Mechanisms of olfactory imprinting and memory retrieval in Pacific salmon
Topic outline
Pacific salmon life history
It is generally accepted that information about some of the odorants in the natal streams of anadromous Pacific salmon is imprinted during seaward migration, and that Pacific salmon use olfaction to identify their natal streams during spawning migration. However, little is known about the functional molecules that are important for olfactory imprinting and memory retrieval in the salmon brain.
Salmon olfactory nerve system and brain
In salmon, the peripheral olfactory organ, known as the olfactory rosette, is composed of olfactory lamellae covered with olfactory epithelia. As in other vertebrates, salmon olfactory receptor neurons (ORNs) relay olfactory information to the brain. In teleosts, three morphological distinct ORN types, ciliated and microvillous ORNs, and crypt neurons, exist according to the ultrastructure of the olfactory knob in the dendrite of the ORNs. Olfactory bulb is the first relay brain area and receives the primary input from olfactory nerve which is the axons from each ORNs project to each restricted glomerulus in the glomerular layer (GL) of olfactory bulb. Terminals of the olfactory nerves organize the synaptic contacts with the dendrite of the mitral cell (M) which in turn sends processed output signals to several target areas in higher central nervous system, i.e., telencephalon, by the axons from the mitral cells. Olfactory center consists of the olfactory bulb and the telencephalon in salmon. Telencephalon in teleost is thought to be homologous to the pallium in mammals and dorsal lateral region of the telencephalon includes structures are possibly analogous to the mammalian hippocampus. It is believed that the hippocampus plays an important role in the formation of learning and memory in mammals.
Synaptic exocytosis-related molecules (SNARE complex)
Generally, memory formation consists of synaptogenesis and a continuous increase in synaptic transmission in the neuronal network. However, little is known about the molecular mechanisms of synaptic transmission associated with olfactory imprinting and memory retrieval in the salmon brain. Exocytosis of pre-synaptic neurotransmitter vesicles is regulated by the soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) complex. Synaptosome-associated protein-25 (SNAP-25), syntaxin-1 (STX-1), and vesicle associated membrane protein 2 (VAMP2) constitute a SNARE complex, which regulates synaptic transmission and neuronal plasticity, and is associated with learning or memorization in mammals. Our present objective was to reveal the relationship between the expression of SNARE molecules and olfactory imprinting/memory retrieval in anadromous Pacific salmon. In this study, we analyzed expression changes during imprinting and/or memory retrieval periods for the natal stream using molecular biological techniques (quantitative PCR).