Field EPSP Slope Research Articles

Deep brain stimulation effects on learning, memory and glutamate and GABAA receptor subunit gene expression in kindled rats.

Epileptic seizures are accompanied by learning and memory impairments. In this study, the effect of low frequency stimulation (LFS) on spatial learning and memory was assessed in kindled animals and followed for one month. Fully kindled rats received LFS at 4 times (immediately, 6 h, 24 h and 30 h following the final kindling stimulation). Applying LFS improved kindled animals' performance in the Barnes maze test. This LFS action was accompanied by a decrease in NR2B gene expression, an increase in the gene expression of the α subunit of calcineurin A and an increased NR2A/NR2B ratio in kindled animals. In addition, the gene expression of the GABAA receptor γ2 subunit increased at 2-3 h after applying LFS. The increase in NR2A/NR2B ratio was also observed 1 week after LFS. No significant changes were observed one month after LFS administration. Field potential recordings in the hippocampal CA1 area showed that kindling-induced potentiation of the field EPSP slope returned to near baseline when measured 2-3 h after applying LFS. Therefore, it may be postulated that applying LFS in kindled animals reduced the seizure-induced learning and memory impairments, albeit time-dependently. In tandem, LFS prevented kindling-induced alterations in gene expression of the described proteins, which are potentially important for synaptic transmission and/or potentiation. Moreover, a depotentiation-like phenomenon may be a possible mechanism underlying the LFS action. Epileptic seizures are accompanied by learning and memory impairments. In this study, the effect of low frequency stimulation (LFS) on spatial learning and memory was assessed in kindled animals and followed for one month. Fully kindled rats received LFS at 4 times (immediately, 6 h, 24 h and 30 h following the final kindling stimulation). Applying LFS improved kindled animals’ performance in the Barnes maze test. This LFS action was accompanied by a decrease in NR2B gene expression, an increase in the gene expression of the α subunit of calcineurin A and an increased NR2A/NR2B ratio in kindled animals. In addition, the gene expression of the GABAA receptor γ2 subunit increased at 2–3 h after applying LFS. The increase in NR2A/NR2B ratio was also observed 1 week after LFS. No significant changes were observed one month after LFS administration. Field potential recordings in the hippocampal CA1 area showed that kindling-induced potentiation of the field EPSP slope returned to near baseline when measured 2–3 h after applying LFS. Therefore, it may be postulated that applying LFS in kindled animals reduced the seizure-induced learning and memory impairments, albeit time-dependently. In tandem, LFS prevented kindling-induced alterations in gene expression of the described proteins, which are potentially important for synaptic transmission and/or potentiation. Moreover, a depotentiation-like phenomenon may be a possible mechanism underlying the LFS action.

DL0410 ameliorates cognitive deficits in APP/PS1 transgenic mice by promoting synaptic transmission and reducing neuronal loss.

At present, few available drugs can be used to either improve pathological features or prevent the progression of Alzheimer’s disease (AD). DL0410 ((1,1′-([1,1′-biphenyl]-4,4′-diyl) bis (3-(piperidin-1-yl) propan-1-one) dihydrochloride) is a multiple-target small molecule that has been found to reverse cognitive impairment in different animal models of AD. In this study we evaluated the cognition-improving effects of DL0410 in APP/PS1 transgenic mice and explored the underlying mechanisms. APP/PS1 transgenic mice were administered DL0410 (3, 10, 30 mg· kg−1· d−1, ig) for 2 months. We found that DL0410 administration significantly ameliorated cognitive deficits in both the nest-building and Morris water maze tests. In electrophysiological analysis of hippocampal slices, we showed that DL0410 administration significantly enhanced the field EPSP slope and HFS-induced LTP in CA1 area. Furthermore, we revealed that DL0410 administration significantly increased the phosphorylation of AKT and the activity of GSK-3β in the hippocampus and cortex. Moreover, DL0410 administration dose-dependently increased the expression level of phosphorylated ERK1/2 in the hippocampus and cortex. In addition, DL0410 dose-dependently decreased the neuronal loss by decreasing the production of Aβ deposition, inhibited glial overactivation, and the production of inflammatory cytokines such as TNF-α, IL-1β, and IL-6. We conclude that DL0410 ameliorates cognitive deficits in APP/PS1 transgenic mice by promoting synaptic transmission via activating the AKT/GSK-3β and MAPK/ERK signaling pathway and reducing neuronal loss. DL0410 may be an effective agent for AD treatment in the future.

Chemogenetic Activation of Excitatory Neurons Alters Hippocampal Neurotransmission in a Dose-Dependent Manner.

Designer receptors exclusively activated by designer drugs (DREADD)-based chemogenetic tools are extensively used to manipulate neuronal activity in a cell type-specific manner. Whole-cell patch-clamp recordings indicate membrane depolarization, coupled with increased neuronal firing rate, following administration of the DREADD ligand, clozapine-N-oxide (CNO) to activate the Gq-coupled DREADD, hM3Dq. Although hM3Dq has been used to enhance neuronal firing in order to manipulate diverse behaviors, often within 30 min to 1 h after CNO administration, the physiological effects on excitatory neurotransmission remain poorly understood. We investigated the influence of CNO-mediated hM3Dq DREADD activation on distinct aspects of hippocampal excitatory neurotransmission at the Schaffer collateral-CA1 synapse in hippocampal slices derived from mice expressing hM3Dq in Ca2+/calmodulin-dependent protein kinase α (CamKIIα)-positive excitatory neurons. Our results indicate a clear dose-dependent effect on field EPSP (fEPSP) slope, with no change noted at the lower dose of CNO (1 μM) and a significant, long-term decline in fEPSP slope observed at higher doses (5–20 μM). Further, we noted a robust θ burst stimulus (TBS) induced long-term potentiation (LTP) in the presence of the lower CNO (1 μM) dose, which was significantly attenuated at the higher CNO (20 μM) dose. Whole-cell patch-clamp recording revealed both complex dose-dependent regulation of excitability, and spontaneous and evoked activity of CA1 pyramidal neurons in response to hM3Dq activation across CNO concentrations. Our data indicate that CNO-mediated activation of the hM3Dq DREADD results in dose-dependent regulation of excitatory hippocampal neurotransmission and highlight the importance of careful interpretation of behavioral experiments involving chemogenetic manipulation.

Maturation- and sex-sensitive depression of hippocampal excitatory transmission in a rat schizophrenia model

Schizophrenia is associated with behavioral and brain structural abnormalities, of which the hippocampus appears to be one of the most consistent region affected. Previous studies performed on the poly I:C model of schizophrenia suggest that alterations in hippocampal synaptic transmission and plasticity take place in the offspring. However, these investigations yielded conflicting results and the neurophysiological alterations responsible for these deficits are still unclear. Here we performed for the first time a longitudinal study examining the impact of prenatal poly I:C treatment and of gender on hippocampal excitatory neurotransmission. In addition, we examined the potential preventive/curative effects of risperidone (RIS) treatment during the peri-adolescence period. Excitatory synaptic transmission was determined by stimulating Schaffer collaterals and monitoring fiber volley amplitude and slope of field-EPSP (fEPSP) in CA1 pyramidal neurons in male and female offspring hippocampal slices from postnatal days (PNDs) 18–20, 34, 70 and 90. Depression of hippocampal excitatory transmission appeared at juvenile age in male offspring of the poly I:C group, while it expressed with a delay in female, manifesting at adulthood. In addition, a reduced hippocampal size was found in both adult male and female offspring of poly I:C treated dams. Treatment with RIS at the peri-adolescence period fully restored in males but partly repaired in females these deficiencies. A maturation- and sex-dependent decrease in hippocampal excitatory transmission occurs in the offspring of poly I:C treated pregnant mothers. Pharmacological intervention with RIS during peri-adolescence can cure in a gender-sensitive fashion early occurring hippocampal synaptic deficits.

A Novel Compound Targeting Kv7.2/3 Channels Relieves Inflammatory and Neuropathic Pain

Encoded by Kv7.2/Kv7.3 subunit assembly, M-channels generate sub-threshold voltage-gated K+ currents that regulate neuronal excitability. Inour quest to target neuronal M-channels, we previously designed noveldiphenylamine carboxylates that are powerful Kv7.2 channel openers. These drugs stabilize the Kv7.2 channel open state by interacting with its voltage sensing domain (VSD). Using the same structural framework, we designed a novel molecule, NH34, to test its activity on neuronal excitability and probe its potency in vivo on inflammatory and neuropathic pain. In transfected CHO cells, NH34 significantly increased Kv7.2 currents, by producing a hyperpolarizing voltage shift of the activation curve (EC50 = 17.5 µM). Interestingly, two S4 mutants, R198A and R207W significantly decreased the opener potency of the drug, suggesting that NH34, like NH29, may interact with the VSD of Kv7.2. NH34 preferentially acts as an opener of Kv7.2 and Kv7.3 channels. NH34 was ineffective on Kv7.4 and Kv7.5 but produced a significant inhibition of TRPV1 cation channels. In CA1 pyramidal neurons from rat hippocampal slices, NH34 reduced the spike discharge frequency evoked by injection of depolarizing current. NH34 significantly depressed the slope of field epsp and the fiber volley amplitude recorded extracellularly in the CA1 pyramidal region following stimulation of the Schaeffer collaterals. NH34 reduced the frequency of evoked spike discharge in dorsal root ganglion neurons. In vivo, NH34 (3-30 mg/kg p.o) significantly relieved the hyperalgesia evoked by mechanical pressure of the paw in a rat model of inflammatory pain. Similarly, NH34 (3-30 mg/kg p.o) was very efficient in alleviating the hyperalgesia evoked by mechanical pressure or cold allodynia in a Seltzer rat model of neuropathic pain. In all, the data suggest that NH34 is a very promising drug template for treating inflammatory and neuropathic pain.

Altered Network Timing in the CA3-CA1 Circuit of Hippocampal Slices from Aged Mice

Network patterns are believed to provide unique temporal contexts for coordinating neuronal activity within and across different regions of the brain. Some of the characteristics of network patterns modeled in vitro are altered in the CA3 or CA1 subregions of hippocampal slices from aged mice. CA3–CA1 network interactions have not been examined previously. We used slices from aged and adult mice to model spontaneous sharp wave ripples and carbachol-induced gamma oscillations, and compared measures of CA3–CA1 network timing between age groups. Coherent sharp wave ripples and gamma oscillations were evident in the CA3–CA1 circuit in both age groups, but the relative timing of activity in CA1 stratum pyramidale was delayed in the aged. In another sample of aged slices, evoked Schaffer collateral responses were attenuated in CA3 (antidromic spike amplitude) and CA1 (orthodromic field EPSP slope). However, the amplitude and timing of spontaneous sharp waves recorded in CA1 stratum radiatum were similar to adults. In both age groups unit activity recorded juxtacellularly from unidentified neurons in CA1 stratum pyramidale and stratum oriens was temporally modulated by CA3 ripples. However, aged neurons exhibited reduced spike probability during the early cycles of the CA3 ripple oscillation. These findings suggest that aging disrupts the coordination of patterned activity in the CA3–CA1 circuit.

Involvement of High-Conductance Calcium-Dependent Potassium Channels in Short-Term Presynaptic Plasticity in the Rat Dentate Gyrus

We examined the involvement of high-conductance calcium-dependent potassium (BK) channels in short-term presynaptic plasticity in the rat dentate gyrus; a paired-pulse stimulation protocol was used for evaluation of this phenomenon. Paired-pulse responses were recorded from the dentate gyrus of rats while stimulating the medial part of the perforant path with different interpulse intervals (IPIs). Iberiotoxin (IbTX), a selective blocker of BK channels, at doses of 50 and 100 μg/kg was i.p. administered 30 min presurgery. The population spike (PS) amplitude ratio and field EPSP (fEPSP) amplitude and slope ratios were measured at IPIs 10, 20, 30, and 50 msec as indices of synaptic facilitation and/or depression. At IPIs of 10 and 20 msec, there was no significant increase in the PS amplitude ratio after IbTX. However, at longer IPIs (30 and 50 msec), there was a significant dose-dependent increase in this ratio vs the vehicle group (P < 0.05 and P < 0.01, respectively). There were a slight decreases in the fEPSP amplitude ratio at short IPIs (10, 20, and 30 msec) in rats pretreated with IbTX, while mild incrases in the fEPSP amplitude ratio were observed at longer IPIs (50 msec). With respect to the fEPSP slope ratio, IbTX dose-dependently and insignificantly increased it. In addition, longer IPIs did not provide significant changes in the fEPSP slope ratio. Thus, high-conductance calcium-dependent potassium channels in the rat dentate gyrus have a modulatory (inhibitory) and (apparently) regulatory role in short-term presynaptic plasticity at relatively long ISIs, and blocking of these channels leads to paired-pulse facilitation.

Experimental Hypothyroidism Delays Field Excitatory Post‐Synaptic Potentials and Disrupts Hippocampal Long‐term Potentiation in the Dentate Gyrus of Hippocampal Formation and Y‐Maze Performance in Adult Rats

Manipulations of thyroid hormones have been shown to influence learning and memory. Although a large body of literature is available on the effect of thyroid hormone deficiency on learning and memory functions during the developmental stage, electrophysiological and behavioural findings, particularly on propylthiouracil administration to adult normothyroid animals, are not satisfactory. The experiments in the present study were carried out on 12 adult male Wistar rats aged 6-7 months. Hypothyroidism was induced by administering 6-n-propyl-2-thiouracil in their drinking water for 21 days at a concentration of 0.05%. The spatial learning performance of hypothyroid and control rats was studied on a Y-maze. The rats were then placed in a stereotaxic frame under urethane anaesthesia. A bipolar tungsten electrode was used to stimulate the medial perforant path. A glass micropipette was inserted into the granule cell layer of the ipsilateral dentate gyrus to record field excitatory post-synaptic potentials. After a 15-min baseline recording of field potentials, long-term potentiation was induced by four sets of tetanic trains. The propylthiouracil-treated rats showed a significantly attenuated input-output (I/O) relationship when population spike (PS) amplitudes and field excitatory post-synaptic potentials (fEPSP) were compared. fEPSP and PS latencies were found to be longer in the hypothyroid group than in the control group. The PS amplitude and fEPSP slope potentiations in the hypothyroid rats were not statistically different from those in the control rats, except for the field EPSP slope measured in the post-tetanic and maintenance phases. The hypothyroid rats also showed lower thyroxine levels and poor performance in the spatial memory task. The present study provides in vivo evidence for the action of propylthiouracil leading to impaired synaptic plasticity, which might explain deficit in spatial memory tasks in adult hypothyroid rats.

Long-Term Plasticity Is Proportional to Theta-Activity

BackgroundTheta rhythm in the hippocampal formation is a main feature of exploratory behaviour and is believed to enable the encoding of new spatial information and the modification of synaptic weights. Cyclic changes of dentate gyrus excitability during theta rhythm are related to its function, but whether theta epochs per se are able to alter network properties of dentate gyrus for long time-periods is still poorly understood.Methodology/Principal FindingsWe used low-frequency stimulation protocols that amplify the power of endogenous theta oscillations, in order to estimate the plasticity effect of endogenous theta oscillations on a population level. We found that stimulation-induced augmentation of the theta rhythm is linked to a subsequent increase of neuronal excitability and decrease of the synaptic response. This EPSP-to-Spike uncoupling is related to an increased postsynaptic spiking on the positive phases of theta frequency oscillations. Parallel increase of the field EPSP slope and the population spike occurs only after concurrent pre- and postsynaptic activation. Furthermore, we observed that long-term potentiation (>24 h) occurs in the dentate gyrus of freely behaving adult rats after phasic activity of entorhinal afferents in the theta-frequency range. This plasticity is proportional to the field bursting activity of granule cells during the stimulation, and may comprise a key step in spatial information transfer. Long-term potentiation of the synaptic component occurs only when the afferent stimulus precedes the evoked population burst, and is input-specific.Conclusions/SignificanceOur data confirm the role of the dentate gyrus in filtering information to the subsequent network during the activated state of the hippocampus.

Involvement of Protein Synthesis and Degradation in Long-Term Potentiation of Schaffer Collateral CA1 Synapses

Expression of synaptic plasticity involves the translation of mRNA into protein and, probably, active protein degradation via the proteasome pathway. Here, we report on the rapid activation of synthesis and degradation of a probe protein with the induction of long-term potentiation (LTP) in the hippocampal Schaffer collateral CA1 pathway. The proteasome inhibitor MG132 significantly reduced the field EPSP slope potentiation and LTP maintenance without acutely affecting basal synaptic transmission. To visualize protein dynamics, CA1 pyramidal cells of hippocampal slices were transfected with Semliki Forest virus particles expressing a recombinant RNA. This RNA contained the coding sequence for a degradable green fluorescence protein with a nuclear localization signal (NLS-d1EGFP) followed by a 3'- untranslated region dendritic targeting sequence. NLS-d1EGFP fluorescence remained stable in the low-frequency test stimulation but increased with LTP induction in the cell body and in most dendritic compartments of CA1 neurons. Applying anisomycin, a protein synthesis inhibitor, caused NLS-d1EGFP levels to decline; a proteasome inhibitor MG132 reversed this effect. In the presence of anisomycin, LTP induction accelerated the degradation of NLS-d1EGFP. When both inhibitors were present, NLS-d1EGFP levels remained unaffected by LTP induction. Moreover, LTP-induced acceleration of NLS-d1EGFP synthesis was blocked by rapamycin, which is consistent with the involvement of dendritic mammalian target of rapamycin in LTP-triggered translational activity. Our results clearly demonstrate that LTP induction not only leads to a rapid increase in the rate of protein synthesis but also accelerates protein degradation via the proteasome system.

Locus Ceruleus Activation Initiates Delayed Synaptic Potentiation of Perforant Path Input to the Dentate Gyrus in Awake Rats: A Novel β-Adrenergic- and Protein Synthesis-Dependent Mammalian Plasticity Mechanism

Norepinephrine, acting through beta-adrenergic receptors, is implicated in mammalian memory. In in vitro and in vivo studies, norepinephrine produces potentiation of the perforant path-dentate gyrus evoked potential; however, the duration and dynamics of norepinephrine-induced potentiation have not been explored over extended time periods. To characterize the long-term effects of norepinephrine on granule cell plasticity, the present study uses glutamatergic activation of the locus ceruleus (LC) to induce release of norepinephrine in the hippocampus of the awake rat and examines the subsequent modulation of the dentate gyrus evoked potential for 3 hr (short term) and 24 hr (long term) after LC activation. LC activation initiates a potentiation of the field EPSP slope observed 24 hr later. This late-phase potentiation of the synaptic potential is not preceded by early phase potentiation, although spike potentiation can be seen both immediately after, and 24 hr after, LC activation. Intracerebroventricular infusion of the beta-adrenergic antagonist, propranolol, or the protein synthesis inhibitor, anisomycin, before LC activation blocks the potentiation of perforant path input observed at 24 hr. The initiation of late-phase synaptic potentiation observed at 24 hr but not at the 3 hr after LC activation parallels the observation of a cAMP- and protein synthesis-dependent long-lasting synaptic facilitation in Aplysia that is not preceded by short-term synaptic facilitation. Locus ceruleus-initiated synaptic potentiation may selectively support long-term, rather than short-term, memory. The observation of selective initiation of long-term synaptic facilitation in a mammalian brain, as in invertebrates, is additional evidence that these two forms of memory depend on separable biological mechanisms.

Effects of stimulus frequency and age on bidirectional synaptic plasticity in the dentate gyrus of freely moving rats

We investigated the frequency-dependent transition from homosynaptic long-term depression (LTD) to long-term potentiation (LTP) at the lateral perforant pathway/dentate gyrus synapse in adult (90 days of age) and immature (15 days of age) awake, freely moving rats. Dentate-evoked field potentials were recorded and analyzed using the population spike amplitude and the field EPSP slope measures following sustained stimulation (900 pulses) of the lateral perforant pathway at various frequencies (1, 3, 7, 30, 50, or 200 Hz). Our results indicate that both the strength and the direction (LTP or LTD) of synaptic plasticity vary as a function of activation frequency: sustained low-frequency stimulation ranging from 1 to 7 Hz results in depression of activated synapses, whereas high-frequency stimulation (30–200 Hz) produces potentiation. In addition, a significant ( P < 0.01) ontogenetic shift in the frequency of transition from LTD to LTP was observed; the transition frequency in immature animals was significantly lower than that obtained in adult animals. These observations agree strongly with the prediction of the Bienenstock–Cooper–Munro theory of synapse modification, indicating perhaps a neurophysiological basis for this theoretical model of learning in the dentate gyrus of awake behaving rats.

Unilateral inner ear damage results in lasting changes in hippocampal CA1 field potentials in vitro.

We investigated the effects of a surgical lesion of one vestibular inner ear (unilateral vestibular damage [UVD]) on the field potential responses of CA1 neurons in vitro. Hippocampal slices were removed from rats at 4-6 weeks or 5-6 months post-UVD, and the field responses of CA1 neurons to electrical stimulation of the Schaffer collateral commissural pathway were analyzed. Compared with slices from sham and naive control animals, slices from UVD animals at 5-6 months post-UVD exhibited decreases in the population spike amplitude, the somal field excitatory postsynaptic potential (sfEPSP) slope, and the field EPSP (fEPSP) slope. For the population spike amplitude and fEPSP slope, this effect was observed in both CA1 ipsilateral and contralateral to the UVD. On both the ipsilateral and contralateral sides, paired-pulse testing showed increases in paired-pulse inhibition at the shortest interstimulus intervals (ISIs), with increases in paired-pulse facilitation at longer ISIs. This study provides the first evidence that peripheral vestibular damage can produce long-term changes in hippocampal electrophysiological activity in vitro.

Long-term suppression of synaptic transmission by tetanization of a single pyramidal cell in the mouse hippocampus in vitro.

1. The consequences of stimulating a single pyramidal cell in the CA1 area of the hippocampus for synaptic transmission in the stratum radiatum were investigated. 2. Tetanic activation of single pyramids caused by depolarizing current injection, but not an equal number of distributed action potentials, reduced excitatory transmission by 20 %, with a delayed onset, for more than 1 h. 3. EPSPs in the tetanized pyramidal cells were increased for equally long periods but this was not the cause of the field EPSP reduction. Spontaneous somatic IPSPs were not affected; evoked IPSPs were decreased in the tetanized cell. 4. Paired pulse facilitation of the field EPSPs was unchanged. 5. The field EPSP reduction was markedly diminished by a knife cut along the base of pyramidal cells in CA1. 6. The addition of antagonists of GABA, NMDA and metabotropic glutamate receptors blocked or diminished the field EPSP slope reduction evoked by intracellular stimulation. 7. Simultaneous recordings revealed long-lasting excitations of interneurons located in the outer oriens layer as a result of single pyramid tetanization. 8. Intense firing of small numbers of pyramidal cells can thus persistently inhibit mass transmission through the hippocampus. This effect involves activation of interneurons by glutamate receptors.

Potentiation of synaptic transmission by (S)-3,5-dihydroxy phenylglycine in the rat dentate gyrus in vitro: A role for voltage dependant calcium channels and protein kinase C

1. 1. The authors have previously shown that direct activation of metabotropic glutamate receptors (mGluRs) by (S)-3,5-dihydroxyphenylglycine ((S)-DHPG) can induce a long-lasting potentiation of synaptic transmission in the rat dentate gyrus in vitro. Here the authors provide further characterisation of this agonist-induced potentiation. 2. 2. Field excitatory post-synaptic potentials were recorded from the dentate gyrus of rat hippocampal slices prepared by standard methods. 3. 3. (S)-DHPG (40 μM) induced a significant potentiation of the field EPSP slope (148. 6 ± 4. 3% compared to controls, n = 5), which occluded tetanically-induced LTP. 4. 4. This potentiation was inhibited by the PKC inhibitors staurosporine (0. 1 μM) and H-7 (100 μM) and by the voltage dependent Ca 2+ channel (VDCC) blockers NiCl 2 (50 μM) and nifedipine (20 μM). 5. 5. The mGluR5 specific agonist (RS)-2-Chloro-5-Hydroxyphenylglycine (CHPG) did not induce a potentiation when applied to slices at concentrations from 20 μM to 1 mM indicating that the (S)-DHPG potentiation may be mediated through group I subtype 1 mGluRs. 6. 6. In conclusion the (S)-DHPG-induced potentiation observed in our studies may be PKC dependent and is likely to be mediated through both T/L subtype VDCC and mGluR1 subtype receptors.

Proline-induced inhibition of glutamate release in hippocampal area CA1

Concentrations of proline typical of human CSF have been shown to potentiate transmission at Schaffer collateral–commissural synapses on CA1 pyramidal cells of the rat hippocampus. This study tested the hypothesis that proline enhances excitatory synaptic transmission by increasing glutamate release. Two concentrations of proline were used: a concentration typical of normal human CSF (3 μM) and a concentration typical of CSF in persons with the genetic disorder hyperprolinemia type II (30 μM). Continuous exposure of hippocampal slices to either concentration of proline potentiated Schaffer collateral–commissural synaptic transmission. Proline shifted the plot of field EPSP slope against fiber volley amplitude upward. Contrary to the original hypothesis, neither concentration of proline reduced paired-pulse facilitation; 30 μM proline enhanced paired-pulse facilitation, whereas 3 μM proline had no effect. In line with its enhancement of paired-pulse facilitation, 30 μM proline reduced both the K +-evoked release of glutamate and aspartate from CA1 slices and the release of glutamate and aspartate from CA1 synaptosomes evoked by 4-aminopyridine. These results suggest that the proline-induced potentiation of Schaffer collateral–commissural synaptic transmission probably involves a postsynaptic, rather than a presynaptic, mechanism. Concentrations of proline normally found in human CSF little affect glutamate release. However, proline-induced inhibition of glutamate release may contribute to the neuropsychiatric disorders associated with hyperprolinemia type II.

Metabotropic Glutamate Receptor Mediated Long-term Depression in Developing Hippocampus

The effects of bath application of the metabotropic glutamate receptor (mGluR) agonist 1 S,3 R-1-aminocyclopentane-1,3-dicarboxylic acid (ACPD, 10 μM) were studied at the Schaffer collateral-CA1 synapse in hippocampal slices from rats of 8–33 days postnatal age. In immature animals (8–12 days) ACPD induced a biphasic response characterized by an acute decrease in field EPSP slope (∼50–60% of baseline) in the presence of the agonist, followed by long-term depression (LTD, ∼75–80% of baseline) after washout. In animals older than 20 days, ACPD induced a slow onset potentiation or minimal change. Both the acute depression and LTD were blocked by the mGluR antagonist α-methyl-4-carboxyphenyl glycine (MCPG). ACPD-induced LTD was blocked by the N-methyl- d-aspartate receptor (NMDAR) antagonists d(−)-2-amino-5 phosphopentanoic acid (AP5) and dizocilpine maleate (MK-801), and by ethanol. Glutamic pyruvic transaminase, an enzyme that selectively metabolizes endogenous extracellular glutamate, also blocked LTD suggesting that the requisite NMDA currents were tonically activated by extracellular rather than synaptically released glutamate. ACPD-induced LTD was blocked by staurosporine, indicating a requirement for serine-threonine kinase activation, and was unaffected by the L-type voltage sensitive calcium channel blocker nitrendipine and the A1 adenosine receptor antagonist 8-cyclopentyl-1,3-dimethylxanthine (CPT). Because mGluR-mediated LTD was observed only in immature CA1, mGluRs may play a role in hippocampal development, perhaps by contributing to synapse pruning in a temporally restricted fashion. © 1997 Elsevier Science Ltd.

(±)CPP, an NMDA receptor antagonist, blocks the induction of commissural-CA3 LTP in the anesthetized rat

Commissural CA3-CA3 (cCA3) long-term potentiation (LTP) was investigated in the anesthetized rat treated with the highly selective NMDA-receptor antagonist d, l-3[(±)-2-carboxypiperazin-4-yl]-propyl-1-phosphonic acid (CPP). Intraperitoneal injections of CPP did not significantly affect baseline test responses for either field EPSP slope or amplitude measures but did reduce LTP in a dose-dependent manner, with 3.2 mg/kg as the lowest effective dose. EPSP variability following tetanization was also significantly reduced in both the 3.2 mg/kg and 10.0 mg/kg groups. We interpret these results to suggest that a 3.2 mg/kg dose of CPP may be sufficient for studying the behavioral effects of this NMDA receptor antagonist.

Brain temperature- and behavior-related changes in the dentate gyrus field potential during sleep, cold water immersion, radiant heating, and urethane anesthesia

The field potential evoked in the dentate gyrus (DG) by stimulation of the perforant path (PP) is known to vary with ongoing behavior and with brain temperature. To further study these phenomena chronic stimulating and recording electrodes were implanted into the PP and DG of rats, and a thermistor was implanted into the contralateral homotopic DG. Field potentials and brain temperature records were made during (1) slow wave sleep (SWS), (2) radiant heating, (3) immersion in cool water, (4) a control session during which no manipulations were made, and (5) under urethane anesthesia. In another group of rats field potentials were recorded during (1) baseline immobile wakefulness, (2) SWS, (3) before SWS or after gentle awakening from SWS (eyes open and presence of intermittent slow waves in the EEG), (4) immobile wakefulness, and (5) 24 h later. Findings were that field EPSP slope decreased and population spike (PS) amplitude increased by up to 60% of baseline values during conditions in which brain temperature was reduced (SWS, immersion in cool water, urethane anesthesia). Conversely, EPSP slope increased and PS amplitude decreased by up to 100% of baseline values during conditions in which brain temperature increased (awakening from SWS, radiant heating, and warming after immersion in cool water or urethane anesthesia). Product moment correlations between brain temperature and field potential measures confirmed the statistical reliability of these findings and accounted for up to 77% of the variance. These findings confirm the robust effect on hippocampal field potentials of brain temperature changes due to exogenous heating and cooling, and extend this effect to anesthetic- and sleep-induced brain temperature changes. They also identify a state that behaviorally resembles quiet wakefulness but resembles SWS in terms of neocortical EEG, brain temperature, and hippocampal field potential measures. The findings indicate the need to control for brain temperature-mediated changes in hippocampal research that uses the dentate gyrus field potential as a dependent measure.

Reversal of long-term potentiation (depotentiation) induced by tetanus stimulation of the input to CA1 neurons of guinea pig hippocampal slices

The reduction of the long-term potentiated response induced by tetanus (depontentiation (DP) of LTP) was investigated by the delivery of a train of low-frequency afferent stimuli (depotentiating stimulation; DPS) after the tetanus (100 Hz, 100 pulses) in CA1 neurons of the guinea pig's hippocampal slice. The parameters of DPS (frequencies of 1, 2, 5, and 10 Hz; number of pulses of 200 and 1000; and the time-lag after tetanus of 20 and 100 min) were altered systematically and their effects on LTP were evaluated through the analysis of the slope of field EPSP (S-EPSP) and amplitude and peak latency of population spike (A- and L-PS). DPS of 1 Hz, 1000 pulses, given 20 min after tetanus, reduced the potentiated component of S-EPSP, A-PS and L-PS by 68.5%, 80.1% and 56.1%, respectively (mean, n = 6), whereas it reduced the control response by 4.3%, 7.1%, and 1.9%, respectively ( n = 6. Significantly less effectiveness was observed for DPS at higher frequencies (2–10 Hz), with smaller numbers of pulses, featuring a longer time-lag after tetanus and under APV administration. When DPS was applied before tetanus, significantly less robust LTP was observed. However, these effects were blocked by the administration of APV during DPS.