Effects of endogenous and exogenous D1/D5 dopamine receptor activation on LTP in ventral and dorsal CA1 hippocampal synapses

Hippocampus is importantly involved in dopamine‐dependent behaviors and dopamine is a significant modulator of synaptic plasticity in the hippocampus. Moreover, the dopaminergic innervation appears to be disproportionally segregated along the hippocampal longitudinal (dorsoventral) axis with unknown consequences for synaptic plasticity. In this study we examined the actions of endogenously released dopamine and the effects of exogenous D1/D5 dopamine receptor agonists on theta‐burst stimulation‐induced long‐term potentiation (LTP) of field excitatory synaptic potential (fEPSP) at Schaffer collateral‐CA1 synapses in slices from dorsal (DH) and ventral hippocampus (VH). Furthermore, we quantified D1 receptor mRNA and protein expression levels in DH and VH. We found that blockade of D1/D5 receptors by SCH 23390 (20 μM) significantly reduced the magnitude of LTP in both DH and VH similarly suggesting that dopamine endogenously released during TBS, presumably mimicking low activity of DA neurons, exerts a homogeneous modulation of LTP along the hippocampal long axis. Moderate to high concentrations of the selective partial D1/D5 receptor agonist SKF 38393 (50‐150 μM) did not significantly change LTP in either hippocampal segment. However, the full D1 receptor selective agonist SKF 82958 (10 μM) significantly enhanced LTP in VH but not DH. Furthermore, the expression of D1 receptor mRNA and protein was considerably higher in VH compared with DH. These results suggest that the dynamic range of D1/D5 receptor‐mediated dopamine effects on LTP may be higher in VH than DH and that VH may be specialized to acquire information about behaviorally relevant strong stimuli signaled by the dopamine system.     

Critical Role of Presynaptic Nicotinic ACh Receptor in the Formation of Long-Term Potentiation: Implications for Development of Anti-Dementia Drugs

Background : Since neuronal nicotinic ACh receptors are involved in the cognitive function, they have been studied as a target of anti‐dementia drugs. The present study was designed to understand the role of nicotinic ACh receptors in the expression of long‐term potentiation (LTP), a cellular model of learning and memory. Methods : The ultrastructural localization of neuronal nicotinic ACh receptors in the rat hippocampus was examined electron‐immunohistochemically using an antibody against the α7 subunit, forming a brain‐type nicotinic ACh receptor. Miniature excitatory postsynaptic currents (mEPSCs) were monitored in cultured rat hippocampal neurons. Schaffer collateral‐CA1 LTP and perforant path LTP were analyzed by recording field excitatory postsynaptic potentials (fEPSPs) and population spikes (PSs) in the CA1 region and the dentate gyrus of rat hippocampal slices or in the intact mouse hippocampus. Results : α7 receptors are preferentially localized on presynaptic terminals, where the receptors are employed in the release of the excitatory neurotransmitter, glutamate. The probability of LTP development was markedly reduced in the presence of the neuronal nicotinic ACh receptor antagonists, α‐bungarotoxin and mecamylamine, in both the CA1 region and the dentate gyrus of rat hippocampal slices. Perforant path LTP was never induced in slices with selective cholinergic denervation using 192 IgG‐saporin, while it was not affected by atropine, a selective muscarinic ACh receptor antagonist, in normal slices. Nicotine facilitated hippocampal neurotransmission with the saturation occluding the potentiation induced by tetanic stimulation, and vice versa. A similar occlusion was also obtained with an intact mouse hippocampus. These types of LTP, which are dependent upon N‐methyl‐D‐aspartate (NMDA) receptors, were still induced by treatment with nicotine in the presence of D‐2‐amino‐5‐phosphonovaleric acid (APV), a selective NMDA receptor antagonist. Conclusion : The results of the present study suggest that presynaptic nicotinic ACh receptors play a critical role as a target of retrograde messengers in the formation of NMDA receptor‐dependent LTP. This may account for the involvement of nicotinic ACh receptors in cognitive function. Drugs enhancing the activity of neuronal nicotinic ACh receptors, therefore, are capable of expressing LTP, conversely, ameliorating dementia.     

Intrinsic connectivity of the rat subiculum: II. Properties of synchronous spontaneous activity and a demonstration of multiple generator regions.

Brain structures that can generate epileptiform activity possess excitatory interconnections among principal cells and a subset of these neurons that can be spontaneously active ("pacemaker" cells). We describe electrophysiological evidence for excitatory interactions among rat subicular neurons. Subiculum was isolated from presubiculum, CA1, and entorhinal cortex in ventral horizontal slices. Nominally zero magnesium perfusate, picrotoxin (100 microM), or NMDA (20 microM) was used to induce spontaneous firing in subicular neurons. Synchronous population activity and the spread of population events from one end of subiculum to the other in isolated subicular subslices indicate that subicular pyramidal neurons are coupled together by excitatory synapses. Both electrophysiological classes of subicular pyramidal cells (bursting and regular spiking) exhibited synchronous activity, indicating that both cell classes are targets of local excitatory inputs. Burst firing neurons were active in the absence of synchronous activity in field recordings, indicating that these cells may serve as pacemaker neurons for the generation of epileptiform activity in subiculum. Epileptiform events could originate at either proximal or distal segments of the subiculum from ventral horizontal slices. In some slices, events originated in both proximal and distal locations and propagated to the other location. Finally, propagation was supported over axonal paths through the cell layer and in the apical dendritic zone. We conclude that subicular burst firing and regular spiking neurons are coupled by means of glutamatergic synapses. These connections may serve to distribute activity driven by topographically organized inputs and to synchronize subicular cell activity.     

Presynaptic Ultrastructural Plasticity Along CA3→CA1 Axons During Long‐Term Potentiation in Mature Hippocampus

In area CA1 of the mature hippocampus, synaptogenesis occurs within 30 minutes after the induction of long‐term potentiation (LTP); however, by 2 hours many small dendritic spines are lost, and those remaining have larger synapses. Little is known, however, about associated changes in presynaptic vesicles and axonal boutons. Axons in CA1 stratum radiatum were evaluated with 3D reconstructions from serial section electron microscopy at 30 minutes and 2 hours after induction of LTP by theta‐burst stimulation (TBS). The frequency of axonal boutons with a single postsynaptic partner was decreased by 33% at 2 hours, corresponding perfectly to the 33% loss specifically of small dendritic spines (head diameters <0.45 μm). Docked vesicles were reduced at 30 minutes and then returned to control levels by 2 hours following induction of LTP. By 2 hours there were fewer small synaptic vesicles overall in the presynaptic vesicle pool. Clathrin‐mediated endocytosis was used as a marker of local activity, and axonal boutons containing clathrin‐coated pits showed a more pronounced decrease in presynaptic vesicles at both 30 minutes and 2 hours after induction of LTP relative to control values. Putative transport packets, identified as a cluster of less than 10 axonal vesicles occurring between synaptic boutons, were stable at 30 minutes but markedly reduced by 2 hours after the induction of LTP. APV blocked these effects, suggesting that the loss of axonal boutons and presynaptic vesicles was dependent on N‐methyl‐D‐aspartic acid (NMDA) receptor activation during LTP. These findings show that specific presynaptic ultrastructural changes complement postsynaptic ultrastructural plasticity during LTP. J. Comp. Neurol. 521:3898–3912, 2013. © 2013 Wiley Periodicals, Inc.     

The contribution of dopamine to the functioning of the hippocampus during spatial learning (a hypothetical mechanism)

A hypothetical mechanism for the influence of dopamine on the formation of neuronal representations of “object–place” associations in the hippocampus is proposed for spatial learning. According to this mechanism, dopamine that is released in a new situation or during expectation of reinforcement improves conditions for the development of homosynaptic long-term potentiation (LTP) of the input to the dentate gyrus granule cells from the medial entorhinal cortex, which transmits information about spatial location of objects and characteristics of objects. The effect occurs due to the activation of D1/D5 receptors on granule cells and D2 receptors on inhibitory interneurons. Heterosynaptic depression is simultaneously developed in inputs that were not activated. As a result, a contrasting representation of the learned “object–place” association is formed on neurons of the dentate gyrus. From these neurons, information about the association via the CA3 field is transmitted to the radial layer of the CA1 field by Schaffer collaterals, whereas the stratum lacunosum-moleculare receives signals directly from the entorhinal cortex and thalamic nucleus reuniens, which connects the hippocampus with the prefrontal cortex. The sign of the modulatory influence of dopamine on the efficacy of excitatory inputs to pyramid neurons of the CA1 field depends on the relationship between excitation and inhibition of these neurons, as well as the dopamine concentration. By acting on D1/D5 receptors on the pyramidal neurons of the CA1 field, dopamine can promote LTP induction in Schaffer collaterals simultaneously with LTP induction in the relatively strong perforant input, whereas relatively weak perforant input, as well as the input from the nucleus reuniens, become depressed. This depression is promoted by the activation of D1/D5 receptors on the inhibitory interneurons of the CA1 field, induction of LTP in these neurons, and the following enhancement of afferent inhibition of pyramidal cells. As a consequence, neuronal representation of the learned “object–place” association in the CA1 field is distorted more weakly by non-relevant information that comes from the entorhinal cortex and thalamus. As a result, the error probability during the performance of spatial task decreases. Because activation of D1/D5 receptors on pyramidal neurons of the prefrontal cortex promotes LTP induction in the input from the CA1 field, dopamine must improve the goal-directed performance of spatial tasks. The proposed mechanism explains the results of some experimental studies that seemed to be contradictory or incomprehensible.     

Dopamine D1/D5 receptor activation fails to initiate an activity-independent late-phase LTP in rat hippocampus

The role of dopamine in the hippocampus remains poorly defined. Numerous studies have suggested that it acts as a neuromodulator of late-phase long-term potentiation (L-LTP) in CA1, while other reports controversially indicate that D1/D5 receptor (D1/D5R) activation may directly initiate activity-independent LTP. We have further investigated this putative role of dopamine in area CA1 in rat hippocampal slices using field potential recording techniques. Application of the dopamine D1/D5 receptor agonists SKF 38393 and 6-bromo-APB at 100 microM for 20 min did not induce an activity-independent L-LTP. Varying the incubation conditions still did not permit either SKF 38393 or an alternative D1/D5R agonist, 6-chloro-PB, to induce L-LTP. To further determine if intracellular mechanisms, which may act to limit the expression of LTP, were preventing D1/D5R-induced L-LTP expression, we inhibited protein phosphatase 1 activity by reducing cyclin-dependent kinase 5 (cdk5) inhibition of inhibitor 1. Inhibition of cdk5 by roscovitine (10 microM, 40 min) did not facilitate the ability of SKF 38393 to induce L-LTP in CA1. Biochemical experiments confirmed that the concentration of agonist used significantly elevated intracellular cAMP levels, suggesting that effective D1/D5R activation was achieved. Furthermore, coactivation with NMDA receptors (NMDAR) resulted in a synergistic increase in cAMP. These findings demonstrate that D1/D5R activation in CA1 initiates intracellular second messenger accumulation, but that this is insufficient to induce an activity-independent L-LTP.     

NMDA Receptor-dependent Long-term Potentiation in the Hippocampal Afferent Fibre System to the Prefrontal Cortex in the Rat

This study investigated the role of the N -methyl- d -aspartate (NMDA) subtype of glutamate receptor in the induction of long-term potentiation (LTP) in the hippocampal-prefrontal cortex pathway in vivo. Field potentials evoked by electrical stimulation of the CA1/subicular region were recorded in the prelimbic area of the prefrontal cortex under continuous perfusion of artificial cerebrospinal fluid in anaesthetized rats. High-frequency stimulation of the CA1/subicular region induced LTP of the evoked response in the prelimbic area of the prefrontal cortex. LTP was completely blocked when the selective NMDA receptor antagonist d -(-)2-amino-5- phosphonopentanoic acid ( d -AP5; 200 μM), was perfused during the tetanus. Perfusion of D-AP5 did not affect normal transmission or pre-established LTP. These results demonstrate that induction of LTP in the hippocampal-prefrontal cortex pathway is an NMDA receptor-dependent process.     

Dopamine modulation of excitatory currents in the striatum is dictated by the expression of D1 or D2 receptors and modified by endocannabinoids

Striatal medium‐sized spiny neurons (MSSNs) receive glutamatergic inputs modulated presynaptically and postsynaptically by dopamine. Mice expressing the gene for enhanced green fluorescent protein as a reporter gene to identify MSSNs containing D1 or D2 receptor subtypes were used to examine dopamine modulation of spontaneous excitatory postsynaptic currents (sEPSCs) in slices and postsynaptic N‐methyl‐d ‐aspartate (NMDA) and α‐amino‐3‐hydroxy‐5‐methyl‐4‐isoxazolepropionic acid (AMPA) currents in acutely isolated cells. The results demonstrated dopamine receptor‐specific modulation of sEPSCs. Dopamine and D1 agonists increased sEPSC frequency in D1 receptor‐expressing MSSNs (D1 cells), whereas dopamine and D2 agonists decreased sEPSC frequency in D2 receptor‐expressing MSSNs (D2 cells). These effects were fully (D1 cells) or partially (D2 cells) mediated through retrograde signaling via endocannabinoids. A cannabinoid 1 receptor (CB1R) agonist and a blocker of anandamide transporter prevented the D1 receptor‐mediated increase in sEPSC frequency in D1 cells, whereas a CB1R antagonist partially blocked the decrease in sEPSC frequency in D2 cells. At the postsynaptic level, low concentrations of a D1 receptor agonist consistently increased NMDA and AMPA currents in acutely isolated D1 cells, whereas a D2 receptor agonist decreased these currents in acutely isolated D2 cells. These results show that both glutamate release and postsynaptic excitatory currents are regulated in opposite directions by activation of D1 or D2 receptors. The direction of this regulation is also specific to D1 and D2 cells. We suggest that activation of postsynaptic dopamine receptors controls endocannabinoid mobilization, acting on presynaptic CB1Rs, thus modulating glutamate release differently in glutamate terminals projecting to D1 and D2 cells.     

Effects of asenapine on prefrontal N‐methyl‐D‐aspartate receptor‐mediated transmission: Involvement of dopamine D1 receptors

Asenapine is a novel psychopharmacologic agent being developed for schizophrenia and bipolar disorder. Like clozapine, asenapine facilitates cortical dopaminergic and N‐methyl‐D ‐aspartate (NMDA) receptor‐mediated transmission in rats. The facilitation of NMDA‐induced currents in cortical pyramidal cells by clozapine is dependent on dopamine and D₁ receptor activation. Moreover, previous results show that clozapine prevents and reverses the blockade of NMDA‐induced currents and firing activity in the pyramidal cells by the noncompetitive NMDA receptor antagonist phencyclidine (PCP). Here, we investigated the effects of asenapine in these regards using intracellular electrophysiological recording in vitro. Asenapine (5 nM) significantly facilitated NMDA‐induced currents (162 ± 15% of control) in pyramidal cells of the medial prefrontal cortex (mPFC). The asenapine‐induced facilitation was blocked by the D₁ receptor antagonist SCH23390 (1 μM). Furthermore, the PCP‐induced blockade of cortical NMDA‐induced currents was effectively reversed by 5 nM asenapine. Our results demonstrate a clozapine‐like facilitation of cortical NMDA‐induced currents by asenapine that involves prefrontal dopamine and activation of D₁ receptors. Asenapine and clozapine also share the ability to reverse functional PCP‐induced hypoactivity of cortical NMDA receptors. The ability of asenapine to increase both cortical dopaminergic and NMDA receptor‐mediated glutamatergic transmission suggests that this drug may have an advantageous effect not only on positive symptoms in patients with schizophrenia, but also on negative and cognitive symptoms. Synapse 64:870–874, 2010. © 2010 Wiley‐Liss, Inc.     

Galantamine enhancement of long‐term potentiation is mediated by calcium/calmodulin‐dependent protein kinase II and protein kinase C activation

Galantamine, a novel Alzheimer's drug, is known to inhibit acetylcholinesterase activity and potentiate nicotinic acetylcholine receptor (nAChR) in the brain. We previously reported that galantamine potentiates the NMDA‐induced currents in primary cultured rat cortical neurons. We now studied the effects of galantamine on long‐term potentiation (LTP) in the rat hippocampal CA1 regions. The field excitatory postsynaptic potentials (fEPSPs) were induced by stimulation of the Schaffer collateral/commissural pathways in the hippocampal CA1 region. Treatment with 0.01–10 μM galantamine did not affect the slope of fEPSPs in the CA1 region. Galantamine treatment increased calcium/calmodulin‐dependent protein kinase II (CaMKII) and protein kinase Cα (PKCα) activities with a bell‐shaped dose–response curve peaked at 1 μM, thereby increasing the phosphorylation of AMPA receptor, myristoylated alanine‐rich protein kinase C, and NMDA receptor as downstream substrates of CaMKII and/or PKCα. By contrast, galatamine treatment did not affect protein kinase A activity. Consistent with the bell‐shaped CaMKII and PKCα activation, galantamine treatment enhanced LTP in the hippocampal CA1 regions with the same bell‐shaped dose–response curve. Furthermore, LTP potentiation induced by galantamine treatment at 1 μM was closely associated with both CaMKII and PKC activation with concomitant increase in phosphorylation of their downstream substrates except for synapsin I. In addition, the enhancement of LTP by galantamine was accompanied with α7‐type nAChR activation. These results suggest that galantamine potentiates NMDA receptor‐dependent LTP through α7‐type nAChR activation, by which the postsynaptic CaMKII and PKC are activated. © 2009 Wiley‐Liss, Inc.     

Long-term potentiation of single subicular neurons in mice

Subicular neurons receive direct afferent connections from the vast majority of CA1 pyramidal cells and send their axons to the various brain areas. Because of this strategic position, subicular cells can modulate output of the hippocampus and, thus, play a significant part in memory, spatial processing, and seizure amplification and propagation from the hippocampus. Despite its important role as a hippocampal interface with different brain regions, present knowledge of the subiculum and the plastic properties of the synapses on the subicular neurons is rather limited. By using IR-DIC videomicroscopy and whole-cell patch-clamp recordings in mouse hippocampal slices, I demonstrated that long-term potentiation (LTP) in CA1-subicular cell synapses can be readily induced by high-frequency stimulation (HFS) of the afferents, but not by pairing of low-frequency stimulation with depolarization of postsynaptic cells. This tetanus-induced LTP is input specific, insensitive to the N-methyl-D-aspartate (NMDA) receptor antagonist 3-[(R)-2Carboxipiperazin-4-yl]-propyl-1-phosphonic acid (R-CPP), and reduces paired-pulse facilitation in potentiated synapses. Subsequent morphologic analysis of the recorded cells, which were filled either with Lucifer Yellow or Biocytin, revealed pyramidal-shaped neurons localized predominantly in the deep layer of the subiculum, close to the CA1 border. Axons of the majority of these neurons extended to the alveus and on toward the hippocampus, probably exiting it via the fornix. These data indicate that CA1-subicular cell synapses in mice exhibit LTP, which can be expressed presynaptically, and its induction does not require NMDA-receptor activation. The observed activity-dependent plasticity might play an important role in the integrative mechanisms of the subiculum and may influence transfer of information from the hippocampus to subcortical and cortical brain areas.     

Antagonism of D1/D5 receptors prevents long‐term depression (LTD) and learning‐facilitated LTD at the perforant path–dentate gyrus synapse in freely behaving rats

Hippocampal synaptic plasticity, in the form of long‐term potentiation (LTP) and long‐term depression (LTD), enables spatial memory formation, whereby LTP and LTD are likely to contribute different elements to the resulting spatial representation. Dopamine, released from the ventral tegmental area particularly under conditions of reward, acts on the hippocampus, and may specifically influence the encoding of information into long‐term memory. The dentate gyrus (DG), as the “gateway” to the hippocampus is likely to play an important role in this process. D1/D5 dopamine receptors are importantly involved in the regulation of synaptic plasticity thresholds in the CA1 region of the hippocampus and determine the direction of change in synaptic strength that occurs during novel spatial learning. Here, we explored whether D1/D5‐receptors influence LTD that is induced in the DG following patterned afferent stimulation of the perforant path of freely behaving adult rats, or influence LTD that occurs in association with spatial learning. We found that LTD that is induced by afferent stimulation, and LTD that is facilitated by learning about novel landmark configurations, were both prevented by D1/D5‐receptor antagonism, whereas agonist activation of the D1/D5‐receptor had no effect on basal tonus or short‐term depression. Other studies have reported that in the DG, D1/D5‐receptor agonism or antagonism do not affect LTP, but agonism prevents depotentiation. These findings suggest that the dopaminergic system, acting via D1/D5‐receptors, influences information gating by the DG and modulates the direction of change in synaptic strength that underlies information storage in this hippocampal substructure. Information encoded by robust forms of LTD is especially dependent on D1/D5‐receptor activation. Thus, dopamine acting on D1/D5‐receptors is likely to support specific experience‐dependent encoding, and may influence the content of hippocampal representations of experience. © 2014 The Authors. Hippocampus Published by Wiley Periodicals, Inc.     

Long‐term depression of inhibitory synaptic transmission induced by spike‐timing dependent plasticity requires coactivation of endocannabinoid and muscarinic receptors

The precise timing of pre‐postsynaptic activity is vital for the induction of long‐term potentiation (LTP) or depression (LTD) at many central synapses. We show in synapses of rat CA1 pyramidal neurons in vitro that spike timing dependent plasticity (STDP) protocols that induce LTP at glutamatergic synapses can evoke LTD of inhibitory postsynaptic currents or STDP‐iLTD. The STDP‐iLTD requires a postsynaptic Ca²⁺ increase, a release of endocannabinoids (eCBs), the activation of type‐1 endocananabinoid receptors and presynaptic muscarinic receptors that mediate a decreased probability of GABA release. In contrast, the STDP‐iLTD is independent of the activation of nicotinic receptors, GABA_BRs and G protein‐coupled postsynaptic receptors at pyramidal neurons. We determine that the downregulation of presynaptic Cyclic adenosine monophosphate/protein Kinase A pathways is essential for the induction of STDP‐iLTD. These results suggest a novel mechanism by which the activation of cholinergic neurons and retrograde signaling by eCBs can modulate the efficacy of GABAergic synaptic transmission in ways that may contribute to information processing and storage in the hippocampus. © 2013 Wiley Periodicals, Inc.     

A grease-gap method for studying the excitatory amino acid pharmacology of CA1 hippocampal pyramidal cells

A grease-gap method for studying the pharmacology of CA1 hippocampal pyramidal cells was developed with use of rat hippocampal slices that included only area CA1 and the retrohippocampal area. These slices were transferred to a two-compartment superfusion chamber and the pyramidal cell bodies in area CA1 were separated from their axons in the subiculum with a grease barrier. The CA1 pyramidal cells were depolarized relative to their axons by superfusion with N-methyl-D-aspartate (NMDA), (RS)-alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA), kainate and L-glutamate. NMDA was unusually potent in the CA1-subiculum slice compared to other preparations. The NMDA receptor antagonists D(-)-2-amino-5-phosphonovalerate (D-AP5), phencyclidine and Mg2+ shifted the NMDA dose-response curve to the right in a parallel manner. Similarly, the quisqualate receptor antagonist pentobarbitone shifted the AMPA dose-response curve to the right. Schild plots for these antagonists had slopes insignificantly different from 1. These results are consistent with the presence of a substantial NMDA receptor reserve on CA1 pyramidal cells. They are also in line with the high density of excitatory amino acid receptors on CA1 hippocampal pyramidal cells and with the known pharmacological properties of these receptors. Grease-gap studies on the CA1-subiculum slice fill the need for a means of obtaining quantitative pharmacological data on CA1 pyramidal cells.     

Frequency-Dependent Synaptic Dynamics Differentially Tune CA1 and CA2 Pyramidal Neuron Responses to Cortical Input

Entorhinal cortex neurons make monosynaptic connections onto distal apical dendrites of CA1 and CA2 pyramidal neurons through the perforant path (PP) projection. Previous studies show that differences in dendritic properties and synaptic input density enable the PP inputs to produce a much stronger excitation of CA2 compared with CA1 pyramidal neurons. Here, using mice of both sexes, we report that the difference in PP efficacy varies substantially as a function of presynaptic firing rate. Although a single PP stimulus evokes a 5- to 6-fold greater EPSP in CA2 compared with CA1, a brief high-frequency train of PP stimuli evokes a strongly facilitating postsynaptic response in CA1, with relatively little change in CA2. Furthermore, we demonstrate that blockade of NMDARs significantly reduces strong temporal summation in CA1 but has little impact on that in CA2. As a result of the differences in the frequency- and NMDAR-dependent temporal summation, naturalistic patterns of presynaptic activity evoke CA1 and CA2 responses with distinct dynamics, differentially tuning CA1 and CA2 responses to bursts of presynaptic firing versus single presynaptic spikes, respectively.SIGNIFICANCE STATEMENT Recent studies have demonstrated that abundant entorhinal cortical innervation and efficient dendritic propagation enable hippocampal CA2 pyramidal neurons to produce robust excitation evoked by single cortical stimuli, compared with CA1. Here we uncovered, unexpectedly, that the difference in efficacy of cortical excitation varies substantially as a function of presynaptic firing rate. A burst of stimuli evokes a strongly facilitating response in CA1, but not in CA2. As a result, the postsynaptic response of CA1 and CA2 to presynaptic naturalistic firing displays contrasting temporal dynamics, which depends on the activation of NMDARs. Thus, whereas CA2 responds to single stimuli, CA1 is selectively recruited by bursts of cortical input.     

Immunohistochemical localization of N-methyl-D-aspartate receptor subunits in the adult murine hippocampal formation: evidence for a unique role of the NR2D subunit

NMDA receptors were immunopurified from adult mouse forebrain and screened by immunoblotting. NR1 was co-associated with NR2A, NR2B and NR2D but not NR2C, nor was NR2C detected in adult mouse hippocampal membranes. The anatomical distribution of NR1, 2A, 2B and 2D was mapped in the adult murine hippocampal formation. NR1-like immunoreactivity was localised to cell bodies of pyramidal neurons, granule cells and hilar cells of the dentate gyrus. Apical dendrites of the CA subfields and hilar cells were also immunopositive. NR2A- and NR2B-like immunoreactivity essentially co-localised with that of NR1 implying co-assembly of all three subunits in this brain structure. NR2D-like immunoreactivity was distinct, being totally excluded from pyramidal, granule and hilar cell bodies. Strong, punctate staining was restricted to the oriens layer of CA1 and the stratum lucidum of CA3 consistent with labelling of presynaptic receptors. Less intense staining was also observed in the internal third of the molecular layer of the dentate gyrus.     

Role of N-methyl-D-aspartate receptors in the induction of synaptic potentiation by burst stimulation patterned after the hippocampal theta-rhythm

Short bursts of high frequency stimulation produce maximal long-term potentiation (LTP) at Schaffer-commissural synapses on CA1 neurons in hippocampal slices when the bursts are spaced 200 ms apart. A burst to one input (S1) does not induce LTP but 'primes' the postsynaptic neurons such that 200 ms later the postsynaptic response to a burst to a second input (S2) is greatly enhanced and LTP is induced. The role of N-methyl-D-aspartate (NMDA) receptors in this response enhancement and LTP induction was studied by perfusing slices with the NMDA antagonist, 2-amino-5-phosphonovalerate (AP5). AP5 (100 microM) had no effect on the field excitatory postsynaptic potential evoked by single pulse stimulation, but completely eliminated both the decremental short-term potentiation (lasting less than 10 min) and stable LTP effects elicited by burst stimulation. AP5 reduced the response to a non-primed burst by about 10% and reduced the relative enhancement of a primed burst response by about 35%. These results indicate that part of the postsynaptic response to a primed burst is mediated by NMDA receptors and that this component is necessary for all forms of synaptic potentiation (including LTP) resulting from burst stimulation. The similarity of the short bursts with the complex-spike discharges of hippocampal neurons as well as the 200 ms optimal interval with the period of the hippocampal theta-rhythm suggest links between theta and the NMDA receptor in the induction of hippocampal synaptic plasticity.     

Inhibition of L-type voltage dependent calcium channels causes impairment of long-term potentiation in the hippocampal CA1 region in vivo

Long-term potentiation (LTP), in the hippocampal CA1 region is dependent on postsynaptic calcium influx. It is generally accepted that calcium influx occurs via activation of the NMDA receptor channel complex. However, studies in vitro using a high-frequency stimulus protocol (> or =200 Hz) demonstrated previously an NMDA receptor-independent form of LTP that is dependent upon activation of L-type voltage-dependent calcium channels (VDCCs). Here we have investigated a role for L-type VDCCs in LTP in vivo. Two structurally different, L-type VDCC blockers, verapamil (1, 3 and 10 mg/kg) and diltiazem (1, 10 and 20 mg/kg), depressed the induction of LTP in a dose-dependent manner. Increased activation of L-type VDCCs by Bay K 8644, an L-type agonist, however, did not enhance LTP. The NMDA receptor antagonist D-AP5 (5 and 20 mM injected i.c.v) impaired, but failed to block fully LTP in vivo. A reduced level of LTP could still be recorded following co-administration of verapamil and D-AP5. The level of LTP recorded was similar to that observed in the presence of either verapamil (10 mg/kg) or D-AP5 alone. These results suggest that activation of the NMDA receptor/channel and L-type VDCCs are involved in the induction of LTP in area CA1 in vivo. However, it appears that activation of other receptor/channels may also play a role in this form of LTP.     

Involvement of NMDA receptors and voltage-dependent calcium channels on augmentation of long-term potentiation in hippocampal CA1 area of morphine dependent rats

The involvement of NMDA receptors and voltage-dependent calcium channels on augmentation of long-term potentiation (LTP) was investigated at the Schaffer collateral–CA1 pyramidal cell synapses in hippocampal slices of morphine dependent rats, using primed-bursts tetanic stimulation. The amplitude of population spike was measured as an index of increase in postsynaptic excitability. d,l-AP5 and nifedipine were used as NMDA receptor antagonist and voltage-dependent calcium channel blocker, respectively. The amount of LTP of orthodromic population spike amplitude was higher in slices from dependent rats. Perfusion of slices from control or dependent rats with ACSF containing either d,l-AP5 (25 μM) or nifedipine (10 μM) and delivering tetanic stimulation, showed that d,l-AP5 completely blocked LTP of OPS in slices from both control and dependent rats, while nifedipine attenuated the amount of LTP of OPS in dependent slices and had no effect on control ones. The results suggest that the enhanced LTP of OPS in the CA1 area of hippocampal slices from morphine dependent rats is primarily induced by the NMDA receptors activity and the voltage-dependent calcium channels may also be partially involved in the phenomenon.