Many developing circuits require experience,
in the form of ongoing patterned activity, to become fully
functional. Identifying the cellular plasticity mechanisms
that underlie this refinement is critical for understanding
the role of early experience in normal and abnormal development.
Until recently the field had largely concentrated on the
role of rapid, synapse-specific forms of synaptic plasticity
such as long-term potentiation (LTP) and depression (LTD).
However, theoretical considerations suggest that such
"Hebbian" forms of plasticity are highly unstable and
cannot function efficiently without complementary forms
of plasticity that stabilize neuronal and network tiring
properties. Over the past decade a body of work has identified
synaptic scaling, a form of homeostatic synaptic plasticity,
as a critical mechanism that provides stability to developing
neurons and circuits. This form of plasticity scales neuronal
synaptic strengths up or down in the right direction to
stabilize firing and appears to operate on the entire
distribution of a neuron's synaptic weights. In addition
to stabilizing Hebbian plasticity, synaptic scaling likely
plays an important role in balancing excitation and inhibition
within highly recurrent cortical microcircuits.
Dr. Turrigiano began by discussing the expression
mechanism(s) of synaptic scaling at excitatory synapses,
with a focus on the role of postsynaptic changes in glutamate
receptor accumulation. She contrasted the molecular expression
mechanisms of synaptic scaling with those underlying LTP
and suggested that there are several pathways for regulating
receptor accumulation that operate over different temporal
and spatial scales. She then turned to a discussion of
the induction mechanism and the role of presynaptic vs.
postsynaptic firing in generating a signal for synaptic
scaling. Her data suggest that synaptic scaling is a function
of postsynaptic firing and is a mechanism that allows
neurons to sense their own activity and adjust synaptic
strengths to keep this activity relatively constant.
