Acute nicotine exposure and modulation of a spinal motor circuit in embryonic zebrafish.

The zebrafish model system is ideal for studying nervous system development. Ultimately, one would like to link the developmental biology to various aspects of behavior. We are studying the consequences of nicotine exposure on nervous system development in zebrafish and have previously shown that chronic nicotine exposure produces paralysis. We also have made observations that the embryos moved in the initial minutes of the exposure as the bend rates of the musculature increased. This nicotine induced behavior manifests as an increase in the rate of spinal musculature bends, which spontaneously begin at approximately 17 h post fertilization. The behavioral observations prompted the systematic characterization of nicotine-induced modulation of zebrafish embryonic motor output; bends of the trunk musculature. We first characterized embryonic motor output in zebrafish embryos with and without their chorions. We then characterized the motor output in embryos raised at 28 degrees C and 25 degrees C. The act of dechorionation along with temperature influenced the embryonic bend rate. We show that nicotine exposure increases embryonic motor output. Nicotine exposure caused the musculature bends to alternate in a left-right-left fashion. Nicotine was able to produce this phenotype in embryos lacking supraspinal input. We then characterize the kinetics of nicotine influx and efflux and demonstrate that nicotine as low as 1 microM can disrupt embryonic physiology. Taken together, these results indicate the presence of nicotinic acetylcholine receptors (nAChRs) associated with embryonic spinal motor circuits early in embryogenesis.

Uncoupling nicotine mediated motoneuron axonal pathfinding errors and muscle degeneration in zebrafish.

Zebrafish embryos offer a unique opportunity to investigate the mechanisms by which nicotine exposure impacts early vertebrate development. Embryos exposed to nicotine become functionally paralyzed by 42 hpf suggesting that the neuromuscular system is compromised in exposed embryos. We previously demonstrated that secondary spinal motoneurons in nicotine-exposed embryos were delayed in development and that their axons made pathfinding errors (Svoboda, K.R., Vijayaraghaven, S., Tanguay, R.L., 2002. Nicotinic receptors mediate changes in spinal motoneuron development and axonal pathfinding in embryonic zebrafish exposed to nicotine. J. Neurosci. 22, 10731-10741). In that study, we did not consider the potential role that altered skeletal muscle development caused by nicotine exposure could play in contributing to the errors in spinal motoneuron axon pathfinding. In this study, we show that an alteration in skeletal muscle development occurs in tandem with alterations in spinal motoneuron development upon exposure to nicotine. The alteration in the muscle involves the binding of nicotine to the muscle-specific AChRs. The nicotine-induced alteration in muscle development does not occur in the zebrafish mutant (sofa potato, [sop]), which lacks muscle-specific AChRs. Even though muscle development is unaffected by nicotine exposure in sop mutants, motoneuron axonal pathfinding errors still occur in these mutants, indicating a direct effect of nicotine exposure on nervous system development.