The chemoarchitecture of the forebrain of lampreys: Evolutionary implications by comparisons with gnathostomes

The distribution of substance P-, leucine-enkephalin-, tyrosine hydroxylase -, and serotonin-like immunoreactivities was studied in the forebrain of th e silver lamprey and compared to other vertebrate groups. In silver lamprey s, substance P and leucine-enkephalin provide a clear distinction between t he pallium and subpallium as they do for gnathostomes. The pallium consists of three subdivisions which show striking similarities to the medial, dors al and lateral pallia of gnathostomes. Boundaries of the preoptic area, the hypothalamus and posterior tubercle can be well defined by histochemical c haracteristics. These data refute the existence of a dorsal hypothalamic su bdivision in silver lampreys and suggest rostral and caudal divisions of th e hypothalamus.