| Summary: | <p>Abstract</p> <p>Background</p> <p>We wanted to determine if changes in the expression of serotonin 2A receptor (5HT2<sub>A </sub>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 <it>vs </it>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.</p> <p>Results</p> <p>In RSD ewes, density of 5HT2<sub>A </sub>receptor mRNA (by in situ hybridization) was significantly higher in the high LH group (25.3 ± 1.4 <it>vs </it>21.4 ± 1.5 grains/neuron, P < 0.05) and <sup>3</sup>H-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 5HT2<sub>A </sub>receptor mRNA and <sup>3</sup>H-Ketanserin binding were both significantly higher in the high LH group (20.8 ± 1.6 <it>vs </it>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).</p> <p>Conclusions</p> <p>We conclude that these higher 5HT2<sub>A </sub>receptor gene expression and binding activity of 5HT2<sub>A </sub>receptor in the premammillary hypothalamus are associated with stimulation of LH pulsatility expressed before the development of refractoriness to short days and prior to the decline of reproductive neuroendocrine activity during expression of the endogenous circannual rhythm.</p>
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