How hamsters keep time: the 6 pm to 6 am light-sensitive period

A recent review of the pineal literature revealed that when hamsters are exposed to 24-hour light:dark (LD) cycles with less than 12 hours of darkness (summerlike, SLD), the nightly period of pineal melatonin synthesis (PPMS) begins close to the midpoint of the dark period ("midnight") and ends at lights-on irrespective of the length of the dark period or time of day presented. New evidence based on the onset of behavioral estrus every 4 days indicated that the 24-hour hamster clock controlling timing of estrus (4:30 PM) and the PPMS has a 12-hour light-sensitive period (LSP) set to 6 PM-6 AM in LD 16:8 (dark 8 PM-4 AM, SLD) by a balance in opposing actions of evening and morning light [Alleva: Pineal Research Reviews, Volume 5, Alan R. Liss, Inc., New York, 1987]. Present experiments focus on how this balance is maintained. When lights-off was advanced to 6 PM in SLDs ranging from LD 12.5:11.5 (dark 6:15 PM-5:45 AM) to 18:6 (dark 9 PM-3 AM), the onset of estrus later that day was advanced in every SLD. However, when lights-on was delayed to or beyond 6 AM, the onset of estrus was unaffected. Thus, the balance is maintained by a resistive force (blocking without a delaying action) of evening light and an advancing action of morning light. In this balancing process all evening light from 6 PM to lights-off but only the first 5 minutes or less of morning light were involved. The advancing action of morning light was characterized in LD 13:11 and 18:6 by imposing on the night before estrus a 5:30 PM-6:30 AM dark period scanned with a 15-minute light pulse. Shifts in onset of estrus later that day were plotted vs. time of the light pulse. The resulting phase response curves (PRCs) were similar and comprised only an advancing curve, which rose about 10 PM, peaked at 2 AM, and returned gradually to normal at 6 AM. In contrast, a PRC obtained from LD 12:12 (dark 6 PM-6 AM) was sinelike, comprising a 6 PM-9 PM delaying curve followed by an advancing curve similar to those from SLD. An hypothesis based on these findings is presented to explain how hamsters would keep constant AM-PM time throughout summer.