Key points
Lightâresponsive neurones in the rat suprachiasmatic nucleus discharge with a harmonic distribution of interspike intervals, whereas unresponsive neurones seldom do.This harmonic patterning has a fundamental frequency of close to 30Â Hz, and is the same in lightâon cells as in lightâoff cells, and is unaffected by exposure to light.Lightâon cells are more active than lightâoff cells in both subjective day and subjective night, and both lightâon cells and lightâoff cells respond more strongly to changes in light intensity during the subjective night than during the subjective day.Paired recordings indicate that the discharge of adjacent lightâresponsive cells is very tightly synchronized.The gap junction inhibitor carbenoxolone increases the spontaneous activity of suprachiasmatic nucleus neurones but does not block the harmonic discharge patterning.
AbstractThe suprachiasmatic nucleus (SCN) of the hypothalamus has an essential role in orchestrating circadian rhythms of behaviour and physiology. In the present study, we recorded from single SCN neurons in urethaneâanaesthetized rats, categorized them by the statistical features of their electrical activity and by their responses to light, and examined how activity in the light phase differs from activity in the dark phase. We classified cells as lightâon cells or lightâoff cells according to how their firing rate changed in acute response to light, or as nonâresponsive cells. In both sets of lightâresponsive neurons, responses to light were stronger at subjective night than in subjective day. Neuronal firing patterns were analysed by constructing hazard functions from interspike interval data. For most lightâresponsive cells, the hazard functions showed a multimodal distribution, with a harmonic sequence of modes, indicating that spike activity was driven by an oscillatory input with a fundamental frequency of close to 30Â Hz; this harmonic pattern was rarely seen in nonâresponsive SCN cells. The frequency of the rhythm was the same in lightâon cells as in lightâoff cells, was the same in subjective day as at subjective night, and was unaffected by exposure to light. Paired recordings indicated that the discharge of adjacent lightâresponsive neurons was very tightly synchronized, consistent with electrical coupling.