Immunoreactivity for progesterone in the giant Rohde cells of the amphioxus, Branchiostoma belcheri.

In higher animals, it is now generally accepted that neurosteroids are steroids that are synthesized in the brain itself, but it remains unclear where, in terms of the phylogeny of chordates, such neurosteroids are first synthesized? We have tried to detect progesterone immunohistochemically in the central nervous system of the amphioxus, Branchiostoma belcheri, an ancient species of chordate. We found immunoreactivity specific for progesterone in the giant neurons known as Rohde cells, at sites that included the perikaryon, in axons and in a thick coarse axon in addition to the gonads. Thus, the present progesterone-like substance appeared to be a phylogenetically ancient and ancestral neurosteroid. Rohde cells are known to be the source of Mauthner cells and to act as interneurons and, therefore, it seems possible that progesterone-like substance might be involved in primitive sigmoid movement, acting as a chemical signal. The presence of progesterone-like substance in amphioxus suggests that animals have evolved by effectively exploiting a rather limited number of active compounds. This report is the first, to our knowledge, to demonstrate that the Rohde cells in the spinal cord of amphioxus have progesterone-like substance, which might be involved in swimming.