INTEGRATION OF EXCITATORY POSTSYNAPTIC POTENTIALS IN DENDRITES OF MOTONEURONS OF RAT SPINAL-CORD SLICE CULTURES

We examined the attenuation and integration of spontaneous excitatory postsynaptic potentials (sEPSPs) in the dendrites of presumed motoneur ons (MNs) of organotypic rat spinal cord cultures. Simultaneous whole cell recordings in current-clamp mode were made from either the soma a nd a dendrite or from two dendrites. Direct comparison of the two volt age recordings revealed that the membrane potentials at the two record ing sites followed each other very closely except for the fast-rising phases of the EPSPs. The dendritic recording represented a low-pass fi ltered version of the somatic recording and vice versa. A computer-ass isted method was developed to fit the sEPSPs with a generalized cu-fun ction for measuring their amplitudes and rise times ( 10-90%). The mea n EPSP peak attenuation between the two recording electrodes was deter mined by a maximum likelihood analysis that extracted populations of s imilar amplitude ratios from. the fitted events at each electrode. For each pair of recordings, the amplitude attenuation ratio for EPSP tra veling from dendrite to soma was larger than that traveling from soma to dendrite. The Linear relation between mean In attenuation and dista nce between recording electrodes was used to map Ile attenuations into units of distance (mu m). For EPSPs with typical time course travelin g from the somatic to the dendritic recording electrode, the mean 1 /e attenuation corresponded to 714 mu m; for EPSPs traveling in the oppo site direction, the mean 1/e attenuation corresponded to 263 mu m. As predicted from cable analysis, fast EPSPs attenuated more in both the somatofugal and somatopetal direction than did slow EPSPs. For EPSPs w ith rise times shorter than similar to 2.0 ms, the attenuation factor increased steeply. Compartmental computer modeling of the experiments with biocytin-filled and reconstructed MNs that used passive membrane properties revealed amplitude attenuation ratios of the EPSP traveling in both the somatofugal and somatopetal direction that were comparabl e to those observed in real experiments. The modeling of a barrage of sEPSPs further confirmed that the somato-dendritic compartments of a M N are virtually isopotential except for the fast-rising phase of EPSPs . Large, transient differences in membrane potential are locally confi ned to the site of EPSP generation. Comparing the modeling results wit h the experiments suggests that the observed attenuation ratios are ad equately explained by passive membrane properties alone.