Clonidine reduces the excitability of spinal dorsal horn neurones

Comparison of Oral versus Intramuscular Clonidine for Prolongation of Bupivacaine Spinal Anesthesia in Patients Undergoing Total Abdominal Hysterectomy

Background:

Clonidine is a commonly used agent for premedication through oral, intravenous, and intramuscular route. Very few studies mentioned intramuscular clonidine as premedication.

Aims and Objectives:

The aim of the present study is to compare oral and intramuscular clonidine as predication agent in bupivacaine spinal anesthesia patients.

Materials and Methods:

In our study, recruited patients were randomly allocated in three groups of 32 each. All patients received intrathecal bupivacaine heavy 3 mL with oral 150 μg clonidine in Group 1, intramuscular 150 μg clonidine in Group 2, and oral placebo tablet in Group 3 1 h before taking the patient in operation theater. We have assessed for duration of sensory block, duration of motor block, duration of analgesia, sedation score, and hemodynamic changes in groups.

Statistical Analysis:

The parametric data were expressed as mean ± standard deviation. Primary analysis of parametric data between the two groups was done by student's t-test, and among three groups, analysis of variance was used.

Results:

Duration of motor block was found significantly high in Group 2 than Group 1 (208.06 ± 9.48 vs. 200.25 ± 9.42; P < 0.05). Duration of sensory block was also found significantly high in Group 2 than Group 1 (219.69 ± 9.44 vs. 210.25 ± 9.68; P < 0.05). Time to give first dose of analgesia was also found greater in Group 2 than Group 1 (234.66 ± 11.76 vs. 217.75 ± 10.09; P < 0.05). Sedation score and other side effects were found statistically nonsignificant between Group 1 and 2.

Conclusion:

We can conclude that preoperative intramuscular clonidine is a better alternate of oral clonidine for bupivacaine spinal anesthesia in terms of long duration of motor and sensory block and less requirement of analgesic with clinically insignificant side effects.

Clonidine as an Additive to Local Anesthetics in Caudal Block for Postoperative Analgesia in Pediatric Surgery: A Systematic Review and Meta-Analysis

Background: Clonidine is an anesthetic with favorable efficacy and safety profiles for caudal epidural block, but comparisons with other adjuvants need to be confirmed in pediatric patients. Aim: To investigate the effects of clonidine as an adjuvant in caudal epidural block to improve the intraoperative and postoperative analgesia in pediatric surgery. Methods: PubMed, Embase, and the Cochrane Library were searched for available papers published up to February 2021. The outcomes were pain score, duration of analgesia, complications, and number of analgesic requirements. The meta-analysis was performed using random-effects models. Results: Fifteen randomized controlled trials (RCTs) were included. There were no differences between clonidine and the control drug regarding the duration of analgesia (SMD = -0.71, 95%CI: -1.64, 0.23; I ² = 95.5%, P_{heterogeneity} < 0.001), pain score (SMD = 0.35, 95%CI: -0.28, 0.98; I ² = 80.8%, P_{heterogeneity} < 0.001), and requirement for additional analgesia (OR = 8.77, 95%CI: 0.70, 110.58, I ² = 81.9%, P_{heterogeneity} = 0.004), but using clonidine resulted in fewer complications than the control drugs (OR = 0.33, 95%CI: 0.20, 0.54, I ² = 21.8%, P_{heterogeneity} = 0.217). The sensitivity analysis showed that the results were robust. A publication bias was observed. Conclusion: Clonidine has the same efficacy as the other adjuvants for caudal epidural block for pediatric surgery but fewer complications. These results support clonidine as an adjuvant to local anesthetic, but additional studies should be conducted.

Caudal epidural blocks in paediatric patients: a review and practical considerations

Caudal epidural blockade in children is one of the most widely administered techniques of regional anaesthesia. Recent clinical studies have answered major pharmacodynamic and pharmacokinetic questions, thus providing the scientific background for safe and effective blocks in daily clinical practice and demonstrating that patient selection can be expanded to range from extreme preterm births up to 50 kg of body weight. This narrative review discusses the main findings in the current literature with regard to patient selection (sub-umbilical vs mid-abdominal indications, contraindications, low-risk patients with spinal anomalies); anatomical considerations (access problems, age and body positioning, palpation for needle insertion); technical considerations (verification of needle position by ultrasound vs landmarks vs 'whoosh' or 'swoosh' testing); training and equipment requirements (learning curve, needle types, risk of tissue spreading); complications and safety (paediatric regional anaesthesia, caudal blocks); local anaesthetics (bupivacaine vs ropivacaine, risk of toxicity in children, management of toxic events); adjuvant drugs (clonidine, dexmedetomidine, opioids, ketamine); volume dosing (dermatomal reach, cranial rebound); caudally accessed lumbar or thoracic anaesthesia (contamination risk, verifying catheter placement); and postoperative pain. Caudal blocks are an efficient way to offer perioperative analgesia for painful sub-umbilical interventions. Performed on sedated children, they enable not only early ambulation, but also periprocedural haemodynamic stability and spontaneous breathing in patient groups at maximum risk of a difficult airway. These are important advantages over general anaesthesia, notably in preterm babies and in children with cardiopulmonary co-morbidities. Compared with other techniques of regional anaesthesia, a case for caudal blocks can still be made.

Effect of clonidine on the cutaneous silent period during spinal anesthesia

AIM

To investigate the effect of clonidine on the cutaneous silent period (CSP) during spinal anesthesia.

METHODS

A total of 67 adult patients were included in this randomized, prospective, single-center, double-blind trial. They did not have neurological disorders and were scheduled for inguinal hernia repair surgery. This trial was registered on ClinicalTrials.gov (NTC03121261). The patients were randomized into two groups with regards to the intrathecally administered solution: (1) 15 mg of 0.5% levobupivacaine with 50 µg of 0.015% clonidine, or (2) 15 mg of 0.5% levobupivacaine alone. There were 34 patients in the levobupivacaine-clonidine (LC) group and 33 patients in the levobupivacaine (L) group. CSP and its latency were measured four times: prior to the subarachnoid block (SAB), after motor block regression to the 0 level of the Bromage scale, with ongoing sensory blockade, and both 6 and 24 h after SAB.

RESULTS

Only data from 30 patients in each group were analyzed. There were no significant differences between the groups investigated preoperatively and after 24 h. The CSP of the L group at the time point when the Bromage scale was 0 was 44.8 ± 8.1 ms, while in the LC group it measured 40.2 ± 3.8 ms (P = 0.007). The latency in the L group at the time point when the Bromage scale was 0 was 130.3 ± 10.2 ms, and in the LC group it was 144.7 ± 8.3 ms (P < 0.001). The CSP of the L group after 6 h was 59.6 ± 9.8 ms, while in the LC group it was 44.5 ± 5.0 ms (P < 0.001). The latency in the L group after 6 h was 110.4 ± 10.6 ms, while in LC group it was 132.3 ± 9.7 ms (P < 0.001).

CONCLUSION

Intrathecal addition of clonidine to levobupivacaine for SAB in comparison with levobupivacaine alone results in a diminished inhibitory tonus and shortened CSP.

Sustained analgesic effect of clonidine co-polymer depot in a porcine incisional pain model

Background

Previous research suggests that the α₂ adrenergic agonist clonidine, a centrally acting analgesic and antihypertensive, may also have direct effects on peripheral pain generators. However, aqueous injections are limited by rapid systemic absorption leading to off target effects and a brief analgesic duration of action.

Purpose

The aim of this study was to examine the efficacy of a sustained-release clonidine depot, placed in the wound bed, in a pig incisional pain model.

Methods

The depot was a 15 mm ×5 mm ×0.3 mm poly(lactide-co-caprolactone) polymer film containing 3% (w/w) clonidine HCl (MDT3). Fifty-two young adult mix Landrace pigs (9–11 kg) were divided into seven groups. All subjects received a 6 cm, full-thickness, linear incision into the left lateral flank. Group 1 served as a Sham control group (Sham, n=8). Group 2 received three placebo strips (PBO, n=8), placed end-to-end in the subcutaneous wound bed before wound closure. Group 3 received one MDT3 and two PBO (n=8), Group 4 received two MDT3 and one PBO (n=8), and Group 5 received three MDT3 (n=8). Positive control groups received peri-incisional injections of bupivacaine solution (Group 6, 30 mg/day bupivacaine, n=8) or clonidine solution (Group 7, 225 µg/day, n=4).

Results

The surgical procedure was associated with significant peri-incisional tactile allodynia. There was a dose-dependent effect of MDT3 in partially reversing the peri-incisional tactile allodynia, with maximum pain relief relative to Sham at 72 hours. Daily injections of bupivacaine (30 mg), but not clonidine (up to 225 µg), completely reversed allodynia within 48 hours. There was a statistically significant correlation between the dose of MDT3 and cumulative withdrawal threshold from 4 hours through the conclusion of the study on day 7.

Conclusion

These data suggest that a sustained-release clonidine depot may be a viable nonopioid, nonamide anesthetic therapy for the treatment of acute postsurgical nociceptive sensitization.

The impact of neuraxial clonidine on postoperative analgesia and perioperative adverse effects in women having elective Caesarean section-a systematic review and meta-analysis

Neuraxial clonidine improves postoperative analgesia in the general surgical population. The efficacy and safety of neuraxial clonidine as a postoperative analgesic adjunct in the Caesarean section population still remains unclear. This systematic review and meta-analysis aims to evaluate the effect of perioperative neuraxial clonidine on postoperative analgesia in women having Caesarean section under neuraxial anaesthesia. We included randomized controlled trials comparing the analgesic efficacy of the perioperative administration of neuraxial clonidine alone or in combination with a local anaesthetic and/or opioids in women having elective Caesarean section under neuraxial anaesthesia when compared with placebo. PubMed, the Cochrane Central Register of Controlled Trials, and EMBASE were searched until February 2017. Eighteen studies were included in the meta-analysis. Neuraxial clonidine reduced 24 h morphine consumption [mean difference (MD): -7.2 mg; 95% confidence interval (CI): -11.4, -3.0 mg; seven studies] and prolonged time to first analgesic request (MD: 135 min; 95% CI: 102, 168 min; 16 studies) when compared with the control group. Neuraxial clonidine increased intraoperative hypotension [odds ratio (OR): 2.849; 95% CI: 1.363, 5.957], intraoperative sedation (OR: 2.355; 95% CI: 1.016, 5.459), but reduced the need for intraoperative analgesic supplementation (OR: 0.224; 95% CI: 0.076, 0.663). The effect of clonidine on intraoperative bradycardia, intraoperative and postoperative nausea and vomiting, postoperative sedation, and pruritus were inconclusive. Neuraxial clonidine did not negatively impact neonatal umbilical artery pH or Apgar scores. This review demonstrates that neuraxial clonidine enhances postoperative analgesia in women having Caesarean section with neuraxial anaesthesia, but this has to be balanced against increased maternal adverse effects.

Clonidine administration during intraoperative monitoring for pediatric scoliosis surgery: Effects on central and peripheral motor responses

OBJECTIVE

To study the effect of clonidine administrated as a co-analgesic during scoliosis surgery, on the neuromonitoring of spinal motor pathways.

METHODS

Using standardized intraoperative monitoring, we compared the time course of peripherally and transcranially electrically evoked motor potentials (TcEMEPs) before and after injection of a single bolus of clonidine in children under total intravenous anesthesia (TIVA). MEP data were obtained from 9 patients and somatosensory evoked potentials (SSEPs) were obtained from 2 patients. The potential effect of clonidine on mean blood pressure (BP) was controlled.

RESULTS

TcEMEPs from upper and lower limbs rapidly showed significant drops in amplitude after the injection of clonidine. Amplitudes reached minimal values within five minutes and remained very weak for at least 10-20minutes during which monitoring of the central motor pathways was severely compromised. SSEPs were not altered during maximal amplitude depression of the TcEMEPS.

CONCLUSIONS

This is the first report showing that clonidine severely interferes with neuromonitoring of the spinal cord motor pathways. The results are discussed in light of the literature describing the effects of dexmedetomidine, another α-2 adrenergic agonist. The experimental and literature data point to central mechanisms taking place at both the spinal and cerebral levels. Therefore, clonidine as well as other α-2 adrenergic agonists should be used with extreme caution in patients for whom neuromonitoring of the motor pathways is required during surgery.

Noradrenaline Modulates the Membrane Potential and Holding Current of Medial Prefrontal Cortex Pyramidal Neurons via β1-Adrenergic Receptors and HCN Channels

The medial prefrontal cortex (mPFC) receives dense noradrenergic projections from the locus coeruleus. Adrenergic innervation of mPFC pyramidal neurons plays an essential role in both physiology (control of memory formation, attention, working memory, and cognitive behavior) and pathophysiology (attention deficit hyperactivity disorder, posttraumatic stress disorder, cognitive deterioration after traumatic brain injury, behavioral changes related to addiction, Alzheimer's disease and depression). The aim of this study was to elucidate the mechanism responsible for adrenergic receptor-mediated control of the resting membrane potential in layer V mPFC pyramidal neurons. The membrane potential or holding current of synaptically isolated layer V mPFC pyramidal neurons was recorded in perforated-patch and classical whole-cell configurations in slices from young rats. Application of noradrenaline (NA), a neurotransmitter with affinity for all types of adrenergic receptors, evoked depolarization or inward current in the tested neurons irrespective of whether the recordings were performed in the perforated-patch or classical whole-cell configuration. The effect of noradrenaline depended on β1- and not α1- or α2-adrenergic receptor stimulation. Activation of β1-adrenergic receptors led to an increase in inward Na+ current through hyperpolarization-activated cyclic nucleotide-gated (HCN) channels, which carry a mixed Na+/K+ current. The protein kinase A- and C-, glycogen synthase kinase-3β- and tyrosine kinase-linked signaling pathways were not involved in the signal transduction between β1-adrenergic receptors and HCN channels. The transduction system operated in a membrane-delimited fashion and involved the βγ subunit of G-protein. Thus, noradrenaline controls the resting membrane potential and holding current in mPFC pyramidal neurons through β1-adrenergic receptors, which in turn activate HCN channels via a signaling pathway involving the βγ subunit.

[Unilateral spinal anesthesia : Literature review and recommendations]

Unilateral spinal anesthesia is a cost-effective and rapidly performed anesthetic technique. An exclusively unilateral block only affects the sensory, motor and sympathetic functions on one side of the body and offers the advantages of a spinal block without the typical adverse side effects seen with a bilateral block. The lack of hypotension, in particular, makes unilateral spinal anesthesia suitable for patients with cardiovascular risk factors e. g. aortic valve stenosis or coronary artery disease. Increasing numbers of surgical procedures are now being performed on an outpatient basis. Until now, spinal anesthesia has been considered unsuitable for this, not only because of the high incidence of intraoperative hypotension and postoperative urinary retention but also because of the prolonged postoperative stay before home discharge. This is not the case with unilateral spinal anesthesia: motor function returns rapidly, the incidence of urinary retention is extremely low, and patients are usually eligible for home discharge sooner than after bilateral spinal anesthesia or general anesthesia. The success of the technique depends on a number of factors. In addition to the local anesthetic, its concentration and dose, and the baricity of the injected solution, the shape of the spinal needle, the injection speed, the patient's position during injection, and the time the patient remains in this position after injection are equally important parameters. A number of intrathecally applied adjuvant drugs are used to give a more intense and/or longer-lasting block. For this review, we collated the published data on unilateral spinal anesthesia from journals with an impact factor greater than 1.0 and defined an optimized method for performing the technique. In order to achieve an exclusively unilateral block one should use 0.5 % hyperbaric bupivacaine injected at a rate of 0.33 ml/min or slower. During the injection and the following 20 min the patient should lie in the lateral decubitus position on the side intended for surgery with knees drawn to the chest. An injection of 5 mg (1 ml) hyperbaric bupivacaine 0.5 % provides an hour-long block to T 12, and a dose of 7.5 to 10 mg (1.5-2.0 ml) extends the block to T 6. Adding clonidine (0.5 to 1.0 µg/kg BW) to the injection prolongs the duration of the block to approximately two to three hours. During the 20-minute fixation period, the cephalad spread of the block can be influenced to a certain extent by raising or lowering the head of the table.

Oral Clonidine vs Oral Pregabalin Premedication to Attenuate Pressor Response to Direct Laryngoscopy in Patients Undergoing Laparoscopic Cholecystectomy: A Randomized Double Blind Study

INTRODUCTION

Direct laryngoscopy and tracheal intubation has adverse effects like tachycardia, hypertension, myocardial ischemia and cerebral haemorrhage. There are several studies on various pharmacological agents to attenuate this response.

AIM

This study was designed to compare efficacy and safety of oral clonidine and oral pregabalin premedication to attenuate stress response in patients undergoing laparoscopic cholecystectomy.

MATERIALS AND METHODS

Total 80 patients of ASA grade I and II, aged between 20-60 years of both sexes scheduled for elective laparoscopic cholecystectomy were included in the study. All the patients were randomized into two groups. Group A received oral clonidine 0.3mg and group B received oral pregabalin 150mg, 60 minutes before surgery. Anaesthesia technique was standardized. Systolic Blood Pressure (SBP), Diastolic Blood Pressure (DBP), mean arterial pressure (MAP) and heart rate were recorded preoperatively, after premedication, immediately after intubation, then at 1 min, 3 min, 5 min, 10 min and 15 min after intubation. Level of sedation, postoperative pain scores and any adverse effects were also noted and compared.

RESULTS

Oral clonidine 0.3mg as well as oral pregabalin 150mg were effective in blunting haemodynamic stress response to laryngoscopy and tracheal intubation. Clonidine was found to be better than pregabalin in lowering of systolic blood pressure, diastolic blood pressure, mean arterial pressure and heart rate changes associated with laryngoscopy. We also found that bradycardia was common with both the drugs, more so in clonidine group. Post-operative analgesia was better in pregabalin group as compared to clonidine group. Both the drugs cause sedation, but it was more with the use of pregabalin.

CONCLUSION

Both the drugs can be used as an effective premedicant to attenuate the sympathetic response to laryngoscopy and tracheal intubation without much side effects and the added advantage of intraoperative and postoperative analgesia.

Effect of epidural clonidine on characteristics of spinal anaesthesia in patients undergoing gynaecological surgeries: A clinical study

Background and Aims:

Combined spinal–epidural (CSE) anaesthesia is being increasingly used for effective post-operative analgesia. This study was designed to evaluate the effect of epidural clonidine on characteristics of spinal anaesthesia for gynaecological surgeries.

Methods:

This was a prospective randomised, double-blind, controlled study involving sixty patients belonging to American Society of Anesthesiologists Physical Status I and II who underwent gynaecological surgeries were randomly divided into clonidine (C) group and saline (S) group of thirty each. All patients received CSE anaesthesia. Ten minutes before subarachnoid block (SAB), Group C received clonidine 150 μg diluted to 5 ml in normal saline (NS) and Group S received NS epidurally. Hyperbaric bupivacaine (15 mg) was administered intrathecally for both groups after epidural injection. Sensory and motor block characteristics, analgesia, sedation and haemodynamics were observed. Statistical analysis was performed using appropriate tests.

Results:

Epidural clonidine produced faster onset (37.83 ± 8.58 s in Group C compared to 50.33 ± 8.80 s in Group S, P = 0.001) and prolonged duration of sensory block (241.17±18.65 minutes in group C compared to 150.33±19.16 minutes in group S, P = 0.001). Time for two segment regression of sensory block was193.67 ± 19.82 min in Group C and 109.33 ± 18.56 min Group S (P < 0.001). The duration of analgesia was 299.00 ± 43.38 min in Group C and 152.50 ± 21.04 min in Group S (P < 0.001). Haemodynamics and sedation scores were comparable between two groups.

Conclusion:

Administration of clonidine epidurally, 10 min before SAB, caused early onset and prolonged duration of motor blockade and analgesia, without any significant post-operative complication.

Prospective multicentre randomised, double-blind, equivalence study comparing clonidine and midazolam as intravenous sedative agents in critically ill children: the SLEEPS (Safety profiLe, Efficacy and Equivalence in Paediatric intensive care Sedation) study

BACKGROUND

Children in paediatric intensive care units (PICUs) require analgesia and sedation but both undersedation and oversedation can be harmful.

OBJECTIVE

Evaluation of intravenous (i.v.) clonidine as an alternative to i.v. midazolam.

DESIGN

Multicentre, double-blind, randomised equivalence trial.

SETTING

Ten UK PICUs.

PARTICIPANTS

Children (30 days to 15 years inclusive) weighing ≤ 50 kg, expected to require ventilation on PICU for > 12 hours.

INTERVENTIONS

Clonidine (3 µg/kg loading then 0-3 µg/kg/hour) versus midazolam (200 µg/kg loading then 0-200 µg/kg/hour). Maintenance infusion rates adjusted according to behavioural assessment (COMFORT score). Both groups also received morphine.

MAIN OUTCOME MEASURES

Primary end point Adequate sedation defined by COMFORT score of 17-26 for ≥ 80% of the time with a ± 0.15 margin of equivalence. Secondary end points Percentage of time spent adequately sedated, increase in sedation/analgesia, recovery after sedation, side effects and safety data.

RESULTS

The study planned to recruit 1000 children. In total, 129 children were randomised, of whom 120 (93%) contributed data for the primary outcome. The proportion of children who were adequately sedated for ≥ 80% of the time was 21 of 61 (34.4%) - clonidine, and 18 of 59 (30.5%) - midazolam. The difference in proportions for clonidine-midazolam was 0.04 [95% confidence interval (CI) -0.13 to 0.21], and, with the 95% CI including values outside the range of equivalence (-0.15 to 0.15), equivalence was not demonstrated; however, the study was underpowered. Non-inferiority of clonidine to midazolam was established, with the only values outside the equivalence range favouring clonidine. Times to reach maximum sedation and analgesia were comparable hazard ratios: 0.99 (95% CI 0.53 to 1.82) and 1.18 (95% CI 0.49 to 2.86), respectively. Percentage time spent adequately sedated was similar [medians clonidine 73.8% vs. midazolam 72.8%: difference in medians 0.66 (95% CI -5.25 to 7.24)]. Treatment failure was 12 of 64 (18.8%) on clonidine and 7 of 61 (11.5%) on midazolam [risk ratio (RR) 1.63, 95% CI 0.69 to 3.88]. Proportions with withdrawal symptoms [28/60 (46.7%) vs. 30/58 (52.6%)] were similar (RR 0.89, 95% CI 0.62 to 1.28), but a greater proportion required clinical intervention in those receiving midazolam [11/60 (18.3%) vs. 16/58 (27.6%) (RR 0.66, 95% CI 0.34 to 1.31)]. Post treatment, one child on clonidine experienced mild rebound hypertension, not requiring intervention. A higher incidence of inotropic support during the first 12 hours was required for those on clonidine [clonidine 5/45 (11.1%) vs. midazolam 3/52 (5.8%)] (RR 1.93 95% CI 0.49 to 7.61).

CONCLUSIONS

Clonidine is an alternative to midazolam. Our trial-based economic evaluation suggests that clonidine is likely to be a cost-effective sedative agent in the PICU in comparison with midazolam (probability of cost-effectiveness exceeds 50%). Rebound hypertension did not appear to be a significant problem with clonidine but, owing to its effects on heart rate, specific cardiovascular attention needs to be taken during the loading and early infusion phase. Neither drug in combination with morphine provided ideal sedation, suggesting that in unparalysed patients a third background agent is necessary. The disappointing recruitment rates reflect a reluctance of parents to provide consent when established on a sedation regimen, and reluctance of clinicians to allow sedation to be studied in unstable critically ill children. Future studies will require less exacting protocols allowing enhanced recruitment.

TRIAL REGISTRATION

Current Controlled Trials ISRCTN02639863.

FUNDING

This project was funded by the NIHR Health Technology Assessment programme and will be published in full in Health Technology Assessment; Vol. 18, No. 71. See the NIHR Journals Library website for further project information.

Response to low-dose intrathecal clonidine in septuagenarians undergoing sub-umbilical surgeries: A study

Clonidine, an alpha-2-adrenergic agonist, may have a clinically relevant analgesic action but also a hypotensive action, when administered spinally.

Clonidine Reduces Nociceptive Responses in Mouse Orofacial Formalin Model: Potentiation by Sigma-1 Receptor Antagonist BD1047 without Impaired Motor Coordination

Although the administration of clonidine, an alpha-2 adrenoceptor agonist, significantly attenuates nociception and hyperalgesia in several pain models, clinical trials of clonidine are limited by its side effects such as drowsiness, hypotension and sedation. Recently, we determined that the sigma-1 receptor antagonist BD1047 dose-dependently reduced nociceptive responses in a mouse orofacial formalin model. Here we examined whether intraperitoneal injection of clonidine suppressed the nociceptive responses in the orofacial formalin test, and whether co-administration with BD1047 enhances lower-dose clonidine-induced anti-nociceptive effects without the disruption of motor coordination and blood pressure. Formalin (5%, 10 µL) was subcutaneously injected into the right upper lip, and the rubbing responses with the ipsilateral fore- or hind-paw were counted for 45 min. Clonidine (10, 30 or 100 µg/kg) was intraperitoneally administered 30 min before formalin injection. Clonidine alone dose-dependently reduced nociceptive responses in both the first and second phases. Co-localization for alpha-2A adrenoceptors and sigma-1 receptors was determined in trigeminal ganglion cells. Interestingly, the sub-effective dose of BD1047 (3 mg/kg) significantly potentiated the anti-nociceptive effect of lower-dose clonidine (10 or 30 µg/kg) in the second phase. In particular, the middle dose of clonidine (30 µg/kg) in combination with BD1047 produced an anti-nociceptive effect similar to that of the high-dose clonidine, but without a significant motor dysfunction or hypotension. In contrast, mice treated with the high dose of clonidine developed severe impairment in motor coordination and blood pressure. These data suggest that a combination of low-dose clonidine with BD1047 may be a novel and safe therapeutic strategy for orofacial pain management.

Effects of the selective sigma-1 receptor antagonist S1RA on formalin-induced pain behavior and neurotransmitter release in the spinal cord in rats

We have previously shown that the selective sigma-1 receptor (σ1 R) antagonist S1RA (E-52862) inhibits neuropathic pain and activity-induced spinal sensitization in various pre-clinical pain models. In this study we characterized both the behavioral and the spinal neurochemical effects of S1RA in the rat formalin test. Systemic administration of S1RA produced a dose-related attenuation of flinching and lifting/licking behaviors in the formalin test. Neurochemical studies using concentric microdialysis in the ipsilateral dorsal horn of awake, freely moving rats revealed that the systemic S1RA-induced antinociceptive effect occurs concomitantly with an enhancement of noradrenaline levels and an attenuation of formalin-evoked glutamate release in the spinal dorsal horn. Intrathecal pre-treatment with idazoxan prevented the systemic S1RA antinociceptive effect, suggesting that the S1RA antinociception depends on the activation of spinal α2 -adrenoceptors which, in turn, could induce an inhibition of formalin-evoked glutamate release. When administered locally, intrathecal S1RA inhibited only the flinching behavior, whereas intracerebroventricularly or intraplantarly injected also attenuated the lifting/licking behavior. These results suggest that S1RA supraspinally activates the descending noradrenergic pain inhibitory system, which may explain part of its antinociceptive properties in the formalin test; however, effects at other central and peripheral sites also account for the overall effect. Formalin-induced nociceptive effect occurs concomitantly with an enhancement of glutamate (Glu) level in the dorsal horn spinal cord. The selective σ1 receptor antagonist S1RA results in inhibition of formalin-evoked Glu release, no modification of GABA levels, and enhancement of noradrenaline (NA) levels. This increased spinal NA activates spinal α2-adrenoceptors producing the attenuation of the formalin-induced pain behaviour.

Central α-adrenoceptors contribute to mustard oil-induced central sensitization in the rat medullary dorsal horn

Our previous studies have demonstrated that application of the inflammatory irritant mustard oil (MO) to the tooth pulp produces trigeminal central sensitization that includes increases in mechanoreceptive field size and responses to noxious stimuli and decrease in activation threshold in brainstem nociceptive neurons of trigeminal subnucleus caudalis (the medullary dorsal horn, MDH). The aim of the present study was to test if central noradrenergic processes are involved in the central sensitization of MDH neurons and if α1-adrenoceptors or α2-adrenoceptors or both are involved. In urethane/α-chloralose-anesthetized rats, the activity of extracellularly recorded and functionally identified single nociceptive neurons in the MDH was studied. Continuous intrathecal (i.t.) superfusion of the adrenergic modulator guanethidine and α-adrenoceptor blocker phentolamine or selective α1-adrenoceptor antagonist prazosin over the medulla strongly attenuated all three MO-induced parameters of central sensitization in the MDH nociceptive neurons, compared to phosphate-buffered saline (as vehicle control). In contrast, i.t. superfusion of the selective α2-adrenoceptor antagonist yohimbine had little effect on the mechanoreceptive field expansion and the decreased mechanical activation threshold, and indeed facilitated responses to noxious stimuli of sensitized nociceptive neurons. Superfusion of each of the four chemicals alone did not affect baseline nociceptive neuronal properties. These findings provide the first documentation of the involvement of central noradrenergic processes in MDH in the development of the central sensitization, and that α1- and α2-adrenoceptors may be differentially involved.

Dexmedetomidine and clonidine inhibit the function of Na(v)1.7 independent of α(2)-adrenoceptor in adrenal chromaffin cells

PURPOSE

Besides being administered systemically for sedation and analgesia, α(2)-agonists such as dexmedetomidine and clonidine have been administered with intrathecal, epidural, or perineural injections, leading to an antinociceptive effect at the spinal cord or peripheral nerve level. However, the mechanism for this remains unclear. In the present study, we examined whether dexmedetomidine and clonidine could inhibit the function of tetrodotoxin-sensitive Na(+) channels, which play important roles in the generation of pain.

METHODS

Cultured bovine adrenal chromaffin cells expressing the tetrodotoxin-sensitive Na(v)1.7 Na(+) channel isoform were incubated in KRP buffer containing 2 μCi (22)NaCl for 5 min without or with dexmedetomidine or clonidine in the absence or presence of veratridine, α-scorpion venom, β-scorpion venom, Ptychodiscus brevis toxin-3 or ouabain. Cells were then washed and counted radioactively.

RESULTS

Dexmedetomidine and clonidine reduced veratridine-induced (22)Na(+) influx via Na(v)1.7 in a concentration-dependent manner (EC(50) = 50 μM and 530 μM), even in the presence of ouabain, an inhibitor of Na(+), K(+)-ATPase. Dexmedetomidine and clonidine shifted the concentration-response curve of veratridine for (22)Na(+) influx downward without altering the EC(50) of veratridine. Atipamezole and yohimbine, α(2)-antagonists, did not prevent the inhibition of veratridine-induced (22)Na(+) influx by dexmedetomidine. Dexmedetomidine and clonidine combined with lidocaine induced more inhibition of veratridine-induced (22)Na(+) influx than each drug did individually. Atipamezole and yohimbine did not prevent the lidocaine-enhancing effect of dexmedetomidine and clonidine.

CONCLUSION

Dexmedetomidine and clonidine inhibit the function of Na(v)1.7 independent of α(2)-adrenoceptor. These results may lead to a deeper understanding of the peripheral antinociceptive effects of α (2)-agonists.

Norepinephrine can act via alpha(2)-adrenoceptors to reduce the hyper-excitability of spinal dorsal horn neurons following chronic nerve injury

BACKGROUND/PURPOSE

Rats display behavioral signs of neuropathic pain lasting for months in the chronic constriction injury (CCI) model. During intrathecal anesthesia, the administered drugs mainly diffuse directly into the superficial neurons in the spinal dorsal horn. This study aimed to investigate the effect of bath application of norepinephrine on whole cell patch clamp recordings from spinal cord slices of CCI rats with allodynia.

METHODS

An assessment of paw withdrawal threshold in response to mechanical stimulation was performed on the operated side on the day before surgery and was repeated after recovery from anesthesia and on the 7(th) and 14(th) days after surgery. Spinal cord slice preparations containing dorsal horn neurons were obtained from both sham-operated rats and CCI rats (after the 14(th) postoperative day behavior test).

RESULTS

Compared with normal controls, CCI rats had significantly lower levels of both hyperpolarization and spike threshold in single action potentials recorded from lamina I and II neurons of the spinal dorsal horn. In contrast, a series of action potential recordings showed that the percentage of spiking neurons of the spinal dorsal horn of CCI rats were significantly higher than those of normal controls. The CCI-induced reduction in hyperpolarization, as well as the increased numbers of spinal dorsal horn spiking neurons could be significantly reduced by norepinephrine application. The norepinephrine-induced increased hyperpolarization and input resistance could be abolished by the application of an alpha(2)-adrenoceptor antagonist (idazoxan; 200 nM).

CONCLUSION

The results suggest that chronic nerve injury may induce neuropathic pain by increasing the excitability of spinal dorsal horn neurons. This excitability can be reduced by norepinephrine.

Unilateral spinal anaesthesia for outpatient surgery: a comparison between hyperbaric bupivacaine and bupivacaine-clonidine combination

BACKGROUND

Low-dose hyperbaric bupivacaine has been used to produce unilateral spinal anaesthesia for outpatient surgery. Unilateral spinal anaesthesia is associated with reduction of hypotension, faster recovery and increased patient satisfaction. Small doses of clonidine have shown effectiveness in intensifying spinal anaesthesia. We investigated the effect of adding 15 microg of clonidine to 5 mg hyperbaric bupivacaine on unilaterality.

METHODS

Sixty patients undergoing outpatient knee arthroscopy were randomly allocated to receive either 1.2 ml (6 mg) of hyperbaric bupivacaine or a 1.2 ml solution containing 1.0 ml (5 mg) hyperbaric bupivacaine, 0.1 ml (75 microg) clonidine and 0.1 ml sterile water. The motor block was assessed by a modified Bromage scale and the sensory block by a pinprick.

RESULTS

There was a significant difference in the spread of anaesthesia between the operated and contralateral sides in both groups. Seventy-seven per cent of the blocks were unilateral in group B and 73% in group B-C. There was no significant difference between the groups, in unilaterality. The motor block was prolonged in group B-C but it did not affect home-readiness. Patients receiving clonidine needed more vasopressors. There was a significant difference in blood pressures between the groups, being lower in group B-C after 1 h 45 min.

CONCLUSION

Using 5 mg hyperbaric bupivacaine with 15 microg of clonidine, the unilaterality can be achieved and spinal anaesthesia intensified without affecting home-readiness. More vasopressors are needed in the beginning, but after the surgery patients experienced less pain.

The analgesic effect of epidural clonidine after spinal surgery: a randomized placebo-controlled trial

BACKGROUND

Clonidine is an alpha(2) adrenoreceptor and imidazoline receptor agonist, which has analgesic, sedative, and minimum alveolar anesthetic concentration-sparing effects. It has been used orally, IV, and epidurally. In spinal surgery, there is a reluctance to use local anesthetic-based epidural analgesia postoperatively because of fears of masking important signs of nerve root or spinal cord injury.

METHODS

We randomized 66 patients undergoing uncomplicated decompressive spinal surgery to receive an epidural infusion of either clonidine (Group C) or saline placebo (Group P) postoperatively. Morphine consumption by patient-controlled analgesia device was recorded for 36 h.

RESULTS

Morphine consumption was significantly lower in Group C. The mean consumption at 36 h was 35 mg (95% confidence interval 21-50 mg) in Group C, compared with 61 mg (95% confidence interval 48-74 mg) in the control group. Nausea was significantly reduced in Group C (6.5%), when compared with placebo (38.2%).

CONCLUSION

Low-dose epidural clonidine significantly reduced the demand for morphine and reduced postoperative nausea with few side effects.

Intrathecal clonidine suppresses phosphorylation of the N-methyl-D-aspartate receptor NR1 subunit in spinal dorsal horn neurons of rats with neuropathic pain

BACKGROUND

Intrathecal (IT) administration of the alpha-2 adrenoceptor agonist, clonidine, produces significant analgesic effects. Although several mechanisms underlying clonidine-induced analgesia have been proposed, the possible interaction with N-methyl-D-aspartate (NMDA) receptors as a major antinociceptive mechanism has not been addressed. We designed the present study to determine whether clonidine or other analgesics can affect spinal NMDA receptor activation in rats with chronic constriction injury (CCI)-induced neuropathy.

METHODS

Rats underwent unilateral CCI, and received IT clonidine (1, 5, 20 microg/rat), [D-Ala2, NMe-Phe4, Gly-ol5]-enkephalin (DAMGO, mu opioid receptor agonist, 1 microg/rat), gabapentin (anticonvulsant, 100 microg/rat) or vehicle 2 wks later. After drug injection, we measured the pain response to thermal or mechanical stimuli and used immunohistochemistry to evaluate spinal cord phosphorylated NMDA-receptor subunit 1 (pNR1) expression.

RESULTS

Two weeks after CCI surgery, rats displayed significant mechanical allodynia and thermal hyperalgesia, and the spinal cord dorsal horn showed a significant increase in the number of pNR1 immunoreactive neurons. IT injection of clonidine (20 microg/rat), DAMGO and gabapentin potently reduced mechanical allodynia and thermal hyperalgesia. Importantly, IT clonidine, but not IT DAMGO or gabapentin, dose-dependently reduced CCI-induced pNR1 expression in all lamina of the spinal cord dorsal horn by 30 min after injection. In addition, IT injection of the alpha-2 adrenoceptor antagonist, idazoxan (40 microg/rat) 10 min before clonidine injection completely reversed clonidine's antihyperalgesic and antiallodynic effects, as well as clonidine's suppressive effect on CCI-induced NR1 phosphorylation in the spinal cord dorsal horn.

CONCLUSIONS

Our data indicate that IT clonidine's antihyperalgesic/antiallodynic effect on neuropathic pain is associated with a significant reduction in spinal NMDA receptor phosphorylation and suggests a potentially novel mechanism of clonidine's action.