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1 Department of Physiology, Oxford University, Oxford, United Kingdom; School of Biosciences, Cardiff University, Cardiff, United Kingdom
2 Royal School of Veterinary Sciences, Edinburgh University, Edinburgh, Scotland, United Kingdom
3 School of Biosciences, Cardiff University, Cardiff, United Kingdom
4 Department of Physiology, Oxford University, Oxford, United Kingdom
* To whom correspondence should be addressed. E-mail: sbinrh{at}cardiff.ac.uk.
Paired neuronal activity is known to induce changes in synaptic strength which result in the synapse in question having different properties to unmodified synapses. Here we show that in paired layer 2/3 excitatory connections in young adult rat cortex paired activity acts to normalize the strength and quantal parameters of connections. Paired action potential firing produces long term potentiation (LTP) in only a third of connections, while a third remain with their amplitude unchanged and a third exhibit long term depression (LTD). Furthermore, the direction of plasticity can be predicted by the initial strength of the connection: weak connections potentiate and strong connections depress. A quantal analysis reveals that changes in synaptic efficacy were predominantly presynaptic in locus, and that the key determinant of the direction and magnitude of synaptic modification was the initial release probability (Pr), which correlated inversely with change in Pr after pairing. Furthermore, distal synapses also exhibited postsynaptic increases in efficacy, while more proximal inputs did not. This may represent a means by which distal synapses preferentially increase their efficacy in order to achieve equal weighting at the soma. Paired activity thus acts to normalise synaptic strength, via both pre and post synaptic mechanisms.
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