Abstract
Many studies have shown that juvenile salmon imprint olfactory memory of natal stream odors during downstream migration, and adults recall this stream-specific odor information to discriminate their natal stream during upstream migration for spawning. The odor information processing of the natal stream in the salmon brain, however, has not been clarified. We applied blood oxygenation level-dependent (BOLD) functional magnetic resonance imaging to investigate the odor information processing of the natal stream in the olfactory bulb and telencephalon of lacustrine sockeye salmon (Oncorhynchus nerka). The strong responses to the natal stream water were mainly observed in the lateral area of dorsal telencephalon (Dl), which are homologous to the medial pallium (hippocampus) in terrestrial vertebrates. Although the concentration of L-serine (1 mM) in the control water was 20,000-times higher than that of total amino acid in the natal stream water (47.5 nM), the BOLD signals resulting from the natal stream water were stronger than those by L-serine in the Dl. We concluded that sockeye salmon could process the odor information of the natal stream by integrating information in the Dl area of the telencephalon.
Highlights
Salmon have an amazing ability to migrate several thousands of kilometers from the ocean to their natal stream for spawning
Some biphasic blood oxygenation level-dependent (BOLD) responses to Lserine and the natal stream water were observed in the olfactory bulb (Fig. 2A) and the Dlv part (Fig. 2C), respectively
We established an in vivo BOLD Functional magnetic resonance imaging (fMRI) (7 Tesla) for salmon that survived for several hours under our fMRI set-up
Summary
Salmon have an amazing ability to migrate several thousands of kilometers from the ocean to their natal stream for spawning. Many researchers have supported this hypothesis [4,5,6,7], and artificial chemicals (e.g., b-phenylethyl alcohol (PEA) or morpholine) as well as natural chemicals (e.g., dissolved free amino acid (DFAA)) have been applied for imprinting and discriminating experiments using coho salmon (Oncorhynchus kisutch) [8,9,10,11] and masu salmon (O. masou) [12]. Recent studies in our laboratory have suggested that DFAA in the natal stream water is a possible odorant substance for such chemical cues in anadromous chum salmon (O. keta) [13] and lacustrine sockeye salmon (O. nerka) [14]. Most of the studies mentioned above have utilized electrophysiological recordings, which can measure the temporal dynamics of odor information processing in small regions of the central nervous system. Because of the spatial limitations of electrophysiological techniques, the mechanisms underlying information processing of the natal stream odor in the central nervous system of salmon, especially in the telencephalon, has not been completely elucidated
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