D1 dopamine receptor immunoreactivity in human and monkey cerebral cortex: predominant and extrasynaptic localization in dendritic spines.

Antibodies to the D1 dopamine receptor were used to localize this protein in several areas of human and monkey cerebral cortex with light and electron microscopy. In addition to cell body labeling in monkeys, all areas of humans and monkeys had a neuropil label with a laminar distribution predicted by previous D1 receptor autoradiography studies. Using electron microscopy, this neuropil label was seen in numerous dendritic spines, in dendritic shafts, and in occasional axon terminals. While labeled spines were common, they represented only a subset of all cortical spines. Serial sectioning through labeled spines showed that the diaminobenzidine reaction product was usually not at postsynaptic densities but instead was displaced to the side of the large asymmetric (presumed glutamatergic) synapse. Furthermore, most labeled spines did not receive synapses with dopaminergic features, suggesting that many D1 receptors are at extrasynaptic sites, possibly receiving dopamine via diffusion in the neuropil. Similarly, double labeling failed to reveal D1 labeling at synapses of tyrosine hydroxylase immunoreactive axons. Localization to numerous dendritic spines suggests that a primary role of D1 receptors is modulation of glutamatergic input to cortical pyramidal cells.     

Fear and safety learning differentially affect synapse size and dendritic translation in the lateral amygdala

Fear learning is associated with changes in synapse strength in the lateral amygdala (LA). To examine changes in LA dendritic spine structure with learning, we used serial electron microscopy to re-construct dendrites after either fear or safety conditioning. The spine apparatus, a smooth endoplasmic reticulum (sER) specialization found in very large spines, appeared more frequently after fear conditioning. Fear conditioning was associated with larger synapses on spines that did not contain a spine apparatus, whereas safety conditioning resulted in smaller synapses on these spines. Synapses on spines with a spine apparatus were smaller after safety conditioning but unchanged with fear conditioning, suggesting a ceiling effect. There were more polyribosomes and multivesicular bodies throughout the dendrites from fear conditioned rats, indicating increases in both protein synthesis and degradation. Polyribosomes were associated with the spine apparatus under both training conditions. We conclude that LA synapse size changes bidirectionally with learning and that the spine apparatus has a central role in regulating synapse size and local translation.     

CD4+ T-cell synapses involve multiple distinct stages

One very striking feature of T-cell recognition is the formation of an immunological synapse between a T cell and a cell that it is recognizing. Formation of this complex structure correlates with cytotoxicity in the case of killer (largely CD8(+)) T-cell activity, or robust cytokine release and proliferation in the case of the much longer lived synapses formed by helper (CD4(+)) T cells. Here we have used electron microscopy and 3D tomography to characterize the synapses of antigen-specific CD4(+) T cells recognizing B cells and dendritic cells at different time points. We show that there are at least four distinct stages in synapse formation, proceeding over several hours, including an initial stage involving invasive T-cell pseudopodia that penetrate deeply into the antigen-presenting cell, almost to the nuclear envelope. This must involve considerable force and may serve to widen the search for potential ligands on the surface of the cell being recognized. We also show that centrioles and the Golgi complex are always located immediately beneath the synapse and that centrioles are significantly shifted toward the late contact zone with either B lymphocytes or bone marrow-derived dendritic cells such as antigen-presenting cells, and that there are dynamic, stage-dependent changes in the organization of microtubules beneath the synapse. These data reinforce and extend previous data on cytotoxic T cells that one of the principal functions of the immunological synapse is to facilitate cytokine secretion into the synaptic cleft, as well as provide important insights into the overall dynamics of this phenomenon.     

Neuronal nonlinearity explains greater visual spatial resolution for darks than lights

Astronomers and physicists noticed centuries ago that visual spatial resolution is higher for dark than light stimuli, but the neuronal mechanisms for this perceptual asymmetry remain unknown. Here we demonstrate that the asymmetry is caused by a neuronal nonlinearity in the early visual pathway. We show that neurons driven by darks (OFF neurons) increase their responses roughly linearly with luminance decrements, independent of the background luminance. However, neurons driven by lights (ON neurons) saturate their responses with small increases in luminance and need bright backgrounds to approach the linearity of OFF neurons. We show that, as a consequence of this difference in linearity, receptive fields are larger in ON than OFF thalamic neurons, and cortical neurons are more strongly driven by darks than lights at low spatial frequencies. This ON/OFF asymmetry in linearity could be demonstrated in the visual cortex of cats, monkeys, and humans and in the cat visual thalamus. Furthermore, in the cat visual thalamus, we show that the neuronal nonlinearity is present at the ON receptive field center of ON-center neurons and ON receptive field surround of OFF-center neurons, suggesting an origin at the level of the photoreceptor. These results demonstrate a fundamental difference in visual processing between ON and OFF channels and reveal a competitive advantage for OFF neurons over ON neurons at low spatial frequencies, which could be important during cortical development when retinal images are blurred by immature optics in infant eyes.Light and dark stimuli are separately processed by ON and OFF channels in the retina and visual thalamus. Surprisingly, although most textbooks assume that ON and OFF visual responses are balanced throughout the visual system, recent studies have identified a pronounced overrepresentation of the OFF visual responses in primary visual cortex (area V1) (1–3). This recent discovery resonates with pioneering studies by Galilei (4) and von Helmholtz (5) who noticed that visual spatial resolution was higher for dark than light stimuli. Galilei (4) related the difference in resolution to the observation that a light patch on a dark background appears larger than the same sized dark patch on a light background, an illusion that von Helmholtz (5) named the “irradiation illusion.” Although this illusion has been studied in the past (6, 7), its underlying neuronal mechanisms remain unknown. It has been suggested that the perceived size differences could be caused by the light scatter in the optics of the eye followed by a neuronal nonlinearity (6, 7), but there are no neuronal measurements of a nonlinearity that fits the explanation. Previous studies revealed differences in response linearity between ON and OFF retinal ganglion cells (8, 9) and horizontal cells (10). However, a main conclusion from these studies was that ON retinal ganglion cells were roughly linear and less rectified than OFF retinal ganglion cells (8, 9), which is exactly the opposite of what would be needed to explain the irradiation illusion. Moreover, it remains unclear if ON/OFF retinal differences in response linearity and response gain propagate from retina to visual cortex. To investigate the neuronal mechanisms of the irradiation illusion, we recorded neuronal activity in the visual thalamus and cortex of anesthetized cats, local field potentials in awake monkeys, and visually evoked potentials in humans. We show that OFF neurons in thalamus and cortex increase their responses roughly linearly with luminance contrast, independently of the background luminance. In contrast, ON neurons saturate their responses with small increases in luminance, and approach the linearity of the OFF neurons only on bright backgrounds that make ON responses weaker. We also show that a simple model that uses an early retinal nonlinearity can explain several seemingly unrelated ON/OFF spatial asymmetries, including the difference in spatial resolution between darks and lights, the spatial frequency dependence of OFF dominance in visual cortex, and the difference in receptive field size between ON and OFF retinal ganglion cells. Moreover, because the asymmetry between ON and OFF neurons is present both at the receptive field center and surround of thalamic neurons, our results strongly suggest that it originates at the level of photoreceptors.     

Estrogen increases synaptic connectivity between single presynaptic inputs and multiple postsynaptic CA1 pyramidal cells: a serial electron-microscopic study

Dendritic spines are sites of the vast majority of excitatory synaptic input to hippocampal CA1 pyramidal cells. Estrogen has been shown to increase the density of dendritic spines on CA1 pyramidal cell dendrites in adult female rats. In parallel with increased spine density, estrogen has been shown also to increase the number of spine synapses formed with multiple synapse boutons (MSBs). These findings suggest that estrogen-induced dendritic spines form synaptic contacts with preexisting presynaptic boutons, transforming some previously single synapse boutons (SSBs) into MSBs. The goal of the current study was to determine whether estrogen-induced MSBs form multiple synapses with the same or different postsynaptic cells. To quantify same-cell vs. different-cell MSBs, we filled individual CA1 pyramidal cells with biocytin and serially reconstructed dendrites and dendritic spines of the labeled cells, as well as presynaptic boutons in synaptic contact with labeled and unlabeled (i.e., different-cell) spines. We found that the overwhelming majority of MSBs in estrogen-treated animals form synapses with more than one postsynaptic cell. Thus, in addition to increasing the density of excitatory synaptic input to individual CA1 pyramidal cells, estrogen also increases the divergence of input from individual presynaptic boutons to multiple postsynaptic CA1 pyramidal cells. These findings suggest the formation of new synaptic connections between previously unconnected hippocampal neurons.     

Scene statistics and noise determine the relative arrangement of receptive field mosaics

Many sensory systems utilize parallel ON and OFF pathways that signal stimulus increments and decrements, respectively. These pathways consist of ensembles or grids of ON and OFF detectors spanning sensory space. Yet, encoding by opponent pathways raises a question: How should grids of ON and OFF detectors be arranged to optimally encode natural stimuli? We investigated this question using a model of the retina guided by efficient coding theory. Specifically, we optimized spatial receptive fields and contrast response functions to encode natural images given noise and constrained firing rates. We find that the optimal arrangement of ON and OFF receptive fields exhibits a transition between aligned and antialigned grids. The preferred phase depends on detector noise and the statistical structure of the natural stimuli. These results reveal that noise and stimulus statistics produce qualitative shifts in neural coding strategies and provide theoretical predictions for the configuration of opponent pathways in the nervous system.

Across many sensory systems, neurons encode information about either increments or decrements of stimuli in the environment, so-called ON and OFF signals. This division between ON and OFF signaling has been observed in visual (1, 2), thermosensory (3), auditory (4), olfactory (5), and electrosensory (6) systems. This organization has the advantage that neurons can be tasked with signaling increments or decrements in steady-state stimulus levels with fewer spikes, thereby resulting in more efficient neural codes (7, 8). Moreover, when the number of potential stimuli is large, neurons often specialize; for example, they only respond to a small region of visual space or a narrow auditory frequency band. The combination of these coding strategies raises two questions. First, how should a particular set of detectors, either the ON or OFF cells, arrange themselves most efficiently to cover stimulus space? Second, what is the optimal relative arrangement of ON and OFF detector grids? For one system, the retina, the answer to the first question is clear from previous work: Detectors of a particular type tile stimulus space and exhibit overlap near the 1-sigma boundary of a Gaussian receptive field (9–13). The answer to the second question, what might be called the “sensor alignment problem,” has received comparatively little attention and is the focus of this study.Conceptually, there are three general possibilities for how the sensor alignment problem could be solved. One possibility is that the grids of sensors are statistically independent, meaning the locations of receptive fields in one grid provide no information about the receptive field locations in the other grid. A second possibility is that the two grids are aligned, meaning the receptive field centers in one grid are closer than expected by chance. The third possibility is that the two grids are antialigned, meaning the receptive field centers in the two grids are further apart than expected by chance. On general information theory grounds, the optimal solution is likely to depend on noise in the encoding process and the statistics of the encoded stimuli (14, 15).While most anatomical studies of retinal mosaics indicate they are statistically independent (16–18, but see ref. 19), we have recently shown that grids of ON and OFF receptive field (henceforth called “mosaics”) formed by retinal ganglion cells (RGCs) are antialigned when those cells encode similar visual features (20). Here, we show how these results can be explained through the lens of efficient coding theory (7). This theory argues that sensory systems should aim to reduce the redundancy present in sensory input while minimizing metabolic costs, thereby reliably encoding natural stimuli with fewer spikes. Efficient coding theory has been successful at explaining many aspects of sensory processing and retinal physiology, including center-surround receptive fields, the formation of mosaics, and a greater proportion of OFF than ON cells (7, 11, 15, 21, 22). Thus, we asked whether efficient coding theory might predict the optimal spatial arrangement of ON and OFF receptive field mosaics within the retina. Our approach to this question involved optimizing a model that approximates the processing performed by many RGCs (21). By maximizing the mutual information between an (input) library of natural images and (output) spike rates, we examined the effects of image statistics and encoding noise on the optimal arrangement of ON and OFF mosaics.In this model, we found that the optimal spatial arrangement was a pair of approximately hexagonal mosaics of ON and OFF receptive fields. However, surprisingly, the relative alignment of these mosaics depended on the input noise, output noise, and the statistics of the natural image set. When output noise was low, the mosaics were aligned, with ON and OFF receptive fields centered at nearby locations more often than expected by chance. When output noise was relatively high, antialignment became the favored arrangement. Surprisingly, the content of the image set also strongly influenced the transition between aligned and antialigned mosaics. In particular, when image sets contained more “outlier” images with particularly large luminance or contrast values, antialignment became the favored state for fixed input and output noise. We demonstrate analytically and confirm computationally that as noise parameters or stimulus statistics vary, mutual information changes smoothly, while the optimal mosaic arrangements undergo a sudden, qualitative shift. Finally, we confirm these predictions by showing that systematic manipulations of the training dataset change the phase boundary in a manner predicted by an analytical model. These findings underscore the crucial role played by both noise and the statistics of natural stimuli for understanding specialization and coordination in sensory processing.     

Dendritic spikes mediate negative synaptic gain control in cerebellar Purkinje cells

Dendritic spikes appear to be a ubiquitous feature of dendritic excitability. In cortical pyramidal neurons, dendritic spikes increase the efficacy of distal synapses, providing additional inward current to enhance axonal action potential (AP) output, thus increasing synaptic gain. In cerebellar Purkinje cells, dendritic spikes can trigger synaptic plasticity, but their influence on axonal output is not well understood. We have used simultaneous somatic and dendritic patch-clamp recordings to directly assess the impact of dendritic calcium spikes on axonal AP output of Purkinje cells. Dendritic spikes evoked by parallel fiber input triggered brief bursts of somatic APs, followed by pauses in spiking, which cancelled out the extra spikes in the burst. As a result, average output firing rates during trains of input remained independent of the input strength, thus flattening synaptic gain. We demonstrate that this "clamping" of AP output by the pause following dendritic spikes is due to activation of high conductance calcium-dependent potassium channels by dendritic spikes. Dendritic spikes in Purkinje cells, in contrast to pyramidal cells, thus have differential effects on temporally coded and rate coded information: increasing the impact of transient parallel fiber input, while depressing synaptic gain for sustained parallel fiber inputs.     

Spontaneous and evoked synaptic rewiring in the neonatal neocortex

The local microcircuitry of the neocortex is structurally a tabula rasa, with the axon of each pyramidal neuron having numerous submicrometer appositions with the dendrites of all neighboring pyramidal neurons, but is functionally highly selective, with synapses formed onto only a small proportion of these targets. This design leaves a vast potential for the microcircuit to rewire without extensive axonal or dendritic growth. To examine whether rewiring does take place, we used multineuron patch-clamp recordings on 12- to 14-day-old rat neocortical slices and studied long-term changes in synaptic connectivity within clusters of neurons. We found pyramidal neurons spontaneously connecting and disconnecting from each other and that exciting the slice with glutamate greatly increases the number of new connections established. Evoked emergence of new synaptic connections requires action potential activity and activation of metabotropic glutamate receptor 5, but not NMDA receptor or group II or group III metabotropic glutamate receptor activation. We also found that it is the weaker connections that are selectively eliminated. These results provide direct evidence for spontaneous and evoked rewiring of the neocortical microcircuitry involving entire functional multisynaptic connections. We speculate that this form of microcircuit plasticity enables an evolution of the microcircuit connectivity by natural selection as a function of experience.     

HIV-1 activates Cdc42 and induces membrane extensions in immature dendritic cells to facilitate cell-to-cell virus propagation

HIV-1 cell-to-cell transmission confers a strong advantage as it increases efficiency of transfer up to 100-fold compared with a cell-free route. Mechanisms of HIV-1 cell-to-cell transmission are still unclear and can in part be explained by the presence of actin-containing cellular protrusions. Such protrusions have been shown to facilitate cell-to-cell viral dissemination. Using fluorescence microscopy, electron tomography, and ion abrasion scanning electron microscopy we show that HIV-1 induces membrane extensions in immature dendritic cells through activation of Cdc42. We demonstrate that these extensions are induced after engagement of DC-SIGN by HIV-1(env) via a cascade that involves Src kinases, Cdc42, Pak1, and Wasp. Silencing of Cdc42 or treatment with a specific Cdc42 inhibitor, Secramine A, dramatically reduced the number of membrane protrusions visualized on the cell surface and decreased HIV-1 transfer via infectious synapses. Ion abrasion scanning electron microscopy of cell-cell contact regions showed that cellular extensions from immature dendritic cells that have the appearance of thin filopodia in thin section images are indeed extended membranous sheets with a narrow cross section. Our results demonstrate that HIV-1 binding on immature dendritic cells enhances the formation of membrane extensions that facilitate HIV-1 transfer to CD4(+) T lymphocytes.     

AMPA receptors regulate experience-dependent dendritic arbor growth in vivo

The size and shape of neuronal dendritic arbors affect the number and type of synaptic inputs, as well as the complexity and function of brain circuits. In the intact brain, dendritic arbor growth and the development of excitatory glutamatergic synapse are concurrent. Consequently, it has been difficult to resolve whether synaptic inputs drive dendritic arbor development. Here, we test the role of AMPA receptor (AMPAR)-mediated glutamatergic transmission in dendrite growth by expressing peptides corresponding to the intracellular C-terminal domains of AMPAR subunits GluR1 (GluR1Ct) and GluR2 (GluR2Ct) in optic tectal neurons of the Xenopus retinotectal system. These peptides significantly reduce AMPAR synaptic transmission in transfected neurons while leaving visual system circuitry intact. Daily in vivo imaging over 5 days revealed that GluR1Ct or GluR2Ct expression dramatically impaired dendrite growth, resulting in less complex arbors than controls. Time-lapse images collected at 2-h intervals over 6 h show that both GluR1Ct and GluR2Ct decrease branch lifetimes. Ultrastructural analysis indicates that synapses formed onto neurons expressing the GluRCt are less mature than synapses onto control neurons. These data suggest that the failure to form complex arbors is due to reduced stabilization of new synapses and dendritic branches. Although visual stimulation increases dendritic arbor growth rates in control tectal neurons, a weak postsynaptic response to visual experience in GluRCt-expressing cells leads to retraction of branches. These results indicate that AMPAR-mediated transmission underlies experience-dependent dendritic arbor growth by stabilizing branches, and support a competition-based model for dendrite growth.     

From the Cover: 3D visualization of HIV transfer at the virological synapse between dendritic cells and T cells

The efficiency of HIV infection is greatly enhanced when the virus is delivered at conjugates between CD4⁺ T cells and virus-bearing antigen-presenting cells such as macrophages or dendritic cells via specialized structures known as virological synapses. Using ion abrasion SEM, electron tomography, and superresolution light microscopy, we have analyzed the spatial architecture of cell-cell contacts and distribution of HIV virions at virological synapses formed between mature dendritic cells and T cells. We demonstrate the striking envelopment of T cells by sheet-like membrane extensions derived from mature dendritic cells, resulting in a shielded region for formation of virological synapses. Within the synapse, filopodial extensions emanating from CD4⁺ T cells make contact with HIV virions sequestered deep within a 3D network of surface-accessible compartments in the dendritic cell. Viruses are detected at the membrane surfaces of both dendritic cells and T cells, but virions are not released passively at the synapse; instead, virus transfer requires the engagement of T-cell CD4 receptors. The relative seclusion of T cells from the extracellular milieu, the burial of the site of HIV transfer, and the receptor-dependent initiation of virion transfer by T cells highlight unique aspects of cell-cell HIV transmission.     

A stable proportion of Purkinje cell inputs from parallel fibers are silent during cerebellar maturation

Cerebellar Purkinje neurons integrate information transmitted at excitatory synapses formed by granule cells. Although these synapses are considered essential sites for learning, most of them appear not to transmit any detectable electrical information and have been defined as silent. It has been proposed that silent synapses are required to maximize information storage capacity and ensure its reliability, and hence to optimize cerebellar operation. Such optimization is expected to occur once the cerebellar circuitry is in place, during its maturation and the natural and steady improvement of animal agility. We therefore investigated whether the proportion of silent synapses varies over this period, from the third to the sixth postnatal week in mice. Selective expression of a calcium indicator in granule cells enabled quantitative mapping of presynaptic activity, while postsynaptic responses were recorded by patch clamp in acute slices. Through this approach and the assessment of two anatomical features (the distance that separates adjacent planar Purkinje dendritic trees and the synapse density), we determined the average excitatory postsynaptic potential per synapse. Its value was four to eight times smaller than responses from paired recorded detectable connections, consistent with over 70% of synapses being silent. These figures remained remarkably stable across maturation stages. According to the proposed role for silent synapses, our results suggest that information storage capacity and reliability are optimized early during cerebellar maturation. Alternatively, silent synapses may have roles other than adjusting the information storage capacity and reliability.

Typical central excitatory synapses are formed onto dendritic spines, the distinctive morphology of which enables their unambiguous identification (1–3). It has generally been assumed that the presence of a dendritic spine equates to the existence of a functional excitatory transmission (4). Based on this assumption, the observation of spine motility in several brain areas (motor cortex and somatosensory cortex) has been considered to reflect synaptic plasticity (5). Indeed, a number of studies have established a correlation between learning and spine formation (6–8) or pruning (9, 10). However, in some conditions, morphological and synaptic plasticity have been shown to be dissociated (11), in line with the view that morphology does not provide all the information necessary to infer synaptic function.The cerebellum contains the majority of brain neurons (12, 13) and the predominant excitatory synapses found in this structure connect granule cells (GC) to Purkinje cells (PC). These synapses are formed on typical spines borne by PC dendrites (14), the majestic shape of which is likely related to the huge amount of independent inputs they receive. The GC-to-PC synapse is generally acknowledged to be an essential site for plasticity (15–19). However, in sharp contrast to other parts of the brain such as motor and somatosensory cortices, PC spines appear to be constitutive (20, 21), i.e., they appear to be an inherent property of PCs, independent of external factors. Indeed, pruning of these synapses has not been reported. Novel spine formation has been reported, but likely as a result of dendritic tree expansion (22, 23). The high density of spines along PC dendritic branchlets (5 to 17 per linear micrometer in rat; refs. 14 and 24–27) and their regular ordering in a helical pattern (28) support the idea that they optimize space occupancy with little room for spine addition, in accordance with their constitutive nature.The apparent morphological homogeneity of PC spines is in sharp contrast with the spectacular heterogeneity observed in the strength of GC-to-PC synapses. An in vivo study has reported that the receptive field of a PC was much smaller than that of the GCs putatively connecting to it (29), suggesting that most GC-to-PC synapses are electrically silent. This has been confirmed by an in vitro report (30) showing that synaptic transmission between paired-recorded GC and PC was detected nearly 10 times less frequently than expected from the occurrence of morphologically defined synaptic connections predicted by anatomical data (14, 31–33). Taken together, these two studies conducted in adult rats suggest that most (85% according to ref. 30) morphologically and molecularly defined GC-to-PC synapses are silent, i.e., they do not transmit any detectable electrical signal.If silent synapses do not transmit information, what is their role? Are they a reserve for additional information storage? Or do they result from information storage optimization (34)? According to this latter proposal (17, 34), since the requirement for optimized information storage is more and more critical as the amount of learned information increases, one might expect that the proportion of silent synapses increases with the amount learning. As previously suggested (34), this hypothesis could be tested by comparing the proportion of silent synapses in young versus adult animals. Indeed, the mouse cerebellar circuitry is not fully in place until the third postnatal week. Then, for at least 3 wk, the mouse acquires basic skills (eating, walking, and social interactions), adapts to changes in muscle strengths and sensitivity to stimuli, and improves its agility (35). Although the amount of cerebellar learning occurring over this maturation period is unclear, it can be reasonably assumed that cerebellar operation continuously optimizes. Here, we investigate how the proportion of silent synapses changes over this period of maturation.We determine the proportion of silent GC-to-PC synapses by a method based on the determination of the average postsynaptic response per activated synapse (average synaptic weight, w¯) in superfused acute slices. Thanks to the geometrical and repetitive architecture of the cerebellar cortex, calcium imaging is used to quantitatively map GC inputs. This mapping, combined with postsynaptic recording of transmission, and the determination of two cerebellar anatomical features (the average synapse density and spacing between PC planar dendritic trees) enables the determination of w¯. By comparison with the properties of synapses that produce an electrical postsynaptic response (investigated by paired recording and quantal analysis), we show that the proportion of silent synapses is higher than 70% and stable between the postnatal stages of interest. This suggests that cerebellar maturation has insignificant impact on the proportion of silent synapses.     

Structural changes in AMPA-receptive neurons in the nucleus of the solitary tract of spontaneously hypertensive rats

The baroreceptor reflex is critical for homeostatic regulation of blood pressure and is initiated centrally by glutamate release from baroreceptive afferents onto neurons in the nucleus of the solitary tract that activates AMPA-type glutamate receptors. The GluR1 subunit of the AMPA receptor is located at postsynaptic sites within the nucleus of the solitary tract, particularly in dendritic spines, which are important sites for synaptic plasticity. We tested whether the distribution of GluR1 changes after sustained hypertension, which alters baroreceptor afferent activity. We examined the distribution of GluR1 in the nucleus of the solitary tract of both hypertensive (spontaneously hypertensive) and normotensive (Wistar-Kyoto) rats at the light microscopic and electron microscopic levels. There were more GluR1-containing dendritic spines in the nucleus of the solitary tract of hypertensive rats compared with normotensive rats, which was attributable to an increase in the proportion of dendritic spines containing GluR1 as well as an increase in the total number of dendritic spines. The differences were only seen after the development of hypertension and were not seen in rostral regions of the nucleus of the solitary tract. In the spontaneously hypertensive rat, many synapses on GluR1-containing dendrites had the morphological features of synapses undergoing dynamic changes, including the presence of perforated synapses. These results suggest that changes in afferent activity to the nucleus of the solitary tract during sustained hypertension alter both the dendritic structure and AMPA receptor content of some neurons. These structural changes may be a substrate for central resetting of the baroreceptor reflex.     

CD2 engagement induces dendritic cell activation: implications for immune surveillance and T-cell activation

We have shown previously that primary dendritic cells and monocytes express equal levels of CD14 but are distinguishable by the presence of CD2 on dendritic cells. CD2 is known to mediate the activation of T and natural killer (NK) cells through its interaction with CD58. CD2 epitopes recognized by anti-T111, -T112, and -T113 monoclonal antibodies (mAbs) are present on dendritic cells. Here we show that CD2 engagement significantly increases class II, costimulatory (CD40, CD80, CD86), adhesion (CD54, CD58), and CCR7 molecule expression on primary dendritic cells. Conversely, minimal or no change in the expression of the above antigens occurs on monocyte-derived dendritic cells, because these molecules are already maximally expressed. However, both kinds of dendritic cells release interleukin-1beta (IL-1beta) and IL-12 after CD2 engagement. Lastly, interference with dendritic cell CD2-T-cell CD58 engagement decreases naive CD4+CD45RA+ T-cell proliferation. Collectively, our results suggest another role of the CD2-CD58 pathway that allows nonimmune and immune cells to interact directly with dendritic cells and initiate innate and adaptive immune responses.     

Glycolipid alpha-C-galactosylceramide is a distinct inducer of dendritic cell function during innate and adaptive immune responses of mice

alpha-Galactosylceramide (alpha-GalCer) is the prototype compound for studying the presentation of glycolipids on CD1d molecules to natural killer T (NKT) lymphocytes. A single i.v. dose of glycolipid triggers a cascade of events involving the production of several cytokines over the course of a day, a short-lived activation of NKT and natural killer (NK) cells, and a more prolonged adaptive T cell immune response if certain antigens are given together with alpha-GalCer. We find that a recently described analogue, alpha-C-galactosylceramide (alpha-C-GalCer), more potently induces these innate and adaptive immune responses in mice. alpha-C-GalCer acts as a more effective trigger for IL-12 and IFN-gamma production, although it minimally elicits IL-4 and TNF-alpha release into the serum. Also, alpha-C-GalCer better mobilizes NKT and natural killer cells to resist B16 melanoma. To help understand these effects, we find that alpha-C-GalCer binds more stably to dendritic cells than alpha-GalCer and that dendritic cells loaded with alpha-C-GalCer induce larger and more long lasting NKT cell responses in vivo. When glycolipid is targeted to dendritic cells in spleen together with antigens in dying cells, such as irradiated tumor cells, alpha-C-GalCer is active as an adjuvant for T cell-mediated immunity at lower doses, just 20 ng per mouse, where it is also able to up-regulate the required CD40L costimulatory molecule on NKT cells. Therefore, alpha-C-GalCer represents a glycolipid that binds more stably to dendritic cells and acts as a more effective link between innate and adaptive immunity in vivo.     

Quantitative fine-structural analysis of olfactory cortical synapses

To determine the extent to which hippocampal synapses are typical of those found in other cortical regions, we have carried out a quantitative analysis of olfactory cortical excitatory synapses, reconstructed from serial electron micrograph sections of mouse brain, and have compared these new observations with previously obtained data from hippocampus. Both superficial and deep layer I olfactory cortical synapses were studied. Although individual synapses in each of the areas-CA1 hippocampus, olfactory cortical layer Ia, olfactory cortical area Ib-might plausibly have been found in any of the other areas, the average characteristics of the three synapse populations are distinct. Olfactory cortical synapses in both layers are, on average, about 2.5 times larger than their hippocampal counterparts. The layer Ia olfactory cortical synapses have fewer synaptic vesicles than do the layer Ib synapses, but the absolute number of vesicles docked to the active zone in the layer Ia olfactory cortical synapses is about equal to the docked vesicle number in the smaller hippocampal synapses. As would be predicted from studies on hippocampus that relate paired-pulse facilitation to the number of docked vesicles, the synapses in layer 1a exhibit facilitation, whereas the ones in layer 1b do not. Although hippocampal synapses provide as a good model system for central synapses in general, we conclude that significant differences in the average structure of synapses from one cortical region to another exist, and this means that generalizations based on a single synapse type must be made with caution.     

Protein kinase C mediates transient spinule-type neurite outgrowth in the retina during light adaptation.

Light and dark adaptation of the teleost retina is accompanied by a remarkable morphological rearrangement of the synaptic connections between photoreceptors and second-order neurons: during light adaptation, numerous new neurites, the so-called spinules, arise from the terminal dendrites of horizontal cells invaginating the cone pedicle, and during dark adaptation, these spinules are retracted. The formation of these spinules is paralleled by the appearance of color opponency in horizontal and ganglion cells, which led to the suggestion that these spinules are the site of the inhibitory synapses in the negative feedback loop between cones and horizontal cells. The formation of the spinules in the light and their disappearance in darkness have a time course of minutes and are modulated by the neurotransmitters dopamine and glutamate, respectively. Neurotransmitters can modulate neuronal processing through a variety of second messengers that activate protein kinases, resulting most commonly in protein phosphorylation. Herein we report that activation of protein kinase C by phorbol esters promotes the formation of new horizontal-cell spinules in animals kept in the dark. Partial inhibition of protein kinase C activation with sphingosines prevents the formation of new spinules during light adaptation but does not affect established spinules. The spinule-forming effect of phorbol esters is not mediated by dopaminergic neurons, since the effect is also seen in retinas depleted of dopaminergic neurons. Phorbol esters also initiate the formation of spinules in synaptically isolated horizontal cells, demonstrating that they have a direct action on these cells. In addition, isolated horizontal cells have substrate proteins that are phosphorylated in a protein kinase C-dependent manner.     

Electrical synapses in retinal ON cone bipolar cells: subtype-specific expression of connexins

Retinal bipolar cells are known to form a complex, interconnecting network through electrical synapses that are either heterologous (with amacrine cells) or homologous (with other bipolar cells). These electrical synapses can be functionally as important as chemical synapses because their distinct properties provide a different character for the network. Much less is known, however, about electrical synapses in retinal bipolar cells than about chemical synapses. Here we report the molecular basis for electrical synapses in retinal bipolar cells, particularly ON cone bipolar cells. We have found variable connexin 36 (cx36) expression in different types of ON cone bipolar cells: cx36 message was found in some, but not all, ON cone bipolar cells (4 of 14 cells). In one specific type of ON cone bipolar cell (BPGus-GFP), however, cx36 was detected in 17 of 19 cells. Moreover, we have located cx36 puncta at the axonal terminals of BPGus-GFP cells, and we have found that these BPGus-GFP-associated cx36 puncta always colocalized with AII amacrine cell processes. Molecular and immunocytochemical evidence obtained in this study also shows that connexin 45 (cx45) is not present in BPGus-GFP cells. Taken together, our results suggest that connexins are expressed in bipolar cells in a neuronal subtype-specific manner and that cx36/cx36 gap junctions form the heterologous electrical synapses between AII amacrine cells and BPGus-GFP cells. Our findings imply that visual information can be differently processed by distinct subtypes of ON cone bipolar cells via electrical synapses.     

Defective killing of dendritic cells by autologous natural killer cells from acute myeloid leukemia patients

At the frontier between innate and adaptive immunity, dendritic cells (DCs) secrete numerous cytokines and express costimulatory molecules that initiate or enhance natural killer (NK) and T-lymphocyte responses. NK cells also regulate DC physiology by killing immature DCs (iDCs), thus limiting inflammation and inappropriate T-lymphocyte tolerization. In a previous study, we have reported that NK cells from acute myeloid leukemia patients (AML-NK cells) have deficient natural cytotoxicity receptor (NCR) expression. Herein, we analyzed the consequences of such a defect regarding the regulatory role of AML-NK cells in DC physiology. We show that NK cells display poor cytolytic capacities against DCs derived from healthy donor monocytes or derived from autologous leukemic blasts. These data point to a novel defect in the regulation of adaptive immune responses initiated by DCs in AML patients. This may lead to specific T-lymphocyte tolerization by spontaneous or ex vivo expanded iDCs expressing leukemia-derived antigens.     

INAUGURAL ARTICLE by a Recently Elected Academy Member:Parallel information processing channels created in the retina

In the retina, several parallel channels originate that extract different attributes from the visual scene. This review describes how these channels arise and what their functions are. Following the introduction four sections deal with these channels. The first discusses the “ON” and “OFF” channels that have arisen for the purpose of rapidly processing images in the visual scene that become visible by virtue of either light increment or light decrement; the ON channel processes images that become visible by virtue of light increment and the OFF channel processes images that become visible by virtue of light decrement. The second section examines the midget and parasol channels. The midget channel processes fine detail, wavelength information, and stereoscopic depth cues; the parasol channel plays a central role in processing motion and flicker as well as motion parallax cues for depth perception. Both these channels have ON and OFF subdivisions. The third section describes the accessory optic system that receives input from the retinal ganglion cells of Dogiel; these cells play a central role, in concert with the vestibular system, in stabilizing images on the retina to prevent the blurring of images that would otherwise occur when an organism is in motion. The last section provides a brief overview of several additional channels that originate in the retina.