Long-term potentiation in the prefrontal cortex following stimulation of the hippocampal CA1/subicular region

Cooperativity between hippocampal–prefrontal short-term plasticity through associative long-term potentiation

The hippocampal-medial prefrontal cortex (mPFC) pathway provides highly convergent input to the mPFC in rats and shows two types of short-term plasticity in terms of paired-pulse facilitation (PPF) of the field potential under urethane anesthesia. We now report that stimulating either the dorsal or ventral subregions of the posterior hippocampus elicited PPF (by about 335 and 120%, respectively) of field potentials recorded in the mPFC at 100 ms interpulse interval. This PPF-like interaction occurred when projections were stimulated in the ventral-dorsal order (by about 200% of the single-pulsed response), but not vice versa. When weak long-term potentiation (LTP) of the dorsal projection was evoked simultaneously with strong LTP of the ventral projection, an associative effect was revealed (about +55%), although the magnitudes of LTP in each projection were not correlated. Even when the impermutable PPF-like facilitation was further enhanced (by about +120%), the enhancement was not correlated with either form of LTP, but exhibited the interaction of changes in the dorsal PPF, rather than in the heterotopic priming effect through the ventral projection. Moreover, this change was correlated with the associated LTP ratio of dorsal to ventral projection LTP (i.e., LTP associativity). Larger increases in LTP associativity correlated with greater impermutable PPF-like facilitation; in addition, there was hardly attenuation of the response to the dorsal projection by subsequent electrolytic lesions of the ventral subregion. These results indicate that the mPFC functionally integrates discrete sources of hippocampal information via cooperativity between short- and long-term plasticity.

Interactions among the medial prefrontal cortex, hippocampus and midline thalamus in emotional and cognitive processing in the rat

The medial prefrontal cortex (mPFC) participates in several higher order functions including selective attention, visceromotor control, decision making and goal-directed behaviors. We discuss the role of the infralimbic cortex (IL) in visceromotor control and the prelimbic cortex (PL) in cognition and their interactions in goal-directed behaviors in the rat. The PL strongly interconnects with a relatively small group of structures that, like PL, subserve cognition, and together have been designated the 'PL circuit.' These structures primarily include the hippocampus, insular cortex, nucleus accumbens, basolateral nucleus of the amygdala, the mediodorsal and reuniens nuclei of the thalamus and the ventral tegmental area of the midbrain. Lesions of each of these structures, like those of PL, produce deficits in delayed response tasks and memory. The PL (and ventral anterior cingulate cortex) (AC) of rats is ideally positioned to integrate current and past information, including its affective qualities, and act on it through its projections to the ventral striatum/ventral pallidum. We further discuss the role of nucleus reuniens of thalamus as a major interface between the mPFC and the hippocampus, and as a prominent source of afferent limbic information to the mPFC and hippocampus. We suggest that the IL of rats is functionally homologous to the orbitomedial cortex of primates and the prelimbic (and ventral AC) cortex to the lateral/dorsolateral cortex of primates, and that the IL/PL complex of rats exerts significant control over emotional and cognitive aspects of goal-directed behavior.

Adult-onset hypothyroidism impairs paired-pulse facilitation and long-term potentiation of the rat dorsal hippocampo-medial prefrontal cortex pathway in vivo

Thyroid hormones are critical for the maturation and function of the central nervous system. Insufficiency of thyroid hormones in the adulthood causes a wide range of cognitive dysfunctions, including deficits in learning and memory. The present study investigated whether adult-onset hypothyroidism would alter synaptic functions in the dorsal hippocampo-medial prefrontal cortex (mPFC) pathway, a neural pathway important for learning and memory. Adult hypothyroidism was induced by oral administration of 1% (g/l) antithyroid acting drug 6-n-propyl-2-thiouracil (PTU) to adult male Sprague-Dawley rats for 4 weeks. Postsynaptic potentials (PSP) were recorded in the mPFC by stimulating the dorsal hippocampal CA1 region in vivo. Basal synaptic transmission was evaluated by comparing input-output relationships. Paired-pulse facilitation and long-term potentiation were recorded to examine short- and long-term synaptic plasticity. Adult-onset hypothyroidism did not change the basal synaptic transmission, but significantly reduced paired-pulse facilitation and long-term potentiation of PSP. These inhibitions can be restored by thyroid hormone replacement. The results suggest that such alterations in synaptic plasticity of the dorsal hippocampo-mPFC pathway might contribute to understanding basic mechanisms underlying learning and memory deficits associated with adult-onset hypothyroidism.

Systems consolidation requires postlearning activation of NMDA receptors in the medial prefrontal cortex in trace eyeblink conditioning

The importance of the hippocampus in declarative memory is limited to recently acquired memory, and remotely acquired memory is believed to be stored somewhere in the neocortex. However, it remains unknown how the memory network is reorganized from a hippocampus-dependent form into a neocortex-dependent one. We reported previously that the medial prefrontal cortex (mPFC) is important for this neocortex-dependent remote memory in rat trace eyeblink conditioning. Here, we investigate the involvement of NMDA receptors in the mPFC in this reorganization and determine the time window of their contribution using chronic infusion of an antagonist into the mPFC, specifically during the postlearning consolidation period. The rats with blockade of the mPFC NMDA receptors during the first 1 or 2 weeks after learning showed a marked impairment in memory retention measured 6 weeks after learning, but relearned normally with subsequent conditioning. In contrast, the same treatment had no effect if it was performed during the third to fourth weeks or during the first day just after learning. The specificity of NMDA receptor blockade was confirmed by the reduced long-term potentiation in the hippocampal-prefrontal pathway in these rats. These results suggest that successful establishment of remotely acquired memory requires activation of NMDA receptors in the mPFC during at least the initial week of the postlearning period. Such NMDA receptor-dependent processes may mediate the maturation of neocortical networks that underlies permanent memory storage and serve as a way to reorganize memory circuitry to the neocortex-dependent form.

Search for a two-input model for future investigations of ‘synaptic tagging’ in freely moving animals in vivo

Processes of "synaptic tagging" guarantee synaptic input specificity after the induction of a protein synthesis-dependent late long-term potentiation (late-LTP). Distinct high-frequency stimulation can set a transient "synaptic tag" at the activated synapses, which captures plasticity-related proteins (PRPs) synthesized synapse-non-specifically in dendritic branches/compartments or the somata. Thus, only those synapses, which expressed a "tag", are also able to express late-LTP. Additionally, it was shown that the synthesis of PRPs is triggered by heterosynaptic, non-glutamatergic requirements during LTP-induction in tissue from adult animals. All these experiments were performed in hippocampal slices in vitro so far. Two questions now arise: first, is it possible to describe processes of 'synaptic tagging' in the intact, freely moving animal and second, is the stimulation of glutamatergic inputs sufficient to induce 'tagging' or is the co-activation of a modulatory-heterosynaptic input, also required for the process? We have first developed a technique, which allows us now to induce distinct forms of LTP at the ipsilateral CA1 site by specifically stimulating glutamatergic hippocampal structures at the contralateral site in the intact, freely moving rat. Thus, the used stimulation protocol allowed us to activate two separate synaptic inputs to the same neuronal stimulation, a pre-requisite for tagging-experiments to be investigated in vivo.

Effects of ventral and dorsal CA1 subregional lesions on trace fear conditioning

Recent lines of research have focused on dissociating function between the dorsal and ventral hippocampus along space and anxiety dimensions. In the dorsal hippocampus, the CA1 subregion has been implicated in the acquisition of contextual fear as well as in the trace interval in trace fear conditioning. The present study was designed to test the relative contributions of dorsal (dCA1) and ventral CA1 (vCA1) in trace fear conditioning. Long-Evans rats received ibotenate lesions of the ventral CA1 (n=7), dorsal CA1 (n=9), or vehicle control lesions (n=8) prior to trace fear conditioning acquisition. Results suggest dCA1 and vCA1 groups show no significant deficits during acquisition when compared to control groups. dCA1 and vCA1 both show deficits in the retention of contextual fear when tested 24 h post-acquisition (P<.05 and P<.01, respectively), and vCA1 was impaired relative to dCA1 (P<.05). This is suggestive of a graded involvement in contextual retention between the dorsal and ventral aspects of CA1. dCA1 showed no deficit for retention of conditioned fear to the tone or the trace when tested 48 h post-acquisition, whereas vCA1 did show a significant deficit for the trace interval and a slight, non-significant reduction in freezing to the tone, when compared to the control group (p<.05). Overall the data are suggestive of a graded involvement in retention of fear conditioning between the dorsal and ventral aspects of CA1, but it is likely that vCA1 may be critically involved in retention of trace fear conditioning.

Essential role of D1 but not D2 receptors in methamphetamine-induced impairment of long-term potentiation in hippocampal-prefrontal cortex pathway

Methamphetamine (MA) abuse induces deficits in cognitive performance that are related to dysfunction of the prefrontal cortex (PFC). The medial portion of the prefrontal cortex (mPFC) in rats that is crucial for cognitive function has been shown to undergo long-term potentiation (LTP) in the projections from the hippocampus. However, no study has been performed to evaluate the influence of MA on synaptic plasticity in the hippocampal-mPFC pathways. In the present experiments, we investigated the effects of repeated MA administration on hippocampal-mPFC LTP, together with MA-induced stereotyped behaviors. Repeated MA administration produced behavioral sensitization and LTP impairment in the hippocampal-mPFC pathways. The MA-induced impairment of hippocampal-mPFC LTP was prevented by the pretreatment of dopamine 1 (D1) but not dopamine 2 (D2) receptor antagonists, while D1 and D2 receptor antagonists attenuated the MA-induced stereotyped behaviors. These findings suggest that D1 receptors are crucial for the MA-induced deterioration of synaptic plasticity in the hippocampal-mPFC circuits. Impairment of LTP associated with D1 receptor dysfunction may underlie cognitive deficits in MA-dependent subjects.

The subiculum comes of age

The subiculum has long been considered as a simple bidirectional relay region interposed between the hippocampus and the temporal cortex. Recent evidence, however, suggests that this region has specific roles in the cognitive functions and pathological deficits of the hippocampal formation. A group of 20 researchers participated in an ESF-sponsored meeting in Oxford in September, 2005 focusing on the neurobiology of the subiculum. Each brought a distinct expertise and approach to the anatomy, physiology, psychology, and pathologies of the subiculum. Here, we review the recent findings that were presented at the meeting.

Excitatory actions of the ventral midline thalamus (rhomboid/reuniens) on the medial prefrontal cortex in the rat

The medial prefrontal cortex (mPFC) has been associated with diverse functions including attentional processes, visceromotor activity, decision making, goal directed behavior, and working memory. The present report examined the effects of stimulation of the midline thalamus, concentrating on ventral nuclei of the midline thalamus, on evoked activity at the mPFC. The nucleus reuniens (RE) of the ventral midline thalamus is a major source of projections to the hippocampus and to the mPFC, and has been shown to exert pronounced excitatory effects on the hippocampus. No previous study has systematically examined the actions of the ventral midline thalamus on the mPFC. We showed that stimulation of the dorsal and ventral midline thalamus, but not of an intermediate region lying between them (null zone), produced short latency, large amplitude evoked potentials throughout the dorsoventral extent of the medial PFC. The largest effects were elicited with ventral midline stimulation (rhomboid/reuniens nuclei) at the ventral mPFC--the prelimbic (PL) and infralimbic (IL) cortices. Specifically, stimulation of RE produced evoked potentials (early negative component, N2) at the PL cortex at a mean latency of 22.6 msec and mean amplitude of 0.85 mV, indicative of monosynaptic effects. In addition, we showed that paired pulse stimulation of RH/RE produced strong facilitatory actions (paired pulse facilitation) at IL (83%) and PL (75%). These findings indicate that RE exerts strong direct excitatory effects on the mPFC, and coupled with the demonstration that RE produces similar actions on the hippocampus, indicates that RE is in a position to influence and possibly coordinate the activity of these two forebrain structures subserving memory.

Temporal processing of information: The role of the medial prefrontal cortex and hippocampus: Theoretical comment on gilmartin and mcechron (2005)

M. R. Gilmartin and M. D. McEchron (2005) reported that single cells recorded in the prelimbic cortex of rats during the acquisition of trace fear conditioning display multiple patterns of neuronal firing during the trace. These finding are discussed in the context of the role of the prelimbic cortex in processing temporal information during trace conditioning and delayed matching- or nonmatching-to-sample paradigms based on both electrophysiology and lesion evidence. In addition, evidence is provided for a role of the hippocampus in supporting temporal processing of information and its potential interaction with the prelimbic cortex.

Prefrontal Phase Locking to Hippocampal Theta Oscillations

The interactions between cortical and hippocampal circuits are critical for memory formation, yet their basic organization at the neuronal network level is not well understood. Here, we demonstrate that a significant portion of neurons in the medial prefrontal cortex of freely behaving rats are phase locked to the hippocampal theta rhythm. In addition, we show that prefrontal neurons phase lock best to theta oscillations delayed by approximately 50 ms and confirm this hippocampo-prefrontal directionality and timing at the level of correlations between single cells. Finally, we find that phase locking of prefrontal cells is predicted by the presence of significant correlations with hippocampal cells at positive delays up to 150 ms. The theta-entrained activity across cortico-hippocampal circuits described here may be important for gating information flow and guiding the plastic changes that are believed to underlie the storage of information across these networks.

Distinct modulatory effects of sleep on the maintenance of hippocampal and medial prefrontal cortex LTP

Both human and animal studies support the idea that memory consolidation of waking experiences occurs during sleep. In experimental models, rapid-eye-movement (REM) sleep has been shown to be necessary for cortical synaptic plasticity and for the acquisition of spatial and nonspatial memory. Because the hippocampus and medial prefrontal cortex (mPFC) play distinct and important roles in memory processing, we sought to determine the role of sleep in the maintenance of long-term potentiation (LTP) in the dentate gyrus (DG) and mPFC of freely behaving rats. Animals were implanted with stimulating and recording electrodes, either in the medial perforant path and DG or CA1 and mPFC, for the recording of field potentials. Following baseline recordings, LTP was induced and the animals were assigned to three different groups: REM sleep-deprived (REMD), total sleep-deprived (TSD) and control which were allowed to sleep (SLEEP). The deprivation protocol lasted for 4 h and the recordings were made during the first hour and at 5, 24 and 48 h following LTP induction. Our results show that REMD impaired the maintenance of late-phase (48-h) LTP in the DG, whereas it enhanced it in the mPFC. Sleep, therefore, could have distinct effects on the consolidation of different forms of memory.

Prefrontal cortex function, quasi-physiological stimuli, and synaptic plasticity

The prelimbic area of rat medial frontal cortex may be functionally analogous to human/primate dorsolateral prefrontal cortex. This area may be involved in selective attention to the external stimuli and the coupling of the attention to a repertory of actions. It was suggested that this function may rely on a form of long-term memory [Biol. Rev. 77 (2002) 563]. Indeed, during learning of this type of behavior, a portion of prelimbic neurons persistently change their firing characteristics [Prog. Brain Res. 126 (2000) 287]. It is therefore important to study long-term potentiation (LTP) and depression (LTD) in rat prelimbic neurons. In this article, the author first briefly reviews recent findings on the prefrontal cortex function and discusses that the prefrontal cortex may be involved in long-term memory. Second, the author will show some new results which indicate that quasi-physiological patterns of stimuli mimicking prelimbic neuronal activity during behavior can induce LTP in prelimbic pyramidal neuron synapses. These results suggest that prelimbic neuronal activity during behavior may lastingly modify prelimbic synaptic efficacy.

Long-term potentiation and memory

One of the most significant challenges in neuroscience is to identify the cellular and molecular processes that underlie learning and memory formation. The past decade has seen remarkable progress in understanding changes that accompany certain forms of acquisition and recall, particularly those forms which require activation of afferent pathways in the hippocampus. This progress can be attributed to a number of factors including well-characterized animal models, well-defined probes for analysis of cell signaling events and changes in gene transcription, and technology which has allowed gene knockout and overexpression in cells and animals. Of the several animal models used in identifying the changes which accompany plasticity in synaptic connections, long-term potentiation (LTP) has received most attention, and although it is not yet clear whether the changes that underlie maintenance of LTP also underlie memory consolidation, significant advances have been made in understanding cell signaling events that contribute to this form of synaptic plasticity. In this review, emphasis is focused on analysis of changes that occur after learning, especially spatial learning, and LTP and the value of assessing these changes in parallel is discussed. The effect of different stressors on spatial learning/memory and LTP is emphasized, and the review concludes with a brief analysis of the contribution of studies, in which transgenic animals were used, to the literature on memory/learning and LTP.

Plasticity at hippocampal to prefrontal cortex synapses is impaired by loss of dopamine and stress: importance for psychiatric diseases

The direct hippocampal to prefrontal cortex pathway and its changes in synaptic plasticity is a useful framework for investigating the functional operations of hippocampal-prefrontal cortex communication in cognitive functions. Synapses on this pathway are modifiable and synaptic strength can be turned up or down depending on specific patterns of activity in the pathway. The objective of this review will be to summarize the different studies carried out on this topic including very recent data and to underline the importance of animal models for the development of new and effective medications in psychiatric diseases. We have shown that long-term potentiation (LTP) of hippocampal-prefrontal synapses is driven by the level of mesocortical dopaminergic (DA) activity and more recently that stress is also an environmental determinant of LTP at these cortical synapses. Stimulation of the ventral tegmental area at a frequency known to evoke DA overflow in the prefrontal cortex produces a long-lasting enhancement of the magnitude of hippocampal-prefrontal cortex LTP whereas a depletion of cortical DA levels generates a dramatic decrease in this LTP. Moreover, hippocampal stimulation induces a transient but significant increase in DA release in the prefrontal cortex and an optimal level of D1 receptor activation is essential for LTP expression. We recently investigated the impact of stress on hippocampal-prefrontal LTP and demonstrated that exposure to an acute stress causes a remarkable and long-lasting inhibition of LTP. Furthermore, we demonstrated that tianeptine, an antidepressant which has a unique mode of action, and clozapine an atypical antipsychotic when administered at doses normally used in human testing are able to reverse the impairment in LTP. Stressful life events have a substantial causal association with psychiatric disorders like schizophrenia and depression and recent imaging studies have shown an important role of the limbic-cortical circuit in the pathophysiology of these illnesses. Therefore, we proposed that agents capable of reversing the impairment of plasticity at hippocampal to prefrontal cortex synapses have the potential of becoming new therapeutic classes of antidepressant or antipsychotic drugs.

Convergence and interaction of hippocampal and amygdalar projections within the prefrontal cortex in the rat

The orbital and medial prefrontal cortex (OMPFC) receives inputs from the CA1/subicular (CA1/S) region of the ventral hippocampus and the basolateral nucleus of the amygdala (BLA). Despite many studies about these projections, little is known as to how CA1/S and BLA inputs converge and interact within the OMPFC. Extracellular recordings of single-unit activity in the OMPFC were performed in sodium pentobarbitone-anesthetized rats. OMPFC neurons driven by CA1/S or BLA stimulation were more frequently encountered in the ventral portion of the prelimbic (v-PrL) and infralimbic cortex (IL). OMPFC neurons showing excitatory convergence of both inputs from the CA1/S and BLA were also located predominantly in the v-PrL and IL. The excitatory latencies of these neurons from both the CA1/S and BLA revealed almost identical values. Excitatory responses of OMPFC neurons to CA1/S (or BLA) stimulation were markedly augmented by simultaneous BLA (or CA1/S) stimulation, whereas the inhibitory influence of the BLA (or CA1/S) on CA1/S-induced (or BLA-induced) excitation was apparent when BLA (or CA1/S) stimulation was given 20-40 msec before CA1/S (or BLA) stimulation. Similar results were also observed when reciprocal connections between the CA1/S and BLA were severed to exclude the influences of these connections on one another. From these studies, we concluded that excitatory and inhibitory inputs from the hippocampus and amygdala converge and interact in the v-PrL and IL. Furthermore, the results indicate that simultaneous activation of hippocampal and amygdalar neurons may be important for amplification of OMPFC neuronal activity.

Long-term potentiation and evoked spike responses in the cingulate cortex of freely mobile rats

Long-term potentiation of synaptic efficiency is regarded as a major candidate for the role of the physiological mechanism of long-term memory. However, the limited development of concepts of the cellular and subcellular characteristics of the induction of long-term potentiation in animals in conditions of free behavior does not correspond to the importance of this question. The present study was undertaken to determine whether the characteristics of potentiation in the cingulate cortex in response to stimulation of fibers of the subiculo-cingulate tract are truly long-term, i.e., develop through all known phases and last at least 24 h, in freely moving animals. In addition, the study aims included identification of the effects of application of blockers of different types of glutamate receptors on the development of long-term potentiation and identification of the characteristics of spike responses of single cingulate cortex neurons to stimulation of the subiculo-cingulate tract. Long-term potentiation, lasting more than 24 h, was obtained in freely moving adult rats not treated with GABA blockers. Injection of glutamate NMDA synapse blockers led to significant decreases in evoked cingulate cortex potentials in response to test stimulation. Activatory short-latency spike responses were characterized by a low probability of spike generation, and this increased with increases in the stimulation current. These data demonstrated that it is methodologically possible to compare, in freely moving rats, the involvement of individual neurons in the mechanisms involved in learning one or another type of adaptive behavior and the dynamics of their evoked spike activity during the formation of long-term potentiation.

Time-dependent reorganization of the brain components underlying memory retention in trace eyeblink conditioning

Many studies have confirmed the time-limited involvement of the hippocampus in mnemonic processes and suggested that there is reorganization of the responsible brain circuitry during memory consolidation. To clarify such reorganization, we chose trace classical eyeblink conditioning, in which hippocampal ablation produces temporally graded retrograde amnesia. Here, we extended the temporal characterization of retrograde amnesia to other regions that are involved in acquisition during this task: the medial prefrontal cortex (mPFC) and the cerebellum. At a various time interval after establishing the trace conditioned response (CR), rats received an aspiration of one of the three regions. After recovery, the animals were tested for their CR retention. When ablated 1 d after the learning, both the hippocampal lesion and the cerebellar lesion group of rats exhibited a severe impairment in retention of the CR, whereas the mPFC lesion group showed only a slight decline. With an increase in interval between the lesion and the learning, the effect of the hippocampal lesion diminished and that of the mPFC lesion increased. When ablated 4 weeks after the learning, the hippocampal lesion group exhibited as robust CRs as its corresponding control group. In contrast, the mPFC lesion and the cerebellar lesion groups failed to retain the CRs. These results indicate that the hippocampus and the cerebellum, but only marginally the mPFC, constitute a brain circuitry that mediates recently acquired memory. As time elapses, the circuitry is reorganized to use mainly the mPFC and the cerebellum, but not the hippocampus, for remotely acquired memory.

Differences between paired-pulse facilitation and long-term potentiation in the dorsal and ventral hippocampal CA1-prefrontal pathways of rats

We studied the interaction between paired-pulse facilitation (PPF) and long-term potentiation (LTP) in the hippocampo-prefrontal cortex (PFC, prelimbic area) pathway, stimulating the ventral or posterior dorsal CA1 region (vCA1 or pdCA1). In the vCA1-PFC, the group averaged PPF did not change after the LTP induction, and there was a negative correlation between the post-LTP PPF change and LTP magnitude. In contrast, the post-LTP PPF of the pdCA1-PFC appeared to decrease significantly, and the PPF change was independent of the LTP magnitude. We found that there were at least two mechanisms of PPF regulation following LTP induction in the pathway resulting from extensive CA1 projections into the prelimbic area. The results imply that the CA1-PFC pathway regulates the PFC PPF quantitatively in LTP-dependent and independent manners, which depend on the local properties of the CA1 regions.

Dynamics of population code for working memory in the prefrontal cortex

Some neurons (delay cells) in the prefrontal cortex elevate their activities throughout the time period during which the animal is required to remember past events and prepare future behavior, suggesting that working memory is mediated by continuous neural activity. It is unknown, however, how working memory is represented within a population of prefrontal cortical neurons. We recorded from neuronal ensembles in the prefrontal cortex as rats learned a new delayed alternation task. Ensemble activities changed in parallel with behavioral learning so that they increasingly allowed correct decoding of previous and future goal choices. In well-trained rats, considerable decoding was possible based on only a few neurons and after removing continuously active delay cells. These results show that neural activity in the prefrontal cortex changes dynamically during new task learning so that working memory is robustly represented and that working memory can be mediated by sequential activation of different neural populations.

Long-term potentiation in visual cortical projections to the medial prefrontal cortex of the rat

In order to investigate neural mechanisms by which the prefrontal cortex adaptively modifies its activities based on past experience, we examined whether or not sensory cortical projections to the medial prefrontal cortex support long-term potentiation (LTP) in rats. Monosynaptic projections from the secondary visual cortex, mediomedial area (V2MM) to the infralimbic cortex were confirmed by orthodromic as well as antidromic activation of single units. High-frequency stimulation (50 Hz, 2 s) induced LTP (approximately 45% increase over the baseline) in the V2MM projection to the infralimbic cortex. LTP induction in this pathway was completely blocked by an injection (i.p.) of CPP, an N-methyl-D-aspartate receptor antagonist. LTP was also induced in the ventral hippocampal projection to the infralimbic cortex by the same high-frequency stimulation. The present results suggest that modification of synaptic weights of afferent sensory cortical projections is one mechanism underlying learning-induced changes in prefrontal cortical neural activities.

Compatibility of bidirectional synaptic plasticity on hippocampo-prefrontal cortex pathway in rats

The hippocampo-prefrontal cortex pathway reportedly expresses long-term potentiation (LTP) and depression (LTD) in anesthetized rats. We examined whether there were any effects governing the induction of LTD after prior induction of LTP, or vice versa. Induction in sequence of LTP and LTD resulted in significantly stable changes of about 140 and 70% of a common control for 1 h each. The reversed sequence, LTD and LTP, showed a mirror image of about 65 and 135% of control, which were not different from the respective changes in the first sequence (P>0.3 for each). The correlation coefficient between changes was significantly positive in the first sequence and weakly negative in the reverse. These results indicate that this pathway can express compatibility of bidirectional synaptic plasticity while historical changes remain covert.

Evoked prefrontal gamma oscillation by hippocampal train stimulation in anesthetized rats

We previously reported a difference in short-term synaptic plasticity between the rat posterior dorsal CA1 (pdCA1)-prefrontal cortex (PFC) and ventral CA1 (vCA1)-PFC pathways. Here, to determine the effects of hippocampal train stimulation on the local field potential in the medial PFC, we recorded the PFC field potential with brief 250-Hz train stimulation (1, 3, 5, 7, and 9 pulses) of pdCA1 or vCA1 in anesthetized rats. Analysis of the gamma-band (40-100 Hz) power revealed stimulation-evoked gamma oscillation in the pdCA1-PFC, but not in the vCA1-PFC. These results indicate that these pathways have different responses to train stimulation. The function of the pdCA1-PFC may differ from that of the vCA1-PFC, a well-known hippocampus-PFC pathway.

The serotonergic modulation of synaptic plasticity in the rat hippocampo-medial prefrontal cortex pathway

The ability of the serotonergic (5-HTergic) system to affect the hippocampo-medial prefrontal cortex (mPFC) synaptic properties was examined in rats with lesions of 5-HTergic neurons. Intracerebroventricular injections of 5,7-dihydroxytryptamine (5,7-DHT) resulted in selective depletion of 5-HT and 5-hydroxyindoleacetic acid in the cerebral cortex, hippocampus and raphe regions. The 5,7-DHT-lesioned rat had no changes in basal synaptic transmission in the hippocampo-mPFC pathway. Conversely, we observed the augmentation of short-term synaptic plasticity, i.e. paired-pulse facilitation, when compared with sham-operated rats in this pathway. The magnitude of long-term potentiation (LTP) was significantly augmented in 5,7-DHT-lesioned rats. This augmentation of hippocampo-mPFC LTP had a significant correlation with the degree of cortical 5-HT levels. These results suggest that the 5-HTergic system may modulate plastic properties at the hippocampal-mPFC synapses in vivo.

Effects of clozapine, haloperidol and iloperidone on neurotransmission and synaptic plasticity in prefrontal cortex and their accumulation in brain tissue: an in vitro study

The mode of action of the antipsychotic drugs clozapine, haloperidol and iloperidone was investigated in layer V of prefrontal cortex slices using extracellular field potential, intracellular sharp-electrode as well as whole-cell voltage clamp recording techniques. Intracellular investigations on a broad range of concentrations revealed that the typical neuroleptic haloperidol at higher concentrations significantly depressed the excitatory postsynaptic component induced by electrical stimulation of layer II. This was not seen with the atypical neuroleptics clozapine and iloperidone. None of the three compounds had any effect on the resting membrane potential, spike amplitude or input resistance at relevant concentrations. Synaptic plasticity was assessed by means of extracellular field potential recordings. Clozapine significantly facilitated the potentiation of synaptic transmission, whereas haloperidol and iloperidone showed no effects. In line with its facilitating effect on synaptic plasticity, it could be demonstrated by whole-cell voltage clamp recordings that clozapine increased N-methyl-D-aspartic acid receptor-mediated excitatory postsynaptic currents in the majority of prefrontal cortical neurones. These investigations were made with neuroleptic drugs applied to the bath in the micromolar concentration range in order to approach clinical brain concentrations that are reached after administration of therapeutic doses. The drug concentrations reached in the slices after the experiments were assessed by means of high-pressure liquid chromatography coupled with mass-spectrometric detection. Surprisingly, drug accumulation in the in vitro preparation was of similar degree as reported in vivo. In conclusion, the typical neuroleptic haloperidol significantly depressed excitatory synaptic transmission in layer V neurones of the prefrontal cortex. In contrast, the two atypical neuroleptics iloperidone and clozapine revealed no depressing effects. This feature of the atypical neuroleptics might be beneficial since a hypofunctionality of this brain area is thought to be linked with the pathophysiology of schizophrenia. Additionally, clozapine facilitated long-term potentiation, which might be linked with the clinically observed beneficial effects on certain cognitive parameters. The clozapine-induced increase of N-methyl-D-aspartic acid receptor-mediated currents suggests that clozapine facilitates the induction of long-term potentiation. Furthermore, the present study points to the importance of considering the significant accumulation of neuroleptic drugs in in vitro studies.

Potentiation of prelimbic field potentials during and seconds after trains of excitations in the rat hippocampo-prefrontal pathway

Field potentials were recorded in the prelimbic cortex of anaesthetized rats after excitations of the hippocampo-prefrontal pathway. Stimuli were delivered to the hippocampal CA1 region and short-term changes of field potential amplitudes were observed in two situations. (1) Amplitudes were monitored during trains of stimulations given at frequencies between 1 and 20 Hz. Within trains, potentiation was followed by depression. Both types of changes were frequency dependent. (2) The time course of recovery from within-train plasticity was obtained from field potentials evoked at varying intervals after trains. This revealed a post-train potentiation having a maximum after 2-4 s and lasting for approximately 10 s. The maximal post-train potentiation was nearly independent of the frequency of the preceding train.

Dopamine: a potential substrate for synaptic plasticity and memory mechanisms

It is only recently that a number of studies on synaptic plasticity in the hippocampus and other brain areas have considered that a heterosynaptic modulatory input could be recruited as well as the coincident firing of pre- and post-synaptic neurons. So far, the strongest evidence for such a regulation has been attributed to dopaminergic (DA) systems but other modulatory pathways have also been considered to influence synaptic plasticity. This review will focus on dopamine contribution to synaptic plasticity in different brain areas (hippocampus, striatum and prefrontal cortex) with, for each region, a few lines on the distribution of DA projections and receptors. New insights into the possible mechanisms underlying these plastic changes will be considered. The contribution of various DA systems in certain forms of learning and memory will be reviewed with recent advances supporting the hypothesis of similar cellular mechanisms underlying DA regulation of synaptic plasticity and memory processes in which the cyclic adenosine monophosphate/protein kinase A (cAMP/PKA) pathway has a potential role. To summarize, endogenous DA, which depends on the activity patterns of DA midbrain neurons in freely moving animals, appears as a key regulator in specific synaptic changes observed at certain stages of learning and memory and of synaptic plasticity.

Learning and memory of cue-reward association meaning by modifications of synaptic efficacy in dentate gyrus and piriform cortex

This article begins with a review of recent experiments investigating the synaptic efficacy changes occurring in rat dentate gyrus and piriform cortex during an associative olfactory task. In all these experiments, animals were trained to discriminate among an artificial cue, a patterned electrical stimulation distributed to the lateral olfactory tract associated with a water reward, and a natural odor associated with a flash of light. Monosynaptic field potential responses evoked by single electrical stimuli to the lateral olfactory tract were recorded in the ipsilateral piriform cortex before and just after each training session. Monosynaptic field and polysynaptic field potentials evoked by single electrical stimuli applied respectively to the lateral perforant pathway and lateral olfactory tract were also recorded in ipsilateral dentate gyrus. The results showed an increase in synaptic efficacy subsequent to the first training session in the dentate gyrus network when compared with piriform cortex at the later stage of the learning. The early increase of monosynaptic response in the dentate gyrus was observed immediately after the first learning session but disappeared 24 h later. Inversely, a synaptic depression developed across sessions, becoming significant at the onset of the last (fifth) session. The polysynaptic potential recorded in this structure increased substantially when rats began to discriminate the leaming cues, usually after the second or third learning session. Then, from the third to the fifth session, an LTP like-phenomenon appeared in piriform cortex when rats perfectly mastered the associations. Experiments using high-frequency stimulation to prevent changes in gyrus dentatus indicated that the onset of the observed depression was necessary for the learning of the olfactory associations. The fact that hippocampal and cortical neuronal networks exhibited different timing in synaptic efficacy changes could physiologically explain learning and memory processes.

Long-term potentiation as a substrate for memory: evidence from studies of amygdaloid plasticity and Pavlovian fear conditioning

Recent reports have raised concerns about the ability of long-term potentiation (LTP) to account for associative learning and memory. In this paper, we review the many mechanistic similarities between one form of associative learning, Pavlovian fear conditioning, and amygdaloid LTP. We then address many of the criticisms levied against LTP within the framework of fear conditioning. We believe that many of the apparent discrepancies between LTP and behavior can be generally accounted for by a failure to appreciate that learned behavior is supported by multiple synapses in an extensive network of brain structures. We conclude that LTP remains a viable substrate for memory.

Memory trace in prefrontal cortex: theory for the cognitive switch

The dorsolateral prefrontal cortex in human and non-human primates functions as the highest-order executor for the perception-action cycle. According to this view, when perceptual stimuli from the environment are novel or complex, the dorsolateral prefrontal cortex serves to set consciously a goal-directed scheme which broadly determines an action repertory to meet the particular demand from the environment. In this respect, the dorsolateral prefrontal cortex is a short-term activation device with the properties of a cognitive switch', because it couples a particular set of perceptual stimuli to a particular set of actions. Here, I suggest that, in order for the organism to react systematically to the environment, neural traces for the switch function must be stored in the brain. Thus, the highest-order, perception-action interface function of the dorsolateral prefrontal cortex per se depends on permanently stored neural traces in the dorsolateral prefrontal cortex and related structures. Such a memory system may be located functionally between two of the well-documented memory systems in the brain: the declarative memory system and the procedural memory system. Finally, based on available neurophysiological data, the possible mechanisms underlying the formation of cognitive switch traces are proposed.

Effects of hippocampus-induced prefrontal long-term depression on gamma-band local field potential in anesthetized rats

To determine whether long-term depression (LTD) affects cortical gamma-band local field potential (40-100 Hz), we conducted a LTD induction experiment in the hippocampus-prefrontal cortex (PFC) pathway of an anesthetized rat. The LTD induction increased the spontaneous level of PFC gamma-band power of 70-100 Hz, which was not affected after the long-term potentiation (LTP) induction in our previous experiment. In addition, the LTD induction increased the evoked PFC gamma-band power at 900 ms after hippocampal test stimulation; this latency appeared to differ from that (500-700 ms) observed in our previous LTP experiment. The results indicate that the PFC field potential increases its gamma-band power following both LTP and LTD in the hippocampus-PFC pathway, which is involved in working memory. Particularly, the sustained increase by LTD may reflect a representation of working memory.

Changes in synaptic plasticity in the rat hippocampo-medial prefrontal cortex pathway induced by repeated treatments with fluvoxamine

The present studies were conducted to examine the effects of single and repeated treatments with fluvoxamine, which is a selective serotonin reuptake inhibitor (SSRI), on the synaptic efficacy and synaptic plasticity in the rat hippocampo-medial prefrontal cortex (mPFC) pathway in vivo. It has been reported that the projections arising from the hippocampal structures to the mPFC are involved in the execution of higher cognitive functions in rats. The evoked potentials were recorded in the mPFC by stimulation of the CA1/subicular region of the ventral hippocampus in halothane-anesthetized rats. Single administration of fluvoxamine (10 and 30 mg/kg, i.p.) enhanced synaptic efficacy in the hippocampo-mPFC pathway in a dose-dependent manner. Although repeated treatments with fluvoxamine (30 mg/kg, i.p. after 30 mg/kg/dayx21 days, p.o.) caused an enhancement of synaptic efficacy, there was no significant difference between single and repeated treatments. The input/output characteristics showed hypersensitivity to stimulation intensity in the group with repeated fluvoxamine treatments. The establishment of long-term potentiation (LTP) in the hippocampo-mPFC pathway after a single administration of fluvoxamine was not different from that in the saline-injected group. On the other hand, the hippocampo-mPFC LTP was significantly augmented by repeated treatments with fluvoxamine when compared to a single treatment. These findings suggest that the serotonergic system could modulate the synaptic plasticity at hippocampal-mPFC synapses. The present study, furthermore, suggests that the enhancement of LTP in the hippocampo-mPFC pathway produced by repeated treatments with fluvoxamine may be implicated in the SSRI-induced therapeutic effect on psychiatric disorders.

Activation of AP5-sensitive NMDA Receptors is Not Required to Induce LTP of Synaptic Transmission in the Lateral Perforant Path

The role of the N-methyl-d-aspartate (NMDA) type of glutamate receptor in long-term potentiation (LTP) of the medial (MPP) and lateral (LPP) divisions of the perforant path - granule cell system was investigated in urethane-anaesthetized rats. A stimulating electrode was positioned in the dorsomedial or ventrolateral aspect of the angular bundle for selective activation of either the MPP or LPP, respectively. A push - pull cannula served to focally perfuse artificial cerebrospinal fluid (ACSF) across the perforant path synaptic zone, while evoked potentials were monitored in the dentate hilus. Identification of LPP and MPP responses was based on (1) differences in population excitatory postsynaptic potential (EPSP) waveform obtained during stimulus depth profiles, and (2) differential sensitivity of evoked EPSPs to the glutamate receptor agonist l-aminophosphonobutyrate (AP4), and the antagonist gamma-d-glutamylglycine (DGG). High-frequency stimulation (400 Hz, 8 bursts of 8 pulses) applied to the lateral and medial perforant path elicited LTP of the EPSP and population spike in rats perfused with standard medium. In the MPP, LTP was almost completely blocked when d-aminophosphonopentanoate (AP5; 100 microM), a selective NMDA receptor antagonist, was perfused during the tetanus. Surprisingly, in the LPP experiments, AP5 did not impair induction of the 'synaptic' EPSP component of LTP. This occurred despite the ability of AP5 to block LTP of the LPP evoked population spike. The results suggest the existence of a novel, NMDA receptor-independent form of synaptic LTP in the lateral perforant path.

Excitatory Amino Acid Pathway from the Hippocampus to the Prefrontal Cortex. Contribution of AMPA Receptors in Hippocampo-prefrontal Cortex Transmission

Previous experiments in the rat have demonstrated that field CA1 and the subiculum project to the prefrontal cortex and that this direct unilateral pathway is excitatory. In the present study, anatomical and electrophysiological approaches were used to determine the transmitter mediating the excitatory responses in prefrontal cortex neurons to low-frequency stimulation of the hippocampus. The method of selective retrograde d-[3H]aspartate labelling was used to identify putative glutamatergic and/or aspartatergic hippocampal afferent fibres to the prefrontal cortex. Unilateral microinjection of d-[3H]aspartate into the prelimbic area of the prefrontal cortex resulted in the retrograde labelling of a fraction of hippocampal neurons. Some labelled cell bodies were distributed in field CA1 and the subiculum but larger numbers of neurons were detected in the ventral and intermediary subiculum. In a second series of experiments, the excitatory transmission from the hippocampus to the prefrontal cortex was pharmacologically analysed to provide further evidence for the involvement of glutamate and/or aspartate in the pathway. All prefrontal cortex neurons responding to the stimulation of the hippocampus were activated by selective agonists of the glutamate receptor subtypes alpha-amino-3-hydroxy-5-methyl-4-isoxazole-propionic acid (AMPA) and N-methyl-d-aspartate (NMDA), and these effects were selectively antagonized by 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) and 2-amino-5-phosphonopentanoic acid (APV) respectively. Most of the excitatory responses of prefrontal cortex neurons to single and paired-pulse stimulation of the hippocampus were antagonized by CNQX. APV only affected the excitatory response in a few cells. These results suggest that the hippocampal input to the prefrontal cortex utilizes glutamate and/or aspartate as a transmitter. Even though prefrontal cortex neurons responding to the stimulation of the hippocampus appear to have both AMPA and NMDA receptors, low-frequency stimulation of the hippocampo-prefrontal cortex pathway activates cortical neurons mostly through AMPA receptors.

Is LTP in the hippocampus a useful model for learning-related alterations in gene expression?

It is well established that the formation of long-term memory requires de novo protein synthesis. Altered gene expression is therefore critical in the signal transduction cascade activated by the learning experience. Long-term potentiation (LTP) is a mnemonic model in which particular patterns of activation of incoming excitatory fibers (representing the learning experience) may induce long-lasting enhancement of the communication between the involved pre- and post-synapses (representing the memory). Therefore, cellular and molecular mechanisms of LTP have been extensively studied under the assumption that their understanding will contribute to our comprehension of the mechanisms underlying memory formation. In recent years, however, this analogy has been challenged by reports of inconsistency between LTP and memory. Here we assess LTP in the hippocampus as a model system to study spatial memory-related alterations in gene expression. We focus on three molecular families that are likely to play a role in synaptic plasticity: (1) synaptic communication related proteins; (2) signal transduction machinery; and (3) growth factors. Reviewing first the literature on LTP and then behavioral research we found both consistent and inconsistent findings regarding the LTP/memory linkage. The importance of restricting the discussion to both a learning phase and a brain (sub)structure, as well as of incorporating more physiological LTP stimulation protocols, is discussed. We conclude that while LTP is indeed limited as a model of memory, a careful use of it as a model system of synaptic plasticity is fruitful and productive in screening out candidate memory-related genes.

Plasticity in the projection from the anterior thalamic nuclei to the anterior cingulate cortex in the rat in vivo: paired-pulse facilitation, long-term potentiation and short-term depression

Several neurophysiological and computational theories of the rodent navigational system suggest that the differing cortices of the frontal lobe and thalamus share information and therefore undergo changes in synaptic strength. We examine here for the first time three forms of synaptic plasticity in the projection from the anterior thalamic nuclei to the anterior cingulate cortex: we demonstrate that this projection is capable of expressing paired-pulse facilitation, long-term potentiation, and short-term depression. Furthermore, input/output curves show that field excitatory post-synaptic potential amplitude increased at all stimulus intensities following high-frequency stimulation. These findings add important information to our understanding of synaptic plasticity in this important pathway, which has been widely hypothesized to play important roles in memory and spatial representation in the rodent.

Local properties of CA1 region in hippocampo-prefrontal synaptic plasticity in rats

We studied paired-pulse facilitation and long-term potentiation/depression in anesthetized rats to determine whether the hippocampal CA1 region inhibits local differences in short-term and long-term synaptic plasticity in its projections to the prefrontal cortex. We compared projections with the PFC from the posterior dorsal and ventral hippocampal CA1 regions (pdCA1 and vCA1 respectively). The two pathways displayed similar properties. However, the PPF properties of the pdCA1, projections differed dramatically from those of the pdCA1 projections. The pdCA1 projections showed the opposite of facilitation (i.e. suppression) at 25-50 ms intervals and more pronounced facilitation at 100-400 ms intervals. These results suggest that there are functional differences between these pathways.

Analysis of projections from the medial prefrontal cortex to the thalamus in the rat, with emphasis on nucleus reuniens

The medial prefrontal cortex (mPFC) is involved in high-order cognitive processes, including, but not limited to, decision making, goal directed behavior, and working memory. Although previous reports have included descriptions of mPFC projections to the thalamus in overall examinations of mPFC projections throughout the brain, no previous study has comprehensively examined mPFC projections to the thalamus. The present report compares and contrasts projections from the four divisions of the mPFC, i.e., the infralimbic, prelimbic, anterior cingulate and medial agranular cortices, to the thalamus in the rat by using the anterograde anatomic tracer Phaseolus vulgaris-leucoagglutinin. We showed that (1) the infralimbic, prelimbic, anterior cingulate cortices distribute heavily and selectively to midline/medial structures of the thalamus, including the paratenial, paraventricular, interanteromedial, anteromedial, intermediodorsal, mediodorsal, reuniens, and the central medial nuclei; (2) the medial agranular cortex distributes strongly to the rostral intralaminar nuclei (central lateral, paracentral, central medial nuclei) as well as to the ventromedial and ventrolateral nuclei of thalamus; and (3) all four divisions of the mPFC project densely to the nucleus reuniens (RE) of the thalamus. The nucleus reuniens is the major source of thalamic afferents to the hippocampal formation. There are essentially no direct projections from the mPFC to the hippocampus. The present demonstration of pronounced mPFC projections to RE suggests that the nucleus reuniens is a critical relay in the transfer of information from the medial prefrontal cortex to the hippocampus. Our further demonstration of strong mPFC projections to several additional thalamic nuclei, particularly to the mediodorsal nucleus, suggests that these thalamic nuclei, like RE, represent important output stations (or gateways) for the actions of mPFC on diverse subcortical and cortical structures of the brain.

Effects of rat medial prefrontal cortex temporal inactivation on a delayed alternation task

To determine the involvement of the medial prefrontal cortex (mPFC) in operant-type delayed alternation, microinjections of muscimol into the mPFC were used for temporal inactivation during behavioral tests in rats. The temporal mPFC inactivation showed effects related to both dorsal (decreased delay-dependent correct ratio, indicating working memory-related deficits) and ventral hippocampus inactivation (increased tendency to repeat errors) reported in our recent paper, without motor or sensory effects. These findings suggest that the mPFC integrates information from different hippocampal regions during a delayed alternation task.

Hypothalamic-pituitary-adrenal axis function and corticosterone receptor expression in behaviourally characterized young and aged Long-Evans rats

In the current investigation, hypothalamic-pituitary-adrenal (HPA) axis function was examined in young and aged male Long-Evans rats that were initially assessed on a version of the Morris water maze sensitive to cognitive impairment during ageing. In behaviourally characterized rats, a 1-h restraint stress paradigm revealed that plasma corticosterone concentrations in aged cognitively impaired rats took significantly longer to return to baseline following the stressor than did those in young or aged cognitively unimpaired rats. No differences in basal or peak plasma corticosterone concentrations, however, were observed between young or aged rats, irrespective of cognitive status. Using ribonuclease protection assays and in situ hybridization, we evaluated mineralocorticoid receptor (MR) and glucocorticoid receptor (GR) mRNA abundance in young and aged rats characterized on the spatial task. Abundance of MR mRNA was decreased as a function of age in stratum granulosum but not hippocampus proper, and the decrease in MR mRNA was largely unrelated to cognitive status. However, GR mRNA was significantly reduced in several hippocampal subfields (i.e. stratum granulosum and temporal hippocampus proper) and other related cortical structures (medial prefrontal and olfactory regions) of aged cognitively impaired rats compared to either young or aged cognitively unimpaired cohorts, and was significantly correlated with spatial learning ability among the aged rats in each of these brain regions. In agreement with previous stereological data from this ageing model, no changes were detected in neuron density in the hippocampus of the rats used in the in situ hybridization analysis. These data are the first to describe a coordinated decrease in GR mRNA in a functional brain system including hippocampus and related cortical areas that occurs in tandem with impairments of the HPA response to stress and cognitive decline in ageing.

Electrical stimuli patterned after the theta-rhythm induce multiple forms of LTP

The induction of long-term potentiation (LTP) by high-frequency stimulation is considered an acceptable model for the study of learning and memory. In area CA1 calcium influx through N-methyl-D-aspartate receptors (NMDARs; nmdaLTP) and/or L-type voltage-dependent calcium channels (vdccLTP) results in distinct forms of LTP. In the light of significant accumulation of knowledge about patterns of naturally occurring activity in the intact animal, we examined whether the application of stimuli patterned after natural activity induced nmdaLTP and/or vdccLTP. In rat hippocampal slices we examined LTP induced by three types of patterned stimulation short (S-TBS), long (L-TBS), and high-intensity long theta-patterned stimulation (HL-TBS). The patterns of stimulation were applied in control, nifedipine (blocks vdccLTP), D,L-2-amino-5-phosphonovaleric acid (APV; blocks nmdaLTP), or APV and nifedipine containing media. We found that S-TBS resulted in LTP that was completely attenuated in the presence of APV but was unaffected by nifedipine. Thus S-TBS results in the selective induction of nmdaLTP. L-TBS resulted in LTP that was completely blocked by APV and only partially blocked by nifedipine. Therefore L-TBS results in a compoundLTP consisting of both nmdaLTP and vdccLTP components. In the presence of APV, HL-TBS resulted in vdccLTP, and when APV and nifedipine were both present, LTP was completely blocked. Thus HL-TBS results in a vdccLTP in isolation when APV is present. We also examined saturation of S-TBS-induced LTP (nmdaLTP) by applying S-TBS at short intervals. When nifedipine was present, multiple S-TBS trains resulted in a substantially smaller final LTP as compared with controls. We conclude that multiple bursts of S-TBS eventually summate to result in compoundLTP. Stimuli patterned after innate rhythms in the hippocampus effectively induce nmdaLTP (S-TBS), compoundLTP (L-TBS), or vdccLTP (HL-TBS).

Effect of long-term potentiation induction on gamma-band electroencephalograms in prefrontal cortex following stimulation of rat hippocampus in vivo

We examined whether long-term potentiation (LTP) affects cortical gamma-band electroencephalograms (EEG) in the hippocampo-prefrontal cortex (PFC) pathway of anesthetized rats. The LTP induction increased the evoked PFC gamma-band EEG power (40-100 Hz) to 120-135% at 500-700 ms after test stimulation. A simple increment of stimulus intensity, instead of LTP induction, did not reveal this evoked increase. Neither LTP induction nor the intensity increment changed significantly the magnitude of an evoked decrease at around 100 ms or the spontaneous prestimulation gamma-band power. These results indicate that LTP in PFC specifically increases the evoked gamma-band EEG power, which may reflect a phasic mode of plastic neurotransmission through the hippocampo-PFC pathway in vivo.

Mouse hippocampo-prefrontal paired-pulse facilitation and long-term potentiation in vivo

To confirm neural plasticity of the mouse hippocampo-prefrontal cortex (PFC) pathway, paired-pulse facilitation (PPF) and long-term potentiation (LTP) induction were determined in the pathway. In addition, we tested whether the plasticity differs in projections of the pathway from the dorsal (upper) and ventral (lower) parts of the temporal hippocampus. The results showed PPF and LTP of this pathway, and these differed between the projections. The projection from the upper part showed stronger PPF and weaker LTP compared with that from the lower part. These results suggest that the mouse hippocampo-PFC pathway is involved in learning and memory, and contains projections related to different functions.

Hippocampo-prefrontal cortex pathway: anatomical and electrophysiological characteristics

The hippocampus, the prefrontal cortex, and interconnected neural circuits are implicated in several aspects of cognitive and memory processes. The present review is dedicated to the description of the anatomo-functional characteristics of the hippocampo-prefrontal pathway and related neuronal circuits in the rat. This pathway, which originates from the hippocampal CA1/subiculum fields, innervates the prelimbic/medial orbital areas of the prefrontal cortex (PL/MO). Its synaptic influence on cortical pyramidal neurons consists in an early monosynaptic excitation followed by an inhibition and, in some cases, a late excitation. These later effects are likely due to the subsequent activation of the local cortical network. PL/MO areas and the CA1/subiculum both send projections to the nucleus accumbens, a region of the ventral striatum which is particularly implicated in goal-directed behavior. Therefore, emphasis is placed on respective projections from PL/MO areas and from the CA1/subiculum on the "core" and the "shell" regions of the nucleus accumbens, as well as on their interconnected circuits. Signals which are directed to the prefrontal cortex through these circuits might modulate hippocampo-prefrontal inputs. Finally, the direct and/or indirect relationships of the hippocampus, prefrontal cortex, and nucleus accumbens with the ventral tegmental area/substantia nigra pars compacta complex (VTA/SNC) (where dopamine neurons are located) will also be described, because these neurons are known to modulate synaptic transmission and plasticity in their target structures and to play a fundamental role in motivational processes.

Plasticity at hippocampal to prefrontal cortex synapses: dual roles in working memory and consolidation

The involvement of the hippocampus and the prefrontal cortex in cognitive processes and particularly in learning and memory has been known for a long time. However, the specific role of the projection which connects these two structures has remained elusive. The existence of a direct monosynaptic pathway from the ventral CA1 region of the hippocampus and subiculum to specific areas of the prefrontal cortex provides a useful model for conceptualizing the functional operations of hippocampal-prefrontal cortex communication in learning and memory. It is known now that hippocampal to prefrontal cortex synapses are modifiable synapses and can express different forms of plasticity, including long-term potentiation, long-term depression, and depotentiation. Here we review these findings and focus on recent studies that start to relate synaptic plasticity in the hippocampo-prefrontal cortex pathway to two specific aspects of learning and memory, i.e., the consolidation of information and working memory. The available evidence suggests that functional interactions between the hippocampus and prefrontal cortex in cognition and memory are more complex than previously anticipated, with the possibility for bidirectional regulation of synaptic strength as a function of the specific demands of tasks.

Comparative induction of long-term depression between dorsal and ventral hippocampal CA1 in the anesthetized rat

We studied whether a protocol reported as in vivo prefrontal long-term depression (LTD) induction protocol, also induced LTD in the anesthetized rat hippocampal CA1, and whether differences in LTD induction existed between dorsal and ventral CA1, by low-frequency stimulation (LFS) (1 Hz, 900) or low-frequency burst stimulation (LFBS) (5-pulse burst at 4 ms interpulse intervals at 1 Hz, 900), hippocampo-prefrontal LTD induction protocol. Though LFS failed to induce stable LTD in dorsal or ventral CA1, LFBS reliably induced LTD in the ventral not dorsal CA1. This similarity between ventral hippocampal and hippocampo-prefrontal LTD induction thus implies their serial integration process, ventral CA3-CA1-prefrontal cortex pathway and observed dorsal and ventral differences involved in behavioral functions such as learning.

Activation of the retrohippocampal region in the rat causes dopamine release in the nucleus accumbens: disruption by fornix section

There is a well-described projection from the retrohippocampus (subiculum and entorhinal cortex) to the nucleus accumbens that is involved in the control of psychomotor behaviour, and is implicated in the aetiology of schizophrenia. Cortical abnormalities are widely reported in the brains of schizophrenic patients, but it is unclear whether they are the cause or consequence of those changes in subcortical systems that are treated with neuroleptic drugs. We have, therefore, conducted a series of microdialysis experiments in anaesthetized rats to determine whether infusion of the excitotoxin, N-methyl-D-aspartate, into the retrohippocampus increases mesolimbic dopamine release. We found a clear and reproducible increase in extracellular dopamine in the nucleus accumbens following N-methyl-D-aspartate (2.5 microg), that was abolished when we sectioned the fimbria-fornix. Furthermore, inhibition of gamma-aminobutyric acid receptors following retrohippocampus administration of bicuculline (4 microg), also increased dopamine in the nucleus accumbens. The dopamine response to bicuculline was accompanied by an increase in dopamine metabolism, was long lasting, and also reduced by fornix section.The response to both N-methyl-D-aspartate and bicuculline depends on the integrity of the projection from the retrohippocampus to the nucleus accumbens. The results provide an underlying mechanism whereby a primary insult in the temporal cortex, caused by excessive N-methyl-D-aspartate receptor stimulation, can produce a hyperdopaminergic state. In addition, an increase in subiculo-accumbens activity evoked by bicuculline may also explain why patients with limbic epilepsy can develop a psychosis.

Long-term potentiation in the reciprocal corticohippocampal and corticocortical pathways in the chronically implanted, freely moving rat

The hippocampus and adjacent cortical structures, including the entorhinal, perirhinal, and parahippocampal cortices, appear to serve as an integrated memory system. This extended hippocampal system is believed to influence memory and consolidation through an extensive set of reciprocal connections with widespread areas of the neocortex. Long-term potentiation (LTP) has been well-examined in the intrinsic connections of the hippocampus and neocortex. However, LTP in the pathways and structures thought to convey information between the hippocampus and neocortex has received little attention. If these pathways and structures are involved in information storage, and if LTP reflects a general synaptic encoding mechanism, then these systems are also likely to support LTP. In this paper we discuss a series of experiments aimed at investigating LTP in the efferents between the hippocampus and neocortex in chronically implanted animals. In the first experiment, the efferents of the perirhinal cortex were stimulated. LTP in the dentate gyrus (DG) reached asymptote more slowly than is typically seen following perforant path stimulation, whereas the frontal area (M1) reached asymptote more quickly than reported following corticocortical stimulation. The DG and M1 LTP was long-lasting, but entorhinal cortex LTP had decayed to baseline levels after a week. In the second experiment, the hippocampal efferents were stimulated. The perirhinal, entorhinal, and frontal cortex showed a similar slow potentiation, with only the perirhinal cortex levels returning to baseline after a week. In the third experiment, the projections from M1 were tested. The perirhinal cortex and hippocampus showed a long-lasting LTP. Although LTP was found in all pathways examined, there were differences in the induction and decay rate, and these properties may correspond to differences in learning rate and longevity of information storage.