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

Efficacy of Ultrasound-Guided Caudal Epidural Calcitonin for Patients with Failed Back Surgery Syndrome

Context:

Pain resulting from failed back surgery syndrome (FBSS) is generally resistant to physiotherapy and pharmacological treatment.

Objective:

The aim of this study is to evaluate the effect of adding calcitonin to local anesthetic and steroids during ultrasound-guided caudal epidural injection for patients suffering from FBSS.

Patients and Methods:

Fifty-six patients were randomly allocated into two equal groups. All patients underwent ultrasound-guided caudal epidural injection. Group A received 40 mg (1 mL) methylprednisolone +9 mL lidocaine 0.5% + 1500 iu hyaluronidase in 10 mL normal saline, whereas Group B received 40 mg (1 mL) methylprednisolone +9 mL of 0.5% lidocaine + 1500 iu hyaluronidase in 5 mL normal saline + 50 iu calcitonin in 5 mL normal saline.

Results:

A statistically significant decrease in visual analog scale, Oswestry Disability Index, and analgesic consumption was recorded in Group B as compared to Group A at 1, 2, and 3 months interval. No side effects were reported in Group A, whereas patients belong to Group B experienced nausea (2 cases), and diuresis for 24 h was detected in two cases.

Conclusion:

The addition of calcitonin to epidural steroid and local anesthetic injection resulted in better Oswestry disability scale, diminished pain intensity, and less analgesic consumption.

Mice Lacking Serotonin 2C Receptors Have increased Affective Responses to Aversive Stimuli

Although central serotonergic systems are known to influence responses to noxious stimuli, mechanisms underlying serotonergic modulation of pain responses are unclear. We proposed that serotonin 2C receptors (5-HT2CRs), which are expressed within brain regions implicated in sensory and affective responses to pain, contribute to the serotonergic modulation of pain responses. In mice constitutively lacking 5-HT2CRs (2CKO mice) we found normal baseline sensory responses to noxious thermal, mechanical and chemical stimuli. In contrast, 2CKO mice exhibited a selective enhancement of affect-related ultrasonic afterdischarge vocalizations in response to footshock. Enhanced affect-related responses to noxious stimuli were also exhibited by 2CKO mice in a fear-sensitized startle assay. The extent to which a brief series of unconditioned footshocks produced enhancement of acoustic startle responses was markedly increased in 2CKO mice. As mesolimbic dopamine pathways influence affective responses to noxious stimuli, and these pathways are disinhibited in 2CKO mice, we examined the sensitivity of footshock-induced enhancement of startle to dopamine receptor blockade. Systemic administration of the dopamine D2/D3 receptor antagonist raclopride selectively reduced footshock-induced enhancement of startle without influencing baseline acoustic startle responses. We propose that 5-HT2CRs regulate affective behavioral responses to unconditioned aversive stimuli through mechanisms involving the disinhibition of ascending dopaminergic pathways.

In vitro binding affinities of a series of flavonoids for μ-opioid receptors. Antinociceptive effect of the synthetic flavonoid 3,3-dibromoflavanone in mice

The pharmacotherapy for the treatment of pain is an active area of investigation. There are effective drugs to treat this problem, but there is also a need to find alternative treatments free of undesirable side effects. In the present work the capacity of a series of flavonoids to bind to the μ opioid receptor was evaluated. The most active compound, 3,3-dibromoflavanone (31), a synthetic flavonoid, presented a significant inhibition of the binding of the selective μ opioid ligand [(3)H]DAMGO, with a Ki of 0.846 ± 0.263 μM. Flavanone 31 was further synthesized using a simple and cheap procedure with good yield. Its in vivo effects in mice, after acute treatments, were studied using antinociceptive and behavioral assays. It showed no sedative, anxiolytic, motor incoordination effects or inhibition of the gastrointestinal transit in mice at the doses tested. It evidenced antinociceptive activity on the acetic acid-induced nociception, hot plate and formalin tests (at 10 mg/kg and 30 mg/kg). The results showed that the 5-HT2 receptor and the adrenoceptors seem unlikely to be involved in its antinociceptive effects. Naltrexone, a nonselective opioid receptors antagonist, totally blocked compound 31 antinociceptive effects on the hot plate test, but naltrindole (δ opioid antagonist) and nor-binaltorphimine (κ opioid antagonist) did not. These findings demonstrated that 3,3-dibromoflavanone (31), at doses that did not interfere with the motor performance, exerted clear dose dependent antinociception when assessed in the chemical and thermal models of nociception in mice and it seems that its action is related to the activation of the μ opioid receptor.

Calcitonin for Acute Neuropathic Pain Associated with Spinal Cord Injury

Neuropathic pain associated with spinal cord injury is caused by complex neural mechanisms and is often refractory to standard therapy. Salmon calcitonin was an effective treatment for neuropathic symptoms in this case series of three patients with recent spinal cord injury. Salmon calcitonin is already used to help manage pain after limb amputation and also after vertebral fractures and it is perhaps surprising that it has not been trialled previously for spinal cord injury pain. Calcitonin is thought to exert its effect by modulation of the serotonergic system and is generally well tolerated and convenient to administer. This underutilised drug may be a very useful adjuvant for neuropathic pain associated with spinal cord injury.

Salmon calcitonin in the treatment of post herpetic neuralgia

A 78-year-old man with severe chronic obstructive pulmonary disease presented to our pain medicine clinic for treatment of post herpetic neuralgia. Pharmacotherapy with tricyclic antidepressants, anticonvulsants, tramadol and traditional analgesics had failed, primarily due to adverse drug effects, particularly sedation, dizziness and nausea. Consequently, intravenous salmon calcitonin was administered, based on evidence of efficacy in the treatment of other neuropathic pain syndromes and its relatively benign side-effects profile. The patient reported immediate and sustained improvement in his post herpetic neuralgia for over two months, without adverse effects from the calcitonin therapy.

Possible involvement of opioidergic systems in the antinociceptive effect of the selective serotonin reuptake inhibitors in sciatic nerve-injured mice

The involvement of opioid receptor activation in the antinociceptive effect of either fluvoxamine, a selective serotonin reuptake inhibitor, or serotonin (5-HT) on thermal hyperalgesia and mechanical allodynia in a model of neuropathic pain in mice induced by sciatic nerve ligation was examined. The experiments were conducted 2 or 6 weeks after unilateral sciatic nerve ligation. Ipsilateral thermal hyperalgesia and mechanical allodynia were observed both 2 and 6 weeks after sciatic nerve ligation. Neither s.c. fluvoxamine nor i.t. 5-HT affected sciatic nerve ligation-induced thermal hyperalgesia or mechanical allodynia in mice 2 weeks after sciatic nerve ligation. However, the same dose of either fluvoxamine or 5-HT significantly reduced mechanical allodynia but not thermal hyperalgesia in sciatic nerve ligated mice 6 weeks after surgery. The antinociceptive effect of fluvoxamine on sciatic nerve ligation-induced mechanical allodynia in mice 6 weeks after surgery was completely abolished by pretreatment with either naloxone, a nonselective opioid receptor antagonist, or beta-funaltrexamine, a selective mu-opioid receptor antagonist. Furthermore, pretreatment with naltrindole, a selective delta-opioid receptor antagonist, partially but significantly inhibited the antinociceptive effect of fluvoxamine in sciatic nerve ligated mice at the 6th postoperative week. The antinociceptive effect induced by i.t. 5-HT was also completely antagonized by either naloxone or beta-funaltrexamine, and partially inhibited by naltrindole. However, pretreatment with nor-binaltorphimine, a selective kappa-opioid receptor antagonist, had no effect against either s.c. fluvoxamine- or i.t. 5-HT-induced antinociception. These results suggest that the antinociceptive effect of s.c. fluvoxamine or i.t. 5-HT in the chronic state of sciatic nerve ligation-induced neuropathic pain may be related to opioidergic activity, mainly through the activation of spinal mu- and delta-opioid receptors.

Role of the histamine system in nefopam-induced antinociception in mice

The present study explored the role of the histaminergic system in nefopam analgesia based on the structural relationship between nefopam and diphenhydramine. In vitro binding assays revealed that nefopam possesses moderate affinity for histamine H1 and H2 receptor subtypes, with IC50 of 0.8 and 6.9 microM, respectively, but no affinity for histamine H(3) receptor subtype until 100 microM. Subcutaneous nefopam administration dose-dependently inhibited pain in acetic acid-induced writhing (1-30 mg/kg) and formalin (1-10 mg/kg) tests in the mouse. Pretreatment with the histamine-depleting agent alpha-fluoromethylhistidine (alpha-FMH, 50 mg/kg), the histamine H1 receptor antagonist pyrilamine (3 or 10 mg/kg), or the histamine H2 receptor antagonists cimetidine (100 mg/kg) and zolantidine (10 or 30 mg/kg) did not significantly modify nefopam antinociception in both tests. The histamine H3 receptor agonist R(-)alpha-methylhistamine (RAMH, 10 mg/kg) did not significantly modify the nefopam analgesic activity in the writhing test. At 25 mg/kg, RAMH inhibited nefopam antinociception at 3 mg/kg, but not at 10 mg/kg in the formalin test. However, pretreatment with the histamine H3 receptor antagonist thioperamide (25 mg/kg) inhibited nefopam antinociception in the writhing test, but not in the formalin test. In conclusion, nefopam analgesic activity is not mediated by histamine H1 or H2 receptors, but can be slightly modulated by histamine H3 receptors in mouse pain tests.

Possible involvement of opioidergic and serotonergic mechanisms in antinociceptive effect of paroxetine in acute pain

Antidepressant drugs, especially tricyclics have been widely used in the treatment of chronic pain, but not in acute pain. Because of numerous undesirable side effects, the selective serotonin reuptake inhibitors (SSRIs), with their favorable side effect profile, are preferred nowadays. An activation of the endogenous opioid mechanisms or potentiation of the analgesic effect mediated by serotonergic and/or noradrenergic pathways are thought to be involved in the antinociceptive action of SSRIs. In this study, the potential antinociceptive effect of paroxetine and its interaction with opioidergic system and serotonin receptors were evaluated. The antinociceptive effect of paroxetine was tested using a hot plate test in mice. Paroxetine, a SSRI antidepressant drug, induced an antinociceptive effect following i.p. administration. This antinociception was significantly inhibited by naloxone, an opioid receptor antagonist, suggesting the involvement of opioidergic mechanisms. While ondansetron (a 5-HT(3)-receptor antagonist) inhibited the effect of paroxetine, ketanserin (a 5-HT(2)-receptor antagonist) could not. In conclusion, paroxetine-induced antinociception, similar to morphine, suggests an involvement of direct or indirect action (via an increase in release of endogenous opioid peptide(s)) at opioid receptor sites and an involvement of serotonergic mechanisms mainly at the receptor level.

Buspirone-induced antinociception is mediated by L-type calcium channels and calcium/caffeine-sensitive pools in mice

RATIONALE

Previous studies have shown that buspirone, a partial 5-HT(1A) receptor agonist, produces antinociceptive effects in rats and mice; Ca(2+) plays a critical role as a second messenger in mediating nociceptive transmission. 5-HT(1A) receptors have been proven to be coupled functionally with various types of Ca(2+) channels in neurons, including N-, P/Q-, T-, or L-type. It was of interest to investigate the involvement of extracellular/intracellular Ca(2+) in buspirone-induced antinociception.

OBJECTIVES

To determine whether central serotonergic pathways participate in the antinociceptive processes of buspirone, and investigate the involvement of Ca(2+) mechanisms, particularly L-voltage-gated Ca(2+) channels and Ca(2+)/caffeine-sensitive pools, in buspirone-induced antinociception.

METHODS

Antinociception was assessed using the hot-plate test (55 degrees C, hind-paw licking latency) in mice treated with either buspirone (1.25-20 mg/kg i.p.) alone or the combination of buspirone and fluoxetine (2.5-10 mg/kg i.p.), 5-HTP (25 mg/kg i.p.), nimodipine (2.5-10 mg/kg i.p.), nifedipine (2.5-10 mg/kg i.p.), CaCl(2) (25-200 nmol per mouse i.c.v.), EGTA (5-30 nmol per mouse i.c.v.), or ryanodine (0.25-2 nmol per mouse i.c.v.).

RESULTS

Buspirone dose dependently increased the licking latency in the hot-plate test in mice. This effect of buspirone was enhanced by fluoxetine, 5-HTP, nimodipine, and nifedipine. Interestingly, central administration of Ca(2+) reversed the antinociceptive effects of buspirone. In contrast to these, ryanodine or EGTA administered centrally potentiated buspirone-induced antinociception.

CONCLUSIONS

Decreasing neuronal Ca(2+) levels potentiated buspirone-induced antinociception; conversely, increasing intracellular Ca(2+) abolished the antinociceptive effects of buspirone. These results suggest that Ca(2+) influx from extracellular fluid and release of Ca(2+) from Ca(2+)/caffeine-sensitive microsomal pools may be involved in buspirone-induced antinociception.

Ontogeny of calcitonin receptor mRNA and protein in the developing central nervous system of the rat

In this study, the expression of receptors for calcitonin (CTR), the CTR C1a and C1b isoforms, was investigated during development of the fetal rat central nervous system (CNS) by using in situ hybridization and immunohistochemistry. Coincident expression with both techniques was evident. Immunohistochemical evidence for the expression of the C1a isoform alone was found. Expression was first observed at embryonic day 12/13 (E12/E13) within and adjacent to the ventricular zones known to include primary matrices of proliferation, in regions of the preoptic area, anterior and posterior hypothalamus, anterior and posterior pons, medulla, and spinal cord. At later times, with the decline in the density of immunoreactivity at these loci (E15), expression in primary matrices was found later at distinct loci within the ventricular zones of cerebellum (E17), and at E19, the tectum, lateral ventricle, and cortical subplate. By E19, the density of staining had increased and was widespread throughout the expanding CNS. In the rostral domains, moderate to high density was found in the external plexiform layer; the medial preoptic area and nucleus; the ventromedial, dorsomedial, and arcuate hypothalamic nuclei; and the lateral and posterior hypothalamic areas. In the midbrain, similar levels of expression were noted in the central nucleus of raphe; the deep mesencephalic, dorsal raphe, and laterodorsal tegmental nuclei; and the ventral periaqueductal gray. In the pons, positive loci included the locus coeruleus and the gigantocellular and pontine reticular nuclei. In the medulla, high expression was evident in the gigantocellular, intermediate, magnocellular, and medullary reticular, spinal trigeminal and cuneate nuclei; and the nucleus tractus solitarius. In the spinal cord, moderate to high density of staining was found in the ventral, dorsal, and lateral horns, and in the ventral, dorsal, and cuneate funiculi. On the other hand, transitory expression was found in the diagonal band, bed nucleus of the stria terminalis, amygdala, and the lateral mamillary and anterobasal nuclei of the hypothalamus. These studies indicate a role for CTR in the activation of some premigratory neuroblasts in the CNS as well as a possible role later in an undefined function associated with mature neurons of particular nuclei.

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.