Synaptic plasticity induced in single neurones of the primary somatosensory cortex in vivo

Experiments carried out in urethane-anaesthetized rats in which single neurones were recorded extracellularly from primary somatosensory (SI) cortex employed a procedure in which one of two vibrissal inputs was temporally paired with iontophoretic applications of glutamate. Following the pairing procedure. 31% of 49 neurones studied displayed some form of synaptic plasticity, in that responses to one or both vibrissal stimuli were altered. Homosynaptic potentiation occurred in 4 neurones, and these were recorded in layers II/III only. Homosynaptic depression occurred in 6 neurones and were mainly recorded in layer IV. Heterosynaptic depression was observed in 3 neurones. Non-selective depression was observed in 2 neurones. The duration of the induced plastic changes typically exceeded 15 min, and often lasted as long as stable recordings continued. The results from experiments in which repeated glutamate applications were given alone (without synaptic input) confirmed that the non-selective changes were due to repeated glutamate applications and not the temporal pairing with synaptic responses per se. Dual recordings confirmed that plasticity was restricted to the neurone at which pairings were made, and (at the other neurone) that synaptic responses remained stable over the course of study. In some neurones homosynaptic potentiation and depression were shown to occur to the early response component (< 10 ms), suggesting that direct thalamocortical synapses are modifiable.