Aspartate-containing neurons of the brainstem and rostral spinal cord of the sea lamprey Petromyzon marinus: distribution and comparison with γ-aminobutyric acid.

The amino acid L-aspartate (ASP) is one of the most abundant excitatory neurotransmitters in the mammalian brain, but its distribution in other vertebrates has not yet been well characterized. We investigated the distribution of ASP in the brainstem and rostral spinal cord of the adult sea lamprey by using ASP immunohistochemistry. Our results indicate that ASP is accumulated in specific neurons, but not in glia (tanycytes). ASP-immunoreactive neuronal populations were rather similar as the glutamatergic populations reported in the adult sea lamprey (Villar-Cerviño et al. [2013] J Comp Neurol 521:522-557), although some important differences were noted. Characteristically, the largest reticular neurons of the lamprey brainstem (Müller cells) showed ASP immunoreactivity in perikarya and processes, in contrast to the absence or faint glutamate immunoreactivity reported in these perikarya. We also compared the distribution of ASP and γ-aminobutyric acid (GABA) in brainstem neurons by using double immunofluorescence methods. In regions such as the midbrain tectum, dorsal isthmus, and motor nuclei, ASP and GABA immunoreactivity was mostly located in different neurons, whereas in other nuclei (torus semicircularis, octavolateralis area, parvocellular reticular formation), many of the ASP-immunonegative neurons displayed colocalization with GABA. These results, together with those of our previous studies of colocalization of glutamate and GABA, suggest that some lamprey neurons may co-release both excitatory and inhibitory neurotransmitters. Further investigation is needed to elucidate the pathways of uptake and release of ASP by ASP-immunoreactive neurons. Our results indicate that ASP is a neurotransmitter in the central nervous system representative of agnathans, the earliest vertebrate group.