Effects of the high-efficacy 5-HT1A receptor agonist, F 13640 in the formalin pain model: a c-Fos study.

We studied the effects of the high-efficacy 5-hydroxytryptamine1A (5-HT1A) receptor agonist, F 13640 on both formalin-induced spinal cord c-Fos protein expression and pain behaviours in the rat. Replicating earlier data, F 13640 (0.63 mg/kg, i.p.; t(-15 min)) completely inhibited the elevation and licking of the formalin-injected paw. In the same animals, and in spite of the agent as in earlier data increasing the number of c-Fos labelled nuclei when it was administered alone, F 13640 markedly reduced the number of formalin-induced c-Fos labelled nuclei. This was found in both the superficial (I-II) and deep (V-VI) dorsal horn laminae (2 h post-injection: 72+/-2% and 92+/-1% of reduction, respectively; P<0.001 in either case), spinal areas that contain neurons responsive to nociceptive stimulation. Co-operation occurred so that after the co-administration of F 13640 and formalin, c-Fos expression was inferior to that induced when either stimulation was administered alone. The data provide initial evidence for the agent's inhibitory effects on noxiously evoked c-Fos expression. The results indicate that co-operation between 5-HT1A receptor activation and nociceptive stimulation powerfully inhibits responses to severe, tonic nociception.

Effects of morphine and naloxone on basal and evoked Fos-like immunoreactivity in lumbar spinal cord neurons of arthritic rats.

We have previously shown, on the one hand, that the number of Fos-like immunoreactive (Fos-LI) neurons in the lumbar spinal cord observed during the development of adjuvant-induced arthritis in the rat correlates with the clinical and behavioral scores and, on the other hand, that the number of Fos-LI neurons induced by repeated mechanical pressure to the ankle was greater in arthritic animals than in healthy ones. In non-stimulated arthritic rats, Fos-LI neurons were mainly present in the neck of the dorsal horn and in the ventral horn of L3-L5, whereas following stimulation they were numerous in the superficial laminae. The aim of this study was to evaluate Fos-LI following morphine injection (1) in arthritic animals in the absence of any stimulation, (2) in arthritic rats after ankle stimulation pretreated with morphine or by the combination of morphine and naloxone, and (3) following naloxone treatment in non-stimulated and stimulated polyarthritic rats. In non-stimulated arthritic rats, a single morphine injection (1-9 mg/kg, i.v.) or a single naloxone injection (1-3 mg/kg, i.v.) induced no change in the basal Fos-LI present in lumbar spinal neurons. In contrast, Fos-LI evoked by noxious pressure was strongly depressed by morphine. In the superficial laminae pretreatment with a single morphine injection of either 0.5 or 1 mg/kg, i.v., reduced by more than 50% the number of Fos-LI neurons and at 3 mg/kg completely abolished the labeling evoked by the stimulation. Similar effects were obtained in the neck of the dorsal horn. These effects were reversed by naloxone (morphine 3 mg/kg and naloxone 0.3 mg/kg). Pretreatment with naloxone (1 mg/kg) did not change Fos labeling. This study which is based on mechanical stimulation in arthritic rats confirms and extends previous investigations and demonstrates that the use of Fos-LI is a suitable method to reveal the efficacy of opioid analgesic. However, the lack of effects of opioids on basal labeling suggests that long-term drug treatment should be used to study the effects of various putative analgesics on chronic pain models.

Chronic treatments with aspirin or acetaminophen reduce both the development of polyarthritis and Fos-like immunoreactivity in rat lumbar spinal cord.

We have previously shown that during the development of adjuvant-induced arthritis (AIA), and without any peripheral stimulation, the number of Fos-like immunoreactive (Fos-LI) neurons in lumbar spinal cord increases in parallel with the clinical and behavioral signs of the disease and peaks 3 weeks after the inoculation which corresponds to the maximal stage of hyperalgesia (Abbadie and Besson 1992a). The aim of this study was to evaluate the suitability of the Fos-LI technique to gauge the effects of the two most prescribed analgesics, aspirin and acetaminophen (paracetamol), on spinal cord neurons of polyarthritic rats. The effects of the two drugs were tested on the "evoked" Fos-LI induced by peripheral mechanical noxious stimulus, as well as the effects of a chronic treatment on "basal" Fos-LI appearing during the development of polyarthritis in the absence of any intentional stimulation. We showed that: (1) Fos-LI evoked by ankle stimulation was not modified by either aspirin (150 mg/kg i.v.) or pro-acetaminophen (300 mg/kg i.v.) injection or by a 10-day chronic treatment with acetaminophen (250 or 500 mg/kg/day). (2) Despite the fact that the clinical signs of arthritis were reduced, basal Fos-LI induced by AIA disease was not changed after a 2-week chronic treatment with either aspirin (300 mg/kg/day) or acetaminophen (500 mg/kg/day) starting 3 weeks after AIA inoculation, i.e., at the maximal stage of hyperalgesia and when Fos-LI is maximal. This observation questions the suitability of Fos-LI technique to gauge the effects of mild analgesics. (3) In contrast, when the same chronic treatment was applied during the development of AIA, i.e., 1 week after inoculation, the number of Fos-LI nuclei was significantly decreased (about 50%) in aspirin- and acetaminophen-treated groups as compared to vehicle-treated groups. In parallel, the clinical signs of AIA disease were blocked by the two drug treatments. In addition, 2 weeks after the end of treatment, neither the clinical signs nor the number of Fos-LI increased again. The fact that the two drugs are able to prevent c-fos expression during development of arthritis, but not to interfere with already existing c-fos expression, suggests that for pharmacological investigations this technique should be used with caution. Thus, the potential use of Fos-LI to gauge the effects of non-steroidal antinociceptive drugs and other mild analgesics during chronic disease such as arthritis is discussed.

Electrophysiological characteristics of dorsal horn cells in rats with cutaneous inflammation resulting from chronic arthritis.

(1) Electrophysiological properties of dorsal horn neurones have been investigated in decerebrate, immobilized spinal rats rendered polyarthritic by intradermal injection of Freund's adjuvant. Since arthritis is associated with pronounced erythema and oedema of the foot sole and ankle areas, this particular study was devoted to the induced modifications of responses of units driven by cutaneous inputs. It allowed comparison with previous studies performed in healthy animals. (2) Superficial dorsal horn cells could be separated in: those driven from non-oedematous skin and which had properties similar to those observed in healthy animals; and those driven from oedematous skin and which characterized by unclassical electrophysiological properties such as: (a) a relatively high level of background activity, frequently with bursting pattern and sometimes exhibiting dramatic increases; these neurones are normally silent in healthy animals; (b) a high degree of responsivity to light mechanical stimuli associated with a clear decrease in threshold. It is possible that units which initially only responded to high intensity or frankly noxious stimuli shifted to become highly or maximally responsive to gentle stimuli. (3) Similar modifications were also observed for some of the neurones located in deeper areas of the spinal cord where it is suggested that some units initially responding to both non-noxious and noxious stimuli (class 2 cells) in healthy rats shifted their characteristics to resemble units maximally driven by non-noxious stimuli (class 1 cells) in arthritic animals. (4) The numerous structural alterations reported in cutaneous tissues compared to the relatively normal structure of peripheral nerves and central nervous systems suggest that these observations can be related to peripheral sensitization of the sensory endings of thin peripheral fibres induced by the release of chemalgogens in inflammatory exudates. (5) For both superficial and deeper dorsal horn neurones, arthritis also affected the late responses of neurones to transcutaneous electrical stimulation and their responses to radiant heat. Responses to radiant heat showed an increase in threshold, sometimes associated with a very high degree of adaptation. (6) Another group of neurones displayed fading responses to repetitive mechanical stimuli. All were located in the deeper two-thirds of the dorsal horn. (7) These data indicate that chronic pathological conditions related to articular and/or cutaneous diseases strongly modify the responses of dorsal horn neurones, some being at the origin of ascending tracts. This model seems to be a promising approach to study the fundamental mechanisms involved in chronic pain in order to approach these problems in human beings.

Attempts to gauge the relative importance of pre- and postsynaptic effects of morphine on the transmission of noxious messages in the dorsal horn of the rat spinal cord.

Both pre- and postsynaptic mechanisms have been proposed as an explanation of the depressive effects of opioids on the activity of nociceptive dorsal horn neurons. In order to gauge the importance of the two mechanisms, we studied the effect of morphine on the spontaneous hyperactivity of nociceptive dorsal horn neurons in the spinalized decerebrated deafferented rat (C5-Th1). In this preparation, intravenous morphine was shown to depress spontaneous firing rate in a dose-dependent fashion. A comparative analysis of the effect of the same dose of morphine (2 mg/kg i.v.) in the intact spinalized decerebrated arthritic rat, in which dorsal horn convergent neurons also display high spontaneous activity, revealed that systemic morphine is twice as potent when primary afferent fibers are left intact. These results can explain why the analgesic effect of morphine is more marked against pains due to an excess of nociception than against pains arising from deafferentation.

Aspirin and acetaminophen reduced both Fos expression in rat lumbar spinal cord and inflammatory signs produced by carrageenin inflammation.

This study, performed in freely moving rats, evaluates the effects of the two most prescribed analgesics, aspirin and acetaminophen, on carrageenin inflammation and the associated c-Fos expression in the rat lumbar spinal cord. Maximal dorsal horn c-Fos expression is observed 3 h after carrageenin (6 mg/150 microliters of saline), with Fos-like (Fos-LI) neurones being predominantly located in laminae I-II and V-VI (41 +/- 3% and 39 +/- 5% of the total number of Fos-LI neurones per section for the control group, respectively) of the dorsal horn. Pretreatment with aspirin (75 or 150 mg/kg, i.v.) reduced the number of Fos-LI neurones induced by carrageenin-inflammation (28 +/- 2% and 45 +/- 1% reduction, respectively; P < 0.001 for both). Acetaminophen (75 or 105 mg/kg, i.v.) reduced the number of Fos-LI neurones (19 +/- 1% and 43 +/- 1% reduction, respectively; P < 0.001 for both). When considering the lower dose (75 mg/kg), the effects of aspirin were significantly more marked than those of acetaminophen (P < 0.001). There was a tendency for both aspirin and acetaminophen to have a more pronounced effect on the number of Fos-LI neurones located in deeper laminae, these differential effects being significant for 75 mg/kg of aspirin (P < 0.01) and 150 mg/kg of acetaminophen (P < 0.01). Both the two doses of aspirin and acetaminophen greatly reduced the inflammatory signs associated with the intraplantar injection of carrageenin. Furthermore there was a positive correlation between the effects of aspirin and acetaminophen on the number of Fos-LI neurones and the inflammatory signs which developed after carrageenin. Our results suggest that the effects of both drugs are mainly due to peripheral site of action without rejecting an additional central site of action of systemic non-steroidal anti-inflammatory drugs (NSAIDs) and acetaminophen. In addition, our results suggest that the approach we used could be a useful tool to evaluate systematically and quantitatively the effects of NSAIDs. Finally, the effects obtained with the low dose of acetaminophen question the classical view of textbooks claiming that such a compound had no anti-inflammatory effect and are in agreement with previous observations in humans.

Intravenous morphine does not modify dorsal horn touch-evoked allodynia in the mononeuropathic rat: a Fos study.

In a model of mononeuropathic pain (chronic constriction injury of the sciatic nerve, CCI), we have demonstrated that light touch stimuli (stroking) to the paw induced Fos-like immunoreactivity (Fos-LI) in the superficial and deep dorsal horn of the rat spinal cord (Catheline et al., Pain 80 (1999a) 347). The efficacy of opioids in neuropathic pain being controversial, we have tested the effects of morphine (0.3, 1 and 3 mg/kg intravenous, i.v.) on this spinal Fos-LI evoked by light tactile stimuli, which could be related to mechanical allodynia. Morphine did not change the level of spinal Fos-LI observed following light touch stimuli in the CCI rats (43 +/- 3, 38 +/- 7, and 37 +/- 4 Fos-LI neurones/40 microm L4-L5 section, respectively, for the three doses versus 32 +/- 4 in the control group). In contrast, the administration of 3 mg/kg of i.v. morphine reduced by 30% the number of Fos-LI neurones induced by heat stimulation (52 degrees C, 15 s duration) in CCI rats (P < 0.05) as in sham-operated rats. These effects were reversed by the systemic administration of naloxone. The lack of effect of morphine on touch-evoked Fos-LI in the superficial dorsal horn reinforces the assertion that dynamic mechanical allodynia is related to information transmitted by A-beta fibres, since opioid receptors are mainly located on thin primary afferent fibres. Our results provide a basis for a certain form of allodynia that is insensitive to morphine.

Complex temporal changes in 5-hydroxytryptamine synthesis in the central nervous system induced by experimental polyarthritis in the rat.

The purpose of this study was to investigate modifications of 5-HT synthesis in a chronic pain model, the arthritic rat, at different times after the inoculation with Freund's adjuvant. This study confirms our previous findings that experimental induced polyarthritis is associated with a marked increase in free tryptophan levels in serum. During the acute phase of the disease (15-21 days after the adjuvant), the general increase in 5-HT synthesis observed in the CNS appeared to be related to an increase in tryptophan availability due to the elevation of free tryptophan in serum. During the post-acute phase of the disease (28-42 days after the adjuvant), the level of free tryptophan in the serum remained markedly increased but the levels in the CNS tended to return to normal values in all areas examined. At 42 days, 5-HT synthesis in the brain had also returned to normal values but was further increased at the spinal level. In addition, although 5-HT levels and 5-HT synthesis were increased in the dorsal as well as in the ventral part of the cord, an increase in the rate of disappearance of the amine after blockade of the decarboxylase (benserazide) was only observed in the dorsal part. This result tends to suggest that the descending serotonergic system projecting to the dorsal horn is preferentially activated during chronic pain.

Benzodiazepines and pain: effects of midazolam on the activities of nociceptive non-specific dorsal horn neurons in the rat spinal cord.

The high density of GABA-benzodiazepine receptors in the superficial dorsal horn suggests a possible involvement of benzodiazepines (BZs) in the modulation of spinal pain processes. In this electrophysiological study we have examined the effects of midazolam (MZ), a water-soluble short-acting BZ, on the activities of 57 nociceptive non-specific dorsal horn cells, one in each animal. Recordings were performed at lumbar level in unanesthetized decerebrate spinal rats before and following intravenous injection of MZ (1, 2 or 5 mg/kg). The spontaneous activity was weakly and significantly but not dose dependently reduced by MZ. For the total neuronal population MZ induced no significant effect on C-fiber evoked responses, whatever the dose used. More precise analysis shows that for 45/55 neurons the responses were slightly depressed, but this effect was not dose dependent. In contrast, A delta-fiber evoked responses were markedly and dose dependently depressed. These effects of MZ were reversed by intravenous administration of the antagonist flumazenil (FZ). Despite the fact that MZ displays a very weak effect on responses due to C-fiber stimulation, the possible involvement of BZs in the modulation of nociceptive transmission at the level of the dorsal horn is discussed on the basis of clinical and experimental findings, taking into account the role of GABAergic mechanisms in sensory events.

Total and free serum tryptophan levels and brain 5-hydroxytryptamine metabolism in arthritic rats.

In rats suffering from experimentally induced arthritis produced by Freund's adjuvant, there is a marked decrease in total serum tryptophan levels and a marked increase in plasma-free tryptophan levels at both 15 and 21 days after the administration of the adjuvant. Tryptophan, 5-hydroxytryptamine and 5-hydroxyindoleacetic acid levels are increased in the brain and the spinal cord at 15 days. However, 21 days after administration of the adjuvant these levels returned to normal values in the brain, but remained increased in the spinal cord. These results are in agreement with investigations suggesting the possible involvement of the raphe-spinal system in response to pain stimuli but are contrary to the observation that there is a large decrease in plasma-free tryptophan levels in arthritic human patients. These opposite results still question the hypothesis of a relationship between changes in plasma-free tryptophan levels and the severity of pain.

Diffuse noxious inhibitory controls (DNIC). I. Effects on dorsal horn convergent neurones in the rat.

(1) Sixty-eight convergent dorsal horn neurones have been recorded at the lumbar level in anaesthetized intact rats. All cells received prominent A alpha and C fibre afferents and correspondingly could be activated by high and low threshold stimuli applied to the peripheral excitatory receptive field. (2) The activity of 67/68 of these neurones was powerfully inhibited by noxious stimuli applied to various parts of the body. Since non-noxious stimuli were ineffective in this respect, the term "diffuse noxious inhibitory controls" (DNIC) is proposed. (3) DNIC could be evoked by noxious pinch applied to the tail, the contralateral hind paw, the forepaws, the ears and the muzzle; the most effective areas were the tail and muzzle. Noxious heat applied to and transcutaneous electrical stimulation of the tail were extemely effective in eliciting DNIC as was the intraperitoneal injection of bradykinin. (4) DNIC strongly depressed by 60-100% both the C fibre response following suprathreshold transcutaneous electrical stimulation and the responses to noxious radiant heat. (5) The spontaneous activity and the responses to low threshold afferents induced either by A alpha threshold electrical or natural stimulation were also powerfully inhibited. (6) In the majority of cases, long lasting post-effects directly related to the duration of conditioning painful stimulus were observed.

Diffuse noxious inhibitory controls (DNIC). II. Lack of effect on non-convergent neurones, supraspinal involvement and theoretical implications.

(1) Diffuse noxious inhibitory controls (DNIC) were tested for their effect on noxious only, non-noxious and proprioceptive cells in the dorsal horn of the intact anaesthetized rat. Unlike convergent neurones, as described in the previous paper, there was no effect of DNIC on these neurones. It is concluded that convergent neurones are specifically inhibited by DNIC. (2) The effect of DNIC could not be demonstrated for convergent neurones in the spinal animal. Thus the neuronal substrate for DNIC must involve supraspinal structures. (3) Because of the level of firing in convergent neurones induced by hair and touch receptors, presumably constantly and randomly activated in the freely moving animal, a noxious message arriving at higher centres may be partly masked by this background noise. On the basis of the known role of convergent neurones in nociception, we propose the following mechanism which may interpret this paradoxical convergence: two pools of convergent neurones are influenced by a painful peripheral stimulation, one segmental pool being activated whilst the remaining population of cells is inhibited; the "contrast" between the messages from these two pools may well produce a significant pain signalling output from the convergent dorsal horn cells. (4) These results and their theoretical implications are discussed with regard to the concept of the "analgesic system", certain clinical observations and the paradoxical pain relieving effects of counterirritation and some forms of acupuncture.

Do acute or chronic tricyclic antidepressants modify morphine antinociception in arthritic rats?

In a chronic pain model, the arthritic rat, tricyclic antidepressants (TCAs) have been shown to clearly reduce behavioural signs of nociception. In the present work, using a test of acute nociception (vocalization threshold to graded foot pressure) in the same model, we evaluated the possible potentiation of morphine analgesia by 2 TCAs: amitriptyline (AMIT) and imipramine (IMIP). Using this test of acute nociception, we failed to demonstrate any analgesic effect of AMIT or IMIP given either acutely or chronically. We also failed to demonstrate any potentiation of morphine by these compounds. On the contrary, we found a significant decrease of morphine antinociception after acute AMIT administration and a tendency towards diminution with both TCAs given chronically. These results appear to temper enthusiasm for human application of this combination. They also indicate that careful further studies in a chronic pain model using behaviour evaluations are necessary before definite conclusions can be drawn concerning TCAs/opiate interaction.

Enhanced effects of co-administered dexamethasone and diclofenac on inflammatory pain processing and associated spinal c-Fos expression in the rat.

This study determines the effects of dexamethasone versus co-administered dexamethasone and diclofenac, on carrageenan-evoked spinal c-Fos expression and peripheral oedema in the freely moving rat. Drugs were administered intravenously 25 min before intraplantar injection of carrageenan (6 mg/150 microliters of saline). Three hours later the number of spinal c-Fos-LI neurones and peripheral oedema were assessed. The total number of control carrageenan-evoked c-Fos-LI neurones in the lumbar spinal cord was 121 +/- 5 labelled neurones per section, segments L4-L5, which were predominantly located in the superficial and deep laminae (41 +/- 3% and 40 +/- 2% of the total number of c-Fos-LI neurones per section, respectively) of the dorsal horn of the spinal cord. Pre-administered dexamethasone (0.05, 0.10 and 0.50 mg/kg i.v.) dose-dependently reduced the total number of c-Fos-LI neurones (30 +/- 4%, 52 +/- 3% and 58 +/- 2% reduction, respectively), with effects of the higher doses being strongest on the deep laminae c-Fos-LI neurones. The effects of dexamethasone on the total number of c-Fos-LI neurones and the peripheral oedema were positively correlated. Co-administration of low doses of dexamethasone and diclofenac (0.025 + 1.5 mg/kg i.v. respectively), which had negligible effects when administered separately, greatly reduced both the total number of carrageenan-evoked c-Fos-LI neurones (61 +/- 5% reduction as compared to control value) and the peripheral oedema (80 +/- 8% and 60 +/- 5% reduction for ankle and paw oedema, respectively). The attenuation by co-administered dexamethasone and diclofenac, of both c-Fos expression and the peripheral oedema, was significantly greater than the effect of dexamethasone alone (P < 0.001 for both) and diclofenac alone (P < 0.001 for both). Our study illustrates enhanced attenuating effects of co-administered dexamethasone and diclofenac on both inflammatory oedema and the associated spinal expression of c-Fos, an indicator of nociceptive transmission at the spinal level. The apparent interactions between the mechanisms of action of NSAIDs and steroids suggest that co-therapy may produce beneficial inflammatory and pain relief in the absence of excessive side effects.

Reduction of arthritis and pain behaviour following chronic administration of amitriptyline or imipramine in rats with adjuvant-induced arthritis.

Tricyclic antidepressants (TCAs) are used extensively to treat chronic pain in man without an adequate explanation for their activity. The purpose of the present study was to investigate this problem by testing the effect of chronic TCAs in an animal pain model: the arthritic rat. Sprague-Dawley rats with adjuvant-induced arthritis were injected daily for 4 weeks with amitriptyline (10 mg/kg) or imipramine (10 mg/kg) or saline, beginning 21 days after the induction of arthritis. Baseline evaluations were made prior to the injection series and at 4 weeks, 24 h after the last injection. Both TCAs significantly reduced 'scratching' and increased 'exploring' behaviour, without changing the response to graded foot pressure. In addition clinical signs of arthritis (ankle circumference, swelling, conjunctivitis, balanitis ...) were significantly reduced, while mobility was increased. This study shows that both amitriptyline and imipramine decrease pain behaviour and arthritis in this chronic pain model. Possible 'antiinflammatory' effects of TCAs and their eventual 'analgesic' effect will be discussed.

A limited arthritic model for chronic pain studies in the rat.

Freund's adjuvant induced polyarthritis in rats has been used extensively to study pain processes of long duration. There are limitations of this model for chronic studies of pain/arthritis since the severe systemic changes provoke ethical concerns and also affect behaviour, physiology and biochemistry. Attempts to limit adjuvant-induced arthritis by plantar injection of the inoculum have been made. In this model, however, the process evolved to produce widespread polyarthritis if followed for the 6-plus-weeks necessary for chronic studies. Therefore, although it offers the researcher a reliable limited model of inflammation and nociception at the outset, for longer studies it may have all the disadvantages of the polyarthritic rat. The purpose of the present study was to produce a limited arthritic process in rats, stable over 6 weeks and suitable for behavioural and neurochemical studies of various chronic pain treatment methods. Injection (0.05 ml) of complete adjuvant containing 300 micrograms Mycobacterium butyricum in the tibio-tarsal joint produces a predictable monoarthritis, stable clinically and behaviourly from weeks 2 through 6 post injection. As revealed by clinical observations and X-ray examinations, the arthritis produced was limited anatomically, pronounced, prolonged and stable. A marked increase in sensitivity to paw pressure was seen in the affected limb. Animals gained weight and remained active, indicating little systemic disturbance as opposed to polyarthritic rats. We propose this limited model of arthritis as a suitable alternative to the polyarthritic rat for prolonged studies.

Chronic pain induces a paradoxical increase in growth hormone secretion without affecting other hormones related to acute stress in the rat.

In several diseases chronic pain is associated with long-lasting pathophysiological responses which differ strongly from those observed in acute situations. When persisting, acute pain often results in physical and psychological stress which may in turn aggravate the initial pathological state. In the present work we examined the secretory patterns of pituitary hormones related to acute stress (growth hormone (GH), prolactin (PRL) and beta-endorphin (beta-END)) in rats during the phase of Freund adjuvant-induced arthritis (AIA, a model used for chronic pain studies) when chronic pain is maximum (14 and 21 days, postinoculation (PI)). Using radio-immunoassay hormones were measured in plasma samples taken every 30 min for 7 h in free-moving rats 14 and 21 days after Freund adjuvant or vehicle injection and in control animals. The total amount of GH secretion was higher at 14 and 21 days PI in AIA rats as compared to vehicle-treated and control animals, and the pulsatility of GH secretory pattern was not modified by AIA. PRL and beta-END secretion were not significantly different in arthritic rats as compared to controls. These results show that GH, PRL and beta-END responses induced by acute stress are not observed during the AIA phase when chronic pain is maximum. Thus, in our experimental conditions, beta-END and PRL do not seem to be good plasma markers of chronic pain.(ABSTRACT TRUNCATED AT 250 WORDS)

The novel analgesic and high-efficacy 5-HT1A receptor agonist, F 13640 induces c-Fos protein expression in spinal cord dorsal horn neurons.

The very-high-efficacy, selective 5-HT(1A) receptor agonist, F 13640 produces uniquely powerful analgesia in rat models of chronic pain by novel neuroadaptive mechanisms (inverse tolerance and co-operation with nociception) [Neuropharmacology 43 (2002) 945-958]. A signal transduction theory and evidence suggest that F 13640 initiates these mechanisms, paradoxically, by mimicking the central effects of nociceptive stimulation. We report that the i.p. injection of F 13640 induces c-Fos protein expression in the L3-L5 segments of the spinal cord. Some 65% of c-Fos protein immunoreactive (c-Fos-IR) nuclei occurred bilaterally in the dorsal horn laminae I-II and V-VI, spinal areas that contain neurons responsive to nociceptive stimulation. This pattern is not unlike that found earlier in arthritic rats, a model of somatotopically widespread nociception. Dose-response studies indicated that c-Fos protein expression was induced at doses (0.63 and 2.5 mg/kg, i.p.) at which previous studies had found F 13640 to produce hyperalgesia. Time-response studies found that c-Fos-IR nuclei appeared within 1-4 h after 0.63 mg/kg of F 13640, with a maximum at 2 h. This parallels literature evidence that c-Fos expression reaches peak late after, and outlasts, nociceptive stimulation. Similar to opioids counteracting noxiously induced c-Fos expression, 10 mg/kg (s.c.) of morphine reduced the number of c-Fos-IR nuclei induced by 0.63 mg/kg of F 13640 (by 45+/-5%; P<0.001). The induction by F 13640 of c-Fos protein expression may relate to the initial hyperalgesia which earlier data indicate the agent to produce early upon its administration.

Mu and delta opioid receptor-like immunoreactivity in the cervical spinal cord of the rat after dorsal rhizotomy or neonatal capsaicin: an analysis of pre- and postsynaptic receptor distributions.

Opioid compounds have powerful analgesic properties when administered to the spinal cord. These effects are exerted through mu and delta opioid receptors, and both pre- and postsynaptic mechanisms have been implicated. To specifically address the relative pre- and postsynaptic contribution to spinal opioid analgesia, we have quantitatively assessed the pre- vs. postsynaptic distribution of the mu-opioid (MOR-1, MOP(1)) and delta-opioid receptors (DOR-1, DOP(1)). We also examined the rostro-caudal arborization of MOR-1 and DOR-1 immunoreactive primary sensory neurons, using an isolated dorsal root preparation. These results were compared to those obtained by labeling for calcitonin gene-related peptide (CGRP), a neuropeptide whose expression in the spinal cord is restricted to the terminals of small diameter primary sensory neurons. We estimate that approximately one half of MOR-1 and two thirds of DOR-1 immunoreactivity in the cervical spinal cord is located on primary afferent fibers. These fibers have a broad rostro-caudal distribution, extending at least three segments rostral and caudal to their segment of entry. Regardless of marker used, the rostral projection was greatest, however, the distribution of CGRP-immunoreactive fibers differed somewhat in that they had a much smaller projection to the most caudal segments examined. Our results suggest that presynaptic delta opioid actions predominate, but that there are mixed pre- and postsynaptic inhibitory effects exerted by opioid analgesics that act at the spinal cord mu opioid receptor.

Further evidence for the interaction of mu- and delta-opioid receptors in the antinociceptive effects of the dual inhibitor of enkephalin catabolism, RB101(S). A spinal c-Fos protein study in the rat under carrageenin inflammation.

We have previously shown that RB101, a dual inhibitor of enkephalin-degrading enzymes, decreased carrageenin-evoked c-Fos protein expression at the spinal cord level in awake rats. Moreover, we have also shown that c-Fos expression is a useful marker of the possible direct or indirect interactions between neural pathways, such as opioid and cholecystokinin systems. We now investigated the respective roles of the three main types of opioid receptors (mu, delta, or kappa) and their possible interactions, in the depressive effects of RB101 in inflammatory nociceptive conditions induced by intraplantar carrageenin (6 mg/150 microl of saline). We used beta-funaltrexamine (beta-FNA), naltrindole (NTI), and nor-binaltorphimine (BNI) as specific antagonists for mu, delta- and kappa-opioid receptors, respectively. c-Fos protein-immunoreactivity (c-Fos-IR) was evaluated as the number of c-Fos-IR nuclei in the lumbar spinal cord 90 min after carrageenin. c-Fos-IR nuclei were preferentially located in the superficial (I-II) and deep (V-VI) laminae of segments L4-L5 (areas containing numerous neurons responding exclusively, or not, to nociceptive stimuli). RB101(S) (30 mg/kg, i.v.) significantly reduced the total number of carrageenin-evoked c-Fos-IR nuclei (30% reduction, P<0.01). This effect was completely blocked by beta-FNA (10 mg/kg, i.v.), or NTI (1 mg/kg, i.v.). In contrast, BNI (2.5 mg/kg, i.v.) did not reverse the reducing effects of RB101(S) on carrageenin-evoked c-Fos protein expression. These results suggest that functional interactions occur between mu- and delta-opioid receptors in enkephalin-induced antinociceptive effects.