Sympathetic-dependence in bradykinin-induced synovial plasma extravasation is dose-related

From neuroimunomodulation to bioelectronic treatment of rheumatoid arthritis

Neuronal stimulation is an emerging field in modern medicine to control organ function and reestablish physiological homeostasis during illness. The nervous system innervates most of the peripheral organs and provides a fine tune to control the immune system. Most of these studies have focused on vagus nerve stimulation and the physiological, cellular and molecular mechanisms regulating the immune system. Here, we review the new results revealing afferent vagal signaling pathways, immunomodulatory brain structures, spinal cord-dependent circuits, neural and non-neural cholinergic/catecholaminergic signals and their respective receptors contributing to neuromodulation of inflammation in rheumatoid arthritis. These new neuromodulatory networks and structures will allow the design of innovative bioelectronic or pharmacological approaches for safer and low-cost treatment of arthritis and related inflammatory disorders.

Effects on hemodynamic variables and echocardiographic parameters after a stellate ganglion block in 15 healthy volunteers

BACKGROUND

The sympathetic nervous system has an important role in generating pain. Various pathomechanisms are involved that respond well to the application of local anesthetics (LA), for example to the stellate ganglion block (SGB).

OBJECTIVES

We wanted to know more about the effects of SGB on cardiovascular parameters.

METHODS

We included 15 healthy volunteers; another 15 healthy volunteers as a control group (sham injection of LA). In order to produce a more precise SGB, we employed only a small volume of LA (3mL), a LA with a lower permeability (procaine 1%), and a modified injection technique. Systolic and diastolic blood pressure (SBP, DBP), heart rate (HR), and echocardiographic parameters were recorded before and after SGB. We also investigated whether there are side differences (left and right SBG).

RESULTS

At baseline all parameters were within the normal range. After performing right and left SGB DBP significantly increased (on the right side from 68.73±8.61 to 73.53±11.10, p=0.015; on the left side from 70.66±13.01 to 77.93±10.40, p=0.003). In the control group no increase in DBP was observed. No side-specific differences were found, except a significant reduction in the maximum velocity of myocardial contraction during the systole with left-sided SGB.

CONCLUSIONS

Even with our methods we could not prevent the simultaneous occurrence of a partial parasympatholytic effect. For this reason, the SGB has only minor hemodynamic effects, which is desirable as it enhances the safety of the SGB.

Acute inflammation in the joint: its control by the sympathetic nervous system and by neuroendocrine systems

Inflammation of tissues is under neural control involving neuroendocrine, sympathetic and central nervous systems. Here we used the acute experimental inflammatory model of bradykinin-induced plasma extravasation (BK-induced PE) of the rat knee joint to investigate the neural and neuroendocrine components controlling this inflammation. 1. BK-induced PE is largely dependent on the sympathetic innervation of the synovium, but not on activity in these neurons and not on release of norepinephrine. 2. BK-induced PE is under the control of the hypothalamo-pituitary-adrenal (HPA) system and the sympatho-adrenal (SA) system, activation of both leading to depression of BK-induced PE. The inhibitory effect of the HPA system is mediated by corticosterone and dependent on the sympathetic innervation of the synovium. The inhibitory effect of the SA system is mediated by epinephrine and β2-adrenoceptors. 3. BK-induced PE is inhibited during noxious stimulation of somatic or visceral tissues and is mediated by the neuroendocrine systems. The nociceptive-neuroendocrine reflex circuits are (for the SA system) spinal and spino-bulbo-spinal. 4. The nociceptive-neuroendocrine reflex circuits controlling BK-induced PE are under powerful inhibitory control of vagal afferent neurons innervating the defense line (connected to the gut-associated lymphoid tissue) of the gastrointestinal tract. This inhibitory link between the visceral defense line and the central mechanisms controlling inflammatory mechanisms in body tissues serves to co-ordinate protective defensive mechanisms of the body. 5. The circuits of the nociceptive-neuroendocrine reflexes are under control of the forebrain. In this way, the defensive mechanisms of inflammation in the body are co-ordinated, optimized, terminated as appropriate, and adapted to the behavior of the organism.

Function of the sympathetic supply in acute and chronic experimental joint inflammation

Joints are densely innervated by postganglionic sympathetic nerve fibers. These fibers control the blood flow in the joint and vascular permeability, either directly or indirectly, in cooperation with leukocytes. Chemical sympathectomy or suppression of adrenergic signaling significantly reduces inflammatory processes in the initial acute state of inflammation whereas the same procedures may increase inflammation at later stages. These findings indicate that the sympathetic nervous system supports the development of inflammation but may reduce inflammation at more chronic stages. During chronic inflammation the density of sympathetic nerve fibers in synovial tissue is reduced but other tyrosine hydroxylase-positive cells secreting noradrenaline appear in the inflamed joint. In addition to local vascular effects in the joint, the sympathetic nervous system influences numerous immune processes in the joint and in lymphoid organs. Hence the net effect of the sympathetic nervous system on inflamed tissue results from local sympathetic effects in the joint as well as from sympathetic influences on major systemic immune processes.

Sympathectomy decreases size and invasiveness of tongue cancer in rats

The sympathetic nervous system plays a role in carcinogenesis wherein locally released sympathetic neurotransmitters affect proliferation, angiogenesis, vessel permeability, lymphocyte traffic and cytokine production. The present in vivo study was designed to investigate whether surgical sympathectomy, both unilateral and bilateral, had an effect on tumor growth, interstitial fluid pressure (IFP) and lymphatics in rat tongue cancer. We used 4-nitroquinoline-1-oxide (4-NQO) in drinking water for 19 weeks to induce tongue cancer in 20 Dark Agouti rats. After 11 weeks, one group underwent unilateral sympathectomy and another underwent bilateral sympathectomy, while the third group underwent sham surgery. By 19 weeks, tumors in the bilaterally sympathectomized (BL-SCGx) rats were significantly smaller (P<0.05), more diffuse in appearance and less invasive (P<0.05) compared with the large exophytic tumors in the sham-operated rats. The relative lymphatic area was significantly decreased (P<0.05) in tumors in the BL-SCGx rats compared with the sham group. Interestingly, the tumors in rats that underwent unilateral or bilateral sympathectomy had a significantly lower (P<0.05) IFP than those in sham rats. Lack of tyrosine hydroxylase (TH) immunoreactive nerves and few neuropeptide Y (NPY) positive fibers indicate absence of sympathetic nerve fibers in the bilateral sympathectomized group. The peritumoral lymph vessel area was correlated with the tumor size (P<0.001), depth of invasion (P<0.001), weight of rats (P<0.005) and IFP (P<0.05). In conclusion, the present study presents evidence that deprivation of sympathetic nerves decreases tumor growth in rat tongue, probably caused by decreasing IFP and lymph vessel area.

Complex regional pain syndrome

Complex regional pain syndrome (CRPS) may develop after limb trauma and is characterized by pain, sensory-motor and autonomic symptoms. Most important for the understanding of the pathophysiology of CRPS are recent results of neurophysiological research. Major mechanism for CRPS symptoms, which might be present subsequently or in parallel during the course of CRPS, are trauma-related cytokine release, exaggerated neurogenic inflammation, sympathetically maintained pain and cortical reorganisation in response to chronic pain (neuroplasticity). The recognition of these mechanisms in individual CRPS patients is the prerequisite for a mechanism-oriented treatment.

Complex regional pain syndrome: mystery explained?

Complex regional pain syndrome (CRPS) is the result of changes to the somatosensory systems that process noxious, tactile, and thermal information; to the sympathetic systems that innervate skin (blood vessels, sweat glands); and to the somatomotor systems. The changes suggest that the CNS representations of the systems have been altered. Patients with CRPS also have peripheral changes (eg, oedema, signs of inflammation, sympathetic-afferent coupling [the basis for sympathetically maintained pain], and trophic changes) that cannot be explained by central changes. On the basis of clinical observation and research in human beings and animals, we hypothesise that CRPS is a systemic disease involving the CNS and peripheral nervous system. The most important question for future research is what causes CRPS? In this article, we suggest a change to the focus of research efforts and treatment. We also suggest there be diagnostic reclassification and redefinition of CRPS.

Blockade of nociceptive inhibition of plasma extravasation by opioid stimulation of the periaqueductal gray and its interaction with vagus-induced inhibition in the rat

We have previously shown that stimulation of cutaneous or visceral nociceptors suppresses inflammation measured as bradykinin-induced synovial plasma extravasation in the knee joint of the rat. This suppression occurs through the activation of a spinal as well as a supraspinal reflex pathway leading to activation of the adrenal medullae and probably the release of epinephrine. These nociceptive-neuroendocrine reflex pathways are tonically inhibited by activity in abdominal vagal afferents acting through an inhibitory descending pathway projecting through the dorsolateral funiculus (DLF) ipsilateral to the cutaneous afferent nociceptive input. Here we investigated whether the descending inhibitory pathway acted upon by vagal afferents is also modulated by the periaqueductal gray (PAG), similar to other bulbo-spinal pathways acting on spinal nociceptive transmission. Injection of morphine sulfate (10 nmol) in the ventrolateral PAG significantly inhibited the nociceptive-neuroendocrine reflex pathways, an effect that was significantly less after removal of vagal afferents (i.e. after release from tonic inhibition maintained by vagal afferents). Interruption of the DLF ipsilateral to the nociceptive input removed the inhibitory effect of vagal afferents and partly reduced the inhibition produced by morphine injected in the PAG. From these investigations we conclude that PAG-induced inhibition of the nociceptive-neuroendocrine reflex pathways is mediated through the DLF ipsilateral to the nociceptive input, involving the same descending inhibitory pathway that relays afferent vagal inhibition, and through other spinal and possibly supraspinal pathways.

Sympathetic modulation of acute cutaneous flare induced by intradermal injection of capsaicin in anesthetized rats

Much of the acute cutaneous neurogenic inflammation after intradermal injection of capsaicin (CAP) in rats is mediated by dorsal root reflexes (DRRs), which cause the release of inflammatory agents from primary afferent terminals. Sympathetic efferents modulate neurogenic inflammation by interaction with primary afferent terminals. In this study, we examined if DRR-mediated flare after CAP injection is subject to sympathetic modulation. Changes in cutaneous blood flow on the plantar surface of the foot were measured using a laser Doppler flow meter. After CAP injection, cutaneous flare spread more than 20 mm away from the site of CAP injection. However, this CAP-induced flare was significantly reduced after surgical sympathectomy. Decentralization of postganglionic neurons did not affect the flare induced by CAP injection. If the foot of sympathectomized rats was pretreated with an alpha(1)-adrenoceptor agonist (phenylephrine) by intra-arterial injection, the spread of flare induced by CAP injection could be restored. However, if the spinal cord was pretreated with a GABA(A) receptor antagonist, bicuculline, to prevent DRRs, phenylephrine no longer restored the CAP-evoked flare. An alpha(2)-adrenoceptor agonist (UK14,304) did not affect the CAP-evoked flare in sympathectomized rats. In sympathetically intact rats, blockade of peripheral alpha(1)-adrenoceptors with terazosin profoundly reduced the flare induced by CAP injection, whereas blockade of peripheral alpha(2)-adrenoceptors by yohimbine did not obviously affect the flare. Therefore the pathogenesis of acute neurogenic inflammation in the intradermal CAP injection model depends in part on intact sympathetic efferents and alpha(1)-adrenoceptors. Peripheral alpha(1)-adrenoceptors thus modulate the ability of capsaicin sensitive afferents to evoke the release of inflammatory agents from primary afferents by DRRs.

The effects of sympathectomy on capsaicin-evoked fos expression of spinal dorsal horn GABAergic neurons

Electrophysiological studies have suggested that activity of spinal GABAergic interneurons can be enhanced following intradermal injection of capsaicin (CAP). This activity is proposed to be involved in the generation of dorsal root reflexes (DRRs) that contribute to neurogenic inflammation. We have recently reported that NMDA or non-NMDA antagonists by intrathecal pretreatment attenuate the increased Fos expression in spinal dorsal horn GABAergic neurons after intradermal injection of CAP in rats. Sympathetic efferents have been suggested to modulate inflammatory pain possibly by interactions with primary afferent terminals. In electrophysiological studies by our group, enhancement of the CAP-induced DRRs could be prevented by surgical sympathectomy and blocked by intraarterial pretreatment of the foot with alpha(1)- but not by alpha(2)-adrenoceptor antagonists. In order to determine morphologically if surgical sympathectomy changes the expression of Fos in GABAergic neurons in the lumbosacral spinal cord induced by CAP injection, further experiments were performed using immunofluorescence double-labeling staining at 30 min following CAP or vehicle injection into the glabrous skin of one hind paw of anesthetized rats both in sham-operated and sympathectomized animals. Our results showed that the proportion of Fos-positive GABAergic neuronal profiles was significantly increased following CAP injection (48.8+/-4.76%) compared to vehicle injection (23.8+/-5.1%) in laminae I-V on the ipsilateral side (P<0.05). However, when sympathetic efferents were removed surgically 7-10 days prior to the experiment (n=6), only 32.07+/-9.03% of GABA-immunoreactive neuronal profiles were stained for Fos following CAP injection, a significant reduction in the CAP-evoked Fos-staining of GABAergic neurons after surgical sympathectomy. These findings support our previous electrophysiological studies that GABAergic neurons take part in nociceptive processing within the spinal dorsal horn and suggest that sympathetic efferents may affect nociceptive transduction in the periphery.

Spino-bulbo-spinal pathway mediating vagal modulation of nociceptive-neuroendocrine control of inflammation in the rat

Stimulation of nociceptors by intradermal capsaicin produces depression of bradykinin (BK)-induced synovial plasma extravasation (PE) that is markedly enhanced by subdiaphragmatic vagotomy. This depression is mediated by the adrenal medullae, a propriospinal pathway between the afferent nociceptive input and preganglionic neurones projecting to the adrenal medullae, and a spino-bulbo-spinal pathway. Here we investigated the role of spinal ascending and descending pathways in the interaction between noxious and vagal afferent inputs, leading to inhibition of BK-induced PE mediated by the adrenal medullae. Nociceptors in the paw were activated by capsaicin and depression of BK-induced PE was measured in rats with intact or cut subdiaphragmatic vagus nerves. After cutting the dorsolateral funiculus (DLF) contralateral to the stimulated hindpaw (segmental level C5/C6 and T8/T9), depression of BK-induced PE was weak or absent both in rats with intact vagus nerves and in vagotomised rats, suggesting that an ascending excitatory pathway was interrupted. After cutting the DLF ipsilateral to the stimulated hindpaw, depression of BK-induced PE was already markedly enhanced, even in the absence of vagotomy. Ipsilateral DLF lesion (L2/L3) below the level of the spinal output to the adrenal medullae produced the same effect, suggesting interruption of a descending inhibitory pathway that relays the effect of vagal activity to the level of the capsaicin-induced nociceptive input. Contralateral and ipsilateral hemisection of the spinal cord (C5/C6) produced the same changes as the corresponding DLF lesions. Ipsi- or contralateral lesion of the dorsal funiculus at the spinal level T8/T9 had no effect on depression of BK-induced PE generated by cutaneous noxious stimulation of the forepaw. We suggest that noxious stimulation activates an ascending pathway of the spino-bulbo-spinal excitatory circuit which projects through the DLF contralateral to the nociceptive input, and that the inhibitory pathway which is activated by vagal afferent activity projects through the DLF ipsilateral to the nociceptive input.

Nociceptive neuroendocrine negative feedback control of neurogenic inflammation activated by capsaicin in the rat paw: role of the adrenal medulla

Recently we have found that inhibition of bradykinin-induced synovial plasma extravasation by transcutaneous electrical stimulation at strengths which excite unmyelinated afferent axons is mediated by the hypothalamo-pituitary-adrenal axis. Here we tested whether stimulation of nociceptors in the rat paw by intradermally injected capsaicin inhibits bradykinin-induced synovial plasma extravasation and whether this inhibition is mediated by the hypothalamo-pituitary-adrenal or sympatho-adrenal axis. Furthermore, we tested whether inhibition of bradykinin-induced plasma extravasation generated by intraperitoneally injected capsaicin, which preferentially excites visceral afferents, is mediated by the hypothalamo-pituitary-adrenal or sympatho-adrenal axis. We used normal rats, subdiaphragmatically vagotomized rats, rats with denervated adrenal medullae and rats with acutely transected spinal cords at the segmental levels T1/T2 or T12/L1. Injection of capsaicin into the plantar or palmar surface of the paws produced a depression of bradykinin-induced plasma extravasation. The inhibition elicited from the forepaw was larger than that from the hindpaw. The inhibition of bradykinin-induced plasma extravasation elicited from both paws was potentiated by subdiaphragmatic vagotomy. Denervation of the adrenal medullae abolished the inhibitory effect of intradermal capsaicin in vagus-intact and in vagotomized animals. After spinalization at the segmental level T1/T2, capsaicin injected into the forepaw did not depress bradykinin-induced plasma extravasation either in vagus-intact or in vagotomized animals. Capsaicin injected into the hindpaw in these spinalized animals produced a small depression. After spinalization at the segmental level T12/L1 no depression was produced by capsaicin injected into the hindpaw. Depression of bradykinin-induced plasma extravasation generated by intraperitoneal injection of capsaicin in vagus-intact and in vagotomized animals was also abolished or attenuated after denervation of the adrenal medullae. This shows that this depression was also largely dependent on the activation of the sympatho-adrenal system. We conclude that depression of bradykinin-induced plasma extravasation during stimulation of nociceptors by capsaicin is mediated predominantly by the sympathoadrenal pathway. This finding differs from the inhibitory mechanism of depression of bradykinin-induced plasma extravasation generated by cutaneous electrical stimulation, which is mediated by the hypothalamo-pituitary-adrenal axis.

The role of vagal visceral afferents in the control of nociception

We have shown that activity in subdiaphragmatic vagal afferents modulates mechanical hyperalgesic behavior in the rat. Subdiaphragmatic vagotomy decreases paw-withdrawal threshold to mechanical stimulation (baseline and after intradermal injection of bradykinin), thus enhancing mechanical hyperalgesic behavior. Most of this decrease is generated by an endocrine signal released by the adrenal medullae because denervation or removal of the adrenal medullae prevents or reverses these changes. This novel mechanism may imply that: (a) the brain is able to regulate sensitivity of nociceptors all over the body by a neuroendocrine mechanisms, (b) sensitivity of nociceptors can be influenced by changes in parts of the body which are remote from the location of the sensitized nociceptors and (c) circulating catecholamines can influence nociceptors in a way which is different from those reported so far (see Jänig and McLachlan, 1994; Jänig, 1996a; Jänig et al., 1996).

Dorsal root reflexes and cutaneous neurogenic inflammation after intradermal injection of capsaicin in rats

The role of dorsal root reflexes (DRRs) in acute cutaneous neurogenic inflammation induced by intradermal injection of capsaicin (CAP) was examined in anesthetized rats. Changes in cutaneous blood flow (flare) on the plantar surface of the foot were measured using a laser Doppler flowmeter, and neurogenic edema was examined by measurements of paw thickness. To implicate DRRs in neurogenic inflammation after CAP injection, the ipsilateral sciatic and femoral nerves were sectioned, dorsal rhizotomies were performed at L(3-)-S(1), and antagonists of GABA or excitatory amino acid receptors were administered intrathecally. Intradermal injection of CAP evoked a flare response that was largest at 15-20 mm from the injection site and that spread >30 mm. Acute transection of the sciatic and femoral nerves or dorsal rhizotomies nearly completely abolished the blood flow changes 15-20 mm from the CAP injection site, although there was only a minimal effect on blood flow near the injection site. These procedures also significantly reduced neurogenic edema. Intrathecal bicuculline, 6-cyano-7-nitroquinoxaline-2,3-dione, (CNQX) or D(-)-2-amino-7-phosphonoheptanoic acid (AP7), but not phaclofen, also reduced dramatically the increases in blood flow 15-20 mm from the CAP injection site, but had only a minimal effect on blood flow near the injection site. Neurogenic edema was reduced by the same agents that reduced blood flow. Multiunit DRRs recorded from the central stumps of cut dorsal rootlets in the lumbar spinal cord were enhanced after CAP injection. This enhanced DRR activity could be reduced significantly by posttreatment of the spinal cord with bicuculline, CNQX or AP7, but not phaclofen. It is concluded that peripheral cutaneous inflammation induced by intradermal injection of CAP involves central nervous mechanisms. DRRs play a major role in the development of neurogenic cutaneous inflammation, although a direct action of CAP on peripheral nerve terminals or the generation of axon reflexes also may contribute to changes in the skin near the injection site.

Endocrine and vagal controls of sympathetically dependent neurogenic inflammation

Recently the very significant role of the postganglionic sympathetic neuron (PGSN) terminal in the production of neurogenic inflammation has been appreciated. An important model of this sympathetically dependent inflammation is venular plasma extravasation (PE) and neutrophil attraction produced by local intra-articular injection of the potent inflammatory mediator bradykinin (BK). Sympathetic-dependent PE in the synovium has been proposed as a protective mechanism in arthritis. In a recent series of studies, a novel mechanism has been discovered by which activation of primary afferent nociceptors exerts a potent feedback inhibition of PGSN-dependent PE. Activation of nociceptive afferents was shown to be involved in this feedback system. Such a negative feedback control of the acute inflammatory response would have survival value; the inflammatory response, as initiated by a high degree of positive feedback, and the inflammatory process itself when persisting can result in significant tissue injury. If indeed HPA axis activity plays a significant physiological role in the modulation of neurogenic inflammation, then physiological processes that modulate the HPA axis would be expected to influence neurogenic inflammation. A dramatic effect of this kind has been demonstrated, in the rat, for vagal afferent activity. In the presence of subdiaphragmatic (or celiac branch) vagotomy, the potency of nociceptive afferent activity to inhibit sympathetically dependent, BK-induced PE was increased by four orders of magnitude compared to vagus-intact animal. Hypoactivity or hyperactivity of these vagally mediated mechanisms could contribute to diseases characterized by either an inadequate or an exaggerated inflammatory response.

Modulation of bradykinin-induced mechanical hyperalgesia in the rat by activity in abdominal vagal afferents

Bradykinin-induced plasma extravasation and mechanical hyperalgesia are sympathetic-dependent components of inflammation. Noxious stimulation has been found to inhibit bradykinin-induced plasma extravasation by activating the hypothalamo-pituitary-adrenal axis. The sensitivity of this nociceptive-neuroendocrine feedback control of inflammation is modulated by activity in subdiaphragmatic vagal afferents. In the present study, we tested the hypothesis that activity in the subdiaphragmatic vagus also modifies bradykinin-induced mechanical hyperalgesia in the rat, using the Randall-Selitto method. Following subdiaphragmatic vagotomy, the baseline paw-withdrawal threshold to mechanical stimulation decreased and bradykinin-induced mechanical hyperalgesia was enhanced. Mechanical hyperalgesia produced by prostaglandin E2, a direct-acting hyperalgesic agent, was not significantly affected by vagotomy. The effect of subdiaphragmatic vagotomy on bradykinin-induced hyperalgesia, but not on baseline paw-withdrawal threshold, was mimicked by coeliac branch vagotomy. Indomethacin blocked the hyperalgesia in normal rats, but not in vagotomized rats, suggesting that bradykinin-induced hyperalgesia in normal rats is mediated by prostaglandins, whose role was unexpectedly diminished after vagotomy. Bradykinin-induced hyperalgesia in normal rats was abolished by lumbar sympathectomy but not by sympathetic decentralization (cutting the preganglionic axons). In rats that were both vagotomized and sympathectomized, hyperalgesia induced by low-dose bradykinin was no longer present. These results demonstrate that vagotomy induces a decrease in baseline mechanical paw-withdrawal threshold and an enhancement of bradykinin-induced mechanical hyperalgesia and suggest that these phenomena are generated by actions in peripheral tissues.

Inhibition of bradykinin-induced plasma extravasation produced by noxious cutaneous and visceral stimuli and its modulation by vagal activity

Intrathecally applied nicotine reduces bradykinin-induced plasma extravasation (BK-induced PE) in the rat knee joint. This depression is mediated by the hypothalamo-pituitary-adrenal (HPA) axis and is enhanced by interruption of impulse traffic in afferents of the abdominal vagus nerve. Like intrathecal nicotine, electrical stimulation of unmyelinated cutaneous fibers also depresses BK-induced PE, which is also dependent on an intact HPA axis. In this study, we investigated whether the inhibitory effect of intrathecal nicotine can be mimicked by noxious stimulation of skin and of viscera. Furthermore we determined whether this depression is potentiated after subdiaphragmatic vagotomy. Stimulation of visceral afferents in the peritoneum, by intraperitoneal capsaicin injection, dose-dependently decreased BK-induced PE. The capsaicin dose-response function was shifted by 1.5-2 orders of magnitude to the left after vagotomy. Stimulation of visceral afferents in the urinary bladder by capsaicin also dose-dependently reduced BK-induced PE, which similarly was potentiated after vagotomy. Transcutaneous stimulation of unmyelinated nociceptive afferents from the plantar skin of the paw depressed BK-induced PE. This depression had a threshold of approximately 0.25 Hz and was maximal at a stimulation frequency of approximately 1 Hz. After subdiaphragmatic vagotomy, the stimulus response function shifted to the left and the inhibition was significantly larger than in control, in the range of 0.125-1 Hz stimulation. These results show that noxious stimulation of skin and viscera depressed BK-induced PE and that such depression was potentiated after subdiaphragmatic vagotomy in a manner similar to that of intrathecally applied nicotine. Based on these observations, we hypothesize that intrathecal nicotine depresses BK-induced PE by exciting spinal nociceptive neurons or the central projections of nociceptive primary afferent neurons.

Negative feedback neuroendocrine control of inflammatory response in the rat is dependent on the sympathetic postganglionic neuron

Negative feedback control of inflammation is mediated by activation of nociceptive afferents that in turn activates the hypothalamic-pituitary-adrenal axis to release corticosteroids. Plasma extravasation (PE) produced by the potent inflammatory mediator, bradykinin (BK), but not that induced by another potent inflammatory mediator, platelet-activating factor (PAF), is inhibited by released corticosterone. Because bradykinin, but not PAF, produces PE by a mechanism that is, in part, dependent on the sympathetic postganglionic neuron (SPGN) terminal, we tested the hypothesis that the negative feedback control of inflammation is dependent on the SPGN terminal in the inflamed tissue. In sympathectomized rats, the residual (i.e., SPGN-independent) PE in the knee joint produced by BK was not inhibited by noxious electrical stimulation. Furthermore, intravenous administration of corticosterone potently inhibited, with a similar time-course, the SPGN-dependent, but not the SPGN-independent, component of BK-induced PE. Neither electrical stimulation nor corticosterone inhibited PAF-induced PE. Finally, corticosterone's actions do not appear to be mediated by release of norepinephrine from the SPGN terminal, because neither the alpha-adrenergic receptor antagonist phentolamine nor the beta2-adrenergic receptor antagonist ICI 118, 551 antagonized the inhibition of BK-induced PE by corticosterone. We conclude that in the rat knee joint, negative feedback control of the inflammatory response is dependent on the presence of the SPGN terminal. Further, our data suggest that a significant component of corticosteroid-induced inhibition of PE produced by inflammatory mediators is SPGN-dependent.

Vagal branches involved in inhibition of bradykinin-induced synovial plasma extravasation by intrathecal nicotine and noxious stimulation in the rat

1. Stimulation of cutaneous and spinal visceral nociceptive afferents and intrathecal nicotine reduces bradykinin-induced plasma extravasation (BK-induced PE) in the knee joint of the rat. This depression is mediated by the hypothalamo-pituitary-adrenal (HPA) axis and is potentiated by subdiaphragmatic vagotomy. It is believed that activity in vagal afferents tonically inhibits ascending impulse transmission in the neuraxis projecting to the hypothalamus. Vagotomy, by removing such inhibition, allows greater depression of BK-induced PE. In this study we determined whether the vagal afferents which negatively regulate activities of the HPA axis are present in all branches of the abdominal vagus nerves or only in specific branches. 2. We measured the depression of BK-induced PE elicited by graded stimulation of spinal visceral afferents with intraperitoneal capsaicin and by intrathecal nicotine in vagus-intact rats and in rats in which specific vagal branches were selectively interrupted. (i) Interruption of the coeliac branches mimicked the effect of total subdiaphragmatic vagotomy in potentiating the depression of BK-induced PE generated by intrathecal nicotine and by stimulation of spinal visceral afferents. (ii) Interruption of the gastric and hepatic branches of the abdominal vagus nerves together or individually did not affect the depression of BK-induced PE generated by the two stimuli. 3. These results indicate that afferent activity in coeliac and accessory coeliac vagal branches is involved in the regulation of the nociceptive system-initiated depression of BK-induced PE. The afferent fibres in these vagal branches involved probably monitor physiological events in abdominal visceral organs.

Contribution of adrenal hormones to nicotine-induced inhibition of synovial plasma extravasation in the rat

1. In this study, we examined the mechanism(s) by which s.c. nicotine inhibits synovial plasma extravasation. We found that nicotine dose-dependently inhibited bradykinin (BK)- and platelet activating factor (PAF)-induced plasma extravasation. 2. The effect of nicotine on both BK- and PAF-induced plasma extravasation was attenuated by adrenal medullectomy. ICI-118,551 (a selective beta 2-adrenoceptor blocker) (30 micrograms ml-1, intra-articularly) significantly attenuated the inhibitory action of high-dose (1 mg kg-1) nicotine on BK-induced plasma extravasation without affecting the inhibition by low- (0.01 microgram kg-1) dose nicotine or that on PAF-induced plasma extravasation by nicotine at any dose. This suggested that beta 2-adrenoceptors mediate the inhibitory actions of high-dose, but not low-dose, nicotine. We also found that systemic naloxone (an opioid receptor antagonist) (two hourly injections of 1 mg kg-1, i.p.) attenuated the inhibitory action produced by all doses of nicotine on BK- or PAF-induced plasma extravasation, suggesting the contribution of endogenous opioids. 3. RU-38,486 (a glucocorticoid receptor antagonist) (30 mg kg-1, s.c.), and metyrapone (a glucocorticoid synthesis inhibitor) (two hourly injections of 100 mg kg-1, i.p.) both attenuated the action of high-dose nicotine without affecting that of low-dose nicotine. 4. Spinal mecamylamine (a nicotinic receptor antagonist) (0.025 mg kg-1, intrathecally, i.t.) attenuated the action of high-dose, but not low-dose, nicotine, suggesting that part of the action of high-dose nicotine is mediated by spinal nicotinic receptors. 5. Combined treatment with ICI-118,551, naloxone and RU-38,486 attenuated the action of low-dose nicotine by an amount similar to that produced by naloxone alone but produced significantly greater attenuation of the effect of high-dose nicotine when compared to the action of any of the three antagonists alone.

Tachyphylaxis develops to bradykinin-induced plasma extravasation in the rat

Bradykinin, an inflammatory mediator produced from plasma kallikreins, has potent effects on vascular functions, including increasing plasma extravasation and vasodilation. Attenuation in the response (desensitization to maintained exposure or tachyphylaxis to repeated administration) to bradykinin actions on synovial vasculature, a critical variable with respect to the role of bradykinin in sustained or chronic synovial inflammation, has not been elucidated. In the present study, we determined if tachyphylaxis and desensitization for bradykinin-induced plasma extravasation in the knee joint occur. Bradykinin-induced plasma extravasation into the knee joint cavity was determined spectrophotometrically by measuring the concentration of Evans blue dye extravasation into the joint perfusate. To examine for the development of tachyphylaxis, perfusion of bradykinin (160 ng/ml) was repeated after a 40-min wash with normal saline. Continuous intra-articular perfusion of bradykinin produced an increase in plasma extravasation that remained relatively stable with only a small, approximately 15 percent, decrease over 170 min. On the other hand, the levels of plasma extravasation produced by intermittent perfusion of bradykinin were dramatically lower than that induced by the first exposure (i.e., tachyphylaxis). We conclude that bradykinin-induced plasma extravasation develops marked tachyphylaxis but only minimal desensitization.

Interactions of sympathetic and primary afferent neurons following nerve injury and tissue trauma

Sympathetic post-ganglionic neurons may be involved in the generation of pain, hyperalgesia and inflammation under pathophysiological conditions. Two categories of influence of the sympathetic neuron on afferent neurons can be distinguished and this distinction seems to be related to whether the coupling between afferent and sympathetic neuron develops after nerve lesion or after tissue trauma with inflammation (Fig. 15): A. Peripheral nerve lesion generates plastic changes of the afferent and sympathetic postganglionic neurons, depending on the type of nerve lesion (e.g. complete, partial). Both afferent and post-ganglionic neurons exhibit degenerative and regenerative changes and unlesioned neurons may show collateral sprouting in the periphery as well as in the dorsal root ganglion. This reorganization of the peripheral neurons may lead to chemical coupling between sympathetic and afferent neurons. The coupling is responsible for sensitization and/or activation of primary afferent neurons by the sympathetic neurons. The mediator probably is norepinephrine, but other substances cannot be excluded. The afferent neuron expresses or upregulates functional adrenoceptors. The type of adrenoceptor involved is probably alpha 2. The coupling may occur at different sites of the primary afferent neuron, e.g. at the lesion site, remote from the lesion site in the dorsal root ganglion or between nonlesioned sympathetic and afferent neurons which show collateral sprouting. The biochemical signals which trigger these changes probably are neurotrophic substances, their receptors which are synthesized by the peripheral neurons, Schwann cells and other cells in response to the peripheral lesions. B. Sympathetic nerve terminals in peripheral tissues may serve as mediator elements in hyperalgesia and inflammation following tissue trauma without nerve lesion. Experiments show that these functions are largely independent of activity in the sympathetic neurons and independent of vesicular release of transmitter substances (such as norepinephrine). Sensitization of nociceptive afferents for mechanical stimuli and venular plasma extravasation in the synovium which are induced by the inflammatory mediator bradykinin are, at least in part, dependent on the sympathetic terminal. The signal to venules and afferent receptors is synthesized and released from the sympathetic terminal or in association with it. It is a prostaglandin (probably PGE2). Sympathetically mediated (neurogenic) inflammation and neurogenic inflammation mediated by afferents may interact reciprocally and enhance the inflammatory process as well as the sensitization of nociceptive afferents. Norepinephrine may also lead to sensitization of nociceptive afferents under inflammatory conditions. This sensitization is presumably mediated by alpha 2-adrenoceptors in the sympathetic varicosities and by a prostaglandin (probably PGI2) which is synthesized and released by or in association with the sympathetic varicosities.