Salmon are an ideal model system for examining the mechanisms of olfactory learning and memory. These fish have an acute sense of smell and imprint and home to experimentally controllable olfactory cues that are learned during a well-defined period of development called the parr-smolt transformation. The parr-smolt transformation is driven in part by distinct fluctuations in plasma hormone levels, particularly thyroid hormone. A major focus of the proposed research is to determine how thyroid hormone regulates peripheral olfactory function and neural development in salmon during this sensitive period for olfactory imprinting. Patch clamp recordings of isolated olfactory receptor neurons clearly suggest that olfactory imprinting involves a tuning of olfactory receptor neurons to specific stream odorants, and that this tuning is hormonally driven. According to this model: 1) Thyroid hormone promotes a proliferation of olfactory receptor neurons that are sensitive to a wide variety of odor molecules. 2) Active olfactory receptor cells survive, while others die. Selective survival may involve competition for synaptic targets. 3) This punctuated proliferation and selective survival of olfactory receptor cells triggers a reorganization of glomerular structures within the olfactory bulb. The proposed research aims to test this model using a multidisciplinary approach including anatomical, extracellular electrical recording and fine-scale electrical recording techniques. This project has three main objectives: 1) to establish the dynamics of olfactory receptor cell proliferation during smolting and in response to elevated plasma thyroid hormone levels, 2) to investigate what physiological changes in the sensitivity of olfactory receptor cells accompany both natural ontogeny and hormonal manipulations, and 3) to assess the structure of glomerular layer of the olfactory bulb before and after smolting and in response to thyroid hormone manipulations. Results will extend our understanding of how hormones influence neurogenesis both in the context of olfaction and in the broader context of how memories are acquired and maintained.
