Fig. 1.

Temporary cessation of test stimulation disturbs LTP maintenance. A, Experimental arrangement of stimulating (s1 and s2) and recording electrodes (r), with the leftfigures and a dashed line indicating cortical layers and surgical cuts, respectively. B, C, LTP was maintained in cells in which test stimulation (0.l Hz) was continued after HFS. B, Top(a) and middle(b) traces show superimposed average (n = 6) responses in a cell before and after HFS for conditioned (left) and unconditioned (right) pathways, respectively. Bottom traces (a, b) are thetop and middle traces superimposed. Recorded time of the traces is indicated in C. Resting membrane potential was −54 and −55 mV before and 100 min after HFS, respectively. Input resistance (42 MΩ) was unchanged before and 100 min after HFS. C, The initial falling slope of IPSPs (percent of the mean baseline) plotted against the time after HFS for 12 cells. Squares and triangles(mean ± SEM) represent responses for conditioned and unconditioned pathways, respectively. No significant difference (p > 0.4; n = 12) was found in either resting membrane potential or input resistance before (−55 ± 2 mV, 44 ± 4 MΩ) and 100 min after HFS (−56 ± 3 mV, 44 ± 5 MΩ), which was also the case for other experimental groups of cells to which HFS was applied.D, E, Similar to B andC, but test stimulation for the conditioned pathway (s1) was stopped for the period indicated by thehorizontal bar in E. D,Left and right superimposedtraces represent responses in two cells that did not and did maintain LTP after cessation of test stimulation, respectively.E, Squares and circlesrepresent responses for conditioned pathways that did (n = 3) and did not maintain LTP (n = 5), respectively, and trianglesindicate unconditioned pathways (n = 8).