Composition of the excitatory postsynaptic potentials recorded from rat visual cortical neurons in layer. II/III: An...

Intracellular recordings were performed on layer II/III neurons of rat brain slices. Neuronal responses to electrical stimulation of the white matter was analyzed pharmacologically using D-2-amino-5-phosphonovalerate (APV), a specific antagonist of the N-methyl-D-aspartate (NMDA) receptor, and kynurenate (Kyn), a broad-spectrum antagonist of both the NMDA and the non-NMDA (kainate and quisqualate) receptors. Fifty-five neurons produced an excitatory postsynaptic potential (EPSP) in response to a single shock. In forty-eight neurons (87%) of them, the EPSP consisted of an APV-sensitive component and an APV-insensitive component. While in seven neurons (13%), it consisted of only the APV-insensitive component. The APV-sensitive component was preceded by the APV-insensitive component and was enhanced in amplitude and duration by the following procedures: a) applying repetitive stimulations to the white matter, b) reducing Mg2+ concentration in the bathing solution and c) depolarizing cell membrane. The APV-insensitive component was affected by neither repetitive stimulation nor reduction of Mg2+ concentration. Both APV-sensitive and APV-insensitive components were reduced by the treatment of Kyn. These suggest that in bathing medium containing Mg2+, the APV-sensitive (i.e. the NMDA receptor-mediated) EPSP is generated by the non-NMDA receptor-mediated depolarization, which removes the Mg2+ blockade in NMDA receptor-gated channel.