The contribution of peripheral bradykinin B2 receptors to carrageenan-evoked oedema and spinal c-Fos expression in rats

A Role for Translational Regulation by S6 Kinase and a Downstream Target in Inflammatory Pain

BACKGROUND AND PURPOSE

Translational controls pervade neurobiology. Nociceptors play an integral role in the detection and propagation of pain signals. Nociceptors can undergo persistent changes in their intrinsic excitability. Pharmacologic disruption of nascent protein synthesis diminishes acute and chronic forms of pain-associated behaviors. Yet, the targets of translational controls that facilitate plasticity in nociceptors are unclear.

EXPERIMENTAL APPROACH

We used ribosome profiling to probe the translational landscape in DRG neurons after treatment of the inflammatory mediators NGF and IL-6. We validated the expression dynamics of c-Fos using immunoblotting and immunohistochemistry. Given that inflammation is known to stimulate mTOR signaling, we reasoned that downstream factors (e.g., ribosomal protein S6 kinase 1, S6K1) might control c-Fos levels. We utilized small-molecule inhibitors of S6K1 (DG2) or c-Fos (T-5224) to probe their effects on nociceptor activity in vitro using multi-electrode arrays (MEAs) and pain behavior in vivo using a hyperalgesic priming model.

KEY RESULTS

We demonstrate that c-Fos is expressed in sensory neurons. Inflammatory mediators that promote pain in both humans and rodents promote c-Fos translation. We demonstrate that the mTOR effector S6K1 is essential for c-Fos biosynthesis. Inhibition of S6K1 or c-Fos with small molecules diminish mechanical and thermal hypersensitivity in response to inflammatory cues. Additionally, both inhibitors reduce evoked nociceptor activity.

CONCLUSION

Our data reveal a novel role of S6K1 in modulating rapid response to inflammatory mediators, with c-Fos being one key downstream target. Targeting the S6 kinase pathway or c-Fos is an exciting new avenue for pain-modulating compounds.

Amygdalin promotes the activity of T cells to suppress the progression of HBV-related hepatocellular carcinoma via the JAK2/STAT3 signaling pathway

Background

Hepatitis B virus (HBV) infection is a high-risk factor of hepatocellular carcinoma (HCC). Cellular immune responses are essential for HCC development, and the CD4+ and CD8+ T subtypes are identified as the primary anti-tumor immune cells. In the study, we investigated the effect and mechanism of amygdalin in the cellular immune response in HBV-related HCC and HCC progression.

Methods

The cell proliferation was examined by MTT analysis. Cells metastasis ability was detected by Invasion and migration assays. Quantification of apoptotic cells was performed with Flow cytometer assay. The protein levels of p-STAT3, STAT3, p-JAK2, JAK2, caspase-3, cleaved caspase-3 were detected by performing immunoblotting assays.

Results

We demonstrate that amygdalin treatment could rescue the HBV-T cell viability and IFN-γ and TNF-αproduction. In HBV-T cells, the MFI levels of CD8⁺ are lower than that in NC-T cells. Moreover, the phosphorylation levels of STAT3 and JAK2 are higher in HBV-T cells, compared to those in NC-T cells, and then reduced by amygdalin treatment. Co-culture with HBV-T cells could reduce IFN-γ and TNF-α, production while increase IL-6 and IL-10 production in HepG2.2.15 cells; these alterations could be partially reversed by amygdalin pretreatment. Finally, co-culture with HBV-T cells significantly promoted the cell viability, inhibited the apoptosis, and promoted the migration of HepG2.2.15 cells, and these alterations could be partially reversed by amygdalin treatment.

Conclusion

Our findings provide a rationale for further studies on the functions and mechanism of amygdalin inhibiting HBV-related HCC cell proliferation, invasion, and migration via T cell-mediated tumor immunity.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12879-020-05713-0.

Anti-inflammatory and anti-hypersensitive effects of the chalcone isocordoin and its semisynthetic derivatives in mice

Isocordoin (1), a chalcone isolated from different plants, has been found to present a range of interesting biological properties. This study aimed to evaluate the anti-hypersensitive and anti-inflammatory effects of isocordoin (1) and several natural and semisynthetic derivatives (2-10). Initial evaluation of (1), dihydroisocordoin (2) and six semisynthetic derivatives (3-8) in the inhibition of abdominal writhes induced by acetic acid model showed that only isocordoin dimethylether (5) caused more than 70% of inhibition. Further evaluation of 5 for its anti-oedematogenic activity and anti-hypersensitivity effect induced by carrageenan, lipopolysaccharide (LPS), bradykinin (BK), prostaglandin E2 (PGE2), and epinephrine showed that isocordoin dimethylether (5) presented a discrete inhibition of carrageenan- and LPS-induced hypersensitivity, and of carrageenan-induced paw oedema, and that it was able to significantly reduce both the oedema and hypersensitivity induced by BK. Furthermore, when tested in the PGE2 model, 5 interfered only with the paw-oedema, without showing any effect against the paw-hypersensitivity. Evaluation of the natural isocordoin (1), together with the semisynthetic derivatives isocordoin dimethylether (5), isocordoin methylether (9), and dihydroisocordoin methylether (10) in the BK-induced oedema and hypersensitivity showed that the monoalkylated derivatives 10 and 9 had the strongest antinociceptive activity. The results of this investigation indicate that both monoalkylation of the C-4' phenolic hydroxyl group and reduction of the double bond in the α,β-unsaturated system of the chalcone skeleton favor activity.

Synthetic chalcones as potential tool for acute- and chronic-pain control

The purpose of this study was to validate the potential anti-hypersensitive activity of two chalcones, (2E)-1-(4-aminophenyl)-3-(4-nitrophenyl)prop-2-en-1-one (ANCh) and N-{4-[(2E)-3-(4-nitrophenyl)prop-2-enoil]phenyl}acetamide (AcANCh), by different models of acute and persistent pain in mice, besides in silico analysis. Molecules computational investigation for prediction of Lipinki's and Veber's rules to determine solubility, % absorption, drug likeness and toxicity liabilities was performed. Male and female C57BL/6 mice (20-30 g, n = 6) were used. Firstly, mice were pre-treated with the compounds ANCh or AcANCh and then submitted to the models of acute hypersensitivity by the intraplantar injection of different phlogistic agents. The mechanical sensitivity was assessed using von Frey hairs (0.6 g). The obtained data shows that both compounds presented important inhibitory effects on mechanical hypersensitivity induced by carrageenan (with oral bioavailability). The anti-hypersensitive effect was also accompanied by the interference in leukocyte migration, interleukin-1β (IL-1β) and tumour necrosis factor (TNF) levels reduction and by the absence of unspecific effects. Added to the in vivo results, the in silico analysis presented none violation in Lipinski's or Veber's rules, good probability to cell membrane permeability and oral bioavailability, positive values of drug likeness and few risk of computational toxicity. ANCh partially reduced the hypersensitivity induced by IL-1β and TNF, epinephrine and prostaglandin E₂ (PGE₂). AcANCh had similar effect, except for the absent of inhibition in PGE₂-injected mice. Both compounds were capable of reducing the mechanical hypersensitivity presented in all persistent models of hypersensitivity (inflammatory pain, chronic nerve constriction and cancer pain), with emphasis for ANCh. These results suggest that both chalcones could represent good strategies for the control of acute and chronic pain, without important side effects. ANCh seems to involve cell migration and cytokines production as the main mechanism, together with interference in PGE2 neuronal sensitization pathway. In vivo and in silico analyses reinforce the potential characteristics of the compounds to become future drugs.

Molecular mechanism of amygdalin action in vitro: review of the latest research

Amygdalin, named as 'laetrile' and 'vitamin B-17' was initially supposed to be a safe drug for cancer treatment and was recognized by followers of natural medicine since it has been considered to be hydrolyzed only in cancer cells releasing toxic hydrogen cyanide (HCN), and thus destroying them. Unfortunately, current studies have shown that HCN is also released in normal cells, therefore it may not be safe for human organism. However, there have still been research works conducted on anti-cancer properties of this compound. In vitro experiments have shown induction of apoptosis by amygdalin as a result of increased expression of Bax protein and caspase-3 and reduced expression of antiapoptotic BcL-2protein. Amygdalin has also been shown to inhibit the adhesion of breast cancer cells, lung cancer cells and bladder cancer cells by decreased expression of integrin's, reduction of catenin levels and inhibition of the Akt-mTOR pathway, which may consequently lead to inhibition of metastases of cancer cells. It has also been revealed that amygdalin in renal cancer cells increased expression of p19 protein resulting in inhibition of cell transfer from G1-phase to S-phase, and thus inhibited cell proliferation. Other studies have indicated that amygdalin inhibits NF-kβ and NLRP3 signaling pathways, and consequently has anti-inflammatory effect due to reducing the expression of proinflammatory cytokines such as pro-IL-1β. Moreover, the effect of amygdalin on TGFβ/CTGF pathway, anti-fibrous activity and expression of follistatin resulting in activation of muscle cells growth has been reported. This compound might be applicable in the treatment of various cancer cell types.

Pain fiber anesthetic reduces brainstem Fos after tooth extraction

We recently demonstrated that pain-sensing neurons in the trigeminal system can be selectively anesthetized by co-application of QX-314 with the TRPV1 receptor agonist, capsaicin (QX cocktail). Here we examined whether this new anesthetic strategy can block the neuronal changes in the brainstem following molar tooth extraction in the rat. Adult male Sprague-Dawley rats received infiltration injection of anesthetic 10 min prior to lower molar tooth extraction. Neuronal activation was determined by immunohistochemistry for the proto-oncogene protein c-Fos in transverse sections of the trigeminal subnucleus caudalis (Sp5C). After tooth extraction, c-Fos-like immunoreactivity (Fos-LI) detected in the dorsomedial region of bilateral Sp5C was highest at 2 hrs (p < .01 vs. naïve ipsilateral) and declined to pre-injury levels by 8 hrs. Pre-administration of the QX cocktail significantly reduced to sham levels Fos-LI examined 2 hrs after tooth extraction; reduced Fos-LI was also observed with the conventional local anesthetic lidocaine. Pulpal anesthesia by infiltration injection was confirmed by inhibition of the jaw-opening reflex in response to electrical tooth pulp stimulation. Our results suggest that the QX cocktail anesthetic is effective in reducing neuronal activation following tooth extraction. Thus, a selective pain fiber 'nociceptive anesthetic' strategy may provide an effective local anesthetic option for dental patients in the clinic.

Electroacupuncture Attenuates 5'-Guanidinonaltrindole-Evoked Scratching and Spinal c-Fos Expression in the Mouse

The present study was undertaken to investigate the influence of electroacupuncture (EA) on compulsive scratching in mice and c-Fos expression elicited by subcutaneous (s.c.) administration of a known puritogen, 5'-guanidinonaltrindole (GNTI) to the neck. Application of EA to Hegu (LI4) and Quchi (LI11) acupoints at 2 Hz, but not 100 Hz, attenuated GNTI-evoked scratching. In mice pretreated with the µ opioid receptor antagonist naloxone, EA 2 Hz did not attenuate GNTI-evoked scratching, whereas EA at 2 Hz did attenuate GNTI-evoked scratching in mice pretreated with the κ opioid receptor antagonist nor-binaltorphimine. Moreover, intradermal (i.d.) administration of the selective µ opioid receptor agonist [d-Ala2, N-Me-Phe4, Gly5-ol]-enkephalin acetate (DAMGO) attenuated GNTI-evoked scratching behavior, while s.c. administration of DAMGO was ineffective. GNTI provoked c-Fos expression on the lateral side of the superficial layer of the dorsal horn of the cervical spinal cord. Application of 2 Hz EA to LI4 and LI11 decreased the number of c-Fos positive nuclei induced by GNTI. It may be concluded that application of 2 Hz EA to LI4 and LI11 attenuates scratching behavior induced by GNTI in mice and that the peripheral µ opioid system is involved, at least in part, in the anti-pruritic effects of EA.

Electroacupuncture reduces cocaine-induced seizures and mortality in mice

The aims of this study were to characterize the protective profile of electroacupuncture (EA) on cocaine-induced seizures and mortality in mice. Mice were treated with EA (2 Hz, 50 Hz, and 100 Hz), or they underwent needle insertion without anesthesia at the Dazhui (GV14) and Baihui (GV20) acupoints before cocaine administration. EA at 50 Hz applied to GV14 and GV20 significantly reduced the seizure severity induced by a single dose of cocaine (75 mg/kg; i.p.). Furthermore, needle insertion into GV14 and GV20 and EA at 2 Hz and 50 Hz at both acupoints significantly reduced the mortality rate induced by a single lethal dose of cocaine (125 mg/kg; i.p.). In the sham control group, EA at 50 Hz applied to bilateral Tianzong (SI11) acupoints had no protective effects against cocaine. In addition, EA at 50 Hz applied to GV14 and GV20 failed to reduce the incidence of seizures and mortality induced by the local anesthetic procaine. In an immunohistochemistry study, EA (50 Hz) pretreatment at GV14 and GV20 decreased cocaine (75 mg/kg; i.p.)-induced c-Fos expression in the paraventricular thalamus. While the dopamine D₃ receptor antagonist, SB-277011-A (30 mg/kg; s.c), did not by itself affect cocaine-induced seizure severity, it prevented the effects of EA on cocaine-induced seizures. These results suggest that EA alleviates cocaine-induced seizures and mortality and that the dopamine D₃ receptor is involved, at least in part, in the anticonvulsant effects of EA in mice.

Peripheral inhibition of glutaminase reduces carrageenan-induced Fos expression in the superficial dorsal horn of the rat

In inflamed tissue, the levels of the excitatory amino acid glutamate are increased. Glutamate sensitizes peripheral axons of primary afferent neurons during inflammation leading to decreased firing threshold and hyperexcitability. One proposed source of glutamate is the primary afferent. Antagonizing glutamate receptors on peripheral axons of primary afferents during inflammation provides analgesia in animals and humans. The enzyme glutaminase is used by primary sensory neurons to convert glutamine to glutamate, and peripheral inhibition of glutaminase with 6-diazo-5-oxo-l-norleucine (DON) provides long-lasting analgesia during inflammation. In this study, we measured the effects of glutaminase inhibition on carrageenan-induced spinal Fos expression. Rats were given intraplantar injections of carrageenan and treated locally with either vehicle or DON. After 3h of inflammation, hind paw swelling and spinal expression of Fos were examined. CellProfiler was used to automate Fos nuclei counting in five laminar groupings in the spinal cord (I-II, III-IV, V-VI, VII-IX, X). Carrageenan increased hind paw thickness by approximately 70% and spinal Fos expression in superficial (I-II) and deep (V-VI) laminae by 10-fold and 5-fold, respectively. Treatment with DON reduced hind paw swelling by approximately 13% and suppressed Fos expression in the laminae I-II by approximately 54%, but not the deep laminae. Our results further support the notion of glutamate as a peripheral inflammatory mediator and indicate that glutaminase should be considered as a novel therapeutic target for treatment of inflammatory pain.

Anti-edematogenic effects of velutinol A isolated from Mandevilla velutina: evidence for a selective inhibition of kinin B1 receptor-mediated responses

This study assesses the effects of compound velutinol A obtained from M. velutina in the rat paw edema induced by several phlogistic agents. Attempts were made to analyze how velutinol A is able to inhibit kinin B(1) receptor-mediated inflammatory responses. Velutinol A (100 nmol/paw) partially reduced (about 30%) the edema evoked by carrageenan (300 microg/paw). However, velutinol A (100 nmol/paw) failed to affect the edema induced by histamine (200 nmol/paw), substance P (30 nmol/paw), PAF (10 nmol/paw) or BK (3 nmol/paw). Interestingly, the edema caused by the selective kinin B(1) receptor agonist des-Arg(9)-BK (100 nmol/paw) in animals pre-treated with PAF or LPS was significantly inhibited by velutinol A (100 nmol/paw) (48 and 46%, respectively). A similar inhibition of des-Arg(9)-BK-induced edema after pre-treatment with PAF was obtained with the non-peptidic and selective B(1) receptor antagonist SSR 240612 (60 nmol/paw) (46%). In addition, the systemic administration of velutinol A (10 mg/kg, i.p.) or SSR 240612 (1 mg/kg, i.p.) also caused a significant reduction of des-Arg(9)-BK (100 nmol/paw)-induced edema in PAF-treated rats (51 and 43%, respectively). The results provide convincing evidence that velutinol A selectively blocks the edema responses mediated by B(1) receptor activation in vivo. This compound might represent a new non-peptidic and selective antagonist for kinin B(1) receptors.

Selective activation of cannabinoid CB(2) receptors suppresses spinal fos protein expression and pain behavior in a rat model of inflammation

Activation of cannabinoid CB(2) receptors attenuates thermal nociception in untreated animals while failing to produce centrally mediated effects such as hypothermia and catalepsy [Pain 93 (2001) 239]. The present study was conducted to test the hypothesis that activation of CB(2) in the periphery suppresses the development of inflammatory pain as well as inflammation-evoked neuronal activity at the level of the CNS. The CB(2)-selective cannabinoid agonist AM1241 (100, 330 micrograms/kg i.p.) suppressed the development of carrageenan-evoked thermal and mechanical hyperalgesia and allodynia. The AM1241-induced suppression of carrageenan-evoked behavioral sensitization was blocked by the CB(2) antagonist SR144528 but not by the CB(1) antagonist SR141716A. Intraplantar (ipl) administration of AM1241 (33 micrograms/kg ipl) suppressed hyperalgesia and allodynia following administration to the carrageenan-injected paw but was inactive following administration in the contralateral (noninflamed) paw, consistent with a local site of action. In immunocytochemical studies, AM1241 suppressed spinal Fos protein expression, a marker of neuronal activity, in the carrageenan model of inflammation. AM1241 suppressed carrageenan-evoked Fos protein expression in the superficial and neck region of the dorsal horn but not in the nucleus proprius or the ventral horn. The suppression of carrageenan-evoked Fos protein expression induced by AM1241 was blocked by coadministration of SR144528 in all spinal laminae. These data provide evidence that actions at cannabinoid CB(2) receptors are sufficient to suppress inflammation-evoked neuronal activity at rostral levels of processing in the spinal dorsal horn, consistent with the ability of AM1241 to normalize nociceptive thresholds and produce antinociception in inflammatory pain states.

A peripheral cannabinoid mechanism suppresses spinal fos protein expression and pain behavior in a rat model of inflammation

The present studies were conducted to test the hypothesis that systemically inactive doses of cannabinoids suppress inflammation-evoked neuronal activity in vivo via a peripheral mechanism. We examined peripheral cannabinoid modulation of spinal Fos protein expression, a marker of neuronal activity, in a rat model of inflammation. Rats received unilateral intraplantar injections of carrageenan (3%). In behavioral studies, carrageenan induced allodynia and mechanical hyperalgesia in response to stimulation with von Frey monofilaments. The cannabinoid agonist WIN55,212-2 (30 microg intraplantarly), administered concurrently with carrageenan, attenuated carrageenan-evoked allodynia and hyperalgesia relative to control conditions. In immunocytochemical studies, WIN55,212-2 suppressed the development of carrageenan-evoked Fos protein expression in the lumbar dorsal horn of the spinal cord relative to vehicle treatment. The same dose administered systemically or to the noninflamed contralateral paw failed to alter either carrageenan-evoked allodynia and hyperalgesia or carrageenan-evoked Fos protein expression, consistent with a peripheral site of action. The suppressive effects of WIN55,212-2 (30 microg intraplantarly) on carrageenan-evoked Fos protein expression and pain behavior were blocked by local administration of either the CB(2) antagonist SR144528 (30 microg intraplantarly) or the CB(1) antagonist SR141716A (100 microg intraplantarly). WIN55,212-3, the enantiomer of the active compound, also failed to suppress carrageenan-evoked Fos protein expression. These data provide direct evidence that a peripheral cannabinoid mechanism suppresses the development of inflammation-evoked neuronal activity at the level of the spinal dorsal horn and implicate a role for CB(2) and CB(1) in peripheral cannabinoid modulation of inflammatory nociception.

Bee venom pretreatment has both an antinociceptive and anti-inflammatory effect on carrageenan-induced inflammation

Although the injection of bee venom (BV) has been reported to evoke tonic pain and hyperalgesia, there is conflicting evidence in the literature indicating that BV can also exert an anti-inflammatory and antinociceptive effects on inflammation. In this regard, BV has been traditionally used in Oriental medicine to relieve pain and to treat chronic inflammatory diseases such as rheumatoid arthritis. The present study was designed to test the hypothesis that BV induces acute nociception under normal conditions, but that it can serve as a potent anti-inflammatory and antinociceptive agent in a localized inflammatory state. The experiments were designed to evaluate the effect of BV pretreatment on carrageenan (CR)-induced acute paw edema and thermal hyperalgesia. In addition, spinal cord Fos expression induced by peripheral inflammation was quantitatively analyzed. In normal animals subcutaneous BV injection into the hindlimb was found to slightly increase Fos expression in the spinal cord without producing detectable nociceptive behaviors or hyperalgesia. In contrast pretreatment with BV (0.8 mg/kg) 30 min prior to CR injection suppressed both the paw edema and thermal hyperalgesia evoked by CR. In addition, there was a positive correlation between the percent change in paw volume and the expression of Fos positive neurons in the spinal cord. These results indicate that BV pretreatment has both antinociceptive and anti-inflammatory effects in CR-induced inflammatory pain. These data also suggest that BV administration may be useful in the treatment of the pain and edema associated with chronic inflammatory diseases.

Carrageenan-induced thermal hyperalgesia in the mouse: role of nerve growth factor and the mitogen-activated protein kinase pathway

NGF is an important link between inflammation and hyperalgesia and interacts with many different mediators of inflammation, including the MAPK signaling pathway. In these studies, carrageenan-induced thermal hyperalgesia was evaluated in the mouse and the role of NGF and the MAPK pathway investigated. Carrageenan induced a time-dependent inflammation and thermal hyperalgesia, which was maximal 4 h post administration. Both indomethacin (0.3, 1.0 and 10 mg/kg s.c., 30 min pre-carrageenan) and morphine (0.4, 1.2, 4.0 mg/kg; s.c., 30 min pre-hyperalgesia measurement) significantly inhibited carrageenan-induced thermal hyperalgesia and indomethicin inhibited paw inflammation, demonstrating the model as suitable for the assessment of anti-hyperalgesic and anti-inflammatory agents. Anti-NGF (0.67 mg/kg sc, 60 min pre-carrageenan) produced a significant inhibition of thermal hyperalgesia, but not inflammation. NGF itself produced a time-dependent hyperalgesia, but not inflammation, following intraplantar injection. The specific MAPK pathway inhibitor, PD98059 (0.1, 0.3 and 1 mg/kg sc, 30 min pre-carrageenan) significantly inhibited carrageenan-induced hyperalgesia, but not inflammation. These data demonstrate a role for both NGF and the MAPK signaling pathway in the production of thermal hyperalgesia, but not inflammation, in the mouse.

The induction of pain: an integrative review

The highly disagreeable sensation of pain results from an extraordinarily complex and interactive series of mechanisms integrated at all levels of the neuroaxis, from the periphery, via the dorsal horn to higher cerebral structures. Pain is usually elicited by the activation of specific nociceptors ('nociceptive pain'). However, it may also result from injury to sensory fibres, or from damage to the CNS itself ('neuropathic pain'). Although acute and subchronic, nociceptive pain fulfils a warning role, chronic and/or severe nociceptive and neuropathic pain is maladaptive. Recent years have seen a progressive unravelling of the neuroanatomical circuits and cellular mechanisms underlying the induction of pain. In addition to familiar inflammatory mediators, such as prostaglandins and bradykinin, potentially-important, pronociceptive roles have been proposed for a variety of 'exotic' species, including protons, ATP, cytokines, neurotrophins (growth factors) and nitric oxide. Further, both in the periphery and in the CNS, non-neuronal glial and immunecompetent cells have been shown to play a modulatory role in the response to inflammation and injury, and in processes modifying nociception. In the dorsal horn of the spinal cord, wherein the primary processing of nociceptive information occurs, N-methyl-D-aspartate receptors are activated by glutamate released from nocisponsive afferent fibres. Their activation plays a key role in the induction of neuronal sensitization, a process underlying prolonged painful states. In addition, upon peripheral nerve injury, a reduction of inhibitory interneurone tone in the dorsal horn exacerbates sensitized states and further enhance nociception. As concerns the transfer of nociceptive information to the brain, several pathways other than the classical spinothalamic tract are of importance: for example, the postsynaptic dorsal column pathway. In discussing the roles of supraspinal structures in pain sensation, differences between its 'discriminative-sensory' and 'affective-cognitive' dimensions should be emphasized. The purpose of the present article is to provide a global account of mechanisms involved in the induction of pain. Particular attention is focused on cellular aspects and on the consequences of peripheral nerve injury. In the first part of the review, neuronal pathways for the transmission of nociceptive information from peripheral nerve terminals to the dorsal horn, and therefrom to higher centres, are outlined. This neuronal framework is then exploited for a consideration of peripheral, spinal and supraspinal mechanisms involved in the induction of pain by stimulation of peripheral nociceptors, by peripheral nerve injury and by damage to the CNS itself. Finally, a hypothesis is forwarded that neurotrophins may play an important role in central, adaptive mechanisms modulating nociception. An improved understanding of the origins of pain should facilitate the development of novel strategies for its more effective treatment.