Effects of cholestasis and drug dependence on anxiety behaviors
The present results indicated that the induction of cholestasis and injection of morphine and tramadol increased %OAT and %OAE while did not affect the locomotor activity in the sham-operated and BDL mice, suggesting an anxiolytic behavior. On the other hand, the application of naloxone, a µ-opioid receptor antagonist, decreased %OAT and %OAE whereas did not change locomotor activity in the sham-operated and BDL mice showing the anxiogenic behavior of this antagonist. According to our results, research revealed that cholestasis and the application of morphine cause anxiolytic behaviors in rodents (Eslimi et al., 2011, Nasehi et al., 2013a, Reza Zarrindast et al., 2013). As stated before, acute and chronic liver failure for instance cholestasis, vary the level of activity of many neurotransmitter systems, for example, the opioidergic system (Zhang et al., 2004). The plasma level of the endogenous opioid peptide augmented in the cholestatic patients and rodents (Gholipour et al., 2008). Opioids are involved in the pathophysiology and manifestation of cholestasis (Bergasa et al., 2000). Furthermore, cholestasis could modify the release of corticotrophin-releasing hormone (Burak et al., 2002) which may be related to the cognitive and non-cognitive behaviors induced by cholestasis (Reza Zarrindast et al., 2013).
Opioid addiction prompts disturbances in mood and triggers anxiety and cognitive deficiencies (Swendsen and Merikangas, 2000). Moreover, Motaghinejad and coworkers (2016) reported that regular standard application of morphine in a continuous pattern supports the initiation of dependency but a controlled treatment declines morphine dependency and diminishes the severity of withdrawal syndrome. Also, these researchers reported that numerous dosage treatments of morphine can weaken morphine-prompted anxiety and cognition deficits (Motaghinejad et al., 2016). Changing several morphine regimens reduced morphine-produced anxiety behavior as determined by decreased levels of cortisol and augmented frequency of entrances and time spent in the open arm of the EPM (Motaghinejad et al., 2014).
According to these studies, we suggested that anxiolytic behaviors observed in cholestatic and addicted mice may be related to the enhancement of the plasma levels of opioid peptides in cholestatic mice (Gholipour et al., 2008) and administration of numerous dosage treatments of morphine (Motaghinejad et al., 2016) which initiated the enhancement of %OAT and %OAE in the sham-operated and BDL mice. Another possibility for these results may be due to the reduction of cortisol levels in cholestatic and addicted mice (Burak et al., 2002, Motaghinejad et al., 2014).
Effects of cholestasis and drug dependence on pain behaviors
The current study found that generation of cholestasis and addiction to opioid drugs (morphine and tramadol) prolonged the tail-flick latency, showing the analgesic effect. However, treatment with naloxone reverses the analgesic effect produced via cholestasis and opioid drugs. We proposed that alone administration of morphine or tramadol, as well as co-administration of them, together resulted in the activation of µ-opioid receptors as well as activation of catecholamine and serotonergic receptors (Gholami et al., 2015, Esmaeili et al., 2017, Subedi et al., 2019) which caused an analgesic effect in cholestatic and addicted mice. This is in agreement with other studies that the administration of morphine and tramadol produced an anti-nociceptive effect through the activation of µ-opioid receptors (Fazli-Tabaei et al., 2005, Zeng et al., 2013, Sousa and Ashmawi, 2015). The analgesic effects of opioid drugs may stem from the opposing effects of µ-opioid receptors on the GABA inhibitory neurons (Heinricher et al., 1994) and glutamate neurotransmission (Schepers et al., 2008) in the central nervous system (CNS). According to our findings, Nelson et al. (2006) reported that cholestatic rodents exhibit anti-nociceptive behavior in response to a thermal stimulus, which is reversed through the opioid receptor antagonist (naloxone) (Nelson et al., 2006). Moreover, the anti-nociceptive effect has been reported in cholestatic patients. So, Tian and coworkers (2016) reported that cholestatic patients indicated analgesia response. These researchers displayed that patients experiencing the orthotopic liver transplantation for cholestatic diseases exhibited lesser postoperative pain scores and needed less morphine for analgesia in comparison to patients undergoing liver resection (Tian et al., 2016).
The opioid drugs were used therapeutically for numerous centuries, understanding that the opioid drugs are also very addictive (Rivat and Ballantyne, 2016). The opioid analgesics act on the brain via coupling to the opioid receptors in the brain, spinal cord, and other parts of the body and decrease the pain sensation. Opioid category drugs, for example, morphine is a very potent analgesic drug, however, is used less because of its adverse effects of addiction, dependency, and constipation (Gholami et al., 2015, Subedi et al., 2019). Also, tramadol is used for the treatment of pain and has a potent analgesic effect (Bravo et al., 2017). The analgesic influence of tramadol is ten times lesser than morphine nonetheless is preferred to be safe in comparison to morphine. Tramadol is described as safe since it does not prompt addiction when compared with other opioid analgesics (Preston et al., 1991, Subedi et al., 2019). It prompts an analgesia property by presenting the agonistic activity to the µ-opioid receptors as well as catecholamine and serotonergic receptors (Gholami et al., 2015, Esmaeili et al., 2017, Subedi et al., 2019). Moreover, the combination of a μ-opioid agonist with another non-μ-opioid analgesic may have augmented analgesic effectiveness and/or a better safety profile (Smith, 2008). Additional support comes from the findings that the analgesic drugs with double mechanisms of function (μ-opioid receptor agonist and a second mechanism) tend to have improved therapeutic properties. For example, tramadol is a μ-opioid receptor agonist which increases serotonin and norepinephrine transmission, too (Reeves and Burke, 2008, Thorn et al., 2011).
In conclusion, due to the effects of s.c. injection of morphine (significant augmentation of anxiolytic and anti-nociceptive responses in the cholestatic and addicted mice), the effects of i.p. injection of tramadol (increasing of anxiolytic and analgesic behaviors in the cholestatic and addicted mice), and co-injection of morphine and tramadol (induction of anxiolytic and enhanced analgesic effects in the cholestatic and addicted mice), it can be proposed that µ-opioid receptors play a key in the modulation of anxiety and pain behaviors in cholestatic and addicted mice. The findings of the present investigation also support the main role of µ-opioid receptors in the pathophysiology of cholestasis as well as the modulation of anxiety and pain behaviors. Based on the findings of the present research, it can be proposed that co-administration of morphine and tramadol increased their effectiveness of them for induction of anxiolytic response as well as analgesic effect both in cholestatic and addicted mice. Nevertheless, more experiments are required to clarify the exact mechanisms of action between morphine and tramadol on the modulation of anxiety and pain behaviors.