Serotonin and GABA are colocalized in restricted groups of neurons in the larval sea lamprey brain: insights into the early evolution of neurotransmitter colocalization in vertebrates.
Colocalization of the classic neurotransmitters serotonin (5-HT) and gamma-aminobutyric acid (GABA) (or the enzyme that synthesizes the latter, glutamate decarboxylase) has been reported in a few neurons of the rat raphe magnus-obscurus nuclei. However, there are no data on the presence of neurochemically similar neurons in the brain of non-mammalian vertebrates. Lampreys are the oldest extant vertebrates and may provide important data on the phylogeny of neurochemical systems. The colocalization of 5-HT and GABA in neurons of the sea lamprey brain was studied using antibodies directed against 5-HT and GABA and confocal microscopy. Colocalization of the neurotransmitters was observed in the diencephalon and the isthmus. In the diencephalon, about 87% of the serotonergic cells of the rostral tier of the dorsal thalamus (close to the zona limitans) exhibited GABA immunoreactivity. In addition, occasional cells double-labelled for GABA and 5-HT were observed in the hypothalamic tuberal nucleus and the pretectum. Of the three serotonergic isthmic subgroups already recognized in the sea lamprey isthmus (dorsal, medial and ventral), such double-labelled cells were only observed in the ventral subgroup (about 61% of the serotonergic cells in the ventral subgroup exhibited GABA immunoreactivity). An equivalence between these lamprey isthmic cells and the serotonergic/GABAergic raphe cells of mammals is suggested. Present findings suggest that serotonergic/GABAergic neurons are more extensive in lampreys than in the rat and probably appeared before the separation of agnathans and gnathostomes. Cotransmission by release of 5-HT and GABA by the here-described lamprey brain neurons is proposed.
Barreiro-Iglesias
Antón
A
Department of Cell Biology and Ecology, Faculty of Biology, University of Santiago de Compostela, 15782 Santiago de Compostela, Spain.
Cornide-Petronio
María Eugenia
ME
Anadón
Ramón
R
Rodicio
María Celina
MC
D000818:Animals; D005075:Biological Evolution; D001923:Brain Chemistry / Q000235:genetics; D018613:Microscopy, Confocal; D009474:Neurons / Q000737:chemistry*; D048428:Petromyzon / Q000235:genetics / Q000378:metabolism*; D012701:Serotonin / Q000032:analysis*; D014714:Vertebrates / Q000235:genetics / Q000378:metabolism; D005680:gamma-Aminobutyric Acid / Q000032:analysis*
D012701:Serotonin; D005680:gamma-Aminobutyric Acid