Projections of ventromedial hypothalamic neurons to the midbrain central gray: an ultrastructural study

Ventromedial hypothalamic projections to the midbrain central gray may be involved in the mediation of female reproductive behavior. In order to demonstrate and examine projections of the ventromedial nucleus in the midbrain central gray, in the rat, electrolytic lesions were placed in the ventromedial nucleus and the midbrain central gray was examined for ultrastructural signs of degeneration at various intervals, i.e. 27.5 h and two, four, six and eight days following the lesions. The fine structure of the midbrain central gray of unlesioned animals was also examined to characterize its normal morphology and to establish a baseline with which to compare the effects of the lesion. In unlesioned animals, the neuropil of midbrain central gray contained several synaptic types, with axodendritic synapses appearing to be the most predominant. Dendrites contained well-preserved microtubules. Synaptic endings contained many clear, round vesicles and some contained dense-cored vesicles as well. Neuropil synapses were both asymmetric and symmetric. Cell bodies were characterized by light cytoplasm and had asymmetric and symmetric synapses on their surface. Following electrolytic lesions in the ventromedial nucleus, various types of degenerative patterns were seen in the midbrain central gray, including electron-dense, flocculent, watery, and pinocytotic degeneration. Specific characteristics of degeneration included shrunken, dense axons and endings, clumped synaptic vesicles, abnormally large, dark mitochondria, membranous sacs of various sizes, swollen endings with reduced numbers of synaptic vesicles, and endings and processes containing large numbers of coated vesicles. Some of these signs were already evident at 27.5 h following the lesion. In addition, degenerating postsynaptic processes and cell bodies were seen in the midbrain central gray. At 27.5 h survival time, degenerating dendritic processes often appeared swollen, devoid of microtubules, and contained enlarged mitochondria. At longer survival times neuronal degeneration was observed in the midbrain central gray, characterized by electron-dense cell bodies and pycnotic nuclei. Both degenerating pre- and postsynaptic elements appeared to be engulfed by glial processes. Control lesions in non-hypothalamic regions which project to the midbrain central gray, i.e. nucleus gigantocellularis and pontine reticular formation and in a non-projecting region, i.e. parietal cortex, were performed.(ABSTRACT TRUNCATED AT 400 WORDS)     

Spatial distribution of pre- and postsynaptic sites of axon terminals in the dorsal horn of the frog spinal cord

Axon terminals which could be interpreted as dorsal root boutons, were photographed from a series of 98 ultrathin sections with a Jeol 100B electron microscope. A total of 13 boutons were recovered for computer reconstruction. Two of them were terminal boutons, eight en passant boutons and three boutons were only partially recovered. All boutons contained multiple synaptic sites (maximum 33 and minimum seven) at which axodendritic and axoaxonic synapses were established. Axodendritic synapses were of the asymmetric type and they were directed toward adjacent dendrites. In axoaxonic synapses, which were of the symmetric type, the boutons were invariably on the postsynaptic side. Among the presynaptic profiles axons with spherical and pleomorphic vesicles and dendrites with flattened vesicles could be discerned. On average, each 2.67-microns2 bouton surface area contained one presynaptic site at which an axodendritic synapse was established, and each 7-microns2 surface area contained one postsynaptic site for an axoaxonic (or dendroaxonic) contact. A tendency of grouping of synaptic sites was observed. Distance measurements between the closest neighbours of all synaptic sites were made in four combinations in boutons with the original and with a random distribution of synaptic sites. The arithmetic mean of distances measured between the presynaptic and the closest postsynaptic sites was almost twice as big as that measured in the reverse direction. The difference between these values became greatly reduced in the case of random distribution. The arithmetic mean of distances between the closest neighbours of presynaptic sites was about the same as that between the closest neighbours of postsynaptic sites. This latter value was considerably increased with randomly distributed synaptic sites. The results suggest a non-random distribution of synaptic sites on the surface of boutons. The analysis of cluster formation of synaptic sites performed with a numerical taxonomy technique revealed that the majority of the 153 synaptic sites were comprised in 27 clusters containing both pre- and postsynaptic sites within the 1-micron similarity level. All postsynaptic sites were within 1 micron of one or more presynaptic sites. On the basis of the assumption that the postsynaptic sites are occupied by inhibitory axoaxonic synapses, it is suggested that the transmitter release from the presynaptic sites can be individually controlled in this structural arrangement. A probable mechanism of this function may be the passive invasion of the bouton by the impulse propagating actively along the dorsal root fibre.     

Compensation in the number of presynaptic dense projections and synaptic vesicles in remaining parallel fibres following cerebellar lesions

Summary Our previous investigations demonstrated an increase in the size of remaining synaptic sites as an intermediate or possible alternative to sprouting plasticity. The total amount of postsynaptic contact area remained relatively constant for each target neuron even though there was a marked decrease in the number of sites on these neurons. In addition, enlarged boutons containing numerous synaptic vesicles were positioned adjacent to enlarged postsynaptic sites.The question posed by this study was to determine whether dense projections, parts of the presynaptic grids of the remaining parallel fibres, spread to cover the enlarged postsynaptic sites, or if the number of these densities increased on each site to maintain the structural organization of the presynaptic grid. In addition, the number of synaptic vesicles per bouton was quantitated to determine whether they compensated by increasing their number in relationship to the increased area of the presynaptic grid.The number of parallel fibre synapses on Purkinje cells was reduced by transection of a narrow bundle of parallel fibres accompanied by a small lesion undercutting the molecular layer to destroy granule cells contributing to this bundle. The number of presynaptic dense projections was quantitated in control and lesioned preparations (using ethanolic phosphotungstic acid staining) in order to determine their correlation to the area of each site. In addition, the average number of synaptic vesicles in boutons was compared to the average size of boutons and the average contact area of the synaptic sites. At 3 to 7 days following partial deafferentation of Purkinje cells in adult rats, the density of dense projections of parallel fibre synapses on Purkinje cell spines remained uniform. This occurred throughout a range of reduction in the number of synapses in conjunction with a reciprocal increase in the size of sites. The finding of a uniform density of these projections and an increase in the size of sites implies that each granule cell axon must gain dense projections. In addition, the remaining presynaptic boutons had a uniform density of synaptic vesicles even though the volume of the boutons and the area of the synaptic contact doubled. Thus, the number of synaptic vesicles gained in proportion to the total enlargement of the contact site and the bouton size.These results strongly suggest that deficits or losses in synaptic connections of parallel fibre on Purkinje cell spines produces a compensation in the total number of synaptic vesicles and presynaptic dense projections of the remaining boutons. An enlargement of the presynaptic grid occurs in concert with redistribution of the constant total area of membrane occupied by macromolecules (or insertion of new ones) on remaining postsynaptic sites. These compensations could be facilitating efficacy of neuronal connections after lesions or neuronal attrition by re-establishing available transmitter and release sites in proportion to the constant amount of receptor area.     

Electron microscopic study of GABA-immunoreactive neuronal processes in the superficial gray layer of the rat superior colliculus: their relationships with degenerating retinal nerve endings

Summary GABA-immunoreactive neuronal elements were detected in the stratum griseum superficiale or superficial gray layer of the rat superior colliculus in an electron microscopic study, using postembedding immunocytochemistry with protein A-gold as a marker. In addition to neuronal somata, two types of GABA-immunoreactive neuronal processes were observed. Numerous profiles of axon terminals (1 m in diameter) with clear round or pleomorphic synaptic vesicles and mitochondria were found to establish mostly symmetrical synaptic contacts with GABA-immunonegative dendrites of various diameters. Some axosomatic synapses could also be observed. The gold particle density in this axon terminal compartment was between seven and 13 times the background level. The stratum griseum superficiale also included GABA-immunoreactive dendrites, some of which contained clear synaptic vesicles. These dendritic profiles always formed the presynaptic component of dendrodendritic synaptic contacts. The density of the gold particles in the dendritic compartment, taken as a whole, was between three and 13 times the background level. Furthermore, the relationship between the GABA-immunoreactive neuronal elements and degenerating retinal nerve endings identified in the left stratum griseum superficiale following enucleation of the right eye was investigated after a 7-day survival period. The profiles of degenerating retinal nerve endings (0.7 m in diameter) were found to be devoid of any specific labelling. Most of the retinal boutons established axodendritic synapses of the asymmetrical type with an immunonegative dendrite, which was also contacted in some cases by a GABA-immunopositive axon terminal. Other retinal endings were presynaptic to GABA-immunopositive dendritic profiles with synaptic vesicles, some of which were found to contact in turn an unlabelled dendrite, thereby completing serial synaptic relationships. More rarely, retinal endings formed the presynaptic component of possible axoaxonic synapses with GABA-positive terminals presumed to be axonic in nature. It can be concluded that the retinal input to the superficial gray layer often converges with a GABAergic axonal input on a dendritic target, the neurotransmitter specificity of which is unknown. In other cases, retinal terminals synaptically contact GABA-immunolabelled conventional and presynaptic dendrites and probably also some axon terminals; this might provide an anatomical substrate for the control of GABA release from these GABAergic processes. These results indicate that transmitter GABA plays an important role in retinocollicular transmission.     

Electron microscopy of synapses in reptile spinal cord

The spinal cord of the reptile Anolis carolinensis was examined by electron microscopy. Motor neurons appear as multipolar cells 30-60 micrometer in diameter. Two types of synaptic endings are endings are present on motor neurons. The first type is characterized by distinct synaptic clefts measuring 15-20 nm between pre- and postsynaptic membranes, and by clear presynaptic vesicles. The second type of synapse, which is less common, is characterized by gap junctions between pre- and postsynaptic membranes. At these synapses, there are also clusters of clear vesicles close to the presynaptic membrane adjacent to the gap junction. These findings indicate that both chemical and electrical synaptic transmission are present in the spinal cord of Anolis.     

大白鼠下丘脑腹内侧核突触的电镜观察

本文报告大白鼠下丘脑腹内侧核内突触的类型和突触亚显微结构的形态特征。共观察了1005个突触,其中轴-树突触占96.6%;轴-体突触占2.8%;轴-轴突触占0.5%。在下丘脑腹内侧核中还观察到1例嵴突触,以往未见报道。嵴突触的突触后成分来自树突,呈杵指状突起。突触后膜明显增厚,具有突触下致密小体。两个内含圆形清亮小泡的轴突并列于嵴的两侧壁上,构成嵴突触。在轴-体、轴-树突触中尚观察到并联突触(包括突触复合体)、切线突触及串联突触等复杂的联接形式。本文对某些复合突触的机能也作了讨论。     

Synaptic organization of the dorsal lateral geniculate nucleus in the adult hamster

Summary The synaptic organization of the sector of the dorsal lateral geniculate nucleus has been examined by electron microscopy in normal adult hamsters and in adult hamsters subjected to unilateral eye enucleation or intravitreal injection of horseradish peroxidase.Two types of neuropil are apparent. Islands of complex neuropil partially enclosed by astrocyte processes (synaptic glomeruli) are surrounded by a sea of simpler non-glomerular neuropil. The latter is dominated by small axon terminals with spherical synaptic vesicles and Gray type 1 axodendritic contacts (SR-boutons) and also contains axon terminals with flattened synaptic vesicles (F-boutons). The glomerular neuropil contains (i) exclusively postsynaptic dendrites and dendritic protrusions of presumptive projection cells; (ii) pre- and postsynaptic pleomorphic-vesiclecontaining P-boutons (interpreted as appendages of the dendrites of interneurons); (iii) large axon terminals containing spherical synaptic vesicles and large pale mitochondria (R-boutons) which were experimentally identified as retinal terminals and which are presynaptic to both projection cell dendrites and P-boutons at Gray type 1 contacts; (iv) F-boutons (minority component). F-boutons and P-boutons are presynaptic to both projection cell dendrites and P-boutons and P-boutons are the intermediate elements of various serial synapses including triplet (triadic) synapses. Medium-large terminals with spherical synatpic vesicles and dark mitochondria (RLD-boutons) which were commonly invaginated by dendritic spines of projection cells in small glomerulus-like formations were also identified. The origin of RLD-boutons is unknown but SR-boutons probably derive chiefly from ipsilateral visual cortex and possibly also from superior colliculus, and non-glomerular F-boutons probably originate in the ipsilateral thalamic reticular nucleus.No differences in synaptic organization were found between the part of the nucleus which receives uncrossed retinal input and the part which receives crossed input, nor were differences seen in the size, fine structure or relationships between the terminals of identified crossed and uncrossed retinal axons.     

三叉神经尾侧脊束核内P物质纤维终末的超微结构

目的　进一步探讨三叉神经尾侧脊束核内SP免疫反应阳性纤维在感觉传递中的可能作用。方法　SP采用免疫细胞化学方法和电子显微镜方法 ,观察大鼠三叉神经尾侧脊束核内SP阳性标记纤维的超微结构和突触联系。结果　SP轴突终末分布于树突间 ,这些轴突终末含有大量的透明小泡、少量大致密芯小泡和线粒体。经过秋水仙素处理后 ,可见到SP免疫反应阳性树突。多数SP轴突终末与非标记树突 ,以及个别SP轴突终末与SP树突形成轴 树突触。含SP的突触复合体较为多见 ,为会聚型。其中可见SP轴突终末与中心的非标记树突形成GrayⅡ型轴 树突触 ;另有非标记的轴突终末与中心SP树突形成 (扁平小泡形 )F型轴 树突触。结论　三叉神经尾侧脊束核接受多种纤维传入 ,SP纤维只是多种传入纤维中的一种。形态学证明 ,在痛调制活动中 ,三叉神经尾侧脊束核内有SP纤维构成的突触后抑制类型 (GrayⅡ )突触参与。     

Competitive Hebbian learning through spike-timing-dependent synaptic plasticity

Hebbian models of development and learning require both activity-dependent synaptic plasticity and a mechanism that induces competition between different synapses. One form of experimentally observed long-term synaptic plasticity, which we call spike-timing-dependent plasticity (STDP), depends on the relative timing of pre- and postsynaptic action potentials. In modeling studies, we find that this form of synaptic modification can automatically balance synaptic strengths to make postsynaptic firing irregular but more sensitive to presynaptic spike timing. It has been argued that neurons in vivo operate in such a balanced regime. Synapses modifiable by STDP compete for control of the timing of postsynaptic action potentials. Inputs that fire the postsynaptic neuron with short latency or that act in correlated groups are able to compete most successfully and develop strong synapses, while synapses of longer-latency or less-effective inputs are weakened.     

猫孤束核胶状质亚核的突触构筑学研究

用透射电镜对猫的孤束核胶状质亚核(SNG)的突触型式进行了观察,除看到已报导的轴—树突触、轴—体突触、树—树突触外,还发现该核内含有轴—轴突触及突触球等结构。SNG内轴—树突触最常见,而轴—体突触、轴—轴突触和树—树突触则较少。各类突触中的突触囊泡多为圆形清亮囊泡,而扁平清亮囊泡和大颗粒囊泡较少。扁平清亮囊泡多与圆形清亮囊泡共存于同一轴—轴突触终末内。轴—轴突触均为对称型突触,有时与树突或胞体相连形成轴—轴—树突触或轴—轴—体突触。突触球多为以树突和棘为中心的中心树突型突触球。此外在SNG内还观察到嵴突触,并联突触等连接形式。SNG内突触的复杂性表明传入冲动在该核中可能经过扩散、汇聚和突触前抑制等多种复杂的整合过程调节内脏活动。     

Light and electron microscopic observations on the inner plexiform layer of the rabbit retina

By comparison of electron micrographs with light microscopical specimens impregnated with the Golgi technique, the large endings of the rod bipolar cells have been identified in the innermost region of the inner plexiform layer of the rabbit retina. The rod bipolar endings contain ribbons and synaptic vesicles, do not synapse with the perikaryon of the ganglion cells, are presynaptic to ganglion cell dendrites and to nerve processes which contain synaptic vesicles but lack ribbons. In these synaptic contacts a ribbon is closely associated with the presynaptic membrane and a dense web of fuzzy material is adherent to the cytoplasmic aspect of the postsynaptic membrane. Commonly, one of these synaptic contacts involves a rod bipolar ending and two postsynaptic processes. The postsynaptic process which is provided with synaptic vesicles is often, in turn, presynaptic to the same rod bipolar ending. This synaptic contact is characterized by the presence of a cluster of vesicles closely related to the presynaptic membrane, whereas the postsynaptic membrane lacks a definite subsynaptic web. In the intermediate and scleral regions of the inner plexiform layer endings containing ribbons and synaptic vesicles show with neighboring nerve processes a synaptic pattern similar to the rod bipolar endings. Nerve processes containing synaptic vesicles but lacking ribbons are presynaptic to the perikaryon and dendrites of the ganglion cells; the synaptic contact shows a cluster of vesicles adherent to the presynaptic membrane. Bipolar cells are proposed as the source of the ribbon containing processes while amacrine cells are proposed as the source of the processes devoid of ribbons and presynaptic to both bipolar endings and ganglion cell dendrites and perikarya.     

The fine structure of synapses in relation to the large spherical neurons in the anterior ventral cochlear of the cat

Summary The fine structure of synapses in relation to the large spherical neurons, the principal neurons in the anterior ventral cochlear nucleus (AVCN) of the cat, was studied. Axo-somatic synapses were classified into three types based on their size and shape and the distribution of synaptic vesicles. Axo-dendritic synapses were also classified into three types with respect to synaptic contact, cytoplasmic densities associated with the pre- and postsynaptic membranes and the shape of the synaptic vesicles. Mossy fibre-like endings and preterminal axons containing dense-core vesicles were observed in this study. Serial or triadic synapses were also found in cat AVCN. We occasionally found a nematosome in the cytoplasm of the principal neuron.     

Opioid neurons and pain modulation: an ultrastructural analysis of enkephalin in cat superficial dorsal horn

To clarify the circuitry through which opioid compounds modulate spinal and trigeminal nociceptive transmission, we have examined the synaptic associations formed by leucine-enkephalin-containing (enkephalin) neurons in the superficial dorsal horn of the cat. As described previously, punctate enkephalin immunoreactivity is concentrated in the marginal layer (lamina I) and in both the outer and inner layers of the substantia gelatinosa (lamina IIo and IIi). In colchicine treated cats, enkephalin perikarya are most numerous in lamina I and at the border between laminae I and II. Ultrastructural analysis reveals that enkephalin cells receive a diverse afferent input. The majority of afferent inputs are presynaptic to the enkephalin dendrites; few axosomatic synapses are seen. Among these presynaptic axonal profiles are unlabeled axons which resemble primary afferent terminals, including the characteristic central axonal varicosity. Enkephalin dendrites are also postsynaptic to enkephalin immunoreactive axons. Two types of enkephalin axonal profiles appear in the superficial dorsal horn. Class I profiles are only found in lamina I. These are large profiles which form few synapses; those synapses made are axodendritic. Class II enkephalin axons are smaller and are distributed in both layers I and II. While Class II axons most commonly form axo-dendritic synapses, they also form axo-axonic synapses with flat vesicle-containing profiles; the latter are generally presynaptic to the enkephalin terminals. Serial analysis further revealed that both the enkephalin and the flat vesicle-containing profile synapse onto a common dendrite. Although enkephalin axons frequently lie adjacent to round vesicle-containing profiles, anatomical evidence that opioid axons form synapses with this type of ending was not found. An additional type of enkephalin vesicle containing-profile is found in layer IIi; its morphological features do not clearly distinguish its axonal or dendritic origin. These endings are typically postsynaptic to unlabelled central endings, and provide minimal presynaptic input to other elements in the neuropil. Like some class II axons, these labelled profiles contain vesicles which cluster at the membrane immediately adjacent to unlabelled central axons. These results indicate that spinal enkephalin neurons receive a variety of synaptic inputs. These include inputs which may derive from primary afferent axons. Enkephalin neurons, in turn, influence nociceptive transmission predominantly through postsynaptic mechanisms. Finally, while we did not observe enkephalin terminals presynaptic in an axoaxonic relationship, the possibility that enkephalin neurons modulate the excitability of fine fiber nociceptive and nonnociceptive afferents via "nonsynaptic interactions" is discussed.     

The development of the inferior olivary complex in preweanling opossums

Summary Pre- and postsynaptic elements within the developing inferior olive (IO) of both control and experimental opossums were examined via electron microscopy. Electron dense boutons identified di-/mesencephalic, cerebellar and spinal afferents within the IO of 8–71 day old animals, which survived 4–48 hours following either midbrain hemisections or spinal transections.During its initial stage of development (3–22 days) the neuropil of the IO is segregated into fields of small diameter neurites or flocculent profiles. Within the fields of flocculent profiles, synaptic interactions are established, which are both infrequent and immature. Although some flocculent profiles are presynaptic, most are postsynaptic and emanate from olivary somata and dendrites. Synaptic contacts also occur with olivary somata, dendritic shafts, spines and dendritic varicosities. Clear round vesicles (crv's; 40 m) predominate within all boutons, normal ones as well as those which degenerate after di-/mesencephalic, cerebellar and spinal lesions; however, larger (70 m) dense cored vesicles (dcv's) are occasionally observed within some boutons. Degenerating terminals from all three sources primarily contact flocculent profiles and dendritic shafts.As the opossum matures (42 days) dramatic increases occur in the number and complexity of both pre- and postsynaptic elements. Marked variations are observed in the matrix density of dendritic shafts. Although all terminal boutons predominantly contain crv's, the number of dcv's within the population of presynaptic elements increases markedly. Concurently, olivary neurons are profusely studded with spines. Simple dendritic spines and spiny appendages as well as dendritic shafts are the most frequent postsynaptic structures within the principal nucleus (PO). Olivary somata and their spines, however, are postsynaptic to degenerating de-/mesencephalic afferents within the PO. Flocculent profiles, which persist within the accessory nuclei, and dendritic shafts are postsynaptic to degenerating spinal boutons.By 70 days of age synaptic contacts appear more mature and more nearly approximate those seen in the adult (King 1980). Few somatic contacts, opaque dendrites, dendritic varicosities, and flocculent profiles are evident within the PO. Dendritic shafts and spines are the principal postsynaptic structures. Many di-/mesencephalic and cerebellar afferents synapse within maturing synaptic clusters on spines between which a rare gap junction is observed. Other di-/mesencephalic and cerebellar endings in the PO as well as spinal endings in the accessory nuclei are presynaptic to dendritic shafts and spines external to synaptic clusters. This predilection for contacting more specific loci on olivary neurons provides good evidence for synaptic remodeling.As the olivary nuclei develop further, the incidence of gap junctions increases and pleomorphic vesicles appear within boutons. The glial investment of neuronal elements, including synaptic clusters, also becomes more extensive.In conclusion, early di-/mesencephalic, cerebellar and spinal synaptic contacts appear qualitatively uniform in their synaptic features and postsynaptic interactions. As olivary development proceeds, however, the distinguishing synaptic features of the nuclear complex become more apparent. Synaptic remodeling occurs as some midbrain and cerebellar terminals are localized within synaptic clusters. The ultrastructural features characteristic of the adult IO are finally achieved by 80 days of age.This research was supported by N.I.H. Research Grant NS-08798     

The relationship between some measures of synaptic ultrastructure as a function of distance from the soma on lamprey reticulospinal neurons

Summary Synaptic junctions located on the dendrites of lamprey (Petromyzon marinus) reticulospinal neurons labelled with intracellularly-injected horseradish peroxidase were studied. The normal ultrastructure of the synaptic junctions was defined and several quantitative measures made from each junction in order to test the hypothesis that distally-located synapses are ultrastructurally different from those located at proximal dendritic sites. A total of 820 contacts from one neuron and 279 from a second neuron ranging from 20 to 340 m from the soma were quantified. The vast majority of the presynaptic endings contained round, clear-cored vesicles and formed an asymmetrical membrane differentiation with the postsynaptic dendrite. A small fraction of the population contained flattened or pleomorphic vesicles and these synapses were equally distributed with respect to distance from the soma. Many of the terminals contained a few large dark- and clear-cored vesicles. Four quantitative measures of each synaptic contact were made. These included vesicle number, length of differentiated membrane, vesicle area and terminal area. Four ratios relating the different quantitative measures were also calculated. Each ratio or measurement from the synaptic junctions was plotted as a function of distance from the soma to determine if differences existed at any distance. It was found that synaptic junctions are uniformly similar and that distal junctions did not differ significantly (P > 0.05) from those at proximal dendritic sites. It is concluded that if distal synapses do compensate for their remote location they do this in some other way, possibly by increasing the number of synaptic contacts made by each presynaptic axon.     

Ultrastructure of cortical synapses after failure of presynaptic activity in ischemia,

The fine structure of cerebral cortical synapses was investigated during failure of synaptic transmission produced by ischemia. Presynaptic and postsynaptic potentials evoked in the anterior sigmoid gyrus of the cat by stimulation of the nucleus ventralis lateralis and the DC potential between cortical layer IV and the white matter were recorded with micropipettes during cerebral ischemia produced by arterial hemorrhage in paralyzed, artificially ventilated animals. After failure of the spontaneous electrocorticogram and postsynaptic responses, the presynaptic volley failed with development of depolarization of intracortical fiber terminals and loss of axon terminal excitability. The gyrus was then biopsied and fixed in collidine-buffered OsO₄. An altered pattern of distribution of synaptic vesicles was observed after presynaptic afferent fiber terminal activity was abolished by 3.5 to 4.0 minutes of cerebral ischemia. Clumping of vesicles in a region away from the cynaptic cleft was seen in about 10% of synaptic endings, and there was more than a two-fold increase in the number of presynaptic profiles devoid of vesicles in ischemic cortex.     

家兔下橄榄背侧副核的超微结构特点——电镜研究

本实验对7只家兔的下橄揽背侧副核进行了电镜观察。除发现已报道过的轴树突触、轴体突触及以树突为中心的突触球外,还见此核内有轴轴突触、轴突与胞体棘形成的突触,以轴突为中心的突触球及核内微纤维束。轴树突触最常见,突触后成分为树突或棘。轴体突触较少见。有一轴突终末与一胞体棘形成突触,同时还与一树突形成突触。轴轴突触前、后成分中均含圆形囊泡,有时形成轴-轴-树突触串。家兔下橄榄背侧副核内见有两种突触球,一种以树突为中心,另一种以轴突为中心。还发现两个轴突终末同时与另一轴突终末形成轴→轴→轴突触。下橄榄背侧副核内的复杂突触形式,说明传入冲动在其中可能经过扩散、会聚、突触前抑制及整合等复杂的过程。     

Hippocampal synaptic transmission and plasticity are preserved in myosin Va mutant mice

Recent studies have identified myosin Va as an organelle motor that may have important functions in neurons. Abundantly expressed at the hippocampal postsynaptic density, it interacts with protein complexes involved in synaptic plasticity. It is also located in presynaptic terminals and may function to recruit vesicles in the reserve pool to the active zone. Dilute-lethal mice are spontaneous myosin Va mutants and have severe neurological symptoms. We studied hippocampal physiology at CA3-CA1 excitatory synapses in dilute-lethal mutant mice to test the hypothesis that myosin Va plays a role in pre- or postsynaptic elements of synaptic transmission. In all assays performed, the mutant synapses appeared to be functioning normally, both pre- and postsynaptically. These data suggest that myosin Va is not essential for the synaptic release machinery, postsynaptic receptor composition, or plasticity at this synapse, but does not exclude significant roles for myosin Va in other cell types nor potential compensation by other myosin V isoforms.     

The cholinergic innervation of the rat substantia nigra: a light and electron microscopic immunohistochemical study

Summary Putative cholinergic axons and synaptic endings were demonstrated in the substantia nigra (SN) of the rat by light and electron microscopy on the basis of the localization of choline acetyltransferase (ChAT) immunoreactivity. The distribution of ChAT immunoreactivity in the SN as demonstrated by light microscopy revealed a modest network of ChAT-immunoreactive beaded axons in the SNc, in comparison to a relatively sparse distribution in the SNr. These axonal profiles were most dense in the middle of the rostral-caudal extent of the SNc and appeared to be concentrated in the middle third of the medial-lateral extent. By electron microscopy, unmyelinated, small diameter (0.25 m) ChAT-immuno-reactive axons were observed interspersed among numerous other non-immunoreactive axons in the SNc. ChAT-immunoreactive synaptic endings were observed in juxtaposition to small caliber (0.5 m) non-immunoreactive dendrites, and contained numerous spheroidal synaptic vesicles and occasional mitochondria. Synaptic contact zones were characterized by an accumulation of synaptic vesicles along the presynaptic membrane, and a prominent postsynaptic densification producing an asymmetrical pre-/postsynaptic membrane profile typical of excitatory synapses. These findings provide direct evidence for a cholinergic innervation of the SN, and suggest that this input may have an excitatory effect on neuronal elements in the SNc.