An optimized procedure for robust volitional cocaine intake in mice.

Substance use disorder (SUD) is a behavioral disorder characterized by volitional drug consumption. Mouse models of SUD allow for the use of molecular, genetic, and circuit-level tools, providing enormous potential for defining the underlying mechanisms of this disorder. However, the relevance of results depends on the validity of the mouse models used. Self-administration models have long been the preferred preclinical model for SUD as they allow for volitional drug consumption, thus providing strong face validity. While previous work has defined the parameters that influence intravenous cocaine self-administration in other species-such as rats and primates-many of these parameters have not been explicitly assessed in mice. In a series of experiments, we showed that commonly used mouse models of self-administration, where behavior is maintained on a fixed-ratio schedule of reinforcement, show similar levels of responding in the presence and absence of drug delivery-demonstrating that it is impossible to determine when drug consumption is and is not volitional. To address these issues, we have developed a novel mouse self-administration procedure where animals do not need to be pretrained on sucrose and behavior is maintained on a variable-ratio schedule of reinforcement. This procedure increases rates of reinforcement behavior, increases levels of drug intake, and results in clearer delineation between drug-reinforced and saline conditions. Together, these data highlight a major issue with fixed-ratio models in mice that complicates subsequent analysis and provide a simple approach to minimize these confounds with variable-ratio schedules of reinforcement. (PsycInfo Database Record (c) 2021 APA, all rights reserved).

Nicotine disrupts safety learning by enhancing fear associated with a safety cue via the dorsal hippocampus.

Learned safety, a learning process in which a cue becomes associated with the absence of threat, is disrupted in individuals with post-traumatic stress disorder (PTSD). A bi-directional relationship exists between smoking and PTSD and one potential explanation is that nicotine-associated changes in cognition facilitate PTSD emotional dysregulation by disrupting safety associations. Therefore, we investigated whether nicotine would disrupt learned safety by enhancing fear associated with a safety cue. In the present study, C57BL/6 mice were administered acute or chronic nicotine and trained over three days in a differential backward trace conditioning paradigm consisting of five trials of a forward conditioned stimulus (CS)+ (Light) co-terminating with a footshock unconditioned stimulus followed by a backward CS- (Tone) presented 20 s after cessation of the unconditioned stimulus. Summation testing found that acute nicotine disrupted learned safety, but chronic nicotine had no effect. Another group of animals administered acute nicotine showed fear when presented with the backward CS (Light) alone, indicating the formation of a maladaptive fear association with the backward CS. Finally, we investigated the brain regions involved by administering nicotine directly into the dorsal hippocampus, ventral hippocampus, and prelimbic cortex. Infusion of nicotine into the dorsal hippocampus disrupted safety learning.

Long-term effects of chronic nicotine on emotional and cognitive behaviors and hippocampus cell morphology in mice: comparisons of adult and adolescent nicotine exposure.

Nicotine dependence is associated with increased risk for emotional, cognitive and neurological impairments later in life. This study investigated the long-term effects of nicotine exposure during adolescence and adulthood on measures of depression, anxiety, learning and hippocampal pyramidal cell morphology. Mice (C57BL/6J) received saline or nicotine for 12 days via pumps implanted on postnatal day 32 (adolescent) or 54 (adults). Thirty days after cessation of nicotine/saline, mice were tested for learning using contextual fear conditioning, depression-like behaviors using the forced swim test or anxiety-like behaviors with the elevated plus maze. Brains from nicotine- or saline-exposed mice were processed with Golgi stain for whole neuron reconstruction in the CA1 and CA3 regions of the hippocampus. Results demonstrate higher depression-like responses in both adolescent and adult mice when tested during acute nicotine withdrawal. Heightened depression-like behaviors persisted when tested after 30 days of nicotine abstinence in mice exposed as adolescents, but not adults. Adult, but not adolescent, exposure to nicotine resulted in increased open-arm time when tested after 30 days of abstinence. Nicotine exposure during adolescence caused deficits in contextual fear learning indicated by lower levels of freezing to the context as compared with controls when tested 30 days later. In addition, reduced dendritic length and complexity in the apical CA1 branches in adult mice exposed to nicotine during adolescence were found. These results demonstrate that nicotine exposure and withdrawal can have long-term effects on emotional and cognitive functioning, particularly when nicotine exposure occurs during the critical period of adolescence.

Acute nicotine enhances spontaneous recovery of contextual fear and changes c-fos early gene expression in infralimbic cortex, hippocampus, and amygdala.

Exposure therapy, which focuses on extinguishing fear-triggering cues and contexts, is widely used to treat post-traumatic stress disorder (PTSD). Yet, PTSD patients who received successful exposure therapy are vulnerable to relapse of fear response after a period of time, a phenomenon known as spontaneous recovery (SR). Increasing evidence suggests ventral hippocampus, basolateral amygdala, and infralimbic cortex may be involved in SR. PTSD patients also show high rates of comorbidity with nicotine dependence. While the comorbidity between smoking and PTSD might suggest nicotine may alter SR, the effects of nicotine on SR of contextual fear are unknown. In the present study, we tested the effects of acute nicotine administration on SR of extinguished contextual fear memories and c-fos immediate early gene immunohistochemistry in mice. Our results demonstrated that acute nicotine enhanced SR of extinguished fear whereas acute nicotine did not affect retrieval of unextinguished contextual memories. This suggests that the effect of acute nicotine on SR is specific for memories that have undergone extinction treatment. C-fos immunoreactive (IR) cells in the ventral hippocampus and basolateral amygdala were increased in the nicotine-treated mice following testing for SR, whereas the number of IR cells in the infralimbic cortex was decreased in the same group. Overall, this study suggests that nicotine may adversely affect context-specific relapse of fear memories and this effect is potentially mediated by the suppression of cortical regions and increased activity in the ventral hippocampus and amygdala.

Neuro-anatomic mapping of dopamine D1 receptor involvement in nicotine self-administration in rats.

Dopaminergic signaling has long been known to be a critical factor in nicotine addiction, as well as other drugs of abuse. Dopaminergic projections from the VTA to the nucleus accumbens and prefrontal cortex have been well established to be critical to the reinforcing effects of these drugs. However, other projections of dopamine neurons are likely to have significant roles in this process. Also, the relative contributions of D1 and D2 dopamine receptors in drug addiction and its treatment remain to be fully understood. In this study, we examined the effects of blocking D1 and D2 receptors in the nucleus accumbens shell (AcS), anterior cingulate cortex (ACC), and parietal association cortex (PtA) on nicotine self-administration in rats. Female Sprague-Dawley rats were fitted with jugular catheters and allowed to self-administer nicotine (0.03 mg/kg/infusion) on an FR1 schedule. Rats were fitted with bilateral infusion cannulae to allow infusion of D1 or D2 antagonists (SCH-23390 or haloperidol) into each targeted brain area. Acute local infusions of SCH-23390 (1-4 µg/side) into the AcS and PtA significantly reduced nicotine self-administration by up to 75%. SCH-23390 infusion into the ACC was less effective with only suggestive non-significant reductions of nicotine self-administration. Acute, local infusions of haloperidol (0.5-2 µg/side) in any of the brain regions targeted did not have significant effects on nicotine self-administration. These results demonstrate a more significant role for D1 receptor mechanisms in the process of nicotine reinforcement and help provide a more detailed neuroanatomic map of nicotine dependence in the brain.

Withdrawal From Chronic Nicotine Reduces Thyroid Hormone Levels and Levothyroxine Treatment Ameliorates Nicotine Withdrawal-Induced Deficits in Hippocampus-Dependent Learning in C57BL/6J Mice.

INTRODUCTION: Cigarette smoking alters a variety of endocrine systems including thyroid hormones. Altered thyroid hormone signaling may lead to a subclinical or overt hypothyroid condition that could contribute to nicotine withdrawal-related symptoms, such as cognitive deficits. Thus, normalizing thyroid hormone levels may represent a novel therapeutic target for ameliorating nicotine withdrawal-associated cognitive deficits. METHODS: The current studies conducted an analysis of serum thyroid hormone levels after chronic and withdrawal from chronic nicotine treatment in C57BL/6J mice using an enzyme-linked immunosorbent assay. The present studies also evaluated the effect of synthetic thyroid hormone (levothyroxine) on contextual and cued memory. RESULTS: The current studies found that nicotine withdrawal reduces secreted thyroid hormone levels by 9% in C57BL/6J mice. Further, supplemental thyroid hormone not only enhanced memory in naïve animals, but also ameliorated deficits in hippocampus-dependent learning associated with nicotine withdrawal. CONCLUSIONS: These results suggest that smokers attempting to quit should be monitored closely for changes in thyroid function. If successfully treated, normalization of thyroid hormone levels may ameliorate some deficits associated with nicotine withdrawal and this may lead to higher rates of successful abstinence.

Strain-dependent performance in nicotine-induced conditioned place preference.

Nicotine addiction is most likely a result of a combination of factors including the rewarding effects of the drug; these effects, however, might be influenced by genetic background. Using a conditioned place preference (CPP) paradigm and 8 inbred mouse strains, we conducted an initial examination of the role of genetic background in the rewarding effects of nicotine. Following habituation and initial place preference test, inbred strains (A/J, BALB/cByJ, C3H/HeJ, C57BL/6J, CBA/J, DBA/1J, DBA/2J, and 129/SvEv) were trained and tested in CPP for nicotine (0.35 mg/kg). Although several strains (C57BL/6J, CBA/J, and 129/SvEv) showed nicotine-induced CPP, 1 strain (DBA/1J) showed conditioned place aversion (CPA), and other strains (A/J, BALB/cByJ, C3H/HeJ, and DBA/2J) did not show CPP. Overall, these results indicate that nicotine's rewarding effects tested in CPP are differentially affected by the genetic background, and this trait has a relatively high heritability (42%-57%). This initial investigation lays the foundation for future studies examining the genetic substrates of nicotine reward.

The effects of acute nicotine on contextual safety discrimination.

Anxiety disorders, such as post-traumatic stress disorder (PTSD), may be related to an inability to distinguish safe versus threatening environments and to extinguish fear memories. Given the high rate of cigarette smoking in patients with PTSD, as well as the recent finding that an acute dose of nicotine impairs extinction of contextual fear memory, we conducted a series of experiments to investigate the effect of acute nicotine in an animal model of contextual safety discrimination. Following saline or nicotine (at 0.0275, 0.045, 0.09 and 0.18 mg/kg) administration, C57BL/6J mice were trained in a contextual discrimination paradigm, in which the subjects received presentations of conditioned stimuli (CS) that co-terminated with a foot-shock in one context (context A (CXA)) and only CS presentations without foot-shock in a different context (context B (CXB)). Therefore, CXA was designated as the 'dangerous context', whereas CXB was designated as the 'safe context'. Our results suggested that saline-treated animals showed a strong discrimination between dangerous and safe contexts, while acute nicotine dose-dependently impaired contextual safety discrimination (Experiment 1). Furthermore, our results demonstrate that nicotine-induced impairment of contextual safety discrimination learning was not a result of increased generalized freezing (Experiment 2) or contingent on the common CS presentations in both contexts (Experiment 3). Finally, our results show that increasing the temporal gap between CXA and CXB during training abolished the impairing effects of nicotine (Experiment 4). The findings of this study may help link nicotine exposure to the safety learning deficits seen in anxiety disorder and PTSD patients.

Translation of associative learning models into extinction reminders delivered via mobile phones during cue exposure interventions for substance use.

Despite experimental findings and some treatment research supporting the use of cues as a means to induce and extinguish cravings, interventions using cue exposure have not been well integrated into contemporary substance abuse treatments. A primary problem with exposure-based interventions for addiction is that after learning not to use substances in the presence of addiction cues inside the clinic (i.e., extinction), stimuli in the naturalistic setting outside the clinic may continue to elicit craving, drug use, or other maladaptive conditioned responses. For exposure-based substance use interventions to be efficacious, new approaches are needed that can prevent relapse by directly generalizing learning from the therapeutic setting into naturalistic settings associated with a high risk for relapse. Basic research suggests that extinction reminders (ERs) can be paired with the context of learning new and more adaptive conditioned responses to substance abuse cues in exposure therapies for addiction. Using mobile phones and automated dialing and data collection software, ERs can be delivered in everyday high-risk settings to inhibit conditioned responses to substance-use-related stimuli. In this review, we describe how associative learning mechanisms (e.g., conditioned inhibition) can inform how ERs are conceptualized, learned, and implemented to prevent substance use when delivered via mobile phones. This approach, exposure with portable reminders of extinction, is introduced as an adjunctive intervention that uses brief automated ERs between clinic visits when individuals are in high-risk settings for drug use.

Acute nicotine delays extinction of contextual fear in mice.

Smoking is linked to post-traumatic stress disorder (PTSD) which suggests smoking is either a risk factor or an attempt at self-medication. The ability to reduce or extinguish fear-related memories may be altered in patients with PTSD and it is possible that nicotine modulates this. Although there are numerous studies examining the effects of nicotine on acquisition of fear learning, the effects of nicotine on extinction of contextual fear are not well understood. In the present study, we examined the effects of acute nicotine (0.18 mg/kg) on extinction of contextual fear in C57BL/6J mice. Animals were first trained in a background contextual fear conditioning paradigm using a white noise as a conditioned stimulus (CS), which co-terminated with a 2 s 0.57 mA unconditioned foot-shock stimulus (US). Animals were then administered either nicotine or saline and exposed to either the training context or a novel context in order to measure freezing to the context during extinction. Our results demonstrate that nicotine administration during extinction delays extinction of contextual freezing while nicotine did not affect cued freezing or freezing to the novel context.

Role of insular cortex D1 and D2 dopamine receptors in nicotine self-administration in rats.

The insular cortex has been associated with the processing of rewarding stimuli and with the neural bases of drug addiction. Ischemic damage to the insula has been associated with decreased desire to smoke cigarettes. Which component of insular function is involved in the neural basis of cigarette smoking is not clear. Dopamine systems are crucial for the reinforcing value of addictive drugs. The DA projection from the ventral tegmental area to the nucleus accumbens (NAc) has been shown to be a vital pathway for the primary reinforcement caused by taking a variety of abused drugs. In the current set of studies, the roles of D1 and D2 receptors in the insular cortex in the self-administration of nicotine by rats were assessed. Adult female Sprague-Dawley rats were fitted with jugular catheters and given access to self-administer nicotine. Bilateral local infusion cannulae were implanted into the agranular insular cortex to locally administer D1 and D2 antagonists (SCH-23390 and haloperidol). Acute local infusions of the D1 antagonist SCH-23390 into the insula (1-2 µg/side) significantly decreased nicotine self-administration by more than 50%. Repeated infusions of SCH-23390 into the agranular insula caused continuing decreases in nicotine self-administration without signs of tolerance. In contrast, local infusions of the D2 antagonist haloperidol 0.5-2 µg/side did not have any discernable effect on nicotine self-administration. These studies show the importance of DA D1 systems in the insula for nicotine reward.

Solving Pavlov's puzzle: attentional, associative, and flexible configural mechanisms in classical conditioning.

This article introduces a new "real-time" model of classical conditioning that combines attentional, associative, and "flexible" configural mechanisms. In the model, attention to both conditioned (CS) and configural (CN) stimuli are modulated by the novelty detected in the environment. Novelty increases with the unpredicted presence or absence of any CS, unconditioned stimulus (US), or context. Attention regulates the magnitude of the associations CSs and CNs form with other CSs and the US. We incorporate a flexible configural mechanism in which attention to the CN stimuli increases only after the model has unsuccessfully attempted learn input-output combinations with CS-US associations. That is, CSs become associated with the US and other CSs on fewer trials than they do CNs. Because the CSs activate the CNs through unmodifiable connections, a CS can become directly and indirectly (through the CN) associated with the US or other CSs. In order to simulate timing processes, we simply assume that a CS is formed by a temporal spectrum of short-duration CSs that are activated by the nominal CS trace. The model accurately describes 94 % of the basic properties of classical conditioning, using fixed model parameters and simulation values in all simulations.

Classical conditioning mechanisms can differentiate between seeing and doing in rats.

We show that the attentional-associative SLG model of classical conditioning, based on the 1996 research of Schmajuk, Lam, and Gray, correctly describes experimental results regarded as evidence of causal learning in rats: (a) interventions attenuate responding following common-cause training but do not interfere on subsequent responding during observation, and (b) interventions do not affect responding after direct-cause training or (c) causal-chain training. According to the model, responding to the weakly attended test stimulus is strongly inhibited by the intervention in the common-cause case. Instead, in the direct-cause and causal-chain cases, the strongly attended test stimulus becomes inhibitory, thereby overshadowing the inhibitory effect of interventions. Most importantly, the model predicted that with relatively few test trials (a) the 2008 results of Experiment 3 by Leising, Wong, Waldmann, and Blaisdell should be similar to those of Dwyer, Starns, and Honey's 2009 Experiment 1, showing that interventions equally affect responding after common-cause and direct-cause training; and (b) the 2006 results of Experiment 2a by Blaisdell, Sawa, Leising, and Waldmann should be similar to those of Dwyer, Starns, and Honey's 2009 Experiment 2, showing that interventions equally affect responding after common-cause and causal-chain training. When those data were made available to us, we confirmed those predictions. In agreement with the SLG associative model, but not with causal model theory, this evidence supports the notion that the attenuation of responding by interventions only following common-cause training is the consequence of well-known learning processes-latent inhibition, sensory preconditioning, conditioned inhibition, protection from extinction, and overshadowing.

Latent inhibition and compound conditioning: a reply to Holmes and Harris (2009).

Schmajuk, Lam, and Gray (SLG, 1996) presented a neural network model of classical conditioning that addresses the multiple properties of latent inhibition (LI). According to the model, LI is the result of the decreased attention to the target stimulus during preexposure and testing. Recently, Holmes and Harris (2009) suggested that, although the model was able to describe their experimental results showing that LI to a preexposed stimulus disappears with extended compound conditioning, it could not describe the fact that LI is not affected by a delay following compound conditioning. However, computer simulations demonstrate that the SLG model describes and explains both results. Because the model also explains both the deleterious and the facilitating effects on LI of a delay following simple conditioning, the SLG model seems unique in explaining the complete range of reported effects of temporal delays on LI as well as most of the properties of LI.