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Journal of Physiology (2001), 532.3, pp. 811-822
© Copyright 2001 The Physiological Society
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ABSTRACT |
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INTRODUCTION |
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Stimulation of nociceptors in the rat hindpaw or forepaw, by capsaicin, leads to depression of neurogenic inflammation, such as in the model of bradykinin (BK)-induced plasma extravasation (PE) in the knee joint of the rat (Miao et al. 1996a,b). This depression is mediated by the adrenal medullae, a propriospinal pathway between afferent nociceptive inflow and preganglionic neurones projecting to the adrenal medullae, and a spino-bulbo-spinal pathway (Miao et al. 2000). The spinal and spino-bulbo-spinal pathways involved in the noxious stimulus-induced activation of the preganglionic neurones innervating the adrenal medullae are normally inhibited by activity in vagal afferents innervating abdominal viscera. Interruption of the abdominal vagal afferents releases these reflex pathways from the ongoing inhibition, and enhances the reflex activation of the preganglionic neurones innervating the adrenal medullae by noxious stimulation and subsequently the depression of BK-induced PE (Miao et al. 2000).
The dorso- and ventrolateral funiculi contain the ascending tracts, in addition to the dorsal columns, to the brainstem and the descending excitatory and inhibitory tracts from the brainstem which are involved in the putative spino-bulbo-spinal reflex loop and its inhibitory control (McMahon & Wall, 1988; Rees & Roberts, 1993; Urban & Gebhart, 1997, 1999; Zhou & Gebhart, 1997). Some of these pathways are lateralised with respect to the nociceptive afferent input to the spinal cord (Swett et al. 1985; McMahon & Wall, 1988; Hylden et al. 1989; Kitamura et al. 1993; Rees & Roberts, 1993; Fields et al. 1995; Li et al. 1998). In this study we investigated the depression of BK-induced PE generated by activation of the sympatho-adrenal system during stimulation of nociceptive afferents by injection of capsaicin into the plantar skin of the hindpaw or palmar skin of the forepaw in order to elucidate the responsible ascending and descending spinal pathways. Controlled spinal cord lesions were performed by cutting the ipsilateral and/or contralateral dorsolateral funiculi or by ipsilateral or contralateral hemisection of the spinal cord in animals with intact vagus nerves and in vagotomised animals. The results demonstrate (1) that an excitatory spino-bulbo-spinal pathway is involved, (2) that the ascending limb of this reflex loop projects through the dorsolateral spinal funiculus contralateral to the spinal afferent input, (3) that the inhibition maintained by activity in vagal afferents occurs in the spinal cord, probably in the segments close to the nociceptive afferent input, and (4) that the descending inhibitory pathway, which is modulated by activity in vagal afferents, projects through the dorsolateral spinal funiculus ipsilateral to the nociceptive spinal afferent input. Thus, the excitatory spino-bulbo-spinal pathway establishes an endogenous positive feedback loop between nociceptive afferent input and sympathetic preganglionic output to the adrenal medullae, which is under inhibitory control from the viscera via activity in vagal afferents. The results suggest a novel form of integration between nociceptive and sympatho-adrenal systems in the control of BK-induced inflammation.
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METHODS |
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The experiments were performed on male Sprague-Dawley rats (300-400 g). All surgical procedures as well as the experiments were carried out under pentobarbital anaesthesia (65 mg kg-1, Abbott Lab, Chicago, IL, USA). Animal care and use conformed to the guidelines of the National Institutes of Health for the care and use of experimental animals. Experimental protocols were approved by the University of California at San Francisco Committee on Animal Research. At the end of the experiment the rats were killed by administration of I.P. sodium pentobarbital at a dose of 200 mg kg-1. All animals also underwent bilateral thoracotomy after injection of pentobarbital.
Perfusion of the knee joint
Knee joint perfusion was performed as previously described (Miao et al. 1996b). In brief, after incision of the skin and connective tissue overlying the anterior aspect of the knee and the saphenous vein, Evans blue dye (50 mg kg-1) was administered intravenously in the saphenous vein. Ten minutes after injection of the dye, a 30 gauge needle was inserted into the cavity of the knee joint for the infusion of fluid (250 µl min-1, controlled by a syringe pump from Sage Instruments, Model 351, Cambridge, MA, USA). After infusion of an initial volume of 100-200 µl of vehicle, a second needle (25 gauge) was inserted into the knee joint, approximately 3 mm from the inflow needle. This second needle served as an outflow cannula. Fluid was withdrawn from the joint through the outflow cannula using a second syringe pump. The fluid was infused and withdrawn at a constant rate of 250 µl min-1. Perfusate samples were collected every 5 min for up to 120 min. Samples were analysed for the amount of Evans blue dye by spectrophotometric measurement of absorbance at 620 nm. The absorbance at this wavelength is linearly related to the dye concentration (Carr & Wilhelm, 1964).
After a baseline perfusion period of 15 min with vehicle (normal saline), plasma extravasation into the knee joint was stimulated by adding bradykinin (BK; 160 ng ml-1, i.e. 0.15 µM) to the perfusion fluid.
Noxious stimulation of primary afferents by intra-plantar capsaicin
Spinal afferents from the hindpaw were excited by intra-plantar capsaicin. For some experiments in which the function of supraspinal sites in noxious-stimulus-induced inhibition of PE was tested, afferents from the forepaw were activated by intra-palmar capsaicin. Capsaicin was cumulatively injected in the hindpaw or forepaw at doses of 3-30 µg (in volumes of 10 µl each), the dose increasing by one-half log unit every 20 min.
Surgical procedures
Lesion of spinal tracts. Lesions of spinal cord tracts were performed at the segmental levels of T8/T9, C5/C6 or L2/L3. The T8/T9 and C5/C6 lesions are rostral to the segmental location of most or all preganglionic neurones which innervate the adrenal medullae (Strack et al. 1988). Eight days before the experiments the spinal cord was exposed under pentobarbital anaesthesia by laminectomy, removing the arch and dorsal spinal processes of two segments. The spinal cord lesions were done under a stereomicroscope using Dumont No. 5 jeweller's forceps. The dorsolateral funiculus (DLF) was lesioned with blunt forceps without opening the dura by crushing the dorsolateral part of the spinal cord lateral to the dorsal root entry zone, between two segmental dorsal roots. For hemisection of the spinal cord the dura and spinal cord were pierced through the mid-line using sharpened forceps and half of the spinal cord was crushed. The dorsal funiculi were interrupted without opening the dura using sharpened forceps. Dura and spinal cord were pierced between the lateral edges of the dorsal columns and the dorsal funiculi were crushed. After the lesions the spinal cord was covered by gelfoam and the spinal canal was closed by suturing the muscles, fascia and skin in layers. Recovery of the rats was uneventful and without complications.
The following lesions were performed (Fig. 1A): unilateral lesion of the DLF either ipsi- or contralateral to the nociceptive input from the hindpaw at the segmental level T8/T9 or C5/C6; DLF lesion ipsilateral to the nociceptive input from the hindpaw at the segmental level L2/L3; unilateral lesion of the DLF either ipsi- or contralateral to the nociceptive input from the forepaw at the segmental level T8/T9; hemisection of the spinal cord at the segmental level C5/C6 either ipsilateral or contralateral to the nociceptive input from the hindpaw; and bilateral lesion of the dorsal funiculi at the segmental level T8/T9.
The extent of the spinal cord lesions was mapped histologically. After the experiment the spinal segment with the lesion together with the next most rostral and the next most caudal segment was removed and fixed in formalin. Transverse sections (50 µm thick) of the lesion site were prepared and stained with haematoxylin-eosin. The location and extent of the lesions were determined under a microscope at a magnification of 
40 or 100, reconstructed and documented (Fig. 1B and C). DLF lesions were usually confined to the DLF extending sometimes into the grey matter, but never extending into the region of the dorsal columns or of the ventral quadrants and never affecting the contralateral side.
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Figure 1. Diagrams of transverse spinal cord sections A, the limits of each of the 4 main spinal funiculi have been delineated: dorsal funiculus (DF), dorsolateral funiculus (DLF), ventrolateral funiculus (VLF), ventromedial funiculus (VMF). B and C, the extent of the lesion of the DLF or of hemisection of the spinal cord is represented schematically. The largest lesion within each group is delimited by the thick line, inside of which a dashed line marks the extent of the smallest lesion within that group. The lesions made in other animals in each group fell within these ranges. Abbreviations: dcs, dorsal corticospinal tract; iml, intermediolateral cell column.
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Vagotomy. Bilateral subdiaphragmatic vagotomy was performed as previously described (Miao et al. 1994, 1997a,b). Experiments were done acutely after vagotomy.
Experimental procedures and statistics
Results are based on different experimental interventions, each conducted on at least eight knee joints (see Table 1 for data summary). Data are presented as means ± S.E.M.; significant differences between pairs of time-effect curves were determined by two-way (group time) repeated measures analysis of variance (ANOVA) followed by Fisher's post hoc test. Significant differences between pairs of means were determined by Student's unpaired t test. Differences were considered statistically significant at a P value of < 0.05. Overall two-way ANOVA showed significant difference among groups (F = 31.42, P < 0.01), which allows multiple comparisons between groups across figures.
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RESULTS |
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BK infused into the rat knee joint cavity increased PE by approximately five times baseline level. This increase in PE lasted throughout the BK infusion with a decay of about 10 % by 90 min (crosses in Fig. 2). Stimulation of cutaneous nociceptors by injection of capsaicin at doses of 3-30 µg into the plantar aspect of the hindpaw revealed a weak (compare open circles with crosses in Fig. 2A: F = 1.65, P > 0.05, two-way ANOVA) but significant decrease in BK-induced PE in the contralateral knee joint at a dose of 30 µg (P < 0.05, Student's unpaired t test). After acute subdiaphragmatic vagotomy this inhibition of BK-induced PE was strongly potentiated, with a significant inhibition at 3 µg (compare filled circles with open circles in Fig. 2A: F = 116.36, P < 0.01). This inhibition of BK-induced PE is mediated by activation of the sympatho-adrenal system (Miao, 2000). Here we describe which ascending and descending spinal pathways mediate the depression of BK-induced PE generated by noxious stimulation and its inhibitory modulation by activity in abdominal vagal afferents. Values of maximal BK-induced PE and relative changes of BK-induced PE following injection of 10 µg capsaicin into the hindpaw or forepaw under the various experimental conditions are listed in Table 1.
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Figure 2. Effect of ipsi- and contralateral DLF lesion at the segmental level of T8/T9 on depression of BK-induced PE generated by intradermal capsaicin (3, 10 and 30 µg) in the plantar skin of the hindpaw in sham-vagotomised and vagotomised rats A, depression of BK-induced PE during stimulation of nociceptors in the plantar skin of the hindpaw by capsaicin in sham-vagotomised animals (
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Effects of DLF lesion on depression of BK-induced PE during stimulation of hindpaw nociceptors
After cutting the DLF at the segmental level T8/T9 contralateral to the nociceptive afferent input from the hindpaw, no depression of BK-induced PE could be elicited by stimulation of hindpaw nociceptors, in animals with intact vagus nerves and in vagotomised animals (open inverted triangles and open squares in Fig. 2B), when compared to controls (no-capsaicin control: compare open inverted triangles with crosses in Fig. 2B: F = 2.03, P > 0.05; compare open squares with crosses in Fig. 2B: F = 2.37, P > 0.05; sham-vagotomy control: compare open inverted triangles in Fig. 2B with open circles in Fig. 2A: F = 0.07, P > 0.05; compare open squares in Fig. 2B with open circles in Fig. 2A: F = 0.02, P > 0.05 in Fig. 2A). After cutting the DLF at the segmental level T8/T9 ipsilateral to the nociceptive afferent input from the hindpaw, the depression of BK-induced PE was large in animals with intact vagus nerves and in vagotomised animals, with no significant difference between the two (filled squares and filled inverted triangles in Fig. 2C: F = 0.09, P > 0.05). This depression was already significant at 3 µg with respect to the control (no capsaicin) and with respect to the depression of BK-induced PE in animals with lesioned contralateral DLF (compare open squares in Fig. 2B with filled squares in Fig. 2C: F = 47.27, P < 0.01). It was slightly but significantly smaller than the depression seen in vagotomised animals without spinal tract lesion (compare filled squares in Fig. 2C with filled circles in Fig. 2A: F = 5.67, P < 0.05). Almost the same results were obtained in animals with intact vagus nerves when the ipsi- or contralateral DLF was lesioned at the C5/C6 cervical level. After DLF lesion contralateral to the nociceptive input, capsaicin injected into the hindpaw generated only a small depression of BK-induced PE, which was not significantly different from BK-induced PE in sham-vagotomised rats (compare open inverted triangles with open circles in Fig. 3A: F = 2.04, P > 0.05). Ipsilateral DLF lesion at the segmental level C5/C6 was followed by a depression which was similar to the depression seen when the ipsilateral DLF was lesioned at the segmental level T8/T9 (compare filled inverted triangles in Fig. 3A and filled inverted triangles in Fig. 2C: F = 0.34, P > 0.05). Again this depression was slightly but significantly smaller than the depression obtained in vagotomised animals without spinal tract lesion (compare filled inverted triangles with filled circles in Fig. 3A: F = 77.85, P < 0.01).
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Figure 3. Effect of ipsi- and contralateral DLF lesion at the segmental levels of C5/C6 or L2/L3 on depression of BK-induced PE generated by intradermal capsaicin (3, 10 and 30 µg) in the plantar skin of the hindpaw in sham-vagotomised and vagotomised rats A, lesion of ipsi- (
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After cutting the DLF ipsilateral to the nociceptive afferent input from the hindpaw at the segmental level L2/L3 the results were qualitatively the same as those obtained after ipsilateral DLF lesions at the thoracic and cervical spinal level. However, ipsilateral DLF lesion at the segmental level L2/L3 now rendered quantitatively the same depression of BK-induced PE during noxious stimulation of the hindpaw as vagotomy without spinal tract lesion (compare filled inverted triangles with filled circles in Fig. 3B: F = 0.17, P > 0.05). This can be explained by the fact that DLF lesion at the thoracic and cervical level also interrupts some descending excitatory axons of presympathetic neurones which innervate preganglionic neurones to the adrenal medullae (see Discussion).
When DLF were cut bilaterally, depression of BK-induced PE by noxious stimulation of the hindpaw was absent and not significantly different from the depression seen in animals with intact vagus nerves (data not shown).
Effects of hemisection of the spinal cord and lesion of the dorsal funiculi on depression of BK-induced PE during hindpaw nociceptor stimulation
After hemisection of the spinal cord at the segmental level C5/C6, noxious stimulation of the plantar skin produced the same changes of BK-induced PE in rats with intact vagus nerves as seen after lesioning of the DLF (for contralateral lesion compare open triangles with open inverted triangles in Fig. 4A: F = 0.67, P > 0.05; for ipsilateral lesion compare filled triangles with filled inverted triangles in Fig. 4A: F = 1.06, P > 0.05). After spinal hemisection contralateral to the nociceptive afferent input from the hindpaw, a small depression of BK-induced PE was only observed at 30 µg capsaicin (compare open triangles with crosses in Fig. 4A: F = 5.91, P < 0.05). After spinal hemisection ipsilateral to the nociceptive afferent input from the hindpaw, the depression of BK-induced PE was large in animals with intact vagus nerves (compare filled inverted triangles with crosses in Fig. 4A: F = 75.00, P < 0.01).
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Figure 4. Effect of spinal hemisection vs. DLF lesion on depression of BK-induced PE generated by intradermal capsaicin A, effect of ipsi- (
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Bilateral lesion of the dorsal funiculi at the spinal level T8/T9 had almost no effect on BK-induced PE generated by stimulation of cutaneous nociceptors (data not shown). A small depression was observed at 30 µg capsaicin, being comparable to the depression of BK-induced PE in sham-vagotomised rats (cf. Fig. 2A).
Depression of BK-induced PE generated by stimulation of cutaneous nociceptors in animals with lesion of the ventrolateral funiculus (VLF) of the spinal cord was not studied since it can be predicted from the hemisection experiments showing that there is no significant difference in depression of BK-induced PE between animals with spinal hemisection and animals with DLF section.
Effects of DLF lesion on depression of BK-induced PE during forepaw nociceptor stimulation
Capsaicin injected in the palm of the forepaw elicits a depression of BK-induced PE in vagus-intact rats which is stronger than the depression elicited from the hindpaw (compare open squares in Fig. 4B with open circles in Fig. 2A, F = 12.51, P < 0.01). Lesion of the ipsilateral or contralateral DLF at the segmental level T8/T9, i.e. caudal to the spinal afferent inflow from the forepaw, had no effect on the depression of BK-induced PE generated from the forepaw (compare open inverted triangles with open squares in Fig. 4B: F = 0.57, P < 0.05; compare filled inverted triangles with open squares in Fig. 4B: F = 0.18, P < 0.05).
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DISCUSSION |
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BK-induced PE is depressed during stimulation of cutaneous nociceptors. This depression is potentiated after subdiaphragmatic vagotomy and abolished after denervation of the adrenal medullae; thus the depression is mediated by the adrenal medullae and probably by adrenaline released by this endocrine gland (Miao et al. 2000). The cellular mechanisms underlying the depression of BK-induced synovial PE generated by the signal from the adrenal medullae is unknown. They involve the innervation of the synovia by sympathetic postganglionic fibres (Miao et al. 1996a,b; Green et al. 1997) but not activity in these fibres (Miao et al. 1996b). Most likely it does not involve antidromic activity ('dorsal root reflexes') in peptidergic primary afferent neurones with unmyelinated fibres (for review see Willis, 1999) for two reasons. (1) Denervation of the adrenal medulla abolishes the depression of BK-induced PE generated by noxious stimulation in normal and in vagotomised rats (Miao et al. 2000). (2) There is no evidence for a continuous antidromic impulse traffic in unmyelinated afferent fibres to the knee joint under normal conditions which could be depressed reflexly during noxious stimulation of skin leading, at least theoretically, in this way to depression of BK-induced PE.
We have postulated (Miao et al. 2000) that the reflex activation of the adrenal medullae by noxious stimulation is relayed by a propriospinal pathway to the preganglionic neurones in the segments T4 to T12, which innervate the adrenal medullae (Strack et al. 1988), and a spino-bulbo-spinal pathway. Spino-bulbo-spinal and spinal pathways are partially or completely inhibited by ongoing activity in abdominal vagal afferents and released from inhibition after subdiaphragmatic vagotomy (Miao et al. 2000). The spinal pathway alone is weak in activating the preganglionic neurones supplying the adrenal medulla and requires the stimulation-induced facilitation via the supraspinal reflex loop. In the experiments reported here we studied the location of the ascending and descending spinal pathways which are involved in the depression of BK-induced PE generated by noxious stimulation and mediated by the sympatho-adrenal system and in its inhibitory modulation by vagal activity.
A hypothesis explaining the results
On the basis of our present results we hypothesise that the supraspinal reflex loop, the spinal pathway and their inhibitory control have the following characteristics (Fig. 5).
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Figure 5. Schematic diagram showing the proposed neural circuits in spinal cord and brainstem which modulate synovial BK-induced PE via the adrenal medullae Cutaneous noxious stimulation by capsaicin leads to activation of preganglionic neurones (pre) to the adrenal medullae via a spinal and a spino-bulbo-spinal excitatory circuit. The ascending limb of this spino-bulbo-spinal reflex loop (neurone 1) projects through the contralateral DLF. The descending limb of this reflex loop (neurone 2) projects through the dorsal quadrants, but not through the ventrolateral funiculi and not or not exclusively through the ipsilateral DLF. The inhibition of this circuit by activity in abdominal vagal afferents is exerted at the level of the spinal cord. The descending limb of this inhibitory pathway (neurone 3) projects through the ipsilateral DLF. Lesion of the contralateral DLF abolishes depression of BK-induced PE generated by the nociceptive-neuroendocrine negative feedback loop; lesion of the ipsilateral DLF enhances the feedback loop. The levels of contra- and ipsilateral DLF lesions are indicated for the respective segmental levels. At the segmental levels T8/T9 and C5/C6 also some presympathetic axons projecting to the preganglionic neurones which innervate the adrenal medullae are interrupted (dotted lines). DLF lesion at the segmental level L2/L3 does not interrupt presympathetic axons which innervate the preganglionic neurones to the adrenal medulla. IN, spinal interneurone; NTS, nucleus of the solitary tract. For details see text.
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(1) An ascending limb projects through the DLF contralateral to the nociceptive input. When this ascending pathway is lesioned, depression of BK-induced PE, which is mediated by activation of the sympatho-adrenal system, can no longer be generated by noxious stimulation of the hindpaw in vagotomised rats. This suggests that ongoing vagally induced inhibition is only effective when the supraspinal reflex loop is activated by the nociceptive input.
(2) When the DLF ipsilateral to the nociceptive input has been lesioned the reflex system generating the depression of BK-induced PE behaves almost as if the animal has been vagotomised (see below), i.e. depression of BK-induced PE by noxious stimulation is large. This suggests (a) that the descending pathway, which is involved in the inhibition generated by activity in vagal afferents, is projecting through the DLF ipsilateral to the nociceptive input, (b) that this inhibition occurs at the spinal level, and (c) that tonic activity in vagal afferents produces tonic activity in the descending inhibitory pathway projecting through the ipsilateral DLF. This last point is supported by preliminary experiments showing that the 
-adrenoceptor antagonist phentolamine and/or the opioid-receptor antagonist naloxone, applied intrathecally at the lumbar spinal cord, have the same effect as vagotomy or interruption of the ipsilateral DLF (F. J.-P. Miao, L. Jasmin, W. Jänig & J. D. Levine, unpublished observations); however, these pharmacological interventions do not prove which descending system is involved in generating the vagus-maintained inhibition. The strong enhancement of depression of BK-induced PE generated by noxious stimulation after ipsilateral DLF lesion virtually excludes the possibility that vagal inhibition occurs at the brainstem level.
(3) The depression of BK-induced PE generated by noxious stimulation of the hindpaw in rats with the ipsilateral DLF lesioned at the segmental level C5/C6 or T8/T9 was significantly smaller than the depression in vagotomised rats (see Figs 2A and C and 3A). This difference can be explained by the fact that excitatory descending axons of 'presympathetic' neurones in the brainstem which project directly to the preganglionic neurones innervating the adrenal medullae have been interrupted (see dotted pathways in Fig. 5). The supraspinal synaptic input to preganglionic neurones innervating the adrenal medullae projects largely through the DLF and is probably bilateral (Strack et al. 1989a,b).
(4) The depression of BK-induced PE generated by noxious stimulation of the hindpaw in rats with lesioned ipsilateral DLF at the segmental level L2/L3 was identical to the depression in vagotomised rats (see Fig. 3B). This finding supports the idea that the vagus-mediated inhibition occurs in the lumbar segments close to the afferent inflow from the hindpaw (as indicated in Fig. 5).
(5) The descending excitatory pathway of the spino-bulbo-spinal reflex loop facilitates the spinal pathway leading to activation of the preganglionic neurones during noxious stimulation of the hindpaw. This facilitation is normally prevented by inhibition which is maintained by activity in abdominal vagal afferents and which is mediated by neurones projecting through the ipsilateral DLF to the lumbar segments. Facilitation and inhibition may occur at the same interneurones in the lumbar segments which project to the preganglionic neurones innervating the adrenal medullae as shown in Fig. 5. Alternatively they may occur directly at the second-order neurones in the dorsal horn (not shown in Fig. 5). This is consistent with the finding that after acute spinalisation at the segmental level T1/T2 (which interrupts the descending excitatory and inhibitory pathways) the depression of BK-induced PE generated by noxious stimulation was weak (Miao et al. 2000). The location of the descending facilitatory pathway is unknown; however, it does not project through the ipsilateral DLF alone (otherwise ipsilateral DLF lesion would not enhance depression of BK-induced PE by removal of inhibition) but either bilaterally or through the contralateral DLF or through other tracts of the dorsal quadrants of the spinal cord.
(6) The finding that ipsilateral and contralateral hemisection of the spinal cord has the same effects as DLF lesions, in rats with intact vagus nerves, suggests that neither the ascending excitatory nor the descending inhibitory pathways involved in the nociceptive- neuroendocrine feedback control of synovial plasma extravasation project through the VLF. Furthermore, the absence of an effect from cutting the dorsal funiculi suggests that both spinal pathways do not project through these funiculi.
Evidence supporting the lateralisation of ascending excitatory and descending inhibitory pathways
In the rat about 80-90 % of the lamina I neurones projecting through the DLF do so contralaterally and the rest ipsilaterally, to parabrachial nuclei (Hylden et al. 1989; Kitamura et al. 1993), to the midbrain (Swett et al. 1985; McMahon & Wall, 1988; Li et al. 1998) and to the anterior pretectal nucleus (for review see Rees & Roberts, 1993). Descending pathways involved in inhibitory control of nociceptive impulse transmission in the dorsal horn pass largely through the DLF ipsilateral to the nociceptive input in the rat (McMahon & Wall, 1988; Fields et al. 1995). These inhibitory descending pathways have their origin in the rostral ventromedial medulla (which includes the nucleus raphe magnus and the nucleus paragigantocellularis; many of the neurones being serotonergic) and in the dorsolateral pontine tegmentum (many of the neurones being catecholaminergic) (for review see Basbaum & Fields, 1984; Fields et al. 1991).
Evidence supporting the existence of the spino-bulbo-spinal reflex loop
Strong support for an excitatory spino-bulbo-spinal reflex loop is given by the experimental work of Rees, Roberts and co-workers (Rees & Roberts, 1993). These authors have shown that the anterior pretectal nucleus and the parabrachial nuclei are involved in a powerful spino-bulbo-spinal reflex loop which excites lamina I neurones, most of them being nociceptive specific, and inhibits wide dynamic range neurones in lamina V and deeper in the dorsal horn (Rees & Roberts, 1987; Rees et al. 1995). The ascending limb of this reflex loop projects through the contralateral DLF, probably to the parabrachial nuclei, and the descending limb projects through the ipsilateral DLF. The origin of the descending (excitatory and inhibitory) pathways is unknown, but the inhibitory one may originate in the parabrachial and ventrolateral regions of the medulla (Terenzi et al. 1991, 1992; Rees & Roberts, 1993), i.e. they are different from the descending inhibitory pathway related to the periaqueductal gray and rostral ventromedial system (Basbaum & Fields, 1984; Fields et al. 1991). Similar findings were reported earlier by McMahon & Wall (1988) showing that the lateral reticular formation of the mesencephalon is involved in the spino-bulbo-spinal reflex loops which enhance evoked activity in some lamina I neurones and inhibit the evoked activity in deeper dorsal horn neurones. Anatomical support for this idea is given by Craig (1995) showing that lamina I neurones at all segmental levels project predominantly through the contralateral lateral funiculus in the cat (which is equivalent to the DLF in rats) to the parabrachial and mesencephalic nuclei.
McMahon & Wall (1988) and Rees & Roberts (1993) argue that the excitatory spino-bulbo-spinal reflex loop is an amplifying circuit which maintains stimulus-evoked descending inhibition in spinothalamic wide dynamic range neurones in lamina V and deeper. Thus, activation of nociceptive lamina I neurones by continuous nociceptive input leads to an enhancement of the activation of these lamina I neurones, via an excitatory spino-bulbo-spinal reflex loop having an excitatory pathway descending in the DLF. This excitatory positive feedforward loop maintains an inhibition of wide dynamic range neurones in lamina V and deeper via a pathway also descending in the DLF. This spino-bulbo-spinal reflex loop involves the anterior pretectal nucleus, parabrachial nuclei and the ventrolateral medulla and operates relatively independently of the antinociceptive system represented in the periaqueductal grey (PAG) and rostral ventromedial medulla (RVM). This idea is supported by several lines of evidence. (1) Lesioning of the contralateral or ipsilateral DLF or of the contralateral parabrachial area (where lamina I neurones terminate) accelerates the onset of autotomy behaviour of rats with sectioned sciatic and saphenous nerves (Wall et al. 1988; Saade et al. 1990). (2) Lesion of the anterior pretectal nucleus enhances autotomy behaviour of rats in response to dorsal rhizotomy (Rees et al. 1995). (3) Formalin injected subcutaneously increases c-fos expression in lamina I and deep dorsal horn spino-parabrachial projection neurones. The formalin-induced c-fos expression in the deep dorsal horn projection neurones but not in the lamina I projection neurones is significantly reduced during morphine-induced antinoception (Jasmin et al. 1994). Here we hypothesise that a similar excitatory spino-bulbo-spinal pathway is involved in the activation of preganglionic neurones innervating the adrenal medullae, generated by noxious stimulation, and in its inhibitory modulation by activity in abdominal vagal afferents.
Evidence supporting the existence of descending excitatory pathways
Is there support for a descending excitatory pathway which is involved in control of nociceptive impulse transmission in the dorsal horn and which is a component of the excitatory spino-bulbo-spinal reflex loop? There are several pieces of evidence showing that nociceptive impulse transmission in the dorsal horn as well as nociceptive behaviour may be facilitated by descending excitatory influences. Fields et al. (1983, 1995) have shown that so-called on-cells in the RVM have an excitatory effect on nociceptive impulse transmission in the dorsal horn; these RVM neurones project through the DLF. Gebhart and co-workers have shown that neurones in the RVM may enhance nociceptive impulse transmission in the dorsal horn and nociceptive behaviour (tail flick reflex). These neurones project through the VLF/VF (Urban & Gebhart, 1997; Zhuo & Gebhart, 1997). Wei et al. (1999) have shown that persistent thermal hyperalgesic behaviour and expression of the immediate early gene c-fos in dorsal horn neurones during experimental inflammation of the hindlimb is facilitated by activity in the neurones of the nucleus reticularis gigantocellularis, which projects to the dorsal horn. Lesion of the nucleus gigantocellularis (including pars alpha) reduces thermal hyperalgesic behaviour and expression of c-fos in dorsal horn neurones during inflammation generated by injection of Freund's adjuvant. Various groups have shown that initiation and maintenance of sensitisation of dorsal horn neurones and secondary hyperalgesia generated by tissue injury involve spino-bulbo-spinal mechanisms (for review see Urban & Gebhart, 1999). Finally, Gebhart and co-workers have shown that enhancement of nociceptive impulse transmission in the spinal dorsal horn and of nociceptive behaviour (tail flick reflex) generated by electrical stimulation of myelinated abdominal vagal afferents are mediated by descending tracts in the VLF of the spinal cord (for review see Gebhart & Randich, 1992; Randich & Gebhart, 1992).
Interruption of presympathetic axons cannot explain the effects of DLF lesion
It is unlikely that our results can be explained by interruption of the presympathetic axons which project through the DLF to the preganglionic neurones innervating the adrenal medullae (see dotted pathways in Fig. 5) since these axons originate from ipsi- and contralateral areas in the brainstem (rostral ventrolateral and ventromedial medulla, caudal raphe nucleus, A5 cell group and paraventricular hypothalamic nuclei; Strack et al. 1989a,b). Also, similar results were obtained with DLF lesions at the spinal segmental level C5/C6, T8/T9 and L2/L3. DLF lesion at the segmental level L2/L3 does not interrupt presympathetic axons innervating preganglionic neurones to the adrenal medulla whereas unilateral DLF lesion at the segmental level T8/T9 or C5/C6 interrupts some presympathetic axons to the preganglionic neurones. This may explain why (1) depression of BK-induced PE during noxious stimulation of the hindpaw after ipsilateral DLF lesion at the segmental level L2/L3 is as strong as after vagotomy and (2) depression of BK-induced PE after DLF lesion at T8/T9 or C5/C6 is slightly but significantly weaker than depression after vagotomy.
The enigma: stimulation of cutaneous afferent C-fibres by capsaicin or by electrical stimulation depresses BK-induced PE by two different mechanisms
Previously we have described that transcutaneous electrical stimulation of unmyelinated afferents leads also to depression of BK-induced PE, which is also enhanced by subdiaphragmatic vagotomy (Green et al. 1995; Miao et al. 1997a,b). However this depression of BK-induced PE is mediated by activation of the hypothalamo-pituitary-adrenal system (Green et al. 1995, 1997) and not by activation of the sympatho-adrenal system. Which physiological stimuli acting in the body or on the body surface lead to the selective activation of the HPA axis? What are the essential differences between the two ways of activation of afferents innervating the paw? Capsaicin activates many C-afferents and some A
-afferents (Szolcsanyi et al. 1988) asynchronously, but not A
-afferents and not most A-afferents. Transcutaneous electrical stimulation activates indiscriminately all afferents synchronously.
Conclusion
Noxious cutaneous stimulation by capsaicin leads to depression of BK-induced PE in the rat knee joint which is generated by activation of the sympatho-adrenal system, dependent on a spinal and spino-bulbo-spinal excitatory reflex circuit and under powerful inhibitory control maintained by vagal activity (Miao et al. 2000). Here we have shown that the ascending pathway of the excitatory spino-bulbo-spinal reflex loop projects through the DLF contralateral to the nociceptive input, that the inhibition exerted by vagal activity occurs at the spinal cord level close to the nociceptive afferent input and that the descending inhibitory pathway projects through the DLF ipsilateral to the nociceptive afferent input. The spino-bulbo-spinal pathway establishes an endogenous positive feedback loop between nociceptive afferent input and sympathetic preganglionic output to the adrenal medulla. The results suggest a novel form of integration between nociceptive and sympatho-adrenal systems in the control of inflammation.
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REFERENCES |
|---|
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TopAbstractIntroductionMethodsResultsDiscussionReferences |
|---|
| BASBAUM A. I. & FIELDS, H. L. (1984). Endogenous pain control systems: brainstem spinal pathways and endorphin circuitry. Annual Review of Neuroscience 7, 309-338 | [Medline] |
| CARR J. & WILHELM, D. L. (1964). The evaluation of increased vascular permeability in the skin of guinea pigs. Journal of Experimental Biology and Medical Science 42, 511-522 | |
| CRAIG A. D. (1995). Distribution of brainstem projections from spinal lamina I neurones in the cat and the monkey. Journal of Comparative Neurology 361, 225-248 | [Medline] |
| FIELDS H. L., BRY, J., HENTALL, I. & ZORMAN, G. (1983). The activity of neurones in the rostral medulla of the rat during withdrawal from noxious heat. Journal of Neuroscience 3, 2545-2552 | [Abstract] |
| FIELDS H. L., HEINRICHER, M. M. & MASON, P. (1991). Neurotransmitters in nociceptive modulatory circuits. Annual Review of Neuroscience 14, 219-245 | [Medline] |
| FIELDS H. L., MALICK, A. & BURSTEIN, R. (1995). Dorsal horn projection targets of ON and OFF cells in the rostral ventromedial medulla. Journal of Neurophysiology 74, 1742-1759 | [Medline] |
| GEBHART G. F. & RANDICH, A. (1992). Vagal modulation of nociception. American Pain Society Journal 1, 26-32 | |
| GREEN P. G., JÄNIG, W. & LEVINE, J. D. (1997). Negative feedback neuroendocrine control of inflammatory response in the rat is dependent on the sympathetic postganglionic neuron. Journal of Neuroscience 17, 3234-3238 | |
| GREEN P. G., MIAO, F. J.-P., JÄNIG, W. & LEVINE, J. D. (1995). Negative feedback neuroendocrine control of the inflammatory response in rats. Journal of Neuroscience 15, 4678-4686 | [Abstract] |
| HYLDEN J. L., ANTON, F. & NAHIN, R. L. (1989). Spinal lamina I projection neurones in the rat: collateral innervation of parabrachial area and thalamus. Neuroscience 28, 27-37 | [Medline] |
| JASMIN L., WANG, H., TARCZY-HORNOCH, K., LEVINE, J. D. & BASBAUM, A. I. (1994). Differential effects of morphine on noxious stimulus-evoked fos-like immunoreactivity in subpopulations of spinoparabrachial neurones. Journal of Neuroscience 14, 7252-7260 | [Abstract] |
| KITAMURA T., YAMADA, J., SATO, H. & YAMASHITA, K. (1993). Cells of origin of the spinoparabrachial fibers in the rat: a study with fast blue and WGA-HRP. Journal of Comparative Neurology 328, 449-461 | [Medline] |
| LI H. S., MONHEMIUS, R., SIMPSON, B. A. & ROBERTS, M. H. (1998). Supraspinal inhibition of nociceptive dorsal horn neurones in the anaesthetized rat: tonic or dynamic? Journal of Physiology 506, 459-469. , | [Abstract/Full Text] |
| MCMAHON S. B. & WALL, P. D. (1988). Descending excitation and inhibition of spinal cord lamina I projection neurones. Journal of Neurophysiology 59, 1204-1219 | [Medline] |
| MIAO F. J.-P., GREEN, P. G., CODERRE, T. J., JÄNIG, W. & LEVINE, J. D. (1996a). Sympathetic-dependence in bradykinin-induced synovial plasma extravasation is dose-related. Neuroscience Letters 205, 165-168 | [Medline] |
| MIAO F. J.-P., JÄNIG, W., DALLMAN, M. F., BENOWITZ, N. L., HELLER, P. H., BASBAUM, A. I. & LEVINE, J. D. (1994). Role of vagal afferents and spinal pathways modulating inhibition of bradykinin-induced plasma extravasation by intrathecal nicotine. Journal of Neurophysiology 72, 1199-1207 | [Medline] |
| MIAO F. J.-P., JÄNIG, W., GREEN, P. G. & LEVINE, J. D. (1997a). Inhibition of bradykinin-induced plasma extravasation by noxious cutaneous and visceral stimuli and its modulation by vagal activity. Journal of Neurophysiology 78, 1285-1292 | [Abstract/Full Text] |
| MIAO F. J.-P., JÄNIG, W. & LEVINE, J. D. (1996b). Role of sympathetic postganglionic neurons in synovial plasma extravasation induced by bradykinin. Journal of Neurophysiology 75, 715-724 | [Medline] |
| MIAO F. J.-P., JÄNIG, W. & LEVINE, J. D. (1997b). Vagal branches involved in inhibition of bradykinin-induced synovial plasma extravasation by intrathecal nicotine and noxious stimulation in the rat. Journal of Physiology 498, 473-481 | [Abstract] |
| MIAO F. J.-P., JÄNIG, W. & LEVINE, J. D. (2000). Nociceptive neuroendocrine negative feedback control of neurogenic inflammation activated by capsaicin in the rat paw: role of the adrenal medulla. Journal of Physiology 527, 601-610 | [Abstract/Full Text] |
| RANDICH A. & GEBHART, G. F. (1992). Vagal afferent modulation of nociception. Brain Research Reviews 17, 77-99 | [Medline] |
| REES H. & ROBERTS, M. H. (1987). Anterior pretectal stimulation alters the responses of spinal dorsal horn neurones to cutaneous stimulation in the rat. Journal of Physiology 385, 415-436 | [Abstract] |
| REES H. & ROBERTS, M. H. (1993). The anterior pretectal nucleus: a proposed role in sensory processing. Pain 53, 121-135 | [Medline] |
| REES H., TERENZI, M. G. & ROBERTS, M. H. (1995). Anterior pretectal nucleus facilitation of superficial dorsal horn neurones and modulation of deafferentation pain in the rat. Journal of Physiology 489, 159-169 | [Abstract] |
| SAADE N. E., ATWEH, S. F., JABBUR, S. J. & WALL, P. D. (1990). Effects of lesions in the anterolateral columns and dorsolateral funiculi on self-mutilation behaviour in rats. Pain 42, 313-321 | [Medline] |
| STRACK A. M., SAWYER, W. B., HUGHES, J. H., PLATT, K. B. & LOEWY, A. D. (1989a). A general pattern of CNS innervation of the sympathetic outflow demonstrated by transneuronal pseudorabies viral infection. Brain Research 491, 156-162 | [Medline] |
| STRACK A. M., SAWYER, W. B., HUGHES, J. H., PLATT, K. B. & LOEWY, A. D. (1989b). CNS cell groups regulating the sympathetic outflow to adrenal gland as revealed by transneuronal cell body labeling with pseudorabies virus. Brain Research 491, 274-296 | [Medline] |
| STRACK A. M., SAWYER, W. B., MARUBIO, L. M. & LOEWY, A. D. (1988). Spinal origin of sympathetic preganglionic neurones in the rat. Brain Research 455, 187-191 | [Medline] |
| SWETT J. E., MCMAHON, S. B. & WALL, P. D. (1985). Long ascending projections to the midbrain from cells of lamina I and nucleus of the dorsolateral funiculus of the rat spinal cord. Journal of Comparative Neurology 238, 401-416 | [Medline] |
| SZOLCSANYI J., ANTON, F., REEH, P. W. & HANDWERKER, H. O. (1988). Selective excitation by capsaicin of mechano-heat sensitive nociceptors in rat skin. Brain Research 446, 262-268 | [Medline] |
| TERENZI M. G., REES, H., MORGAN, S. J., FOSTER, G. A. & ROBERTS, M. H. (1991). The antinociception evoked by anterior pretectal nucleus stimulation is partially dependent upon ventrolateral medullary neurones. Pain 47, 231-239 | [Medline] |
| TERENZI M. G., REES, H. & ROBERTS, M. H. (1992). The pontine parabrachial region mediates some of the descending inhibitory effects of stimulating the anterior pretectal nucleus. Brain Research 594, 205-214 | [Medline] |
| URBAN M. O. & GEBHART, G. F. (1997). Characterization of biphasic modulation of spinal nociceptive transmission by neurotensin in the rat rostral ventromedial medulla. Journal of Neurophysiology 78, 1550-1562 | [Abstract/Full Text] |
| URBAN M. O. & GEBHART, G. F. (1999). Supraspinal contributions to hyperalgesia. Proceedings of the National Academy of Sciences of the USA 96, 7687-7692 | [Abstract/Full Text] |
| WALL P. D., BERY, J. & SAADE, N. (1988). Effects of lesions to rat spinal cord lamina I cell projection pathways on reactions to acute and chronic noxious stimuli. Pain 35, 327-339 | [Medline] |
| WEI F., DUBNER, R. & REN, K. (1999). Nucleus reticularis gigantocellularis and nucleus raphe magnus in the brain stem exert opposite effects on behavioral hyperalgesia and spinal Fos protein expression after peripheral inflammation. Pain 80, 127-141 | [Medline] |
| WILLIS W. D. (1999). Dorsal root potentials and dorsal root reflexes: a double-edged sword. Experimental Brain Research 124, 395-421 | [Medline] |
| ZHUO M. & GEBHART, G. F. (1997). Biphasic modulation of spinal nociceptive transmission from the medullary raphe nuclei in the rat. Journal of Neurophysiology 78, 746-758 | [Abstract/Full Text] |
Note added in proof
Since this paper was submitted, H. Bester, C. De Felipe & S. P. Hunt (Journal of Neuroscience 21, 1039-1046) have provided evidence that NK1 receptor stimulation is involved in the activation of ascending pathways responsible for nociceptive stimulation-induced descending inhibition. Since many NK1 receptors are present on spinobrachial neurons, it is conceivable that substance P neurotransmission is involved in the ascending loop of the spino-bulbo-spinal circuit proposed here.
Acknowledgements
This work was supported by grants from TRDRP (8RT-0032) and NIH (AR-32634).
Corresponding author
F. J.-P. Miao: NIH Pain Center, Box 0440, University of California at San Francisco, 521 Parnassus Avenue, Room C-522, San Francisco, CA 94143-0440, USA.
Email: fjmiao{at}itsa.ucsf.edu
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) and in animals in which the vagus nerves were acutely cut subdiaphragmatically (
). B and C, effect of lesioning of the contralateral (B) and of the ipsilateral (C) DLF in animals with intact vagus nerves (
, contralateral DLF; 
, ipsilateral DLF) and in vagotomised rats (
, contralateral DLF; 
, ipsilateral DLF) on depression of BK-induced PE. Experiments performed 8 days after DLF lesion and acutely after vagotomy. 
) and contralateral (
) spinal hemisection at the segmental level C5/C6 on depression of BK-induced PE generated by intradermal capsaicin (3, 10 and 30 µg) in the plantar skin of the rat hindpaw. B, effect of ipsi- (