Prostaglandins inhibit endogenous pain control mechanisms by blocking transmission at spinal noradrenergic synapses

Spinal intrathecal injections of the nonsteroidal antiinflammatory analgesics (NSAIAs) indomethacin and acetylsalicylic acid, which inhibit prostaglandin synthesis, cause dose-dependent hypoalgesia in the rat. Intrathecal injections of prostaglandin-E2 (PGE2) produce dose-dependent hyperalgesia. To determine whether this action of prostaglandins on the central nervous system is mediated through pain-generating or analgesia pathways, we studied the effect of intrathecal PGE2 on endogenous opioid-induced analgesia. Intrathecal PGE2 antagonized the analgesia produced by both brain stimulation and intracerebroventricular morphine. In contrast, the NSAIAs synergized with brain stimulation and morphine-induced analgesia. The alpha-adrenergic antagonist phentolamine and the catecholaminergic selective neurotoxin 6-hydroxydopamine, used to block tonic catecholamine activity in endogenous opioid-mediated analgesia systems, prevented the hyperalgesia induced by intrathecal PGE2. Phentolamine did not, however, block the hyperalgesia caused by intradermal PGE2. These findings suggest that prostaglandins can block endogenous opioid-mediated analgesia systems by inhibiting the bulbospinal noradrenergic component of this analgesia pathway.

Generation and recognition of leukotriene mediators of hypersensitivity and inflammation

The potent mediators generated by the 5- and 15-lipoxygenation of arachidonic acid have diverse effects on smooth muscles, blood vessels, leukocytes, epithelial cells and glands, and sensory neurons, which suggest possible roles in the initiation and regulation of physiological and biochemical events. The responses to leukotrienes and related mediators are attributable to binding by stereospecific cellular receptors and consequent activation of biochemical transductional sequences analogous to those characteristic of other receptor systems. The elevated concentrations of these mediators in lesional fluids and tissues of inflammatory bowel disease and other hypersensitivity and inflammatory states are, in some instances, clearly related to the time course of development of the disease process. Systematic application of specific inhibitors and antagonists that are becoming available will define more clearly the involvement of leukotrienes in health and disease and possibly lead to new therapeutic approaches.

Hyperalgesia onset latency suggests a hierarchy of action

Hyperalgesia onset latencies of inflammatory mediators were quantified by measuring the threshold of the nociceptive flexion reflex in the rat at 1 min intervals after intradermal injection. Prostaglandin E2 and 8(R), 15(S)-dihydroxyicosa-(5E,9,11,13Z)-tetraenoic acid induced hyperalgesia with short onset latencies, compatible with a direct action on primary afferent nociceptors. Bradykinin, norepinephrine and leukotriene B4 induced hyperalgesia with a significant delay in onset, compatible with their known indirect mechanisms of producing hyperalgesia. We propose that use of this approach, rapid frequent measurement of nociceptive threshold, can be used to determine the hierarchy of action of mediators in hyperalgesic mechanisms.

Contribution of the nervous system to the pathophysiology of rheumatoid arthritis and other polyarthritides

Some clinical features of rheumatoid arthritis (RA) (for example, preferential joint involvement and bilateral symmetry), taken together with the strong evidence of neurogenic inflammatory processes, suggest that the nervous system contributes to the inflammatory component of RA and other polyarthritides. The authors propose that the increased risk and severity of disease in particular joints reflects a greater innervation of those joints by unmyelinated afferent and sympathetic efferent fibers. Release of the proinflammatory peptide, substance P, from the peripheral terminals of nociceptive joint afferent fibers, through interactions with many nonneural cells, exacerbates the inflammatory process. Release of mediators from sympathetic efferents (including norepinephrine) also contributes to the inflammation, either through an independent mechanism or by acting in concert with the nociceptive afferent-derived substances. Therapies directed at interruption of the nervous system contribution to the pathophysiology of these diseases should offer a new direction to treatment.

Leukotriene and prostaglandin sensitization of cutaneous high-threshold C- and A-delta mechanonociceptors in the hairy skin of rat hindlimbs

Single C- and A-delta fibers were isolated from dissected filaments of the saphenous nerve in pentobarbital anesthetized rats and the corresponding cutaneous receptive fields mapped with calibrated von Frey hairs. Nociceptors were characterized by their responses to noxious mechanical, thermal and chemical stimuli, including intradermal injections of leukotriene B4, prostaglandin E2, bradykinin and capsaicin. Leukotriene B4 decreased the mean mechanical threshold by a maximum of 80% within 10 min and for more than 3 h after intradermal injection of 75 ng of leukotriene B4. The degrees of sensitization of a fiber by leukotriene B4 and prostaglandin E2 were highly correlated. A potentiation effect also was observed, in that injection of prostaglandin E2 or leukotriene B4 1 h after the other eicosanoid further lowered the mechanical threshold of a sensitized fiber, whereas fibers that were not sensitized by leukotriene B4 were unaffected by prostaglandin E2. The sensitizing action of leukotriene B4 and prostaglandin E2 was directed to multiple classes of cutaneous nociceptors including 73% of C-polymodal, 60% of C-mechano-heat, 42% of C-mechano-cold nociceptors and 70% of A-delta high-threshold mechanonociceptors. The pain-evoking substances bradykinin and capsaicin activated 81% and 88%, respectively, of the sensitized C-polymodal nociceptors, 17% and 84% of the sensitized-C-mechano-heat nociceptors, 12% and 37% of the sensitized C-mechano-cold nociceptors, and 17% and none of the sensitized A-delta high-threshold mechanociceptors. The responses of C-fibers to bradykinin and capsaicin were highly correlated.(ABSTRACT TRUNCATED AT 250 WORDS)

Lupus anticoagulant and cerebral infarction: therapeutic implications

The clinical hematological and radiological findings in a patient with stroke and the lupus anticoagulant are presented, and therapeutic alternatives are reviewed. The importance of recognizing this association is stressed because of its potential therapeutic implications.

Clinical response to regional intravenous guanethidine in patients with rheumatoid arthritis

A novel therapy for rheumatoid arthritis, regional sympathetic blockade using guanethidine, was investigated in 24 patients with active disease. In a randomized double blind short-term (14 days) study, we evaluated the effect of therapy on subjective responses, change in pain, stiffness, and morning stiffness and no objective responses, change in pinch strength, grip strength, and joint tenderness. Compared to placebo, guanethidine produced a decrease in pain (p less than 0.025) and an increase in pinch strength (less than 0.025) over the 2-week duration of the study. The therapeutic effect of guanethidine may be mediated by an interruption of the proinflammatory effects of the sympathetic nervous system.

Contribution of sensory afferents and sympathetic efferents to joint injury in experimental arthritis

We used pharmacological and surgical methods to determine the contribution of several neural components to joint injury in rats with adjuvant-induced arthritis. Both neonatal administration of capsaicin, which eliminates small-diameter afferents, and peripheral sympathectomy, which depletes catecholamines, attenuated joint injury. In contrast, the arthritis was more severe in spontaneously hypertensive rats, which have increased sympathetic tone. To address the contribution of the central vs peripheral afferent terminal selectively, a group of rats underwent unilateral dorsal rhizotomy. These rats developed a more severe arthritis in the deafferented limb. The increase in arthritis severity produced by dorsal rhizotomy could be reduced by prior sympathectomy or, less effectively, by prior treatment with capsaicin. The latter observation suggests that large-diameter afferents that are cut during dorsal rhizotomy also influence inflammation. Finally, intracerebroventricular injection of morphine attenuated the severity of arthritis, possibly through activation of bulbospinal sympathoinhibitory circuits. Taken together, these data indicate that no one class of nerve fiber is wholly responsible for the neurogenic component of inflammation in experimental arthritis but that large- and small-diameter afferents, sympathetic efferents, and CNS circuits that modulate those fiber systems all influence the severity of joint injury in arthritic rats.

Circadian activity rhythms in rats with midbrain raphe lesions

The dorsal and median mesencephalic raphe nuclei provide a robust projection to the hypothalamic suprachiasmatic nucleus, the site of a putative neuronal circadian pacemaker. Although it has been suggested that the raphe may play a role in the circadian timing system, this role has not yet been specified. In the present report, we examined the circadian activity patterns of rats with large mid-brain lesions aimed at the median and dorsal raphe nuclei under conditions of light-dark entrainment, and while free-running in constant light and constant darkness. The results indicate that midbrain raphe lesions may interfere with the expression of free-running circadian activity rhythms.

Desipramine enhances opiate postoperative analgesia

In a double-blind, placebo-controlled study the analgesic efficacy of the combination of a tricyclic antidepressant and morphine was investigated. One of two tricyclic antidepressants (either amitriptyline, a relatively selective serotonin uptake inhibitor or desipramine, a relatively selective noradrenaline uptake inhibitor) or a placebo, was given for 1 week prior to surgery, followed by a single postoperative dose of morphine. Desipramine, but not amitriptyline, both increased and prolonged morphine analgesia. Neither tricyclic antidepressant reduced dental postoperative pain in the absence of morphine. We propose that desipramine enhances opiate analgesia by enhancing a noradrenergic component that contributes to endogenous opioid-mediated analgesia systems.

Noradrenaline hyperalgesia is mediated through interaction with sympathetic postganglionic neurone terminals rather than activation of primary afferent nociceptors

In hyperalgesic states, observed commonly as a major symptom of tissue inflammation or after central or peripheral nerve injury, non-noxious stimuli produce pain and noxious stimuli are perceived as more painful than usual. The mechanisms underlying the generation of hyperalgesia are not known. In patients with causalgia (burning pain and severe hyperalgesia after a nerve injury) activation of sympathetic post-ganglionic neurones or application of noradrenaline to painful skin exacerbates pain and hyperalgesia while sympathectomy may afford complete relief. One suggestion is that noradrenaline released from sympathetic post-ganglionic neurons increases the discharge of damaged small-diameter afferents by a direct action on the primary afferents. Here we present a new model for noradrenaline-sensitive hyperalgesia and demonstrate that the site of action of noradrenaline is not on the primary afferents but rather is presynaptic on the sympathetic post-ganglionic terminals.

The contribution of pain to disability in experimentally induced arthritis

We examined the differential effects of disease severity and pain on morbidity (as measured by weight loss and decrease in activity) in rats with adjuvant-induced arthritis. We found that eliminating nociceptive messages from affected extremities, although not significantly affecting the course of the disease, attenuated the morbidity observed in arthritic rats. When pain was factored out, the severity of the arthritis had no significant effect on the same measures of morbidity. These findings suggest that treatment of pain may reduce morbidity, and thereby may have significant benefits beyond that provided by slowing of the disease process.

The neurotoxic effect of gold sodium thiomalate on the peripheral nerves of the rat. Insights into the antiinflammatory actions of gold therapy

Although gold is one of the few therapeutic agents that has been proven effective in producing remission in patients with rheumatoid arthritis, its mechanism of action is unknown. Since nociceptive afferent and sympathetic efferent fibers of the peripheral nervous system contribute to the pathophysiology of inflammation, and since a known side effect of gold therapy is a polyneuropathy, we tested the hypothesis that gold is toxic to small-diameter peripheral nerve fibers in the rat. We found that prolonged treatment with gold, at the same dosage reported to be effective against adjuvant-induced arthritis in the rat, produced a significant decrease in the numbers of unmyelinated, but not of myelinated, axons. Gold treatment also elevated nociceptive thresholds in both articular and nonarticular structures. These results suggest that gold produces an antiinflammatory effect on arthritis by a neurotoxic effect on the peripheral nerves involved in neurogenic inflammation.

Hyperalgesic properties of 15-lipoxygenase products of arachidonic acid

Induction of hyperalgesia by leukotriene B4 (LTB4), a potent chemotactic factor for polymorphonuclear leukocytes (PMNLs), depends on the generation by cutaneous PMNLs of mediators that are probably derived from the 15-lipoxygenation of arachidonic acid. The capacity of dihydroxyeicosatetraenoic acid (diHETE) products of the 15-lipoxygenation of arachidonic acid in PMNL to elicit hyperalgesia was evaluated by assessing the effects of intradermal injection of synthetic diHETEs on the pressure nociceptive threshold in rats. (8R,15S)-Dihydroxyeicosa-(5E-9,11,13Z)-tetraenoic acid [(8R,15S)-diHETE] produced a dose-dependent hyperalgesia, as measured by decrease in threshold for paw withdrawal. The isomer (8S,15S)-diHETE antagonized in a dose-dependent manner this hyperalgesia due to (8R,15S)-diHETE but did not suppress prostaglandin E2-induced hyperalgesia. (8S,15S)-DiHETE produced a dose-dependent hypoalgesia, as reflected by an increase in nociceptive threshold, suggesting a contribution of endogenous (8R,15S)-diHETE to normal nociceptive threshold. The hypoalgesic effect of (8S,15S)-diHETE was blocked by corticosteroids but not by the cyclooxygenase inhibitor indomethacin. Neither (8R,15S)-dihydroxyeicosa-(5,15E-9,11Z)-tetraenoic acid nor (8R,15S)-dihydroxyeicosa-(5,11E-9,13Z)-tetraenoic acid exhibited any hyperalgesic or hypoalgesic activity. The stereospecificity of the effect of (8R,15S)-diHETE suggests that the induction of hyperalgesia is a receptor-dependent phenomenon and that (8S,15S)-diHETE may be an effective receptor-directed antagonist. The (8R,15S)-diHETE and (8S,15S)-diHETE from PMNL, keratinocytes, and other epithelial cells may modulate normal primary afferent function and contribute to inflammatory hyperalgesia.

Indomethacin blocks central nociceptive effects of PGF2 alpha

Intrathecal administration of PGE2 and PGF2 alpha and intradermal administration of PGE2 but not PGF2 alpha, in low doses, produce hyperalgesia as measured by the Randall-Selitto paw-withdrawal test. Indomethacin (2 mg/kg, i.p.) prevented intrathecal PGF2 alpha, but not PGE2-induced hyperalgesia. We propose that the central nociceptive effects of PGF2 alpha are mediated, indirectly, by effecting the cyclo-oxygenation of arachidonic acid in the central nervous system.

Method of administration determines the effect of naloxone on pain

The opiate antagonist, naloxone, produces dose-dependent biphasic changes in clinical pain. The mechanism of the analgesia produced by low dose naloxone is unknown. To study the analgesic effect of naloxone, we have used a programmable infusion pump, which eliminates placebo-induced endorphin-mediated analgesia, to administer different doses of naloxone. We report that use of machine infusion of naloxone exclusively produces analgesia. The implications of this finding to the mechanism of naloxone-induced analgesia are discussed.

The role of the polymorphonuclear leukocyte in hyperalgesia

The results of recent studies of the mechanism of leukotriene B4-induced hyperalgesia suggest a dependence on polymorphonuclear leukocytes (PMNLs). In this study, we addressed the contribution of PMNLs to hyperalgesia evoked by the peptide chemotactic factors N-formyl-methionyl-leucyl-phenylalanine (fMLP) and the anaphylatoxin fragment of the fifth component of the complement pathway (C5a). Local injection of glycogen, which attracts but does not activate PMNLs, produced a marked shift to the left (toward lower concentrations) in the concentration dependence curve of fMLP-induced hyperalgesia. In addition, PMNL repletion by transfusion with syngeneic PMNLs reestablished fMLP-induced hyperalgesia in PMNL-depleted rats. Finally, supernatants from rat and human PMNLs, that had been stimulated with fMLP in vitro, produced hyperalgesia in PMNL-depleted rats. Preliminary characterization of the hyperalgesia-inducing activity released by stimulated PMNLs indicated that it is lipid in nature. The nonsteroidal anti-inflammatory indomethacin did not attenuate C5a and fMLP-induced hyperalgesia. Thus, the hyperalgesia produced by fMLP and C5a is similar to that produced by leukotriene B4 in that it is dependent on PMNLs and independent of the cyclo-oxygenation of arachidonic acid. Taken together, these data suggest that structurally diverse PMNL-chemotactic factors produce hyperalgesia by a novel mechanism, involving PMNL-derived factors.

The contribution of neurogenic inflammation in experimental arthritis

The release of the peptide neurotransmitter substance P from the peripheral terminals of nociceptive afferent neurons and the release of catecholamines from postganglionic sympathetic efferent neurons produce physiologic changes associated with acute inflammation. The contribution of these neurogenic mechanisms to inflammatory diseases has not been determined. Activation of central neural circuits elicits similar physiologic changes, and lesions of the peripheral and central nervous system are associated with alteration in activity of inflammatory diseases. We have evaluated the contribution of neurogenic inflammation to the severity of joint injury in experimentally induced arthritis in the rat. The finding of a greater density of substance P-containing nociceptive afferents in a joint that develops more severe arthritis (ankle) suggests a role of substance P in joint injury. Direct evidence that the proinflammatory factor released from these nociceptors is substance P is provided by the finding that the injection of substance P into a joint which normally develops less severe arthritis (knee) increases the severity of arthritis in that joint. A contribution of catecholamines to the severity of joint injury was suggested by the finding that both guanethidine-induced sympathectomy and reserpine-induced depletion of catecholamines attenuated the severity of joint injury. Finally, a contribution of central neural circuits to inflammatory processes was studied in a model in which activation of nociceptive afferents elicited swelling and tenderness at a remote site. This reflex neurogenic inflammation was inhibited by intracerebroventricular injections of morphine, which also attenuated the severity of arthritis. These studies provide evidence that elements of the peripheral afferent and sympathetic efferent neurons and of descending supraspinal, opioid-mediated, circuits in the central nervous system modulate the severity of joint injury in experimental arthritis in the rat.

Reflex neurogenic inflammation. I. Contribution of the peripheral nervous system to spatially remote inflammatory responses that follow injury

Recent studies of the mechanism of neurogenic inflammation have focused on the contribution of neuropeptides released from peripheral terminals of primary afferent sensory neurons. In this study we addressed the contribution of humoral and neural factors to the hyperalgesia and swelling that are produced contralateral to an injured hindpaw, a phenomenon which we refer to as reflex neurogenic inflammation. The contralateral inflammatory response develops gradually, over a period of hours, and shows no tachyphylaxis with repeated application of the same stimulus. Denervation of either limb significantly attenuated the contralateral responses. Selective lesions of small-diameter, presumed nociceptive afferent fibers with capsaicin, or of sympathetic postganglionic efferents by immunosympathectomy, also reduced swelling and hyperalgesia of the uninjured paw. Interruption of venous circulation to the injured limb by vein ligation did not alter the response in the contralateral paw. Taken together, these data suggest that reflex neurogenic inflammation is neurally mediated, via connections across the spinal cord.

Influence of the method of drug administration on analgesic response

The appropriate control group in studies of placebo-induced analgesia has not been established. A traditional control has been a 'no treatment' or natural-history group. In some studies, the natural-history group receives a hidden infusion of vehicle, a physiologically inactive substance such as saline solution, to eliminate differences in expectation of the outcome on the part of the experimenter. To evaluate whether 'hidden' as well as open infusion of vehicle can elicit a placebo response, we have now tested a different natural-history group, one which received an infusion of vehicle from a syringe pump controlled by a programmable timer. A comparison of these two control groups provides evidence that hidden infusion of vehicle can elicit a placebo response. Use of this new control group also permitted a clear distinction between a naloxone-antagonizable component of placebo analgesia and naloxone antagonism of endorphin-mediated analgesia induced by surgical stress. Our study underscores the power of the placebo and emphasizes that even the most subtle cues can elicit a placebo response.

Intraneuronal substance P contributes to the severity of experimental arthritis

There is evidence that substance P is a peptide neurotransmitter of some unmyelinated primary afferent nociceptors and that its release from the peripheral terminals of primary afferent fibers mediates neurogenic inflammation. The investigators examined whether substance P also contributes to the severity of adjuvant-induced arthritis, an inflammatory disease in rats. They found that, in the rat, joints that developed more severe arthritis (ankles) were more densely innervated by substance P-containing primary afferent neurons than were joints that developed less severe arthritis (knees). Infusion of substance P into the knee increased the severity of arthritis; injection of a substance P receptor antagonist did not. These results suggest a significant physiological difference between joints that develop mild and severe arthritis and indicate that release of intraneuronal substance P in joints contributes to the severity of the arthritis.

Pain--mechanics and management

In the past two decades there has been remarkable progress in understanding the neural mechanisms of pain. However, chronic pain is poorly understood and, by definition, poorly managed. In addition to hyperactivity of the sympathetic nervous system and damage to normal inhibitory mechanisms, social and psychological factors play a major role in producing the disability of chronic pain. New approaches to manage chronic pain include nonopiate drugs, transcutaneous electral nerve stimulation and psychological and behavioral methods. A nervous system network has recently been described that suppresses pain. This analgesic action is mediated by endogenous opioid peptides (endorphins) and by biogenic amines. The analgesia network can be activated either by electral stimulation or by opiates such as morphine or methadone.

Leukotriene B4 produces hyperalgesia that is dependent on polymorphonuclear leukocytes

Leukotriene B4, at the same intracutaneous doses as bradykinin, reduced the nociceptive threshold in the rat paw. The mechanism of leukotriene B4-induced hyperalgesia was distinguished from that of the hyperalgesia elicited by prostaglandin E2 and bradykinin by its dependence on polymorphonuclear leukocytes and independence of the cyclooxygenation of arachidonic acid.