  In sheep as in many vertebrates, the seasonal pattern of reproduction is
timed by the annual photoperiodic cycle, characterized by seasonal changes in
the daylength. The photoperiodic information is translated into a circadian
profile of melatonin secretion. After multiple neuronal relays (within the
hypothalamus), melatonin impacts GnRH (gonadotrophin releasing hormone)
secretion that in turn controls ovarian cyclicity. The pattern of GnRH
secretion is mirrored into that of LH (luteinizing hormone) secretion, whose
plasmatic level can be easily measured. We addressed the question of whether
there exists an endogenous circannual rhythm in a tropical sheep population
that exhibits clear seasonal ovarian activity when ewes are subjected to
temperate latitudes. We based our analysis on LH time series collected in the
course of 3 years from ewes subjected to a constant photoperiodic regime. Due
to intra- and inter- animal variability and unequal sampling times, the
existence of an endogenous rhythm is not straightforward. We have used
time-frequency signal processing methods to extract hidden rhythms from the
data. To further investigate the LF (low frequency) and HF (high frequency)
components of the signals, we have designed a mathematical model of LH
plasmatic level accounting for the effect of experimental sampling times. The
model enables us to confirm the existence of an endogenous circannual rhythm,
to investigate the action mechanism of photoperiod on the pulsatile pattern of
LH secretion (control of the interpulse interval) and to conclude that the HF
component is mainly due to the experimental sampling protocol.
