Stratum Radiatum Interneurons Research Articles

Dendritic nicotinic receptors modulate backpropagating action potentials and long‐term plasticity of interneurons

Stratum radiatum interneurons, unlike pyramidal cells, are rich in nicotinic acetylcholine receptors (nAChRs); however, the role of these receptors in plasticity has remained elusive. As opposed to previous physiological studies, we found that functional alpha7-subunit-containing nAChRs (alpha7-nAChRs) are abundant on interneuron dendrites of rats. Moreover, dendritic Ca2+ transients induced by activation of alpha7-nAChRs increase as a function of distance from soma. The activation of these extrasynaptic alpha7-nAChRs by cholinergic agonists either facilitated or depressed backpropagating action potentials, depending on the timing of alpha7-nAChR activation. We have previously shown that dendritic alpha7-nAChRs are involved in the regulation of synaptic transmission, suggesting that alpha7-nAChRs may play an important role in the regulation of the spike timing-dependent plasticity. Here we provide evidence that long-term potentiation is indeed boosted by stimulation of dendritic alpha7-nAChRs. Our results suggest a new mechanism for a cholinergic switch in memory encoding and retrieval.

Convergence of entorhinal and CA3 inputs onto pyramidal neurons and interneurons in hippocampal area CA1—An anatomical study in the rat

The entorhinal cortex (EC) conveys information to hippocampal field CA1 either directly by way of projections from principal neurons in layer III, or indirectly by axons from layer II via the dentate gyrus, CA3, and Schaffer collaterals. These two pathways differentially influence activity in CA1, yet conclusive evidence is lacking whether and to what extent they converge onto single CA1 neurons. Presently we studied such convergence. Different neuroanatomical tracers injected into layer III of EC and into CA3, respectively, tagged simultaneously the direct entorhino-hippocampal fibers and the indirect innervation of CA1 neurons by Schaffer collaterals. In slices of fixed brains we intracellularly filled CA1 pyramidal cells and interneurons in stratum lacunosum-moleculare (LM) and stratum radiatum (SR). Sections of these slices were scanned in a confocal laser scanning microscope. 3D-reconstruction was used to determine whether boutons of the labeled input fibers were in contact with the intracellularly filled neurons. We analyzed 12 pyramidal neurons and 21 interneurons. Perforant path innervation to pyramidal neurons in our material was observed to be denser than that from CA3. All pyramidal neurons and 17 of the interneurons received contacts of both perforant pathway and Schaffer input on their dendrites and cell bodies. Four interneurons, which were completely embedded in LM, received only labeled perforant pathway input. Thus, we found convergence of both projection systems on single CA1 pyramidal and interneurons with dendrites that access the layers where perforant pathway fibers and Schaffer collaterals end.

Age-dependent changes in the functional expression of two nicotinic receptor subtypes in CA1 stratum radiatum interneurons in the rat hippocampus

Protein density measurements and mRNA analysis have provided valuable information on age-dependent changes in the distribution of different nicotinic receptor (nAChR) subtypes in various areas of the rat brain, including the hippocampus. However, very little is known regarding the functional expression of nAChRs in individual neuron types at various ages. Likewise, there is paucity of information regarding the functional and pharmacological profile of nAChRs in the mature rat hippocampus. To address these issues, we used the whole-cell patch-clamp technique to record nicotinic responses from CA1 stratum radiatum (SR) interneurons in hippocampal slices from rat pups (5–19 days old) and adult rats (2–5 months old). As previously observed in the hippocampus of rat pups, CA1 SR interneurons in the hippocampus of adult rats responded to choline (10 mM, 12 s) with whole-cell currents that decayed to the baseline within the agonist pulse, were sensitive to inhibition by methyllycaconitine (10 nM) or α-bungarotoxin (50 nM), and were, therefore, mediated by α7* 1 1 According to the nomenclature for nAChRs and their subunits [Lukas RJ, Changeux JP, Le Novere N, Albuquerque, EX, Balfour DJ, Berg, DK et al. International Union of Pharmacology. XX. Current status of the nomenclature for nicotinic acetylcholine receptors and their subunits. Pharmacol Rev 1999; 51: 397–401.], the asterisk next to nAChR subunits throughout text is meant to indicate that the exact subunit composition is not known. [1] nAChRs. Likewise, as previously observed in the hippocampus of young rats, in the adult rat hippocampus excitatory postsynaptic currents (EPSCs) were recorded from SR interneurons in response to a pulse of ACh (0.1 mM, 12 s) applied in the presence of the GABA A receptor antagonist bicuculline. ACh-triggered EPSCs were inhibited by mecamylamine (1 μM) or choline (1 mM) and were, therefore, likely to have resulted from activation of α3β4β2* nAChR. The magnitude of α7* nAChR-mediated responses increased with the age of the animals. In contrast, the magnitude of α3β4β2* nAChR-mediated responses was highest at the second postnatal week. The distinct age dependency of functional expression of α7* and α3β4β2* nAChRs strongly suggests that the excitability of CA1 SR interneurons is differentially regulated by the nicotinic cholinergic system in the hippocampus of rat pups and adult rats.

Presynaptic Inhibition of GABAA Receptor-Mediated Unitary IPSPs by Cannabinoid Receptors at Synapses Between CCK-Positive Interneurons in Rat Hippocampus

There is growing evidence to link cholecystokinin (CCK)-positive interneurons and anxiety disorders. Despite this, little is known about the physiology and pharmacology of synaptic interactions between CCK-positive interneurons. This study aims to investigate the local circuit connections among CCK-positive Schaffer collateral associated (SCA) interneurons in stratum radiatum (SR) and their modulatory interactions using paired whole cell recordings combined with biocytin and double immunofluorescence labeling in slices of rat hippocampus. The cell bodies of SCA interneurons were located in SR, and their sparsely spiny dendrites projected toward s. pyramidale (SP) and along SR. Their axons innervated SR, SP, and s. oriens (SO) with predominant ramification in SR. These cells were immunopositive for CCK and immunonegative for parvalbumin (PV). SCA interneurons often displayed an accommodating firing pattern with or without a "sag" in response to hyperpolarizing current injection. Pairs of these cells exhibited electrical coupling and reciprocal chemical connections in which inhibitory postsynaptic potentials (IPSPs) displayed powerful frequency-dependent facilitation and augmentation. The synaptic connections were modulated by the endogenous cannabinoid receptor (CB) agonist, anandamide and by depolarization-induced suppression of inhibition (DSI), both of which reduced the amplitude of unitary IPSPs to 50% of control and increased the number of apparent failures of transmission. These effects were blocked by the CB1 receptor antagonist, AM-251. I suggest that synaptic facilitation between CCK-positive SCA interneurons may modify the onset of CB1 receptor-mediated regulation of inhibition, thereby affecting spike timing, and that this process could influence the expression of anxiety.

Intracellular Ca2+ dynamics of hippocampal interneurons following nicotinic acetylcholine receptor activation

Ca2+ permeability of central nicotinic acetylcholine receptors (nAChRs), especially the α7 subunits, are exceptionally high and this important feature provide a special functional importance for these receptors at the system level. Although studies at the cellular level extensively characterized the molecular properties of Ca2+ influx following nAChR activation, much less is known about the time-related Ca2+ dynamics during nicotine administration in integration units of neurons. Such studies are of particular relevance to understanding in situ nonsynaptic actions of nicotine. Puff ejection of drugs produce a rapid drug delivery and elimination from the cell surface allowing the activation of extrasynaptic receptors within desensitization time-frame. In this report we provide evidence that rapid nicotine application is able to produce irregular Ca2+ transients in the dendrites of stratum radiatum interneurons in the hippocampal CA1 region. Potential components and mechanisms of nAChR-mediated Ca2+ influx are discussed in details to demonstrate the unique feature of activation of nAChRs involved in nonsynaptic function in interneurons as compared to other types of nicotinic activity.

Cholinergic Axons Modulate GABAergic Signaling among Hippocampal Interneurons via Postsynaptic α7 Nicotinic Receptors

Homopentameric alpha7 nicotinic receptors have a high affinity for acetylcholine (ACh), are permeable to Ca2+ ions, and are abundant in hippocampal interneurons. Although nicotinic agonists evoke inward currents and Ca2+ transients in stratum radiatum interneurons, the role of endogenous ACh in modulating synaptic integration by interneurons is incompletely understood. Many cholinergic axonal varicosities do not have postsynaptic specializations, but alpha7 receptors frequently occur close to synaptic GABA(A) receptors. These observations raise the possibility that alpha7 nicotinic receptors activated by ACh released from cholinergic axons modulate GABAergic transmission in interneurons. We show that agonists of alpha7 receptors profoundly depress GABAergic IPSCs recorded in stratum radiatum interneurons in the CA1 region of the hippocampus. This depression is accompanied by a small increase in GABA release. Alpha7 nicotinic receptor agonists also depress GABA- or muscimol-evoked currents in interneurons, indicating that the major effect is a postsynaptic modulation of GABA(A) receptors. The depression of GABA-evoked currents is abolished by chelating Ca2+ in the recorded interneuron and attenuated by inhibitors of PKC. We also show that stimuli designed to release endogenous ACh from cholinergic axons evoke an alpha7 receptor-dependent heterosynaptic depression of GABAergic IPSCs in interneurons. This heterosynaptic modulation is amplified by blocking cholinesterases. These results reveal a novel mechanism by which cholinergic neurons modulate information processing in the hippocampus.

In the developing rat hippocampus a tonic GABAA‐mediated conductance selectively enhances the glutamatergic drive of principal cells

In the adult hippocampus, two different forms of GABA(A) receptor-mediated inhibition have been identified: phasic and tonic. The first is due to the activation of GABA(A) receptors facing the presynaptic releasing sites, whereas the second is due to the activation of receptors localized away from the synapses. Because of their high affinity and low desensitization rate, extrasynaptic receptors are persistently able to sense low concentrations of GABA. Here we show that, early in postnatal life, between postnatal day (P) 2 and P6, CA1 and CA3 pyramidal cells but not stratum radiatum interneurons, express a tonic GABA(A)-mediated conductance. Block of the neuronal GABA transporter GAT-1 slightly enhanced the persistent GABA conductance in principal cells but not in GABAergic interneurons. However, in adulthood, a tonic GABA(A)-mediated conductance could be revealed in stratum radiatum interneurons, indicating that the ability of these cells to sense ambient GABA levels is developmentally regulated. Pharmacological analysis of the tonic conductance in principal cells demonstrated the involvement of beta2/beta 3, alpha 5 and gamma 2 GABA(A) receptor subunits. Removal of the tonic depolarizing action of GABA with picrotoxin, reduced the excitability and the glutamatergic drive of principal cells but did not modify the excitability of stratum radiatum interneurons. The increased cell excitability and synaptic activity following the activation of extrasynaptic GABA(A) receptors by ambient GABA would facilitate the induction of giant depolarizing potentials.

Strain-Specific Nicotinic Modulation of Glutamatergic Transmission in the CA1 Field of the Rat Hippocampus: August Copenhagen Irish Versus Sprague-Dawley

Prepulse inhibition (PPI), a measure of sensorimotor gating impaired in patients with schizophrenia, is more sensitive to disruption by apomorphine in prepubertal August Copenhagen Irish (ACI) than Sprague-Dawley (SD) rats. In brain regions including the hippocampus, PPI is modulated by alpha7* nicotinic receptors (nAChRs) and kynurenic acid (KYNA), a kynurenine metabolite that blocks alpha7 nAChRs. Here, KYNA levels and nAChR activities were measured in the hippocampi of 10- to 23-day-old ACI and SD rats of both sexes. Hippocampal KYNA levels were not different between ACI and SD rats. In hippocampal slices from both rat strains, choline (10 mM) evoked alpha7* nAChR-mediated type IA currents in CA1 stratum radiatum (SR) interneurons. In the presence of alpha7 nAChR antagonists, acetylcholine (ACh, 1 mM) evoked alpha4beta2* nAChR-mediated type II currents. ACh also triggered excitatory postsynaptic currents (EPSCs) that resulted from alpha3beta4* nAChR activation in glutamatergic neurons/axons synapsing onto the interneurons. The magnitude of the nicotinic responses did not differ significantly between male and female rats. Only the magnitude of alpha3beta4* nAChR responses and the frequency of spontaneous EPSCs recorded from CA1 SR interneurons differed between the rat strains, being significantly larger in ACI than SD rats. These results indicate that the alpha3beta4* nAChR activity in glutamatergic neurons/axons and the number of glutamatergic terminals synapsing onto CA1 SR interneurons are larger in prepubertal ACI than SD rats. The differential sensitivity of these rats to PPI disruption by apomorphine may result from strain-specific levels of glutamatergic activity and its strain-specific modulation by alpha3beta4* nAChRs in the hippocampus.

Thyrotropin‐releasing hormone increases GABA release in rat hippocampus

Thyrotropin-releasing hormone (TRH) is a tripeptide that is widely distributed in the brain including the hippocampus where TRH receptors are also expressed. TRH has anti-epileptic effects and regulates arousal, sleep, cognition, locomotion and mood. However, the cellular mechanisms underlying such effects remain to be determined. We examined the effects of TRH on GABAergic transmission in the hippocampus and found that TRH increased the frequency of GABAA receptor-mediated spontaneous IPSCs in each region of the hippocampus but had no effects on miniature IPSCs or evoked IPSCs. TRH increased the action potential firing frequency recorded from GABAergic interneurons in CA1 stratum radiatum and induced membrane depolarization suggesting that TRH increases the excitability of interneurons to facilitate GABA release. TRH-induced inward current had a reversal potential close to the K+ reversal potential suggesting that TRH inhibits resting K+ channels. The involved K+ channels were sensitive to Ba2+ but resistant to other classical K+ channel blockers, suggesting that TRH inhibits the two-pore domain K+ channels. Because the effects of TRH were mediated via Galphaq/11, but were independent of its known downstream effectors, a direct coupling may exist between Galphaq/11 and K+ channels. Inhibition of the function of dynamin slowed the desensitization of TRH responses. TRH inhibited seizure activity induced by Mg2+ deprivation, but not that generated by picrotoxin, suggesting that TRH-mediated increase in GABA release contributes to its anti-epileptic effects. Our results demonstrate a novel mechanism to explain some of the hippocampal actions of TRH.

Excitation of rat hippocampal interneurons via modulation of endogenous agonist activity at the α7 nicotinic ACh receptor

The alpha7 subtype of the nicotinic acetylcholine receptor (alpha7 nAChR) is prominently expressed in the hippocampus where it is thought to play a role in the regulation of cognitive function. In this study, we have investigated the effects of 5-hydroxyindole (5-HI), a positive modulator of the alpha7 nAChR, on GABAergic activity in hippocampal CA1 stratum radiatum interneurons in acute rat brain slices. Superfusion of 5-HI (100 microM) increased the mean frequency and amplitude of spontaneous IPSCs (sIPSCs). The potentiation was occluded by pretreatment of slices with: (1) a high concentration of the broad-spectrum agonist nicotine to desensitize the alpha7 receptor, (2) an alpha7 nAChR antagonist, and (3) tetrodotoxin to block action potential firing. These results indicate that facilitation by 5-HI was mediated by the alpha7 nAChR and required neuronal excitation. In contrast, 5-HI had no effect on sIPSCs recorded in hippocampal slices from younger animals, even though the expression of functional alpha7 nAChRs was confirmed by agonist application experiments. In these slices, 5-HI only enhanced sIPSCs after pretreatment with the acetylcholinesterase inhibitor Bw284c51. Taken together, our results suggest that 5-HI facilitates GABAergic transmission via excitation of the alpha7 nAChR, and that this effect requires the presence of the endogenous agonist ACh in the extracellular environment of the receptor.

Subtype-Specific Inhibition of Nicotinic Acetylcholine Receptors by Choline: A Regulatory Pathway

Choline is an essential nutrient and a precursor of neurotransmitter acetylcholine (ACh) and is produced at synapses during depolarization, upon hydrolysis of ACh via acetylcholinesterase, and under conditions of injury and trauma. Animal studies have shown that supplementation with choline during early development results in long-lasting improvement in memory in adults; however, the mechanisms underlying this effect are poorly defined. Previous studies revealed that choline interacts with type IA (alpha7*) nicotinic acetylcholine receptors (nAChRs) as a full agonist and as a desensitizing agent and is a weak agonist of type III (alpha3beta4*) nAChRs. Because nAChRs play a role in learning and memory and are generally inhibited by agonists at low concentrations, we investigated in this study the inhibitory effects of choline on non-alpha7 nAChRs such as type II (alpha4beta2*) and type III nAChRs. Using whole-cell patch-clamp recordings from neurons of rat hippocampal and dorsal striatal slices, we demonstrate that choline inhibited type III nAChR-mediated glutamate excitatory postsynaptic currents (EPSCs). Choline inhibited ACh-induced N-methyl-D-aspartate (NMDA) EPSCs in CA1 stratum radiatum (SR) interneurons of rat hippocampal slices with an IC50 of approximately 15 microM. Choline did not inhibit NMDA or alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors in CA1 SR interneurons. Choline inhibited type II nAChRs in CA1 SR interneurons with an IC50 of approximately 370 microM. The present results reveal an order of inhibitory potency for choline type III>type IA>type II nAChRs. It is concluded that brain nAChRs, but not glutamate receptors, are the primary targets for the regulatory actions of choline.

Septal innervation regulates the function of α7 nicotinic receptors in CA1 hippocampal interneurons

The hippocampus receives substantial input from the medial septum/diagonal band of broca (MS/DB) via the fibria-fornix (FF). Projections from the MS/DB innervate hippocampal interneurons that express α7 nicotinic receptors and regulate excitation in principal cell populations. In the present report we used stereotaxic surgery, whole-cell patch clamping, and immunohistochemical techniques to evaluate the effects of FF and MS/DB lesions on α7 nicotinic receptors in stratum radiatum interneurons. Focal somatic application of ACh (1 mM) evoked methyllycaconitine (MLA)-sensitive currents that were markedly reduced following aspirative lesions of the FF. Reductions in current amplitudes were prevented or restored to levels not significantly different from controls following in vivo treatment with the α7-selective agonist GTS-21, and GTS-21 treatment did not change current amplitudes measured in tissue from unlesioned animals. MS/DB injections of the selective cholinergic neurotoxin 192 IgG-saporin did not affect α7 receptor currents, although MS/DB ChAT and hippocampal AChE immunolabeling were significantly reduced. In contrast, kainic acid lesions of the MS/DB, potentially more selective for GABAergic projection neurons, produced significant reductions in current amplitudes. These findings are the first to show functional changes in α7 receptors following hippocampal denervation and suggest that MS/DB hippocampal innervation regulates functional aspects of hippocampal α7 receptors. The results confirm hippocampal α7 nicotinic receptors as viable therapeutic targets in diseases that involve degradation of the septohippocampal pathway and may indicate that GABAergic MS/DB hippocampal input plays a more substantial role in the regulation of α7 nicotinic receptor function than MS/DB hippocampal cholinergic input.

Cell‐type specific GABA synaptic transmission and activity‐dependent plasticity in rat hippocampal stratum radiatum interneurons

Abstract In hippocampal pyramidal cells, the efficacy of synaptic transmission at gamma-aminobutyric acid (GABA)ergic synapses, is modulated by activity. However, whether such plasticity occurs at inhibitory synapses on interneurons remains largely unknown. Using whole-cell voltage-clamp recordings of inhibitory postsynaptic currents (IPSCs) in Sprague-Dawley rat hippocampal slices, we examined whether GABA synapses of stratum radiatum interneurons were affected by stimulation protocols known to alter efficacy at inhibitory synapses of CA1 pyramidal cells. Monosynaptically evoked IPSCs (eIPSCs) exhibited different properties with significantly faster kinetics, higher coefficients of variation, a current-voltage (I-V) relationship shifted to depolarized values and a smaller paired-pulse depression, in interneurons than in pyramidal cells. GABA synapses on interneurons also showed a different capacity for plasticity. Indeed, theta-burst stimulation induced a long-term potentiation of eIPSCs in both cell types, but the induction mechanisms differed in interneurons, as it was not affected by antagonists of GABAB receptors and group I/II metabotropic glutamate receptors (mGluRs). Furthermore, 100-Hz tetanization selectively elicited a short-term depression of eIPSCs in pyramidal cells. A postsynaptic depolarization produced a transient suppression of eIPSCs (depolarization-induced suppression of inhibition) in pyramidal cells but not in interneurons. Spontaneous IPSCs were similarly reduced following depolarization in pyramidal cells, but not in interneurons. These results indicate that GABA synapses of stratum radiatum interneurons exhibit different properties and capacity for activity-dependent plasticity than those of pyramidal cells. This cell-type specific mode of transmission and adaptive regulation of GABA synapses may contribute to hippocampal plasticity and functions.

Hebbian LTP in feed-forward inhibitory interneurons and the temporal fidelity of input discrimination

Cortical information processing requires a delicate balance of excitatory and inhibitory signaling. How is this balance preserved during hippocampal memory encoding, which involves NMDA receptor-dependent long term potentiation (LTP)? This form of LTP occurs at synapses between pyramidal neurons but has not been detected in feed-forward inhibitory interneurons. We show that paired pre- and postsynaptic activity evokes pathway-specific LTP in half of rat stratum radiatum interneurons if cytoplasmic integrity is preserved. LTP occurs in aspiny feed-forward interneurons and propagates to pyramidal neurons as an enhancement of disynaptic inhibition. We also show that when LTP is restricted to synapses on pyramidal neurons, the temporal fidelity of synaptic integration and action potential generation in pyramidal cells is compromised. However, when LTP also occurs at synapses on feed-forward interneurons, temporal fidelity is preserved. We propose that Hebbian LTP at synapses driving disynaptic inhibition is necessary to maintain information processing without degradation during memory encoding.

Endogenous Ca2+Buffer Concentration and Ca2+Microdomains in Hippocampal Neurons

Ca2+-binding proteins are ubiquitously expressed throughout the CNS and serve as valuable immunohistochemical markers for certain types of neurons. However, the functional role of most Ca2+-binding proteins has to date remained obscure because their concentration in central neurons is not known. In this study, we investigate the intracellular concentration of the widely expressed Ca2+-binding protein calbindin-D28k in adult hippocampal slices using patch-clamp recordings and immunohistochemistry. First, we show that calbindin-D28k freely exchanges between patch pipette and cytoplasm during whole cell patch-clamp recordings with a time constant of approximately 10 min. Substituting known concentrations of recombinant calbindin-D28k in patch pipettes enabled us to determine the endogenous calbindin-D28k concentration by postrecording immunohistochemistry. Using this calibration procedure, we find that mature granule cells (doublecortin-) contain approximately 40 microm, and newborn granule cells (doublecortin+) contain 0-20 microm calbindin-D28k. CA3 stratum radiatum interneurons and CA1 pyramidal cells enclose approximately 47 and approximately 45 microm calbindin-D28k, respectively. Numerical simulations showed that 40 microm calbindin-D28k is capable of tuning Ca2+ microdomains associated with action potentials at the mouth of single or clustered Ca2+ channels: calbindin-D28k reduces the increment in free Ca2+ at a distance of 100 and 200 nm by 20 and 35%, respectively, and strongly accelerates the collapse of the Ca2+ gradient after cessation of Ca2+ influx. These data suggest that calbindin-D28k equips hippocampal neurons with approximately 160 microm mobile, high-affinity Ca2+-binding sites (kappa(S) approximately 200) that slow and reduce global Ca2+ signals while they enhance the spatiotemporal fidelity of submicroscopic Ca2+ signals.

AMPA receptor modulators have different impact on hippocampal pyramidal cells and interneurons

Positive modulators of AMPA receptors enhance synaptic plasticity and memory encoding. Facilitation of AMPA receptor currents not only results in enhanced activation of excitatory neurons but also increases the activity of inhibitory interneurons by up-modulating their excitatory input. However, little is known about the effects of these modulators on cells other than pyramidal neurons and about their impact on local microcircuits. This study examined the effects of members from three subfamilies of modulators (mainly CX516, CX546 and cyclothiazide) on excitatory synaptic responses in four classes of hippocampal CA1 neurons and on excitatory and disynaptically induced inhibitory field potentials in hippocampal slices. Effects on excitatory postsynaptic currents (EPSCs) were examined in pyramidal cells, in two types of inhibitory interneurons located in stratum radiatum and oriens, and in stratum radiatum giant cells, a novel type of excitatory neuron. With CX516, increases in EPSC amplitude in pyramidal cells were two to three times larger than in interneurons and six times larger than in radiatum giant cells. The effects of CX546 on response duration similarly were largest in pyramidal cells. However, this drug also strongly differentiated between stratum oriens and radiatum interneurons with increases being four times larger in the latter. In contrast, cyclothiazide had similar effects on response duration in all cell types. In field recordings, CX516 was several times more potent in enhancing excitatory postsynaptic potentials (EPSPs) than feedback or feedforward circuits, as expected from its larger influence on pyramidal cells. In contrast, BDP-20, a CX546 analog, was more potent in enhancing feedforward inhibition than either EPSPs or feedback inhibition. This preference for feedforward over feedback circuits is probably related to its higher potency in stratum radiatum versus oriens interneurons. Taken together, AMPA receptor modulators differ substantially in their potency and/or efficacy across major classes of neurons which is likely to have consequences with regard to their impact on circuits and behavior.

Voltage-controlled plasticity at GluR2-deficient synapses onto hippocampal interneurons.

High-frequency stimulation of pyramidal cell inputs to developing (P9-12) hippocampal stratum radiatum interneurons expressing GluR2-lacking, Ca(2+)-permeable AMPA receptors produces long-term depression of synaptic transmission, if N-methyl-d-aspartate (NMDA) receptors are blocked. Here we show that these same synapses display a remarkably versatile signal integration if postsynaptic NMDA receptors are activated. At synapses expressing GluR2-deficient AMPA receptors, tetanic stimulation that activates NMDA receptors triggered long-term potentiation or depression (LTP or LTD) depending on membrane potential. LTP was elicited at most synapses when the interneuron was held at -30 mV during the stimulus train but was typically prevented by postsynaptic hyperpolarization to -70 mV, by strong depolarization to 0 mV, by d-2-amino-5-phosphonovaleric acid, or by postsynaptic injection of the Ca2+ chelator bis-(o-aminophenoxy)-N,N,N',N'-tetraacetic acid. At synapses with predominantly GluR2-containing AMPA receptors, repetitive stimulation did not change synaptic strength regardless of whether NMDA receptors were activated. The interactions among GluR2 expression, NMDA receptor expression, and membrane potential thus confer on hippocampal interneurons a distinctive means for differential decoding of high-frequency inputs, resulting in enhanced or depressed transmission depending on the functional state of the interneuron.

Targeted deletion of the kynurenine aminotransferase ii gene reveals a critical role of endogenous kynurenic acid in the regulation of synaptic transmission via alpha7 nicotinic receptors in the hippocampus.

It has been postulated that endogenous kynurenic acid (KYNA) modulates alpha7* nicotinic acetylcholine receptor (nAChR) and NMDA receptor activities in the brain.a To test this hypothesis, alpha7* nAChR and NMDA receptor functions were studied in mice with a targeted null mutation in the gene encoding kynurenine aminotransferase II (mKat-2-/- mice), an enzyme responsible for brain KYNA synthesis. At 21 postnatal days, mKat-2-/- mice had lower hippocampal KYNA levels and higher spontaneous locomotor activity than wild-type (WT) mice. At this age, alpha7* nAChR activity induced by exogenous application of agonists to CA1 stratum radiatum interneurons was approximately 65% higher in mKat-2-/- than WT mice. Binding studies indicated that the enhanced receptor activity may not have resulted from an increase in alpha7* nAChR number. In 21-d-old mKat-2-/- mice, endogenous alpha7* nAChR activity in the hippocampus was also increased, leading to an enhancement of GABAergic activity impinging onto CA1 pyramidal neurons that could be reduced significantly by acute exposure to KYNA (100 nM). The activities of GABA(A) and NMDA receptors in the interneurons and of alpha3beta4* nAChRs regulating glutamate release onto these neurons were comparable between mKat-2-/- and WT mice. By 60 d of age, KYNA levels and GABAergic transmission in the hippocampus and locomotor activity were similar between mKat-2-/- and WT mice. Our findings that alpha7* nAChRs are major targets for KYNA in the brain may provide insights into the pathophysiology of schizophrenia and Alzheimer's disease, disorders in which brain KYNA levels are increased and alpha7* nAChR functions are impaired.

Distance-Dependent Scaling of Calcium Transients Evoked by Backpropagating Spikes and Synaptic Activity in Dendrites of Hippocampal Interneurons

Although interactions between backpropagating action potentials and synaptic stimulations have been extensively studied in pyramidal neurons, dendritic propagation and the summation of these signals in interneurons are not nearly as well known. In this study, two-photon imaging was used to explore the basic properties of dendritic calcium signaling in CA1 stratum radiatum interneurons. In contrast to hippocampal pyramidal neurons, the backpropagating action potential-evoked calcium transients in dendrites of interneurons underwent a distance-dependent increment. Although, in proximal dendrites, an increment could be attributed to a smaller dendrite diameter, distal dendrites did not show such dependence. Calcium responses in interneurons had a smaller amplitude, slower rise time, and decay than in pyramidal neurons. To explore the factors underlying the difference, we compared the calcium-binding capacity in interneurons and in pyramidal neurons. Our finding that endogenous calcium buffers had a higher level in interneurons may primarily explain the different kinetics and amplitudes of calcium transients. Synaptic stimulation-evoked calcium transients were also larger at distant dendritic locations. The spread of these signals was restricted to 12-13 microm long dendritic compartments. Supporting the reported lack of long-term potentiation in these interneurons, we found only sublinear or linear summations of calcium responses to coincident synaptic inputs and backpropagating spikes.

Functional Mapping and Ca2+Regulation of Nicotinic Acetylcholine Receptor Channels in Rat Hippocampal CA1 Neurons

Diverse subtypes of nicotinic acetylcholine receptors (nAChRs), including fast-desensitizing alpha7-containing receptors thought to be Ca2+-permeable, are expressed in the CNS, where they appear to regulate cognitive processing and synaptic plasticity. To understand the physiological role of nAChRs in regulating neuronal excitability, it is important to know the distribution of functional receptors along the surface of neurons, whether they can increase [Ca2+]i, and/or are regulated by Ca2+. We mapped the distribution of receptors on the membrane of rat hippocampal CA1 stratum radiatum interneurons and pyramidal cells in acute slices by recording nAChR-mediated currents elicited by local UV laser-based photolysis of caged carbachol in patch-clamped neurons. The local application (approximately 7 microm patches) allowed mapping of functional nAChRs along the soma and dendritic tree, whereas the fast uncaging minimized the effects of desensitization of alpha7-containing nAChRs and allowed us to measure the kinetics of responses. The alpha7-containing nAChRs were the predominant subtype on interneurons, and were located primarily at perisomatic sites (<70 microm from the soma; in contrast to the more uniform distribution of glutamate receptors); no currents were detectable on pyramidal neurons. The activation of nAChRs increased [Ca2+]i, indicating that these native receptors in acute slices are significantly Ca2+-permeable, consistent with previous observations made with recombinant receptors. In addition, they exhibited strong desensitization, the rate of recovery from which was controlled by [Ca2+]i. Our results demonstrate the strategic location and Ca2+ regulation of alpha7-containing nAChRs, which may contribute to understanding their involvement in hippocampal plasticity.