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

Post-activation potentiation in the neocortex. III. Kindling-induced potentiation in the chronic preparation

Previous experiments have shown the neocortex to be very resistant to the induction of long-term potentiation in chronic preparations. We show here that kindling-induced potentiation effects can be reliably produced in the neocortex of awake, freely moving rats. These effects develop rather slowly. In sites contralateral to the stimulation electrode, potentiation effects did not become clear until the animals had received about 5 days or more of stimulation. Ipsilateral sites required even longer (approximately 10 days), and both sites required more than 13 days to reach asymptotic levels of potentiation. Both monosynaptic and polysynaptic components were present in the neocortical field potentials. When population spikes were absent, the surface negative monosynaptic EPSP component tended to show a potentiation effect. If population spikes were present, they were generally enhanced while the monosynaptic population EPSP tended to be depressed. Consequently, the apparent depression may have been due to competing field currents. The later polysynaptic components (15-28 ms latency to peak) always showed a potentiation effect with 5 or more kindling stimulations and is presumed to result from activation of cortico-cortical associational fibers. All of these effects were long-lasting, showing little decay over a period of several weeks.

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.

Amplification of EPSPs by axosomatic sodium channels in neocortical pyramidal neurons

Simultaneous somatic and dendritic recordings were made from the same neocortical layer V pyramidal neuron, and current injection via the dendritic recording pipette was used to simulate the voltage change that occurs during an EPSP. At the soma, these simulated EPSPs increased nonlinearly with the amplitude of the dendritic current injection and with depolarization of the membrane potential. Bath application of the sodium channel blocker TTX decreased large (> 5 mV) EPSPs and also blocked amplification of EPSPs at depolarized membrane potentials, whereas calcium channel blockers had little effect. Local application of TTX to the soma and axon blocked EPSP amplification, whereas dendritic application had little effect. Simultaneous somatic and axonal recordings demonstrated that EPSP amplification was largest in the axon. These results show that EPSPs are amplified by voltage-activated sodium channels located close to the soma and in the axon.

Effect of sensory deafferentation on immunoreactivity of GABAergic cells and on GABA receptors in the adult cat visual cortex

To investigate the effects of sensory deafferentation on the cortical GABAergic circuitry in adult cats, glutamic acid decarboxylase (GAD) and gamma-aminobutyric acid (GABA) immunoreactivity and GABA receptor binding were studied in the visual cortex of normal cats and compared with cats that had received restricted binocular central lesions of the retina and had survived for 2 weeks postlesion in a normal visual environment. In the visual cortex of lesioned cats, two changes were observed in the number of GAD-immunoreactive elements in the regions affected by the retinal lesions: the number of GAD-positive puncta decreased, whereas that of GAD-immunoreactive somata increased. In contrast, no detectable changes were measured in the number of GABA-immunopositive somata or puncta. At the receptor level, we observed no differences in either the laminar distribution or the affinity of cortical GABAA and GABAB receptors labeled with [3H]-muscimol and [3H]-baclofen, respectively, in the lesioned versus normal cats. We present the hypothesis that sensory deafferentation in these adult cats (1) leads to a reduction of cortical GABAergic inhibition in the deafferented region, and (2) that this decreased inhibition may permit changes in efficiency of synapses and (3) that these changes may represent a first stage of events underlying the retinotopic reorganization preceeding the structural changes.

Simultaneous expression of long-term depression of NMDA and long-term potentiation of AMPA receptor-mediated synaptic responses in the CA1 area of the kainic acid-lesioned hippocampus

This study investigates the plasticity of the excitatory synapses in an experimental model of epilepsy, the kainic acid-lesioned rat hippocampus. Stimulation of afferents in the CA1 area of lesioned hippocampi produced an epileptiform burst of action potentials, with an underlying synaptic potential composed of mixed alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA; 80%) and N-methyl-D-aspartate (NMDA; 20%) receptor-mediated components. Tetanic stimulation yielded a long-term potentiation (LTP) of the mixed AMPA/NMDA receptor-mediated population excitatory postsynaptic potentials. However, the same type of tetanus resulted in a long-term depression (LTD) of pharmacologically isolated NMDA receptor-mediated responses. This LTD occurred independently of the antagonism of AMPA receptors. This suggests that tetanic stimulation produced LTP of AMPA and LTD of NMDA receptor-mediated responses simultaneously. Finally, both LTP and LTD were shown to be NMDA dependent. This property has profound functional implications for the control of excitatory networks in temporal lobe epilepsy.

Involvement of serotonin in developmental plasticity of kitten visual cortex

During a critical period of postnatal development, neuronal connections in the kitten visual cortex are susceptible to experience-dependent modifications. These modifications are facilitated by the neuromodulators noradrenaline and acetylcholine. To address the question of whether serotonin (5-hydroxytryptamine; 5-HT), the other major neuromodulator in the cerebral cortex, also plays a role in developmental plasticity, we investigated whether interference with serotoninergic transmission in the kitten visual cortex affects ocular dominance (OD) plasticity. The serotonin neurotoxin 5,7-dihydroxytryptamine or the serotonin receptor blockers ketanserin and methysergide were infused into the visual cortex of kittens undergoing monocular deprivation. We found that both methods of disrupting serotoninergic transmission reduced OD plasticity. However, to be effective, the receptor blockers ketanserin and methysergide had to be applied in combination, suggesting that coactivation of serotonin receptor subtypes of both the 5-HT1 and 5-HT2 families have a permissive function in OD plasticity. Since activation of 5-HT2 receptors stimulates phosphoinositide hydrolysis, our data suggest that second messengers from the phospholipid pathway may play an important role in developmental plasticity of visual cortex.

Multideterminant role of calcium in hippocampal synaptic plasticity

Hippocampal CA1 cells possess several varieties of long-lasting synaptic plasticity: two different forms of long-term potentiation (LTP) and at least one form of long-term depression (LTD). All forms of synaptic plasticity are induced by afferent activation, all involve Ca2+ influx, all can be blocked by Ca2+ chelators, and all activate Ca(2+)-dependent mechanisms. The question arises as how different physiological responses can be initiated by activation of the same second messenger. We consider two hypotheses which could account for these phenomena: voltage-dependent differences in cytosolic Ca2+ concentration acting upon Ca2+ substrates of differing Ca2+ affinities and compartmentalization of the Ca2+ and its substrates.

Long-term depression requiring tACPD-receptor activation and NMDA-receptor blockade

In slices from the rat visual cortex, application of the metabotropic glutamate receptor (mGluR) agonist trans-1-aminocyclo-pentane-1,3-dicarboxylic acid (tACPD), whether combined with tetanization or not, produced only a reversible depression but not long-term depression (LTD) of synaptic transmission. In the presence of both tACPD and the NMDA receptor antagonist D-(-)-2-amino-5-phosphonopentanoate, tetanization induced LTD. These findings suggest requirement of tACPD-sensitive mGluR subtypes for inducing a form of LTD in the visual cortex.

Effects of maternal lead exposure on functional plasticity in the visual cortex and hippocampus of immature rats

We examined the amount of long-term potentiation (LTP) in slices from the visual cortex and hippocampus of pre- and postnatally lead-exposed rats and controls at postnatal days (PND) 12-20. A dietary lead intake of 750 ppm by the dams resulted in a mean blood lead concentration in the suckling offspring of about 17' micrograms/dl. While high-frequency stimulation (HFS) of the white matter induced LTP of the field potentials in layers II/III in cortical slices of ten out of the 14 control rats, only three of the twelve lead-exposed rats showed a small amount of LTP. However, in slices from seven of the twelve lead-exposed rats a long-term depression was found following HFS. Furthermore, paired-pulse inhibition was weaker in cortical slices from the lead-exposed as compared to the control rats. In the CA1 hippocampal region the amount of LTP was significantly reduced in the lead-exposed group only in slices taken from rats at PND 16-20, while no differences were seen in slices from younger animals. It is concluded that even low level lead exposure impairs functions of the visual cortex in the immature rat. We suggest that the developing hippocampus is able to compensate for lead-induced functional deficits in the 2nd postnatal week, being more vulnerable at older ages.

Potassium-induced long-term potentiation in area CA1 of the hippocampus involves phospholipase activation

Previous studies have shown that potassium-induced long-term potentiation (LTP) of the Schaffer collateral/commissural synapses in area CA1 of the hippocampus shares common properties with tetanus-induced LTP. In the present investigation, we performed electrophysiological and binding experiments on CA1 hippocampal slices to evaluate the location and nature of the changes underlying potassium-induced LTP. Paired-pulse facilitation, which represents an index of transmitter release, was markedly reduced by potassium-induced LTP. In addition, KCl-induced LTP was associated with an increase in 3H-AMPA ([3H]-amino-3-hydroxy-5-methylisoxazole-4-propionate) binding to CA1 synaptic membranes when measured 40 min after high-potassium exposure; however, no changes were detected in binding of an antagonist ([3H]-6-cyano-7-nitroquinoxaline-2,3-dione; 3H-CNQX) to AMPA receptors in slices expressing KCl-induced LTP. Administration of the phospholipase A2 (PLA2) inhibitor bromophenacyl bromide (BPB) prior to potassium application prevented LTP formation as well as the changes in paired-pulse facilitation and 3H-AMPA binding that characterized this type of potentiation. Taken together, these data indicate that potassium-induced LTP may be related to modifications in both pre- and postsynaptic properties and confirm the hypothesis that PLA2 activation is an important mechanism in long-term changes of synaptic operation.

N-methyl-D-aspartate receptor mediated component of field potentials evoked in horizontal pathways of rat motor cortex

To identify potential sites of synaptic modification of intrinsic cortical circuits, the contribution of the N-methyl-D-aspartate type of glutamate receptors to field potentials evoked in horizontal and oblique intracortical pathways was examined in rat motor cortex slice preparations. Presumably monosynaptic, short latency responses with a prominent negativity (-0.4 to -2.0 mV) were recorded in both superficial (across layer III) and deep (across layer V) horizontal pathways at a distance of approximately equal to 500 microns lateral to electrical stimulation sites and in oblique V-III pathway (-0.3 to -1.6 mV). Bath application of the N-methyl-D-aspartate receptor antagonist D,L-2-amino-5-phosphonovaleric acid (100 microM) reversibly decreased field potentials. Although decreases were observed in all components of the waveform, the most pronounced effect was on the late phase of the response. D,L-2-Amino-5-phosphonovaleric acid produced on average a 22% decrease in area, 12% in initial slope and 11% in peak amplitude of responses. Combined application of 100 microM D,L-2-amino-5-phosphonovaleric acid and a non-N-methyl-D-aspartate glutamate receptor antagonist, 6-cyano-7-nitro- or 6,7-dinitro-quinoxaline-2,3- dione (10-20 microM), eliminated all but a small, early and presumably non-synaptic response. In 18 of 23 cases, the relative contribution of the D,L-2-amino-5-phosphonovaleric acid-sensitive component was unrelated to field potential magnitude, suggesting that this component is present in all fiber classes. It is concluded that glutamate is the major transmitter of horizontal connections of layers II/III and layer V, as well as in the oblique V-III pathway. While most glutamatergic transmission is relayed by other glutamate receptor subtypes, N-methyl-D-aspartate receptor activation contributes a small but consistent part of ordinary transmission in each of these pathways in vitro. The results further suggest that a potential for N-methyl-D-aspartate receptor-mediated synaptic modification exists in intrinsic horizontal pathways of both superficial and deep layers of rat motor cortex.

Cellular localization and laminar distribution of NMDAR1 mRNA in the rat cerebral cortex

N-methyl-D-aspartate (NMDA) receptors, which play a critical role in many cortical functions, are composed of a heteromeric assembly of different subunits: of these, the NMDA receptor subunit 1 (NMDAR1) is a constant component of, and thus an excellent marker for, NMDA receptors. In this study, we have investigated the cellular localization and laminar distribution of NMDAR1 mRNA in the cerebral cortex of adult rats by in situ hybridization histochemistry with a 35S-labeled cRNA probe. Specificity and background levels were determined in adjacent sections incubated with a 35S-labeled sense RNA. In sections incubated with the antisense RNA probe, specific hybridization signal was observed in a large number of cells. Some cells, however, did not appear to contain NMDAR1 mRNA. The vast majority of these unlabeled cells were small, suggesting that they are astrocytes or other small nonneuronal cells. Double-labeling studies with in situ hybridization histochemistry and immunocytochemistry with antibodies to glial fibrillary acidic protein (GFAP) showed that about 95.7% of the GFAP-positive cells did not express NMDAR1 mRNA, indicating that virtually all astrocytes do not contain this transcript. A semiquantitative evaluation of cortical neurons, defined as those cells larger than the GFAP-positive astrocytes, revealed that about 80% were associated with silver grains. The number of silver grains associated with every neuron was determined from sections exposed for 15 days, the background level was subtracted, and all labeled neurons were grouped into five groups: A (< or = 10 grains), B (11-20 grains), C (21-30 grains), D (31-40 grains), and E (> 40 grains). The number of neurons belonging to each group was then evaluated according to their occurrence in each cortical layer. In layer I all labeled neurons were in group A, whereas in layers II-III and V-VI positive neurons were in group A-E. In layer IV most neurons were in groups A and B, whereas only a few were in group E. These observations indicate that 1) virtually all cortical cells containing NMDAR1 mRNA in adult rats are neurons; 2) about 80% of all cortical neurons express NMDAR1 mRNA; and 3) labeled neurons can be divided into several groups on the basis of NMDAR1 mRNA levels expressed, which presumably reflect the number of NMDA receptors. The existence of neurons with a different number of receptors may be a critical factor for determining the physiological effect of NMDA receptor activation.(ABSTRACT TRUNCATED AT 400 WORDS)

Post-activation potentiation and depression in the neocortex of the rat: II. Chronic preparations

Although long-term potentiation (LTP) has been demonstrated in a number of subcortical sites in chronic preparations, there have been no demonstrations of LTP in the neocortex of chronic preparations. Even neocortical slice and acute preparations often require a drug-induced suppression of inhibition before LTP effects can be reliably induced. We have attempted to induce LTP in neocortical sites in 7 different experiments using chronically prepared adult rats. We were unable to obtain any evidence, even a trend, for the induction of LTP. The following manipulations were tested: (1) standard stimulation train parameters that have been shown to be highly effective in subcortical and hippocampal sites; (2) a 10-fold increase in the intra-train pulse durations; (3) variations in train pulse frequency (1 Hz to 300 Hz) and train duration (100 ms to 15 min); (4) co-activation of multiple inputs by stimulation of combinations of cortical sites or cortical and thalamic sites; (5) reduction of inhibition by administration of picrotoxin; 5) Housing of animals in an enriched environment; (6) utilization of the neocortical stimulation trains as a cue in a learning task; (7) application of pilocarpine to co-activate cholinergic systems. Although none of these manipulations produced LTP, the application of pilocarpine did facilitate the induction of a long-lasting depression effect. These findings contrast with the results obtained from anesthetized rats and from studies using brain slices, where LTP can be reliably induced. These results are discussed in light of other recent findings with respect to LTP and LTD effects.

Post-activation potentiation in the neocortex: I. Acute preparations

Long-term potentiation is widely studied as a memory model, and has been demonstrated in a number of subcortical sites in both acute and chronic preparations. In the neocortex, however, most of the demonstrations of LTP have been in neocortical slice or acute preparations, and even these have often required a drug-induced attenuation of inhibition before the LTP could be reliably expressed. In this paper we show that LTP can be reliably expressed in adult rats in a number of neocortical sites, both ipsilateral and contralateral to the site of callosal stimulation. We also show that, when recording field potentials, LTP is expressed roughly equally at all cortical depths. In a third experiment, we monitored input/output (I/O), paired-pulse inhibition and short-term potentiation effects over the course of LTP induction. The ipsilateral responses were, as expected, of shorter latency and larger amplitude than contralateral responses. They also showed small spike-like components that correlated with cell discharge. Nevertheless, the contralateral responses tended to show the largest LTP effects. The paired pulse effect was mainly depression, lasting for up to 3000 ms, at both ipsilateral and control sites. The short-term potentiation components were best fit by two summed exponentials with time constants of about 70 s and 12 min. The LTP effect lasted at least two h which was the longest period monitored in these experiments.

Neuronal tuning and associative mechanisms in form representation

We examine the hypothesis that the form representation in the anterior inferotemporal (AIT) cortex is acquired through learning. According to this hypothesis, perceptual aspects of the temporal association area are closely related to its visual representation, in that the response selectivity of AIT neurons can be influenced by visual experience. On the basis of the neurophysiological evidence, we summarize two neuronal mechanisms that subserve the acquisition of form selectivity in AIT neurons. The first mechanism is neuronal tuning to particular stimuli that were learned in a cognitive task. The second mechanism is association, with which relevant information can be retrieved from other stored memories. On the grounds that long-term memory of objects is acquired and organized by at least these two neuronal mechanisms in the temporal association area, we further present a model of the cognitive memory system that unifies perception and imagery.

Long-term depression of excitatory synaptic transmission and its relationship to long-term potentiation

In many brain areas, including the cerebellar cortex, neocortex, hippocampus, striatum and nucleus accumbens, brief activation of an excitatory pathway can produce long-term depression (LTD) of synaptic transmission. In most preparations, induction of LTD has been shown to require a minimum level of postsynaptic depolarization and a rise in the intracellular Ca2+ concentration [Ca2+]i in the postsynaptic neurone. Thus, induction conditions resemble those described for the initiation of associative long-term potentiation (LTP). However, data from structures susceptible to both LTD and LTP suggest that a stronger depolarization and a greater increase in [Ca2+]i are required to induce LTP than to initiate LTD. The source of Ca2+ appears to be less critical for the differential induction of LTP and LTD than the amplitude of the Ca2+ surge, since the activation of voltage- and ligand-gated Ca2+ conductances as well as the release from intracellular stores have all been shown to contribute to both LTD and LTP induction. LTD is induceable even at inactive synapses if [Ca2+]i is raised to the appropriate level by antidromic or heterosynaptic activation, or by raising the extracellular Ca2+ concentration [Ca2+]o. These conditions suggest a rule (called here the ABS rule) for activity-dependent synaptic modifications that differs from the classical Hebb rule and that can account for both homosynaptic LTD and LTP as well as for heterosynaptic competition and associativity.

Distribution of seven major neurotransmitter receptors in the striate cortex of the New World monkey Callithrix jacchus

The distribution of seven different binding sites for the transmitters L-glutamate (L-glutamate binding sites and N-methyl-D-aspartate receptor), GABA (GABAA receptor), noradrenaline (alpha 1 receptor), acetylcholine (muscarinic M1 and M2 receptors) and serotonin (5-hydroxytryptamine1 receptor) are analysed in the primary visual cortex (area 17) of the common marmoset, Callithrix jacchus, using quantitative autoradiography. All binding sites show a well-defined laminar pattern, which changes sharply at the cytoarchitectonic border to area 18. The quantitative data show that the distribution of different receptors is relatively invariant across the cortical layers. Almost all receptors show a maximum in supragranular layers, low densities in layers IVA/IVB and a second maximum in layer IVC. Statistical analysis of these similarities in laminar distribution patterns of different receptors (co-distribution) reveals, as in other brain regions and species, that L-glutamate binding sites are co-distributed with N-methyl-D-aspartate, GABAA, and muscarinic M1 and M2 receptors. This may reflect the structural basis of a possible interaction between these receptors and their respective transmitters on the level of single cortical layers. Further co-distributions are found between N-methyl-D-aspartate, GABAA and M1, as well as between alpha 1 and M1 and finally between M1 and M2 receptors. Since not all receptors are co-distributed, the similarities in laminar patterns reveal specific aspects of the neurochemical organization of the cortex when receptors of different transmitter systems are analysed in the same brain.(ABSTRACT TRUNCATED AT 250 WORDS)

Nicotinic and muscarinic modulations of excitatory synaptic transmission in the rat prefrontal cortex in vitro

The importance of the cholinergic innervation of the neocortex in cognitive functions has been shown in a number of clinical and animal studies. Until recently, attempts to study the mode of action of acetylcholine in the neocortex have concentrated on muscarinic effects, whereas cholinergic actions mediated by nicotinic receptors have been difficult to demonstrate. The present work was undertaken to study the mechanism of action of nicotinic agents on cortical neurons and compare it to muscarinic effects by means of intracellular recordings in a slice preparation. The study was performed in the prelimbic area of the rat prefrontal cortex, a cortical region particularly involved in cognitive processes. Recordings were made from pyramidal cells located in layers II/III and synaptic potentials were evoked by stimulation of superficial cortical layers. Iontophoretic applications of nicotinic agonists (nicotine, dimethylphenylpiperazinium, cytisine) increased the amplitude of the monosynaptic excitatory postsynaptic potential mediated by non-N-methyl-D-aspartate glutamate receptors in 14% (22/159) of cells. This effect was abolished by the selective nicotinic blocker, neuronal bungarotoxin (IC50 = 0.6-0.7 microM) and by dihydro-beta-erythroidine (IC50 = 20-30 microM), whereas hexamethonium, mecamylamine, curare and alpha-bungarotoxin were ineffective. The nicotinic agonists did not change resting membrane potential, input resistance or current-voltage relationship. They also did not affect the depolarizations produced by glutamate applied by iontophoresis in the somatic or dendritic area. In contrast, the muscarinic agonists (muscarine, acetyl-beta-methylcholine) decreased the amplitude of the excitatory postsynaptic potential in 100% of the neurons tested. Atropine was more effective (IC50 = 0.08 microM) than pirenzepine (IC50 = 2 microM) to antagonize the muscarinic action. These effects were observed in the absence of any direct postsynaptic change in membrane potential or input resistance, provided that the site of the iontophoretic application was more than 100 microM distant from the soma. The muscarinic agonists did not influence the actions of iontophoretically applied glutamate. These results suggest that nicotinic and muscarinic agonists modulate excitatory synaptic transmission mediated at dendritic sites by non-N-methyl-D-aspartate glutamate receptors, possibly through a presynaptic action. Thus ascending cholinergic systems may take part in information processing in the prefrontal cortex through the control of ongoing excitation to pyramidal cells.

Reduced Mg2+ block of N-methyl-D-aspartate receptor-mediated synaptic potentials in developing visual cortex

Molecular cloning has demonstrated a diversity of artificially expressed N-methyl-D-aspartate (NMDA) receptors, implying a similar diversity of naturally occurring NMDA receptors in situ. Particularly significant was the success in expression of NMDA receptor classes exhibiting various sensitivities to Mg2+ block, a voltage-dependent channel blockade by Mg2+ that is essential to NMDA receptor functioning. Release from Mg2+ block often allows or facilitates the occurrence of long-term potentiation, a form of synaptic plasticity. Here we show that in the immature visual cortex, which is more susceptible to long-term potentiation than adult visual cortex, synaptically activated NMDA receptors, unlike those in the adult, have varying but clearly reduced sensitivities to Mg2+ block. We propose that the initially expressed, later-eliminated NMDA receptors exhibiting a reduced Mg2+ block may underlie the greater susceptibility to plasticity in the immature neocortex.

Comparison of associative and non-associative conditioning procedures in the induction of LTD in CA1 of the hippocampus

Recent reports have indicated that weak activity in a test input, negatively correlated (out-of-phase) with tetanization of a separate, converging input, produces an NMDA-independent, associative long-term depression (LTD) of the test input synapses, in hippocampal field CA1 (Stanton and Sejnowski, 1989; Stanton et al., 1991). Associative LTD has also been observed in the dentate gyrus, in vivo, but only following "priming" of the test path with 5 Hz stimulation prior to associative conditioning (Christie and Abraham, 1992b). We have used these stimulus protocols, in vitro, in order to compare the induction of non-associative and associative LTD in field CA1 of the adult rat hippocampus. Stimulation in normal solution evoked a small non-associative LTD, but no associative LTD. Addition of picrotoxin to the medium facilitated the induction of NMDA-dependent non-associative LTD, but not associative LTD. Previously potentiated pathways were not different from naive pathways in expression of LTD of either kind. Finally, 'priming' stimulation (5 Hz) of the test pathway produced a weak, selective enhancement of associative LTD that was, however, not significantly greater than non-associative LTD. These results indicate that, for our experimental conditions, negatively correlated co-activity during afferent tetanization does not induce a substantial associative LTD in area CA1.

Facilitatory effects of substance P on the susceptibility to long-term potentiation in the visual cortex of adult rats

We studied whether substance P (SP) facilitates induction of long-term potentiation (LTP) in adult rat visual cortex slices, using intracellular recordings of postsynaptic potentials (PSPs) elicited by white matter stimulation. Tetanic stimulation in normal medium induces no change of the PSP amplitude. In SP-containing medium, by contrast, tetanization potentiated the PSP amplitude in 5 over 7 cells examined. Although the other 2 cells underwent no change, the overall effect of SP was an increase in the probability of LTP induction. This effect of SP was canceled out by bath-application of the non-peptide antagonist CP96,345 (n = 7) or intracellular application of guanosine 5'-beta-thio-diphosphate (GDP beta S; n = 7). These results suggest that SP increased LTP susceptibility of neurons in the adult rat visual cortex through SP receptor-mediated mechanisms.

Neurons in rat cerebral cortex that synthesize nitric oxide: NADPH diaphorase histochemistry, NOS immunocytochemistry, and colocalization with GABA

Neurons that stain for NADPH diaphorase, which colocalizes with nitric oxide synthase (NOS), are scattered uniformly across neocortex, and denser in entorhinal cortex. In the primary sensorimotor cortex, 0.5-2% of neurons contain NOS. These are most numerous in layers II-III, whereas NOS-positive fibers are concentrated in layers IV and VI. Most stained neurons are aspiny bipolar cells. Some in deep layers are multipolar; very few are pyramidal-shaped. In layer IV, NOS-positive neurons and their dendrites are confined to the septa between barrels. Retrograde tracing experiments demonstrate that NOS-positive cells are local circuit neurons. Double staining demonstrates that NOS-positive neurons also contain GABA.

Common forms of synaptic plasticity in the hippocampus and neocortex in vitro

Activity-dependent synaptic plasticity in the superficial layers of juvenile cat and adult rat visual neocortex was compared with that in adult rat hippocampal field CA1. Stimulation of neocortical layer IV reliably induced synaptic long-term potentiation (LTP) and long-term depression (LTD) in layer III with precisely the same types of stimulation protocols that were effective in CA1. Neocortical LTP and LTD were specific to the conditioned pathway and, as in the hippocampus, were dependent on activation of N-methyl-D-aspartate receptors. These results provide strong support for the view that common principles may govern experience-dependent synaptic plasticity in CA1 and throughout the superficial layers of the mammalian neocortex.

N-methyl-D-aspartate stimulates the dephosphorylation of the microtubule-associated protein 2 and potentiates excitatory synaptic pathways in the rat hippocampus

We have studied the effect of brief (50-150 s) applications of N-methyl-D-aspartate (10-100 microM) on the phosphorylated state of the microtubule-associated protein 2 in slices of rat hippocampus. Following a similar experimental protocol we also studied the pattern of excitatory postsynaptic potentials intracellularly recorded in CA1 pyramidal cells elicited by stimulation of the Schaffer collateral-commissural pathway. N-Methyl-D-aspartate treatment produced a marked and specific dephosphorylation of the cytoskeletal microtubule-associated protein 2, which was not due to enhanced proteolytic activity. Dephosphorylation of the microtubule-associated protein 2 affects mainly the tubulin-binding domain of the molecule and seems to be a consequence of the activation of the Ca2+/calmodulin-dependent phosphatase calcineurin, as it is partially inhibited by calmidazolium but not by okadaic acid. A few minutes after N-methyl-D-aspartate treatment we observed a 23 +/- 17% increase in the amplitude of the monosynaptic excitatory postsynaptic potential recorded in the cells and the appearance of a large polysynaptic excitatory postsynaptic potential. Both effects lasted for several tens of minutes. The late polysynaptic potential was not observed when the CA3 and CA1 subfields were surgically separated. Our results indicate that the N-methyl-D-aspartate receptor activation leads to the dephosphorylation of the microtubule-associated protein 2 via a Ca2+/calmodulin phosphatase, probably calcineurine. This may, in turn, participate in the potentiation of synaptic efficacy.

Tetanization during GABAA receptor activation induces long-term depression in visual cortex slices

In rat visual cortex slices, long-term depression of synaptic transmission was induced by tetanic stimulation to white matter during bath-application of the GABAA agonist muscimol. This result supports the view that a combination of presynaptic activity and postsynaptic inhibition leads to a depression of synaptic efficiency.

Fluctuations in pyramid-pyramid excitatory postsynaptic potentials modified by presynaptic firing pattern and postsynaptic membrane potential using paired intracellular recordings in rat neocortex

Single axon excitatory connections between pairs of neocortical pyramidal neurons were studied using paired intracellular recordings in layers II/III and IV of coronal slices of adult rat somatosensory/motor cortex. Excitatory postsynaptic potentials evoked with different presynaptic firing patterns and at different postsynaptic membrane potentials were compared. Two methods of statistical analysis were used in attempts to determine whether changes in mean excitatory postsynaptic potential amplitude were due to presynaptic or postsynaptic modifications. Analysis of the decrease in mean excitatory postsynaptic potential amplitude associated with increases in presynaptic firing rate were consistent with a change in probability of transmitter release. Paired pulse depression appeared to exhibit both presynaptic and postsynaptic components when the interspike interval was < 10 ms, but could be explained simply by a decrease in probability of release with interspike intervals between 10 and 80 ms. Previous studies had demonstrated that these excitatory postsynaptic potentials are partially mediated by N-methyl-D-aspartate receptors. In contrast to the apparently presynaptic effects of firing pattern, postsynaptic membrane depolarization appeared to produce an increase in quantal amplitude. In addition to this increase at low frequencies, a form of frequency-dependent, self-potentiation involving the recruitment of an additional, longer-latency postsynaptic component occurred at higher presynaptic firing rates. The possibility is discussed that two different mechanisms are involved in the replacement of vesicles at release sites. Over a few tens of milliseconds (paired-pulse depression) availability of releasable transmitter may be determined by the rate of replacement of discharged vesicles from a readily releasable pool of vesicles. Over longer periods of firing at 0.33-2 Hz, the readily releasable pool may become exhausted and require replenishment. Postsynaptic depolarization increases the duration of these excitatory postsynaptic potentials, facilitating summation and enables two components of excitatory postsynaptic potential enhancement at N-methyl-D-aspartate receptor-mediated synapses; one that is present at all firing rates and relates simply to voltage dependent events and one that occurs at higher firing rates and involves a gradual, time dependent event. These data also indicate that the optimal pyramidal firing pattern if another pyramid is to be activated is a tonic, or brief burst pattern at relatively low repetition rates. Long bursts of many presynaptic spikes recruit little that is not activated by pairs of spikes. This situation is in stark contrast to the results obtained in the following paper in which excitatory inputs from pyramids to non-pyramids are described.

Dependence of long-term depression on postsynaptic metabotropic glutamate receptors in visual cortex

Long-term depression (LTD) is held relevant to memory and learning. Its induction is known to require postsynaptic calcium increases. However, the source of these calcium increases remains unclear. In visual cortex slices, LTD was induced by tetanization after blockade of N-methyl-D-aspartate (NMDA) and non-NMDA ionotropic glutamate receptors. LTD induced under this condition was prevented by an intracellular injection of each of the following drugs into the postsynaptic neuron: (i) guanosine 5'-[beta-thio]diphosphate, which competitively inhibits the binding of GTP to GTP-binding regulatory proteins; (ii) heparin, which antagonizes 1,4,5-inositol triphosphate binding; and (iii) calcium chelators. Moreover, LTD was induced without tetanization by applying quisqualate (10 microM), a metabotropic glutamate receptor agonist, but not another agonist, trans-aminocyclopentane-1,3-dicarboxylic acid (10 microM). Together, these results suggest that activation of 1,4,5-inositol trisphosphate-linked subtypes of metabotropic glutamate receptor is responsible for the increase in postsynaptic calcium concentration, which results in homosynaptic LTD.

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.

Synaptic N-methyl-D-aspartate receptors in the neonatal rat visual cortex are less sensitive to MK-801 than in adult

The excitatory postsynaptic potential mediated by N-methyl-D-aspartate (NMDA) receptors (NMDA-EPSP) was intracellularly obtained in slices from the visual cortex of adult and 2-week-old rats. Bath application of 0.3 microM (+)-MK-801, an NMDA receptor open-channel blocker, completely blocked NMDA-EPSPs in slices from adult rats. By contrast, in slices from the younger rats, a 10 microM solution was needed for the complete blockade. This difference in sensitivity to an open-channel blocker implies a major developmental change in the conformation of NMDA receptors in situ.

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.

The effects of selective glutamate receptor antagonists on synchronized firing bursts in layer III of rat visual cortex

In the rat visual cortex in vitro, single-shock stimulations applied to the border between layer VI and the white matter evoke synchronized burst-firing by units in layer III. We have examined the effects of glutamate receptor antagonists on this activity, with antagonists applied via the bath to allow correlation of effects with concentrations. All synaptically driven components (recorded extracellularly as field potential 'S2' spikes, dipoles 'W1' and 'W2', and coinciding single-unit spikes) were inhibited by greater than 90% in 1.0 mM kynurenic acid and in 3 or 10 microM 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX, which selectively blocks AMPA/kainate receptors). S2 spike amplitudes were reduced by half in 0.7 microM CNQX. 2-Amino-5-phosphonovalerate (APV), a specific blocker of NMDA receptors, did not prevent S2 spike burst or horizontal spread of bursting within layer III. However, APV reduced the duration of synchronized bursts and the slower potentials which followed. In Mg(2+)-free medium, new components appeared which were APV-sensitive: (1) low amplitude spikes, distributed spatially like S2 spike, but recurring more slowly, and (2) slow potentials, distributed spatially like W1 and W2 potentials, but lasting for hundreds of milliseconds. The amplitudes of these spikes were reduced by half in 3 microM D-APV. Our data imply that: (1) glutamate receptors play a major role in mediating local, excitatory neurotransmission in the supragranular layers of neocortex, with NMDA and AMPA/kainate subtypes each subserving somewhat different functions; (2) AMPA/kainate receptors mediate rapid excitatory transmission between layer III neurons, responsible for driving the first 15 ms of synchronized bursts; (3) currents gated by NMDA receptors determine the duration of coherent firing bursts, and drive asynchronous neuronal firing following bursts; and (4) under conditions which circumvent block by extracellular Mg2+, activation of NMDA receptors greatly enhances and prolongs the response to single-shock stimulations. In vivo, activation of layer III neurons is likely to depend significantly upon currents gated by NMDA receptors whenever repetitively firing excitatory inputs summed over several tens of milliseconds provide enough depolarization to lift block by extracellular Mg2+.

Development of neural connections between visual cortex and transplanted lateral geniculate nucleus in rats

The development of neural connections between transplanted lateral geniculate nucleus (LGN) and host visual cortex (VC) was studied in slice preparations obtained from rat brain in which a fetal (embryonic day 15-17) rat LGN was transplanted to the white matter underlying the VC of a neonate rat (postnatal day 0-1). Placing a fluorescent dye (DiI) in the transplant of the fixed slices revealed that retrogradely labeled cortical cells projecting to the transplant were broadly distributed through layers II to VI at 1 week after transplantation. Three weeks after transplantation, these cells were virtually confined to layer VI. Likewise, anterograde labeling showed that cells in the transplant sent axons up to layer I with a few branches at 1 week after transplantation, while the axons were found to terminate at layer IV with many arborizations at 3 weeks after transplantation. These observations were supported by electrophysiological studies. Analysis of the antidromic responses of the cortical cells to stimulation of the transplant showed that the efferent cells projecting to the transplant were broadly distributed in layers II-VI at 1 week after transplantation, while they were virtually restricted to layer VI at 3 weeks after transplantation. Current source-density analysis of the field potentials and intracellular analysis of the synaptic potentials in the cortical cells demonstrated that geniculocortical connections were broadly established in layers II-VI at 1 week after transplantation, and were localized to layer IV and VI at 3 weeks after transplantation. These results suggest that the development of neural connections between transplanted LGN and host VC is characterized by an initial broad distribution of afferent and efferent connections without laminar specificity, and by later selection of appropriate connections to yield lamina-specific connections comparable to those in normal adult VC.