Expression of GalR1 and GalR2 galanin receptor messenger ribonucleic acid in proopiomelanocortin neurons of the rat arcuate nucleus: effect of testosterone.

Abstract

Previous studies have shown that galanin-containing fibers make synaptic contacts with POMC neurons in the arcuate nucleus. However, the ability of POMC neurons to express galanin receptors has never been assessed. The present study was designed to investigate whether POMC neurons express galanin receptor messenger RNA (mRNA) and whether testosterone could modulate galanin receptor gene expression. A dual-labeling in situ hybridization histochemistry, using 35S-labeled (galanin receptors GalR1 or GalR2) and digoxigenin-labeled (POMC) riboprobes, was performed on brain sections from intact, castrated, and testosterone-replaced adult male rats. For analysis, the arcuate nucleus was divided into four rostro-caudal areas. The results revealed that both GalR1 and GalR2 mRNAs were expressed in POMC neurons. Most POMC neurons expressing galanin receptor mRNAs were found in the rostral parts of the nucleus. Castration reduced the labeling density of galanin receptor mRNAs in POMC neurons, and testosterone prevented the effects of castration in all rostro-caudal subdivisions of the arcuate nucleus. Taken together, these data indicate that galanin can directly modulate the activity of POMC neurons, via an action on GalR1 or GalR2 receptors, particularly in the rostral-arcuate nucleus. In addition, testosterone can modulate the expression of GalR1 and GalR2. Because POMC neurons located in the rostral part of the nucleus are known to project preferentially to the preoptic area, POMC neurons expressing the galanin receptor genes may play an important role in the regulation of the GnRH neuroendocrine axis.