A cold plate apparatus was designed to test the responses of unrestrained rats to low temperature stimulation of the plantar aspect of the paw. At plate temperatures of 10 degrees C and 5 degrees C, rats with either chronic constriction injury (CCI) of the sciatic nerve or complete Freund's adjuvant (CFA) induced inflammation of the hindpaw displayed a stereotyped behavior. Brisk lifts of the treated hindpaw were recorded, while no evidence of other nociceptive behaviors could be discerned. The most consistent responses were obtained with a plate temperature of 5 degrees C in three 5-min testing periods, separated by 10-min intervals during which the animals were returned to a normal environment. Concomitantly to cold testing, the rats were evaluated for their response to heat (plantar test) and mechanical (von Frey hairs) stimuli. In both injury models, while responses to heat stimuli had normalized at 60 days post-injury, a clear lateralization of responses to cold was observed throughout the entire study period. Systemic lidocaine, clonidine, and morphine suppressed responses to cold in a dose-related fashion. At doses that did not affect motor or sensory behavior, both lidocaine and its quaternary derivative QX-314 similarly reduced paw lifts, suggesting that cold hyperalgesia is in part due to peripheral altered nociceptive processing. Clonidine was more potent in CCI then in CFA rats in reducing the response to cold. Paradoxically, clonidine increased the withdrawal latencies to heat in the CCI hindpaw at 40 days and thereafter, at a time when both hindpaws had the same withdrawal latencies in control animals. Morphine was also more potent on CCI than CFA cold responses, indicating that, chronically, CFA-induced hyperalgesia might be opiate resistant. Evidence for tonic endogenous inhibition of cold hyperalgesia was obtained for CFA rats, when systemic naltrexone significantly increased the number of paw lifts; this was not found in rats with CCI. At 60 days, neither morphine nor naltrexone affected cold-induced paw lifting in CFA rats, suggesting that the neuronal circuit mediating cold hyperalgesia in these animals had become opiate insensitive. In conclusion, the cold plate was found to be a reliable method for detecting abnormal nociceptive behavior even at long intervals after nerve or inflammatory injuries, when responses to other nociceptive stimuli have returned to near normal. The results of pharmacological studies suggest that cold hyperalgesia is in part a consequence of altered sensory processing in the periphery, and that it can be independently modulated by opiate and adrenergic systems.

Noradrenaline (NA), a key neurotransmitter of the endogenous pain inhibitory system, acutely inhibits nociceptive transmission (including that mediated by substance P), potentiates opioid analgesia, and underlies part of the antinociceptive effects of the widely prescribed tricyclic antidepressants. Lesions of noradrenergic neurons, however, result in either normal or reduced pain behavior and variable changes in morphine antinociception, undermining the proposed association between noradrenaline (NA) deficiency and chronic pain (hyperalgesia). We used mice lacking the gene coding for dopamine ␤-hydroxylase, the enzyme responsible for synthesis of NA from dopamine, to reexamine the consequences of a lack of NA on pain behavior. Here, we show that absence of NA in the central nervous system results in a substance P-mediated chronic hyperalgesia (decreased nociceptive threshold) to thermal, but not mechanical, stimuli and decreased efficacy of morphine. Contrary to studies that show substance P-mediated hyperalgesia requires intense stimuli, we found that even a mild stimulus is sufficient to evoke substance P-dependent hyperalgesia in the NA-deficient mice. Restoring central NA normalized both the nociceptive threshold and morphine efficacy, which is consistent with a tonic inhibitory effect of NA on nociceptive transmission. Unexpectedly, however, antagonists to the substance P receptor (the NK1 receptor) could achieve the same effect as NA replacement. We conclude that when unopposed by NA, substance P acting at the NK1 receptor causes chronic thermal hyperalgesia, and that the reduced opioid efficacy associated with a lack of NA is due to increased NK1-receptor stimulation. N oradrenaline (NA) is an essential neurotransmitter of the endogenous pain inhibitory system (1, 2) that tonically and phasically inhibits spinal nociceptive transmission, including that mediated by substance P (3, 4). By stimulating central nervous system (CNS) ␣ 2 adrenoreceptors, NA increases threshold and latency to noxious stimuli without affecting responses to innocuous stimuli, and potentiates the antinociceptive effects of opiates (5-10).Based on the above understanding, it has been suggested that noradrenergic dysfunction is a key component of certain chronic intractable pain disorders (11). A major caveat is that this hypothesis is based primarily on experimental studies that use acute rather than chronic pain. Although acute lesions or inhibition of noradrenergic spinal afferents produce a state of hyperalgesia and reduced antinociceptive effects of opiates (2, 10, 12), the increased pain behavior abates over time. In fact, long-term effects of noradrenergic denervation have been reported to result in decreased pain behavior, whereas the antinociceptive effect of morphine is either reduced, unchanged, or increased (13-17). These conflicting results can be caused in part by the variability in the extent of the removal of noradrenergic neurons or terminals which, when using a neurotoxin, is both incomplete and nonselective. Moreov...