DIFFERENT FORMS OF SYNAPTIC PLASTICITY IN SOMATOSENSORY AND MOTOR AREAS OF THE NEOCORTEX

We have studied vertical synaptic pathways in two cytoarchitectonicall y distinct areas of rat neocortex-the granular primary somatosensory ( SI) area and the agranular primary motor (MI) area-and tested their pr opensity to generate long-term potentiation (LTP), long-term depressio n (LTD), and related forms of synaptic plasticity. Extracellular and i ntracellular responses were recorded in layer II/III of slices in vitr o while stimulating in middle cortical layers (in or around layer IV). Under control conditions, 5 Hz theta-burst stimulation produced LTP i n the granular area, but not in the agranular area. Agranular cortex d id generate shortterm potentiation that decayed within 20 min, Varying the inter-burst frequency from 2 Hz to 10 Hz reliably yielded LTP of 21-34% above control levels in granular cortex, but no lasting changes were induced in agranular cortex, However, the agranular cortex was c apable of generating LTP if a GABA, receptor antagonist was applied lo cally at the recording site during the induction phase. In contrast to LTP, an identical form of homosynaptic LTD could be induced in both g ranular and agranular areas by applying low frequency stimulation (1 H z for 15 min) to the middle layers, Under control conditions, both LTP and LTD were synapse-specific; theta-burst or low-frequency stimulati on in the vertical pathway did not induce changes in responses to stim ulation of a layer II/III horizontal pathway. Application of the NMDA receptor antagonist D-2-amino-5-phosphonovaleric acid (AP5) blocked th e induction of both LTP and LTD in granular and agranular cortex, In t he presence of AP5, low-frequency conditioning stimuli yielded a short -term depression in both areas that decayed within 10-15 min. Nifedipi ne, which blocks L-type, voltage-sensitive calcium channels, slightly depressed the magnitudes of LTP and LTD but did not abolish them. Syna ptic responses evoked during theta-burst stimulation were strikingly d ifferent in granular and agranular areas, Responses in granular cortex were progressively facilitated during each sequence of 10 theta-burst s, and from sequence-to-sequence; in contrast, responses in agranular cortex were stable during an entire theta-burst tetanus. The results s uggest that vertical pathways in primary somatosensory cortex and prim ary motor cortex express several forms of synaptic plasticity, They we re equally capable of generating LTD, but the pathways in somatosensor y cortex much more reliably generated LTP, unless inhibition was reduc ed, LTP may be more easily produced in sensory cortex because of the p ronounced synaptic facilitation that occurs there during repetitive st imulation of the induction phase, In addition, both cortical areas gen erated short-term potentiation and short-term depression under certain conditions, Thus, the ability to generate short- and long-term forms of synaptic plasticity may vary in similar synaptic pathways of differ ent neocortical areas.