Indolemine-containing nerve terminals in the rat median eminence

Immunodetection of the serotonin transporter protein is a more valid marker for serotonergic fibers than serotonin

Tracking serotonergic pathways in the brain through immunodetection of serotonin has widely been used for the anatomical characterization of the serotonergic system. Immunostaining for serotonin is also frequently applied for the visualization of individual serotonin containing fibers and quantification of serotonin positive fibers has been widely used to detect changes in the serotonergic innervation. However, particularly in conditions with enhanced serotonin metabolism the detection level of serotonin may lead to an underestimation of the true number of serotonergic fibers. The serotonin transporter (SERT) protein, on the other hand, is less liable to metabolism and for that reason we hypothetized that SERT immunostaining is a more stable marker of serotonergic fibers. Rats were pretreated with monoamine oxidase (MAO) inhibitor and compared with placebo treated rats. Brains were double immunostained for serotonin and SERT protein and colocalization was quantified in several brain areas by confocal microscopy. In comparison with untreated rats, MAO inhibitor treated rats had a significantly higher number (almost 200% increase) of serotonin immunopositive fibers whereas no difference was observed in the number of the SERT positive fibers. Colocalization between serotonin and SERT positive fibers was close to 100% in MAO inhibitor treated animals but only 30% in untreated rats. We conclude that the rapid metabolism of serotonin leads to an underestimation of immunodetected serotonergic fibers and that in many instances, SERT immunostaining may be a better indicator of serotonergic fibers.

Ultrastructural, biochemical and electrophysiological changes induced by 5,6-dihydroxytryptamine in the CNS of the snail Helix pomatia L

The serotonin neurotoxin, 5,6-dihydroxytryptamine (5,6-DHT), was injected into the body cavity of snails. Changes induced in the central nervous system (CNS) by the neurotoxin were studied by morphological, electrophysiological and biochemical techniques for up to 90 days following injection. The neurotoxin induced a variety of ultrastructural alterations during the early phase (1st to 6th days) following treatment. On day 6 after treatment, membranous structures first appeared in the synaptic-like areas and apparently migrated to cell bodies where they were detected by day 14. Their number increased with time. Neurotoxin-induced structural alterations were found in neuronal processes and cell bodies of the serotonergic metacerebral giant cells injected intracellularly with horseradish peroxidase and in serotonin immunoreactive axons. These findings suggest that the toxin-induced alterations are rather selective for the serotonin-containing neuronal elements. The neurotoxin decreased the concentration of 5-HT in and [3H]5-HT uptake into cerebral and pedal ganglia, with a maximum effect between the 3rd and 5th day following drug administration. 5-HT levels and 5-HT uptake returned to normal by 19-21 days after treatment. The concentration of dopamine and of [3H]DA uptake capacity were reduced between 3-5 days after injection of 5,6-DHT by 6-7 days following treatment. The transmission from identified serotonergic synapses to targets was reduced beyond day 5 after 5,6-DHT administration. By 15 days after treatment, synaptic transmission between the metacerebral giant cell (MGC) and buccal followers was blocked. Transmission recovered by day 21 after 5,6-DHT. Comparison of the time-course of functional and structural recovery indicates that while functional recovery takes place within 21 days after treatment, certain structural alterations, e.g. the membranous structures and dense particles, remain in the nerve fibres and cell bodies. These may serve as specific intracellular markers of the serotonin-containing neuronal elements long after functional recovery from the effect of 5,6-DHT.

Immunocytochemical localization of serotonin in the rat dentate gyrus following raphe transplants

The ultrastructural features of the serotoninergic innervation of the rat dentate gyrus in normal adults and in animals receiving raphe nuclear area transplants was investigated using an antibody to serotonin (5-HT). Neonatal rats received a lesion of the fimbria-fornix and entorhinal cortex. Three days later, a portion of embryonic (E-16-18) raphe nuclear area was transplanted to the entorhinal cavity and the animals were allowed to survive for 60 days. Animals were processed for the immunocytochemical localization of 5-HT using the peroxidase-antiperoxidase method. Light microscopic observation showed that 5-HT-containing fibers from transplanted raphe neurons densely innervated the hilar and molecular zones of the dentate gyrus. Electron microscopic analysis showed that 5-HT immunoreactivity was contained only in axons and axon varicosities. There were no differences in the ultrastructural characteristics of axons and axon terminals between normal animals and those which had received raphe transplants. A mixture of both conventional synaptic junctions and non-synaptic axonal swellings were found in both groups.

Origin and termination of dorsal raphe-median eminence projection

The origin and termination of the dorsal raphe (DR)-median eminence (ME) projection were studied by combining well-limited small electrolytic lesions of different parts of the DR, the axonal degeneration and the serotonin content in the median eminence. Our results show that the rostral third of the DR is a source of serotonergic innervation to the ME and that the rest of the nucleus does not contribute to this projection. The light microscopy study revealed that degenerating fibers occupy the internal layer of the ME. These results were confirmed by electron microscopy which in addition showed degenerating terminals.

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.

Localization of serotonin-like immunoreactivity in the central nervous system and pituitary of the rat, with special references to the innervation of the hypothalamus

The localization and distribution of serotonin (5-HT) has in the rat brain been studied with the indirect immunofluorescence technique of Coons (1958) using a newly developed, specific antibody to 5-HT. This paper contains a detailed description of the distribution of 5-HT in the hypothalamus and hypophyis and a brief report on the localization of 5-HT containing cell bodies. Principally, 5-HT perikarya were present in neuron systems with a distribution similar to that revealed by the mapping of Dahlström & Fuxe (1964) with the formaldehyde-induced-fluorescence technique (FIF). However, in addition to the nine areas originally described, several other areas in the mesencephalon and rhombencephalon appeared to contain 5-HT cell bodies. In the hypothalamus no 5-HT positive neuronal perikarya could be observed. However, in the infundibulum and in the median eminence some 5-HT positive tanycytes and mast cells were observed. Serotonergic fibers and terminals are present throughout the hypothalamus. Particularly dense plexus of the fibers and terminals have been observed in the lateral hypothalamic area, nucleus mamillaris medialis, the nucleus perifornicalis, the nucleus suprachiasmaticus and the nucleus ventromedialis hypothalami. All remaining hypothalamic areas demonstrate various densities of 5-HT immunoreactive nerve fibers. In the pituitary 5-HT-like immunoreactivity have been found in the pars nervosa.

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.

Quantative radioautographic light and electron microscopic analysis of the localization of monoamines in the median eminence of the rat

Serotonin containing structures in the median eminence of the rat have been studied by quantitative light and electron microscopic radioautography following intraventricular infusion of tritiated 5-hydroxytryptophan. One hour after injection of the tracer the highest density of silver grains was recorded in the ependymal and external zones, especially in the lateral palisade zone. The proportion of labelled neurosecretory terminals was also larger in the lateral palisade zone (29%) as compared with the medial palisade zone (13%), although the mean number of developed silver grains per one terminal was higher in the latter. On the average, 16% of neurosecretory terminals sequestered radiolabelled 5-hydroxytryptophan in the external zone of the rat median eminence. It is suggested that serotonin, like catecholamines, is discharged from neurosecretory terminals localized in the external zone and via the portal circulation affects the function of the anterior pituitary. The sites of origin of serotoninergic structures of the median eminence as well as the possible role of monoamine (catecholamine and indolamine) neurohormones in a dual peptidergic and monoaminergic control of anterior pituitary functions are discussed.

Serotoninergic synapses on ependymal and hypendymal cells of the rat subcommissural organ

The nervous input to the subcommissural organ (SCO) of the rat has been investigated with Falck-Hillarp fluorescence histochemistry and electron microscopical techniques. Previous fluorescence histochemical observations of a dense plexus of serotoninergic nerve fibres in relation to the basal SCO were confirmed. Electron microscopically, unmyelinated fine varicose axons ranging in size from 0.1--0.6 micrometer were observed to penetrate into the SCO hypendyma. Boutons and presynaptic varicosities filled with a diversity of round and elongated clear vesicles, and occasional large dense cored vesicles establish asymmetric (Gray's type I) synaptic contacts with the basal processes and somata of the SCO ependymal and hypendymal cells. A typical varicosity in synaptic contact with an SCO cell contains a population of approximately 85% clear, elongated vesicles 45 X 60 nm in diameter, 15% clear, round vesicles 50 nm in diameter, and 1--2% large dense cored vesicles with a vesicle diameter of about 85 nm and a dense core diameter of 50--55 nm. The mean length of the postsynaptic membrane specialization was found to be 0.5 micrometer. Experiments with specific neurotoxic drugs revealed that the nerve terminals in synaptic contact with the SCO cells are identical to the fibres of the serotoninergic plexus identified fluorescence histochemically. Thus, an intraventricular injection of either 5,6-dihydroxytryptamine or 5,7-dihydroxytryptamine induced typical degenerative changes in most of the boutons in synaptic contact with the SCO cells, and also a disappearance of the yellow fluorescent nerve plexus. It is concluded that the SCO of the rat receives a dense plexus of serotonin-containing nerve fibres which form typical synaptic contacts with the specialized ependymal cells of the SCO and that these fibres may constitute the only direct nervous input to the organ. The degeneration of the serotoninergic synapses elicited a long-lasting, pronounced increase in the secretory activity of the SCO. Despite long survival times after the treatment with neurotoxic drugs, we found no evidence of regenerative restitution of the serotoninergic innervation nor normalization of the secretory activity of the SCO. The observed inverse relationship between secretory activity and serotoninergic innervation is in line with previous observations which indicate that the 5-hydroxytryptamine input to the SCO ependymal and hypendymal cells exerts a powerful inhibition on their protein synthetic machinary.

Serotonin neurons of the midbrain raphe: ascending projections

The ascending projections of serotonin neurons of the midbrain raphe were analyzed in the rat using the autoradiographic tracing method. Axons of raphe serotonin neurons ascend in the ventral tegmental area and enter the medial forebrain bundle. A number of fibers leave the major group to ascend along the fasciculus retroflexus. Some fibers enter the habenula but the majority turn rostrally in the internal medullary lamina of the thalamus to innervate dorsal thalamus. Two additional large projections leave the medial forebrain bundle in the hypothalamus; the ansa peduncularis-ventral amygdaloid bundle system turns laterally through the internal capsule into the striatal complex, amygdala and the external capsule to reach lateral and posterior cortex, and another system of fibers turns medially to innervate medial hypothalamus and median eminence and form a contrelateral projection via the supraoptic commissures. Rostrally the major group in the medial forebrain bundle divides into several components: fibers entering the stria medullaris to terminate in thalamus; fibers entering the stria terminalis to terminate in the amygdala; fibers traversing the fornix to the hippocampus; fibers running through septum to enter the cingulum and terminate in dorsal and medial cortex and in hippocampus; fibers entering the external capsule to innervate rostral and lateral cortex; and fibers continuing forward in the medial olfactory stria to terminate in the anterior olfactory nucleus and olfactory bulb.

Localization of serotonin within tanycytes of the rat median eminence

Formaldehyde and glyoxylic acid histochemical methods were employed to examine monoamine fluorescence of the rat median eminence. Tanycytes of the median eminence contained a yellow histofluorescence which was verified with microspectrofluorometry as due to the presence of serotonin. Catecholamine-containing varicosities, arranged in linear profiles throughout the depth of the median eminence, were observed. These linear profiles appeared to follow the contours of serotonin-containing tanycytes. Organ-culture experiments supported the hypothesis that the serotonin associated with tanycytes is localized within the tanycytes and does not arise from an extrahypothalamic source of nerve terminals. These data provide evidence that a tanycytic catecholamine-indoleamine morphological juxtaposition occurs in a manner reminiscent of that of another circumventricular organ, the pineal.

Monosodium glutamate induced lesions of the arcuate nucleus. I. Endocrine deficiency and ultrastructure of the median eminence

Monosodium glutamate was administered daily on days 5 through 10 postnatally at a dose of 2.5 mg/gm body weight. Counts of remaining perikarya in the arcuate nucleus of adult mice indicated approximately an 80% decrease in the number of perikarya. The arcuate lesion resulted in endocrine deficits; reporductive capacity was reduced, animals were smaller in stature and obese, and the weights of the anterior pituitary, ovaries and testes were significantly decreased while adrenals were unaffected. Light microscopic studies revealed no significant changes in thickness or general histological appearance of the median eminence. At the electron microscope level, there were no alterations in the number of nerve terminals or dense core vesicles per unit area in the contact zone. These observations suggest that afferents to the median eminence from the arcuate nucleus may form a relatively small portion of its total nerve terminal population.

Specific effect of 5,6-dihydroxytryptamine on the monoamine fluorophore of the frog's gustatory cells

A specific formaldehyde-induced yellow fluorescence, suggesting the presence of serotonin-like monoamine has been demonstrated in the gustatory cells of the frog. The fungiform papillae of frogs were examined fluorescence-histochemically after intraperitoneal injection of 5,6-dihydroxytryptamine. The results indicated that the fluorophore of gustatory cells was affected selectively by the drug injection: the yellow fluorescence was transiently enhanced 3 hours after the drug injection, thereafter being reduced rapidly. The effect of 5,6-dihydroxytryptamine was long-lasting with the reduction of the yellow fluorophore persisting at least for the experimental duration of 14 days. A single injection of 6-hydroxydopamine induced a complete depletion of noradrenaline fluorescence from adrenergic nerve terminals, while the fluorescence of gustatory cells was not affected by a high dose of the drug. The present results with pharmacologic treatments further support the view that the gustatory cell of the frog contains a serotonin-like monoamine.