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 reg ulation of gonadotropin releasing hormone (GnRH) release. At present, it is not known if this role is exerted by direct or indirect interactions betwe en galanin producing neurones and GnRH neurones. The objective of this stud y was to determine whether or not GnRH neurones could express galanin recep tor Gal-R1 mRNA, Dual in-situ hybridization experiments were carried out wi th digoxigenin-labelled cRNA probes encoding GnRH in combination with S-35- labelled riboprobe encoding the galanin receptor Gal-R1. In order to detect possible variations in the expression of the Gal-R1 mRNA under different p hysiological conditions, male rats, intact female rats throughout the phase s of the oestrous cycle, ovariectomized (OVX) and steroid-treated rats were analysed. The results show that many cells expressing Gal-R1 mRNA were pre sent throughout the preoptic area. Gal-R1 mRNA-expressing cells were observ ed 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 observe d 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-l abelled GnRH neurones were seen in OVX animals but not in oestrogen plus pr ogesterone-treated ones. In the male rat, colocalization of GnRH mRNA and G al-R1 receptor mRNA was not observed. In the medial preoptic area, no doubl e-labelled GnRH neurones were detected regardless of the endocrine conditio ns. These results suggest that, in addition to a possible indirect action o f galanin on GnRH cells via neurones located at close proximity, the effect s of galanin on GnRH can be mediated by direct activation of galanin recept ors 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 st eroids.