Apraclonidine attenuates the increases in spinal excitatory amino acid release in rats with adjuvant-induced inflammation

The release of excitatory amino acids (EAAs), nitric oxide, and prostaglandins plays a critical role in the development of peripheral tactile and thermal hypersensitivity after the induction of knee joint inflammation. In this study, we used a model of chronic spinal microdialysis to examine the effect of complete Freund's adjuvant (CFA)-induced inflammation on the spinal release of EAAs and also assessed the antinociceptive effect of a new alpha(2)-adrenergic agonist, apraclonidine, by using this model. Male Sprague-Dawley rats were implanted with microdialysis catheters. CFA was injected into the plantar surface of the left hindpaw to induce inflammation. Concentrations of amino acids in dialysate and thermal and tactile withdrawal latency were evaluated for 1 wk. Intraplantar injection of CFA evoked a significant release of glutamate, aspartate, and citrulline for 6 days. Three milligrams of intraperitoneal apraclonidine significantly suppressed the release of EAAs and citrulline. Apraclonidine was given intraperitoneally 2--3 days after CFA injection. Prominent thermal and tactile allodynia was observed for 6 days. Our results show that the significant modulatory effect of the alpha(2)-adrenergic agonist apraclonidine on the release of EAAs may account for its antinociceptive properties in adjuvant-induced inflammation. IMPLICATIONS: This study showed a novel finding that the hypersensitivity state seems to be dependent on increased release of spinal excitatory amino acids (EAAs), and the significant modulatory effect of the alpha(2)-adrenergic agonist apraclonidine on the release of spinal EAAs accounts for its analgesic properties in adjuvant-induced inflammation.     

Comparison of the visceral antinociceptive effects of spinally administered MPV-2426 (fadolmidine) and clonidine in the rat

BACKGROUND: The authors determined the visceral antinociceptive effect induced by MPV-2426 (fadolmidine), a selective alpha 2 -adrenoceptor agonist, in rats with and without inflammation of the colon. They also determined whether the sympathetic nervous system or intact descending pathways are critical for the alpha 2 -adrenoceptor-induced visceral antinociception. METHODS: Spinal neuronal responses evoked by colorectal distension were recorded in pentobarbitone-anesthetized rats. MPV-2426 was administered onto the spinal cord. Clonidine was used as a reference alpha 2 -adrenoceptor agonist. Inflammation of the colon was induced by turpentine. Sympathectomy was induced by 6-hydroxydopamine. A midthoracic transection of the spinal cord was performed to study the role of descending pathways. RESULTS: Spinal administration of MPV-2426 produced a dose-dependent attenuation of responses evoked by colorectal distension, and this effect was of the same percentual magnitude in inflamed as in noninflamed animals. Clonidine and MPV-2426 induced equipotent visceral antinociception. The effect by spinally administered MPV-2426 was enhanced by a chemical sympathectomy but not influenced by spinal transection. CONCLUSIONS: Spinally administered MPV-2426 produces a dose-dependent visceral antinociception as well in animals with an inflammation of the colon as in controls. The visceral antinociceptive effect induced by spinal MPV-2426 is equipotent to that of spinal clonidine. An intact sympathetic nervous system or intact brainstem-spinal pathway is not critical for the MPV-2426-induced visceral antinociception.     

Experimental arthritis in the rat does not alter the analgesic potency of intrathecal or intraarticular morphine.

To explore further the role of inflammatory processing on peripheral opioid pharmacology, we examined whether the potency of intraarticular (i.a.) or intrathecal (i.t) morphine in tests of thermal and mechanical nociception changed during the induction of experimental arthritis in the rat. Thermal nociception by i.t. morphine (3, 10, and 50 micrograms) or i.a. morphine (100, 1000, and 3000 micrograms) was assessed by means of a modified Hargreaves box ever) 28 h. Mechanical antinociception was determined for the largest applied doses of morphine using von Frey hairs. Morphine produced dose-dependent thermal antinociception after i.t. or i.a. administration: a 50% increase in maximum antinociceptive thermal response (50% effective dose) was produced by i.t. doses of 9.7 micrograms at the start and 9.1 micrograms at the end of this 28-h observational interval, whereas after i.a. administration, 50% effective dose values were 553 micrograms at the start and 660 micrograms at the end. The largest applied dose of either i.t. or i.a. morphine produced mechanical antinociception. On Day 1, the antinociceptive effect for mechanical nociception (expressed as the area under the curve of the percentage of maximal possible effect values at 0.5, 1, 2, and 4 h) was 68% for i.t. morphine 50 micrograms and 53% for i.a. morphine 3000 micrograms. Neither result differed from the corresponding area under the curve values on Day 2. Naloxone administered either i.t. or i.a. abolished the antinociceptive action of morphine given at the same site. We conclude that, although morphine has a peripheral analgesic site of action in a rat arthritis model, its potency for both i.a. and i.t. routes of administration does not change during the onset of arthritis. Implications: In this animal study, we showed that the administration of morphine modulates thermal and mechanical antinociception at central and peripheral sites in inflammatory pain.     

Antinociceptive interactions between alpha 2-adrenergic and opiate agonists at the spinal level in rodents

This study was undertaken to evaluate the antinociceptive interactions of alpha 2 adrenergic and opiate receptors at the spinal level. Morphine and clonidine were administered intrathecally (i.t.) by lumbar puncture to rats either alone or in the presence of either i.t. yohimbine, an alpha 2 antagonist, or systemic naloxone, an opioid antagonist. The effect of tolerance to systematically administered morphine on responses to i.t. morphine and clonidine was examined in mice. Antinociception was determined by observing the response to a clamp applied to the tail (Haffner test) in mice and by the tail-flick test in rats; log dose-response curves for antinociception were generated for morphine, clonidine, and each drug combination. Morphine and clonidine both produced dose-dependent antinociception when given i.t. in both species. The i.t. administration of yohimbine attenuated the antinociceptive effect of both clonidine and morphine, but naloxone attenuated only the response to morphine. Further, a sub-analgetic dose of i.t. clonidine potentiated the effect of i.t. morphine. In morphine-tolerant mice, i.t. morphine was not efficacious whereas clonidine retained full efficacy, although potency was slightly diminished. Thus, it appears that alpha 2 adrenoceptor-mediated antinociception is independent of opiate receptor mechanisms. Clinical use of intrathecal combinations of alpha 2 adrenergic and opiate receptor agonists to increase analgesia and use of intrathecal alpha 2 agonists for pain relief in patients tolerant to opiates might deserve evaluation.     

Antinociceptive and hemodynamic effects of a novel alpha2-adrenergic agonist, MPV-2426, in sheep.

BACKGROUND: alpha2-Adrenergic agonists produce analgesia primarily by a spinal action and hypotension and bradycardia by actions at several sites. Clonidine is approved for epidural use in the treatment of neuropathic pain, but its wider application is limited by hemodynamic side effects. This study determined the antinociceptive and hemodynamic effects of a novel alpha2-adrenergic agonist, MPV-2426, in sheep. METHODS: Forty sheep of mixed Western breeds with indwelling catheters were studied. In separate studies, antinociception to a mechanical stimulus, hemodynamic effects, arterial blood gas tensions, cerebrospinal fluid pharmacokinetics, and spinal cord blood flow was determined after epidural, intrathecal, and intravenous injection of MPV-2426. RESULTS: MPV-2426 produced antinociception with greater potency intrathecally (ED50 = 49 microg) than epidurally (ED50 = 202 microg), whereas intravenous administration had no effect. Intrathecal injection, in doses up to three times the ED95, failed to decrease systemic or central arterial blood pressures or heart rate, whereas larger doses, regardless of route, increased systemic arterial pressure. Bioavailability in cerebrospinal fluid was 7% after epidural administration and 0.17% after intravenous administration. Intrathecal MPV-2426, in an ED95 dose and three times this dose, produced a dose-independent reduction in thoracic and lumbar spinal cord blood flow. CONCLUSIONS: MPV-2426 shares many characteristics of other alpha2-adrenergic agonists examined in sheep, but differs from clonidine and dexmedetomidine by lack of antinociception and minimal reduction in oxygen partial pressure after large intravenous and epidural injections. No hemodynamic depression was observed after intrathecal injection at antinociceptive doses. These results suggest this compound may be an effective spinal analgesic in humans with less hypotension than clonidine, although its relative potency to cause sedation was not tested in this study.     

Antinociceptive Interaction of Intrathecal alpha sub 2 -adrenergic Agonists, Tizanidine and Clonidine, with Lidocaine in Rats

Background: The intrathecal alpha2 -adrenergic agonist, clonidine, has been shown to have considerable antinociceptive effect, although clonidine causes hypotension and bradycardia The combination of intrathecal clonidine and local anesthetics enhances analgesic effects, whereas the combination may cause marked hypotension and motor blockade, which may limit the clinical application of the combination. Tizanidine, another alpha2 -adrenergic agonist, has also provided antinociception without producing pronounced hemodynamic changes. This study was designed to evaluate the antinociceptive and hemodynamic interactions of tizanidine and clonidine with lidocaine.

Methods: Male Sprague Dawley rats were chronically implanted with lumbar intrathecal catheters. The tail-flick test was used to assess the thermal nociceptive threshold. The ability of intrathecal tizanidine, clonidine, lidocaine, or the combinations of alpha2 -adrenergic agonist and lidocaine to alter the tail-flick latency was examined. To characterize the antinociceptive interaction, the isobolographic analysis was applied. Additionally, the motor function, blood pressure and heart rate after intrathecal administration of drugs and combinations were also monitored.

Results: Intrathecal tizanidine, clonidine, or the combinations increased the tail-flick latency in dose- and time-dependent fashion without affecting motor function. The order potencies (dose producing a 50% of peak effect, in micro gram) of tizanidine and clonidine were 1.8 and 0.75, respectively. With isobolographic analysis, tizanidine with lidocaine and clonidine with lidocaine showed significantly synergistic antinociceptive interaction. Potency ratio analysis and fractional analysis also confirmed the synergistic interaction. At the doses in the combinations showing comparable antinociception, tizanidine with lidocaine, unlike clonidine with lidocaine, did not affect motor function or blood pressure.     

Peripheral nerve injury alters the alpha2 adrenoceptor subtype activated by clonidine for analgesia

BACKGROUND: Previous studies suggest that the alpha adrenoceptor subtype is the target for spinally administered alpha -adrenergic agonists, clonidine, for pain relief. However, ST 91, a preferential alpha adrenoceptor subtype agonist, induces antinociception, and intrathecally administered alpha antisense oligodeoxynucleotide decreases antinociception induced by clonidine in the rat, suggesting non-A sites may be important as well. Therefore, the authors examined the subtype of alpha adrenoceptor activated by clonidine and ST 91 in normal rats and those with nerve injury-induced hypersensitivity. METHODS: The same mechanical stimulus was applied to normal rats and those following spinal nerve ligation, and the effect of intrathecal clonidine and ST 91 on withdrawal threshold to the stimulus was determined. To further examine subtypes, animals were spinally pretreated with vehicle, BRL 44408 (an alpha subtype-preferring antagonist), and ARC 239 (an alpha subtype-preferring antagonist). RESULTS: In normal animals, clonidine's effect was diminished by pretreatment with either antagonist, whereas ST 91's antinociceptive effect was solely blocked by pretreatment with ARC 239. In nerve-injured animals, the antihypersensitivity action of both clonidine and ST 91 was blocked by administration of ARC 239, whereas BRL 44408 was ineffective. CONCLUSIONS: These data agree with previous studies supporting that the alpha adrenoceptor is important to the antinociceptive effect of clonidine in normal animals. Nerve injury alters this and results in a total reliance on alpha adrenoceptors.     

Dextromethorphan potentiates morphine antinociception at the spinal level in rats

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

Peripheral Nerve Injury Alters the α2 Adrenoceptor Subtype Activated by Clonidine for Analgesia

Background: Previous studies suggest that the [alpha]2A adrenoceptor subtype is the target for spinally administered [alpha]2-adrenergic agonists, i.e., clonidine, for pain relief. However, ST 91, a preferential [alpha]2 NON-A adrenoceptor subtype agonist, induces antinociception, and intrathecally administered [alpha]2C antisense oligodeoxynucleotide decreases antinociception induced by clonidine in the rat, suggesting non-A sites may be important as well. Therefore, the authors examined the subtype of [alpha]2 adrenoceptor activated by clonidine and ST 91 in normal rats and those with nerve injury-induced hypersensitivity.

Methods: The same mechanical stimulus was applied to normal rats and those following spinal nerve ligation, and the effect of intrathecal clonidine and ST 91 on withdrawal threshold to the stimulus was determined. To further examine subtypes, animals were spinally pretreated with vehicle, BRL 44408 (an [alpha]2A subtype-preferring antagonist), and ARC 239 (an [alpha]2 NON-A subtype-preferring antagonist).

Results: In normal animals, clonidine's effect was diminished by pretreatment with either antagonist, whereas ST 91's antinociceptive effect was solely blocked by pretreatment with ARC 239. In nerve-injured animals, the antihypersensitivity action of both clonidine and ST 91 was blocked by administration of ARC 239, whereas BRL 44408 was ineffective.     

Epidural clonidine produces antinociception, but not hypotension, in sheep

Intrathecally administered clonidine produces analgesia, but also produces hypotension. To assess the effects of epidural administration, the authors inserted lumbar epidural catheters in seven nonpregnant ewes, and injected, on separate days, clonidine (50-750 mcg), morphine (5-10 mg), and a clonidine-morphine combination (clonidine 150 mcg + morphine 5 mg). Clonidine produced dose-dependent antinociception and sedation, with the lowest maximally effective antinociceptive dose being 300 mcg. Morphine produced less intense antinociception than clonidine, and did not potentiate clonidine's effect. Antinociception, but not sedation, following clonidine injection was reversed by epidural injection of the alpha 2-adrenergic antagonist, idazoxan. Epidurally administered naloxone and prazosin did not reverse clonidine's antinociceptive effect, nor did intravenously administered idazoxan. Epidurally administered clonidine did not decrease blood pressure or heart rate or affect arterial blood gas tensions or spinal cord histology. These data suggest that epidurally administered clonidine produces analgesia by a local, alpha 2-adrenergic mechanism. In sheep, epidurally administered clonidine does not produce hypotension.     

Intrathecal ketorolac enhances antinociception from clonidine

Although both alpha2-adrenergic agonists and cyclooxygenase inhibitors produce analgesia, their exact sites of action and interaction remain unclear. A previous report demonstrated a surprising inhibition of antinociception in rats from intrathecal clonidine by co-administered ketorolac. There are no other reports of interaction between these two classes of analgesics. We therefore reexamined this interaction, determining the effect of intrathecal clonidine and ketorolac alone and in combination in normal rats. Clonidine, but not ketorolac, produced antinociception to noxious hind paw thermal stimulation. The addition of ketorolac significantly enhanced the effect of clonidine, indicating a synergistic interaction for analgesia. Although the reasons for the discrepancy between this and the previous report are unclear, these results are consistent with previous studies that indicate an antinociceptive action of intrathecal alpha2-adrenergic agonists in the normal condition, a lack of such effect for cyclooxygenase inhibitors, and positive reinforcing effects of these two systems when co-stimulated. IMPLICATIONS: Spinal injection of the alpha2-adrenergic agonist clonidine and the cyclooxygenase inhibitor ketorolac results in a synergistic interaction for antinociception in normal animals, suggesting that the combination of these drugs will enhance rather than detract from the analgesia of either alone.     

Production of paradoxical sensory hypersensitivity by alpha 2-adrenoreceptor agonists

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

Production of Paradoxical Sensory Hypersensitivity by α2-Adrenoreceptor Agonists

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

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

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

Comparison of the Visceral Antinociceptive Effects of Spinally Administered MPV-2426 (Fadolmidine) and Clonidine in the Rat

Background: The authors determined the visceral antinociceptive effect induced by MPV-2426 (fadolmidine), a selective [alpha]2-adrenoceptor agonist, in rats with and without inflammation of the colon. They also determined whether the sympathetic nervous system or intact descending pathways are critical for the [alpha]2-adrenoceptor-induced visceral antinociception.

Methods: Spinal neuronal responses evoked by colorectal distension were recorded in pentobarbitone-anesthetized rats. MPV-2426 was administered onto the spinal cord. Clonidine was used as a reference [alpha]2-adrenoceptor agonist. Inflammation of the colon was induced by turpentine. Sympathectomy was induced by 6-hydroxydopamine. A midthoracic transection of the spinal cord was performed to study the role of descending pathways.

Results: Spinal administration of MPV-2426 produced a dose-dependent attenuation of responses evoked by colorectal distension, and this effect was of the same percentual magnitude in inflamed as in noninflamed animals. Clonidine and MPV-2426 induced equipotent visceral antinociception. The effect by spinally administered MPV-2426 was enhanced by a chemical sympathectomy but not influenced by spinal transection.     

Pharmacological evidence for different alpha 2-adrenergic receptor sites mediating analgesia and sedation in the rat

Alpha2-adrenergic agonists given intrathecally result in antinociception and intracerebroventricularly (ICV) in sedation. To examine whether different alpha2-adrenergic receptor subtypes differentially mediate antinociception and sedation, we measured the relative potency of three alpha2-adrenergic agonists, dexmedetomidine (DMET), clonidine (CLON) and UK-14.304 (UK), after spinal and ICV administration. Each agonist was given either alone or in the presence of systemically administered yohimbine, which acts as a competitive alpha2-antagonist in unanaesthetized rats. Intrathecal delivery of the agonists alone resulted in a dose-dependent antinociceptive effect (ED50 (nmol): DMET = 1.2, UK = 1.7, CLON = 5.6) with little sedative effect at the lower doses. Yohimbine pretreatment resulted in a rightward shift of the dose-response curves (DMET > CLON > UK). ICV alpha2-adrenergic agonists produced a dose-dependent sedation (ED50 (nmol): DMET = 10.5; UK = 28.7; CLON = 126), with little antinociceptive action. Again, yohimbine pretreatment produced a right shift of the ICV sedation dose-response curves (UK > DMET > CLON). Thus, we conclude that the spinal analgesic effects of DMET, CLON and UK appear to be mediated by two sites. After ICV delivery, DMET, CLON and UK appear to act at a common supra-spinal site to produce sedation and this site resembles that acted upon by UK in the spinal cord.      

Pharmacologic interaction between cannabinoid and either clonidine or neostigmine in the rat formalin test

BACKGROUND: Although spinal cannabinoid receptor agonist (WIN 55,212-2) has been shown to encounter various models of pain, the role of two subtypes of cannabinoid receptor for the antinociceptive effect of cannabinoids has not been investigated at the spinal level. Spinal alpha 2 receptor agonist (clonidine) and cholinesterase inhibitor (neostigmine) are also active in the modulation of nociception. The authors examined the properties of drug interaction after coadministration of WIN 55,212-2-clonidine, and intrathecal WIN 55,212-2-neostigmine, and further clarified the role of cannabinoid 1 and 2 receptors in cannabinoid-induced antinociception at the spinal level. METHODS: Catheters were inserted into the intrathecal space of male Sprague-Dawley rats, and 50 microl of 5% formalin solution was injected into the hind paw to evoke the pain. Isobolographic analysis was used for evaluation of pharmacologic interaction. RESULTS: Intrathecal 55,212-2, clonidine, and neostigmine dose-dependently suppressed the flinching observed during phase 1 and 2 in the formalin test. Isobolographic analysis revealed a synergistic interaction after intrathecal delivery of WIN 55,212-2-clonidine or WIN 55,212-2-neostigmine mixture in both phases. The antinociceptive effect of WIN 55,212-2 was antagonized by cannabinoid 1 receptor antagonist (AM 251) but not by cannabinoid 2 receptor antagonist (AM 630). No antinociceptive effect was seen after intrathecal administration of cannabinoid 2 receptor agonist (JWH 133). CONCLUSIONS: Intrathecal 55,212-2, clonidine, and neostigmine attenuate the facilitated state and acute pain. WIN 55,212-2 interacts synergistically with either clonidine or neostigmine. The antinociception of WIN 55,212-2 is mediated through the cannabinoid 1 receptor, but not the cannabinoid 2 receptor, at the spinal level.     

The contribution of alpha-1 and alpha-2 adrenoceptors in peripheral imidazoline and adrenoceptor agonist-induced nociception

We evaluated the effects of activation of peripheral adrenoceptors (AR) and imidazoline receptors on nociception and the contribution of alpha-1 and alpha-2 AR receptors in agonist-induced nociception by using the tail-flick test in mice. Clonidine (alpha-2 AR agonist), agmatine (imidazoline receptor and alpha-2 AR agonist), noradrenaline (mixed alpha-1 and alpha-2 AR agonist), phenylephrine (alpha-1 AR agonist), or 0.9% saline was given by intradermal injection (10 microL) into the tail. The intradermal injection of clonidine (1, 3, and 10 microg) and agmatine (3, 30, and 50 microg) produced dose-dependent antinociception, whereas noradrenaline (1, 10, and 30 microg) and phenylephrine (1, 10 and 30 microg) produced dose-dependent thermal hyperalgesia. Clonidine (10 microg) and agmatine (50 microg)-induced peripheral antinociception were antagonized by pretreatment with yohimbine (2.5 mg/kg IP), a selective alpha-2 AR antagonist, but not by prazosin (1 mg/kg IP), a selective alpha-1 AR antagonist. Noradrenaline (30 microg) and phenylephrine (30 mug)-induced thermal hyperalgesia were antagonized by prazosin (1 mg/kg IP) but not by yohimbine (2.5 mg/kg IP). Our results suggest that local thermal hyperalgesic effects of noradrenaline and phenylephrine are linked to alpha-1 AR and the peripheral antinociceptive action of clonidine and agmatine are linked to alpha-2 AR.     

Intra-articular injection of ketorolac in the rat knee joint: effect on articular cartilage and synovium

We have investigated the effects of intra-articular (i.a.) administration of ketorolac in the rat knee joint. Thirty Sprague- Dawley rats were given 0.25 ml of a standard preparation of ketorolac trometamol (10 mg ml-1) by injection into the right knee joint and 0.25 ml of 0.9% physiological saline solution by injection into the left knee as a control. Ten rats were killed at 24 h, 10 at 48 h and 10 at 5 days after injection. The joints were prepared and sectioned for histological examination. There was significantly more inflammation in those knees that had received i.a. ketorolac at all times of examination, with the most severe changes occurring 5 days after injection. A further group of 10 rats were given 0.25 ml of 10% w/v ethanol in physiological saline (similar to the vehicle for parenteral ketorolac) injected into the knee joint, with a 0.9% saline control injected in the other knee. These rats were then killed at 5 days (as this was the time interval after which we found the maximum inflammatory response in the earlier phase of our study). The joints were prepared and examined histologically. We feel that the absence of inflammatory changes in these joints make it unlikely that ethanol was responsible for the inflammation produced by ketorolac injection.      

Spinal Adrenergic and Cholinergic Receptor Interactions Activated by Clonidine in Postincisional Pain

Background: Previous pharmacologic and molecular studies suggest that the [alpha]2-adrenoceptor subtype A is the target for spinally administered [alpha]2-adrenergic agonists, i.e., clonidine, for pain relief. However, intrathecally administered [alpha]2 C antisense oligodeoxynucleotide was recently reported to decrease antinociception induced by clonidine in the rat, suggesting non-A sites may be important as well. The current study sought to determine the subtype of [alpha]2 adrenoceptors activated by clonidine in a rodent model for human postoperative pain, and to examine its interaction with spinal cholinergic receptors.

Methods: Postoperative hypersensitivity was produced in rats by plantar incision of the hind paw and punctuate mechanical stimuli were applied around the wound 24 h after surgery. Effects of intrathecal clonidine and 2-(2,6-diethylphenylamino)-2-imidazoline (ST91) on withdrawal thresholds to the stimulus were determined. To examine the adrenoceptor subtype and its interaction with spinal cholinergic receptors, animals were intrathecally pretreated with vehicles BRL44408 (an [alpha]2 A subtype-preferring antagonist), ARC239 (an [alpha]2 non-A subtype-preferring antagonist), atropine (a muscarinic antagonist), and mecamylamine (a nicotinic antagonist).

Results: Intrathecal ST91 showed a significantly greater efficacy when compared with clonidine. The analgesic effect of clonidine was diminished by pretreatment with either adrenoceptor antagonist, whereas the effect of ST91 was solely blocked by ARC239 pretreatment. Atropine and mecamylamine abolished the effect of clonidine effect but not the effect of ST91.     

Antinociceptive effect of riluzole in rats with neuropathic spinal cord injury pain

Symptoms of neuropathic spinal cord injury (SCI) pain include cutaneous hypersensitivity and spontaneous pain below the level of the injury. Riluzole, an FDA-approved drug for the treatment of amyotrophic lateral sclerosis, has been demonstrated to attenuate neural excitotoxicity by blocking the effects of the excitatory amino acid glutamate on glutamate receptors and by inhibiting voltage-gated Na(+) and Ca(2+) channels. Neuropathic pain in rat models of SCI is thought to be mediated by dysfunctional ion channels and glutamate receptors expressed on CNS neurons. Thus riluzole's mechanism of action could be relevant in treating neuropathic SCI pain. The current study evaluated the antinociceptive potential of riluzole in rats following a SCI. Four weeks after a brief compressive injury to the mid-thoracic spinal cord, rats displayed significantly decreased hind paw withdrawal thresholds, suggestive of below-level cutaneous hypersensitivity. A single systemic dose of riluzole (8?mg/kg) injected intraperitoneally (i.p.) reversed cutaneous hypersensitivity in SCI rats. To identify riluzole's CNS site of action, riluzole was injected intrathecally (i.t.) and intracerebroventricularly (i.c.v.) in SCI rats. Significant antinociceptive effects were obtained following i.c.v., but not i.t., injection. Systemic riluzole was also antinociceptive in uninjured rats, increasing the latency to respond to an acute noxious thermal stimulus in the tail flick test. Unlike in SCI rats, however, riluzole was not effective when administered directly into the CNS, indicating a peripherally mediated antinociceptive mechanism. Although riluzole appears to have a general antinociceptive effect, the site of action may be model dependent. In total, these data indicate that riluzole may be an effective clinical analgesic for the treatment of below-level neuropathic SCI pain. Although the exact mechanism of action is not clear, there is a predominant supraspinal component of riluzole-induced antinociception in SCI rats.