New insights on the neuropeptide Y system in the larval lamprey brain: neuropeptide Y immunoreactive neurons, descending spinal projections and comparison with tyrosine hydroxylase and GABA immunoreactivities.

Lampreys are useful models for studying the evolution of the nervous system of vertebrates. Here we used immunofluorescence and tract-tracing methods to study new aspects of the neuropeptide Y-immunoreactive (NPY-ir) system in larval sea lampreys. NPY-ir neurons were observed in brain nuclei that contain NPY-ir cells in other lamprey species. Moreover, a group of NPY-ir cells that migrated away the periventricular layer was observed in the lateral part of the dorsal hypothalamus, which suggests a role for NPY in feeding behavior in lampreys. We also report NPY-ir cells in the dorsal column nucleus, which appears to be unique among vertebrates, and in the habenula. A combination of tract-tracing and immunohistochemical labeling demonstrated the presence of spinal projecting NPY-ir reticular cells in the anterior rhombencephalic reticular formation, and the relationships between the NPY-ir system and the reticulospinal nuclei and some afferent systems. The colocalization of catecholamines and GABA in lamprey NPY-ir neurons was investigated by double immunofluorescence methods. Colocalization of tyrosine hydroxylase (TH) and NPY immunoreactivities was not observed in any brain neuron, although reported in amphibians and mammals. The frequent presence of NPY-ir terminals on TH-ir cells suggests that NPY modulates the activity of some dopaminergic nuclei in lampreys. Colocalization of NPY and GABA immunoreactivities was frequently observed in neurons of different rhombencephalic and diencephalic NPY-ir populations. These results in lampreys suggest that the coexpression of NPY and GABA in neurons appeared early on in the brains of vertebrates.