Systemic clonidine differentially modulates the abnormal reactions to mechanical and thermal stimuli in rats with peripheral mononeuropathy

Therapeutic effect of melittin on a rat model of chronic prostatitis induced by Complete Freund's Adjuvant

OBJECTIVES

The present study was aimed to establish a model of chronic prostatitis in rat with the use of intraprostatic injection of Complete Freund's Adjuvant, and to examine the anti-inflammatory and analgesic effects of melittin on the newly-developed chronic prostatic pain model.

METHODS

Adult male Sprague-Dawley rats were injected with Complete Freund's Adjuvant (CFA) into the prostate. Twelve days after model rats of the treatment group were injected melittin into the prostate, while those of the control group received sterile saline injection. The nociceptive effects of CFA were evaluated by using a behavior approach (i.e. mechanical pain threshold measurement) on the day of CFA injection and 6, 12, and 18days after CFA injection. After the in-live study was done, the prostate was collected for histological examination of inflammatory cell infiltration. Levels of cyclooxygenase (COX)-2 in prostate and glial fibrillary acidic protein (GFAP) in spinal cord were determined using immunohistochemistry. Rats of the sham control group received intraprostatic injection of sterile saline and were studied using the same methods RESULTS: Intraprostatic CFA injection induced local allodynia that lasted over at least 2 weeks. The pain behavior of rat was associated with increases in inflammatory cell infiltration into the prostate. Levels of COX-2 in prostate and GFAP in spinal cord were also elevated. Treatment with melittin significantly raised pain threshold, decreased inflammatory infiltrates, and suppressed COX-2 and GFAP expression.

CONCLUSION

Intraprostatic injection of CFA induced neurogenic prostatitis and prostatic pain. The established model will be useful to the study of CP/CPPS pathogenesis. Melittin demonstrated profound anti-inflammatory and analgesic effects on the chronic prostatic pain model, suggesting melittin may hold promise as a novel therapeutic for treatment of CP/CPPS.

Effects of clonidine on bilateral pain behaviors and inflammatory response in rats under the state of neuropathic pain

This study was conducted to investigate the effects of clonidine on bilateral pain behaviors and inflammatory responses in neuropathic pain induced by partial sciatic nerve ligation (PSNL), and to better understand whether the antinociception of clonidine was related to α(2)-adrenoceptor mechanisms. Rats were divided randomly into five groups: sham-operation with saline, i.p.; PSNL with clonidine (0.2mg/kg) or saline, i.p.; PSNL with yohimbine (2mg/kg) followed by clonidine (0.2mg/kg), i.p.; and PSNL with naloxone (0.3mg/kg) followed by clonidine (0.2mg/kg), i.p. On post-operative days 1, 3, 7, 14, and 21, both ipsilateral and contralateral pain behaviors were measured. In rats receiving antagonists, bilateral behavioral changes were measured on day 14. Bilateral paw pressure threshold and paw withdrawal latencies were measured, and the extent of glial activation was dertermined by measuring macrophage antigen complex-1 (Mac-1) and glial fibrillary acidic protein (GFAP). Additionally, the levels of tumor necrosis factor α (TNF-α) and interleukin (IL)-6 were determined. PSNL induced bilateral behavioral hyperalgesia, with the ipsilateral level displaying a higher extent of behavior changes than the contralateral side. In addition, the glial activation markers and cytokine production were augmented bilaterally. Clonidine caused significant attenuation of bilateral mechanical allodynia and thermal hyperalgesia, accompanied by inhibition of glial activation and the expression of cytokines. The effects of clonidine were blocked by the α(2)-adrenoceptor antagonist yohimbine and partially reversed by the μ-opioid receptor antagonist naloxone. These data suggest that the bilateral antinoceptive effects of clonidine might mediate through immunomodulation by acting on α(2)-adrenoceptor in rats undergoing neuropathic pain.

A comparison of sedation with dexmedetomidine or propofol during shockwave lithotripsy: a randomized controlled trial

BACKGROUND

Dexmedetomidine, because it has both sedative and analgesic properties, may be suitable for conscious sedation during painful procedures. Extracorporeal shockwave lithotripsy (ESWL) is a minimal to mildly painful procedure that requires conscious sedation. We thus evaluated the utility of dexmedetomidine compared with propofol during an ESWL procedure.

METHODS

Forty-six patients were randomly allocated into two groups to receive either dexmedetomidine or propofol for elective ESWL. Dexmedetomidine was infused at 6 microg x kg(-1) x h(-1) for 10 min followed by an infusion rate of 0.2 microg x kg(-1) x h(-1). Propofol was infused at 6 mg x kg(-1) x h(-1) for 10 min followed by an infusion of 2.4 mg x kg(-1) x h(-1). Fentanyl 1 microg/kg IV was given to all patients 10 min before ESWL. Pain intensity was evaluated with a visual analog scale at 5-min intervals during ESWL (10-35 min). Sedation was determined using the Observer's Assessment of Alertness/Sedation. The Observer's Assessment of Alertness/ Sedation scores and hemodynamic and respiratory variables were recorded regularly during ESWL (35 min) and up to 85 min after.

RESULTS

Forty patients were evaluated. Visual analog scale values with dexmedetomidine were significantly lower than those with propofol only at the 25-35 min assessments (P < 0.05). During sedation, the respiratory rate with dexmedetomidine was significantly slower but Spo2 was significantly higher than with propofol (P < 0.05). Other clinical variables were similar (P > 0.05).

CONCLUSION

A combination of dexmedetomidine with fentanyl can be used safely and effectively for sedation and analgesia during ESWL.

Sympathetic vasoconstrictor antagonism and vasodilatation relieve mechanical allodynia in rats with chronic postischemia pain

Chronic pain that responds to antisympathetic treatments and alpha-adrenergic antagonists is clinically referred to as sympathetically maintained pain. Animal models of neuropathic pain have shown mixed results in terms of antinociceptive effectiveness of antisympathetic agents. The effectiveness of these agents have not been yet investigated in animal models of complex regional pain syndrome-type 1 (CRPS-I). In this study, we examined the effectiveness of antisympathetic agents and sympathetic vasoconstrictor antagonists, as well as agents that are vasodilators, in relieving mechanical allodynia in a recently developed animal model of CRPS-I (chronic postischemia pain or CPIP) produced by 3 hours of hind paw ischemia-reperfusion injury. Systemic guanethidine, phentolamine, clonidine, and prazosin are effective in reducing mechanical allodynia particularly at 2 days after reperfusion, and less so at 7 days after reperfusion. A nitric oxide donor vasodilator, SIN-1, also reduces mechanical allodynia more effectively at 2 days after reperfusion, but not at 7 days after reperfusion. These results suggest that the pain of CPIP, and possibly also CRPS-I, is relieved by reducing sympathetically mediated vasoconstriction, or enhancing vasodilatation.

PERSPECTIVE

The results of this study indicate that sympathetic block, or administration of alpha(1)-adrenergic antagonists, clonidine, or a nitric oxide donor, relieve allodynia in an animal model of CRPS-I. Thus, the pain of CRPS-I may depend on enhanced vasoconstrictor responsiveness, which may be relieved by blocking sympathetic efferent-dependent vasoconstriction, or by enhancing nitric oxide-dependent vasodilatation.

Intraoperative infusion of dexmedetomidine reduces perioperative analgesic requirements

PURPOSE

This prospective, randomized, double-blind study was designed to assess whether intraoperative infusion of dexmedetomidine provides effective postoperative analgesia. Postoperative pain scores and morphine consumption were compared in a treated group and a placebo group, both of which received patient-controlled morphine after total abdominal hysterectomy.

METHODS

Fifty women were randomly assigned to two groups. Group D (n = 25) received a loading dose of dexmedetomidine 1 mug.kg(-1) iv during induction of anesthesia, followed by a continuous infusion at a rate of 0.5 mug.kg(-1).hr(-1) throughout the operation. Group P (n = 25) received a volume-matched bolus and infusion of placebo (0.9% saline). For each case, heart rate, peripheral oxygen saturation, and systolic and diastolic blood pressure were recorded intraoperatively and for 48 hr postoperatively. Patients used a patient-controlled analgesia device to receive bolus doses of morphine after surgery. Total morphine consumption, pain scores, and sedation scores were recorded for the first 48 hr (two hours in the postanesthesia care unit and 46 hr on the ward).

RESULTS

The groups were similar with respect to mean times to extubation of the trachea. Pain and sedation scores were also similar between groups at all corresponding times throughout the 48-hr period of observation. Group D patients consumed significantly less morphine in the postanesthesia care unit and on the ward (P < 0.05 and P < 0.01, respectively). Fewer patients in Group D experienced itching or nausea/vomiting (P < 0.05).

CONCLUSION

Continuous iv dexmedetomidine during abdominal surgery provides effective postoperative analgesia, and reduces postoperative morphine requirements without increasing the incidence of side effects.

Activation of alpha2-adrenoceptors in the trigeminal region produces sex-specific modulation of nociception in the rat

Sex-related differences in the sensitivity to pain and in the response to analgesics have been reported including higher perceptual responses to experimentally induced pain and the higher prevalence of many pain syndromes in women compared with men. This study examines whether alpha2-adrenoceptor-mediated antinociceptive effects are reduced by estrogen which could account for the sex-related differences in pain perception and modulation. Clonidine, an alpha2-adrenoceptor agonist, has been shown to inhibit noxious stimulus-evoked nociceptive behavior as well as the responses of nociceptive neurons in the medullary dorsal horn. Intracisternal microinjection of clonidine (7 microg/5 microl) through the implanted PE-10 cannulae dorsal to the trigeminal region in male, ovariectomized (OVX), and diestrous (DiE) Sprague-Dawley rats produced a strong antinociceptive effect on N-methyl-D-aspartic acid (NMDA)-induced nociceptive scratching behavior and heat-induced face withdrawal nociceptive tests. However, it failed to produce any inhibition in the estradiol-treated ovariectomized (OVX+E) group regardless of the dose of estradiol (1, 10 or 100 microg/100 microl sesame oil) or in the proestrous (ProE) group. Further, clonidine produced dose-dependent effects in male and OVX groups but not in the OVX+E group on the NMDA-induced nociceptive behavior. Finally, the effect of clonidine was reversed by yohimbine, an alpha2-adrenoceptor antagonist, in male and OVX groups on thermal nociceptive test. These results lead us to conclude that activation of alpha2-adrenoceptors produces sex-specific, estrogen dependent modulation of nociception in the trigeminal region of the rat. A decreased alpha2-adrenoceptor-mediated inhibition could be one of the factors responsible for the higher prevalence of pain syndromes in females.

The effects of alpha2-adrenoceptor agents on anti-hyperalgesic effects of carbamazepine and oxcarbazepine in a rat model of inflammatory pain

In this study, the effects of yohimbine (alpha2-adrenoceptor antagonist) and clonidine (alpha2-adrenoceptor agonist) on anti-hyperalgesia induced by carbamazepine and oxcarbazepine in a rat model of inflammatory pain were investigated. Carbamazepine (10-40 mg/kg; i.p.) and oxcarbazepine (40-160 mg/kg; i.p.) caused a significant dose-dependent reduction of the paw inflammatory hyperalgesia induced by concanavalin A (Con A, intraplantarly) in a paw pressure test in rats. Yohimbine (1-3 mg/kg; i.p.) significantly depressed the anti-hyperalgesic effects of carbamazepine and oxcarbazepine, in a dose- and time-dependent manner. Both drug mixtures (carbamazepine-clonidine and oxcarbazepine-clonidine) administered in fixed-dose fractions of the ED50 (1/2, 1/4 and 1/8) caused significant and dose-dependent reduction of the hyperalgesia induced by Con A. Isobolographic analysis revealed a significant synergistic (supra-additive) anti-hyperalgesic effect of both combinations tested. These results indicate that anti-hyperalgesic effects of carbamazepine and oxcarbazepine are, at least partially, mediated by activation of adrenergic alpha2-receptors. In addition, synergistic interaction for anti-hyperalgesia between carbamazepine and clonidine, as well as oxcarbazepine and clonidine in a model of inflammatory hyperalgesia, was demonstrated.

A Behavioral and Pharmacological Validation of the Acetone Spray Test in Gerbils with a Chronic Constriction Injury

Cold and mechanical allodynia are important symptoms in patients with neuropathic pain. The study of cold allodynia in animals can help us to understand the underlying pathophysiological mechanisms of neuropathic pain and to validate drugs. The evaluation of cold allodynia in gerbils with a chronic constriction injury of the sciatic nerve on the cold plate is not always stable. We developed a new application method of acetone using a specific spray technique with an Eppendorf multistepper pipette. The chronic constriction injury of the sciatic nerve in gerbils resulted in a long-lasting mechanical and acetone spray-induced hyperreactivity throughout the testing period, which is clearly different from what was seen in sham-operated animals. The acetone spray test incorporates a multimodal stimulus different from direct cold stimulation. The reactivity to the acetone spray coincides in time and strength with the reactivity observed in mechanical allodynia in gerbils and with mechanical and thermal allodynia in other species. Furthermore, a pharmacological validation of the acetone spray test by different reference compounds was performed. Different compounds effective in neuropathic pain models in rodents influence the hyperreactivity to the acetone spray after acute and chronic administration. This study indicates that the multimodal acetone spray test is a valuable tool in the study of neuropathic pain in rodents.

IMPLICATIONS

The acetone spray test is a multimodal and valuable tool in the evaluation of neuropathic pain behavior in gerbils.

Effects of the novel analgesic, cizolirtine, in a rat model of neuropathic pain

Cizolirtine (5-9[(N,N-dimethylaminoethoxy)phenyl]methyl0-1-methyl-1H-pyrazol citrate) is a centrally acting analgesic with a currently unknown mechanism of action, whose efficacy has been demonstrated in various models of acute and inflammatory pain in rodents. Further studies were performed in order to assess its potential antinociceptive action in a well-validated model of neuropathic pain, i.e. that produced by unilateral sciatic nerve constriction in rats. Animals were subjected to relevant behavioural tests based on mechanical (vocalization threshold to paw pressure) and thermal (struggle latency to paw immersion in a cold (10 degrees C) water bath) stimuli, 2 weeks after sciatic nerve constriction, when pain-related behaviour was fully developed. Acute pretreatment with 2.5-10 mg/kg p.o. of cizolirtine reversed both mechanical and thermal allodynia. These effects were antagonized by prior injection of the alpha(2)-adrenoceptor antagonist idazoxan (0.5 mg/kg i.v.), but not the opioid receptor antagonist naloxone (0.1 mg/kg i.v.). On the other hand, cizolirtine (10 mg/kg p.o.) produced no motor deficits in animals using the rotarod test. Our study showed that cizolirtine suppressed pain-related behavioural responses to mechanical and cold stimuli in neuropathic rats, probably via an alpha(2)-adrenoceptor-dependent mechanism. These results suggest that cizolirtine may be useful for alleviating some neuropathic somatosensory disorders, in particular cold allodynia, with a reduced risk of undesirable side effects.

Electrophysiologic characterization of the antinociceptive actions of S18616, a novel and potent alpha2-adrenoceptor agonist, after acute and persistent pain states

alpha (2)-Adrenoceptor (AR) agonists are active in behavioral models of persistent pain involving tissue and nerve damage. We evaluated the spinal effect of a novel, potent, and selective alpha (2)-AR agonist, [7,8](2-chlorobenzo)-2-amino-1-aza-3-oxa[4,5]spirodeca-1,7-diene (S18616), on the responses of dorsal horn neurons in halothane-anesthetized rats. Intrathecal administration of S18616 (0.1 to 3.0 microg) dose-dependently suppressed C- and A delta-fiber evoked responses but not the A beta-fiber evoked response. Drug effects were reversed by the alpha (2)-AR antagonists, atipamezole and idazoxan (100 microg). In rats with unilateral spinal nerve (L5-L6) ligation performed 2 weeks before study, S18616 (0.1 to 3.0 microg) dose-dependently suppressed the C- and A delta-fiber evoked responses and blocked "wind-up" in these neurons. The potency was comparable between nerve-injured and sham-operated rats, and S18616 was equally effective against responses to thermal and high-intensity mechanical stimuli. Interestingly, the effectiveness of S18616 on the low-intensity mechanical evoked response was significantly enhanced after nerve injury. Finally, S18616 (0.3 and 3.0 microg) reduced the neuronal responses produced by intraplantar injection of formalin. In conclusion, S18616 dose-dependently and potently inhibits the responses of dorsal horn neurons to peripheral stimulation in normal, inflamed, and neuropathic rats. These data support the use of spinal S18616 and other alpha (2)-AR agonists in the management of clinical pain.

Descending control of pain

Upon receipt in the dorsal horn (DH) of the spinal cord, nociceptive (pain-signalling) information from the viscera, skin and other organs is subject to extensive processing by a diversity of mechanisms, certain of which enhance, and certain of which inhibit, its transfer to higher centres. In this regard, a network of descending pathways projecting from cerebral structures to the DH plays a complex and crucial role. Specific centrifugal pathways either suppress (descending inhibition) or potentiate (descending facilitation) passage of nociceptive messages to the brain. Engagement of descending inhibition by the opioid analgesic, morphine, fulfils an important role in its pain-relieving properties, while induction of analgesia by the adrenergic agonist, clonidine, reflects actions at alpha(2)-adrenoceptors (alpha(2)-ARs) in the DH normally recruited by descending pathways. However, opioids and adrenergic agents exploit but a tiny fraction of the vast panoply of mechanisms now known to be involved in the induction and/or expression of descending controls. For example, no drug interfering with descending facilitation is currently available for clinical use. The present review focuses on: (1) the organisation of descending pathways and their pathophysiological significance; (2) the role of individual transmitters and specific receptor types in the modulation and expression of mechanisms of descending inhibition and facilitation and (3) the advantages and limitations of established and innovative analgesic strategies which act by manipulation of descending controls. Knowledge of descending pathways has increased exponentially in recent years, so this is an opportune moment to survey their operation and therapeutic relevance to the improved management of pain.

Systemic tizanidine hydrochloride (Zanaflex) relieves thermal hyperalgesia in rats with an experimental mononeuropathy

We sought to determine whether tizanidine, an alpha2-agonist, relieved thermal hyperalgesia in rats with surgically induced neuropathic pain. We used a Sprague-Dawley rat model in which a chronic constriction of the sciatic nerve caused the rats to develop postural changes, mechanical allodynia, and thermal hyperalgesia. Thermal hyperalgesia was verified through paw withdrawal latency (PWL). PWL was tested before surgery, after surgery, and after injections with tizanidine (0.5, 1.0, or 2.0 mg/kg) or normal saline. Ambulatory and total movements were evaluated by placing the rats in activity cages. Thermal hyperalgesia was induced in all rats after surgery. Tizanidine, but not saline, caused a significant improvement in PWL (P < 0.05), with complete reversal of thermal hyperalgesia at all doses on postoperative Day 6. Rats who received tizanidine 2 mg/kg maintained complete reversal of thermal hyperalgesia through postoperative Day 9. Some sedation was observed with tizanidine 2 mg/kg, but not with smaller doses. We conclude that tizanidine effectively reversed thermal hyperalgesia in a rat model.

IMPLICATIONS

This study was conducted to determine whether tizanidine could attenuate the thermal hyperalgesia that occurs in rats with surgically induced chronic constriction of the sciatic nerve. Tizanidine was effective in reducing sensitivity to heat, as measured by paw withdrawal latency, and did not cause sedation at smaller doses.

α-1 and α-2 Adrenergic Antagonists Relieve Thermal Hyperalgesia in Experimental Mononeuropathy from Chronic Constriction Injury

Phentolamine, a nonspecific alpha 1- and alpha 2-adrenergic antagonist, relieves pain in patients with reflex sympathetic dystrophy. We sought to determine whether phentolamine, prazosin (alpha 1 antagonist), or SKF86466 (alpha 2 antagonist) relieve thermal hyperalgesia in rats with neuropathic pain. Four days after producing a chronic constriction injury (CCI), thermal hyperalgesia was tested by measuring paw withdrawal latency (PWL). After injection of phentolamine, prazosin, or SKF86466 each at doses of 1, 2, or 5 mg/kg, PWL tests were measured at 5 min and repeated at 15-min intervals for 1 h. Phentolamine, prazosin, and SKF86466 1, 2, and 5 mg/kg provided statistically significant analgesia in rats with CCI for at least 65 min. PWL did not return to baseline levels after 1 or 2 mg/kg of prazosin or SKF86466 but did so after 35 min after phentolamine 2 mg/kg. After 5 mg/kg, PWL returned to preoperative values between 5 and 50 min for phentolamine, at 35 and 65 min for prazosin, and at 50 min for SKF86466. We conclude that both alpha1 and alpha2 peripheral receptors of the sympathetic nervous system are involved in the thermal hyperalgesia caused by CCI and that thermal hyperalgesia can be reversed by both alpha1 and alpha2 antagonists in a dose-dependent manner.

The alpha(2A) adrenoceptor and the sympathetic postganglionic neuron contribute to the development of neuropathic heat hyperalgesia in mice

We have addressed the role of the sympathetic nervous system in the development and maintenance of neuropathic pain. Using a new neuropathic mouse model, we examined the development of hyperalgesia in transgenic mice lacking functional alpha(2A) adrenoceptors and in sympathectomized wild-type mice, to determine if sympathetic-sensory coupling generates hyperalgesia. The development of neuropathic heat hyperalgesia required the presence of both the alpha(2A) adrenoceptor and the sympathetic postganglionic neuron (SPGN), but the development of mechanical hyperalgesia did not require either the alpha(2A) adrenoceptor or the SPGN, indicating different mechanisms of sensitization. These results suggest that the development of neuropathic heat hyperalgesia, but not mechanical hyperalgesia, requires sympathetic-sensory coupling in the peripheral nervous system. Nerve injury enhanced the analgesic efficacy of the alpha(2) adrenoceptor agonist dexmedetomidine, and paradoxically also induced an analgesic response to alpha(2) adrenoceptor antagonists. The alpha(2) agonist-evoked analgesia to mechanical stimuli was mediated by activating central alpha(2A) adrenoceptors, possibly at the spinal level. The peripherally restricted alpha(2) antagonist L659,066 evoked analgesia for heat, but not for mechanical stimuli, findings which support the hypothesis that the peripheral alpha(2) adrenoceptor plays a role in both the development and the maintenance of neuropathic heat hyperalgesia. The alpha(2) antagonist-evoked analgesia for heat stimuli was mediated by blocking peripheral and probably central alpha(2) adrenoceptors, while the analgesia for mechanical stimuli was mediated by blocking central alpha(2A) adrenoceptors. Intradermal injections with an alpha(2) agonist or antagonist had no effect on nociceptive thresholds, indicating that sympathetic-sensory coupling at the level of the cutaneous nociceptor did not contribute to the maintenance of neuropathic hyperalgesia.

Antinociceptive effect of (S)-N-desmethyl trimebutine against a mechanical stimulus in a rat model of peripheral neuropathy

Trimebutine (2-dimethylamino-2-phenylbutyl 3,4,5-trimethoxybenzoate, hydrogen maleate) relieves abdominal pain in humans. In the present study, the antinociceptive action of systemic (S)-N-desmethyl trimebutine, a stereoisomer of N-monodesmethyl trimebutine, the main metabolite of trimebutine in humans, was studied in a rat model of neuropathic pain produced by chronic constriction injury to the sciatic nerve. Mechanical (vocalization threshold to hindpaw pressure) stimulus was used. Experiments were performed two weeks after surgery when the pain-related behaviour has fully developed. (S)-N-desmethyl trimebutine (1, 3, 10 mg/kg s.c.) produced dose-dependent antinociceptive effects on the nerve-injured and the contralateral hindpaw. The effect of the lowest dose (1 mg/kg s.c.) of (S)-N-desmethyl trimebutine on the nerve-injured paw was equal to that seen after a ten time stronger dose on the contralateral paw. The effect of (S)-N-desmethyl trimebutine (1 mg/kg) was not naloxone reversible. The results suggest that systemic (S)-N-desmethyl trimebutine may be useful in the treatment of some aspects of neuropathic pain.

The antinociceptive effect of combined systemic administration of morphine and the glycine/NMDA receptor antagonist, (+)-HA966 in a rat model of peripheral neuropathy

1. We evaluated the ability of the functional antagonist at the glycine site of the N-methyl-D-aspartate (NMDA) receptor complex, (+)-(1-Hydroxy-3-aminopyrrolodine-2-one) ((+)-HA966), to modulate the antinociceptive action of systemic morphine in a rat model of neuropathic pain produced by chronic constriction injury to the sciatic nerve. Mechanical (vocalization threshold to hindpaw pressure) and thermal (struggle latency to hindpaw immersion into a water bath) stimuli were used. 2. In the mechanical test, morphine (0.05, 0.1 and 0.3 mg kg(-1), i.v.) alone produced dose-dependent effects in both neuropathic and uninjured rats. Likewise, morphine (0.1, 0.3 and 1 mg kg(-1), i.v.) dose-dependently increased struggle latencies of the nerve-injured hindpaw in the hot noxious (46 degrees C) test but was ineffective in the non-noxious warm (44 degrees C) and cold (10 degrees C) test. 3. Pretreatment with (+)-HA966 (2.5 mg kg(-1), s.c.) dose-dependently enhanced the effect of morphine in the mechanical test with the relative potency being nerve-injured hindpaw > contralateral hindpaw > uninjured rat. 4. Likewise, (+)-HA966 dose-dependently enhanced the effect of morphine against a hot (46 degrees C) stimulus and produced, in combination with morphine, a dose-dependent effect against a warm (44 degrees C) stimulus. In the cold (10 degrees C) test, (+)-HA966 reversed the ineffectiveness of the highest dose of morphine. 5. Naloxone blocked the effect of the combination of (+)-HA966 with morphine in all tests. The drug combination produced no motor deficits in animals using the rotarod test. 6. These findings suggest that combined administration of antagonists, acting at the glycine site of the NMDA receptor complex and morphine may be a promising approach in the treatment of neuropathic and acute pain.

alpha2-adrenoceptors modulate NMDA-evoked responses of neurons in superficial and deeper dorsal horn of the medulla

Extracellular single unit recordings were made from neurons in the superficial and deeper dorsal horn of the medulla (trigeminal nucleus caudalis) in 21 male rats anesthetized with urethan. NMDA produced an antagonist-reversible excitation of 46 nociceptive as well as nonnociceptive neurons. Microiontophoretic application of a preferential alpha2-adrenoceptor (alpha2AR) agonist, (2-[2, 6-dichloroaniline]-2-imidazoline) hydrochloride (clonidine), reduced the NMDA-evoked responses of 86% (6/7) of nociceptive-specific (NS) neurons, 82% (9/11) of wide dynamic range (WDR) neurons, and 67% (4/6) of low-threshold (LT) neurons in the superficial dorsal horn. In the deeper dorsal horn, clonidine inhibited the NMDA-evoked responses of 94% (16/17) of NS and WDR neurons and 60% (3/5) of LT neurons. Clonidine facilitated the NMDA-evoked responses in 14% (1/17) of NS, 9% (1/11) of WDR, and 33% (2/6) of LT neurons in the superficial dorsal horn. Idazoxan, an alpha2AR antagonist, reversed the inhibitory effect of clonidine in 90% (9/10) of neurons, whereas prazosin, an alpha1-adrenoceptor antagonist with affinity for alpha2BAR, and alpha2CAR, were ineffective. We suggest that activation of alpha2ARs produces a predominantly inhibitory modulation of the NMDA-evoked responses of nociceptive neurons in the medullary dorsal horn.

The analgesic potency of dexmedetomidine is enhanced after nerve injury: a possible role for peripheral alpha2-adrenoceptors

This study investigated the analgesic potency and site of action of systemic dexmedetomidine, a selective alpha2-adrenoceptor (alpha2AR) agonist, in normal and neuropathic rats. Ligation of the L5-6 spinal nerves produced a chronic mechanical and thermal neuropathic hyperalgesia in rats. von Frey fibers and a thermoelectric Peltier device were used to measure mechanical and heat withdrawal thresholds over the hindpaw. Systemic dexmedetomidine dose-dependently increased the mechanical and thermal thresholds in the control animals (50% effective dose [ED50] 144 and 180 microg/kg intraperitoneally [i.p.], respectively). Neuropathic animals responded to much smaller doses of dexmedetomidine with mechanical and thermal ED50 values of 52 and 29 microg/kg i.p., respectively. There was no difference between the control and neuropathic animals with respect to dexmedetomidine-evoked sedation, as determined by decreased grid crossings in an open-field activity chamber (ED50 12 and 9 microg/kg i.p., respectively). Atipamezole, a selective alpha2AR antagonist, blocked the analgesic and sedative actions of dexmedetomidine inboth the neuropathic and control animals. However, L-659,066, a peripherally restricted alpha2AR antagonist, could only block the analgesic actions of dexmedetomidine in the neuropathic rats, with no effect in control animals. In conclusion, nerve injury enhanced the analgesic but not the sedative potency of systemic dexmedetomidine and may have shifted the site of alpha2 analgesic action to outside the blood-brain barrier.

IMPLICATIONS

We tested the analgesic efficacy of the alpha2 agonist dexmedetomidine in normal and nerve-injured rats. The analgesic potency of dexmedetomidine was enhanced after nerve injury with a site of action outside the central nervous system. Peripherally restricted alpha2 agonists may be useful in the management of neuropathic pain.

Inhibition of a hyperpolarization-activated current by clonidine in rat dorsal root ganglion neurons

Whole cell voltage- and current-clamp recordings were carried out to investigate the effects of clonidine, an alpha 2-adrenoceptor agonist, in L4 and L5 dorsal root ganglion (DRG) neurons of the rat. In voltage-clamp mode, application of 20 microM clonidine reversibly reduced the inward current evoked by hyperpolarizing voltage steps. The "clonidine-sensitive current" was obtained by subtracting the current during clonidine application from the control current, and its properties were as follows. 1) It was a slowly activating inward current evoked by hyperpolarization. 2) The reversal potential in the standard extracellular solution ([K+]o = 5 mM, [Na+]o = 151 mM) was -38.3 mV, and reduction of [Na+]o shifted it to a more negative potential, whereas an increase of [K+]o shifted it to a more positive potential, indicating that the current was carried by Na+ and K+ (PNa/PK = 0.22). 3) The relationship between the chord conductance underlying the clonidine-sensitive current and voltage could be fitted by a Boltzmann equation. These results indicate that the clonidine-sensitive current corresponds to a hyperpolarization-activated current (Ih), i.e., clonidine inhibits Ih in rat DRG neurons. DRG neurons were classified as small (15.9-32.9 microns diam), medium-sized (33-42.9 microns), and large (43-63.6 microns), and 7 of 19, 24 of 25, and 22 of 22 of these types exhibited Ih with mean +/- SE clonidine-induced inhibition values of 36.1 +/- 3.5% (n = 7), 43.1 +/- 3.7% (n = 24), and 35.1 +/- 2.7% (n = 22), respectively. Clonidine application to L4 and L5 DRG neurons excised from rats the sciatic nerves of which had been transected 14-35 days previously (transected DRG neurons) also reduced Ih. In current-clamp mode, 9 of 13 intact and 4 of 6 transected medium-sized DRG neurons that exhibited Ih responded to clonidine with hyperpolarization (> 2 mV). Some medium-sized DRG neurons exhibited repetitive action potentials in response to a depolarizing current pulse, and clonidine reduced the firing discharge frequencies in 8 of 11 intact and 3 of 4 transected neurons tested. Injection of a hyperpolarizing current pulse produced time-dependent rectification in DRG neurons that exhibited Ih, and clonidine blocked this rectification in all intact and transected neurons tested. These results suggest that inhibition of Ih due to alpha 2-adrenoceptor activation contributes to modulation of DRG neuronal activity in rats. On the basis of our findings, we discuss the possible mechanisms whereby sympathetically released norepinephrine modulates the abnormal activity of DRG neuronal cell bodies after nerve injury.

The cold plate as a test of nociceptive behaviors: description and application to the study of chronic neuropathic and inflammatory pain models

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.

Peripherally administered alpha2-adrenoceptor agonist in the modulation of chronic allodynia induced by spinal nerve ligation in the rat

We studied whether a peripherally administered alpha2-adrenoceptor agonist modulates mechanical allodynia caused by unilateral ligation of two spinal nerves in the rat. Medetomidine, an alpha2-adrenoceptor agonist, atipamezole, an alpha2-adrenoceptor antagonist, or saline (control) was administered into the footpad of either the allodynic or the contralateral hindpaw. Medetomidine (1-10 microg/kg in 50 microL) reversed the unilateral allodynia in a dose-dependent fashion independent of the site of administration. At this dose range, medetomidine did not influence the heat-induced tail-flick response. The antiallodynic effect of medetomidine was completely reversed by a dose of atipamezole that alone was ineffective (30 microg/kg). At the largest dose used (100 microg/kg in 50 microL), atipamezole decreased the latency of the heat-induced tail flick and had an ipsilateral allodynic effect in the injected paw when administered into the control side. The atipamezole-induced mechanical allodynia was not attenuated by medetomidine. The results indicate that an alpha2-adrenoceptor agonist at a subantinociceptive dose may significantly attenuate allodynia produced by spinal nerve ligation. This antiallodynic effect is not due to a peripheral action, but rather to action on central (presumably spinal) alpha2-adrenoceptors. The allodynic effect of a high concentration of atipamezole (100 microg/kg in 50 microL) in the injected paw can be explained by peripheral nonadrenergic mechanisms (e.g., local irritation).

IMPLICATIONS

The present behavioral results indicate that a selective alpha2-adrenoceptor agonist at a subantinociceptive dose effectively attenuates mechanical allodynia induced by an experimental model of chronic neuropathy in the rat. This antiallodynic action can be explained by central, rather than peripheral, alpha2-adrenergic mechanisms.

In mononeuropathic rats, the enhancement of morphine antinociception by L-365,260, a selective CCK(B) receptor antagonist, depends on the dose of systemic morphine and stimulus characteristics

The ability of the selective cholecystokinin(B) (CCK(B)) receptor antagonist L-365,260 (0.2 mg/kg s.c.) to modulate the antinociceptive action of relatively low doses of systemic morphine (0.1, 0.3 and 1.0 mg/kg i.v.) was evaluated using a well established rat model of peripheral unilateral mononeuropathy. Behavioural tests based on both mechanical (vocalization threshold to paw pressure) and thermal (struggle latency after immersion of the paw into a cold (10 degrees C), warm (44 degrees C) or hot (46 degrees C) water bath) stimuli were used. Experiments were performed 2 weeks after the surgery when the pain-related behaviour has fully developed. We demonstrated a differential effect of L-365,260 depending both on the dose of morphine and the test used. Pretreatment with the CCK(B) receptor antagonist (0.2 mg/kg) inverted the ineffectiveness of the lowest dose (0.1 mg/kg i.v.) of morphine against the noxious (46 degrees C) thermal stimulus, and the effect of the combination was equal to that seen after the dose 0.3 mg/kg of morphine alone. Likewise, in the mechanical test, the already enhanced effect of this dose (0.1 mg/kg) of morphine on the nerve-injured paw was further increased (by 4-fold) after L-365,260 pretreatment. These effects were abolished by naloxone (0.01 mg/kg i.v.). However, the effects of the higher doses (0.3 and 1.0 mg/kg i.v.) of morphine against the mechanical or noxious thermal stimuli were not significantly enhanced by pretreatment with the CCK(B) receptor antagonist. Further, L-365,260 was found to be completely ineffective in modulating the responses to morphine at 10 degrees C and at 44 degrees C.

Effects of intrathecal vs. systemic clonidine in treating chronic allodynia-like response in spinally injured rats

A chronic pain-like response to innocuous mechanical stimuli (allodynia) was observed in rats after severe spinal cord ischemia, which resembled some painful conditions observed in spinally injured patients. The present studies examined the effects of clonidine, an alpha 2-adrenoceptor agonist, on this allodynia-like response. Intrathecal (i.t.) clonidine dose-dependently relieved allodynia and doses up to 10 micrograms did not induce motor deficits or sedation, but slightly increased systemic blood pressure. The anti-allodynic effect of i.t. clonidine was reversed by the selective alpha 2-adrenoceptor antagonist atipamezole. In contrast, 50 and 100 micrograms/kg intraperitoneal (i.p.) clonidine did not relieve the chronic allodynia, although the higher dose induced some motor deficits and sedation. Allodynic behavior was abolished after 200 micrograms/kg, i.p. clonidine, which, however, caused strong sedative and motor impairment. The present data suggested that spinal, but not systemic, alpha 2-adrenoceptor agonists may have therapeutic value in treating mechanical allodynia in patients with neuropathic pain of spinal origin.

Influence of the sympathetic nervous system in the development of abnormal pain-related behaviours in a rat model of neuropathic pain

This study evaluated the effect of surgical sympathectomy on pain-related behaviours in a well established model of peripheral mononeuropathy produced by loose ligatures around the common sciatic nerve in the rat. Behavioural abnormalities include spontaneous abnormal position of the hindpaw after the nerve constriction, indicative of "spontaneous pain", and changes in responses to mechanical or thermal stimuli applied to this paw. These changes are usually maximal at week 2 after the surgery, stable until weeks 3-4, and disappear between weeks 8 and 12. To assess the role of the sympathetic nervous system in the development and persistence of these abnormalities, four groups of rats were behaviourally tested: (i) rats receiving a complete sham surgery, (ii) rats with a sciatic nerve constriction produced by loose ligatures around the common nerve trunk plus a sham sympathectomy, (iii) rats receiving a lumbar sympathectomy with a sham nerve ligature, and (iv) rats receiving a simultaneous surgical lumbar sympathectomy and a sciatic nerve constriction. The efficacy of the sympathectomy was assessed by the measure of the noradrenaline level in the sciatic nerve. Sympathectomy reduced selectively or even prevented the abnormal reaction to cold temperature and to heat (45 degrees C) in rats with a peripheral mononeuropathy. In contrast, the abnormal reaction to mechanical pressure was not influenced, and the behavioural abnormalities indicating spontaneous pain were still present. Sympathectomy alone resulted in a reduction of the vocalization threshold to pressure on both hindpaws, but also a short-lasting increased tolerance to cold immersion. This study confirms the selective role of the sympathetic nervous system in affecting the development and maintenance of some abnormal pain-related behaviours to thermal stimuli in rats with a moderate, but persistent, constriction of one sciatic nerve.