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Evidence for Expression of Galanin Receptor Gal‐R1 mRNA in Certain Gonadotropin Releasing Hormone Neurones of the Rostral Preoptic Area

Previous studies have shown that galanin plays an important role in the regulation of gonadotropin releasing hormone (GnRH) release. At present, it is not known if this role is exerted by direct or indirect interactions between galanin producing neurones and GnRH neurones. The objective of this study was to determine whether or not GnRH neurones could express galanin receptor Gal‐R1 mRNA. Dual in‐situ hybridization experiments were carried out with digoxigenin‐labelled cRNA probes encoding GnRH in combination with 35S‐labelled riboprobe encoding the galanin receptor Gal‐R1. In order to detect possible variations in the expression of the Gal‐R1 mRNA under different physiological conditions, male rats, intact female rats throughout the phases of the oestrous cycle, ovariectomized (OVX) and steroid‐treated rats were analysed. The results show that many cells expressing Gal‐R1 mRNA were present throughout the preoptic area. Gal‐R1 mRNA‐expressing cells were observed in very close proximity with GnRH neurones. In the female rat, some GnRH neurones located in the rostral preoptic area/vascular organ of the lamina terminalis expressed Gal‐R1 mRNA. These double‐labelled cells were observed at all times of the oestrous cycle, except during diestrus at 08.00 h and pro‐oestrus at 18.00 h. Conspicuously, at oestrus 1800 h, we found that 21.6% of rostral GnRH neurones expressed the Gal‐R1 mRNA. In addition, dual‐labelled GnRH neurones were seen in OVX animals but not in oestrogen plus progesterone‐treated ones. In the male rat, colocalization of GnRH mRNA and Gal‐R1 receptor mRNA was not observed. In the medial preoptic area, no double‐labelled GnRH neurones were detected regardless of the endocrine conditions. These results suggest that, in addition to a possible indirect action of galanin on GnRH cells via neurones located at close proximity, the effects of galanin on GnRH can be mediated by direct activation of galanin receptors in rostral GnRH neurones. This study also shows that expression of Gal‐R1 mRNA in GnRH cells is influenced by the levels of circulating gonadal steroids.