INDUCTION OF LONG-TERM POTENTIATION WITHOUT PARTICIPATION OF N-METHYL-D-ASPARTATE RECEPTORS IN KITTEN VISUAL-CORTEX

1. Intracellular recording was made from layer II-III cells in slice preparations of kitten (30-40 days old) visual cortex. Low-frequency (0.1 Hz) stimulation of white matter (WM) usually evoked an excitatory postsynaptic potential (EPSP) followed by an inhibitory postsynaptic potential (IPSP). The postsynaptic potentials (PSPs) showed strong dependence on stimulus frequency. Early component on EPSP and IPSP evoked by weak stimulation both decreased monotonically at frequencies > 0.5-1 Hz. Strong stimulation similarly depressed the early EPSP at higher frequencies ( > 2 Hz) and replaced the IPSP with a late EPSP, which had a maximum amplitude in the stimulus frequency range of 2-5 Hz. 2. Very weak WM stimulation sometimes evoked EPSPs in isolation from IPSPs. The falling phase of the EPSP revealed voltage dependence characteristic to the response mediated by N-methyl-D-aspartate (NMDA) receptors and was depressed by application of an NMDA antagonist DL-2-amino-5-phosphonovalerate (APV), whereas the rising phase of the EPSP was insensitive to APV. 3. The early EPSPs followed by IPSPs were insensitive to APV but were replaced with a slow depolarizing potential by application of a non-NMDA antagonist 6,7-dinitro-quinoxaline-2,3-dione (DNQX), indicating that the early EPSP is mediated by non-NMDA receptors. The slow depolarization was mediated by NMDA receptors because it was depressed by membrane hyperpolarization or addition of APV. 4. The late EPSP evoked by higher-frequency stimulation was abolished by APV, indicating that it is mediated by NMDA receptors, which are located either on the recorded cell or on presynaptic cells to the recorded cells. 5. Long-term potentiation (LTP) of EPSPs was examined in cells perfused with solutions containing 1 mu-M bicuculline methiodide (BIM), a gamma-aminobutyric acid (GABA) antagonist. WM was stimulated at 2 Hz for 15 min as a conditioning stimulus to induce LTP, and the resultant changes were tested by low-frequency (0.1 Hz) stimulation of WM. 6. LTP of early EPSPs occurred in more than one-half of the cells (8/13) after strong conditioning stimulation. The rising slope of the EPSP was increased 1.6 times on average. 7. To test involvement of NMDA receptors in the induction of LTP in the early EPSP, the effect of conditioning stimulation was studied in a solution containing 100 mu-M APV, which was sufficient to block completely synaptic transmission mediated by NMDA receptors. LTP occurred in the same frequency and magnitude as in control solution. 8. This intracellular observation that NMDA receptor is unnecessary for induction of LTP was supported by the study of LTP in field potentials evoked in layer II-III. There was no difference in the frequency of occurrence and the magnitude of LTP in solutions containing APV (100 or 400 mu-M) or another potent NMDA antagonist, MK-801 (20 mu-M) compared to control solutions. 9. To test the involvement of non-NMDA receptors in the induction of LTP, conditioning stimulation was given during DNQX application. Two stimulating electrodes, s1 and s2, were placed in WM to stimulate different axons; s1 was for testing the effect of conditioning stimulation and s2 for confirming that DNQX was washed out. After conditioning stimulation, responses evoked by s1 never exceeded the responses before conditioning, although the responses evoked by s2 recovered completely. 10. These results indicate that the LTP of synaptic transmission mediated by non-nmda receptors requires activation of the non-NMDA receptors but not of the NMDA receptors during conditioning stimulation.