Publications

Cues associated with rewards acquire the ability to engage the same brain systems as rewards themselves. However, reward cues have multiple properties. For example, they not only act as predictors of reward capable of evoking conditional responses (CRs), but they may also acquire incentive motivational properties. As incentive stimuli they can evoke complex emotional and motivational states. Here we sought to determine whether the predictive value of a reward cue is sufficient to engage brain reward systems, or whether the cue must also be attributed with incentive salience. We took advantage of the fact that there are large individual differences in the extent to which reward cues are attributed with incentive salience. When a cue (conditional stimulus, CS) is paired with delivery of food (unconditional stimulus, US), the cue acquires the ability to evoke a CR in all rats; that is, it is equally predictive and supports learning the CS–US association in all. However, only in a subset of rats is the cue attributed with incentive salience, becoming an attractive and desirable incentive stimulus. We used in situ hybridization histochemistry to quantify the ability of a food cue to induce c-fos mRNA expression in rats that varied in the extent to which they attributed incentive salience to the cue. We found that a food cue induced c-fos mRNA in the orbitofrontal cortex, striatum (caudate and nucleus accumbens), thalamus (paraventricular, intermediodorsal and central medial nuclei), and lateral habenula, only in rats that attributed incentive salience to the cue. Furthermore, patterns of “connectivity” between these brain regions differed markedly between rats that did or did not attribute incentive salience to the food cue. These data suggest that the predictive value of a reward cue is not sufficient to engage brain reward systems—the cue must also be attributed with incentive salience.

The current study uses an extended access rat model of cocaine self-administration (5-h session per day, 14 days), which elicits several features manifested during the transition to human addiction, to study the neural adaptations associated with cocaine withdrawal. Given that the hippocampus is thought to have an important role in maintaining addictive behavior and appears to be especially relevant to mechanisms associated with withdrawal, this study attempted to understand how extended access to cocaine impacts the hippocampus at the cellular and molecular levels, and how these alterations change over the course of withdrawal (1, 14, and 28 days). Therefore, at the cellular level, we examined the effects of cocaine withdrawal on cell proliferation (Ki-67+ and NeuroD+ cells) in the DG. At the molecular level, we employed a ‘discovery’ approach with gene expression profiling in the DG to uncover novel molecules possibly implicated in the neural adaptations that take place during cocaine withdrawal. Our results suggest that decreased hippocampal cell proliferation might participate in the adaptations associated with drug removal and identifies 14 days as a critical time-point of cocaine withdrawal. At the 14-day time-point, gene expression profiling of the DG revealed the dysregulation of several genes associated with cell fate regulation, highlighting two new neurobiological correlates (Ascl-1 and Dnmt3b) that accompany cessation of drug exposure. Moreover, the results point to Fas-Associated protein with Death Domain (FADD), a molecular marker previously associated with the propensity to substance abuse and cocaine sensitization, as a key cell fate regulator during cocaine withdrawal. Identifying molecules that may have a role in the restructuring of the hippocampus following substance abuse provides a better understanding of the adaptations associated with cocaine withdrawal and identifies novel targets for therapeutic intervention.

Metabolic function is integrally related to an individual's susceptibility to, and progression of, disease. Selective breeding for intrinsic treadmill running in rats has produced distinct lines of high- or low-capacity runners (HCR and LCR, respectively) that exhibit numerous physiological differences. To date, the role of intrinsic aerobic capacity on behavior and stress response in these rats has not been addressed and was the focus of these studies. HCR and LCR rats did not differ in their locomotor response to novelty or behavior in the light/dark box. In contrast, immobility in the forced swim test was higher in LCR rats compared with HCR rats, regardless of desipramine treatment. Although both HCR and LCR rats responded to cat odor with decreased exploration and increased risk assessment, HCR rats showed greater contextual conditioning to cat odor. HCR rats exhibited higher expression of corticotropin-releasing hormone in the central nucleus of the amygdala, as well as heavier adrenal and thymus weight. Corticosterone was comparable among HCR and LCR rats at light/dark transitions, and in response to unavoidable cat odor. HCR rats, however, exhibited a greater corticosterone response following the light/dark box. These experiments show that the LCR phenotype associates with decreased risk assessment in response to salient danger signals and passive coping. In contrast, HCR rats show a more naturalistic strategy in that they employ active coping and a more vigilant and cautious response to environmental novelty and salient danger signals. Within this context, we propose that intrinsic aerobic capacity is a central feature mechanistically linking complex metabolic disease and behavior.