A method for measuring colocalization of presynaptic markers with anatomically labeled axons using double label immunofluorescence and confocal microscopy

Neurotrophin NT-4/5 Promotes Structural Changes in Neurons of the Developing Visual Cortex

Current hypotheses on the mechanisms underlying the development and plasticity of the ocular dominance system through competitive interactions between pathways serving the two eyes strongly suggest the involvement of neurotrophins and their high affinity receptors. In the cat, infusion of the tyrosine kinase B ligand (trkB), neurotrophin-4/5 (NT-4/5), abolishes ocular dominance plasticity that follows monocular deprivation (Gillespie et al., 2000), while tyrosine kinase A and C ligands (trkA and trkC) do not have this effect. One interpretation of this finding is that NT-4/5 causes overgrowth and sprouting of thalamocortical and/or corticocortical terminals, leading to promiscuous neuronal connections which override the experience-dependent fine tuning of connections based on correlated activity. The present study tested whether neurons in cortical regions infused with NT-4/5 showed anatomical changes compatible with this hypothesis. Cats at the peak of the critical period received chronic infusion NT-4/5 into visual cortical areas 17/18 via an osmotic minipump. Visual cortical neurons were labeled in fixed slices using the DiOlistics methods (Gan et al., 2000) and analyzed in confocal microscopy. Infusion of NT-4/5 induced a significant increase of spine-like processes on primary dendrites and a distinctive sprouting of protuberances from neuronal somata in all layers. The increase of neuronal membrane was paralleled by an increase in density of the presynaptic marker synaptophysin in infused areas, suggesting an increase in the numbers of synapses. A contingent of these newly formed synapses may feed into inhibitory circuits, as suggested by an increase of GAD-65 immunostaining in NT-4/5 affected areas. These anatomical changes are consistent with the physiological changes in such animals, suggesting that excess trkB neurotrophin can stimulate the formation of promiscuous connections during the critical period.

Co-Density Distribution Maps for Advanced Molecule Colocalization and Co-Distribution Analysis

Cellular and subcellular spatial colocalization of structures and molecules in biological specimens is an important indicator of their co-compartmentalization and interaction. Presently, colocalization in biomedical images is addressed with visual inspection and quantified by co-occurrence and correlation coefficients. However, such measures alone cannot capture the complexity of the interactions, which does not limit itself to signal intensity. On top of the previously developed density distribution maps (DDMs), here, we present a method for advancing current colocalization analysis by introducing co-density distribution maps (cDDMs), which, uniquely, provide information about molecules absolute and relative position and local abundance. We exemplify the benefits of our method by developing cDDMs-integrated pipelines for the analysis of molecules pairs co-distribution in three different real-case image datasets. First, cDDMs are shown to be indicators of colocalization and degree, able to increase the reliability of correlation coefficients currently used to detect the presence of colocalization. In addition, they provide a simultaneously visual and quantitative support, which opens for new investigation paths and biomedical considerations. Finally, thanks to the coDDMaker software we developed, cDDMs become an enabling tool for the quasi real time monitoring of experiments and a potential improvement for a large number of biomedical studies.

Quantitative and Qualitative Evaluation of Photoreceptor Synapses in Developing, Degenerating and Regenerating Retinas

Quantitative and qualitative evaluation of synapses is crucial to understand neural connectivity. This is particularly relevant now, in view of the recent advances in regenerative biology and medicine. There is an urgent need to evaluate synapses to access the extent and functionality of reconstructed neural network. Most of the currently used synapse evaluation methods provide only all-or-none assessments. However, very often synapses appear in a wide spectrum of transient states such as during synaptogenesis or neural degeneration. Robust evaluation of synapse quantity and quality is therefore highly sought after. In this paper we introduce QUANTOS, a new method that can evaluate the number, likelihood, and maturity of photoreceptor ribbon synapses based on graphical properties of immunohistochemistry images. QUANTOS is composed of ImageJ Fiji macros, and R scripts which are both open-source and free software. We used QUANTOS to evaluate synaptogenesis in developing and degenerating retinas, as well as de novo synaptogenesis of mouse iPSC-retinas after transplantation to a retinal degeneration mouse model. Our analysis shows that while mouse iPSC-retinas are largely incapable of forming synapses in vitro, they can form extensive synapses following transplantation. The de novo synapses detected after transplantation seem to be in an intermediate state between mature and immature compared to wildtype retina. Furthermore, using QUANTOS we tested whether environmental light can affect photoreceptor synaptogenesis. We found that the onset of synaptogenesis was earlier under cyclic light (LD) condition when compared to constant dark (DD), resulting in more synapses at earlier developmental stages. The effect of light was also supported by micro electroretinography showing larger responses under LD condition. The number of synapses was also increased after transplantation of mouse iPSC-retinas to rd1 mice under LD condition. Our new probabilistic assessment of synapses may prove to be a valuable tool to gain critical insights into neural-network reconstruction and help develop treatments for neurodegenerative disorders.

Region-Specific Differences in Morphometric Features and Synaptic Colocalization of Astrocytes During Development

It is well established that astrocytes play pivotal roles in neuronal synapse formation and maturation as well as in the modulation of synaptic transmission. Despite their general importance for brain function, relatively little is known about the maturation of astrocytes during normal postnatal development, especially during adolescence, and how that maturation may influence astroglial-synaptic contact. The medial prefrontal cortex (mPFC) and dorsal hippocampus (dHipp) are critical for executive function, memory, and their effective integration. Further, both regions undergo significant functional changes during adolescence and early adulthood that are believed to mediate these functions. However, it is unclear the extent to which astrocytes change during these late developmental periods, nor is it clear whether their association with functional synapses shifts as adolescent and young adult maturation proceeds. Here we utilize an astrocyte-specific viral labeling approach paired with high-resolution single-cell astrocyte imaging and three-dimensional reconstruction to determine whether mPFC and dHipp astrocytes have temporally distinct maturation trajectories. mPFC astrocytes, in particular, continue to mature well into emerging adulthood (postnatal day 70). Moreover, this ongoing maturation is accompanied by a substantial increase in colocalization of astrocytes with the postsynaptic neuronal marker, PSD-95. Taken together, these data provide novel insight into region-specific astrocyte-synapse interactions in late CNS development and into adulthood, thus raising implications for the mechanism of post-adolescent development of the mPFC.

Sustained anti-BCR-ABL activity with pH responsive imatinib mesylate loaded PCL nanoparticles in CML cells

IM–chitosan complex encapsulated poly(ε-caprolactone) (PCL) nanoparticles are proposed for their potential in enabling more intelligent controlled release and enhancing chemotherapeutic efficiency of IM.

Dark rearing maintains tyrosine hydroxylase expression in retinal amacrine cells following optic nerve transection

The present study examined changes in retinal tyrosine hydroxylase (TH) expression in rats having undergone optic nerve transection and housed under a normal day/night cycle or in the dark. The aim was to investigate the effects of amacrine cells on axonal regeneration in retinal ganglion cells and on the synapses that transmit visual signals. The results revealed that retinal TH expression gradually decreased following optic nerve transection in rats housed under a normal day/night cycle, reaching a minimum at 5 days. In contrast, retinal TH expression decreased to a minimum at 1 day following optic nerve transection in dark reared rats, gradually increasing afterward and reaching a normal level at 5–7 days. The number of TH-positive synaptic particles correlated with the TH levels, indicating that dark rearing can help maintain TH expression during the synaptic degeneration stage (5–7 days after optic nerve injury) in retinal amacrine cells.

Substance P, tyrosine hydroxylase and serotonin terminals in the rat caudal nucleus ambiguus

Substance P (SP), tyrosine hydroxylase (TH) and serotonin inputs onto laryngeal motoneurons (LMNs) are known to exist, but the distribution of their terminals in the caudal nucleus ambiguus (NA), remains unclear. Using immunofluorescence and confocal microscopy, we assessed simultaneously the distribution of SP, TH, serotonin and synaptophysin immunoreactive (ir) terminals in the caudal NA. SP, TH and serotonin-ir varicosities were considered to represent immunoreactive synapses if, using confocal microscopy, they were co-localized with the presynaptic protein, synaptophysin. Relative to the total number of synapses, we found only a modest number of SP, TH or serotonin-ir synaptic terminals in the caudal NA. The density of SP-ir synaptic terminals was higher than that of TH-ir and serotonin-ir synaptic terminals. Our results suggest that SP, TH, and serotonin-ir inputs may play only a modest role in regulating the activity of LMN. We conclude that SP, TH and serotonin are not always co-localized in terminals forming inputs with LMN and that they arise from separate subpopulations of neurons.

The lateral hypothalamus as integrator of metabolic and environmental needs: from electrical self-stimulation to opto-genetics

As one of the evolutionary oldest parts of the brain, the diencephalon evolved to harmonize changing environmental conditions with the internal state for survival of the individual and the species. The pioneering work of physiologists and psychologists around the middle of the last century clearly demonstrated that the hypothalamus is crucial for the display of motivated behaviors, culminating in the discovery of electrical self-stimulation behavior and providing the first neurological hint accounting for the concepts of reinforcement and reward. Here we review recent progress in understanding the role of the lateral hypothalamic area in the control of ingestive behavior and the regulation of energy balance. With its vast array of interoceptive and exteroceptive afferent inputs and its equally rich efferent connectivity, the lateral hypothalamic area is in an ideal position to integrate large amounts of information and orchestrate adaptive responses. Most important for energy homeostasis, it receives metabolic state information through both neural and humoral routes and can affect energy assimilation and energy expenditure through direct access to behavioral, autonomic, and endocrine effector pathways. The complex interplays of classical and peptide neurotransmitters such as orexin carrying out these integrative functions are just beginning to be understood. Exciting new techniques allowing selective stimulation or inhibition of specific neuronal phenotypes will greatly facilitate the functional mapping of both input and output pathways.

Experience-induced plasticity of thalamocortical axons in both juveniles and adults

We examined the effect of sensory deprivation on thalamocortical (TC) projections to the rat primary somatosensory cortex at different postnatal ages ranging from P0 to P96. Rats had their whiskers clipped off with one or two vibrissae spared. TC axons innervating barrel cortex were specifically labeled by injecting virus expressing fluorescent proteins into the corresponding primary (VPM) and/or secondary (POm) thalamic nuclei. The density of VPM axons in deprived columns was ≈34% lower relative to spared columns with a concomitant decrease in bouton density, suggesting a deprivation-induced retraction of VPM axons. Axonal changes were reversible upon regrowth of the clipped whiskers and independent of age at deprivation, indicating the absence of a critical period for anatomical plasticity. The POm projection was not obviously altered by sensory deprivation. We suggest that retraction and regrowth of TC axons substantially contribute to long-term deprivation-dependent functional plasticity.

Rapid structural remodeling of thalamocortical synapses parallels experience-dependent functional plasticity in mouse primary visual cortex

Monocular lid closure (MC) causes a profound shift in the ocular dominance (OD) of neurons in primary visual cortex (V1). Anatomical studies in both cat and mouse V1 suggest that large-scale structural rearrangements of eye-specific thalamocortical (TC) axons in response to MC occur much more slowly than the shift in OD. Consequently, there has been considerable debate as to whether the plasticity of TC synapses, which transmit competing visual information from each eye to V1, contributes to the early functional consequences of MC or is simply a feature of long-term deprivation. Here, we used quantitative immuno-electron microscopy to examine the possibility that alterations of TC synapses occur rapidly enough to impact OD after brief MC. The effect of short-term deprivation on TC synaptic structure was examined in male C57BL/6 mice that underwent 3 and 7 d of MC or monocular retinal inactivation (MI) with tetrodotoxin. The data show that 3 d of MC is sufficient to induce substantial remodeling of TC synapses. In contrast, 3 d of MI, which alters TC activity but does not shift OD, does not significantly affect the structure of TC synapses. Our results support the hypothesis that the rapid plasticity of TC synapses is a key step in the sequence of events that shift OD in visual cortex.

Retinal afferents synapse with relay cells targeting the middle temporal area in the pulvinar and lateral geniculate nuclei

Considerable debate continues regarding thalamic inputs to the middle temporal area (MT) of the visual cortex that bypass the primary visual cortex (V1) and the role they might have in the residual visual capability following a lesion of V1. Two specific retinothalamic projections to area MT have been speculated to relay through the medial portion of the inferior pulvinar nucleus (PIm) and the koniocellular layers of the dorsal lateral geniculate nucleus (LGN). Although a number of studies have demonstrated retinal inputs to regions of the thalamus where relays to area MT have been observed, the relationship between the retinal terminals and area MT relay cells has not been established. Here we examined direct retino-recipient regions of the marmoset monkey (Callithrix jacchus) pulvinar nucleus and the LGN following binocular injections of anterograde tracer, as well as area MT relay cells in these nuclei by injection of retrograde tracer into area MT. Retinal afferents were shown to synapse with area MT relay cells as demonstrated by colocalization with the presynaptic vesicle membrane protein synaptophysin. We also established the presence of direct synapes of retinal afferents on area MT relay cells within the PIm, as well as the koniocellular K1 and K3 layers of the LGN, thereby corroborating the existence of two disynaptic pathways from the retina to area MT that bypass V1.

Regulation of endosomal motility and degradation by amyotrophic lateral sclerosis 2/alsin

Dysfunction of alsin, particularly its putative Rab5 guanine-nucleotide-exchange factor activity, has been linked to one form of juvenile onset recessive familial amyotrophic lateral sclerosis (ALS2). Multiple lines of alsin knockout (ALS2(-/-)) mice have been generated to model this disease. However, it remains elusive whether the Rab5-dependent endocytosis is altered in ALS2(-/-) neurons. To directly examine the Rab5-mediated endosomal trafficking in ALS2(-/-) neurons, we introduced green fluorescent protein (GFP)-tagged Rab5 into cultured hippocampal neurons to monitor the morphology and motility of Rab5-associated early endosomes. Here we report that Rab5-mediated endocytosis was severely altered in ALS2(-/-) neurons. Excessive accumulation of Rab5-positive vesicles was observed in ALS2(-/-) neurons, which correlated with a significant reduction in endosomal motility and augmentation in endosomal conversion to lysosomes. Consequently, a significant increase in endosome/lysosome-dependent degradation of internalized glutamate receptors was observed in ALS2(-/-) neurons. These phenotypes closely resembled the endosomal trafficking abnormalities induced by a constitutively active form of Rab5 in wild-type neurons. Therefore, our findings reveal a negatively regulatory mechanism of alsin in Rab5-mediated endosomal trafficking, suggesting that enhanced endosomal degradation in ALS2(-/-) neurons may underlie the pathogenesis of motor neuron degeneration in ALS2 and related motor neuron diseases.

Transiently increased colocalization of vesicular glutamate transporters 1 and 2 at single axon terminals during postnatal development of mouse neocortex: a quantitative analysis with correlation coefficient

Vesicular glutamate transporter 1 (VGLUT1) and VGLUT2 show complementary distribution in neocortex; VGLUT1 is expressed mainly in axon terminals of neocortical neurons, whereas VGLUT2 is located chiefly in thalamocortical axon terminals. However, we recently reported a frequent colocalization of VGLUT1 and VGLUT2 at a subset of axon terminals in postnatal developing neocortex. We here quantified the frequency of colocalization between VGLUT1 and VGLUT2 immunoreactivities at single axon terminals by using the correlation coefficient (CC) as an indicator in order to determine the time course and spatial extent of the colocalization during postnatal development of mouse neocortex. The colocalization was more frequent in the primary somatosensory (S1) area than in both the primary visual (V1) and the motor areas; of area S1 cortical layers, colocalization was most evident in layer IV barrels at postnatal day (P) 7 and in adulthood. CC in layer IV showed a peak at P7 in area S1, and at P10 in area V1 though the latter peak was much smaller than the former. These results suggest that thalamocortical axon terminals contained not only VGLUT2 but also VGLUT1, especially at P7-10. Double fluorescence in situ hybridization confirmed coexpression of VGLUT1 and VGLUT2 mRNAs at P7 in the somatosensory thalamic nuclei and later in the thalamic dorsal lateral geniculate nucleus. As VGLUT1 is often used in axon terminals that show synaptic plasticity in adult brain, the present findings suggest that VGLUT1 is used in thalamocortical axons transiently during the postnatal period when plasticity is required.

Nociceptive craniofacial muscle primary afferent neurons synapse in both the rostral and caudal brain stem

Limited information is available on muscle afferent neurons with fine fibers despite their presumed participation in musculoskeletal disorders, including temporomandibular disorders. To study these neurons, intracellular recordings were made from the central axons of slowly conducting muscle afferent neurons in anesthetized rats. After intraaxonal impalement, axons were characterized by masseter nerve stimulation, receptive field testing, muscle stretching and intramuscular injection of hypertonic saline. Intracellular recordings were made from 310 axons (conduction velocity: 6.5-60(M)/s, mean = 27.3(M)/s; following frequency: 27-250 Hz, mean = 110Hz). No neurons responded to cutaneous palpation or muscle stretching. Some axons (n = 34) were intracellularly stained with biotinamide. These neurons were classified as group II/III noxious mechanoreceptors because their mechanical threshold exceeded 15 mN, and conduction velocities ranged from 12 to 40.2(M)/s (mean = 25.3(M)/s). Two morphological types were recognized by using an object-based, three-dimensional colocalization methodology to locate synapses. One type (IIIHTM(Vp-Vc)) possessed axon collaterals that emerged along the entire main axon and synapsed in the trigeminal principal sensory nucleus and spinal trigeminal subnuclei oralis (Vo), interpolaris (Vi), and caudalis (Vc). A second type (IIIHTM(Vo-Vc)) possessed axon collaterals that synapsed only in caudal Vo, Vi, and Vc. Our previous studies show that muscle spindle afferent neurons are activated by innocuous stimuli and synapse in the rostral and caudal brain stem; here we demonstrate that nociceptive muscle mechanoreceptor afferent axons also synapse in rostral and caudal brain stem regions. Traditional dogma asserts that the most rostral trigeminal sensory complex exclusively processes innocuous somatosensory information, whereas caudal portions receive nociceptive sensory input; the data reported here do not support this paradigm.

Developmental period for N-methyl-D-aspartate (NMDA) receptor-dependent synapse elimination correlated with visuotopic map refinement

During a short perinatal interval, N-methyl-D-aspartate receptor (NMDAR) function is essential to a process in which spontaneous retinal waves focus retinal axon arbors in the superficial layers of the rodent superior colliculus (sSC). Here we provide evidence that this NMDAR-dependent axonal refinement occurs through elimination of uncorrelated retinal synapses arising from disparate loci, rather than stabilization of topographically appropriate inputs. The density of synaptic release sites within fluorescently labeled retinal terminals was counted in double-labeling experiments using confocal microscopy and antibodies against synaptophysin or synapsin-1. Chronic NMDAR blockade from birth increased retinal axon synapse density at postnatal days (P) 6, 8, and 10, suggesting that NMDAR currents reduce synapse density during the refinement period. With assay at P14, after focal arborization has been established, the effect disappeared. Conversely, chronic NMDA treatment, known to induce functional synaptic depression in the sSC, decreased retinocollicular synapse density at P14, but not earlier, during the refinement period (P8). Thus during the development of retinocollicular topographic order, there is a period when NMDAR activity predominantly eliminates retinal axon synapses. We were able to extend this period by using retinal lesions to reduce synaptic density in a defined zone. Synapse density on intact retinocollicular axons sprouting into this zone was increased by NMDAR blockade, even when examined at P14. Thus, the period of NMDAR-dependent synaptic destabilization is terminated by a factor related to the density and refinement of retinal arbors.

Orexin (hypocretin) innervation of the paraventricular nucleus of the thalamus

The paraventricular nucleus of the thalamus (PVT) is a midline thalamic nucleus with projections to limbic forebrain areas such as the nucleus accumbens and amygdala. The orexin (hypocretin) peptides are synthesized in hypothalamic neurons that project throughout the CNS. The present experiments were done to describe the extent of orexin fiber innervation of the PVT in comparison to other midline and intralaminar thalamic nuclei and to establish the location and proportion of orexin neurons innervating the PVT. All aspects of the anteroposterior PVT were found to be densely innervated by orexin fibers with numerous enlargements that also stained for synaptophysin, a marker for synaptic vesicle protein associated with pre-synaptic sites. Small discrete injections of cholera toxin B into the PVT of rats resulted in the retrograde labeling of a relatively small number of orexin neurons in the medial and lateral hypothalamus. The results also showed a lack of topographical organization among orexin neurons projecting to the PVT. Previous studies indicate that orexin neurons and neurons in the PVT appear to be most active during periods of arousal. Therefore, orexin neurons and their projections to the PVT may be part of a limbic forebrain arousal system.

NMDA receptor currents suppress synapse formation on sprouting axons in vivo

NMDA receptors (NMDARs) play an important role in the structural maintenance and functional strength of synapses. The causal relationship between these anatomical and functional roles is poorly defined. Using quantitative confocal microscopy, synaptic vesicle immunoreactivity, and differential label of retinal projections, we measured axon volume and synapse density along ipsilateral retinal axons (ipsi axons) sprouting into the superficial visual layers of the superior colliculus (sSC) deafferented by a contralateral retinal lesion (a scotoma) 8 d earlier. When retinal lesions were made at postnatal day 6 (P6), glutamatergic synaptic currents on neurons within the scotoma were significantly reduced. Both ipsi axon sprouting and synapse density were increased by chronic d-AP-5 antagonism of NMDARs. Conversely, ipsi axon sprouting and synapse density were reduced by chronic exposure to the agonist, NMDA, known to functionally depress glutamate transmission in this system. After P11 lesions, however, NMDAR blockade had no effect on sprouting or synapse density. Developmental changes in NMDAR current kinetics could not account for this difference in the structural effects of NMDAR function. Also, synaptic current frequencies within the scotoma were not affected after the P11 lesions. The corticocollicular projection matures during the P11 survival interval and, as indicated by previous work, it is a source of competition for synaptic space and probably of maintained activity in the older sSC. Thus, our results suggest that during early development, NMDAR currents predominantly destabilize nascent synapses. As the neuropil matures, however, competition for synaptic space suppresses axon sprouting and synapse formation regardless of NMDAR function.

The organization of projections from the amygdala to visual cortical areas TE and V1 in the macaque monkey

We examined the organization of amygdaloid projections to visual cortical areas TE and V1 by injecting anterograde tracers into the amygdaloid complex of Macaca fascicularis monkeys. The magnocellular and intermediate divisions of the basal nucleus of the amygdala gave rise to heavy projections to both superficial layers (border of I/II) and deep layers (V/VI) throughout the rostrocaudal extent of area TE. Although most of the injections led to heavier fiber and terminal labeling in the superficial layers of area TE, the most dorsal injections in the basal nucleus produced denser labeled fibers and terminals in the deep layers of area TE. Area V1 received projections primarily from the magnocellular division of the basal nucleus, and these terminated exclusively in the superficial layers. As in area TE, projections from the amygdala to area V1 were distributed throughout its rostrocaudal and transverse extents. Labeled axons demonstrated 11.67 varicosities/100 microm on average in the superficial layers of area TE and 8.74 varicosities/100 microm in the deep layers. In area V1 we observed 8.24 varicosities/100 microm. Using confocal microscopy, we determined that at least 55% of the tracer-labeled varicosities in areas TE and V1 colocalized synaptophysin, a marker of synaptic vesicles, indicating that they are probably synaptic boutons. Electron microscopic examination of a sample of these varicosities confirmed that labeled boutons formed synapses in areas TE and V1. These feedback-like projections from the amygdala have the potential of modulating key areas of the visual processing system.

VGluT2 immunochemistry identifies thalamocortical terminals in layer 4 of adult and developing visual cortex

A vesicular glutamate transporter, VGluT2, has been suggested to be the transporter utilized in the thalamocortical pathway. We examined the reliability of this marker in identifying and discriminating thalamic terminals in adult and developing ferret visual cortex. We studied brain sections stained for the transporter protein and/or anterogradely filled thalamocortical or intracortical axons, by using light, confocal, and electron microscopy. Under light microscopy, VGluT2 immunoreactivity (ir) in adult animals [past postnatal day (P)90] and in neonatal animals as early as P27 formed a dense band in layer 4 and appeared as scattered puncta in layers 6 and 1. Confocal dual-labeling analyses of P46 and adult striate cortices indicated that VGluT2 was present in thalamocortical axons, suggesting that thalamic projections utilize this transporter during postnatal development as well as adulthood. In contrast, extracellularly filled intracortical axons failed to colocalize with VGluT2-ir, suggesting that no significant terminal population originating in cortex contained VGluT2 in layer 4. Electron microscopic analysis revealed that, in adult layer 4, VGluT2-ir was present in large terminals, forming asymmetric synapses. Similar to anterogradely labeled thalamocortical terminals, VGluT2-ir synaptic terminals were different from their unlabeled counterparts in terms of terminal area (0.6 vs. 0.3 microm), synaptic length (486 vs. 353 nm), and preference for synapsing on spines (77% vs. 59%). Moreover, no significant differences were found between VGluT2-ir and anterogradely labeled thalamocortical terminals. Comparable similarities were also demonstrated at P46. These results indicate that thalamocortical terminals in layer 4 of visual cortex utilize VGluT2 and suggest that this marker can be used to identify thalamic axons specifically in adult and developing animals.

Automated high through-put colocalization analysis of multichannel confocal images

The laser scanning confocal microscope (LSCM) generates images of multiple labelled fluorescent samples. Colocalization of fluorescent labels is frequently examined. Here we present an example where localization of fluorescent analogues of cloned protein were referenced to fluorescent antibodies directed against the proteins of cellular compartments. Colocalization is usually evaluated by visual inspection of signal overlap or by using commercially available software tools, but there are limited possibilities to automate the analysis of large amounts of data. We developed a simple tool using Matlab to automate the colocalization procedure and to exclude the biased estimations resulting from visual inspections of images. The script in Matlab language code automatically imports confocal images and converts them into arrays. The contrast of all images is uniformly set by linearly reassigning the values of pixel intensities to use the full 8-bit range (0-255). Images are binarized on several threshold levels. The area above a certain threshold level is summed for each channel of the image and for colocalized regions. As a result, count of pixels above several threshold levels in any number of images is saved in an ASCII file. In addition Pearson's r correlation coefficient is calculated for fluorescence intensities of both confocal channels. Using this approach quick quantitative analysis of colocalization of hundreds of images is possible. In addition, such automated procedure is not biased by the examiner's subject visualization.

Synergistic effects of brain-derived neurotrophic factor and chondroitinase ABC on retinal fiber sprouting after denervation of the superior colliculus in adult rats

Damage to the adult CNS often causes devastating and permanent deficits because of the limited capacity of the brain for anatomical reorganization. The finding that collateral sprouting of uninjured fiber tracts mediates recovery of function prompts the search for experimental strategies that stimulate axonal plasticity after CNS trauma. Here we characterize treatments that promote the sprouting of undamaged retinal afferents into the denervated superior colliculus (SC) after a partial retinal lesion in the adult rat. Delivery of brain-derived neurotrophic factor (BDNF) was performed to enhance the intrinsic potential of retinal ganglion cells to reelongate their axons. Reduction of the neurite growth-inhibitory properties of the adult SC was accomplished via treatment with chondroitinase ABC (C-ABC), which degrades chondroitin sulfate proteoglycans. Retinal axons were labeled via intraocular injections of fluorescently tagged cholera toxin B subunit, and fiber sprouting within the denervated SC was measured by quantitative laser-scanning confocal microscopy 1 week after the retinal lesion. We found that both the administration of BDNF and the injection of C-ABC induce significant sprouting of retinal afferents into the collicular scotoma. Remarkably, the combined treatment with BDNF and C-ABC showed synergistic effects on axon growth. Colocalization analysis with anti-synapsin antibodies demonstrated synapse formation by the sprouting axons. These results suggest that the combined treatment with BDNF and C-ABC can be relevant in therapies for the repair of the damaged adult CNS.

Regulation by glycogen synthase kinase-3beta of the arborization field and maturation of retinotectal projection in zebrafish

The retinotectal projection is one of the best systems to study the molecular basis of synapse formation in the CNS because of the well characterized topographic connections and activity-dependent refinement. Here, we developed a presynaptic neuron-specific gene manipulation system in the zebrafish retinotectal projection in vivo using the nicotinic acetylcholine receptor beta3 (nAChRbeta3) gene promoter. Enhanced green fluorescent protein (EGFP) expression signals in living transgenic zebrafish lines carrying the nAChRbeta3 gene promoter-directed EGFP expression vector visualized the development of entire retinal ganglion cell (RGC) axon projection to the tectum. Microinjection of the nAChRbeta3 gene promoter-driven double-cassette vectors directing the expression of both dominant-negative glycogen synthase kinase-3beta (dnGSK-3beta) and EGFP enabled us to follow the development of individual RGCs and to examine the effect of the molecule on the axonal arborization and maturation of the same neurons in living zebrafish. We found that the expression of the dominant-negative form of zebrafish GSK-3beta suppressed the arborization field of RGC axon terminals in the tectum as estimated by the reduction of arbor branch length and arbor areas. Furthermore, the suppression of GSK-3beta activity increased the size of vesicle-associated membrane protein 2-EGFP puncta in RGC axon terminals at the early stage of innervation to the tectum. These results suggest that GSK-3beta regulates the arborization field and maturation of RGC axon terminals in vivo.

Tracking sodium channels in live cells: confocal imaging using fluorescently labeled toxins

One particularly important class of ion channels in excitable cells are the voltage-dependent sodium channels (VDSC). Knowledge of the distribution of VDSC in living cells is important for studies of neuronal excitability, development, and plasticity. Here, we demonstrate a new method for visualizing the spatial distribution of VDSC in living cells. To illustrate the capabilities of the technique, the distribution of VDSC in GH3 cells was revealed with fluorescent derivatives of the alpha-type and beta-type scorpion toxins in conjunction with laser scanning confocal microscopy. Cells exhibited fluorescent hot spots on the surface of the membrane. This characteristic staining pattern was prevented by pre-incubation with unlabeled native toxins and blocked by membrane depolarization for alpha-type toxins. Labeling was not observed in cells lacking sodium channels (HEK 293) after incubation with fluorescent-labeled toxins. Image processing techniques were applied to identify the location of each cluster of labeled VDSC in these cells. The proposed method eliminates artefacts commonly introduced during sample preparation for immunostaining and should prove to be a valuable research tool for investigating VDSC distribution in living specimens.

The glial glutamate transporter GLT-1 is localized both in the vicinity of and at distance from axon terminals in the rat cerebral cortex

Glutamate transporter-1 (GLT-1) is responsible for the largest proportion of glutamate transport in the brain and the density of GLT-1 molecules inserted in the plasma membrane is highest in regions of high demand. Previous electron microscopic studies in the hippocampus and cerebellum have shown that GLT-1 is concentrated both in the vicinity of and at considerable distance from the synaptic cleft [Chaudry et al., Neuron 15 (1995) 711-721], but little is known about its distribution in the neocortex. We therefore studied the spatial relationships between elements expressing the presynaptic marker synaptophysin and those containing GLT-1 in the rat cerebral cortex using confocal microscopy. Preliminary studies confirmed that GLT-1 positive puncta were exclusively astrocytic processes; moreover, they showed that in most cases GLT-1 positive processes either completely surrounded asymmetric synapses or had no apparent relationship with synapses; occasionally, they were apposed to terminals containing pleomorphic vesicles. In sections double-labeled for GLT-1 and synaptophysin, codistribution analysis revealed that 61.2% of pixels detecting fluorescent emission for GLT-1 immunoreactivity overlapped with pixels detecting synaptophysin. The percentages of GLT-1/synaptophysin codistribution were significantly different from controls. In sections double-labeled for GLT-1 and the vesicular GABA transporter, codistribution analysis revealed that 27% of pixels detecting GLT-1 overlapped with those revealing the vesicular GABA transporter.The remarkable 'synaptic' localization of GLT-1 provides anatomical support for the hypothesis that in the cerebral cortex GLT-1 contributes to shaping fast, point-to-point, excitatory synaptic transmission. Moreover, the considerable fraction of GLT-1 immunoreactivity localized at sites distant from axon terminals supports the notion that glutamate spillout occurs also in the intact brain and suggests that 'extrasynaptic' GLT-1 regulates the diffusion of glutamate escaped from the cleft.

TrkB-like immunoreactivity is present on geniculocortical afferents in layer IV of kitten primary visual cortex

Exogenous administration of the neurotrophins brain-derived neurotrophic factor (BDNF) or neurotrophin-4/5 (NT-4/5), or blockade of their endogenous actions, have been reported to affect the anatomic organization and physiological responses of neurons in developing mammalian primary visual cortex. Experimental alteration of levels of these neurotrophic factors can also influence the morphology of the geniculocortical afferents that project from the lateral geniculate nucleus (LGN) to primary visual cortex. BDNF and NT-4/5 are ligands of the TrkB tyrosine kinase receptor. Although multiple populations of cortical neurons express TrkB, it is not known whether geniculocortical afferents express this receptor on their axon branches in visual cortex. We have anatomically labeled geniculocortical afferents of postnatal day 40 kittens with the anterograde neuronal tracer Phaseolus vulgaris leucoagglutinin (PHA-L) and performed double-label immunofluorescence with a panel of anti-TrkB antibodies. Confocal microscopy and object-based colocalization analysis were used to measure levels of TrkB-like immunoreactivity (IR) on geniculocortical afferents in layer IV of primary visual cortex. By using a conservative analysis involving a comparison of measured colocalization with the amount of colocalization expected based on random overlap of TrkB puncta and PHA-L--labeled afferents, 3 of 5 anti-TrkB antibodies tested showed significant colocalization with the geniculocortical axons. Results for the other two antibodies were indeterminate. The indices obtained for colocalization of TrkB and geniculocortical afferents were also compared with the equivalent index obtained for GAD65, a protein that has a similar overall expression pattern to that of TrkB but is not expressed on geniculocortical axons. This analysis indicated that TrkB was present on geniculocortical axons for all five TrkB antibodies tested. TrkB-like IR was also observed on neuronal somata in the LGN. These results indicate that TrkB receptors on geniculocortical afferents are potential mediators of the actions of BDNF and NT-4/5 in developing visual cortex.

Synaptic density in geniculocortical afferents remains constant after monocular deprivation in the cat

Monocular eyelid closure in cats during a critical period in development produces both physiological plasticity, as indicated by a loss of responsiveness of primary visual cortical neurons to deprived eye stimulation, and morphological plasticity, as demonstrated by a decrease in the total length of individual geniculocortical arbors representing the deprived eye. Although the physiological plasticity appears maximal after 2 d of monocular deprivation (MD), the shrinkage of deprived-eye geniculocortical arbors is less than half-maximal at 4 d and is not maximal until 7 d of deprivation, at which time the deprived arbors are approximately half their previous size. To study this form of plasticity at the level of individual thalamocortical synapses rather than arbors, we developed a new double-label colocalization technique. First, geniculocortical afferent arbors serving either the deprived or nondeprived eye were labeled by injection of the anterograde tracer Phaseolus vulgaris leucoagglutinin into lamina A of the lateral geniculate nucleus. Then, using antibodies to synaptic vesicle proteins, we identified presynaptic terminals within the labeled arbors in layer IV of the primary visual cortex. Analysis of serial optical sections obtained using confocal microscopy allowed measurement of the numerical density of presynaptic sites and the relative amounts of synaptic vesicle protein in geniculocortical afferents after both 2 and 7 d of MD. We found that the density of synapses in geniculocortical axons was similar for deprived and nondeprived afferents, suggesting that this feature of the afferents is conserved even during periods in which synapse number is reduced by half in deprived-eye arbors. These results are not consistent with the hypothesis that a rapid loss of deprived-eye geniculocortical presynaptic sites is responsible for the prompt physiological effects of MD.