The effect of imidazoline receptors and alpha2-adrenoceptors on the anesthetic requirement (MAC) for halothane in rats

Regulation of I1-imidazoline receptors on the sedation effect of dexmedetomidine in mice

Dexmedetomidine has been used as a sedative drug in the clinic for a long time. Many studies demonstrated that the sedative mechanism of dexmedetomidine might be related to the activation of α2-adrenoceptor (α2AR). In addition, it was reported that dexmedetomidine had some affinity for the I1-imidazoline receptor (I1R); however, the role of I1R in dexmedetomidine-induced sedative effects and its possible mechanism are poorly studied. In the present study, we found that agmatine, an I1R agonist, was able to enhance the sedative effect of dexmedetomidine in mice. Efaroxan, an α2AR and I1R antagonist, could prevent and rescue the sedative action of dexmedetomidine in mice, and its preventive effect was better than atipamezole, the specific α2AR antagonist. Knockout of imidazoline receptor antisera-selected (IRAS), the functional I1R candidate protein, suppressed the dexmedetomidine-induced sedation. Moreover, IRAS knockout led to the inhibition of agmatine and efaroxan in regulating dexmedetomidine-induced sedative effects in mice, but not of atipamezole. We then used CHO cell lines that stably expressed α2AR and IRAS to investigate the possible molecular mechanism of IRAS in regulating the dexmedetomidine-induced sedative effect. The results showed that IRAS expression significantly up-regulated dexmedetomidine-induced ERK phosphorylation, which was enhanced by agmatine and inhibited by efaroxan at low concentrations. Therefore, by taking advantage of pharmacological and genetic approaches, our finding revealed the evidence that IRAS plays an important role in the sedative effects of dexmedetomidine, and the ERK signal pathway may be involved in the mechanism of IRAS in regulating dexmedetomidine-induced sedation. This study may offer valuable insights for the advancement of novel anesthetic adjuvants.

Alternatives to Opioid Analgesia in Small Animal Anesthesia: Alpha-2 Agonists

Alpha-2 agonists have potent analgesic effects, in addition to their sedative actions. Alpha-2 agonists provide analgesia through any of several routes of administration, including parenteral, oral, epidural or intrathecal and intraarticular, because of spinal and supraspinal actions. Systemic doses are short acting, whereas local administration at the site of action result in longer analgesic effects. The potent cardiovascular and respiratory effects of alpha-2 agonists should be considered when used as analgesics.

Does premedication with dexmedetomidine provide perioperative hemodynamic stability in hypertensive patients?

Background

Perioperative hemodynamic fluctuations are seen more often in hypertensive patients than in normotensive patients. The purpose of our study was to investigate the perioperative hemodynamic effects of dexmedetomidine and midazolam used for premedication in hypertensive patients relative to each other and in comparison to normotensive patients.

Methods

One-hundred-forty female, normotensive or hypertensive patients undergoing myomectomies or hysterectomies. They were randomly enrolled into the subgroups: Group ND (normotensive-dexmedetomidine); Group HD (hypertensive-dexmedetomine); Group NM (normotensive-midazolam); Group HM (hypertensive- midazolam). Dexmedetomidine was administered at a concentration of 0.5 μg.kg⁻¹, and midazolam was administered at a concentration of 0.025 μg.kg⁻¹ via intravenous (IV) infusion before the induction of anaesthesia. Haemodynamic parameters were recorded at several times (T_beginning, T_{preop5 min}, T_{preop 10 min}, T_induction, T_intubation, T_{intubation5 min}, T_{initial surgery}, T_{surgery 15 min}, T_{surgery 30 min}, T_extubation, T_{extubation 5 min}). Propofol amount for induction, time between induction and initial surgery, demand of antihypertensive therapy, rescue atropine were recorded. Quantitative clinical and demographic characteristics were compared using One Way ANOVA. The values were compared using One-way Analysis of Variance. Additionally periodic variations were examined by One way Repeated Measures Analysis of Variance for groups separately.

Results

SBP was significantly different between normotensive and hypertensive groups at the following time points: T_{preop 5 min}, T_{preop 10 min}, T_induction, T_intubation, T_{intubation 5 min} and T_{initial surgery}. MBP was significantly different in the hypertensive groups at T_induction, T_intubation, T_{intubation 5 min,} T_{initial surgery}, T_{surgery 15 min}, T_{surgery 30 min,} T_extubation and T_{extubation 5 min}. The perioperative requirements for antihypertensive drugs were significantly higher in Group HM.

Conclusion

In the hypertensive patients, dexmedetomidine premedication provides better hemodynamic stability compared with midazolam, and because it decreases the antihypertensive requirements, its use might be beneficial.

Trial registration

Trial registration: Clinicaltrials.gov identifier: NCT02058485.

Isoflurane but not halothane minimum alveolar concentration-sparing response of dexmedetomidine is enhanced in rats chronically treated with selective α2-adrenoceptor agonist

Halothane minimum alveolar concentration (MAC)-sparing response is preserved in rats rendered tolerant to the action of dexmedetomidine. It has been shown that halothane and isoflurane act at different sites to produce immobility. The authors studied whether there was any difference between halothane and isoflurane MAC-sparing effects of dexmedetomidine in rats after chronic administration of a low dose of this drug. Twenty-four female Wistar rats were randomly allocated into four groups of six animals: two groups received 10 μg/kg intraperitoneal dexmedetomidine for five days (treated groups) and the other two groups received intraperitoneal saline solution for five days (naive groups) prior to halothane or isoflurane MAC determination (one treated and one naive group of halothane and one treated and one naive group of isoflurane). Halothane or isoflurane MAC determination was performed before (basal) and 30 min after an intraperitoneal dose of 30 μg/kg of dexmedetomidine (post-dex) from alveolar gas samples at the time of tail clamp. Administration of an acute dose of dexmedetomidine to animals that had chronically received dexmedetomidine resulted in a MAC-sparing effect that was similar to that seen in naive animals for halothane; however, the same treatment increased the MAC-sparing response of dexmedetomidine for isoflurane. Isoflurane but not halothane MAC-sparing response of acutely administered dexmedetomidine is enhanced in rats chronically treated with this drug.

Effect of dexmedetomidine on the minimum alveolar concentration of isoflurane in cats

This study reports the effects of dexmedetomidine on the minimum alveolar concentration of isoflurane (MAC(iso) ) in cats. Six healthy adult female cats were used. MAC(iso) and dexmedetomidine pharmacokinetics had previously been determined in each individual. Cats were anesthetized with isoflurane in oxygen. Dexmedetomidine was administered intravenously using target-controlled infusions to maintain plasma concentrations of 0.16, 0.31, 0.63, 1.25, 2.5, 5, 10, and 20 ng/mL. MAC(iso) was determined in triplicate at each target plasma dexmedetomidine concentration. Blood samples were collected and analyzed for dexmedetomidine concentration. The following model was fitted to the concentration-effect data: [Formula in text] where MAC(iso.c) is MAC(iso) at plasma dexmedetomidine concentration C, MAC(iso.0) is MAC(iso) in the absence of dexmedetomidine, I(max) is the maximum possible reduction in MAC(iso), and IC(50) is the plasma dexmedetomidine concentration producing 50% of I(max). Mean ± SE MAC(iso.0), determined in a previous study conducted under conditions identical to those in this study, was 2.07 ± 0.04. Weighted mean ± SE I(max), and IC(50) estimated by the model were 1.76 ± 0.07%, and 1.05 ± 0.08 ng/mL, respectively. Dexmedetomidine decreased MAC(iso) in a concentration-dependent manner. The lowest MAC(iso) predicted by the model was 0.38 ± 0.08%, illustrating that dexmedetomidine alone is not expected to result in immobility in response to noxious stimulation in cats at any plasma concentration.

High concentrations of dexmedetomidine inhibit compound action potentials in frog sciatic nerves without alpha(2) adrenoceptor activation

BACKGROUND AND PURPOSE

Dexmedetomidine, an alpha(2)-adrenoceptor agonist, exhibits anti-nociceptive actions at the spinal cord and enhances the effect of local anaesthetics in the peripheral nervous system. Although the latter action may be attributed in part to inhibition of nerve conduction produced by dexmedetomidine, this has not been fully examined yet.

EXPERIMENTAL APPROACH

We examined the effects of various adrenoceptor agonists including dexmedetomidine, and tetracaine, a local anaesthetic, on compound action potentials (CAPs) recorded from the frog sciatic nerve, using the air-gap method.

KEY RESULTS

Dexmedetomidine reversibly and concentration-dependently reduced the peak amplitude of CAPs (IC(50)= 0.40 mmol x L(-1)). This action was not antagonized by two alpha(2)-adrenoceptor antagonists, yohimbine and atipamezole; the latter antagonist itself reduced CAP peak amplitude. Clonidine and oxymetazoline, two other alpha(2)-adrenoceptor agonists, also inhibited CAPs; the maximum effect of clonidine was only 20%, while oxymetazoline was less potent (IC(50)= 1.5 mmol x L(-1)) than dexmedetomidine. On the other hand, (+/-)-adrenaline, (+/-)-noradrenaline, alpha(1)-adrenoceptor agonist (-)-phenylephrine and beta-adrenoceptor agonist (-)-isoprenaline (each 1 mmol x L(-1)) had no effect on CAPs. Tetracaine reversibly reduced CAP peak amplitude (IC(50) of 0.014 mmol x L(-1)).

CONCLUSIONS AND IMPLICATIONS

Dexmedetomidine reduced CAP peak amplitude without alpha(2)-adrenoceptor activation (at concentrations >1000-fold higher than those used as alpha(2) adrenoceptor agonist), with a lower potency than tetracaine. CAPs were inhibited by other alpha(2) adrenoceptor agonists, oxymetazoline and clonidine, and also an alpha(2) adrenoceptor antagonist atipamezole. Thus, some drugs acting on alpha(2) adrenoceptors are able to block nerve conduction.

Perioperative use of α2-adrenoceptor agonists and the cardiac patient

The centrally acting alpha2-adrenoceptor agonists clonidine and dexmedetomidine have been used with success to provide haemodynamic stability for patients undergoing surgery. Particularly in the case of patients with overt or underlying cardiac disease the actions of alpha2-adrenoceptor agonists, which include maintenance of stable systemic blood pressure and low heart rate and a reduction in overall oxygen consumption, can be expected to reduce the risk of procedure-related cardiac events. This expectation has been corroborated in clinical trials with clonidine, dexmedetomidine and mivazerol and meta-analyses; additional large controlled trials would be instructive in establishing a robust estimate of the scale of the benefit. In addition, alpha2-adrenoceptor agonists used as premedication have been shown to substantially reduce anaesthetic requirements among surgical patients, and the use of these agents has been associated with a reduced risk of postoperative delirium, which may be expected to improve considerably the postoperative course for at-risk patients. Dexmedetomidine is the only alpha2-adrenoceptor agonist currently approved for use in the intensive care unit. A distinctive feature of dexmedetomidine in that setting is that in addition to haemodynamic stability it confers a distinctive and advantageous quality of sedation: patients are tranquil but responsive to requests from attending staff. This review examines the pharmacological principles underlying the use of alpha2-adrenoceptor agonists as adjuncts to surgery and clinical experience in that indication.

Alpha-adrenergic stimulation of ERK phosphorylation in astrocytes is alpha(2)-specific and may be mediated by transactivation

The highly specific alpha(2)-adrenergic agonist, dexmedetomidine, has hypnotic-sedative, anesthetic-sparing and analgesic effects, and it protects neurons against ischemia. The alpha(1)-adrenergic agonist, phenylephrine, does not share dexmedetomidine's pharmacological properties, although both dexmedetomidine and phenylephrine increase free cytosolic Ca(2+) ([Ca(2+)](i)) in astrocytes, and most of dexmedetomidine's actions in the brain are exerted on postjunctional receptors. alpha(2)-Adrenergic receptors are abundant on astrocytes. Dexmedetomidine-mediated 'down-streamn' signal transduction was therefore investigated in primary cultures of mouse astrocytes and contrasted with that of phenylephrine. The cultures were incubated with dexmedetomidine concentrations known to be pharmacologically active and to act specifically on alpha(2)-adrenergic receptors (25-100 nM). ERK(1/2) phosphorylation was measured using specific antibodies. Peak increases of ERK(1/2) phosphorylation occurred at 50 nM dexmedetomidine, with less effect at higher concentrations. Phenylephrine caused ERK phosphorylation only at a concentration high enough to exert non subtype-specific effects (10 microM), and this effect was counteracted by the alpha(2)-adrenergic antagonist atipamezole. The phosphorylation of ERK was reduced by tyrphostin AG1478, an inhibitor of phosphorylation of the epidermal growth factor receptor (EGFR), and by heparin, which neutralizes heparin-binding epithelial growth factor (HB-EGF), suggesting the involvement of a transactivation process, in which alpha(2)-adrenergic stimulation leads to proteolytic shedding of HB-EGF (and perhaps other EGFR agonists) from transmembrane-spanning precursors.

A correlation between dexmedetomidine-induced biphasic increases in free cytosolic calcium concentration and energy metabolism in astrocytes

The alpha(2)-adrenergic agonist, dexmedetomidine, increases free cytosolic calcium concentration ([Ca(2+)](i)) in astrocytes, but not in neurons. The present study was performed to characterize the origin of the increased Ca(2+) in mouse astrocytes cultured from the cerebral cortex, the dose dependence of the effect, and its functional consequences. The increase in [Ca(2+)](i) was independent of extracellular Ca(2+), but was inhibited by dantrolene, showing that it is derived from intracellular stores; two peaks in [Ca(2+)](i) were demonstrated-one around 100 nM dexmedetomidine and the other in the low micromolar range. A similar dose dependence was found for pyruvate dehydrogenation, the initial metabolic reaction of oxidative degradation of pyruvate, suggesting that the these events are interrelated. The alpha(2)-adrenergic antagonist, yohimbine, abolished the metabolic stimulation at both peaks. However, whereas the increase in [Ca(2+)] (i) at 100 nM is abolished by yohimbine, increase in the micromolar range was partly inhibited by yohimbine and partly by idazoxan, an inhibitor at the imidazoline-preferring site. The stimulation of energy metabolism in cerebrocortical astrocytes may explain the repeated finding that dexmedetomidine does not decrease oxidative metabolism in the brain in vivo. The functional importance of the additional imidazoline receptor-mediated increase in [Ca(2+)](i) at large dexmedetomidine concentrations is unknown.

IMPLICATIONS

Cytosolic calcium concentration and metabolism were measured in cultured astrocytes, the predominant glial cells. The results suggest that dexmedetomidine may owe its anesthetic effects to a Ca(2+)-dependent increase in astrocytic energy metabolism, allowing these cells to more effectively remove extracellular glutamate and potassium ions, and thus, decreasing neuronal excitability.

Volatile anesthetics modulate the binding of guanine nucleotides to the alpha subunits of heterotrimeric GTP binding proteins

The effects of volatile anesthetics on guanine nucleotide binding to the purified alpha subunits of heterotrimeric GTP binding (G) proteins were studied. At sub-anesthetic doses, halothane, isoflurane, enflurane and sevoflurane inhibit exchange of GTPgammaS for GDP bound to Galpha subunits and markedly enhance the dissociation of GTPgammaS, but fail to suppress GDPbetaS release. Nucleotide exchange from non-myristoylated Galpha(i1) is similarly inhibited in the absence of any membrane lipid or detergent. The degrees of inhibition of GDP/GTPgammaS exchange and enhancement of GTPgammaS dissociation are in the same order: alpha(i2)alpha(i1)alpha(i3)alpha(s). By contrast, Galpha(o), which is closely related to Galpha(i), is completely insensitive to anesthetics. We conclude that volatile agents, at clinically relevant doses, have a direct effect on the conformation and stability of the GTP/Mg(2+) bound state of some, but not all Galpha subunits. By destabilizing this state, volatile agents may uncouple metabotropic and other heptahelical receptors from pathways modulating neuronal excitation.

Increase in the threshold of pain and touch sensation in the human face with clonidine plus 30% nitrous oxide

OBJECTIVE

This study was undertaken to assess the effects of clonidine combined with 30% nitrous oxide on tactile and pain sensations in the human face.

STUDY DESIGN

Thirty-three subjects were involved in the study. The subjects were divided into 4 groups: 100% oxygen with placebo; 30% N2O with placebo; 100% oxygen with clonidine (0.075 mg), and 30% N2O with clonidine. Three tests for the threshold of pain sensation and tactile sensation were made at 60 minutes before and 0, 15, and 30 minutes during N2O or O2 inhalation.

RESULTS

(1) The N2O with clonidine significantly increased the threshold of pain and tactile sensation in comparison with the other 3 treatments. (2) In terms of pain sensation, both N2O and clonidine showed significant increases in threshold of pain in comparison with the control values.

CONCLUSIONS

These results indicate that the analgesic effects of 30% nitrous oxide are enhanced when use of the gas is combined with prior clonidine administration.