Organization of alpha-transducin immunoreactive system in the brain and retina of larval and young adult Sea Lamprey (Petromyzon marinus), and their relationship with other neural systems.

We employed an anti-transducin antibody (Gαt-S), in combination with other markers, to characterize the Gαt-S-immunoreactive (ir) system in the CNS of the sea lamprey, Petromyzon marinus. Gαt-S immunoreactivity was observed in some neuronal populations and numerous fibers distributed throughout the brain. Double Gαt-S- and opsin-ir neurons (putative photoreceptors) are distributed in the hypothalamus (postoptic commissure nucleus, dorsal and ventral hypothalamus) and caudal diencephalon, confirming results of García-Fernández et al. (Cell and Tissue Research, 288, 267-278, 1997). Singly Gαt-S-ir cells were observed in the midbrain and hindbrain, increasing the known populations. Our results reveal for the first time in vertebrates the extensive innervation of many brain regions and the spinal cord by Gαt-S-ir fibers. The Gαt-S innervation of the habenula is very selective, fibers densely innervating the lamprey homologue of the mammalian medial nucleus (Stephenson-Jones et al., Proceedings of the National Academy of Sciences of the United States of America, 109, E164-E173, 2012), but not the lateral nucleus homologue. The lamprey neurohypophysis was not innervated by Gαt-S-ir fibers. We also analyzed by double immunofluorescence the relation of this system with other systems. A dopaminergic marker (TH), serotonin (5-HT) or GABA do not co-localize with Gαt-S-ir neurons although codistribution of fibers was observed. Codistribution of Gαt-S-ir fibers and isolectin-labeled extrabulbar primary olfactory fibers was observed in the striatum and hypothalamus. Neurobiotin retrograde transport from the spinal cord combined with immunofluorescence revealed spinal-projecting Gαt-S-ir reticular neurons in the caudal hindbrain. Present results in an ancient vertebrate reveal for the first time a collection of brain targets of Gαt-S-ir neurons, suggesting they might mediate non-visual modulation by light in many systems.