Electrical properties of morphologically characterized neurons in the intergeniculate leaflet of the rat thalamus

The intergeniculate leaflet (IGL) is a flat thalamic nucleus that responds to retinal illumination, but also to non-photic input from many brain areas. Its only known function is to modulate the circadian rhythm generated by the suprachiasmatic nucleus. Previously, the firing behavior of cells in IGL has been investigated with extra-cellular recordings, but intracellular recordings from morphologically identified mammalian IGL neurons are lacking. We recorded from and labeled IGL cells in rat brain slices to characterize their basic membrane properties and cell morphology. A high fraction of neurons (82.5%) were spontaneously active. The silent cells were identified as neurons by electrophysiological techniques. The spontaneous activity was due to intrinsic membrane properties, and not driven by rhythmic synaptic input. Most spontaneously active cells had a very regular firing pattern with a coefficient of variation of the spike intervals <0.12 in more than 50% of the cells. Rebound depolarization after a hyperpolarizing pulse, usually with one fast action potential on top, was observed in 80% of the cells. The silent neurons had a range of resting membrane potentials and spike thresholds overlapping with the active ones. This suggests that spontaneous activity was controlled by several, yet undetermined factors in addition to membrane potential. Within the IGL we found a broad range of morphologies without apparent categories and no significant correlation with activity. However, the spontaneous, usually regular, spiking and the rebound depolarization of IGL cells is typical a feature that distinguish them from neurons in ventral and from interneurons in the dorsal lateral geniculate nuclei.