Baclofen selectively inhibits excitatory synaptic transmission in the hippocampus

The effect of baclofen was investigated on mossy fiber, Schaffer collateral and perforant path synaptic transmission in hippocampal slices. Baclofen completely inhibits mossy fiber and Schaffer collateral synaptic transmission with an IC50 of 3.8 microM. The lateral perforant path is insensitive to baclofen, while the response in the medial zone was partially blocked. Baclofen does not appear to act in a GABA-like manner.     

Short-term plasticity in excitatory synapses of the rat medial preoptic nucleus

The medial preoptic nucleus (MPN) regulates sexual behavior which is subject to experience-dependent modifications. Such modifications must depend on functional plasticity in the controlling neural circuits. Thus, MPN synapses are likely candidates for the site of alterations. The present work is a first systematic study of functional synaptic plasticity at glutamatergic synapses in the MPN. Short-term activity-dependent plasticity was investigated using a slice preparation from young male rats. The average efficacy of AMPA/kainate-receptor-mediated synaptic transmission was activity-dependent, showing a peak at a steady stimulation rate of 2 Hz. The variation in efficacy was attributed to mainly presynaptic factors since the average response amplitude was roughly paralleled by the response probability. Upon paired-pulse stimulation, paired-pulse facilitation as well as paired-pulse depression was observed. In some cases, paired-pulse facilitation as well as paired-pulse depression was recorded from an individual neuron depending on the interval between the paired stimuli. On average, paired-pulse facilitation was observed at intervals <500 ms, and paired-pulse depression at intervals in the range 1-4 s. The findings thus reveal complex activity-dependent short-term plasticity of the functional synaptic properties in the medial preoptic nucleus.     

GABAB receptors modulate synaptically-evoked responses in the rat dentate gyrus, in vivo

We assessed the effects of systemically injected baclofen, a GABA_B agonist, on single and paired-pulse responses in the dentate gyrus of urethane-anesthetized rats, in vivo. Baclofen (10 mg/kg) significantly increased the duration of single excitatory responses. This increase was blocked by the GABA_B receptor antagonist, CGP 35348, indicating that baclofen was acting through GABA_B receptors. To determine the mechanism underlying this increase in response duration, the NMDA antagonist,d-2-amino-5-phosphonopentanoic acid (d-APV), was administered intracerebroventricularly (i.c.v.) after baclofen.d-APV by itself had no effect on the duration of the population excitatory post-synaptic potential (EPSP). However, when infused after baclofen,d-APV blocked the baclofen induced increase in EPSP duration. This indicates the prolonged EPSP duration caused by baclofen resulted from an enhancement of an NMDA receptor mediated component of the response. We then examined the effect of baclofen on population responses to paired stimuli. Baclofen attenuated paired-pulse inhibition of population spike amplitudes at a 25 ms interstimulus interval. CGP-35348 reduced the effect of baclofen on paired-pulse inhibition, indicating that baclofen suppressed paired-pulse inhibition by acting on GABA_B receptors. In contrast to its disinhibitory effect at the 25 ms interval, baclofen had an inhibitory effect on responses evoked at a 150 ms interstimulus interval. Under control conditions, we observed that when stimuli were delivered 150 ms apart, both the EPSP duration and population spike amplitude evoked by the second stimulus were enhanced. Baclofen suppressed this enhancement. We conclude that GABA_B receptor activation by baclofen reduces short interval inhibition and this reduction of inhibition leads to an enhanced NMDA-receptor mediated response. We also conclude that by reducing inhibition, baclofen occludes paired-pulse disinhibition found at longer interstimulus intervals. In contrast, CGP-35348 directly blocks paired-pulse disinhibition. Thus, GABA_B receptor activation can regulate synaptic inhibition, in turn influencing NMDA receptor activation and synaptic transmission. Thus, pharmacologic manipulation of GABA_B receptors may provide a powerful approach for regulating synaptic transmission in the hippocampus.     

Regulation of synaptic plasticity by mGluR1 studied in vivo in mGluR1 mutant mice

The role of the metabotropic glutamate receptor 1 (mGluR₁) in synaptic plasticity was investigated in vivo in the intact hippocampus of mutant mice lacking this receptor. In a previous study we showed reduced long-term potentiation (LTP) in the dentate gyrus of mGluR₁ −/− mice in vivo, but not when LTP was studied in a slice preparation. A possible explanation of this difference is that dentate neurons receive more inhibitory synaptic drive in vivo than in slice preparation where many inhibitory axon collaterals are lost. We report here that another form of synaptic plasticity, paired-pulse depression of the population spike, is also abnormal in the dentate gyrus of mGluR₁-deficient mice when tested in vivo. In wild-type mice, stimulation of the medial perforant path produced paired-pulse depression of inter-pulse intervals (IPIs) up to 30 ms. Mutant mGluR₁, on the other hand, showed a significantly longer IPI depression, up to 50 ms. Paired-pulse depression results from the activation of inhibitory interneurons. The GABA_B agonist baclofen, acting presynaptically on the GABA interneurons, attenuated paired-pulse depression and allowed for a normal and stable LTP in mGluR₁ mutant mice. These findings suggest an indirect role for mGluR₁ in synaptic plasticity via a regulation of GABA inhibition.     

Effects of prenatal protein malnutrition on kindling-induced alterations in dentate granule cell excitability. II. Paired-pulse measures

The effects of prenatal protein malnutrition on kindling-induced changes in inhibitory modulation of dentate granule cell activity were examined by analysis of extracellular field potentials recorded from the granule cell layer of the dentate gyrus in response to paired-pulse stimulation of the perforant pathway in freely-moving rats. Since we have shown that kindling results in enhanced synaptic transmission at the level of the perforant path/granule cell synapse (see preceding paper), we sought to determine if the kindling process might induce changes in inhibitory modulation of granule cell excitability which could be involved in the slower acquisition of the kindled state we have previously reported in malnourished animals. Beginning at 120-150 days of age, the response of dentate granule cells to paired-pulse stimulation of the perforant path was examined at interpulse intervals (IPIs) ranging from 20-1000 ms. A paired-pulse index (PPI) was constructed based on the mean percent change in population spike amplitudes of the two responses resulting from application of the pulse pair. PPI measures obtained during the kindling process were compared with individual prekindling measures to determine the mean percent change in excitatory/inhibitory modulation of granule cell activity. Significant inhibition of the second population response was apparent at all IPIs tested for both diet groups following the first kindled afterdischarge.(ABSTRACT TRUNCATED AT 250 WORDS)     

Pathway specificity of noradrenergic plasticity in the dentate gyrus

Previous experiments have described highly specific effects of noradrenergic agonists on synaptic transmission in the dentate gyrus (DG). For example, perfusion of hippocampal slices with the beta-noradrenergic agonist isoproterenol induces a long-lasting potentiation (LLP) of extracellularly recorded responses following stimulation of the medial perforant path (PP), and long-lasting depression (LLD) of responses evoked by stimulation of the lateral PP (Dahl D, Sarvey JM, 1989, Proc Natl Acad Sci USA 86:4776–4780). To examine the possible interactions of LLP, LLD, and long-term potentiation induced by tetanic stimulation (LTP), the authors recorded extracellular field potentials evoked in the DG by stimulation of the lateral or medial perforant path following LTP and LLP or LLD, invoked in different orders. After establishment of LLP or LLD by path application of isoproternol, subsequent tetanization of the respective afferents resulted in additional potentiation of the medial PP-evoked response and return of the lateral PP-evoked response to baseline levels. In other slices application of isoproterenol after establishment of LTP resulted in further potentiation of medial PP-evoked responses but no change in the potentiated response evoked by lateral PP stimulation. Thus the pathway specificity was maintained irrespective of the history of previous potentiation or depression. Experiments using the specific beta₁ antagonist metoprolol further confirmed pathway specificity. Perfusion with 20 μM of metoprolol appeared to reduce LTP evoked by stimulation of the medial but not lateral PP. In a subsequent experiment, metoprolol in the absence of tetanization produced LLD of the medial PP-evoked response and LLP of the lateral PP-evoked response, opposite to the effects of ISO. These results confirm the impressive extent of pathway specificity in the DG and reveal the persistent capacity for synaptic modification as exemplified by the processes of LLP, LLD, and LTP. © 1994 Wiley-Liss, Inc.     

Afferent-specific modulation of short-term synaptic plasticity by neurotrophins in dentate gyrus

Neurotrophins modulate synaptic transmission and plasticity in the adult brain. We here show a novel feature of this synaptic modulation, i.e. that two populations of excitatory synaptic connections to granule cells in the dentate gyrus, lateral perforant path (LPP) and medial perforant path (MPP), are differentially influenced by the neurotrophins BDNF and NT-3. Using field recordings and whole-cell patch-clamp recordings in hippocampal slices, we found that paired-pulse (PP) depression at MPP-granule cell synapses was impaired in BDNF knock-out (+/-) mice, but PP facilitation at LPP synapses to the same cells was not impaired. In accordance, scavenging of endogenous BDNF with TrkB-IgG fusion protein also impaired PP depression at MPP-granule cell synapses, but not PP facilitation at LPP-granule cell synapses. Conversely, in NT-3+/- mice, PP facilitation was impaired at LPP-granule cell synapses whilst PP depression at MPP-granule cell synapses was unaffected. These deficits could be reversed by application of exogenous neurotrophins in an afferent-specific manner. Our data suggest that BDNF and NT-3 differentially regulate the synaptic impact of different afferent inputs onto single target neurons in the CNS.     

Interactions between septal and entorhinal inputs to the rat dentate gyrus: Facilitation effects

We examined facilitation effects between the medial septum and perforant path inputs to the dentate gyrus for the four possible combinations of paired-pulse activation. Facilitation effects occurred in all cases. The largest facilitation effects occurred when the septal pulse served as the conditioning pulse for the population spike subsequently evoked by a perforant path pulse. Using 3 pulses, we also examined the influence of septal activation on paired-pulse facilitation of the perforant path-granule cell population spike. A septal stimulation pulse, applied 6-10 ms prior to the onset of the population spike evoked by a perforant path conditioning pulse, did not affect the perforant path-dentate test response at any interpulse interval. If the septal pulse occurred immediately prior to population spike onset, however, there was a significantly greater depression of the test response from 70-3000 ms, but no effect at early intervals (20-50 ms). The effect of the septal pulse appears more consistent with a direct action of the septal terminals on granule cells than with an indirect action via the recurrent inhibitory interneurons.     

Long-lasting reduction of dentate paired-pulse depression following LTP-inducing tetanic stimulations of perforant path

Effects of high-frequency stimulations of the perforant path on the dentate paired-pulse depression were examined in urethane-anesthetized rats. The tetanic stimulations produced a long-term potentiation (LTP) of the excitatory synaptic transmission at the perforant path-dentate granule cell synapses in almost all animals examined. The strength of the early paired-pulse depression at an inter-pulse interval (IPI) of 20 ms decreased significantly for at least 60 min after the tetanic stimulations, whereas the late paired-pulse depression at an IPI of 2 s remained almost unchanged. The reduction of the early paired-pulse depression was stepwise augmented by each of successive tetanic stimulations given at an interval of 10 min. A preceding antidromic stimulation of the mossy fibers depressed the population spike amplitude of perforant path response at an interval of 5-9 ms. The strength of the antidromic depression of population spike also decreased following the perforant path tetanic stimulations. These results suggest that tetanic stimulations of the perforant path produce a long-lasting reduction of the GABAergic recurrent inhibition in the dentate area associated with LTP. The possible mechanisms of the decrease in GABAergic inhibition produced by tetanic stimulations and its possible effects on the development of LTP with succeeding tetanic stimulations were discussed.     

Effects of baclofen and bicuculline on inhibition in the fascia dentata and hippocampus regio superior

The effects of microiontophoretically applied baclofen, bicuculline and phaclofen were studied on evoked field responses, paired-pulse (PP) plasticity and single-unit activity of dentate granule cells (DGCs) and CA1 pyramidal cells (PCs) in anesthetized rats. The GABAB agonist, baclofen, increased population spike (PS) amplitudes in the dentate evoked by perforant path stimulation but decreased PS amplitudes in CA1 evoked by Schaffer collateral stimulation, whereas the GABAA antagonist, bicuculline, increased PS amplitudes in both regions. Neither baclofen nor bicuculline had significant effects on dendritically recorded population excitatory postsynaptic potentials (EPSPs) in the dentate or CA1 evoked by stimulation of their respective afferents. Control PP curves in the dentate revealed a triphasic response of inhibition/potentiation/inhibition, whereas control PP curves in CA1 manifested a biphasic response of inhibition/potentiation of test/conditioned PS amplitudes. Baclofen and bicuculline reversed the early and late phases of PP inhibition in the dentate and the early phase of PP inhibition in CA1. The GABAB antagonist, phaclofen, selectively reversed the effects of baclofen on PP inhibition in both the dentate and CA1. Whereas baclofen had no effect, bicuculline incre sed and phaclofen decreased DGC single-unit spontaneous firing rate, while baclofen decreased and bicuculline and phaclofen increased PC firing rate. These results support and extend studies suggesting that GABAergic feedback inhibition of DGCs and PCs is mediated by postsynaptic GABAA receptors and feedback inhibition of PCs is mediated by postsynaptic GABAB receptors. Our results also provide significant new evidence suggesting that postsynaptic inhibition in the dentate is not regulated by GABAB receptors and that feedback and feedforward inhibition of DGCs and PCs is regulated by presynaptic GABAB receptors located on GABAergic interneurons.     

Effect of low frequency stimulation of perforant path on kindling rate and synaptic transmission in the dentate gyrus during kindling acquisition in rats

Low frequency stimulation (LFS) has an inhibitory effect on kindling acquisition. In the present study the effect of the perforant path LFS on induction of rapid perforant path kindled seizures and synaptic transmission in the dentate gyrus was investigated. Animals were kindled by perforant path stimulation in a rapid kindling manner (12 stimulations per day). In one group of animals LFS (0.1 ms pulse duration at 1 Hz, 200 pulses, and 50-150 microA) was applied to perforant path, immediately after termination of each rapid kindling stimulation. Application of LFS significantly retarded the kindling acquisition and increased the number of stimulations to achieved different kindled seizure stages. LFS also prevented an increment in the slope of field excitatory postsynaptic potentials and population spike amplitude during kindling. In addition, LFS significantly reduced the marked increase in early (10-50 ms intervals) and late (300-1000 ms intervals) paired-pulse depression induced by kindling. According to obtained results, it may be suggested that LFS of perforant path has a significant antiepileptogenic effect through inhibition of synaptic transmission in dentate gyrus. Meanwhile, LFS prevents an increase in the paired-pulse depression during kindling acquisition.     

Hippocampal long-term depression as an index of spatial working memory

Long-term potentiation (LTP), a form of synaptic plasticity in the hippocampus, is a cellular model for the neural basis of learning and memory, but few studies have investigated the contribution of long-term depression (LTD), a counterpart of LTP. To address the possible relationship between hippocampal LTD and spatial performance, the spatial cognitive ability of a rat was assessed in a spontaneous alternation test and, thereafter, LTD in response to low-frequency burst stimulation (LFBS) was monitored in the dentate gyrus of the same rat under anaesthesia. To enhance a divergence in the ability for spatial performance, some of the animals received fimbria-fornix (FF) transection 14 days before the experiments. LTD was reliably induced by application of LFBS to the medial perforant path of intact rats, while no apparent LTD was elicited in rats with FF lesions. The behavioural parameters of spatial memory showed a significant correlation with the magnitude of LTD. We found no evidence that the cognitive ability correlated with other electrophysiological parameters, e.g. basal synaptic responses, stimulus intensity to produce half-maximal responses, paired-pulse facilitation or paired-pulse depression. These results suggest that the magnitude of LTD in the dentate gyrus serves as a reliable index of spatial cognitive ability, providing insights into the functional significance of hippocampal LTD.     

Fimbrial control of bidirectional synaptic plasticity of medial perforant path-dentate transmission

Lesions of the fimbria-fornix (FF) tract cause profound impairments of cognitive ability in animals. Our previous study showed that spatial performance correlates with long-term potentiation (LTP) of the dentate gyrus (DG), but not of the CA1 region, in rats with bilateral FF lesions, suggesting that FF lesions selectively inhibited LTP in the DG. The cortical input to the DG is anatomically and physiologically divided into two types of afferents, i.e., the medial perforant path (MPP) and the lateral perforant path (LPP), which show distinct synaptic properties. To elucidate the difference in the FF modulation of these two inputs, field responses were recorded from MPP- or LPP-DG synapses in anesthetized rats. MPP-DG synapses of rats with FF lesions displayed neither LTP in response to theta-burst stimulation nor long-term depression (LTD) in response to low-frequency burst stimulation. In contrast to the MPP, LPP-DG synapses showed normal LTP in rats with FF lesions. The low-frequency burst stimulation could not induce LTD at LPP-DG synapses in either intact or FF-lesioned rats. These results suggest that the FF pathway selectively supports the mechanisms of bidirectional synaptic plasticity at MPP-DG synapses. This study provides new insights into external control of information processing in the hippocampus.     

GABA(B) receptor-mediated heterosynaptic depression of excitatory synaptic transmission in rat frontal neocortex

Neocortical synapses display several forms of short-term plasticity including paired-pulse facilitation and depression. The mechanisms underlying this diversity are unclear. Synaptic currents in response to paired stimulation were recorded from layer II/III pyramidal neurons in rat frontal neocortical slices using the whole-cell patch-clamp method. Both paired-pulse facilitation (PPF) and paired-pulse depression (PPD) were observed in control saline. In the presence of 10 microM bicuculline (BIC), prominent PPD was consistently elicited. The maximal depression of the second EPSC occurred around 100 ms although PPD was still observed at intervals up to 1500 ms. Manipulations that reduced the probability of transmitter release significantly affected PPD. Both conditioning (C)- and test (T)-EPSCs were reduced when the extracellular Ca(2+) concentration was lowered from 3 to 1 mM. The decrease was greater in the C-EPSC resulting in a decrease in PPD. The gamma-aminobutyric acid (GABA)(B) receptor agonist baclofen (10 microM) reduced the amplitude of both evoked EPSCs and changed PPD to PPF. In the presence of the GABA(B) antagonists 2(OH)-saclofen (200-400 microM) or SCH50911 (10 microM), PPF was commonly observed. The metabotropic glutamate receptor antagonist MCPG (500 microM) had no effect on neocortical PPD. Brief stimulus trains induced a progressive depression that was insensitive to GABA(B) antagonists. Paired-pulse depression of excitatory synaptic transmission is a prominent phenomenon in frontal neocortex. At least two components of depression were observed. They may play an important role in regulating the balance between excitation and inhibition, therefore maintaining stability in cortical circuits.     

Effects of bicuculline and baclofen on paired-pulse depression in the dentate gyrus of epileptic patients

Paired-pulse field responses were recorded from the granule cell layer of the dentate gyrus in brain slices from temporal lobe epileptic patients. Paired-pulse depression (PPD) was examined using perforant path stimulation of low to moderate intensity at an inter-stimulus interval (ISI) of 20 ms. The paired-pulse ratio (PS₂/PS₁) was expressed as the population spike amplitude of the second response (PS₂) relative to that of the first response (PS₁). Representative tissue responses from each patient biopsy were divided into two groups that were significantly different based on the magnitude of the highest paired-pulse ratio recorded for each biopsy specimen: the strong paired-pulse depression group (PS₂/PS₁ = 0.12 ± 0.03; n = 15) and the weak paired-pulse depression group (PS₂/PS₁ = 0.68 ± 0.06; n = 13). Paired-pulse ratios from the strong PPD group were relatively independent of stimulus intensity, whereas, PPD was dependent on stimulus intensity in the weak PPD group; i.e., PPD was greatest at the lowest intensity and reached a plateau at higher intensities. Bicuculline (20 μM) and low concentrations of baclofen (0.1–0.2 μM) reduced paired-pulse depression in the strong PPD group, but did not significantly change the paired-pulse ratio in the weak PPD group. Paired-pulse facilitation was observed in some cases after inhibition was blocked pharmacologically. The number of population spikes was increased in the presence of bicuculline but was unchanged by baclofen. In the strong PPD group, baclofen significantly altered the EPSP-population spike (E-S) relationship by increasing the slope of the relationship for the second response, without having an effect on the slope of the first response. Baclofen had no effect on the E-S relationship of either response in the weak PPD group. The data are consistent with (1) less inhibition in the weak PPD group compared to the strong PPD group, (2) reduction of feedback inhibition in the strong PPD group by bicuculline and by low concentrations of baclofen, and (3) the occurrence of paired-pulse facilitation when inhibition was pharmacologically reduced in the dentate gyrus of temporal lobe epileptic patients. The results are also consistent with the presence of GABA_B receptors on human inhibitory interneurons that, when activated by baclofen, result in disinhibition of granule cells through feedback circuits. Although inhibition may be compromised in some epileptic human biopsy specimens, the presence of strong inhibition in other patients' biopsy material suggests the re-evaluation of the role of inhibition in epilepsy.     

The perforant path projection from the medial entorhinal cortex layer III to the subiculum in the rat combined hippocampal–entorhinal cortex slice

Intracellular recordings were performed to examine the perforant path projection from layer III of the entorhinal cortex to the subiculum in rat combined hippocampal–entorhinal cortex slices. Electrical stimulation in the medial entorhinal cortex layer III caused short latency combined excitatory and inhibitory synaptic responses in subicular cells. In the presence of the GABA_A antagonist bicuculline and the GABA_B antagonist CGP-55845 A inhibition was blocked and isolated AMPA- or NMDA receptor-mediated EPSPs could be elicited. After application of the non-NMDA antagonist NBQX and the NMDA antagonist APV excitatory responses were completely blocked indicating a glutamatergic input from the neurons of the medial entorhinal cortex layer III. By stimulation from a close (< 0.2 mm) position in the presence of NBQX and APV and either CGP-55845 A or bicuculline we could record monosynaptic fast GABA_A or slow GABA_B receptor-mediated IPSPs, respectively. We compared synaptic responses in subicular cells induced by stimulation in the medial entorhinal cortex layer III with responses elicited by stimulation of afferent fibres in the alveus. The EPSPs of subicular cells induced by stimulation of alvear fibres could be significantly augmented by simultaneous activation of perforant path fibres originating in the medial entorhinal cortex layer III, while delayed activation of alvear fibres after stimulation of the perforant path resulted in a weak inhibition of the alveus evoked EPSPs. Thus, the perforant path projection activates monosynaptic excitation of subicular neurons. Therefore the entorhinal cortex does not only function as an important input structure of the hippocampal formation but is also able to modulate the hippocampal output via the entorhinal–subicular circuit.     

Analysis of short-term plasticity at the perforant path-granule cell synapse

Short-term plasticity was investigated at the perforant path-granule cell synapse in the hippocampal slice preparation. A successive decrement in the amplitude of the extracellular EPSP was obtained at all stimulus frequencies above 0.05 Hz. This effect of repetitive stimulation has previously been shown to fulfill the requirements for habituation processes. If each stimulus within an habituation train was followed by a second identical test stimulus the response to the test stimulus was larger than that to the paired conditioning stimulus. This short-term plasticity has been called paired pulse potentiation. The test response potentiated only with respect to the paired conditioning response and not with respect to previous test responses. Neither form of plasticity appeared to result from changes in the amplitude of the afferent fiber volley. Both habituation and paired pulse potentiation result from an interaction of at least three changes in the efficacy of transmission after a conditioning stimulus: (1) an initial depression, (2) an intermediate relative potentiation and (3) a late depression which decays slowly. Paired pulse potentiation could be demonstrated only if the interpair interval corresponded to the period of maximal late depression and the interstimulus interval to the period of relative potentiation. The amplitudes of intermediate relative potentiation and late depression (and inhibition of transmission by 2-amino-4-phosphonobutyric acid (APB)) were inversely related to the control response amplitude. This relationship likely derives from nonlinear stimulation of postsynaptic ionic currents at higher stimulus intensities. In contrast, the initial depression increased with response amplitude. This is consistent with a mechanism dependent on the postsynaptic membrane potential, such as refractoriness to succeeding stimuli. When the response amplitudes in the presence and absence of 2.5 mM APB were equalized by adjusting the stimulus intensity, no difference was found in the magnitude of either form of plasticity. Since APB probably inhibits transmission at this site through competitive antagonism at the postsynaptic receptor, this observation suggests that habituation and paired pulse potentiation are generated presynaptically.     

Kynurenic acid as an antagonist of hippocampal excitatory transmission

Kynurenate, an endogenous tryptophan metabolite, was bath-applied to hippocampal slices while recording extracellular synaptic field potentials. Kynurenate antagonized the medial and lateral entorhinal projections to dentate granule cells, the Schaffer collateral projections to CA1 pyramidal cells, and inputs to the CA3 stratum radiatum of regio inferior with similar potencies. Concentration-response curves for these pathways paralleled theoretical antagonist curves with a Hill coefficient of 1, and the KdS were in the range of 130-400 microM. Projections to the stratum lucidum of regio inferior were much less sensitive to kynurenate. Inputs to CA3 pyramidal cells showed varying sensitivities to kynurenate, L-2-amino-4-phosphonobutanoic acid (L-APB), and (-)-baclofen depending on the placements of the stimulating and recording electrodes. When both electrodes were located in area CA3, outside the hilus of area dentata, all responses were insensitive to inhibition by L-APB. Under these conditions, responses recorded within the stratum radiatum were sensitive to inhibition by kynurenate and baclofen, while responses recorded within the stratum lucidum were insensitive to these drugs. When the stimulating electrode was placed within the hilus of area dentata, variable patterns of sensitivity to APB, baclofen, and kynurenate were observed from recording electrodes in area CA3. These results suggest that stimulation in the hilus, while recording in the stratum lucidum, produces responses that show composite effects resulting both from direct stimulation of mossy fibers and from stimulation of neuronal elements in the hilus which produce outputs to mossy fibers.     

Long-term Potentiation and Field EPSPs in the Lateral and Medial Perforant Paths in the Dentate Gyrus In Vitro: a Comparison

The entorhinal cortex projects monosynaptically to the granule cells in the dentate gyrus via the lateral and medial perforant paths. These two subdivisions of the perforant path differ with respect to synaptic properties, and recent studies suggest that they also differ with respect to long-term potentiation (LTP). In the present study, using the in vitro slice preparation of the guinea-pig hippocampus, field excitatory postsynaptic potentials (EPSPs) and LTP in the lateral and medial perforant paths were compared. The two pathways were distinguished on the basis of their different termination in the dendritic layer, their different pharmacology and short-term synaptic facilitation. The field EPSP [obtained in the presence of γ-aminobutyric acid (GABA) A and B receptor antagonists] consisted of a non- N -methyl- d -aspartate (NMDA) component with different time characteristics in the two pathways, the decay being monoexponential in the lateral perforant path and biexponential in the medial one. In addition, the field EPSP in both pathways contained a small NMDA-mediated component that could also be observed after complete blockade of the non-NMDA one. LTP induction in both lateral and medial perforant paths was facilitated by blockade of GABA_A inhibition, showed associative properties, and was blocked by NMDA receptor antagonists. Following the induction event, LTP in both pathways developed to a peak value within 30–40 s, and the stability of LTP was correlated with the amount of postsynaptic, but not presynaptic, activity during the induction event. During blockade of GABA_A inhibition the opioid receptor antagonist naloxone and the β-adrenergic antagonist timolol had no effect on the magnitude or stability of LTP. It is concluded that LTP in the lateral and medial perforant paths does not differ with respect to induction mechanisms and early temporal characteristics.     

Impairment of in vivo theta-burst long-term potentiation and network excitability in the dentate gyrus of synaptopodin-deficient mice lacking the spine apparatus and the cisternal organelle

The function of the spine apparatus in dendritic spines and the cisternal organelles in axon initial segments is little understood. The actin-associated protein, synaptopodin, is essential for the formation of these organelles which are absent in synaptopodin -/- mice. Here, we used synaptopodin -/- mice to explore the role of the spine apparatus and the cisternal organelle in synaptic plasticity and local circuit excitability in response to activation of the perforant path input to the dentate gyrus in vivo. We found impaired long-term potentiation following theta-burst stimulation, whereas tetanus-evoked LTP was unaffected. Furthermore, paired-pulse inhibition of the population spike was reduced and granule cell excitability was enhanced in mutants, hence revealing an impairment of local network inhibition. In summary, our data represent the first electrophysiological evidence that the lack of the spine apparatus and the cisternal organelle leads to a defect in long-term synaptic plasticity and alterations in local circuit control of granule cell excitability under adult in vivo conditions.