Effects of substance P analogues in the rat tail-flick test

Five substance P (SP) analogues which antagonize tachykinins on isolated organs were studied using the tail-flick test. None antagonized the nociceptive effect of 1.9 nmol of physalaemin when studied at the concentrations of 6.5 and 65.0 nmol. [D-alpha Npa7,9,10]SP (65.0 nmol) produced a flaccid paralysis of the hindlimbs and the tail in 2 out of 9 rats; however, [D-Pro4,Lys6,D-Trp7,9,10,Phe11]SP-(4-11) (6.5 nmol) produced this type of paralysis in 4 out of 6 rats so treated. The [D-Trp]SP analogue appears to be the most toxic. The results also suggest that the receptor for physalaemin in the spinal cord is different from that in peripheral tissues.