The journal of pain : official journal of the American Pain Society
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Devices designed for mechanical pain threshold studies are often difficult to implement. The purpose of this study was to investigate a simple tool based on calibrated forceps to induce quantifiable mechanical stimulation in the rat on a linear scale. The most suitable protocol was tested by determining the effects of 3 repetitive measurements on both hind paws, respectively, during long-term (9 days), mid-term (1 day), and short-term (2 hours). Only threshold increase related to weight gain over long-term was observed, suggesting that moderate rat training can be used. The capacity of the device to reveal hyperalgesia was tested in a model of carrageenan-induced inflammation in the hind paw. The hyperalgesia was maximal 6 hours after carrageenan injection and progressively decreased. Similar, although more variable, responses were observed with von Frey filaments. Morphine-induced analgesia resulted in a dose-dependent increase of paw threshold. Tolerance to morphine administrated on a once daily schedule (10 mg/kg) during 5 days was revealed by a significant decrease in analgesia by day 3. Taken together, these results demonstrated accuracy of this device for easy, fast, and reproducible measure of mechanical pain threshold on rat limbs. Moreover, it allows the performance of rat testing with minimal constraint, which reduces data variability. ⋯ The calibrated forceps is an easy to use device well-suited to rapidly test mechanical pain threshold with accuracy. It is well-designed for preclinical behavioral screening of noxious or analgesic properties of molecules.
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Randomized Controlled Trial
Effects of focusing and distraction on cold pressor-induced pain in chronic back pain patients and control subjects.
Previous studies showed equivocal findings regarding the efficacy of focused attention and distraction to experimental pain. This study examined the relative efficacy of these strategies on perception of cold pressor pain in 41 chronic back pain patients and 41 healthy control participants. Participants were randomized to the 2 strategies and then completed a 7-minute cold pressor test. Pain intensity and discomfort ratings were obtained during the task. Participants who completed the first task were asked to complete a second cold pressor task without instructions. Pain and discomfort ratings differed by condition across time. In the distraction condition, pain levels started low but continued to rise throughout the cold pressor immersion, whereas in the focused attention condition, pain levels started higher, rose less quickly, and then decreased from the middle of the task. Focused attention was associated with higher pain and lower completion rates in chronic pain patients compared with healthy control subjects. Focused attention might therefore not be an effective intervention strategy for individuals with chronic back pain. Finally, in the second cold pressor test, patients' pain reports rose more rapidly than those of healthy control subjects. The results of this study can be explained in terms of differences in cognitive appraisal between pain patients and healthy control participants. ⋯ Marked differences were found between chronic back pain patients and control participants regarding focused attention as compared with distraction as a means of coping with cold pressor-induced pain. These differences underline the importance of taking into account previous experience with pain when recommending strategies to cope with painful procedures.
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Morphine is one of the main analgesics in cancer-induced bone pain (CIBP). To investigate the efficacy of morphine in CIBP and alteration in dorsal horn pathophysiology, systemic morphine was administered (3 mg/kg) bi-daily between days 11 and 15 after MRMT-1 carcinoma cell injections (compared with a single injection (3 mg/kg) of morphine on day 15, and acute spinal morphine (0.1, 1, 10 microg/50 microL). The chronic systemic morphine schedule significantly attenuated pain behavior (von Frey 15 g; P < .01) to a greater extent than acute systemic morphine (von Frey 15 g; P < .05). In vivo electrophysiology (day 15 chronic systemic morphine) showed an attenuation of hyperexcitable wide dynamic range (WDR) neurons, but the abnormal raised WDR to nociceptive specific neuronal ratio remained. Acute spinal morphine attenuated electrical and natural WDR neuronal response in shams at a lower dose (1 microg) compared with cancer (10 microg). Chronic morphine is more effective at attenuating pain-related behaviors than single doses, although the dorsal horn retains a pathophysiologic characterization. ⋯ This study confirms the resemblance of the rat model to human CIBP with respect to the efficacy of morphine and further suggests that adjuvant therapy is required to reverse the dorsal horn pathophysiology.