The Involvement of N‐Methyl‐D‐Aspartate Receptors in Induction and Maintenance of Long‐Term Potentiation in Rat Visual Cortex

Pyramidal neurons from layers II and III of rat visual cortex slices were studied with intracellular recordings. The involvement of N‐methyl‐D‐aspartate (NMDA) receptors was investigated: (1) in the synaptic response to white matter stimulation; (2) in the induction of long‐term potentiation (LTP); and (3) in the maintenance of LTP. Bath application of 25 μM of 2‐amino‐5‐phosphonovalerate (APV), an NMDA receptor antagonist, caused a slight (< 10%) reduction of the amplitude of the synaptic response elicited by white matter stimulation. The APV‐sensitive excitatory postsynaptic potential (EPSP) had a longer peak latency and duration than the APV‐resistant EPSP. Bath application of 10 μM of 6‐cyano‐7‐nitroquinoxaline‐2,3‐dione (CNQX), a non‐NMDA glutamate receptor antagonist, revealed a CNQX‐resistant EPSP in response to white matter stimulation which was APV‐sensitive. The time course of the CNQX‐resistant EPSP was similar to that of the APV‐sensitive EPSP and its onset latency was similar to that of the synaptic response in normal medium. Bath application of the GABA‐A antagonist bicuculline (0.1 to 0.5 μM) led to a progressive enhancement of the amplitude of the APV‐sensitive EPSP. At bicuculline concentrations above 0.3 μM the amplitude of this EPSP increased with membrane depolarization as was the case for the CNQX‐resistant EPSP implying that the NMDA receptors were located on the recorded neuron. The susceptibility of the cells to undergo LTP was tested at various concentrations of bicuculline. The effectiveness of bicuculline treatment was quantified by comparing the amplitudes of the synaptic response to just subthreshold stimuli at two post‐stimulus delays: (i) at 22 ms, which corresponds to the time to peak of both the initial inhibitory postsynaptic potential and the APV‐sensitive EPSP; and (ii) at 8–11 ms post‐stimulus, which corresponds to the peak of the postsynaptic potential (PSP) in normal medium. Bath application of APV, 20 min after the conditioning tetanus, allowed the authors to measure the amplitude of the APV‐sensitive EPSP in the potentiated response. In normal medium, the ratio of the late over the early PSP amplitude was 33.6 ± 4.1% and tetanic stimulation failed to induce LTP. The conditions remained the same at bicuculline concentrations of 0.1 to 0.2 μM. At higher concentrations of bicuculline the amplitude ratio of late versus early PSP increased and tetanic stimulation induced LTP. In cells, in which bicuculline had caused small ratio increases, only the APV‐sensitive EPSP underwent LTP. In cells in which bicuculline had caused large ratio changes, both the APV‐resistant and the APV‐sensitive EPSP showed LTP. Together with the previous finding that blockade of NMDA receptors prevents LTP (Artola and Singer, 1987) these results suggest that there is a threshold for LTP induction, which is only reached if NMDA receptor‐gated channels are sufficiently activated. The data indicate further that the NMDA receptor‐mediated EPSP is itself susceptible to LTP whereby its LTP threshold is lower than that of the APV‐resistant EPSP. Given the different LTP thresholds of the APV‐resistant and APV‐sensitive EPSPs, the possibility is raised that their potentiation depends on different mechanisms.