The ultrastructure of neuronal contacts

Structure of peripheral synapses: autonomic ganglia

Final motor neurons in sympathetic and parasympathetic ganglia receive synaptic inputs from preganglionic neurons. Quantitative ultrastructural analyses have shown that the spatial distribution of these synapses is mostly sparse and random. Typically, only about 1%-2% of the neuronal surface is covered with synapses, with the rest of the neuronal surface being closely enclosed by Schwann cell processes. The number of synaptic inputs is correlated with the dendritic complexity of the target neuron, and the total number of synaptic contacts is related to the surface area of the post-synaptic neuron. Overall, most neurons receive fewer than 150 synaptic contacts, with individual preganglionic inputs providing between 10 and 50 synaptic contacts. This variation is probably one determinant of synaptic strength in autonomic ganglia. Many neurons in prevertebral sympathetic ganglia receive additional convergent synaptic inputs from intestinofugal neurons located in the enteric plexuses. The neurons support these additional inputs via larger dendritic arborisations together with a higher overall synaptic density. There is considerable neurochemical heterogeneity in presynaptic boutons. Some synapses apparently lack most of the proteins normally required for fast transmitter release and probably do not take part in conventional ganglionic transmission. Furthermore, most preganglionic boutons in the ganglionic neuropil do not form direct synaptic contacts with any neurons. Nevertheless, these boutons may well contribute to slow transmission processes that need not require conventional synaptic structures.

The mammalian central nervous synaptic cleft contains a high density of periodically organized complexes

Cryo-electron microscopy of vitreous section makes it possible to observe cells and tissues at high resolution in a close-to-native state. The specimen remains hydrated; chemical fixation and staining are fully avoided. There is minimal molecular aggregation and the density observed in the image corresponds to the density in the object. Accordingly, organotypic hippocampal rat slices were vitrified under high pressure and controlled cryoprotection conditions, cryosectioned at a final thickness of approximately 70 nm and observed below -170 degrees C in a transmission electron microscope. The general aspect of the tissue compares with previous electron microscopy observations. The detailed analysis of the synapse reveals that the density of material in the synaptic cleft is high, even higher than in the cytoplasm, and that it is organized in 8.2-nm periodic transcleft complexes. Previously undescribed structures of presynaptic and postsynaptic elements are also described.

The structure of the fourth abdominal ganglion of the crayfish, Procambarus clarki (Girard). II. Synaptic neuropils

Four discrete regions of synaptic neuropil in the crayfish fourth abdominal ganglion are described by light and electron microscopy. The largest is the horseshoe neuropil, a horseshoe-shaped mass of synaptic glomeruli that lies horizontally in the ventral ganglionic core. This neuropil has a substructure of three rings of fused glomeruli associated with the entry of small axons from the first and second nerve roots. The lateral neuropils are large, paired bulges of neuropil that define the sides of the ganglionic core. They contain neuronal profiles of various sizes, filled with clear or dense-cored vesicles. The neurons are randomly oriented except for occasional dendritic bundles. The tract neuropil is ultrastructurally similar to the lateral neuropils but it is distributed among the largest axons of the through-tracts and commissures. The midline neuropils are small, U-shaped clumps of uniformly sized neuronal profiles that contain large numbers of dense-cored vesicles and distinctive lamellar inclusions.

Preliminary characterization of synaptic vesicle/calmodulin interaction

The association of calmodulin with brain synaptic vesicle proteins was analyzed. Scatchard analysis of [125I]calmodulin binding to brain synaptic vesicles revealed one high-affinity, low-binding-capacity, Kd = 1.0 (+/- 0.15) nM, Bmax = 4.1 (+/- 0.6) pmol/mg, and one low-affinity high-binding-capacity site, Kd = 177. (+/- 12.0) nM and Bmax = 202 (+/- 15.0) pmol/mg. Triton X-100 solubilization of synaptic vesicle proteins and subsequent elution on a Sepharose-4B-CNBr-calmodulin affinity column demonstrated that two protein doublets of approximate MrS 55 K and 30 K were the major synaptic vesicle calmodulin binding proteins. In addition there were two minor calmodulin binding singlet polypeptides with MrS 62 K and 40 K. Calmodulin stimulated endogenous synaptic vesicle protein kinase, Ca2+, Mg2+-ATPase and Ca2+ uptake activities. Phosphorylation assays coupled with immunological studies using affinity-purified antibodies suggested that the synaptic vesicle Ca2+/calmodulin-dependent protein kinase migrated in the 30 K Mr region.

An ultrastructural study of serially sectioned Renshaw cells. II. Synaptic types

Boutons and synaptic contacts on 17 presumed Renshaw cells were studied ultrastructurally. All 17 neurons were postsynaptic to axon collateral boutons of intracellularly HRP-stained triceps and surae alpha-motoneurons and located in lamina VII, ventromedially to the main motor nuclei. The boutons and synaptic contacts could be classified into two main categories on the basis of synaptic vesicles: S-type boutons with spherical synaptic vesicles and F-type boutons with flattened vesicles, the alpha-motoraxon collateral boutons falling into the S-category. In addition, some S-type boutons containing neurofilaments and some being apposed by small presynaptic boutons were observed. The results are discussed to earlier observations on the synaptology of central neurons, particularly spinal alpha-motoneurons.

An ultrastructural study of the synaptic contacts of alpha 1-motoneuron axon collaterals. II. Contacts in lamina VII

Horseradish peroxidase (HRP) was injected intracellularly in triceps surae alpha-motoneurons. The axons and axon collaterals of these neurons were traced light and electron microscopically. Synaptic boutons of collaterals in the ventral part of Rexed's lamina VII were studied ultrastructurally. The boutons exhibited spherical synaptic vesicles and made synaptic contacts with cell bodies and proximal dendrites of neurons assumed to be Renshaw cells and with dendrites of unknown origin. The observations are discussed in relation to earlier qualitative and quantitative studies on the other known synaptic contacts of the alpha-motor axons, both in the central and peripheral nervous system.