Distribution of Galanin Receptor-2 Immunoreactive Neurones in the Ovine Hypothalamus: No Evidence for Involvement in the Control of Gonadotrophin-Releasing Hormone Secretion

Division of Cell Sciences, Institute of Comparative Medicine, Faculty of Veterinary Medicine, University of Glasgow, UK.
Journal of Neuroendocrinology (Impact Factor: 3.14). 01/2008; 19(12):966-73. DOI: 10.1111/j.1365-2826.2007.01609.x
Source: PubMed


Galanin is a small neuropeptide that mediates its effects via three receptor isoforms: galanin receptor-1, galanin receptor-2 and galanin receptor-3 (Gal-R1, Gal-R2 and Gal-R3). Galanin is thought to be an important intermediate in signalling in the hypothalamic-pituitary-gonadal axis and has been widely detected in the ovine hypothalamus. The expression of galanin and Gal-R1 has been reported to fluctuate during the reproductive cycle. Although the distribution of Gal-R1 has been determined in the ovine hypothalamus, the distribution of Gal-R2 was hitherto unknown. Using immunohistological and immunofluorescence techniques, we have mapped the distribution of Gal-R2 in the ovine hypothalamus, collected during the follicular phase of the oestrous cycle and examined colocalisation of Gal-R2 with oestrogen receptor alpha (ERalpha) and gonadotrophin-releasing hormone (GnRH). Gal-R2 was expressed in several regions of the hypothalamus (supraoptic nucleus, paraventricular nucleus, ventromedial nucleus, arcuate nucleus) but not as widely expressed as Gal-R1. Areas of Gal-R2 expression overlapped with those reported for Gal-R1. We observed that, in certain defined regions of the hypothalamus, up to 50% of neurones that express Gal-R2 also express ERalpha. No neurones coexpressed Gal-R2 and GnRH. Thus, we conclude that, in follicular phase animals, this receptor plays little or no role in direct intermediary signal transmission in GnRH-mediated control of the reproductive cycle.

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    • "Previous studies in sheep have reported that galanin is expressed within the hypothalamus and pituitary gland (Tempel et al. 1988, Leibowitz et al. 1998, Chaillou et al. 1999), and GALR1 is expressed within the hypothalamus (Tempel et al. 1988, Leibowitz et al. 1998, Chaillou et al. 1999), is co-expressed within GnRH1 neurones and is dynamically regulated by oestradiol (Merchenthaler et al. 1993, Rossmanith et al. 1994, Mitchell et al. 1999, Dufourny & Skinner 2005, Tourlet et al. 2005). GALR2- immunopositive neurones have also been reported to be present within the ovine hypothalamus and a proportion of the GALR2-immunopositive cells have been reported to be oestradiol sensitive (Chambers et al. 2007). However, to date, no studies have characterised age-related changes in mRNA expression for galanin and its three receptor isoforms in the hypothalamo-pituitary axis of both male and female animals. "
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