Round Vesicles Research Articles

Colocalization of somatostatin with GABA or glutamate in distinct afferent terminals presynaptic to the Mauthner cell

The presence of somatostatin in afferent fibers impinging on the goldfish Mauthner (M-) cell was determined using immunohistochemical methods, combined with confocal and electron microscopy, and the relationship of this peptide with inhibitory and excitatory terminals was studied. Somatostatin-reactive boutons were present only on the distal part of the M-cell's lateral dendrite. Somatostatin immunoreactivity was observed in typical large myelinated club endings (LMCEs) corresponding to mixed (electrical and chemical) eighth nerve primary afferent fibers. The axoplasm of these fibers contained dense-core vesicles (DCVs) dispersed among round vesicles. We have made a novel finding that the excitatory transmitter glutamate is present in LMCEs. Colocalization of this amino acid with somatostatin was detected in 75% of these endings using postembedding staining with gold particles of various sizes. The other structures labeled by somatostatin antibody were found to be small vesicle boutons (SVBs), which establish symmetrical synapses and contain a population of pleiomorphic vesicles with DCVs scattered among them. Double labeling with antibodies against glutamic acid decarboxylase and GABA allowed the definition of three types of biochemically characterized terminals: [somatostatin-GABA], [GABA], and [somatostatin]. However, the occurrence of DCVs in SVBs stained for GABA alone suggests that neuropeptides other than somatostatin may also coexist with GABA in this class of boutons. The coexistence of somatostatin with both inhibitory and excitatory neurotransmitters acting on the same region of a postsynaptic cell is discussed in relation to the role postulated for this peptide in synaptic plasticity.

Light and electron microscope distribution of the NMDA receptor subunit NMDAR1 in the rat nervous system using a selective anti-peptide antibody

NMDA receptors play key roles in synaptic plasticity and neuronal development, and may be involved in learning, memory, and compensation following injury. A polyclonal antibody that recognizes four of seven splice variants of NMDAR1 was made using a C-terminus peptide (30 amino acid residues). NMDAR1 is the major NMDA receptor subunit, found in most or all NMDA receptor complexes. On immunoblots, this antibody labeled a single major band migrating at M(r) = 120,000. The antibody did not cross-react with extracts from transfected cells expressing other glutamate receptor subunits, nor did it label non-neuronal tissues. Immunostained vibratome sections of rat tissue showed labeling in many neurons in most structures in the brain, as well as in the cervical spinal cord, dorsal root and vestibular ganglia, and in pineal and pituitary glands. Staining was moderate to dense in the olfactory bulb, neocortex, striatum, some thalamic and hypothalamic nuclei, the colliculi, and many reticular, sensory, and motor neurons of the brainstem and spinal cord. The densest stained cells included the pyramidal and hilar neurons of the CA3 region of the hippocampus, Purkinje cells of the cerebellum, supraoptic and magnocellular paraventricular neurons of the hypothalamus, inferior olive, red nucleus, lateral reticular nucleus, peripheral dorsal cochlear nucleus, and motor nuclei of the lower brainstem and spinal cord. Ultrastructural localization of immunostaining was examined in the hippocampus, cerebral cortex, and cerebellar cortex. The major staining was in postsynaptic densities apposed by unstained presynaptic terminals with round or mainly round vesicles, and in associated dendrites. The pattern of staining matched that of previous in situ hybridization but differed somewhat from that of binding studies, implying that multiple types of NMDA receptors exist. Comparison with previous studies of localization of other glutamate receptor types revealed that NMDAR1 may colocalize with these other types in many neurons throughout the nervous system.

The nucleus basalis of Meynert of the human brain: a Golgi and electron microscope study.

The nucleus basalis of Meynert of normal brains, aged from 15 to 73 years was studied in Golgi preparations and in electron microscopy. The nucleus is composed of large triangular, polyhedral and bipolar cells which are intermixed with numerous small or medium-sized spiny neurons. All of the neurons form a dense three dimensional dendritic arborization, with numerous secondary and tertiary dendritic branches studded with spines. The ultrastructural analysis revealed numerous axodendritic and axosomatic synapses between the spines. The ultrastructural analysis revealed numerous axodendritic and axosomatic synapses between the spiny neurons and the large triangular and polyhedral neurons. The presynaptic axonic profiles are plenty of ellipsoid and round synaptic vesicles. Large presynaptic terminals are seen frequently surrounded by numerous dendritic spines forming synaptic glomeruli, in all the areas of the nucleus basalis of Meynert. An age depended decrease of the number of neurons was noticed affecting mainly the population of the spiny neurons. Although in senile and presenile dementias an impressive loss of the cholinergic neurons of the nucleus basalis was reported, in normal aging the large cholinergic neurons of the nucleus basalis seems to be intact, whereas the medium and small shaped spiny neurons are decreased in number suggesting that the GABA-ergic neurons are principally affected.

Synaptic organization of the gustatory NST

A fundamental problem in taste research is to determine how gustatory signals are processed and disseminated in the mammalian central nervous system. An important first step toward understanding information processing is the identification of cell types in the nucleus of the solitary tract (NST) and their synaptic relationships with oral primary afferent terminals. Facial and glossopharyngeal (LIX) terminals in the hamster were labelled with HRP, examined with EM, and characterized as containing moderate concentrations of medium-sized round vesicles, and engaging in asymmetrical synaptic junctions. Ultrastructurally the endings resemble excitatory synapses in other brain regions.Labelled facial afferent endings in the RC subdivision synapse almost exclusively with distal dendrites and dendritic spines of NST cells. Most synaptic relationships between the facial synapses and the dendrites are simple. However, 40% of facial endings engage in complex synaptic relationships within glomeruli containing unlabelled axon endings particularly ones termed "SP" endings. SP endings are densely packed with small, pleomorphic vesicles and synapse with both the facial endings and their postsynaptic dendrites by means of nearly symmetrical junctions.

The peripheral and central projections of the Edinger-Westphal nucleus in the rat. A light and electron microscopic tracing study

The peripheral and central efferent projections of the rostral part of the Edinger-Westphal nucleus in the rat were investigated at the light and electron microscopic level by means of iontophoretic injections of the anterograde tracer Phaseolus vulgaris-leucoagglutinin and retrograde tracer injections of Fast blue and Nuclear yellow into the facial nucleus and into the principal olive. Two pathways leaving the rostral part of the Edinger-Westphal nucleus were studied, a peripheral and a central descending pathway. Fluorescent experiments demonstrated that the central pathway fibers originated from distinct individual Edinger-Westphal neurons. These neurons were mainly distributed throughout the rostral part of the Edinger-Westphal nucleus and had fusiform cell bodies. The neurons rarely form collateral projections. The central descending pathway left the Edinger-Westphal nucleus medially and terminated bilaterally in the principal olive, in the subnuclei A, B and C of the inferior olive and ipsilaterally in the medial accessory olive. The central pathway also terminated contralaterally in the lateral parabrachial nucleus, the facial nucleus, the trigeminal brainstem nuclear complex, the lateral reticular nucleus and the rostroventral reticular nucleus. The projection to the facial nucleus provides evidence for the existence of a polysynaptic loop forming the central part of the corneal blink reflex. Projections from the Edinger-Westphal nucleus to the cerebellar cortex or the deep nuclei, as described in cat and primate, could not be confirmed. The peripheral pathway left the Edinger-Westphal nucleus ventrally and terminated on dendrites of ciliary ganglion cells, along smooth muscle cells of ciliary ganglion associated arterioles and in the proximity of ciliary ganglion associated venules. The central and peripheral terminals that originate in the Edinger-Westphal nucleus all had similar ultrastructural features: clear, round vesicles and electron dense mitochondria. The terminals originating from the central descending pathway were often found to be arranged in glomerular-like structures. The central and peripheral terminals made asymmetric synaptic membrane specializations (Gray type one), except terminals innervating the ciliary ganglion associated vessels, which showed no synaptic contacts.

Convergent projections of trigeminal afferents from the principal nucleus and subnucleus interpolaris upon rat ventral posteromedial thalamic neurons

Synaptic boutons originating from the trigeminal principal sensory nucleus (PrV) and the subnucleus interpolaris (SpI) which contact rat ventral posteromedial (VPM) neurons are similar in appearance. They are large boutons and contain a moderate packing density of round synaptic vesicles and establish asymmetric (Gray type I) synaptic contacts principally on dendrites and occasionally on somata. These boutons are similar to RL boutons of dorsal column nuclei and spinothalamic tract origin found in the ventral posterolateral thalamic nucleus. The boutons of PrV and SpI origin have an overlapping distribution on proximal dendrites of VPM neurons. Double labeling using degeneration and WGA-HRP shows that boutons from PrV and SpI contact the same VPM neuron confirming there is convergence of trigeminal afferents in VPM.

Ramification pattern and ultrastructural characteristics of the serotonin-immunoreactive neuron in the antennal lobe of the moth Manduca sexta: a laser scanning confocal and electron microscopic study.

The two antennal lobes, the primary olfactory centers of the brain, of the moth Manduca sexta each contain one neuron that displays serotonin immunoreactivity. The neuron projects out of the antennal lobe and sends branches into ipsi- and contralateral protocerebral areas. An axon-like process extends from the contralateral protocerebrum to, and terminates in, the contralateral antennal lobe. In order to begin to investigate the possible role of this unique neuron in olfactory information processing, we have used laser scanning confocal microscopic and electron microscopic immunocytochemical techniques to study the ramification pattern, ultrastructural characteristics, and synaptic connections of the neuron in the antennal lobes of female adult Manduca sexta. The neuron ramifies extensively in the antennal lobe contralateral to the cell body. The ramifications, mainly in the base and center of each glomerulus, do not overlap with those of the sensory axons from the antenna. This finding suggests that the serotonin-immunoreactive neuron may not receive direct input from sensory neurons, and that it may modulate the activity of the neurons of the antennal lobe rather than that of the sensory neurons. In the electron microscope, the neuron exhibits large dense-cored vesicles and small, clear round vesicles. In the antennal lobe ipsilateral to the cell body, the primary neurite of the serotonin-immunoreactive neuron is unbranched and lacks detectable synaptic connections. The ramifications in the contralateral antennal lobe, however, participate in synaptic connections. At very low frequency, contralateral branches form synapses onto unlabeled processes and also receive synapses from unidentified neurons in the glomeruli, indicating that the neuron may participate directly in synaptic processing of olfactory information. The high ratio of output to input synapses made by the serotonin-immunoreactive processes in the contralateral antennal lobe is consistent with the idea that this neuron may receive synaptic input via its bilateral branches in the protocerebrum and then send information to the contralateral antennal lobe where the neuron may exert feedback or modulatory influences on olfactory information processing in the glomeruli.

The rat bladder tumor model system RBT resembles phenotypically and cytogenetically human superficial transitional cell carcinoma.

A cohort of 300 ACI rats was kept under standard laboratory conditions. After 30 months or upon natural death, complete autopsy was performed. In the genitourinary tract four kidney and five bladder tumors were found. Two of these bladder tumors, RBT323 and RBT157, are serially transplantable. In the fifth transplant generation the RBT323 tumor becomes metastatic to the lungs in more than 90% of animals. The metastatic ability of the RBT157 tumor changes from low to intermediate (50% of the rats have lung metastases) in the fourth passage. Histologically, the initial passages of the RBT323 and 157 tumors are grade II transitional cell carcinoma (TCC). The histological pattern of the RBT157 tumor remains essentially unchanged, whereas the RBT323 tumor progresses to a grade III tumor in the third passage. Electron microscopical studies reveal oblong elliptical and round vesicles lined by an asymmetrical unit membrane in the tumor cells, which stresses the urothelial origin of the tumors. Immunohistochemically both tumors show expression of cytokeratin 5, 7, 8 and 18. The progression of the tumors to a metastatic phenotype, however, is not associated with a specific change in the morphological characteristics. Cytogenetic analysis shows that both tumors are peridiploid with few marker chromosomes. Interestingly, both of these independently arising tumors exhibit a loss of chromosome 5. Rat chromosome 5 is syntenic to the major portion of human chromosome 9 (p23-qter). Loss of chromosome 9 is a cytogenetic trait of human superficial TCC, hence the RBT model is also in cytogenetic respect similar to human TCC.(ABSTRACT TRUNCATED AT 250 WORDS)

Parvalbumin‐containing neurons in the cerebral cortex of the lizard Podarcis hispanica: Morphology, ultrastructure, and coexistence with GABA, somatostatin, and neuropeptide Y

The morphology, fine structure, and degree of colocalization with GABA, somatostatin, and neuropeptide Y of parvalbumin-containing cells were studied with immunocytochemistry in the cerebral cortex of the lizard Podarcis hispanica. Parvalbumin-containing cells make up a morphologically heterogeneous population of spine-free neurons, displaying the morphological features of nonprincipal cells previously described in Golgi studies. Electron microscopically, parvalbumin-immunoreactive cell bodies are similar in all cortical areas and layers. The perisomatic input is moderate in number, and boutons with either round clear vesicles or flattened vesicles were observed making asymmetric or symmetric synaptic contacts, respectively. Parvalbumin-immunoreactive dendrites are smooth and almost completely covered with synaptic boutons of different types, most of which establish asymmetric contacts. Parvalbumin-immunoreactive boutons are concentrated around cell bodies of principal cells. They are large, containing abundant mitochondria and small pleomorphic vesicles, and establishing symmetric synaptic contacts with somata, proximal dendritic shafts, and axon initial segments of principal cells. Colocalization studies revealed that all the parvalbumin-containing cells are GABA-immunoreactive, representing only a fraction of the GABA-immunopositive cell population, and that parvalbumin- and peptide- (somatostatin and neuropeptide Y) containing cells show a negligible overlap. These results demonstrate that in the cerebral cortex of the lizard Podarcis hispanica, parvalbumin-containing cells represent a subset of nonprincipal GABAergic neurons largely involved in perisomatic inhibition, which are different from the peptide-containing cells, and suggest that they may include both axosomatic and axoaxonic cells.

Influence of low density lipoproteins on cytosolic free Ca 2+ concentration and dense tubular system in human platelets

The cytosolic free Ca 2+ concentration [Ca 2+]i and dense tubular system (DTS) of washed human platelets were affected by low density lipoproteins (LDL) of 25 μg/ml at 37 0 C for 10 minutes. After the incubation with LDL, the [Ca 2+]i increased from 115±29 nM to 141±24 nM. LDL promoted the increase of [Ca 2+]i (471±31 nM) induced by thrombin (0.03 U/ml) as compared to that which thrombin did alone (240±11 nM) ( p<0.05). The increased Ca 2+ influx from the extracellular space is thought to be the reason of the increase in [Ca 2+]i, since the effect of LDL was abolished by removal of external Ca 2+ by EGTA. The DTS changed primarily from thin elongated forms to rounded vesicles. No evidence was noticed that LDL caused a mobilization of Ca 2+ from the DTS.

Synapses between slowly adapting lung stretch receptor afferents and inspiratory beta-neurons in the nucleus of the solitary tract of cats: a light and electron microscopic analysis.

Previous neuroanatomic and physiologic studies indicated that afferent fibres from slowly adapting pulmonary stretch receptors (SAR) project to the nuclei of the solitary tract and terminate on inspiratory beta-neurons. In the present study we combined electrophysiologic and morphologic approaches to verify the presumed monosynaptic connections between SARs and beta-neurons. Single identified beta-neurons and single identified SAR afferent fibres were labelled intrasomally and intraaxonally, respectively, with horseradish peroxidase (HRP) in the same anesthetized cats. Under the light microscope, we analyzed the morphology of beta-neurons and their dendritic fields and of the terminal projection pattern of fibres from SARs and identified potential synaptic connections between boutons of SAR afferent fibres and the soma and dendrites of beta-neurons. The identified tissue was then processed further for electron microscopic analysis. On average, beta-neurons had 6 primary dendrites that bifurcated 3-8 times. The dendritic trees extended 1.5 mm both rostrocaudally in the ventrolateral nucleus of the solitary tract and medially into the intermediate subnucleus. Axons of beta-neurons curved toward the midline and no collateral branches were evident over its stained length (2.5-3.4 mm). Axodendritic synaptic contacts between SAR fibres and beta-neurons were identified electron microscopically in four of six tissue samples chosen by light microscopy. In addition, we located 2 axodendritic and 2 axosomatic synaptic contacts that were not observed under light microscopic screening. The boutons of SAR fibres contained clear, round vesicles and formed asymmetrical synapses with beta-neurons. Multiple synaptic connections were found between collaterals of a single SAR and single beta-neurons, indicating a dense terminal projection of single SAR afferent fibres onto beta-neurons. These morphologic data prove monosynaptic connections between electrophysiologically identified SAR afferent fibres and beta-neurons.

Acetylcholinesterase activity is associated with efferent endings in the sensory epithelia of the utricle and semicircular canals of the rainbow trout inner ear

The mechanosensory hair cells of the utricle and semicircular canals of the trout inner ear are morphologically similar to type II hair cells of the avian and mammalian vestibular end organs. These cells are innervated by two types of nerve terminals. The nonvesiculated terminals are considered to be afferent, and the vesiculated endings are presumed to be efferent. The presumptive efferent endings contain numerous clear, round vesicles and a few that are dense-cored. Histocytochemical, electron microscopic analysis has localized acetylcholinesterase activity to plasma membranes of vesiculated, presumptive efferent nerve terminals in sensory epithelia of the utricle and semicircular canals. No reaction product was observed at the receptoneural synapse or found in nonvesiculated, presumptive afferent endings. Control specimens incubated in the presence of eserine sulfate, an inhibitor of acetylcholinesterase, were devoid of reaction product. These results support the tenet that vesiculated nerve endings in the sensory epithelia of the utricle and semicircular canals of the trout are cholinergic.

Axon hillocks and initial segments in spinal trigeminal nucleus with emphasis on synapses including axo-axo-axonic contacts

As a part of a continuing study of the feline spinal trigeminal nucleus, the fine structure and synaptic arrangements on the axon hillock and axon initial segment of neurons in this region are described here. Transmission electron microscopy has been used to characterize qualitatively the axon hillock and initial segment and associated synapses in pars interpolaris. Axon hillocks and initial segments are easily identified in continuity with somata or as isolated profiles in the neuropil, and they receive synaptic contacts: these we regard as axo-axonic. The presynaptic terminals contain either mainly round or mainly flattened synaptic vesicles and have Type I (asymmetric) or Type II (symmetric) thickenings respectively at their contacts with the axon hillock or initial segment. I report here also the unusual arrangement of three separate axons in a serial synaptic complex. Some of the round vesicle Type I contacts onto the axon hillock-initial segment region also receive Type II contacts from one or more flattened vesicle terminals, thus forming an axo-axo-axonic complex. These flattened vesicle terminals lack the usual features of a presynaptic dendrite. It has been shown that in this nucleus some round vesicle terminals, especially those postsynaptic to flattened vesicle terminals, are primary afferents from the periphery. Therefore the round vesicle terminal presynaptic to the axon hillock-initial segment region, some of which are included in the axo-axo-axonic complex may also be a primary afferent directly contacting the spike generator area of the relay neuron and under presynaptic control of a flattened vesicle synapse. The latter may possibly be an intrinsic contact. This strategic situation of round vesicle terminals and the axo-axo-axonic complex at the axon hillock or initial segment has major implications relevant to the overall output of these neurons.

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.

Descending projections from the medullary dorsal reticular nucleus make synaptic contacts with spinal cord lamina I cells projecting to that nucleus: An electron microscopic tracer study in the rat

An ultrastructural study is made of the synaptic contacts occurring between structures labelled anterogradely and retrogradely in the superficial dorsal horn following injections of cholera toxin subunit B or horseradish peroxidase in the dorsal reticular nucleus of the medulla oblongata of the rat. Both tracers revealed labelled axonal boutons in lamina I with round synaptic vesicles and a few large granular vesicles making asymmetrical synaptic contacts upon labelled somata and dendrites. After injections of Phaseolus vulgaris leucoagglutinin in the dorsal reticular nucleus, labelled boutons identical to those revealed by the two other tracers were presynaptic to unlabelled somata and dendrites. In addition, dorsoreticular neurons were labelled retrogradely following injections of cholera toxin subunit B into the superficial dorsal horn of the cervical enlargement. These observations show the occurrence of a reciprocal connection between dorsal reticular and lamina I neurons. Considering the putative excitatory nature of the axodendritic contacts in lamina I, a positive feedback circuit is suggested, whereby the nociceptive signals transmitted to the dorsal medullary reticular formation by marginal neurons are intensified.

Formation of synapses between basal forebrain afferents and cerebral cortex neurons: an electron microscopic study in organotypic slice cultures.

Co-cultures of rat basal forebrain and cerebral cortex were maintained from 1 to 5 weeks in vitro with serum-free defined medium. The formation of synaptic connections between basal forebrain afferent fibres and cortical neurons was studied by specific labelling with three staining techniques, including (i) neuronal tract tracing with the fluorescent dye 1,1'-dioctodecyl-3,3,3'3'- tetramethylindocarbocyanine perchlorate, (ii) acetylcholinesterase histochemistry, and (iii) choline acetyltransferase immunocytochemistry. Both basal forebrain and cerebral cortex tissue displayed organotypic characteristics in culture. Cerebral cortex revealed a dense innervation by axonal projections from the basal forebrain. All three labelling techniques produced similar results at the light microscopic level, with densest innervation located in the marginal zone. At the fine structural level, the 1,1'-dioctodecyl-3,3,3'3'-tetramethylindocarbocyanine perchlorate-, acetylcholinesterase- and choline acetyltransferase-stained basal forebrain afferents all revealed a number of synaptic contacts with cortical neurons. The contacts displayed consistent synaptic features, including presynaptic accumulation of small round vesicles, cleft widening, and postsynaptic densities forming symmetric synapses. These morphological characteristics of connections formed in vitro are similar to basal forebrain cholinergic projections to cerebral cortex in normal brain. Based on these results, this tissue culture model appears to be an useful tool for investigations of the development of cholinergic innervation of cerebral cortex.

New aspects of dopaminergic interplexiform cell organization in the goldfish retina.

Dopaminergic interplexiform cells (DA-IPCs) in the goldfish retina have been reexamined by light and electron microscopic immunocytochemistry with antisera against dopamine (DA) or tyrosine hydroxylase (TH). Successful immunostaining with a specific anti-DA antiserum offers further direct support for DA-IPCs. Anti-DA immunocytochemistry in combination with [3H]-DA autoradiography shows 92% colocalization of the two markers, indicating that [3H]-DA autoradiography is a reliable technique for identification of DA-IPCs. Incubations with anti-TH antiserum show that immunoreactive DA-IPCs have a homogeneous distribution, with an average frequency of 71 +/- 8 cells/mm2 in retinas of 14-15 cm long goldfish. Their arrangement is distinctly nonrandom. Electron microscopy of TH-immunoreactive cell processes confirms that horizontal cell axons synapse onto DA-IPCs and adds the following junctional arrangements to the circuit diagram of the DA-IPC: 1) adjacent serial synapses between DA-IPCs, external horizontal cells, and putative glycinergic interplexiform cells, 2) junctional appositions between DA-IPCs and photoreceptor cells, 3) junctional appositions between neighbouring DA-IPCs, and 4) the "gap junctional complex," typically consisting of a DA-IPC process juxtaposed with a gap junction between horizontal cell axons. The gap junction is flanked by clusters of small, round vesicles and groups of electron-dense structures resembling intermediate filaments. These morphological results support the functional involvement of DA-IPCs in adaptive retinomotor movements and in horizontal cell gap junction modulation and/or dynamics. They also suggest particular interaction between the dopaminergic and the glycinergic IPC system in the outer plexiform layer of goldfish retina.

Organization and synaptic connections of cholinergic fibers in the cat superior colliculus.

The cat superior colliculus (SC) receives a dense cholinergic input from three brainstem nuclei, the pedunculopontine tegmental nucleus, the lateral dorsal tegmental nucleus, and the parabigeminal nucleus (PBG). The tegmental inputs project densely to the intermediate gray layer (IGL) and sparsely to the superficial layers. The PBG input probably projects only to the superficial layers. In the present study, the morphology of choline acetyltransferase (ChAT)-immunoreactive axons and synaptic endings in the superficial and deep layers of the SC was examined by light and electron microscopy to determine whether these cholinergic afferents form different types of synapses in the superficial and deep layers. Two types of fibers were found within the zonal (ZL) and upper superficial gray layers (SGL): small diameter fibers with few varicosities and larger diameter fibers with numerous varicosities. Quantitative analysis demonstrated a bimodal distribution of axon diameters, with one peak at approximately 0.3-0.5 micron and the other at 0.9-1.0 micron. On the other hand, ChAT-immunoreactive fibers in the IGL were almost all small and formed discrete patches within the IGL. Two types of ChAT-immunoreactive synaptic profiles were observed within the ZL and upper SGL using the electron microscope. The first type consisted of small terminals containing predominantly round synaptic vesicles and forming asymmetric synaptic contacts, mostly on dendrites. The second type was comprised of varicose profiles that also contained round synaptic vesicles. Their synaptic contacts were always symmetric in profile. ChAT-immunoreactive terminals in the IGL patches contained round or pleomorphic synaptic vesicles, and the postsynaptic densities varied from symmetric to asymmetric, including intermediate forms. However, no large varicose profiles were observed. This study suggests that cholinergic fibers include at least two different synaptic morphologies: small terminals with asymmetric thickenings and large varicose profiles with symmetric terminals. The large varicose profile in the superficial layers is absent in the IGL. This result suggests that the cholinergic inputs that innervate the superficial layers and the patches in the IGL of the cat SC differ in their synaptic organization and possibly also in their physiological actions.

Quantitative ultrastructural analysis of the periaqueductal gray in the rabbit

The fine structure of the periaqueductal gray (PAG) of the rabbit was examined using the transmission electron microscope. On the basis of synaptic polarity, vesicle size, and the nature of the pre- and post-synaptic elements, 10 essentially different synaptic types could be discerned (6 axo-dendritic, 2 axo-somatic, 1 axo-axonic, and 1 dendro-dendritic). Synaptic contacts on the soma of PAG neurons were small and covered, on average, only 1.6% of the soma surface. The most striking feature of the synaptic structure of the PAG was that more than 94.1% of all synapses were axo-dendritic. Of these, 83.5% were of the symmetrical type. Most of these contacts occurred on buttons of small to medium size, and contained either round vesicles of medium size or pleomorphic vesicles of medium size. Boutons containing only flattened vesicles were quite rare. Boutons contacting larger dendrites were generally small-to-medium in size, made asymmetric-type synaptic contacts, and contained pleomorphic vesicles of medium-to-large size. Medium-sized dendrites were contacted principally by small boutons exhibiting either symmetrical or asymmetrical junctions containing medium-sized pleomorphic vesicles, and in addition a few of these boutons contained both large, and small, round vesicles. Dendritic spines were generally provided with only one synaptic contact, stretching the entire width of the spinous process. Boutons and the spines on dendrites were approximately the same size. Synapses between two vesicle-containing structures (axo-axonic or dendro-dendritic synapses) were rare (1.4%). They were generally asymmetric and contained round vesicles of medium size. Complex synapses, where a glial sheet enclosed the synapse, were occasionally observed. Also seen were multiple synapses, with up to 11 contacts on a single dendritic profile. Large dense-core vesicle were seen in approximately 40% of all synapses, whereas small dense-core vesicles were only found in about 3%. Data is provided on how different synaptic features relate to ventral, lateral, dorsal, and medial PAG. Principally this is in relation to neuron size, glia cell content, axonal characterization, and vesicular type.

Comparative sperm ultrastructure and spermiogenesis in basal heterobranch gastropods (Valvatoidea, Architectonicoidea, Rissoelloidea, Omalogyroidea, Pyramidelloidea) (Mollusca)

Three distinct groups of basal heterobranch gastropods (=‘Allogastropoda/Heterostropha’) can be distinguished on the basis o f sperm and spermiogenic features: (1) Valvatoidea; (2) Architectonicoidea; (3) Rissoelloidea + Omalogyroidea + Pyramidelloidea. Sperm of pentaganglionate heterobranchs (Opisthobranchia sensu stricto+ Pulmonata) conform in all respects to the pattern shown by group (3). Heterobranchia are united, and differentiated from other gastropods, by a shared suite of sperm and spermiogenic features (rounded acrosomal vesicle, nucleus usually helical, highly modified mitochondrial derivative, spermatid acrosome associated with a dense nuclear plaque, formation of the mitochondrial derivative through fusion of numerous small mitochondria along the length of the axoneme). Sperm autapomorphies of the Architectonicoidea and the Valvatoidea suggest that these two superfamilies arc distinct but terminal offshoots from ancestral heterobranchs.