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

Incremental induction of NMDAR-STP and NMDAR-LTP in the CA1 area of ventral hippocampal slices relies on graded activation of discrete NMDA receptors

N-methyl-d-aspartate receptor (NMDAR)-dependent short- and long-term types of potentiation (STP and LTP, respectively) are frequently studied in the CA1 area of dorsal hippocampal slices (DHS). Far less is known about the NMDAR dependence of STP and LTP in ventral hippocampal slices (VHS), where both types of potentiation are smaller in magnitude than in the DHS. Here, we first briefly review our knowledge about the NMDAR dependence of STP and LTP and some other forms of synaptic plasticity. We then show in new experiments that the decay of NMDAR-STP in VHS, similar to dorsal hippocampal NMDAR-STP, is not time- but activity-dependent. We also demonstrate that the induction of submaximal levels of NMDAR-STP and NMDAR-LTP in VHS differs from the induction of saturated levels of plasticity in terms of their sensitivity to subunit-preferring NMDAR antagonists. These data suggest that activation of distinct NMDAR subtypes in a population of neurons results in an incremental increase in the induction of different phases of potentiation with changing sensitivity to pharmacological agents. Differences in pharmacological sensitivity, which arise due to differences in the levels of agonist-evoked biological response, might explain the disparity of the results concerning NMDAR subunit involvement in the induction of NMDAR-dependent plasticity.This article is part of a discussion meeting issue 'Long-term potentiation: 50 years on'.

Some distorted thoughts about ketamine as a psychedelic and a novel hypothesis based on NMDA receptor-mediated synaptic plasticity

Ketamine, a channel blocking NMDA receptor antagonist, is used off-label for its psychedelic effects, which may arise from a combination of several inter-related actions. Firstly, reductions of the contribution of NMDA receptors to afferent information from external and internal sensory inputs may distort sensations and their processing in higher brain centres. Secondly, reductions of NMDA receptor-mediated excitation of GABAergic interneurons can result in glutamatergic overactivity. Thirdly, limbic cortical disinhibition may indirectly enhance dopaminergic and serotonergic activity. Fourthly, inhibition of NMDA receptor mediated synaptic plasticity, such as short-term potentiation (STP) and long-term potentiation (LTP), could lead to distorted memories. Here, for the first time, we compared quantitatively the effects of ketamine on STP and LTP. We report that ketamine inhibits STP in a double sigmoidal fashion with low (40 nM) and high (5.6 μM) IC50 values. In contrast, ketamine inhibits LTP in a single sigmoidal manner (IC50 value ∼ 15 μM). A GluN2D-subunit preferring NMDA receptor antagonist, UBP145, has a similar pharmacological profile. We propose that the psychedelic effects of ketamine may involve the inhibition of STP and, potentially, associated forms of working memory. This article is part of the Special Issue entitled 'Psychedelics: New Doors, Altered Perceptions'.

A Re-Examination of Hebbian-Covariance Rules and Spike Timing-Dependent Plasticity in Cat Visual Cortex in vivo

Spike timing-dependent plasticity (STDP) is considered as an ubiquitous rule for associative plasticity in cortical networks in vitro. However, limited supporting evidence for its functional role has been provided in vivo. In particular, there are very few studies demonstrating the co-occurrence of synaptic efficiency changes and alteration of sensory responses in adult cortex during Hebbian or STDP protocols. We addressed this issue by reviewing and comparing the functional effects of two types of cellular conditioning in cat visual cortex. The first one, referred to as the “covariance” protocol, obeys a generalized Hebbian framework, by imposing, for different stimuli, supervised positive and negative changes in covariance between postsynaptic and presynaptic activity rates. The second protocol, based on intracellular recordings, replicated in vivo variants of the theta-burst paradigm (TBS), proven successful in inducing long-term potentiation in vitro. Since it was shown to impose a precise correlation delay between the electrically activated thalamic input and the TBS-induced postsynaptic spike, this protocol can be seen as a probe of causal (“pre-before-post”) STDP. By choosing a thalamic region where the visual field representation was in retinotopic overlap with the intracellularly recorded cortical receptive field as the afferent site for supervised electrical stimulation, this protocol allowed to look for possible correlates between STDP and functional reorganization of the conditioned cortical receptive field. The rate-based “covariance protocol” induced significant and large amplitude changes in receptive field properties, in both kitten and adult V1 cortex. The TBS STDP-like protocol produced in the adult significant changes in the synaptic gain of the electrically activated thalamic pathway, but the statistical significance of the functional correlates was detectable mostly at the population level. Comparison of our observations with the literature leads us to re-examine the experimental status of spike timing-dependent potentiation in adult cortex. We propose the existence of a correlation-based threshold in vivo, limiting the expression of STDP-induced changes outside the critical period, and which accounts for the stability of synaptic weights during sensory cortical processing in the absence of attention or reward-gated supervision.

The kinetic profile of intracellular calcium predicts long-term potentiation and long-term depression

Efficiency of synaptic transmission within the neocortex is regulated throughout life by experience and activity. Periods of correlated or uncorrelated presynaptic and postsynaptic activity lead to enduring changes in synaptic efficiency [long-term potentiation (LTP) and long-term depression (LTD), respectively]. The initial plasticity triggering event is thought to be a precipitous rise in postsynaptic intracellular calcium, with higher levels inducing LTP and more moderate levels inducing LTD. We used a pairing protocol in visual cortical brain slices from young guinea pigs with whole-cell recording and calcium imaging to compare the kinetic profiles of calcium signals generated in response to individual pairings along with the cumulative calcium wave and plasticity outcome. The identical pairing protocol applied to layer 2/3 pyramidal neurons results in different plasticity outcomes between cells. These differences are not attributable to variations in the conditioning protocol, cellular properties, inter-animal variability, animal age, differences in spike timing between the synaptic response and spikes, washout of plasticity factors, recruitment of inhibition, or activation of different afferents. The different plasticity outcomes are reliably predicted by individual intracellular calcium transients in the dendrites after the first few pairings. In addition to the differences in the individual calcium transients, the cumulative calcium wave that spreads to the soma also has a different profile for cells that undergo LTP versus LTD. We conclude that there are biological differences between like-type cells in the dendritic calcium signals generated by coincident synaptic input and spiking that determine the sign of the plasticity response after brief associations.

Distribution of nitric oxide synthase immunoreactivity in the mouse brain

Nitric oxide (NO) is a gaseous intercellular messenger with a wide range of neural functions. NO is synthesized by activation of different isoforms of nitric oxide synthases (NOS). At present NOS immunoreactivity has been described in mouse brain in restricted and definite areas and no detailed mapping studies have yet been reported for NOS immunoreactivity. We have studied the distribution of neuronal NOS-containing neurons in the brain of three months male mice, using a specific commercial polyclonal antibody against the neuronal isoform of nitric oxide synthase (nNOS). Neuronal cell bodies exhibiting nNOS immunoreactivity were found in several distinct nuclei throughout the brain. The neurons that were positively stained exhibited different intensities of reaction. In some brain areas (i.e., cortex, striatum, tegmental nuclei) neurons were intensely stained in a Golgi-like fashion. In other regions, immunoreactive cells are moderately stained (i.e., magnocellular nucleus of the posterior commissure, amygdaloid nucleus, interpeduncular nucleus, lateral periaqueductal gray) or weakly stained (i.e., vascular organ of the lamina terminalis, hippocampus, inferior colliculus, reticular nucleus). In the mouse, the NO-producing system appears well developed and widely diffused. In particular, nNOS immunoreactive neurons seem chiefly present in several sensory pathways like all the nuclei of the olfactory system, as well as in many regions of the lymbic system. These data suggest a widespread role for the NO system in the mouse nervous system.

A morphometric study of the progressive changes on NADPH diaphorase activity in the developing rat's barrel field

The distribution of NADPH diaphorase (NADPH-d)/nitric oxide synthase (NOS) neurons was evaluated during the postnatal development of the primary somatosensory cortex (SI) of the rat. Both cell counts and area measurements of barrel fields were carried out throughout cortical maturation. In addition, NADPH-d and cytochrome oxidase (CO) activities were also compared in both coronal and tangential sections of rat SI between postnatal days (P) 10 and 90. Throughout this period, the neuropil distributions of both enzymes presented a remarkable similarity and have not changed noticeably. Their distribution pattern show the PMBSF as a two-compartmented structure, displaying a highly reactive region (barrel hollows) flanked by less reactive regions (barrel septa). The number of NADPH-d neurons increased significantly in the barrel fields between P10 and P23, with peak at P23. The dendritic arborization of NADPH-d neurons became more elaborated during barrel development. In all ages evaluated, the number of NADPH-d cells was always higher in septa than in the barrel hollows. Both high neuropil reactivity and differential distribution of NADPH-d neurons during SI development suggest a role for nitric oxide throughout barrel field maturation.

Acetylcholine-dependent potentiation of temporal frequency representation in the barrel cortex does not depend on response magnitude during conditioning

The response properties of neurons of the postero-medial barrel sub-field of the somatosensory cortex (the cortical structure receiving information from the mystacial vibrissae can be modified as a consequence of peripheral manipulations of the afferent activity. This plasticity depends on the integrity of the cortical cholinergic innervation, which originates at the nucleus basalis magnocellularis (NBM). The activity of the NBM is related to the behavioral state of the animal and the putative cholinergic neurons are activated by specific events, such as reward-related signals, during behavioral learning. Experimental studies on acetylcholine (ACh)-dependent cortical plasticity have shown that ACh is needed for both the induction and the expression of plastic modifications induced by sensory-cholinergic pairings. Here we review and discuss ACh-dependent plasticity and activity-dependent plasticity and ask whether these two mechanisms are linked. To address this question, we analyzed our data and tested whether changes mediated by ACh were activity-dependent. We show that ACh-dependent potentiation of response in the barrel cortex of rats observed after sensory-cholinergic pairing was not correlated to the changes in activity induced during pairing. Since these results suggest that the effect of ACh during pairing is not exerted through a direct control of the post-synaptic activity, we propose that ACh might induce its effect either pre- or post-synaptically through activation of second messenger cascades.

Substrates for coincidence detection and calcium signaling for induction of synaptic potentiation in the neonatal visual cortex

Regulation of the efficacy of synaptic transmission by activity-dependent processes has been implicated in learning and memory as well as in developmental processes. We previously described transient potentiation of excitatory synapses onto layer 2/3 pyramidal neurons in the visual cortex that is induced by coincident presynaptic stimulation and postsynaptic depolarization. In the adult visual cortex, activation of N-methyl-d-aspartate (NMDA) glutamate receptors is necessary to induce this plasticity. These receptors act as coincidence detectors, sensing presynaptic glutamate release and postsynaptic depolarization, and cause an influx of Ca(2+) that is necessary for the potentiation. In the neurons of the neonatal visual cortex, on the other hand, coincident presynaptic stimulation and postsynaptic depolarization induce stable long-term potentiation (LTP). In addition, reduced but significant LTP can be induced in many neurons in the presence of the NMDA receptor (NMDAR) antagonist, 2-amino-5-phosphonovaleric acid despite the Ca(2+) requirement. Therefore there must be an alternative postsynaptic Ca(2+) source and coincidence detection mechanism linked to the LTP induction mechanism in the neonatal cortex operating in addition to NMDARs. In this study, we find that in layer 2/3 pyramidal neurons, release of Ca(2+) from inositol trisphosphate (InsP(3)) receptor-mediated intracellular stores and influx through voltage-gated Ca(2+) channels (VGCCs) provide alternative postsynaptic Ca(2+) sources. We hypothesize that InsP(3)Rs are coincidence detectors, sensing presynaptic glutamate release through linkage with group I metabotropic glutamate receptors (mGluRs), and depolarization, through VGCCs. We also find that the downstream protein kinases, PKA and PKC, have a role in potentiation in layer 2/3 pyramidal neurons of the neonatal visual cortex.

Spatio-temporal plasticity of cortical receptive fields in response to repetitive visual stimulation in the adult cat

Many psychophysical experiments on perceptual learning in humans show increases of performance that are most probably based on functions of early visual cortical areas. Long-term plasticity of the primary visual cortex has so far been shown in vivo with the use of visual stimuli paired with electrical or pharmacological stimulation at the cellular level. Here, we report that plasticity in the adult visual cortex can be achieved by repetitive visual stimulation. First, spatial receptive field profiles of single units (n=38) in area 17 or 18 of the anesthetized cat were determined with optimally oriented flashing light bars. Then a conditioning protocol was applied to induce associative synaptic plasticity. The receptive field center and an unresponsive region just outside the excitatory receptive field were synchronously stimulated ('costimulation', repetition rate 1 Hz; for 10-75 min). After costimulation the receptive field and its adjacent regions were mapped again. We observed specific increases of the receptive field size, changes of the receptive field subfield structure as well as shifts in response latency. In 37% of the cells the receptive field size increased specifically towards the stimulated side but not towards the non-stimulated opposite side of the receptive field. In addition, changes in the relative strength and size of the on and off subfield regions were observed. These specific alterations were dependent on the level of neuronal activity during costimulation. During recovery, the new responses dropped down to 120% of the preconditioning value on average within 103 min; however, the decay times significantly depended on the response magnitude after costimulation. In the temporal domain, the latency of new responses appeared to be strongly influenced by the latency of the response during costimulation.Twenty-nine percent of the units displayed no receptive field enlargement, most likely because the activity during costimulation was significantly lower than in the cases with enlarged receptive fields. An unspecific receptive field enlargement towards both the stimulated and non-stimulated side was observed in 34% of the tested cells. In contrast to the cells with specifically enlarged receptive fields, the unspecific increase of receptive field size was always accompanied by a strong increase of the general activity level. We conclude that the receptive field changes presumably took place by strengthening of synaptic inputs at the recorded cells in a Hebbian way as previously shown in the visual cortex in vitro and in vivo. The observed receptive field changes may be related to preattentive perceptual learning and could represent a basis of the 'filling in' of cortical scotomas obtained with specific training procedures in human patients suffering from visual cortex lesions.

LTD induction in adult visual cortex: role of stimulus timing and inhibition

One Hertz stimulation of afferents for 15 min with constant interstimulus intervals (regular stimulation) can induce long-term depression (LTD) of synaptic strength in the neocortex. However, it is unknown whether natural patterns of low-frequency afferent spike activity induce LTD. Although neurons in the neocortex can fire at overall rates as low as 1 Hz, the intervals between spikes are irregular. This irregular spike activity (and thus, presumably, irregular activation of the synapses of that neuron onto postsynaptic targets) can be approximated by stimulation with Poisson-distributed interstimulus intervals (Poisson stimulation). Therefore, if low-frequency presynaptic spike activity in the intact neocortex is sufficient to induce a generalized LTD of synaptic transmission, then Poisson stimulation, which mimics this spike activity, should induce LTD in slices. We tested this hypothesis by comparing changes in the strength of synapses onto layer 2/3 pyramidal cells induced by regular and Poisson stimulation in slices from adult visual cortex. We find that regular stimulation induces LTD of excitatory synaptic transmission as assessed by field potentials and intracellular postsynaptic potentials (PSPs) with inhibition absent. However, Poisson stimulation does not induce a net LTD of excitatory synaptic transmission. When the PSP contained an inhibitory component, neither Poisson nor regular stimulation induced LTD. We propose that the short bursts of synaptic activity that occur during a Poisson train have potentiating effects that offset the induction of LTD that is favored with regular stimulation. Thus, natural (i.e., irregular) low-frequency activity in the adult neocortex in vivo should not consistently induce LTD.

The effects of forelimb deafferentation on the post-natal development of synaptic plasticity in the hippocampus

The effects of partial deafferentation of the forelimb on the development of long-term potentiation in the hippocampus of rats aged 13-18 days were studied. Long-term potentiation in hippocampus field CA1 was of greater amplitude and duration in control rats at 16-18 days of post-natal ontogenesis than in adult animals. Partial deafferentation by section of the median nerve in the forelimb on the 13th day of life led to the disappearance of this excess at 16-18 days. The peak in synaptic plasticity occurred later in operated animals--on day 17--and was much less marked than in controls. The decreases in the amplitude and duration of long-term potentiation in hippocampal field CA1 in operated animals provides evidence for a decrease in the sensitivity and/or number of NMDA receptors. This suggests that partial deafferentation of one limb may lead not to a decrease but to an increase in spike and synaptic activity in the hippocampus, which in normal conditions may affect the maturation of the plastic properties of synaptic transmission associated with the expression and positions of NMDA receptors. The level of long-term potentiation in sham-operated rats was significantly greater than in controls of the same age. This significant increase in NMDA-dependent long-term potentiation may be explained by a decrease in the level of activation due to anesthesia. It is suggested that the decrease in the spike activity of cells receiving signals from the median nerve may be compensated for by activation of other specific and non-specific inputs.

Cooperative changes in GABA, glutamate and activity levels: the missing link in cortical plasticity

Different intracortical mechanisms have been reported to contribute to the substantial topographic reorganization of the mammalian primary visual cortex in response to matching lesions in the two retinas: an immediate expansion of receptive fields followed by a gradual shift of excitability into the deprived area and finally axonal sprouting of laterally projecting neurons months after the lesion. To gain insight into the molecular mechanisms of this adult plasticity, we used immunocytochemical and bioanalytical methods to measure the glutamate and GABA neurotransmitter levels in the visual cortex of adult cats with binocular central retinal lesions. Two to four weeks after the lesions, glutamate immunoreactivity was decreased in sensory-deprived cortex as confirmed by HPLC analysis of the glutamate concentration. Within three months normal glutamate immunoreactivity was restored. In addition, the edge of the unresponsive cortex was characterized by markedly increased glutamate immunoreactivity 2-12 weeks postlesion. This glutamate immunoreactivity peak moved into the deprived area over time. These glutamate changes corresponded to decreased spontaneous and visually driven activity in unresponsive cortex and to strikingly increased neuronal activity at the border of this cortical zone. Furthermore, the previously reported decrease in glutamic acid decarboxylase immunoreactivity was found to reflect decreased GABA levels in sensory-deprived cortex. Increased glutamate concentrations and neuronal activity, and decreased GABA concentrations, may be related to changes in synaptic efficiency and could represent a mechanism underlying the retinotopic reorganization that occurs well after the immediate receptive field expansion but long before the late axonal sprouting.

Nitric oxide modulates retinal ganglion cell axon arbor remodeling in vivo

Nitric oxide (NO) has been postulated to act as an activity-dependent retrograde signal that can mediate multiple aspects of synaptic plasticity during development. In the visual system, a role for NO in activity-dependent structural modification of presynaptic arbors has been proposed based on NO's ability to prune inappropriate projections and segregate axon terminals. However, evidence demonstrating that altered NO signaling does not perturb ocular dominance map formation leaves unsettled the role of NO during the in vivo refinement of visual connections. To determine whether NO modulates the structural remodeling of individual presynaptic terminal arbors in vivo we have: 1. Used NADPH-diaphorase histochemistry to determine the onset of NO synthase (NOS) expression in the Xenopus visual system. 2. Used in vivo time-lapse imaging to examine the role of NO during retinal ganglion cell (RGC) axon arborization. We show that NOS expression in the target optic tectum is developmentally regulated and localized to neurons that reside in close proximity to arborizing RGC axons. Moreover, we demonstrate that perturbations in tectal NO levels rapidly and significantly alter the dynamic branching of RGC arbors in vivo. Tectal injection of NO donors increased the addition of new branches, but not their stabilization in the long term. Tectal injection of NOS inhibitors increased the dynamic remodeling of axonal arbors by increasing branch addition and elimination and by lengthening pre-existing branches. Thus, these results indicate that altering NO signaling significantly modifies axon branch dynamics in a manner similar to altering neuronal activity levels (Cohen-Cory, 1999). Consequently, our results support a role for NO during the dynamic remodeling of axon arbors in vivo, and suggest that NO functions as an activity-dependent retrograde signal during the refinement of visual connections.

The role of dendritic filtering in associative long-term synaptic plasticity

Several forms of synaptic plasticity in the neocortex and hippocampus depend on the temporal coincidence of presynaptic activity and postsynaptic trains of action potentials (APs). This requirement is consistent with the Hebbian, or correlational, type of cellular learning rule used in many studies of associative synaptic plasticity. Recent experimental evidence suggests that APs initiated in the axosomatic area are actively back-propagated to the dendritic arborization of neocortical and pyramidal cells. High-frequency trains of postsynaptic APs that are used as conditioning stimuli for the induction of Hebbian-like plasticity in both neocortical and hippocampal pyramidal cells display attenuation of the dendritic AP amplitude during the train. This attenuation has been shown to be modulated by neurotransmitters and by electrical activity. We suggest here that both spike train attenuation in the dendrite and its modulation by neurotransmitters and electrical activity may have important functional consequences on the magnitude and/or the sign of the synaptic plasticity induced by a Hebbian pairing procedure.

Arginine analogs modify signal detection by neurons in the visual cortex

Nitric oxide (NO) modulates neurotransmitter release, induction of long-term synaptic potentiation and depression, and activity levels of neurons. However, it is not known whether NO contributes to the ability of the CNS to distinguish sensory signals from background noise and/or extract sensory information with greater reliability. We addressed these questions in the visual cortex, in vivo, using electrophysiological recording and analysis of signal detection from individual neurons. This was combined with microiontophoretic application of arginine analogs that either upregulate or downregulate the brain's endogenous NO-generating pathways or compounds that produce exogenous NO. Protocols that enhance NO levels generally increased the number of action potentials per trial evoked by visual stimuli, improved signal detection, and decreased the coefficient of variation of visually evoked responses, whereas NO-reducing protocols predominantly had complementary effects. Control experiments demonstrate that these effects are likely attributable to the specific ability of these arginine compounds to modify NO levels versus other nonspecific effects. Differential effects between neighboring cells and between single-cell receptive subfields suggest that these actions have a significant direct neural component versus exclusively operating indirectly on neurons through the central vascular actions of NO.

A web-accessible digital atlas of the distribution of nitric oxide synthase in the mouse brain

We have produced a digital atlas of the distribution of nitric oxide synthase (NOS) in the mouse brain as a reference source for our studies on the roles of nitric oxide in brain development and plasticity. NOS was labeled using nicotinamide adenine dinucleotide phosphate diaphorase (NADPHd) histochemistry. In addition, choline acetyltransferase (ChAT) immunocytochemistry was used to identify cholinergic cells because many of the NADPHd positive cells were thought to colocalize acetylcholine. Some sections were also labeled with antibodies to either the neuronal (nNOS) or endothelial (eNOS) isoforms of NOS. Series of sections from 11 C57/BL6 mice were collected and labeled for NADPHd and/or ChAT. We collected two types of data from this material: color digital photographs illustrating the density of cell and fiber labeling, and computer/microscope plots of the locations of all the labeled cells in selected sections. The data can be viewed as either a series of single-section maps produced by combining the plots with the digital images, or as 3-D views derived from the cell plots. The atlas of labeled cell maps, together with selected color photographs and 3-D views, is available for viewing via the World Wide Web (http:@nadph.anatomy.lsumc.edu). Examination of the atlas data has revealed several points about the distribution of NOS throughout the mouse brain. Firstly, different populations of NADPHd-positive neurons can be distinguished by different patterns of staining. In some brain areas neurons are intensely stained by the NADPHd technique where label fills the cell bodies and much of the dendritic trees. In other brain regions labeling is much lighter, is principally confined to the cytoplasm of the cell soma, and extends only a short distance within proximal dendrites. Intense labeling is typical of neurons in the caudate/putamen and mesopontine tegmental nuclei. Most of the labeled neurons in the cortex also stain this way. Lighter, "granular" label is found in many other nuclei, including the medial septum, hippocampus, and cerebellum. In addition to staining pattern, we have also noted that different subpopulations of NOS-neurons can be distinguished on the basis of colocalization with ChAT. Substantial overlap of the distributions of these two substances was observed although very little colocalization was found in most cholinergic cell groups except the mesopontine tegmental nuclei. Other points of interest arising from this project include the apparent lack of NADPHd labeling in the CA1 pyramidal cells of the hippocampus or the Purkinje neurons in the cerebellum. This observation is especially relevant given that synaptic plasticity in these regions is reported to be nitric-oxide dependent.

Dynamic modulation of cerebral cortex synaptic function by nitric oxide

Our experiments demonstrate that NO exerts several actions in the cerebral cortex (see Fig. 4). Its production is mediated by neuronal activity through at least two pathways, NMDA receptors and AMPA receptors. By virtue of its diffusion in extracellular space, NO can interact with synapses that are near the production site but not necessarily anatomically connected to the NO source by a conventional synaptic linkage. NO's primary action is amplification of the release of the excitatory neurotransmitter, L-glutamate, thus effectively creating a positive feed-forward gain system. However, a number of effective brakes, presumably activated under physiological conditions, serve to limit the cascade. These include NO's ability to inhibit NMDA receptors, its negative feedback on the rate limiting enzyme, NOS (Rengasamy and Johns, 1993; Park et al., 1994; Ravichandran et al., 1995) and other inhibitory actions (Figs. 3H and L). Under conditions of extremely strong activation or curtailment of the inhibitory feedback mechanisms, as might occur with a change in the local redox milieu (see Lipton, this volume), the amplification cascade may proceed unchecked leading to neurotoxicity (see Dawson, this volume). NO's ability to modulate synaptic function is indicated by both its positive and negative modulatory role in a form of activity-dependent synaptic plasticity, covariance-induced synaptic potentiation. These opposing effects may be due to NO's ability to amplify glutamate release and inhibit NMDA receptors, respectively. The actions of endogenous NO in vivo are primarily facilitatory in visual cortex (Fig. 4). However, inhibitory actions also occur in vivo. The targets for NO in vivo, are potentially more diverse including the neurotransmitter release process, NMDA receptors, other receptors and ion channels and the cerebral vasculature. However, regardless of the signaling pathways, the net result of endogenous NO production in the intact visual cortex is a potent modulation of cells' responses to visual stimulation. Thus, it is likely that this signal plays an important role in ongoing information processing in the mature cerebral cortex, dynamically altering the effective strength of cortical networks.

Establishment of patterned thalamocortical connections does not require nitric oxide synthase

Subplate neurons are early-generated neurons that project into the overlying neocortex and are required for the formation of ocular dominance columns. A subset of subplate neurons express nitric oxide synthase (NOS) and produce nitric oxide (NO), a neuronal messenger thought to be involved in adult hippocampal synaptic plasticity and also in the establishment of certain specific connections during visual system development. Here, we examine whether the NOS-containing subplate neurons are involved in ocular dominance column formation in the ferret visual system. Ocular dominance columns form in ferrets between postnatal day 35 (P35) and P60. NOS expression in the visual subplate is low at birth, increases to a maximum at the onset of ocular dominance column formation, and falls thereafter. Nevertheless, blockade of NOS with daily injections of nitroarginine from P14 to P56 fails to prevent the formation of ocular dominance columns, although NOS activity is reduced by >98%. To test further a requirement for NOS in the patterning of connections during CNS development, we examined the cortical barrels in the somatosensory system of mice carrying targeted disruptions of NOS that also received injections of nitroarginine; cortical barrels formed normally in these animals. In addition, barrel field plasticity induced by whisker ablation at birth was normal in nitroarginine-injected NOS knock-out mice. Thus, despite the dynamic regulation of NOS in subplate neurons, NO is unlikely to be essential for the patterning of thalamocortical connections either in visual or somatosensory systems.

Endogenous NMDA-receptor activation regulates glutamate release in cultured spinal neurons

N-methyl--aspartate (NMDA) receptor activation plays a fundamental role in the genesis of electrical activity of immature neurons and may participate in activity-dependent aspects of CNS development. A recent study has suggested that NMDA-receptor-mediated glutamatergic neurotransmission might occur in the developing spinal cord via activation of nonsynaptic receptors, but the details of NMDA-receptor activation in the developing CNS are not yet well understood. We describe here a model of cultured spinal neurons that display ongoing alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor activity characterized by spontaneous excitatory postsynaptic currents (EPSCs), with NMDA-receptor activity detectable only as single channel events. -2-amino-5-phosphonovaleric acid (100 microM) and tetrodotoxin (TTX) 100 nM each reduced the occurrence of spontaneous AMPA EPSCs; quantal analysis showed a decrease in the number of released quanta but no changes in quantal size, indicating that NMDA-receptor activation and Na+ channel activity affect the generation of spontaneous AMPA EPSCs, at least in part, via mechanisms that impinge on the presynaptic terminal. Once the Mg2+-block was released, activity of NMDA receptors dramatically increased the release of quantal and multiquantal amounts of glutamate, indicating that the NMDA receptors are physiologically coupled to glutamate release. In Mg2+-free solution, TTX application elicited an increase in the number of quantal AMPA EPSCs and a reduction in the number of multiquantal EPSCs, consistent with an effect of NMDA-receptor activation on presynaptic terminals. Our results suggest that endogenous activity at a small number of NMDA receptors can regulate the release of neurotransmitters at developing AMPA synapses.

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.