Painful stimulation suppresses joint inflammation by inducing shedding of L-selectin from neutrophils.

Although the inflammatory response is essential for protecting tissues from injury and infection, unrestrained inflammation can cause chronic inflammatory diseases such as arthritis, colitis and asthma. Physiological mechanisms that downregulate inflammation are poorly understood. Potent control might be achieved by regulating early stages in the inflammatory response, such as accumulation of neutrophils at the site of injury, where these cells release chemical mediators that promote inflammatory processes including plasma extravasation, bacteriocide and proteolysis. To access an inflammatory site, neutrophils must first adhere to the vascular endothelium in a process mediated in part by the leukocyte adhesion molecule L-selectin. This adhesion is prevented when L-selectin is shed from the neutrophil membrane. Although shedding of L-selectin is recognized as a potentially important mechanism for regulating neutrophils, its physiological function has not been demonstrated. Shedding of L-selectin may mediate endogenous downregulation of inflammation by limiting neutrophil accumulation at inflammatory sites. Here we show that activation of nociceptive neurons induces shedding of L-selectin from circulating neutrophils in vivo and that this shedding suppresses an ongoing inflammatory response by inhibiting neutrophil accumulation. These findings indicate a previously unknown mechanism for endogenous feedback control of inflammation. Failure of this mechanism could contribute to the etiology of chronic inflammatory disease.

Benzene forms hydrogen bonds with water.

Fully rotationally resolved spectra of three isotopic species of 1:1 clusters of benzene with water (H(2)O, D(2)O, and HDO) were fit to yield moments of inertia that demonstrate unambiguously that water is positioned above the benzene plane in nearly free internal rotation with both hydrogen atoms pointing toward the pi cloud. Ab initio calculations (MP2 level of electron correlation and 6-31 G(**) basis set with basis set superposition error corrections) predict a binding energy D(e) greater, similar 1.78 kilocalories per mole. In both the experimental and theoretical structures, water is situated nearly 1 angstrom within the van der Waals contacts of the monomers, a clear manifestation of hydrogen bond formation in this simple model of aqueous-pi electron interactions.

A novel nociceptor signaling pathway revealed in protein kinase C epsilon mutant mice.

There is great interest in discovering new targets for pain therapy since current methods of analgesia are often only partially successful. Although protein kinase C (PKC) enhances nociceptor function, it is not known which PKC isozymes contribute. Here, we show that epinephrine-induced mechanical and thermal hyperalgesia and acetic acid-associated hyperalgesia are markedly attenuated in PKCepsilon mutant mice, but baseline nociceptive thresholds are normal. Moreover, epinephrine-, carrageenan-, and nerve growth factor- (NGF-) induced hyperalgesia in normal rats, and epinephrine-induced enhancement of tetrodotoxin-resistant Na+ current (TTX-R I(Na)) in cultured rat dorsal root ganglion (DRG) neurons, are inhibited by a PKCepsilon-selective inhibitor peptide. Our findings indicate that PKCepsilon regulates nociceptor function and suggest that PKCepsilon inhibitors could prove useful in the treatment of pain.

Role of interleukin-6 in chronic muscle hyperalgesic priming.

After recovery from acute muscle pain even minor subsequent muscle use can initiate recurrence of the same mechanical hyperalgesia months or years after the initial injury. We have recently developed a model of this chronic latent hyperalgesia in the rat. In this study, we have examined the possibility that interleukin-6 (IL-6), an inflammatory mediator produced during acute muscle inflammation, can mediate the production of this chronic latent hyperalgesic state in which subsequent exposure to inflammatory mediators produces a markedly prolonged mechanical hyperalgesia. We now report that i.m. injection of IL-6 produced mechanical hyperalgesia, lasting several hours, that was prevented by intrathecal injection of antisense to glycoprotein 130 (gp130), an IL-6 receptor subunit. Furthermore, following complete recovery from i.m. IL-6-induced hyperalgesia, i.m. prostaglandin E(2) produced a mechanical hyperalgesia that was remarkably prolonged compared with naïve controls, indicating the presence of chronic latent hyperalgesia. This ability of IL-6 to produce chronic latent hyperalgesia was prevented by intrathecal administration of antisense for gp130. Furthermore, gp130 antisense also prevented chronic latent hyperalgesia produced by i.m. injection of the inflammogen, carrageenan. These results identify a role for IL-6 in acute inflammatory muscle pain and as a potential target against which therapies might be directed to treat chronic muscle pain.

Mechanisms mediating nitroglycerin-induced delayed-onset hyperalgesia in the rat.

Nitroglycerin (glycerol trinitrate, GTN) induces headache in migraineurs, an effect that has been used both diagnostically and in the study of the pathophysiology of this neurovascular pain syndrome. An important feature of this headache is a delay from the administration of GTN to headache onset that, because of GTN's very rapid metabolism, cannot be due to its pharmacokinetic profile. It has recently been suggested that activation of perivascular mast cells, which has been implicated in the pathophysiology of migraine, may contribute to this delay. We reported that hyperalgesia induced by intradermal GTN has a delay to onset of ∼ 30 min in male and ∼ 45 min in female rats. This hyperalgesia was greater in females, was prevented by pretreatment with the anti-migraine drug, sumatriptan, as well as by chronic pretreatment with the mast cell degranulator, compound 48/80. The acute administration of GTN and compound 48/80 both induced hyperalgesia that was prevented by pretreatment with octoxynol-9, which attenuates endothelial function, suggesting that GTN and mast cell-mediated hyperalgesia are endothelial cell-dependent. Furthermore, A-317491, a P2X3 antagonist, which inhibits endothelial cell-dependent hyperalgesia, also prevents GTN and mast cell-mediated hyperalgesia. We conclude that delayed-onset mechanical hyperalgesia induced by GTN is mediated by activation of mast cells, which in turn release mediators that stimulate endothelial cells to release ATP, to act on P2X3, a ligand-gated ion channel, in perivascular nociceptors. A role of the mast and endothelial cell in GTN-induced hyperalgesia suggests potential novel risk factors and targets for the treatment of migraine.

Sex steroid regulation of the inflammatory response: sympathoadrenal dependence in the female rat.

To investigate the role of sex steroids in sex differences in the response of rats to the potent inflammatory mediator bradykinin (BK), we evaluated the effect of sex steroid manipulation on the magnitude of BK-induced synovial plasma extravasation (PE). The magnitude of BK-induced PE is markedly less in females. Ovariectomy of female rats increased BK-induced PE, and administration of 17beta-estradiol to ovariectomized female rats reconstituted the female phenotype. Castration in male rats decreased BK-induced PE, and administration of testosterone or its nonmetabolizable analog dihydrotestosterone reconstituted the male phenotype. The results of these experiments strongly support the role of both male and female sex steroids in sex differences in the inflammatory response. Because the stress axes are sexually dimorphic and are important in the regulation of the inflammatory response, we evaluated the contribution of the hypothalamic-pituitary-adrenal and the sympathoadrenal axes to sex differences in BK-induced PE. Neither hypophysectomy nor inhibition of corticosteroid synthesis affected BK-induced PE in female or male rats. Adrenal denervation in females produced the same magnitude increase in BK-induced PE as adrenalectomy or ovariectomy, suggesting that the adrenal medullary factor(s) in females may account for the female sex steroid effect on BK-induced PE. Furthermore, we have demonstrated that in female but not male rats, estrogen receptor alpha immunoreactivity is present on medullary but not cortical cells in the adrenal gland. These data suggest that regulation of the inflammatory response by female sex steroids is strongly dependent on the sympathoadrenal axis, possibly by its action on estrogen receptors on adrenal medullary cells.

Homocysteine-induced attenuation of vascular endothelium-dependent hyperalgesia in the rat.

We have recently demonstrated a role of the vascular endothelium in peripheral pain mechanism by disrupting endothelial cell function using intravascular administration of octoxynol-9, a non-selective membrane active agent. As an independent test of the role of endothelial cells in pain mechanisms, we evaluated the effect of homocysteine, an agent that damages endothelial cell function. Mechanical stimulus-induced enhancement of endothelin-1 hyperalgesia in the gastrocnemius muscle of the rat was first prevented then enhanced by intravenous administration of homocysteine, but was only inhibited by its precursor, methionine. Both homocysteine and methionine significantly attenuated mechanical hyperalgesia in two models of ergonomic muscle pain, induced by exposure to vibration, and by eccentric exercise, and cutaneous mechanical hyperalgesia in an ischemia-reperfusion injury model of Complex Regional Pain Syndrome type I, all previously shown responsive to octoxynol-9. This study provides independent support for a role of the endothelial cell in pain syndromes thought to have a vascular basis, and suggests that substances that are endothelial cell toxins can enhance vascular pain.

Opioid inhibition of formalin-induced changes in plasma extravasation and local blood flow in rats.

Hindpaw injection of dilute formalin produces brief (Phase 1) and persistent (Phase 2) nociceptive responses in the rat. We recently showed that systemically-administered remifentanil during Phase 1 interacted with peripheral opioid receptors to delay the onset and termination of Phase 2 (Taylor et al., 1997b). To test the hypothesis that opioid inhibition of proinflammatory events during Phase 1 contributed to this delay, we evaluated the effects of remifentanil on the time course of formalin-induced inflammation. We found that formalin increased paw thickness (edema), plasma extravasation and local blood flow within minutes of its injection, i.e. during Phase 1. Each of these responses was blocked during remifentanil administration (30 microg/kg i.v. bolus, followed 90 s later with a 15 microg/kg/min infusion for 13.5 min), indicating that opioids inhibit Phase 1 inflammation. Opioid blockade of the blood flow response could be reversed with a peripherally-acting opioid antagonist, naloxone methiodide, indicating that remifentanil acted upon peripheral opioid receptors. Although the administration of remifentanil during Phase 1 did not reduce the magnitude of inflammatory responses during Phase 2, it did delay the onset and termination of edema during Phase 2. As this corresponds to the effects of remifentanil on nociceptive responses during Phase 2, we suggest that opioid analgesics act upon peripheral sites to inhibit inflammation during Phase 1, leading to a delay in the temporal profile of inflammatory (and likely nociceptive) responses during Phase 2.

Delta- and kappa-opioid agonists inhibit plasma extravasation induced by bradykinin in the knee joint of the rat.

We used an experimental model of neurogenic inflammation, plasma extravasation induced by bradykinin or capsaicin, to study the effect of receptor-selective opioid agonists on plasma extravasation. Plasma extravasation was induced in the knee joint of the rat by continuous perfusion of either bradykinin (160 ng/ml), an inflammatory mediator produced at sites of tissue injury, that produces plasma extravasation significantly dependent on the sympathetic postganglionic neuron, or capsaicin (5 mg/ml), a C-fiber excitotoxin, that induces plasma extravasation that is dependent on both primary afferents and sympathetic post-ganglionic neurons. When selective delta-((d-Pen2,5)-enkephalin) or kappa-(trans-3,4-dichloro-N-methyl-N[2-(- pyrolidinyl)cyclohexyl]benzeneacetamide; U50,488H) opioid agonists were perfused with bradykinin, plasma extravasation was significantly attenuated. Co-perfusion of the non-selective opioid antagonist naloxone (1 microM), reversed this opioid-induced inhibition of bradykinin-induced plasma extravasation. In contrast, co-perfusion of a selective mu-opioid agonist (Tyr-d-Ala-Gly-NMe-Phe-Gly-ol) did not reduce bradykinin-induced plasma extravasation. Tyr-d-Ala-Gly-NMe-Phe-Gly-ol was, however, able to completely inhibit the plasma extravasation produced by capsaicin. These results suggest that delta- and kappa-, but not mu-selective opioids inhibit bradykinin-stimulated plasma extravasation, while a mu-selective opioid inhibits primary afferent-dependent plasma extravasation. Therefore, inhibition of neurogenic plasma extravasation by receptor-selective opioids may depend on the relative contribution to plasma extravasation of unmyelinated afferent and sympathetic postganglionic neuron terminals. Our findings can also explain, in part, the variation in anti-inflammatory effects of receptor-selective opioids reported in different inflammatory conditions.

Comparison of prostaglandin E1- and prostaglandin E2-induced hyperalgesia in the rat.

We have studied prostaglandin E1-induced mechanical hyperalgesia in the rat hindpaw, by assessing paw-withdrawal thresholds, before and after injecting prostaglandin E1 alone or with other agents, in normal and streptozotocin-induced diabetic rats. In normal and diabetic rats, prostaglandin E1 (1-1000 ng) produced a dose-dependent decrease in mechanical nociceptive threshold. In diabetic rats, prostaglandin E1 was more potent than in normal rats, in producing hyperalgesia, whereas prostaglandin E2 hyperalgesia was not changed in normal and diabetic rats. Prostaglandin E1-induced hyperalgesia was not inhibited by E-type 1 prostaglandin receptor antagonists, SC19220 or SC51089, either in normal or diabetic rats. In fact, in the presence of SC19220, prostaglandin E1 produced enhanced hyperalgesia, in normal rats. Prostaglandin E1 hyperalgesia was not significantly modified by sympathectomy or indomethacin. Unlike prostaglandin E2, prostaglandin E1 hyperalgesia was not blocked by the inhibitor of the stimulatory guanine nucleotide-binding regulatory protein, guanosine 5'-O-(2-thiodiphosphate). It is suggested that prostaglandin E1 decreases primary afferent nociceptive threshold directly, by activating a prostaglandin receptor other than the E-type 1 prostaglandin receptor, and that this receptor is not coupled to a stimulatory guanine nucleotide-binding regulatory protein.

Neurogenic and non-neurogenic mechanisms of plasma extravasation in the rat.

We describe two distinct mechanisms for the enhancement of plasma extravasation in the knee joint of the rat. One is activated by bradykinin and is neurogenic; the other is activated by platelet-activating factor and is non-neurogenic. Bradykinin-induced synovial plasma extravasation is known to be dependent on the sympathetic postganglionic neuron terminal, and to involve prostaglandins, ATP, adenosine A2 receptor action, and the attraction and activation of neutrophils. In this study we found that bradykinin-induced plasma extravasation also involves endothelium-derived relaxing factor; specifically we found that bradykinin-induced plasma extravasation was antagonized stereospecifically by the inhibitor of endothelium-derived relaxing factor synthesis, NG-monomethyl-L-arginine. Perfused alone, platelet-activating factor produced an increase in synovial plasma extravasation which was markedly reduced by the platelet-activating factor receptor antagonists BN 52021 and WEB 2086 (these antagonists did not affect bradykinin-induced plasma extravasation). Platelet-activating factor-induced plasma extravasation was not affected by NG-monomethyl-L-arginine, indomethacin (a prostaglandin synthesis inhibitor), phenol 3-(5H-thiozolo[2,3b]quinazolin) (an A2 receptor adenosine antagonist), dextran sulfate (an inhibitor of leukocyte rolling), hydroxyurea (a depletor of leukocytes), chronic sympathectomy or the depletion of unmyelinated afferent fibers. Of note, the magnitude of platelet-activating factor-induced plasma extravasation was increased by co-perfusion with prostaglandin E2 and attenuated by co-perfusion with L-arginine; that is, two of the mediators involved in neurogenic bradykinin-induced plasma extravasation exerted an influence on non-neurogenic plasma extravasation. Separate mechanisms for bradykinin and platelet-activating factor plasma extravasation were further demonstrated in the streptozotocin-treated diabetic rat, in which there is a peripheral neuropathy.(ABSTRACT TRUNCATED AT 250 WORDS)

Modulation of bradykinin-induced plasma extravasation in the rat knee joint by sympathetic co-transmitters.

We describe the contribution of various sympathetic post-ganglionic neuron mediators to bradykinin-induced plasma extravasation in the knee joint of the rat. Co-perfusion of the sympathetic post-ganglionic neuron mediators, norepinephrine or neuropeptide Y with bradykinin resulted in diminished plasma extravasation. In contrast, the putative sympathetic post-ganglionic neuron mediators of bradykinin-induced plasma extravasation, namely prostaglandin E2, ATP, the selective adenosine A2-receptor agonist, CGS21680 or the endothelium-derived relaxing factor (as its precursor L-arginine) all greatly enhanced bradykinin-induced plasma extravasation, but produced little or no increase in plasma extravasation administered alone. The data show that sympathetic post-ganglionic neuron-derived mediators may either inhibit or enhance plasma extravasation induced by bradykinin, and we hypothesize that differential release of mediators from the sympathetic post-ganglionic neuron terminal, in response to varying stimuli, regulates local plasma extravasation during inflammation.

Further substantiation of a significant role for the sympathetic nervous system in inflammation.

This study provides significant new evidence substantiating a role of the postganglionic sympathetic neuron in plasma extravasation in the knee-joint of the rat. Increased plasma extravasation produced by the potent inflammatory mediator bradykinin was mimicked by 6-hydroxydopamine, a selective stimulator of sympathetic fibers. Various treatments (chemical sympathectomy, co-perfusion with the local anesthetic lidocaine, or co-perfusion with depolarizing concentrations of potassium) similarly modulated plasma extravasation induced by both bradykinin and 6-hydroxydopamine, but not that produced by platelet activating factor. We also showed that bradykinin is able to release norepinephrine in the knee-joint, indicating action on the sympathetic postganglionic neuron. In summary, these experiments provide substantial additional evidence supporting a significant contribution of the sympathetic post-ganglionic neuron terminal to inflammatory plasma extravasation.

Sensory neuropeptide interactions in the production of plasma extravasation in the rat.

We used an experimental model of neurogenic inflammation to study the contribution of the primary afferent peptides substance P, calcitonin gene-related peptide, galanin and somatostatin to plasma extravasation in rat synovium. Perfusion of the C-fiber excitotoxin, capsaicin (1.6 mM), through the knee joint of the pentobarbital anesthetized rat, increased plasma extravasation transiently (< 30 min). Perfusion of substance P (1 microM) or calcitonin gene-related peptide (100 nM), two primary afferent neuropeptides that are released by acute capsaicin administration, had no significant effect on plasma extravasation. Co-perfusion of these two neuropeptides, however, evoked an increase in plasma extravasation that was greater than that produced by capsaicin remaining above 250% of the baseline level by the end of the perfusion period (55 min). Capsaicin co-perfused with either galanin (100 nM) or somatostatin (1 microM) failed to increase plasma extravasation. Neither galanin nor somatostatin significantly affected increase in plasma extravasation induced by co-perfusion of substance P plus calcitonin gene-related peptide. Therefore, we suggest that galanin and somatostatin inhibit, presynaptically, the release of substance P and calcitonin gene-related peptide from primary afferent terminals. The interactions among these four neuropeptides provide a novel mechanism for the regulation of primary afferent neurogenic inflammation.

Peripheral nociceptive effects of alpha 2-adrenergic receptor agonists in the rat.

We have previously shown that norepinephrine can produce hyperalgesia via an alpha 2-adrenergic receptor mechanism. The alpha 2-adrenergic receptor agonist clonidine has, however, also been shown to produce peripheral analgesia. In view of the multiple alpha 2-subtypes currently known (i.e. alpha 2A, alpha 2B and alpha 2C), we evaluate the alpha 2-receptor subtypes mediating norepinephrine-induced peripheral hyperalgesia and clonidine analgesia. Norepinephrine and the alpha 2-adrenergic agonists clonidine and UK 14,304 (1-1000 ng), when co-injected with the calcium ionophore A23187 (1000 ng) produced dose-dependent hyperalgesia in the Randall-Selitto paw withdrawal test. Norepinephrine (100 ng) hyperalgesia was dose-dependently antagonized by alpha 2-adrenergic receptor antagonists. From the estimated ID50, the rank order of potency was: SK&F 104856 (alpha 2B) approximately imiloxan (alpha 2B) > rauwolscine (alpha 2C) >> BRL 44408 (alpha 2A). Norepinephrine hyperalgesia was not significantly affected by pertussis-toxin treatment. Prostaglandin E2 (100 ng) hyperalgesia was inhibited dose-dependently, by clonidine and UK 14,304. Rauwolscine was more potent in reversing the inhibitory effect of clonidine on prostaglandin E2 than imiloxan while BRL 44408 was ineffective. The inhibitory effect of clonidine on prostaglandin E2 hyperalgesia was reversed by pertussis toxin. These data suggest that alpha 2B-subtype receptors mediate (norepinephrine hyperalgesia while the antinociceptive effect of alpha 2-agonist is mediated by the alpha 2C-subtype receptor. Differential coupling of these receptor subtypes to second messenger systems and location on different cell types in the rat paw may explain, at least in part, their differential responses to alpha 2-agonist stimulation, leading to hyperalgesia and analgesia.

Is there more than one prostaglandin E receptor subtype mediating hyperalgesia in the rat hindpaw?

Five synthetic prostaglandin E analogs (11-deoxyPGE1, 17-phenyl-ol-trinor prostaglandin E2, enisoprost, MB28767 and misoprostol) have been evaluated for their ability to produce mechanical hyperalgesia in rats. The Randall-Selitto paw withdrawal model of mechanical hyperalgesia was used. Following intradermal injections (2.5 microliters) into the dorsal surface of the hindpaw, each prostaglandin E analog produced a dose-dependent (1-1000 ng) decrease in nociceptive threshold (i.e. hyperalgesia). Hyperalgesia produced by 17-phenyl-ol-trinor prostaglandin E2 and MB28767, was inhibited by the prostaglandin E1 antagonist SC19220 (7.5 ng), while the hyperalgesia produced by 11-deoxyprostaglandin E1, enisoprost and misoprostol was not inhibited by this antagonist. Hyperalgesia produced by all five analogs was significantly attenuated or completely blocked by inhibiting stimulatory guanine nucleotide-binding regulatory protein with guanosine 5'-O-(2-thiodiphosphate), adenylyl cyclase with 2'5'-dideoxyadenosine and protein kinase A with WIPTIDE. These results suggest the presence of more than one prostaglandin E-receptor subtype, which mediate hyperalgesia, predominantly via the cAMP second messenger system, in the hindpaw of the rat.

Neutrophils contribute to sympathetic nerve terminal-dependent plasma extravasation in the knee joint of the rat.

Infusion of bradykinin or 6-hydroxydopamine into the knee joint of the rat activates sympathetic postganglionic nerve terminals and increases plasma extravasation, a major sign of acute inflammation. Since bradykinin attracts and activates neutrophils in vivo and since neutrophils can release factors leading to plasma extravasation, we evaluated the contribution of the neutrophil to bradykinin-induced plasma extravasation. We report that perfusion of bradykinin into the rat knee joint produces a prolonged increase in plasma extravasation which is markedly reduced not only by sympathectomy (chronic pretreatment with systemic 6-hydroxydopamine) but also by depletion of circulating polymorphonuclear leukocytes (intravenous infusion of hydroxyurea combined with intraperitoneal glycogen). Depletion of polymorphonuclear leukocytes also reduced the plasma extravasation induced by intra-articular infusion of 6-hydroxydopamine, which acutely activates sympathetic postganglionic terminals. We next tested whether attraction of neutrophils into the joint, in the absence of bradykinin, was sufficient to enhance plasma extravasation. Although the classical neutrophil attractant glycogen attracted neutrophils into the knee joint, it did not increase plasma extravasation. Co-infusion of bradykinin and glycogen into the knee joint, however, provoked plasma extravasation that was significantly greater than that produced by bradykinin alone. We hypothesize, therefore, that bradykinin not only attracts neutrophils but also activates them, by an as yet undefined mechanism that requires the sympathetic terminal. The activated neutrophils release factors that lead to plasma extravasation. The next series of studies evaluated the role of the sympathetic nervous system in neutrophil attraction in vivo by bradykinin and glycogen. Since quantification of neutrophil attraction was not possible in the knee joint, we performed these studies in the peritoneal cavity, a site where neutrophils are readily attracted.(ABSTRACT TRUNCATED AT 250 WORDS)

Central terminals of nociceptors are targets for nicotine suppression of inflammation.

Spinal intrathecal administration of nicotine inhibits bradykinin-induced plasma extravasation, a component of the inflammatory response, in the knee joint of the rat in a dose-related fashion. Nociceptors contain nicotinic receptors and activation of a nociceptor at its peripheral terminal, by capsaicin, also produces inhibition of inflammation. Therefore the aim of this study was to test the hypothesis that the spinal target for this effect of nicotine is the central terminal of the primary afferent nociceptor. Intrathecal administration of the neurokinin-1 receptor antagonist, (3aR,7aR)-7,7-diphenyl-2-(1-imino-2(2-methoxyphenyl)-ethyl) perhydroisoindol-4-1 hydrochloride or the N-methyl-D-aspartate receptor antagonist, DL-2-amino-5-phosphonovaleric acid, both antagonists of the action of primary afferent neurotransmitters, markedly attenuated the inhibition of bradykinin-induced plasma extravasation produced by both intrathecal nicotine and intraplantar capsaicin.Conversely, intrathecal administration of an alpha-adrenoceptor antagonist, phentolamine or an opioid receptor antagonist, naloxone, to block descending antinociceptive controls, which provide inhibitory input to primary afferent nociceptors, enhanced the action of both nicotine and capsaicin. These findings support the hypothesis that the central terminal of the primary afferent nociceptor is a CNS target at which nicotine acts to inhibit inflammation.

Fasting is a physiological stimulus of vagus-mediated enhancement of nociception in the female rat.

The vagus nerve modulates nociception by a mechanism dependent upon gonadal hormones and the adrenal medulla. In the present study we tested the hypothesis that this modulation is dynamically controlled by physiological stimulation of structures innervated by the subdiaphragmatic vagus. Specifically, food deprivation (fasting) was employed to increase activity in the subdiaphragmatic vagus, and the experiments were performed mainly in female rats because our previous observations suggested that baseline activity in the pathway is lower in females than in males. Consistent with the hypothesis, after a 48-h fast, female rats exhibited increased nociceptive behavior in the formalin test. In contrast, fasting had no effect on formalin-evoked nociceptive behavior in male rats. The fasting-induced effect on nociception appears to be mediated by the vagus nerve since it is prevented by subdiaphragmatic vagotomy. Also similar to the previously characterized vagus-mediated modulation, the effect of fasting in the female is blocked by gonadectomy or adrenal medullectomy, and hormone replacement with 17beta-estradiol in gonadectomized female rats restored the effect of fasting. Decreased glucose metabolism apparently does not play a significant role in the effect of fasting on nociception, since the effect was unchanged when 5% glucose was provided in the drinking water throughout the fasting period. On the other hand, increasing the bulk content of the stomach (without providing nutrients) by infusion of petrolatum significantly attenuated the effect of fasting during the interphase period of the formalin response, suggesting that decreased gut distention, and possibly motility, are important in fasting-induced enhancement of nociception. These results indicate that fasting is a physiological activator of the vagus-mediated pain modulation pathway. This suggests the possibility that, especially in females, natural periodic changes in gut distention and motility may control an ongoing vagus-mediated adjustment in the organism's nociceptive sensitivity.

Mu-opioid agonist enhancement of prostaglandin-induced hyperalgesia in the rat: a G-protein beta gamma subunit-mediated effect?

Guanine nucleotide-binding regulatory protein stimulation of adenylyl cyclase has been shown to be an important second messenger system for many processes, including mechanical hyperalgesia. Recently, interactions between guanine nucleotide-binding regulatory protein subunits and adenylyl cyclase affecting the level of cyclic adenosine 3',5'-monophosphate accumulation have been demonstrated. In this study we evaluated such an interaction by measuring paw-withdrawal thresholds to mechanical stimuli in Sprague-Dawley rats in the presence of two direct-acting hyperalgesic agents, prostaglandin E2 and the adenosine A2-agonist, CGS21680. The effects of two agents expected to liberate inhibitory guanine nucleotide-binding regulatory protein subunits were also studied: [D-Ala2,N-Me-Phe4,Gly5-ol]enkephalin (a mu-opioid receptor agonist) and N6-cyclopentyladenosine (an A1-adenosine agonist). Injection of [D-Ala2,N-Me-Phe4,Gly5-ol]enkephalin immediately before prostaglandin E2 or CGS21680 significantly attenuated the hyperalgesia subsequently induced by these agents, i.e. the sensitivity to these hyperalgesic agents was decreased. On the other hand, injection of [D-Ala2,N-Me-Phe4,Gly5-ol]enkephalin 5 min after prostaglandin E2 or CGS21680 significantly enhanced the hyperalgesia observed. Injection of the adenosine A1-agonist N6-cyclopentyladenosine immediately before and 5 min after prostaglandin E2 or CGS21680 had a similar effect to [D-Ala2,N-Me-Phe4,Gly5-ol]enkephalin. The decrease in sensitivity to prostaglandin E2- and CGS21680-induced hyperalgesia by preadministration of [D-Ala2,N-Me-Phe4,Gly5-ol]enkephalin or N6-cyclopentyladenosine and the enhancement by postadministration were all reversed by pertussis toxin, an inhibitor of inhibitory guanine nucleotide-binding regulatory protein, suggesting the involvement of an inhibitory guanine nucleotide-binding regulatory protein.(ABSTRACT TRUNCATED AT 250 WORDS)