Noradrenergic and purinergic involvement in spinal antinociception by 5-hydroxytryptamine and 2-methyl-5-hydroxytryptamine

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.

Possible involvement of cholinergic and opioid receptor mechanisms in fluoxetine mediated antinociception response in streptozotocin-induced diabetic mice

Clinical and experimental studies have been reported that antidepressant drugs can be used as co-analgesics in the management of neuropathic pain. However, the mechanism through which they alleviate pain still remains unclear. The aim of the present study was to investigate the possible mechanism of action of fluoxetine-induced antinociceptive effect in streptozotocin-induced diabetic mice, especially the involvement of non-serotonergic neurotransmitters and their receptors. Diabetes was induced in male Laka mice with a single intraperitoneal injection of streptozotocin (200 mg/kg). Four weeks after streptozotocin, diabetic mice were tested for pain responses in the tail-immersion and hot-plate assays. Diabetic mice exhibited significant hyperalgesia as compared with control mice. Fluoxetine (10 and 20 mg/kg, i.p) injected into diabetic mice produced an antinociceptive effect in both tail-immersion and hot-plate assays. The antinociceptive effect of fluoxetine in diabetic mice was significantly lower as compared with that in control mice. Pretreatment with a muscarinic receptor antagonist, atropine (2 and 5 mg/kg, i.p) and an opioid receptor antagonist, naloxone (2 and 5 mg/kg, i.p), but not the alpha(2)-adrenoreceptor antagonist, yohimbine (2 and 5 mg/kg, i.p) reversed the antinociceptive effect of fluoxetine (20 mg/kg). These results suggest that apart from serotonin pathway, muscarinic and opioid receptors also participate in fluoxetine-induced antinociception in diabetic neuropathic pain.

Joint manipulation reduces hyperalgesia by activation of monoamine receptors but not opioid or GABA receptors in the spinal cord

Joint manipulation has long been used for pain relief. However, the underlying mechanisms for manipulation-related pain relief remain largely unexplored. The purpose of the current study was to determine which spinal neurotransmitter receptors mediate manipulation-induced antihyperalgesia. Rats were injected with capsaicin (50 microl, 0.2%) into one ankle joint and mechanical withdrawal threshold measured before and after injection. The mechanical withdrawal threshold decreases 2 h after capsaicin injection. Two hours after capsaicin injection, the following drugs were administered intrathecally: bicuculline, blocks gamma-aminobutyric acid (GABAA) receptors; naloxone, blocks opioid receptors; yohimbine blocks, alpha2-adrenergic receptors; and methysergide, blocks 5-HT(1/2) receptors. In addition, NAN-190, ketanserin, and MDL-72222 were administered to selectively block 5-HT1A, 5-HT2A, and 5-HT3 receptors, respectively. Knee joint manipulation was performed 15 min after administration of drug. The knee joint was flexed and extended to end range of extension while the tibia was simultaneously translated in an anterior to posterior direction. The treatment group received three applications of manipulation, each 3 min in duration separated by 1 min of rest. Knee joint manipulation after capsaicin injection into the ankle joint significantly increases the mechanical withdrawal threshold for 45 min after treatment. Spinal blockade of 5-HT(1/2) receptors with methysergide prevented, while blockade of alpha2-adrenergic receptors attenuated, the manipulation-induced antihyperalgesia. NAN-190 also blocked manipulation-induced antihyperalgesia suggesting that effects of methysergide are mediated by 5-HT1A receptor blockade. However, spinal blockade of opioid or GABAA receptors had no effect on manipulation induced-antihyperalgesia. Thus, the antihyperalgesia produced by joint manipulation appears to involve descending inhibitory mechanisms that utilize serotonin and noradrenaline.

Interactions of 5-HT2 receptor agonists with acetylcholine in spinal analgesic mechanisms in rats with neuropathic pain

Serotonin type 2 (5-HT(2)) receptors reportedly inhibit neuropathic pain in the spinal cord, but little is known about how spinal 5-HT(2) receptors might act against such abnormal sensitivity. We examined whether the cholinergic and tachykinin systems were involved in the antiallodynic effect of intrathecally administered 5-HT(2) receptor agonists in rats with nerve injury. Allodynia was produced by tight ligation of the left L5 and L6 spinal nerves, and determined by applying von Frey hairs to the left hindpaw. Effects of intrathecal pretreatment with 5-HT(2) receptor antagonists (ketanserin and RS-102221), muscarinic receptor antagonists (atropine and scopolamine), a choline uptake blocker (hemicholium-3), and an NK(1) receptor antagonist (L-706336) were assessed in rats subsequently given a 100- micro g intrathecal dose of a 5-HT(2) receptor agonist either alpha-methyl-5-HT or iododimethoxy aminopropane (DOI). Antiallodynic effects of 5-HT(2) receptor agonists were attenuated by the 5-HT(2A) receptor antagonist ketanserin (30 micro g), but not by the 5-HT(2C) receptor antagonist RS-102221 (40 micro g). Muscarinic receptor antagonists (30 micro g each), the choline uptake blocker (10 micro g), and the NK(1) receptor antagonist (30 micro g) also inhibited the antiallodynic effects of 5-HT(2) receptor agonists. Antiallodynic effects of intrathecally administered 5-HT(2) receptor agonists may be mediated by spinal release of acetylcholine induced via 5-HT(2A) and NK(1) receptors.

Possible involvement of a muscarinic receptor in the anti-allodynic action of a 5-HT2 receptor agonist in rats with nerve ligation injury

Intrathecal administration of 5-HT(2) receptor agonists produces an anti-allodynic effect in a rat model of neuropathic pain. Several non-serotonergic neurotransmitters have been implicated these anti-nociceptive effects. In the present study, intrathecal pre-treatment with the muscarinic receptor antagonist atropine (10 and 30 microg) and pirenzepine (10 microg) reversed the anti-allodynic effect of the 5-HT(2) receptor agonist alpha-methyl-5-hydroxytryptamine, unlike various other antagonists. Thus, muscarinic receptors may be involved in the anti-allodynic action of intrathecally injected 5-HT(2) receptor agonist.

Descending control of pain

Upon receipt in the dorsal horn (DH) of the spinal cord, nociceptive (pain-signalling) information from the viscera, skin and other organs is subject to extensive processing by a diversity of mechanisms, certain of which enhance, and certain of which inhibit, its transfer to higher centres. In this regard, a network of descending pathways projecting from cerebral structures to the DH plays a complex and crucial role. Specific centrifugal pathways either suppress (descending inhibition) or potentiate (descending facilitation) passage of nociceptive messages to the brain. Engagement of descending inhibition by the opioid analgesic, morphine, fulfils an important role in its pain-relieving properties, while induction of analgesia by the adrenergic agonist, clonidine, reflects actions at alpha(2)-adrenoceptors (alpha(2)-ARs) in the DH normally recruited by descending pathways. However, opioids and adrenergic agents exploit but a tiny fraction of the vast panoply of mechanisms now known to be involved in the induction and/or expression of descending controls. For example, no drug interfering with descending facilitation is currently available for clinical use. The present review focuses on: (1) the organisation of descending pathways and their pathophysiological significance; (2) the role of individual transmitters and specific receptor types in the modulation and expression of mechanisms of descending inhibition and facilitation and (3) the advantages and limitations of established and innovative analgesic strategies which act by manipulation of descending controls. Knowledge of descending pathways has increased exponentially in recent years, so this is an opportune moment to survey their operation and therapeutic relevance to the improved management of pain.

Interactions of descending serotonergic systems with other neurotransmitters in the modulation of nociception

The effects of 5-hydroxytryptamine (5-HT) and ligands selective for particular 5-HT receptor subtypes on the transmission of nociceptive information in the spinal cord are complex. In these studies, we have focused on their interactions with two endogenous mediators of pain suppression, noradrenaline (NA) and adenosine. Spinal antinociception by 5-HT is blocked by alpha-adrenoreceptor antagonists and depletion of endogenous NA by 6-hydroxydopamine, while it is potentiated by blockade of NA reuptake with desipramine. These observations provide evidence for a 5-HT receptor-mediated increase in the release of NA from the spinal cord. This action appears to be due to activation of a 5-HT1-like receptor as it is mimicked by some 5-HT1 receptor ligands (mCPP, TFMPP and 5-Me-O-DMT), but not by DOI (5-HT2) or 2-Me-5-HT (5-HT3). An additional component of 5-HT action is via release of adenosine. Antinociception by 5-HT is blocked by the adenosine receptor antagonist 8-phenyltheophylline, and 5-HT has been shown to release adenosine from the spinal cord in in vitro and in vivo paradigms. Methylxanthine-sensitive antinociception is seen with some 5-HT1 receptor ligands (CGS 12066B, mCPP), but not with others or with DOI or 2-Me-5-HT. Further characterization of the 5-HT receptor subtype involved in adenosine release will require the use of additional approaches.

Adenosine kinase and adenosine deaminase inhibition modulate spinal adenosine- and opioid agonist-induced antinociception in mice

Endogenous purinergic systems mediating antinociception, and their interactions with opioids, were characterized following intrathecal (i.t.) administration of inhibitors of adenosine clearance in mice. 5'-Amino,5'-deoxyadenosine (5'-NH2dAdo), an inhibitor of adenosine kinase, induced significant antinociception after i.t. injection and enhanced antinociception induced by selected opioids (i.t.). Isobolographic analysis of antinociception following coadministration (i.t.) of 5'-NH2dAdo with opioids revealed additive interactions with mu-, and synergistic interactions with delta-, opioid receptor-selective agonists. Inhibitors of adenosine deaminase, deoxycoformycin and erythro-9-(2-hydroxy-3nonyl) adenine (EHNA), generally failed to induce antinociception when administered (i.t.) alone or to enhance opioid (i.t.)-induced antinociception, however, was significantly enhanced by either 5'-NH2dAdo or deoxycoformycin. These results confirm different physiologic roles for adenosine kinase and adenosine deaminase in spinal purinergic systems. 5'-NH2dAdo interactions with opioid receptor-selective agonists demonstrate significant, but heterogeneous interactions between endogenous adenosine and opioid spinal systems mediating antinociception.

Spinal supersensitivity to 5-HT1, 5-HT2 and 5-HT3 receptor agonists following 5,7-dihydroxytryptamine

The present study examined functional supersensitivity to 5-hydroxytryptamine (5-HT) and 5-HT ligands selective for 5-HT1, 5-HT2 and 5-HT3 receptors in two tests for nociception following the spinal administration of 5,7-dihydroxytryptamine (5,7-DHT). Intrathecal pretreatment with 5,7-DHT 30-100 micrograms (following desipramine) produced a selective depletion of spinal cord 5-HT levels of > 80% and augmented the antinociceptive action of 5-HT in the tail flick and hot plate tests. The tail flick test was the more sensitive test for expression of this action. Supersensitivity was observed with the 5-HT1 receptor ligands CGS 12066B (7-trifluoromethyl-4-(4-methyl-1-piperazinyl-pyrrolo[1,2-a] quinoxalinedimaleate), RU 24969 (5-methoxy-3-(1,2,4,6-tetrahydro-4-pyridinyl)1H indole succinate), TFMPP (m-trifluoromethylphenyl-piperazine HCl), mCPP (1-(3-chlorophenyl)piperazine dihydrochloride) and 5-Me-ODMT (5-methoxy-N,N-dimethyltryptamine hydrogen oxalate) but not with the 5-HT2 receptor ligand DOI ((+/-)-1-(4-iodo-2,5-dimethoxyphenyl)-2-aminopropane HCl) or the 5-HT3 receptor ligand 2-Me-5-HT (2-methyl-5-hydroxytryptamine maleate) in the tail flick test. In the hot plate test, supersensitivity was observed only with 5-Me-ODMT. Intrathecal pretreatment with fluoxetine, a 5-HT uptake inhibitor, potentiated the action of 5-HT but not any of the other 5-HT1 receptor ligands examined. These results indicate that supersensitivity occurs with 5-HT and 5-HT1 receptor ligands but not with 5-HT2 or 5-HT3 receptor ligands. Both the loss of uptake sites and receptor upregulation may contribute to enhanced activity of 5-HT, but for other ligands, only the latter mechanism appears to occur.

Partial sciatic nerve ligation results in an enlargement of the receptive field and enhancement of the response of dorsal horn neurons to noxious stimulation by an adenosine agonist

In this study we examined the effect of partial sciatic nerve ligation (PSNL) on the receptive field size, the baseline firing rate (BFR) and the response of spinal dorsal horn (DH) neurons to mechanical stimulation. In addition, we tested the effect of adenosine agonist, 5'-N-ethylcarboxamide-adenosine (NECA), and the adenosine antagonist caffeine on these parameters. Adult male Sprague-Dawley animals were used. One-third to one-half of the right sciatic nerve was tightly ligated. Unanesthetized animals were tested for their response to mechanical stimulation using Von Frey filaments and a blunt probe. The mean force that produced a paw withdrawal response in the operated animals was significantly less than the force that produced withdrawal in unoperated animals (median: 103.5 vs. 259.7; P < 0.001 for the paw ipsilateral to the ligation). Extracellular recordings were made from nociceptive-specific DH neurons located in laminal I-V of chloral hydrate-anesthetized rats. Recordings were made from 38 neurons in the right and 29 cells in the left DH of unoperated and 40 cells in right and 41 cell in the left DH of operated animals. The BFRs of neurons recorded in the operated animals were not significantly different from those recorded in normal animals. The mean receptive field size (RFS) of neurons (both ipsilateral and contralateral to the ligation) in the operated animals was significantly larger than the RFS of unoperated animals (right side: 180 +/- 2.8 mm2 compared to 66 +/- 2.3 mm2; left side: 93 +/- 31 compared to 65 +/- 21). Twenty-four percent of all neurons in the operated group had bilateral receptive fields; in contrast, only 3% of the neurons in the control animals showed bilateral receptive fields. To examine the effects of adenosine agonist and antagonist, NECA and caffeine were applied next to the recording electrode.(ABSTRACT TRUNCATED AT 250 WORDS)

Intrathecal 5-methoxy-N,N-dimethyltryptamine in mice modulates 5-HT1 and 5-HT3 receptors

The antinociceptive effects of intrathecally administered 5-methoxy-N,N-dimethyltryptamine (5-MeO-DMT), a potent 5-HT receptor agonist, were studied in three behavioral tests in mice: the tail-flick test and the intrathecal substance P and N-methyl-D-aspartic acid (NMDA) assays. Intrathecal administration of 5-MeO-DMT (4.6-92 nmol/mouse) produced a significant prolongation of the tail-flick latency. This action was blocked by 5-HT3 and gamma-aminobutyric acidA (GABAA) receptor antagonists but not by 5-HT2, 5-HT1A, 5-HT1B or 5-HT1S receptor antagonists. Binding studies indicated that 5-MeO-DMT had very low affinity for 5-HT3 receptors. 5-MeO-DMT inhibited biting behavior while increasing scratching behavior induced by intrathecally administered substance P. The inhibition of biting behavior was antagonized by intrathecal co-administration of 5-HT1B and GABAA receptor antagonists while 5-HT1A, 5-HT1S, 5-HT2 and 5-HT3 receptor antagonists had no effect. 5-MeO-DMT-enhanced scratching behavior was inhibited by all the antagonists used except ketanserin and bicuculline, suggesting the involvement of 5-HT1A, 5-HT1B, 5-HT1S, 5-HT3 and GABAA receptors. NMDA-induced biting behavior was inhibited by 5-MeO-DMT pretreatment; this action was antagonized by 5-HT1B, 5-HT3 and GABAA receptor antagonists. The involvement of these receptors in 5-MeO-DMT action suggests that it may promote release of 5-HT (5-hydroxytryptamine, serotonin).

Intrathecal coadministration of serotonin and morphine differentially modulates the tail-flick reflex of intact and spinal rats

In a previous study, we found that the antinociceptive effect of IT-administered morphine on the tail-flick (TF) reflex of rats was potentiated within 1 day after spinal transection. This suggested that the analgesic effect of spinal morphine in the intact animal was tonically suppressed, presumably by the release of a transmitter(s) from descending supraspinal pathway(s), and that the potency of IT morphine was increased because these inputs were removed by spinalization. Because spinally projecting serotonin [5-hydroxytryptamine (5-HT)] fibers are known to be involved in modulating nociception at this site, the present studies examined the possibility that 5-HT might be the proposed "antiopiate" at the spinal cord. Separate groups of intact and spinal rats were pretested on the TF and then injected IT with either morphine (intact: 0.25-5.0 micrograms, spinal: 0.0312-0.5 microgram) or 5-HT (1-200 micrograms), or combinations of these two agents, in a single solution. All rats were then retested 15 min later and the difference in latency was used to compare the effect of these treatments. The results confirmed that the antinociceptive effect of IT morphine was significantly increased by spinalization, whereas the antinociceptive effect of 5-HT was essentially abolished. In intact rats, morphine-induced analgesia was potentiated by a low (10 micrograms) dose of 5-HT but not by higher doses. However, in the spinal rat morphine-induced antinociception was antagonized by the same (10 micrograms) dose. The data suggest that IT 5-HT promotes antinociception in intact rats but acts pro-nociceptively in spinal rats.(ABSTRACT TRUNCATED AT 250 WORDS)

Noradrenergic mediation of spinal antinociception by 5-hydroxytryptamine: characterization of receptor subtypes

The present study examined the involvement of spinal noradrenergic mechanisms in spinal antinociception by the 5-hydroxy-tryptamine (5-HT) receptor-selective agonists CGS 12066B (5-HT1B; 7-trifluoromethyl-4(4-methyl-1-piperazinyl)-pyrrolo[1,2-a]quinoxaline), TFMPP (5-HT1C; M-trifluoromethylphenyl-piperazine) and DOI (5-HT2; 1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane) using the rat hot plate test. Effects of alpha-adrenoreceptor antagonists (phentolamine, yohimbine), the adrenergic neurotoxin 6-hydroxydopamine, and the selective noradrenergic uptake blocker desipramine were determined. CGS 12066B, TFMPP and DOI produced dose-related antinociception. The antinociceptive effect of each agent was reduced by pretreatment with both phentolamine and yohimbine (15-60 micrograms). Pretreatment with 6-hydroxydopamine (100 micrograms, intrathecal) for 7-10 days, which reduced spinal cord levels of noradrenaline by 87%, inhibited the action of TFMPP (and 5-HT), but not CGS 12066B or DOI. Pretreatment with desipramine (25 mg/kg, systemic) potentiated the action of TFMPP but not CGS 12066B or DOI (or 2-methyl-5-HT). These results suggest that antinociception by TFMPP is dependent on release of endogenous noradrenaline from the spinal cord, while that produced by CGS 12066B and DOI is not. As TFMPP exhibits a close similarity to 5-HT in these experiments, the 5-HT receptor subtype being activated to induce noradrenaline release may either be a 5-HT1C or a 5-HT1S subtype. Other mechanisms account for the observed blockade of the action of CGS 12066B and DOI by alpha-adrenoreceptor antagonists.

Desipramine potentiates spinal antinociception by 5-hydroxytryptamine, morphine and adenosine

The effects of pretreatment with desipramine, a selective noradrenaline (NA) uptake blocker, on spinal antinociception by 5-hydroxytryptamine (5-HT), morphine and an adenosine analog (NECA) in the rat hot-plate test were examined to determine if endogenous NA is involved in the spinal action of these agents. Desipramine, 25 mg/kg, had no significant intrinsic effect in the hot-plate test but potentiated spinal antinociception by NA and 5-HT. Potentiation was more prominent at higher doses of NA and 5-HT. Desipramine also enhanced the action of morphine and NECA, but, in these instances, the greatest enhancement occurred at lower doses. These results, in conjunction with others, suggest that 5-HT releases NA from the spinal cord while morphine and NECA interact synergistically with endogenously released NA.