Sedative but Not Analgesic α2 Agonist Tolerance Is Blocked by NMDA Receptor and Nitric Oxide Synthase Inhibitors

Background: Studies show that the sedative and analgesic effects of [alpha]2 adrenergic agonists decrease over time, which is a form of synaptic plasticity referred to as tolerance. Because both the N-methyl-d-aspartate (NMDA) receptor complex and nitric oxide synthase are pivotal for some forms of synaptic plasticity, their role in tolerance to the hypnotic and analgesic effects of [alpha]2 agonists was investigated.

Methods: After institutional approval, rats were made tolerant to the hypnotic or analgesic action of an [alpha]2 agonist, dexmedetomidine. The hypnotic response to dexmedetomidine was assessed by the duration of loss of righting reflex, and the analgesic response to dexmedetomidine was assessed by the tail-flick assay. In separate cohorts, either the NMDA receptors or nitric oxide synthase was antagonized by coadministration of MK-801, ketamine, or NO2-arginine, respectively, during induction of tolerance. In a separate series of experiments, after tolerance was induced, the hypnotic and analgesic responses to dexmedetomidine were assessed in the presence of acutely administered MK-801 or NO2-arginine.

Results: Induction of tolerance to the hypnotic effect of dexmedetomidine is blocked by coadministration of MK-801, ketamine, and NO2-arginine. However, after tolerance developed, acute administration of MK-801, ketamine, or NO2-arginine did not prevent the expression of tolerance. Coadministration of MK-801 or NO2-arginine neither prevents the development nor reverses the expression of tolerance to the analgesic action of dexmedetomidine.     

Regional Anesthesia and Pain

PURPOSE: Not all bone cancer pain can be effectively treated with current therapies. In the present study, the effects of ip administration of alpha-2 agonists (dexmedetomidine and clonidine), N-methyl-D-aspartate (NMDA) antagonists (MK-801 and ketamine), an N-acetylaspartylglutamate peptidase inhibitor (ZJ-43), and morphine were examined in a mouse bone cancer pain model. METHODS: A bone cancer pain model was produced by injection of murine sarcoma cells into the medullary cavity of the distal femur. To estimate the level of bone cancer pain, the number of pain-related behaviours induced by repeated applications of a von Frey monofilament (0.166 g) to the site of tumour cells implantation was counted. Drugs were administered two weeks after the implantation. RESULTS: Morphine produced a significant analgesic effect (P < 0.001). The alpha-2 agonists produced analgesic effects (P < 0.001) with an efficacy similar to that of morphine, but only at doses that produced severe sedation. MK-801 had only limited analgesic effects, while ketamine produced an analgesic effect (P < 0.001) with the same efficacy as morphine. ZJ-43 (100 mg.kg(-1)) had a significant analgesic effect (P < 0.05) and the effect of ZJ-43 was antagonized by the selective group II metabotropic glutamate receptor (mGluR) antagonist. CONCLUSION: These data suggest that alpha-2 agonists produce an analgesic effect only at a sedative dose and that ketamine, but not MK-801, is associated with an analgesic response without overt side effects. The effect of ZJ-43 is mediated by activating group II mGluRs.     

Ketamine inhibits nitric oxide production in mouse-activated macrophage- like cells

We have investigated the effects of ketamine on nitric oxide produced by activated macrophages using a murine macrophage-like cell line, J774. Cells were incubated for 18 h under stimulation with lipopolysaccharide and interferon-gamma or lipoteichoic acid and interferon-gamma, with various concentrations of ketamine (6-600 microgramsmol litre-1). Nitric oxide production was assessed by measuring nitrite, a stable by-product of nitric oxide breakdown, in the medium. Other N- methyl-D-aspartate receptor antagonists, MK-801 (150 microgramsmol litre-1) and dextromethorphan (150 microgramsmol litre-1) were also tested. In addition, we studied the effects of ketamine on production of tumour necrosis factor- alpha by activated macrophages. Ketamine inhibited nitrite production dose-dependently with both lipopolysaccharide- and lipoteichoic acid- activated macrophages by up to approximately 65% at the highest ketamine concentration (600 microgramsmol litre-1). Neither MK-801 nor dextromethorphan had an inhibitory effect. Ketamine also suppressed production of tumour necrosis factor-alpha. The data show that ketamine inhibited nitric oxide production by activated macrophages probably, in part, via inhibition of production of tumour necrosis factor-alpha, an autocrine stimulatory factor for nitric oxide production, but not via the NMDA receptor pathway, which is involved in neuronal nitric oxide production.      

Desensitization to the Behavioral Effects of alpha sub 2 -Adrenergic Agonists in Rats

Background: The analgesic and sedative-hypnotic utility of the alpha2 agonists clonidine and dexmedetomidine are currently being investigated. Both compounds exert their behavioral responses by activating central alpha2 adrenoceptors, albeit with different selectivities and efficacies. Furthermore, the analgesic and hypnotic behavioral responses are produced at different sites and may be affected independently of one another. A series of studies was conducted in rats to determine (1) whether tolerance and cross-tolerance develop to the analgesic actions of clonidine or dexmedetomidine; (2) how the number of available alpha2 adrenoceptors affects the analgesic response to dexmedetomidine and clonidine; and (3) how the number of available alpha sub 2 adrenoceptor affects the hypnotic response to dexmedetomidine.

Methods: Rats were administered equianalgesic doses of dexmedetomidine or clonidine continuously, subcutaneously by osmotic minipumps. After 7 days the analgesic response to acutely administered dexmedetomidine or clonidine at median effective analgesic doses was assessed by the tail-flick latency response. The number of alpha2 adrenoceptors in the spinal cord was diminished in a dose-dependent manner by covalent modification with a noncompetitive receptor blocker, N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline (EEDQ). Recovery of the tail-flick latency response to clonidine and dexmedetomidine was determined and correlated to the recovery of receptor density as assessed by radiolabeled-ligand binding studies. The alpha2 adrenoceptor population in the locus ceruleus of rats was depleted with EEDQ, and recovery of the hypnotic response (as assessed by the loss of righting reflex) to dexmedetomidine was determined and correlated to the recovery of receptor density.

Results: After 7 days of chronic treatment with dexmedetomidine, analgesic responses to dexmedetomidine and clonidine remained unaltered. However, chronic treatment with clonidine significantly decreased the analgesic effect of clonidine, whereas the analgesic effect to dexmedetomidine was unaffected. In the EEDQ experiments, the analgesic response to dexmedetomidine was restored to normal when 44% of the alpha sub 2 adrenoceptors in the spinal cord were available for agonist binding; comparatively more alpha2 adrenoceptors (77%) were required for the analgesic response to clonidine to be restored. The recovery of the hypnotic response to dexmedetomidine after EEDQ treatment was retarded when compared with the recovery of the analgesic response to that compound. Greater than 77% of alpha2 adrenoceptors in the locus ceruleus must be available for the hypnotic response to alpha2 agonists to be expressed.     

Production of paradoxical sensory hypersensitivity by alpha 2-adrenoreceptor agonists

BACKGROUND: Administration of opioid receptor agonists is followed by paradoxical sensory hypersensitivity. This hypersensitivity has been suggested to contribute to the antinociceptive tolerance observed with opioids. The authors hypothesized that alpha 2-adrenoreceptor agonists, which also produce antinociceptive tolerance, would produce sensory hypersensitivity. METHODS: alpha 2-Adrenoreceptor agonists were administered to male Sprague-Dawley rats as a single subcutaneous injection, a continuous subcutaneous infusion, a single intrathecal injection, or a continuous intrathecal infusion. Thermal sensitivity was determined using latency to withdrawal of the hind paw from radiant heat. Tactile sensitivity was determined using withdrawal threshold to von Frey filaments. Spinal dynorphin content was measured by enzyme immunoassay. RESULTS: Single systemic or intrathecal injections of clonidine or dexmedetomidine produced antinociception followed by delayed thermal and tactile hypersensitivity. Six-day systemic or intrathecal infusion of clonidine produced tactile and thermal hypersensitivity observed even during clonidine infusion. Sensory hypersensitivity was prevented by coadministration of the alpha 2-adrenoreceptor-selective antagonist idazoxan or the N-methyl-D-aspartate receptor-selective antagonist MK-801. Six-day infusion of intrathecal clonidine increased dynorphin content in dorsal lumbar spinal cord. MK-801 and dynorphin antiserum reversed clonidine-induced sensory hypersensitivity. CONCLUSIONS: alpha 2-Adrenoreceptor agonists produce sensory hypersensitivity that may be analogous to that produced by opioids. Sensory hypersensitivity was prevented by idazoxan, demonstrating that it is mediated by alpha 2 receptors. Clonidine infusion increased spinal dynorphin content. Sensory hypersensitivity was prevented or reversed by MK-801 and dynorphin antiserum, implicating N-methyl-D-aspartate receptors and spinal dynorphin in its production. Clinicians should be mindful of the possibility of drug-induced hyperalgesia in patients treated with alpha 2-adrenoreceptor agonists.     

Production of Paradoxical Sensory Hypersensitivity by α2-Adrenoreceptor Agonists

Background: Administration of opioid receptor agonists is followed by paradoxical sensory hypersensitivity. This hypersensitivity has been suggested to contribute to the antinociceptive tolerance observed with opioids. The authors hypothesized that [alpha]2-adrenoreceptor agonists, which also produce antinociceptive tolerance, would produce sensory hypersensitivity.

Methods: [alpha]2-Adrenoreceptor agonists were administered to male Sprague-Dawley rats as a single subcutaneous injection, a continuous subcutaneous infusion, a single intrathecal injection, or a continuous intrathecal infusion. Thermal sensitivity was determined using latency to withdrawal of the hind paw from radiant heat. Tactile sensitivity was determined using withdrawal threshold to von Frey filaments. Spinal dynorphin content was measured by enzyme immunoassay.

Results: Single systemic or intrathecal injections of clonidine or dexmedetomidine produced antinociception followed by delayed thermal and tactile hypersensitivity. Six-day systemic or intrathecal infusion of clonidine produced tactile and thermal hypersensitivity observed even during clonidine infusion. Sensory hypersensitivity was prevented by coadministration of the [alpha]2-adrenoreceptor-selective antagonist idazoxan or the N-methyl-d-aspartate receptor-selective antagonist MK-801. Six-day infusion of intrathecal clonidine increased dynorphin content in dorsal lumbar spinal cord. MK-801 and dynorphin antiserum reversed clonidine-induced sensory hypersensitivity.     

Alpha-2 Adrenoceptor Agonists Decrease Cyclic Guanosine 3',5'-Monophosphate in the Mouse Brain

Background: In the central nervous system neurotransmitters, drugs or conditions that excite increase cyclic guanosine 3',5'-monophosphate (cGMP), an effect mediated by the neuromodulator nitric oxide, whereas those that sedate decrease cGMP. Volatile anesthetics were shown to decrease cerebellar cGMP, an effect that correlates with their anesthetic and anticonvulsant effect. Because alpha-2 adrenoceptor agonists have anesthetic properties, the role of the nitric oxide-cGMP pathway in the action of the alpha-2 adrenoceptor agonists clonidine and dexmedetomidine was investigated.

Methods: Groups of mice were given, intraperitoneally, one dose of either 30-600 micro gram/kg clonidine, or 3-300 micro gram/kg D-medetomidine (dexmedetomidine) or L-medetomidine. The alpha-2 adrenoceptor antagonists, 0.3-5 mg/kg yohimbine or 1 mg/kg atipamezole, 1 mg/kg of the alpha-1 antagonist prazosin, and 10-300 mg/kg of the nitric oxide synthase inhibitors, N omega-nitro-l-arginine methylester and N omega-nitro-l-arginine, were given 10-20 min before the agonist. The mice were killed by microwave radiation focused to the head. Cyclic GMP was measured by radioimmunoassay in deproteinized extracts from different brain areas.

Results: Clonidine and dexmedetomidine, at sedative doses, dose-dependently decreased cerebellar cGMP (ED50: 100 and 50 micro gram/kg for clonidine and dexmedetomidine, respectively). This effect was inhibited by yohimbine and atipamezole, but not by prazosin, confirming the alpha-2 nature of the response to the agonists. L-medetomidine, which has no sedative/hypnotic effect, did not decrease cGMP. Pretreatment of the mice with a maximum dose of 100 mg/kg of a nitric oxide synthase antagonist abolished the cGMP response to the agonists. Similar results were obtained in the cerebral cortex, hippocampus and caudate nucleus.     

Ketamine Inhibits Glutamate-, N-Methyl-D-Aspartate-, and Quisqualate-stimulated cGMP Production in Cultured Cerebral Neurons

Background: Glutamatergic signaling has been linked to the recently discovered neurotransmitter/neuromodulator nitric oxide (NO), and several classes of anesthetics block some step in glutamatergic signaling. This study was designed to determine whether or not ketamine would prevent NO-dependent cGMP production stimulated by glutamate (GLU) and the GLU analogs NMDA, quisqualate (QUIS), and kainate (KAIN).

Methods: Primary cultures of cortical neurons and glia (prepared from 16-day gestational rat fetuses) were used after 12-16 days in culture. Reactions were carried out in magnesium-free buffer containing 100 micro Meter 3-isobutyl-l-methylxanthine, and cGMP content of cultures was used as a bioassay of NO production.

Results: Cyclic GMP production stimulated by sodium nitroprusside (100 micro Meter) occurred predominately in neurons and not in glia. Neurons were spontaneously active in these cultures; basal cGMP production was decreased by 50% in the presence of 1 micro Meter tetrodotoxin (TTX). Glutamate (100 micro Meter), NMDA (100 micro Meter), QUIS (300 micro Meter), and KAIN (100 micro Meter) each increased cGMP content of neuronal cultures. L-NMMA (100 micro Meter), a NO synthase inhibitor, prevented the stimulation of cGMP production by GLU or its analogs. Pretreatment with MK-801 (1 micro Meter) or ketamine (10-100 micro Meter) inhibited GLU-, NMDA-, and QUIS-stimulated cGMP production. Quisqualate-stimulated responses were the most sensitive to inhibition by ketamine and NMDA-stimulated responses were the least sensitive to inhibition. MK-801 and ketamine did not significantly inhibit KAIN-stimulated cGMP production. CNQX (10 micro meter) blocked KAIN-stimulated cGMP production only.     

Dexmedetomidine Enhances Analgesic Action of Nitrous Oxide: Mechanisms of Action

Background: Nitrous oxide and dexmedetomidine are thought to mediate analgesia (antinociception in a noncommunicative organism) via [alpha]2B- and [alpha]2A-adrenergic receptor subtypes within the spinal cord, respectively. Nitrous oxide and dexmedetomidine exert diametrically opposite effects on neuronal activity within the locus ceruleus, a pivotal site for modulation of analgesia. Because of these differences, the authors explored whether the two analgesics in combination would provide satisfactory analgesia.

Methods: The analgesic effects of nitrous oxide and dexmedetomidine given both intraperitoneally and intrathecally were evaluated using the tail-flick latency test in rats. For investigation of the interaction, rats were pretreated with dexmedetomidine, either intraperitoneally or intrathecally, immediately before nitrous oxide exposure such that peak antinociceptive effects of each drug coincided. For assessment of the effect on tolerance, dexmedetomidine was administered as tolerance to nitrous oxide developed. Expression of c-Fos was used to assess neuronal activity in the locus ceruleus.

Results: Nitrous oxide and dexmedetomidine increased tail-flick latency with an ED50 (mean +/- SEM) of 55.0 +/- 2.2% atm for nitrous oxide, 27.6 +/- 5.1 for [mu]g/kg intraperitoneal dexmedetomidine, and 2.9 +/- 0.1 [mu]g for intrathecal dexmedetomidine. Combinations of systemically administered dexmedetomidine and nitrous oxide produced an additive analgesic interaction; however, neuraxially administered dexmedetomidine interacted synergistically with nitrous oxide. Tolerance to nitrous oxide was reversed by coadministration of dexmedetomidine. Prazosin, the [alpha]1-/[alpha]2B-adrenoceptor antagonist, attenuated the analgesic effect of nitrous oxide and prevented dexmedetomidine-induced reversal of tolerance to nitrous oxide. Nitrous oxide-induced increase of neuronal activity in the locus ceruleus was reversed by dexmedetomidine.     

一氧化氮及一氧化氮合酶与吗啡耐受关系的新进展

疼痛治疗中长期给予吗啡易导致严重的耐受问题.多年来,针对耐受机制的研究表明NMDA/NO级联反应参与耐受的发生及发展.一氧化氮(nitric oxide,NO)主要是由一氧化氮合酶(nitric oxide synthase,NOS)催化其惟一前体--L-精氨酸生成NO和瓜氨酸.研究者们证实了大鼠鞘内吗啡耐受后脊髓内NOS尤其是nNOS的表达增高,耐受机制主要通过N-甲基-天门冬氨酸(N-methyL-D-aspartate,NMDA)受体的激活以及胞内钙离子浓度的升高来调节NOS的活性而触发NMDA/NO级联反应,继而影响耐受的发展.诸多研究给予吗啡的同时给予NOS抑制剂可以阻止耐受的发生,甚至在耐受形成后应用NOS抑制剂也可以翻转已经建立的耐受.但证实NOS各亚型在耐受中的具体作用仍不明确,需开展相关的研究进一步阐述其间的关系.     

Dexmedetomidine enhances analgesic action of nitrous oxide: mechanisms of action

BACKGROUND: Nitrous oxide and dexmedetomidine are thought to mediate analgesia (antinociception in a noncommunicative organism) via alpha 2B- and alpha 2A-adrenergic receptor subtypes within the spinal cord, respectively. Nitrous oxide and dexmedetomidine exert diametrically opposite effects on neuronal activity within the locus ceruleus, a pivotal site for modulation of analgesia. Because of these differences, the authors explored whether the two analgesics in combination would provide satisfactory analgesia. METHODS: The analgesic effects of nitrous oxide and dexmedetomidine given both intraperitoneally and intrathecally were evaluated using the tail-flick latency test in rats. For investigation of the interaction, rats were pretreated with dexmedetomidine, either intraperitoneally or intrathecally, immediately before nitrous oxide exposure such that peak antinociceptive effects of each drug coincided. For assessment of the effect on tolerance, dexmedetomidine was administered as tolerance to nitrous oxide developed. Expression of c-Fos was used to assess neuronal activity in the locus ceruleus. RESULTS: Nitrous oxide and dexmedetomidine increased tail-flick latency with an ED50 (mean +/- SEM) of 55.0 +/- 2.2% atm for nitrous oxide, 27.6 +/- 5.1 for microg/kg intraperitoneal dexmedetomidine, and 2.9 +/- 0.1 microg for intrathecal dexmedetomidine. Combinations of systemically administered dexmedetomidine and nitrous oxide produced an additive analgesic interaction; however, neuraxially administered dexmedetomidine interacted synergistically with nitrous oxide. Tolerance to nitrous oxide was reversed by coadministration of dexmedetomidine. Prazosin, the alpha 1-/alpha 2B-adrenoceptor antagonist, attenuated the analgesic effect of nitrous oxide and prevented dexmedetomidine-induced reversal of tolerance to nitrous oxide. Nitrous oxide-induced increase of neuronal activity in the locus ceruleus was reversed by dexmedetomidine. CONCLUSION: The synergistic analgesic interaction between nitrous oxide and dexmedetomidine within the spinal cord is obscured by a supraspinal antagonism when dexmedetomidine is administered systemically in the pretolerant state. After tolerance to nitrous oxide develops, supraspinal functional antagonism no longer obtains exposing the synergistic action at the level of the spinal cord, which expresses itself as a reversal of the tolerant state. The authors speculate that the addition of dexmedetomidine to nitrous oxide is likely to provide enhanced and more durable analgesia in settings in which nitrous oxide is currently used alone (e.g., labor and dental surgery).     

The effects of nitric oxide synthase inhibitors on the sedative effect of clonidine.

The mechanism underlying the Niteroi, Rio de Janeiro sedative effect of clonidine, an alpha2-adrenoceptor agonist, remains uncertain. Because activation of alpha2-adrenoceptors induces release of nitric oxide (NO), we tested the hypothesis that the sedative effect of clonidine depends on NO-related mechanisms. The effect of 7-nitro indazole on the sleeping time induced by clonidine was studied in Wistar rats. In addition, we examined the effect of clonidine, alpha-methyldopa, and midazolam on the thiopental-induced sleeping time in rats pretreated with N(G)-nitro-L-arginine-methyl-ester (L-NAME). The sleeping time induced by clonidine was significantly decreased by 7-nitro indazole. Thiopental sleeping time was increased by clonidine, alpha-methyldopa, and midazolam. L-NAME reduced the prolongation effect of clonidine and alpha-methyldopa, but did not alter the effect of midazolam on the thiopental-induced sleeping time. The inhibitory effect of L-NAME on clonidine-dependent prolongation of thiopental-induced sleeping time was reversed by L-arginine. These results suggest that NO-dependent mechanisms are involved in the sedative effect of clonidine. In addition, this effect seems to be specific for the sedative action of alpha2-adrenoceptors agonists. IMPLICATIONS: Clonidine, an antihypertensive drug, is also a sedative. This sedative effect, although an adverse event in the treatment of hypertensive patients, can be helpful for sedation of surgical patients. The mechanism of this effect, however, is unknown. In this study, we show that the sedative effect of clonidine is mediated by nitric oxide, because it could be prevented by pretreatment with nitric oxide synthase inhibitors.     

Dextromethorphan potentiates morphine antinociception at the spinal level in rats

PURPOSE: Morphine is an effective analgesic, but adverse effects limit its clinical use in higher doses. The non-opioid antitussive, dextromethorphan (DM), can potentiate the analgesic effect of morphine and decrease the dose of morphine in acute postoperative pain, but the underlying mechanism remains unclear. We previously observed that DM increases the serum concentration of morphine in rats. Therefore, we investigated the effects of drugs administered at the spinal level to exclude possible pharmacokinetic interactions. As DM has widespread binding sites in the central nervous system [such as N-methyl-D-aspartate (NMDA) receptors, sigma receptors and alpha(3)ss(4) nicotinic receptors], we investigated whether the potentiation of morphine antinociception by DM at the spinal level is related to NMDA receptors. METHODS: We used MK-801 as a tool to block the NMDA channel first, and then studied the interaction between intrathecal (i.t.) morphine and DM. The tail-flick test was used to examine the antinociceptive effects of different combinations of morphine and other drugs in rats. RESULTS: DM (2-20 microg) or MK-801 (5-15 microg) showed no significant antinociceptive effect by themselves. The antinociceptive effect of morphine (0.5 microg, i.t.) was significantly enhanced by DM and reached the maximal potentiation (43.7%-50.4%) at doses of 2 to 10 microg. Pretreatment with MK-801 (5 or 10 microg, i.t.) significantly potentiated morphine antinociception by 49.9% or 38.7%, respectively. When rats were pretreated with MK-801, DM could not further enhance morphine antinociception (45.7% vs 50.5% and 43.3%). CONCLUSION: Our results suggest that spinal NMDA receptors play an important role in the effect of DM to potentiate morphine antinociception.     

Dexmedetomidine exerts dose-dependent age-independent antinociception but age-dependent hypnosis in Fischer rats

Dexmedetomidine (Dex), an alpha(2)-adrenoceptor agonist, is an effective analgesic and sedative drug in adults; however, little information is available about its efficacy in pediatric populations. Some anesthetics exhibit an age-dependent analgesic effect, e.g., nitrous oxide, being relatively ineffective in newborn rats. We investigated the analgesic and hypnotic efficacy of Dex using 6 cohorts of Fischer rats aged 7, 15, 19, 23, and 29 days and adults exposed to either Dex (10 or 50 microg/kg) or saline subcutaneously. Formalin plantar testing was used to mimic inflammatory pain, and its effect was assessed using immunohistochemical (c-Fos staining) and behavioral methods. The hypnotic action of Dex was assessed by loss of righting reflex. Formalin administration produced a typical nociceptive response in each age group; these nociceptive responses were significantly attenuated by Dex 50 microg/kg at all ages (P < 0.05), whereas Dex 10 microg/kg had little effect. Neonatal rats showed the greatest hypnotic sensitivity to Dex (P < 0.05).     

The alpha2-adrenoceptor agonist dexmedetomidine converges on an endogenous sleep-promoting pathway to exert its sedative effects

BACKGROUND: The authors investigated whether the sedative, or hypnotic, action of the general anesthetic dexmedetomidine (a selective alpha -adrenoceptor agonist) activates endogenous nonrapid eye movement (NREM) sleep-promoting pathways. METHODS: c-Fos expression in sleep-promoting brain nuclei was assessed in rats using immunohistochemistry and hybridization. Next, the authors perturbed these pathways using (1) discrete lesions induced by ibotenic acid, (2) local and systemic administration of gamma-aminobutyric acid receptor type A (GABA ) receptor antagonist gabazine, or (3) alpha2-adrenoceptor antagonist atipamezole in rats, and (4) genetic mutation of the alpha -adrenoceptor in mice. RESULTS: Dexmedetomidine induced a qualitatively similar pattern of c-Fos expression in rats as seen during normal NREM sleep, a decrease in the locus ceruleus (LC) and tuberomammillary nucleus (TMN) and an increase in the ventrolateral preoptic nucleus (VLPO). These changes were attenuated by atipamezole and were not seen in mice lacking functional alpha2a-adrenoceptors, which do not show a sedative response to dexmedetomidine. Bilateral VLPO lesions attenuated the sedative response to dexmedetomidine, and the dose-response curve to dexmedetomidine was shifted right by gabazine administered systemically or directly into the TMN. VLPO lesions and gabazine pretreatment altered c-Fos expression in the TMN but in not the LC after dexmedetomidine administration, indicating a hierarchical sequence of changes. CONCLUSIONS: The authors propose that endogenous sleep pathways are causally involved in dexmedetomidine-induced sedation; dexmedetomidine's sedative mechanism involves inhibition of the LC, which disinhibits VLPO firing. The increased release of GABA at the terminals of the VLPO inhibits TMN firing, which is required for the sedative response.     

Experimental neoplastic spinal cord compression: effect of ketamine and MK-801 on edema and prostaglandins

Excitotoxin-induced neural tissue damage is mediated through specific receptors. We studied the in vivo effect of two selective N-methyl-D-aspartate receptor antagonists on the compressed spinal cord segments of rats harboring a thoracolumbar epidural tumor. The effect of a single intramuscular treatment with either MK-801 (3 mg/kg) or ketamine (110 mg/kg) given at the onset of paraplegia was evaluated 30 hours later. In saline-treated control animals, significant increases in water content, prostaglandin E2, and 6-keto-prostaglandin F1 alpha were evident. Treatment with either agent resulted in a normal water content in the compressed segments but had no effect on prostaglandin synthesis. Evaluation of the effect of treatment on the course of the disease required dose reduction by 45% for ketamine and by 30% for MK-801, to avoid the excessive sedative effect. Treatment was started at the first appearance of neurological dysfunction (Grade 1) and continued to paraplegia (Grade 5). The mean time interval between Grades 1 and 5 was 2.1 +/- 0.3 days in saline-treated control animals, and it was not significantly altered by either ketamine or MK-801. Our study indicates that in the end stage of epidural compression, when ischemia is present, excitotoxins probably participate in the evolution of a cytotoxic edema. It is suggested that treatment initiated at the onset of paraplegia may still reduce the cytotoxic edema, but its potential clinical value requires further investigations.     

The Effects of Nitric Oxide Supplementation and Inhibition on Bacterial Translocation in Bile Duct Ligated Rats

Obstructive jaundice promotes bacterial translocation from the gut, but the role of nitric oxide is controversial in this process. We studied the effects of nitric oxide synthase substrate, L-arginine, and nitric oxide synthase inhibitor, NG-nitro-¿-arginine methyl ester, on bacterial translocation in bile duct ligated rats. The animals were randomized into five groups; control, sham, common bile duct ligation alone, nitric oxide inhibition, and nitric oxide supplementation. Obstructive jaundice was performed with common bile duct ligation. ¿-arginine or NG-nitro-¿-arginine methyl ester was injected once daily for 14 days. Blood bilirubin level, liver histology, and bacterial translocation to the mesenteric lymph nodes as well as to the liver were assessed. The ¿-arginine supplemented group had the lowest bacterial translocation rate, but the most prominent hepatic fibrosis. Nitric oxide inhibition increased bacterial translocation to the mesenteric lymph nodes. Therefore, the administration of nitric oxide donor or inhibitor acts as a significant regulatory factor for bacterial translocation in obstructive jaundice.     

Lack of Effect of Intrathecally Administered N-methyl-D-aspartate Receptor Antagonists in a Rat Model for Postoperative Pain

Background: Evidence from experiments by others indicates an important role for excitatory amino acids activating spinal n-methyl-d-aspartate (NMDA) receptors in models of persistent pain. The purpose of this study was to examine the effect of intrathecal (+)-5-methyl-10,11-dihydro-5H-dibenzo(a,d)cyclohepten-5,10-imine (MK-801), a noncompetitive NMDA receptor antagonist, 2-amino-5-phosphonovaleric acid (AP5), a competitive NMDA receptor antagonist, and N-G-nitro-L-arginine methyl ester (L-Name), a nitric oxide synthase inhibitor, on pain behaviors in a rat model of postoperative pain.

Methods: Rats with intrathecal catheters were anesthetized and underwent a plantar incision. Withdrawal threshold to punctate stimulation applied adjacent to the wound, response frequency to application of a nonpunctate stimulus applied directly to the wound, and nonevoked pain behaviors were measured before and after intrathecal administration of MK-801 or AP5. The effect of intrathecal L-Name on mechanical hyperalgesia was also examined.

Results: Mechanical hyperalgesia increased and was persistent after plantar incision and was not decreased by intrathecal administration of 4, 14, or 40 nmol MK-801 or 10 nmol AP5. Only the greatest dose of AP5, 30 nmol, caused a small decrease in punctate and nonpunctate hyperalgesia. Intrathecal L-Name had no effect. Neither intrathecal MK-801 nor intrathecal AP5 affected nonevoked pain behaviors. The greatest doses caused motor deficits.     

MK-801 enhances gabaculine-induced loss of the righting reflex in mice, but not immobility

PURPOSE: gamma-Aminobutyric acid (GABA) and N-methyl-D-aspartate (NMDA) receptors are important targets for anesthetic action at the in vitro cellular level. Gabaculine is a GABA-trans-aminase inhibitor that increases endogenous GABA in the brain, and enhances GABA activity. We have recently shown that unconsciousness is associated with the enhanced GABA activity due to gabaculine, but that immobility is not. MK-801 is a selective NMDA channel blocker. In this study, we examined behaviourally whether gabaculine in combination with MK-801 could produce these components of the general anesthetic state. We further compared the effect of MK-801 with ketamine, another NMDA channel blocker. METHODS: All drugs were administered intraperitoneally to adult male ddY mice. To assess the general anesthetic components, two endpoints were used. One was loss of the righting reflex (LORR; as a measure of unconsciousness) and the other was loss of movement in response to tail-clamp stimulation (as a measure of immobility). RESULTS: Large doses of MK-801 alone (10-50 mg.kg(-1)) induced neither LORR nor immobility in response to noxious stimulation. However, even a small dose (0.2 mgxkg(-1)) significantly enhanced gabaculine-induced LORR (P < 0.05), although gabaculine in combination with MK-801 (0.2-10 mgxkg(-1)) produced no immobility. However, gabaculine plus a subanesthetic dose of ketamine (30 mgxkg(-1)), which acts on NMDA, opioid and nicotinic acetylcholine receptors and neuronal Na(+) channels, suppressed the pain response, but did not achieve a full effect. Ketamine alone dose-dependently produced both LORR and immobility. CONCLUSION: These findings suggest that gabaculine-induced LORR is modulated by blocking NMDA receptors, but that immobility is not mediated through GABA or NMDA receptors.