ReviewOrigin and functional role of the extracellular serotonin in the midbrain raphe nuclei
Introduction
Many of the milestones regarding chemical transmission were first discovered using isolated nerve-muscle preparations. This enabled it to be established that action potentials were generated at the somatodendritic level of the neuron, and conveyed downstream along the axon, eventually triggering the release of the neurotransmitter from the nerve terminal. This traditional view, however, has been somehow challenged with the finding that some transmitters such as the monoamines serotonin (5-hydroxytryptamine, 5-HT), dopamine and noradrenaline are present in the extracellular space of the nuclei containing their cell bodies in the central nervous system (CNS). Does this mean that they are released from their respective somata and/or dendrites? Not necessarily. Monoaminergic neurons form clusters, which are concentrated in small nuclei of the brainstem. However, these nuclei also contain the proximal portion of the axonal processes as well as axonal varicosities and terminals. For this purpose varicosities are defined as axonal swellings endowed with clusters of synaptic vesicles whereas intervaricose segments contain only few vesicles [224]. Therefore, transmitter molecules can also be released from these ‘terminal-like’ structures similarly to what occurs in nerve endings of projection areas.
The last three decades have witnessed an enormous effort to understand the many factors that control the firing rate of neurons of different transmitter systems. However, the role of such factors on the release of transmitters in their cell body areas has received far less attention. This reflects in part the earlier availability of electrophysiological techniques endowed with superior space and temporal resolution that allowed detailed assessments of neuronal function to be made. In the present article, we will review the mechanisms that regulate the release of 5-HT in the rostral raphe nuclei, i.e. dorsal and median raphe nuclei (DR and MnR, respectively). Both nuclei are the origin of the great majority of serotonergic fibers that innervate most forebrain structures of the CNS [22], [136], [143]. The DR of the rat has been extensively studied. It contains ∼10,000–12,000 5-HT neurons, which represents approximately one third of the total number of cells of the nucleus [68], [82], [143]. In contrast, much less attention has been paid to the MnR, probably because only a minor proportion of its cells is serotonergic [329].
Section snippets
Anatomical structure of rostral raphe nuclei
An early histological study by Ramón y Cajal [241] provided the first outline of the neuronal structure of both DR and MnR (Fig. 1). Neurons in the DR showed a relatively condensed arrangement whereas those in the MnR appeared more loosely organized. The internal architecture and cell types found in the DR of different species exhibit relatively little variation, which suggests a feature that is well preserved along evolution. Several immunohistochemical studies have revealed that the raphe
Studies on 5-HT release in raphe nuclei: historical perspective
The first studies on the release of 5-HT in raphe area used an in vitro preparation in which the potassium-elicited release of previously accumulated [3H]5-HT from slices was measured. Such evoked 5-HT release was shown to be reduced by some 5-HT agonists [85] and the removal of Ca2+ ions [155]. The earliest evidence of an in vivo release of 5-HT in the raphe nuclei came from the work of Hery and coworkers [29], [126]. These authors utilized the push–pull perfusion technique in anesthetized
Methodological considerations
Although intracerebral microdialysis is routinely used at present to sample neurotransmitters released to the extracellular space, several limitations of the technique must be taken into consideration for an appropriate interpretation of the results. For instance, the concentration of 5-HT and other monoamine transmitters in the extracellular space is very low because they are readily removed via a reuptake transporter. This is the reason why the tissue/extracellular fluid ratio for 5-HT is
Intrinsic ionic mechanisms
In the pioneering work of Hery and coworkers, the spontaneous release of 5-HT in the raphe area was shown to be calcium dependent [126]. Furthermore, N- rather than L-type voltage sensitive Ca2+ channels seem to be involved in this release [23]. The release of 5-HT in the raphe area was also enhanced after potassium stimulation and markedly reduced after tetrodotoxin (TTX) treatment in vivo but not in vitro [127]. It is generally assumed that the dependence of calcium ions and TTX implies an
Afferent pathways and their receptors
The afferent connections of the raphe nuclei provide the neuroanatomical substrate for the interaction between different brain areas and serotonergic neurons. Elucidation of the neurotransmitters and receptors involved has been and continues to be an active area of research. A general scheme of the main afferent pathways to the DR and the transmitters involved is shown in Fig. 6. Taking into account the widespread implication of 5-HT systems in health and disease, a better knowledge of these
Functional relevance of extracellular 5-HT in the raphe nuclei
Due to several anatomical and physiological characteristics, the overall activity of the raphe-based, ascending serotonergic systems that innervate the forebrain is mainly determined at midbrain level. First, as already mentioned (see Section 2), 5-HT fibers originate from a small number of cell bodies confined to the DR and MnR and spread profusely, giving rise to an extensive arborization of axons in terminal fields [208], [214], [282]. Second, serotonergic neurons fire at a slow and regular
Concluding remarks
Although further research is needed to provide clear-cut evidence of the origin of extracellular 5-HT in the DR and MnR, we have reviewed in detail the multiple factors that regulate the release of 5-HT in the rostral raphe nuclei. These can be distinctly divided into two main classes: intrinsically serotonergic factors and factors involving other neurotransmitter afferents and/or their receptors. Among the former, the most important are the autoreceptors of the 5-HT1A and 5-HT1B subtypes.
Acknowledgements
This work was supported by grants from the CICYT (SAF2001-2133) and Fundació la Marató de TV3. The following drug companies have supported some of the studies reviewed: Bayer S.A. and Lilly S.A. The skilful technical assistance of Leticia Campa is gratefully acknowledged. The authors are also keenly indebted to two anonymous reviewers for their helpful comments on this manuscript.
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