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 the silver lamprey and compared to other vertebrate groups. In silver lampreys, substance P and leucine-enkephalin provide a clear distinction between the pallium and subpallium as they do for gnathostomes. The pallium consists of three subdivisions which show striking similarities to the medial, dorsal and lateral pallia of gnathostomes. Boundaries of the preoptic area, the hypothalamus and posterior tubercle can be well defined by histochemical characteristics. These data refute the existence of a dorsal hypothalamic subdivision in silver lampreys and suggest rostral and caudal divisions of the hypothalamus.