Red LED light treatment promotes cognitive learning through up-regulation of trpm4 in zebrafish

Highlights

• The monochromatic LED light treatment changes zebrafish learning behaviour.

• The LED light treatment has a long-termed effects on zebrafish cognitive learning.

• The red light (630 nm) treatment promotes cognitive learning and memory recall.

• The up regulation of trpm4 may play an important role for maintaining learning and memory in zebrafish.

Abstract

Although light emitting diodes (LEDs) are widely used in our daily lives, there is little research regarding LED light’s possible effects on biological functions. We used a zebrafish animal model to investigate the long-term effects of white, blue and red LED lights on cognitive learning and memory recall. Our data suggest that these treatments had not only an impact on learning but also surprisingly long-lasting effects, particularly with regard to individuals treated with red light. The qPCR results revealed that the expression levels of trpm4, trpa1b, grin2aa and dlg4 in the skin were increased after monochromatic light treatment. Furthermore, the up-regulation of trpm4 in the brain may correlate to enhanced learning and memory following red-light treatment. Our results identify a light-based stimulation system for enhancing zebrafish learning, which has the potential to provide important insights into the relationship between LED lighting and animal behaviour.

Introduction

Although light from light emitting diodes (LEDs) has become a very common and important environmental factor in our lives, its effects have not been widely studied. LED technology has revolutionized our lives through lowering power usage and electricity costs, and extending the useful life of light bulbs. Furthermore, LEDs can be manufactured to output specific, narrow-band wavelengths, which can be used to test physiological reactions to different wavelengths through light treatments. Studies that have tried to understand the effects of LED light on aquatic animals have suggested both positive and negative outcomes, for example green (530 nm) light regulates the hypothalamic–pituitary–gonadal axis, enhancing sexual maturation in goldfish [1]; infrared light (840 nm) does not influence the circadian rhythm of zebrafish [2]; red light (630 nm) induces oxidative stress of yellow clownfish [3]; blue light (455 nm) significantly decreases ocular melatonin within 30 min of exposure in tropical damselfish [4]; and blue light reduces stress in Nile tilapia [5]. These studies focused on the short-term effects of LED treatment and affirm the importance of environmental lighting on fish physiology and behaviour. However, the long-term effects have not been closely examined.

The zebrafish provides an ideal animal model for this because it is small in size, has a simple husbandry requirement, produces a large number of transparent eggs [6], and has its whole genome sequenced. It has been widely used in various biomedical research fields, including developmental biology, toxicology, and drug development/screening, as well as in genetic and behavioural studies. In recent years, the zebrafish has become the third most genetically well characterized animal species, after the mouse and the fruit fly [7,8]. Assessment of zebrafish (larvae and adult fish) learning behaviour is also becoming popular due to the availability of genetic techniques and commercially available behavioural-assessment systems, including video-tracking and behavioural-analysis programs. Some models have been developed to measure the capacity of adaptive learning in zebrafish through contraptions such as T-mazes [9], Plus mazes [8], Y-Mazes [10] and conditioned place preference setups [9]. Although models are tailored to the needs of the study, a common positive conclusion has been made: the zebrafish provides a good model for analysing complex behavioural phenomena, namely learning and memory [8,[10], [11], [12], [13]], visual discrimination [14,15] and colour preference [[16], [17], [18], [19]]. Using either reward or aversive-based training, zebrafish are able to recognize conditioned situations, performing associative learning and memory recording. We examined some genes that may respond to light treatment, such as the transient receptor potential (TRP) family and genes related to circadian rhythm, pigmentation, mitochondrial biogenesis, behaviour, synaptic plasticity, growth and stress factors. Through analysing the gene-expression patterns of these chosen genes, we expect to gather a preliminary idea of how LED-light treatment influences zebrafish behaviour and what kinds of genes could be affected after treatment. In this study we used the zebrafish as an animal model to research the long-term effects of different LED lights on fish behaviour at different time courses. We aim to better understand the influence of light wavelengths emitted by LED lights, in particular, the commonly seen white, blue and red lights on zebrafish behaviour as well as the gene expression that may be affected.