SUMMARY1. Long-term potentiation (LTP) of synaptic transmission following tetanic stimulation of the white matter was studied by recording extracellular field potentials and intracellular synaptic potentials from layer II/III of visual cortical slices from young rats ranging in age from 21 to 40 days.2. Single shocks applied to the white matter at 0-1 Hz, used as test stimuli, elicited field potentials that consisted of primary and secondary components. The removal of Ca2" ions from the perfusate allowed identification of the secondary component as originating postsynaptically and the primary one as reflecting a mixture of antidromic and postsynaptic potentials.3. Tetanic stimulation at 5 Hz for 60 s was delivered to the white matter and field potentials were observed for 20 min to 9 h after the tetanus. LTP was defined as being present when the response displayed more than a 20% increase in amplitude of the Ca2+-sensitive components 20 min after the tetanus. LTP was induced in twelve of twenty-three slices tested, and this potentiation lasted throughout the period of observation. The average magnitude of potentiation was 147-8 + 28-4 % of the control value for the twelve slices.4. Administration of D,L-2-amino-5-phosphonovalerate (APV), an antagonist selective for N-methyl-D-aspartate (NMDA)-preferring receptors, slightly reduced the amplitudes of Ca2 -sensitive components of the field potentials. The average magnitude of reduction was 80-2+15-3% of the pre-drug control values. In the presence of APV, LTP was induced in only one slice of twelve tested.5. Stable intracellular recordings were obtained from twenty-three cells from layer II/III. Excitatory postsynaptic potentials (EPSPs) evoked by white matter stimulation had mean onset and peak latencies of 4.1 and 11-3 ms, respectively. In some cells these fast EPSPs were followed by another slow EPSP with a much longer latency and higher amplitude. Administration of APV revealed further that the fast EPSPs consisted of two components, i.e. early and late components.6. Tetanization of the white matter induced long-lasting enhancement of EPSPs in eight of twelve cells tested. In five of these eight cells, fast EPSPs were enhanced * Present address:

Actions of excitatory amino acid (EAA) antagonists on the responses of cells in layers II/III and IV of the cat's visual cortex to stimulation of layer VI and the underlying white matter were studied in slice preparations. Antagonists used were 2-amino-5-phosphonovalerate (APV), a selective antagonist for the N-methyl-D-aspartate (NMDA) type of EAA receptors, and kynurenate, a broad-spectrum antagonist for the three types of EAA receptors. In extracellular recordings it was demonstrated that most of the layer II/III cells were sensitive to APV, while the great majority of the layer IV cells were not. By contrast, kynurenate suppressed the responses completely in both layers. Excitatory post-synaptic potentials (EPSPs) evoked by stimulation of layer VI and the while matter were recorded intracellularly from layer II/III neurons. To determine whether the EPSPs were elicited mono- or polysynaptically, the synaptic delay for each EPSP was calculated from a pair of onset latencies of EPSPs evoked by stimulation of the two sites. Forty-two percent of the layer II/III cells were classified as having monosynaptic EPSPs. In 60% of these monosynaptic cells, the rising slope of the EPSPs was reduced by APV while in the other 40%, it was not. In the former (APV-sensitive cells), subtraction of the APV-sensitive component from the total EPSP indicated that the onset latency of the NMDA receptor-mediated component was roughly equal to that of the non-NMDA component. In the latter (APV-resistant cells), only the slowly-decaying component was in part mediated by NMDA receptors. The conduction velocities of the afferent fibers innervating APV-resistant cells were slower than those of the APV-sensitive cells, suggesting that both types of cells are innervated by different types of afferents. The polysynaptic EPSPs of almost all layer II/III cells were sensitive to APV. The subtraction method indicated that the NMDA component had about the same magnitude as the non-NMDA components. When the slices were superfused by a Mg2+-free solution, the EPSPs were potentiated dramatically, but this potentiation was reduced to the control level during the administration of APV. Similarly, APV-sensitive components were potentiated during the administration of bicuculline, a selective antagonist for gamma-aminobutyric acid receptors of A type. These results suggest that NMDA receptors participate, at varying degrees, in excitatory synaptic transmission at most layer II/III cells in the cat's visual cortex, and their actions appear to be regulated by intracortical inhibition.

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Actions of antagonists for excitatory amino-acid (EAA) receptors on extracellularly and intracellularly recorded responses of layer II/III cells to electrical stimulation of the underlying white matter were studied in a slice preparation of rat's visual cortex. Antagonists used were 2-amino-5-phosphonovalerate (APV) and 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX), which are selective antagonists for EAA receptors of N-methyl-D-aspartate (NMDA) and quisqualate/kainate (non-NMDA) type, respectively. In extracellular recordings, it was found that responses of almost all of the cells were suppressed by CNQX. In contrast, sensitivity to APV was different between cells with short- and long-latency responses; 81% of the former responses were not suppressed by APV, while about a half of the latter were suppressed. Excitatory postsynaptic potentials (EPSPs) evoked by white-matter stimulation were recorded intracellularly from 42 neurons. Most of polysynaptically elicited EPSPs were sensitive to APV, whereas the majority of monosynaptic EPSPs were not. CNQX almost completely suppressed EPSPs irrespective of monosynaptically or polysynaptically evoked, but in some cases slow EPSPs with low amplitude were spared. These CNQX-resistant EPSPs were elicited polysynaptically and had an anomalous voltage dependence, a characteristic of NMDA receptors. It is suggested that non-NMDA receptors contribute dominantly to first-order synaptic transmission while NMDA receptors participate substantially in second-order transmission so as to serve as a booster of outputs from visual cortex.