Induction of NMDA receptor-independent long-term potentiation (LTP) in visual cortex of adult rats

NMDA receptor-independent LTP in mammalian nervous system

Long-term potentiation (LTP) of synaptic transmission is a form of activity-dependent synaptic plasticity that exists at most synapses in the nervous system. In the central nervous system (CNS), LTP has been recorded at numerous synapses and is a prime candidate mechanism associating activity-dependent plasticity with learning and memory. LTP involves long-lasting increase in synaptic strength with various underlying mechanisms. In the CNS, the predominant type of LTP is believed to be dependent on activation of the ionotropic glutamate N-methyl-D-aspartate receptor (NMDAR), which is highly calcium-permeable. However, various forms of NMDAR-independent LTP have been identified in diverse areas of the nervous system. The NMDAR-independent LTP may require activation of glutamate metabotropic receptors (mGluR) or ionotropic receptors other than NMDAR such as nicotinic acetylcholine receptor (α7-nAChR), serotonin 5-HT3 receptor or calcium-permeable AMPA receptor (CP-AMPAR). In this review, NMDAR-independent LTP of various areas of the central and peripheral nervous systems are discussed.

Calcium signalling through L-type calcium channels: role in pathophysiology of spinal nociceptive transmission

L-type voltage-gated calcium channels are ubiquitous channels in the CNS. L-type calcium channels (LTCs) are mostly post-synaptic channels regulating neuronal firing and gene expression. They play a role in important physio-pathological processes such as learning and memory, Parkinson's disease, autism and, as recognized more recently, in the pathophysiology of pain processes. Classically, the fundamental role of these channels in cardiovascular functions has limited the use of classical molecules to treat LTC-dependent disorders. However, when applied locally in the dorsal horn of the spinal cord, the three families of LTC pharmacological blockers - dihydropyridines (nifedipine), phenylalkylamines (verapamil) and benzothiazepines (diltiazem) - proved effective in altering short-term sensitization to pain, inflammation-induced hyperexcitability and neuropathy-induced allodynia. Two subtypes of LTCs, Ca_v 1.2 and Ca_v 1.3, are expressed in the dorsal horn of the spinal cord, where Ca_v 1.2 channels are localized mostly in the soma and proximal dendritic shafts, and Ca_v 1.3 channels are more distally located in the somato-dendritic compartment. Together with their different kinetics and pharmacological properties, this spatial distribution contributes to their separate roles in shaping short- and long-term sensitization to pain. Ca_v 1.3 channels sustain the expression of plateau potentials, an input/output amplification phenomenon that contributes to short-term sensitization to pain such as prolonged after-discharges, dynamic receptive fields and windup. The Ca_v 1.2 channels support calcium influx that is crucial for the excitation-transcription coupling underlying nerve injury-induced dorsal horn hyperexcitability. These subtype-specific cellular mechanisms may have different consequences in the development and/or the maintenance of pathological pain. Recent progress in developing more specific compounds for each subunit will offer new opportunities to modulate LTCs for the treatment of pathological pain with reduced side-effects.

LINKED ARTICLES

This article is part of a themed section on Recent Advances in Targeting Ion Channels to Treat Chronic Pain. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v175.12/issuetoc.

Prolonged rote learning produces delayed memory facilitation and metabolic changes in the hippocampus of the ageing human brain

BACKGROUND

Repeated rehearsal is one method by which verbal material may be transferred from short- to long-term memory. We hypothesised that extended engagement of memory structures through prolonged rehearsal would result in enhanced efficacy of recall and also of brain structures implicated in new learning. Twenty-four normal participants aged 55-70 (mean = 60.1) engaged in six weeks of rote learning, during which they learned 500 words per week every week (prose, poetry etc.). An extensive battery of memory tests was administered on three occasions, each six weeks apart. In addition, proton magnetic resonance spectroscopy (1H-MRS) was used to measure metabolite levels in seven voxels of interest (VOIs) (including hippocampus) before and after learning.

RESULTS

Results indicate a facilitation of new learning that was evident six weeks after rote learning ceased. This facilitation occurred for verbal/episodic material only, and was mirrored by a metabolic change in left posterior hippocampus, specifically an increase in NAA/(Cr+Cho) ratio.

CONCLUSION

Results suggest that repeated activation of memory structures facilitates anamnesis and may promote neuronal plasticity in the ageing brain, and that compliance is a key factor in such facilitation as the effect was confined to those who engaged fully with the training.

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.

Chronic exposure to trichloroethylene affects neuronal plasticity in rat hippocampal slices

Inhalational exposure to organic solvents is known to exert neurotoxic effects. Using the new multielectrode dish system (Panasonic) the effects of chronic exposure to trichloroethylene (TCE) on neuronal plasticity were assessed in different regions of the adult rat brain. Two groups of Long-Evans rats were exposed to 0 ppm or 500 ppm TCE, respectively, 6 h/day, 5 days/week for 6 months. Long-term potentiation (LTP) as well as paired-pulse potentiation/inhibition were assessed in slices from the visual cortex and the hippocampus. In addition, several behavioral tests were performed. Trichloroethanol concentrations were measured in blood and trichloroacetic acid concentrations were determined in urine. While TCE exposure impaired LTP as well as paired-pulse potentiation in hippocampal slices, no effects were seen in cortical slices. Our data demonstrate brain region specific functional changes following TCE exposure with the hippocampus being more vulnerable than the visual cortex. The behavioral measurements revealed no TCE related effects.

Metabotropic glutamate receptors mediate expression of LTP in slices of rat visual cortex

Long-term-potentiation (LTP) can be induced by application of a standard theta-burst stimulation protocol in slice preparations of the neocortex. This type of LTP is known to be dependent on the activation of NMDA receptors. The present study used specific experimental conditions to evoke a non-NMDA receptor mediated type of LTP. By use of weak theta-burst stimulation (wTBS) we describe a non-NMDA receptor dependent LTP in rat visual cortex in vitro, which is sensitive to an antagonist of metabotropic glutamate receptors (mGluR). In slices of the visual cortex we stimulated ascending inputs in cortical layer IV and recorded extracellular field potentials (FPs) from cortical layers II/III. In disinhibited slices (with 1 microm picrotoxin), a wTBS induced LTP to 138% of control. The expression of this potentiation was insensitive to the NMDA-receptor antagonist, D-AP5, but could be abolished by application of the mGluR antagonist MCPG. These data suggest an NMDA-independent mechanism for LTP induction in the visual cortex which can be observed in layer II/III neurons.

Long-term potentiation of thalamocortical transmission in the adult visual cortex in vivo

It has been suggested that NMDA receptor-dependent synaptic strengthening, like that observed after long-term potentiation (LTP), is a mechanism by which experience modifies responses in the neocortex. We report here that patterned (theta burst) stimulation of the dorsal lateral geniculate nucleus reliably induces LTP of field potentials (FPs) evoked in primary visual cortex (Oc1) of adult rats in vivo. The response enhancement is saturable, long-lasting, and dependent on NMDA receptor activation. To determine the laminar locus of these changes, current source density (CSD) analysis was performed on FP profiles obtained before and after LTP induction. LTP was accompanied by an enhancement of synaptic current sinks located in thalamorecipient (layer IV and deep layer III) and supragranular (layers II/III) cell layers. We also examined immunocytochemical labeling for the immediate early gene zif-268 1 hr after induction of LTP. In concert with the laminar changes observed in CSD analyses, we observed a significant increase in the number of zif-268-immunopositive neurons in layers II-IV that occurred over a wide extent of Oc1. Last, we investigated the functional consequences of LTP induction by monitoring changes in visually evoked potentials. After LTP, we observed that the cortical response to a full-field flash was significantly enhanced and that responses to grating stimuli were increased across a range of spatial frequencies. These findings are consistent with growing evidence that primary sensory cortex remains plastic into adulthood, and they show that the mechanisms of LTP can contribute to this plasticity.

The role of N-methyl-D-aspartate receptors in synaptic plasticity of rat visual cortex in vitro: effect of sensory experience

We examined the role of N-methyl-D-aspartate (NMDA) receptors in synaptic plasticity of visual cortex of light (LR) and dark (DR) reared adult rats in vitro. Layer IV stimulation resulted in field potentials in layer II/III, consisting of two excitatory postsynaptic potentials (EPSP) called EPSP1 and EPSP2. Tetanic stimulation induced long-term potentiation (LTP) in EPSP2 of both LR and DR visual cortices. NMDA receptor antagonist D, L-2-amino-5-phosphono-valeric acid (AP5) completely blocked the LTP of EPSP2 in DR visual cortex while it reduced slightly the extent of LTP of EPSP2 in LR ones. Another NMDA receptor antagonist ketamine blocked potentiation of EPSP1 as well as EPSP2 in both groups. Our findings demonstrate that dependency of LTP on NMDA receptors and/or sensitivity of these receptors to the antagonists are different in LR and DR animals.

Retrograde signalling with nitric oxide at neocortical synapses

Long-term changes of synaptic transmission in slices of rat visual cortex were induced by intracellular tetanization: bursts of short depolarizing pulses applied through the intracellular electrode without concomitant presynaptic stimulation. Long-term synaptic changes after this purely postsynaptic induction were associated with alterations of release indices, thus providing a case for retrograde signalling at neocortical synapses. Both long-term potentiation and long-term depression were accompanied by presynaptic changes, indicating that retrograde signalling can achieve both up- and down-regulation of transmitter release. The direction and the magnitude of the amplitude changes induced by a prolonged intracellular tetanization depended on the initial properties of the input. The inputs with initially high paired-pulse facilitation (PPF) ratio, indicative of low release probability, were most often potentiated. The inputs with initially low PPF ratio, indicative of high release probability, were usually depressed or did not change. Thus, prolonged postsynaptic activity can lead to normalization of the weights of nonactivated synapses. The dependence of polarity of synaptic modifications on initial PPF disappeared when plastic changes were induced with a shorter intracellular tetanization, or when the NO signalling pathway was interrupted by inhibition of NO synthase activity or by application of NO scavengers. This indicates that the NO-dependent retrograde signalling system has a relatively high activation threshold. Long-term synaptic modifications, induced by a weak postsynaptic challenge or under blockade of NO signalling, were nevertheless associated with presynaptic changes. This suggests the existence of another retrograde signalling system, additional to the high threshold, NO-dependent system. Therefore, our data provide a clear case for retrograde signalling at neocortical synapses and indicate that multiple retrograde signalling systems, part of which are NO-dependent, are involved.

Do Ca2+ channels share NMDA receptors in plasticity of synaptic transmission in the rat visual cortex?

We examined the involvement of Ca2+ channels in LTP of responses in rat visual cortex slices. Stimulating layer IV, field potentials including EPSP1 and EPSP2 from layer II/III were recorded. L-type Ca2+ channel blocker nifedipine did not have a considerable effect on LTP of the responses. T-type Ca2+ channel blocker Ni2+ decreased potentiation of EPSP1 and almost blocked that of EPSP2. Effect of visual experience on the function of the channels is also considered. These results indicate that T-type Ca2+ channels play a real role in stable LTP of EPSP2. Also the function of the channels was almost the same in dark and light reared visual cortices.

Input-and layer-dependent synaptic plasticity in the rat perirhinal cortex in vitro

The perirhinal cortex is crucially important in several forms of memory. Whilst it is important to understand the underlying mechanisms of this role in memory, little is known about the synaptic physiology or plasticity of this region of transitional cortex. In this study, we recorded evoked field potentials in superficial layers (approximately layer I) of the perirhinal cortex in vitro. One stimulating electrode was placed on the temporal side and the other on the entorhinal side of the rhinal sulcus in either the superficial or intermediate layers (approximately layers II/III). Paired stimuli resulted in depression of the second response. Paired-pulse depression was maximal at a 200-ms interpulse interval. Low-frequency stimulation resulted in synaptic depression, which returned to baseline within 60 min. The magnitude of both paired-pulse depression and low-frequency stimulation-induced depression was significantly greater at synapses activated from the temporal intermediate pathway than the other three pathways. Long-term potentiation, stable for at least 60 min, was induced by high-frequency stimulation of intermediate but not superficial pathways. Long-lasting depression (depotentiation) was induced by low-frequency stimulation following the induction of long-term potentiation. The induction of both long-term potentiation and depotentiation was N-methyl-D-aspartate receptor dependent. The group I/II metabotropic glutamate receptor antagonist (S)-alpha-methyl-4-carboxyphenylglycine was without effect on either of these forms of plasticity. Thus, both long- and short-lasting forms of synaptic plasticity exist at synapses in the perirhinal cortex, and these may mediate the changes in neuronal responses associated with visual recognition memory.

Use of brain slices to study long-term potentiation and depression as examples of synaptic plasticity

Brain slices have been responsible for the majority of advances in our understanding of the cellular aspects of altered synaptic strength underlying memory, long-term potentiation (LTP) and long-term depression (LTD), and increases and decreases, respectively, in synaptic strength at glutamatergic synapses. Our current understanding of LTP and LTD has come largely from studies in hippocampal slices. We consider the strengths and limitations of brain slice technology applied to this subject and conclude that they will continue to have an important role in future studies into the cellular machinery underlying changes in synaptic strength.

Impairment of neocortical long-term potentiation in mice deficient of endothelial nitric oxide synthase

The role of the possible retrograde messenger nitric oxide (NO) in the induction of long-term potentiation (LTP) was studied in supragranular layers of somatosensory cortical slices obtained from adult mice. High-frequency stimulation produced a slowly rising, long-lasting (50 min) and significant (P < 0.001) increase in the extracellular synaptic response by 23%. The induction of LTP was independent from activation of N-methyl-D-aspartate (NMDA) receptors, but prevented by bath application of NG-nitro-L-arginine methyl ester (L-NAME), indicating that one or several of the different NO synthases (NOS) produced NO within the postsynaptic neuron. No LTP could be induced in knockout mice lacking the endothelial NOS (eNOS) isoform. These data suggest that eNOS is involved in an NMDA receptor-independent form of LTP in the rodent cerebral cortex.

The cell cycle gene SKP1 is regulated by light in postnatal rat brain

During the early postnatal phase of high neuronal plasticity, an altered visual input leads to great modifications of visual cortex organization [Y. Frégnac, M. Imbert, Development of neuronal selectivity in primary visual cortex of cat, Physiol. Rev., 64 (1984) 375-434; D.H. Hubel, T.N. Wiesel, S. LeVay, Plasticity of ocular dominance columns in monkey striate cortex, Philos. Trans. R. Soc. London, Ser. B, 278 (1977) 377-409.]. We used refined differential screening of an organized cDNA library to identify the genes that may participate in this plasticity. We isolated a candidate plasticity gene encoding for a 163 aa protein that is closely related to the human and yeast Skp1p, a key factor in cell cycle progression [C. Baï, K. Hofman, L. Ma, M. Goebl, J.W. Harper, S.J. Elledge, SKP1 connects cell cycle regulators to the ubiquitin proteolysis machinery through a novel motif, the F-box, Cell, 86 (1996) 263-274; C. Connelly, P. Hieter, Budding yeast SKP1 encodes an evolutionary conserved kinetochore protein required for cell cycle progression, Cell, 86 (1996) 275-285; H. Zhang, R. Kobayashi, K. Galaktionov, D. Beach, p19Skp1 and p45Skp2 are essential elements of the cyclin A-CDK2 S phase kinase, Cell, 82 (1995) 915-925.]. Northern blot analysis showed that the expression of SKP1 (Skp1p gene) dramatically decreased after 2 h of light stimulation in the visual cortex of young dark-reared rats. This down regulation lasted at least 72 h. It was specific for the critical period as we did not observe any significant regulation of SKP1 mRNA by light in adult dark-reared rat brain. The down regulation was observed in the superior colliculus but also in the frontal cortex and in the hippocampus. The fact that this down regulation was not restricted to the visual system, suggested that it could be produced by dark rearing-induced hormonal changes. The significance of SKP1 expression in the brain and its regulation are discussed.

Transient synaptic potentiation in the visual cortex. II. Developmental regulation

In our previous study, pairing-induced transient synaptic potentiation in supragranular layers of the visual cortex was described in mature guinea pigs. In the present study, the development of this type of synaptic plasticity and the underlying cellular mechanisms that mediate it were evaluated in animals from postnatal day (PND) 5 to 180. Potentiation is more reliably evoked in younger animals (likelihood: 75%, PND 5-30; 51%, PND > or = 34), and the magnitude of the effect is greater (+40 +/- 3%, mean +/- SE, PND 5-30; +26 +/- 3%, PND > or = 34). Similar to data obtained from the mature animals, visual cortical transient synaptic potentiation in the immature cortex occurs at excitatory synaptic sites directly activated by the stimulation, and activation by local recurrent cortical circuits is not necessary for the induction of this potentiation. This is demonstrated by 1) experiments in which action potential output from the paired neuron was blocked by Lidocaine, N-ethyl bromide quaternary salt applied into the neuron (5 of 5), and 2) experiments in which the contribution to the compound postsynaptic potential by inhibitory synapses was eliminated by selective, intracellular blockade by gamma-aminobutyric acid-mediated inhibitory postsynaptic potentials only onto the recorded neuron (7 of 11). Thus these perturbations do not reduce the likelihood or magnitude of this synaptic potentiation. In contrast to the N-methyl-D-aspartate (NMDA) receptor dependence for induction of this synaptic potentiation in the cortex of mature animals, in the young animals' cortices (PND 11-27) potentiation is readily induced during blockade of NMDA receptors (72%, 13 of 18, did not different from control: 75%, 40 of 53). Thus the NMDA receptor becomes functionally linked to a synaptic potentiation cascade during development, replacing another 2-amino-5-phosphonovaleric acid (APV)-insensitive potentiation process in the neonatal cortex. Postsynaptic intracellular calcium has a critical role in the induction of this form of synaptic potentiation in all ages studied. Synaptic potentiation was prevented (8 of 11 cases) or was replaced by synaptic depression (3 of 11 cells) in experiments in which postsynaptic calcium levels were reduced by intracellular application of 1,2-bis-2-aminophenoxy ethane-N,N,N',N'-tetraacetic acid (BAPTA) in the cortex of young (PND 7-14) animals, or in which the extracellular calcium concentrations was lowered. Inhibition of postsynaptic calcium-induced calcium release blocked synaptic potentiation (4 of 4 cells). Prolonged superfusion (3 h) of the nitric oxide synthase inhibitor L-nitro-arginine (LNA) did not significantly affect the likelihood (in LNA, 81%; 13 of 16 cells), or the magnitude (+38 +/- 7% increase in LNA vs. +40 +/- 3% in control cases) of potentiation, in contrast to its effects in the mature cortex.

Protein and RNA synthesis-dependent and -independent LTPs in developing rat visual cortex

Multiple forms of synaptic potentiation have been described, but their involvement in development versus learning is unknown. To address this, we examined whether long-term potentiation (LTP) in visual cortex requires protein or RNA synthesis using slice preparations. Theta-burst stimulation of white matter induced two distinct types of LTP in layer 4. A slowly developing LTP, preferentially induced in juveniles, was blocked by protein and RNA synthesis inhibitors and was L-type calcium channel dependent. A quickly developing LTP, induced in juveniles and adults, was independent of macromolecular synthesis and required N-methyl-D-aspartate receptor activation. Thus, slow LTP might account for developmental plasticity in visual cortex including the activity-dependent refinement of neural circuitry while fast LTP might underlie the changes in synaptic strength that may participate in visual learning and memory.

Long-term changes, induced by microstimulation of the neocortex, in the efficiency of excitatory postsynaptic transmission in the thalamocortical networks

Neuronal networks with synaptic plasticity, consisting of cells located in various loci of the AC and the MGB, were investigated. It was demonstrated that, as a result of MS applied in the region of cortical elements of the network, connections could alter between all elements of the cortex-thalamus-cortex neuronal network. Changes were manifested in the form of LTP and/or LTD of the efficiency of excitatory connections, as well as in the form of intensification or attenuation of the action of the "common source'; the changes were maintained for tens of minutes. The number of connections between stimulated and non-stimulated elements of the network increased. Neurons in which more favorable conditions for LTP developed were distinguished in the networks. The character of the modification of synapses formed by the axons of several cells on one of the elements of the network could vary. Synapses formed by axonal collaterals of one cell on several elements of the network could also be modified variously.

Long-term potentiation and N-methyl-D-aspartate receptors: foundations of memory and neurologic disease?

Understanding the physiology of learning and memory is one of the great challenges of neuroscience. The discovery in recent years of long-term potentiation (LTP) of synaptic transmission and the elaboration of the mechanisms involved, in particular the NMDA receptor, offers the prospect not only of improving our understanding of normal memory storage and retrieval, but may also yield insights about various neurological and psychiatric clinical disorders. In this review, we begin by examining the different forms, properties, and methods of inducing LTP, followed by a description of molecular mechanisms thought to underlie the phenomenon. Molecular structure of the receptor is discussed, along with the roles of Ca2+ second messenger systems, synaptic morphology changes, and retrograde messengers in LTP. Finally, implications of the NMDA receptor and LTP in learning, memory, and certain clinical conditions such as epilepsy, Alzheimer's disease, and schizophrenia are discussed.

The development of MK-801, kainate, AMPA, and muscimol binding sites in cat visual cortex

Previous work using homogenate binding has shown that the development of (+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]-cyclohepten-5,10imin e maleate (MK-801) binding in cat visual cortex increases from 21 days to 42 days, the height of the plastic period, and decreases in adulthood. We have studied the generality of this finding by examining the development of NMDA binding sites in several brain regions and by examining the development of other binding sites in the visual cortex. After confirming the original finding, we extended it by showing that the sensitivity of MK-801 binding sites to glutamate and glycine decreases when the cat becomes an adult. We then examined the regional specificity of MK-801 binding. Retinal binding did not change significantly with age. Binding in both visual cortex and hippocampus increased significantly from 7 days to 42 days regardless of whether binding was measured per milligram wet weight or per milligram protein. The decline from 42 days to adulthood was less dramatic in the hippocampus than in the visual cortex and was statistically significant only when binding was measured per milligram protein. Saturation analyses also showed a difference in the two structures. Bmax in the visual cortex, but not in the hippocampus, decreased from 42 days to adulthood. To determine whether these developmental changes were specific to MK-801 binding sites, we compared the age-dependent binding of MK-801, kainate, alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA), and muscimol. Like MK-801, kainate binding increased from 7 days to 42 days and decreased from 42 days to adulthood. AMPA and muscimol binding showed a similar increase in binding from 7 days to 42 days but did not decrease significantly from 42 days to adulthood. Displacement experiments suggest that AMPA and kainate bind to separate sites. The 42-day peak in NMDA and kainate binding suggests that their associated receptors may have a role in determining the plastic period 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.

The origin of synaptic neuroplasticity: crucial molecules or a dynamical cascade?

What is neuroplasticity and what are its origins? These questions have been the subject of intense theoretical and experimental research in the neurosciences for decades. Basically, the term neuroplasticity refers to the ability of neurons to alter some functional property in response to alterations in input. Traditional definitions, however, are often imprecise and restricted to particular 'model' neural systems. In the present article we will consider several of the most widely studied models of synaptic-level neuroplasticity including alterations in response properties of two types of invertebrate sensory neurons, long-term potentiation (LTP) in mammalian hippocampus and cortex, and ocular dominance shifts in cat visual cortex. While many other forms of neuroplasticity have been studied, these examples typify the diversity of the subject, as well as illustrate our contention that no unitary model of the phenomena is possible for all conditions. This last point is of particular importance for the mammalian literature, since many hypotheses concerning the mechanism(s) underlying the initiation of neuroplasticity have proposed a single crucial molecular element as the primary causal agent. A closer examination of these various hypotheses, in concert to several examples from the invertebrate literature, leads, however, to the conclusion that synaptic neuroplasticity must arise from a series of inter-related molecular events of a particular form, a cascade, in which individual elements may differ radically from system to system. We next provide an overview of our studies of age-dependent regulation of excitatory and inhibitory ionotropic neurotransmitter receptor populations in cortex in response to agonist and depolarizing stimulation. We provide evidence that such regulation for ionotropic receptors is under the control of ionically driven receptor kinase and phosphatase activity which is also age-dependent in function. These data provide the basis for a cascade model of receptor regulation. Based on this qualitative model, we describe a quantitative computer simulation of certain age-dependent stages in the receptor regulatory cascade which may interact to produce LTP-like effects. While such a model is not exclusive, it nevertheless provides a demonstration that elements in the proposed cascade may comprise the necessary and sufficient conditions for some forms of neuroplasticity. We also propose some of the principles underlying our model as a means of unifying much of the diverse phenomenology reported in the literature. Finally, we make a series of explicit predictions which are testable with current experimental techniques.(ABSTRACT TRUNCATED AT 250 WORDS)

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.

Activation of NMDA receptors in hippocampal area CA1 by low and high frequency orthodromic stimulation and their contribution to induction of long-term potentiation

N-methyl-D-aspartate (NMDA) receptors are important in many instances of synaptic plasticity. In hippocampal area CA1, long-term potentiation (LTP) can be induced by both NMDA receptor-dependent and -independent mechanisms. Using intracellular recordings and single-electrode voltage clamp, we isolated and characterized NMDA receptor-mediated synaptic responses. NMDA receptor-mediated responses evoked by low frequency orthodromic stimulation were inhibited in a dose-dependent manner by the competitive antagonist D,L-2-amino-5-phosphonovaleric acid (APV). High frequency (tetanic) stimulation, which facilitates synaptic release of glutamate, failed to overcome the blockade of NMDA receptors by APV. Using extracellular recordings of field potentials, we studied the contribution of NMDA receptors to LTP induced by different patterns of tetanic stimulation. LTP was inhibited in a dose-dependent manner by APV, but was more sensitive to APV than were NMDA receptor-mediated synaptic responses. This most likely reflects a threshold for NMDA receptor activation in LTP induction. A component of LTP that resisted blockade by APV was induced by high (200 Hz), but not low (25 Hz), frequency tetanization. This NMDA receptor-independent component of LTP persisted for > 4 hours and accounted for approximately half the potentiation induced by 200 Hz tetanization. Procedures necessary to induce LTP at the Schaffer collateral/commissural synapses in area CA1 by both NMDA receptor-dependent and -independent mechanisms are now well characterized. Using the same neuronal population, it will be possible to ask if processes involved in the maintenance of LTP are shared even when LTP is induced through two different mechanisms.

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.

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.

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.