Immunohistochemical study on the development of serotoninergic neurons in the chick: II. Distribution of cell bodies and fibers in the spinal cord

The serotonin reuptake blocker citalopram destabilizes fictive locomotor activity in salamander axial circuits through 5-HT1A receptors

Serotoninergic (5-HT) neurons are powerful modulators of spinal locomotor circuits. Most studies on 5-HT modulation focused on the effect of exogenous 5-HT and these studies provided key information about the cellular mechanisms involved. Less is known about the effects of increased release of endogenous 5-HT with selective serotonin reuptake inhibitors. In mammals, such molecules were shown to destabilize the fictive locomotor output of spinal limb networks through 5-HT_1A receptors. However, in tetrapods little is known about the effects of increased 5-HT release on the locomotor output of axial networks, which are coordinated with limb circuits during locomotion from basal vertebrates to mammals. Here, we examined the effect of citalopram on fictive locomotion generated in axial segments of isolated spinal cords in salamanders, a tetrapod where raphe 5-HT reticulospinal neurons and intraspinal 5-HT neurons are present as in other vertebrates. Using electrophysiological recordings of ventral roots, we show that fictive locomotion generated by bath-applied glutamatergic agonists is destabilized by citalopram. Citalopram-induced destabilization was prevented by a 5-HT_1A receptor antagonist, whereas a 5-HT_1A receptor agonist destabilized fictive locomotion. Using immunofluorescence experiments, we found 5-HT-positive fibers and varicosities in proximity with motoneurons and glutamatergic interneurons that are likely involved in rhythmogenesis. Our results show that increasing 5-HT release has a deleterious effect on axial locomotor activity through 5-HT_1A receptors. This is consistent with studies in limb networks of turtle and mouse, suggesting that this part of the complex 5-HT modulation of spinal locomotor circuits is common to limb and axial networks in limbed vertebrates.NEW & NOTEWORTHY Little is known about the modulation exerted by endogenous serotonin on axial locomotor circuits in tetrapods. Using axial ventral root recordings in salamanders, we found that a serotonin reuptake blocker destabilized fictive locomotor activity through 5-HT_1A receptors. Our anatomical results suggest that serotonin is released on motoneurons and glutamatergic interneurons possibly involved in rhythmogenesis. Our study suggests that common serotoninergic mechanisms modulate axial motor circuits in amphibians and limb motor circuits in reptiles and mammals.

Neuroanatomical Distribution of the Serotonergic System in the Brain and Retina of Holostean Fishes, The Sister Group to Teleosts

Among actinopterygian fishes, holosteans are the phylogenetically closest group to teleosts but they have been much less studied, particularly regarding the neurochemical features of their central nervous system. The serotonergic system is one of the most important and conserved systems of neurotransmission in all vertebrates. By means of immunohistochemistry against serotonin (5-hydroxytryptamine), we have conducted a comprehensive and complete description of this system in the brain and retina of representative species of the 3 genera of holostean fishes, belonging to the only 2 extant orders, Amiiformes and Lepisosteiformes. Serotonin-immunoreactive cell groups were detected in the preoptic area, the hypothalamic paraventricular organ, the epiphysis, the pretectal region, the long and continuous column of the raphe, the spinal cord, and the inner nuclear layer of the retina. Specifically, the serotonergic cell groups in the preoptic area, the epiphysis, the pretectum, and the retina had never been identified in previous studies in this group of fishes. Widespread serotonergic innervation was observed in all main brain regions, but more abundantly in the subpallium, the hypothalamus, the habenula, the optic tectum, the so-called cerebellar nucleus, and the area postrema. The comparative analysis of these results with those in other groups of vertebrates reveals some extremely conserved features, such as the presence of serotonergic cells in the retina, the pineal organ, and the raphe column, while other characteristics, like the serotonergic populations in the preoptic area, the paraventricular organ, the pretectum, and the spinal cord are generally present in all fish groups, but have been lost in most amniotes.

Intraspinal serotonergic signaling suppresses locomotor activity in larval zebrafish

Serotonin (5HT) is a modulator of many vital processes in the spinal cord (SC), such as production of locomotion. In the larval zebrafish, intraspinal serotonergic neurons (ISNs) are a source of spinal 5HT that, despite the availability of numerous genetic and optical tools, has not yet been directly shown to affect the spinal locomotor network. In order to better understand the functions of ISNs, we used a combination of strategies to investigate ISN development, morphology, and function. ISNs were optically isolated from one another by photoconverting Kaede fluorescent protein in individual cells, permitting morphometric analysis as they developed in vivo. ISN neurite lengths and projection distances exhibited the greatest amount of change between 3 and 4 days post-fertilization (dpf) and appeared to stabilize by 5 dpf. Overall ISN innervation patterns were similar between cells and between SC regions. ISNs possessed rostrally-extending neurites resembling dendrites and a caudally-extending neurite resembling an axon, which terminated with an enlarged growth cone-like structure. Interestingly, these enlargements remained even after neurite extension had ceased. Functionally, application of exogenous 5HT reduced spinally-produced motor nerve bursting. A selective 5HT reuptake inhibitor and ISN activation with channelrhodopsin-2 each produced similar effects to 5HT, indicating that spinally-intrinsic 5HT originating from the ISNs has an inhibitory effect on the spinal locomotor network. Taken together this suggests that the ISNs are morphologically mature by 5 dpf and supports their involvement in modulating the activity of the spinal locomotor network. © 2018 Wiley Periodicals, Inc. Develop Neurobiol, 2018.

Light on a sensory interface linking the cerebrospinal fluid to motor circuits in vertebrates

The cerebrospinal fluid (CSF) is circulating around the entire central nervous system (CNS). The main function of the CSF has been thought to insure the global homeostasis of the CNS. Recent evidence indicates that the CSF also dynamically conveys signals modulating the development and the activity of the nervous system. The later observation implies that cues from the CSF could act on neurons in the brain and the spinal cord via bordering receptor cells. Candidate neurons to enable such modulation are the cerebrospinal fluid-contacting neurons (CSF-cNs) that are located precisely at the interface between the CSF and neuronal circuits. The atypical apical extension of CSF-cNs bears a cluster of microvilli bathing in the CSF indicating putative sensory or secretory roles in relation with the CSF. In the brainstem and spinal cord, CSF-cNs have been described in over two hundred species by Kolmer and Agduhr, suggesting an important function within the spinal cord. However, the lack of specific markers and the difficulty to access CSF-cNs hampered their physiological investigation. The transient receptor potential channel PKD2L1 is a specific marker of spinal CSF-cNs in vertebrate species. The transparency of zebrafish at early stages eases the functional characterization of pkd2l1⁺ CSF-cNs. Recent studies demonstrate that spinal CSF-cNs detect spinal curvature via the channel PKD2L1 and modulate locomotion and posture by projecting onto spinal interneurons and motor neurons in vivo. In vitro recordings demonstrated that spinal CSF-cNs are sensing pH variations mainly through ASIC channels, in combination with PKD2L1. Altogether, neurons contacting the CSF appear as a novel sensory modality enabling the detection of mechanical and chemical stimuli from the CSF and modulating the excitability of spinal circuits underlying locomotion and posture.

The serotonergic system in fish

Neurons using serotonin (5-HT) as neurotransmitter and/or modulator have been identified in the central nervous system in representatives from all vertebrate clades, including jawless, cartilaginous and ray-finned fishes. The aim of this review is to summarize our current knowledge about the anatomical organization of the central serotonergic system in fishes. Furthermore, selected key functions of 5-HT will be described. The main focus will be the adult brain of teleosts, in particular zebrafish, which is increasingly used as a model organism. It is used to answer not only genetic and developmental biology questions, but also issues concerning physiology, behavior and the underlying neuronal networks. The many evolutionary conserved features of zebrafish combined with the ever increasing number of genetic tools and its practical advantages promise great possibilities to increase our understanding of the serotonergic system. Further, comparative studies including several vertebrate species will provide us with interesting insights into the evolution of this important neurotransmitter system.

Serotonergic innervation of the caudal spinal stump in rats after complete spinal transection: effect of olfactory ensheathing glia

Spinal cord injury studies use the presence of serotonin (5-HT)-immunoreactive axons caudal to the injury site as evidence of axonal regeneration. As olfactory ensheathing glia (OEG) transplantation improves hindlimb locomotion in adult rats with complete spinal cord transection, we hypothesized that more 5-HT-positive axons would be found in the caudal stump of OEG- than media-injected rats. Previously we found 5-HT-immunolabeled axons that spanned the transection site only in OEG-injected rats but detected labeled axons just caudal to the lesion in both media- and OEG-injected rats. Now we report that many 5-HT-labeled axons are present throughout the caudal stump of both media- and OEG-injected rats. We found occasional 5-HT-positive interneurons that are one likely source of 5-HT-labeled axons. These results imply that the presence of 5-HT-labeled fibers in the caudal stump is not a reliable indicator of regeneration. We then asked if 5-HT-positive axons appose cholinergic neurons associated with motor functions: central canal cluster and partition cells (active during fictive locomotion) and somatic motor neurons (SMNs). We found more 5-HT-positive varicosities in lamina X adjacent to central canal cluster cells in lumbar and sacral segments of OEG- than media-injected rats. SMNs and partition cells are less frequently apposed. As nonsynaptic release of 5-HT is common in the spinal cord, an increase in 5-HT-positive varicosities along motor-associated cholinergic neurons may contribute to the locomotor improvement observed in OEG-injected spinal rats. Furthermore, serotonin located within the caudal stump may activate lumbosacral locomotor networks.

Axonal projections originating from raphe serotonergic neurons in the developing and adult zebrafish, Danio rerio, using transgenics to visualize raphe-specific pet1 expression

Serotonin is a major central nervous modulator of physiology and behavior and plays fundamental roles during development and plasticity of the vertebrate central nervous system (CNS). Understanding the developmental control and functions of serotonergic neurons is therefore an important task. In all vertebrates, prominent serotonergic neurons are found in the superior and inferior raphe nuclei in the hindbrain innervating most CNS regions. In addition, all vertebrates except for mammals harbor other serotonergic centers, including several populations in the diencephalon. This, in combination with the intricate and wide distribution of serotonergic fibers, makes it difficult to sort out serotonergic innervation originating from the raphe from that of other serotonergic cell populations. To resolve this issue, we isolated the regulatory elements of the zebrafish raphe-specific gene pet1 and used them to drive expression of an eGFP transgene in the raphe population of serotonergic neurons. With this approach together with retrograde tracing we 1) describe in detail the development, anatomical organization, and projection pattern of zebrafish pet1-positive neurons compared with their mammalian counterparts, 2) identify a new serotonergic population in the ventrolateral zebrafish hindbrain, and 3) reveal some extent of functional subdivisions within the zebrafish superior raphe complex. Together, our results reveal for the first time the specific innervation pattern of the zebrafish raphe and, thus, provide a new model and various tools to investigate further the role of raphe serotonergic neurons in vertebrates.

Monoaminergic systems in the brainstem and spinal cord of the turtlePseudemys scripta elegansas revealed by antibodies against serotonin and tyrosine hydroxylase

With the aim of gaining more insight into the monoaminergic regulation of spinal motor systems in the turtle, we have studied the distribution of 5-HT (5-HTir) and tyrosine hydroxylase immunoreactivity (THir) in the brainstem and spinal cord of Pseudemys scripta elegans. 5-HTir cell bodies were located in the midline in nucleus raphe inferior, nucleus raphe superior, and laterally in nuclei reticularis superior and inferior and nucleus reticularis isthmi. THir cell bodies were located in the commissural nucleus, nucleus tractus solitarii, the locus coeruleus-subcoeruleus complex, nuclei reticularis superior and inferior, the pretectal area, and substantia nigra. 5-HTir and THir tracts were found in lateral and ventral bundles superficially in the brainstem. 5-HTir fibers in the spinal cord were located in a large dorsolateral and a smaller ventrolateral tract. In the gray matter, a high concentration of 5-HTir fibers were observed in areas I-IV and in the lateral motor column of cervical and lumbar enlargements. Areas V-VIII and area X were less intensively innervated, with the lowest fibre concentration in areas VII-VIII and area X. Throughout the spinal cord, THir nerve fibres were located in the same areas but with a lower density. Small bipolar 5-HTir and THir cell bodies were found ventromedially to the central canal especially in cervical and lumbosacral segments. Large THir cells were found in area IX in the caudal sacral and coccygeal spinal cord. THir cerebrospinal fluid-contacting cells were also found in the most caudal part of the brainstem and the upper cervical spinal cord. The well developed spinal 5-HT system and the less developed THir system provides an anatomical explanation for the monoaminergic modulation of turtle motoneuron membrane properties, which has been observed in electrophysiological experiments.

Lower rhombic lip-derived cells undergo transmedian tangential migration followed by radial migration in the chick embryo brainstem

Migration behaviour and fate of cells originated from the lower rhombic lip (LRL) was examined in the chick embryo hindbrain. LRL-derived cells tangentially migrate along the pial surface of the brainstem and form a transient subpial migratory stream. In the initial stages of migration, LRL-derived cells appose each other or axon-like processes, which is indicative of mode of homophilic chain migration and/or axophilic migration. Some LRL-derived cells relocate rostroventrally towards the pontine region, although the majority of them migrate circumferentially to the ventral medulla oblongata. Depending on the stage of generation, LRL-derived cells undergo transmedian migration; late-generated LRL-derived cells preferentially colonize the contralateral brainstem compared with early generated cells. Thus, latecomer neuron precursors may migrate past their predecessors in the migratory stream. When LRL-derived cells leave the subpial migratory stream, they change their migratory direction to a radial one and relocate inwardly, with a profile that resembles a tangential-to-radial change seen in cerebellar granule cell precursors. After they enter the parenchymal region of the brainstem, they exhibited morphological differentiation, and some differentiate into excitatory neurons. The present results suggest that LRL-derived cells migrate across boundaries such as midline or rhombomere, which may facilitate to build up cellular and functional architectures of the hindbrain.

Ontogeny of descending serotonergic innervation and evidence for intraspinal 5-HT neurons in the mouse spinal cord

Neuronal networks in the mouse spinal cord express serotonin (5-HT)-induced rhythmic motor activity at early developmental stages (embryonic day (E) 12.5). Later in development, by post-natal day (P) 10, the 5-HT-evoked rhythmic motor activity matures and acquires an adult locomotor-like pattern. With the view to establishing a relationship between the ontogeny of locomotor networks and the maturation of spinal 5-HT systems, we have traced 5-HT immunoreactivity in the mouse spinal cord from E12.5 to PN10. By E12.5, descending 5-HT immunoreactive (5-HT-ir) fibers that likely originate from raphe nuclei were detected in the ventral and lateral funiculi, at anterior cervical spinal levels, but not at more caudal levels. Descending 5-HT-ir axons reached thoracic levels at E14.5 and lumbar levels at E16.5. Some 5-HT-ir fibers could be detected in the ventral and intermediate gray matter by E16.5, whereas the dorsal gray matter was not invaded before PN0. At PN10, a dense serotonergic innervation was restricted to the gray matter with a high concentration of 5-HT-ir fibers in three areas: dorsal horn, ventral horn (where motoneurons are located) and intermediate area. Surprisingly, from E16.5 to PN10, 5-HT-ir intraspinal neurons were found, exclusively at sacral levels. Their somata lay in the gray matter around the central canal and preferentially in the ventro-median part of the ventral horn. The functional significance of these sacral 5-HT-ir neurons is discussed.

Ontogeny of modulatory inputs to motor networks: early established projection and progressive neurotransmitter acquisition

Modulatory information plays a key role in the expression and the ontogeny of motor networks. Many developmental studies suggest that the acquisition of adult properties by immature networks involves their progressive innervation by modulatory input neurons. Using the stomatogastric nervous system of the European lobster Homarus gammarus, we show that contrary to this assumption, the known population of projection neurons to motor networks, as revealed by retrograde dye migration, is established early in embryonic development. Moreover, these neurons display a large heterogeneity in the chronology of acquisition of their full adult neurotransmitter phenotype. We performed retrograde dye migration to compare the neuronal population projecting to motor networks located in the stomatogastric ganglion in the embryo and adult. We show that this neuronal population is quantitatively established at developmental stage 65%, and each identified projection neuron displays the same axon projection pattern in the adult and the embryo. We then combined retrograde dye migration with FLRFamide-like, histamine, and GABA immunocytochemistry to characterize the chronology of neurotransmitter expression in individual identified projection neurons. We show that this early established population of projection neurons gradually acquires its neurotransmitter phenotype complement. This study indicates that (1) the basic architecture of the known population of projection inputs to a target network is established early in development and (2) ontogenetic plasticity may depend on changes in neurotransmitter phenotype expression within preexisting neurons rather than in the addition of new projection neurons or fibers.

Developmental changes in leg coordination of the chick at embryonic days 9, 11, and 13: uncoupling of ankle movements

To understand changes in motor behavior during development, kinematic measurements were made of the right leg during embryonic motility in chicks on embryonic (E) days 9, 11, and 13. This is an interesting developmental period during which the embryo first becomes large enough to be physically constrained by the shell. Additionally, sensory systems are incorporated at that time into the spinal motor circuitry. Previous electromyographic (EMG) recordings have shown that the basic pattern of muscle activity seen at E9, composed of half-center-type alternation of extensor and flexor activation, breaks down by E13. This breakdown in organization could be because of disruption of motor patterns by the immature sensory system and/or new spatial constraints on the embryo. The current article describes several changes in leg movement patterns during this period. Episodes of motility increase in duration and the intervals of time between episodes of motility decrease in length. The range of motion of the leg increases, but the overall posture of the leg becomes more flexed. It was found that in-phase coordination of movement among the hip, knee, and ankle decreased between E9 and E13 in agreement with the previous EMG recordings. However, it was also found that the decrease of in-phase coordination among the three joints was accompanied by an increase in the time any two joints were moving in the same manner. Furthermore, examination of in-phase coordination within pairs of joints showed that all three pairs were well coordinated at E9, but that at E13 the in-phase coordination of the ankle with the other two joints decreased, whereas the knee and hip coordination was maintained. This suggests that the hip-knee synergy was closely coupled and that coupling of the ankle with other joints was more labile. The authors conclude that embryos respond to the reduction of free space in the egg during this period not by decreasing the amplitude or coordination of leg movements in general, but instead by differentially controlling the movements of the ankle from those of the hip and knee. Additionally, the changes in movement patterns do not represent a decrease in organization, but rather an alteration of motor coordination possibly as the result of information from the newly acquired sensory systems. These data also support theories that limb central pattern generators (CPGs) are composed of unit CPGs for each joint that can be modulated individually and that this organization is already established early in embryogenesis.

Ethanol exposure alters the development of serotonergic neurons in chick spinal cord

Exposure to ethanol is known to alter the development of the serotonergic system. However, previous studies have examined large populations of cells and have not determined the effects of ethanol on individual serotonergic neurons. In the present study, the effects of various concentrations of ethanol on the development of single serotonergic neurons in the chick embryo spinal cord were determined using immunohistochemical techniques. Between embryonic day 7 (E7) and E14, ethanol administrations produced in ovo alcohol concentrations of: a) low dose, 30-60 mg/dl, b) medium dose, 150-200 mg/dl or c) high dose, 240-300 mg/dl. In animals exposed to the medium and high ethanol doses, the normal developmental increase in cross-sectional area of the somata was not observed. At all stages examined, the numbers of primary and nonprimary processes were significantly lower in ethanol-treated groups compared to controls. These data indicate that ethanol exposure induces dose-dependent alterations in the development of identified spinal cord neurons. The ethanol-induced changes may be involved in the motor dysfunction observed after embryonic ethanol exposure.

Excessive stimulation of serotonin2 (5-HT2) receptors during late development of chicken embryos causes decreased embryonic motility, interferes with hatching, and induces herniated umbilici

The existence and functional significance of 5-HT2 receptors in chicken embryos was studied by injecting the selective agonist dimethoxyiodophenylaminopropane (DOI), alone or in conjunction with the selective 5-HT2 antagonist ritanserin (RIT), into domestic chicken eggs with embryos of varying ages. DOI caused dose-dependent reductions in hatchability and herniated umbilici in hatchlings. These effects were observed after injection early, mid, or late during embryonic development, with evidence of the toxic effects of DOI being greater in older embryos, probably due to 5-HT2 receptor activation late in development, even after injecting DOI as early as on day 3 of embryogenesis. This is based upon the fact that embryos in eggs injected with DOI early continued to develop apparently normally, failing to hatch, often after pipping their shells. Additionally, those that hatched often did so with herniated umbilici, as did late-exposed embryos, indicating that DOI's effects upon this organ were most likely mediated during the prehatching period (i.e., days 18-20). The agonist's selectivity was confirmed by the capacity of RIT to dose dependently block both of these toxic effects of DOI. Reduced embryonic motility monitored on day 19, after injection of DOI on the evening of day 18, suggests that excessive activation of 5-HT2 receptors late during development of this species interferes with some normal embryonic behaviors and physiological changes necessary for inducing and/or maintaining the hatching process.

Immunohistochemical examination of intraspinal serotonin neurons and fibers in the chicken lumbar spinal cord and coexistence with Leu-enkephalin

Intraspinal serotonin-positive cells and fibers were examined in the chicken lumbar spinal cord following removal of descending serotonin fibers by spinal transection. Co-localization of Leu-enkephalin immunoreactivity in intraspinal serotonin cells was also examined using a double immunofluorescence labeling technique. By one or two weeks after spinal transection, virtually all supraspinal serotonin fibers were eliminated. Intraspinal serotonin cells were located ventral or ventrolateral to the central canal corresponding to laminae VII, VIII, and IX, and the anterior funiculus. Intraspinal serotonin cells sent fibers to (1) the pia mater on the ventral or ventrolateral surface of the spinal cord; (2) vessels in the spinal cord; (3) sympathetic preganglionic column of Terni; (4) other intraspinal serotonin neurons; (5) the central canal. Some 30%-50% of the intraspinal serotonin cells co-localized with Leu-enkephalin. Intraspinal serotonin fibers co-containing Leu-enkephalin were observed in the pia mater located on the most lateral surface of the spinal cord.

Cerebrospinal fluid-contacting neurons in the paraventricular organ and in the spinal cord of the quail embryo: a fluorescence-histochemical study

Although the cerebrospinal fluid-contacting neurons of the avian paraventricular organ exhibit considerable amounts of catecholamines, they show no tyrosine hydroxylase immunoreactivity. In the quail embryo, the development of these neurons has been studied using the paraformaldeyde-glutaraldeyde method for the fluorescence-histochemical localization of catecholamines. The timing of the appearance of catecholamine fluorescence in cerebrospinal fluid-contacting neurons and that in catecholamine-containing neurons of the brainstem have been compared. The first neurons displaying catecholamine fluorescence are found within the locus coeruleus and the nucleus subcoeruleus ventralis on the 5.5th day of incubation. Catecholaminergic neuronal groups of the medulla and mesencephalon can be identified by embryonic day 7, and fluorescent cerebrospinal fluid-contacting neurons of the hypothalamic paraventricular organ can be first recognized at the 8th day of incubation. If the catecholamine content of cerebrospinal fluid-contacting neurons that lack tyrosine hydroxylase depends upon an uptake mechanism, it may be significant that, in fluorescence-histochemical preparations, these neurons can be identified 1-3 days later than those in which catecholamines are synthesized and from which catecholamines are released at an earlier developmental stage. Moreover, cerebrospinal fluid-contacting neurons that have previously been shown to be tyrosine-hydroxylase immunoreactive, and that lie at the spinal-medullary junction display a different developmental pattern. By fluorescence histochemistry, they can be detected only by embryonic day 10.5. The chemical, developmental and topographical differences suggest that the catecholamine-containing cerebrospinal fluid-contacting elements of the paraventricular organ and those of the spinal cord represent two different subsets of cerebrospinal fluid-contacting neurons whose respective functional roles remain to be investigated.

Role of neuropeptide Y projection on the development of serotonergic innervation in the suprachiasmatic nucleus of the rat, shown by triple intraocular grafts

In our previous paper, the intraocular double grafts of fetal mesencephalic raphe and suprachiasmatic nucleus (SCN) demonstrated that the serotonergic fibers from raphe tissue did not show a dense innervation of SCN [28]. To examine the influence of NPY innervation from lateral geniculate nucleus (LGN) on the development of serotonergic fibers in the SCN, fetal mesencephalic raphe, SCN and LGN tissues were transplanted together into the eye chamber of adult rat. 6 weeks after transplantation, triple grafts were immunohistochemically examined. The SCN cell cluster was recognized by vasoactive intestinal polypeptide (VIP)- and arginine vasopressin (AVP)-immunoreactive neurons and The SCN cell cluster also contained a large number of serotonin-immunoreactive fibers from raphe tissue and a moderate number of neuropeptide Y (NPY)-immunoreactive fibers from LGN tissue. The present results provide information on possible NPY-serotonin interactions in the developing SCN.

Synaptic loss following removal of serotoninergic fibers in newly hatched and adult chickens

Neurotransmitters such as serotonin (5HT) may have nontransmitter, trophic-like functions in the developing and adult nervous system. In order to examine this possibility in the avian spinal cord, we have quantified synapse numbers on spinal neurons following treatment with drugs that result in the destruction of 5HT positive axons. Either p-chlorophenylalanine or reserpine was injected into newly hatched or adult chickens. Following treatment for 7 days the density of nonserotoninergic synapses was considerably decreased in the targets of 5HT fibers. By contrast, neither change was observed in the dendritic structures of spinal motoneurons or in the distribution of substance P and enkephalin positive fibers. These data suggest that 5HT may play an important role in the normal increase and maintenance of synapses in developing and adult animals. A lesion of 5HT neurons may not only alter neurochemistry but also alter the general synaptic structures of the brain. While 5HT containing fibers were depleted in a dose-dependent fashion we cannot rule out the possibility that other neurotransmitter systems were depleted at higher dose of PCPA and reserpine.

Developmental changes in the serotoninergic innervation of hindlimb extensor motoneurons in neonatal rats

The postnatal development of quadriceps femoris motoneurons (Q-MNs) and serotonin (5-HT) nerve terminals in rat spinal cord were studied using retrograde neurotracing techniques combined with 5-HT immunohistochemistry. We attempted to elucidate the 5-HT-ergic innervation to the Q-MNs by counting the number of 5-HT-immunoreactive varicosities that were in close apposition to the Q-MNs. The following results were obtained: (1) Q-MNs possessed, at birth, few if any very short dendrites. The size of these somata was relatively uniform and small. During postnatal periods lasting from 1 to 30 days, the mean cell size of Q-MNs increased with the development of dendrites. From 5 to 14 days after birth, in particular, cell size increased markedly. (2) 5-HT-immunopositive fibers were, at birth, already observed in the ventral horn of the lumbar spinal cord. The density of these fibers increased gradually with age. (3) At birth, only a few 5-HT terminals and varicosities showed close apposition with about half the Q-MNs examined. At 5-days postnatally, such close apposition was found in all Q-MNs. By the first two postnatal weeks, Q-MNs grew quickly and the 5-HT innervation to the Q-MNs appeared to have been established. Based on these results, the significance of 5-HT innervation to developing Q-MNs is discussed in relation to the postnatal development of motor function.

Development of serotoninergic system in the brain and spinal cord of the chick

(1) Development of serotonin positive cells and fibers was immunohistochemically studied by the use of an antibody against serotonin. (2) Serotoninergic neurons were first observed in the immature rohmbencephalon raphe nuclei on embryonic day (E)4, where two clusters of serotonin positive neurons were located: one observed at the rostral part of the rohmbencephalon corresponding to the dorsal raphe nuclei had many serotonin positive cells: the other located at the caudal part of the rohmbencephalon corresponding to the medullary raphe nuclei of the adult animals had only a small number of serotoninergic cells. (3) By E8 the number of serotonin positive cells in the brain stem increased, and virtually all the raphe nuclei found in an adult animal were located. (4) Serotonin positive fibers in the marginal layer reached up to the diencephalon and telencephalon on E6 and E8, respectively. (5) Serotonin positive cells were found beside the midline regions in the ventral part of the spinal cord of the embryonic as well as posthatching chick. (6) Because almost all the serotoninergic fibers in the spinal cord originated from the brain stem raphe nuclei, propriospinal serotonin positive cells were considered as phylogenetic vestiges. (7) Serotoninergic fibers were first found in the marginal layer of the cervical and lumbar spinal cord on E6 and E8, respectively. (8) There was a waiting period of a few days before they penetrated into the mantle layer. (9) Terminal arbolization of the serotoninergic fibers started from late embryonic periods (E16 less than), and was maximized within one week of hatching. (10) Thereafter the density of serotonin positive fibers decreased in all the regions of the spinal cord. (11) Developmental changes of the density of serotonin determined with a high performance liquid chromatography were the same as those determined through immunohistochemistry. Namely the density of serotonin increased linearly from E6 to hatching period, and reached the maximum value one week posthatching. (12( The density of the serotonin in the adult spinal cord was about half of the maximum value. (13) It is to say that the densities of serotonin and serotoninergic fibers transiently increased around one week posthatching. (14) Following the transient increase serotoninergic fibers were eliminated from the neuropil, the fibers were localized in the specific regions of the motor nucleus: motor neuron pools of extensor muscles of the hip joint in the lumbosacral spinal cord.(ABSTRACT TRUNCATED AT 400 WORDS)

Immunohistochemical study of tyrosine-hydroxylase-positive cells and fibers in the chicken spinal cord

Tyrosine hydroxylase (TH)-positive cells and fibers were examined by immunohistochemistry in the chick spinal cord. TH-positive cells, which were located in laminae I, V and X, were most frequently found in the rostral part of the cervical spinal cord, with fewer cells being found in more caudal levels of the spinal cord. TH-positive cells located in lamina X, which were bipolar in shape, were mainly found in regions lateral as well as just ventral to the central canal. They had processes reaching to the central canal. The terminals of these cerebrospinal-fluid-contacting cells were oval in shape, and were most frequently found at the ventral wall of the central canal. There were dense clusters of TH-positive fibers in lamina X. A meshwork-like structure of TH-positive fibers was found over the lateral wall of the central canal. A high density of TH-positive fibers was also found in the medial part of laminae V-VII. In lamina IX, small numbers of TH-positive fibers were observed in the lateral motor column of the brachial spinal cord, and in the medial and lateral motor columns of the lumbosacral spinal cord. However, within the medial motor column of the brachial spinal cord TH-positive fibers were densely distributed around somal as well as dendritic profiles. Similar to our previous observations on serotoninergic fibers. TH-positive fibers were also differentially distributed in the ventral horn of the chicken spinal cord: a high density of TH-positive fibers was localized to specific regions of the spinal motor nucleus.

Axonal projections and synaptogenesis by supraspinal descending neurons in the spinal cord of the chick embryo

Following the injection of horeseradish peroxidase (HRP) into the brachial spinal cord of the chick on embryonic day (E)4.5, retrogradely labeled neurons can be found in the brainstem (Okado and Oppenheim: Journal of Comparative Neurology 232: 143-161, 1985). By contrast, following high cervical spinal transection, functional (behavioral) deficits are not observed until E10 (Oppenheim: Journal of Comparative Neurology 160: 37-50, 1975). To determine whether this temporal difference between projections and function reflects a delay in synaptogenesis, we looked for the presence of anterogradely HRP-labeled pre-synaptic terminals in brachial cord following injection of HRP into the boundary between brainstem and spinal cord at ages between E3.5 and E7. HRP-labeled fibers were observed in the branchial cord by E4.5 and were diffusely distributed in the ventral and lateral marginal zones (presumptive ventral and lateral funiculi, respectively). Although some axo-dendritic and axo-somatic synapses were observed in the brachial cord prior to E6, the presynaptic profiles were always unlabeled by HRP and thus must originate from propriospinal sources. The first HRP-labeled supraspinal synapses were found in the ventral and lateral funiculi on E6. They contained several clear spherical synaptic vesicles and were axo-dendritic in nature. The cells of origin of the postsynaptic dendrites were determined by injecting HRP into the wing-bud to label the brachial motoneurons retrogradely and the presynaptic component was identified as supraspinal by HRP injections into the brainstem/spinal cord boundary to orthogradely label the descending fibers. Several double-labeled axo-dendritic synapses were found in the ventral and lateral funiculi of E6 brachial cord. Therefore, at least some descending supraspinal fibers make synapses directly onto motoneuron dendrites. We conclude that 1) there is a delay of about 1.5 days between the arrival of supraspinal fibers and synapse formation in the brachial cord, 2) the earliest synapses are axo-dendritic in nature, 3) at least some supraspinal fibers make direct contact with motoneuron dendrites as early as E6, and 4) synaptogenesis from propriospinal sources precedes that from supraspinal descending axons. These observations provide evidence indicating that the temporal difference between the onset of projections of supraspinal descending fibers and the onset of their function may be partly owing to delayed synaptogenesis.

Activation of the serotonergic system in chick spinal cord during hatching

The objectives of the present study were to determine the levels of serotonin (5-HT), its major catabolic metabolite, 5-hydroxyindoleacetic acid (5-HIAA), and norepinephrine (NE) in chick spinal cord before, during, and after hatching and also to determine if changes in the levels of these chemicals are directly related to the hatching behavior. The levels of 5-HT, 5-HIAA, and NE were measured by high performance liquid chromatography with electrochemical detection in whole spinal cords of 20-day-old "pre-hatching" embryos, 21-day-old "normal hatching" embryos, 0-day-old "post-hatching" chicks, and 0-day-old "glass egg hatching" chicks. NE was measured but no significant differences were found in NE levels among experimental groups. The concentration of 5-HT was elevated in chick embryo spinal cords during normal hatching compared to pre-hatching embryos and post-hatching chicks. The concentration of 5-HIAA was elevated during and after normal hatching compared to pre-hatching embryos. However, neither 5-HT nor 5-HIAA levels were found to be elevated in chick spinal cords during glass egg hatching compared to pre-hatching embryos or post-hatching chicks. Therefore, there appears to be an activation of the serotonergic system in chick spinal cord related to the specific event of hatching but this activation is not directly related to the movements common to both hatching and glass egg hatching.

The serotoninergic system in the brain of the Japanese quail. An immunohistochemical study

The presence and topographical localization of the serotoninergic system in the brain of the Japanese quail (Coturnix coturnix japonica) have been studied by means of peroxidase-anti-peroxidase immunocytochemistry. The perimeter, diameter, area, and shape factor of immunoreactive cells have been recorded and analyzed morphometrically for intra- and interspecies comparison. The data reported here confirm and extend results previously obtained in the brain of other avian species. Serotonin-immunoreactive neurons of the quail are mainly located in the hypothalamic paraventricular organ and adjacent areas, and in the brainstem where they form three separate groups. The first of these groups consists of small-sized neurons located in the ventro-rostral mesencephalon. The second group is composed of medium-sized neurons located in the dorsal mesencephalo-pontine region. The third group is also formed by medium-sized neurons, and is located ventrally in the ponto-medullary region. In the quail brain, serotoninergic neurons are not restricted to nuclei located in the vicinity of the midsagittal plane, but show some lateralization, especially in the brainstem. The organization of the different groups of immunoreactive neurons based on this topographical distribution and morphometric analysis has been compared with descriptions of the serotoninergic system in other birds. Serotonin-immunoreactive nerve fibers are widely distributed throughout the brain, but appear to be particularly abundant in regions involved in the control of reproductive activities, such as the septal region, the medial preoptic nucleus, the nucleus intercollicularis, and the external zone of the median eminence. The data reported here have allowed the drawing of a map of serotonin-immunoreactive structures.

Developmental expression of serotonin-like immunoreactivity in the sympathoadrenal system of the chicken

The avian sympathoadrenal system has been used as a model to examine the differentiation of cells expressing neuroactive substances derived from the neural crest. Previous studies have dealt with the expression of the "classical" neurotransmitters acetylcholine and catecholamines and of the neuropeptides somatostatin and vasoactive intestinal polypeptide. We have used immunocytochemistry to examine the developmental expression of the monoamine serotonin (5HT) in the chicken sympathoadrenal system. 5HT-like immunoreactivity (5HT-LI) was found to be transiently expressed by cells of the sympathetic ganglia very early in development (E-5 to E-8), disappearing almost entirely at more advanced embryonic stages (E-10 to E-19) and post-hatched chickens where only a population of cells similar to mammalian small intensely fluorescent cells express immunoreactivity to the amine. In contrast, in the adrenal gland of embryos and post-hatched chickens, most chromaffin cells also express 5HT-LI. Double labeling experiments show that in both the adrenal gland and the sympathetic ganglia catecholaminergic properties and somatostatin immunoreactivity are co-expressed with 5-HT-LI. Moreover, the cells that transiently express 5HT-LI in sympathetic ganglia also transiently express somatostatin. The catecholaminergic cells expressing serotonin and somatostatin appear to define a biochemical phenotype common to some chromaffin cells, small intensely fluorescent cells and early sympathoblasts.

Rearrangement of serotonin-immunoreactive fibers in the denervated rat suprachiasmatic nucleus after transplantation of fetal raphe tissue

Pieces of fetal midbrain raphe tissue were transplanted into the third ventricle or the ventral hypothalamic region near the suprachiasmatic nucleus (SCN) of adult host rats that had previously been denervated by treatment with 5,6-dihydroxytryptamine. The ability of grafted serotonin neurons to reinnervate the SCN in the host rats was studied by means of immunohistochemistry 1 and 3 months after transplantation. In both the intraventricular and intraparenchymal transplant experiments, reinnervation by outgrowing serotonin fibers was observed in the hypothalamus of host rats at 1 and 3 months after surgery. At both survival periods, there was no abundant arborization of serotonin fibers in the SCN, while the preoptic and periventricular areas of the host rats displayed a pattern of serotonergic innervation resembling that in normal (untreated) rats. It is suggested that within the SCN the regenerating serotonin fibers may be exposed to an inhibitory environment.

Evidence for two kinds of serotoninergic fibers in the ventral horn of spinal cord of the newly hatched chick

We observed two types of serotonin-positive (5-HT) fibers in the ventral horn of the lumbar spinal cord of the newly hatched chicken: the first was composed of fine 5-HT fibers, which were increased transiently in the neuropil at 1 week after hatching (transient type); the second type consisted of thick fibers, which were densely localized around motoneuron somata in the motoneuron pools innervating extensor muscles of the hip joint (adult type). Pharmacological perturbation experiments demonstrated that these two types of 5-HT fibers may have different functions: the adult type of fibers may act to maintain an erect standing posture, whereas the transient type may act to induce or facilitate dendrogenesis (or dendritic elongation) of motoneurons. Thus, we concluded that 5-HT fibers may modulate neuronal transmission and serve as a kind of inductive agent for dendritic development in the chick spinal cord.

Developmental changes in serotonin levels in the chick spinal cord and brain

Developmental changes in 5-hydroxytryptamine (5-HT) and 5-hydroxyindoleacetic acid (5-HIAA) in the developing chick spinal cord and brain were examined using high-performance liquid chromatography with electrochemical detection and immunohistochemistry. On embryonic day (E)6 only small amounts of 5-HT (0.086 ng) and 5-HIAA (0.0144 ng) were found in the spinal cord. By contrast, large amounts of 5-HT (x30) and 5-HIAA (x60) were detected in non-neuronal tissue outside the spinal cord; a similar distribution of 5-HT was also detected by immunohistochemistry. Up to E10, the highest concentrations of 5-HT in the spinal cord were found in the cervical region, followed by the thoracic and lumbar regions. In embryos older than E16, as well as in posthatched chicks, however, the highest and lowest concentrations of 5-HT were found in the lumbar and thoracic spinal cord, respectively. The concentration of spinal cord 5-HT reached maximal values on posthatching day (P)7, after which there was a marked decrease. By P120, 5-HT levels in the spinal cord decreased to the same level as on E10-E16. Concentrations in the brain, however, gradually increased with development. The basic pattern of development of 5-HIAA was similar to that of 5-HT.

Immunohistochemical distribution of serotonin in spinal autonomic nuclei: II. Early and late postnatal ontogeny in the rat

These studies reveal that the postnatal ontogeny of serotonin (5HT) in the sympathetic nuclei of the rat spinal cord is protracted; the adult complement of 5HT-immunoreactive fibers is not achieved until at least 60 days of age. As descending serotonin fibers innervate and demarcate the distribution of preganglionic sympathetic nuclei, rostral-caudal and temporal gradients exist. Additionally, a heterogeneous segmental 5HT ontogenetic pattern is observed in sympathetic nuclei. Most serotonin fibers in laminae VII and X are unorganized at birth except for some sympathetic nuclei in high thoracic regions where the 5HT sympathetic pattern is being initiated. By postnatal day 6 the framework of the 5HT pattern is established in all sympathetic nuclei, and by postnatal day 16 a pattern is formed, which develops into the compact adult state by postnatal day 60. The protracted period of sympathetic 5HT development corresponds with the length of time it takes for the autonomic nervous system to mature. In addition, 5HT intraspinal cell bodies are observed at all time points examined, except for the day of birth, and are found in the same regions as adult 5HT neurons, i.e., dorsal or lateral to the central canal in laminae VII and X and in all spinal segments except cervical levels. Many of the 5HT neurons are pericanalicular and bipolar in appearance. Multipolar 5HT neurons are first observed on postnatal day 45.

Differential innervation of specific motor neuron pools by serotoninergic fibers in the chick spinal cord

A new technique in which cholera toxin subunit B conjugated to horseradish peroxidase is injected in chick muscles followed by perfusion with Zamboni's fixative for serotonin immunocytochemistry allows one to visualize immunoreactive fibers and retrogradely labelled motoneurons in alternate sections. Using these procedures, we have found that there is a differential innervation by serotoninergic fibers of motoneuron pools that project to specific muscles or muscle groups. Dense clusters of serotonin-positive fibers were located in the motoneuron pools of extensors of the hip joint consisting of the lateral iliotibialis, ischioflexorius, iliofibularis, accessorius and caudilioflexorius muscles.

The pattern of distribution of serotoninergic fibers in the anterior horn of the chick spinal cord

The pattern of distribution of serotonin positive fibers in the motor nuclei of the chick spinal cord was examined immunohistochemically by using an antiserum against serotonin. A dense aggregation of serotoninergic fibers was located around anterior horn cells in the cervical spinal cord. In the brachial spinal cord, serotoninergic fibers were densely aggregated in the medial motor column and in the parts of the lateral motor column. There were two regions of serotonin immunoreactivity in the lateral motor column of the brachial spinal cord; one located in the ventromedial regions where a dense aggregation of serotoninergic fibers was found, and the reminder of the lateral motor column where only a few serotoninergic fibers were observed. The region containing a dense cluster of serotoninergic fibres around profiles of motoneuron somata and proximal dendrites appears to correspond to motor neuron pools of flexor muscles. In the thoracic spinal cord a high density of serotoninergic fibers was found in the motor nucleus. In the lumbosacral spinal cord (segments LS1-LS8) serotoninergic fibers were not observed in the medial motor column. However, there were five regions in the lateral motor column, where a high density of serotoninergic fibers was found. These very likely correspond to motor neuron pools of muscles which extend the hip joint.

Presence of serotonin in early chick embryos

With biochemical analysis and with autoradiography based on injection of 5-[3H]hydroxytryptophan, it was possible to demonstrate the presence of serotonin (5-hydroxytryptamine) in early chick embryos as early as the pre-streak stage. The biochemical analysis which covered the early developmental period (0.5-6 days of incubation) revealed an elevated concentration of serotonin at gastrulation; from then it stayed at a lower and fairly even level. Autoradiographs of embryos at the pre-streak stage, the primitive streak stage, the head fold stage and the 4-6 somites stage indicated the presence of serotonin in intracellular yolk granules and in cell nuclei. Moreover, the amine appeared associated with microfilaments and microtubules, particularly in developing neural cells. Notably the elevated concentration of serotonin at gastrulation, but also the intracellular distribution of the amine during early organogenesis, indicates a prominent role for it in cell-shape changes and morphogenesis in the early chick embryo.

The morphology and distribution of rat serotoninergic intraspinal neurons: an immunohistochemical study

An immunohistochemically derived morphological description of a diverse population of rat lamina VII and X intraspinal 5HT neurons is provided. These bipolar or multipolar neurons occur most frequently in lamina X, dorsal or dorsolateral to the central canal, in thoracolumbar, sacral, and coccygeal spinal segments. These 5HT intraspinal neurons are found in normal rat spinal cords as well as in spinal cords that have been hemisected or transected 60 days prior to serotonin immunostaining. Therefore, 5HT intraspinal neurons are the probable source of the biochemically detectable 5HT that remains in the spinal cord distal to a spinal transection. In the rat, serotonin intraspinal neurons are most often associated with spinal autonomic nuclei but it is unknown if they are preganglionic in nature.

Developmental changes in density and distribution of serotoninergic fibers in the chick spinal cord

Developmental changes of serotoninergic innervation in the chick spinal cord (third lumbosacral segment) were examined with an immunohistochemical technique using an antiserum to serotonin. In the 1-day-old hatched chick, serotoninergic fibers were located in laminae I, II, VII, IX, and X. A large number of serotonin-positive fibers and terminals were found around somal profiles of large neurons and in the neuropil of the medial and lateral parts of the lateral motor column (LMC). In the 1-week-old chick, the density of serotoninergic fibers was greatly increased in the posterior columns, and serotoninergic fibers were most densely aggregated in the dorsolateral part of the LMC. In the 2-week-old chick, a considerable decrease in the density of serotoninergic fibers was observed in the lateral funiculus and the gray matter (laminae I, II, VII, IX, and X). In the LMC, serotonin-positive fibers and terminals were largely absent from the neuropil, but were found preferentially around the somal profiles of large neurons. Between 1 and 2 weeks after hatching the density of varicosities and terminals in the neuropil of the dorsolateral and medial parts of the LMC decreased by 33% and 56%, respectively. In the 3-month-old chick, the density of serotoninergic fibers in laminae I, II, V, VII, and X had increased compared to younger ages. Serotonin-positive fibers were not evenly distributed in the LMC of the adult chicken; rather, they were densely aggregated around the soma and proximal dendrites of motoneurons in the dorsolateral LMC. Many neuronal soma in the medial and intermediate regions of the LMC lacked serotoninergic fibers.

Pathway formation and the terminal distribution pattern of the spinocerebellar projection in the chick embryo

Pathway formation and the terminal distribution pattern of spinocerebellar fibers in the chick embryo were examined by means of an anterograde labelling technique with wheat germ agglutinin conjugated horseradish peroxidase (WGA-HRP). Spinocerebellar fibers, which originate in the lumbar spinal cord and are located in the marginal layer of the spinal cord, reach the dorsal part of the cerebellar plate on embryonic day (E)8. On the way to the cerebellum the fibers form one distinct bundle, that suggests that gross projection errors probably do not occur during the formation of the spinocerebellar pathway. On E10, labelled fibers are located mostly in the medullary zone of the anterior lobe. By E12, the number of labelled fibers increases greatly in the inner granular and molecular layers. In transverse sections labelling was distributed throughout the mediolateral extent of the medullary zone. By E14, sagittal strips of labelling were clearly recognized in lobules II-IV; however, labelled terminals were present throughout lobule I. Although the adult pattern of terminal distribution is attained by E14, the mossy fiber terminals are still quite immature. The density of labelling decreased greatly by E16, and small terminal varicosities were first recognized. Structural differentiation of mossy fiber terminals continues to the end of the embryonic or the newly posthatched period.

Immunohistochemical study on the development of serotoninergic neurons in the chick: I. Distribution of cell bodies and fibers in the brain

The ontogenetic development of the serotoninergic system in the embryonic as well as in posthatching chick brain was studied with an indirect immunohistochemical technique with the aid of a specific antibody to serotonin (5-hydroxytryptamine). By embryonic day 4, rostral and caudal groups of serotonin-immunoreactive cell populations appeared in the mesencephalon and rostral and caudal rhombencephalon. At this stage, the rostral group had a considerable number of labelled cells that sent axons toward more rostral parts of brain, whereas the caudal group consisted of a small number of scattered serotonin-immunoreactive cells. The number of serotonin-positive cells increased with development, such that by embryonic day 8 almost all the serotoninergic cell groups found in the adult chick were already present. Serotoninergic-positive cells appeared in the paraventricular organ of the diencephalon as early as embryonic day 10. Judging from the cytoarchitectural organization of serotonin-immunoreactive cells, all of the serotoninergic cell groups in the chick brain seemed to be fully developed by embryonic day 16. On embryonic day 4, serotonin-immunoreactive fibers were found to enter into the marginal layer of the mesencephalon. Subsequently, serotonin-positive fibers ascended in the marginal layer of the brainstem up to the levels of the diencephalon and to the telencephalon on embryonic day 6 and 8, respectively. Serotonin-positive fibers, which first began to penetrate into the mantle layer on embryonic day 8, reached to the rostral pole of the telencephalon by embryonic day 10. In general, serotonin fibers were found in almost all brain regions by embryonic day 16. However, "terminal formation" in some nuclei did not seem to begin until the late embryonic or posthatching period. These observations indicate that the initial development of serotoninergic cell groups occurs during the first half of the 20th day of the incubation period of the chick. However, a longer time, ranging from early embryonic to posthatching stages, is necessary for the complete development of the serotoninergic projections.