Interactions between serotonin and substance P in the spinal regulation of nociception

Dezocine exhibits antihypersensitivity activities in neuropathy through spinal μ-opioid receptor activation and norepinephrine reuptake inhibition

Dezocine is the number one opioid painkiller prescribed and sold in China, occupying 44% of the nation’s opioid analgesics market today and far ahead of the gold-standard morphine. We discovered the mechanisms underlying dezocine antihypersensitivity activity and assessed their implications to antihypersensitivity tolerance. Dezocine, given subcutaneously in spinal nerve-ligated neuropathic rats, time- and dose-dependently produced mechanical antiallodynia and thermal antihyperalgesia, significantly increased ipsilateral spinal norepinephrine and serotonin levels, and induced less antiallodynic tolerance than morphine. Its mechanical antiallodynia was partially (40% or 60%) and completely (100%) attenuated by spinal μ-opioid receptor (MOR) antagonism or norepinephrine depletion/α₂-adrenoceptor antagonism and combined antagonism of MORs and α₂-adenoceptors, respectively. In contrast, antagonism of spinal κ-opioid receptors (KORs) and δ-opioid receptors (DORs) or depletion of spinal serotonin did not significantly alter dezocine antiallodynia. In addition, dezocine-delayed antiallodynic tolerance was accelerated by spinal norepinephrine depletion/α₂-adenoceptor antagonism. Thus dezocine produces antihypersensitivity activity through spinal MOR activation and norepinephrine reuptake inhibition (NRI), but apparently not through spinal KOR and DOR activation, serotonin reuptake inhibition or other mechanisms. Our findings reclassify dezocine as the first analgesic of the recently proposed MOR-NRI, and reveal its potential as an alternative to as well as concurrent use with morphine in treating pain.

S-(+)-fenfluramine-induced nociceptive behavior in mice: Involvement of interactions between spinal serotonin and substance P systems

Intrathecal (i.t.) administration into mice of S-(+)-fenfluramine (0.01-0.1nmol), a serotonin (5-hydroxytryptamine, 5-HT) releaser, produced a behavioral response consisting of scratching, biting and licking. Here, we report the behavioral characteristics and the involvement of interactions between 5-HT and substance P (SP) systems in the S-(+)-fenfluramine-induced behavioral response. The S-(+)-fenfluramine-induced behavioral response peaked at 5-15min and almost disappeared at 20min after injection. The behavior induced by S-(+)-fenfluramine (0.1nmol) was dose-dependently inhibited by an intraperitoneal injection of morphine (0.02-0.5mg/kg), suggesting that the behavioral response is related to nociception. The S-(+)-fenfluramine-induced nociceptive behavior was significantly inhibited by pretreatment with 5-HT antiserum and co-administration of ketanserin, a selective 5-HT2 receptor antagonist. However, WAY-100635, a selective 5-HT1A receptor antagonist, and ramosetron, a selective 5-HT3 receptor antagonist, were not active. On the other hand, SP antiserum and RP67580, a selective neurokinin-1 (NK1) receptor antagonist, significantly inhibited S-(+)-fenfluramine-induced nociceptive behavior. These results suggest that i.t.-administered S-(+)-fenfluramine releases SP through the activation of 5-HT2 receptors subsequent to 5-HT release, and, as a result, produces nociceptive behavior.

Serotonergic activity contributes to analgesic overuse in chronic tension-type headache

OBJECTIVE

To evaluate the possible existence of a genetically determined innate factor that could exert a profound influence on the development of analgesic overuse in chronic tension-type headache (CTTH).

BACKGROUND

Many patients with CTTHs report the regular use of analgesics. Continuous use of analgesics results in the ultimate worsening of headaches. The factors related to development of analgesic overuse, however, remain poorly understood. The genetic factors for serotonin metabolism and the harm avoidance (HA) personality dimension are known to be associated with various substance abuse patterns.

DESIGN

We performed serotonin transporter protein (5-HTT) gene-linked polymorphic region (5-HTTLPR) genotype polymorphism analyses, and investigated serotonin-related personality traits by assessing the HA dimension using tridimensional personality questionnaire, given to 48 patients with CTTHs and reported analgesic overuse (CTTH-AO), in 50 patients with CTTHs without analgesic overuse (CTTH-NO), and in 100 healthy controls. We compared their headache characteristics using standardized questionnaires.

RESULTS

We discovered an excess frequency of 5-HTTLPR short allele and a different genotypic distribution in patients with CTTH-AO. The S/S genotype frequency was significantly higher in patients with CTTH-AO (83%) than in CTTH-NO (72%) and control (59%; P= .010) groups. Patients with CTTH-AO exhibited the highest HA scores (23.3 +/- 5.4), as compared to CTTH-NO (19.9 +/- 6.7) and control (16.3 +/- 6.1) groups. Individuals with the S/S genotype showed a greater tendency toward analgesic overuse (13.3 +/- 11.3 days per month) than did those with the non-S/S genotype (7.0 +/- 8.6 days per month: P= .02).

CONCLUSIONS

Our data suggest that serotonergic activity may be involved in the development of analgesic overuse in CTTH and that 5-HTTLPR might be one of the genetically contributing factors.

Experimental evaluation of an altered tryptophan metabolism in fibromyalgia

Fibromyalgia (FM) is a prevalent syndrome with chronic pain and a hypothesised underlying disturbance of the tryptophan (TRP) metabolism. We performed a tryptophan depletion (TD) test in 17 FM patients and 17 controls. TRP, 5-hydroxyindoleacetic acid (5-HIAA), kynurenine (KYN), and Interleukin-6 (IL-6) were measured. Additionally pain perception was monitored in the FM patients. FM patients and controls exhibited a decrease of TRP and KYN during TD. 5-HIAA levels also decreased in all controls and in 11 FM patients, but showed a marked increase in 6 FM patients. IL-6 significantly increased during TD in the patients, but not in the controls. Pain perception was not affected in the FM patients. These data demonstrate an altered TRP metabolism in a subgroup of FM patients, where the TD seems to activate 5-HT metabolism and IL-6 production. Our findings may have diagnostic as well as therapeutic implications in the field of fibromyalgia.

Serotonergic signaling inhibits hyperalgesia induced by spinal cord damage

Although dysesthesia is one of the most serious problems in patients with spinal cord injury, most of them being unresponsive to conventional treatments. In this study, we established a rat thoracic spinal cord mild-compression model that revealed thermal hyperalgesia in the hind limb. The thoracic spinal cord was compressed gently, using a 20 g weight for 20 min. The withdrawal latency of the thermal stimulation of the bilateral hind-limb was monitored using Hargreaves' Plantar test apparatus. In this model, thermal-hyperalgesia was observed for 1 week after the injury. The spinal cord injury-induced thermal-hyperalgesia was mimicked by the intrathecal application of metergoline, a non-selective 5-HT antagonist, 1-(2-methoxyphenyl)-4-[4-(2-phthalimido) butyl]-piperazine hydrobromide (NAN190), a selective 5-HT1 antagonist, and 3-tropanyl-3,5-dichlorobenzoate (MDL72222), a selective 5-HT3 antagonist. Intraperitoneal application of fluvoxamine maleate, a selective serotonin reuptake inhibitor, reduced the intensity of hyperalgesia induced by spinal cord injury. The inhibitory effect of fluvoxamine maleate on thermal hyperalgesia was prevented by the application of the aforementioned nonselective or selective 5-HT receptor antagonists. Intrathecal application of fluvoxamine maleate and selective 5-HT receptor agonists, i.e., 8-hydroxy-2-(di-n-proplyamino)-tetralin hydrobromide (8-OH-DPAT: 5HT-1 selective) and 2-methyl-5-hydroxytryptamine maleate (2-m-5-HT: 5HT-3 selective), inhibited the spinal cord injury-induced hyperalgesia. These results suggest that the change in the descending serotonergic signal plays an important role in hyperalgesia after the spinal cord injury, and that the application of selective serotonin reuptake inhibitors will be one of the candidates for new therapeutic methods against post-spinal cord injury dysesthesia.

FR143166 attenuates spinal pain transmission through activation of the serotonergic system

We investigated the antinociceptive effect of 1-(4-fluorophenyl)-3-methyl-5-[4-(methylsulfinyl)phenyl]pyrazole (FR143166) in the tail-pinch test in mice. The p.o. and i.t. injection of FR143166 exerted dose-dependent antinociceptive actions with ED(50) values of 24 mg/kg and 15 micro g/mouse, respectively. However, i.c.v. injection of FR143166 at a maximum dose of 128 micro g/mouse did not show any antinociceptive effect. The antinociceptive effect of FR143166 injected i.t. was abolished by co-administration of the nonselective serotonin (5-hydroxytryptamine, 5-HT) receptor antagonist, methysergide, but not by the adrenoceptor antagonists, phentolamine and propranolol. Moreover, the effect of FR143166 was also reversed by the 5-HT(2A) receptor antagonist, ketanserin, and the 5-HT(3) receptor antagonist, MDL-72222 (3-tropanyl-3,5-dichlorobenzoate). The effect of FR143166 was attenuated by p-chlorophenylalanine, but not by 6-hydroxydopamine plus nomifensine pretreatment. These results suggest that the descending serotonergic system, especially spinal 5-HT(2A) and 5-HT(3) receptors, is involved in the antinociceptive activity of spinally administered FR143166 on noxious mechanical stimuli.

Expression of 5-HT1A receptor mRNA in rat lumbar spinal dorsal horn neurons after peripheral inflammation

The present study observed the expression of the 5-hydroxytryptamine (5-HT) (1A) receptor mRNA in the lumbar spinal dorsal horn neurons following carrageenan inflammation using in situ hybridization (ISH). We also studied the co-localization of 5-HT(1A) receptor mRNA and gamma-amino butyric acid (GABA) or enkephalin (ENK) immunoreactivities using a combined fluorescent ISH and immunofluorescent histochemical double-staining technique. The finding of this study demonstrated that 5-HT(1A) receptor mRNA was widely distributed in the spinal dorsal horn with the highest density in laminae III-VI. Following carrageenan-induced inflammation, the 5-HT(1A) receptor mRNA expression in all layers of ipsilateral dorsal horn was significantly enhanced, and the peak occurred after 8h. Furthermore, the number of 5-HT(1A) receptor mRNA and GABA or ENK immunoreactive double-labeled cells was also markedly increased 8h after carrageenan injection. These findings suggested that following peripheral inflammation, the synthesis of 5-HT(1A) receptor was increased in the lumbar spinal dorsal horn neurons, especially in spinal GABA and ENK neurons.

2002 Wolff Award. 5 -HT2A receptor activation and nitric oxide synthesis: a possible mechanism determining migraine attacks

OBJECTIVE

To determine the effect of the 5-HT2A receptor in control of spinal nociception, cerebral circulation, and nitric oxide synthase (nNOS) expression in trigeminovascular neurons.

BACKGROUND

The plasticity of the 5-HT2A receptor is a possible factor determining the course of migraine. Up-regulation of this receptor has been demonstrated to correlate with the increasing frequency of migraine attacks and may underlie the development of chronic daily headache.

METHODS

Adult male Wistar rats were divided into groups receiving the 5-HT2A agonist, 1,2,5-dimethoxy-4-iodophenyl-2-aminopropane (DOI), nitroglycerin, or normal saline. The tail flick test and chemical nociception-evoked Fos-expression in dorsal horn neurons were used as indicators of nociception. Regional cerebral blood flow was monitored using laser Doppler flowmetry. Expression of Fos and nNOS was studied using immunohistochemical method.

RESULTS

Administration of DOI led to the shortening of tail flick latency (1.3 +/- 0.2 and 7.2 +/- 0.6 seconds for DOI-treated and control groups, respectively). The number of Fos-immunoreactive neurons was also greater in the DOI-treated group compared with the control group. DOI also produced long-lasting cerebral hyperemia (123% of baseline value) associated with the enlargement of perivascular nNOS-immunoreactive nerve fibers and increased nNOS-immunoreactive neurons in trigeminal ganglia and trigeminal nucleus caudalis. These findings resembled those observed in the rats exposed to nitroglycerin.

CONCLUSION

Our results suggest that activation of the 5-HT2A receptor leads to an enhancement of NO production in trigeminovascular pathway. NO may trigger migraine attacks by inducing cerebral vasodilation and sensitizing the perivascular nociceptors and central nociceptive neurons in trigeminovascular system. Up-regulation of this pronociceptive receptor can increase headache attacks and contributes to the development of chronic daily headache.

Chronic daily headache: a scientist's perspective

Certain features of chronic daily headache, namely, increased headache frequency, expansion of headache area, and cutaneous allodynia, may imply sensitization of central nociceptive neurons in the trigeminal pathway. Repetitive activation of the trigeminal nerve can lead to a biologic and functional change in trigeminal nucleus caudalis neurons, characterized by a decrease in nociceptive threshold and receptive field expansion. Suppression of the endogenous pain control system can facilitate the process of central sensitization. Evidence of such suppression in patients with chronic daily headache includes decreased platelet serotonin, up-regulation of 5-HT2A receptors, increased platelet nitric oxide production, and increased levels of substance P and nerve growth factor in the cerebrospinal fluid. Results from a number of animal experiments have indicated that chronic analgesic exposure leads to changes in serotonin content and density of 5-HT2A receptors in the central nervous system. This plasticity of the serotonin-dependent pain control system may accelerate the process of sensitization; a biologic outcome that is expressed clinically by the development of chronic daily headache associated with analgesic overuse.

Pathophysiology of chronic daily headache

Despite no clear explanation of the mechanism underlying chronic daily headache, sensitization of central nociceptive neurons is one possibility. Either prolonged activation of peripheral nociceptors or any factors that can alter the endogenous pain control system can trigger this process. A decrease in platelet serotonin has been observed in patients with chronic tension-type headache as well as migraine patients with medication-induced headache. It was also shown that chronic analgesic exposure led to changes in the serotonin content and the density of the 5-HT(2A) receptor in the cerebral cortex. The plasticity of the serotonin-dependent pain control system may facilitate the process of sensitization and results in the development of chronic daily headache.

The role of 5-hydroxytryptamine1A and 5-hydroxytryptamine1B receptors in modulating spinal nociceptive transmission in normal and carrageenan-injected rats

Single unit extracellular recordings from the dorsal horn neurons were obtained with glass micropipettes in pentobarbital-anesthetized rats. A total of 115 wide dynamic range (WDR) neurons were studied in 94 rats. In normal rats, the size of nociceptive receptive fields (RFs) of WDR neurons was approximately 123.3 +/- 8.21 mm2 (n = 88). Following carrageenan-induced inflammation, the RFs were markedly enlarged (332.4 +/- 30.1 mm2, n = 27, P < 0.001). The frequency of background activity of the WDR neurons in carrageenan-injected rats (11.3 +/- 2.1 imp/s, n = 27) was greater than that in normal rats (7.1 +/- 0.8 imp/s, n = 88, P < 0.05). In 82% of WDR neurons in normal rats, there was a separation between the A- and C-responses. In contrast, in 67% of the neurons in carrageenan-injected rats, the response to suprathreshold electrical stimuli was a long train with no separation between the A- and C-responses. In carrageenan-injected rats, the magnitude and duration of the nociceptive responses were significantly increased compared to those in normal rats, and the average C-response threshold (7.7 +/- 1.1 mA, n = 27) was lower than that in normal rats (10.4 +/- 0.7 mA, n = 88, P < 0.05). Intrathecal injection of the 5-hydroxytryptamine(1A) (5-HT1A) receptor agonist 8-hydroxy-DPAT hydroxybromide (8-OH-DPAT) (0.305, 1.525, 3.05, and 15.25 mM) dose-dependently increased Adelta- and C-responses and post-discharge in most of the WDR neurons. Following carrageenan-induced inflammation, the 8-OH-DPAT-induced facilitatory effect on Adelta- and C-responses and post-discharge was significantly enhanced (P < 0.05). Intrathecal injection of the 5-hydroxytryptamine1B (5-HT1) receptor agonist CGS12066A (0.222, 1.11, 2.22, and 11.1 mM) dose-dependently enhanced the C-response and post-discharge without influencing the Adelta-response. In carrageenan-injected rats, CGS12066A not only enhanced the facilitatory effect on the C-response and post-discharge, but also facilitated the Adelta-response. Intrathecal injection of the 5-HT(1A) receptor antagonist NAN-190 (0.2 mM) alone did not influence Adelta- and C-responses and post-discharge of WDR neurons in normal rats. When 0.2 mM NAN-190 was co-administered with 3.05 mM 8-OH-DPAT, the facilitatory effect of 8-OH-DPAT on Adelta- and C-responses and post-discharge was completely antagonized, whereas CGS12066A-induced facilitation on the C-response and post-discharge was not influenced by co-administration of 0.2 mM NAN-190 and CGS12066A. These data suggest that 5-HT1A and 5-HT1B receptor subtypes mediate the facilitatory effect of 5-HT on nociceptive processing in the spinal cord of rats. The excitability of dorsal horn WDR neurons and the sensitivity of the neurons to intrathecal 5-HT1A and 5-HT1B receptor agonists might increase following carrageenan-induced inflammation.

Effect of intrathecal administration of serotonin in chronic pain models in rats

The present study examined the effects of intrathecal (i.t.) administration of 5-hydroxytryptamine (5-HT; 0.1-100 microg) on mechanical hyperalgesia associated with neuropathic pain (chronic constriction of the sciatic nerve model and diabetic model) and inflammatory pain (carrageenan and polyarthritic models) in rats. Results demonstrated that the hyperalgesia observed in the mononeuropathic and diabetic rats was attenuated by 5-HT; the active dose, however, was 100- to 1000-fold higher than that required in normal rats, and was moderately effective. In the two experimental models of inflammatory pain, 5-HT was not markedly or similarly active. In the carrageenan model, 5-HT at the highest dose was only weakly effective whereas in the polyarthritic model it was inactive. Together, these results show that 5-HT has antinociceptive effects in several rat pain models, except in the model of diffuse pain (polyarthritic rats). Its antinociceptive effects in these models, however, are slight and differ from those observed in normal rats.

Effect of chronic analgesic exposure on the central serotonin system: a possible mechanism of analgesic abuse headache

OBJECTIVE

To investigate the effects of chronic analgesic exposure on the central serotonin system and the relationship between the serotonin system and the analgesic efficacy of nonnarcotic analgesics.

METHODS

Paracetamol was administered daily to adult male Wistar rats for a period of 15 or 30 days. Analgesic efficacy was measured by the tail flick test. After completion of the treatment protocol, the rats were humanely killed, and the frontal cortex and brain stem were isolated. Characteristics of the specific binding of the 5-HT2A serotonin receptor and the serotonin transporter were studied using a radioligand binding technique. Platelet serotonin was determined by high-performance liquid chromatography.

RESULTS

Chronic paracetamol administration resulted in a significant decrease in the maximum number of 5-HT2A binding sites and an increase in the maximum number of 5-HT transporter binding sites in frontal cortical membrane (P<.001). Changes in the central 5-HT system were associated with a rise in platelet 5-HT levels. The degree of receptor downregulation, as well as transporter upregulation, became less evident after more prolonged drug administration. Readaptation of serotonin receptors and transporters coincided with the decrease in the analgesic efficacy of paracetamol, as well as a fall in platelet 5-HT levels.

CONCLUSIONS

These findings provide further evidence in support of an involvement of the 5-HT system in the antinociceptive activity of simple nonnarcotic analgesics. Plasticity of this neurotransmitter system after chronic analgesic exposure may lead to the loss of analgesic efficacy and, in its more extreme form, may produce analgesic-related painful conditions, for example, analgesic abuse headache.

Acetaminophen-induced antinociception via central 5-HT(2A) receptors

Acetaminophen is one of the most widely used analgesic drugs. Although the mechanism of analgesic action of acetaminophen is still not known, the involvement of the central serotonin (5-hydroxytryptamine: 5-HT) system is one possibility. In the present study, we examined the antinociceptive effect of acute and chronic intraperitoneally (i.p.) administered acetaminophen by tail flick latency measurements in the rat. A significantly increased tail flick latency was observed in acute and 15-day acetaminophen-treated rats, but not in 30-day acetaminophen-treated rats, at a dose of 400 mg/kg/day. To investigate the plasticity of receptors at postsynaptic membrane, we conducted a series of experiments by radioligand binding method on frontal cortex and brainstem membrane. The technique involved radioligand binding with [phenyl-4-3H]spiperone and ketanserin for studying 5-HT(2A) receptor characteristics. A significant decrease in the maximum number of 5-HT(2A) binding sites (Bmax) was demonstrated in all treatment groups with acetaminophen 300 and 400 mg/kg on frontal cortex membrane, whereas the value of the dissociation equilibrium constant (Kd) remained unchanged. The down-regulation of 5-HT(2A) binding sites in frontal cortex was of a lesser magnitude after 30 days of treatment and the tail flick latency was as in the control animals. These results suggest that down-regulation of 5-HT(2A) receptor in response to 5-HT release is a major step in the mechanism underlying analgesia produced by this agent. On the contrary, chronic use of acetaminophen may result in 5-HT depletion, which in turn produces re-adaptation of postsynaptic 5-HT(2A) receptors. These data provide further evidence for a central 5-HT-dependent antinociceptive effect of acetaminophen.

Relationship of substance P, 5-hydroxyindole acetic acid and tryptophan in serum of fibromyalgia patients

The serotonergic system has repeatedly been discussed to be involved in the pathophysiology of fibromyalgia (FM), which is a syndrome of widespread pain and sleep disturbance. Elevated levels of substance P (SP), a mediator of nociception, have been described in FM. In this study the possible relationship between SP and serotonin (5-HT) together with its precursor tryptophan (TRP) and its metabolite 5-hydroxyindoleacetic acid (5-HIAA) was evaluated in 51 serum samples of fibromyalgia patients. These parameters were compared with clinical data such as pain intensity or sleep quality. A strong negative correlation between SP and 5-HIAA (P = .000) as well as between SP and TRP (P = .009) could be demonstrated. High serum concentrations of 5-HIAA and TRP showed a significant relation to low pain scores (5-HIAA: P = .030; TRP: P = .014). Moreover, 5-HIAA was strongly related to good quality of sleep (P = .000), while SP was related to sleep disturbance (P = .005). These data are valid to support the hypothesis of a systemic involvement of 5-HT and SP in fibromyalgia.

Advances in fibromyalgia: possible role for central neurochemicals

The neurophysiologic term allodynia has been applied to fibromyalgia because people with that disorder experience pain from pressure stimuli which are not normally painful. The nociceptive neurotransmitters of animal studies are now relevant to this human model of chronic, widespread pain. Evidence is presented to implicate several chemical pain mediators (including serotonin, substance P, nerve growth factor, and dynorphin A) in the pathogenesis of fibromyalgia. This perception is hopeful because it offers many new options for the development of innovative therapy.

Serotonin receptor adaptation in patients with analgesic-induced headache

Analgesic abuse has recently been recognized as a cause of deterioration in primary headache patients. Although the pathogenesis of this headache transformation is still obscure, alteration of serotonin receptor function is one possible mechanism. To assess the plasticity of 5HT2 serotonin receptors in this condition, we investigated receptor binding by the platelet membrane in patients with analgesic-induced headache (AIH), migraine and non-headache controls. The technique involved radioligand binding with (phenyl-4-3H)spiperone and ketanserin. A greater density of receptor numbers (Bmax) was found in patients with AIH and in non-headache controls (96.47 +/- 10.21 and 92.01 +/- 13.15 fmol/mg protein), as compared to migraine patients (49.52 +/- 5.14 fmol/mg protein). The value of dissociation equilibrium constant (KD) remained unchanged (3.07 +/- 0.49, 2.24 +/- 0.24 and 2.91 +/- 0.42 nM for patients with AIH, migraine and non-headache controls, respectively). Based on these findings, we suggest that up-regulation of 5HT2 serotonin receptors may be a possible mechanism of headache transformation in patients with AIH.

Involvement of opioid receptors in the antinociception produced by intracerebroventricularly administered spantide in mice

The antinociceptive effect of intracerebroventricularly (i.c.v.) administered [D-Arg1, D-Trp7,9, Leu11]-substance P (spantide), a non-selective tachykinin antagonist, was examined using the mouse formalin test. Licking behaviour induced by 2% formalin solution in the hindpaw of mice had two peaks, 0-5 min (first phase) and 10-30 min (second phase). I.c.v. spantide produced a dose-dependent antinociception during the first and second phases. The ID50 values were 2.95 (1.59-5.46) nmol for the first phase and 2.87 (1.49-5.52) nmol for the second phase. The antinociceptive effect in the first phase, but not in the second phase produced by spantide was antagonized by pretreatment with naloxone (1.0 mg/kg, i.p.), an opioid receptor antagonist. An opioid binding study using [3H]naloxone revealed that spantide was able to inhibit [3H]naloxone binding to mouse brain membrane preparations. These results suggest that opioid receptor systems in the mouse brain are involved in spantide-induced antinociception during the first phase, but not during the second phase of the formalin-induced nociceptive behaviour.

BDNF produces analgesia in the formalin test and modifies neuropeptide levels in rat brain and spinal cord areas associated with nociception

Previous studies have demonstrated an antinociceptive effect of brain-derived neurotrophic factor (BDNF) following infusion into the midbrain, near the periaqueductal grey and dorsal raphe nuclei. BDNF administration attenuated the behavioural response in the tail-flick and hot-plate tests, two models employing a phasic, thermal high-intensity nociceptive stimulus; the present studies extend our previous findings to include a model of moderate, continuous pain resulting from a chemical stimulus, the formalin test. Midbrain infusion of BDNF decreased the behavioural paw flinch response to subcutaneous formalin injection in both the early and late phases of the test. As our previous studies showed that BDNF-induced analgesia was reversible by naloxone, we have examined the effects of BDNF administration on brain and spinal cord levels of neuropeptides involved in the modulation of nociceptive information, including the endogenous opioid peptides, met-enkephalin and beta-endorphin, as well as substance P and neuropeptide Y (NPY). At the site of infusion, within the PAG and dorsal raphe, BDNF increased the level of beta-endorphin by 63%, but had no effect on substance P, metenkephalin or NPY levels. In the dorsal spinal cord, substance P (113% increase), beta-endorphin (97% increase) and NPY (64% increase) were elevated, although ventral spinal cord levels of these peptides remained unchanged. These studies demonstrate a modulatory effect of BDNF on relevant neuropeptides within areas of the brain and spinal cord involved in the processing of nociceptive information.

Illness-induced hyperalgesia is mediated by spinal neuropeptides and excitatory amino acids

The spinal cord dorsal horn contains neural mechanisms which can greatly facilitate pain. We have recently shown that 'illness'-inducing agents, such as intraperitoneally administered lipopolysaccharide (LPS; bacterial endotoxin), can produce prolonged hyperalgesia. This hyperalgesic state is mediated at the level of the spinal cord via activation of the NMDA-nitric oxide cascade. However, prolonged neuronal depolarization is required before such a cascade can occur. The present series of experiments were aimed at identifying spinal neurotransmitters which might be responsible for creating such a depolarized state. These studies show that LPS hyperalgesia is mediated at the level of the spinal cord by substance P, cholecystokinin and excitatory amino acids acting at non-NMDA sites. No apparent role for serotonin or kappa opiate receptors was found.

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)

The role of 5-hydroxytryptamine (5-HT) receptor subtypes and plasticity in the 5-HT systems in the regulation of nociceptive sensitivity

This review shows that the role of 5-hydroxytryptamine (5-HT) in the regulation of nociception depends on the 5-HT receptor subtypes involved and on long-term functional changes in the 5-HT receptors. Stimulation of the 5-HT1 receptors, as well as of the 5-HT2 and 5-HT3 receptors, may reduce nociceptive sensitivity. In addition, activation of 5-HT2 and 5-HT3 receptors may also enhance nociceptive sensitivity. Up- or down-regulation of the 5-HT receptors may result in long-lasting changes, plasticity, in the 5-HT systems. Lesioning of 5-HT neurons induces denervation supersensitivity to 5-HT, and prolonged stimulation of 5-HT receptors may produce subsensitivity to 5-HT. In the spinal cord denervation supersensitivity to 5-HT may depend on reduced release of substance P (SP). An increase in the release of SP, on the other hand, may reduce the effects of 5-HT receptor activation. Long-term treatment with antidepressants which are used in clinical pain therapy appears to up-regulate the 5-HT1 receptors and to down-regulate the 5-HT2 receptors.

Regional distribution of serotonin and substance P co-existing in nerve fibers and terminals in the brainstem of the rat

Two-color fluorescence immunohistochemistry was used to identify and map the distribution of nerve processes immunoreactive for both serotonin and substance P in the rat brainstem. Doubly labeled fibers were observed throughout the brainstem, but tended to be densest in cranial nerve motor nuclei and in reticular regions of the ventral medulla. In the trigeminal motor nucleus, the facial nucleus and the spinal accessory nucleus, the majority of serotonergic varicosities also appeared to contain substance P; in the occulomotor nucleus and the hypoglossal nucleus the numbers of double-labeled and single-labeled serotonergic varicosities were roughly equal. Thus, co-existence of substance P with serotonin was common in many cell groups innervating skeletal muscle. The proportion of double-labeled varicosities was significantly lower in the nucleus of the solitary tract, wherein single-labeled varicosities were much more common. Double-labeled fibers and varicosities were also significantly less common in the spinal trigeminal nucleus. In addition, double-labeling appeared to be uncommon in regions involved in the processing of special sensory information (e.g. auditory, vestibular and visual pathways). These results demonstrate a subpopulation of serotonergic fibers immunoreactive for substance P in the brainstem of the rat. The consistently high density of double-labeled processes in cranial nerve motor nuclei suggests that, as may be the case in the spinal cord, neurons containing serotonin and substance P regulate the activity of motoneurons that innervate skeletal muscle. In addition, they may be involved in other aspects of the function of the brainstem.

Naloxone-reversible effect of spantide on the spinally mediated behavioural response induced by neurokinin-2 and -3 receptor agonists

[D-Arg1, D-Trp7,9, Leu11]-substance P (spantide) was tested for antagonism against the licking, biting and scratching response induced by various neurokinin (NK) receptor agonists and bombesin (Bom) in mice. When co-administered with substance P (SP) intrathecally, spantide reduced the SP-induced behavioural responses in a dose-dependent manner. The duration of this antagonistic effect was approximately 30 min. Behavioural responses induced by physalaemin (Phy), [pGlu6, L-Pro9]-SP (6-11) (septide), [pGlu6, D-Pro7]-SP (6-11) (D-septide) and eledoisin (Ele) were also dose-dependently decreased by relatively small doses of spantide. Higher doses of spantide were needed to reduce the behavioural responses induced by [Sar9, Met (O2)11]-SP, neurokinin A (NK A) and neurokinin B (NK B). No significant effect of spantide was observed against the behavioural responses elicited by Bom. Pretreatment with naloxone, an opioid antagonist, resulted in a reversible effect on the behavioural reduction of NK-2 and NK-3 receptor agonists produced by spantide. However, the effect of spantide on the NK-1 receptor agonist-induced response was unchanged by naloxone. In homogenates of mouse spinal cord, competition studies confirmed that the binding of the opioid ligand [3H]naloxone was displaced by spantide with a low but measurable affinity. These results suggest that the behavioural response to NK-2 and NK-3 receptor agonists may be partially inhibited by spantide through the activation of opioid system in the mouse spinal cord.