A local circuit neocortical synapse that operates via both NMDA and non-NMDA receptors.

1. In slices of rat neocortex, spike triggered averaging was employed to record in one neurone the excitatory postsynaptic potential (e.p.s.p.) generated by a spike in another, neighbouring neurone. When recorded at different membrane potentials, some of these e.p.s.ps exhibited a voltage relation typical of neuronal responses to N-methyl-D-aspartate (NMDA). 2. Selective NMDA antagonists reduced the amplitude of these e.p.s.ps, but had little effect on their early rising phase. In contrast, a less selective excitatory amino acid antagonist reduced all phases of the e.p.s.p. 3. By analyzing single axon e.p.s.ps we have been able to establish that the synaptic input to one cortical cell, delivered by a single presynaptic cortical cell, operates simultaneously via NMDA and non-NMDA amino acid receptors.