Lesioning the fimbria-fornix impairs basolateral amygdala induced reinforcement of LTP in the dentate gyrus

Basolateral amygdala stimulation plus water maze training restore dentate gyrus LTP and improve spatial learning and memory

Synaptic plasticity is a key mechanism of neural plasticity involved in learning and memory. A reduced or impaired synaptic plasticity could lead to a deficient learning and memory. On the other hand, besides reducing hipocampal dependent learning and memory, fimbria-fornix lesion affects LTP. However, we have consistently shown that stimulation of the basolateral amygdala (BLA) 15 min after water maze training is able to improve spatial learning and memory in fimbria fornix lesioned rats while also inducing changes in the expression of plasticity-related genes expression in memory associated brain regions like the hippocampus and prefrontal cortex. In this study we test that hypothesis: whether BLA stimulation 15 min after water maze training can improve LTP in the hippocampus of fimbria-fornix lesioned rats. To address this question, we trained fimbria-fornix lesioned rats in water maze for four consecutive days, and the BLA was bilaterally stimulated 15 min after each training session.Our data show that trained fimbria-fornix lesioned rats develop a partially improved LTP in dentated gyrus compared with the non-trained fimbria-fornix lesioned rats. In contrast, dentated gyrus LTP in trained and BLA stimulated fimbria-fornix lesioned rats improved significantly compared to the trained fimbria-fornix lesioned rats, but was not different from that shown by healthy animals. BLA stimulation in non-trained FF lesioned rats did not improve LTP; instead produces a transient synaptic depression. Restoration of the ability to develop LTP by the combination of training and BLA stimulation would be one of the mechanisms involved in ameliorating memory deficits in lesioned animals.

Amygdala stimulation ameliorates memory impairments and promotes c-Fos activity in fimbria-fornix-lesioned rats

Recently we provided data showing that amygdala stimulation can ameliorate spatial memory impairments in rats with lesion in the fimbria-fornix (FF). The mechanisms for this improvement involve early gene expression and synthesis of BDNF, MAP-2, and GAP43 in the hippocampus and prefrontal cortex. Now we have studied which brain structures are activated by the amygdala using c-Fos as a marker of neural activation. First, we studied neuronal activation after tetanic stimulation to the amygdala in intact rats. We then carried out a second study in FF-lesioned rats in which the amygdala was stimulated 15 min after daily spatial memory training in the water maze. Our results showed that amygdala stimulation produces widespread brain activation, that includes cortical, thalamic, and brain stem structures. Activation was particularly intense in the dentate gyrus and the prefrontal cortex. Training in the water maze increased c-Fos positive nuclei in the dentate gyrus of the hippocampus and in medial prefrontal cortex. Amygdala stimulation to trained FF-lesioned rats induced an increase of neural activity in the dentate gyrus and medial prefrontal cortex relative to the FF-lesioned, but not stimulated group, like the c-Fos activity seen in trained control rats. Based on these and previous results we explain the mechanisms of amygdala reinforcement of neural plasticity and the partial recovery of spatial memory deficits.

Frequency-dependent electrical stimulation of fimbria-fornix preferentially affects the mesolimbic dopamine system or prefrontal cortex

BACKGROUND

The fimbria/fornix fiber system is an essential part of the hippocampal-VTA loop, and therefore activities that are propagated through this fiber system control the activity of the mesolimbic dopamine system.

OBJECTIVES/HYPOTHESIS

We hypothesized that stimulation of the fimbria/fornix with an increasing number of electrical pulses would cause increasing activity of the mesolimbic dopamine system, which coincides with concurrent changes in neuronal activities in target regions of the mesolimbic dopaminergic system.

METHODS

Right fimbria/fornix fibers were electrically stimulated with different pulse protocols. Stimulus-induced changes in neuronal activities were visualized with BOLD-fMRI, whereas stimulus-induced release of dopamine, as measured for the activity of the mesolimbic dopamine system, was determined in the nucleus accumbens with in vivo fast-scan cyclic voltammetry.

RESULTS

Dependent on the protocol, electrical fimbria/fornix stimulation caused BOLD responses in various targets of the mesolimbic dopamine system. Stimulation in the low theta frequency range (5 Hz) triggered significant BOLD responses mainly in the hippocampal formation, infralimbic cortex, and septum. Stimulation in the beta frequency range (20 Hz) caused additional activation in the medial prefrontal cortex (mPFC), nucleus accumbens, striatum, and VTA. Stimulation in the high-gamma frequency range (100 Hz) caused further activation in the hippocampus proper and mPFC. The strong activation in the mPFC during 100 Hz stimulations depended not only on the number of pulses but also on the frequency. Thus, short bursts of 5 or 20 high-frequency pulses caused stronger activation in the mPFC than continuous 5 or 20 Hz pulses. In contrast, high-frequency burst fimbria/fornix stimulation did not further activate the mesolimbic dopamine system when compared to continuous 5 or 20 Hz pulse stimulation.

CONCLUSIONS

There exists a frequency-dependent dissociation between BOLD responses and activation of the dopaminergic system. Low frequencies were more efficient to activate the mesolimbic dopamine system, whereas high frequencies were more efficient to trigger BOLD responses in target regions of the mesolimbic dopamine system, particularly the mPFC.

Amygdala stimulation promotes recovery of behavioral performance in a spatial memory task and increases GAP-43 and MAP-2 in the hippocampus and prefrontal cortex of male rats

The relationships between affective and cognitive processes are an important issue of present neuroscience. The amygdala, the hippocampus and the prefrontal cortex appear as main players in these mechanisms. We have shown that post-training electrical stimulation of the basolateral amygdala (BLA) speeds the acquisition of a motor skill, and produces a recovery in behavioral performance related to spatial memory in fimbria-fornix (FF) lesioned animals. BLA electrical stimulation rises bdnf RNA expression, BDNF protein levels, and arc RNA expression in the hippocampus. In the present paper we have measured the levels of one presynaptic protein (GAP-43) and one postsynaptic protein (MAP-2) both involved in synaptogenesis to assess whether structural neuroplastic mechanisms are involved in the memory enhancing effects of BLA stimulation. A single train of BLA stimulation produced in healthy animals an increase in the levels of GAP-43 and MAP-2 that lasted days in the hippocampus and the prefrontal cortex. In FF-lesioned rats, daily post-training stimulation of the BLA ameliorates the memory deficit of the animals and induces an increase in the level of both proteins. These results support the hypothesis that the effects of amygdala stimulation on memory recovery are sustained by an enhanced formation of new synapses.

Associations of hippocampal subfields in the progression of cognitive decline related to Parkinson's disease

Objective

Hippocampal atrophy has been associated with mild cognitive impairment (MCI) in Parkinson's disease (PD). However, literature on how hippocampal atrophy affects the pathophysiology of cognitive impairment in PD has been limited. Previous studies assessed the hippocampus as an entire entity instead of their individual subregions. We studied the progression of cognitive status in PD subjects over 18 in relation to hippocampal subfields atrophy.

Methods

65 PD subjects were included. Using the MDS task force criteria, PD subjects were classified as either having no cognitive impairment (PD-NCI) or PD-MCI. We extended the study by investigating the hippocampal subfields atrophy patterns in those who converted from PD-NCI to PD-MCI (PD-converters) compared to those who remained cognitively stable (PD-stable) over 18 months. Freesurfer 6.0 was used to perform the automated segmentation of the hippocampus into thirteen subregions.

Results

PD-MCI showed lower baseline volumes in the left fimbria, right CA1, and right HATA; and lower global cognition scores compared to PD-NCI. Baseline right CA1 was also correlated with baseline attention. Over 18 months, decline in volumes of CA2–3 and episodic memory were also seen in PD-converters compared to PD-stable. Baseline volumes of GC-DG, right CA4, left parasubiculum, and left HATA were predictive of the conversion from PD-NCI to PD-MCI.

Conclusion

The findings from this study add to the anatomical knowledge of hippocampal subregions in PD, allowing us to understand the unique functional contribution of each subfield. Structural changes in the hippocampus subfields could be early biomarkers to detect cognitive impairment in PD.

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Hippocampal subfields atrophy could detect cognitive impairment in PD.

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Each hippocampal subfields has a unique functional contribution.

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Baseline hippocampal subfields volumes predicted conversion to from NCI to MCI.

Priming locus coeruleus noradrenergic modulation of medial perforant path-dentate gyrus synaptic plasticity

Priming phenomenon, in which an earlier exposure to a stimulus or condition alters synaptic plasticity in response to a subsequent stimulus or condition, known as a challenge, is an example of metaplasticity. In this review, we make the case that the locus coeruleus noradrenergic system-medial perforant path-dentate gyrus pathway is a neural ensemble amenable to studying priming-challenge effects on synaptic plasticity. Accumulating evidence points to a tyrosine hydroxylase-dependent priming effect achieved by pharmacological (nicotine and antipsychotics) or physiological (septal theta driving) manipulations of the locus coeruleus noradrenergic system that can facilitate noradrenaline-induced synaptic plasticity in the dentate gyrus of the hippocampus. The evidence suggests the hypothesis that behavioural experiences inducing tyrosine hydroxylase expression in the locus coeruleus may be sufficient to prime this form of metaplasticity. We propose exploring this phenomenon of priming and challenge physiologically, to determine whether behavioural experiences are sufficient to prime the locus coeruleus, enabling subsequent pharmacological or behavioural challenge conditions that increase locus coeruleus firing to release sufficient noradrenaline to induce long-lasting potentiation in the dentate gyrus. Such an approach may contribute to unravelling mechanisms underlying this form of metaplasticity and its importance in stress-related mnemonic processes.

The effects of tetanic stimulation on plasticity of remote synapses in the hippocampus-perirhinal cortex-amygdala network

In the temporal lobe, multiple synaptic pathways reciprocally link different structures. These multiple pathways play an important role in the integrity of the function of the temporal lobe and malfunction in this network has been suggested to underlie some neurological disorders such as epilepsy. To test whether the induction of long-term potentiation (LTP) in one temporal lobe structure would modulate functional synaptic plasticity in other structures of this network, tetanic stimulation was applied to the white matter of the perirhinal cortex, Schaffer collaterals of the hippocampus, or the external capsule in combined rat amygdala-hippocampus-cortex slices. This tetanic stimulation was accompanied by enhancement of the evoked field potential slope in the third layer of perirhinal cortex, hippocampal CA1 area, and the lateral amygdala. Induction of LTP in each of these structures was concomitant with increased evoked field potentials in the neighboring structures. Surgical disconnection of anatomical pathways between these structures inhibited this concomitant enhancement of the evoked field potential slope. Both NMDA and AMPA glutamate sub-receptors were involved in changes of synaptic plasticity elicited by induction of LTP in the neighboring structures. The present data indicate a reciprocal control among the perirhinal cortex, the amygdala, and the hippocampus plasticity. This could be important for the formation and retention of the medial temporal lobe-dependent memory and may play a role in the involvement of all different regions of the temporal lobe in pathological conditions such as epilepsy that affect this brain structure.

Basolateral amygdala regulation of adult hippocampal neurogenesis and fear-related activation of newborn neurons

Impaired regulation of emotional memory is a feature of several affective disorders, including depression, anxiety and post-traumatic stress disorder. Such regulation occurs, in part, by interactions between the hippocampus and the basolateral amygdala (BLA). Recent studies have indicated that within the adult hippocampus, newborn neurons may contribute to support emotional memory, and that regulation of hippocampal neurogenesis is implicated in depressive disorders. How emotional information affects newborn neurons in adults is not clear. Given the role of the BLA in hippocampus-dependent emotional memory, we investigated whether hippocampal neurogenesis was sensitive to emotional stimuli from the BLA. We show that BLA lesions suppress adult neurogenesis, while lesions of the central nucleus of the amygdala do not. Similarly, we show that reducing BLA activity through viral vector-mediated overexpression of an outwardly rectifying potassium channel suppresses neurogenesis. We also show that BLA lesions prevent selective activation of immature newborn neurons in response to a fear-conditioning task. These results demonstrate that BLA activity regulates adult hippocampal neurogenesis and the fear context-specific activation of newborn neurons. Together, these findings denote functional implications for proliferation and recruitment of new neurons into emotional memory circuits.

Roles of hippocampal N-methyl-D-aspartate receptors and calcium/calmodulin-dependent protein kinase II in amphetamine-produced conditioned place preference in rats

This study investigates the roles of hippocampal N-methyl-D-aspartate (NMDA) glutamate receptors and CaMKII (calcium/calmodulin-dependent protein kinase II) in amphetamine-produced conditioned place preference (AMPH-CPP) in rats. An earlier report showed that AMPH-CPP resulted in the enhancement of hippocampal CaMKII activity. In this study, AMPH-CPP significantly increased hippocampal GluR1 receptors, though AMPH-CPP was impaired by either blockade of NMDA receptors (AP5) or inhibition of CaMKII (KN-93) during conditioning. These treatments also impaired CPP if administered before testing, but CPP recovered during the next testing session. Therefore, these treatments had no effect on the extinction of CPP. If the conditioned rats were, however, reexposed to AMPH-CPP after a hippocampal-infusion of AP5 or KN-93, the extinction of the original CPP was greater than that seen in the controls. The hippocampal-infusion of D-cycloserine before CPP testing enhanced the extinction of CPP. These results, taken together, indicate that NMDA receptor activation and CaMKII activity are essential for the AMPH-CPP. AMPH-CPP reexposure is similar to the memory reconsolidation process, being disrupted by either a blockade of the NMDA receptor or an inhibition of CaMKII. Furthermore, the extinction of CPP resembles new learning, which is an active process and is facilitated by a partial NMDA agonist.

Amphetamine-induced place preference and conditioned motor sensitization requires activation of tyrosine kinase receptors in the hippocampus

The environmental context in which abused drugs are taken contribute to the drug experience and is a powerful and persistent stimulus to elicit memories of that experience even in the abstinent addict. Using amphetamine (AMPH) as the unconditioned stimulus, the present study compared two popular context-dependent paradigms in rats, conditioned motor sensitization (CMS) and conditioned place preference (CPP), to ascertain whether particular brain regions were differentially involved. The neuronal substrates underlying these context-dependent behaviors are poorly understood, but regulators of the neuronal plasticity that accompany learning, such as neurotrophic factors and their cognate tyrosine kinase receptors (e.g., TrkB), are credible candidates. We found a significant elevation of TrkB-like immunoreactivity specifically in CA3/dentate gyrus (DG) subregions of the hippocampus after AMPH (0.3 mg/kg)-induced CPP, but not in the delayed-paired (control) AMPH condition. A higher AMPH dose (1.0 mg/kg) induced both CPP and CMS and elevated TrkB in the CA3/DG as well as in the nucleus accumbens shell. The development of both conditioned behaviors was blocked by intra-CA3/DG infusion of the Trk inhibitor K-252a. These findings reveal that CPP and CMS are induced by different doses of AMPH and are associated with TrkB changes in particular brain regions. Moreover, Trk receptors in the hippocampus are critical mediators of the neuronal changes necessary for inducing both forms of conditioning. Thus, although these two conditioning models are distinct, because they are commonly regulated by the hippocampal Trk system, these receptors may be a therapeutic target for attenuating the significance of contextual cues that otherwise strengthen the addictive properties of abused drugs.

Bidirectional modulation of hippocampal long-term potentiation under stress and no-stress conditions in basolateral amygdala-lesioned and intact rats

Hippocampal long-term potentiation (LTP) is widely considered as a cellular model for learning and memory formation. We have shown previously that protein synthesis-independent, early dentate gyrus (DG) LTP, lasting approximately 4-5 h, can be transformed into a late-LTP with a duration of > or = 24 h by a brief acute swim stress experience (high-stress condition). This reinforcement requires the activation of mineralocorticoid receptors and protein synthesis. The basolateral amygdala (BLA) is known to modulate glucocorticoid effects on the consolidation of spatial/contextual memory via a beta-adrenergic mechanism. Interestingly, hippocampal DG-LTP can also be indirectly modulated by beta-adrenergic and cholinergic/muscarinergic processes. Here, we show that the reinforcement of early-DG-LTP under high-stress conditions depends on the processing of novel spatial/contextual information. Furthermore, this reinforcement was blocked in BLA-lesioned animals compared with sham-operated and intact controls; however, it was not dependent on beta-adrenergic or cholinergic/muscarinergic receptor activation. In contrast, under low-stress conditions, the induction of late-LTP in BLA-lesioned animals is facilitated, and this facilitation, again, was dependent on beta-adrenergic activation. The data suggest that DG-LTP maintenance can be influenced by the BLA through different mechanisms: a short-lasting corticosterone-dependent and beta-adrenergic-independent mechanism and a long-lasting mechanism that facilitated hippocampal beta-adrenergic mechanisms.

Subcortical deafferentation impairs behavioral reinforcement of long-term potentiation in the dentate gyrus of freely moving rats

Long-term potentiation is a form of neural functional plasticity which has been related with memory formation and recovery of function after brain injury. Previous studies have shown that a transient early-long-term potentiation can be prolonged by direct stimulation of distinct brain areas, or behavioral stimuli with a high motivational content. The basolateral amygdala and other subcortical structures, like the medial septum and the locus coeruleus, are involved in mediating the reinforcing effect. We have previously shown that the lesion of the fimbria-fornix--the main entrance of subcortical afferents to the hippocampus--abolishes the reinforcing basolateral amygdala-effects on long-term potentiation in the dentate gyrus in vivo. It remains to be investigated, however, if such subcortical afferents may also be important for behavioral reinforcement of long-term potentiation. Young-adult (8 weeks) Sprague-Dawley male rats were fimbria-fornix-transected under anesthesia, and electrodes were implanted at the dentate gyrus and the perforant path. One week after surgery the freely moving animals were studied. Fimbria-fornix-lesion reduced the ability of the animals to develop long-term potentiation when a short pulse duration was used for tetanization (0.1 ms per half-wave of a biphasic stimulus), whereas increasing the pulse duration to 0.2 ms per half-wave during tetanization resulted in a transient early-long-term potentiation lasting about 4 h in the lesioned animals, comparable to that obtained in non-lesioned or sham-operated control rats. In water-deprived (24 h) control animals, i.e. in non-lesioned and sham-operated rats, early-long-term potentiation could be behaviorally reinforced by drinking 15 min after tetanization. However, in fimbria-fornix-lesioned animals long-term potentiation-reinforcement by drinking was not detected. This result indicates that the effect of behavioral-motivational stimuli to reinforce long-term potentiation is mediated by subcortical, heterosynaptic afferents.

Long-term potentiation in the dentate gyrus in freely moving rats is reinforced by intraventricular application of norepinephrine, but not oxotremorine

Growing evidence suggests that processes of synaptic plasticity, such as long-term potentiation (LTP) occurring in one synaptic population, can be modulated by consolidating afferents from other brain structures. We have previously shown that an early-LTP lasting less than 4 h (E-LTP) in the dentate gyrus can be prolonged by stimulating the basolateral amygdala, the septum or the locus coeruleus within a specific time window. Pharmacological experiments have suggested that noradregeneric (NE) and/or cholinergic systems might be involved in these effects. We have therefore investigated whether the direct intraventricular application of agonists for NE- or muscarinic receptors is able to modulate synaptic plasticity. E-LTP was induced at the dentate gyrus of freely moving rats using a mild tetanization protocol that induces only an E-LTP. NE or oxotremorine (OXO) were applied icv 10 min after the tetanus. Results show that low doses of NE (1.5 and 5 nM) effectively prolong LTP. A higher dose (50 nM) was not effective. None of the OXO doses employed (5, 25, and 50 nM) showed similar effects. These results stress the importance of transmitter-specific modulatory influences on the time course of synaptic plasticity, in particular NE whose application mimics the reinforcing effect of directly stimulating limbic structures on LTP.

Effect of LTP-reinforcing paradigms on neurotransmitter release in the dentate gyrus of young and aged rats

Long-term potentiation (LTP) is considered a cellular correlate of memory processing. A short-lasting early-LTP can be prolonged into a late-L TP (>4h) by stimulation of the basolateral amygdala (BLA) or motivational behavioral stimuli in young, but not in aged, cognitively impaired rats. We measured the changes in transmitter release-induced by BLA or behavioral reinforcement-in young and aged cognitively impaired rats, after implanting a microdialysis cannula at the dentate gyrus. Samples were taken under baseline conditions and during stimulation of BLA. Rats were water deprived and tested again next day, taking samples after allowing access to water. Higher concentrations of choline, HIAA, aspartate, glutamate, and glycine were found in baseline samples from young animals compared to aged. In young animals, BLA stimulation increased the levels of ACh and reduced norepinephrine and serotonine, while behavioral reinforcement reduced the levels of glutamate and glycine. These effects were absent among aged rats, suggesting that this reduced neurochemical response might be linked to the impaired LTP-reinforcement reported previously.

Amygdala stimulation modulates hippocampal synaptic plasticity

Experience-dependent synaptic plasticity is a fundamental feature of neural networks involved in the encoding of information, and the capability of synapses to express plasticity is itself activity-dependent. Here, we introduce a "low-frequency burst stimulation" protocol, which can readily induce both long-term potentiation (LTP) and long-term depression (LTD) at in vivo medial perforant path-dentate gyrus synapses. By varying stimulation parameters, we were able to build a stimulus-response map of synaptic plasticity as a LTP-LTD continuum. The response curve displayed a bidirectional shift toward LTP and LTD, depending on the degree and timing of neural activity of the basolateral amygdala. The range of this plastic modulation was also modified by past activity of the basolateral amygdala, suggesting that the amygdala can arrange its ability to regulate the dentate plastic responses. The effects of the BLA activation were replicated by stimulation of the lateral perforant path and, hence, BLA stimulation may recruit the lateral entorhinal cortex. These results represent a high-order dimension of heterosynaptic modulations of hippocampal synaptic plasticity.

Emotional tagging of memory formation--in the search for neural mechanisms

Memory-related areas, such as the hippocampus, should be able to sort out the more significant from the less relevant aspects of an experience in order to transform only the earlier into long-term memory. We have recently suggested the Emotional Tagging concept, according to which the activation of the amygdala in emotionally arousing events mark the experience as important and aids in enhancing synaptic plasticity in other brain regions. Here, we review evidence from both human and animal studies that lend support to the Emotional Tagging hypothesis and to the central role the amygdala may play in its formation. We further speculate on potential neural mechanisms that may underlie emotional tagging. Long-term memory formation is considered to involve lasting alterations in synaptic efficacy, known as synaptic plasticity. It has been suggested that two factors are crucial for obtaining a synapse-specific long-term plasticity: (a) the successful activation of a synapse-specific, protein synthesis-independent tag, and (b) the activation of synapse-non-specific protein synthesis. The activation of protein synthesis can then induce lasting plasticity only in those synapses marked by a tag. Interestingly and relevant to the Emotional Tagging hypothesis, it has been recently shown that the activation of the amygdala could transform transient into long-lasting plasticity. These recent findings seem to fit well with the Emotional Tagging hypothesis. It seems reasonable to assume that the activation of the amygdala triggers neuromodulatory systems, which in turn reduce the threshold for the activation of the synaptic tag, and by this facilitate the transformation of early- into late-phase memory.

The amygdala is part of the behavioural reinforcement system modulating long-term potentiation in rat hippocampus

Long-term potentiation (LTP) in the dentate gyrus can be modulated and prolonged by emotional/motivational influences when concurrently activated. A similar effect on LTP can be obtained by stimulating the amygdala, suggesting that this limbic structure might be part of the neural system involved in behavioural reinforcement. To confirm this we have performed a series of experiments in which the basolateral amygdala was either temporary inactivated by injection of lidocaine or permanently lesioned electrolytically. Both manipulations completely blocked the reinforcing effect of a motivational stimulus (drinking after 24-h deprivation) on LTP at the perforant pathway-dentate gyrus synapses, whilst leaving intact the non-reinforced potentiation. These results demonstrate that the basolateral amygdala is a key structure within the system involved in the modulatory interaction between the affective status of the animal and the mechanisms of functional plasticity.

Modulation of late phases of long-term potentiation in rat dentate gyrus by stimulation of the medial septum

The prolonged maintenance of hippocampal long-term potentiation (LTP) seems to require heterosynaptic events during its induction. We have previously shown that stimulation of the basolateral nucleus of the amygdala (BLA) within a distinct time window can reinforce a transient early-LTP into a long-lasting late-LTP in the dentate gyrus (DG) in freely moving rats. We have shown that this reinforcement was dependent on beta-adrenergic and/or muscarinergic receptor activation and protein synthesis. However, since the BLA does not directly stimulate the DG the question remained by which inputs such heterosynaptic processes are triggered. We have now directly stimulated the medial septal pathway 15 min after induction of early-LTP in the DG and show that this input is capable of reinforcing early into late-LTP in a frequency-dependent manner. This septal reinforcement of DG LTP was dependent on beta-adrenergic receptor activation and protein synthesis. We suggest that the reinforcing effect of the BLA stimulation can, potentially, be mediated via the septal input to the DG, though it differs in its ability to induce or modulate functional plasticity.

Aging impairs amygdala-hippocampus interactions involved in hippocampal LTP

Aging impairs amygdala-hippocampus interactions involved in hippocampal LTP. NEUROBIOL. AGING. We have recently shown that the stimulation of the basolateral nucleus of the amygdala (BLA) is able to prolong early-LTP (<4h) into late-LTP (>4h) in the dentate gyrus. To study whether aging affects this interaction, aged (24-27 months) rats were used, classified as cognitively impaired (I), or non-impaired (N) by means of their results in the Morris water maze. Paired pulses (30-90 ms interval) showed no differences among age groups. Among young controls, the early-LTP induced in the dentate gyrus by stimulation of the perforant path (PP) was prolonged in a late-LTP when the BLA was stimulated 15 min later. In aged-impaired rats the stimulation of the PP induced a reduced LTP, decaying to baseline in less than 2 h. BLA stimulation was without effect. Aged non-impaired rats showed an early-LTP identical to that of young animals; however, stimulation of the BLA showed no effect. These results suggest that deficient synaptic plasticity and memory functions in aged animals might be caused, in part by impaired mechanisms of heterosynaptic reinforcement.

Behavioral reinforcement of long-term potentiation is impaired in aged rats with cognitive deficiencies

Behavioral stimuli with emotional/motivational content can reinforce long-term potentiation in the dentate gyrus, if presented within a distinct time window. A similar effect can be obtained by stimulating the basolateral amygdala, a limbic structure related to emotions. We have previously shown that aging impairs amygdala-hippocampus interactions during long-term potentiation. In this report we show that behavioral reinforcement of long-term potentiation is also impaired in aged rats with cognitive deficits. While among young water-deprived animals drinking 15 min after induction of long-term potentiation leads to a significant prolongation of potentiation, cognitively impaired aged rats are devoid of such reinforcing effects. In contrast, a slight but statistically significant depression develops after drinking in this group of animals. We suggest that an impaired mechanism of emotional/motivational reinforcement of synaptic plasticity might be functionally related to the cognitive deficits shown by aged animals.

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.

Modulation of hippocampal long-term potentiation by the amygdala: a synaptic mechanism linking emotion and memory

Why are emotionally arousing experiences well-remembered? Since the amygdala and hippocampus play pivotal roles in emotion and memory, respectively, the interaction between these brain regions may underlie the formation of enhanced memory for emotionally arousing events. Behavioral experiments using animals have demonstrated that lesions of the amygdaloid nuclei or infusions of drugs into the amygdaloid nuclei impair or enhance hippocampal-dependent learning. In addition, we have obtained direct evidence that neural inputs from the amygdala modulate synaptic plasticity in the hippocampus, through electrophysiological experiments using anesthetized rats. Electrical stimulation of the basolateral amygdala evoked synaptic potentials in the dentate gyrus of the hippocampus, indicating that there is a neural connection from the amygdala to the hippocampus. Lesion of the basolateral or basomedial, but not central, amygdala resulted in attenuation of long-term potentiation (LTP) at the perforant path-dentate gyrus granule cell synapses. High-frequency stimulation of the basolateral or basomedial amygdala alone did not induce LTP in the dentate gyrus, but facilitated the induction of LTP when applied at the same time as tetanic stimulation of the perforant path. The activity-dependent facilitation of hippocampal LTP by the basomedial and basolateral amygdala may be a synaptic mechanism underlying memory enhancement associated with emotions.