Effects of intrathecal substance P and a substance P antagonist on a reflex to noxious heat are independent of changes in tail skin temperature.

Substance P or the substance P receptor antagonist (D-Arg1,D-Trp7.9,Leu11)-substance P (Spantide) was injected into the lumbar subarachnoid space in mice, and the ability to change the tail-flick reflex and the tail skin temperature was investigated. Tail-flick latency (the time needed to evoke the tail-flick reflex by noxious radiant heat) was reduced for 1-4 min after intrathecal administration of substance P (5 micrograms), but the tail skin temperature was not significantly changed. Nor was the tail skin temperature significantly changed after intrathecal injection of Spantide (5 micrograms), but this compound significantly increased tail-flick latencies 5-30 min after injection. Analysis of co-variance showed that the effects of substance P or Spantide on tail-flick latency were significant, whereas the influence of tail skin temperature on tail-flick latency was non-significant. Thus, intrathecal substance P induces a short-lasting increase in nociceptive sensitivity, and intrathecal Spantide produces an antinociceptive effect of longer duration. The results seem not to be the result of changes in tail skin temperature.