DL Conditions Research Articles

Circadian rhythm of plasma corticosteroid in adult female rats: chronological shifts in abnormal lighting regimens and connection with oestrous cycle.

The circadian rhythm of plasma corticosteroid (CS) levels in adult female rats was studied chronologically under the following conditions: normal light-dark (LD), inverted light-dark (DL), constant dark (DD) and constant light (LL). Animals were accustomed to LD condition for 7 days before exposure to each abnormal lighting regimen. Normal circadian rhythm established under LD condition was clearly inverted on the third day of DL regimen, and the inverted rhythm persisted thereafter under DL condition. The circadian CS rhythm persisted essentially unchanged throughout DD condition, but lost its regular periodicity showing "free running" and changed day by day under LL condition. The average CS levels over a 24 h period were higher under LL than under DD condition. Plasma CS levels in each lighting regimen exhibited diurnal variations regardless of the vaginal smear patterns of autopsied animals. Exposure of rats to LL for 21 days made the circadian CS rhythm flat, but induced persistent oestrus in only a few animals. The data suggest that (1) an unexpectedly rapid inversion of the circadian CS rhythm occurs if animals are exposed to inverted light-dark environment; (2) constant darkness seems to be a near-natural environment for rats, and changes little of the pre-established circadian CS rhythm; (3) constant light, on the contrary, is assumed to be a stress for rats, and disrupts the circadian CS rhythm and elevates CS levels; (4) the change in circadian CS rhythm in adult female rats is not mediated by a change in gonadal function and the two conditions may not be connected directly with each other.

Circadian rhythms and seizure susceptibility: Effects of manipulations of light cycles on susceptibility to audiogenic seizures and on levels of 5-hydroxytryptamine and norepinephrine in brain

Susceptibility to audiogenic seizures was determined in 26-day-old DBA/2J mice raised under either normal (LD) or reverse-cycle (DL) lighting conditions. Reverse-cycle lighting conditions completely reversed the circadian rhythm of susceptibility to audiogenic seizures in these mice. In another experiment, mice were placed into either LD, DL, complete light (LL) or complete dark (DD) conditions at noon of either Days 21, 23, 25 or 26. These animals were then tested for susceptibility to audiogenic seizures at either 1700 or 2000 hr of Day 26. Reverse-cycle lighting conditions for 5 days almost reversed the audiogenic seizure susceptibility pattern, and both DD and LL conditions sharply increased susceptibility to audiogenic seizures. In a third experiment, DBA/2J mice were placed into either LD, DL, DD or LL conditions at noon of Day 21 and sacrificed at 1700 hr on Day 26. Levels of 5-hydroxytryptamine and norepinephrine were measured in cortex, diencephalon and brain stem of these mice. Increased susceptibility to audiogenic seizures was found to be directly related to lower levels of 5-hydroxytryptamine and norepinephrine in brain. The relation between susceptibility to audiogenic seizures and levels of these biogenic amines in brain is discussed.