Serotonin-immunoreactive varicosities in the cell body region and neural sheath of the snail, Helix pomatia, ganglia: an electron microscopic immunocytochemical study

The distribution and connections of serotonin-immunoreactive fibers in the cell body region and neural sheath of the central ganglia of the snail, Helix pomatia, have been examined. The cell body region of the ganglia is supplied by an extremely dense network of varicose serotonin-immunoreactive fibers which surround neuronal perikarya in the ganglia. Immunoreactive processes also run to the neural sheath of both the ganglia and the peripheral nerve roots, forming a dense network. Electron microscopy revealed five different connections of serotonin-immunoreactive varicosities, according to their target: (i) non-specialized contacts with neuronal perikarya; (ii) non-specialized contacts with axon processes on the surface of the peripheral nerve roots; (iii) non-specialized neuromuscular connections with smooth muscle fibers in the neural sheath; (iv) varicosities engulfed by glial processes in both the cell body region and neural sheath; (v) varicosities embedded in the connective tissue elements of the sheath either partly or completely free of glial processes. In all cases of appositions no membrane specializations could be observed on either site of the contacts. These observations provide morphological evidence for non-synaptic regulatory actions of serotonin-containing neurons in Helix central nervous system: (i) modulation of the activity of neuronal perikarya; (ii) involvement in neuromuscular regulation; (iii) neurohormonal modulation of peripheral processes by release through the neural sheath.     

Light and electron microscopic immunocytochemical localization of vasoactive intestinal polypeptide VIP-like activity in the rat small intestine

Vasoactive intestinal polypeptide nerve processes and cell bodies were identified by electron microscopic immunocytochemistry in the rat small intestine. Labeled nerve processes were numerous in the inner circular smooth muscle coat and mainly in the mucosa, but were absent in the longitudinal muscle layer. Submucosal blood vessels were often surrounded by immunoreactive vasoactive intestinal polypeptide positive nerves, in close associations (distance less than 40 mn) to blood vessel basement membranes and to smooth muscle cells. In the ganglia of the myenteric and submucous plexuses, labeled fibers surrounded unstained neural cell bodies. The synaptic vesicles of vasoactive intestinal polypeptide positive terminals were 35-40 nm in diameter and some dense core vesicles (80-120 nm in diameter) were also observed in the same profiles. These observations suggest that vasoactive intestinal polypeptide nerves may participate in regulating smooth muscle activity and local blood flow in the small intestine.     

A histochemical and ultrastructural study of serotonin-containing nerves in cerebral blood vessels of the lamprey

Lamprey, Entosphenus japonicus, cerebral blood vessel autonomic nerve supply was studied with fluorescence and cholinesterase histochemistry and electron microscopy. Nerve fibers emitting a yellow fluorescence characteristic of serotonin (Exc./Em. max.; 380/530 nm) were found on the major cerebral and pial arteries, but not acetylcholinesterase (AChE)-positive ones. Single ganglion cells also emitting a strong yellow fluorescence were seen in the artery adventitia. On rare occasions these cells were observed in pairs. Terminal varicosities of central catecholamine-containing nerves (Exc./Em. max.; 410/475 nm) were observed on parenchymal capillaries, but not central AChE-positive nerve terminals. In ganglion cells, dense cored vesicles (ca. 130 nm in average diameter; DCV) were abundant in the Golgi area, suggesting their formation at this site. Two types of DCV were observed; one with a homogeneous dense core and the other with a granular core. DCV were numerous in axons as well, axons in which many small clear vesicles (40–60 nm in diameter) as well as DCV were occasionally observed. The question of whether the small clear vesicles or the DCV contained serotonin could not be resolved.     

Serotonergic innervation of the foot of the pond snail Lymnaea stagnalis (L.)

The aminergic innervation of the foot of Lymnaea stagnalis was investigated using electron microscopy, immunocytochemistry, and HPLC. The foot was found to contain large amounts of serotonin and dopamine, though at lower concentrations than are found in nervous tissue. Serotonin containing tissue was concentrated in the ventral surface of the foot, under ciliated areas of the epidermis where it occurred in varicosities, with fine tracts joining these varicosities. Varicosities also occurred in deeper tissues, probably adjacent to mucus cells. Positive fluorescence for serotonin in axons was found in nerves innervating the foot, but few neuronal cell bodies containing serotonin were detected, indicating that most of the innervation was coming from the central ganglia. Axon varicosities were found using TEM on ciliated cells, mucus cells, and muscle cells as well as interaxonal junctions (possibly non-synaptic) within nerves. The neuronal varicosities contacting the ciliated cells and mucus cells contained mostly dense-cored vesicles of between 60 and 100 nm in diameter. Smaller, lucent vesicles also occurred in these terminals. The origin and significance of this innervation is discussed. It is suggested that both serotonin and dopamine may play a large role in controlling ciliary gliding by the foot.     

Ultrastructural demonstration of serotonin-immunoreactivity in the nervous system of an insect (Calliphora erythrocephala)

Serotonin (5-HT) immunocytochemistry, was performed on the whole dissected nervous system of the blowfly. Employing the peroxidase-antiperoxidase technique and osmium postfixation, it was possible to examine 5-HT-immunoreactive neuronal elements first light microscopically in 25 microns sections, and, after re-embedding, to analyze the same sections electron microscopically in ultrathin sections. We describe the ultrastructure of 5-HT-positive terminals in the neural sheath of peripheral nerves and in the optic lobes. The immunoreactivity was observed in large (100 nm) granular vesicles, on membranes of clear vesicles, along neurotubules, and along the internal surface of the plasmalemma.     

Ultrastructural localization of neurotensin immunoreactivity in the stellate ganglion of the cat

Summary The morphological features and cellular relationships of neurotensin-containing axon terminals were studied at light and electron microscopic levels in the cat stellate ganglion using peroxidase and immunogold immunocytochemistry. By light microscopy, neurotensin immunoreactivity was detected within thin varicose fibres distributed throughout the ganglion. Immunoreactive fibres were no longer apparent following chronic deafferentation of the ganglion indicating that they were of extrinsic origin. Ultrastructural analysis of peroxidase immunostained material confirmed the presence of neurotensin immunoreactivity within a subpopulation of axonal varicosities which made synaptic contacts with the dendrites of ganglion cells. Within labelled varicosities neurotensin immunoreactivity was found by both immunoperoxidase and immunogold methods to be concentrated within large dense core vesicles 80–120 nm in diameter. These large dense core vesicles were characteristically distant from the active zone, in keeping with a possible extrasynaptic release of the peptide.     

On certain fluorescent axon terminals containing granular synaptic vesicles in the cerebellar nucleus lateralis

Summary Thin, unmyelinated, green fluorescent fibers bearing fine beads or varicosities have been found in the neuropil and near blood vessels of the nucleus lateralis. In electron micrographs these fibers are identifiable as a class of axons, the CAT fibers, which contain large and small granular synaptic vesicles and agranular vesicles in their varicosities. There are two types of CAT fiber. 1) The CAT₁ terminals contain many large and elongated vesicles, 700–1700 Å in size, with dark, homogeneously dense centers; a few small granular vesicles each with an intensely osmiophilic particle, and small agranular vesicles. These terminals have not been seen in synaptic contact with other elements of the neuropil. 2) The CAT₂ terminals have a very thin unmyelinated connecting thread between small varicosities. The varicosities contain small agranular synaptic vesicles and small granular ones containing either a single dense particle, or an elliptical, intensely osmiophilic droplet flanked by lighter semicircular particles. Large granular vesicles, 750–950 Å each, with a variably dense center, are also found. These terminals form conventional axodendritic synapses with Gray's type 1 synaptic junctions and the subsynaptic specialization of Taxi, as well as synapses on thorns of spiny neurons.It is suggested that the CAT₁ and CAT₂ fibers may be the electron microscope equivalents of norepinephrine- and 5-hydroxytryptamine-containing fluorescent axons. These fibers probably have extrinsic origins since no fluorescent cells or perikarya with small or large granular vesicles have been found in the lateral nucleus. Their origins, however, are unknown. The proximity of these fluorescent fibers to blood vessels is discussed, and their functions are the subject of some speculation.Supported in part by U.S. Public Health Service grants NS10536, NS03659, Training grant NS05591 from the National Institute of Neurological Diseases and Stroke and a William F, Milton Fund Award from Harvard University.     

The ontogeny and fine structure of calcitonin gene-related peptide (CGRP)-immunoreactive nerve fibers in the celiac ganglion of rats

Calcitonin gene-related peptide (CGRP)-like immunoreactivity has been reported to occur in both varicose and non-varicose nerve fibers among and around principal ganglion cells in the celiac ganglion of rats. The present electron microscopic immunohistochemistry revealed a small number of myelinated nerve fibers immunoreactive for CGRP, although most of the immunoreactive fibers were unmyelinated. In the fetal development, CGRP-immunoreactive nerve fibers were first revealed at day 18 of gestation and thereafter gradually increased in number. Both the density and distribution of the immunoreactive fibers within the ganglion similar to those in adult rats were attained by postnatal day 14. This developmental change was compared with that of enkephalin-immunoreactive nerve fibers in the same ganglion. In electron microscopy, the immunoreactive fibers with varicosities were characterized by abundant small clear vesicles mixed with large granular vesicles. These formed numerous axo-dendritic and several axo-somatic synapses with the principal ganglion cells, whereby the immunoreactive fibers were presynaptic. The immunoreactive material was localized in the core of the large granular vesicles as well as in the axoplasm. On the other hand, the immunoreactive fibers without varicosities were characterized by neurofilaments, neurotubules and small mitochondria; small clear or large granular vesicles were rarely found in the immunoreactive fibers. They lay in no direct apposition to adjacent neuronal elements and were, therefore, regarded either as fibers passing through the ganglion or as preterminal portions of the CGRP-immunoreactive varicose terminals.     

Neurons with 5-hydroxytryptamine-like immunoreactivity in the enteric nervous system: Their projections in the guinea-pig small intestine

Changes in the distribution of 5-hydroxytryptamine-like immunoreactivity have been examined in enteric neurons at various times after microsurgical lesions of the enteric plexuses. In the myenteric plexus, varicose immunoreactive nerve fibres disappeared or were reduced in number in ganglia anal to an interruption of the myenteric plexus. Up to about 2 mm on the anal side, all varicose immunoreactive fibres disappeared from the ganglia. At about 14–16 mm below an interruption, there were about 50% of the normal number of fibres in the myenteric ganglia and at about 24 mm the innervation was normal. In the submucosa, fibres immunoreactive for 5-hydroxytryptamine were absent from an area on the anal side following interruption of the myenteric plexus. From consideration of the pattern of disappearance, it is deduced that some myenteric nerve cell bodies send immunoreactive axons in an anal direction to supply submucous ganglia. The axons run for about 8 mm in the myenteric plexus, enter the submucosa and then run for a further 4 mm approximately.Thus, varicose fibres immunoreactive for 5-hydroxytryptamine, which occur around the enteric ganglion cells of both plexuses arise from nerve cell bodies in the myenteric ganglia that send their axons in an anal direction.     

Properties of nerve endings with small granular vesicles in the distal colon and rectum of the guinea-pig

The appearance and distribution of nerve endings (varicosities) containing small granular vesicles have been studied in the distal colon and rectum of the guinea-pig with the electron microscope. Two types of varicosity were recognised. They were distinguished by differences in their synaptic vesicles and in their distribution in the layers of the gut wall. The first type resembled noradrenergic nerves in having predominantly (92%) small vesicles and few (8%) large granular vesicles (90 nm diameter). This type was common in the plexuses and at the medial-adventitial border of arteries and arterioles but was sparsely distributed in the muscle coat. The second type had a lower proportion of small vesicles (69%) and a higher proportion (31%) of large granular vesicles (132 nm diameter). This type was absent in Auerbach's plexus, well represented in the muscle coat and Meissner's plexus and not associated with blood vessels. The first type was labelled with 5-hydroxydopamine, a specific marker for noradrenergic nerves, and disappeared after extrinsic denervation. a procedure which causes degeneration of noradrenergic nerves in the gut. The second type was unaffected by 5-hydroxydopamine and extrinsic denervation.It is concluded that the two types of small granular vesicle-containing varicosities belong to different axons and that the first type is noradrenergic. The second type of nerve axon has not been described in the gut before and is intrinsic to it. From the distribution and numbers of these axons in the circular muscle it would seem that they play an important role in gut motility.     

Noradrenergic and non-noradrenergic nerves containing small granular vesicles in Auerbach's plexus of the guinea-pig: evidence against the presence of noradrenergic synapses

The appearance and distribution of varicosities containing small granular vesicles in Auerbach's plexus of the guinea-pig ileum, distal colon and rectum has been studied with the electron-microscope. Two types of varicosity were recognised. The first type was located predominantly at the surface of the plexus and did not form synapses on intrinsic neurons. This type became labelled with 5-hydroxydopamine, a specific marker for noradrenergic axons, and was destroyed by 6-hydroxydopamine and extrinsic denervation, procedures which lead to degeneration of noradrenergic nerves in the gut. The second type formed axodendritic and axosomatic synapses on intrinsic neurons and the morphology of its synaptic vesicles differed subtly from that of the first type. The second type was unaffected by 5-hydroxydopamine, 6-hydroxydopamine, or extrinsic denervation. It is concluded that the two types of small granular vesicle-containing varicosities belong to different neurons and that the first type is noradrenergic. Noradrenergic varicosities do not, therefore, form synapses in Auerbach's plexus. This conclusion is in accord with the electrophysiological findings. The second type of small granular vesicle-containing varicosity is not noradrenergic although it was formerly thought to be so. It is intrinsic to the gut and is resistant to the serotoninergic neurotoxin, 5,6-dihydroxytryptamine.     

Cholecystokinin-like immunoreactivity in the dorsal horn of the spinal cord of the rat: a light and electron microscopic study

Cholecystokinin-like immunoreactivity was investigated with an indirect immunoperoxidase technique in the whole spinal cord with the light microscope and in the dorsal horn with the electron microscope. Intraparenchymal injections of colchicine were performed to allow the detection of cholecystokinin-like immunoreactive cell bodies. Rats treated at birth with capsaicin were also studied at the light microscope. Numerous cholecystokinin-like immunoreactive fibres and varicosities were found in the two superficial layers of the dorsal horn and in the intermedio-medial nucleus; cholecystokinin-like immunoreactive cell bodies were also present in these two regions. After neonatal capsaicin treatment, the number of cholecystokinin-like immunoreactive fibres and varicosities was strongly reduced in the dorsal horn. At the electron microscope level, cholecystokinin-like immunoreactivity was localized in numerous neurites often filled with vesicles (axon terminals and dendrites containing vesicles) and in few cell bodies and dendrites. The immunoreaction was found mainly associated with ribosomes, granular reticulum, neurotubules and vesicles. Large granular vesicles were filled with the reaction product whereas small and medium-sized vesicles showed a varying degree of immunoprecipitate around their membrane. In addition dense "granules" of precipitate were observed in numerous presynaptic neurites. Cholecystokinin-like immunoreactive axons were of small calibre and mostly unmyelinated. Cholecystokinin-like immunoreactive axon terminals made asymmetric synaptic contacts with generally unlabelled dendrites or dendritic spines. A single labelled nerve terminal could contact several different dendrites in structures resembling glomeruli. Few axo-somatic synapses but a relatively high number of axo-axonic contacts were seen. About half of these axo-axonic contacts involved pre- and postsynaptic profiles. Both light and electron microscopic observations led us to the conclusion that some of the cholecystokinin-like immunoreactive fibres of the dorsal horn originate in the spinal ganglia via capsaicin-sensitive C afferents; and some from intrinsic neurons, particularly islet cells. Other fibres may come from supraspinal centres, other local neurons or capsaicin-insensitive afferents from the spinal ganglia. The results are discussed with regard to data in the literature, particularly those concerned with the specificity of the cholecystokinin antibodies; it is hypothesized that several types of cholecystokinin-like immunoreactive peptides may be present in the dorsal horn, depending on their origin (supraspinal, intrinsic or peripheral).(ABSTRACT TRUNCATED AT 400 WORDS)     

The organization of serotonin fibers in the mammalian superior colliculus

Summary The distribution of serotonin immunoreactivity in the superior colliculus (SC) of the rat, hamster, chipmunk, cat, and monkey was studied using a sensitive immunohistochemical method. In all of these animals, serotonin immunoreactivity formed a dense network of varicose fibers throughout the SC. These fibers had a characteristic arrangement corresponding to the laminar structures of the SC. Except in the chipmunk, serotonergic fibers were more dense in the stratum griseum superficiale than in the other layers. In the SC of the chipmunk, these fibers appeared evenly distributed.To explore the degree of scrotonergic innervation in each layer, a semi-quantitative assay of serotonin immunoreactive varicosities was conducted in the rat, chipmunk, cat, and monkey. Peaks in varicose density were seen in the stratum griseum superficiale, the stratum griseum intermedium and the stratum griseum profundum. In the rat, cat, and monkey, the highest density of these varicosities was in the stratum griseum superficiale. On the other hand, the stratum griseum intermedium of the chipmunk SC received the greatest innervation of serotonergic varicose fibers.Supported by a grant from the Ministry of Education, Science, and Culture of Japan (No. 57214028)     

Termination pattern and fine structural characteristics of GABA- and [Met]enkephalin-containing nerve fibers and synapses in the superior cervical ganglion of adult rat.

Morphological features of nerve fibers and synapses containing GABA and [Met]enkephalin were studied at the light and electron microscopic levels in the superior cervical ganglia of rats by pre- and postembedding immunohistochemistry. Both GABA and [Met]enkephalin immunoreactivities were found in varicose nerve fibers, forming diffuse networks which were denser in the rostral than in the caudal part of each ganglion. For both antigens rich and basket-like innervation was observed around some of the principal neurons. The GABA-immunoreactive fibers were evenly stained, while in case of [Met]enkephalin-positive nerve fibers the varicosities showed intensive immunopositivity only. Postembedding immunochemistry revealed that both inhibitory substances were located in axon varicosities which established asymmetric synapses of Gray I type. Fine structural investigation revealed that GABA-like immunoreactivity was confined in the nerve endings to the clear synaptic vesicles of 40 nm diameter, whereas the immunogold particles, indicating the occurrence of [Met]enkephalin, were located over the large dense-cored vesicles of 120 nm diameter. The clear and dense-cored vesicles were, however, mixed in the nerve endings labeled by either neurotransmitter substance. Interestingly, the [Met]enkephalin-immunopositive axon terminals were found, consequently, in synaptic contacts with dendrites containing dense bodies in a row underlying the postsynaptic membrane thickening. Since nerve terminals with GABA-like immunoreactivity established synapses of Gray I type without such subjunctional bodies, one can reasonably assume that, in spite of similarities in termination pattern, there is no co-existence of GABA and enkephalin in the axons in the superior cervical ganglion.     

Ultrastructural immunocytochemical study of the dopaminergic innervation of the rat lateral septum with anti-dopamine antibodies

The dopaminergic innervation of the rat lateral septum has been investigated at ultrastructural level by immunocytochemistry using the unlabelled peroxidase-anti-peroxidase method with anti-dopamine antibodies. The specificity of the reaction has been carefully checked by immunological and histochemical controls. A strong immunoreaction was observed in fibres of the lateral septum as well as in their cells of origin in the ventral tegmental area. In the lateral septum, dopamine-immunoreactive fibres were localized in two distinct areas. A first area, located ventrally in the anterior part of the septum was characterized by a high density of immunoreactive varicosities with barely visible intervaricose segments. A more dorsal area, extending throughout the anteroposterior region of the septum, was characterized by immunoreactive fibres in pericellular arrangements. Electron microscopic observations revealed no difference in the ultrastructure of dopamine-immunoreactive profiles in the different areas. Reaction product was found in vesicles, linked to microtubules and in the cytoplasm. Three types of vesicles were seen: (i) small vesicles (30-50 nm) with varying intensity of immunoreaction, filling up the varicosities; (ii) rare large clear vesicles (50-80 nm) with no internal immunoreaction; (iii) very rare large dense vesicles (50-100 nm) with a strong dopamine immunoreactivity. Labelled profiles were observed in clearly defined asymmetrical synaptic contacts with somata and dendrites. Due to the lack of previous work dealing with the use of anti-dopamine antibodies for electron microscope immunocytochemistry, our observations are compared to previous data obtained by more indirect labelling techniques.     

脊髓侧角区5-HT、TH、SP和L-ENK免疫反应纤维和终末的超微结构

本文应用免疫细胞化学ABC法,在电镜下观察脊髓侧角区单胺能和某些肽能纤维及末梢的突触组合。大鼠侧角内的5-HT、TH、SP和L-ENK免疫反应纤维均为无髓纤维。在侧角细胞簇内,这些纤维穿行于胞体之间,有的与胞体相邻,但很少与胞体形成轴-体突触。这些单胺和肽类纤维也与树突伴行,在树突束内数量最多。有时一小束无髓纤维都含同一种免疫反应物质。轴-树突触是各种免疫反应纤维终末所形成突触的主要形式。各种纤维终末所含的小泡多为圆清亮小泡,或兼有少数大颗粒泡。SP和L-ENK纤维膨体内的小泡与其终末内者不同,大颗粒泡较多,有时约占半数。各种免疫反应终末所组成参与的突触,对称或非对称型均不显现优势。     

High-resolution radioautographic study of the serotonin innervation of the rat corpus striatum after intraventricular administration of [3H]5-hydroxytryptamine

In the rat striatum, identification of the serotonin-containing nerve endings at the ultrastructural level has been done by radioautography after ventriculodsternal perfusion of [³H]serotonin. Only the ventricular border of the middle part of the nucleus caudatus-putamen (neostriatum) was studied and, in addition, a small dorsal part of the globus pallidus.For every 100 μm²of thin sections of the neostriatum, 0.5 labelled structures were found, which were either axons (14%), preterminal enlargements (21.5%) or nerve terminals (64.5%). Three kinds of labelled nerve endings could be recognized according to the size and shape of their synaptic vesicles, although all contained occasional large granular vesicles. The first type (36%) was characterized by small and often granular, ovoid vesicles mixed with tubular structures of the same diameter (20 nm). The second type (28%) contained crowded spherical vesicles of 45 nm in diameter which were mostly clear. The third type (36%) contained a mixture of both types of synaptic vesicles. The labelled nerve terminals in the neostriatum showed a few synaptic contacts (mean of 14%) which were, however, more numerous in the second type of nerve endings (25–30%) and mainly of the asymmetric type. In the globus pallidus the serotonin-labelled nerve terminals resembled those in the neostriatum but one additional type was observed, which was characterized by only a few typical synaptic vesicles and by numerous large granular vesicles.The observations made in this study indicate that the serotonin-containing varicosities of the striatum are very similar to those described in some other nuclei. They exhibit characteristic large granular vesicles and the synaptic vesicles often have an eccentric tiny granule after administration of exogenous serotonin and/or inhibition of monoamine oxidase. The peculiar type of nerve endings with tubular vesicles described by others in the cerebellum and in the locus coeruleus was also present in the neostriatum. The morphological heterogeneity of serotonin-containing nerve endings within a single nucleus or from one nucleus to another, is discussed in relation to the amine content, the stage of formation of the synaptic vesicles and their origin in different raphe nuclei.     

Supraependymal nerve fibres in human brain: Correlative transmission and scanning electron microscopical and fluorescence histochemical studies

Supraependymal, varicose nerve fibres were observed in human post-mortem brain when ventricle surfaces were examined by transmission and scanning electron microscopy. Transmission electron microscopy revealed the presence of numerous profiles of nerve terminals, containing small electron-lucent vesicles and large dense-cored vesicles, and axons with their characteristic neurotubules. Occasionally, desmosome-like junctions were found between the nerve terminals and the adjacent ependymal cells. Scanning electron microscopy demonstrated the topographical distribution of varicose nerve fibres traversing among clusters of cilia and numerous microvilli on the ventricle surface. These nerve fibres, which were most frequently observed in the interventricular foramen, often coursed in bundles. In one of the five cases examined, a discrete formaldehyde-induced fluorescence typical of 5-hydroxytryptamine (yellow colour and rapid fading upon violet-blue irradiation) was observed on the floor of the fourth ventricle.It is concluded that supraependymal nerve fibres in human brain are probably identical to similar nerves previously characterized as serotoninergic in rat brain and occurring in several other mammalian brains.     

Localization of tritiated norepinephrine in the renal arteriolar nerves

The innervation of the glomerular arterioles was investigated by light and electron microscopy autoradiography for localization of exogenous tritiated norepinephrine. By light microscopy accumulations of grains were seen associated with afferent arterioles and in lesser numbers with efferent arterioles and neighboring tubules. Accumulations of grains were noted to be in contact with juxtaglomerular granular cells. Electron microscopy autoradiography revealed that nearly two-thirds of the silver grains were on axons. Most of the label was on varicosities packed with small, clear and dense-cored, vesicles. Most varicosities, including those in contact with smooth muscle, juxtaglomerular granular or tubular cells, were labeled. Some varicosities which appeared unlabeled in a given section were labeled in subsequent sections. These findings are consistent with the notion that the glomerular arterioles are innervated mainly by adrenergic nerves. This view is supported by the previously reported observations of the concomitant virtual disappearance of fluorescent and acetylcholinesterase-positive nerves from the region of the glomerular arterioles after two injections of six-hydroxydopamine (a drug which selectively destroys adrenergic nerves) and the presence of small dense-cored vesicles in all axons of the juxtaglomerular region when examined by serial section electron microscopy.