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

Predifferentiated embryonic stem cells prevent chronic pain behaviors and restore sensory function following spinal cord injury in mice

Embryonic stem (ES) cells have been investigated in repair of the CNS following neuronal injury and disease; however, the efficacy of these cells in treatment of postinjury pain is far from clear. In this study, we evaluated the therapeutic potential of predifferentiated mouse ES cells to restore sensory deficits following spinal cord injury (SCI) in mice. The pain model used unilateral intraspinal injection of quisqualic acid (QUIS) into the dorsal horn between vertebral levels T13 and L1. Seven days later, 60,000 predifferentiated ES cells or media were transplanted into the site of the lesion. Histological analysis at 7, 14, and 60 days post-transplantation revealed that animals receiving ES cell transplants suffered significantly less tissue damage than animals receiving media alone. Transplanted cells provided immediate effects on both spontaneous and evoked pain behaviors. Treatment with ES cells resulted in 0% (n = 28) excessive grooming behavior versus 60% (18 of 30) in media-treated animals. In the acetone test (to assess thermal allodynia), mice recovered to preinjury levels by 12 days after ES cell transplant, whereas control animals injected with media after SCI did not show any improvement up to 60 days. Similarly, the von Frey test (to assess mechanical allodynia) and the formalin test (to assess nociceptive hyperalgesia) showed that transplantation of predifferentiated ES cells significantly reduced these pain behaviors following injury. Here we show that predifferentiated ES cells act in a neuroprotective manner and provide antinociceptive and therapeutic effects following excitotoxic SCI.

In vivo quantification by SPECT of [123I] ADAM bound to serotonin transporters in the brains of rabbits

BACKGROUND

A novel radioiodine ligand [(123)I] ADAM (2-((2-((dimethylamino)methyl)phenyl)thio)-5-iodophenylamine) has been suggested as a promising serotonin transporter (SERT) imaging agent for the central nervous system. In this study, the biodistribution of SERTs in the rabbit brain was investigated using [(123)I] ADAM and mapping images of the same animal produced by both single-photon emission computed tomography (SPECT) and microautoradiography. A semiquantification method was adopted to deduce the optimum time for SPECT imaging, whereas the input for a simple fully quantitative tracer kinetic model was provided from arterial blood sampling data.

METHODS

SPECT imaging was performed on female rabbits postinjection of 185 MBq [(123)I] ADAM. The time-activity curve obtained from the SPECT images was used to quantify the SERTs, for which the binding potential was calculated from the kinetic modeling of [(123)I] ADAM. The kinetic data were analyzed by the nonlinear least squares method. The effects of the selective serotonin reuptake inhibitors fluoxetine and p-chloroamphetamine (PCA) on rabbits were also evaluated. After scanning, the same animal was sacrificed and the brain was removed for microautoradiography. Regions-of-interest were analyzed using both SPECT and microautoradiography images. The SPECT images were coregistered manually with the corresponding microautoradiography images for comparative study.

RESULTS

During the time interval 90-100 min postinjection, the peak specific binding levels in different brain regions were compared and the brain stem was shown to have the highest activity. The target-to-background ratio was 1.89+/-0.02. Similar studies with fluoxetine and PCA showed a background level for SERT occupation. Microautoradiography demonstrated a higher level of anatomical details of the [(123)I] ADAM distribution than that obtained by SPECT imaging of the rabbit brain.

CONCLUSION

SPECT imaging of the rabbit brain with [(123)I] ADAM showed high affinity, high specificity, and favorable kinetics. The time-activity curve showed that the accumulation of the [(123)I] ADAM in the brain stem reached a maximum between 90 and 100 min postinjection. The microautoradiography provides high-resolution images of the rabbit brain. Our results for the [(123)I] ADAM biodistribution in the rabbit brains demonstrate that this new radioligand is suitable as a selective SPECT imaging agent for SERTs.

Microautoradiography of [123I]ADAM in mice treated with fluoxetine and serotonin reuptake inhibitors

A radiopharmaceutical, (123)I-labeled 2-((2-((dimethylamino)methyl)phenyl)thio)-5-iodophenylamine ([(123)I]ADAM), has been developed recently for evaluation of how serotonin transporters (SERT) function in the brain. However, the detailed biodistribution and specific binding in certain brain areas are not well investigated. In this study, both phosphor plate imaging and microautoradiography were applied to explore the binding characteristics of [(123)I]ADAM in SERT neurons. The effect of two psychotropics and one narcotic on the binding of [(123)I]ADAM to SERT was also studied. Fluoxetine and desipramine, both are psychotropics and specific SERT ligands and decreased the affinity of [(123)I]ADAM, while p-chloroamphetamine (PCA), a narcotic, destroyed most of serotonergic neurons, as well as reducing the concentration of serotonin and the number of SERT in the brain as shown by the biodistribution of [(123)I]ADAM. Significant and selective accumulation of [(123)I]ADAM in the areas from midbrain to brain stem in normal mice with maximum target-to-background ratio was found at 90 minutes postinjection. A rapid clearance of [(131)I]ADAM at 120 minutes postinjection was found in the CA1, CA3 and ThN brain areas. In addition, the inhibition effect on binding ability of [(123)I]ADAM to SERT by the psychotropics and the narcotic was found to have the order of: PCA > fluoxetine > desipramine.

Involvement of the 5-HT(2) receptor in hyperthermia induced by p-chloroamphetamine, a serotonin-releasing drug in mice

The effects of a serotonin (5-hydroxytryptamine, 5-HT)-releasing drug, p-chloroamphetamine (PCA), on body temperature were investigated in mice. PCA induced hyperthermia in mice. PCA-induced hyperthermia was inhibited by the 5-HT(2A/2B/2C) receptor antagonist, 4-isopropyl-7-methyl-9-(2-hydroxy-1-methyl-propoxycarbonyl)-4,6A,7 , 8,9,10,10A-octahydro-indolo[4,3-FG]quinolone maleate (LY53857). The 5-HT(2A) receptor antagonist, ketanserin, reduced the PCA-induced hyperthermia, while the 5-HT(2B/2C) receptor antagonist, N-3-pyridinyl-3,5-dihydro-5-methyl-benzo[1,2-b:4, 5-b']dipyrrole-1(2H)-carboxamide (SB 206553), enhanced it. LY 53857, ketanserin and SB 206553 did not affect hyperactivity in mice treated with PCA. These results suggest that PCA-induced hyperthermia in mice is mediated by 5-HT(2A) receptors and is not related to changes in locomotor activity.

Study of mechanisms of calcitonin analgesia in mice. Involvement of 5-HT3 receptors

The analgesic effect of calcitonin when serotonin (5-HT) concentration is increased and the involvement of some 5-HT receptors were studied using the writhing test in mice. 5-hydroxytryptophan (5-HTP) administration increased both 5-HT levels in the central nervous system (CNS) and calcitonin analgesia. The 5-HT(1A) agonist (+/-)-8-hydroxy-2-dipropylaminotetralin hydrobromide (8-OH-DPAT) diminished calcitonin analgesia, this effect being antagonised by the 5-HT(1A) antagonist (WAY 100, 135). As the stimulation of 5-HT(1A) autoreceptors reduces the turnover of 5-HT, the effect of 8-OH-DPAT on calcitonin analgesia may be attributed to this decrease. The 5-HT(2A-2C) agonist (+/-)-1-(4-iodo-2,5-dimethoxyphenyl)-2-aminopropane hydrochloride (DOI) diminished calcitonin analgesia. A sub-analgesic dose of the 5-HT(2A) antagonist ketanserin failed to prevent this effect. The 5-HT(3) agonist (+/-)-2-methyl-5-hydroxytryptamine maleate (2-methyl-5-HT) potentiated calcitonin analgesia, whereas it was significantly reduced by the 5-HT(3) antagonist tropisetron. The effect of 2-methyl-5-HT on calcitonin analgesia was also reversed by tropisetron, This result suggests that the 5-HT(3) receptor may play an important role in the relationship between calcitonin and the serotonergic system. Tropisetron also reversed the analgesia induced by calcitonin plus 5-HTP corroborating importance of the 5-HT(3) receptors.

Involvement of central serotonergic pathways in analgesia elicited by salmon calcitonin in the mouse

The contribution of central serotonergic pathways to the analgesic activity induced by salmon calcitonin in the writhing test was investigated. Salmon calcitonin was administered to mice after lesioning of the ascending and descending serotonergic pathways by means of i.p. administration of p-chloroamphetamine (40 mg/kg, for 2 days) or p-chlorophenylalanine (300 mg/kg, for 3 days). The analgesic effect induced by salmon calcitonin at the doses of 10 and 20 IU/kg was not evident in mice previously treated with p-chloroamphetamine or p-chlorophenylalanine. However, the analgesic effect of salmon calcitonin 40 IU/kg was not significantly modified by p-chloroamphetamine or p-chlorophenylalanine pretreatment. Salmon calcitonin did not alter the depletion of 5-hydroxytryptamine (5-HT) and 5-hydroxyindoleacetic acid after p-chloroamphetamine or p-chlorophenylalanine administration. Similarly, this hormone did not change the NSD 1015-induced accumulation of 5-hydroxytryptophan or the tranylcypromine-induced accumulation of 5-HT. These results indicate that although salmon calcitonin does not influence the synthesis and metabolism of 5-HT, it does require the integrity of the serotonergic system in order to cause analgesia.

The formalin test: an evaluation of the method

The formalin test for nociception, which is predominantly used with rats and mice, involves moderate, continuous pain generated by injured tissue. In this way it differs from most traditional tests of nociception which rely upon brief stimuli of threshold intensity. In this article we describe the main features of the formalin test, including the characteristics of the stimulus and how changes in nociceptive behaviour may be measured and interpreted. The response to formalin shows an early and a late phase. The early phase seems to be caused predominantly by C-fibre activation due to the peripheral stimulus, while the late phase appears to be dependent on the combination of an inflammatory reaction in the peripheral tissue and functional changes in the dorsal horn of the spinal cord. These functional changes seem to be initiated by the C-fibre barrage during the early phase. In mice, the behavioural response in the late phase depends on the ambient temperature. We argue that the peripheral tissue temperature as well as other factors influencing the peripheral inflammation may affect the response, possibly confounding the results obtained with the test. Furthermore, we discuss the methods of recording the response and the value of observing more than one aspect of behaviour. Scoring of several behavioural variables provides a means of assessing motor or sensorimotor function as possible causes for changes in behaviour. In conclusion, the formalin test is a valuable addition to the battery of methods available to study nociception.

Effects of p-chloroamphetamine on brain serotonin neurons

p-Chloroamphetamine (PCA) is a useful pharmacologic tool for selectively increasing brain serotonin function acutely by release of serotonin into the synaptic cleft. PCA produces behavioral, neurochemical and neuroendocrine effects believed due to serotonin release after doses in the range of 0.5-5 mg/kg. At higher doses and at longer times, PCA causes depletion of brain serotonin. The mechanisms of this depletion are not well understood but require the serotonin uptake carrier. Antagonism of PCA-induced depletion of brain serotonin is a useful means of assessing the ability of a compound to block the serotonin uptake carrier on brain serotonin neurons. PCA can also be used as a neurotoxic agent to deplete brain serotonin in functional studies, apparently by destroying some serotonergic nerve terminals. Used in this way, PCA has an advantage over 5,6- and 5,7-dihydroxytryptamines in being effective by systemic injection, and it affects brain serotonergic projections with a different neuroanatomic specificity than the dihydroxytryptamines.

Dopamine receptor subtypes and formalin test analgesia

The role played by dopamine D1 and D2 receptors in formalin test analgesia was explored by challenging D-amphetamine- and morphine-induced analgesia with mixed and selective D1 and D2 antagonists, and by examining the relative analgesic activity of mixed and selective D1 and D2 agonists. The mixed D1/D2 dopamine antagonist cis-flupenthixol (0.5 mg/kg), the D2 antagonist pimozide (0.5 mg/kg), and the D1 antagonist SCH 23390 (0.1 mg/kg) attenuated both D-amphetamine and morphine analgesia. The mixed D1/D2 agonist apomorphine and the selective D2 agonist quinpirole produced dose-dependent analgesia while the selective D1 agonist SKF 38393 was without effect. These data suggest that D1 receptors play an "enabling" role in D2 receptor-mediated analgesia in the formalin test.

Intrathecal coadministration of clonidine with serotonin receptor agonists produces supra-additive visceral antinociception in the rat

The intrathecal (i.t.) coadministration of sub-antinociceptive doses of clonidine, an alpha 2-adrenoceptor agonist, with DOI or RU-24969 (5-HT2 or 5-HT1B receptor agonists, respectively) produced dose-dependent supra-additive antinociceptive effects in a model of visceral pain. The enhanced attenuation of responses to noxious colorectal distension produced by the coadministration of these drugs is evidenced by significant leftward shifts in the dose-response curves as compared to those of each drug alone and by isobolographic analysis. The supra-additive antinociceptive effects produced following the i.t. coadministration of clonidine with RU-24969 were antagonized by i.t. pretreatment with phentolamine; the coadministration of phentolamine with methysergide produced no greater antagonism of effects. The supra-additive antinociceptive effects produced by i.t. coadministration of clonidine with DOI were antagonized by i.t. pretreatment with methysergide; the coadministration of methysergide with yohimbine produced no greater antagonism of effects. These data suggest that receptors acted upon by descending bulbospinal neurons interact to modulate the rostrad transmission of visceral nociceptive transmission.

Formalin nociception in the mouse does not lead to increased spinal serotonin turnover

The mouse formalin test is a model of tonic (continuous), chemical/inflammatory nociception. To test the hypothesis that bulbospinal serotonergic pathways modulate such nociception, whole spinal cords from mice pretreated with probenecid and sacrificed at 15, 30, 45 and 60 min after injection of 5% formalin or 0.9% saline in the hindpaw were assayed by high performance liquid chromatography with electrochemical detection for the serotonin metabolite, 5-hydroxyindoleacetic acid, as an index of turnover. No difference in serotonin turnover was found between formalin and saline groups, indicating that increased spinal serotonin release is not a normal response to formalin nociception in the mouse.

The effects of p-chloroamphetamine, a depletor of brain serotonin, on the performance of rats in two types of positively reinforced complex spatial discrimination tasks

Learning in male Sprague-Dawley rats was assessed in two types of positively reinforced complex spatial discrimination tasks (Stone 14-unit T-maze and eight-arm radial-arm maze) following cytotoxic lesions of central serotonergic terminal projection fields with p-chloroamphetamine (PCA). Learning, as expressed as mean number of errors per day and mean number of trails required to reach criterion, was significantly enhanced in the PCA-lesioned animals trained in the Stone maze. On the other hand, the performance of the PCA-lesioned animals trained in the eight-arm radial-arm maze was not found to differ significantly from that of saline-injected animals. The improved acquisition of the PCA-lesioned rats trained in the Stone maze was completely abolished following pretreatment with the selective serotonergic reuptake inhibitor norzimeldine. Neurochemical analyses of the brains of representative animals revealed that the levels of serotonin and its major metabolite, 5-hydroxy-3-indoleacetic acid, were both significantly reduced by PCA in all regions examined. While it is clear from these and other studies that the serotonergic nervous system plays an important role in the processes underlying learning and memory, these results further underscore the selective role of this neurotransmitter system in the way information is processed by the brain.

Mechanisms of orphenadrine-induced antinociception in mice: a role for serotonergic pathways

The possible involvement of central serotonergic pathways in the mechanism of action of orphenadrine citrate was investigated in male albino mice. Orphenadrine (20 mg/kg) did not alter the concentration of 5-hydroxytryptamine (5-HT) or its metabolite 5-hydroxyindole acetic acid in the frontal cortex or spinal cord, nor did it, in moderate concentrations, inhibit the uptake of [14C]5-HT, [3H]noradrenaline ([3H]NA) or [3H]dopamine ([3H]DA) into crude synaptosomal preparations from the cortex. The antinociceptive effect of orphenadrine was studied in the formalin test and in the increasing temperature hot plate test. No sensorimotor impairment was observed for doses of 30 mg/kg or lower. A general depletion of serotonin by means of p-chlorophenylalanine significantly reduced the effect of orphenadrine in both tests, while lesion of the ascending serotonergic systems by means of p-chloroamphetamine did not affect the analgesia. It is concluded that the antinociceptive effect of orphenadrine may be mediated in part via the raphe-spinal serotonergic systems.

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.

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.

5-HT depletion with 5,7-DHT, PCA and PCPA in mice: differential effects on the sensitivity to 5-MeODMT, 8-OH-DPAT and 5-HTP as measured by two nociceptive tests

Depletion of 5-hydroxytryptamine (5-HT) in mice was produced by intracerebroventricular injection of 5,7-dihydroxytryptamine (5,7-DHT, 80 micrograms) or by systemic injections of p-chloroamphetamine (PCA, 3 X 40 or 4 X 40 mg/kg), p-chlorophenylalanine (PCPA, 5 X 400 or 14 X 400 mg/kg) or combined PCA (3 X 40 mg/kg) + PCPA (11 X 400 mg/kg). Neither of the pretreatments altered nociception in the increasing temperature hot-plate test, whereas hyperalgesia was demonstrated in 5,7-DHT lesioned animals in the tail-flick test. 5,7-DHT-pretreatment enhanced the antinociceptive effect of the 5-HT agonists 5-methoxy-N,N-dimethyltryptamine (5-MeODMT), 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) and 5-hydroxytryptophan (5-HTP). This effect was observed after 2, 5 and 8 days in the tail-flick test and after 5 and 8 days in the hot-plate test. However, pretreatment with PCPA or PCA failed to alter the antinociception elicited by the 5-HT agonists, although a tendency towards enhancement of antinociception was found after combined treatment with PCA and PCPA. It is suggested that the injection of 5,7-DHT induces denervation supersensitivity of post-synaptic 5-HT receptors. The lack of such supersensitivity after PCPA-pretreatment which induces similar 5-HT depletion to 5,7-DHT, may suggest that other factors than the absence of 5-HT may contribute to the development of denervation supersensitivity. Alternatively, the three 5-HT depleting agents may produce a qualitatively different reduction of 5-HT.

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.

Increased sensitivity to intrathecal substance P following chronic administration of zimelidine

The behavioural response to intrathecal substance P (SP) was evaluated following acute and chronic administration of the selective serotonin reuptake inhibitor zimelidine. A single dose of zimelidine (10 mg/kg) attenuated the response to SP by approximately 40%, in agreement with previous findings that acute administration of zimelidine reduces nociceptive behaviour. Twenty-four hours following the withdrawal of long-term treatment with zimelidine (10 mg/kg X 2 daily for 14 days) the behavioural response to SP was increased by 116%. This may indicate the development of supersensitivity to SP, or may reflect an increased responsiveness to noxious stimuli due to reduced serotonergic inhibition.

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.