Luminance-dependent decrease in vasoactive intestinal polypeptide in the rat suprachiasmatic nucleus.

Light responsiveness of the vasoactive intestinal polypeptide (VIP) content in the suprachiasmatic nucleus (SCN) of the rat with pupils dilated by atropine was examined by enzyme immunoassay. After exposure to 6 h light at 3-1000 lux VIP levels in the SCN decreased as a monotonic function with a working range from 3 to 300 lux. At 12 h, 30 lux light decreased the VIP content to the minimum level that was attained by 300 lux light exposure in 6 h, suggesting that brighter illumination decreases VIP levels more rapidly, but light at a luminance of 0.05 lux for 3 days did not suppress VIP levels. These results suggest that VIP in the SCN codes visual information on luminance with a small working range and a relative high threshold.