Tail-flick Response Research Articles

The Effect of a Selective α 2-Adrenoceptor Antagonist on Pain Behavior of the Rat Varies, Depending on Experimental Parameters

Effects of atipamezole, an α 2-adrenoceptor antagonist, in various acute pain tests were studied in the rat. Atipamezole (at doses ⩾0.1 mg/kg IP) and idazoxan, another α 2-adrenoceptor antagonist (2.5 mg/kg, IP), increased licking latency in the hot-plate test. Bilateral administration of atipamezole (10 μg) into the locus coeruleus did not increase licking latency in the hot-plate test. Medetomidine (an α 2-adrenoceptor agonist; 1–3 mg/kg) or repeated preexposures to the testing apparatus reversed the effect of atipamezole (1.5 mg/kg) in the hot-plate test. Atipamezole also increased the latency to mechanically induced licking/biting response at a dose of 1.5 mg/kg, but not at lower doses. In the heat-induced tail-flick test, in contrast, atipamezole at doses of 0.1 and 1.5 mg/kg produced a medetomidine-reversible decrease of response latencies. This facilitation of the tail-flick response disappeared if the intensity of the heat stimulus was high. At a dose range from 0.03 to 1.5 mg/kg atipamezole did not significantly alter the paw withdrawal latency to noxious mechanical stimulation, nor pain behavior in the formalin test. Responses to nociceptive spinal dorsal horn neurons were not modulated by atipamezole (1 mg/kg) in anesthetized spinalized rats. The results indicate that an α 2-adrenoceptor antagonist may have variable effects in behavioral pain tests, depending on habituation of the experimental animals to the testing conditions, the dose of the drug, the type of behavioral response and the submodality or the intensity of the noxious test stimulus. The atipamezole-induced changes in pain behavior observed in this study may rather be explained due to action on motor expression of pain than due to modulation of nociception.

Effects of Peptidase Inhibitors on the Enkephalin-Induced Anti-nociception in Rats

The intra-third-ventricular (i.t.v.) administration of [Met5]-enkephalin (enk) to rats pretreated i.t.v. with three peptidase inhibitors (Pis), amastatin, captopril and phosphoramidon, inhibited the tail-flick response. The enk-induced inhibition was augmented by increasing the doses of the three Pis, with the maximum inhibition being attained at the doses of 10 nmol each. The enk-induced inhibition in rats pretreated with any combination of two Pis, however, were markedly smaller than that in rats pretreated with all three Pis, indicating that three kinds of enzymes all played important roles in the inactivation of enk. The inhibitory effect of enk on the tail-flick response in rats pretreated with the three Pis at doses of 10 nmol each was approximately tenfold higher than that of morphine. The relative anti-nociceptive potencies of enk and morphine were similar to the relative inhibitory potencies obtained previously with the isolated guinea pig ileum pretreated with the three Pis, indicating that the hydrolysis of the i.t.v. administered enk was largely prevented by the three Pis. However, the magnitude of the enk-induced inhibition in rats pretreated s.c. with the three Pis indicated that the hydrolysis of enk injected i.t.v. was not largely prevented by the s.c. administration of three Pis at doses up to 10 μmol each/kg.

Heroin Antinociception Changed From Mu to Delta Receptor in Streptozotocin-Treated Mice

CD-1 mice were treated intravenously with streptozotocin, 200 mg/kg, and tested 2 weeks later or treated with 60 mg/kg and tested 3 days later. Both treatments changed the tail flick response of heroin and 6-monoacetylmorphine (6 MAM) given intracerebroventricularly from a μ- to δ-opioid receptor-mediated action as determined by differential effects of opioid receptor antagonists. The response to morphine remained μ. Heroin and 6 MAM responses involved δ1 (inhibited by 7-benzylidenenaltrexone) and δ2 (inhibited by naltriben) receptors, respectively. These δ-agonist actions did not synergize with the μ-agonist action of morphine in the diabetic mice. The expected synergism between the δ agonist, [D-Pen2-D-Pen5]enkephalin (DPDPE), and morphine was not obtained in diabetic mice. Thus, diabetes disrupted the purported μ/δ-coupled response. In nondiabetic CD-1 mice, heroin and 6 MAM produced a different μ-receptor response (not inhibited by naloxonazine) from that of morphine (inhibited by naloxonazine). Also, these μ actions, unlike that of morphine, did not synergize with DPDPE. The unique receptor actions and changes produced by streptozotocin suggest that extrinsic in addition to genetic factors influence the opioid receptor selectivity of heroin and 6 MAM.

Nociceptin peptide-receptor system in the brainstem auditory regions

The antinociceptive potency of nociceptin/orphanin FQ, an opioid-like orphan receptor agonist, was examined using the tail-flick test and the formalin-induced nociception test in diabetic mice. Nociceptin/orphanin FQ, at doses of 0.1 to 10 nmol, intrathecal (i.t.), produced a marked and dose-dependent inhibition of the tail-flick response in both non-diabetic and diabetic mice. The antinociceptive effect of nociceptin/orphanin FQ in the tail-flick test in diabetic mice was greater than that in non-diabetic mice. The antinociceptive effect of nociceptin/orphanin FQ was not antagonized by pretreatment with either β-funaltrexamine, a selective μ-opioid receptor antagonist, naltrindole, a selective δ-opioid receptor antagonist, or nor-binaltorphimine, a selective κ-opioid receptor antagonist. The antinociceptive effects of nociceptin/orphanin FQ in diabetic, but not in non-diabetic mice, were abolished when mice were pretreated with capsaicin i.t. 24 h before testing. In the formalin test, nociceptin/orphanin FQ also produced a marked and dose-dependent antinociceptive effect on the first-phase response, but not the second phase-response, in both diabetic and non-diabetic mice. Furthermore, nociceptin/orphanin FQ significantly and dose-dependently reduced the flinching responses to i.t.-administered substance P in diabetic mice, but not in non-diabetic mice. The results of the present experiments clearly indicate that the antinociceptive potency of nociceptin/orphanin FQ is significantly greater in diabetic mice than in non-diabetic mice. Furthermore, the results of this study suggest that the reduction of substance P-mediated nociceptive transmission in the spinal cord may be responsible for the antinociceptive effect of nociceptin/orphanin FQ.

Effects of ginsenosides injected intrathecally or intracerebroventricularly on antinociception induced by morphine administered intracerebroventricularly in the mouse

1. 1. Total saponin fraction at doses of 0.1 to 1.0 μg, which administered IT alone did not affect the latencies of the tail-flick threshold, dose dependently attenuated inhibition of the tail-flick response induced by ICV-administered morphine (2 μg). 2. 2. Total saponin fraction at doses of 1 to 20 μg, which administered ICV alone did not affect the latencies of the tail-flick response, did not affect ICV-administered morphine-induced antinociception. 3. 3. The duration of antagonistic action of the total saponin fraction against morphine-induced antinociception lasted for at least 6 hrs. 4. 4. Various doses of ginsenosides R b2, R c, R d and R g1, but not R b1 and R e, injected IT dose dependently attenuated antinociception induced by morphine administered ICV. 5. 5. In summary, ginsenosides R b2, R c, R d and R g1 injected spinally appear to have antagonistic action against the antinociception induced by supraspinally applied morphine. On the other hand, the total ginseng fraction, at supraspinal sites, may not have an antagonistic action against the antinociception induced by morphine administered supraspinally.

Peripherally administered alpha2-adrenoceptor agonist in the modulation of chronic allodynia induced by spinal nerve ligation in the rat.

We studied whether a peripherally administered alpha2-adrenoceptor agonist modulates mechanical allodynia caused by unilateral ligation of two spinal nerves in the rat. Medetomidine, an alpha2-adrenoceptor agonist, atipamezole, an alpha2-adrenoceptor antagonist, or saline (control) was administered into the footpad of either the allodynic or the contralateral hindpaw. Medetomidine (1-10 microg/kg in 50 microL) reversed the unilateral allodynia in a dose-dependent fashion independent of the site of administration. At this dose range, medetomidine did not influence the heat-induced tail-flick response. The antiallodynic effect of medetomidine was completely reversed by a dose of atipamezole that alone was ineffective (30 microg/kg). At the largest dose used (100 microg/kg in 50 microL), atipamezole decreased the latency of the heat-induced tail flick and had an ipsilateral allodynic effect in the injected paw when administered into the control side. The atipamezole-induced mechanical allodynia was not attenuated by medetomidine. The results indicate that an alpha2-adrenoceptor agonist at a subantinociceptive dose may significantly attenuate allodynia produced by spinal nerve ligation. This antiallodynic effect is not due to a peripheral action, but rather to action on central (presumably spinal) alpha2-adrenoceptors. The allodynic effect of a high concentration of atipamezole (100 microg/kg in 50 microL) in the injected paw can be explained by peripheral nonadrenergic mechanisms (e.g., local irritation). The present behavioral results indicate that a selective alpha2-adrenoceptor agonist at a subantinociceptive dose effectively attenuates mechanical allodynia induced by an experimental model of chronic neuropathy in the rat. This antiallodynic action can be explained by central, rather than peripheral, alpha2-adrenergic mechanisms.

Opiate receptors in the periaqueductal gray mediate analgesic effect of nitrous oxide in rats

The site of action and the pathways which are activated by nitrous oxide (N2O) to produce an analgesic effect are not well defined. Experiments were designed to determine whether N2O produces analgesia by activating opiate receptors or α2-adrenoceptors in periaqueductal gray. The analgesic effect of N2O was determined using the tail flick response to noxious radiant heat in lightly anesthetized rats. Different antagonists were bilaterally microinjected into ventrolateral periaqueductal gray to determine whether the analgesic effect produced by N2O was reversed. The increase in the tail flick latencies produced by N2O was reversed by bilateral microinjection into the ventrolateral part of periaqueductal gray with the opiate receptor antagonist naloxone 2.5 μg/0.5 μl, but not with the α2-adrenoceptors antagonist yohimbine 1.5 μg/0.5 μl. These results indicate that the N2O analgesic effect is mediated by activation of opiate receptors, but not α2-adrenoceptors, in the periaqueductal gray. Combined with the previous experiments that the N2O analgesic effect is reversed by intrathecal injection of an α2-adrenoceptor antagonist but not by an opiate receptor antagonist, it seems likely that N2O causes activation of the opiate receptors in the periaqueductal gray, which in turn activate the noradrenergic descending pathways to the spinal cord to produce the analgesic effect.

A Comparative Study of the Antinociceptive Action of Xenon and Nitrous Oxide in Rats

We attempted to clarify the mechanism of antinociceptive action induced by xenon and nitrous oxide.Eighty percent of nitrous oxide or 80% xenon was applied to rats inside enclosed clear plastic glass cylinders with their tails protruding for assessment of the tail-flick response to radiant heat. With repeated testing, there was a rapid reduction to nitrous oxide antinociception within 90 min, which was interpreted as development of tolerance, but not to xenon antinociception. Nitrous oxide antinociception was blocked by the intraperitoneal administration of 0.1 or 1.0 mg/kg yohimbine, but not by 1.0 or 5.0 mg/kg L659-066 or by 5.0 or 10 mg/kg naloxone. Xenon antinociception was not affected by any of these drugs. Yohimbine and L659-066 are characterized as alpha2-adrenoceptor antagonists. Although yohimbine penetrates the blood-brain barrier after systemic administration, L659-066 does not penetrate it and act peripherally. Therefore, the results indicate that alpha2-adrenoceptors, but not opioid receptors, may play a key role in antinociception induced by nitrous oxide in the central nervous system. Furthermore, the mechanism of xenon antinociception differs from that of nitrous oxide because it does not involve either alpha2 or opioid receptors. Implications: The precise mechanism of antinociceptive action of nitrous oxide and xenon remains unknown. It is still controversial whether an opioid system plays a role in antinociception induced by nitrous oxide. The results of the study showed that antagonism of central alpha2-adrenoceptors, but not opioid receptors, reverses the antinociception induced by nitrous oxide but not by xenon, which indicates that alpha2-adrenoceptors may play a key role in nitrous oxide antinociception. (Anesth Analg 1997;85:931-6)

Neuroanatomical aspects of mydriatic action of morphine in rats.

Morphine causes mydriasis in rats. In order to investigate whether this effect is due to direct inhibition of preganglionic pupilloconstrictor neurons in the Edinger-Westphal nucleus (EWN), we injected opiate agonists into the EWN in male albino Charles River rats. Bilateral stereotactic microinjections of morphine (10, 20, 30, 40 micrograms/side) inhibited spinal nociceptive reflexes and caused pronounced catalepsy, but had no effect on pupillary size. The powerful opiate agonist, fentanyl, also elicited analgesia and catalepsy, when given in doses of 5 and 10 micrograms/side, but no dose of fentanyl up to 10 micrograms/side induced mydriasis. Naloxone (10 micrograms/side), given into the EWN, effectively antagonized inhibition of the tail-flick response induced by subcutaneously administered morphine (30 micrograms/kg), but had no effect on the cataleptic and mydriatic actions of systemic morphine. These results indicate that, in the rat, morphine-induced mydriasis is not accounted for by a direct action on the EWN.

Differential effects of adenosine receptor antagonists injected intrathecally on antinociception induced by morphine and β-endorphin administered intracerebroventricularly in the mouse

A previous study reported that β-endorphin and morphine administered supraspinally produce antinociception by activating different descending pain inhibitory systems. The present study was designed to investigate the blocking effects of A 1 or A 2 adenosine receptors in the spinal cord on antinociception induced by supraspinally administered μ- and ε-opioid receptor agonists. The effects of 1,3-dipropyl-8-(2-amino-4-chloro-phenyl)-xanthine (PACPX; an A 1 adenosine receptor antagonist) or 3,7-dimethyl-1-propargylxanthine (DMPX; an A 2 adenosine receptor antagonist) on the antinociception induced by morphine (a μ-opioid receptor agonist) or β-endorphin (an ε-opioid receptor agonist) administered intracerebroventricularly (i.c.v.) were studied. The antinociception was assayed by the tail-flick test. DMPX at doses of 1–40 μg (which administered intrathecally alone did not affect the latencies of tail-flick thresholds), attenuated dose-dependently the inhibition of the tail-flick response induced by i.c.v. administered morphine (0.5 μg) or β-endorphin (1 μg). PACPX at doses of 1–40 μg (which administered intrathecally alone did not affect the latencies of tail-flick thresholds), attenuated dose-dependently the inhibition of the tail-flick response induced by i.c.v. administered β-endorphin but not morphine. These results suggest that A 2 but not A 1 adenosine receptors in the spinal cord may be involved in the antinociception induced by supraspinally administered morphine, while the antinociception induced by supraspinally administered β-endorphin appears to be mediated by spinal A 1 and A 2 adenosine receptors. These results support the hypothesis that morphine and β-endorphin administered supraspinally produce antinociception by different neuronal mechanisms.

Effects of intrathecal injection of nimodipine, ω-conotoxin GVIA, calmidazolium, and KN-62 on the antinociception induced by cold water swimming stress in the mouse

The present study was designed to determine if spinal calcium channels, calmodulin, and calcium/calmodulin-dependent protein kinase II were involved in the production of antinociception induced by cold water swimming stress (CWSS). The effects of intrathecal (i.t.) injection of nimodipine, ω-conotoxin GVIA, calmidazolium, or (S)-5-isoquinolinesulfonic acid, 4-[2-[(5-isoquinolinyl-sulfonyl)methylamino]-3-oxo-3-(4-phenyl-1-piperazinyl)-propyl]phenyl ester (KN-62) on CWSS-induced antinociception were studied in ICR mice. The antinociception was assessed by the tail-flick test. CWSS produced inhibition of the tail-flick response. Various doses of nimodipine (10–40 ng), ω-conotoxin GVIA (5–40 ng), calmidazolium (10–40 ng), or KN-62 (5–40 ng) injected i.t. alone did not show any antinociceptive effect in the tail-flick test. I.t. pretreatment with ω-conotoxin GVIA, calmidazolium, or KN-62 dose dependently attenuated the CWSS-induced inhibition of the tail-flick response. However, i.t. pretreatment with nimodipine did not affect the inhibition of the tail-flick response induced by CWSS. Our results suggest that spinal N-type calcium channel, calmodulin and calcium/calmodulin-dependent protein kinase II may be involved in the production of antinociception induced by CWSS. On the other hand, CWSS-induced antinociception appears not to be mediated via the spinal L-type calcium channel.

Involvement of nitric oxide and serotonin in modulation of antinociception and pressor responses evoked by stimulation in the dorsolateral region of the periaqueductal gray matter in the rat

In rats anaesthetized with alphaxalone/alphadolone, electrical stimulation in the periaqueductal gray matter in the region lying lateral and dorsolateral to the aqueduct produced a pressor response and an increase in the latency of the tail flick response to noxious heat applied to the tail. The antinociception and the pressor response were significantly attenuated following microinjection of 15 nmol 5-hydroxytryptamine at the site of stimulation in the periaqueductal gray matter. Microinjection of an equal volume of 165 mM saline had no effect. The inhibitory effects of 5-hydroxytryptamine were blocked by prior intracerebroventricular administration of 100 μg of the nitric oxide synthase inhibitor l-nitroarginine methyl ester. Neither 5-hydroxytryptamine or l-nitroarginine methyl ester had any effect on resting arterial pressure or on the baseline latency of the tail flick reflex. It is suggested that the inhibitory effects of 5-hydroxytryptamine in the dorsolateral periaqueductal gray matter are normally dependent on the functional integrity of local nitric oxide synthase-containing interneurons. Nitric oxide may act in association with 5-hydroxytryptamine to control the excitability of the aversive system in the midbrain.

Met]enkephalin in the spinal cord is involved in the antinociception induced by intracerebroventricularly-administered etorphine in the mouse

We have recently reported that the antinociception induced by etorphine given i.c.v. is mediated in part by the stimulation of both μ- and ϵ-opioid receptors and the activation of both monoaminergic and opioidergic descending pain control systems. [29]Since the opioid ϵ-receptor-mediated antinociception induced by β-endorphin is mediated by the release of [Met]enkephalin and subsequent stimulation of δ-opioid receptors in the spinal cord, the present studies were designed to determine if β-endorphin-like action is also involved in etorphine-induced antinociception. The tail-flick test was used to assess the antinociceptive response performed in male ICR mice. Etorphine at doses from 5–20 ng given i.c.v. produced a dose-dependent inhibition of the tail-flick response. The inhibition of the tail-flick response induced by etorphine given i.c.v. was antagonized by intrathecal pretreatment for 60 min with antiserum against [Met]enkephalin (10 μg), but not with antiserum against [Leu]enkephalin (10 μg) or dynorphin A (1–13) (10 μg). Desensitization of δ-opioid receptors in the spinal cord by intrathecal pretreatment with [Met]enkephalin (5 μg) for 60 min attenuated i.c.v. administered etorphine-induced tail-flick inhibition. However, intrathecal pretreatment with [Leu]enkephalin (5 μg) or dynorphin A (1–17) (0.1 μg) for 60 min did not attenuate i.c.v. administered etorphine-induced tail-flick inhibition. The results indicate that antinociception induced by etorphine given i.c.v. is mediated in part by the stimulation of the ϵ-opioid receptor at the supraspinal sites and by the release of [Met]enkephalin which subsequently stimulates δ-opioid receptors in the spinal cord.

Effects of spinally and supraspinally injected 3-isobutyl1-methylxanthine, cholera toxin, and pertussis toxin on cold water swimming stress-induced antinociception in the mouse

1. 1. The cold (4°C) water swimming stress (CWSS) for 3 min significantly increased the inhibition of the tail-flick response in ICR mice. 2. 2. Pertussis toxin (PTX, 0.05–0.5 μg) in mice pretreated intrathecally (IT) for 6 days attenuated the inhibition of the tail-flick response induced by CWSS. However, intracerebroventricular (ICV) pretreatment with PTX at the same doses did not affect CWSS-induced inhibition of the tail-flick inhibition. 3. 3. 3-Isobutyl-l-methylxanthine (IBMX, 0.01–1 ng) in mice pretreated IT for 10 min dose-dependently attenuated the inhibition of the tail-flick response induced by CWSS. However, IBMX in mice ICV pretreated ICV at the same doses was not effective in attenuating the CWSS-induced inhibition of the tail-flick response. 4. 4. Neither IT nor ICV pretreatment with cholera toxin (CTX, 0.05–0.5 μg) for 24 hr affected the inhibition of the tail-flick response induced by CWSS. 5. 5. The ICV or IT injection of PTX, CTX, or IBMX did not affect the basal tail-flick response latency. 6. 6. It is concluded that spinal, but not supraspinal, PTX-sensitive G-proteins and cAMP phosphodiesterase may be involved in the antinociception produced by CWSS. However, neither spinal nor supraspinal CTX-sensitive G-proteins appear to be involved in mediating the antinociception induced by CWSS.

Comparison of relative potencies of i.v. and i.c.v. administered 8-OH-DPAT gives evidence of different sites of action for hypothermia, lower lip retraction and tail flicks

8-Hydroxy-2-(di- n-propylamino)tetralin (8-OH-DPAT)-induced temperature reduction, lower lip retraction and tail flick responses are widely used models of 5-HT 1A receptor function. To obtain information about the sites of receptors mediating these effects we measured these responses, parallel over wide dose ranges after intracerebroventricular (i.c.v., 0.6–67 μg/kg) and intravenous (i.v., 3–500 μg/kg) administration. Analysis of the dose–response curves provided evidence for a 9.8-fold ratio of the potency of 8-OH-DPAT following i.c.v. compared to i.v. administration on body temperature reduction (ED 50 values are 5.1 and 50 μg/kg, after i.c.v. and i.v. administration, respectively) and a 2.9-fold ratio in potency for lower lip retraction (ED 50 values are 29 and 86 μg/kg, after i.c.v. and i.v. administration, respectively). 8-OH-DPAT was less potent in the induction of tail flicks than of the other responses and had a lower potency after i.c.v. than after i.v. administration (ED 50 values, the first one extrapolated, are 526 and 246 μg/kg, after i.c.v. and i.v. administration, respectively). In addition, the i.c.v. ED 50 for temperature reduction was significantly lower than those for lower lip retraction or tail flick responses. The relative potency, that is, the ratio of i.v. and i.c.v. ED 50, was significantly higher for temperature reduction than for lower lip retraction or tail flick responses (ED 50 i.v./ED 50 i.c.v. values are 9.8, 2.9, and 0.47, respectively). These data provide evidence that distinct sites of action are involved in these models. Temperature reduction is mediated mainly by postsynaptic receptors in the close vicinity of the lateral ventricle. Receptors that mediate lower lip retraction are located more distantly in the brain, supporting previous evidence that they are somatodendritic autoreceptors, and receptors in the spinal cord are probably responsible for tail flick responses. © 1997 Elsevier Science B.V. All rights reserved.

Supraspinal δ 1-opioid receptor-mediated antinociceptive properties of (−)-TAN-67 in diabetic mice

The antinociceptive potencies of the enantiomorphs of TAN-67 (2-methyl-4 aα-(3-hydroxyphenyl)-1,2,3,4,4 a,5,12,12 aα-octahydro-quinolino[2,3,3,– g]isoquinoline), (−)-TAN-67 and (+)-TAN-67, given intracerebroventricularly (i.c.v.) on the antinociceptive response were studied in streptozotocin-induced diabetic mice using the tail-flick test. (−)-TAN-67 at doses of 3–10 μg given i.c.v. produced dose-dependent inhibition of the tail-flick response in both non-diabetic and diabetic mice. The antinociceptive effect of (−)-TAN-67 in the tail-flick test in diabetic mice was greater than that in non-diabetic mice. The antinociceptive effect of (−)-TAN-67 was not antagonized by pretreatment with either β-funaltrexamine, a selective μ-opioid receptor antagonist, or nor-binaltorphimine, a selective κ-opioid receptor antagonist. When 7-benzylidenenaltrexone, a selective δ 1-opioid receptor antagonist, was administered 10 min before treatment with (−)-TAN-67, the antinociceptive effect of (−)-TAN-67 was significantly antagonized. However, naltriben, a selective δ 2-opioid receptor antagonist, had no significant effect on the antinociceptive effect of (−)-TAN-67. On the other hand, in the tail-flick test, (+)-TAN-67 at doses of 3–30 μg given i.c.v. did not produce dose-dependent inhibition of the tail-flick response in either non-diabetic or diabetic mice. In conclusion, (−)-TAN-67, but not its enantiomer (+)-TAN-67, produced an antinociceptive effect through the activation of δ 1-opioid receptors. © 1997 Elsevier Science B.V. All rights reserved.

Inhibition of spinal nitric oxide synthase by Nω-nitro- l-arginine blocks the release of met-enkephalin and antinociception induced by supraspinally administered β-endorphin in the rat

The antinociception induced by β-endorphin given supraspinally has been demonstrated previously to be mediated by the release of Met-enkephalin acting on δ 2-opioid receptors in the spinal cord. The present study was designed to determine the role of nitric oxide in the spinal cord on β-endorphin-induced release of Met-enkephalin and antinociception. The experiments were performed in pentobarbital-anesthetized rats. The release of Met-enkephalin was performed using a spinal cord perfusion technique and the Met-enkephalin released in the spinal perfusates was measured by radioimmunoassay. Antinociception was assessed by the tail-flick test. β-Endorphin (2 μg) given intraventricularly induced the release of Met-enkephalin from the spinal cord. The release of Met-enkephalin was dose-dependently attenuated by N ω -nitro- l-arginine (0.1 nM–1 μM) added into spinal perfusates and the attenuation was reversed by intrathecally applied l-arginine. The stereoisomer N ω -nitro- d-arginine given intrathecally, however, did not inhibit the release of Met-enkephalin induced by intraventricularly administered β-endorphin. β-Endorphin (4 μg) given intraventricularly produced antinociception in rats pretreated intrathecally with saline. The antinociception induced by β-endorphin was blocked by intrathecally administered N ω -nitro- l-arginine (5 μg) and the blockade of antinociception was reversed by intrathecal injection of l-arginine (50 μg). N ω -Nitro- d-arginine (5 μg) given intrathecally did not block the intraventricularly administered β-endorphin-induced antinociception. N ω -Nitro- l-arginine (10 μg) given intraventricularly did not affect intraventricularly administered β-endorphin-induced Met-enkephalin release nor did it affect intraventricular β-endorphin-induced antinociception, indicating that the effect of N ω -nitro- l-arginine is not at supraspinal sites. Intrathecal pretreatment with N ω -nitro- l-arginine did not affect intrathecally administered [ d-Ala 2]deltorphin II-induced antinociception. Our results indicate that N ω -nitro- l-arginine given intrathecally attenuates intraventricular β-endorphin-administered inhibition of the tail-flick response by presynaptically inhibiting the release of Met-enkephalin.

Hyperalgesic effect induced by barbiturates, midazolam and ethanol: pharmacological evidence for GABA-A receptor involvement.

The involvement of GABA-A receptors in the control of nociception was studied using the tail-flick test in rats. Non-hypnotic doses of the barbiturates phenobarbital (5-50 mg/kg), pentobarbital (17-33 mg/kg), and thiopental (7.5-30 mg/kg), of the benzodiazepine midazolam (10 mg/kg) or of ethanol (0.4-1.6 g/kg) administered by the systemic route reduced the latency for the tail-flick response, thus inducing a 'hyperalgesic' state in the animals. In contrast, non-convulsant doses of the GABA-A antagonist picrotoxin (0.12-1.0 mg/kg) administered systemically induced an increase in the latency for the tail-flick response, therefore characterizing an 'antinociceptive' state. Previous picrotoxin (0.12 mg/kg) treatment abolished the hyperalgesic state induced by effective doses of the barbiturates, midazolam or ethanol. Since phenobarbital, midazolam and ethanol reproduced the described hyperalgesic effect of GABA-A-specific agonists (muscimol, THIP), which is specifically antagonized by the GABA-A antagonist picrotoxin, our results suggest that GABA-A receptors are tonically involved in the modulation of nociception in the rat central nervous system.

Analgesic and Anti-inflammatory Activities of the Aqueous Extract of Plantago major L.

This study aimed to validate the pharmacological effects of Plantago major L., a medicinal plant used to decrease pain and inflammation. The aqueous extract (AE) was prepared from the dried ground leaves. Pretreatment with AE (1 g/kg, p.o.) decreased writhing induced by acetic acid in mice, but it did not change the tail flick responses to heat nociceptive stimuli. In rats, AE (1 g/kg, p.o.) reduced the paw edema and pleurisy induced by carrageenin, but it did not alter the paw edema induced by dextran. The effect of AE on the carrageenin inflammatory responses was more intense than that obtained on the ear edema induced by croton oil in mice. In addition, daily treatment with AE (1 g/kg/day during 8 days, p.o.) inhibited the exudative process induced by croton oil injected into the air pouch of rats. The results indicate that the aqueous extract of Plantago major is endowed with effective antiinflammatory and analgesic activities.

Spinal gabapentin is antinociceptive in the rat formalin test

Gabapentin is a novel anticonvulsant that may be of value for the relief of clinical pain. To determine whether gabapentin is antinociceptive after spinal administration, the drug was given via an intrathecal catheter in doses from 6 to 200 μg/rat 10 min prior to intraplantar formalin. Five percent formalin injected subcutaneously in the right hind paw produced a biphasic reaction consisting of flinching and licking behaviors (phase 1, 0–10 min; phase 2, 10–60 min). Gabapentin dose-dependently reduced the numbers of flinches and the duration of licking during phase 2 of the formalin test. The highest dose of gabapentin (200 μg/rat) did not affect the tail-flick response. These results demonstrate that spinal gabapentin is antinociceptive in the formalin test.