Volume 17, Number 12,
Issue of June 15, 1997
pp. 4820-4828
Copyright ©1997 Society for Neuroscience
Studies on Long-Term Depression in Area CA1 of the Anesthetized
and Freely Moving Rat
Received Dec. 23, 1996; revised March 24, 1997; accepted March 27, 1997.
Ursula Staubli and
Joey Scafidi
Center for Neural Science, New York University, New York, New York
10003
Homosynaptic long-term depression (LTD) is reported to occur in
field CA1 of hippocampal slices collected from immature brains. Because
the effect has been postulated to be a memory storage mechanism, it is
of interest to test for its presence in adult, awake animals.
Unfortunately, not only has hippocampal LTD proved difficult to obtain
reliably in vivo, but the few successful studies vary
with respect to protocols and evidence that the depression is
input-specific. The present study tested for input-specific (homosynaptic) LTD in field CA1 after application of various
stimulation protocols to the Schaffer collateral/commissural
projections in freely moving, adult rats. The results indicate that
although low-frequency trains do induce decrements in synaptic
transmission lasting for hours to several days, the success rate of
eliciting input-specific LTD in the awake rat is very modest compared
with the ease with which stable potentiation is obtained in the same synapses. Moreover, it is questionable that the effective protocols represent patterns of activity likely to occur during behavior. The
stronger the afferent activation during low-frequency stimulation, the
greater was the probability of eliciting LTD accompanied by persistent
heterosynaptic depression. Clear evidence for the occurrence of LTD,
irrespective of stimulation protocol and current intensity, could not
be obtained in rats under barbiturate anesthesia. In all, the results
do not accord with the suggestion that LTD occurs routinely in the
hippocampus in vivo as part of memory encoding.
Key words:
hippocampus;
CA1;
long-term depression;
homosynaptic;
heterosynaptic;
low-frequency stimulation;
long-term potentiation
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