Long-term potentiation in frontal cortex: role of NMDA-modulated polysynaptic excitatory pathways

Long-term potentiation of glycinergic synapses by semi-natural stimulation patterns during tonotopic map refinement

Before the onset of hearing, cochlea-generated patterns of spontaneous spike activity drive the maturation of central auditory circuits. In the glycinergic sound localization pathway from the medial nucleus of the trapezoid body (MNTB) to the lateral superior olive (LSO) this spontaneous activity guides the strengthening and silencing of synapses which underlies tonotopic map refinement. However, the mechanisms by which patterned activity regulates synaptic refinement in the MNTB-LSO pathway are still poorly understood. To address this question, we recorded from LSO neurons in slices from prehearing mice while stimulating MNTB afferents with stimulation patterns that mimicked those present in vivo. We found that these semi-natural stimulation patterns reliably elicited a novel form of long-term potentiation (LTP) of MNTB-LSO synapses. Stimulation patterns that lacked the characteristic high-frequency (200 Hz) component of prehearing spike activity failed to elicit potentiation. LTP was calcium dependent, required the activation of both g-protein coupled GABA_B and metabotropic glutamate receptors and involved an increase in postsynaptic glycine receptor-mediated currents. Our results provide a possible mechanism linking spontaneous spike bursts to tonotopic map refinement and further highlight the importance of the co-release of GABA and glutamate from immature glycinergic MNTB terminals.

Transcriptional evidence for the role of chronic venlafaxine treatment in neurotrophic signaling and neuroplasticity including also Glutamatergic [corrected] - and insulin-mediated neuronal processes

OBJECTIVES

Venlafaxine (VLX), a serotonine-noradrenaline reuptake inhibitor, is one of the most commonly used antidepressant drugs in clinical practice for the treatment of major depressive disorder (MDD). Despite being more potent than its predecessors, similarly to them, the therapeutical effect of VLX is visible only 3-4 weeks after the beginning of treatment. Furthermore, recent papers show that antidepressants, including also VLX, enhance the motor recovery after stroke even in non depressed persons. In the present, transcriptomic-based study we looked for changes in gene expressions after a long-term VLX administration.

METHODS

Osmotic minipumps were implanted subcutaneously into Dark Agouti rats providing a continuous (40 mg/kg/day) VLX delivery for three weeks. Frontal regions of the cerebral cortex were isolated and analyzed using Illumina bead arrays to detect genes showing significant chances in expression. Gene set enrichment analysis was performed to identify specific regulatory networks significantly affected by long term VLX treatment.

RESULTS

Chronic VLX administration may have an effect on neurotransmitter release via the regulation of genes involved in vesicular exocytosis and receptor endocytosis (such as Kif proteins, Myo5a, Sv2b, Syn2 or Synj2). Simultaneously, VLX activated the expression of genes involved in neurotrophic signaling (Ntrk2, Ntrk3), glutamatergic transmission (Gria3, Grin2b and Grin2a), neuroplasticity (Camk2g/b, Cd47), synaptogenesis (Epha5a, Gad2) and cognitive processes (Clstn2). Interestingly, VLX increased the expression of genes involved in mitochondrial antioxidant activity (Bcl2 and Prdx1). Additionally, VLX administration also modulated genes related to insulin signaling pathway (Negr1, Ppp3r1, Slc2a4 and Enpp1), a mechanism that has recently been linked to neuroprotection, learning and memory.

CONCLUSIONS

Our results strongly suggest that chronic VLX treatment improves functional reorganization and brain plasticity by influencing gene expression in regulatory networks of motor cortical areas. These results are consonant with the synaptic (network) hypothesis of depression and antidepressant-induced motor recovery after stroke.

Long-term potentiation of excitatory synapses on neocortical somatostatin-expressing interneurons

Synaptic plasticity has been extensively studied in principal neurons of the neocortex, but less work has been done on GABAergic interneurons. Interneurons consist of multiple subtypes and their synaptic properties vary between subtypes. In the present study, we have examined long-term potentiation (LTP) of excitatory synapses on somatostatin (SS)-expressing interneurons in neocortex using transgenic mice that express enhanced green fluorescent protein in these interneurons. We found that a strong theta burst stimulation was required to induce LTP in SS interneurons. LTP was associated with a reduction in paired-pulse facilitation and was not blocked by an N-methyl-d-aspartate receptor (NMDAR) antagonist. LTP was not affected by chelating postsynaptic Ca(2+) with BAPTA, a fast Ca(2+) chelator, and blocking L-type voltage-dependent Ca(2+) channels with nimodipine. Application of forskolin, an activator of adenylate cyclase that increases cyclic adenosine monophosphate (cAMP) concentration, enhanced synaptic transmission and occluded subsequent induction of LTP. Finally, we found that LTP was blocked by protein kinase A (PKA) inhibitors. Our results suggest that excitatory synapses on SS interneurons express a presynaptic form of LTP that is not dependent on NMDARs or postsynaptic Ca(2+) rise but is dependent on the cAMP-PKA signaling pathway.

NR2B antagonists restrict spatiotemporal spread of activity in a rat model of cortical dysplasia

Freeze-lesion-induced focal cortical dysplasia in rats closely resembles human microgyria, a neuronal migration disorder associated with drug-resistant epilepsy. Alterations in expression of N-methyl-D-aspartate receptors (NMDARs) containing NR2B subunits have been suggested to play a role in the hyperexcitability seen in this model. We examined the effect of NMDAR antagonists selective for NR2B subunits (Ro 25-6981 and ifenprodil) on activity evoked by intracortical stimulation in brain slices from freeze-lesioned rat neocortex. Whole-cell voltage-clamp recordings showed that Ro 25-6981 (1 microM) significantly reduced the response area of evoked postsynaptic currents in pyramidal cells from the paramicrogyral area whereas responses were unaffected in slices from control (sham operated) animals. Voltage-sensitive dye imaging was used to examine spatiotemporal spread of evoked activity in lesioned and control cortices. The imaging experiments revealed that peak amplitude, duration, and lateral spread of evoked activity in the paramicrogyral area was reduced by bath application of Ro 25-6981 (1 microM) and ifenprodil (10 microM). Ro 25-6981 had no effect on evoked activity in neocortical slices from control animals. The non-selective NMDAR antagonist d-2-amino-5-phosphonvaleric acid (APV, 20 microM) reduced activity evoked in presence of 50 microM 4-aminopyridine (known to increase excitability by enhancing neurotransmitter release) in neocortical slices from control animals whereas Ro 25-6981 (1 microM) did not. These results suggest that NR2B subunit-containing NMDARs contribute significantly to the enhanced spatiotemporal spread of paroxysmal activity observed in vitro in the rat freeze-lesion model of focal cortical dysplasia.

The Involvement of N-Methyl-D-Aspartate Receptors in Induction and Maintenance of Long-Term Potentiation in Rat Visual Cortex

Pyramidal neurons from layers II and III of rat visual cortex slices were studied with intracellular recordings. The involvement of N-methyl-D-aspartate (NMDA) receptors was investigated: (1) in the synaptic response to white matter stimulation; (2) in the induction of long-term potentiation (LTP); and (3) in the maintenance of LTP. Bath application of 25 microM of 2-amino-5-phosphonovalerate (APV), an NMDA receptor antagonist, caused a slight (< 10%) reduction of the amplitude of the synaptic response elicited by white matter stimulation. The APV-sensitive excitatory postsynaptic potential (EPSP) had a longer peak latency and duration than the APV-resistant EPSP. Bath application of 10 microM of 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX), a non-NMDA glutamate receptor antagonist, revealed a CNQX-resistant EPSP in response to white matter stimulation which was APV-sensitive. The time course of the CNQX-resistant EPSP was similar to that of the APV-sensitive EPSP and its onset latency was similar to that of the synaptic response in normal medium. Bath application of the GABA-A antagonist bicuculline (0.1 to 0.5 microM) led to a progressive enhancement of the amplitude of the APV-sensitive EPSP. At bicuculline concentrations above 0.3 microM the amplitude of this EPSP increased with membrane depolarization as was the case for the CNQX-resistant EPSP implying that the NMDA receptors were located on the recorded neuron. The susceptibility of the cells to undergo LTP was tested at various concentrations of bicuculline. The effectiveness of bicuculline treatment was quantified by comparing the amplitudes of the synaptic response to just subthreshold stimuli at two post-stimulus delays: (i) at 22 ms, which corresponds to the time to peak of both the initial inhibitory postsynaptic potential and the APV-sensitive EPSP; and (ii) at 8 - 11 ms post-stimulus, which corresponds to the peak of the postsynaptic potential (PSP) in normal medium. Bath application of APV, 20 min after the conditioning tetanus, allowed the authors to measure the amplitude of the APV-sensitive EPSP in the potentiated response. In normal medium, the ratio of the late over the early PSP amplitude was 33.6 +/- 4.1% and tetanic stimulation failed to induce LTP. The conditions remained the same at bicuculline concentrations of 0.1 to 0.2 microM. At higher concentrations of bicuculline the amplitude ratio of late versus early PSP increased and tetanic stimulation induced LTP. In cells, in which bicuculline had caused small ratio increases, only the APV-sensitive EPSP underwent LTP. In cells in which bicuculline had caused large ratio changes, both the APV-resistant and the APV-sensitive EPSP showed LTP. Together with the previous finding that blockade of NMDA receptors prevents LTP (Artola and Singer, 1987) these results suggest that there is a threshold for LTP induction, which is only reached if NMDA receptor-gated channels are sufficiently activated. The data indicate further that the NMDA receptor-mediated EPSP is itself susceptible to LTP whereby its LTP threshold is lower than that of the APV-resistant EPSP. Given the different LTP thresholds of the APV-resistant and APV-sensitive EPSPs, the possibility is raised that their potentiation depends on different mechanisms.

Activation of AP5-sensitive NMDA Receptors is Not Required to Induce LTP of Synaptic Transmission in the Lateral Perforant Path

The role of the N-methyl-d-aspartate (NMDA) type of glutamate receptor in long-term potentiation (LTP) of the medial (MPP) and lateral (LPP) divisions of the perforant path - granule cell system was investigated in urethane-anaesthetized rats. A stimulating electrode was positioned in the dorsomedial or ventrolateral aspect of the angular bundle for selective activation of either the MPP or LPP, respectively. A push - pull cannula served to focally perfuse artificial cerebrospinal fluid (ACSF) across the perforant path synaptic zone, while evoked potentials were monitored in the dentate hilus. Identification of LPP and MPP responses was based on (1) differences in population excitatory postsynaptic potential (EPSP) waveform obtained during stimulus depth profiles, and (2) differential sensitivity of evoked EPSPs to the glutamate receptor agonist l-aminophosphonobutyrate (AP4), and the antagonist gamma-d-glutamylglycine (DGG). High-frequency stimulation (400 Hz, 8 bursts of 8 pulses) applied to the lateral and medial perforant path elicited LTP of the EPSP and population spike in rats perfused with standard medium. In the MPP, LTP was almost completely blocked when d-aminophosphonopentanoate (AP5; 100 microM), a selective NMDA receptor antagonist, was perfused during the tetanus. Surprisingly, in the LPP experiments, AP5 did not impair induction of the 'synaptic' EPSP component of LTP. This occurred despite the ability of AP5 to block LTP of the LPP evoked population spike. The results suggest the existence of a novel, NMDA receptor-independent form of synaptic LTP in the lateral perforant path.

Adenosine preferentially suppresses serotonin2A receptor-enhanced excitatory postsynaptic currents in layer V neurons of the rat medial prefrontal cortex

Serotonin induces 'spontaneous' (non-electrically evoked) excitatory postsynaptic currents in layer V pyramidal neurons in the prefrontal cortex. This is likely due to a serotonin2A receptor-mediated focal release of glutamate onto apical dendrites. In addition, activation of the serotonin2A receptor selectively enhances late components of electrically evoked excitatory postsynaptic currents. In this study, using in vitro intracellular and whole-cell recording in rat brain slices, we examined the role of adenosine in modulating serotonin2A-enhanced 'spontaneous' and electrically evoked excitatory postsynaptic currents in layer V pyramidal neurons in the medial prefrontal cortex. Adenosine and N6-cyclopentyladenosine, an A1 adenosine agonist, markedly suppressed the serotonin2A-induced ('spontaneous') excitatory postsynaptic currents. However, adenosine had no effect on spontaneous miniature (tetrodotoxin-insensitive) postsynaptic potentials. Adenosine also blocked the late excitatory postsynaptic currents induced by the serotonin2A/2C agonist R(-)-2,5-dimethoxy-4-iodoamphetamine hydrochloride. Surprisingly, in contrast to other regions, adenosine had a relatively small effect on electrically evoked fast excitatory postsynaptic currents. These findings represent a novel demonstration of adenosine's ability to preferentially modulate serotonin2A-mediated synaptic events in the medial prefrontal cortex. As the serotonin2A receptor is closely linked with the effects of atypical antipsychotics and hallucinogens, further understanding of the modulators of this receptor such as adenosine may provide useful therapeutic applications.

Primed-burst potentiation in adult rat visual cortex in vitro

The effectiveness of &theta; pattern primed-bursts (PBs) on development of PB potentiation was investigated in layer II/III of the adult rat visual cortex in vitro. Experiments were carried out in the visual cortical slices. Population excitatory postsynaptic potentials (pEPSPs) were evoked in layer II/III by stimulation of either white matter or layer IV. To induce long-term potentiation (LTP), eight episodes of PBs were delivered at 0.1 Hz. Regardless of stimulation site, field potential recorded in layer II/III consisted of two components: a short latency and high amplitude response called pEPSP1, and a long latency and low amplitude response called pEPSP2. The incidence of LTP produced by PBs of layer IV was higher than that of the white matter tetanization. In contrast, PBs of both layer IV and white matter reliably produced LTP of pEPSP2 in layer II/III. It is concluded that PBs, as a type of activity pattern, of either white matter or layer IV can gain access to the modifiable synapses that are related to pEPSP2 in layer II/III, but accessibility of the modifiable synapses that are related to pEPSP1 depends on tetanization site. Relevancy of the results to the plasticity gate hypothesis is also discussed.

Effect of 2-amino-5-phosphopentanoic acid (AP5), a glutamate NMDA receptor blocker, on neuron activity in the cat motor cortex during performance of a paw placement conditioned reflex

Two types of stimulus-associated response were recorded in the contralateral motor cortex during performance of a condition reflex consisting of placing the forepaw on a support in response to a short electrical stimulus (4 msec, 500 Hz) applied to the contralateral parietal cortex (field 5). Primary short-latency responses (peak latent period about 10 msec, duration 30-50 msec) showed little sensitivity to the application of AP5, a blocker of glutamate NMDA receptors; secondary long-latency responses (peak latent period 65 msec, duration 150-200 msec) were inhibited in 44% of cases. Excitatory neuron responses associated with movement were inhibited by AP5 in 18% of cases. Increases in the latent period of the movement itself were seen in 19% of cases. AP5 decreased background activity in 46% of background-active neurons. The number of cases in which individual components of the response and neuron background activity were increased and latent periods of movement were decreased after application of AP5 was no more than expected from a random spread of data.

Initiation of spontaneous epileptiform activity in the neocortical slice

Cortical local circuitry is important in epileptogenesis. Voltage-sensitive dyes and fast imaging were used to visualize the initiation of spontaneous paroxysmal events in adult rat neocortical slices. Although spontaneous paroxysmal events could start from anywhere in the preparation, optical imaging revealed that all spontaneous events started at a few confined initiation foci and propagated to the whole preparation. Multielectrode recording over hundreds of spontaneous events revealed that often two or three initiation foci coexisted in each preparation (n = 10). These foci took turns being dominant; the dominant focus initiated the majority of the spontaneous paroxysmal events during that period. The dominant focus and dynamic rearrangement of foci suggest that the initiation of spontaneous epileptiform events involves a local multineuronal process, perhaps with potentiated synapses.

Patchy distribution of NMDAR1 subunit immunoreactivity in developing visual cortex

Development of ocular dominance columns is dependent on patterned retinal activity, and yet patterned activity alone cannot explain all aspects of cortical column development. Features intrinsic to the cortex have been proposed to interact with activity to guide the patterning of cortical columns (), and the NMDA receptor, because of its role in experience-dependent plasticity, is an obvious candidate. Using immunohistochemical techniques, we found a transiently patchy distribution of the NMDA receptor 1 (NMDAR1) subunit in kitten visual cortex. Regularly spaced patches of NMDAR1-immunoreactive neurons were found at the top of the cortical plate in the developing visual cortex at 2 weeks of age. At 4-5 weeks of age, the radial extent of the NMDAR1 patches spanned the supragranular layers, and by 12 weeks of age, this nonuniform pattern of NMDAR1 immunostaining was no longer apparent. Monocular visual experience prevented the expression of the NMDAR1 patches, but just 4 d of subsequent binocular visual experience was sufficient to promote expression of the patches. Furthermore, the NMDAR1 patches tended to be associated with the borders of ocular dominance columns. These results suggest that the degree of plasticity associated with NMDA-mediated mechanisms is elevated in local regions across the tangential extent of the visual cortex and that the NMDAR1 patches may participate in sculpting the overall arrangement of visual cortical columns.

Transient synaptic potentiation in the visual cortex. I. Cellular mechanisms

The cellular mechanisms that underlie transient synaptic potentiation were studied in visual cortical slices of adult guinea pigs (> or = age 5 wk postnatal). Postsynaptic potentials (PSPs) elicited by stimulation of the white matter/layer VI border were recorded with conventional intracellular techniques from layer II/III neurons. Transient potentiation (average duration 23 +/- 3 min, mean +/- SE) was evoked by 60 low-frequency (0.1 Hz) pairings of weak afferent stimulation with coincident intracellular depolarizing pulses (80 ms) of the postsynaptic cell. Fifty-one percent (47 of 92) of the pairing protocols led to significant enhancement (+26 +/- 3%) of the PSP peak amplitude. Blockade of action potential output from the recorded neuron during pairing with Lidocaine, N-ethyl bromide quaternary salt in the recording micropipette did not reduce the likelihood of potentiation (7 of 14 protocols = 50%). Thus transient synaptic potentiation does not require action potential output from the paired cell or recurrent synaptic activation in the local cortical circuit. Rather, the modification occurs at synaptic sites that directly impinge onto the activated neuron. Intracellular postsynaptic blockade of inhibitory PSPs only onto the paired cell with the chloride channel blocker 4,4'-dinitro-stilbene-2,2'-disulfonic acid and the potassium channel blocker cesium in he micropipette also did not reduce the likelihood of induction of potentiation (6 of 9 protocols = 67%). These results suggest that the potentiation is due to a true upregulation of excitatory synaptic transmission and that it does not require a reduction of inhibitory components of the compound PSP for induction. Chelation of postsynaptic intracellular calcium with 1,2-bis-2-aminophenoxy ethane-N,N,N',N'-tetraacetic acid (BAPTA) in all cases effectively blocked the induction of potentiation (no change in the PSP, 9 of 13 protocols; induction of synaptic depression, 4 of 13 protocols), suggesting that a rise in the intracellular postsynaptic calcium level is critical for the pairing-induced synaptic potentiation to occur. Bath application of the N-methyl-D-aspartate (NMDA) receptor antagonist 2-amino-5-phosphonovaleric acid (APV) reversibly blocked potentiation of the PSP peak amplitude in most cells (14 of 16) that were capable of significant potentiation of control solution. Blockade of nitric oxide production with bath application of the competitive inhibitor of nitric oxide synthase, L-nitro-arginine (LNA), did not significantly affect the likelihood of synaptic potentiation (11 of 20 cells). It did, however, block subsequent enhancement for several cells (2 of 4) that had previously had their inputs potentiated. Moreover, LNA increased the overall average magnitude of synaptic potentiation (with an additional +28%) when induction was successful. These results suggest that endogenous cortical nitric oxide production can both positively and negatively modulate this NMDA receptor-mediated type of synaptic plasticity.

Quantitative localization of NMDAR1 receptor subunit immunoreactivity in inferotemporal and prefrontal association cortices of monkey and human

The cellular and synaptic localization of immunoreactivity for the N-methyl-D-aspartate (NMDA) receptor subunit, NMDAR1, was investigated in inferotemporal and prefrontal association neocortices of monkeys and humans. In all monkey association areas examined, the laminar distribution patterns of NMDAR1 immunoreactivity were similar, and characterized by predominant pyramidal-like neuronal labeling in layers II, III, V and VI and a dense neuropil labeling consisting of intensely stained puncta and fine-caliber processes present throughout layers I-III, and V-VI. Layer IV, in contrast, contained only very lightly immunostained neurons which mostly lacked extensive dendritic staining. The laminar distribution of NMDAR1 immunolabeling in human association cortex was similar to that observed in monkeys. Electron microscopy of monkey areas 46 and TE1 confirmed that intensely immunoreactive asymmetrical postsynaptic densities were present throughout all cell-dense layers of prefrontal and inferotemporal association cortex. Quantitative analyses of the laminar proportions of immunoreactive synapses demonstrated that in both areas examined, the percentages of immunolabeled synapses were mostly similar across superficial layers, layer IV and infragranular layers. Finally, quantitative double-labeling immunofluorescence for non-NMDA receptor subunits or calcium-binding proteins demonstrated that virtually all GluR2/3 or GluR5/6/7-immunoreactive neurons were also labeled for NMDAR1, while regionally-specific subsets of parvalbumin-, calbindin- and calretinin-immunoreactive neurons were co-labeled. These data indicate that in primate association cortex, NMDA receptors are heterogeneously distributed to subsets of functionally distinct types of neurons and subsets of excitatory synapses, suggesting a critical and highly specific role in mediating the activity of excitatory connectivity which converges on cortical association areas.

Post-activation potentiation in the neocortex. IV. Multiple sessions required for induction of long-term potentiation in the chronic preparation

The neocortex in chronically prepared rats is very resistant to the induction of long-term potentiation (LTP). In the first of two experiments described in this paper, we tried unsuccessfully to induce neocortical LTP within one session by coactivating basal forebrain cholinergic and cortical inputs to our neocortical recording site. In the second experiment, we tested a new procedure which involved the application of repeated conditioning sessions over several days. This procedure was suggested by our finding that kindling-induced potentiation (KIP) of cortical field potentials could be reliably triggered but was slow to develop. We administered 30 high frequency trains per day to the corpus callosum for 25 days. LTP in callosal-neocortical field potentials became clear after about 5 days of stimulation and reached asymptotic levels by about 15 days. After the termination of treatment, LTP persisted for at least 4 weeks, the duration of our post-stimulation test period. As in previous experiments on kindling-induced potentiation, the potentiation effects were clear in both early population spike components and in a late (probably disynaptic) component. The monosynaptic EPSP component was often depressed, but this may have been due to competing field currents generated by the enhanced population spike activity. We discuss these results in the context of theories emphasizing slower but more permanent memory storage in neocortex compared to the hippocampus.

Pharmacology of memory: cholinergic-glutamatergic interactions

Both acetylcholine and glutamate are now thought to play important roles in memory. Recent evidence suggests that the interaction of these two neurotransmitters may be important for some forms of memory, and that acetylcholine, in particular, may function to facilitate glutamate activity by coordinating states of acquisition and recall in the cortex and hippocampus.

NMDA receptor-dependent long-term potentiation in the hippocampal afferent fibre system to the prefrontal cortex in the rat

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

Thyrotropin-releasing hormone enhances excitatory postsynaptic potentials in neocortical neurons of the rat in vitro

Several lines of evidence suggest a modulatory effect of thyrotropin-releasing hormone (TRH) on synaptic transmission in the mammalian neocortex. In the present study, the effects of this tripeptide on intracellularly recorded neocortical pyramidal neurons were investigated using rat in vitro brain slice preparations. TRH (5 microM and 50 microM) added to the perfusion medium concentration-dependently increased the excitability of pyramidal neurons, reflected by the number of spikes evoked by a depolarizing current pulse and by the augmentation of the time integral of glutamatergic excitatory postsynaptic potentials (EPSPs). TRH increased preferentially the time integrals of the late components of EPSPs (1-EPSPs) and increased their voltage-dependence. The early components of the EPSPs (e-EPSPs) were changed to much lesser extent. Iontophoretically applied D-2-amino-5-phosphonovalerate (D-APV) antagonized the TRH-induced increase of the 1-EPSPs. TRH also markedly enhanced the depolarizing responses evoked by iontophoretically applied N-methyl-D-aspartate (NMDA), while the depolarizing responses evoked by (R,S)-alpha-amino-3-hydroxy-5-methyl-4-isoxazole-propionic acid (AMPA) and L-glutamate were not significantly affected. The depolarizing inward rectification present in all neurons studied was augmented by the higher concentration of TRH. The effects of TRH were incited after about 5 min and were long-lasting. In most neurons the effects of TRH on neuronal excitability did not completely recover during the 45 min washout period. The present data suggest that some of the non-hormonal actions of TRH in the neocortex may be due to an enhancement of glutamatergic synaptic transmission.(ABSTRACT TRUNCATED AT 250 WORDS)

The NMDA receptor antagonist CPP suppresses long-term potentiation in the rat hippocampal-accumbens pathway in vivo

Excitation of afferent fibres originating in the ventral subiculum of the hippocampus through stimulation of the fimbria elicits field potentials in the nucleus accumbens. When recorded in the dorsomedial aspect of the nucleus accumbens, the evoked field responses consisted of an early, negative-going component (N1) with a peak latency of 8-10 ms, followed by a second negative-going peak (N2) with a latency of 22-24 ms. The N1 response reflects monosynaptic activation of nucleus accumbens neurons; the N2 component appears to be polysynaptic in origin. In control rats, high-frequency stimulation of the fimbria (three trains at 250 Hz, 250 ms, delivered at 50 min intervals) resulted in a long-lasting potentiation of both the N1 and N2 components. The magnitude of potentiation exhibited by the polysynaptic N2 response was typically greater than that of the monosynaptically evoked N1 response. Following delivery of the first train, the amplitude of the N1 and N2 components was increased by approximately 20 and 50% respectively. Administration of the competitive N-methyl-D-aspartate (NMDA) receptor antagonist 3-[(+-)-2-carboxypiperazin-4-yl]-propyl-1-phosphonic acid (CPP, 10 mg/kg i.p.) had no significant effects on the evoked nucleus accumbens responses. High-frequency stimulation failed to produce a significant increase in the amplitude of either the N1 or the N2 response when delivered 45-60 min after CPP administration. To test whether the suppressant effects of CPP were time-dependent, two further high-frequency trains were applied 90 and 180 min after administration of the drug.(ABSTRACT TRUNCATED AT 250 WORDS)

Use-dependent modification of a slow NMDA receptor-mediated synaptic potential in rat amygdalar slices

A single stimulus applied to the endopyriform nucleus evoked in 35 of the 101 basolateral amygdaloid (BLA) neurons a slow excitatory postsynaptic potential (s-EPSP) of varying latencies. The s-EPSP could be graded by changing the stimulus intensity and, on reaching the threshold, triggered action potentials. At stimulus intensity just subthreshold for evoking a spike, the s-EPSP has an average amplitude of 16.3 +/- 1.4 mV, a time to peak of 25.7 +/- 3.8 ms, and a duration of 124 +/- 14 ms. The s-EPSP was reversibly blocked by DL-2-amino-5-phosphonovaleate (DL-APV) or ketamine, indicating its mediation through N-methyl-D-aspartate (NMDA) receptor activation. However, the s-EPSP was not able to follow stimulus frequency of 1 Hz, suggesting that APV-sensitive s-EPSP is probably generated by a polysynaptic pathway. The s-EPSP was greatly enhanced by synaptic stimulation in the presence of bicuculline or in Mg(++)-free solution leading to the genesis of paroxysmal depolarizing shift (PDS). The s-EPSP can undergo robust long-term potentiation (LTP) following tetanic stimulation. These results suggest that the NMDA receptor-mediated s-EPSP may play an important role in epileptogenesis and synaptic plasticity in the amygdala.

Neocortical long-term potentiation

LTP is a form of activity-dependent synaptic plasticity that has been investigated mainly in the hippocampus. It is considered likely that similar mechanisms may also account for aspects of naturally occurring plasticity in the neocortex. Consequently, an increasing number of studies have been devoted to the investigation of neocortical LTP. Recent results suggest that at least two forms of LTP coexist in layer III of the neocortex. One depends on NMDA-receptor activation and resembles the LTP observed in hippocampal field CA1. A second form is independent of NMDA receptors and requires activation of voltage-sensitive Ca2+ channels.

Long-term retention of learning-induced receptive-field plasticity in the auditory cortex

Brief learning experience (classical conditioning) induces frequency-specific receptive-field (RF) plasticity in the auditory cortex, characterized as increased response to the frequency of the conditioned stimulus and decreased responses to most other frequencies, including the pretraining best frequency. This experiment asked (i) whether learning-induced RF plasticity, established in the waking state, can be expressed under general anesthesia and if so (ii) whether it exhibits long-term retention. Pure-tone-frequency RFs were obtained from adult guinea pigs under general anesthesia (sodium pentobarbital or ketamine) before and repeatedly after (1 hr-8 weeks) a 20- to 30-trial session of pairing a non-best-frequency tone with mild footshock. Conditioned-stimulus-specific RF plasticity was expressed under both types of anesthesia and included shifts of the pretraining best frequency toward or even to the frequency of the conditioned stimulus. Moreover, this RF plasticity exhibits long-term retention, being evident 1-8 weeks after training. This satisfies a criterion for the long-term storage of information in the auditory cortex.

Dihydropyridine-sensitive calcium channels are involved in the induction of N-methyl-D-aspartate receptor-independent long-term potentiation in visual cortex of adult rats

It was recently shown that induction of long-term potentiation (LTP) in the visual cortex of adult rats does not require suppression of inhibition or N-methyl-D-aspartate (NMDA) receptor activation. In the present study we examined the role of dihydropyridine-sensitive Ca2+ channels in the induction of this form of LTP. In visual cortical slices from 60 to 90-day-old rats tetanic stimulation (100 Hz 0.2 s, every 5 s for 10 min) of the white matter in the control medium or in the presence of D,L-2-amino-5-phosphonovalerate (100-200 microM) induced LTP of the field potential in layer III. Tetanic stimulation in the presence of nifedipine (50-100 microM) or nimodipine (10 microM) prevented induction of LTP in most of the slices. It appears that the known reduction of NMDA receptor activity in the mature neocortex is accompanied by a diminished role of NMDA receptors and an increased importance of voltage-gated Ca2+ channels in maintaining synaptic plasticity.

Long-term changes in synaptic strength along specific intrinsic pathways in the cat visual cortex

1. The dense system of horizontal connections that arise and course within the striate cortex are thought to inform single cells about stimuli arising in disparate points in visual space and to modulate responses evoked from within the receptive field. To learn whether or not the strength of the horizontal connections could vary over the long term, and if such changes could affect the integration of vertical, interlaminar inputs, we have recorded intracellularly from the superficial layers in slices of the adult cat's visual cortex. 2. The monosynaptic EPSP evoked by stimulating horizontal fibres showed long-term facilitation in twelve of the twenty cells that were conditioned by repetitively pairing synaptic responses with depolarizing pulses of current; the maximum increase observed was 200%. Strong inhibition present in the postsynaptic response usually indicated that facilitation would not occur. 3. In instances where horizontal input evoked both mono- and polysynaptic EPSPs, both early and late events showed facilitation, with the most dramatic enhancement contributed by the polysynaptic components. 4. For the twenty-eight cells whose responses to stimulation of interlaminar as well as horizontal pathways were assessed, all were found to receive non-overlapping inputs from each source. Conditioning produced long-term changes in the strength of the interlaminar inputs. 5. Changes in synaptic strength were usually confined to the conditioned pathway, though in four out of twenty-six times we observed heterosynaptic facilitation of polysynaptic EPSPs. 6. The conditioning protocol led to lasting depression rather than facilitation in three out of eleven instances; the reduction was only observed in the multisynaptic components. 7. We suggest that the synaptic changes observed here may be related to certain dynamic changes in receptive field properties that have been characterized in vivo.

Actions of vasoactive intestinal polypeptide (VIP) on neocortical neurons of the rat in vitro

The action of vasoactive intestinal polypeptide (VIP) was studied using intracellular recording techniques in neocortical neurons of the rat in vitro. When added to the perfusion medium, VIP enhanced direct excitability and increased the amplitude of the excitatory postsynaptic potential (EPSP). Effects of VIP were long-lasting and in most cases not fully reversible within the observation period. The present results suggest a role for VIP in long-lasting peptidergic modulation of excitatory synaptic components as well as direct excitability of neocortical pyramidal neurons of the adult rat.

Properties of vertebrate glutamate receptors: calcium mobilization and desensitization

Glutamate is now recognized as a major excitatory neurotransmitter in the vertebrate CNS, participating in a number of physiological and pathological processes. The importance of glutamate in the mobilization of intracellular Ca2+ as well as the relationship between excitatory and toxic properties has made it important to understand factors that regulate the responsivity of glutamate receptors. In recent years considerable insight has been gained about regulatory sites on NMDA receptors, with the recognition that these receptors are modulated by multiple endogenous and exogenous agents. Less is known about the regulation of responses mediated by AMPA, kainate, ACPD or APB receptors. Desensitization represents a potentially powerful means by which glutamate responses may be regulated. Indeed, two agents closely linked to the physiology of NMDA receptors, glycine and Ca2+, appear to modulate different types of desensitization. In the case of glycine, alteration of a rapid form of desensitization may be important in the role of this amino acid as a necessary cofactor for NMDA receptor activation. Additionally, changes in the affinity of the receptor complex for glycine may underlie the use-dependent decline in NMDA responses under certain conditions. Likewise, Ca2+ is a crucial player in the synaptic and toxic effects mediated by NMDA receptors, and is involved in a slower form of desensitization, in effect helping to regulate its own influx into neurons. The site and mechanism of the Ca2+ regulatory effects remain uncertain with evidence supporting both intracellular and ion channel sites of action. A clear role for Ca(2+)-dependent desensitization in the function of NMDA receptors under physiological conditions has not yet been demonstrated. AMPA receptor desensitization has been an area of intense investigation in recent years. The rapidity and degree of this process, coupled with its apparent rapid recovery, has suggested that desensitization is a key mechanism for the short-term regulation of responses mediated by these receptors. Furthermore, rapid desensitization appears to be one factor determining the time course and efficacy of fast excitatory synaptic transmission mediated by AMPA receptors, highlighting the physiological relevance of the process. The molecular mechanisms underlying desensitization remain uncertain. Traditionally, desensitization, like inactivation of voltage-gated channels, has been thought to represent a conformational change in the ion channel complex (Ochoa et al., 1989). However, it is unknown to what extent desensitization, in particular rapid AMPA receptor desensitization, has mechanistic features in common with inactivation. In voltage-gated channels, conformational changes in the channel protein restrict ion flow through the channel (Stuhmer, 1991).(ABSTRACT TRUNCATED AT 400 WORDS)

Pretreatment with caerulein protects against memory impairment induced by protein kinase C inhibitors in the rat

The effect of subcutaneously injected caerulein (CER) on memory impairment induced by protein kinase C (PKC) inhibitors, H-7 and melittin, was examined in rats. Intracerebroventricular injection of PKC inhibitors caused marked memory impairment in one-trial passive avoidance response and Morris water tank tasks. However, when rats were pretreated with CER at a subcutaneous dose of 1 microgram/kg 3 hours before the training trials, the reduced latency of the passive avoidance response was significantly increased, and in the Morris water pool tasks the memory deficit induced by PKC inhibitors completely disappeared. These results indicate that CER can offer protection against the effect of PKC inhibitors at least from the viewpoint of the memory processes.

Blockade of NMDA receptors unmasks a long-term depression in synaptic efficacy in rat prefrontal neurons in vitro

All the experiments were carried out in slices of rat prefrontal cortex maintained in vitro. The effect of 2-amino-5-phosphonovalerate (APV) was tested on the postsynaptic potential (PSP) recorded in layer V pyramidal cells, in response to single or high frequency stimulation of the superficial layers I-II. Wash-out of Mg2+ increased the amplitude and duration of the PSPs. This effect resulted from activation of N-methyl-D-aspartate (NMDA) receptors since it was suppressed by bath application of APV. Furthermore, in every cell tested in Mg2+ containing medium (N = 16), exposure to APV reversibly reduced both mono- and polysynaptic components of the PSPs, indicating that, even in the control solution, activation of NMDA-coupled channels contributed to these synaptic events. Finally, the anomalous voltage-dependence of the EPSP in the presence of Mg2+ and its sensitivity to APV suggests that at least a fraction of the NMDA receptors are postsynaptically located. Tetanization was applied to the afferents of cells bathed in control- or APV-medium. Long-term potentiation (LTP) or long-term depression (LTD) is defined as an increase or a decrease respectively, of the PSPs peak amplitude or initial slope, lasting 20 min. In the control medium, LTP in synaptic efficacy was observed in 34% of the cells and LTD in 48% (N = 23). When exposed to APV, none of the cells tested (N = 16) showed LTP of the response. In contrast, the tetanus induced a LTD of the PSP amplitude or slope in 14 out of these 16 cells. The percentage of cells showing LTD in synaptic efficacy (87%) when the NMDA receptors activation was blocked was significantly higher than that in control-medium.

Developmental changes in the susceptibility to long-term potentiation of neurones in rat visual cortex slices

We investigated with intracellular recordings from rat visual cortex slices whether the susceptibility to undergo long-term potentiation (LTP) is age-dependent and whether it is correlated with the expression of synaptic responses mediated by N-methyl-D-aspartate (NMDA) receptors. Test and tetanic stimuli were applied to the white matter and post-tetanic modifications of the amplitude of postsynaptic potentials (PSPs) were assessed in regular spiking cells of supragranular layers. At 2 weeks of age, the amplitudes of early (8-10 ms post-stimulus) and late (20 ms post-stimulus) PSP-components increased after tetanic stimulation to 137.1 +/- 13.4% and 141.3 +/- 12.1% of the pretetanic controls, respectively. At 3 weeks, potentiation of both PSP-components was less pronounced but still significant, the late component being on average more potentiated than the early one. At 4 weeks, PSPs were no longer potentiated. Bath application of 25 microM DL-2-amino-5-phosphonovalerate (APV), an NMDA receptor antagonist, blocked LTP induction both at 2 and at 3 weeks. We also studied developmental changes of two synaptic responses known to influence the susceptibility of cortical neurones to LTP, the NMDA receptor-mediated excitatory PSP (EPSP) and the initial inhibitory PSP (iIPSP). The amplitude of the APV-sensitive EPSP decreased with age and reached adult values in 4-week-old animals. The iIPSPs were pronounced already at 2 weeks and showed no marked change during further development. The results suggest a close correlation between the susceptibility to undergo LTP and the extent to which NMDA receptor-gated conductances contribute to the synaptic response.(ABSTRACT TRUNCATED AT 250 WORDS)

Ketamine blocks the plasticity associated with prefrontal cortex self-stimulation

Intracranial self-stimulation (ICSS) at sites within the medial prefrontal cortex (MFC) is acquired slowly but can be hastened by prior exposure to a regimen of noncontingent stimulation delivered to the MFC ICSS electrode. The facilitatory effects of noncontingent MFC stimulation on subsequent ICSS acquisition were blocked by pretreatment with ketamine, a noncompetitive antagonist of the N-methyl-D-aspartate (NMDA) receptor. These findings provide further support for the view that the NMDA receptor is importantly involved in mechanisms of neural plasticity.

NMDA depolarizations and long-term potentiation are reduced in the aged rat neocortex

N-Methyl-D-aspartate (NMDA) responses were recorded intracellularly in layer V neocortical neurons in in vitro slices taken from young (4-6 months) and aged (27-29 months) Fischer 344 rats. Increasing amounts of NMDA produced membrane depolarizations in both groups of cells. The regression analysis showed significantly reduced sensitivity to NMDA in old neurons compared to young. A significant long-term potentiation of the field potential evoked by subcortical white matter stimulation was present in young but not in old slices. These results suggest that aging results in a decreased sensitivity to NMDA and impaired synaptic plasticity in the neocortex.

Different voltage-dependent thresholds for inducing long-term depression and long-term potentiation in slices of rat visual cortex

In the hippocampus and neocortex, high-frequency (tetanic) stimulation of an afferent pathway leads to long-term potentiation (LTP) of synaptic transmission. In the hippocampus it has recently been shown that long-term depression (LTD) of excitatory transmission can also be induced by certain combinations of synaptic activation. In most hippocampal and all neocortical pathways studied so far, the induction of LTP requires the activation of N-methyl-D-aspartate (NMDA) receptor-gated conductances. Here we report that LTD can occur in neurons of slices of the rat visual cortex and that the same tetanic stimulation can induce either LTP or LTD depending on the level of depolarization of the postsynaptic neuron. By applying intracellular current injections or pharmacological disinhibition to modify the depolarizing response of the postsynaptic neuron to tetanic stimulation, we show that the mechanisms of induction of LTD and LTP are both postsynaptic. LTD is obtained if postsynaptic depolarization exceeds a critical level but remains below a threshold related to NMDA receptor-gated conductances, whereas LTP is induced if this second threshold is reached.

Use-dependent changes in synaptic efficacy in rat prefrontal neurons in vitro

1. An in vitro slice preparation of rat prefrontal cortex was used to analyse the responses of layer V pyramidal cells to electrical stimulation of layer II. We also studied the long-lasting modifications of synaptic efficacy following high-frequency stimulation of the same region. 2. Stable intracellular recordings were obtained from forty-three regular spiking pyramidal cells. The input resistance was 56 +/- 18 M omega (mean +/- S.D.) at a resting membrane potential of -71 +/- 4 mV. 3. At rest, a single stimulus of increasing strength evoked a monophasic, purely depolarizing postsynaptic potential (PSP) of increasing amplitude. In neurons recorded with potassium acetate-filled micropipettes, membrane depolarization disclosed an excitatory-inhibitory (EPSP-IPSP) sequence (onset latency of the EPSP, 3.6 +/- 0.6 ms). 4. Superfusion with the non-N-methyl-D-aspartate (NMDA) receptor antagonist, 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) reduced the EPSP and suppressed the IPSP. The small EPSP which remained was blocked by the NMDA receptor antagonist, D,L-2-amino-5-phosphonovalerate (APV). 5. In five cells, administration of 0.5 mumol l-1 bicuculline revealed a postsynaptic NMDA component in the evoked response as evidenced by its anomalous voltage dependence in the presence of Mg2+ and its sensitivity to APV. In these cells the latency of the APV-sensitive EPSP was the same as that of the APV-insensitive EPSP. 6. In six cells superfused with a high-Mg2+, low-Ca2+ artificial cerebrospinal fluid (ACSF) a small monosynaptic EPSP remained which had the same latency as the PSP recorded in control ACSF. 7. Patterned high-frequency stimulation (50-100 Hz) was applied to the afferents of twenty-eight neurons (twenty-three of them were recorded in the presence of bicuculline). During the train the membrane potential depolarized some 20 mV and each stimulus evoked a small PSP. The tetanic stimulation was followed by a short-term enhancement of the PSP amplitude and a slight increase in membrane input resistance. 8. Out of the twenty-eight cells, twenty-four showed long-lasting (over 30 min) modifications of the PSP. Long-term depression (LTD) of the evoked PSP was observed in fourteen cells and long-term potentiation (LTP) in ten cells. There was no significant change in the steady-state membrane properties and in the latency of the response. 9. In 64% of the cells that showed LTD and 70% of those that showed LTP of synaptic efficacy, the latency of the enhanced or depressed component of the PSP was the same as the control.(ABSTRACT TRUNCATED AT 400 WORDS)

Long-term potentiation in the prefrontal cortex following stimulation of the hippocampal CA1/subicular region

We have examined single cell activity and field potentials in the prelimbic area of the prefrontal cortex of the rat to electrical stimulation of the CA1/subicular region of the temporal hippocampus. Excitatory unit responses were found in 50 out of 120 neurons recorded in the prelimbic area. Paired-pulse facilitation was found for both single cell responses and field potentials. High-frequency, tetanic stimulation of the temporal hippocampus produced a significant and persistent potentiation of prelimbic field potentials. The evidence suggests that the direct pathway from the temporal hippocampus to the prelimbic area of the prefrontal cortex in the rat is excitatory and can undergo long-term potentiation (LTP).