Synaptic terminal degeneration and remodeling at the rat neuromuscular junction resulting from a single exposure to acrylamide

Repetitive exposure to low doses of acrylamide results in extensive pathological changes at the neuromuscular junction (NMJ), but it remains undetermined if a single exposure to a larger dose will produce a similar neuropathological outcome. In the present study, morphometric and ultrastructural analyses of rat soleus NMJ were performed to determine early pathological effects of an intraperitoneal injection of 100 mg/kg acrylamide. Widespread nerve terminal degeneration, terminal sprouting, and endplate lengthening were evident as early as 4 days after injection. Degenerating terminal branches were swollen and exhibited enhanced argyrophilia. Ultrastructurally, the majority of terminals exhibited axolemmal abnormalities, neurofilament accumulations, and a paucity of synaptic vesicles; occasional swollen terminals lacked neurofilaments but contained increased numbers of tubulovesicular profiles. This early morphological pattern of nerve terminal changes suggests that acrylamide may disrupt both synaptic vesicle recycling and neurofilament degradation. These findings indicate that a single high dose of acrylamide triggers pathological lesions and remodeling in motor nerve terminals virtually identical to those resulting from multiple low doses.

Ultrastructural observations on regenerating hair cells in the chick basilar papilla

This experiment was designed to investigate cellular and subcellular maturational changes in regenerated immature sensory cells and support cells of the chick basilar papilla following gentamycin treatment. Scanning and transmission electron microscopy were used. The experimental animals received one subcutaneous injection of gentamycin sulfate daily (50 mg/kg) for five or 10 days. The animals receiving five days of injection were sacrificed the following day. The remaining animals were allowed to survive either seven or 28 days before sacrifice and preparation for electron microscopy. The initial lesion consisted of total degeneration of hair cells within 500 microns of the proximal tip providing the opportunity to study a 'pure' population of regenerating sensory cells. Sensory cell regeneration could be identified by one day after terminating gentamycin treatment. Early in development sensory cell precursors were morphologically very similar to supporting cells. A density gradient, based on cytoplasmic staining characteristics, was established which increased from cells displaying low density at the base of the supporting cell layer to high density cells at the luminal surface. These changes in density were equated to increase in number of and types of cytoplasmic organelles. In contrast to the empty appearing cytoplasm of the support cell, the cytoplasm of the hair cell precursor contained numerous mitochondria, clusters of ribosomes, and vesicles. As the cell approached the surface, mitochondria became more numerous as did smooth and coarse endoplasmic reticulum and Golgi apparatus. This gradient suggested that determination of the cellular phenotype occurred at the level of the basal membrane followed by migration to the surface, during which time differentiation was characterized by an increase in number and complexity of cellular organelles. Luminal surface modifications occurred as soon as the cell erupted. The development of stereocilia, rootlet, cuticular plate and cellular polarization followed the normal embryogenetic pattern. At 28 days, stereocilia organization was still incomplete as was the orientation of the bundle. To the extent that proper orientation of hair cells or bundles is necessary for normal transduction, mature function at 28 days would not be anticipated. Innervation of the presumptive hair cell precursors could be observed one day after treatment, early in the course of hair cell differentiation. Synaptogenesis followed the normal embryogenetic sequence; however, afferent and efferent nerve terminals remained immature appearing at 28 days. This observation may have physiological implications manifested by delay of hearing

End structure and neuropeptide immunoreactivity of axons in sciatic neuromas following nerve section in neonatal rats

The formation of neuromas after neonatal nerve injury was studied in rats. In neonatal pups, the sciatic nerve was cut and tightly ligated, and a portion of the distal stump was removed. After 6-10 weeks, a nerve-end neuroma had formed in about 70% of the animals. In the remaining animals the nerve had grown on the side of the ligature. The end structure of the neuroma axons was studied using anterogradely transported WGA-HRP injected into the L5 dorsal root ganglion. HRP labeling occurred in the entire proximal sciatic nerve. In the neuroma, labeled fibers branched profusely and either terminated with minor end swellings or turned in the retrograde direction. Immunohistochemistry showed that the fibers which projected into the neuroma presented a moderate immunoreactivity to substance P and neuropeptide Y and a strong reactivity to calcitonin gene-related peptide. The results show that many sensory and sympathetic sciatic nerve fibers survive chronic axotomy in the newborn and contribute to the formation of nerve-end neuromas. There are, however, important structural differences between adult and neonatally induced neuromas.

Activity-driven sharpening of the regenerating retinotectal projection: effects of blocking or synchronizing activity on the morphology of individual regenerating arbors

Both blocking activity with intraocular tetrodotoxin (TTX) and synchronizing activity with a xenon strobe light (1 Hz) prevent retinotopic sharpening of regenerating optic projection in goldfish (Meyer, 1983; Schmidt, 1985; Cook and Rankin, 1986). In this study, we tested, in both normal and regenerating projections, the effects of these two treatments on individual optic arbors. Arbors were stained via anterograde transport of HRP, drawn in camera lucida from tectal whole mounts, and analyzed for spatial extent in the plane of the retinotopic map, order of branching, number of branch endings, depth of termination, and the caliber of the parent axon. In normal tectum, fine, medium, and coarse caliber axons gave rise to small, medium, and large arbors, which averaged 127 microns, 211 microns and 275 microns in horizontal extent, and terminated at characteristic depths. All three classes averaged roughly 21 branch endings. Optic arbors that regenerated with normal patterns of activity returned to a roughly normal appearance by 6-11 weeks postcrush: the same three calibers of axons gave rise to the same three sizes of arbors at the same depths, but they were much less stratified and well on average about 16% larger in horizontal extent. At this time point, arbors regenerated under TTX or strobe were on the average 71 and 119% larger, respectively, than the control-regenerated arbors (larger in all classes), although they had approximately the same number of branch endings and were equally poorly stratified. Synapses formed under strobe were also normal in appearance. Thus the only significant effect of both strobe and TTX treatment was to enlarge the spatial extent of arbor branches. Arbors that were not regenerating were very slightly (but significantly) enlarged by TTX block of activity or strobe illumination. As previous staining showed that regenerating axons initially make widespread branches and later retract many of those branches (Schmidt, Turcotte, Buzzard, and Tieman, 1988; Stuermer, 1988), the present findings support the idea that blocking activity or synchronizing activity prevents retinotopic sharpening by interfering with the elimination of some of the errant branches.

Tenotomy-induced motor endplate alterations in rat soleus muscle

The effects of tenotomy on the ultrastructure of rat soleus muscle motor endplates were examined both qualitatively and quantitatively. Rat soleus muscle was studied 2 weeks following tenotomy and compared with normal littermates. The motor endplates from the tenotomized muscles were found to exhibit both degenerative and regenerative changes. Degeneration consisted of postjunctional fold breakdown, exposed junctional folds, myelin-like bodies within the sub-junctional sarcoplasm, and dense bodies within the Schwann cell cytoplasm. The regenerative changes consisted of several small nerve terminals occurring within the same primary synaptic cleft and several axons wrapped by the same Schwann cell. The results demonstrate that tenotomy induces denervation-like changes at endplates that lead to terminal sprouting within the neuromuscular junctional area and remodelling.

Ultrastructural evidence for serotonin-immunoreactive terminals contacting phrenic motoneurons in the cat

The innervation of the phrenic motor nucleus in the cat by serotonin-containing neurons has been studied using retrograde tracing combined with immunohistochemistry at the electron microscope level. It was found that phrenic motoneuron cell bodies and dendrites are contacted by serotonin-immunoreactive synaptic terminals. This finding suggests that the activity of phrenic motoneurons is directly affected by serotonergic neurons.

Fine structure of noradrenergic terminals and their synapses in the rat spinal dorsal horn: an immunohistochemical study

Noradrenergic fibers in the spinal dorsal horn originate from neurons in the A5-7 cell groups, and may participate in the modulation of pain. Here we studied the fine structure of noradrenergic terminals in the rat by immunohistochemistry using antiserum against dopamine-beta-hydroxylase (DBH). We also investigated the relationship between such terminals and primary afferent terminals. DBH-like immunoreactive terminals were found in lamina I and the outer layer of lamina II of the dorsal horn and they contained many clear round vesicles and some large granular vesicles. More than half of these terminals made synaptic contact with other neuronal elements with membrane specialization. Most of the postsynaptic structures of these terminals were small dendrites (69%); 28% were spines, and no synaptic contact was made with primary afferent terminals. These findings suggest that noradrenaline acts on the spinal dorsal horn neurons postsynaptically mainly via a direct synaptic mechanism.

Effects of colchicine on central and peripheral nerve terminals of the locust Schistocerca gregaria

The treatment of locusts (Schistocerca gregaria) with colchicine leads to various changes in the ultrastructure of both central and peripheral nerve terminals. These changes include a marked reduction in synaptic vesicle density, which is further enhanced following stimulation. There are also increased numbers of coated pits and membraneous cisternae together with enlarged terminal areas and disruption of mitochondrial ultrastructure. The capacity of isolated central nerve terminals (synaptosomes) to take up the radiolabelled transmitter precursor [3H]-choline is enhanced following in vivo treatment of locusts with colchicine, whilst in vitro treatment leads to a reduction in choline uptake. The results are discussed with regard to the postulated effects of colchicine on axoplasmic transport.

Increased density of glutamic acid decarboxylase-containing terminals in the medial preoptic nucleus and the area surrounding the paraventricular hypothalamic nucleus is associated with deoxycorticosterone acetate (DOCA)-salt hypertension

gamma-Aminobutyric acid (GABA) is a major inhibitory neurotransmitter and has been shown to exert considerable influence on the neural control of the cardiovascular function. It is not clear, however, which GABAergic systems are involved in salt-induced hypertension. This study was designed to investigate the GABAergic neurons in specific regions of the brain possibly linked to salt-induced hypertension. After 4 weeks of deoxycorticosterone acetate (DOCA) and salt treatments, the rats developed cardiac hypertrophy. All of the animals were sacrificed for immunocytochemical localization of GABAergic terminals using specific antibodies to glutamic acid decarboxylase (GAD). GAD-positive GABAergic terminal densities in discrete regions of the brain were determined by using morphometric quantitation. Results showed that GABAergic terminal densities in the medial preoptic nucleus and the area lateral to the paraventricular hypothalamic nucleus were significantly increased in DOCA-salt-treated rats 4 weeks after the experiment as compared with 4 week controls. This study provides new evidence to support further the idea that central GABAergic neurons are closely associated with pathogenesis of salt-induced hypertension. Different hypertensive mechanisms between salt-induced hypertension and genetic hypertension are also discussed.

The synaptic connections of basket cell axons in the developing rat hippocampal formation

Recent studies have indicated that hippocampal basket cells in both the dentate gyrus and Ammon's horn develop their somal and dendritic features during the first two postnatal weeks in rats. Their axon terminals form exclusively symmetric synapses that are found as early as 5 postnatal days in both regions. The present study used Golgi-electron microscopic material from 10 and 16 day old rats to demonstrate that the axon terminals of basket cells form synapses not only with somata, dendrites, and dendritic spines as reported for adult material but also with axon initial segments. However, the terminals forming synapses with axon initial segments and dendritic spines represent only a minor portion of the total number of basket cell terminals. Quantitative results indicate that 36-62% of the total number of these terminals form axosomatic synapses and 32-50% form axodendritic synapses depending on the analyzed cell. These data indicate that hippocampal basket cells have an axonal distribution similar to that found for cortical basket cells.

Termination patterns of serotoninergic medullary raphespinal fibers in the rat lumbar spinal cord: an anterograde immunohistochemical study

Electrical and chemical stimulation given in the ventral medullary raphe nuclei inhibits spinal nociceptive reflexes and spinal nociceptive transmission; serotoninergic receptors have been demonstrated to partially mediate that inhibition. In the present study, the termination patterns of raphespinal fibers in the rat lumbar spinal cord demonstrating serotonin-like immunoreactivity were examined by using the anterograde tracer Phaseolus vulgaris leucoagglutinin (PHA-L) in combination with immunohistochemistry. Fibers and terminations from the ventral medullary raphe nuclei (raphe magnus and raphe pallidus) demonstrating both PHA-L- and serotonin-like immunoreactivity were identified in all laminae of the dorsal horn and the ventral horn. Networks of large fibers, characterized by large boutons, and which did not demonstrate serotonin-like immunoreactivity, were identified in deeper laminae of the dorsal horn. The heterogeneous morphology of raphespinal fibers identified in the dorsal horn suggests that these fibers also may be heterogeneous in neurochemistry and function. Medial medullary sites outside the raphe nuclei were found to innervate the ventral horn and all laminae of the dorsal horn, with the exception of lamina I. Descending fibers and terminations also demonstrating serotonin-like immunoreactivity were identified in deep laminae (III, IV, V, VI) of the dorsal horn and in the ventral horn. Similarly, large fiber networks were identified which did not demonstrate serotonin-like immunoreactivity.

Ultrastructural study on cell differentiation of the rabbit carotid body

The development of the carotid body in both fetuses from 13 to 30 days of gestation and newborn rabbits was studied by light and electron microscopy. The carotid body anlage first appeared as a cellular aggregation close to the ventral wall of the third branchial artery on the 14th day of gestation. Type-I cells were recognized as such by the presence of dense-cored vesicles in the cytoplasm on the 16th day of gestation, whereas cells destined to develop into Type-II cells became clearly distinguishable by their typical relationship to the Type-I cells on the 20th day of gestation. Afferent and efferent synapses as well as reciprocal-like synapses between Type-I cells and nerve endings were also observed in perinatal fetuses. Although cell differentiation was almost finished by birth, Type-I cells and their innervation did not seem to be fully matured. Thus, it was concluded that ultrastructurally, the rabbit carotid body during the fetal and perinatal periods was relatively inactive.

Demonstration of a releasable pool of glutamate in cerebellar mossy and parallel fibre terminals by means of light and electron microscopic immunocytochemistry

The chemical substance(s) responsible for the fast signalling in the mossy fibre to granule cell synapses in the cerebellum has not been identified, although recent studies suggest that glutamate is a strong candidate. In the present investigation, this issue was explored by means of a quantitative electron microscopic immunocytochemical procedure. Ultrathin sections of plastic-embedded rat cerebella were treated with an antiserum specific for glutaraldehyde-fixed glutamate, followed by a secondary antibody coupled to colloidal gold particles. The gold particle density over mossy fibre terminals was assessed in tissue that had been rapidly fixed by perfusion, as well as in tissue that had been incubated in artificial cerebrospinal fluid in vitro before immersion fixation. In both preparations the mossy fibres appeared as the most intensely glutamate-immunoreactive profile type in the cerebellar cortex, and the parallel fibre terminals were also strongly labelled. Corresponding results were obtained at the light microscopic level. Most of the immunoreactivity in the mossy and parallel fibre terminals could be depleted in a Ca(+)-dependent manner by depolarization with a high K+ concentration. These data suggest that the mossy and parallel fibre terminals contain a glutamate pool that behaves as a transmitter pool.

The morphological variability of neuromuscular junctions in the rat extraocular muscles: a scanning electron microscopical study

Six morphologically distinct types of neuromuscular junctions were identified by scanning electron microscopy in the rat extraocular muscles: two diffuse and four focal types. The diffuse junctions, spreading out extensively over the muscle fiber surface, were characterized by two types of varicose swellings (or terminal varicosities) of nerve endings. One type consisted of several ramifying nerve endings and shallow postsynaptic depressions with poorly-developed junctional folds. The other type consisted of a single axon and formed many synaptic contacts along the long axis of a muscle fiber. Round synaptic depressions facing the varicosities contained several junctional folds. The focal junctions, confined to an oval area on the muscle fiber surface, were characterized by the complexity and variability of their subneural apparatuses. Four different types of apparatuses, i.e., focal junctions, were found: 1) an apparatus consisting of labyrinthine gutters with numerous slit-like junctional folds, 2) apparatuses consisting of a large number (more than 20) of cup-like depressions with either a small number of pit-like junctional folds or 3) numerous slit-like ones, and 4) an apparatus consisting of a small number (about 10) of cup-like depressions with a few junctional folds. The findings indicate that the rat extraocular muscles contain six different types of muscle fibers.

Organization of geniculocortical projections in turtles: isoazimuth lamellae in the visual cortex

The projection from the dorsal lateral geniculate complex to the visual cortex in Pseudemys and Chrysemys turtles was examined by using the anterograde transport of horseradish peroxidase (HRP) in vitro and the retrograde transport of HRP in vivo. In vitro HRP injections into the lateral forebrain bundle were used to fill geniculocortical axons anterogradely, which were then analyzed in cortical wholemount preparations. Geniculocortical axons gain access to the visual cortex along its entire rostral-caudal extent. They course in slightly curved trajectories for up to 2 mm from the lateral edge of the cortex through both the lateral (or pallial thickening) and medial parts of Desan's cortical area D2. Single axons are of fine caliber. They tend to cross each other and sometimes branch in the pallial thickening, but are generally unbranched in the medial part of D2. They bear small, fusiform varicosities at irregular intervals along their lengths. Although axons show small variations in the number of varicosities per 100 microns segment, no consistent variation in varicosity number as a function of distance could be detected. These results indicate that geniculocortical axons project to the visual cortex in an orderly pattern. The retrograde transport experiments provide some clue as to the significance of this pattern. Small, ionotophoretic injections of HRP in the visual cortex retrogradely labeled neurons in the dorsal lateral geniculate complex. Injections in the rostral visual cortex retrogradely labeled neurons in the caudal pole of the geniculate complex. Injections at progressively more caudal loci within the visual cortex labeled neurons at progressively more rostral loci within the geniculate complex. Thus, there is a representation of the rostral-caudal axis of the geniculate complex along the caudal-rostral axis of the visual cortex. Consistent with the anterograde transport experiments that showed individual geniculocortical axons coursing through both lateral and medial parts of the visual cortex, HRP injections restricted to the medial edge of the visual cortex retrogradely labeled neurons along the entire dorsal-ventral axis of the geniculate complex at the appropriate rostral-caudal position. The neurophysiological studies of Mazurskaya ('72: J. Evol. Biochem. Physiol. 8:550-555; respond to a small, moving stimulus anywhere in visual space, implying a convergence of inputs from all points in visual space somewhere along the retinogeniculocortical pathway. The experiments reported here suggest a convergence in the geniculocortical projections of information along the vertical meridians, or azimuth lines, of visual space onto neurons lying along lateral to medial transects through the visual cortex.(ABSTRACT TRUNCATED AT 400 WORDS)

Functional morphology of the telson-uropod stretch receptor in the sand crab Emerita analoga

The telson-uropod stretch receptor in Emerita analoga belongs to a segmental array of axial-coxal receptors having sensory neurons with central somata. It consists of an elastic strand innervated by four giant, nonspiking, mechanoreceptive neurons. We examined the elastic (sensory) strand by light and electron microscopy to discover whether 1) functional differences between the four sensory neurons could be surmised from their peripheral terminations and 2) the process of mechano-electric transduction in this type of receptor might be revealed by the ultrastructure of the dendritic terminals. The elastic strand is divided into nonoverlapping domains by the mechanoreceptive neurons. We discerned no differences other than their serial order along the strand. Each neuron has three morphological zones in the periphery. In the zone of dendrite entry, the dendrite bifurcates into 40-60 microns primary branches that enter the strand at acute to right angles and turn rostrally and caudally along the strand's axis. The zone of branching consists of 4-35 microns diameter dendritic branches within the strand. Approximately 21,000 dendritic tips (diameters, 0.1-0.7 micron; lengths, 3-20 microns) per sensory neuron arise via short stubs from all levels of the zone of branching and constitute the zone of dendrite termination. The repeated bifurcations of dendrites within the zone of branching take place by radial intrusion of longitudinally oriented, partitions of extracellular matrix each of which extends the length of the pair of dendritic branches it divides. Tips project into these partitions. Since they change shape when the elastic strand is stretched, which compresses the extracellular matrix of the partitions, they are probably the site of mechanosensory transduction. Cross-sectional profiles of dendritic tips are significantly larger but less numerous in stretched than in relaxed receptors. We propose a model in which stretch compresses distal portions of tips until they are too small to be recognized. The large profiles are the proximal portions of tips that have been expanded by hydrostatic pressure. The very large number of dendritic tips/nonspiking mechanoreceptor may be what endows each neuron with 1) high sensitivity to stretch of the elastic strand and 2) ability to respond to a broader range of tensions than is usual for single sensory cells (the entire range experienced by the stretch receptor).

Hippocampal and midline thalamic fibers and terminals in relation to the choline acetyltransferase-immunoreactive neurons in nucleus accumbens of the rat: a light and electron microscopic study

The synaptic interactions between terminals of allocorticostriatal and thalamostriatal fibers and the cholinergic neurons in the nucleus accumbens were investigated using degeneration and dual labelling immunocytochemistry in Wistar rats. The presumptive cholinergic neurons were labelled with antibodies directed against choline acetyltransferase and the afferent fibers were labelled anterogradely with Phaseolus vulgaris-leucoagglutinin. Fibers from the subiculum of the hippocampal formation and from the midline and intralaminar thalamus project densely into the medial nucleus accumbens where they overlap a relatively dense population of choline acetyltransferase-immunoreactive neurons. Varicosities containing Phaseolus vulgaris-leucoagglutinin juxtapose the immunoreactive neurons. To study the possibility that the cholinergic neurons could be the synaptic targets of these incoming fibers, the subiculum, the fornix, and the midline/intralaminar thalamus were lesioned in separate animals and brain sections were immunoprocessed for choline acetyltransferase and studied with the electron microscope. In addition, dual-labelling electron microscopic immunocytochemistry was employed. In total, 164 synaptic terminals from the subiculum/hippocampus and 130 from the midline/intralaminar thalamus were examined; all formed asymmetrical synaptic specializations. No hippocampal endings were seen to contact the somata or primary dendrites of the choline acetyltransferase-immunoreactive neurons; however, three were found in synaptic contact with distal, immunolabelled dendritic shafts. Most hippocampal terminals established contacts with unlabelled spines. Fifteen percent of the thalamic endings were found to synapse on the somata and the primary and distal dendrites of the choline acetyltransferase-immunoreactive neurons. The remaining thalamic terminals established synaptic junctions with small unlabelled dendrites or spines. These findings have important implications not only for our understanding of the synaptic organization of the hippocampal and thalamic projections to the nucleus accumbens, but also for the contribution of the cholinergic neurons to the circuitry of this nucleus.

Catecholamine-containing axon terminals in the hypoglossal nucleus of the rat: an immuno-electronmicroscopic study

A correlative light and electron microscopic investigation was undertaken to determine the morphology and distribution of catecholamine (CA)-containing axon terminals in the hypoglossal nucleus (XII) of the rat. This was accomplished immunocytochemically with antibody to tyrosine hydroxylase (TH). The major findings in this study were the following: 1) Immunoreactive profiles were found throughout XII and included unmyelinated axons, varicosities, axon terminals and dendrites; 2) Nonsynaptic immunoreactive profiles (preterminal axons, varicosities) were more frequently observed (55.2%) than synaptic profiles (43.5%); 3) CA-containing axon terminals ending on dendrites were more numerous (71.8%) than those synapsing on somata (25.4%) or non-labeled axon terminals (2.7%); 4) The morphology of labeled axon terminals was variable. Axodendritic terminals typically contained numerous small, round agranular vesicles, a few large dense-core vesicles and were associated with either a symmetric or no synaptic specialization, axosomatic terminals were often associated with a presynaptic membrane thickening or a symmetric synaptic specialization and contained small, round and a few elliptical-shaped vesicles, while axoaxonic synapses formed asymmetric postsynaptic specializations; and 5) CA-positive dendritic processes were identified in XII. These findings confirm the CA innervation of XII, and suggest a complex, multifunctional role for CA in controlling oro-lingual motor behavior.

Correlation between retinal afferent distribution, neuronal size, and functional activity in the guinea pig medial terminal accessory optic nucleus

The intrinsic morpho-functional organization of the medial terminal nucleus of the accessory optic system was investigated in the guinea pig. The distribution of the retinal afferents, as assessed by the axoplasmic transport of 14C-valine, showed a remarkable asymmetry within the nucleus. Thus, while the retinal terminal field covered the entire medial terminal nucleus, by far the largest density of labeled retinofugal axon terminals was found within its dorsal division. In this same portion of the nucleus, we found the greatest density of large cells and the maximum intensity of functional activation, this latter as estimated by the increase in metabolic activity of neurons using the 14C-2-deoxyglucose autoradiographic method in the vertical and horizontal whole-field movement in the visual field.

Immunolocalization of cytosolic aspartate aminotransferase (cAAT) in axon terminals that form synapses in the rat cerebellar cortex. A study at the electron microscopic level

This study was conducted to determine the ultrastructural localization of cytosolic aspartate aminotransferase (cAAT)-like immunoreactivity in the cerebellar cortex in the rat. The isoenzyme was found both in excitatory and inhibitory axon terminals, but not in the climbing fibers of the molecular layer. These findings suggest that cAAT may have a different role in the excitatory and inhibitory synapses, and that climbing fibers of the molecular layer do not appear to use aspartate as neurotransmitter.

Anatomical variation in mast cell nerve associations in the rat small intestine, heart, lung, and skin. Similarities of distances between neural processes and mast cells, eosinophils, or plasma cells in the jejunal lamina propria

Several studies have indicated that mast cells occur in close proximity to enteric nerves in the gastrointestinal tract of rats, man, and other mammalian species, and such intimate associations have been proposed as one of the anatomical bases of communication between the immune and the nervous systems. However, the specificity of anatomical associations between enteric nerves and mast cells, as opposed to other bone marrow-derived or lymphoid cells normally present in mucosal sites, is unclear. We used transmission electron microscopy to quantify the distances between mast cells and neural processes (nerve terminals or axons) in the small intestinal mucosa, right atrium, skin, and pulmonary parenchyma of normal rats, and in the small intestinal mucosa and lung parenchyma of rats that had undergone hyperplasia of the mast cell populations in these sites as a result of infection with the nematode Nippostrongylus brasiliensis. In the jejunal mucosa of normal rats, 8.0% of mast cells occurred within 100 nm of neural processes and an additional 11.0% between 101 and 500 nm of these structures; the corresponding figures for eosinophils were 3.3% (N.S. vs. mast cell value) and 23.3% (p less than 0.05 vs. mast cell value) and for plasma cells were 8.5% and 14.6% (N.S. vs. mast cell values). In the right atrium, 1.2% of mast cells occurred within 100 nm and an additional 13.4% within 101 and 500 nm of neural processes, whereas no mast cells were observed within 500 nm of neural processes in the pulmonary parenchyma or ear skin. Infection with N. brasiliensis increased by 61% the proportion of mast cells within 500 nm of neural processes in the jejunal mucosa and resulted in the appearance of mast cells in close association with these structures in the jejunal muscularis propria, but had no effect on the proportion of mast cells within 100 or 500 nm of neural processes in the pulmonary parenchyma. Acetylcholine esterase staining demonstrated dense networks of neural processes in the three sites where some mast cells were closely associated with these structures (jejunal mucosa and muscularis, right atrium) but not in the pulmonary parenchyma or ear skin. Taken together, our findings indicate that mast cells occur in close proximity to neural processes in sites where these structures are abundant, but that anatomical associations as close as those between mast cells and neural processes can also occur between such structures and other bone marrow-derived cells (eosinophils) or lymphoid cells (plasma cells) resident in the small intestinal mucosa.(ABSTRACT TRUNCATED AT 400 WORDS)

Afferent and efferent synaptic connections of somatostatin-immunoreactive neurons in the rat fascia dentata

The aim of this study was to determine whether somatostatin (SS)-immunoreactive neurons of the rat fascia dentata are involved in specific excitatory circuitries that may result in their selective damage in models of epilepsy. Synaptic connections of SS-immunoreactive neurons were determined at the electron microscopic level by using normal and colchicine pretreated rats. Vibratome sections prepared from both fascia dentata of control animals and from rats that had received an ipsilateral lesion of the entorhinal cortex 30-36 hours before sacrifice were immunostained for SS by using a monoclonal antibody (SS8). Correlated light and electron microscopic analysis demonstrated that many SS-immunoreactive neurons in the hilus send dendritic processes into the outer molecular layer of the fascia dentata, and dendrites of the same neurons occupy broad areas in the dentate hilar area. The majority of SS-immunoreactive axon terminals form symmetric synapses with the granule cell dendrites in the outer molecular layer and also innervate deep hilar neurons. Via their dendrites in the outer molecular layer, the SS-immunoreactive neurons receive synaptic inputs from perforant pathway axons which were identified by their anterograde degeneration following entorhinal lesions. The axons from the entorhinal cortex are the first segment of the main hippocampal excitatory loop. The hilar dendrites of the same SS-immunoreactive cells establish synapses with the mossy axon collaterals which represent the second member in this excitatory neuronal chain. These observations suggest that SS-immunoreactive neurons in the dentate hilar area may be driven directly by their perforant path synapses and via the granule cells which are known to receive a dense innervation from the entorhinal cortex. These observations demonstrate that SS-immunoreactive neurons in the hilar region are integrated in the main excitatory impulse flow of the hippocampal formation.

GABA immunoreactivity of calyceal nerve endings in the vestibular system of the guinea pig

Neurotransmitters involved in the vestibular system are largely uncharacterized. On the basis of results of earlier electrophysiological and immunohistochemical experiments, glutamate and gamma-amino-butyric acid (GABA) have been proposed in both mammalian and non-mammalian species as afferent transmitters between the sensory cell and the afferent dendrite. GABA is also suspected to act as an efferent neurotransmitter in the cochlea. We describe in this study the immunocytochemical localization of GABA within the vestibular end organs in the guinea pig. GABA immunoreactivity was found in the calyceal nerve endings surrounding type I hair cells of the vestibular epithelia. The most significant labelings were obtained in the crista ampullaris. Labeling was more difficult to observe in the utricular and saccular macula. These results contribute to the recent proposal that the calyx has a secretory function, and suggest that GABA may have a modulatory influence upon the type I hair cells.

Termination patterns of calcitonin gene-related peptide-immunoreactive nerve fibers in the dorsal horn of the human spinal cord

CGRP-immunoreactive varicose nerve fibers displayed three kinds of termination patterns in the cervical, thoracic and lumbar segments of the human spinal cord. Bundles of immunoreactive fibers formed a loose network in lamina I. A homogenous band of immunoreactive fibers filled lamina II. Multiple bundles of CGRP-positive fibers coursed through the superficial laminae towards deep portions of the grey matter. In the lumbar segments, in contrast to the cervical and thoracic segments, the bundles could be followed deep into the dorsal funiculus. Bundles of varicose immunoreactive fibers were seen to twine around the dendrites of neurons located in lamina I, in the dorsal funiculus of the lumbar segments and deep in the dorsal horn (laminae III-V). The corresponding types of large and medium-sized neurons were found in silver impregnated adjacent spinal cord sections. It is suggested that neurons in the above locations preferentially receive multiple contacts from CGRP-containing nerve fibers along their extensive dendritic arborizations (CGRP-target neurons).