Review
The human raphe nuclei and the serotonergic system

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Abstract

The raphe nuclei are distributed near the midline of the brainstem along its entire rostro-caudal extension. The serotonergic neurons are their main neuronal components, although a proportion of them lie in subdivisions of the lateral reticular formation. They develop from mesopontine and medullary primordia, and the resulting grouping into rostral and caudal clusters is maintained into adulthood, and is reflected in the connectivity. Thus, the mesencephalon and rostral pons, neurons within the rostral raphe complex (caudal linear, dorsal raphe, and median raphe nuclei) project primarily to the forebrain. By contrast, in the caudal pons and medulla oblongata, neurons within the caudal raphe complex (raphe magnus, raphe obscurus, raphe pallidus nuclei and parts of the adjacent lateral reticular formation) project to the brainstem nuclei and to the spinal cord. The median raphe and dorsal raphe nuclei provide parallel and overlapping projections to many forebrain structures with axon fibers exhibiting distinct structural and functional characteristics. The caudal group of the serotonergic system projects to the brainstem, and, by three parallel projections, to the dorsal, intermediate and ventral columns in the spinal cord. The serotonergic axons arborize over large areas comprising functionally diverse targets. Some projections form classical chemical synapses while many do not, thus contributing to the so-called paracrine or volume transmission. The serotonergic projections participate in the regulation of different functional (motor, somatosensory, limbic) systems; and have been associated with a wide range of neuropsychiatric and neurological disorders. Finally, recent experimental data support the role of serotonin in modulating brain development, such that a dysfunction in serotonergic transmission during early life could lead to long lasting structural and functional alterations.

Introduction

The involvement of serotonergic neurotransmission in a number of neurological and psychiatric conditions has been widely documented in recent years. In parallel, anatomical and developmental studies of the mammalian serotonergic system, including in humans, have significantly enlarged our understanding on its functional organization. This review aims to place experimental data relevant to human neurobiology in the perspective of clinical studies, to better specify the contribution of the serotonergic neurons to brain functions in health and disease.

The raphe nuclei are collections of neurons, with poorly defined cytoarchitectonic limits. They flank the midline, along the rostrocaudal extension of the brainstem, both in animals (Meessen and Olszewsky, 1949, Taber et al., 1960) and human (Olszewski and Baxter, 1954). They contain heterogeneous populations of neurons, with distinct morphologies, projections and neurochemical characteristics. The serotonergic neurons are the major constituents of the raphe nuclei, and most of the modern literature dealing with these nuclei refers to their serotonergic neuronal components. The first histological localization of monoamines was performed by histofluorescence techniques, which revealed the preferential localization of the serotonergic neurons near the midline (Dahlström and Fuxe, 1964). The cellular and ultrastructural analysis of serotonergic neurons was subsequently refined by the development of immunohistochemical techniques for the visualization of either the biosynthetic enzyme, tryptophan hydroxylase (Joh et al., 1975), specific for the serotonergic neurons, or serotonin itself (Steinbusch, 1981), or the serotonin transporter (Ovalle et al., 1995, Sur et al., 1996, Zhou et al., 1996). These techniques revealed a great similarity in the distribution and projections of serotonergic cells for all mammalian species studied so far. A higher proportion of neurons positioned laterally to the midline is a characteristic feature of primate species, including man (Nobin and Bjorklund, 1973, Hubbard and Di Carlo, 1974, Schoefield and Everitt, 1981, Schoefield and Dixon, 1982, Felten and Sladek, 1983, Azmitia and Gannon, 1986, Takahashi et al., 1986, Hornung and Fritschy, 1988). The histological preparation of the material presented in this review has been described in detail in a series of publications describing the cyto- and chemoarchitecture of the human raphe nuclei (Halliday et al., 1988a, Halliday et al., 1990, Tork and Hornung, 1990, Baker et al., 1991a, Baker et al., 1991b). The illustrations of the raphe nuclei histology, their three-dimensional reconstruction and the counts of serotonergic neurons were obtained from serial coronal sections of a normal adult human brainstem alternately stained with the following techniques: Nissl stain for the nuclei, Loyez stain for myelin, tryptophan hydroxylase (TPH)—and tyrosine hydroxylase (TH) immunocytochemistry, to reveal the serotonergic and catecholaminergic neurons, respectively. A complete series of TPH-immunoreacted sections, spaced by 400 μm, were outlined and the location of each immunoreactive neuron recorded and displayed in three-dimensional reconstructions using Neurolucida software (Hornung, 2003). This data set was used for the calculation of the numbers of serotonergic neurons in the raphe nuclei reported in Section 2 (see below). The serotonergic innervation of the human cerebral cortex was studied in biopsies from patients surgically treated for brain tumors (Hornung and de Tribolet, 1995).

Section snippets

Divisions and connections of the human raphe nuclei

The serotonergic neuron clusters may be allocated, on the basis of their distribution and main projections, into two groups: the rostral group, confined to the mesencephalon and rostral pons, with major projections to the forebrain, and the caudal group, extending from the caudal pons to the caudal portion of the medulla oblongata, with major projections to the caudal brainstem and to the spinal cord. This antero-posterior division in two populations has been recently corroborated by genetic

Functional organization of the human raphe nuclei and involvement in pathological conditions

The above description of the serotonergic neurons outlines the principle of organization of this neuronal population: cell bodies clustered around midline brainstem nuclei, two patterns of termination (with small or large varicosities) with partial overlap in both the rostral and the caudal groups. This system has widely distributed projections, capable of broadly modulating the activity of the neurons in all brain divisions. Yet, a closer examination of the serotonergic system reveals separate

Modulatory role of serotonin during brain development

The early synthesis of serotonin and the early growth of the axons in developing raphe neurons, when cell division, migration and differentiation are still prominent in their target areas, have led to the hypothesis that serotonin has a modulatory function in brain development (Lauder, 1993, Turlejski, 1996). The following examples illustrate the mechanisms observed at the cellular, system and behavioral levels. Depletion of serotonin by p-chlorophenylalanine (an inhibitor of tryptophan- and

Summary and conclusions

The development of histological and imaging techniques has demonstrated the great homologies between the organizations of the serotonergic system in many mammalian species including human. With a total number of about half a million in the human raphe nuclei, serotonin neurons have a dense and divergent projection extending to all divisions of the brain, but still with a specific areal and cellular targeting in different circuits of the sensory, motor or limbic systems. Several neurological and

Acknowledgements

The author is grateful to Drs. R.G.H. Cotton and I. Jennings for their generous gift of monoclonal antibody to label 5-HT neurons, to A. Nicolet, C. Bezençon and R. Kraftsik for their excellent technical and scientific assistance, and to Dr. P.G.H. Clarke for his comments on the manuscript. This work was supported by the Swiss National Foundation grants 31-50565.97 and 31-62113.00.

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