Molecular Adaptations in the Rat Dorsal Striatum and Hippocampus Following Abstinence-Induced Incubation of Drug Seeking After Escalated Oxycodone Self-Administration

Molecular Neurobiology https://doi.org/10.1007/s12035-018-1318-z Molecular Adaptations in the Rat Dorsal Striatum and Hippocampus Following Abstinence-Induced Incubation of Drug Seeking After Escalated Oxycodone Self-Administration Christopher A. Blackwood 1 & Reece Hoerle 1 & Michael Leary 1 & Jennifer Schroeder 1 & Martin O. Job 1 & Michael T. McCoy 1 & Bruce Ladenheim 1 & Subramaniam Jayanthi 1 & Jean Lud Cadet 1 Received: 7 May 2018 / Accepted: 14 August 2018 # The Author(s) 2018 Abstract Repeated exposure to the opioid agonist, oxycodone, can lead to addiction. Here, we sought to identify potential neurobiological consequences of withdrawal from escalated and non-escalated oxycodone self-administration in rats. To reach these goals, we used short-access (ShA) (3 h) and long-access (LgA) (9 h) exposure to oxycodone self-administration followed by protracted forced abstinence. After 31 days of withdrawal, we quantified mRNA and protein levels of opioid receptors in the rat dorsal striatum and hippocampus. Rats in the LgA, but not the ShA, group exhibited escalation of oxycodone SA, with distinction of two behavioral phenotypes of relatively lower (LgA-L) and higher (LgA-H) oxycodone takers. Both LgA, but not ShA, phenotypes showed timedependent increases in oxycodone seeking during the 31 days of forced abstinence. Rats from both LgA-L and LgA-H groups also exhibited decreased levels of striatal mu opioid receptor protein levels in comparison to saline and ShA rats. In contrast, mu opioid receptor mRNA expression was increased in the dorsal striatum of LgA-H rats. Moreover, hippocampal mu and kappa receptor protein levels were both increased in the LgA-H phenotype. Nevertheless, hippocampal mu receptor mRNA levels were decreased in the two LgA groups whereas kappa receptor mRNA expression was decreased in ShA and LgA oxycodone groups. Decreases in striatal mu opioid receptor protein expression in the LgA rats may serve as substrates for relapse to drug seeking because these changes occur in rats that showed incubation of oxycodone seeking. Keywords Oxycodone . Opioid receptors . Protein . mRNA . Incubation . Dorsal striatum . Hippocampus Introduction Addiction to opioid agonists is a public health menace [1, 2]. This is related to the over-prescription and illicit use of these agents, including oxycodone, for the treatment of various pain syndromes [3, 4]. Patients treated with these drugs usually increase their intake of opioids as they become tolerant to the clinical effects of the drugs or suffer from withdrawal signs and symptoms and go through repeated relapses [5]. Electronic supplementary material The online version of this article (https://doi.org/10.1007/s12035-018-1318-z) contains supplementary material, which is available to authorized users. * Jean Lud Cadet 1 Molecular Neuropsychiatry Research Branch, NIH/NIDA Intramural Research Program, 251 Bayview Boulevard, Baltimore, MD 21224, USA Unfortunately, efforts to treat some of these clinical manifestations of abuse have not always been met with a great degree of success and may have complicated the clinical history of oxycodone by promoting drug craving, one of the driving forces of repeated relapses marred by compulsive drug seeking and uncontrollable use [5]. Attempts to treat opioid drug craving have targeted classical opioid receptors [5, 6]. These include mu (OPRM1), delta (OPRD1), and kappa (OPRK1) opioid receptors [7–9]. Opioid receptors are members of the G protein coupled receptors family that can form homo- and heterodimeric complexes and signal via kinase cascades [10, 11]. These receptors may also play relevant and diverse roles in the signs and symptoms of opioid withdrawal. For example, Src-dependent phosphorylation of mu receptors is a prerequisite for naloxone-induced withdrawal in mice treated chronically with morphine [12] while constitutive mu receptor activation is also enhanced in the ventral tegmental area (VTA) of animals undergoing morphine withdrawal [13]. Moreover, antagonism of kappa receptors with nor- Mol Neurobiol binaltorphine (nor-BNI) was reported to reduce morphine withdrawal symptoms in rats [14]. Thus, in order to develop more logical therapeutic approaches against addiction to opioids and other drugs, it is essential to understand the biochemical and molecular neurobiology of drug seeking after long periods of abstinence from drug self-administration (SA) [4, 15]. In the case of oxycodone addiction, it has been shown that animals, given various lengths of access to the drug during SA experiments, will show various degrees of escalation of oxycodone intake and exhibit compulsive drug seeking [16–18], behaviors that may be secondary to neuroadaptive changes in striatum-dependent habitual behaviors [19–21] and/or hippocampus-mediated mnemonic properties [22–25]. These circuits and their potential roles in addictive processes have been reviewed extensively [26]. At present, however, very little is known about the biochemical and molecular consequences of long-term oxycodone exposure to the brain. In a first attempt to fill some of these gaps, we have used both short- and longaccess oxycodone self-administration to identify potential outcomes of forced abstinence on drug-seeking behaviors. We also wanted to identify the effects of withdrawal from oxycodone self-administration on biochemical and molecular markers of opioid circuitries in the rat dorsal striatum and hippocampus in animals that showed variable cue-induced behavioral responses. To achieve these goals, we have measured the mRNA and protein expression of the three opioid receptors in these two brain regions that express the three receptors [27–29]. Parenthetically, oxycodone has also been reported to interact with the three relevant opioid receptor proteins [30–33]. Materials and Methods Subjects Male Sprague Dawley rats, (Charles River, Raleigh, NC, USA) weighing 350–400 g before surgery, were used in our experiments. Rats were maintained on a 12-h reversed light/ dark cycle with food and water available ad libitum. All procedures followed the guidelines outlined in the National Institutes of Health (NIH) Guide for the Care and Use of Laboratory Animals (eighth Edition, https://guide-for-thecare-and-use-of-laboratory-animals.pdf) and were approved by the local NIDA (National Institute of Drug Abuse) Animal Care and Use Committee. Intravenous Surgery Surgical implantations of intravenous catheter were done as previously described [34]. Briefly, we anesthetized the rats with an intraperitoneal injection of ketamine (50 mg/kg) and xylazine (5 mg/kg) and inserted polyurethane catheters (SAI Infusion Technologies, Lake Villa, IL) into the jugular vein. One end of the catheters was in the jugular vein while the other end was attached to modified 22-gauge cannulas that were mounted to the rats’ backs with dental cement to serve as catheter externalized infusion ports. The catheter infusion ports were close (...truncated)