Role of pertussis toxin-sensitive G-protein, K+ channels, and voltage-gated Ca2+ channels in the antinociceptive effect of inosine

Abstract

Inosine is the first metabolite of adenosine. It exerts an antinociceptive effect by activating the adenosine A 1 and A 2A receptors. We have previously demonstrated that inosine exhibits antinociceptive properties in acute and chronic mice models of nociception. The aim of this study was to investigate the involvement of pertussis toxinsensitive G-protein-coupled receptors, as well as K + and Ca 2+ channels, in the antinociception promoted by inosine in the formalin test. Mice were pretreated with pertussis toxin (2.5 μg/site, i.t., an inactivator of G i/0 protein); after 7 days, they received inosine (10 mg/kg, i.p.) or morphine (2.5 mg/kg, s.c., used as positive control) immediately before the formalin test. Another group of animals received tetraethylammonium (TEA) or 4-aminopyridine (4-AP) (1 μg/site, i.t., a non-specific voltage-gated K + channel blockers), apamin (50 ng/site, i.t., a small conductance Ca 2+ -activated K + channel blocker), charybdotoxin (250 pg/site, i.t., a largeconductance Ca 2+ -activated K + channel blocker), glibenclamide (100 μg/site, i.t., an ATP-sensitive K + channel blocker) or CaCl 2 (200 nmol/site, i.t.). Afterwards, the mice received inosine (10 mg/kg, i.p.), diclofenac (10 mg/kg, i.p., a positive control), or morphine (2.5 mg/kg, s.c., a positive control) immediately before the formalin test. The antinociceptive effect of inosine was reversed by the pre-administration of pertussis toxin (2.5 μg/site, i.t.), TEA, 4-aminopyridine, charybdotoxin, glibenclamide, and CaCl 2 , but not apamin. Further, all K + channel blockers and CaCl 2 reversed the antinociception induced by diclofenac and morphine, respectively. Taken together, these data suggest that the antinociceptive effect of inosine is mediated, in part, by pertussis toxinsensitive G-protein coupled receptors and the subsequent activation of voltage gated K + channel, large conductance Ca 2+ -activated and ATP-sensitive K + channels or inactivation of voltage-gated Ca 2+ channels. Finally, small conductance Ca 2+ -activated K + channels are not involved in the antinociceptive effect of inosine.

Long-lasting antinociceptive effects of green light in acute and chronic pain in rats

“…Our findings uncovered proteins enriched in tissues from GLED exposed rats that are reportedly linked to antinociception (Table 3). For example, purine nucleoside phosphorylase breaks down adenosine into inosine, which has antinociceptive properties via actions on the Adenosine A1 receptor [57] and via pertussis toxin-sensitive G-protein coupled receptors and the subsequent inactivation of calcium channels [49]. Aspartyl aminopeptidase, which converts angiotensin II to angiotensin III, is reported to activate inhibitory pain descending pathways from the PAG [65].…”

The antidepressant-like effect of inosine in the FST is associated with both adenosine A1 and A2A receptors

“…The antidepressant-like effects of inosine observed in the FST persisted for 2 h after systemic administration. Indeed, literature data have shown that intraperitoneal or even oral administration of inosine may elicit several biological effects, including antinociceptive responses in different models of nociception [16,17], neuroprotective activity against hypoxic-ischemic brain damage [30], and axonal regrowth [31]. Inosine has a high safety profile and is devoid of any particular side effects.…”

“…Indeed, the activation of these receptors produces antidepressant-like effects in the FST [21] and is responsible, at least in part, for the antidepressant-like effects of compounds like adenosine [21] and zinc chloride [34]. However, recent works have indicated that inosine might have other potential targets like the other adenosine receptors, the protein kinase B (Akt), the nuclear enzyme poly ADP-ribose polymerase, K + channels, and voltage-gated Ca 2+ channels [17,35,36]. Inosine may also partially act via its breakdown product, uric acid, which has been consistently shown to be a scavenger of oxyradicals and peroxynitrite [37,38].…”

“…In spite of the originally described lack of biological effects, it is increasingly clear that inosine can act as a potent immunomodulatory and neuroprotective compound [10,13]. Several works emerged indicating that inosine attenuates the production of pro-inflammatory cytokines [14,15]; induces antinociceptive, antiallodynic, and antihyperalgesic effects [16,17]; and is protective in animal models of sepsis, ischemia, and autoimmunity [10]. In addition, inosine preserves the viability of glial and neuronal cells during hypoxia, and stimulates axonal regrowth after injury [7,9,18].…”

Signaling pathways underlying the antidepressant-like effect of inosine in mice

“…It has been suggested that inosine elicits neuroprotective effects in neurons and astrocytes subjected to hypoxia, glucose-oxygen deprivation, and oxidative damage [18,19]; stimulates neurite outgrowth in vitro [20]; and induces axonal outgrowth in vivo and in vitro studies [21][22][23][24][25]. Moreover, systemic administration of inosine causes antinociceptive, antiallodynic, and antihyperalgesic effects in mice [26,27]. Evidence from clinical studies has suggested antioxidant and protective properties of inosine in multiple sclerosis and Parkinson's disease patients [28][29][30].…”