br Conflict of interest br Acknowledgment The

Conflict of interest
Acknowledgment The authors gratefully acknowledge the financial support from the National Natural Sciences Foundation of China (81070220 and 81170278), and the Aid Program for Science and Technology Innovative Research Team in Higher Educational Institutions of Human Province, China (2008-244).
Introduction To date various Malonic acid mg of painkillers, including opioids such as morphine, heroin, codeine, methadone (Lledo-Fernandez and Banks, 2011), nonsteroidal anti-inflammatory drugs (NSAIDs) such as aspirin, ibuprofen, naproxen and indomethacin (Warden, 2010) have been identified to get rid of pain. Unfortunately, continuous usage of these drugs causes analgesic tolerance, respiratory depression and stomach ulcer, respectively (Lynch and Watson, 2006). Tolerance is a process which goes on by neuroadaptations and reduction of drug effects due to receptor down regulation (Pradhan et al., 2010). Apelin is an endogenous peptide which isolated from bovine stomach extract first in 1998. The apelin receptor (APJ), as G-protein-coupled receptors (GPCRs) with 377 amino-acid, cloned from human genomic DNA by O'Dowd and colleagues. The receptor resembles (54% in the transmembrane regions) angiotensin II receptor type 1 (AT1), but angiotensin II does not bind to it (O'Dowd et al., 1993). However, activated or non-activated APJ may affect Ang II-AT1 signaling (Sun et al., 2011). Apelin/APJ system also are distributed in central nerve system, spanal cord (Matsumoto et al., 1996, O’Carroll et al., 2000, Reaux et al., 2001) hypothalamus, cortex, cerebellum, hippocampus, midbrain, striatum, pituitary (Lee et al., 2000, Medhurst et al., 2003), amygdale, raphe nucleus (Reaux et al., 2002) and peripheral areas such as heart, kidney, skeletal muscle, liver, lung, intestine, colon and glands (Habata et al., 1999, Hekmat et al., 2011, Hosoya et al., 2000, Kawamata et al., 2001, Najafipour et al., 2012, O’Carroll et al., 2000, Wang et al., 2004). Interestingly, apelin plays an important role in the neuronal signaling pathway (O'Carroll et al., 2000). The apelin peptide has been also shown that stimulate hypothalamic–pituitary–adrenal (HPA) axis in vivo and in vitro studies (Newson et al., 2009, Taheri et al., 2002). It has been demonstrated that central apelin-induced elevated levels of plasma corticosterone is partially blocked by corticotrophin releasing factor (CRF) antagonist (Jaszberenyi et al., 2004). On the other hand, many studies have indicated that addictive drugs (e.g. opiates, ethanol, cannabinoids, nicotine, cocaine, amphetamines) can activate HPA axis (Armario, 2010). Lyengar et al. reported that opioid-induced alterations of HPA function is involved in morphine tolerance in rats (Iyengar et al., 1987). Recently, it has been shown that apelin has central antinociceptive effects. In addition, μ-opioid receptor potentially involved in the analgesic effect of apelin (Lv et al., 2012b, Xu et al., 2009). Although, many studies have been demonstrated the unwanted side effect of analgesic tolerance in human and animals (Furlan et al., 2006), it has still remained unspecified whether antinociceptive tolerance can be induced by chronic administration of apelin-13. In addition, potential affinity of APJ with μ opioid receptor (Befort et al., 2008) and HPA axis (Bülbül et al., 2016) prompts us to evaluate the possible involvement of opioid receptor in apelin analgesic tolerance.
Materials and methods
Result
Discussion It has been previously reported that neuropeptide apelin induces potent antinociceptive effects (Lv et al., 2012b, Lv et al., 2013, Xu et al., 2009). In contrast, it has also been reported that apelin can produce hyperalgesic effects (Chen and Bai, 2008, Lv et al., 2013). In this study, acute intrathecal injection of apelin could induce analgesic effects which were disappeared following chronic administration. It is well known that opioid receptors (μ, δ and κ) play crucial roles in pain perception, addiction, neuroprotection and cardiovascular regulation (Chen et al., 2005, Kao et al., 2008, Qi and Smith, 2007, Wang et al., 2006). It seems that apelin has either direct or indirect effects on acute and phasic pain models which have been demonstrated with pharmacological studies (Lv et al., 2013). For instance, the blockage of apelin analgesic tolerance by apelin receptor antagonist, F13-A, showed that this phenomenon is mediated directly by apelin receptor signaling. In addition, partial inhibition of apelin tolerance by naloxone indicated that there is also an indirect opioid pathway.