Identification of a seasonal elevation in daytime melatonin levels associated with the rut in fallow bucks (Dama dama): the effect of day length and exogenous melatonin.
The timing of the seasonal reduction in voluntary food intake and hormonal changes associated with the rut were investigated in mature fallow bucks subjected to artificial long-day exposure with and without superimposed melatonin-releasing implants. Circulating testosterone profiles and the period of inappetence, signalling the onset of the period of reproductive activity in untreated bucks under natural photoperiod, were phase-advanced in both treatment groups. In the long-day treated group, these profiles were phase-advanced by 1 week, while in the long-day + melatonin group, the phase advance was 6 weeks compared to the controls. Circulating daytime melatonin, elevated in all three groups, corresponds with seasonal changes in circulating testosterone levels associated with the rut. Prolactin profiles followed ambient photoperiod, being high during natural or artificial long days and low during natural short days. Exogenous melatonin administered during long-day exposure initiated a rapid and reversible decrease in circulating prolactin levels. The elevation in plasma melatonin could not be separated temporally from the seasonal testosterone peak by exposure to long days or to exogenous melatonin. However, this daytime melatonin profile was abolished by the ablation of circulating testosterone following immunization against luteinizing hormone releasing hormone (LHRH). The presence of this endogenous melatonin during daylight was observed only during the rut, as measured in two radioimmunoassays and confirmed by mass spectrometry. It is likely that both testosterone and melatonin influence the onset of the seasonal rut in fallow bucks.
Newman
R E
RE
CSIRO, Division of Animal Production, Blacktown, NSW, Australia.
Foldes
A
A
Maxwell
C A
CA
Rigby
R D
RD
Wynn
P C
PC
D000818:Animals; D002940:Circadian Rhythm / Q000502:physiology*; D003670:Deer / Q000502:physiology*; D004435:Eating; D008401:Gas Chromatography-Mass Spectrometry; D015918:Infusion Pumps, Implantable; D008297:Male; D008550:Melatonin / Q000008:administration & dosage / Q000097:blood*; D011388:Prolactin / Q000097:blood; D011863:Radioimmunoassay; D012621:Seasons; D012726:Sexual Behavior, Animal / Q000502:physiology*; D013739:Testosterone / Q000097:blood
D013739:Testosterone; D011388:Prolactin; D008550:Melatonin