Rapid phase adjustment of melatonin and core body temperature rhythms following a 6-h advance of the light/dark cycle in the horse

Abstract

Background: Rapid displacement across multiple time zones results in a conflict between the new cycle of light and dark and the previously entrained program of the internal circadian clock, a phenomenon known as jet lag. In humans, jet lag is often characterized by malaise, appetite loss, fatigue, disturbed sleep and performance deficit, the consequences of which are of particular concern to athletes hoping to perform optimally at an international destination. As a species renowned for its capacity for athletic performance, the consequences of jet lag are also relevant for the horse. However, the duration and severity of jet lag related circadian disruption is presently unknown in this species. We investigated the rates of re-entrainment of serum melatonin and core body temperature (BT) rhythms following an abrupt 6-h phase advance of the LD cycle in the horse.

Methods: Six healthy, 2 yr old mares entrained to a 12 h light/12 h dark (LD 12:12) natural photoperiod were housed in a light-proofed barn under a lighting schedule that mimicked the external LD cycle. Following baseline sampling on Day 0, an advance shift of the LD cycle was accomplished by ending the subsequent dark period 6 h early. Blood sampling for serum melatonin analysis and BT readings were taken at 3-h intervals for 24 h on alternate days for 11 days. Disturbances to the subsequent melatonin and BT 24-h rhythms were assessed using repeated measures ANOVA and analysis of Cosine curve fitting parameters.

Results: We demonstrate that the equine melatonin rhythm re-entrains rapidly to a 6-h phase advance of an LD12:12 photocycle. The phase shift in melatonin was fully complete on the first day of the new schedule and rhythm phase and waveform were stable thereafter. In comparison, the advance in the BT rhythm was achieved by the third day, however BT rhythm waveform, especially its mesor, was altered for many days following the LD shift.

Conclusion: Aside from the temperature rhythm disruption, rapid resynchronization of the melatonin rhythm suggests that the central circadian pacemaker of the horse may possess a particularly robust entrainment response. The consequences for athletic performance remain unknown.

Figure 1

Response of equine circadian melatonin (MT) and body temperature (BT) rhythms to a 6-h phase advance of LD12:12. The LD cycle with 6-h phase advance is depicted above each graph with white bars representing light and black bars times of darkness. The abscissa represents light cycle time (ZT) in hours, where ZT 0 corresponds to lights on and ZT 12 to lights off of a 12 h photoperiod. Through Day 0 (curves at left) and for one additional day, lights were on 7:30 AM to 7:30 PM (ZT0-12). As indicated by the arrow, the photoperiod was advanced 6 h on Day1 (+Δφ = 6 h, from ZT18 to ZT 0) to give new lights on from 1:30 AM to 1:30 PM (Days 1–11). Baseline curves (mean+/-SEM) for MT (top) and BT (bottom) are plotted on the left from ZT 0 to ZT 24. Curves for Day 1 through 11 are plotted on the right with point symbols and SEM removed for clarity. The dotted lines retrace the Day 0 curve assuming no phase shift. Asterisks represent Day 0 mean acrophase times.

Figure 2

Time course of reentrainment of MT (left) and BT (right) rhythms to a 6-h phase advance of LD12:12. The Figure 1 curves for Days 1 through Day 11 are re-plotted in a vertical array to show mean+/-SEM in relation to the Day 0 curve, which has been advanced 6 h for comparison. Dotted lines connect the upper and lower 95% confidence limits of the phase shifted Day 0 means. Without exception, Day 1 through Day 11 melatonin curves closely paralleled the zeitgeber time adjusted (6 h advanced) baseline Day 0 curve. By contrast, BT curves on Days 7, 9 and 11, deviate significantly from baseline at individual ZT times marked by adjacent black asterisks (* p < .05; ** p < .01; *** p < .001). Large red asterisks beneath each curve represent cosine fitted acrophases (ZT), which for BT, are notably phase delayed in association with the waveform distortions on Days 7–11 (see Figure 3A). Other conventions as in Figure 1.

Figure 3

Time course of changes in cosine acrophase, amplitude and mesor for equine MT and BT rhythms with a 6-h phase advance of LD12:12 between Day 0 and Day 1. A. Change in acrophase clock time (mean +/- SEM) over Days 0 through 11 of the experiment. B. mean +/- SEM melatonin amplitude and mesor values. C. Means +/- SEM body temperature amplitude and mesor (right y axis) (Range in degrees centigrade: 37.8°C – 38.9°C). Dotted horizontal lines in A represent a 6 h phase advance relative to baseline (upper, MT; lower BT). Note that the consistent steep rise in MT between ZT 12 and 15 on all days suggests a rapid MT shift that is essentially complete (+ 6 h) on Day 1 and stable thereafter (Figure 2A).