Supraspinal Antinociceptive Effects Research Articles

The In Vivo Antinociceptive and Antiinflammatory Effects of Verbascum exuberans Hub.-Mor.

Safe and effective drugs are still lacking for many pain therapies. In recent years, growing interest has been devoted thus on herbal drugs as an option to identify new pain killers. Based on this, extensive researches are carried out on Verbascum L. genus due to its therapeutic potency on pain and inflammation therapy. In this study, among Verbascum species, the antinociceptive effect of Verbascum exuberans Hub.-Mor., and its contributions to nitrergic, serotonergic, or opioidergic pathways as well as its antiinflammatory acitivity were investigated. Tail clip, tail flick, and hot plate tests were used to determine the central (spinal and supraspinal) antinociceptive effect, while an acetic acid-induced writhing test was used to measure the peripheral antinociceptive effect of the extract (250 and 500 mg/kg). The extract (250 mg/kg) was then combined with nω-nitro-L-arginine methyl ester, cyproheptadine, and naloxone to evaluate its involvement in nitrergic, serotonergic, or opioidergic pathways, respectively. Carrageenan-induced hind paw edema model was used to determine the antiinflammatory effect of the extract (250 mg/kg). The extract shows central spinal but not central supraspinal antinociceptive effect, and presents peripheral antinociceptive effect. The antinociceptive actions of the extract is largely regulated via targeting the nitrergic pathway, while the opioidergic pathway is partly involved. Further, the extract shows antiinflammatory effect due to the significant inhibitions on the time dependent edema progression and the cytokine (tumor necrosis factor-alfa and interleukin-1beta) productions. V. exuberans could be stated as a new source with a high beneficial potential in alleviating pain and inflammation.

The Role of Glutamatergic and Dopaminergic Neurons in the Periaqueductal Gray/Dorsal Raphe: Separating Analgesia and Anxiety.

The periaqueductal gray (PAG) is a significant modulator of both analgesic and fear behaviors in both humans and rodents, but the underlying circuitry responsible for these two phenotypes is incompletely understood. Importantly, it is not known if there is a way to produce analgesia without anxiety by targeting the PAG, as modulation of glutamate or GABA neurons in this area initiates both antinociceptive and anxiogenic behavior. While dopamine (DA) neurons in the ventrolateral PAG (vlPAG)/dorsal raphe display a supraspinal antinociceptive effect, their influence on anxiety and fear are unknown. Using DAT-cre and Vglut2-cre male mice, we introduced designer receptors exclusively activated by designer drugs (DREADD) to DA and glutamate neurons within the vlPAG using viral-mediated delivery and found that levels of analgesia were significant and quantitatively similar when DA and glutamate neurons were selectively stimulated. Activation of glutamatergic neurons, however, reliably produced higher indices of anxiety, with increased freezing time and more time spent in the safety of a dark enclosure. In contrast, animals in which PAG/dorsal raphe DA neurons were stimulated failed to show fear behaviors. DA-mediated antinociception was inhibitable by haloperidol and was sufficient to prevent persistent inflammatory pain induced by carrageenan. In summary, only activation of DA neurons in the PAG/dorsal raphe produced profound analgesia without signs of anxiety, indicating that PAG/dorsal raphe DA neurons are an important target involved in analgesia that may lead to new treatments for pain.

Activation of delta-opioid receptor contributes to the antinociceptive effect of oxycodone in mice

Oxycodone has been used clinically for over 90 years. While it is known that it exhibits low affinity for the multiple opioid receptors, whether its pharmacological activities are due to oxycodone activation of the opioid receptor type or due to its active metabolite (oxymorphone) that exhibits high affinity for the mu-opioid receptors remains unresolved. Ross and Smith (1997) reported the antinociceptive effects of oxycodone (171nmol, i.c.v.) are induced by putative kappa-opioid receptors in SD rat while others have reported oxycodone activities are due to activation of mu- and/or delta-opioid receptors. In this study, using male mu-opioid receptor knock-out (MOR-KO) mice, we examined whether delta-opioid receptor was involved in oxycodone antinociception. Systemic subcutaneous (s.c.) administration of oxycodone (above 40mg/kg) could induce a small but significant antinociceptive effect in MOR-KO mice by the tail flick test. Delta-opioid receptor antagonist (naltrindole, 10mg/kg or 20mg/kg, i.p.) could block this effect. When oxycodone was injected directly into the brain of MOR-KO mice by intracerebroventricular (i.c.v.) route, oxycodone at doses of 50nmol or higher could induce similar level of antinociceptive responses to those observed in wild type mice at the same doses by i.c.v. Delta-opioid receptor antagonists (naltrindole at 10nmol or ICI 154,129 at 20μg) completely blocked the supraspinal antinociceptive effect of oxycodone in MOR-KO mice. Such oxycodone antinociceptive responses were probably not due to its active metabolites oxymorphone because (a) the relative low level of oxymorphone was found in the brain after systemically or centrally oxycodone injection using LC/MS/MS analysis; (b) oxymorphone at a dose that mimics the level detected in the mice brain did not show any significant antinocieption effect; (c) oxycodone exhibits equal potency as oxymorphone albeit being a partial agonist in regulating [Ca2+]I transients in a clonal cell line expressing high level of mu-opioid receptor. These data suggest that oxycodone by itself can activate both the mu- and delta-opioid receptors and that delta-opioid receptors may contribute to the central antinociceptive effect of oxycodone in mice.

Enhanced GABAergic synaptic transmission at VLPAG neurons and potent modulation by oxycodone in a bone cancer pain model.

Background and PurposeWe demonstrated previously that oxycodone has potent antinociceptive effects at supraspinal sites. In this study, we investigated changes in neuronal function and antinociceptive mechanisms of oxycodone at ventrolateral periaqueductal gray (VLPAG) neurons, which are a major site of opioid action, in a femur bone cancer (FBC) model with bone cancer-related pain.Experimental ApproachWe characterized the supraspinal antinociceptive profiles of oxycodone and morphine on mechanical hypersensitivity in the FBC model. Based on the disinhibition mechanism underlying supraspinal opioid antinociception, the effects of oxycodone and morphine on GABAA receptor-mediated inhibitory postsynaptic currents (IPSCs) in VLPAG neurons were evaluated in slices from the FBC model.Key ResultsThe supraspinal antinociceptive effects of oxycodone, but not morphine, were abolished by blocking G protein-gated inwardly rectifying potassium1 (Kir3.1) channels. In slices from the FBC model, GABAergic synaptic transmission at VLPAG neurons was enhanced, as indicated by a leftward shift of the input–output relationship curve of evoked IPSCs, the increased paired-pulse facilitation and the enhancement of miniature IPSC frequency. Following treatment with oxycodone and morphine, IPSCs were reduced in the FBC model, and the inhibition of presynaptic GABA release by oxycodone, but not morphine was enhanced and dependent on Kir3.1 channels.Conclusion and ImplicationsOur results demonstrate that Kir3.1 channels are important for supraspinal antinociception and presynaptic GABA release inhibition by oxycodone in the FBC model. Enhanced GABAergic synaptic transmission at VLPAG neurons in the FBC model is an important site of supraspinal antinociception by oxycodone via Kir3.1 channel activation.

HINT1 protein: A new therapeutic target to enhance opioid antinociception and block mechanical allodynia

In the nervous system, the glutamate N-methyl-d-aspartate receptor (NMDAR) restricts the activity of the mu-opioid receptor (MOR). Both receptors are present in midbrain periaqueductal grey (PAG) neurons, an area that plays a central role in the supraspinal antinociceptive effects of opioids. The cross-talk that occurs between these receptors is sustained by the MOR-associated histidine triad nucleotide binding protein 1 (HINT1), which displays nucleoside phosphoramidase and acyl-AMP hydrolase activity. Here we report that the inhibitor of HINT1 enzymatic activity guanosine-5′-tryptamine carbamate (TpGc) significantly enhanced morphine antinociception while preventing the development of tolerance. At the molecular level, TpGc reduced the capacity of MORs to recruit NMDAR activity to negatively regulate opioid signaling. In mice suffering from chronic constriction injury concurrent with increased NMDAR activity, a single intracerebroventricular administration of TpGc attenuated NMDAR function and alleviated mechanical allodynia for several days. These data suggest a potential therapeutic role for HINT1 inhibitors in the clinical management of acute and neuropathic pain.

The antinociceptive effect and mechanisms of action of pregabalin in mice

BackgroundPregabalin, a potent anticonvulsant agent, is used in treatment-resistant epileptic patients. It is reported that pregabalin also has analgesic effect in different pain syndromes. However, there is limited data on its antinociceptive mechanisms of action. We aimed to investigate the central and peripheral antinociceptive effects of pregabalin and the contribution of nitrergic, serotonergic, and opioidergic pathways in mice. MethodsWe used tail flick, tail clip and hot plate tests to investigate the central antinociceptive effects and acetic acid-induced writhing test to assess peripheral antinociceptive effects of pregabalin (10, 30, 100mg/kg). We also combined pregabalin (100mg/kg) with, a nonspecific nitric oxide synthase inhibitor l-NAME (100mg/kg), a serotonin receptor antagonist cyproheptadine (50μg/kg), and an opioid receptor antagonist naloxone (1mg/kg). ResultsPregabalin 30mg/kg enhanced the percentage of maximal possible effect (% MPE) in tail flick test. Pregabalin 100mg/kg significantly increased % MPE in tail clip and tail flick tests and decreased the number of writhings. Pregabalin made no significant alteration in hot plate test at all doses. The combined use of pregabalin 100mg/kg with l-NAME, cyproheptadine, and naloxone showed that % MPE was reduced only in the combination of pregabalin with naloxone and solely in tail clip test while no significant difference was observed in writhing test. ConclusionsWe suggest that pregabalin (30 and 100mg/kg) presents central spinal but not central supraspinal antinociceptive effect and pregabalin 100mg/kg shows peripheral antinociceptive effect. The opioidergic pathway seems to mediate the central spinal antinociceptive effect of pregabalin while nitrergic and serotonergic pathways are not involved.

Opposite effects of neuropeptide FF on central antinociception induced by endomorphin-1 and endomorphin-2 in mice.

Neuropeptide FF (NPFF) is known to be an endogenous opioid-modulating peptide. Nevertheless, very few researches focused on the interaction between NPFF and endogenous opioid peptides. In the present study, we have investigated the effects of NPFF system on the supraspinal antinociceptive effects induced by the endogenous µ-opioid receptor agonists, endomorphin-1 (EM-1) and endomorphin-2 (EM-2). In the mouse tail-flick assay, intracerebroventricular injection of EM-1 induced antinociception via µ-opioid receptor while the antinociception of intracerebroventricular injected EM-2 was mediated by both µ- and κ-opioid receptors. In addition, central administration of NPFF significantly reduced EM-1-induced central antinociception, but enhanced EM-2-induced central antinociception. The results using the selective NPFF1 and NPFF2 receptor agonists indicated that the EM-1-modulating action of NPFF was mainly mediated by NPFF2 receptor, while NPFF potentiated EM-2-induecd antinociception via both NPFF1 and NPFF2 receptors. To further investigate the roles of µ- and κ-opioid systems in the opposite effects of NPFF on central antinociception of endomprphins, the µ- and κ-opioid receptors selective agonists DAMGO and U69593, respectively, were used. Our results showed that NPFF could reduce the central antinociception of DAMGO via NPFF2 receptor and enhance the central antinociception of U69593 via both NPFF1 and NPFF2 receptors. Taken together, our data demonstrate that NPFF exerts opposite effects on central antinociception of endomorphins and provide the first evidence that NPFF potentiate antinociception of EM-2, which might result from the interaction between NPFF and κ-opioid systems.

Cav3.2 calcium channels: The key protagonist in the supraspinal effect of paracetamol

To exert its analgesic action, paracetamol requires complex metabolism to produce a brain-specific lipoamino acid compound, AM404, which targets central transient receptor potential vanilloid receptors (TRPV1). Lipoamino acids are also known to induce analgesia through T-type calcium-channel inhibition (Cav3.2). In this study we show that the antinociceptive effect of paracetamol in mice is lost when supraspinal Cav3.2 channels are inhibited. Therefore, we hypothesized a relationship between supraspinal Cav3.2 and TRPV1, via AM404, which mediates the analgesic effect of paracetamol. AM404 is able to activate TRPV1 and weakly inhibits Cav3.2. Interestingly, activation of TRPV1 induces a strong inhibition of Cav3.2 current. Supporting this, intracerebroventricular administration of AM404 or capsaicin produces antinociception that is lost in Cav3.2−/− mice. Our study, for the first time, 1) provides a molecular mechanism for the supraspinal antinociceptive effect of paracetamol; 2) identifies the relationship between TRPV1 and the Cav3.2 channel; and 3) suggests supraspinal Cav3.2 inhibition as a potential pharmacological strategy to alleviate pain.

Supraspinal antinociceptive effect of apelin-13 in a mouse visceral pain model

Apelin, as the endogenous ligand of the APJ receptor, is a novel identified neuropeptide whose biological functions are not fully understood. APJ receptor mRNA was found in several brain regions related to descending control system of pain, such as amygdala, hypothalamus and dorsal raphe nucleus (DRN). The present study was designed to determine whether supraspinal apelin-13 may produce antinociceptive effect observed in the acetic acid-induced writhing test, a model of visceral pain. Apelin-13 not only significantly produced preemptive antinociception at the dose of 0.3, 0.5, 1 and 3μg/mouse when injected intracerebroventricularly (i.c.v.) before acetic acid, but also significantly induced antinociception at a dose of 0.5, 1 and 3μg/mouse when injected i.c.v. after acetic acid. And i.c.v. apelin-13 did not influence 30-min locomotor activity counts in mice. Intrathecal (i.t.) administration of apelin-13 (1 and 3μg/mouse) significantly decreased the number of writhes, however, intraperitoneal (i.p.) injection of apelin-13 (10–100μg/mouse) had no effect on the number of writhes in the writhing test. The specific APJ receptor antagonist apelin-13(F13A), no-specific opioid receptor antagonist naloxone and μ-opioid receptor antagonist β-funaltrexamine hydrochloride (β-FNA) could significantly antagonize the antinociceptive effect of i.c.v. apelin-13, suggesting APJ receptor and μ-opioid receptor are involved in this process. Central low dose of apelin-13 (0.3μg/mouse, i.c.v.) could significantly potentiate the analgesic potencies of modest and even relatively ineffective doses of morphine administrated at supraspinal level. This enhanced antinociceptive effect was reversed by naloxone, suggesting that the potentiated analgesic response is mediated by opioid-responsive neurons.

Antinociceptive action of (±)- cis-(6-ethyl-tetrahydropyran-2-yl)-formic acid in mice

The objective of this study was to investigate spinal and supraspinal antinociceptive effects of a new synthetic compound, (±)- cis-(6-ethyl-tetrahydropyran-2-yl)-formic acid (tetrahydropyran derivative). Its activity was compared with those from morphine. In peripheral models of inflammation and hyperalgesia, tetrahydropyran derivative significantly reduced nociceptive effect induced by acetic acid or formalin in mice. Tetrahydropyran derivative developed antinociceptive effect on the tail-flick and hot-plate tests with a long-acting curve maintaining the effect for 4 h longer than morphine. The opioid receptor antagonist naloxone totally reverted tetrahydropyran derivative effects on both models. Morphine as well as tetrahydropyran derivative induced tolerance and sedation in mice. However, tetrahydropyran derivative-induced tolerance had its onset retarded and the sedative activity was lower when compared to that induced by morphine. These results indicate that this new substance develops an antinociceptive activity and may be used in the future as a substitute for traditional opioids.

PIII-1Determinant role of CYP2D6 and CYP3A4 pathways on the antinociceptive effects of oxycodone

BACKGROUND Oxycodone's (OX) major and minor metabolic routes (N- and O- demethylation) involve CYP3A4 and 2D6, respectively. We investigated their relative contribution on the antinociceptive effects of OX. METHODS Randomized, double-blind, placebo-controlled, crossover, 5-arm, PK-PD study in 10 healthy genotyped males (1PM, 7EM, 2UM for CYP2D6, AmpliChip™ Roche). They received orally placebo (P), single OX dose (0,2 mg/kg), OX 2h after CYP2D6 blockade with 100 mg quinidine (Q), OX 2h after CYP3A4 blockade with ketoconazole (K), or OX+Q+K. Naloxone (NX) iv was given 1.75h later. CYP2D6 (dextromethorphan) and CYP3A4 (midazolam) activity were assessed at each session. PK was followed for 24h and PD for 6h. Quantitative sensory testing using (thermal), nociceptive flexion reflex (electrical) (Viking IV, Madison USA), cold pressor test, and digital pupillometry (NeurOptics PLR-100) were performed. RESULTS After OX alone, the peak supraspinal antinociceptive effect increased in EM (37% SEM19), the PM did not experience significant effect (4%) whereas both UM showed a clear-cut increase (49 & 108%, p<0.05). Antinociceptive effects were inversely correlated with pupil diameter. NX reversed all OX PD effects. CYP3A4 inhibition by K enhanced (∼2-fold) the magnitude of OX effect. Adverse events (nausea, vomiting, drowsiness, headache, hallucinations) were observed with K or in UM only. CONCLUSION Both CYP2D6 genotype and CYP3A4 activity exert a determinant role on PK, PD, and safety of OX. Clinical Pharmacology & Therapeutics (2005) 79, P57–P57; doi: 10.1016/j.clpt.2005.12.209

Disruption of the type 1 inositol 1,4,5-trisphosphate receptor gene suppresses the morphine-induced antinociception in the mouse

The present study was designed to investigate whether endoplasmic inositol 1,4,5-trisphosphate (IP 3) receptor-mediated intracellular signaling pathway could be involved in the morphine-induced antinociception in the mouse. An intracerebroventricular (i.c.v.) pretreatment with xestospongin C (10–100 pmol per mouse, i.c.v.), a membrane permeable and selective antagonist for IP 3 receptor, produced a dose-dependent reduction in the supraspinal antinociceptive effect produced by i.c.v. administration of morphine (0.3–10 nmol). In addition, the dose-response curve for morphine-induced antinociception was significantly shifted to the right by i.c.v. pretreatment with xestospongin C at 100 pmol. In the present study, we confirmed that the IP 3R1 mRNA in opisthonos ( opt) heterozygote mice was approximately 50% reduced as compared to that in wild-type mice. Under these conditions, a significant antinociception produced by subcutaneous (s.c.) injection of morphine (5 mg/kg) observed in wild-type mice was markedly reduced in opt heterozygote mice. These findings suggest that IP 3 receptor-, especially type 1 IP 3 receptor-mediated intracellular signaling pathway may be implicated in the expression of antinociceptive effect induced by morphine in mice.

Antinociceptive, antiinflammatory and acute toxicity effects of Salvia leriifolia Benth seed extract in mice and rats.

The antinociceptive and antiinflammatory effects as well as the acute toxicity of Salvia leriifolia aqueous seed extract were studied in mice and rats. Antinociceptive activity was assessed using the hot-plate and tail flick tests. The effect on acute inflammation was studied using vascular permeability increased by acetic acid and xylene-induced ear oedema in mice. The activity against chronic inflammation was assessed using the cotton pellet test in rats. The LD(50) of the extract was found to be 19.5 g/kg (i.p.) in mice. The aqueous seed extract showed significant and dose-dependent (1.25-10 g/kg) antinociceptive activity over 7 h, and was inhibited by naloxone pretreatment. Significant and dose-dependent (2.5-10 g/kg) activity was observed against acute inflammation induced by acetic acid and in the xylene ear oedema test. In the chronic inflammation test the extract (2.5-5 g/kg) showed significant and dose-dependent antiinflammatory activity. The aqueous seed extract of S. leriifolia may therefore have supraspinal antinociceptive effects which may be mediated by opioid receptors, and showed considerable effects against acute and chronic inflammation.

Supraspinal antinociceptive effects of mu and delta agonists involve modulation of adenosine uptake.

The modulation of extracellular adenosine concentration by opioids provides evidence that the antinociceptive effects of these compounds involve endogenous adenosine. The aim of this study was to determine whether there is a relation between the inhibition of brain synaptosome adenosine uptake by opioid agonists and the analgesic effects of these compounds. The authors used the hot plate and tail-pinch tests to evaluate in mice (C57BL/6 females; weight, 25-30 g) the effects of caffeine, a nonspecific adenosine receptor antagonist, on the antinociceptive effect induced by the intracerebroventricular administration of oxymorphone as a mu agonist, SNC80 as a delta agonist, or U69593 as a kappa agonist. They also investigated the effect of these opioid receptor agonists on the uptake of adenosine by whole brain synaptosomes. Caffeine decreased the analgesic effects induced by oxymorphone or SNC80 but not those induced by U69593. Oxymorphone and SNC80 inhibited adenosine uptake by brain cells, but U69593 did not. The antinociceptive effects obtained with mu or delta (but not kappa) agonists administered supraspinally were indicative of the involvement of modulation of adenosine uptake.

Antisense oligodeoxynucleotide targeting distinct exons of the cloned mu-opioid receptor distinguish between endomorphin-1 and morphine supraspinal antinociception in mice.

Antisense oligodeoxynucleotides (ODN) were used to investigate the supraspinal antinociceptive effects of endomorphin-1, an endogenous peptide whose analgesic profile suggests that it is a ligand at the mu-opioid receptor. To selectively restrict the expression of this receptor, five ODN targeting distinct exons of the gene sequence were injected subchronically by the intracerebroventricular route (i.c.v.) into mice. The antinociception induced by endomorphin-1 was greatly reduced in animals receiving the ODN directed to nucleotides 677-697, which code for a sequence located on the second extracellular loop of the mu receptor. ODN-mu(un), one of the two antisense ODN directed to exon 1, also impaired endomorphin-1 antinociception. ODN targeting exons 2 and 4 were totally inactive. In contrast, all five ODN blocked the antinociception induced by morphine and beta-casomorphin. The analgesic potency of endomorphin-1, morphine, and beta-casomorphin remained unaltered by administration of an ODN to nucleotides 29-46 of the murine delta-opioid receptor gene sequence of a random-sequence ODN. This suggest the existence of diverse molecular forms for the mu-opioid receptor that mediate the antinociceptive effects of endomorphin-1 and morphine/beta-casomorphin.

Antinociception produced by receptor selective opioids. Modulation of supraspinal antinociceptive effects by spinal opioids

This study evaluated the antinociceptive effects produced when different combinations of supraspinal μ- and δ-opioid agonist were co-administered with spinal μ-, δ-, and κ-opioid agonist. Using the Randall-Selitto paw-withdrawal test, in the rat, changes in nociceptive thresholds were measured following co-administration of sequentially increasing i.c.b. doses of either DAMGO or DPDPE with a low-antinociceptive dose of intrathecal DAMGO, DPDPE, or U50,488H. Antinociceptive synergy (i.e. a more than additive antinociceptive effect) was demonstrated with all of the combinations tested except for supraspinal DPDPE co-administered with spinal DAMGO. The results of this study provide support for the suggestion that supraspinal and spinal antinociceptive mechanisms share, in part, common neural circuits. Marked differences in the overall magnitude of the antinociceptive effects produced by the various combinations of opioid agonists were demonstrated through a secondary analysis of the data. When sequentially increasing i.c.v. doses of DAMGO were administered, significantly larger increases in nociceptive thresholds were observed with co-administration of intrathecal injections of low antinociceptive doses of either DAMGO or U50,488H compared to DPDPE. In contrast, when DPDPE was administered supraspinally, the largest increases in nociceptive thresholds were demonstrated with co-administration of DPDPE at the spinal site. The results of the secondary analysis provide support for the hypothesis that descending antinociceptive control systems activated by supraspinal administration of selective μ- and δ-opioid agonists interact, differently, with spinal μ-, δ, and κ-opioidergic mechanisms.

G X/Z and G i2 transducer proteins on μ/δ opioid-mediated supraspinal antinociception

Intracerebroventricular (i.c.v.) administration of immune sera raised against G i2α subunits to mice, significantly reduced the supraspinal antinociceptive effect of opioids when evaluated 24 h later in the tail-flick test. Antisera directed against G i1α subunits did not modify this opioid activity. In mice injected with sera anti-G X/Zα, the μ-preferential agonists, DAMGO and morphine, and the endogenous μ/δ opioid peptide β-endorphin-(1–31) displayed a reduced antinociceptive activity, whereas, the potency of the δ-selective agonists DPDPE and [D-Ala 2] Deltorphin II, was not altered. This reduction was present for 3 to 7 days and returned to the control values after 10 days. Anti-G i2α and anti-G X/Zα, but not anti-G i1α, reduced the e specific binding of [ 3H]DAMGO to the opioid receptor in PAG. These results suggest the ability of the μ receptor to interact in vivo with different classes of G transducer proteins (G X/Z/G i2) to produce an effect. This work also indicates a functional role of the pertussis toxin insensitive G X/Z protein, on the μ-mediated (but not δ-mediated) supraspinal antinociception in mice.

The noradrenergic component contributing to spinal fentanyl-induced antinociception is supraspinally mediated

1. 1. Male Sprague-Dawley rats were fitted with intrathecal (i.t.) and intracerebroventricular (i.c.v.) catheters. Fentanyl was injected either i.t. or i.c.v., and the antinociceptive efficacy of fentanyl was evaluated using the tail-flick analgesiometric assay. 2. 2. Fentanyl dose-dependently elevated tail-flick latency (TFL) following i.c.v. or i.t. administration. The antinociceptive effects of fentanyl were reversed by naltrexone. 3. 3. Experiments were also designed to evaluate the effects of serotonin and α-adrenoceptor antagonists on i.t. or i.c.v. fentanyl-induced elevations in TFL. 4. 4. Phentolamine administered i.t. reversed both the spinal and supraspinal antinociceptive effects of fentanyl, whereas i.t. methysergide did not significantly alter the i.t. or i.c.v. effects of the μ agonist. 5. 5. These data suggest that fentanyl-induced antinociception does not rely on local serotonergic neuronal activation. Due to the highly lipophilic nature of fentanyl, it is possible that the noradrenergic component contributing to spinal fentanyl-induced analgesia is supraspinally-mediated.