Alpha 2-adrenoceptor-mediated inhibition of capsaicin-evoked release of glutamate from rat spinal dorsal horn slices

Intrathecal Morphine versus Morphine-Dexmedetomidine Combination for Postoperative Pain Control After Total Knee Replacement: A Randomized Controlled Trial

Objective

This prospective study aimed to compare the analgesic efficacy and adverse effects of intrathecal morphine, dexmedetomidine, and a combination of both in patients undergoing total knee replacement (TKR).

Patients and Methods

This randomized prospective study was carried out in Tanta university hospital in orthopedic surgery for 6 months on 105 adult patients with American Society of Anesthesiologists Physical Status Class II and III, aged > 50 years, and scheduled for total knee replacement surgery randomly allocated into morphine group received 0.5% heavy bupivacaine plus 0.1 mg of morphine, morphine/ dexmedetomidine group, received 0.5% heavy bupivacaine plus 0.1 mg of morphine and 5 mcg of dexmedetomidine and dexmedetomidine group received 0.5% heavy bupivacaine plus 5 mcg of dexmedetomidine. The time of the first required analgesia, postoperative pain severity, the total dose of morphine, postoperative complication, and the patient's level of sedation were recorded.

Results

About half of the patients in the dexmedetomidine group requested first rescue analgesia 6 hours after the operation, significantly shorter than the other two groups. On the other hand, the other two groups show no significant difference between them regarding the first required analgesia. At rest, the dexmedetomidine group have significantly higher VAS with a significant increase in patients who required morphine as rescue analgesia than the other two groups. While at movement, patients in the dexmedetomidine group felt pain at 4 hrs postoperatively with significantly higher VAS than the other two groups. At the same time, the sedation score was significantly lower in the dexmedetomidine group than in the other two groups. 22.2% of cases in the morphine group developed nausea and vomiting with a significant difference between the three groups.

Conclusion

Despite the absence of substantial side effects, our findings did not suggest enhanced analgesia with the combination of intrathecal morphine and dexmedetomidine.

Acetaminophen Exerts an Analgesic Effect on Muscular Hyperalgesia in Repeated Cold-Stressed Rats through the Enhancement of the Descending Pain Inhibitory System Involving Spinal 5-HT3 and Noradrenergic α2 Receptors

Musculoskeletal and psychological complaints have increased with the widespread use of visual display terminals, and musculoskeletal pain is known to be closely related to stress. One method of experimentally inducing persistent muscle pain is repeated cold stress (RCS), and animals exposed to such stress exhibit a dysfunction in the descending pain inhibitory system. Acetaminophen (N-acetyl-p-aminophenol; APAP) is widely used to relieve several types of pain, including musculoskeletal pain, and is available as an OTC drug. However, the mechanism underlying its analgesic action has not yet been fully elucidated. In this study, we compared the analgesic effect of APAP on RCS-induced muscular hyperalgesia with those of other analgesics to identify its mechanism of action. The daily oral administration of APAP significantly suppressed the decrease in the mechanical withdrawal threshold caused by RCS, similar to the results for neurotropin but not for the cyclooxygenase inhibitor ibuprofen (IBP). Moreover, the intrathecal administration of antagonists of the 5-hydroxytryptamine (5-HT)3 receptor or α2-adrenoceptor significantly abolished the analgesic effect of APAP but not of IBP. These results suggest that the analgesic effect of APAP on RCS-induced muscular pain might be exerted due to the activation of the descending pathways involving the spinal 5-HT3 receptor or α2-adrenoceptor.

Involvement of spinal α2 -adrenoceptors in prolonged modulation of hind limb withdrawal reflexes following acute noxious stimulation in the anaesthetized rabbit

The role of spinal α₂‐adrenoceptors in mediating long‐lasting modulation of hind limb withdrawal reflexes following acute noxious chemical stimulation of distant heterotopic and local homotopic locations has been investigated in pentobarbitone‐anaesthetized rabbits. Reflexes evoked in the ankle extensor muscle medial gastrocnemius (MG) by electrical stimulation of the ipsilateral heel, and reflexes elicited in the ankle flexor tibialis anterior and the knee flexor semitendinosus by stimulation at the base of the ipsilateral toes, could be inhibited for over 1 h after mustard oil (20%) was applied to either the snout or into the contralateral MG. The heel–MG response was also inhibited after applying mustard oil across the plantar metatarsophalangeal joints of the ipsilateral foot, whereas this homotopic stimulus facilitated both flexor responses. Mustard oil also caused a significant pressor effect when applied to any of the three test sites. The selective α₂‐adrenoceptor antagonist, RX 821002 (100–300 μg, intrathecally), had no effect on reflexes per se, but did cause a decrease in mean arterial blood pressure. In the presence of the α₂‐blocker, inhibitory and facilitatory effects of mustard oil on reflexes were completely abolished. These data imply that long‐lasting inhibition of spinal reflexes following acute noxious stimulation of distant locations involves activation of supraspinal noradrenergic pathways, the effects of which are dependent on an intact α₂‐adrenoceptor system at the spinal level. These pathways and receptors also appear to be involved in facilitation (sensitization) as well as inhibition of reflexes following a noxious stimulus applied to the same limb.

Clonidine, an alpha-2 adrenoceptor agonist relieves mechanical allodynia in oxaliplatin-induced neuropathic mice; potentiation by spinal p38 MAPK inhibition without motor dysfunction and hypotension

Cancer chemotherapy with platinum-based antineoplastic agents including oxaliplatin frequently results in a debilitating and painful peripheral neuropathy. We evaluated the antinociceptive effects of the alpha-2 adrenoceptor agonist, clonidine on oxaliplatin-induced neuropathic pain. Specifically, we determined if (i) the intraperitoneal (i.p.) injection of clonidine reduces mechanical allodynia in mice with an oxaliplatin-induced neuropathy and (ii) concurrent inhibition of p38 mitogen-activated protein kinase (MAPK) activity by the p38 MAPK inhibitor SB203580 enhances clonidine's antiallodynic effect. Clonidine (0.01-0.1 mg kg(-1), i.p.), with or without SB203580(1-10 nmol, intrathecal) was administered two weeks after oxaliplatin injection(10 mg kg(-1), i.p.) to mice. Mechanical withdrawal threshold, motor coordination and blood pressure were measured. Postmortem expression of p38 MAPK and ERK as well as their phosphorylated forms(p-p38 and p-ERK) were quantified 30 min or 4 hr after drug injection in the spinal cord dorsal horn of treated and control mice. Clonidine dose-dependently reduced oxaliplatin-induced mechanical allodynia and spinal p-p38 MAPK expression, but not p-ERK. At 0.1 mg kg(-1), clonidine also impaired motor coordination and decreased blood pressure. A 10 nmol dose of SB203580 alone significantly reduced mechanical allodynia and p-p38 MAPK expression, while a subeffective dose(3 nmol) potentiated the antiallodynic effect of 0.03 mg kg(-1) clonidine and reduced the increased p-p38 MAPK. Coadministration of SB203580 and 0.03 mg kg(-1) clonidine decreased allodynia similar to that of 0.10 mg kg(-1) clonidine, but without significant motor or vascular effects. These findings demonstrate that clonidine treatment reduces oxaliplatin-induced mechanical allodynia. The concurrent administration of SB203580 reduces the dosage requirements for clonidine, thereby alleviating allodynia without producing undesirable motor or cardiovascular effects.

Clonidine Reduces Nociceptive Responses in Mouse Orofacial Formalin Model: Potentiation by Sigma-1 Receptor Antagonist BD1047 without Impaired Motor Coordination

Although the administration of clonidine, an alpha-2 adrenoceptor agonist, significantly attenuates nociception and hyperalgesia in several pain models, clinical trials of clonidine are limited by its side effects such as drowsiness, hypotension and sedation. Recently, we determined that the sigma-1 receptor antagonist BD1047 dose-dependently reduced nociceptive responses in a mouse orofacial formalin model. Here we examined whether intraperitoneal injection of clonidine suppressed the nociceptive responses in the orofacial formalin test, and whether co-administration with BD1047 enhances lower-dose clonidine-induced anti-nociceptive effects without the disruption of motor coordination and blood pressure. Formalin (5%, 10 µL) was subcutaneously injected into the right upper lip, and the rubbing responses with the ipsilateral fore- or hind-paw were counted for 45 min. Clonidine (10, 30 or 100 µg/kg) was intraperitoneally administered 30 min before formalin injection. Clonidine alone dose-dependently reduced nociceptive responses in both the first and second phases. Co-localization for alpha-2A adrenoceptors and sigma-1 receptors was determined in trigeminal ganglion cells. Interestingly, the sub-effective dose of BD1047 (3 mg/kg) significantly potentiated the anti-nociceptive effect of lower-dose clonidine (10 or 30 µg/kg) in the second phase. In particular, the middle dose of clonidine (30 µg/kg) in combination with BD1047 produced an anti-nociceptive effect similar to that of the high-dose clonidine, but without a significant motor dysfunction or hypotension. In contrast, mice treated with the high dose of clonidine developed severe impairment in motor coordination and blood pressure. These data suggest that a combination of low-dose clonidine with BD1047 may be a novel and safe therapeutic strategy for orofacial pain management.

Noradrenergic modulation of itch transmission in the spinal cord

Inhibition of both itching and scratching is important in the treatment of chronic pruritic diseases, because itching has a negative impact on quality of life and vigorous scratching worsens skin conditions. Pharmacological modulation of itch transmission in the dorsal horn is an effective way to inhibit both itching and scratching in pruritic diseases. Pruriceptive transmission in the spinal dorsal horn undergoes inhibitory modulation by the descending noradrenergic system. The noradrenergic inhibition is mediated by excitatory α₁-adrenoceptors located on inhibitory interneurons and inhibitory α₂-adrenoceptors located on central terminals of primary sensory neurons. The descending noradrenergic system and α-adrenoceptors in the dorsal horn are potential targets for antipruritic drugs.

Plasticity of α2-adrenergic spinal antinociception following nerve injury: selective, bidirectional interaction with the delta opioid receptor

Interactions of opioid receptors with other receptor families can be made use of to improve analgesia and reduce adverse effects of opioid analgesics. We investigated interactions of the α2-adrenergic receptor (α2AR) with opioid receptors of the mu (MOR) and delta (DOR) types in the spinal dorsal horn in an animal model of neuropathic pain induced by spinal nerve ligation. Nine days after nerve injury, immunoreactivity for the α2AR subtype A (α2AAR) was increased both in tissue homogenates and at pre- and post-synaptic sites in transverse sections. The efficacy of spinally administered α2AAR agonist guanfacine at reducing C-fiber-evoked field potentials was increased in nerve-ligated rats. This reducing effect was impaired by simultaneous administration of DOR antagonist naltrindole, but not MOR antagonist CTOP, suggesting that concurrent DOR activation was required for α2AAR-mediated inhibition. While DOR agonist deltorphin II and MOR agonist DAMGO both effectively depressed C-fiber-evoked spinal field potentials, DOR- but not MOR-mediated depression was enhanced by subclinical guanfacine. In conscious, nerve-ligated rats, chronically administered deltorphin II produced stable thermal and mechanical antinociception over the 9 following days after nerve injury without apparent signs of habituation. Such an effect was dramatically enhanced by co-administration of a low dose of guanfacine, which reversed thermal and mechanical thresholds to levels near those prior to injury. The results suggest that spinal, α2AAR-mediated antinociception is increased after nerve injury and based on DOR co-activation. We demonstrate in vivo that α2AAR/DOR interaction can be exploited to provide effective behavioral antinociception during neuropathic pain.

[A memoir of my research on pain and analgesia for 39 years]

This review describes my research for the past 39 years regarding the pharmacology of pain and analgesia. We have demonstrated that the descending noradrenergic system is involved in the analgesic effect of morphine injected into the nucleus reticularis gigantocellularis, and that noradrenaline exerts antinociception mediated by α-adrenoceptors. We have found that noxious mechanical and thermal stimuli to the skin increase the release of substance P and somatostatin, respectively, from the dorsal horn in situ, and that noradrenaline inhibits the release of substance P and glutamate from primary afferents. We developed an animal model of cancer pain using melanoma cells. We have shown that the suppression of cancer pain results in the inhibition of tumor growth and lung metastasis, and that melanoma cells release several algogenic substances including ATP, endothelin-1, and bradykinin. We investigated neuropathic allodynia induced by the chemotherapeutic drugs paciltaxel, oxaliplatin, vincristine, and bortezomib. Single administration of these drugs caused allodynia with similar time-courses. However, antiallodynic actions of adjuvant analgesics, including gabapentin and limaprost, were dependent on the chemotherapeutic drugs used. Limaprost experiments have revealed that a decrease in peripheral blood flow is involved in allodynia exacerbation after the administration of paciltaxel and oxaliplatin. We have developed animal models of herpetic pain and postherpetic neuralgia using herpes simplex virus 1. We have demonstrated that nitric oxide, prostaglandin E2, and galectin-3 are involved in herpetic allodynia, that risk factors associated with postherpetic allodynia include severe herpetic pain, nociceptin, and major histocompatibility complex, and that deafferentation and nitric oxide are involved in postherpetic allodynia.

Effects of the selective sigma-1 receptor antagonist S1RA on formalin-induced pain behavior and neurotransmitter release in the spinal cord in rats

We have previously shown that the selective sigma-1 receptor (σ1 R) antagonist S1RA (E-52862) inhibits neuropathic pain and activity-induced spinal sensitization in various pre-clinical pain models. In this study we characterized both the behavioral and the spinal neurochemical effects of S1RA in the rat formalin test. Systemic administration of S1RA produced a dose-related attenuation of flinching and lifting/licking behaviors in the formalin test. Neurochemical studies using concentric microdialysis in the ipsilateral dorsal horn of awake, freely moving rats revealed that the systemic S1RA-induced antinociceptive effect occurs concomitantly with an enhancement of noradrenaline levels and an attenuation of formalin-evoked glutamate release in the spinal dorsal horn. Intrathecal pre-treatment with idazoxan prevented the systemic S1RA antinociceptive effect, suggesting that the S1RA antinociception depends on the activation of spinal α2 -adrenoceptors which, in turn, could induce an inhibition of formalin-evoked glutamate release. When administered locally, intrathecal S1RA inhibited only the flinching behavior, whereas intracerebroventricularly or intraplantarly injected also attenuated the lifting/licking behavior. These results suggest that S1RA supraspinally activates the descending noradrenergic pain inhibitory system, which may explain part of its antinociceptive properties in the formalin test; however, effects at other central and peripheral sites also account for the overall effect. Formalin-induced nociceptive effect occurs concomitantly with an enhancement of glutamate (Glu) level in the dorsal horn spinal cord. The selective σ1 receptor antagonist S1RA results in inhibition of formalin-evoked Glu release, no modification of GABA levels, and enhancement of noradrenaline (NA) levels. This increased spinal NA activates spinal α2-adrenoceptors producing the attenuation of the formalin-induced pain behaviour.

Suppression of the cough reflex by α 2-adrenergic receptor agonists in the rabbit

The α₂-adrenergic receptor agonist clonidine has been shown to inhibit citric acid-induced cough responses in guinea pigs when administered by aerosol, but not orally. In contrast, oral or inhaled clonidine had no effect on capsaicin-induced cough and reflex bronchoconstriction in humans. In addition, intravenous administration of clonidine has been shown to depress fentanyl-induced cough in humans. We investigated the effects of the α₂-adrenergic receptor agonists, clonidine and tizanidine, on cough responses induced by mechanical and chemical (citric acid) stimulation of the tracheobronchial tree. Drugs were microinjected (30–50 nL) into the caudal nucleus tractus solitarii (cNTS) and the caudal ventral respiratory group (cVRG) as well as administered intravenously in pentobarbital sodium-anesthetized, spontaneously breathing rabbits. Bilateral microinjections of clonidine into the cNTS or the cVRG reduced cough responses at 0.5 mmol/L and abolished the cough reflex at 5 mmol/L. Bilateral microinjections of 0.5 mmol/L tizanidine into the cNTS completely suppressed cough responses, whereas bilateral microinjections of 5 mmol/L into the cVRG only caused mild reductions in them. Depressant effects on the cough reflex of clonidine and tizanidine were completely reverted by microinjections of 10 mmol/L yohimbine. Intravenous administration of clonidine (80–120 μg/kg) or tizanidine (150–300 μg/kg) strongly reduced or completely suppressed cough responses. These effects were reverted by intravenous administration of yohimbine (300 μg/kg). The results demonstrate that activation of α₂-adrenergic receptors in the rabbit exerts potent inhibitory effects on the central mechanism generating the cough motor pattern with a clear action at the level of the cNTS and the cVRG.

Milnacipran inhibits itch-related responses in mice through the enhancement of noradrenergic transmission in the spinal cord

We investigated whether milnacipran, a serotonin-noradrenaline reuptake inhibitor, exhibits an antipruritic effect through the spinal action in mice. Intrathecal injections of milnacipran (0.1 - 10 μg/site) significantly suppressed serotonin-induced biting, which is an itch-related response. However, such an effect was not observed with fluvoxamine (10 μg/site), which is a selective serotonin reuptake inhibitor. Furthermore, an intraperitoneal injection of milnacipran (10 mg/kg) inhibited serotonin-induced biting. When phentolamine (1.0 μg/site), a non-selective α-adrenoceptor antagonist, was intrathecally injected, it inhibited the above response of milnacipran. These results suggest that milnacipran suppresses itching through the inhibition of noradrenaline reuptake in the spinal cord.

The interaction of octopamine and neuropeptides to slow aversive responses in C. elegans mimics the modulation of chronic pain in mammals

Octopamine (OA) appears to function as the invertebrate counterpart of norepinephrine (NE) in the modulation of a number of key behaviors. In C. elegans, OA signaling is complex, mediated by at least three distinct α-adrenergic-like receptors and appears to activate more global peptidergic signaling cascades that have the potential to dramatically amplify the octopaminergic signal. These OA-dependent peptidergic signaling cascades involve an array of neuropeptides that activate receptors throughout the nervous system and have the potential to both directly and indirectly modulate locomotory decision-making. In this commentary we highlight the use of C. elegans as a model to expand our understanding of noradrenergic signaling in mammals, specifically as it relates to the role of NE in anti-nociception.

Inhibition of noradrenaline release by clonidine in the ventral bed nucleus of the stria terminalis attenuates pain-induced aversion in rats

Pain is an unpleasant sensory and emotional experience. The neural systems underlying the sensory component of pain have been studied extensively, but we are only beginning to understand those underlying the affective component of pain. Previously, we showed the pivotal role of noradrenergic transmission in the ventral part of the bed nucleus of the stria terminalis (vBNST) in the negative affective component of pain using a conditioned place paradigm. In this study, we examined the effect of local administration of clonidine, an α(2)-adrenoceptor agonist, into the vBNST on noradrenaline release and on conditioned place aversion (CPA) induced by an intraplantar formalin injection in male Sprague-Dawley rats. In vivo microdialysis showed that the formalin-induced increase in the extracellular noradrenaline level within the vBNST was significantly suppressed by clonidine (100 μM) perfusion through a microdialysis probe. Bilateral intra-vBNST injections of clonidine (1 and 10 nmol/side) dose-dependently attenuated formalin-induced CPA without reducing nociceptive behaviors. These results suggest that clonidine inhibits noradrenaline release by acting on α(2)-adrenoceptors located in the vBNST and thereby attenuates pain-induced aversion. α(2)-adrenoceptors in the vBNST play a pivotal role in the regulation of negative affective, but not the sensory, component of pain.

Dexmedetomidine and ST-91 analgesia in the formalin model is mediated by alpha2A-adrenoceptors: a mechanism of action distinct from morphine

BACKGROUND AND PURPOSE

Intrathecal administration of alpha(2)-adrenoceptor agonists produces potent analgesia. This study addressed the subtype of spinal alpha(2)-adrenoceptor responsible for the analgesic effects of i.t. dexmedetomidine and ST-91 in the formalin behavioural model and their effects on primary afferent substance P (SP) release and spinal Fos activation.

EXPERIMENTAL APPROACH

The analgesic effects of i.t. dexmedetomidine and ST-91 (alpha(2) agonists) were tested on the formalin behavioural model. To determine the subtype of alpha(2)-adrenoceptor involved in the analgesia, i.t. BRL44408 (alpha(2A) antagonist) or ARC239 (alpha(2B/C) antagonist) were given before dexmedetomidine or ST-91. Moreover, the ability of dexmedetomidine and ST-91 to inhibit formalin-induced release of SP from primary afferent terminals was measured by the internalization of neurokinin(1) (NK(1)) receptors. Finally, the effects of dexmedetomidine on formalin-induced Fos expression were assessed in the dorsal horn.

KEY RESULTS

Intrathecal administration of dexmedetomidine or ST-91 dose-dependently reduced the formalin-induced paw-flinching behaviour in rats. BRL44408 dose-dependently blocked, whereas ARC239 had no effect on the analgesic actions of dexmedetomidine and ST-91. Dexmedetomidine and ST-91 had no effect on the formalin-induced NK(1) receptor internalization, while morphine significantly reduced the NK(1) receptor internalization. On the other hand, both dexmedetomidine and morphine diminished the formalin-induced Fos activation. The effect of dexmedetomidine on formalin-induced Fos activation was reversed by BRL44408, but not ARC239.

CONCLUSION AND IMPLICATIONS

These findings suggest that alpha(2A)-adrenoceptors mediate dexmedetomidine and ST-91 analgesia. This effect could be through a mechanism postsynaptic to primary afferent terminals, distinct from that of morphine.

Intrathecal clonidine suppresses phosphorylation of the N-methyl-D-aspartate receptor NR1 subunit in spinal dorsal horn neurons of rats with neuropathic pain

BACKGROUND

Intrathecal (IT) administration of the alpha-2 adrenoceptor agonist, clonidine, produces significant analgesic effects. Although several mechanisms underlying clonidine-induced analgesia have been proposed, the possible interaction with N-methyl-D-aspartate (NMDA) receptors as a major antinociceptive mechanism has not been addressed. We designed the present study to determine whether clonidine or other analgesics can affect spinal NMDA receptor activation in rats with chronic constriction injury (CCI)-induced neuropathy.

METHODS

Rats underwent unilateral CCI, and received IT clonidine (1, 5, 20 microg/rat), [D-Ala2, NMe-Phe4, Gly-ol5]-enkephalin (DAMGO, mu opioid receptor agonist, 1 microg/rat), gabapentin (anticonvulsant, 100 microg/rat) or vehicle 2 wks later. After drug injection, we measured the pain response to thermal or mechanical stimuli and used immunohistochemistry to evaluate spinal cord phosphorylated NMDA-receptor subunit 1 (pNR1) expression.

RESULTS

Two weeks after CCI surgery, rats displayed significant mechanical allodynia and thermal hyperalgesia, and the spinal cord dorsal horn showed a significant increase in the number of pNR1 immunoreactive neurons. IT injection of clonidine (20 microg/rat), DAMGO and gabapentin potently reduced mechanical allodynia and thermal hyperalgesia. Importantly, IT clonidine, but not IT DAMGO or gabapentin, dose-dependently reduced CCI-induced pNR1 expression in all lamina of the spinal cord dorsal horn by 30 min after injection. In addition, IT injection of the alpha-2 adrenoceptor antagonist, idazoxan (40 microg/rat) 10 min before clonidine injection completely reversed clonidine's antihyperalgesic and antiallodynic effects, as well as clonidine's suppressive effect on CCI-induced NR1 phosphorylation in the spinal cord dorsal horn.

CONCLUSIONS

Our data indicate that IT clonidine's antihyperalgesic/antiallodynic effect on neuropathic pain is associated with a significant reduction in spinal NMDA receptor phosphorylation and suggests a potentially novel mechanism of clonidine's action.

Modulation of pain transmission by G-protein-coupled receptors

The heterotrimeric G-protein-coupled receptors (GPCR) represent the largest and most diverse family of cell surface receptors and proteins. GPCR are widely distributed in the peripheral and central nervous systems and are one of the most important therapeutic targets in pain medicine. GPCR are present on the plasma membrane of neurons and their terminals along the nociceptive pathways and are closely associated with the modulation of pain transmission. GPCR that can produce analgesia upon activation include opioid, cannabinoid, alpha2-adrenergic, muscarinic acetylcholine, gamma-aminobutyric acidB (GABAB), groups II and III metabotropic glutamate, and somatostatin receptors. Recent studies have led to a better understanding of the role of these GPCR in the regulation of pain transmission. Here, we review the current knowledge about the cellular and molecular mechanisms that underlie the analgesic actions of GPCR agonists, with a focus on their effects on ion channels expressed on nociceptive sensory neurons and on synaptic transmission at the spinal cord level.

Electroacupuncture-induced analgesia in a rat model of ankle sprain pain is mediated by spinal alpha-adrenoceptors

In a previous study, we showed that electroacupuncture (EA) applied to the SI-6 point on the contralateral forelimb produces long-lasting and powerful analgesia in pain caused by ankle sprain in a rat model. To investigate the underlying mechanism of EA analgesia, the present study tested the effects of various antagonists on known endogenous analgesic systems in this model. Ankle sprain was induced in anesthetized rats by overextending their right ankle with repeated forceful plantar flexion and inversion of the foot. When rats developed pain behaviors (a reduction in weight-bearing of the affected hind limb), EA was applied to the SI-6 point on the contralateral forelimb for 30 min under halothane anesthesia. EA significantly improved the weight-bearing capacity of the affected hind limb for 2h, suggesting an analgesic effect. The alpha-adrenoceptor antagonist phentolamine (2mg/kg, i.p. or 30 microg, i.t.) completely blocked the EA-induced analgesia, whereas naloxone (1mg/kg, i.p.) failed to block the effect. These results suggest that EA-induced analgesia is mediated by alpha-adrenoceptor mechanisms. Further experiments showed that intrathecal administration of yohimbine, an alpha(2)-adrenergic antagonist, reduced the EA-induced analgesia in a dose-dependent manner, whereas terazosin, an alpha(1)-adrenergic antagonist, did not produce any effect. These data suggest that the analgesic effect of EA in ankle sprain pain is, at least in part, mediated by spinal alpha(2)-adrenoceptor mechanisms.

Noradrenergic pain modulation

Norepinephrine is involved in intrinsic control of pain. Main sources of norepinephrine are sympathetic nerves peripherally and noradrenergic brainstem nuclei A1-A7 centrally. Peripheral norepinephrine has little influence on pain in healthy tissues, whereas in injured tissues it has variable effects, including aggravation of pain. Its peripheral pronociceptive effect has been associated with injury-induced expression of novel noradrenergic receptors, sprouting of sympathetic nerve fibers, and pronociceptive changes in the ionic channel properties of primary afferent nociceptors, while an interaction with the immune system may contribute in part to peripheral antinociception induced by norepinephrine. In the spinal cord, norepinephrine released from descending pathways suppresses pain by inhibitory action on alpha-2A-adrenoceptors on central terminals of primary afferent nociceptors (presynaptic inhibition), by direct alpha-2-adrenergic action on pain-relay neurons (postsynaptic inhibition), and by alpha-1-adrenoceptor-mediated activation of inhibitory interneurons. Additionally, alpha-2C-adrenoceptors on axon terminals of excitatory interneurons of the spinal dorsal horn possibly contribute to spinal control of pain. At supraspinal levels, the pain modulatory effect by norepinephrine and noradrenergic receptors has varied depending on many factors such as the supraspinal site, the type of the adrenoceptor, the duration of the pain and pathophysiological condition. While in baseline conditions the noradrenergic system may have little effect, sustained pain induces noradrenergic feedback inhibition of pain. Noradrenergic systems may also contribute to top-down control of pain, such as induced by a change in the behavioral state. Following injury or inflammation, the central as well as peripheral noradrenergic system is subject to various plastic changes that influence its antinociceptive efficacy.

Noradrenergic modulation of XII motoneuron inspiratory activity does not involve α2-receptor inhibition of the Ih current or presynaptic glutamate release

Norepinephrine has powerful and diverse modulatory effects on hypoglossal (XII) motoneuron activity, which is important in maintaining airway patency. The objective was to test two hypotheses that α2-adrenoceptor-mediated, presynaptic inhibition of glutamatergic inspiratory drive (Selvaratnam SR, Parkis MA, and Funk GD. Brain Res 805: 104–115, 1998) and postsynaptic inhibition of the hyperpolarization-activated inward current ( Ih) (Parkis MA and Berger AJ. Brain Res 769: 108–118, 1997) modulate XII inspiratory activity. Nerve and whole cell recordings were applied to rhythmic medullary slice preparations from neonatal rats ( postnatal days 0–4) to monitor XII inspiratory burst amplitude and motoneuron properties. Application of an α2-receptor agonist (clonidine, 1 mM) to the XII nucleus reduced inspiratory burst amplitude to 71 ± 3% of control but had no effect on inspiratory synaptic currents. It also reduced the Ih current by ∼40%, but an Ih current blocker (ZD7288), at concentrations that blocked ∼80% of Ih, had no effect on inspiratory burst amplitude. The clonidine inhibition was unaffected by the GABAA antagonist (+)bicuculline but attenuated by the α2-antagonist rauwolscine and the imidazoline 1 (I1) antagonist efaroxan. The I1 agonist rilmenidine, but not the α2-agonist UK14304, inhibited XII output. Clonidine also reduced action potential amplitude or impaired repetitive firing. Although a contribution from α2, and in particular I1, receptors remains possible, results demonstrate that 1) noradrenergic modulation of XII inspiratory activity is unlikely to involve α2-receptor-mediated presynaptic inhibition of glutamate release or modulation of Ih; 2) inhibition of repetitive firing is a major factor underlying the inhibition of XII output by clonidine; and 3) Ih is present in neonatal XII motoneurons but does not contribute to shaping their inspiratory activity.

Interactions between the cardiovascular and pain regulatory systems: an updated review of mechanisms and possible alterations in chronic pain

Endogenous pain regulatory system dysfunction appears to play a role in the maintenance of chronic pain. An important component of the pain regulatory process is the functional interaction between the cardiovascular and pain regulatory systems, which results in an association between elevated resting blood pressure (BP) and diminished acute pain sensitivity. This BP/pain sensitivity relationship is proposed to reflect a homeostatic feedback loop helping restore arousal levels in the presence of painful stimuli. Evidence is emerging that this normally adaptive BP/pain sensitivity relationship is significantly altered in chronic pain conditions, affecting responsiveness to both acute and chronic pain stimuli. Several mechanisms that may underlie this adaptive relationship in healthy individuals are overviewed, including endogenous opioid, noradrenergic, and baroreceptor-related mechanisms. Theoretical models are presented regarding how chronic pain-related alterations in the mechanisms above and increased pain facilatory system activity (central sensitization) may contribute to altered BP/pain sensitivity interactions in chronic pain. Clinical implications are discussed.

Alpha-2 adrenoceptor mediating the facilitatory effect of norepinephrine on the glycine response in the spinal dorsal horn neuron of the rat

Effects of norepinephrine (NE) on the glycine-mediated inhibitory response were investigated in neurons acutely dissociated from the rat spinal dorsal horn, using nystatin perforated patch recording mode under voltage-clamp conditions. NE reversibly and concentration dependently facilitated Cl(-) current induced by 3 x 10(-5) M glycine. NE neither changed the reversal potential of the glycine response nor affected the affinity of glycine to its receptor. This effect could be mimicked by clonidine (10(-7) M) and blocked by yohimbine (10(-6) M), respectively. N-[2(methylamino)ethyl]-5-isoquinoline sulfonamide dihydrochloride (H-89), an inhibitor of protein kinase A, effectively mimicked the effect of NE on glycine response, whereas chelerythrine (an inhibitor of protein kinase C) failed. NE further enhanced glycine response even in the presence of chelerythrine or stearoylcarnitine chloride (another inhibitor of protein kinase C) or chelerythrine together with stearoylcarnitine chloride. The present results suggest that alpha2-adrenoceptor is involved in the potentiation of NE on glycine response in freshly isolated spinal dorsal horn neurons. Activation of alpha2-adrenoceptor down-regulates the activity of protein kinase A that results in the potentiation of the glycinergic inhibitory effects within the spinal dorsal horn.

Norepinephrine depresses the capsaicin-evoked miniature excitatory postsynaptic currents in substantia gelatinosa of the rat spinal cord

Capsaicin selectively excites nociceptive primary afferent fibers and increases the frequency of glutaminergic miniature excitatory postsynaptic currents (mEPSCs) in the substantia gelatinosa of the spinal dorsal horn. The whole-cell voltage-clamp recording technique was used to examine the effect of norepinephrine (NE) on the capsaicin-induced increase in the frequency of mEPSCs. In the presence of tetrodotoxin, bath application of capsaicin (1 microM) remarkably enhanced the frequency of mEPSCs (295+/-52% of control). Following pretreatment with NE (10 microM), the capsaicin-induced frequency facilitation of mEPSCs was significantly depressed to 151+/-17% of the control. NE-induced depression in capsaicin action was blocked by yohimbine, a selective alpha(2)-adrenergic receptor antagonist, indicating that NE exerts depression by activating the alpha(2)-adrenergic receptor. As the postsynaptic action of NE has been precluded in the present study, the results suggest that NE inhibits nociceptive input at a presynaptic site, the primary afferent terminal, during the nociceptive transmission in the spinal dorsal horn.

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.

Inhibition of glutamatergic synaptic input to spinal lamina II(o) neurons by presynaptic alpha(2)-adrenergic receptors

Activation of spinal alpha(2)-adrenergic receptors by the descending noradrenergic system and alpha(2)-adrenergic agonists produces analgesia. However, the sites and mechanisms of the analgesic action of spinally administered alpha(2)-adrenergic receptor agonists such as clonidine are not fully known. The dorsal horn neurons in the outer zone of lamina II (lamina II(o)) are important for processing nociceptive information from C-fiber primary afferents. In the present study, we tested a hypothesis that activation of presynaptic alpha(2)-adrenergic receptors by clonidine inhibits the excitatory synaptic input to lamina II(o) neurons. Whole cell voltage-clamp recordings were performed on visualized lamina II(o) neurons in the spinal cord slice of rats. The miniature excitatory postsynaptic currents (mEPSCs) were recorded in the presence of tetrodotoxin, bicuculline, and strychnine. The evoked EPSCs were obtained by electrical stimulation of the dorsal root entry zone or the attached dorsal root. Both mEPSCs and evoked EPSCs were abolished by application of 6-cyano-7-nitroquinoxaline-2,3-dione. Clonidine (10 microM) significantly decreased the frequency of mEPSCs from 5.8 +/- 0.9 to 2.7 +/- 0.6 Hz (means +/- SE) without altering the amplitude and the decay time constant of mEPSCs in 25 of 27 lamina II(o) neurons. Yohimbine (2 microM, an alpha(2)-adrenergic receptor antagonist), but not prazosin (2 microM, an alpha(1)-adrenergic receptor antagonist), blocked the inhibitory effect of clonidine on the mEPSCs. Clonidine (1-20 microM, n = 8) also significantly attenuated the peak amplitude of evoked EPSCs in a concentration-dependent manner. The effect of clonidine on evoked EPSCs was abolished in the presence of yohimbine (n = 5). These data suggest that clonidine inhibits the excitatory synaptic input to lamina II(o) neurons through activation of alpha(2)-adrenergic receptors located on the glutamatergic afferent terminals. Presynaptic inhibition of glutamate release from primary afferents onto lamina II(o) neurons likely plays an important role in the analgesic action produced by activation of the descending noradrenergic system and alpha(2)-adrenergic agonists.

Gastrointestinal afferents as targets of novel drugs for the treatment of functional bowel disorders and visceral pain

An intricate surveillance network consisting of enteroendocrine cells, immune cells and sensory nerve fibres monitors the luminal and interstitial environment in the alimentary canal. Functional bowel disorders are characterized by persistent alterations in digestive regulation and gastrointestinal discomfort and pain. Visceral hyperalgesia may arise from an exaggerated sensitivity of peripheral afferent nerve fibres and/or a distorted processing and representation of gut signals in the brain. Novel strategies to treat these sensory bowel disorders are therefore targeted at primary afferent nerve fibres. These neurons express a number of molecular traits including transmitters, receptors and ion channels that are specific to them and whose number and/or behaviour may be altered in chronic visceral pain. The targets under consideration comprise vanilloid receptor ion channels, acid-sensing ion channels, sensory neuron-specific Na(+) channels, P2X(3) purinoceptors, 5-hydroxytryptamine (5-HT), 5-HT(3) and 5-HT(4) receptors, cholecystokinin CCK(1) receptors, bradykinin and prostaglandin receptors, glutamate receptors, tachykinin and calcitonin gene-related peptide receptors as well as peripheral opioid and cannabinoid receptors. The utility of sensory neuron-targeting drugs in functional bowel disorders will critically depend on the compounds' selectivity of action for afferent versus enteric or central neurons.

The synergistic antinociceptive interactions of endomorphin-1 with dexmedetomidine and/or S(+)-ketamine in rats

Spinal administration of the endogenous mu-opioid agonist peptide, endomorphin-1, results in antinociception in rodents, but there are few data about its interaction with other antinociceptive drugs. We investigated the antinociceptive interactions at the spinal level of endomorphin-1 with the N-methyl-D-aspartate antagonist S(+)-ketamine, the alpha2-adrenoceptor agonist dexmedetomidine, or both in awake rats. Nociception was assessed by the tail-flick test. Dose-response curves were determined for endomorphin-1 (0.6-50 microg), for dexmedetomidine (0.1-10 microg), for mixtures of S(+)-ketamine (30 or 100 microg) with endomorphin-1 (2-18 microg) or of endomorphin-1 with dexmedetomidine in a fixed ratio (4:1), and for the triple combination of the three drugs after intrathecal administration. Endomorphin-1 and dexmedetomidine both produced dose-dependent antinociception. The coadministration of 100 microg S(+)-ketamine significantly enhanced the antinociceptive effect of 6 microg endomorphin-1. Isobolographic analysis of the combinations of endomorphin-1 and dexmedetomidine revealed a synergistic interaction between these drugs. The 80% effective dose for the triple combination was significantly less than that for either binary combination. These data indicate that S(+)-ketamine and dexmedetomidine, acting via different receptors, produce synergistic antinociceptive interaction with endomorphin-1 at the spinal level. Furthermore, the triple combination of an opioid agonist, an alpha2-adrenoceptor agonist, and an N-methyl-D-aspartate receptor antagonist shows potent antinociceptive activity.

IMPLICATIONS

The coadministration of the N-methyl-D-aspartate antagonist receptor antagonist, S(+)-ketamine, or the specific alpha2-adrenoceptor agonist, dexmedetomidine, significantly enhances the antinociceptive effect of the endogenous mu-opioid agonist, endomorphin-1, at the spinal level. The triple combination of the three drugs causes a further improved antinociception.

alpha2-adrenoceptors modulate NMDA-evoked responses of neurons in superficial and deeper dorsal horn of the medulla

Extracellular single unit recordings were made from neurons in the superficial and deeper dorsal horn of the medulla (trigeminal nucleus caudalis) in 21 male rats anesthetized with urethan. NMDA produced an antagonist-reversible excitation of 46 nociceptive as well as nonnociceptive neurons. Microiontophoretic application of a preferential alpha2-adrenoceptor (alpha2AR) agonist, (2-[2, 6-dichloroaniline]-2-imidazoline) hydrochloride (clonidine), reduced the NMDA-evoked responses of 86% (6/7) of nociceptive-specific (NS) neurons, 82% (9/11) of wide dynamic range (WDR) neurons, and 67% (4/6) of low-threshold (LT) neurons in the superficial dorsal horn. In the deeper dorsal horn, clonidine inhibited the NMDA-evoked responses of 94% (16/17) of NS and WDR neurons and 60% (3/5) of LT neurons. Clonidine facilitated the NMDA-evoked responses in 14% (1/17) of NS, 9% (1/11) of WDR, and 33% (2/6) of LT neurons in the superficial dorsal horn. Idazoxan, an alpha2AR antagonist, reversed the inhibitory effect of clonidine in 90% (9/10) of neurons, whereas prazosin, an alpha1-adrenoceptor antagonist with affinity for alpha2BAR, and alpha2CAR, were ineffective. We suggest that activation of alpha2ARs produces a predominantly inhibitory modulation of the NMDA-evoked responses of nociceptive neurons in the medullary dorsal horn.

Mivazerol inhibits intrathecal release of glutamate evoked by halothane withdrawal in rats

BACKGROUND

Mivazerol is a new and selective alpha 2-adrenergic receptor agonist devoid of hypotensive effects (1, 2). Previous studies have demonstrated that mivazerol prevents hemodynamic instability during emergence from halothane anesthesia in rats (3). The present study was to determine whether glutamate and aspartate are involved in this action of mivazerol, at the second to third thoracic segments (T2-T3) of the spinal cord.

METHODS

In vivo microdialysis in combination with high-performance liquid chromatography (HPLC) was employed in the study. Blood pressure (BP) and heart rate (HR) were recorded along with intrathecal (i.t.) microdialysis perfusion.

RESULTS

BP, HR and i.t. release of glutamate (GLU, pmol/microliter) were stable in the rats under 1.1% halothane anesthesia. However, halothane withdrawal immediately increased BP, HR, and i.t. release of GLU, and remained elevated for at least 2 h after withdrawal of halothane. Thirty minutes prior to halothane withdrawal, intravenous (i.v.) infusion of mivazerol (15 micrograms.kg-1.h-1) almost completely prevented the increases in HR (delta 18 +/- 7 vs delta 79 +/- 7 beats/min), and in the i.t. release of GLU (delta 10.3 +/- 3.7 vs delta 30.6 +/- 5.9; 112% vs 167%). Local i.t. microinjection of mivazerol (2.5 micrograms/kg) 2 min prior to withdrawal of halothane also blocked the HR responses, as well as on the i.t. release of GLU following halothane withdrawal.

CONCLUSION

The present study demonstrates that emergence from halothane anesthesia increases i.t. release of GLU, and that mivazerol has an inhibitory effect on the above, through its direct action on the spinal cord.

Na+ channel block prevents the ischemia-induced release of norepinephrine from spinal cord slices

The principal finding of the present study with rat spinal cord slices was the novel demonstration of the [Ca2+]o-independent effect of ischemia on norepinephrine release and its antagonism by tetrodotoxin and low temperature (10 degrees C). Our finding that tetrodotoxin antagonized the effects of glucose deprivation on norepinephrine release in a [Ca2+]o-independent way suggests that Na+ channel block alone, i.e., the prevention of Na+ accumulation, may account for the protective action. Low temperature completely prevented the effect of ischemia on norepinephrine release but did not change the release associated with axonal activity. This finding is in good agreement with the observation that small changes in brain temperature critically determine the extent of neuronal injury from ischemia and suggests that both [Ca2+]o-independent release and cell injury are associated with the norepinephrine membrane carrier. It is suggested, therefore, that drugs able to attenuate the increase in [Na+]i during ischemia may be useful agents to protect against ischemic damage if given before the insult.

Clonidine potentiates the neuropathic pain-relieving action of MK-801 while preventing its neurotoxic and hyperactivity side effects

Antagonists of NMDA glutamate receptors have been shown to alleviate neuropathic pain in rats and humans. However, NMDA antagonists can cause significant side effects ranging from behavioral disturbances to injury of neurons in the posterior cingulate/retrosplenial (PC/RS) cortex. We have found that alpha-2 adrenergic agonists prevent the PC/RS neurotoxic side effects of NMDA antagonists. In the present study of adult female rats subjected to sciatic nerve ligation (Bennett neuropathic pain model) and tested for paw withdrawal latency (PWL) following a thermal stimulus, we evaluated the ability of the NMDA antagonist, MK-801, to alleviate neuropathic pain either by itself or when administered together with the alpha-2 adrenergic agonist, clonidine. We found that MK-801, at a dose (0.05 mg/kg s.c.) that is known to cause mild hyperactivity but is subthreshold for producing PC/RS neurotoxic changes, relieved the neuropathic pain state associated with sciatic nerve ligation. However, the relief at this dose was very transient, and no neuropathic pain-relieving effect was observed at a lower dose (0. 025 mg/kg s.c.) of MK-801. Clonidine, at a dose (0.05 mg/kg s.c.) that prevents the cerebrocortical neurotoxic effects of MK-801, decreased sensitivity to the thermal stimulus equally under all conditions (ligated, sham ligated, unoperated), but did not specifically relieve neuropathic pain in the ligated limb. Combining this dose of clonidine with an ineffective dose (0.025 mg/kg s.c.) of MK-801 provided specific, complete and long lasting (up to 4 h) relief from neuropathic pain. Rats receiving this drug combination did not display hyperactivity or any other behavioral disturbance typically associated with MK-801 treatment, nor show neurotoxic changes in cerebrocortical neurons. In separate experiments on normal unoperated rats, we found that clonidine (0.05 mg/kg s.c.) counteracted the hyperactivity induced by MK-801 (0.05 mg/kg s.c.) and returned activity levels to a normal range. These findings signify that clonidine, which does not specifically relieve neuropathic pain, can potentiate the neuropathic pain-relieving action of MK-801, while also protecting against neurotoxicity and hyperactivity side effects of MK-801. The potentiation is of a sufficient magnitude that it permits cutting the MK-801 dose requirement in half, thereby achieving prolonged neuropathic pain relief while doubling the margin of safety against any type of side effect that might be mediated by blockade of NMDA receptors.

Effect of repeated cold stress on capsaicin-evoked release of glutamate from rat spinal dorsal horn slices

We investigated the effect of repeated cold stress (RCS) on the capsaicin-evoked release of glutamate from the primary afferent fibers of the rat, and compared this with the effect of inoculation of complete Freund's adjuvant (adjuvant inoculation). The release of glutamate was measured using a fluorometric on-line continuous monitoring system in which the immobilized glutamate dehydrogenase column was connected to an in vitro superfusion system. In the presence of 0.3 microM tetrodotoxin, the application of 1 microM capsaicin to spinal dorsal horn slices evoked glutamate release (18.6 +/- 1.2 pmol mg(-1) protein, n = 11). In rats subjected to RCS (RCS rats), the release of glutamate evoked by 1 microM capsaicin was markedly increased to 272% (n = 6, P < 0.05) of the value for the control group, although the basal release was not significantly altered (n = 6, P > 0.05). Adjuvant inoculation produced a significant increase in the basal and capsaicin (1 microM) evoked release of glutamate to 141 and 344% (n = 6, P < 0.05) of the value for the control group, respectively. The present results suggest that the facilitated release of glutamate from capsaicin-sensitive primary afferent terminals in the spinal dorsal horn is, at least in part, involved in the hyperalgesia of RCS rats as well as the complete Freund's adjuvant-induced hyperalgesia.

Alpha2-adrenoreceptor agonist, dexmedetomidine, alters acute gene expression after global ischemia in gerbils

The effect of dexmedetomidine, a selective alpha2-adrenoreceptor agonist and neuroprotective agent on the expression of immediate early genes and heat shock protein hsp70, was studied using quantitative in situ hybridization in a global ischemia model. At the dose previously shown to be neuroprotective dexmedetomidine inhibited the expression of c-fos and hsp70 mRNA, did not affect jun-B mRNA, and enhanced the induction of NGFI-A mRNA in the postischemic gerbil hippocampus. The reduced gene expression of c-fos and hsp70 was detected in the CA1 pyramidal cells which are prone to ischemic degeneration, whereas the increased gene expression of NGFI-A was measured from the CA3 and dentate gyrus, areas relatively resistant to ischemia. These alterations in early gene expression possibly reflect the mechanisms mediating the neuroprotective effects of alpha2-adrenoreceptor agonists.

alpha 2A subtype of presynaptic alpha 2-adrenoceptors modulates the release of [3H]-noradrenaline from rat spinal cord

The modulation of noradrenaline (NA) release from rat spinal cord slices was investigated and the subtype of presynaptic alpha 2-adrenoceptors involved in the negative feedback modulation was characterized using in vitro perfusion experiments. Rat spinal cord slices were loaded with [3H]NA and the release of radioactivity at rest and in response to field stimulation was determined. The alpha 2-adrenoceptor agonists, clonidine and dexmedetomidine inhibited the stimulation-evoked release of NA from spinal cord slices, whereas alpha 2-adrenoceptor antagonists yohimbine and CH-38083 (7,8-(methylenedioxy)-14-alpha-hydroxyalloberbane HCI), enhanced it. ARC 239, a selective alpha 2B-antagonist, had no effect on the release. In contrast, BRL 44408 a selective alpha 2A-antagonist increased the release of NA. Our results indicate that the negative feedback modulation of NA release from noradrenergic fibres in the spinal cord is mediated via alpha 2A subtype of alpha 2-adrenoceptors.

Evidence for the participation of glutamate in reflexes involving afferent, substance P-containing nerve fibres in the rat

1. Responses mediated, either peripherally or centrally, by substance P-containing primary afferent C-fibres were investigated in the rat following impairment of axonal transport by colchicine (120 micrograms kg-1, i.p., daily for 3 days), and after treatment with the tachykinin antagonist SR-140333 (10-100 micrograms kg-1, i.v.) or the N-methyl-D-aspartate (NMDA) antagonist MK-801 (100 micrograms kg-1). 2. Peripheral effects mediated by afferent C-fibres were measured by plasma protein extravasation (Evans blue method), following antidromic stimulation of the sciatic nerve, topical application of mustard oil and, as control, i.v. injection of substance P. SR-140333 (100 micrograms kg-1) reduced the effects by 86%, 75% and 74%, respectively. Colchicine reduced the effects of the first two stimuli by 31% and 33% and, as expected not the effect of substance P. The increase of paw skin temperature following capsaicin i.v. was inhibited by SR-140333, but not by colchicine. MK-801 had no effect on the plasma protein extravasation following antidromic sciatic nerve stimulation or on the rise of paw skin temperature induced by capsaicin i.v., thus excluding an effect of MK-801 on peripheral terminals of afferent neurones. 3. Depressor reflexes, which are known to be mediated by capsaicin-sensitive afferent neuones, such as those elicited (A) by a stimulating dose of 30 ng capsaicin i.a., (B) by distension of the ascending colon or (C) by afferent sciatic nerve stimulation were studied. Colchicine significantly reduced depressor reflexes A and B, but had no effect on reflex C. None of the reflexes was affected by SR-140333. MK-801 significantly inhibited all three reflexes. 4. Capsaicin, injected either i.v. (200 micrograms kg-1) or into the nucleus caudatus/putamen (i.c., 30 micrograms), induced an increase in paw skin temperature and a decrease in colon temperature. The rise in fore paw skin temperature (delta t = 2.3 +/- 0.4 degrees C) evoked by capsaicin i.v. was almost completely blocked by SR-140333 (100 micrograms kg-1, i.v.), but no inhibition was observed with MK-801, indicating that capsaicin had brought about a release of substance P from peripheral nerve terminals. Colchicine did not influence heat dissipation induced by i.v. capsaicin. 5. When capsaicin was injected i.c., the rise in paw skin temperature in colchicine- and SR-140333-pretreated groups did not differ from that of the control group. MK-801 totally prevented the heat loss reaction to i.c. capsaicin administration. Colchicine did not change the effects of i.v. or i.c. injected capsaicin: this excludes the involvement of a mechanism dependent on axonal transport of neurotransmitters. 6. The reduction of axonal transport by colchicine reduced plasma extravasation induced by mustard oil and antidromic sciatic nerve stimulation (peripheral functions) and depressor reflexes evoked by i.a. capsaicin and colon distension (central functions). It can be argued that afferent stimulation of the sciatic nerve includes the stimulation of A-fibres, which might be less sensitive to colchicine. SR-140333 was effective only on peripherally mediated responses. 7. The recent evidence for the concomitant release of glutamate and substance P from central terminals of afferent C-fibres, known to mediate reflexes abolished after capsaicin treatment allows the following conclusions: (a) the inhibition by MK-801 indicates an essential role for glutamate in the central transmission of these reflexes; (b) tachykinin antagonists such as SR-140333 do not affect these responses when administered systemically. Centrally released substance P could be involved in functions of the CNS other than those investigated here unless the access of neurokinin antagonists to their receptors in the CNS is insufficient.