Synaptogenic effect of estrogen on the hypothalamic arcuate nucleus of the adult female rat

Summary In order to examine the effect of estrogen on the synaptic structures in the hypothalamic arcuate nucleus (ARCN), semi-quantitative studies were performed by counting synapses in an 18,000 m² area in the middle part of the ARCN in each brain. In ovariectomized female rats injected with 2 g of estradiol benzoate (EB) for three weeks, the mean numbers of axodendritic and axosomatic synapses were not significantly different from those in the intact and ovariectomized controls. When the medial basal hypothalamus (MBH) including the ARCN was isolated by use of a Halász knife (MBH island), the mean number of axodendritic synapses was decreased to about half of the controls. However, EB treatment for three weeks from the day of surgery effectively restored the axodendritic synaptic population of the deafferented ARCN. This may suggest that estrogen has a facilitatory effect on axodendritic synapse formation in the deafferented ARCN, presumably by stimulating axonal sprouting and synaptic regeneration of intact axons in the MBH island.Supported by grants from the Ministry of Education of Japan     

Distribution of synaptic boutons in the mesencephalic trigeminal nucleus of the rat--a quantitative electron-microscopical study.

The distribution of synapses and synaptic bouton types in the mesencephalic trigeminal (Me5) nucleus was examined in a quantitative electron-microscopical study. Of 588 terminal boutons that were counted in the compact caudal part of the Me5 nucleus, less than 8% formed synapses on the somata of the predominantly unipolar Me5 neurons. About 79% formed synapses on fibres located between the Me5 somata, while about 13% of the vesicle-containing terminals had no clear synaptic specialization. All of these non-synaptic terminals were G type boutons, with pleomorphic and large characteristic dense-core vesicles. Approximately 60% of the axosomatic synapses were of the S type, containing spherical vesicles and an asymmetrical or symmetrical synaptic specialization. About 20, respectively 15% of the axosomatic synapses, were of the F, respectively P type; both are symmetrical synapse types containing either a majority of flat or pleomorphic vesicles. Less than 10% of the axosomatic synapses were of the G type. Although some proportional differences were noted, an almost similar bouton type distribution pattern was found for the axodendritic synapses suggesting that the axosomatic and axodendritic synapses in the Me5 nucleus are part of the same afferent fibre plexus covering the Me5 nucleus.     

Ultrastructural organization of the synapses in ganglia of the hen intestinal nerve under various conditions of its activity

The interneuronal connections in ganglia of the caudal part of the hen intestinal nerve of Remak are presented as axodendritic and axosomatic synapses and symmetric axo-axonal, dendro-dendritic and axodendritic contacts, often forming complicated complexes. Under conditions of preliminary decentralization or under certain disturbances of nervous connections with the intestine, a part of synapses remains, and a part of them degenerates, this demonstrates participation of peripheral afferent neurons in formation of the synaptic apparatus of the ganglia mentioned. The axonal terminals differentiate by composition of the synaptic vesicles: some contain mainly light agranular vesicles, others--a large amount of granular ones. The characteristic peculiarities of the hen intestinal nerve ganglia, in contrast to analogous mammalian ganglia, are abundant axosomatic synapses in some neurons, and presynaptic terminals, containing a large number of granular vesicles.     

Ultrastructure of synapses and cellular relationships in the oculomotor nucleus of the rhesus monkey

Summary The fine structure of synapses and of cellular relations was examined in the somatic efferent portion of the oculomotor nucleus of the adult rhesus monkey. Axosomatic and axodendritic synapses are characterized by distinct synaptic clefts which usually measure 20–30 nm between pre- and postsynaptic membranes. Cytoplasmic thickenings of pre- and postsynaptic membranes are often observed. Subjunctional bodies are present at both axosomatic and axodendritic synapses. Somatic and dendritic spine synapses are present. Serial synapses are also found, suggesting the operation of presynaptic inhibition in this nucleus. At some synapses the extracellular gap between pre- and postsynaptic membranes is reduced to 5–9 nm. However, junctions similar to the latter are also present between neurons and glia, and at the junctions between adjacent glial elements. The present results provide no evidence for a clear morphological substrate for electrotonic transmission in the somatic efferent portion of the primate oculomotor nucleus.Supported in part by grants from the National Institutes of Health (NS-15320), the Kroc Foundation, and by the Medical Research Service of the Veterans Administration (to S.G.W.), and from the National Science Foundation (BNS 77-28493) and the Muscular Dystrophy Association of America (to G.D.P.)     

Ultrastructural changes in the interneuronal contacts of the upper layers of the cerebral cortex in cats when aminazine is applied

The fine structure of upper layer of axodendritic synapses of the cat cerebral cortex was studied during 10, 45 and 90 minute applications of aminazine, and also 2.5 and 5 hours after the cessation of the 90 minute application. During the 10 and 45 minute applications no clear-cut changes are observed in the ultrastructure of the synapses. 90 minutes after the onset of application osmophilic changes are enhanced in the pre- and postsynaptic areas. During the drug application, the number of synaptic vesicles and the area of the axonal terminals decrease. The above morphological changes are reversible: restoration of the number of synaptic vesicles commences, but the area of the terminal is still shrunken.     

Structure of synapes in the supraoesophageal ganglion of the water beetle (Dytiscus marginalis).

1. Light- and electron microscopic investigations prove that synapses, without any exception, are confined to the neuropil. 2. Under the light-microscope, synapses display the shape of terminal boutons at the ends of the nerve fibres; in some cases they appear as smaller or larger plates. Electron microscopic investigations suggest that also varicosities of nerve fibres can be regarded as synapses, though these might have possible arisen from axoplasmatic peristalsis. 3. Electron microscopically the overwhelming majority of the synapses are axodendritic contacts; axo-axonic contacts occur less often. 4. The generally accepted characteristics of synapses are defective. Membrane thickenings and intersynaptic spaces are missing. Accordingly, synapses in the supraoesophageal ganglion of the water beetle differ markedly from those described in Vertebrates. 5. Synaptic vesicles sometimes fill the axoplasm of the nerve fibre completely. In other cases, clusters of synaptic vesicles can be seen, on both sides of the contact. 6. Synaptic vesicles are mixed with neurosecretory granules. Synaptic vesicles may appear also in the dendrites.     

Long-term effects of alloxan-induced diabetes on the nucleus ventralis posterolateralis in the thalamus of the rat.

The present paper describes the long-term ultrastructural changes in the nucleus ventralis posterolateralis of the thalamus of male Wistar rats after alloxan-induced diabetes. Degenerating dendrites were characterized by an electron-dense cytoplasm with scattered endoplasmic reticulum and ribosomes. Degenerating axon terminals were characterized by an electron-dense cytoplasm and clustering of small spherical agranular vesicles. Degenerating axon terminals formed axosomatic synapses with seemingly normal cell bodies and axodendritic synapses with normal as well as degenerating dendrites. Degenerating axons (both myelinated and unmyelinated) were readily encountered in the neuropil. Activated microglial and astrocytic cells in the neuropil were in the process of phagocytosis or had residua in their cytoplasm.     

Synaptology of three peptidergic neuron types in the central nucleus of the rat amygdala

The synaptology of neurotensin (NT)-, somatostatin (SS)- and vasoactive intestinal polypeptide (VIP)-immunoreactive neurons was studied in the central nucleus of the rat amygdala (CNA). Three types of axon terminals formed synaptic contacts with peptide-immunoreactive neurons in the CNA: Type A terminals containing many round or oval vesicles; Type B terminals containing many pleomorphic vesicles; and Type C terminals containing fewer, pleomorphic vesicles. Peptide-immunoreactive terminals were type A. All three types of terminals formed symmetrical axosomatic and asymmetrical axodendritic contacts. However, type B and peptide-immunoreactive terminals frequently formed symmetrical axodendritic synaptic contacts. VIP-immunoreactive terminals also formed asymmetrical axodendritic contacts. SS- and NT-immunoreactive terminals commonly formed symmetrical contacts on SS- and NT-immunoreactive cell bodies, respectively. VIP-immunoreactive axon terminals were postsynaptic to nonreactive terminals. Type B terminals appeared more frequently on VIP neurons than on NT or SS neurons.     

Communicating synapses: types and functional interpretation. Exceptions to Cajal's neuron theory.

The neurons of the dorsal periaqueductal nucleus of the mesencephalon and their synaptic contacts were observed under a transmission electron microscope. We found various types of synapses which constituted an exception to Cajal's neuron theory (law of neuron independence). Some of these synapses had an open communicating or continuity 'passage' between the presynaptic bouton of a neuron (first neuron) and the postsynaptic portion of another neuron (second neuron). The 'communicating' passage (located in the synaptosome) is formed by the continuity of the presynaptic and postsynaptic membrane, and its limits or rims are the reflexion points of the membranes. When only two neurons intervene they could be termed 'simple communicating synapses'. We found three types: I = communicating axosomatic synapses; II = communicating axodendritic synapses, and III = communicating axoaxonic synapses'. When three neurons intervene in the synaptic contact, they could be termed 'complex communicating synapses'. In these, the first and second neurons form a normal synapse, but the lateral portion of the presynaptic bouton of the first neuron also enters into contact with a third neuron, with which it establishes an open communicating or continuity passage. The points of these passages are collateral to the synapse, and may be in the presynaptic or pre-postsynaptic portions simultaneously, communicating collaterally with the third neuron. We found a further three types: IV = complex communicating axosomatic and dendritic synapses; V = complex communicating axoaxonic and somatic synapses, and VI = complex communicating axodendritic and double-somatic synapses. It is suggested that communicating synapses may constitute an exception to Cajal's neuron theory, representing functional states for the acceleration, retardation or modulation of the synaptic function. The neurotransmitters would pass en masse through the communicating passage and the depolarization wave would pass through the rims without being retarded. In the simple communicating synapses, their action would be intensifying. In the complex communicating synapses, their action would be modulating or retarding, since the collateral communicating passage would function as an 'escape valve' through which part of the impulse reaching the presynaptic bouton would escape.     

Synapsoarchitectonics of the septum of the cat brain

In the medial and lateral septal nuclei, 4 types of axonal terminals are distinguished. Type I contains spherical vesicles and forms asymmetric synapses on small and middle stems and spines of the dendrites; type I terminals comprise 63% in the medial nucleus of the total number of axons, and in the lateral one--52%. Type II contains polymorphic vesicles and forms symmetrical synapses on the soma and large dendrites. In the medial nucleus they comprise 6%, and in the lateral one--3%. Type III contains either clear spherical (IIIa), or polymorphic (IIIb) vesicles, as well as 1-2 vesicles with a dense core. They form axodendritic, axospine and axosomatic synapses. In the medial nucleus they comprise 25% and 3%, respectively, in the lateral one--40% and 2%. Type IV contains a great number of vesicles with a dense core. These terminals in both septal nuclei comprise 3% and do not participate in formation of active contacts.     

Synaptogenesis in the neural ganglia of the heart in the human embryo

Electron microscopy was used to study synapse development in the cardiac ganglia of human fetuses ranging from 8 to 27 weeks of ovulation time. Staining with ethanolic phosphotungstic acid was used for analysis of synaptic active zones. Specialization of interneuronal links begins with the appearance of electron dense material on plasmalemmas of nerve cells in the places of simple contacts. First synapses with single synaptic vesicles and short osmiophilic zones were found in cardiac ganglia in 8-week-old fetuses. Large granular vesicles and mitochondria vesicles are formed from cisternae of agranular endoplasmic reticulum in the preterminal parts of axons and moved by axoplasmic transport to the osmiophilic zones of future synapses. Axodendritic synapses appeared earlier in the cardiac ganglia than axosomatic ones, the latter were observed from the middle of gestation. Transient neuroglial synapse-like contacts were found in the cardiac ganglia. Staining with phosphotungstic acid made it possible to distinguish the degree of synapse maturation according to active synaptic zones. The peculiarities of synaptic development in cardiac ganglia in comparison with that in the central nervous system may be accounted for by different origins of the neural tube and of neural crest and by the level of their phylogenic development.     

Synaptic processes in red nucleus neurons produced by stimulating the entopeduncular nucleus and globus pallidus in the cat

Synaptic processes in red nucleus neurons produced by stimulating the entoped-uncular nucleus and the globus pallidus were investigated during acute experiments on cats using intracellular recording techniques. It was found that stimulating these structures produces mono- and polysynaptic excitation of rubrospinal neurons. Analysis of the time parameters of the EPSP recordings obtained suggested that they were produced by activation of the axosomatic and axodendritic synapses of rubrospinal neurons. Mechanisms of basal ganglia involvement in the integrating of red nucleus activity are examined.L. A. Orbeli Institute of Physiology, Academy of Sciences of the Armenian SSR, Erevan. Translated from Neirofiziologiya, Vol. 17, No. 6, pp. 809–817, November–December, 1985.     

Synaptic components of cerebellar electrocortical activity evoked by various afferent pathways

Electrical responses evoked in different regions of the cerebellar cortex of cat by stimulating various cerebello-petal pathways have been analyzed for their component postsynaptic potentials (p.s.p.'s). The principal analytical tools of the present work were pharmacological agents; the selective inactivator of depolarizing (excitatory) axodendritic synapses, γ-aminobutyric acid (GABA, or C₄); the homologous C₆ and C₈ ω-amino acids, which inactivate selectively the hyperpolarizing (inhibitory) axodendritic synapses; and the general inactivator of inhibitory synapses, strychnine. Some experiments employed the analytical possibilities of activity cycles. The potentials evoked in one cerebellar region by different exciting pathways may differ markedly in their responses to drugs or may show different types of activity cycle. Also, the potentials evoked in various cortical regions by one cerebello-petal pathway are acted upon differently by the testing drugs. These differences are believed to be due to involvement of different proportions of excitatory and inhibitory, axosomatic and axodendritic p.s.p.'s. The analyses of a number of different responses confirm an earlier conclusion, that the cerebellar cortex is relatively lacking in inhibitory axodendritic p.s.p.'s in comparison with the cerebral cortex. Only the cortex of the paramedian lobule appears to be endowed with a considerable proportion of inhibitory p.s.p.'s, a finding which correlates with other data.     

Fine structure of the paraventricular magnocellular neurons in the domestic fowl

The morphology of the PVN of the domestic fowl (Gallus domesticus) was studied by the Golgi method and the electron microscope. The magnocellular neurons of the PVN are periventricularly scattered, in the region caudal to the anterior commissure, and they are situated in small clusters. Their ultrastructural features are the presence of a large amount of cytoplasm containing also few dense-core vesicles (1100--1500 A in diameter). In the neuropil axosomatic and axodendritic synapses are seen. Some of these show also dense-core vesicles (600--900 A).     

Ultrastructural features of presumptive vasopressinergic synapses in the hypothalamic magnocellular secretory nuclei of the rat

Despite convincing physiological evidences for vasopressin (VP) autoregulation in the supraoptic (SON) and paraventricular (PVN) nuclei, the morphological demonstration of VP synapses has lagged behind. The present work investigates the possible existence of such synapses in the SON and PVN of the rat. Electron microscopy of sections immunostained with VP antibody (1:5,000) and conjugated with avidin-biotin demonstrated presynaptic terminals containing neurosecretory granule (NSG)-like bodies, 80-100 nm in diameter. The terminals formed axodendritic, axosomatic and axoaxonic synapses, though the postsynaptic elements remained largely unidentified. Other ultrastructural features of synaptic specialization were evident. The NSG-like bodies exhibited a varying and dynamic relationship to the presynaptic membrane, suggesting their involvement in synaptic mechanisms.     

Electrotonic synapses in the mammalian spinal cord

By means of light and electron microscopy methods structural peculiarities of motor nuclei have been studied in the rat spinal cord (17 animals) on the 1st-3d and on the 10th-18th days of postnatal ontogenesis. Synaptic junctions of the gap type are revealed; they are considered as electrotonic synapses. Dendro-somatic and dendrodendritic synaptic junctions of the gap type are found. Together with the electrotonic synapses, morphologically mixed synapses of axo-somatic and axo-axonal types are disclosed; they contain, besides organells, specific for chemical synapses, close opposition areas of pre- and postsynaptic membranes of the gap junction type. Morphologically mixed synapses occur in neuropil of the motor nuclei of the spinal cord in young rats of all age groups studied. Homologous synapses are detected in the motor nuclei of the white mouse spinal cord. Synaptic junctions of the gap type in the mammalian spinal cord could be a substrate of electrical interaction between its motor neurons.     

Fine structural characteristics and synaptic connections of trigeminocerebellar projection neurons in rat trigeminal nucleus oralis

In order to classify the presynaptic terminals contacting trigeminocerebellar projection neurons (TCPNs) in rat trigeminal nucleus oralis (Vo), electron-microscopic examination of sequential thin sections made from TCPNs located in the border zone (BZ) of Vo, labeled by the retrograde transport of horseradish peroxidase, was undertaken. The use of BZ TCPNs, labeled in Golgi-like fashion so that many of their dendrites and axons were visible, allowed for the determination of the distribution of each bouton type along the soma and dendrites, as well as for the characterization of the morphology and synaptic relations of the labeled axon and its terminals. Three types of axon terminals contacting labeled BZ TCPNs have been recognized, depending upon whether they contain primarily spherical-shaped, agranular synaptic vesicles (S endings); predominantly flattened, agranular synaptic vesicles (F endings); or a population of pleomorphic-shaped, agranular synaptic vesicles (P endings). The S endings represent the majority of axon terminals contacting labeled BZ TCPNs and establish asymmetrical axosomatic and axodendritic synaptic contacts. Many S endings are situated in one of two types of synaptic glomeruli. One type of glomerulus has a large S ending at its core, whereas the other contains a small S ending. Large-S-ending glomeruli include only labeled distal dendrites of BZ TCPNs; small-S-ending glomeruli contain either a labeled soma, proximal dendrite, or distal dendritic shaft. The remaining S endings are extraglomerular, synapsing on distal dendrites. P endings are less frequently encountered and establish intermediate axosomatic and axodendritic synapses. These endings exhibit a generalized distribution along the entire somatodendritic tree. F endings make symmetrical axodendritic synapses with distal dendrites, are only found in glomeruli containing small S endings, and are the least frequently observed ending contacting labeled BZ TCPNs. The majority of axonal endings synapsing on labeled BZ TCPNs are located along distal dendrites, with only a relatively few synapsing terminals situated on proximal dendrites and somata. The axons of labeled BZ TCPNs arise from the cell body and generally give rise to a single short collateral near their points of origin. This collateral remains unbranched and generates several boutons within BZ, while the parent axon acquires a myelin sheath and, without branching further, travels dorsolaterally toward the inferior cerebellar peduncle. The collateral boutons resemble extraglomerular S endings. They contain agranular, spherical-shaped synaptic vesicles and make asymmetrical axodendritic synapses with small-diameter unlabeled dendritic shafts in the BZ neuropil.     

Electron microscopy of the indoleamine-accumulating neurons in the retina of the rabbit

Summary The recently discovered indoleamine-accumulating retinal neurons were studied electron microscopically after destruction of the dopaminergic retinal neurons and subsequent labeling with 5,6-dihydroxytryptamine. These observations confirm earlier fluorescence microscopical studies on the distribution of the indoleamine-accumulating neurons in the rabbit retina. Their perikarya are known to be located in the inner nuclear layer (INL) among the amacrine cell bodies. Their processes are found only in the inner plexiform layer (IPL), most of them in the innermost third part of that layer. The indoleamine-accumulating terminals are pre- and postsynaptic to bipolar neurons in the innermost sublayer of the IPL. Reciprocal synapses are probably the rule. The synaptic vesicles of indoleamine-accumulating synapses onto bipolar cells are arranged in globular clusters around a central electron dense, round body. A number of synapses formed by unlabeled amacrine neurons with postsynaptic indoleamine-accumulating elements were also detected. These synapses were mainly found in the outermost third of the IPL. Synaptic contacts between presynaptic indoleamine-accumulating neurons and postsynaptic unlabeled processes of amacrine cells are very rare.     

Synaptogenesis of hippocampal neurons in primary cell culture

Hippocampal neurons in dissociated cell culture are one of the most extensively used model systems in the field of molecular and cellular neurobiology. Only limited data are however available on the normal time frame of synaptogenesis, synapse number and ultrastructure of excitatory synapses during early development in culture. Therefore, we analyzed the synaptic ultrastructure and morphology and the localization of presynaptic (Bassoon) and postsynaptic (ProSAP1/Shank2) marker proteins in cultures established from rat embryos at embryonic day 19, after 3, 7, 10, 14, and 21 days in culture. First excitatory synapses were identified at day 7 with a clearly defined postsynaptic density and presynaptically localized synaptic vesicles. Mature synapses on dendritic spines were seen from day 10 onward, and the number of synapses steeply increased in the third week. Fenestrated or multiple synapses were found after 14 or 21 days, respectively. So-called dense-core vesicles, responsible for the transport of proteins to the active zone of the presynaptic specialization, were seen on cultivation day 3 and 7 and could be detected in axons and especially in the presynaptic subcompartments. The expression and localization of the presynaptic protein Bassoon and of the postsynaptic molecule ProSAP1/Shank2 was found to correlate nicely with the ultrastructural results. This regular pattern of development and maturation of excitatory synapses in hippocampal culture starting from day 7 in culture should ease the comparison of synapse number and morphology of synaptic contacts in this widely used model system.