Unpredictability as a modulator of drug self-administration: Relevance for substance-use disorders

Unpredictable, intermittent access to sucrose or water promotes increased reward pursuit in rats

Reward uncertainty can sensitize reward pathways, promoting increased reward-seeking and -taking behaviours. This is relevant to human conditions such as pathological gambling, eating disorders and drug addiction. In the context of addiction, preclinical self-administration procedures have been developed to model the intermittency of human drug use. These intermittent-access (IntA) procedures involve intermittent but predictable access to drug during self-administration sessions. However, human drug use typically involves intermittent and unpredictable drug access. We introduce a new procedure modeling unpredictable, intermittent access (UIntA) to a reinforcer, and we compare it to procedures that provide predictable reinforcer availability; continuous (ContA) or intermittent (IntA) access. Female rats self-administered water or liquid sucrose in daily hour-long sessions. IntA and ContA rats had access to a fixed volume of water or sucrose (0.1 ml), under a fixed ratio 3 schedule of reinforcement. IntA rats had predictable 5-min reinforcer ON and 25-min reinforcer OFF periods. ContA rats had 60 min of reinforcer access during each session. For UIntA rats, variation in the length of ON and OFF periods (1, 5 or 9 min/period), response requirement (variable ratio 3 schedule of reinforcement), reinforcer probability (50%) and quantity (0, 0.1 or 0.2 ml) introduced reward uncertainty. Following 14 daily self-administration sessions, UIntA rats showed the highest levels of responding for water or sucrose under progressive ratio conditions, responding under extinction conditions, and cue-induced reinstatement of sucrose seeking. Thus, unpredictable, intermittent reward access promotes increased reward pursuit. This has implications for modeling addiction and other disorders of increased reward seeking.

Choice between food and cocaine or fentanyl reinforcers under fixed and variable schedules in female and male rhesus monkeys

RATIONALE

Illicit drugs may be unpredictable in terms of the time and effort required to obtain them, and this can be modeled with variable- (VR) vs. fixed-ratio (FR) schedules. In a recent experiment (Zamarripa et al. 2023), the potency of cocaine to maintain choice was greatest under a VR (compared with a FR) when food was available under a FR schedule.

OBJECTIVES

The goal of the current study was to extend prior choice results with VR vs. FR schedules to a more efficient procedure with cocaine or fentanyl vs. food. Furthermore, the FR schedule of food delivery was manipulated to determine whether increased drug choice under a VR (compared with a FR) schedule depends on the size of the schedule of nondrug reinforcement.

METHODS

Adult female (n = 2) and male (n = 4) monkeys chose between cocaine (0-30 µg/kg/injection) or fentanyl (0-1.0 µg/kg/injection) and food (2 pellets/delivery) under a 5-component procedure. In different conditions, food was available under a FR 25, 50, or 100 and cocaine or fentanyl were available under FR or VR 100 schedules.

RESULTS

Cocaine's potency to maintain choice was greatest under a VR 100 (compared with FR 100) when food was available under a FR 50 or 100, and fentanyl's potency to maintain choice was generally greatest under a VR 100 (compared with FR 100) when food was available under a FR 25 or 100. However, outcomes between FR and VR schedules with fentanyl were less robust compared with cocaine.

CONCLUSION

Variability in the time and effort required to obtain illicit drugs could contribute to excessive allocation of behavior toward drug use at the expense of more predictable nondrug alternatives, supporting treatment or policies aimed at making drug access more predictable through agonist medications or a safe supply. The impact of variable requirements on drug choice may be reduced if nondrug reinforcers are relatively less costly, supporting the use of low-cost reinforcers in behavioral therapies like contingency management.

Environmental Uncertainty and Substance Use Disorders: A Behavior Analytic Perspective

Substance use disorder (SUD) and drug overdose deaths represent major economic, health, and safety issues in the United States. The psychology of uncertainty provides a mechanism for understanding, reducing, and controlling the damage from substance misuse. Illicit drugs (such as heroin or cocaine) are uncertain in their availability, quality, and acquisition (the time and effort required to obtain them) compared with nondrug-related alternatives (such as consumable goods, hobbies, or paychecks). Furthermore, the severity and likelihood of negative outcomes associated with drug use likewise are uncertain. Such uncertainties worsen substance use outcomes. The current review conveys what is known about the impact of uncertainty on substance use: laboratory investigations of uncertain time and effort required to obtain a substance and uncertain drug quality show uncertainty exacerbates harm. Furthermore, uncertain negative outcomes are not likely to deter substance use in individuals with a SUD. Finally, several policy implications include access to agonist medications; creating a safer drug supply; access to clean syringes/needles, naloxone, and safe-injection sites; and ending incarceration for substance use.

Altered expression of somatostatin signaling molecules and clock genes in the hippocampus of subjects with substance use disorder

Substance use disorders are a debilitating group of psychiatric disorders with a high degree of comorbidity with major depressive disorder. Sleep and circadian rhythm disturbances are commonly reported in people with substance use disorder and major depression and associated with increased risk of relapse. Hippocampal somatostatin signaling is involved in encoding and consolidation of contextual memories which contribute to relapse in substance use disorder. Somatostatin and clock genes also have been implicated in depression, suggesting that these molecules may represent key converging pathways involved in contextual memory processing in substance use and major depression. We used hippocampal tissue from a cohort of subjects with substance use disorder (n = 20), subjects with major depression (n = 20), subjects with comorbid substance use disorder and major depression (n = 24) and psychiatrically normal control subjects (n = 20) to test the hypothesis that expression of genes involved in somatostatin signaling and clock genes is altered in subjects with substance use disorder. We identified decreased expression of somatostatin in subjects with substance use disorder and in subjects with major depression. We also observed increased somatostatin receptor 2 expression in subjects with substance use disorder with alcohol in the blood at death and decreased expression in subjects with major depression. Expression of the clock genes Arntl, Nr1d1, Per2 and Cry2 was increased in subjects with substance use disorder. Arntl and Nr1d1 expression in comparison was decreased in subjects with major depression. We observed decreased expression of Gsk3β in subjects with substance use disorder. Subjects with comorbid substance use disorder and major depression displayed minimal changes across all outcome measures. Furthermore, we observed a significant increase in history of sleep disturbances in subjects with substance use disorder. Our findings represent the first evidence for altered somatostatin and clock gene expression in the hippocampus of subjects with substance use disorder and subjects with major depression. Altered expression of these molecules may impact memory consolidation and contribute to relapse risk.