Persistent inward currents in rat ventral horn neurones

Throughout the mammalian spinal cord, interneurones have been shown to exhibit distinct firing patterns in response to a step of injected current. In this study of ventral horn interneurones in a thick slice preparation of the lumbar cord of 11–19‐day‐old‐rats, four distinct firing patterns were observed and classified as repetitive‐firing, repetitive/burst, initial‐burst or single‐spiking. The hypothesis that a persistent sodium current was the predominant determinant of cell firing behaviour was investigated. A slow voltage ramp was used to assess persistent inward currents (PICs). Cells with repetitive‐firing patterns had significantly larger PICs than cells displaying repetitive/burst, initial‐burst or single‐spiking patterns. Repetitive‐firing, repetitive/burst and initial‐burst‐firing cells were reduced to a single‐spiking pattern with the application of riluzole, which also markedly reduced the persistent sodium current. Persistent sodium current was found to account for most of the PIC with only a small contribution from L‐type calcium current. These results suggest that the persistent sodium current plays a major role in determining firing patterns in these cells.