The effects of Lysergic Acid Diethylamide (LSD) on the Positive Valence Systems: A Research Domain Criteria (RDoC)-Informed Systematic Review.

BACKGROUND AND OBJECTIVES: The renewed interest in psychedelic research provides growing evidence of potentially unique effects on various aspects of reward processing systems. Using the Research Domain Criteria (RDoC) framework, as proposed by the National Institute of Mental Health, we aim to synthesize the existing literature concerning the impact of lysergic acid diethylamide (LSD) on the RDoC's Positive Valence Systems (PVS) domain, and to identify potential avenues for further research. METHODS: Two LSD-related terms (lysergic acid diethylamide and LSD) and 13 PVS-related terms (reward, happiness, bliss, motivation, reinforcement learning, operant, conditioning, satisfaction, decision making, habit, valence, affect, mood) were used to search electronic databases such as PubMed, Scopus, PsychINFO, and Web of Science for relevant articles. A manual search of the reference list resulted in nine additional articles. After screening, articles and data were evaluated and included based on their relevance to the objective of investigating the effects of LSD on the PVS. Articles and data were excluded if they did not provide information about the PVS, were observational in nature, lacked comparators or reference groups, or were duplicates. A risk of bias assessment was performed using the National Toxicology Program's Office of Health Assessment and Translation (NTP OHAT) risk of bias (RoB) tool. Data from the included articles were collected and structured based on the RDoC bio-behavioral matrix, specifically focusing on the PVS domain and its three constituent constructs: reward responsiveness, reward learning, and reward valuation. RESULTS: We reviewed 28 clinical studies with 477 participants. Lysergic acid diethylamide, assessed at self-report (23 studies), molecular (5 studies), circuit (4 studies), and paradigm (3 studies) levels, exhibited dose-dependent mood improvement (20 short-term and 3 long-term studies). The subjective and neural effects of LSD were linked to the 5-HT2A receptor (molecular). Animal studies (14 studies) suggested LSD could mildly reinforce conditioned place preference without aversion and reduce responsiveness to other rewards. Findings on reward learning were inconsistent but hinted at potential associative learning enhancements. Reward valuation measures indicated potential reductions in effort expenditure for other reinforcers. CONCLUSION: Our findings are consistent with our previous work, which indicated classical psychedelics, primarily serotonin 2A receptor agonists, enhanced reward responsiveness in healthy individuals and patient populations. Lysergic acid diethylamide exhibits a unique profile in the reward learning and valuation constructs. Using the RDoC-based framework, we identified areas for future research, enhancing our understanding of the impact of LSD on reward processing. However, applying RDoC to psychedelic research faces limitations due to diverse study designs that were not initially RDoC-oriented. Limitations include subjective outcome measure selection aligned with RDoC constructs and potential bias in synthesizing varied studies. Additionally, some human studies were open-label, introducing potential bias compared to randomized, blinded studies.

Effects of predictive and incentive value manipulation on sign- and goal-tracking behavior.

When a neutral stimulus is repeatedly paired with an appetitive reward, two different types of conditioned approach responses may develop: a sign-tracking response directed toward the neutral cue, or a goal-tracking response directed toward the location of impending reward delivery. Sign-tracking responses have been postulated to result from attribution of incentive value to conditioned cues, while goal-tracking reflects the assignment of only predictive value to the cue. We therefore hypothesized that sign-tracking rats would be more sensitive to manipulations of incentive value, while goal-tracking rats would be more responsive to changes in the predictive value of the cue. We tested sign- and goal-tracking before and after devaluation of a food reward using lithium chloride, and tested whether either response could be learned under negative contingency conditions that precluded any serendipitous reinforcement of the behavior that might support instrumental learning. We also tested the effects of blocking the predictive value of a cue using simultaneous presentation of a pre-conditioned cue. We found that sign-tracking was sensitive to outcome devaluation, while goal-tracking was not. We also confirmed that both responses are Pavlovian because they can be learned under negative contingency conditions. Goal-tracking was almost completely blocked by a pre-conditioned cue, while sign-tracking was much less sensitive to such interference. These results indicate that sign- and goal-tracking may follow different rules of reinforcement learning and suggest a need to revise current models of associative learning to account for these differences.

Subregional Differences in Medium Spiny Neuron Intrinsic Excitability Properties between Nucleus Accumbens Core and Shell in Male Rats.

The nucleus accumbens (NAc) is known for its central role in reward and motivation (Day and Carelli, 2007; Floresco, 2015; Salgado and Kaplitt, 2015). Decades of research on the cellular arrangement, density, and connectivity of the NAc have identified two main subregions known as the core and shell (Záborszky et al., 1985; Berendse and Groenewegen, 1990; Zahm and Heimer, 1990). Although anatomically and functionally different, both the NAc core and shell are mainly comprised of GABAergic projection neurons known as medium spiny neurons (MSNs) (Matamales et al., 2009). Several studies have identified key morphologic differences between core and shell MSNs (Meredith et al., 1992; Forlano and Woolley, 2010) but few studies have directly addressed how core and shell MSNs differ in their intrinsic excitability (Pennartz et al., 1992; O'Donnell and Grace, 1993). Using whole-cell patch-clamp recordings in slices prepared from naive and rewarded male rats, we found that MSNs in the NAc shell were significantly more excitable than MSNs in the NAc core in both groups. In the shell, MSNs had significantly greater input resistance, lower cell capacitance, and a greater sag. This was accompanied by a lower action potential current threshold, a greater number of action potentials, and faster firing frequency compared with core MSNs. These subregional differences in intrinsic excitability could provide a potential physiological link to the distinct anatomic characteristics of core and shell MSNs and to their distinct functional roles in reward learning (Zahm, 1999; Ito and Hayen, 2011; Saddoris et al., 2015; West and Carelli, 2016).

Nicotinic and muscarinic acetylcholine receptor antagonism dose-dependently decreases sign- but not goal-tracking behavior in male rats.

RATIONALE: Acetylcholinergic antagonists have shown some promise in reducing addiction-related behaviors in both preclinical and clinical studies. However, the psychological mechanisms by which these drugs are able to affect addictive behavior remain unclear. A particular key process for the development of addiction is the attribution of incentive salience to reward-related cues, which can be specifically measured in animals using a Pavlovian conditioned approach procedure. When confronted with a lever that predicts food delivery, some rats engage with the lever directly (i.e., they sign track), indicating attribution of incentive-motivational properties to the lever itself. In contrast, others treat the lever as a predictive cue and approach the location of impending food delivery (i.e., they goal track), without treating the lever itself as a reward. OBJECTIVES: We tested whether systemic antagonism of the either nicotinic or muscarinic acetylcholine receptors would selectively affect sign- or goal-tracking behavior, indicating a selective effect on incentive salience attribution. METHODS: A total of 98 male Sprague Dawley rats were either given the muscarinic antagonist scopolamine (100, 50, or 10 µg/kg i.p.) or the nicotinic antagonist mecamylamine (0.3, 1.0, or 3 mg/kg i.p.) before being trained on a Pavlovian conditioned approach procedure. RESULTS: Scopolamine dose-dependently decreased sign tracking behavior and increased goal-tracking behavior. Mecamylamine reduced sign-tracking but did not affect goal-tracking behavior. CONCLUSIONS: Antagonism of either muscarinic or nicotinic acetylcholine receptors can reduce incentive sign-tracking behavior in male rats. This effect appears to be specifically due to a reduction in incentive salience attribution since goal-tracking either increased or was not affected by these manipulations.

Genetic characterization of outbred Sprague Dawley rats and utility for genome-wide association studies.

Sprague Dawley (SD) rats are among the most widely used outbred laboratory rat populations. Despite this, the genetic characteristics of SD rats have not been clearly described, and SD rats are rarely used for experiments aimed at exploring genotype-phenotype relationships. In order to use SD rats to perform a genome-wide association study (GWAS), we collected behavioral data from 4,625 SD rats that were predominantly obtained from two commercial vendors, Charles River Laboratories and Harlan Sprague Dawley Inc. Using double-digest genotyping-by-sequencing (ddGBS), we obtained dense, high-quality genotypes at 291,438 SNPs across 4,061 rats. This genetic data allowed us to characterize the variation present in Charles River vs. Harlan SD rats. We found that the two populations are highly diverged (FST > 0.4). Furthermore, even for rats obtained from the same vendor, there was strong population structure across breeding facilities and even between rooms at the same facility. We performed multiple separate GWAS by fitting a linear mixed model that accounted for population structure and using meta-analysis to jointly analyze all cohorts. Our study examined Pavlovian conditioned approach (PavCA) behavior, which assesses the propensity for rats to attribute incentive salience to reward-associated cues. We identified 46 significant associations for the various metrics used to define PavCA. The surprising degree of population structure among SD rats from different sources has important implications for their use in both genetic and non-genetic studies.

Sex- and stress-dependent effects of a single injection of ketamine on open field and forced swim behavior.

Ketamine has emerged as a novel treatment for common psychiatric conditions such as Major Depressive Disorder (MDD) and anxiety disorders, many of which can be initiated and exacerbated by psychological stress. Sex differences in the frequency of both anxiety and depressive disorders are well known and could be due to sex differences in neuroendocrine responses to stress. Ketamine is known to modulate the hormonal response to stress, specifically corticosterone. It is not clear if the acute effect of ketamine on corticosterone differs by sex, or what role this could play in subsequent behavior. Here we test whether a single injection of (R,S)-ketamine (30 mg/kg, i.p.), administered either with or without unpredictable chronic stress (UCS), has different sustained effects on open field test (OFT), elevated zero maze (EZM) or forced swim test (FST) behavior in female versus male C57BL/6J mice. In the OFT (24 h post-injection), ketamine increased center square exploration in males but not females. In contrast, in the FST (72 h post-injection), females showed a trend toward a decrease in immobility after ketamine whereas males were not strongly modulated. These behavioral effects of ketamine were stronger in the presence of UCS than in unstressed animals. UCS animals also showed lower corticosterone after injection than unstressed animals, and in the presence of UCS ketamine increased corticosterone; these effects were similar in both sexes. Corticosterone post-injection did not predict subsequent behavior. These findings complement a growing preclinical literature suggesting both stress-dependency and sex differences in OFT and FST behavioral responses to ketamine.LAY SUMMARYIn humans, it is known that major depression and anxiety disorders, which can be caused or made worse by exposure to psychological stress, occur roughly twice as frequently in women than in men, but the underpinnings of these effects are not well characterized. In the current study, we explored how sex interacts with stress and ketamine (a rapidly acting antidepressant) by assessing both open field and forced swim behavior in mice after chronic mild stress. We report the novel finding that male mice exhibit greater exploration of the aversive center square in the open field after ketamine, whereas females trended toward lower immobility (often interpreted as an antidepressant-like effect) in the forced swim test after this drug, and these effects were amplified by prior stress exposure.

The lateral hypothalamus and orexinergic transmission in the paraventricular thalamus promote the attribution of incentive salience to reward-associated cues.

RATIONALE: Prior research suggests that the neural pathway from the lateral hypothalamic area (LHA) to the paraventricular nucleus of the thalamus (PVT) mediates the attribution of incentive salience to Pavlovian reward cues. However, a causal role for the LHA and the neurotransmitters involved have not been demonstrated in this regard. OBJECTIVES: To examine (1) the role of LHA in the acquisition of Pavlovian conditioned approach (PavCA) behaviors, and (2) the role of PVT orexin 1 receptors (OX1r) and orexin 2 receptors (OX2r) in the expression of PavCA behaviors and conditioned reinforcement. METHODS: Rats received excitotoxic lesions of the LHA prior to Pavlovian training. A separate cohort of rats characterized as sign-trackers (STs) or goal-trackers (GTs) received the OX1r antagonist SB-334867, or the OX2r antagonist TCS-OX2-29, into the PVT, to assess their effects on the expression of PavCA behavior and on the conditioned reinforcing properties of a Pavlovian reward cue. RESULTS: LHA lesions attenuated the development of sign-tracking behavior. Administration of either the OX1r or OX2r antagonist into the PVT reduced sign-tracking behavior in STs. Further, OX2r antagonism reduced the conditioned reinforcing properties of a Pavlovian reward cue in STs. CONCLUSIONS: The LHA is necessary for the development of sign-tracking behavior; and blockade of orexin signaling in the PVT attenuates the expression of sign-tracking behavior and the conditioned reinforcing properties of a Pavlovian reward cue. Together, these data suggest that LHA orexin inputs to the PVT are a key component of the circuitry that encodes the incentive motivational value of reward cues.

Effects of the cannabinoid receptor agonist CP-55,940 on incentive salience attribution.

RATIONALE: Pavlovian conditioned approach paradigms are used to characterize the nature of motivational behaviors in response to stimuli as either directed toward the cue (i.e., sign-tracking) or the site of reward delivery (i.e., goal-tracking). Recent evidence has shown that activity of the endocannabinoid system increases dopaminergic activity in the mesocorticolimbic system, and other studies have shown that sign-tracking behaviors are dependent on dopamine. OBJECTIVES: Therefore, we hypothesized that administration of a cannabinoid agonist would increase sign-tracking and decrease goal-tracking behaviors. METHODS: Forty-seven adult male Sprague-Dawley rats were given a low, medium, or high dose of the cannabinoid agonist CP-55,940 (N = 12 per group) or saline (N = 11) before Pavlovian conditioned approach training. A separate group of rats (N = 32) were sacrificed after PCA training for measurement of cannabinoid receptor type 1 (CB1) and fatty acid amide hydrolase (FAAH) using in situ hybridization. RESULTS: Contrary to our initial hypothesis, CP-55,940 dose-dependently decreased sign-tracking and increased goal-tracking behavior. CB1 expression was higher in sign-trackers compared with that in goal-trackers in the prelimbic cortex, but there were no significant differences in CB1 or FAAH expression in the infralimbic cortex, dorsal or ventral CA1, dorsal or ventral CA3, dorsal or ventral dentate gyrus, or amygdala. CONCLUSIONS: These results demonstrate that cannabinoid signaling can specifically influence behavioral biases toward sign- or goal-tracking. Pre-existing differences in CB1 expression patterns, particularly in the prelimbic cortex, could contribute to individual differences in the tendency to attribute incentive salience to reward cues.

Mechanisms of Shared Vulnerability to Post-traumatic Stress Disorder and Substance Use Disorders.

Psychoactive substance use is a nearly universal human behavior, but a significant minority of people who use addictive substances will go on to develop an addictive disorder. Similarly, though ~90% of people experience traumatic events in their lifetime, only ~10% ever develop post-traumatic stress disorder (PTSD). Substance use disorders (SUD) and PTSD are highly comorbid, occurring in the same individual far more often than would be predicted by chance given the respective prevalence of each disorder. Some possible reasons that have been proposed for the relationship between PTSD and SUD are self-medication of anxiety with drugs or alcohol, increased exposure to traumatic events due to activities involved in acquiring illegal substances, or addictive substances altering the brain's stress response systems to make users more vulnerable to PTSD. Yet another possibility is that some people have an intrinsic vulnerability that predisposes them to both PTSD and SUD. In this review, we integrate clinical and animal data to explore these possible etiological links between SUD and PTSD, with an emphasis on interactions between dopaminergic, adrenocorticotropic, GABAergic, and glutamatergic neurobehavioral mechanisms that underlie different emotional learning styles.

Individual variation in the attribution of incentive salience to social cues.

Research on the attribution of incentive salience to drug cues has furthered our understanding of drug self-administration in animals and addiction in humans. The influence of social cues on drug-seeking behavior has garnered attention recently, but few studies have investigated how social cues gain incentive-motivational value. In the present study, a Pavlovian conditioned approach (PCA) procedure was used to identify rats that are more (sign-trackers; STs) or less (goal-trackers; GTs) prone to attribute incentive salience to food reward cues. In Experiment 1, a novel procedure employed social 'peers' to compare the tendency of STs and GTs to attribute incentive salience to social reward cues as well as form a social-conditioned place preference. In Experiment 2, social behavior of STs and GTs was compared using social interaction and choice tests. Finally, in Experiment 3, levels of plasma oxytocin were measured in STs and GTs seven days after the last PCA training session, because oxytocin is known to modulate the mesolimbic reward system and social behavior. Compared to GTs, STs attributed more incentive salience to social-related cues and exhibited prosocial behaviors (e.g., social-conditioned place preference, increased social interaction, and social novelty-seeking). No group differences were observed in plasma oxytocin levels. Taken together, these experiments demonstrate individual variation in the attribution of incentive salience to both food- and social-related cues, which has important implications for the pathophysiology of addiction.

Sign-tracking behavior is difficult to extinguish and resistant to multiple cognitive enhancers.

The attribution of incentive-motivational value to drug-related cues underlies relapse and craving in drug addiction. One method of addiction treatment, cue-exposure therapy, utilizes repeated presentations of drug-related cues in the absence of drug (i.e., extinction learning); however, its efficacy has been limited due to an incomplete understanding of extinction and relapse processes after cues have been imbued with incentive-motivational value. To investigate this, we used a Pavlovian conditioned approach procedure to screen for rats that attribute incentive-motivational value to reward-related cues (sign-trackers; STs) or those that do not (goal-trackers; GTs). In Experiment 1, rats underwent Pavlovian extinction followed by reinstatement and spontaneous recovery tests. For comparison, a separate group of rats underwent PCA training followed by operant conditioning, extinction, and tests of reinstatement and spontaneous recovery. In Experiment 2, three cognitive enhancers (sodium butyrate, D-cycloserine, and fibroblast growth factor 2) were administered following extinction training to facilitate extinction learning. STs but not GTs displayed enduring resistance to Pavlovian, but not operant, extinction and were more susceptible to spontaneous recovery. In addition, none of the cognitive enhancers tested affected extinction learning. These results expand our understanding of extinction learning by demonstrating that there is individual variation in extinction and relapse processes and highlight potential difficulties in applying extinction-based therapies to drug addiction treatment in the clinic.

Divergent effects of repeated cocaine and novel environment exposure on locus coeruleus c-fos expression and brain catecholamine concentrations in rats.

INTRODUCTION: Chronic administration of cocaine causes a disinhibited, hyperexploratory response to novel environments. As the norepinephrine (NE) system regulates exploration and is dysregulated following cocaine exposure, we hypothesized that this cocaine-mediated hyperexploratory response is associated with increased locus coeruleus (LC) reactivity. METHODS: To test this hypothesis, we used dual fluorescent in situ hybridization immunofluorescence to analyze novelty-induced c-fos and tyrosine hydroxylase expression in the LC and high-pressure liquid chromatography to measure dopamine (DA) and NE concentrations in key catecholamine projection regions following exposure to cocaine. RESULTS: Repeated cocaine exposure followed by a 14-day drug-free period increased exploration of novel environments, replicating previous findings. Novelty exposure increased LC c-fos expression, increased anterior cingulate NE, and decreased ventral tegmental area DA. Cocaine exposure decreased amygdala (AMY) DA, but had no effect on LC c-fos expression or NE in any tested brain region. No interactions between cocaine and novelty were found. Open arm exploration was positively correlated with LC c-fos expression and NE concentrations in both the anterior cingulate and nucleus accumbens, and negatively correlated with AMY DA concentration. CONCLUSIONS: Our findings confirm that exposure to novel environments increases LC activity and NE in the anterior cingulate cortex, that long-term exposure to cocaine dysregulates AMY DA, and that disinhibited exploration in novel environments correlates with NE and DA in regions that modulate risk-taking and avoidance behavior. Further studies investigating the effects of cocaine on brain catecholamine systems are important in understanding the long-lasting effects of cocaine on brain function.

Single prolonged stress decreases sign-tracking and cue-induced reinstatement of cocaine-seeking.

Exposure to prolonged, uncontrollable stress reduces reward-seeking behavior, resulting in anhedonia in neuropsychiatric disorders, such as posttraumatic stress disorder. However, it is unclear to what degree stressed subjects lose interest in rewards themselves or in reward-related cues that instigate reward-seeking behavior. In the present study, we investigated the effects of single prolonged stress (SPS) on cue-directed behavior in two different procedures: Pavlovian conditioned approach (PCA) and cue-induced reinstatement of cocaine-seeking. In Experiment 1, rats were exposed to SPS and tested for the acquisition of sign-tracking (cue-directed) and goal-tracking (reward-directed) behaviors during a PCA procedure. In Experiment 2, rats were exposed to SPS and tested for the expression of sign- and goal-tracking as well as cue-induced reinstatement of cocaine-seeking. Because dopaminergic activity in the nucleus accumbens is known to play a central role in many cue-directed behaviors, including both sign-tracking and cue-induced reinstatement, Experiment 3 used in vivo microdialysis to measure the effect of SPS on baseline and evoked dopamine levels in the nucleus accumbens. SPS decreased sign-tracking and increased goal-tracking during the acquisition of PCA behavior without affecting reward consumption. In addition, SPS decreased cue-induced reinstatement without affecting cocaine self-administration. Finally, SPS decreased evoked but not baseline levels of dopamine in the nucleus accumbens. These results suggest that SPS decreases the motivational, but not consummatory, aspects of reward-seeking behavior, which may result from long-term, SPS-induced reductions in dopamine release in the nucleus accumbens.

Isopropyl Alcohol Swabs as a Preferred Substance of Abuse.

The authors describe the treatment of a 36-year-old woman who presented with excessive consumption of antiseptic alcohol swabs. Her behavior was diagnosed as a severe substance use disorder with isopropyl alcohol as the primary drug of choice. Diagnostic and treatment challenges involved in the case are discussed in detail, with particular focus on the differential diagnosis for this unusual presentation, including obsessive compulsive disorder, pica, and intentional self-injurious behavior. A brief review of the literature on non-ethanol ingestion for the purposes of intoxication is included, as well as clinical information about the potential dangers posed by isopropyl alcohol consumption.

Thalamic mast cell activity is associated with sign-tracking behavior in rats.

Mast cells are resident immune cells in the thalamus that can degranulate and release hundreds of signaling molecules (i.e., monoamines, growth factors, and cytokines) both basally and in response to environmental stimuli. Interestingly, mast cell numbers in the brain show immense individual variation in both rodents and humans. We used a Pavlovian conditioned approach (PCA) procedure to examine whether mast cells are associated with individual variation in the attribution of incentive-motivational value to reward-related cues. During the PCA procedure, a lever response-independently predicts the delivery of a food pellet into a magazine, and over training sessions three conditioned responses (CRs) develop: sign-tracking (lever-directed CRs), goal-tracking (magazine-directed CRs), and an intermediate response (both CRs). In Experiment 1, we measured thalamic mast cell number/activation using toluidine blue and demonstrated that sign-trackers have increased degranulated (activated) but not granulated (inactive) mast cells. In Experiment 2, we infused the mast cell inhibitor, cromolyn (200µg/rat; i.c.v.), immediately before five daily PCA training sessions and demonstrated that mast cell inhibition selectively impairs the acquisition of sign-tracking behavior. Taken together, these results demonstrate that thalamic mast cells contribute to the attribution of incentive-motivational value to reward-related cues and suggest that mast cell inhibition may be a novel target for addiction treatment.

Subanesthetic ketamine decreases the incentive-motivational value of reward-related cues.

The attribution of incentive-motivational value to reward-related cues contributes to cue-induced craving and relapse in addicted patients. Recently, it was demonstrated that subanesthetic ketamine increases motivation to quit and decreases cue-induced craving in cocaine-dependent individuals. Although the underlying mechanism of this effect is currently unknown, one possibility is that subanesthetic ketamine decreases the incentive-motivational value of reward-related cues. In the present study, we used a Pavlovian conditioned approach procedure to identify sign-trackers, rats that attribute incentive-motivational value to reward-related cues, and goal-trackers, rats that assign only predictive value to reward-related cues. This model is of interest because sign-trackers are more vulnerable to cue-induced reinstatement of drug-seeking behavior and will persist in this drug-seeking behavior despite adverse consequences. We tested the effect of subanesthetic ketamine on the expression of Pavlovian conditioned approach behavior and the conditioned reinforcing properties of a reward-related cue in sign- and goal-trackers. We found that subanesthetic ketamine decreased sign-tracking and increased goal-tracking behavior in sign-trackers, though it had no effect on conditioned reinforcement. These results suggest that subanesthetic ketamine may be a promising pharmacotherapy for addiction that acts by decreasing the incentive-motivational value of reward-related cues.

Lesions of the ventral hippocampus attenuate the acquisition but not expression of sign-tracking behavior in rats.

Individual variation in the attribution of motivational salience to reward-related cues is believed to underlie addiction vulnerability. Pavlovian conditioned approach measures individual variation in motivational salience by identifying rats that are attracted to and motivated by reward cues (sign-trackers) or motivationally fixed on the reward itself (goal-trackers). Previously, it has been demonstrated that sign-trackers are more vulnerable to addiction-like behavior. Moreover, sign-trackers release more dopamine in the nucleus accumbens than goal-trackers in response to reward-related cues, and sign- but not goal-tracking behavior is dopamine-dependent. In the present study, we investigated whether the ventral hippocampus, a potent driver of dopaminergic activity in the nucleus accumbens, modulates the acquisition and expression of Pavlovian conditioned approach behavior. In Experiment 1, lesions of the ventral, but not dorsal or total hippocampus, decreased sign-tracking behavior. In Experiment 2, lesions of the ventral hippocampus did not affect the expression of sign- or goal-tracking behaviors nor conditioned reinforcement. In addition, temporary inactivation of the ventral subiculum, the main output pathway of the ventral hippocampus, did not affect the expression of sign- or goal-tracking behaviors. High-pressure liquid chromatography of nucleus accumbens tissue punches revealed that ventral hippocampal lesions decreased levels of homovanillic acid and the homovanillic acid/dopamine ratio (a marker of dopamine release and metabolism) in only sign-trackers, and decreased accumbal norepinephrine levels in both sign- and goal-trackers. These results suggest that the ventral hippocampus is important for the acquisition but not expression of sign-tracking behavior, possibly as a result of altered dopamine and norepinephrine in the nucleus accumbens. © 2016 Wiley Periodicals, Inc.

Neurobiology of Adolescent Substance Use Disorders.

There are many facets of the neurobiology of substance use that are distinct in adolescence as compared with adulthood. The adolescent brain is subject to intense subcortical reward processes, but is left with an immature prefrontal control system that is often unable to resist the pull of potentially exciting activities like substance use, even when fully aware of the dangers involved. Peer influences serve only to magnify these effects and foster more sensation-seeking, risky behavior. The unique aspects of neurobiology should be taken into consideration when designing prevention programs and clinical interventions for adolescent substance use disorders.

Pavlovian Conditioned Approach Training in Rats.

Cues that are contingently paired with unconditioned, rewarding stimuli can acquire rewarding properties themselves through a process known as the attribution of incentive salience, or the transformation of neutral stimuli into attractive, "wanted' stimuli capable of motivating behavior. Pavlovian conditioned approach (PCA) develops after the response-independent presentation of a conditioned stimulus (CS; e.g., a lever) that predicts the delivery of an unconditioned stimulus (US; e.g., a food pellet) and can be used to measure incentive salience. During training, three patterns of conditioned responses (CRs) can develop: sign-tracking behavior (CS-directed CR), goal-tracking behavior (US-directed CR), and an intermediate response (both CRs). Sign-trackers attribute incentive salience to reward-related cues and are more vulnerable to cue-induced reinstatement of drug-seeking as well as other addiction-related behaviors, making PCA a potentially valuable procedure for studying addiction vulnerability. Here, we describe materials and methods used to elicit PCA behavior from rats as well as analyze and interpret PCA behavior in individual experiments.