Time-dependent deficits of rat's memory consolidation induced by tetrodotoxin injections into the caudate-putamen, nucleus accumbens, and globus pallidus

Intense training prevents the amnestic effect of inactivation of dorsomedial striatum and induces high resistance to extinction

A large body of evidence has shown that treatments that interfere with memory consolidation become ineffective when animals are subjected to an intense learning experience; this effect has been observed after systemic and local administration of amnestic drugs into several brain areas, including the striatum. However, the effects of amnestic treatments on the process of extinction after intense training have not been studied. Previous research demonstrated increased spinogenesis in the dorsomedial striatum, but not in the dorsolateral striatum after intense training, indicating that the dorsomedial striatum is involved in the protective effect of intense training. To investigate this issue, male Wistar rats, previously trained with low, moderate, or high levels of foot shock, were used to study the effect of tetrodotoxin inactivation of dorsomedial striatum on memory consolidation and subsequent extinction of inhibitory avoidance. Performance of the task was evaluated during seven extinction sessions. Tetrodotoxin produced a marked deficit of memory consolidation of inhibitory avoidance trained with low and moderate intensities of foot shock, but normal consolidation occurred when a relatively high foot shock was used. The protective effect of intense training was long-lasting, as evidenced by the high resistance to extinction exhibited throughout the extinction sessions. We discuss the possibility that increased dendritic spinogenesis in dorsomedial striatum may underly this protective effect, and how this mechanism may be related to the resilient memory typical of post-traumatic stress disorder (PTSD).

Brain circuitry underlying the ABC model of anxiety

Anxiety Disorders are prevalent and often chronic, recurrent conditions that reduce quality of life. The first-line treatments, such as serotonin reuptake inhibitors and cognitive behavioral therapy, leave a significant proportion of patients symptomatic. As psychiatry moves toward targeted circuit-based treatments, there is a need for a theory that unites the phenomenology of anxiety with its underlying neural circuits. The Alarm, Belief, Coping (ABC) theory of anxiety describes how the neural circuits associated with anxiety interact with each other and domains of the anxiety symptoms, both temporally and spatially. The latest advancements in neuroimaging techniques offer the ability to assess these circuits in vivo. Using Neurosynth, a large open-access meta-analytic imaging database, the association between terms related to specific neural circuits was explored within the ABC theory framework. Alarm-related terms were associated with the amygdala, anterior cingulum, insula, and bed nucleus of stria terminalis. Belief-related terms were associated with medial prefrontal cortex, precuneus, bilateral temporal poles, and hippocampus. Coping-related terms were associated with the ventrolateral and dorsolateral prefrontal cortices, basal ganglia, and anterior cingulate. Neural connections underlying the functional neuroanatomy of the ABC model were observed. Additionally, there was considerable interaction and overlap between circuits associated with the symptom domains. Further neuroimaging research is needed to explore the dynamic interaction between the functional domains of the ABC theory. This will pave the way for probing the neuroanatomical underpinnings of anxiety disorders and provide an evidence-based foundation for the development of targeted treatments, such as neuromodulation.

Inactivation of endopeduncular nucleus impaired fear conditioning and hippocampal synaptic plasticity in rats

The internal globus pallidus (GPi) is one part of basal ganglion nucleuses which play fundamental role in motor function. Recent studies indicated that GPi could modulate emotional processing and learning, but the possible mechanism remains still unknown. In this study, the effects of endopeduncular nucleus (EP, a rodent homolog of GPi) on fear conditioning were tested in rats. GABA_A receptor agonist muscimol was bilaterally delivered into the EP 15 min before or immediately after fear conditioning in rats. We found that EP inactivation impaired the acquisition but not consolidation of fear memory in rats. Furthermore, the long-term potentiation (LTP) in hippocampal CA1 area was impaired, and the learning related phosphorylation of α-amino-3-hydroxy-5-methyl-4-isoxazole-propionic acid receptor (AMPAR) subunit 1 (GluA1) at the Ser845 site in hippocampus was decreased in muscimol treated group. These results demonstrated that dysfunction of EP impaired hippocampal dependent learning and memory in rats.

Contributions of the Nucleus Accumbens Shell in Mediating the Enhancement in Memory Following Noradrenergic Activation of Either the Amygdala or Hippocampus

The nucleus accumbens shell is a site of converging inputs during memory processing for emotional events. The accumbens receives input from the nucleus of the solitary tract (NTS) regarding changes in peripheral autonomic functioning following emotional arousal. The shell also receives input from the amygdala and hippocampus regarding affective and contextual attributes of new learning experiences. The successful encoding of affect or context is facilitated by activating noradrenergic systems in either the amygdala or hippocampus. Recent findings indicate that memory enhancement produced by activating NTS neurons, is attenuated by suppressing accumbens functioning after learning. This finding illustrates the significance of the shell in integrating information from the periphery to modulate memory for arousing events. However, it is not known if the accumbens shell plays an equally important role in consolidating information that is initially processed in the amygdala and hippocampus. The present study determined if the convergence of inputs from these limbic regions within the nucleus accumbens contributes to successful encoding of emotional events into memory. Male Sprague-Dawley rats received bilateral cannula implants 2 mm above the accumbens shell and a second bilateral implant 2 mm above either the amygdala or hippocampus. The subjects were trained for 6 days to drink from a water spout. On day 7, a 0.35 mA footshock was initiated as the rat approached the spout and was terminated once the rat escaped into a white compartment. Subjects were then given intra-amygdala or hippocampal infusions of PBS or a dose of norepinephrine (0.2 μg) previously shown to enhance memory. Later, all subjects were given intra-accumbens infusion of muscimol to functionally inactivate the shell. Muscimol inactivation of the accumbens shell was delayed to allow sufficient time for norepinephrine to activate intracellular cascades that lead to long-term synaptic modifications involved in forming new memories. Results show that memory improvement produced by infusing norepinephrine in either the amygdala or hippocampus is attenuated by interrupting neuronal activity in the shell 1 or 7 7 h following amygdala or hippocampus activation. These findings suggest that the accumbens shell plays an integral role modulating information initially processed by the amygdala and hippocampus following exposure to emotionally arousing events. Additionally, results demonstrate that the accumbens is involved in the long-term consolidation processes lasting over 7 h.

Flashing Lights Induce Prolonged Distortions in Visual Cortical Responses and Visual Perception

The primary sensory neocortex generates an internal representation of the environment, and its circuit reorganization is thought to lead to a modification of sensory perception. This reorganization occurs primarily through activity-dependent plasticity and has been well documented in animals during early developmental stages. Here, we describe a new method for the noninvasive induction of long-term plasticity in the mature brain: simple transient visual stimuli (i.e., flashing lights) can be used to induce prolonged modifications in visual cortical processing and visually driven behaviors. Our previous studies have shown that, in the primary visual cortex (V1) of mice, a flashing light stimulus evokes a long-delayed response that persists for seconds. When the mice were repetitively presented with drifting grating stimuli (conditioned stimuli) during the flash stimulus-evoked delayed response period, the V1 neurons exhibited a long-lasting decrease in responsiveness to the conditioned stimuli. The flash stimulus-induced underrepresentation of the grating motion was specific to the direction of the conditioned stimuli and was associated with a decrease in the animal's ability to detect the motion of the drifting gratings. The neurophysiological and behavioral plasticity both persisted for at least several hours and required N-methyl-d-aspartate receptor activation in the visual cortex. We propose that flashing light stimuli can be used as an experimental tool to investigate the visual function and plasticity of neuronal representations and perception after a critical period of neocortical plasticity.

Learning induced epigenetic modifications in the ventral striatum are necessary for long-term memory

Epigenetic modifications such as histone acetylation in cortical or allocortical regions have been shown to be necessary for the formation of long-term memories. Here we investigated whether similar changes were occurring also in the ventral striatum and whether they are necessary for the consolidation of aversive memory. To this purpose we performed immediate post-training focal administrations of the histone deacetylase inhibitor suberoylanilide hydroxamic acid (SAHA, 5, 10 or 15 μg/side) or the DNA methyltransferase (DNMT) inhibitor, 5-aza-2'-deoxycytidine (5-AZA, 0.0625 or 0.125 μg/side) in the ventral striatum of mice trained in one-trial inhibitory avoidance task. Intra-ventral striatal SAHA administrations, immediately after training, improved memory retention. Opposite effects were found with 5-AZA. We also found that training in the one-trial inhibitory avoidance is accompanied by increased acetylation of specific residues that can be further increased by intra-VS SAHA administrations. Intra-VS 5-AZA administrations on the other hand reduced training-induced histones acetylation at the same residues. These findings imply the occurrence of histone acetylation in the ventral striatum in order to store aversive memory. Moreover, they suggest that the effects induced by the DNMT inhibitor 5-AZA may at least partially due to blockade of H3 and H4 acetylation. These results suggest that the contemporary activation of similar molecular mechanisms might be needed in different brain regions to enable the formation of long-term memories.

Genetic background modulates behavioral impairments in R6/2 mice and suggests a role for dominant genetic modifiers in Huntington’s disease pathogenesis

Variability and modification of the symptoms of Huntington’s disease (HD) are commonly observed in both patient populations and animal models of the disease. Utilizing a stable line of the R6/2 HD mouse model, the present study investigated the role of genetic background in the onset and severity of HD symptoms in a transgenic mouse. R6/2 congenic C57BL/6J and C57BL/6J × DBA/2J F1 (B6D2F1) mice were evaluated for survival and a number of behavioral phenotypes. This study reports that the presence of the DBA/2J allele results in amelioration or exacerbation of several HD-like phenotypes characteristic of the R6/2 mouse model and indicates the presence of dominant genetic modifiers of HD symptoms. This study is the first step in identifying genes that confer natural genetic variation and modify the HD symptoms. This identification may lead to novel targets for treatment and help elucidate the molecular mechanisms of HD pathogenesis.

Behavioral and neurotransmitter specific roles for the ventral tegmental area in reinforcer-seeking and intake

BACKGROUND

The ventral tegmental area (VTA) is a pivotal relay site within the reinforcement circuit that has been shown to play a role in ethanol (EtOH)-motivated behaviors. The primary dopamine projections within this system originate in the VTA and innervate several areas including the nucleus accumbens (NAc) and prefrontal cortex (PFC), and the PFC has afferent glutamate projections to the VTA and the NAc. The following studies utilized 2 different operant paradigms, one focusing on reinforcer-seeking and the other on reinforcer drinking (both with an EtOH and a sucrose reinforcer solution), to elucidate regulation of these behaviors by the posterior VTA, and the specific roles of dopamine and glutamate in this region.

METHODS

The present experiments assessed the effects of microinjections of the glutamate (AMPA/kainate) antagonist CNQX and the dopamine D1-like antagonist SCH23390 in the posterior VTA, as well as transient chemical inactivation of this region using tetrodotoxin (TTX). In 4 separate experiments (2 dopamine, 2 glutamate, both with TTX), male Long Evans rats were trained to complete a single response requirement that resulted in access to 10% EtOH or 2% sucrose for a 20-minute drinking period.

RESULTS

Prior to microinjections, EtOH-reinforced subjects were consuming approximately 0.45 to 0.65 g/kg EtOH and making approximately 50 responses during intermittent nonreinforced artificial cerebrospinal fluid sessions (Sucrose groups had similar baseline response levels). Overall, TTX inactivation of the VTA consistently decreased reinforcer-seeking but not intake in all experiments. CNQX also dose-dependently decreased EtOH-seeking, with no significant effect on sucrose-seeking or reinforcer intake. SCH23390 had no significant effects on reinforcer-seeking, and very moderately decreased intake of both EtOH and sucrose.

CONCLUSIONS

Inactivation of the posterior VTA implicated this region in reinforcer-seeking as opposed to reinforcer intake. Overall, the present findings provide support for the importance of posterior VTA glutamate activity specifically in EtOH-seeking behavior in animals consuming pharmacologically relevant amounts of EtOH.

Onset and progression of behavioral and molecular phenotypes in a novel congenic R6/2 line exhibiting intergenerational CAG repeat stability

In the present study we report on the use of speed congenics to generate a C57BL/6J congenic line of HD-model R6/2 mice carrying 110 CAG repeats, which uniquely exhibits minimal intergenerational instability. We also report the first identification of the R6/2 transgene insertion site. The relatively stable line of 110 CAG R6/2 mice was characterized for the onset of behavioral impairments in motor, cognitive and psychiatric-related phenotypes as well as the progression of disease-related impairments from 4 to 10 weeks of age. 110Q mice exhibited many of the phenotypes commonly associated with the R6/2 model including reduced activity and impairments in rotarod performance. The onset of many of the phenotypes occurred around 6 weeks and was progressive across age. In addition, some phenotypes were observed in mice as early as 4 weeks of age. The present study also reports the onset and progression of changes in several molecular phenotypes in the novel R6/2 mice and the association of these changes with behavioral symptom onset and progression. Data from TR-FRET suggest an association of mutant protein state changes (soluble versus aggregated) in disease onset and progression.

Dorsal hippocampal regulation of memory reconsolidation processes that facilitate drug context-induced cocaine-seeking behavior in rats

Exposure to a cocaine-paired context increases the propensity for relapse in cocaine users and prompts cocaine-seeking behavior in rats. According to the reconsolidation hypothesis, upon context re-exposure, established cocaine-related associations are retrieved and can become labile. These associations must undergo reconsolidation into long-term memory to effect enduring stimulus control. The dorsal hippocampus (DH), dorsolateral caudate-putamen and dorsomedial prefrontal cortex are critical for the expression of context-induced cocaine seeking, and these brain regions may also play a role in the reconsolidation of cocaine-related memories that promote this behavior. To test this hypothesis, rats were trained to press a lever for unsignaled cocaine infusions (0.2 mg/infusion, i.v.) in a distinct environmental context (cocaine-paired context), followed by extinction training in a different context (extinction context). Rats were then re-exposed to the cocaine-paired context for 15 min in order to reactivate cocaine-related memories or received comparable exposure to a novel unpaired context. Immediately thereafter, rats received bilateral microinfusions of the protein synthesis inhibitor anisomycin, the sodium channel blocker tetrodotoxin or vehicle into one of the above brain regions. After additional extinction training in the extinction context, reinstatement of cocaine-seeking behavior (i.e., non-reinforced lever presses) was assessed in the cocaine-paired context. Tetrodotoxin, but not anisomycin, administered into the DH inhibited drug context-induced cocaine-seeking behavior in a memory reactivation-dependent manner. Other manipulations failed to alter this behavior. These findings suggest that the DH facilitates the reconsolidation of associative memories that maintain context-induced cocaine-seeking behavior, but it is not the site of anisomycin-sensitive memory restabilization per se.

The dorsal subiculum mediates the acquisition of conditioned reinstatement of cocaine-seeking

Contextual stimuli present during a single lifetime cocaine self-administration experience acquire occasion-setting actions sufficient to persistently elicit cocaine-seeking behavior in rats, with effects lasting nearly 1 year. The goal of this study was to identify neural substrates mediating the acquisition of drug-related conditioning taking place during a single cocaine self-administration experience with focus on the subicular formation, a brain site that has been implicated in associative learning relevant for conditioned reward-seeking including conditioned reinstatement. Male Wistar rats were given 2 h of response-contingent access to intravenous cocaine or saline in the presence of distinct stimuli that served as contextual stimuli associated with the availability and subjective effects of cocaine (S(+)) vs saline (S(-)). Before onset of the sessions, rats received bilateral microinjections of tetrodotoxin (TTX) into the ventral subiculum (VSUB) or dorsal subiculum (DSUB). Following extinction of responding by withholding cocaine, rats were subjected to reinstatement tests in which exposure to the cocaine- (but not saline) associated stimulus produced strong recovery of responding. This effect was completely abolished in rats with transient TTX inactivation of the DSUB during the conditioning session. TTX inactivation of the VSUB during conditioning did not alter the response-reinstating effects of the cocaine cue. The results suggest that functional integrity of the DSUB, but not VSUB, is critical for the acquisition of conditioned cocaine-seeking controlled by contextual stimuli under conditions where such learning occurs during a single conditioning trial.

Spatial deficits in a mouse model of Parkinson disease

RATIONALE

Accumulating evidence in humans demonstrated that visuo-spatial deficits are the most consistently reported cognitive abnormalities in Parkinson disease (PD). These deficits have been generally attributed to cortical dopamine degeneration. However, more recent evidence suggests that dopamine loss in the striatum is responsible for the visuo-spatial abnormalities in PD. Studies based on animal models of PD did not specifically address this question.

OBJECTIVES

Thus, the first goal of this study was to analyze the role of dopamine within the dorsal striatum in spatial memory. We tested bilateral 6-OHDA striatal lesioned CD1 mice in an object-place association spatial task. Furthermore, to see whether the effects were selective for spatial information, we measured how the 6-OHDA-lesioned animals responded to a non-spatial change and learned in the one-trial inhibitory avoidance task.

RESULTS

The results demonstrated that bilateral (approximately 75%) dopamine depletion of the striatum impaired spatial change discrimination. On the contrary, no effect of the lesion was observed on non-spatial novelty detection or on passive avoidance learning.

CONCLUSIONS

These results confirm that dopamine depletion is accompanied by cognitive deficits and demonstrate that striatal dopamine dysfunction is sufficient to induce spatial information processing deficits.

Substantia nigra role in fear conditioning consolidation

The substantia nigra (SN) is known to be involved in the memorization of several conditioned responses. To investigate the role of the SN in fear conditioning consolidation this neural site was subjected to fully reversible tetrodotoxin (TTX) inactivation during consolidation in adult male Wistar rats which had undergone fear training to acoustic CS and context. TTX was stereotaxically administered to different groups of rats at increasing intervals after the acquisition session. Memory was assessed as conditioned freezing duration measured during retention testing, always performed 72 and 96 h after TTX administration. In this way there was no interference with normal SN function during either acquisition or retrieval phases, so that any amnesic effect could be due only to consolidation disruption. The results show that SN functional integrity is necessary for contextual fear response consolidation up to the 24-h after-acquisition delay. On the contrary SN functional integrity was shown not to be necessary for the consolidation of acoustic CS fear responses. The present findings help to elucidate the role of the SN in memory consolidation and better define the neural circuits involved in fear memories.

Persistent disruption of an established morphine conditioned place preference

In human addicts, craving and relapse are frequently evoked by the recall of memories connected to a drug experience. Established memories can become labile if recalled and can then be disrupted by several interfering events and pharmacological treatments, including inhibition of protein synthesis. Thus, reactivation of mnemonic traces provides an opportunity for disrupting memories that contribute to pathological states. Here, we tested whether the memory of a drug experience can be weakened by inhibiting protein synthesis after the reactivation of its trace. We found that an established morphine conditioned place preference (mCPP) was persistently disrupted if protein synthesis was blocked by either anisomycin or cycloheximide after the representation of a conditioning session. Unlike other types of memories, an established mCPP did not become labile after contextual recall, but required the concomitant re-experience of both the conditioning context and the drug. An established mCPP was disrupted after the conditioning session if protein synthesis was blocked selectively in the hippocampus, basolateral amygdala, or nucleus accumbens but not in the ventral tegmental area. This disruption seems to be permanent, because the preference did not return after further conditioning. Thus, established memories induced by a drug of abuse can be persistently disrupted after reactivation of the conditioning experience.

The role of the amygdala and rostral anterior cingulate in encoding expected outcomes during learning

Successful passive avoidance learning is thought to require the use of learned stimulus-reinforcement associations to guide decision making [Baxter, M.G., Murray, E.A., 2002. The amygdala and reward. Nature Reviews. Neuroscience 3, 563-573]. The current experiment investigated the neural correlates of successful passive avoidance learning in 19 healthy adults. Behaviorally, subjects showed a distinct pattern of performance: early indiscriminate responding to stimuli (pre-criterion performance), followed by relatively rapid learning before a plateau of successful performance (post-criterion performance). Neural responses to post-criterion correct responses were compared with neural responses to both incorrect responses and pre-criterion correct responses. Post-criterion correct responding was associated with increased activation in regions including rostral anterior cingulate, insula, caudate, hippocampal regions, and the amygdala.

Tetrodotoxin inactivation of pontine regions: Influence on sleep–wake states

Studies using various methodologies have implicated n. reticularis pontis oralis (RPO) and n. subcoeruleus (SubC) in the generation of rapid eye movement sleep (REM). In rats, electrolytic lesions in these regions may give rise to the phenomenon of REM without atonia (REM-A), in which the electrophysiological features of REM are normal except that atonia is absent and elaborate behaviors may be exhibited. However, electrolytic lesions damage both cell bodies and fibers of passage, and the neural reorganization and adaptation that can occur post-lesion can complicate interpretation. Tetrodotoxin (TTX) is a sodium channel blocker that temporarily inactivates both neurons and fibers of passage and thus may be functionally equivalent to an electrolytic lesion, but without allowing time for neural adaptation. In this study, we examined the influence of microinjections of TTX into RPO and SubC on sleep in freely behaving rats. Rats (90 day old male Sprague-Dawley) were implanted with electrodes for recording EEG and EMG. Guide cannulae were implanted aimed into RPO or SubC. Each animal received one unilateral microinjection (TTXUH: 5.0 ng/0.2 microl) and two bilateral microinjections (TTXBL: 2.5 ng/0.1 microl; TTXBH: 5.0 ng/0.2 microl) of TTX, and control microinjections of saline alone (SAL). The injections were made 2 h following lights on, and sleep was recorded for the subsequent 22 h. Sleep was scored from computerized records in 10 s epochs. Recordings from the 10-h light period and the 12-h dark period were examined separately. TTX inactivation of RPO could decrease REM and non-REM (NREM), whereas inactivation of SubC produced relatively more specific decreases in REM with smaller effects on NREM. The results complement studies that have implicated RPO and SubC in REM generation. REM-A was not observed, suggesting that REM-A is a complex phenomenon that requires time for reorganization of the nervous system after insult.

Temporary inactivation of the nucleus accumbens disrupts acquisition and expression of fear-potentiated startle in rats

Recent research suggests that in addition to its prominent role in appetitive learning, the nucleus accumbens (NAC) may also be involved in fear conditioning. In the present study, we investigated whether temporary inactivation of the NAC, by injection of tetrodotoxin (TTX), affects acquisition and expression of conditioned fear, as measured by fear-potentiated startle (FPS). TTX injection into the NAC totally blocked acquisition and markedly decreased expression of conditioned fear to a discrete visual conditioned stimulus (CS). Interestingly, temporary inactivation of the NAC did not affect shock sensitization of startle, indicating that both the perception of the shock and short-term contextual conditioning was not affected by intra-accumbal TTX injection. Taken together, these results show that the NAC is crucial for acquisition and expression of long-term conditioned fear, as measured by fear-potentiated startle, to discrete CSs, but not short-term conditioned fear to a context.

The role of the dorsomedial prefrontal cortex, basolateral amygdala, and dorsal hippocampus in contextual reinstatement of cocaine seeking in rats

The present study tested the hypothesis that separate neural substrates mediate cocaine relapse elicited by drug-associated contextual stimuli vs explicit conditioned stimuli (CSs) and cocaine. Specifically, we investigated the involvement of the dorsal hippocampus (DH), basolateral amygdala (BLA), and dorsomedial prefrontal cortex (dmPFC) in contextual reinstatement of cocaine-seeking behavior and the involvement of the DH in explicit CS- and cocaine-induced reinstatement. Rats were trained to self-administer cocaine in a distinct context or in the presence of CSs paired explicitly with cocaine infusions. Responding of context-trained rats was then extinguished in the previously cocaine-paired or an alternate context, whereas responding of explicit CS-trained rats was extinguished in the absence of the CSs. Subsequently, the target brain regions or anatomical control regions were functionally inactivated using tetrodotoxin (0 or 5 ng/side), and cocaine-seeking behavior (ie, nonreinforced responses) was assessed in the cocaine-paired context, in the alternate context, in the presence of the explicit CSs, or following cocaine priming (10 mg/kg, i.p.). DH inactivation abolished contextual, but failed to alter explicit CS- or cocaine-induced, reinstatement of cocaine-seeking behavior. BLA or dmPFC inactivation also abolished contextual reinstatement. Conversely, inactivation of the control brain regions failed to alter contextual reinstatement. In conclusion, the DH, BLA, and dmPFC play critical roles in contextual reinstatement. Previous findings suggest that the BLA is critical for explicit CS-induced, but not cocaine-primed, reinstatement and the dmPFC is critical for both explicit CS-induced and cocaine-primed reinstatement. Thus, distinct but partially overlapping neural substrates mediate context-induced, explicit CS-induced, and cocaine-primed reinstatement of extinguished cocaine-seeking behavior.

Tetrodotoxin infusions into the dorsal hippocampus block non-locomotor place recognition

The hippocampus is critical for navigation in an open field. One component of this navigation requires the subject to recognize the target place using distal cues. The experiments presented in this report tested whether blocking hippocampal function would impair open field place recognition. Hungry rats were trained to press a lever on a feeder for food. In Experiment 1, they were passively transported with the feeder along a circular trajectory. Lever pressing was reinforced only if the feeder was passing through a 60 degrees -wide sector. Thus, rats preferentially lever pressed in the vicinity of the reward sector indicating that they recognized its location. Tetrodotoxin (TTX) infusions aimed at the dorsal hippocampi caused rats to substantially increase lever pressing with no preference for any region. The aim of Experiment 2 was to determine whether the TTX injections caused a loss of place recognition or a general increase of lever pressing. A separate group of rats was conditioned in a stationary apparatus to press the lever in response to a light. The TTX injections did not abolish preferential lever pressing in response to light. Lever pressing increased less than half as much as the TTX-induced increase in Experiment 1. When these animals with functional hippocampi could not determine the rewarded period because the light was always off, lever pressing increased much more and was similar to the TTX-induced increase in Experiment 1. We conclude that the TTX inactivation of the hippocampi impaired the ability to recognize the reward place.

Amphetamine injections into the nucleus accumbens affect neither acquisition/expression of conditioned fear nor baseline startle response

The acoustic startle response is enhanced during states of fear and attenuated during pleasant ones. Our question was whether pharmacological stimulation of the reward system disrupts the learning and retrieval of conditioned fear as measured by fear-potentiated startle. We therefore injected the dopamine agonist amphetamine into the nucleus accumbens (NAC) immediately before either acquisition or expression of conditioned fear and measured the effect of these injections on fear-potentiated startle and baseline startle response. This study clearly showed that amphetamine injections into the NAC had no effect on baseline startle amplitude and acquisition/expression of conditioned fear. In contrast, amphetamine injections into the nucleus accumbens clearly enhanced spontaneous motor activity. These results suggest that dopamine within the NAC is not involved in modulation of fear-potentiated startle and baseline startle.

Distinct roles of the different ionotropic glutamate receptors within the nucleus accumbens in passive-avoidance learning and memory in mice

Research on the role of the nucleus accumbens in behaviour has been largely focused on the functions of this structure in conditioning to appetitive stimuli. It has been suggested that a network comprising the nucleus accumbens and its convergent inputs might mediate dissociable functions in the acquisition, the consolidation and the retrieval of information. However, findings related to a role of this structure in aversive conditioning are somewhat contradictory, and its involvement in this form of learning is still under debate. Moreover, very little evidence is available on the step of information processing mediated by the accumbens. Thus the purpose of this study was to investigate the effects of the blockade of the AMPA and NMDA glutamate receptors, which have been suggested to mediate the transmission of information from the limbic system to this structure, on a classical aversive conditioning task - the one-trial step through inhibitory avoidance paradigm (24 h interval between training and testing). Intra-accumbens focal injections of AP-5 and DNQX (NMDA and AMPA antagonists, respectively) were performed immediately after training, before training and before testing in mice. The NMDA antagonist (37.5, 75 and 150 ng per side) impaired animal performance only if administered immediately after but not before training or before testing. Conversely, DNQX (0.5, 1.0 and 5.0 ng per side) reduced the step through latencies when administered before training and before testing. These findings suggest that NMDA receptor activation within the accumbens is necessary in formation but not expression of memory for inhibitory avoidance. AMPA receptors, instead, are necessary for the acquisition and the expression but not consolidation of inhibitory avoidance memory.

Systems mediating acute glucocorticoid effects on memory consolidation and retrieval

It is well established that glucocorticoid hormones, secreted by the adrenal cortex after a stressful event, influence cognitive performance. This article reviews recent findings from this laboratory on the acute effects of glucocorticoids in rats on specific memory phases, i.e., memory consolidation and memory retrieval. Posttraining activation of glucocorticoid-sensitive pathways involving glucocorticoid receptors (GRs) enhances memory consolidation in a dose-dependent manner. Glucocorticoid influences on memory consolidation depend on noradrenergic activation of the basolateral complex of the amygdala (BLA) and interactions of the BLA with other brain regions. By contrast, memory retrieval processes are usually impaired with high circulating levels of glucocorticoids or following infusions of GR agonists into the hippocampus. Although the BLA does not appear to be a site of glucocorticoid action in influencing memory retrieval, an intact BLA is required for enabling glucocorticoid effects on memory retrieval. The BLA appears to be a key structure in a memory-modulatory system that regulates, in concert with other brain regions, stress and glucocorticoid effects on both memory consolidation and memory retrieval.

The nucleus accumbens shell is critical for normal expression of pup-retrieval in postpartum female rats

The nucleus accumbens (NA) plays an important modulatory role in the control of normal expression of maternal behavior (termed maternal performance). The present study investigated the relative functions of two subregions of the NA (the shell and core) in maternal performance. Electrolytic lesions of the shell or core were performed either before parturition or immediately after a varying amount of maternal experience (none, 1 or 24h) during the immediate postpartum period. Maternal performance was tested on Day 1 postpartum and re-tested 9 days later using a pup sensitization technique. Results show that lesions of the shell, but not the core, significantly disrupted pup-retrieval: the shell-lesioned rats took significantly longer to finish retrieving all test pups, but their retrieval latency for the first pup was not affected. Neither lesion affected other components of maternal behavior (pup licking, nest building and nursing). These findings suggest that the shell, but not the core, is critical for the normal expression of pup-retrieval behavior possibly through its role in maintaining maternal motivation or attention.

Differential involvement of NMDA and AMPA receptors within the nucleus accumbens in consolidation of information necessary for place navigation and guidance strategy of mice

Recent evidence now points to a role of glutamate transmission within the nucleus accumbens (Nacc) in spatial learning and memory. Unfortunately, the role of the distinct classes of glutamate receptors within this structure in mediating the different steps of the memorization process is not clear. The aim of this study therefore was to further investigate this issue, trying to assess the involvement of the two classes of glutamate receptors within the Nacc in consolidation of spatial information using an associative spatial task, the water maze. For this purpose, focal injections of the NMDA antagonist, AP-5, and of the AMPA antagonist, DNQX, have been performed immediately after the training phase, and mice have been tested for retention 24 h later. Two different versions of the water-maze task have been used: In the place version, animals could learn the position of the platform using visual distal cues, and in the cue version, the location of the platform was indicated by a single proximal cue. The results demonstrated that posttraining NMDA receptor blockade affects mice response in the place but not in the cue water-maze task. On the contrary, AMPA receptor blockade induced no effect in either version of the task. These data confirm a functional dissociation between glutamate receptors located in the Nacc in modulating spatial memory consolidation and indicate that they are specifically involved in consolidation of information necessary to acquire a place but not to a guidance strategy.

Differential involvement of nucleus accumbens shell and core subregions in maternal memory in postpartum female rats

Maternal memory refers to the long-term retention of maternal responsiveness as a consequence of animals' prior experiences with their young. This study examined the relative roles of 2 subregions of the nucleus accumbens (NA; shell and core) in maternal memory in rats. NA shell lesions either before or immediately after a short experience significantly disrupted maternal memory, but lesions after a 24-hr maternal experience had no effect. NA core lesions had no significant impact on maternal memory. Cycloheximide (a protein synthesis inhibitor) at a high dose (25 micrograms/microliter) infused in the NA shell immediately after 1 hr of maternal experience also significantly disrupted maternal memory, whereas infusions in the medial preoptic area had no effect. It was concluded that the NA shell, but not the NA core, is involved in the consolidation of maternal memory.

Neural encoding in ventral striatum during olfactory discrimination learning

A growing body of evidence implicates the ventral striatum in using information acquired through associative learning. The present study examined the activity of ventral striatal neurons in awake, behaving rats during go/no-go odor discrimination learning and reversal. Many neurons fired selectively to odor cues predictive of either appetitive (sucrose) or aversive (quinine) outcomes. Few neurons were selective when first exposed to the odors, but many acquired this differential activity as rats learned the significance of the cues. A substantial proportion of these neurons encoded the cues' learned motivational significance, and these neurons tended to reverse their firing selectivity after reversal of odor-outcome contingencies. Other neurons that became selectively activated during learning did not reverse, but instead appeared to encode specific combinations of cues and associated motor responses. The results support a role for ventral striatum in using the learned significance, both appetitive and aversive, of predictive cues to guide behavior.

Learning and memory functions of the Basal Ganglia

Although the mammalian basal ganglia have long been implicated in motor behavior, it is generally recognized that the behavioral functions of this subcortical group of structures are not exclusively motoric in nature. Extensive evidence now indicates a role for the basal ganglia, in particular the dorsal striatum, in learning and memory. One prominent hypothesis is that this brain region mediates a form of learning in which stimulus-response (S-R) associations or habits are incrementally acquired. Support for this hypothesis is provided by numerous neurobehavioral studies in different mammalian species, including rats, monkeys, and humans. In rats and monkeys, localized brain lesion and pharmacological approaches have been used to examine the role of the basal ganglia in S-R learning. In humans, study of patients with neurodegenerative diseases that compromise the basal ganglia, as well as research using brain neuroimaging techniques, also provide evidence of a role for the basal ganglia in habit learning. Several of these studies have dissociated the role of the basal ganglia in S-R learning from those of a cognitive or declarative medial temporal lobe memory system that includes the hippocampus as a primary component. Evidence suggests that during learning, basal ganglia and medial temporal lobe memory systems are activated simultaneously and that in some learning situations competitive interference exists between these two systems.

Patterns of interference in sequence learning and prism adaptation inconsistent with the consolidation hypothesis

The studies reported here used an interference paradigm to determine whether a long-term consolidation process (i.e., one lasting from several hours to days) occurs in the learning of two implicit motor skills, learning of a movement sequence and learning of a visuo-motor mapping. Subjects learned one skill and were tested on that skill 48 h later. Between the learning session and test session, some subjects trained on a second skill. The amount of time between the learning of the two skills varied for different subjects. In both the learning of a movement sequence and the learning of a visuo-motor mapping, we found that remote memories were susceptible to interference, but the passage of time did not afford protection from interference. These results are inconsistent with the long-term consolidation of these motor skills. A possible difference between these tasks and those that do show long-term consolidation is that the present tasks are not dynamic motor skills.

Cerebellar role in fear-conditioning consolidation

Some cerebellar structures are known to be involved in the memorization of several conditioned responses. The role of the interpositus nucleus (IN) and the vermis (VE) in fear-conditioning consolidation was investigated by means of a combined behavioral and neurophysiological technique. The IN and VE were subjected to fully reversible tetrodotoxin (TTX) inactivation during consolidation in adult male Wistar rats that underwent acoustic conditioned stimulus (CS) and context fear training. TTX was injected in different groups of rats at increasing intervals after the acquisition session. Memory was assessed as conditioned freezing duration measured during retention testing, always performed 72 and 96 h after the stereotaxic TTX administration. This schedule ensures that there is no interference with normal cerebellar function during either the acquisition or the retrieval phase so that any amnesic effect may be due only to consolidation disruption. Our results show that IN functional integrity is necessary for acoustic CS fear response memory formation up to the 96-h after-acquisition delay. VE functional integrity was shown to be necessary for memory formation of both context (up to the 96-h after-acquisition delay) and acoustic CS (up to the 192-h after-acquisition delay) fear responses. The present findings help to elucidate the role of the cerebellum in memory consolidation and better define the neural circuits involved in fear memories.

Basolateral amygdala-nucleus accumbens interactions in mediating glucocorticoid enhancement of memory consolidation

Systemic or intracerebral administration of glucocorticoids enhances memory consolidation in several tasks. Previously, we reported that these effects depend on an intact basolateral nucleus of the amygdala (BLA) and efferents from the BLA that run through the stria terminalis (ST). The BLA projects directly to the nucleus accumbens (NAc) via this ST pathway. The NAc also receives direct projections from the hippocampus and, therefore, may be a site of convergence of BLA and hippocampal influences in modulating memory consolidation. In support of this view, we found previously that lesions of either the NAc or the ST also block the memory-modulatory effect of systemically administered glucocorticoids. The present experiments examined the effects of lesions of the NAc or the ST on the memory-modulatory effects of intracerebral glucocorticoids on inhibitory avoidance training. Microinfusions of the specific glucocorticoid receptor agonist 11beta,17beta-dihydroxy-6,21-dimethyl-17alpha-pregna-4,6-trien-20yn-3-one (RU 28362; 1.0 or 3.0 ng) into either the BLA or the hippocampus of male Sprague Dawley rats administered immediately after training enhanced the 48 hr retention performance in a dose-dependent manner. Bilateral lesions of the NAc or the ST alone did not affect retention performance but blocked the memory enhancement induced by intra-BLA or intrahippocampal glucocorticoid receptor agonist administration. These findings indicate that the BLA-NAc pathway plays an essential role in mediating glucocorticoid effects on memory consolidation and suggest that the BLA interacts with hippocampal effects on memory consolidation via this pathway.

NMDA and AMPA antagonist infusions into the ventral striatum impair different steps of spatial information processing in a nonassociative task in mice

Most of the research on ventral striatal functions has been focused on their role in modulating reward and motivation. More recently, a possible role of this structure in cognitive functions has been suggested. However, very little information is available on the involvement of the nucleus accumbens in the different stages of the consolidation process. In this study, the effect of focal injections of AP-5 and DNQX, competitive antagonists at the NMDA and AMPA receptors, respectively, was examined in a nonassociative task designed to estimate the ability of mice to react to spatial changes. The task consists of placing the animals in an open field containing five objects; after three sessions of habituation, their reactivity to object displacement was examined 24 hr later. AP-5 injections administered after training impaired the ability of mice to detect the spatial novelty but did not affect response when injected 120 min after training or before testing. On the contrary, DNQX did not affect response when administered immediately or 120 min after training but did impair spatial discrimination when administered before training or testing. These data demonstrate a double dissociation between glutamate receptor subtypes, such that accumbens NMDA receptors are important for consolidation and not ongoing discrimination of spatial information, whereas AMPA receptors have an opposite role in these processes.

A new subdivision, marginal division, in the neostriatum of the monkey brain

A new subdivision, the "marginal division" (MrD), was discovered at the caudal border of the striatum and surrounds the rostral edge of the globus pallidus in the rat brain in our previous studies. The neuronal somata of the MrD are mostly fusiform in shape with their long axes lining dorsoventrally. The MrD is more densely filled with substance P (SP)-, Leucine-enkephalin (L-Enk)-, dynorphin B-, neurotensin-, somatostatin- and cholecystokinin (CCK)-immunoreactive fibers and terminal-like structures than the rest of the striatum. The MrD was confirmed in the cat neostriatum as well. The present study intended to explore whether the MrD exists in the monkey neostriatum (putamen) with Nissl, histochemical and immunohistochemical methods. A band of fusiform neurons were obviously identified at the caudomedial edge of the putamen. These neurons lie outside the lateral medullary lamina and indirectly surround the rostrolateral border of the globus pallidus. The abundance of SP-, L-Enk-, neuropeptide Y-, CCK-, dopamine- and serotonin-positive fibers and terminal-like structures with a few positive fusiform neurons accumulating at the caudomedial border of the putamen obviously distinguishes this zone from the rest of neostriatum and globus pallidus. The acetylcholinesterase (AChE) positive and nicotinamide adenine dinucleotide phosphate diaphorase (NADPH-d) containing fusiform neurons are distinctly visualized in the same zone. The morphological figure and the location of these neurons, and the histochemical and immunohistochemical characteristics of this area coincide well with those of the MrD in the rat and cat striatum. This study thus convincingly identifies the existence of the MrD in the monkey neostriatum. It is fairly asserted that the MrD is a universal structure in the mammalian brain.

Involvement of a basolateral amygdala complex-nucleus accumbens pathway in glucocorticoid-induced modulation of memory consolidation

Systemic or intracerebral administration of glucocorticoids modulates memory consolidation in several tasks. Previously, we have shown that these memory-modulatory effects depend on an intact basolateral complex of the amygdala (BLC) and efferents from the BLC that run through the stria terminalis. It is currently unknown, however, what BLC efferent structures mediate these effects. The present experiments were designed to determine whether the nucleus accumbens (NA), which receives BLC efferents through the stria terminalis and is involved in several BLC-dependent behaviours, is involved in glucocorticoid-induced modulation of memory consolidation. In experiment 1, rats with bilateral sham or N-methyl-D-aspartate (NMDA)-induced lesions of the NA were trained on a one-trial, footshock-motivated inhibitory avoidance task, and given immediate post-training injections of either the synthetic glucocorticoid dexamethasone (0.3 or 1.0 mg/kg, s.c.) or vehicle. Testing 48 h later revealed that dexamethasone significantly enhanced retention in sham-lesioned rats but that the enhancing effect was blocked in NA-lesioned rats. An asymmetrical, or crossed-lesion design was employed in experiment 2. Rats with a unilateral NMDA-induced lesion of the BLC and a unilateral lesion of either the ipsilateral or contralateral NA were trained as in experiment 1. Testing 48 h later revealed that dexamethasone enhanced retention in ipsilaterally lesioned rats, but that this effect was blocked in contralaterally lesioned rats. These findings indicate that an intact BLC-NA pathway is critical for the enhancing effects of glucocorticoids on memory consolidation, and are consistent with the view that the BLC regulates memory consolidation in other brain regions.

Auditory thalamus, dorsal hippocampus, basolateral amygdala, and perirhinal cortex role in the consolidation of conditioned freezing to context and to acoustic conditioned stimulus in the rat

On the basis of previous experimental evidence, it is known that the auditory thalamus (AT), the dorsal hippocampus (DH), the basolateral amygdala (BLA), and the perirhinal cortex (PC) are involved in the mnemonic processing of conditioned freezing. In particular, BLA and PC appear to be involved both in conditioned stimulus (CS) and context conditioned freezing. Through AT, the auditory CS is sent to other sites, whereas DH is involved in context conditioning. Nevertheless, the existing evidence does not make it possible to assess AT, DH, BLA, and PC involvement during the consolidation phase of conditioned freezing. To address this question, fully reversible tetrodotoxin (TTX) inactivation was performed on adult male Wistar rats having undergone CS and context fear training. Anesthetized animals were injected stereotaxically with TTX (either 5 or 10 ng in 0.5 or 1.0 microliter of saline, according to site dimensions) at increasing post-acquisition delays. Context and CS freezing durations were measured during retention testing, always performed 48 and 72 hr after TTX administration. The results showed that AT inactivation does not disrupt consolidation of either contextual or auditory fear memories. In contrast, inactivation of the other three structures disrupted consolidation. For the DH, this disruption was specific to contextual cues and only occurred when inactivation was performed early (up to 1.5 hr) after training. The BLA and PC were shown to be involved in the consolidation of both contextual and auditory fear. Their involvement persisted for longer periods of time (2d for BLA and 8 d for PC). These findings provide information to build a temporal profile for the post-training processing of fear memories in structures known to be important for this form of learning. The results are discussed in relation to previous studies on conditioned freezing and other aversive conditioned response neural correlates.

Neural topography and chronology of memory consolidation: a review of functional inactivation findings

Findings on the role of subcortical and cortical structures in mnemonic processes, obtained by means of the reversible functional inactivation technique, are reviewed. The main advantage of this method (subcortical or cortical administration of local anesthetics or tetrodotoxin) is that it provides information not only on "where" but also "when" and for "how long" these processes take place, thus adding to the topographical dimension the chronological one. The review covers several types of memory (e.g., passive avoidance and spatial memory) studies examining the neural substrates of memory consolidation on the basis of the functional inactivation of the nucleus of the solitary tract, parabrachial nuclei, substantia nigra, hippocampus (dorsal and ventral), nucleus basalis magnocellularis, amygdala, medial septal area, striatum, olfactory bulb, and neocortex. The data are discussed in relation to earlier research and with respect to the anatomical and functional connectivity of the examined centers.

Entorhinal cortex and fimbria-fornix role in rat's passive avoidance response memorization

The stereotaxic administration of tetrodotoxin (TTX) was employed to induce the fully reversible inactivation of the fimbria-fornix complex (FF) and of the entorhinal cortex (EC), in order to ascertain the role of these structures in the memorization of a passive avoidance response (PAR). On permanently cannulated rats TTX (5 ng in 0.5 microliter saline) or saline (0.5 microliter) was injected uni- or bilaterally, respectively, in the FF and in the EC, 60 min before PAR acquisition, immediately after PAR acquisition and 60 min before PAR retrieval, always performed 48 h after the acquisition trial. It was shown that EC unilateral or bilateral pre-acquisition inactivation was followed by amnesia, while TTX inactivation in post-acquisition and pre-retrieval had no effects. Identical results were obtained by TTX administration in FF. The experimental evidence indicates that both EC and FF play a role during acquisition of PAR engram. The results are discussed in comparison with previous ones concerning dorsal and ventral hippocampus TTX inactivation effects on rat's PAR, and in relation to hippocampal and medial septal area connectivity.

Differential effects of unilateral lidocaine infusion into the globus pallidus on consolidation and performance of inhibitory avoidance

The striatum is involved in memory consolidation; also involved in this process is one of its two major efferent targets, namely, the substantia nigra. It is not clear, however, if the other target, the globus pallidus, participates in storage and/or performance of learned information. To examine this problem, male Wistar rats were trained in an inhibitory avoidance task and tested for retention 24 h afterward. Independent groups were infused, unilaterally, with 2% lidocaine in the pallidus either 2 min after training or 2 min before testing. No disturbances of memory were detected with posttraining infusion, but a significant deficit in retention was observed as a consequence of pretest infusion. Infusion of isotonic saline into the globus pallidus, or of lidocaine before testing into the parietal cortex, after training into the ventral thalamic nucleus, and both before training and testing into this thalamic nucleus were without effect. Taken together, the data indicate that unilateral inactivation of the GP interferes with retrieval of information derived from inhibitory avoidance training, but not with the early stages of memory consolidation of this task, and other work indicates that the pallidus may be involved in a late phase of this process.

Analysis of mnemonic processing by means of totally reversible neural inactivations

The irreversible lesions technique precludes the analysis of the possibly critical role played by discrete brain sites in the several distinct stages of mnemonic processing (acquisition, consolidation, retrieval) during which these may be specifically but transiently active. On the contrary, the reversible functional inactivation techniques, by means of stereotaxic local microinjection of active compounds, make it possible to suppress the neuronal function of a discrete volume of nervous tissue, for a pre-determined time, with the assurance of complete functional recovery within a known duration. This technique makes it possible to block the neural activity of a chosen neural site at a given stage of memory processing without any interference with the function of the same structure either during earlier or later stages of the same process. Thus, the reversible ablation results may provide information not only on the qualitative topographical but also on the quantitative temporal dimension of learning and memory. The technique employed to cause totally reversible neural inactivation is detailed. The employment of several agents to obtain functional inactivation is discussed. Of these, perhaps the safest and most manageable is tetrodotoxin when a fairly long functional inactivation (e.g., 1 h) is desired. The effects of a reversible inactivation can be quite easily and accurately assessed by observing the severity of the amnesic disruption, if any, of a conditioned response. In order to do this as well as possible, it is advantageous to employ a very simple behavioral paradigm. The passive avoidance response in the light-dark box apparatus fulfills this requirement. Moreover, this paradigm, being one-trial, provides the necessary condition of a single well-defined temporal beginning. The present protocol has been successfully employed in learning and memory research, to assess when the functional integrity of a given neural structure is necessary in order that a conditioned response may be acquired, consolidated or retrieved. The employment of this protocol in relation to the intrinsic functional characteristics of a given subcortical neural site is discussed.

The nucleus accumbens and learning and memory

Recent research on the nucleus accumbens (NA) indicates that this brain region is involved in learning and memory processes in a way that is separable from its other well-known roles in behavior, such as motivation, reward, and locomotor activity. These findings have suggested that 1) the NA may be involved in declarative, or hippocampal formation-dependent learning and memory, and not in several other non-declarative forms of learning and memory, and 2) the NA may be selectively involved in certain stages of learning and memory. These characteristics suggest that the NA may be part of a larger striatal system which subserves acquisition and consolidation, but is not a site of long-term storage, of different forms of learning and memory.

Mnemonic functions of the basal ganglia

A synthesis of older and recent work on mnemonic functions of the basal ganglia in rats, monkeys and humans emphasizes a reciprocal relationship of the caudate nucleus and putamen with the cerebral cortex, which mediates the memory of consistent relationships between stimuli and responses (sometimes called habits) that often involve relationships between the individual and its environment (egocentric memory). Evidence at several levels of analysis (including neuroplastic synaptic changes, activity of single neurons, and behavioral changes caused by lesions or neurochemical manipulations) implicate dopamine release from nigro-striatal neurons in the reinforcement, or strengthening, of neural representations in the basal ganglia.

Role of dorsal hippocampus in acquisition, consolidation and retrieval of rat's passive avoidance response: a tetrodotoxin functional inactivation study

By means of local administration of tetrodotoxin (TTX) fully reversible functional inactivation of rat's dorsal hippocampus (DH) was obtained in order to define the role of this structure in the memorization of a conditioned passive avoidance response (PAR). In Experiment 1, on permanently cannulated animals, TTX (10 ng in 1.0 microliter saline) or saline (1.0 microliter) was injected uni- or bilaterally in the DH, respectively 1 h before PAR acquisition, immediately after PAR acquisition, and 1 h before PAR retrieval, always performed 48 h after the acquisition trial. It was shown that both pre-acquisition and pre-retrieval DH uni- or bilateral blockades were followed by significant PAR retention impairment, while in post-acquisition only the bilateral blockade determined PAR retention impairment. In Experiment 2, on three different groups of rats, TTX (10 ng in 1 microliter) saline) was bilaterally administered, under general ketamine anesthesia (100 mg/kg), into the DH at different post-acquisition delays (0.25, 1.5, 6 h). Retrieval testing, 48 h after treatment, showed that post-acquisition bilateral DH blockade caused PAR impairment only when performed 0.25 or 1.5 h after acquisition. The results indicate a well defined mnemonic role of DH during the acquisition, consolidation and retrieval of PAR engram. The experimental evidence is discussed in relation to other reports and to DH connectivity with the medial septal area and the amygdala.