Experience-dependent modification of synaptic plasticity in visual cortex.
In many regions of the cerebral cortex, Ca2+ influx through NMDA (N-methyl-D-aspartate) sensitive glutamate receptors (NMDA receptors) can trigger two forms of synaptic plasticity: long-term depression (LTD) and long-term potentiation (LTP). LTD is induced by low levels of postsynaptic NMDA-receptor activation, for instance in response to low-frequency stimulation, whereas LTP is induced by the stronger activation that occurs following high-frequency stimulation. Theoretical studies have shown that the properties of synaptic LTD and LTP can account for many aspects of experience-dependent plasticity in the developing visual cortex, provided that the LTD-LTP crossover point (the modification threshold, theta(m)) varies as a function of the history of cortical activity. Here we provide direct experimental evidence that the value of theta(m) depends on sensory experience. We find in visual cortex of light-deprived rats that LTP is enhanced and LTD diminished over a range of stimulation frequencies, and that these effects can be reversed by as little as two days of light exposure. Our findings support the idea that a variable synaptic modification threshold allows synaptic weights in neural networks to achieve a stable equilibrium.
Kirkwood
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A
Department of Neuroscience and Howard Hughes Medical Institute, Brown University, Providence, Rhode Island 02912, USA.
Rioult
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MC
Bear
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MF
D000818:Animals; D005074:Evoked Potentials, Visual; D008027:Light; D017774:Long-Term Potentiation / Q000502:physiology; D009473:Neuronal Plasticity / Q000502:physiology*; D051381:Rats; D012683:Sensory Deprivation; D012684:Sensory Thresholds; D013569:Synapses / Q000502:physiology; D014793:Visual Cortex / Q000502:physiology*