OCTOPAMINE REGULATES GLOWWORMS' GLOW

Abstract

Dotted across cave ceilings like a starlit night, Arachnocampa glowworms lure flies and other passing victims into their sticky snares with their brilliant glow. According to Lisa Rigby and David Merritt from the University of Queensland, Australia, these Antipodean glowworms produce light from a modified section of their Malpighian tubules that glows when oxygen reacts with a chemical called luciferin. The duo explains that most glowing creatures regulate the oxygen supply to the light-emitting organ to vary its intensity by releasing neurotransmitters. Rigby and Merritt also point out that anaesthetised glowworms burn brighter than the brightest glowworms in a cave, suggesting that the larvae actively switch off their glow during daylight and regulate the intensity after dark. However, little was known about the mechanism of light regulation in one of Arachnocampa's Australian cousins, Arachnocampa flava, so Rigby and Merritt decided to investigate the innervation of the A. flava light organ to find out which neurotransmitter regulates the larvae's glow (p. 3286).Collecting glowworms from the Springbrook National Park south of Brisbane, the duo analysed the structure of the light-emitting organ at the end of the Malpighian tubule and found four tubules surrounded by a reflector comprised of air-filled trachea. The light organ was also well supplied with neural processes from the local nervous system.After identifying the nerve supply to the light organ, Rigby and Merritt used an ingenious and non-invasive way of administering biogenic amine neurotransmitters to the larvae to identify the neurotransmitter that regulates their glow. They placed a vinegar fly laced with a specific dose of neurotransmitter in the snare of each individual larva and monitored the glowworm's light production after the larva consumed the snack.Having supplied larvae with a dose of dopamine, serotonin or tyramine, the team found no significant change in the intensity of the larvae's glow from the previous night. The biogenic amines were not regulating the glowworms' intensity. However, when the team provided the glowworms with octopamine, the larvae lit up, suggesting that octopamine regulates their bioluminescence to make them glow brighter.Also, when the glowworms ingested the octopamine antagonists phentolamine and mianserin, the larvae's glow became even more powerful. Pointing out that anaesthetised glowworms also glow more brightly when they can no longer suppress their glow, the duo suggests that the antagonists blocked the octopamine receptors to turn up the intensity.The team says, ‘Experimental exposure to octopamine suggests that it is involved in the regulation of repression, either by acting directly on the cells of the light organ or through regulation of oxygen access to the cells.’