Márcio M. Coelho

B vitamins induce an antinociceptive effect in the acetic acid and formaldehyde models of nociception in mice.

The effect of some B vitamins in chemical and thermal models of nociception in mice was investigated. The association thiamine/pyridoxine/cyanocobalamin (TPC, 20-200 mg/kg, i.p. or per os), thiamine, pyridoxine (50-200 mg/kg, i.p.) or riboflavin (3-100 mg/kg, i.p) induced an antinociceptive effect, not changed by naloxone (10 mg/kg, i.p.), in the acetic acid writhing model. Treatment for 7 days with thiamine/pyridoxine/cyanocobalamin (100 or 200 mg/kg, i.p.), thiamine (50 or 100 mg/kg) or pyridoxine (50 or 100 mg/kg) or acute treatment with riboflavin (6 or 12 mg/kg, i.p) inhibited the nociceptive response induced by formaldehyde. The B vitamins did not inhibit the nociceptive response in the hot-plate model. Both 7-day thiamine/pyridoxine/cyanocobalamin (100 mg/kg, i.p.) or acute riboflavin (25 or 50 mg/kg, i.p.) treatment partially reduced formaldehyde-induced hindpaw oedema. The B vitamins antinociceptive effect may involve inhibition of the synthesis and/or action of inflammatory mediators since it was not observed in the hot-plate model, was not reversed by naloxone, only the second phase of the formaldehyde-induced nociceptive response was inhibited, and formaldehyde-induced hindpaw oedema was reduced.

In vivo evidence for a role of protein kinase C in peripheral nociceptive processing

The present study was designed to characterize the nociceptive response induced by protein kinase C (PKC) peripheral activation and to investigate if this biochemical event is important for the nociceptive response induced by formaldehyde, and bradykinin (BK). Intraplantar injection of phorbol‐12,13‐didecanoate (PDD; 0.01, 0.1 or 1 μg), a PKC activator, but not of 4α‐PDD (inactive analogue), dose‐dependently induced thermal hyperalgesia in rats. This response was not observed at the contralateral hindpaw. Intraplantar injection of PDD (0.01, 0.1 or 1 μg) also induced mechanical allodynia. In mice, injection of PDD (0.1 or 1 μg) into the dorsum of the hindpaw induced a spontaneous licking behaviour. Intraplantar co‐injection of chelerythrine (10 or 50 μg), a PKC inhibitor, attenuated the thermal hyperalgesia induced by PDD (0.1 μg) in rats. The second phase of the nociceptive response induced by the injection of formaldehyde (0.92%, 20 μl) into the dorsum of mice hindpaws was inhibited by ipsi‐, but not contralateral, pre‐treatment with chelerythrine (1 μg). Intraplantar injection of BK (10 μg) induced mechanical allodynia in rats. Ipsi‐ but not contralateral injection of bisindolylmaleimide I (10 μg), a PKC inhibitor, inhibited BK‐induced mechanical allodynia. In conclusion, this study demonstrates that PKC activation at peripheral tissues leads to the development of spontaneous nociceptive response, thermal hyperalgesia and mechanical allodynia. Most importantly, it also gives in vivo evidence that peripheral PKC activation is essential for the full establishment of the nociceptive response induced by two different inflammatory stimuli.

Drug Repositioning: Playing Dirty to Kill Pain

The number of approved new molecular entity drugs has been decreasing as the pharmaceutical company investment in research and development is increasing. As we face this painful crisis, called an innovation gap, there is increasing awareness that development of new uses of existing drugs may be a powerful tool to help overcome this obstacle because it takes too long, costs too much and can be risky to release drugs developed de novo. Consequently, drug repositioning is emerging in different therapeutic areas, including the pain research area. Worldwide, pain is the main reason for seeking healthcare, and pain relief represents an unmet global clinical need. Therefore, development of analgesics with better efficacy, safety and cost effectiveness is of paramount importance. Despite the remarkable advancement in research on cellular and molecular mechanisms underlying pain pathophysiology over the past three decades, target-based therapeutic opportunities have not been pursued to the same extent. Phenotypic screening remains a more powerful tool for drug development than target-based screening so far. It sounds somewhat heretical, but some multi-action drugs, rather than very selective ones, have been developed intentionally. In the present review, we first critically discuss the utility of drug repositioning for analgesic drug development and then show examples of ‘old’ drugs that have been successfully repositioned or that are under investigation for their analgesic actions. We conclude that drug repositioning should be more strongly encouraged to help build a bridge between basic research and pain relief worldwide.

Tetracyclines and pain.

Tetracyclines are natural or semi-synthetic bacteriostatic agents which have been used since late 1940s against a wide range of gram-positive and gram-negative bacteria and atypical organisms such as chlamydia, mycoplasmas, rickettsia, and protozoan parasites. After the discovery of the first tetracyclines, a second generation of compounds was sought in order to improve water solubility for parenteral administration or to enhance bioavailability after oral administration. This approach resulted in the development of doxycycline and minocycline in the 1970s. Doxycycline was included in the World Health Organization Model List of Essential Medicines either as antibacterial or to prevent malaria or to treat patients with this disease. Additional development led to the third generation of tetracyclines, being tigecycline the only medicine of this class to date. Besides antibacterial activities, the anti-inflammatory, antihypernociceptive and neuroprotective activities of tetracyclines began to be widely studied in the late 1990s. Indeed, there has been an increasing interest in investigating the effects induced by minocycline as this liposoluble derivative is known to cross the blood–brain barrier to the greatest extent. Minocycline induces antihypernociceptive effects in a wide range of animal models of nociceptive, inflammatory and neuropathic pain. In this study, we discuss the antihypernociceptive activity of tetracyclines and summarise its underlying cellular and molecular mechanisms.

Anti-inflammatory, antinociceptive and ulcerogenic activity of a zinc-diclofenac complex in rats

We investigated the anti-inflammatory, antinociceptive and ulcerogenic activity of a zinc-diclofenac complex (5.5 or 11 mg/kg) in male Wistar rats (180-300 g, N = 6) and compared it to free diclofenac (5 or 10 mg/kg) and to the combination of diclofenac (5 or 10 mg/kg) and zinc acetate (1.68 or 3.5 mg/kg). The carrageenin-induced paw edema and the cotton pellet-induced granulomatous tissue formation models were used to assess the anti-inflammatory activity, and the Hargreaves model of thermal hyperalgesia was used to assess the antinociceptive activity. To investigate the effect of orally or intraperitoneally (ip) administered drugs on cold-induced gastric lesions, single doses were administered before exposing the animals to a freezer (-18 degrees C) for 45 min in individual cages. We also evaluated the gastric lesions induced by multiple doses of the drugs. Diclofenac plus zinc complex had the same anti-inflammatory and antinociceptive effects as diclofenac alone. Gastric lesions induced by a single dose administered per os and ip were reduced in the group treated with zinc-diclofenac when compared to the groups treated with free diclofenac or diclofenac plus zinc acetate. In the multiple dose treatment, the complex induced a lower number of the most severe lesions when compared to free diclofenac and diclofenac plus zinc acetate. In conclusion, the present study demonstrates that the zinc-diclofenac complex may represent an important therapeutic alternative for the treatment of rheumatic and inflammatory conditions, as its use may be associated with a reduced incidence of gastric lesions.

Antinociceptive and anti-inflammatory activities of nicotinamide and its isomers in different experimental models

Although there is evidence for the anti-inflammatory activity of nicotinamide, there is no evaluation of its effects in models of nociceptive and inflammatory pain. In addition, there is no information about the potential anti-inflammatory and antinociceptive activities of the nicotinamide isomers, picolinamide and isonicotinamide. Per os (p.o.) administration of nicotinamide (1000 mg/kg, -1h) inhibited the first and second phases of the nociceptive response induced by formalin in mice. In the model of nociceptive pain, exposure of mice to a hot-plate (50°C), nicotinamide (1000 mg/kg, -1h) also presented antinociceptive activity. Nicotinamide (500 mg/kg, -1 and 3h) also inhibited the mechanical allodynia induced by carrageenan in rats, a model of inflammatory pain. In addition to inhibiting the nociceptive response, nicotinamide (500 or 1000 mg/kg, -1 and 3h) inhibited the paw edema induced by carrageenan in mice and rats. P.o. administration of picolinamide (125 mg/kg, -1h) and isonicotinamide (500 or 1000 mg/kg, -1h) inhibited the second phase of the nociceptive response induced by formalin in mice. The paw edema induced by carrageenan in mice was also inhibited by isonicotinamide (500 or 1000 mg/kg, -1h) and picolinamide (125 mg/kg, -1h and 3h). The results represent the first demonstration of the activity of nicotinamide and its isomers in models of nociceptive and inflammatory pain and provide support to their anti-inflammatory activity. The demonstration of new activities for nicotinamide is important as it may contribute to expand its use in the treatment of other pathological conditions.

Antiedematogenic activity and phytochemical composition of preparations from Echinodorus grandiflorus leaves

The leaves of Echinodorus grandiflorus (Alismataceae) are traditionally used in Brazil to treat inflammatory conditions. The aim of the present study was to evaluate the antidematogenic activity of crude aqueous, dichloromethane and hydroethanolic extracts from E. grandiflorus leaves using the carrageenan-induced paw edema model in mice, along with of fractions enriched in diterpenes, flavonoids and hydroxycinnamoyltartaric acids (HCTA). Significant inhibitions of paw edema were elicited by the 50% and 70% EtOH extracts (1000 mg/kg, p.o.), as well as by the fractions enriched in diterpenes (70-420 mg/kg, p.o.) and flavonoids (7.2-36 mg/kg, p.o.). Isovitexin, isoorientin, trans-aconitic and chicoric acids were identified in all extracts by HPLC analysis. Trans-aconitic acid itself exhibited significant antiedematogenic effect (270 mg/kg, p.o.). The biological activity correlated positively with the contents of flavonoids and diterpenes, but negatively with HCTA concentrations, demonstrating the participation of the two classes of compounds in the antiedematogenic activity of E. grandiflorus.

A minocycline derivative reduces nerve injury-induced allodynia, LPS-induced prostaglandin E2 microglial production and signaling via toll-like receptors 2 and 4

Many studies have shown that minocycline, an antibacterial tetracycline, suppresses experimental pain. While minocyclines positive effects on pain resolution suggest that clinical use of such drugs may prove beneficial, minocyclines antibiotic actions and divalent cation (Ca(2+); Mg(2+)) chelating effects detract from its potential utility. Thus, we tested the antiallodynic effect induced by a non-antibacterial, non-chelating minocycline derivative in a model of neuropathic pain and performed an initial investigation of its anti-inflammatory effects in vitro. Intraperitoneal minocycline (100mg/kg) and 12S-hydroxy-1,12-pyrazolinominocycline (PMIN; 23.75 mg/kg, 47.50mg/kg or 95.00 mg/kg) reduce the mechanical allodynia induced by chronic constriction injury of mouse sciatic nerve. PMIN reduces the LPS-induced production of PGE2 by primary microglial cell cultures. Human embryonic kidney cells were transfected to express human toll-like receptors 2 and 4, and the signaling via both receptors stimulated with PAM3CSK4 or LPS (respectively) was affected either by minocycline or PMIN. Importantly, these treatments did not affect the cell viability, as assessed by MTT test. Altogether, these results reinforce the evidence that the anti-inflammatory and experimental pain suppressive effects induced by tetracyclines are neither necessarily linked to antibacterial nor to Ca(2+) chelating activities. This study supports the evaluation of the potential usefulness of PMIN in the management of neuropathic pain, as its lack of antibacterial and Ca(2+) chelating activities might confer greater safety over conventional tetracyclines.

Nicotinic acid induces antinociceptive and anti-inflammatory effects in different experimental models

Although in vitro studies have shown that nicotinic acid inhibits some aspects of the inflammatory response, a reduced number of in vivo studies have investigated this activity. To the best of our knowledge, the effects induced by nicotinic acid in models of nociceptive and inflammatory pain are not known. Per os (p.o.) administration of nicotinic acid (250, 500 or 1000 mg/kg, -1 h) inhibited the first and the second phases of the nociceptive response induced by formalin in mice. Nicotinic acid (250 or 500 mg/kg, -1 and 3 h) also inhibited the mechanical allodynia induced by carrageenan in rats, a model of inflammatory pain. However, in a model of nociceptive pain, exposure of mice to a hot-plate, nicotinic acid was devoid of activity. In addition to inhibiting the nociceptive response in models of inflammatory pain, nicotinic acid (250 or 500 mg/kg, p.o., -1 and 3 h) inhibited paw edema induced by carrageenan in mice and rats. Picolinic acid (62.5 or 125 mg/kg, p.o., -1 h), a nicotinic acid isomer, inhibited both phases of the nociceptive response induced by formalin, but not paw edema induced by carrageenan in mice. The other nicotinic acid isomer, isonicotinic acid, was devoid of activity in these two models. In conclusion, our results represent the first demonstration of the activity of nicotinic acid in experimental models of nociceptive and inflammatory pain and also provide further support to its anti-inflammatory activity. It is unlikely that conversion to nicotinamide represents an important mechanism to explain the antinociceptive and anti-inflammatory activities of nicotinic acid. The demonstration of new activities of nicotinic acid, a drug that has already been approved for clinical use and presents a positive safety record, may contribute to raise the interest in conducting clinical trials to investigate its usefulness in the treatment of painful and inflammatory diseases.

Peripheral 5-HT1B and 5-HT2A receptors mediate the nociceptive response induced by 5-hydroxytryptamine in mice

While the role of 5-hydroxytryptamine (5-HT, serotonin) in the nociceptive processing has been widely investigated in the central nervous system, information regarding its role in peripheral tissues is still lacking. Noteworthy, 5-HT induces phenotypic changes of nociceptors and peripheral injection induces pain in humans and nociceptive response in rodents. However, local receptors involved in 5-HT effects are not well characterized. Thus, we aimed to investigate the role of 5-HT and some of its receptors in the peripheral nociceptive processing in mice. Intraplantar injection of 5-HT (10, 20 or 40 μg) into the hind-paw of mice induced paw licking behavior, which was inhibited by previous intraplantar treatment with cyproheptadine (5-HT(1) and 5-HT(2) antagonist; 0.5 or 5 μg), mianserin (5-HT(2) and 5-HT(6) antagonist; 0.1 μg), isamoltane (5-HT(1B) antagonist; 0.5 or 5 μg) and ketanserin (5-HT(2A) antagonist; 0.1 or 1 μg), but not by BRL 15572 (5-HT(1D) antagonist; 1 or 10 μg), ondansetron (5-HT(3) antagonist; 1, 5, 10 or 20 μg) and SB 269970 (5-HT(7) antagonist; 2.5 and 25 μg). Altogether, these results indicate the local involvement of 5-HT(1), 5-HT(2) and 5-HT(6), especially 5-HT(1B) and 5-HT(2A), in the nociceptive response induced by 5-HT in mice, thus contributing to a better understanding of 5-HT role in the peripheral nociceptive processing. In addition, they also point to important species differences and the need of a wide evaluation of the peripheral nociceptive processing in mice as these animals have been increasingly used in studies investigating the cellular and molecular mechanisms mediating the nociceptive response.