Chronic Extended Access (18-H) Intravenous Cocaine Self-Administration Procedure: Pattern of Cocaine Intake and Role of the Kappa Opioid Receptor.

Background: Cocaine addiction is a chronic brain disease characterized by compulsive drug intake and dysregulation of brain reward and stress systems. Addicted patients typically undergo cycles of active cocaine use, abstinence and relapse. Few preclinical studies have modeled the natural longitudinal course of cocaine addiction and its molecular consequences. Furthermore, the precise role of exacerbated cocaine intake over time (“escalation”) remains unclear. Identifications of changes in the pattern of cocaine intake during the development of addictive-like states may allow better treatments for vulnerable subjects. Extended access self-administration protocols are powerful methodologies to model the advanced stages of addiction, which may include changes in the daily pattern and amount of total intake. Several protocols have been published in literature; however, few have a duration of drug access greater than 12 hours per day, potentially resulting in limited construct validity. We therefore characterized the behavioral pattern of intake during our recently developed 18-hour intravenous cocaine self-administration protocol, analyzing behavioral difference between high- and low-intake rats, across chronic exposure (14 consecutive daily sessions). Moreover, since a promising pharmacological target to relieve distress during abstinence/ withdrawal, thus potentially preventing relapse, is the kappa opioid receptor (KOPr), we tested the effect of its blockade in relieving neuroendocrine signs of stress and depression, such as serum corticosterone and brain-derived neurotrophic factor in rats in early (30-hour) withdrawal from chronic cocaine self-administration and compared it to yokedsaline rats. Methods: Adult male Sprague Dawley rats were implanted with a catheter in the jugular vein under isoflurane anesthesia. Upon recovery, rats were trained to press a lever to receive 0.5 mg/kg cocaine unit doses, associated with a light cue (FR1). Training sessions lasted 2h; rats then underwent a 14-day self-administration paradigm, in which they had free access to cocaine for 18 hours/day. Analysis of daily intake and hourly active lever presses during the 18-h self-administration procedure was performed. Yoked-saline rats were run concurrently and paired with cocaine rats, and received an equal number of infusions of physiological saline. Cocaine and yoked-saline rats were sacrificed 30 hours in withdrawal, their brains were dissected and brain regions were analyzed through custom microarray analysis as well as qPCR gene expression. An additional cohort was devoted to the serum CORT and BDNF quantification comparing cocaine-exposed and yoked-saline rats that received acute systemic administration of a selective short-acting kappa opioid receptor antagonist (LY2444296; 0, 3 mg/kg). Data were analyzed with factorial or repeated measures two-way ANOVA, followed by the Student Neumann Keul’s test for multiple comparisons. Significance was set at po0.05. Results: Over 14 days of 18-h self-administration sessions, the hourly pattern of intake changed between the early sessions and the later ones. Thus during the first week of access rats showed greater intake during the first hours of each operant session, whereas during the second week of self-administration rats showed the greater intake during the last hours of the sessions. This change in behavior across days was particularly pronounced in high-intake rats, compared to low-intake rats. Additionally, rats in early withdrawal from chronic extended access cocaine selfadministration showed depressive and anxiety-like behavior compared to yoked-saline rats, as well as an overall reduction in circulating serum CORT. Administration of a selective short-acting kappa opioid receptor antagonist affected serum CORT level. Rats re-exposed to the self-administration sessions after 2 weeks of abstinence showed cocaine intake comparable to the last session before withdrawal. Of note, there was a significant positive correlation between rat cocaine intake during the last day of withdrawal and the first session of re-exposure. Conclusions: Taken together, these results demonstrate that the pattern of self-infusions of cocaine changes over time during chronic extended access 18h self-administration sessions, as well as a characteristic pattern of within-day circadian changes of self-exposure. A single dose of KOPr antagonist with “medication-like” duration of action prevented cocaine withdrawal-induced neuroendocrine changes. Further studies are needed to determine the effectiveness in preventing relapse-like and depressant-like or anxiety like behaviors in specific addictive diseases.