Assessment of the reproducibility of intradermal administration of capsaicin as a model for inducing human pain

The reproducibility and tolerability of intradermal (i.d.) administration of capsaicin as a method for eliciting human pain was assessed in healthy male volunteers (n = 12). The primary endpoints for assessing pain were spontaneous pain response and areas of allodynia, pinprick hyperalgesia and neurogenic inflammation. These were recorded before, immediately after, and at regular intervals following each of four doses (250 microg) of capsaicin (two per trial day). Within- and between-subject variability to the technique was assessed by measuring the maximum recorded values (max), time to maximum value (t(max)) and area under the curve (AUC(0-1 h)) of each of the endpoints. Tolerability to the technique was addressed by recording adverse events. Reproducibility of the i.d. capsaicin model was demonstrated for each type of capsaicin-induced pain. Following each dose, the magnitude and profile of response and overall AUC values were similar for each parameter although some decrease in pinprick hyperalgesia was observed over time. For spontaneous pain, evidence of a period effect was observed in mean AUC data, with values increasing following the second dose of each trial day. This effect was confounded by the possibility of an arm effect, with the non-dominant arm appearing to be more sensitive to pain than the dominant arm. The data were not sufficient to confirm the existence of these effects. Between-subject variability and within-day, within-subject variability accounted for most of the variability observed in the trial. By optimising study design to eliminate these sources of variability, it was estimated that spontaneous pain and the area of allodynia would be the least variable endpoints. A positive correlation was found between the area of allodynia and area of pinprick hyperalgesia (r(2) = 0.835). Overall, the model was well tolerated with no reports of adverse events. We conclude that the tolerability profile, and variability of i.d. capsaicin-induced pain is acceptable for pharmacological profiling of novel anti-nociceptive agents, with limited number of subjects.

Local injection of a selective endothelin-B receptor agonist inhibits endothelin-1-induced pain-like behavior and excitation of nociceptors in a naloxone-sensitive manner

We showed previously that subcutaneous injection of the injury-associated peptide mediator endothelin-1 (ET-1) into the rat plantar hindpaw produces pain behavior and selective excitation of nociceptors, both through activation of ET(A) receptors likely on nociceptive terminals. The potential role of ET(B) receptor activation in these actions of ET-1-has not been examined. Therefore, in these experiments, we studied the effect of blocking or activating ET(B) receptors on ET-1-induced hindpaw flinching and excitation of nociceptors in rats. An ET(B) receptor-selective antagonist, BQ-788 (3 mm), coinjected with ET-1 (200 microm) reduced the time-to-peak of flinching and significantly enhanced the average maximal flinch frequency (MFF). In contrast, coinjection of an ET(B) receptor selective agonist, IRL-1620 (100 or 200 microm), with ET-1 reduced the average MFF and the average total number of flinches. Interestingly, this unexpected inhibitory effect of IRL-1620 was prevented by the nonselective opioid receptor antagonist naloxone (2.75 mm). To confirm these inhibitory actions, we studied the effects of IRL-1620 on ET-1-induced spike responses in single, physiologically characterized nociceptive C-fibers. IRL-1620 suppressed spike responses to ET-1 in all (n = 12) C-units, with mean and maximum response frequencies of 0.08 +/- 0.02 and 1.5 +/- 0.4 impulses/sec versus 0.32 +/- 0.07 and 4.17 +/- 0.17 impulses/sec for ET-1 alone. In additional support of the behavioral results, coinjection of naloxone (2.75 mm) completely prevented this inhibitory action of IRL-1620. These results establish that ET(B) receptor activation inhibits ET-1-induced pain behavior and nociception in a naloxone-sensitive manner and point to a previously unrecognized dual modulation of acute nociceptive signaling by ET(A) and ET(B) receptors in cutaneous tissues.

Prevention of respiratory syncytial virus infection among Puerto Rican infants

Respiratory syncytial virus (RSV) is the leading cause of lower respiratory illness in children. Prevention of this infection is available with the use of intravenous immunoglobulin or an intramuscular humanized monoclonal antibody (palivizumab). Palivizumab has been available in Puerto Rico since 1999. The objective of this study was to follow-up infants who received RSV prophylaxis with palivizumab in Puerto Rico to assess its efficacy and safety. A total of 230 infants who received RSV prophylaxis during the 2000-2001 and 2001-2002 seasons were followed-up. Adverse events from injections were minimal including erythema (2%), fever (5%), pain (4%), and rash (2%). In none of the patients prophylaxis was discontinued due to side effects. Forty-four infants (19%) had at least one respiratory hospitalization throughout the season, with RSV confirmed in seven (3%). Most hospitalizations occurred in the month of August when infants had received only one dose of palivizumab and on December, a peak month for RSV infections. Five infants (2.2%) required admission to an intensive care unit. In none of them, RSV was confirmed. This study confirms that monthly intramuscular administration of palivizumab is effective in preventing serious RSV infections in high risk infants.

Studies on the anti-inflammatory and anti-nociceptive effects of melatonin in the rat

The present study aimed to evaluate the anti-inflammatory and anti-nociceptive effects of melatonin in the rat. Acute inflammation was induced by sub-plantar injection of carrageenan (1%) in the rat hind paw. The rats received vehicle or drug 30 min before carrageenan administration and were evaluated for paw oedema at 1, 2, 3, and 4 h post-carrageenan. The induced inflammation and the formation of oedema were determined by measurement of the paw thickness. Nociception was tested by determining vocalization following electrical stimulation of the tail. Given intraperitoneally (i.p.) 30 min before carrageenan, melatonin caused significant and a dose-dependent reduction of hind paw swelling induced by carrageenan. At doses of 0.5 and 1 mg kg(-1), melatonin inhibited the carrageenan-induced oedema by 20.5 and 29.6% versus control values at 4 h post-carrageenan, respectively. Melatonin (0.5 and 1 mg kg(-1), i.p.) 30 min beforehand displayed anti-nociceptive effect in the electric stimulation of the rat tail test, increasing nociceptive thresholds to electrically-induced pain at 4 h post-treatment by 29.6 and 39.5%, respectively. Melatonin given simultaneously with the non-selective COX-1 and COX-2 inhibitor indomethacin (5 mg kg(-1), i.p.) 30 min prior to carrageenan, enhanced the anti-inflammatory effect of the latter in the carrageenan-induced paw oedema model by 23%. Melatonin (0.5 mg kg(-1), i.p.) increased the anti-nociceptive effect of indomethacin (5 mg kg(-1), i.p.). Meanwhile, the anti-inflammatory and anti-nociceptive effect of the highly selective COX-2 inhibitor rofecoxib (2.25 mg kg(-1), i.p.) was only slightly increased by melatonin administration at 0.5 mg kg(-1). Melatonin enhanced the anti-inflammatory effect of cysteamine (300 mg kg(-1), s.c.) in the carrageenan-induced paw oedema. Melatonin (20 and 40 microg per paw) given prior to carrageenan into the rat hind paw was devoid of anti-inflammatory effect. These results indicate that melatonin possesses anti-inflammatory and anti-nociceptive properties in the rat and enhance those of indomethacin. This effect is likely to be centrally mediated.

Modulation of formalin-induced behaviors and edema by local and systemic administration of dextromethorphan, memantine and ketamine

The present study examined the effects of local peripheral and systemic administration of three clinically used excitatory amino acid receptor antagonists (dextromethorphan, memantine, ketamine) on pain behaviors and edema produced by formalin (1.5% and 5%) in rats. Peripheral administration of dextromethorphan produced a locally mediated suppression of flinching behaviors induced by 1.5% and 5% formalin, but biting/licking behaviors were not affected. Memantine and ketamine had no effect on either of these behaviors. All three agents augmented edema produced by 1.5% and 5% formalin. When administered alone, dextromethorphan, memantine and ketamine produced an intrinsic paw swelling response, and this was blocked by the biogenic amine receptor antagonists mepyramine, phentolamine, methysergide and ketanserin. Following systemic administration, all three agents suppressed biting/licking behaviors, had no effect on flinching behaviors, and suppressed paw swelling induced by 5% formalin to varying degrees. These results provide evidence for a peripherally mediated antinociceptive action of dextromethorphan in the rat formalin test, but this may not necessarily be due to block of excitatory amino acid receptors as it is not observed with memantine or ketamine. All three agents produce a peripherally mediated paw swelling, which is likely due to blockade of biogenic amine reuptake. Systemic administration of all three agents produces antinociceptive and anti-inflammatory actions that may be due to block of excitatory amino acid receptors in the spinal cord.

Painful ejaculation and urinary hesitancy in association with antidepressant therapy: relief with tamsulosin

Painful ejaculation has been reported in association with a variety of antidepressants such as the tricyclic antidepressants (TCAs, e.g. clomipramine, imipramine, desipramine, protriptyline, amoxapine), the selective serotonin reuptake inhibitors (SSRIs, e.g. fluoxetine), venlafaxine and the MAOIs. Apart from lowering the dose and changing the antidepressant, no strategies are available to treat this side effect. In this paper, painful ejaculation following the administration of reboxetine is described in two patients. Both patients were treated concomitantly with the selective alpha(1A)-adrenoceptor antagonist, tamsulosin. A re-challenge was performed in one patient. The Hamilton Depression Rating Scale (HAM-D), the American Urological Association symptom index, a (dis)satisfaction item score and the Udvalg for Kliniske Undersoegelser (UKU-side effect rating scale) were used to assess the treatment. Tamsulosin rapidly and completely resolved the painful ejaculation and urinary hesitancy in both patients. A re-challenge in one patient resulted in a prompt reappearance of both side effects. Tamsulosin resolved the problem of painful ejaculation in these patients; however, larger studies are needed to confirm these results.

Causative agent of vascular pain among photodegradation products of dacarbazine

The photodegradation products of the anticancer drug, dacarbazine, cause adverse reactions including local venous pain when injected intravenously. In this study, we attempted to identify which of these products is responsible. We synthesized or purchased five photodegradation products of dacarbazine (dimethylamine, 5-diazoimidazole-4-carboxamide (Diazo-IC), 4-carbamoylimidazolium-5-olate, 5-carbamoyl-2-(4-carbamoylimidazol-5-ylazo)imidazolium-5-olate and 2-azahypoxanthine) and examined the pain reaction induced by their intraperitoneal administration in mice using an abdominal stretching or constriction assay. Only Diazo-IC clearly induced pain reaction in mice in a dose-dependent manner, the other products caused no pain reaction. The threshold concentration for pain reaction in mice was estimated to be about 0.1 mg mL-1. While diclofenac sodium significantly reduced acetic-acid-induced pain reaction in mice, it did not influence those induced by Diazo-IC. This result suggests that the mechanism of Diazo-IC-induced pain is different from that of acetic-acid-induced inflammatory pain. Dacarbazine itself produced marked relaxation of rat thoracic aorta strips in a concentration-dependent manner, but there was no difference between the activity of dacarbazine and its photo-exposed solution, so constriction or relaxation of blood vessels is unlikely to be a factor in the pain reaction. In conclusion, Diazo-IC generated by photodegradation of dacarbazine solution causes the side-effect of venous pain. Dacarbazine solution that has turned pink should not be used, because Diazo-IC is an intermediate in the formation of the reddish product, 5-carbamoyl-2-(4-carbamoylimidazol-5-ylazo)imidazolium-5-olate. Drip infusion preparations of dacarbazine should be shielded from light.

Comparison of effects of lidocaine hydrochloride, buffered lidocaine, diphenhydramine, and normal saline after intradermal injection

STUDY OBJECTIVE

To evaluate pain and the spread of analgesia when local anesthetics are given as an intradermal injection into the dorsal aspect of the hand.

DESIGN

Randomized, double-blinded, placebo-controlled study.

SETTING

University medical center.

PATIENTS

40 consenting adult volunteers.

INTERVENTIONS

Volunteers were randomly assigned to receive a 0.25-mL injection of either lidocaine hydrochloride (1%), buffered lidocaine, diphenhydramine (1%), or placebo (0.9% sodium chloride solution) into the dorsal aspect of both hands.

MEASUREMENTS

The volunteers used a visual analog scale to compare the pain of needle insertion and solution injection. Then at 1, 2, 5, 10, 20, and 30 minutes after intradermal injection, the extent of the analgesic area was marked on a strip of tape placed horizontally across the hand. Then at 32 minutes after intradermal injection, the extent of the analgesic area was marked on a strip of tape placed vertically across the hand. The volunteers were called each day and asked the duration of their numbness or hyperesthesia until their hands were no longer numb or sore.

MAIN RESULTS

Buffered lidocaine during intradermal infiltration was found to be significantly (p < 0.05) less painful than either lidocaine hydrochloride or diphenhydramine and equivalent to placebo. Diphenhydramine and lidocaine hydrochloride during intradermal infiltration induced significantly (p < 0.05) more pain than buffered lidocaine or placebo. Lidocaine hydrochloride displayed a significantly (p < 0.05) larger diameter of analgesia than placebo by 1 minute after the injection, buffered lidocaine by 2 minutes after injection, and diphenhydramine by 5 minutes after injection. By 20 minutes after injection, diphenhydramine diameter of analgesia was significantly (p < 0.05) larger than placebo but significantly less than buffered lidocaine. By 30 minutes after injection, diphenhydramine diameter of analgesia was equivalent to placebo whereas buffered lidocaine and lidocaine diameters were still significantly (p < 0.05) larger than placebo. Diphenhydramine injection resulted in numbness that lasted significantly (p < 0.05) longer than other study solutions whereas buffered lidocaine and lidocaine injections resulted in numbness that lasted significantly longer than placebo. Diphenhydramine injection resulted in hyperesthesia that lasted for 2 or more days in 12 of the volunteers.

CONCLUSION

There is a reduction of infiltration pain using buffered lidocaine as opposed to lidocaine and diphenhydramine. Although lidocaine injection resulted in a slightly faster spread of analgesic diameter, buffered lidocaine was equivalent to lidocaine from minute 2 until minute 30. Therefore, to obtain optimal anesthetic conditions, we recommend that buffered lidocaine be given 2 minutes before performing catheterization, whereas diphenhydramine should be given 5 minutes before catheterization, but only when buffered lidocaine cannot be used.

A randomised, double-blind trial comparing raloxifene HCl and continuous combined hormone replacement therapy in postmenopausal women: effects on compliance and quality of life

OBJECTIVE

To compare continuous combined hormone replacement therapy (ccHRT) and raloxifene with respect to compliance and quality of life, which were predefined secondary endpoints of a large, prospective study designed to investigate the uterine effects of both treatments.

DESIGN

Double-blind, randomised controlled trial of six-month duration.

SETTING

One hundred and twenty-nine gynaecology hospital departments, clinics or practices specialised in women's healthcare, located in Europe, South Africa and Israel.

POPULATION

Healthy postmenopausal women (n = 1008).

MAIN OUTCOME MEASURES

Changes in quality of life using the Women's Health Questionnaire (WHQ) and compliance using a compliance questionnaire and pill count. Adverse event and early discontinuation rates and satisfaction with treatment using a visual analogue scale (VAS).

RESULTS

Women taking raloxifene reported greater satisfaction with their treatment as assessed on the VAS (P = 0.004), and a lower proportion, as compared with ccHRT, reported being worried by the treatment (9.6% vs 20.2%, P < 0.01). Women taking ccHRT reported greater deterioration in scores from the WHQ for depressed mood and menstrual symptoms than those taking raloxifene (P < 0.01). For memory, vasomotor symptoms and sexual behaviour, the ccHRT group reported significantly greater mean improvements (P < 0.05). Over half (58.8%) of those taking raloxifene noticed no effect, 37.7% felt better and 3.4% felt worse as measured using the compliance questionnaire. Fifty percent of the women taking ccHRT felt better, 37.8% noticed no effect but over 10% felt worse. More women on raloxifene (94.6%) than on ccHRT (85.9%) reported that they were taking their double-blinded medication regularly (P < 0.01).

CONCLUSIONS

A lower rate of adverse event-related discontinuations, the lack of negative effects on quality of life and a smaller proportion of women being worried by the drug treatment were associated with higher treatment satisfaction and better compliance in postmenopausal women taking ccHRT or raloxifene.

Release of algesic substances in human experimental muscle pain

OBJECTIVE

We employed the 'delayed onset of muscle soreness' (DOMS) and the 'hypertonic saline' muscle pain models in combination with muscle microdialysis to evaluate the role of potentially algesic substances (lactate, glutamate, prostaglandin E2 (PGE2), nitric oxide (NO) and substance P (SP)) in the development of human muscle pain.

METHODS

DOMS was induced by 2 sets of 50 concentric/eccentric contractions of the calf muscles 24 h before the start of microdialysis. During microdialysis pain was stimulated through calf muscle contractions (dorsal and plantar flexions of the foot). Hypertonic saline was injected into the biceps muscle (5 x 200 microl 5.8% NaCl, 2 min interval) during dialysis. The calf (no treatment) and biceps (normal saline) of the other side was used as control.

RESULTS

Both models reliably induced muscle pain with similar intensities as assessed by visual analog scale. The DOMS exercise caused an increase of lactate in serum and the calf muscles of the DOMS leg. In addition, glutamate, PGE2 and substance P dialysate concentrations increased following contraction-induced pain stimulation (peak concentrations 125 +/- 20 microM, 239 +/- 45 pg/ml and 60 +/- 11 pg/ml for glutamate, PGE2 and SP, respectively). This increase did not occur in the control leg (peak concentrations 97 +/- 12 microM, 114 +/- 26 pg/ml and 46 +/- 9 pg/ml for glutamate, PGE2 and SP, respectively). Concentrations of nitric oxide were lower in the DOMS than control leg, particularly during the first 4h of microdialysis. Injection of hypertonic saline into the biceps muscle caused a significant increase of dialysate glutamate concentrations (peak 50 +/- 3 microM) whereas glutamate remained constant after injection of normal saline (mean 26 +/- 1 microM). Injection of hypertonic saline had no effect on lactate, PGE2 or NO levels.

CONCLUSION

Our data support the notion that an inflammatory reaction may be involved in muscle soreness following eccentric exercise, whereas the injection of hypertonic saline into the muscle probably directly stimulates muscle nociceptors and causes glutamate release.

Differential effects of NMDA and AMPA/KA receptor antagonists on c-Fos or Zif/268 expression in the rat spinal dorsal horn induced by noxious thermal or mechanical stimulation, or formalin injection

The involvement of N-methyl-D-aspartate (NMDA) and alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA)/kainate (KA) receptors in the induction of c-Fos and Zif/268 expression in spinal dorsal horn neurons following noxious thermal or mechanical stimulation, or formalin injection into the rat hind paw was examined by intrathecal administration of a competitive NMDA receptor antagonist, 2-amino-5-phosphonopentanoic acid (APV) or an AMPA/KA receptor antagonist, 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX), or both, 30 min prior to noxious stimulation. APV caused a significant reduction in the level of c-Fos expression in the superficial layer induced by each of these three noxious stimuli. The effects of APV on Zif/268 expression or of CNQX on c-Fos or Zif/268 expression in the superficial layer induced by these three noxious stimuli were dependent on the type of stimulus applied to the rat hind paw. The noxious thermal stimulus-evoked c-Fos expression level was reduced by APV and/or CNQX, while Zif/268 expression was hardly changed. Both c-Fos and Zif/268 expressions following formalin injection were reduced by APV alone and APV+CNQX, but not by CNQX alone. Zif/268 expression following noxious mechanical stimulation was significantly reduced only by APV+CNQX although APV or CNQX alone did not affect the expression, while c-Fos expression was reduced by APV and APV+CNQX but not by CNQX alone. These findings suggest that NMDA and AMPA/KA receptors are differentially involved in c-Fos and Zif/268 expression in the spinal dorsal horn following noxious thermal, formalin and mechanical stimulation.

Bioactivity studies on beta-sitosterol and its glucoside

Beta-sitosterol and beta-sitosteryl-beta-D-glucoside were isolated as analgesic constituents from the leaves of Mentha cordifolia Opiz. The acetic acid-induced writhing test showed that beta-sitosterol and beta-sitosteryl-beta-D-glucoside decreased the number of squirms induced by acetic acid by 70.0% and 73.0%, respectively, at a dose of 100 mg / kg mouse. Statistical analysis using the Kruskall Wallis one-way analysis of variance by ranks showed that these isolates approximate the analgesic activity of mefenamic acid at a 0.001 level of significance. The hot plate method confirmed their analgesic activities, as beta-sitosterol and beta-sitosteryl-beta-D-glucoside exhibited a 300% and 157% increase in pain tolerance, respectively, while mefenamic acid, a known analgesic, showed a 171% increase. Neither isolate exhibited antiinflammatory activity using the carrageenan-induced mouse paw oedema assay. Beta-sitosterol also exhibited anthelminthic and antimutagenic activities. In vitro tests using live Ascaris suum as test animals showed that the behaviour of worms treated with beta-sitosterol approximated that of the positive controls, Combantrin and Antiox. An in vivo micronucleus test showed that beta-sitosterol inhibited the mutagenicity of tetracycline by 65.3% at a dose of 0.5 mg /kg mouse. At the same dose, it did not exhibit chromosome-breaking activity.

Evaluation of the analgesic and anti-inflammatory activities of some thiazolo[4,5-d]pyrimidines

Some thiazolo[4,5-d]pyrimidine-7(6H)-one derivatives were evaluated in vivo for their analgesic and anti-inflammatory activities. The results were compared with that of acetyl salicylic acid and phenylbutazone. Compounds 3b and 3h were the most active in the anti-inflammatory paw edema inhibition test. In terms of the analgesic activity (acetic acid writhing test), the most active compound was 2a followed by 31. The most active members of the series were investigated for their ED50 values and ulcerogenic potential.

Reduced aversion to oral capsaicin following neurotoxic destruction of superficial medullary neurons expressing NK-1 receptors

Aversion to capsaicin (0.1-10 ppm) was assessed using a two-bottle paired preference paradigm, before and after intracisternal injection of substance P conjugated to saporin (SP-SAP) to ablate neurons in superficial medullary and cervical dorsal horn that express NK-1 receptors. Before SP-SAP, there was a concentration-dependent decrease in consumption of capsaicin with a threshold of 0.1-0.3 ppm. Following SP-SAP, significantly more capsaicin solution was consumed at 1- and 10-ppm concentrations. These results support a role for substance P in the mediation of high, but not low, levels of capsaicin-induced oral irritation.

Neurolytic blockade of the obturator nerve for intractable spasticity of adductor thigh muscles

Neurolytic blockade is one of the therapeutic possibilities to treat spasticity of various muscles. In patients with spasticity of the adductor thigh muscles, a percutaneous approach to the obturator nerve is often difficult. We describe a new approach to the obturator nerve and we examine its feasibility. The second objective was to assess the efficacy of obturator neurolysis for the management of adductor thigh muscle pain and spasticity associated with hemiplegia or paraplegia. Nerve blocks were performed via a combined approach using fluoroscopy and nerve stimulation to identify the obturator nerve. Neurolysis was performed by injection of 65% ethanol. We performed 27 blocks in 23 patients. Technical evaluation was achieved in terms of number of attempted needle insertions, time to accurate location of the nerve and success rate. The efficacy of the block was assessed using four scores: degree of alleviation of muscle spasm and triple flexion of the lower limb, improvement of gait and facilitation of hygienic care. Success rate of the technique was 100% with a time to accurate nerve location of 130+/-35 s. Compared with scores measured immediately before the block, all studied parameters were significantly improved. Efficiency was significant on adductor muscle spasticity (p<0.001 at 1 day and p<0.01 at 60 and 120 months). Triple flexion was also significantly improved (p<0.05 from 1 to 120 days), as well as gait (p<0.02) and hygiene (p<0.01) scores. No complications occurred. The combined approach of the obturator nerve represents a new technique which proved to be accurate, fast, simple, highly successful and reproducible. Obturator neurolysis was confirmed as an efficient and cost-effective technique to reduce adductor muscle spasm and related pain and to improve gait and hygienic care in patients with neurological sequelae of stroke, head trauma or any lesion of the motor neurone.

Effect of MK-801 on the antinociceptive effect of [D-Ala(2),N-MePhe(4), Gly-ol(5)]enkephalin in diabetic mice

The role of N-methyl-D-aspartate (NMDA) receptors in supraspinal and spinal sites on the reduction of supraspinal micro-opioid receptor-induced antinociception in diabetic mice was examined. The antinociceptive effect of i.c.v. [D-Ala(2), N-MePhe(4), Gly-ol(5)]enkephalin (DAMGO, 20 pmol) in diabetic mice was significantly less than that in non-diabetic mice. The antinociceptive effect of i.c.v. DAMGO (20 pmol) was significantly and dose dependently reduced by i.c.v. (+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-imine maleate (MK-801) in both non-diabetic (0.03-0.3 nmol) and diabetic mice (0.1-3.0 nmol). While the antinociceptive effect of i.c.v. DAMGO (10 pmol) was significantly enhanced by i.c.v NMDA (0.01-0.1 nmol) in non-diabetic mice, the same doses of i.c.v. NMDA had no significant effect on the antinociceptive effect of i.c.v. DAMGO (20 pmol) in diabetic mice. In non-diabetic mice, the antinociceptive effect of DAMGO (20 pmol, i.c.v.) was dose dependently reduced by intrathecal administration of MK-801 (0.1-1.0 nmol). The antinociceptive effect of DAMGO (20 pmol, i.c.v.) was dose-dependently enhanced by MK-801 (0.1-1.0 nmol, i.t.) in diabetic mice. Furthermore, NMDA (0.1 nmol, i.t.) significantly enhanced the antinociceptive effect of DAMGO (10 pmol, i.c.v.) in non-diabetic mice. However, in non-diabetic mice, the antinociceptive effect of DAMGO (40 pmol, i.c.v.) was dose dependently reduced by NMDA (0.03-0.3 nmol, i.t.). These results suggest that NMDA receptor function in supraspinal and spinal sites appear to be modulated differently by the diabetic state, and this functional modulation may play an important role in the reduction of supraspinal micro-opioid receptor-induced antinociception in diabetic animals.

Effect of sympathetic muscle vasoconstrictor activity on capsaicin-induced muscle pain

Neuropathic pain syndromes with sympathetically maintained pain are often associated with a deep somatic pain component. An adrenergic interaction between sympathetic vasoconstrictor neurons and cutaneous afferents has been demonstrated. To determine whether a sympathetic-afferent interaction exists in deep somatic tissues, we investigated the effect of sympathetic muscle vasoconstrictor activity on experimentally induced pain. In 12 healthy volunteers, capsaicin was infused into the anterior tibial muscle. Intensity and quality of muscle and referred pain were assessed. The analyses were performed during the presence of low sympathetic muscle vasoconstrictor activity induced by breathing 100% O(2) gas (normocapnia), and during high activity induced by inspiration of 95% O(2) and 5% CO(2) (hypercapnia). The degree of sympathetic muscle vasoconstrictor discharge was monitored indirectly by measuring systemic blood pressure and end-expiratory CO(2) and by performing duplex sonography of muscle resistance vessels. The intensity, quality, and spatial distribution of muscle and referred pain were not significantly different during resting and increased sympathetic muscle vasoconstrictor discharge, indicating that such activity does not influence pain after intramuscular infusion of capsaicin.

Antinociceptive and antidiarrhoeal activity of Zanthoxylum rhetsa

The methanolic extract of the Zanthoxylum rhetsa Roxb. stem bark, given by oral route to mice at doses of 250 and 500 mg/kg, significantly reduced the abdominal contraction induced by acetic acid and the diarrhoeal episodes induced by castor oil in mice.

Elevated spinal cyclooxygenase and prostaglandin release during hyperalgesia in diabetic rats

Diabetic rats display exaggerated hyperalgesic behavior in response to noxious stimuli that may model aspects of painful diabetic neuropathy. This study examined the contribution of spinal prostaglandin production to this exaggerated hyperalgesic behavior. Rats were implanted with spinal dialysis probes and received noxious stimulation to the hind paw by subcutaneous injection of 0.5% formalin solution. Prostaglandin E(2) (PGE(2)) was measured in dialysates of lumbar spinal cerebrospinal fluid concurrent with behavioral responses to formalin injection. In separate experiments, formalin-evoked behavioral responses were measured after intrathecal delivery of either a cyclooxygenase inhibitor or an EP(1) receptor antagonist, and cyclooxygenase protein was measured in spinal cord homogenates. Diabetic rats exhibited exaggerated behavioral responses to paw formalin injection and a concurrent prolongation of formalin-evoked PGE(2) release. Formalin-evoked behavioral responses were dose-dependently reduced in diabetic rats by spinal delivery of a cyclooxygenase inhibitor or an EP(1) receptor antagonist. Protein levels of cyclooxygenase-2 were elevated in the spinal cord of diabetic rats, whereas cyclooxygenase-1 protein was reduced. Hyperalgesic behavior in diabetic rats is associated with both increased cyclooxygenase-2 protein and cyclooxygenase-mediated PGE(2) release. Spinal delivery of selective inhibitors of cyclooxygenase-2 or antagonists of prostaglandin receptors may have therapeutic potential for treating painful diabetic neuropathy.

Pronociceptive effects of spinal dynorphin promote cannabinoid-induced pain and antinociceptive tolerance

Recent studies indicate that sustained opioid administration produces increased expression of spinal dynorphin, which promotes enhanced sensitivity to non-noxious and noxious stimuli. Such increased "pain" may manifest behaviorally as a decrease in spinal antinociceptive potency. Here, the possibility of similar mechanisms in the antinociception of spinal cannabinoids was explored. Response thresholds to non-noxious mechanical and noxious thermal stimuli were assessed. Antinociception was determined using the 52 degrees C tail-flick test. Mice received repeated WIN 55,212-2, its inactive enantiomer, WIN 55,212-3 or vehicle (i.th., bid, 5 days). WIN 55,212-2, but not WIN 55,212-3 or vehicle, produced a time-related increased sensitivity to non-noxious and noxious stimuli. WIN 55,212-2, but not WIN 55,212-3 or vehicle, elicited a significant increase in lumbar spinal dynorphin content at treatment day 5. Increased sensitivity to mechanical and thermal stimuli produced by WIN 55,212-2 was reversed to baseline levels by i.th. MK-801 or dynorphin antiserum; control serum had no effect. WIN 55,212-2, but not WIN 55,212-3 or vehicle, produced dose-related antinociception and repeated administration resulted in antinociceptive tolerance. While MK-801 and dynorphin antiserum did not alter acute antinociception produced by WIN 55,212-2, these substances significantly blocked antinociceptive tolerance when given immediately prior to WIN 55,212-2 challenge on day 5. Daily MK-801 pretreatments, prior to WIN 55,212-2 injection, also produced a significant block of antinociceptive tolerance. These data suggest that like opioids, repeated spinal administration of a cannabinoid CB1 agonist elicits abnormal pain, which results in increased expression of spinal dynorphin. Manipulations that block cannabinoid-induced pain also block the behavioral manifestation of cannabinoid tolerance.

Modulation of trigeminal laser evoked potentials and laser silent periods by homotopical experimental pain

Cutaneous laser stimulation activates predominantly the A-delta and C mechano-heat nociceptors. Applied to the perioral region, low intensity CO(2)-laser pulses evoke reproducible trigeminal cortical evoked potentials (LEPs). High intensity CO(2)-laser stimuli induce a reflex response in the contracted jaw-closing muscle, the so-called laser silent period (LSP). Both LEPs and LSP provide a useful tool to study the physiology of the trigeminal nociceptive system. In ten healthy subjects we recorded the subjective ratings of the perioral laser stimulation and the trigeminal LEPs and LSP before, during and after homotopic experimental tonic muscle (infusion of hypertonic saline into the masseter muscle) and tonic skin pain (topical application of capsaicin to the cheek). LEPs were recorded from the vertex at two stimulus intensities: low (1.1 x pain threshold, PTh) and high (1.5 x PTh). LSP from masseter and temporalis muscles were recorded bilaterally through surface electromyographic (EMG) electrodes. CO(2)-laser pulses were applied to the perioral region (V2/V3) on the painful and non-painful side. The amplitude of LEPs increased with higher stimulus intensities (P<0.0001), but were suppressed by 42.3+/-5.3% during experimental muscle pain (P<0.0001) and by 41.6+/-3.2% during skin pain (P<0.0001). No pain-related effects were observed for the N and P latency of the LEPs (P> 0.20). The LSP in the masseter and temporalis muscles had similar onset-latency (80+/-5 ms), offset-latency (111+/-5 ms) and duration (31+/-4 ms). Experimental pain had no effect on the onset- and offset-latency (P>0.05). Experimental pain, whether from muscle or from skin, reduced the degree of suppression (P<0.01) and the area under the EMG curve (P< 0.005) of the LSP. The LSP was still suppressed during the post-pain recordings when the skin pain had disappeared (P<0.05). In all experiments experimental tonic pain decreased the subjective ratings of the perioral laser stimulation (P< 0.001). Experimental tonic pain, either from muscle or from skin, induced bilateral inhibitory effects on the trigeminal laser evoked potentials and brainstem reflex responses and on the subjective ratings of the laser pulses. These effects could be mediated through the activation of segmental and suprasegmental inhibitory systems that may function interdependently.