Angela Neitz

More than a retrograde messenger: nitric oxide needs two cGMP pathways to induce hippocampal long-term potentiation.

Although nitric oxide (NO) has been implicated as a messenger molecule in hippocampal long-term potentiation (LTP) for almost 20 years, its precise function has not been elucidated because presynaptic and/or postsynaptic actions of NO have been reported. Most of the effects of NO as a signaling molecule are mediated by the NO receptor guanylyl cyclases (NO-GCs), two heme-containing enzymes with pronounced homology in which cGMP-forming activity is stimulated on NO binding. Here we report on knock-out (KO) mice in which either one of the NO-GC receptors has been genetically deleted. By measuring NO-induced cGMP levels, similar quantities of both NO-GC receptors were determined in the hippocampus. Surprisingly, hippocampal LTP was abolished in either one of the KO strains, demonstrating that both NO-GC receptors are required in the course of LTP. Expression of LTP was restored with a cGMP analog in one of the KO strains but did not recover in the other one. Moreover, single-cell recordings of paired pulse facilitation revealed a presynaptic role of one of the NO-GC isoforms in neurotransmitter release, confirming different roles of the NO-GC receptors in LTP. Because neither one of the NO/cGMP-induced responses by itself is sufficient for LTP, two divergent, possibly presynaptically and postsynaptically localized NO-stimulated cGMP pathways are apparently required for the expression of LTP. The unexpected role of cGMP at two sites of the synaptic cleft explains many of the controversial results in former NO research in LTP and demonstrates the necessity of presynaptic and postsynaptic changes for LTP expression.

Oligodendrocyte Precursor Cells Modulate the Neuronal Network by Activity-Dependent Ectodomain Cleavage of Glial NG2

This study shows that the activity of neurons can trigger shedding of a protein, NG2, from the surface of oligodendrocyte precursor cells; this protein in turn modulates synaptic transmission, revealing a two-way conversation between neurons and glia.

Presynaptic nitric oxide/cGMP facilitates glutamate release via hyperpolarization‐activated cyclic nucleotide‐gated channels in the hippocampus

In hippocampal neurons, synaptic transmission is affected by a variety of modulators, including nitric oxide (NO), which was proposed as a retrograde messenger as long as two decades ago. NO signals via two NO‐sensitive guanylyl cyclases (NO‐GCs) (NO‐GC1 and NO‐GC2) and the subsequent increase in cGMP. Lack of long‐term potentiation in mice deficient in either one of the two NO‐GCs demonstrates the involvement of both NO‐GCs in synaptic transmission. However, the physiological consequences of NO/cGMP and the cellular mechanisms involved are unknown. Here, we analyzed glutamatergic synaptic transmission, most likely reflecting glutamate release, in the hippocampal CA1 region of NO‐GC knockout mice by single‐cell recording, and found glutamate release to be reduced under basal and stimulated conditions in the NO‐GC1 knockout mice, but restorable to wild‐type‐like levels with a cGMP analog. Conversely, an inhibitor of NO/cGMP signaling, ODQ, reduced glutamate release in wild‐type mice to knockout‐like levels; thus, we conclude that presynaptic cGMP formed by NO‐GC1 facilitates glutamate release. In this pathway, NO is supplied by endothelial NO synthase. In search of a cGMP target, we found that two mechanistically distinct blockers of hyperpolarization‐activated cyclic nucleotide‐gated (HCN) channels (ZD7288 and DK‐AH269) abolished the cGMP‐induced increase in glutamate release, suggesting that cGMP either directly or indirectly signals via HCN channels. In summary, we unravel a presynaptic role of NO/cGMP most likely in glutamate release and propose that HCN channels act as effectors for cGMP.

Focal laser-lesions activate an endogenous population of neural stem/progenitor cells in the adult visual cortex

CNS lesions stimulate adult neurogenic niches. Endogenous neural stem/progenitor cells represent a potential resource for CNS regeneration. Here, we investigate the response to unilateral focal laser-lesions applied to the visual cortex of juvenile rats. Within 3 days post-lesion, an ipsilateral increase of actively cycling cells was observed in cortical layer one and in the callosal white matter within the lesion penumbra. The cells expressed the neural stem/progenitor cell marker Nestin and the 473HD-epitope. Tissue prepared from the lesion area by micro-dissection generated self-renewing, multipotent neurospheres, while cells from the contralateral visual cortex did not. The newly formed neural stem/progenitor cells in the lesion zone might support neurogenesis, as suggested by the expression of Pax6 and Doublecortin, a marker of newborn neurons. We propose that focal laser-lesions may induce the emergence of stem/progenitor cells with neurogenic potential. This could underlie the beneficial effects of laser application in neurosurgery.

Postsynaptic NO/cGMP Increases NMDA Receptor Currents via Hyperpolarization-Activated Cyclic Nucleotide-Gated Channels in the Hippocampus

The nitric oxide (NO)/cyclic guanosine monophosphate (cGMP) signaling cascade participates in the modulation of synaptic transmission. The effects of NO are mediated by the NO-sensitive cGMP-forming guanylyl cyclases (NO-GCs), which exist in 2 isoforms with indistinguishable regulatory properties. The lack of long-term potentiation (LTP) in knock-out (KO) mice deficient in either one of the NO-GC isoforms indicates the contribution of both NO-GCs to LTP. Recently, we showed that the NO-GC1 isoform is located presynaptically in glutamatergic neurons and increases the glutamate release via hyperpolarization-activated cyclic nucleotide (HCN)-gated channels in the hippocampus. Electrophysiological analysis of hippocampal CA1 neurons in whole-cell recordings revealed a reduction of HCN currents and a hyperpolarizing shift of the activation curve in the NO-GC2 KOs associated with reduced resting membrane potentials. These features were mimicked in wild-type (WT) neurons with an NO-GC inhibitor. Analysis of glutamate receptors revealed a cGMP-dependent reduction of NMDA receptor currents in the NO-GC2 KO mice, which was mimicked in WT by HCN channel inhibition. Lowering extracellular Mg(2+) increased NMDA receptor currents in the NO-GC2 KO and allowed the induction of LTP that was absent at physiological Mg(2+). In sum, our data indicate that postsynaptic cGMP increases the N-methyl-D-aspartate (NMDA) receptor current by gating HCN channels and thereby is required for LTP.

NO regulates the strength of synaptic inputs onto hippocampal CA1 neurons via NO-GC1/cGMP signalling

GABAergic interneurons are the predominant source of inhibition in the brain that coordinate the level of excitation and synchronization in neuronal circuitries. However, the underlying cellular mechanisms are still not fully understood. Here we report nitric oxide (NO)/NO-GC1 signalling as an important regulatory mechanism of GABAergic and glutamatergic synaptic transmission in the hippocampal CA1 region. Deletion of the NO receptor NO-GC1 induced functional alterations, indicated by a strong reduction of spontaneous and evoked inhibitory postsynaptic currents (IPSCs), which could be compensated by application of the missing second messenger cGMP. Moreover, we found a general impairment in the strength of inhibitory and excitatory synaptic inputs onto CA1 pyramidal neurons deriving from NO-GC1KO mice. Finally, we disclosed one subpopulation of GABAergic interneurons, fast-spiking interneurons, that receive less excitatory synaptic input and consequently respond with less spike output after blockage of the NO/cGMP signalling pathway. On the basis of these and previous findings, we propose NO-GC1 as the major NO receptor which transduces the NO signal into cGMP at presynaptic terminals of different neuronal subtypes in the hippocampal CA1 region. Furthermore, we suggest NO-GC1-mediated cGMP signalling as a mechanism which regulates the strength of synaptic transmission, hence being important in gating information processing between hippocampal CA3 and CA1 region.

Focal Cortical Lesions Induce Bidirectional Changes in the Excitability of Fast Spiking and Non Fast Spiking Cortical Interneurons

A physiological brain function requires neuronal networks to operate within a well-defined range of activity. Indeed, alterations in neuronal excitability have been associated with several pathological conditions, ranging from epilepsy to neuropsychiatric disorders. Changes in inhibitory transmission are known to play a key role in the development of hyperexcitability. However it is largely unknown whether specific interneuronal subpopulations contribute differentially to such pathological condition. In the present study we investigated functional alterations of inhibitory interneurons embedded in a hyperexcitable cortical circuit at the border of chronically induced focal lesions in mouse visual cortex. Interestingly, we found opposite alterations in the excitability of non fast-spiking (Non Fs) and fast-spiking (Fs) interneurons in acute cortical slices from injured animals. Non Fs interneurons displayed a depolarized membrane potential and a higher frequency of spontaneous excitatory postsynaptic currents (sEPSCs). In contrast, Fs interneurons showed a reduced sEPSCs amplitude. The observed downscaling of excitatory synapses targeting Fs interneurons may prevent the recruitment of this specific population of interneurons to the hyperexcitable network. This mechanism is likely to seriously affect neuronal network function and to exacerbate hyperexcitability but it may be important to protect this particular vulnerable population of GABAegic neurons from excitotoxicity.

Physiological Properties of Supragranular Cortical Inhibitory Interneurons Expressing Retrograde Persistent Firing

Neurons are polarized functional units. The somatodendritic compartment receives and integrates synaptic inputs while the axon relays relevant synaptic information in form of action potentials (APs) across long distance. Despite this well accepted notion, recent research has shown that, under certain circumstances, the axon can also generate APs independent of synaptic inputs at axonal sites distal from the soma. These ectopic APs travel both toward synaptic terminals and antidromically toward the soma. This unusual form of neuronal communication seems to preferentially occur in cortical inhibitory interneurons following a period of intense neuronal activity and might have profound implications for neuronal information processing. Here we show that trains of ectopically generated APs can be induced in a large portion of neocortical layer 2/3 GABAergic interneurons following a somatic depolarization inducing hundreds of APs. Sparsely occurring ectopic spikes were also observed in a large portion of layer 1 interneurons even in absence of prior somatic depolarization. Remarkably, we found that interneurons which produce ectopic APs display specific membrane and morphological properties significantly different from the remaining GABAergic cells and may therefore represent a functionally unique interneuronal subpopulation.

NO signalling in synaptic transmission

Background The NO/cGMP signaling cascade has been proposed to play a role in long-term potentiation (LTP) and the modulation of synaptic transmission. Nitric oxide is formed enzymatically by NO synthases (NOS); two NOS, the endothelial and neuronal isoform (eNOS, nNOS) produce NO as a signalling molecule. Functionally, NO has been reported to act as a retrograde messenger that is generated postsynaptically to increase the neurotransmitter release presynaptically. The NO effects are mediated by the NO-sensitive guanylyl cyclases (NO-GC), which by the formation of cGMP transduce the NO signal. Two functionally indistinguishable isoforms of the NO-GCs, NO-GC1 and NO-GC2 exist.

PDE2, a component of the NO/cGMP signalling in the hippocampus

Background NO/cGMP-mediated signal transduction is involved in synaptic plasticity in various brain regions. NO effects are transduced by the NO receptor guanylyl cyclase (NO-GC) that exists in two isoforms, NO-GC1 and NO-GC2, with indistinguishable regulatory properties. Mice deficient in either NO-GC1 or NO-GC2 revealed that both NO-GC isoforms are required for LTP indicating the existence of two separated NO/cGMP pathways. Recently, we demonstrated a presynaptic role of NO/cGMP in facilitation of glutamate release and indentified eNOS and NO-GC1 as the participating enzymes. Yet, the involved cGMP-hydrolysing phosphodiesterases (PDE) remained unknown.