Sign-trackers have elevated myo-inositol in the nucleus accumbens and ventral hippocampus following Pavlovian conditioned approach

Individual Variation in Intrinsic Neuronal Properties of Nucleus Accumbens Core and Shell Medium Spiny Neurons in Animals Prone to Sign- or Goal-Track

The "sign-tracking" and "goal-tracking" model of individual variation in associative learning permits the identification of rats with different cue-reactivity and predisposition to addiction-like behaviors. Certainly, compared to "goal-trackers" (GTs), "sign-trackers" (STs) show more susceptibility traits such as increased cue-induced 'relapse' of drugs of abuse. Different cue- and reward-evoked patterns of activity in the nucleus accumbens (NAc) have been a hallmark of the ST/GT phenotype. However, it is unknown whether differences in the intrinsic neuronal properties of NAc medium spiny neurons (MSNs) in the core and shell subregions are also a physiological correlate of these phenotypes. We performed whole-cell slice electrophysiology in outbred male rats and found that STs exhibited the lowest excitability in the NAc core, with lower number of action potentials and firing frequency as well as a blunted voltage/current relationship curve in response to hyperpolarized potentials in both the NAc core and shell. Although firing properties of shell MSNs did not differ between STs and GTs, intermediate responders that engage in both behaviors showed greater excitability compared to both STs and GTs. These findings suggest that intrinsic excitability in the NAc may contribute to individual differences in the attribution of incentive salience.

Significance Statement

During associative learning, cues acquire predictive value, but in some instances, they also acquire incentive salience, meaning they take on some of the motivational properties of the reward. The propensity to attribute cues with incentive salience varies between individuals, and excessive attribution can lead to maladaptive behaviors. The "sign-and goal-tracking" model allows us to isolate these two properties and disambiguate the neurobiological processes that govern them. To our knowledge this is the first study characterizing passive and active membrane properties of MSNs in the NAc core and shell of STs and GTs, as well as IRs. These findings are meant to better inform investigations of the distinct role of the NAc in reward learning, particularly in the attribution of incentive salience and addiction predisposition.

1H Nuclear Magnetic Resonance (NMR)-Based Metabolic Changes in Nucleus Accumbens and Medial Prefrontal Cortex Following Administration of Morphine in Mice

INTRODUCTION

It is well known that opiate addiction is a neurobiological disease associated with dysregulation of multiple neurotransmitters and neurochemicals. Previous ex-vivo 1H nuclear magnetic resonance (NMR) studies have yielded mixed findings concerning opiate-induced neurometabolic changes at key reward-addiction sites. Whether such changes reflect the conditions in a live animal remains unknown. The present study was therefore designed to fill this knowledge gap by determining the effects of morphine-induced neurometabolic changes under in-vivo conditions.

METHODS

In-vivo 1H NMR spectroscopy (SA Instruments, Stony Brook, NY) was used to measure neurochemical changes in nucleus accumbens (NAc) and medial prefrontal cortex (mPFC) of mice, subjected to twice-daily injections of morphine (10 mg kg-1 s.c.) for five days.

RESULTS

Morphine induced significant changes in the concentrations of a number of metabolites in both mPFC and NAc. The glutamine component of the glutamine-glutamate-GABA excitatory-inhibitory cycle, increased in both mPFC and NAc. Significant increase in glutamate was also observed at mPFC, but not in NAc. The phosphocreatine, marker for energy metabolism, and the N-acetylaspartate marker for neuronal viability and energy metabolism decreased significantly in both mPFC and NAc. Glycerophosphocholine + phosphocholine, markers for cell membrane integrity, increased significantly in both NAc and mPFC after morphine. The antioxidant neurometabolites taurine and glutathione increased significantly in NAc; however, taurine decreased, and glutathione was unchanged in mPFC after morphine. Inositol, a marker for neuroinflammation, increased significantly in NAc.

CONCLUSION

The present study is the first in-vivo 1H NMR spectroscopy in mice to demonstrate morphine-induced dysregulation of multiple metabolites and neurochemicals within the reward-addiction neurocircuitry.

A mechanical task for measuring sign- and goal-tracking in humans: A proof-of-concept study

Cue-based associative learning (i.e., Pavlovian conditioning) is a foundational component of behavior in almost all forms of animal life and may provide insight into individual differences in addiction liability. Cues can take on incentive-motivational properties (i.e., incentive salience) through Pavlovian learning. Extensive testing with non-human animals (primarily rats) has demonstrated significant variation among individuals in the behaviors this type of learning evokes. So-named "sign-trackers" and "goal-trackers" have been examined in many studies of non-human animals, but this work in humans is still a nascent area of research. In the present proof-of-concept study, we used a Pavlovian conditioned approach task to investigate human sign- and goal-tracking in emerging adults. Conditioned behaviors that developed over the course of the task were directed toward the reward-cue and toward the reward location. Participants' eye-gaze and behavior during the task were submitted to a latent profile analysis, which revealed three groups defined as sign-trackers (n = 10), goal-trackers (n = 4), and intermediate responders (n = 36). Impulsivity was a significant predictor of the sign-tracking group relative to the goal-tracking group. The present study provides preliminary evidence that a simple procedure can produce learned Pavlovian conditioned approach behavior in humans. Though further investigation is required, findings provide a promising step toward the long-term goal of translating important insights gleaned from basic research into treatment strategies that can be applied to clinical populations.

Neural circuits linking sleep and addiction: Animal models to understand why select individuals are more vulnerable to substance use disorders after sleep deprivation

Individuals differ widely in their drug-craving behaviors. One reason for these differences involves sleep. Sleep disturbances lead to an increased risk of substance use disorders and relapse in only some individuals. While animal studies have examined the impact of sleep on reward circuitry, few have addressed the role of individual differences in the effects of altered sleep. There does, however, exist a rodent model of individual differences in reward-seeking behavior: the sign/goal-tracker model of Pavlovian conditioned approach. In this model, only some rats show the key behavioral traits associated with addiction, including impulsivity and poor attentional control, making this an ideal model system to examine individually distinct sleep-reward interactions. Here, we describe how the limbic neural circuits responsible for individual differences in incentive motivation overlap with those involved in sleep-wake regulation, and how this model can elucidate the common underlying mechanisms. Consideration of individual differences in preclinical models would improve our understanding of how sleep interacts with motivational systems, and why sleep deprivation contributes to addiction in only select individuals.

Adolescent cocaine exposure enhances goal-tracking behavior and impairs hippocampal cell genesis selectively in adult bred low-responder rats

RATIONALE

Environmental challenges during adolescence, such as drug exposure, can cause enduring behavioral and molecular changes that contribute to life-long maladaptive behaviors, including addiction. Selectively bred high-responder (bHR) and low-responder (bLR) rats represent a unique model for assessing the long-term impact of adolescent environmental manipulations, as they inherently differ on a number of addiction-related traits. bHR rats are considered "addiction-prone," whereas bLR rats are "addiction-resilient," at least under baseline conditions. Moreover, relative to bLRs, bHR rats are more likely to attribute incentive motivational value to reward cues, or to "sign-track."

OBJECTIVES

We utilized bHR and bLR rats to determine whether adolescent cocaine exposure can alter their inborn behavioral and neurobiological profiles, with a specific focus on Pavlovian conditioned approach behavior (i.e., sign- vs. goal-tracking) and hippocampal neurogenesis.

METHODS

bHR and bLR rats were administered cocaine (15 mg/kg) or saline for 7 days during adolescence (postnatal day, PND 33-39) and subsequently tested for Pavlovian conditioned approach behavior in adulthood (PND 62-75), wherein an illuminated lever (conditioned stimulus) was followed by the response-independent delivery of a food pellet (unconditioned stimulus). Behaviors directed toward the lever and the food cup were recorded as sign- and goal-tracking, respectively. Hippocampal cell genesis was evaluated on PND 77 by immunohistochemistry.

RESULTS

Adolescent cocaine exposure impaired hippocampal cell genesis (proliferation and survival) and enhanced the inherent propensity to goal-track in adult bLR, but not bHR, rats.

CONCLUSIONS

Adolescent cocaine exposure elicits long-lasting changes in stimulus-reward learning and enduring deficits in hippocampal neurogenesis selectively in adult bLR rats.