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Comparative analysis of the serotonergic systems in the CNS of two lungfishes, Protopterus dolloi and Neoceratodus forsteri


The organization of the serotonergic system, one of the most important neurotransmitter systems in the brain, has been carefully analyzed in most vertebrate groups, and major shared characteristics have been described, although traits particular to each vertebrate class have also been found. The present study is the first that provides a comprehensive and detailed map of the serotonergic structures in the brain of two representative species of lungfishes, the African lungfish (Protopterus dolloi) and the Australian lungfish (Neoceratodus forsteri), as revealed by immunohistochemistry against serotonin (5-HT). Lungfishes are currently considered the closest living relatives of tetrapods and represent an interesting group for the study of evolutionary traits in the transition from fishes to tetrapods. Distinct groups of serotonin immunoreactive cells were observed in the preoptic area, nucleus of the periventricular organ, pretectum, optic tectum and the long column of the raphe. Fiber labeling was widely distributed in all main brain subdivisions but was more abundant in regions such as the striatum, septum, amygdaloid complex, preoptic area, suprachiasmatic nucleus, lateral hypothalamic area, prethalamus, thalamus, mesencephalic tegmentum and rhombencephalic reticular formation. Comparison of these results with those from other classes of vertebrates highlights numerous common traits shared by most groups of fishes but also reveals that the serotonergic system in lungfishes largely resembles those of amphibians and other tetrapods.

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Cerebellar auricle


Anterior commissure


Octavolateral area


Bed nucleus of the stria terminalis


Corpus cerebelli


Cerebellar commissure


Central canal


Central amygdala


Dorsal part of the thalamus


Dorsal column nucleus


Dorsal funiculus


Dorsal pallium




Fasciculus longitudinalis medialis


Fasciculus retroflexus


Ganglion cell layer of the retina


Glomerular layer of the olfactory bulb




Habenular commissure




Internal granular layer of the olfactory bulb


Inner nuclear layer of the retina


Interpeduncular nucleus


Inner plexiform layer of the retina


Interpeduncular neuropile


Isthmic nucleus


Juxtacommissural pretectal nucleus


Lateral amygdala


Locus coeruleus


Laterodorsal tegmental nucleus


Lateral funiculus


Lateral forebrain bundle


Lateral pallium


Lateral septum


Median part of the thalamus


Mammillary region


Mauthner cell


Mauthner cell axon


Medial amygdala


Mitral layer of the olfactory bulb


Medial pallium


Dorsal part of the medial pallium


Ventral part of the medial pallium


Medial septum


Facial nerve


Neoceratodus forsteri


Anterior nerve of the lateral line


Nucleus of the periventricular organ


Spino-occipital nerve


Nucleus of the solitary tract


Vagal nerve


Olfactory bulb


Optic chiasm


Outer nuclear layer of the retina


Optic tectum






Purkinje cells of cerebellum


Precommissural pretectal nucleus


Protopterus dolloi

p 1–3:

Prosomeres 1–3


Preoptic area





r 0–8:

Rhombomeres 0–8


Inferior raphe nucleus


Median raphe nucleus


Superior raphe nucleus


Inferior reticular nucleus


Dorsal infundibular recesses


Inferior dorsal infundibular recesses


Retromammillary region


Median reticular nucleus


Superior reticular nucleus




Suprachiasmatic nucleus


Subcommissural organ


Granular layer of the cerebellum


Stratum griseum centrale of the mesencephalic tectum


Stratum griseum periventriculare of the mesencephalic tectum


Somatomotor neurons of spinal cord


Molecular layer of the cerebellum


Superficial nucleus


Substantia nigra


Spino-occipital motor nucleus


Solitary tract


Stratum opticum of the mesencephalic tectum




Supraoptoparaventricular nucleus








Torus semicircularis


Posterior tubercle


Tuberal hypothalamus


Ventral part of the thalamus




Ventral funiculus


Ventral pallium


Ventral tegmental area/substantia nigra complex


Oculomotor nucleus


Trochlear nucleus


Trigeminal motor nucleus


Facial motor nucleus


Glossopharyngeal motor nucleus


Vagal motor nucleus


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We deeply thank Dr. Glenn R. Northcutt for providing the brains of Neoceratodus forsteri and for his continuous help and advise during many phases of the research. This research was supported by the Spanish Ministry of Science and Technology (Grant numbers: BFU2009-12315/BFI and BFU2012-31687).

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The authors declare that they have no conflict of interest.

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Correspondence to Agustín González.

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López, J.M., González, A. Comparative analysis of the serotonergic systems in the CNS of two lungfishes, Protopterus dolloi and Neoceratodus forsteri . Brain Struct Funct 220, 385–405 (2015).

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  • Serotonin
  • Immunohistochemistry
  • Hypothalamus
  • Raphe
  • Lungfish
  • Brain evolution