Delayed nociceptive response following cold-water swim in the formalin test: possible mechanisms of action

The changes of nociception and the signal molecules expression in the dorsal root ganglia and the spinal cord after cold water swimming stress in mice

Several studies have previously reported that exposure to stress provokes behavioral changes, including antinociception, in rodents. In the present study, we studied the effect of acute cold-water (4°C) swimming stress (CWSS) on nociception and the possible changes in several signal molecules in male ICR mice. Here, we show that 3 min of CWSS was sufficient to produce antinociception in tail-flick, hot-plate, von-Frey, writhing, and formalin-induced pain models. Significantly, CWSS strongly reduced nociceptive behavior in the first phase, but not in the second phase, of the formalin-induced pain model. We further examined some signal molecules' expressions in the dorsal root ganglia (DRG) and spinal cord to delineate the possible molecular mechanism involved in the antinociceptive effect under CWSS. CWSS reduced p-ERK, p-AMPKα1, p-AMPKα2, p-Tyk2, and p-STAT3 expression both in the spinal cord and DRG. However, the phosphorylation of mTOR was activated after CWSS in the spinal cord and DRG. Moreover, p-JNK and p-CREB activation were significantly increased by CWSS in the spinal cord, whereas CWSS alleviated JNK and CREB phosphorylation levels in DRG. Our results suggest that the antinociception induced by CWSS may be mediated by several molecules, such as ERK, JNK, CREB, AMPKα1, AMPKα2, mTOR, Tyk2, and STAT3 located in the spinal cord and DRG.

Development of antinociceptive tolerance and changes of opioid receptor ligand binding in central nervous system of the mouse forced to single and repeated swimming in the cold water

The present study was designed to characterize underlying mechanism involved in the development of tolerance in the production of antinociceptive effect induced by repeated cold water swimming stress (CWSS) using the tail-flick test. Mice were forced to swim at 4 degrees C for 3min and the tail-flick test was performed 5, 10, 20, and 30min after the swimming. The profound antinociception was induced by a single CWSS. However, when the mice were exposed to CWSS repeatedly seven times, they showed a tolerance in the production of antinociception. In the opioid receptor binding study, Bmax of delta opioid receptor (DOR) was decreased in the brainstem, midbrain and the spinal cord areas by both a single and repeated CWSS, without altering Kd value. However, Bmax values of mu opioid receptor (MOR) and kappa opioid receptor (KOR) were increased in the brainstem, midbrain and spinal cord regions by repeated CWSS, without changing Kd values. Our results suggest that the development of tolerance in the production of antinociception in mice forced to the repeated CWSS may be, at least, due to the reduction of DOR number in the brainstem, midbrain and the spinal cord regions.

Reduction of stress-induced analgesia following ethanol exposure in mice

In the present study, we examined the effects of ethanol treatment on the subsequent expression of opioid and nonopioid forms of swim stress-induced analgesia (SSIA). In Experiment 1, mice were injected with ethanol (2.5 g/kg, i.p.) or an equal volume of saline once a day for two days. Animals received no treatment on day 3. On day 4, the animals were tested for opioid (3-min swim in water maintained at 32 degrees C) or nonopioid (3-min swim in water maintained at 20 degrees C) SSIA in the hotplate test (52 degrees C). Mice pretreated with ethanol injections showed a decrease in nonopioid SSIA, but not in opioid SSIA. In Experiment 2, mice were given an ethanol solution (10%) or tap water to drink for 15 days. On day 16, all animals were given tap water to drink. On day 17, the animals were tested for opioid or nonopioid SSIA. Neither form of SSIA was modified in mice that drank the ethanol solution. These results show that ethanol pretreatment can modify nonopioid endogenous analgesic responses in mice. Further, the route of administration influences the effects of ethanol pretreatment on SSIA.

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 measured by the formalin test. One group of rats was exposed to a 3.5-min swim in 2 degrees C water immediately prior to the formalin injection, 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, subjects which received repeated CWS had dramatically altered formalin pain responses. Formalin responses began just over 3 h postformalin injection, peaked at 4 h, and were still present at 5 h. Inspection of individual responses revealed a substantial degree of variability in the onset of responses, although the magnitude and duration of the formalin pain response remained at the same levels as those of control subjects. The lack of a decrease in the magnitude and duration of the delayed formalin responses indicates that repeated CWS does not produce analgesia for tonic pain. The period of stress, therefore, produces pain suppression but not loss of the mechanisms that subsequently underlie the pain. Earlier controls have ruled out peripheral mechanisms (such as retention of the formalin in the paw tissue). Rather, a memory mechanism appears to have been indicated and it is not lost, but persists until it can be manifested. Further research is needed to study the mechanisms responsible for the DNR.