Responses of cells in the rat suprachiasmatic nucleus in vivo to stimulation of afferent pathways are different at different times of the light/dark cycle.

Conventional extracellular recordings were made from single cells in the suprachiasmatic nucleus (SCN) region of the anaesthetized rat. Each cell was tested for its response to stimulation at three sites; the contralateral optic nerve, the ipsilateral supraoptic nucleus (SON) or the ipsilateral arcuate nucleus (ARC) to determine whether the behaviour of the synapses in the SCN was different at different times. Responses to stimulation were tested once each hour and assessed by creating peristimulus time histograms. Excitatory, inhibitory or complex (consisting of more than one component) responses were seen. The responses of some cells that were recorded for several hours changed with time. Changes were seen in the responses of SCN cells to stimulation of the ARC (31/91 cells) and the SON (26/90 cells) regions, but only rarely to stimulation of the optic nerve (2/72 cells). Such differences in proportion are unlikely to have occurred by chance (P < 0.001; chi-square test). Changes seen included the appearance of both excitatory and inhibitory responses in cells that were initially unresponsive. In some cells, one component of a complex response remained constant while another component changed with time. When the cells in the SCN were treated as a group, the proportion of excitatory, inhibitory or complex responses to ARC stimulation did not remain constant throughout the light/dark cycle (P = 0.014; chi-square test). The proportion of excitatory, inhibitory or complex responses to SON and optic nerve stimulation showed no significant variation with the light/dark cycle. If a change in response can be interpreted as a change in the behaviour of a neural connection, the results imply that some of the projections to the SCN from within the hypothalamus change at different times of the light/dark cycle, whereas no change could be seen in the input from the optic nerve. Thus, some of the connections of the SCN appear not to be hard wired, but change rapidly with time.