Failure to respond to endogenous or exogenous melatonin may cause nonphotoresponsiveness in Harlan Sprague Dawley rats

Abstract

BackgroundResponsiveness to changing photoperiods from summer to winter seasons is an important but variable physiological trait in most temperate-zone mammals. Variation may be due to disorders of melatonin secretion or excretion, or to differences in physiological responses to similar patterns of melatonin secretion and excretion. One potential cause of nonphotoresponsiveness is a failure to secrete or metabolize melatonin in a pattern that reflects photoperiod length.MethodsThis study was performed to test whether a strongly photoresponsive rat strain (F344) and strongly nonphotoresponsive rat strain (HSD) have similar circadian urinary excretion profiles of the major metabolite of melatonin, 6-sulfatoxymelatonin (aMT6s), in long-day (L:D 16:8) and short-day (L:D 8:16) photoperiods. The question of whether young male HSD rats would have reproductive responses to constant dark or to supplemental melatonin injections was also tested. Urinary 24-hour aMT6s profiles were measured under L:D 8:16 and L:D 16:8 in young male laboratory rats of a strain known to be reproductively responsive to the short-day photoperiod (F344) and another known to be nonresponsive (HSD).ResultsBoth strains exhibited nocturnal rises and diurnal falls in aMT6s excretion during both photoperiods, and the duration of the both strains' nocturnal rise was longer in short photoperiod treatments. In other experiments, young HSD rats failed to suppress reproduction or reduce body weight in response to either constant dark or twice-daily supplemental melatonin injections.ConclusionThe results suggest that HSD rats may be nonphotoresponsive because their reproductive system and regulatory system for body mass are unresponsive to melatonin.

Differential response of type 2 deiodinase gene expression to photoperiod between photoperiodic Fischer 344 and nonphotoperiodic Wistar rats

“…This hypothesis would be distinct from the case of F344 and Sprague-Dawley rats, because the reproductive system and regulatory system for body mass of Sprague-Dawley rats are unresponsive to melatonin signals, even if melatonin patterns are identical in both strains (25). The differences between Wistar and Sprague-Dawley rats are also obvious in the fact that gonadal development of Wistar rats is inhibited by constant darkness or melatonin injections (15,30), whereas that of Sprague-Dawley rats does not respond to these treatments (25). Instead, our data lead to the possibility that patterns of melatonin secretion under short-and long-day conditions may differ between Wistar and F344 strains.…”

Variation in nocturnality and circadian activity rhythms between photoresponsive F344 and nonphotoresponsive Sprague Dawley rats

“…HSD rats have relatively low amplitude 6-sulfatoxymelatonin excretion patterns when adjusted for body mass [ 32 ] and a long duration tau (24.05 +/- 0.02; Figure 3f ), similar to nonphotoperiodic Siberian hamsters [ 10 , 11 ]. However, HSD rats produce 6-sulfatoxymelatonin excretion rhythms that adjust to night length and are similar to those of F344 rats in SD [ 32 ], and HSD rats successfully decompress activity to a long-duration pattern when transferred from LD to SD (Figure 3b ). We have not tested directly the ability of HSD rats to phase delay, but successful entrainment to SD suggests that HSD rats have the ability to phase delay to achieve entrainment to SD.…”

“…F344 rats are becoming a model for mechanisms of photoresponsiveness [ 31 ], and may be useful models for the study of correlated genetic variation in rhythms, regulation of appetite and body mass, and reproduction. A recent comparison of the rhythm of excretion of the major metabolite of melatonin, 6-sulfatoxymelatonin, between photoperiodic F344 rats and nonphotoperiodic HSD rats suggests that both strains have long duration melatonin secretion in short photoperiod and short duration melatonin secretion in long photoperiod [ 32 ]. Thus, both photoresponsive F344 and nonphotoresponsive HSD rats were similar in pattern of melatonin production to photoresponsive rather than nonphotoresponsive Siberian hamsters [ 10 ].…”

“…For this study, a companion study to Price et al [ 32 ], we tested two competing hypotheses. One hypothesis is that differences in photoresponsiveness between F344 and HSD rats are caused by differences in circadian clock function and/or clock outputs cause differences in photoresponsiveness.…”

“…One hypothesis is that differences in photoresponsiveness between F344 and HSD rats are caused by differences in circadian clock function and/or clock outputs cause differences in photoresponsiveness. A competing hypothesis is that differences in photoresponsiveness are caused by differences at the level of melatonin secretion and responses to melatonin [ 32 ], rather than by differences in clock function. We tested for circadian differences between young male F344 and young male HSD rats by measuring running wheel activity in short photoperiod, long photoperiod, and constant dark (DD) to assess circadian traits and the degree of nocturnality.…”