Worm Breeder's Gazette 13(3): 68 (June 1, 1994)

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.

Analysis of the neural circuit regulating thermotactic responses of C. elegans

Ikue Mori, Yasumi Ohshima

Department of Biology, Faculty of Science, Kyushu University, Fukuoka 812, Japan

We have been conducting laser ablation experiments to identify neurons mediating thermotaxis of C. elegans. Through the ultrastructural analysis of ttx-1 mutant, AFD neuron in the amphid was thought to be thermosensory. (1, 2) To confirm this result, we first killed a pair of AFD neurons. We found that killing AFD often resulted in cryophilic phenotype (3), but further laser ablation revealed that some fraction of AFD-killed animals exhibited athermotactic phenotype. These abnormal ttx phenotypes were independent of cultivation temperatures. Despite the variation in phenotype, the most striking consequence of AFD-killing is the loss of isothermal movement. If we suppose that isothermal movement involves processes such as 1) storage and maintenance, and 2) assessment on a thermal gradient, of thermal information, AFD might be also essential for these processes in addition to its critical role for thermosensation. As for other 11 amphid sensory neurons, we so far failed to identify any neurons that caused significant defects in thermotaxis when killed. When all amphid sensory neurons except AFD were killed, the TTX phenotype of the operated animal looked normal.

We then started to look for interneurons functional for thermotaxis. At the EM level, AFD is electrically connected to AIB interneuron and presynaptic to AIY interneuron (4). AIB-killed animals showed normal thermotaxis: they were able to move isothermally and responded almost normally to different cultivation temperatures on a radial thermal gradient. When AIY was killed, however, the operated animals showed clear cryophilic phenotype, and like AFD-killed animals, they were unable to move isothermally. One of the major postsynaptic partners of AIY is AIZ interneuron (4). To our surprise, the TTX phenotype of the putative AIZ-killed animals seemed to be almost indistinguishable from that of thermophilic mutants present to date: the operated animals seek warmer temperature than their cultivation temperature, but their ability to move isothermally is not abolished, although isothermal movement is somewhat abnormal. At this point, we have to say that our AIZ-killing results are still preliminary, since we are not completely confident about the identification of AIZ neuron at L1 stage under Nomarski. We are planning to confirm AIZ kills by staining the operated worms using anti- UNC-86 antibody (5).

What can we draw from the results of killing AIY and AIZ? AIY-killed animals mimic cryophilic mutant phenotype with no isothermal tracking and AIZ-killed animals mimic thermophilic mutant phenotype. The interesting model is that AIY and AIZ regulate one or a set of neurons in opposite directions, i.e., AIY activates (or inhibits) and AIZ inhibits (or activates) the neuron(s) essential for thermotaxis. Accumulation to temperature might be accomplished by counterbalancing the activities of these two neurons, depending on the set point of the adaptation temperature. To understand more about the neural regulation of thermotaxis, we want to know the effect of double kills of AFD, AIY and AlZ neurons.

Literature Cited:

(1) Hedgecock and Russell (1975), PNAS 72:4061

(2) Perkins et. al. (1986), Dev. Bio. 117:456

(3) Mori et. al. (1993), The ninth C. elegans meeting Abst:319

(4) White et. al. (1986), Philos.Trans. R. Soc. London Ser B 314:1

(5) Finney and Ruvkun (1990), Cell 63:895