Retrieval and savings of contextual fear memories across an extended retention interval in juvenile and adult male and female rats.

Adult rodents exhibit an exceptional ability to retrieve context fear memories across lengthy retention intervals. In contrast, these memories established in younger rodents are susceptible to significant forgetting. The present study aimed to examine the persistence of contextual fear memories established in juvenile and adult Long-Evans male and female rats. Testing 1-day after conditioning, adult males exhibited evidence for greater conditioning than juvenile males, while in females, conditioning did not differ between juvenile and adult rats. In adults, males displayed greater conditioning than females, while in juveniles, males and females reached similar conditioning levels. At the 60-day retention interval, adult sex differences were maintained; however, juvenile rats failed to retrieve this remote contextual fear memory. Next, we examined whether a savings test procedure could recover these remotely established juvenile memories. Following a 60-day retention test, the now adult rats were presented with an additional context-shock pairing to assess the level of savings. While this procedure produced greater conditioning in males than females, the relative savings of this early life memory were similar in males and females. The results of these experiments indicate that adult sex differences in contextual fear memory are maintained across an extended retention interval, while in juveniles, there were no significant sex differences. A novel finding in the present study was that both male and female rats failed to retrieve an initial juvenile memory following an extended retention interval. However, these memories were recovered with a single reminder of the original juvenile experience. (PsycInfo Database Record (c) 2023 APA, all rights reserved).

Contributions of gonadal hormones in the sex-specific organization of context fear learning.

It is widely established that gonadal hormones are fundamental to modulating and organizing the sex-specific nature of reproductive behaviors. Recently we proposed that context fear conditioning (CFC) may emerge in a sex-specific manner organized prior to the pubertal surge of gonadal hormones. Here we sought to determine the necessity of male and female gonadal hormones secreted at critical periods of development upon context fear learning. We tested the organizational hypothesis that neonatal and pubertal gonadal hormones play a permanent role in organizing contextual fear learning. We demonstrate that the postnatal absence of gonadal hormones by neonatal orchiectomy (oRX) in males and ovariectomy (oVX) in females resulted in an attenuation of CFC in adult males and an enhancement of CFC in adult females. In females, the gradual introduction of estrogen before conditioning partially rescued this effect. However, the decrease of CFC in adult males was not rescued by introducing testosterone before conditioning. Next, at a further point in development, preventing the pubertal surge of gonadal hormones by prepubertal oRX in males resulted in a reduction in adult CFC. In contrast, in females, prepubertal oVX did not alter adult CFC. However, the adult introduction of estrogen in prepubertal oVX rats reduced adult CFC. Lastly, the adult-specific deletion of gonadal hormones by adult oRX or oVX alone or replacement of testosterone or estrogen did not alter CFC. Consistent with our hypothesis, we provide initial evidence that gonadal hormones at early periods of development exert a vital role in the organization and development of CFC in male and female rats.

Know thy SEFL: Fear sensitization and its relevance to stressor-related disorders.

Extreme stress can cause long-lasting changes in affective behavior manifesting in conditions such as post-traumatic stress disorder (PTSD). Understanding the biological mechanisms that govern trauma-induced behavioral dysregulation requires reliable and rigorous pre-clinical models that recapitulate multiple facets of this complex disease. For decades, Pavlovian fear conditioning has been a dominant paradigm for studying the effects of trauma through an associative learning framework. However, severe stress also causes long-lasting nonassociative fear sensitization, which is often overlooked in Pavlovian fear conditioning studies. This paper synthesizes recent research on the stress-enhanced fear learning (SEFL) paradigm, a valuable rodent model that can dissociate associative and nonassociative effects of stress. We discuss evidence that the SEFL paradigm produces nonassociative fear sensitization that is distinguishable from Pavlovian fear conditioning. We also discuss key biological variables, such as age and sex, neural circuit mechanisms, and crucial gaps in knowledge. We argue that nonassociative fear sensitization deserves more attention within current PTSD models and that SEFL provides a valuable complement to Pavlovian conditioning research on trauma-related pathology.

Contextual processing elicits sex differences in dorsal hippocampus activation following footshock and context fear retrieval.

Establishing a contextual representation of an environment places specific spatial-temporal processing demands on the mammalian hippocampus, a region showing sex-differences in processing capabilities. However, evidence for sex differences in these processing demands during contextual fear learning remains limited. Here, we examined the relationship among contextual processing, timing of footshock, and activation of the dorsal hippocampus and basolateral amygdalar nuclei (BLA) in male and female mice (C57Bl/6 J). We modified the initial exposure time to the conditioning context prior to administration, or not, of a single footshock. We then quantified Fos- ir neurons activated by acquisition or retrieval of contextual fear memories in the rostral half of the dorsal CA1 (proximal - distal regions), CA3, Dentate Gyrus and basolateral amygdalar nuclei corresponding to atlas levels of the Allen Reference Atlas. In experiment 1, we found that sex differences in context elicited freezing were evident at the longest context placement-to-shock interval and that context fear retrieval with increasing contextual exposure periods increased CA1 Fos-ir in males, but not females. In experiment 2, we observed that an aversive footshock in males potentiated CA1 activation, while it downregulated CA1 activation in females. We also found that an aversive footshock independent of sex moderated Dentate Gyrus activation that normally showed increased activation with greater context exposure periods. Lastly, we identified a heightened responsiveness in BLA neurons to both footshock and length of context exposure in females compared to males. Overall, our findings suggest that sex differences in contextual fear conditioning may arise from marked sex differences in the contextual processing demands of the dorsal hippocampus subfields largely modulated by aversive outcomes.

Contextual fear retrieval-induced Fos expression across early development in the rat: An analysis using established nervous system nomenclature ontology.

The neural circuits underlying the acquisition, retention and retrieval of contextual fear conditioning have been well characterized in the adult animal. A growing body of work in younger rodents indicates that context-mediated fear expression may vary across development. However, it remains unclear how this expression may be defined across the full range of key developmental ages. Nor is it fully clear whether the structure of the adult context fear network generalizes to earlier ages. In this study, we compared context fear retrieval-induced behavior and neuroanatomically constrained immediate early-gene expression across infant (P19), early and late juvenile (P24 and P35), and adult (P90) male Long-Evans rats. We focused our analysis on neuroanatomically defined subregions and nuclei of the basolateral complex of the amygdala (BLA complex), dorsal and ventral portions of the hippocampus and the subregions of the medial prefrontal cortex as defined by the nomenclature of the Swanson (2004) adult rat brain atlas. Relative to controls and across all ages tested, there were greater numbers of Fos immunoreactive (Fos-ir) neurons in the posterior part of the basolateral amygdalar nuclei (BLAp) following context fear retrieval that correlated statistically with the expression of freezing. However, Fos-ir within regions having known connections with the BLA complex was differentially constrained by developmental age: early juvenile, but not adult rats exhibited an increase of context fear-dependent Fos-ir neurons in prelimbic and infralimbic areas, while adult, but not juvenile rats displayed increases in Fos-ir neurons within the ventral CA1 hippocampus. These results suggest that juvenile and adult rodents may recruit developmentally unique pathways in the acquisition and retrieval of contextual fear. This study extends prior work by providing a broader set of developmental ages and a rigorously defined neuroanatomical ontology within which the contextual fear network can be studied further.

Sexual differentiation of contextual fear responses.

Development and sex differentiation impart an organizational influence on the neuroanatomy and behavior of mammalian species. Prior studies suggest that brain regions associated with fear motivated defensive behavior undergo a protracted and sex-dependent development. Outside of adult animals, evidence for developmental sex differences in conditioned fear is sparse. Here, we examined in male and female Long-Evans rats how developmental age and sex affect the long-term retention and generalization of Pavlovian fear responses. Experiments 1 and 2 describe under increasing levels of aversive learning (three and five trials) the long-term retrieval of cued and context fear in preadolescent (P24 and P33), periadolescent (P37), and adult (P60 and P90) rats. Experiments 3 and 4 examined contextual processing under minimal aversive learning (1 trial) procedures in infant (P19, P21), preadolescent (P24), and adult (P60) rats. Here, we found that male and female rats display a divergent developmental trajectory in the expression of context-mediated freezing, such that context fear expression in males tends to increase toward adulthood, while females displayed an opposite pattern of decreasing context fear expression toward adulthood. Longer (14 d) retention intervals produced an overall heightened context fear expression relative to shorter (1 d) retention intervals an observation consistent with fear incubation. Male, but not Female rats showed increasing generalization of context fear across development. Collectively, these findings provide an initial demonstration that sexual differentiation of contextual fear conditioning emerges prior to puberty and follows a distinct developmental trajectory toward adulthood that strikingly parallels sex differences in the etiology and epidemiology of anxiety and trauma- and stressor-related disorders.

Conditioning- and time-dependent increases in context fear and generalization.

A prominent feature of fear memories and anxiety disorders is that they endure across extended periods of time. Here, we examine how the severity of the initial fear experience influences incubation, generalization, and sensitization of contextual fear memories across time. Adult rats were presented with either five, two, one, or zero shocks (1.2 mA, 2 sec) during contextual fear conditioning. Following a recent (1 d) or remote (28 d) retention interval all subjects were returned to the original training context to measure fear memory and/or to a novel context to measure the specificity of fear conditioning. Our results indicate rats that received two or five shocks show an "incubation"-like enhancement of fear between recent and remote retention intervals, while single-shocked animals show stable levels of context fear memory. Moreover, when fear was tested in a novel context, 1 and 2 shocked groups failed to freeze, whereas five shocked rats showed a time-dependent generalization of context memory. Stress enhancement of fear learning to a second round of conditioning was evident in all previously shocked animals. Based on these results, we conclude that the severity or number of foot shocks determines not only the level of fear memory, but also the time-dependent incubation of fear and its generalization across distinct contexts.

Retrieval and Reconsolidation Accounts of Fear Extinction.

Extinction is the primary mode for the treatment of anxiety disorders. However, extinction memories are prone to relapse. For example, fear is likely to return when a prolonged time period intervenes between extinction and a subsequent encounter with the fear-provoking stimulus (spontaneous recovery). Therefore there is considerable interest in the development of procedures that strengthen extinction and to prevent such recovery of fear. We contrasted two procedures in rats that have been reported to cause such deepened extinction. One where extinction begins before the initial consolidation of fear memory begins (immediate extinction) and another where extinction begins after a brief exposure to the consolidated fear stimulus. The latter is thought to open a period of memory vulnerability similar to that which occurs during initial consolidation (reconsolidation update). We also included a standard extinction treatment and a control procedure that reversed the brief exposure and extinction phases. Spontaneous recovery was only found with the standard extinction treatment. In a separate experiment we tested fear shortly after extinction (i.e., within 6 h). All extinction procedures, except reconsolidation update reduced fear at this short-term test. The findings suggest that strengthened extinction can result from alteration in both retrieval and consolidation processes.

Sensitization of fear learning to mild unconditional stimuli in male and female rats.

Stress-enhanced fear learning (SEFL) refers to the long-lasting nonassociative sensitization produced by intense stress (e.g., repeated and unpredictable footshock) that results in increased fear learning to a mild conditioning regimen (e.g., one shock). SEFL experiments suggest that one component of posttraumatic behavior is inappropriately strong fear conditioning occurring to relatively mild stressors. Past reports of SEFL have used the same intensity (1 mA) of footshock to cause both the sensitization and conditioning of new fear. SEFL would be a particularly problematic component of posttrauma behavior if intense stress results in substantial fear conditioning under conditions that would not normally support conditioning. Therefore, we determined if SEFL occurred when the conditioning shock was substantially milder than the SEFL-inducing shock. The results indicate that exposure to a sensitizing regimen of shock can convert a mild footshock that normally does not support measurable levels of fear conditioning into one that causes substantial learned fear. Moreover, as the intensity of single footshock increases, so does the capacity of the prior stressor to contribute to the sensitization of fear responses. Consistent with prior studies, males acquired and retained a greater level of fear conditioning than female rats, however the level of sensitization did not differ between sexes.

Localization and characterization of an essential associative memory trace in the mammalian brain.

We argue here that we have succeeded in localizing an essential memory trace for a basic form of associative learning and memory - classical conditioning of discrete responses learned with an aversive stimulus - to the anterior interpositus nucleus of the cerebellum. We first identified the entire essential circuit, using eyelid conditioning as the model system, and used reversible inactivation, during training, of critical structures and activation of pathways to localize definitively the essential memory trace. This discovery and the associated studies have: 1) shown that the essential cerebellar circuit applies equally to all mammals studied, including humans; 2) shown that this cerebellar circuit holds for the learning of any discrete behavioral response elicited by an aversive US, not just eyelid closure; 3) identified the essential circuit and process for reinforcement for this form of learning; 4) shown that this form of learning and its essential cerebellar circuitry is phylogenetically very old; 5) solved the long-standing puzzle of where memory traces are formed in the brain when the CS is electrical stimulation of the cerebral cortex in conditioning; 6) shown that this cerebellar circuitry forms the essential neural substrate for the behavioral phenomenon of "blocking", and hence, 7) provides the first clear neural instantiation of the Rescorla-Wagner learning algorithm; 8) shown that the fundamental neural process underlying this form of learning is a strengthening of preexisting pathways, and 9) shown that the basic mechanism underlying this strengthening is the formation of new excitatory synapses. This article is part of a Special Issue entitled SI: Brain and Memory.

Amnesia for early life stress does not preclude the adult development of posttraumatic stress disorder symptoms in rats.

BACKGROUND: Traumatic experience can result in life-long changes in the ability to cope with future stressors and emotionally salient events. These experiences, particularly during early development, are a significant risk factor for later life anxiety disorders such as posttraumatic stress disorder (PTSD). However, because traumatic experience typically results in strong episodic memories, it is not known whether such long-term memories are necessary for particular features of PTSD, such as enhanced fear and anxiety. Here, we used a fear conditioning procedure in juvenile rats before maturation of the neural systems supporting declarative memory to assess the necessity of early memory to the later life development of PTSD-related symptoms. METHODS: Nineteen-day old rats were exposed to unpredictable and inescapable footshocks, and fear memory for the shock context was assessed during adulthood. Thereafter, adult animals were either exposed to single-trial fear conditioning or elevated plus maze or sacrificed for basal diurnal corticosterone and quantification of neuronal glucocorticoid and neuropeptide Y receptors. RESULTS: Early trauma exposed rats displayed stereotypic footshock reactivity, yet by adulthood, hippocampus-dependent contextual fear-related memory was absent. However, adult rats showed sensitized fear learning, aberrant basal circadian fluctuations of corticosterone, increased amygdalar glucocorticoid receptors, decreased time spent in the open arm of an elevated plus maze, and an odor aversion associated with early-life footshocks. CONCLUSIONS: These results suggest that traumatic experience during developmental periods of hippocampal immaturity can promote lifelong changes in symptoms and neuropathology associated with human PTSD, even if there is no explicit memory of the early trauma.

Concussive brain injury enhances fear learning and excitatory processes in the amygdala.

BACKGROUND: Mild traumatic brain injury (cerebral concussion) results in cognitive and emotional dysfunction. These injuries are a significant risk factor for the development of anxiety disorders, including posttraumatic stress disorder. However, because physically traumatic events typically occur in a highly emotional context, it is unknown whether traumatic brain injury itself is a cause of augmented fear and anxiety. METHODS: Rats were trained with one of five fear-conditioning procedures (n = 105) 2 days after concussive brain trauma. Fear learning was assessed over subsequent days and chronic changes in fear learning and memory circuitry were assessed by measuring N-methyl-D-aspartate receptor subunits and glutamic acid decarboxylase, 67 kDa isoform protein levels in the hippocampus and basolateral amygdala complex (BLA). RESULTS: Injured rats exhibited an overall increase in fear conditioning, regardless of whether fear was retrieved via discrete or contextual-spatial stimuli. Moreover, injured rats appeared to overgeneralize learned fear to both conditioned and novel stimuli. Although no gross histopathology was evident, injury resulted in a significant upregulation of excitatory N-methyl-D-aspartate receptors in the BLA. There was a trend toward decreased γ-aminobutyric acid-related inhibition (glutamic acid decarboxylase, 67 kDa isoform) in the BLA and hippocampus. CONCLUSIONS: These results suggest that mild traumatic brain injury predisposes the brain toward heightened fear learning during stressful postinjury events and provides a potential molecular mechanism by which this occurs. Furthermore, these data represent a novel rodent model that can help advance the neurobiological and therapeutic understanding of the comorbidity of posttraumatic stress disorder and traumatic brain injury.

Compensation in the neural circuitry of fear conditioning awakens learning circuits in the bed nuclei of the stria terminalis.

The basolateral amygdala (BLA) is thought to be essential for fear learning. However, extensive training can overcome the loss of conditional fear evident following lesions and inactivation of the BLA. Such results suggest the existence of a primary BLA-dependent and a compensatory BLA-independent neural circuit. We tested the hypothesis that the bed nuclei of the stria terminalis (BST) provides this compensatory plasticity. Using extensive context-fear conditioning, we demonstrate that combined BLA and BST lesions prevented fear acquisition and expression. Additionally, protein synthesis in the BST was critical only for consolidation of BLA-independent but not BLA-dependent fear. Moreover, fear acquired after BLA lesions resulted in greater activation of BST regions that receive hippocampal efferents. These results suggest that the BST is capable of functioning as a compensatory site in the acquisition and consolidation of context-fear memories. Unlocking such neural compensation holds promise for understanding situations when brain damage impairs normal function or failure to regulate compensatory sites leads to anxiety disorders.

Persistence of fear memory across time requires the basolateral amygdala complex.

Mammals evolved a potent fear-motivated defensive system capable of single-trial fear learning that shows no forgetting over the lifespan of the animal. The basolateral amygdala complex (BLA) is considered an essential component of this conditional fear learning system. However, recent studies challenge this view and suggest that plasticity within other brain regions (i.e., central nucleus of the amygdala) may be crucial for fear conditioning. In the present study, we examine the mnemonic limits of contextual fear conditioning in the absence of the BLA using overtraining and by measuring remote fear memories. After excitotoxic lesions of the BLA were created, animals underwent overtraining and were tested at recent and remote memory intervals. Here we show that animals with BLA lesions can learn normal levels of fear. However, this fear memory loses its adaptive features: it is acquired slowly and shows substantial forgetting when remote memory is tested. Collectively, these findings suggest that fear-related plasticity acquired by brain regions outside of the BLA, unlike those acquired in the intact animals, do so for a relatively time-limited period.

Disruption of cerebellar cortical inhibition in the absence of learning promotes sensory-evoked eyeblink responses.

Theories of cerebellar learning propose that alterations in synaptic plasticity resulting in decreases in cerebellar cortical inhibition and increases in sensory activation of interpositus nuclei underlie the development of adaptively timed conditioned motor responses. The authors found that with concurrent pharmacological disconnection of the cerebellar cortex and intense sensory stimulation in the untrained rabbit, eyeblink responses were generated. Neither sensory stimulation nor disconnection alone generated significant eyeblink responses. These results are consistent with dual plasticity models of cerebellar learning and strongly support the general hypothesis that conditioned responses are the result of strengthening of preexisting connections in the nervous system.

Decremental effects of context exposure following delay eyeblink conditioning in rabbits.

The conditioning context arises from the relatively static features of the training environment. In rabbit eyeblink conditioning, procedures that retard acquisition (conditioned stimulus [CS] preexposure, unconditioned stimulus preexposure, blocking manipulations) are attenuated by context changes. In this article the authors investigate the effect of context exposure after initial delay conditioning. After conditioned responses (CRs) were established, one group received 6 sessions of context exposure, whereas control groups either remained in their home cages or received exposure to handling and a novel context. Thereafter, all groups received CS-alone testing. The expression of CRs was substantially reduced following context exposure relative to any retention loss in the home-cage control. Exposure to handling and a novel context facilitated the CRs rather than reducing them.

The neuroscience of mammalian associative learning.

Mammalian associative learning is organized into separate anatomically defined functional systems. We illustrate the organization of two of these systems, Pavlovian fear conditioning and Pavlovian eyeblink conditioning, by describing studies using mutant mice, brain stimulation and recording, brain lesions and direct pharmacological manipulations of specific brain regions. The amygdala serves as the neuroanatomical hub of the former, whereas the cerebellum is the hub of the latter. Pathways that carry information about signals for biologically important events arrive at these hubs by circuitry that depends on stimulus modality and complexity. Within the amygdala and cerebellum, neural plasticity occurs because of convergence of these stimuli and the biologically important information they predict. This neural plasticity is the physical basis of associative memory formation, and although the intracellular mechanisms of plasticity within these structures share some similarities, they differ significantly. The last Annual Review of Psychology article to specifically tackle the question of mammalian associative learning ( Lavond et al. 1993 ) persuasively argued that identifiable "essential" circuits encode memories formed during associative learning. The next dozen years saw breathtaking progress not only in detailing those essential circuits but also in identifying the essential processes occurring at the synapses (e.g., Bi & Poo 2001, Martinez & Derrick 1996 ) and within the neurons (e.g., Malinow & Malenka 2002, Murthy & De Camilli 2003 ) that make up those circuits. In this chapter, we describe the orientation that the neuroscience of learning has taken and review some of the progress made within that orientation.

Effects of a corneal anesthetic on extinction of the classically conditioned nictitating membrane response in the rabbit (Oryctolagus cuniculus).

Rabbits (Oryctolagus cuniculus) were presented with 7 daily sessions of tone-alone training after conditioning. Before the beginning of each of the first 4 extinction sessions, an artificial tear solution or tetracaine hydrochloride was administered to the cornea of rabbits in the control group (n = 6) and experimental group (n = 7), respectively. There were no between-group differences in the percentage of conditioned responses between both groups. However, the amplitude of the conditioned response was notably reduced in the tetracaine group (M = 0.40, SEM +/- 0.216) relative to the control group (M = 1.32, SEM +/- 0.639) early in extinction. Results seem to suggest that although motor output has been found to play an important role in extinction, corneal sensory feedback is not necessary.

Brain mechanisms of extinction of the classically conditioned eyeblink response.

It is well established that the cerebellum and its associated circuitry are essential for classical conditioning of the eyeblink response and other discrete motor responses (e.g., limb flexion, head turn, etc.) learned with an aversive unconditioned stimulus (US). However, brain mechanisms underlying extinction of these responses are still relatively unclear. Behavioral studies have demonstrated extinction as an active learning process distinct from acquisition. Experimental data in eyeblink conditioning suggest that plastic changes specific to extinction may play an important role in this process. Both cerebellar and hippocampal systems may be involved in extinction of these memories. The nature of this phenomenon and identification of the neural substrates necessary for extinction of originally learned responses is the topic of this review.