Evidence for postsynaptic induction and expression of NMDA receptor independent LTP

Whole cell/patch-clamp and extracellular field potential recordings were used to study the induction and expression of N-methyl-D-aspartate (NMDA) receptor independent long-term potentiation (LTP) in area CA1 of the in vitro rat hippocampus. Induction of NMDA receptor independent LTP was prevented by manipulations that inhibited postsynaptic depolarization during tetanic stimulation: direct hyperpolarization of postsynaptic neurons and bath application of an alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA) and kainate receptor antagonist. NMDA receptor independent LTP also was blocked by intracellular application of the lidocaine derivative, N-(2,6-dimethylphenylcarbamoylmethyl)triethylammonium bromide (QX-314), to CA1 pyramidal neurons. These results complement the previous findings that NMDA receptor independent LTP was inhibited by postsynaptic injections of the calcium chelator 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid and also was inhibited by a L-type voltage-dependent calcium channel antagonist (nifedipine). Collectively, these data make a strong case for the postsynaptic induction of this form of LTP. This paper also provides evidence for postsynaptic expression of NMDA receptor independent LTP. In an experiment where AMPA- and NMDA-receptor-mediated excitatory postsynaptic potentials (EPSPs) were isolated pharmacologically, LTP was found for only the AMPA-receptor-mediated EPSPs. In a separate experiment, paired-pulse facilitation (PPF) was measured during NMDA receptor independent LTP. Although there was an initial decrease in PPF, suggesting a posttetanic increase in the probability of glutamate release, the change in PPF decayed within 30-40 min of the tetanic stimulation, whereas the magnitude of the LTP was constant over this same time period. In addition, the LTP, but not the corresponding change in PPF, was blocked by the metabotropic glutamate receptor antagonist (+/-)-alpha-methyl-4-carboxyphenylglycine. These results are accounted for most easily by a selective increase in postsynaptic AMPA receptor function, but one type of presynaptic modification-an increase in the number of release sites without an overall change in the probability of release-also could account for these results (assuming that the level of glutamate release before LTP induction fully saturated NMDA, but not AMPA, receptors). One possible presynaptic modification, an increase in axon excitability, was ruled out by analysis of the presynaptic fiber volley, which was not increased at any time after LTP induction.     

Repeated tetanic stimulation in piriform cortex in vitro: epileptogenesis and pharmacology

1. Focal cortical epilepsy was investigated by applying tetanic stimulation repeatedly (100 Hz. 2 s in duration, once every 10 min, 10 episodes) to layer III association fibers in rat piriform cortex slices and recording both extracellular and intracellular responses from the endopiriform nucleus. To promote excitability, piriform slices were incubated in artificial cerebrospinal fluid (ACSF) containing 0.9 mM Mg2+ and 5 mM K+, at an initial temperature of 10-12 degrees C, which was allowed to warm passively to room temperature. 2. Responses recorded extracellularly in the endopiriform nucleus consisted of two types: weak stimulation evoked an early-occurring, small-amplitude, negatively deflecting potential; strong stimulation evoked a more complex response comprising both an early potential of maximal amplitude and a later-occurring epileptiform potential of greater amplitude and longer duration. Late-occurring epileptiform potentials were not observed in slices incubated in ACSF at room temperature. 3. Both the early potential and the late-occurring epileptiform responses were abolished by the non-N-methyl-D-aspartic acid (non-NMDA) subtype of glutamate receptor blocker, 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX; 10 microM). Application of D(-)-2-amino-5-phosphonopentanoic acid (APV; 50 microM) to block NMDA receptors was without effect on the early potential but diminished the late-occurring epileptiform potential. The late-occurring potential was unable to follow stimulation delivered at a frequency of 1 Hz. These results suggest that the early potential was generated monosynaptically and dependent solely on the activation of non-NMDA receptors, whereas the late-occurring epileptiform potential was polysynaptic in origin and possessed both a CNQX- and an APV-sensitive component. 4. Responses increased progressively in both amplitude and duration after tetanic stimulation. The threshold intensity required to evoke the complex dual-component potential was reduced by tetanic stimulation. An increase in multiunit spiking activity, indicating an increase in synchronous discharges, was also observed. A residual potential could be evoked in the presence of CNQX (10 microM) after the tetanic stimulation procedure. 5. Spontaneous discharges occurred as early as after the first episode of tetanic stimulation and persisted for the duration of the experiment. Spontaneous discharges were abolished by either CNQX or by a fourfold increase in extracellular Mg2+ concentration, the latter reversibly. APV reduced the frequency of spontaneous discharges by 38.6 +/- 9.3% (mean +/- SE). The conventional anticonvulsant drug 5,5-diphenylhydantoin, the benzodiazepine receptor agonist midazolam, and the benzodiazepine receptor antagonist flumazenil were without effect on the frequency of spontaneous discharges. Evoked responses were also unaffected by either 5,5-diphenylhydantoin or midazolam. Slices not exposed to cold ACSF, although demonstrating potentiation of evoked responses after tetanization did not produce spontaneous epileptiform discharges. 6. Intracellular recordings from endopiriform neurons revealed the cellular correlates of the extracellular responses. Weak stimulation evoked a small-amplitude depolarizing potential. Increasing the intensity of stimulation increased the amplitude of this response and also evoked a second depolarizing potential of greater amplitude occurring at variable latencies. Maximal stimulation evoked an action potential. After tetanic stimuli, responses resembling a paroxysmal depolarizing shift consisting of a depolarizing potential with superimposed multiple action potentials were evoked reliably. Passive membrane properties after repeated tetanic stimulation were not different when compared with control. 7. This novel model of in vitro focal cortical epilepsy has many features characteristic of conventional kindling including 1) progressive nature; 2) reduced threshold to evoke discharges; and 3) persist     

Anoxic LTP is mediated by the redox modulatory site of the NMDA receptor

1. The effects of redox reagents, 5,5'-dithiobis-2-nitrobenzoic acid (DTNB) and tris(carboxyethyl)phosphine (TCEP), on anoxia-induced long-term potentiation (LTP) were investigated in CA1 hippocampal neurons using extracellular recording techniques. Experiments were performed in the presence of 0.1 mM MgCl2 and 10 microM 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) to pharmacologically isolate N-methyl-D-aspartate (NMDA) receptor-mediated responses. 2. DTNB (200 microM), a thiol oxidizing reagent, reduces by 52 +/- 9% (mean +/- SE) (n = 9/9) NMDA-receptor field potentials evoked by electrical stimulation of Schaffer collaterals and this effect could not be reversed by extensive washing. Nearly the same reduction of the initial response was obtained with different concentrations of DTNB (100 and 500 microM), but the time required to reach the maximal inhibition was concentration-dependent. 3. In keeping with an earlier study oxygen and glucose deprivation for 2-3 min induced a long-term potentiation (LTP) of the NMDA receptor response (+65 +/- 16%, n = 4/6). This potentiation was reversed by DTNB (100-500 microM) (-47 +/- 18%; n = 4/4) and the initial LTP could not be restored upon extensive washing of the drug. 4. TCEP (200 microM), a reagent which reduces S-S bond, amplified the electrically evoked NMDA-receptor EPSP (+27 +/- 12%; n = 3). In addition, TCEP (200 microM), nearly completely reversed the effect of DTNB (200 microM) on anoxia-induced LTP (+56 +/- 19%; n = 3/3). Preliminary results also indicate that TCEP occlude anoxic-LTP (n = 3/4). 5. Following DTNB (200 microM) treatment, oxygen and glucose deprivation did not generate anoxic LTP and extensive washing did not restore a potentiated NMDA field potential. 6. These observations strongly suggest that the redox site of the NMDA receptor is involved in the induction and the maintenance of the anoxic LTP of the NMDA receptor-mediated response in CA1.     

Longitudinal involvement of the spinal cord in a patient with lupus related transverse myelitis

Arachidonic acid is suggested to play a role in the expression of long-term potentiation (LTP), a synaptic analog of memory and learning. However, it is unknown whether this free fatty acid is actually released during LTP or not. To address this question, we assayed arachidonic acid with an HPLC system using 9-anthryldiazomethane (ADAM) as a fluorescent probe. High frequency stimulation (tetanic stimulation) to a hippocampal slice from the guinea pig brain caused a huge increase in the release of glutamate from presynaptic terminals and in turn, a gradual increase in the release of arachidonic acid. A similar increase in the release of arachidonic acid was induced by application of glutamate and the increase was inhibited by either the selective AMPA/kainate receptor antagonist, DNQX, or to a lesser extent by the selective NMDA receptor antagonist, APV. These findings suggest that arachidonic acid is produced by activation of ionotropic glutamate receptors involving expression of LTP. Arachidonic acid exerted a long-lasting facilitatory action on synaptic transmission in the CA1 region of rat hippocampal slices and the facilitation occluded the tetanic LTP. Arachidonic acid, thus, appears to be a significant factor for the expression of LTP.     

Intracellular injection of a Ca2+ chelator prevents generation of anoxic LTP

1. The effects of intracellular injection of Ca2+ chelator 1,2-bis (2-aminophenoxy) ethane N,N,N',N'-tetra-acetic acid (BAPTA, 50 mM) on anoxia-aglycemia-induced long-term potentiation (LTP) were investigated in the CA1 region of hippocampal slices with the use of extra- and intracellular recording techniques. Experiments were performed in artificial cerebrospinal fluid (ACSF) containing 10 microM bicuculline and 10 microM 6-cyano-7-nitroquinoxaline- 2,3-dione (CNQX) to pharmacologically isolate N-methyl-D-aspartate (NMDA)-receptor-mediated responses. NMDA-receptor-mediated excitatory postsynaptic potentials (EPSPs) and field potentials were evoked by stimulation of the Schaffer collateral/commissural pathway in the presence of 0.3 mM MgCl2 and 10 microM glycine to promote NMDA-receptor-mediated responses. Under these conditions, application of 50 microM D-2-amino-phosphono-valerate (D-APV) abolished EPSPs and field potentials. 2. Anoxic-aglycemic (AA) episodes (duration 2-2.5 min) potentiated the initial slope (measured within 3 ms from the onset of the synaptic responses) of EPSPs by 108 +/- 14.3% (mean +/- SE, P = 0.0012, n = 7). We refer to this LTP of NMDA-receptor-mediated synaptic responses as anoxic LTP. 3. Intracellular injection of the Ca2+ chelator BAPTA (with the intracellular recording electrode filled with 50 mM BAPTA in 3 M KCl) prevented anoxic LTP. Thirty to 40 min after the AA episode, in BAPTA-loaded cells, the initial slope of the EPSPs was not significantly changed (+7.12 +/- 5%, P = 0.35, n = 5). In contrast, the initial slope of the field potentials, measured at the same time in the same slices, was persistently increased (+49 +/- 2.8%, P = 0.0022, n = 5). 4. High-frequency tetanic stimulation (100 Hz for 500 ms, 2 times, 30 s apart) of the Schaffer collateral/commissural pathway, applied > 0.5 h after the AA episode, induced an additional significant and persistent increase in the initial slope of the field potential (tetanic LTP, +35.4 +/- 9.8%, P = 0.012, n = 5). In BAPTA-loaded cells, there was no further change in the initial slope of the EPSP (+3.9 +/- 3.4%, P = 0.205, n = 5) after the tetanic stimulation. 5. We also report that AA episodes or tetanic stimulation induced a persistent increase in a late synaptic component that was blocked by 50 microM D-APV. This late component was mediated polysynaptically, because its time to peak decreased with increasing stimulation intensities and it was strongly reduced by high-divalent-cation superfusate (ACSF containing 7 mM Ca2+). This component, which had a delay of approximately 8-30 ms, contaminated mainly the peak amplitude and the decay of the monosynaptic response without affecting its initial slope. Thus the measure of the initial slope takes into account only the early phase of the monosynaptic response. 6. We conclude that 1) a rise in intracellular Ca2+ is necessary to generate anoxic LTP of NMDA-receptor-mediated responses, as is the case for tetanic LTP; and 2) in the presence of bicuculline and low extracellular Mg2+, AA episodes and tetanic stimulations induced a long-lasting enhancement of a polysynaptic component mediated or controlled by NMDA receptors.     

5-HT1A receptor-mediated inhibition of long-term potentiation in rat visual cortex

We investigated the effect of 8-hydroxy-2-(N,N-dipropylamino)tetralin (8-OH-DPAT), a 5-HT1A receptor agonist, on the induction of long-term potentiation in rat visual cortex slices. Perfusion of 8-OH-DPAT (0.1-10 microM) did not affect layer II/III field potentials evoked by test stimulation of layer IV, but significantly reduced long-term potentiation induced by tetanic stimulation. The inhibitory effect of 8-OH-DPAT was blocked by the 5-HT1A receptor antagonist, pindolol (10 microM), but not by the 5-HT2,7 receptor antagonist, ritanserin (100 microM), nor by the 5-HT3,4 receptor antagonist, MDL72222 (100 microM). These results suggest that the rat visual cortex long-term potentiation is inhibited by 5-HT1A receptor stimulation.     

The pharmacology of mesocortical dopamine neurons: a dual-probe microdialysis study in the ventral tegmental area and prefrontal cortex of the rat brain

The development of receptor function at corticothalamic synapses during the first 20 days of postnatal development is described. Whole cell excitatory postsynaptic currents (EPSCs) were evoked in relay neurons of the ventral posterior nucleus (VP) by stimulation of corticothalamic fibers in in vitro slices of mouse brain from postnatal day 1 (P1). During P1-P12, excitatory postsynaptic conductances showed strong voltage dependence at peak current and at 100 ms after the stimulus and were almost completely antagonized by -2-amino-5-phosphonopentoic acid (APV), indicating that N-methyl--aspartate (NMDA) receptor-mediated currents dominate corticothalamic EPSCs at this time. After P12, in 42% of cells, excitatory postsynaptic conductances showed no voltage-dependence at peak current but still showed voltage-dependence 100-ms poststimulus. This voltage-dependent conductance was antagonized by APV. The nonvoltage-dependent component was APV resistant, showed fast decay, and was antagonized by the nonNMDA antagonist, 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX). In the remaining 58% of cells after P12, excitatory postsynaptic conductances showed moderate voltage dependence at peak conductance and strong voltage dependence 100 ms after the stimulus. Analysis of EPSCs before and after APV showed a significant increase in the relative contribution of the non-NMDA conductance after the second postnatal week. From P1 to P16, there was a significant decrease in the time constant of decay of the NMDA EPSC but no change in the voltage dependence of the NMDA response. After P8, slow EPSPs, 1.5-30 s in duration and mediated by metabotropic glutamate receptors (mGluRs), could be evoked by high-frequency stimulation of corticothalamic fibers in the presence of APV and CNQX. Similar slow depolarizations could be evoked by local application of the mGluR agonist (+/-)-1-aminocyclopentane-trans-1,3-dicarboxylic acid (t-ACPD) but from P0. Both conductances were blocked by the mGluR antagonist, (RS)-alpha-methyl-4-carboxyphenylglycine. Hence functional mGluR receptors are present on VP cells from birth, but their synaptic activation at corticothalamic synapses can only be detected after P8. In voltage clamp, the extrapolated reversal potential of the t-ACPD current, with potassium gluconate-based internal solution, was +12 +/- 10 (SE) mV, and the measured reversal potential with cesium gluconate-based internal solution was 1.5 +/- 9.9 mV, suggesting that the mGluR-mediated depolarization was mediated by a nonselective cation current. Replacement of NaCl in the external solution caused the reversal potential of the current to shift to -18 +/- 2 mV, indicating that Na+ is a charge carrier in the current. The current amplitude was not reduced by application of Cs+, Ba2+, and Cd2+, indicating that the t-ACPD current was distinct from the hyperpolarization-activated cation current (IH) and distinct from certain other previously characterized mGluR-activated, nonselective cation conductances.     

Colorectal cancer in the Royal Navy--an opportunity to intervene?

Whole-cell recordings were made in the nucleus tractus solitarii (NTS) in transverse brainstem slices from rats. Monosynaptic GABAA-receptor-mediated inhibitory postsynaptic currents (IPSCs) or potentials (IPSPs) were evoked (0.1-0.2 Hz) by electrical stimulation within and medial to the tractus solitarius in the presence of the ionotropic glutamate receptor antagonists 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX; 10 microM) or 6,7-dinitroquinoxaline-2,3-dione (DNQX; 10 microM) and D-amino-5-phosphonopentanoic acid (APV; 50 microM). A brief period of tetanic stimulation (20 Hz, 2 s) resulted in posttetanic (< 5 min, 69 of 73 recordings) and sustained potentiation (> 15 min, 31 of 73 recordings) of the IPSP/Cs. Sustained potentiation was not due to alterations in the reversal potential of IPSP/Cs. Both pre- and post-tetanus IPSP/Cs were completely blocked by the GABAA antagonist bicuculline (10 microM). Postsynaptic responses to pressure ejection of the GABAA-receptor agonist muscimol were unaltered in cells displaying sustained potentiation. Sustained potentiation of IPSP/Cs could be induced by tetanus in the presence of either metabotropic glutamate receptor antagonists or bicuculline. However, sustained potentiation could not be induced in the presence of the GABAB-receptor antagonists 2-OH-saclofen (400 microM) or CGP35348 (3-amino-propyl-(diethoxymethyl)phosphinic acid, 100 microM), although a subsequent tetanus following washout induced sustained potentiation. Posttetanic potentiation was unaffected by GABAB-receptor antagonists. These data suggest that neuronal or terminal excitability of GABAergic interneurons in the NTS is enhanced following brief periods of increased frequency of activation in vitro. This novel phenomenon within the rat medulla may be involved in the temporal modulation of autonomic reflex sensitivity observed during certain behavioral states, such as the defense reaction.     

Glutamate receptor activity is required for normal development of tectal cell dendrites in vivo

Glutamatergic retinotectal inputs mediated principally by NMDA receptors can be recorded from optic tectal neurons early during their morphological development in Xenopus tadpoles. As tectal cell dendrites elaborate, retinotectal synaptic responses acquire an AMPA receptor-mediated synaptic component, in addition to the NMDA component. Here, we tested whether glutamatergic activity was required for the elaboration of dendritic arbors in Xenopus optic tectal neurons. In vivo time-lapse imaging of single DiI-labeled neurons shows that the NMDA receptor antagonist APV (100 microM) blocked the early development of the tectal cell dendritic arbor, whereas the AMPA receptor antagonist CNQX (20 microM) or the sodium channel blocker TTX (1 microM) did not. The decreased dendritic development is attributable to failure to add new branches and extend preexisting branches. These observations indicate that NMDA-type glutamatergic activity promotes the initial development of the dendritic arbor. At later stages of tectal neuron development when AMPA receptor-mediated synaptic transmission is strong, both APV and CNQX decrease dendritic arbor branch length, consistent with a role for glutamatergic synaptic transmission in maintaining dendritic arbor structure. These results indicate that AMPA and NMDA receptors can differentially influence dendritic growth at different stages of neuronal development, in correlation with changes in the relative contribution of the receptor subtype to synaptic transmission.     

Evidence for NMDA and mGlu receptor-dependent long-term potentiation of mossy fiber-granule cell transmission in rat cerebellum

Long-term potentiation (LTP) is a form of synaptic plasticity that can be revealed at numerous hippocampal and neocortical synapses following high-frequency activation of N-methyl--aspartate (NMDA) receptors. However, it was not known whether LTP could be induced at the mossy fiber-granule cell relay of cerebellum. This is a particularly interesting issue because theories of the cerebellum do not consider or even explicitly negate the existence of mossy fiber-granule cell synaptic plasticity. Here we show that high-frequency mossy fiber stimulation paired with granule cell membrane depolarization (-40 mV) leads to LTP of granule cell excitatory postsynaptic currents (EPSCs). Pairing with a relatively hyperpolarized potential (-60 mV) or in the presence of NMDA receptor blockers [5-amino--phosphonovaleric acid (APV) and 7-chloro-kynurenic acid (7-Cl-Kyn)] prevented LTP, suggesting that the induction process involves a voltage-dependent NMDA receptor activation. Metabotropic glutamate receptors were also involved because blocking them with (+)-alpha-methyl-4-carboxyphenyl-glycine (MCPG) prevented potentiation. At the cytoplasmic level, EPSC potentiation required a Ca2+ increase and protein kinase C (PKC) activation. Potentiation was expressed through an increase in both the NMDA and non-NMDA receptor-mediated current and by an NMDA current slowdown, suggesting that complex mechanisms control synaptic efficacy during LTP. LTP at the mossy fiber-granule cell synapse provides the cerebellar network with a large reservoir for memory storage, which may be needed to optimize pattern recognition and, ultimately, cerebellar learning and computation.     

Serotonin inhibits the induction of long-term potentiation in rat primary visual cortex

1. The effect of serotonin (5-hydroxytryptamine; 5-HT) on the induction of long-term potentiation (LTP) in rat visual cortex was investigated by using slice preparations in vitro. 2. Bath application of 5-HT (0.1-10 microM) did not affect the baseline synaptic potentials evoked by single-pulse test stimulation, but inhibited the induction of LTP in a concentration-dependent manner. 3. The effect of 5-HT was blocked by the 5-HT1 receptor antagonist pindolol or the 5-HT2,7 receptor antagonist ritanserin, but not by the 5-HT3,4 receptor antagonist MDL72222. 4. These results suggest that 5-HT plays a role in suppressing the induction of LTP in the rat visual cortex.     

Imaging the induction and spread of seizure activity in the isolated brain of the guinea pig: the roles of GABA and glutamate receptors

1. The induction and spread of seizure activity was studied using imaging and electrophysiological techniques in the isolated whole brain of the guinea pig. We examined the role of GABA and glutamate receptor subtypes in controlling the spread of seizure activity across the olfactory cortex from a focus in the entorhinal cortex. Seizure spread was monitored by video imaging of intrinsic optical signals (reflectance changes) combined with multiple extracellular recordings. Both the unilateral and bilateral spread of seizure activity was monitored in different experiments. 2. Electrical stimulation of the lateral entorhinal cortex (10-15 V, 5 Hz, 5-10 s) evoked seizure activity that originated in the entorhinal cortex/hippocampus and later spread preferentially toward the posteromedial cortical amygdaloid nucleus ipsilaterally and bilaterally. The pattern of seizure spread in a given brain was highly reproducible. 3. The influence of gamma-aminobutyric acid (GABA) receptors on the spread of seizure activity was monitored at higher resolution on one side of the brain. Perfusion of a low concentration of the GABAA antagonist bicuculline methiodide (20 microM) resulted in spontaneous seizures that spread to the posteromedial cortical amygdaloid nucleus more rapidly than electrically evoked seizures [spread times: 5.5 +/- 3.7 s vs. 15.5 +/- 2.7 s, respectively (means +/- SE)]. Seizure spread was also more extensive in the presence of bicuculline involving the posterior perirhinal cortex and larger areas over the medial amygdala. Higher concentrations of bicuculline (100 microM) resulted in even more widespread propagation of spontaneous seizure activity throughout the olfactory cortex as well as to the perirhinal, insular, and occipital cortices. This concentration of bicuculline also further reduced the time required for seizure activity to spread from the entorhinal cortex to the posteromedial cortical amygdaloid nucleus (spread time = 2.3 +/- 1.7 s). The GABAB antagonist, CGP 35348 (200 microM), in contrast, had no significant effect of seizure induction or propagation. 4. The role of glutamate receptor subtypes in seizure propagation was studied by examining the bilateral spread of seizures. Perfusion of the kainate/alpha-amino-3-hydroxy-5-methyl-4-isoxazole proprionic acid (K/A) receptor antagonist (6-cyano-7-nitroquinoxaline-2,3-dione, CNQX, 20 microM) completely and reversibly suppressed stimulus-evoked seizure activity as detected electrophysiologically and optically. CNQX also reduced the magnitudes of field potentials recorded in the isolated brain in a reversible manner by an average of 70.8 +/- 2.21% of control. The N-methyl-D-aspartate (NMDA) receptor antagonist dibenzocyclohepteneimine (MK-801) did not significantly alter the magnitudes or shapes of field potentials recorded in the isolated brain nor did it significantly alter seizure activity measured optically or electrophysiologically. 5. Perfusion of the metabotropic glutamate receptor agonist [trans-1-amino-(IS,3R)-cyclopentanedicarboxylic acid (trans-ACPD), 150 microM] completely and reversibly suppressed stimulus-evoked seizure activity as detected electrophysiologically and optically. The magnitudes of field potentials recorded in the isolated brain also were reduced by trans-ACPD an average of 75.4 +/- 5.39% of control values. 6. These results demonstrate that GABAA-mediated transmission is functionally present and may play an important role in epileptic tissue in limiting the spread of seizure activity from the entorhinal cortex to the posteromedial cortical amygdaloid nucleus and in creating functional pathways or preferential routes of seizure spread. GABAB-mediated postsynaptic inhibition played no significant role in the induction or spread of seizure activity in this study. K/A receptors but not NMDA receptors are necessary for the induction and subsequent spread of seizure activity originating in the entorhinal cortex/hippocampus.     

Reversal of age-related alterations in synaptic plasticity by blockade of L-type Ca2+ channels

The role of L-type Ca2+ channels in the induction of synaptic plasticity in hippocampal slices of aged (22-24 months) and young adult (4-6 months) male Fischer 344 rats was investigated. Prolonged 1 Hz stimulation (900 pulses) of Schaffer collaterals, which normally depresses CA3/CA1 synaptic strength in aged rat slices, failed to induce long-term depression (LTD) during bath application of the L-channel antagonist nifedipine (10 microM). When 5 Hz stimulation (900 pulses) was used to modify synaptic strength, nifedipine facilitated synaptic enhancement in slices from aged, but not young, adult rats. This enhancement was pathway-specific, reversible, and impaired by the NMDA receptor (NMDAR) antagonist DL-2-amino-5-phosphonopentanoic acid (AP5). Induction of long-term potentiation (LTP) in aged rats, using 100 Hz stimulation, occluded subsequent synaptic enhancement by 5 Hz stimulation, suggesting that nifedipine-facilitated enhancement shares mechanisms in common with conventional LTP. Facilitation of synaptic enhancement by nifedipine likely was attributable to a reduction ( approximately 30%) in the Ca2+-dependent K+-mediated afterhyperpolarization (AHP), because the K+ channel blocker apamin (1 microM) similarly reduced the AHP and promoted synaptic enhancement by 5 Hz stimulation. In contrast, apamin did not block LTD induction using 1 Hz stimulation, suggesting that, in aged rats, the AHP does not influence LTD and LTP induction in a similar way. The results indicate that, during aging, L-channels can (1) facilitate LTD induction during low rates of synaptic activity and (2) impair LTP induction during higher levels of synaptic activation via an increase in the Ca2+-dependent AHP.     

The NMDA receptor antagonist CPP impairs conditioned taste aversion and insular cortex long-term potentiation in vivo

It has been proposed that long-term potentiation (LTP) a form of activity-dependent modification of synaptic efficacy, may be a synaptic mechanism for certain types of learning. Recent studies on the insular cortex (IC) a region of the temporal cortex implicated in the acquisition and storage of conditioned taste aversion (CTA), have demonstrated that tetanic stimulation of the basolateral nucleus of the amygdala (Bla) induce an N-methyl-d-aspartate (NMDA) dependent LTP in the IC of adult rats in vivo. Here we present experimental data showing that intracortical administration of the NMDA receptor competitive antagonist CPP (-3(-2 carboxipiperazin-4-yl)-propyl-1-phosphonic acid) disrupts the acquisition of conditioned taste aversion, as well as, the IC-LTP induction in vivo. These findings are of particular interest since they provide support for the view that the neural mechanisms underlying NMDA dependent neocortical LTP, constitute a possible mechanism for the learning related functions performed by the IC.     

Importance of polysynaptic inputs and horizontal connectivity in the generation of tetanus-induced long-term potentiation in the rat auditory cortex

Supragranular pyramidal neurons in the adult rat auditory cortex (AC) show marked long-term potentiation (LTP) of population spikes after tetanic white matter stimulation (TS). For determination of whether this marked LTP is specific to AC, LTP in rat AC slices was compared with LTP in slices of the visual cortex (VC). The amplitude of TS-induced LTP in AC was twice that in VC. LTP of EPSPs was also studied with perforated patch or whole-cell recording. Although the amplitude of TS-induced LTP of EPSPs in AC was larger that in VC, no cortical difference was found in LTP elicited by low-frequency stimulation paired with current injection. Neocortical LTP is dependent on the activation of NMDA receptors, and induction of LTP requires postsynaptic depolarization for removal of Mg2+ blockade of NMDA receptors. The postsynaptic depolarization elicited by TS in supragranular pyramidal neurons in AC was significantly larger than that in VC. Cutting of supragranular horizontal connections resulted in a decrease in the depolarization amplitude in AC but an increase in the depolarization amplitude in VC. The cortical difference in TS-induced LTP was diminished in the slices in which horizontal connections in supragranular layers were cut. The estimated density of horizontal axon collaterals of supragranular pyramidal neurons in AC was approximately twice that in VC. These results strongly suggest that the marked polysynaptic and postsynaptic depolarization during TS and the resulting marked LTP in AC are attributed to well developed horizontal axon collaterals of supragranular pyramidal neurons in AC.     

Concurrent blockade of beta-adrenergic and muscarinic receptors suppresses synergistically long-term potentiation of population spikes in the rat hippocampal CA1 region

The muscarinic acetylcholine receptor antagonist scopolamine, but not the beta-adrenoceptor antagonist propranolol or atenolol, suppressed tetanus-induced long-term potentiation (LTP) of population spikes in the rat hippocampal CA1 region. When scopolamine was coapplied with propranolol or atenolol, a synergistic effect in preventing LTP generation was observed. On the other hand, the coapplication of scopolamine and atenolol failed to affect tetanus-induced LTP of field EPSP. These findings suggest that cooperative mechanisms via muscarinic and beta-adrenergic receptor activation might contribute to LTP induction in terms of the EPSP-spike potentiation, i.e., an increase in the excitability of hippocampal CA1 pyramidal cells after tetanic stimulation, but are independent of the tetanus-evoked potentiation of a synaptic component.     

Aging differentially alters forms of long-term potentiation in rat hippocampal area CA1

Long-term potentiation (LTP) of the Schaffer collateral/commissural inputs to CA1 in the hippocampus was shown to consist of N-methyl-D-aspartate receptor (NMDAR) and voltage-dependent calcium channel (VDCC) dependent forms. In this study, the relative contributions of these two forms of LTP in in vitro hippocampal slices from young (2 mo) and old (24 mo) Fischer 344 rats were examined. Excitatory postsynaptic potentials (EPSP) were recorded extracellularly from stratum radiatum before and after a tetanic stimulus consisting of four 200-Hz, 0.5-s trains given 5 s apart. Under control conditions, a compound LTP consisting of both forms was induced and was similar, in both time course and magnitude, in young and old animals. NMDAR-dependent LTP (nmdaLTP), isolated by the application of 10 microM nifedipine (a voltage-dependent calcium channel blocker), was significantly reduced in magnitude in aged animals. The VDCC dependent form (vdccLTP), isolated by the application of 50 microM D,L-2-amino-5-phosphonvalerate (APV), was significantly larger in aged animals. Although both LTP forms reached stable values 40-60 min posttetanus in young animals, in aged animals vdccLTP increased and nmdaLTP decreased during this time. In both young and old animals, the sum of the two isolated LTP forms approximated the magnitude of the compound LTP, and application of APV and nifedipine or genestein (a tyrosine kinase inhibitor) together blocked potentiation. These results suggest that aging causes a shift in synaptic plasticity from NMDAR-dependent mechanisms to VDCC-dependent mechanisms. The data are consistent with previous findings of increased L-type calcium current and decreased NMDAR number in aged CA1 cells and may help explain age-related deficits in learning and memory.     

LTP in the lateral perforant path is beta-adrenergic receptor-dependent

Norepinephrine induces an activity-independent long-lasting depression of synaptic transmission in the lateral perforant path input to dentate granule cells, whereas high frequency stimulation induces activity-dependent long-term potentiation (LTP). We investigated the role of endogenous activation of beta-adrenergic receptors in LTP of the lateral and medial perforant paths under conditions affording selective stimulation of these pathways in the rat hippo-campal slice. Propranolol (1 microM), a beta-receptor antagonist, blocked LTP induction of both lateral and medial perforant path-evoked field excitatory postsynaptic potentials. The results indicate a broad requirement for norepinephrine in different types of synaptic plasticity, including activity-independent depression and activity-dependent LTP in the lateral perforant path.     

NMDA receptor-mediated control of presynaptic calcium and neurotransmitter release

Before action potential-evoked Ca2+ transients, basal presynaptic Ca2+ concentration may profoundly affect the amplitude of subsequent neurotransmitter release. Reticulospinal axons of the lamprey spinal cord receive glutamatergic synaptic input. We have investigated the effect of this input on presynaptic Ca2+ concentrations and evoked release of neurotransmitter. Paired recordings were made between reticulospinal axons and the neurons that make axo-axonic synapses onto those axons. Both excitatory and inhibitory paired-cell responses were recorded in the axons. Excitatory synaptic inputs were blocked by the AMPA receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX; 10 microM) and by the NMDA receptor antagonist 2-amino-5-phosphonopentanoate (AP-5; 50 microM). Application of NMDA evoked an increase in presynaptic Ca2+ in reticulospinal axons. Extracellular stimulation evoked Ca2+ transients in axons when applied either directly over the axon or lateral to the axons. Transients evoked by the two types of stimulation differed in magnitude and sensitivity to AP-5. Simultaneous microelectrode recordings from the axons during Ca2+ imaging revealed that stimulation of synaptic inputs directed to the axons evoked Ca2+ entry. By the use of paired-cell recordings between reticulospinal axons and their postsynaptic targets, NMDA receptor activation was shown to enhance evoked release of transmitter from the axons that received axoaxonic inputs. When the synaptic input to the axon was stimulated before eliciting an action potential in the axon, transmitter release from the axon was enhanced. We conclude that NMDA receptor-mediated input to reticulospinal axons increases basal Ca2+ within the axons and that this Ca2+ is sufficient to enhance release from the axons.     

High-frequency stimulation of the basolateral amygdala facilitates the induction of long-term potentiation in the dentate gyrus in vivo

We investigated the effect of high-frequency stimulation of the basolateral amygdala (BLA) on the induction of long-term potentiation (LTP) in the medial perforant path (PP)-dentate gyrus (DG) synapses of anesthetized rats. A conditioning stimulation (100 pulses at 100 Hz) of the ipsilateral BLA did not change the DG synaptic potential. However, when the BLA conditioning stimulation was applied at the same time as a weak tetanic stimulation of PP (20 pulses at 20 Hz) which alone did not induce LTP, robust DG LTP was induced. Simultaneous application of contralateral BLA stimulation and PP weak tetanus did not induce LTP. Moreover, the ipsilateral BLA stimulation enhanced the magnitude of LTP induced by a moderate tetanic stimulation of PP (30 pulses at 60 Hz), but did not further enhance the LTP induced by a strong tetanic stimulation of PP (100 pulses at 100 Hz). These results suggest that the ipsilateral BLA neurons modulate the induction of DG LTP in vivo.