EXPERIENCE-DEPENDENT MODIFICATION OF SYNAPTIC PLASTICITY IN VISUAL-CORTEX

IN many regions of the cerebral cortex, Ca2+ influx through NMDA (N-me thyl-D-aspartate) sensitive glutamate receptors (NMDA receptors) can t rigger two forms of synaptic plasticity: long-term depression (LTD) an d long-term potentiation (LTP)(1). LTD is induced by low levels of pos tsynaptic NMDA-receptor activation, for instance in response to low-fr equency stimulation, whereas LTP is induced by the stronger activation that occurs following high-frequency stimulation(2-4). Theoretical st udies have shown that the properties of synaptic LTD and LTP can accou nt for many aspects of experience-dependent plasticity in the developi ng visual cortex, provided that the LTD-LTP crossover point (the modif ication threshold, theta(m)) varies as a function of the history of co rtical activity(5-7). Here we provide direct experimental evidence tha t the value of theta(m) depends on sensory experience. We find in visu al cortex of light-deprived rats that LTP is enhanced and LTD diminish ed 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 allow s synaptic weights in neural networks to achieve a stable equilibrium.