Evidence for a role for bulbospinal pathways in the spinal antinociceptive effect of systemically administered vapreotide in normal rats

Summary— Numerous neurotransmitters are involved in nociceptive transmission or regulation. Several reports have shown the analgesic effects of somatostatin and its analogues. Somatostatin, when given intrathecally, markedly reduced pain in cancer patients. Somatostatin analogues that possess a longer half-life time are more convenient for therapeutic use. Vapreotide, a somatostatin analogue, was shown to induce a long-lasting antinociceptive effect in rats. We studied the site and the mechanism of action of vapreotide in rats using the paw pressure test. Intrathecal administration of vapreotide induced no antinociception. Systemically administered vapreotide-induced antinociception was inhibited by several intrathecal (it) administered antagonists (yohimbine, naloxone and to a lesser degree tropisetron). These results show a lack of spinal effect and suggest a supraspinal site of action with an involvement of noradrenergic and to a lesser degree serotonergic bulbospinal pathways. In addition, spinal opioid receptors also seem to be involved.     

The antinociceptive effects of stimulating the pretectal nucleus of the rat

Changes in the tail-flick latency to noxious heat were studied following electrical stimulation of the dorso-medial thalamus of the rat. Brief (15 sec), low intensity (35 microA) stimulation of the anterior pretectal nucleus caused no escape behavior or motor deficits but increased tail-flick latency for more than 45 min. Responses to non-noxious stimuli were enhanced but the animals were not hyperactive. The anterior pretectal nucleus does not receive retinal or accessory visual inputs like other parts of the pretectal complex but is known to receive axons from somatosensory cortex and project to the perirubral mesencephalic reticular formation and the periaqueductal gray (PAG). The antinociceptive effects of anterior pretectal stimulation were much longer lasting than those of PAG, less disrupting to motor performance and the stimulation was not aversive.     

Effects of a single dose of methamphetamine and iprindole on the serotonergic and dopaminergic system of the rat brain

A single dose (17.5 mg/kg i.p.) of methamphetamine was administered to iprindole-treated (10 mg/kg i.p.) rats. Forebrain concentrations of methamphetamine and amphetamine were significantly increased in iprindole-treated rats 1 and 6 hr after injection; in contrast to rats pretreated with saline, both amines were also detected after 18 hr. Three and 7 days after injection, significant decreases were seen in tryptophan hydroxylase (TPH) activity and serotonin concentrations in the cerebral cortex, neostriatum and hypothalamus. Hypothalamic TPH activity had recovered by 14 days. Neostriatal tyrosine hydroxylase activity and dopamine concentrations were significantly depressed at all time points examined. Iprindole alone produced a significant increase in cortical TPH activity after 1 day. After 3 days, TPH activity was significantly decreased when compared with control, whereas serotonin and 5-hydroxyindoleacetic acid concentrations were significantly increased. This study demonstrates that persistence of methamphetamine and/or amphetamine at the site of action is important for neurotoxicity.     

Adrenergic and serotonergic receptor binding in rat brain after chronic desmethylimipramine treatment.

The effects of chronic administration of the tricyclic antidepressant agent desmethylimipramine (DMI) on brain adrenergic and serotonergic receptor binding processes were studied. We examined the kinetic properties of alpha adrenergic, beta adrenergic and serotonergic receptor binding sites in cortical and subcortical brain regions of rats treated chronically for various time periods with DMI(6 mg/kg i.p. daily). After 1 week of daily injections, beta receptor binding density in the cortex was significantly decreased. The reduced density of the cortical beta receptors was evident throughout a 12-week administration period. It was not until after 6 weeks of DMI administration that a significant reduction in the subcortical beta receptors was evident. Compared to saline-injected controls, chronic DMI administration lowered [3H]dihydroalprenolol binding in the hippocampus but not in the striatum. After 12 weeks of DMI we detected no differences in alpha adrenergic binding characteristics in the cortex or subcortical forebrain using [3H]dihydroergocryptine as the binding ligand. There was no consistent alteration in the cortical serotonin receptor densities throughout the 12 weeks of DMI administration, and DMI had no effect on the serotonergic binding characteristics in the subcortical forebrain region. We conclude that chronic DMI administration selectively decreases the density of beta adrenergic receptors in rat brain.     

Oral ketamine is antinociceptive in the rat formalin test: role of the metabolite, norketamine.

The present study was designed to evaluate the oral efficacy and bioavailability of ketamine. Antinociceptive efficacy was determined with the rat formalin test and oral bioavailability by the measurement of plasma and brain concentrations of ketamine and its major metabolite, norketamine. Oral ketamine in a dose range from 30 to 180 mg/kg or saline was given prior to intraplantar formalin and the flinching behavior was measured. Oral ketamine dose-dependently reduced the flinching during phase 2, while flinching during phase 1 was reduced only with the highest dose given. Following oral ketamine at 100 mg/kg, blood and brain samples were obtained and plasma and brain ketamine and norketamine levels were measured using high-performance liquid chromatography (HPLC). The average concentration ratio of norketamine/ketamine, as expressed by the area under the curve (AUC) value, was 6.4 for plasma and 2.9 for brain. These results demonstrate that a significant amount of norketamine is formed by first pass biotransformation of ketamine and is distributed to the brain. Competition binding assays for the [3H]MK-801-labeled non-competitive site of the N-methyl-D-aspartate receptor (NMDA) receptor revealed that both norketamine and ketamine displaced [3H]MK-801 at low micromolar concentrations with Ki values of 2.5 and 0.3 mM in the forebrain, and 4.2 and 1.0 mM in the spinal cord, respectively. Spinal norketamine was approximately equipotent to ketamine in producing antinociceptive effects during phase 2 of the formalin test. Thus, norketamine appears to contribute to the antinociceptive effects of oral ketamine through its NMDA receptor antagonist activity.     

Neurotensin activation of the NTR1 on spinally-projecting serotonergic neurons in the rostral ventromedial medulla is antinociceptive

Microinjection of neurotensin (NT) in the rostral ventromedial medulla (RVM) produces dose-dependent antinociception. The NTR1 (Neurotensin Receptor Subtype 1) may mediate part of this response, however definitive evidence is lacking, and the spinal mediators of NTR1-induced antinociception are unknown. In the present study, we used immunohistochemical techniques to show that the NTR1, but not the NTR2 is expressed by spinally projecting serotonergic neurons of the RVM. We also show that microinjection of NT or the NTR1-selective agonist PD149163 in the RVM both produce dose-dependent antinociception in the tail-flick test that is blocked by the NTR1-selective antagonist SR48692. The antinociception produced by NT or PD149163 is also blocked by intrathecal administration of the non-selective serotonergic receptor antagonist methysergide. The results of these experiments provide anatomical and behavioral evidence that activation of NTR1-expressing spinally projecting neurons in the RVM produces antinociception through release of serotonin in the spinal dorsal horn. These results support the conclusion that the NTR1 plays an important role in the central modulation of nociception.     

The role of 5-HT(1A)-receptors in fentanyl-induced bulbospinal inhibition of a spinal withdrawal reflex in the rabbit

The sural to gastrocnemius withdrawal reflex is inhibited after injection of the OP(3) (micro)-receptor-selective opioid fentanyl into the fourth ventricle of decerebrated rabbits. This effect is abolished by complete section of the spinal cord but not by the selective alpha(2)-adrenoceptor antagonist RX 821002 (Clarke RW, Parry-Baggott C, Houghton AK, Ogilvie J. The involvement of bulbo-spinal pathways in fentanyl-induced inhibition of spinal withdrawal reflexes in the decerebrated rabbit. Pain 1998;78:197-207). We have now investigated the role of 5-HT(1A) receptors in mediating the descending inhibition activated by intraventricular fentanyl. In the control state, intraventricular fentanyl (3-30 microgram/kg) inhibited gastrocnemius reflex responses to a median of 34% of pre-drug levels. After intrathecal administration of the selective 5-HT(1A) receptor antagonist WAY-100635 (100 microgram), fentanyl reduced reflex responses to 83% of pre-fentanyl values, significantly less inhibition than in the control state. In a separate group of experiments, intravenous fentanyl (0.3-30 microgram/kg) depressed the sural-gastrocnemius reflex to 17% of pre-drug controls. This inhibition was not affected by intrathecal WAY-100635 (100 microgram), but combined administration of the 5-HT(1A) antagonist with RX 821002 (100 microgram) significantly reduced the effectiveness of i.v. fentanyl. After the highest dose reflexes were 37% of pre-fentanyl levels. These data show that the bulbospinal inhibition activated by fentanyl is mediated, at least in part, by activation of spinal 5-HT(1A) receptors. That blockade of these receptors failed to influence the inhibition induced by i.v. fentanyl might be taken to mean that the brain-stem action of fentanyl does not contribute significantly to the systemic actions of this opioid. A more probable explanation is that, in the preparation used in the present study, the bulbospinal and direct spinal actions of fentanyl occlude each other to produce an overall inhibition that is less than the sum of the two effects.     

Hexose monophosphate pathway activity in normal and hypoxic rat brain

Experiments were undertaken to define the role of the alternative route of glucose metabolism in the hexose monophosphate pathway (HMP) during energy balanced, mild brain hypoxia. In similar hypoxic model in spite of the lack of the deficit of high energy compounds, the significant acceleration of glycolysis and inhibition of macromolecular syntheses (lipid, proteins, and nucleic acids) were previously observed. The HMP activity, although directly coupled to intracellular synthetic processes, has not been defined and little is known about the mechanisms of its regulation under brain hypoxia. HMP activity was examined in the rat brain in vivo by estimation of the increment of 6-phosphogluconate concentration after inhibition of its oxidation as achieved by injection of 6-aminonicotinamide. The activity of this alternative route of glucose metabolism was estimated to be 0.4 mmol/h/kg w.w. in the brain cortex and 0.7 mmol/h/kg w.w. in the brain stem. During 2 h of mild hypoxia (7% O2 in N2) the HMP activity dropped to 30% of control level, whereas during first hour of reoxygenation increased to 200% of control. Increased activity of HMP in posthypoxic brain during reoxygenation also was observed in vitro by measuring the rate of [1-14C]- and [6-14C]glucose conversion to 14CO2 in cerebral cortical slices. The possible mechanism of the rapid changes in the activity of HMP induced by hypoxia is discussed. The results suggest that the brain glucose metabolism under mild hypoxia is reoriented toward energy producing pathway (glycolysis) partially at the expense of HMP. The mechanism of this regulation seems not to be directly triggered by energy deficit. Activity of HMP in the brain is in accord with the intracellular synthetic processes and their demands on the metabolites produced by this pathway. Relying upon that, the posthypoxic stimulation of HMP would indicate the metabolic recovery during reoxygenation.     

鞘内注射脂氧素A4对切口疼痛大鼠的抗伤害性感受作用

目的观察脊髓水平阿司匹林诱生型脂氧素A4(ATL)对切口疼痛大鼠热痛敏、机械痛敏的影响,探讨其是否具有抗伤害性感受作用及其作用机制。方法雄性SD大鼠72只制作动物切口疼痛模型,随机分为4组:生理盐水组(NS组),ATL组,脂氧素受体激动剂组(BML-111组),脂氧素受体拮抗剂组(BOC-2组),每组18只,于手术前(T0),术后2 h(T2)、4 h(T4)、6 h(T6)、8 h(T8)、10 h(T10)通过热板实验、探针刺激实验评估对大鼠热痛敏、机械痛敏的影响。实时PCR检测脊髓炎症因子(TNF-α、IL-6)m RNA的表达,Western blot检测腰段椎体p-ERK的表达。结果组内与T0比较发现,四组切口疼痛模型后2 h,大鼠TWL、MWT均减少(P<0.05),ATL和BML-111组内T4、T6、T8、T10与T2比较,TWL、MWT均增加(P<0.05),NS和Boc-2组内T4、T6、T8、T10与T2比较,TWL、MWT无明显变化。ATL和BML-111两组与NS组组间比较,T4、T6、T8、T10的TWL、MWT均增加(P<0.05)。RT-PCR显示,与NS组、Boc-2组比较,ATL组和BML-111组TNF-α、IL-6脊髓m RNA表达明显减少(P<0.05);Western blot结果显示,与NS组、Boc组相比,ATL组、BML-111组p-ERK蛋白表达降低;Boc组与NS组表达无差异。结论切口模型术后后足出现热缩足潜伏期缩短、机械缩足反应阈值减低,鞘内注射阿司匹林诱生型脂氧素A4可以减轻痛觉过敏,降低脊髓背角神经元中促炎症因子的表达,抑制脊髓p-ERK的表达。     

The role of TRPV1 receptors in the antinociceptive effect of anandamide at spinal level

While it is well known that the endogenous cannabinoid receptor ligand anandamide also activates the transient receptor potential vanilloid1 (TRPV1) receptors, there has been no in vivo study indicating the role of the TRPV1 receptors in the antinociceptive effect of anandamide at spinal level. The goal of this study was to determine the effect of inhibition of TRPV1 receptors by capsazepine on the antinociceptive potency of anandamide after intrathecal administration. Anandamide alone (1, 30 or 100 microg) dose-dependently decreased carrageenan-induced thermal hyperalgesia, however, the highest dose caused temporary excitation and vocalization, suggesting the pain-inducing potential of anandamide. Capsazepine (10 or 20 microg) by itself did not change the pain sensitivity markedly, but the lower dose increased it, and the higher dose decreased the antinociceptive effect of 30 microg anandamide. Furthermore, both doses of capsazepine decreased the efficacy of the largest dose of anandamide. These results show that TRPV1 receptor activation plays a substantial role in the antinociceptive effects of anandamide at spinal level. The effect of the inhibition on TRPV1 receptors depended on the dose applied. We presume that coactivation of the cannabinoid and TRPV1 receptors by anandamide provides elevated antinociception through the release of antinociceptive endogenous ligands at spinal level.     

经典的Wnt信号通路在大鼠肝脏卵圆细胞增殖和分化中的作用

目的探讨激活经典的Wnt信号转导通路在大鼠肝脏卵圆细胞增殖、分化中的作用。方法不同浓度的重组Wnt3a纯化蛋白(20,40,80,160,200ng/ml)在无血清培养条件下作用WB-F344细胞24h,Brdu掺入法检测细胞增殖情况。160ng/ml重组Wnt3a纯化蛋白在无血清条件下作用WB-F344细胞,作用24h后通过免疫荧光观察β-catenin亚细胞定位的变化,Western-blot观察β-catenin蛋白表达的变化;半定量RT-PCR观察下游靶基因CyclinD1mRNA表达的变化。作用72h后通过RT-PCR观察CK-19,AFP,ALB,HNF-6,HNF-4αmRNA表达的变化,Western-blot观察CK-19,AFP蛋白表达的变化。以同期未做处理的WB-F344细胞作为空白对照。结果不同浓度的重组Wnt3a纯化蛋白作用后,处理组WB-F344细胞以剂量依赖方式增殖,Wnt3a处理组明显高于空白对照组(P<0.05),160ng/ml作用时,增殖达到最高峰。Wnt3a(160ng/ml)作用24h后,通过免疫荧光共聚焦观察β-catenin的亚细胞定位发现,处理组β-catenin在WB-F344主要在细胞核中表达;而未处理组主要在核周表达;Western-blot发现处理组β-catenin蛋白表达轻度增加;半定量RT-PCR发现下游靶基因CyclinD1mRNA的表达显著增加(P<0.01)。72h后,RT-PCR发现两组中均有AFP,CK-19,HNF-6,HNF-4α基因表达,而ALB在两组中均不表达;同时通过Western-blot发现AFP,CK-19蛋白在两组中均有表达。结论激活经典的Wnt信号转导通路促进大鼠肝脏卵圆细胞系WB-F344的增殖和自我更新。     

Bile formation in the rat: the role of the paracellular shunt pathway.

Transepithelial movement of water and solute occurs both through the cell membrane as well as across the intercellular junctional complex (paracellular shunt pathways). Permeability of paracellular shunt pathways is increase by transmucosal osmotic gradients, and in certain epithelia these changes are associated with bullous-like deformations (blisters) of the zonula occludens and localization of lanthanum within junctional complexes. Although bile acids increase biliary secretion by osmotic forces, the source of this water movement into bile is not known. In the present studies we examined whether a choleretic infusion of sodium dehydrocholic acid (DHC) or its taurine conjugate, taurodehydrocholate, altered the solute permeability characteristics and morphologic appearance of the junctional complexes of rat hepatocytes. Animals were continuously infused for 1 hr with 1% albumin--0.9% NaCl alone or 120 mumol of DHC and bile flow and biliary clearance of [14C]sucrose, an indirect marker of biliary permeability were measured. The number of intercellular blisters adjacent to the bile canaliculus were counted in an unbiased manner from photographs obtained with scanning electron microscopy. Bile flow and the biliary sucrose clearance remained unchanged in control animals whereas DHC infusions resulted in a progressive increase in the biliary clearance of [14C]sucrose during the 60 min of infusion even though the choleretic response to DHC was stable during the final 30 min of infusion. DHC infusions produced surface invaginations, or blisters, (0.1--0.7 micrometer in diameter) which were located immediately adjacent to the hemi-bile canaliculus and occurred with a frequency of 1.62 +/- 0.08 per hepatocyte surface, which was fivefold greater than observed in controls. In separate groups of animals 5 mM ionic lanthanum chloride was perfused intraportally after taurodehydrocholate infusions, and the number of junctional complexes that contained the electron dense marker were quantitated by transmission electron microscopy. Localization of lanthanum in the junctional complexes of fasted control animals was not observed, whereas approximately equal to 50% of the zonula occludens in DHC-infused animals contained lanthanum which was also occasionally identified within the lumen of the bile canaliculus. These results indicate that infusions of DHC cause blisters adjacent to the junctional complex of rat hepatocytes in association with changes in solute conductivity of the zonula occludens to cations such as ionic lanthanum chloride, and presumably to larger solutes such as sucrose. Qualitatively similar morphologic findings were also observed during the infusion of sodium taurocholate at physiologic rate (40 mumol/h). These studies suggest that the paracellular shunt pathway in the liver is an important site for bile acid-induced water and solute movement into bile.     

Contractile serotonergic receptor in rat stomach fundus

Serotonin (5HT) is a potent agonist in contracting the rat stomach fundus although the nature of the receptor mediating the response has not been established. The present study was designed to explore the possibility that 5HT-induced contractions in the rat stomach fundus were mediated by interaction with receptors identical with either 5HT1A, 5HT1B or 5HT1C binding sites or 5HT3 receptors. Contractile concentration-response curves for several 5HT agonists [5-carboxamidotryptamine, TR3369, MK212, quipazine, RU 24969, 8-hydroxy-2-(di-n-propylamino)tetralin, TVXQ7821 and BEA 1654CL] were obtained in the rat stomach fundus. However, neither the potency nor maximum response of these agonists in contracting the rat stomach fundus correlated with the affinity of agonists at 5HT1A, 5Ht1B or 5HT1C binding sites. These agonists were interacting with 5HT receptors in the fundus based on the ability of 1-(1-naphthyl) piperazine (10(-7) M) to antagonize the contractile response of the relatively potent agonists. TVXQ7821 and BEA 1654Cl did not produce a marked contractile response in the fundus and also did not antagonize the contractile response to 5HT, suggesting that these agents have little, if any, affinity for the serotonergic receptor-mediating contraction in the fundus. The putative 5HT1A-selective receptor antagonists, WB4101 and spiroxatrine, did not block 5HT-induced contractions in the rat stomach fundus in concentrations consistent with their affinity at 5HT1A binding sites. The nonselective 5HT1A and 5HT1B receptor antagonist, cyanopindolol, also did not block 5HT-induced contractions in the rat stomach fundus.(ABSTRACT TRUNCATED AT 250 WORDS)     

Fenfluramine selectively and differentially decreases the density of serotonergic nerve terminals in rat brain: evidence from immunocytochemical studies

Fenfluramine is an amphetamine derivative which is used primarily as an anorectic agent in the treatment of obesity. High doses of fenfluramine have been reported to cause long-term decreases in brain serotonin (5-HT) levels and density of high-affinity 5-HT uptake sites, actions characteristic of a "neurotoxic" effect of the drug. In view of these neurochemical changes, we used immunocytochemistry to assess, in detail, the effects of fenfluramine treatment on the morphology and density of 5-HT-like immunoreactive neurons in rat brain. Twelve to 18 hr after high dose dl-fenfluramine HCl treatment (24 mg/kg s.c., twice daily for 4 days), there was a profound regional decrease in density of fine-caliber 5-HT-like immunoreactive fibers and terminals in brain. This effect was especially apparent in cerebral cortex, hippocampus, cerebellum and striatum and less striking decreases were noted in septum, locus ceruleus and hypothalamus. On the other hand, 5-HT-like immunoreactive somata in midbrain nuclei and fibers and terminals in spinal cord appeared unaffected after fenfluramine treatment. Remaining 5-HT-like immunoreactive fibers and terminals displayed morphology characteristic of degenerating axons (thickening, swollen varicosities and fragmentation). Two weeks after the 4-day treatment regimen, patterns of 5-HT-like immunostaining appeared similar to those noted immediately (i.e., 18 hr) after drug treatment; however, the presence of grossly deformed fibers and terminals seen shortly after drug treatment was lacking. Tyrosine hydroxylase-like immunoreactivity, used to assess changes in catecholamine-containing neurons, appeared unaffected by drug treatment. These data suggest that, in rats, high s.c. doses of fenfluramine may be neurotoxic to some 5-HT-like immunoreactive axons and terminals. The relevance of these observations to the continued therapeutic use in humans of smaller p.o. doses of fenfluramine remains to be determined.     

Chronic treatment with SCH 23390 and haloperidol: effects on dopaminergic and serotonergic mechanisms in rat brain

Effects of chronic administration (18 days) with SCH 23390 (0.1 or 0.5 mg/kg/day s.c.) and haloperidol (1 mg/kg/day s.c.) on dopamine and serotonin synthesis and metabolism in discrete dopaminergic and serotonergic nuclei of rat brain were studied. Additionally, the effects of these treatments on dopamine D-1 and D-2 receptor characteristics in rat caudate-putamen were investigated. Chronic administration with both dose regimens of SCH 23390 decreased DA metabolism significantly (basal homovanillic acid concentrations) in nucleus caudatus. In another set of experiments dopamine synthesis (rate of accumulation of 3,4-dihydroxyphenylalanine after 3,4-dihydroxyphenylalanine-decarboxylase inhibition) was reduced significantly only in nucleus accumbens after the higher SCH 23390 dose regimen. In turn, chronic administration with haloperidol decreased basal dopamine metabolism and synthesis in nucleus caudatus and nucleus accumbens. Chronic haloperidol, but not SCH 23390, treatment induced a clear-cut increase in [3H]spiperone binding in caudate-putamen. Interestingly, neither SCH 23390 nor haloperidol treatments affected [3H]SCH 23390 binding in caudate-putamen. SCH 23390 and haloperidol had no significant effects on serotonin synthesis and metabolism in serotonergic and dopaminergic areas. In conclusion, the classical antipsychotic drug, haloperidol, clearly decreases dopamine turnover in nigrostriatal and mesolimbic dopaminergic systems. The D-1 antagonist, SCH 23390, also decreases dopaminergic activity in nigrostriatal and mesolimbic systems although DA synthesis and metabolism are affected to different degrees in nucleus caudatus and nucleus accumbens. Therefore, we suggest that if D-1 antagonists such as SCH 23390 show antipsychotic activity in clinical studies, they may not be free of extrapyramidal side-effects.     

The role of the dorsal column pathway in visceral nociception

Neurosurgeons have successfully used punctate midline myelotomy to relieve visceral cancer pain in human patients. Animal experiments demonstrate a visceral nociceptive pathway in the posterior column that is more effective than the spinothalamic tract in activating thalamic neurons, eliciting behavioral responses and triggering increases in regional cerebral blood flow. This visceral nociceptive pathway involves postsynaptic dorsal column neurons in the central, visceral processing region of the spinal cord. Axons from the sacral cord ascend near the midline and from the thoracic cord at the junction of the gracile and cuneate fasciculi.     

Nrf2/ARE信号系统在黄体酮对急性脑创伤多效神经保护中的作用

近年来大量研究发现黄体酮(progesterone,PROG)对急性脑创伤(traumatic brain injury,TBI)、脑卒中等神经系统疾病有减轻脑水肿,抑制过度炎症反应,细胞凋亡,兴奋性神经毒性等多效神经保护作用[1-2].但PROG多效神经保护作用机制尚不清楚.Nrf2/ARE信号系统是重要的细胞保护机制,是许多保护性转录分子的共同上游调节系统[3].本研究旨在探讨Nrf2/ARE信号系统在PROG多效神经保护中作用,进一步阐明PROG多效神经保护作用机制.     

Pharmacokinetic/pharmacodynamic modeling of the antinociceptive effects of (+)-tramadol in the rat: role of cytochrome P450 2D activity

In this study the role of cytochrome P450 2D (CYP2D) in the pharmacokinetic/pharmacodynamic relationship of (+)-tramadol [(+)-T] has been explored in rats. Male Wistar rats were infused with (+)-T in the absence of and during pretreatment with a reversible CYP2D inhibitor quinine (Q), determining plasma concentrations of Q, (+)-T, and (+)-O-demethyltramadol [(+)-M1], and measuring antinociception. Pharmacokinetics of (+)-M1, but not (+)-T, was affected by Q pretreatment: early after the start of (+)-T infusion, levels of (+)-M1 were significantly lower (P < 0.05). However, at later times during Q infusion those levels increased continuously, exceeding the values found in animals that did not receive the inhibitor. These results suggest that CYP2D is involved in the formation and elimination of (+)-M1. In fact, results from another experiment where (+)-M1 was given in the presence and in absence of Q showed that (+)-M1 elimination clearance (CL(ME0)) was significantly lower (P < 0.05) in animals receiving Q. Inhibition of both (+)-M1 formation clearance (CL(M10)) and CL(ME0) were modeled by an inhibitory E(MAX) model, and the estimates (relative standard error) of the maximum degree of inhibition (E(MAX)) and IC(50), plasma concentration of Q eliciting half of E(MAX) for CL(M10) and CL(ME0), were 0.94 (0.04), 97 (0.51) ng/ml, and 48 (0.42) ng/ml, respectively. The modeling of the time course of antinociception showed that the contribution of (+)-T was negligible and (+)-M1 was responsible for the observed effects, which depend linearly on (+)-M1 effect site concentrations. Therefore, the CYP2D activity is a major determinant of the antinociception elicited after (+)-T administration.