Light responsiveness of the suprachiasmatic nucleus within the island with the retino-hypothalamic tract spared

To investigate daily variation in responsiveness of the suprachiasmatic nucleus (SCN) to exposure to light, the evoked change in multiple unit activity of the SCN within the retino-hypothalamic island with the retino-hypothalamic tract (RHT) spared, was measured in the rat at 6 h intervals across a 24 h day. It is found that SCN multiple unit discharge rates increase during a 1 h exposure to light which interrupts constant darkness irrespective of the time of day. A non-parametric statistical analysis did not indicate a significant daily variation in light-responsiveness of the SCN. The present results suggest that the phase-dependent shift of the SCN rhythm after a light exposure is induced not by a rhythm in light-responsiveness of the SCN mediated by the RHT, but by a more complicated machinery within the SCN.