In Vivo Whole-Cell Patch-Clamp Recordings from Purkinje Cells and Interneurons. Synaptic Integration of Peripheral Input
In extracellular recordings, peripheral activation of parallel fiber input in Purkinje cells and interneurons indicates that only a small percentage of parallel fiber synapses are electrically effective. These synapses are all driven from a small receptive field that is specific to the receptive field of the local climbing fiber. This suggests that the synaptic integration performed by these neurons may be much simpler than we generally assume. The scope of the current study was to analyze this synaptic integration using peripheral stimulation.
In cats decerebrated at the intercollicular level, whole-cell patch-clamp recordings in the current clamp mode were obtained from interneurons and somata and dendrites of Purkinje cells in the forelimb area of the C3 zone. Blood pressure, end-expiratory CO2, and body temperature were continuously monitored and maintained within physiologic limits. Patch-clamp electrodes contained 140 mM K-gluconate and 7 mM Cl- as well as other electrolytes. Cells were generally held slightly below (−60 to −80 mV) resting potential to facilitate analysis of synaptic potentials without contaminating spike activity. Synaptic potentials evoked by manual peripheral stimulation and direct electrical stimulation in the inferior olive and in parallel fiber beams were investigated.
In the spontaneous activity of molecular layer interneurons, there are only a few large excitatory postsynaptic potentials (EPSPs). Peripheral stimulation of the receptive field activates many large EPSPs, including the climbing fiber EPSP, as well as inhibitory postsynaptic potentials (IPSPs). Outside the receptive field, few or no postsynaptic potentials are evoked. In Purkinje cell recordings from putative stem dendrites, all-or-nothing calcium spikes are evoked from both the climbing fiber and the parallel fiber receptive fields, but not from any other cutaneous areas. Somatic Purkinje cell recordings are dominated by large excitatory events of various amplitudes. Several characteristics suggest that they are all attenuated calcium spikes generated in stem dendrites.