The effect of alpha-trinositol (D-myo-inositol 1,2,6-trisphosphate) on formalin-evoked spinal amino acid and prostaglandin E2 levels

Mechanisms involved in the antinociceptive and anti-inflammatory effects of bis selenide in mice

Objectives

The present study examined the mechanisms involved in the antinociceptive effects of bis selenide [(Z)-2,3-bis(4-chlorophenylselanyl)prop-2-en-1-ol].

Methods

The effects of oral bis selenide were tested against licking behaviour and oedema in mice induced by formalin, serotonin, histamine, glutamate, phorbol 12-myristate 13-acetate (PMA), 8-bromoadenosine 3′,5′-cyclic monophosphate (8-BrcAMP) and pros-taglandin E2. The effects of a variety of receptor antagonists on the antinociceptive activity were tested to determine the likely mechanism of action of bis selenide.

Key findings

Bis selenide caused antinociception on the first and second phases of the formalin test, with mean ID50 values of 34.21 (29.66–39.45) and 15.86 (12.17–20.67) mg/kg and maximal inhibition of 65 ± 3% and 90 ± 1%, respectively. At 50 mg/kg bis selenide significantly inhibited (31 ± 2%) paw oedema induced by intraplantar injection of formalin. At 25 mg/kg given 5 min after the formalin injection, bis selenide caused a significant inhibition (42 ± 5%) in the second phase of the formalin test, whereas the prophylactic treatment caused more intense inhibition (64 ± 3%). Oral administration of bis selenide reduced licking and paw oedema induced by serotonin, histamine, glutamate, PGE2, PMA and 8-BrcAMP. The antinociceptive effect of bis selenide (25 mg/kg, p.o.) on the formalin test was reversed by i.p. administration of p-chlorophenylalanine methyl ester (an inhibitor of serotonin synthesis), ketanserin (a selective 5-HT2a receptor antagonist), ondansetron (a 5-HT3 receptor antagonist) and ranitidine (a histamine H2-receptor antagonist).

Conclusions

Glutamatergic, prostaglandin E2, serotonergic (5-HT2a and 5-HT3) and histamine H2 receptors are involved in the antinociceptive effects of bis selenide in mice. The interaction of bis selenide with protein kinase C and A signalling pathways was also demonstrated.

Immune and glial cell factors as pain mediators and modulators

A decade ago the attention of pain scientists was focused on a small number of molecules such as prostaglandin and bradykinin as peripheral pain mediators or modulators. These factors were known to be produced by tissue damage or inflammation, and considered responsible for the activation and sensitization of peripheral pain signaling sensory neurons. A small number of molecules were also identified as central pain mediators, most notably glutamate and substance P released from central nociceptive nerve terminals, and, starting at that time, appreciation that nitric oxide might be produced by dorsal horn neurons and act as a diffusible transmitter to increase excitability of central pain circuits. During the last decade evidence has emerged for many novel pain mediators. The old ones have not disappeared, although their roles have been redefined in some cases. Prostaglandin E2 (PGE2), for instance, is now recognized as playing a prominent role in CNS as well as peripheral tissues. The newly identified mediators include a variety of factors produced and released from nonneuronal cells-predominantly immune and glial cells. The evidence is now growing apace that these are important mediators of persistent pain states and can act at a number of loci. Here we review the actions of several of these factors-the pro-inflammatory cytokines, some chemokines, and some neurotrophic factors, which, in addition to their traditionally recognized roles, are all capable of changing the response properties of peripheral and central pain signaling neurons. We review these actions, first in periphery, where a substantial literature has accumulated, and then in spinal cord, where the role of factors from nonneuronal cells has only recently been identified as of considerable importance.

Microdialysis in pain research

In vivo microdialysis has been used in preclinical pain research for more than a decade. This valuable tool allows correlations between nociceptive behavior and neurotransmitter release in pain-related CNS sites. However, several methodological issues must be considered to adequately interpret microdialysis data. Thus, the aim of this review is to describe key considerations, potential pitfalls, and important control experiments. We focus on animal experiments which evaluate the effects of noxious stimulation on CNS neurotransmitter release, particularly those that address clinically relevant problems in patients with long-lasting painful conditions.

Spinal administration of prostaglandin E(2) or prostaglandin F(2alpha) primarily produces mechanical hyperalgesia that is mediated by nociceptive specific spinal dorsal horn neurons

The effects of intrathecal (i.t.) administration of prostaglandin E2 (PGE2) and prostaglandin F2 (PGF2) on behavioral and spinal neuronal responses to mechanical and thermal stimuli were examined in rats. i.t. Administration of either PGE2 (1-100 nmol) or PGF2 (1-100 nmol) produced a robust, dose-dependent mechanical hyperalgesia, but only a weak thermal hyperalgesia and touch-evoked allodynia. Spinal administration of either PGE2 (100 pmol-100 nmol) or PGF2 (1-100 nmol) produced dose-dependent increases in responses of nociceptive specific (NS) neurons to mechanical stimuli, but only modest increases in wide dynamic range (WDR) neurons to mechanical stimuli. Spinal administration of PGE2 produced a bi-directional, dose-response effect on thermally-evoked responses of both WDR and NS neurons when prostaglandin-induced changes in background discharges were controlled for. Thermally evoked responses of WDR and NS neurons were decreased at lesser doses of PGE2, but this trend reversed with greater doses, such that responses of WDR neurons were significantly increased at the greatest dose tested at some test temperatures. PGF2 generally produced non-significant increases in thermally evoked neuronal responses, and this trend occurred primarily in WDR neurons. Both PGE2 and PGF2 produced increases in background discharges of WDR and NS neurons, although this effect was most consistently observed with WDR neurons and PGE2. These behavioral and electrophysiological data suggest that mechanical hyperalgesia induced by spinal administration of PGE2 and PGF2 is mediated mainly by changes in NS neurons. The weak thermal hyperalgesia may reflect changes in WDR neurons.

The major role of peripheral release of histamine and 5-hydroxytryptamine in formalin-induced nociception

Formalin injected subcutaneously into the paw is a widely used model of pain. This procedure evokes a short-lasting period of flinching (phase 1) and a long-lasting period of intense flinching (phase 2) following a very short period of quiescence. Phase 2 has been extensively used to support the involvement of central (spinal cord) sensitization in inflammatory hyperalgesia. The present study evaluated the contribution of stimulation of peripheral nociceptors by the release of endogenous mediators at the site of lesion. The participation of histamine and 5-hydroxytryptamine was demonstrated by the treatment of the rat hindpaws with selective histamine H1 (pyrilamine and meclizine) and histamine H2 (cimetidine) receptor antagonists or selective 5-hydroxytryptamine(1A) (WAY100,135) and 5-hydroxytryptamine(4/3) (tropisetron) receptor antagonists. The co-administration of pyrilamine or meclizine with formalin (1%) significantly reduced phases 1 and 2, while cimetidine had no effect. Pyrilamine administration during the period of quiescence (10min after formalin administration) caused strong dose-related inhibition of phase 2. The co-administration of tropisetron with formalin caused a blockade of both phases, while with WAY100,135 caused only inhibition of the phase 2. In contrast, tropisetron administrated during the period of quiescence did not cause antinociception. Histamine and 5-hydroxytryptamine receptors could be strongly activated in naïve animals by administration of a mixture of both agonists or compound 48/80 (2microg/paw) which is known to release both mediators from mast cells. Pretreatment of the paws with a mast cell stabilizer, sodium cromoglycate, significantly reduced the second phase of the formalin injection model. From these results we suggest that phases 1 and 2 of the formalin test are dependent upon the ongoing afferent input. Furthermore, while histamine H1 participates in both phases, 5-hydroxytryptamine(4/3) participates in phase 1 and 5-hydroxytryptamine(1A) in phase 2.

D-myo inositol 1,2,6, triphosphate (alpha-trinositol, pp56): selective antagonist at neuropeptide Y (NPY) Y-receptors or selective inhibitor of phosphatidylinositol cell signaling?

1. D-myo inositol 1,2,6 trisphosphate (alpha-trinositol, pp56), an isomer of the second messenger substance, D-myo inositol 1,4,5 trisphosphate, has an interesting pharmacological profile that includes anti-inflammatory and analgesic effects and antagonism of neuropeptide Y (NPY)-mediated cellular responses. 2. However, not all responses elicited by this neuropeptide are sensitive to antagonism by pp56. Evidence is emerging, at least in certain tissues, that other receptor populations, in addition to those for NPY, are also sensitive to inhibition by pp56. 3. A direct or allosteric interaction of pp56 at receptors for NPY is now considered unlikely and it is more probable that pp56 might interfere at some point in the phosphatidylinositol signaling pathway, possibly at the level of the plasmalemmal inositol 1,3,4,5, tetrakisphosphate receptor. 4. Full realization of the therapeutic potential of this novel compound, however, must await a thorough characterization of the cellular mechanism(s) associated with the various pharmacological effects of pp56.