The prelimbic cortex (PrL) plays a critical role in reward-related memory processes. While its functional interactions with the ventral tegmental area (VTA) and cornu ammonis 1 (CA1) are implicated in reward circuitry, the specific oscillatory dynamics between these regions during the expression of morphine-associated memory remain poorly understood. In this study, local field potentials (LFPs) were recorded from the PrL, CA1, and VTA to investigate the PrL-CA1 and PrL-VTA pathways in a rat model of morphine-induced conditioned place preference (CPP). Recordings were analyzed during both pre-conditioning and post-conditioning (expression) phases. Coherence and Granger causality (GC) analyses evaluated functional synchronization and directional information flow across these pathways. A comparison of relative power between saline- and morphine-treated rats during the post-conditioning phase revealed significant alterations across all LFP sub-bands in the VTA. Coherence analysis demonstrated a significant increase in PrL-VTA synchronization during the expression phase, particularly within the theta and alpha bands. In contrast, coherence within the PrL-CA1 pathway remained unaltered during either phase. Furthermore, GC analysis indicated distinct, band-specific directional changes: a significant increase in delta-band causality in both directions of the PrL-VTA pathway, alongside specific reductions in delta, beta, and low-gamma information flow from the PrL to CA1. These results indicate that the expression of morphine-associated reward memory recruits the PrL-VTA pathway through increased functional synchronization and bidirectional information flow, while overall PrL-CA1 coherence remains unaffected. These findings highlight pathway-specific oscillatory dynamics underlying reward conditioning and memory retrieval.
The hippocampus plays a fundamental role in contextual fear conditioning (CFC). Studies have found that its ventral portion is mostly related to emotional processing, while the dorsal portion is related to the processing of contextual memories. The prelimbic cortex (PL) is also associated with contextual fear conditioning with temporal discontinuity (CFC-5s), in which a brief time gap separates context exposure and the aversive stimulus. Studies have shown that PL is involved in tasks that require the maintenance of information over short time intervals, such as working memory. The aim of the present study is to verify whether pharmacological manipulations of the dorsal hippocampus (dHPC) unilaterally in conjunction with contralateral inactivation of the PL interfere with the acquisition of CFC-5s. For this, Male Wistar rats were divided into three groups: asymmetric manipulation (CONTRA), ipsilateral manipulation (IPSI) and saline administration (SALINE). In Experiment 1, AP5 was infused into the dHPC and muscimol into the PL; in Experiment 2, muscimol was infused into both regions. Rats were trained and tested to a contextual fear conditioning protocol with temporal discontinuity. The results show that infusion of AP5 in the dorsal hippocampus impairs CFC-5s, whereas muscimol infusion does not. These findings indicate that NMDA receptor-dependent plasticity in the dHPC, rather than transient neural activity, is critical for the interaction between dHPC and PL during the acquisition of temporally discontinuous contextual memories.
The role of adult-born neurons in the hippocampal dentate gyrus and its significance in memory processes is a field of high interest. To add to this literature, we investigated the feasibility of ivermectin-based chemogenetic manipulation of recently born neurons in the hippocampus in altering spatial and contextual memory retrieval. The adult-born neurons in male mice were targeted using a nestin-cre mouse model, employing lentivirus-mediated transduction of inhibitory chloride channel constructs sensitive or insensitive to ivermectin in order to be able to silence them during different developmental stages. Two mouse age groups were compared: 5 weeks and 5 months old at the time of lentivirus injection. Behavioural assessments involving the Morris water maze and contextual fear conditioning were conducted. Unexpectedly, ivermectin administration alone led to apparent memory impairments regardless of the virus construct, confounding behavioural test outcomes. This study highlights the challenge of distinguishing specific effects of targeting adult-born neuronal activity from nonspecific effects of ivermectin on cognition.
Understanding the neural mechanisms underlying the formation, retrieval, and extinction of taste memory is crucial for elucidating learning and memory processes. Using the conditioned taste aversion (CTA) model, we investigated the roles of the amygdala and gustatory cortex (GC), key structures consistently implicated in taste memory. Our study focused on the plastic changes associated with both appetitive and aversive taste memories, emphasizing the importance of extinction in promoting adaptive behavior and counteracting maladaptive responses. Memory processes elicit synaptic plasticity, often reflected in increased expression of neuronal remodeling markers such as growth-associated protein 43 (GAP-43) and synaptophysin (SYN). We examined the involvement of GAP-43 and SYN in taste memory by immunohistochemical analysis in male rats subjected to the formation of appetitive and aversive memories. During memory retrieval, GAP-43 expression was significantly upregulated in the basolateral amygdala (BLA), particularly under aversive conditions. In contrast, memory extinction induced a robust increase in GAP-43 expression in the central amygdala of overtrained animals, accompanied by elevated SYN expression in both the basolateral and central nuclei. These findings highlight the critical roles of GAP-43 and SYN in mediating synaptic changes during memory extinction, supporting the view of extinction as a dynamic, plastic process essential for behavioral adaptation. Our results advance understanding of the molecular substrates of taste memory and suggest potential targets for interventions aimed at modifying maladaptive memory traces, with implications for therapeutic strategies addressing aversive experiences and enhancing adaptive learning.
Genetic tracing revealed that subsets of neurons in the medial amygdala (MeA) were labeled with the transneuronal tracer tWGA-DsRed originating from bitter taste receptor cells in male mice, suggesting the locations of bitter taste-relaying neurons in the MeA. Although not many studies have reported the involvement of MeA neurons in taste information processing, several studies clearly indicated that the MeA neurons play an important role in taste neophobia that refers to a reduction in consumption of a novel taste. On the other hand, several amygdaloid nuclei of the amygdala also operate to acquire conditioned taste aversion (CTA), which taste novelty determines the efficiency and strength for, while processing the conditioned stimulus (CS) followed by the unconditioned one (US) to elicit behavioral aversion to the CS. Here we combined genetic tracing and immunohistochemical analyses to examine whether subpopulations of tWGA-DsRed-labeled MeA neurons which inherently receive aversive bitter input can induce changes in responses to the novelty of saccharin and the CS saccharin during CTA learning. Immunohistochemical detection of Zif268 induction revealed that saccharin activated more tWGA-DsRed-labeled MeA neurons of mice that experienced saccharin at the first time than it did in mice experiencing saccharin multiple times. The CS saccharin activated a larger number of tWGA-DsRed-labeled MeA neurons after mice acquired CTA memory, compared with those detected in mice without CTA. Our results also suggest that the increased population of the CS-activated neurons among tWGA-DsRed-labeled MeA neurons may remain unchanged and activated by the CS after the subsequent extinction of CTA memory.
Remembering the identity and the location of distinct spatial elements is crucial for an animal's successful exploration of their environment. Locating food and shelter are but a few behaviors that rely on forming effective associations between the identity and location of spatial elements. Object-in-place (OiP) tasks are commonly used in rats to assess this identity-location association. While the availability of genetic resources has made the mouse an increasingly used animal model in neuroscience research, very few studies have successfully assessed OiP in mice. To address this limitation, we tested three distinct experimental designs of the spontaneous OiP preference task in adult C57/129 J and C57BL/6 J mice. C57BL/6 J, but not C57/129 J mice, displayed OiP preference in the four-object version of the OiP task. In contrast, mice of both strains successfully performed the two-object OiP task design, with retention intervals of five minutes and one hour. To broaden our task validation, we uncovered the ontogenetic profile of two-object OiP in C57/129 J mice, which emerges between postnatal day (P)25 and P28 in this mouse strain. Our data establish robust guidelines for successful assessment of OiP preference in mice across the lifespan, expanding the available behavioral toolbox for spatial memory research in mice.
Since Ivan Pavlov first demonstrated classical conditioning by pairing a neutral stimulus with a reinforcer, researchers have applied this approach to understand not only direct associations, but also more complex learning processes, such as sensory preconditioning and second-order conditioning. Despite their relative immaturity, very young mammals such as newborn rabbits exhibit robust classical and higher-order conditioning. However, the neuromodulatory systems involved in these different types of neonatal conditioning remain to be identified. Here, we compared the role played by the noradrenergic and the endocannabinoid systems in classical conditioning, sensory preconditioning and second-order conditioning in newborn rabbits. Intraperitoneal injections of Propranolol, an antagonist of beta-adrenergic receptors, blocked classical conditioning promoted by the mammary pheromone in newborn rabbits but had no effect on sensory preconditioning or second-order conditioning. Conversely, intraperitoneal injections of Rimonabant, an antagonist of the main cannabinoid receptor CB1, had no effect on classical conditioning but blocked sensory preconditioning and second-order conditioning, indicating a specific impact on unreinforced association. Moreover, an effect of Rimonabant on memory reconsolidation was also revealed. These findings demonstrate a double dissociation in the role of noradrenergic and endocannabinoid modulations in first- and higher-order conditioning in newborn rabbits. Whereas our results indicate the noradrenergic system specifically promotes reinforced association in pups, they also establish that the endocannabinoid system selectively mediates higher-order conditioning by regulating unreinforced association in newborns. This also highlights that the rabbit is an excellent model for further investigating the neurobiology of neonatal first- and higher-order memory.
Nearly 100 years ago, Ivan Pavlov's research into learning processes provided a framework for understanding how the brain makes behaviorally impactful associations between events. His seminal findings on excitatory and inhibitory conditioning established the scientific and conceptual foundation for the later discovery of Pavlovian occasion setting. That is, a higher-order association that modulates whether or not a conditioned stimulus will evoke a conditioned response-feature positive and feature negative occasion setters, respectively. While much of this research has used exteroceptive stimuli (tone, lights), interoceptive stimuli (intoxication, hunger) can also acquire occasion setting functions. Research using the drug discriminated goal-tracking (DDGT) task demonstrates that interoceptive drug stimuli serve reliably as occasion setters. A wide range of drug stimuli across pharmacological classes function as occasion setters (e.g., nicotine, chlordiazepoxide, morphine) in the DDGT task. Moreover, interoceptive drug occasion setters exhibit similar characteristics as exteroceptive occasion setters. These include resistance to simple extinction procedures, generalization across stimulus (drug) class with shared function as an occasion setters, and a drug feature-negative occasion setter acquiring conditioned inhibitory effects. We explore future avenues of inquiry utilizing the DDGT task to expand our understanding of occasion setting. We highlight methodological strengths that recommend the DDGT task for probing the neuropharmacological and neuroanatomical mechanisms of drug occasion setters. Pavlov recognized the importance of interoception, and the role conditioning processes play in health and well-being. We invite readers to consider how their research intersects with the topic of interoceptive conditioning. By working together, we can build our understanding of basic interoceptive Pavlovian conditioning processes and realize the solutions this knowledge holds to societal issues.
Adaptive behavior often relies on learning associations between stimuli that were never directly paired with reinforcement. Sensory preconditioning provides a powerful paradigm to investigate such indirect learning: when neutral stimuli A and X are paired during preconditioning, and X is subsequently paired with an unconditioned stimulus (US) during conditioning, stimulus A elicits conditioned response at test despite never being directly paired with the US. While the hippocampus is known to be critical for this process, its involvement in unimodal sensory preconditioning remains unclear. Here, we investigated the role of the hippocampus (dorsal and ventral) during different phases of gustatory preconditioning using chemogenetic approach in rats. We demonstrated that disruption of the hippocampus during either preconditioning or test selectively impaired the mediated aversion to A while leaving the direct conditioned aversion to X intact. These findings provide the first evidence that the hippocampus is necessary for unimodal gustatory preconditioning, extending previous demonstrations of hippocampal involvement in polymodal protocols. Our work therefore clarifies the critical role of the hippocampus in forming and retrieving purely gustatory associations to guide behavior, highlighting its function as a central hub for memory integration.
In this work, we develop a mathematical model that captures both the early and late phases of Long-Term Potentiation (LTP) and Long-Term Depression (LTD), incorporating NMDAR-dependent induction and changes in AMPAR conductance. The model combines multiple essential properties. First, it emphasizes a detailed representation of biochemical processes within the postsynaptic neuron, thereby illustrating the interaction between LTD and distinct forms of LTP. Second, the dynamic modulation of postsynaptic AMPA receptor conductance is represented through nonlinear differential equations and algebraic relations. Third, the model incorporates input specificity, associativity, and cooperativity, allowing synaptic changes at one site to influence the strength of neighboring synapses. These features provide a comprehensive description of synaptic dynamics, allowing the simulation of plasticity at both the cellular and the network levels. Overall, the model offers a valuable framework for studying NMDAR-dependent LTP and LTD by explicitly incorporating changes in AMPAR conductance. We believe that this model provides deeper insights into the molecular mechanisms of synaptic plasticity and paves the way for the construction of network-level models by linking multiple cells through AMPA receptor conductance. Significance statement: We present a comprehensive mathematical framework that integrates early (E-LTP), late (L-LTP), and LTD by incorporating NMDAR-dependent signaling and changes in AMPAR conductance. By combining and extending established biochemical models, our approach links molecular signaling, receptor trafficking, and postsynaptic membrane dynamics to changes in synaptic strength. The model reproduces key experimental phenomena, including input specificity, associativity, and cooperativity, and clarifies how pathways, such as CaMKII and PKA govern the stability of synaptic modifications. By capturing both cellular- and network-level properties, this framework provides a foundation for building scalable neural models grounded in the biophysics of learning and memory.
Translational inhibition has been suggested to impair long-term memory (LTM) while leaving short-term memory (STM) unaffected. However, intracranial infusions of protein synthesis inhibitors such as anisomycin (ANI), cycloheximide, and emetine have been shown to profoundly suppress both spontaneous and evoked neural activity, likely due to impairments of mitochondrial function and cellular metabolic loss. Given this neural suppressive effect, we wanted to re-examine the influence of translational inhibition on acquiring and retaining novel information with a direct comparison to agents that simply inactivate neural activity (without affecting protein synthesis). Our model was the widely-used cued-fear conditioning paradigm which is known to be dependent on the basolateral amygdala complex (BLA). Using male rats, we compared the behavioral deficits in cue fear conditioning induced by intra-BLA infusions of ANI to other commonly used neural inactivators, including the GABAA agonist muscimol (MUSC) and the sodium channel blocker tetrodotoxin (TTX). We confirmed that intra-BLA infusions of ANI and MUSC suppressed neural activity by examining effects on spiking activity in the BLA. In behavioral experiments, pre-training bilateral BLA infusions of ANI impaired both short-term (2 h) and long-term (24 h) cue fear memory similarly to the effects observed with both MUSC and TTX. Our results suggest that reductions in neural activity, whether via GABAA agonism, sodium channel blockade, or translational inhibition, are essential for both short- and long-term memory. Consequently, the influence of translational inhibitors may be better understood as being mediated via neural inactivation rather than solely the result of the absence of de novo synthesis of plasticity-related proteins. Previous work that interprets results using translational inhibitors as being purely protein-synthesis-dependent must to be reconsidered. SIGNIFICANCE STATEMENT: The axiomatic idea that protein synthesis can distinguish between phases of memory, namely short- versus long-term forms, is entirely based on work purporting to show differential disruptive effects of translational inhibitors on retrieval tested at long, but not short-term delays. However, protein synthesis inhibitors profoundly suppress neural activity as well, which should have an equivalent detrimental effect on both short- and long-term memory. In this study, we demonstrate that the translational inhibitor anisomycin suppresses neural activity in the basolateral amygdala which results in deficits in both short- and long-term memory for conditioned fear, similarly to agents that ONLY block neural activity. These findings are consistent with an important and under-appreciated role of neural activity in memory processes and argue for a critical re-evaluation of the strict de novo protein hypothesis of memory consolidation.
Rapid eye movement (REM) phases of sleep are consistently disrupted after trauma and in post-traumatic stress disorder (PTSD). REM disruption impairs cued fear learning, a core element of trauma processing linked to PTSD risk and symptom persistence. Configural threat learning depends on the hippocampus and may be influenced by REM sleep, though its role in humans remains unclear. Here, 55 healthy participants were randomly assigned to REM sleep fragmentation or a control group without sleep disruption for two nights. On the final day, participants completed a configural threat learning task while electrodermal activity (EDA) and behavioral threat expectancy ratings were recorded. REM fragmentation group (REM frag group) had significantly lower threat responses as measured by EDA to the threat configuration relative to the control group. Relative to the REM frag group, the control group showed a significantly greater increase in threat discrimination from the first to the second block of learning. Polysomnography revealed that weighted REM efficiency, but not slow wave sleep or total sleep time, was significantly associated with configural threat learning. REM sleep fragmentation did not affect mnemonic pattern separation, a non-threat measure of hippocampal learning. These results provide evidence that REM sleep is a critical component of configural threat. These findings set the stage for future investigations into how REM sleep disruptions may impair the neural processes supporting threat generalization and discrimination in PTSD, with implications for targeted sleep-based interventions aimed at modulating trauma-related learning and memory.
BACKGROUND:Insufficient sleep is prevalent in youth and associated with elevated anxiety risk, yet the neurobiological mechanisms linking sleep to anxiety remain poorly understood. One potential pathway involves impaired fear extinction recall-a process critical for effective fear regulation. Further, fear extinction recall relies on corticolimbic circuitry and has been shown to change dynamically during adolescence. Here, we examined whether acute sleep duration was associated with conditioned fear responses and corticolimbic activation during recall in youth. METHODS:One hundred youth (ages 10-17) completed a validated Pavlovian fear extinction and recall paradigm across two days. Acute sleep duration was self-reported. Conditioned fear was assessed via skin conductance responses (SCRs), subjective fear ratings, and behavioral avoidance. A subsample (n = 64) underwent functional magnetic resonance imaging (fMRI) during the recall phase. RESULTS:Shorter sleep duration was associated with higher anxiety symptoms, elevated SCRs, and reduced behavioral avoidance during recall, controlling for age, sex, and prior fear learning. Neuroimaging analyses revealed that shorter sleep duration was linked to lower activation in the amygdala and right postcentral gyrus during recall. CONCLUSIONS:Inadequate sleep may disrupt physiological, behavioral, and neural processes supporting fear regulation, possibly through impaired consolidation and/or retrieval of extinction memories and altered behavioral avoidance. These findings identify fear extinction recall as a potential pathway linking insufficient sleep to anxiety vulnerability in youth, and highlight the importance of sleep for the consolidation and retrieval of therapeutic learning (e.g., during exposure-based treatments).
Pavlov's research on how arbitrary stimuli elicit conditioned reflexes has revolutionized the field of learning. His original work with the orienting response set the foundation for further investigations on the nature of Pavlovian conditioned responding.Where Pavlov's research may have lacked the ecological significance of Pavlovian conditioning, the behavior systems approach emerged later to capture how conditioning is embedded into a broader spectrum of naturalistic behaviors.The behavior systems approach proposes that behaviors evolved as organized systems designed to solve specific adaptive problems, and that learning interacts with these systems. The approach emphasizes the arrangement of responses to stimuli in the environment on a temporal and spatial continuum from an appetitive general search behavior at one end to focal and consummatory acts at the other end.During Pavlovian conditioning, a conditioned stimulus may become integrated along the continuum.The current paper describes several studies of Pavlovian conditioning that support the behavior systems approach with more detail in studies on sexual conditioning in male quail. Collectively, these studies demonstrate the importance of the form and relevance of the CS and the CS-US interval in determining where along the continuum the CS becomes integrated and thereby determine the nature or topography of the conditioned response.These studies, and many others, should serve as a reminder of how Pavlov's research set the framework for the conceptualization of behavior systems.
Pavlov’s research on conditioning, perhaps due to his place in founding the field, was limited to identifying a small number of core principles. This may explain why most of his findings have proven to be of broad generality. A century later, associative learning, including Pavlovian conditioning, is viewed as encompassing a multitude of principles, with most being influenced by a number of independent variables, and we are still far from identifying all the independent variables for each principle. However, it is already clear that many behavioral principles share consequential independent variables, making it difficult to examine any given principle in isolation. This precludes making generalizable predictions, except for further studies in the same laboratory setting where most independent variables can be held constant across experiments. When exported to other laboratories or applied situations, this is usually not feasible. While there is theoretical value in identifying how select independent variables influence behavioral phenomena when other independent variables are held constant, I assert that generalizing the findings to different laboratory settings or applied situations is highly prone to being unsuccessful because these ‘other variables’ will differ from those of the initial laboratory. Nonetheless, academic and funding structures reward researchers who claim great generality concerning behavioral phenomena in the abstract (e.g., ‘blocking’ as the result of simply stated procedures devoid of specific parameters). The central contribution of this paper is to suggest factors that have led behavioral psychology to develop this penchant for claiming broad generality and to advocate for more conceptual replications, so we are less apt to be distracted by phenomena of limited generality.
Pavlovian conditioning is typically defined as a form of learning in which a neutral stimulus, through repeated pairing with an unconditional stimulus (US) that elicits an unconditional response (UR), comes to evoke a conditional response (CR) that is related to the UR. I argue that each substantive element of this common definition is empirically and conceptually flawed and that it obscures conditioning’s central role in everyday adaptive behavior. I develop an alternative framework in which Pavlovian conditioning is the process by which organisms learn to recognize biologically significant objects (e.g., foods, mates, predators, toxins) and generate phylogenetically appropriate responses via functional behavior systems organized along imminence continua. Within this view, CSs and USs are features of objects, with the difference that a US serves to tag the object as belonging to a specific behavior system. CRs consist of behavior system mode-specific behaviors and broad changes in reactivity (Pavlovian sensitization). This reconceptualization provides new definitions of CS, US, CR, and UR and situates Pavlovian conditioning as a pervasive mechanism for immediate adaptive action rather than a narrow laboratory phenomenon.
The common marmoset is rapidly emerging as a powerful nonhuman primate model in neuroscience, yet the development of scalable, mechanistically informative cognitive paradigms has lagged behind advances in neural recording and genetic tools. Here, we introduce and validate a touchscreen-based spatial working memory task designed for direct cross-species translation between marmosets and humans. The paradigm independently manipulates retention delay and spatial separation between test choice stimuli, enabling parametric control over maintenance and interference demands within a single framework. Twelve marmosets and seventy-one human participants performed a Delayed Non-Match-to-Position task in which memory delay (1, 5, 10 s) and angular separation between target and distractor locations were systematically varied. Across species, accuracy declined as delay increased and as spatial separation decreased, demonstrating robust sensitivity to both maintenance demands and similarity-based interference. Critically, delay and separation interacted in both species, indicating that these had additive effects. Choice latency analyses further supported interpretation of performance, with slower responses on incorrect trials in both groups. Together, these findings establish a scalable and translationally aligned spatial working memory paradigm that captures interacting maintenance and interference processes. This task provides a powerful platform for circuit-level investigation and offers a sensitive cognitive assay for future studies of aging, neurodegenerative disease, and therapeutic intervention in the marmoset model.
One hundred years ago, Pavlov observed that omitting the reinforcer that was previously paired with a conditional stimulus resulted in a decrease in the conditional response evoked by that stimulus, a phenomenon labeled extinction. Notably, Pavlov found that extinction was not permanent and the conditional response could recover under a range of conditions. As extinction is the basis of many modern therapies, the aim of much current work is to identify ways to enhance its longevity. To this end, one strategy is compound extinction, where a combination of previously reinforced stimuli is presented together for the first time during extinction training. This treatment has been shown to enhance extinction learning, evidenced by reduced future spontaneous recovery. However, the mechanisms are not fully understood. Experiment 1 assessed whether the compound extinction effect is the result of increased expectation of reward generated by the compound of stimuli that is then violated, driving further learning, or more simply, whether the novelty of the compound is able to re-engage attention and thus promotes extinction without increasing prediction error. Experiment 2 tested whether the effects of the noradrenaline β-receptor antagonist propranolol, shown elsewhere to reduce the compound extinction effect, relate to prediction error or novelty. We found that, when equating the novelty of the stimulus compound, larger prediction error resulted in better extinction evidenced as reduced spontaneous recovery. Further, we found that propranolol reduced this effect suggesting that prediction error rather than novelty recruits noradrenergic signalling. Together our results identify behavioural and pharmacological strategies that can improve the long-term expression of extinction.
