The central analgesia induced by antimigraine drugs is independent from Gi proteins: superiority of a fixed combination of indomethacin, prochlorperazine and caffeine, compared to sumatriptan, in an in vivo model

Abstract

A hypofunctionality of Gi proteins has been found in migraine patients. The fixed combination of indomethacin, prochlorperazine and caffeine (Indoprocaf) is a drug of well-established use in the acute treatment of migraine and tension-type headache. The aim of this study was to investigate if Indoprocaf was able to exert its central antinociceptive action when Gi proteins activity is abolished by pertussis toxin (PTX), compared to its single active ingredients and to sumatriptan. The mice model of abdominal constriction test induced by an i.p. injection of a 0.6% solution of acetic acid was used. The study showed that Indoprocaf (a fixed combination of indomethacin 1 mg/kg, prochlorperazine 1 mg/kg and caffeine 3 mg/kg, s.c.) and sumatriptan (20 mg/kg, s.c.) exert their central antinociceptive action independently from the Gi proteins. In addition, the antinociceptive efficacy of Indoprocaf in this study was statistically superior to that of sumatriptan. This study also showed that the single active ingredients of Indoprocaf, indomethacin (1 mg/kg, s.c.), prochlorperazine (1 mg/kg, s.c.) and caffeine (3 mg/kg, s.c.), were able to exert their central antinociceptive action independently from the Gi proteins. However, Indoprocaf at analgesic doses was able to abolish almost completely the abdominal constrictions, with a statistically higher efficacy compared to the single active ingredients, showing an important synergic effect of Indoprocaf. This synergic effect was evident not only when Gi proteins activity was abolished by PTX, but also under control condition, when Gi proteins were active. This study suggests that the central antinociceptive action induced by antimigraine drugs is independent from Gi proteins.

Introduction

It is worldwide recognized that specific drugs are the most effective in the acute treatment of migraine, in particular in patients with moderate or severe pain [1]. The fixed combination of indomethacin, prochlorperazine and caffeine (hereinafter Indoprocaf) is a drug of well-established use in Italy, with a recognized efficacy and a good level of safety both in the acute treatment of migraine [2, 3] and tension-type headache [4].

In contrast to other non-steroidal anti-inflammatory drugs (NSAIDs), indomethacin has an indole nucleus which is similar to serotonin and to the 5HT_1B/1D agonists known as triptans. The efficacy of indomethacin in the treatment of migraine and other primary headaches might be due to its central analgesic activity [5], its ability to abolish peripheral and central hyperalgesia [6–8], its vasoconstrictive effect on cerebral vessels [9–11] and its ability to inhibit neurogenic inflammation [12]. The mechanism of action of prochlorperazine in migraine could be attributed to its central cholinergic analgesic effect [13] and to its ability to abolish hyperalgesia [6, 7]. Caffeine is used as an effective analgesic adjuvant in various analgesic combinations. It is a non-selective antagonist of adenosine A₁ and A₂ receptors [14]. Its positive effects in the symptomatic treatment of headache are attributed to an increased analgesic action of other drugs [14], a central cholinergic analgesic effect [15], its ability to abolish hyperalgesia [6] and a vasoconstrictive activity [16].

Indoprocaf, administered i.p. at sub-analgesic doses in mice, showed to be able to reverse hyperalgesia induced by morphine withdrawal and to be superior to each single active ingredient in abolishing hyperalgesia induced by a 0.3% solution of acetic acid i.p. injected [6]. Moreover, Indoprocaf, but not sumatriptan, administered i.p. at sub-analgesic doses in mice, was able to abolish peripheral and central sensitization [7].

Nearly all inhibitory neurotransmitters which are able to enhance the pain threshold use Gi proteins as signal transduction system. Gi proteins reduce neurones excitability through the inhibition of adenylate cyclase activity and modulation of several K⁺ and Ca²⁺ channels [17]. A hypofunctionality of Gi proteins has been found in lymphocytes of patients with migraine and cluster headache [17]. The intracerebroventricular administration of pertussis toxin (PTX) selectively inhibits Gi proteins and blocks the antinociceptive action of common analgesic drugs such as opioids (morphine), histamine H₁ receptor antagonists (diphenhydramine), GABA_B agonists (baclofen), tricyclic antidepressants (clomipramine), but not of cholinesterase inhibitors (physostigmine), in the mouse hot plate test [18, 19].

The aim of this study was to investigate if Indoprocaf was able to exert its central antinociceptive action when Gi proteins activity is inactivated by PTX, compared to its single active ingredients and to sumatriptan, in the mice model of abdominal constriction test induced by a 0.6% solution of acetic acid i.p. injected.

Methods

Animals

Male albino mice (20–25 g) from Morini breeding farm (San Polo d’Enza—Italy) were used. All experiments were carried out according to the guidelines of the European Community Council Directive dated November 24, 1986 (86/609/EEC) for experimental animal care. All efforts were made to minimize suffering and to reduce the number of animals used.

Pertussis toxin administration

Mice were randomly assigned to the intracerebroventricular administration of PTX (0.25 μg per mouse) or to a saline solution 9 days before the abdominal constriction test. Intracerebroventricular administration was performed under ether anaesthesia using isotonic saline as a solvent, according to the method of Haley and McCormick [20]. During anaesthesia, mice were grasped firmly by the loose skin behind the head. A 0.4-mm external diameter hypodermic needle attached to a 10-μl syringe was inserted perpendicularly through the skull and no more than 2 mm into the brain of the mouse, where 5 μl were then administered. The injection site was 1 mm to the right or left from the midpoint on a line drawn through to the anterior base of the ears. Injections were performed into the right or left ventricle randomly. To ascertain that the drugs were administered exactly into the cerebral ventricle, some mice were injected with 5 μl of Indian ink diluted 1:10 and their brains were examined macroscopically after sectioning.

Drugs

The following drugs were used: indomethacin (non-steroidal anti-inflammatory drug—Sigma, Milan, Italy), prochlorperazine (D₂-antagonist—Sigma, Milan, Italy), caffeine (adenosine antagonist—Sigma, Milan, Italy), sumatriptan (5-HT_1B/1D agonist—GlaxoSmithKline) and PTX (RBI).

The doses of the tested drugs were chosen on the basis of their analgesic efficacy in the 0.6% solution of acetic acid-induced abdominal constriction test: indomethacin (1 mg/kg, s.c.), prochlorperazine (1 mg/kg, s.c.) [13], caffeine (3 mg/kg, s.c.) [15] and sumatriptan (20 mg/kg, s.c.) [21] (Table 1). A fixed combination of indomethacin (1 mg/kg), prochlorperazine (1 mg/kg) and caffeine (3 mg/kg) was s.c. administered.

Table 1 Analgesic and antihyperalgesic doses of indomethacin, prochlorperazine and caffeine as Indoprocaf, and sumatriptan

According to their pharmacokinetics, in order to reach the analgesic peak in correspondence of the abdominal constriction test, the tested drugs were s.c. administered 30 min (prochlorperazine and sumatriptan) or 15 min (indomethacin, caffeine and Indoprocaf) before the acetic acid injection. A control group was treated with a saline solution.

Abdominal constriction test

Mice were intraperitoneally injected with a 0.6% solution of acetic acid (10 ml/kg) according to the method of Koster [22]. The analgesic effect was evaluated through the count of abdominal constrictions. The number of stretching movements was counted for 10 min, starting 5 min after the acetic acid injection. Ten animals per group were used.

Statistics

Results from abdominal constriction tests are given as the mean ± SEM. Analysis of variance (ANOVA), followed by Fisher’s PLSD procedure for post-hoc comparison, was used to verify the significance between two means. P-values of less than 0.05 were considered significant. Data were analyzed with the StatView program (version 4.01) for the Macintosh computer.

Results

In the abdominal constriction test, indomethacin (1 mg/kg, s.c.), prochlorperazine (1 mg/kg, s.c.) and caffeine (3 mg/kg, s.c.) induced a statistically significant central antinociceptive effect also when the Gi proteins were inactivated (Table 2), due to the administration of PTX, in comparison with the control group.

Table 2 Effect of indomethacin, prochlorperazine, caffeine, Indoprocaf and sumatriptan in mouse abdominal constriction test, with and without pertussis toxin (PTX) (mean ± SEM)

Also sumatriptan (20 mg/kg, s.c.) was able to reduce significantly the abdominal constrictions when the Gi proteins were inactivated by the PTX (Table 2; Fig. 1).

Fig. 1

Effect of sumatriptan (20 mg/kg, s.c.) in mouse abdominal constriction test, with and without pertussis toxin (PTX) (mean ± SEM); *P < 0.01 vs. saline only treated mice, °P < 0.01 vs. PTX/saline treated mice

The combination of indomethacin (1 mg/kg), prochlorperazine (1 mg/kg) and caffeine (3 mg/kg), s.c. administered, induced a statistically significant reduction of abdominal constrictions even when the Gi proteins were inactivated by the PTX (Table 2; Fig. 2). The reduction induced by the combination was statistically superior to that of each of its single active ingredients and to that of sumatriptan (Table 2; Fig. 3). Moreover, also when used as analgesic in the control group (saline + drug), the combination of indomethacin, prochlorperazine and caffeine induced a superior antinociceptive effect than that obtained with the same analgesic dose of each single active ingredient and than sumatriptan (Table 2).

Fig. 2

Effect of Indoprocaf (indomethacin 1 mg/kg, prochlorperazine 1 mg/kg, caffeine 3 mg/kg, s.c.) in mouse abdominal constriction test, with and without pertussis toxin (PTX) (mean ± SEM); *P < 0.01 vs. saline only treated mice, °P < 0.01 vs. PTX/saline treated mice

Fig. 3

Effect of indomethacin, prochlorperazine, caffeine, sumatriptan and Indoprocaf in mouse abdominal constriction test with pertussis toxin (PTX) (mean ± SEM); *P < 0.01 vs. indomethacin or prochlorperazine or caffeine or sumatriptan treated mice

Discussion

The aim of this study was to investigate if Indoprocaf was able to exert its central antinociceptive action when Gi proteins activity is abolished by PTX, compared to its single active ingredients and to sumatriptan, in the mice model of abdominal constriction test induced by an i.p. injection of 0.6% solution of acetic acid. This study showed that Indoprocaf and sumatriptan at analgesic doses exert their central antinociceptive action independently from the Gi proteins. In addition, the antinociceptive efficacy of Indoprocaf in this study was statistically superior to that of sumatriptan.

This study also showed that the single active ingredients of Indoprocaf, indomethacin, prochlorperazine and caffeine, were able to exert their central antinociceptive action independently from the Gi proteins.

Considering that prochlorperazine and caffeine induce a central cholinergic analgesia [13, 15] and that the central cholinergic system was proven to have a role in the antinociception induced by sumatriptan in rodents [21], these results are consistent with previous data showing that PTX had no effect on cholinergic drugs-induced analgesia [19]. To our knowledge, no other NSAID besides indomethacin has been tested in in vivo models of PTX-induced Gi proteins inactivation.

Moreover, this study showed that Indoprocaf at analgesic doses was able to abolish almost completely the abdominal constrictions, with a statistically superior efficacy compared to the single active ingredients, showing an important synergic effect of Indoprocaf. This synergic effect was evident not only when Gi proteins activity was abolished by PTX, but also under control condition, when Gi proteins were active. A synergic action of Indoprocaf at sub-analgesic doses, compared to its single active ingredients, was also proven in mice in the hyperalgesia model induced by the intraperitoneal injection of a 0.3% solution of acetic acid [6].

The dosages used for Indoprocaf in this experimental model are in the range of effective analgesic doses [13, 15, 23] and are 10 times higher than those shown to be able to revert hyperalgesia [6, 7]. The dose of sumatriptan used in this experimental model is in the range of effective analgesic doses [21] and is 20 times higher than that shown to revert hyperalgesia [24] (Table 1). From a clinical point of view, the therapeutic doses (orally administered) of indomethacin (25 mg), prochlorperazine (2 mg) and caffeine (75 mg) contained in Indoprocaf as antimigraine drug are much lower than the range of doses of indomethacin (50–200 mg daily) when used to treat rheumatoid arthritis or pain of various aetiologies, of prochlorperazine (15–50 mg daily) when used as antiemetic and of anhydrous caffeine (100–200 mg) when used in analgesic preparations (Table 1). These dosages are consistent with the concept that migraine is a particular pain characterized by neuronal hyperexcitability and that migraine should be treated with specific drugs at dosages that could be sub-analgesic, but are antihyperalgesic [7]. Moreover, this study shows first that Indoprocaf is superior to its single active ingredients also at analgesic doses, supporting that, when used in the clinical practice, the doses of active ingredients in Indoprocaf could be lower than those of the single active ingredients when administered independently.

The central antinociceptive action exerted by Indoprocaf and sumatriptan independently from Gi proteins might be clinically important considering that it has been shown that in migraine patients there is a hypofunctionality of Gi proteins [17].

A hypofunctionality of Gi protein system has been reported also in patients suffering from fibromyalgia [25], and a high incidence of fibromyalgia was found among female migraine patients [26]. Many chronic pain syndromes, as fibromyalgia and chronic tension-type headache, show evidence of central nervous system hyperexcitability related to central sensitization [27, 28].

The efficacy of Indoprocaf both independently from the Gi protein functionality and in abolishing the central sensitization induced by N-methyl-d-aspartate (NMDA) in the experimental animal model [7] could imply a potential therapeutic value not only in migraine and tension-type headache but also in fibromyalgia, three conditions sharing some common features as an hyperalgesic state showing as increased pain sensitivity [29, 30].

Conclusion

This study suggests that the central antinociceptive action exerted by Indoprocaf and sumatriptan is independent from Gi proteins. The superiority of Indoprocaf compared to sumatriptan in this in vivo model supports the recent clinical evidence that Indoprocaf most likely has similar therapeutic efficacy to sumatriptan [31, 32], recognizing therefore the role of Indoprocaf in the acute treatment of migraine.