Resetting the rat circadian clock by ultra-short light flashes

We examined the effects that ultra-brief, intense, light flashes have on th e rat circadian clock, the suprachiasmatic nucleus of the hypothalamus (SCN ), We found that as few as five intense flashes, each 10-mu s in duration ( 1 per s), can produce both phase shifts in free-running activity rhythms an d Fos expression in the SCN in rats kept in constant darkness. After pre-ex posure to such flashes, phase shifts in response to a continuous light puls e delivered 2 h later were potentiated, but Fos expression in the SCN was d ecreased as following pre-exposure to continuous light. These results show that flashes induce behavioral and cellular effects indicative of clock res etting similar to those induced by light stimuli of longer duration. Extrem ely brief but intense, light stimuli may be much more important to clock re setting than had been previously known. (C) 1999 Elsevier Science ireland L td. All rights reserved.