Antinociception produced by capsaicin: spinal or peripheral mechanism?

We have studied the effects of capsaicin, administered at concentrations found to be antinociceptive in behavioural tests, on nociceptive responses evoked both in spinal dorsal horn neurons in vivo and in spinal ventral roots in vitro. In halothane anesthetized rats, C-fibre evoked input produced by transcutaneous electrical stimulation in the peripheral receptive field was recorded from single wide dynamic range neurons located in superficial and deep dorsal horn of the lumbar spinal cord. This input was reduced by systemic administration of capsaicin at an antinociceptive dose (20 mumol/kg s.c.). Intradermal injections of capsaicin localized to the peripheral receptive field produced a transient increase in C-fibre evoked activity followed by a prolonged period of localized insensitivity to C-fibre stimulation. Spinal i.t. administered capsaicin also produced a rapid but reversible attenuation of peripherally evoked C-fibre input. In a neonatal rat spinal cord-tail preparation maintained in vitro, superfusion of the spinal cord with capsaicin (100-500 nM) produced a transient depolarization which was followed by an attenuation of responses to peripheral noxious heat and to spinal administration of substance P. Similar activity was produced by a prolonged superfusion of the spinal cord with substance P (50-200 nM). An HPLC method was used to estimate the concentration of capsaicin in a number of tissues following s.c. administration at an antinociceptive dose. In addition capsaicin concentrations were determined in the spinal cord following an i.t. administration.(ABSTRACT TRUNCATED AT 250 WORDS)     

Activation of a bradykinin receptor in peripheral nerve and spinal cord in the neonatal rat in vitro.

In an in vitro preparation of the neonatal rat spinal cord with attached tail, administration of bradykinin (Bk) to the spinal cord or to the tail produced depolarization of a ventral root (L3-L5). The effect of Bk at each site was selectively and reversibly antagonized by D-Arg [Hyp2, Thi5,8 D-Phe7]-Bk but could not be mimicked or antagonized by the B1-receptor ligands [des-Arg9]-Bk or Leu8[des-Arg9]-Bk, respectively. Peripherally evoked noxious responses produced by capsaicin or heat, were unaffected by either antagonist administered to the spinal cord. These data suggest that Bk-evoked responses in the spinal cord and at peripheral nociceptors were mediated via a receptor which by definition is of the B2-type. Additionally Bk is unlikely to be a physiological mediator of acute nociception in the spinal cord.     

Actions of capsaicin on peripheral nociceptors of the neonatal rat spinal cord-tail in vitro: dependence of extracellular ions and independence of second messengers.

1. We have tested the hypothesis that capsaicin-induced activation, desensitization and impairment of peripheral nociceptor function is mediated by separate mechanisms. This was investigated by use of an in vitro preparation of the neonatal rat spinal cord with the functionally attached tail in which the cord and tail were separately superfused with physiological solution. Activation of peripheral fibres by noxious (capsaicin, bradykinin, 5-hydroxytrptamine, heat, pinch) and innocuous (light brush) stimuli was assessed by recording the depolarization of a spinal ventral root (L3-L5). 2. Brief administration of capsaicin produced dose-related depolarizing responses (EC50 = 280 nM). These responses could be reproduced for many hours following the repeated application of capsaicin at a submaximal concentration. Prolonged application of 0.5-2.0 microM capsaicin induced a selective desensitization to subsequent brief administrations of capsaicin. Prolonged administration at 20-50 microM produced an additional non-selective reduction in responses to all noxious stimuli without changing innocuous brush responses. 3. Removal of extracellular calcium from the tail superfusate did not reduce the response to capsaicin or prevent capsaicin-induced desensitization. However, high concentrations of capsaicin no longer induced a non-specific reduction of responses to other noxious stimuli. The response to a brief administration of capsaicin was unaffected by calcium channel blocking drugs including nifedipine, cadmium or omega-conotoxin. On the other hand high extracellular calcium increased the incidence of the non-selective reduction of responses to all noxious stimuli produced by high concentrations of capsaicin. 4. Replacement of extracellular sodium with choline blocked peripheral nerve conduction but did not prevent the desensitization produced by capsaicin.(ABSTRACT TRUNCATED AT 250 WORDS)     

Muscarinic excitatory and inhibitory mechanisms involved in afferent fibre-evoked depolarization of motoneurones in the neonatal rat spinal cord.

1. The involvement of acetylcholine and muscarinic receptors in spinal synaptic responses evoked by electrical and noxious sensory stimuli was investigated in the neonatal rat spinal cord in vitro. 2. Potentials were recorded extracellularly from a ventral root (L3-L5) of the isolated spinal cord, spinal cord-cutaneous nerve, and spinal cord-skin preparations of 1- to 4-day-old rats. Spinal reflexes were elicited by electrical stimulation of the ipsilateral dorsal root or the cutaneous saphenous nerve, or by noxious skin stimulation. 3. Single shock stimulation of supramaximum intensity of a dorsal root induced a mono-synaptic reflex in the corresponding ventral root. Bath-application of the muscarinic agonists, muscarine (0.3-30 microM) and (+)-cis-dioxolane (0.1-100 microM), produced an inhibition of the mono-synaptic reflex and a depolarization of motoneurones. Other muscarinic agonists, arecoline (10 nM-10 microM) and oxotremorine (10 nM-1 microM), inhibited the mono-synaptic reflex with little or no depolarization of motoneurones. Repetitive stimulation of the saphenous nerve at C-fibre strength induced a slow depolarizing response lasting about 30 s of the L3 ventral root. This slow ventral root potential (VRP) was also inhibited by arecoline (10 nM-10 microM) and oxotremorine (10 nM-1 microM). 4. In the spinal cord-saphenous nerve-skin preparation, a slow VRP was evoked by application of capsaicin (0.5 microM), bradykinin (3 microM), or noxious heat (47 degrees C) to skin. This slow VRP was depressed by the muscarinic agonists, arecoline (3 microM) and oxotremorine (1 microM). 5. Of the (+)-cis-dioxolane-induced inhibition of mono-synaptic reflex and motoneurone depolarization, the M2 antagonists, AF-DX 116 (0.1-1 microM) and methoctramine (100-300 nM), preferentially blocked the former response, whereas the M3 antagonists, 4-DAMP (3-10 nM) and p-F-HHSiD (0.3-3 microM), preferentially blocked the latter response. AF-DX 116 (0.1-1 microM) and methoctramine (100-300 nM) also effectively antagonized the arecoline- and oxotremorine-induced inhibition of the slow VRP. The pA2 values of AF-DX 116 and methoctramine against the arecoline-induced inhibition of the mono-synaptic reflex were both 6.79, and that of 4-DAMP against the (+)-cis-dioxolane-induced motoneurone depolarization was 8.16. 6. In the spinal cord-cutaneous nerve preparation, the saphenous nerve-evoked slow VRP was augmented by the anticholinesterase, edrophonium (5 microM). AF-DX 116 (1 microM) and methoctramine (100 nM) also potentiated the slow VRP, whereas 4-DAMP (10 nM) depressed the response.(ABSTRACT TRUNCATED AT 400 WORDS)     

Resiniferatoxin, a potent capsaicin-like stimulator of peripheral nociceptors in the neonatal rat tail in vitro.

1. A spinal ventral root response was measured following the activation of peripheral fibres by noxious (heat at 48 degrees C, capsaicin, bradykinin) and innocuous (brush) stimuli in a preparation of the neonatal rat spinal cord-tail maintained in vitro. 2. Following superfusion of the tail with 0.1-1.0 nM of the potent irritant, resiniferatoxin (RTX), brief, irregular depolarization and a selective loss of capsaicin sensitivity was produced. RTX 10-100 nM evoked a tonic response, initiated transient irregular depolarizations and densitization to further applications of RTX and capsaicin but not to other stimuli. Following RTX 1 microM a prolonged loss of sensitivity to all noxious stimuli was produced. 3. When a selective densitization to capsaicin was produced by a long application of capsaicin, RTX was also ineffective. 4. Superfusion of the tail with 4 beta-phorbol, 12, 13-dibutyrate (PDBu), a protein kinase C activator, stimulated capsaicin-sensitive peripheral fibres. Prolonged administration of PDBu attenuated or abolished further responses to PDBu and bradykinin but responses to RTX and capsaicin were unchanged. The protein kinase C inhibitor staurosporine (50-200 nM), attenuated the effects of PDBu and bradykinin but not those of RTX or capsaicin. 5. The present data suggest that neither RTX nor capsaicin act on peripheral nociceptors via a phorbol ester-like stimulation of protein kinase C. Rather, RTX acts on nociceptors by a similar mechanism to capsaicin. These effects may be the basis for the irritant properties of RTX and may further relate to the antinociceptive actions observed in vivo.(ABSTRACT TRUNCATED AT 250 WORDS)     

NE-19550 and NE-21610, antinociceptive capsaicin analogues: studies on nociceptive fibres of the neonatal rat tail in vitro

When applied to peripheral fibres in a neonatal rat tail/spinal cord preparation in vitro, capsaicin (0.2-50 microM) induced an activation, selective desensitization and reduced responses to other noxious stimuli (heat, bradykinin). Similar concentrations of the antinociceptive analogues NE-19550 and NE-21610, did not affect peripheral fibre responsiveness but induced cross desensitization to capsaicin. At 500 microM both analogues produced similar effects to capsaicin. Capsaicin analogues may induce analgesia without initial activation of nociceptors.     

Effects of ruthenium red and capsazepine on C-fibres in the rabbit iris.

1. We have investigated the effects of ruthenium red and capsazepine on a C-fibre-smooth muscle preparation (the rabbit isolated iris sphincter muscle). 2. Like capsaicin, ruthenium red and capsazepine were found to produce contractions in a concentration-dependent manner. C-fibre activation was held to be responsible since the contractions could be inhibited by tachykinin receptor blockade. 3. Both ruthenium red and capsazepine inhibited capsaicin-induced contractions concentration-dependently; the pIC50 values were 5.1 and 4.9, respectively. The contractions induced by bradykinin, which, like capsaicin, acts by releasing tachykinins from C-fibres, were also inhibited by ruthenium red and capsazepine in a concentration-dependent manner; the pIC50 values were 4.1 and 4.6, respectively. 4. Electrically evoked, tachykinin-mediated contractions were inhibited by ruthenium red and capsazepine in a concentration-dependent manner; the pIC50 values were 4.3 and 4.5, respectively. 5. The contractile response to neurokinin A (NKA) was inhibited by capsazepine (and by capsaicin), but not by ruthenium red, in a concentration-dependent manner; the pIC50 value was 4.3. 6. The results suggest that, besides their ability to antagonize capsaicin, ruthenium red and capsazepine possess a weak capsaicin-like effect. Conceivably, capsazepine interacts with binding sites for capsaicin, acting as a partial agonist/antagonist, while ruthenium red interacts with capsaicin-operated cation channels. The inhibition of electrically evoked- or bradykinin-induced responses by capsazepine and ruthenium red suggests that capsaicin/capsazepine binding sites and capsaicin-operated cation channels play a role in the process of transmitter release in response not only to capsaicin but also to other C-fibre stimuli.(ABSTRACT TRUNCATED AT 250 WORDS)     

The excitatory and inhibitory modulation of primary afferent fibre-evoked responses of ventral roots in the neonatal rat spinal cord exerted by nitric oxide.

1. We investigated the role of nitric oxide (NO) in modulating spinal synaptic responses evoked by electrical and noxious sensory stimuli in the neonatal rat spinal cord in vitro. 2. Potentials were recorded extracellularly from a ventral root (L3-L5) of the isolated spinal cord preparation or spinal cord-saphenous nerve-skin preparation of 0- to 2-day-old rats. Spinal reflexes were elicited by electrical stimulation of the ipsilateral dorsal root or by noxious skin stimulation. 3. In the spinal cord preparation, single shock stimulation of a dorsal root at C-fibre strength induced mono-synaptic reflex followed by a slow depolarizing response lasting about 30 s (slow ventral root potential; slow VRP) in the ipsilateral ventral root of the same segment. Bath-application of NO gas-containing medium (10(-4)- 10(-2) dilution of saturated medium) and NO donors, 1-hydroxy-2-oxo-3-(N-ethyl-2-aminoethyl)-3-ethyl-1-triazene (NOC12, 3-300 microM), S-nitroso-N-acetyl-D,L-penicillamine (SNAP, 3-300 microM) and S-nitroso-L-glutathione (GSNO, 3-300 microM), produced an inhibition of the slow VRP and a depolarization of ventral roots. Another NO donor, 3-morpholinosydononimine (SIN-1, 30-300 microM), also depressed the slow VRP but did not depolarize ventral roots. These agents did not affect the mono-synaptic reflex. 4. In the spinal cord-saphenous nerve-skin preparation, application of capsaicin (0.1-0.2 microM) to skin evoked a slow depolarizing response of the L3 ventral root. This slow VRP was depressed by NOC12 (10-300 microM) and SIN-1 (100-300 microM). When the concentration of NOC12 was increased to 1 mM, spontaneous synaptic activities were augmented and the depressant effect of NOC12 on the slow VRP became less pronounced. 5. A NO-scavenger, 2-(4-carboxyphenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide( carboxy- PTIO, 100-300 microM) prevented the depressant effect on the dorsal root-evoked slow VRP and ventral root depolarizing effects of NO donors. Carboxy-PTIO increased spontaneous synaptic activities and markedly potentiated the slow VRP. A NO synthase (NOS) inhibitor, N omega-nitro-L-arginine methyl ester (L-NAME, 0.03-1 microM), but not D-NAME (0.03-1 microM), also markedly potentiated the slow VRP and this effect was reversed by L-arginine (300 microM). 6. 8-Bromo-cyclic guanosine 3': 5'-monophosphate (8-Br-cyclic GMP, 100-300 microM) produced both an inhibition of the slow VRP and a depolarization of ventral roots. A cyclic GMP-dependent protein kinase inhibitor, KT5823 (0.3 microM), partly inhibited the depressant effects of NO donors and 8-Br-cyclic GMP on the dorsal root-evoked slow VRP. In contrast, KT5823 did not inhibit the depolarizing effects of NO donors. 7. Perfusion of the spinal cord with medium containing tetrodotoxin (0.3 microM) and/or low Ca2+ (0.1 mM)-high Mg2+ (10 mM) markedly potentiated the depolarizing effect of NO donors. The SNAP-evoked depolarization in the tetrodotoxin-containing low Ca(2+)-high Mg2+ medium was significantly inhibited by excitatory amino acid receptor antagonists D-(-)-2-amino-5-phosphonovaleric acid (30 microM) and 6-cyano-7-nitroquinoxaline-2,3-dione (10 microM). 8. The present study suggests that inhibitory and excitatory mechanisms meditated by the NO-cyclic GMP cascade are involved in the primary afferent fibre-evoked nociceptive transmission in the neonatal rat spinal cord. The inhibitory mechanism, but not the excitatory mechanism, appears to be partly mediated by cyclic GMP-dependent protein kinase. It is also suggested that Ca(2+)-independent release of excitatory amino acid neurotransmitters contributes to the depolarizing response to NO of ventral roots.     

Bradykinin-induced depolarization of primary afferent nerve terminals in the neonatal rat spinal cord in vitro.

1. Application of bradykinin (BK) to the spinal cord of the neonatal rat evoked depolarizations which could be recorded via either the dorsal or ventral roots. However, responses recorded via the ventral root were abolished by removal of extracellular Ca2+ or the addition of Cd2+, while responses recorded via the dorsal root were unaffected. 2. The response recorded via the ventral root was inhibited by the substance P antagonist spantide, while responses recorded via the dorsal root were unaffected. 3. Depolarizations recorded via the dorsal root were concentration-dependent with an EC50 of 30 nM. These responses were not antagonized by the BK1 selective antagonist Leu8des-Arg9BK, but were antagonized by D-Arg0Hyp3Thi5,8D-Phe7BK with a pA2 of 6.8 +/- 0.6, which is similar to the values determined for other BK2-mediated responses. 4. Application of phorbol dibutyrate (PDBu) to the spinal cord also evoked a depolarization with respect to the dorsal root. This response to PDBu was enhanced by removal of extracellular Ca2+, while the response to BK was unaffected. 5. The potent protein kinase inhibitor staurosporine reduced the response to PDBu, but did not affect the response to BK. 6. These results suggest that BK by acting on BK2 receptors can depolarize the central terminals of primary afferent nerve fibres. This response to BK does not appear to be mediated via the activation of protein kinase C. The depolarization to BK recorded via the ventral root of the spinal cord is indirect and may be secondary to the action of BK on the primary afferent terminals.     

Depression of primary afferent-evoked responses by GR71251 in the isolated spinal cord of the neonatal rat.

1. The pharmacological profile of GR71251, a new tachykinin receptor antagonist, and its effect on the responses evoked by stimulation of primary afferent fibres were studied in isolated spinal cord preparations of neonatal rats. Potential changes were recorded extracellularly from a lumbar ventral root (L3-L5). 2. Bath-application of substance P (SP), neurokinin A (NKA) and neurokinin B (NKB) at 0.01-3 microM to the spinal cord induced depolarization of the ventral root in normal artificial cerebrospinal fluid (CSF). The NK1 agonist, acetyl-Arg6-septide, and the NK3 agonist, senktide, at 0.01-3 microM, also had potent depolarizing actions whereas two NK2 agonists, beta-Ala8NKA4-10 and Nle10NKA4-10, showed little depolarizing effects at 1 microM. 3. GR71251 (0.3-3 microM) caused a rightward shift of the concentration-response curves for SP, acetyl-Arg6-septide and NKA with pA2 values of 6.14, 6.75 or 6.70, respectively. The effects of GR71251 were readily reversible within 15-30 min after its removal. By contrast, GR71251 (1-5 microM) had little effect on the depolarizing responses to NKB and senktide. 4. GR71251 (1-3 microM) did not depress the depolarizing responses to bombesin, neuromedin B and gastrin-releasing peptide in normal artificial CSF. 5. Application of capsaicin to the spinal cord induced a depolarizing response, which was partially depressed by GR71251 (3-10 microM). 6. In the isolated spinal cord preparation, intense electrical stimulation of a dorsal root evoked a slow depolarizing response of the contralateral ventral root of the same segment.(ABSTRACT TRUNCATED AT 250 WORDS)     

Bradykinin-induced activation of nociceptors: receptor and mechanistic studies on the neonatal rat spinal cord-tail preparation in vitro.

1. The effects of bradykinin on nociceptors have been characterized on a preparation of the neonatal rat spinal cord with functionally connected tail maintained in vitro. Administration of bradykinin to the tail activated capsaicin-sensitive peripheral fibres and evoked a concentration-dependent (EC50 = 130 nM) depolarization recorded from a spinal ventral root (L3-L5). 2. The response to bradykinin was unaffected by the peptidase inhibitors, bestatin (0.4 mM), thiorphan (1 microM), phosphoramidon (1 microM) and MERGETPA (10 microM) or by the presence of calcium blocking agents, cadmium (200 microM) and nifedipine (10 microM). 3. Inhibition of cyclo-oxygenase with indomethacin (1-5 microM), aspirin (1-10 microM) and paracetamol (10-50 microM) consistently attenuated responses to bradykinin. 4. The effect of bradykinin was mimicked by the phorbol ester PDBu, an activator of protein kinase C. The response to bradykinin was attenuated following desensitization to PDBu but desensitization to bradykinin did not induce a cross-desensitization to PDBu. The protein kinase C inhibitor staurosporine (10-500 nM) consistently attenuated the effects of PDBu and bradykinin. 5. Bradykinin responses were reversibly enhanced by dibutyryl cyclic AMP (100 microM). However dibutyryl cyclic GMP (0.5 mM) and nitroprusside (10 microM) produced prolonged block of responsiveness to bradykinin. Prolonged superfusion with pertussis toxin did not affect responses to bradykinin. 6. The B1-receptor agonist des Arg9-bradykinin (10-100 microM) was ineffective alone or after prolonged exposure of the tail to lipopolysaccharide (100 ng ml-1) or epidermal growth factor (100 ng ml-1) to induce B1 receptors.(ABSTRACT TRUNCATED AT 250 WORDS)     

High affinity antagonists of the vanilloid receptor

The vanilloid receptor VR1 has attracted great interest as a sensory transducer for capsaicin, protons, and heat, and as a therapeutic target. Here we characterize two novel VR1 antagonists, KJM429 [N-(4-tert-butylbenzyl)-N'-[4-(methylsulfonylamino)benzyl]thiourea] and JYL1421 [N-(4-tert-butylbenzyl)-N'-[3-fluoro-4-(methylsulfonylamino)benzyl]thiourea], with enhanced activity compared with capsazepine on rat VR1 expressed in Chinese hamster ovary (CHO) cells. JYL1421, the more potent of the two novel antagonists, inhibited [(3)H]resiniferatoxin binding to rVR1 with an affinity of 53.5 +/- 6.5 nM and antagonized capsaicin-induced calcium uptake with an EC(50) of 9.2 +/- 1.6 nM, reflecting 25- and 60-fold greater potencies than capsazepine. Both JYL1421 and KJM429 antagonized RTX as well as capsaicin and their mechanism was competitive. The responses to JYL1421 and KJM429 differed for calcium uptake by rVR1 induced by heat or pH. JYL1421 antagonized the response to both pH 6.0 and 5.5, whereas KJM429 antagonized at pH 6.0 but was an agonist at lower pH (<5.5). For heat, JYL1421 fully antagonized and KJM429 partially antagonized. Capsazepine showed only weak antagonism for both pH and heat. Responses of rVR1 to different activators could thus be differentially affected by different ligands. In cultured dorsal root ganglion neurons, JYL1421 and KJM429 likewise behaved as antagonists for capsaicin, confirming that the antagonism is not limited to heterologous expression systems. Finally, JYL1421 and KJM429 had little or no effect on ATP-induced calcium uptake in CHO cells lacking rVR1, unlike capsazepine. We conclude that JYL1421 is a competitive antagonist of rVR1, blocking response to all three of the agonists (capsaicin, heat, and protons) with enhanced potency relative to capsazepine.     

Bradykinin activates peripheral capsaicin-sensitive fibres via a second messenger system

A preparation of the neonatal rat spinal cord with attached tail was maintained in vitro and was used to study the mechanism of action of bradykinin on peripheral nociceptors. Spinal ventral root depolarization was used as an index of peripheral fibre activation. Capsaicin-sensitive fibres in the tail were activated by bradykinin and a phorbol ester. The protein kinase C inhibitor staurosporine attenuated the effect of bradykinin and phorbol. [Des-Arg9]-bradykinin was inactive but bradykinin responses were reversibly antagonized by D-Arg0-Hyp2-Thi5,8-D Phe7-bradykinin. These data suggest that bradykinin activates nociceptors via a bradykinin B2-receptor coupled to protein kinase C.     

Capsazepine reversal of the antinociceptive action of capsaicin in vivo.

1. This study was designed to investigate the antinociceptive activity of capsaicin in acute and persistent nocifensor behavioural models and to determine whether this was mediated via a specific receptor interaction, by use of the antagonist, capsazepine. 2. Capsaicin administered systemically in low doses produced antinociception in the knee joint hyperalgesia, rat paw pressure, rat and mouse tail flick and mouse hot plate nocifensor models with similar efficacy in all tests. 3. The novel competitive capsaicin antagonist, capsazepine, prevented the capsaicin-induced antinociception when administered systemically in the range 50-100 mumol kg-1. 4. Capsazepine administered by itself had no antinociceptive actions. 5. These data describe a joint hyperalgesic model in the rat which does not rely on extreme nocifensor behavioural endpoints and it is suggested this is a useful model for investigating mechanisms in persistent pain. 6. This is the first demonstration of antagonism by capsazepine of the behavioural antinociceptive properties of capsaicin and provides further evidence that capsaicin acts to reduce nociceptive thresholds via a specific receptor.     

B1 bradykinin receptors and sensory neurones.

1. The location of the B1 bradykinin receptors involved in inflammatory hyperalgesia was investigated. 2. No specific binding of the B1 bradykinin receptor ligand [3H]-des-Arg10-kallidin was detected in primary cultures of rat dorsal root ganglion neurones, even after treatment with interleukin-1 beta (100 iu ml-1). 3. In dorsal root ganglion neurones, activation of B2 bradykinin receptors stimulated polyphosphoinositidase C. In contrast, B1 bradykinin receptor agonists (des-Arg9-bradykinin up to 10 microM and des-Arg10-kallidin up to 1 microM) failed to activate polyphosphoinositidase C, even in neurones that had been treated with interleukin-1 beta (100 iu ml-1), prostaglandin E2 (1 microM) or prostaglandin I2 (1 microM). 4. Dorsal root ganglion neurones removed from rats (both neonatal and 14 days old) that had been pretreated with inflammatory mediators (Freund's complete adjuvant, or carrageenan) failed to respond to B1 bradykinin receptor selective agonists (des-Arg9-bradykinin up to 10 microM and des-Arg10-kallidin up to 1 microM). 5. Bradykinin (25 nM to 300 nM) evoked ventral root responses when applied to peripheral receptive fields or central terminals of primary afferents in the neonatal rat spinal cord and tail preparation. In contrast, des-Arg9-bradykinin (50 nM to 500 nM) failed to evoke ventral root depolarizations in either control rats or in animals that developed inflammation following ultraviolet irradiation of the tail skin. 6. The results of the present study imply that the B1 bradykinin receptors that contribute to hypersensitivity in models of persistent inflammatory hyperalgesia are located on cells other than sensory neurones where they may be responsible for releasing mediators that sensitize or activate the nociceptors.     

Inhibitory action of nociceptin on spinal dorsal horn neurones of the rat,in vivo.

Intrathecally administered nociceptin (5, 50, 225 micrograms) dose-relatedly inhibited the C-fibre evoked wind-up and post-discharge of dorsal horn neurones, but not the baseline C-fibre evoked responses. Spinal naloxone 50 micrograms, but not 10 micrograms, reversed the effects of nociceptin. Thus the antinociceptive role of nociceptin in the spinal cord differs from that of classical opioids.     

The cannabinoid CB1 receptor antagonist, SR141716A, selectively facilitates nociceptive responses of dorsal horn neurones in the rat.

The effect of spinal administration of the selective cannabinoid CB1 receptor antagonist, SR141716A, and the selective CB2 receptor antagonist, SR144528, on innocuous versus noxious evoked responses of dorsal horn neurones in the spinal cord of the anaesthetized rat was investigated. SR141716A (0.001-1 ng 50 microl(-1)) dose-relatedly facilitated the non-potentiated component of the electrical C-fibre mediated neuronal response (120+/-6, 156+/-13, 192+/-33 and 192+/-31% of control respectively; n=6). In contrast, SR144528 (0.001-1 ng 50 microl(-1)) did not influence the non-potentiated component of the C-fibre evoked neuronal response (n=5). The electrical evoked Abeta-fibre mediated neuronal responses were not influenced by SR141716A or SR144528. The results of this study provide evidence that tonic cannabinoid CB1 receptor activation, but not CB2 receptor activation, attenuates acute nociceptive transmission, at the level of the spinal cord. These results suggest a selective antinociceptive role of the endogenous cannabinoids at spinal CB1 receptors.     

Capsaicin and nociception in the rat and mouse

Summary Newborn or adult rats and mice were treated with capsaicin. The effect of systemic or intrathecal treatment on thermonociception, chemonociception, content and release of immunoreactive substance P (I-SP) was investigated.Treatment of two day old rats caused a small, but life-long elevation of the hot plate or tail withdrawal latency. Treatment of adult rats led to a large increase in the reaction time on the hot plate for 4–10 days but the tail withdrawal latency was only slightly elevated for not more than 1–2 days.Mice treated on the 2nd day of life had normal reaction times on the hot plate and a small and inconsistent prolongation of the tail withdrawal latency. In contrast, mice treated on day 7, 10 or as adults had greatly prolonged latencies in both tests for at least 3 months. The changes in latencies were not affected by naloxone or methysergide.Responses to noxious chemical stimuli were moderately inhibited in mice treated on the 2nd day of life, but almost abolished in mice treated on day 7, 10 or as adults.Neonatal capsaicin treatment of rats resulted in a depletion of I-SP in spinal cord and sciatic nerve for 20 months. Capsaicin-evoked release of I-SP from rat spinal cord was reduced by 93% after neonatal treatment, but only by 69% 2 weeks after adult treatment.Treatment of mice on day 2 caused a similar decrease of the I-SP content in spinal cord and of the capsaicin-evoked I-SP release (88%) as treatment on day 4 or 7 although behavioural changes were different. After treatment of adult mice release of I-SP was reduced by 93%.Capsaicin administered intrathecally to rats or mice depleted I-SP in the spinal cord but not in the sciatic nerve. The animals were almost insensitive to noxious heat (tail withdrawal test) and to local application of mustard oil or capsaicin to the hindpaw. Chemosensitivity of the eye, however, remained unchanged.The experiments indicate that systemic or intrathecal capsaicin treatment of rats or mice affects thermo- and chemonociception but species differences were found. It appears, furthermore, that changes in substance P alone cannot explain all the observed behavioral effects after capsaicin treatment.     

Neuropharmacological mechanisms of capsaicin and related substances.

Capsaicin activates poorly myelinated primary afferent neurons, many of which are polymodal nociceptors. Activation is accompanied by membrane depolarization and the opening of a unique, cation-selective, ion channel which can be blocked by the polyvalent dye ruthenium red. The capsaicin-induced activation is mimicked by resiniferatoxin, a potent analogue, and by low pH. Activation is mediated by a specific membrane receptor which can be selectively and competitively antagonized by capsazepine. Repetitive administration of capsaicin produces a desensitization and an inactivation of sensory neurons. Several mechanisms are involved including receptor inactivation, block of voltage activated calcium channels, intracellular accumulation of ions leading to osmotic changes, and activation of proteolytic enzyme processes. Systemic and topical capsaicin produces a reversible antinociceptive and anti-inflammatory action after an initial undesirable algesic effect. Capsaicin analogues, such as olvanil, have similar properties with minimal initial algesic activity. Antinociception produced by capsaicin does not involve neurotoxicity, sensory neuropeptide depletion or activity at peripheral receptors; rather, systemic capsaicin produces antinociception by activating capsaicin receptors on afferent nerve terminals in the spinal cord. Spinal neurotransmission is blocked by a prolonged inactivation of sensory neurotransmitter release. However, local or topical applications of capsaicin block C-fibre conduction and inactive neuropeptide release from peripheral nerve endings. These mechanisms account for localized antinociception and the reduction of neurogenic inflammation, respectively.