Mechanisms of operant conditioning in the zebrafish larval whole brain: cells, circuits, and behavior
The ability to shape behavior based on the consequences of actions is fundamental for the survival of animals in complex environments. The neural mechanisms underlying this type of operant learning have been studied intensely in mammals, and are thought to be dysfunctional in a number of neurological and neuropsychiatric disorders in humans including addiction, Parkinson’s disease, and autism. Prior studies have identified cortico-basal ganglia circuits as an important locus of operant learning function in the brain. The striatum, the major input structure of the basal ganglia (BG), receives inputs from a broad set of regions, including, in mammals, most of the cortex and thalamus, as well as subcortical areas, and a prominent feature of BG circuits is the existence of parallel loops in which the outputs are connected back to the areas from which their inputs originated. A comprehensive understanding of BG contributions to operant learning would benefit greatly, therefore, from the ability to observe simultaneously activity across a diverse set of brain areas during behavior. Additionally, knowledge of the identity of neural circuit elements beyond what is reflected in their anatomical location is essential for building an accurate circuit-level understanding of learning.
Zebrafish, a model organism with a substantial toolbox of genetic methods, is very well suited to such integrative approaches. At early life stages, they show a variety of robust innate and learned behaviors, while their brain, which has one million times fewer neurons than a human?s, and is less than a billionth of the size, already follows the basic vertebrate blueprint. Recent advances in optical and genetic technologies have made it possible to image activity, in real-time and with cellular resolution, non-invasively from throughout the entire brain. Since zebrafish are capable of operant learning, they can provide a powerful system to investigate its underlying circuit mechanisms.
In mammals, striatal projection neurons fall into two main types based on immunohistochemistry and anatomy. Direct pathway neurons express D1 dopamine (DA) receptors and substance P and project directly to BG output nuclei. Indirect pathway neurons express D2 DA receptors, enkephalin, and largely send information to BG output nuclei indirectly, with a sign inversion. It was recently shown in mice that activation of direct and indirect pathway neurons produces opposite effects on reinforcement. Stimulation of direct/indirect pathway neurons positively/aversively reinforced actions respectively. Teleost fish possess a similar divergent circuit architecture in the homologous structure to the mammalian striatum. We aim to interrogate these circuits using a combination of genetic methods, whole-brain calcium imaging and optogenetics to better understand their functional organization at a cellular level and to elucidate basic mechanisms of operant learning.
