REPEATED COLD-WATER SWIM PRODUCES DELAYED NOCICEPTIVE RESPONSES, BUT NOT ANALGESIA, FOR TONIC PAIN IN THE RAT

Earlier studies have demonstrated that cold water swim (CWS) produces stress-induced analgesia in tests of brief, phasic pain and produces a delayed nociceptive response (DNR) for more prolonged tonic pain. The present study reports the effect of repeated CWS on tonic pain, as me asured by the formalin test, One group of rats was exposed to a 3.5-mi n swim in 2 degrees C water immediately prior to the formalin injectio n, to a 1.5-min swim at 50 min, and to another 1.5-min swim at 100 min postformalin injection, Compared to the no-swim control group, subjec ts which received repeated CWS had dramatically altered formalin pain responses. Formalin responses began just over 3 h postformalin injecti on, peaked at 4 h, and were still present at 5 h. Inspection of indivi dual responses revealed a substantial degree of variability in the ons et of responses, although the magnitude and duration of the formalin p ain response remained at the same levels as those of control subjects. The lack of a decrease in the magnitude and duration of the delayed f ormalin responses indicates that repeated CWS does not produce analges ia for tonic pain. The period of stress, therefore, produces pain supp ression but not loss of the mechanisms that subsequently underlie the pain. Earlier controls have ruled out peripheral mechanisms (such as r etention of the formalin in the paw tissue). Rather, a memory mechanis m appears to have been indicated and it is not lost, but persists unti l it can be manifested. Further research is needed to study the mechan isms responsible for the DNR. (C) 1997 Academic Press.