The modulation of methamphetamine-induced behaviours by oxytocin in the nucleus accumbens core and subthalamic nucleus

The psychostimulant methamphetamine (METH) is an addictive illicit drug, which is commonly abused on a global scale. Repeat administration of the drug is associated with a range of long-term adverse effects and effective pharmacotherapies for METH dependence are currently lacking. The neuropeptide oxytocin has been identified as a potential pharmacotherapeutic agent due to the ability of systemic administration of this peptide to modulate METH-related reward and METH-seeking behaviour. This modulation is thought to occur through the attenuation of dopamine activity and release. Recent findings identified the nucleus accumbens (NAc) core and subthalamic nucleus (STh) as key regions involved in oxytocin modulation of acute METH-related reward. However, little is known about the mechanisms by which oxytocin modulates METH-related behaviours, which in turn limits a thorough understanding of the implications of using oxytocin as a pharmacotherapy for METH dependence. Taken together, this thesis aimed to examine the ability of oxytocin to modulate METH-related reward and relapse to METH seeking within the NAc core and STh through the utilisation of animal models of reward and addiction, incorporating pharmacological, cellular, and biochemical investigation. In the first experimental chapter of this thesis (Chapter 2), oxytocin In the first experimental chapter of this thesis (Chapter 2), oxytocin modulation of dopamine-related reward in the STh was investigated using the conditioned place preference paradigm. Following a single conditioning session, male Sprague Dawley rats formed a place preference for the context paired with a microinjection of dopamine (100 nmol/side) into the STh (200 nl/side) and this preference was blocked by the co-administration of oxytocin (0.6 pmol/side). In addition, the inhibitory effect of oxytocin on dopamine place preference formation was reversed by the concurrent administration of desGly-NH₂d(CH₂)₅[DTyr², Thr⁴]OVT (3 nmol/side), a selective oxytocin receptor (OTR) antagonist into the STh. These findings suggest that oxytocin modulated dopamine-related reward within the STh through the OTR. Considering the detrimental impact that METH abuse has on the individual, the following two chapters examined the effect of exogenous oxytocin on METH reinforcement. More specifically, the ability of oxytocin to modulate relapse to METH-seeking behaviour when microinjected into either the NAc core (Chapter 3) or STh (Chapter 4) was examined using the drug-primed reinstatement model of intravenous drug self-administration. Chapter 3 determined in male Sprague Dawley rats that oxytocin (0.5 pmol, 1.5 pmol, 4.5 pmol/side) microinjected into the NAc core (500 nl/side) reduced METH-primed reinstatement (1 mg/kg, i.p.) and that the co-administration of the selective OTR antagonist used in Chapter 2 surprisingly had a non-specific effect on reinstatement to METH-seeking behaviour. When oxytocin was locally administered into the STh (200 nl/side; Chapter 4), the highest dose tested (3.6 pmol/side) decreased reinstatement to METH-seeking behaviour. Similar to the findings reported in Chapter 3, co-administration of the selective OTR antagonist did not specifically reverse the inhibitory effect of oxytocin on reinstatement to METH-seeking behaviour. The findings of Chapters 3 and 4 suggest that oxytocin mediated METH-primed reinstatement through the activation of receptors beyond the OTR. In light of the findings from Chapters 3 and 4, and the lack of reporting on the role of the OTR in oxytocin attenuation of psychostimulant-induced behaviours, the final experimental chapter (Chapter 5) primarily investigated whether there are cellular changes to the endogenous oxytocin system in the NAc core and STh, as well as changes to oxytocin plasma levels following chronic METH intravenous self-administration (IVSA) and after behavioural extinction. Male Sprague Dawley rats that self-administered METH had higher oxytocin plasma levels, and decreased OTR-immunoreactive fibres in the NAc core than yoked saline rats. After behavioural extinction, oxytocin plasma levels remained elevated, OTR-immunoreactive fibre density increased in the STh that exceeded baseline yoked control levels, and a trend towards normalisation of OTR-immunoreactive fibre density to baseline yoked levels was evident in the NAc core in rats that were previously experienced at METH IVSA compared to yoked controls. These findings demonstrate that the oxytocin system, both centrally within the NAc core and STh, and peripherally through plasma measures, are dysregulated following METH abuse. Chapter 6 discusses implications for the research findings of this thesis and future research directions. The results of this thesis show a direct modulatory role of oxytocin at the NAc core and STh on METH administration. These effects may incorporate the activity of oxytocin at receptors other than the OTR as previous studies have demonstrated interactions with the amino acids glutamate and gamma-aminobutyric acid (GABA) to mediate METHrelated behaviours. Further, oxytocin is known to act through the V1a receptor of the structurally similar neuropeptide arginine vasopressin to modulate prosocial and autonomic effects, suggesting that V1a receptors may be involved in regulating psychostimulant abuse. As such, a more complex interplay incorporating dopamine, glutamate, GABA and vasopressin in oxytocin modulation of METH reward and abuse is discussed. Lastly, a regulated endogenous oxytocin system has been proposed to increase resilience to addiction. This may be through the regulation of the circuits impacted by drug abuse. Intranasal oxytocin administration may then help replenish depleted oxytocin levels in drug-addicted individuals, potentially reducing engagement in drug-seeking and taking behaviours. In conclusion, the results of this thesis demonstrate that oxytocin modulation of METH reward and abuse incorporates the NAc core and STh. In terms of reward, oxytocin attenuates dopamine-driven reward in the STh through activation of the OTR. Oxytocin administration to either the NAc core or STh reduced relapse to METH-seeking behaviour however, additional receptors to the OTR were involved. Regardless of the weak effect of OTR antagonism to alter oxytocin reductions in METH-seeking behaviour, the density of OTR-ir fibres in the NAc core and STh were differentially affected by METH IVSA and following a period of extinction, despite a constant increase in blood plasma levels of oxytocin. These results provide insight into the neurobiological processes of oxytocin and its receptor in regulating METH abuse, with the NAc core and STh as primary brain substrates. Future studies should determine additional receptor interactions by oxytocin in these and other brain regions affected by chronic METH exposure. Overall, the current body of research has important implications for the development of oxytocin-based compounds for pharmacological treatment of METH abuse and dependence.