Test-dependent antinociceptive effect of spinal serotonin release induced by intrathecal p-chloroamphetamine in mice

Fos-like immunoreactivity in rat dorsal raphe nuclei induced by alkaloid extract of Mitragyna speciosa

Mitragyna speciosa (MS) has been traditionally used for medicinal purposes especially in southern Thailand. Previously, an alkaloid extract of this plant was demonstrated to mediate antinociception, partly, through the descending serotonergic system. The present study investigated the stimulatory effect of the MS extract on the dorsal raphe nucleus and its antidepressant-like activity. The MS extract containing approximately 60% mitragynine as a major indole alkaloid was used to treat the animals. The stimulatory effect of the MS extract was determined by detecting the expression of the immediate early gene, cfos, in the dorsal raphe nucleus of male Wistar rats. The immunohistochemistry was used to detect Fos protein, the protein product of cfos gene. The present data show that a significant increase in Fos expression was observed following long-term administration of the MS extract (40 mg/kg) for 60 consecutive days. In addition, the antidepressant-like activity of the MS extract was determined by using the forced swimming test (FST) in male mice. The results show that a single injection (either 60 or 90 mg/kg doses) significantly decreased immobility time in the FST. These findings indicate that the MS extract has a stimulatory effect on the dorsal raphe nucleus and an antidepressant-like activity. Stimulation of this brain area has been known to cause antinociception. These findings suggest that the MS extract might produce antinociceptive and/or antidepressive actions partly through activation of the dorsal raphe nucleus. Moreover, the dorsal raphe nucleus may be one of site of MS action in the central nervous system.

Light and electron microscopic studies of the effects of p-chloroamphetamine on the monoaminergic innervation of the rat spinal cord

A previous report has shown that small diameter serotoninergic (5-HT) axons innervating the forebrain are selectively eliminated by treatment with an amphetamine derivative, (+/-)p-chloroamphetamine (PCA; Mamounas et al., [1991] J. Comp. Neurol. 314:558-586). It is well known that the spinal cord is the target of numerous monoaminergic fibers of different types. We have previously shown that the dorsal and ventral horns and the intermediolateral cell column are innervated by numerous serotonin-, noradrenaline- and dopamine-containing axons, including both thin axons with small varicosities and beaded axons with large varicosities. In all these regions, the large majority of fine indolaminergic fibers do not establish synaptic contacts, contrasting with the beaded axons which mostly exhibit synapses. The present work was conducted to study the effect of PCA on the monoaminergic innervation of the adult rat spinal cord. Animals received two subcutaneous doses of PCA 24 hours apart and were perfused 3 weeks later. Immunocytochemistry was performed to detect 5-HT, noradrenergic and dopaminergic fibers by using light and electron microscopy. Light microscopy revealed that PCA treatment caused a marked and selective elimination of the fine 5-HT-immunoreactive fibers, mainly found in the dorsal horn, but spared all other larger axons. This selective effect on the dorsal horn innervating thin 5-HT fibers was confirmed with the electron microscope by calculating the synaptic incidence(s) of monoaminergic innervation. These results suggest that fine and beaded 5-HT axons correspond to two anatomically, biochemically and pharmacologically different types of fibers, which could arise from two subpopulations of brainstem neurons. In addition, this drug could be used to provide an experimental animal, devoid of 5-HT nonsynaptic fibers, thereby facilitating a study on the role of dorsal horn nonsynaptic system in pain modulation.

Role of spinal serotonin1 receptor subtypes in thermally and mechanically elicited nociceptive reflexes

The ability of 5-HT1A and 5-HT1B agonists to alter a spinal animal's nociceptive threshold was examined using two analgesiometric tests. In the spinal withdrawal reflex test, administration of the selective 5-HT1A agonists ipsapirone, gepirone and PAPP resulted in significant dose-dependent increases in receptive field (RF) area for withdrawal reflexes when compared to predrug baseline values, indicating an increase in nociceptive sensitivity. The average overall percent maximal increase in RF area following administration of 5-HT1A selective compounds was: 80 +/- 16% for the ventroflexion reflex, 90 +/- 6% for the dorsiflexion reflex and 87 +/- 8% for the lateral flexion reflex. Similar to the effects noted with 5-HT1A agonists, administration of 5-HT1B agonists RU24969, mCPP and TFMPP resulted in a hyperalgesic response with an overall percent maximal increase of 43 +/- 6% for the ventroflexion reflex, 51 +/- 6% for the dorsiflexion reflex and 38 +/- 9% for the lateral flexion reflex. In the tail-flick analgesiometric test, administration of the 5-HT1A agonists 8-OH-DPAT and ipsapirone and the 5-HT1B agonists RU24969 and mCPP resulted in a significant dose-dependent increase in tail-flick latencies when compared to predrug baseline values, indicating a decrease in nociceptive sensitivity to noxious thermal stimuli. No differences in magnitude of the effect of the two receptor subtypes were found, indicating that stimulation of either 5-HT1A or 5-HT1B receptors was equipotent in producing the antinociceptive tail-flick response.(ABSTRACT TRUNCATED AT 250 WORDS)

Lesions of bulbo-spinal serotonergic or noradrenergic pathways reduce nociception as measured by the formalin test

Intrathecal administration of the neurotoxins 5,6-dihydroxytryptamine (5,6-DHT) and 6-hydroxydopamine (6-OHDA) in rats selectively lesioned the descending spinal serotonergic and noradrenergic pathways, respectively. Four days after neurotoxin administration the behaviour was evaluated in the formalin test. Several behavioural variables were recorded. The statistical analysis of the results was supplemented using a multivariate statistical method (partial least squares projection to latent variables, PLS) in addition to traditional analysis of variance. The described methods for recording and statistical analysis of behaviour appear to be useful in describing drug-induced differences in behavioural patterns in the formalin test. Both types of lesion reduced the pain-related behaviour in the formalin test (protection of the paw, biting and licking). The results indicate that the descending monoaminergic pathways are parts of a network which maintains adequate nociceptive responses to a chemical stimulus, or to stimuli lasting several minutes, as in the formalin test.

Antinociceptive effect of paracetamol in rats is partly dependent on spinal serotonergic systems

The possible involvement of bulbo-spinal monoaminergic pathways in the antinociceptive effect of paracetamol was investigated in rats. Serotonergic pathways were lesioned with intrathecal 5,6-dihydroxytryptamine (5,6-DHT), and noradrenergic pathways with 6-hydroxydopamine (6-OHDA). Intact and lesioned rats were tested in the formalin test after i.p. paracetamol (400 mg/kg) or vehicle. Behaviour was scored for 1 h after the dorsal injection of 100 microliters of 5% formalin into one hind paw. Behavioural variables were evaluated with a multivariate statistical procedure, as well as an analysis of variance. Paracetamol itself reduced pain-related behaviour and increased normal motor activity. This antinociceptive effect was reduced in rats lesioned with 5,6-DHT. In lesioned rats paracetamol caused a change in nociceptive behaviour from active, focused behaviour towards passive, protective and non-focused behaviour in the early phase of the formalin test. No significant effect of lesioning with 6-OHDA upon the paracetamol effect was found. These results show that activation of spinal serotonergic systems is involved in the antinociceptive effect of paracetamol. The relative importance of this mechanism in the central effect of paracetamol and the mechanisms that cause the activation remain to be determined.

Pharmacological manipulation with the descending serotonergic system or transection of the mouse spinal cord has no effect on ependymal ultrastructure

In order to investigate an effect of descending nerve fibres on mouse spinal cord ependymal ultrastructure, pharmacological manipulation with the serotonergic system or transection of the spinal cord was done. Biochemical analysis showed an 83% reduction of serotonin content in spinal cord tissue after p-chlorophenylalanine injections and a 93% reduction after transection. However, none of the experimental animals showed changes in ependymal ultrastructure compared to control animals as revealed by electron microscopy.

Apparent hyperalgesia in the mouse tail-flick test due to increased tail skin temperature after lesioning of serotonergic pathways

The relationship between tail skin temperature and responsiveness to noxious radiant heat in the tail-flick test was investigated in mice. A significant negative correlation between tail skin temperature and tail-flick latency was found when the tail skin temperature was increased by elevating the ambient temperature. After intracerebroventricular injection of the serotonin neurotoxin 5,7-dihydroxytryptamine (5,7-DHT, 80 micrograms) tail skin temperatures were increased and tail-flick latencies reduced. In contrast, administration of the tryptophan hydroxylase inhibitor p-chlorophenylalanine (PCPA, 400 mg kg-1 for 5 consecutive days) lead to a slight lowering of tail temperatures and a tendency towards elevation of tail-flick latencies. The results show that factors which affect tail skin temperature also influence the tail-flick test in mice. The divergent effects of 5,7-DHT and PCPA on tail-flick responsiveness may be due to the different effects of these compounds on the tail skin temperature. The results suggest that the reduced tail-flick latency after partial destruction of serotonergic pathways by 5,7-DHT is due primarily to the increased tail skin temperature. The dependence of tail-flick latency on tail skin temperature limits the usefulness of the tail-flick test unless changes in tail skin temperature are controlled for.

The relative contribution of ascending and descending serotonergic pathways in p-chloroamphetamine-induced antinociception

Systemic administration in rats of p-chloroamphetamine (PCA; 2 x 10 mg/kg) reduced the in vitro uptake of 14C-5-hydroxytryptamine (14C-5-HT) in cortical synaptosomes by 76% and in spinal cord synaptosomes by 35%. Intrathecal injection of 5,6-dihydroxytryptamine (20 micrograms/rat) selectively lesioned the descending serotonergic pathways (83% reduction in uptake of 14C-5-HT in spinal synaptosomes, no significant change in uptake in cortical synaptosomes). Administration of PCA or 5,6-DHT did not significantly alter the uptake of 3H-noradrenalin into cortical or spinal synaptosomes. The response thresholds of the rats in the increasing temperature hot plate test (1 to 7 days after administration) were unaffected by either type of lesion. Interference with the antinociceptive effect of PCA (2.5 mg/kg) was evaluated 7 days after administration of the neurotoxins. PCA pretreatment strongly reduced the peak of the PCA-induced antinociception while 5,6-DHT reduced its duration. Thus, both ascending and descending serotonergic pathways contribute to PCA-induced antinociception.

Intrathecal substance P modulates the depressant effect of 5-methoxy-N,N-dimethyltryptamine on a reflex response to radiant heat in mice

The effect of intrathecal (i.th.) substance P (SP) on antinociception elicited by the serotonin (5-HT) receptor agonist 5-methoxy-N,N-dimethyltryptamine (5-MeODMT) was investigated in mice by means of the tail-flick method. Substance P (0.07, 0.7 or 7 micrograms) induced a behavioral syndrome for 1-2 min, but had no apparent toxic or neurologic effects and did not alter the tail-flick response to noxious radiant heat 30 min after injection. The depressant effect of 5-MeODMT (3 mg/kg) on tail-flick responses was, however, markedly attenuated when administered 30 min after SP. The tail skin temperatures of vehicle- and SP-injected mice were nearly identical 30 min after i.th. injection as well as after administration of 5-MeODMT. The results indicate a functional interaction between SP and 5-HT in spinal nociceptive processes, and it is suggested that i.th. SP modulates the function of 5-HT receptors.

Apparent hyperalgesia after lesions of the descending serotonergic pathways is due to increased tail skin temperature

It has been suggested that the descending serotonergic pathways exercise a tonic inhibition on nociception in the spinal cord. In this study 5,6-dihydroxytryptamine (5,6-DHT, 20 micrograms base) injected intrathecally in rats reduced spinal serotonin concentration to 3.5% of control levels without significantly affecting spinal noradrenaline. The lesion reduced the mean tail-flick latency by approximately 35% and increased the mean tail skin temperature by approximately 3.5 degrees C; both parameters gradually returned to normal values within 2-3 weeks. Both in controls and in lesioned animals there was a highly significant negative correlation between tail skin temperature and tail-flick latency. Multiple regression analysis showed that the effect of lesioning on tail-flick latency was non-significant when the effect of skin temperature was taken into account. Thus the reduced tail-flick latency after lesions of descending serotonergic pathways, usually interpreted as increased nociception, may be due to changes in tail skin temperature.

Increased behavioural response to intrathecal substance P after intracerebroventricular 5,7-dihydroxytryptamine but not after p-chlorophenylalanine administration

The behavioural response to intrathecally injected substance P (SP, 1.25 ng) was investigated in mice after lesioning of serotonergic (5-HT) pathways by intracerebroventricular 5,7-dihydroxytryptamine (5,7-DHT, 80 micrograms base/mouse) and after 5-HT synthesis inhibition by p-chlorophenylalanine (PCPA, 400 mg kg-1 for 6 consecutive days). Pretreatment with 5,7-DHT and PCPA reduced the 5-HT level in the spinal cord to 6 and 7% of controls and the noradenaline (NA) level to 69 and 84% of controls, respectively. Intrathecally injected SP produced a response consisting of vigorous biting, licking and scratching of the caudal part of the body. The response to SP was significantly increased 5 days after injection of 5,7-DHT, but only a non-significant tendency towards enhancement of the response was found after 24 h. There was no change in the response to SP 24 h after the last injection of PCPA. It is suggested that 5,7-DHT but not PCPA induces receptor supersensitivity to SP, and that reduction in spinal SP by 5,7-DHT may be a factor in this change in receptor sensitivity.

Acute and chronic treatment with selective serotonin uptake inhibitors in mice: effects on nociceptive sensitivity and response to 5-methoxy-N,N-dimethyltryptamine

The tail-flick and increasing temperature hot-plate tests were employed to study the effects of acute or chronic treatment with zimelidine, alaproclate or chlorimipramine on nociception and response to 5-methoxy-N,N-dimethyltryptamine (5-MeODMT) in mice. A single dose of the serotonin (5-HT) uptake inhibitors produced antinociception in the hot-plate test but not in the tail-flick test. After chronic administration, reduced tail-flick latencies were demonstrated 24, 48, 72 and 144 h after withdrawal of zimelidine treatment, 48 h after withdrawal of alaproclate and 48 and 96 h after withdrawal of chlorimipramine treatment. The hot-plate response temperatures were slightly lowered after chronic zimelidine treatment but not after treatment with alaproclate or chlorimipramine. The response to 5-MeODMT was not altered by a single dose of the 5-HT uptake inhibitors, however, after withdrawal of chronic treatment this response was increased in the tail-flick test but not in the hot-plate test. It was concluded that acute and chronic treatment with 5-HT uptake inhibitors modulate nociception differently, and that chronic treatment induces supersensitivity of spinal postsynaptic 5-HT receptors. Different modulation of different 5-HT receptor subpopulations by these compounds may possibly contribute to the test-dependent results.

Test-dependent changes in nociception after administration of the putative serotonin antagonist metitepin in mice

Intraperitoneal administration of the putative serotonin receptor antagonist metitepin (0.06-1.0 mg/kg) in mice induced dose-dependent antinociception in the increasing temperature hot-plate test and the formalin test, but elicited hyperalgesia in the tail-flick test. Reduced motor activity was observed after injection of the largest dose of metitepin, but did not influence the behavioural responses in the tests. Selective lesions of ascending serotonergic pathways induced by administration of the neurotoxin p-chloroamphetamine 5 and 6 days before testing (40 mg/kg each day) did not directly affect the responsiveness in any of the tests but enhanced the metitepin-induced antinociception in the hot-plate and formalin tests. The hyperalgesia in the tail-flick test was not affected by the lesions. The results suggest that metitepin may alter nociception in mice by exhibiting both agonist and antagonist properties on central serotonergic receptors.

Involvement of central serotonergic pathways in nefopam-induced antinociception

The possible involvement of central serotonergic pathways in the mechanism of action of nefopam was investigated in male albino mice. Nefopam (15 mg/kg i.p.) did not alter the concentration of serotonin or its metabolite 5-hydroxyindole acetic acid in frontal cortex or spinal cord. Lesions of the ascending serotonergic pathways were made by systemic administration of p-chloroamphetamine (PCA). Serotonin depletion in all serotonergic systems was obtained by means of p-chlorophenylalanine (PCPA). Two different nociceptive assays were used, the formalin test and the increasing temperature hot plate test. PCPA pretreatment significantly reduced the effect of nefopam (15 mg/kg) in the formalin test. In contrast, nefopam-induced analgesia was not affected by PCA pretreatment, either in the formalin test or in the increasing temperature hot plate test. In conclusion, the data suggest that descending serotonergic pathways are involved in nefopam-induced antinociception.

Effects of the putative 5-HT1A receptor agonist 8-OH-2-(di-n-propylamino)tetralin on nociceptive sensitivity in mice

The ability of 8-OH-2-(di-n-propylamino)tetralin (8-OH-DPAT) to alter nociceptive sensitivity in mice was studied using the tail-flick, hot-plate and formalin tests. Subcutaneous (SC) administration of 8-OH-DPAT (0.63-1.0 mg/kg) dose-dependently increased the temperature at which hindpaw lick occurred in a hot-plate test using slowly rising temperature and increased the latencies to hindpaw lick, but reduced the latencies to jump in a conventional hot-plate test. Intracerebroventricular (ICV) injections (0.25-1.0 microgram) produced similar results in the conventional hot plate test. Following intrathecal (ITH) injections (0.25-1.0 microgram), however, the latencies to hindpaw lick were elevated without any change in jump latencies. In the formalin test a low systemic dose of 8-OH-DPAT (0.063 mg/kg) elicited hyperalgesia, while hypoalgesia was found after a high dose (1.0 mg/kg). ICV injection of 1.0 microgram produced hypoalgesia in the formalin test while the same dose injected ITH was without effect. 8-OH-DPAT did not alter tail-flick latencies, either by SC, ICV or ITH administration. Previous studies have shown that 8-OH-DPAT stimulates central serotonergic receptors, and shows selectivity for the 5-HT1A recognition site. The present findings indicate an involvement of 5-HT1A receptors in the processing of nociceptive information both at spinal and supraspinal sites. However, stimulation of 5-HT1A receptors does not seem to affect spinal, nociceptive reflexes.

A modified hot-plate test sensitive to mild analgesics

The present study compares a modified hot-plate test in which the temperature is slowly increased from non-noxious levels with a standard constant temperature hot-plate test. In both tests, hindpaw lick was found to be a more reliable criterion response than forepaw lick, and was employed throughout the experiments. In the constant temperature hot-plate test, 1-min exposure to the test apparatus the day before testing significantly reduced the response latencies of both rats and mice. No effect of pre-exposure was found in the increasing temperature test. In both tests and in both species, sessions of repeated testing were conducted with only insignificant alterations in responsiveness between trials. In both rats and mice, dose-related increases in response temperature were obtained in the increasing temperature hot-plate test after administration of morphine, paracetamol and acetylsalicylic acid, whereas only morphine had consistent effects in the constant temperature test. Thus, the increasing temperature hot-plate is a useful analgesimetric test in both rats and mice, superior to the conventional hot-plate test with regard to consistency of results and sensitivity to non-narcotic analgesics.

Lesions of the ascending serotonergic pathways and antinociceptive effects after systemic administration of p-chloroamphetamine in mice

The present study reports a method for lesioning of the ascending serotonergic system. The neurotoxic substance p-chloroamphetamine (PCA) was given IP on 2 consecutive days (40 mg/kg/day). After each injection, the animals were kept at 4 degrees C for 4 hr since a lower dose of PCA (25 mg/kg) induced severe hyperthermia. The mortality rate was 12%, considerably lower than previously reported in similar studies. Evaluated 9 days after the last injection of PCA, the uptake of 14C-5-HT into cortical and hippocampal crude synaptosomal preparations was reduced by 50 and 60%, respectively, while the uptake into spinal synaptosomes was unaffected. The uptake of 3H-NA was not significantly altered in any of the structures. Measurements of PCA performed 30 min to 4 hr after IP injections of 5 to 40 mg/kg demonstrated higher concentrations in the cortex than in the lumbar spinal cord. Administration of PCA (5 mg/kg) had an acute antinociceptive effect in the hot-plate and formalin tests, but not in the tail-flick test. Prior treatment with neurotoxic doses of PCA prevented the antinociception but had in itself no effect on the responsiveness in any of the tests. Thus systemic administration of PCA produces highly selective and functional lesions of the ascending serotonergic pathways in mice.