Worm Breeder's Gazette 15(2): 24 (February 1, 1998)
These abstracts should not be cited in bibliographies. Material contained herein should be treated as personal communication and should be cited as such only with the consent of the author.
Genzentrum der Ludwig-Maximilians-Universitaet, Feodor-Lynen-Str. 25, D-81377 Muenchen, Germany
Animals can respond to temperature in two different ways. The perception of thermal stimuli within the physiological temperature range of an organism results in regulatory reactions, like sweating or the active search for more comfortable temperatures (thermotaxis). The perception of noxious thermal stimuli, that is a temperature that might result in cell damage upon longer exposure, represents a fundamentally different quality. Perception of painful thermal stimuli triggers reflexive responses in an organism, like, for example, a withdrawal reaction. The molecular mechanism of this response is not understood very well. Extensive pharmacological studies have been performed in vertebrates to chemically manipulate nociceptive heat response, but no genetic model system is established to analyse nociception at the molecular level. We want to use C. elegans as a model system to identify the molecules and circuits involved in this behaviour.
Thermotaxis response in C.elegans has been described at the genetic and molecular level. But in addition to the movements on a temperature gradient, the worms are also responding to exposure upon heat stimuli, e.g. applied by a hot scalpel blade, with a very stereotypic backward movement. We have been analysing this behaviour which we refer to as thermal avoidance (TAV). For this purpose, we developed a diode laser system (special thanks to the Department of Electronic Engineering at the Genzentrum) coupled to a dissecting scope which allows us to selectively heat small areas (30 um diameter) on the worm plates to about 35 C. The worms strongly respond to the laser beam hitting the head of the animal, whereas no response was seen after targeting the other body parts. This suggests that the receptive neurons for this behaviour are located only in the head. This is further supported by the lack of response of worms with mutations in vab-3, which ! affect mostly head neurons. Preliminary results indicate that the worms do not adapt to nociceptive heat, but are rather sensitised by repeated innervations. We are currently using a video documentation system to analyse the response in detail.
To ensure that what we are looking at is not mediated by the same neural circuit Ikue Mori and colleagues have already described we tested mutants which should eliminate the signalling through AIY and AIZ interneurons. unc-86 mutants do not make the AIZ, and ttx-3 mutants eliminate AIY function in response to temperature (Oliver Hobert et al., 1997). A unc-86(n846)III; ttx-3(ky55)X double mutant does not show thermotaxis, but still responds in our TAV assay like wild type, as do the other tax and ttx mutants we tested so far. Therefore, we believe that noxious temperature signals are not transmitted through the same neural circuit.
The analysis of more than 150 mutant strains with various defects in the nervous system further support this notion. The results indicate that the neurotransmitters dopamine, serotonin, acetylcholine, and GABA probably do not play a substantial role in mediating TAV response, because mutants affecting the production or activity of these neurotransmitters behave like wild type. In addition, we performed several screens for new mutants that do not respond in our TAV assay and are currently mapping the genes affected. We are also preparing to do laser ablations to identify neurons involved in the TAV response, and are currently knocking out several candidate genes, for which a function in heat perception is being discussed in vertebrates.
We hope that our work will lead to a better understanding of how painful stimuli are perceived and transmitted.
Special thanks to the Plasterk lab for its hospitality and help during the knock-out screens.