Fangchinoline inhibited the antinociceptive effect of morphine in mice

Fangchinoline (FAN), a non-specific calcium antagonist, is a major alkaloidal component of the creeper Stephania tetrandra S. Moore (or fenfangji). It has been shown to possess antagonistic activity on morphine-induced antinociception in mice. This study was undertaken to assess the antagonistic mechanism. The results demonstrated that FAN (IP) attenuated morphine (SC)-induced antinociception in a dose-dependent manner with significant effect at doses of 30 and 60mg/kg body wt. (IP) in the tail-flick test but not the tail-pinch tests, carried out in mice. This antagonism was abolished by pretreatment with a serotonin precursor, 5-hydroxytryptophan (5-HTP, IP), but not by pretreatment with a noradrenaline precursor, L-dihydroxyphenylalanine (L-DOPA, IP) in the tail-flick test. On the other hand, the development of morphine-induced analgesic tolerance was not prevented by FAN. These results suggest that the serotonergic pathway may be involved in the antagonism of morphine-induced antinociception by FAN and, in agreement with other reports, also indicates the possible dissociation of the morphine analgesic effect from its tolerance-development mechanism.     

Possible involvement of mu1-opioid receptors in the fentanyl- or morphine-induced antinociception at supraspinal and spinal sites

Fentanyl has been shown to be a potent analgesic with a lower propensity to produce tolerance and physical dependence in the clinical setting. The present study was designed to investigate the mechanisms of fentanyl- or morphine-induced antinociception at both supraspinal and spinal sites. In the mouse tail-flick test, the antinociceptive effects induced by both fentanyl and morphine were blocked by either the mu1-opioid receptor antagonist naloxonazine or the mu1/mu2-opioid receptor antagonist beta-funaltrexamine (beta-FNA) after s.c., i.c.v. or i.t. injection. In contrast, both fentanyl and morphine given i.c.v. or i.t. failed to produce antinociception in mu1-deficient CXBK mice. These findings indicate that like morphine, the antinociception induced by fentanyl may be mediated predominantly through mu1-opioid receptors at both supraspinal and spinal sites in mice. We also determined the ED50 values for s.c.-, i.c.v.- and i.t.-administered fentanyl- or morphine-induced antinociception in mice. The ED50 values for s.c.-, i.c.v.- and i.t.-administered fentanyl-induced antinociception were 73.7, 18.5 and 1.2-fold lower than that of morphine, respectively. The present data clearly suggest the usefulness of peripheral treatment with fentanyl for the control of pain.     

Modification of Morphine-induced Hyperlocomotion and Antinociception in Mice by Clorgyline,a Monoamine Oxidase-A Inhibitor

We evaluated the effects of pretreatment with clorgyline, an irreversible monoamine oxidase (MAO)-A inhibitor, on morphine-induced hyperlocomotion and antinociception. A single administration of morphine (30 mg/kg, i.p.) to male ICR mice induced a hyperlocomotion. ANOVA analysis revealed the statistical significance of the morphine effect on horizontal locomotion and of the clorgyline pretreatment × morphine interaction effect, but not of the effect of clorgyline pretreatment. The initial (5 min after challenge) phase of morphine actions vs. saline challenge appeared as if morphine had a strong inhibitory effect on locomotor activity in combination with different doses of clorgyline. The mice administered with morphine in combination of clorgyline (1 and 10 mg/kg) did not show any stereotypic behaviors. Clorgyline at a dose of 0.1 mg/kg but not other doses tested significantly potentiated morphine-induced antinociception evaluated by tail flick but not hot plate test. During the measurements of locomotor activity and antinociception, clorgyline at doses of 1 and 10 mg/kg significantly inhibited monoamine metabolism through MAO. These results suggest that clorgyline showed an inhibitory effect on morphine-induced hyperlocomotion, but not antinociception, through MAO inhibition. There is not a possibility that clorgyline pretreatment enhanced morphine action on motor activity, resulting in the abnormal behavior from hyperlocomotion to stereotypic movements.     

Antinociceptive mechanisms of orally administered decursinol in the mouse

Antinociceptive profiles of decursinol were examined in ICR mice. Decursinol administered orally (from 5 to 200 mg/kg) showed an antinociceptive effect in a dose-dependent manner as measured by the tail-flick and hot-plate tests. In addition, decursinol attenuated dose-dependently the writhing numbers in the acetic acid-induced writhing test. Moreover, the cumulative response time of nociceptive behaviors induced by an intraplantar formalin injection was reduced by decursinol treatment during the both 1st and 2nd phases in a dose-dependent manner. Furthermore, the cumulative nociceptive response time for intrathecal (i.t.) injection of TNF-alpha (100 pg), IL-1 beta (100 pg), IFN-gamma (100 pg), substance P (0.7 microg) or glutamate (20 microg) was dose-dependently diminished by decursinol. Intraperitoneal (i.p.) pretreatment with yohimbine, methysergide, cyproheptadine, ranitidine, or 3,7-dimethyl-1-propargylxanthine (DMPX) attenuated inhibition of the tail-flick response induced by decursinol. However, naloxone, thioperamide, or 1,3-dipropyl-8-(2-amino-4-chloro-phenyl)-xanthine (PACPX) did not affect inhibition of the tail-flick response induced by decursinol. Our results suggests that decursinol shows an antinociceptive property in various pain models. Furthermore, antinociception of decursinol may be mediated by noradrenergic, serotonergic, adenosine A(2), histamine H(1) and H(2) receptors.     

Modulation of acute morphine tolerance by corticotropin-releasing factor and dynorphin A in the mouse spinal cord.

Previously, we have demonstrated that intrathecally (i.t.) administered corticotropin-releasing factor (CRF) in mice produces stimulus-specific antinociception and modulation of morphine-induced antinociception by mechanisms involving spinal kappa opioid receptors. Recently, we also have found that CRF releases immunoreactive dynorphin A, a putative endogenous kappa opioid receptor agonist, from superfused mice spinal cords in vitro. Dynorphin A administered intracerebroventricularlly (i.c.v.) to mice has been shown to modulate the expression of morphine tolerance. In the present study, the possible modulatory effects of i.t. administered CRF as well as dynorphin A on morphine tolerance were studied in an acute tolerance model. Subcutaneous administration of 100 mg/kg of morphine sulfate (MS) to mice caused an acute tolerance to morphine-induced antinociception. The antinociceptive ED50 of MS was increased from 4.4 mg/kg (naive mice) to 17.9 mg/kg (4 hours after the injection of 100 mg/kg MS). To study the modulatory effects of spinally administered CRF and dynorphin A on the expression of morphine tolerance, CRF and dynorphin A were injected i.t. at 15 min and 5 min, respectively, before testing the tolerant mice by the tail-flick assay. The antinociceptive ED50 of MS in tolerant mice was decreased to 8.8 mg/kg and 7.1 mg/kg, respectively, after i.t. administration of CRF (0.1 nmol) and dynorphin A (0.2 nmol). In contrast, 0.5 nmol of alpha-helical CRF (9-41), a CRF antagonist and 0.4 nmol of norbinaltorphimine, a highly selective kappa opioid receptor antagonist, when administered i.t. at 15 min before the tail-flick test in tolerant mice, increased the antinociceptive ED50 of MS to 56.6 mg/kg and 88.8 mg/kg, respectively. These data confirmed the modulatory effect of dynorphin A on morphine tolerance and suggested that CRF, which releases dynorphin A in several central nervous system regions, also plays a modulatory role in the expression of morphine tolerance.     

Withania somnifera root extract prolongs analgesia and suppresses hyperalgesia in mice treated with morphine

Previous studies demonstrated that Withania somnifera Dunal (WS), a safe medicinal plant, prevents the development of tolerance to the analgesic effect of morphine.In the present study, we investigated whether WS extract (WSE) (100 mg/kg, i.p.) may also modulate the analgesic effect induced by acute morphine administration (2.5, 5, 10 mg/kg, s.c.) in the tail-flick and in the hot plate tests, and if it may prevent the development of 2.5 mg/kg morphine-induced rebound hyperalgesia in the low intensity tail-flick test. Further, to characterize the receptor(s) involved in these effects, we studied, by receptor-binding assay, the affinity of WSE for opioid (μ, δ, k), cannabinoid (CB₁, CB₂), glutamatergic (NMDA), GABAergic (GABA_A, GABA_B), serotoninergic (5HT_2A) and adrenergic (α₂) receptors.The results demonstrated that (i) WSE alone failed to alter basal nociceptive threshold in both tests, (ii) WSE pre-treatment significantly protracted the antinociceptive effect induced by 5 and 10 mg/kg of morphine only in tail-flick test, (iii) WSE pre-treatment prevented morphine-induced hyperalgesia in the low intensity tail-flick test, and (iv) WSE exhibited a high affinity for the GABA_A and moderate affinity for GABA_B, NMDA and δ opioid receptors.WSE prolongs morphine-induced analgesia and suppresses the development of morphine-induced rebound hyperalgesia probably through involvement of GABA_A, GABA_B, NMDA and δ opioid receptors. This study suggests the therapeutic potential of WSE as a valuable adjuvant agent in opioid-sparing therapies.     

Contribution of spinal mu(1)-opioid receptors and dynorphin B to the antinociception induced by Tyr-d-Arg-Phe-Sar

The antinociceptive effect of Tyr-d-Arg-Phe-Sar (TAPS) at the spinal level was characterized with the mouse tail-flick test. Intrathecal (i.t.) administration of TAPS produced a dose-dependent antinociception. The antinociception induced by TAPS was completely blocked by i.t. pretreatment with the mu-opioid receptor antagonist beta-funaltrexamine, the mu(1)-opioid receptor antagonist naloxonazine or the kappa-opioid receptor antagonist nor-binaltorphimine, but not with the delta-opioid receptor antagonist naltrindole. Moreover, TAPS-induced antinociception was dose-dependently attenuated by i.t. pretreatment with an antiserum against dynorphin B, but not against dynorphin A, alpha-neo-endorphin, [Met(5)]enkephalin, or [Leu(5)]enkephalin. In mice lacking prodynorphin, TAPS-induced antinociception was significantly reduced compared to that in wild-type mice. These results suggest that TAPS mainly stimulates mu(1)-opioid receptors, which subsequently induce the release of dynorphin B, which then acts on kappa-opioid receptors to produce antinociception.     

A Gi2α antisense oligonucleotide differentiates morphine antinociception,constipation and acute dependence in mice

In the same mice in which the intracerebroventricular (i.c.v.) administration of antisense oligodeoxyribonucleotide (oligo) directed against the G_i2α (but not G_i1α, G_i3α or G_sα) G-protein subunits attenuated i.c.v. morphine-induced antinociception in the tail-flick test, none of the oligos altered naloxone-precipitated jumping (acute dependence). Likewise, none of the oligos significantly altered morphine-induced constipation. Hence, i.c.v. morphine-induced antinociception might be preferentially mediated via transduction pathway(s) different from constipation or acute dependence, offering novel opportunities for drug discovery.     

Cholera and pertussis toxins inhibit differently hypothermic and anti-opioid effects of neuropeptide FF

In mice pretreated intracerebroventricularly (i.c.v.) with pertussis or cholera toxins, effects of neuropeptide FF (NPFF), on hypothermia and morphine-induced analgesia, were assessed. NPFF and a potent NPFF agonist, 1DMe (0.005-22 nmol) injected into the lateral ventricle decreased morphine analgesia and produced naloxone (2.5 mg x kg(-1), s.c.)-resistant hypothermia after administration into the third ventricle. Cholera toxin (CTX 1 microg, i.c.v.) pretreatment (24 or 96 h before) inhibited the effect of 1DMe on body temperature, but failed to reverse its anti-opioid activity in the tail-flick test. CTX reduced hypothermia induced by a high dose of morphine (8 nmol, i.c.v.) but not the analgesic effect due to 3 nmol morphine. Pertussis toxin (PTX) pretreatment inhibited both morphine-hypothermia and -analgesia but did not modify hypothermia induced by 1DMe. The present results suggest that NPFF-induced hypothermia depends on the stimulation of Gs (but not Gi) proteins. In contrast, anti-opioid effects resulting from NPFF-receptor stimulation do not involve a cholera toxin-sensitive transducer protein.     

Analgesic effect of tianeptine in mice

The effects of tianeptine, a novel and unusual tricyclic antidepressant drug, on tail-flick and hot-plate tests, which are two thermal analgesia evaluating methods, have been investigated in mice. Tianeptine (5 and 10 mg/kg), para-chlorophenylalanine (pCPA) (100 mg/kg) and a combination of pCPA and tianeptine (10 mg/kg) or saline were injected to mice intraperitoneally. pCPA (100 mg/kg) was injected 24 h before tianeptine or saline treatment when it was combined with tinaeptine (10 mg/kg) or tested alone. The tail-flick latencies and hot-plate reaction times of the mice were measured between 15th and 180th minutes following injections. Tianeptine (10 mg/kg) exhibited a significant antinociceptive activity that could be measured by both tests as compared to groups which were treated with saline or pCPA alone between 15th and 180th min of the observation period. The lower dose of tianeptine (5 mg/kg) or pCPA (100 mg/kg) did not produce any significant changes on tail-flick latency or hot-plate reaction time of the mice. However, pretreatment with pCPA completely blocked the antinociceptive effect induced by tianeptine (10 mg/kg) in both tests used in the present study. Furthermore, tianeptine (10 mg/kg) did not cause any significant impairment effects on rotarod performance of the mice. Our results suggested that tianeptine has a prominent thermal antinociceptive activity in mice and that increased serotonergic activity may be responsible for the analgesic effect of tianeptine.     

Clonidine analgesia: tolerance and cross-tolerance to autoanalgesia

Clonidine induced significant antinociception in rats, as assessed by radiant-heat tail-flick procedures. Continued administration of 0 5 mg/kg and 1.0 mg/kg of the drug for 7 days induced significant analgesic tolerance in a dose-dependent manner. Significant cross-tolerance to antinociception elicited by acute footshock or classical conditioning procedures was observed, with the greatest cross-tolerance being exhibited by the 1.0 mg/kg group. These data suggest similar noradrenergic mechanisms may subserve clonidine antinociception and autoanalgesia.     

Immobility response elicited by clamping the neck induces antinociception in a "tonic pain" test in mice

Clamping the neck followed by body inversion to a supine position in mice elicits an immobility response called immobility by clamping the neck (ICN). The noxious pinch to the scruff of the neck produces antinociception in "phasic pain" models (e.g. tail-flick test). Here, a "tonic pain" model was used to test the antinociception associated with the ICN, and naloxone was used to determine the role of opioids in such antinociception. Mice were injected intraperitoneally with 0.3 mL of 0.4% acetic acid to produce writhing responses that were measured for one hour. ICN was induced every five minutes for one hour. Naloxone (5 mg/kg ip) was injected 10 min before acetic acid administration. There was a control group, sham clamping (SCLA). These mice were handled and restricted every five minutes as in the ICN but without real clamping. The repetitive inductions of ICN were able to reduce the writhing behavior; this antinociception was blocked by the naloxone pretreatment. In the SCLA group antinociception was not observed. These findings indicate that as in the "phasic pain" model, ICN also was able to elicit antinociception in this "tonic pain" model, and such antinociception seems to be mediated by opioids.     

Colonic bead expulsion time in normal and mu-opioid receptor deficient (CXBK) mice following central (ICV) administration of mu- and delta-opioid agonists

A method utilizing the insertion of a 3 mm glass bead into the distal colon was used to evaluate the activity of intracerebroventricularly (ICV) administered mu- and delta-opioid agonists on colonic bead expulsion time in mice. Specifically, the ability of two mu-opioid receptor agonists, morphine and [D-Ala2,NMePhe4, Gly-ol5]-enkephalin (DAGO) and a selective delta-opioid receptor agonist, [D-Pen2,L-Pen5]-enkephalin (DPLPE), to inhibit colonic bead expulsion time was measured in normal (Swiss) and mu-opioid deficient (CXBK) mice. All three compounds maximally inhibited colonic bead expulsion time in normal mice. All three compounds also inhibited colonic bead expulsion time in CXBK mice, but none maximally. These results are in contrast to previous work in which clear differential analgesic sensitivity of CXBK mice to centrally administered mu- and delta-opioid receptor agonists was observed in the tail-flick test. Taken together, the results suggest (a) that mu-, and possibly delta-, opioid receptors can mediate supraspinal inhibition of colonic bead expulsion in mice and (b) that the genetic deficits of mu-receptor number or genetically-induced alteration in receptor function in CXBK mice do not equally affect inhibition of colonic bead expulsion and tail-flick antinociception.     

Circadian-Rhythm-dependent Effects of L-NG-Nitroarginine Methyl Ester (L-Name) on Morphine-Induced Analgesia

Circadian changes in the interactions between L-N^G-nitroarginine methyl ester (L-NAME), a nitric oxide synthase (NOS) inhibitor, and morphine-induced antinociception were investigated by the mouse hot-plate test. Born the basal pain sensitivity and morphine-induced analgesia undergo significant 24h variations. L-NAME (40 mg/kg, ip) alone did not show any antinociceptive activity, but potentiated morphine-induced analgesia when combined with morphine at all injection times. In terms of percentage absolute potentiation (%AP), L-NAME dramatically augmented the analgesic effect of morphine in the late dark period at 19 hours after lights on (HALO). It is concluded that nitric oxide (NO) is involved in the modulation of the analgesic effect of morphine; thus, the L-NAME and morphine combination might be beneficial in alleviating pain.     

Glycyl-glutamine inhibits the respiratory depression, but not the antinociception, produced by morphine

Glycyl-glutamine (Gly-Gln; beta-endorphin(30-31)) is an endogenous dipeptide that is synthesized through the posttranslational processing of beta-endorphin in brain stem regions that control respiration and autonomic function. This study tested the hypothesis that Gly-Gln administration to conscious rats will prevent the respiratory depression caused by morphine without affecting morphine antinociception. Rats were administered Gly-Gln (1-100 nmol) or saline (10 microl) intracerebroventricularly followed, 5 min later, by morphine (40 nmol icv). Arterial blood gases and pH were measured immediately before Gly-Gln and 30 min after morphine injection. Gly-Gln pretreatment inhibited morphine-induced hypercapnia, hypoxia, and acidosis significantly. The response was dose dependent and significant at Gly-Gln doses as low as 1 nmol. In contrast, Gly-Gln (1-300 nmol) had no effect on morphine-evoked antinociception in the paw withdrawal test. When given alone to otherwise untreated animals, Gly-Gln did not affect nociceptive latencies or blood gas values. These data indicate that Gly-Gln inhibits morphine-induced respiratory depression without compromising morphine antinociception.     

Spinal δ2 but not δ1 opioid receptors are involved in intracereboventricular β-endorphin-induced antinociception in the mouse

The antinociception induced by β-endorphin given intracerebroventricularly (i.c.v.) has been previously demonstrated to be mediated by the release of Met-enkephalin and subsequent stimulation of δ receptors in the spinal cord for antinociception. The present study was designed to determine what type of opioid receptor, δ1 or δ2, in the spinal cord is involved in i.c.v. β-endorphin-induced antinociception. Antinociception was assessed by the tail-flick test in male ICR mice. NTB (0.2–20 nmol) and NTI0 (0.22–2.2 nmol),selective δ2 receptor antagonists, given intrathecally (i.t.) dose-dependently attenuated i.c.v. β-endorphin-induced inhibition of the tail-flick response. On the other hand, BNTX (0.02–2.2 nmol), a selective δ1 receptor antagonist, given i.t., did not block i.c.v. β-endorphin-induced antinociception. The tail-flick inhibition induced by DAMGO, a μ receptor agonist, or U50,488H, a к receptor agonist, was not blocked by i.t. BNTX, NTB or NTI. It is concluded that δ2 but not δ1 receptors in the spinal cord are involved in i.c.v. β-endorphin-induced antinociception.     

Involvement of spinal release of α-neo-endorphin on the antinociceptive effect of TAPA

The antinociceptive effect of i.t.-administered Tyr-d-Arg-Phe-β-Ala (TAPA), an N-terminal tetrapeptide analog of dermorphin, was characterized in ddY mice. In the mouse tail-flick test, TAPA administered i.t. produced a potent antinociception. The antinociception induced by TAPA was significantly attenuated by i.t. pretreatment with the κ-opioid receptor antagonist nor-binaltorphimine, as well as by the μ-opioid receptor antagonist β-funaltrexamine and the μ₁-opioid receptor antagonist naloxonazine. TAPA-induced antinociception was also significantly suppressed by co-administration of the μ₁-opioid receptor antagonist Tyr-d-Pro-Phe-Phe-NH₂ (d-Pro²-endomorphin-2) but not by co-administration of the μ₂-opioid receptor antagonists Tyr-d-Pro-Trp-Phe-NH₂ (d-Pro²-endomorphin-1) and Tyr-d-Pro-Trp-Gly-NH₂ (d-Pro²-Tyr-W-MIF-1). In CXBK mice whose μ₁-opioid receptors were naturally reduced, the antinociceptive effect of TAPA was markedly suppressed compared to the parental strain C57BL/6ByJ mice. Moreover, the antinociception induced by TAPA was significantly attenuated by i.t. pretreatment with antiserum against the endogenous κ-opioid peptide α-neo-endorphin but not antisera against other endogenous opioid peptides. In prodynorphin-deficient mice, the antinociceptive effect of TAPA was significantly reduced compared to wild-type mice. These results suggest that the spinal antinociception induced by TAPA is mediated in part through the release of α-neo-endorphin in the spinal cord via activation of spinal μ₁-opioid receptors.     

Stereoselective effect of morphine on antinociception and endogenous opioid peptide levels in plasma but not cerebrospinal fluid of dogs.

Morphine releases endogenous opioids into the circulation of dogs. To test the stereospecificity of this effect, as well as to determine whether morphine also releases endogenous opioids centrally, which might be involved in its antinociceptive action, the effects of (-)-morphine sulfate (10 mg/kg, sc) or (+)-morphine hydrobromide on antinociception in a dog tail-flick test, on semi-quantified morphine-induced signs of salivation, emesis, defecation and ataxia, and on the plasma and cerebrospinal fluid (CSF) levels of endogenous opioid peptides were studied. Plasma and CSF levels of immunoreactive beta-endorphin (i-BE), met-enkephalin (i-ME), leu-enkephalin (i-LE), and dynorphin (i-DY) were quantified by radioimmunoassay in octadecylsilyl-silica cartridge extracts. Immunoreactive morphine (i-M) levels were measured in unextracted samples. (-)-Morphine treatment significantly increased antinociception, morphine-induced signs, i-M levels in plasma and CSF, and i-BE, i-ME, and i-LE levels in plasma, but not CSF. Levels of i-DY remained constant in plasma and CSF. (+)-Morphine treatment did not alter any of these parameters, indicating that the effects of morphine on nociception, behavioral signs, and plasma endogenous opioids in dogs were stereoselective. It is concluded that morphine does not cause an increase in immunoreactive endogenous opioid peptides in the CSF at the time of its peak antinociceptive effect.     

Lack of antinociceptive cross-tolerance between intracerebroventricularly administered beta-endorphin and morphine or DPDPE in mice

Antinociceptive tolerance and cross-tolerance to intracerebroventricular (i.c.v.) beta-endorphin, morphine, and DPDPE (D-Pen2-D-Pen5-enkephalin) induced by a prior i.c.v. administration of beta-endorphin, morphine and DPDPE, respectively, were studied in mice. Acute tolerance was induced by i.c.v. pretreatment with beta-endorphin (0.58 nmol), morphine (6 nmol) and DPDPE (31 nmol) for 120, 180 and 75 min, respectively. Various doses of beta-endorphin, morphine or DPDPE were then injected. The tail-flick and hot-plate tests were used as antinociceptive tests. Pretreatment of mice with beta-endorphin i.c.v. reduced inhibition of the tail-flick and hot-plate responses to i.c.v. administered beta-endorphin, but not morphine and DPDPE. Pretreatment of mice with morphine i.c.v. reduced inhibition of the tail-flick and hot-plate responses to morphine but not beta-endorphin. Pretreatment of mice with DPDPE reduced inhibition of the tail-flick and hot-plate responses to DPDPE but not beta-endorphin. The results indicate that one injection of beta-endorphin, morphine or DPDPE induces acute antinociceptive tolerance to its own distinctive opioid receptor and does not induce cross-tolerance to other opioid agonists with different opioid receptor specificities. The data support the hypothesis that beta-endorphin, morphine and DPDPE produce antinociception by stimulating specific epsilon, mu- and delta-opioid receptors, respectively.     

Synthesis, nicotinic acetylcholine receptor binding, and pharmacological properties of 3'-(substituted phenyl)deschloroepibatidine analogs

A series of 3'-(substituted phenyl)deschloroepibatidine analogs (5a-j) were synthesized. The alpha4beta2( *) and alpha7 nicotinic acetylcholine receptor (nAChR) binding properties and functional activity in the tail-flick, hot-plate, locomotor, and body temperature tests in mice of 5a-j were compared to those of the nAChR agonist, nicotine (1), epibatidine (4), and deschloroepibatidine (13), the partial agonist, varenicline (3), and the antagonist 2'-fluoro-3'-(substituted phenyl)deschloroepibatidine analogs (7a-j). Unlike epibatidine and deschloroepibatidine, which are potent agonists in the tail-flick test, 5a-k show no or very low antinociceptive activity in the tail-flick or hot-plate test. However, they are potent antagonists in nicotine-induced antinociception in the tail-flick test, but weaker than the corresponding 2'-fluoro-3'-(substituted phenyl)deschloroepibatidines.