Effects of an opiate on cold-induced pain and the CNS in healthy volunteers

A cerebral blood flow study on tonic pain activation in man

This study examined brain areas involved in tonic pain perception. Cerebral blood flow was assessed by dynamic Xenon-133 inhalation single-photon emission tomography (SPET) in 7 healthy right-handed male volunteers undergoing the cold pressor test (CPT). In single experimental sessions, each subject was scanned twice, once in the resting state and once while immersing the left hand in freezing water (0 degrees C +/- 1). Immersion of the hand induced severe pain (visual analogue scale: 6.9 +/- 1.9) in all subjects. After correction for pCO2, cerebral blood flow was analyzed by placing a template of square regions of interest (ROIs) over 5 selected tomographic slices. Relative to the resting-state values, during the CPT, flow determinations revealed a 7-8% regional blood flow increase in the contralateral frontal lobe and bilateral temporal regions and a 15% flow increase in a ROI located over the primary sensorimotor cortex in the tomogram at 80 mm above the orbito-meatal line (corresponding to the cortical somatotopic representation of the hand) contralateral to the stimulated side. The tonic pain induced by the CPT thus appears to activate the contralateral frontal and bilateral temporal regions and more prominently, the primary sensorimotor cortex. This pattern of activation suggests that tonic painful stimuli activate the cortex partly via complex circuits and partly via direct somatosensory pathways.

Subjective, behavioral and physiological responses to intravenous meperidine in healthy volunteers

Meperidine is a mu opiate agonist that is frequently used to treat pain. We examined in healthy volunteers (N = 10) the effects of intravenous meperidine (0, 0.25, 0.5, and 1.0 mg/kg) on mood and psychomotor performance. A randomized, placebo-controlled, crossover design was used in which subjects were injected with meperidine or saline in a double-blind fashion. Subjects completed several subjective effects questionnaires commonly used in abuse liability testing studies before drug injection and at periodic intervals for up to 5 h after drug injection. Subjects also completed several psychomotor tests. Meperidine produced a constellation of subjective effects in a dose-related fashion, including increases in ratings of "sedated," "coasting or spaced out" and "feel drug effect" ratings. Many of the drug's subjective effects persisted up to 4 or 5 h after administration of the 1.0 mg/kg dose. Drug liking ratings assessed on a visual analog scale were increased after meperidine injection in about half of the subjects (P = 0.09). Eye-hand coordination was affected slightly by meperidine but other indices of psychomotor functioning were unaffected. Miosis increased in a dose-related fashion. Other physiological parameters, such as vital signs, were not affected by meperidine. We conclude that meperidine in healthy volunteers has robust and long-lasting effects on mood, but may have weaker effects on psychomotor performance.

Acute tryptophan depletion blocks morphine analgesia in the cold-pressor test in humans

The effects of depletion of the serotonin precursor, L-tryptophan, on the threshold and tolerance to cold pressor pain, and the analgesic effect of morphine (10 mg intramuscularly), were tested in a double blind trial on human volunteers. Effects on mood were also assessed using the Profile of Mood States and the Addiction Research Center Inventory (ARCI) Scales. To deplete tryptophan, subjects were fed a tryptophan-deficient amino acid mixture 4.5 h before morphine was administered. Controls received the mixture with tryptophan, which is equivalent to a nutritionally balanced protein. The tryptophan-deficient meal reduced plasma tryptophan more than 70% but had no effect on threshold or tolerance to cold pressor pain. After morphine, tolerance to cold pressor pain increased in controls. Tryptophan depletion abolished this analgesic effect. Pain threshold was not altered by morphine. In subjects with normal tryptophan, the analgesic effect of morphine was predicted by the level of plasma morphine-6-glucuronide, but not by the level of morphine. Morphine increased scores on the LSD scale of the ARCI, but had no effect on other measures of mood. Tryptophan depletion also failed to alter mood in these subjects, who had unusually low depression scores before tryptophan depletion.

Subjective and behavioral responses to intravenous fentanyl in healthy volunteers

Fentanyl is a mu opiate agonist which is occasionally abused by medical personnel who have ready access to the drug. We examined in healthy volunteers (N = 13) the subjective and psychomotor-impairing effects of intravenous fentanyl (0-100 micrograms/70 kg). A randomized, placebo-controlled, crossover design was used in which subjects were injected with 0, 25 (N = 6), 50 and 100 micrograms/70 kg fentanyl in a double-blind fashion. Subjects completed several questionnaires commonly used in abuse liability testing studies before drug injection and at periodic intervals for up to 3 h after drug injection. Subjects also completed several psychomotor tests at these times. Some aspects of psychomotor functioning (e.g., eye-hand coordination) were impaired by fentanyl. Fentanyl produced dose-related increases in ratings of "high" and "sedated," but also tended to produce dysphoria and somatic symptomatology. Most subjects reported liking the effects of the two higher doses of fentanyl for at least a brief time after injection, but they varied widely in their linking ratings across the 3-h post-drug injection period. Despite the transient increases in liking ratings, fentanyl did not increase scores on a widely-used measure of drug-induced euphoria (morphine-benzedrine group scale of the Addiction Research Center Inventory). The present results suggest that some medical personnel who experiment with fentanyl may like it, and thus be at increased risk for abusing the drug in the future.

Analgesic efficacy of controlled-release dihydrocodeine. A comparison of 60, 90 and 120 mg tablets in cold-induced pain

A prospective, double-blind, single-dose placebo-controlled four-part crossover study of 12 healthy volunteers was carried out to compare the analgesic efficacy of controlled-release dihydrocodeine tablets 60, 90 and 120 mg (DHC Continus tablets, Napp Laboratories) in cold-induced pain. Subjects received each of the four treatments in a random order using a latin square design. On each of the four study days, the volunteers performed cold pressor tests, before dose and again at 4, 8 and 12 hours after dose. Subjects rated their pain continuously over a 120-second period using a visual analogue scale. At 4 hours there was a significant reduction in pain in subjects who received 120 mg or 90 mg tablets compared with placebo, and in subjects who received 120 mg tablets compared with those who received 60 mg tablets. At 8 hours, 120 mg and 90 mg dihydrocodeine were still better than placebo. There was no significant difference in side effects between treatments.

Blood pressure reactivity and perception of pain during the forehead cold pressor test

The relationship between blood pressure reactivity and the perception of pain was examined during a series of three forehead cold pressor tests given every other day to a group of 18 male college students. Subjects classified as high reactors on the basis of peak increases in mean blood pressure during cold pressor tests perceived the cold pressor stimulus as more painful than subjects classified as low reactors. The propensity to rate the cold pressor stimulus as painful was positively correlated with the individual level of blood pressure reactivity (baseline-free partial r = .62). Intra-individual correlations between pain and blood pressure responses were unrelated to subjects' reactivity status. Across the 3-min test, correlations between pain and blood pressure reactivity (with the effects of baseline blood pressure levels partialled out) were significant only during periods when levels of responses were relatively high. The heart rate responses were unrelated to pain ratings. Generalizability theory was applied to the analysis of temporal stability of cold pressor reactions. Both blood pressure and pain responses were highly reproducible across three sessions, appearing to express stable individual differences. The efficacy of 800 mg oral ibuprofen in controlling the cold pressor pain was also tested. Analgesic activity of the drug during the cold pressor test could not be demonstrated.

Action of neurotropin on cold-induced pain in normal volunteers: a double-blind placebo study

The analgesic action of neurotropin, a biological compound widely used in Japan with a record of very limited side-effects, was tested in 8 Caucasian normal volunteers. For the pain test, the subjects were requested to immerse the right hand in ice-cold water and to report the appearance of the following sensations: pain threshold (PThr) and tolerance to pain (PTol). Pain sensitivity range (PSR) was calculated by difference between PThr and PTol expressed in s from beginning of immersion. Neurotropin tablets or an indistinguishable inert placebo were administered according to a randomized double-blind cross-over design and their influence in the pain test was investigated during 2 daily sessions. The effects of both treatments were assessed by calculating the difference between initial and post-medication values for each pain response parameter. Under placebo the means of all effects were found to be negative, reflecting the appearance of hyperalgesia under repetitive pain conditions, with neurotropin, the effects on PTol and PSR were positive and significantly different from those of placebo. These results are discussed in terms of possible mechanisms of action: neurotropin analgesic activity could be linked to its kinin release-inhibiting properties or to a reduction in vaso-constriction and hyperalgesia related to an effect on catecholamines.

Analgesic, central, cardiovascular and endocrine effects of the enkephalin analogue Tyr-D.Arg-Gly-Phe(4NO2)-Pro-NH2 (443C81) in healthy volunteers

443C81 is a synthetic enkephalin thought to act on peripheral opiate receptors. The analgesic, central, cardiovascular and endocrine effects of two i.v. doses of 443C81 were investigated in 12 healthy male volunteers. Its effects were compared with those of placebo and the classical opiate dipipanone given orally using a double dummy design. 443C81 produced dose-related analgesia; dipipanone 10 mg had a greater effect than the high dose 443C81. In contrast to dipipanone, 443C81 did not cause significant miosis or reduce minute volume on rebreathing CO2 and there was no evidence of sedation. Dry mouth was reported frequently and associated with reduced salivation after all active treatments. Both 443C81 and dipipanone increased circulating prolactin and growth hormone and reduced cortisol levels. This novel enkephalin appears to possess analgesic activity and some other properties of opiates but is devoid of clinically relevant narcotic effects.

Morphine and ibuprofen compared using the cold pressor test

The analgesic efficacy of single doses of oral morphine sulphate solution (10 mg) and ibuprofen 600 mg was compared in 12 volunteers using a double-blind, double-dummy, placebo-controlled design on the cold pressor experimental pain model. Measurement of pain intensity was made before medication and then at 30, 60, 90, 120 and 180 min; blood samples were taken at these times for measurement of morphine and glucuronide metabolites by radioimmunoassay. Sessions were at least 5 days apart. Correlations were sought between analgesic effect and plasma concentrations of either morphine or morphine-6-glucuronide. Morphine produced significant reduction in both peak pain intensity and area under the pain intensity curve compared with placebo; the threshold time was significantly increased by morphine compared with placebo. Ibuprofen was statistically indistinguishable from placebo on all three measures of analgesia. Analgesic effect and plasma concentrations of morphine showed significant correlation (P = 0.053). The study confirmed reports of the opiate sensitivity of the cold pressor model, and the apparent insensitivity of the model to non-steroidal anti-inflammatory drugs.

Dipipanone and nifedipine in cold induced pain; analgesia not due to skin warming

The mechanism of the pain relief produced by opiates in normal volunteers in the cold induced pain test has been investigated. In a double-blind placebo controlled study, hand skin temperature during a 3 min immersion in water at 1 degree C was not affected by either the opioid dipipanone 8 mg or the vasodilator nifedipine 10 and 20 mg. During this immersion, dipipanone produced significant pain relief. Nifedipine reduced pre-immersion blood pressures and raised heart rates, however, it did not significantly alter pain scores. It is concluded that vasodilatation and local warming do not play a role in the relief of pain by opiates in the cold immersion test.

Indomethacin: effects on cold-induced pain and the nervous system in healthy volunteers

The sensitivity of the cold-induced pain (CP) model to the non-steroidal anti-inflammatory drug (NSAID) indomethacin was studied in healthy volunteers. Effects on the central nervous system were also sought. Subjects received single oral doses of indomethacin 50 and 100 mg, dipipanone 8 mg and placebo, according to a double-blind, randomised, balanced, cross-over design with an interval of 7 days between occasions. A test battery was performed before each treatment and then at 45, 105 and 165 min post treatment. Pain scores were unaltered by indomethacin at either dose, but the drug certainly affected the CNS, increasing respiratory drive and changing self-assessed mood. It is concluded that the CP model is insensitive to indomethacin, even in doses which have clear-cut CNS effects. The respiratory stimulant action of indomethacin may deserve further study.

Effects of triprolidine and dipipanone in the cold induced pain test, and the central nervous system of healthy volunteers

1 Twelve healthy volunteers took part in a study of the interaction between the antihistamine triprolidine and the opioid dipipanone in the cold induced pain (CP) test and tests of sedation. They received placebo, triprolidine 2.5 mg, dipipanone 8 mg or the combination of the two active treatments according to a double-blind, randomised, balanced, crossover design. 2 Antihistamine activity was demonstrated by triprolidine reducing the size of wheals and flares produced by intradermal histamine 1.6 micrograms. However, triprolidine produced no analgesia in the CP test, nor did it enhance the analgesia produced by dipipanone alone. 3 Neither treatment alone produced statistically significant sedation, assessed by visual analogue scales (VAS), side effect check list, body sway and reaction times. However, the combination did cause significant sedation. 4 Dipipanone reduced pupil size, depressed respiration, and decreased salivation. Triprolidine had no effects on pupil size and respiration, but reduced salivation slightly. It was concluded that histaminergic (H1) mechanisms are unlikely to be involved in pain produced by cold.