Gene expression of neuronal soluble N-ethylmaleimide-sensitive factor attachment protein receptor complex components in the olfactory organ and brain during seaward and homeward migration by pink salmon (Oncorhynchus gorbuscha)

Messenger RNA transcription levels of neuronal soluble N-ethylmaleimide-sensitive factor attachment protein receptor complex components in the olfactory nerve system of the anadromous Pacific salmon, masu salmon Oncorhynchus masou

Anadromous Pacific salmon (genus Oncorhynchus) are known for homing behavior to their natal rivers based on olfactory imprinted memories during seaward migration. The SNARE complex is a regulator of vesicle exocytosis from the presynaptic membrane. Our previous study suggested that its component genes (Snap25, Stx1, and Vamp2) are more highly expressed in the olfactory nervous system (ONS) during the migration stages associated with olfactory imprinting in the evolutionary species of Pacific salmon, such as chum (O. keta) and pink (O. gorbuscha) salmon. Masu salmon (O. masou) has a significantly different life history from these species, living longer in rivers and being a more primitive Pacific salmon species. In this study, the transcription of snare mRNAs in the ONS was analyzed using mainly male wild masu salmon. Five cDNAs encoding masu salmon SNAREs, which are well conserved among vertebrates, were isolated and sequenced. Each snare mRNA was highly expressed in age 1+ (yearling) parr prior to smoltification, particularly in the olfactory bulb. Their transcription status was significantly different from that of chum and pink salmon, which showed high expression in earlier under-yearling juveniles. The present results and our previous studies indicate that snare mRNAs are highly transcripted until the seaward migration, reflecting neural development and neuroplasticity of the ONS for olfactory imprinting.

Ontogenetic shifts in olfactory rosette morphology of the sockeye salmon, Oncorhynchus nerka

Sockeye salmon, Oncorhynchus nerka, are anadromous, semelparous fish that breed in freshwater-typically in streams, and juveniles in most populations feed in lakes for 1 or 2 years, then migrate to sea to feed for 2 or 3 additional years, before returning to their natal sites to spawn and die. This species undergoes important changes in behavior, habitat, and morphology through these multiple life history stages. However, the sensory systems that mediate these migratory patterns are not fully understood, and few studies have explored changes in sensory function and specialization throughout ontogeny. This study investigates changes in the olfactory rosette of sockeye salmon across four different life stages (fry, parr, smolt, and adult). Development of the olfactory rosette was assessed by comparing total rosette size (RS), lamellae number, and lamellae complexity from scanning electron microscopy images across life stages, as a proxy for olfactory capacity. Olfactory RS increased linearly with lamellae number and body size (p < .001). The complexity of the rosette, including the distribution of sensory and nonsensory epithelia and the appearance of secondary lamellar folding, varied between fry and adult life stages. These differences in epithelial structure may indicate variation in odor-processing capacity between juveniles imprinting on their natal stream and adults using those odor memories in the final stages of homing to natal breeding sites. These findings improve our understanding of the development of the olfactory system throughout life in this species, highlighting that ontogenetic shifts in behavior and habitat may coincide with shifts in nervous system development.

Olfactory behavioural and neural responses of planktivorous lacustrine sockeye salmon (Oncorhynchus nerka) to prey odours

Fish use a variety of sensory systems when foraging. Salmonids are generally considered visual feeders. However, some species feed on zooplanktons under dark conditions, suggesting they also detect prey using nonvisual cues. Under experimental conditions, hatchery-reared rainbow trout (Oncorhynchus mykiss) have been shown to use olfaction when searching for food pellets, but olfactory foraging has not been documented in wild salmonids. In the present study, to examine their behavioural response and neural activity in the olfactory nervous system using c-fos expression as a neural molecular marker, immature wild-caught lacustrine sockeye salmon (Oncorhynchus nerka) in a flow-through aquarium were exposed to zooplanktons (Daphnia spp.) extract including zooplanktons odorant and to dimethyl sulfide. The salmon exposed to zooplanktons odour increased their total swimming distance and time, numbers of turns and ascents, and c-fos expression in the olfactory bulb, suggesting that they can detect zooplanktons extract to locate prey in the laboratory experiments. However, no response was seen in those exposed to dimethyl sulfide. The results of this study suggest that prey odour may serve as a chemosensory cue for wild immature salmonids.

Gene expression levels of synaptic exocytosis regulator synaptophysin in the brain and the olfactory organ of anadromous salmon

Anadromous Pacific salmon (genus Oncorhynchus) are known for their homing behavior based on olfactory imprinting, which is formed during their seaward migration. Soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE/Snare) complex is a minimum unit of vesicle exocytosis from the pre-synaptic membrane. Its component genes (synaptosome-associated protein 25, syntaxin 1, and vesicle-associated membrane protein 2) are more strongly expressed in the olfactory nervous system (olfactory epithelium, olfactory bulb, and telencephalon) at the migration stages related to olfactory imprinting and/or retrieval in salmon. This study focused on the mRNA synthesis of synaptophysin (Syp), one of the Snare regulatory factors. syp is strongly expressed in chum salmon (Oncorhynchus keta) olfactory nervous system during the seaward migration and temporarily increased during the homeward migration. In reference to our previous studies, these expression changes were similar to the snare genes in the chum salmon. Therefore, syp and Snare component genes were synchronously expressed reflecting the development and short-term plasticity of the olfactory nervous system that is essential for olfactory imprinting.