Integrative mechanisms for protopathic sensation including pain and g ravitropism

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Neuronal mechanisms of nociceptive and gravitational responses were investigated in c. elegans. Wild type c.eleganse was stimulated by noxious or high gravity stimulation. Noxious stimulations were maechanical (von Frey type apparatus), chemical, heat, pH and osmomic ones. Gravity stimulation was high- gravity of 1.2-2.0G. Main results were as follows: (1)Threshold values of mechanical stimulation for avoidance reflex showing escape from the stimulus site were lower than that evoked pain sensation in man. A typical behavior of avoidance reflex is peristalsis or coiling motion (2)Pain producing substances, such as Bradykinin, Histamine, 5-HT were not effective on the avoidance reflex in C. elegans. (3)Low pH and low osmomic stimulation were quite effective for evoking coiling reflex.Next, we investigated interaction between noxious and gravitational stimuli. For this purpose, nociceptive threshold before and after gravitational stimulation were compared. Centrifugal apparatus were used to apply high gravity. For 30 min. For mechanical threshold, it was higher after gravitational stimulation. Degree of threshold changes by gravitational simulation was dose-response fashon For example, 2G was more effective than 1.2 G stimulation on the von Frey-induced escape response. Chemical threshold to evoke nociceptive response were not changed even after gravitational stimulation. 2G stimulation enhanced low osmimic-induced escape behavior. The present results suggest that prtopathic sensation may be integrated in the central nervous system. Both nociceptive and gravitational sensations are critical for the preservation of the species in animals.