Serotonin axons in the supra- and subependymal plexuses and in the leptomeninges; their roles in local alterations of cerebrospinal fluid and vasomotor activity

Occurrence of unmyelinated axon profiles at distal, middle and proximal levels in the ventral root L7 of cats and kittens

The proportion of unmyelinated axon profiles (UAP) was estimated electronmicroscopically at different proximo-distal levels in the ventral root L7 of 7 normal adult cats and 2 kittens. In 3 of the adult cats the occurrence of UAP was also determined in the juxtamedullary root fascicles, including the CNS/PNS transitional region. Three additional adult (male) cats, which had been subjected to prepubertal castration, were examined at mid-root level. Prepubertal castration was found not to interfere with the increase in L7 ventral root UAP that takes place during puberty. In the normal animals the proportion of UAP was highest distally in the ventral root and decreased as the spinal cord was approached. The same trend was seen in cats and kittens, but the values were obviously higher in adults. The content of UAP decreased markedly in the immediate vicinity of the CNS/PNS border. Very few unmyelinated axons and no myelinated CNS-type axons with sizes below the gamma range were found on the CNS side of this border. In addition groups of PNS-type unmyelinated and small myelinated axons occurred in the pia mater outside some juxtamedullary root fascicles.

The distribution and origin of serotonin-containing fibers in the septal area: a combined immunohistochemical and fluorescent retrograde tracing study in the rat

The distribution of 5-hydroxytryptamine (serotonin, 5-HT)-containing nerve fibers and terminals in the septal area of the rat was studied by using immunohistochemistry with specific antibodies to 5-HT in combination with fluorescent retrograde tracing methods. The 5-HT innervation of the septum is heterogeneous with regard to both the morphology of individual processes and the density of distribution in different parts of the septum. Three major classes of 5-HT like immunoreactive processes can be distinguished: 1) thin, convoluted fibers with small, round or elongated varicosities; 2) thick and relatively straight fibers with few varicosities; and 3) pericellular plexuses with large varicosities in close association with perikarya in the lateral septum. Three areas of the septum receive a prominent innervation by 5-HT processes: the diagonal band of Broca, the ventral part of the lateral septum, and an area bordering the medial edge of the islands of Calleja (insula magna). Whereas the two latter areas contain dense terminal networks, the diagonal band of Broca is occupied primarily by 5-HT fibers en route to other parts of the septum. Intraseptal injections of HRP or fluorescent dyes (granular blue, propidium iodide) resulted in retrograde labeling of neuronal cell bodies in several nuclei of the brainstem which are known to contain 5-HT neurons: the dorsal raphe, the median raphe, the nucleus reticularis tegmenti pontis, the raphe pontis, and the raphe magnus. Where fluorescent retrograde tracing was performed with 5-HT immunohistochemistry on the same tissue section, a prominent 5-HT containing pathway and a non-5-HT-containing pathway from the raphe nuclei to the septum were revealed. Finally, double retrograde fluorescent labeling after injections of granular blue or propidium iodide into the septum and entorhinal area respectively of the same rat revealed extensive branching of the raphe efferents. Thus, individual raphe neurons may simultaneously connect with septum and the entorhinal area, two structures essential for normal hippocampal function.

Radioautographic investigation of monoaminergic neurons: an evaluation

This review outlines the most relevant contributions currently available on the detection and ultrastructural characterization of monoaminergic neurons by radioautography after administration of radiolabeled monoamines. It includes methodological considerations and then a critical analysis of the diagnostic value of the radioautographic method for catecholaminergic and serotoninergic neurons, emphasizing in particular its recent applications to the visualization of dopaminergic axon terminals. An attempt is then made to evaluate the method in terms of specificity, sensitivity and resolution and its possibilities with regard to quantitative analysis. Lastly, its value for approaching the dynamic and metabolic properties of monoaminergic neurons is stressed.

Supra-ependymal serotoninergic nerves in mammalian brain: morphological, pharmacological and functional studies

Supra-ependymal nerves in mammals (mainly rats) have been shown to contain serotonin (5-hydroxytryptamine, 5-HT) by combined Falck-Hillarp fluorescence histochemistry, ultrastructural monoamine cytochemistry and pharmacology as well as by immunohistochemistry and autoradiography. Supra-ependymal 5-HT cells do not occur. At least in rats, virtually all supra-ependymal nerves contain 5-HT and in our opinion the occasionally described non-5-HT supra-ependymal nerve cells and their processes contribute little to the supra-ependymal nerve plexus (with the possible exception of those cells above the median eminence). The cells of origin of the supra-ependymal 5-HT nerves are situated in raphe nuclei. The axons and terminals (varicosities) contain small and large dense core vesicles in both of which 5-HT is stored. A co-transmitter has not been found among the candidates investigated so far (leu- and met-enkephalin, substance P and gamma-aminobutyric acid (GABA)). The nerves possess uptake mechanisms specific for 5-HT and possibly GABA. Occasionally desmosome-like junctions are observed between 5-HT nerve terminals and ependymal cells but no true synapses. The function of these nerves is not known. They do not appear to regulate ciliary movement, but might influence the shape of ependymal cells.

Dihydroxytryptamines as tools to study the neurobiology of serotonin

The neurotoxins 5,6- and 5,7-dihydroxytryptamine are accepted tools for "chemical degeneration" of serotonergic (5-HT) axons in the CNS (for reviews, see [11, 12, 15, 20] ). Optimum application of these substances requires knowledge of their chemical properties, disposition in the biophase and mechanism of action. Current knowledge and concepts on this issue are described and results of recent studies utilizing 5,7-DHT uptake as a tool for localizing 5-HT neurons neuroanatomically are reviewed.

Effect of morphine on 3H-thymidine incorporation in the subependyma of the rat: an autoradiographic study

Following morphine treatment, an autoradiographic study investigated the uptake of 3H-thymidine by the subependymal cells in the rat brain. 3H-thymidine was administered subcutaneously to adult, male Sprague-Dawley rats 30 minutes after saline or morphine (19 mg/kg) injection. The animals were sacrified 1 hour after 3H-thymidine administration. In some experiments the opioid antagonist, naloxone, was given alone 45 minutes before 3H-thymidine or 125 minutes before morphine treatment. Three areas of the subependyma were evaluated in terms of the percentage labeled cells and number of grains per nucleus, and a dorsal-to-ventral gradiant was described. Morphine treatment significantly increased the number of 3H-thymidine labeled subependymal cells and number of grains/nucleus within labeled cells. Examination of the distribution of grains/nucleus showed that morphine-treated animals had significantly more cells labeled with 30 or more grains than did saline-injected controls. Prior administration of naloxone blocked the increased 3H-thymidine uptake in morphine-treated animals but had no significant influence on cell proliferation when administered alone. The data are discussed in terms of morphine's possible dual influence on mechanisms which enhance cell transition from G to S phase and/or which accelerate DNA synthesis once these cells have entered the S phase of cell replication.

Immunohistochemical evidence of indolamine neurons in monkey spinal cord

In this study, we have identified over 150 cells in the adult macaque spinal cord that are immunoreactive when stained with an antibody to 5-HT by means of the PAP method. The staining was blocked by preabsorption of the primary antibody by the 5-HT-BSA conjugate (used to generate the antibody) and by 5-HT, while BSA, dopamine, norepinephrine, 5-methoxytryptamine and tryptamine preabsorption did not block staining. In addition, the Falck-Hillarp histofluorescence technique was applied to the lumbosacral cord from one of the three monkeys studied; results confirm the presence of cells with yellow fluorescence and rapid fading to brown characteristic for serotoninergic cells. The neuronal identity of these immunoreactive cells is based on light and electron microscopic morphology and the presence of synaptic terminals in contact with labeled somata and dendrites. Most of these neurons were small (10-25 micrometers) and located ventral to the central canal in lamina X. Some processes of the cells extended into the intermediate gray and the ventromedial area of the ventral horn; other processes wrapped around large blood vessels in lamina X or around the wall of the central canal. Cells were most frequent in the cervical cord (approximately 6-7/mm length of cord) and less frequent in the thoracic (1.5/mm), lumbar (3/mm), and sacral (2/mm) cord. A few cells were also found in the marginal and gelatinosa regions of the dorsal horn of the sacral cord. Examination of sections from the medulla-spinal cord junction (obex level) indicates that the spinal cells may be an extension of cell groups located near the raphe obscurus in the gray region around the IVth ventricle. The indolamine spinal cells may act as interneurons in spinal circuits, control spinal blood flow through vessel innervation, or play a role in CSF composition through central canal innervation.

Electron microscopic immunocytochemical localization of tyrosine hydroxylase in the area postrema of rat

The ultrastructural morphology and specialized neuronal, vascular, and ventricular associations of tyrosine hydroxylase-labeled neurons are examined within the area postrema of rat brain. Specific antiserum to the purified enzyme is localized throughout the rostrocaudal and dorsoventral extent of the area postrema by means of the peroxidase-antiperoxidase technique. In all regions, peroxidase immunoreactivity for tyrosine hydroxylase is distributed throughout the cytoplasm of selectively labeled neuronal perikarya and processes. The perikarya contain a large nucleus, infolded nuclear membrane, numerous cytoplasmic organelles, and form axosomatic synapses with unlabeled terminals. The majority of the labeled processes are dendrites, which contain ribosomes, microtubules, mitochondria, and scattered vesicles. These dendrites are postsynaptic to unlabeled axon terminals and show membrane specializations with other labeled dendrites and perikarya. In contrast to dendrites, peroxidase-labeled profiles clearly distinguished as axons or axon terminals are sparse and never show membrane specializations with other neuronal or nonneuronal structures within the area postrema. Numerous large processes which could be either axons or dendrites are associated with blood vessels and the ventricular surface of the area postrema. With respect to blood vessels, processes are located either in direct apposition to the external glial membrane, or less frequently, within the perivascular space. The ventricular processes are either associated with blood vessels in the subpial space or distributed among the cilia and villi at the anterior margins of the area postrema. The neuronal and nonneuronal associations of the tyrosine hydroxylase-labeled processes are consistent with a receptor or chemosensor function for catecholamines in this circumventricular organ.

Identification of serotonin and non-serotonin-containing neurons of the mid-brain raphe projecting to the entorhinal area and the hippocampal formation. A combined immunohistochemical and fluorescent retrograde tracing study in the rat brain

We have studied the localization of serotonin- and non-serotonin-containing cell bodies in the midbrain raphe nuclei that project to the entorhinal area and the hippocampal formation in the rat brain, using the technique of combined retrograde fluorescent tracing and immunohistochemistry on the same tissue section. The branching properties of these neurons were studied by retrograde double labelling using two fluorochromes which emit fluorescence with different spectral characteristics. After injections of granular blue or propidium iodide into the medial entorhinal area, retrogradely-labelled cells were found situated bilaterally in the caudal half of the dorsal raphe nucleus, the medial part of the median raphe and throughout the rostrocaudal extension of the nucleus reticularis tegmentipontis. Injections placed successively more laterally in the entorhinal area labelled progressively less cells contralaterally in the dorsal raphe and the reticular tegmental nucleus of the pons. After fluorochrome injections into the dorsal part of the hippocampal formation, retrogradely-labelled cells were found in the caudal part of the dorsal raphe, in the peripheral part of the median raphe and to a minor extent in the medial part of this nucleus, but not in the nucleus reticularis tegmentipontis. The experiments with double retrograde fluorescent tracing showed that the raphe nuclei do not send bilateral projections to the entorhinal area in spite of the fact that many of these cells are located contralateral to the injected hemisphere in single labelling experiments. Injections of the fluorochromes into the entorhinal area and hippocampal formation showed that at least 10% of the raphe cells project to both areas simultaneously. Analysis of sections incubated with antiserum to serotonin showed that a majority of the retrogradely-labelled versus serotonin-immunoreactive cells was found to vary within different parts of the individual raphe nuclei: the ventromedial part of the dorsal, the medial part of the median and the nucleus reticularis tegmentipontis being the highest. The findings indicate that both serotonin- and non-serotonin-containing neurons in the raphe innervate the hippocampal region, that these projections may be crossed but not bilateral, and that the same neuron in the raphe may influence the neural activity in the entorhinal area and the hippocampus simultaneously.

Nerve endings and pharmacological receptors in cerebral vessels

1. The cerebral vessels possess adrenergic, cholinergic, serotonergic and peptidergic innervations. 2. The cerebral vessels have alpha- and beta-adrenergic, cholinergic, serotonergic, histaminergic H1 and H2 and dopaminergic receptors whose activation by different agents causes vasomotor responses. 3. The induced effects by noradrenaline, adrenaline and serotonin are characterized by a cerebral vasoconstriction clearly manifested in man, the awake or anesthetized animal and isolated vessels. The vasoconstrictor response generally obeys an activation of specific receptors. 4. Under experimental conditions acetylcholine, histamine, dopamine and isoproterenol relax these vessels, in which cholinergic, H2-histaminergic, dopaminergic and beta-adrenergic (beta 1) receptors are implicated, respectively.

The organization of afferent projections to the midbrain periaqueductal gray of the rat

The retrograde transport technique was utilized in the present study to investigate the afferent projections to the periaqueductal gray of the rat. Iontophoretic injections of horseradish peroxidase were made into the periaqueductal gray of 22 experimental animals and into regions adjacent to the periaqueductal gray in 6 control animal. Utilization of the retrograde transport method permitted a quantitative analysis of the afferent projections not only to the entire periaqueductal gray, but also to each of its four intrinsic subdivisions. The largest cortical input to this midbrain region arises from areas 24 and 32 in the medial prefrontal cortex. The basal forebrain provides a significant input to the periaqueductal gray and this arises predominantly from the ipsilateral lateral and medial preoptic areas and from the horizontal limb of the diagonal band of Broca. The hypothalamus was found to provide the largest descending input to the central gray. Numerous labeled cells occurred in the ventromedial hypothalamic nucleus, the lateral hypothalamic area, the posterior hypothalamic area, the anterior hypothalamic area, the perifornical nucleus and the area of the tuber cinereum. The largest mesencephalic input to the periaqueductal gray arises from the nucleus cuneiformis and the substantia nigra. The periaqueductal gray was found to have numerous intrinsic connections and contained a significant number of labeled cells both above and below the injection site in each case. Other structures containing significant label in the midbrain and isthmus region included the nucleus subcuneiformis, the ventral tegmental area, the locus coeruleus and the parabrachial nuclei. The medullary and pontine reticular reticular formation provide the largest input to the periaqueductal gray from the lower brain stem. The midline raphe magnus and superior central nucleus also supply a significant fiber projection to the central gray. Both the trigeminal complex and the spinal cord provide a minor input to this region of the midbrain. The sources of afferent projections to the periaqueductal gray are extensive and allow this midbrain region to be influenced by motor, sensory and limbic structures. In addition, evidence is provided which indicates that the four subdivisions of the central gray receive differential projections from the brain stem as well as from higher brain structures.

Immunohistochemical studies on the serotonergic innervation of the pia mater

The direct innervation of the pial blood vessels by serotonin neurons has been demonstrated with a modified peroxidase-antiperoxidase technique in the mammalian central nervous system. The pia mater covering the ventrolateral surface of the medulla oblongata is innervated by numerous varicose serotonin fibers originating from the serotonin neurons of the lower brainstem. Scattered serotonin fibers were observed in the pia mater in every part of the brain and spinal cord.

Aminergic neurons: state control and plasticity in three model systems

Aminergic neurons have particular functions in many systems, and in this review their role is discussed and compared in three systems: those parts of the central nervous system controlling sleep and waking in the cut; the superior cervical ganglion: and the isolated nervous system of Aplysia. In the cat the aminergic neurons are most important in a waking state during which time external information is received, processed, and can be retrieved, and during which time habituation and sensitization occur. Aminergic neurons appear to have similar roles in state control in plasticity in both the Aplysia nervous system and the superior cervical ganglion. The striking similarities in the role of aminergic neurons in these three systems support the speculation that aminergic neurons have uniquely important roles in regulation of the plastic properties of neurons.

Emergence of supraependymal cells in rat third ventricle after administration of p-chloroamphetamine

Administration of single or multiple 10 mg/kg doses of parachloroamphetamine hydrochloride, a serotonin-neurotoxic drug, to adult male rats, leads to emergence of clusters of supraependymal cells in the third ventricle above the median eminence. Transmission EM of specimens previously examined with scanning electron optics reveals that the cell clusters have characteristics of neurons. Two weeks after the initial drug injection, the ventricular surfaces of some animals lack the cell clusters, suggesting a "transience" in their appearance. The neuron-like elements may appear in response to alterations in brain serotonin levels, and their presence suggests an ability of adult nerve cells to migrate in response to chemical changes in neural tissue or cerebrospinal fluid.

Supraependymal cell clusters in the rat brain

The histological patterns of supraependymal cell clusters (CC) in rats of different ages (untreated, androgenized, and treated with monosodium glutamate) were investigated with light (LM)-, scanning- and transmission electron microscopy (SEM, TEM). These clusters were a frequent but not a constant finding. In 18 day- and older embryos, CC were always found in the recess of the olfactory bulb immediately prior to its obliteration. All other CC appear in the infundibular recess between the 3rd and the 6th postnatal day. Independent of age, all cell clusters exhibit small aggregates of subependymal tissue protruding through the ependyma. Both neurons (light cells) and neuroglia (dark cells) were found in the CC. By use of SEM, in the region of the infundibular recess it is possible to distinguish four forms of supraependymal cell clusters according to localization, size, number of cells, and presence of intraventricular axons. CC may be 1) receptors or have an additional secretory function; 2) Manifestations of a pathological type of reaction of the ventricular wall; 3) possible excrescences of the neural matrix, or 4) modifications of the ventricular wall in relation to the obliteration of the ventricular recesses. The first two interpretations are not tenable based on the present observations.

The ventricular surface of the monkey mediobasal hypothalamus. A scanning electron microscopic study

The ventricular lining of the infundibular recess of monkey brains of both sexes was investigated with SEM. Based on the distribution patterns of surface profiles regional differences in the tanycytic ependyma are described. Variations in the number of surface profiles were observed in females during the ovarian cycle. These variations may be an indication not only of absorptive and/or secretory activities, but may also be related to cell membrane redistribution and membrane turnover in dependence of endocrine factors. In the males such variations were not registered. An essental sexual dimorphism does not seem to exist in the tanycytic ependyma of the monkey, only the reaction pattern is different. Supraependymal macrophages (SEC) were more numerous in the ovulating females than in the males or menstruating females. In addition to these SEC some monkeys had a second SEC type which was arranged in sheets. These are interpreted to be glioses. A variable number of axons can also be found in the lower regions of the infundibular recess. Their morphology suggests the presence of several types of axons, but they do not appear to constitute an organized network. No significant sexual differences were registered in relation to the axons.

The suprahabenular recess in the rat: a quantitative study of ciliated cells, supraependymal cells and some specific features of supraependymal fibres

Using light microscopy, scanning and transmission electron microscopy, an extensive quantitative analysis of ciliated ependymal cells, supraependymal cells and supraependymal fibres in the suprahabenular recess of twenty-two adult Wistar rats was carried out. In addition, six animals were used to determine the origin of supraependymal fibres and their relationship to ependyma. The findings were as follows: ciliated cells-mean 3.32 +/- 0.26 per 1000 micron 2 (range 0-10); supraependymal cells-5.91 +/- 1.37 per recess (range 2-16). Average number of cilia per cell in different animals ranged from 31 to 33. Sex did not influence the density of ciliated ependyma or supraependymal cells. Evidence was obtained that some supraependymal fibres are closely associated with aberrant commissural fibres. The supraependymal fibres enter the suprahabenular recess and form gap junctions, and other types of junctions, with subjacent ependyma.

Biochemistry and ultrastructure of serotonergic nerve endings in the lobster: serotonin and octopamine are contained in different nerve endings

In this article we report that the distribution of serotonin in the lobster nervous system parallels the distribution of octopamine and that the same tissues that contain endogenous serotonin can synthesize it from tryptophan. Octopamine and serotonin are highly concentrated in a neurosecretory region of the second thoracic roots in association with a group of neurosecretory cells. The roots possess separate high-affinity uptake systems for both serotonin and tryptophan. Radioactive serotonin, accumulated in tissues during incubations with either tritiated serotonin or tritiated tryptophan, can be released, in a calcium-dependent manner, by depolarization with potassium. A detailed morphological examination of the second thoracic roots shows four distinct categories of nerve endings in the vicinity of the neurosecretory cells. Octopamine is synthesized in one of these types of endings and serotonin in another. The high-affinity uptake systems for serotonin and tryptophan are found only in association with the endings that make serotonin. These endings and all the biochemical parameters of serotonin metabolism in the roots are selectively destroyed by previous injection of animals with the neurotoxin 5,7-dihydroxytryptamine.

Autonomic nerves in the mammalian choroid plexus and their influence on the formation of cerebrospinal fluid

The choroid plexuses of all ventricles receive a well-developed adrenergic and cholinergic innervation reaching both the secretory epithelium and the vascular smooth muscle cells. Also peptidergic nerves, containing vasoactive intestinal polypeptide, are present but primarily associated only with the vascular bed. A sympathetic inhibitory effect on the plexus epithelium has been indicated in determinations of carbonic anhydrase activity and by studies of various aspects of active transport in isolated plexus tissue. Pharmacological analysis in vitro has shown the choroidal arteries to possess both vasoconstrictory alpha-adrenergic and vasodilatory beta-adrenergic receptors. Electrical stimulation of the sympathetic nerves, which originate in the superior cervical ganglia, induces as much as 30% reduction in the net rate of cerebrospinal fluid (CSF) production, while sympathectomy results in a pronounced increase, about 30% above control, in the CSF formation. There is strong reason to believe that the choroid plexus is under the influence of a considerable sympathetic inhibitory tone under steady-state conditions. From pharmacological and biochemical experiments it is suggested that the sympathomimetic reduction in the rate of CSF formation is the result of a combined beta-receptor-mediated inhibition of the secretion from the plexus epithelium and a reduced blood flow in the choroid plexus tissue resulting from stimulation of the vascular alpha-receptors. The choroid plexus probably also represents an important inactivation site and gate mechanism for sympathomimetic amines, as evidenced by considerable local activity of catechol-O-methyl transferase and monoamine oxidase, primarily type B. The CSF production rate is also reduced by cholinomimetic agents, suggesting the presence of muscarinic-type cholinergic receptors in the choroid plexus.

Evidence for the coexistence of serotonin and substance P in single raphe cells and fiber plexuses: combined immunocytochemistry and autoradiography

Combined immunocytochemistry and autoradiography after in vivo injections of monoclonal antibody to Substance P and 3H-serotonin provide convincing evidence that the soma and processes of a single neuron can contain serotonin and immunoreactivity with another putative peptide transmitter Substance P in a single permanent preparation suitable for study with light and electron microscopy. The serotonin-Substance P cells have uptake systems for low molarity 3H-serotonin that are not affected by reserpine treatment. They are sensitive to serotonin uptake inhibitors and monoamine oxidase inhibitors and the cells are destroyed by 5,6-dihydroxytryptamine. They contain endogenous stores of serotonin detectable by the Falck-Hillarp technique and microspectrofluorimetry and immunofluorescence by antibody to serotonin. Their Substance P content is identified by specific binding with monoclonal antibody, and by animal antisera against Substance P. The raphe pallidus neurons are a mixed population -- some cells have a majority of Substance P immunoreactivity with little detectable serotonin, others have considerable quantities of both Substance P immunoreactivity and serotonin, and still others have only serotonin but are innervated by Substance P immunoreactive processes. These subtle differences in neuronal content of the two compounds are made more obvious by avoiding the use of colchicine, a drug known to inhibit fast axoplasmic transport. Such differences may be an expression of the dynamic or cyclic functions of neurons with multiple putative transmitter substances. Fluctuating levels of one or both substances may depend upon parameters of rhythm, demands for one or the other mediator during specific types or phases of activity. The fundamental scheme of simultaneous injection of multiple labels, one radioactive, another a specific characterized monoclonal antibody for the detection of multiple, separate transmitter systems opens vast avenues for future investigation.

Retrograde axonal transport following injection of [3H]-serotonin into the olfactory bulb. II. Radioautographic study

Radioautography was used to study the intraneuronal distribution of [3H]-serotonin (5-HT) and/or its derivatives selectively taken up by the olfactory bulb (OB) serotonergic terminals and subsequently transported to their parent cell bodies in the midbrain raphe nuclei. This was done 24 h after injection of [3H]5-HT into the main OB of rats either pretreated or not with monoamine oxidase (MAO) inhibitor. A prior mechanical obstruction of the rostral ventricular cavities prevented diffusion of the tracer towards cerebrospinal fluid. Heavily labelled nerve cell bodies were found mainly in the ipsilateral raphe dorsalis nucleus (RDN) and to a lesser extent in the raphe centralis nucleus. The radioautographic reaction often extended to dendritic processes while sparing the nucleus. A diffuse reaction was also observed but limited to the raphe area. The supraependymal 5-HT fibers were found to be free of labelling. Neither local destruction of catecholaminergic terminals with 6-OHDA, nor absence of MAO inhibition, impaired this radioautographic pattern, while destruction of serotonergic terminals with 5,6-dihydroxytryptamine in OB resulted in the disappearance of labelled axonal varicosities and neurons in the OB and the RDN respectively. At the electron microscopy level, labelled cell bodies in the RDN were medium-sized (12-15 micrometers). Silver grains were localized mainly on mitochondria and, to a lesser extent, on lysosomes and endoplasmic reticulum but spared the nucleus and the nucleolus. Silver grains were also found near the nuclear membrane and outside the neuronal membrane. The observation of heavy metal impregnated thick sections confirmed the preferential localization of silver grains on mitochondria with or without inhibition of MAO. These results could account for the subcellular compartments involved in the retrograde axonal transport of [3H]5-HT and its subsequent degradation and/or dendritic release.

Fluorescence and electron microscopical observations on the amine-accumulating neurons of the cebus monkey retina

The organization of the Cebus monkey regina was analysed after the intraocular injection of 5,6-dihydroxytryptamine. This amine was taken up not only by the previously known dopaminergic neurons, but also by a set of indoleamine-accumulating neurons, whose processes are confined to the inner plexiform layer. The synaptic contacts of the dopaminergic neurons were analysed in the electron microscope after the processes of the indoleamine-accumulating neurons were destroyed by the intravitreal injection of the neurotoxic indoleamine, 5,7-dihydroxytryptamine. The subsequent injection of 5,6-dihydroxytryptamine induces certain changes in the dopaminergic neurons which accumulate the substance: electron-dense cores appear in the synaptic vesicles, and increased electron-density of mitochodrial and cellular membranes is often observed. The dopaminergic neurons were found to be presynaptic to amacrine cell perikarya and processes in the inner plexiform layer. In the outer plexiform layer they were presynaptic to both bipolar and horizontal cells, but they did not contact photoreceptors. The dopaminergic neurons received synapses only in the inner plexiform layer, from amacrine cell processes. It is inferred that in Cebus most dopaminergic neurons belong to a special class of retinal neuron, the interplexiform cells, which appear to transmit information centrifugally within the retina, from the inner to the outer plexiform layers. There are considerable similarities between the synaptology of the dopaminergic interplexiform neurons in the Cebus monkey and the goldfish retina, and the function of interplexiform neurons may therefore be similar in these two species.

Cells of origin of the afferent fibers to the median eminence in the rat

The cell bodies of origin of axons terminating in the median eminence have been identified by retrograde axonal transport of horseradish peroxidase (HRP) or 125I-wheat germ agglutinin (WGA). The tracers were injected into the median eminence by pressure and under direct visual control, using a ventral surgical approach. The retrogradely labeled cells are exclusively located within the hypothalamus. The most heavily labeled cells are parvocellular neurons in the arcuate nucleus, the periventricular area, the medial part of the paraventricular nucleus, and the rostral paraventricular nucleus; a few cells are also located in the rostral part of the preoptic area immediately lateral and dorsal to the organum vasculosum of the lamina terminalis (OVLT). Less heavily located cells are found in the magnocellular neurosecretory nuclei, including the lateral parts of the paraventricular and rostral paraventricular nuclei, the supraoptic nucleus, and the accessory magnocellular nuclei. Retrogradely labeled cells are not found in the ventromedial hypothalamic nucleus, except for a few lightly labeled cells in the posterior division of the nucleus. However, if the injected tracer spreads into the arcuate nucleus, labeled cells are present throughout the ventromedial nucleus. Labeled cells are not found in other parts of the hypothalamus, including lateral and posterior portions of the preoptic area, the anterior hypothalamic area, and the suprachiasmatic nucleus, or in catecholaminergic cell groups of the midbrain, pons, and medulla. Control injections of HRP into the posterior pituitary and the ventromedial nucleus produce patterns of cell labeling which are very distinct from that seen with injections into the median eminence. Following injections into the posterior pituitary, the cells of the magnocellular neurosecretory nuclei are all heavily labeled, but small cells in the parvocellular neuronal groups are not labeled. Direct injections into the ventromedial nucleus resulted in labeled cells in widespread parts of the hypothalamus, as well as in the bed nucleus of the stria terminalis and the lateral septum, in parts of the amygdaloid complex and the subiculum, and in several cell groups in the midbrain, pons, and medulla.

Resting and K+-evoked release of serotonin and norephinephrine in vivo from the rat and cat spinal cord

Using a spinal superfusion system, the release of serotonin (5-HT) and norepinephrine (NE) from rat and cat spinal cord was measured using a high performance liquid chromatograph with an electrochemical detector. Resting levels of 5-HT and NE in the chloralose-urethanized rat were of the order of 0.8 and 0.4 ng/ml, respectively. Potassium (40 or 80 mM, in excess) produced a dose-dependent increase in the levels of both monoamines in the superfusate. The substitution of cobalt blocked this evoked release. Depletion of spinal 5-HT or NE by the prior intrathecal administration of 5,6-dihydroxytryptamine (5,6-DHT) or 6-hydroxydopamine (6-OHDA) reduced the resting levels of 5-HT and NE, respectively, and prevented the increase produced by excess potassium. These experiments indicate the ability to reliably collect and measure monoamines which overflow from spinal terminals and which reflect the degree of activation of the spinal monoamine system.

Neurotransmitter receptor binding in bovine cerebral microvessels

Purified preparations of microvessels from bovine cerebral cortex contain substantial levels of alpha-adrenergic, beta-adrenergic, and histamine 1 receptor binding sites but only negligible serotonin, muscarinic cholinergic, opiate, and benzodiazepine receptor binding. Norepinephrine and histamine may be endogenous regulators of the cerebral microcirculation at the observed receptors.

Neurons and synaptic patterns in the deep layers of the superior colliculus of the cat. A Golgi and electron microscopic study

Golgi and electron microscopic observations were made on the neurons in the deep layers (below the stratum opticum) of the cat superior colliculus. Large neurons, 35-60 micrometers in somal diameter, occur mainly in the lateral two-thirds of the colliculus. They have numerous somatic and dendritic spines and receive a large number of axon terminals (bouton covering ratio: more than 70%). The medium-sized neurons (20-30 micrometer), with a moderate number of dendritic spines, show a lower bouton covering ratio (25-30%). The ratio for small neurons (8-15 micrometers), with very few dendritic spines, is less than 10%. The medium-sized and small neurons are distributed throughout the colliculus and show marked variability in the dendritic arrangement. Seven different types of axon terminals were distinguished: types I, II, V, and VII form asymmetrical and types III, IV, and VI symmetrical synapses. Type I terminals represent small boutons containing predominantly spherical vesicles, and are in contact mainly with small dendritic profiles. Type II terminals are medium-sized and slender, contain a mixture of spherical and slightly oval vesicles, and make synaptic contacts with small to medium-sized dendrites and somatic spines. This type of terminal is occasionally presynaptic to vesicle-containing dendrites (type VIII). Type III terminals are small, contain flattened vesicles predominantly, and are presynaptic to a wide variety of neuronal elements in the deep layers of the superior colliculus. Type IV terminals are represented by medium to large-sized boutons that contain pleomorphic vesicles and make synaptic contacts chiefly with the large neurons. Type V and VI terminals exhibit a quite dense axoplasmic matrix and mainly contact the large neurons. Type VII terminals are often in the form of boutons en passant and contain numerous large granular vesicles. Pleomorphic vesicle-containing dendrites (type VIII terminals) are also observed to participate in the axodendrodendritic serial synapses.

Ultrastructural characterization of axonal endings in the substantia gelatinosa which take up [3H]serotonin

Serotonergic axonal endings in layers I and II of the dorsal horn of the medulla were identified by autoradiography. In adult cats, pretreated with a monoamine oxidase inhibitor, tritiated serotonin ([3H]5-HT) was topically applied onto the surface of the caudal medulla. Light autoradiographs from 1 micrometer sections demonstrated silver grains in both layers I and II. In EM autoradiographs, two categories of axonal endings were labeled by [3H]5-HT uptake: dome-shaped endings which form a single synapse and scalloped endings which form multiple synapses. Each category was further divided into several types based on morphological criteria. The [3H]5-HT-labeled endings synapse primarily on small caliber dendritic shafts and spines, with the dome-shaped endings forming both symmetrical and asymmetrical synapses and the scalloped endings forming only asymmetrical synapses. Dome-shaped endings were most common and two types were found in layers I and II while a third type was found only in layer II. Layer I contained a single type of scalloped ending while layer II contained three types of scalloped endings. In a series of experiments designed to provide another approach to identifying serotonergic endings, 5,6-dihydroxytryptamine, a serotonin neurotoxin, was either topically applied onto the caudal medulla or injected into the fourth ventricle. Following treatment with the neurotoxin, blackened degenerating dome-shaped and scalloped endings similar to those labeled in the [3H]5-HT uptake experiments were found in layers I and II. The presence of serotonergic endings in layer I suggests that some of these endings synapse on the dendrites of layer I projection neurons where they may inhibit the output of the projection neuron directly. Serotonergic endings in layer II may modulate the activity of layer II interneurons by synapsing directly on these interneurons. The interneurons in layer II may function by mediating the transfer of inputs from primary endings in these layers to layer I projection neurons.

Serotonergic intraventricular axons in the habenular region. Phagocytosis after induced degeneration

Both intracerebroventricular injection of 5,7-dihydroxytryptamine and electrolytical midbrain-raphe lesions in rats induce degeneration of supraependymal axons (SEAs) normally occurring in large numbers upon the ependyma of the medial habenular nucleus and habenular commissure. It is concluded that the intraventricular axon plexus in the epithalamic region is comprised of serotonergic (5-HT) fibers originating in the dorsal and/or median raphe nuclei. Besides the elimination of SEAs, conspicious features were a marked reduction in the number of cilia, degenerative signs in the habenular ependyma, and the emergence of large numbers of supraependymal macrophages, being most probably involved in phagocytosis of the axonal debris. It is suggested that the nucleus habenulae medialis is influenced serotonergically by the midbrain raphe via (1) a direct projection upon its neurons and (2) an indirect projection by way of the intraventricular axon plexus. The origin of intraventricular macrophages is discussed in relation to recent data in the literature.

The effects of sleep on neurons in isolated cerebral cortex

Slabs of cat parietal cortex with some 2 mm of underlying white matter were surgically isolated from the rest of the nervous system, without interference with the superficial blood supply. Wire micro-recording electrodes were inserted into the isolated cortex; bone, muscle and skin wounds were repaired and the animal allowed to recover from anaesthesia. The adequacy of surgical isolation was examined histologically 8--12 weeks after operation. Only one of the six preparations reported here showed surviving neural connections with the rest of the brain. Soon after operation, spontaneous bursts of neural activity appeared within the isolated area. These became more frequent until neural discharge was continuous but irregular. Our records were made from this time onwards. The interval distributions obtained from neurons within the isolated area did not differ significantly from log-normal curves. When the unrestrained animal fell asleep, there was no significant alteration in the model interval or geometric standard deviation of interval distributions recorded from cells in isolated cortex. The interval distributions of neurons in isolated cerebral cortex resembled those of neurons in the intact cortex of an alarmed animal. It is concluded that the reduction of modal interval that is shown by neurons in intact cortex when an animal falls asleep is probably due to the neural influence of infracortical structures.

Serotonin neurons project to small blood vessels in the brain

Electrolytic lesions of the nucleus raphe dorsalis and medianus reduce the concentration of serotonin (5-hydroxytryptamine) within rat brain intraparenchymal blood vessels. The concentration of serotonin within these vessels increases or decreases after the administration of drugs that modify the biosynthesis and degradation of serotonin or destroy nerve terminals by an uptake-dependent mechanism. These studies provide evidence for the existence of a serotonin-containing pathway seemingly analogous to the neuronal projection that terminates on small parenchymal blood vessels from noradrenergic neurons of the locus coeruleus.