Influence of behavioral traits in the inter-individual variability of nociceptive, emotional and cognitive manifestations of neuropathic pain.

Neuropathic pain is a complex disorder associated with emotional and cognitive deficits that may impair nociceptive manifestations. There is high inter-individual variability in the manifestations of human neuropathic pain, which largely depends on personality traits. We aim to identify the influence of different behavioral traits in the inter-individual vulnerability to neuropathic pain manifestations using behavioral, electrophysiological and genetic approaches. We first selected mice with extreme social and emotional traits and look for correlation with the spontaneous neuronal activity in the central amygdala. Neuropathic pain was induced to these mice to evaluate the influence of behavioral traits on nociceptive manifestations and gene expression profiles in the amygdala. Our results show an association of the spontaneous central amygdala neuronal activity with the sociability behavior. We demonstrate that low sociable, high anxious and low depressive phenotypes develop enhanced nociceptive hypersensitivity after nerve injury. However, greater emotional alterations and cognitive impairment are observed in high sociable, anxious-like and depressive-like mice, indicating that nociceptive, emotional and cognitive manifestations of neuropathic pain do not correlate with each other. Gene analyses identify high Pdyn and Il6 levels in the amygdala as indicative of enhanced nociceptive hypersensitivity and reveal an association between high Gadd45 expression and attenuated emotional and cognitive manifestations of neuropathic pain.

P2X7 receptor-mediated analgesia in cancer-induced bone pain.

Pain is a common and debilitating complication for cancer patients significantly compromising their quality of life. Cancer-induced bone pain involves a complex interplay of molecular events, including mechanisms observed in inflammatory and neuropathic pain states, but also changes unique for cancer-induced bone pain. The P2X7 receptor (P2X7R) is involved in a variety of cellular functions and has been linked to both inflammatory and neuropathic pain. Here we study the analgesic potential of P2X7R antagonism in a rat model of cancer-induced bone pain. In cancer-bearing animals, the P2X7R antagonist A839977 attenuated dorsal horn neuronal responses in a modality and intensity-specific way. Spinal application of 0.4-mg/kg and 1.2-mg/kg A839977 significantly reduced the evoked responses to high-intensity mechanical and thermal stimulation, whereas no effect was seen in response to low-intensity or electrical stimulation. In contrast, A839977 had no effect on the tested parameters in naïve or sham animals. In awake animals, 40-mg/kg A839977 (i.p.) significantly reduced both early- and late-stage pain behavior. In contrast, no effect was observed in sham or vehicle-treated animals. The results suggest that the P2X7R is involved in the mechanisms of cancer-induced bone pain, and that P2X7R antagonism might be a useful analgesic target. No effect was observed in sham or naïve animals, indicating that the P2X7R-mediated effect is state-dependent, and might therefore be an advantageous target compared to traditional analgesics.

Randall Selitto pressure algometry for assessment of bone-related pain in rats.

BACKGROUND: Deep pain is neglected compared with cutaneous sources. Pressure algometry has been validated in the clinic for assessment of bone-related pain in humans. In animal models of bone-related pain, we have validated the Randall Selitto behavioural test for assessment of acute and pathological bone pain and compared the outcome with more traditional pain-related behaviour measures. METHODS: Randall Selitto pressure algometry was performed over the anteromedial part of the tibia in naïve rats, sham-operated rats, and rats inoculated with MRMT-1 carcinoma cells in the left tibia, and the effect of morphine was investigated. Randall Selitto measures of cancer-induced bone pain were supplemented by von Frey testing, weight-bearing and limb use test. Contribution of cutaneous nociception to Randall Selitto measures were examined by local anaesthesia. RESULTS: Randall Selitto pressure algometry over the tibia resulted in reproducible withdrawal thresholds, which were dose-dependently increased by morphine. Cutaneous nociception did not contribute to Randall Selitto measures. In cancer-bearing animals, compared with sham, significant differences in pain-related behaviours were demonstrated by the Randall Selitto test on day 17 and 21 post-surgery. A difference was also demonstrated by von Frey testing, weight-bearing and limb use tests. CONCLUSION: Our results indicate that pressure applied by the Randall Selitto algometer on a region, where the bone is close to the skin, may offer a way to measure bone-related pain in animal models and could provide a supplement to the traditional behavioural tests and a means to study deep pain.

Spinal neuronal correlates of tapentadol analgesia in cancer pain: a back-translational approach.

BACKGROUND: Pain is a common and highly debilitating complication for cancer patients significantly compromising their quality of life. Cancer-induced bone pain involves a complex interplay of multiple mechanisms including both inflammatory and neuropathic processes and also some unique changes. Strong opioids are a mainstay of treatments but side effects are problematic and can compromise optimal pain control. Tapentadol is a novel dual-action drug, both stimulating inhibitory µ-opioid receptors (MOR) and mediating noradrenaline reuptake inhibition (NRI) leading to activation of the inhibitory α-2 adrenoceptor. It has been demonstrated to treat effectively both acute and chronic pain. We here demonstrate the efficacy in a model of cancer-induced bone pain. METHODS: MRMT-1 mammary carcinoma cells were inoculated into the tibia of 6-week-old rats and 2 weeks after, the neuronal responses to a wide range of peripheral stimulation were evaluated. The recordings were made from wide-dynamic range neurons in lamina V of the dorsal horn before and after administration of tapentadol as well as antagonists of the two mechanisms, naloxone or atipamezole. RESULTS: We found marked inhibitions of the neuronal activity with efficacy against mechanical, thermal and electrically evoked activity following tapentadol administration. In addition, the effects of the drug were fully reversible by naloxone and partly by atipamezole, supporting the idea of MOR-NRI dual actions. CONCLUSIONS: These findings add to the mechanistic understanding of cancer-induced bone pain and support the sparse clinical data indicating a possible use of the drug as a therapeutic alternative for cancer patients with metastatic pain complication.

Neuropathic pain in cancer.

Neuropathic pain in cancer arises following injury to peripheral or central neurons, in a similar manner to such pain arising from a non-cancer injury. Much of our knowledge of neuropathic pain is based on peripheral originating events with little known about central neuropathic pain. This article explores some of the similarities and differences between cancer and non-cancer-related neuropathic pain. The neural pathways, ion channels, receptors and neurotransmitters that potentially can be altered in both neuropathies are the same; however the nature of the injury, the timing, repeated injuries and the co-existence of simultaneous non-neuropathic pain states lead to potential unique constellations of neuroreceptor and neurotransmitter expression in the context of cancer pain. This in turn may lead to different clinical presentation of pain sensations and potentially lead to specific treatment options.

Anti-convulsants and anti-depressants.

Damage to a nerve should only lead to sensory loss. While this is common, the incidence of spontaneous pain, allodynia and hyperalgesia indicate marked changes in the nervous system that are possible compensations for the loss of normal function that arises from the sensory loss. Neuropathic pain arises from changes in the damaged nerve which then alter function in the spinal cord and the brain and lead to plasticity in areas adjacent to those directly influenced by the neuropathy. The peripheral changes drive central compensations so that the mechanisms involved are multiple and located at a number of sites. Nerve damage increases the excitability of both the damaged and undamaged nerve fibres, neuromas and the cell bodies in the dorsal root ganglion. These peripheral changes are substrates for the ongoing pain and the efficacy of excitability blockers such as carbamazepine, lamotrigine and mexiletine, all anti-convulsants. A better understanding of ion channels at the sites of injury has shown important roles of particular sodium, potassium and calcium channels in the genesis of neuropathic pain. Within the spinal cord, increases in the activity of calcium channels and the receptors for glutamate, especially the N-methyl-D-aspartate (NMDA) receptor, trigger wind-up and central hyperexcitability. Increases in transmitter release, neuronal excitability and receptive field size result from the damage to the peripheral nerves. Ketamine and gabapentin/pregabalin, again with anti-convulsant activity, may interact with these mechanisms. Ketamine acts on central spinal mechanisms of excitability whereas gabapentin acts on a subunit of calcium channels that is responsible for the release of pain transmitters into the spinal cord. In addition to these spinal mechanisms of hyperexcitability, spinal cells participate in a spinal-supraspinal loop that involves parts of the brain involved in affective responses to pain but also engages descending excitatory and inhibitory systems that use the monoamines. These pathways become more active after nerve injury and are the site of action of anti-depressants. This chapter reviews the evidence and mechanisms of drugs, both anti-depressants and anti-convulsants, that are believed to be effective in pain control, with a major emphasis on the neuropathic state.

Superficial dorsal horn neuronal responses and the emergence of behavioural hyperalgesia in a rat model of cancer-induced bone pain.

Animal models of cancer-induced bone pain have revealed a unique neurochemical reorganisation of segments of dorsal horn of the spinal cord receiving nociceptive input from the affected bone, and altered spinal neuronal response patterns. The aim of this investigation was determine correlations between the behavioural hyperalgesia and allodynia observed in these animals and the dorsal horn neuronal changes. The results show that in this rat model of cancer-induced bone pain, behavioural manifestations of pain emerge in parallel with the altered superficial dorsal horn neuronal response. Thus, in this model, the alterations in neuronal responses are a viable substrate for pharmacological studies on suprathreshold stimuli. In addition the clear temporal link between behavioural hyperalgesia and altered neuronal responses may provide an opportunity to investigate changes in dorsal horn gene expression in hyperalgesia.

Nerve injury-induced changes in opioid modulation of wide dynamic range dorsal column nuclei neurones.

In the present study we investigated the effects of spinal morphine on the electrically and naturally evoked responses of gracile nuclei neurones in a rat model of neuropathy, induced by the tight ligation of lumbar L5/6 spinal nerves. Two weeks after surgery, animals were prepared for electrophysiological recordings and neuronal responses were characterised to a range of controlled natural (brush, low- and high-intensity von Frey filaments, heat 45 degrees C) and peripheral electrical stimuli. Morphine (0.1, 0.25, 1 and 5 microg) was applied spinally and its effect was compared to that in sham-operated or naive animals. Following surgery, all neuropathic rats exhibited signs of mechanical allodynia. Nerve injury induced a significant increase in the receptive field size of gracile nuclei neurones, and also produced a non-significant increase in the proportion and level of spontaneous activity in these neurones. The baseline electrical and natural evoked responses remained unaltered. Spinal morphine reduced both the Adelta-fibre- and C-fibre-evoked responses of gracile nuclei neurones, and similarly inhibited the heat-evoked responses of neuropathic, sham-operated and naive rats. Morphine, however, produced only minor reductions (<30% inhibition of pre-drug control responses) of the Abeta-fibre- and brush-evoked responses of gracile nuclei neurones. These drug effects were similar in all animal groups. In complete contrast, morphine produced a marked inhibition of the low-intensity punctate mechanical evoked responses (von Freys 2 and 9 g) after nerve injury, an effect that was totally lacking in the sham-operated or naive animal groups. This dramatic shift was selective for the low-intensity punctate mechanical stimuli and such an effect was not seen with the noxious mechanical punctate stimulus (von Frey 75 g) where there was a modest inhibition in all groups. Our results suggest that there is plasticity in the opioid modulation of dorsal column projection pathways following spinal nerve ligation and these alterations appear to interact with sensory pathways conveying low-threshold punctate stimuli.

The effect of ABT-702, a novel adenosine kinase inhibitor, on the responses of spinal neurones following carrageenan inflammation and peripheral nerve injury.

1. Adenosine (ADO) receptor activation modulates sensory transmission in the dorsal horn. Little is known about the circumstances underlying release of the purine. The present study was conducted to investigate the effect of a novel and potent non-nucleoside adenosine kinase (AK) inhibitor, ABT-702, on the responses of dorsal horn neurones to selected peripheral stimuli. ABT-702 is orally effective to reduce behavioural signs of nociception in models of acute, inflammatory, and neuropathic pain. 2. Electrophysiological recordings were made from wide dynamic range (WDR) neurones in halothane-anaesthetized rats. ABT-702 was given subcutaneously following either carrageenan inflammation or peripheral nerve injury (L5/L6 spinal nerve ligation). Comparisons were made between carrageenan and uninjected control animals, and similarly between spinal nerve ligated (SNL) and sham operated animals. 3. ABT-702 produced inhibition of the postdischarge, wind-up and C-fibre evoked responses in both carrageenan and nerve-injured animals. Furthermore, the mechanical and thermal evoked responses were similarly reduced in SNL rats. Overall, ABT-702 produced a significantly greater inhibition of these responses in SNL rats as compared to sham controls. Similarly ABT-702 tended to produce greater effects after carrageenan inflammation, however this did not reach significance. 4. Protection of endogenous adenosine by ABT-702 therefore produces a marked inhibition of the noxious evoked neuronal activity in inflamed and neuropathic rats. Our results demonstrate a plasticity in the endogenous adenosine-mediated inhibitory system following SNL and provide a possible basis for the use of this compound for the treatment of neuropathic and other persistent pain states.

Comparison of the effects of MK-801, ketamine and memantine on responses of spinal dorsal horn neurones in a rat model of mononeuropathy.

Selective ligation of the L5/L6 spinal nerves produces a partial denervation of the hindpaw and has proved to be a useful model for studying the mechanisms underlying neuropathic pain. Two weeks after surgery, in vivo electrophysiological studies were performed in sham operated and nerve injured rats and the responses of spinal dorsal horn neurones to controlled electrical and natural (mechanical and heat) stimuli were recorded. The systemic effects of three N-methyl-D-aspartate receptor (NMDA) antagonists, ketamine (1-10 mg/kg), memantine (1-20 mg/kg) and MK-801 (0.1-5 mg/kg) were compared. Ketamine a clinically available NMDA receptor antagonist, produced greater reductions of the postdischarge, thermal (10 mg/kg, P=0.02), and mechanical evoked responses in spinal nerve ligated (SNL) rats (von Frey 9 g, 1 mg/kg, P=0.04; 5 mg/kg, P=0.01; 10 mg/kg, P=0.05; von Frey 50 g, 5 mg/kg, P=0.02; 10 mg/kg, P=0.003). The inhibition of wind-up was comparable in both animal groups. Memantine produced powerful inhibitions of wind-up after nerve injury with little effect in sham controls (5 mg/kg, P=0.02). The postdischarge, mechanical and thermal evoked responses were reduced to similar extents by memantine in both experimental groups. The effects of MK-801 were comparable between SNL and sham operated rats for all neuronal measures (wind-up, postdischarge, thermal and noxious mechanical evoked responses). The differential blocking abilities of these antagonists on the various neuronal responses may relate to the characteristics of their voltage-dependent blockage of the channel associated with the receptor. The favourable side effect profile of memantine supports its potential use for the treatment of neuropathic pain.

Different increase in C-fibre evoked responses after nociceptive conditioning stimulation in sham-operated and neuropathic rats.

Mechanisms of neuropathic pain are not clear. Recently we showed long-term potentiation (LTP) of wide dynamic range (WDR) neurones after electrical conditioning stimulation of the sciatic nerve in normal rats. In this study we investigated the effects of the same conditioning on both the evoked responses of WDR cells and on vital signs in neuropathic rats. Nerve injury was produced by tight ligation of the L5 and L6 spinal nerves and a control group received sham surgery. The electrical conditioning gave a significantly smaller LTP in neuropathic rats as compared to controls. Despite giving a smaller LTP in neuropathic rats, the rise in mean arterial blood pressure and heart rate induced by the conditioning was exactly the same as in the control group. The threshold for activating C-fibres was significantly lower and the baseline tended to be higher in neuropathic rats. Thus, it is suggested that the nerve injury itself is likely to induce an LTP-like state in the neurones studied. Further studies using the LTP-inducing model can provide new information on the mechanisms of neuropathic pain.

Effects of spinally administered P2X receptor agonists and antagonists on the responses of dorsal horn neurones recorded in normal, carrageenan-inflamed and neuropathic rats.

1. The function and role of P2X receptors in the spinal transmission of nociception was investigated using the selective P2X receptor agonists, alpha,beta-methylene ATP (alpha,beta-me ATP) and beta, gamma-methylene-L-ATP (beta,gamma-me-L-ATP) and the P2X receptor antagonists pyridoxal-phosphate-6-azophenyl-2',4'-disulphonate (PPADS) and suramin. 2. Intrathecal administration of 5 and 50 microg of beta,gamma-me-L-ATP produced a significant facilitation of the C-fibre evoked response and a tendency towards increased excitability of the post-discharge, but not Abeta-fibre evoked response of dorsal horn neurones recorded in normal animals. Administration of similar doses of alpha,beta-me ATP did not produce an overall change in the response of the neuronal population. 3. Peripheral administration of 20 microg of these agonists into the paw of the rat evoked firing in the dorsal horn neurones. 4. Intrathecal administration of the antagonists, suramin (50 and 500 microg) and PPADS (5, 50 and 500 microg), to normal animals and to animals with a model of neuropathy induced by spinal nerve ligation did not alter the evoked neuronal responses. In contrast, intrathecal administration of 500 microg of suramin to animals 3 h after the induction of carrageenan inflammation produced a significant inhibition of the C-fibre evoked response of the neurones. Similar inhibitions were also seen following high doses of intrathecal PPADS, although this did not reach significance. 5. These results suggest that spinal P2X receptors may play a role in the modulation of spinal nociceptive transmission following the development of inflammation, but that these receptors play at most a minor role in spinal nociceptive processing in normal and neuropathic animals.

Evidence that [Phe1 psi(CH2-NH)Gly2]nociceptin-(1-13)-NH2, a peripheral ORL-1 receptor antagonist, acts as an agonist in the rat spinal cord.

[Phe1 psi(CH2-NH)Gly2]nociceptin-(1-13)-NH2, a pseudopeptide analogue of nociceptin is an antagonist in peripheral assays. Here, using in vivo electrophysiological recordings of dorsal horn neurones, [Phe1 psi(CH2-NH)Gly2]nociceptin-(1-13)-NH2 appears to have agonist activity after spinal administration. The noxious evoked activity of the neurones was inhibited by [Phe1 psi(CH2-NH)Gly2]nociceptin-(1-13)-NH2, which was as potent as nociceptin itself.

Spinal effects of bicuculline: modulation of an allodynia-like state by an A1-receptor agonist, morphine, and an NMDA-receptor antagonist.

Single-unit recordings were made in the intact anesthetized rat of the responses of dorsal horn neurons to C-, Adelta-, and Abeta-fiber stimulation. The postdischarge and windup responses of the same cells along with responses to innocuous stimuli, prod and brush, also were measured. The effects of (-)-bicuculline-methobromide (0.5, 5, 50, and 250 microg) were observed on these neuronal responses. The C- and Adelta-fiber-evoked responses were facilitated significantly in a dose-dependent manner. The input was facilitated, but as the final overall response was not increased by the same factor, windup appeared to be reduced. However, postdischarge, resulting from the increase in the excitability produced by windup, tended to be facilitated. After doses of >/=5 microg bicuculline, stimulation at suprathreshold Abeta-fiber-evoked activity caused enhanced firing, mainly at later latencies corresponding to Adelta-fiber-evoked activity in normal animals. Few cells responded consistently to brush and so no significant change was observed. Responses evoked by innocuous pressure (prod) always were observed in cells that concurrently responded to electrical stimulation with a C-fiber response. This tactile response was facilitated significantly by bicuculline. The effects of N6-cyclopentyladenosine (N6-CPA), an adenosine A1-receptor agonist, was observed after pretreatment with 50 microg bicuculline, as were the effects of morphine and 7-chlorokynurenate (7-CK). N6-CPA inhibited prod, C- and Adelta-fiber-evoked responses as well as the initial and overall final response to the train of C-fiber strength stimuli. Inhibitions were reversed with 8(p-sulphophenyl) theophylline. Morphine, the mu-receptor agonist, also inhibited the postbicuculline responses to prod, C-, and Adelta-fiber responses and initial and final responses to a train of stimuli. Inhibitory effects of morphine were reversed partly by naloxone. 7-CK, an antagonist at the glycine site on the N-methyl-D-aspartate-receptor complex, inhibited the responses to C- and Adelta-fiber-evoked activity as well as prod. The postdischarges were inhibited by this drug. Again both the initial and overall responses of the cell were inhibited. To conclude, bicuculline caused an increase in the responses of deep dorsal horn cells to prod, Adelta-fiber-evoked activity, increased C-fiber input onto these cells along with the appearance of responses at latencies normally associated with Adelta fibers, but evoked by suprathreshold Abeta-fiber stimulation. These alterations may be responsible for some aspects of the clinical phenomenon of allodynia and hyperalgesia. These altered and enhanced responses were modulated by the three separate classes of drugs, the order of effectiveness being 7-CK, N6-CPA, and then morphine.

Broad-spectrum, non-opioid analgesic activity by selective modulation of neuronal nicotinic acetylcholine receptors.

Development of analgesic agents for the treatment of severe pain requires the identification of compounds that are devoid of opioid receptor liabilities. A potent (inhibition constant = 37 picomolar) neuronal nicotinic acetylcholine receptor (nAChR) ligand called ABT-594 was developed that has antinociceptive properties equal in efficacy to those of morphine across a series of diverse animal models of acute thermal, persistent chemical, and neuropathic pain states. These effects were blocked by the nAChR antagonist mecamylamine. In contrast to morphine, repeated treatment with ABT-594 did not appear to elicit opioid-like withdrawal or physical dependence. Thus, ABT-594 may be an analgesic that lacks the problems associated with opioid analgesia.

The pharmacology of excitatory and inhibitory amino acid-mediated events in the transmission and modulation of pain in the spinal cord.

1. The aim of this review is to consider the relative roles of inhibitory and excitatory amino acid receptor-mediated events in the processes leading to pain transmission in the spinal cord. 2. Emphasis will be on the roles of the inhibitory and excitatory amino acids, GABA and glutamate, and how the relative balance between activity in these systems appears to determine the level of pain transmission. 3. The N-methyl-D-aspartate (NMDA) receptor for glutamate has been implicated in the generation and maintenance of central (spinal) states of hypersensitivity. It has been shown that activation of this receptor underlies wind-up, whereby the level of transmission of noxious messages is potentiated. Antagonists at this receptor-channel complex prevent or block enhanced (hyperalgesic) pain states induced by tissue damage, inflammation, nerve damage and ischemia. 4. Information concerning amplification systems in the spinal cord, such as the NMDA receptor, is a step toward understanding why and how a painful response is not always matched to the stimulus. Such events have parallels with other plastic events such as long-term potentiation (LTP) in the hippocampus. 5. However, the roles of inhibitory transmitter systems can also change insofar as opioid, adenosine and GABA transmission in the spinal cord can vary in different pain states. 6. Changes in GABA systems have been well-documented and discussion will center on whether this has clinical implications. 7. In addition to behavioral and electrophysiological approaches to the pharmacology of pain the current status of the use of markers of early onset genes such as c-fos, as monitors of activity, will be discussed. 8. Hyperalgesia would appear to be balanced by inhibitions during inflammatory conditions but not in neuropathic states, pains due to nerve damage. In the latter case, events reminiscent of LTP may predominate, whereas they are held in check by inhibitions under conditions of inflammation.

NMDA receptor antagonists: interactions with opioids.

The N-methyl-D-aspartate (NMDA) receptor for glutamate has been implicated in the generation and maintenance of central (spinal) states of hypersensitivity. Thus antagonists at this receptor-channel complex have the potential not to totally abolish pain, but to prevent or block hyperalgesic states induced by tissue damage, inflammation, nerve damage and ischaemia. Information on amplification systems in the spinal cord such as the NMDA receptor is a step towards understanding why and how a painful stimulus is not always related to the response. There are now sufficient controlled clinical studies with agents such as ketamine to believe that, in humans, NMDA mediated events are critical in pathological and/or prolonged pain states. Consequently, use of antagonists may aid the treatment of difficult clinical pains when given alone or in combination with opioids. The combinations of opioids and NMDA antagonists may be especially helpful since some NMDA mediated events can be difficult to control with opioids alone (eg. neuropathic pain states). In addition, the ability of opioids to act presynaptically on C-fibre terminals to reduce transmitter release produces synergistic inhibitions with postsynaptically acting NMDA receptor antagonists. Consequently, low dose combinations may be possible with low side-effect liability. Finally, NMDA receptor activation would appear to be responsible for the generation of nitric oxide and prostanoids which further enhance pain transmission whereas adenosine release acts to control these NMDA mediated events. Thus, further targets for the indirect control of NMDA induced activity are possible.

The roles of spinal adenosine receptors in the control of acute and more persistent nociceptive responses of dorsal horn neurones in the anaesthetized rat.

1. We describe here the effects of intrathecal selective adenosine receptor agonists on acute and more persistent evoked responses of dorsal horn nociceptive neurones recorded in intact rats anaesthetized with halothane. 2. The effects of the A1 receptor agonist, N6-cyclopentyladenosine and the non-selective agonist 2-chloroadenosine as well as the A2a receptor agonist, 2-p-(2-carboxyethyl) phenethylamino-5'-N-ethylcarboxamidoadenosine hydrochloride were gauged on the C-, A delta-, A beta-fibre, post-discharge and wind-up responses produced by peripheral tanscutaneous stimulation. The antagonists, theophylline and 8(p-sulphophenyl) theophylline were also tested alone and to reverse the agonist effects. 3. Subcutaneous formalin (5%) was used to produce a more prolonged nociceptive response initiated by peripheral inflammation. 4. Both N6-cyclopentyladenosine and 2-chloroadenosine produced inhibitions of the C-fibre evoked responses, wind-up and post-discharge of the neurones with no significant effects on the A beta responses. By contrast, the A delta evoked responses were facilitated over the same time course and dose-range as the inhibitions. N6-cyclopentyladenosine was more potent and effective than 2-chloroadenosine. In marked contrast to these agonists, the A2a agonist produced only weak non-specific inhibitions. Theophylline and 8(p-sulphophenyl) theophylline alone had no effect on the acute responses but prevented or reversed inhibitory effects of N6-cyclopentyladenosine. 5. The formalin response was markedly inhibited by spinal N6-cyclopentyladenosine with both the acute first phase and more prolonged second phase being dose-dependently inhibited. N6-cyclopentyladenosine was considerably more potent on the formalin response than on the other neuronal measures. 6 The results suggest a role of adenosine Al receptors in the modulation of both acute and inflammatory nociception in the spinal cord.

Electrophysiological study on spinal antinociceptive interactions between adenosine and morphine in the dorsal horn of the rat.

The effects of intrathecal N6-cyclopentyladenosine (N6-CPA), an A1-receptor agonist, and morphine, a mu opioid-receptor agonist were studied on the electrically evoked responses of deep dorsal horn neurones in the rat. Both caused inhibitions, when applied alone, of the C-fibre evoked response and post-discharges. Enhanced inhibitory effects were observed when N6-CPA (5 micrograms) was given with morphine (1 microgram). Increasing the dose of morphine (5 micrograms) with the same dose of N6-CPA occluded the expected inhibitions. By contrast, the A delta-fibre evoked response, inhibited by morphine alone, was significantly facilitated by the combination. Parallel activation of purinergic and opioid inhibitory systems within the dorsal horn as well as complex interactions between them are likely.