Galanin-Like Peptide (GALP) mRNA Expression Is Restricted to Arcuate Nucleus of Hypothalamus in Adult Male Rat Brain

Abstract

Galanin-like peptide (GALP) is a novel 60-amino acid neuropeptide, isolated from porcine hypothalamus and subsequently identified in rats and humans, which has reported selectivity for the Gal-R2 galanin receptor [Ohtaki T et al: J Biol Chem 1999; 274: 37041–37045]. In the current study, the regional and cellular distribution of GALP mRNA in rat brain has been investigated by in situ hybridization of [³⁵S]-labelled oligonucleotide probes. In a thorough screening of adult male rat brain, GALP mRNA expression was detected only throughout the rostrocaudal extent of the arcuate nucleus (ARC) with the most abundant hybridization signal in the posterior, periventricular zones. GALP mRNA-positive neurons were mostly localized in the ventromedial division of the ARC, with many closely adjacent to the wall of the third ventricle. Smaller numbers of labelled neurons were also found in ventrolateral areas. The distribution of GALP mRNA was somewhat complementary to that for galanin (GAL) mRNA in the ARC, but contrasted with the broad distribution of this transcript throughout the hypothalamus. GAL mRNA was also distributed along the rostrocaudal extent of the ARC, but was most abundant in the anterior to middle levels and in ventrolateral regions. Interestingly, somatostatin mRNA expression appeared to overlap the distribution of GALP mRNA in posterior, ventromedial regions of the ARC. Thus, in adult rat brain GALP mRNA expression was restricted to a discrete subpopulation of neurons in the ARC, with a unique localization pattern unlike GAL or many other known peptide- and transmitter-containing cells in this region. GALP could, however, be co-expressed in sub-populations of other neuronal phenotypes (e.g. somatostatin neurons) or within cells that express Gal-R2 receptors. In view of the established anatomy and function of the ARC and the restricted localization of GALP mRNA, this novel peptide is likely to play a role in regulation of anterior pituitary hormone secretion, or in regulation of other hypothalamic peptide and transmitter systems.