Cntnap4 differentially contributes to GABAergic and dopaminergic synaptic transmission.

Although considerable evidence suggests that the chemical synapse is a lynchpin underlying affective disorders, how molecular insults differentially affect specific synaptic connections remains poorly understood. For instance, Neurexin 1a and 2 (NRXN1 and NRXN2) and CNTNAP2 (also known as CASPR2), all members of the neurexin superfamily of transmembrane molecules, have been implicated in neuropsychiatric disorders. However, their loss leads to deficits that have been best characterized with regard to their effect on excitatory cells. Notably, other disease-associated genes such as BDNF and ERBB4 implicate specific interneuron synapses in psychiatric disorders. Consistent with this, cortical interneuron dysfunction has been linked to epilepsy, schizophrenia and autism. Using a microarray screen that focused upon synapse-associated molecules, we identified Cntnap4 (contactin associated protein-like 4, also known as Caspr4) as highly enriched in developing murine interneurons. In this study we show that Cntnap4 is localized presynaptically and its loss leads to a reduction in the output of cortical parvalbumin (PV)-positive GABAergic (γ-aminobutyric acid producing) basket cells. Paradoxically, the loss of Cntnap4 augments midbrain dopaminergic release in the nucleus accumbens. In Cntnap4 mutant mice, synaptic defects in these disease-relevant neuronal populations are mirrored by sensory-motor gating and grooming endophenotypes; these symptoms could be pharmacologically reversed, providing promise for therapeutic intervention in psychiatric disorders.

Spiers Memorial Lecture. Breeding and building molecular spies.

To circumvent the limited spatial resolution of fluorescent protein imaging, we are developing genetically encoded tags for electron microscopy (EM).

Cytochrome c is released in a single step during apoptosis.

Release of cytochrome c from mitochondria is a central event in apoptotic signaling. In this study, we utilized a cytochrome c fusion that binds fluorescent biarsenical ligands (cytochrome c-4CYS (cyt. c-4CYS)) as well as cytochrome c-green fluorescent protein (cyt. c-GFP) to measure its release from mitochondria in different cell types during apoptosis. In single cells, the kinetics of cyt. c-4CYS release was indistinguishable from that of cyt. c-GFP in apoptotic cells expressing both molecules. Lowering the temperature by 7 degrees C did not affect this corelease, but further separated cytochrome c release from the subsequent decrease in mitochondrial membrane potential (DeltaPsi(m)). Cyt. c-GFP rescued respiration in cells lacking endogenous cytochrome c, and the duration of cytochrome c release was approximately 5 min in a variety of cell types induced to die by various forms of cellular stress. In addition, we could observe no evidence of caspase-dependent amplification of cytochrome c release or changes in DeltaPsi(m) preceding the release of cyt. c-GFP. We conclude that there is a general mechanism responsible for cytochrome c release that proceeds in a single step that is independent of changes in DeltaPsi(m).

Differential expression of alpha 1 and beta subunits of voltage dependent Ca2+ channel at the neuromuscular junction of normal and P/Q Ca2+ channel knockout mouse.

Voltage-dependent calcium channels (VDCC) have a key role in neuronal function transforming the voltage signals into intracellular calcium signals. They are composed of the pore-forming alpha(1) and the regulatory alpha(2)delta, gamma and beta subunits. Molecular and functional studies have revealed which alpha(1) subunit gene product is the molecular constituent of each class of native calcium channel (L, N, P/Q, R and T type). Electrophysiological and immunocytochemical studies have suggested that at adult mouse motor nerve terminal (MNT) only P/Q type channels, formed by alpha(1A) subunit, mediate evoked transmitter release. The generation of alpha(1A)-null mutant mice offers an opportunity to study the expression and localization of calcium channels at a synapse with complete loss of P/Q calcium channel. We have investigated the expression and localization of VDCCs alpha(1) and beta subunits at the wild type (WT) and knockout (KO) mouse neuromuscular junction (NMJ) using fluorescence immunocytochemistry. The alpha(1A) subunit was observed only at WT NMJ and was absent at denervated muscles and at KO NMJ. The subunits alpha(1B), alpha(1D) and alpha(1E) were also present at WT NMJ and they were over- expressed at KO NMJ suggesting a compensatory expression due to the lack of the alpha(1A). On the other hand, the beta(1b), beta(2a) and beta(4) were present at the same levels in both genotypes. The presence of other types of VDCC at WT NMJ indicate that they may play other roles in the signaling process which have not been elucidated and also shows that other types of VDCC are able to substitute the alpha(1A) subunit, P/Q channel under certain pathological conditions.

Imaging single synaptic vesicles undergoing repeated fusion events: kissing, running, and kissing again.

At synapses of the mammalian central nervous system, release of neurotransmitter occurs at rates transiently as high as 100 Hz, putting extreme demands on nerve terminals with only tens of functional vesicles at their disposal. Thus, the presynaptic vesicle cycle is particularly critical to maintain neurotransmission. To understand vesicle cycling at the most fundamental level, we studied single vesicles undergoing exo/endocytosis and tracked the fate of newly retrieved vesicles. This was accomplished by minimally stimulating boutons in the presence of the membrane-fluorescent styryl dye FM1-43, then selecting for terminals that contained only one dye-filled vesicle. We then observed the kinetics of dye release during single action potential stimulation. We found that most vesicles lost only a portion of their total dye during a single fusion event, but were able to fuse again soon thereafter. We interpret this as direct evidence of "kiss-and-run" followed by rapid reuse. Other interpretations such as "partial loading" and "endosomal splitting" were largely excluded on the basis of multiple lines of evidence. Our data placed an upper bound of <1.4 s on the lifetime of the kiss-and-run fusion event, based on the assumption that aqueous departitioning is rate limiting. The repeated use of individual vesicles held over a range of stimulus frequencies up to 30 Hz and was associated with neurotransmitter release. A small percentage of fusion events did release a whole vesicle's worth of dye in one action potential, consistent with a classical picture of exocytosis as fusion followed by complete collapse or at least very slow retrieval.

Single synaptic vesicles fusing transiently and successively without loss of identity.

Vesicle fusion and recycling are particularly critical for ongoing neurotransmitter release in the small nerve terminals of the brain, which typically contain about 30 functional vesicles. However, the modes of exocytosis and endocytosis that operate at synapses of the central nervous system are incompletely understood. Here we show real-time visualization of a single vesicle fusing at a small synapse of the central nervous system, made possible by highly intensified charge-coupled device imaging of hippocampal synaptic terminals, in which a single vesicle was labelled with the fluorescent membrane marker FM1-43 (ref. 6). In a small number of cases, full loss of fluorescent membrane dye was elicited by a single action potential, consistent with classical complete collapse. In most cases, however, action potentials triggered only partial loss of fluorescence, suggesting vesicular retention of membrane marker, consistent with 'kiss-and-run' vesicle cycling. An alternative hypothesis of independent fusion of partially stained vesicles arising from endosomal splitting could be excluded by observations on the size and timing of successive fusion events. Thus, our experimental evidence supports a predominance of kiss-and-run fusion events and rapid vesicular re-use.

Calmodulin priming: nuclear translocation of a calmodulin complex and the memory of prior neuronal activity.

The neuronal nucleus plays a vital role in information processing, but whether it supports computational functions such as paired-pulse facilitation, comparable to synapses, is unclear. Ca(2+)-dependent movement of calmodulin (CaM) to the nucleus is highly responsive to Ca(2+) entry through L-type channels and promotes activation of the transcription factor CREB (cAMP-responsive element binding protein) through phosphorylation by CaM-sensitive kinases. We characterized key features of this CaM translocation and its possible role in facilitation of nuclear signaling. Nuclear CaM was elevated within 15 s of stimulus onset, preceding the first signs of CREB phosphorylation in hippocampal pyramidal neurons. Depolarization-induced elevation of nuclear CaM also was observed in cerebellar granule cells, neocortical neurons, and dentate gyrus granule cells. Nuclear translocation of CaM was not blocked by disruption of actin filaments or microtubules, or by emptying endoplasmic reticulum Ca(2+) stores with thapsigargin. Translocation of fluorescently tagged CaM was prevented by fusing it with the Ca(2+)/CaM binding peptide M13, suggesting that nuclear CaM accumulation depends on association with endogenous Ca(2+)/CaM binding proteins. To determine whether increased nuclear [CaM] might influence subsequent nuclear signal processing, we compared responses to two consecutive depolarizing stimuli. After a weak "priming" stimulus that caused CaM translocation, CREB phosphorylation caused by a subsequent stimulus was significantly faster, more sensitive to Ca(2+) elevation, and less specifically dependent on Ca(2+) influx through L-type channels. CaM translocation not only supports rapid signaling to the nucleus, but also could provide a "memory" for facilitatory effects of repeated neural activity, seen in altered phosphorylated CREB dynamics and Ca(2+) channel dependence.

Genetically encoded reporters of protein kinase A activity reveal impact of substrate tethering.

The complexity and specificity of many forms of signal transduction are widely suspected to require spatial microcompartmentation of protein kinase and phosphatase activities, yet current relevant imaging methods such as phosphorylation-specific antibodies or fluorescent peptide substrates require fixation or microinjection and lack temporal or spatial resolution. We present a genetically encoded fluorescent reporter for protein kinase A (PKA) consisting of fusions of cyan fluorescent protein, a phosphoamino acid binding domain (14-3-3tau), a consensus substrate for PKA, and yellow fluorescent protein. cAMP elevations cause 25-50% changes in the ratios of yellow to cyan emissions in live cells caused by phosphorylation-induced changes in fluorescence resonance energy transfer. The reporter response was accelerated by tethering to PKA holoenzyme and slowed by localization to the nucleus. We demonstrate that deliberate redistribution of a substrate or colocalizing a substrate and PKA can modulate its susceptibility to phosphorylation by the kinase. The successful design of a fluorescent reporter of PKA activity and its application for studying compartmentalized and dynamic modulation of kinases lays a foundation for studying targeting and compartmentation of PKA and other kinases and phosphatases.

Genetically encoded fluorescent reporters of protein tyrosine kinase activities in living cells.

The complexity and specificity of many forms of signal transduction are widely believed to require spatial compartmentation of protein kinase and phosphatase activities, yet existing methods for measuring kinase activities in cells lack generality or spatial or temporal resolution. We present three genetically encoded fluorescent reporters for the tyrosine kinases Src, Abl, and epidermal growth factor (EGF) receptor. The reporters consist of fusions of cyan fluorescent protein (CFP), a phosphotyrosine binding domain, a consensus substrate for the relevant kinase, and yellow fluorescent protein (YFP). Stimulation of kinase activities in living cells with addition of growth factors causes 20-35% changes in the ratios of yellow to cyan emissions because of phosphorylation-induced changes in fluorescence resonance energy transfer (FRET). Platelet-derived growth factor (PDGF) stimulated Abl activity most strongly in actin-rich membrane ruffles, supporting the importance of this tyrosine kinase in the regulation of cell morphology. These results establish a general strategy for nondestructively imaging dynamic protein tyrosine kinase activities with high spatial and temporal resolution in single living cells.

Increased expression of alpha 1A Ca2+ channel currents arising from expanded trinucleotide repeats in spinocerebellar ataxia type 6.

The expansion of polyglutamine tracts encoded by CAG trinucleotide repeats is a common mutational mechanism in inherited neurodegenerative diseases. Spinocerebellar ataxia type 6 (SCA6), an autosomal dominant, progressive disease, arises from trinucleotide repeat expansions present in the coding region of CACNA1A (chromosome 19p13). This gene encodes alpha(1A), the principal subunit of P/Q-type Ca(2+) channels, which are abundant in the CNS, particularly in cerebellar Purkinje and granule neurons. We assayed ion channel function by introduction of human alpha(1A) cDNAs in human embryonic kidney 293 cells that stably coexpressed beta(1) and alpha(2)delta subunits. Immunocytochemical analysis showed a rise in intracellular and surface expression of alpha(1A) protein when CAG repeat lengths reached or exceeded the pathogenic range for SCA6. This gain at the protein level was not a consequence of changes in RNA stability, as indicated by Northern blot analysis. The electrophysiological behavior of alpha(1A) subunits containing expanded (EXP) numbers of CAG repeats (23, 27, and 72) was compared against that of wild-type subunits (WT) (4 and 11 repeats) using standard whole-cell patch-clamp recording conditions. The EXP alpha(1A) subunits yielded functional ion channels that supported inward Ca(2+) channel currents, with a sharp increase in P/Q Ca(2+) channel current density relative to WT. Our results showed that Ca(2+) channels from SCA6 patients display near-normal biophysical properties but increased current density attributable to elevated protein expression at the cell surface.

Visualizing recycling synaptic vesicles in hippocampal neurons by FM 1-43 photoconversion.

Exo-endocytotic turnover of synaptic vesicles (SVs) at synapses between hippocampal neurons in culture was examined by electron microscopy (EM). We carried out photoconversion (PC) of the fluorescent endocytotic marker FM 1-43 by using 3,3'-diaminobenzidine to convert the dye signal into an electron-dense product. Electron-dense products were located almost exclusively in SVs, whose densities were bimodally distributed in two sharply demarcated populations, PC-positive (PC+) and PC-negative (PC-). The median densities of these populations did not vary with the proportion of vesicles stained within a presynaptic terminal (bouton). The proportion of PC+ SVs remained constant across consecutive thin sections of single boutons, but varied greatly from one bouton to another, indicating marked heterogeneity in exo-endocytotic activity. Our experiments indicated that only a minority of SVs were stained in most boutons after stimuli known to cause complete turnover of the functional vesicular pool. A direct spatial correlation was found between FM 1-43 fluorescent spots seen with light microscopy and PC+ boutons by EM. The correlation was clearer in isolated boutons than in clusters of boutons. Photoconversion in combination with FM dyes allows clarification of important aspects of vesicular traffic in central nervous system nerve terminals.

Limited numbers of recycling vesicles in small CNS nerve terminals: implications for neural signaling and vesicular cycling.

The tiny nerve terminals of central synapses contain far fewer vesicles than preparations commonly used for analysis of neurosecretion. Photoconversion of vesicles rendered fluorescent with the dye FM1-43 directly identified vesicles capable of engaging in exo-endocytotic recycling following stimulated Ca(2+) entry. This recycling pool typically contained 30-45 vesicles, only a minority fraction (15-20% on average) of the total vesicle population. The smallness of the recycling pool would severely constrain rates of quantal neurotransmission if classical pathways were solely responsible for vesicle recycling. Fortunately, vesicles can undergo rapid retrieval and reuse in addition to conventional slow recycling, to the benefit of synaptic information flow and neuronal signaling.

Fluorescent sensors for Zn(2+) based on a fluorescein platform: synthesis, properties and intracellular distribution.

Two new fluorescent sensors for Zn(2+) that utilize fluorescein as a reporting group, Zinpyr-1 and Zinpyr-2, have been synthesized and characterized. Zinpyr-1 is prepared in one step via a Mannich reaction, and Zinpyr-2 is obtained in a multistep synthesis that utilizes 4',5'-fluorescein dicarboxaldehyde as a key intermediate. Both Zinpyr sensors have excitation and emission wavelengths in the visible range ( approximately 500 nm), dissociation constants (K(d1)) for Zn(2+) of <1 nM, quantum yields approaching unity (Phi = approximately 0.9), and cell permeability, making them well-suited for intracellular applications. A 3- to 5-fold fluorescent enhancement is observed under simulated physiological conditions corresponding to the binding of the Zn(2+) cation to the sensor, which inhibits a photoinduced electron transfer (PET) quenching pathway. The X-ray crystal structure of a 2:1 Zn(2+):Zinpyr-1 complex has also been solved, and is the first structurally characterized example of a complex of fluorescein substituted with metal binding ligands.

Photo-mediated gene activation using caged RNA/DNA in zebrafish embryos.

We report a new and simple technique for photo-mediated temporal and spatial control of gene activation in zebrafish embryos as an alternative to the gene 'knockdown' approach using antisense, morpholino-modified oligonucleotides (morpholinos). The synthetic compound 6-bromo-4-diazomethyl-7-hydroxycoumarin (Bhc-diazo) forms a covalent bond with the phosphate moiety of the sugar-phosphate backbone of RNA, a process known as caging. The 6-bromo-7-hydroxycoumarin-4-ylmethyl (Bhc) group binds to approximately 30 sites on the phosphate moieties per 1 kb of RNA sequence. Bhc-caged mRNA undergoes photolysis (uncaging) when exposed to long-wave ultraviolet light (350 to 365 nm). We show that Bhc-caged green fluorescent protein (Gfp) mRNA has severely reduced translational activity in vitro, whereas illumination of Bhc-caged mRNA with ultraviolet light leads to partial recovery of translational activity. Bhc-caged mRNA is highly stable in zebrafish embryos. In embryos injected with Bhc-caged Gfp mRNA at the one-cell stage, GFP protein expression and fluorescence is specifically induced by ultraviolet light. We also show that, consistent with results obtained using other methods, uncaging eng2a (which encodes the transcription factor Engrailed2a) in the head region during early development causes a severe reduction in the size of the eye and enhanced development of the midbrain and the midbrain-hindbrain boundary at the expense of the forebrain.

Fluorescence resonance energy transfer analysis of cell surface receptor interactions and signaling using spectral variants of the green fluorescent protein.

BACKGROUND: Fluorescence resonance energy transfer (FRET) is a powerful technique for measuring molecular interactions at Angstrom distances. We present a new method for FRET that utilizes the unique spectral properties of variants of the green fluorescent protein (GFP) for large-scale analysis by flow cytometry. METHODS: The proteins of interest are fused in frame separately to the cyan fluorescent protein (CFP) or the yellow fluorescent protein (YFP). FRET between these differentially tagged fusion proteins is analyzed using a dual-laser FACSVantage cytometer. RESULTS: We show that homotypic interactions between individual receptor chains of tumor necrosis factor receptor (TNFR) family members can be detected as FRET from CFP-tagged receptor chains to YFP-tagged receptor chains. Noncovalent molecular complexation can be detected as FRET between fusions of CFP and YFP to either the intracellular or extracellular regions of the receptor chains. The specificity of the assay is demonstrated by the absence of FRET between heterologous receptor pairs that do not biochemically associate with each other. Interaction between a TNFR-like receptor (Fas/CD95/Apo-1) and a downstream cytoplasmic signaling component (FADD) can also be demonstrated by flow cytometric FRET analysis. CONCLUSIONS: The utility of spectral variants of GFP in flow cytometric FRET analysis of membrane receptors is demonstrated. This method of analyzing FRET allows probing of noncovalent molecular interactions that involve both the intracellular and extracellular regions of membrane proteins as well as proteins within the cells. Unlike biochemical methods, FRET allows the quantitative determination of noncovalent molecular associations at Angstrom level in living cells. Moreover, flow cytometry allows quantitative analyses to be carried out on a cell-by-cell basis on large number of cells. Published 2001 Wiley-Liss, Inc.

Reducing the environmental sensitivity of yellow fluorescent protein. Mechanism and applications.

Yellow mutants of the green fluorescent protein (YFP) are crucial constituents of genetically encoded indicators of signal transduction and fusions to monitor protein-protein interactions. However, previous YFPs show excessive pH sensitivity, chloride interference, poor photostability, or poor expression at 37 degrees C. Protein evolution in Escherichia coli has produced a new YFP named Citrine, in which the mutation Q69M confers a much lower pK(a) (5.7) than for previous YFPs, indifference to chloride, twice the photostability of previous YFPs, and much better expression at 37 degrees C and in organelles. The halide resistance is explained by a 2.2-A x-ray crystal structure of Citrine, showing that the methionine side chain fills what was once a large halide-binding cavity adjacent to the chromophore. Insertion of calmodulin within Citrine or fusion of cyan fluorescent protein, calmodulin, a calmodulin-binding peptide and Citrine has generated improved calcium indicators. These chimeras can be targeted to multiple cellular locations and have permitted the first single-cell imaging of free [Ca(2+)] in the Golgi. Citrine is superior to all previous YFPs except when pH or halide sensitivity is desired and is particularly advantageous within genetically encoded fluorescent indicators of physiological signals.

Mechanisms of pH regulation in the regulated secretory pathway.

A precise pH gradient between organelles of the regulated secretory pathway is required for sorting and processing of prohormones. We studied pH regulation in live endocrine cells by targeting biotin-based pH indicators to cellular organelles expressing avidin-chimera proteins. In AtT-20 cells, we found that steady-state pH decreased from the endoplasmic reticulum (ER) (pH(ER) = 7.4 +/- 0.2, mean +/- S.D.) to Golgi (pH(G) = 6.2 +/- 0.4) to mature secretory granules (MSGs) (pH(MSG) = 5.5 +/- 0.4). Golgi and MSGs required active H(+) v-ATPases for acidification. ER, Golgi, and MSG steady-state pH values were also dependent upon the different H(+) leak rates across each membrane. However, neither steady-state pH(MSG) nor rates of passive H(+) leak were affected by Cl(-)-free solutions or valinomycin, indicating that MSG membrane potential was small and not a determinant of pH(MSG). Therefore, our data do not support earlier suggestions that organelle acidification is primarily regulated by Cl(-) conductances. Measurements of H(+) leak rates, buffer capacities, and estimates of surface areas and volumes of these organelles were applied to a mathematical model to determine the H(+) permeability (P(H+)) of each organelle membrane. We found that P(H+) decreased progressively from ER to Golgi to MSGs, and proper acidification of Golgi and MSGs required gradual decreases in P(H+) and successive increases in the active H(+) pump density.

Molecular basis of calmodulin tethering and Ca2+-dependent inactivation of L-type Ca2+ channels.

Ca(2+)-dependent inactivation (CDI) of L-type Ca(2+) channels plays a critical role in controlling Ca(2+) entry and downstream signal transduction in excitable cells. Ca(2+)-insensitive forms of calmodulin (CaM) act as dominant negatives to prevent CDI, suggesting that CaM acts as a resident Ca(2+) sensor. However, it is not known how the Ca(2+) sensor is constitutively tethered. We have found that the tethering of Ca(2+)-insensitive CaM was localized to the C-terminal tail of alpha(1C), close to the CDI effector motif, and that it depended on nanomolar Ca(2+) concentrations, likely attained in quiescent cells. Two stretches of amino acids were found to support the tethering and to contain putative CaM-binding sequences close to or overlapping residues previously shown to affect CDI and Ca(2+)-independent inactivation. Synthetic peptides containing these sequences displayed differences in CaM-binding properties, both in affinity and Ca(2+) dependence, leading us to propose a novel mechanism for CDI. In contrast to a traditional disinhibitory scenario, we suggest that apoCaM is tethered at two sites and signals actively to slow inactivation. When the C-terminal lobe of CaM binds to the nearby CaM effector sequence (IQ motif), the braking effect is relieved, and CDI is accelerated.

Spaced stimuli stabilize MAPK pathway activation and its effects on dendritic morphology.

Memory storage in mammalian neurons probably depends on both biochemical events and morphological alterations in dendrites. Here we report an activity-dependent stabilization of the MAP kinase (MAPK) pathway, prominent in hippocampal dendrites. The longevity of the signal in these dendrites was increased to hours when multiple spaced stimuli were used. Likewise, spaced stimuli and MAPK activation were critical for protrusion of new dendritic filopodia that also remained stable for hours. Our experiments define a new role for stimulus-specific responses of MAPK signaling in activity-dependent neuronal plasticity. The local biochemical signaling in dendrites complements MAPK signaling in gene expression. Together, these processes may support long-lasting behavioral changes.

Spatiotemporal dynamics of guanosine 3',5'-cyclic monophosphate revealed by a genetically encoded, fluorescent indicator.

To investigate the dynamics of guanosine 3',5'-cyclic monophosphate (cGMP) in single living cells, we constructed genetically encoded, fluorescent cGMP indicators by bracketing cGMP-dependent protein kinase (cGPK), minus residues 1-77, between cyan and yellow mutants of green fluorescent protein. cGMP decreased fluorescence resonance energy transfer (FRET) and increased the ratio of cyan to yellow emissions by up to 1.5-fold with apparent dissociation constants of approximately 2 microM and >100:1 selectivity for cGMP over cAMP. To eliminate constitutive kinase activity, Thr(516) of cGPK was mutated to Ala. Emission ratio imaging of the indicators transfected into rat fetal lung fibroblast (RFL)-6 showed cGMP transients resulting from activation of soluble and particulate guanylyl cyclase, respectively, by nitric oxide (NO) and C-type natriuretic peptide (CNP). Whereas all naive cells tested responded to CNP, only 68% responded to NO. Both sets of signals showed large and variable (0.5-4 min) latencies. The phosphodiesterase (PDE) inhibitor 3-isobutyl-1-methylxanthine (IBMX) did not elevate cGMP on its own but consistently amplified responses to NO or CNP, suggesting that basal activity of guanylate cyclase is very low and emphasizing the importance of PDEs in cGMP recycling. A fraction of RFL cells showed slowly propagating tides of cGMP spreading across the cell in response to delocalized application of NO. Biolistically transfected Purkinje neurons showed cGMP responses to parallel fiber activity and NO donors, confirming that single-cell increases in cGMP occur under conditions appropriate to cause synaptic plasticity.