Abstract

Nociceptive response induced by 0.5% Formalin in the hindpaw of mice had two peaks, 0-5 min (first phase) and 15-20 min (second phase). By using the distinct biphasic response, the nature of the transmitter systems activated by Formalin in the spinal cord was studied for the purpose of determining the difference of the role of substance P (SP) and somatostatin (SST). The injection of (D-Pro2, D-Trp7,9)SP, (D-Arg1, D-Pro2, D-Trp7,9, Leu11)SP and SP antiserum inhibited only the first phase response. The i.t. injection of -Aminoheptanoyl-Phe-D-Trp-Lys-(OBz)-Thr- (an SST antagonist), SST antiserum and cysteamine (an SST depletor) inhibited only the second phase. This result indicates that SP is involved in the transmission of the first phase, and SST is involved in the transmission of the second phase of the Formalin-induced nociceptive response. With regard to other nociceptive stimuli, two i.t. SP antagonists produced a significant analgesia in the hot plate and tail pinch tests but had no effect in the acetic acid writhing test. However, i.t. SST antagonist and cysteamine produced a significant analgesia in the writhing test but had no effect in the hot plate and tail pinch test. These results suggest that SP participates in the transient pain induced by such acute stimuli as hot plate, tail pinch and the first phase of Formalin response and that SST participates in the prolonged and inflammatory pain induced by stimuli such as acetic acid and the second phase response.