Mechanisms involved in the antinociception caused by agmatine in mice

Involvement of I2-imidazoline binding sites in positive and negative morphine analgesia modulatory effects

Some studies, suggesting the involvement of I(2)-imidazoline binding sites (I(2)-IBS) in morphine analgesia modulation, prompted us to examine on mice antinociceptive assays the effect produced by 1 (phenyzoline), that in view of its high I(2)-IBS affinity and high I(2)-IBS selectivity with regard to I(1)-IBS, alpha(2)-adrenoreceptors and mu-opioid receptors might be considered the first interesting I(2)-IBS ligand. The study was also applied to its ortho phenyl derivative 2 (diphenyzoline), designed and prepared in order to produce a possible modification of the biological profile of 1. Diphenyzoline (2) retains a significant I(2)-IBS selectivity with regard to I(1)-IBS, alpha(2)-adrenoreceptors and mu-opioid receptors. Moreover, by the functional assays 1 and 2 proved inactive at all alpha(2)-adrenoreceptors subtypes up to 10(-3) M. As expected, phenyzoline and diphenyzoline, which are structurally related, highlighted an interesting "positive" or "negative", respectively, morphine analgesia modulatory effect. In fact, 1 (s.c. 10 mg/kg) enhanced morphine analgesia (60% and 40% in mouse tail-flick and mouse hot-plate, respectively), while 2 (s.c. 10 mg/kg) decreased it (-41% and -20%, respectively). The ability to decrease morphine analgesia had never been observed before in I(2)-IBS ligands. These effects were not affected by i.p. treatment of animals with yohimbine (a selective alpha(2)-adrenoreceptor antagonist, 0.625 mg/kg) or efaroxan (an I(1)-IBS/alpha(2)-adrenoreceptor antagonist, 1.0 mg/kg). In contrast, they were completely reversed by i.p. treatment of animals with idazoxan (an I(2)-IBS/alpha(2)-adrenoreceptor antagonist, 2 mg/kg). Moreover, compound 2, in mouse tail-flick test, was able to potentiate by 23% the naloxone-induced decrease of morphine analgesia. Therefore, the results of this study indicate the crucial involvement of I(2)-IBS in the morphine analgesia modulatory effects of 1 and 2.

The Contribution of Alpha-1 and Alpha-2 Adrenoceptors in Peripheral Imidazoline and Adrenoceptor Agonist-Induced Nociception

We evaluated the effects of activation of peripheral adrenoceptors (AR) and imidazoline receptors on nociception and the contribution of alpha-1 and alpha-2 AR receptors in agonist-induced nociception by using the tail-flick test in mice. Clonidine (alpha-2 AR agonist), agmatine (imidazoline receptor and alpha-2 AR agonist), noradrenaline (mixed alpha-1 and alpha-2 AR agonist), phenylephrine (alpha-1 AR agonist), or 0.9% saline was given by intradermal injection (10 microL) into the tail. The intradermal injection of clonidine (1, 3, and 10 microg) and agmatine (3, 30, and 50 microg) produced dose-dependent antinociception, whereas noradrenaline (1, 10, and 30 microg) and phenylephrine (1, 10 and 30 microg) produced dose-dependent thermal hyperalgesia. Clonidine (10 microg) and agmatine (50 microg)-induced peripheral antinociception were antagonized by pretreatment with yohimbine (2.5 mg/kg IP), a selective alpha-2 AR antagonist, but not by prazosin (1 mg/kg IP), a selective alpha-1 AR antagonist. Noradrenaline (30 microg) and phenylephrine (30 mug)-induced thermal hyperalgesia were antagonized by prazosin (1 mg/kg IP) but not by yohimbine (2.5 mg/kg IP). Our results suggest that local thermal hyperalgesic effects of noradrenaline and phenylephrine are linked to alpha-1 AR and the peripheral antinociceptive action of clonidine and agmatine are linked to alpha-2 AR.

Agmatine: biological role and therapeutic potentials in morphine analgesia and dependence

Agmatine is an amine that is formed by decarboxylation of L-arginine by the enzyme arginine decarboxylase (ADC) and hydrolyzed by the enzyme agmatinase to putrescine. Agmatine binds to several target receptors in the brain and has been proposed as a novel neuromodulator. In animal studies, agmatine potentiated morphine analgesia and reduced dependence/withdrawal. While the exact mechanism is not clear, the interactions with N-methyl-D-aspartate (NMDA) receptors, alpha2-adrenergic receptors, and intracellular cyclic adenosine monophosphate (cAMP) signaling have been proposed as possible targets. Like other monoamine transmitter molecules, agmatine is rapidly metabolized in the periphery and has poor penetration into the brain, which limits the use of agmatine itself as a therapeutic agent. However, the development of agmatinase inhibitors will offer a useful method to increase endogenous agmatine in the brain as a possible therapeutic approach to potentiate morphine analgesia and reduce dependence/withdrawal. This review provides a succinct discussion of the biological role/therapeutic potential of agmatine during morphine exposure/pain modulation, with an extensive amount of literature cited for further details.

Contribution of spinal glutamatergic receptors to the antinociception caused by agmatine in mice

This study was designed to evaluate the role of spinal glutamatergic receptors in the antinociception elicited by agmatine in mice. Intraperitoneal (i.p.) administration of agmatine (1.0-100.0 mg/kg) dose dependently inhibited the nociceptive response induced by intrathecal (i.t.) injection of glutamate, N-methy-D-aspartate (NMDA) and (+/-)-1-aminocyclopentane-trans-1,3-dicarboxylic acid (trans-ACPD), with mean ID(50) values of 16.7, 6.8 and 27.0 mg/kg, respectively. However, agmatine completely failed to affect the nociception induced by alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) or kainic acid (kainate). Agmatine injected by i.t. route (10-100 microg/site) also produced dose-related inhibition of NMDA- and trans-ACPD-induced biting response with mean ID(50) values of 29.6 and 36.0 mug/site, respectively. The nitric oxide synthase inhibitor N(omega)-nitro-L-arginine (l-NOARG) (75.0 mg/kg, i.p.) also consistently inhibited glutamate-, NMDA- and trans-ACPD-induced nociception (41 +/- 13, 100 and 83 +/- 6%, of inhibition, respectively) but had no effect on the same response caused by AMPA and kainate agonists. The selective NMDA receptor antagonist (5S,10R)-(+)-5-Methyl-10,11-dihydro-5H-dibenzo[a,d] (MK-801) at a low dose (0.05 mg/kg, i.p.) inhibited the nociceptive response caused by both glutamate and NMDA agonists (inhibitions of 35 +/- 1 and 72 +/- 2%, respectively). At a high dose, MK-801 (0.5 mg/kg, i.p.) significantly inhibited the biting response induced by i.t. administration of all the glutamatergic agonists tested: glutamate, AMPA, NMDA, kainate and trans-ACPD, with inhibitions of 49 +/- 8, 84 +/- 16, 84 +/- 3, 76 +/- 8 and 97 +/- 2%, respectively. Together, these results provide experimental evidence indicating that agmatine given systemically and spinally produce marked antinociception at spinal sites in mice. Furthermore, an interaction with glutamate receptors, namely NMDA and trans-ACPD, metabotropic and NMDA-ionotropic origin, by a mechanism similar to that of nitric oxide (NO) inhibitors, seems to account for the agmatine antinociceptive action.

Antinociceptive properties of the ethanolic extract and of the triterpene 3β,6β,16β-trihidroxilup-20(29)-ene obtained from the flowers of Combretum leprosum in mice

The present study examined the antinociceptive effects of the ethanolic extract (EE) and of the triterpene 3beta,6beta,16beta-trihidroxilup-20(29)-ene obtained from the flowers of Combretum leprosum in chemical and thermal behavioural models of pain in mice. The EE (10-1000 mg/kg) given orally (p.o.), 1 h prior to testing, produced dose-dependent inhibition of acetic acid-induced visceral pain, with mean ID50 value of 131.9 mg/kg. In the formalin test, the EE (10-300 mg/kg, p.o.) also caused significant inhibition of both the early (neurogenic pain) and the late (inflammatory pain) phases of formalin-induced licking, however, it was more potent and efficacious in relation to the late phase of the formalin test, with mean ID50 values for the neurogenic and the inflammatory phases of approximately 300 and 88.8 mg/kg, respectively. The EE (10-1000 mg/kg, p.o.) also caused significant and dose-dependent inhibition of capsaicin- and glutamate-induced pain, with mean ID50 values of 160.5 and 38.3 mg/kg, respectively. Furthermore, the triterpene 3beta,6beta,16beta-trihidroxilup-20(29)-ene (1-30 mg/kg), given p.o., 1 h prior to testing, also produced dose-related inhibition of glutamate-induced pain, with a mean ID50 value of 5.6 mg/kg. When assessed in a thermal model of pain, the EE (10-300 mg/kg, p.o.) and fentanyl (100 microg/kg, s.c.) caused a significant and marked increase in the latency response on the hot-plate test (50 degrees C). The antinociception caused by EE (100 mg/kg, p.o.) in the glutamate test was significantly attenuated by intraperitoneal (i.p.) treatment of mice with naloxone (opioid receptor antagonist, 1 mg/kg), pindolol (a 5-HT 1A/1B receptor/beta adrenoceptor antagonist, 1 mg/kg), WAY100635 (a 5-HT 1A receptor antagonist, 0.7 mg/kg) or ketanserin (a 5-HT 2A receptor antagonist, 0.3 mg/kg). In contrast, EE (100 mg/kg, p.o.) antinociception was affected neither by L-arginine (precursor of nitric oxide, 600 mg/kg) nor by ondansetron (a 5-HT3 receptor antagonist, 0.5 mg/kg) i.p. treatment. It was not associated with non-specific effects such as muscle relaxation or sedation. Together, these results indicate that EE produces dose-related antinociception in several models of chemical and thermal pain through mechanisms that involve an interaction with opioid and serotonergic (i.e., through 5-HT 1A/1B and 5-HT 2A receptors) systems.