Regulation of contextual conditioning by the median raphe nucleus

The median raphe nucleus (MRN) has been suggested as the origin of a behavioral inhibition system that projects to the septum and hippocampus. Electrical stimulation of this mesencephalic area causes behavioral and autonomic manifestations characteristic of fear such as, freezing, defecation and micturition. In this study we extend these observations by analyzing the behavioral and autonomic responses of rats with lesions in the MRN submitted to a contextual conditioning paradigm. The animals underwent electrolytic or sham lesions of the median raphe nucleus. One day (acute) or 7 days (chronic) later they were tested in an experimental chamber where they received 10 foot-shocks (0.7 mA, 1 s with 20-s interval). The next day, sham and MRN-lesioned animals were tested again either in the same or in a different experimental chamber. During this, the duration of freezing, rearings, bouts of micturition and number of fecal boli were recorded. Sham-operated rats placed in the same chamber showed more freezing than rats exposed to a different context. This freezing behavior was clearly suppressed in rats with acute or chronic lesions in the MRN. MRN lesions also reduced the bouts of micturition and number of fecal boli. These rats showed a reduced number of rearings than sham-lesioned rats. This effect is probably the result of the displacement effect provoked by freezing since no significant differences in the number of rearings could be observed between these animals and the NMR-lesioned rats tested in an open field. This lesion produced higher horizontal locomotor activity in this test than the controls (sham-lesioned rats). These results point to the importance of the median raphe nucleus in the processing of fear conditioning with freezing being the most salient feature of it. Behavioral inhibition is also under control of MRN but its neural substrate seems to be dissociated from that of contextual fear.     

Hippocampal lesions, contextual retrieval, and autoshaping in pigeons

Both pigeons and rats with damage to the hippocampus are slow to acquire an autoshaped response and emit fewer overall responses than control animals. Experiment 1 explored the possibility that the autoshaping deficit was due to an impairment in contextual retrieval. Pigeons were trained for 14 days on an autoshaping task in which a red stimulus was followed by reinforcement in context A, and a green stimulus was followed by reinforcement in context B. On day 15, the subjects were given a context test in which the red and green stimuli were presented simultaneously in context A and then later in context B. Both control and hippocampal animals showed context specificity, that is, they responded more to the red stimulus in context A and to the green stimulus in context B. In Experiment 2 we video-recorded the control and hippocampal animals performing the autoshaping task. Hippocampal animals tended to miss-peck the key more often than control animals. In addition, the number of missed pecks increased across days for hippocampal animals but not for control animals, suggesting that while the control animals increased their pecking accuracy, the hippocampal animals actually decreased their pecking accuracy. Our findings suggest that impairments in moving through space may underlie the hippocampal autoshaping deficit.     

Contextual fear, gestalt memories, and the hippocampus

This review examines the relationship between exploration and contextual fear conditioning. The fear acquired to places or contexts associated with aversive events is a form of Pavlovian conditioning. However, an initial period of exploration is necessary to allow the animal to form an integrated memory of the features of the context before conditioning can take place. The hippocampal formation plays a critical role in this process. Cells within the dorsal hippocampus are involved in the formation, storage and consolidation of this integrated representation of context. Projections from the subiculum to the nucleus accumbens regulate the exploration necessary for the acquisition of information about the features of the context. This model explains why electrolytic but not excitotoxic lesions of the dorsal hippocampus cause enhanced exploratory activity but both cause deficits in contextual fear. It also explains why retrograde amnesia of contextual fear is greater than anterograde amnesia.     

Temporary inactivation of the dorsal hippocampus induces a transient impairment in retrieval of aversive memory

The reconsolidation hypothesis, which predicts that consolidated memories become labile and sensitive to amnestic agents upon reactivation, has been one of the most debated topics in memory research over recent years. One of the main criticisms to this hypothesis is the fact that some studies have shown the effects of "reconsolidation blockade" to be transient. Here we show that muscimol inactivation of the dorsal hippocampus following memory reactivation produces a reversible impairment of step-down inhibitory avoidance memory in rats. Moreover, we show that the reversal of this effect is dependent on the passage of time, and not on repeated testing. The implications of the findings to the interpretation of the phenomenon of transient retrieval impairment induced by post-reactivation pharmacological interventions on memory systems are discussed.     

Estrogen modulates sexually dimorphic contextual fear conditioning and hippocampal long-term potentiation (LTP) in rats

The present study examined the role of ovarian steroids in contextual fear conditioning and hippocampal synaptic plasticity in female rats. In experiment 1, adult female rats were ovariectomized and submitted to contextual fear conditioning, a procedure in which rats received unsignaled footshock in a novel observation chamber; freezing behavior served as the measure of conditional fear. Ovariectomized female rats froze at levels comparable to male rats, both of which froze significantly more than sham-operated female rats. In experiment 2, estrogen replacement in ovariectomized female rats reduced fear conditioning to a level comparable to that of sham-operated females in experiment 1. In experiment 3, the influence of estrogen on the induction of long-term potentiation (LTP) at perforant path-dentate granule cell synapses in ovariectomized female rats was examined. Estrogen decreased both population spike LTP and EPSP-spike potentiation at perforant path synapses. Taken together, these experiments indicate that ovarian steroids regulate both sexually dimorphic behavior and hippocampal plasticity in a fear-conditioning paradigm.     

Is the hippocampus necessary for contextual fear conditioning?

The hippocampus is widely believed to be essential for learning about the context in which conditioning occurs. This view is based primarily on evidence that lesions of the dorsal hippocampus disrupt freezing to contextual cues after fear conditioning. However, lesions that disrupt freezing produce no effect on fear-potentiated startle, a second measure of contextual fear. Moreover, hippocampal lesions also do not disrupt the contextual ‘blocking’ phenomenon, which provides an indirect measure of contextual fear. In these paradigms, at least, it appears that hippocampal lesions disrupt the expression of freezing, rather than contextual fear itself. This interpretation is supported by the finding that rats showing preserved contextual blocking after hippocampal lesions show deficits not only in contextual freezing, but also in unconditioned freezing. These findings are consistent with a growing body of data from other conditioning paradigms that contextual learning is spared after lesions of the dorsal hippocampus. Nonetheless, there remain some reports of impaired contextual fear conditioning after hippocampal lesions that cannot be attributed easily to a disruption of freezing. Thus, it is concluded that the hippocampus may be involved in contextual learning under certain – as yet, unspecified – circumstances, but is not critical for contextual learning in general.     

The form and function of hippocampal context representations

Context is an essential component of learning and memory processes, and the hippocampus is critical for encoding contextual information. However, connecting hippocampal physiology with its role in context and memory has only recently become possible. It is now clear that contexts are represented by coherent ensembles of hippocampal neurons and new optogenetic stimulation studies indicate that activity in these ensembles can trigger the retrieval of context appropriate memories. We interpret these findings in the light of recent evidence that the hippocampus is critically involved in using contextual information to prevent interference, and propose a theoretical framework for understanding contextual influence on memory retrieval. When a new context is encountered, a unique hippocampal ensemble is recruited to represent it. Memories for events that occur in the context become associated with the hippocampal representation. Revisiting the context causes the hippocampal context code to be re-expressed and the relevant memories are primed. As a result, retrieval of appropriate memories is enhanced and interference from memories belonging to other contexts is minimized.     

The hippocampus integrates context and shock into a configural memory in contextual fear conditioning

Contextual fear conditioning involves forming a representation for the context and associating it with a shock, which were attributed by the prevailing view to functions of the hippocampus and amygdala, respectively. Yet our recent evidence suggested that both processes require integrity of the dorsal hippocampus (DH). In view of the DH involvement in uniting multiple stimuli into a configuration, this study examined whether the DH would integrate context and shock into a shocked‐context representation. Male Wistar rats were trained on a two‐phase training paradigm of contextual fear conditioning. They explored a novel context on the first day to acquire a contextual representation, and received a shock in that context on the second day to form the context–shock memory. Tests of conditioned freezing given on the following days revealed two properties of configural memory—direct and mediated pattern completion: First, the contextual fear memory was retrieved in a novel context by a cue embedded in the configural set—a shock that did not elicit significant freezing on its own. Second, freezing was also elicited in a novel context by a transportation chamber that was not directly paired with the shock but could activate the fear memory inferentially. The effects were specific to the cue and not due to context generalization. Infusion of lidocaine into the DH, but not the amygdala, immediately after context–shock training impaired conditioned freezing elicited through either type of pattern completion. Our data suggest that the DH in contextual fear conditioning associates context and shock in parallel with the amygdala by incorporating the shock into an otherwise neutral context representation and turning it into a shocked‐context representation. © 2016 Wiley Periodicals, Inc.     

The hippocampus and appetitive Pavlovian conditioning: effects of excitotoxic hippocampal lesions on conditioned locomotor activity and autoshaping

The hippocampus (HPC) is known to be critically involved in the formation of associations between contextual/spatial stimuli and behaviorally significant events, playing a pivotal role in learning and memory. However, increasing evidence indicates that the HPC is also essential for more basic motivational processes. The amygdala, by contrast, is important for learning about the motivational significance of discrete cues. This study investigated the effects of excitotoxic lesions of the rat HPC and the basolateral amygdala (BLA) on the acquisition of a number of appetitive behaviors known to be dependent on the formation of Pavlovian associations between a reward (food) and discrete stimuli or contexts: (1) conditioned/anticipatory locomotor activity to food delivered in a specific context and (2) autoshaping, where rats learn to show conditioned discriminated approach to a discrete visual CS+. While BLA lesions had minimal effects on conditioned locomotor activity, hippocampal lesions facilitated the development of both conditioned activity to food and autoshaping behavior, suggesting that hippocampal lesions may have increased the incentive motivational properties of food and associated conditioned stimuli, consistent with the hypothesis that the HPC is involved in inhibitory processes in appetitive conditioning.     

The modulation of Pavlovian memory

Exposure to stressful experiences as well as sex differences in the brain are known to influence the acquisition of new memories. This review focuses on acquisition of two types of Pavlovian learning paradigms: hippocampal-independent delay conditioning and hippocampal-dependent trace conditioning and their modulation by exposure to stressful experience and sex differences in the brain. We concentrate on two sets of findings: the first is that exposure to an acute stressful experience enhances Pavlovian conditioning in the male rat, while exposure to the very same experience dramatically impairs conditioning in female rat. The sexually-opposed effects of stress on conditioning are mediated by differing hormonal substrates (adrenal versus ovarian steroids) and possibly by differing anatomical and biochemical pathways. The second set of findings is that training with hippocampal-dependent trace conditioning enhances the survival of newly generated neurons in the adult hippocampal formation. The same amount of training with hippocampal-independent delay conditioning does not affect their survival. In addition, females acquire the trace task faster than males and generate more new neurons. As with the stress effects on learning, these sex effects are influenced by hormonal status. It is our contention that identifying the hormonal and neuronal processes that modulate associative memory formation will provide insight into the processes of memory formation itself.     

Encoding and immediate retrieval tasks in patients with epilepsy: A functional MRI study of verbal and visual memory

Purpose

Medial lobe temporal structures and more specifically the hippocampus play a decisive role in episodic memory. Most of the memory functional magnetic resonance imaging (fMRI) studies evaluate the encoding phase; the retrieval phase being performed outside the MRI. We aimed to determine the ability to reveal greater hippocampal fMRI activations during retrieval phase.

Production of the Fos protein after contextual fear conditioning of C57BL/6N mice

Male C57BL/6N mice were chosen to determine Fos production during acquisition of context-dependent fear and after re-exposure to the conditioning context. Fear-conditioning was induced by a single exposure of mice to a context followed by an electric shock. Control groups consisted of mice exposed to context only (Context group) or to an immediate electric shock. When contextual retention was measured 24 h after conditioning (retention test 1), significant contextual generalization was observed. However, when animals were exposed to a different context from days 2–5 after conditioning and then tested for retention on day 6 (retention test 2), generalization was markedly reduced. After the training, the fear-conditioned mice produced higher Fos levels than mice exposed to an immediate shock in the hippocampus, medial amygdaloid nucleus and parietal somatosensory cortex. Both shock groups produced significantly more Fos than the Context group in the central nucleus of the amygdala. After retention test 1, fear-conditioned mice generated more Fos in the hippocampus and central amygdaloid nucleus than the two control groups. However, all groups exhibited similarly low Fos production after retention test 2. The results demonstrated that simultaneous Fos production in the hippocampus, central and medial nuclei of amygdala and somatosensory parietal cortex closely paralleled the ability of mice to acquire conditioned fear. In contrast, Fos production after the retention tests did not correlate with the expression of conditioned fear.     

Enhancement of acoustic prepulse inhibition by contextual fear conditioning in mice is maintained even after contextual fear extinction

Prepulse inhibition (PPI) of the acoustic startle response is one of the few and major paradigms for investigating sensorimotor gating systems in humans and rodents in a similar fashion. PPI deficits are observed not only in patients with schizophrenia, but also in patients with anxiety disorders. Previous studies have shown that PPI in rats can be enhanced by auditory fear conditioning. In this study, we evaluated the effects of contextual fear conditioning (FC) for six times a day and fear extinction (FE) for seven days on PPI in mice. C57BL/6J mice (male, 8–12 weeks) were divided into three groups; no-FC (control), FC and FC + FE. We measured PPI at the following three time points, (1) baseline before FC, (2) after FC, and (3) after FE. The results showed that PPI was increased after FC. Moreover, the enhanced PPI following FC was observed even after FE with decreased freezing behaviors. These results suggested contextual fear conditioning could enhance acoustic PPI, and that contextual fear extinction could decrease freezing behaviors, but not acoustic PPI.     

Different implications of the dorsal and ventral hippocampus on contextual memory retrieval after stress

This study assessed the relative contributions of dorsal (dHPC) and ventral (vHPC) hippocampus regions in mediating the rapid effects of an acute stress on contextual memory retrieval. Indeed, we previously showed that an acute stress (3 electric footschocks; 0.9 mA each) delivered 15 min before the 24 h‐test inversed the memory retrieval pattern in a contextual discrimination task. Specifically, mice learned in a four‐hole board two successive discriminations (D1 and D2) varying by the color and texture of the floor. Twenty‐four hours later, nonstressed animals remembered accurately D1 but not D2 whereas stressed mice showed an opposite memory retrieval pattern, D2 being more accurately remembered than D1. We showed here that, at the time of memory testing in that task, stressed animals exhibited no significant changes neither in pCREB activity nor in the time‐course evolution of corticosterone into the vHPC; in contrast, a significant decrease in pCREB activity and a significant increase in corticosterone were observed in the dHPC as compared to nonstressed mice. Moreover, local infusion of the anesthetic lidocaine into the vHPC 15 min before the onset of the stressor did not modify the memory retrieval pattern in nonstress and stress conditions whereas lidocaine infusion into the dHPC induced in nonstressed mice an memory retrieval pattern similar to that observed in stressed animals. The overall set of data shows that memory retrieval in nonstress condition involved primarily the dHPC and that the inversion of memory retrieval pattern after stress is linked to a dHPC but not vHPC dysfunction.     

Increased inhibition and enhancement of memory retrieval are associated with reduced hippocampal volume

Putative control of encoding and retrieval processes have been linked to communication between the lateral prefrontal cortex (LPFC) and the hippocampus. Moreover, correlations between the LPFC (e.g., MFG) and hippocampus have predicted individuals' ability to inhibit memory retrieval. Anatomically, differences in volume of the hippocampus have been related to changes in long-term episodic memories. Although the relationship between these ideas is clear, few studies have examined the association of how anatomy may affect the role of control over brain regions involved in distint memory processes. The current study sought to examine hippocampal volume and its relationship to LPFC control over the hippocampus. Using an automated cortical/subcortical segmentation technique (FIRST) on brain imaging gata from the Think/No-Think task, we show that hippocampal volume is associated to changes in both enhancement and inhibitory processes of memory retrival.     

Time-limited role of the hippocampus in the memory for trace eyeblink conditioning in mice

We examined the role of the hippocampus in memory retention after trace eyeblink conditioning in mice. After establishing the conditioned response (CR) in the trace paradigm, mice received a bilateral aspiration of the dorsal hippocampus and its overlying neocortex on the next day (1-day group) or after 4 weeks (4-week group). Control mice received a neocortical aspiration on the same schedule as the hippocampal-lesion group. After 2 weeks of recovery, these groups received additional conditioning for 3 days. Frequency of the CR of the 1-day group was as low as spontaneous values on the first day in the post-lesion session and never reached pre-surgical level during the post-lesion sessions, while that of the control group did reach pre-surgical level during the post-lesion sessions although there was a transient decline just after lesion. In contrast to the 1-day group, the 4-week-hippocampal lesion group retained the CR and showed a further increase, without significant difference from the control group. The temporal pattern of the CR also was unchanged by the hippocampal lesion 4 weeks after learning. These results suggest a time-limited role for the hippocampus in memory retention after trace conditioning in mice: the CR acquired recently requires an intact hippocampus for its retention, but the CR acquired remotely does not. This is similar to the result reported in rabbits. Therefore, the mechanism and time course of memory consolidation after trace eyeblink conditioning may be similar in mice and rabbits.     

Selective involvement of GABAergic mechanisms of the dorsal periaqueductal gray and inferior colliculus on the memory of the contextual fear as assessed by the fear potentiated startle test

The inferior colliculus (IC) together with the dorsal periaqueductal gray (dPAG), the amygdala and the medial hypothalamus make part of the brain aversion system, which has mainly been related to the organization of unconditioned fear. However, the involvement of the IC and dPAG in the conditioned fear is still unclear. It is certain that GABA has a regulatory role on the aversive states generated and elaborated in these midbrain structures. In this study, we evaluated the effects of injections of the GABA-A receptor agonist muscimol (1.0 and 2.0 nmol/0.2 μL) into the IC or dPAG on the freezing and fear-potentiated startle (FPS) responses of rats submitted to a context fear conditioning. Intra-IC injections of muscimol did not cause any significant effect on the FPS or conditioned freezing but enhanced the startle reflex in non-conditioned animals. In contrast, intra-dPAG injections of muscimol caused significant reduction in FPS and conditioned freezing without changing the startle reflex in non-conditioned animals. Thus, intra-dPAG injections of muscimol produced the expected inhibitory effects on the anxiety-related responses, the FPS and the freezing whereas these injections into the IC produced quite opposite effects suggesting that descending inhibitory pathways from the IC, probably mediated by GABA-A mechanisms, exert a regulatory role on the lower brainstem circuits responsible for the startle reflex.     

The role of hippocampus in memory: A hypothesis

The role of the hippocampus in memory storage in the mammalian brain is examined. The intrinsic anatomical organization of the hippocampus is such that a multidimensional mapping of other brain regions is represented. Emerging knowledge of the cortico-limbic-subcortical anatomy suggests that the hippocampal representations preserve the topological features of the targets and possess reciprocal connectivity. Long-Term Potentiation (LTP) is a prominent physiological characteristic of hippocampal synapses and is a promising candidate mnemonic device. The hypothesis is advanced that the pattern, or index, of specific neocortical (and other) areas activated by an experiential event is represented, or indexed, in the hippocampus by means of LTP. This hypothesis, termed the Memory Indexing Theory, suggests that experiential events are initially stored in an index of neocortical locations maintained in hippocampus. Subsequently, other regions, notably neocortex itself, permanently encode these experiential events and the interrelationships between them.     

Hippocampal mGluR1 and consolidation of contextual fear conditioning

The effects of post-training intra-hippocampal injections of group I mGluR agonists and antagonists, were examined in the contextual fear test, in rats. It was found that (S)-3,5-dihydroxyphenylglycine (DHPG) (a mGluR1-5 agonist) decreased, and (RS)-1-aminoindan-1,5-dicarboxylic acid (AIDA) (a mGluR1 antagonist) increased fear conditioning (a freezing reaction), examined 24h after conditioning session. (RS)-2-Chloro-5-hydroxyphenylglycine (CHPG) (a mGluR5 agonist), and 2-methyl-6-(phenylethynyl)pyridine hydrochloride (MPEP) (a mGluR5 antagonist) did not cause any effect. In the immunocytochemical study, the post-conditioning administration of AIDA decreased the c-Fos induction in the dentate gyrus and CA1 layer of the hippocampus proper, 2h after exposure of animals to the aversive context, and 24h after conditioning session. It is suggested that overactivation of glutamatergic transmission in the critical for memory trace formation structure and period of time, may result in an attenuation of memory consolidation. On the other hand, reduction of an exaggerated glutamatergic tone can facilitate learning and memory processes. The immunocytochemical study and factor analysis of experimental data revealed that hippocampal mGlu1 receptors significantly influence the memory consolidation in a way dependant on the level of glutamatergic activity. Furthermore, they indicate that changes of glutamatergic activity within brain limbic structures can affect the threshold for the induction of the long-term neuronal plasticity, involved in some forms of learning and memory.