Melatonin concentrations in the two jugular veins, and relationship with the seasonal reproductive activity in goats L.A. Zarazaga a, *, I. Celi a , J.L. Guzmán a , B. Malpaux b a Departamento de Ciencias Agroforestales, Universidad de Huelva, Carretera de Palos de la Frontera s/n, 21819, Palos de la Frontera, Huelva, Spain b Neuroendocrinologie Sexuelle, INRA-PRMD, URA CNRS 1291, 37380, Nouzilly, France. Received 11 September 2009; received in revised form 1 February 2010; accepted 5 February 2010 Abstract The authors investigated whether melatonin concentrations vary between the two jugular veins and whether absolute (nocturnal) or relative (nocturnal/diurnal ratio) plasma melatonin concentrations are associated with seasonal reproductive activity measured by oestrus or ovulatory activity in Payoya goats. Thirty-two adult Payoya goats were penned under natural photoperiod. Oestrus activity was tested daily using aproned males—twice a week plasma was sampled for progesterone. Melatonin plasma concentrations were studied at each equinox and solstice of the year in jugular samples taken simultaneously by venipuncture. Nocturnal and diurnal plasma melatonin concentrations from each jugular vein were assessed in 3 and 2 plasma samples per goat, respectively, taken at hourly intervals in each period. No differences in melatonin concentrations between the two veins were observed, but there was a significant interaction (P  0.001) between jugular vein and animal in nocturnal melatonin concen- trations. There was no effect of sampling period on melatonin concentrations and the coefficient of correlation between sampling periods was very high. The analyses performed indicated that neither absolute nor relative melatonin concentrations were related with the dates of onset or end of ovulatory/oestrus activity. Therefore, we concluded that in goats (1) melatonin concentrations are highly variable between jugular veins in the same individual but not in the general population, (2) melatonin concentrations are highly repeatable for each individual, and (3) absolute and relative amplitudes of melatonin concentrations are not linked to the seasonal breeding activity in Mediterranean goats. © 2010 Elsevier Inc. All rights reserved. Keywords: Goat; Melatonin; Jugular; Seasonality; Reproduction 1. Introduction Reproductive activity in goats shows a clear annual pattern, with a period of breeding activity that begins in the late summer or autumn, when daylight is decreas- ing, and the onset of seasonal anoestrus in late winter or at the start of spring, with increasing daylight. This seasonal breeding cycle in goats is not directly driven by the photoperiod, rather it appears to be a reflection of the expression of a self-sustained endogenous rhythm that is synchronised or entrained by the photo- period [1]. The reproductive response to photoperiod is mediated by the pineal gland via changes in the daily secretion of its main secretory product, melatonin [2] that is read by a large variety of tissues in the whole organism [3]. After its synthesis, melatonin is secreted via the vasculature of the pineal that flows into the vein of Galen or vena cerebri magna. This cerebral vein drains into the sinus rectus, and venous blood is delivered to the internal jugular veins via the lateral sinus [4]. An * Corresponding author: Tel.: +34 959217523; Fax: +34 959217304. E-mail address: zarazaga@uhu.es (L.A. Zarazaga) Available online at www.sciencedirect.com Theriogenology 74 (2010) 221–228 www.theriojournal.com 0093-691X/$ – see front matter © 2010 Elsevier Inc. All rights reserved. doi:10.1016/j.theriogenology.2010.02.005