Endogenous adenosine systems have largely been implicated in

Endogenous adenosine systems have largely been implicated in drug actions by the demonstration that caffeine (non-selective A1R and A2AR antagonist) (see Section The issue of caffeine), in doses up to 7.5–10mg/kg, inhibits antinociception, and when this occurs, it INCB28060 is generally also seen with a selective A1R antagonist (DPCPX most commonly used). Thus, caffeine and/or DPCPX, given systemically, inhibit antinociception by systemic amitriptyline (Esser and Sawynok, 2000, Ulugol et al., 2002, Sawynok et al., 2008), carbamazepine and oxcarbazepine (Tomić et al., 2004, Sawynok et al., 2010), venlafaxine (Yaba et al., 2006, but see Abed et al., 2015), acetaminophen (Godfrey et al., 2006, Sawynok and Reid, 2012), allopurinol (Schmidt et al., 2009, Essawy and Elbaz, 2013), tramadol (Sawynok et al., 2013), levetiracetam (Tomić et al., 2015), sumatriptan (Tomić et al., 2015), and gabapentin (Martins et al., 2015b), all of which are currently used clinically. Caffeine and/or DPCPX also inhibit the action of: (1) experimental agents such as cizoliritine (Aubel et al., 2007), inosine (Nascimento et al., 2010), guanosine (Schmidt et al., 2010), and selenide compounds (Savegnago et al., 2008, Luchese et al., 2010, Marcondes Sari et al., 2014), as well as (2) agents considered to be supplements or herbal remedies such as linalool (Peana et al., 2006), fructose 1,6-bisphosphate (Valério et al., 2009), paeoniflorin (main active ingredient in root of Paeonia lactiflora Pall) (Zhang et al., 2009), Artemia scopara extract (Habib and Waheed, 2013), Daniella oliveri extract (Boyce et al., 2013; theophylline was used instead of caffeine), norisoboldine from Radix Linderae (Gao et al., 2014), and Muntingia calabura extract (Zakaria et al., 2014). In some cases, selective A2AR antagonists (Peana et al., 2006, Nascimento et al., 2010, Essawy and Elbaz, 2013, Marcondes Sari et al., 2014) or A2BR antagonists (Savegnago et al., 2008) also inhibited the antinociception being examined. Some studies have undertaken a compartmental analysis to identify sites of interaction with endogenous adenosine systems by administering the analgesic agent systemically and the adenosine receptor antagonist locally. Thus, i.t. DPCPX (or i.t. caffeine) inhibits antinociception by systemic oxcarbazepine (Sawynok et al., 2010), acetaminophen (Sawynok and Reid, 2012), amitriptyline (Liu et al., 2013a) and tramadol (Sawynok et al., 2013), and these observations implicate spinal sites in the adenosine involvement. In addition, intraplantar (i.pl.) DPCPX (or caffeine) reverses systemic antinociception by oxcarbazepine (Sawynok et al., 2010), acetaminophen (Sawynok and Reid, 2012), amitriptyline (Liu et al., 2013a) and tramadol (Sawynok et al., 2013), further implicating peripheral sites in the action of the systemic agent. Both the i.t. and i.pl. DPCPX reversal of systemic antinociception is no longer observed in A1R knockout mice, confirming A1R involvement in such actions (Liu et al., 2013a, Liu et al., 2013b). Details of mechanisms by which these various pharmacological agents recruit endogenous adenosine systems (e.g. receptor interactions, increased generation or release from the cell, decreased metabolism or reuptake into the cell) are not clear. However, for amitriptyline and acetaminophen, the interaction in the spinal cord appears to involve adenosine release secondary to activation of 5HT7 receptors; these receptors are potentially on sensory afferents and result in increased cyclic AMP production (Liu et al., 2013a, Liu et al., 2013b). In some studies, analgesics are also given locally into the hindpaw to which the nociceptive stimulus is applied, and the drug is deemed to act peripherally if injection into the contralateral hindpaw is without effect. Using this approach, i.pl. caffeine or DPCPX attenuates the peripheral antinociceptive action of amitriptyline (Esser and Sawynok, 2000, Ulugol et al., 2002), carbamazepine (Vučković et al., 2006), oxcarbazepine (Tomić et al., 2006), and levetiracetam (Stepanović-Pertović et al., 2012). These observations implicate peripheral A1Rs in locally mediated effects, and are relevant to their potential actions as topical analgesics.