High-throughput automated detection and analyses of locomotor and hunting sequences in larval zebrafish unveil the role of a conserved dopaminergic diencephalospinal tract in locomotor development and goal-directed behavior

Abstract

The dopaminergic diencephalospinal tract (DDT), and its source orthopedia- specified dopaminergic (DAergic) population, is the most conserved part of the vertebrate DAergic system. The source somata of the DDT have widespread ascending and descending projections that span and have potential to integrate the entire rostro-caudal axis of the central nervous system, from telencephalon to spinal cord. Mammalian studies confirm that the extensive DDT network is multifunctional, even via its direct influence within the spinal cord. While specific mechanosensory and nociceptive functions of the DDT acting in the spinal cord in vivo have been elucidated in adult mammals, whether or not the DDT also exerts locomotor influences in the spinal cord in vivo, as well as whether the DDT plays an early role during development, has remained unknown despite suggestive in vitro studies. My thesis explored the role of the vertebrate DDT in locomotor development and goal-directed behavior in zebrafish larvae, a premiere model to elucidate the neural bases of such behaviors at organismal, systems, circuit, cellular, and subcellular levels in vivo. To this aim, I developed new methodologies for high-throughput, unbiased, automated detection and analyses of locomotion and goal-directed hunting. This approach was combined with peripheral nerve recordings of the neural locomotor output of the spinal cord and employed during: DAergic pharmacological perturbations, demarcated transections at varying locations of the nervous system, selective chemogenetic ablation of orthopedia neurons, and laser ablations of the DDT. Collectively, this thesis reveals that the DDT acts specifically through endogenous dopamine receptor 4 (D4R) signaling to mediate locomotor development and provide a multifunctional modulation of multiple locomotor parameters in a separable manner, by putatively influencing disparate neuronal targets concurrently. Moreover, this thesis elucidates that endogenous D4R signaling is crucial to goal-directed prey capture via a specific motor-centric role in shifting prey- directed motor strategies and providing precision of speed control during execution of advancing hunting maneuvers. From the collective elucidations of my thesis, I posit the existence of a modular organization subserving a versatile locomotor network, wherein separate neural modules are recruited spontaneously and during hunting and that the hunting module is further subdivided into separable orienting and advancing regimes. Integrated into this granular network, endogenous D4R signaling, perhaps through a widespread integrative impetus at disparate regions via the extensive DDT network, differentially influences multiple modules concurrently. These findings, integrated with the mammalian literature, suggest that the conserved vertebrate DDT is crucial for locomotor development, as well as motor planning and execution of goal-directed behavior.