Connectivity, plasticity, and function of neuronal circuits in the zebrafish olfactory forebrain

For most living animals such as worms, insects, fishes, rodents and humans, chemical cues from the environment (odorants) play critical roles in guiding behaviors important for survival, including preying, mating, breeding, and escaping. How those odorants are detected, identified, learned, remembered, and used by the nervous system is a longstanding interest for neuroscientists. An animal that is well-suited to study the processing of odor information at the level of neuronal circuits is the zebrafish (Danio rerio) because its small brain size allows for exhaustive quantitative measurements of neuronal activity patterns.
In vertebrates, odorants are detected by olfactory sensory neurons in the nose and transmitted to the first olfactory processing center in the brain, the olfactory bulb
(OB), as patterns of neuronal activities. In the OB, neuronal activity patterns from the nose are transformed into odor-specific spatiotemporal activity patterns across second order neurons, the mitral cells. These discrete neuronal activity patterns are broadcast to various target areas. The largest of these higher brain areas is piriform cortex or its teleost homolog, the posterior zone of dorsal telencephalon (Dp). In this higher brain region, an odor-encoding neuronal activity pattern from the OB is thought to be encoded as a "gestalt", or "odor object", and possibly stored in memory by specific modifications of functional connections between distributed neuronal ensembles. Such neuronal ensembles are also thought to be connected with other brain regions that involved in the control of different behaviors. Therefore, by inducing a specific activity pattern in the OB, which then retrieves related neuronal ensemble activities in a higher brain region, an odor cue (or even partial cue) recalls an odor object memory that may further trigger a specific set of behavioral responses in the animal.
The mechanisms by which odor object memory is synthesized, stored, and recalled is of major interest in neuroscience because it may provide fundamental insights into associative memory functions. However, dissecting higher brain functions such as associative memory will first require basic understanding of connectivity, plasticity, and related modulating factors for the underlying neuronal circuits. In this inaugural dissertation, I present an approach to study the connectivity, plasticity, and cholinergic modulation of the neural circuits in Dp and present new insights into the synaptic organizations of this neuronal network.
In results part one, I show that transgenes can be introduced directly into the adult zebrafish brain by herpes simplex type I viruses (HSV-1) or electroporation. I developed a new procedure to target electroporation to defined brain areas, e.g. Dp, and identified promoters that produced strong long-term expression. These new methods fill an important gap in the spectrum of molecular tools for zebrafish and are likely to have a wide range of applications. In results part two, I used a combination of electroporation, optogenetics, electrophysiology, and pharmacology to study the intrinsic connectivity and plasticity in neural circuits of Dp. I found that connectivity between any pair of excitatory neurons in Dp is extremely sparse (connection probability < 1.5 %). The connection probability of inhibitory synapses is also sparse but slightly higher (< 2.5 %). Furthermore, I found that connectivity can be functionally modified by activity-dependent synaptic plasticity including spike timing-dependent long-term potentiation. Moreover, I show that cholinergic agonists differentially modulate excitatory and inhibitory synaptic transmissions in Dp, consistent with the notion that cholinergic neuromodulation controls experience-dependent changes in functional connectivity. These findings show that the synaptic organization of Dp is similar to mammalian piriform cortex and provide quantitative insights into the functional organization of a brain area that is likely to be involved in associative memory.