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Organization, plasticity and perceptual functions of neuronal circuits in a higher olfactory forebrain area

Final Report Summary - ZEBRAFISHFOREBRAIN (Organization, plasticity and perceptual functions of neuronal circuits in a higher olfactory forebrain area)

How are associations formed between the diverse sensory stimuli that an individual encounters, and their significance? While evolution can allow organisms to innately prefer and avoid certain stimuli, neuronal plasticity is required to refine and complement these preferences through life experience. While major advances have been made in understanding learning-related processes at the molecular and cellular level, we still lack a comprehensive understanding of learning at the circuit level. How do groups of neurons within specific circuits change their activity as a result of learning? The objective of this project is to elucidate circuit mechanisms of learning and plasticity in the olfactory forebrain of the zebrafish. The zebrafish brain provides unique opportunities for addressing this question, because it is on the one hand a vertebrate brain, with well-established homologies to the mammalian (and hence human) brain, while on the other hand being small, allowing modern imaging techniques to monitor the activity of a large fraction of the neurons in the relevant circuits.

The specific goals of this project have been:
1. To develop a behavioural paradigm that would allow zebrafish to associate neutral odours with both positive (food) and negative (mild shock) outcomes. Developing such a paradigm is crucial for studying olfactory learning-induced changes in neuronal circuits.
2. To monitor the activity of neuronal circuits in the olfactory forebrain in response to an array of odours, some of which have previously been associated with food reward or electric shock, and some which were presented without any associated outcome (control odours).
3. To relate the circuit activity in the olfactory forebrain to its connectivity with the brain area from which it receives input (the olfactory bulb). This part of the project, which is only partially related to the previous goals, required the establishment of a viral technique (trans-synaptic labelling) which marks connected neurons within circuits.

Behavioural training of adult zebrafish: A custom behavioural arena was developed which allows zebrafish to swim freely in a two-compartment chamber, in which each compartment is perfused independently with a different odour. During odour presentation, both electric shock and food pellets can be delivered to the individual compartments, allowing the fish to form specific odour-outcome associations. The strength of the associations can be assessed by monitoring the compartment which the fish inhabits during the presentation of specific odours. During the period of the research, the behavioural setup and paradigm continuously evolved. The two major changes over time have been an improvement of the chamber design, which turned out to be crucial for lowering fish stress levels; and a recent major hardware-software development by the researcher. The custom-built software for controlling all aspects of the experiment and data acquisition is important for high-throughput training of fish. This turned out to be crucial, as there is large across-fish variability in training results.

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