Low-frequency stimulation induces long-term depression and slow onset long-term potentiation at perforant path-dentate gyrus synapses in vivo.

The expression of homosynaptic long-term depression (LTD) is thought to mediate a crucial role in sustaining memory function. Our in vivo investigations of LTD expression at lateral (LPP) and medial perforant path (MPP) synapses in the dentate gyrus (DG) corroborate prior demonstrations that PP-DG LTD is difficult to induce in intact animals. In freely moving animals, LTD expression occurred inconsistently among LPP-DG and MPP-DG responses. Interestingly, following acute electrode implantation in anesthetized rats, low-frequency stimulation (LFS; 900 pulses, 1 Hz) promotes slow-onset LTP at both MPP-DG and LPP-DG synapses that utilize distinct induction mechanisms. Systemic administration of the N-methyl-d-aspartate (NMDA) receptor antagonist (+/-)-cyclopiperidine-6-piperiperenzine (CPP; 10 mg/kg) 90 min before LFS selectively blocked MPP-DG but not LPP-DG slow onset LTP, suggesting MPP-DG synapses express a NMDA receptor-dependent slow onset LTP whereas LPP-DG slow onset LTP induction is NMDA receptor independent. In experiments where paired-pulse LFS (900 paired pulses, 200-ms paired-pulse interval) was used to induce LTD, paired-pulse LFS of the LPP resulted in rapid onset LTP of DG responses, whereas paired-pulse LFS of the MPP induced slow onset LTP of DG responses. Although LTD observations were very rare following acute electrode implantation in anesthetized rats, LPP-DG LTD was demonstrated in some anesthetized rats with previously implanted electrodes. Together, our data indicate in vivo PP-DG LTD expression is an inconsistent phenomenon that is primarily observed in recovered animals, suggesting perturbation of the dentate through surgery-related tissue trauma influences both LTD incidence and LTP induction at PP-DG synapses in vivo.

Endogenous opioid peptides contribute to associative LTP in the hippocampal CA3 region.

The medial and lateral perforant path projections to the hippocampal CA3 region display distinct mechanisms of long-term potentiation (LTP) induction, N-methyl-d-aspartate (NMDA) and opioid receptor dependent, respectively. However, medial and lateral perforant path projections to the CA3 region display associative LTP with coactivation, suggesting that while they differ in receptors involved in LTP induction they may share common downstream mechanisms of LTP induction. Here we address this interaction of LTP induction mechanisms by evaluating the contribution of opioid receptors to the induction of associative LTP among the medial and lateral perforant path projections to the CA3 region in vivo. Local application of the opioid receptor antagonists naloxone or Cys2-Tyr3-Orn5-Pen7-amide (CTOP) normally block induction of lateral perforant path-CA3 LTP. However, these opioid receptor antagonists failed to block associative LTP in lateral perforant path-CA3 synapses when it was induced by strong coactivation of the medial perforant pathway which displays NMDAR-dependent LTP. Thus strong activation of non-opioidergic afferents can substitute for the opioid receptor activation required for lateral perforant path LTP induction. Conversely, medial perforant path-CA3 associative LTP was blocked by opioid receptor antagonists when induced by strong coactivation of the opioidergic lateral perforant path. These data indicate endogenous opioid peptides contribute to associative LTP at coactive synapses when induced by strong coactivation of an opioidergic afferent system. These data further suggest that associative LTP induction is regulated by the receptor mechanisms of the strongly stimulated pathway. Thus, while medial and lateral perforant path synapses differ in their mechanisms of LTP induction, associative LTP at these synapses share common downstream mechanisms of induction.

Modulation of CA3 afferent inputs by novelty and theta rhythm.

Models of hippocampal function suggest that the modulation of CA3 afferent input during theta rhythm allows for a rapid alternation between encoding and retrieval states, with each phase enhancing either extrinsic or intrinsic CA3 afferents, favoring either encoding or retrieval, respectively. Here, we show that during the initial exploration of a novel environment by rats, intrinsic CA3-CA3 synaptic inputs are attenuated on CA3 theta peaks, favoring extrinsic CA3 inputs, whereas extrinsic perforant path-CA3 synaptic inputs are attenuated on CA3 theta troughs, favoring intrinsic CA3 inputs. This modulation is absent when animals are re-exposed to the same environment 2 or 48 h later and thus habituates with familiarity, suggesting a process involved in learning. Modulation of CA3 synaptic inputs during novelty was blocked by atropine at a dose that blocks type 2 theta rhythm. Re-exposure to the same novel environment 48 h later in the absence of atropine did not result in habituation, but instead modulated CA3 synaptic responses as though the environment were novel and explored for the first time. The NMDA receptor antagonist (+/-)-3-(2-carboxypiperazin-4-yl)propyl-1-phosphonic acid (CPP), administered in a dose that blocks long-term potentiation induction, did not alter CA3 synaptic modulation during initial exploration. However, like atropine, CPP blocked the habituation of synaptic modulation normally observed with re-exposure, as though the environment were novel and explored for the first time. Thus, as predicted theoretically, recurrent and cortical CA3 afferents are differentially modulated during phases of theta rhythm. This modulation is atropine sensitive and habituates in an NMDA receptor-dependent manner, suggesting an NMDA receptor-dependent process that, in conjunction with theta rhythm, contributes to encoding of novel information in the hippocampus.

NMDA receptor antagonists sustain LTP and spatial memory: active processes mediate LTP decay.

Although long-term potentiation (LTP) is long-lasting, it is not permanent and decays within weeks after its induction. Little is known about the processes underlying this decay. Here we assessed the contribution of synaptic activity to LTP decay by determining the effect of the competitive NMDA receptor antagonist CPP on the decay of perforant path-dentate LTP. CPP blocked decay over a one-week period when administered daily following the induction of LTP, and blocked decay of the late, protein-synthesis-dependent phase of LTP when administered two days after LTP induction. CPP administered for a five-day period following spatial memory training enhanced subsequent memory retention. These data suggest that LTP is normally a persistent process that is actively reversed by NMDA receptor activation, and that both the early and late phases of LTP are dynamic processes regulated by NMDA receptors. These data also support the view that LTP is involved in maintaining spatial memory.

Plastic processes in the dentate gyrus: a computational perspective.

The dentate gyrus has the capacity for numerous types of synaptic plasticity that use diverse mechanisms and are thought essential for the storage of information in the hippocampus. Here we review the various forms of synaptic plasticity that involve afferents and efferents of the dentate gyrus, and, from a computational perspective, relate how these plastic processes might contribute to sparse, orthogonal encoding, and the selective recall of information within the hippocampus.

Long-term Potentiation at Temporoammonic Path-CA1 Synapses in Freely Moving Rats.

Hippocampal area CA1 receives direct entorhinal layer III input via the temporoammonic path (TAP) and recent studies implicate TAP-CA1 synapses are important for some aspects of hippocampal memory function. Nonetheless, as few studies have examined TAP-CA1 synaptic plasticity in vivo, the induction and longevity of TAP-CA1 long-term potentiation (LTP) has not been fully characterized. We analyzed CA1 responses following stimulation of the medial aspect of the angular bundle and investigated LTP at medial temporoammonic path (mTAP)-CA1 synapses in freely moving rats. We demonstrate monosynaptic mTAP-CA1 responses can be isolated in vivo as evidenced by observations of independent current sinks in the stratum lacunosum moleculare of both areas CA1 and CA3 following angular bundle stimulation. Contrasting prior indications that TAP input rarely elicits CA1 discharge, we observed mTAP-CA1 responses that appeared to contain putative population spikes in 40% of our behaving animals. Theta burst high frequency stimulation of mTAP afferents resulted in an input specific and N-methyl-D-aspartate (NMDA) receptor-dependent LTP of mTAP-CA1 responses in behaving animals. LTP of mTAP-CA1 responses decayed as a function of two exponential decay curves with time constants (τ) of 2.7 and 148 days to decay 63.2% of maximal LTP. In contrast, mTAP-CA1 population spike potentiation longevity demonstrated a τ of 9.6 days. To our knowledge, these studies provide the first description of mTAP-CA1 LTP longevity in vivo. These data indicate TAP input to area CA1 is a physiologically relevant afferent system that displays robust synaptic plasticity.

Identification of upregulated SCG10 mRNA expression associated with late-phase long-term potentiation in the rat hippocampal Schaffer-CA1 pathway in vivo.

The maintenance of long-term potentiation (LTP) depends on alteration of gene transcription. By screening a subtracted cDNA library that is enriched in upregulated transcripts in rat hippocampus 3 hr after Schaffer-CA1 LTP induction in vivo, we identified a neural growth-associated protein SCG10 (superior cervical ganglia clone 10) gene. The semiquantitative reverse transcription-PCR and Northern blot experiments confirmed that SCG10 mRNA levels were elevated in tetanized rat hippocampi compared with those of sham controls that received only low-frequency stimulation. Both 1 and 2 kb forms of SCG10 mRNAs contributed to the increased expression. Using a riboprobe with a sequence specific to the 3'-untranslated region of rat SCG10 mRNA, in situ hybridization further revealed a significant increase of the SCG10 mRNA 2 kb form in the ipsilateral CA3 and CA1 regions of LTP animals. In addition, we systemically injected the competitive NMDA receptor antagonist d,l-3[(+/-)-2-carboxypiperazine-4-yl]-propyl-1-phosphonic acid (CPP) to determine whether the alteration of SCG10 expression depends on NMDA receptor activation or tetanus alone. Administration of CPP 1 hr before tetanus completely blocked LTP induction and the increase of SCG10 mRNA levels. Thus, these results suggest that the transcription of SCG10 in vivo is regulated by long-lasting synaptic activity and may contribute to the maintenance of long-term synaptic plasticity via a presynaptic remodeling mechanism.

Novel environments enhance the induction and maintenance of long-term potentiation in the dentate gyrus.

The induction of long-term potentiation (LTP) in the hippocampal formation can be modulated by different behavioral states. However, few studies have addressed modulation of LTP during behavioral states in which the animal is likely acquiring new information. Here, we demonstrate that both the induction and the longevity of LTP in the dentate gyrus are enhanced when LTP is induced during the initial exploration of a novel environment. These effects are independent from locomotor activity, changes in brain temperature, and theta rhythm. Previous exposure to the novel environment attenuated this enhancement, suggesting that the effects of novelty habituate with familiarity. LTP longevity also was enhanced when induced in familiar environments containing novel objects. Together, these data indicate that both LTP induction and maintenance are enhanced when LTP is induced while rats investigate novel stimuli. We suggest that novelty initiates a transition of the hippocampal formation to a mode that is particularly conducive to synaptic plasticity, a process that could allow for new learning while preserving the stability of previously stored information. In addition, LTP induced in novel environments elicited a sustained late LTP. This suggests that a single synaptic population can display distinct profiles of LTP maintenance and that this depends on the animal's behavioral state during its induction. Furthermore, the duration of LTP enhanced by novelty parallels the time period during which the hippocampal formation is thought necessary for memory, consistent with the view that dentate LTP is of a duration sufficient to sustain memory in the hippocampal formation.

NMDA receptor antagonists block heterosynaptic long-term depression (LTD) but not long-term potentiation (LTP) in the CA3 region following lateral perforant path stimulation.

High-frequency stimulation of lateral perforant path is accompanied by a heterosynaptic long-term depression (LTD) of medial perforant path synaptic responses in both the dentate gyrus and the CA3 region of the hippocampus. We reported previously that LTP induction at lateral perforant path-CA3 synapses is unaffected by NMDA antagonists. However, it is not known if heterosynaptic LTD that is observed in the CA3 region following lateral perforant path stimulation also is independent from NMDA receptors. We address this question in anesthetized adult rats using systemic administration of the competitive NMDA receptor antagonist CPP. Induction of lateral perforant path-CA3 LTP produced a sustained heterosynaptic depression of medial perforant path-CA3 responses. Systemic administration of CPP (10 mg/kg) was ineffective in blocking the induction of LTP at lateral perforant path-CA3 responses. However, heterosynaptic LTD of medial perforant path-CA3 responses was blocked completely by CPP. These data indicate that NMDA receptors are not required for the induction of lateral perforant path-CA3 LTP, but are involved in the induction of heterosynaptic LTD that accompanies lateral perforant path activity. The requirement for NMDA receptors for heterosynaptic LTD suggests one functional role of NMDA receptors at termination fields of the lateral perforant path.

Time-course study of SCG10 mRNA levels associated with LTP induction and maintenance in the rat Schaffer-CA1 pathway in vivo.

The maintenance of long-term potentiation (LTP) depends on altered gene expression. Previously, we found the expression of neuronal growth associated protein SCG10, which is involved in neurite outgrowth and neural regeneration, was up-regulated by LTP induction in the rat hippocampal Schaffer-collateral CA1 pathway. Here we studied the temporal expression pattern of SCG10 mRNA after LTP induction using permanently implanted electrodes in the same CA1 pathway. The real-time RT-PCR showed that both SCG10 mRNA 1 and 2 kb forms were increased at the 3 h, but not at 1 or 24 h. In situ hybridization revealed an increase of SCG10 2 kb mRNA level in ipsilateral CA3 and CA1 areas, but not their contralateral counterparts or either side of dentate gyrus. These results suggest that SCG10 may play a role in the maintenance of synaptic plasticity through a transient regulation of microtubule dynamics, which facilitates the structural remodeling of the presynaptic element during the consolidation period.

Associative long-term potentiation (LTP) among extrinsic afferents of the hippocampal CA3 region in vivo.

Monosynaptic perforant path projections to the CA3 region of the hippocampus are anatomically and physiologically substantial pathways that relay cortical input directly to the hippocampus proper. Despite the suggested relevance of these direct pathways in models of information processing within the CA3 region, surprisingly few studies have characterized synaptic plasticity in these direct cortical projections to the CA3 region. We assessed the ability of perforant path projections, and commissural/associational projections to the hippocampal CA3 region to both induce or display associative LTP in vivo. In pentobarbital-anesthetized adult rats, trains delivered to either the medial or lateral perforant pathway at current intensities normally insufficient to induce LTP displayed associative LTP when these same trains were delivered in conjunction with high-intensity trains to the alternate perforant pathway. Similarly, associative LTP is induced at intrinsic commissural/associational-CA3 (C/A-CA3) synapses when weak C/A trains were delivered in conjunction with high-intensity trains to either the medial or lateral perforant pathway. Associative LTP also was observed at medial and lateral perforant path-CA3 synapses when weak perforant path trains were tetanized in conjunction with high-intensity trains delivered to C/A-CA3 synapses. Thus direct perforant path-CA3 synapses and commissural/associational-CA3 synapses can modify and be modified by other CA3 afferents in an associative manner, verifying a requirement for synaptic plasticity explicit in models of autoassociative information processing in the CA3 region.

5-HT1a receptor antagonists block perforant path-dentate LTP induced in novel, but not familiar, environments.

Numerous studies suggest roles for monoamines in modulating long-term potentiation (LTP). Previously, we reported that both induction and maintenance of perforant path-dentate gyrus LTP is enhanced when induced while animals explore novel environments. Here we investigate the contribution of serotonin and 5-HT1a receptors to the novelty-mediated enhancement of LTP. In freely moving animals, systemic administration of the selective 5-HT1a antagonist WAY-100635 (WAY) attenuated LTP in a dose-dependent manner when LTP was induced while animals explored novel cages. In contrast, LTP was completely unaffected by WAY when induced in familiar environments. LTP was also blocked in anesthetized animals by direct application of WAY to the dentate gyrus, but not to the median raphe nucleus (MRN), suggesting the effect of systemic WAY is mediated by a block of dentate 5-HT1a receptors. Paradoxically, systemic administration of the 5-HT1a agonist 8-OH-DPAT also attenuated LTP. This attenuation was mimicked in anesthetized animals following application of 8-OH-DPAT to the MRN, but not the dentate gyrus. In addition, application of a 5-HT1a agonist to the dentate gyrus reduced somatic GABAergic inhibition. Because serotonergic projections from the MRN terminate on dentate inhibitory interneurons, these data suggest 5-HT1a receptors contribute to LTP induction via inhibition of GABAergic interneurons. Moreover, activation of raphe 5-HT1a autoreceptors, which inhibits serotonin release, attenuated LTP induction even in familiar environments. This suggests that serotonin normally contributes to dentate LTP induction in a variety of behavioral states. Together, these data suggest that serotonin and dentate 5-HT1a receptors play a permissive role in dentate LTP induction, particularly in novel conditions, and presumably, during the encoding of novel, hippocampus-relevant information.

In vivo recordings of long-term potentiation and long-term depression in the dentate gyrus of the neonatal rat.

Previous in vitro studies demonstrated that long-term potentiation (LTP) could be elicited at medial perforant path (MPP) synapses onto hippocampal granule cells in slices from 7-day-old rats. In contrast, in vivo studies suggested that LTP at perforant path synapses could not be induced until at least days 9 or 10 and then in only a small percentage of animals. Because several characteristics of the oldest granule cells are adult-like on day 7, we re-examined the possibility of eliciting LTP in 7-day-old rats in vivo. We also recorded from 8- and 9-day-old rats to further elucidate the occurrence and magnitude of LTP in neonates. With halothane anesthesia, all animals in each age group exhibited synaptic plasticity of the excitatory postsynaptic potential following high-frequency stimulation of the MPP. In 7-day-old rats, LTP was elicited in 40% of the animals and had an average magnitude of 143%. Long-term depression (LTD) alone (magnitude of 84%) was induced in 40% of the animals, while short-term potentiation (STP) alone (magnitude of 123%) was induced in 10%. STP followed by LTD was elicited in the remaining 10%. Data were similar for all ages combined. In addition, the N-methyl-d-aspartate (NMDA) antagonist (R,S)-3-(2-carboxypiperazin-4-yl)propyl-1-phosphonic acid (CPP) blocked the occurrence of LTP at each age and doubled the percentage of animals expressing LTD alone for all ages combined. These results demonstrate that tetanic stimulation can elicit LTP or LTD at MPP synapses in 7-day-old rats, supporting our premise that at least a portion of the dentate gyrus is functional at this early age.

Long-term potentiation in direct perforant path projections to the hippocampal CA3 region in vivo.

The perforant path constitutes the primary projection system relaying information from the neocortex to the hippocampal formation. Long-term synaptic potentiation (LTP) in the perforant path projections to the dentate gyrus is well characterized. However, surprisingly few studies have addressed the mechanisms underlying LTP induction in the direct perforant path projections to the hippocampus. Here we investigate the role of N-methyl-D-aspartate (NMDA) and opioid receptors in the induction of LTP in monosynaptic medial and lateral perforant path projections to the CA3 region in adult pentobarbital sodium-anesthetized rats. Similar to LTP observed at the medial perforant path-dentate gyrus synapse, medial perforant path-CA3 synapses display LTP that is blocked by both local and systemic administration of the competitive NMDA receptor antagonist (+/-)-3-(2-carboxypiperazin-4-yl) propyl-1-phosphonic acid [(+/-)-CPP]. By contrast, LTP induced at the lateral perforant path-CA3 synapses is not blocked by either local or systemic administration of this NMDA receptor antagonist. The induction of LTP at lateral perforant path-CA3 synapses, which is blocked by the opioid receptor antagonist naloxone, is also blocked by the selective mu opioid receptor antagonist Cys(2)-Tyr(3)-Orn(5)-Pen(7)-amide (CTOP), but not the selective delta opioid receptor antagonist naltrindole (NTI). CTOP was without effect on the induction of medial perforant path-CA3 LTP. The selective sensitivity of lateral perforant path-CA3 LTP to mu-opioid receptor antagonists corresponds with the distribution of mu-opioid receptors within the stratum lacunosum-moleculare of area CA3 where perforant path projections to CA3 terminate. These data indicate that both lateral and medial perforant path projections to the CA3 region display LTP, and that LTP induction in medial and lateral perforant path-CA3 synapses are differentially sensitive to NMDA receptor and mu-opioid receptor antagonists. This suggests a role for opioid, but not NMDA receptors in the induction of LTP at lateral perforant path projections to the hippocampal formation.