Pharmacological validation of early and late phase of rat mono-iodoacetate model using the Tekscan system.

BACKGROUND: Previous pharmacological validations of the rat mono-iodoacetate (MIA)-induced chronic joint pain model were mostly performed by measuring weight-bearing (WB) deficit with an incapacitance tester. However, conventional incapacitance testers have several drawbacks including restrain stress on animal and sole use of hind limbs WB. OBJECTIVES: The aim of the present study was to compare pharmacological sensitivity of the early (up to 1 week after MIA) versus late (between 2 and 4 weeks after MIA) phase of the rat MIA model using a highly sensitive tactile pressure measurement system (Tekscan(®)), which can measure weight borne by all four limbs and the tail in a non-restrained animal. METHODS: The Tekscan(®) WB measurement system was used in MIA rats to examine the acute and chronic dosing effects of drugs that targeted different mechanisms. Electrophysiological recordings from joint afferents and biochemical analysis of synovial fluid were also performed. RESULTS: Dexamethasone, duloxetine and morphine significantly alleviated WB deficits in the Tekscan(®) system during both early and late phase of the MIA model while celecoxib and naproxen alleviated WB deficit only during the early phase. Similarly, naproxen was able to inhibit spontaneous neuronal activity from MIA joint afferents only during the early phase. Finally, concentrations of prostaglandin E(2) in synovial fluid were elevated only during the early phase of the rat MIA model. CONCLUSIONS: Our pharmacological validation studies using the Tekscan(®) system along with electrophysiological and biochemical results suggest different mechanisms for early and late phase of MIA-induced chronic joint pain in rat.

Tactile-induced ultrasonic vocalization in the rat: a novel assay to assess anti-migraine therapies in vivo.

A pharmacological model of migraine is described using ultrasound vocalization (USV) of rats following central inflammation-induced sensitization to tactile stimulation. Central inflammation induced by intracerebroventricular injection of lipopolysaccharide (LPS) increased USV induced by an air current focused on the head and this was abolished by morphine and ketorolac, suggesting a nociceptive component. USV in naive rats were unaffected. Diazepam reduced USV in both inflamed and naive rats. The triptans, zolmitriptan and sumatriptan, both reduced USV in inflamed but not in naive rats, as did dihydroergotamine, and the calcitonin gene-related peptide (CGRP) antagonists alphaCGRP(8-37) and BIBN4096BS. The neurokinin-1 antagonist L-733-060 had no effect in either inflamed or naive rats when given after induction of inflammation, but when given with the LPS it prevented the augmentation of USV. This profile of activity of agents proven to be effective in the clinic suggests this model can be used to predict novel therapeutic agents for migraine.

Selective depression of nociceptive responses of dorsal horn neurones by SNC 80 in a perfused hindquarter preparation of adult mouse.

Detailed electrophysiological characterisation of spinal opioid receptors in the mouse has been limited due to various technical difficulties. In this study, extracellular single unit recordings were made from dorsal horn neurones in a perfused spinal cord with attached trunk-hindquarter to investigate the role of delta-opioid receptor in mediating nociceptive and non-nociceptive transmission in mouse. Noxious electrical shock, pinch and heat stimuli evoked a mean response of 20.8+/-2.5 (n=10, P<0.005), 30.1+/-5.4 (n=58, P<0.005) and 40.9+/-6.3 (n=29, P<0.005) spikes per stimulus respectively. In 5 of 22 cells, repetitive noxious electrical stimuli applied to the hindpaw for 20 s produced a progressive increase in spike number, the phenomenon known as 'wind-up' and/or hyperactivity. When the selective delta-opioid receptor agonist (+)-4-[(alpha R)-alpha-((2S,5R)-4-allyl-2,5-dimethyl-1-piperazinyl)-3-methoxybenzyl]-N,N-diethylbenzamide (SNC 80) was perfused for 8-10 min, these evoked nociceptive responses were reversibly depressed. SNC 80 (2 microM) depressed the nociceptive responses evoked by electrical shock, pinch and heat by 74.0+/-13.7% (n=8, P<0.01), 66.5+/-16.6% (n=10, P<0.01) and 74.1+/-17.0% (n=10, P<0.01) respectively. The maximum depression by 5 microM SNC 80 was 92.6+/-6.8% (n=3). SNC 80 at 5 microM also completely abolished the wind-up and/or hypersensitivity (n=5). The depressant effects of SNC 80 on the nociceptive responses were completely blocked by 10 microM naloxone (n=5) and 3 microM 17-(cyclopropylmethyl)-6,7-dehydro-4,5 alpha-epoxy-14 beta-ethoxy-5 beta-methylindolo [2',3':6',7'] morphinan-3-ol hydrochloride (HS 378, n=8), a novel highly selective delta-opioid receptor antagonist. Interestingly, HS 378 (3 microM) itself potentiated the background activity and evoked responses to pinch and heat by 151.8+/-38.4% (P<0.05, n=8), 34.2+/-6.1% (P<0.01, n=7) and 45.5+/-11.8% (P<0.05, n=5) respectively. In contrast, the responses of non-nociceptive dorsal horn neurones were not inhibited by SNC 80 at a dose of up to 10 microM (n=5). These data demonstrate that delta-opioid receptor modulate nociceptive, but not non-nociceptive, transmission in spinal dorsal horn neurones of the adult mouse. The potentiation of neuronal activity by HS 378 may reflect an autoregulatory role of the endogenous delta-opioid in nociceptive transmission in mouse.

The effects of delta agonists on locomotor activity in habituated and non-habituated rats.

The effects of the delta agonists SNC80 and deltorphin II on ambulation and rearing activity were measured in habituated and non-habituated rats. SNC80 (30, 100, 200, 400 nmol, i.c.v.) and deltorphin II (3, 15, 30, 60 nmol, i.c.v.) induced similar, dose-dependent biphasic locomotor effects in non-habituated subjects. An initial decrease in exploratory activity was associated with anxiogenic signs such as pilo-erection, freezing behaviour and pupil dilation for each drug. Pre-treatment with the delta antagonist naltrindole (10 nmol, i.c.v.) inhibited the depressant effect, but not the subsequent stimulant effect, on locomotor activity in response to 30 nmol deltorphin II in this assay (P<0.05). In habituated rats, deltorphin II (0.03, 0.1, 0.3, 3 nmol, i.c.v.) caused significant, naltrindole-reversible increases in locomotor activity (P<0.05 for all doses) at 1,000-fold lower doses than those required for a similar response to SNC80 (10, 30, 100, 300 nmol, i.c.v.). Pharmacokinetic studies suggest that these compounds penetrate the brain to similar extents following i.c.v. injection. The substantial potency difference between deltorphin II and SNC80 in stimulating locomotor activity in habituated rats suggests pharmacological heterogeneity for these delta opioid receptor agonists.

Antihyperalgesic effects of delta opioid agonists in a rat model of chronic inflammation.

Opioid receptors in the brain activate descending pain pathways to inhibit the nociceptive response to acute noxious stimuli. The aim of the present study was to clarify the role of supraspinal opioid receptors in modulating the nociceptive response to persistent inflammation in rats. Subcutaneous administration of 50 microl of complete Freund's Adjuvant (CFA) into the plantar surface of the hindpaw induced a significant decrease in paw withdrawal latency to thermal stimuli (P<0.01) at 24 h post-injection. Intracerebroventricular (i.c.v.) administration of the mu opioid receptor agonists, DAMGO and morphine, and the delta opioid receptor agonists, deltorphin II and SNC80, significantly reversed the hyperalgesic response associated with peripheral inflammation in a dose-dependent manner (P<0.0001). The mu and delta agonists also significantly attenuated the antinociceptive response to acute thermal stimulation in rats (P<0.001). However, deltorphin II and SNC80 were less potent, and in the case of SNC80 less efficacious, in modulating the response to acute thermal nociception in comparison to hyperalgesia associated with persistent inflammation. These results indicate that mu and delta opioid receptors in the brain modulate descending pain pathways to attenuate the nociceptive response to acute thermal stimuli in both normal and inflamed tissues. The heightened response to delta agonists in the hyperalgesia model suggests that delta opioid receptors in the brain are promising targets for the treatment of pain arising from chronic inflammation.

Bradyzide, a potent non-peptide B(2) bradykinin receptor antagonist with long-lasting oral activity in animal models of inflammatory hyperalgesia.

Bradyzide is from a novel class of rodent-selective non-peptide B(2) bradykinin antagonists (1-(2-Nitrophenyl)thiosemicarbazides). Bradyzide has high affinity for the rodent B(2) receptor, displacing [(3)H]-bradykinin binding in NG108-15 cells and in Cos-7 cells expressing the rat receptor with K(I) values of 0.51+/-0.18 nM (n=3) and 0.89+/-0.27 nM (n=3), respectively. Bradyzide is a competitive antagonist, inhibiting B(2) receptor-induced (45)Ca efflux from NG108-15 cells with a pK(B) of 8.0+/-0.16 (n=5) and a Schild slope of 1.05. In the rat spinal cord and tail preparation, bradyzide inhibits bradykinin-induced ventral root depolarizations (IC(50) value; 1.6+/-0.05 nM (n=3)). Bradyzide is much less potent at the human than at the rodent B(2) receptor, displacing [(3)H]-bradykinin binding in human fibroblasts and in Cos-7 cells expressing the human B(2) receptor with K(I) values of 393+/-90 nM (n=3) and 772+/-144 nM (n=3), respectively. Bradyzide inhibits bradykinin-induced [(3)H]-inositol trisphosphate (IP(3)) formation with IC(50) values of 11.6+/-1.4 nM (n=3) at the rat and 2.4+/-0.3 microM (n=3) at the human receptor. Bradyzide does not interact with a range of other receptors, including human and rat B(1) bradykinin receptors. Bradyzide is orally available and blocks bradykinin-induced hypotension and plasma extravasation. Bradyzide shows long-lasting oral activity in rodent models of inflammatory hyperalgesia, reversing Freund's complete adjuvant (FCA)-induced mechanical hyperalgesia in the rat knee joint (ED(50), 0.84 micromol kg(-1); duration of action >4 h). It is equipotent with morphine and diclofenac, and 1000 times more potent than paracetamol, its maximal effect exceeding that of the non-steroidal anti-inflammatory drugs (NSAIDs). Bradyzide does not exhibit tolerance when administered over 6 days. In summary, bradyzide is a potent, orally active, antagonist of the B(2) bradykinin receptor, with selectivity for the rodent over the human receptor. British Journal of Pharmacology (2000) 129, 77 - 86

Substance P and capsaicin-induced mechanical hyperalgesia in the rat knee joint; the involvement of bradykinin B1 and B2 receptors.

1. Substance P (SP) and capsaicin induced a mechanical hyperalgesia when injected into rat knee joints. 2. The NK1 receptor antagonists CP 99994 (10-100 nmol) and RP 67580 (0.1-1 nmol) blocked the development of, and also reversed, SP-induced hyperalgesia. Capsaicin (10 nmol)-induced hyperalgesia was blocked by capsazepine (0.5-5 nmol). 3. Capsaicin-induced hyperalgesia was prevented and reversed by the NK1 receptor antagonists CP 99994 (100 nmol) and RP 67580 (1 nmol). 4. The bradykinin B2 receptor antagonist icatibant (5 pmol) blocked the development of both SP and capsaicin-induced hyperalgesia. Icatibant (100 pmol kg-1, i.v.) also reversed an established SP and capsaicin-induced hyperalgesia. 5. Both low dose SP (1 nmol) and capsaicin (1 nmol)-induced hyperalgesia were potentiated by the kininase II inhibitor captopril (100 micrograms). 6. The B1 receptor antagonists desArg9Leu8-bradykinin (BK) (0.5-5 nmol) and desArg10[Hoe 140] (5-50 pmol) only blocked the development of SP-induced hyperalgesia for 30 min after administration. desArg9Leu8-BK (10 nmol kg-1 i.v.) did not reverse an established SP-induced hyperalgesia. 7. Capsaicin-induced hyperalgesia was blocked by desArg9Leu8-BK (0.5 nmol) and this antagonist also reversed an established capsaicin-induced hyperalgesia. 8. Interleukin-1 receptor antagonist (IL-1ra 0.1 microgram) reduced the development of SP-induced hyperalgesia up to 4 h after administration, but did not reverse an established hyperalgesia. IL-1ra (0.1 microgram) also blocked the development of and reversed an established capsaicin-induced hyperalgesia. 9. Indomethacin pretreatment (1 mg kg-1, s.c.) did not reduce the development of either SP- or capsaicin-induced hyperalgesia but following indomethacin-pretreatment desArg9Leu8-BK (10 nmol kg-1, i.v.) failed to reverse a capsaicin-induced hyperalgesia. 10. In conclusion, both SP and capsaicin can induce behavioural hyperalgesia when injected into the knee joint of rats. In addition, blockade of NK1, bradykinin B1, B2 and IL-1 beta receptors can substantially modulate this hyperalgesia.

Bradykinin B1 and B2 receptor mechanisms and cytokine-induced hyperalgesia in the rat.

The involvement of bradykinin (BK) B1 and B2 receptors in cytokine-induced hyperalgesia has been studied in the rat. Intraplantar injections of interleukin (IL) 1 beta and tumor necrosis factor alpha (TNF alpha) induced thermal hyperalgesia, with in the the case of IL-1 beta, contralateral hyperalgesia also present. Subsequent to administration of IL-1 beta, but not after TNF alpha, des-Arg9-BK reduced the withdrawal latency in both ipsi- and contra-lateral paws. Mechanical hyperalgesia was also induced by IL-1 beta, IL-2, and IL-8 when injected into rat knee joints, whereas IL-6 and TNF alpha were without effect. Coadministration of des-Arg9,Leu8-BK prevented the development of the cytokine-induced hyperalgesia for the duration of the experiment (6 h), but HOE-140 only reversed the hyperalgesia for the 1st h. At 3.5 h after IL-1 beta, IL-2, or IL-8, administration of des-Arg9,Leu8-BK or HOE-140 (iv) completely reversed the hyperalgesia. Twenty-four hours after pretreatment with IL-1 beta, injection of des-Arg9-BK into the joint produced opposite effects, depending on the dose: at 50 pmol the hyperalgesia was reversed, but at 0.5 nmol there was further hyperalgesia. Both responses were blocked by B1 but not B2 receptor antagonists. These data suggest that both B1 and B2 receptors are involved in the induction and maintenance of cytokine-induced hyperalgesia. B1 receptors appear to play a more important role than B2 receptors in the development of mechanical hyperalgesia.

The role of bradykinin B1 receptors in the maintenance of intra-articular plasma extravasation in chronic antigen-induced arthritis.

1. The role of bradykinin B1 and B2 receptors in bradykinin- and des-Arg9-bradykinin-induced plasma extravasation in normal and inflamed rat knee joints was investigated by use of an antigen-induced model of chronic arthritis. A modification of an Evans blue extraction technique allowed the unstimulated (basal) plasma extravasation to be assessed in this model. The contributions of bradykinin B1 and B2 receptors towards basal synovial plasma extravasation were determined. 2. In normal knees, intra-articular injection of bradykinin (BK) induced plasma extravasation in a potent, dose-dependent manner with a threshold of 0.01 nmol and an ED50 of 0.1 nmol. In day 5 arthritic knees, basal plasma extravasation was substantially enhanced. Lower doses of BK had no demonstrable effect and increases above basal extravasation were first observed at 0.1 nmol. Thereafter the dose-response mirrored the response in normal knees and the maximal response was unaltered. 3. The B1 agonist, des-Arg9-BK, induced slight but significant plasma extravasation in normal knees but was less potent than bradykinin. This response was inhibited by the B1 receptor antagonist, des-Arg9, [Leu8]-BK. Lower doses of des-Arg9-BK bradykinin did not significantly increase basal extravasation in day 5 arthritic knees but, in contrast to BK, the maximal response was significantly enhanced. 4. The B2 antagonist, Hoe 140, inhibited BK-induced plasma extravasation in normal joints over a dose-range of 0.1-1.0 nmol but was relatively inactive in day 5 inflamed knees. The B1 receptor antagonist, des-Arg9, [Leu8]-BK, was relatively inactive in normal joints but showed increased potency against BK-induced plasma extravasation in day 5 arthritic joints.5. Hoe 140 and des-Arg9,[Leu8]-BK both inhibited basal extravasation in arthritic joints on days 1 and 5 post-challenge in a dose-dependent fashion. Whilst Hoe 140 was the more potent inhibitor on day 1, it was less potent than des-Arg9,[Leu8]-BK on day 5.6. Although the majority of responses to BK in normal tissue are mediated via B2 receptors, a small population of B1 receptors may exist in normal joint tissues. The data presented in this study suggest an evolving role for B1 receptors in the mediation of plasma extravasation in inflamed joint tissues. A role for BK antagonists in the treatment of arthritis is also suggested.

The involvement of bradykinin B1 and B2 receptor mechanisms in cytokine-induced mechanical hyperalgesia in the rat.

1. Interleukin-1 beta (IL-1 beta), IL-2 and IL-8 induced a mechanical hyperalgesia following intra-articular (i.artic.) injection into rat knee joints, whereas IL-6 and tumour necrosis factor alpha (TNF-alpha) were without effect. 2. Co-administration of IL-1 receptor antagonist (0.1 micrograms) with IL-1 beta (1 mu), IL-2 (10 mu) or IL-8 (0.1 mu) prevented the subsequent development of the hyperalgesia. 3. Co-administration of desArg9Leu8BK (0.5-5 nmol) with IL-1 beta (1 mu), IL-2 (10 mu) or IL-8 (0.1 mu) reduced the level of hyperalgesia at 1, 4 and 6 h post administration, whereas Hoe 140 (5 pmol) antagonized the hyperalgesia only at the 1 h time point. 4. Intravenous administration of desArg9Leu8BK (10 nmol kg-1) or Hoe 140 (100 pmol kg-1) following IL-1 beta (1 mu), IL-2 (10 mu), or IL-8 (0.1 mu) reversed the subsequent hyperalgesia. 5. Administration of desArg9BK into joints 24 h after pre-treatment with IL-1 beta (1 mu) produced analegsia at low doses (50 pmol) and hyperalgesia at a higher dose (0.5 nmol). Both these effects were blocked by desArg9Leu8BK (0.5 nmol). 6. Administration of desArg9BK (0.5 nmol i.artic.) to animals 24 h after pre-treatment with IL-2 (1-100 mu) or IL-8 (0.1-10 mu) had no effect on the load tolerated by the treated joint. 7. Administration of indomethacin (1 mg kg-1, s.c.) prior to IL-1 beta (1 mu i.artic.) prevented the development of hyperalgesia. Administration of desArg9BK (5 pmol-0.5 nmol, i.artic.) to animals 24 h after indomethacin and IL-1 beta pretreatment had no effect on the load tolerated by the treated joint. 7. Administration of indomethacin (1 mg kg-1, s.c.) prior to IL-1beta (1 u i.artic.) prevented the development of hyperalgesia. Administration of desArg9BK (5 pmol-0.5 nmol, i.artic.) to animals 24 h after indomethacin and IL-1 beta pretreatment had no effect on the load tolerated by the treated joint.8. These data suggest that both bradykinin B1 and B2 receptors are involved in the induction and maintenance of cytokine-induced hyperalgesia. They also show that the induction of B1 receptor-mediated hyperalgesia requires both cyclo-oxygenase products and IL-1 in vivo.

The induction of des-Arg9-bradykinin-mediated hyperalgesia in the rat by inflammatory stimuli.

1. The B1 receptor agonist des-Arg9-BK does not induce mechanical hyperalgesia when injected into the rat knee joint at 1-100 nmol, or thermal hyperalgesia when injected intravenously up to 1 mumol/kg. 2. Bradykinin (BK), administered into the joint, (1 nmol-1 mumol) induces a mechanical hyperalgesia, which is maximal by 4 h. Co-administration of BK with Hoe 140 (5 pmol) blocked development of the hyperalgesia, whereas des-Arg9-Leu8-BK (0.5 nmol) had no effect. Intravenous BK had no effect on thermal paw withdrawal latencies up to 1 mumol/kg. 3. Following joint inflammation induced by local Freund's complete adjuvant, intraarticular injection of des-Arg9-BK (0.05-10 nmol) and BK (0.5-100 nmol) caused a reduction in tolerated load. Co-administration of des-Arg9-Leu8-BK (0.5 nmol) with des-Arg9-BK (0.5 nmol) blocked development of the hyperalgesia, whereas Hoe 140 (5 pmol) had no effect. BK (1 nmol)-induced hyperalgesia was blocked by Hoe 140 but not des-Arg9-Leu8-BK. 4. Following UV irradiation of the paw, intravenous des-Arg9-BK and BK reduced paw withdrawal latencies to a noxious thermal stimulus indicating thermal hyperalgesia. The latency reduction induced by des-Arg9-BK and BK was prevented with co-administration of des-Arg9-Leu8-BK 200 nmol/kg, but not with Hoe 140 0.5 mumol/kg. 5. After interleukin-1 beta pre-treatment (1 unit into the joint or paw) des-Arg9-BK induced both thermal and mechanical hyperalgesia. Co-administration of des-Arg9-Leu8-BK 0.5 nmol with des-Arg9-BK 0.5 nmol into the joint prevented development of hyperalgesia and co-administration of des-Arg9-Leu8-BK (200 nmol/kg, iv) with des-Arg9-BK 10 nmol/kg prevented reduction of thermal withdrawal latencies. 6. These data suggest that after an inflammatory insult B1 receptors may play a role in the transduction of nociceptive information.

Interleukin-1 beta induced-desArg9bradykinin-mediated thermal hyperalgesia in the rat.

The induction of desArg9Bk-mediated thermal hyperalgesia has been studied in the rat. Intraplantar injections of interleukin-1 beta, (IL-1 beta, 1-100 Units, U) produced a 30-40% decrease in withdrawal latency to noxious heat which lasted for up to 24 hr in the treated paw. IL-1 beta-induced thermal hyperalgesia was also present in the contralateral paw for up to 1 hr following IL-1 beta injection. Intraplantar tumour necrosis factor-alpha (TNF alpha, 500 U) also reduced the withdrawal latency in the u.v.-treated paw with recovery by 18 hr but with no contralateral hyperalgesia. Forty-eight hr after IL-1 beta but not after TNF alpha administration, desArg9Bk (10 nmol/kg i.v.) reduced the withdrawal latency in both the ipsi- and contra-lateral paws by 30-49%. In naive animals, desArg9Bk was without effect. These data suggest that, in the rat, IL-1 beta but not TNF alpha can lead to the induction of bradykinin B1 receptors mediating thermal hyperalgesia.

Induction of B1 receptors in vivo in a model of persistent inflammatory mechanical hyperalgesia in the rat.

The induction of mechanical hyperalgesia by B1 and B2 receptor agonists following an inflammatory insult has been studied in the rat. Intra-articular injection of bradykinin, but not desArg9 bradykinin, into naïve joints induced mechanical hyperalgesia with a reduction of up to 38% in load tolerated by the treated leg. Following intra-articular injection of Freund's complete adjuvant (FCA) the load tolerated by the treated leg was depressed for three days. The development of FCA-induced hyperalgesia was reduced by desArg9Leu8BK or Hoe 140. Bradykinin, desArg9BK and the metabolically stable B1 receptor agonist SarDPhe8desArg9BK were all hyperalgesic upon intra-articular administration 3 days after FCA administration. The hyperalgesia induced by desArg9BK and SarDPhe8desArg9BK was prevented by co-administration of desArg9Leu8BK but not by Hoe 140. At low doses (< 10 nmol) bradykinin-induced hyperalgesia was presented by Hoe 140 but not by desArg9Leu8BK. At higher doses of bradykinin (> 100 nmol) the hyperalgesia was reduced by both Hoe 140 and desArg9Leu8BK. These data suggest that following an inflammatory insult both bradykinin B1 and B2 receptor mechanisms are involved in the accompanying hyperalgesia.

Induction of bradykinin B1 receptors in vivo in a model of ultra-violet irradiation-induced thermal hyperalgesia in the rat.

1. The role of bradykinin B1 receptors in the thermal hyperalgesia following unilateral ultra-violet (u.v.) irradiation of the hindpaw of rats has been investigated. 2. In non-irradiated (naive) animals the B1 receptor agonist des-Arg9-bradykinin and bradykinin (BK) (up to 1 mumol kg-1 i.v.) had no effect on withdrawal latency to a noxious heat stimulus when administered 60 min before testing. 3. Following exposure of one hindpaw to strong u.v. irradiation the withdrawal latency of the u.v.-treated paw to radiant noxious heat fell by a maximum of 50% after 48 h. There was no reduction in latency in the contralateral paw. 4. des-Arg9-BK (1-100 nmol kg-1 i.v.) administered 24 h after u.v. exposure caused a further dose-dependent fall (50 +/- 4% reduction from saline injected animals at 100 nmol kg-1 i.v.) in withdrawal latency in the u.v.-treated paw when measured 60 min after injection. The withdrawal latency of the contralateral paw was also reduced but to a lesser extent following des-Arg9-BK (100 nmol kg-1 i.v.) with a maximum reduction of 19 +/- 3%. 5. Bradykinin also induced a further reduction in withdrawal latency (33 +/- 5% reduction at 1 mumol kg-1) although it was not as effective as des-Arg9-BK. Bradykinin did not reduce the withdrawal latency in the contralateral paw. 6. The hyperalgesic action of both des-Arg9-BK (10 nmol kg-1 i.v.) and bradykinin (100 nmol kg-1 i.v.)were antagonized by the B, receptor antagonist, des-Arg9,Leu8-BK (200 nmol kg-1 i.v.) but not by the B2 receptor antagonist, HOE 140 (0.5 .micromol kg-1 i.v.).7. The results suggest that in conditions of inflammatory hyperalgesia bradykinin B1 receptors are induced both locally and distant to the inflamed area, activation of which leads to further thermal hyperalgesia. In addition, in these conditions bradykinin appears to act predominantly via B1 receptors,presumably after degradation to des-Arg9-BK.

Activity of deep dorsal horn neurons in the anaesthetized rat during hyperalgesia of the hindpaw induced by ultraviolet irradiation.

Thermal hyperalgesia was induced by UV irradiation of the glabrous skin of the hindpaw of adult female Sprague-Dawley rats. We have recorded single cell activity and studied excitability changes in wide dynamic range neurons in the lumbar spinal segments during the early phase (days 1-3) and late phase (days 5-7) of thermal hyperalgesia in animals under urethane anaesthesia. The proportion of spontaneously active wide dynamic range cells was increased following UV irradiation and the degree of spontaneous activity was enhanced during the course of hyperalgesia. In addition there was a significant increase in the total number of spikes evoked by standardized mechanical and noxious heat stimuli when tested at days 1-3 and days 5-7. The duration of the evoked responses was also significantly prolonged in both UV-treated groups. The noxious temperature threshold to radiant heat stimulation was significantly decreased on the UV-treated but not on the contralateral hindpaw. The average size of the receptive fields on the UV-treated paws was expanded in comparison to control. To differentiate between possible central and peripheral components of the hyperactivity of wide dynamic range cells we performed in situ dorsal rhizotomy during the recording. Cutting the dorsal roots (L2-5) evoked a significantly larger and more prolonged discharge in wide dynamic range cells in both UV-treated groups in comparison to control. Spontaneous activity in spinal wide dynamic range neurons was reduced after rhizotomy in each group. However, the decrease was only significant at days 1-3 (P < 0.05) but not at days 5-7.(ABSTRACT TRUNCATED AT 250 WORDS)

Analogues of capsaicin with agonist activity as novel analgesic agents; structure-activity studies. 2. The amide bond "B-region".

A series of compounds incorporating replacements for the amide bond "B-region" moiety of capsaicin have been synthesized, including vanillylamides and esters, homovanillic acid amides and esters, ureas, and thioureas. These have been tested in an in vitro assay for agonism (45Ca2+ influx into dorsal root ganglia neurones), which is predictive of analgesic activity, to investigate the requirements in this region of capsaicin for activity. N-(4-Hydroxy-3-methoxybenzyl)-N'-octylthiourea (14a) emerged as the most potent analogue (EC50 = 0.06 microM). An operational model based on multiple hydrogen-bonding interactions is proposed to explain the structure-activity profile observed. In combination with studies on the other regions of the capsaicin molecule these results describe a picture of the molecular interactions of capsaicin with its putative receptor.

Analogues of capsaicin with agonist activity as novel analgesic agents; structure-activity studies. 1. The aromatic "A-region".

A series of analogues of capsaicin, the pungent principle of chilli peppers, was synthesized and tested in assays for capsaicin-like agonism in vitro. The results of these assays were compared with activities in an acute nociceptive model and a correlation was observed which established that the results of these in vitro assays were predictive of analgesia. Using a modular approach the structure-activity profile of specific regions of capsaicin congeners was established using an in vitro assay measuring 45Ca2+ uptake into neonatal rat dorsal root ganglia neurones. Substituted benzylnonanamides 2a-z and N-octyl-substituted phenylacetamides 4a-v were made to test the requirements for activity in the aromatic "A-region" of the molecule. Compounds with the natural substitution pattern (2b and 4c) and the corresponding catechols (2i and 4g) were the most potent, although the catechols were less potent in vivo. Other substitution patterns have reduced activity. These results have established stringent structural requirements for capsaicin-like activity in this part of the molecule.

Analogues of capsaicin with agonist activity as novel analgesic agents; structure-activity studies. 3. The hydrophobic side-chain "C-region".

Structural variants of the hydrophobic side chain ("C region") of the capsaicin molecule have been incorporated into a series of vanillylamides and vanillylthioureas. These compounds have been tested in an in vitro assay for agonism (45Ca2+ influx into dorsal root ganglia neurones), previously shown to be predictive of analgesic activity. The results of this study have established the requirement for a hydrophobic substituent of limited size (molar refractivity, MR, < 55) in order to obtain high potency. Combination of the information gained here about the "C-region" of the capsaicin molecule with the studies described in the preceding two papers provides a rational basis for the design of compounds of increased potency.

The effect of capsaicin and conventional analgesics in two models of monoarthritis in the rat.

Systemic administration of capsaicin has been shown to be antinociceptive in a number of acute nociceptive models. We present here two models of prolonged hyperalgesia in the rat induced by intra-articular injection of uric acid or Freund's complete adjuvant into the knee. The time course of hyperalgesia lasts up to 48 h with uric acid and 5 to 10 days with Freund's adjuvant. The hyperalgesia was maximal in both models after 18 h. At this time, the weight tolerated by the injected leg was reduced by 70-80% in the Freund's-treated and 50-60% in the uric-treated animals, compared to the uninjected leg. The hyperalgesia was reversed in both models by indomethacin and ibuprofen (1-100 mg kg-1 s.c.), paracetamol (50-500 mg kg-1 s.c.) and morphine (2-16 mg kg-1 s.c.) in a dose-dependent manner. The morphine reversal was blocked by coadministration with naloxone (2.5 mg kg-1 s.c.). Capsaicin (3-9 mg kg-1 s.c.) also reversed the hyperalgesia in both models.