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| Jean-Pierre Hornung |
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Institut de Biologie Cellulaire et de Morphologie, Rue du Bugnon 9, 1005 Lausanne, Switzerland
e-mail: jean-pierre.hornung@ibcm.unil.ch |
Presentation: |
| 2002-10-06, 11:30-12:10 |
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| Raphe nuclei – The serotonin system. |
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The raphe nuclei are organized along the midline of the entire rostrocaudal extent of the brainstem tegmentum, except for one, the dorsal raphe nucleus, which lies in the ventral quadrant of the periaqueductal gray. The raphe nuclei contain a mixed neuronal population. Although the term raphe is often used to name the nuclei of origin of the serotonergic system, one should bear in mind that only a proportion varying from 30 to 80 percent of the neurons of the raphe nuclei synthesize serotonin, and also that some serotonergic neurons are located in paramedian and lateral divisions of the reticular formation. The raphe nuclei are subdivided in two groups. The rostral group, with efferent projections mainly directed rostrally, includes the caudal linear (CLi), the median raphe (MnR) and the dorsal raphe (DR) nucelus. The caudal group, projecting to the caudal brainstem and spinal cord, is constituted by the raphe magnus (RMg), the raphe obscurus (ROb), the raphe pallidus (RPa) nuclei and a collection of neurons in the lateral reticular formation of the myelencephalon. The axonal arborization of a single serotonergic neuron could cover one or several large territories overlapping with different functional subdivisions. This feature leaded to the concept of diffuse system shared with the other aminergic projections of the brain. However, one should be aware that within the serotonergic system, subdivisions of raphe nuclei have distinct afferent and efferents connections. An alternative concept to the entity of a single chemically-defined serotonergic system would be a parcellation and integration of raphe nuclei or subnuclei in functional systems (such as pain or muscle tone control, or central autonomic regulation). The concept of diffuseness of the serotonergic projection was reinforced by the observation that the varicosities along the terminal axonal branches, where serotonin is released from vesicular stores, were not associated with morphologically differentiated chemical synapses, except for a population of large size serotonergic varicosities such as those of the cortical and hippocampal MnR projections. At the cellular level, the complexity of serotonin neurotransmission is potentiated by the existence of more than 15 types of serotonin receptors, each with specific localization and effector pathways.
The mature serotonergic function could be altered in number of pathological conditions such as ischemia affecting selectively midline region of the brainstem, increased cell death of serotonergic neurons in certain neurodegenerative diseases, or selective neurotoxic effects following chronic elevated alcohol consumption or repetitive use of drug of abuse such as metamphetamines (ectasy). Finally alteration of serotonergic transmission during development has been shown to affect brain differentiation and maturation. The study of transgenic animals with deficient serotonin neurotransmission revealed abnormalities in the axonal arborization of several central sensory pathways, a feature associated with the transitory expression of the cytoplamic serotonin transporter by neurons of these pathways. During the embryonic development, similar transitory expression of these transporters has been reported in non-human primates and in human, suggesting that at least some of the phenotype regulated by serotonin in experimental mammalian species during development is also prone to alteration during embryogenesis in human. |
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