The neuronal endoplasmic reticulum: its cytochemistry and contribution to the endomembrane system. I. Cell bodies and dendrites

Electrical signals in the ER are cell type and stimulus specific with extreme spatial compartmentalization in neurons

The endoplasmic reticulum (ER) is a tortuous organelle that spans throughout a cell with a continuous membrane containing ion channels, pumps, and transporters. It is unclear if stimuli that gate ER ion channels trigger substantial membrane potential fluctuations and if those fluctuations spread beyond their site of origin. Here, we visualize ER membrane potential dynamics in HEK cells and cultured rat hippocampal neurons by targeting a genetically encoded voltage indicator specifically to the ER membrane. We report the existence of clear cell-type- and stimulus-specific ER membrane potential fluctuations. In neurons, direct stimulation of ER ryanodine receptors generates depolarizations that scale linearly with stimulus strength and reach tens of millivolts. However, ER potentials do not spread beyond the site of receptor activation, exhibiting steep attenuation that is exacerbated by intracellular large conductance K+ channels. Thus, segments of ER can generate large depolarizations that are actively restricted from impacting nearby, contiguous membrane.

Morphological Heterogeneity of the Endoplasmic Reticulum within Neurons and Its Implications in Neurodegeneration

The endoplasmic reticulum (ER) is a multipurpose organelle comprising dynamic structural subdomains, such as ER sheets and tubules, serving to maintain protein, calcium, and lipid homeostasis. In neurons, the single ER is compartmentalized with a careful segregation of the structural subdomains in somatic and neurite (axodendritic) regions. The distribution and arrangement of these ER subdomains varies between different neuronal types. Mutations in ER membrane shaping proteins and morphological changes in the ER are associated with various neurodegenerative diseases implying significance of ER morphology in maintaining neuronal integrity. Specific neurons, such as the highly arborized dopaminergic neurons, are prone to stress and neurodegeneration. Differences in morphology and functionality of ER between the neurons may account for their varied sensitivity to stress and neurodegenerative changes. In this review, we explore the neuronal ER and discuss its distinct morphological attributes and specific functions. We hypothesize that morphological heterogeneity of the ER in neurons is an important factor that accounts for their selective susceptibility to neurodegeneration.

STED and parallelized RESOLFT optical nanoscopy of the tubular endoplasmic reticulum and its mitochondrial contacts in neuronal cells

The classic view of organelle cell biology is undergoing a constant revision fueled by the new insights unraveled by fluorescence nanoscopy, which enable sensitive, faster and gentler observation of specific proteins in situ. The endoplasmic reticulum (ER) is one of the most challenging structure to capture due the rapid and constant restructuring of fine sheets and tubules across the full 3D cell volume. Here we apply STED and parallelized 2D and 3D RESOLFT live imaging to uncover the tubular ER organization in the fine processes of neuronal cells with focus on mitochondria-ER contacts, which recently gained medical attention due to their role in neurodegeneration. Multi-color STED nanoscopy enables the simultaneous visualization of small transversal ER tubules crossing and constricting mitochondria all along axons and dendrites. Parallelized RESOLFT allows for dynamic studies of multiple contact sites within seconds and minutes with prolonged time-lapse imaging at ~50 nm spatial resolution. When operated in 3D super resolution mode it enables a new isotropic visualization of such contacts extending our understanding of the three-dimensional architecture of these packed structures in axons and dendrites.

The moonlighting protein c-Fos activates lipid synthesis in neurons, an activity that is critical for cellular differentiation and cortical development

Differentiation of neuronal cells is crucial for the development and function of the nervous system. This process involves high rates of membrane expansion, during which the synthesis of membrane lipids must be tightly regulated. In this work, using a variety of molecular and biochemical assays and approaches, including immunofluorescence microscopy and FRET analyses, we demonstrate that the proto-oncogene c-Fos (c-Fos) activates cytoplasmic lipid synthesis in the central nervous system and thereby supports neuronal differentiation. Specifically, in hippocampal primary cultures, blocking c-Fos expression or its activity impairs neuronal differentiation. When examining its subcellular localization, we found that c-Fos co-localizes with endoplasmic reticulum markers and strongly interacts with lipid-synthesizing enzymes, whose activities were markedly increased in vitro in the presence of recombinant c-Fos. Of note, the expression of c-Fos dominant-negative variants capable of blocking its lipid synthesis-activating activity impaired neuronal differentiation. Moreover, using an in utero electroporation model, we observed that neurons with blocked c-Fos expression or lacking its AP-1-independent activity fail to initiate cortical development. These results highlight the importance of c-Fos-mediated activation of lipid synthesis for proper nervous system development.

NeurodegenERation: The Central Role for ER Contacts in Neuronal Function and Axonopathy, Lessons From Hereditary Spastic Paraplegias and Related Diseases

The hereditary spastic paraplegias (HSPs) are a group of inherited neurodegenerative conditions whose characteristic feature is degeneration of the longest axons within the corticospinal tract which leads to progressive spasticity and weakness of the lower limbs. Though highly genetically heterogeneous, the majority of HSP cases are caused by mutations in genes encoding proteins that are responsible for generating and organizing the tubular endoplasmic reticulum (ER). Despite this, the role of the ER within neurons, particularly the long axons affected in HSP, is not well understood. Throughout axons, ER tubules make extensive contacts with other organelles, the cytoskeleton and the plasma membrane. At these ER contacts, protein complexes work in concert to perform specialized functions including organelle shaping, calcium homeostasis and lipid biogenesis, all of which are vital for neuronal survival and may be disrupted by HSP-causing mutations. In this article we summarize the proteins which mediate ER contacts, review the functions these contacts are known to carry out within neurons, and discuss the potential contribution of disruption of ER contacts to axonopathy in HSP.

Models of Intracellular Transport: Pros and Cons

Intracellular transport is one of the most confusing issues in the field of cell biology. Many different models and their combinations have been proposed to explain the experimental data on intracellular transport. Here, we analyse the data related to the mechanisms of endoplasmic reticulum-to-Golgi and intra-Golgi transport from the point of view of the main models of intracellular transport; namely: the vesicular model, the diffusion model, the compartment maturation–progression model, and the kiss-and-run model. This review initially describes our current understanding of Golgi function, while highlighting the recent progress that has been made. It then continues to discuss the outstanding questions and potential avenues for future research with regard to the models of these transport steps. To compare the power of these models, we have applied the method proposed by K. Popper; namely, the formulation of prohibitive observations according to, and the consecutive evaluation of, previous data, on the basis on the new models. The levels to which the different models can explain the experimental observations are different, and to date, the most powerful has been the kiss-and-run model, whereas the least powerful has been the diffusion model.

Proteome analysis reveals a strong correlation between olfaction and pollen foraging preference in honeybees

To gain a deeper understanding of the molecular basis of pollen foraging preference, we characterized the proteomes of antennae and brains of bees foraging on pear and rapeseed flowers, and the volatile compounds from nectar, anther, and inflorescence of both plants. Bees foraging on the pollen of the two plants have shaped the distinct proteome arsenals in the antenna and brain to drive olfactory and brain function. In antennae, bees foraging on pear (PA) pollen pathways associated with protein metabolism were induced to synthesize new proteins for modulation of synaptic structures via stabilizing and consolidating specific memory traces. Whereas, bees foraging on rapeseed (BA) pollen pathways implicated in energy metabolism were activated to provide metabolic fuels critical for neural activity. These findings suggest that the distinct biochemical route is functionally enhanced to consolidate the divergent olfaction in PA and BA. In brain, although the uniquely induced pathways in bees forging on both plants are likely to cement selective roles in learning and memory, pollen foraging preference in bees is mainly drived by olfaction. Furthermore, both plants have shaped different repertoires of signal odors and food rewards to attract pollinators. The suggested markers are potentially useful for selection of bees to improve their olfaction for better pollination of the plants.

Axonal Activation of the Unfolded Protein Response Promotes Axonal Regeneration Following Peripheral Nerve Injury

Adult mammalian peripheral neurons have an intrinsic regrowth capacity in response to axonal injury. The induction of calcium ion (Ca²⁺) oscillations at an injured site is critical for the regulation of regenerative responses. In polarized neurons, distal axonal segments contain a well-developed endoplasmic reticulum (ER) network that is responsible for Ca²⁺ homeostasis. Although these characteristics implicate the relevance among injury-induced Ca²⁺ dynamics, axonal ER-derived signaling, and regenerative responses propagated along the axons, the details are not fully understood. In the present study, we found that Ca²⁺ release from the axonal ER was accelerated in response to injury. Additionally, axonal injury-dependent Ca²⁺ release from the ER activated unfolded protein response (UPR) signaling at injured sites. Inhibition of axonal UPR signaling led to fragmentation of the axonal ER and disrupted growth cone formation, suggesting that activation of axonal UPR branches following axonal injury promotes regeneration via regulation of ER reconstruction and formation of growth cones. Our studies revealed that local activation of axonal UPR signaling by injury-induced Ca²⁺ release from the ER is critical for regeneration. These findings provide a new concept for the link between injury-induced signaling at a distant location and regulation of organelle and cytoskeletal formation in the orchestration of axonal regeneration.

Targeting Cellular Stress Mechanisms and Metabolic Homeostasis by Chinese Herbal Drugs for Neuroprotection

Traditional Chinese medicine has been practiced for centuries in East Asia. Herbs are used to maintain health and cure disease. Certain Chinese herbs are known to protect and improve the brain, memory, and nervous system. To apply ancient knowledge to modern science, some major natural therapeutic compounds in herbs were extracted and evaluated in recent decades. Emerging studies have shown that herbal compounds have neuroprotective effects or can ameliorate neurodegenerative diseases. To understand the mechanisms of herbal compounds that protect against neurodegenerative diseases, we summarize studies that discovered neuroprotection by herbal compounds and compound-related mechanisms in neurodegenerative disease models. Those compounds discussed herein show neuroprotection through different mechanisms, such as cytokine regulation, autophagy, endoplasmic reticulum (ER) stress, glucose metabolism, and synaptic function. The interleukin (IL)-1β and tumor necrosis factor (TNF)-α signaling pathways are inhibited by some compounds, thus attenuating the inflammatory response and protecting neurons from cell death. As to autophagy regulation, herbal compounds show opposite regulatory effects in different neurodegenerative models. Herbal compounds that inhibit ER stress prevent neuronal death in neurodegenerative diseases. Moreover, there are compounds that protect against neuronal death by affecting glucose metabolism and synaptic function. Since the progression of neurodegenerative diseases is complicated, and compound-related mechanisms for neuroprotection differ, therapeutic strategies may need to involve multiple compounds and consider the type and stage of neurodegenerative diseases.

Neuronal activity-dependent local activation of dendritic unfolded protein response promotes expression of brain-derived neurotrophic factor in cell soma

Unfolded protein response (UPR) has roles not only in resolving the accumulation of unfolded proteins owing to endoplasmic reticulum (ER) stress, but also in regulation of cellular physiological functions. ER stress transducers providing the branches of UPR signaling are known to localize in distal dendritic ER of neurons. These reports suggest that local activation of UPR branches may produce integrated outputs for distant communication, and allow regulation of local events in highly polarized neurons. Here, we demonstrated that synaptic activity‐ and brain‐derived neurotrophic factor (BDNF)‐dependent local activation of UPR signaling could be associated with dendritic functions through retrograde signal propagation by using murine neuroblastoma cell line, Neuro‐2A and primary cultured hippocampal neurons derived from postnatal day 0 litter C57BL/6 mice. ER stress transducer, inositol‐requiring kinase 1 (IRE1), was activated at postsynapses in response to excitatory synaptic activation. Activated dendritic IRE1 accelerated accumulation of the downstream transcription factor, x‐box‐binding protein 1 (XBP1), in the nucleus. Interestingly, excitatory synaptic activation‐dependent up‐regulation of XBP1 directly facilitated transcriptional activation of BDNF. BDNF in turn drove its own expression via IRE1‐XBP1 pathway in a protein kinase A‐dependent manner. Exogenous treatment with BDNF promoted extension and branching of dendrites through the protein kinase A‐IRE1‐XBP1 cascade. Taken together, our findings indicate novel mechanisms for communication between soma and distal sites of polarized neurons that are coordinated by local activation of IRE1‐XBP1 signaling. Synaptic activity‐ and BDNF‐dependent distinct activation of dendritic IRE1‐XBP1 cascade drives BDNF expression in cell soma and may be involved in dendritic extension.

Cover Image for this issue: doi. 10.1111/jnc.14159.

Neuronal activity-dependent local activation of dendritic unfolded protein response promotes expression of brain-derived neurotrophic factor in cell soma

Unfolded protein response (UPR) has roles not only in resolving the accumulation of unfolded proteins owing to endoplasmic reticulum (ER) stress, but also in regulation of cellular physiological functions. ER stress transducers providing the branches of UPR signaling are known to localize in distal dendritic ER of neurons. These reports suggest that local activation of UPR branches may produce integrated outputs for distant communication, and allow regulation of local events in highly polarized neurons. Here, we demonstrated that synaptic activity- and brain-derived neurotrophic factor (BDNF)-dependent local activation of UPR signaling could be associated with dendritic functions through retrograde signal propagation by using murine neuroblastoma cell line, Neuro-2A and primary cultured hippocampal neurons derived from postnatal day 0 litter C57BL/6 mice. ER stress transducer, inositol-requiring kinase 1 (IRE1), was activated at postsynapses in response to excitatory synaptic activation. Activated dendritic IRE1 accelerated accumulation of the downstream transcription factor, x-box-binding protein 1 (XBP1), in the nucleus. Interestingly, excitatory synaptic activation-dependent up-regulation of XBP1 directly facilitated transcriptional activation of BDNF. BDNF in turn drove its own expression via IRE1-XBP1 pathway in a protein kinase A-dependent manner. Exogenous treatment with BDNF promoted extension and branching of dendrites through the protein kinase A-IRE1-XBP1 cascade. Taken together, our findings indicate novel mechanisms for communication between soma and distal sites of polarized neurons that are coordinated by local activation of IRE1-XBP1 signaling. Synaptic activity- and BDNF-dependent distinct activation of dendritic IRE1-XBP1 cascade drives BDNF expression in cell soma and may be involved in dendritic extension. Cover Image for this issue: doi. 10.1111/jnc.14159.

Endoplasmic reticulum stress in the pathogenesis of hypertension

What is the topic of this review? This review highlights the emerging role of disruptions in endoplasmic reticulum (ER) function, namely ER stress, as a contributor to hypertension. What advances does it highlight? This review presents an integrative view of ER stress in cardiovascular control systems, including systems within the brain, kidney and peripheral vasculature, as related to development of hypertension. The endoplasmic reticulum (ER) is a cellular organelle specialized in the synthesis, folding, assembly and modification of proteins. In situations of increased protein demand, complex signalling pathways, termed the unfolded protein response, influence a series of cellular feedback loops to control ER function strictly. Although this is initially a compensatory attempt to maintain cellular homeostasis, chronic activation of the unfolded protein response, known as ER stress, leads to sustained changes in cellular function. A growing body of literature points to ER stress in diverse cardioregulatory systems, including the brain, kidney and vasculature, as central to the development of hypertension. Here, these recent findings from essential and obesity-related forms of hypertension are highlighted in an integrative manner, with discussion of the potential upstream causes and downstream consequences of ER stress. Given that hypertension is a leading medical and socio-economic global challenge, emerging findings suggest that targeting ER stress might represent a viable strategy for the treatment of hypertensive disease.

Contacts between the endoplasmic reticulum and other membranes in neurons

Close appositions between the membrane of the endoplasmic reticulum (ER) and other intracellular membranes have important functions in cell physiology. These include lipid homeostasis, regulation of Ca²⁺ dynamics, and control of organelle biogenesis and dynamics. Although these membrane contacts have previously been observed in neurons, their distribution and abundance have not been systematically analyzed. Here, we have used focused ion beam-scanning electron microscopy to generate 3D reconstructions of intracellular organelles and their membrane appositions involving the ER (distance ≤30 nm) in different neuronal compartments. ER-plasma membrane (PM) contacts were particularly abundant in cell bodies, with large, flat ER cisternae apposed to the PM, sometimes with a notably narrow lumen (thin ER). Smaller ER-PM contacts occurred throughout dendrites, axons, and in axon terminals. ER contacts with mitochondria were abundant in all compartments, with the ER often forming a network that embraced mitochondria. Small focal contacts were also observed with tubulovesicular structures, likely to be endosomes, and with sparse multivesicular bodies and lysosomes found in our reconstructions. Our study provides an anatomical reference for interpreting information about interorganelle communication in neurons emerging from functional and biochemical studies.

Endoplasmic Reticulum-Localized Transmembrane Protein Dpy19L1 Is Required for Neurite Outgrowth

The endoplasmic reticulum (ER), including the nuclear envelope, is a continuous and intricate membrane-bound organelle responsible for various cellular functions. In neurons, the ER network is found in cell bodies, axons, and dendrites. Recent studies indicate the involvement of the ER network in neuronal development, such as neuronal migration and axonal outgrowth. However, the regulation of neural development by ER-localized proteins is not fully understood. We previously reported that the multi-transmembrane protein Dpy19L1 is required for neuronal migration in the developing mouse cerebral cortex. A Dpy19L family member, Dpy19L2, which is a causative gene for human Globozoospermia, is suggested to act as an anchor of the acrosome to the nuclear envelope. In this study, we found that the patterns of exogenous Dpy19L1 were partially coincident with the ER, including the nuclear envelope in COS-7 cells at the level of the light microscope. The reticular distribution of Dpy19L1 was disrupted by microtubule depolymerization that induces retraction of the ER. Furthermore, Dpy19L1 showed a similar distribution pattern with a ER marker protein in embryonic mouse cortical neurons. Finally, we showed that Dpy19L1 knockdown mediated by siRNA resulted in decreased neurite outgrowth in cultured neurons. These results indicate that transmembrane protein Dpy19L1 is localized to the ER membrane and regulates neurite extension during development.

Specialization of biosynthetic membrane trafficking for neuronal form and function

Neuronal growth and synaptic transmission require the continuous production of adhesion molecules, neurotransmitter receptors, ion-channels, and secreted trophic factors, and thus critically relies on the secretory pathway-the series of intracellular organelles including the endoplasmic reticulum (ER) and the Golgi apparatus (GA), where membrane lipids and proteins are synthesized. Commensurate with the gigantic size of the neuronal membrane and its compartmentalization by thousands of synapses with distinct compositions and activities, the neuronal secretory pathway has evolved to both traffic synaptic components over very long distances, and locally control the composition of specified segments of dendrites. Here we review new insights into the distribution and dynamics of dendritic secretory organelles and their impact on postsynaptic compartments.

Lens opacities in glycogenoses type I and III

OBJECTIVE

The glycogen storage diseases (GSD) or glycogenoses comprise several inherited diseases caused by abnormalities of the enzymes that regulate the synthesis or degradation of glycogen. This report presents lens opacities not previously described in patients with type I or III GSD.

PARTICIPANTS

Eleven patients with type I and III GSD.

METHODS

We examined a series of 11 consecutive patients (aged 13-40 years) with type I and III GSD by full ophthalmologic examination.

RESULTS

We found changes of the lens on 7 of 11 patients (aged 23-40 years) with glycogenoses I and III. In 6 patients, the lens showed multiple, bilateral, punctate, and peripheral opacities; only 1 patient showed a posterior subcapsular opacity in both eyes. We did not observe changes in the cornea and the posterior pole correlated to the accumulation of glycogen and lipids.

CONCLUSIONS

In this series, we found that 60% of patients with type I and III GSD show lens opacities. These opacities are bilateral, peripheral, multiple, and small; they do not give any visual disturbance. Considering that subjects with age ranging from 13 to 23 years had no lens opacities, we postulate that they could progressively develop over time because of exposure to recurrent attacks of hypoglycemia, which lead to a progressive depletion of hexokinase.

Diversifying the secretory routes in neurons

Nervous system homeostasis and synaptic function need dedicated mechanisms to locally regulate the molecular composition of the neuronal plasma membrane and allow the development, maintenance and plastic modification of the neuronal morphology. The cytoskeleton and intracellular trafficking lies at the core of all these processes. In most mammalian cells, the Golgi apparatus (GA) is at the center of the biosynthetic pathway, located in the proximity of the microtubule-organizing center. In addition to this central localization, the somatic GA in neurons is complemented by satellite Golgi outposts (GOPs) in dendrites, which are essential for dendritic morphogenesis and are emerging like local stations of membranes trafficking to synapses. Largely, GOPs participation in post-ER trafficking has been determined by imaging the transport of the exogenous protein VSVG. Here we review the diversity of neuronal cargoes that traffic through GOPs and the assortment of different biosynthetic routes to synapses. We also analyze the recent advances in understanding the role of cytoskeleton and Golgi matrix proteins in the biogenesis of GOPs and how the diversity of secretory routes can be generated.

Signaling Over Distances

Neurons are extremely polarized cells. Axon lengths often exceed the dimension of the neuronal cell body by several orders of magnitude. These extreme axonal lengths imply that neurons have mastered efficient mechanisms for long distance signaling between soma and synaptic terminal. These elaborate mechanisms are required for neuronal development and maintenance of the nervous system. Neurons can fine-tune long distance signaling through calcium wave propagation and bidirectional transport of proteins, vesicles, and mRNAs along microtubules. The signal transmission over extreme lengths also ensures that information about axon injury is communicated to the soma and allows for repair mechanisms to be engaged. This review focuses on the different mechanisms employed by neurons to signal over long axonal distances and how signals are interpreted in the soma, with an emphasis on proteomic studies. We also discuss how proteomic approaches could help further deciphering the signaling mechanisms operating over long distance in axons.

ATF6 and caspase 12 expression in Purkinje neurons in acute slices from adult, ethanol-fed rats

The purpose of this study was to determine, whether previously reported ethanol-induced alterations to the smooth endoplasmic reticulum (SER), predispose Purkinje neurons (PN) to thapsigargin-induced endoplasmic reticulum (ER) stress. Thapsigargin blocks the sarco/endoplasmic Ca(2+) ATPase pump (SERCA 2), depleting the SER of calcium. Forty-one, eight month old Fischer 344 male rats were treated with either the AIN (American Institute of Nutrition) liquid control or ethanol diets for 10 (n=14), 20 (n=10), or 40(n=17) weeks. At the end of treatment, acute cerebellar slices were prepared by standard means. Cerebellar slices were treated with thapsigargin or as controls for three hours in oxygenated (95% CO2, 5% O2) ACSF (artificial cerebrospinal fluid). Slices were then fixed in 4% paraformaldehyde and sectioned on a freezing microtome. Free floating sections were stained with antibodies against activating transcription factor 6 (ATF6) or activated caspase 12 and calbindin. Results showed a significant increase in the activated caspase+PN dendrites in the EF rats along with a significant interaction due to enhanced expression of activated caspase 12 at 20 weeks. The density of ATF6 labeling was not different between the EF and PF groups and was confined to the PN soma. The finding of activated caspase and ATF6 expression in PN within both the EF and PF groups supports the finding of thapsigargin-induced ER stress. The finding of increased activated caspase 12 in the dendrites supports an increased tendency to ER stress and other dendritic deficits in the ethanol rats.

Transport along the dendritic endoplasmic reticulum mediates the trafficking of GABAB receptors

In neurons, secretory organelles within the cell body are complemented by the dendritic endoplasmic reticulum (ER) and Golgi outposts (GOPs), whose role in neurotransmitter receptor trafficking is poorly understood. γ-aminobutyric acid (GABA) type B metabotropic receptors (GABABRs) regulate the efficacy of synaptic transmission throughout the brain. Their plasma membrane availability is controlled by mechanisms involving an ER retention motif and assembly-dependent ER export. Thus, they constitute an ideal molecular model to study ER trafficking, but the extent to which the dendritic ER participates in GABABR biosynthesis has not been thoroughly explored. Here, we show that GABAB1 localizes preferentially to the ER in dendrites and moves long distances within this compartment. Not only diffusion but also microtubule and dynein-dependent mechanisms control dendritic ER transport. GABABRs insert throughout the somatodendritic plasma membrane but dendritic post-ER carriers containing GABABRs do not fuse selectively with GOPs. This study furthers our understanding of the spatial selectivity of neurotransmitter receptors for dendritic organelles.

The axonal endoplasmic reticulum and protein trafficking: Cellular bootlegging south of the soma

Neurons are responsible for the generation and propagation of electrical impulses, which constitute the central mechanism of information transfer between the nervous system and internal or external environments. Neurons are large and polarized cells with dendrites and axons constituting their major functional domains. Axons are thin and extremely long specializations that mediate the conduction of these electrical impulses. Regulation of the axonal proteome is fundamental to generate and maintain neural function. Although classical mechanisms of protein transport have been around for decades, a variety newly identified mechanisms to control the abundance of axonal proteins have appeared in recent years. Here we briefly describe the classical models of axonal transport and compare them to the emerging concepts of axonal biosynthesis centered on the endoplasmic reticulum. We review the structure of the axonal endoplasmic reticulum, and its role in diffusion and trafficking of axonal proteins. We also analyze the contribution of other secretory organelles to axonal trafficking and evaluate the potential consequences of axonal endoplasmic reticulum malfunction in neuropathology.

Neuronal protein trafficking: emerging consequences of endoplasmic reticulum dynamics

The highly polarized morphology and complex geometry of neurons is determined to a great extent by the structural and functional organization of the secretory pathway. It is intuitive to propose that the spatial arrangement of secretory organelles and their dynamic behavior impinge on protein trafficking and neuronal function, but these phenomena and their consequences are not well delineated. Here we analyze the architecture and motility of the archetypal endoplasmic reticulum (ER), and their relationship to the microtubule cytoskeleton and post-translational modifications of tubulin. We also review the dynamics of the ER in axons, dendrites and spines, and discuss the role of ER dynamics on protein mobility and trafficking in neurons.

Cellular and subcellular distributions of delta opioid receptor activation sites in the ventral oral pontine tegmentum of the cat

The ventral division of the reticular oral pontine nucleus (vRPO) is a pontine tegmentum region critically involved in REM sleep generation. Previous reports of morphine microinjections in the cat pontine tegmentum have shown that opioid receptor activation in this region modulates REM sleep. Even though opiate administration has marked effects on sleep-wake cycle architecture, the distribution of opioid receptors in vRPO has only been partially described. Using an antiserum directed against delta opioid receptor (DOR), to which morphine binds, in the present study, we use (1) light microscopy to determine DOR cellular distribution in the rostral pontine tegmentum and (2) electron microscopy to determine DOR subcellular distribution in the cat vRPO. In the dorsal pons, DOR immunoreactivity was evenly distributed throughout the neuropil of the reticular formation and was particularly intense in the parabrachial nuclei and locus coeruleus; the ventral and central areas of the RPO and locus coeruleus complex were especially rich in DOR-labeled somata. Within the vRPO, DOR was localized mainly in the cytoplasm and on plasma membranes of medium to large dendrites (47.8% of DOR-labeled profiles), which received both symmetric and asymmetric synaptic contacts mainly from non-labeled (82% of total inputs) axon terminals. Less frequently, DOR was distributed presynaptically in axon terminals (19% of DOR-labeled profiles). Our results suggest that DOR activation in vRPO regulates REM sleep occurrence by modulating postsynaptic responses to both excitatory and inhibitory afferents. DOR activation in vRPO could have, however, an additional role in direct modulation of neurotransmitter release from axon terminals.

Subcellular distribution of M2 muscarinic receptors in relation to dopaminergic neurons of the rat ventral tegmental area

Acetylcholine can affect cognitive functions and reward, in part, through activation of muscarinic receptors in the ventral tegmental area (VTA) to evoke changes in mesocorticolimbic dopaminergic transmission. Among the known muscarinic receptor subtypes present in the VTA, the M2 receptor (M2R) is most implicated in autoregulation and also may play a heteroreceptor role in regulation of the output of the dopaminergic neurons. We sought to determine the functionally relevant sites for M2R activation in relation to VTA dopaminergic neurons by examining the electron microscopic immunolabeling of M2R and the dopamine transporter (DAT) in the VTA of rat brain. The M2R was localized to endomembranes in DAT-containing somatodendritic profiles but showed a more prominent, size-dependent plasmalemmal location in nondopaminergic dendrites. M2R also was located on the plasma membrane of morphologically heterogenous axon terminals contacting unlabeled as well as M2R- or DAT-labeled dendrites. Some of these terminals formed asymmetric synapses resembling those of cholinergic terminals in the VTA. The majority, however, formed symmetric, inhibitory-type synapses or were apposed without recognized junctions. Our results provide the first ultrastructural evidence that the M2R is expressed, but largely not available for local activation, on the plasma membrane of VTA dopaminergic neurons. Instead, the M2R in this region has a distribution suggesting more indirect regulation of mesocorticolimbic transmission through autoregulation of acetylcholine release and changes in the physiological activity or release of other, largely inhibitory transmitters. These findings could have implications for understanding the muscarinic control of cognitive and goal-directed behaviors within the VTA.

Ethanol-Related Smooth Endoplasmic Reticulum Dilation in Purkinje Dendrites of Aging Rats

BACKGROUND

Long-term ethanol consumption in aging rats results in degeneration and regression of the Purkinje neuron (PN) dendritic arbor. One marked ethanol-related change in Purkinje dendrite ultrastructure is dilation of the smooth endoplasmic reticulum (SER) within PN dendritic shafts. The purpose of this study was to determine a time course for ethanol-related dendritic regression in PN dendritic shafts and spines.

METHODS

One-hundred eighty aging, male Fischer 344 rats were used. Four durations of treatment (5, 10, 20, and 40 weeks) and 3 dietary treatment groups (60 rats/treatment group) were studied. Ethanol-fed rats received a liquid ethanol diet (35% of dietary calories from ethanol). Pair-fed rats received an isocaloric liquid control diet and chow-fed rats received rat chow and water ad libitum. After each duration of treatment, 45 rats (15/treatment) were euthanized and 2 posterior cerebellar lobules/rat were viewed with electron microscopy and photographed. Diameters of SER profiles within PN shafts and spines were measured with image analysis.

RESULTS

Ethanol-related SER dilation in dendritic shafts occurred following 40 weeks of treatment. Ethanol-related SER dilation was not detected in PN dendritic spines.

CONCLUSIONS

These results confirm that ethanol-related dilation of SER profiles in PN dendritic shafts occurs following the same duration of treatment as the dendritic regression previously reported in other studies. Degenerating bodies that may be linked to dendritic regression were also identified in PN dendrites.

Vesicular monoamine transporter-2 and aromatic L-amino acid decarboxylase gene therapy prevents development of motor complications in parkinsonian rats after chronic intermittent L-3,4-dihydroxyphenylalanine administration

Motor complications after chronic L-3,4-dihydroxyphenylalanine (L-DOPA) therapy occur partly because of the sensitization to dopaminergic agents resulting from pulsatile dopaminergic stimulation. The loss of presynaptic storage contributes to short duration of action by dopamine. Vesicular monoamine transporter-2 (VMAT-2) controls intraneuronal dopamine storage by packaging dopamine into synaptic vesicles, thereby allowing exocytotic release of dopamine. Using primary fibroblast doubly transduced with VMAT-2 and aromatic L-amino acid decarboxylase (AADC) genes, we previously demonstrated the beneficial effects of such double gene transduction in the production, storage, and gradual release of dopamine in vitro and in vivo. In this study, we further evaluate the effect of achieving sustained level of dopamine within the striata by VMAT-2 gene on behavioral response of parkinsonian rats after chronic intermittent L-DOPA administration. Primary fibroblast (PF) cells were genetically modified with AADC and VMAT-2 genes. We grafted primary fibroblast cells, PF with AADC (PFAADC), or doubly transduced PF with AADC and VMAT-2 (PFVMAA) (n = 6 for each group) into parkinsonian rat striata and administered L-DOPA (25 mg/kg/day) intermittently for 4 weeks. For behavioral study, we employed a model of akinesia using forepaw adjusting steps (FAS) that have been well characterized to reflect the effect of the lesion and the antiparkinsonian effect of dopaminergic drugs and transplants. The duration of FAS response to L-DOPA was sustained for a longer duration in rats grafted with PFVMAA cells than in those grafted with either control cells or cells with AADC alone. In PFVMAA-grafted animals, prolonged duration of FAS responses to L-DOPA was sustained even 6 weeks after discontinuation of 4-week intermittent L-DOPA treatment. These findings suggest that the restoration of dopamine storage capacity could enhance the efficacy of L-DOPA therapy and attenuate the motor fluctuations that result from chronic intermittent L-DOPA administration. The gene therapy expressing AADC and VMAT-2 along with systemic L-DOPA therapy could provide a novel treatment strategy to prevent motor fluctuations.

Translational control at the synapse

The strength of synaptic connections can undergo long-lasting changes, and such long-term plasticity is thought to underlie higher brain functions such as learning and memory. De novo synthesis of proteins is required for such plastic changes. This model is now supported by several lines of experimental data. Components of translational machinery have been identified in dendrites, including ribosomes, translation-al factors, numerous RNAs, and components of posttranslational secretory pathways. Various RNAs have been shown to be actively and rapidly transported to dendrites. Dendritic RNAs typically contain transport-specifying elements (dendritic targeting elements). Such dendritic targeting elements associate with trans-acting factors to form transport-competent ribonucleoprotein particles. It is assumed that molecular motors mediate transport of such particles along dendritic cytoskeletal elements. Once an mRNA has arrived at its dendritic destination site, appropriate spatiotemporal control of its translation, for example, in response to transsynaptic activity, becomes vital. Such local translational control, recent evidence indicates, is implemented at different levels and through various pathways. In the default state, translation is assumed to be repressed, and several mechanisms, some including small untranslated RNAs, have been proposed to contribute to such repression. Translational control at the synapse thus provides a molecular basis for the long-term, input-specific modulation of synaptic strength.

Histochemical localization of acid mannose 6-phosphatase activity in the newborn rat hepatocytes

The localization of acid mannose 6-phosphatase activity in newborn rat hepatocytes was demonstrated at the electron microscopic level by using a histochemical method based on the work of Robinson and Karnovsky. Reaction product was virtually restricted to the lysosomes. Most of them exhibited various grades of reactivity. Some were devoid of activity. Our observations suggested that this histochemical method could be used to differentiate distinct subpopulations of lysosomes on the basis of their acid mannose 6-phosphatase activity.

Region-specific targeting of dopamine D2-receptors and somatodendritic vesicular monoamine transporter 2 (VMAT2) within ventral tegmental area subdivisions

Throughout the ventral tegmental area (VTA), dopamine is packaged within subcellular organelles by the vesicular monoamine transporter-2 (VMAT2). Somatodendritically released dopamine in this region binds to the D2 receptor (D2R) to modulate ongoing neurotransmission. Although autoregulation of mesocortical dopaminergic neurons in the parabrachial VTA (PB-VTA) is known to be less efficacious than that of mesolimbic dopaminergic neurons in the paranigral (PN-VTA), the cellular basis for this regional heterogeneity is not known. For this reason, we used electron microscopic immunocytochemistry to determine the subcellular localization of the dopamine storage vesicles (identified by the presence of VMAT2) in relation to the D2R in these VTA subdivisions. In both regions, D2R immunoreactivity was principally located on extrasynaptic dendritic plasma membranes near excitatory-type synapses. Equivalent percentages (72 and 74%) of the D2R-labeled dendrites in each region contained VMAT2-immunoreactive tubulovesicles. Of the total VMAT2-labeled dendrites, however, a significantly lower percentage in the PB-VTA (26%) than in the PN-VTA (38%) contained D2R labeling. In contrast, a significantly higher number of VMAT2 immunogold-silver deposits was seen within individual dendrites in the PB-VTA than in PN-VTA. In both regions, D2R immunoreactivity was also detected in VMAT2-negative axon terminals that formed synapses on dendrites containing VMAT2. Our results are the first to demonstrate that within VTA neurons and their afferents the D2R is strategically positioned for activation by dopamine released from dendritic storage vesicles. These findings also suggest that the potential for D2R activation may affect the expression levels of VMAT2 in VTA dendrites.

The endoplasmic reticulum as an integrating signalling organelle: from neuronal signalling to neuronal death

The endoplasmic reticulum is one of the largest intracellular organelles represented by continuous network of cisternae and tubules, which occupies the substantial part of neuronal somatas and extends into finest neuronal processes. The endoplasmic reticulum controls protein synthesis as well as their post-translational processing, and generates variety of nucleus-targeted signals through Ca(2+)-binding chaperones. The normal functioning of the endoplasmic reticulum signalling cascades requires high concentrations of free calcium ions within the endoplasmic reticulum lumen ([Ca(2+)](L)), and severe alterations in [Ca(2+)](L) trigger endoplasmic reticulum stress response, manifested by either unfolded protein response (UPR) or endoplasmic reticulum overload response (EOR). At the same time, the endoplasmic reticulum is critically involved in fast neuronal signalling, by producing local or global cytosolic calcium signals via Ca(2+)-induced Ca(2+) release (CICR) or inositol-1,4,5-trisphosphate-induced Ca(2+) release (IICR). Both CICR and IICR are important for synaptic transmission and synaptic plasticity. Several special techniques allowing real-time [Ca(2+)](L) monitoring were developed recently. Video-imaging of [Ca(2+)](L) in neurones demonstrates that physiological signalling triggers minor decreases in overall intraluminal Ca(2+) concentration due to strong activation of Ca(2+) uptake, which prevents severe [Ca(2+)](L) alterations. The endoplasmic reticulum lumen also serves as a "tunnel" which allows rapid transport of Ca(2+) ions within highly polarised nerve cells. Fluctuations of intraluminal free Ca(2+) concentration represent a universal mechanism, which integrates physiological cellular signalling with protein synthesis and processing. In pathological conditions, fluctuations in [Ca(2+)](L) may initiate either adaptive or fatal stress responses.

Plasmalemmal mu-opioid receptor distribution mainly in nondopaminergic neurons in the rat ventral tegmental area

Opiate-evoked reward and motivated behaviors reflect, in part, the enhanced release of dopamine produced by activation of the mu-opioid receptor (muOR) in the ventral tegmental area (VTA). We examined the functional sites for muOR activation and potential interactions with dopaminergic neurons within the rat VTA by using electron microscopy for the immunocytochemical localization of antipeptide antisera raised against muOR and tyrosine hydroxylase (TH), the synthesizing enzyme for catecholamines. The cellular and subcellular distribution of muOR was remarkably similar in the two major VTA subdivisions, the paranigral (VTApn) and parabrachial (VTApb) nuclei. In each region, somatodendritic profiles comprised over 50% of the labeled structures. MuOR immunolabeling was often seen at extrasynaptic/perisynaptic sites on dendritic plasma membranes, and 10% of these dendrites contained TH. MuOR-immunoreactivity was also localized to plasma membranes of axon terminals and small unmyelinated axons, none of which contained TH. The muOR-immunoreactive axon terminals formed either symmetric or asymmetric synapses that are typically associated with inhibitory and excitatory amino acid transmitters. Their targets included unlabeled (30%), muOR-labeled (25%), and TH-labeled (45%) dendrites. Our results suggest that muOR agonists in the VTA affect dopaminergic transmission mainly indirectly through changes in the postsynaptic responsivity and/or presynaptic release from neurons containing other neurotransmitters. They also indicate, however, that muOR agonists directly affect a small population of dopaminergic neurons expressing muOR on their dendrites in VTA and/or terminals in target regions.

Mu-opioid receptors in the ventral tegmental area are targeted to presynaptically and directly modulate mesocortical projection neurons

Mesocorticolimbic projections originating from dopaminergic and GABAergic neurons in the ventral tegmental area (VTA) play a critical role in opiate addiction. Activation of mu-opioid receptors (MOR), which are located mainly within inhibitory neurons in the VTA, results in enhanced dopaminergic transmission in target regions, including the medial prefrontal cortex (mPFC). We combined retrograde tract-tracing and electron microscopic immunocytochemistry to determine if neurons in the VTA that project to the mPFC contain MOR or receive input from MOR-containing terminals. Rats received unilateral injections of the retrograde tracer Fluoro-Gold (FG) into the mPFC. Tissue sections throughout the VTA were then processed for electron microscopic examination of FG and MOR. Immunoperoxidase labeling for FG was present in VTA cell bodies that contained immunogold-silver particles for MOR that often were contacted by profiles exclusively immunoreactive for MOR, including somata and axon terminals. The majority of dually labeled profiles were dendrites that received convergent input from unlabeled axon terminals forming either symmetric or asymmetric type synapses. Within retrogradely labeled cell bodies and proximal dendrites, MOR immunoreactivity was mainly sequestered within the cytoplasm. In contrast, distal retrogradely labeled dendrites contained MOR gold particles located along the plasma membranes. These data suggest that opiates active at MOR in the VTA modulate cortical activity through 1) presynaptic actions on MOR in terminals contacting mesocortical cell bodies, and 2) direct activation of MOR in distal dendrites of projection neurons.

Negligible glucose-6-phosphatase activity in cultured astroglia

2-Deoxy[14C]glucose-6-phosphate (2-[14C]DG-6-P) dephosphorylation and glucose-6-phosphatase (G-6-Pase) activity were examined in cultured rat astrocytes under conditions similar to those generally used in assays of glucose utilization. Astrocytes were loaded with 2-[14C]DG-6-P by preincubation for 15 min in medium containing 2 mM glucose and 50 microM 2-deoxy[14C]glucose (2-[14C]DG). The medium was then replaced with identical medium including 2 mM glucose but lacking 2-[14C]DG, and incubation was resumed for 5 min to diminish residual free 2-[14C]DG levels in the cells by either efflux or phosphorylation. The medium was again replaced with fresh 2-[14C]DG-free medium, and the incubation was continued for 5, 15, or 30 min. Intracellular and extracellular 14C contents were measured at each time point, and the distribution of 14C between 2-[14C]DG and 2-[14C]DG-6-P was characterized by paper chromatography. The results showed little if any hydrolysis of 2-[14C]DG-6-P or export of free 2-[14C]DG from cells to medium; there were slightly increasing losses of 2-[14C]DG and 2-[14C]DG-6-P into the medium with increasing incubation time, but they were in the same proportions found in the cells, suggesting they were derived from nonadherent or broken cells. Experiments carried out with medium lacking glucose during the assay for 2-deoxyglucose-6-phosphatase activity yielded similar results. Evidence for G-6-Pase activity was also sought by following the selective detritiation of glucose from the 2-C position when astrocytes were incubated with [2-3H]glucose and [U-14C]glucose in the medium. No change in the 3H/14C ratio was found in incubations for as long as 15 min. These results indicate negligible G-6-Pase activity in cultured astrocytes.

Dendritic and axonal targeting of the vesicular acetylcholine transporter to membranous cytoplasmic organelles in laterodorsal and pedunculopontine tegmental nuclei

Autoregulation of cholinergic neurons in the laterodorsal tegmental (LDT) and pedunculopontine (PPT) nuclei has been implicated in many functions, most importantly in drug reinforcement and in the pathophysiology of schizophrenia. This autoregulation is attributed to the release of acetylcholine, but neither the storage or release sites are known. To determine these sites, we used electron microscopy for the immunocytochemical localization of antipeptide antiserum raised against the vesicular acetylcholine transporter (VAchT) that is responsible for the uptake of acetylcholine into storage vesicles. The cellular and subcellular distribution of VAchT was remarkably similar in the two regions by by using each of two methods, immunogold and immunoperoxidase. In both PPT and LDT nuclei, VAchT labeling was seen mainly on membranous organelles including the trans-Golgi network in many somata. VAchT-immunoreactive tubulovesicles resembling saccules of smooth endoplasmic reticulum were often seen near the plasma membrane in dendrites. The VAchT-containing dendrites comprised almost 50% of the labeled profiles (1027/2129) in PPT and LDT nuclei. The remaining VAchT-immunoreactive profiles were primarily small unmyelinated axons and axon terminals. In axon terminals, VAchT was densely localized to membranes of small synaptic vesicles. The VAchT-immunoreactive axon terminals formed either symmetric or asymmetric synapses. The postsynaptic targets of these axon terminals included dendrites that were with (36/110) or without (74/110) VAchT immunoreactivity. Our results suggest that dendrites, as well as axon terminals, have the potential for storage and release of acetylcholine in the LDT and PPT nuclei. The released acetylcholine is likely to play a major role in autoregulation of mesopontine cholinergic neurons.

Mu-opioid and NMDA-type glutamate receptors are often colocalized in spiny neurons within patches of the caudate-putamen nucleus

The patch compartments of the caudate-putamen nucleus (CPN) are enriched in mu-opioid receptors (MORs) and have been recently implicated in reward-related behaviors. This function has been established more clearly in the nucleus accumbens, where physiological and anatomical studies show reward-associated interactions involving MORs and N-methyl-D-aspartate-type glutamate receptors (NMDARs). We examined the immunolabeling for MOR and NMDAR subunit NR1 in patches of the rat CPN to determine the potential relevance of dual activation of the respective receptors. Electron microscopy showed the presence of MOR and/or NR1 immunoreactivity (IR) in many perikarya, dendrites, and spines and in morphologically heterogeneous axon terminals. In each 1,000-microm(2) area, the dually labeled dendrites and spines constituted 65% (37/57) and 37% (9/25) of the total NR1-labeled and 34% (37/109) and 13% (9/71) of the total MOR-labeled dendritic profiles. Dually labeled spines received asymmetric excitatory-type synapses from terminals, which were generally unlabeled, but also occasionally contained MOR and/or NR1. The asymmetric synapses comprised the majority (81%) of the total 263 synaptic contacts between MOR- and NR1-labeled neuronal profiles. In dendrites and spines, MOR-IR was localized mainly along nonsynaptic plasma membranes, whereas NR1-IR was more often associated with asymmetric postsynaptic densities and cytoplasmic organelles. In contrast to dendrites, 6% (1.3/22) of NR1-IR and 4% (1.3/33) of MOR-IR axon terminals were dually labeled in each 1,000-microm(2) area. Most singly or dually labeled terminals formed asymmetric synapses with MOR- or NR1-labeled spines. Our results suggest that opioids acting through MOR and excitatory neurotransmitters through NMDAR dually regulate the output of single spiny neurons and some of their excitatory afferents in the CPN.

Cellular sites for dynorphin activation of kappa-opioid receptors in the rat nucleus accumbens shell

The nucleus accumbens (Acb) is prominently involved in the aversive behavioral aspects of kappa-opioid receptor (KOR) agonists, including its endogenous ligand dynorphin (Dyn). We examined the ultrastructural immunoperoxidase localization of KOR and immunogold labeling of Dyn to determine the major cellular sites for KOR activation in this region. Of 851 KOR-labeled structures sampled from a total area of 10,457 microm2, 63% were small axons and morphologically heterogenous axon terminals, 31% of which apposed Dyn-labeled terminals or also contained Dyn. Sixty-eight percent of the KOR-containing axon terminals formed punctate-symmetric or appositional contacts with unlabeled dendrites and spines, many of which received convergent input from terminals that formed asymmetric synapses. Excitatory-type terminals that formed asymmetric synapses with dendritic spines comprised 21% of the KOR-immunoreactive profiles. Dendritic spines within the neuropil were the major nonaxonal structures that contained KOR immunoreactivity. These spines also received excitatory-type synapses from unlabeled terminals and were apposed by Dyn-containing terminals. These results provide ultrastructural evidence that in the Acb shell (AcbSh), KOR agonists play a primary role in regulating the presynaptic release of Dyn and other neuromodulators that influence the output of spiny neurons via changes in the presynaptic release of or the postsynaptic responses to excitatory amino acids. The cellular distribution of KOR complements those described previously for the reward-associated mu- and delta-opioid receptors in the Acb shell.

Dendritic and postsynaptic protein synthetic machinery

There is a growing body of evidence that local protein synthesis beneath synapses may provide a novel mechanism underlying plastic phenomena. In vivo and in vitro biochemical data show that dendrites can perform translation and glycosylation. Using antibodies directed against the eukaryotic protein synthetic machinery, we sought to identify the structures implicated in nonperinuclear translation, namely dendritic and postsynaptic protein synthesis. We performed a morphological and immunocytochemical analysis of ventromedial horn rat spinal cord neurons using both light and electron microscopy. We show at the cellular level that, in vivo, protein synthesis macrocomplexes (ribosomes and eIF-2) as well as the endomembranous system implicated in cotranslational and posttranslational modifications (endoplasmic reticulum and Golgi cisternae) penetrated some dendrites. Membrane-limited organelles of different shape and size are present close to the postsynaptic differentiations of most synapses, independently of their localization on the neuronal surface. We demonstrate (1) that some cisternae are immunoreactive for antibodies against ribosomal proteins and eIF-2, and (2) that markers of endoplasmic reticulum (BiP), intermediate compartment, and Golgi complex (rab1, CTR433, TGN38) label subsets of these subsynaptic organelles. Therefore, these findings indicate that synapses are equipped with the essential elements required for the synthesis and insertion of a well folded and glycosylated transmembrane protein.

Immunogold localization of the dopamine transporter: an ultrastructural study of the rat ventral tegmental area

The dopamine transporter (DAT) plays an important role in the plasmalemmal reuptake of dopamine and, thus, in the termination of normal dopaminergic neurotransmission. DAT is also a major binding site for cocaine and other stimulants, the psychoactive effects of which are associated primarily with the inhibition of dopamine reuptake within mesocorticolimbic dopaminergic neurons. We used electron microscopy with an anti-peptide antiserum directed against the N-terminal domain of DAT to determine the subcellular localization of this transporter in the rat ventral tegmental area (VTA), the region that contains the cell bodies and dendrites of these dopaminergic neurons. We show that in the VTA, almost 95% of the DAT immunogold-labeled profiles are neuronal perikarya and dendrites, and the remainder are unmyelinated axons. Within perikarya and large proximal dendrites, almost all of the DAT immunogold particles are associated with intracellular membranes, including saccules of Golgi and cytoplasmic tubulovesicles. In contrast, within medium- to small-diameter dendrites and unmyelinated axons, most of the DAT gold particles are located on plasma membranes. In dually labeled tissue, peroxidase reaction product for the catecholamine-synthesizing enzyme tyrosine hydroxylase is present in DAT-immunoreactive profiles. These findings suggest that intermediate and distal dendrites are both the primary sites of dopamine reuptake and the principal targets of cocaine and related psychostimulants within dopaminergic neurons in the VTA.

Immunogold localization of the dopamine transporter: an ultrastructural study of the rat ventral tegmental area

The dopamine transporter (DAT) plays an important role in the plasmalemmal reuptake of dopamine and, thus, in the termination of normal dopaminergic neurotransmission. DAT is also a major binding site for cocaine and other stimulants, the psychoactive effects of which are associated primarily with the inhibition of dopamine reuptake within mesocorticolimbic dopaminergic neurons. We used electron microscopy with an anti-peptide antiserum directed against the N-terminal domain of DAT to determine the subcellular localization of this transporter in the rat ventral tegmental area (VTA), the region that contains the cell bodies and dendrites of these dopaminergic neurons. We show that in the VTA, almost 95% of the DAT immunogold-labeled profiles are neuronal perikarya and dendrites, and the remainder are unmyelinated axons. Within perikarya and large proximal dendrites, almost all of the DAT immunogold particles are associated with intracellular membranes, including saccules of Golgi and cytoplasmic tubulovesicles. In contrast, within medium- to small-diameter dendrites and unmyelinated axons, most of the DAT gold particles are located on plasma membranes. In dually labeled tissue, peroxidase reaction product for the catecholamine-synthesizing enzyme tyrosine hydroxylase is present in DAT-immunoreactive profiles. These findings suggest that intermediate and distal dendrites are both the primary sites of dopamine reuptake and the principal targets of cocaine and related psychostimulants within dopaminergic neurons in the VTA.

Three-dimensional organization of smooth endoplasmic reticulum in hippocampal CA1 dendrites and dendritic spines of the immature and mature rat

Recent studies have shown high levels of calcium in activated dendritic spines, where the smooth endoplasmic reticulum (SER) is likely to be important for regulating calcium. Here, the dimensions and organization of the SER in hippocampal spines and dendrites were measured through serial electron microscopy and three-dimensional analysis. SER of some form was found in 58% of the immature spines and in 48% of the adult spines. Less than 50% of the small spines at either age contained SER, suggesting that other mechanisms, such as cytoplasmic buffers, regulate ion fluxes within their small volumes. In contrast, >80% of the large mushroom spines of the adult had a spine apparatus, an organelle containing stacks of SER and dense-staining plates. Reconstructed SER occupied 0.001-0.022 microm3, which was only 2-3.5% of the total spine volume; however, the convoluted SER membranes had surface areas of 0.12-2.19 microm2, which were 12 to 40% of the spine surface area. Coated vesicles and multivesicular bodies occurred in some spines, suggesting local endocytotic activity. Smooth vesicles and tubules of SER were found in continuity with the spine plasma membrane and margins of the postsynaptic density (PSD), respectively, suggesting a role for the SER in the addition and recycling of spine membranes and synapses. The amount of SER in the parent dendrites was proportional to the number of spines and synapses originating along their lengths. These measurements support the hypothesis that the SER regulates the ionic and structural milieu of some, but not all, hippocampal dendritic spines.

Ultrastructural localization of 5-hydroxytryptamine1A receptors in the rat brain

5-Hydroxytryptamine1A (5-HT1A) receptors have been visualized at the electron microscopic level in selected areas (dorsal raphe nucleus, hippocampus, septum) of the rat brain using specific anti-peptide antibodies. 5-HT1A receptor immunoreactivity was found almost exclusively in the somatodendritic compartment of neurons and was very rarely observed within processes possibly belonging to glial cells. The immunoenzymatic reaction product was associated exclusively with dendritic spines in the dorsal hippocampus, whereas in the dorsal raphe nucleus and the septal complex, immunoreactivity was found in both dendritic processes and somata. Although some immunolabeling was observed within the cytoplasm of cell bodies, 5-HT1A receptor immunoreactivity was essentially confined to the plasma membrane where it was unevenly distributed. It was frequently associated with synapses (except in the dorsal raphe nucleus), but was also found extrasynaptically in both somata and dendrites. These data suggest that the action of serotonin via 5-HT1A receptor could occur through junctional as well as nonjunctional transmission.

Ultrastructural localization of the vesicular monoamine transporter-2 in midbrain dopaminergic neurons: potential sites for somatodendritic storage and release of dopamine

Midbrain dopaminergic neurons are known to release dopamine from somata and/or dendrites located in the substantia nigra (SN) and the ventral tegmental area (VTA). There is considerable controversy, however, about the subcellular sites for somatodendritic dopamine storage in these regions. In the present study, we used dual-labeling electron microscopic immunocytochemistry to localize the vesicular monoamine transporter-2 (VMAT2), a novel marker for sites of intracellular monoamine storage, within identified dopaminergic (tyrosine hydroxylase-containing) neurons in the rat SN and VTA. In dopaminergic perikarya, immunogold labeling for VMAT2 was localized to the Golgi apparatus, tubulovesicles that resembled smooth endoplasmic reticulum (SER), and the limiting membranes of multivesicular bodies. In dopaminergic dendrites, VMAT2 was extensively localized to tubulovesicles that resembled saccules of SER, and less frequently localized to isolated small synaptic vesicles (SSVs) or large dense-core vesicles (DCVs). In rare cases, VMAT2-immunoreactive SSVs were clustered within the cytoplasm of an SN or a VTA dendrite. Dopaminergic dendrites in the VTA contained a significantly higher number of immunogold particles for VMAT2 per unit than those in the SN. Together, these observations support the proposal that dopamine is stored in and may be released from dendritic SSVs and DCVs, but suggest that the SER is the major site of dopamine storage within midbrain dopaminergic neurons. In addition, they provide new evidence that dopaminergic dendrites in the VTA may have greater potential for reserpine-sensitive storage and release of dopamine than those in the SN.

Ultrastructural immunocytochemical localization of mu-opioid receptors in rat nucleus accumbens: extrasynaptic plasmalemmal distribution and association with Leu5-enkephalin

mu-Opioid receptors and their endogenous ligands, including Leu5-enkephalin (LE), are distributed abundantly in the nucleus accumbens (NAC), a region implicated in mechanisms of opiate reinforcement. We used immunoperoxidase and/or immunogold-silver methods to define ultrastructural sites for functions ascribed to mu-opioid receptors and potential sites for activation by LE in the NAC. An antipeptide antibody raised against an 18 amino acid sequence of the cloned mu-opioid receptor (MOR) C terminus showed that MOR-like immunoreactivity (MOR-LI) was localized predominantly to extrasynaptic sites along neuronal plasma membranes. The majority of neuronal profiles containing MOR-LI were dendrites and dendritic spines. The dendritic plasma membranes immunolabeled for MOR were near sites of synaptic input from LE-labeled terminals and other unlabeled terminals forming either inhibitory or excitatory type synapses. Unmyelinated axons and axon terminals were also intensely but less frequently immunoreactive for MOR. Observed sites for potential axonal associations with LE included coexistence of MOR and LE within the same terminal, as well as close appositions between differentially labeled axons. Astrocytic processes rarely contained detectable MOR-LI, but also were sometimes observed in apposition to LE-labeled terminals. We conclude that in the rat NAC, MOR is localized prominently to extrasynaptic neuronal and more rarely to glial plasma membranes that are readily accessible to released LE and possibly other opioid peptides and opiate drugs. The close affiliation of MOR with spines receiving excitatory synapses and dendrites receiving inhibitory synapses provides the first direct morphological evidence that MOR selectively modulates postsynaptic responses to cortical and other afferents.

Protein synthesis within dendrites: glycosylation of newly synthesized proteins in dendrites of hippocampal neurons in culture

There is increasing evidence that certain mRNAs are present in dendrites and can be translated there. The present study uses two strategies to evaluate whether dendrites also possess the machinery for protein glycosylation. First, precursor labeling techniques were used to conjunction with autoradiography to visualize glycosyltransferase activities that are characteristic of the rough endoplasmic reticulum (RER) (mannose) or the Golgi apparatus (GA) (galactose and fucose) in dendrites that had been separated from their cell bodies and in intact neurons treated with brefeldin A or low temperature. Second, immunocytochemical techniques were used to define the subcellular distribution of proteins that are considered markers of the RER (ribophorin I) and GA (p58, alpha-mannosidase II, galactosyltransferase, and TGN38/41). Autoradiographic analysis revealed that isolated dendrites incorporated sugar precursors in a tunicamycin-sensitive and protein synthesis-dependent manner. Moreover, when intact neurons were pulse-labeled with 3H-labeled sugars at low temperature or after treatment with brefeldin A, labeling was distributed over proximal and sometimes distal dendrites. Immunolabeling for RER markers was predominantly localized in cell bodies but extended for a considerable distance into dendrites of all neurons. Immunolabeling for GA markers was confined to the cell body in approximately 70% of the neurons, but in 30% of the neurons, the staining extended into proximal and middle dendrites. These results indicate that the machinery for glycosylation extends well into dendrites in many neurons.