Serotonin-immunoreactive and dopamine-immunoreactive neurones in the terminal ganglion of the cricket,Acheta domestica: Light- and electron-microscopic immunocytochemistry

Summary The distribution and ultrastructure of serotonin- and dopamine-immunoreactive (5-HTi and DAi) neurones have been investigated in the terminal ganglion of the cricket, Acheta domestica, using a pre-embedding chopper technique. Special attention has been paid to the immunoreactive structures in the neuropil. 5-HTi structures are extensively distributed and densely packed throughout the 5 neuromeres of the terminal ganglion and originate from several interneurones and efferent neurones. In contrast, DAi fibres are distributed sparsely although they extend to all neuromeres of the ganglion and originate from 6 interneurons only. For both 5-HTi and DAi neurones characteristic axonal projections and branching patterns can be distinguished. The 5-HTi axons exhibit rich varicose arborizations, whereas DAi neurones possess fewer varicosities in the neuropil. Electron microscopy shows that 5-HTi varicosities contain small ( 60 nm) and large ( 100 nm) agranular vesicles, and large ( 100 nm) granular vesicles, whereas in DAi varicosities small ( 60 nm) agranular and large ( 100 nm) granular vesicles are seen. Both 5-HTi and DAi varicosities form synaptic contacts. We conclude that both serotonin and dopamine may be used as neurotransmitters in the terminal ganglion of the cricket.Fellow of the Alexander von Humboldt-Stiftung     

Serotonin fiber innervation of cerebellar cell suspensions intraparenchymally grafted to the cerebellum ofpcd mutant mice

One aspect of integration of implanted neurons into the neuronal circuitry of a defective host brain is the re-establishment of a host-to-graft afferent innervation. We addressed this issue by using the adult cerebellum of Purkinje cell degeneration (pcd) mutant mice, which lack virtually all Purkinje cells after postnatal day (P) 45. Purkinje cells constitute one of the cerebellar cell types being innervated by axons of raphé serotonin (5-HT) neurons. In normal mice, 5-HT-immunoreactive fibers are distributed to all cerebellar folia. Following Purkinje cell loss inpcd mice, cerebellar 5-HT-immunoreactive fibers persist. Cerebellar cell suspensions were prepared from embryonic day (E) 11–13 normal mouse embryos and were intraparenchymally grafted into the cerebellum ofpcd mutants either directly or after pre-treatment with 5, 7-dihydroxytryptamine (5,7-DHT) to selectively remove 5-HT cells of donor origin. The state of Purkinje cells and 5-HT axons was monitored in alternate sections by 28-kDa Ca²⁺-binding protein (CaBP) and 5-HT immunocytochemistry, respectively. Serotonin-immunoreactive axons were seen in the grafts from 5 to 32 days after transplantation. In some of the grafts which had not been pre-treated with 5,7-DHT, a small number of 5-HT-immunoreactive cell bodies was found, indicating that part of the 5-HT fiber innervation of the graft could actually derive from donor cells. On the other hand, in grafts pre-treated with 5,7-DHT, no 5-HT cell bodies were seen in the grafted cerebellum; 5-HT fibre innervation of the grafts occurred, but it appeared to be slightly less robust compared to situations of co-grafted 5-HT cell bodies. These findings suggest that host 5-HT fibers are able to provide afferent innervation to donor cerebellar tissue; the presence of co-grafted 5-HT cells may augment such an innervation.Special issue dedicated to Dr. Morris H. Aprison.     

Nerve-purkinje fiber relationship in the moderator band of bovine and caprine heart

Summary The moderator band in the heart of the ox and goat contains bundles of Purkinje fibers and nerve fibers separated by connective tissue. The axons are mostly unmyelinated and embedded in the cytoplasm of Schwann cells.Small bundles of axons run close to the Purkinje fibers. The axons dilate into varicosities 0.5 to 1.6 in diameter (mean 0.95 ), containing three types of vesicles: 1) agranular vesicles with a diameter of 400–500 Å, 2) large dense-cored vesicles with a diameter of 800–1200 Å, 3) small dense-cored vesicles with a diameter of 500 Å. Most varicosities contain agranular vesicles together with a few large dense-cored vesicles.The gap between the varicosities and the nearest Purkinje fiber is unusually wide and normally varies between 0.3 and 0.8 . No intimate nerve-Purkinje fiber contacts, with a cleft of 200 Å, were observed.     

Innervation of the intestine in the bivalve mollusc Chione stutchburyi

Summary The innervation of the gut of the venerid bivalve mollusc, Chione stutchburyi, has been examined by fluorescence histochemistry, electron microscopy and autoradiography. Specific green and yellow varicose fluorescent fibres indicate the presence of dopaminergic and serotonergic axons, respectively. Three different types of axons can be distinguished by the morphological characteristics of their vesicles. Type I axons contain predominantly small granular vesicles (average diameter 65 nm), Type II axons possess large granular vesicles (average diameter 100 nm) and Type III axons contain large opaque vesicles (average diameter 150 nm). The granular vesicles in both Types I and II axons react positively to dichromate, and their granularity is reduced by reserpine indicating that they are monoaminergic. Only Type I axons accumulate tritiated dopamine and are selectively damaged by 6-hydroxydopamine. It is concluded that Type I axons are dopaminergic. Type II axons are serotonergic: they alone take up tritiated 5-hydroxytryptamine, and 5,7-dihydroxytryptamine selectively causes degenerative changes in these axons. Type III axons contain an unidentified neurotransmitter substance. The large opaque vesicles of these axons do not react to dichromate and are unaffected by reserpine, 6-hydroxydopamine or 5,7-dihydroxytryptamine.     

The ultrastructure of the innervation of the intestinal wall in the teleosts Myoxocephalus scorpius and Pleuronectes platessa

Summary The innervation of the intestinal wall in the teleosts Myoxocephalus and Pleuronectes was examined electron microscopically. Two classes of axons can be identified. The first, which is in the majority, contains numerous 50–150 nm granular vesicles as well as some 40–50 nm agranular vesicles while the second contains predominantly the 40–50 nm agranular vesicles. Chromate/dichromate staining methods suggest that the first type is aminergic. Both types lie in close association with the perikarya of intrinsic myenteric neurons but only axons containing predominantly agranular vesicles have synaptic membrane specialisations. No axon bundles pass into the longitudinal muscle layer in Myoxocephalus gut and though some do in Pleuronectes, they do not closely approach the smooth muscle cells. Axons containing large granular vesicles lie in intimate contact with the myocytes of the circular muscle layer. Both axon types pass through the submucosa to form a plexus underneath the mucosal epithelium. Varicosities containing agranular or granular vesicles are separated from the epithelial cells by a gap of about 200 nm in which lies a basal lamina.     

Morphology and central synaptic connections of the efferent neurons innervating the crayfish hindgut

Summary The distribution, morphology and synaptic connections of the hindgut efferent neurons in the last (sixth) abdominal ganglion of the crayfish, Orconectes limosus, have been investigated using light and electron microscopy in conjunction with retrograde cobalt/nickel and HRP labeling through the intestinal nerve. The hindgut efferent neurons occur singly and in clusters, and are unipolar. Their axonal projections are uniform and consist of a thick primary neurite with typical lateral projections and limited arborization of varicose fibers in the ganglionic neuropil. They also send lower order axon processes to the ganglionic neural sheath, where they arborize profusely, forming a network of varicose fibers. The majority of the efferent neurons project to the anterior part of the hindgut. HRP-labeled axon profiles are found in both pre- and postsynaptic position in the neuropil of the ganglion. HRP-labeled axon profiles also establish pre- and postsynaptic contacts in the intestinal nerve root. All hindgut efferent terminals contain similar synaptic vesicle populations: ovoid agranular vesicles (50–60 nm) and a few large granular vesicles (100–200 nm). It is suggested that the hindgut efferent neurons in the last abdominal ganglion are involved in: (1) innervation of the hindgut; (2) central integrative processes; (3) en route synaptic modification of efferent and afferent signals in the intestinal nerve; (4) neurohumoral modulation of peripheral physiological processes.Fellow of the Alexander von Humboldt Stiftung     

Ultrastructural characterisation of vasopressinergic terminals in the lateral septum of murine brains by use of monoclonal anti-neurophysins

Summary Synapses in the lateral septum of the murine brain have been investigated by ultrastructural immunocytochemistry, using monoclonal anti-neurophysins in both immunoperoxidase and immunogold techniques. In the region shown by light microscopy to be rich in vasopressinergic innervation, synaptic boutons containing 30 nm clear vesicles and occasional 100 nm dense-cored granules (granules) were stained by pre-embedding immunoperoxidase procedures with antisera to vasopressin-neurophysin, but not oxytocin-neurophysin; reaction product was diffusely distributed in the terminals. Terminals were symmetrical, and both axosomatic and axodendritic in type. Postembedding immunogold procedures by use of anti-vasopressin-neurophysin labeled only the 100 nm diameter granules in the terminals. Sodium meta-periodate treatment bleached immunoreactive granules, indicating the presence of a carbohydrate residue. The quantum of peptide packaged in the granules appears to be smaller than that in magnocellular neurones; nevertheless, the results indicate that, as in the magnocellular neurosecretory system, vasopressin and its neurophysin are packaged exclusively in granules, and that vasopressin in the septum is likely to be derived from a precursor comprising vasopressin, vasopressin-neurophysin and a glycosylated residue.     

Varicose axons bearing “synaptic” vesicles on the basal lamina of the human seminiferous tubules

Summary Normal (infant and adult) and pathological testes were examined by electron microscopy in order to study testicular innervation. Nerves composed of non-myelinated fibres were abundant in the tunica vasculosa of the tunica albuginea. These nerves penetrated into the testicular septa reaching the interstitial tissue. This showed numerous non-myelinated nerve fibres running among the Leydig cells and blood vessels. Single axons or small groups of them, partially surrounded by Schwann cells, approached: 1) the Leydig cells, 2) the interstitial blood vessels, and 3) the seminiferous tubules. Single naked axons were also observed primarily in the proximity of the seminiferous tubules. These axons showed varicosities containing both small and large synaptic vesicles. The latter were less numerous and contained a central dense core. Small vesicles were agranular. Some varicose axons ran across the myofibroblast layer of the tunica propria reaching the basal lamina of the seminiferous tubules at the level of the Sertoli cells but not at the level of the spermatogonia. The intercellular space between Sertoli cell and axon membrane was about 150–200 nm.Profesor Agregado de Histología y EmbriologíaProfesor Adjunto de Citología e HistologíaProfesor Adjunto de Histología y Embriología     

Characterization of the photosynthetic electron transport chain in normal and photobleachedAnabaena cylindrica by flash spectroscopy

Electron transport of normal and photobleachedAnabaena cylindrica was studied using spectral and kinetic analyses of absorbance transients induced by single turnover flashes. Between 500 and 600 nm two positive bands (540 and 566 nm) and two negative bands (515 and 554 nm) were found. Absorbance changes at 515 and 540 nm were partly characterized. None of these absorbance changes represent an electrochromic shift. Absorbance changes at 554 and 566 nm correspond to the oxidation of cytochromef and the reduction of cytochromeb ₅₆₃, respectively. We found a very slight 3-(3,4-dichlorophenyl)-1, 1-dimethylurea (DCMU) sensitivity of cytochromef in normal cells, while DCMU was completely ineffective for cytochromef reduction in photobleached cells. The absorbance change of cytochromeb ₅₆₃ increased, while the absorbance change of cytochromef was smaller than in normal cells. The increased O₂ evolution in photobleached cells and the negligible electron transport via cytochromef suggest the participation of other electron acceptor(s) in the electron-transport chain of photobleachedAnabaena cylindrica.     

Membrane differentiation in the cytoplasmic-tubule system of the intestinal absorptive cells of the lamprey,Lampetra japonica

Summary Specific membrane differentiation occurs in the cytoplasmic-tubule system of the absorptive cells lining the mucosa of the lamprey anterior intestine. The absorptive cells are characterized by the presence of abundant mitochondria and a system of well-developed cytoplasmic tubules (120 nm in diameter). The cytoplasmic tubules open on to the basolateral cell surface and contain numerous lipoprotein particles (50–100 nm diam.) in their lumina. Lipoprotein particles are also observed in the endoplasmic reticulum and the Golgi complex, and they are transfered to the lateral intercellular space and lamina propria by way of the cytoplasmic tubules. Spirally-wound parallel rows of particles are found in the luminal surface of the cytoplasmic tubules. The rows are 17 nm apart and are wound spirally at a pitch of 210 nm. Freeze-fracture images of the tubule membranes also show spiral arrays of particles (9 nm in diameter) on the P-face, and complementary shallow grooves on the E-face. From these observations, it is suggested that the cytoplasmic-tubule system of the intestinal absorptive cells serves as a channel for the transport of synthesized lipoprotein into the interstitium, and is also the site of the ion and water exchange essential for the maintenance of ionic homeostasis.     

Location and major postembryonic changes of identified 5-HT-immunoreactive neurones in the terminal ganglion of a cricket (Acheta domesticus)

Summary In the terminal ganglion of the cricket, Acheta domesticus, the somata of certain interneurones and efferent neurones consistently react to 5-HT immunohistochemistry. There are serially homologous pairs of bilateral interneurones seen in the neuromeres of the 7th to the 10th segment and hindgut neurones with their somata located at the posterior median end of the ganglion. In adult crickets, pairs of large efferent neurones with lateral somata supply specific genital muscles in the 8th and the 9th segment of females. In males, only one pair of these efferent neurones supplies genital muscles of the 9th segment only. These identified 5-HT-immunoreactive neurones are not detected in larval crickets before development of the genital apparatus.     

Organization and ultrastructural identification of the catecholamine nerve terminals in the neural lobe and pars intermedia of the rat pituitary

Summary The central catecholamine innervation of the pituitary neural lobe and pars intermedia of the rat have been identified ultrastructurally and their organization has been investigated in a combined fluorescence histochemical and electron microscopical study. The dopamine analogues, 5-hydroxydopamine and 6-hydroxydopamine, were used to label the catecholamine terminals, and to enable the direct correlation between the fluorescence microscopical and the electron microscopical pictures.The fibre type that was identified as catecholamine-containing was ultrastructurally chiefly characterized by dense-cored vesicles, 500–1200 Å in diameter, intermingled with varying numbers of small empty vesicles. 5-hydroxydopamine was selectively accumulated in these fibres and caused an increased electron density of the granular vesicles as well as of some small normally agranular vesicles, and systemically administered 6-hydroxydopamine caused a selective degeneration of these fibres, most prominently within the neural lobe. The dopaminergic terminals of the neural lobe showed frequent close contacts (80–120 Å), without real membrane thickenings, to neurosecretory axons and to pituicyte processes. It is suggested that these close contacts might signify a direct dopaminergic influence on the neurosecretory axons and/or on the pituicyte processes. The identified central catecholamine fibres were also found to make common synapse-like contacts on the pars intermedia cells, whereas the innervation by neurosecretory fibres was very rare. This suggests that the direct central nervous control of the rat pars intermedia is exerted by the catecholamine neurons. A very special feature of the catecholamine fibres in the pituitary is the occurrence of peculiar, large dopamine-filled droplet-like swellings. Electron microscopically, such large axonal swellings (more than 2 in diameter) were found to contain, in addition to the characteristic vesicles and organelles, strongly osmiophilic lamellated membrane complexes resembling myelin bodies and multivesicular bodies encircling disintegrated vesicles, suggesting that these droplet fibres represent dilated stumps of spontaneously degenerating dopaminergic axons. It is suggested that the dopaminergic neural lobe fibres are undergoing continuous reorganization through degeneration—regeneration cycles, a phenomenon previously suggested for the neurosecretory axons of the neural lobe.Supported by the Deutsche Forschungsgemeinschaft.Supported by Svenska Livförsäkringsbolags Nämnd för Medicinsk Forskning, by The Medical Faculty, University of Lund and by the Ford Foundation.     

The hypothalamo-hypophysial system of the frog Rana temporaria

Summary The median eminence (ME) of the adult frog, Rana temporaria, was studied by means of electron microscopy including quantitative electron-microscopic autoradiography. In frogs captured in May and June numerous peptidergic neurosecretory fibres extending via the internal zone to the pars nervosa display large swellings containing few granules, mitochondria, neurotubules and cisternae of the smooth endoplasmic reticulum. In addition, few secretory globules up to 1.5 m in diameter occur in these varicosities. In animals collected during the autumn period many of these neurosecretory swellings filled with neurosecretory granules and polymorphic inclusions resemble Herring bodies. Three types of granule-containing neurosecretory fibres were observed in the external zone (EZ) of the ME of adult R. temporaria. Peptidergic A₁- and A₂-type fibres are characterized by granules 150–220 nm and 100–160 nm in diameter, respectively. Monoaminergic fibres of type B with granules approximately 100 nm in diameter represent 50% of all neurosecretory elements in the EZ of the frog ME; 12% of the total number of granule-bearing axons in the EZ actively taking up radiolabelled 5-hydroxytryptophan are thought to be serotoninergic terminals. Neurosecretory terminals of all types and glial vascular endfeet establish direct contacts with the perivascular space of the primary portal capillaries. Some neurosecretory terminals are separated from the lumen of the third ventricle by a thin cytoplasmic lamella of tanycytes. The possible physiological significance of this structural pattern is discussed.     

Spectroscopic studies of bound cytochrome c and an iron-sulfur center in a purified reaction center complex from the green sulfur bacterium Chlorobium tepidum

Flash-induced optical kinetics at room temperature of cytochrome (Cyt) c ₅₅₁ and an Fe-S center (CF_A/CF_B) bound to a purified reaction center (RC) complex from the green sulfur photosynthetic bacterium Chlorobium tepidum were studied. At 551 nm, the flash-induced absorbance change decayed with a t _1/2 of several hundred ms, and the decay was accelerated by 1-methoxy-5-methylphenazinium methyl sulfate (mPMS). In the blue region, the absorbance change was composed of mPMS-dependent (Cyt) and mPMS-independent component (CF_A/CF_B) which decayed with a t _1/2 of 400–650 ms. Decay of the latter was effectively accelerated by benzyl viologen (Em –360 mV) and methyl viologen (–440 mV), and less effectively by triquat (–540 mV). The difference spectrum of Cyt c had negative peaks at 551, 520 and 420 nm, with a positive rise at 440 to 500 nm. The difference spectrum of CF_A/CF_B resembled P430 of PSI, and had a broad negative peak at 430435 nm.Abbreviations (B)Chl (bacterio)chlorophyll - Cyt cytochrome - F_A, F_B and F_X iron-sulfur center A, B and X of Photosystem I - CF_A, CF_B and CF_X F_A-,F_B- and F_X-like Fe-S center of Chlorobium - mPMS 1-methoxy-5-methylphenazinium methyl sulfate - PSI Photosystem I - RC reaction center     

Synaptic organisation of the pelvic ganglion in the guinea-pig

Summary A semi-quantitative electron-microscopic study of neuronal cell bodies, nerve profiles and synapses in the anterior pelvic ganglia of the guinea-pig has been carried out following in vivo labelling of adrenergic nerve endings with 5-hydroxydopamine. Ganglion cells of three main types have been distinguished: 1) the majority (about 70%) not containing granular vesicles, probably cholinergic elements; 2) those containing large granular vesicles of uniform size (80–110 nm), with granules of medium density and prominent halos; and 3) those containing vesicles of variable size (60–150 nm), with very dense eccentrically placed granular cores. Some non-neuronal granule-containing cells were present, mainly near small blood vessels. Some 95% of the total axon profiles within the ganglia were cholinergic, the remaining 5% were adrenergic. However, 99% of synapses (i.e. axons within 50 nm of nerve cell membrane with pre-and post-synaptic specialisations) were cholinergic, and 1 % were adrenergic. Only three examples of nerve cell bodies exhibiting both cholinergic and adrenergic synapses were observed. Unlike the para-and prevertebral ganglia, the pelvic ganglia contained large numbers of axo-somatic synapses. As many as 20% of the nucleated neuronal cell profiles displayed two distinct nuclei.     

A microscopic investigation of enamel in wombat (Vombatus ursinus)

Summary A study of the enamel of continuously growing Vombatus ursinus molars was carried out using the techniques of light microscopy, hardness testing, scanning electron microscopy and transmission electron microscopy. From the erupted end to within 8 mm of the growing end, mature enamel was observed and it was found that between comparable areas there were no significant ultrastructural differences in enamel; however, small (12nm diameter), loosely packed needle-like crystals characteristic of developing enamel were observed near the growing end. Mature enamel was found to consist of three optically-translucent regions interleaved with two opaque regions. Opaque enamel was softer than translucent enamel. The opacity and relative softness characteristic of two of the enamel regions was not related to prism pattern or orientation; it was, however, related to the presence of voids (28 nm diameter) in these regions.     

Catecholaminergic nerves in the embryonic chick ovary: co-localization with β2-adrenoceptor-bearing steroidogenic cells

Summary The present study investigates the innervation of the embryonic chick ovary with regard to (i) development and compartmentalization of catecholaminergic nerves, and (ii) presence of adrenoceptors on steroidogenic target cells of catecholaminergic nerve terminals. Catecholaminergic nerve fibers visualized by glyoxylic acid-induced histofluorescence first appeared at embryonic day (E) 13. From E15 through E21 the density of fluorescent aminergic nerves increased markedly in parallel with the concentration of catecholamines and numbers of nerve bundles and single axons seen at the electron-microscopic level. Catecholaminergic nerves were confined to the ovarian medulla and closely associated with interstitial cells. Nerve terminals approached interstitial cells up to a distance of 20 nm and, in their majority, exhibited uptake of the false adrenergic transmitter 5-hydroxydopamine. Although adrenaline amounted to 14% of the total catecholamine content at E21, adrenaline immunoreactivity was only detected in adrenal chromaffin cells, but not in nerve fibers or cell bodies within the ovary. Interstitial cells structurally matured between E15 and E21 as documented by an increase of smooth endoplasmic reticulum and tubular mitochondria. Monoclonal antibodies mAB 120 and BRK 2 raised against avian ₁ and mammalian ₂-adrenergic receptors revealed the presence of ₂-adrenoceptor-like immunoreactivity on the surface of interstitial cells, but not on any other cell type.The results are consistent with the notion of a dense adrenergic innervation of the embryonic chick ovarian medulla and its steroidogenic interstitial cells, and suggest the chick ovary as an excellent model for elucidating the functional role of a neural input to steroidogenic cells during development.     

Helical structure in the apical tubules of several absorbing epithelia

Summary Unique and highly ordered structures were discovered in the so-called apical tubules of several absorbing epithelia (kidney proximal tubule, visceral yolk sac and ductuli efferentes) fixed in situ with a mixture of formaldehyde, glutaraldehyde and osmium tetroxide. The apical tubules were especially numerous in the apical cytoplasm, in addition to the invaginations of the apical plasma membrane, newly formed endocytic vesicles and large endocytic vacuoles. They showed a cylindrical structure (80 nm in diameter) limited by a smooth membrane. Helically wound parallel rows of particles (11 nm in diameter) were found in the apical tubules in close proximity to their limiting membrane. The structure of the helix was determined by following the rows through serial sections and semithin sections, and was found to be a left-handed quadruple helix. These particles surround an electron-lucent cylinder (35 nm in diameter), containing at its center a single row of particles (9 nm in diameter). The apical tubules with the luminal specializations were not seen in continuity with the apical plasma membrane, but were frequently connected with the large endocytic vacuoles, which were present in the deeper levels of the apical cytoplasm. From these observations, it is suggested that the apical tubules are not derivatives of the apical plasma membrane; rather, they represent an intracellular compartment, which is morphologically related to the large endocytic vacuoles.     

Dense-core vesicles in motor nerve terminals

Summary Motor nerve terminals on white and intermediate muscle fibers of the Atlantic hagfish (Myxine glutinosa, L.) contain translucent synpatic vesicles and about 1–2% dense-core vesicles. Terminals on red muscle fibers contain up to 40% dense-core vesicles with diameter 800–1100 Å. Examinations for formaldehyde-induced fluorescence indicate yellow fluorescence (5-HT ?) apparently corresponding with terminal axons on red muscle fibers in craniovelar muscles. Possibly red muscle fibers of Myxine receive monoaminergic innervation.The author is indebted to Dr. Finn Walvig, Biological station, University of Oslo, Drøbak, for supply of hagfishes.     

Electron microscopic analysis of tyrosine hydroxylase,dopamine-β-hydroxylase and phenylethanolamine-N-methyltransferase immunoreactive innervation of the hypothalamic paraventricular nucleus in the rat

Summary The catecholaminergic innervation of the hypothalamic paraventricular nucleus (PVN) of the rat was studred by preembedding immunocytochemical methods utilizing specific antibodies which were generated against catecholamine synthesizing enzymes. Phenylethanolamine-N-methyltransferase (PNMT)-immunoreactive terminals contained 80–120 nm dense core granules and 30–50 nm clear synaptic vesicles. The labeled boutons terminated on cell bodies and dendrites of both parvo- and magnocellular neurons of PVN via asymmetric synapses. The parvocellular subnuclei received a more intense adrenergic innervation than did the magnocellular regions of the nucleus. Dopamine--hydroxylase (DBH)-immunopositive axons were most numerous in the periventricular zone and the medial paryocellular subnucleus of PVN. Labeled terminal boutens contained 70–100 nm dense granules and clusters of spherical, electron lucent vesicles. Dendrites, perikarya and spinous structures of paraventricular neurons were observed to be the postsynaptic targets of DBH axon terminals. These asymmetric synapses frequently exhibited subsynaptic dense bodies. Paraventricular neurons did not demonstrate either PNMT or DBH immunoreactivity. The fibers present within the nucleus which contained these enzymes are considered to represent extrinsic afferent connections to neurons of the PVN.Tyrosine hydroxylase (TH)-immunoreactivity was found both in neurons and neuronal processes within the PVN In TH-cells, the immunolabel was associated with rough endoplasmic reticulum, free ribosomes and 70–120 nm dense granules. Occasionally, nematosome-like bodies and cilia were observed in the TH-perikarya. Unlabeled axons established en passant and bouton terminaux type synapses with these TH-immunopositive cells. TH-immunoreactive axons terminated on cell bodies as well as somatic and dendritic spines of paraventricular parvocellular neurons. TH-containing axons were observed to deeply invaginate into both dendrites and perikarya of magnocellular neurons.These observations provide ultrastructural evidence for the participation of central catecholaminergic neuronal systems in the regulation of the different neuronal and neuroendocrine functions which have been related to hypothalamic paraventricular neurons.Supported by NIH Grant NS 19266 to W.K. Paull