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

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'.

Linking Network Activity to Synaptic Plasticity during Sleep: Hypotheses and Recent Data

Research findings over the past two decades have supported a link between sleep states and synaptic plasticity. Numerous mechanistic hypotheses have been put forth to explain this relationship. For example, multiple studies have shown structural alterations to synapses (including changes in synaptic volume, spine density, and receptor composition) indicative of synaptic weakening after a period of sleep. Direct measures of neuronal activity and synaptic strength support the idea that a period of sleep can reduce synaptic strength. This has led to the synaptic homeostasis hypothesis (SHY), which asserts that during slow wave sleep, synapses are downscaled throughout the brain to counteract net strengthening of network synapses during waking experience (e.g., during learning). However, neither the cellular mechanisms mediating these synaptic changes, nor the sleep-dependent activity changes driving those cellular events are well-defined. Here we discuss potential cellular and network dynamic mechanisms which could underlie reductions in synaptic strength during sleep. We also discuss recent findings demonstrating circuit-specific synaptic strengthening (rather than weakening) during sleep. Based on these data, we explore the hypothetical role of sleep-associated network activity patterns in driving synaptic strengthening. We propose an alternative to SHY—namely that depending on experience during prior wake, a variety of plasticity mechanisms may operate in the brain during sleep. We conclude that either synaptic strengthening or synaptic weakening can occur across sleep, depending on changes to specific neural circuits (such as gene expression and protein translation) induced by experiences in wake. Clarifying the mechanisms underlying these different forms of sleep-dependent plasticity will significantly advance our understanding of how sleep benefits various cognitive functions.

Plasticity of horizontal connections at a functional border in adult rat somatosensory cortex

Horizontal connections in superficial cortical layers integrate information across sensory maps by connecting related functional columns. It has been hypothesized that these connections mediate cortical reorganization via synaptic plasticity. However, it is not known if the horizontal connections from discontinuous cortical regions can undergo plasticity in the adult. Here we located the border between two discontinuous cortical representations in vivo and used either pairing or low-frequency stimulation to induce synaptic plasticity in the horizontal connections surrounding this border in vitro. Individual neurons revealed significant and diverse forms of synaptic plasticity for horizontal connections within a continuous representation and discontinuous representations. Interestingly, both enhancement and depression were observed following both plasticity paradigms. Furthermore, plasticity was not restricted by the border's presence. Depolarization in the absence of synaptic stimulation also produced synaptic plasticity, but with different characteristics. These experiments suggest that plasticity of horizontal connections may mediate functional reorganization.

Postnatal development and age-related changes in the distribution of nitric oxide synthase-immunoreactive neurons in the visual system of rats

We have determined postnatal development and age-related changes in the distribution of neuronal nitric oxide synthase (nNOS) in the visual system including superior colliculus (SC), dorsal nucleus of lateral geniculate body (dLGB) and visual cortex of rats. In SC, the number and intensity of immunoreactive cells were increased until postnatal day 21 (P21), and were comparably decreased during the following days. Both neurons and neuropil were intensely stained in dLGB at P21, and the staining intensity was decreased in aged rats. The number of nNOS-immunoreactive cells was increased from P7 to P28 and was decreased in aged rats. The first demonstration of different staining patterns in SC, dLGB and visual cortex during the postnatal days and aging suggests that NO may mediate both postnatal refinement and age-associated visual deficits in some brain regions.

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.

Nitric oxide, impulse activity, and neurotrophins in visual system development(1)

Topographic refinement of synaptic connections within the developing visual system involves a variety of molecules which interact with impulse activity in order to produce the precise retinotopic maps found in the adult brain. Nitric oxide (NO) has been implicated in this process, as have various growth factors. Within the subcortical visual system, we have recently shown that nitric oxide contributes to pathway refinement in the superior colliculus (SC). Long-term potentiation (LTP) and long-term depression (LTD) are also expressed in SC during the time that this pathway undergoes refinement. The role of NO has been demonstrated by showing that refinement of ipsilateral fibers in the retinocollicular pathway is significantly delayed in gene knockout mice in which both the endothelial and neuronal isoforms of nitric oxide synthase (NOS) have been disrupted. The effect also depends upon Ca(2+) channels because refinement of both the ipsilateral retinocollicular and retinogeniculate pathways is disrupted in genetic mutants in which the beta3 subunit of the Ca(2+) channel has been deleted. LTD may also be involved in this process, because the time course of its expression correlates with that of pathway refinement and LTD magnitude is depressed by nitrendipine, an L-type Ca(2+) channel blocker. LTP is also expressed during early postnatal development in the LGN and SC and may contribute to synaptic stabilization. The role of neurotrophins in pathway refinement in the visual system is also reviewed.

Distinct temporal profiles of activity-dependent calcium increase in pyramidal neurons of the rat visual cortex

1. Using fluo-3-based fluorometry, we studied variation in depolarization-induced calcium increases in the proximal dendrites or soma of pyramidal neurons in layer II/III of the rat visual cortex. 2. Depolarization for all durations tested (0.1-2 s; 0.5 nA) evoked a train of action potentials and a small increase in calcium signal (mean 26 %) which peaked within 1 s of the onset of depolarization. With depolarization for longer than 1 s, this small increase was often followed by a larger increase (73 %). This later phase of calcium increase occurred without sudden changes in action potential firing. 3. Application of ryanodine, which suppresses intracellular calcium release, abolished the second phase without affecting the early phase in a use-dependent manner. Meanwhile, no major changes were observed in the pattern of action potential firing. 4. In calcium-free medium, both the early and late phases were almost undetectable, although action potential firing was still evoked by injection of depolarizing currents. Since the late phase depended on intracellular calcium release, this effect of calcium-free medium on the late phase is likely to be indirect through an influence on the early phase. 5. This two-phase profile was observed with somatic depolarization or with antidromic action potentials induced by tetanization. Neocortical pyramidal neurons can thus recruit calcium from different sources, even without chemical sensitization, generating temporally diverse profiles of intracellular calcium signal in response to action potential firing. 6. Such variety in the mechanisms of calcium increase may be relevant to the role of calcium as a versatile second messenger for various types of synaptic plasticity.

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.

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.