Rapid lesioning of large numbers of identified vertebrate neurons: applications in zebrafish

Establishing a causal role between the activity of specific individual nerv e cells and the behaviors they produce (or the neural computations they exe cute) is made difficult in vertebrate animals because of the large numbers of neurons involved. Traditional techniques for establishing causal roles, such as tract cutting and electrolytic lesions, are limited because they pr oduce damage that affects a variety of different cell types, invariably int ermingled, and often of uncertain identity. We propose here an alternative lesioning technique in which large numbers of neurons are lesioned, but the lesioned neurons are specifically identified by fluorescent labeling. We u se the locomotor control system of the larval zebrafish to illustrate this approach. In this example, the technique involves injection of fluorescent dextrans into far-rostral spinal cord to label descending nerve fibers. Suc h injections appear to interrupt the descending nerve fibers, and therefore their accompanying locomotor control signals. This protocol is shown to pr oduce significant behavioral deficits. Because the CNS of the larval zebraf ish is transparent, the entire population of lesioned cells can be imaged i n vivo and reconstructed using confocal microscopy. This large-scale lesion ing technique is important, even in this relatively 'simple' vertebrate ani mal, because the ablation of smaller numbers of neurons, using more precise laser-ablation techniques, often fails to produce observable behavioral de ficits. While this technique is most readily applied in simpler and transpa rent vertebrate animals, the approach is general in nature and might, in pr inciple, be applied to any vertebrate nerve tract. (C) 2001 Elsevier Scienc e B.V. All rights reserved.