Glutamatergic neuronal populations in the forebrain of the sea lamprey, Petromyzon marinus: an in situ hybridization and immunocytochemical study.

Despite the importance of glutamate as a major excitatory neurotransmitter in the brain, the distribution of glutamatergic populations in the brain of most vertebrates is still unknown. Here, we studied for the first time the distribution of glutamatergic neurons in the forebrain of the sea lamprey (Petromyzon marinus), belonging to the most ancient group of vertebrates (agnathans). For this, we used in situ hybridization with probes for a lamprey vesicular glutamate transporter (VGLUT) in larvae and immunofluorescence with antiglutamate antibodies in both larvae and adults. We also compared glutamate and γ-aminobutyric acid (GABA) immunoreactivities in sections using double-immunofluorescence methods. VGLUT-expressing neurons were observed in the olfactory bulb, pallium, septum, subhippocampal lobe, preoptic region, thalamic eminence, prethalamus, thalamus, epithalamus, pretectum, hypothalamus, posterior tubercle, and nucleus of the medial longitudinal fascicle. Comparison of VGLUT signal and glutamate immunoreactivity in larval forebrain revealed a consistent distribution of positive cells, which were numerous in most regions. Glutamate-immunoreactive cell populations were also found in similar regions of the adult forebrain. These include mitral-like cells of the olfactory bulbs and abundant cells in the lateral pallium, septum, and various diencephalic regions, mainly in the prethalamus, thalamus, habenula, pineal complex, and pretectum. Only a small portion of the glutamate-immunoreactive cells showed colocalization with GABA, which was observed mainly in the olfactory bulb, telencephalon, hypothalamus, ventral thalamus, and pretectum. Comparison with glutamatergic cells observed in rodent forebrains suggests that the regional distribution of glutamatergic cells does not differ greatly in lampreys and mammals.