The vertebrate olfactory system utilizes odorant receptors to receive and discriminate thousands of different chemical stimuli. An understanding of how these receptors encode information about an odorant's molecular structure requires a characterization of their ligand specificities. We employed an expression cloning strategy to identify a goldfish odorant receptor that is activated by amino acids-potent odorants for fish. Structure-activity analysis indicates that the receptor is preferentially tuned to recognize basic amino acids. The receptor is a member of a multigene family of G protein-coupled receptors, sharing sequence similarities with the calcium sensing, metabotropic glutamate, and V2R class of vomeronasal receptors. The ligand tuning properties of the goldfish amino acid odorant receptor provide information for unraveling the molecular mechanisms underlying olfactory coding.

It is well known that salmon home to their natal rivers for spawning, but the spatial scale of homing within a river basin is poorly understood and the interaction between natal site fidelity and habitatbased spawning site selection has not been elucidated. Understanding the complex trade-offs among homing to the natal site, spawning site selection, competition for sites, and mate choice is especially important in the context of hatchery supplementation efforts to reestablish self-sustaining natural spawning populations. To address these questions, we examined the homing patterns of supplemented Yakima River spring Chinook salmon Oncorhynchus tshawytscha released from satellite acclimation facilities after common initial rearing at a central facility. Final spawning location depended strongly on where fish were released as smolts within the upper Yakima River basin, but many fish also spawned in the vicinity of the central rearing hatchery, suggesting that some fish imprinted to this site. While homing was clearly evident, the majority (55.1%) of the hatchery fish were recovered more than 25 km from their release sites, often in spawning areas used by wild conspecifics. Hatchery and wild fish displayed remarkably similar spawning distributions despite very different imprinting histories, and the highest spawning densities of both hatchery and wild fish occurred in the same river sections. These results suggest that genetics, environmental and social factors, or requirements for specific spawning habitat may ultimately override the instinct to home to the site of rearing or release.

The remarkable homing ability of salmon relies on olfactory cues, but its cellular basis is unknown. To test the role of peripheral olfactory receptors in odorant memory retention, we imprinted coho salmon (Oncorhynchus kisutch) to micromolar concentrations of phenyl ethyl alcohol during parr-smolt transformation. The following year, we measured phenyl ethyl alcohol responses in the peripheral receptor cells using patch clamp. Cells from imprinted fish showed increased sensitivity to phenyl ethyl alcohol compared either to cells from naive fish or to sensitivity to another behaviorally important odorant (L-serine). Field experiments verified an increased behavioral preference for phenyl ethyl alcohol by imprinted salmon as adults. Thus, some component of the imprinted olfactory homestream memory appears to be retained peripherally. Despite the extensive behavioral evidence linking olfactory imprinting to salmon homing, attempts to trace the neurobiological basis of homestream memory and recognition have proven inconclusive. Studies using electroolfactogram recordings have reported that imprinted fish show significantly greater responses to specific imprinted odorants than do nonimprinted fish (6, 7), but other researchers have claimed that these "imprinted" odorants consistently fail to elicit increased electrical activity at the level of the olfactory bulb in imprinted as compared to nonimprinted fish (8-10).In this study, we have combined patch clamp recording and behavioral imprinting assays to test directly the hypothesis that a change in the sensitivity of peripheral olfactory receptor cells contributes to establishing an olfactory memory in salmon. Our results demonstrate that plasticity in the peripheral olfactory system at the receptor cell level is linked to olfactory imprinting (11,12). METHODSAll investigations were performed on coho salmon spawned from the University of Washington's School of Fisheries stock. Details of imprinting paradigms, electrophysiological techniques, and behavioral methods are given below.Imprinting Procedures. Determination ofparr-smolt transformation. To achieve parr-smolt transformation by the first spring (0-age), fish were maintained at slightly elevated water temperatures (14-160C) and fed ad libitum. Fish were handculled to avoid exposing them to harsh drugs or other chemicals commonly used as disease preventatives in salmon culture. Parr-smolt transformation was determined by coloration changes, a behavioral tendency to orient downstream, and the ability of fish to maintain blood plasma Na+ levels to within 5% during a 48-hr exposure to salt water (freshwater exposure: 152.9 ± 1.2 meq of Na+ per liter of blood plasma, n = 10; saltwater exposure: 159.0 ± 1.7 meq ofNa+ per liter ofblood plasma, n = 8) (13-15). To determine plasma Na+ levels, salmon were transferred from their rearing hatchery (130C) to holding tanks containing either aerated fresh filtered hatchery or sea water (27 ppt; 120C). After 48 hr, blood was collected using heparinized capillary tubes and...

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