['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 per paradigm): (a) refractoriness (low LH secretion) or not (high LH secretion) to short days in pineal-intact Ile-de-France ewes (RSD) and (b) endogenous circannual rhythm (ECR) in free-running pinealectomized Suffolk ewes in the active or inactive stage of their reproductive rhythm.', 'In RSD ewes, density of 5HT2A receptor mRNA (by in situ hybridization) was significantly higher in the high LH group (25.3 +/- 1.4 vs 21.4 +/- 1.5 grains/neuron, P < 0.05) and 3H-Ketanserin binding (a specific radioligand) of the median part of the premammillary hypothalamus tended to be higher in the high group (29.1 +/- 4.0 vs 24.6 +/- 4.2 fmol/mg tissu-equivalent; P < 0.10). In ECR ewes, density of 5HT2A receptor mRNA and 3H-Ketanserin binding were both significantly higher in the high LH group (20.8 +/- 1.6 vs 17.0 +/- 1.5 grains/neuron, P < 0.01, and 19.7 +/- 5.0 vs 7.4 +/- 3.4 fmol/mg tissu-equivalent; P < 0.05, respectively).'] Are changes in the 5-HT2A receptor system in the pre-mammillary hypothalamus of the ewe related to regulation of LH pulsatile secretion by an endogenous circannual rhythm?