Localization of mRNA encoding thyrotropin-releasing hormone precursor in the brain of sockeye salmon

The distribution of mRNA encoding thyrotropin-releasing hormone (TRH) precu rsor in the brain of sockeye salmon was studied by in situ hybridization hi stochemistry using digoxigenin-labeled riboprobes as a basis to investigate its physiological functions in the salmon brain. Since seasonal variation in TRH gene expression was expected in relation to smelting or maturation, fish were sampled in February and October. In both groups, TRH mRNA was wid ely distributed in discrete brain regions including the internal cellular l ayer (ICL) of the olfactory bulb, postcommissural nucleus of area ventralis telencephali (Vp), nucleus preopticus parvocellularis anterioris (PPa), nu cleus preopticus magnocellularis, dorsal zone of periventricular hypothalam us (Hd), torus semicircularis, and also the motor nucleus of vagus nerve in the medulla oblongata. TRH neurons in ICL and Hd are round and small with diameters of 5-10 mu m. In contrast, TRH neurons in the ventral telencephal on and the preoptic area are medium-sized (10-20 mu m), and appear to have multiple processes. Most of these cells are restrictively localized along t he lateral margin of the preoptic nuclei. The number of TRH neurons in Vp a nd PPa was smaller in February than in October, suggesting a seasonal chang e of TRH neurons in the preoptic area. In the medulla oblongata, a cluster of large oval-shaped cells (20-30 mu m) showed signals for TRH mRNA. The pr esent results suggest that TRH may function as a neurotransmitter or neurom odulator involved in olfactory activity and also autonomic motor integratio n, in addition to neurohormonal control of secretion of pituitary hormones.