Effects of light-deprivation on development of photopositive behavior in Xenopus laevis tadpoles

A second-generation device for automated training and quantitative behavior analyses of molecularly-tractable model organisms

A deep understanding of cognitive processes requires functional, quantitative analyses of the steps leading from genetics and the development of nervous system structure to behavior. Molecularly-tractable model systems such as Xenopus laevis and planaria offer an unprecedented opportunity to dissect the mechanisms determining the complex structure of the brain and CNS. A standardized platform that facilitated quantitative analysis of behavior would make a significant impact on evolutionary ethology, neuropharmacology, and cognitive science. While some animal tracking systems exist, the available systems do not allow automated training (feedback to individual subjects in real time, which is necessary for operant conditioning assays). The lack of standardization in the field, and the numerous technical challenges that face the development of a versatile system with the necessary capabilities, comprise a significant barrier keeping molecular developmental biology labs from integrating behavior analysis endpoints into their pharmacological and genetic perturbations. Here we report the development of a second-generation system that is a highly flexible, powerful machine vision and environmental control platform. In order to enable multidisciplinary studies aimed at understanding the roles of genes in brain function and behavior, and aid other laboratories that do not have the facilities to undergo complex engineering development, we describe the device and the problems that it overcomes. We also present sample data using frog tadpoles and flatworms to illustrate its use. Having solved significant engineering challenges in its construction, the resulting design is a relatively inexpensive instrument of wide relevance for several fields, and will accelerate interdisciplinary discovery in pharmacology, neurobiology, regenerative medicine, and cognitive science.

Visual avoidance in Xenopus tadpoles is correlated with the maturation of visual responses in the optic tectum

The optic tectum is central for transforming incoming visual input into orienting behavior. Yet it is not well understood how this behavior is organized early in development and how it relates to the response properties of the developing visual system. We designed a novel behavioral assay to study the development of visually guided behavior in Xenopus laevis tadpoles. We found that, during early development, visual avoidance-an innate, tectally mediated behavior-is tuned to a specific stimulus size and is sensitive to changes in contrast. Using in vivo recordings we found that developmental changes in the spatial tuning of visual avoidance are mirrored by changes in tectal receptive field sharpness and the temporal properties of subthreshold visual responses, whereas contrast sensitivity is affected by the gain of the visual response. We also show that long- and short-term perturbations of visual response properties predictably alter behavioral output. We conclude that our assay for visual avoidance is a useful functional measure of the developmental state of the tectal circuitry. We use this assay to show that the developing visual system is tuned to facilitate behavioral output and that the system can be modulated by neural activity, allowing it to adapt to environmental changes it encounters during development.

Ontogeny of circadian and light regulation of melatonin release in Xenopus laevis embryos

The retinal photoreceptors of Xenopus laevis contain a circadian clock that controls the synthesis and release of melatonin, resulting in high levels during the night and low levels during the day. Light is also an important regulator of melatonin synthesis and acts directly to acutely suppress melatonin synthesis during the day and indirectly to entrain the circadian clock. We examined the development of circadian and light regulation of melatonin release in Xenopus retinas and pineal glands. Pineal glands are capable of making measurable melatonin in culture soon after they evaginate from the diencephalon at stage 26. In cyclic light, the melatonin rhythms are robust, with higher overall levels and greater amplitudes than in constant darkness. However, the rhythm of melatonin release damps strongly and quickly toward baseline in constant darkness. Similar results are observed in older (stage 47) embryos, indicating that cyclic light has a positive effect on melatonin synthesis in this tissue. Optic vesicles dissected at stage 26 do not release melatonin in culture until the second or third day. It is weakly rhythmic in cyclic light, but in constant dark it is released at constitutively high levels throughout the day. By stage 41, the eyes release melatonin rhythmically in both cyclic light and constant darkness with similar amplitude. Our results show that Xenopus embryos develop a functional, photoresponsive circadian clock in the eye within the first few days of life and that rhythmic melatonin release from the pineal gland at comparable stages is highly dependent on a light-dark cycle.