Enhancement of pentazocine analgesia by clonidine

Involvement of imidazoline and opioid receptors in the enhancement of clonidine-induced analgesia by sulfisoxazole

Clonidine, an α2-adrenergic agonist, has been demonstrated to produce significant analgesia and potentiate morphine analgesia. Endothelin (ETA) receptor antagonists have also been found to potentiate the antinociceptive response to morphine. Clonidine and ET have been reported to have cardiovascular interactions involving the sympathetic nervous system, but it is not known whether ETA receptor antagonist affects clonidine analgesia. This study examined the influence of sulfisoxazole (ETA receptor antagonist) on clonidine analgesia. Male Swiss Webster mice were used to determine antinociceptive response of drugs by measuring tail-flick latency. The effect of clonidine (0.3, 1.0, and 3.0 mg/kg, i.p.) alone or in combination with sulfisoxazole (25, 75, and 225 mg/kg, p.o.) on analgesia and body temperature was determined. Clonidine produced a dose-dependent analgesia and hypothermia. Sulfisoxazole (25, 75, and 225 mg/kg), when administered with clonidine (0.3 mg/kg), significantly potentiated (31% increase in area under the curve (AUC)) the analgesic effect of clonidine. Yohimbine (α2-adrenergic receptor antagonist) did not affect analgesic effect of clonidine plus sulfisoxazole. Idazoxan (I1-imidazoline and α2-adrenergic receptor antagonist) reduced (47% decrease in AUC) the analgesic effect of clonidine plus sulfisoxazole. Treatment with naloxone reduced (46% decrease in AUC) the analgesic effect of clonidine plus sulfisoxazole. The effect of another ETA receptor antagonist, BMS-182874 (2, 10, and 50 µg, i.c.v.) was studied, and it was found that the dose of 10 µg significantly potentiated (26% increase in AUC) the analgesic effect of clonidine. These results indicate that sulfisoxazole, an ETA receptor antagonist, potentiates the analgesic effect of clonidine, which could be mediated through I1-imidazoline receptors and opioid receptors.

The caudal ventrolateral medulla as an important inhibitory modulator of pain transmission in the spinal cord

The caudal ventrolateral medulla (VLM) has emerged during the last decade as one of the main components of the endogenous pain control system. Profound and long-lasting analgesia is produced by mild stimulation of the VLM. The VLMlat, the reticular formation located between the spinal trigeminal nucleus and the lateral reticular nucleus (LRt), appears to play a major role in that antinociceptive action. The projections to spinal cord laminae involved in nociceptive transmission originate exclusively in the VLMlat. The VLMlat participates in a disynaptic pathway involving spinally projecting pontine A5 noradrenergic neurons, which appears to convey alpha(2)-adrenoreceptor-mediated analgesia produced from the VLM. Neurons in the VLMlat and in lamina I are reciprocally connected by a closed loop that is likely to mediate feedback control of supraspinal nociceptive transmission. On the other hand, the LRt, which is targeted by ventral (lamina VII) and deep dorsal (laminae IV to V) horn inputs, projects to the premotor lamina VII. Nociceptive input ascending from the cord and increases in blood pressure are discussed as possible physiologic triggers of the analgesia produced by the VLM. The overall role of the VLM as a center for integration of nociceptive, cardiovascular, and motor functions is discussed. The putative therapeutic benefits of manipulating the VLM for the control of chronic pain are envisaged.

Effects of yohimbine on the antinociceptive and place conditioning effects of opioid agonists in rodents

1. The pharmacological modulation of opioid actions by drugs acting on heterologous mechanisms could be useful to overcome some of the main problems associated with the use of opiate agonists. Based on previous findings on the interactions between yohimbine and opioid drugs, we have further studied the effects of yohimbine on the antinociceptive and positive-negative reinforcing effects of morphine (mu opioid receptor-preferring agonist), U-50,488 (kappa agonist) and SNC80 (delta agonist). 2. Pretreatment with yohimbine completely blocked the antinociception provided by the three opioid agonists in the mouse tail-immersion test. 3. A similar blockade of SNC80 and U-50,488-induced antinociception was observed with yohimbine in the mouse hot plate test at the same doses. In this paradigm, the effect of the kappa agonist was very slight and the actions of yohimbine rather variable. 4. In place conditioning experiments with SD (Sprague -- Dawley) male rats, yohimbine alone was inactive but it limited the preference induced by morphine and SNC80 and the aversive effect of U-50,488. Impaired novelty preference was also observed with the combination of yohimbine and U-50,488. 5. It is concluded that yohimbine tends to limit opioid antinociception and the addictive potential of mu and delta opioid agonists. More selective drugs could help to understand the mechanisms involved in these actions.

[Systemic administration of alpha(2)-adrenoceptor agonists for postoperative pain reliefagonists for postoperative pain relief.]

The alpha(2)-adrenoceptor agonist clonidine has analgesic properties comparable to those of opioids after systemic administration. It also has antihypertensive, antiemetic, anxiolytic, sedative and antisialogogue effects and reduces the incidence of shivering. Thus, the pharmacodynamic profile of clonidine seems to suit it quite well for the special problems related to recovery from anaesthesia. The respiratory depression is weak, particularly in comparison with opioids. This paper reviews previous experience with systemic administration of alpha(2)-adrenoceptor agonists for postoperative pain relief. Especially in combination with low-dose opioids, clonidine leads to a similar or even better level of pain relief with significantly reduced adverse side effects compared with opioid mono-therapy, perhaps because different sites of action are addressed and influence nociception. Therefore, balanced postoperative analgesia including alpha(2)-adrenoceptor agonists, therefore seems to be a beneficial addition to differentiated postoperative pain relief.

Pain mechanisms and management: an update

1. Recent findings have further helped to elucidate the mechanisms involved in the transmission and modulation of pain. It is now known that pain, inflammation and nervous system damage results in a number of changes in peripheral nerves, spinal cord and supraspinal structures. These changes themselves may be responsible for the development and maintenance of chronic pain syndromes. 2. In response to these findings, new agents and techniques have been applied in the clinical setting and new approaches have been developed to use existing agents more effectively. This review presents some of the findings from recent studies and the implications they have for the management of pain.

Recent advances in pain management

Over the last 20 years there has been an upsurge of interest in the basic mechanisms of pain. The findings that have arisen as a result of this interest have flowed through to the clinical area and have seen applications in a variety of settings. Identification of receptors and processes that are involved in the transmission of pain has led to the use of new agents in pain management. New techniques have provided new and more effective approaches in managing pain. These include the use of pre-emptive analgesia, postoperative pain management with patient controlled analgesia and use of techniques such as intrathecal drug administration and epidural spinal cord stimulation. This review presents some of the findings from basic research which have led to these developments, in particular those that relate to the changes that occur following inflammation and nerve injury, and the implications that these findings have had on pain management.

Spinal opioid systems in inflammation

Until recently, basic science studies, both behavioural and electrophysiological, have concentrated on the antinociceptive actions of opioids primarily gauged against acute nociceptive responses. However, of more relevance to clinical situations are the actions of opioids in more persistent/prolonged pain states. This review sets out to examine the central actions of opioids against nociception of inflammatory origins. The first section deals with the response of the endogenous opioid system to the development of an inflammatory state and the second examines the ability of exogenous opioids to modulate inflammatory nociception. There are complex changes in the roles of endogenous opioids, in particular dynorphin, at the spinal level after inflammation although the physiological consequences remain unclear. With regard to exogenous opioids, the effectiveness of spinal morphine is rapidly enhanced after inflammation, likely to be due to changes in the interaction between the peptide cholecystokinin and the mu opioid receptor. The ability of inflammatory processes to alter both endogenous opioids and morphine analgesia at the spinal level illustrates the considerable degree of plasticity observed in opioid function.

Enhancement of morphine analgesia by the alpha 2-adrenergic antagonist yohimbine

Although interactions between opioids and adrenergic agonists in the treatment of pain have been demonstrated in humans, the contribution of specific adrenergic receptors in this interaction remains to be clarified. In a double-blind, placebo-controlled study in male patients with postoperative dental pain, we investigated the effect of preoperative administration of the alpha 2-adrenergic antagonist, yohimbine, on analgesia produced by postoperative intravenous morphine. Although yohimbine by itself did not affect the pain, the overall analgesic effect of morphine was significantly enhanced in the presence of yohimbine. This report is the first to demonstrate that an alpha 2-adrenergic antagonist enhances opiate analgesia in humans.

Intrathecal morphine and clonidine in the management of spinal cord injury pain: a case report

Neuropathic pain following spinal cord injury (SCI) can be difficult to manage using currently available pain management techniques. We describe a case of chronic pain following SCI which failed to respond to a variety of approaches including intrathecal administration of morphine. Use of clonidine in addition to the morphine resulted in a marked decrease in pain. The use of intrathecal clonidine with or without opioids may present an effective alternative in the management of intractable SCI pain and other forms of neuropathic pain.

Sympathetic suppression attenuates anomalous responses to morphine in unexplained pain after cholecystectomy

Anomalous responses to morphine are common in patients with unexplained pain in the upper abdomen after cholecystectomy and may be linked to activation of the sympathetic nervous system. The hypothesis that sympathetic suppression would attenuate anomalous responses to morphine was tested by a randomized, cross-over trial using a standard challenge with morphine, with and without pretreatment with clonidine (300 micrograms orally, 1 h prior to the administration of morphine). In 13 of the 15 patients who completed the study, pre-treatment with clonidine decreased plasma concentrations of noradrenaline, dopamine and adrenaline by 56, 15 and 25% respectively. This was associated with a significant reduction in morphine-induced pain (p = 0.02) and nausea (p = 0.04) and attenuated increases in plasma aspartate aminotransferase (AST) activity (p = 0.03). Clonidine attenuates anomalous responses to morphine, perhaps through effects on sympathetic nervous activity or plasma concentrations of catecholamines.

Interactions between fluoxetine and opiate analgesia for postoperative dental pain

In a double-blind placebo-controlled study we investigated the analgesic efficacy of combinations of the serotonergic tricyclic antidepressant fluoxetine with either the mu-opiate morphine or the kappa-opiate pentazocine. Administration of oral fluoxetine (10 mg p.o. daily for 7 days pre-operatively) had no effect on the immediate postoperative pain level. However, pre-operative administration of fluoxetine was found, compared to placebo, to antagonize analgesia seen after administration of morphine (6 mg, i.v.) in the immediate postoperative period. Attenuation of morphine analgesia consisted essentially of a shortening of the duration of action of the dose of morphine administered. Similar administration of fluoxetine had no effect on the analgesia produced by the kappa-opiate pentazocine (45 mg, i.v.). This effect probably results from alteration in the known serotonergic circuits in endogenous pain-modulating systems.

Temporal factors in the enhancement of morphine analgesia by desipramine

Administration of desipramine, the tricyclic noradrenergic agent, for 7 days pre-operatively, had been found to potentiate postoperative morphine analgesia. In this study we investigated the necessary timing of administration of desipramine in its action to potentiate morphine analgesia. We report that the administration of desipramine for only 3 days, starting 7 days before surgery, also potentiated postoperative morphine analgesia and that the analgesia observed was not different from that in patients receiving a full 7 days of desipramine pre-operatively. The potentiation of morphine analgesia observed was most evident as a prolongation of the analgesic response. Patients who also received desipramine for only 3 days, but starting 3 days pre-operatively had an analgesic response to postoperative morphine that was the same as that in patients receiving placebo. The ability of the administration of desipramine early in the pre-operative week to interact with postoperative morphine and the lack of response when desipramine was given late in the week does not have an explanation at present. However, it may reflect the known latency in humans to the onset of the central effects of tricyclic antidepressants (TCAs).