Serotonergic Fibers Research Articles

Noradrenergic and serotonergic fibers innervate complementary layers in monkey primary visual cortex: an immunohistochemical study.

Antisera directed against human dopamine beta-hydroxylase or serotonin were used to characterize the noradrenergic and serotonergic innervation patterns within the primary visual cortex of the squirrel monkey. The noradrenergic and serotonergic projections exhibit a high degree of laminar complementarity: layers V and VI receive a dense noradrenergic projection and a very sparse serotonergic projection, whereas layer IV receives a very dense serotonergic projection and is largely devoid of noradrenergic fibers. In addition, the noradrenergic fibers manifest a geometric order that is not so readily apparent in the distribution of serotonergic fibers. These patterns of innervation imply that the two transmitter systems affect different stages of cortical information processing--the raphe-cortical serotonergic projection preferentially innervates the spiny stellate cells of layers IVa and IVc, whereas the ceruleo-cortical noradrenergic projection innervates pyramidal cells.

Modulation of the hippocampal α-adrenoceptor population by lesion of the serotonergic raphe-hippocampal pathway in rats

Electrolytic lesion of the ascending serotonergic fibers in the median raphe nucleus or in both the median raphe nucleus and dorsal raphe nucleus caused after 18 days more than 80% depletion of serotonin in the hippocampus and frontal cortex, respectively, without affecting norepinephrine and acetylcholine contents. α 1-Adrenoceptor binding of ( 3H) WB-4101 was increased in the hippocampus but not in the frontal cortex. Scatchard analysis revealed that the increase in ( 3H) WB-4101 binding in the lesioned hippocampus was the result of an elevated density of α 1-adrenergic receptors of about 65%. This phenomenon began 8 days postlesion and persisted for at least 90 days postlesion. Similar qualitative and quantitative results were obtained following chemical lesion of the serotonergic cells of origin in the median raphe nucleus with 5,7-dihydroxytryptamine. Selectivity of the phenomenon was further demonstrated as β-adrenoceptor binding with ( 3H) dihydroalprenolol and cholinergic muscarinic receptor binding with ( 3H) dexetimide were not significantly affected in the hippocampus. By comparison, when norepinephrine in the hippocampus was depleted by more than 90% by bilateral lesion of the ascending noradrenergic fibers with 6-hydroxydopamine (18 days), the α 1-adrenoceptor number was significantly increased by only about 20% while the β-adrenoceptor number was enhanced by 40%. The area-selective increase in α 1-adrenoceptor number in the hippocampus in the presence of unchanged norepinephrine content and in the absence of serotonin probably signifies that serotonin actively participates in the modulation of the noradrenergic receptor population.

Noradrenaline inhibits central serotonin release through alpha 2-adrenoceptors located on serotonergic nerve terminals.

The effect of noradrenaline on the depolarization-evoked release of 3H-5-hydroxytryptamine was investigated in superfused synaptosomes prepared from rat cortex and hippocampus and prelabelled with the radioactive indoleamine. Noradrenaline reduced in a concentration-dependent way the release of 3H-5-hydroxytryptamine elicited by 15 mM KCl. The inhibition was counteracted by the alpha-adrenoceptor antagonists phentolamine or yohimbine, but not by prazosin. The results indicate that, in rat brain, the inhibition of 5-hydroxytryptamine release by noradrenaline is mediated by adrenoceptors of the alpha 2-type localized on the terminal serotonergic fibres.

Bilateral serotonergic projections to the dorsal hippocampus of the rat: simultaneous localization of 3H-5HT and HRP after retrograde transport.

Horseradish peroxidase (HRP, Sigma VI, 30-70 nl of a 10-15% solution in saline) or 3H-5HT (30 Ci/mmole, 2.5 X 10 -3 M containing 3.3 X 10(-3) M norepinephrine in saline, 50-100 nl) was injected unilaterally into the dorsal hippocampus in separate groups of rats. HRP-labeled cells were seen in the hippocampus, medial septal nucleus, nucleus of the diagonal band, supramammilary nucleus, median raphe nucleus, interfascicular portion of the dorsal raphe nucleus, and the locus coeruleus. In contrast, 3H-5HT-labeled cells were largely restricted to the raphe nuclei. In this nucleus an equal number of ipsilateral and bilateral cells were found. Occasionally, these labeled cells stretched across the midline (bridge pattern). In another series, the 3H-5HT and HRP were injected into the same hippocampus either as a mixture or sequentially. This resulted in double labeling of the median and dorsal raphe neurons. A final group of rats received injections of 3H-5HT and HRP into opposing hippocampi. Double-labeled cells accounted for 10% of the neurons labeled. In addition, closely paired neurons composed of an HRP- and 3H-5HT-containing cell were found. In summary, the serotonergic fibers may play a key role in harmonizing the electrical activity of the hippocampi by use of bilateral projections, paired neurons with differential projections, and bridging neurons stretching across the midline but with unilateral projections.

Fetal raphe and hippocampal transplants into adult and aged C57BL/6N mice: A preliminary immunocytochemical study

Fetal mouse raphe and hippocampal tissue (from embryos with crown-rump length (CRL) 13–16 mm) was transplanted into adult and aged isogenetic mice to study the growth of serotonergic fibers between host and donor tissue. A specific antibody against serotonin (5-HT) was used to immunocytochemically visualize 5-HT containing cell bodies and fibers. Unilateral fetal transplants into the hippocampi of adult (4–6 mo.) or aged (24 mo.) mice matured and sent out processes which very densely innervated the transplant tissue itself and extended into the host hippocampus. The termination of these fibers was consistent with the known 5-HT-hippocampal lamination pattern in normal animals. Qualitative comparisons suggested that the density of outgrowth into adult hippocampus was greater than into aged hippocampus. Conversely, adult 5-HT neurons send sprouts into fetal hippocampal tissue transplanted into the lateral ventricle. Therefore, immunocytochemical procedures can be used to monitor outgrowth from the fetal tissue to the host and ingrowth from the adult host to the fetal tissue. Furthermore, the apparent normal 5-HT lamination pattern produced by fetal raphe axons in adult hippocampus is consistent with reports that neuronal transplantation is effective in reversing the anatomical and behavioral deficits produced by homotypic denervation of a terminal field.

Experimentally induced supersensitivity of neocortical neurons to microiontophoretically administered serotonin

Central serotonergic fiber systems of the rat were selectively lesioned by intraventricular injection of the neurotoxin 5,7-dihydroxytryptamine (5,7-DHT). At various times thereafter, the sensitivity of rostral cortical neurons to microiontophoretically administered serotonin (5-HT) was compared in groups of lesioned and sham-operated animals pretreated with the 5-HT uptake inhibitor CGP 6085. Twenty-four hours after the injection of 5,7-DHT, at which time the cortical 5-HT and 5-hydroxyindoleacetic acid (5-HIAA) levels were both reduced by 40%, there was no significant difference in the sensitivity of cortical neurons to 5-HT. However, 3 days after such treatment, when the cortical 5-HT and 5-HIAA levels were reduced by 52% and 53% respectively, pronounced supersensitivity to 5-HT was noted. The depressant action of 5-HT on neuronal firing was potentiated with regard to both maximal firing depression and duration of the firing inhibition. A similar potentiation of the 5-HT responses was observed 7 days after lesioning. Supersensitivity thus appears to develop between 1 and 3 days after the injection of 5,7-DHT. Seven days after lesioning, the sensitivity of rostral cortical neurons to γ-aminobutyric acid was unchanged compared to that observed in sham-operated animals.

The serotonergic somatosensory projection to the tectum of normal and eyeless salamanders.

The spinotectal somatosensory projection was compared in normal, genetically eyeless, and embryonically manipulated salamanders. In normal animals, serotonin fluorescence was restricted to the intermediate tectalneuropil. This same region showed both high levels of serotonin uptake and somatosensory single unit electrical activity. In mutant eyeless salamanders and in normal animals enucleated early in development, serotonin fluorescence, serotonin uptake, and somatosensory activity were present in the superficial tectal neuropil. One-eyed animals, either genetically normal axolotls with one eye enucleated embryonically or genetically eyeless animals in which a normal eye had been transplanted, showed normal intermediate serotonin fluroescence and somatosensory physiology in the visually innervated half-tectum. In the visually uninnervated half-tectum, they showed superficial serotonin fluorescence and somatosensory physiology. In normal animals, 5,7-dihydroxytryptamine (5,7-DHT), a specific poison for serotonergic fibers, eliminated physiological responses in the contralateral somatosensory tectal region. The 5,7-DHT poisoning also abolished U.V.-induced serotonin fluorescence in the intermediate tectal neuropil. These results are discussed in terms of (1) evidence for serotonin as a central neurotransmitter for somatosensory information in the tectum, (2) the effects of eyelessness on tectal organization, and (3) related results in other animals.

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.

Nucleus raphe magnus modulation of response of rat dorsal horn neurons to unmyelinated fiber inputs: partial involvement of serotonergic pathways

1. In the rat under N2O-halothane anesthesia, stimulation of the nucleus raphe magnus (NRM) with stimulus parameters similar to those used to obtain analgesia in freely moving animals strongly inhibits the responses of dorsal horn convergent neurons due to A-delta- and C-afferents. 2. Responses to noxious radiant heat were also depressed, and pronounced post-effects were frequently observed. 3. Comparison between coupled sites of stimulation in NRM and in adjacent bulbar reticular formation (BRF) on responses to C-fibers revealed the preeminent effects of NRM; these were systematically encountered (93% of neurons), much more pronounced, and of longer duration. 4. The latency of these inhibitory effects (around 20 ms) suggests the participation of myelinated axons in such descending action and, consequently, we question the involvement of unmyelinated serotonergic fibers. 5. However, descending inhibitory influences from NRM on responses to C-fibers are reduced after 5-hydroxytryptamine (5-HT) depletion by p-chlorophenylalinine (pCPA), thus demonstrating the implication of both serotonergic and nonserotonergic pathways. 6. In addition, after pCPA pretreatment, long-lasting and sustained excitatory effects from NRM were observed in 35% of convergent neurons; their possible origin is discussed.

Specific uptake of tritiated serotonin in the adult rat pancreas: evidence for the presence of serotonergic fibers.

Fragments of adult rat pancreas were incubated in vitro with tritiated serotonin at concentrations from 10(-8) to 10(-7) M. The pancreas exhibited an uptake of serotonin which was saturable, with an uptake constant (Km) of 8.75 x 10(-7) M, and a Vmax of 873 pmoles per gram. Specificity was determined by the addition of fluoxetine or norepinephrine to the reaction mixture, both at 10(-5) M. Fluoxetine significantly reduced the 3H-5HT uptake, whereas norepinephrine was without effect. Metergoline (10(-6) M), a specific 5-HT postsynaptic receptor blocker, similarly had no effect on the serotonin uptake in the pancreas. Radioautography of the fragments following uptake of tritiated serotonin (5 x 10(-8) M) revealed silver- grain aggregates dispersed along blood vessels in the interstitial spaces of the exocrine and endocrine pancreas, areas known to be traversed by nerve fibers. There were no silver- grain aggregates over the exocrine or islet parenchymal cells. These data support the hypothesis that the pancreas is innervated by serotonergic fibers. Further evidence for this hypothesis was provided by a preliminary study demonstrating the presence of tryptophan hydroxylase in pancreatic homogenates. These serotonergic fibers may be involved in the regulation of pancreatic secretion.

Neuronal and extraneuronal effects of intracisternally administered 6-hydroxydopamine on the developing rat brain.

Abstract: Intracisternal administration of 100 μg 6‐OHDA to newborn rats causes permanent defects, not only of the monoaminergic neuron system, but also of extraneuronal tissue elements. The long noradrenergic fibre tracts are irreversibly destroyed, while the short projections recover and regenerate after a transient period of injury. In the major noradrenergic cell group, the locus coeruleus, most of the cells in the caudal and middle parts degenerate, while a small dorsorostral group survives and forms the source of the regenerating fibres. Dopaminergic and serotonergic fibre tracts are also affected. The 6‐OHDA treatment also damages granule and dial cells of the cerebellar cortex as well as the mesenchymal cells of the pial coverings of the cerebellum, leading to primitive foliation, absence of fissuration, and defective migration of granule cells and resulting in a marked reduction of cerebellar size, area, and granule cell number.

Harmaline-induced activation of the olivo-cerebellar system in young rabbits: Further evidence for a transient multiinnervation of purkinje cells by climbing fibres

Ontogenetic evolution of behavioural and electrophysiological responses to harmaline was studied in the maturing rabbit. As regards behavioural effects, harmaline-induced tremor could not be elicited before the second post-natal week, but as soon as it appeared, no significant difference was seen in its intensity compared with the adult animal. Electrophysiological studies of cerebellar Purkinje cell activity revealed a similar age-dependence since no rhythmic firing of climbing fibre responses was found before the 8th post-natal day. In addition, harmaline activation revealed a transient multiple innervation of Purkinje cells by climbing fibres. It is suggested that the presence of serotonergic fibres is critical for the effects of harmaline to develop.

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.

Noradrenergic and serotonergic mediation of spinal analgesia mechanisms

The effects of α-adrenergic and serotonergic receptor agonist administration on spinal analgesia mechanisms (D'Amour-Smith analgesia assay) were examined in the spinal rat from 6 h to 3 weeks following transection. Analgesia (increased response latency)_was observed following administration of the α-adrenergic agonists, clonidine or xylazine at all times after transection. This dose-response-related analgesia was blocked by pretreatment with the α-adrenergic receptor blocker phenoxybenzamine but not by the serotonergic receptor blocker metergoline. Biochemical asay of spinal cord noradrenaline (NA) caudal to the level of transection indicated that the above drugs were effective agonists even when NA levels were not detectable suggesting a mechanism of action via direct Receptor stimulation. Administration of the serotonergic receptor stimulants quipazine or 5-methoxy-N,N-dimethyltryptamine (5-MeODMT) was analgesic only during the first few postoperative days. Specificity of these drugs for the serotonergic receptor is supported by the observations that the drug induced analgesia was blocked by pretreatment with metergoline but not by phenoxybenzamine and that while quipazine or 5-MeODMT administration was only analgesic immediately following transection, α-adrenergic agonists were effective at all times. The implications of the present data for the involvement of descending NA and serotonergic fiber systems in the mediation of narcotic analgesia is discussed. The present data suggest that C.N.S. active α-adrenergic agonists may represent a class of non-narcotic analgesics of future clinical importance.

Twenty-four hour pattern of prolactin levels in female rats subjected to transection of the mesencephalic raphe or ablation of the suprachiasmatic nuclei.

The present study was undertaken to evaluate the 24-hour periodicity in serum prolactin levels subsequent to transecting the ascending serotonergic system or ablating the suprachiasmatic nuclei (SCN) of adult (185 g) female rats. The ascending serotonergic system was transected at the level of the rostral mesencephalon using a 1.5 mm wide knife and the SCN was ablated with a modified Halász-Pupp knife. The effect of raphe transection (RT) or SCN ablation on the circadian rhythm in nonstress serum prolactin levels was assessed 45 days after surgery by measuring with radioimmunoassay the serum prolactin concentration in serial blood samples obtained from the tail vein of lightly restrained rats. Serum prolactin concentration in control and RT animals showed circadian periodicity; peak levels occurred during the midafternoon, 4 h prior to the dark phase of the lighting regime (lights on at 4 a.m., off at 6 p.m.). The amplitude of the fluctuations of both groups varied markedly and were related to the estrous cycle. However, RT animals showed a reduced amplitude in the rhythm. In controls peak prolactin levels on days of proestrus and estrus were 15-20 times higher than on days of diestrus. In contrast, serum prolactin levels in SCN-ablated rats did not vary with a circadian periodicity but rather showed random, low amplitude fluctuations. Reproductive cyclicity was also abolished by SCN ablation, i.e., SCN-ablated rats presented persistent vaginal cornification. These data indicate that circadian periodicity in serum prolactin levels is not compromised by sectioning the ascending serotonergic fibers but is abolished after ablating the SCN.

Effects of mesencephalic central gray and raphe nuclei lesions on hypothalamically induced escape

Central gray (CG) lesions, in particular those located in its anterior part, provoked in about half of the lesioned rats a decrease in switch-off responses (SOR) induced by medial hypothalamic stimulation. Such a decrease in performance, followed by a partial or even total recovery, occurred only when the stimulation site was located in the postero-medial part of the medial hypothalamus. Occurrence and duration of the decrease in SOR did not depend on a possible severance of noradrenergic fibers that ascend from the locus coeruleus and course along the CG, since bilateral destruction of the locus coeruleus did not alter SOR. On the other hand, a lasting facilitation of SOR was observed in those rats in which the CG lesion extended more caudally and involved the dorsal raphé nucleus. This facilitation cannot be fully explained by an interruption of serotonergic fibers ascending from the raphé nuclei since lesions limited to the dorsal nucleus as well as combined lesions of the dorsal and medial raphé nuclei induced only a transient facilitation, whereas a lasting one was provoked both by raphé lesions that encroached upon the ventro-medial tegmentum and by CG lesions that only partially destroyed the dorsal raphé nucleus.

Regeneration of monoaminergic and cholinergic neurons in the mammalian central nervous system

Regeneration of monoaminergic and cholinergic neurons in the mammalian central nervous system

Involvement of the Serotonergic System in Depression of the Caudate Spindle

The involvement of the serotonergic system on the caudate spindle recorded from the posterior sigmoid gyrus was examined in cats. L-5-Hydroxytryptophan injected intravenously inhibited the caudate spindle significantly. On the other hand, the caudate spindle was inhibited by high frequency stimulation of the dorsal raphe nucleus where many serotonergic fibers originate: this inhibitory effect was antagonized by cyproheptadine but not by atropine. These results suggest an involvement of serotonergic mechanisms in depression of the caudate spindle.

Failure of medial forebrain bundle or raphe ablation to alter the daily temperature rhythm of the rat

The data presented in the present study suggest that neither the ascending noradrenergic fibres confined to the MFB nor the serotonergic fibres originating in or passing through the mesencephalic raphe are essential for periodicity in body temperature. Both control and experimental groups, i.e., rats subjected to medial forebrain bundle or raphe ablation presented circadian periodicity in body temperature and neither the phase, amplitude or overall mean of experimentals differed significantly from controls.

Differential projections of cat medullary raphe neurons demonstrated by retrograde labelling following spinal cord lesions.

Neurons of the medullary raphe nuclei in cats were retrogradely labelled following injection of horseradish peroxidase (HRP) into the L6 spinal cord segment. Brainstems were cut in sagittal section to facilitate examination of the rostral-caudal extent of raphe neurons projecting to the spinal cord. Large numbers of HRP-labelled neurons were found in nucleus raphe magnus, nucleus raphe pallidus, and nucleus raphe obscurus (as well as a few neurons in nucleus raphe pontis). Dorsal or ventral hemisections at the T12-L1 level restricted HRP retrograde transport to those pathways within the intact portion of spinal cord, allowing a determination of the part of the cord through which raphe neurons project to the lumbar enlargement. Neurons of nucleus raphe magnus were found to project primarily in dorsolateral fasciculus. A significant number of neurons of nucleus reticularis gigantocellularis also project in dorsolateral fasciculus. Nucleus raphe obscurus neurons were found to project primarily in ventral funiculus, while nucleus raphe pallidus neurons project in the ventrolateral fasciculi and ventral funiculus. The serotonergic (5HT) fibers described by Dahlström and Fuxe ('65) to terminate in the dorsal horn, intermediolateral cell column, and ventral horn are likely to coincide with the raphe-spinal projections documented in this work.