An immunohistochemical study of the telencephalon and the diencephalon in a Myxinoid jawless fish, the Pacific hagfish, Eptatretus stouti.

The forebrain of adult hagfishes (jawless craniates, Myxinoidea) displays a unique morphology. The forebrain is thick-walled and well-differentiated cytoarchitecturally but lacks a well-developed ventricular system, and there is a pronounced compression of the entire brain along the longitudinal axis. This combination of characters obscures the boundaries between the major subdivisions of the forebrain; thus, some elementary morphological issues, such as the location of the pallial-subpallial and the telencephalic-diencephalic boundaries, have remained a matter of dispute over the past 100 years. In an attempt to resolve some of these issues, we investigated the chemoarchitecture of the forebrain of the Pacific hagfish, Eptatretus stouti. Because a number of previous studies of other craniates, mainly gnathostomes, have shown that the spatial distribution of some neuroactive substances and enzymes mirrors the main subdivisions of the forebrain, we localized acetylcholinesterase, enkephalins, substance P, tyrosine hydroxylase, and alpha-melanocyte stimulating hormone by means of histochemistry. Surprisingly, our data show that there are very limited chemoarchitectural similarities shared by hagfishes and other craniates. Some striking similarities occur in the organization of the catecholaminergic systems. Hagfishes, as well as other craniates, possess catecholaminergic cell groups at the ventral junction of the mesencephalon and diencephalon that give rise to a catecholaminergic innervation of the basal forebrain. The distribution of all other substances examined is dissimilar to that found in other craniates. In particular, there are many neurons positive for acetylcholinesterase in the pallium; the subpallium contains relatively small amounts of neuroactive peptides; and the highest densities of structures positive for neuroactive peptides and acetylcholinesterase occur in the central prosencephalic nucleus. Therefore, a comparative chemoarchitectural analysis proved to be of limited value in revealing homologies among cell groups of hagfishes, lampreys, and gnathostomes. We conclude that some chemoarchitectural features that appear to be well conserved within gnathostomes, such as the histochemical differences between the pallium and subpallium, may result from evolutionary change early in craniate history.