An In Vitro Model for Identifying Cardiac Side Effects of Anesthetics.

The understanding of anesthetic side effects on the heart has been hindered by the lack of sophisticated clinical models. Using micropatterned human-induced pluripotent stem cell-derived cardiomyocytes, we obtained cardiac muscle depressant profiles for propofol, etomidate, and our newly identified anesthetic compound KSEB01-S2. Propofol was the strongest depressant among the 3 compounds tested, exhibiting the largest decrease in contraction velocity, depression rate, and beating frequency. Interestingly, KSEB01-S2 behaved similarly to etomidate, suggesting a better cardiac safety profile. Our results provide a proof-of-concept for using human-induced pluripotent stem cell-derived cardiomyocytes as an in vitro platform for future drug design.

A newly developed anesthetic based on a unique chemical core.

Intravenous anesthetic agents are associated with cardiovascular instability and poorly tolerated in patients with cardiovascular disease, trauma, or acute systemic illness. We hypothesized that a new class of intravenous (IV) anesthetic molecules that is highly selective for the slow type of γ-aminobutyric acid type A receptor (GABAAR) could have potent anesthetic efficacy with limited cardiovascular effects. Through in silico screening using our GABAAR model, we identified a class of lead compounds that are N-arylpyrrole derivatives. Electrophysiological analyses using both an in vitro expression system and intact rodent hippocampal brain slice recordings demonstrate a GABAAR-mediated mechanism. In vivo experiments also demonstrate overt anesthetic activity in both tadpoles and rats with a potency slightly greater than that of propofol. Unlike the clinically approved GABAergic anesthetic etomidate, the chemical structure of our N-arylpyrrole derivative is devoid of the chemical moieties producing adrenal suppression. Our class of compounds also shows minimal to no suppression of blood pressure, in marked contrast to the hemodynamic effects of propofol. These compounds are derived from chemical structures not previously associated with anesthesia and demonstrate that selective targeting of GABAAR-slow subtypes may eliminate the hemodynamic side effects associated with conventional IV anesthetics.

External validation of simulation-based assessments with other performance measures of third-year anesthesiology residents.

INTRODUCTION: There has been interest in the use of high-fidelity medical simulation to evaluate performance. We hypothesized that technical and nontechnical performance in the simulated environment is related to other various criterion measures, providing evidence to support the validity of the scores from the performance-based assessment. METHODS: Twelve third-year anesthesia residents participated in a series of 6 short 5-minute scenarios and 1 longer 30-minute scenario. The short scenarios measured technical skills, whereas the longer one focused on nontechnical skills. Two independent raters scored subjects using analytic and holistic ratings. Short scenarios involved acute hemorrhage, blocked endotracheal tube, bronchospasm, hyperkalemia, tension pneumothorax, and unstable ventricular tachycardia. The long scenario concerned management of myocardial ischemia/infarction leading to cardiac arrest. Scores from the simulations were correlated with (a) rankings generated from an Internet-based global ranking instrument that categorized residents based on overall clinical ability and (b) residency board scores. RESULTS: There were moderate correlations between various participant scores from the simulation-based assessment and aggregate rankings based on the global ranking instrument and residency examination scores. CONCLUSIONS: The associations between simulator performance, both for technical and nontechnical skills, and other markers of ability provide some evidence to support the validity of simulation-based assessment scores. Replication studies with larger numbers of residents are warranted.

Association of admission hematocrit with 6-month and 1-year mortality in intensive care unit patients.

BACKGROUND: This study examined the association of hematocrit (Hct) levels measured upon intensive care unit (ICU) admission and red blood cell transfusions to long-term (1-year or 180-day) mortality for both surgical and medical patients. STUDY DESIGN AND METHODS: Administrative and laboratory data were collected retrospectively on 2393 consecutive medical and surgical male patients admitted to the ICU between 2003 and 2009. We stratified patients based on their median Hct level during the first 24 hours of their ICU stay (Hct < 25.0%, 25% ≤ Hct < 30%, 30% ≤ Hct < 39%, and 39.0% and higher). An extended Cox regression analysis was conducted to identify the time period after ICU admission (0 to <180, 180 to 365 days) when low Hct (<25.0) was most strongly associated with mortality. The unadjusted and adjusted relationship between admission Hct level, receipt of a transfusion, and 180-day mortality was assessed using Cox proportional hazards regression modeling. RESULTS: Patients with an Hct level of less than 25% who were not transfused had the worst mortality risk overall (hazard ratio [HR], 6.26; 95% confidence interval [CI], 3.05-12.85; p < 0.001) during the 6 months after ICU admission than patients with a Hct level of 39.0% or more who were not transfused. Within the subgroup of patients with a Hct level of less than 25% only, receipt of a transfusion was associated with a significant reduction in the risk of mortality (HR, 0.40; 95% CI, 0.19-0.85; p = 0.017). CONCLUSION: Anemia of a Hct level of less than 25% upon admission to the ICU, in the absence of a transfusion, is associated with long-term mortality. Our study suggests that there may be Hct levels below which the transfusion risk-to-benefit imbalance reverses.

Feasibility of an internet-based global ranking instrument.

BACKGROUND: Single-item global ratings are commonly used at the end of undergraduate clerkships and residency rotations to measure specific competencies and/or to compare the performances of individuals against their peers. We hypothesized that an Internet-based instrument would be feasible to adequately distinguish high- and low-ability residents. MATERIALS AND METHODS: After receiving Institutional Review Board approval, we developed an Internet-based global ranking instrument to rank 42 third-year residents (21 in 2008 and 21 in 2009) in a major university teaching hospital's department of anesthesiology. Evaluators were anesthesia attendings and nonphysicians in 3 tertiary-referral hospitals. Evaluators were asked this ranking question: "When it comes to overall clinical ability, how does this individual compare to all their peers?" RESULTS: For 2008, 111 evaluators completed the ranking exercise; for 2009, 79 completed it. Residents were rank-ordered using the median of evaluator categorizations and the frequency of ratings per assigned relative performance quintile. Across evaluator groups and study years, the summary evaluation data consistently distinguished the top and bottom resident cohorts. DISCUSSION: An Internet-based instrument, using a single-item global ranking, demonstrated feasibility and can be used to differentiate top- and bottom-performing cohorts. Although ranking individuals yields norm-referenced measures of ability, successfully identifying poorly performing residents using online technologies is efficient and will be useful in developing and administering targeted evaluation and remediation programs.

Use of medical simulation to explore equipment failures and human-machine interactions in anesthesia machine pipeline supply crossover.

BACKGROUND: High-fidelity medical simulation can be used to explore failure modes of technology and equipment and human-machine interactions. We present the use of an equipment malfunction simulation scenario, oxygen (O(2))/nitrous oxide (N(2)O) pipeline crossover, to probe residents' knowledge and their use of anesthetic equipment in a rapidly escalating crisis. METHODS: In this descriptive study, 20 third-year anesthesia residents were paired into 10 two-member teams. The scenario involved an Ohmeda Modulus SE 7500 anesthetic machine with a Datex AS/3 monitor that provided vital signs and gas monitoring. Before the scenario started, we switched pipeline connections so that N(2)O entered through the O(2) pipeline and vice versa. Because of the switched pipeline, the auxiliary O(2) flowmeter delivered N(2)O instead of O(2). Two expert, independent raters reviewed videotaped scenarios and recorded the alarms explicitly noted by participants and methods of ventilation. RESULTS: Nine pairs became aware of the low fraction of inspired O(2) (Fio(2)) alarm. Only 3 pairs recognized the high fraction of inspired N(2)O (Fin(2)o) alarm. One group failed to recognize both the low Fio(2) and the high Fin(2)o alarms. Nine groups took 3 or more steps before instigating a definitive route of oxygenation. Seven groups used the auxiliary O(2) flowmeter at some point during the management steps. CONCLUSIONS: The fact that so many participants used the auxiliary O(2) flowmeter may expose machine factors and related human-machine interactions during an equipment crisis. Use of the auxiliary O(2) flowmeter as a presumed external source of O(2) contributed to delays in definitive treatment. Many participants also failed to notice the presence of high N(2)O. This may have been, in part, attributable to 2 facts that we uncovered during our video review: (a) the transitory nature of the "high N(2)O" alert, and (b) the dominance of the low Fio(2) alarm, which many chose to mute. We suggest that the use of high-fidelity simulations may be a promising avenue to further examine hypotheses related to failure modes of equipment and possible management response strategies of clinicians.

Gabrb3 gene deficient mice exhibit increased risk assessment behavior, hypotonia and expansion of the plexus of locus coeruleus dendrites.

Gabrb3 gene deficient (gabrb3(-/-)) mice, control littermates (gabrb3(+/+)) and their progenitor strains C57Bl/6J and 129/SvJ were assessed for changes in the morphology of the main noradrenergic nuclei, the locus coeruleus (LC) and LC-associated behaviors including anxiety and muscle tone. While the area defined by the cell bodies of the LC was found not to differ between gabrb3(-/-) mice and controls, the pericoerulear dendritic zone of the LC was found to be significantly enlarged in gabrb3(-/-) mice. Relative to controls, gabrb3(-/-) mice were also found to be hypotonic, as was indicated by poor performance on the wire hanging task. Gabrb3(-/-) mice also exhibited a significant increase in stretch-attend posturing, a form of risk assessment behavior associated with anxiety. However, in the plus maze, a commonly used behavioral test for assessing anxiety, no significant difference was observed between gabrb3(-/-) and control mice. Lastly, relative to controls, gabrb3(-/-) mice exhibited significantly less marble burying behavior, a method commonly used to assess obsessive-compulsive behavior. However, the poor marble burying performance of the gabrb3(-/-) mice could be associated with the hypotonic condition exhibited by these mice. In conclusion, the results of this study indicate that the gabrb3 gene contributes to LC noradrenergic dendrite development with the disruption of this gene in mice resulting in an enlarged plexus of LC dendrites with a concurrent reduction in muscle tone and marble burying behavior, an increase in risk assessment behavior but no change in the plus maze parameters that are commonly used for assessing anxiety.

The role of the craniospinal nerves in mediating the antinociceptive effect of transcranial electrostimulation in the rat.

Transcranial electrostimulation (TES) has been reported to elicit significant analgesia, but its mechanism of action has not been elucidated. In a recently introduced clinically relevant rat model of TES we have validated and characterized the TES antinociceptive effect, suggesting involvement of the sensory nerves of the rat's scalp in mediating that effect. In this study, we have further investigated the role of the craniospinal nerves by attempting to block the TES antinociceptive effect with local anesthetic injected under the TES electrodes. We also applied different transcutaneous electrical nerve stimulation modalities through the TES electrodes and compared the elicited antinociceptive effect to that of TES. The antinociceptive effect was assessed by measuring nociceptive thresholds in the tail-flick latency test in awake, unrestrained male rats. Data were analyzed by one-way analysis of variance followed by the Bonferroni t-test. The TES antinociceptive effect was significantly reduced after local anesthetic injection, and administration of 100 Hz transcutaneous electrical nerve stimulation was, over time, capable of eliciting the same degree of antinociceptive effect as TES. We conclude that sensory craniospinal nerves play a critical role in mediating the TES antinociceptive action and offer a hypothesis on the underlying mechanism(s) responsible for this action.

Comparative analgesic and mental effects of increasing plasma concentrations of dexmedetomidine and alfentanil in humans.

BACKGROUND: In animals, systemic and intrathecal administration of the alpha2 -adrenergic receptor agonist dexmedetomidine results in robust antinociceptive effects in models of heat pain. In humans, systemically administered dexmedetomidine is approved for sedating patients in the intensive care unit. However, whether systemic administration of dexmedetomidine in humans produces significant analgesia at doses causing sedation but not unconsciousness remains controversial. METHODS: This study in human volunteers used a placebo-controlled, double-blind, and randomized design to examine whether dexmedetomidine at doses causing mild to severe sedation produces analgesia in experimental models of heat and electrical pain. Results were compared to the effects of the mu-opioid receptor agonist alfentanil. A computer-controlled infusion provided four median step-up plasma concentrations of dexmedetomidine (0.09, 0.24, 0.54, and 1.23 ng/ml) and alfentanil (13.4, 33.8, 67.8, and 126.1 ng/ml). RESULTS: Sedative and cognitive effects of dexmedetomidine were dose-dependent, resulting in a median sedation score of 95 of 100 and slowing of cognitive speed (reaction time, trail-making test) by a factor of about two at the highest plasma concentration. Dexmedetomidine did not attenuate heat or electrical pain. Alfentanil caused severe sedation (median sedation score 88 of 100) and slowed cognitive speed by a factor of approximately 1.4 at the highest plasma concentration. Alfentanil attenuated heat and electrical pain dose dependently. CONCLUSION: This study documents that systemic dexmedetomidine lacks analgesic efficacy for heat and electrical pain at doses causing mild to severe sedation. These results provide further evidence suggesting that systemic administration of dexmedetomidine lacks broad analgesic activity in models of acute pain at doses not rendering humans unconscious.

The antinociceptive effect of transcranial electrostimulation with combined direct and alternating current in freely moving rats.

UNLABELLED: Transcranial electrostimulation (TES) has been reported to elicit significant analgesia, allowing a substantial reduction of intraoperative opioids. Acceptance of TES into clinical practice is hampered by lack of controlled clinical trials and inconclusive animal data regarding the TES antinociceptive action. This inconclusive data may be explained, in part, by failure in rat experiments to simulate the variables used in humans when TES electrodes are positioned on the skin. In this study we validated the TES antinociceptive effect in a novel animal model of cutaneously administered TES, when the stimulating conditions mimic the ones used in clinical practice. The antinociceptive effect was assessed by measuring nociceptive thresholds in the tail-flick and hot-plate latency tests in awake, unrestrained male rats. Data were analyzed by analysis of variance and mixed-effects population modeling. The administration of TES at 2.25 mA produced an almost immediate, sustained, frequency-dependent (40-60 Hz) antinociceptive effect, reaching approximately 50% of the maximal possible value. We conclude that an antinociceptive effect of cutaneously administered TES can be demonstrated in the rat. Some characteristics of the effect suggest an important role of the sensory nerves of the rat's scalp in mediating the TES antinociceptive response. IMPLICATIONS: Transcranial electrostimulation produces a significant, frequency-dependent antinociceptive effect that may be mediated by cutaneous nerves of the scalp.

Activation of alpha2 adrenergic receptors suppresses fear conditioning: expression of c-Fos and phosphorylated CREB in mouse amygdala.

alpha(2) adrenergic agonists such as dexmedetomidine generally suppress noradrenergic transmission and have sedative, analgesic, and antihypertensive properties. Considering the importance of the neurotransmitter norepinephrine in forming memories for fearful events, we have investigated the acute and chronic effects of dexmedetomidine on discrete cue and contextual fear conditioning in mice. When administered before training, dexmedetomidine (10-20 microg/kg, i.p.) selectively suppressed discrete cue fear conditioning without affecting contextual memory. This behavioral change was associated with a decrease in memory retrieval-induced expression of c-Fos and P-CREB in the lateral, basolateral, and central nuclei of the amygdala. Dexmedetomidine's action on discrete cue memory did not occur in alpha(2A) adrenoceptor knockout (KO) mice. When dexmedetomidine was administered after training, it suppressed contextual memory, an effect that did not occur in alpha(2A) adrenoceptor KO mice. We conclude that dexmedetomidine, acting at alpha(2A) adrenoceptors, must be present during the encoding process to decrease discrete cue fear memory; however, its ability to suppress contextual memory is likely the result of blocking the consolidation process. The ability of alpha(2) agonists to suppress fear memory may be a valuable property clinically in order to suppress the formation of memories during stressful situations.

A substance P receptor (NK1) antagonist enhances the widespread osteoporotic effects of sciatic nerve section.

The long-term effects of sciatic nerve section on bone mineral density (BMD) were studied using dual-energy X-ray absorptiometry (DEXA) in skeletally mature rats. Unilateral sciatic neurectomy caused the rapid loss of cancellous bone in the proximal and distal femur and tibia in the ipsilateral hindlimb and, to a lesser extent, in the contralateral intact hindlimb. The reduction in BMD rapidly progressed for 4 weeks after sciatic section and then gradually stabilized with no evidence of recovery at 12 weeks. The development of osteoporosis in the contralateral intact hindlimb was a novel finding. There was no evidence of disuse in the normal contralateral hindlimb after unilateral sciatic section; grid-crossing activity over a 24-h interval was unchanged and there was no reduction in weight bearing on the contralateral normal hindpaw during the stance phase of ambulation. Unilateral peripheral nerve lesions have well-documented effects on substance P content and function in the corresponding contralateral intact nerve. We hypothesized that after sciatic section a reduction in substance P signaling might contribute to bone loss in the contralateral hindlimb. Daily administration of the substance P receptor (NK1) antagonist LY303870 for 2 weeks caused significant loss of cancellous bone in the denervated and the contralateral hindlimb, evidence that substance P signaling sustained bone density after nerve section. After sciatic neurectomy there was a 33% reduction in sciatic nerve stimulation-evoked extravasation in the contralateral intact hindlimb, indicating transmedian inhibition of substance P signaling after nerve injury. Furthermore, there was a 50% reduction in the substance P content in both tibias after unilateral sciatic section. Collectively, these data support the hypothesis that a widespread reduction in substance P content in bone contributes to the osteoporotic effects of sciatic neurectomy and that residual substance P signaling maintains bone integrity after nerve section in both the denervated and contralateral intact hindlimb.

Augmentation of the noradrenergic system in alpha-2 adrenergic receptor deficient mice: anatomical changes associated with enhanced fear memory.

We have investigated sensitivity to the conditioned fear procedure of mice is influenced by the genetic deletion of alpha2A adrenoceptors (ARs). We observed a heightened freezing response in the discrete cue memory test in alpha2A AR knockout (alpha2A AR KO) mice and in D79N mice, a transgenic mouse strain with functionally impaired alpha2A ARs. No significant differences in contextual memory were observed between control and alpha2A AR KO or D79N mice suggesting a minimal role for the noradrenergic system in contextual memory. We speculated that the increased freezing response of the alpha2A AR KO and D79N mice in the discrete cue setting was due to increased release of norepinephrine evoked by the unconditioned footshock stimulus. In alpha2A AR KO mice we measured a doubling in the number of noradrenergic neurons in the locus coeruleus (LC) and a large increase in the cell volume of tyrosine hydroxylase positive neurons, likely due to selective preservation of large, multipolar neurons in the subcoeruleus. Hyperplasia of the noradrenergic neurons in the nucleus tractus solitarius, A5 and A7, was also observed. Alpha2A AR KO mice exhibit greater c-Fos expression in the LC compared to wild type mice suggesting that the LC neurons in the alpha2A AR KO mice were spontaneously more active. This study suggests that alpha2A ARs are involved in the development of the central noradrenergic system and raises the possibility that alterations in alpha2A AR expression may contribute to variations in fear and stress responses.

A substance P receptor (NK1) antagonist can reverse vascular and nociceptive abnormalities in a rat model of complex regional pain syndrome type II.

Sciatic nerve section in rats evokes chronic hindlimb edema, pain behavior, and hyperalgesia, a syndrome resembling complex regional pain syndrome (CRPS II) in man. Furthermore, there is an increase in spontaneous protein extravasation in the hindpaw skin of rats after sciatic transection, similar to the increased protein extravasation observed in the edematous limbs of CRPS patients. Now we demonstrate that sciatic nerve section also generates chronic hindlimb warmth, distal articular tenderness, allodynia, and periarticular osteoporosis, sequelae of nerve injury resembling those observed in CRPS. We postulated that facilitated substance P signaling may contribute to these vascular and nociceptive abnormalities and attempted to reverse these changes with the long acting substance P receptor (NK(1)) antagonist LY303870. Hindpaw spontaneous extravasation was inhibited by LY303870. Systemic administration of LY303870 also reversed hindpaw edema and cutaneous warmth. Intrathecal, but not systemic administration of LY303870 reversed soft tissue and articular mechanical hyperalgesia in the hindpaw. Collectively, these data further support the hypothesis that the sciatic nerve transection model closely resembles CRPS and that substance P contributes to the spontaneous extravasation, edema, warmth, and mechanical hyperalgesia observed in this model.

Dexmedetomidine fails to cause hyperalgesia after cessation of chronic administration.

UNLABELLED: Hyperalgesia occurring after the cessation of chronic opioid administration occurs in humans and has been modeled in rodents with chronic systemic and intrathecal administration paradigms. It is, however, unclear if this type of postanalgesic hyperalgesia is unique to opioids. The alpha(2)-adrenergic receptor agonist, dexmedetomidine (Dex), is similar to opioids in that it is an analgesic that interacts with cell-surface receptors linked to the inhibition of adenylate cyclase and the modulation of ion channel activity. In these studies, we first constructed antinociceptive dose-response curves for Dex and morphine (MSO4). The 50% effective doses for Dex and MSO4 administered intraperitoneally to C57Bl/6 mice were 75 micro g/kg and 5.2 mg/kg, respectively. Using equally effective doses, we treated separate groups of mice with twice-daily injections of Dex or MSO4 for 5 days. Tolerance to these drugs was documented after this period. In the 16-72 h after cessation of administration, MSO4-treated mice demonstrated both thermal hyperalgesia and mechanical allodynia. However, the Dex-treated mice showed no changes in their thermal or mechanical withdrawal thresholds. We conclude that using this experimental paradigm, opioids but not an alpha(2)-adrenergic agonist, cause hyperalgesia and allodynia after cessation of chronic administration. IMPLICATIONS: The cessation of the administration of opioids is associated with hyperalgesia in both humans and other animals. However, antinociceptive dexmedetomidine does not seem to be associated with this type of hyperalgesia syndrome during periods of abstinence.

Meperidine exerts agonist activity at the alpha(2B)-adrenoceptor subtype.

BACKGROUND: The opioid agonist meperidine has actions, such as antishivering, that are more pronounced than those of other opioid agonists and that are not blocked with nonselective opioid antagonists. Agonists at the alpha(2) adrenoceptors, such as clonidine, are very effective antishivering drugs. Preliminary evidence also indicates that meperidine interacts with alpha(2) adrenoceptors. The authors therefore studied the ability of meperidine to bind and activate each of the alpha(2)-adrenoceptor subtypes in a transfected cell system. METHODS: The ability of meperidine to bind to and inhibit forskolin-stimulated cyclic adenosine monophosphate formation as mediated by the three alpha(2)-adrenoceptor subtypes transiently transfected into COS-7 cells has been tested. The ability of the opioid antagonist naloxone and the alpha(2)-adrenoceptor antagonists yohimbine and RX821002 to block the analgesic action of meperidine in the hot-plate test was also assessed. The ability of meperidine to fit into the alpha(2B) adrenoceptor was assessed using molecular modeling techniques. RESULTS: Meperidine bound to all alpha2-adrenoceptor subtypes, with alpha(2B) having the highest affinity (alpha(2B), 8.6 +/- 0.3 microm; alpha(2C), 13.6 +/- 1.5 microm, P < 0.05; alpha(2A), 38.6 +/- 0.7 microm). Morphine was ineffective at binding to any of the receptor subtypes. Meperidine inhibited the production of forskolin-stimulated cyclic adenosine monophosphate mediated by all receptor subtypes but was most effective at the alpha(2B) adrenoceptor (alpha(2B), 0.6 microm; alpha(2A), 1.3 mm; alpha(2C), 0.3 mm), reaching the same level of inhibition (approximately 70%) as achieved with the alpha2-adrenoceptor agonist dexmedetomidine. The analgesic action of meperidine was blocked by naloxone but not by the alpha 2-adrenoceptor antagonists yohimbine and RX821002. The modeling studies demonstrated that meperidine can fit into the alpha(2B)-adrenoceptor subtype. CONCLUSION: Meperidine is a potent agonist at the alpha2 adrenoceptors at its clinically relevant concentrations, especially at the alpha(2B)-adrenoceptor subtype. Activation of the alpha(2B) receptor does not contribute significantly to the analgesic action of meperidine. This raises the possibility that some of its actions, such as antishivering, are transduced by this mechanism.

Capsaicin sensitive afferents mediate the development of heat hyperalgesia and hindpaw edema after sciatic section in rats.

Sciatic section in rats evokes chronic hyperalgesia, autotomy pain behavior, and hindpaw edema, a constellation resembling complex regional pain syndrome (CRPS) in man. Glucocorticoid treatment inhibits these sequelae of sciatic section and also blocks neurogenic extravasation. Small diameter afferent neurons release substance P (SP), a mediator of both hyperalgesia and neurogenic extravasation. Now, we show that pre-emptive destruction of the small diameter fibers prevents neurogenic extravasation, and prevented the development of heat hyperalgesia and hindpaw edema after sciatic section. Thus, capsaicin sensitive primary afferent neurons which release SP are required for the development of heat hyperalgesia and hindpaw edema in this animal model of CRPS.

Isoflurane and nociception: spinal alpha2A adrenoceptors mediate antinociception while supraspinal alpha1 adrenoceptors mediate pronociception.

BACKGROUND: The authors recently established that the analgesic actions of the inhalation anesthetic nitrous oxide were mediated by noradrenergic bulbospinal neurons and spinal alpha2B adrenoceptors. They now determined whether noradrenergic brainstem nuclei and descending spinal pathways are responsible for the antinociceptive actions of the inhalation anesthetic isoflurane, and which alpha adrenoceptors mediate this effect. METHODS: After selective lesioning of noradrenergic nuclei by intracerebroventricular application of the mitochondrial toxin saporin coupled to the antibody directed against dopamine beta hydroxylase (DbetaH-saporin), the antinociceptive action of isoflurane was determined. Antagonists for the alpha1 and alpha2 adrenoceptors were injected at spinal and supraspinal sites in intact and spinally transected rats to identify the noradrenergic pathways mediating isoflurane antinociception. Null mice for each of the three alpha2-adrenoceptor subtypes (alpha2A, alpha2B, and alpha2C) and their wild-type cohorts were tested for their antinociceptive response to isoflurane. RESULTS: Both DbetaH-saporin treatment and chronic spinal transection enhanced the antinociceptive effects of isoflurane. The alpha1-adrenoceptor antagonist prazosin also enhanced isoflurane antinociception at a supraspinal site of action. The alpha2-adrenoceptor antagonist yohimbine inhibited isoflurane antinociception, and this effect was mediated by spinal alpha2 adrenoceptors. Null mice for the alpha2A-adrenoceptor subtype showed a reduced antinociceptive response to isoflurane. CONCLUSIONS: The authors suggest that, at clinically effective concentrations, isoflurane can modulate nociception via three different mechanisms: (1) a pronociceptive effect requiring descending spinal pathways, brainstem noradrenergic nuclei, and supraspinal alpha1 adrenoceptors; (2) an antinociceptive effect requiring descending noradrenergic neurons and spinal alpha2A adrenoceptors; and (3) an antinociceptive effect mediated within the spinal cord for which no role for adrenergic mechanism has been found.