Synaptic Plasticity in Rat Hippocampal Slice Cultures: Local "Hebbian" Conjunction of Pre- and Postsynaptic Stimulation Leads to Distributed Synaptic Enhancement
A central theme in neurobiology is
the search for the mechanisms underlying learning and memory. Since the
seminal work, first of Cajal and later of Hebb, the synapse is thought
to be the basic "storing unit." Hebb proposed that information is
stored by correlation: synapses between neurons, which are often
coactive, are enhanced. Several recent findings suggest that such a
mechanism is indeed operative in the central nervous system. Pairing of
activity on presynaptic fibers with strong postsynaptic depolarization
results in synaptic enhancement. While there is substantial evidence in
favor of a postsynaptic locus for detection of the synchronous pre- and
postsynaptic event and subsequent initiation of synaptic enhancement,
the locus of this enhancement and its ensuing persistence is still
disputed: both pre- and postsynaptic contributions have been suggested.
In all previous studies, the enhancement was presumed to be specific to
the synapses where synchronous pre- and postsynaptic stimulation was
applied. We report here that two recording techniques--optical
recording, using voltage-sensitive dyes, and double intracellular
recordings--reveal that synaptic enhancement is not restricted to the
stimulated cell. Although we paired single afferent volleys with
intracellular stimulation confined to one postsynaptic cell, we found
that strengthening also occurred on synapses between the stimulated
presynaptic fibers and neighboring cells. This suggests that synaptic
enhancement by the "paired-stimulation paradigm" is not local on the
presynaptic axons and that, in fact, the synapses of many neighboring
postsynaptic cells are enhanced.
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