Medial Perforant Path Synapses Research Articles

The Amyloid Precursor Protein Regulates Synaptic Transmission at Medial Perforant Path Synapses.

The perforant path provides the primary cortical excitatory input to the hippocampus. Because of its important role in information processing and coding, entorhinal projections to the dentate gyrus have been studied in considerable detail. Nevertheless, synaptic transmission between individual connected pairs of entorhinal stellate cells and dentate granule cells remains to be characterized. Here, we have used mouse organotypic entorhino-hippocampal tissue cultures of either sex, in which the entorhinal cortex (EC) to dentate granule cell (GC; EC-GC) projection is present, and EC-GC pairs can be studied using whole-cell patch-clamp recordings. By using cultures of wild-type mice, the properties of EC-GC synapses formed by afferents from the lateral and medial entorhinal cortex were compared, and differences in short-term plasticity were identified. As the perforant path is severely affected in Alzheimer's disease, we used tissue cultures of amyloid precursor protein (APP)-deficient mice to examine the role of APP at this synapse. APP deficiency altered excitatory neurotransmission at medial perforant path synapses, which was accompanied by transcriptomic and ultrastructural changes. Moreover, presynaptic but not postsynaptic APP deletion through the local injection of Cre-expressing adeno-associated viruses in conditional APPflox/flox tissue cultures increased the neurotransmission efficacy at perforant path synapses. In summary, these data suggest a physiological role for presynaptic APP at medial perforant path synapses that may be adversely affected under altered APP processing conditions.SIGNIFICANCE STATEMENT The hippocampus receives input from the entorhinal cortex via the perforant path. These projections to hippocampal dentate granule cells are of utmost importance for learning and memory formation. Although there is detailed knowledge about perforant path projections, the functional synaptic properties at the level of individual connected pairs of neurons are not well understood. In this study, we investigated the role of APP in mediating functional properties and transmission rules in individually connected neurons using paired whole-cell patch-clamp recordings and genetic tools in organotypic tissue cultures. Our results show that presynaptic APP expression limits excitatory neurotransmission via the perforant path, which could be compromised in pathologic conditions such as Alzheimer's disease.

Prolonged development of long-term potentiation at lateral entorhinal cortex synapses onto adult-born neurons.

Critical period plasticity at adult-born neuron synapses is widely believed to contribute to the learning and memory functions of the hippocampus. Experience regulates circuit integration and for a transient interval, until cells are ~6 weeks old, new neurons display enhanced long-term potentiation (LTP) at afferent and efferent synapses. Since neurogenesis declines substantially with age, this raises questions about the extent of lasting plasticity offered by adult-born neurons. Notably, however, the hippocampus receives sensory information from two major cortical pathways. Broadly speaking, the medial entorhinal cortex conveys spatial information to the hippocampus via the medial perforant path (MPP), and the lateral entorhinal cortex, via the lateral perforant path (LPP), codes for the cues and items that make experiences unique. While enhanced critical period plasticity at MPP synapses is relatively well characterized, no studies have examined long-term plasticity at LPP synapses onto adult-born neurons, even though the lateral entorhinal cortex is uniquely vulnerable to aging and Alzheimer's pathology. We therefore investigated LTP at LPP inputs both within (4-6 weeks) and beyond (8+ weeks) the traditional critical period. At immature stages, adult-born neurons did not undergo significant LTP at LPP synapses, and often displayed long-term depression after theta burst stimulation. However, over the course of 3-4 months, adult-born neurons displayed increasingly greater amounts of LTP. Analyses of short-term plasticity point towards a presynaptic mechanism, where transmitter release probability declines as cells mature, providing a greater dynamic range for strengthening synapses. Collectively, our findings identify a novel form of new neuron plasticity that develops over an extended interval, and may therefore be relevant for maintaining cognitive function in aging.

Hippocampal subfield‐specific Homer1a expression is triggered by learning‐facilitated long‐term potentiation and long‐term depression at medial perforant path synapses

Learning about general aspects, or content details, of space results in differentiated neuronal information encoding within the proximodistal axis of the hippocampus. These processes are tightly linked to long-term potentiation (LTP) and long-term depression (LTD). Here, we explored the precise sites of encoding of synaptic plasticity in the hippocampus that are mediated by information throughput from the perforant path. We assessed nuclear Homer1a-expression that was triggered by electrophysiological induction of short and long forms of hippocampal synaptic plasticity, and compared it to Homer1a-expression that was triggered by LTP and LTD enabled by different forms of spatial learning. Plasticity responses were induced by patterned stimulation of the perforant path and were recorded in the dentate gyrus (DG) of freely behaving rats. We used fluorescence in situ hybridization to detect experience-dependent nuclear encoding of Homer1a in proximodistal hippocampal subfields. Induction of neither STP nor STD resulted in immediate early gene (IEG) encoding. Electrophysiological induction of robust LTP, or LTD, resulted in highly significant and widespread induction of nuclear Homer1a in all hippocampal subfields. LTP that was facilitated by novel spatial exploration triggered similar widespread Homer1a-expression. The coupling of synaptic depression with the exploration of a novel configuration of landmarks resulted in localized IEG expression in the proximal CA3 region and the lower (infrapyramidal) blade of the DG. Our findings support that synaptic plasticity induction via perforant path inputs promotes widespread hippocampal information encoding. Furthermore, novel spatial exploration promotes the selection of a hippocampal neuronal network by means of LTP that is distributed in an experience-dependent manner across all hippocampus subfields. This network may be modified during spatial content learning by LTD in specific hippocampal subfields. Thus, long-term plasticity-inducing events result in IEG expression that supports establishment and/or restructuring of neuronal networks that are necessary for long-term information storage.

Long-term potentiation expands information content of hippocampal dentate gyrus synapses

An approach combining signal detection theory and precise 3D reconstructions from serial section electron microscopy (3DEM) was used to investigate synaptic plasticity and information storage capacity at medial perforant path synapses in adult hippocampal dentate gyrus in vivo. Induction of long-term potentiation (LTP) markedly increased the frequencies of both small and large spines measured 30 minutes later. This bidirectional expansion resulted in heterosynaptic counterbalancing of total synaptic area per unit length of granule cell dendrite. Control hemispheres exhibited 6.5 distinct spine sizes for 2.7 bits of storage capacity while LTP resulted in 12.9 distinct spine sizes (3.7 bits). In contrast, control hippocampal CA1 synapses exhibited 4.7 bits with much greater synaptic precision than either control or potentiated dentate gyrus synapses. Thus, synaptic plasticity altered total capacity, yet hippocampal subregions differed dramatically in their synaptic information storage capacity, reflecting their diverse functions and activation histories.

Redistribution of Ionotropic Glutamate Receptors Detected by Laser Microdissection of the Rat Dentate Gyrus 48 h following LTP Induction In Vivo

The persistence and input specificity of long-term potentiation (LTP) make it attractive as a mechanism of information storage. In its initial phase, both in vivo and in vitro studies have shown that LTP is associated with increased membrane localization of AMPA receptor subunits, but the molecular basis of LTP maintenance over the long-term is still unclear. We have previously shown that expression of AMPA and NMDA receptor subunits is elevated in whole homogenates prepared from dentate gyrus 48 h after LTP induction in vivo. In the present study, we utilized laser microdissection (LMD) techniques to determine whether AMPA and NMDA receptor upregulation occurs specifically in the stimulated regions of the dentate gyrus dendritic arbor. Receptor proteins GluN1, GluA1 and GluA2, as well as postsynaptic density protein of 95 kDa and tubulin were detected by Western blot analysis in microdissected samples. Gradients of expression were observed for GluN1 and GluA2, decreasing from the inner to the outer zones of the molecular layer, and were independent of LTP. When induced at medial perforant path synapses, LTP was associated with an apparent specific redistribution of GluA1 and GluN1 to the middle molecular layer that contains these synapses. These data indicate that glutamate receptor proteins are delivered specifically to dendritic regions possessing LTP-expressing synapses, and that these changes are preserved for at least 48 h.

Glycogen Synthase Kinase-3 Inhibitors Reverse Deficits in Long-term Potentiation and Cognition in Fragile X Mice

Identifying feasible therapeutic interventions is crucial for ameliorating the intellectual disability and other afflictions of fragile X syndrome (FXS), the most common inherited cause of intellectual disability and autism. Hippocampal glycogen synthase kinase-3 (GSK3) is hyperactive in the mouse model of FXS (FX mice), and hyperactive GSK3 promotes locomotor hyperactivity and audiogenic seizure susceptibility in FX mice, raising the possibility that specific GSK3 inhibitors may improve cognitive processes. We tested if specific GSK3 inhibitors improve deficits in N-methyl-D-aspartate receptor-dependent long-term potentiation at medial perforant path synapses onto dentate granule cells and dentate gyrus-dependent cognitive behavioral tasks. GSK3 inhibitors completely rescued deficits in long-term potentiation at medial perforant path-dentate granule cells synapses in FX mice. Furthermore, synaptosomes from the dentate gyrus of FX mice displayed decreased inhibitory serine-phosphorylation of GSK3β compared with wild-type littermates. The potential therapeutic utility of GSK3 inhibitors was further tested on dentate gyrus-dependent cognitive behaviors. In vivo administration of GSK3 inhibitors completely reversed impairments in several cognitive tasks in FX mice, including novel object detection, coordinate and categorical spatial processing, and temporal ordering for visual objects. These findings establish that synaptic plasticity and cognitive deficits in FX mice can be improved by intervention with inhibitors of GSK3, which may prove therapeutically beneficial in FXS.

Plasticity of NMDA receptor‐mediated excitatory postsynaptic currents at perforant path inputs to dendrite‐targeting interneurons

Synaptic plasticity of NMDA receptors (NMDARs) has been recently described in a number of brain regions and we have previously characterised LTP and LTD of glutamatergic NMDA receptor-mediated EPSCs (NMDAR-EPSCs) in granule cells of dentate gyrus. The functional significance of NMDAR plasticity at perforant path synapses on hippocampal network activity depends on whether this is a common feature of perforant path synapses on all postsynaptic target cells or if this plasticity occurs only at synapses on principal cells. We recorded NMDAR-EPSCs at medial perforant path synapses on interneurons in dentate gyrus which had significantly slower decay kinetics compared to those recorded in granule cells. NMDAR pharmacology in interneurons was consistent with expression of both GluN2B- and GluN2D-containing receptors. In contrast to previously described high frequency stimulation-induced bidirectional plasticity of NMDAR-EPSCs in granule cells, only LTD of NMDAR-EPSCs was induced in interneurons in our standard experimental conditions. In interneurons, LTD of NMDAR-EPSCs was associated with a loss of sensitivity to a GluN2D-selective antagonist and was inhibited by the actin stabilising agent, jasplakinolide. While LTP of NMDAR-EPSCs can be readily induced in granule cells, this form of plasticity was only observed in interneurons when extracellular calcium was increased above physiological concentrations during HFS or when PKC was directly activated by phorbol ester, suggesting that opposing forms of plasticity at inputs to interneurons and principal cells may act to regulate granule cell dendritic integration and processing.

Cooperation and competition between lateral and medial perforant path synapses in the dentate gyrus

It has been suggested that non-spatial and spatial pieces of information are transmitted to the dentate gyrus from entorhinal cortex layer II through the lateral and medial perforant paths (LPP and MPP), which establish synapses on granule cell dendrites in the outer and middle one-thirds of the dentate molecular layer, respectively. In the present paper, we first investigated cooperation and competition between MPP and LPP synapses being subject to STDP rules, using a four-compartmental granule cell model. MPP and LPP were stimulated simultaneously by periodic and random pulse trains, respectively. Both synapses were gradually enhanced by cooperation between those synapses in the early stage, and then either the MPP or the LPP synapse was rapidly enhanced through synaptic competition in the following stage, depending on their initial synaptic conductances. The dominant cause of synaptic competition is that the distance between the MPP synapse and the soma is shorter than that between the LPP synapse and the soma. These results suggest that the LPP and MPP synapses tend to be enhanced in the dentate supra- and infrapyramidal blades, respectively, taking account of the thickness of each of the LPP and MPP fiber laminae in the blades. The dentate gyrus may select spatial and non-spatial pieces of information through synaptic cooperation, and may open a gate for each piece of information through synaptic competition. Then we investigated the role of inhibitory local circuits in synaptic competition in the dentate gyrus. The feed-forward GABA(B) inhibition suppressed unusual high-frequency firing of the granule cell, and consequently prevented excessive synaptic depression due to synaptic competition through STDP. The feed-forward and feedback GABA(A) inhibitions tend to reduce synaptic conductance fluctuations resulting from large increments and decrements due to very small spike-timings happening occasionally.

Mineralocorticoid receptor activation restores medial perforant path LTP in diabetic rats

In the hippocampus, glucocorticoids bind to two types of receptors: the mineralocorticoid receptor, which binds corticosterone with high affinity and is tonically occupied; and the glucocorticoid receptor, which is occupied during stress and at certain phases in the circadian cycle. Diabetes mellitus increases levels of glucocorticoids in both humans and animal models. To explore the contributions of hippocampal corticosteroid receptors to the diabetes-induced suppression of neuroplasticity, we manipulated these receptors in hippocampal slices from streptozocin-diabetic rats, a model of Type 1 diabetes mellitus. STZ-diabetes reduced long-term potentiation (LTP) at medial perforant path synapses in the dentate gyrus, and induced a bias in favor of long-term depression following intermediate stimulation frequencies. Bath application of the mineralocorticoid receptor agonist aldosterone restored LTP in slices from diabetic animals. These results suggest additional mechanisms for diabetes-induced functional alterations and support a restorative role for dentate gyrus mineralocorticoid receptors.

Anti-Glucocorticoid Gene Therapy Reverses the Impairing Effects of Elevated Corticosterone on Spatial Memory, Hippocampal Neuronal Excitability, and Synaptic Plasticity

Moderate release of the major stress hormones, glucocorticoids (GCs), improves hippocampal function and memory. In contrast, excessive or prolonged elevations produce impairments. Enzymatic degradation and reformation of GCs help to maintain optimal levels within target tissues, including the brain. We hypothesized that expressing a GC-degrading enzyme in hippocampal neurons would attenuate the negative impact of an excessive elevation in GC levels on synaptic physiology and spatial memory. We tested this by expressing 11-beta-hydroxysteroid dehydrogenase (type II) in dentate gyrus granule cells during a 3 d GC treatment followed by examination of synaptic responses in hippocampal slices or spatial performance in the Morris water maze. In adrenalectomized rats with basal GC replacement, additional GC treatments for 3 d reduced synaptic strength and promoted the expression of long-term depression at medial perforant path synapses, increased granule cell and CA1 pyramidal cell excitability, and impaired spatial reference memory (without influencing learning). Expression of 11-beta-hydroxysteroid dehydrogenase (type II), mostly in mature dentate gyrus granule cells, reversed the effects of high GC levels on granule cell and pyramidal cell excitability, perforant path synaptic plasticity, and spatial memory. These data demonstrate the ability of neuroprotective gene expression limited to a specific cell population to both locally and trans-synaptically offset neurophysiological disruptions produced by prolonged increases in circulating stress hormones. This report supplies the first physiological explanation for previously demonstrated cognitive sparing by anti-stress gene therapy approaches and lends additional insight into the hippocampal processes that are important for memory.

Enduring Medial Perforant Path Short-Term Synaptic Depression at High Pressure

The high pressure neurological syndrome develops during deep-diving (>1.1 MPa) involving impairment of cognitive functions, alteration of synaptic transmission and increased excitability in cortico-hippocampal areas. The medial perforant path (MPP), connecting entorhinal cortex with the hippocampal formation, displays synaptic frequency-dependent-depression (FDD) under normal conditions. Synaptic FDD is essential for specific functions of various neuronal networks. We used rat cortico-hippocampal slices and computer simulations for studying the effects of pressure and its interaction with extracellular Ca2+ ([Ca2+]o) on FDD at the MPP synapses. At atmospheric pressure, high [Ca2+]o (4–6 mM) saturated single MPP field EPSP (fEPSP) and increased FDD in response to short trains at 50 Hz. High pressure (HP; 10.1 MPa) depressed single fEPSPs by 50%. Increasing [Ca2+]o to 4 mM at HP saturated synaptic response at a subnormal level (only 20% recovery of single fEPSPs), but generated a FDD similar to atmospheric pressure. Mathematical model analysis of the fractions of synaptic resources used by each fEPSP during trains (normalized to their maximum) and the total fraction utilized within a train indicate that HP depresses synaptic activity also by reducing synaptic resources. This data suggest that MPP synapses may be modulated, in addition to depression of single events, by reduction of synaptic resources and then may have the ability to conserve their dynamic properties under different conditions.

Epileptogenesis Is Associated With Enhanced Glutamatergic Transmission in the Perforant Path

The perforant path provides the main excitatory input into the hippocampus and has been proposed to play a critical role in the generation of temporal lobe seizures. It has been hypothesized that changes in glutamatergic transmission in this pathway promote the epileptogenic process and seizure generation. We therefore asked whether epileptogenesis is associated with enhanced glutamatergic transmission from the perforant path to dentate granule cells. We used a rat model of temporal lobe epilepsy in which spontaneous seizures occur after an episode of pilocarpine-induced status epilepticus. Whole cell patch-clamp recordings were obtained from dentate granule cells in hippocampal slices from control and epileptic animals 3 wk after pilocarpine-induced status epilepticus. The paired pulse ratio of perforant path-evoked AMPA receptor-mediated excitatory postsynaptic currents (EPSCs) was reduced in tissue obtained from epileptic rats. This is consistent with an increase in release probability. N-methyl-d-aspartate (NMDA) receptor-mediated EPSCs were also prolonged. This prolongation could not be accounted for by decreased activity of glutamate transporters or by a change in NMDA receptor subunit composition in dentate granule cells, implying a change in NMDA receptor kinetics. This change in NMDA receptor kinetics was associated with the emergence of significant synaptic cross-talk, detected as a use-dependent block of receptors activated by medial perforant path synapses after lateral perforant path stimulation in MK-801. Enhanced glutamatergic transmission and the emergence of cross-talk among perforant path-dentate granule cell synapses may contribute to lowering seizure threshold.

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.

Comparative morphometric analysis of lateral and medial perforant path synapses in the dentate gyrus of adult rats

Comparative morphometric analysis of lateral and medial perforant path synapses in the dentate gyrus of adult rats

Differential involvement of group II and group III mGluRs as autoreceptors at lateral and medial perforant path synapses

1. Previous reports have shown that group III metabotropic glutamate receptors (mGluRs) serve as autoreceptors at the lateral perforant path, but to date there has been no rigorous determination of the roles of other mGluRs as autoreceptors at this synapse. Furthermore, it is not known which of the mGluR subtypes serve as autoreceptors at the medial perforant path synapse. With the use of whole cell patch-clamp and field excitatory postsynaptic potential (fEPSP) recording techniques, we examined the groups of mGluRs that act as autoreceptors at lateral and medial perforant path synapses in adult rat hippocampal slices. 2. Consistent with previous reports, the group III mGluR agonist (D,L)-2-amino-4-phosphonobutyric acid reduced fEPSPs and excitatory postsynaptic currents (EPSCs) in the dentate gyrus. However, the group-II-selective agonist (2S,1'R,2'R,3'R)-2-(2,3-dicarboxycyclopropyl)glycine (DCG-IV) also reduced fEPSPs and EPSCs, suggesting that multiple mGluR subtypes may serve as autoreceptors at perforant path synapses. 3. Selective activation of either medial or lateral perforant pathways revealed that micromolar concentrations of (L)-2-amino-4-phosphonobutyric acid (L-AP4) reduce fEPSPs in lateral but not medial perforant path, suggesting group III involvement at the lateral perforant pathway. Conversely, DCG-IV and 2R, 4R-4-aminopyrrolidine-2,4-dicarboxylate, another group-II-selective mGluR agonist, potently reduced fEPSPs at the medial but not lateral perforant path, suggesting that a group II mGluR may act as an autoreceptor at the medial perforant path-dentate gyrus synapse. 4. Antagonist studies with group-selective antagonists such as (2S,3S,4S)-2-methyl-2-(carboxycyclpropyl)glycine (MCCG; group II) and alpha-methyl-L-AP4 (MAP4; group III) suggest differential involvement of each group at these synapses. The effect of L-AP4 at the lateral perforant path synapse was blocked by MAP-4, but not MCCG. In contrast, the effect of DCG-IV was blocked by application of MCCG, but not MAP4. 5. Previous studies suggest that the effect of L-AP4 at the lateral perforant path synapse is mediated by a presynaptic mechanism. In the present studies, we found that concentrations of DCG-IV that reduce transmission at the medial perforant path synapse reduce paired-pulse depression and do not reduce kainate-evoked currents recorded from dentate granule cells. This is consistent with the hypothesis that DCG-IV also acts by a presynaptic mechanism.

Opioid receptor-dependent long-term potentiation at the lateral perforant path-CA3 synapse in rat hippocampus

The involvement of opioid receptors in the induction of long-term potentiation (LTP) was investigated in the lateral and medial perforant path projections to area CA3 of the hippocampus in anesthetized rats. The opioid receptor antagonist naloxone (10 nmol), applied to the hippocampal CA3 region 10 min prior to tetanization, blocked the induction lateral perforant path-CA3 LTP induced by high-frequency stimulation. By contrast, LTP induction in medial perforant path-CA3 was not attenuated by a 10 nmol quantity of naloxone. (+)-Naloxone (10 nmol), the inactive stereoisomer of naloxone, was without effect on the induction of lateral perforant path-CA3 LTP. Naloxone applied l h following LTP induction did not reverse established lateral perforant path-CA3 LTP, indicating that opioid receptors are involved in the induction but not the maintenance of LTP in this pathway. LTP of medial perforant path responses developed immediately, while LTP of lateral perforant path responses was slow to develop. The latter pattern is similar to the time course of the development of LTP observed at the mossy fiber-CA3 synapse and suggests that lateral and medial perforant path synapses may use distinct mechanisms of both induction and expression of LTP. These data extend previous findings demonstrating opioid receptor-dependent mechanisms of LTP induction at both the mossy fiber-CA3 synapse and the lateral perforant path-dentate gyms synapse. We suggest that lateral perforant path and mossy fiber synapses may utilize similar, opioid receptor-dependent, mechanisms of LTP induction and expression.

Muscarinic depression of synaptic transmission and blockade of norepinephrine-induced long-lasting potentiation in the dentate gyrus

Bath application of the muscarinic receptor agonist, muscarine, produced a concentration-dependent depression of synaptic activity in the dentate gyrus of hippocampal slices. A concentration of 10 μM muscarine produced a reversible depression that could be competitively antagonized by the muscarinic receptor antagonist pirenzepine. However, other muscarinic receptor subtype (M1-M3) antagonists could also block the effects of muscarine. The rank order of antagonist potency was: 4-diphenylacetoxy- N-methyl-piperidine methiodide (M3/M1 antagonist) > pirenzepine (M1) > AFDX-116 (M2). The depression produced by 10 μM muscarine was not affected by in vivo pretreatment with pertussis toxin, and therefore was not mediated by a pertussis toxin-sensitive G-protein. In addition, high concentrations of muscarine did not affect either basal or isoproterenol-stimulated accumulation of cyclic AMP from slices of dentate gyrus. Muscarine also produced a concentration-dependent blockade of the induction of norepinephrine-induced long-lasting potentiation in the dentate gyrus. Norepinephrine-induced long-lasting potentiation is a form of long-lasting plasticity induced in medial perforant path synapses by β-adrenergic agonists such as isoproterenol. The muscarinic blockade of norepinephrine-induced long-lasting potentiation was also prevented by pretreatment with pirenzepine. Based on these pharmacological data, we conclude that muscarinic depression of evoked responses, as well as blockade of norepinephrine-induced long-lasting potentiation, involves activation of either M3 or M1, but not M2, muscarinic receptors. These data also demonstrate that in addition to modulating normal synaptic transmission, muscarinic receptors may also play an important role in modulating synaptic plasticity.