Oxytocin Reduces Sensitized Stress-Induced Alcohol Relapse in a Model of Posttraumatic Stress Disorder and Alcohol Use Disorder Comorbidity

Abstract

Background: There is high comorbidity of posttraumatic stress disorder (PTSD) and alcohol use disorder with few effective treatment options. Animal models of PTSD have shown increases in alcohol drinking, but effects of stress history on subsequent vulnerability to alcohol relapse have not been examined. Here we present a mouse model of PTSD involving chronic multimodal stress exposure that resulted in long-lasting sensitization to stress-induced alcohol relapse, and this sensitized stress response was blocked by oxytocin (OT) administration.

Methods: Male and female mice trained to self-administer alcohol were exposed to predator odor (TMT) + yohimbine over 5 consecutive days or left undisturbed. After reestablishing stable alcohol responding/intake, mice were tested under extinction conditions, and then all mice were exposed to TMT or context cues previously associated with TMT before a reinstatement test session. Separate studies examined messenger RNA expression of Oxt and Oxtr in hypothalamus following chronic stress exposure. A final study examined the effects of systemic administration of OT on stress-induced alcohol relapse in mice with and without a history of chronic stress experience.

Results: Chronic stress exposure produced long-lasting sensitization to subsequent stress-induced alcohol relapse that also generalized to stress-related context cues and transcriptional changes in hypothalamic OT system. OT injected before the reinstatement test session completely blocked the sensitized stress-induced alcohol relapse effect.

Conclusions: Collectively, these results provide support for the therapeutic potential of OT, along with highlighting the value of utilizing this model in evaluating other pharmacological interventions for treatment of PTSD/alcohol use disorder comorbidity.

Keywords: Alcohol relapse; Alcohol use disorder; Comorbidity; Oxytocin; Posttraumatic stress disorder; Stress.

Figure 1:. Sensitized Stress-Induced Alcohol Relapse-Like Behavior in Mice with a History of Chronic Stress Exposure.

(A) Experimental procedures and timeline for alcohol self-administration, chronic stress exposure, extinction testing, and reinstatement testing phases of the study. Lever responses for (B, C) males and (D, E) females during each phase of the study for mice in the chronic stress group or no-stress control (CTL) condition. Red arrow denotes 5-day stress exposure period. Values are mean ± s.e.m. active lever responses during baseline alcohol self-administration (last 5 days prior to chronic stress exposure and last 5-days prior to extinction testing), the last 5 days of extinction responding, and responses during the reinstatement test. Acute predator odor (TMT) exposure significantly increased alcohol-seeking behavior in CTL male and female mice above their level of responding during extinction, and this effect was significantly greater in mice with a history of chronic stress exposure. *** significantly differs from respective extinction responding (ps< 0.001); ## significantly differs from TMT-induced reinstatement responding in CTL group (p< 0.01).

Figure 2:. Sustained Stress/Emotional Reactivity During Repeated Stress Exposure Sessions.

Mean ± s.e.m. defecation droppings for (A) male and (B) female mice during each of five daily consecutive stress exposure sessions. Stress-exposed mice received yohimbine (2 mg/kg) 15-min being placed in a chamber for 15-min exposure to TMT. CTL mice were injected with saline and then placed in a control chamber.. *** significantly differs from control group (main effect of TMT exposure; ps< 0.001); ^ significantly differs from control group at each respective exposure session (ps< 0.01).

Figure 3:. Long-Lasting Sensitization of Stress-Induced Alcohol Relapse-Like Behavior in Mice with a History of Chronic Stress Exposure.

(A) Experimental procedures and timeline for alcohol self-administration, chronic stress exposure, extinction testing, and reinstatement testing phases of the study. (B) Mean ± s.e.m. active lever responses during each phase of the study for male mice in the chronic stress group or no-stress control (CTL) condition. Red arrow denotes 5-day multimodal stress exposure period. During the first reinstatement test, acute TMT exposure significantly increased alcohol relapse-like responding in both CTL and Stress mice above their respective levels of extinction responding, and this effect was significantly greater in mice with a history of chronic stress exposure. After re-establishing stable baseline alcohol self-administration followed by extinction responding, the same profile of results was observed during a second reinstatement test, which occurred about 60 days following chronic stress exposure. * significantly differs from CTL group extinction responding (ps< 0.05); ***significantly differs from Stress group extinction responding (ps< 0.001); ## significantly differs from TMT-induced reinstatement responding in CTL group (ps< 0.01). (C) Lever responding for individual male mice in Stress and CTL groups during the final extinction session preceding stress (TMT)-induced reinstatement testing at ~30 days and ~60 days following the 5-day chronic stress treatment.

Figure 4:. Exposure to Context Cues Previously Associated with Chronic Stress Enhances Alcohol Relapse-Like Behavior in Male and Female Mice.

(A) Experimental procedures and timeline for alcohol self-administration, chronic stress exposure, extinction testing, and reinstatement testing phases of the study. Note: during reinstatement testing mice were exposed to environmental (context) cues in the absence of TMT. Mean ± s.e.m. lever responses for (B) male and (C) female mice during each phase of the study for Stress and CTL groups. Red arrow denotes 5-day yohimbine+TMT exposure in unique environmental context. Male and female CTL mice exposed to the environmental cues for the first time prior to the reinstatement test session showed significantly increased alcohol-seeking behavior while male and female mice exposed to the context cues previously associated with chronic stress exposure (Stress group) evidenced a significantly greater alcohol relapse-like response. ** significantly differs from CTL group extinction responding (ps< 0.01); *** significantly differs from Stress group extinction responding (ps< 0.001); ### significantly differs from stress-induced reinstatement responding in CTL group (ps< 0.001). Lever responding for individual (D) male and (E) female mice in Stress and CTL groups during the final extinction session preceding context cue-induced reinstatement testing.

Figure 5:. Chronic Stress Exposure Produces Long-Lasting Changes Hypothalamic Oxt and Oxtr mRNA Expression.

(A) Schematic representation of tissue punch used for determination of transcriptional changes in oxytocin and its receptor in the hypothalamus. Mice were sacrificed and brain tissue was collected at 48-hr or 60-days following chronic (5-day) stress exposure. (B) Oxt mRNA expression in the hypothalamus of alcohol-naïve male and female mice was significantly reduced 48-hr following the final stress exposure relative to no-stress controls, and this reduction persisted for at least 60-days following the chronic stress treatment. ** significantly differs from CTL group (ps< 0.01). (C) Oxtr mRNA expression in the hypothalamus was significantly elevated in male mice but reduced (trend) in females relative to the controls at 48-hr following the chronic stress procedure. ** significantly differs from respective CTL group (p< 0.01). Oxtr mRNA levels were significantly elevated in both male and female mice relative to CTL levels at 60-days post-stress treatment. * significantly differs from respective CTL group (ps< 0.05).

Figure 6:. Oxytocin Blocks Sensitized Stress-Induced Alcohol Relapse-Like Behavior in Mice with a History of Chronic Stress Exposure.

(A) Experimental procedures and timeline for alcohol self-administration, chronic stress exposure, extinction testing, and reinstatement testing phases of the study. Mean ± s.e.m. lever responses for (B) male and (C) female mice during each phase of the study for Stress and CTL groups treated with OT (1 mg/kg) or Vehicle prior to reinstatement testing. In Vehicle-treated CTL male and female mice, acute TMT exposure significantly increased alcohol-seeking behavior above their level of responding during extinction, and this effect was significant greater in mice with a history of chronic stress exposure. ** significantly differs from extinction responding in CTL-Veh group (p< 0.01); *** significantly differs from extinction responding in Stress-Veh group (p< 0.001); ### significantly differs from TMT-induced reinstatement responding in CTL group (p< 0.001). OT blocked TMT-induced alcohol-seeking behavior in CTL and Stress male and female groups. ^^^ significantly differs from Vehicle-treated CTL and Stress groups (p< 0.001). Lever responding for individual (D) male and (E) female mice in Stress and CTL groups during the final extinction session prior to reinstatement testing when mice were pretreated with Vehicle or OT (1 mg/kg).