Morphology and distribution of the synapses to the spinal motoneuron of the frog

Endoplasmic Reticulum-Plasma Membrane Contact Sites

The endoplasmic reticulum (ER) has a broad localization throughout the cell and forms direct physical contacts with all other classes of membranous organelles, including the plasma membrane (PM). A number of protein tethers that mediate these contacts have been identified, and study of these protein tethers has revealed a multiplicity of roles in cell physiology, including regulation of intracellular Ca²⁺ dynamics and signaling as well as control of lipid traffic and homeostasis. In this review, we discuss the cross talk between the ER and the PM mediated by direct contacts. We review factors that tether the two membranes, their properties, and their dynamics in response to the functional state of the cell. We focus in particular on the role of ER-PM contacts in nonvesicular lipid transport between the two bilayers mediated by lipid transfer proteins.

Ultrastructural characteristics of zebrafish spinal motoneurons innervating glycolytic white, and oxidative red and intermediate muscle fibers

Spinal motoneurons in the zebrafish were classified using morphological criteria. Dorsomedial white motoneurons which innervate the fast, glycolytic white muscle fiber compartment were distinguished from ventrolateral red and intermediate motoneurons which innervate the slow, oxidative, red and intermediate muscle fiber compartments. Synapses on cell somata and cell organelles were studied in detail. The motoneurons which innervate white muscle fibers (W motoneurons) are considerably larger than those which innervate red and intermediate muscle fibers (RI motoneurons; W > RI). Significant differences were also found in the size of the nucleus (W > RI) and in the ratio size nucleus/size soma (W < RI); small differences were found regarding endoplasmic reticulum (W > RI) and mitochondria (W < RI). There were no differences in synaptic apposition length or percentage of terminals with flat vesicles. Small differences were discerned with regard to covering percentages (W < RI) and percentage of terminals with round vesicles (W > RI). Terminals with dense cored vesicles appeared on W motoneuron somata only. Within the motoneuron population, there was a positive correlation between the coverage of terminals containing flat vesicles and the perimeter of the cell soma. In RI motoneurons, there was a positive correlation between the perimeter of the cell and the amount of endoplasmic reticulum. A negative correlation was found between the RI cell perimeter and mitochondria, which is in line with a high succinate dehydrogenase activity in small cells.

Evidence for the cholinergic nature of C-terminals associated with subsurface cisterns in alpha-motoneurons of rat

C-terminals can be distinguished at the ultrastructural level from other types of nerve endings on motoneurons by their prominent and regularly occurring postsynaptic specializations termed subsurface cisterns (SSC). We have previously shown (Yamamoto et al., 1991) that an antibody directed against a sequence within the gap junction protein connexin32 immunolabels these motoneuronal SSCs and can therefore serve as a immunohistochemical tool to visualize indirectly the location of C-terminals on motoneurons at the light microscope level. Here we have used this anti-SSC antibody in combination with antibodies against choline acetyltransferase (ChAT) and acetylcholinesterase (AChE) to determine whether C-terminals on motoneurons contain these cholinergic enzyme markers. In sections at all major spinal cord levels and in several cranial motor nuclei examined, motoneuronal cell bodies and their proximal dendrites were studded with large ChAT-immunoreactive (ChAT-IR) boutons. Boutons having a similar distribution and appearance on motoneurons were also immunolabeled for AChE. In addition, motoneurons were surrounded by a dense plexus of AChE-immunoreactive (AChE-IR) varicose fibers and fine preterminal axons. In double-labeled sections, AChE-IR boutons corresponded to those immunolabeled for ChAT. In sections processed for simultaneous immunofluorescence detection of ChAT and SSCs, ChAT-IR boutons were very often found in apposition to immunolabeled SSCs. In sections processed for simultaneous labeling of AChE and SSCs. AChE-IR boutons were again frequently seen abutting labeled SSCs. These results provide the first strong evidence at the LM level that a large proportion, if not the entirety, of C-terminals are cholinergic and show that these terminals consist in part of relatively large varicosities along highly varicose axons that form en passant type contacts on motoneurons. At the same time, our results substantially narrow possibilities regarding the as yet undetermined source of C-terminals, which can now be considered to originate from cholinergic neurons, such as those located in the brainstem and/or the spinal cord.

Subsurface cisterns in alpha-motoneurons of the rat and cat: immunohistochemical detection with antibodies against connexin32

A monoclonal antibody against amino acids 224-234 of the gap junction protein connexin32 was found by immunohistochemistry to label subsurface cisterns (SSCs) in alpha-motoneurons of the rat (Yamamoto et al., 1990) and was used here to document by light (LM) and electron microscopy (EM) the appearance of immunoreactive SSCs in motoneurons of the rat and cat. This antibody and a polyclonal antibody against connexin32 labelled gap junctions in rat liver as well as SSCs in facial motoneurons. By LM, SSCs were seen as labelled puncta on motoneuronal perikarya and proximal dendrites. In the rat, they appeared to be present on all motoneurons at cranial and spinal levels, but varied considerably in size and number among motor nuclei. Labelled SSCs were the smallest and most sparse in motoneurons of the dorsal vagal motor nucleus, moderate in size and most numerous in the trochlear, oculomotor, and trigeminal motor nuclei, and largest though less densely distributed in spinal motoneurons. Dendrites were seen to contain SSCs for distances of up to 230 micron from their somal origin. Labelling within individual SSCs seen en face consisted of either numerous small puncta or linear arrays of immunoreactivity. By EM, labelled SSCs in the rat facial nucleus were always seen beneath a cluster of C-terminals. Immunolabelling was most dense in the space between the plasma membrane and SSC, which we define as the subsurface cisternal cleft. The SSCs were usually intermittently labelled along their length and exhibited a narrow luminal space ranging from 2 to 5 nm. On the basis of structural analogies between SSCs in neurons and the sacroplasmic reticulum terminal cistern/T-tubule complex in muscle, SSCs have previously been suggested to be important sites of calcium mobilization. The constant association of C-terminal with SSCs in motoneurons may represent a useful model in which to study SSC function as well as to investigate the possible presence of a connexinlike protein at regions of SSCs that form a narrow lumen similar to that at gap junctions.

Pinealocyte subsurface cisterns. I: Cytological aspects including three-dimensional structure

Subsurface cisterns (ssc) in the pineal gland of Meriones unguiculatus have been characterized with transmission electron microscopy, freeze-fracture, morphometry, and three-dimensional reconstruction. Subsurface cisterns (which are observed only in pinealocytes and never in gliocytes) are cisterns of the endoplasmic reticulum (ER) approaching the plasmalemma at a distance of 15-20 nm. Freeze-fracture preparations show that except for some ribosomes at the cytoplasmic face, the cistern membranes as well as the related portions of the plasmalemma are free of special or specifically arranged particles or pores. All ssc have a lumen of 15-20 nm width and underlie 5.6% of the plasmalemma in a single layer; neither collapsed types nor stacks of ssc could be observed. As seen from reconstructions, large ssc are fenestrated and are situated preferentially in regions where the neighboring pinealocyte also bears large ssc. As a consequence, double-sided ssc, which can be observed in sections of (mostly large) ssc, are not a random phenomenon. In regions of the large ssc, adhering junctions are also concentrated. Tubules of 20-nm diameter link ssc with deeper parts of the ER, particularly with the perinuclear cistern and with the tubulo-cisternal network at the trans-side of the Golgi apparatus. Besides ssc proper, a lot of small endplates of ER-tubules are seen close to the plasmalemma. It is suggested that ssc form by the widening and fusing of such ER-tubules and decrease by retraction of ER-tubules from the cell surface.

Small vesicle bouton synapses on the distal half of the lateral dendrite of the goldfish Mauthner cell: freeze-fracture and thin section study

To understand principles of synaptic integration, it is necessary to define the types of synapses on a particular neuron and their distribution. Thin sectioning and double replica freeze-fracture techniques were employed to characterize the small vesicle bouton (SVB) synapses on the distal half of the Mauthner (M) cell lateral dendrite, which probably mediate a remote dendritic inhibition. Three morphologically distinct SVB synapses, types A, B, and C, were found. These three SVB synapses form roughly 90% of the synapses on the distal half of the lateral dendrite, with types A and B being most common. The SVB A synapse is characterized by mostly oval and round synaptic vesicles, a discrete presynaptic active zone with a highly variable shape, and a postsynaptic active zone with no apparent particle aggregate in either the E or P face. At the SVB B synapse, most of the synaptic vesicles are flat. A very high particle density is present throughout the presynaptic P face, and vesicle attachment sites are dispersed over much of the presynaptic membrane. Postsynaptic P face particle aggregates are subjacent to the presynaptic vesicle attachment sites, and are often large and anastomosing. The SVB C synapse is characterized by synaptic vesicle profiles that vary from flattened to round. The SVB C cytoplasm was unclouded by the flocculent material that characterized SVBs A and B. The presynaptic active zones at the SVB C synapse are discrete, and macular or oblong. No particle aggregates are apparent in the postsynaptic active zone. Small, macular particle aggregates were found in nonactive zone regions of the postsynaptic E face of all three types of SVBs. Small subsurface cisterns were also observed underlying the M cell membrane at all three types of SVB synapses. Neither the postsynaptic E face aggregates nor the subsurface cisterns were ever observed directly subjacent to presynaptic active zones, but were often seen adjacent to active zones. Short, straight rows of particles and short cylinders were often seen in both pre- and postsynaptic surrounding zone regions of SVB A and C synapses. These structures are thought to represent tight junctions.

Three-dimensional analysis of dendritic spines. I. Quantitative observations related to dendritic spine and synaptic morphology in cerebral and cerebellar cortices

A total of 212 dendritic spines (108 from the visual and 104 from cerebellar cortices of the mouse) were analyzed in serial sections. Dendritic spines (DS) and synaptic active zones (SAZ) were classified according to their shape, and the following quantitative data were measured: DS stalk and bulb diameters, DS length and volume, number of cisterns of the spine apparatus, DS and SAZ surface areas and their mutual proportions. Quantitative relationships between the spine apparatus and the size of DS and SAZ, between the volume and surface area of DS and between the size of DS and the size of SAZ were studied. Thin, mushroom-shaped and stubby DS with simple (circular or oval), complex (perforated, annulate or horseshoe-shaped) and multifocal SAZ were found on terminal branches of pyramidal cell apical dendrites and club-shaped DS with simple (circular or oval) SAZ on spiny branchlets of Purkinje cells. Statistically significant differences were found between all values measured on various DS types in the visual cortex. Linear dependencies of the DS surface area on DS volume and of the SAZ surface area on the DS surface area were established. Only a limited area of DS plasma membrane (7-10%) was occupied by SAZ. This finding indicates a possible functional importance of the SAZ/DS (and possibly also of the total SAZ/total postsynaptic membrane) surface ratio.

Fine structure of the sensory epithelium of guinea-pig organ of Corti: subsurface cisternae and lamellar bodies in the outer hair cells

The fine structure of subsurface cisternae and lamellar bodies in the outer hair cells of the guinea-pig organ of Corti was studied with thin sections and freeze-fracture replicas. Subsurface cisternae in the outer hair cells consist of multilayers along the lateral plasma membrane of the cell. The outermost layer is a flattened cistern in the upper part of the supranuclear region, but comprises a series of tubules in the lower part. Deeper layers are fenestrated cisternae in which disc-like areas are found in the upper part of the supranuclear region. Lamellar bodies consist of concentric layers of fenestrated cisternae and are located in the apical cytoplasm beneath the cuticular plate. They are continuous with the subsurface cisternae. In the supranuclear cytoplasm, bulges of the subsurface cisternae and the lamellar bodies are found. Dilated cisternae are also present. Some dilated cisternae contain many small vesicles, which display acid phosphatase activity. The dilated cisternae are considered as forms of the bulges undergoing transformation into multivesicular bodies. The possible role of the lamellar bodies, and the origin and fate of the subsurface cisternae are discussed.

An analysis of the epileptogenic potency of CO2+- its ability to induce acute convulsive activity in the isolated frog spinal cord

The action of Co2+ on the isolated frog spinal cord was studied by extracellular application of the ion in the superfusing solution. A complete and reversible blockade of chemical synaptic transmission by Co2+ (3 mmol/l) could be achieved after a superfusion period of 20-30 min. During continued Co2+ application (greater than 60 min) the following effects upon the motoneuron membrane, dorsal root and ventral root fibres were observed. Motoneurons and ventral root fibers: 1. prolongation of initial segment action potential to a maximum of 30 ms, 2. blockade of the long afterhyperpolarization, 3. abolition of adaptation, 4. increased duration of fibre action potential in the ventral root, 5. backfiring after ventral root stimulation. Dorsal root fibres: 1. prolongation of the extraspinal fibre action potential, 2. marked prolongation of the action potential of the terminal region, 3. backfiring of multiple action potentials after dorsal root stimulation. Even in the presence of Co2+, when synaptic transmission was completely blocked, strong convulsive reactions of the isolated spinal cord were observed. Intracellular injection of Co2+ into motoneurons did not affect the action potential, but led to a shift of the EIPSP towards the membrane potential. The results indicate that the induction of convulsive reactions by Co2+ is mainly due to a prolongation of action potentials. The plateau-like deformation of the action potential of the initial segment membrane and presumably of the terminal region of nerve endings results in retrograde propagation of action potentials and in some cases induces oscillatory discharge of single neurons.

Subsurface cisterns and lamellar bodies in the granule cells of the guinea-pig fascia dentata

Subsurface cisterns (SSC's) and less frequent lamellar bodies (LB's) were identified in the granule cells of the guinea-pig fascia dentata. Both structures, composed of flattened or collapsed agranular cisterns, are continuous with the regular rough endoplasmic reticulum and occasionally connected with each other forming LB-SSC complexes. The SSC's are apposed to glia, synaptic boutons, and nerve cell processes as well as to neighbouring granule cells appearing here singly and in confronting pairs. The quantitative analysis of the various cisternal appositions compared to the distribution of the tissue components on the granule cell soma shows that the overwhelming majority of SSC's are related to glial cells.