Morphological correlates of electrotonic coupling in the magnocellular mesencephalic nucleus of the weakly electric fish Gymnotus carapo.

The magnocellular mesencephalic nucleus (MMN) of Gymnotus carapo was studied by electron microscopy. This particular nucleus, characteristic of weakly electric fish, contains two principal classes of neuron. (1) Large neurons (25-35 mum): these are rounded unipolar cells, with the perikaryon partially covered by a sheath of compact myelin. The axon leaves the neuron as a short thick unmyelinated process not resembling the initial segment of multipolar neurons. The axon branches profusely and becomes myelinated very close to its origin. The perikaryal surface not covered by the myelin sheath receives abundant club endings. The synaptic interface between club endings and large neurons is characterized by alternating gap junctions and attachment plaques. In addition, at the periphery of the club endings, "active" zones are generally present, and this synapse is therefore a "mixed" synapse. (2) Small neurons (5-12 mum): these are uni- or bipolar cells, scattered throughout the nucleus, and occasionally, grouped in small clusters. Gap junctions were not observed between neuronal perikarya in such clusters. The synaptic investment of small neurons is formed by long cup endings which almost completely encircle the perikarya. The synaptic interface between cup endings and the perikarya of small neurons is characterized by large areas of gap junctions. A single cup ending establishing gap junctions with two small neurons within the plane of the section was frequently observed and this arrangement provides a morphological basis for electrotonic coupling between small neurons by way of presynaptic fibres. In the neuropil of the MMN, there are abundant synaptic islands constituted by a large axon terminal in synaptic contact with small unidentified profiles; both synaptic elements are surrounded by numerous thin glial lamellae. At the synaptic interface, in the islands, both gap junctions and "active" zones are present. The synaptic islands must also be considered as "mixed" synapses. The morphological results presented here correlate with electrophysiological data (Szabo et al., 1975). The very short delay (0.8-1.3 ms) of the MMS response to the fish's own electric organ discharge can only be explained by the existence of electrotonic transmission along the neuronal chain of the electrosensory pathway. The presence of gap junctions between club endings and large neurons provides a morphological basis for electrotonic transmission at the mesencephalic level of the electrosensory pathway.     

Large myelinated club endings on the Mauthner cell in the goldfish

Summary Thin sectioning and freeze-fracturing have revealed the distribution of gap junctions and chemical synapses in the synaptic interface of the large myelinated club endings on the lateral dendrite of the goldfish Mauthner cell. In 12 samples of club endings fractured completely or nearly completely, the apposed synaptic membrane area averaged 39.090 m², of which 16.6% was occupied by gap junctions and about 4 to 5% by the active zones of chemical synapses. The numerical profile density (number per unit area of the synaptic membrane) of gap junctions varied greatly, from 1.78 to 6.30, and was mostly in inverse proportion to their size. The chemical synapses were located mainly in two places: in the circumferential rim of the synaptic membrane next to the widened extracellular space, and in the margins of intraterminal invaginations of the synaptic cleft. The axoplasm of the preterminal axon, just after losing its myelin sheath, was filled with microtubules, among which neurofilaments gathered into many small bundles. The correlation between the areas of gap junctions and the chemical synapses and the amplitude of the excitatory postsynaptic potentials (EPSP) is discussed.     

Synaptology of the medullary command (pacemaker) nucleus of the weakly electric fish (Apteronotus leptorhynchus) with particular reference to comparative aspects

Summary The general organization and synaptology of the medullary command (pacemaker) nucleus (MCN) was investigated in the high frequency weakly electric fish, Apteronotus leptorhynchus. This study was undertaken in order to establish differences and similarities between the MCN of A. leptorhynchus and that of the closely related species, Apteronotus albifrons which has been studied previously. The basic morphology and synaptology of the MCN in A. leptorhynchus is similar to that of A. albifrons. The MCN of A. leptorhynchus consists of large (relay) and small (pacemaker) cells; both cell types receive synaptic input or large club endings with electrotonic gap junctions and bouton-like terminals with polarized chemical synapses. Club endings originate from thick meyelinated fibres belonging to the small (pacemaker) cells, whereas the bouton-like terminals issue from thin myelinated fibers of extranuclear origin. Via their club endings, the small (pacemaker) cells are connected both to each other and to the large (relay) cells. Besides the similarities, there are distinct and characteristic differences between the MCN of the two species, which mainly concern the synaptology of the nucleus. In A. leptorhynchus, the large (relay) cells possess long dendritic processes, covered exclusively with bouton-like terminals; the axon initial segment of large (relay) cells receives boutons, in addition to club endings. Small (pacemaker) cells have short dendritic protrusions receiving input from club endings and boutons; furthermore, the small pacemaker cells axon initial segment receives both club endings and bouton-like terminals. These differences are discussed in terms of the functional organization of the MCN in certain gymnotoids and draw attention to the fact that the morphological and ultrastructural aspects of the central command of the electric organ discharge reveal several differences not only between different gymnotoid fish (Apteronotus and Eigenmannia) but also between closely related species such as A. albifrons and A. leptorhynchus.     

The mormyrid brainstem--II. The medullary electromotor relay nucleus: an ultrastructural horseradish peroxidase study

The medullary relay nucleus of the mormyrid weakly electric fish Gnathonemus petersii is a stage in the command pathway for the electric organ discharge. It receives input from the presumed command or pacemaker nucleus and projects to the electromotoneurons in the spinal cord. Its fine structure and synaptology were investigated by electron microscopy. The origin of the terminals contacting the cell membrane of the neurons of this nucleus was determined by horseradish peroxidase (HRP) injections into different brain structures, namely into the bulbar command- and mesencephalic command-associated nuclei. Twenty-five to thirty large cells of about 45 micron in diameter constitute the medullary electromotor relay. Each cell has a kidney-shaped, lobulated nucleus, a large myelinated axon with a short initial segment and several long, richly arborizing primary dendrites. Many, if not all, cells are interconnected with large somatosomatic or dendrosomatic, dendrodendritic and dendroaxonic gap junctions. These junctions often occur in serial or triadic arrangements. The relay cells receive large club endings as well as small boutons. The club endings are found mainly on the soma and primary dendrites and are morphologically mixed synapses. The boutons are characterized by synapses which are only chemical and are distributed all over the cell membrane, but with a definitely higher frequency on secondary dendrites and more distal parts of dendritic processes. Horseradish peroxidase injections into the mesencephalic command-associated nucleus reveal a large number of labelled boutons on the secondary dendrites of the relay cells. Injections into the bulbar command-associated nucleus label the same type of boutons as mesencephalic injections, but also label club endings on relay cell soma and primary dendrites. The results support the conclusion made on the basis of previous light microscopical observations that boutons originate from the bulbar command-associated nucleus, whereas the club endings issue from the presumed pacemaker nucleus (nucleus c). The club endings of the bifurcating axons of this nucleus are labelled by retro- and anterograde transport of horseradish peroxidase; the bifurcating axons project simultaneously to the bulbar command-associated nucleus and the medullary relay nucleus.     

Anatomical study and HRP identification of electromotoneurons and motoneurons in the spinal cord of Gymnarchus niloticus

Two types of neurons were identified by light and electron microscopy in the spinal cord of Gymnarchus niloticus following HRP injections at different peripheral sites: one, situated in the mediodorsal region, is labelled after injection into the electric organ; the other, situated in the lateroventral region, is labelled in addition after injections extending beyond to the lateral muscle. The cells of the first type, thus shown to be electromotoneurons (EMNs), are spherical and do not have dendritic processes; some of them are connected by somato-somatic gap junctions. The EMNs are surrounded by dense glial processes and embedded in a network of myelinated axons of large diameter. Synaptic contacts with the EMNs are of axosomatic type, exhibiting mostly gap junctions and, less frequently, mixed or chemical junctions. Some endings establish gap junctions simultaneously, with two electromotoneurons, the somatic membrane of which may be joined. The initial segment does not bear any endings. The second type of cells, identified as motoneurons, are pyriform and have large dendritic processes. The motoneurons are surrounded by myelinated axons of small diameter. They receive axosomatic endings with chemical synapses.     

Junctional systems in the pineal gland of the Wistar rat (Ratus ratus). A freeze-fracture and thin section study

Intercellular junctions between neighbouring pinealocytes, glial cells, glial cells and pinealocytes as well as between nerve endings and parenchymal cells of the pineal gland of Wistar rats were investigated on freeze-fracture replicas and thin sections by transmission electron microscopy. Gap junctions, tight junctions and the annular gap junctions have been revealed. In addition, chemical synapses between nerve endings and pinealocytes have been observed.     

(+)-epi-Clausenamide, but not (-)-epi-clausenamide, showed more potential than (-)-clausenamide on facilitating synaptic transmission in CA1 region of hippocampal synapses

In the rabbit retina, there are two types of horizontal cell (HC). The axonless A-type HCs form a coupled network via connexin 50 (Cx50) gap junctions in the outer plexiform layer (OPL). The axon-bearing B-type HCs form two independently coupled networks; the dendritic network via gap junctions consisted of unknown Cx and the axon terminal network via Cx57. The present study was conducted to examine the localization and morphological features of Cx50 and Cx57 gap junctions in rabbit HCs at cellular and subcellular levels. The results showed that each gap junction composed of Cx50 or Cx57 showed distinct features. The larger Cx50 gap junctions were located more proximally than the smaller Cx50 gap junctions. Both Cx50 plaques formed symmetrical homotypic gap junctions, but some small ones had an asymmetrical appearance, suggesting the presence of heterotypic gap junctions or hemichannels. In contrast, Cx57 gap junctions were found in the more distal part of the OPL but never on the axon terminal endings entering the rod spherules, and they were exclusively homotypic. Interestingly, about half of the Cx57 gap junctions appeared to be invaginated. These distinct features of Cx50 and Cx57 gap junctions show the variety of HC gap junctions and may provide insights into the function of different types of HCs.     

Synaptology of the command (pacemaker) nucleus in the brain of the weakly electric fish, Sternarchus (Apteronotus) albifrons

The high frequency electric emission of the weakly electric fish Sternarchus (Apteronotus) albifrons depends on the pacemaker activity of a specific brainstem nucleus located in the ventral part of the rhombencephalic reticular formation. The general morphology and fine structure of this nucleus has been investigated, with particular reference to its synaptic connections.Three neuronal components could be distinguished in the nucleus; namely large cells of 80–100 μm diameter, small cells of 30–50 μm diameter and bundles of thin, myelinated fibres. These elements are embedded in a network of thick myelinated fibres. The large cells have a few small and short dendrites whereas the small neurons have long branching dendrites. Large and small neurons possess thick myelinated axons, but only those of the latter show branching patterns and send collaterals which have intranuclear courses only. Two types of synaptic terminals have been found on both neurons: large club endings exclusively with gap junctions and small bouton-like terminals with polarized chemical synapses. Serial semi-thin and ultra-thin sections revealed that the large club endings belong to the pacemaker cells, whereas the small terminals are found in the thin myelinated axons of extranuclear origin.The findings indicate that the small neurons are connected 1) to each other and 2) to the large neurons, by way of their large myelinated axons. Both, small (pacemaker) as well as large (relay), neurons receive chemical synapses from myelinated fine fibers probably originating from higher encephalic centers. Thus, electric organ discharge rhythm can be modulated at the level of pacemaker as well as of the relay cells. No somatosomatic, dendrodendritic or dendrosomatic connections were found between large, small or large and small cells.     

The mormyrid brainstem--III. Ultrastructure and synaptic organization of the medullary "pacemaker" nucleus

The ultrastructure and synaptic organization of the presumed medullary pacemaker nucleus, nucleus c of the weakly electric mormyrid fish, Gnathonemus petersii has been investigated. Nucleus c consists of about 12-15 small (20-25 micron) neurones (P-cells), which form a group situated ventrally to the medullary relay nucleus and embedded in a neuropil of myelinated fibres and dendritic processes. The P-cells often exhibit an enhanced electron density of their cytoplasm and dendroplasm. They possess several dendrites of different diameter, a short, thin axon initial segment and a thickly myelinated axon running in dorsal direction. The pacemaker neurons are interconnected by complex electronic coupling, established by somatosomatic, dendrosomatic and dendrodendritic gap junctions. Perikarya and dendrites are frequently interconnected serially by gap junctions; dendrites showed sometimes triadic gap-junction arrangement. It is suggested that this high degree of electrotonic coupling amongst the pacemaker cells represents the first level of the highly ordered synchronization processes which characterize the electric discharge command system of Gnathonemus. Pacemaker cells receive synaptic input from club endings with mixed synapses and from bouton-like terminals with chemical synapses, both of them originating from medium-sized myelinated fibres and contacting mainly neuronal perikarya and dendritic processes. The axon initial segment receives only few synaptic inputs. Bouton-like terminals were found to be of two types according to their vesicle content, namely, boutons with ovoid, clear synaptic vesicles forming Gray type-1 synapses and boutons with pleomorphic clear synaptic vesicles forming Gray type-2 synapses. Different functional roles for the two types of boutons in modulating pacemaker cell activity are suggested.     

Neuronal and synaptic organization of the outer plexiform layer of the pigeon retina

The organization of the outer plexi-form layer (OPL) of the pigeon retina is described by electron microscopy and Golgi impregnation. Six types of photoreceptor, four types of horizontal cell, eight types of bipolar cell, and an interplexiform cell type were found by Golgi impregnation. The OPL was tri-stratified due to the endings of the photoreceptors at three different levels. This stratification was reflected in the laminar arrangement of the dendrites of the horizontal and bipolar cells. Electron microscopy showed that the synaptic endings of the photoreceptors made ribbon synapses, both triads and dyads, and basal junctions with the process of second-order neurons. Horizontal cells formed conventional chemical synapses, while horizontal cell axon terminals were extensively linked by gap junctions.     

Ultrastructural observations on intercellular contacts of nigral dendrites.

Possible morphological correlates of dendritic release sites for dopamine have been investigated in the rat substantia nigra. Electronmicroscopic and freeze etching studies show that nigral dendrites fail to form dendrodendritic contacts but on the contrary are always separated by glial elements, which in turn are connected by multiple gap junctions. No dendro-axonic synapses are present but large specialized symmetrical junctions could provide a possible morphological specialization for a dopamine transfer from presynaptic dendrites to axon endings.     

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.     

Plexus muscularis profundus and associated interstitial cells. II. Ultrastructural studies of mouse small intestine

The ultrastructure of plexus muscularis profundus (PMP) of the mouse small intestine was investigated subsequent to vascular perfusion with ruthenium red-containing and routine aldehyde fixatives. Four types of nerve terminals were revealed. Type I: numerous 500-Å agranular vesicles and few 1,000-Å granular vesicles. Type II: predominantly large (1,000–1,500 Å), granular vesicles and fewer 500-Å agranular vesicles. Type III: an abundance of mitochondria and many flattened vesicles (300 Å × 700-1,300 Å). Type IV was identified by abundant smooth cisternae 200 Å in width. Types I-III formed close (200 Å), synapselike contacts to interstitial cells of Cajal (ICC-III). Presynaptic densities were frequent in type I endings. A direct innervation of muscle cells via PMP was only very occasionally suggested. ICC-III possessed a basal lamina and numerous caveolae associated with subsurface SER-cisternae. Mitochondria were very abundant in ICC-III-processes. ICC-III formed multiple, large gap junctions with outer circular-muscle cells and with other ICC-III. Also reflexive gap junctions were observed. Fibroblastlike cells (FLC) were distinguished by their prominent GER, the frequent presence of lipid droplets, and the lack of caveolae and a basal lamina. FLC never participated in synaptic arrangements or gap junctions. Macrophagelike cells were occasionally encountered. It is concluded that possible efferent and afferent nerve terminals in PMP may chiefly, if not exclusively, innervate ICC-III, the ultrastructure of which is compatible with efferent and/or afferent modulatory actions.     

Electric synapses in the carotid body-nerve complex

Slices of rat carotid bodies, or cultured glomus cells, were used to study intercellular coupling. This phenomenon occurs because gap junctions allow passage of currents and dyes from one cell to another. There is a two-way resistive coupling between glomus cells (GC/GC coupling), which is accompanied by activity of intercellular channels. Coupling between glomus cells and nerve endings is more complex. Coupling is mostly resistive from cell to nerve (GC/NE) but it is mostly capacitive in the opposite direction (NE/GC). Thus, slow electric events originating in the glomus cells can be transferred to the nerve endings. But, only electric transients can pass from nerve to cell. There is also coupling between nerve endings (NE/NE), which is mostly capacitive in either direction. Chemoreceptor stimulants (acute and chronic hypoxia, hypercapnia, acidity, cholinergic agents and dopamine) uncouple most glomus cells, accompanied by cell depolarization and decreased amplitude of junction channels. Chronic hypobaric hypoxia increases GC/NE, NE/GC and NE/NE coupling. GC/GC uncoupling seems related to transmitter secretion. Transmission across chemical synapses is aided by increased coupling from glomus cell to nerve ending.     

大鼠表皮角化细胞分裂活性的拓扑学特征

为了探讨大鼠躯干背部皮肤皮表角化细胞的分裂活性是否因部位而有区别，常规石蜡切片和ＨＥ染色，通过核型观察计数分裂中期细胞，结果显示：在所研究的范围内，分裂活性随表皮的部位而异，分裂活性高低不同的角化细胞各自聚集成群，细胞群的大小约为１０＾３ｍ数量级，大于已报告的变异尺度。结果提示，肾上腺素能神经末梢和缝隙连接的密度分布可是这种表皮角化细胞分裂活性拓扑学特征形成的原因。     

Abundance and ultrastructural diversity of neuronal gap junctions in the OFF and ON sublaminae of the inner plexiform layer of rat and mouse retina

Neuronal gap junctions are abundant in both outer and inner plexiform layers of the mammalian retina. In the inner plexiform layer (IPL), ultrastructurally-identified gap junctions were reported primarily in the functionally-defined and anatomically-distinct ON sublamina, with few reported in the OFF sublamina. We used freeze-fracture replica immunogold labeling and confocal microscopy to quantitatively analyze the morphologies and distributions of neuronal gap junctions in the IPL of adult rat and mouse retina. Under "baseline" conditions (photopic illumination/general anesthesia), 649 neuronal gap junctions immunogold-labeled for connexin36 were identified in rat IPL, of which 375 were photomapped to OFF vs. ON sublaminae. In contrast to previous reports, the volume-density of gap junctions was equally abundant in both sublaminae. Five distinctive morphologies of gap junctions were identified: conventional crystalline and non-crystalline "plaques" (71% and 3%), plus unusual "string" (14%), "ribbon" (7%) and "reticular" (2%) forms. Plaque and reticular gap junctions were distributed throughout the IPL. However, string and ribbon gap junctions were restricted to the OFF sublamina, where they represented 48% of gap junctions in that layer. In string and ribbon junctions, curvilinear strands of connexons were dispersed over 5 to 20 times the area of conventional plaques having equal numbers of connexons. To define morphologies of gap junctions under different light-adaptation conditions, we examined an additional 1150 gap junctions from rats and mice prepared after 30 min of photopic, mesopic and scotopic illumination, with and without general anesthesia. Under these conditions, string and ribbon gap junctions remained abundant in the OFF sublamina and absent in the ON sublamina. Abundant gap junctions in the OFF sublamina of these two rodents with rod-dominant retinas revealed previously-undescribed but extensive pathways for inter-neuronal communication; and the wide dispersion of connexons in string and ribbon gap junctions suggests unique structural features of gap junctional coupling in the OFF vs. ON sublamina.     

Dual transmission at morphologically mixed synapses: Evidence from postsynaptic cobalt injections

Two experimental approaches have been utilized to test the possibility that morphologically mixed synaptic terminals of the eighth nerve fibers mediate both electrotonic and chemical excitation of the goldfish Mauthner cell. First, the spatial distributions of electrotonic and chemical postsynaptic potentials, evoked by stimulation of the eighth nerve, have been determined with intracellular recordings from the Mauthner cell soma and several locations along the lateral dendrite. In some instances, both synaptic components were maximal at distal dendritic recording sites. In that region, it appears that the only presynaptic terminals with morphological characteristics consistent with excitatory chemical transmission are the large myelinated club endings, which actually establish mixed synapses with the lateral dendrite. Second, we have analyzed the effects of postsynaptic Co²⁺ injections on these synaptic responses. With high iontophoretic currents, there was a rapid uncoupling of the electrotonic component. However, with smaller current intensities, uncoupling is accompanied, or preceded, by a transient reduction in the later chemically mediated postsynaptic potentials. This latter effect on chemical transmission is only observed if the postsynaptic potentials are associated with electrotonic synaptic inputs. We speculate that Co²⁺ diffuses across the gap junctions and into the presynaptic terminals, acting there to reduce evoked transmitter release.The results of these two experimental approaches support the hypothesis that mixed synapses on the lateral dendrite of the Mauthner cell do actually mediate transmission by both chemical and electrical modes.     

The lateral vestibular nucleus of the toadfish Opsanus tau: Ultrastructural and electrophysiological observations with special reference to electrotonic transmission

The lateral vestibular nucleus of the toadfish Opsanus tau was localized by means of axonal iontophoresis of Procion Yellow. The ultrastructure of the lateral vestibular nucleus neurons was then correlated with their electrophysiological properties. The lateral vestibular nucleus consists of neurons of various sizes which are distributed in small clusters over a heavily myelinated neuropil. The perikarya and main dendrites of the large and the small neurons are surrounded by a synaptic bed, which is separated from the neighboring neuropil by a layer of thin astrocytic processes. The synaptic bed contains three main classes of axon terminals, club endings, large and small terminals, the first being quite infrequent. All the large terminals as well as the occasionally observed club endings contain a pure population of rounded synaptic vesicles. In some of the small axon terminals there are also rounded vesicles; however, the majority contain flattened vesicles or a pleomorphic population. These data indicate that the small terminals originate from different afferent sources. The synaptic interfaces of the large boutons and of the club endings bear three types of junctional complexes: attachment plates, gap junctions and active zones. Those showing both gap junctions and active zones were designated as morphologically ‘mixed synapses’. Gap junctions, although in large number, have only been observed at the synaptic interfaces between terminals with rounded vesicles and the perikarya or the dendrite of the lateral vestibular nucleus neurons. Therefore electrotonic coupling would only be possible by way of presynaptic fibers. Some axons observed in the neuropil were found to establish gap junctional complexes with two different dendritec profiles and this observation is in favour of electrotonic coupling by way of presynaptic terminals.Field and intracellular potentials were recorded in the lateral vestibular nucleus. The field potential evoked by stimulation of the vestibular nerve consisted of an early positive-negative wave followed by a slow negativity, and that evoked by spinal cord stimulation was composed of an antidromic potential followed by a slow negative wave. Vestibulo-spinal neurons were identified by their antidromic spikes. In these cells, stimulation of the ipsilateral vestibular nerve evoked an excitatory postsynaptic potential with two components. The short delay of the first component of this excitatory postsynaptic potential and its ability to follow paired stimulation at close intervals without reduction of the second response suggest that it is transmitted electrotonically from primary vestibular afferent fibers. By contrast the latency of the second peak of the vestibular evoked excitatory postsynaptic potential and its sensitivity to high stimulus frequencies are compatible with monosynaptic chemically mediated transmission from primary vestibular afferents. Spinal stimulation evoked graded antidromic depolarizations in vestibulo-spinal neurons. The latency of these potentials was too short to allow for chemical transmission through afferents or recurrent collaterals and suggests electrotonic spread of antidromic activity from neighboring neurons. An important finding is that the graded antidromic depolarizations can initiate spikes; thus coupling between neurons in the lateral vestibular nucleus is sufficiently close that a cell can be excited by activity spread from neighboring cells. Similar graded depolarizations were recorded in identified primary vestibular afferents; their latencies and time course indicate that they were brought about by electrotonic spread of postsynaptic potentials and spikes to the impaled presynaptic fibers; this confirms the morphological evidence that coupling between lateral vestibular nucleus neurons occurs, at least in part, by way of presynaptic vestibular axons. As the spinal stimulus strength was increased, these graded depolarizations became large enough to initiate spikes which presumably propagate to the vestibular receptors. Thus antidromic invasion of the presynaptic terminals may provide negative feedback by preventing their re-excitation at short intervals after a synchronous discharge of an adequate number of postsynaptic cells. Excitatory inputs to the neurons of the lateral vestibular nucleus were identified from the spinal cord and from the contralateral vestibular nerve. Long latency excitatory postsynaptic potentials large enough to excite the cells were recorded following spinal stimulation; the threshold intensity for evoking them was consistently higher than that adequate to generate the graded antidromic depolarizations. Field potentials recorded after stimulation of the contra lateral vestibular nerve consisted of an initial positive negative wave followed by a slow negative wave. the stimulus intensity for evoking these potentials was the same or slightly above the threshold for those evoked in the lateral vestibular nucleus on the stimulated side. Also lateral vestibular nucleus neurons exhibited excitatory postsynaptic potentials large enough to excite the cells following stimulation of the contralateral vestibular nerve. but no inhibitory postsynaptic potentials were detected. This lack of commissural inhibition indicates a qualitative difference between the central organization of these cells in the toadfish and in mammals.The presence of neurons in the lateral vestibular nucleus which send their axons to the labyrinth was confirmed by their heavy staining with Procion Yellow following axonal iontophoresis. In a number of vestibular neurons. abruptly rising spikes were evoked at short latencies after adequate stimulation of the ipsilateral vestibular nerve. Graded stimuli applied to the vestibular nerve evoked graded short latency depolarizations as well as long latency excitatory postsynaptic potentials in these presumed efferent neurons to the labyrinth; the former could indicate electrotonic coupling of the efferent cells or electrotonic transmission from primary afferents, resulting in a short latency feedback loop.From these studies, the synaptic organization of the lateral vestibular nucleus neurons is compared with that of the Mauthner cells of teleosts, and the possibility of a dual mode of transmission, electrical and chemical, by primary vestibular afferents is discussed.     

Tissue-specific granularity of gap junction cytoplasmic surface revealed by rapid-freeze,deep-etch replicas

Previous rapid-freeze, deep-etch replica studies have revealed the differences between heart and liver gap junctions; cytoplasmic surfaces of in situ and phenylmethylsulfnoyl fluoride (PMSF)-unproteolyzed isolated cardiac gap junctions (MW 47 kD) have a particulate substructure, which is absent both in the proteolyzed heart junctions (MW 29 kD) and in the liver junctions isolated with PMSF (MW 28 kD). The present deep-etch replica studies of gap junction cytoplasmic surface (CS) membranes in several tissues of rats and mice were performed to examine whether or not this difference between liver and heart is typical of variations in gap junction proteins from tissue to tissue. In surface mucous cells of the stomach, intestinal epithelial cells, and kidney tubule cells, these epithelial gap junctions always showed smooth cytoplasmic surfaces, similar to the liver gap junctions. In contrast, in the atrial myocardium, aortic endothelium, and the ciliary process, cytoplasmic surface membranes of the gap junctions consistently revelaed particulate patterns. Close examinations disclosed that those granular structures were not merely attached to the membrane surface, but they also protruded from the membrane interior as an integral component of gap junctions particles. Furthermore, in the pregnant rat uterus at term, cytoplasmic surface membrances of myometrial smooth muscle gap junctions were particulate, but those of endometrial epithelium were smooth. The present observations strongly suggest that tissue specificity exists in cytoplasmic surface structures of gap junctions between the “true” epithelial and the noneepithelial tissues: the nonepithelial gap junctions contain the additional cytoplasmic surface domain that is absent in the gap junctions of “true” epithelial origin.