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Melatonin transduces the effect of day length on LH secretion by acting on the hypothalamus. However, the precise hypothalamic site is unknown. Two studies were undertaken to clarify where melatonin acts in the hypothalamus. Using autoradiographic methods, the hypothalami of 5 ewes were screened to determine whether specific regional densities in melatonin(More)
Experiments were designed to localize the central sites of action of melatonin involved in the control of seasonal reproduction. Ewes were exposed to long days and received microimplants of melatonin in the preoptic area (n = 9), anterior hypothalamus (n = 4), dorsolateral hypothalamus (n = 4), or mediobasal hypothalamus (n = 12). The effects of implants(More)
BACKGROUND Seasonality of ovulatory activity is observed in European sheep and goat breeds, whereas tropical breeds show almost continuous ovulatory activity. It is not known if these tropical breeds are sensitive or not to temperate photoperiod. This study was therefore designed to determine whether tropical Creole goats and Black-Belly ewes are sensitive(More)
This study was undertaken to assess the influence of photoperiod on growth hormone (GH) secretion in rams and its possible influence on body weight. Twenty young adult rams were divided into two groups. One was subjected to an annual (AR) and the other to a semestral (SR) light regime during the same 18-month period. In both groups, daylength (DL) varied(More)
The mechanisms by which melatonin controls seasonal reproduction are poorly understood. The use of a large animal model, namely the sheep, has allowed progress in the understanding of these mechanisms, and is the subject of this review. Firstly, the contribution made by large animal models to demonstrating that melatonin acts in the hypothalamus and the(More)
Melatonin regulation of reproductive functions in sheep is mediated by action in the premammillary hypothalamus (PMH). The aim of this study was to identify the high-affinity melatonin-receptor subtypes expressed in this structure. To achieve this, we used reverse transcription-polymerase chain reaction (RT-PCR) and developed in situ hybridization(More)
In order to determine individual variability in the amplitude and duration of the nycthemeral rhythm of the plasma melatonin concentration, 12 ovariectomized estradiol-treated Ile-de-France ewes were submitted to a sequence of 34 long days (LD1, 16L:8D), 21 short days (SD, 8L:16D), and 21 LD (LD2). Intensive blood sampling (hourly from 1 hr before dusk to 1(More)
The time of appearance of plasma LH and testosterone peaks through the day determined in 75 Préalpes du Sud and 41 Ile-de-France rams in December and in 44 Préalpes du Sud and 11 Ile-de-France rams in June. The distribution of peaks throughout the day was non-random for the two hormones in the two breeds and for both times of the year (P less than 0.01 at(More)
An experiment was designed to determine whether the pars tuberalis is the site of action of melatonin involved in the photoperiodic control of LH and prolactin secretion in sheep. In an attempt to produce a 'short-day' effect on these hormones (i.e. stimulation of LH secretion and inhibition of prolactin release), microimplants of melatonin were placed(More)
BACKGROUND We wanted to determine if changes in the expression of serotonin 2A receptor (5HT2A receptor) gene in the premammillary hypothalamus are associated with changes in reproductive neuroendocrine status. Thus, we compared 2 groups of ovariectomized-estradiol-treated ewes that expressed high vs low LH pulsatility in two different paradigms (2 groups(More)