The ability to sense and react to noxious high environmental temperature is critical for animals’ survival. C. elegans uses distinct escape and avoidance regimes to minimize exposure to noxious heat. It has been shown that C. elegans heat avoidance requires several polymodal nociceptors, such as the FLP neurons, ASH neurons and others, but many of the key molecules involved in C. elegans thermosensation must be identified (Liu et al., 2012). Moreover the neural circuit mechanisms by which thermosensory behavior is generated remain incompletely understood especially with respect to heat avoidance (Liu et al., 2012). Therefore we proposed that C. elegans behavior response to noxious heat includes adaptive changes in the activity of cholinergic synapses in neural circuit regulating worm’s movement (Jospin et al., 2009).
To test this hypothesis we used pharmacological analysis of noxious heat effect on steady state of cholinergic synapses of C. elegans in conditions of moderate rise in temperature tolerated by worms. Pharmacological analysis of steady state of C. elegans cholinergic synapses in vivo usually consists in measuring of locomotion sensitivity to ACh-esterase inhibitor aldicarb and nAChRs’ agonists levamisole and nicotine. In both cases enormous rise in ACh content caused by aldicarb or overactivation of nAChRs by their agonists induced locomotion disturbances. In numerous genetic investigations this analysis was used to identify molecular mechanisms of neuromuscular synapses’ function. In all these investigations aldicarb or levamisole sensitivity was measured by registration of worms’ full inability to move on agar plate after slight touch by needle (“paralysis”). In contrast, in our experiments performed in liquid medium (NG buffer) we registered swimming disturbances, such as failure of muscle contraction in undulatory pattern or inability to forward swimming during 10 seconds, induced by aldicarb, levamisole and nicotine in concentrations which are subthreshold for nematode paralysis. Aldicarb concentration effective to induce behavior disturbances in 50% of worms after 20-minutes exposure at 23°C was 32 µM, while such for 20-minutes exposure at 33°C was 15 µM.
C. elegans exposition to temperature 33°C didn’t cause disturbances in worms’ swimming induced by mechanical stimulus, but induced rapid (less than 30 minutes) rise in this behavior sensitivity to partial inhibition of ACh-esterase by aldicarb. These data indicate that noxious heat caused a rise of steady state cholinergic synaptic transmission in cholinergic synapses, involved in regulation of C. elegans swimming.
Temperature rise up to 33°C strongly elevated swimming sensitivity not only to aldicarb, but also to nAChRs’ agonists levamisole and nicotine. Concentrations of levamisole and nicotine effective to induce behavior disturbances in 50% of worms after 20-minutes exposure were respectively 62 µM and 710 µM at 23°C and respectively 15 µM and 320 µM at 33°C. Therefore it is evident that moderate heat stress activates steady state of synaptic transmission by sensitization of nAChRs, but not by stimulation of ACh release from neurons. Sensitization of nAChRs by noxious heat can be involved in rise of running away speed caused by heat stress.
Jospin M, Qi YB, Stawicki TM, Boulin T, Schuske KR, Horvitz HR, Bessereau J-L, Jorgensen EM, and Jin Y. (2009). A neuronal acetylcholine receptor regulates the balance of muscle excitation and inhibition in Caenorhabditis elegans. PLoS Biol. 7, e32360.