Development of methods for in vivo studies of network adaptation and synaptic plasticity upon repetitive visual sensory conditioning in the zebrafish larva

The brain is the most complex organ of animals and serves them as to aid and guide rapid interactions with the environment in an adaptive manner. One of the most intricate feats of the brain is memory formation, storage and recall. Non-invasive imaging and electrophysiological methods in humans have helped deepen our understanding of brain function and the areas involved in memorization but lack the far more detailed resolution of invasive methods thus making imperative the use of animal models.
A variety of animal models have provided important insights into memory processes on multiple levels from the molecular to behavioral systems and the most compelling general theory we have to date is the synaptic plasticity and memory hypothesis. Synapses, the contacting points between neurons, far exceed the already astronomical number of neurons in even the “simplest” model organisms and they are dynamically changing their properties as a result of experience. Detailed unified theories of memorization are lacking due to this tremendous complexity. There is thus a need to develop in vivo preparations where theories about memorization can be tested and combined with vast physiological measurements on multiple levels along with monitoring of behavioral readout in order to uncover the relation between synaptic plasticity and memory.
The Negative Image Model is one of the most complete synaptic network models having strong explanatory power about one of the simplest forms of memory, habituation, along with other more complex types of memory. It describes synaptic network alterations as a result of experience with an emphasis on inhibitory neuron function and plasticity and is supported by scientific evidence gathered from a wide spectrum of animal models.
Here, I use the larval zebrafish which concentrates many advantages that would make it a favored model organism for studying memory formation. Its transparency allows monitoring the activity of hundreds to thousands of neurons simultaneously with modern imaging methods while its behavior is composed by well studied sets of movements that can be tracked in detail. Additionally, decades work on genome sequence and developmental studies have made possible genetic engineering methods to create mutant or transgenic lines in a straight forward manner. I characterize a novel transgenic line expressing a neural activity sensor for an extended period in the fish’s lifespan and use it to test predictions about alterations in local network activity as a result of patterned sensory experience. I find that the neuropil of the Optic Tectum (OT), the most complex visual center in teleost fish, exhibits network adaptation motifs to repetitive patterned visual stimulation which are in can be interpreted with the Negative Image Model of habituation on 2 different sensory stimulation contexts.
In parallel, I attempted to develop transgenesis tools in order to tag synapse strength modifications in living zebrafish. Such tools should serve as to monitor in vivo synaptic strength alterations during and after memorization. Although this effort did not produce clear results, it outlines a strategy which could be used to convert the larval zebrafish from an excellent vertebrate model to study sensory-motor transformations, to an excellent vertebrate model of simple memory formation, retention and expression.