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SummaryBackground: Excitatory synapses in the CNS are highly dynamic structures that can show activity-dependent remodeling and stabilization in response to learning and memory. Synapses are enveloped with intricate processes of astrocytes known as perisynaptic astrocytic processes (PAPs). PAPs are motile structures displaying rapid actin-dependent movements and are characterized by Ca 2+ elevations in response to neuronal activity. Despite a debated implication in synaptic plasticity, the role of both Ca 2+ events in astrocytes and PAP morphological dynamics remain unclear. Results: In the hippocampus, we found that PAPs show extensive structural plasticity that is regulated by synaptic activity through astrocytic metabotropic glutamate receptors and intracellular calcium signaling. Synaptic activation that induces long-term potentiation caused a transient PAP motility increase leading to an enhanced astrocytic coverage of the synapse. Selective activation of calcium signals in individual PAPs using exogenous metabotropic receptor expression and two-photon uncaging reproduced these effects and enhanced spine stability. In vivo imaging in the somatosensory cortex of adult mice revealed that increased neuronal activity through whisker stimulation similarly elevates PAP movement. This in vivo PAP motility correlated with spine coverage and was predictive of spine stability. Conclusions: This study identifies a novel bidirectional interaction between synapses and astrocytes, in which synaptic activity and synaptic potentiation regulate PAP structural plasticity, which in turn determines the fate of the synapse. This mechanism may represent an important contribution of astrocytes to learning and memory processes.

The ligand 2,6-bis( 1 '-ethyl-5'-methylbenzimidazol-2'-yl)pyridine ( L5) reacts with lanthanide perchlorate in acetonitrile to give the mononuclear triple-helical complexes [Ln( L5),I3+ (Ln = Eu, Gd or Tb). The crystal structure of [Eu( L5),] [ClO4],*4MeCN has been determined, which shows three unco-ordinated perchlorate anions and an [Eu( L5),I3+ cation where the three tridentate ligands are wrapped around a pseudo-C, axis. The co-ordination sphere around Eu"' may be best described as a slightly distorted trigonal-tricapped prism where the six benzimidazole nitrogen atoms occupy the vertices of the prism and the three pyridine nitrogen atoms occupy the capping positions. A detailed geometrical analysis showed that the ethyl groups in L5 produce a slide of the strands which is responsible for the distortion of the triple-helical structure as exemplified by the low symmetry for the Eu"' site in the luminescence spectra of [ Eu( L5),I3+. Proton N M R spectra in acetonitrile indicate that the triple-helical structure is maintained for [Ln(Li),I3+ {Ln = Eu or Tb; L = 2,6-bis(l '-R-benzimidazol-2'-yl)pyridine [R = Me L'. Et Lz. Pr L3 or CH,C,H,(OMe),-3,5 L4] or L5} on the N M R time-scale, but the stability of the complexes together with the structural arrangement of the ligands depend on the size of the substituents bound to the benzimidazole nitrogen atoms. Photophysical studies of [ Eu( Li),I3+ show that these steric effects affect the quantum yield in solution and that methyl groups bound to the 5 positions of the benzimidazole rings in L5 shift the n d n * transitions centred on the ligand, but do not strongly modify the emission properties of [Eu( L5),I3+. Extended Huckel calculations give a qualitative insight into the factor controlling the n -n* transitions of the ligands and complexes.The development of stable luminescent lanthanide complexes is a subject of increasing interest mainly due to their potential uses as fluorescent sensors in natural,' medical,2 analytical and bioinorganic sciences, and probes based on Eu"' and Tb"' are of special interest because of the particularly suitable spectroscopic properties of these ions. To obtain strongly luminescent complexes, the lanthanide metal ion should be bound to chromophoric ligands which are able (i) to absorb energy and then transfer it efficiently to the cation and (ii) to protect the lanthanide ion from external interactions which usually quench the luminescen~e.~,~ Macrocyclic, macrobicyclic ' or podand-type ligands containing heterocyclic aromatic donor groups have been extensively used for this purpose, but it has been shown recently that much simpler linear oligo-multidentate ligands, such as L' or L8, selfassemble with Ln"' to produce stable mono-" and di-nuclear " triple-stranded helicates which possess well defined and protected metallic sites. In these molecular light-conversion devices the Ln"' ions are co-ordinated by nine nitrogen atoms in a pseudo-tricapped trigonal-prismatic arrangement leading to a pseudo-D, symmetry for the cations [Eu(L1),...

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The possibility of wavelength-selective cleavage of seven photolabile caging groups from different families has been studied. Amine-, thiol-, and carboxylic-terminated organosilanes were caged with o-nitrobenzyl (NVOC, NPPOC), benzoin (BNZ), (coumarin-4-yl)methyl (DEACM), 7-nitroindoline (DNI, BNI), and p-hydroxyphenacyl (pHP) derivatives. Caged surfaces modified with the different chromophores were prepared, and their photosensitivity at selected wavelengths was quantified. Different pairs, trios, and quartets of chromophore combinations with wavelength-selective photoresponse were identified. Our results show, for the first time, the possibility of generating surfaces with up to four different and independently addressable functional levels. In addition, this manuscript presents the first systematic comparison of the photolytic properties of different photolabile groups under different irradiation conditions.