The opioid antagonist naloxone does not alter discharges of nociceptive afferents from the acutely inflamed knee joint of the cat

Recent studies have shown peripheral antinociceptive effects of opiates in inflamed tissue. To test whether the afferent activity during an acute inflammation may also be suppressed by endogenous opioids, we studied whether the application of the opioid antagonist naloxone would alter the afferent discharges from the cat knee joint inflamed by kaolin and carrageenin. After i.a. bolus administration of naloxone (3 micrograms/kg and 1 mg/kg) close to the joint, neither the ongoing activity nor the responses to noxious and innocuous movements significantly changed in group III or group IV units. Since naloxone did not unmask opioidergic activity under these conditions, we conclude that the development of increased activity in joint afferents during an acute kaolin/carrageenin-induced inflammation is not tonically suppressed by endogenous opioids.

Fine sensory innervation of the knee joint capsule by group III and group IV nerve fibers in the cat

Afferent group III and IV nerve fibers of the knee joint markedly differ in their responsiveness to mechanical stimulation, which may be reflected in the structure and location of their terminals. Therefore, in sympathectomized cats, the fine afferent innervation of the knee joint capsule was studied via ultrastructural three-dimensional reconstructions over distances of up to 300 microns. Small peripheral nerves and "free" (noncorpuscular) sensory nerve endings were found in a superficial layer of the outer fibrous part of the capsule, in the patellar retinaculum, and in the outer and inner surface layers of the medial collateral and patellar ligaments. Group III nerve fibers showed a proximal myelinated portion inside the nerve, an intermediate portion that lacks a myelin sheath and is only surrounded by perineurium, and a distal portion outside of the perineurium that forms the sensory ending proper. Group IV fibers showed only two distinct portions, an intraperineurial (proximal) and an extraperineurial (distal) portion without any further morphological differences. Outside of the perineurium, a network formed by Schwann cells ("Schwann cell reticulum") provides a pathway for the distal portion of the sensory axons. No distinct subgroups of the sensory terminal fibers could be defined according to the configuration of the Schwann cells and the nerve fiber terminals. Sensory terminals were located adjacent to different structures such as venous and lymphatic vessels, fat cells, and collagenous fibers. Distinct parts of the same terminal nerve fiber were found in close contact to a vessel wall; others were surrounded by dense collagenous tissue. Close to sensory endings, mast cells and mast cell-like cells were frequently found, indicating a functional relationship.

An intact nerve preparation for monitoring inputs from single joint afferent fibres

A preparation is described that permits the monitoring of activity from individual joint afferent nerve fibres in an intact peripheral nerve of the cat. The joint nerve used was the medial articular nerve (MAN) that supplies the medial and anteromedial aspects of the knee joint. This nerve is sufficiently fine that if freed from nearby tissue over a length of 2-5 cm and placed over a platinum hook electrode it is possible to identify and monitor, from the intact nerve, the impulse activity from each group II joint afferent fibre activated by mechanical stimulation of the joint capsule. The signal-to-noise ratio exceeds 5:1 and in most cases was approximately 10:1. With this preparation it is now possible to examine the central actions and security of transmission at central synaptic targets for single, identified group II joint afferent fibres.

Responsiveness of slowly conducting articular afferents to bradykinin: effects of an experimental arthritis

Bradykinin (BK), an important inflammatory mediator and potent algogenic substance, is supposed to contribute to the generation of arthritic hyperalgesia and pain. The present study was undertaken to examine if an experimental kaolin/carrageenan arthritis sensitizes articular afferents to BK in the cat's knee joint using two different approaches. First, the proportion of afferent units activated by BK was assessed in fully inflamed joints and compared with corresponding data of normal knee joints. BK (injected i.a. as a bolus close to the joint) at the dose of 2.6 micrograms activated 60% of the units of groups II-IV in the inflamed state, compared to 71% in normal joints. The proportions of low- and high-threshold afferents activated by BK were similar, but more spontaneously active units than units without ongoing activity responded to BK both in inflamed and normal knee joints. Second, the responsiveness of individual afferent units to BK was examined during the development of inflammation. Units not activated by BK remained unresponsive after inflammation. From 11 units activated by BK, 3 units lost their responsiveness and in 4 other units the response to BK was reduced within 2-6 h after the onset of inflammation. Only in 4 units was the BK response increased in the inflamed joint. It is concluded that desensitizing rather than sensitizing processes are involved to change the response behavior of articular afferents to BK during acute experimental inflammation.

Responses of lateral thalamic neurons to algesic chemical stimulation of the cat knee joint

In order to gain insight into the representation of articular pain of the knee at the supraspinal level, recordings were made from lateral thalamic neurons receiving input from afferent fibres of the knee joint in chloralose-anaesthetized cats. Dorsoventral penetrations were made through the ventral posterior lateral nucleus (VPL) using high intensity electrical stimulation of the medial articular nerve (MAN), which contains a high proportion (80%) of A delta and C afferent fibres. All recording sites were verified histologically. Close retrograde injections (300 microliters over 6 s) into geniculate artery of KCl (2 x isotonic), bradykinin (BK, 2.6 or 26 micrograms) and capsaicin (200 microM) were used to test the response properties of thalamic neurons. Of the 50 MAN-positive units tested, 20 showed a response to intra-arterial KCl; of these 20, 12 had a response to BK; 8 of these 12 units were additionally tested with capsaicin and all responded. KCl and capsaicin injections had similar mean response latencies (4.5 and 6.8 s), whereas BK had a longer mean latency (18.6 s). The mean peak response was greatest for capsaicin (168 impulses/s), then KCl (87.5 imp/s) and least with BK (36.4 imp/s). The mean response duration was longest with capsaicin (118 s), followed by BK (67.5 s) and least with KCl (27.9 s). Most of these were convergent wide dynamic range (WDR) neurons with a deep receptive field in the knee joint and hindlimb muscle and/or cutaneous distal hind limb digit, located to the dorsal or ventral periphery of the lateral division of the VPL, the VPL1. In addition, 8 neurons showed inhibitory responses to KCl and/or BK injections. The background activity of VPL1 neurons activated by saphenous nerve stimulation was inhibited by the nociceptive articular stimulus with a magnitude and time course which mirrored the excitatory responses in the periphery of VPL1. These results support the concept that the lateral thalamus plays an important role in mediating discriminative aspects of joint pain.

Electrophysiological evidence for a spinal antinociceptive action of dipyrone

Electrophysiological experiments in anesthetized cats and rats were performed in order to study the effects of dipyrone on single afferent fibers from the knee joint and on spinal cord neurons with knee joint input. The neurons were activated and/or rendered hyperexcitable by an acute inflammation in the knee joint. In the joint nerve in cats, intravenous dipyrone (25-100 mg/kg) reduced ongoing activity in 10/12 thinly myelinated afferents but only in 1/10 unmyelinated afferents; the responses to movements of the inflamed knee were reduced in 8/10 thinly myelinated but only in 3/10 unmyelinated units. The reduction of activity was significant 20-30 min after application and was maximal at 60-180 min. In the spinal cord of spinalized cats, intravenous dipyrone (25-100 mg/kg) reduced ongoing activity and/or responses to pressure onto the inflamed knee in 14/16 neurons and in non-spinalized rats similar effects were seen in 10/11 neurons. Effects on spinal cord neurons started 5-10 min after application and were maximal after 20-40 min. These data show pronounced suppression of inflammation-induced nociception by dipyrone and they suggest that the spinal cord is a major site of action of this compound.

Mitochondria in fine afferent nerve fibres of the knee joint in the cat: a quantitative electron-microscopical examination

The distribution of mitochondria, their content and concentration (expressed as the ratio of the mean volume of mitochondria and the surface of the sensory axon) were determined in group-III and -IV nerve fibres innervating the knee joint capsule in the cat. Mitochondria mainly accumulated in axonal swellings ("beads") and end bulbs of the terminal branches. Between single nerve fibres, marked differences in the content and the concentration of mitochondria were obtained in proximal portions (inside of the perineurium) and in distal portions (unmyelinated sensory endings). In group-III nerve fibres, the mitochondrial concentration ranged from 0.005 to 0.030 microns 3/microns 2 (proximal portion) and from 0.016 to 0.080 microns 3/microns 2 (distal portion). In unmyelinated group-IV nerve fibres, the values also showed a broad variation ranging from 0.001 to 0.011 microns 3/microns 2 (proximal portion) and from 0.003 to 0.019 microns 3/microns 2 (distal portion). The wide range of mitochondrial concentrations may reflect different energy consumption during receptive processes: nerve fibres with a low mechanical threshold and a high probability of excitatory events may be rich in mitochondria, whereas fibres with a high mechanical threshold and a low probability of excitatory events may be poor in mitochondria.

Bradykinin B1 and B2 receptor antagonists do not change the ongoing activity of slowly conducting articular afferents in the inflamed knee joint of the cat

Arthritic hyperalgesia and pain result from an increased activity of nociceptive afferents that may be induced and maintained by inflammatory mediators like bradykinin (BK). The B1 receptor antagonist des-Arg9-Leu8-BK and the B2 receptor antagonists Thi5,8-D-Phe7-BK and Hoe 140 were used to study the involvement of BK receptors in the generation of ongoing afferent activity in the cat's knee joint that was inflamed by kaolin and carrageenin. After i.a. bolus administration of BK receptor antagonists (26-260 micrograms) close to the joint, the ongoing activity did not significantly vary in any group III or group IV unit. We conclude that activation of BK receptors by endogenous BK is probably not the mechanism that is responsible for the increased ongoing activity of articular afferents in the inflamed joint.

Primary afferent depolarization of muscle afferents elicited by stimulation of joint afferents in cats with intact neuraxis and during reversible spinalization

1. In the anesthetized and artificially ventilated cat, stimulation of the posterior articular nerve (PAN) with low strengths (1.2-1.4 x T) produced a small negative response (N1) in the cord dorsum of the lumbosacral spinal cord with a mean onset latency of 5.2 ms. Stronger stimuli (> 1.4 x T) produced two additional components (N2 and N3) with longer latencies (mean latencies 7.5 and 15.7 ms, respectively), usually followed by a slow positivity lasting 100-150 ms. With stimulus strengths above 10 x T there was in some experiments a delayed response (N4; mean latency 32 ms). 2. Activation of posterior knee joint nerve with single pulses and intensities producing N1 responses only, usually produced no dorsal root potentials (DRPs), or these were rather small. Stimulation with strengths producing N2 and N3 responses produced distinct DRPs. Trains of pulses were clearly more effective than single pulses in producing DRPs, even in the low-intensity range. 3. Cooling the thoracic spinal cord to block impulse conduction, increased the DRPs and the N3 responses produced by PAN stimulation without significantly affecting the N2 responses. Reversible spinalization also increased the DRPs produced by stimulation of cutaneous nerves. In contrast, the DRPs produced by stimulation of group I afferents from flexors were reduced. 4. Conditioning electrical stimulation of intermediate and high-threshold myelinated fibers in the PAN depressed the DRPs produced by stimulation of group I muscle and of cutaneous nerves. 5. Analysis of the intraspinal threshold changes of single Ia and Ib fibers has provided evidence that stimulation of intermediate and high threshold myelinated fibers in the posterior knee joint nerve inhibits the primary afferent depolarization (PAD) of Ia fibers, and may either produce PAD or inhibit the PAD in Ib fibers, in the same manner as stimulation of cutaneous nerves. In 7/16 group I fibers the inhibition of the PAD was increased during reversible spinalization. 6. The results obtained suggest that intermediate and high-threshold myelinated fibers in the PAN have the same actions on Ia and Ib fibers as intermediate and high-threshold cutaneous afferents and may therefore be considered as belonging to the same functional system. They further indicate that in anesthetized preparations the pathways mediating the PAD of group I fibers, as well as the pathways mediating the inhibition of the PAD, may be subjected to a descending control that is removed by spinalization.

Innervation of the dura mater encephali of cat and rat: ultrastructure and calcitonin gene-related peptide-like and substance P-like immunoreactivity

Ultrastructural, immunocytochemical, and immunoelectron microscopical examinations are reported that describe the morphology of putative sensory nerve endings in the dura mater encephali of the rat and the cat. Morphometrical measurements and reconstructions showed that in the cat the mean diameter of axons, the bare area of axolemma, and the content of mitochondria and vesicles are highly variable in dural nerve endings. Nerve fibers with a high volume density of mitochondria are thought to be sensory, while nerve fibers containing many small vesicles are considered autonomic. There is, however, a broad overlap of mitochondria-rich and vesicle-rich nerve fibers in the dura, so that discrimination between sensory and autonomic endings by these characteristics frequently fails. Whole-mount preparations treated cytochemically for detection of substance P- and calcitonin gene-related peptide-like immunoreactivity in the rat and the cat showed a network of immunopositive nerve fibers in the vicinity of dural blood vessels. Most of these peptidergic and probably sensory nerve fibers were found terminating in the dural connective tissue far from vessels. Calcitonin gene-related peptide-positive nerve fibers were much more abundant than substance P-positive fibers. Immunoelectron microscopic preparations revealed that calcitonin gene-related peptide- and substance P-like immunoreactivity is found in a small proportion of generally thin unmyelinated nerve fibers. These proportions were very similar in the rat and the cat. Summarizing the recent literature, the morphological characteristics of putative sensory nerve fibers in the dura mater are discussed in relation to their possible functional significance for neurogenic inflammation and nociception.

Small reduction of capsaicin-induced neurogenic inflammation in human forearm skin by the glucocorticoid prednicarbate

Capsaicin applied to human skin provokes a response known as neurogenic inflammation. Neuropeptides (substance P, CGRP), released from afferent C-fiber terminals and histamine, secondarily released from mast cells, are supposed to participate in this reaction. We investigated the contribution of arachidonic acid and metabolic products to neurogenic inflammation, using a potent topically applied glucocorticoid and the corresponding vehicle. Arachidonic acid is liberated from membrane phospholipids by phospholipase A2, an enzyme that can be blocked by glucocorticoids. In 12 healthy volunteers, neurogenic inflammation was induced by capsaicin 1% on both upper forearms after 16 h of topical pretreatment with either prednicarbate or vehicle. Neurogenic inflammation was assessed by laser Doppler flowmetry and by planimetry of flare sizes. Prednicarbate significantly reduced the laser Doppler flow values inside the flare responses, as well as the flare sizes themselves. These results show that to some extent glucocorticoids reduce capsaicin-induced neurogenic inflammation.

Cyclooxygenase inhibitors acetylsalicylic acid and indomethacin do not affect capsaicin-induced neurogenic inflammation in human skin

Neurogenic inflammation is evoked by neuropeptides released from primary afferent terminals and, presumably, by other secondarily released inflammatory mediators. This study examines whether prostaglandins might participate in the development of neurogenic inflammation in humans and whether cyclooxygenase inhibitors have any anti-inflammatory effect on this type of inflammation. In healthy volunteers, neurogenic inflammation was elicited by epicutaneously applied capsaicin (1%), after systemic pretreatment with acetylsalicylic acid, or topically applied indomethacin compared to pretreatment with saline or vehicle, respectively. The extent of neurogenic inflammation was quantified by planimetry of visible flare size and recording the increase of superficial cutaneous blood flow (SCBF) with a laser Doppler flowmeter. Capsaicin-induced flare sizes and outside SCBF (both representing neurogenically evoked inflammation) were unaffected by acetylsalicylic acid or indomethacin. Only the capsaicin-induced increase of inside SCBF was attenuated by local pretreatment with indomethacin, reflecting the participation of prostaglandins in the inflammatory response of those areas which were in direct contact with capsaicin.

The effects of phorbol ester on slowly conducting afferents of the cat's knee joint

The effects of beta-phorbol 12,13-dibutyrate (PDBu) on the discharge properties of slowly conducting knee joint afferents (group III and group IV fibers) were studied to determine the role of protein kinase C in nociception. Extracellular single unit recordings were made from small filaments dissected from the medial articular nerve in cats anesthetized with alpha-chloralose. PDBu was applied intra-arterially close to the joint in concentrations of 10(-6) up to 10(-4) M. The afferents were classified as low-threshold and high-threshold units with regard to their sensitivity to passive noxious and innocuous movements of the knee joint. Following PDBu application, an excitation occurred in 28% of the group III and in 40% of the group IV fibers. An enhancement of responses to passive movements of the joint (sensitization) occurred in 37% of group III and 19% of group IV afferents. In summary, 37.5% of the low-threshold and 50% of the high-threshold fibers proved to be sensitive to PDBu. Most of the PDBu-positive units responded also to bradykinin, whereas only a few PDBu-positive units were sensitive to prostaglandin I2 and E2. We conclude from these results that, in a distinct population of slowly conducting joint afferents, protein kinase C is likely to be involved in the process of transduction. Thus, pain and hyperalgesia may be mediated at least partly by intracellular mechanisms that are linked to protein kinase C.

Depending on the mode of application morphine enhances or depresses somatocardiac sympathetic A- and C-reflexes in anesthetized rats

The effects of morphine on the reflex discharges in sympathetic efferents recorded from branches of the inferior cardiac nerve (ICN) were studied in rats anesthetized with alpha-chloralose and urethane. Somatocardiac sympathetic A- and C-reflexes were elicited by single shock electrical stimulation of myelinated (A) and unmyelinated (C) afferent fibers of the tibial nerve, respectively. Application of morphine either into the femoral vein or into the subarachnoid space of the cisterna magna enhanced both the A- and C-reflexes in a dose-dependent manner, while application of morphine into the intrathecal space of the lumbar spinal cord selectively inhibited C-reflexes. All effects of morphine were antagonized by naloxone. Application of morphine via the internal carotid artery to central nervous structures above the brainstem had no effect on the somatocardiac sympathetic reflexes. It is concluded that in the anesthetized rat morphine in a dose-dependent and naloxone-reversible manner (1) depresses spinal transmission of C-afferent activity, whereas (2) at the brainstem it enhances the transmission of somatocardiac sympathetic A- and C-reflexes.

Inflammatory mediators and nociception in the joint: excitation and sensitization of slowly conducting afferent fibers of cat's knee by prostaglandin I2

The effects of prostaglandin I2 on the discharge properties of fine articular afferents (group III and group IV fibers) in the cat were examined by extracellular recordings from single units dissected from the medial articular nerve of the knee joint. Prostaglandin I2 was applied intra-arterially close to the joint in doses of 0.3-30 micrograms per 0.3 ml bolus injection, and its effects on the spontaneous activity as well as on discharges evoked by mechanical and chemical stimulation (bradykinin) were monitored. Prostaglandin E2 was also applied and the effects of prostaglandins I2 and E2 on particular units were compared. An excitatory effect of prostaglandin I2 was observed in 49% of 37 group III and in 37% of 27 group IV units. A sensitization to passive movements of the joint occurred in 71% of 31 group III and 48% of 21 group IV units. Sixty-seven per cent of 32 units (groups III and IV) were both excited and sensitized by prostaglandin I2 to movements of 27% were sensitized but not excited. In 64% of 11 group III and 63% of eight group IV units studied the responses to bradykinin were enhanced by prostaglandin I2. Prostaglandin E2 had qualitatively similar effects as prostaglandin I2 but excited and sensitized a lower proportion of articular afferents. Forty-one per cent of the units were sensitive to both prostaglandins but 26% of the fibers were only sensitive to prostaglandin I2. None of the units was exclusively sensitive to prostaglandin E2. In general, the excitatory and sensitizing effects of prostaglandin E2 had a longer duration than those exerted by prostaglandin I2. We conclude that prostaglandin I2 increases the sensitivity to mechanical stimuli as well as to chemical stimulation by bradykinin in the majority of articular group III and group IV fibers. Moreover, in a large proportion of articular afferents, prostaglandin I2 had an excitatory effect. Thus, prostaglandin I2 may be an inflammatory mediator which is important for inflammation-evoked activity in slowly conducting afferents and it may participate in the development of arthritic hyperalgesia and pain.

Neurokinin A-like immunoreactivity in articular afferents of the cat

Dorsal root ganglion cells with axons innervating the cat's knee joint via the medial articular nerve were retrogradely labelled with Fast blue. Neurokinin A-like immunoreactivity was found in 4.5 +/- 1.1% (mean +/- S.D. of 5 nerves and 695 cells) of the articular afferents. Colchicine treatment of the ganglia increased the percentage of immunopositive cells to 8.5 +/- 0.7% (mean +/- S.D. of 6 nerves and 554 cells) after 3-22 h. The diameter distribution of the immunopositive somata ranged from 20 to 50 microns with a maximum at 26-30 microns. Comparing the proportions of neurokinin A-immunopositive cells with those of substance P, it can be calculated on the basis of mRNA encoding that neurokinin A is synthetized in about half of the substance P-containing primary articular afferents.

Activation of normal and inflamed fine articular afferent units by serotonin

In cats anesthetized with alpha-chloralose, extracellular recordings were made from fine afferent units belonging to the medial articular nerve (MAN) of the knee joint. The excitatory and sensitizing effects on articular afferents of serotonin (5-HT) applied intra-arterially close to the joint were examined. The joints were either normal or an experimental arthritis had been induced some hours before the recording session. Bolus injections of 1.35-135 micrograms 5-HT excited about 43% of group III (CV: 2.5-20 m/sec) and 73% of group IV units (CV: less than 2.5 m/sec) from normal joints. The latency was usually between 10 and 30 sec, and the duration and size of the responses were dose-dependent. Fast group III units (CV: greater than 16 m/sec) and group II units (CV: greater than 20 m/sec) were never excited by 5-HT. Repetitive administration led to pronounced tachyphylaxis of the 5-HT response. Inflammation induced an enhanced sensitivity of group III articular afferent units to close intra-arterial application of 5-HT. In particular the total duration of each response was considerably prolonged (4-10 min against 1-2 min under normal conditions). At the same time the tachyphylaxis seen under normal conditions was greatly reduced. In contrast, group IV articular afferent units did not become sensitized to 5-HT in the course of inflammation. In normal joints 5-HT did not sensitize fine afferent units for movement-induced responses. However, after inflammation, a distinct sensitization to such movements by 5-HT application could be observed both in group III and group IV fiber ranges. The sensitization had a short time course not exceeding 7 min. The tonic component of the movement-induced response was more enhanced than the phasic one. The bolus application of 5-HT led to temporary vasoconstriction of the knee joint vessels. This vasoconstriction was especially pronounced in inflamed joints and impeded the access of subsequently applied substances to the terminal regions of the afferent units under observation. It is concluded that the present results support the notion that 5-HT may participate in the mediation of pain from inflamed tissue such as an arthritic joint by exciting and sensitizing fine afferent units. During inflammation group III units are particularly sensitive to 5-HT and, thus, may carry the bulk of the 5-HT-induced nociceptive messages.

Knee joint input into the peripheral region of the ventral posterior lateral nucleus of cat thalamus

1. Experiments were carried out in chloralose-anesthetized cats to study the responses of neurons in the lateral thalamus to excitation of afferent fibres from the knee joint. 2. Single- and multi-unit recordings were made with tungsten electrodes in dorsoventral penetrations through the ventral posterior lateral nucleus (VPL) during electrical stimulation of the medial articular nerve (MAN) of the cat's knee joint at an intensity sufficient to excite slowly conducting unmyelinated fibers. The locations of the recording sites were verified by recovering electrolytic lesion sites in histological sections (Nissl and cytochrome oxidase staining). 3. The average earliest latency for excitation of thalamic responses was 19.1 +/- 8.5 (SD) ms (n = 50). The threshold for excitation of most thalamic units was found to correspond to peripheral joint afferent fibers of the A-delta group. 4. The majority of neurons responding to MAN stimulation were found to be dorsal or ventral to the low-threshold cutaneous hindlimb region of the lateral division of the VPL (stereotaxic coordinates: AP 9.0-11.5; ML 7.0-9.5). In the ventral periphery of the VPL, most neurons responding to MAN stimulation (11/14) were wide dynamic range (WDR) with a discrete cutaneous receptive field on the hindpaw digits. Six WDR neurons were found dorsal to the hindlimb VPL with a convergent receptive field on the hindlimb (but not hindpaw digits). No nociceptive-specific knee joint units were found. 5. Other neurons were found dorsal to the hindlimb VPL with large receptive fields often encompassing the whole contralateral leg, including skin and deep hindlimb structures, possibly in a region described as the dorsal portion of the posterior complex (POd). Some neurons were found with no receptive field. 6. This study provides the first observations on the responses of lateral thalamic neurons to stimulation of the MAN of the cat knee joint. These results demonstrate a central pathway conveying impulses from specific deep joint afferents of the MAN to the peripheral region of the VPL and overlying region known as the POd, regions implicated in the transmission of nociceptive information.

Changes in tonic descending inhibition of spinal neurons with articular input during the development of acute arthritis in the cat

1. In 15 alpha-chloralose-anesthetized cats we studied the presence of tonic descending inhibition (TDI) of spinal neurons with input from the knee and its modulation during an acute inflammation of this joint. TDI of spinal neurons with articular input was assessed by applying reversible cold blocks to the lower thoracic cord. The amount of descending inhibition was estimated from the induction and/or increase of resting discharges and of the responses to mechanical stimuli to the knee and other structures during the transitory and reversible blocks. In each experiment one or a few neurons were investigated while the joint was in normal condition [altogether 15 nociceptive-specific (NS) and 6 wide-dynamic-range (WDR) neurons]. One of the neurons was then selected for long-term recordings during which an acute inflammation in the knee was induced by the intra-articular injection of kaolin and carrageenan. Before and during developing arthritis, cold blocks were applied to examine whether the amount of TDI would change during the inflammatory process. 2. The neurons with input from the normal knee were under TDI because application of the cold block induced or increased resting discharges and the responses to noxious compression of the knee and the adjacent thigh and lower leg. In 10 of 15 NS neurons, the response threshold was lowered into the innocuous range. In 9 of 17 cells tested, the excitatory receptive field expanded to the ipsilateral paw, and 4 neurons became inhibited by paw compression. Seven of 18 neurons tested revealed inhibitory receptive fields on the contralateral leg during cold block. The neurons were located in laminae IV-VII. 3. Fourteen neurons were continuously monitored during development of inflammation, and changes in the effectiveness of TDI were assessed by blocking the cord before and during the development of arthritis. In most neurons baseline resting activity in the intact state of the cord increased while the arthritis developed. This inflammation-evoked enhancement of resting discharges was more pronounced during periods of spinalization. Consequently, the differences between the resting discharges in the cold-blocked and the intact state were progressively enhanced in arthritis. 4. After induction of arthritis, the responses to compression of the knee joint increased in the intact state as well as during cold blocks. In 11 of 14 neurons, the differences between the responses in the spinal and intact state were progressively enlarged during the development of inflammation. A similar result was obtained for flexion of the injected knee.(ABSTRACT TRUNCATED AT 400 WORDS)

Nociception and pain

In recent years, progress in the treatment of pain has been strongly influenced by new insights into the mechanisms underlying pain and nociception. The following article briefly reviews some recent reports which make a significant contribution to our knowledge of the structure and function of nocisensors, the neuropeptides involved in the nociceptive system, and the spinal and supraspinal central mechanisms of nociception.

Tonic descending inhibition of spinal cord neurones driven by joint afferents in normal cats and in cats with an inflamed knee joint

In ten cats, single unit electrical activity was recorded in the lumbosacral spinal cord from neurones driven by stimulation of afferent fibres from the ipsilateral knee joint. Tonic descending inhibition (TDI) on the responses of these cells was measured as increases in resting and evoked activity of the neurones following reversible spinalization of the animals with a cold block at upper lumbar level. Acute inflammation of the knee joint was induced in five of the cats by the injection of kaolin and carrageenan into the joint. TDI was observed in 25 of 33 neurones recorded in normal animals (76%) and in 36 of 40 (90%) neurones recorded in animals with acute knee joint inflammation. In both kinds of preparation TDI was more pronounced in neurones recorded in the deep dorsal horn and in the ventral horn than in those recorded in the superficial dorsal horn. There was a tendency in the whole sample for TDI to be greater in neurones with input from inflamed knees. We conclude that the spinal processing of afferent information from joints is under tonic descending influences and that the amount of TDI can be altered during acute arthritis.

Substance P- and calcitonin gene-related peptide immunoreactivity in primary afferent neurons of the cat's knee joint

The distribution of substance P and calcitonin gene-related peptide was determined in primary afferent neurons of the medial and posterior articular nerve of the cat's knee joint. Perikarya of articular afferents were visualized by retrograde labelling with the fluorescent dye Fast Blue which was applied at the transected end of the peripheral nerves. Substance P was found in about 17% of labelled medial articular afferents and in about 16% of labelled posterior articular afferents, respectively, whereas calcitonin gene-related peptide was present in about 35 and 32% of the medial and posterior articular nerve cells, respectively. Taking into account that these neuropeptides are known to be co-localized, probably not more than one-third of the joint afferents contain substance P and/or calcitonin gene-related peptide. Quantification of cell diameters revealed that substance P was found only in small- or intermediate-sized perikarya (less than 50 microns) indicating that this peptide is predominantly found in unmyelinated neurons. Calcitonin gene-related peptide was present mainly in small- and intermediate- but also in some large-sized neurons (greater than 50 microns) providing evidence that this peptide is found in unmyelinated and to a lesser extent in myelinated neurons. This is consistent with previous studies that show that substance P and calcitonin gene-related peptide are present primarily in unmyelinated and thinly myelinated primary afferents. When the portion of substance P-positive neurons of the medial articular nerve is compared to the number of articular afferents displaying a nociceptive function as determined in earlier electrophysiological studies, it can be calculated that at most 30% of the nociceptive-specific articular afferents contain this neuropeptide.(ABSTRACT TRUNCATED AT 250 WORDS)

Response properties of thick myelinated group II afferents in the medial articular nerve of normal and inflamed knee joints of the cat

This study investigated the responses to innocuous and noxious mechanical stimuli of joint mechanoreceptors with thick myelinated articular afferents (conduction velocities 21-65 m/sec) in the medial articular nerve of the cat's knee. In nine experiments, we examined whether acute arthritis would modify the discharge properties. The vast majority of the group II afferents were excited by gentle local stimuli and by movements in the working range of the knee. Although they encoded pressure and particular movement stimuli up to the noxious range, their responses were more closely related to the particular type of stimulus (e.g., a movement in a specific direction) than to its intensity (innocuous vs. noxious). In inflamed joints, the response patterns of group II units were similar with regard to local mechanical thresholds, thresholds for passive movements, and patterns of responses to passive movements. In both situations, most units had no resting activity. These results suggest that articular group II afferents do not play a significant role in nociception. Rather, they subserve proprioceptive functions such as deep pressure sensation and kinesthesia in normal as well as inflamed joints.

Capsaicin inhibits responses of fine afferents from the knee joint of the cat to mechanical and chemical stimuli

In adult anaesthetized cats we studied the effects of capsaicin on the responses of single slowly conducting afferents of the knee joint to mechanical and chemical stimuli (bradykinin). An intra-arterial bolus injection of capsaicin into the joint reduced or abolished the responses to passive movements of the joint in 8 of 19 afferents and the responses to intra-arterially administered bradykinin in 10 of 11 units. Capsaicin was usually effective at 10(-4) to 10(-3) M. The inhibition was predominantly observed in nociceptive afferents, whereas most low threshold units were not desensitized.