THE EFFECT OF REPEATED PULSES OF LIGHT AT THE SAME TIME ON PERIOD RESPONSES OF THE RAT CIRCADIAN PACEMAKER

Abstract

A consequence of simple velocity-based models is that, inresponse to light pulses, the circadian period should adjust inversely tophase. In addition, because of the interaction of circadian period and phaseresponse, earlier circadian period changes should modify later circadian periodchanges. The literature contains few mentions of response curves of circadianperiod responses following light pulses. Rats were exposed to four pulsesof light (60 minutes, 1000 lux) at the same circadian time, a minimum of 26days apart; we assessed period responses and possible bias in the period-responsecurve. Modulation of circadian period following light-induced phase responseswas examined by assessing the period of running wheel activity onset. Phaseand circadian period were not consistently found to share an inverse relationship.Moreover, biases in initial period tended to be increased by the experimentalprotocol regardless of circadian time of pulse. Rats with a short initial(high-velocity) period had a lengthened period, while rats with a long initialperiod (low velocity) tended to have a reduce period. However, rats with along initial period were phase delay biased, not phase advance biased. Theseresults do not support a simple velocity model of the pacemaker. (Chronobiology International, 18(2), 187–201, 2001)

• Circadian rhythms
• Light pulses
• Multiple light pulses
• Period responses
• Phase response curves