Direct synaptic projections to esophageal motoneurons in the nucleus ambiguus from the nucleus of the solitary tract of the rat

Neurons of the nucleus of the solitary tract (NTS) serve as interneurons in swallowing. We investigated the synaptology of the terminals of these neurons and whether they project directly to the esophageal motoneurons in the compact formation of the nucleus ambiguus (AmC). Following wheat germ agglutinin conjugated horseradish peroxidase (WGA-HRP) injection into the NTS, many anterogradely labeled axodendritic terminals were found in the neuropil of the AmC. The majority of labeled axodendritic terminals (89%) contained round vesicles and made asymmetric synaptic contacts (Gray's type I), but a few (11%) contained pleomorphic vesicles and made symmetric synaptic contacts (Gray's type II). More than half of the labeled terminals contacted intermediate dendrites (1-2 μm diameter). There were no retrogradely labeled medium-sized motoneurons, but there were many retrogradely labeled small neurons having anterogradely labeled axosomatic terminals. A combined retrograde and anterograde transport technique was developed to verify the direct projection from the NTS to the esophageal motoneurons. After the esophageal motoneurons were retrogradely labeled by cholera toxin subunit B conjugated HRP, the injection of WGA-HRP into the NTS permitted ultrastructural recognition of anterogradely labeled axosomatic terminals contacting directly labeled esophageal motoneurons. Serial sections showed that less than 20% of the axosomatic terminals were labeled in the esophageal motoneurons. They were mostly Gray's type I, but a few were Gray's type II. In the small neurons, more than 30% of axosomatic terminals were labeled, which were exclusively Gray's type I. These results indicate that NTS neurons project directly not only to the esophageal motoneurons, but also to the small neurons which have bidirectional connections with the NTS. J. Comp. Neurol. 381:18-30, 1997. © 1997 Wiley-Liss, Inc.     

Synaptology of the direct projections from the nucleus of the solitary tract to pharyngeal motoneurons in the nucleus ambiguus of the rat

During the pharyngeal phase of the swallowing reflex, the nucleus of the solitary tract (NTS) receives peripheral inputs from the pharynx by means of the glossopharyngeal ganglion and is the location of premotor neurons for the pharyngeal (PH) motoneurons. The semicompact formation of the nucleus ambiguus (AmS) is composed of small and medium-sized neurons that do not project to the pharynx, and large PH motoneurons. We investigated whether the neurons in the NTS projected directly to the PH motoneurons or to the other kinds of neurons in the AmS by using the electron microscope. When wheat germ agglutinin-conjugated horseradish peroxidase (WGA-HRP) was injected into the NTS after cholera toxin subunit B-conjugated HRP (CT-HRP) injections into the pharyngeal muscles of male Sprague-Dawley rats, many nerve terminals anterogradely labeled with WGA-HRP were found to contact PH motoneurons retrogradely labeled with CT-HRP. Most of the labeled axodendritic terminals (63%) contained pleomorphic vesicles with symmetric synaptic contacts (Gray's type II), and the remaining ones contained round vesicles with asymmetric synaptic contacts (Gray's type I). About 14% of the axosomatic terminals on PH motoneuron in a sectional plane were anterogradely labeled, and about 70% of the labeled axosomatic terminals were Gray's type II. Observations of serial ultrathin sections revealed that both the small and the medium-sized neurons received only a few labeled axosomatic terminals that were exclusively Gray's type I. These results indicate that the NTS neurons may send mainly inhibitory as well as a few excitatory inputs directly to the PH motoneurons in the AmS. J. Comp. Neurol. 393:391–401, 1998. © 1998 Wiley-Liss, Inc.     

Distribution and ultrastructure of dopaminergic neurons in the dorsal motor nucleus of the vagus projecting to the stomach of the rat

Almost all parasympathetic preganglionic motor neurons contain acetylcholine, whereas quite a few motor neurons in the dorsal motor nucleus of the vagus (DMV) contain dopamine. We determined the distribution and ultrastructure of these dopaminergic neurons with double-labeling immunohistochemistry for tyrosine hydroxylase (TH) and the retrograde tracer cholera toxin subunit b (CTb) following its injection into the stomach. A few TH-immunoreactive (TH-ir) neurons were found in the rostral half of the DMV, while a moderate number of these neurons were found in the caudal half. Most of the TH-ir neurons (78.4%) were double-labeled for CTb in the half of the DMV caudal to the area postrema, but only a few TH-ir neurons (5.5%) were double-labeled in the rostral half. About 20% of gastric motor neurons showed TH-immunoreactivity in the caudal half of the DMV, but only 0.3% were TH-ir in the rostral half. In all gastric motor neurons, 8.1% were double-labeled for TH. The ultrastructure of the TH-ir neurons in the caudal DMV was determined with immuno-gold-silver labeling. The TH-ir neurons were small (20.4 x 12.4 microm), round or oval, and contained numerous mitochondria, many free ribosomes, several Golgi apparatuses, a round nucleus and a few Nissl bodies. The average number of axosomatic terminals per section was 4.0. More than half of them contained round synaptic vesicles and made asymmetric synaptic contacts (Gray's type I). Most of the axodendritic terminals contacting TH-ir dendrites were Gray's type I (90%), but a few contained pleomorphic vesicles and made symmetric synaptic contacts (Gray's type II).     

Ultrastructure of the mammillotegmental projections to the ventral tegmental nucleus of Gudden in the rat.

This study examines the termination pattern of axons from the medial mammillary nucleus within the ventral tegmental nucleus of Gudden (TV) in rats by using anterograde transport of horseradish peroxidase conjugated with wheat germ agglutinin (WGA-HRP) and visualized with tetramethylbenzidine. The neuropil of TV contains three classes of axodendritic terminals, that is, terminals containing round, flat, and pleomorphic synaptic vesicles. These types make up 55.6%, 26.1%, and 18.3%, respectively, of all normal axodendritic terminals. Injection of WGA-HRP into the medial mammillary nucleus permits ultrastructural recognition of anterogradely labeled terminals within the TV. More than 80% of the labeled terminals contain round synaptic vesicles and form asymmetric synaptic contacts, whereas about 16% contain flat synaptic vesicles with symmetric synaptic contacts. There are a few labeled terminals with pleomorphic vesicles and only a few axosomatic terminals. Almost all labeled terminals are small, having diameters of less than 1.5 microns. Compared with the distributions of normal and labeled terminals with round vesicles, there is an increase of the percentage of labeled terminals with round vesicles on the intermediate dendrites (1-2 microns diameter) and a decrease on the distal dendrites (less than 1 micron diameter). Anterogradely labeled axon terminals often contact retrogradely labeled dendrites. These results suggest that the medial mammillary neurons send mainly excitatory as well as a few inhibitory inputs to the dendrites of TV and have direct reciprocal contacts with the TV neurons.     

The ultrastructural characteristics of the pyramidal callosal neurons of layer III in the primary auditory area (A1) of the cat cortex]

An electron microscope study of retrogradely labelled pyramidal neurons in layer III of the primary auditory cortex (AI) after HRP injections into the contralateral AI has been carried out in cats. From 4 to 10 synapses were usually revealed on somatic profiles of these callosal neurons. Synapses occupied 20.0% of the somatic surface of these neurons. All of the revealed synapses on the somata of callosal neurons had symmetric contacts and were formed by axon terminals with small elongated synaptic vesicles. Average length of these synaptic contacts was 1.6 microns. In layer III anterogradely labelled terminals of callosal fibres were also revealed. The majority of them contained large round synaptic vesicles and formed asymmetric contacts on spines. Three labelled axon terminals with small elongated vesicles were found to form symmetric axo-somatic synapses on callosal neurons of layer III.     

A light and electron microscopic study of the dorsal motor nucleus of the vagus nerve in the cat

The morphology of the dorsal motor nucleus of the vagus nerve (DMV) in the cat was studied with the aid of light and electron microscopy. In frozen sections stained by the Kluver-Barrera method or stained to show retrograde labeling in the DMV following injections of horseradish peroxidase (HRP) in the cervical vagus nerve and the stomach wall a range of sizes of DMV neurons was observed but it was not possible to distinguish separate types. In contrast, two distinct types of neurons, one medium-sized and the other small, were identified with the light microscope in Golgi-Cox and 1-μm Epon sections and with the electron microscope in ultrathin sections. The medium-sized neurons had a range of sizes but generally measured 18 × 25 μm and possessed three to four proximal dendrites which branched two or three times. Spines were observed occasionally on the soma and on dendrites. These neurons contained a well-developed cytoplasm and a noninvaginated round to oval nucleus. The small neurons generally measured 9 × 14 μm and were round or slightly elongated in shape. Their dendritic processes were fewer and thinner than those of the medium-sized neurons and extended for shorter lengths. Their soma contained scanty cytoplasm and an invaginated nucleus. The medium-sized neurons outnumbered the small neurons by more than three to one but both neuronal types were distributed evenly throughout the nucleus. The medium-sized neurons seemed to correspond in size to the parasympathetic efferent neurons of the viscera as indicated by the HRP studies. Axosomatic synapses on both types of neurons and axodendritic synapses were observed in the DMV. Terminals containing mainly small clear round vesicles and making asymmetrical contact with the postsynaptic membrane were involved in the majority of synapses on both the soma and dendrites. Terminals containing predominantly pleomorphic vesicles and making symmetrical contact with the postsynaptic membrane were also common, comprising up to one-third of all synapses observed. Serial sections revealed that most synaptic terminals contained varying numbers of large (75–110 nm) dense-cored vesicles. Smaller dense-cored vesicles (45–55 nm) were sometimes observed, often close to the area of synaptic contact. Terminals 1–2 μm in diameter which contacted dendrites 1–3 μm in diameter formed the most common synaptic combination throughout the rostral to caudal extent of the DMV. No distinct regional differences were observed with respect to distribution of synaptic types.     

Ultrastructure and synaptic organization of axon terminals from brainstem structures to the mediodorsal thalamic nucleus of the rat.

The ultrastructural characteristics and synaptic organization of afferent terminals from the brainstem to the mediodorsal thalamic nucleus (MD) of the rat have been studied with the electron microscope, by means of anterograde transport of wheat germ agglutinin-horseradish peroxidase (WGA-HRP). Labeled fibers were seen predominantly in the lateral portion of MD after the injections of WGA-HRP into the substantia nigra pars reticulata (SNr), the superior colliculus (SC), and the dorsal tegmental region (DT). The boutons arising from the SC were relatively small (less than 1.5 microns in diameter), formed asymmetric synaptic contacts with small dendrites and dendritic spines, and contained round synaptic vesicles. The axon terminals from the DT were mostly large boutons (2-4.5 microns) with asymmetric synaptic specializations and round vesicles. These boutons and their postsynaptic targets formed synaptic glomeruli that were entirely or partially ensheathed by glial lamellae. The ultrastructural features are almost identical to those of boutons in the medial and central segments of MD that were previously shown to originate from the basal amygdaloid nucleus and the piriform cortex. The boutons from the SNr had a wide range in size, but the majority were medium-sized to large (1.5-4 microns). The nigral boutons established symmetric synaptic contacts with dendritic shafts and occasionally with somata, and contained pleomorphic vesicles. However, like the DT terminals, they participated in glomerular formations. The nigral terminals closely resemble previously described terminals in the medial part of MD from the ventral pallidum, except that the nigral terminals formed en passant and axosomatic synapses as well as axodendritic synapses. A combined immunohistochemistry and WGA-HRP tracing study revealed that the nigral inputs were immunoreactive for glutamic acid decarboxylase and the axon terminals from the DT were immunoreactive for choline acetyltransferase. In a separate study, the colliculothalamic fibers have been shown to take up and transport the transmitter specific tracer [3H]-D-aspartate, and are therefore putatively glutamatergic and/or aspartatergic. Taken together with this, the present results suggest that the collicular afferents are excitatory and glutamatergic and/or aspartatergic, that the inputs from the DT are also excitatory and cholinergic, while the nigral inputs are inhibitory and GABAergic.     

The ultrastructure of the subnucleus gelatinosus of the nucleus of the tractus solitarius in the cat

The dorsomedial region of the nucleus of the tractus solitarius termed the subnucleus gelatinosus (SNG) was studied at the light and electron microscopic level in the cat. In cresyl violet and luxol fast blue stained sections the SNG contained small neuronal somata that were scattered throughout a pale-staining neuropil containing few myelinated fibers. These neurons were difficult to impregnate with Golgi staining techniques, but in successful impregnations the somata were observed to be 10--19 micrometers in diameter and bore few sparsely branching primary dendrites. Spines were present on the dendrites of some neurons and were more numerous on distal portions of the dendritic tree. Ultrastructural examination of the SNG revealed that the neuronal complement consisted of round, oval, or spindle shaped neurons with little or no organized Nissl substance. Rare myelin-like ensheathments of neuronal perikarya were also observed. Bundles of fine unmyelinated axons that coursed mainly longitudinally were a prominent feature of the area. The most common type of axon terminal observed contained mainly round clear vesicles, approximately 31 nm in diameter, and made asymmetrical synaptic contact with a dendritic profile. Pleomorphic vesicle-containing terminals involved in symmetrical synaptic contact were also commonly seen. Axodendritic and axosomatic synapses were associated with terminals containing either round clear vesicles or pleomorphic vesicles. Less commonly, dendrodendritic and dendrosomatic synapses were seen, the presynaptic elements of which contained pleomorphic vesicles. Following removal of a nodose ganglion, degenerating terminals of vagal afferent fibers were observed throughout the neuropil. Such terminals contained round, clear vesicles with an occasional large, dense-cored vesicle, and made axodendritic and axosomatic synaptic contacts.     

Characteristics of the synaptic apparatus of the parietal associative cortex (area 5b) in the cat brain

An electron microscopic examination of the associative cerebral cortex (area 5b) in cat was performed. The average density of axonal terminal profiles in this area was 263 +/- 16 terminals per 1000 micron2 of the slice area. 75.5% of axonal terminals contained synaptic vesicles and had asymmetric or symmetric contacts with visible active zones. 8.4% of axonal terminals had contacts without visible active zones. 24.5% of axonal terminals contained synaptic vesicles, but had no visible contacts with neighbouring structures. 84.9% of axonal terminals contained round or slightly elongated vesicles, 7.8% --a mixture of round and elongated vesicles and 7.3%--thin elongated vesicles. Among the axonal terminals with visible synaptic contacts 46.6% were of the axo-spine type, 50%--of axo-dendritic type and 3.4%--of axo-somatic type. 77% of axo-somatic terminals contained elongated vesicles and had symmetric contacts and 23% contained round vesicles and had asymmetric contacts.     

Ultrastructure and synaptology of the paratrigeminal nucleus in the rat: primary pharyngeal and laryngeal afferent projections

The paratrigeminal nucleus (PTN) receives primary afferent projections from the aerodigestive tract and orofacial regions and plays a role in the integration of visceral and somatic information. This study describes the fine structure of the rat PTN and the synaptology of primary afferent projections from the pharynx and larynx. Injections of wheat germ agglutinin-horseradish peroxidase (WGA-HRP) or cholera toxin-HRP (CT-HRP) were made into the wall of the pharynx or larynx to label primary afferent axon terminals. Light microscopic observations demonstrated that afferent axons terminated bilaterally in overlapping fields in the PTN. Electron microscopic observations of the PTN revealed that there were three distinct classes of neurons, based on morphology and axosomatic contacts. The most abundant neurons, Type 1, were fusiform in shape and received very few or no axosomatic contacts. Type 2 neurons contained prominent Nissl substance (rough endoplasmic reticulum) and few axosomatic contacts, while Type 3 neurons had many axosomatic synapses. Terminals containing round, clear vesicles and forming asymmetric contacts (round asymmetric, RA) with dendrites were the predominant synaptic type in the PTN. Primary afferent terminals from the pharynx and larynx were of the RA type and formed synaptic contacts with small-diameter (<1 microm) dendrites. Visceral primary afferent inputs from the pharynx and larynx overlap with trigeminal somatic afferents in the PTN and have similar synaptic morphology. The results support the concept that the PTN provides an anatomical substrate for mediating viscerovisceral and somatovisceral reflexes via efferent connections with autonomic centers in the brainstem.     

Fine structure of the supramammillary nucleus of the rat: Analysis of the ultrastructural character of dopaminergic neurons

The supramammillary nucleus projecting to widespread regions contains dopaminergic and non-dopaminergic neurons. The present study provided a comprehensive electron microscopic analysis of these dopaminergic and non-dopaminergic neurons in the supramammillary nucleus of the rat. The normal supramammillary nucleus was composed of round spindle-shaped, small and medium-sized neurons (12.7 × 8.0 μm, 78.0 μm²) containing a light oval nucleus with invaginated envelop, mitochondria, Golgi apparatus, lysosomes, less-developed rough endoplasmic reticulum, and no Nissl bodies, The majority of terminals (more than 70%) in the normal neuropil were small (diameter less than 1.0 μm) and contained round vesicles forming asymmetric synaptic contacts. The terminals often contained dense-cored vesicles. To determine the morphological features of dopaminergic neurons, we examined the ultrastructural localization of tyrosine hydroxylase (TH) immunoreactivity, which is the synthetic enzyme of dopamine, and compared TH-immunoreactive neurons to non-TH-immunoreactive neurons. Their shape and size were similar. The average number of axosomatic terminals in a sectional plane was 5.0 in TH-neurons and 2.4 in non-TH-neurons; the bouton covering ratio was 16.5% in the former and 8.6% in the latter. Both numbers were significantly larger in TH-neurons than in non-TH-neurons. Serial ultrathin sections of these neurons revealed that the average total number of axosomatic terminals was 55.7 in the TH-neuron and 28.4 in the non-TH-neuron. Characteristic lamellar bodies and subsurface cisternae were often present in TH neurons. There were no TH-labeled terminals. These results indicate that dopaminergic neurons receive more inputs than neurons containing other neurotransmitters. © 1994 Wiley-Liss, Inc.     

Ultrastructure of the rostral ventral respiratory group neurons in the ventrolateral medulla of the rat

The neurons in the ventrolateral medulla that project to the spinal cord are called the rostral ventral respiratory group (rVRG) because they activate spinal respiratory motor neurons. We retrogradely labeled rVRG neurons with Fluoro-Gold (FG) injections into the fourth cervical spinal cord segment to determine their distribution. The rostral half of the rVRG was located in the area ventral to the semicompact formation of the nucleus ambiguus (AmS). A cluster of the neurons moved dorsally and intermingled with the palatopharyngeal motor neurons at the caudal end of the AmS. The caudal half of the rVRG was located in the area including the loose formation of the nucleus ambiguus caudal to the AmS. We also labeled the rVRG neurons retrogradely with wheat germ agglutinin-horseradish peroxidase (WGA-HRP) to determine their ultrastructural characteristics. The neurons of the rVRG were medium to large (38.1 x 22.1 microm), oval or ellipsoid in shape, and had a dark cytoplasm containing numerous free ribosomes, rough endoplasmic reticulum (rER), mitochondria, Golgi apparatuses, lipofuscin granules and a round nucleus with an invaginated nuclear membrane. The average number of axosomatic terminals in a profile was 33.2. The number of axosomatic terminals containing round vesicles and making asymmetric synaptic contacts (Gray's type I) was almost equal to those containing pleomorphic vesicles and making symmetric synaptic contacts (Gray's type II). The axodendritic terminals were large (1.55 microm), and about 60% of them were Gray's type I. The rVRG neurons have ultrastructural characteristics, which are different from the palatopharyngeal motor neurons or the prorpiobulbar neurons.     

Fine structure of the lateral mammillary projection to the dorsal tegmental nucleus of Gudden in the rat.

The synaptic organization of projections from the lateral mammillary neurons within the dorsal tegmental nucleus of Gudden is studied in the rat with the aid of anterograde transport of horseradish peroxidase conjugated with wheat germ agglutinin (WGA-HRP) and visualized with tetramethylbenzidine. The dorsal tegmental nucleus consists of the pars ventralis (TDV) and the pars dorsalis (TDD). The normal neuropil of the dorsal tegmental nucleus contains three classes of axodendritic terminals, that is, terminals containing round, flat, and pleomorphic vesicles. They make up 44%, 5%, and 51%, respectively, of all axodendritic terminals in the TDV, and 62%, 1%, and 37% in the TDD. Injection of WGA-HRP into the lateral mammillary nucleus permits ultrastructural recognition of many anterograde labeled terminals within both the TDV and TDD. In the TDV, 81% of the labeled terminals contain round synaptic vesicles and make asymmetric synaptic contacts. A few of the labeled terminals contain pleomorphic vesicles and make symmetric synaptic contacts. More than 50% of the labeled terminals contact intermediate dendrites (1-2 microns diameter). In the TDD, almost all labeled terminals are small, contain round vesicles, and make asymmetric synaptic contacts. These terminals mainly contact intermediate as well as distal (less than 1 micron diameter) dendrites. There are only a few labeled terminals with pleomorphic vesicles and no terminals with flat vesicles. The termination pattern of the lateral mammillary neurons in the TDV is similar to that in the TDD. Anterograde labeled axon terminals often contact retrograde labeled dendrites in the TDV. No reciprocal connections are present in the TDD. These results suggest that the TDV and the TDD receive mainly excitatory and a few inhibitory inputs from the lateral mammillary nucleus. The TDV neurons also have direct reciprocal connections with the lateral mammillary neurons.     

Ultrastructure of chemically defined neuron systems in the dorsal horn of the monkey. I. Substance P immunoreactivity

The ultrastructural localization of substance P-like immunoreactivity (SPLI) in lamina I (marginal zone) and lamina II₀ (outer substantia gelatinosa) of the dorsal horn of the macaque monkey was examined by the indirect antibody peroxidase-antiperoxidase method. SPLI was found in small unmyelinated and finely myelinated axons and a variety of terminal types. The majority of SPLI terminals contained a few to many large granular vesicles (mean diameter 90 nm) in addition to a population of small clear round vesicles. A very few terminals contained mainly small round vesicles. SPLI terminals were presynaptic in axosomatic, axodendritic and axospinous contacts forming, in all but the axosomatic junctions, asymmetrical synapses. Some axosomatic junctions were symmetrical. SPLI terminals also formed the center of glomeruli with unlabeled dendrites and dendritic spines; some of the unlabeled dendrites contained a few small scattered vesicles and large dense-core vesicles. In more complex formations 2 to 4 SPLI terminals were associated with one another and linked by desmosomal contacts. The individual terminals in the complexes or ‘congregate terminals’ were simple large granular vesicle containing terminals (LGV), LGV-central terminals of associated glomeruli, or terminals containing mainly small round vesicles. In the apical region of lamina I an unlabeled terminal was found occasionally in contact with an SPLI terminal, which in turn synapsed onto a dendrite. These contacts have some synaptic characteristics and the SPLI terminal was possibly postsynaptic. Most of the types of SPLI terminals resemble closely terminal types shown to be of primary afferent origin. These terminals which make direct contact with dorsal horn dendrites may be the morphological substrate for postsynaptic excitation of dorsal horn neurons by substance P. The contacts of unlabeled terminals with SPLI terminals may represent a morphological substrate by which other neurochemical substances such as enkephalin or serotonin may modulate the substance P effects on dorsal horn neuronal activity.     

The fine structure of the lateral superior olivary nucleus of the cat

The synaptic organization of the lateral superior olivary nucleus of the cat was analyzed under the electron microscope. The predominant cell type, the fusiform cell, has dendrites that extend from opposite poles of the cell body toward the margins of the nucleus, where they terminate in spinous branches. The fusiform cells are contacted by three types of synaptic terminals that can be distinguished by the size and shape of their synaptic vesicles. The somatic and proximal dendritic surfaces are apposed by synaptic terminals containing small, flat synaptic vesicles. Further from the cell body, the dendrites form numerous synaptic contacts with terminals containing large round vesicles as well as with the terminals containing small, flat vesicles. The most distal dendritic branches and their spiny appendages appear to form synapses almost exclusively with the terminals with large, round vesicles. A relatively rare type of terminal that contains small, round vesicles may form synapses with either the somatic or dendritic surfaces. A few small cells are interspersed among the fusiform cells, but they are more commonly located around the margins of the nucleus. The small cells form few axosomatic contacts. The simplest interpretation of the findings is that the terminals with small, flat vesicles arise in the medial nucleus of the trapezoid body and are inhibitory in function, whereas the terminals with large, round vesicles arise in the anteroventral cochlear nucleus and are excitatory; however, this remains to be demonstrated experimentally. In any case, the differential distribution of these two types of inputs on the somatic and dendritic surfaces must be an important determinant of the physiological response properties of the fusiform cells to binaural acoustic stimuli.     

Ultrastructural single- and double-label immunohistochemical studies of substance P-containing terminals and dopaminergic neurons in the substantia nigra in pigeons.

The vast majority of striatonigral projection neurons in pigeons contain substance P (SP), and the vast majority of SP-containing fibers terminating in the substantia nigra arise from neurons in the striatum. To help clarify the role of striatonigral projection neurons, we conducted electron microscopic single- and double-label immunohistochemical studies of SP+ terminals and/or dopaminergic neurons (labeled with either anti-dopamine, DA, or anti-tyrosine hydroxylase, TH) in pigeons to determine: (1) the synaptic organization of SP+ terminals, (2) the synaptic organization of TH+ perikarya and/or dendrites, and (3) the synaptic relationship between SP+ terminals and TH+ neurons in the substantia nigra. Tissue single-labeled for SP revealed numerous SP+ terminals contacting thin unlabeled dendrites in the substantia nigra, but few SP+ terminals were observed contacting perikarya or large-diameter dendrites. SP+ terminals contained round, densely packed, clear vesicles, and often contained one or more dense-core vesicles. Synaptic junctions between SP+ terminals and their targets were more often symmetric (86%) than asymmetric. In tissue single-labeled for DA, we observed few terminals contacting DA+ perikarya, whereas terminals contacting DA+ dendrites were more abundant. Terminals contacting DA+ structures comprised at least four different morphologically distinct types based on the morphology of the clear synaptic vesicles and the type of synaptic junction. One type of terminal contained round clear vesicles and made symmetric synapses, and thus resembled the predominant type of SP+ terminal. The second type contained round clear vesicles and made asymmetric synapses, the third type contained medium-size pleomorphic clear vesicles and made symmetric synapses, and the fourth type contained small pleomorphic clear vesicles and made symmetric synapses. The presence of contacts between SP+ terminals and dopaminergic dendrites in the substantia nigra was directly demonstrated in tissue double-labeled for SP (by the peroxidase-antiperoxidase procedure, or PAP, with diaminobenzidine) and TH (by either the silver-intensified immunogold procedure or the PAP procedure with benzidine dihydrochloride). SP+ terminals commonly contacted thin TH+ dendrites in the substantia nigra, but few SP+ terminals contacted large-diameter TH+ dendrites or perikarya. Synapses between SP+ terminals and TH+ neurons were always symmetric. TH+ dendrites also were contacted by terminals not labeled for SP, which were more abundant than were SP+ terminals. Non-TH+ neurons were also contacted by both SP+ terminals and non-SP+ terminals.(ABSTRACT TRUNCATED AT 400 WORDS)     

An electron microscopic study of the origins of the projections of the first auditory area of the cortex (AI) to the medial geniculate body in cats

An electron microscope study of retrogradely labelled neurons in layer VI of the primary auditory cortex (AI) after injection of the horseradish peroxidase to the medial geniculate body was carried out in cats. Three-eight synapses (4.6 +/- 0.6 at an average) were revealed on the somata profiles of these retrogradely labelled cortico-geniculate neurons. Synapses occupied 10.8 +/- 1.0% of the somatic profile of cortico-geniculate neurons. Almost all (98.7%) of these axosomatic synapses had symmetrical contacts and were formed by axonal terminals with small elongated synaptic vesicles. HRP retrogradely labelled axonal terminals of geniculo-cortical fibres were revealed in neuropil of layer VI. They contained large round synaptic vesicles and formed asymmetrical synapses, mainly on spines. The role of axo-somatic synapses in regulation of the activity of cortico-geniculate neurons was discussed.     

Ultrastructural characteristics of callosal neurons in deep layers of the primary auditory cortex (AI) in cat]

An electron microscope study of retrogradely labelled nonpyramidal neurons has been carried out in layers V-VI of the primary auditory cortex (AI) after HRP injections into the contralateral AI of cats. From 2 to 9 synapses were usually revealed on somatic profiles of these callosal neurons. Synapses occupied 15.8 +/- 1.7% (on the average) of the somatic surface of these neurons. All of the revealed synapses on the somata of these callosal neurons had symmetric contacts and were formed by axon terminals with small elongated synaptic vesicles. An average length of these synaptic contacts in sections was 1.6 +/- 0.1 mm. HRP-labelled axon terminals of callosal fibres in layers V-VI contained round synaptic vesicles and formed asymmetric synapses on spines and dendrites. Possible functional significance of axo-somatic synapses in formation of impulsation patterns of the callosal neurons is discussed.     

Synaptic organization of projections from basal forebrain structures to the mediodorsal thalamic nucleus of the rat.

The synaptic organization of the mediodorsal thalamic nucleus (MD) in the rat was studied with the electron microscope, and correlated with the termination of afferent fibers labeled with wheat germ agglutinin conjugated to horseradish peroxidase (WGA-HRP). Presynaptic axon terminals were classified into four categories in MD on the basis of the size, synaptic vesicle morphology, and synaptic membrane specializations: 1) small axon terminals with round synaptic vesicles (SR), which made asymmetrical synaptic contacts predominantly with small dendritic shafts; 2) large axon terminals with round vesicles (LR), which established asymmetrical synaptic junctions mainly with large dendritic shafts; 3) small to medium axon terminals with pleomorphic vesicles (SMP), which formed symmetrical synaptic contacts with somata and small-diameter dendrites; 4) large axon terminals with pleomorphic vesicles (LP), which made symmetrical synaptic contacts with large dendritic shafts. Synaptic glomeruli were also identified in MD that contained either LR or LP terminals as the central presynaptic components. No presynaptic dendrites were identified. In order to identify terminals arising from different sources, injections of WGA-HRP were made into cortical and subcortical structures known to project to MD, including the prefrontal cortex, piriform cortex, amygdala, ventral pallidum and thalamic reticular nucleus. Axons from the amygdala formed LR terminals, while those from the prefrontal and insular cortex ended exclusively in SR terminals. Fibers labeled from the piriform cortex formed both LR and SR endings. Based on their morphology, all of these are presumed to be excitatory. In contrast, the axons from the ventral pallidum ended as LP terminals, and those from the thalamic reticular nucleus formed SMP terminals. Both are presumed to be inhibitory. At least some terminals from these sources have also been identified as GABAergic, based on double labeling with anterogradely transported WGA-HRP and glutamic acid decarboxylase (GAD) immunocytochemistry.     

Fine structure of calcitonin gene-related peptide immunoreactive synaptic contacts in the thalamus of the rat

Recent studies have shown a prominent calcitonin gene-related peptide immunoreactive (CGRP-ir) pathway extending from the external medial and external lateral para-brachial nuclei to the area surrounding and including the gustatory nuclei in the thalamus, and the cortex and amygdala. The function of the CGRP-ir pathway is not completely understood, but may be involved with the processing of both nociceptive and gustatory information in the thalamus. The purpose of this study was to characterize the nature of the CGRP-ir synaptic contacts in the gustatory nucleus. Electron microscopic examination of CGRP-ir synaptic contacts revealed two classes of CGRP-ir terminals. One class, which was large, formed asymmetric synaptic contacts on dendritic appendages, had many small, round synaptic vesicles, and heavy patches of reaction product which obscured any underlying organelles. Since similar terminals in unstained tissue contained large numbers of dense-cored vesicles, it was concluded that CGRP-ir was contained predominantly in dense-cored vesicles. A second class of CGRP-ir terminals was smaller and made either asymmetric or symmetric synaptic contacts. Both symmetric and asymmetric small terminals contained small, round synaptic vesicles and fewer patches of dense reaction product. Several of the CGRP-ir terminals making symmetric contacts also contained pleomorphic vesicles. There were very few contacts on cell bodies. There were no contacts on other CGRP-ir elements, somal or dendritic, or on axon terminals. None of the CGRP-ir terminal elements were postsynaptic to unlabeled terminals. Axons containing CGRP-ir were primarily unmyelinated, but a few myelinated axons were also seen. © 1993 Wiley-Liss, Inc.