Antinociceptive interaction between spinal clonidine and lidocaine in the rat formalin test: an isobolographic analysis

Potent suppression of stretch reflex activity after systemic or spinal delivery of tizanidine in rats with spinal ischemia-induced chronic spastic paraplegia

BACKGROUND

Spasticity and rigidity are serious complications associated with spinal traumatic or ischemic injury. Clinical studies show that tizanidine (Tiz) is an effective antispasticity agent; however, the mechanism of this effect is still not clear. Tiz binds not only to α2-adrenoreceptors (AR) but also to imidazoline (I) receptors. Both receptor systems (AR+I) are present in the spinal cord interneurons and α-motoneurons. The aim of the present study was to evaluate the therapeutic potency of systematically or spinally (intrathecally [IT]) delivered Tiz on stretch reflex activity (SRA) in animals with ischemic spasticity, and to delineate supraspinal or spinal sites of Tiz action.

EXPERIMENTAL PROCEDURES

Animals were exposed to 10 min of spinal ischemia to induce an increase in SRA. Increase in SRA was identified by simultaneous increase in recorded electromyography (EMG) activity and ankle resistance measured during computer-controlled ankle dorsiflexion (40°/3 s) in fully awake animals. Animals with increased SRA were divided into several experimental subgroups and treated as follows: (i) Tiz administered systemically at the dose of 1 mg kg(-1), or IT at 10 μg or 50 μg delivered as a single dose; (ii) treatment with systemic Tiz was followed by the systemic injection of vehicle, or by nonselective AR antagonist without affinity for I receptors; yohimbine (Yoh), α2A AR antagonist; BRL44408 (BRL), α2B AR antagonist; ARC239 (ARC), nonselective AR and I(1) receptor antagonist; efaroxan (Efa), or nonselective AR and I(2) receptor antagonist; idazoxan (Ida); (iii) treatment with IT Tiz was followed by the IT injection of selective α2A AR antagonist; atipamezole (Ati). In a separate group of spastic animals the effect of systemic Tiz treatment (1 mg/kg) or isoflurane anesthesia on H-reflex activity was also studied.

RESULTS

Systemic and/or IT treatment with Tiz significantly suppressed SRA. This Tiz-mediated anti-SRA effect was reversed by BRL (5 mg kg(-1)), Efa (1 mg kg(-1)), and Ida (1 mg kg(-1)). No reversal was seen after Yoh (3 mg kg(-1)) or ARC (5 mg kg(-1)) treatment. Anti-SRA induced by IT Tiz (50 μg) was reversed by IT injection of Ati (50 μg). Significant suppression of H-reflex was measured after systemic Tiz treatment (1 mg/kg) or isoflurane (2%) anesthesia, respectively. Immunofluorescence staining of spinal cord sections taken from animals with spasticity showed upregulation of α2A receptor in activated astrocytes.

CONCLUSIONS

These data suggest that α2A AR and I receptors, but not α2B AR, primarily mediate the Tiz-induced antispasticity effect. This effect involves spinal and potentially supraspinal sites and likely targets α2A receptor present on spinal neurons, primary afferents, and activated astrocytes. Further studies using highly selective antagonists are needed to elucidate the involvement of specific subtypes of the AR and I receptors in the antispasticity effect seen after Tiz treatment.

Clonidine--a potent analgesic adjuvant

PURPOSE OF REVIEW

The alpha(2)-adrenoceptor agonist clonidine is one of the most widely investigated substances in anaesthesia and pain therapy. Recently, numerous experimental and clinical studies have allowed a better understanding of its underlying mechanisms of action and interactions with other analgesic drugs.

RECENT FINDINGS

Several experimental studies have shown that clonidine may improve the analgesic effect of anti-inflammatory agents and also have significant peripheral antinociceptive effects. Volunteer and clinical studies have demonstrated analgesic effects of clonidine after systemic administration, whereas local administration of plain clonidine (e.g. intra-articular, intravenous regional) showed only limited clinical efficacy. The major clinical place of clonidine may thus be as an adjuvant to other analgesics, as shown in a number of studies in which clonidine has been investigated in combination with local anaesthetics, opioids and ketamine. An increasing number of studies have now investigated clonidine in paediatric patients and have demonstrated that the effects are generally similar to those in adults. Furthermore, it seems that in paediatric patients the side-effects of clonidine are predictable and of limited clinical importance.

SUMMARY

During the past decade clonidine has been investigated as an adjuvant for general and regional anaesthesia and in the postoperative period. There is no doubt that clonidine improves analgesia after systemic, spinal or peripheral opioids, and prolongs the analgesic action of most local anaesthetics. The side-effects of usual doses of clonidine are predictable. Given the clinical experience of an increasing number of hospitals, clonidine should no longer be considered an experimental drug, but a useful addendum to the pharmacological armamentarium.

Intrathecal clonidine decreases spinal nitric oxide release in a rat model of complete Freund's adjuvant induced inflammatory pain

A long-lasting antihyperalgesic effect has been demonstrated for intrathecal (IT) clonidine, an alpha2-adrenergic agonist. In the present study, the mechanism and antihyperalgesic effects of IT clonidine were examined post-treatment in a rat model of Complete Freund's Adjuvant (CFA)-induced inflammatory hyperalgesia. Using a chronic model of spinal cord dialysis, we examined the effect of the adjuvant-induced inflammation on spinal release of nitric oxide (NO) and the development of chronic pain and assessed the antinociceptive effects and mechanisms of the alpha2-adrenergic agonist, clonidine (IT). Chronic, persistent inflammatory pain was induced by left hind paw injection of 0.3 ml CFA prepared in a mixture with Mycobacterium butyricum. Rats were randomly assigned to groups receiving IT clonidine in discrete doses of 1, 10 or 50 microg, 3 or 24 hr post-inflammation. Measurement of total NOx (NO + NO2- + NO3-) was used to determine NO release into the cerebrospinal fluid. Rat thermal antinociception was assessed using a radiant heat thermal hyperalgesia model. CFA injection resulted in significant thermal hyperalgesia throughout the four days of observation. A dose-dependent suppression of thermal hyperalgesia and spinal NO release was observed after IT clonidine treatment. Evidence from this CFA-induced inflammatory pain model suggests that clonidine's spinal antihyperalgesic mechanisms act through inhibition of spinal NO release.

Descending control of pain

Upon receipt in the dorsal horn (DH) of the spinal cord, nociceptive (pain-signalling) information from the viscera, skin and other organs is subject to extensive processing by a diversity of mechanisms, certain of which enhance, and certain of which inhibit, its transfer to higher centres. In this regard, a network of descending pathways projecting from cerebral structures to the DH plays a complex and crucial role. Specific centrifugal pathways either suppress (descending inhibition) or potentiate (descending facilitation) passage of nociceptive messages to the brain. Engagement of descending inhibition by the opioid analgesic, morphine, fulfils an important role in its pain-relieving properties, while induction of analgesia by the adrenergic agonist, clonidine, reflects actions at alpha(2)-adrenoceptors (alpha(2)-ARs) in the DH normally recruited by descending pathways. However, opioids and adrenergic agents exploit but a tiny fraction of the vast panoply of mechanisms now known to be involved in the induction and/or expression of descending controls. For example, no drug interfering with descending facilitation is currently available for clinical use. The present review focuses on: (1) the organisation of descending pathways and their pathophysiological significance; (2) the role of individual transmitters and specific receptor types in the modulation and expression of mechanisms of descending inhibition and facilitation and (3) the advantages and limitations of established and innovative analgesic strategies which act by manipulation of descending controls. Knowledge of descending pathways has increased exponentially in recent years, so this is an opportune moment to survey their operation and therapeutic relevance to the improved management of pain.

Alpha2 receptors and agonists in pain management

Alpha(2) agonists have been in clinical use for decades, primarily in the treatment of hypertension. In recent years, alpha(2) agonists have found wider application, particularly in the fields of anesthesia and pain management. It has been noted that these agents can enhance analgesia provided by traditional analgesics, such as opiates, and may result in opiate-sparing effects. This has important implications for the management of acute postoperative pain and chronic pain states, including disorders involving spasticity or myofascial pain, neuropathic pain, and chronic daily headaches. The clinical utility of these agents is ever expanding, as they are gaining broader use in neuraxial analgesia, and new applications are continuously under investigation. The alpha(2) agonists that are currently employed in anesthesia and pain management include clonidine, tizanidine, and dexmedetomidine. Moxonidine and radolmidine, which are not currently in clinical use in humans, may offer favorable side-effect profiles when compared with traditional alpha(2) agonists, and may thereby allow for more widespread pain management applications.