recordings, single bins after axis break). For rise time, single events
measuring °14 ms were recorded (an average of 7.6 mEPSCs with 10–85% rise times ú7 ms in control and 0.7 mEPSCs
in g-irradiated recordings, single bins after axis break). Amplitude and 10–85% rise-time histograms are significantly
different at P õ 0.002 and P õ 0.005, respectively (2-tailed Kolmogorov-Smirnov test). Insets: control vs. g-irradiated
cumulative probability histograms. mEPSCs from g-irradiated slices display narrower cumulative distributions of amplitude
and rise time.
1082
D. A. HENZE, J. P. CARD, G. BARRIONUEVO, AND Y. BEN-ARI
to express dynorphin, this is unlikely because the residual
MF boutons in g-irradiated tissue appear to express dynorphin at control levels. These data are in agreement with
previous reports showing a drastic reduction in Timm staining in the stratum lucidum after g-irradiation (Repressa et
al. 1991). Timm staining reveals Zn 2/ , which has been
shown to exist at elevated levels in the mossy fiber terminals.
With respect to changes in nonglutamatergic neurotransmission, we have observed an increase in cholinergic innervation of the g-irradiated hippocampus (data not shown),
as has been observed for colchicine-induced DG lesions (Repressa and Ben-Ari 1989). We determined that the increased
cholinergic innervation did not play a role in causing the
observed changes in the mEPSC amplitude and rise-time
histograms because adding the cholinergic antagonist scopolamine had no effect on mEPSCs recorded from g-irradiated tissue (data not shown).
A possible nonspecific effect of the g-irradiation treatment
is that it could decrease the amplitude of non-MF mEPSCs.
This is highly unlikely for at least three reasons. First, Gaiarsa
/ 9k0e$$mr09 J448-6
et al. (1994) have shown that both excitatory AMPA- and
NMDA-receptor-mediated neurotransmission at non-MF synapses are unchanged with respect to control. In addition, they
have shown that both g-aminobutyric acid A (GABAA )- and
GABAB-mediated neurotransmission are unaffected. Second,
the irradiation treatment induces the lesion by disrupting the
mitotic process of granule cells (Hicks and D’Amato 1966).
Because most of the non-MF synapses are formed after the girradiation treatment, there can be no direct effect of the treatment on the synapses. Finally, previous work also has shown
that the only change in the dendritic morphology of CA3 pyramidal cells after the g-irradiation lesion is the loss of the large
thorny excrescences that normally receive the MF input (Gaiarsa et al. 1992; Repressa et al. 1991). It is not likely that the
loss of the thorny excrescences could, in and of itself, account
for the observations in this study because the loss of the extra
membrane and channels within that membrane would only
serve to increase input resistance and therefore increase the
observed amplitude and decrease the observed rise time of
distal synaptic signals.
09-02-97 13:43:39
neupa
LP-Neurophys
Downloaded from http://jn.physiology.org/ by 10.220.32.247 on August 12, 2017
FIG . 7. Amplitude and rise-time distributions for mEPSCs removed by g-irradiation. Amplitude (A) and 10–85% risetime (C) distributions of the mEPSCs removed by g-irradiation-induced lesions were obtained by subtracting the distributions
for g-irradiated tissue from the distributions for control tissue. Respective amplitude and 10–85% rise-time cumulative
probability distributions are shown in B and D. These distributions nominally represent distribution of MF mEPSC amplitude
and rise time.
MOSSY FIBER
Calculated MF and non-MF mEPSC amplitude and 10–
85% rise-time distributions
We have taken advantage of the specificity of the g-irradiation induced lesion to estimate the separate distributions of
mEPSC amplitudes and 10–85% rise times for the MF and
non-MF synapses. The MF mEPSCs are represented by the
mEPSCs that were subject to removal by the irradiation
treatment (see subtracted histograms of Fig. 7). The non-MF
mEPSCs are represented by the mEPSCs that are observed
following the irradiation treatment (see g-irradiated histograms in Fig. 6). Unfortunately, these two histograms do
not account for the 15% of the MF synapses that are not
removed by the g-irradiation treatment. As a result, the subtracted histograms of Fig. 7 are 15% smaller than would be
expected and the g-irradiated histograms of Fig. 6 contain
the 15% of the mEPSCs missing from the histograms of Fig.
7. Figure 9, A and C, illustrates what the mossy and nonMF mEPSC amplitude and 10–85% rise-time histograms
would look like if they are corrected for the residual nonle-
/ 9k0e$$mr09 J448-6
1083
sioned MFs (see figure legend for details of calculations).
The relative probability of an mEPSC of a given amplitude
or rise time arising from either the MF or non-MF synapses
is illustrated in Fig. 9, B and D.
Examination of these calculated histograms indicate that,
on average, MF mEPSCs are larger and slower than nonMF mEPSCs although there is extensive overlap of the distributions for these two classes of synapse. The mean MF
mEPSC peak amplitude is twice that of non-MF mEPSCs
(24.6 vs. 12.2 pA corresponding to 307.5 vs. 152.5 pS).
The median MF mEPSC amplitude is also twice that of the
non-MF mEPSC (17.5 vs. 9.0 pA corresponding to 218 vs.
112 pS). The two amplitude distributions are significantly
different (P ! 0.001) using a one-tailed, large sample Kolmogorov-Smirnov test (1-pA bin size). The distributions
also indicate that, on average, the MF mEPSCs are slower
rising than non-MF mEPSCs. The mean MF mEPSC 10–
85% rise time is twice that of the non-MF mEPSCs (2.16
vs. 1.26 ms). The median MF mEPSC 10–85% rise time
is also twice that of the non-MF mEPSC (1.8 vs. 0.97 ms).
The two amplitude distributions are significantly different
(P ! 0.001) using a one-tailed, large sample KolmogorovSmirnov test (0.1-ms bin size).
The mean MF mEPSC amplitude reported above is larger
than that (16.1 pA) previously reported by Jonas et al.
(1993). This difference is best explained by the fact that in
that report, mEPSCs where determined to be of MF origin
using a 20–80% rise-time criteria of õ0.8 ms. This approach
to identify MF mEPSCs would exclude many MF mEPSCs
that have longer rise times, which can be very large in amplitude (see Fig. 8).
As expected, the mEPSC amplitude histogram for nonMF synapses is very similar to published values for mEPSCs
recorded from CA1 pyramidal cells (Malgaroli and Tsien
1992; Manabe et al. 1992). This was expected for two reasons. First, collaterals of the same CA3 pyramidal cell axons
provide input to both CA1 and CA3, and second, the morphological structure of CA1 and CA3 pyramidal cells, while
not identical, is similar with respect to the overall electronic
locations of synapses, particularly for proximal synapses.
We found that the maximal non-MF mEPSC amplitude was
Ç30 pA, similar to values reported for recordings from area
CA1 pyramidal cells (Chavez-Noriega and Stevens 1994;
Malgaroli and Tsien 1992; Manabe et al. 1992). Thus it is
impossible that the approximate 10-fold difference in maximal mEPSC amplitude observed between MF and non-MF
mEPSCs is strictly due to the dendritic locations of the synapses. Instead, large mEPSCs recorded from CA3 pyramidal
cells are due to some special property of neurotransmission
at the MF synapse (see below).
Properties of mossy fiber mEPSCs
The apparent quantitative mismatch between an 85% reduction in MF synaptic input and the lack of a statistically
significant change in overall mEPSC frequency can be explained by the anatomy and biophysics of CA3 pyramidal
cells and their inputs. Each CA3 pyramidal cell receives
Ç46 MF boutons (Amaral et al. 1990) and each bouton has
Ç15 release sites (Chicurel and Harris 1992); thus each
CA3 pyramidal cell receives Ç700 MF release sites. This
09-02-97 13:43:39
neupa
LP-Neurophys
Downloaded from http://jn.physiology.org/ by 10.220.32.247 on August 12, 2017
FIG . 8. Examples of monophasic mEPSCs of large amplitude. Three
examples of large mEPSCs arising from putative MF synapses. A: 2 events
detected, larger measured at 229 pA and 10–85% rise time of 2.2 ms. B:
2 events detected, larger measured at 277 pA and 10–85% rise time of 5.5
ms. C: 1 event detected, measured at 207 pA and 10–85% rise time of 5.5
ms. D: 1 event detected, measured at 100 pA and 10–85% rise time of 1.5
ms. All 4 examples are from a recording with a 12-MV series resistance.
MEPSCS
1084
D. A. HENZE, J. P. CARD, G. BARRIONUEVO, AND Y. BEN-ARI
is in contrast to the estimated 12,000 release sites from the
commissural/associational collaterals and the perforant path
(Amaral et al. 1990). This works out to be that õ4% of the
total synaptic input to a typical CA3 pyramidal cell is of
MF origin. In light of the anatomically small contribution
of the MFs to the total number of synapses on a CA3 cell,
one might also question why any effect of the g-irradiation
was seen at all. Whereas synapse numbers favor the detection
of non-MF mEPSCs, the electrotonic filtering properties of
the dendrites favor the detection of MF mEPSCs by reducing
the contribution of the distal non-MF synaptic inputs to below detectable levels. The interaction between the number
of synapses for MFs and non-MFs and the differential electrotonic filtering leads to the observed relative contribution
by each of these two types of synapses to the overall level
of mEPSC activity.
An unexpected outcome of this study is that MFs give
/ 9k0e$$mr09 J448-6
rise to the slowest rising mEPSCs. Because of its close electrotonic location, it has long been held that evoked MF synaptic EPSCs should be of large amplitude and fast rise time,
whereas evoked non-MF EPSCs should be smaller and
slower (Brown and Johnston 1983; Johnston and Brown
1983; Rall et al. 1967). It is important to point out that the
mEPSCs we attribute to the MFs are not necessarily large
and slow because both large and fast, as well as, large and
slow mEPSCs are reduced by g-irradiation treatment. This
observation is supported by the lack of a correlation between
mEPSC amplitude and rise time when plotted against one
another (data not shown). The lack of a significant shapeindex correlation suggests that the contribution of electrotonic filtering to variations in mEPSC size and shape is
relatively minor. Due to the problems inherent in whole
cell recording from large neurons of the CNS (e.g., access
resistance and dendritic cable filtering), the reported abso-
09-02-97 13:43:39
neupa
LP-Neurophys
Downloaded from http://jn.physiology.org/ by 10.220.32.247 on August 12, 2017
FIG . 9. Amplitude and 10–85% rise time distributions for MF- and non-MF-mediated mEPSCs. Distributions of mEPSC
amplitude and 10–85% rise time were calculated by subtracting raw distributions from all 10 g-irradiated recordings from
raw distributions from all 10 control recordings. Difference distributions then were scaled by multiplying by 1.176 to correct
for MF mEPSCs that were not removed by g-irradiation. Individual non-MF distributions were calculated by subtracting
15% of scaled MF distributions from raw g-irradiated distributions. This was to correct for those MFs that were not removed
by g-irradiation. MF and non-MF mEPSC amplitude (A) and rise-time (C) distributions are significantly different (P !
0.001, 1-tailed large sample Kolmogorov-Smirnov test using x 2 sampling distribution with 2 df). Relative contribution of
both MF and non-MF synapses to mEPSCs of a given amplitude or rise time are shown in B and D. MF mEPSCs (B, ● )
are Ç80% of the mEPSCs recorded in 30-pA bin whereas non-MF mEPSCs comprise 20%. For mEPSC amplitudes ú45
pA, ¢95% of the mEPSCs are of MF origin. MF mEPSCs (D, ● ) are Ç80% of mEPSCs recorded in 3-ms bin whereas
non-MF mEPSCs comprise 20%. For mEPSC 10–85% rise times ú4.5 ms, ¢95% of the mEPSCs are of MF origin.
MOSSY FIBER
Summary
The major goal of this study was to test the hypothesis
that the large mEPSCs observed in recordings from CA3
pyramidal cell arise from synapses made by the MFs of the
dentate gyrus granule cells. We have shown that ¢95% of
mEPSCs with amplitudes ú45 pA are from the g-irradiation-sensitive MF synapses. The data also suggest that 95%
of the events with 10–85% rise times ú4.5 ms are from the
MF synapses. These data provide evidence for yet another
unique feature of neurotransmission at the MF synapse, specifically that the MFs give rise to a population of unusually
large and/or slow mEPSCs. Further work elucidating the
mechanisms that give rise to these maxi-mEPSCs will determine the relevance of these events.
We gratefully acknowledge the contribution of J. Cabarlo to the anatomic
portion of this investigation.
This work was supported by National Institutes of Health Grants NS24288 and predoctoral fellowship MH-10474, Institut National de la Santé
et de la Recherche Médicale, and a fellowship awarded by the Samuel and
Emma Winters Foundation. J. Cabarlo was supported by grants from the
Howard Hughes Foundation and National Institute of Mental Health to
promote undergraduate research training.
Address for reprint requests: G. Barrionuevo, Dept. of Neuroscience, 446
Crawford Hall, University of Pittsburgh, Pittsburgh, PA 15260.
Received 5 June 1996; accepted in final form 5 November 1996.
REFERENCES
AGHAJANIAN, G. K. AND RASMUSSEN, K. Intracellular studies in the facial
nucleus illustrating a simple new method for obtaining viable motoneurons in adult rat brain slices. Synapse 3: 331–338, 1989.
AMARAL, D. G. Synaptic extensions from the mossy fibers of the Fascia
Dentata. Anat. Embryol. 155: 241–251, 1979.
/ 9k0e$$mr09 J448-6
1085
AMARAL, D. G., ISHIZUKA, N., AND CLAIBORNE, B. Neurons, numbers, and
the hippocampal network. Prog. Brain Res. 83: 1–11, 1990.
AMARAL, D. G. AND WITTER, M. P. The three-dimensional organization of
the hippocampal formation: a review of anatomical data. Neuroscience
31: 571–591, 1989.
ANKRI, N., LEGENDRE, P., FABER, D. S., AND KORN, H. Automatic detection
of spontaneous synaptic responses in central neurons. J. Neurosci. Methods 52: 87–100, 1994.
BAYER, S. A. AND ALTMAN, J. Radiation-induced interference with postnatal
hippocampal cytogenesis in rats and its long-term effects on the acquisition of neurons and glia. J. Comp. Neurol. 163: 1–20, 1975a.
BAYER, S. A. AND ALTMAN, J. The effects of X-irradiation on the postnatally-forming granule cell populations in the olfactory bulb, hippocampus,
and cerebellum of the rat. Exp. Neurol. 48: 167–174, 1975b.
BLACKSTAD, T. W. Commissural connections of the hippocampal region in
the rat, with special reference to their mode of termination. J. Comp.
Neurol. 105: 417–537, 1956.
BLACKSTAD, T. W., BRINK, K., HEM, J., AND JEUNE, B. Distribution of
hippocampal mossy fibers in the rat. An experimental study with silver
impregnation methods. J. Comp. Neurol. 138: 433–450, 1970.
BLACKSTAD, T. W. AND KJAERHEIM, A. Special axo-dendritic synapses in
the hippocampal cortex: electron and light microscopic studies on the
layer of mossy fibers. J. Comp. Neurol. 117: 133–146, 1961.
BLANTON, M. G., LOTURCO, J. J., AND KRIEGSTEIN, A. R. Whole cell recording from neurons in slices of reptilian and mammalian cerebral cortex. J. Neurosci. Methods 30: 203–210, 1989.
BROWN, T. H. AND JOHNSTON, D. Voltage-clamp analysis of mossy fiber
synaptic input to hippocampal neurons. J. Neurophysiol. 50: 487–507,
1983.
BROWN, T. H., WONG, R.K.S., AND PRINCE, D. A. Spontaneous miniature
synaptic potentials in hippocampal neurons. Brain Res. 177: 194–199,
1979.
CARD, J. P. AND ENQUIST, L. W. The use of pseudorabies virus for definition
of synaptically linked population of neurons. In: Methods in Molecular
Genetics, edited by K. W. Adolph. Orlando, FL: Academic, 1994, p.
363–382.
CASTILLO, P. E., WEISSKOPF, M. G., AND NICOLL, R. A. The role of Ca 2/
channels in hippocampal mossy fiber synaptic transmission and longterm potentiation. Neuron 12: 261–269, 1994.
CHAVEZ-NORIEGA, L. AND STEVENS, C. F. Increased transmitter release at
excitatory synapses produced by direct activation of adenylate cyclase
in rat hippocampal slices. J. Neurosci. 14: 310–317, 1994.
CHICUREL, M. E. AND HARRIS, K. M. Three-dimensional analysis of the
structure and composition of CA3 branched dendritic spines and their
synaptic relationships with mossy fiber boutons in the rat hippocampus.
J. Comp. Neurol. 325: 169–182, 1992.
DESSI, F., REPRESSA, A., AND BEN-ARI, Y. Effects of neonatal gamma-ray
irradiation on rat hippocampus. II. Development of excitatory amino acid
binding sites. Neuroscience 42: 151–157, 1991.
GAIARSA, J. L., BEAUDOIN, M., AND BEN-ARI, Y. Effect of neonatal degranulation on the morphological development of rat CA3 pyramidal neurons:
inductive role of mossy fibers on the formation of thorny excrescences.
J. Comp. Neurol. 321: 612–625, 1992.
GAIARSA, J.-L., ZAGREAN, L., AND BEN-ARI, Y. Neonatal irradiation prevents the formation of hippocampal mossy fibers and the epileptic action
of kainate on rat CA3 pyramidal neurons. J. Neurophysiol. 71: 204–215,
1994.
GALL, C. Seizures induce dramatic and distinctly different changes in enkephalin, dynorphin, and CCK immunoreactivities in mouse hippocampal
mossy fibers. J. Neurosci. 8: 1852–1862, 1988.
HAMLYN, L. H. The fine structure of the mossy fibre endings in the hippocampus of the rabbit. J. Anat. 96: 112–126, 1962.
HICKS, S. P. AND D’AMATO, C. J. Effect of ionizing radiation on mammalian
development. Adv. Teratol. 1: 196–250, 1966.
HJORTH-SIMONSEN, A. Some intrinsic connections of the hippocampus in
the rat: an experimental analysis. J. Comp. Neurol. 147: 145–162, 1973.
HJORTH-SIMONSEN, A. AND JEUNE, B. Origin and termination of the hippocampal perforant path in the rat studied by silver impregnation. J. Comp.
Neurol. 144: 215–231, 1972.
HSU, S. M., RAINE, L., AND FANGER, H. The use of avidin-biotin-peroxidase
complex (ABC) in immunoperoxidase techniques: a comparison between
ABC and unlabeled antibody (PAP) procedures. J. Histochem. Cytochem. 29: 577–580, 1981.
09-02-97 13:43:39
neupa
LP-Neurophys
Downloaded from http://jn.physiology.org/ by 10.220.32.247 on August 12, 2017
lute values for mEPSC rise times should be considered with
care. However, due to the comparative nature of our study,
the validity of the conclusion that g-irradiation removes a
population of slower rising mEPSCs is unaffected by these
technical issues.
Another explanation for the presence of long MF rise
times is that our automated detection method cannot resolve
individual mEPSCs that have overlapping rising phases,
which it then would count as a single large slow event. This,
however, cannot account for the data because we found that
the detection algorithm misses relatively few mEPSCs (57
missed, 1,341 detected per 60-s period) that we would have
included when inspected by eye with no bias toward missing
overlapping events. Although the algorithm does fail to resolve some overlapping mEPSCs, when the data traces are
inspected visually, there do exist many monophasic mEPSCs
with amplitudes ú250 pA and/or rise times ú5 ms as shown
in Fig. 8.
A possible presynaptic mechanism by which the MF synapse could give rise to the observed large and/or slow
mEPSCs (‘‘maxi’’-mEPSCs) could involve multiquantal release from the multiple active zones of the presynaptic bouton. This proposed mechanism requires the presence of a
synchronizing signal that would cause the simultaneous or
near-simultaneous release of multiple quanta. Experiments
searching for the mechanism of the unique maxi-mEPSCs
are currently underway.
MEPSCS
1086
D. A. HENZE, J. P. CARD, G. BARRIONUEVO, AND Y. BEN-ARI
/ 9k0e$$mr09 J448-6
excitatory synaptic transmission by 6-cyano-7-nitroquinoxaline-2,3-dione
(CNQX) in the hippocampus in vitro. Neurosci. Lett. 96: 64–68, 1988.
RALL, W., BURKE, R. E., SMITH, T. G., NELSON, P. G., AND FRANK, K.
Dendritic location of synapses and possible mechanisms for the monosynaptic EPSP in motoneurons. J. Neurophysiol. 30: 1169–1193, 1967.
REPRESA, A. AND BEN-ARI, Y. Effects of colchicine treatment on the cholinergic septohippocampal system. Exs 57: 288–294, 1989.
REPRESA, A., DESSI, F., BEAUDOIN, M., AND BEN-ARI, Y. Effects of neonatal
g-ray irradiation on rat hippocampus. I. Postnatal maturation of hippocampal cells. Neuroscience 42: 137–150, 1991.
SPRUSTON, N., JAFFE, D. B., WILLIAMS, S. H., AND JOHNSTON, D. Voltageand space-clamp errors associated with the measurement of electrotonically remote synaptic events. J. Neurophysiol. 70: 781–802, 1993.
URBAN, N. N. AND BARRIONUEVO, G. Induction of hebbian and non-hebbian
mossy fiber long-term potentiation by distinct patterns of high-frequency
stimulation. J. Neurosci. 16: 4293–4299, 1996.
WATSON, R. E., WIEGAND, S. T., CLOUGH, R. W., AND HOFFMAN, G. E. Use
of cryoprotectant to maintain long-term peptide immunoreactivity and
tissue morphology. Peptides 7: 155–159, 1986.
XIANG, Z., GREENWOOD, A. C., KAIRISS, E. W., AND BROWN, T. H. Quantal
mechanism of long-term potentiation in hippocampal mossy-fiber synapses. J. Neurophysiol. 71: 2552–2556, 1994.
ZALUTSKY, R. A. AND NICOLL, R. A. Comparison of two forms of longterm potentiation in single hippocampal neurons. Science Wash. DC 248:
1619–1624, 1990.
09-02-97 13:43:39
neupa
LP-Neurophys
Downloaded from http://jn.physiology.org/ by 10.220.32.247 on August 12, 2017
JOHNSTON, D. AND BROWN, T. H. Interpretation of voltage-clamp measurements in hippocampal neurons. J. Neurophysiol. 50: 464–486, 1983.
JOHNSTON, D., WILLIAMS, S., JAFFE, D., AND GRAY, R. NMDA-receptorindependent long-term potentiation. Annu. Rev. Physiol. 54: 489–505,
1992.
JONAS, P., MAJOR, G., AND SAKMANN, B. Quantal components of unitary
EPSCs at the mossy fibre synapse on CA3 pyramidal cells of rat hippocampus. J. Physiol. Lond. 472: 615–663, 1993.
KATSUKI, H., KANEKO, S., TAJIMA, A., AND SATOH, M. Separate mechanisms of long-term potentiation in two input systems to CA3 pyramidal
neurons of rat hippocampal slices as revealed by the whole-cell patchclamp technique. Neurosci. Res. 12: 393–402, 1991.
LANGDON, R. B., JOHNSON, J. W., AND BARRIONUEVO, G. Posttetanic potentiation and presynaptically induced long-term potentiation at the mossy
fiber synapse in rat hippocampus. J. Neurobiol. 26: 370–385, 1995.
MC LEAN, I. W. AND NAK ANE, P. K. Periodate-lysine-paraformaldehyde
fixative. A new fixative for immunoelectron microscopy. J. Histochem.
Cytochem. 22: 1077–1083, 1974.
MALGAROLI, A. AND TSIEN, R. W. Glutamate-induced long-term potentiation of the frequency of miniature synaptic currents in cultured hippocampal neurons. Nature Lond. 357: 134–139, 1992.
MANABE, T., RENNER, P., AND NICOLL, R. A. Postsynaptic contribution to
long-term potentiation revealed by the analysis of miniature synaptic
currents. Nature Lond. 355: 50–55, 1992.
NEUMAN, R. S., BEN-ARI, Y., GHO, M., AND CHERUBINI, E. Blockade of

Similar documents