Hongjoo J. Lee

Amygdala Is Critical for Stress-Induced Modulation of Hippocampal Long-Term Potentiation and Learning

Stress is a biologically significant factor shown to influence synaptic plasticity and memory functioning in the hippocampus. This study examined the role of the amygdala, a brain structure implicated in coordinating stress behaviors and modulating memory consolidation, in mediating stress effects on hippocampal long-term potentiation (LTP) and memory in rats. Electrolytic lesions of the amygdala effectively blocked the adverse physiological and behavioral effects of restraint and tailshock stress, without impeding the increase in corticosterone secretion to stress. Physiologically, hippocampal slices from stressed animals exhibited impaired LTP relative to slices from unstressed control animals, whereas hippocampal slices from stressed animals with amygdalar lesions exhibited normal LTP. Behaviorally, stressed animals were impaired in retention of a hippocampal-dependent hidden platform version of the Morris water maze task, and this impairment was blocked by amygdalar lesions. In a fixed location–visible platform water maze task that can be acquired by independent hippocampal and nonhippocampal memory systems, stress enhanced the use of nonhippocampal-based memory to acquire the task. These results indicate that an intact amygdala is necessary for the expression of the modulatory effects of stress on hippocampal LTP and memory.

Amygdalar NMDA Receptors are Critical for New Fear Learning in Previously Fear-Conditioned Rats

NMDA receptors in the amygdala seem to be critical for fear conditioning in naive rats. Recent spatial-learning studies suggest that previous learning protected animals from the amnesic effect of NMDA antagonists on new learning (of a similar behavioral task). Therefore, the present study examined whether blocking of NMDA receptors in the basolateral nucleus of the amygdala (BLA) prevents new fear learning in previously fear-conditioned rats, as measured by freezing behavior. Intra-BLA infusions of the NMDA receptor antagonistdl-2-amino-5-phosphonovaleric acid (APV) completely blocked fear conditioning to a tone stimulus in animals that had previously been fear-conditioned to a light stimulus. Similar results were obtained with intra-BLA infusions of APV before contextual fear conditioning in rats that had been fear-conditioned to a different context. Additional experiments showed that intra-BLA APV infusions substantially interfere with the expression and extinction of conditioned fear to tone, light, and context stimuli. Together, these results indicate that NMDA receptors in the BLA are crucial for the encoding of new fear memories (i.e., the formation of specific conditioned stimulus–unconditioned stimulus association), the expression of conditioned fear responses, and the extinction of acquired fear.

Amygdalar Inactivation Blocks Stress-Induced Impairments in Hippocampal Long-Term Potentiation and Spatial Memory

Electrolytic lesions to the amygdala, a limbic structure implicated in stress-related behaviors and memory modulation, have been shown to prevent stress-induced impairments of hippocampal long-term potentiation (LTP) and spatial memory in rats. The present study investigated the role of intrinsic amygdalar neurons in mediating stress effects on the hippocampus by microinfusing the GABAA receptor agonist muscimol into the amygdala and examining stress effects on Schaffer collateral/commissural-CA1 LTP and spatial memory. The critical period of the amygdalar contribution to stress effects on hippocampal functions was determined by applying muscimol either before stress or immediately after stress. Our results indicate that intra-amygdalar muscimol infusions before uncontrollable restraint-tailshock stress effectively blocked stress-induced physiological and behavioral effects. Specifically, hippocampal slices prepared from vehicle-infused stressed animals exhibited markedly impaired LTP, whereas slices obtained from muscimol-infused stressed animals demonstrated robust LTP comparable with that of unstressed animals. Correspondingly, vehicle-infused stressed animals displayed impaired spatial memory (on a hidden platform version of the Morris water maze task), whereas muscimol-infused stressed animals revealed unimpaired spatial memory. In contrast to prestress muscimol effects, however, immediate poststress infusions of muscimol into the amygdala failed to interfere with stress impairments of LTP and spatial memory. Together, these results suggest that the amygdalar neuronal activity during stress, but not shortly after stress, is essential for the emergence of stress-induced alterations in hippocampal LTP and memory.

Neurogenesis in a rat model of age-related cognitive decline.

Age‐related decrements in hippocampal neurogenesis have been suggested as a basis for learning impairment during aging. In the current study, a rodent model of age‐related cognitive decline was used to evaluate neurogenesis in relation to hippocampal function. New hippocampal cell survival was assessed approximately 1 month after a series of intraperitoneal injections of 5‐bromo‐2′‐deoxyuridine (BrdU). Correlational analyses between individual measures of BrdU‐positive cells and performance on the Morris water maze task provided no indication that this measure of neurogenesis was more preserved in aged rats with intact cognitive abilities. On the contrary, among aged rats, higher numbers of BrdU‐positive cells in the granule cell layer were associated with a greater degree of impairment on the learning task. Double‐labelling studies confirmed that the majority of the BrdU+ cells were of the neuronal phenotype; the proportion of differentiated neurons was not different across a broad range of cognitive abilities. These data demonstrate that aged rats that maintain cognitive function do so despite pronounced reductions in hippocampal neurogenesis. In addition, these findings suggest the interesting possibility that impaired hippocampal function is associated with greater survival of newly generated hippocampal neurons at advanced ages.

Early life stress impairs fear conditioning in adult male and female rats.

We demonstrated that neonatal isolation (1-h pup isolation; postnatal days 2-9) impairs context-induced fear conditioning in adult male rats and tends to enhance this effect and foot shock sensitivity in females. In this study, we examine the effects of brief (i.e., handling; 15 min) and prolonged (3 h) maternal separations (postnatal days 1-21) on fear conditioning and foot shock sensitivity in adult male and female rats. Identical training and test conditions from our prior study were employed so comparisons of the three early life stressors could be made. Context- and cue-elicited freezing and ultrasonic vocalizations (USVs; 22 kHz) were measured after 10 tone-shock training trials in Experiment 1. In Experiment 2, foot shock responses (flinch, jump, sonic vocalizations) to escalating shock levels were assessed. Brief maternal separation impaired context- and cue-conditioned fear in rats of both sexes as assessed by USVs. Prolonged maternal separation only impaired context fear in female rats. There were no effects on foot shock sensitivity. Results of this and other studies suggest that early life stress impairs fear conditioning in adult rats whereas stress experienced in adulthood has the opposite effect. These opposing effects may reflect developmental differences on stress-induced alterations on hippocampal regulation of the hypothalamic-pituitary-adrenal axis.

Role of Amygdalo-Nigral Circuitry in Conditioning of a Visual Stimulus Paired with Food

The amygdala central nucleus (CeA) plays an important part in associative learning. Although most research has focused on functions of its descending projections to brainstem areas involved in autonomic and somatomotor responses, the ascending projections of CeA also play critical roles in learning. For example, a CeA-nigrostriatal pathway is important for acquiring orienting responses (ORs) to conditioned stimuli (CSs) that signal food delivery. In this study, the function of this CeA-nigrostriatal pathway in appetitive conditioning of rats was considered in more detail. In experiment 1, we combined anatomical tracing and methods for detecting neuronal activation to examine whether CeA neurons that project to the substantia nigra pars compacta (SNc) are activated by a visual CS for food. After injection of the retrograde tracer Fluoro-Gold (FG) into SNc, the rats received pairings of a visual CS with food. After a test with the CS alone, the brains were prepared to assess FG labeling and CS-induced Fos expression in CeA with immunohistochemical procedures. Colocalization of Fos and FG in CeA neurons was visualized with confocal-fluorescence microscopy. The CS induced Fos expression in CeA, and a majority of these Fos-positive neurons were also FG positive, indicating activation of the CeA-SNc pathway by the CS. In experiment 2, lesions that disconnected CeA and SNc prevented the acquisition of conditioned ORs but did not affect the acquisition of conditioned food-related responses or the display of unconditioned ORs. These experiments demonstrate a role for amygdalo-nigral circuitry in learned modulation of attention to signals for biologically significant events.

Stress-Induced Alterations in Hippocampal Plasticity, Place Cells, and Spatial Memory

Acute, inescapable, and unpredictable stress can profoundly modify brain and cognition in humans and animals. The present study investigated the ensuing effects of 2-h variable “audiogenic” stress on three related levels of hippocampal functions in rats: long-term potentiation, place cell activity, and spatial memory. In agreement with prior findings, we observed that stress reduced the magnitude of Schaffer collateral/commissural–Cornu Ammonis field 1 long-term potentiation in vitro, and selectively impaired spatial memory on a hidden platform version of the Morris water maze task. We also observed that stress impaired the stability of firing rates (but not firing locations) of place cells recorded from dorsal Cornu Ammonis field 1 in rats foraging freely on a novel open-field platform located in a familiar surrounding room. These findings suggest that stress-induced modifications in synaptic plasticity may prevent the storage of stable “rate maps” by hippocampal place cells, which in turn may contribute to spatial memory impairments associated with stress.

Sex-selective effects of neonatal isolation on fear conditioning and foot shock sensitivity

Our previous work demonstrates enduring effects of the early life stress of neonatal isolation (ISO). ISO facilitates appetitive response learning in adult female, but not male rats, and enhances corticosterone levels and stress responsivity in infant and juvenile rats of both sexes. Corticosterone acts at brain areas such as hippocampus that are rich in glucocorticoid receptors, differentiate postnatally, are sexually dimorphic, and involved in learning. Thus, ISO is hypothesized to alter aversive learning in adult rats in a sex-specific manner. This study tests this hypothesis using context and cue fear conditioning. Pups were isolated for 1h a day on postnatal (PN) days 2-9 or were non-handled and were then tested in adulthood (PN70-90). In Experiment 1, context- and cue-elicited freezing and ultrasonic vocalizations (USVs; 22 kHz range) were measured. Experiments 2-4, respectively, examined three unconditioned foot shock responses (flinch, jump, vocalization), unconditioned fear (time in center of an open, novel arena), and appetitive (stroking-induced; 50 kHz range) USVs. ISO had a sex-selective effect on context-induced USVs that may reflect changes in foot shock sensitivity. ISO increases foot shock sensitivity and tends to enhance context-induced fear in female rats, whereas ISO tends to impair context-induced fear in male rats. Overall, male rats show greater conditioned fear, and female rats show greater unconditioned fear as well as enhanced responses to the aversive and appetitive stimuli. The sex-specific effect of ISO on context fear may reflect neuronal reorganization in stress responsive areas and/or sex differences in some unconditioned responses.

Neonatal handling alters learning in adult male and female rats in a task-specific manner

We demonstrated that early life manipulations (neonatal isolation, neonatal handling, maternal separation) impaired fear conditioning in adult rats [Kosten, T.A., Miserendino, M.J.D., Bombace, J.C., Lee, H.J., Kim, J.J., 2005. Sex-selective effects of neonatal isolation on fear conditioning and foot shock sensitivity. Behav. Brain Res. 157, 235-244.; Kosten, T.A., Lee, H.J. and Kim, J.J., 2006. Early life stress impairs fear conditioning in adult male and female rats. Brain Res. 1087, 142-150.]. Although we found few effects on somatic responses to footshock, deficits in conditioned fear may reflect altered emotional reactivity to aversive stimuli not learning deficits. Here we test neonatal handling effects on learning and memory tasks that vary by aversive stimuli. Neonatal handling was chosen because it alters emotional reactivity in adult rats. Litters of Sprague-Dawley rats were assigned to neonatal handling (15-min separation from dam and nest on postnatal days 1-21) or control (nonseparated) conditions. Adult male and female rats with or without neonatal handling experience were compared on: (1) inhibitory avoidance that involves footshock; (2) a circular maze task that involves escape from bright light; and (3) object recognition that presumably does not involve aversive stimuli. Neonatal handling impaired inhibitory avoidance but enhanced object recognition. There were no differences in circular maze performance. In addition, sex differences emerged in both the inhibitory avoidance and object recognition tasks; female rats perform better in inhibitory avoidance and worse in object recognition compared to male rats. These data suggest that neonatal handling alters learning and memory in a task-specific manner that may reflect alterations in emotional reactivity or differential effects of the manipulation on unknown neurohormonal mechanisms.

Role of Substantia Nigra–Amygdala Connections in Surprise-Induced Enhancement of Attention

Coding of prediction error by midbrain dopamine neurons has been examined extensively in the framework of associative learning theory. Most of this research has focused on the role of prediction error in determining the reinforcement value of unconditioned stimuli: poorly predicted (“surprising”) outcomes are more effective reinforcers and produce a greater dopamine response than well predicted outcomes. However, surprise also enhances attention to cues that signal poorly predicted outcomes. Previous reports from our laboratories demonstrated that circuitry, including the amygdala central nucleus (CeA), the cholinergic neurons of the substantia innominata/nucleus basalis region, and their innervation of the posterior parietal cortex, is critical to these surprise-induced enhancements of attention in associative learning. The present study considered the origin of prediction error information important for the operation of this system by examining the effects of disrupting communication between the midbrain substantia nigra pars compacta (SNc) and the CeA. Rats received unilateral lesions of the SNc and lesions of the CeA in either the contralateral or ipsilateral hemisphere. Contralateral lesions eliminated the surprise-induced enhancement of attention and learning that was displayed by rats with ipsilateral control lesions. These results show that SNc–CeA communication is critical to mechanisms by which the coding of prediction error by midbrain dopamine neurons is translated into enhancement of attention and learning modulated by the cholinergic system.