The synaptic organization of the cerebello-olivary circuit

Section of the superior cerebellar peduncle just rostral to the deep cerebellar nuclei results in degenerating axon terminals within the contralateral inferior olive. The nuclear origin of this fiber system and its distribution within the subdivisions of the inferior olive were described in a companion study (Martin et al., 1976). Precise localization of these degenerating terminals within the nucleus was accomplished by the examination of 1 mu plastic sections cut from each tissue block prior to thin sectioning. Degenerating axon terminals are present in all the nuclear subdivisions and when seen with the electron microscope they frequently are localized in the previously described synaptic clusters (King, 1976). These terminals demonstrate an electron dense reaction at survival times of 2 and 3 days. By day 4, they are shrunken and irregular in shape, and typically are surrounded by astrocyte processes. Cerebello-olivary axon terminals measure 1-3 mu, contain spherical, clear synaptic vesicles and typically contact spiny appendages within the synaptic clusters (glomeruli). Thus, we have demonstrated that one of the primary axon systems which terminates within the synaptic clusters is from the cerebellar nuclei. We have yet to determine the origins of the remaining terminals within the synaptic clusters which include endings with either smaller spherical, pleomorphic or numerous dense core vesicles.     

Quantitative aspects of the synaptic organization of the superior colliculus in control and dark-reared rabbits

Fine structural and quantitative aspects of the synaptic organization of the superior colliculus of the rabbit were studied. The synaptic density, the frequency of the distinct types of terminals, the length of the contact zones, the percentage of serial contacts and the number of synaptic vesicles per terminal were estimated in control, dark-reared and dark-reared-recovered animals. The ultrastructure of the synaptic terminals and the complex interconnectivity correspond to that described for other species. The depth distribution of terminals with round synaptic vesicles agrees quite well with the distribution of cortical and retinal afferents found in lesion experiments. Dark-rearing has little effect on the quantitative organization of the synaptic terminals. In contrast with previous observations in the visual cortex of the same animals37 no changes in the density of synaptic vesicles is observed in the superior colliculus after long term dark-rearing.     

Cat intraamygdaloid inhibitory network: ultrastructural organization of parvalbumin-immunoreactive elements

Projection neurons of the basolateral (BL) amygdaloid complex are regulated by an intrinsic inhibitory network. To improve our understanding of this inhibitory circuit, we studied the synaptology of parvalbumin-immunopositive (PV+) elements as this calcium-binding protein is localized in a subpopulation of gamma-aminobutyric acid (GABA)-ergic interneurons. Two populations of PV+ cells were identified on the basis of soma shape (ovoid, type A vs. polygonal, type B). In the lateral and BL nuclei, the majority of boutons in contact with PV+ cells formed asymmetric synapses (types 1-3; 94%), whereas a minority (type 4, 6%) established symmetric synaptic contacts and resembled GABAergic terminals. In both nuclei, type B PV+ perikarya were more densely innervated than were type A neurons. However, the pattern of synaptic innervation of type B PV+ neurons differed in the two nuclei: in the lateral nucleus, they were almost exclusively innervated by a population of small, presumed excitatory terminals (type 1), whereas the four categories of terminals contributed more equally to their innervation in the BL nucleus. PV+ boutons belonged to a single category of terminals that was enriched with GABA and formed symmetric synapses mostly with the proximal part of PV neurons. The proportion of axosomatic synapses was significantly higher in the lateral nucleus than in the BL nucleus (33% vs. 18%). The reverse was true for the contacts with proximal dendrites (33% in the lateral nucleus vs. 46% in the BL nucleus). The remaining terminals formed synapses with distal dendrites (23-28%) and spines (8-12%). These results indicate that PV+ interneurons receive massive excitatory inputs and that PV+ terminals are strategically located to exert a powerful inhibitory control of amygdala neurons.     

UDP-glucuronosyltransferase UGT1A7 induced in rat small intestinal mucosa by oral administration of 2-naphthoflavone

Cerebellar basket cells form highly specialized inhibitory synaptic contacts with Purkinje cells, namely the pericellular basket and pinceau nerve terminal structures, wrapping around the Purkinje cell somatic and axon hillock regions. These inhibitory synaptic contacts are ideally located to control the ultimate output of the cerebellar cortex. Previous immunohistochemical studies have shown that these synaptic structures possess a very high density of the dendrotoxin (DTX)-sensitive potassium channel subunit, Kv1.2. We have taken advantage of this unique anatomical arrangement offering a high concentration of identified Kv channel subunits by combining whole-cell patch-clamp recording and fluorescence microscopy to establish a novel preparation and perform the first recordings from unambiguously identified mammalian CNS inhibitory presynaptic terminals. We report that DTX-sensitive potassium channels are present in basket cell terminals but not in the basket cell soma. This selective cellular distribution suggests that these channels play an important role in modulating cerebellar inhibitory synaptic transmission.     

Neurons and synaptic patterns in the deep layers of the superior colliculus of the cat. A Golgi and electron microscopic study

Golgi and electron microscopic observations were made on the neurons in the deep layers (below the stratum opticum) of the cat superior colliculus. Large neurons, 35-60 micrometers in somal diameter, occur mainly in the lateral two-thirds of the colliculus. They have numerous somatic and dendritic spines and receive a large number of axon terminals (bouton covering ratio: more than 70%). The medium-sized neurons (20-30 micrometer), with a moderate number of dendritic spines, show a lower bouton covering ratio (25-30%). The ratio for small neurons (8-15 micrometers), with very few dendritic spines, is less than 10%. The medium-sized and small neurons are distributed throughout the colliculus and show marked variability in the dendritic arrangement. Seven different types of axon terminals were distinguished: types I, II, V, and VII form asymmetrical and types III, IV, and VI symmetrical synapses. Type I terminals represent small boutons containing predominantly spherical vesicles, and are in contact mainly with small dendritic profiles. Type II terminals are medium-sized and slender, contain a mixture of spherical and slightly oval vesicles, and make synaptic contacts with small to medium-sized dendrites and somatic spines. This type of terminal is occasionally presynaptic to vesicle-containing dendrites (type VIII). Type III terminals are small, contain flattened vesicles predominantly, and are presynaptic to a wide variety of neuronal elements in the deep layers of the superior colliculus. Type IV terminals are represented by medium to large-sized boutons that contain pleomorphic vesicles and make synaptic contacts chiefly with the large neurons. Type V and VI terminals exhibit a quite dense axoplasmic matrix and mainly contact the large neurons. Type VII terminals are often in the form of boutons en passant and contain numerous large granular vesicles. Pleomorphic vesicle-containing dendrites (type VIII terminals) are also observed to participate in the axodendrodendritic serial synapses.     

Ultrastructural features of tyrosine-hydroxylase-immunoreactive afferents and their targets in the rat amygdala

Interrelations of tyrosine-hydroxylase-immunoreactive afferent fibres with neuronal elements were studied in central, basal and intercalated nuclei of the rat amygdaloid complex. Comparison with dopamine-beta-hydroxylase-immunoreacted and phenylethanolamine-N-methyltransferase-immunoreacted parallel sections indicated that the tyrosine-hydroxylase immunoreaction labelled preferentially dopaminergic axons. At the electron-microscopic level, the majority of tyrosine-hydroxylase-immunoreactive axons possessed small boutons containing small clear vesicles and contacting dendrites, spines or somata of amygdala neurons, forming mostly symmetric synapses. They were often directly apposed to or in the vicinity of unlabelled terminals synapsing on the same structure. Synaptic density was highest in the central lateral part of the central nucleus. In the central and basal nuclei labelled axons synapsed preferentially on small dendrites and dendritic spines, and on somata of a few neurons. A detailed study of the neuronal ultrastructure showed that innervated somata possessed the differential characteristics displayed by the predominant neuron types in the medial and central lateral central nucleus and resembled the typical projection neurons in the basal nuclei. In the paracapsular intercalated cell groups the majority of neurons possessed intense perisomatic innervation by immunoreactive terminals. The results suggest that tyrosine-hydroxylase-immunoreactive, predominantly dopaminergic amygdaloid afferent fibres preferentially modulate the effect of extrinsic inputs into neurons of the central and basal nuclei, while a nonselective regulation is exerted upon the output of paracapsular intercalated neurons. It is suggested that this innervation pattern may be important for the coordinated integration of extrinsic and intraamygdaloid connections and thus for balanced output of the structure.     

Ultrastructural immunocytochemical localization of neurotensin and the dopamine D2 receptor in the rat nucleus accumbens

The neuroleptic-like effects of neurotensin (NT) are thought to be due to interactions with dopamine (DA) acting primarily at D2 receptors within the nucleus accumbens septi (Acb). Using electron microscopic dual labeling immunocytochemistry, we sought to demonstrate cellular substrates for functional interactions involving NT and DA D2 receptors in the adult rat Acb. Peroxidase reaction product representing D2 receptor-like immunoreactivity (D2-LI) was seen along membranes of Golgi lamellae and multivesicular bodies of perikarya containing immunogold labeling representing NT-LI. Dually labeled somata usually contained highly indented nuclei, a characteristic of aspiny neurons. Dendrites also occasionally colocalized the two immunomarkers. Other somata, dendrites, and all axon terminals were singly labeled with either NT-LI or D2-LI. In distinct sets of terminals, NT-LI was commonly associated with large, dense-cored vesicles, whereas D2-LI was found along the plasmalemma and over nearby small clear vesicles. Each type of terminal comprised approximately 20% of synaptic input to NT-immunoreactive dendrites. Similar proportions of terminals containing NT-LI or D2-LI contacted unlabeled (approximately 55%) or NT-labeled (approximately 35%) dendrites and, occasionally, were observed converging onto common dendrites. Terminals containing NT-LI or D2-LI also were often closely apposed. These findings provide the first ultrastructural evidence that: (1) NT and D2 receptors are colocalized in aspiny neurons and dendrites, (2) NT may produce a direct postsynaptic effect on neurons receiving input from terminals which are presynaptically modulated by DA via D2 receptors, and (3) NT and DA acting at D2 receptors may interact through presynaptic modulation of common axon terminals.     

Recognition and management of childhood cardiac arrhythmias

Axonal connections between the amygdala and the hypothalamic paraventricular nucleus were examined by combined anterograde-retrograde tract tracing. Iontophoretic injections of the retrograde tracer Fluorogold were placed in the paraventricular nucleus, and the anterograde tracer PHA-L in the ipsilateral central or medial amygdaloid nuclei. Single and double-label immunohistochemistry were used to detect tracers. Single label anterograde and retrograde tracing suggest limited evidence for direct connections between the central or medial amygdala and the paraventricular nucleus. In general, scattered PHA-L-positive terminals were seen in autonomic subdivisions of the paraventricular nucleus (lateral parvocellular, dorsal parvocellular and ventral medial parvocellular subnuclei) following central or medial amygdaloid nucleus injection. Double-label studies indicate that central and medial amygdaloid nucleus efferents contact paraventricular nucleus-projecting cells in several forebrain nuclei. In the case of central nucleus injections, PHA-L positive fibers occasionally contacted Fluorogold-labeled neurons in the anteromedial, ventromedial and preoptic subnuclei of the bed nucleus of the stria terminalis. Overall, such contacts were quite rare, and did not occur in the bed nucleus of the stria terminalis regions showing greatest innervation by the central amygdaloid nucleus. In contrast, medial amygdala injections resulted in a significantly greater overlap of PHA-L labeling and Fluorogold-labeled neurons, with axosomatic appositions observed in medial divisions of the bed nucleus of the stria terminalis, anterior hypothalamic area and preoptic area. The results provide anatomical evidence that a substantial proportion of amygdaloid connections with hypophysiotrophic paraventricular nucleus neurons are likely multisynaptic, relaying in different subregions of the bed nucleus of the stria terminalis and hypothalamus.     

Endothelin in perivascular nerves. An electron-immunocytochemical study of rat basilar artery

The endothelium is recognized as a major source of endothelin in the vasculature. In this report we show for the first time that endothelin can be demonstrated with immunoelectronmicroscopy in perivascular nerves. About 36% of the axon profiles (varicosity/intervaricosity) examined in the rat basilar artery showed immunolabelling with a polyclonal antibody to endothelin-1. Endothelin-labelled axon varicosities were characterized by the presence of small spherical agranular vesicles (42 +/- 1 nm); some varicosities also contained large granular vesicles (92 +/- 5 nm) with labelled cores. Immunolabelling was mostly distributed in the axoplasm and in association with the membrane of synaptic vesicles (the lumen of the small agranular vesicles was immuno-negative). The presence of endothelin in perivascular nerves of the basilar artery suggests a neuronal as well as endothelial role in the physiological control of the vessel wall.     

An ultrastructural study of p75 neurotrophin receptor-immunoreactive fiber terminals in the reticular thalamic nucleus of young rats

The reticular thalamic nucleus (RT) receives cholinergic fibers from both the basal forebrain and the brainstem. Recent studies have shown that the p75 neurotrophin receptor (p75NTR) is synthesized in cholinergic neurons in the basal forebrain but not in those in the brainstem. In this study, to identify cholinergic fibers originating from the basal forebrain, we used a monoclonal antibody against p75NTR (192-IgG) and characterized the ultrastructure of the immunoreactive fiber terminals in the rostral part of the RT in 3-week-old rats. Light microscopy revealed that p75NTR-immunoreactive fine fibers and varicosities were distributed throughout the nucleus. From electron micrographs, three types of labeled terminals were identified. The first type of labeled fiber terminals (63 out of 106) was consistently small, contained densely packed vesicles, and established asymmetrical synaptic contacts with heavy and bushy postsynaptic thickening on distal dendritic profiles; the second type (18 out of 106) established asymmetrical synaptic contacts with very slight postsynaptic thickening; and the third type (25 out of 106) of labeled terminals contained pleomorphic vesicles and established symmetrical synaptic contacts with more proximal dendritic surfaces than the first two types. In addition to the above, labeled dendritic profiles receiving non-labeled asymmetrical and symmetrical synaptic contacts were identified. These findings suggest that the basal forebrain cholinergic system establishes a variety of synaptic connections in the RT and influences cortical activity indirectly via thalamocortical pathways, as well as via direct projections to the cortex.     

Amygdaloid corticotropin-releasing factor targets locus coeruleus dendrites: substrate for the co-ordination of emotional and cognitive limbs of the stress response

Corticotropin-releasing factor (CRF), the neurohormone that initiates the endocrine limb of the stress response via its actions on the anterior pituitary, also acts as a neurotransmitter in the noradrenergic locus coeruleus (LC) to activate this system during stress. Because the central nucleus of the amygdala contains numerous CRF-immunoreactive neurones, the present study examined whether CRF projections from the central nucleus of the amygdala target LC dendrites, thereby providing a mechanism for limbic-CRF modulation of brain noradrenergic activity. Retrograde tracers injected into the rostrolateral pericoerulear region, where CRF-immunoreactive fibres are dense, labelled numerous CRF-immunoreactive neurones in the central nucleus of the amygdala. Consistent with this, ultrastructural analysis of the rostrolateral pericoerulear region in sections that were dually labelled for an anterograde tracer (biotinylated dextran amine, BDA) injected into the central nucleus of the amygdala and CRF immunoreactivity revealed that a substantial percentage (35%) of amygdaloid axon terminals were CRF-immunoreactive. These terminals formed synaptic specializations with unlabelled dendrites that were more often of the asymmetric (excitatory) type. Additionally, ultrastructural analysis of sections that were dually labelled to visualize CRF-and tyrosine hydroxlase-immunoreactivity demonstrated synaptic specializations between CRF-immunoreactive terminals and LC dendrites in the rostrolateral peri-LC, which were also frequently asymmetric. Taken together with previous ultrastructural findings that LC dendrites in the rostrolateral pericoerulear region are targeted by anterogradely labelled terminals from the central nucleus of the amygdala, the present results implicate this nucleus as a source of CRF that can impact on LC activity via effects on dendrites in the rostrolateral pericoerulear region. This cellular substrate for amygdaloid-CRF modulation of brain noradrenergic activity may serve as a mechanism for the integration of emotional and cognitive responses to stress.     

Diffuse transmission by acetylcholine in the CNS

Recent immunoelectron microscopic studies have revealed a low frequency of synaptic membrane differentiations on ACh (ChAT-immunostained) axon terminals (boutons or varicosities) in adult rat cerebral cortex, hippocampus and neostriatum, suggesting that, besides synaptic transmission, diffuse transmission by ACh prevails in many regions of the CNS. Cytological analysis of the immediate micro-environment of these ACh terminals, as well as currently available immunocytochemical data on the cellular and subcellular distribution of ACh receptors, is congruent with this view. At least in brain regions densely innervated by ACh neurons, a further aspect of the diffuse transmission paradigm is envisaged: the existence of an ambient level of ACh in the extracellular space, to which all tissue elements would be permanently exposed. Recent experimental data on the various molecular forms of AChE and their presumptive role at the neuromuscular junction support this hypothesis. As in the peripheral nervous system, degradation of ACh by the prevalent G4 form of AChE in the CNS would primarily serve to keep the extrasynaptic, ambient level of ACh within physiological limits, rather than totally eliminate ACh from synaptic clefts. Long-lasting and widespread electrophysiological effects imputable to ACh in the CNS might be explained in this manner. The notions of diffuse transmission and of an ambient level of ACh in the CNS could also be of clinical relevance, in accounting for the production and nature of certain cholinergic deficits and the efficacy of substitution therapies.     

2 types of neuromuscular junction in the pharyngeal retractor muscle of snails

The pharyngeal retractor muscle of Helix lucorum is innervated by two symmetrical nerves which contain axons of two types forming myoneural junctions with the muscle cells. Type I junctions correspond to thick axons. These axon terminals which contain a large number of spherical, clear vesicles (41 +/- 5 nm) and a smaller number of dense-cored vesicles (67 +/- 3 nm) make contacts mainly with noncontractile sarcoplasmic processes of muscle cells. Type II junctions correspond to thin axons containing many of granules. Their axon terminals contact with contractile parts of muscle cells and contain a heterogenous population of vesicles: small spherical clear vesicles (44 +/- 2 nm), dense-cored vesicles and numerous irregularly outlined granules with fine-granular content (135 +/- 5 nm). Space between muscle cell is usually wide (50 nm and more) with the exception of sarcoplasmic processes where the gap may be less than 10 nm.     

Immunohistochemical localization of metabotropic glutamate receptors, mGluR7a and mGluR7b, in the central nervous system of the adult rat and mouse: a light and electron microscopic study

The distributions of two alternative splicing variants of metabotropic glutamate receptor mGluR7, mGluR7a and mGluR7b, were examined immunohistochemically in the rat and mouse by using variant-specific antibodies raised against C-terminal portions of rat mGluR7a and human mGluR7b. Many regions throughout the central nervous system (CNS) showed mGluR7-like immunoreactivities (LI). The distribution patterns of mGluR7-LI in the rat were substantially the same as those in the mouse, although some species differences were observed in a few regions. Intense mGluR7a-LI was seen in the main and accessory olfactory bulbs, anterior olfactory nucleus, islands of Calleja, superficial layers of the olfactory tubercle, piriform cortex and entorhinal cortex, periamygdaloid cortex, amygdalohippocampal area, hippocampus, layer I of the neocortical regions, globus pallidus, superficial layers of the superior colliculus, locus coeruleus, and superficial layers of the medullary and spinal dorsal horns. The distribution of mGluR7b was more restricted. It was intense in the islands of Calleja, substantia innominata, hippocampus, ventral pallidum, and globus pallidus. The medial habenular nucleus also showed intense mGluR7a-LI in the rat but not in the mouse. For both mGluR7a- and mGluR7b-LI, localization in the active zones of presynaptic axon terminals was confirmed electron microscopically at synapses of both the asymmetrical and symmetrical types. It is noteworthy that mGluR7a-LI is seen preferentially in relay nuclei of the sensory pathways and that both mGluR7a- and mGluR7b-LI are observed not only in presumed glutamatergic axon terminals, but also in non-glutamatergic axon terminals including presumed inhibitory ones. Thus, mGluR7 may play roles not only as an autoreceptor in glutamatergic axon terminals, but also as a presynaptic heteroreceptor in non-glutamatergic axon terminals in various CNS regions.     

Light and electron microscopy of the ground squirrel retina: functional considerations

Light and electron microscopy of Golgi-impregnated ground squirrel retinas have revealed a range of morphological subtypes of bipolar, amacrine, and ganglion cells. There are at least seven subtypes of bipolar cells. Those subtypes in which the somata were high (sclerad) in the inner nuclear layer (3 subtypes) had axon terminals low (vitread) in the inner plexiform layer, and those with somata low in the inner nuclear layer (4 subtypes) had axon terminals high in the inner plexiform layer. The bipolar subtypes with high axon terminals made flat contacts with receptor cells, whereas all but one of the bipolar subtypes with low axon terminals made ribbon-related contacts with receptor cells. There are at least five subtypes of amacrine cells. The two subtypes which the Golgi method revealed most frequently were a broad-field, unistratified neuron with a dendritic spread in excess of 1,000 mum and a narrow-field, diffuse neuron with a dendritic spread of about 30 mum. The broad-field, unistratified cell had the lowest proportion of amacrine vs. bipolar cell synaptic input of the amacrine subtypes (43%), whereas the narrow-field, diffuse cell had one of the greatest proportions of amacrine cell input (96%). There are at least 15 subtypes of ganglion cells. The proportion of synaptic inputs to these cells ranged from 21% to 100% amacrine cell synapses. An attempt has been made to relate this new knowledge of retinal circuitry to the physiological output of the ganglion cells.     

Development and fine structure of murine Purkinje cells in dissociated cerebellar cultures: dendritic differentiation, synaptic maturation, and formation of cell-class specific features

Cerebellar Purkinje cells (PC) display a highly distinctive form of polarity. We have cultured murine PCs from dissociated E16 cerebellar anlagen for 1 week to investigate the early stages of neuronal compartmentalization and synaptic interactions, features which are important for the establishment of neuronal polarity. To unequivocally identify the PCs we utilized light and electron microscopic immunocytochemistry with an anti-serum to the cell class-specific marker L7/pcp2 gene product. The PCs typically show a single, long axon, numerous short appendages classified as filopodia and protospines, and a small number of protodendrites. The nucleus is positioned asymmetrically in both the horizontal and vertical axes of the soma. The Golgi apparatus, coated and uncoated vesicles, and mitochondria are prominent ultrastructural features, while the endoplasmic reticulum is highly fragmented. The cell body receives rudimentary synapses on its smooth surfaces and appendages and no consistent morphological differences were detected between these elementary contacts. The axon is clearly identifiable; it emanates from either the cell body or a protodendrite, bifurcates at predominantly right angles, forms beaded collaterals, and terminates with relatively large growth cones. The varicosities of the PC axon contain pleomorphic synaptic vesicles and form rudimentary synapses primarily with the dendritic shafts of immunonegative neurons. The protodendrites are short, quickly tapering and sparsely branched; they emit numerous filopodia and immature spines and terminate with small growth cones. Rudimentary synapses are received on the proximal dendritic shafts and filopodia, and more mature synapses occur frequently on protospines. With few exceptions, PCs lie atop an astrocytic bed layer and glial processes are apposed to the various aspects of the PC body left free by the afferent axons. By contrast, PC processes are largely free of glial sheaths. We conclude that the "stellate stage" of PC development in situ is replicated rather faithfully in culture and that PCs have established polarity and have begun to form intercellular contacts by 1 week in vitro. Moreover, the PCs are already morphologically distinct from other cell types in the 1 week cultures, although they have yet to develop the differentiated features that distinguish mature PCs.     

Measurement of digestive disorders in the piglet at weaning and related risk factors

Inputs from the amygdaloid and extraamygdaloid areas terminate in various divisions of the central nucleus. To elucidate the interconnections between the different regions of the central nucleus and its connectivity with the other amygdaloid areas, we injected the anterograde tracer, Phaseolus vulgaris-leucoagglutinin (PHA-L) into the capsular, lateral, intermediate, and medial divisions of the central nucleus in rat. There were a number of labeled terminals near the injection site within each division. The intrinsic connections between the various divisions of the central nucleus were organized topographically and originated primarily in the lateral division, which projected to the capsular and medial divisions. Most of the connections were unidirectional, except in the capsular division, which received a light reciprocal projection from its efferent target, the medial division. The intermediate division did not project to any of the other divisions of the central nucleus. Extrinsic projections from the central nucleus to the other amygdaloid nuclei were meager. Light projections were observed in the parvicellular division of the basal nucleus, the anterior cortical nucleus, the amygdalohippocampal area, and the anterior amygdaloid area. No projections to the contralateral amygdala were found. These data show that the central nucleus has a dense network of topographically organized intradivisional and interdivisional connections that may integrate the intraamygdaloid and extraamygdaloid information entering the different regions of the central nucleus. The sparse reciprocal connections to the other amygdaloid nuclei suggest that the central nucleus does not regulate the other amygdaloid regions but, rather, executes the responses evoked by the other amygdaloid nuclei that innervate the central nucleus.     

Large dense-core vesicle exocytosis and membrane recycling in the mossy fibre synapses of the rabbit hippocampus during epileptiform seizures

The ultrastructure of the hippocampal mossy fibre layer was studied in ultrathin sections and freeze-fracture preparations of rabbits under deep Nembutal anaesthesia, after recovery from ether anaesthesia, and 40 min after a single injection of methoxypyridoxine, that is, during the second generalized seizure discharge. The giant mossy fibre boutons contain two types of vesicles: evenly distributed, small round clear vesicles (50 nm) and a few scattered large dense-core vesicles (100 nm). In rare instances fusion of dense-core vesicles with the presynaptic membrane was observed. No differences in the morphology of the mossy fibre synapses were found between anaesthetized and unanaesthetized animals. During epileptiform seizures, however, the size and shape of clear and dense-core vesicles varied greatly. The active synaptic zones were covered with large, core-containing omega profiles or bumps and indentations. Only dense-core vesicles seem to undergo exocytosis. A fusion of clear vesicles with presynaptic membrane was not observed. Various explanations for the fact that only dense-core vesicles seem to undergo exocytosis are discussed. The hypothesis is put forward that in the mossy fibre bouton two morphologically and functionally distinct populations of synaptic vesicles exist and that only one of them undergoes visible irreversible exocytosis, whereas the majority, that is, the small vesicles discharge their transmitter by reversible fusion. After MP injection features of membrane retrieval were also prominent. Frequently, at the borders of the active synaptic zones coated membrane convolutes of both pre- and postsynaptic membranes had invaded the terminals as well as the postsynaptic spine. Thus, in contrast to electrical stimulation, the self-sustained seizures allows energy-expensive processes such as extensive membrane internalization to take place during the interictal pauses.     

Morphologically heterogeneous met-enkephalin terminals form synapses with tyrosine hydroxylase-containing dendrites in the rat nucleus locus coeruleus

Physiological and anatomical studies have suggested that the endogenous opioid peptide, methionine-enkephalin (ENK), may directly modulate noradrenergic neurons. Additionally, chronic opiate administration has been shown to increase the levels of a number of G-proteins and phosphoproteins including the catecholamine synthesizing enzyme, tyrosine hydroxylase (TH). We combined immunogold-silver localization of tyrosine hydroxylase and immunoperoxidase labeling for ENK in single sections through the nucleus locus coeruleus (LC) in the rostral pons to determine potential substrates for the divergent actions of this opioid peptide. Light microscopic analysis of ENK immunoreactivity in the LC area indicated that ENK fibers are dense and highly varicose. In coronal sections, ENK-immunoreactive processes were punctate and appeared to envelop LC-cell bodies. More rostrally, in the region of catecholamine-immunoreactive extranuclear dendrites, ENK-immunoreactive varicose processes were interdigitated with TH-labeled processes. Electron microscopy of this rostral region revealed that ENK-immunoreactive axon terminals contained small clear as well as large dense core vesicles. The large dense core vesicles (1-10/terminal) were consistently the most immunoreactive and were identified toward the periphery of the axon terminal distal to the active zone of the synapse. Unlabeled axon terminals and glial processes were the most commonly observed elements located adjacent to the plasmalemma of axons containing the labeled dense core vesicles. Axon terminals containing ENK immunoreactivity varied in size (0.3 micron to 2.0 microns) as well as formation of synaptic specializations (i.e., asymmetric versus symmetric). The ENK-labeled terminals formed synapses with dendrites with and without detectable TH immunoreactivity. These results provide the first direct ultrastructural evidence that morphologically heterogeneous terminals containing ENK immunoreactivity form synapses with catecholamine dendrites within the LC. The formation of asymmetric and symmetric synaptic specializations suggests that the opioid peptide, ENK, may be colocalized with other neurotransmitters. Furthermore, the distribution of ENK immunoreactivity in axon terminals apposed to other unlabeled afferents or astrocytic processes suggests that actions of ENK may also include presynaptic modulation of other transmitters and/or effects on astrocytes.     

The ventral lateral geniculate nucleus, pars lateralis of the rat. Synaptic organization and conditions for axonal sprouting

The synaptic organization of the pars lateralis portion of the ventral lateral geniculate nucleus is similar to that of other thalamic nuclei. There are four types of synaptic knobs (RL, RS, F1, F2). RL knobs are large and irregularly shaped, contain round synaptic vesicles and make multiple asymmetrical junctions. They are found primarily in "synaptic islands" making contact with gemmules, spines, small dendrites, and other synaptic profiles containing pleiomorphic synaptic vesicles (F2). Smaller RS knobs contain round vesicles and make asymmetrical junctions with the same type of elements as RL knobs, with the exception of the F2 profiles, but are seldom found in synaptic islands. F1 knobs contain flattened synaptic vesicles and form symmetrical junctions with F2 knobs, gemmules, spines, and small-medium dendrites in synaptic islands, throughout the neuropil, and on the proximal dendrites and soma of the largest type of neuron. F2 knobs are irregularly shaped, contain pleiomorphic synaptic vesicles and make symmetrical junctions primarily with gemmules and spines in synaptic islands. They are postsynaptic to RL and F1 knobs. Occipital decortication indicates that cortical terminals are of the RS type. Bilateral enucleation indicates that retinal terminals are of both the FL and RS type. The large amount of geographic overlap of retinal and cortical terminals on gemmules, spines, and small dendrites found in the neuropil outside of synaptic islands logically would maximize axonal sprouting between these two sources.