Clonidine and dexmedetomidine produce antinociceptive synergy in mouse spinal cord

Assessment of the Efficacy of Nalbuphine as an Adjuvant to Intrathecal Bupivacaine in Endoscopic Urological Surgeries for the Prolongation of Postoperative Analgesia

Background In anaesthesiology, intrathecal drugs play pivotal roles in spinal anaesthesia. Despite their ability to induce a high sensory block, bupivacaine alone may not be adequate for postoperative analgesia. It often requires a substantial dose of postoperative rescue analgesia to manage pain effectively. Thus, we studied the efficacy of nalbuphine 1.5 mg injected intrathecally as an adjuvant in endoscopic urological surgery. Materials and methods Sixty patients undergoing endoscopic urological surgery were equally divided into two study groups: group B (injection 0.5% hyperbaric bupivacaine 15 mg (3 ml) plus sterile NS 0.15 ml) and group N (injection 0.5% hyperbaric bupivacaine 15 mg (3 ml)+nalbuphine 1.5 mg (0.15 ml)). The first appearance of the sensory and motor blockages and duration required to attain complete sensory and motor threshold was noted. All vitals were recorded. After surgery, it was recorded when the patient first needed rescue analgesia (injection paracetamol 1 gm IV). Any adverse effects were recorded and addressed. The statistical analysis was conducted using IBM SPSS Statistics for Windows, V. 22.0 (IBM Corp., Armonk, NY), with p<0.05 indicating significance in both groups' operations. Results Intrathecal nalbuphine as an adjuvant caused an earlier onset of sensory and motor inhibition, delayed two-segment regression, and prolonged postoperative anaesthesia. The control group experienced sensory block at 3.33±0.61 minutes, while the nalbuphine group had a mean onset of 2.66±0.92 minutes (p=0.001). The patient who received nalbuphine had a mean regression time of 119.60±14.549 minutes, whereas the bupivacaine group had a mean regression time of 88.43±17.196 minutes. Group N had a considerably longer duration of postoperative analgesia, lasting 264.97 minutes, compared to group B's 198.50 minutes (p<0.001). Intrathecal nalbuphine did not influence vital indicators such as heart rate, respiration rate, and oxygen saturation. Conclusion To conclude, endoscopic urological surgery patients who received a subarachnoid block with 1.5 mg (0.15 ml) of nalbuphine hydrochloride with 0.5% hyperbaric bupivacaine 15 mg (3 ml) had longer postoperative pain relief than those who received 3 ml of intrathecal bupivacaine (15 mg). Urinary retention and pruritus were absent. Intrathecal nalbuphine with hyperbaric 0.5% bupivacaine is deemed safe with minimal side effects in endoscopic urology surgery.

Dexmedetomidine relieves inflammatory pain by enhancing GABAergic synaptic activity in pyramidal neurons of the anterior cingulate cortex

Pyramidal neuron (Pyn) hyperactivity in the anterior cingulate cortex (ACC) is involved in the modulation of pain. Previous studies indicate that the activation of α2 adrenoceptors (α2-ARs) by dexmedetomidine (DEX) is a safe and effective means of alleviating multiple types of pain. Here, we showed that systemically administered DEX can ameliorate the inflammatory pain induced by hindpaw injection of formalin (FA) and further examined the molecular and synaptic mechanisms of this DEX-elicited antinociceptive effect. We found that FA caused an increase in c-Fos expression in contralateral layer 2/3 (L2/3) ACC, and that intra-ACC infusion of DEX could also relieve phase 2 inflammatory pain behavior. DEX elicited an increase in the amplitude and frequency of miniature inhibitory post-synaptic currents (mIPSCs) and evoked IPSC amplitude, as well as a reduction in the hyperexcitability and both paired-pulse and excitation/inhibition ratios in contralateral L2/3 ACC Pyns of FA mice. These electrophysiological effects were associated with the upregulation of GABA A receptor (GABAAR) subunits. The interaction of phosphorylated Akt (p-Akt) with GABAAR subunits increased in the ACC following administration of DEX. These results suggest that DEX treatment reduces hyperactivity and enhances GABAergic inhibitory synaptic transmission in ACC Pyns, which produces analgesic effects by increasing GABAAR levels and activating the Akt signaling pathway.

Incorporating inter-individual variability in experimental design improves the quality of results of animal experiments

Inter-individual variability in quantitative traits is believed to potentially inflate the quality of results in animal experimentation. Yet, to our knowledge this effect has not been empirically tested. Here we test whether inter-individual variability in emotional response within mouse inbred strains affects the outcome of a pharmacological experiment. Three mouse inbred strains (BALB/c, C57BL/6 and 129S2) were behaviorally characterized through repeated exposure to a mild aversive stimulus (modified Hole Board, five consecutive trials). A multivariate clustering procedure yielded two multidimensional response types which were displayed by individuals of all three strains. We show that systematic incorporation of these individual response types in the design of a pharmacological experiment produces different results from an experimental pool in which this variation was not accounted for. To our knowledge, this is the first study that empirically confirms that inter-individual variability affects the interpretation of behavioral phenotypes and may obscure experimental results in a pharmacological experiment.

The influence of dexmedetomidine on remifentanil-induced hyperalgesia and the sex differences

In the clinical settings, patients often develop opioid-induced hyperalgesia (OIH) after utilization of high dose intra-operative remifentanil. Systemic α2 agonists, including dexmedetomidine, are believed to reduce pain and opioid requirements after surgery, thus decreasing the incidence of hyperalgesia. The present study aimed to investigate the effect of dexmedetomidine on remifentanil-induced hyperalgesia and explored the sex differences. A total of 48 patients (24 male, 24 female) with an American Society of Anesthesiologists physical status of I-II that were undergoing thyroidectomy were randomly assigned to one of the following six groups: Male controlled group (MC) and female controlled group (FC) (group MC, n=8 and group FC, n=8), which received a preoperative placebo of 0.2 µg.kg^-1 normal saline and intraoperative remifentanil 0.2 µg.kg^-1.min^-1; male and female group with low-dose dexmedetomidine (group MD1, n=8 and group FD1, n=8), which received preoperative dexmedetomidine 0.2 µg.kg^-1 and intraoperative remifentanil 0.2 µg.kg^-1.min^-1; and male and female groups with high-dose dexmedetomidine (group MD2, n=8 and group FD2, n=8), which received dexmedetomidine 0.6 µg.kg^-1 and intraoperative remifentanil 0.2 µg.kg^-1.min^-1. Result indicated that the visual analog scale (VAS) scores and morphine dosing frequency were significantly higher in MC and FC groups compared with the other same sex groups. Furthermore, the mechanical hyperalgesia threshold and patients' analgesia satisfaction score after surgery were significantly lower in MC and FC groups. Notably, the frequency of post-operative chills, nausea and vomiting were significantly lower in groups MD1, MD2, FD1 and FD2. The present findings indicated that low- and high-dose dexmedetomidine injection significantly decreased the patient's risk of enhanced pain intensity and increased postoperative morphine dosing caused by remifentanil-induced hyperalgesia. These findings suggest that the influence of dexmedetomidine displayed minimal significant differences between sex. Trial registration no., IRB2018-YX-001 (Name of registry: Institutional Medical Ethics Committee of Tianjin Medical University General Hospital; date of registration: February 1, 2016).

Bivalent ligand that activates mu opioid receptor and antagonizes mGluR5 receptor reduces neuropathic pain in mice

The mu opioid receptor (MOR) and metabotropic glutamate receptor 5 (mGluR5) are well-established pharmacological targets in the management of chronic pain. Both receptors are expressed in the spinal cord. MMG22, a bivalent ligand containing 2 pharmacophores separated by 22 atoms, which simultaneously activates MOR and antagonizes mGluR5, has been shown to produce potent reversal of tactile hypersensitivity in rodent models of lipopolysaccharide (LPS)-and bone cancer-induced chronic pain. This study assessed whether intrathecal MMG22 also is effective in reducing pain of neuropathic origin. Furthermore, we theorized that MMG22 should reduce hyperalgesia in nerve-injured mice in a manner consistent with a synergistic interaction between MOR and mGluR5. Several weeks after spared nerve injury, tactile hypersensitivity was reversed in mice by the intrathecal injection of MMG22 (0.01-10 nmol) but also by its shorter spacer analog, MMG10, with similar potency. The potencies of the bivalent ligands were 10- to 14-fold higher than those of the compounds upon which the bivalent structure was based, the MOR agonist oxymorphone and the mGluR5 antagonist MPEP. Coadministration of oxymorphone and MPEP demonstrated analgesic synergism, an interaction confirmed by isobolographic analysis. This study indicates that in the spared nerve injury-induced model of neuropathic pain, the 2 pharmacophores of the bivalent ligands MMG22 and MMG10 target MOR and mGluR5 as separate receptor monomers. The observed increase in the potency of MMG22 and MMG10, compared with oxymorphone and MPEP, may reflect the synergistic interaction of the 2 pharmacophores of the bivalent ligand acting at their respective separate receptor monomers.

Truncated mu opioid GPCR variant involvement in opioid-dependent and opioid-independent pain modulatory systems within the CNS

The clinical management of severe pain depends heavily on opioids acting through mu opioid receptors encoded by the Oprm1 gene, which undergoes extensive alternative splicing. In addition to generating a series of prototypic seven transmembrane domain (7TM) G protein-coupled receptors (GPCRs), Oprm1 also produces a set of truncated splice variants containing only six transmembrane domains (6TM) through which selected opioids such as IBNtxA (3'-iodobenzoyl-6β-naltrexamide) mediate a potent analgesia without many undesirable effects. Although morphine analgesia is independent of these 6TM mu receptor isoforms, we now show that the selective loss of the 6TM variants in a knockout model eliminates the analgesic actions of delta and kappa opioids and of α2-adrenergic compounds, but not cannabinoid, neurotensin, or muscarinic drugs. These observations were confirmed by using antisense paradigms. Despite their role in analgesia, loss of the 6TM variants were not involved with delta opioid-induced seizure activity, aversion to the kappa drug U50, 488H, or α2-mediated hypolocomotion. These observations support the existence of parallel opioid and nonopioid pain modulatory systems and highlight the ability to dissociate unwanted delta, kappa1, and α2 actions from analgesia.

Intraoperative Dexmedetomidine Improves the Quality of Recovery and Postoperative Pulmonary Function in Patients Undergoing Video-assisted Thoracoscopic Surgery: A CONSORT-Prospective, Randomized, Controlled Trial

Video-assisted thoracoscopic surgery (VATS) has been known to be a stressful event for patients, and dexmedetomidine is known to attenuate surgery-induced sympathetic responses and potentiate analgesia in perioperative periods. The present was designed to evaluate the effects of intraoperative dexmedetomidine administration on the quality of recovery (QoR) and pulmonary function after VATS. Patients with lung cancer undergoing VATS were randomized to Dex group (loading of 1.0 μg/kg for 20 minutes before the termination of surgery, n = 50) or Control group (comparable volume of normal saline, n = 50). The QoR-40 questionnaire assesses postoperative recovery and validates the overall surgical and general anesthesia outcomes. The QoR-40 scores, forced expiratory volume for 1 second (FEV1) on postoperative day (POD) 1 and 2, and emergence agitation were evaluated. The global QoR-40 score (162.3 ± 17.8 vs 153.3 ± 18.7, P = 0.016 on POD 1; 174.3 ± 16.0 vs 166.8 ± 16.7, P = 0.028 on POD 2) and FEV1 (2.1 ± 0.4 vs 1.9 ± 0.5 L, P = 0.034 on POD 1; 2.2 ± 0.5 vs 2.0 ± 0.4 L, P = 0.030 on POD 2) were significantly higher in the Dex group compared with the Control group on POD1 and POD 2. The score of emergence agitation was lower in the Dex group compared with the Control group (3 [2-5] vs 5 [3-7], P < 0.001). The number of patients indicating severe emergence agitation was shorter in the Dex group than Control group (0 [0%] vs 7 [14%], P = 0.048). The length of hospital stay was significantly shorter (6.7 [3-9] vs 8.4 [4-9] days, P = 0.045) in the Dex group compared with the Control group. Intraoperative dexmedetomidine administration improved QoR, postoperative pulmonary function, and emergence agitation in patients undergoing VATS. Consequently, intraoperative dexmedetomidine administration could improve postoperative outcomes and reduced the length of hospital stay in patients undergoing VATS.

Role of dexmedetomidine in IL-4 and IFN-γ expression in rats with multiple organ dysfunction syndrome induced by postpartum bleeding

Bleeding-induced multiple organ dysfunction syndrome (MODS) is one of the major causes of death in pregnant women. MODS is thought to result from an inappropriate generalized host inflammatory response to a variety of acute insults. In this study we established a MODS model in postpartum rats, in which MODS was induced by the combination of induced hypotension for 60 min and clamping of the superior mesenteric artery for a period of 40 min. We sacrificed all the rats 24 h after dexmedetomidine (DEX) treatment. Thymus, spleen, and mesenteric lymph node tissue were collected to detect interferon-γ (IFN-γ) and interleukin-4 (IL-4) protein expression; lung and intestine tissue were collected to measure IFN-γ and IL-4 gene expression. In the present study, IFN-γ and IL-4 mRNA were increased in the lungs and intestines of the MODS rats. DEX administration decreased IFN-γ and IL-4 mRNA expression. IFN-γ and IL-4 expression for the thymus, spleen, and mesenteric lymph nodes were higher in the MODS postpartum rats relative to control rats, and these expression levels decreased upon DEX administration, But there were no significant differences between DEX doses. In conclusion DEX administration appeared to reduce IFN-γ and IL-4 protein expression in thymus, spleen, and mesenteric lymph node tissue and reduce IFN-γ and IL-4 gene expression in the lungs and intestines in the MODS postpartum rats but was not dose-dependent.

Neurotoxicity of intrathecal injections of dexmedetomidine into the rat spinal dorsal horn

To investigate the neurotoxicity of intrathecal injections of dexmedetomidine, Sprague-Dawley rats were intrathecally injected with dexmedetomidine at doses of 0.75, 1.50 and 3.00 μg/kg into the spinal dorsal horn. We found that c-Fos expression in the rat spinal dorsal horn peaked at 7 hours following the 3.00 μg/kg dexmedetomidine injection, while the levels of c-Fos expression following 0.75 and 1.50 μg/kg dexmedetomidine were similar to those in the spinal dorsal horn of normal rats. At 48 hours following administration, the level of c-Fos expression was similar to normal levels. In addition, the intrathecal injections of dexmedetomidine increased paw withdrawal mechanical thresholds and prolonged thermal tail flick latencies. These results indicate that dexmedetomidine has pronounced antinociceptive effects. However, dexmedetomidine appears to have neurotoxic effects in the spinal cord because it increased c-Fos expression in the spinal dorsal horn within 7 hours following administration.

Putative kappa opioid heteromers as targets for developing analgesics free of adverse effects

It is now generally recognized that upon activation by an agonist, β-arrestin associates with G protein-coupled receptors and acts as a scaffold in creating a diverse signaling network that could lead to adverse effects. As an approach to reducing side effects associated with κ opioid agonists, a series of β-naltrexamides 3-10 was synthesized in an effort to selectively target putative κ opioid heteromers without recruiting β-arrestin upon activation. The most potent derivative 3 (INTA) strongly activated KOR-DOR and KOR-MOR heteromers in HEK293 cells. In vivo studies revealed 3 to produce potent antinociception, which, when taken together with antagonism data, was consistent with the activation of both heteromers. 3 was devoid of tolerance, dependence, and showed no aversive effect in the conditioned place preference assay. As immunofluorescence studies indicated no recruitment of β-arrestin2 to membranes in coexpressed KOR-DOR cells, this study suggests that targeting of specific putative heteromers has the potential to identify leads for analgesics devoid of adverse effects.

Dexmedetomidine decreases requirement of thiopentone sodium and pentazocine followed with improved recovery in patients undergoing laparoscopic cholecystectomy

Background:

Pain relief remains the most fundamental and consequential aspect of surgery for patients throughout perioperative period. Dexmedetomidine has created an interest in α2-adrenoceptor agonists in the management of pain and hence the aim of this study was to evaluate the effectiveness of the drug in hilly population of North India.

Materials and Methods:

Patients, irrespective of gender, were randomly allocated to two groups, control and test, each having 30 patients. Test group received an infusion of dexmedetomidine at a rate of 0.5 μg/kg/h 30 min before induction and 0.6 μg/kg/h after inducing anesthesia. Control patients received a volume-matched infusion of normal saline as placebo. Approximately 2 min before induction, analgesia was provided in the form of pentazocine, 0.5 mg/kg in control and 0.3 mg/kg in the test group. Induction was performed by 2 mg/kg thiopentone sodium supplemented with intravenous boluses of 25 mg thiopentone sodium every 15 s until loss of eyelid reflex (determined every 15 s). Induction dose of thiopentone sodium and total pentazocine dose were recorded. Recovery was assessed on the clinical recovery score (CRS) scale.

Results:

Infusion of dexmedetomidine decreased the induction dose of thiopentone approximately by 33% and of pentazocine dose by approximately 39% in patients undergoing laparoscopic cholecystectomy. Moreover, incidence of pain was also decreased significantly. Improved CRS from 4.33 to 6.87 was noticed immediately post-operatively in dexmedetomidine group of patients.

Conclusion:

Infusion of dexmedetomidine during the laparoscopic cholecystectomy decreases the requirement of thiopentone sodium and pentazocine and leads to early recovery of patients.

Dexmedetomidine reduced cytokine release during postpartum bleeding-induced multiple organ dysfunction syndrome in rats

Dexmedetomidine (DEX) is an α2-adrenergic agonist. It decreases the levels of norepinephrine release, resulting in a reduction of postsynaptic adrenergic activity. In the present study, the effects of DEX on postpartum bleeding-induced multiple organ dysfunction syndrome (BMODS) were studied in rats in which BMODS was induced by the combination of hypotension and clamping of the superior mesenteric artery. We evaluated the role of dexmedetomidine (DEX) in cytokine release during postpartum BMODS in rats. In summary, the present study demonstrated that DEX administration reduced IFN-r and IL-4 release and decreased lung injury during postpartum BMODS. It is possible that DEX administration decreased inflammatory cytokine production in BMODS by inhibiting inflammation and free radical release by leukocytes independent of the DEX dose.

Negative reinforcement reveals non-evoked ongoing pain in mice with tissue or nerve injury

Patients with chronic pain experience spontaneous or ongoing pain as well as enhanced sensitivity to evoked stimuli. Spontaneous or ongoing pain is rarely evaluated in preclinical studies. In fact, it remains controversial whether ongoing or spontaneous pain even develops in mice after tissue or nerve injury. This study tested a hypothesis that negative reinforcement can be used to unmask the presence of pain in mice with tissue or nerve injury. We found that spinal administration of clonidine or lidocaine did not elicit conditioned place preference (CPP) in uninjured or sham-operated mice. However, these agents produced CPP in mice with chronic inflammation induced by complete Freund's adjuvant (CFA) or following L5/L6 spinal nerve ligation (SNL). These data indicate the presence of non-evoked (ie, stimulus-independent) ongoing pain in mice with chronic inflammation (CFA) or following nerve injury (SNL). In addition, this study validates the use of negative reinforcement to unmask non-evoked ongoing pain in mice. Given the existence of a large collection of transgenic and knockout mice, our data show the application of this approach to elucidate molecular mechanisms underlying non-evoked pain and to contribute to drug discovery for pain.

PERSPECTIVE

We demonstrated the presence of non-evoked ongoing pain in mice with chronic inflammation or following nerve injury. The study also validates the use of negative reinforcement to unmask non-evoked pain in mice. We propose to apply this approach to identify molecular mechanisms and effective drugs for chronic pain.

Central modulation of pain

It has long been appreciated that the experience of pain is highly variable between individuals. Pain results from activation of sensory receptors specialized to detect actual or impending tissue damage (i.e., nociceptors). However, a direct correlation between activation of nociceptors and the sensory experience of pain is not always apparent. Even in cases in which the severity of injury appears similar, individual pain experiences may vary dramatically. Emotional state, degree of anxiety, attention and distraction, past experiences, memories, and many other factors can either enhance or diminish the pain experience. Here, we review evidence for "top-down" modulatory circuits that profoundly change the sensory experience of pain.

Pharmacological profiles of alpha 2 adrenergic receptor agonists identified using genetically altered mice and isobolographic analysis

Endogenous, descending noradrenergic fibers impose analgesic control over spinal afferent circuitry mediating the rostrad transmission of pain signals. These fibers target alpha 2 adrenergic receptors (alpha(2)ARs) on both primary afferent terminals and secondary neurons, and their activation mediates substantial inhibitory control over this transmission, rivaling that of opioid receptors which share a similar pattern of distribution. The terminals of primary afferent nociceptive neurons and secondary spinal dorsal horn neurons express alpha(2A)AR and alpha(2C)AR subtypes, respectively. Spinal delivery of these agents serves to reduce their side effects, which are mediated largely at supraspinal sites, by concentrating the drugs at the spinal level. Targeting these spinal alpha(2)ARs with one of five selective therapeutic agonists, clonidine, dexmedetomidine, brimonidine, ST91 and moxonidine, produces significant antinociception that can work in concert with opioid agonists to yield synergistic antinociception. Application of several genetically altered mouse lines had facilitated identification of the primary receptor subtypes that likely mediate the antinociceptive effects of these agents. This review provides first an anatomical description of the localization of the three subtypes in the central nervous system, second a detailed account of the pharmacological history of each of the six primary agonists, and finally a comprehensive report of the specific interactions of other GPCR agonists with each of the six principal alpha(2)AR agonists featured.