Analgesic mechanisms of ketamine in the presence and absence of peripheral inflammation

The effect and mechanism of low-dose esketamine in neuropathic pain-related depression-like behavior in rats

BACKGROUND

Clinical evidence suggests that Esketamine (ESK) is an effective treatment for depression. However, the effects of Esketamine in treating depression-like behavior induced by neuropathic pain is unclear. The underlying molecular mechanisms require further investigation to provide new therapeutic targets for the treatment of clinical neuropathic pain-related depression.

METHODS

A neuropathic pain-related depression model was established in rats with spared nerve injury (SNI). Male Sprague-Dawley rats were randomly divided into four groups: Sham Group, SNI group, SNI + Normal Saline (NS) Group and SNI + ESK5mg/kg Group. Mechanical pain thresholds were measured to assess pain sensitivity in SNI rats. On the 14th day after surgery a forced swim test and sucrose preference test were used to evaluate the depressive-like behavior of rats in each group. Further, a proteomic analysis was used to quantify differentially expressed proteins. The Gene Onotology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) pathways were analyzed to explore the main protein targets of SNI in the medial prefrontal cortex. The expression of proteins was detected by Western blotting.

RESULTS

A neuropathic pain-related depression model was established. Compared with the Sham group, the mechanical pain threshold was decreased significantly (13.2 ± 1.0 vs. 0.7 ± 0.01 g n = 8), while immobility on the forced swim test was also decreased (93.1 ± 7.4 vs. 169.5 ± 9.6 s n = 8), and sucrose preference rate was significantly increased (98.8 ± 0.3 vs. 73.1 ± 1.4n = 7) in SNI group rats. Compared with the SNI + NS group, the mechanical pain threshold was not statistically significant, while immobility on the forced swim test was clearly decreased (161.1 ± 11.6 vs. 77.9 ± 5.0 s n = 8), and sucrose preference rate was significantly increased (53.1 ± 8.9 vs. 96.1 ± 1.4n = 7) in SNI + ESK5mg/kg group rats. To further investigate the underlying mechanism, we employed proteomics to identify proteins exhibiting more than a 1.2-fold difference (P < 0.05) in expression levels within each group for subsequent analysis. Relative to the Sham group, 88 downregulated and 104 up-regulated proteins were identified in the SNI group, while 120 and 84 proteins were up- and down-regulated in the Esketamine treatment group compared with the SNI + NS group. Compared with Sham group, the expressions of mGluR5 and Homer1a were up-regulated in the medial prefrontal cortex (mPFC) in SNI group (mGluR5:0.97 ± 0.05 vs 1.47 ± 0.15, Homer1a:1.03 ± 0.06 vs 1.46 ± 0.16n = 6), and down-regulated after intervention with Esketamine (mGluR5:1.54 ± 0.11 vs 1.06 ± 0.07, Homer1a:1.51 ± 0.13 vs 1.12 ± 0.34n = 6).

CONCLUSIONS

Low-dose Esketamine appeared to relieve depression-like behavior induced by neuropathic pain. The Homer1a-mGluR5 signaling pathway might be the mechanism of antidepressant effect of Esketamine.

Chronic Neuropathic Pain and Comorbid Depression Syndrome: From Neural Circuit Mechanisms to Treatment

Chronic neuropathic pain and comorbid depression syndrome (CDS) is a major worldwide health problem that affects the quality of life of patients and imposes a tremendous socioeconomic burden. More than half of patients with chronic neuropathic pain also suffer from moderate or severe depression. Due to the complex pathogenesis of CDS, there are no effective therapeutic drugs available. The lack of research on the neural circuit mechanisms of CDS limits the development of treatments. The purpose of this article is to provide an overview of the various circuits involved in CDS. Notably, activating some neural circuits can alleviate pain and/or depression, while activating other circuits can exacerbate these conditions. Moreover, we discuss current and emerging pharmacotherapies for CDS, such as ketamine. Understanding the circuit mechanisms of CDS may provide clues for the development of novel drug treatments for improved CDS management.

Effect of esketamine on postoperative analgesia and postoperative delirium in elderly patients undergoing gastrointestinal surgery

OBJECTIVE

To investigate the analgesic effect of esketamine combined with low-dose sufentanil in elderly patients after gastrointestinal surgery, and whether the anti-inflammatory effect of esketamine is involved in the mechanism of postoperative delirium.

METHOD

We enrolled sixty elderly patients (age ≥ 65 years old, American Society of Anesthesiologists (ASA) grade I-III) who underwent gastrointestinal surgery. Patients were randomly assigned to Group C (control group) who received sufentanil 2 ug/kg, and Group E (experimental group) who received sufentanil 1.5 ug/kg + esketamine 1 mg/kg, with 30 patients in each group. All patients underwent total intravenous anesthesia during the surgery and were connected to a patient-controlled intravenous analgesia (PCIA) pump after surgery. The primary outcome was the evaluation of pain at 4, 24, 48 h after surgery which was evaluated by NRS scores. In secondary outcomes, inflammation was assessed by measuring IL-6 levels using ELISA. The postoperative delirium and the occurrence of adverse reactions were observed on the 1st and 3rd day after surgery.

RESULTS

The NRS scores at 4, 24, and 48 h after surgery in the experimental group [(4.53 ± 1.22), (3.46 ± 0.73), (1.37 ± 0.99)] were lower than that in the control group [(5.23 ± 1.16), (4.46 ± 0.77), (2.13 ± 0.78)] (P < 0.05). The concentration of serum IL-6 in the experimental group at 24 and 48 h after operation [(15.96 ± 4.65), (11.8 ± 3.24)] were lower than that in the control group [(23.07 ± 4.86), (15.41 ± 4.01)] (P < 0.05); the incidence of postoperative delirium in the experimental group was less than that in the control group (P < 0.05); there was no significant difference in the incidence of postoperative nausea and vomiting between the two groups (P > 0.05), and neither group had nightmares or delirium.

CONCLUSION

Esketamine may enhance postoperative pain management compare with sufentanil, and esketamine has anti-inflammatory effects that reduce the incidence of postoperative delirium.

TRIAL REGISTRATION

Full name of the registry: Chinese Clinical Trial Registry.

TRIAL REGISTRATION NUMBER

ChiCTR2300072374. Date of registration:2023/06/12.

Effects of a Single Sub-Anesthetic Dose of Ketamine on Postoperative Emotional Responses and Inflammatory Factors in Colorectal Cancer Patients

Objective: To investigate the effect of a single sub-anesthetic dose of ketamine on postoperative anxiety, depression, and inflammatory factors in patients with colorectal cancer. Methods: A total of 104 patients undergoing selective colorectal surgery in our hospital from Jan 2015 to Oct 2017 were included and randomly assigned (1:1:1:1) into a 0.1 mg kg^-1 ketamine group (K1 group), 0.2 mg kg^-1 ketamine group (K2 group), 0.3 mg kg^-1 ketamine group (K3 group), or control group (C group). Corresponding doses of ketamine were given intravenously in the K groups (K1, K2, and K3 groups) 5 min before operation, and the same amount of normal saline was given in the C group. The intravenous analgesia program was identical in the four groups. The patients' emotional reactions (anxiety and depression) were assessed by the Hospital Anxiety and Depression Scale (HAD), the quality of postoperative recovery was evaluated by the Quality of Recovery-40 (QoR-40) questionnaire, and the levels of IL-6, IL-8, and TNF-α in peripheral blood were detected by enzyme-linked immunosorbent assay (ELISA) on the day before operation and within 24, 48, and 72 h post-operation respectively. Pain was estimated by the visual analog scale (VAS), and sedation was assessed with Ramsay score 30 min after extubation. The time points of anesthetic end and extubation were recorded. The complications during anesthesia and recovery such as cough and agitation 30 min after extubation were recorded. Results: The anxiety score (HAD-A) and depression score (HAD-D) of the K3 group were significantly lower than those of the C group post-operation (p < 0.05). The QoR-40 score of the K3 group was significantly higher than that of the C group (p < 0.05). The serum levels of IL-6, IL-8, and TNF-α in the K3 group were significantly lower than those in the C group (p < 0.05 and p < 0.01). There were no significant differences in HAD-A, HAD-D, and QoR-40 scores or serum levels of IL-6, IL-8, and TNF-α between the K1 and K2 groups and the C group. There were no significant differences in VAS pain score or Ramsay sedation score among the four groups 30 min after extubation. There were no significant differences in extubation time, postoperative cough, emergence agitation, or delirium among the four groups. Dizziness, nausea, vomiting, diplopia, or other adverse reactions were not found 30 min after extubation. Conclusion: A single sub-anesthetic dose (0.3 mg kg^-1) of ketamine can significantly improve the postoperative anxiety and depression of colorectal cancer patients and reduce the levels of IL-6, IL-8, and TNF-α.

Increased burst coding in deep layers of the ventral anterior cingulate cortex during neuropathic pain

Neuropathic pain induces changes in neuronal excitability and synaptic connectivity in deep layers of the anterior cingulate cortex (ACC) that play a central role in the sensory, emotional and affective consequences of the disease. However, how this impacts ACC in vivo activity is not completely understood. Using a mouse model, we found that neuropathic pain caused an increase in ACC in vivo activity, as measured by the indirect activity marker c-Fos and juxtacellular electrophysiological recordings. The enhanced firing rate of ACC neurons in lesioned animals was based on a change in the firing pattern towards bursting activity. Despite the proportion of ACC neurons recruited by noxious stimuli was unchanged during neuropathic pain, responses to noxious stimuli were characterized by increased bursting. Thus, this change in coding pattern may have important implications for the processing of nociceptive information in the ACC and could be of great interest to guide the search for new treatment strategies for chronic pain.

Participation of the descending noradrenergic inhibitory system in the anti-hyperalgesic effect of acetaminophen in a rat model of inflammation

AIMS

This study investigated the relationship between the analgesic efficacy of acetaminophen and the descending noradrenergic systems using rodent models of inflammatory pain.

MAIN METHODS

Inflammatory pain models were established by carrageenan injection into rats' paws. The models were defined as acute (4 h after carrageenan injection), subacute (24 h after carrageenan injection), and late (1 week after carrageenan injection) phase. To evaluate intravenous acetaminophen treatment, the withdrawal threshold to mechanical stimuli was assessed simultaneously with in vivo microdialysis assay of noradrenaline levels in the locus coeruleus (LC). Further analyses were performed to observe the effect of yohimbine on the treatment and the impact of AM404 treatment, a metabolite of acetaminophen, on noradrenaline levels in the LC.

KEY FINDINGS

In all phases, intravenous acetaminophen had a significant anti-hyperalgesic effect (p < 0.05). There was a significant time-dependent increase in the noradrenaline concentration within the LC (acetaminophen versus saline treatment; at 30 min, p < 0.001; 60 min, p < 0.01) in the subacute pain model, but not in the acute and late phase pain models. Intrathecal pre-injection of yohimbine attenuated the anti-hyperalgesic effect after acetaminophen injection only in the subacute model (p < 0.05). In the subacute pain model, intracerebroventricular administration of AM404 showed the same trend in noradrenaline levels as acetaminophen administration (AM404 versus vehicle group at 30 min, p < 0.001).

SIGNIFICANCE

We found the descending noradrenergic inhibitory system is involved in the antinociceptive action of acetaminophen in the subacute phase of inflammatory pain.

Dissociative and Analgesic Properties of Ketamine Are Independent

BACKGROUND

Ketamine is a dissociative anesthetic with analgesic properties. Ketamine's analgesic properties have been suggested to result from its dissociative properties. To the authors' knowledge, this postulate is unsubstantiated. The authors hypothesize that the dissociative and analgesic properties of ketamine are independent.

METHODS

The authors conducted a single-site, open-label study of ketamine anesthesia (2 mg/kg) in 15 healthy subjects. Midazolam was administered at a prespecified time point to attenuate dissociation. The authors longitudinally assessed precalibrated cuff pain intensity and quality using Patient-Reported Outcomes Measurement Information System questionnaires, and dissociation, using the Clinician Administered Dissociative States Scale. Mixed effects models were used to assess whether dissociation accounted for the effect of ketamine on pain intensity and quality.

RESULTS

The dissociation model demonstrated an inverted U-shaped quadratic relationship between time and dissociation scores. Additive to this effect, midazolam reduced the dissociation adjusted means by 10.3 points (95% CI, 3.4 to 17.1; P = 0.005). The pain intensity model also demonstrated a U-shaped quadratic relationship between time and pain intensity. When the pain intensity model was reanalyzed with dissociation scores as an additional covariate, the dissociation term was not retained in the model, and the other effects were preserved in direction and strength. This result was conserved for nociceptive and neuropathic pain quality.

CONCLUSIONS

Ketamine's analgesic properties are not exclusively caused by dissociation. Thus, ketamine may be used as a probe to advance our knowledge of dissociation independent neural circuits that encode pain.

EDITOR’S PERSPECTIVE

I. Antidepressants and sexual behavior: Weekly ketamine injections increase sexual behavior initially in female and male rats

The present study characterized the effects of weekly ketamine injections on sexual behavior and anxiety in female and male rats, using a dosing protocol that mimics human therapeutic treatment for depression. In Experiment 1A, ketamine (10 mg/kg, i.p.) or saline was injected once per week for four consecutive weeks. The partner preference paradigm was used to measure sexual motivation 30 min after each weekly injection. Briefly, subjects were first given a 10-min test during which they could choose to spend time in the vicinity of a sexually receptive female stimulus or a sexually experienced male stimulus, however physical contact was restricted (no-contact). Immediately after, subjects were given unrestricted access to the stimulus animals (contact). After a washout period, subjects received four additional weekly injections of ketamine or saline, and then were tested for anxiety-like behavior on the elevated plus maze (EPM) after the last injection (Experiment 1B). For Experiment 2, similar procedures were used to test the effects of weekly ketamine injections on sexual motivation (Experiment 2A) and anxiety (Experiment 2B) in male subjects. In female subjects, ketamine increased sexual motivation as measured by greater time spent with the male stimulus, decreased likelihood of leaving after receiving mounts, and shorter return latencies after receiving intromissions, when compared to saline controls. In male subjects, ketamine shortened latency to first mount and first intromission, as well as increased time spent with the female stimulus. Very little anxiety was observed in either group (ketamine or saline) of female or male subjects when tested on the EPM. In conclusion, even after four weeks of ketamine exposure, sexual dysfunction did not emerge in either females or males. In contrast, ketamine increased sexual motivation in both females and males, with an initial robust response. However, as both groups gained sexual experience, the impact of ketamine diminished.

KEA-1010, a ketamine ester analogue, retains analgesic and sedative potency but is devoid of Psychomimetic effects

Background

Ketamine, a widely used anaesthetic and analgesic agent, is known to improve the analgesic efficacy of opioids and to attenuate central sensitisation and opioid-induced hyperalgesia. Clinical use is, however, curtailed by unwanted psychomimetic effects thought to be mediated by N-methyl-D-aspartate (NMDA) receptor antagonism. KEA-1010, a ketamine ester-analogue designed for rapid offset of hypnosis through hydrolysis mediated break-down, has been shown to result in short duration sedation yet prolonged attenuation of nociceptive responses in animal models. Here we report on behavioural effects following KEA-1010 administration to rodents.

Methods

KEA-1010 was compared with racemic ketamine in its ability to produce loss of righting reflex following intravenous injection in rats. Analgesic activity was assessed in thermal tail flick latency (TFL) and paw incision models when injected acutely and when co-administered with fentanyl. Tail flick analgesic assessment was further undertaken in morphine tolerant rats. Behavioural aberration was assessed following intravenous injection in rats undergoing TFL assessment and in auditory pre-pulse inhibition models.

Results

KEA-1010 demonstrated an ED₅₀ similar to ketamine for loss of righting reflex following bolus intravenous injection (KEA-1010 11.4 mg/kg [95% CI 10.6 to 12.3]; ketamine (racemic) 9.6 mg/kg [95% CI 8.5–10.9]). Duration of hypnosis was four-fold shorter in KEA-1010 treated animals. KEA-1010 prolonged thermal tail flick responses comparably with ketamine when administered de novo, and augmented morphine-induced prolongation of tail flick when administered acutely. The analgesic effect of KEA-1010 on thermal tail flick was preserved in opioid tolerant rats. KEA-1010 resulted in increased paw-withdrawal thresholds in a rat paw incision model, similar in magnitude yet more persistent than that seen with fentanyl injection, and additive when co-administered with fentanyl. In contrast to ketamine, behavioural aberration following KEA-1010 injection was largely absent and no pre-pulse inhibition to acoustic startle was observed following KEA-1010 administration in rats.

Conclusions

KEA-1010 provides antinociceptive efficacy in acute thermal and mechanical pain models that augments standard opioid analgesia and is preserved in opioid tolerant rodents. The NMDA channel affinity and psychomimetic signature of the parent compound ketamine is largely absent for KEA-1010.

Involvement of serotonergic and opioidergic systems in the antinociceptive effect of ketamine-magnesium sulphate combination in formalin test in rats

BACKGROUND

Ketamine and magnesium sulphate showed synergic interaction in the tail-immersion test and additive interaction in the rat formalin test. Aim of study was to evaluate the influence of serotonergic and opioidergic system of this combination in the formalin test in rats.

METHODS

Antinociceptive activity was assessed by the formalin test in male Wistar rats (200-250 g). Antagonists (naloxone and methysergide) were administrated 5 min before and magnesium sulphate 5 min after ketamine injection. Formalin (2.5%, 100 μL) was injected into the right hind paw surface (intraplantar) of rats 5 min after ketamine/magnesium combination. Data were recorded as the total time spent in pain related behavior after the injection of formalin or vehicle (0.9% NaCl).

RESULTS

In the intermediate phase of the formalin test, methysergide at a dose of 0.2 mg/kg did not have any effect, but at doses of 0.5 and 1 mg/kg it had a pronociceptive effect. Methysergide (0.2, 0.5 and 1 mg/kg) inhibited the antinociceptive effect of ketamine-magnesium sulphate combination. In the intermediate phase, naloxone at a dose of 0.2 mg/kg did not have any effect, but at a dose of 3 mg/kg it produced a pronociceptive effect. Naloxone (0.2 and 3 mg/kg) antagonized the antinociceptive effect of the ketamine (5 mg/kg)-magnesium sulphate (5 mg/kg) combination.

CONCLUSION

The results of the present study suggest that serotonergic and opioidergic systems are involved, at least in part, in the antinociceptive effect of the ketamine-magnesium sulphate combination in the model of inflammatory pain in rats.

Effects of dopamine and serotonin synthesis inhibitors on the ketamine-, d-amphetamine-, and cocaine-induced locomotor activity of preweanling and adolescent rats: sex differences

The pattern of ketamine-induced locomotor activity varies substantially across ontogeny and according to sex. Although ketamine is classified as an NMDA channel blocker, it appears to stimulate the locomotor activity of both male and female rats via a monoaminergic mechanism. To more precisely determine the neural mechanisms underlying ketamine's actions, male and female preweanling and adolescent rats were pretreated with vehicle, the dopamine (DA) synthesis inhibitor ∝-methyl-_DL-p-tyrosine (AMPT), or the serotonin (5-HT) synthesis inhibitor 4-chloro-_DL-phenylalanine methyl ester hydrochloride (PCPA). After completion of the pretreatment regimen, the locomotor activating effects of saline, ketamine, d-amphetamine, and cocaine were assessed during a 2 h test session. In addition, the ability of AMPT and PCPA to reduce dorsal striatal DA and 5-HT content was measured in male and female preweanling, adolescent, and adult rats. Results showed that AMPT and PCPA reduced, but did not fully attenuate, the ketamine-induced locomotor activity of preweanling rats and female adolescent rats. Ketamine (20 and 40 mg/kg) caused a minimal amount of locomotor activity in male adolescent rats, and this effect was not significantly modified by AMPT or PCPA pretreatment. When compared to ketamine, d-amphetamine and cocaine produced different patterns of locomotor activity across ontogeny; moreover, AMPT and PCPA pretreatment affected psychostimulant- and ketamine-induced locomotion differently. When these results are considered together, it appears that both dopaminergic and serotonergic mechanisms mediate the ketamine-induced locomotor activity of preweanling and female adolescent rats. The dichotomous actions of ketamine relative to the psychostimulants in vehicle-, AMPT-, and PCPA-treated rats, suggests that ketamine modulates DA and 5-HT neurotransmission through an indirect mechanism.

An Update on Nonopioids: Intravenous or Oral Analgesics for Perioperative Pain Management

Despite an appreciation for many unwanted physiologic effects from inadequate postoperative pain relief, moderate to severe postoperative pain remains commonplace. Although treatment options have evolved in recent years, the use of nonopioid analgesics agents can reduce acute pain-associated morbidity and mortality. This review focuses on the importance of effective postoperative nonopioid analgesic agents, such as acetaminophen, nonsteroidal anti-inflammatory agents, gabapentinoid agents, NMDA antagonists, alpha 2 agonists, and steroids, in opioid sparing and enhancing recovery. A careful literature review focusing on these treatment options, potential benefits, and side effects associated with these strategies is emphasized in this review.

Antinociceptive and Anti-Inflammatory Effects of Ketamine and the Relationship to Its Antidepressant Action and GSK3 Inhibition

Ketamine (KET), a NMDA antagonist, exerts an antidepressant effect at subanaesthetic doses and possesses analgesic and anti-inflammatory activities. We evaluated the involvement of KET antinociceptive and anti-inflammatory effects with its antidepressant action. Male Swiss mice were subjected to formalin, carrageenan-induced paw oedema and forced swimming tests, for assessing antinociceptive, anti-inflammatory and antidepressant effects. The treatment groups were as follows: control, KET (2, 5 and 10 mg/kg), lithium (LI: 5 mg/kg) and KET2 + LI5 combination. Immunohistochemistry analyses (TNF-α, iNOS, COX-2 and GSK3) in oedematous paws were performed. KET5 and KET10 reduced licking times in neurogenic (22 and 38%) and inflammatory (67 and 78%) phases of the formalin test, respectively, as related to controls. While LI5 inhibited the second phase by 24%, the licking time was inhibited by 26 and 59% in the KET2 + LI5 group (first and second phases). Furthermore, oedema volumes were reduced by 37 and 45% in the KET5 and KET10 groups, respectively. Oedema reductions were 29% in the LI5 group and 48% in the KET2 + LI5 group. In the forced swimming test, there were 23, 38 and 53% decreases in the immobility time in KET2, KET5 and KET10 groups, respectively. While LI5 caused no significant effect, decreases of 52% were observed with KET2 + LI5. KET also decreased TNF-α, iNOS, COX-2 and GSK3 immunostainings in oedematous paws, effects intensified with KET2 + LI5. We showed that KET presents antinociceptive and anti-inflammatory effects associated with its antidepressant response. Furthermore, our results indicate the close involvement of GSK3 inhibition and blockade of inflammatory responses, in the antidepressant drug effect.

NMDA Receptor Antagonists, Gabapentinoids, α-2 Agonists, and Dexamethasone and Other Non-Opioid Adjuvants: Do They Have a Role in Plastic Surgery?

BACKGROUND

Inadequate pain control and opioid-related adverse effects result in delayed patient recovery and discharge times. Adjuvants help to improve the quality of analgesia and decrease opioid consumption, consequently decreasing opioid-related effects, such as nausea and vomiting, sedation, ileus, and respiratory depression. We review the mechanisms and clinical evidence for nonopioid adjuvants.

METHODS

MEDLINE, EMBASE, and the Cochrane Register were searched for meta-analyses, systematic reviews, and randomized, controlled trials that compared the adjuvants ketamine, gabapentin, pregabalin, dexmedetomidine, clonidine, and dexamethasone with placebo. Keywords used in the search included "plastic surgery," "reconstructive surgery," "opioid," "pain," "analgesia," and the names of each adjuvant. The references of included studies were searched for additional relevant studies.

RESULTS

Ketamine was found in 6 meta-analyses to have a significant reduction in opioid requirements and may reduce the hyperalgesia associated with opioids. This seems to be most beneficial in surgeries where high postoperative pain is expected. Multiple robust trials have demonstrated that the gabapentinoids and α-2 agonists significantly improve quality of analgesia and decrease opioid consumption. Two recent meta-analyses found that a single low-dose of dexamethasone used for postoperative nausea and vomiting prophylaxis may also improve postoperative analgesia. There is also emerging evidence for the use of low-dose naloxone, adenosine, and neuraxial neostigmine and acupuncture as part of a successful multimodal pain management regimen.

CONCLUSIONS

Although there is a lack of studies specifically focused in the plastic and reconstructive surgery patient population, the existing literature provides information about when the above adjuvants are likely to have the greatest impact.

Nitric oxide synthase modulates the antihyperalgesic effect of the NMDA receptor antagonist MK-801 on Carrageenan-induced inflammatory pain in rats

The N-methyl-D-aspartate (NMDA) receptor, an ionotropic glutamate receptor, may play a significant role in the development and maintenance of an inflammatory pain. Activation of NMDA receptors may cause nitric oxide (NO) release through activation of NO synthase (NOS). MK-801, a noncompetitive NMDA receptor antagonist is commonly used as a neuropharmacological tool. The interaction between MK-801 and NOS in the inflammatory pain has not been evaluated before. We investigated whether MK-801 affects inflammatory pain and whether NOS modulates the effect of MK-801. Carrageenan-induced hyperalgesia was evaluated by measuring the withdrawal response to mechanical stimuli, using an electronic version of the von Frey anesthesiometer in Wistar rats. MK-801 given subcutaneously (0.5-20 μg/kg) or intraplantarly (0.1 and 0.15 μg/paw) significantly reduced mechanical hyperalgesia. Intraplantarly given MK-801 exerted a local antihyperalgesic effect, because when applied to the contralateral side it did not reduce mechanical sensitivity in the ipsilateral side. N-nitro-L-arginine methyl ester hydrochloride (5 and 10 mg/kg), a non-selective NOS inhibitor, significantly reduced the effects of MK-801. N-ω-Propyl-L-arginine hydrochloride (0.5-2 mg/kg), a selective inhibitor of neuronal NOS, increased the antihyperalgesic effect of MK-801, whereas S-methylisothiourea (5-15 μg/kg), a selective inhibitor of inducible NOS, lowered the antihyperalgesic effect of MK-801. Importantly, each NOS inhibitor given alone did not affect carrageenan-induced hyperalgesia. In conclusion, MK-801 is effective against inflammatory pain and its antihyperalgesic effect is modulated in a different ways by NOS, being enhanced by a neuronal NOS inhibitor but reduced by an inducible NOS inhibitor.

Intravenous acetaminophen is superior to ketamine for postoperative pain after abdominal hysterectomy: results of a prospective, randomized, double-blind, multicenter clinical trial

BACKGROUND

In recent years, intravenously (IV) administered acetaminophen has become one of the most common perioperative analgesics. Despite its now-routine use, IV acetaminophen's analgesic comparative efficacy has never been compared with that of ketamine, a decades-old analgesic familiar to obstetricians, gynecologists, and anesthesiologists alike. This doubleblind clinical trial aimed to evaluate the analgesic effects of ketamine and IV acetaminophen on postoperative pain after abdominal hysterectomy.

METHODS

Eighty women aged 25-70 years old and meeting inclusion and exclusion criteria were randomly allocated into two groups of 40 to receive either IV acetaminophen or ketamine intraoperatively. Postoperatively, each patient had patient-controlled analgesia. Pain and sedation (Ramsay Sedation Scale) were documented based on the visual analog scale in the recovery room and at 4 hours, 6 hours, 12 hours, and 24 hours after the surgery. Hemodynamic changes, adverse medication effects, and the need for breakthrough meperidine were also recorded for both groups. Data were analyzed by repeated-measures analysis of variance.

RESULTS

Visual analog scale scores were significantly lower in the IV acetaminophen group at each time point (P<0.05), and this group required significantly fewer doses of breakthrough analgesics compared with the ketamine group (P=0.039). The two groups had no significant differences in terms of adverse effects.

CONCLUSION

Compared with ketamine, IV acetaminophen significantly improved postoperative pain after abdominal hysterectomy.

Spinal-supraspinal and intrinsic μ-opioid receptor agonist-norepinephrine reuptake inhibitor (MOR-NRI) synergy of tapentadol in diabetic heat hyperalgesia in mice

Tapentadol is a μ-opioid receptor (MOR) agonist and norepinephrine reuptake inhibitor (NRI) with established efficacy in neuropathic pain in patients and intrinsic synergistic interaction of both mechanisms as demonstrated in rodents. In diabetic mice, we analyzed the central antihyperalgesic activity, the occurrence of site-site interaction, as well as the spinal contribution of opioid and noradrenergic mechanisms in a hotplate test. Tapentadol (0.1-3.16 µg/animal) showed full efficacy after intrathecal as well as after intracerebroventricular administration (ED50 0.42 µg/animal i.t., 0.18 µg/animal i.c.v.). Combined administration of equianalgesic doses revealed spinal-supraspinal synergy (ED50 0.053 µg/animal i.t. + i.c.v.). Morphine (0.001-10 µg/animal) also showed central efficacy and synergy (ED50 0.547 µg/animal i.t., 0.004 µg/animal i.c.v., 0.014 µg/animal i.t. + i.c.v.). Supraspinal potencies of tapentadol and morphine correlated with the 50-fold difference in their MOR affinities. In contrast, spinal potencies of both drugs were similar and correlated with their relative systemic potencies (ED50 0.27 mg/kg i.p. tapentadol, 1.1 mg/kg i.p. morphine). Spinal administration of the opioid antagonist naloxone or the α2-adrenoceptor antagonist yohimbine before systemic administration of equianalgesic doses of tapentadol (1 mg/kg i.p.) or morphine (3.16 mg/kg i.p.) revealed pronounced influence on opioidergic and noradrenergic pathways for both compounds. Tapentadol was more sensitive toward both antagonists than was morphine, with median effective dose values of 0.75 and 1.72 ng/animal i.t. naloxone and 1.56 and 2.04 ng/animal i.t. yohimbine, respectively. It is suggested that the antihyperalgesic action of systemically administered tapentadol is based on opioid spinal-supraspinal synergy, as well as intrinsic spinally mediated MOR-NRI synergy.

Chemical analgesia for velvet antler removal in deer

There is a legal requirement to provide analgesia for velvet antler removal in New Zealand. Currently, this is achieved using local anaesthetic blockade, with or without systemically administered sedative/analgesic agents, or by compression in 1-year-old stags. Lignocaine hydrochloride 2% is most commonly used and is most effective when administered as a high-dose ring block. Combinations of various amino-amide local anaesthetic agents can achieve rapid onset and prolonged duration of analgesia, though concerns about drug residues and carcinogenic potential of a lignocaine metabolite have led to consideration of the amino-ester family of local anaesthetics as alternatives. Systemically administered analgesics, including opioids, alpha-2-adrenergic agents and ketamine provide dose-dependent sedation and analgesia. However, none are sufficient, alone or in combination, to produce surgical analgesia at currently recommended dose rates and when reversal agents are given, analgesic effects are usually reversed as well as sedation. Thus, local anaesthetic blockade is still indicated, though the potential for drug or drug-metabolite residues in velvet antler remains a concern. The need for and effectiveness of non-steroidal anti-inflammatory drugs (NSAIDs) for post-operative analgesia requires investigation. Amitriptyline, locally administered opioid agonists, tramadol and other systemically administered agents may warrant future investigation for surgical and post-operative analgesia for velvet antler removal.

The improvement of the anti-hyperalgesic effect of ketamine and of its isomers by the administration of ifenprodil

Intrathecal or epidural administration of NMDA (N-methyl-D-aspartate) receptors antagonists, in special ketamine and ifenprodil are used to control moderate to severe hyperalgesia in humans. Activation of NMDA receptor usually requires binding of two agonists, glutamate and glycine, in different receptor subunits. Ketamine is a NMDA receptor antagonist and acts at phencyclidine site in NR1 subunit while ifenprodil is a selective NR2B subunit antagonist of NMDA receptor. The aim of this study was to investigate the pharmacological interactions between ketamine or its isomers and ifenprodil, when intrathecally co-administrated, to reduce prostaglandin E(2)-induced hyperalgesia in rat's hind paw. The intrathecal administration of ketamine, its isomers R(-) or S(+), or ifenprodil-induced anti-hyperalgesic effects in a dose-related manner. Ifenprodil, in a dose that did not induce significant effect when administrated alone, significantly improved the anti-hyperalgesic effect of ketamine or its isomers. The other way round, ketamine or S(+) ketamine, but not R(-) ketamine, in a dose that did not induce significant effect when administrated alone, improved the anti-hyperalgesic effect of ifenprodil. However, by comparing ED(50)s (half maximal effective doses), ifenprodil-induced potentiation of ketamine was significantly greater than ketamine-induced potentiation of ifenprodil. The findings of this present study suggest that intrathecal administration of very small doses of ifenprodil, just before and ketamine significantly improves its anti-hyperalgesic effect and this association could be useful to control inflammatory pain with less undesirable effects.

Effects of intrathecal ketamine in the neonatal rat: evaluation of apoptosis and long-term functional outcome

BACKGROUND

Systemic ketamine can trigger apoptosis in the brain of rodents and primates during susceptible developmental periods. Clinically, spinally administered ketamine may improve the duration or quality of analgesia in children. Ketamine-induced spinal cord toxicity has been reported in adult animals but has not been systematically studied in early development.

METHODS

In anesthetized rat pups, intrathecal ketamine was administered by lumbar percutaneous injection. Changes in mechanical withdrawal threshold evaluated dose-dependent antinociceptive and carrageenan-induced antihyperalgesic effects in rat pups at postnatal day (P) 3 and 21. After intrathecal injection of ketamine at P3, 7, or 21, spinal cords were examined for apoptosis (Fluoro-Jade C and activated caspase-3), histopathologic change, and glial responses (ionized calcium-binding adapter molecule 1 and glial fibrillary acid protein). After maximal doses of ketamine or saline at P3 or P21, sensory thresholds and gait analysis were evaluated at P35.

RESULTS

Intrathecal injection of 3 mg/kg ketamine at P3 and 15 mg/kg at P21 reverses carrageenan-induced hyperalgesia. Baseline neuronal apoptosis in the spinal cord was greater at P3 than P7, predominantly in the dorsal horn. Intrathecal injection of 3-10 mg/kg ketamine in P3 pups (but not 15 mg/kg at P21) acutely increased apoptosis and microglial activation in the spinal cord and altered spinal function (reduced mechanical withdrawal threshold and altered static gait parameters) at P35.

CONCLUSIONS

Because acute pathology and long-term behavioral change occurred in the same dose range as antihyperalgesic effects, the therapeutic ratio of intrathecal ketamine is less than one in the neonatal rat. This measure facilitates comparison of the relative safety of spinally administered analgesic agents.

Atipamezole changes the antinociceptive effects of butorphanol after medetomidine-ketamine anaesthesia in rats

OBJECTIVE

To investigate the effects of atipamezole administered before butorphanol, on tail-flick latency (TFL) and also following medetomidine-ketamine anaesthesia in rats.

STUDY DESIGN

Prospective, randomized experimental study.

ANIMALS

Thirty-four adult male Sprague-Dawley rats weighing 260-390 g.

METHODS

TFL in 50 degrees C water was used to measure antinociception. In the first experiment, rats received saline (n = 5) or atipamezole (n = 5) followed by butorphanol treatments. In the second experiment, three groups of rats received saline (n = 8), atipamezole (n = 8) or atimpamezole (n = 8) followed by butorphanol 60 minutes after medetomidine-ketamine administration.

RESULTS

In the first experiment, butorphanol significantly increased TFL compared to baseline. Atipamezole significantly decreased this effect. In the second experiment, TFL was significantly increased after recovery from medetomidine-ketamine anaesthesia compared to baseline. This was almost completely blocked by atipamezole. Atipamezole with butorphanol after recovery from the anaesthesia significantly reduced TFL compared to saline but still significantly increased TFL compared to the baseline.

CONCLUSION AND CLINICAL RELEVANCE

Atipamezole attenuated the analgesic effects of butorphanol. When postoperative atipamezole is used to hasten recovery from anaesthesia in rats, it may interfere with the postoperative analgesic effect of butorphanol.

Role of endogenous sleep-wake and analgesic systems in anesthesia

Classical anesthetics of the gamma-aminobutyric acid type A receptor (GABA(A))-enhancing class (e.g., pentobarbital, chloral hydrate, muscimol, and ethanol) produce analgesia and unconsciousness (sedation). Dissociative anesthetics that antagonize the N-methyl-D-aspartate (NMDA) receptor (e.g., ketamine, MK-801, dextromethorphan, and phencyclidine) produce analgesia but do not induce complete loss of consciousness. To understand the mechanisms underlying loss of consciousness and analgesia induced by general anesthetics, we examined the patterns of expression of c-Fos protein in the brain and correlated these with physiological effects of systemically administering GABAergic agents and ketamine at dosages used clinically for anesthesia in rats. We found that GABAergic agents produced predominantly delta activity in the electroencephalogram (EEG) and sedation. In contrast, anesthetic doses of ketamine induced sedation, followed by active arousal behaviors, and produced a faster EEG in the theta range. Consistent with its behavioral effects, ketamine induced Fos expression in cholinergic, monoaminergic, and orexinergic arousal systems and completely suppressed Fos immunoreactivity in the sleep-promoting ventrolateral preoptic nucleus (VLPO). In contrast, GABAergic agents suppressed Fos in the same arousal-promoting systems but increased the number of Fos-immunoreactive neurons in the VLPO compared with waking control animals. All anesthetics tested induced Fos in the spinally projecting noradrenergic A5-7 groups. 6-hydroxydopamine lesions of the A5-7 groups or ibotenic acid lesions of the ventrolateral periaqueductal gray matter (vlPAG) attenuated antinociceptive responses to noxious thermal stimulation (tail-flick test) by both types of anesthetics. We hypothesize that neural substrates of sleep-wake behavior are engaged by low-dose sedative anesthetics and that the mesopontine descending noradrenergic cell groups contribute to the analgesic effects of both NMDA receptor antagonists and GABA(A) receptor-enhancing anesthetics.

Expression of nitric oxide synthase isoforms in the dorsal horn of monoarthritic rats: effects of competitive and uncompetitive N-methyl-D-aspartate antagonists

Chronic pain is associated with N-methyl-D-aspartate (NMDA) receptor activation and downstream production of nitric oxide, which has a pivotal role in multisynaptic local circuit nociceptive processing in the spinal cord. The formation of nitric oxide is catalyzed by three major nitric oxide synthase (NOS) isoforms (neuronal, nNOS; inducible, iNOS; endothelial, eNOS), which are increased in the spinal cord of rodents subjected to some tonic and chronic forms of experimental pain. Despite the important role of NOS in spinal cord nociceptive transmission, there have been no studies exploring the effect of NMDA receptor blockade on NOS expression in the dorsal horn during chronic pain. Furthermore, NOS isoforms have not been fully characterized in the dorsal horn of animals subjected to arthritic pain. The aim of this work was therefore to study the expression of nNOS, iNOS and eNOS in the dorsal horns of monoarthritic rats, and the modifications in NOS expression induced by pharmacological blockade of spinal cord NMDA receptors. Monoarthritis was produced by intra-articular injection of complete Freund's adjuvant into the right tibio-tarsal joint. At week 4, monoarthritic rats were given either the competitive NMDA antagonist (±)-3-(2-carboxypiperazin-4-yl)-propyl-1-phosphonic acid (CPP) or the uncompetitive NMDA antagonist ketamine. After 6 and 24 hours, animals were killed and posterior quadrants of the lumbar spinal cord were dissected. Sample tissues were homogenized and subjected to immunoblotting with anti-nNOS, anti-iNOS or anti-eNOS monoclonal antibodies. The nNOS isoform, but not the iNOS and eNOS isoforms, were detected in the dorsal horns of control rats. Monoarthritis increased the expression of nNOS, iNOS and eNOS in the dorsal horns ipsilateral and contralateral to the inflamed hindpaw. Intrathecal administration of CPP and ketamine reduced nNOS expression in monoarthritic rats but increased the expression of iNOS and eNOS. Results suggest that blockade of spinal cord NMDA receptors produces complex regulatory changes in the expression of NOS isoforms in monoarthritic rats that may be relevant for nitridergic neuronal/glial mechanisms involved in the pathophysiology of monoarthritis and in the pharmacological response to drugs interacting with NMDA receptors.

The clinical role of NMDA receptor antagonists for the treatment of postoperative pain

Recent advances in the understanding of postoperative pain have demonstrated its association with sensitization of the central nervous system (CNS) which clinically elicits pain hypersensitivity. N-methyl-D-aspartate (NMDA) receptors play a major role in synaptic plasticity and are specifically implicated in CNS facilitation of pain processing. Therefore, NMDA receptor antagonists, and specifically ketamine commonly used in clinical practice, have been implicated in perioperative pain management. At subanesthetic (i.e. low) doses, ketamine exerts a specific NMDA blockade and hence modulates central sensitization induced both by the incision and tissue damage and by perioperative analgesics such as opioids. However, the mechanisms underlying ketamine anti-hyperalgesic effect are not totally understood, and neither is the relationship between central sensitization and the risk of developing residual pain after surgery. This chapter examines the role of low doses of ketamine as an adjuvant drug in current perioperative pain management and questions the anti-hyperalgesic mechanisms involved.

Effects of ketamine on acute somatic nociception in wild-type and N-methyl-d-aspartate (NMDA) receptor ɛ1 subunit knockout mice

Although the properties of ketamine appear to be well characterized, there is a lot of ambiguity in the literature regarding its analgesic effects. After careful selection of proper experimental conditions and drug doses, we systematically characterized the effects of systemic ketamine on acute somatic nociception in mice and examined the role of the NMDA receptor epsilon1 subunit in mediating its analgesia. Intraperitoneal administration of ketamine was not analgesic in any of the phasic pain assays (thermal, mechanical, electrical) applied to C57BL/6 (wild-type) and NMDA receptor epsilon1 subunit knockout (mutant) mice. Surprisingly, rather than being analgesic for thermal nociception, ketamine showed pronociceptive properties in case of low-intensity heat stimulation in wild-type mice. In the formalin test (tonic pain), ketamine significantly reduced phase 2 nociceptive behavior in both wild-type and mutant mice. These data indicate that in wild-type mice ketamine has no analgesic effect on phasic pain in normal somatic tissues, but alleviates tonic pain after inflammation. Such analgesic spectrum of ketamine can be fully explained by its NMDA receptor antagonist properties. The results for the mutant mice suggest that the epsilon1 subunit of the NMDA receptor does not mediate the analgesic effects of ketamine in tonic pain.

Ketamine attenuates liver injury attributed to endotoxemia: role of cyclooxygenase-2

BACKGROUND

Endotoxic shock can cause end-organ dysfunction and liver injury. Critically ill patients frequently require surgical intervention under general anesthesia for source control. However, the effects of anesthetics on organ function during sepsis and their influence on inflammatory mediators such as cyclooxygenase-2 (COX-2) and inducible nitric oxide synthase (iNOS) remain to be fully elucidated. Because ketamine anesthesia has anti-inflammatory effects in some tissues, we hypothesized that it would attenuate lipopolysaccharide (LPS)-induced liver injury.

METHODS

Adult rats were given no anesthesia (saline), continuous isoflurane inhalation, or intraperitoneal (i.p.) injection of ketamine 70 mg/kg. One hour later, the rats received saline or LPS (20 mg/kg i.p.) for 5 hours. The rats were killed, and serum hepatocellular enzymes, liver COX-2, iNOS protein (Western immunoblot), and nuclear factor kappa B (NF-kappaB)-binding activity (electrophoretic mobility shift assay) determined. In a separate study, the role of COX-2 in LPS-induced liver injury was examined by pretreating rats with the selective COX-2 inhibitor NS-398 (3 mg/kg, i.p.) and the role of iNOS examined with the use of the selective inhibitor aminoguanidine (45 mg/kg, i.p.) 1 hour before LPS.

RESULTS

LPS increased serum aspartate aminotransferase and alanine aminotransferase levels, hepatic iNOS and COX-2 protein, and nuclear factor NF-kappaB. Ketamine, but not isoflurane, attenuated these effects caused by LPS. COX-2 inhibition with NS-398 as well as iNOS inhibition with aminoguanidine diminished LPS-induced changes in aspartate aminotransferase and alanine aminotransferase levels.

CONCLUSIONS

These data indicate that anesthetics differ in their effects on liver injury caused by LPS. Ketamine has hepatoprotective effects, while isoflurane does not. Moreover, the protective effects of ketamine are mediated, at least in part, through a reduction in COX-2 and iNOS protein that could be regulated via changes in NF-kappaB-binding activity.

Systemic ketamine inhibits hypersensitivity after surgery via descending inhibitory pathways in rats

PURPOSE

Systemic ketamine suppresses several types of chronic pain. Although ketamine is used as a general anesthetic agent, the analgesic effect of systemic ketamine for early-stage postoperative pain is not clear. We investigated the efficacy and mechanism of systemic ketamine in a rat model of postoperative pain.

METHODS

An incision was made in the plantar aspect of the left hind paw in male Wistar rats. Mechanical hypersensitivity was measured using calibrated von Frey filaments. The anti-hypersensitivity effect of systemic or intrathecal administration of ketamine was determined every hour after making the incision. We examined the effects of intrathecal pretreatment with yohimbine, an alpha2-adrenoceptor antagonist, and methysergide, a serotonergic receptor antagonist, on the anti-hypersensitivity effect of ketamine. We also examined the effect of systemic ketamine on the c-fos immunoreactivity in the spinal cord.

RESULTS

Systemic administration of ketamine at doses from 3 to 30 mg.kg(-1) produced anti-hypersensitivity effects in a dose-dependent manner. Intrathecal administration of ketamine had no effect. There was no significant difference between effects of pre- and post-incisional administration. Intrathecal pretreatment with yohimbine (10 microg) or methysergide (15 microg) completely reversed the anti-hypersensitivity effects of systemic ketamine. Systemic ketamine reduced fos expression in laminae I-II in the dorsal horn of the lumbar spinal cord ipsilateral to the paw incision.

CONCLUSIONS

The results suggest that systemic administration of ketamine perioperatively suppresses early-stage postoperative pain via monoaminergic descending inhibitory pathways.

The persisting analgesic effect of low-dose intravenous ketamine after spinal anaesthesia for Caesarean section

BACKGROUND AND OBJECTIVES

To compare the analgesic effects of intrathecal fentanyl and low-dose intravenous ketamine as adjuvants to intrathecal bupivacaine for Caesarean section.

METHODS

Ninety elective Caesarean section patients were randomized into three groups. Spinal anaesthesia was performed with 15 mg hyperbaric bupivacaine in all groups. Ketamine (0.15 mg kg(-1)) or an equal volume of normal saline was given intravenously immediately after initiating spinal anaesthesia in the ketamine and control group, respectively. In the fentanyl group, 10 microg fentanyl was added to the intrathecal bupivacaine. Arterial pressures, heart rate values, adverse effects, the time of first request for postoperative analgesia, visual analogue pain scores, total analgesic consumptions at 24 and 48 h were recorded in all patients.

RESULTS

The time to first request for analgesia was significantly longer in the ketamine (197 min) and fentanyl (165 min) groups compared to the control group (144 min). Postoperative pain scores were significantly lower in the ketamine group than in both other groups. Although the analgesic requirements during first 24 h were significantly lower in the ketamine group, there was no significant difference between the groups during the following 24 h.

CONCLUSION

Intravenous low-dose ketamine combined with intrathecal bupivacaine for Caesarean section provides longer postoperative analgesia and lower postoperative analgesic consumption than bupivacaine alone suggesting a pre-emptive effect.

Differential effects of anesthetics on endotoxin-induced liver injury

BACKGROUND

The liver is both a source and a target of inflammatory and anti-inflammatory mediators during sepsis. The oxidative stress proteins inducible nitric oxide synthase (iNOS) and heme oxygenase-1 (HO-1) are upregulated in the liver during sepsis but have opposite roles. Upregulation of HO-1 has hepatoprotective effects, whereas iNOS has injurious effects to the liver. Although recent studies indicate that ketamine anesthesia has anti-inflammatory effects during sepsis, the effects of other anesthetics are unknown. We hypothesized that ketamine, but not isoflurane, would attenuate lipopolysaccharide (LPS)-induced liver injury through differential modulation of iNOS and HO-1.

METHODS

Adult rats were given no anesthesia (saline), continuous isoflurane inhalation, or intraperitoneal ketamine (70 mg/kg). One hour later, saline or LPS (20 mg/kg intraperitoneally) was given for 5 hours. Rats were killed, serum prepared for determination of hepatocellular enzymes, and the liver assessed for iNOS and HO-1 by Western immunoblot.

RESULTS

LPS significantly increased serum aspartate aminotransferase levels, iNOS, and HO-1 immunoreactivity in the liver. Ketamine but not isoflurane attenuated LPS-induced liver injury, upregulated HO-1, and downregulated iNOS.

CONCLUSION

These data indicate that anesthetics differ in their effects on the liver in a rat model of sepsis with LPS. Ketamine has hepatoprotective effects against LPS-induced liver injury that appear to be mediated, at least in part, by differential modulation of the oxidative stress proteins iNOS and HO-1. Thus, ketamine may be the anesthetic agent of choice for septic patients requiring anesthesia.

The effects of racemic ketamine on painful stimulation of skin and viscera in human subjects

Evidence suggests that NMDA receptors may have a differential role in the modulation of visceral and somatic pain. Specifically, animal data indicate an analgesic role of NMDA-R antagonists in acute visceral but not acute somatic pain. In humans analgesic effects are documented in acute somatic pain, while the role of NMDA-R antagonists in acute visceral pain is still questionable. We, therefore, conducted a study in humans comparing the analgesic effects of ketamine in an experimental model of visceral and cutaneous pain. In a double-blind, randomized, cross-over study, 11 healthy volunteers (3M, 8F) participated in two experimental sessions in which they evaluated perceptions induced by balloon distention of the distal esophagus and contact heat on the upper chest during continuous computer-controlled i.v. infusion of either ketamine (60 and 120 ng/mL) or saline. Two stimulus intensities producing non-painful and painful sensation were used for each stimulus modality. Subjects reported maximum pain intensity and unpleasantness on visual analog scales (VAS). For noxious visceral stimulation, low dose ketamine produced significant attenuation of both pain intensity and unpleasantness. In contrast, for noxious cutaneous stimulation, ketamine reduced pain unpleasantness, but not perceived intensity. In addition, ketamine did not alter the perception of innocuous stimuli in either modality. Our results confirm the analgesic effects of low-dose ketamine, with minimal side effects, on acute visceral pain and indicate a similar but smaller effect on acute cutaneous pain. A decrease in the unpleasantness but not in the intensity of cutaneous pain may reflect the differential effect of NMDA-R antagonists for the two pain states observed in animal models.

An analgesic evaluation of isoxsuprine in horses

Isoxsuprine is used clinically to treat navicular disease and laminitis in horses. Although it is thought to increase digital and laminar blood flow, isoxsuprine's mechanism of action remains controversial, and analgesia has been suggested recently as such possible mechanism. This research investigated the analgesic potential of isoxsuprine in healthy horses submitted to a mechanical nociceptive test. Isoxsuprine (1.2 mg/kg), xylazine (1.1 mg/kg), distilled water : ethanol 95% (2 : 1, v/v, 20 ml) and saline (0.9%, 20 ml) were injected intravenously, and nociceptive thresholds were measured over 90 min. Only xylazine significantly increased nociceptive thresholds, confirming that alpha(2)-adrenoceptor agonists produce analgesia in horses. Our results do not support an analgesic mechanism of action for isoxsuprine in horses, suggesting that other mechanisms might account for the clinical efficacy of this drug or that mechanical nociceptive testing may not be sufficiently sensitive to demonstrate an analgesic effect for this drug.

Low-dose anesthesia for corneal transplantation in mice

BACKGROUND

Despite technical difficulties caused by the small dimensions, scientific and economic considerations stimulated activities in the murine keratoplasty model. Delicate surgery requires stable anesthesia with rapid postoperative waking.

METHODS

For corneal transplantation, all animals were intraperitoneally injected with 0.08 ml of a 1:8 mixture of 2% xylazine and 2.5% esketamine hydrochloride. Thus they all received 0.18 mg of xylazine and 1.8 mg of ketamine. In other words, 4.5 mg/kg BW of xylazine and 45 mg/kg BW of ketamine were injected in a mouse weighing 25 g. The inhalation anesthetic isoflurane was used for short interventions like removing lid sutures or in the case of an animal awaking before the end of the operation. A small but effective set of apparatus comprising a glass bottle, a test tube and several interconnected syringes was developed to create a closed system and visualize the amount of isoflurane used in the animal. Air bubbles showed the amount of isoflurane escaping from the opened respiratory mask.

RESULTS

The dose reduction of systemic anesthesia requires additional inhalation anesthesia with isoflurane in nearly all cases. The equipment developed is easy to handle and allows systemic anesthesia with a very low dose.

CONCLUSION

This murine anesthesia model may be expected to help other surgeons performing corneal transplantations in mice.

Pre-Emptive Epidural Ketamine or S(+)-Ketamine in Post-incisional Pain in Dogs: A Comparative Study

OBJECTIVE

To compare the pre-emptive analgesic effects of epidural ketamine or S(+)-ketamine on post-incisional hyperalgesia.

STUDY DESIGN

Prospective randomized study.

ANIMALS

Twenty-four mongrel dogs (1-5 years, weighing 11.9+/-1.8 kg).

METHODS

Dogs were anesthetized with propofol (5 mg/kg intravenously) and a lumbosacral epidural catheter was placed. Dogs were randomly allocated to 3 groups, each with 8 dogs. The control group (CG) was administered saline solution (0.3 mL/kg); the ketamine group (KG) ketamine (0.6 mg/kg); and the S(+)-ketamine group (SG) S(+)-ketamine (0.6 mg/kg). The final volume was adjusted to 0.3 mL/kg in all groups. Five minutes after the epidural injection a surgical incision was made in the common pad of the right hind limb and was immediately closed with simple interrupted nylon suture. Respiratory (RR) and heart (HR) rates, rectal temperature (T), sedation (S), lameness score, and mechanical nociceptive threshold by von Frey filaments were evaluated before the propofol anesthesia and at 15, 30, 45, 60, 75, and 90 minutes and then at 2, 4, 6, 8, 12, and 24 hours after epidural injection.

RESULTS

There were no differences in RR, HR, T, or S between groups. Motor blockade of the hind limbs was observed during 20+/-3.6 minutes in KG and during 30.6+/-7.5 minutes in SG (mean+/-SD). Mechanical force applied to obtain an aversive response was higher from 45 minutes to 12 hours in KG and from 60 to 90 minutes in SG, when compared with CG.

CONCLUSIONS

Pre-emptive epidural ketamine induced no alterations in RR and HR, and reduced post-incisional hyperalgesia for a longer time than did S(+) ketamine.

CLINICAL RELEVANCE

Although anesthetic and analgesic potency of S(+) ketamine is twice that of ketamine, the racemic form is seemingly better for post-incisional hyperalgesia.

Alfentanil potentiates anaesthetic and electroencephalographic responses to ketamine in the rat

The interactions between mu-opioid and N-methyl-D-aspartate (NMDA) receptors have important implications for clinical pain management. We recently examined the pharmacokinetics of ketamine in rats following i.v. infusion of ketamine (racemate, 50 mg/kg/5 min) and found increased central nervous system distribution of ketamine in the presence of low constant plasma alfentanil concentrations (approximately 50 ng/ml). We now report on the effects of low plasma alfentanil concentrations on the duration of anaesthetic and electroencephalographic (EEG) responses to i.v. infusion of ketamine. Compared to ketamine alone, alfentanil significantly increased both the duration of anaesthesia (by 130%, P=0.00022) and the processed EEG power (microV(2)/s) (by 48%, P=0.0040). The plasma ketamine concentration producing half-maximal EEG effect was significantly reduced (by 60%, P<0.0001) in the presence of alfentanil. The results indicate that low plasma alfentanil concentrations potentiate the anaesthetic and EEG effects produced by ketamine.

Ketamine: In the past thirty years and in the future

Introduction There are two isomers of ketamine: S(+)ketamine and R(-) ketamine. Effects of this drug are mediated by N-methyl-D-aspartate (NMDA), opioid and muscarinic receptors. Pharmacokinetics Due to its high lipid solubility and low protein binding, ketamine is extensively distributed in the body. Ketamine metabolism is mediated by hepatic microsomal enzymes. Effects on body systems Ketamine causes increased intracranial pressure, bronchodilation and stimulation of cardiovascular system. Clinical use Anesthesia It is used for premedication, sedation, induction and maintenance of general anesthesia. Ketamine is an ideal anesthetic agent for trauma victims, patients with hypovolemic and septic shock, patients with pulmonary diseases. Analgesia Even subanesthetic doses of ketamine have analgesic effects, so ketamine is also recommended for postoperative analgesia. Recent results show that ketamine is not always effective as an analgesic in treatment of chronic pain and that some side effects limit its use in patients with chronic pain. Intensive care Combination of ketamine and midazolam can be very useful and safe for sedation and pain relief in intensive care patients, especially during ventilator management. Ketamine may be the anesthetic of choice for painful procedures in intensive care units. Emergency medicine Ketamine is recommended for critically ill patients with sedative, inotropic and bronchodilatatory effects. New clinical use Future studies will reveal if ketamine can be used for outpatient anesthesia, intravenous regional anesthesia and in treatment of depression. S(+)ketamine is a more effective anesthetic with significantly less psychomotor effects which provides rapid recovery from anesthesia in regard to racemate or R(-)ketamine. Conclusion Increasing interest in use of ketamine has resulted from current knowledge. In the future we expect new clinical uses in treatment of acute and chronic pain, intensive care and emergency medicine.

Modulation of formalin-induced behaviors and edema by local and systemic administration of dextromethorphan, memantine and ketamine

The present study examined the effects of local peripheral and systemic administration of three clinically used excitatory amino acid receptor antagonists (dextromethorphan, memantine, ketamine) on pain behaviors and edema produced by formalin (1.5% and 5%) in rats. Peripheral administration of dextromethorphan produced a locally mediated suppression of flinching behaviors induced by 1.5% and 5% formalin, but biting/licking behaviors were not affected. Memantine and ketamine had no effect on either of these behaviors. All three agents augmented edema produced by 1.5% and 5% formalin. When administered alone, dextromethorphan, memantine and ketamine produced an intrinsic paw swelling response, and this was blocked by the biogenic amine receptor antagonists mepyramine, phentolamine, methysergide and ketanserin. Following systemic administration, all three agents suppressed biting/licking behaviors, had no effect on flinching behaviors, and suppressed paw swelling induced by 5% formalin to varying degrees. These results provide evidence for a peripherally mediated antinociceptive action of dextromethorphan in the rat formalin test, but this may not necessarily be due to block of excitatory amino acid receptors as it is not observed with memantine or ketamine. All three agents produce a peripherally mediated paw swelling, which is likely due to blockade of biogenic amine reuptake. Systemic administration of all three agents produces antinociceptive and anti-inflammatory actions that may be due to block of excitatory amino acid receptors in the spinal cord.

The involvement of nitric oxide in the analgesic effects of ketamine

We investigated the contribution of NO-cyclic GMP (cGMP) pathway to the antinociceptive effects of ketamine in mice by using the nitric oxide synthase inhibitor, nitro(g)- L-arginine methyl ester (L-NAME). Intraperitoneal (i.p.) (1, 5 or 10 mg/kg) or intrathecal (i.th.) (10, 30 or 60 microg/mouse) administration of ketamine produced dose-dependent antinociceptive effects in the acetic acid-induced writhing and formalin tests but not in the tail-flick nor in hot-plate tests. Pretreatment of mice with L-NAME (10 mg/kg, i.p.) which produced no antinociception on its own, significantly inhibited the antinociceptive effect of ketamine (1, 5 or 10 mg/kg, i.p.). However, L-NAME (30 microg/mouse) was given intrathecally, it neither modified the antinociceptive effect of i.th. ketamine (10, 30 or 60 microg/mouse) nor did it produce an antinociceptive effect alone. These data suggest that the activation of the NO-cGMP pathway probably at the supraspinal level, but not spinal level, contributes to the antinociceptive effects of ketamine.

Concurrent ketamine and alfentanil administration: pharmacokinetic considerations

BACKGROUND

A ketamine-alfentanil combination has been suggested for total i.v. anaesthesia. We determined the pharmacokinetics of ketamine and alfentanil, alone and together, in three groups of adult male rats, to assess any pharmacokinetic interaction.

METHODS

Group 1 animals were infused with i.v. ketamine for 5 min; in group 2, constant low plasma concentrations of alfentanil were maintained by computer-controlled infusion; in group 3, the treatments were combined. Serial plasma and terminal tissue concentrations were measured by high performance liquid chromatography or gas chromatography-mass spectrometry.

RESULTS

In the presence of alfentanil, the mean plasma ketamine concentration-time area under the curve (AUC) value was significantly lower (by 13%, P<0.05), while clearance (CIT) and volume of distribution (Vss) were significantly higher (by 16 and 28%, respectively, both P<0.05). Tissue:plasma distribution coefficients for ketamine in the presence of alfentanil were significantly higher in forebrain (by 128%, P<0.005), hindbrain (by 207%, P<0.01), gut (by 254%, P<0.005), and fat (by 344%, P<0.0001). Mean AUC values for alfentanil did not differ significantly in the presence of ketamine, but alfentanil tissue concentrations were significantly lower in forebrain (by 77%, P<0.0001), hindbrain (by 28%, P<0.01), heart (by 33%, P<0.01), lung (30%, P<0.05), and gut (by 21%, P<0.05). Corresponding tissue:plasma distribution coefficients were significantly lower for forebrain (by 69%, P<0.0001) alone.

CONCLUSIONS

The finding that the distribution of ketamine into the brain was increased by low plasma concentrations of alfentanil could have important clinical applications for pain management.

'Balanced analgesia' in the perioperative period: is there a place for ketamine?

We investigated whether intraoperative 'subanesthetic doses' of ketamine have a postoperative anti-hyperalgesic and an analgesic effect and which is the preferential route of administration, either systemic (intravenous, i.v.) or epidural. One hundred patients scheduled for rectal adenocarcinoma surgery under combined epidural/general anesthesia were included. Before skin incision all the patients received an epidural bolus followed by an infusion of continuous bupivacaine/sufentanil/clonidine mixture. They were randomly assigned to receive no ketamine (group 1), i.v. ketamine at the bolus dose of 0.25 mg/kg followed by an infusion of 0.125 mg/kg per h (group 2), 0.5 mg/kg and 0.25 mg/kg per h (group 3), epidural ketamine 0.25 mg/kg and 0.125 mg/kg per h (group 4), or 0.5 mg/kg and 0.25 mg/kg per h (group 5). All i.v. and epidural analgesics were stopped at the end of surgery and patients were connected to an i.v. morphine patient-controlled analgesia (PCA) device. Short-term postoperative analgesia (72 h) was assessed by pain visual analog scale scores at rest, cough, and movements as well as by PCA requirements. Wound mechanical hyperalgesia was evaluated and residual pain was assessed by asking the patients at 2 weeks, and 1, 6, and 12 months. The area of hyperalgesia and morphine PCA requirements were significantly reduced in group 3. These patients reported significantly less residual pain until the sixth postoperative month. These observations support the theory that subanesthetic doses of i.v. ketamine (0.5 mg/kg bolus followed by 0.25 mg/kg per h) given during anesthesia reduce wound hyperalgesia and are a useful adjuvant in perioperative balanced analgesia. Moreover, they show that the systemic route clearly is the preferential route.