Uridine attenuates morphine-induced conditioned place preference and regulates glutamate/GABA levels in mPFC of mice

Highlights

• Uridine blocked the rewarding effects of morphine.

• Levels of glutamate and GABA in the mPFC may be related to stress-induced relapse.

• Uridine depressed the glutamate overflow in the stress-induced relapse.

• Uridine increased the GABA levels in the stress-induced relapse.

Abstract

Several lines of evidence suggest that uridine, as a neuromodulator, plays an important role in drug addiction. We previously found that uridine circumvents morphine-induced behavioral sensitization by decreasing the extracellular dopamine levels in the dorsal striatum. In the present study, the effects of uridine on morphine-induced conditioned place preference (CPP) and the possible roles of glutamate and GABA in the stress-induced reinstatement of CPP were investigated. First, the effects of uridine (1, 10 and 100 mg/kg, i.p.) on the four defined phases - acquisition, expression, extinction and reinstatement (drug priming and restraint stress) - of morphine-induced CPP were studied. The results showed that pretreatment with uridine significantly blocked the acquisition and expression phases of CPP. Additionally, uridine also facilitated CPP extinction and inhibited stress-induced CPP reinstatement, although it failed to affect drug-induced CPP reinstatement. Since glutamatergic and GABAergic systems are both involved in CPP reinstatement, the extracellular levels of glutamate and GABA in the mPFC during the stress-induced CPP reinstatement were determined using in vivo microdialysis. The results showed that uridine attenuated the stress-induced glutamate increase in the mPFC without influencing the basal glutamate levels, and increased the levels of extracellular GABA in the mPFC both under normal physiological conditions and after the stress stimulus. Thus, our results indicate that uridine depresses the stress-induced reinstatement of CPP, simultaneously regulating glutamatergic and GABAergic neurotransmission in the mPFC. The present work provides further understanding of the role of uridine in morphine-induced neurobehavioral changes.

Introduction

Uridine, a pyrimidine nucleoside, is responsible for the synthesis of RNA, glycogen and biological membranes. It has been established that oral administration of uridine can increase uridine levels in the plasma as well as in the brain, and that uridine exerts extensive effects in the central nervous system (CNS), for example through its sleep-promoting and anti-epileptic properties, and by influencing neuronal plasticity (Cansev et al., 2005, Dobolyi et al., 2011, Guarneri et al., 1985). Beyond that, several lines of evidence suggest that uridine also plays an important role in drug addiction. For instance, uridine administration significantly inhibits the locomotor activity induced by amphetamine and reduces the rotation induced by methamphetamine in 6-OHDA-treated rats (Myers et al., 1995, Myers et al., 1993). We have previously reported that morphine significantly increases the spontaneous uridine release from mouse dorsal striatum in a transporter-dependent manner (Song et al., 2013). Furthermore, exogenous uridine was able to circumvent the acute morphine-induced hyperactivity as well as the established behavioral sensitization by reducing the extracellular dopamine levels in the dorsal striatum (Liu et al., 2014).

Conditioned place preference (CPP), or place conditioning, has been widely used to assess the rewarding effects of a variety of drugs, including opiates (Tzschentke, 2007). The CPP paradigm involves four major phases - acquisition, expression, extinction and reinstatement - which simulate the activation, expression, withdrawal and relapse, respectively, of drug abusers (Maldonado et al., 2007, Shoblock et al., 2005). Specifically, the reinstatement paradigm has been used to evaluate the effect of interventions on blocking the relapse-like behaviors which can be caused by a small dose of the drug or various social pressures in humans (Shaham et al., 2003). Just as CPP provides an animal model for the formation of drug-associated cues, it has been used to uncover the neurochemical mechanisms involved in rewarding and addiction development (Tomazi et al., 2017).

The mPFC has received considerable attention because of its involvement in drug rewarding behaviors, such as self-administration, CPP and behavioral sensitization (Goeders et al., 1986, Hao et al., 2007b, Zavala et al., 2003). The glutamatergic projection from the mPFC has been demonstrated to be closely related to its role in drug abuse. Bishop et al. reported that the dysfunction of glutamate receptors in the mPFC can amplify the behavioral reward of opiates (Bishop et al., 2011). Likewise, our previous studies demonstrated that the glutamatergic system was involved in morphine dependence and withdrawal (Hao et al., 2005, Hao et al., 2007b). GABA and its receptors have also been proved to play critical roles in the processes underlying substance dependence (Davies, 2003, Zhou et al., 2010). We also reported the dysregulation of glutamatergic and GABAergic neurotransmission in methamphetamine-treated mice (Han et al., 2012, Han et al., 2014, Qi et al., 2012, Qi et al., 2009). Thus, both glutamatergic and GABAergic neurotransmission can occur in response to rewarding drugs.

Although our previous study clearly indicated that uridine may reduce the morphine-induced stimulant effects using the model of behavioral sensitization, the effects of uridine on morphine craving have not been clarified. Thus, in the present study, we aimed at investigating whether uridine was able to intervene in the different stages of morphine-induced CPP. Moreover, we employed in vivo microdialysis to clarify the effect of uridine on changing the extracellular glutamate and GABA levels in the mPFC of mice during stress-induced CPP reinstatement.