Roles of serotonin receptor subtypes for the antinociception of 5-HT in the spinal cord of rats

The contribution of 5-HT (5-hydroxytryptamine) receptor subtypes to the antinociception produced by intrathecal 5-HT in the formalin test was investigated in rats. Intrathecal 5-HT suppressed both phases of behaviors produced by 5% formalin, and this was blocked by antagonists for 5-HT(1B) (3-[3-(Dimethylamino)propyl]-4-hy-droxy-N-[4-(4-pyridinyl)phenyl]benzamide dihydrochloride, GR 55562), 5-HT(2C) (N-ormethylclozapine/8-Chloro-11-(1-piperazinyl)-5H-dibenzo[b,e][1,4]diazepine, D-MC), 5-HT3 (1-Methyl-N-(8-methyl-8-azabicyclo[3.2.1]-oct-3-yl)-1H-indazole-3-carboxamide maleate, LY-278,584) and 5-HT4 receptors (4-Amino-5-chloro-2-metho-xy-benzoic acid 2-(diethylamino)ethyl ester hydrochloride, SDZ-205,557), but not the 5-HT(1D) receptor antagonist 3-[4-(4-Chlorophenyl)piperazin-1-yl]-1,1-diphenyl-2-propanol hydrochloride (BRL 15572). The 5-HT(1A) receptor antagonist N-[2-[4-(2-Methoxyphenyl)-1-piperazinyl]-N-2-pyridinyl-cyclohexanecarboxamide maleate (WAY-100635) decreased only the second phase antinociception of 5-HT. Intrathecal administration of agonists for 5-HT(1A) (3-(N,N-Dipropylaminoethyl)-1H-indole-5-carboxamide maleate, Dipropyl-5CT), 5-HT(1B) (7-Trifluoromethyl-4(4-met-hyl-1-piperazinyl)-pyrrolo[1,2-a]quinoxaline maleate, CGS-12066A), 5-HT(2C) (6-Ch-loro-2-(1-piperazinyl)pyrazine hydrochloride, MK 212), 5-HT3 (N-(3-Chlorophenyl)imidodicarbonimidic diamide hydrochloride, m-CPBG) and 5-HT4 receptors (2-[1-(4-Piperonyl)piperazinyl]benzothiazole, BZTZ) suppressed both phases of the formalin response. The results of the present study indicate that spinal 5-HT(1B,) 5-HT(2C,) 5-HT3 and 5-HT4 receptors, but not the 5-HT(1D) receptor, mediate antinociception produced by 5-HT in the formalin test. The relevance of the 5-HT(1A) receptor is less clear because of the different effects of antagonist and agonist.     

Involvement of spinal lipoxygenase metabolites in hyperalgesia and opioid tolerance

This study investigated role of spinal lipoxygenase metabolites in induction of hyperalgesia and development of opioid analgesic tolerance. In the rat, nociception was measured using formalin and tail-flick tests. Intrathecal administration of leukotriene receptor agonist (LTB4) augmented the second phase of the formalin response and marginally increased sensitivity to acute thermal stimulation in the tail-flick test, responses suppressed by 6-(6-(3R-hydroxy-1E,5Z-undecadien-1-yl)-2-pyridinyl)-1,5S-hexanediol (U75302), a leukotriene BLT receptor antagonist. Treatment with 15-hydroxyperoxyeicosatetranoic acid (HPETE) increased phase II formalin activity, but had no effect on tail-flick responses. 12-HPETE failed to produce an effect in either nociceptive test. In the second part of this study, chronic spinal morphine for 5 days produced progressive decline in morphine antinociception and loss in analgesic potency. These effects were attenuated by co-administration of morphine with selective and nonselective lipoxygenase inhibitors. These results suggest involvement of lipoxygenase metabolites in both pain modulation and induction of opioid tolerance at the spinal level.     

Intracerebroventricular injection of trazodone produces 5-HT receptor subtype mediated anti-nociception at the supraspinal and spinal levels.

Serotonin (5-HT) mediated anti-nociceptive effects induced by an anti-depressant, trazodone, are related to 5-HT(1A) receptor activities at the supraspinal level. 5-HT(3) receptor activation via the descending anti-nociceptive pathways may contribute to the trazodone mediated anti-nociception at the spinal level. Intracerebroventricular (i.c.v.) injection of trazodone dose-dependently impaired nociceptive responses in the formalin test in mice. Six and 15 microg of trazodone inhibited the early (P<0.05 or 0.01) and the late phases of the formalin test (P<0.05 or 0.01), while 3 microg had no effect. We examined the effects of a selective 5-HT(1A) receptor antagonist, WAY-100635, a single injection of which induced hyperalgesia (P<0.05), and blocked the anti-nociceptive effects of trazodone (P<0.01) when the two were simultaneously injected i.c.v. Intrathecal (i.t.) injection of a selective 5-HT(3) receptor antagonist, 3-tropanylindole-3-carboxylate hydrochloride, blocked the anti-nociceptive effects of i.c.v. trazodone (P<0.01), while WAY-100635 (i.t.) did not impair trazodone mediated anti-nociception. Trazodone mediated anti-nocicepton is related to serotonergic activity at both the supraspinal and the spinal level.     

Synergistic antinociception of intrathecal sildenafil with clonidine in the rat formalin test

Spinal sildenafil (phosphodiesterase 5 inhibitor) and clonidine (alpha-2 adrenoceptor agonist) have shown antinociception. The author examined the properties of drug interaction after concurrent administration of intrathecal sildenafil-clonidine, and further clarified the reciprocity of sildenafil and clonidine. Catheters were inserted into the intrathecal space of male Sprague-Dawley rats. The formalin test was used as a nociceptive test, which was induced by subcutaneous injection of 50 µl of 5% formalin solution into the hindpaw. The pharmacological interaction was characterized using an isobolographic analysis. Intrathecal sildenafil and clonidine dose-dependently suppressed the flinching response observed during phase 1 and phase 2 in the formalin test. Isobolographic analysis revealed a synergistic interaction after intrathecal delivery of sildenafil-clonidine in both phases. Intrathecal yohimbine antagonized the antinociceptive action of intrathecal sildenafil during both phases in the formalin test. However, intrathecal ODQ failed to antagonize the antinociceptive action of intrathecal clonidine. These results suggest that sildenafil and clonidine, and the mixture of the two are effective against acute pain and facilitated pain state at the spinal level. Furthermore, synergism was noted after delivery of sildenafil-clonidine mixture. The antinociception of sildenafil can be modulated by spinal alpha-2 adrenoceptor, while the effect of clonidine is independent on the guanyly cyclase.     

Effects of 5-HT2 and 5-HT3 receptors on the modulation of nociceptive transmission in rat spinal cord according to the formalin test

We used the formalin test to clarify the 5-hydroxytryptamine (5-HT) receptor subtypes involved in the modulation of spinal nociceptive transmission in rats. Intrathecal administration of a 5-HT1A receptor agonist, 8-hydroxy-2-(di-n-propylamino)-tetraline (8-OH-DPAT; 1, 10, and 30 microg), or a 5-HT1B receptor agonist, 1, 4-dihydro-3-(1, 2, 3, 6-tetrahydro-4-pyridinyl)-5H-pyrrol (3, 2-b) pyridin-5-one (CP 93129; 1 and 10 microg), produced no significant change in the number of flinches. A 5-HT(2) receptor agonist, (+/-)-2, 5-dimethoxy-4-iodoamphetamine (DOI; 10, 30, and 100 microg), and a 5-HT3 receptor agonist, 2-methyl-5-HT (100 and 300 microg), produced dose-dependent decreases in the number of flinches in phases 1 (1 to 6 min) and 2 (10 to 61 min) of the test. The antinociceptive effects of DOI and 2-methyl-5-HT were antagonized by intrathecal pretreatment with a 5-HT2 receptor antagonist, ketanserin, and a 5-HT3 receptor antagonist, 3-tropanyl-3, 5-dichlorobenzoate (MDL-72222), respectively. These results suggest that 5-HT2 and 5-HT3 receptors in the spinal cord mediate antinociception to chemical stimuli.     

Evidence for the involvement of spinal endogenous ATP and P2X receptors in nociceptive responses caused by formalin and capsaicin in mice

1. The aim of the present study is to characterize the role of spinal endogenous ATP and P2X receptors in the generation of neurogenic and inflammatory pain. We examined the effects of intrathecal treatment with P2X receptor antagonists on the formalin- and capsaicin-induced nociceptive behaviours in mice. 2. Intrathecal pretreatment with the general P2 receptor antagonist, pyridoxal-phosphate-6-azophenyl-2', 4'-disulphonic acid (PPADS), significantly suppressed both the first and second phases of the formalin-induced nociceptive behaviour. The second phase of the nociceptive response was also suppressed by intrathecal treatment with PPADS after the first phase. Furthermore, pretreatment with the selective antagonist for the P2X1, P2X3 and P2X2+3 receptors, 2',3'-O-(2,4,6-trinitrophenyl)adenosine 5'-triphosphate (TNP-ATP), significantly reduced the first phase, but not the second phase. The second phase was also not suppressed by intrathecal TNP-ATP after the first phase. 3. Capsaicin-induced nociceptive behaviour that has been shown to be a model for neurogenic pain, was also significantly suppressed by intrathecal pretreatment with PPADS or TNP-ATP. 4. Nociceptive behaviour in the first phase of the formalin test and in the capsaicin test were significantly inhibited by intrathecal pretreatment with alpha, beta-methylene ATP (alpha,betameATP: 5 microg mouse-1) 15 min prior to injection of formalin or capsaicin. This treatment has been previously shown to desensitize spinal P2X3 receptor subtypes in vivo. 5. These findings suggest that spinal endogenous ATP may play a role in (1) the formalin- and capsaicin-induced neurogenic pain via the PPADS- and TNP-ATP-sensitive P2X receptors which are also desensitized by alpha,betameATP (perhaps the P2X3 receptor subtype) and (2) formalin-induced inflammatory pain via PPADS-sensitive, TNP-ATP- and alpha,betameATP-insensitive P2X (and/or P2Y) receptors.     

Possible involvement of spinal noradrenergic mechanisms in the antiallodynic effect of intrathecally administered 5-HT2C receptor agonists in the rats with peripheral nerve injury

Intrathecal administration of serotonin type 2C (5-HT(2C)) receptor agonists produces an antiallodynic effect in a rat model of neuropathic pain. In the present study, we characterized this effect pharmacologically. Allodynia was produced by tight ligation of the fifth (L5) and sixth (L6) lumbar spinal nerves on the left side, and was measured by applying von Frey filaments to the left hindpaw. 6-chloro-2-(1-piperazinyl)-pyrazine (MK212; 100 microg) and 1-(m-chlorophenyl)-piperazine (mCPP; 300 microg) were used as 5-HT(2C) receptor agonists. Intrathecal administration of these agonists resulted in an antiallodynic effect. Intrathecal administration of atropine (30 mug), a muscarinic receptor antagonist, and yohimbine (30 microg), an alpha(2)-adrenoceptor antagonist, reversed the effects of 5-HT(2C) receptor agonists. Intrathecal pretreatment with 6-hydroxydopamine, an adrenergic neurotoxin, inhibited the antiallodynic effect of MK212. These results suggest that spinal noradrenergic mechanisms are involved in the antiallodynic effects of intrathecally administered 5-HT(2C) receptor agonists. Previously, we demonstrated that intrathecal administration of 5-HT(2A) receptor agonists also produced antiallodynic effects, and the effects were not reversed by yohimbine. Taken together, these findings suggest that 5-HT(2A) and 5-HT(2C) receptors in the dorsal horn of the spinal cord might be involved in alleviating neuropathic pain by different mechanisms.     

Analgesic profile of centrally administered 2-methylserotonin against acute pain in rats

The present study examined patterns of analgesia by intracerebroventricular (i.c.v.) or intrathecal (i.t.) administration of the serotonin 5-HT3 receptor agonist, 2-methylserotonin (1-100 micrograms) against acute thermal, mechanical or formalin-induced chemo-inflammatory pain in male rats. Neither i.c.v. or i.t. 2-methylserotonin produced motoric, sedative or respiratory effects. I.c.v. 2-methylserotonin was not analgesic at any dose in the pain assays employed. I.t. 2-methylserotonin produced dose-related analgesia in the formalin test with significant effects at 20-100 micrograms doses. In contrast, only the 100 micrograms dose of 2-methylserotonin produced analgesia against thermal pain, and analgesia was not observed at any dose in the mechanical pain test. The effects of 2-methylserotonin (100 micrograms) in the formalin test were attenuated by pretreatment (10 micrograms i.t.) with the 5-HT3 receptor antagonist MDL-72222, opioid antagonist naloxone or GABA antagonist bicuculline; the 5-HT2-receptor antagonist ketanserin or 5-HT1 receptor antagonist mianserin did not affect 2-methylserotonin-induced analgesia. In the thermal test, i.t. pretreatment with MDL-72222, ketanserin, naloxone or bicuculline, but not mianserin, reduced analgesic effects of 2-methylserotonin (100 micrograms i.t.). These findings suggest that spinal 5-HT3, opioid and GABA receptor systems interact to mediate acute chemo-inflammatory pain, and implicate the interaction of these systems with 5-HT2 receptor substrates in analgesia against acute thermal nociception.     

Analgesic effect of intrathecally administered orexin-A in the rat formalin test and in the rat hot plate test

1. Orexin-A and orexin-B (also known as hypocretin-1 and hypocretin-2) are hypothalamic peptides and regulate feeding behaviour, energy metabolism and the sleep-wake cycle. Orexin-A binds equally to both orexin-1 and orexin-2 receptors, while orexin-B has a preferential affinity for orexin-2 receptors. 2. Orexins are also known to be concentrated in superficial laminae of the spinal dorsal horn, and orexin-A and orexin-1 receptors are found in the dorsal root ganglion cells. 3. In the present study, the authors examined the effect of intrathecal injection of either orexin-A or orexin-B in the rat formalin test (a model of inflammatory pain) and in the rat hot plate test. The paw formalin injection induces biphasic flinching (phase 1: 0-6 min; phase 2: 10-60 min) of the injected paw. 4. Intrathecal injection of orexin-A, but not orexin-B, decreased the sum of flinches in phases 1 and 2 in the formalin test and increased the hot plate latency. These effects of orexin-A were completely antagonized by pre-treatment with SB-334867, a selective orexin-1 receptor antagonist. Intrathecal injection of SB-334867 alone had no effect in the formalin test or in the hot plate test. 5. Intrathecal injection of orexin-A suppressed the expression of Fos-like immunoreactivity (Fos-LI), induced by paw formalin injection, in laminae I-II of L4-5 of the spinal cord. 6. These data suggest that the spinal orexin-1 receptor is involved in the nociceptive transmission and that the activation of the spinal orexin-1 receptor produces analgesic effects in the rat formalin test and in the rat hot plate test.     

Interaction between gamma-aminobutyric acid GABAB and cannabinoid CB1 receptors in spinal pain pathways in rat

Antinociceptive effects of cannabinoids are mediated, in part, at the spinal level. Cannabinoid CB1 receptors are co-localized with dorsal horn interneurons containing gamma-aminobutyric acid (GABA). In this study, we investigated the interaction between intrathecally administered cannabinoid and GABA(B) receptor agonists and antagonists in the modulation of formalin-induced pain at the spinal level. Intrathecal pretreatment of rats with a cannabinoid receptor antagonist [N-(piperidin-1-yl)-5-(4-chlorophenyl)-1-(2,4-dichlorophenyl)-4-methyl-1-H-pyrazole-3-carboxamide] (SR141716A, 30 microg) decreased the analgesic effect of the intrathecal administration of the GABA(B) receptor agonist, baclofen (0.125 microg and 0.25 microg). Intrathecal administration of the GABA(B) receptor antagonist, saclofen (30 microg), 10 min before administration of the cannabinoid receptor agonist (-)-cis-3-[2-hydroxy-4-(1,1-dimethylheptyl)-phenyl]-trans-4-(3-hydroxy-propyl)-cyclohexano (CP55940), did not affect the analgesia produced by the cannabinoid receptor agonist. Our results confirm that intrathecal administration of cannabinoid and GABA(B) receptor agonists have analgesic effects and that spinal antinociceptive effects of GABA(B) receptor agonists are likely through endocannabinoid modulation.     

Roles of adenosine receptor subtypes in the antinociceptive effect of intrathecal adenosine in a rat formalin test

The contributions of adenosine receptor subtypes to antinociception produced by adenosine were determined at the spinal level. There are 4 types of adenosine receptors, namely A1, A(2A), A(2B) and A3. The authors investigated the properties of the subtypes of spinal adenosine receptors in terms of nociceptive modulation. The nociceptive state was induced by subcutaneously injecting formalin solution (5%, 50 microl) into the hind paws of male Sprague-Dawley rats. After observing the effect of intrathecal adenosine during the formalin test, the effects of intrathecal adenosine A1 (CPT), A(2A) (CSC), A(2B) (alloxazine) and A3 (MRS 1220) receptor antagonists on the action of adenosine were examined. Intrathecal adenosine inhibited phase 2 flinching response without affecting phase 1 response. CPT, CSC, alloxazine and MRS 1220 antagonized the antinociceptive action of adenosine during phase 2 of the formalin test. These results suggest that spinal adenosine A1, A(2A), A(2B) and A3 receptors may play an important role in the antinociception of adenosine in the formalin-induced facilitated state.     

Interaction between intrathecal morphine and glutamate receptor antagonists in formalin test

The analgesic interaction between intrathecally administered morphine and the NMDA receptor antagonist, ((+/-)-2-amino-5-phosphonopentanoic acid; AP-5), the NMDA receptor glycine site antagonist, (5-nitro-6,7-dichloro-2,3-quinoxaline dion; ACEA 1021), or the AMPA (alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid) receptor antagonist (ACEA 2752) in the formalin test was investigated with a rat model of chronic lumbar intrathecal catheterization. After obtaining dose-response curves for each agent, combinations of morphine and AP-5, ACEA 1021 or ACEA 2752 were tested for their effect on the number of flinches in the formalin test and for associated side-effects, such as motor disturbances, flaccidity, and agitation/allodynia. Using isobolographic analyses, a potent analgesic synergy was observed with decreased side-effects between morphine and ACEA 2752 or AP-5. ACEA 1021 increased the analgesic effect of low-dose morphine. Spinal mu-opioid receptor activation and NMDA or AMPA receptor antagonism showed a synergistic antinociception against tonic pain. These results suggest an important direction in the management of inflammatory pain.     

Intrathecal treatment with sigma1 receptor antagonists reduces formalin-induced phosphorylation of NMDA receptor subunit 1 and the second phase of formalin test in mice

Although previous reports have suggested that the sigma 1 (sigma(1)) receptor may be involved in pain sensation, its specific site of action has not been elucidated. The aim of present study was to determine the role of the spinal sigma(1) receptor in formalin-induced pain behavior, spinal cord Fos expression and phosphorylation of N-methyl-D-aspartate receptor subunit 1 (pNR1). Intrathecal (i.t.) pretreatment with the selective sigma(1) receptor antagonist, BD-1047 (N-[2-(3,4-dichlorophenyl)ethyl]-N-methyl-2-(dimethylamino)ethylamine dihydrobromide) (10-100 nmol) dose dependently reduced formalin-induced pain behaviors in second phase, but not first phase, of the formalin test. I.t. injection of BD-1047 also reduced formalin-evoked Fos expression and pNR1 at the protein kinase C-dependent site, serine-896 (Ser896) and the protein kinase A-dependent site, serine-897 (Ser897) in spinal dorsal horn.i.t. BMY-14802 ((alpha-(4-fluorophenyl)-4-(5-fluoro-2-pyrimidinyl)-1-piperazinebutanol hydrochloride) (10-100 nmol, sigma(1) receptor antagonist and 5-HT(1A) receptor agonist) dose dependently reduced formalin-induced pain behaviors in both phases. However, the 5-HT(1A) receptor might not be involved in the antinociceptive effect of BMY-14802 on the second phase, since i.t. pretreatment with the 5-HT(1A) receptor antagonist propranolol ((S)-1-isopropylamino-3-(1-naphthyloxy)-2-propanol hydrochloride) (injected 10 min prior to i.t. BMY-14802) partially blocked the effect of BMY-14802 on the first phase of the formalin test but did not affect the inhibitory effect of BMY-14802 on the second phase. In addition, i.t. BMY-14802 significantly reduced formalin-evoked Fos expression and pNR1 (Ser896 and Ser897) expression in spinal dorsal horn. The results of this study suggest that selective blockage of spinal sigma(1) receptors can reduce pain behaviors, spinal cord Fos expression and pNR1 (Ser896 and Ser897) expression associated with the second phase of the formalin test.     

Evaluation of interaction between intrathecal adenosine and MK801 or NBQX in a rat formalin pain model

Adenosine and excitatory amino acids have been known to be involved in modulating nociceptive transmission at the spinal level. The authors assessed the characteristics of the interaction of the adenosine-excitatory amino acid antagonist combinations in the spinal cord of rats on the formalin-induced nociception. Intrathecal NMDA antagonist ((5R, 10S)-(+)-5-methyl-10,11-dihydro-(5)H-dibenzo[a[,]d]cyclohepten-5,10-imine hydrogen maleate, MK801, 30 microg) and AMPA antagonist (2,3-dioxo-6-nitro-1,2,3,4-tetrahydrobenzo[F]quinoxaline-7-sulfonamide, NBQX, 3 microg) decreased the total number of flinches during both phases in the formalin test. Intrathecal adenosine (300 microg) had little effect on the phase 1 flinching response, but decreased the phase 2 response. The fixed dose analysis and the isobolographic analysis revealed that adenosine interacts additively with MK801 and NBQX in the spinal cord.     

5-HT(1A) receptors in endogenous regulation of neuropathic hypersensitivity in the rat

The role of medullary and spinal 5-HT(1A) receptors in endogenous regulation of neuropathic hypersensitivity was studied. When administered in the rostroventromedial medulla or subcutaneously, WAY-100635, a 5-HT(1A) receptor antagonist, attenuated mechanical hypersensitivity in rats with a spinal nerve injury. Thermal or mechanical nociception outside of the injured area was not influenced by medial medullary or subcutaneous administration of WAY-100635. Intrathecal administration of WAY-100635 had no significant effect on pain-related behavior. Suppression of mechanical hypersensitivity induced by medial medullary administration of WAY-100635 was reversed by intrathecal administration of WAY-100635 or atipamezole, an alpha2-adrenoceptor antagonist, but not by naloxone, an opioid receptor antagonist. The results indicate that endogenous release of 5-HT, via action on medial medullary 5-HT(1A) receptors, tonically suppresses descending inhibition in neuropathic animals. Following medial medullary administration of a 5-HT(1A) receptor antagonist, descending pain regulatory pathways are disinhibited. This leads to selective attenuation of neuropathic hypersensitivity, due to action on spinal 5-HT(1A) receptors and alpha2-adrenoceptors.     

The behavioural response to intrathecal serotonin is changed by acute but not by repeated treatment with zimelidine or metergoline.

The behavioural response to intrathecal serotonin (5-HT) was examined in mice after acute and after withdrawal of repeated treatment with the 5-HT uptake inhibitor zimelidine or the 5-HT receptor antagonist metergoline. Intrathecal 5-HT elicits a response consisting of biting or licking of the lower part of abdomen and reciprocal hindlimb scratching, indicative of nociceptive stimulation. Acute injection of zimelidine (20 mg/kg) significantly increased the response to intrathecal 5-HT (0.25-1 micrograms) whereas a single dose of metergoline (5 mg/kg) completely blocked the response to intrathecal 5-HT (2 micrograms). The behavioural response to intrathecal 5-HT (0.25-2 micrograms) was not significantly changed 48 hr after withdrawal of repeated treatment with zimelidine (2 x 10 mg/kg/day for 14 days) or metergoline (2 x 2.5 mg/kg/day for 14 days). In the present experiments acute zimelidine appeared to increase nociceptive responsiveness, whereas metergoline had the opposite effect. This study does not provide evidence that long-term treatment with zimelidine or metergoline leads to adaptive changes in the response to spinal cord 5-HT receptor stimulation.     

The Behavioural Response to Intrathecal Serotonin is Changed by Acute but not by Repeated Treatment with Zimelidine or Metergoline

Abstract: The behavioural response to intrathecal serotonin (5-HT) was examined in mice after acute and after withdrawal of repeated treatment with the 5-HT uptake inhibitor zimelidine or the 5-HT receptor antagonist metergoline. Intrathecal 5-HT elicits a response consisting of biting or licking of the lower part of abdomen and reciprocal hindlimb scratching, indicative of nociceptive stimulation. Acute injection of zimelidine (20 mg/kg) significantly increased the response to intrathecal 5-HT (0.25-1 μg) whereas a single dose of metergoline (5 mg/kg) completely blocked the response to intrathecal 5-HT (2 μg). The behavioural response to intrathecal 5-HT (0.25-2 μg) was not significantly changed 48 hr after withdrawal of repeated treatment with zimelidine (2 × 10 mg/kg/day for 14 days) or metergoline (2 × 2.5 mg/kg/day for 14 days). In the present experiments acute zimelidine appeared to increase nociceptive responsiveness, whereas metergoline had the opposite effect. This study does not provide evidence that long-term treatment with zimelidine or metergoline leads to adaptive changes in the response to spinal cord 5-HT receptor stimulation.     

Role of peripheral and spinal 5-HT(3) receptors in development and maintenance of formalin-induced long-term secondary allodynia and hyperalgesia

The role of peripheral and spinal 5-HT₃ receptors in formalin-induced secondary allodynia and hyperalgesia in rats was assessed. Formalin produced acute nociceptive behaviors (flinching and licking/lifting) followed by long-term secondary mechanical allodynia and hyperalgesia in both paws. In experiments where the test drug was anticipated to augment or antagonize the response, 0.5 or 1% formalin, respectively, was used for injection. Peripheral ipsilateral, but not contralateral, pre-treatment (− 10 min) with serotonin (5-HT, 10-100 nmol/paw) and the selective 5-HT₃ receptor agonist 1-(m-chlorophenyl)-biguanide (m-CPBG, 10-300 nmol/paw) increased 0.5% formalin-induced secondary allodynia and hyperalgesia in both paws. Moreover, spinal pre-treatment with m-CPBG (10-300 nmol/rat) increased 0.5% formalin-induced secondary hyperalgesia but not allodynia in both paws. Accordingly, peripheral ipsilateral (30-300 nmol/paw), but not contralateral (300 nmol/paw), and spinal (10-100 nmol) pre-treatment with the selective 5-HT₃ receptor antagonist ondansetron prevented 1% formalin-induced secondary mechanical allodynia and hyperalgesia in both paws. The peripheral pronociceptive effects of 5-HT (100 nmol/paw) and m-CPBG (300 nmol/paw) as well as the spinal effect of m-CPBG (300 nmol/rat) were completely prevented by the peripheral (10 nmol/paw) and spinal (1 nmol/rat) injection, respectively, of ondansetron. At these doses, ondansetron did not modify per se formalin-induced nociceptive behaviors. Spinal (30-300 nmol/rat), but not peripheral (300 nmol/paw), post-treatment (on day 6) with ondansetron reversed established formalin-induced secondary mechanical allodynia and hyperalgesia in both paws. Results suggest that a barrage of afferent input induced by 5-HT at peripheral 5-HT₃ receptors participates in the development of formalin-induced long-term secondary allodynia and hyperalgesia in the rat. In addition, our data suggest that spinal 5-HT₃ receptors play an important role during development and maintenance of these evoked long-term behaviors.     

Effects of a 5-HT2A receptor antagonist, sarpogrelate on thermal or inflammatory pain

The effects of intrathecally and systemically administered 5-hydroxytriptamine (5-HT)(2A) receptor antagonist, sarpogrelate on acute thermal or formalin induced pain were examined. Male Sprague-Dawley rats with lumbar intrathecal catheters were tested with their tail withdrawal response to thermal stimulation (tail flick test) or their paw flinching and shaking response by subcutaneous formalin injection into the hind paw (formalin test) after intrathecal or intraperitoneal administration of sarpogrelate. 5-HT(2A) receptor agonist was used to antagonize the effects of sarpogrelate. In the tail flick test, only intraperitoneal administration induced analgesia. In the formalin test, both intrathecal and intraperitoneal administration were analgesic. The analgesic effects were inhibited by pretreatment with 5-HT(2A) receptor agonist. Motor disturbance and behavioral side effects were not observed. In conclusion, sarpogrelate might be analgesic on inflammatory induced acute and facilitated pain by intrathecal or systemic administration. However, only systemic administration could be effective on thermal induced acute pain.     

Involvement of 5-HT2 receptors in the behaviours produced by intrathecal administration of selected 5-HT agonists and the TRH analogue (CG 3509) to rats.

1. The behavioural effects of the intrathecal injection of a thyrotrophin-releasing hormone (TRH) analogue L-orotyl-L-histidyl-prolineamide (CG 3509, 0.5 micrograms), the non-selective 5-HT1 and 5-HT2 receptor agonist 5-methoxy-N,N'-dimethyltryptamine (5-MeODMT, 2-100 micrograms) and the selective 5-HT2 receptor agonist 2,5-dimethoxy-alpha,4-dimethyl-benzene ethamine hydrochloride (DOM, 2-25 micrograms) were compared with the response of systemically administered 5-MeODMT (2 mg kg-1, i.p.) in rats, to establish whether the agonist-induced behaviours were mediated by bulbospinal 5-HT1 or 5-HT2 receptors. 2. Intrathecal injection of 5-MeODMT or DOM produced dose-related back muscle contractions (a previously undocumented behaviour) and wet-dog shakes which were both markedly attenuated by ritanserin pretreatment (1 mg kg-1, i.p.) indicating the involvement of 5-HT2 receptors. In contrast, reciprocal forepaw treading, flat body posture and Straub-tail were evoked by 5-MeODMT but not by DOM indicating that these behaviours were not produced by 5-HT2 receptor activation alone. However, as ritanserin pretreatment reduced the reciprocal forepaw treading induced by intrathecal 5-MeODMT, this behaviour may be facilitated by 5-HT2 receptor activation. 3. Intrathecal 5,7-dihydroxytryptamine (5,7-DHT, 2 x 150 micrograms) treatment decreased thoraco-lumbar spinal cord 5-HT (-95%) and potentiated the back muscle contractions produced by intrathecal DOM injection without altering the wet-dog shake behaviour. None of the components of the 5-HT syndrome produced by 5-MeODMT (2 mg kg-1, i.p.), with the exception of a small increase in wet-dog shakes, was significantly altered by intrathecal 5,7-DHT (which reduced thoraco-lumbar spinal cord 5-HT by 84%). Taken together these data suggest that the only 5-HT agonist-induced behaviour mediated by the activation of 5-HT2 receptors located postsynaptic to bulbospinal 5-hydroxytryptaminergic (5-HTergic) neurones was back muscle contractions. 4. The wet-dog shake and forepaw licking behaviors produced by intrathecal CG 3509 (0.5 micrograms) were attenuated when ritanserin was administered intrathecally 30 min before, but not when it was given at the same time as CG 3509 and neither behaviour was altered by intrathecal 5,7-DHT. This suggests that bulbospinal 5-HTergic neurones are not involved in the production of these TRH analogue-induced behaviours and that the 5-HT2 receptors which mediate these behaviours are not located in the spinal cord.