LONG-TERM POTENTIATION: PHYSIOLOGY AND PHARMACOLOGY 111
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533.13
533.14
TIME-COURSE AND PLASTICITY OF RAT HIPPOCAMPAL CA3 PYRAMIDAL
ISOLATED MOSSY-FIBER SYNAPTIC CURRENTS IN CA3
PYRAMIDAL NEURONS. 5
A.C. Nobre. and T.H. Brown.
Dept. of Psychology, Yale University, N e w Haven, CT 06520.
CELL RESPONSES TO INPUT PROM RECURRENT COLLATERALS
EXAMINED BY WHOLE-CELL RECORDINGS. R. B. Lanedon. 1. Johnson. and
G.&rrionuevo. Departments of Behavioral Neuroscience and Psychiatry, University
of Pittsburgh, Pgh., PA.
pyramidal cells in the CA3 field of hippocampus make direct excitatory synapses
onto each other. We have studied these (1) as a mechanism whereby neuronal activity is synchronized, and (2) because studies of LTP of responses in CA3 depend on
discrimination between EPSCs evoked via collaterals and via mossy fibers; the
latencies and rise-times of the former have not been well-studied and may provide
criteria for such discriminations. To evoke EPSCs via collaterals in CA3h pyamidal
cells we applied stimuli to the alveus of region CA3c, with, interposed near the
CA3bICA3c border, a cut extending 125 to 200 pm radially from the alveus. In 5
out of 11 cells, the rising phase (10 to 90%) of the EPSC lasted between 1.4 to 3.0
msec. EPSCs with longer rising phases often exhibited multiple inflections, suggesting temporal dispersion of inputs. The mean latency to EPSC onset was 2.7 msec
(S.D. = 0.4, range = 1.9 to 3.2) for CA3b cells which were between 570 and 780
pm from the stimulus site (n = 10). By comparison, direct conduction over this
same distance by mossy filiers should require 1.4 to 2.0 msec (at 33' C), based upon
the latencies of antidromic responses ins. granulosum, evoked by stimulationof the
s. lu~idunz.To induce LTP, we applied 100 Hz stimulus trains at least 30 min after
hreak-ln. withcells clamoed at -70 mV:. oresumablv these condit~onsresulted onlv in
LTP expressed polysynapticsllyand not on inputs to the cell recorded. Although late
phases of theEPSC exhibited the greatestpotentiation,nevertheless, the initial 3 msec
of the EPSC was enhanced appreciably. In conclusion, the timecourse of responses
of CA3b pyramidal cells to inputs from collaterals can be rapid, and activily-induced
potentiation of the collateral connections can affect early phases of the EPSC to
collaterals. Supported by NINDS NS24288 and NIMH MH45156.
.
( 6)f l
THURSDAY ppq
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,
The mossy-fiber synapse is particularly suitable for newophysiologicaland optica,
experimentation of plasticity due to its large size and electrotonic proximity to the
soma. Because there are many synaptic systems present in the CA3 region of the
hippocampus, it is difficult to excite the mossy-fiber synapse selectively when using
extracellular microstimulation. For example, if .the stimulating electrode is placed
either in the stratum granulosum or in the stratum lucidum, it is possible to activate not
only mossy fibers, but also fibers of the perfo~antpath, commiswal fibers, CA3
recurrent collaterals,and Imal inhibitorycircuits.
We have used previous characterizations of mossy-fiber synapses (Brown &
Johnston, 1983; Williams &Johnston, 1991) and oyown 0b~eNationSto devisea set
of minimal criteria for identifying isolated mossy-fiber excitatory postsynaptic
currents (epscs). These include: fast rise time (under 3 ms), voltage independence of
the decay time constant, small variability of the epsc latency. and ability to follow
high-frequency slimulation. These criteria are not easy to satisfy. Slower currents of
unkown origin as well as polysynaptic contamination are much more hequently
observed. Using whole-cell recording methods, which has improved our signal-to.
noise ratio by a decade, it is possible to dissect more componentsof evoked epscs. We
have noted that small slow currents. which would be below the noise level using
inuacellular methods, are often evoked in conjunction with currents satisfying the
mossy-fiber criteria. Criteria for isolating mossy-fiber epscs should be employed
regularly, since failure to obtain pure mossy-fiber epscs may lead to significant
interpretationalerrors. (Supported by O M and NM.)
ASSOClATIVB LONG-POTENllATlON IN THE HPPOCAMPUS
USING ANTIDROMlC STIMULATION AS THE CONDITIONING
TETANUS. 1;M,lester and T. I. Seinowski, Computational Newobiology
Laboratory, Salk Institute, La folla, C A 92037.
Associative long-term potentiation (LTP) is a long-lasting increase in
synaptic response produced when a stimulus insufficient to cause LTP is
paired with a conditioning tetanus. In area CAI of the hippocampus,
associative LTP i s mediated by depolarization of the post-synaptic
membrane which relieves the Mg++ block of the N-methyl-Paspartate
(NMDA) type of glutamate receptor. Can antidromically-elicited action
potentials in the soma of CAI pyramidal neurons depolarize the post$naptic membrane in dendrites Gough to prime the NMDA receptor?
Extracellular ficid wtentials were recorded from 400 u m thick slices of
rat hippocampus. ~ i orthodromic
e
test stimulus site (TEST) was in
stratum radiatum of CAI and the antidromic stimuls site (ANTI)was in
the alveus. The ANTI site received a train of 50 bursts of 5 pulses at 100 hz
with a n inter-burst interval of 200 msec. The TEST site received 50 shocks
at 5 Hz so that the shocks arrived simultaneous with the bursts to ANTI.
The conditioning stimulation to ANTI when given alone did not produce
a change in either-population spike or EPSP ofihe test site (0.0 f 3.9%,n=6
PS. -5.4 + 9.6%. n=5 EPSP). nor did the 5 hz stimulation to TEST (-2.5 +
6.32, n=i1, PS and 0.6 + 2.7%, n=ll, EPSP); however, when the ortho- a n d
antidromic stimulations were paired an increase in the population spike
was observed (+24.0 + 9A%, n=5) which lasted at least 30 minutes.
Application of 2-amino-5-phosphonovalerate(AP5) reversibly blocked
this assodative LTP. Thus, antidromically stimulated action potentials
may invade the apical dendrites of CAI hippocampal pyramidal cells
and provide the depolarization necessary for associative LTPa
CURRENT-SOURCE DENSITY ANALYSIS O F LTP INDUCTION
I N HIPPOCAMPAL SLICES. G. Capocchi and M. Zampolini
Clinical Neurology, Univ. Sch. of Med. Pen~eia.Italv
In a p r e v i o u s s t J d y we showed t h a t t h e Long Term
P o t e n t i a t i o n (LTP) i n t h e CAI a r e a of h ~ p p o c a m p u s was
more e a s i l y o b t a i n e d In t h e proximal p o r t i o n of Apical
D e n d r i t e s {PAD) compared t o t h e D l s t a l Portlon (dAD) T h e
Slope i n c r e a s e w a s g r e a t e r In PAD t h a n In dAD In order
t o I n v e s t i g a t e t h e origin of t h e s e differences we
performed a c u r r e n t - s o u r c e d e n s i t y (CSD) a n a l y s ~ s before
a n d a f t e r LTP i n d u c t i o n T h e experiments were c a r r ~ e do u t
o n r a t hlppocampal s l l c e s m a i n t a m e d i n v l t r o
The
stimulating e l e c t r o d e w a s p l a c e d in dAD T h e recordings
were o b t a i n e d w l t h t w o e l e c t r o d e s o n e placed In CAI
s o m a t i c l a y e r t o t e s t t h e c o n s t a n c y of s p i k e p o p u l a t ~ o n
a n d t h e o t h e r u s e d t o explore, w l t h 5 0 m s t e p s , t h e
d e n d r l t l c t r e e T h e d a t a were s t o r e d In t h e computer a n d
one-dimensional CSD w a s c a l c u l a t e d T h e LTP w a s induced
w i t h s h o r t b u r s t (4 p u l s e s a t 1 0 0 Hz), r e p e a t e d 1 0 t u n e s
w i t h 2 0 0 ms of i n t e r v a l In some e x p e r i m e n t s a 5 s e c of
Interburst Interval was used T h e input-output curves
w e r e a l s o c o m ~ u t e dbefore a n d a f t e r LTP
T h e r e s u l t s showed t h a t i n some e x p e r i m e n t s a n e a r l y
s i n k (ES) i n PAD a f t e r i n d u c t i o n of LTP a p p e a r e d
S t i m u l a t i o n a t 5 s e c of i n t e r b u r s t i n t e r v a l showed LTP
a n d ES i n PAD w h e r e a s n o modification w a s o b s e r v e d i n
dAD. T h e modification i n D A w
~ a s n o t i n f l u e n c e d hv DLAPV ( 5 0 /1M) p e r f u s i o n following LTP i n d u c t i o n .
T h e d a t a suggest t h a t t h e patterned stimulation
i n d u c i n g LTP modifies t h e p r o p e r t i e s of PAD i n c r e a s i n g
depolarizlng c u r r e n t s a n d s u p p o r t t h e h y p o t e s i s of a
s e c o n d ( d e n d r i t i c ) component i n t h e e x p r e s s i o n of LTP
T h i s modification i s n o t m a n t a i n e d by NMDA r e c e p t o r .
LONG-TERM POTENTIATION CANNOT BE INDUCED IN THE CAI-FIBLD
OF THE HIPPOCAMPUS OF MICROENCEPHALICRATS.
Ramakm,G.MJ.". Urban, I.J.A.', De Graan. P.N.E.1, Cattabeni. F?. DiLuca, M?,,
and Gisoen. W.H.'
Rudolf Magnus Institute, University of Utrecht, The Netherlands. Institute of
Pharmacological Sciences. Faculty of Pharmacy, University of Milan. Italy.
Injection of methylazoxy methanol (MAM)to pregnant rats produces in offspring
a.0. micmncephaly, hypoplash in the cerebral cortex and CAI field of the
hippocampus, disturbances in cognitive behavior and a reduction in the
phospharylation state of the B501GAP43 protein (a specific substrate for
proteinkinuse C (PKC)) in the hypoplastic brain areas. Since long-term potentiation
in the hippocampus involves a PKCdependent phosphorylation we decided to
examine the hippocampal LTP in slices from the rats with MAM-induced
micmncephaly (25 mgkg of MAM injected on day 15 of gestation (GIs)) and
from control (saline on G15) rats. 'eld potentials (Xs)'
were evoked in the
pynunidal and radiate layers of the CAI field or in the granular layer of the dentate
E maximum intensity) either Schaffer
gyrus (DG)by stimulating (at 0.1 Hz and Y
collaterals or the perforant path. Glassalectmdes (2 M o b , Wed with perfusing
medium) were used for the recording. After U) min of base-line FPs, a tetanic
stimulation (1 sec, 100 Hz) of the same intensity was given. The rwrding of FPs
was continued for the 60 min following the tetanus. The magnitude of the
population spike (PS)and the slope of the BPSPs of aversged FPs (every 5 min;
n=30) were computed. None of the MAM-heated slices (n=7) exhibited LTP in the
CAI field. whereas control slices (n=7) exhibited afler the tetanus a LTP-like
inmase in FPs (increase of PS by >SO% and the sloue of BPSPs by 4540%). In
the DG slices from MAM-mled &d control rats (n=?) showed a long-lasting.(4550%) increase in the slopo of EPSPs. Wether the inability of tetanic slimulalbn to
induce LTP in the CAI field of the minoencephalic rats is due to the profound
neumnal disorganisation found in h i s field of the MAM-treated rats or to abnormal
intrinsic propertics of the CAI neurons is currently investigated.
HETEROSYNAPTIC LONG-TERM POTENTIATION IN THE LATERAL ENTORHINAL
CORTEX: AND EXTRA AND INTRACEUULAR STUDY ON THE ISOIATED IN VKRO
rind d
Dept. ol
BAAIN PREPARATDN. -1.)
Physiology end Biophys~cs.New York University, New York and (') Dept.
Neurophysiology. 1st. Neuroiogico. Milano. Italy.
A study of the properties of LTP in the entorhinei conex (EC)
(Alonso. d e Curtis end Liinas: PNAS: 87. 9280-84. 1990) demonstrated
that a non-Hebbian potentiation can b e evoked in the layer II neurons by
direct manipulating the postsyneptic membrane potential.
Thls
potentiation does not require simultaneous activation of the presynaptic
fibers, suggesting that postsynaptic mechanisms are operant in this form
of LTP. The ability to induce heterosynaptlc LTP to a non-tetanked input
folbwing a high frequency stimulation of e separate input converging on
the s a m e cell populmion h a s been described (Bradier end Barnonuevo;
Synapse, 4, 132. 1989). Utiiizing the isolated brain preparation
meinteined h vltm by arterial perfusion (Liinas. Muhielhaler end Welton;
J. Physioi. 414, 16, 1989). we tested both extra and intraceiiuleriy the
possibility of heterosynaptic LTP induct~on in the splny stellate end
pyremidai neurons of layer I1 in the lateral EC. Two separate rnputs
converging on the apical dendrites of these cells were studred. The EPSPs
evoked by both stimulations were characterized pharmacologicaily. LTP
was induced either by e thete-frequency tetenlc stimulation of the one
input or by intraceliuier postsynaptic manipulations. In both c a s e s
heterosynaptlc and non-Hebbian potentiation of the responses evoked by
both stimuli were obtained.
In contrast. short-term post-tetenic
potentiation (or depression) wee obsewed only in the response to the
tatanized input. Supported by N.I.H. grant NS13742.
533.18
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SOCIETY FOR NEUROSCIENCE ABSTRACTS, VOLUME 17,1991