Differential stress effects on responses to noxious stimuli as measured by tail-flick latency and squeak threshold in rats.

In the present study tail-flick latency (TFL) and squeak threshold (ST) were investigated in different environmental conditions in normotensive Wistar Kyoto rats (WKY) and spontaneously hypertensive rats (SHR). The TFL did not vary significantly during the testing period (120 min). The ST increased gradually and significantly with time when measured in rats restrained in the TFL test tube. Freely moving rats transferred to the TFL test tube showed a marked decrease of the ST. During the stay in the TFL test tube, the ST value increased gradually. Also after transfer from the test tube to the home cage the threshold increased to the same level as freely moving rats. There were no significant differences in these effects between WKY and SHR. Naloxone (1 mg kg-1 i.v.) decreased the ST in both freely moving and restrained animals of either strain. Intravenous injection of morphine (mg kg-1) gave a pronounced threshold increase of ST in both groups. It is suggested that animals restrained in the TFL test tube become hyperalgesic. The adaptation to stress is reflected in the ST but not in the TFL. Squeak threshold appears to be influenced by a tonically active endorphin system.