Maladaptive choices by defeated rats: link between rapid approach to social threat and escalated cocaine self-administration

Intermittent social stress produces different short- and long-term effects on effort-based reward-seeking behavior

Previous studies show that intermittent social defeat (ISD) stress increases self-administration of psychostimulants, which suggests that ISD promotes reward-seeking behavior and, ultimately, increases vulnerability to develop drug abuse. The present study investigates whether ISD alters cost/benefit evaluations to promote reward-seeking behavior and whether these alterations are time-dependent. Male rats performed two different tasks that assessed their motivation to seek and consume food rewards. An effort-discounting task in which rats chose between less and more effortful options (i.e., 1 lever-press versus 2, 5, 10 or 20 lever-presses) associated with low- and high-reward (i.e., 1 sugar pellet versus 3 sugar pellets), respectively; and a progressive ratio task in which rats had to increase their effort (more lever presses) to obtain a sugar pellet. ISD consisted of exposing animals to social defeat once every three days for ten days (4 stress episodes). Rats were tested 24-48 h after stress episodes, and 1 week and 6 weeks after the last stress episode. In the effort-discounting task, stressed animals showed a decrease in their preference for high rewards associated with more effort (i.e., 10 and 20 lever-presses). These effects were transient and not maintained one week after stress. In the progressive ratio task, stressed animals showed an increase in the number of lever presses to obtain rewards that emerged six weeks after the last stress episode. These results suggest different short- and long-term effects on the motivation for rewards after ISD and indicate temporal dynamic adaptations in the function of the brain reward system.

Role of Mesolimbic Brain-Derived Neurotrophic Factor in Depression

Brain-derived neurotrophic factor (BDNF) is widely accepted as being critical for neural and synaptic plasticity throughout the nervous system. Recent work has shown that BDNF in the mesolimbic dopamine (DA) circuit, originating in ventral tegmental area DA neurons that project to the nucleus accumbens, is crucial in the development of depressive-like behaviors following exposure to chronic social defeat stress in mice. Whereas BDNF modulates DA signaling in encoding responses to acute defeat stress, BDNF signaling alone appears to be responsible for the behavioral effects after chronic social defeat stress. Very different patterns are seen with another widely used chronic stress paradigm in mice, chronic mild stress (also known as chronic variable or unpredictable stress), where DA signaling, but not BDNF signaling, is primarily responsible for the behavioral effects observed. This review discusses the molecular, cellular, and circuit basis of this dramatic discrepancy, which appears to involve the nature of the stress, its severity and duration, and its effects on distinct cell types within the ventral tegmental area-to-nucleus accumbens mesolimbic circuit.

Social defeat stress and escalation of cocaine and alcohol consumption: Focus on CRF

Both the ostensibly aversive effects of unpredictable episodes of social stress and the intensely rewarding effects of drugs of abuse activate the mesocorticolimbic dopamine systems. Significant neuroadaptations in interacting stress and reward neurocircuitry may underlie the striking connection between stress and substance use disorders. In rodent models, recurring intermittent exposure to social defeat stress appears to produce a distinct profile of neuroadaptations that translates most readily to the repercussions of social stress in humans. In the present review, preclinical rodent models of social defeat stress and subsequent alcohol, cocaine or opioid consumption are discussed with regard to: (1) the temporal pattern of social defeat stress, (2) male and female protocols of social stress-escalated drug consumption, and (3) the neuroplastic effects of social stress, which may contribute to escalated drug-taking. Neuroadaptations in corticotropin-releasing factor (CRF) and CRF modulation of monoamines in the ventral tegmental area and the bed nucleus of the stria terminalis are highlighted as potential mechanisms underlying stress-escalated drug consumption. However, the specific mechanisms that drive CRF-mediated increases in dopamine require additional investigation as do the stress-induced neuroadaptations that may contribute to the development of compulsive patterns of drug-taking.

Social Defeat Stress, Sex, and Addiction-Like Behaviors

Social confrontation is a form of social interaction in animals where two conspecific individuals confront each other in dispute over territory, during the formation of hierarchies, and during breeding seasons. Typically, a social confrontation involves a prevailing individual and a yielding individual. The prevailing individual often exhibits aggressive postures and launches attacks, whereas the yielding individual often adopts postures of defeat. The yielding or defeated animals experience a phenomenon known as social defeat stress, in which they show exaggerated stress as well as autonomic and endocrine responses that cause impairment of both the brain and body. In laboratory settings, one can reliably generate social defeat stress by allowing a naïve (or already defeated) animal to intrude into a home cage in which its resident has already established a territory or is nursing. This resident-intruder paradigm has been widely used in both males and females to study mechanisms in the brain that underlie the stress responses. Stress has profound effects on drug reward for cocaine, methamphetamine, alcohol, and opioids. Particularly, previous experiences with social defeat can exaggerate subsequent addiction-like behaviors. The extent of these addiction-like behaviors depends on the intensity, duration, frequency, and intermittency of the confrontation episodes. This chapter describes four types of social defeat stress: acute, repeated, intermittent, and chronic. Specifically, it focuses on social defeat stress models used in laboratories to study individual, sex, and animal strain differences in addiction-like behaviors.

Reversal of Stress-Induced Social Interaction Deficits by Buprenorphine

BACKGROUND

Patients with post-traumatic stress disorder frequently report persistent problems with social interactions, emerging after a traumatic experience. Chronic social defeat stress is a widely used rodent model of stress that produces robust and sustained social avoidance behavior. The avoidance of other rodents can be reversed by 28 days of treatment with selective serotonin reuptake inhibitors, the only pharmaceutical class approved by the U.S. Food and Drug Administration for treating post-traumatic stress disorder. In this study, the sensitivity of social interaction deficits evoked by 10 days of chronic social defeat stress to prospective treatments for post-traumatic stress disorder was examined.

METHODS

The effects of acute and repeated treatment with a low dose of buprenorphine (0.25 mg/kg/d) on social interaction deficits in male C57BL/6 mice by chronic social defeat stress were studied. Another cohort of mice was used to determine the effects of the selective serotonin reuptake inhibitor fluoxetine (10 mg/kg/d), the NMDA antagonist ketamine (10 mg/kg/d), and the selective kappa opioid receptor antagonist CERC-501 (1 mg/kg/d). Changes in mRNA expression of Oprm1 and Oprk1 were assessed in a separate cohort.

RESULTS

Buprenorphine significantly reversed social interaction deficits produced by chronic social defeat stress following 7 days of administration, but not after acute injection. Treatment with fluoxetine for 7 days, but not 24 hours, also reinstated social interaction behavior in mice that were susceptible to chronic social defeat. In contrast, CERC-501 and ketamine failed to reverse social avoidance. Gene expression analysis found: (1) Oprm1 mRNA expression was reduced in the hippocampus and increased in the frontal cortex of susceptible mice and (2) Oprk1 mRNA expression was reduced in the amygdala and increased in the frontal cortex of susceptible mice compared to non-stressed controls and stress-resilient mice.

CONCLUSIONS

Short-term treatment with buprenorphine and fluoxetine normalized social interaction after chronic social defeat stress. In concert with the changes in opioid receptor expression produced by chronic social defeat stress, we speculate that buprenorphine's efficacy in this model of post-traumatic stress disorder may be associated with the ability of this compound to engage multiple opioid receptors.

Social Status in Monkeys: Effects of Social Confrontation on Brain Function and Cocaine Self-Administration

Individual differences in response to social stress and environmental enrichment may contribute to variability in response to behavioral and pharmacological treatments for drug addiction. In monkeys, social status influences the reinforcing effects of cocaine and the effects of some drugs on cocaine self-administration. In this study, we used male cynomolgus macaques (n=15) living in established social groups to examine the effects of social confrontation on the reinforcing effects of cocaine using a food-drug choice procedure. On the test day, a dominant or subordinate monkey was removed from his homecage and placed into another social pen; 30 min later he was studied in a cocaine-food choice paradigm. For the group, following social confrontation, sensitivity to cocaine reinforcement was significantly greater in subordinate monkeys compared with dominant animals. Examining individual-subject data revealed that for the majority of monkeys (9/15), serving as an intruder in another social group affected cocaine self-administration and these effects were dependent on the social rank of the monkey. For subordinate monkeys, sensitivity to the reinforcing effects of cocaine increased while sensitivity decreased in dominant monkeys. To investigate potential mechanisms mediating these effects, brain glucose metabolism was studied in a subset of monkeys (n=8) using [18F]fluorodeoxyglucose ([18F]FDG) with positron emission tomography. Dominant and subordinate monkeys displayed distinctly different patterns of brain glucose metabolism in their homecage, including areas associated with vigilance and stress/anxiety, respectively, and during social confrontation. These data demonstrate that, depending on an individual's social status, the same social experience can have divergent effects on brain function and cocaine self-administration. These phenotypic differences in response to social conditions support a personalized treatment approach to cocaine addiction.