Thermal and mechanical antinociceptive action of spinal vs peripherally administered clonidine in the rat inflamed knee joint model

Effects of administration of α2 adrenergic receptor agonist into psoas major muscle on inflammatory pain induced by injection of complete Freund's adjuvant in rats

α₂ adrenergic agonists are widely used in clinical anesthesia and ICU sedation owing to their effective sedative and analgesic effects. Lumbago and leg pain is the most common clinical pain disease. Studies have reported that lumbago and leg pain is associated with dysregulation of paravertebral muscles, especially psoas major muscles. In the present study, a unilateral lower extremity chronic inflammation and pain model was established by subcutaneous administration of low-dose complete Fredrin’s adjuvant (CFA) into the posterior paw of rats. α₂ adrenalin receptor agonist was then injected into the psoas major muscle. Behavioral tests were conducted for 21 days. Psoas major muscle tissue was harvested for evaluation of biochemical indexes related to pain. The effect of α₂ adrenergic receptor agonist injected into psoas major muscle on chronic inflammatory pain of lower extremities in rats was explored. The results showed that injection of α₂ adrenergic receptor agonist into the psoas major muscle relieved CFA-induced mechanical hyperalgesia. Administration of α₂ adrenergic receptor antagonist yohimbine reversed the analgesic effect of α₂ adrenergic receptor agonists. Administration of dexmedetomidine into psoas major muscle downregulated the levels of norepinephrine, interleukin-6 and tumor necrosis factor-α in tissues. The findings of the present study show that administration of α₂ adrenoceptor agonists into the psoas major muscle relieves chronic inflammatory pain induced by CFA. Local injection of dexmedetomidine also exerted anti-inflammatory and anti-sympathetic effect by activating α₂-adrenoceptor in the psoas major muscle.

Adding low concentrations of clonidine to ropivacaine for transversus abdominis plane blocks does not reduce plasma ropivacaine levels, suggesting a lack of vasoconstrictor effect

Clonidine has been used successfully to prolong the duration of action of local anaesthetics in peripheral nerve blocks, but its mechanism of action in this setting remains unclear. Some studies suggest that clonidine exerts a vasoconstrictor effect, limiting the washout of local anaesthetic from its site of deposition. We investigated this potential vasoconstrictor effect, using plasma ropivacaine concentrations as a surrogate measure of vasoconstriction, in patients who received transversus abdominis plane (TAP) blocks with and without clonidine. Eighty women undergoing laparoscopic gynaecological surgery were randomly assigned to receive one of four TAP block solutions: 0.2% ropivacaine (control), ropivacaine with clonidine 2 μg/kg (clonidine), ropivacaine with 1:400,000 adrenaline (adrenaline) or ropivacaine and a subcutaneous injection of clonidine 2 μg/kg (SC clonidine). The primary outcome was total venous plasma ropivacaine concentrations up to 6 h after the block. There were no significant differences in plasma ropivacaine concentrations between the control group and the clonidine group at any timepoint in the study, nor were there differences in either the mean maximum ropivacaine concentration ( C_max) (1.99 μg/mL versus 2.05 μg/mL, P = 0.712) or the time to maximum concentration ( T_max) (51.0 min versus 56.0 min, P = 0.537). The SC clonidine group also did not differ significantly from the controls ( C_max 2.13 μg/mL versus 1.99 μg/mL, P = 0.424; T_max 43.5 min versus 51.0 min, P = 0.201). Plasma ropivacaine concentrations in the adrenaline group were significantly lower than the controls from 10 to 90 min ( P < 0.003 for each comparison), and the C_max was less than that of the control group (1.36 μg/mL versus 1.99 μg/mL, P < 0.001) with a longer T_max (103.5 min versus 51.0 min, P = 0.001). These findings indicate that clonidine at a concentration of 1.35 μg/mL added to ropivacaine for TAP blocks did not produce a reduction in plasma ropivacaine concentrations. This suggests a lack of vasoconstrictor effect during TAP blocks. Further studies should evaluate whether vasoconstriction occurs when clonidine is used at higher concentrations or for other blocks.

Comparison of Nebulized Ketamine and Ketamine with Clonidine in Postoperative Sore Throat

Background:

Various nonpharmacological and pharmacological trials have been used for attenuating postoperative sore throat (POST) with no proven single modality. Ketamine has been used as a gargle or nebulization in the attenuation of POST by its action on peripheral N-methyl-D-aspartate receptors. Topical administration of clonidine elicits antinociception through α2-adrenoceptors without producing undesirable side effect.

Aim:

This study aims to compare between nebulized ketamine and ketamine with clonidine in POST.

Materials and Methods:

The effects of nebulized ketamine and ketamine with clonidine in POST have been compared in 100 patients. Patients were divided into two groups: Group K patients were nebulized with ketamine 1 mL (50 mg) plus normal saline 3 mL and Group KC patients were nebulized with ketamine 1 mL (50 mg) plus clonidine 1 mL (50 mg) plus normal saline 2 mL. Online statistical software was used for analysis of the data.

Results:

At 4 h, seven patients experienced sore throat in Group K while no incidence was reported in group KC (0%). At 8 h, 12 (85.7%) patients experienced sore throat in Group K and 2 (14.28%) patients in Group KC. At 12 h, 13 (81.25%) patients experienced sore throat in Group K and 3 (18.75%) patients of Group KC. At 24 h, 6 (85.71%) patients experienced sore throat in Group K and 1 (14.28%) patient in Group KC. Patients in both groups remained hemodynamically stable with no complaint of nausea, vomiting, sedation, laryngospasm, or any other side effect.

Conclusion:

Nebulization with ketamine plus clonidine preoperatively is more effective in reducing POST with no adverse effects as compared to preoperative nebulization with ketamine.

Antiepileptic drugs as analgesics/adjuvants in inflammatory pain: current preclinical evidence

Inflammatory pain is the most common type of pain that is treated clinically. The use of currently available treatments (classic analgesics - NSAIDs, paracetamol and opioids) is limited by insufficient efficacy and/or side effects/tolerance development. Antiepileptic drugs (AEDs) are widely used in neuropathic pain treatment, but there is substantial preclinical evidence on their efficacy against inflammatory pain, too. In this review we focus on gabapentinoids (gabapentin and pregabalin) and dibenzazepine AEDs (carbamazepine, oxcarbazepine, and recently introduced eslicarbazepine acetate) and their potential for relieving inflammatory pain. In models of somatic, visceral and trigeminal inflammatory pain, that have a translational value for inflammatory conditions in locomotor system, viscera and head/face, AEDs have demonstrated analgesic activity. This activity was mostly consistent, dependent on the dose and largely independent on the site of inflammation and method of its induction, nociceptive stimuli, species, specific drug used, its route of administration and dosing schedule. AEDs exerted comparable efficacy with classic analgesics. Effective doses of AEDs are lower than toxic doses in animals and, when expressed as equivalent human doses, they are largely overlapping with AEDs doses already used in humans for treating epilepsy/neuropathic pain. The main mechanism of antinociceptive/antihyperalgesic action of gabapentinoids in inflammatory pain models seems to be α₂δ-dependent suppression of voltage-gated calcium channels in primary sensory neurons that leads to reduced release of neurotransmitters in the spinal/medullar dorsal horn. The suppression of NMDA receptors via co-agonist binding site primarily at spinal sites, activation of various types of K⁺ channels at spinal and peripheral sites, and activation of noradrenergic and serotonergic descending pain modulatory pathways may also contribute. Inhibition of voltage-gated sodium channels along the pain pathway is probably the main mechanism of antinociceptive/antihyperalgesic effects of dibenzazepines. The recruitment of peripheral adrenergic and purinergic mechanisms and central GABAergic mechanisms may also contribute. When co-administered with classic/other alternative analgesics, AEDs exerted synergistic/additive interactions. Reviewed data could serve as a basis for clinical studies on the efficacy/safety of AEDs as analgesics/adjuvants in patients with inflammatory pain, and contribute to the improvement of the treatment of various inflammatory pain states.

Pain Management After Bone Reconstruction Surgery Using an Analgesic Bone Cement: A Functional Noninvasive In Vivo Study Using Gait Analysis

Postoperative pain after bone reconstruction is a serious complication that could jeopardize the global success of a surgery. This pain must be controlled and minimized during the first 3 to 4 postoperative days to prevent it from becoming chronic. In this study, a critical-size bone defect was created at the femoral distal end of rats and filled by an injectable calcium phosphate cement (CPC) loaded or not with local anesthetics (bupivacaine or ropivacaine). A functional evaluation of the gait was performed using the CatWalk system to compare the postoperative pain relief enhanced by the different CPCs after such a bone filling surgery. The results demonstrated significant pain relief during the short-term postoperative period, as shown by the print area and intensity parameters of the operated paw. At 24hours, the print area decreased by 65%, 42%, and 24%, and the intensity decreased by 25%, 9%, and 1% for unloaded, ropivacaine-loaded, and bupivacaine-loaded CPCs, respectively, compared with the preoperative values. Bupivacaine-loaded CPC provided an earlier return to full functional recovery than ropivacaine-loaded CPC. Moreover, the CPCs retained their biologic and mechanical properties. For all these reasons, anesthetic-loaded CPCs could be part of the global pain management protocol after bone reconstruction surgery such as iliac crest bone grafting procedures.

PERSPECTIVE

Bupivacaine-loaded CPC provided an earlier return to full gait function than ropivacaine-loaded CPC, with preserved bone filling properties. Such analgesic CPCs deserve further in vivo investigation and may be part of the global pain management protocol after bone reconstruction or bone augmentation surgery such as iliac crest bone grafting.

Omega-3-carboxylic acids provide efficacious anti-inflammatory activity in models of crystal-mediated inflammation

This study assesses the efficacy and exposure–response relationship of omega-3-carboxylic acids (OM-3 CA) in models of crystal-based inflammation. Human THP-1 macrophages and primary peripheral blood mononuclear cells exposed to multiple inflammatory crystal types were used to determine the anti-inflammatory potential of omega-3 (OM-3) fatty acids in vitro. Anti-inflammatory effects of OM-3 CA in vivo were tested in rat monosodium urate (MSU) crystal air pouch and rat knee intra-articular MSU injection models. Acute treatment with the OM-3 fatty acid docosahexaenoic acid suppressed MSU-, cholesterol crystal-, and calcium pyrophosphate crystal-mediated interleukin-1β (IL-1β) production in vitro. In vivo, OM-3 CA dose-dependently reduced crystal-mediated cell migration, exudate volume, and levels of IL-1β and prostaglandin E₂. Following intra-articular injection of MSU, treatment with OM-3-CA (1 mL/kg) and indomethacin (1 mg/kg) resulted in similar mean reductions in pain (23% and 41%, respectively) and swelling (58% and 50%, respectively), compared with controls. Additionally, in complex formulations of OM-3 fatty acids, high levels of palmitic acid could reduce the in vivo effect on crystal-mediated IL-1β elevation. OM-3 CA has a broadly efficacious anti-inflammatory effect with a strong exposure–response relationship that could be beneficial in prevention and treatment of crystal arthritis, with potential applications in other IL-1β-mediated diseases.

Activation of α2A-adrenergic signal transduction in chondrocytes promotes degenerative remodelling of temporomandibular joint

This study tested whether activation of adrenoreceptors in chondrocytes has roles in degenerative remodelling of temporomandibular joint (TMJ) and to determine associated mechanisms. Unilateral anterior crossbite (UAC) was established to induce TMJ degeneration in rats. Saline vehicle, α2- and β-adrenoreceptor antagonists or agonists were injected locally into the TMJ area of UAC rats. Cartilage degeneration, subchondral bone microarchitecture and the expression of adrenoreceptors, aggrecans, matrix metalloproteinases (MMPs) and RANKL by chondrocytes were evaluated. Chondrocytes were stimulated by norepinephrine to investigate signal transduction of adrenoreceptors. Increased α2A-adrenoreceptor expression was observed in condylar cartilage of UAC rats, together with cartilage degeneration and subchondral bone loss. Norepinephrine depresses aggrecans expression but stimulates MMP-3, MMP-13 and RANKL production by chondrocytes through ERK1/2 and PKA pathway; these effects were abolished by an α2A-adrenoreceptor antagonist. Furthermore, inhibition of α2A-adrenoreceptor attenuated degenerative remodelling in the condylar cartilage and subchondral bone, as revealed by increased cartilage thickness, proteoglycans and aggrecan expression, and decreased MMP-3, MMP-13 and RANKL expressions in cartilage, increased BMD, BV/TV, and decreased Tb.Sp in subchondral bone. Conversely, activation of α2A-adrenoreceptor intensified aforementioned degenerative changes in UAC rats. It is concluded that activation of α2A-adrenergic signal in chondrocytes promotes TMJ degenerative remodelling by chondrocyte-mediated pro-catabolic activities.

Postoperative analgesic effect of intravenous (i.v.) clonidine compared with clonidine administration in wound infiltration for open cholecystectomy

OBJECTIVES

This randomized double-blind study was designed to compare the postoperative analgesic effect of clonidine administered intravenously or in wound infiltration with bupivacaine.

METHODS

Sixty adults of ASA grade I-II undergoing open cholecystectomy were randomly allocated into three groups. Group 1 (control group) patients received wound infiltration with 30 ml of 0.25% bupivacaine at the end of surgery. Group 2 patients received 3 μg kg(-1) clonidine intravenously after resection of gall bladder plus wound infiltration with 30 ml of 0.25% bupivacaine. Group 3 patients received wound infiltration with 3 μg kg(-1) clonidine with 30 ml of 0.25% bupivacaine. A standard general anaesthesia technique was used. Postoperative analgesia was provided with i.v. diclofenac and morphine on demand. Postoperative pain, number of patients requiring rescue analgesia and total morphine consumption during 24 h after operation was recorded.

RESULTS

Postoperative morphine consumption was significantly less in patients receiving clonidine by either route when compared with the control group (P<0.0001). All patients in the control group required supplemental morphine, with nine patients in the i.v. clonidine group and 11 patients in the wound infiltration group (P<0.002). Pain scores were lower at rest for 12 h and on cough for 6 h in both clonidine groups when compared with the control group (P<0.01). Patients receiving i.v. clonidine had more hypotension (P<0.01) and sedation (P<0.001) compared with other groups.

CONCLUSIONS

Clonidine 3 μg kg(-1) provided effective postoperative analgesia and reduced morphine requirement when administered intravenously or in wound infiltration with bupivacaine. However, the incidence of complications was less with wound infiltration. CLINICAL TRIAL REGISTRY OF INDIA: (www.ctri.nic.in/), registration number CTRI/2012/12/003258.

The mechanisms of antihyperalgesic effect of topiramate in a rat model of inflammatory hyperalgesia

Recent studies have shown that topiramate, a structurally novel anticonvulsant, exerts antinociceptive activity in animal models of neuropathic, acute somatic, and visceral pain. This study was aimed to examine: (i) the effects of systemically and locally peripherally administered topiramate in the rat inflammatory pain model and (ii) the potential role and site(s) of gamma-aminobutyric acid (GABA), opioid, and adrenergic receptors in topiramate's antihyperalgesia. Rats received intraplantar (i.pl.) injections of the pro-inflammatory compound carrageenan. A paw pressure test was used to determine: (i) the effect of systemic and local peripheral topiramate on carrageenan-induced hyperalgesia and (ii) the effects of systemic and local peripheral bicuculline (selective GABAA receptor antagonist), naloxone (nonselective opioid receptor antagonist), and yohimbine (selective α2-adrenergic receptor antagonist) on topiramate-induced antihyperalgesia. Systemic topiramate (40-160 mg/kg; p.o.) produced a significant dose-dependent reduction in the paw inflammatory hyperalgesia induced by carrageenan. The antihyperalgesic effect of systemic topiramate was significantly decreased by systemic bicuculline (0.5-1 mg/kg; i.p.), naloxone (2-5 mg/kg; i.p.), and yohimbine (1-3 mg/kg; i.p.). Local peripheral topiramate (0.03-0.34 mg/paw; i.pl.) also produced significant dose-dependent antihyperalgesia, which was significantly depressed by local peripheral yohimbine (0.05-0.2 mg/paw; i.pl.) but not by local peripheral bicuculline (0.15 mg/paw; i.pl.) or naloxone (0.1 mg/paw; i.pl.). The results suggest that topiramate produces systemic and local peripheral antihyperalgesia in an inflammatory pain model, which is, at least partially, mediated by central GABAA and opioid receptors and by peripheral and most probably central α2-adrenergic receptors. These findings contribute to better understanding of topiramate's action in pain states involving inflammation.

Analgesic properties of calcium phosphate apatite loaded with bupivacaine on postoperative pain

Synthetic calcium-deficient apatites (CDA) are structurally similar to biological apatites and are well known as chemical precursors of biphasic calcium phosphates (BCP). BCP are mixtures of hydroxyapatite and beta-tricalcium phosphate and are widely used as bone substitutes in human surgery. Bupivacaine, a local anesthetic, has been loaded onto CDA using isostatic compaction. The purpose of this study was to evaluate the in vivo performance of such a local release on pain after having previously defined the in vitro release profile of bupivacaine. CDA was loaded with 1%, 4%, and 16% of bupivacaine using an isostatic compaction process. In vitro release profile assays performed indicated the complete release of bupivacaine after 24 h. Wistar male rats received 50 mg implants of CDA associated, respectively, with 0, 1%, 4%, and 16% of bupivacaine into the distal femur. Analgesia was measured using the electronic version of the Von Frey monofilament test, assessing the inflammatory response and a neurological score. During the first postoperative days, a dose-dependent analgesic effect was observed with the bupivacaine adsorbed on the resorbable implant. This combined device system thus appears to release local anesthetic in a manner that prevents or limits postoperative pain following bone surgery. This innovative approach could be integrated into a global pain management program, for example, in the context of bone harvesting where bone reconstruction is required.

Analgesic efficacy of bilateral superficial cervical plexus block administered before thyroid surgery under general anaesthesia

BACKGROUND

The use of regional anaesthesia in thyroid surgery remains controversial. This double-blind, randomized controlled study was conducted to evaluate the analgesic efficacy of bilateral superficial cervical plexus block (BSCPB) performed under general anaesthesia in patients undergoing total thyroidectomy.

METHODS

Eighty-seven consecutive consenting patients were randomized to receive a BSCPB with saline (Group P, n = 29), ropivacaine 0.487% (Group R, n = 29), or ropivacaine 0.487% plus clonidine 5 microg ml(-1) (Group RC, n = 29). Sufentanil was given during the intraoperative period for a 20% increase in arterial mean pressure or heart rate in a patient with a bispectral index between 40 and 60. All patients received 4 g of acetaminophen during the first 24 h after operation. The pain score was checked every 4 h and nefopam was given for pain score >4 on a numeric pain scale.

RESULTS

During surgery, the median sufentanil requirements were significantly reduced in Group RC compared with Groups R and P (0.32 vs 0.47 and 0.62 microg kg(-1); P < 0.0001). After surgery, the number of patients requiring nefopam within 24 h of surgery was significantly lower in Groups R and RC than in Group P (16 and 19 vs 25; P = 0.03). At post-anaesthetic care unit admission, median (range) pain scores were significantly lower in Groups R [3 (0-10)] and RC [3 (0-8)] than in Group P [5 (0-8), P = 0.03]. No major complications of BSCPB occurred during study.

CONCLUSIONS

BSCPB with ropivacaine and clonidine improved intraoperative analgesia. BSCPB with ropivacaine or ropivaciane and clonidine was effective in reducing analgesic requirements after thyroid surgery.

Peripheral Suppression of Arthritic Pain by Intraarticular Fadolmidine, an α2-Adrenoceptor Agonist, in the Rat

BACKGROUND

Earlier results suggest that peripheral alpha(2)-adrenoceptors and opioid receptors may reduce arthritic pain. Fadolmidine is a highly selective alpha(2)-adrenoceptor agonist that has only limited central access after peripheral administration. We assessed the peripheral antinociceptive properties of fadolmidine and the potential contribution of peripheral opioid receptors to its antinociceptive effect in experimental monoarthritis.

METHODS

After induction of monoarthritis in the knee joints of rats, we determined the frequency of vocalization induced by repetitive movement of the knee joint. Fadolmidine and clonidine were administered intraarticularly ipsi- or contralateral to the inflamed joint. Reversal of the fadolmidine-induced effect was attempted with subcutaneous (s.c.) administration of atipamezole, an alpha(2)-adrenoceptor antagonist, and intraarticular administration of naloxone methiodide, an opioid receptor antagonist that does not penetrate the blood-brain barrier.

RESULTS

Fadolmidine produced a dose-dependent attenuation of the vocalization response to movement of the inflamed knee joint, and this effect was significantly stronger after ipsi- than contralateral drug administration. Clonidine also produced a dose-dependent attenuation of the vocalization response, but this effect was not significantly different after ipsi- versus contralateral drug administration. Fadolmidine-induced antinociception was reversed by s.c. administration of atipamezole. Furthermore, intraarticular administration of naloxone methiodide into the inflamed, but not the contralateral, joint reversed the antinociceptive effect of fadolmidine independent of whether fadolmidine was administered into the inflamed or contralateral joint.

CONCLUSIONS

In rats, intraarticular administration of fadolmidine provides a marked suppression of pain-related behavior in arthritis, due to a selective action on peripheral alpha(2)-adrenoceptors and opioid receptors.

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.

The influence of the time course of inflammation and spinalization on the antinociceptive activity of the α2-adrenoceptor agonist medetomidine

The purpose of the present study was to investigate the influence of the time course of inflammation and the implication of spinal and supraspinal sites on the antihyperalgesic effects of the alpha(2)-adrenoceptor agonist medetomidine. Behavioral experiments showed a more intense antihyperalgesia in the phase of maintenance of inflammation than in the early or resolution stages. Maximum effect, without sedation, was observed with a dose of 40 microg/kg (66+/-12% and 76+/-15% reduction of mechanical and thermal hyperalgesia). No change was observed in the paw swelling, indicating that its effects were not secondary to a reduction of inflammation. In electrophysiological experiments, the effect was more pronounced in animals with an intact spinal cord than in spinalized animals (max. effects of 2+/-0.7% vs. 48+/-11% of control, noxious mechanical stimulation). We conclude that the antihyperalgesic effect of medetomidine depends on the time course of inflammation and that it is mainly located supraspinally.

Transcutaneous electrical nerve stimulation activates peripherally located alpha-2A adrenergic receptors

The alpha2A and alpha2C adrenergic receptor (AR) subtypes mediate antinociception when activated by the endogenous ligand norepinephrine. These receptors also produce antinociceptive synergy when activated concurrently with opioid receptor activation. The involvement of the opioid receptors in the mechanisms governing transcutaneous electrical nerve stimulation (TENS) has been well described. While spinal alpha-2 ARs do not appear to be involved in TENS antihyperalgesia in rats, the noradrenergic analgesic system also involves supraspinal and peripheral sites. Thus, a broader evaluation of the potential contribution of alpha-2 AR to TENS is warranted. The current study compared the antihyperalgesic efficacy of high (100 Hz) and low (4 Hz) frequency TENS in mutant mice lacking a functional alpha2A AR against their respective wildtype counterparts. The degree of secondary heat hyperalgesia induced by intra-articular injection of carrageenan/kaolin (3%) mixture did not differ among the experimental groups. However, the antihyperalgesia induced by both low and high frequency TENS was significantly diminished in alpha2A mutant mice compared to controls. The alpha2 adrenergic receptor selective antagonist, SK&F 86466, reversed TENS-mediated antihyperalgesia when delivered intra-articularly, but not when delivered intrathecally or intracerebroventricularly. These data suggest that peripheral alpha2 ARs contribute, in part, to TENS antihyperalgesia. This pharmacodynamic response is consistent with previous anatomical observations that alpha2A ARs are expressed on primary afferent neurons and macrophages near injured tissue.

Roles of alpha-adrenoceptors and sympathetic nerve in acute herpetic pain induced by herpes simplex virus inoculation in mice

Percutaneous inoculation with herpes simplex virus type-1 brings about herpes zoster-like skin lesions, tactile allodynia, and mechanical hyperalgesia in mice. This study was conducted to determine whether the sympathetic nervous system and alpha-adrenoceptors would be involved in these pain-related responses and whether the alpha(2)-adrenoceptor agonist clonidine would suppress these responses. The adrenergic neuron blocker guanethidine and the non-selective alpha-adrenoceptor antagonist phentolamine did not affect the pain-related responses, although these agents suppressed the pain-related responses induced by partial ligation of the sciatic nerve. The pain-related responses induced by herpetic inoculation was suppressed by intraperitoneal and intrathecal injections, but not by intraplantar and intracerebroventricular injections, of clonidine. The suppressive effect of an intraperitoneal injection of clonidine (0.1 mg/kg) was antagonized by intrathecal injections of phentolamine and the alpha(2)-adrenoceptor antagonist yohimbine, but not the alpha(1)-adrenoceptor antagonist prazosin. The results suggest that sympathetic nerves and alpha-adrenoceptors are not involved in the pain-related responses induced by herpetic infection. Clonidine suppresses the responses probably through the action on alpha(2)-adrenoceptors in the dorsal horn.

Topical and peripherally acting analgesics

Acute nociceptive, inflammatory, and neuropathic pain all depend to some degree on the peripheral activation of primary sensory afferent neurons. The localized peripheral administration of drugs, such as by topical application, can potentially optimize drug concentrations at the site of origin of the pain, while leading to lower systemic levels and fewer adverse systemic effects, fewer drug interactions, and no need to titrate doses into a therapeutic range compared with systemic administration. Primary sensory afferent neurons can be activated by a range of inflammatory mediators such as prostanoids, bradykinin, ATP, histamine, and serotonin, and inhibiting their actions represents a strategy for the development of analgesics. Peripheral nerve endings also express a variety of inhibitory neuroreceptors such as opioid, alpha-adrenergic, cholinergic, adenosine and cannabinoid receptors, and agonists for these receptors also represent viable targets for drug development. At present, topical and other forms of peripheral administration of nonsteroidal anti-inflammatory drugs, opioids, capsaicin, local anesthetics, and alpha-adrenoceptor agonists are being used in a variety of clinical states. There also are some clinical data on the use of topical antidepressants and glutamate receptor antagonists. There are preclinical data supporting the potential for development of local formulations of adenosine agonists, cannabinoid agonists, cholinergic ligands, cytokine antagonists, bradykinin antagonists, ATP antagonists, biogenic amine antagonists, neuropeptide antagonists, and agents that alter the availability of nerve growth factor. Given that activation of sensory neurons involves multiple mediators, combinations of agents targeting different mechanisms may be particularly useful. Topical analgesics represent a promising area for future drug development.

Enhanced reduction in hyperalgesia by combined administration of clonidine and TENS

Transcutaneous electrical nerve stimulation (TENS) partially reduces primary hyperalgesia and is frequency dependent such that high frequency TENS produces approximately a 30% reduction in hyperalgesia whereas low frequency TENS has no effect. Both high and low frequency TENS completely reduce secondary hyperalgesia by activation of mu and delta- opioid receptors in the spinal cord and rostral-ventral medulla suggesting an opiate mediated analgesia. Clonidine in combination with opiates produces a synergistic interaction such that there is a potentiated reduction in hyperalgesia. Thus, we tested if combined application of clonidine with TENS would enhance the reduction in primary hyperalgesia. Male Sprague-Dawley rats were inflamed by subcutaneous injection of 3% carrageenan into one hindpaw. Withdrawal latency to radiant heat and withdrawal threshold to mechanical stimuli were assessed before and after inflammation and after administration of clonidine (0.002-2 mg/kg, intraperitoneal (i.p.)) with either low (4 Hz) or high (100 Hz) frequency TENS. Clonidine alone reduced both heat and mechanical hyperalgesia with ED50s of 0.02 and 1.0 mg/kg, respectively. In combination with either low or high frequency TENS, the dose-response curve shifted to the left and was significantly different from clonidine alone. The ED50s for heat and mechanical hyperalgesia following low frequency TENS with clonidine were 0.002 and 0.2 mg/kg, respectively and those following high frequency TENS with clonidine were 0.005 and 0.15 mg/kg, respectively. Thus, combined use of clonidine and TENS enhances the reduction in analgesia produced by TENS and enhances the potency of clonidine. It would thus be expected that one would reduce the side effects of clonidine and enhance analgesic efficacy with combinations of pharmaceutical and non-pharmaceutical treatments.

The analgesic efficacy of bilateral combined superficial and deep cervical plexus block administered before thyroid surgery under general anesthesia

In this study we evaluated the analgesic efficacy of combined deep and superficial cervical plexus block in patients undergoing thyroidectomy under general anesthesia. For this purpose, 39 patients undergoing elective thyroid surgery were randomized to receive a bilateral combined deep and superficial cervical block (14 mL per side) with saline (Group 1; n = 13), ropivacaine 0.5% (Group 2; n = 13), or ropivacaine 0.5% plus clonidine 7.5 microg/mL (Group 3; n = 13). Deep cervical plexus block was performed with a single injection (8 mL) at the C3 level. Superficial cervical plexus block consisted of a subcutaneous injection (6 mL) behind the lateral border of the sternocleidomastoid muscle. During surgery, the number of additional alfentanil boluses was significantly reduced in Groups 2 and 3 compared with Group 1 (1.3 +/- 1.0 and 1.1 +/- 1.0 vs 2.6 +/- 1.0; P < 0.05). After surgery, the opioid and non-opioid analgesic requirements were also significantly reduced in Groups 2 and 3 (P < 0.05) during the first 24 h. Except for one patient in Group 3, who experienced transient anesthesia of the brachial plexus, no side effect was noted in any group. We conclude that combined deep and superficial cervical plexus block is an effective technique to alleviate pain during and immediately after thyroidectomy.

IMPLICATIONS

Combined deep and superficial cervical plexus block is an effective technique to reduce opioid requirements during and after thyroid surgery.

Olanzapine in the management of cancer pain

In cancer patients, cognitive impairment, psychological distress, and anxiety may accompany and aggravate pain. Neuroleptics are frequently used to control these symptoms and may be used to treat pain that has been unresponsive to more conventional approaches. Because of prominent side effects of traditional neuroleptics and conflicting data regarding their analgesic efficacy, their use in the treatment of pain remains controversial. Olanzapine, an atypical neuroleptic, might offer advantages because of its safer side effect profile. It has also been shown to have an independent antinociceptive activity in animals. The use of olanzapine in the management of cancer pain has not been previously described. We prospectively collected the data on 8 cancer patients with severe pain, uncontrolled in spite of aggressive opioid titration, who received olanzapine to treat severe anxiety and mild cognitive impairment. Patients did not meet criteria for delirium and their cognitive impairment was defined as cognitive disorder not otherwise specified (NOS) according to DSM-IV. Patients received 2.5 to 7.5 mg of olanzapine daily. In all patients, opioid requirements had escalated rapidly prior to starting olanzapine. Levels of pain, sedation, and opioid use were measured 2 days before and 2 days after olanzapine was started. Cognitive state was assessed daily. All 8 patients had marked reduction of the daily pain scores. The average daily opioid use decreased significantly in all patients. Cognitive impairment and anxiety resolved within 24 hours of initiating olanzapine. In these 8 patients, decreased pain scores and opioid requirements may have resulted from improvement in cognitive function and the known anxiolytic effect of olanzapine. Other mechanisms may include independent or adjuvant analgesic effects of olanzapine. We conclude that olanzapine may be useful in the treatment of patients with uncontrolled cancer pain associated with cognitive impairment or anxiety. Further studies to evaluate possible analgesic effect of olanzapine are needed.

Pharmacologic treatment of neuropathic pain

Neuropathic pain, or pain after nervous system injury, can be very refractory to pharmacologic interventions. Through a better understanding of the pathophysiology of neuropathic pain, it has been suggested that nonopioid agents, such as antidepressants and anticonvulsants, may be more efficacious in the treatment of neuropathic pain than common analgesics, such as opioids or nonsteroidal anti-inflammatory drugs. However, this has not been consistently demonstrated in clinical studies. Conversely, many confounding factors of neuropathic pain make it difficult to interpret clinical studies. Therefore, we must develop a better understanding of the preclinical models of neuropathic pain to better understand the application of new and old drugs to the human neuropathic pain state. This article provides an overview of the commonly used preclinical neuropathic pain models, followed by a summary of the efficacy of currently available agents in preclinical pain models and human correlates.

Inhibition of the I(h) current in isolated peripheral nerve: a novel mode of peripheral antinociception?

Although the alpha2-adrenergic agonist clonidine has been shown to promote peripheral antinociception, its mechanism of action has not yet been clearly elucidated. By the use of the sucrose-gap method, we have shown that in C fibers of the rabbit vagus nerve, clonidine at micromolar concentrations enhances activity-dependent hyperpolarizations generated by the Na+-K+ pump during and after repetitive stimulation. Similar results were obtained with 10 microM of ZD 7288, a specific blocker of the hyperpolarization-activated cation current (I(h)) and with 2 mM of Ba2+ that blocks the inwardly rectifying potassium current (I(KIR)). Furthermore, clonidine had no added effect on the ZD 7288-induced response, whereas it produced a marked enhancement of Ba2+induced response. From these results, it can be concluded that clonidine enhances activity-dependent hyperpolarization by inhibiting the current I(h). We propose that clonidine, by increasing the threshold for initiating the action potential, induces a slowing or block of conduction and that this mechanism is the origin of the clonidine-induced antinociception. Finally, this study suggests a novel role for inwardly rectifying hyperpolarization-activated conductances in peripherally mediated antinociception.