Immunocytochemical localization of dopamine and its synthetic enzymes in the central nervous system of the lamprey Lampetra fluviatilis.

The distribution of dopamine (DA)-containing cell bodies, fibers, and terminals in the brain and spinal cord of Lampetra fluviatilis was investigated by immunohistochemical means. In order to distinguish dopaminergic neurons from those using other catecholamines as the primary neurotransmitter, the distribution of dopamine-immunoreactive structures was compared to that of cell bodies, fibers, and terminals labelled with antibodies directed against the enzymes tyrosine hydroxylase (TH), aromatic L-amino acid decarboxylase (AADC), dopamine beta-hydroxylase (DBH), and phenylethanolamine-N-methyl transferase (PNMT). We define dopaminergic neurons as those that are simultaneously DA, TH, and AADC immunoreactive and at the same time DBH and PNMT nonreactive. The overall concentrations of dopamine, noradrenaline, and adrenaline and some of their metabolites were also measured via high-performance liquid chromatography of whole-brain extracts. Our results revealed the presence of 10 populations of dopaminergic neurons in the brain of the lamprey in the olfactory bulb, preoptic area, hypothalamus, rhombencephalon, and spinal cord. In addition, uniquely DA-immunoreactive neurons, in contact with the cerebrospinal fluid, were observed in the hypothalamus and spinal cord. Chromatography indicated that dopamine exists in considerably higher concentrations than noradrenaline in the lamprey brain, whereas adrenaline is absent, the latter finding being supported by our failure to observe any PNMT-immunoreactive cell bodies, fibers, or terminals. The dopaminergic system of the lamprey appears to share many features not only with that of other anamniotes but also with that of amniotes; however, as in teleosts, dopaminergic neurons in the midbrain corresponding to the substantia nigra, the retrorubral area, and the ventral tegmental area of other species do not exist in the lamprey.